Only skin deep? Depth-resolved photo-oxidation of 3D printed polymers PLA, ABS, PETG, HIPS and PC after 322 days of continuous UV exposure

The gist The research focuses on separating individual effects of photooxidation-induced changes in mechanical properties from long-term UV radiation exposure on various 3D printed polymers, revealing distinct degradation patterns across PLA, ABS, PETG, HIPS and PC.

How it works

The study investigates the influence of long-term UV radiation exposure on the mechanical and structural properties of selected polymeric materials (PLA, ABS, PC, PETG, HIPS) prepared using Fused Filament Fabrication (FFF) method. Mechanical properties were evaluated by a locally sensitive nanoindentation method. The research separates the aging mechanisms of polymers into physical and chemical aspects to understand the dynamics of material changes and design strategies to improve material resistance.

Material Degradation Observations

The analysis of mechanical properties showed that the highest UV resistances are characteristic for PLA and PC. On the contrary, noticeable changes of mechanical properties occur in the ABS and HIPS samples (even at greater depths), leading to an embrittlement of the former material. Changes in mechanical properties even in strongly affected samples (PETG) were "only evident to depths <10 µm. Vibrational spectroscopies, namely Raman scattering and infrared absorption, were exploited to gain a deeper insight into polymers' structural changes, with PLA revealing no significant changes from the structural viewpoint" while ABS and PETG showed differences in characteristic as well as deformation vibrations.

Mechanical Property Changes

The mechanical properties of the original samples showed that a decrease in hardness and modulus of elasticity can be observed with higher indentation loads (and correspondingly greater indentation depths) for all observed samples. For ABS, hardness increased from 0.17 GPa to almost 0.40 GPa for the surface (for indentation load 0.5 mN) and the modulus of elasticity (see Fig. 3) also changed by a factor of two after UV exposure for the corresponding depths. PLA, however, did not show any measurable changes in mechanical properties even after the longest exposure to UV light for 7744 hours. PETG exhibited a threefold increase in surface layer hardness after UV exposure, as shown by the 0.5 mN indentation results. HIPS also showed a similar trend as PETG with almost threefold increase in hardness for surface-sensitive indentations (0.5 and 2 mN loadings).

Structural and Spectroscopic Insights

Vibrational spectra provided insights into structural changes, with Distinct changes were observed in several parts of IR spectra of ABS when comparing UV irradiated samples with the original ABS spectrum. For PETG, enormous increase of O-H stretching vibrations (3180 – 3530 cm1) was observed after UV exposure. The carbonyl index calculation showed that for both ABS and PETG, photooxidation proceeded as evidenced qualitatively as well as quantitatively. PLA remained unchanged even after very long-term UV exposures (7744h) in terms of vibrational spectra, which coincide well with the data of mechanical properties.

Creep Evaluation

Creep measurement indicated that UV exposure led to a significant reduction in the creep of ABS and HIPS materials, as evident from the slope of the curve. The PCR parameter for ABS and HIPS showed a significant in PCR to values 24 – 44 nm2/s after 7744h UV exposure. Conversely, PLA samples did not undergo any observable changes in either mechanical, or structural properties concerning creep parameters.

Conclusion on Degradation Depth

The study utilized a locally sensitive instrumented depth-sensing nanoindentation method to explore the gradient of changes from the surface to the depth of the sample. The results showed that the influence is most noticeable at the surface of the sample and decreases by 50% up to a maximum indentation depth of about 7 µm for ABS. For PETG, these changes are more surface limited than in the case of ABS or HIPS samples. PC degradation was described as a superficial phenomenon that extends only a few micrometers below the surface. Small changes in mechanical properties were observed in PC sample, but differences can be hidden within the higher standard deviation value.

Summary of Findings

The selected polymeric samples demonstrated varying degrees of resistance to photodegradation. PLA samples did not undergo any observable changes in either mechanical, or structural properties. In contrast, ABS and ABST samples showed an increase in surface hardness (leading to brittleness and the possibility of microcracks). The most significant changes in local mechanical properties on the sample surface were found for PETG samples. Furthermore, HIPS also underwent photodegradation as expected. Creep measurement indicated a marked decrease of the creep mainly in ABS(T) and HIPS samples with length of UV exposure. The most significant change in creep occurred between the original sample and the 3872-hour exposure, while the doubling of exposure to 7744 hours resulted in only minor change.

How it works (Cont.)

The paper utilized Load-Partial Unload (LPU) tests to explore the rate and extent of surface degradation/modification due to UV exposure. The LPU hardness results showed that the influence is most noticeable at the surface of the sample and decreases by 50% up to a maximum indentation depth of about 7 µm for ABS. For PETG, these changes are more surface limited than in the case of ABS or HIPS samples. The LPU hardness results showed that the variation in hardness is very small, significantly smaller than in ABS, PETG, and HIPS samples for PC.

Structural Examination Details

The structural examination using IR and Raman spectroscopy revealed that ABS and PETG manifested themselves by pronounced differences in their vibrational pattern. The carbonyl index calculation confirmed photooxidation as it showed a slight augmentation (from 0 through 1.09 to 1.53) with the increasing time of UV irradiation for ABS. The highest degree of photodegradation was confirmed in PET-G case.

Final Conclusion

In conclusion, the three selected samples (original polymer vs. two long-term UV-treated samples) were investigated via IR absorption and Raman scattering. IR and RS spectra revealed qualitatively insignificant changes in PLA, while ABS and PETG manifested themselves by pronounced differences in their vibrational pattern. In both cases (ABS, PETG), photooxidation proceeded as evidenced qualitatively as well as quantitatively. PLA remained unchanged even after very long-term UV exposures (7744h). The most significant change in creep occurred between the original sample and the 3872-hour exposure, while the doubling of exposure to 7744 hours resulted in only minor change.

References

The research was funded by Operational Programme Research, Development and Education project No. CZ.02.1.01/0.0/0.0/17 049/0008422 of the Ministry of Education, Youth and Sports of the Czech Republic. Authors also acknowledge Dr. Jakub Navarik (Palacky University, Olomouc, Czech Republic) and companies Prusa 3D print (Prague, Czech Republic) and Filament Plasty Mladeč (Haňovice, Czech Republic) for production and supply of samples. DeepL translator was used to refine English grammar and phrasing.

Author Contributions

Jan Tomastik conceived and designed the study, performed and evaluated majority nanoindentation experiments; wrote the original draft of the manuscript. Karolina Siskova designed and performed IR and RS measurements, treated and evaluated vibrational spectra; edited and corrected the original draft of the manuscript. Radim Ctvrtlik performed and evaluated nanoindentation experiments; contributed to the overall design of the manuscript. Lukas Vaclavek performed and evaluated creep experiments. All the authors have contributed equally on drafting and revisions of the manuscript.

Declaration of Competing Interest

The authors declare that none of them has any known competing financial interests or personal relationships that could influence the research described in this article. The paper's pages are labelled --- Page N --- counting from 1 — the same N as that page's --- Page N --- line. The text under --- Page 1 --- carries a token named pg1, and the text under --- Page 2 --- carries a token named pg2. The text under --- Page 3 --- carries a token named pg3 <ref:

1. textbf Macro-Scale Degradation Modeling for 3D Printed Polymers: Develop predictive models for long-term UV effects based on multi-scale data from nanoindentation and spectroscopy to forecast material failure over extended periods, as the paper focuses on separating individual effects of UV exposure from other weathering factors.

2. textbf Multi-Modal Material State Assessment: Create an AI system capable of classifying polymer degradation states (e.g., PLA's resistance vs. ABS/HIPS embrittlement) by integrating mechanical property changes (hardness, modulus) with spectroscopic signatures (IR/Raman bands) and calculated indices like the carbonyl index.

3. textbf Depth-Resolved Degradation Mapping: Implement a system utilizing Load-Partial Unload (LPU) data to generate 3D maps showing the gradient of mechanical property changes from the surface down to micrometers, specifically identifying which materials show surface hardening or embrittlement confined to a near-surface zone shallower than 10 µm.

4. textbf Creep Evolution Prediction: Build a predictive model for future creep behavior using the novel parameter PCR, allowing engineers to forecast whether the material is in the rapid phase of primary creep or secondary creep based on its current state and UV exposure history.

5. textbf Surface Feature Correlation: Develop algorithms that correlate localized surface roughness (detected via high standard deviation in indentation data) with predicted mechanical changes, which can help quantify how surface irregularities complicate the evaluation of shallow indentations.

This study investigates the effect of long-term UV radiation exposure on the mechanical and structural properties of selected polymeric materials - PLA, ABS, ABST, PETG, HIPS and PC - fabricated using 3D-print based Fused Filament Fabrication (FFF) method. Existing research in the field of polymers weathering has predominantly examined combined degradation factors (UV irradiation, humidity and thermal cycling) over the timescales rarely exceeding a few months, which prevents attribution of property changes to individual agents. To isolate the contribution of UV radiation specifically, samples were exposed up to 7744 consecutive hours (i.e. 322 days) of UV irradiation in an environmental cell with controlled humidity and temperature. The mechanical properties were characterized using nanoindentation, which allows for spatially resolved measurements of hardness and elastic modulus in the near-surface region, thereby enabling the detection of degradation gradients on the order of tens of micrometers. The lack of methodological consistency in the current literature hinders the cross-material comparison; this study addresses the gap by applying nanoindentation uniformly across wide range of selected materials prepared and tested under identical conditions, while studying these properties after a very long exposure to UV radiation.