Heat-resistant thermoset resins are critical components of high-performance adhesives, coatings, prepregs, and composites. They are frequently used in the aerospace and defense industry where previously metal and reinforced ceramic matrix composites were the only option. This includes wing and fuselage skins, engine parts, landing gears, and satellite components. Electrical and mechanical aerospace devices often have to withstand continuous service temperatures in excess of 300 - 400°C and temperature spikes in excess of 500 -1000°C. Although high performance polymers have a lower service temperature than metals and ceramics, they have better fatigue resistance than ceramics and are (much) lighter than most metals.
Phenolic resins are the oldest and cheapest high-volume
thermosetting resins. They can be divided into novolacs and resoles.
Novolacs need a cross-linking agent to fully polymerize whereas
resoles are self-curing due to the presence of reactive side groups.
Phenolics are known for their outstanding dimensional stability,
excellent corrosion and temperature resistance, low creep over a wide temperature range, and
excellent price-to-performance characteristic.
Novolacs can be converted to epoxy phenol novolacs by reacting them with epichlorohydrin.
These highly functional resins have better thermal properties and chemical resistance and cure more rapidly than
standard epoxy resins.
Epoxies are the most popular class of high-performance thermosetting resins. They are used across almost every industry due to their reasonable price, good heat and corrosion resistance, and outstanding bond performance. They can be cross-linked either with themselves through catalytic homopolymerization or with a broad range of co-reactants including polyfunctional amines, acid anhydrides, phenols, alcohols and thiols. When cured with a curative, the mechanical and thermophysical properties depend on the type of curative and on the epoxy to curative mix ratio.
Polybismaleimides (BMI) cure by addition rather than by condensation reaction. Their characteristics are similar to those of polyimides. Their heat resistance is usually better than high-performance epoxies but not as good as condensation-cured polyimides. BMIs are often sold as low molecular weight dry powder resins containing imide structures already in the monomer form. They can be polymerized with themselves as well as with other co-monomers through reaction with the double bonds of the maleimide end groups.
Polyimides (PI) are high-performance engineering resins synthesized by condensation of aromatic dianhydrides with diamines. They have some of the best high temperature properties, and thus, are often an excellent choice for very demanding applications where very high mechanical strength in combination with high temperature, corrosion, and wear resistance is required. However, polyimides are rather expensive and require high processing temperatures and, therefore, are only used if cheaper thermoset resins are not an option.
Cyanate esters are monomers/oligomers with reactive cyanate end groups. When heated, they form highly crosslinked polymers that exhibit very high thermal, oxidative, and hydrolytic stability. They are often preferred over epoxy and BMI resins in demanding applications because they have superior high-temperature properties but are less frequently used due to their much higher price. To lower cost and to increase their toughness, they are often blended with elastomers and other resins such as epoxies and novolacs.
Polybenzoxazines are a new class of high-performance thermoset resins that resemble conventional phenolic resins but do not generate volatiles during cure. They are typically stiffer, have better chemical resistance, and less cure shrinkage than bismaleimides, epoxies and phenolics. However, unmodified polybenzoxazines exhibit some deficiencies including high cure temperature, low fracture toughness and higher price. These weaknesses can be overcome by blending benzoxazines with toughening agents and cheaper resins.