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THERMOSETS Introduction Thermosets may be defined as network-forming polymers. Unlike thermoplastic polymers, chemical reactions are involved when producing or fabricating parts. As a result of these reactions the materials cross-link and become “set,” ie, they can no longer flow or dissolve. Cure most often is thermally activated, hence the term thermoset, but network-forming materials whose cure is light or radiation activated are also considered to be thermosets. Some thermosetting adhesives cross-link by a dual cure mechanism, ie, by either heat or light activation. In contrast to cross-linked elastomers or rubbers, the glass-transition temperature (T g ) of thermosets is generally above room temperature. Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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In this article we describe the distinguishing characteristics of thermosetting materials; the more common thermosetting resins including their cure chemistry; thermoset nanocomposites; cure and properties, including cure kinetics; the development of residual stress as a result of curing; cure monitoring; and conclude with a description of selected thermoset processes. Uncured thermosets are mixtures of small reactive molecules, often monomers. They may contain additives such as catalysts to promote or accelerate cure. Most thermosets are used in filled or reinforced form to reduce cost, modify physical properties, act as a binder for particles or fibers, reduce shrinkage during cure, or to provide or enhance flame retardance. In general, thermosets possess good dimensional stability, thermal stability, chemical resistance, and electrical properties. Because of these attributes, they find widespread use in several applications such as adhesives; primary and secondary structural members in aerospace; countertops and floors for manufacturing facilities and homes; printed circuit boards, conductive polymer elements, and encapsulation materials for electronic applications; dental materials, especially adhesives; and recreational products such as tennis racquets, bicycle frames, golf clubs, and fishing rods. Epoxy resins (qv) are probably the best-known members of the thermoset family whose members also include phenolic resins (qv), unsaturated polyester, polyurethanes (qv), dicyanate, bismaleimide, acrylate, and many others (see POLYESTERS, UNSATURATED; ISOCYANATE-DERIVED POLYMERS). Good thermoset references include Rosato (1), Prime (2), Pascault and co-workers (3), and Van Mele and co-workers (4). The first of these is process-oriented and the others are cure- and chemistry-oriented.

Characteristic Behavior of Thermosets Thermosets are distinct from thermoplastic polymers in one major respect: their processing includes the chemical reactions of cure. As illustrated in Figure 1, cure begins by the growth and branching of chains. As the reaction proceeds, the increase in molecular weight accelerates, and eventually several chains become linked together into a network of infinite molecular weight. The abrupt and irreversible transformation from a viscous liquid to an elastic gel or rubber is called the gel point. The gel point (qv) of a chemically cross-linking system can be defined as the instant at which the weight-average molecular weight diverges to infinity (5). Cure is illustrated schematically in Figure 1 for a material with co-reactive monomers such as an epoxy–diamine system. Reaction in the early stages of cure (a to b in Fig. 1) produces larger and branched molecules and reduces the total number of molecules. Macroscopically, the thermoset can be characterized by an increase in its viscosity η (see Fig. 2 below). As the reaction proceeds (1b to 1c in Fig. 1), the increase in molecular weight accelerates and all the chains become linked together at the gel point into a network of infinite molecular weight. The gel point coincides with the first appearance of an equilibrium (or time-independent) modulus as shown in Figure 2. Reaction continues beyond the gel point (1c to 1d in Fig. 1) to complete the network formation. Macroscopically, physical properties such as modulus build to levels characteristic of a fully developed network.

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Fig. 1. Schematic, two-dimensional representation of thermoset cure. For simplicity difunctional and trifunctional co-reactants are depicted. Cure starts with A-stage monomers (a); proceeds via simultaneous linear growth and branching to a B-stage material below the gel point (b); continues with formation of a gelled but incompletely cross-linked network (c); and ends with the fully cured, C-stage thermoset (d). From Ref. 2.

Gelation is the incipient formation of a cross-linked network, and it is the most distinguishing characteristic of a thermoset. A thermoset loses its ability to flow and is no longer able to be processed above the gel point, and therefore gelation defines the upper limit of the work life. As an example, for a Five Minute Epoxy, which can be found in any hardware store, the “five minutes” refers to the gel point. After the two parts are mixed the user must form an adhesive joint within 5 min before the material becomes rubbery, and then keep the repaired part fixtured until cure is sufficiently complete, typically for several hours. A distinction may be drawn between the phenomenon of molecular gelation and its consequence, macroscopic gelation. Molecular gelation occurs at a well-defined stage of the chemical reaction, provided the reaction mechanism is independent of temperature and free of noncross-linking side reactions (7–9). It is dependent on the functionality, reactivity, and stoichiometry of the reactants. Macroscopic consequences of gelation include a rapid approach toward infinite viscosity and development of elastic properties not present in the pregel resin. The gel point

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Fig. 2. Macroscopic development of rheological properties (eg, η0 = zero-shear viscosity) and mechanical properties (eg, Ge = equilibrium shear modulus) during network formation, illustrating the approach to infinite viscosity and the first appearance of an equilibrium modulus at the gel point. From Ref. 6.

may be calculated if the chemistry is known (7,8). Molecular gelation may be detected at the point at which the reacting resin just becomes insoluble, or as the point where the mechanical loss tangent becomes frequency-independent (5–10). Macroscopic means to approximate gelation include the time to reach a specific viscosity, the G = G crossover in a dynamic rheology measurement, and a series of frequency-independent damping peaks accompanied by a small increase in storage modulus in a dynamic mechanical measurement. Gelation in condensation or step-growth systems typically occurs between 50 and 80% conversion (degree of cure α − 0.5–0.8). For high functionality and free-radical initiated or chain-growth systems, gelation may occur at much lower conversions. The degree of conversion at the gel point α gel is constant for a given thermoset, independent of cure temperature, ie gelation is isoconversional. For this reason the time to gel versus temperature can be used to measure the activation energy for cure. Gelation does not usually inhibit cure (eg, the reaction rate remains unchanged on passing through gelation), and it cannot be detected by techniques sensitive only to the chemical reaction, such as differential scanning calorimetry (DSC). Beyond the gel point, the reaction proceeds toward the formation of one infinite network with substantial increase in cross-link density, glass-transition temperature, and ultimate physical properties. Vitrification, a completely distinct phenomenon from gelation, may or may not occur during cure depending on the cure temperature relative to the T g for full cure. Vitrification is glass formation due to T g increasing from below T cure to above T cure as a result of the cure reaction, and is defined as the point where T g = T cure (11). Vitrification can occur anywhere during the reaction to form either an ungelled glass or a gelled glass. It can be avoided by curing at or above T g∞ , the glass-transition temperature for the fully cured network. In the glassy state, the rate of reaction will usually undergo a significant decrease and fall below the chemical reaction rate as the reaction becomes controlled by the diffusion

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of reactants. It is common for complete vitrification to result in a decrease in the rate of reaction by 2–3 orders of magnitude. Unlike gelation, vitrification is reversible by heating, and chemical control of cure may be reestablished by heating to devitrify the partially cured thermoset. Vitrification may be detected by a step increase in heat capacity by modulated-temperature DSC (MTDSC) and by dynamic mechanical analysis (DMA) as a frequency-dependent transition resulting in a glassy modulus, typically >1 GPa (∼150,000 psi). Generally, the shift from chemical control to diffusion control of the reaction may be observed by a decay of the reaction rate, which is often observed when T g reaches 10–15◦ C above T cure (see Figs. 7 and 8). Though not common, it is possible for diffusion to control the cure kinetics prior to vitrification; it is also possible for sluggish reactions to remain under chemical control well into the glassy state (see Refs. 2–4). Isothermal time–temperature-transformation (TTT) cure diagrams, as illustrated in Figure 3, are a useful tool for illustrating the phenomenological changes that take place during cure, such as gelation, vitrification, complete cure, and degradation (11,12). Three critical temperatures are marked on the temperature axis of the TTT cure diagram: T g0 , the glass transition temperature of the completely unreacted thermoset; gel T g , the temperature at which gelation and vitrification coincide; and T g∞ , the glass-transition temperature of the fully cured network (see Table 1). In this generalized diagram, the times of gelation and the times of vitrification are plotted as functions of the cure temperature. At temperatures below T g0 , reaction takes place in the glassy state and is therefore slow to occur. To minimize reaction during storage, unreacted systems should be stored well below T g0 ; 20–50◦ C below T g0 , ie deep into the glassy state, is recommended.

Fig. 3. Generalized time–temperature-transformation (TTT) cure diagram. A plot of the times to gelation and vitrification during isothermal cure versus temperature delineates the regions of four distinct states of matter: liquid, gelled rubber, gelled glass, and ungelled glass. From Ref. 12.

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Table 1. Glossary of Characteristic Cure Parameters α α gel tgel tvit T cure Tg T g0 gel T g T g∞

Chemical conversion (eg, of epoxide or isocyanate groups), degree of cure α at the gel point Time to gelation, gel time Time to vitrification Cure temperature, a process parameter Glass-transition temperature, a material property T g for uncured thermoset with degree of conversion α = 0 T g for thermoset with degree of conversion α gel T g for fully cured thermoset with degree of conversion α = 1

See the later section in this article on Adhesive Processes for comments about storing premixed and frozen adhesives. Between T g0 and gel T g , the liquid resin will react without gelation until its continuously rising glass-transition temperature becomes coincidental with the cure temperature, at which stage vitrification begins and the reaction may become diffusion controlled. Note that gel T g is the temperature at which gelation and vitrification occur simultaneously. At temperatures between gel T g and T g∞ , the viscous liquid changes to a viscoelastic fluid, then to a rubber, and finally to a glass. Gelation precedes vitrification, and a cross-linked rubbery network forms and grows until its glasstransition temperature coincides with the cure temperature, where the reaction may become diffusion controlled. At temperatures above T g∞ , the network remains in the rubbery state after gelation unless other reactions occur, such as thermal degradation or oxidative cross-linking. Note that in the manufacture of carbon–carbon composites, network degradation is part of the process (see PHENOLIC RESINS). The handling and processing of thermosets are very much dependent on gelation and vitrification. For example, thermosets are often identified at three stages of cure: A, B, and C. A-stage refers to an unreacted resin; B-stage to a partially reacted and usually vitrified system, below the gel point, which, upon heating to devitrify, can flow and be processed and cured; and C-stage to the completely cured network. Thus, to B-stage a thermoset requires vitrification prior to gelation, and this can be accomplished by maintaining the reaction temperature below gel T g . B-staging often provides systems that are optimized for processing. In general, thermosets that need to be solid during the precure stage of processing, eg powder coatings, will be B-staged to T g > T process , while thermosets that need to have some flow or tack during precure processing, eg prepregs, will be B-staged to T g ≤ T process . Prepregs are sheets of oriented fibers or fabric that are impregnated with resin and B-staged; typically several layers are laminated together under heat and pressure to fabricate a part. To avoid vitrification and achieve complete cure in a reasonable time generally necessitates cure at temperatures close to or greater than T g∞ .

Thermosetting Resin Systems As previously mentioned, in the uncured state, thermosetting materials are generally mixtures of small reactive molecules that form networks; catalysts, initiators and/or accelerators; and particulate, fiber-based, or nanosize fillers. There

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are several families of thermosetting materials. Here we briefly review some of the more important classes both in terms of volume as well as key engineering applications. Epoxies. Probably the best-known thermoset is epoxy (13) (see EPOXY RESINS). The largest use of epoxies is in protective coatings, with other applications including printed circuit board (PCB) laminates, electronic materials, structural ` composites, flooring and adhesives. The higher cost of epoxy resins vis-a-vis commodity thermosets and thermoplastics is justified by their superior properties and longer service life. Cured epoxies provide excellent mechanical strength and toughness; outstanding chemical, moisture, and corrosion resistance; good thermal, adhesive, and electrical properties; absence of volatiles and low shrinkage on cure; and good dimensional stability. Epoxy resins are characterized by a structure containing the epoxide or oxirane group (see structure 1 below). The most widely used epoxy resins are the diglycidyl ethers of bisphenol A:

Others include brominated bisphenol A resins which impart fire resistance, epoxy phenol novolac (EPN) resins, bisphenol F epoxy (DGEBF) resins, epoxy cresol novolac (ECN) resins, cycloaliphatic epoxy resins, and tetraglycidyl-4,4 diaminodiphenylmethane (TGDDM). The number of repeat units, n, in structure 1 can be as high as 35 but is usually 0–6, 0 being the monomer. Epoxy resins can be cured by a variety of crosslinking agents known as hardeners (reactive comonomers) or by catalysts that promote homopolymerization. Common examples of homopolymerization include the anionic polymerization promoted by Lewis bases such as tertiary amines and imidizoles, and cationic polymerization promoted by Lewis acid BF3 complexes such as boron trifluoride monoethylamine (BF3 ·NH2 C2 H5 ). Photoinitiated cationic cure of epoxies, especially cycloaliphatic epoxies, with photoinitators such as aryldiazonium and diaryliodonium salts provide coatings and adhesives with long shelf life and near-instantaneous cure. But it is more common to cross-link epoxies with a co-reactant such as a diamine, polyamide, or acid anhydride. The low exotherm of epoxy–anhydride systems make them suitable for uses in large mass epoxy cures. Amines account for close to 50% of all epoxy curing agents. For crosslinking to occur, at least one of the reactants must be trifunctional or higher. Epoxy resins are typically difunctional, reacting through the oxirane group, although in some cases, such as homopolymerization or when epoxy is in stoichiometric excess, reaction can occur through the OH groups. Diamines, a well-studied co-reactant with epoxy, are tetrafunctional where each amine hydrogen can react. The reactions of primary amine and secondary amine with epoxide are illustrated in equations 1 and 2, respectively. The heat of reaction H rxn for epoxy–amine is ∼106 kJ/mol (∼25.5 kcal/mol) for reaction with

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both primary amine and secondary amine (2–4). The activation energy for cure can vary from 40–125 kJ/mol (10 to 30 kcal/mol) (2–4).

(1)

(2) For equal reactivity of primary amine and secondary amine with epoxy k2 /k1 = 0.5. But there is usually a substitution effect where the secondary amine is less reactive and k2 /k1 < 0.5, often in the range of 0.40–0.45. When steric hindrance is significant, k2 /k1 can be < 0.1. In general, aliphatic amines are used with bisphenol A resins where roomtemperature cures are desired and heat-deflection temperatures (HDT) or glasstransition temperatures (T g ) below 100◦ C can be tolerated. Aromatic amines can provide HDTs and T g ’s up to 170◦ C but require curing at elevated temperatures. Anhydride cured bisphenol A systems offer long pot life, low exotherm, excellent adhesion and electrical properties, and HDT/T g values between 125 and 170◦ C. A small amount of tertiary amine is frequently used to accelerate the cure reaction, which proceeds by addition esterification and addition etherification. Cycloaliphatic epoxides respond well to acidic hardeners and can have HDT/T g values approaching 200◦ C. Tetraglycidyl-4,4 -diaminodiphenylmethane (TGDDM) is a tetrafunctional epoxide which is often cured with diaminodiphenylsulfone (DDS) to form high temperature, high performance epoxy systems for aerospace applications. Commercial TGDDM/DDS systems, which are typically epoxy-rich, represent materials in which secondary amine hydrogens are significantly less reactive than primary amine hydrogens and the etherification reaction of epoxide with hydroxyl is significant. Phenolic Resins. Phenol–formaldehyde resins are employed in a wide range of applications, including commodity construction materials such as plywood to high technology applications such as honeycomb and carbon–carbon composites (see PHENOLIC RESINS). They are composed of a wide variety of structures based on the reaction products of phenols with formaldehyde. Either resole or novolac resins are formed, depending on the mole ratio of formaldehyde to phenol (F/P) and catalyst. Resole resins are produced from an excess of formaldehyde (F/P > 1) and contain reactive methylol groups that can condense to a network structure. The cross-linking reactions in these one-part systems are exothermic and produce water and formaldehyde as volatile products, as illustrated in equation 3.

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(3) Novolac resins are produced from an excess of phenol (F/P