3D Printed Polymers: Solvent Compatibility Charts Must Be Dedicated Rather Than Simple

Authors Kirill S. Erokhin, Evgeniy G. Gordeev, and Valentine P. Ananikov, researchers from Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, delve into a new level of 3D printing science, outlined in the recently published ‘Revealing interactions of layered polymeric materials at solid-liquid interface for building solvent compatibility charts for 3D printing applications.’

Strength and stability in 3D printed parts is an ongoing area of refinement for users today, and researchers have performed a multitude of experiments on materials to figure out how to get better results, creating new algorithms, studying causes and effects on porosity, and ways to eliminate structural defects. This study is centered around the effects of chemicals, however, as the authors evaluate how thermoplastics respond to a variety of solvents.

While there are the obvious challenges in creating nearly any kind of part or prototype, the benefits can be immense in many applications such as medicine and related areas such as tissue engineering, aerospace, automotive, agriculture, and so much more—with the potential for infinite innovation; however, for functional parts being used in critical applications, stability must be ensured. Microstructures may be changed during the printing process, porosity may affect quality, and exposure to organic solvents may cause chemical breakdown.

Materials like polyether ether ketone (PEEK), polyphenylsulfone (PPSU) and polyetherimide (Ultem) may show chemical resistance along with displaying superior mechanical properties, but for most ‘superconstruction materials,’ customized, expensive 3D printers are required to main the high temperatures (350–400 °C) required during successful fabrication:

“Besides, the materials themselves are expensive (as compared with the conventional production-grade thermoplastics), which makes their routine use not very common. Despite the high demand for strong and chemically resistant FDM materials with low shrinkage, the efforts towards their development are still limited,” explain the researchers.

“As an important limitation, 3D printing is poorly suitable for production of plastic objects to be exposed to liquid media. The limitation is due to susceptibility of the plastics to the action of liquid chemicals.”

Simple adjustment of printing parameters could offer the solution to creating parts that are more structurally sound though, even with significant exposure to solvents. The authors performed a comparative model experiment with a cylindrical part and an identical 3D printed copy.

As they immersed the parts in DCM, the 3D printed part began to lose its integrity quickly.

Representation of stability test of FDM parts in liquid media. (a) PLA parts made by standard extruding technology; (b) PLA parts made in this work by FDM 3D printing; (c) macrophotos of the extruded part (left) and FDM printed part (right) with same diameter 2.85 mm; (d,e) snapshots of the chemical resistance tests in DCM for the 3D-printed part (extrusion multiplier k = 0.9) and for the extruded part, respectively, with a brass cylinder as an indicator of integrity (Supplementary Movies S1, S2); (f) the principle of FDM-based additive manufacturing; (g) layered structure produced by FDM, (h) destruction of the 3D-printed surface due to interaction with a solvent.

Samples were created using a Picaso 3D Designer Pro 250 printer, with the following information provided regarding materials:

“Printing with ABS, SBS, PLA, Nylon, PP, PE, PETG, HIPS, POM, and Primalloy was accomplished at a layer height of 0.2 mm by using a 0.3 mm nozzle. Printing with filled plastics PLA-Cu, Nylon-C, and Ceramo was accomplished by using a 0.5 mm nozzle at a layer height of 0.35 mm.”

Ultimately, 12 different solvents were used:

1. Dichloromethane (DCM)
2. Tetrahydrofurane (THF)
3. Acetone
4. Dimethylformamide (DMF)
5. Toluene
6. Ethyl acetate
7. Triethylamine (TEA)
8. Acetic acid
9. Ethanol
10. Sulfuric acid
11. Sodium hydroxide
12. Water

An example of destruction of ABS part in DCM with a metal bead as an indicator of integrity. (a) A series of snapshots; (b) corresponding curve reflecting increased visible area occupied by 3D-printed blue cylinder due to destruction process (horizontal axis shows experimental time, vertical axis shows actual-to-initial areas ratio); (c) examples of representation in a circular diagram: ABS as an unstable material (red) as well as general notations of more stable materials (blue and green).

“The comparison shows that mode of manufacturing (traditional vs. 3D printing) is highly important for the real performance in a contact with liquid,” explained the authors, leaving them to realize that a ‘simple solvent compatibility chart’ would not suffice.

Other polymeric materials were tested as well, with the highest chemical resistance displayed by the following:  PP, PE, POM, Nylon, and Nylon-C. Alternately, FDM materials such as PLA, ABS, SBS, and HIPS showed less resistance to solvents.

Circular diagrams of change in the FDM part area during 1 h exposure to organic and inorganic solvents. () the material collapses during the experiment (ΔS > 20%): the object loses its shape by dissolution, disintegration and/or delamination; () the material shows moderate stability during the experiment (ΔS = 2–20%), with minor swelling or dissolution of the outer layers, but satisfactory retention of the shape; () the material is stable during the experiment (ΔS < 2%): the object retains its shape, and no dissolution of the outer layers is observed (see Supplementary Movie S3). 1Destruction of Primalloy in THF took just a few seconds.

Primalloy (an elastomer) exhibited ‘moderate resistance,’ while PETG was resistant to acetone and toluene, along with ethyl acetate and DCM.

The main types of destruction noted were disintegration, delamination, molecular dissolution, and swelling—with a ‘destruction scenario’ being noted as the relationship between the thermoplastics and solvents, and differences such as the way PLA becomes dissolved in DCM but delaminated in acetone.

“All tested materials are resistant to water, acidic and basic aqueous solutions, and also to ethanol, which allows their exposure to aqueous reaction media. At the same time, PLA, PLA-Cu, ABS, SBS, Ceramo, HIPS and Primalloy parts are incompatible with acetone, ethyl acetate, toluene, DMF, THF and DCM,” concluded the researchers.

“Resistance of the printed parts to solvents can be increased by three ways. Firstly, the polymers can be modified by additives that would protect them from the solvent action. Secondly, the influence of solvents can be prevented by reasonable choice of 3D printing parameters, as decreased porosity prevents penetration of the solvent. Alternatively, the influence of solvents can be mitigated by adjustment of the part geometry.”

The influence of extrusion multiplier k on structural integrity of FDM parts. (a) change in the wall thickness for a cylindrical part made of PLA depending on the extrusion multiplier; (b) change in the wall structural integrity for a cylindrical part made of PLA; (c) change in the wall thickness for a cylindrical test tube made of ABS; (d) change in the wall structural integrity for a cylindrical test tube made of ABS; (e) a graphic representation of complete G-code for FDM test tube; (f) G-code-defined distribution of the seam points on the FDM test tube wall; (g) structure of a single layer of FDM test tube with the seam position denoted by red arrow.

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