FIBROUS REINFORCEMENT Structure: 1.Continuous bundles of fibers. 2.Woven fabrics. 3.Chopped fiber.

Normally their diameters in the range of 5-15 µm and produce in two or bundles consist of large number of filaments ( ). A critical feature of fibers used for reinforcement is the coating of size applied to the surface during the manufacturing process. Sizing is designed to: 1.Hold the fibre together as a coherent bundle 2.Protect the fibre surface from mechanical and environmental damage. 3.Improve the chemical bonding between fibres and matrix in the composite.

TYPES OF FIBROUS REINFORCEMENT Three types are dominate the reinforced plastics. 1.Glass fibers. 2.Carbon fibers. 3.Oriented polymeric fibers (Kevlar). Figure. 5 Stress strain curves for reinforcing fibers compared with 0.1%plane steel: (a) Stress vs. strain; and (b) specific stress vs. strain.

GLASS FIBRE. Widely used fibre for reinforcement of plastic Moderate prices. Desirable properties for reinforcement Based on silica (SiO 2 ) + some other smaller quantities of other inorganic oxides.

MANUFACTURING Glass fibre are manufactured by extruding molten glass at high velocity through a large number ( ) of holes in a platinum plate. (Bushing). The resulting filaments are then wound up at an even faster rate as they cool through their glass transition and solidity. Fibre are cooled rapidly through Tg during drawing to ensure that there are no crystallinity. Crystals act as stress raisers in glass and greatly weaken the fibers.

ADVANTAGE OF GLASS FIBRE 1.Resistance to high temperature (softening point as about 850 o C). 2.Transparency to visible light. 3.Isotropy (thermal expansion is identical in axial and radical direction).

DISADVANTAGE Very susceptible to surface damage. Such as rubbings or action of moisture. Therefore sizing is important.

CARBON FIBRES Less widely used compared to glass fibre because of their higher cost. Carbon fibers prepared from polyacryonitrile (PAN) by converting the polymer into graphite through a sequence of carefully controlled heat treatment operations. Carbon fibers are preferred due to their high degree of orientation and strong covalent bonding between the carbon atoms.

Advantages: 1.Chemical innerness (resist moisture) 2.High electrical and thermal conductivity 3.Dimensional stability – V. Low thermal expansion. Disadvantage: 1.They are black and impart this colors to the composite.

AREMID POLYMER (FIBRE) Contain both aromatic and amide groups in the molecular chain. Example Kevlar 49. It is the most widely used aramid fibre for reinforcement. n C O C O N H N H Poly (paraphenylene tetephthalamide)

The molecules is rigid because of the benzene rings. No chain folds. The rod-like molecules pack together like pencils and bond firmly to their neighbors by the amide groups present, providing an excellent glue. C = ONH C O N H … Their high degree of orientation is achieve by extruding the Kevlar fibre from solution and stretched in order to a sign the molecules parallel to the fibre axis.

The structure is of a disordered crystal without discrete amorphous regions. The properties of Kevlar fibers show two drawbacks 1.Weakness in axial shear 2.Yellow coloration (imparts thing color to its composite) Because of their high cost their are considered only where their outstanding mechanical properties are really needed.

ADVANTAGES 1.Fatigue resistance (but carbon fibre is better) 2.Elevated temperature resistances 3.Chemical resistance 4.Weathering resistance

PLATELET REINFORCEMENT Commonly are minerals such as talc and mica Talc: 3 MgO.4SiO 2.H 2 O Mica: k 2 O.3Al 2 O 3.6SiO 2.2H 2 O Never obtained in pure form Talc and Mica both are crystalline Their dimension after crushing and grinding are in the order of m across and 1-5 m in thickness.

ADVANTAGE 1.Low price. 2.Stiffness and strength are greater than plastics 3.Provide reinforcement in all directions and not merely in direction as with uniaxially aligned fibre.

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