High-performance thermosets for additive manufacturing

Additive manufacturing (AM) has come a long way since its initial inception. Previously considered a fast prototyping method, it offers significant benefits for use as a method of producing user-end parts that are limited in quantity, customizable, and/or complicated geometries. For AM to be considered in high-performance applications, such as automotive and aerospace, we must consider AM technology and the available and compatible printing materials. Typically only thermoset plastic resins are capable of meeting high-performance specifications, such as sufficiently high strength, stiffness, and toughness, as well as excellent chemical and environmental resistance. This review presents a broad overview of the available high-performance thermoset chemistries and formulations, i.e., resin blends. The base resin chemistries that are covered are: vinyl, epoxy, imides, cyanate ester, urethanes, benzoxazine, and click chemistries (e.g., Michael addition). Subsequently, more application-relevant blends of these base resins are discussed. Each section focuses on resin details such as reaction mechanisms, typical monomer structure, mechanical properties, and applications specific to AM. The review is organized as follows. We begin with an introduction on the state-of-the-art, the challenges still faced by the field, and a benchmark definition of "high performance." This is followed by a discussion of the available AM technologies for thermoset printing, with a focus on their advantages and disadvantages. Next, we cover the details of different resin chemistry, followed by their blends. The following section details the difficulties in developing AM technologies that allow for the incorporation of fillers, such as rheological modifiers and reinforcements. The review ends with a perspective on the future of AM technologies that would bridge the gap between pure resin printing and the much needed composite printing for high-performance applications.