Esters in the allyl family consist principally of diallyl phthalate (DAP) and diallyl isophthalate (DAIP). Both are used as monomers and as prepolymers, which are readily converted to thermoset molding compounds and resins for preimpregnated glass cloth and paper. Allyls are also used as crosslinking agents for unsaturated polyesters.
Compounds based on allyl prepolymers are reinforced with fibers (glass, polyester, or acrylic) and filled with particulate materials to improve properties. Glass fiber imparts maximum mechanical properties, acrylic fiber provides the best electrical properties, and polyester fiber improves impact resistance and strength in thin sections. Compounds can be made in a wide range of colors because the resin is essentially colorless.
Prepregs (preimpregnated glass cloth) based on allyl prepolymers can be formulated for short cure cycles. They contain no toxic additives, and they offer long storage stability and ease of handling and fabrication. Properties such as flame resistance can be incorporated. The allyl prepolymers contribute excellent chemical resistance and good electrical properties.
Properties: Allyl molding compounds do not corrode copper or plated inserts or contacts, even in hot, humid environments. Molded parts maintain their high electrical properties at high temperature (to 370 °F for DAP and 400 °F for DAIP) and humidity levels. Allyl materials are also characterized by excellent dimensional stability and resistance to moisture, chemicals, and liquid oxygen. They withstand strong and weak acids, alkalies, and organic solvents, even at elevated temperatures.
Allyl moldings have low mold shrinkage and postmold shrinkage -- attributed to their nearly complete addition reaction in the mold -- and have excellent stability under prolonged or cyclic heat exposure. Advantages of allyl systems over polyesters are freedom from styrene odor, low toxicity, low-evaporation losses during evacuation cycles, no subsequent oozing or bleed-out, and long-term retention of electrical-insulation characteristics.
Applications: Diallyl phthalate monomer is used as a nonvolatile crosslinker in polyester compounds to improve properties and handling characteristics. Deflection temperature is raised to 400 °F or higher, dimensional stability and electrical properties are upgraded, and flexural properties are retained for long periods at elevated temperatures. DAP is also used in combination with polyester resin systems for low-pressure decorative laminates. Allylic resins in powder and liquid form are used for coatings and for impregnating materials.
Allyl prepolymers are particularly suited for critical electronic components that serve in severe environmental conditions. Chemical inertness qualifies the resins for molded pump impellers and other chemical-processing equipment. Their ability to withstand steam environments permits uses in sterilizing and hot-water equipment. Because of their excellent flow characteristics, diallyl-phthalate compounds are used for parts requiring extreme dimensional accuracy. Modified resin systems are used for encapsulation of electronic devices such as semiconductors and as sealants for metal castings.
A major application area for allyl compounds is electrical connectors, used in communications, computer, and aerospace systems. The high thermal resistance of these materials permits their use in vapor-phase soldering operations. Uses for prepolymers include arc-track-resistant compounds for switchgear and TV components. Other representative uses are insulators, encapsulating shells, potentiometer components, circuit boards, and housings.
Allyl-based prepregs are used to make lightweight, intricate parts such as radomes, printed-circuit boards, tubing, ducting, and aircraft parts. Another use is in copper-clad laminates for high-performance printed-circuit boards.