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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