Thermosets: How to Avoid Incomplete Curing

Oligomers and monomers that react and polymerize to form a rigid network with elevated temperatures are called thermosets. Thermosetting oligomers have been used for several decades to produce composite materials that have superior strength and reduced weight in comparison to metals. A question often heard is: "What is the fastest curing profile that I can use and still get superior polymer properties, i.e., complete cure?"

The curing reaction

Cross-linking reactions are exothermic. In general, the temperature of the reaction, the reaction rate, and the reaction enthalpy (heat of reaction) are the main points of interest. During the reaction, the viscosity, density, and the modulus of elasticity increase due to an increase in the molar mass and cross-linking.

The cross-linking process is complex because different reaction steps are involved. Basically, the growth of the polymer structure can be divided into two mechanisms of chain formation: 1) stepwise growth through elementary reactions of two functional groups, and 2) chain growth through the attachment of monomers to the cross-linking polymer.

In the first process, a covalent bond is created through the reaction of two coreactive centers and thus joins monomers, oligomers, and macromolecules together. The structure of the polymer is mainly determined by the functionality of the monomers and the molar ratios of the coreactive centers. In the second process, only monomers are attached to the chain, and the structure of the polymer is determined by the functionality of the monomer, by the ratio of the reaction rates between initiation and growth, and by the concentrations of monomer and initiator.

The formation of a thermoset can take place between monomers with a functionality of three or more. In the fully reacted polymer network, practically all the constituent units are incorporated covalently in the three-dimensional structure. At the gel point, the infinite network becomes apparent for the first time.

Figure 1 - Schematic diagram illustrating the steps involved in cross-linking: a) starting resin material with low-molecular-mass monomers (A-stage monomers); b) linear growth and branching (B-stage material or prepolymer); c) gelled, but still incomplete network; d) fully cured polymer (C-stage thermoset).

As shown in Figure 1, the reaction begins (region a) with the formation of larger molecules that may also already be branched. This so-called prepolymerization is a way to increase the molecular weight of a polymer to some intermediate value so that there are still enough residual reacting sites to further react in a curing environment. Some reasons for prepolymerization are to increase the viscosity, decrease the toxicity, and reduce reactivity for control of gelation time.

Stoichiometric considerations are important for optimum curing conditions. However, in reality, the mixing ratio may be adapted due to steric hindrance of reacting species to come together and react. Side reactions and chain-stopping contaminants may reduce the calculated number of reactive sites. Cross-links are formed between the chains via trifunctional and multifunctional groups. An elastic gel is then formed from the viscous liquid through cross-linking. In the gel, the cross-linking continues until the network density is practically complete or the reaction comes to a stop due to vitrification.

When two or more polymer networks are formed in a cross-linking reaction, the term "interpenetrating polymer network" (IPN) is used. IPNs are combinations of two or more entangled cross-linked polymers. Ideally, only one glass transition is observed.

Time-temperature transformation

Figure 2 - The curing process illustrated by the TTT diagram. Shown are the three states of the material on curing: liquid, rubbery elastic material, and glass. The continuous S-shaped curve shows the time it takes for the resin to vitrify when the reaction is performed at an isothermal temperature, TR.

The three different physical states that can occur in curing are usually displayed in a time-temperature-transformation (TTT) cure diagram (Figure 2). This is a plot of temperature versus the logarithm of reaction time. It shows the state of the resin after a certain reaction time at a reaction temperature TR. Below Tg0, the resin is in the glassy state and the reaction is practically blocked.

At a curing temperature TR2, the gelation line (dashed line) is reached after a relatively short time, the material gels (gel point G2) and is transformed to the rubbery state, and cross-linking continues until curing is complete. The curing temperature is thus always higher than the maximum possible glass transition temperature Tg∞. With longer reaction times, decomposition can begin.

If a reaction temperature of TR1 is chosen, it takes somewhat longer for the gel point at G1 to be reached. Cross-linking continues in the rubbery elastic material until vitrification occurs at V1. The glass transition temperature increases due to continued cross-linking until it reaches the reaction temperature. At the transition to the glassy state, the reaction stops. The material is hard, and it seems as if curing is complete.

If the temperature TR1 of a thermoset cured in this way is exceeded during use, softening occurs, which can lead to a material defect. In the glassy state, the reaction is not completely hindered but can still proceed at a very low rate. The properties of the material are clearly not stable.

The continuous S-shaped line therefore shows when the resin vitrifies at an isothermal curing temperature. Figure 2 thus characterizes a reactive resin system with regard to reaction temperature and reaction time.

From a technical viewpoint, the gelation time, pot life (processing time), and storage time at a particular temperature are of great importance, irrespective of whether cross-linking is started catalytically, through exposure to light or temperature increase.

At the gel point, the viscosity increases markedly (theoretically infinitely) and thereby ends the possibility of using the resin (casting, coating, pumping) (the B-Time according to DIN 16916).

The pot life is also related to the gel point. It characterizes the time available to process a reacting thermosetting resin formulation under normal conditions after the start of the reaction before the mixture becomes intractable or otherwise difficult to process (e.g., residence time in a molding machine for trouble-free molding and defect-free parts).

Shelf-life can mean an arbitrary time for practical storage of a thermoset system, either a one-component system or a system after mixing the components. Shelf-life is also used to describe the storage stability of unmixed components. For example, some curing agents will lose reactivity due to the uptake of atmospheric moisture.

As already indicated, the handling, processing, and properties of a thermoset depend strongly on gelation and vitrification. A resin in the previously reacted, vitrified state can be easily stored and simply heated to start curing. This simplifies the handling and processing of resins because resin and hardener do not first have to be mixed in the right proportions. The influence of temperature on curing provides another practical production control mechanism, the concept of staging: In the A-stage, the thermoset is ready to react (i.e., after mixing, but cross-linking has not begun); then it goes through the B-stage as time and cross-linking progress. Often, the process is stopped by lowering the temperature sufficiently to achieve an inactive period (up to one year), for example, for reactive coating powders or tacky adhesive tapes for easy application. The C-stage represents the fully cross-linked part in its final configuration.