Allylics are thermosetting materials developed since World War II. The most important of these are diallyl phthalate (DAP) and diallyl iso-phthalate (DAIP), which are currently available in the form of monomers and prepolymers (resins). Both DAP and DAIP are readily converted to thermoset molding compounds and resins for preimpregnated glass cloth and paper. Allyls are also used as cross-linking agents for unsatur-ated polyesters.
DAP resin is the first all-allylic polymer commercially available as a dry, free-flowing white powder. Chemically, DAP is a relatively linear partially polymerized resin that softens and flows under heat and pressure (as in molding and laminating), and cross-links to a three-dimensional insoluble thermoset resin during curing.
Properties
In preparing the resin, DAP is polymerized to a point where almost all the change in specific gravity has taken place. Final cure, therefore, produces very little additional shrinkage. In fact, DAP is cured by polymerization without water formation. The molded material, depending on the filler, has a tensile strength from 30 to 48 MPa, a compressive strength up to 210 MPa, a Rockwell hardness to M108, dielectric strength to 16.9 x 106 V/m, and heat resistance to 232°C.
Allylic resins enjoy certain specific advantages over other plastics, which make them of interest in various special applications. Allylics exhibit superior electrical properties under severe temperature and humidity conditions. These good electrical properties (insulation resistance, low loss factor, arc resistance, etc.) are retained despite repeated exposure to high heat and humidity. DAP resin is resistant to 155 to 180°C temperatures, and the DAIP resin is good for continuous exposures up to 206 to 232°C temperatures. Allylic resins exhibit excellent post-mold dimensional stability, low moisture absorption, good resistance to solvents, acids, alkalis, weathering, and wet and dry abrasion. They are chemically stable, have good surface finish, mold well around metal inserts and can be formulated in pastel colors with excellent color retention at high temperatures.
DAP resin currently finds major use in (1) molding and (2) industrial and decorative laminates. Both applications utilize the desirable combination of low shrinkage, absence of volatiles, and superior electrical and physical properties common to DAP.
Molding Compounds
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 pre-polymers contribute excellent chemical resistance and good electrical properties.
Other molding powders are compounded of DAP resin, DAP monomer, and various fillers like asbestos, Orlon, Dacron, cellulose, glass, and other fibers. Inert fillers used include ground quartz and clays, calcium carbonate, and talc.
Allyl moldings have low mold shrinkage and post-mold 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 sty-rene odor low toxicity, low evaporation losses during evacuation cycles, no subsequent oozing or bleed-out, and long-term retention of electrical-insulation characteristics.
Applications
Uses of such DAP molding compounds are largely for electrical and electronic parts, connectors, resistors, panels, switches, and insulators. Other applications for molding compounds include appliance handles, control knobs, dinnerware, and cooking equipment.
Properties of DAP Molding Compounds
Decorative laminates containing DAP resin can be made from glass cloth (or other woven and nonwoven materials), glass mat, or paper. Such laminates may be bonded directly to a variety of rigid surfaces at lower pressures (50 to 300 psi) than generally required for other plastic laminates. A short hot-to-hot cycle is employed, and press platens are always held at curing temperatures. DAP laminates can, therefore, be used to give a permanent finish to high-grade wood veneers (with a clear overlay sheet) or to upgrade low-cost core materials (by means of a patterned sheet).
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, DAP 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 television components. Other representative uses are for insulators, encapsulating shells, potentiometer components, circuit boards, junction boxes, and housings.
DAP and DAIP 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.