Resins
The primary functions of
the resin are to transfer stress between the reinforcing fibers,
act as a glue to hold the fibers together, and protect the fibers
from mechanical and environmental damage. Resins are divided into
two major groups known as thermoset and thermoplastic.
Thermoplastic resins become soft when heated, and may be shaped or
molded while in a heated semi-fluid state and become rigid when
cooled. Thermoset resins, on the other hand, are usually liquids
or low melting point solids in their initial form. When used to
produce finished goods, these thermosetting resins are “cured”
by the use of a
catalyst, heat or a combination of the two. Once
cured, solid thermoset resins cannot be converted back to their
original liquid form. Unlike thermoplastic resins, cured
thermosets will not melt and flow but will soften when heated (and
lose hardness) and once formed they cannot be reshaped.
Heat
Distortion Temperature (HDT) and the
Glass Transition Temperature
(Tg) is used to measure the softening of a cured resin. Both test
methods (HDT and Tg) measure the approximate temperature where the
cured resin will soften significantly to yield (bend or sag) under
load.
The most common
thermosetting resins used in the composites industry are
unsaturated polyesters, epoxies, vinyl esters and phenolics. There
are differences between these groups that must be understood to
choose the proper material for a specific application.
Polyester
Unsaturated polyester resins
(UPR) are the workhorse of the composites industry and represent approximately 75% of the total resins used. To avoid any
confusion in terms, readers should be aware that there is a family
of thermoplastic polyesters that are best known for their use as
fibers for textiles and clothing. Thermoset polyesters are produced by the
condensation polymerization of dicarboxylic acids and difunctional
alcohols (glycols). In addition, unsaturated polyesters contain an
unsaturated material, such as maleic anhydride or fumaric acid, as
part of the dicarboxylic acid component. The finished polymer is
dissolved in a reactive monomer such as styrene to give a low
viscosity liquid. When this resin is cured, the monomer reacts
with the unsaturated sites on the polymer converting it to a solid
thermoset structure.
A range of raw materials
and processing techniques are available to achieve the desired
properties in the formulated or processed polyester resin.
Polyesters are versatile because of their capacity to be modified
or tailored during the building of the polymer chains. They have
been found to have almost unlimited usefulness in all segments of
the composites industry. The principal advantage of these resins
is a balance of properties (including mechanical, chemical,
electrical)
dimensional
stability, cost and ease of handling or
processing.
Unsaturated
polyesters are divided into classes depending upon the structures
of their basic building blocks. Some common examples would be
orthophthalic (“ortho”),
isophthalic (“iso”),
dicyclopentadiene (“DCPD”) and bisphenol A fumarate resins. In
addition, polyester resins are classified according to end use
application as either general purpose (GP) or specialty
polyesters.
Polyester producers have
proved willing and capable of supplying resins with the necessary
properties to meet the requirements of specific end use
applications. These resins can be formulated and chemically
tailored to provide properties and process compatibility.
Epoxy
Epoxy
resins have a well-established record in a wide range of composite
parts, structures and concrete repair. The structure of the resin
can be engineered to yield a number of different products with
varying levels of performance. A major benefit of epoxy resins
over unsaturated polyester resins is their lower
shrinkage. Epoxy
resins can also be formulated with different materials or blended
with other epoxy resins to achieve specific performance features.
Cure rates can be controlled to match process requirements through
the proper selection of hardeners and/or catalyst systems.
Generally, epoxies are cured by addition of an anhydride or an
amine hardener as a 2-part system. Different
hardeners, as well as
quantity of a hardener produce a different cure profile and give
different properties to the finished composite.
Epoxies are used primarily
for fabricating high performance composites with superior
mechanical properties, resistance to corrosive liquids and
environments, superior electrical properties, good performance at
elevated temperatures, good adhesion to a substrate, or a
combination of these benefits. Epoxy resins do not however, have
particularly good UV resistance. Since the viscosity of epoxy is
much higher than most polyester resin, requires a post-cure
(elevated heat) to obtain ultimate mechanical properties making
epoxies more difficult to use. However, epoxies emit little odor
as compared to polyesters.
Epoxy resins are used with
a number of fibrous reinforcing materials, including glass, carbon
and aramid. This latter group is of small in volume, comparatively
high cost and is usually used to meet high strength and/or high
stiffness requirements. Epoxies are compatible with most composite
manufacturing processes, particularly vacuum-bag molding,
autoclave molding, pressure-bag molding, compression molding,
filament winding and hand lay-up.
Vinyl Ester
Vinyl esters were developed to combine the
advantages of epoxy resins with the better handling/faster cure,
which are typical for unsaturated polyester resins. These resins
are produced by reacting epoxy resin with acrylic or methacrylic
acid. This provides an unsaturated site, much like that produced
in polyester resins when maleic anhydride is used. The resulting
material is dissolved in styrene to yield a liquid that is similar
to polyester resin. Vinyl esters are also cured with the
conventional organic peroxides used with polyester resins. Vinyl
esters offer mechanical toughness and excellent corrosion
resistance. These enhanced properties are obtained without complex
processing, handling or special shop fabricating practices that
are typical with epoxy resins.
Phenolic
Phenolics are a class of
resins commonly based on phenol (carbolic acid) and formaldehyde.
Phenolics are a thermosetting resin that cure through a
condensation reaction producing water that should be removed
during processing. Pigmented applications are limited to red,
brown or black. Phenolic composites have many desirable
performance qualities including high temperature resistance,
creep
resistance, excellent thermal insulation and sound damping
properties, corrosion resistance and excellent fire/smoke/smoke
toxicity properties. Phenolics are applied as adhesives or matrix
binders in engineered woods (plywood), brake linings, clutch
plates, circuit boards, to name a few.
Polyurethane
Polyurethane is a family of polymers with widely
ranging properties and uses, all based on the
exothermic reaction
of an organic polyisocyanates with a polyols (an alcohol
containing more than one hydroxl group). A few basic constituents
of different molecular weights and functionalities are used to
produce the whole spectrum of polyurethane materials. The
versatility of polyurethane chemistry enables the polyurethane
chemist to engineer polyurethane resin to achieve the desired
properties.
Polyurethanes appear in an amazing variety of
forms. These materials are all around us, playing important roles
in more facets of our daily life than perhaps any other single
polymer. They are used as a coating, elastomer, foam, or
adhesive. When used as a coating in exterior or interior finishes,
polyurethane’s are tough, flexible, chemical resistant, and fast
curing. Polyurethanes as an elastomer have superior toughness
and
abrasion is such applications as solid tires, wheels, bumper
components or insulation. There are many formulations of
polyurethane foam to optimize the density for insulation,
structural sandwich panels, and architectural components.
Polyurethanes are often used to bond composite structures
together. Benefits of polyurethane adhesive bonds are that they
have good impact resistance, the resin cures rapidly and the resin
bonds well to a variety of different surfaces such as concrete.
Summary of Resins
The
resins in thermoset composites are an important source of
properties and process characteristics. One of the great design
strengths of composites is the multiple choice of resins. In order
to make effective use of these choices, designers and product
specifiers should be familiar with the properties, advantages and
limitations of each of the common composite resins. It is common
to use the resources of the resin manufacturers laboratories to
determine the best resin or an application.
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Reinforcements
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