Temporal evolution of structure property relationship for UV+RH artificially weathered material extrusion additive manufactured PLA

Temporal evolution of structure property relationship for UV+RH artificially weathered material extrusion additive manufactured PLA | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Temporal evolution of structure property relationship for UV+RH artificially weathered material extrusion additive manufactured PLA Mirza Faizaan, Satish Shenoy Baloor, Srinivas Nunna, Suhas Yeshwant Nayak, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7324138/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 02 Mar, 2026 Read the published version in Scientific Reports → Version 1 posted 11 You are reading this latest preprint version Abstract This study addresses the underreported temporal evolution of weathering on material extrusion additive-manufactured (MEX-AM) polylactic acid (PLA). Overcoming the limitation of arbitrary exposure durations in existing literature, a time-dependent investigation was conducted on MEX-PLA samples subjected to prolonged artificial weathering for up to 2000 hours using a UV-B equipped accelerated weathering chamber with controlled relative humidity. The changes in mechanical, chemical and thermal properties were analysed at 200-hour intervals. The results revealed a time-dependent degradation mechanism characterised by β-chain scission. FTIR analysis confirmed the formation of C = C groups and the progressive loss of H groups, indicating substantial material degradation. Furthermore, DSC and XRD data demonstrated a progressive increase in crystallinity with prolonged exposure, leading to a significant reduction in tensile strength. At the same time, the tensile modulus remained relatively stable for MEX-AM PLA. Physical sciences/Chemistry Physical sciences/Engineering Physical sciences/Materials science 3D Printing Additive Manufacturing FDM PLA Temporal evolution Tensile Properties Artificial Weathering Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1 Introduction Assessing the outdoor conditions of polymers to evaluate their degradation in mechanical, chemical and thermal properties has been gaining interest due to the desire for maximising the lifetime of polymer structures. As weathering conditions are very local and complex to replicate due to the nature of non-controlled variables such as atypical hot and cold days, changing wind patterns, etc., accelerated weathering is considered representative of the outdoor conditions [ 1 ]. The growing awareness of sustainable bio-based polymers has increased the demand for polylactic acid (PLA) across industries [ 2 ]. Owing to its low cost, ease of use, better mechanical properties than other 3D printable plastics and inherent biodegradability, PLA is the most common used thermoplastic in material extrusion additive manufacturing (MEX-AM), widely called Filament Fusion Fabrication (FFF) or Fused Deposition Modelling (FDM). Further, the advent of MEX-AM and rapid prototyping technology has transformed the polymer/plastic accessibility in every household [ 3 ]. Currently, very little literature is focused on the accelerated weathering of PLA and its composites [ 1 ]. We observed that predominantly arbitrary exposure durations have been adopted in the literature. Table_ 1 summarises the tensile and flexural strength degradation caused by ultraviolet (UV) and relative humidity (RH) reported in the literature. The duration of exposure measured irradiance, percentage of decrement in mechanical strength, and testing standards adopted in each study are summarised. Table_ 1 Summary of PLA artificial weathering caused by UV degradation. Exposure Condition [hours; mW/m 2 ] % Decrement Testing Standard Remark Ref. 2000h; 890 FS – 99% ASTM G154 [ 4 ] 600h; 39 TS – 77.6% - [ 5 ] 700h; 60000 TS – 43.5% ISO 4892-2 Irradiance calculation error likely [ 6 ] 500h; 760 TS – 12.5% - [ 7 ] 600h; 39 TS – 74% - [ 8 ] 200h; 490 TS – 61.1% ISO 4892-3 [ 9 ] 300h; 490 FS – 83.2% ISO 4892-3 [ 10 ] 1200h; 890 TS – 66.15% ASTM G154 [ 11 ] 750h; 680 TS – 37.5% ASTM G154 [ 12 ] FS – Flexural Strength; TS – Tensile Strength Notably, PLA is susceptible to UV light and moisture, such as photo-degradation and hydrolytic degradation [ 3 ]. It is noted that PLA absorbs UV below 270 nm, and the degradation rate increases with temperature and RH. PLA polarity favours hydrolysis and photolysis [ 1 ]. The intensity of photodegradation is highest for carbon arc lamps, followed by metal halide lamps, xenon lamps and outdoor exposure [ 13 ]. Photochemical degradation of PLA is widely studied in the literature [ 3 , 14 – 16 ], and the degradation reaction is summarised in Table_ 2. PLA degrades following Norrish Type I and Norrish Type II reactions, beginning with homolysis of CH 3 groups (stage 1) and photochemical cleavage of the polymer chain, indicative of Norrish Type I behaviour (stage 2). The reaction progresses to the continued abstraction of H groups (stage 3), confirming Norrish Type II reaction, leaving free radicals (stage 4) that promote further cross-linking and chain scission (stage 5). Gonzalez-Lopez et al. [ 1 ] reviewed the effect of additives, fillers and reinforcements on the photo-degradation and hydrolytic degradation of PLA and its biocomposites, noting that the degradation rate of PLA depends on the initial molecular weight, sample dimensions, polymer crystallinity and filler/reinforcements. With the increasing interest in PLA for MEX-AM and PLA-based composites as a bio-sourced biodegradable and sustainable alternative, it is imperative to understand the effect of weathering and environmental conditioning on the polymer's mechanical, chemical and thermal properties, especially as a product of MEX-AM. We know from literature that MEX-PLA tends to absorb twice as much water and has a higher crystallinity than injection moulded PLA [ 17 ]. The FDM process-induced voids promote water uptake and induce residual stresses and the formation of cracks [ 18 ]. Chopra et al. exposed MEX-PLA to outdoor conditions in Aurangabad, India and elucidated the degradation mechanism of PLA [ 3 ]. The authors further studied the effect of infill density and pattern on the degradation, confirming that samples printed with less than 100% infill density suffered higher degradation, resulting in the deterioration of their tensile performance. A similar decrease in tensile performance was observed in weathered PLA processed using methods other than FDM [ 1 ]. Changes in thermal properties and crystallinity of artificially weathered PLA note that accelerated weathering forces rearrangement of amorphous PLA chains to highly ordered spherulites, resulting in a significant increase in the crystallinity of the PLA up to 60% [ 9 , 19 – 21 ]. The literature on the weathering of PLA focuses primarily on the effect of biocomposites, polymer blends, and fillers as opposed to neat PLA subjected to prolonged exposure times. Also, the literature adopts arbitrary exposure times to evaluate the degradation of these PLA composites [ 1 ]. A reliable time-based trend representation is still missing in the literature. Furthermore, limited literature is available on the weathering of MEX-AM-based PLA. Thus, an experiment was designed to evaluate the time-dependent degradation of MEX-PLA specimens. This study examines the temporal evolution of UV + RH accelerated weathering on MEX-PLA tensile coupons. The samples were subjected to artificial weathering for up to 2000 hours to establish the trends in polymer properties and mechanical strength degradation that are currently missing in the literature. Fourier transform Infrared Spectroscopy (FTIR) was carried out on the samples before tensile testing, followed by X-Ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC) on the gauge length of the tensile coupons. The percentage of crystallinity and crystallite size were calculated from the XRD spectra, and the thermal behaviour of weathered MEX-PLA was examined using DSC. Finally, the structure-property relations are discussed. 2 Materials and Methods 2.1 Fabrication of tensile test coupons Samples were fabricated on the open-source Creality Ender 3 V2 desktop 3D printer using a commercial 1.75 mm diameter eSun PLA + filament (white). A 0.6 mm nozzle was used to fabricate 100% concentric infill samples following ASTM 638D Type − 1 [ 22 ]. A 0.15 mm layer thickness, 20 mm/s print speed at 210°C with two top/bottom 45° raster layers was selected to minimise voids in the test coupons. Tensile tests were performed on the BISS 50KN UTM with a 5 mm/min strain rate. Three samples were tested every 200 hours, up to 2000 hours of UV + RH exposure, to ensure repeatability, and tensile performance degradation over prolonged exposure times was analysed. The unexposed samples are referred to as ‘0 hours’ for convenience. Thus, 33 samples were fabricated for accelerated weathering, while 30 test coupons were subjected to extreme conditions. 2.2 Accelerated weathering The weathering tests were carried out according to ASTM G151 and G154 standards for ultraviolet (UV) weathering of non-metallic materials [ 23 , 24 ]. An accelerated weathering chamber equipped with fluorescent UV-B lamps rated with a peak spectral intensity of 306 nm and a typical irradiance of approximately 0.49W•m-2 was used for the cyclic 8 hours UV and 4 hours condensation (relative humidity (RH)) at 60°C and 50°C, respectively. Figure_ 1a shows the weathering equipment, and the patented sheet metal tensile sample holder [ 25 ] designed to expose only the gauge length of the test coupons while preventing warping of the samples under prolonged temperature exposure is given in Figure_ 1b. The samples were flipped every 7 days to expose both sides of the tensile test coupons. 2.3 Fourier Transform Infrared Spectroscopy (FTIR) To assess the surface degradation of 3D printed PLA, Fourier Transform Infrared (FTIR) spectroscopy was employed. A diamond Attenuated Total Reflectance (ATR) probe was used on the exposed surface of the gauge length of the tensile coupons. FTIR spectra were collected every 200 hours of cyclic exposure, with 50 scans recorded per sample on a Shimadzu IRSpirit spectrophotometer (Figure_ 1c). A total of 2526 data points were recorded for each FTIR measurement. Subsequently, the recorded spectra were processed on MATLAB, involving baseline correction, normalisation, and smoothing. As is used alternatively in literature, the carbonyl index (CI) was calculated by determining the ratio of both area under the curve and intensity, under the carbonyl (C = O) peak at 1750 cm − 1 and methylene (CH 2 ) peak at ~ 3000 cm − 1 , respectively [ 1 , 26 , 27 ]. The CI is a dimensionless parameter that provides valuable insights into the degree of polymer chain scission and crosslinking, key indicators of material degradation and embrittlement. 2.4 X-Ray Diffraction (XRD) The 50mm gauge length sections were carefully cut using the Struers Accutom-50 precision cutter. XRD measurements were carried out on the X’PERT PRO with a Cu X-Ray tube emitting a wavelength of 1.5406Å for a range of 5–50° with a scan time of 2.5s per step. XRD data collected for every 200 hours of exposure were batch processed and plotted on MATLAB, including calculating crystallinity [%] and crystallite size at different peaks. The per cent crystallinity was calculated by integrating the area under crystalline peaks, and the crystallite size (L) was calculated using the Scherrer equation: $$\:L=\:K\lambda\:/\beta\:cos\theta\:$$ where K = 0.9, λ = 1.5406 Å, β is the full-width half maximum (FWHM), and θ is the scan angle of the respective peaks. 2.5 Differential Scanning Calorimetry (DSC) DSC was performed on a Netzsch DSC 214 Polyma in dynamic mode. The gauge section of the tensile coupon subjected to accelerated weathering conditions was carefully scraped using a scalpel to collect approximately 10mg of the sample. These were heated from 20°C to 300°C with a 10°C/min heating rate. Only the first heating was recorded, and the changes in glass transition (Tg), cold crystallisation (Tc), and melting (Tm) were analysed for all eleven samples, including unexposed (0 hours) up to 2000 hours in steps of 200 hours of UV + RH exposure. 3 Results and Discussion 3.1.1 Fourier Transform Infrared Spectroscopy FTIR spectra were collected using an ATR probe on the gauge length of the tensile coupons exposed to UV + RH weathering in an accelerated weathering chamber. FTIR spectra was captured for every 200 hours of weathering, starting from 0 hours (unexposed) up to 2000 hours of exposure, as seen in Figure_ 2a. The collected data were normalised and smoothed using MATLAB, and peaks were identified. The peak intensity, corresponding functional group and its vibrations are tabulated in Table_ 3. The changes in spectra over continued exposure are illustrated in Figure_ 2b. Table_ 3 Peak assignment for artificially weathered UV + RH FDM-PLA. Wavelength [cm-1] Bond Vibration 3800–3600 -O-H group Stretch 3000–2900 sp3 C-H group Symmetric Stretch 2900–2800 sp2 C-H group Asymmetric Stretch ~ 1750 C = O δ-lactone Stretch 1700–1600 C = C group Stretch ~ 1450 C-H deformation Asymmetric Bend ~ 1350 C-H deformation Symmetric Bend 1250–1000 C-O group Stretch 900 − 850 C-C group Stretch 720–820 C-H group Rocking Like Chopra et al. [ 3 ], new OH group peaks appeared at 3750 cm − 1 , corresponding to free hydroxyl (-OH) groups possibly due to hydrolytic degradation under UV + RH exposure. The weak band at 3750 cm − 1 suggests free non-hydrogen bonded hydroxyl groups at low concentrations often appearing at surfaces, indicative of chain scission and surface oxidation of PLA [ 28 ]. Splitting of the CH and CO bonds was observed at 1370 cm − 1 and 1080 cm − 1 , respectively, and the C = C stretch was shown owing to sp 2 CH. As biopolymers predominantly degrade by Norrish Type I, Norrish Type II, or both reactions when exposed to UV rays, the loss of peak intensity in the C-O stretch (1250–1050 cm − 1 ) and the characteristic C = O δ-lactone (1750 cm − 1 ) are indicative of the α-scission reactions (Norrish Type I) [ 29 ]. The Norrish Type II reactions that present as an abstraction of hydrogen are visible as the loss of intensity of CH groups at 760 cm − 1 , 1450–1350 cm − 1 , and the shift in peaks observed from 2925 to 2850 cm − 1 (sp 3 to sp 2 hybridisation) [ 3 ]. Furthermore, the presence of C = C group (~ 1630 cm − 1 ) [ 30 ] occurring subsequently due to prolonged UV + RH exposure of over 1200 hours, led to β-scission of the alkyl radical that breaks the C-C bonds [900 − 850 cm − 1 ], forming C = C groups. These structural changes can significantly compromise the mechanical properties of the FDM-PLA, leading to decreased tensile strength and embrittlement. 3.1.2 Tensile Performance Three MEX-PLA tensile coupons were tested for every 200 hours of UV + RH exposure at 60°C and 50°C, respectively, up to 2000 hours of exposure. Figure_ 3a illustrates the progressive change in colour and surface roughness of the specimens over prolonged periods of UV exposure, leading to cracks and flaking of the topmost layer (Figure_ 3b). A significant decline in tensile strength (Figure_ 3c) was observed with UV + RH exposure, starting at a 10% loss at 200 hours, followed by a near 5% decrease with every consecutive 200-hour exposure. After 1200 hours, the tensile strength exhibited large deviations, likely due to increased brittleness. The increased brittleness correlates with the observed slowdown or plateau in the rate of carbonyl group formation after a sharp increase up to 800 hours (Figure_ 3d). The subsequent increase and fluctuation in the CI after 1200 hours suggest that the polymer may have reached a state of saturation or alternative degradation mechanisms have become more dominant. Interestingly, the tensile modulus remains unaffected by the prolonged exposure and may be attributed to the increasing crystallinity of the samples, which has been shown to increase the tensile modulus of the weathered samples [ 18 ]. 3.1.3 X-Ray Diffraction (XRD): XRD patterns were scanned in the gauge length of the exposed samples to evaluate the effect of prolonged UV + RH exposure on the crystallisation of semi-crystalline FDM-PLA. Figure_ 4a illustrates the semi-crystalline PLA with a broad amorphous halo and crystalline peak centred around 2θ = 16.5° and 29.5°, respectively, indicative of a predominantly amorphous structure. Upon UV + RH exposure, the amorphous halo resolved into two distinct characteristic α-phase crystalline peaks at 2θ = 16.5° and 18.75° as a result of crystallisation [ 19 ], with increasing intensity for the former while the intensity remains unchanged for the latter. Interestingly, the crystalline peak at 29.45° decreases over continued exposure and is ascribed to the β-crystals [ 31 , 32 ]. Figure_ 4b summarises the crystallinity (%) and crystallite size for the three distinct peaks observed at 2θ = 16.5° (200/110),18.75° (203) and 29.5° (0010), respectively. The near amorphous PLA with 2.24% crystallinity crystallises to 57% with 200 hours of UV + RH exposure and steadily increases up to 71.84% with prolonged exposure up to 2000 hours. Sawpan et al. also noted that the degree of crystallinity for artificially weathered PLA could exceed 50% [ 3 , 19 ]. While the crystallite size for 16.5° and 18.75° peaks shares a similar trend, the peak at 29.5° decreases with exposure times from 24.12nm to 16.35nm. These findings suggest that the UV + RH weathering induces crystallisation in the FDM-PLA samples, leading to a more crystalline and potentially less stable material, which translates to a decline in the tensile performance of these specimens. Owing to the open chain ends created by the chain scission reaction, the content of amorphous segments in the polymer structure increases [ 33 ]. As the broken segments have higher freedom to form more organised structures, they try to crosslink to stabilise the structure, ultimately increasing crystallinity. Although this may increase the modulus [ 3 ], the chain scission is more detrimental than any further cross-linking. However, the exact cause for this behaviour can be complex. DSC was employed to analyse the changes in the polymer’s thermal properties to understand the specific changes in the weathered PLA fully. 3.1.4 Differential Scanning Calorimetry (DSC) FDM-PLA tensile coupons were shaved at the exposed gauge length section and analysed using DSC to understand the changes in thermal behaviour over time of weathering exposure. The DSC curves were baseline corrected and overlayed in Figure_ 5. The cold crystallisation peak (T c ) at 96°C is only visible in the unexposed (0 hours) samples, indicative of the semi-crystalline nature of the polymer. The T c disappears when exposed to UV + RH weathering, confirming the reduced amorphous phase, likely due to the chain scission degradation reaction, increasing the weathered FDM-PLA's crystallinity. The increased crystallinity is confirmed by the XRD patterns obtained for these samples. The glass transition temperature (T g ) at 67°C shifts higher to 72°C and nearly loses the area under the peak. The smaller peak is a common consequence of the crystallisation and Norrish chain scission reactions during weathering. The increased crystallinity and cross-linking limit chain mobility, making it difficult for the polymer to transition from a glassy to a rubbery state. The shift in T g is attributed to a combination of annealing of the amorphous PLA and hydrolytic degradation [ 19 , 34 ]. The melting peak (T m ) initially gets sharper with UV + RH degradation, indicative of increased crystallinity; however, prolonged exposure causes the T m to shift lower from 171°C to 168°C for 1800 hours. The combined effect may suggest a complex mechanism between partial degradation and the impact of any plasticisers. Additionally, the shift in T g accompanied by the subtle exothermic peaks before thermal events indicate higher residual stresses in the exposed samples that may interfere with the T m reading. A second heating is recommended to release stored energy and further understand the change in the thermal behaviour of the polymer samples. 4 Conclusion This study establishes the time-dependent effects of accelerated weathering of MEX-AM PLA. It was noted that prolonged exposure leads to β-scission of the alkyl radical that breaks the C-C bonds, forming C = C groups. Further, the FDM-PLA specimens indicate over 50% crystallisation within 200 hours of cyclic UV + RH, continuing up to 72% at 2000 hours of exposure. These structural changes can significantly compromise the mechanical properties of the FDM-PLA, leading to decreased tensile strength and embrittlement. A significant decline in tensile strength was observed with UV + RH exposure, starting at a 10% loss at 200 hours, followed by a nearly 5% decrease with every 200-hour exposure. The study aimed to establish the time-dependent trend between weathering, mechanical properties, degradation time, crystallinity, and thermal properties. However, these tests were carried out in a simulated environment, and it is not straightforward to correlate these results with natural outdoor conditions. UV chambers have higher irradiance and temperatures, which may not determine MEX-AM PLA's durability in different environments. Declarations Acknowledgements We gratefully acknowledge the support of several individuals who significantly contributed to this work. We thank Dr Padmaraj for his invaluable assistance during the tensile testing experiments and providing accurate tensile test data. Finally, we thank Dr Sitarama Raju Kada, Dr Rusheni Senanayake and Matt Singleton for their support during the XRD and DSC experiments. Their expertise and guidance were instrumental in obtaining reliable thermal property data. Author Contributions Mirza Faizaan: Conceptualization, Data curation, Formal analysis, Methodology, Visualization, Writing – original draft. [email protected] Satish Shenoy Baloor: Conceptualisation, Methodology, Supervision, Writing – review & editing. [email protected] Srinivas Nunna: Conceptualisation, Formal analysis, Supervision, Validation, Writing – review & editing. [email protected] Suhas Yeshwant Nayak: Project administration, resources. [email protected] Rohit Nandakumar Shenoy: Investigation, resources. [email protected] Chandrakant Ramanath Kini: Conceptualisation, Writing – review & editing, Supervision [email protected] Claudia Creighton: Conceptualisation, Writing – review & editing, Supervision, Formal analysis, Validation. [email protected] Funding details: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. Declaration of conflict of interest The authors declare that there is no conflict of interest. ORCID Mirza Faizaan – https://orcid.org/0000-0002-5721-6430 Satish Shenoy Baloor – https://orcid.org/0000-0003-2374-3854 Srinivas Nunna – https://orcid.org/0000-0002-6384-4417 Suhas Yeshwant Nayak – https://orcid.org/0000-0002-8616-2021 Rohit Nandakumar Shenoy – https://orcid.org/0000-0002-8831-1430 Chandrakant Ramanath Kini – https://orcid.org/0000-0003-1540-1686 Claudia Creighton – https://orcid.org/0000-0002-3848-2696 Data Availability The authors declare that the data supporting the findings of this study are available within the article. The generated datasets are also available from the first and the corresponding authors on reasonable request. References González-López, M. E., del Martín, A. S., Robledo-Ortíz, J. R., Arellano, M. & Pérez-Fonseca, A. A. Accelerated weathering of poly(lactic acid) and its biocomposites: A review. Polym. Degrad. 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Influence of different β-nucleation agents on poly(L-lactic acid): structure, morphology, and dynamic mechanical behavior. RSC Adv. 7 , 55364–55370. https://doi.org/10.1039/C7RA10550A (2017). Echeverría, C., Limón, I., Muñoz-Bonilla, A., Fernández-García, M. & López, D. Development of Highly Crystalline Polylactic Acid with β-Crystalline Phase from the Induced Alignment of Electrospun Fibers. Polym. (Basel) . 13. https://doi.org/10.3390/POLYM13172860 (2021). Liu, X., Hua, X. & Wu, H. Degradation Behavior of Poly (Lactic Acid) during Accelerated Photo-Oxidation: Insights into Structural Evolution and Mechanical Properties. J. Polym. Environ. 32 , 3810–3821. https://doi.org/10.1007/S10924-024-03211-X/FIGURES/7 (2024). Gonzalez, M. F., Ruseckaite, R. A. & Cuadrado, T. R. Structural Changes of Polylactic-Acid (PLA) Microspheres under Hydrolytic Degradation. J. Appl. Polym. Sci. 71 , 1223–1230. https://doi.org/10.1002/(SICI)1097-4628(19990222)71:8 (1999). Table 2 Table 2 is available in the Supplementary Files section. Additional Declarations No competing interests reported. Supplementary Files Table2.docx Cite Share Download PDF Status: Published Journal Publication published 02 Mar, 2026 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 08 Oct, 2025 Reviews received at journal 29 Sep, 2025 Reviewers agreed at journal 18 Sep, 2025 Reviewers agreed at journal 18 Sep, 2025 Reviews received at journal 29 Aug, 2025 Reviewers agreed at journal 28 Aug, 2025 Reviewers invited by journal 28 Aug, 2025 Editor assigned by journal 26 Aug, 2025 Editor invited by journal 26 Aug, 2025 Submission checks completed at journal 13 Aug, 2025 First submitted to journal 13 Aug, 2025 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. 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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-7324138","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":507441296,"identity":"f4ad9ec9-f9d0-4eb0-a3a7-d8298fe8d91a","order_by":0,"name":"Mirza Faizaan","email":"","orcid":"","institution":"Manipal Academy of Higher Education","correspondingAuthor":false,"prefix":"","firstName":"Mirza","middleName":"","lastName":"Faizaan","suffix":""},{"id":507441297,"identity":"65450fce-1453-4685-a7bc-c1e49b34e81d","order_by":1,"name":"Satish Shenoy Baloor","email":"data:image/png;base64,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","orcid":"","institution":"Manipal Academy of Higher Education","correspondingAuthor":true,"prefix":"","firstName":"Satish","middleName":"Shenoy","lastName":"Baloor","suffix":""},{"id":507441298,"identity":"45900961-4f53-4075-bb32-e379d1c2038d","order_by":2,"name":"Srinivas Nunna","email":"","orcid":"","institution":"RMIT University","correspondingAuthor":false,"prefix":"","firstName":"Srinivas","middleName":"","lastName":"Nunna","suffix":""},{"id":507441299,"identity":"2280406e-7e1e-4836-9bd1-2436f9153f63","order_by":3,"name":"Suhas Yeshwant Nayak","email":"","orcid":"","institution":"Manipal Academy of Higher Education","correspondingAuthor":false,"prefix":"","firstName":"Suhas","middleName":"Yeshwant","lastName":"Nayak","suffix":""},{"id":507441300,"identity":"33b51877-8b01-406f-b791-01d218137c46","order_by":4,"name":"Rohit Nandakumar Shenoy","email":"","orcid":"","institution":"Manipal Academy of Higher Education","correspondingAuthor":false,"prefix":"","firstName":"Rohit","middleName":"Nandakumar","lastName":"Shenoy","suffix":""},{"id":507441301,"identity":"8d8b6ab2-bfbe-405f-9efa-ac69a7b21580","order_by":5,"name":"Chandrakant Ramanath Kini","email":"","orcid":"","institution":"Manipal Academy of Higher Education","correspondingAuthor":false,"prefix":"","firstName":"Chandrakant","middleName":"Ramanath","lastName":"Kini","suffix":""},{"id":507441302,"identity":"917887a2-242f-47eb-81ae-e6dbe7f2c98c","order_by":6,"name":"Claudia Creighton","email":"","orcid":"","institution":"Deakin University","correspondingAuthor":false,"prefix":"","firstName":"Claudia","middleName":"","lastName":"Creighton","suffix":""}],"badges":[],"createdAt":"2025-08-08 06:38:24","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7324138/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7324138/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41598-026-41192-0","type":"published","date":"2026-03-02T15:58:05+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":90616764,"identity":"654bb29c-4ef2-43b1-a6eb-d74db3d686a8","added_by":"auto","created_at":"2025-09-04 18:48:51","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":314310,"visible":true,"origin":"","legend":"\u003cp\u003e(a) UV+RH weathering chamber used for ageing FDM-PLA. (b) custom-made tensile sample mount to expose only the 50mm gauge length of the tensile test coupons. (c) ATR-FTIR probe on the tensile test coupon gauge section exposed to cyclic UV+RH.\u003c/p\u003e","description":"","filename":"Picture1.png","url":"https://assets-eu.researchsquare.com/files/rs-7324138/v1/e4d8c7ea1a0690889beb67fe.png"},{"id":90617264,"identity":"7c137f21-d9de-4c3f-80ce-68c787e647e3","added_by":"auto","created_at":"2025-09-04 18:56:51","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":376523,"visible":true,"origin":"","legend":"\u003cp\u003e(a) FTIR spectra for weathered samples in steps of 200 hours of exposure up to 2000 hours, and (b) illustrating the change in absorbance intensity of normalised spectra over continued exposure.\u003c/p\u003e","description":"","filename":"Picture2.png","url":"https://assets-eu.researchsquare.com/files/rs-7324138/v1/eaac6a8c35e20fc98d39dee5.png"},{"id":90616585,"identity":"061561c7-84e6-4074-9f21-37a332a33f1d","added_by":"auto","created_at":"2025-09-04 18:40:51","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":802914,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Temporal changes in FDM-PLA tensile test coupons exposed to UV+RH accelerated weathering for up to 2000 hours. (b) Surface cracks were observed on the gauge length of the 2000-hour sample. (c) Degradation of the tensile performance in the weathered test coupons over prolonged exposure. (d) Change in the test coupons' carbonyl index (CI) over prolonged UV+RH exposure.\u003c/p\u003e","description":"","filename":"Picture3.png","url":"https://assets-eu.researchsquare.com/files/rs-7324138/v1/fa904fdffb93d0eda8eb212d.png"},{"id":90616762,"identity":"4b49f707-b347-4e09-992c-aff897463288","added_by":"auto","created_at":"2025-09-04 18:48:51","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":133172,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Temporal change in XRD spectra with identified peaks observed at 2θ = 16.5°,18.75° and 29.45°, respectively, for progressive weathering exposure and (b) Change in per cent crystallinity (blue) and crystallite size (red) at each identified peak for 0 - 2000 hours of accelerated UV+RH weathering.\u003c/p\u003e","description":"","filename":"Picture4.png","url":"https://assets-eu.researchsquare.com/files/rs-7324138/v1/127eb8bd8521cc028c5c149b.png"},{"id":90616582,"identity":"501481c0-0146-457a-afaf-fe7b10af4c18","added_by":"auto","created_at":"2025-09-04 18:40:51","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":185915,"visible":true,"origin":"","legend":"\u003cp\u003eChange in heat flow from differential scanning calorimetry (DSC) of artificially weathered FDM-PLA over prolonged UV+RH exposure.\u003c/p\u003e","description":"","filename":"Picture5.png","url":"https://assets-eu.researchsquare.com/files/rs-7324138/v1/f8a1a05f7af2293ae3d714af.png"},{"id":104252178,"identity":"2e357327-0977-4cca-b48c-bb33873d278c","added_by":"auto","created_at":"2026-03-09 16:17:31","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2641503,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7324138/v1/60b0dec0-5293-4226-83d6-ec1f104a6667.pdf"},{"id":90616579,"identity":"67a9644f-24a2-4a9e-8fc8-bfeb768f55dc","added_by":"auto","created_at":"2025-09-04 18:40:51","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":69294,"visible":true,"origin":"","legend":"","description":"","filename":"Table2.docx","url":"https://assets-eu.researchsquare.com/files/rs-7324138/v1/7c60d223a02023447e9c6c2b.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Temporal evolution of structure property relationship for UV+RH artificially weathered material extrusion additive manufactured PLA","fulltext":[{"header":"1 Introduction","content":"\u003cp\u003eAssessing the outdoor conditions of polymers to evaluate their degradation in mechanical, chemical and thermal properties has been gaining interest due to the desire for maximising the lifetime of polymer structures. As weathering conditions are very local and complex to replicate due to the nature of non-controlled variables such as atypical hot and cold days, changing wind patterns, etc., accelerated weathering is considered representative of the outdoor conditions [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. The growing awareness of sustainable bio-based polymers has increased the demand for polylactic acid (PLA) across industries [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Owing to its low cost, ease of use, better mechanical properties than other 3D printable plastics and inherent biodegradability, PLA is the most common used thermoplastic in material extrusion additive manufacturing (MEX-AM), widely called Filament Fusion Fabrication (FFF) or Fused Deposition Modelling (FDM). Further, the advent of MEX-AM and rapid prototyping technology has transformed the polymer/plastic accessibility in every household [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eCurrently, very little literature is focused on the accelerated weathering of PLA and its composites [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. We observed that predominantly arbitrary exposure durations have been adopted in the literature. Table_ 1 summarises the tensile and flexural strength degradation caused by ultraviolet (UV) and relative humidity (RH) reported in the literature. The duration of exposure measured irradiance, percentage of decrement in mechanical strength, and testing standards adopted in each study are summarised.\u003c/p\u003e\u003cp\u003eTable_ 1 Summary of PLA artificial weathering caused by UV degradation.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"No\" id=\"Taba\" border=\"1\"\u003e\u003ccolgroup cols=\"5\"\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\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eExposure Condition\u003c/p\u003e\u003cp\u003e[hours; mW/m\u003csup\u003e2\u003c/sup\u003e]\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e% Decrement\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eTesting\u003c/p\u003e\u003cp\u003eStandard\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eRemark\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003eRef.\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e2000h; 890\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFS \u0026ndash; 99%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eASTM G154\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e600h; 39\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eTS \u0026ndash; 77.6%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e700h; 60000\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eTS \u0026ndash; 43.5%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eISO 4892-2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003eIrradiance calculation\u003c/p\u003e\u003cp\u003eerror likely\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e500h; 760\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eTS \u0026ndash; 12.5%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e600h; 39\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eTS \u0026ndash; 74%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e200h; 490\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eTS \u0026ndash; 61.1%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eISO 4892-3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e300h; 490\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eFS \u0026ndash; 83.2%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eISO 4892-3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e1200h; 890\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eTS \u0026ndash; 66.15%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eASTM G154\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e750h; 680\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003eTS \u0026ndash; 37.5%\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eASTM G154\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c5\"\u003e\u003cp\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"5\" nameend=\"c5\" namest=\"c1\"\u003e\u003cp\u003eFS \u0026ndash; Flexural Strength; TS \u0026ndash; Tensile Strength\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\u003eNotably, PLA is susceptible to UV light and moisture, such as photo-degradation and hydrolytic degradation [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. It is noted that PLA absorbs UV below 270 nm, and the degradation rate increases with temperature and RH. PLA polarity favours hydrolysis and photolysis [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. The intensity of photodegradation is highest for carbon arc lamps, followed by metal halide lamps, xenon lamps and outdoor exposure [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Photochemical degradation of PLA is widely studied in the literature [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan additionalcitationids=\"CR15\" citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e], and the degradation reaction is summarised in Table_ 2. PLA degrades following Norrish Type I and Norrish Type II reactions, beginning with homolysis of CH\u003csub\u003e3\u003c/sub\u003e groups (stage 1) and photochemical cleavage of the polymer chain, indicative of Norrish Type I behaviour (stage 2). The reaction progresses to the continued abstraction of H groups (stage 3), confirming Norrish Type II reaction, leaving free radicals (stage 4) that promote further cross-linking and chain scission (stage 5). Gonzalez-Lopez et al. [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e] reviewed the effect of additives, fillers and reinforcements on the photo-degradation and hydrolytic degradation of PLA and its biocomposites, noting that the degradation rate of PLA depends on the initial molecular weight, sample dimensions, polymer crystallinity and filler/reinforcements.\u003c/p\u003e\u003cp\u003eWith the increasing interest in PLA for MEX-AM and PLA-based composites as a bio-sourced biodegradable and sustainable alternative, it is imperative to understand the effect of weathering and environmental conditioning on the polymer's mechanical, chemical and thermal properties, especially as a product of MEX-AM. We know from literature that MEX-PLA tends to absorb twice as much water and has a higher crystallinity than injection moulded PLA [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. The FDM process-induced voids promote water uptake and induce residual stresses and the formation of cracks [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Chopra et al. exposed MEX-PLA to outdoor conditions in Aurangabad, India and elucidated the degradation mechanism of PLA [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. The authors further studied the effect of infill density and pattern on the degradation, confirming that samples printed with less than 100% infill density suffered higher degradation, resulting in the deterioration of their tensile performance. A similar decrease in tensile performance was observed in weathered PLA processed using methods other than FDM [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. Changes in thermal properties and crystallinity of artificially weathered PLA note that accelerated weathering forces rearrangement of amorphous PLA chains to highly ordered spherulites, resulting in a significant increase in the crystallinity of the PLA up to 60% [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan additionalcitationids=\"CR20\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe literature on the weathering of PLA focuses primarily on the effect of biocomposites, polymer blends, and fillers as opposed to neat PLA subjected to prolonged exposure times. Also, the literature adopts arbitrary exposure times to evaluate the degradation of these PLA composites [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. A reliable time-based trend representation is still missing in the literature. Furthermore, limited literature is available on the weathering of MEX-AM-based PLA. Thus, an experiment was designed to evaluate the time-dependent degradation of MEX-PLA specimens.\u003c/p\u003e\u003cp\u003eThis study examines the temporal evolution of UV\u0026thinsp;+\u0026thinsp;RH accelerated weathering on MEX-PLA tensile coupons. The samples were subjected to artificial weathering for up to 2000 hours to establish the trends in polymer properties and mechanical strength degradation that are currently missing in the literature. Fourier transform Infrared Spectroscopy (FTIR) was carried out on the samples before tensile testing, followed by X-Ray Diffraction (XRD) and Differential Scanning Calorimetry (DSC) on the gauge length of the tensile coupons. The percentage of crystallinity and crystallite size were calculated from the XRD spectra, and the thermal behaviour of weathered MEX-PLA was examined using DSC. Finally, the structure-property relations are discussed.\u003c/p\u003e"},{"header":"2 Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Fabrication of tensile test coupons\u003c/h2\u003e\u003cp\u003eSamples were fabricated on the open-source Creality Ender 3 V2 desktop 3D printer using a commercial 1.75 mm diameter eSun PLA\u0026thinsp;+\u0026thinsp;filament (white). A 0.6 mm nozzle was used to fabricate 100% concentric infill samples following ASTM 638D Type \u0026minus;\u0026thinsp;1 [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. A 0.15 mm layer thickness, 20 mm/s print speed at 210\u0026deg;C with two top/bottom 45\u0026deg; raster layers was selected to minimise voids in the test coupons. Tensile tests were performed on the BISS 50KN UTM with a 5 mm/min strain rate. Three samples were tested every 200 hours, up to 2000 hours of UV\u0026thinsp;+\u0026thinsp;RH exposure, to ensure repeatability, and tensile performance degradation over prolonged exposure times was analysed. The unexposed samples are referred to as \u0026lsquo;0 hours\u0026rsquo; for convenience. Thus, 33 samples were fabricated for accelerated weathering, while 30 test coupons were subjected to extreme conditions.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Accelerated weathering\u003c/h2\u003e\u003cp\u003eThe weathering tests were carried out according to ASTM G151 and G154 standards for ultraviolet (UV) weathering of non-metallic materials [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. An accelerated weathering chamber equipped with fluorescent UV-B lamps rated with a peak spectral intensity of 306 nm and a typical irradiance of approximately 0.49W\u0026bull;m-2 was used for the cyclic 8 hours UV and 4 hours condensation (relative humidity (RH)) at 60\u0026deg;C and 50\u0026deg;C, respectively. Figure_ 1a shows the weathering equipment, and the patented sheet metal tensile sample holder [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] designed to expose only the gauge length of the test coupons while preventing warping of the samples under prolonged temperature exposure is given in Figure_ 1b. The samples were flipped every 7 days to expose both sides of the tensile test coupons.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Fourier Transform Infrared Spectroscopy (FTIR)\u003c/h2\u003e\u003cp\u003eTo assess the surface degradation of 3D printed PLA, Fourier Transform Infrared (FTIR) spectroscopy was employed. A diamond Attenuated Total Reflectance (ATR) probe was used on the exposed surface of the gauge length of the tensile coupons. FTIR spectra were collected every 200 hours of cyclic exposure, with 50 scans recorded per sample on a Shimadzu IRSpirit spectrophotometer (Figure_ 1c). A total of 2526 data points were recorded for each FTIR measurement. Subsequently, the recorded spectra were processed on MATLAB, involving baseline correction, normalisation, and smoothing. As is used alternatively in literature, the carbonyl index (CI) was calculated by determining the ratio of both area under the curve and intensity, under the carbonyl (C\u0026thinsp;=\u0026thinsp;O) peak at 1750 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and methylene (CH\u003csub\u003e2\u003c/sub\u003e) peak at ~\u0026thinsp;3000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. The CI is a dimensionless parameter that provides valuable insights into the degree of polymer chain scission and crosslinking, key indicators of material degradation and embrittlement.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4 X-Ray Diffraction (XRD)\u003c/h2\u003e\u003cp\u003eThe 50mm gauge length sections were carefully cut using the Struers Accutom-50 precision cutter. XRD measurements were carried out on the X\u0026rsquo;PERT PRO with a Cu X-Ray tube emitting a wavelength of 1.5406\u0026Aring; for a range of 5\u0026ndash;50\u0026deg; with a scan time of 2.5s per step. XRD data collected for every 200 hours of exposure were batch processed and plotted on MATLAB, including calculating crystallinity [%] and crystallite size at different peaks. The per cent crystallinity was calculated by integrating the area under crystalline peaks, and the crystallite size (L) was calculated using the Scherrer equation:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:L=\\:K\\lambda\\:/\\beta\\:cos\\theta\\:$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere K\u0026thinsp;=\u0026thinsp;0.9, λ\u0026thinsp;=\u0026thinsp;1.5406 \u0026Aring;, β is the full-width half maximum (FWHM), and θ is the scan angle of the respective peaks.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.5 Differential Scanning Calorimetry (DSC)\u003c/h2\u003e\u003cp\u003eDSC was performed on a Netzsch DSC 214 Polyma in dynamic mode. The gauge section of the tensile coupon subjected to accelerated weathering conditions was carefully scraped using a scalpel to collect approximately 10mg of the sample. These were heated from 20\u0026deg;C to 300\u0026deg;C with a 10\u0026deg;C/min heating rate. Only the first heating was recorded, and the changes in glass transition (Tg), cold crystallisation (Tc), and melting (Tm) were analysed for all eleven samples, including unexposed (0 hours) up to 2000 hours in steps of 200 hours of UV\u0026thinsp;+\u0026thinsp;RH exposure.\u003c/p\u003e\u003c/div\u003e"},{"header":"3 Results and Discussion","content":"\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\n \u003cdiv class=\"Heading\"\u003e3.1.1 Fourier Transform Infrared Spectroscopy\u003c/div\u003e\n \u003cp\u003eFTIR spectra were collected using an ATR probe on the gauge length of the tensile coupons exposed to UV\u0026thinsp;+\u0026thinsp;RH weathering in an accelerated weathering chamber. FTIR spectra was captured for every 200 hours of weathering, starting from 0 hours (unexposed) up to 2000 hours of exposure, as seen in Figure_ 2a. The collected data were normalised and smoothed using MATLAB, and peaks were identified. The peak intensity, corresponding functional group and its vibrations are tabulated in Table_ 3. The changes in spectra over continued exposure are illustrated in Figure_ 2b.\u003c/p\u003e\n \u003cp\u003eTable_ 3 Peak assignment for artificially weathered UV\u0026thinsp;+\u0026thinsp;RH FDM-PLA.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003ctable id=\"Tabc\" border=\"1\"\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eWavelength [cm-1]\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eBond\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eVibration\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3800\u0026ndash;3600\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-O-H group\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eStretch\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3000\u0026ndash;2900\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003esp3 C-H group\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSymmetric Stretch\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e2900\u0026ndash;2800\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003esp2 C-H group\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAsymmetric Stretch\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e~\u0026thinsp;1750\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eC\u0026thinsp;=\u0026thinsp;O \u0026delta;-lactone\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eStretch\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1700\u0026ndash;1600\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eC\u0026thinsp;=\u0026thinsp;C group\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eStretch\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e~\u0026thinsp;1450\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eC-H deformation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAsymmetric Bend\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e~\u0026thinsp;1350\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eC-H deformation\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eSymmetric Bend\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1250\u0026ndash;1000\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eC-O group\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eStretch\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e900\u0026thinsp;\u0026minus;\u0026thinsp;850\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eC-C group\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eStretch\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e720\u0026ndash;820\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eC-H group\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eRocking\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003cp\u003eLike Chopra et al. [\u003cspan class=\"CitationRef\"\u003e3\u003c/span\u003e], new OH group peaks appeared at 3750 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, corresponding to free hydroxyl (-OH) groups possibly due to hydrolytic degradation under UV\u0026thinsp;+\u0026thinsp;RH exposure. The weak band at 3750 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e suggests free non-hydrogen bonded hydroxyl groups at low concentrations often appearing at surfaces, indicative of chain scission and surface oxidation of PLA [\u003cspan class=\"CitationRef\"\u003e28\u003c/span\u003e]. Splitting of the CH and CO bonds was observed at 1370 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1080 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, respectively, and the C\u0026thinsp;=\u0026thinsp;C stretch was shown owing to sp\u003csup\u003e2\u003c/sup\u003e CH. As biopolymers predominantly degrade by Norrish Type I, Norrish Type II, or both reactions when exposed to UV rays, the loss of peak intensity in the C-O stretch (1250\u0026ndash;1050 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and the characteristic C\u0026thinsp;=\u0026thinsp;O \u0026delta;-lactone (1750 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) are indicative of the \u0026alpha;-scission reactions (Norrish Type I) [\u003cspan class=\"CitationRef\"\u003e29\u003c/span\u003e]. The Norrish Type II reactions that present as an abstraction of hydrogen are visible as the loss of intensity of CH groups at 760 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, 1450\u0026ndash;1350 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, and the shift in peaks observed from 2925 to 2850 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (sp\u003csup\u003e3\u003c/sup\u003e to sp\u003csup\u003e2\u003c/sup\u003e hybridisation) [\u003cspan class=\"CitationRef\"\u003e3\u003c/span\u003e]. Furthermore, the presence of C\u0026thinsp;=\u0026thinsp;C group (~\u0026thinsp;1630 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) [\u003cspan class=\"CitationRef\"\u003e30\u003c/span\u003e] occurring subsequently due to prolonged UV\u0026thinsp;+\u0026thinsp;RH exposure of over 1200 hours, led to \u0026beta;-scission of the alkyl radical that breaks the C-C bonds [900\u0026thinsp;\u0026minus;\u0026thinsp;850 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e], forming C\u0026thinsp;=\u0026thinsp;C groups. These structural changes can significantly compromise the mechanical properties of the FDM-PLA, leading to decreased tensile strength and embrittlement.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section3\"\u003e\n \u003cdiv class=\"Heading\"\u003e3.1.2 Tensile Performance\u003c/div\u003e\n \u003cp\u003eThree MEX-PLA tensile coupons were tested for every 200 hours of UV\u0026thinsp;+\u0026thinsp;RH exposure at 60\u0026deg;C and 50\u0026deg;C, respectively, up to 2000 hours of exposure. Figure_ 3a illustrates the progressive change in colour and surface roughness of the specimens over prolonged periods of UV exposure, leading to cracks and flaking of the topmost layer (Figure_ 3b). A significant decline in tensile strength (Figure_ 3c) was observed with UV\u0026thinsp;+\u0026thinsp;RH exposure, starting at a 10% loss at 200 hours, followed by a near 5% decrease with every consecutive 200-hour exposure. After 1200 hours, the tensile strength exhibited large deviations, likely due to increased brittleness. The increased brittleness correlates with the observed slowdown or plateau in the rate of carbonyl group formation after a sharp increase up to 800 hours (Figure_ 3d). The subsequent increase and fluctuation in the CI after 1200 hours suggest that the polymer may have reached a state of saturation or alternative degradation mechanisms have become more dominant. Interestingly, the tensile modulus remains unaffected by the prolonged exposure and may be attributed to the increasing crystallinity of the samples, which has been shown to increase the tensile modulus of the weathered samples [\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003e3.1.3 X-Ray Diffraction (XRD):\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section3\"\u003e\n \u003cp\u003eXRD patterns were scanned in the gauge length of the exposed samples to evaluate the effect of prolonged UV\u0026thinsp;+\u0026thinsp;RH exposure on the crystallisation of semi-crystalline FDM-PLA. Figure_ 4a illustrates the semi-crystalline PLA with a broad amorphous halo and crystalline peak centred around 2\u0026theta;\u0026thinsp;=\u0026thinsp;16.5\u0026deg; and 29.5\u0026deg;, respectively, indicative of a predominantly amorphous structure. Upon UV\u0026thinsp;+\u0026thinsp;RH exposure, the amorphous halo resolved into two distinct characteristic \u0026alpha;-phase crystalline peaks at 2\u0026theta;\u0026thinsp;=\u0026thinsp;16.5\u0026deg; and 18.75\u0026deg; as a result of crystallisation [\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e], with increasing intensity for the former while the intensity remains unchanged for the latter. Interestingly, the crystalline peak at 29.45\u0026deg; decreases over continued exposure and is ascribed to the \u0026beta;-crystals [\u003cspan class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e32\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eFigure_ 4b summarises the crystallinity (%) and crystallite size for the three distinct peaks observed at 2\u0026theta;\u0026thinsp;=\u0026thinsp;16.5\u0026deg; (200/110),18.75\u0026deg; (203) and 29.5\u0026deg; (0010), respectively. The near amorphous PLA with 2.24% crystallinity crystallises to 57% with 200 hours of UV\u0026thinsp;+\u0026thinsp;RH exposure and steadily increases up to 71.84% with prolonged exposure up to 2000 hours. Sawpan et al. also noted that the degree of crystallinity for artificially weathered PLA could exceed 50% [\u003cspan class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e]. While the crystallite size for 16.5\u0026deg; and 18.75\u0026deg; peaks shares a similar trend, the peak at 29.5\u0026deg; decreases with exposure times from 24.12nm to 16.35nm. These findings suggest that the UV\u0026thinsp;+\u0026thinsp;RH weathering induces crystallisation in the FDM-PLA samples, leading to a more crystalline and potentially less stable material, which translates to a decline in the tensile performance of these specimens.\u003c/p\u003e\n \u003cp\u003eOwing to the open chain ends created by the chain scission reaction, the content of amorphous segments in the polymer structure increases [\u003cspan class=\"CitationRef\"\u003e33\u003c/span\u003e]. As the broken segments have higher freedom to form more organised structures, they try to crosslink to stabilise the structure, ultimately increasing crystallinity. Although this may increase the modulus [\u003cspan class=\"CitationRef\"\u003e3\u003c/span\u003e], the chain scission is more detrimental than any further cross-linking. However, the exact cause for this behaviour can be complex. DSC was employed to analyse the changes in the polymer\u0026rsquo;s thermal properties to understand the specific changes in the weathered PLA fully.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section3\"\u003e\n \u003cdiv class=\"Heading\"\u003e3.1.4 Differential Scanning Calorimetry (DSC)\u003c/div\u003e\n \u003cp\u003eFDM-PLA tensile coupons were shaved at the exposed gauge length section and analysed using DSC to understand the changes in thermal behaviour over time of weathering exposure. The DSC curves were baseline corrected and overlayed in Figure_ 5. The cold crystallisation peak (T\u003csub\u003ec\u003c/sub\u003e) at 96\u0026deg;C is only visible in the unexposed (0 hours) samples, indicative of the semi-crystalline nature of the polymer. The T\u003csub\u003ec\u003c/sub\u003e disappears when exposed to UV\u0026thinsp;+\u0026thinsp;RH weathering, confirming the reduced amorphous phase, likely due to the chain scission degradation reaction, increasing the weathered FDM-PLA\u0026apos;s crystallinity. The increased crystallinity is confirmed by the XRD patterns obtained for these samples.\u003c/p\u003e\n \u003cp\u003eThe glass transition temperature (T\u003csub\u003eg\u003c/sub\u003e) at 67\u0026deg;C shifts higher to 72\u0026deg;C and nearly loses the area under the peak. The smaller peak is a common consequence of the crystallisation and Norrish chain scission reactions during weathering. The increased crystallinity and cross-linking limit chain mobility, making it difficult for the polymer to transition from a glassy to a rubbery state. The shift in T\u003csub\u003eg\u003c/sub\u003e is attributed to a combination of annealing of the amorphous PLA and hydrolytic degradation [\u003cspan class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e34\u003c/span\u003e]. The melting peak (T\u003csub\u003em\u003c/sub\u003e) initially gets sharper with UV\u0026thinsp;+\u0026thinsp;RH degradation, indicative of increased crystallinity; however, prolonged exposure causes the T\u003csub\u003em\u003c/sub\u003e to shift lower from 171\u0026deg;C to 168\u0026deg;C for 1800 hours. The combined effect may suggest a complex mechanism between partial degradation and the impact of any plasticisers. Additionally, the shift in T\u003csub\u003eg\u003c/sub\u003e accompanied by the subtle exothermic peaks before thermal events indicate higher residual stresses in the exposed samples that may interfere with the T\u003csub\u003em\u003c/sub\u003e reading. A second heating is recommended to release stored energy and further understand the change in the thermal behaviour of the polymer samples.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4 Conclusion","content":"\u003cp\u003eThis study establishes the time-dependent effects of accelerated weathering of MEX-AM PLA. It was noted that prolonged exposure leads to β-scission of the alkyl radical that breaks the C-C bonds, forming C\u0026thinsp;=\u0026thinsp;C groups. Further, the FDM-PLA specimens indicate over 50% crystallisation within 200 hours of cyclic UV\u0026thinsp;+\u0026thinsp;RH, continuing up to 72% at 2000 hours of exposure. These structural changes can significantly compromise the mechanical properties of the FDM-PLA, leading to decreased tensile strength and embrittlement. A significant decline in tensile strength was observed with UV\u0026thinsp;+\u0026thinsp;RH exposure, starting at a 10% loss at 200 hours, followed by a nearly 5% decrease with every 200-hour exposure. The study aimed to establish the time-dependent trend between weathering, mechanical properties, degradation time, crystallinity, and thermal properties. However, these tests were carried out in a simulated environment, and it is not straightforward to correlate these results with natural outdoor conditions. UV chambers have higher irradiance and temperatures, which may not determine MEX-AM PLA's durability in different environments.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe gratefully acknowledge the support of several individuals who significantly contributed to this work. We thank Dr Padmaraj for his invaluable assistance during the tensile testing experiments and providing accurate tensile test data. Finally, we thank Dr Sitarama Raju Kada, Dr Rusheni Senanayake and Matt Singleton for their support during the XRD and DSC experiments. Their expertise and guidance were instrumental in obtaining reliable thermal property data.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMirza Faizaan:\u003c/strong\u003e Conceptualization, Data curation, Formal analysis, Methodology, Visualization, Writing \u0026ndash; original draft.\u0026nbsp;\u003c/p\u003e\n\u003cp\[email protected]\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSatish Shenoy Baloor:\u003c/strong\u003e Conceptualisation, Methodology, Supervision, Writing \u0026ndash; review \u0026amp; editing.\u0026nbsp;\u003c/p\u003e\n\u003cp\[email protected]\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSrinivas Nunna:\u003c/strong\u003e Conceptualisation, Formal analysis, Supervision, Validation, Writing \u0026ndash; review \u0026amp; editing.\u003c/p\u003e\n\u003cp\[email protected]\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eSuhas Yeshwant Nayak:\u003c/strong\u003e Project administration, resources. \u0026nbsp;\u003c/p\u003e\n\u003cp\[email protected]\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eRohit Nandakumar Shenoy:\u003c/strong\u003e Investigation, resources.\u0026nbsp;\u003c/p\u003e\n\u003cp\[email protected]\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eChandrakant Ramanath Kini:\u003c/strong\u003e Conceptualisation, Writing \u0026ndash; review \u0026amp; editing, Supervision\u003c/p\u003e\n\u003cp\[email protected]\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eClaudia Creighton:\u003c/strong\u003e Conceptualisation, Writing \u0026ndash; review \u0026amp; editing, Supervision, Formal analysis, Validation.\u003c/p\u003e\n\u003cp\[email protected]\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding details:\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of conflict of interest\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that there is no conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eORCID\u003c/strong\u003e\u003c/p\u003e\n\u003col\u003e\n \u003cli\u003eMirza Faizaan \u0026ndash; https://orcid.org/0000-0002-5721-6430\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eSatish Shenoy Baloor \u0026ndash; https://orcid.org/0000-0003-2374-3854\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eSrinivas Nunna \u0026ndash; https://orcid.org/0000-0002-6384-4417\u003c/li\u003e\n \u003cli\u003eSuhas Yeshwant Nayak \u0026ndash; https://orcid.org/0000-0002-8616-2021\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eRohit Nandakumar Shenoy \u0026ndash; https://orcid.org/0000-0002-8831-1430\u003c/li\u003e\n \u003cli\u003eChandrakant Ramanath Kini \u0026ndash; https://orcid.org/0000-0003-1540-1686\u0026nbsp;\u003c/li\u003e\n \u003cli\u003eClaudia Creighton \u0026ndash; https://orcid.org/0000-0002-3848-2696\u0026nbsp;\u003c/li\u003e\n\u003c/ol\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that the data supporting the findings of this study are available within the article. The generated datasets are also available from the first and the corresponding authors on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eGonz\u0026aacute;lez-L\u0026oacute;pez, M. E., del Mart\u0026iacute;n, A. S., Robledo-Ort\u0026iacute;z, J. R., Arellano, M. \u0026amp; P\u0026eacute;rez-Fonseca, A. A. Accelerated weathering of poly(lactic acid) and its biocomposites: A review. \u003cem\u003ePolym. Degrad. Stab.\u003c/em\u003e 2020;179. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.polymdegradstab.2020.109290\u003c/span\u003e\u003cspan address=\"10.1016/j.polymdegradstab.2020.109290\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eCastro-Casado, D. 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Sci.\u003c/em\u003e \u003cb\u003e71\u003c/b\u003e, 1223\u0026ndash;1230. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1002/(SICI)1097-4628(19990222)71:8\u003c/span\u003e\u003cspan address=\"10.1002/(SICI)1097-4628(19990222)71:8\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e (1999).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Table 2","content":"\u003cp\u003eTable 2 is available in the Supplementary Files section.\u003c/p\u003e"}],"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":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"3D Printing, Additive Manufacturing, FDM, PLA, Temporal evolution, Tensile Properties, Artificial Weathering","lastPublishedDoi":"10.21203/rs.3.rs-7324138/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7324138/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study addresses the underreported temporal evolution of weathering on material extrusion additive-manufactured (MEX-AM) polylactic acid (PLA). Overcoming the limitation of arbitrary exposure durations in existing literature, a time-dependent investigation was conducted on MEX-PLA samples subjected to prolonged artificial weathering for up to 2000 hours using a UV-B equipped accelerated weathering chamber with controlled relative humidity. The changes in mechanical, chemical and thermal properties were analysed at 200-hour intervals. The results revealed a time-dependent degradation mechanism characterised by β-chain scission. FTIR analysis confirmed the formation of C\u0026thinsp;=\u0026thinsp;C groups and the progressive loss of H groups, indicating substantial material degradation. Furthermore, DSC and XRD data demonstrated a progressive increase in crystallinity with prolonged exposure, leading to a significant reduction in tensile strength. At the same time, the tensile modulus remained relatively stable for MEX-AM PLA.\u003c/p\u003e","manuscriptTitle":"Temporal evolution of structure property relationship for UV+RH artificially weathered material extrusion additive manufactured PLA","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-09-04 18:40:46","doi":"10.21203/rs.3.rs-7324138/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-10-08T08:33:07+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-09-29T05:24:47+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"65203474509286394066084592605703792117","date":"2025-09-18T23:16:33+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"88334458724517841078084447029010194831","date":"2025-09-18T16:33:35+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-08-29T11:52:42+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"155713729366045603492205661055278466255","date":"2025-08-28T07:10:09+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-08-28T06:45:41+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-08-26T18:13:43+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-08-26T18:03:36+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-08-13T05:11:30+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-08-13T05:08:30+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"de635f3c-c1c3-4176-8386-048723f8213d","owner":[],"postedDate":"September 4th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":53902359,"name":"Physical sciences/Chemistry"},{"id":53902360,"name":"Physical sciences/Engineering"},{"id":53902361,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2026-03-09T16:15:38+00:00","versionOfRecord":{"articleIdentity":"rs-7324138","link":"https://doi.org/10.1038/s41598-026-41192-0","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2026-03-02 15:58:05","publishedOnDateReadable":"March 2nd, 2026"},"versionCreatedAt":"2025-09-04 18:40:46","video":"","vorDoi":"10.1038/s41598-026-41192-0","vorDoiUrl":"https://doi.org/10.1038/s41598-026-41192-0","workflowStages":[]},"version":"v1","identity":"rs-7324138","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7324138","identity":"rs-7324138","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}