Mechanical study on the impact of an effective solvent support-removal methodology for FDM Ultem 9085 parts

Journal Pre-proof Mechanical study on the impact of an effective solvent support-removal methodology for FDM Ultem 9085 parts Ariadna Chueca de Bruijn, Giovanni Gómez-Gras, Marco A. Pérez

PII: DOI: Reference:

S0142-9418(19)32344-X https://doi.org/10.1016/j.polymertesting.2020.106433 POTE 106433

To appear in:

Polymer Testing

Received date : 13 December 2019 Revised date : 7 February 2020 Accepted date : 11 February 2020 Please cite this article as: A.C de Bruijn, G. Gómez-Gras and M.A. Pérez, Mechanical study on the impact of an effective solvent support-removal methodology for FDM Ultem 9085 parts, Polymer Testing (2020), doi: https://doi.org/10.1016/j.polymertesting.2020.106433. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

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Mechanical study on the impact of an effective solvent support-removal methodology for FDM Ultem 9085 parts Ariadna Chueca de Bruijna , Giovanni Gómez-Grasa,∗, Marco A. Péreza

Abstract

School of Engineering, Universitat Ramon Llull, Via Augusta 390, 08017 Barcelona, Spain

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The use of support material to produce Fused Deposition Modelling parts is often unavoidable. The support removal task tends to be laborious and time-consuming when no soluble support materials are available, which is the case of the high-performance thermoplastic Ultem™9085. This paper investigates the effect of different solvent/solvent mixtures on Ultem’s mechanical properties with the aim to identify a solvent capable of dissolving its support material (a polysulfone) without noticeably damaging the model material. To do so, initial solubility tests have helped narrow the list of solvent candidates. These have been followed by infrared analyses to identify the presence of dissolved polymers in the media, as well as scanning electron microscope micrographs to analyse the surface topography of the treated parts. Finally, tensile and flexural tests have permitted to quantify the change on Ultem’s mechanical properties as a function of the treatment time. Major findings include a reproducible method for softening or eliminating Ultem’s support material with non-significant changes in their mechanical properties. The outcome of this work represents a first step on the lookout for a solution to facilitate the removal of polysulfone and is considered of great interest for the scientific community due to the rise of Ultem as a structural material. Keywords: Additive manufacturing, Fused deposition modeling, Support removal, ULTEM 9085, Mechanical behavior 1. Introduction

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Over the last two decades, additive manufacturing (AM) has experienced a striking technological evolution, thanks to the exceptional versatility, customization capacity, and economic savings it represents in comparison to other traditional methods [1]. This accelerated growth has placed AM at the forefront of manufacturing processes and has created new expectations in a vast number of applications, which, until now, suffered from the limitations of subtractive or conventional bulk-forming processes [2, 3]. Alongside the development of these manufacturing techniques, there have been advances in the obtention of new polymeric materials with enhanced properties, creating a synergy with AM that has enabled its use even in sectors where polymers were not widely accepted. One of these featured materials is the Ultem™9085 (Ultem), a thermoplastic of the polyetherimide family (PEI) with outstanding chemical, mechanical, and thermal properties [4, 5]. Its resistance-to-weight ratio and flame, smoke and toxicity (FST) certification position it as a flame-retardant, non-toxic, and no-smoke emitting material and validate its use for components in sectors with high restrictions, such as the railway or aerospace industry [6, 7]. In general, AM consists of the controlled deposition, layer by layer, of a model material, to form a three-dimensional structure of high geometric complexity. Among all the 3D printing technologies, Fused Deposition Modeling (FDM) stands out, and it ∗ Corresponding

author. Email address: [email protected] (Giovanni Gómez-Gras)

Preprint submitted to Polymer Testing

is widely extended due to its technical affordable and low cost, as well as the vast range of materials that can be used to create end parts using these techniques, including Ultem [8, 9].

Nonetheless, in parallel with these advantages, FDM presents some limitations that must be addressed by the scientific community, until it can be adopted as a broadly used technique by the industrial sector. One of its main disadvantages is, in fact, related to obtaining complex geometries. The need to use another material (in addition to the construction material) that serves as scaffolding has been evidenced when manufacturing cantilevers, cavities, tubular sections, or slender geometries, among other configurations otherwise impossible to obtain without the help of support material [10]. Considering that a substantial amount of scientific research acknowledges that the printing parameters and the construction orientation of the pieces affect their fabrication time and mechanical properties [11–16], these restrictions become critical if the overall performance of the materials and the building process want to be optimized. For dual-extrusion printers, the choice of an appropriate support material depends, among other factors, on the complexity of the end part. The removal of a break-away (or manually extractable) support material usually elongates production time and can potentially cause damage to the part, so this type of support material should only be considered for simpler pieces. In the case of more irregular geometries, a soluble support material facilitates the removal task. Regarding this last type of support materials, successful solutions have been found to work with some widely used FDM February 7, 2020

Journal Pre-proof polymers. It is the case, for example, of polyvinyl alcohol (PVA), an outstanding soluble supports on the market, as it is water-soluble and can be used in combination with polylactic acid (PLA), Nylon and co-polyesters (CPE). Duran et al. [17] have reported PVA’s compatibility with acrylonitrile butadiene styrene (ABS) due to the similar extrusion temperature of ABS and PLA, but have encountered the long exposure time and its sensitivity to environmental humidity to be notable drawbacks regarding its use [17]. Also, for PLA, Stratasys® patented in 2001 the use of poly(2-ethyl-2-oxazoline) (PEO), which is also soluble in water [18], and subsequently proposed to solubilize their commercially available ABS and polycarbonate (PC) support materials with a basic aqueous solution, at a temperature of 70° C with constant agitation [19]. Nonetheless, due to the large diversity of 3D printing polymers, most of them having different thermal and chemical properties, there is no universal support material that can be used in combination with all model materials or removed using the same solvent, and the available options are, in some cases, limited or inexistent. Therefore, all this evidence accelerates the need to find new alternative materials or, ideally, to encounter new thermochemical procedures to interact with the already existent materials that serve as scaffolding to guarantee their complete elimination, without damaging the properties of the building material. In the case of the engineering-grade thermoplastic Ultem, the high working temperatures (380ºC at the extruder and 195ºC inside the chamber) make it more challenging to find compatible polymers that can be used as support materials. Nowadays, a break-away support material specially designed to work with Ultem is the only commercially available solution and it consists of a polymer mixture whose main component is polysulfone (PSF) [20]. Although Ultem and its support material’s compositions are not disclosed, the monomeric units of their main components (polyetherimide (PEI) and PSF, respectively) are shown in Figure 1. It can be observed that both chemical structures are similar, as they present a skeleton of diphenyl propane surrounded by an ether group on each side. The high proportion of aromatic rings provides these polymers with excellent thermal stability [21]. The main difference in their structures is found in the sulfone group of the PSF and the imide group of the PEI. Both polymers present hydrogen bond acceptors such as the oxygens from the ether, the sulfone, or the imide groups, but the overall polarity of PSF is assumed to be higher than that of Ultem due to the presence of the electronegative sulfone group. Nevertheless, the difference in polarity between both structures could not be enough to identify a solvent capable of interacting with the support material and leaving Ultem unaffected, so other more complex dissolving mechanisms are assumed to be involved in the dissolving process. Other researchers have already addressed this issue [22, 23] that have gathered a list of possible candidates to dissolve Ultem’s support material. However, the authors of this work have found that the removal of the support using the suggested solvents is not entirely satisfactory, as in many cases there are softened residues, and there is no guarantee that the model ma-

O

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Figure 1: Chemical structures of the main components of Ultem™9085 (PEI) and its support material (PSF).

terial is not mechanically affected. Accordingly, this work focus on obtaining a reproducible and effective chemical methodology to eliminate Ultem’s support material, aiming to drastically reduce the complexity of the extraction of PSF without significantly affecting the properties of Ultem. To validate the objective of the work, an experimental analysis on the capacity of thirteen solvents/solvent mixtures to dissolve the support material has been performed. From these tests, the adequate candidates have been selected to study their effect on Ultem’s mechanical performance and surface morphology, the results obtained with the different solutions have been compared and discussed.

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2. Methodology

2.1. Materials and solvents The chosen model material for the construction of the parts was Ultem™9085, a polyetherimide alloy (PEI). The support material used was a polysulfone mixture (PSF) provided under the name of Ultem support material. Ten solvents were tested and are listed in Tab. 1 alongside their chemical formula, boiling point (BP), and dipole moment (DM). Toluene and 1-bromopropane were selected as candidate solvents for PSF, as proposed by [22, 23]. These same studies suggested that NMP would be harmful to Ultem parts, so this solvent was chosen as a reference for non-desirable results. Acetone, 1,4-dioxane, tetrahydrofuran, dimethyl sulfoxide (DMSO), tetrachloroethylene, dichloromethane (DCM), and dimethylformamide (DMF) were selected based on available chemical compatibility charts, as well as due to their frequent use as polymer solvents, with different boiling points, dipole moment or solvation mechanisms than the previously mentioned ones. In addition to these pure solvents, given the results that are discussed throughout this paper, three mixtures of equal volumes of 1,4-dioxane and 1-bromopropane, toluene, or acetone were also investigated, and its effects on Ultem and its support material were studied. 2.2. Samples and building parameters The STL output files for the parts’ CAD models were processed with Insight 3D printing software and printed using

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Chemical formula CH3COCH3 C3H7Br C7H8 C5H9NO C 4 H8 O2 C 4 H8 O C2H6OS C2Cl4 CH2Cl2 C3H7NO

Boiling point (◦ C) 56 71 111 204 101 66 191 121 40 153

Dipole moment (D) 2.91 2.12 0.36 4.09 0.45 1.73 3.96 0.00 1.62 3.82

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Solvent Acetone 1-Bromopropane Toluene N-methyl-2-pyrrolidone (NMP) 1,4-Dioxane Tetrahydrofuran (THF) Dimethyl sulfoxide (DMSO) Tetrachloroethylene Dichloromethane (DCM) Dimethylformamide (DMF)

Table 1: List of the solvents considered in the investigation, with its chemical formula, boiling point and dipole moment

Stratasys FDM printer Fortus 400mc. The used printing settings are detailed in Tab. 2. Chamber temperature Model material extruder temperature Support material extruder temperature Slice height Part orientation Part interior style Infill raster angle Number of contours Contour width Support style Model and support tips

The excess solvent was removed from the samples, and were then dried in a vacuum chamber for at least 12 hours before inspection using an electron microscope.

195 ◦ C 380 ◦ C 421 ◦ C 0.254 mm X-Flat Solid 45◦ 1 0.508 mm Sparse T16

2.3. Solubility tests

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Table 2: Printing settings of FDM printer Fortus 400mc.

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The potential capacity to solubilize Ultem’s support material and the effect that the solvent has on Ultem’s superficial appearance were studied using a cubic part with edges 25 mm in length and a cavity of 15×15×20 mm3 , resulting in a support-to-model volume ratio of approximately 1:2. The printing orientation of the part was such that it forced the use of support material inside the cavity. By doing so, the aim was to mimic a realistic scenario where part of the support material would be directly exposed to the solvent, but some would be deeper inside the part and thus be more challenging to reach. Twelve different solvents/solvent mixtures were used, so a total of thirteen identical cubes were printed (one was used as a reference). Each cube was placed into a metallic basket, which was, in turn, placed in a glass beaker containing 100 ml of the solvent. Solvent mixtures were agitated using a magnetic stirrers, so the role of the basket was to avoid damaging of the piece as a result of the stirring process. All experiments were carried out at room temperature and under a chemical fume hood. They were considered finished when a) the support material was dissolved, b) the model material was visually deteriorated or c) a time higher than 8 hours elapsed without apparent dissolution of neither Ultem nor its support material.

2.4. Treatment of test specimens To study the effect of the selected solvent candidates on Ultem’s mechanical properties, tensile and flexural specimens were submerged in a glass box filled with the chosen solvent for a period of 2, 4, 6 and 8 hours. Agitation was kept constant using a magnetic stirrer, and experiments were performed at room temperature and under atmospheric pressure. Given the direct correlation between solubility and temperature, and the indication by previously performed tests that Ultem degradation increases drastically with temperature, this process parameter was monitored throughout all tests and kept constant at 22ºC using a stainless-steel temperature probe with an accuracy of 0.1ºC. This probe was, in turn, connected to a stirring hotplate so that the solvent mixture acted as a thermostatic bath. In a similar way to the procedure followed by the solubility tests, a metallic net was designed to suspend the treated specimens and avoid damaging as a result of the stirring process. After the immersion time, treated samples were dried in a vacuum chamber for at least 12 hours before mechanical testing. Samples were also weighted using an analytic scale pre- and post-treatment to register any mass changes. 2.5. Samples characterization 2.5.1. Infrared spectroscopy The composition of the remaining liquid from the solubility tests was determined using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) (previous evaporation of the solvent to facilitate the detection of dissolved polymer). Powdered forms of extruded Ultem and PSF were also analyzed using this technique for comparative purposes. All experiments were conducted using a infrared spectrometer1 . ATR-FTIR, in transmittance mode, were obtained using a scanning resolution of 1 cm-1 in the range of wavenumbers of 4000650 cm-1 . Spectra were examined using OMNIC™Spectra software. 1 Thermo

3

Fisher Scientific Nicolet™iS™10.

Journal Pre-proof 3 hours of being submerged in this solvent, and the polysulfone support material was completely dissolved. Given these results, three new solvents were introduced, consisting of a mixture of equal volumes of 1,4-dioxane and each of the softening solvents (acetone, toluene, and 1-bromopropane). The aim was to minimize the damage done to the part while still dissolving the polysulfone in an acceptable period. As expected, after 3 to 4 hours of treatment with the solvent mixtures containing toluene and 1-bromopropane, the polysulfone material dissolved, leaving only a viscous residue on the surface of the printed part, and Ultem’s surface seemed less affected than in the case of using pure 1,4-dioxane. Nevertheless, the Ultem part lost its initial color and appeared more damaged after being exposed to the solvent mixture containing acetone.

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2.5.2. Scanning electron microscopy Scanning electron microscope2 equipped with a tungsten hairpin filament as an electron gun was used to study the surface morphology of the immersed cubic samples. Due to the non-conductive nature of Ultem, samples were gold-sputtered using a sputter coater3 prior to imaging. A backscattered electron detector, an accelerating voltage of 10 kV, a spot size of 50 nm, and a working distance of 9 to 10 mm were used to collect the micrographs.

2.5.3. Mechanical testing The mechanical properties in terms of tensile Young’s modulus, ultimate tensile strength, and tensile strain were determined accordingly to ASTM D638 [24] standard by using an electromechanical universal testing machine4 , equipped with a load cell of 10 kN, and a lineal extensometer5 with 25 mm of nominal length. Type IV specimens were manufactured using the printing parameters previously described. Flexural specimens with a cross-sectional area of 4×10 mm2 were printed following ASTM D790 (three-point bending test, procedure A) [25]. Flexural Young’s and flexural strength and strain were determined in the same electromechanical universal testing machine, equipped with a load cell of 1 kN. The displacement rate was calculated accordingly to the standard, with a support span fixed at 64 mm. 3. Results and Discussion 3.1. Solubility tests

Figure 2: From left to right: Untreated Ultem cube, cube after treatment with toluene, cube after treatment with the mixture dioxane-toluene, cube after treatment with NMP.

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A summary of the results obtained in the solubility tests is presented in Tab. 3. The pure solvents used have different effects on the target support material, so they have been categorized into three different groups for easier comprehension. A visual example of these effects can be observed in Fig. 2. The first group comprises acetone, 1-bromopropane, and toluene, as all of them could soften the support material in 8 hours without producing a visual effect on the Ultem’s surface. As a result, the support material was quickly removed from the inner part of the cube using a pair of tweezers. Although the model material was not damaged, the use of these solvents still required some manual intervention, signifying an obstacle in the case of hidden cavities inside the printed part. The second group of solvents included tetrachloroethylene, THF, and DMSO. These solvents did not have a visual effect on either Ultem or the support material, so they were discarded for further analysis. For its part, NMP, DMF, and DCM were able to dissolve the polysulfone in periods ranging from 1 to 3 hours, but they all had a detrimental effect on Ultem’s surface, making them inadequate candidates for this work. However, in the case of using 1,4-dioxane, the printed part showed only minor damage after 2 JEOL

JSM-6460. Scientific Instruments Sputter coater 108 Auto 4 MTS Insight Electromechanical 10kN 5 MTS 634.12F-54 3 Cressington

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3.1.1. Infrared spectroscopy Fourier transform infrared (FTIR) of Ultem and its support material (PSF) are shown in Fig. 3 (a) and (b). Its characteristic peaks were found in agreement with spectra reported in the Nicolet Sampler FTIR Spectral Library for polyetherimide and polysulfone, respectively. Since these peaks appear in a reduced wavenumber range and the rest of the spectrum was not relevant for the present study, experimentally obtained spectra are presented only for wavenumbers from 800 to 2400 cm-1 . A sharp, high-intensity peak at 1720±30 cm-1 corresponds to the carbonyl group (C=O) stretching vibration present in polyetherimides, hence why it was chosen as Ultem’s characteristic peak. Two double peaks at 1140±20 cm-1 and 1325±25 cm-1 correspond to the symmetric and asymmetric stretching of the sulfoxide group (S=O), and both are characteristic of PSF. Below these two base spectra Fig. 3(c) to (j) are the FTIR spectra of samples of the remaining liquid from the most promising solvents / solvent mixtures after the solubility tests: 1-bromopropane, toluene, acetone, 1,4-dioxane and the mixtures dioxane-bromopropane (50%D 50%B), dioxane-toluene (50%D 50%T), and dioxane-acetone (50%D 50%A). The remaining liquid from the test with NMP was also subjected to analysis for comparative reasons. All sample solutions were analyzed after evaporating the solvent, but traces of some solvents containing ketone groups can induce a peak where Ultem’s characteristic peak is situated. It is the case of treatments with acetone, NMP, and dioxane-acetone, where FTIR results are not reliable indicators of the presence of Ultem. From these results, both characteristic peaks of the support material can be identified in all samples, meaning that the chosen solvents have been able to dissolve at least a small portion

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Test time 8h 8h 8h 3h 4.5 h 8h 8h 8h 3h 1h

Ultem™9085 appearance Unaffected Unaffected Unaffected Partially dissolved Lightly affected Unaffected Unaffected Unaffected Partially dissolved Softened

50% 1,4-Dioxane 50% 1-Bromopropane

3h

Slight loss of shine

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50% 1,4-Dioxane 50% Acetone

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Polysulfone appearance Softened Softened Softened Dissolved Dissolved Unaffected Unaffected Unaffected Dissolved Dissolved Dissolved, but viscous residue Dissolved, but viscous residue and cloudy liquid Dissolved, but viscous residue

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Solvent Acetone 1-Bromopropane Toluene N-methyl-2-pyrrolidone (NMP) 1,4-Dioxane Tetrahydrofuran (THF) Dimethyl sulfoxide (DMSO) Tetrachloroethylene Dichloromethane (DCM) Dimethylformamide (DMF)

Table 3: Results from the solubility tests, including test time and appearance of Ultem and its support material after the test.

of the PSF. Ultem’s characteristic peak appears in the infrared spectra of the samples treated with 1-bromopropane, toluene, acetone, NMP, and 1,4-dioxane, but does not appear in samples treated with the solvent mixtures, indicating no detectable concentration of dissolved Ultem. 3.2. Scanning electron microscopy

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Scanning electron microscope (SEM) images shown in Fig. 4 provide an insight into the surface topology of the printed Ultem samples before and after being treated with the chemical solvents during the solubility tests. A feature to emphasize is the smoother surface of the Ultem filaments after being treated with 1-bromopropane, toluene, and acetone (Fig. 4 (d),(e) and (f))when compared to the untreated case (Fig. 4 (a)). This phenomenon could explain the presence of Ultem peaks in the FTIR analyses. In the case of treatment with 1,4-dioxane (Fig. 4(b)), the sample surface also appears smoother, but the inter filament bonding seems slightly degraded. Fig. 4 (c) shows a irregular and more porous surface of the polymer, as well as a more pronounced separation of the filaments, corroborating the hypothesis that NMP deteriorates Ultem. A similar decrease in the rasters’ diameter is observed in samples treated with a mixture of 1,4-dioxane and acetone, but instead of poriferous, the bonding appears smoothened, probably implicating a partial relocation of material in these areas. In chemically treated samples with solvent mixtures containing 1,4-dioxane and 1-bromopropane or toluene Fig. 4 (g), (h) and (i), a slight decrease in the filaments’ diameter is accompanied with a deposition of material between filaments, meaning that, instead of being released to the media, the dissolved Ultem is trapped in the form of a viscous residue and then redeposited during the drying process. As a result, the filaments’ bonding is reinforced in some zones and weakened in others.

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3.3. Mechanical performance Representative stress-versus-strain curves of the tensile tests performed after samples were chemically treated with the chosen solvents are shown in Fig. 5. From these curves, solvents can be separated into three different groups depending on the effect they have on Ultem’s properties. Strain-stress curves of samples treated with toluene, acetone, and 1-bromopropane are similar to that of the pristine sample. Samples treated with 1,4-dioxane and the mixture of this solvent with acetone, 1bromopropane, or toluene present a similar initial slope but lower maximum stress. Finally, samples treated with NMP exhibit much lower maximum stress and strain. Fig. 6 shows the results in the percentage of mass lost as a result of the chemical treatment of the standard probes, as well as the evolution of the mechanical properties after 2, 4, 6 and 8 hours of treatment in terms of tensile modulus, tensile strength, strain at tensile strength, flexural modulus and flexural strength at 5% strain (percentage value suggested by ASTM D790). Overall, the two mechanical properties calculated within the elastic range of the material (tensile and flexural moduli) are the least affected as a result of the use of solvents during the studied time range. This could be explained by the fact that, in fused deposition modeling, failure of a specimen is often due to rupture or separation of rasters, which usually happens at high deformations (outside the elastic range), hence why the material behaves homogeneously at the beginning of the test and properties calculated at higher deformations are more affected by inter-filament degradation. Another aspect to consider when examining these results is that the change in mechanical properties is more noticeable in the tensile test than in the flexural test, a phenomenon probably caused by the difference in shape of these tests’ standard specimens, as tensile probes have a curved shape, with more voids and thus more isolated rasters prone to accumulation of solvent. Regarding the individual results for each investigated solvent, a treatment that negatively stands out is that with NMP. As

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expected from the solubility tests, Ultem exhibits the worst mechanical properties after being treated with this solvent: there is a considerable mass loss from the first 2 hours of treatment (and as much as 68% after 8 hours), accompanied by a marked decrease in all studied mechanical properties. Tensile and flexural specimens were so degraded after 8 hours that mechanical testing could not be performed. These results corroborate the initial idea that the contact of Ultem with NMP is not recommended during short-term exposure, even if NMP can dissolve Ultem’s support material.

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Contrarily, treatment with 1-bromopropane, toluene, and acetone does not affect the studied mechanical properties nor the specimens’ mass over the examined period. Again, this fact correlates with the results obtained from the SEM micrographs, where no deterioration of the surface is observed and reinforces the theory that the weak Ultem peak visible in the FTIR analyses corresponds to a negligible dissolution of this material. Nevertheless, it shall be remembered that none of these solvents can potentially dissolve Ultem’s support material; they are only capable of softening it.

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A less pronounced but equally adverse effect on the mechanical properties of Ultem can be observed when this is treated with 1,4-dioxane. After 4 hours of treatment (which is the estimated immersion time for correct support removal), recorded mass loss is only 1%, but tensile strength and strain at this point fall by nearly 20%, and there is a decrease in flexural strength of 14% when compared to untreated specimens. After 6 hours, this decrease approaches 29% and 17%, respectively.

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A clear difference between the samples treated with 1,4dioxane and acetone and the ones treated with 1,4-dioxane and 1-bromopropane or toluene is that, even though mechanical properties present a similar trend, mass loss increases over time when acetone is added to the mixture, hitting a 10% loss at 8 hours of treatment, whereas in the case2500 of 1,4-dioxane with 1bromopropane or toluene, mass loss does not exceed 1% in the investigated time range. These results are consistent with the reduced filament diameter size seen in2000 the SEM images corresponding to the treatment with 1,4-dioxane with acetone (Fig. 4 (i)).

(j) 50%D 50%A

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Apart from NMP and 1,4-dioxane, the solvents that can dissolve polysulfone are the equal volume mixture of 1,4-dioxane with acetone, 1-bromopropane, or toluene. A remarkable feature that stands out when looking into the corresponding plots (Fig. 6, 50%D 50%B, 50%D 50%T and 50%D 50%A) is that, despite the drop in mechanical properties, these do not seem to follow a decreasing tendency over time. On average, treatment with the mixture of 1,4-dioxane with either 1-bromopropane or toluene has a similar effect on the mechanical properties of Ultem: tensile strength falls by 17%, and flexural strength at 5 percent strain drops by 2%. In terms of strain at tensile strength, samples treated with 1,4-dioxane and 1-bromopropane undergo a more significant deformation than samples treated with 1,4dioxane and toluene. In comparison with the pristine material, the decrease in this property is 4% and 18%, respectively. Flexural and tensile moduli remain equal to the pristine specimen for reasons previously explained in this section.

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ULTEM characteristic peak at 1720 cm -1 PSF characteristic peaks at 1325 cm-1 and 1140 cm-1 Figure 3: FTIR spectra of pulverized Ultem (a) and pulverized PSF (b). Evaporated FTIR of the remaining liquid from the solubility tests(c) to (j).

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Figure 4: SEM micrographs showing the state of the Ultem filaments after the solubility tests. Scale bar is 100 µm.

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Journal Pre-proof Acknowledgements

65 Toluene (T) 60

This work has been supported by the Ministry of Science, Innovation and Universities through the project New Developments in Lightweight Composite Sandwich Panels with 3D Printed Cores (3DPC) - RTI2018-099754-A-I00; and by the RIS3CAT Llavor 3D Community co-financed by the Generalitat de Catalunya (ACCIÓ) through the project TRANSPORT COMRDI16-1-0010 - (2017-2020). The authors are very grateful to Ana Belén Cuenca for constructive suggestions and also would like to gratefully acknowledge their gratitude to Albert Forés and Miquel Otero for the assistance in conducting the experiments.

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Figure 5: Representative stress vs. strain curves showing the effect of the chemical treatment on Ultem’s tensile behaviour.

4. Conclusions

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This paper proposes an effective solvent-removal methodology for PSF support material in FDM Ultem 9085 parts, and experimentally demonstrates the impact on the mechanical performance. The results from the analysis are in agreement with the SEM micrographs, which provide useful information on the acting mechanism of the different solvents on Ultem’s surface.

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Experimental evidence proves that submerging the desired Ultem specimen in an agitated bath of pure 1-bromopropane or toluene for as long as 8 hours does not affect the tensile and flexural properties, and soft the support material allowing for manual extraction. The most remarkable contribution of this paper includes the use of 1,4-dioxane in combination with 1-bromopropane or toluene, which drastically accelerates the dissolving process of the PSF support material allowing for the release of internal or difficult-to-reach support structures, with a quantified impact on the Ultem’s mechanical performance (tensile and flexural strength decreases up to 15% while tensile and flexural modulus remain unaffected.

Future work includes the adjustment of the 1,4-dioxane concentration in the solvent mixture for optimal dissolution time with minimal loss of mechanical properties, as well as the determination of the effect of the proposed solvent mixtures on the mechanical properties of semi-solid (sparse) Ultem pieces. 8

[1] C. W. J. Lim, K. Q. Le, Q. Lu, C. H. Wong, An Overview of 3-D Printing in Manufacturing, Aerospace, and Automotive Industries, IEEE Potentials 35 (2016) 18–22. [2] F. Zhang, M. Wei, V. V. Viswanathan, B. Swart, Y. Shao, G. Wu, C. Zhou, 3D printing technologies for electrochemical energy storage, Nano Energy 40 (2017) 418–431. [3] L. Gasman, Additive aerospace considered as a business, Elsevier, 2019. [4] Stratasys, ULTEM 9085 Production-Grade Thermoplastic for Fortus 3D Printers, Technical Report, 2017. [5] Stratasys, SDS ULTEM 9085 Natural Model Material, Technical Report, 2017. [6] K. C. Chuang, J. E. Grady, R. D. Draper, C. Patterson, T. D. Santelle, Additive manufacturing and characterization of ULTEM polymers and composites. CAMX-The Composites and Advanced Materials Expo. [7] S. C. Joshi, A. A. Sheikh, 3D printing in aerospace and its long-term sustainability, Virtual Phys. Prototy. 10 (2015) 175–185. [8] G. D. Kim, Y. T. Oh, A benchmark study on rapid prototyping processes and machines: Quantitative comparisons of mechanical properties, accuracy, roughness, speed, and material cost, P. I. Mech. Eng. B.-J. Eng. 222 (2008) 201–215. [9] A. D. Valino, J. R. C. Dizon, A. H. Espera, Q. Chen, J. Messman, R. C. Advincula, Advances in 3D printing of thermoplastic polymer composites and nanocomposites, Prog. Polym. Sci. 98 (2019) 1–19. [10] J. Jiang, J. Stringer, X. Xu, R. Y. Zhong, Investigation of printable threshold overhang angle in extrusion-based additive manufacturing for reducing support waste, Int. J. Comput. Integ. M. 31 (2018) 961–969. [11] A. W. Gebisa, H. G. Lemu, Influence of 3D printing FDM process parameters on tensile property of ultem 9085, Procedia Manuf. 30 (2019) 331–338. [12] A. Bellini, S. Güçeri, Mechanical characterization of parts fabricated using fused deposition modeling, Rapid Prototyping J. 9 (2003) 252–264. [13] S. Ding, B. Zou, P. Wang, H. Ding, Effects of nozzle temperature and building orientation on mechanical properties and microstructure of PEEK and PEI printed by 3D-FDM, Polym. Test. 78 (2019) 1–9. [14] D. Popescu, A. Zapciu, C. Amza, F. Baciu, R. Marinescu, FDM process parameters influence over the mechanical properties of polymer specimens: A review, Polym. Test. 69 (2018) 157–166. [15] V. Durga Prasada Rao, P. Rajiv, V. Navya Geethika, Effect of fused deposition modelling (FDM) process parameters on tensile strength of carbon fibre PLA, Mater. Today (2019) 2012–2018. [16] T. D. McLouth, J. V. Severino, P. M. Adams, D. N. Patel, R. J. Zaldivar, The impact of print orientation and raster pattern on fracture toughness in additively manufactured ABS, Addit. Manuf. 18 (2017) 103–109. [17] C. Duran, V. Subbian, M. T. Giovanetti, J. R. Simkins, F. R. Beyette, Experimental desktop 3D printing using dual extrusion and water-soluble polyvinyl alcohol, Rapid Prototyping J. 21 (2015) 528–534. [18] J. Lombardi, D. Popovich, G. Artz, Water soluble rapid prototyping support and mold material, U.S. Patent US20010025073A1, Sep. 2001. [19] Stratasys, FDM Best practices: Support Removal, Technical Report, 2019. [20] Stratasys, SDS ULTEM 9085 Support Material, Technical Report, 2018.

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Figure 6: Ultem’s properties over time as a result of the chemical treatment. From top to bottom: Mass lost in %, tensile modulus in MPa, tensile strength in MPa, strain at tensile strength in %, flexural modulus in MPa and flexural strength at 5% strain in MPa.

[21] I. M. Balashova, R. P. Danner, P. S. Puri, J. L. Duda, Solubility and Diffusivity of Solvents and Nonsolvents in Polysulfone and Polyetherimide, Ind. Eng. Chem. Res. 40 (2001) 3058–3064. [22] R. D. Deshmukh, J. H. Hemond, Article and method of forming an article, U.S. Patent US20170210079A1, Jul. 2017. [23] B. C. Mosher, 3d printed parts with support removal cleaner, U.S. Patent US20170291374A1, Oct. 2017.

[24] ASTM D638-14, Standard Test Method for Tensile Properties of Plastics, Technical Report, ASTM International, West Conshohocken, PA, 2014. [25] ASTM D790-17, Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials, Technical Report, ASTM International, West Conshohocken, PA, 2017.

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Mass loss m (%) m2h m4h 0,019 0,065 0,048 0,023 0,048 0,087 8,328 21,40 0,312 0,676 0,139 0,590 0,203 0,588 0,954 0,161

m6h 0,066 0,454 0,033 52,68 4,867 1,111 1,024 4,459

m8h 0,011 1,197 0,054 66,45 8,809 1,366 0,858 10,14

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Solvent 1-Bromopropane Toluene Acetone NMP 1,4-Dioxane 50%D 50%B 50%D 50%T 50%D 50%A

Table 4: Average mass loss of the Ultem tensile and flexural specimens over time due to the treatment with the listed solvents.

Solvent 1-Bromopropane Toluene Acetone NMP 1,4-Dioxane 50%D 50%B 50%D 50%T 50%D 50%A Reference

Tensile modulus E and standard deviations s (MPa) E2h s2h E4h s4h E6h s6h 2215,2 ±65,2 2270,1 ±52,7 2275,6 ±60,9 2207,6 ±44,5 2313,7 ±136,1 2314,0 ±71,9 2193,5 ±84,5 2164,6 ±6,6 2215,4 ±70,7 1781,3 ±35,1 1565,1 ±259,0 1919,7 ±91,7 2104,8 ±104,8 2103,3 ±100,8 2034,7 ±36,0 2112,0 ±115,7 2130,3 ±89,8 2155,6 ±113,6 2165,7 ±47,3 2063,3 ±100,8 2150,0 ±125,0 2044,4 ±33,5 2126,8 ±45,4 2065,3 ±70,7 2146 ±104

E8h 2215,4 2307,9 2211,4 1895,3 2023,5 2130,0 2179,3 2083,6

s8h ±7,2 ±57,1 ±26,2 ±122,4 ±102,8 ±158,3 ±26,2

Table 5: Average tensile modulus and standard deviations of the Ultem specimens over time due to the treatment with the listed solvents. The last row shows the reference values.

Tensile strength σ and standard deviations s (MPa) σ2h s2h σ4h s4h σ6h s6h 62,37 ±1,49 62,30 ±2,41 64,82 ±0,98 59,78 ±1,29 60,81 ±0,93 62,17 ±1,23 58,89 ±3,37 61,69 ±1,42 60,34 ±4,46 44,09 ±2,79 32,84 ±10,68 39,92 ±4,99 52,58 ±3,16 49,93 ±5,97 44,04 ±3,37 52,82 ±6,38 50,28 ±4,08 51,05 ±1,18 53,23 ±5,66 48,68 ±5,40 52,86 ±8,53 49,75 ±3,71 52,57 ±3,70 50,50 ±7,05 61,21 ±1,04

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σ8h 62,49 64,27 58,76 40,13 41,60 51,10 45,40 56,31

s8h ±2,75 ±2,59 ±8,00 ±3,78 ±9,31 ±8,62 ±3,95

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Table 6: Average tensile strength and standard deviations of the Ultem specimens over time due to the treatment with the listed solvents. The last row shows the reference values.

Strain at tensile strength ε and standard deviations s (%) Solvent ε2h s2h ε4h s4h ε6h s6h ε8h 1-Bromopropane 5,28 ±0,33 5,14 ±0,43 5,52 ±0,37 5,38 Toluene 5,05 ±0,32 5,01 ±0,11 5,16 ±0,15 5,30 Acetone 4,58 ±0,40 5,14 ±0,17 4,74 ±0,77 4,40 NMP 4,36 ±0,77 2,86 ±0,50 3,65 ±0,68 3,55 1,4-Dioxane 4,90 ±0,46 4,14 ±1,06 3,54 ±1,06 3,36 50%D 50%B 4,97 ±0,63 4,79 ±0,81 4,90 ±0,46 5,29 50%D 50%T 4,55 ±0,73 4,51 ±0,84 4,19 ±1,30 3,74 50%D 50%A 4,64 ±0,66 4,94 ±0,86 4,33 ±0,33 4,89 Reference 5,17 ±0,49

s8h ±0,64 ±0,57 ±1,29 ±0,93 ±1,15 ±1,39 ±0,86

Table 7: Average strain at tensile strength and standard deviations of the Ultem specimens over time due to the treatment with the listed solvents. The last row shows the reference values.

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Solvent 1-Bromopropane Toluene Acetone NMP 1,4-Dioxane 50%D 50%B 50%D 50%T 50%D 50%A Reference

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Flexural modulus E f and standard deviations s (MPa) E f 2h s2h E f 4h s4h E f 6h s6h 1909,5 ±14,5 1833,4 ±41,7 1895,6 ±111,7 1937,4 ±96,2 1891,8 ±61,4 1827,9 ±68,9 1819,3 ±44,5 1917,1 ±43,2 1835,9 ±7,8 1616,6 ±211,1 1335,0 ±267,0 1026,5 ±198,1 1788,8 ±36,5 1752,9 ±43,8 1747,9 ±134,8 1898,2 ±23,1 1953,8 ±29,9 1919,8 ±29,6 1917,9 ±38,7 1906,2 ±112,3 2015,4 ±50,6 1947,0 ±31,9 1900,7 ±87,1 1848,9 ±105,5 1921 ±84

E f 8h 1968,9 1959,4 1912,8 695,4 1561,4 1981,7 1972,2 1827,5

s8h ±102,9 ±23,1 ±17,2 ±67,2 ±172,6 ±33,9 ±17,6 ±39,3

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Table 8: Average flexural modulus and standard deviations of the Ultem specimens over time due to the treatment with the listed solvents. The last row shows the reference values.

Flexural strength σ f and standard deviations s (MPa) σ f 2h s2h σ f 4h s4h σ f 6h s6h 80,25 ±2,06 77,31 ±4,14 80,04 ±5,94 83,22 ±2,79 80,28 ±5,26 78,98 ±1,79 77,53 ±3,39 83,94 ±1,95 77,61 ±0,84 58,88 ±12,50 48,16 ±12,22 30,50 ±6,72 70,48 ±1,57 67,60 ±2,68 67,94 ±5,51 77,17 ±0,86 82,57 ±6,03 76,65 ±2,25 79,84 ±4,97 76,83 ±5,75 77,09 ±3,80 68,04 ±1,75 67,79 ±2,34 68,27 ±3,47 78,91 ±0,61

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σ f 8h 86,41 82,91 79,89 17,03 57,04 76,53 76,10 71,37

s8h ±3,10 ±0,68 ±3,27 ±4,31 ±9,31 ±1,93 ±2,11 ±2,16

Table 9: Average flexural strength and standard deviations of the Ultem specimens over time due to the treatment with the listed solvents. The last row shows the reference values.

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Highlights •

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This study proves the viability of chemically eliminating Ultem 9085’s support material. Novel methodology to completely dissolve polysulfone with minor Ultem degradation. The support removal solution uses 1,4-dioxane with 1-bromopropane or toluene. Determination of Ultem’s tensile and flexural behavior has been conducted. SEM micrographs allow for a deeper understanding of the changes in mechanical properties.

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Declaration of interests

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☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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CRediT author statement Ariadna Chueca de Bruijn: Methodology, Validation, Investigation, Writing Original Draft

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Giovanni Gómez-Gras: Conceptualization, Investigation, Writing - Review & Editing, Formal analysis

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Marco A. Pérez: Conceptualization, Investigation, Writing - Review & Editing, Funding acquisition