WITHDRAWN: Influence of resin infiltrants on mechanical and thermal performance in plaster binder jetting additive manufacturing

Journal Pre-proof INFLUENCE OF RESIN INFILTRANTS ON MECHANICAL AND THERMAL PERFORMANCE IN PLASTER BINDER JETTING ADDITIVE MANUFACTURING Timothy J. Ayres, Santosh R. Sama, Sanjay B. Joshi, Guha P. Manogharan

PII:

S2214-8604(19)30204-0

DOI:

https://doi.org/10.1016/j.addma.2019.100798

Reference:

ADDMA 100798

To appear in:

Additive Manufacturing

Received Date:

19 February 2019

Revised Date:

16 July 2019

Accepted Date:

17 July 2019

Please cite this article as: Ayres TJ, Sama SR, Joshi SB, Manogharan GP, INFLUENCE OF RESIN INFILTRANTS ON MECHANICAL AND THERMAL PERFORMANCE IN PLASTER BINDER JETTING ADDITIVE MANUFACTURING, Additive Manufacturing (2019), doi: https://doi.org/10.1016/j.addma.2019.100798

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. © 2019 Published by Elsevier.

INFLUENCE OF RESIN INFILTRANTS ON MECHANICAL AND THERMAL PERFORMANCE IN PLASTER BINDER JETTING ADDITIVE MANUFACTURING Timothy J. Ayres Pennsylvania State University World Campus University Park, PA, USA [email protected] Corresponding Author 817-264-6534 611 N Center St Arlington, TX 76011

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Santosh R. Sama Pennsylvania State University State College, PA, USA [email protected]

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Guha P. Manogharan Department of Mechanical and Nuclear Engineering Pennsylvania State University State College, PA, USA [email protected]

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Sanjay B. Joshi Department of Industrial and Manufacturing Engineering Pennsylvania State University State College, PA, USA [email protected]

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Abstract Plaster Binder Jetting (BJ) is one of the major Additive Manufacturing (AM) technologies which has been in use since the 1990s. It has many advantages such as the ability to print in full color CMY(K), no support structures, and is relatively faster and less expensive when compared to other AM technologies. Since there is no phase transformation (e.g. powder to molten pool in powder bed fusion, directed energy deposition), BJ does not require support structures and enables higher packing density in the build volume. However, relatively lower mechanical strength when compared to other AM processes has mostly limited it to non-functional applications such as prototyping. This paper investigates novel methods to improve the mechanical and temperature performance of plaster BJ additive manufactured parts via improved infiltration processes and incorporation of infiltrants with higher strength. Potential applications include functional end use products, including tooling, jigs and fixtures for higher temperature applications. Three 2-part epoxy resin systems were evaluated as infiltrants in comparison to epoxy and cyanoacrylate (CA) resins recommended by the original equipment manufacturer (OEM). Multiple impregnation methods including hot and wet vacuum were evaluated on their infiltration effectiveness. The best impregnation method was then used to prepare tensile, flexural and compressive samples for additional evaluation of each resin. Statistical analysis was conducted to analyze and compare the data. Both resins and infiltrated samples were individually evaluated using Differential Scanning Calorimetry (DSC) to determine glass transition temperatures and other thermal events. Infiltrated specimens of the best performing resins were evaluated for Heat Deflection Temperature (HDT) performance utilizing Dynamic Mechanical Analysis (DMA). It was found that infiltration is anisotropic, with the higher penetration depth from the sides (between layers) than top and bottom (across layers). Vacuum impregnation resulted in the highest infiltration depth by fully impregnating the 25 mm cubic samples. The best performing epoxy showed a 10% increase in mechanical strength over the OEM epoxy at 76% reduction in cost. The OEM cyanoacrylate had the lowest mechanical strength across all tests. DSC analysis revealed that the plaster and gypsum base material will start to dehydrate above 100°C and will ultimately limit the parts’ high temperature capabilities. The OEM epoxy showed the highest HDT. Keywords: Additive Manufacturing, Binder Jetting, Vacuum Impregnation

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1. INTRODUCTION Binder Jetting, or 3-D Printing (3DP) as originally known, was invented at MIT in the early 1990s. Similar to other AM processes, BJ fabricates parts by creating successive cross-sectional layers of an object. This process uses inkjet nozzle heads to selectively deposit a liquid binder (and in later developments, colored ink) onto a bed of powder to create the shape of the cross section of the part. After each layer is printed, another layer of powder is spread across the bed, and the process repeats until all layers of the part have been printed. Following printing, the parts are removed from the powder bed, and the excess powder is removed using vacuum suction and compressed air. The recovered powder is typically recycled to be used again. The powder used in this study primarily composed of calcium sulfate hemihydrate (CaSO4·0.5H2O), commonly known as plaster of Paris. The powder may also contain proprietary additives to improve its suitability for AM. The binder used is water-based with additional proprietary additives, typically including glycerol and liquid adhesives. The powder and binder used in this study contain graphene added to improve the green strength. However, the effectiveness of this additive is not evaluated in this study. The plaster will fully or partially cure into gypsum, or calcium sulfate dihydrate (CaSO4·2H2O) when it interacts with the binder, similar to a plaster cast. The powder particles are held together by this chemical change along with bonding action provided by additional adhesives in the binder and/or powder. The printing strategy employed by Z Corp is to deposit a high saturation (volume ratio of binder to powder) shell on the outer perimeter of the part and a low saturation core. The shell and core saturation values can be adjusted in the OEM control software, and can be varied to adjust the part properties or for different powder-binder formulations. The resulting “green” plaster parts are highly porous and have low mechanical strength. In order to survive even moderate handling, the parts need to be infiltrated with a liquid that subsequently hardens to improve the structural strength. Typical infiltrants for full color models include wax, cyanoacrylate, and epoxy, with cyanoacrylate as the current de facto industry standard. However, relatively lower mechanical strength when compared to other AM processes due to available material choices, has mostly limited plaster BJ to non-functional applications such as prototyping. Expanding the scope to functional applications requires exploring options to increase strength and material properties. Gypsum plaster is sometimes used in high temperature applications, such as plaster molds for non-ferrous metal casting, and hence it is desirable for BJ printed parts to have superior high temperature properties for this application. This paper investigates methods to improve the mechanical and temperature performance of plaster BJ additively manufactured products for potential applications such as tooling, jigs and fixtures, or parts for hot air and hot water testing. The following methods were evaluated to improve the final part properties:

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 Evaluation of high strength and/or high temperature capable 2-part epoxy resins  Maximizing infiltrant penetration depth using heat to reduce resin viscosity along with a novel wet vacuum impregnation process. Mechanical performance was evaluated by preparing tensile, flexural and compressive samples infiltrated with each resin. Thermal performance was evaluated using Differential Scanning Calorimetry (DSC) and Heat Deflection Temperature (HDT) testing.

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1.1. Literature Review The strength of plaster BJ parts has been studied extensively in the literature. Similar to other AM technologies, parts produced by this process exhibit anisotropic strength properties, both in the green state and after infiltration, as studied by Frascati[1], Fajić et al. [2] & [3], Impens [5] & [6], Shougat et al. [7], and Gharaie et al. [9]. Drying the green parts after printing has been shown to improve green strength per Fajić et al. [3] and is also the OEM recommended practice per 3D Systems [28]. Galeta et al. [4] and Vaezi and Chua [11] found that layer height also has an impact on tensile and flexural strength, with a smaller layer height providing improved tensile strength, but a decrease in flexural strength. Vaezi and Chua also showed that higher saturation level results in more complete curing of the plaster, leading to higher green strength. However, this also results in a loss of dimensional accuracy as the greater amount of binder spreads further within the powder. Overall, infiltration has been demonstrated as the best way to improve mechanical strength. Various infiltrants have been evaluated, including epoxy resins, cyanoacrylates, wax, Epsom salt, and various glues. In the majority of the literature, epoxies have been found to yield the highest final strengths, such as by Frascati, Galeta et al., and Impens. In studies by Garzón et al. [8]and Pilipović et al. [10] cyanoacrylate (CA) produced higher strength parts. However, this could be attributed to higher infiltration depth of CA than epoxy in these studies. Tensile, flexural and compressive strength has been positively correlated with infiltration depth by Frascati, Fajić et al. and Impens, which is expected as the infiltrant is typically much stronger than the plaster base material. Fajić et al. found that wax was stronger than CA, which was attributed to the greater penetration depth of the wax, even though it is a weaker material than CA. In summary, lower viscosity infiltrants are correlated to greater infiltrant penetration, and infiltrant penetration depth is positively correlated with mechanical strength. Based on these prior studies, both increasing the depth of infiltration and the choice of infiltrant are the most promising approaches to improving the mechanical strength of printed parts. Although several infiltrants have already been studied, considering the large number of potential infiltrants on the market there is still substantial research opportunities to investigate novel infiltrants.

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High temperature performance of plaster BJ has not been thoroughly evaluated in the literature. Lowmunkong et al. [13] heated green prints to 300°C in an effort to insolubilize them and found that the samples discolored after heat exposure, and if heated for too long, samples became extremely fragile. However, mechanical strength or other properties were not evaluated. Reyes [14] investigated using printed molds to cast fusible alloys with good results. Plaster molds are also used to cast higher temperature non-ferrous alloys, however no examples of printed parts being used for this purpose were found. Different infiltration methods for BJ have also not been thoroughly evaluated in the literature. A double infiltration method was evaluated by Suwanprateeb [12], which showed improvement in flexural strength in wet conditions, but no measurable differences were observed in dry samples. Lizardo et al. [15] investigated the effect of vacuum infiltration on plaster BJ mechanical properties and found that all properties studied were significantly improved compared to the uninfiltrated parts, in particular with epoxy infiltrant. However, in this study only a single type of epoxy was used, and the improvement of vacuum compared to immersion was not evaluated. Maleksaeedi et al. [16] used vacuum infiltration to infiltrate alumina ceramic BJ parts with highly solid loaded slurries, and found that the amount of solid loading and hence viscosity was inversely related to penetration depth. Vacuum impregnation is also a common process for cast, powder injection molded and powder metallurgy metal parts [17]; MIL-STD-276 [18] is one such standard for this process. This process is typically used for porosity sealing to make the parts pressure tight, however the infiltrant frequently does not add to the structural strength of the material. Conversely, Pelletier et al. [19] found that the resin did have a significant impact on mechanical strength when used with high purity iron powder compacts. For non-metallic powders, Komlev et al. [20] showed that vacuum impregnation of hydroxyapatite ceramics by polymers can increase tensile strength. Vacuum resin infusion is also used for composite construction, such as fiber-reinforced epoxy resin composites. In this process bubble formation, e.g. from dissolved gasses in the resin, is a concern, as this leads to voids and defects, and may also be a concern with BJ parts. Sul et al. [21] proposed a theoretical model to describe bubble growth under vacuum, and Afendi et al. [22] looked at ways to reduce bubbles, such as a degassing step.

2.1. Overview and Hypotheses The following hypotheses were evaluated in this study:

Vacuum infiltration will increase the resin infiltration depth. Using heat to reduce the resin viscosity will increase the resin infiltration depth. High strength epoxy resin infiltrants will result in high strength parts. High temperature epoxy resin infiltrants will result in high temperature resistant parts.

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2. EXPERIMENTAL METHODOLOGY

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The initial experimentation evaluated methods to improve the infiltration depth. Using the best infiltration method, mechanical test specimens (tensile, flexural and compressive) were then produced and tested. Lastly, the temperature performance of the infiltrated specimens was evaluated using Differential Scanning Calorimetry (DSC) and Heat Deflection Temperature (HDT) testing utilizing Dynamic Mechanical Analysis (DMA).

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2.2. Resin Selection Over 20 different epoxy resins were evaluated in the preliminary stage of this research. 5 resins were selected using the criteria indicated with the choices. R1 was selected for its high strength, extremely clear color and non-yellowing properties, which is desirable for infiltrating full color parts, although this resin has a higher viscosity and lower temperature resistance than others. R2 and R3 are the primary resins under investigation, and were both selected for their low viscosity, high strength and high temperature resistance. A long working time (the usable time between mixing the resin and when it starts to gel and harden) is also desirable, as too short of a time will limit the infiltration depth and preclude advanced infiltration techniques such as vacuum. R4 was the OEM high strength epoxy and R5 was the OEM cyanoacrylate, and were included for comparison purposes. The properties of these 5 resins from their technical data sheets are presented in Table 1. All epoxies selected are 2-part systems comprised of a resin and hardener; the cyanoacrylate is a single part system. The reported strength values for the OEM resins (R4 and R5) are for infiltrated parts, and all other values corresponds to resin infiltrants.

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TABLE 1: RESIN INFILTRANTS No.

Manufacturer

Name

Working Max Tensile Flexural Viscosity Time(mi Temp Strength( Strength (cP) n) (°C) MPa) (MPA)

Price($ Ref. /gal)

R1

Art Resin

Art Resin

45-60

R2

POLYGEM

R3

Cotronics

R4 R5

2000

50

55

103

$119

[23]

POLYJECT #1001 LV-HT 60-120

200

>115

56

-

$52

[24]

Duralco 4460

240

600

316

71

-

$338

[25]

3D Systems

StrengthMax

45

<600

71

26

44

$217

3D Systems

ColorBond

5

2

60

14

31

$875

[26], [27]

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2.3. Specimen Design Based on the maximum infiltration depths recorded by Impens ([5], [6]), as well as the OEM’s reported penetration depth of 5-10 mm for R4 [28], 25 mm cubes were expected to be sufficient to measure the infiltration depth of the resins. As there are currently no standards to evaluate composite 3D printed plaster specimens infiltrated with resin, the ASTM standards for polymers were utilized. Tensile specimens were designed as per ASTM D638-14 Type I, §6.1.1, with a gauge area of 50 mm × 13 mm, an overall length and width of 165 mm × 19 mm, transition radius of 76 mm, and a thickness of 3.2 mm. Flexural specimens were designed as per ASTM D790-17 §7.5, with a length and width of 127 mm × 12.7 mm, and a thickness of 3.2 mm. Compressive specimens were designed as per ASTM D695-15 §6.7.1, with a length and width of 12.7 mm × 12.7 mm, and a thickness of 3.2 mm. A thickness of 3.2 mm was used both because it is recommended by D638 and to ensure that the parts would be fully infiltrated, and hence the part properties would be consistent across respective test conditions. The DMA HDT samples were designed similarly to the flexural models except with a length, width and depth of 75 mm × 12.7 mm × 2 mm. Prior to testing the specimens, the sample lengths were trimmed to 55 mm to fit within the machine. DSC testing only requires a small amount of material (~5 mg). For the infiltrated specimens, material was taken from the excess length trimmed from the HDT specimens. For the green specimen, an additional HDT specimen was prepared but not infiltrated. For the resin specimens, the epoxies were cast into small silicone molds, cured, and a small sample was sectioned from the cured block. The machine’s color printing capabilities were utilized to print information on the part surfaces, including axis labels, centerlines, and identification labels as shown in Figures 3 and 4.

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2.4. Specimen Printing and Curing A ZPrinter 450, firmware version 4.164.0.0 (Z Corporation) with a HP11 inkjet printhead for the clear binder and a HP57 tri-color (CMY) printhead for the color shell was used in this study. Software used to slice the models and control the machine was 3DPrint version 1.03.8 [30]. The major BJ process parameters used across all prints are listed in Table 2. For this study the default saturation values were used.

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TABLE 2: 3D PRINTING PROCESS PARAMETERS Process Parameter

Value

Powder

Noble 3D GRAPHENITE™

Binder

Noble 3D GRAPHINK™

Binder Saturation Levels

ZP 130 default values

Anisotropic Scaling

0

Bleed compensation

On

Print Resolution

300 x 450 dpi

Layer Height

.1016 mm (.004")

Build orientation

XY (Figure 1)

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FIGURE 1: MECHANICAL TEST SPECIMEN PRINTING ORIENTATION

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TABLE 3: RESIN CURE SCHEDULES Cure No. Schedule Cure Description No.

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After printing, the parts were dried in the 35°C heated bed before removal and de-powdering. A combination of compressed air and a stiff brush was used to remove loose powder from the parts. The parts were then dried in a convection oven at 55°C ±5°C before infiltration to remove any excess water and increase the green strength. Following infiltration, the parts were cured per the resin manufacturer’s directions shown in Table 3.

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72 hours @ room temperature

R2

-

24 hours @ room temperature

R3-120C

1

4 hours @ 120°C (250°F)

R3-230C

2

For optimum properties, post-cure 1-2 hours @ 175°C (350°F). For use in severe environments, post-cure 16 hours @ 230°C (450°F).

R4

-

2 hours @ 70°C. Alternate: 24 hours @ room temperature.

R5

-

~30 minutes

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R3 is the only epoxy among the evaluated resins that requires a high temperature oven cure. In order to achieve the maximum material properties, R3 samples were initially cured per schedule 2 (identified as R3-230C). Temperature in the curing oven was ramped to 120°C at 1.6°C/min, held for 4 hours, ramped to 175°C at 0.9°C/min, held for 1.5 hours, and finally ramped to 230°C at 0.9°Cmin for 3.5 hours into the final 16 hour hold time @ 230°C. Under this curing regimen, it was observed that the parts were severely warping and curling, and bubbling was seen on the surface of the parts. Curing was halted at that point, and parts were allowed to cool to room temperature in the oven. The cooled parts straightened considerably, however some distortion was still observed in the final parts. Considerable amounts of smoke rose from the resin as it cured, which the manufacturer indicated was normal as volatile components in the resin burnt off. The resin and parts also changed to a dark brown color, which the manufacturer also indicated was normal. However, this made any printed text illegible, as seen in Figure 2. Following these results, it was decided to add an additional evaluation case of R3 specimens cured using the basic schedule 1 directions, with a maximum cure temperature of 120°C (identified as R3-120C). The parts still experienced a small amount of warping and some minimal bubbling on the surface, and darkened to a strong yellow color, however printed text was still legible, see Figure 2.

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FIGURE 2: DSC SPECIMENS (75 mm long) Left to Right: Green×2, R1×4, R2×4, R3-230C×4, R3-120C×4, R4×4, R5×4

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2.5. Design of Experiments Four different processes were evaluated to determine the best method to maximize the depth and amount of infiltration, which was measured by the increase in mass after infiltration and visual examinations. The design of experiments (DOE) consisted of a sequential series of single factor experiments, where each process was designed to evaluate the effect of a single factor, e.g. either heat or vacuum. Possible interaction effects were not evaluated in this study. Two replicates were evaluated for each process. An example of a test cube undergoing infiltration is shown in Figure 3.  Process A – Control. For the control samples, room temperature specimens were submerged in room temperature resin for 15 minutes, wiped of excess resin, and cured. For R5 (cyanoacrylate), due to its short cure time this was the only process used.  Process B - Heated Resin. The resin was pre-heated using a hot air gun to reduce its viscosity and was also repeatedly re-heated during the infiltration. Printed parts which were at room temperature were submerged in the heated resin for 15 minutes, then wiped off and cured. The effect of heating on the resin working time was also evaluated. The use of a hot air gun did not allow for the temperature of the resin to be precisely measured or controlled, as this study was only interested in determining if the application of heat had any measurable effect. Temperature variability may be evaluated in a future study.  Process C - Preheated Part. Printed specimens were taken directly from the 55°C drying oven and infiltrated to reduce processing time. If process B showed an improvement over process A, the resin was also preheated.  Process D - Vacuum Impregnation. The part was submerged in the resin and placed in a vacuum chamber for 15 minutes. Following removal, the part was wiped off and cured. For the 25 mm infiltration test cubes with R3 resin, processes A, B and C were cured per schedule 1 (R3-120C), and process D was cured per schedule 2 (R3-230C).

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FIGURE 3: 25MM CUBE INFILTRATION

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To evaluate the effect of the resins on the mechanical strength of the infiltrated parts, tensile, flexural, and compressive specimens were prepared. A total of 6 different conditions were evaluated as shown in Table 4. All parts were infiltrated using vacuum (except for the R5 cyanoacrylate) to ensure that the parts would be fully infiltrated. As per the ASTM specs, 5 replicates were produced in each configuration. One of the R-120C tensile specimens broke in green state, and so only 4 replicates were produced for this configuration. A sample of the parts after infiltration are shown in Figure 4.

FIGURE 4: MECHANICAL STRENGTH SPECIMENS

DSC was utilized per ASTM D3418 [35] to determine the resin glass transition (T g) temperature and any other thermal transitions. 12 different configurations were evaluated, including 6 infiltrated specimens (R1, R2, R3-120C, R3-230C, R4 and R5), 6 noninfiltrated cured resin samples of the epoxies (R1, R2, R3-120C, R3-230C, and R4), and a green specimen. By evaluating a green specimen, the resin, and an infiltrated specimen, the individual and combined effects were determined. DSC specimens are shown in Figure 2. A polymer’s heat deflection temperature is the temperature at which a polymer deforms a specified amount under a specified load. While it is not considered a fundamental physical property, it is a general indicator of the thermal capability of the material, and is

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considered as a comparative metric. HDT testing per ASTM D648 [32] was preferred since it uses the same flexural specimen design as the mechanical strength specimens and is thus more directly comparable. However, due to equipment availability the similar ASTM E2092 [36] procedure was used, which utilizes a DMA with a thinner specimen. For this testing, only the two best performing resins, R2 and R4, were considered. TABLE 4: TESTING AND INFILTRATION PROCESS SUMMARY

Resin

Cube (Infiltrant Mass)

Tensile

Flexural

Compressive

DSC

HDT

None

-

-

-

D

-

1

A, B, C, D

D

D

D

D

D

2

A, B, C, D

D

D

D

D

D

3-120C

A, B, C

D

D

D

D

-

3-230C

D

D

D

D

D

4

A, B, C, D

D

D

D

D

5

A

A

A

A

A

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Green

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2.6. Testing All specimens were weighed within an accuracy of 0.01 g (American Weigh Scales, model AWS-100) in the green state, after drying, infiltration and curing. The increase in mass due to infiltration was determined using the following equation:

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𝑀𝑖𝑛𝑓𝑖𝑙𝑡𝑟𝑎𝑡𝑒𝑑 − 𝑀𝑔𝑟𝑒𝑒𝑛 𝑀𝑔𝑟𝑒𝑒𝑛 The tensile specimen width and thickness were measured within an accuracy of 0.025 mm (0.001”) in the gauge region using a micrometer (Starrett) at three points along the gauge length. The compressive specimen’s length, width and thickness were measured similarly across each pair of faces. The flexural specimen width and thickness was measured at the center of the specimen. Tensile and compressive testing was performed using an MTS CRITERION_45 machine. Tensile testing was performed at a rate of 5 mm/min, and compressive testing was performed at a rate of 1.27 mm/min. Some of the tensile specimens were very brittle, and broke prematurely, e.g. while loading into the machine. This included one R3-120C specimen, three of the R3-230C specimens, and one R5 specimen. The R5 specimen broke early enough in the study that another batch of specimens was produced and infiltrated, however the other specimens broke too late to produce replacements, and so these have less than the preferred 5 replicates. Flexural testing was performed using an Instron ElectroPuls E3000 machine with an Instron 2810-412 Mini 3-point bending fixture. This fixture had a radius of 2.5 mm. A support span of 51 mm was used for all testing. Testing was performed at a rate of 1.3 mm/min. Prior to DSC testing, the samples were weighed (Mettler Toledo, model MS204S). DSC was performed using a TA Instruments DSC Q2000. Samples were placed inside aluminum sample pans with aluminum lids. The lid was crimped to make good thermal contact but not a hermetic seal. Testing was performed per ASTM D3418-15 §10.1 with a heating and cooling rate of 10°C/min and nitrogen purge gas flow of 50 mL/min. The sample was heated from 20°C to 300°C, then immediately cooled back to 20°C, and then the heating and cooling cycle was repeated. Time, temperature, and heat flow into the sample was recorded. HDT testing was performed as per ASTM E2092-13 using TA Instruments RSA-G2 Solids Analyzer with a 40 mm 3-point bending frame. The HDT specimen width and thickness were measured at the center of the specimen using a micrometer; the resulting area was used to calculate the applied force to generate the desired 0.455 MPa stress. Under this applied constant stress, the samples were heated at 2°C/min until they deflected to a strain of 2.0 mm/m. Two replicates were tested for each resin. 𝑀𝑖𝑛𝑐𝑟𝑒𝑎𝑠𝑒 =

3. RESULTS AND ANALYSIS After infiltration, the cube specimens were sectioned along the XZ plane using an abrasive cutoff disc and were subsequently ground using 200 grit sandpaper to remove cutting marks. The sectioned parts were visually examined to determine infiltrant penetration, as shown in Figure 5. The visual examination revealed that the infiltration is highly anisotropic, with the infiltrant penetrating much more readily in between layers (i.e. in the X and Y directions) than across layers (Z direction). It was also apparent that the vacuum impregnated specimens (process D) we fully or nearly fully infiltrated.

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Resin

Process A

B

C

D

1

2

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FIGURE 5: SECTIONED INFILTRATION SPECIMENS

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A pairwise single factor analysis of variance (ANOVA) was performed on the cube Mincrease results to determine statistical significance of each process. Since this was designed as a single factor analysis, processed B, C, and D were all compared to process A (the control) for each resin. The relative amount of infiltration between different resins was not compared, as the metric used to quantify the amount of infiltration (mass increase) is dependent on the density of the resin, which is not known. The average increase in mass after infiltration and curing is listed in Table 5 and Table 6, along with the ANOVA pairwise single factor analysis indicated by significant (S) and non-significant (NS) interactions.

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TABLE 5: INFILTRATION CUBE % MASS INCREASE AND ANOVA SIGNIFICANCE

Process A B

C

D

1

17.7%

28.6% S

22.5% NS

48.7% S

2

21.9%

23.6% NS

28.6% NS

47.7% S

3

6.7%

13.5% S

11.0% NS

33.5% S

4

24.3%

25.5% NS

25.1% NS

44.8% S

5

12.1%

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Resin

Heating the resin repeatedly (process B) was found to increase infiltration for resins R1 and R3. In particular, heating had a significant effect on reducing the viscosity of R1. Heat was found not to have a significant effect for resins R2 and R4, although their viscosities were observed to reduce as well. For R4, the effect of heat had an undesirable effect of both reducing the working time and increasing the temperature during the exothermic curing reaction. No impact on the working time of the other resins due to the use of heat was observed. Preheating the part (process C) was found not to have a significant effect on infiltration for any of the resins studied. However, it shortened the working time of R4. Vacuum infiltration showed a highly significant effect on infiltration for all epoxy resins, fully or near-fully infiltrating the 25 mm thick cubes. Table 6 list the average increase in mass after infiltration and curing for all of the samples which were vacuum infiltrated, including the process D cubes, as well as the mechanical strength and HDT specimens. It can be seen that for the epoxy resins the percent mass increase of the mechanical specimens is significantly less than that of the cubes.

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TABLE 6: % MASS INCREASE OF VACUUM INFILTRATED SPECIMENS Resin

Cube

Tens.

Flex.

Comp.

HDT

1

48.7%

30.4%

32.1%

34.6%

35.0%

2 3-120C 3-230C

47.7% 33.5%

31.8% 27.9% 12.8%

34.2% 28.9% 17.8%

34.5% 29.1% 15.7%

36.3% 31.0% 15.5%

4

44.8%

30.9%

31.4%

34.1%

36.9%

5

12.1%

15.2%

16.4%

19.8%

26.6%

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The average of the maximum strengths in MPa for each tensile, flexural, and compressive specimen are listed in Table 7 and shown graphically in Fig. 6. Consistently across specimens compressive strength was highest, tensile strength was lowest, and flexural strength was somewhere in between. Following mechanical testing, the max strength values were analyzed using a single factor ANOVA analysis. Due to non-uniform numbers of replicates for each condition, the Tukey-Kramer procedure was used to determine the statistical significance of the results, as shown in Table 8. R5 cyanoacrylate had the lowest strength across all testing conditions. R1 had the lowest strength of the epoxies, with the exception of R3-230C in tensile tests. R2 had a higher average strength than R4 in all test conditions, although this difference was only found to be statistically significant in flexural tests. R3-120C had a higher average strength than R2 in all tests, however this was not found to be significant. R3-230C showed a lower strength than R3-120C in all tests.

Tens.

Flex.

Resin

Avg σ

Avg σ

1

11.2

0.9

21.5

-p

TABLE 7: MAXIUM STRENGTH (MPA) IN TENSILE, 3 POINT FLEXURE, AND COMPRESSION

0.8

33.0

1.3

2

15.9

1.1

29.7

1.3

62.5

2.6

3-120C

16.7

0.9

31.6

2.0

67.1

4.4

3-230C

7.2

2.7

30.0

1.9

52.7

2.8

4

14.5

3.2

26.7

1.0

58.3

1.8

4.8

0.4

13.1

1.5

21.4

2.1

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Tensile

Avg σ

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5

Comp.

Flexural

Compressive

Max Strength (MPa)

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75 60 45 30 15

0 1

2

3-120C 3-230C Resin

Error bars show Standard Deviation

FIGURE 6: MECHANICAL STRENGTH RESULTS

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4

5

TABLE 8: MECHANICAL TESTING ANOVA SIGNIFICANCE R2

R3-120C

R3-230C

R4

R5

R

T

F

C

T

F

C

T

F

C

T

F

C

T

F

C

1

S

S

S

S

S

S

NS

S

S

NS

S

S

S

S

S

NS

NS

NS

S

NS

S

NS

S

NS

S

S

S

S

NS

S

NS

S

S

S

S

S

S

S

NS

NS

S

S

S

S

S

2 3-120C 3-230C 4

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The plots from the DSC testing of the 12 specimens are shown in Fig. 7 (a) through (f). In Fig. 7 (a), the first heating cycle of the green specimen is dominated by a dehydration event. As the sample is heated, water is released from the calcium sulfate hydrate which converts it to its anhydrous form. The extrapolated onset temperature of this dehydration event is 143.1°C, although dehydration begins to occur around 100°C. On the second heating cycle, the sample is now completely anhydrous and inert. This same dehydration event is seen in all of the BJ infiltrated specimens, although with different extrapolated onset temperatures. On the second heating cycle, the BJ specimens are either inert or undergo glass transition. The R1, R2 and R4 resins show exothermic peaks consistent with additional curing and cross-linking, and the Tg increased significantly in the second heating cycle. The R3-120C sample showed a small increase in T g in the second cycle, while R3-230C appeared almost completely inert. The R5 BJ specimen results are highly similar to the green results, with the exception of an additional endothermic peak at 236.6°C during the first heating cycle, which may be due to degradation of cyanoacrylate. If any transitions are present, they are likely obscured by the dehydration event.

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(a) Green and R5

(b) R1 5.0

-2.0 -3.0

138.8

-4.0 236.6

-5.0

179.1

-6.0

4.0

-4.0 Tg 72.7

-2.0

260

300

Tg 48.3 20

(c) R2

0.0

128.6

-1.0

133.0

-2.0

163.3

Tg 47.8

-5.0 20

60

100 140 180 220 Temperature (°C) (e) R3-120C

3.0

-2.0

260

260

-7.0 300

Tg 65.9

0.0

Tg 122.1

-1.0

128.4

-2.0

141.8

-3.0

Tg 46.0

Tg 114.9

-4.0

155.8

-5.0

300

20

60

2.0 1.0

-3.0 -4.0

-5.0 100 140 180 220 Temperature (°C)

260

300

(f) R3-230C

2.0 1.0 0.0

ur na

3.0

Tg 113.2 Tg 126.1

-1.0 -2.0

Tg 93.7

-3.0

Jo

87.7

-4.0 -5.0

20

143.8

2.0

2.0

Resin Heat Flow (mW)

-4.0

Tg 87.7

-1.0

lP

-3.0

1.0 0.0

213.4

(d) R4

re

85.0 Tg 91.4 Tg 55.0

1.0

-6.0

154.2

100 140 180 220 Temperature (°C)

2.0

Resin Heat Flow (mW)

2.0

60

3.0

BJ Heat Flow (mW)

3.0 2.0 1.0 0.0 -1.0 -2.0 -3.0 -4.0 -5.0 -6.0 -7.0 -8.0

-5.0

ro

100 140 180 220 Temperature (°C)

-3.0

-1.0 -3.0

3.0

Resin Heat Flow (mW)

-2.0 125.6

0.0

-4.0 60

-1.0

1.0

183.3 20

0.0

Tg 59.8 Tg 81.2

2.0

-7.0

Heat Flow (mW)

1.0

3.0

1.0

1.0 0.0

0.0

-1.0 Tg 184.9

-2.0

-1.0

-3.0

-2.0

-4.0

95.4 167.1

-5.0 20

60

100 140 180 220 260 300 Temperature (°C) FIGURE 7: DIFFERENTIAL SCANNING CALORIMETRY RESULTS.

BJ Heat Flow (mW)

-1.0

2.0

of

143.1

-p

Heat Flow (mW)

0.0

3.0 163.7

BJ Heat Flow (mW)

1.0

BJ Heat Flow (mW)

6.0

Resin Heat Flow (mW)

2.0

60

100 140 180 220 Temperature (°C)

260

-3.0 300

Thin line = Green Specimen (a) / Resin (b)-(f), Thick Line = BJ Infiltrated Specimen The HDT results are shown in Fig. 8; the glass transition values shown are from the DSC results and are included for reference. The average HDT (where the curves intersect the target of 2 mm/m strain) is also identified. The data shows that initially the measured strain increases, presumably due to thermal expansion of the material. Once R2 is above its glass transition, it shows a sharp decline in

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strain, and continues to decrease until the test limit is reached. Although the glass transition is evident in R4, the strain continues to increase until a sharp drop off above 100°C is observed.

R2

R4

0.5

-0.5

Tg 65.9

Tg 55.0

-1.0 -1.5

131.3

of

Strain (mm/m)

0.0

-2.0

25

50

75 100 Temperature (°C)

ro

111.6

-2.5

125

-p

FIGURE 8: HEAT DEFLECTION TEMPERATURE RESULTS

150

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4. OBSERVATIONS AND DISCUSSION It is theorized that the anisotropic infiltration is due to porosity differences between the XY and the Z planes. During printing the binder is deposited relatively uniformly in the XY plane, leading to a solid high density shell with little porosity that resists infiltration. Conversely, the bonding between layers in the Z plane is a result of binder seeping down from the layer above and is far more porous, allowing the infiltrant to penetrate much more readily. This should be considered during part design and selection of build orientation. In future work the effect of anisotropic infiltration may be studied in more detail, including how more complex geometries are impacted. The hypothesis that vacuum would increase the resin infiltration depth was heavily supported by the data. The 25 mm cubes proved to be too small to adequately judge the maximum infiltration depth possible using this method, and hence the maximum allowable wall thickness for structural parts. Future work should examine even larger specimens to determine the limits of this infiltration method. The hypothesis that heat would increase the resin infiltration depth was only partially supported by the data, and proved to be resin dependent. The viscosity was observed to decrease with the application of heat for all of the epoxies studied, however this only resulted in an improvement in infiltration depth for R1 and R3. Heat may actually be detrimental for some resins, such as R4, which experienced a significantly reduced working time. Thus, it will be necessary to evaluate the effectiveness and impact of heat for each resin considered. The 25 mm cubes gained significantly more mass of resin than the 3 mm or 2 mm thick test specimens. It is theorized that this is due to the greater volume of low saturation core in the cubes compared to the thinner specimens; this core is less dense than the out shell and can accept a higher amount of resin. The impact of this observation on mechanical strength may be examined in future work. It is hypothesized that this may lead to a corresponding increase in strength per volume for thicker parts due to the high resin content, although the core is inherently weaker due to the lower amount of cured plaster. Table 6 also shows that the R3-230C specimens have a significantly lower increase in mass than the R3-120C specimens. Whether the lower mass increase is primarily due to loss of volatiles as the resin cures, loss of bound water due to dehydration, or a combination of effects, could not be determined from the data in this study. It was found that resins have different bubbling characteristics that must also be considered. R1 and R2 experienced substantial bubbling under vacuum which required repeated venting to collapse the bubbles. This venting is undesirable, as it prolongs the vacuum infiltration process, and also cools down the resins (and subsequently, increases its viscosity of which is not favorable). Vacuum degassing of resins prior to introducing the part can help minimize bubbling but does not eliminate it. Both R3 and R4 showed few instances of bubbling and required little or no venting. It was observed that resins will bubble under vacuum, as noted in the literature. This may be caused by entrained gases or by some of the more volatile components of the epoxy mixture flashing off; this later case may have a detrimental effect on the final properties

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if it hinders curing. This also makes it difficult to determine when a part has been fully infiltrated, as the resin will never be completely free of bubbles regardless of the duration of vacuuming. Consistent with the majority of the literature, it was found that the epoxy resins produced significantly stronger parts than the cyanoacrylate. It was also found that the reported strength of the epoxy is not a good indicator of the strength of the infiltrated plastergypsum-resin composite part. For example, R1 and R2 resins had similar reported tensile strengths on their data sheets, however for infiltrated parts R2 significantly outperformed R1. R3 resin had a significantly higher reported tensile strength than any other resin, however the infiltrated parts (in its 120°C cure condition) performed similarly to R2 and R4. The fact that R2, R3-120C and R4 all performed nearly identically may indicate that there is a limit to the maximum strength achievable with this material. It was also found that the tensile strength of the R4 specimens was significantly less than reported by the OEM for infiltrated parts. This may be due to the thinness of the parts tested, as previously noted thicker parts may achieve higher strength levels due to the higher resin content. Overall, the hypothesis that higher strength resins will lead to higher strength parts was not supported by this data. In the DSC thermal testing, the dehydration event seen in every BJ specimen is indicative of chemical changes within the material. The bound water is an integral part of the plaster/gypsum crystal structure and chemistry and is partially responsible for adding strength to the material. As evidenced in the R3 oven curing, once the material starts to dehydrate, it may deform and warp. For these reasons, the max operating temperature of the BJ parts should be kept below 100°C to prevent the onset of dehydration. This finding does not support the hypothesis that high temperature epoxy resin infiltrants will increase the temperature resistance of BJ parts, as the base plaster material will be the limiting factor, especially over long durations. However, a maximum operating temperature of 100°C is sufficient to support parts for hot water applications, and possibly hot air and steam as well depending on the length of high temperature exposure. In R1, R2 and R4, the glass transition of the BJ specimen is higher than the resin alone. It is hypothesized that the additional thermal mass of the plaster reduces the heat flow into the resin, resulting in this apparent shift. The dehydration peaks for the R3 BJ specimens are much shallower than the other specimens, since some dehydration had already occurred during the oven cure, especially for R3-230C. Also interesting is the fact that the apparent dehydration onset temperatures are below 100°C, although it is not possible to confirm from this data that the samples were actually losing mass. The epoxies experienced additional curing as they were heated during the first cycle, resulting in additional cross-linking and a higher glass transition temperature in the second heating cycle. Typically, this additional curing results in improvements to other mechanical properties. R2 is of special interest, as the additional curing occurred at 85°C which is below the onset of dehydration. This additional curing had a significant impact on the glass transition, raising it from 47.8°C to 87.7°C in the resin. This should be explored further to determine if the BJ parts can benefit from a post-cure treatment. For R1 and R4, the exothermic curing peaks occurred well above 100°C, and hence, the part would likely begin dehydrating before curing. It is difficult to determine from the DSC data when mass loss events, e.g. dehydration and possible degradation of the resins is occurring in the material. Repeating the DSC testing using different heating rates would yield additional information. In addition, thermogravimetric analysis (TGA), which measures the mass of the sample as the temperature changes will be considered in future study. R4 outperformed R2 during HDT testing, even above its glass transition temperature. However, as discussed earlier, post-curing R2 may significantly improve its performance in this testing, especially as HDT appears closely related to the Tg in this resin. As noted in the literature, resin infiltration is broadly utilized in traditional manufacturing, most notably for porosity sealing metallic composites and as a matrix for fiber composites. In these applications the base material is typically much stronger than the infiltrating resin; for plaster BJ the opposite is true. While the addition of resin was found to significantly increase the strength of the composite part, the mass increase results show that the final material is still composed primarily of gypsum/plaster, which will ultimately limit the final properties achievable. In future work the infiltration process discussed herein could be applied to other AM materials with a higher base strength. For the BJ process, this could include sand or metal powders, or plaster with strengthening additives such as fibers. Other potential processes that may benefit include polymer material extrusion or polymer powder bed fusion. 5. CONCLUSIONS Vacuum infiltration was found to be highly effective for all resins tested in this study with complete infiltration of 25 mm thick cubes. Infiltration was found to be anisotropic, with the infiltrant penetrating more deeply from the sides than the top and bottom, which should be considered during part design and printing. The chemistry of the calcium sulfate base material limits the maximum temperature of the parts to 100°C. Parts may warp, discolor, and lose strength above this temperature since the material could begin to dehydrate. R5 (OEM CA) was the worst performing resin, showing the least amount of infiltration and lowest strength, but having the highest cost. R1 was the worst performing (lowest strength) of the epoxies evaluated. R3 showed good thermal stability in DSC testing and similar strength to R2, however its requirement for an oven cure above 100°C, brittleness of the resulting parts, and its high cost limits its utility for this application. R4 (OEM epoxy) demonstrated strong performance with the highest strength results, and excellent HDT performance. R2 demonstrated the best price/performance ratio, being the most inexpensive resin evaluated (24% of the cost of R4)

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yet demonstrating higher strength values than R4. Additionally, its DSC results indicate that its performance could be further improved after a post-cure process. Declaration of interest: Timothy Ayres is co-founder and co-owner of 3DLirious, LLC, who provided the 3D printer, 3D printing raw materials, resins, and infiltration equipment for this study.

AUTHOR DECLARATION TEMPLATE We wish to draw the attention of the Editor to the following facts which may be considered as potential conflicts of interest and to significant financial contributions to this work.

of

Timothy Ayres is co-founder and co-owner of 3DLirious, LLC, who provided the 3D printer, 3D printing raw materials, resins, and infiltration equipment for this study.

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We confirm that the manuscript has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. We further confirm that the order of authors listed in the manuscript has been approved by all of us.

-p

We confirm that we have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing we confirm that we have followed the regulations of our institutions concerning intellectual property.

lP

re

We understand that the Corresponding Author is the sole contact for the Editorial process (including Editorial Manager and direct communications with the office). He/she is responsible for communicating with the other authors about progress, submissions of revisions and final approval of proofs. We confirm that we have provided a current, correct email address which is accessible by the Corresponding Author and which has been configured to accept email from [email protected].

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ACKNOWLEDGMENTS This project was sponsored by 3DLirious, LLC, who provided the 3D printer, 3D printing raw materials, resins, and infiltration equipment. The authors acknowledge and thank Bridget Ayres, who discovered that plaster BJ parts could be infiltrated with metal powders. All mechanical and thermal testing was performed at Pennsylvania State University in University Park, PA. The authors gratefully acknowledge and thank Dr. Saurabh Basu and Mustafa Rifat, for their assistance with tensile and compressive testing; Dr. Jing Du and Kangning Su, for their assistance with flexural testing; Dr. Zoubeida Ounaies, Abdulla Al Masud and Albert Foster, for their assistance with DSC and DMA testing.

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[28]. 3D Systems, Inc. (2013). User Guide, StrengthMax High-Strength Epoxy Infiltrant, PN: 22-95042, Rev.A. Rock Hill, SC: 3D Systems, Inc. [29]. 3D Systems, Inc. (2013). User Guide, ColorBond Instant Infiltrant, PN: 22-95041, Rev.A. Rock Hill, SC: 3D Systems, Inc. [30]. 3DPrint. (2017). Rock Hill, SC: 3D Systems, Inc. [online] Available at: http://infocenter.3dsystems.com/projetcjpx60/softwaredownloads [31]. ASTM Standard D638 (2014). Standard Test Method for Tensile Properties of Plastics. West Conshohocken, PA (United States): ASTM International. [32]. ASTM Standard D648 (2016). Standard Test Method for Deflection Temperature of Plastics Under Flexural Load in the Edgewise Position. West Conshohocken, PA (United States): ASTM International. [33]. ASTM Standard D695 (2015). Standard Test Method for Compressive Properties of Rigid Plastics. West Conshohocken, PA (United States): ASTM International. [34]. ASTM Standard D790 (2017). Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials. West Conshohocken, PA (United States): ASTM International. [35]. ASTM Standard D3418 (2015). Standard Test Method for Transition Temperatures and Enthalpies of Fusion and Crystallization of Polymers by Differential Scanning Calorimetry. West Conshohocken, PA (United States): ASTM International. [36]. ASTM Standard E2092 (2013). Standard Test Method for Distortion Temperature in Three-Point Bending by Thermomechanical Analysis. West Conshohocken, PA (United States): ASTM International.

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