Reinforcements are strong, inert fibrous material incorporated in the polymeric mass to improve physical properties.
· GLASS FIBERS:
It is the material originally and most widely used for low pressure moldings. It is used in several forms like chopped strands, rovings, reinforcing mats, yarns, surfacing and overlay mats, woven fabrics, woven rovings, non woven fabrics etc.
Preheating and finishing techniques:
All the glass fibers are treated and finished in a special manner to promote the maximum adhesion of the resin to fibers. First, the original lubricant, (or sizing) used to help in drawing the filament, is removed by the process known as heat cleaning. Some resins do not require a further finishing but for the other resins a finish known as a coupling agent is applied to the fiber, to result in increased adhesion, particularly under high humidity or moisture condition. Some finishes are best suited to particular resin but the most widely used finishes are the chrome complexes and the silanes. However, the type of finish should be determined before using any glass fiber as reinforcements.
Six fibrous material considered as asbestos can be classified under two major groups as:
1. Fibrous serpentine or chrysotile:
It is a hydrated magnesium silicate with double layers of silica tetrahedrons and magnesium hydroxide. Fibers are long and hence it is ideal for the use in laminated plastics.
They are various combinations of Mg, Fe, Ca, and Na hydrated silicates with a basic structure of Si4O11 groups in a double chain configuration. Amphiboles include crocidolite, amosite, anthophillite, actinolite and tremolite. Crocidolite fibers impart high tensile strength, hence are used for general molding. Amosites have limited use in polymer industry because the moldings obtained from it are brittle. It is used in specialized case where acid resistant characteristic is required. Actinolite and tremolite are not significant in commercial plastics.
Mining of asbestos is a difficult process and results in a product having wide range of fiber lengths and includes impurities. Short fibers impart higher modulus and greater hardness to the finished product compared to unfilled system but do not improve other properties.
The moldings produced with asbestos as filler have excellent heat resistance, lower flexibility and tensile strength and increased specific gravity.
Asbestos can be wetted by almost any thermoplastic or thermoset resin. However, when longer fibers agglomerate, it prevents the development of homogenous mixture.
Health problem also results, when asbestos is used. It causes mesothelinoma and lung cancer to the workers; hence the exposure levels must be below the limits prescribed by OSHA. (Occupational safety and health act)
It is available as flock, filament and chopped fabric. It flows at many molding temperature causing considerable mold shrinkage. Hence it is used when special surface is required in the molding e.g. shower stall bases.
· POLYESTER FIBERS:
They can reinforce both thermoplastic and thermoset. The fiber thermoplastic composite have unusually high strength because they serve as nucleating agents for the crystalline thermoplastic. Abrasion and impact resistance of these fibers can be increased by reinforcing it with woven roving of polyethylene terephthalate. These fibers are solvent resistant and high melting and can reinforce thermoplastic, phenolic and epoxy resin system.
· EXOTIC FIBERS:
To prepare the composites using the exotic fibers, only boron, graphite and Kevlar are used.
It is mainly used where the fibers can be laid in relatively straight configuration because these fibers are very stiff and brittle.
· GRAPHITE FIBERS:
They are used in most of the high modulus applications because they are cheaper than boron, have lower specific gravity and are capable of being formed easily into 3-D curves and bent into small radii. Also the parts which are graphite reinforced have a very low coefficient of expansion. This makes such material very useful for high precision parts like optical parts, space antennas etc. In general it is used in articles where high stiffness and light weight are required.
· KEVLAR (POLYARAMID FIBERS):
They can form composites with excellent strength and heat resistance. They also have low densities and hence show high specific strength values.Three types of Kevlar aramid are available. The tensile strength of Kevlar is better than twice that of nylon or polyester and 15% better than E-type fiber glass. Modulus is 20 times greater than nylon, 10 times greater than Polyester and 2 times than E-type fiber glass. Elongation is low and density is 40% less than glass. They are as strong as steel at 1/5 the weight. Their thermal stability is also good and no embrittlement or degradation occurs during cryogenic exposure. Composites made with Kevlar have excellent dimensional stability because it has slightly negative coefficient of thermal expansion in the longitudinal direction. They have good chemicla resistance except to strong acids and alkalis. They have high stress rupture life and fracture toughness, good cyclic tensile fatigue resistance and fair damping characteristic. However they have high moisture absorption, low compressive strength and difficulty of machining.