Evaluating the photooxidative ageing properties of 3D printed plastics: strategies for their use and conservation in cultural heritage contexts

Abstract

3D printing is a fairly ubiquitous term today, due in part to the dissemination of the manufacturing technique to a wide variety of applications. While initially developed as a prototyping tool for product development, enterprising individuals situated outside engineering and concept prototyping have integrated this technology to suit broader applications. The advent of new printing processes and print materials, coupled with the democratisation of this equipment, has introduced this technology to many new end users, including those working within cultural heritage. 3D printing applications in the arts stem from artists who employ the unique printing geometries offered by 3D printers to realise their designs, to curators eager to engage and enhance visitor experience through touch and to conservators who aim to find alternative approaches to treatment when repairing damaged objects. The adaptation of digital technologies within cultural heritage has created extensive opportunities for practitioners within the field; however, there lies an uncertainty in their application and ability to reconstruct missing elements given the recent implementation of 3D-printed material types and the lack of studies detailing their behaviour over time.

Many of the materials employed in the 3D printing process are plastic polymers that have, in other forms, presented long-term stability challenges–the reality of which may prove problematic for professionals working within the cultural heritage sector. These synthetic polymeric materials exhibit susceptibility to thermal, mechanical, photooxidative, hydrolytic, and biodegradation degradation processes through pathways that include hydrolysis, photolysis, thermolysis and oxidation. Of these degradation mechanisms, ultraviolet radiation and visible light represent more significant concerns for longevity within a cultural heritage environment, due to the need for light to facilitate visitor access, as well as the destructive impact photodegradation has on the physical appearance of the material–the primary function of which was originally its aesthetic. Visible light cannot be entirely removed from a cultural heritage environment, and ultraviolet radiation is introduced to the environment through solar radiation and in some artificial light sources. Consequently, visible and ultraviolet regions of the electromagnetic spectrum are likely to deleteriously impact 3D-printed polymers.

To determine the impact of these conditions, this thesis explores the susceptibility of 9 different 3D print plastic polymers to photodegradation through two accelerated ageing experiments designed to 1) induce photooxidation for comparison and 2) simulate material responses to a museum environment over a period of 40 years. Materials selected for evaluation include examples from each of the primary manufacturing processes–extruded thermoplastics, photopolymers, and binding printers–and represent those commonly employed by artists for fabrication, by museum professionals creating replica models and by conservators fashioning missing components for damaged objects. Accelerated ageing experiments also included four conservation-grade materials currently classified as best-practice for use with objects, which function as a benchmark to interpret the aged physical and chemical changes in the 3D printed plastics.

Experimental data revealed a clear delineation in the degradation rates evidenced within individual printed polymers and provided an explanation for the discolouration exhibited following exposure to ultraviolet radiation and visible light. A proclivity for photooxidation was determined to stem primarily from the presence of the unsaturated double-bonded carbon backbone in the butadiene rubber constituent. Additional polymer susceptibility was observed through the photolysis of material bonds and the presence of light-absorbing impurities likely introduced during manufacturing. In all cases, the long-wave UV component precipitated degradation. Across both experimental designs, acrylonitrile butadiene styrene (ABS) performed the poorest, while polylactic acid (PLA) and acrylonitrile styrene acrylate (ASA) exhibited the best resistance to photooxidation. Overall, the experimental results are consistent with the initial hypothesis that 3D-printed polymers are likely susceptible to photodegradation.

Further, in comparison with the 3D printed plastic polymers, the four conservation-grade materials did not exhibit significant material deterioration. These results indicate that materials currently considered best-practice within conservation performed better than the 3D-printed polymers examined in this thesis. Ergo, 3D-printed polymers do not present as an ideal alternative to materials currently employed for loss compensation, unless additional methods are taken to mitigate exposure.

When situated within an a cultural heritage context, these results hold considerable significance for the profession. They inform 3D-printed material selections for artists concerned with the iii

longevity of materials employed in the manufacture of their art; they facilitate future approaches for curators and conservators tasked with the display and storage of 3D-printed artworks; and they guide the approach for integrating 3D print materials with conservation treatments. Recommendations developed as a result of this research include the elimination of UV-producing light sources and the integration UV filters to block solar radiation. Visible light did not appear to significantly impact the polymers tested, however it was shown to fade the cream colourant present in the acrylonitrile styrene acrylate (ASA) sample.