Polymer Design for 3D Printing Elastomers: Recent Advances in Structure, Properties, and Printing

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Abstract

Elastomers are a unique and important class of polymers that enjoy applications in virtually every industry, including healthcare, aerospace, automotive, and apparel. However, due to inherent physical, thermal, and mechanical properties of elastomers, the additive manufacturing (AM) of elastomers remains challenging. These challenges are discussed in the context of various AM processes, including powder bed fusion, material extrusion, and vat photopolymerization. This review provides an in-depth discussion of the current state of silicone and polyurethane polymers for AM, and also discusses polyesters/polycarbonates, liquid crystalline elastomers, and monomer compositions that provide elastomeric properties upon photocuring. Finally, the current state of common, commercially-available elastomers for AM is provided, as well as an outlook for this rapidly progressing field. We expect this review to be useful for any research involved with the additive manufacturing of elastomers.

Introduction

Thermoplastic elastomers and crosslinked rubbers remain an extremely important polymer class and enjoy a global demand expected to rise 5.2 % annually to a total of 6.7 million metric tons in 2019. [1,2] Their versatile properties render them ideal materials for everyday usage in products that include car tires and footwear, medical tubing in health care applications, automotive and aircraft sealants, and soft robots. Named as a portmanteau of the words “elastic” and “polymer,” elastomers are composed of long polymer chains that undergo shape recovery after being subjected to often-large applied strains. While crosslinked rubbers permanently retain their shape as a result of their covalent crosslinks, thermoplastic elastomers may be re-shaped, often by heating and subsequent re-forming above their highest transition temperature. Both classes of materials possess unique strengths and are suited to individual applications.

The advent of additive manufacturing (AM), also known as three-dimensional (3D) printing, inspired academic and industrial researchers to combine elastomeric properties with design freedom and the potential for facile mass customization. This enabled the production of customized articles, e.g. headphone or hearing aid inserts, dental retainers, and footwear. For example, in 2015, a large footwear manufacturer reported the first 3D printed shoe sole utilizing selective laser sintering of thermoplastic polyurethanes. [3] Due to its reliance on polymer deposition only at the desired 3D pixels (voxels) in a 3D design, instead of reliance on standard subtractive manufacturing methods, AM significantly reduces material waste, enables lightweight design through printing of low-density, high-strength truss geometries, and provides the ability for mass customization where each part remains customized to the needs of the individual. Inspired by the potential of 3D printing, two major tire companies recently released their vision of the future tire. [4,5] Starting from recycled or bio-based materials, they propose 3D printing airless and lightweight tires. Their design would provide improved shock-absorbing properties and enhance the safety of a tire, due to improved wet grip and puncture resistance. One of these companies also envisions a customized tread pattern, enabling the customer to reprint the tread pattern at a service station for demanding weather conditions such as ice or snow. [4] Both concepts demonstrate the importance of AM and materials design for a more sustainable future.

This review highlights the recent achievements in additive manufacturing of elastomers and elastomeric-like materials. It aims to elucidate synthetic strategies and required material properties that enable 3D printing of elastomers with a focus on silicone- and polyurethane-based materials. A brief introduction of relevant AM techniques and synthetic methods to prepare elastomers provide the reader with the proper background information for the detailed sections that discuss 3D printing of elastomers. Finally, a short perspective outlines potential future work and inspires the reader to solve ongoing challenges.

Section snippets

Elastomer Properties

Elastomers are crosslinked polymer networks which exhibit low crosslinking density and possess high molecular weight polymer strands between crosslinks. When an external stress is applied to an elastomer, the long polymer strands alter their chain conformation and enable a large deformation of the network, while the crosslinks prevent flow. A typical elastomer may be stretched up to 10 times its original length, reach extremely high elongation at break (e.g. ≥ 400-1000 %), and recovers its

General overview

Here, we introduce the types of additive manufacturing (AM) that are most employed in the 3D printing of elastomers. A general overview of various types of AM technologies is provided in more detail elsewhere. [12]

Vat photopolymerization

Vat photopolymerization (VPP) involves directed energy deposition upon the surface of a photocurable mixture of monomers, oligomers, and/or polymers, termed a photopolymer, inside a container, or vat. [12,13] Multiple literature reports discuss this and other types of additive

Challenges of 3D Printing Elastomers

Challenges related to elastomer processing with various AM technologies are highly dependent on the printing technology. Therefore, many challenges remain specific either to the printer or the combination of the polymer and the printer. One major challenge with vat photopolymerization relates to photopolymer viscosity. [31] Processing of photopolymers with too high viscosity often results in slower print times at best,[15] or warpage of the printed objects at worst. [32] A practical upper

Properties of silicone elastomers

Silicone elastomers are biocompatible, optically transparent, relatively chemically inert, electrically insulating, non-flammable, non-UV absorbing, exhibit low surface tensions, are water-impermeable and possess a high oxygen permeability. Furthermore, they are rather inexpensive and maintain their mechanical and electrical properties over a wide temperature range (−50 °C to +300 °C). [54] Applications below −50 °C are challenging due to the melt crystallization of polydimethyl siloxane (PDMS)

Motivation

AM of silicone elastomers is highly desirable for healthcare applications, e.g. fabricating microfluidic devices, implants and vascular tubes. It would also facilitate the manufacturing of soft active materials, such as actuators, robots, wearable electronics and sensing devices. For example, conventional molding processes require individual fabrication steps for each part and subsequent assembly, which are time-consuming iterative steps and hinder automation. AM of silicone elastomers in

Motivation

Like silicone elastomers, the AM of polyurethanes presents many advantages for healthcare applications (e.g. implants and vasculature), as well as automotive and engineering applications. Printed objects retain characteristic strengths of bulk thermoplastic polyurethanes (TPUs), namely high elongation at break, elastic recovery, recyclability, and high wear resistance. For example, Invisalign® retainers from Align Technologies, Inc. are a series of polyurethane orthodontic retainers that serve

Photopolymers for Vat Photopolymerization or Inkjet Printing

Most photopolymer compositions for vat photopolymerization or inkjet printing remain highly formulated and contain a variety of reactants, such as photocurable oligomers, reactive diluents (e.g. monomers), solvents (e.g. unreactive diluents), photoinitiators (e.g. sensitizers), UV absorbers, chain extenders, and radical inhibitors (e.g. antioxidants). [12] These components are reviewed in great depth elsewhere but are briefly discussed here for context. [12,[150], [151], [152]] Each component

Additive Manufacturing Using Polymer Emulsions

As briefly mentioned in section 7.4, emulsions offer great potential for AM of elastomers. In emulsion polymerization, the water-insoluble monomers (oil-phase) are dispersed in water using surfactants. Radical polymerization yields latex particles suspended in water, which contain high molecular weight polymers. [177] Because the polymers are formed in micelles/colloidal particles, the viscosity of the latexes are close to that of water (ηH2O = 0.001 Pa·s) and do not depend on the molecular

Liquid Crystalline Elastomers (LCE)

Liquid crystalline elastomers (LCEs) are slightly crosslinked, flexible polymers which bear liquid crystalline mesogenic groups in their side- or main-chains (Scheme 24). They combine the ordered and mobile character of liquid crystals (LC) with rubbery elasticity of polymers. [180,181] Details on the synthesis and use of LCEs for harnessing macroscale mechanical responses was recently reviewed by White and Broer. [182] In brief, LCE’s exhibit reversible mechanical and optical properties

Polyesters and Polycarbonates

A small number of 3D printable, polyester- or polycarbonate-based polymers are reported, though the volume of literature does not match that of polyurethanes or poly(dimethyl siloxane)s. In one example, Zhang et al. synthesized and 3D printed unsaturated, low Tg, random copolyester thermoplastic vulcanizates (TPVs),[190] shown in Scheme 26, which are based on some of their previous work. [191,192] These phase-separated TPVs contain a high weight fraction of elastomer and smaller thermoplastic

3D Printing of Commercially Available Elastomer and Elastomeric-Like Materials

Table 3 provides a selective overview of commercially available materials specifically distributed to additively manufacture objects with rubbery-like and flexible characteristics. It lists the polymer type, resulting thermal and mechanical properties and the respective printing technique, which are provided in the data sheets of the material suppliers. While chemical details on the polymer resins/powders/filaments are proprietary, the materials safety data sheets (MSDS) provide some insight,

Conclusions and Outlook

The interest in additive manufacturing of elastomers is rapidly growing because of the wide range of potential applications. Elastomers play an extremely important role in the automotive industry, e.g. for tires and gaskets, and find use in sport equipment and consumer products such as shoe soles, toothbrush or bicycle grips, in health care, e.g. as artificial vascular constructs or implants and also in specialty technologies, e.g. as soft actuators. The ease of manufacturing customized designs

Acknowledgements

The authors thank Emily M. Wilts for proof-reading the manuscript. The authors further acknowledge Philip J. Scott for helpful discussions regarding blocked isocyanates.

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  • Cited by (0)

    1

    Current institution: Department of Chemical and Materials Engineering, University of Kentucky, Lexington, KY 40506, USA

    2

    These authors contributed equally to the work.

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