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.
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.
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.