Epoxy Doped, Nano‐scale Phase‐separated Poly‐Acrylates with Potential in 3D Printing

Abstract

An efficient method to improve the mechanical performance of a commercially available photocure resin is described wherein the resin is modified with a mixture of a cycloaliphatic epoxy and an anhydride curing agent. Photocured samples are thermally treated in a subsequent step to cure the epoxy to obtain an interpenetrated polymer network (IPN) and also complete reaction of the acrylate monomers remaining from the photocure. The latter is accomplished by a thermal radical initiator added earlier into the formulation together with the epoxy‐anhydride. The thermal properties and microstructure of the resulting IPN are analyzed. Uniform and quantitative conversions are obtained, with glass transition temperatures comparable to conventional epoxies. The liquid, uncured samples containing different amounts of epoxy are stable at 30 °C for several weeks. In the fully cured epoxy‐rich materials, nano‐scale phase separation is observed by atomic force microscopy. This is corroborated by the existence of multiple relaxations determined by dynamic mechanical analysis analysis. Specimens from a formulation containing 50% by weight of epoxy‐anhydride are 3D printed in a customized masked image processing stereolithography, thermally treated, and are subjected to compression tests. Results show that Young's modulus increases by 900% over the neat resin.

Cost-Effectively 3D-Printed Rigid and Versatile Interpenetrating Polymer Networks

“…As explained previously, the first curing stage is free-radical acrylate photopolymerization at ambient temperature and the second curing stage is nucleophilically initiated epoxy–anhydride copolymerization carried out at elevated temperature. The overall reaction process is similar to that reported in our previous works [ 17 , 26 ]. A polyacrylate network is formed by radical acrylate homopolymerization initiated by activation of a photoinitiator upon exposure to UV light, and a copolyester network is formed by epoxy–anhydride copolymerization by initiation with a nucleophilic tertiary amine such as DMAP, as seen in Scheme 1 .…”

“…In contrast to our previous work, the use of Spot-E resin A yields rubbery parts that are flexible and thus shape-conformable after 3D printing at ambient temperature [ 17 , 20 ]. This flexibility can be exploited from an economical point of view, by printing simple 2D geometries in significantly fewer layers, and thus in a shorter time.…”

“…At the thermal curing stage, this initiator ensures complete conversion of all unreacted acrylates remaining from the DLP-3D printing stage. For a more detailed discussion of its function, the reader is directed to our earlier work [ 17 , 20 ]. Although present also in the FTIR samples, the initiator is essentially redundant, since the UV irradiation conditions in the FTIR spectrometer ensure full conversion of acrylates at ambient temperature, as will be shown later.…”

“…Once the epoxy–anhydride copolymerization takes place, in certain formulations, the formed network formed a separated phase, evidenced by dynamic mechanical analysis (DMA) and atomic force microscopy (AFM). Although such separation was observed previously in similar systems [ 17 , 20 ], its effective mitigation was not attempted. In this work, by employing a coupling agent bearing carboxyl and acrylate groups, the separation could be suppressed.…”

“…In all these works, researchers employed dual-curing approaches wherein the desirable kinetics of free radical acrylate photopolymerization is coupled with the high rigidity of epoxy-based polymeric networks. Whereas in some works the free radical acrylate photopolymerization took place concurrently with cationic epoxy curing [ 8 , 9 , 10 , 11 , 12 , 13 ], in others, a sequential dual-process was used in which the epoxy was cured in a thermal step, subsequent to acrylate photopolymerization [ 14 , 15 , 16 , 17 ]. Very recently, our group has published a comprehensive review on the recent state-of-art of dual-processing of 3D printing materials [ 18 ].…”