Introduction

Sidney H. Goodman

This book presents an overview of a major class of materials of construction: thermosetting plastics. Using the biological analogy, this class fits into the family of materials as shown in Figure 1-1.

One popular definition of thermosets is:

Another more rigorous definition is found in Whittington’s Dictionary of Plastics (p 239):

This leads to an interesting concept. All too often trade usage confers titles on classes of materials. These titles reflect a nomenclature or jargon that is fully comprehensible to those in the trade. Those new to the trade soon learn the meaning of the terms by association, osmosis, etc. At some point in the technology maturation, someone decides to establish a precise definition of the terms. The true definitions are quickly found to be elusive: no two practitioners define them exactly the same way: the definitions are not “scientific” enough; more exceptions to the rule exist than examples of the rule; and on and on. The term “thermoset” or “thermosetting plastics” is a classic illustration of this phenomenon.

This book is an attempt to collate and present the current practices and technology associated with a group of commercial polymeric materials called “Thermosets.” Everyone who works with these materials has an intuitive understanding of the types of plastics that fall into this category. We know, for example, that chemical crosslinking must occur in order for the resultant product to be called a thermoset. We know that the monomeric precursors may or may not be polymeric in and of themselves, will undergo reaction when the chemical kinetics are right; that these precursors are commonly called thermoset resins because they will participate in a crosslinking reaction.

We also know that under the right conditions many of these resins can polymerize linearly and form a traditional thermoplastic polymer. Vulcanization is a form of crosslinking wherein a rubber is formed, yet rarely do technologists refer to rubber as a thermoset plastic. Biopolymers (amino acid/protein based) are known to crosslink (one theory suggests this as a root cause of aging) and we hardly think of animals as thermosetting plastics.

This book then will be structured based on the commonly perceived “definitions” of thermosetting resins. Both definitions stated earlier remain valid and useful.

This introductory chapter will include a series of basic terms and definitions that will be referred to throughout the individual chapters that follow. Many of the “definitions” will in fact be descriptions of the phenomena which best illustrate the sense of the terms, as opposed to a rigorous definition per se. That these explanations are “common usage” or “trade jargon,” that they are not scientifically precise, does not compromise or lessen their meaning or value.

HISTORY

Goodyear’s (and Hancock in England) discovery of the vulcanization of natural rubber in 1839 could be construed as the first successful commercial venture based on thermosetting polymers. The plastics industry dates the beginning of thermosetting plastics to the development by Leo Baekeland in 1909 of phenolics. In this instance, Baekeland not only produced the first synthetic crosslinked polymer, but as importantly, he discovered the molding process that enabled him to produce homogeneous useful articles of commerce. The Bakelite product line dominated plastics technology for years until the advent of alkyds in 1926 and the aminos in 1928. Table 1-1 lists a synopsis of the various historical milestones in thermosetting resin technology. Progress was made more often as a result of the economical commercialization of key precursor materials rather than as a conscientious result of a chemist’s ability to tailor polymers for specific properties and characteristics. It must be remembered that the acceptance of Staudinger’s heretical concept of macromolecules was not universally accepted until the late 1920s and early 1930s, long after products made from polymeric materials had reached commercial maturity.

DEFINITIONS

The broad classifications of plastics — general purpose, engineering, and specialty — applies to thermosets as well as thermoplastics. General purpose thermosets are characterized by average (for thermosets) mechanical properties, lower resistance to temperature, higher coefficients of expansion, and low cost/commodity-like production and sales (tons/year). Engineering thermosets have higher mechanical properties and temperature resistance and they are perceived to be more durable. They are more expensive with a moderate production volume (pounds/year). Specialty thermosets are useful because of one or more highly specific and unusual property which offsets any lack of other “good” properties. They are usually very expensive and are produced in relatively small quantities (pounds/batch). Overlapping between the three categories often occurs — a general purpose phenolic is often competitive with an engineering polyimide. The individual families of plastics in this book can be loosely classed as shown in Table 1-2.

It is assumed that the reader has a reasonable understanding of the basic principles of polymer science and organic chemistry. These initial discussions therefore, are designed to highlight and review some of the basic concepts in order to establish the proper perspective for the material which follows.

CROSSLINKING AND CURING

A linear polymer is a long continuous chain of carbon-carbon bonds with the remaining two valence bonds attached primarily to hydrogen or another relatively small hydrocarbon moiety. Figure 1-2 shows a schematic representation of some linear polymer configurations.

A network polymer is formed as a result of the chemical interaction between linear polymer chains or the build-up from monomeric resinous reactants of a three-dimensional fish-net configuration [Figures 1-3(a) and 1.3(b)]. The process of interaction is called crosslinking and is the main distinguishing element of a thermosetting material. The “thermo” implies that the crosslinking proceeds through the influence of heat energy input, although, as will be seen in the individual chapters, much crosslinking occurs at room temperature (25°C, 77°F) and below. The “setting” term references the fact that an irreversible reaction has occurred on a macro scale. The network polymer formed has an “infinite” molecular weight with chemical interconnects restricting long chain macromovement or slippage.

Molecular functionality (i.e., number of reactive moieties per mole of reactant) dictates the potential for a crosslinking reaction. A total average functionality between reactant elements greater than two suggests the potential for crosslinking independent of mechanism. In other words, the bifunctional C=C, would, via an addition reaction, normally produce a linear polymer. If, however, other unsaturation is generated or remains in the formed linear chain, crosslinking can yet occur (Figure 1-4).

Similarly for a condensation reaction, a tri- or polyfunctional reactant will form a thermoset structure with a polyfunctional comonomer.

INFLUENCE OF TIME,...