Understanding Thermoset Composites

Thermoset plastics provide a useful material for a variety of applications thanks to their

resistance to heat, ability to insulate and their durability and light weight. Understanding how

these plastics are made and the forms they’re available in will help engineers and

manufacturers make an informed decision whether it is the right material for their project.

Thermoset plastics are polymer materials that permanently cure, in a way making them like a

plastic version of cement. These plastics are typically cured by means of heat, pressure,

chemical reaction or irradiation. Once the plastic cures, it becomes very durable and much more

resistant to heat than other plastics. This form of plastic was pioneered in the early 20th century

by Dr. Leo Baekeland, a Belgian native who later immigrated to America. Thus, the original

thermosetting plastic was known as Bakelite. It found many uses in the nascent electronics

industry of the mid-20th century and spawned development of many more thermoset laminate

grades used in the industrial revolution.

In general, these thermoset laminates have been generically recognized by combined names

characterized by the resin used and the reinforcing material. i.e. Canvas/Phenolic! Over the

years, these thermoset laminates have been defined by standards developed by NEMA

(National Electrical Manufacturers Association) and by Military Standard MIL-I-24768 to assure

quality.

Glass/Epoxy – They are resistant to heat and corrosion and also provide excellent insulation

from electricity. Glass/Epoxy laminates are also known for having a long shelf life, with

anecdotal stories of them being stored for years working well when used. These thermoset

laminates are identified by the aforementioned standards as FR4, G10, G11, and FR5.

Glass/Melamine – A hard form of thermoset plastic, Glass/Melamine laminates are used in

electrical apparatus where resistance to arc tracking and resistance to corrosion are needed-

refrigeration and/or switch assemblies. These grades are identified by G5 and G9.

Glass/Silicone- The most temperature resistant grade of thermoset laminates is used in many

applications in welding equipment and arc furnaces where both heat resistance and electrical

insulation is required. It is identified by G7.

Canvas/Phenolic- These grades combine various weaves of cotton fabric with phenolic resin (

also Linen/phenolic). They are characterized by toughness, impact and wear properties and

find use in many industrial applications including wear bearings, shims, and gaskets along with

some electrical applications as well. These grades are identified by C, CE, L, and LE.

Paper/Phenolics- These grades combine papers (kraft or high alpha content) with phenolic

resin binders to form laminates with good structural integrity, machinability and cost advantages

over the glass fabric grades. They are referred to as XX and XXX (sometimes also as X ). The

higher number of X’s means better resistance to absorption of water.

Glass/Polyester- These grades are combinations of random mat fiberglass and polyester

resins. They offer good structural properties along with electrical insulation at cost advantages

in switchgear applications. They are generally referred to as GPO2 & GPO3.

Thermoset laminates, or composites, may also be combined with other components to develop

special properties that allow flexibility in design of items by engineers. For many devices where

strong resistance to heat or electricity are necessary, thermoset composites can be extremely

helpful, allowing designers a flexible, durable, lightweight option for replacing metals and other

high weight components. When working on project design, engineers and other designers

should inquire with thermoset manufacturers concerning whether this versatile plastic can help

meet their needs.

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