Industrial thermosetting laminates

So-called industrial thermosetting laminates are those in which a reinforcing material has been impregnated with thermosetting resins and the laminates have usually been formed at relatively high pressures.

Resins most commonly used are phenolic, polyester, melamine, epoxy, and silicone; reinforcements are usually paper, woven cotton, asbestos, glass cloth, or glass mat.

NEMA (National Electrical Manufacturers’ Association) has published standards covering many standard grades of laminates. Each manufacturer, in addition to these, normally provides a range of special grades with altered or modified properties or fabricating characteristics. Emphasis here is on the standard grades available.

Laminates are available in the form of sheet, rod, and rolled or molded tubing. Laminated shapes can also be specified; these are generally custom-molded by the laminate producer.

General Properties

Resin binders generally provide the following characteristics: (1) phenolics are low in cost, have good mechanical and electrical properties, and are somewhat resistant to flame; (2) polyesters used with glass mat provide a low-cost laminate for general-purpose uses; (3) epoxy resins are more expensive, but provide a high degree of resistance to acids, alkalies, and solvents, as well as extremely low moisture absorption, resulting in excellent mechanical and electrical property retention under humid conditions; (4) silicones, high in cost, provide optimum heat resistance; and (5) melamines provide resistance to flame, alkalies, arcing, and tracking, as well as good colorability.


Mechanical properties

Tensile strengths of paper, asbestos, and cotton fabric-base laminates vary from about 136 to over 552 MPa; flexural strengths are somewhat higher and the moduli of elasticity are about 7000 to 14,000 MPa.

Other properties

In general, moisture absorption is about 0.3 to 1.3% (24 h, 1.5 mm thickness), depending on resin and reinforcement.

Heat resistance depends on both resin and reinforcements. Most standard laminates are designed for a variety of insulation requirements. General maximum continuous service temperatures are about 121 °C for phenolics reinforced with organic reinforcements, 149°C for phenolics, melamines, and epoxies reinforced with inorganic materials, and 260°C for silicones reinforced with inorganic materials.

Flame retardance of most standard grades other than melamines is relatively poor, but special flame-resistant or self-extinguishing grades are available.

Glass cloth reinforcement provides tensile strength values in the 92 to 255 MPa range; modulus of elasticity can approach 20,600 MPa. Typical flatwise compressive strength values fall in the 160 to 340 MPa range.

Electrical Properties

Dielectric strengths (perpendicular to laminates) generally range between 400 and 700 v/mil, and up to 1000 v/mil for paper-based phenolics. Dissipation factor, as-received and at 106 cps, ranges from a high of about 0.055 for cotton-based phenolics to a low of about 0.0015 for a glass-reinforced silicone laminate.

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