The effect of high-intensity gamma radiation on PETG and ASA polymer-based fused deposition modelled 3D printed parts

There is growing interest in the application of fused deposition modelled (FDM) parts in environments subject to gamma radiation. A few examples of these applications include the sterilisation of surgical equipment [1‐4], deployment of robotics in the nuclear industry [5, 6] and flexible manufacturing during space missions [7, 8]. As interest in additive manufacturing (AM) in these sectors continues to grow, it will become important to understand the chemical and mechanical response of 3D printed parts exposed to intense gamma radiation.

The chemical mechanisms that occur when gamma-ray radiation is applied to polymers manufactured using traditional methods have been well documented [9]. However, few papers have investigated plyometric AM parts subject to intense gamma radiation, which may react differently to traditionally manufactured polymer response. Regarding irradiation of additively manufactured polymetric parts, the AM process introduces control over part porosity, a control aspect that may well influence the response to radiation.

The primary mechanisms that occur in polymers exposed to radiation are cross-linking and chain scission [10]. Cross-linking increases the complexity of the three-dimensional polymer network, ultimately increasing molecular mass with exposure to radiation [11]. This occurs through the free radicalisation of electrons, which induces oxidation and can eventually lead to the material’s embrittlement and degradation [12]. In contrast, chain scission occurs through the breaking of long polymer chains, leading to the material’s softening and loss of elastic properties [13]. Many factors can affect the response of a material to radiation, including molecular structure, temperature, dose rate and dissolved oxygen [10]. While exposure to sufficient radiation can substantially weaken materials, in the context of 3D printing, radiation has been used to reduce the inherent anisotropy of 3D printed parts by inducing cross-linking within the microstructure [14].

The magnitude of radiation exposure varies between applications. A typical unit of measurement for radiation exposure is the gray (or Gy), where one gray is a unit radiation dose equivalent to a joule of energy absorbed per kilogram (or 100 rad) [15]. Depending on the application and duration of deployment, the range of dosages experienced by a workforce and/or apparatus will vary substantially. Example dosages for specific applications are listed in Table 1. These vary from an upper extreme in applications such as nuclear incident clean-up using remotely operated vehicles (ROVs) or operating in the vicinity of spent nuclear fuels, which will, at around 5500 Gy/day, amount to over 2 MGy after a year of operation. Sterilisation of polymeric medical devices are typically sterilised to 25 kGy. However, the cumulative nature of radiation can lead to a rise in total exposure over an extended lifetime with multiple sterilisations. Finally, the lowest cited doses are from ambient exposure during space travel.

The rate of oxygen diffusion in a 3D printed part is primarily determined by the porosity, which can be modified at the design stage via printing parameters, specifically infill percentage and orientation [23, 24]. By controlling the rate of oxygen diffusion within the microstructure, tailoring of the radiation-induced chemical process may be possible, with oxygen abundance promoting these chemical changes. Due to the fabrication process, radiation-induced changes in chemistry within 3D printed parts will differ from those experienced by parts manufactured with more traditional methods. Research into the mechanical and chemical changes that occur within 3D printed parts following exposure to gamma radiation has been steadily increasing since 2016; however, the range of materials investigated has been limited. Previous research has focused on polylactic acid (PLA) [15, 23, 25] and acrylonitrile butadiene styrene (ABS) [23, 26, 27], and historically, these have been the most commonly printed and commercially available materials. However, PLA has been shown to be highly sensitive to even small radiation dosages, making it unsuitable for many long-life operations exposed to radiation. Additionally, ABS has been losing popularity to alternative polymers, due to comparative difficulties in printing compared to the more contemporary materials, with requirements such as actively heated chambers, to reduce warping and cracking, and fume cupboards to remove harmful fumes. Examples of data published on the response of the mechanical properties to gamma radiation (ultimate tensile strength (UTS), maximum strain, Young’s modulus and hardness) of PLA and ABS are summarised in Table 2. PLA showed significant degradation in both UTS and maximum strain at exposure levels above 0.05 MGy, while increases in Young’s modulus and hardness were recorded up to an exposure of 0.2 MGy. Additionally, papers that reported infrared spectroscopy recorded the formation of a new peak in the 3200–3550 wave number range, corresponding to O–H stretch, with this formation attributed to cross-linking. The discrepancies in Young’s modulus between [15] and [23] at exposures above 0.2 MGy could be attributed to the difference in dose rates applied to the materials. In contrast to PLA, ABS has demonstrated a much higher mechanical robustness to gamma radiation. At an equivalent dose of 0.015 MGy, ABS increased in UTS and maximum strain. However, at higher exposures over 1 MGy, large decreases in the observed UTS and significant decreases in the maximum strain were observed. Increases in hardness and Young’s modulus were recorded at all exposure levels.

Polymers containing phenyl groups in the main chain, such as PETG, are known to be more robust to radiation, as the phenyl group absorbs energy, mitigating bond ruptures [12, 13, 28]. Similarly, polymers that contain a benzene ring, such as ASA, are more resistant to radiation than polymers that contain saturated bonds [3, 10, 28, 29]. ASA is well known in the 3D printing community for its excellent weathering resistance, surpassing PLA, ABS and PETG in UV, water and thermal resistivity. ASA is also easier to print than ABS, with less warping and cracking occurring when printing at a larger scale. However, ASA does still require a fume cupboard during printing, as toxic fumes are released due to the generation of styrene.

There is no current research detailing the response of these materials to radiation when used in a 3D printed part. This work therefore investigates the effect of gamma radiation on the mechanical properties of 3D printed parts made from polyethylene terephthalate glycol (PETG) and acrylic styrene acrylonitrile (ASA). Infrared spectroscopy was used to assess chemical changes resulting from radiation exposure. Since 3D printed parts are deposited in layers and exhibit different physical properties depending on whether tension is applied parallel to or normal to the deposited layers, samples were fabricated in both horizontal and vertical builds to focus primarily on material tensile strength and fused layer adhesion strength, respectively.

Infrared analysis suggested that exposure to gamma radiation altered the intermolecular bonding of the PETG and ASA polymers. The spectra suggest evidence of cross-linking occurring in both materials as indicated by the formation of the broad peak located at 3300 cm⁻¹ corresponding to the hydroxyl group OH bond. From the PETG spectra, increases in peak intensity in bond corresponding to CH groups further indicate cross-linking through the radicalisation of hydrogen electrons, reforming new bonds and creating three-dimensional networks of polymer chains. For ASA, the decrease in other peak intensities could be an indication of chain scission breaking bonds along the polymer chain.

Research by others suggests that the AM fabrication process does not alter functional groups present in 3D printed parts when compared to bulk material [38]. However, gamma radiation has been shown to alter the intermolecular bonding within polymers [23], leading to cross-linking and chain scission. Cross-linking is known to induce embrittlement in the material, reducing the maximum strain at fracture yet increasing the Young’s modulus [12]. In contrast, chain scission leads to a suppression of elastic properties, thus reducing Young’s modulus [12]. Mechanical testing supports this cross-linking hypothesis, with embrittlement of the materials evidenced by a decrease in maximum strain but increase in Young’s modulus. Additionally, chain scission is expected to occur alongside cross-linking, evidenced by the decrease in tensile strength with increasing radiation exposure. Furthermore, visual inspection of the fracture surfaces following irradiation indicated the horizontal build failure modes transitioned towards brittle rather than ductile failure. Dose rates are quoted for air, and there will be attenuation of gamma radiation within any printed components depending on material, filament structure and thickness which will ultimately affect the mechanical behaviour. A study by Almeida et al. [43] shows attenuation of approximately 10% of incident gamma for 1 cm thick PLA and ABS printed with 100% infill.

Similar trends of reduction in tensile strength and strain at failure with radiation exposure were observed in the vertical build samples. However, these surfaces did not fundamentally change in the type of failure, being brittle in nature regardless. At the time of writing, there is no research into the effect of high levels of gamma radiation on the layer adhesion strength.

There are differences in mechanical properties measured between the work presented in this study and quoted results from supplier data sheets. All samples were fabricated using printing temperatures recommended by the supplier. However, other printing parameters, such as infill type, layer height, number of walls and print speed, were not indicated and were derived by experimental optimisation. It has been shown that changes in the printing parameters can drastically affect mechanical properties [39], and compared with previous reported work for PETG and ASA, the horizontal (non-irradiated) PETG samples gave similar or lower UTS and only slightly lower maximum strain and Young’s modulus [38, 40]. Previously published work for non-irradiated ASA samples gave similar or slightly higher UTS and higher maximum strain [41, 42] compared this study. Moreover, previous work has identified the difficulties in testing vertical build samples due to their fragile nature and inconsistency in layer adhesion at high Z coordinates [37]. Little research has been conducted on PETG or ASA samples printed in the vertical orientation. At the time of writing, no standard methodology is applied to tensile testing of 3D printed samples in terms of how the samples are produced and parameters such as raster angle and print temperature. While process parameters will affect the magnitude of the measured mechanical properties, the nature of the radiation-imposed changes should not be affected, and this test is a fair comparison of property changes that result from radiation exposure. A notable aspect of the mechanical data was a relatively high variability between many of the repeat measurements. This variability existed between unexposed as well as exposed samples. In some cases, the error bars overlapped, suggesting that there may not be a statistically significant difference between the materials or from the exposure.

Although there are no reported data on the robustness of PETG and ASA to radiation, previous studies exist on the effect of gamma radiation on PLA and ABS which can act as a benchmark. Comparing previous work investigating the radiation response of PLA and ABS to results from PETG and ASA in this study. Research has shown PLA to become unsuitable for functional use at exposure over 0.05 MGy [14, 15, 23]. In comparison, ABS has been shown to be more resistant to radiation, increasing in UTS and maximum strain at exposures lower than 0.05 kGy [26, 27]. However, at higher dosages, above 1 MGy, ABS significantly reduced in UTS and maximum strain. The data presented in this study demonstrate that PETG performs better than both PLA and ABS in mechanical testing after exposure to radiation, and ASA performs better than PLA and similarly to ABS. Note that direct comparison to previous work is difficult due to differences in dose rate and print parameter settings, i.e. work from [26] used thicker 2 mm walls. Ultimately, the results demonstrate the higher robustness of PETG to radiation, especially below 2 MGy, when compared to other similarly priced materials on the market.

The work presented in this paper investigates how the mechanical characteristics of 3D printed polyethylene terephthalate glycol (PETG) and acrylic styrene acrylonitrile (ASA) polymers respond to high doses of gamma radiation. The findings indicate that PETG outperforms ASA as well as PLA and ABS (reported historically). PETG maintains UTS up to 0.75 MGy and appears to be a suitable material from a robustness and cost-effective perspective for deployment in radioactive areas.

Infrared spectroscopy showed the evidence of polymer cross-linking as a result exposure to gamma radiation with the formation of peaks corresponding to –OH and C–H bonds. This is associated with a reduction in ultimate tensile stress and maximum strain in parts made from both polymers, but with PETG retaining greater strength and ductility than ASA, especially at intermediate gamma exposure. Young’s modulus and hardness showed either modest increases or a fairly flat response with exposure. Mechanical properties were heavily dependent on the build structure, with the inherent strength of the material along the filament being several times greater than the fused layer adhesion strength. Non-irradiated samples pulled parallel to the filament direction showed ductile failure which transitioned to brittle failure with exposure to radiation. For samples pulled perpendicular to the filament direction, fractures occurred between layers, indicating brittle failure regardless of whether the samples were irradiated.