The Oddy test applied to the study of fused deposition modelling additive manufacturing materials

Abstract

In the recent years, Additive Manufacturing (AM) technologies ―popularly known as 3D printing― have been introduced in different areas of daily life and nowadays a wide variety of polymeric printing materials are available. The term AM comprises a huge range of fabrication processes mainly characterized by the method employed during the transformation of the raw materials. Amongst them, Extrusion-based technologies, and specifically Fused Deposition Modelling (FDM) have become very popular, affordable and widespread for many applications. Three-dimensional printing has demonstrated a relevant potential to contribute to the conservation and restoration of cultural heritage, as well as to artistic production. Some applications of AM in the conservation landscape are the design of display systems, the creation of storage structures and the reconstruction of missing parts. Thus, the interaction of new AM materials introduced in the proximity of cultural assets in a closed space could represent a risk of damage when gaseous pollutants are emitted from the printing material. In this context, a literature review was conducted, and the Oddy test was performed to assess a selection of 9 FDM PLA materials, evaluating their long-term behaviour in terms of Volatile Organic Compounds (VOCs) emissions and their suitability for use in heritage conservation and artistic production. Polylactic acid (PLA) is a widespread polymer used for FDM due to its cost and the rise of the social awareness concerning the relevance of using waste-recycling and biodegradable materials. This assay was performed both before and after subjecting the 3D printed samples to accelerated ageing in a Xenon-Arc chamber during 1080 h of exposure with 60 W/m2 irradiance through a window glass filter, temperature of 60 °C and 10 % relative humidity. After the Oddy test, the FDM materials were evaluated, and their recommendation of use was classified based on the corrosion observed on the metallic coupons. Simultaneously, crystalline efflorescences emerged from the 3D printed samples exposed to the Oddy Test, motivating their observation through optical microscopy. Additionally, for the chemical characterization, inorganic fillers of the PLA filaments were analysed with Energy Dispersive X-ray spectroscope attached to a Scanning Electron Microscope (SEM-EDX). The results allowed the classification of PLA filaments for their permanent or temporary use in an environment with cultural assets and reject those that released harmful pollutants. It is remarkable the effect of the additives and fillers when comparing the results in terms of corrosive VOC emissions of different composite PLA materials. The lower corrosion of the metals was promoted by PLA containing CaCO3 fillers, while other PLA combinations were completely discarded. Nonetheless, crystalline efflorescences emerged from 3D printed samples containing mineral fillers. It is suggested that the combination of continuous high temperature and saturated relative humidity is a probable cause of these transformations. Thus, the potential for these minerals to grow at ambient conditions over an extended period and migrate to a cultural asset that is in direct contact with the 3D printed material should be also considered for their recommendation or rejection.