Investigation into crosslinking kinetics, physical properties, and thermal conductivity of humic acid epoxy composite

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This paper seeks by a series of FT-IR spectroscopy, gravimetry, and DSC experiments to address the curing behavior, thermo-physical properties (glass transition temperature ( T g)), thermal conductivity (TC), and moisture absorption (MA) of bio-composites made of commercial epoxy resin (E) with humic acid (HA). It is observed that in almost all cases, the prepared composites (HAE) have similar curing behavior to that of (E) with higher activation energy, T g, and MA. In addition, TC decreases for 5 and 10 wt% HAE, while it increases for 20 wt% HAE. Conversely, by increasing the amount of humic acid, the T g and MA ability of the composites increased. Concerning TC, the presence of HA at 5 and 10 wt% results a slight decrease in the TC of the composites. A bio-based and valuable composite have been produced from epoxy and humic acid by commonly simple mixing process which might be easily transferable to commercial production process. Epoxy composite humic acid cross-linking kinetics thermal conductivity DSC Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Due to their advantages combined properties, such as high adhesion strength, low shrinkage, good chemical and thermal resistance, and low volatile emission, epoxy-based composites have been widely studied for decades and are presently being considered as one of the most materials used for industrial applications. Bio-based-fillers or bio-based-reinforced composites are now utilized in building and construction, furniture, food packaging, automotive parts, and more. However, for more economic and sustainability concerns, low CO 2 emission, and high performance, there is a lot of academic research and considerable efforts supported by both industrial companies and government agencies have been made to replace commercial composites with composites that contain renewable resources. Bio-composites that contain at least one ingredient in their main composition derived from natural resources could be the alternative materials for artificial fibers or artificial fillers reinforced-composites [1, 2]. In spite of, the bio-based composites offer many advantages, their development and application still face some challenges and some remarkable issues due to bio-degradability, incompatibility of natural reinforcements with most polymer matrices, and limited water resistance. These limitation factors are due to the hydrophilic nature of fiber or fillers, where they tend to absorb relativity more amount of water from humid environments. As a result of water absorption, the reinforcements will swell and cause poor interfacial bonding between fibers or fillers and polymer matrix which directly deteriorates the mechanical properties of the composites. Incompatibility between the matrix and additives, inferior mechanical performance compared with their neat counterparts. Hygroscopicity and recycling of bio-reinforcements are considered critical parameters that constrain their use as reinforcement for polymeric composites [3]. In the last decade, interest in the utilization of natural materials as additives or reinforcements in composites to reduce the use of artificial additives has been increasing for various industrial applications. Natural fillers and other agricultural byproducts are potential materials for composites. The combination of commercial epoxy resins with bio-fillers shows many advantages including using renewable and inexpensive additives, reducing the cost, availability of renewable resources, and low environmental issues caused by conventional petroleum-based materials, and also improving the processing or final properties of the composites [4–8]. Recently, in addition to plant fibers, natural products or food processing byproducts such as husks, walnut shell powder, lignin, tannins, and others are extensively studied as reinforcements for bio-composites [9–12]. Among many natural products, humic acid (HA) as a natural organic material has attracted considerable academic and agricultural interest. Humic acid is a macromolecule obtained by the natural biological and chemical degradation process of biomass for long periods. The supramolecular architectures with carbon-enriched chemical structures, and plentiful oxygen-containing functional groups (Fig. 1 ) endows humic acid with excellent chelating ability to different ions [13]. Also, HA was evaluated as a charring and flame retardant agent in polymers and epoxy resins in some studies [14–15]. The three-dimensional structure of humic acid based on aliphatic and aromatic units is stabilized by hydrogen, weak hydrophobic, and metal-bridged electrostatic bonds. Reports concerning the utilization of humic acids as fillers or additives in a mixture with thermosetting polymers are scarce and account for only a small percentage of the total bio-based fillers for polymers and composites. A good example of that is the work of Virginia Venezia et al. [16] in which they evaluate the effect of humic acid as a flame retardant additive on the thermal, fire, and mechanical performance of epoxy resin modified with (3-aminopropyl)-triethoxysilane. In this work, it was possible to observe a strong improvement in thermal stability and self-extinguishing capability of the epoxy system by incorporation of humic acid coupled with ammonium polyphosphate and urea and also melt dripping of the epoxy system can prevent during the vertical flame spread tests in the presence of humic acid. Liu Gaungya [17] could develop an environmentally friendly flame retardant with high efficiency through the modification of humic acid with four different metal ions (Fe, Mn, Al, and Cu) and mixing the obtained complex at about 10% wt. with epoxy resin as a polymeric matrix. It was found that the limiting oxygen index (LOI) of the epoxy matrix was increased from 21.2% for pure epoxy resin to 25.3–26.6% for epoxy-metal humates composites. They attributed the effect of metal humates as flame retardant in the epoxy matrix to catalytic charring of this complex which produces char layers and retard the transfer of heat between the matrix and flame zone. Mattia Di Maro et al. [18] prepared composites of ethylene-vinyl acetate (EVA) copolymer incorporated with bio-char which is produced from hemp hurd through a pyrolytic process and commercial humic acid (as sodium salt). The effect of biomas-derived flame retardant formulations (biochar and humic acid) on the fire, morphological features, and thermal and mechanical properties of the composites were thoroughly evaluated. The results of this study show that the thermal and thermo-oxidative stability of the polymeric matrix enhanced in the presence of bio-char, while the addition of humic acid to the mixture can significantly reduce Young's modulus and slightly decrease both peaks of heat release rate and total heat release by 16% and 5%, respectively, with no effect on the degree of crystallinity of the EVA copolymer. All authors of the above studies do not investigate the effect of humic acid on the curing profile of epoxy resin, more over there have been no attempts to determine the kinetics parameters of the production process. In a previous paper [19], humic acid was used as raw material to prepare a bio-based thermal stabilizer for PVC. Calcium and zinc humates were synthesized starting from potassium humate through a single-step reaction with calcium hydroxide and with zinc sulfate, and both the obtained salts were mixed with PVC at different concentrations. The results show that, unlike zinc humate, calcium humate shows a significant stabilization effect and interesting activity for the stabilization of PVC. In general, composites reinforced with plant fibers are more studied than those with natural fillers. In addition, to the best of the author's investigations, most of the previous studies which were carried out on humic acid as a potential filler in epoxy-based composites have never been focused on the curing behavior, kinetics, and physical properties of the produced composites. Moreover, there have been no attempts to investigate of thermal-physical properties, water absorption, and thermal conductivity of the final products. Therefore, the aims of this work are: (i) applied of humic acid as a bio-based filler to prepare a thermosetting composite and (ii) Additionally, the characteristics are investigated to understand the effect of humic acid as an active and high molecular weight filler on the physical, thermal properties, and thermal conductivity of the prepared composites. Materials and Methods 2.1. Materials used and composite fabrication Commercial epoxy resin (hexanediol diglycidyl ether) and it curing agent (a modified cycloaliphatic amine (isophorone diamine) supplied by Fosroc Chemicals limited (Staffordshire. B783TL, UK) were used in this work as a matrix. The epoxy-to-hardener ratio used here was the one recommended by the manufacturer, 100:40 mass units. Humic acid (potassium salt) in granule form was purchased from Humintech GmbH (Germany). The potassium salt of humic acid was dissolved in 25 ml distillation water with a magnetic stirrer for 5 min, then HCl 5% solution was added drop by drop with a magnetic stirrer until the pH of the mixture reached 2–3 at which the pure humic acid will precipitate, filtrated, and washed several times with distillation water. The sample preparation steps for composite samples are presented in Scheme 1 . Pristine epoxy resin without humic acid (noted as 0HAE) was prepared by following the steps reported elsewhere [20]. Firstly, according to the mas ratio 10:4, epoxy resin (A) and its hardener (C) were mixed at room temperature for 5 min, and then the resulting viscous mixture was divided into two parts, the first part was transferred directly to thermal analysis. The second one was poured into a circular plastic container to get a cylindrical shape through the curing and post- curing process. The curing process was carried out at 60 o C for 1 hour, followed by the post-curing process at which the samples were kept for 1 hour at 100 o C, and then at 130 o C for 1 hour. Table 1 shows the formulations of epoxy resin with its hardener and with humic acid. The mixture of epoxy resin with humic acid at mixing ratios 5, 10, and 20% (noted as 5HAE, 10HAE, and 20HAE, respectively) was prepared according to this sequence presented in Scheme 1 . Liquid epoxy resin and humic acid powder in different weight proportions were well stirred at room temperature in a plastic container for 5 min, after the homogeneity was observed in the mixture; the amount of hardener was added and finally re-mixed for 5 min. The resulting systems were directly submitted to DSC tests before curing. Table 1 Formulation of humic acid/epoxy composites at different HA contents. Composite Epoxy resin (g) Curing agent (g) Humic acid (g) (wt.%) 0HAE 1 0.4 0 0 5HAE 1 0.4 0.05 5 10HAE 1 0.4 0.1 10 20HAE 1 0.4 0.2 20 Characterization FT-IR Measurements Fourier-transformed infrared (FTIR) transmittance spectra of HA, cured E, and cured specimens were analyzed using a Jasco-400 spectrometer (Japan). All the spectra were taken over the range of 400–4000 cm − 1 and with a resolution of 4 cm − 1 and 32 scans. Sample of HA was ground and mixed with KBr powder in an agate mortar and pressed to get a fine pellet, while neat E and its composite were tested after full curing as films which were prepared by the casting method according to these steps: (1) non-cured system (pure epoxy-hardener mixture and epoxy-humic acid-hardener) was mixed and poured on a parchment paper and pressed between another layer from parchment paper to get thin-sheets (about 0.01 mm thickness). (2) The samples were submitted to a thermal curing program in an electrical oven at 60 o C for 1 hour, 100 o C for 1 hour, and then post-curing at 130 o C for 1 hour. The obtained sheets were examined by FTIR spectrophotometer. Curing process and physical properties The differential scanning calorimetry (DSC-60, Shimadzu, Japan) was employed to investigate the curing conditions and curing parameters of all composite formulations prepared in the work. Pure epoxy resin was mixed with HA for 5 min then with the curing agent for 5 min. Samples with a mass of 10–12 mg of each prepared formulation were used for thermal analysis. The samples were heated from RT to 250 o C with a scan rate of 20 o C/min under nitrogen atmosphere flows at 30 mL min − 1 . The curing parameters such as initial, optimum, and final curing temperatures, and curing enthalpies were determined from DSC curves using the DSC softwear. To determine the glass transition temperature ( T g) and the curing degree, sample of the fully-cured HAE systems were tested using a DSC scan from 30–200 o C at a heating rate of 20 o C min − 1 and under nitrogen flow of 30 ml min − 1 . Curing kinetics analysis The kinetics of the curing process of the non-polymerized composite was determined using DSC. Specifically, one non-polymerized composite sample, namely, 20HAE was selected to determine the kinetics parameters of humic acid-epoxy composites, and the results were compared with that of pure epoxy resin obtained from literature. The cure reaction kinetics of the 20HAE composites were monitored in non-isothermal mode by the DSC calorimeter. The DSC experiments were performed using a DSC-60 (Shimatzu, Japan). In each DSC run, a small sample mass with a total weight in the range of 10–12 mg of uncured composite was placed in a standard DSC aluminum pan. Each sample was scanned with a temperature range from 25 to 250 o C and at different heating rates, i.e. , 10, 15, 20, and 25 o C min − 1 , continuous flow of dry nitrogen gas purge at a flow rate of 30 ml min − 1 was maintained throughout all experiments. Thermal conductivity The thermal conductivity of the full-polymerized composite samples was measured using DSC following the method of Hakvoort and van Reijen [21] and Flynn and Levin [22]. Pure indium (melting enthalpy = 28.15 J/g; melting temperature = 156.4 o C) was used as a censor for the determination thermal resistance of the composite systems. In this method, the DSC instrument measures the melting behavior of a pure metal placed on the top of the samples. All composite samples for thermal conductivity were prepared by heating the samples in an oven for 1 hr at 100–110 o C to get softening over the T g point then the hot materials were bunched by a leather belt bunch tool. At least two samples for each formulation were prepared. Each sample was a circular disk of polymerized composite system with 4.5 ± 0.1 mm in diameter and a height of 2 ± 0.2 mm. The weight of each sample after bunching was found in the range of 30 ± 0.5 mg. The experimental setup consist of the following two steps: In the first step, an indium metal pellet (15 mg) was placed in the DSC pan, and the melting peak was determined. In the second step, the composite sample with an aluminum crucible was placed in the DSC furnace. An aluminum crucible with a calibration substance (In) was then placed on top of the composite sample, Fig. 2 A. An empty aluminum pan with its aluminum lid was placed in the reference position of the DSC cell. A DSC curve was recorded during the melting of the reference (Fig. 2 B). Nitrogen was used as the purge gas at a rate of 30 mL min − 1 in all tests. The thermal conductivity ( K ) of the samples can be determined from the slope of the melting peak of the sensor both with and without sample, Eq. 1[23]. $$\:K=\frac{\varDelta\:\varPhi\:}{\varDelta\:T}x\frac{h}{A}\dots\:\dots\:\dots\:\left(1\right)$$ Where ΔΦ is the difference between the heat flow (mW) at a melting peak and the heat flow at the beginning of the melting behavior of the sensor, ΔT ( o C) is the difference between the temperature at the melting peak and the temperature of the onset melting behavior. h is the sample height (mm), A is the cross-section area (mm 2 ). Both ΔΦ and ΔT can calculate from the slope of the linear side of the melting curve of the sensor above the sample. High ( h ) and cross-section area ( A ) of the samples were measured with an electronic digital caliper. Moisture content in the composite systems Moisture absorption tests of the composite systems were carried out following ASTM D5229-29M [24, 25]. The basis of this approach is a gravimetric technique by which the dry specimens are weighed before and after being exposed to a high humidity and wet environment at room temperature and under atmospheric pressure for two days. At least two specimens were prepared and tested for each composite formulation. Before the experimentation, the samples were placed in an oven for 24h at 103 o C. Each dry sample was weighed ( W dry ) using a precision scale with a 0.0001 g accuracy. Immediately after the weighing of the composites, they were placed over filter papers which covered plastic cups containing distilled water. Then each sample was covered with another plastic cup to imitate high humidity and wetness conditions. After 24 hours, the composite specimens were periodically taken from the cups and re-weighed to determine the amount of water that they absorbed ( W wet ). Composite specimens were retained in cups and weighed regularly after 48 hours. The percentage of moisture content ( M % ) is measured using the following formula (Eq. 2). $$\:M\left(\%\right)=\frac{Wwet-Wdry}{Wdry}x\:100\%\dots\:\dots\:\left(2\right)\:$$ Where M is the percentage of moisture content, W dry is the mass of the composites before exposition to humid condition (dry), and W wet is the mass of sample after exposition to the humid conditions (humid). Results and Discussion 3.1. FTIR analysis DSC and FTIR analysis were carried out to understand if there is any new bond can be formed between the humic acid and epoxy matrix during the polymerization reaction. The final structures of neat epoxy and its composite with humic acid were cured in an oven to obtain specimens for FTIR analysis. FTIR spectra of pure humic acid (HA), the cured neat epoxy resin (E), and the cured humic acid/epoxy composite containing 20 wt% humic acid (20HAE) were presented in Fig. 3 . In the humic acid spectrum (Fig. 3 A) the broad transmittance band at 3300–33400 cm − 1 is related to hydrogen-bonded hydroxyl groups of carboxylic, phenol, and alcohol stretching. The intense band at wavenumber around 1709 cm − 1 is assigned to the stretching of carbonyl in carboxylic acid groups. Moreover, the observed peak at 1609 cm − 1 corresponds to aromatic skeletal vibration and confirms the presence of aromatic units in the structure of HA [26]. The peak that appears at 1240 cm − 1 may attributed to the C–C, C–O, and C = O stretching. The high-density peak at 1030 cm − 1 in the spectrum of HA could be observed, produced mainly by aromatic C–H in-plane deformation, stretching vibration of C–O in primary alcohols, and C = O stretch. In the spectra of epoxy resin and humic acid-epoxy composite, the signal at 918 cm − 1 associated with oxirane rings is still present, indicating an inefficient curing process that was carried out in an oven at 130 o C. At the spectra of pure epoxy resin and its composites, the peak denoted at 1200 cm − 1 corresponds to the stretching of aliphatic ether which was connected with the polymerization of epoxy groups. The pure humic acid and its composite with epoxy resin have very similar FTIR spectra. The FTIR of the composite system reveals the presence of characteristic absorption peaks for the main functional groups in humic acid and epoxy matrix. the absorption bands at wave numbers at around 3300–3400, 1709, 1240–1246, and 103–1040 cm − 1 are characteristic bands for O–H stretching, C = O stretch in unconjugated ketone, carbonyl and in ester and carboxylic acid groups, C–C, C–O and C = O stretching, aromatic C–H in-plane deformation, C–O deform in primary alcohols and C = O stretch, respectively, for humic acid molecules in the composite system. And those at 3062, 2932, 1608, 1508, and 1036–1040 cm − 1 are assigned to stretching of C–H of the oxirane ring, bending vibration of –C–H in the methylene group, stretching vibration of –C = C in the benzene ring, C–O–C stretching, and stretching C–O–C of ethers for epoxy resin in the composite system, respectively. Despite the high humic acid content in epoxy composite (20%), there is a high comparison between the FTIR spectrum of neat epoxy resin and humic acid/epoxy composite. The FTIR of the composite system suggests no formation of a chemical bonding during the curing process. A possible physical mixture between humic acid and epoxy without significant change in the structure of components 3.2. Crosslinking and thermal characterization The curing behavior of 20HAE composite was studied by DSC analysis. Figure 4 shows the change of the heat flow to the temperature of the curing reaction for humic acid-epoxy composites (5HAE, 10HAE, and 20HAE). Thermal parameters about the curing reaction as the initial curing temperature ( T start ), the peak or maximum temperature ( T max ), the final temperature ( T end ) at which the complete curing, and the total heat of the curing reaction (ΔH) for the sample were calculated from the DSC analyses and all the values summarized in Table 2 . Curing data from the literature were taken for pure epoxy resin because this epoxy resin has already been subject to previous investigations [19, 27]. In Fig. 4 , it can be seen that, the presence of humic acid does not produce any dilation effect to the curing process even when it is at high proportion in the formulation. All the DSC curves of all composite systems have similar shapes. A high exothermic heat flow observed in all samples corresponded to the reaction of an amine with epoxy groups. The addition of humic acid leads to practically the same curing temperatures as the pure epoxy resin, which means that the humic acid has an inert effect in the reaction of epoxy resin with amine. This peak appears in the temperature range of 70–155 o C. This event is ascribed to the curing reaction by which the oxirane rings will open and form high molecular weight and crosslinking networks. The curing temperature ranges for all composite systems are consistent with the usual curing ranges of most composite processing [28, 29]. Conversely, further exothermic or endothermic reactions cannot be observed at higher temperatures. For all composite systems, the curing reaction begins at the same temperature ( T i), and the exothermic peak ( T max ) is equal confirms the similar reactivity of these systems compared to the epoxy resin. The curing parameters calculated for humic acid-epoxy composite (Table 2 ) and peak shapes (Fig. 4 ) show no significant variation with humic acid content. This result confirms that the reaction of epoxy with amine during network formation is not influenced with the presence of the humic acid. This can be explained by the humic acid does not have any catalytic or retardation effect to the curing reaction of the epoxy resin. Figure 4 also shows that the curing and post curing reactions can be completed by heating up to 155 o C in all formulations. Upon continues heating to higher temperatures, the composite systems show chemical and thermal stability, so no reaction or thermal degradation occurs during this stage. Table 2 Calorific characteristics of HAE composite from DSC curves Composites First heating cycle Curing enthalpy for neat epoxy content in total sample mass (J/g) Second heating cycle T a start ( o C) T a max ( o C ) T a end ( o C ) Total curing enthalpy (J/g) T c g ( o C) 0HAE * 66 119 161 61.85 61.85 84 5HAE 66 119 157 49.89 52.63 95 10HAE 66 119 160 44.65 49.61 94.5 20HAE 68 119 156 39.25 49.06 104 a the start, the end, and the maximum temperatures of curing peaks. b The Tg values were determined from the midpoint of glass transition region obtained by 2nd DSC scan. * Literature data [19, 27]. The decrease in the curing enthalpies as the humic acid content increases is a consequence of the decreasing proportion of total oxirane rings in the bulk sample mass. The humic acid-epoxy composite system (20HAE) presents a lower curing enthalpy than the classical value obtained for neat epoxy resin. This means that the total exothermic heat of the curing reaction calculated per neat epoxy percentage in the total composite mass decreases by 10–13 units by increasing the HA content from 5 to 20% in the composite. 3.3. Thermo-physical properties of the composites The thermophysical properties of the composites after full curing were investigated using DSC analysis. Figure 5 shows the thermal curves as a function temperature for fully cured humic acid-epoxy composites. The curves in this figure exhibit single thermal relaxation from the second-order relaxation. The main transition, in these curves is associated with glass transition. With the complete first heating run, and after cooling to room temperature, the composite systems are chemically stable so that residual endo- or exo-thermic reaction from the first-order type could not be observed in the second heating run. This result demonstrates the completeness of the curing reaction in the first heating cycle. Table 2 shows the glass transition temperatures for all samples. A big difference was observed between the T g of the pure epoxy and that of composites. The incorporation of HA into epoxy resin at 20% weight ratio (20HAE) promoted an increase in the T g by about (20 o C) concerning the value recorded to the neat epoxy resin. It is worth pointing out that the presence of humic acid in the epoxy matrix at all loading amounts affects the T g of the composites. It is well known that the fillers when incorporated into polymers caused a reduction in the polymer chains mobility, which increases the T g of the polymers [30]. Additionally, this result also may be explained considering the formation of secondary interactions such as hydrogen bonding the filler and matrix. The high glass transition temperature of humic acid-epoxy composites compared to pure epoxy network reveals that the functional groups in humic acid could produce a reinforcing effect by the increase of aromatic density in the composite matrix. On the other hand, the sold structure of humic acid leads to more rigidity in the final composite, which also leads to a higher T g value. Figure 5 DSC results of HAE composites after second heating cycle 3.4. Kinetics analysis The kinetics of the cross-linking process is a critical factor for the designing-process-cost relationship for the production high-performance composites. The typical DSC curves of the humic acid/epoxy composites (20HAE) curing process at different heating rates are shown in Fig. 5 . The information about curing parameters such as T i , T max , T end , and total curing enthalpy, can be inferred from these curves, these parameters are listed in Table 3 . It can be observed that, at all heating rates there is a smooth curve with a single and broad exothermic peak. For example, the exothermic peak of the uncured sample (20HAE) starts at about 60 o C and ends at around 160 o C. Figure 5 shows that the maximum curing temperature shifted to a higher temperature when the scanning rate rose from 10 to 25 o C/min. this is a normal behavior in most thermosetting resins and the reasons are at a slow heating process, the resin takes more time to get fully cured structure, while at high heating rates, the sample takes short time to get fully cured and most crosslinking reactions will take place at a short period with low heat distribution. There is also a shift to higher initial and final curing temperatures with increasing heating rates. Peak temperature values from DSC experiments at different heating rates were used to calculate the activation energy of the curing reaction for humic acid–epoxy composite, and the Kissinger and Ozawa methods were used for this calculation. The linear plots of ln(β/T 2 ) versus 1/T (Eq. 1) and lnβ versus 1/T (equations 3 and 4) are shown in Fig. 6 . The activation energy E a and pre-exponential factor A of the curing reactions can be calculated from the slop of linear fit and the intercept, respectively, in the Kissinger and Ozawa plots. The values of Kissinger and Ozawa activation energies are given in Table 3 . Where the T p is the temperature at which the maximum curing rate occur, Ea is the activation energy of the reaction, A is the pre-exponential factor, R is the universal gas constant, and the T refers to the temperature in kelvin. Table 3 Data and kinetics parameters of 20HAE composite curing process Heating rate (β) T max ( o K) 1/T max x1000 Tmax 2 β/T max 2 lnβ/T max 2 lnβ Kissinger E (KJ/mol) Ozawa E (KJ/mol) A/min − 1 10 376 2.65 141,376 0.000070733 -9.55 2.3 51.72 57.64 6396.48 15 386 2.59 148,996 0.000100673 -9.20 2.7 20 392 2.55 153,664 0.000130154 -8.94 2.99 25 396 2.52 156,816 0.000159422 -8.744 3.2 Figure 6 gives linear fit plots of Kissinger and Ozawa equations for the 20HAE composite. The kinetics results describe the reactivity of epoxy resin in the presence of humic acid. The activation energies for the humic acid/epoxy system calculated by Kissinger and Ozawa equation are 51.72 and 57.64 KJ/mol, respectively. The activation energy of humic acid epoxy composite shows an increasing to higher value per that of neat epoxy (45.04 and 48.76 KJ/mol) [27], but it stays in the range of activation energies calculated for epoxy resin using other approaches such as Friedman and Ozawa–Flynn–Wall as reported in literature [31,32]. The normal value indicates that the HAE system can cured by similar reaction mechanisms and conditions (temperatures and time). 3.5. Thermal conductivity The thermal conductivity ( K ) of the composites with different weight fractions of humic acid has been determined by the DSC instrument in the presence of indium put on the top of the sample. The DSC melting curves of indium in the presence of composites are shown in Fig. 2 A. The melting behavior of the indium is related to the thermal resistance of the sample ( Rs ) which is determined by Eq. 5 [21, 33–35]. $$\:Rs={R}^{{\prime\:}}-R$$ 5 …… Where R is the thermal resistance between the calorimeter and the sensor metal (indium) without any sample, and R' is the thermal resistance between the calorimeter and the sensor in the presence of sample. R and R' can be calculated from the slope of the indium melting peak with and without the sample. Therefore, the thermal conductivity ( K ) of the sample can be determined from Eq. 1. Table 4 Geometry, temperature range, and thermal conductivity values obtained in this study Samples Length(h)/mm Diameter (D)/mm Cross- section-area (A)/mm 2 ΔΦ(mW) ΔT( o K) K (Wm − 1o K −1 ) Literature values for K (W/m o K) Temp. range o C 0HAE 2.07 4.33 14.71 4 6 0.093 160–170 5HAE 2.08 4.36 14.92 3.6 6 0.083 N.A. 160–170 10HAE 2.14 4.53 16.10 3.4 6 0.075 N.A. 160–170 20HAE 1.74 4.51 15.96 5.4 5 0.11 N.A. 160–166 ΔΦ is the difference between the heat flow; ΔT is the difference between the temperature at the melting peak; h is the sample height (mm); A is the cross-section area (mm 2 ) of the sample. N.A.: not available. Table 4 lists the values of geometry and thermal conductivity of the humic acid-epoxy composites fabricated in this study. It can be seen from Table 4 that the thermal conductivity values obtained for prepared composites, in the measurement temperature range, are in the range of 0.075-0.11Wm − 1 K − 1 . A small decreasing in thermal conductivity with humic acid loading amount (5HAE and 10HAE) was visible for both samples. It is well known that the addition of thermally non-conductive fillers is essential tool to produce low thermal conductive materials. With high filler loading in composite, the fillers are in contact with each other and form thermally insolation walls. However, this effect can be related to the fact that the humic acid has low thermal conductivity when compared to that of pure epoxy resin. Humic acid, as well as common organic materials, can increase the disorder degree of the epoxy matrix produce low conducting system. On the other hand, the results show that, at high humic acid content, the composite system gives the highest value to thermal conductivity. This would be explained at low contents, the better distribution of the filler in the matrix. While at high humic acid content, the filler particles will make large agglomerates with a smaller surface area and hence more interfaces for the phonons to cross the matrix. 3.7. Moisture content Studying the moisture absorption behavior of humic acid-epoxy composites is necessary to understand the durability of composites in outdoor or humid environments. Sample mass increases due to the moisture absorption were measured after 24 and 48 hrs. The resulting moisture percentage-time diagram is represented graphically in Fig. 7 . Humic acid has high moisture absorption properties. The quantity of water contained as moisture inside and outside of the composite can be calculated quantitative by the mass of the sample in a wed environment before and after drying as described in the experimental part. In Fig. 7 the results of moisture content tests of pure epoxy and its composites with humic acid are presented whereas the percentage of moisture test was carried out at room temperature for two days. Pure epoxy samples have the lowest moisture content. In spite of the filler is dried before mixed with epoxy to release complete dry filler in composites, it is clear from the figure, highest moisture level was shown by the humic acid-epoxy composites compared to the pure epoxy specimen. While the higher values of moisture sorption were observed in composites with 10 and 20% humic acid both after 24 and 48 hr. In general, water molecules can penetrate the composite through the interfaces between the fillers and the matrix and by the filler itself via hydrogen bonding. Epoxy materials have hydrophobic and water-resistant nature in which moisture absorption ability is limited. Therefore, the main reason for the high moisture content in the composite is mainly by water absorption ability of humic acid. Water will be absorbed into the composite by the polar OH groups of the filler, causing an increase of the moisture absorption ability of the composite. The free OH groups in humic acid will form hydrogen bonding with water molecules a matter that leads to increase moisture content in composite. Figure 7 also shows the variation in the moisture content of humic acid-epoxy composites as a function of time. The moisture content increases with exposition time, the hydrophobic nature of humic acid results more efficiency to decreasing the hydrophilic characteristics of the composites. It is clear from Fig. 7 that the moisture percentage in the composites percentages increased with exposition time. Humic acid-epoxy composites showed rapid uptake of moisture due to the hydrophilic nature of the filler, accelerating the moisture to penetrate the composite. Conclusions Humic acid was assessed as a polymer additive to epoxy resin and evaluated to understand its influence on the thermal, kinetics, and physical properties of the final composites. At different weight percentages, epoxy reinforced with humic acid was prepared in this study. Structures, thermal, and physical properties of prepared bio-composites were investigated by DSC measurements. A single exothermic peak was observed in all curves, belonging to the cross-linking process of the epoxy matrix. The curing temperatures of the composites reinforced with 5, 10, and 20% humic acid were slightly equal to that of pure epoxy resin. The kinetics analysis was performed. Kinetics parameters obtained by Ozawa and Kissinger approaches such as the activation energies and the pre-exponential factors were calculated. The thermal conductivity of composite mixtures containing humic acid was determined by the DSC approach. For the composites containing high humic acid contents, the measured thermal conductivity was higher than that of pure. Thus, it could be demonstrated that the crosslinking behavior and thermal properties of epoxy resin mixed with humic acid was not significantly different compared with pure epoxy resin. Overall, this study has shown that humic acid can provide promise as a sustainable and inert additive in composite systems. Declarations Funding: This research did not receive any external funding. Conflicts of Interest: The author declares no conflict of interest. References Liang J, Liu L, Qin Z, Zhao X, Li Z, Emmanuel U, Feng J (2023) Experimental study of curing temperature effect on mechanical performance of carbon fiber composites with application to filament winding pressure vessel design. 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Thermochimica Acta 126: 93-100 Simple Determination of the Thermal Conductivity of Polymers by DSC | Thermal Analysis Application No. UC 226 | Application published in METTLER TOLEDO Thermal Analysis User Com 22 https://www.mt.com/sg/en/home/supportive_content/matchar_apps/MatChar_UC226.html ASTM D5229-29M, Standard Test Method for Moisture Absorption Properties and Equilibrium Conditioning of Polymer Matrix Composite Materials. ASTM International: West Conshohocken, PA, USA, 2012. Duncan BC, Broughton WR (2007) Absorption and diffusion of moisture in polymeric materials; National Physical Laboratory: Teddington, UK. Mayans B, Pérez-Esteban J, Escolástico C, Eymar E, Masaguer A (2019) Evaluation of commercial humic substances and other organic amendments for the immobilization of copper through 13C CPMAS NMR, FT-IR, and DSC Analyses. Agronomy 9(11):762. https://doi.org/10.3390/agronomy9110762. Shnawa HA (2021) Curing and thermal properties of tannin-based epoxy and its blends with commercial epoxy resin. Polym Bull 78:1925–1940 https://doi.org/10.1007/s00289-020-03192-6 Liang J, Liu L, Qin Z, Zhao X, Li Z, Emmanuel U, Feng J (2023) Experimental study of curing temperature effect on mechanical performance of carbon fiber composites with application to filament winding pressure vessel design. Polymers 15(4):982. https://doi.org/10.3390/polym15040982 Nascimento LFC, da Luz FS, Costa UO, Braga FO, Lima Júnior ÉP, Monteiro SN (2019) Curing kinetic parameters of epoxy composite reinforced with mallow fibers. Materials (Basel). 12(23):3939. doi:10.3390/ma12233939 Liao KH, Aoyama S, Abdala AA, Macosko C (2014) Does graphene change T g of nanocomposites? Macromolecules 47:8311–8319. Khalafi HR, Ehsani M, Khonakdar HA (2021) Investigation of the cure kinetics and thermal stability of an epoxy system containing cystamine as curing agent. Polym Adv Technol 32:1251–1261. https://doi.org/10.1002/pat.5174 Wudy K, Budde T (2018), Reaction kinetics and curing behavior of epoxies for use in a combined selective laser beam melting process of polymers. J Appl Polym Sci 136:46850. doi: 10.1002/app.46850 Marcus SM, Blaine RL (1994) Thermal conductivity of polymers, glasses and ceramics by modulated DSC. Thermochim Acta 243:231–239. Hu M, Yu D, Wei J (2007) Thermal conductivity determination of small polymer samples by differential scanning calorimetry. Polym Test 26:333–337 Camirand CP (2004) Measurement of thermal conductivity by diferential scanning calorimetry. Thermochim Acta 417:1–4 Scheme 1 Scheme 1 is available in the Supplementary Files section. Supplementary Files Scheme1.png Scheme 1 Schematic illustration of the synthesis protocol for HAE composites Cite Share Download PDF Status: Published Journal Publication published 10 May, 2025 Read the published version in Journal of Polymer Research → Version 1 posted Reviewers agreed at journal 18 Dec, 2024 Reviewers invited by journal 18 Dec, 2024 Editor invited by journal 10 Dec, 2024 Editor assigned by journal 03 Dec, 2024 First submitted to journal 02 Dec, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5569934","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":391930849,"identity":"43df14f6-d804-4da9-9d18-94567e77d3a3","order_by":0,"name":"Hussein Shnawa","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAw0lEQVRIiWNgGAWjYBACAyjNw8/AwEaiFskGUrUwGBwgVou5RPrjDz93WMsY30h+9uBDBYM8v9gB/FosZ+SYSfaeSecxu5FmbjjjDIPhzNkJBBx2I4eNgbftMFBLgpk0bxtDgsFtglrSH3/8C9RiPCP9G7FaEgykQbYYSOQQa8uZN2bSsm3pPBJn3pRJzjgjQYRfjgMd9rbN2p6/PX2bxIcKG3l+aQJaoICZgUEArFKCKOVQLfwHiFY9CkbBKBgFIwwAABqVQAsP5u+4AAAAAElFTkSuQmCC","orcid":"https://orcid.org/0000-0001-9953-5854","institution":"University of Basrah","correspondingAuthor":true,"prefix":"","firstName":"Hussein","middleName":"","lastName":"Shnawa","suffix":""}],"badges":[],"createdAt":"2024-12-03 07:53:22","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5569934/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5569934/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s10965-025-04390-4","type":"published","date":"2025-05-10T15:57:14+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":72291842,"identity":"efb46218-5503-4542-b718-098da762f5a7","added_by":"auto","created_at":"2024-12-24 17:30:17","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":78484,"visible":true,"origin":"","legend":"\u003cp\u003eStructure of humic acid\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5569934/v1/d51f72124304d4191aca3632.png"},{"id":72291838,"identity":"189bb663-ae49-489d-9c66-79a88cee4f03","added_by":"auto","created_at":"2024-12-24 17:30:16","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":144117,"visible":true,"origin":"","legend":"\u003cp\u003e(A) Schematic diagram of the sample arrangement in the DSC pan; (1) Indium without sample, (2) Indium attached on sample. (B) DSC melting peak of indium without sample (blue curve) and DSC curve of indium when it was attached on 20HAE sample (red curve).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5569934/v1/31fe24c3f635b267d7194a4c.png"},{"id":72291847,"identity":"751ad654-de20-49a2-b68b-5544b5c6274e","added_by":"auto","created_at":"2024-12-24 17:30:17","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":303429,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra of humic acid (A), epoxy resin (B), and 20wt% humic acid epoxy composite (20HAE)\u003c/p\u003e","description":"","filename":"31.png","url":"https://assets-eu.researchsquare.com/files/rs-5569934/v1/4fb1484f09e6b1b5a91e39e4.png"},{"id":72291852,"identity":"fc6a03eb-015b-4831-a2ea-e599c9350245","added_by":"auto","created_at":"2024-12-24 17:30:17","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":97636,"visible":true,"origin":"","legend":"\u003cp\u003eDSC curves of epoxy composites modified with 5HAE, 10HAE, and 20HAE composites at 20 \u003csup\u003eo\u003c/sup\u003eC min\u003csup\u003e-1\u003c/sup\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5569934/v1/b5f7f3baf3919edd0576c062.png"},{"id":72291841,"identity":"485018cd-ab7d-46e5-97c6-d13f3cdda7b8","added_by":"auto","created_at":"2024-12-24 17:30:17","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":89380,"visible":true,"origin":"","legend":"\u003cp\u003eDSC results of HAE composites after second heating cycle\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5569934/v1/c8c8ca6f0d706065f3be4184.png"},{"id":72291864,"identity":"a2612587-672e-462d-ab94-928113174ca5","added_by":"auto","created_at":"2024-12-24 17:30:17","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":101560,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFig. 5\u003c/strong\u003e DSC curves of 20HAE at different heating rates\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5569934/v1/a48bc801bc6849d0d4e777c6.png"},{"id":72291881,"identity":"635e1f81-63e7-477b-bb16-7f4f87948b9a","added_by":"auto","created_at":"2024-12-24 17:30:18","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":113580,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFig. 6\u003c/strong\u003e Linear Ozawa and Kissinger plots of \u0026nbsp;20HAE composite\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-5569934/v1/0edf9555bdd692e74ba99f12.png"},{"id":72292400,"identity":"1eb725bd-dd9e-4d37-a216-36831d3783df","added_by":"auto","created_at":"2024-12-24 17:38:18","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":108951,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFig. 7\u003c/strong\u003e Moisture content in pure epoxy resin and humic acid–epoxy composites\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-5569934/v1/2331e7d8937fd999b0f6b6d8.png"},{"id":82537619,"identity":"1fb8d3b3-b555-4e79-bcc2-e101962b954f","added_by":"auto","created_at":"2025-05-12 16:09:22","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1783102,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5569934/v1/19f65a7a-4075-47dc-b211-03ded5ffd783.pdf"},{"id":72291837,"identity":"41db8f7f-a664-41a5-962e-14bf9a42867f","added_by":"auto","created_at":"2024-12-24 17:30:16","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":118155,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eScheme 1\u003c/strong\u003e Schematic illustration of the synthesis protocol for HAE composites\u003c/p\u003e","description":"","filename":"Scheme1.png","url":"https://assets-eu.researchsquare.com/files/rs-5569934/v1/08cff1e2c054dd9d0d8d3ae4.png"}],"financialInterests":"","formattedTitle":"Investigation into crosslinking kinetics, physical properties, and thermal conductivity of humic acid epoxy composite","fulltext":[{"header":"Introduction","content":"\u003cp\u003eDue to their advantages combined properties, such as high adhesion strength, low shrinkage, good chemical and thermal resistance, and low volatile emission, epoxy-based composites have been widely studied for decades and are presently being considered as one of the most materials used for industrial applications. Bio-based-fillers or bio-based-reinforced composites are now utilized in building and construction, furniture, food packaging, automotive parts, and more. However, for more economic and sustainability concerns, low CO\u003csub\u003e2\u003c/sub\u003e emission, and high performance, there is a lot of academic research and considerable efforts supported by both industrial companies and government agencies have been made to replace commercial composites with composites that contain renewable resources. Bio-composites that contain at least one ingredient in their main composition derived from natural resources could be the alternative materials for artificial fibers or artificial fillers reinforced-composites [1, 2].\u003c/p\u003e \u003cp\u003eIn spite of, the bio-based composites offer many advantages, their development and application still face some challenges and some remarkable issues due to bio-degradability, incompatibility of natural reinforcements with most polymer matrices, and limited water resistance. These limitation factors are due to the hydrophilic nature of fiber or fillers, where they tend to absorb relativity more amount of water from humid environments. As a result of water absorption, the reinforcements will swell and cause poor interfacial bonding between fibers or fillers and polymer matrix which directly deteriorates the mechanical properties of the composites. Incompatibility between the matrix and additives, inferior mechanical performance compared with their neat counterparts. Hygroscopicity and recycling of bio-reinforcements are considered critical parameters that constrain their use as reinforcement for polymeric composites [3].\u003c/p\u003e \u003cp\u003eIn the last decade, interest in the utilization of natural materials as additives or reinforcements in composites to reduce the use of artificial additives has been increasing for various industrial applications. Natural fillers and other agricultural byproducts are potential materials for composites. The combination of commercial epoxy resins with bio-fillers shows many advantages including using renewable and inexpensive additives, reducing the cost, availability of renewable resources, and low environmental issues caused by conventional petroleum-based materials, and also improving the processing or final properties of the composites [4–8].\u003c/p\u003e \u003cp\u003eRecently, in addition to plant fibers, natural products or food processing byproducts such as husks, walnut shell powder, lignin, tannins, and others are extensively studied as reinforcements for bio-composites [9–12].\u003c/p\u003e \u003cp\u003eAmong many natural products, humic acid (HA) as a natural organic material has attracted considerable academic and agricultural interest. Humic acid is a macromolecule obtained by the natural biological and chemical degradation process of biomass for long periods. The supramolecular architectures with carbon-enriched chemical structures, and plentiful oxygen-containing functional groups (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) endows humic acid with excellent chelating ability to different ions [13]. Also, HA was evaluated as a charring and flame retardant agent in polymers and epoxy resins in some studies [14–15].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe three-dimensional structure of humic acid based on aliphatic and aromatic units is stabilized by hydrogen, weak hydrophobic, and metal-bridged electrostatic bonds. Reports concerning the utilization of humic acids as fillers or additives in a mixture with thermosetting polymers are scarce and account for only a small percentage of the total bio-based fillers for polymers and composites. A good example of that is the work of Virginia Venezia et al. [16] in which they evaluate the effect of humic acid as a flame retardant additive on the thermal, fire, and mechanical performance of epoxy resin modified with (3-aminopropyl)-triethoxysilane. In this work, it was possible to observe a strong improvement in thermal stability and self-extinguishing capability of the epoxy system by incorporation of humic acid coupled with ammonium polyphosphate and urea and also melt dripping of the epoxy system can prevent during the vertical flame spread tests in the presence of humic acid. Liu Gaungya [17] could develop an environmentally friendly flame retardant with high efficiency through the modification of humic acid with four different metal ions (Fe, Mn, Al, and Cu) and mixing the obtained complex at about 10% wt. with epoxy resin as a polymeric matrix. It was found that the limiting oxygen index (LOI) of the epoxy matrix was increased from 21.2% for pure epoxy resin to 25.3–26.6% for epoxy-metal humates composites. They attributed the effect of metal humates as flame retardant in the epoxy matrix to catalytic charring of this complex which produces char layers and retard the transfer of heat between the matrix and flame zone.\u003c/p\u003e \u003cp\u003eMattia Di Maro et al. [18] prepared composites of ethylene-vinyl acetate (EVA) copolymer incorporated with bio-char which is produced from hemp hurd through a pyrolytic process and commercial humic acid (as sodium salt). The effect of biomas-derived flame retardant formulations (biochar and humic acid) on the fire, morphological features, and thermal and mechanical properties of the composites were thoroughly evaluated. The results of this study show that the thermal and thermo-oxidative stability of the polymeric matrix enhanced in the presence of bio-char, while the addition of humic acid to the mixture can significantly reduce Young's modulus and slightly decrease both peaks of heat release rate and total heat release by 16% and 5%, respectively, with no effect on the degree of crystallinity of the EVA copolymer.\u003c/p\u003e \u003cp\u003eAll authors of the above studies do not investigate the effect of humic acid on the curing profile of epoxy resin, more over there have been no attempts to determine the kinetics parameters of the production process. In a previous paper [19], humic acid was used as raw material to prepare a bio-based thermal stabilizer for PVC. Calcium and zinc humates were synthesized starting from potassium humate through a single-step reaction with calcium hydroxide and with zinc sulfate, and both the obtained salts were mixed with PVC at different concentrations. The results show that, unlike zinc humate, calcium humate shows a significant stabilization effect and interesting activity for the stabilization of PVC.\u003c/p\u003e \u003cp\u003eIn general, composites reinforced with plant fibers are more studied than those with natural fillers. In addition, to the best of the author's investigations, most of the previous studies which were carried out on humic acid as a potential filler in epoxy-based composites have never been focused on the curing behavior, kinetics, and physical properties of the produced composites. Moreover, there have been no attempts to investigate of thermal-physical properties, water absorption, and thermal conductivity of the final products. Therefore, the aims of this work are: (i) applied of humic acid as a bio-based filler to prepare a thermosetting composite and (ii) Additionally, the characteristics are investigated to understand the effect of humic acid as an active and high molecular weight filler on the physical, thermal properties, and thermal conductivity of the prepared composites.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e "},{"header":"Materials and Methods","content":"\u003cp\u003e2.1. Materials used and composite fabrication\u003c/p\u003e\n\u003cp\u003eCommercial epoxy resin (hexanediol diglycidyl ether) and it curing agent (a modified cycloaliphatic amine (isophorone diamine) supplied by Fosroc Chemicals limited (Staffordshire. B783TL, UK) were used in this work as a matrix. The epoxy-to-hardener ratio used here was the one recommended by the manufacturer, 100:40 mass units. Humic acid (potassium salt) in granule form was purchased from Humintech GmbH (Germany). The potassium salt of humic acid was dissolved in 25 ml distillation water with a magnetic stirrer for 5 min, then HCl 5% solution was added drop by drop with a magnetic stirrer until the pH of the mixture reached 2–3 at which the pure humic acid will precipitate, filtrated, and washed several times with distillation water.\u003c/p\u003e\u003cp\u003eThe sample preparation steps for composite samples are presented in Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Pristine epoxy resin without humic acid (noted as 0HAE) was prepared by following the steps reported elsewhere [20]. Firstly, according to the mas ratio 10:4, epoxy resin (A) and its hardener (C) were mixed at room temperature for 5 min, and then the resulting viscous mixture was divided into two parts, the first part was transferred directly to thermal analysis. The second one was poured into a circular plastic container to get a cylindrical shape through the curing and post- curing process. The curing process was carried out at 60 \u003csup\u003eo\u003c/sup\u003eC for 1 hour, followed by the post-curing process at which the samples were kept for 1 hour at 100 \u003csup\u003eo\u003c/sup\u003eC, and then at 130 \u003csup\u003eo\u003c/sup\u003eC for 1 hour. Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the formulations of epoxy resin with its hardener and with humic acid. The mixture of epoxy resin with humic acid at mixing ratios 5, 10, and 20% (noted as 5HAE, 10HAE, and 20HAE, respectively) was prepared according to this sequence presented in Scheme \u003cspan refid=\"Sch1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Liquid epoxy resin and humic acid powder in different weight proportions were well stirred at room temperature in a plastic container for 5 min, after the homogeneity was observed in the mixture; the amount of hardener was added and finally re-mixed for 5 min. The resulting systems were directly submitted to DSC tests before curing.\u003c/p\u003e\u003cdiv class=\"gridtable\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eFormulation of humic acid/epoxy composites at different HA contents.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003c/colgroup\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eComposite\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eEpoxy resin\u003c/p\u003e \u003cp\u003e(g)\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eCuring agent (g)\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003eHumic acid\u003c/p\u003e \u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(g)\u003c/p\u003e \u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003e(wt.%)\u003c/p\u003e \u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0HAE\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5HAE\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.05\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10HAE\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20HAE\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.4\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.2\u003c/p\u003e \u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/table\u003e\u003c/div\u003e\u003cp\u003e \u003cb\u003eCharacterization\u003c/b\u003e \u003c/p\u003e\u003cp\u003e \u003cb\u003eFT-IR Measurements\u003c/b\u003e \u003c/p\u003e\u003cp\u003eFourier-transformed infrared (FTIR) transmittance spectra of HA, cured E, and cured specimens were analyzed using a Jasco-400 spectrometer (Japan). All the spectra were taken over the range of 400–4000 cm\u003csup\u003e− 1\u003c/sup\u003e and with a resolution of 4 cm\u003csup\u003e− 1\u003c/sup\u003e and 32 scans. Sample of HA was ground and mixed with KBr powder in an agate mortar and pressed to get a fine pellet, while neat E and its composite were tested after full curing as films which were prepared by the casting method according to these steps: (1) non-cured system (pure epoxy-hardener mixture and epoxy-humic acid-hardener) was mixed and poured on a parchment paper and pressed between another layer from parchment paper to get thin-sheets (about 0.01 mm thickness). (2) The samples were submitted to a thermal curing program in an electrical oven at 60\u003csup\u003eo\u003c/sup\u003eC for 1 hour, 100\u003csup\u003eo\u003c/sup\u003eC for 1 hour, and then post-curing at 130\u003csup\u003eo\u003c/sup\u003eC for 1 hour. The obtained sheets were examined by FTIR spectrophotometer.\u003c/p\u003e\n\u003ch3\u003eCuring process and physical properties\u003c/h3\u003e\n\u003cp\u003eThe differential scanning calorimetry (DSC-60, Shimadzu, Japan) was employed to investigate the curing conditions and curing parameters of all composite formulations prepared in the work. Pure epoxy resin was mixed with HA for 5 min then with the curing agent for 5 min. Samples with a mass of 10\u0026ndash;12 mg of each prepared formulation were used for thermal analysis. The samples were heated from RT to 250\u003csup\u003eo\u003c/sup\u003eC with a scan rate of 20\u003csup\u003eo\u003c/sup\u003eC/min under nitrogen atmosphere flows at 30 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The curing parameters such as initial, optimum, and final curing temperatures, and curing enthalpies were determined from DSC curves using the DSC softwear. To determine the glass transition temperature (\u003cem\u003eT\u003c/em\u003eg) and the curing degree, sample of the fully-cured HAE systems were tested using a DSC scan from 30\u0026ndash;200 \u003csup\u003eo\u003c/sup\u003eC at a heating rate of 20 \u003csup\u003eo\u003c/sup\u003eC min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and under nitrogen flow of 30 ml min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eCuring kinetics analysis\u003c/h2\u003e \u003cp\u003eThe kinetics of the curing process of the non-polymerized composite was determined using DSC. Specifically, one non-polymerized composite sample, namely, 20HAE was selected to determine the kinetics parameters of humic acid-epoxy composites, and the results were compared with that of pure epoxy resin obtained from literature. The cure reaction kinetics of the 20HAE composites were monitored in non-isothermal mode by the DSC calorimeter. The DSC experiments were performed using a DSC-60 (Shimatzu, Japan). In each DSC run, a small sample mass with a total weight in the range of 10\u0026ndash;12 mg of uncured composite was placed in a standard DSC aluminum pan. Each sample was scanned with a temperature range from 25 to 250 \u003csup\u003eo\u003c/sup\u003eC and at different heating rates, \u003cem\u003ei.e.\u003c/em\u003e, 10, 15, 20, and 25 \u003csup\u003eo\u003c/sup\u003eC min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, continuous flow of dry nitrogen gas purge at a flow rate of 30 ml min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was maintained throughout all experiments.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eThermal conductivity\u003c/h3\u003e\n\u003cp\u003eThe thermal conductivity of the full-polymerized composite samples was measured using DSC following the method of Hakvoort and van Reijen [21] and Flynn and Levin [22]. Pure indium (melting enthalpy\u0026thinsp;=\u0026thinsp;28.15 J/g; melting temperature\u0026thinsp;=\u0026thinsp;156.4 \u003csup\u003eo\u003c/sup\u003eC) was used as a censor for the determination thermal resistance of the composite systems.\u003c/p\u003e \u003cp\u003eIn this method, the DSC instrument measures the melting behavior of a pure metal placed on the top of the samples. All composite samples for thermal conductivity were prepared by heating the samples in an oven for 1 hr at 100\u0026ndash;110 \u003csup\u003eo\u003c/sup\u003eC to get softening over the \u003cem\u003eT\u003c/em\u003eg point then the hot materials were bunched by a leather belt bunch tool. At least two samples for each formulation were prepared. Each sample was a circular disk of polymerized composite system with 4.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 mm in diameter and a height of 2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 mm. The weight of each sample after bunching was found in the range of 30\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mg.\u003c/p\u003e \u003cp\u003eThe experimental setup consist of the following two steps: In the first step, an indium metal pellet (15 mg) was placed in the DSC pan, and the melting peak was determined. In the second step, the composite sample with an aluminum crucible was placed in the DSC furnace. An aluminum crucible with a calibration substance (In) was then placed on top of the composite sample, Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA. An empty aluminum pan with its aluminum lid was placed in the reference position of the DSC cell. A DSC curve was recorded during the melting of the reference (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eB). Nitrogen was used as the purge gas at a rate of 30 mL min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in all tests.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003e\u003c/h3\u003e\n \u003cp\u003eThe thermal conductivity (\u003cem\u003eK\u003c/em\u003e) of the samples can be determined from the slope of the melting peak of the sensor both with and without sample, Eq.\u0026nbsp;1[23].\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:K=\\frac{\\varDelta\\:\\varPhi\\:}{\\varDelta\\:T}x\\frac{h}{A}\\dots\\:\\dots\\:\\dots\\:\\left(1\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003eΔΦ\u003c/em\u003e is the difference between the heat flow (mW) at a melting peak and the heat flow at the beginning of the melting behavior of the sensor, \u003cem\u003eΔT\u003c/em\u003e (\u003csup\u003eo\u003c/sup\u003eC) is the difference between the temperature at the melting peak and the temperature of the onset melting behavior. \u003cem\u003eh\u003c/em\u003e is the sample height (mm), \u003cem\u003eA\u003c/em\u003e is the cross-section area (mm\u003csup\u003e2\u003c/sup\u003e). Both \u003cem\u003eΔΦ\u003c/em\u003e and \u003cem\u003eΔT\u003c/em\u003e can calculate from the slope of the linear side of the melting curve of the sensor above the sample. High (\u003cem\u003eh\u003c/em\u003e) and cross-section area (\u003cem\u003eA\u003c/em\u003e) of the samples were measured with an electronic digital caliper.\u003c/p\u003e\n\u003ch3\u003eMoisture content in the composite systems\u003c/h3\u003e\n\u003cp\u003eMoisture absorption tests of the composite systems were carried out following ASTM D5229-29M [24, 25]. The basis of this approach is a gravimetric technique by which the dry specimens are weighed before and after being exposed to a high humidity and wet environment at room temperature and under atmospheric pressure for two days. At least two specimens were prepared and tested for each composite formulation. Before the experimentation, the samples were placed in an oven for 24h at 103\u003csup\u003eo\u003c/sup\u003eC. Each dry sample was weighed (\u003cem\u003eW\u003c/em\u003e\u003csub\u003e\u003cem\u003edry\u003c/em\u003e\u003c/sub\u003e) using a precision scale with a 0.0001 g accuracy. Immediately after the weighing of the composites, they were placed over filter papers which covered plastic cups containing distilled water. Then each sample was covered with another plastic cup to imitate high humidity and wetness conditions. After 24 hours, the composite specimens were periodically taken from the cups and re-weighed to determine the amount of water that they absorbed (\u003cem\u003eW\u003c/em\u003e\u003csub\u003e\u003cem\u003ewet\u003c/em\u003e\u003c/sub\u003e). Composite specimens were retained in cups and weighed regularly after 48 hours. The percentage of moisture content (\u003cem\u003eM %\u003c/em\u003e) is measured using the following formula (Eq.\u0026nbsp;2).\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:M\\left(\\%\\right)=\\frac{Wwet-Wdry}{Wdry}x\\:100\\%\\dots\\:\\dots\\:\\left(2\\right)\\:$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003eM\u003c/em\u003e is the percentage of moisture content, \u003cem\u003eW\u003c/em\u003e\u003csub\u003e\u003cem\u003edry\u003c/em\u003e\u003c/sub\u003e is the mass of the composites before exposition to humid condition (dry), and \u003cem\u003eW\u003c/em\u003e\u003csub\u003e\u003cem\u003ewet\u003c/em\u003e\u003c/sub\u003e is the mass of sample after exposition to the humid conditions (humid).\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1. FTIR analysis\u003c/h2\u003e \u003cp\u003eDSC and FTIR analysis were carried out to understand if there is any new bond can be formed between the humic acid and epoxy matrix during the polymerization reaction. The final structures of neat epoxy and its composite with humic acid were cured in an oven to obtain specimens for FTIR analysis. FTIR spectra of pure humic acid (HA), the cured neat epoxy resin (E), and the cured humic acid/epoxy composite containing 20 wt% humic acid (20HAE) were presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. In the humic acid spectrum (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eA) the broad transmittance band at 3300\u0026ndash;33400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is related to hydrogen-bonded hydroxyl groups of carboxylic, phenol, and alcohol stretching. The intense band at wavenumber around 1709 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is assigned to the stretching of carbonyl in carboxylic acid groups. Moreover, the observed peak at 1609 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to aromatic skeletal vibration and confirms the presence of aromatic units in the structure of HA [26]. The peak that appears at 1240 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e may attributed to the C\u0026ndash;C, C\u0026ndash;O, and C\u0026thinsp;=\u0026thinsp;O stretching. The high-density peak at 1030 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the spectrum of HA could be observed, produced mainly by aromatic C\u0026ndash;H in-plane deformation, stretching vibration of C\u0026ndash;O in primary alcohols, and C\u0026thinsp;=\u0026thinsp;O stretch.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn the spectra of epoxy resin and humic acid-epoxy composite, the signal at 918 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e associated with oxirane rings is still present, indicating an inefficient curing process that was carried out in an oven at 130\u003csup\u003eo\u003c/sup\u003eC. At the spectra of pure epoxy resin and its composites, the peak denoted at 1200 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponds to the stretching of aliphatic ether which was connected with the polymerization of epoxy groups. The pure humic acid and its composite with epoxy resin have very similar FTIR spectra. The FTIR of the composite system reveals the presence of characteristic absorption peaks for the main functional groups in humic acid and epoxy matrix. the absorption bands at wave numbers at around 3300\u0026ndash;3400, 1709, 1240\u0026ndash;1246, and 103\u0026ndash;1040 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are characteristic bands for O\u0026ndash;H stretching, C\u0026thinsp;=\u0026thinsp;O stretch in unconjugated ketone, carbonyl and in ester and carboxylic acid groups, C\u0026ndash;C, C\u0026ndash;O and C\u0026thinsp;=\u0026thinsp;O stretching, aromatic C\u0026ndash;H in-plane deformation, C\u0026ndash;O deform in primary alcohols and C\u0026thinsp;=\u0026thinsp;O stretch, respectively, for humic acid molecules in the composite system. And those at 3062, 2932, 1608, 1508, and 1036\u0026ndash;1040 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e are assigned to stretching of C\u0026ndash;H of the oxirane ring, bending vibration of \u0026ndash;C\u0026ndash;H in the methylene group, stretching vibration of \u0026ndash;C\u0026thinsp;=\u0026thinsp;C in the benzene ring, C\u0026ndash;O\u0026ndash;C stretching, and stretching C\u0026ndash;O\u0026ndash;C of ethers for epoxy resin in the composite system, respectively. Despite the high humic acid content in epoxy composite (20%), there is a high comparison between the FTIR spectrum of neat epoxy resin and humic acid/epoxy composite. The FTIR of the composite system suggests no formation of a chemical bonding during the curing process. A possible physical mixture between humic acid and epoxy without significant change in the structure of components\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003e3.2. Crosslinking and thermal characterization\u003c/h3\u003e\n\u003cp\u003eThe curing behavior of 20HAE composite was studied by DSC analysis. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows the change of the heat flow to the temperature of the curing reaction for humic acid-epoxy composites (5HAE, 10HAE, and 20HAE). Thermal parameters about the curing reaction as the initial curing temperature (\u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003estart\u003c/em\u003e\u003c/sub\u003e), the peak or maximum temperature (\u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003emax\u003c/em\u003e\u003c/sub\u003e), the final temperature (\u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003eend\u003c/em\u003e\u003c/sub\u003e) at which the complete curing, and the total heat of the curing reaction (ΔH) for the sample were calculated from the DSC analyses and all the values summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Curing data from the literature were taken for pure epoxy resin because this epoxy resin has already been subject to previous investigations [19, 27]. In Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, it can be seen that, the presence of humic acid does not produce any dilation effect to the curing process even when it is at high proportion in the formulation. All the DSC curves of all composite systems have similar shapes. A high exothermic heat flow observed in all samples corresponded to the reaction of an amine with epoxy groups. The addition of humic acid leads to practically the same curing temperatures as the pure epoxy resin, which means that the humic acid has an inert effect in the reaction of epoxy resin with amine.\u003c/p\u003e \u003cp\u003eThis peak appears in the temperature range of 70\u0026ndash;155 \u003csup\u003eo\u003c/sup\u003eC. This event is ascribed to the curing reaction by which the oxirane rings will open and form high molecular weight and crosslinking networks. The curing temperature ranges for all composite systems are consistent with the usual curing ranges of most composite processing [28, 29]. Conversely, further exothermic or endothermic reactions cannot be observed at higher temperatures. For all composite systems, the curing reaction begins at the same temperature (\u003cem\u003eT\u003c/em\u003ei), and the exothermic peak (\u003cem\u003eT\u003c/em\u003e\u003csub\u003emax\u003c/sub\u003e) is equal confirms the similar reactivity of these systems compared to the epoxy resin. The curing parameters calculated for humic acid-epoxy composite (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) and peak shapes (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) show no significant variation with humic acid content. This result confirms that the reaction of epoxy with amine during network formation is not influenced with the presence of the humic acid. This can be explained by the humic acid does not have any catalytic or retardation effect to the curing reaction of the epoxy resin. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e also shows that the curing and post curing reactions can be completed by heating up to 155 \u003csup\u003eo\u003c/sup\u003eC in all formulations. Upon continues heating to higher temperatures, the composite systems show chemical and thermal stability, so no reaction or thermal degradation occurs during this stage.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCalorific characteristics of HAE composite from DSC curves\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eComposites\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c5\" namest=\"c2\"\u003e \u003cp\u003eFirst heating cycle\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eCuring enthalpy for neat epoxy content in total sample mass (J/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eSecond heating cycle\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"1\" nameend=\"c8\" namest=\"c8\"\u003e\u0026nbsp;\u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csup\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sup\u003e\u003csub\u003estart\u003c/sub\u003e (\u003csup\u003eo\u003c/sup\u003eC)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csup\u003ea\u003c/sup\u003e\u003csub\u003emax\u003c/sub\u003e ( \u003csup\u003eo\u003c/sup\u003eC\u003cem\u003e)\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csup\u003e\u003cem\u003ea\u003c/em\u003e\u003c/sup\u003e\u003csub\u003eend\u003c/sub\u003e (\u003csup\u003eo\u003c/sup\u003eC\u003cem\u003e)\u003c/em\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTotal curing enthalpy (J/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e \u003cp\u003e\u003cem\u003eT\u003c/em\u003e\u003csup\u003ec\u003c/sup\u003eg ( \u003csup\u003eo\u003c/sup\u003eC)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0HAE\u003csup\u003e*\u003c/sup\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e119\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e161\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e61.85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e61.85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e \u003cp\u003e84\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5HAE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e119\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e157\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e49.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e52.63\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e \u003cp\u003e95\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10HAE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e66\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e119\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e160\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e44.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e49.61\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e \u003cp\u003e94.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20HAE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e119\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e156\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e39.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e49.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e \u003cp\u003e104\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003csup\u003ea\u003c/sup\u003e the start, the end, and the maximum temperatures of curing peaks.\u003c/p\u003e \u003cp\u003e \u003csup\u003eb\u003c/sup\u003eThe \u003cem\u003eTg\u003c/em\u003e values were determined from the midpoint of glass transition region obtained by 2nd DSC scan.\u003c/p\u003e \u003cp\u003e* Literature data [19, 27].\u003c/p\u003e \u003cp\u003eThe decrease in the curing enthalpies as the humic acid content increases is a consequence of the decreasing proportion of total oxirane rings in the bulk sample mass. The humic acid-epoxy composite system (20HAE) presents a lower curing enthalpy than the classical value obtained for neat epoxy resin. This means that the total exothermic heat of the curing reaction calculated per neat epoxy percentage in the total composite mass decreases by 10\u0026ndash;13 units by increasing the HA content from 5 to 20% in the composite.\u003c/p\u003e \u003cp\u003e \u003cb\u003e3.3. Thermo-physical properties of the composites\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe thermophysical properties of the composites after full curing were investigated using DSC analysis. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the thermal curves as a function temperature for fully cured humic acid-epoxy composites. The curves in this figure exhibit single thermal relaxation from the second-order relaxation. The main transition, in these curves is associated with glass transition. With the complete first heating run, and after cooling to room temperature, the composite systems are chemically stable so that residual endo- or exo-thermic reaction from the first-order type could not be observed in the second heating run. This result demonstrates the completeness of the curing reaction in the first heating cycle. Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the glass transition temperatures for all samples. A big difference was observed between the \u003cem\u003eT\u003c/em\u003eg of the pure epoxy and that of composites. The incorporation of HA into epoxy resin at 20% weight ratio (20HAE) promoted an increase in the \u003cem\u003eT\u003c/em\u003eg by about (20 \u003csup\u003eo\u003c/sup\u003eC) concerning the value recorded to the neat epoxy resin. It is worth pointing out that the presence of humic acid in the epoxy matrix at all loading amounts affects the \u003cem\u003eT\u003c/em\u003eg of the composites. It is well known that the fillers when incorporated into polymers caused a reduction in the polymer chains mobility, which increases the \u003cem\u003eT\u003c/em\u003eg of the polymers [30]. Additionally, this result also may be explained considering the formation of secondary interactions such as hydrogen bonding the filler and matrix. The high glass transition temperature of humic acid-epoxy composites compared to pure epoxy network reveals that the functional groups in humic acid could produce a reinforcing effect by the increase of aromatic density in the composite matrix. On the other hand, the sold structure of humic acid leads to more rigidity in the final composite, which also leads to a higher \u003cem\u003eT\u003c/em\u003eg value.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e DSC results of HAE composites after second heating cycle\u003c/p\u003e\n\u003ch3\u003e3.4. Kinetics analysis\u003c/h3\u003e\n\u003cp\u003eThe kinetics of the cross-linking process is a critical factor for the designing-process-cost relationship for the production high-performance composites. The typical DSC curves of the humic acid/epoxy composites (20HAE) curing process at different heating rates are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e. The information about curing parameters such as \u003cem\u003eT\u003c/em\u003e\u003csub\u003ei\u003c/sub\u003e, \u003cem\u003eT\u003c/em\u003e\u003csub\u003emax\u003c/sub\u003e, \u003cem\u003eT\u003c/em\u003e\u003csub\u003eend\u003c/sub\u003e, and total curing enthalpy, can be inferred from these curves, these parameters are listed in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. It can be observed that, at all heating rates there is a smooth curve with a single and broad exothermic peak. For example, the exothermic peak of the uncured sample (20HAE) starts at about 60 \u003csup\u003eo\u003c/sup\u003eC and ends at around 160 \u003csup\u003eo\u003c/sup\u003eC. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows that the maximum curing temperature shifted to a higher temperature when the scanning rate rose from 10 to 25 \u003csup\u003eo\u003c/sup\u003eC/min. this is a normal behavior in most thermosetting resins and the reasons are at a slow heating process, the resin takes more time to get fully cured structure, while at high heating rates, the sample takes short time to get fully cured and most crosslinking reactions will take place at a short period with low heat distribution. There is also a shift to higher initial and final curing temperatures with increasing heating rates. Peak temperature values from DSC experiments at different heating rates were used to calculate the activation energy of the curing reaction for humic acid\u0026ndash;epoxy composite, and the Kissinger and Ozawa methods were used for this calculation. The linear plots of ln(β/T\u003csup\u003e2\u003c/sup\u003e) versus 1/T (Eq.\u0026nbsp;1) and lnβ versus 1/T (equations 3 and 4) are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe activation energy \u003cem\u003eE\u003c/em\u003ea and pre-exponential factor \u003cem\u003eA\u003c/em\u003e of the curing reactions can be calculated from the slop of linear fit and the intercept, respectively, in the Kissinger and Ozawa plots. The values of Kissinger and Ozawa activation energies are given in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003cimg src=\"https://myfiles.space/user_files/122228_c8a1650c59388082/122228_custom_files/img1734716380.png\"\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWhere the \u003cem\u003eT\u003c/em\u003e\u003csub\u003e\u003cem\u003ep\u003c/em\u003e\u003c/sub\u003e is the temperature at which the maximum curing rate occur, \u003cem\u003eEa\u003c/em\u003e is the activation energy of the reaction, \u003cem\u003eA\u003c/em\u003e is the pre-exponential factor, \u003cem\u003eR\u003c/em\u003e is the universal gas constant, and the \u003cem\u003eT\u003c/em\u003e refers to the temperature in kelvin.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eData and kinetics parameters of 20HAE composite curing process\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"10\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHeating rate (β)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eT\u003csub\u003emax\u003c/sub\u003e (\u003csup\u003eo\u003c/sup\u003eK)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1/T\u003csub\u003emax\u003c/sub\u003e\u003c/p\u003e \u003cp\u003ex1000\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTmax\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eβ/T\u003csub\u003emax\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003elnβ/T\u003csub\u003emax\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003elnβ\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eKissinger E (KJ/mol)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eOzawa E\u003c/p\u003e \u003cp\u003e(KJ/mol)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c10\"\u003e \u003cp\u003eA/min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e376\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e141,376\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.000070733\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-9.55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e2.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e51.72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e57.64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c10\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e6396.48\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e386\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.59\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e148,996\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.000100673\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-9.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e2.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e392\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.55\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e153,664\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.000130154\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-8.94\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e2.99\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e396\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e156,816\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.000159422\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e-8.744\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e3.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e gives linear fit plots of Kissinger and Ozawa equations for the 20HAE composite. The kinetics results describe the reactivity of epoxy resin in the presence of humic acid. The activation energies for the humic acid/epoxy system calculated by Kissinger and Ozawa equation are 51.72 and 57.64 KJ/mol, respectively. The activation energy of humic acid epoxy composite shows an increasing to higher value per that of neat epoxy (45.04 and 48.76 KJ/mol) [27], but it stays in the range of activation energies calculated for epoxy resin using other approaches such as Friedman and Ozawa\u0026ndash;Flynn\u0026ndash;Wall as reported in literature [31,32]. The normal value indicates that the HAE system can cured by similar reaction mechanisms and conditions (temperatures and time).\u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.5. Thermal conductivity\u003c/h2\u003e \u003cp\u003eThe thermal conductivity (\u003cem\u003eK\u003c/em\u003e) of the composites with different weight fractions of humic acid has been determined by the DSC instrument in the presence of indium put on the top of the sample. The DSC melting curves of indium in the presence of composites are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eA. The melting behavior of the indium is related to the thermal resistance of the sample (\u003cem\u003eRs\u003c/em\u003e) which is determined by Eq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e5\u003c/span\u003e [21, 33\u0026ndash;35].\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:Rs={R}^{{\\prime\\:}}-R$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e5\u003c/div\u003e\u003c/div\u003e\u0026hellip;\u0026hellip;\u003c/p\u003e \u003cp\u003eWhere \u003cem\u003eR\u003c/em\u003e is the thermal resistance between the calorimeter and the sensor metal (indium) without any sample, and \u003cem\u003eR'\u003c/em\u003e is the thermal resistance between the calorimeter and the sensor in the presence of sample. \u003cem\u003eR\u003c/em\u003e and \u003cem\u003eR'\u003c/em\u003e can be calculated from the slope of the indium melting peak with and without the sample. Therefore, the thermal conductivity (\u003cem\u003eK\u003c/em\u003e) of the sample can be determined from Eq.\u0026nbsp;1.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eGeometry, temperature range, and thermal conductivity values obtained in this study\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"9\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSamples\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLength(h)/mm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDiameter (D)/mm\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCross- section-area (A)/mm\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eΔΦ(mW)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eΔT(\u003csup\u003eo\u003c/sup\u003eK)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eK (Wm\u003csup\u003e\u0026minus;\u0026thinsp;1o\u003c/sup\u003eK\u003csup\u003e\u0026minus;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eLiterature values\u003c/p\u003e \u003cp\u003efor K (W/m \u003csup\u003eo\u003c/sup\u003eK)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eTemp. range \u003csup\u003eo\u003c/sup\u003eC\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e0HAE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.33\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e14.71\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.093\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e160\u0026ndash;170\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e5HAE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e14.92\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.083\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eN.A.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e160\u0026ndash;170\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e10HAE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e16.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e3.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.075\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eN.A.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e160\u0026ndash;170\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e20HAE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1.74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e4.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e15.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e5.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\"\u003e \u003cp\u003eN.A.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c9\"\u003e \u003cp\u003e160\u0026ndash;166\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003cem\u003eΔΦ is the difference between the heat flow; ΔT is the difference between the temperature at the melting peak; h is the sample height (mm); A is the cross-section area (mm\u003c/em\u003e \u003csup\u003e \u003cem\u003e2\u003c/em\u003e \u003c/sup\u003e \u003cem\u003e) of the sample. N.A.: not available.\u003c/em\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e lists the values of geometry and thermal conductivity of the humic acid-epoxy composites fabricated in this study. It can be seen from Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e that the thermal conductivity values obtained for prepared composites, in the measurement temperature range, are in the range of 0.075-0.11Wm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003eK\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. A small decreasing in thermal conductivity with humic acid loading amount (5HAE and 10HAE) was visible for both samples.\u003c/p\u003e \u003cp\u003eIt is well known that the addition of thermally non-conductive fillers is essential tool to produce low thermal conductive materials. With high filler loading in composite, the fillers are in contact with each other and form thermally insolation walls. However, this effect can be related to the fact that the humic acid has low thermal conductivity when compared to that of pure epoxy resin. Humic acid, as well as common organic materials, can increase the disorder degree of the epoxy matrix produce low conducting system.\u003c/p\u003e \u003cp\u003eOn the other hand, the results show that, at high humic acid content, the composite system gives the highest value to thermal conductivity. This would be explained at low contents, the better distribution of the filler in the matrix. While at high humic acid content, the filler particles will make large agglomerates with a smaller surface area and hence more interfaces for the phonons to cross the matrix.\u003c/p\u003e \u003cp\u003e \u003cb\u003e3.7. Moisture content\u003c/b\u003e \u003c/p\u003e \u003cp\u003eStudying the moisture absorption behavior of humic acid-epoxy composites is necessary to understand the durability of composites in outdoor or humid environments. Sample mass increases due to the moisture absorption were measured after 24 and 48 hrs. The resulting moisture percentage-time diagram is represented graphically in Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e. Humic acid has high moisture absorption properties. The quantity of water contained as moisture inside and outside of the composite can be calculated quantitative by the mass of the sample in a wed environment before and after drying as described in the experimental part. In Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e the results of moisture content tests of pure epoxy and its composites with humic acid are presented whereas the percentage of moisture test was carried out at room temperature for two days.\u003c/p\u003e \u003cp\u003ePure epoxy samples have the lowest moisture content. In spite of the filler is dried before mixed with epoxy to release complete dry filler in composites, it is clear from the figure, highest moisture level was shown by the humic acid-epoxy composites compared to the pure epoxy specimen. While the higher values of moisture sorption were observed in composites with 10 and 20% humic acid both after 24 and 48 hr. In general, water molecules can penetrate the composite through the interfaces between the fillers and the matrix and by the filler itself via hydrogen bonding.\u003c/p\u003e \u003cp\u003eEpoxy materials have hydrophobic and water-resistant nature in which moisture absorption ability is limited. Therefore, the main reason for the high moisture content in the composite is mainly by water absorption ability of humic acid. Water will be absorbed into the composite by the polar OH groups of the filler, causing an increase of the moisture absorption ability of the composite. The free OH groups in humic acid will form hydrogen bonding with water molecules a matter that leads to increase moisture content in composite. Figure\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e also shows the variation in the moisture content of humic acid-epoxy composites as a function of time. The moisture content increases with exposition time, the hydrophobic nature of humic acid results more efficiency to decreasing the hydrophilic characteristics of the composites.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIt is clear from Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e that the moisture percentage in the composites percentages increased with exposition time. Humic acid-epoxy composites showed rapid uptake of moisture due to the hydrophilic nature of the filler, accelerating the moisture to penetrate the composite.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusions","content":"\u003cp\u003eHumic acid was assessed as a polymer additive to epoxy resin and evaluated to understand its influence on the thermal, kinetics, and physical properties of the final composites. At different weight percentages, epoxy reinforced with humic acid was prepared in this study. Structures, thermal, and physical properties of prepared bio-composites were investigated by DSC measurements. A single exothermic peak was observed in all curves, belonging to the cross-linking process of the epoxy matrix. The curing temperatures of the composites reinforced with 5, 10, and 20% humic acid were slightly equal to that of pure epoxy resin. The kinetics analysis was performed. Kinetics parameters obtained by Ozawa and Kissinger approaches such as the activation energies and the pre-exponential factors were calculated. The thermal conductivity of composite mixtures containing humic acid was determined by the DSC approach. For the composites containing high humic acid contents, the measured thermal conductivity was higher than that of pure. Thus, it could be demonstrated that the crosslinking behavior and thermal properties of epoxy resin mixed with humic acid was not significantly different compared with pure epoxy resin. Overall, this study has shown that humic acid can provide promise as a sustainable and inert additive in composite systems.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding:\u0026nbsp;\u003c/strong\u003eThis research did not receive any external funding.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of Interest:\u003c/strong\u003e The author declares no conflict of interest.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eLiang J, Liu L, Qin Z, Zhao X, Li Z, Emmanuel U, Feng J (2023) Experimental study of curing temperature effect on mechanical performance of carbon fiber composites with application to filament winding pressure vessel design. Polymers 15: 982. https://doi.org/10.3390/ polym15040982\u003c/li\u003e\n\u003cli\u003eWang D, Onawumi P, Ismail S, Dhakal H, Popov I, Silberschmidt V, Roy A (2019) Machinability of natural-fibre-reinforced polymer composites: Conventional vs ultrasonically-assisted machining. Compos Part A Appl Sci Manuf 119:188\u0026ndash;195.\u003c/li\u003e\n\u003cli\u003ePicard MC, Rodriguez-Uribe A, Thimmanagari M, Misra M, Mohanty, AK (2020) Sustainable biocomposites from poly(butylene succinate) and apple pomace: A study on compatibilization performance. 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Thermochim Acta 417:1\u0026ndash;4\u003c/li\u003e\n\u003c/ol\u003e\n"},{"header":"Scheme 1","content":"\u003cp\u003eScheme 1 is available in the Supplementary Files section.\u003c/p\u003e\n"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-polymer-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jpol","sideBox":"Learn more about [Journal of Polymer Research](https://www.springer.com/journal/10965)","snPcode":"10965","submissionUrl":"https://www.editorialmanager.com/jpol/","title":"Journal of Polymer Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Epoxy composite, humic acid, cross-linking kinetics, thermal conductivity, DSC","lastPublishedDoi":"10.21203/rs.3.rs-5569934/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5569934/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eNatural and renewable materials, such as lignin, for high value products has become part of the research of increasing importance due to the phenomenon of global warming there has been a lot of interest in the development of composites based on renewable resources for a variety of applications. This paper seeks by a series of FT-IR spectroscopy, gravimetry, and DSC experiments to address the curing behavior, thermo-physical properties (glass transition temperature (\u003cem\u003eT\u003c/em\u003eg)), thermal conductivity (TC), and moisture absorption (MA) of bio-composites made of commercial epoxy resin (E) with humic acid (HA). It is observed that in almost all cases, the prepared composites (HAE) have similar curing behavior to that of (E) with higher activation energy, \u003cem\u003eT\u003c/em\u003eg, and MA. In addition, TC decreases for 5 and 10 wt% HAE, while it increases for 20 wt% HAE. Conversely, by increasing the amount of humic acid, the \u003cem\u003eT\u003c/em\u003eg and MA ability of the composites increased. Concerning TC, the presence of HA at 5 and 10 wt% results a slight decrease in the TC of the composites. A bio-based and valuable composite have been produced from epoxy and humic acid by commonly simple mixing process which might be easily transferable to commercial production process.\u003c/p\u003e","manuscriptTitle":"Investigation into crosslinking kinetics, physical properties, and thermal conductivity of humic acid epoxy composite","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-12-24 17:30:11","doi":"10.21203/rs.3.rs-5569934/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2024-12-18T14:31:00+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-12-18T13:56:20+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Journal of Polymer Research","date":"2024-12-10T16:52:42+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-12-04T00:52:08+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Polymer Research","date":"2024-12-03T02:42:00+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-polymer-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jpol","sideBox":"Learn more about [Journal of Polymer Research](https://www.springer.com/journal/10965)","snPcode":"10965","submissionUrl":"https://www.editorialmanager.com/jpol/","title":"Journal of Polymer Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"967da077-3a7e-40c0-a35b-75667e16c932","owner":[],"postedDate":"December 24th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-05-12T16:04:52+00:00","versionOfRecord":{"articleIdentity":"rs-5569934","link":"https://doi.org/10.1007/s10965-025-04390-4","journal":{"identity":"journal-of-polymer-research","isVorOnly":false,"title":"Journal of Polymer Research"},"publishedOn":"2025-05-10 15:57:14","publishedOnDateReadable":"May 10th, 2025"},"versionCreatedAt":"2024-12-24 17:30:11","video":"","vorDoi":"10.1007/s10965-025-04390-4","vorDoiUrl":"https://doi.org/10.1007/s10965-025-04390-4","workflowStages":[]},"version":"v1","identity":"rs-5569934","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5569934","identity":"rs-5569934","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}