RESIN:
Function:
The
resin in RP holds the fibers together and transmits the loads from fiber to
fiber.
Requirements
of an ideal resin in RP:
1. The
uncured resin must wet the reinforcement well and should be cured at room
temperature or by heat. Also, when cured, it should have good adherence to the
fibers, good heat resistance and good electrical and chemical properties.
2. The
resin should be available in a wide range of viscosities, should be compatible with
pigments, filler material and if possible, must be cured at temperature ranging
from ambient to 1500C.
Conventional
resins:
1. Polyester:
It is the 1st resin used in the low pressure process and is the most
widely used polymer because it possesses almost all the requirements of an
ideal resin.
a. Polyester
resins are available in liquid forms in many viscosities, so good fiber
impregnation is relatively easy.
b. Using
different catalysts the resin can be cured at room temperature or at an
elevated temperature or by ultra violet (uv) light. This allows a part
to be worked on all day without its setting up and then be cured overnight in
an oven, or by using uv radiation from a lamp (sunlight can also be
used)
c. Cured
materials have excellent electrical and chemical properties.
d. Cured
materials have good strength to weight ratios.
e. Costs
are relatively low.
f. Disadvantages
of polyester resin: (1) Relatively high shrinkage occurs when curing takes
place. (2) Under high humidity or moisture condition, less adherence (than
ideal) to the fibers occurs hence fibers are specially treated to upgrade the
performance of laminates. (3)For thin fiberglass reinforced parts, relatively
high brittleness occurs.
g. Special
formulations are produced which show increased toughness. Vinyl ester resins
are tougher and have higher temperature resistance than the conventional
polyester resins.
2. Epoxies:
These materials have all good properties of the polyester. The epoxy resins
also have some special properties of their own, which makes them superior to
the polyesters like: Low shrinkages and excellent adhesion to most material
which gives higher strength to weight ratio to the laminates.
3. Other resins: Some of the older resins are
modified so that they can be used with the new low pressure technique. Some new
resin types are also developed. Today there are a number of thermoset resin
each having unique characteristic e.g. Phenolics, Silicones, Melamines,
Polyimides, Diallyl phthalates.
CURING
TECHNIQUES:
The
resins were cured at pressures ranging from 0-15 psi but now pressures up to
50-100 psi are used. The use of high pressures will result in more highly
densified part because no volatiles are given off in the polymer. The
densification takes place by compression of the dissolved air. This results in stronger
parts.
