Thermosets and thermoset resins are raw, unshaped polymer materials that have been irreversibly cured. They are generally stronger than thermoplastics due to polymer cross-linking and have a higher resistance to heat. Cured thermoset resins may soften when heated, but do not melt or flow. They tend to be more brittle than thermoplastics and many cannot be recycled due to irreversibility.

Thermoset molding compounds are designed for processes such as reaction injection molding (RIM) and resin transfer molding (RTM). Composite thermoset materials consist of a matrix and a dispersed, fibrous, or continuous second phase. Casting resins include a catalyst or hardener. Thermoset electrical resins and electronic-grade products are used in potting or encapsulating compounds, conductive adhesives, and dielectric sealants.

Thermal compounds are designed to form a thermally conductive layer on a substrate, either between components or within a finished electronic product. Thermoset purging compounds are used to clean molding machines between runs of different colors or compositions. Gap filling products are used to fill in gaps or spaces between two surfaces to be bonded or sealed.

Types of Thermosets and Thermoset Resins

Information and listings on the types of thermoset chemical systems can be found on the Engineering360 SpecSearch Database. The most common types are epoxy, phenolics, bismaleimide, and fluoropolymers.

Epoxy resins exhibit high strength and low shrinkage during curing. They are known for their toughness and resistance to chemical and environmental damage. Most are two-part resins cured at room temperature. Depending on the formulation, epoxy resins are used as casting resins, potting agents, resin binders, or laminating resins in fiberglass or composite construction. They are also used as encapsulates, electrical conductors in microelectronic packaging, and adhesives in structural bonding applications. 

Phenolics are thermosetting molding compounds and adhesives that offer strong bonds and provide good resistance to high temperatures. Phenolic resin adhesives made from chemicals of the phenol group and formaldehyde are generally the most durable. Phenolic resins are available in liquid, powder, and film form. Special phenolic resins are available that harden at moderate temperatures when mixed with suitable accelerators. Urea formaldehyde resins can harden rapidly at moderate temperatures, but generally do not have the properties of phenolic resins. Melamine resins have excellent dielectric properties. 

Bismaleimide (BMI) resins are aromatic polymides used in high performance structural composites requiring higher temperature use and increased toughness. Epoxy blends of BMI have exhibited use temperatures of 205° to 245 °C and increased toughness. Bismaleimide (BMI) resins have processing characteristics similar to epoxy resins and are used as laminating resins, prepregs, adhesives, and other composite applications. 

Fluoropolymers including polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) are used in applications requiring superior chemical resistance. PTFE is used in applications requiring superior chemical resistance or low friction.

Filled or reinforced products consist of resins and modifiers such as pigments, plasticizers, or chopped fibers. By contrast, unfilled resins, base polymers, and raw materials do not contain additives. Typically, raw materials are available as pellets, powders, granules, or liquids.

Fibers can be either chopped or wound, and commonly include glass, fiberglass, or cloth. Some products contain solid lubricant fillers such as graphite or molybdenum disulfide. Others contain aramid fibers, metal powders, or inorganic fillers with ceramics and silicates. 

Vulcanized Thermosets

Vulcanization is a thermosetting reaction that uses a crosslinked compound or catalyst. In rubber-like materials, vulcanization results in greatly increased strength, stability, and elasticity. Traditionally, sulfur is used as the vulcanizing agent for natural rubber. Thermosets and thermoset materials may contain filler materials such as powders or fibers to provide improved strength and/or stiffness.  

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Buehler, an ITW Company