Constituents of Composites

CONSTITUENTS OF COMPOSITES

Composites consist of two important constituents.

(i) Matrix phase (or) Matrix resin

(ii) Dispersed phase (or) Reinforcement.

1. Matrix Phase (or) Matrix resin

Matrix phase is the continuous body constituent, which encloses the composite. Matrix phase may be metals, ceramics (or) polymers (liquid resins), composites using these matrix are known as

(i) Metal Matrix Composites (MMC)

(ii) Ceramic Matrix Composites (CMC)

(iii) Polymer Matrix Composites (PMC) respectively.

Liquid Resins

Polymers used as adhesives, potting compounds, sealants, etc., in a liquid form are called as liquid resins.

Examples : Epoxy adhesives, polysulphides are typical commercially available adhesives.

Cure reactions

Formation of cross-linking in certain polymers is known as cure reaction.

Cure reactions are done by using a curing agent. These uncross-linked linear polymers contain functional groups (or) double bonds. They are usually not high molecular weight resin and are in the form of liquid resins. They are also called as "pre-polymers”. The liquid resins will be cured in the moulding to get solid mass of cross linked products.

 

2. Dispersed Phase (or) Reinforcement

Dispersed phase is the structural constituent, which determines the internal structure of composite.

Examples :  Fibres, Particulates, Flakes, Whiskers.

Reinforcement

It is a process of improving the characteristics of the plastic matrix by adding reinforcing agents. The reinforcing agents may be organic (or) inorganic in nature. They may be in the form of powder, flakes, fibres. These materials are characterised by high strength to weight ratio, excellent resistance to erosion, corrosion, etc., and ease of fabrication. For effective structural performance, there should be a strong adhesive bond between the matrix phase (or) matrix resin and the dispersed phase (or) reinforcing agents.

1. Fibres

Fibres are long and thin filament of any macromolecular substance such as polymer, metal (or) ceramic having high length to diameter ratio at least 100:1.

The fibre (reinforcement material) can be used in variety of forms like continuous rovings, cloth, chopped strands, etc.,

Characteristics of Fibre

(i) Fibre possesses high tensile strength.

(ii) It possesses high stiffness.

(iii) It lowers overall density of composites.

Classification of Fibres

The fibres used in reinforcements are classified as

(i) Natural fibres: These are derived from naturally available sources.

Examples : Cotton, wool, silk, sisal, jute and coir.

(ii) Semi-synthetic fibres: These are produced by modifying the naturally occurring materials.

Examples : Rayon, cellulosic.

(iii) Synthetic fibres: These are produced exclusively by the laboratory processes.

Examples : Polyester fibre, polypropylene and carbon fibre (poly acrylonitrile), glass fibres, aramid fibres.

(iv) Aramid fibres: These are aromatic polyamides.

Example : Kevlar.

(i) Important Natural Fibres

(a) Cotton Fibres: Cotton is a vegetable fibre and is made of cellulose molecule, which is a linear polymer of B-glucose. Cotton fibres provide a low cost fibrous reinforcements for the use in dough moulding compounds (DMC). Mostly phenolic resin based moulding compounds.

The painted fabrics can be used to provide a decorative finish for applications like wall panels.

(b) Wool Fibre: Wool is an animal protein fibre. Like other proteins, wool is also made up of a-aminoacids. These aminoacids are joined end-to-end in the form of peptide chains forming long chain molecules. Many of the physical and chemical properties of wool are due to the disulphide bonds formed by the covalent cross-linking of the cystein molecule. Any chemical that affect the disulphide bonds may cause structural break down of the fibre. The elastic properties of the wool fibre is due to the presence of hydrogen bond between various peptide chain.

(c) Silk: Silk is a natural protein fibre produced by the insect, silk worm. It possesses all the desirable qualities of fibre such as softness, strength, elongation, etc. It has a bright lusture and it is an excellent insulator of heat, hence it is warm in winter and cool in summer.

(d) Sisal: Sisal is another low-cost fibrous material, which is also used in phenolic based dough moulding compounds.

(e) Jute and Coir: They are also low-cost fibrous material for reinforcement for polyester resin. They can be used as woven cloth (or) in yarn form for filament winding. A variety of objects like houses, fishing boats have been developed.

(ii) Synthetic Fibres

(a) Polyethylene terephthalate (PET) (or) Terylene (or) Dacron

PET is produced by the condensation of ethylene glycol (EG) and terephthalic acid (TPA). This fibre is used as surface tissues which can be used to provide improved chemical and abrasion resistance. They may be used in light weight woven fabrics. They are rarely used as the main reinforcement, although they give laminates with high impact resistance, excellent chemical and abrasion resistance. Generally they are used in combination with glass fibres.

(b) Poly Propylene: It is obtained by polymerisation of propylene. These fibres possess better hardness, strength, stiffness. These fibres are used in making ropes, carpets, blankets, hand bags, apparels, etc.

(c) Carbon fibres: Carbon fibres are obtained as a continuous filament by the pyrolysis of organic fibres in an inert atmosphere such as cellulose, polyacrylonitrile (PAN)

Properties

The carbon fibres are widely used because of their specific strength, stiffness, high modules even at elevated temperatures. Also they are resistant to moisture, acids, bases and a number of solvents.

PAN

Uses

PAN fibre is used to make surfacing tissues to provide improved chemical resistance. Tissues may be used in conventional constructions (or) light weight woven fabrics.

(d) Nylon

Nylon fabrics are used as reinforcements for epoxide resin systems to give flexible laminates with high impact, abrasion and chemical resistance. They are sometimes combined with glass fibre reinforcements for specialized applications.

(iii) Aramid Fibres

The wholly aromatic polyamides are called as aramids. They are made by spinning liquid crystal aramid oligomers (Kevlar).

The meta-linked poly-m-phenylene isophthalimide is called Nomex, where as para-linked poly-p-phenylene phthalimide is called Kevlar.

Properties

These are used as high performance fibers because of their high modules, tensile strength, stability at high temperatures, wide temperature range (- 200 to 200°C) for retaining mechanical properties, excellent toughness and impact resistance.

Uses

Aramid fibres are used in reinforcements, widely used in tyre card reinforcement, bullet proof clothing, race drivers, astronaut suits, etc.,

2. Particulates

Particulates are small pieces of hard solid materials. They may be metallic (or) non-metallic. Generally distribution of particles in a given matrix is random, so the resulting composites are usually isotropic. Due to the inherent hardness, the particles in a composite cause constraints on the plastic deformation on the matrix.

Effect of particulate in particulate composite

(i) Surface hardness gets increased.

(ii) Wear and abrasion resistance are increased.

(iii) Performance at elevated temperature is improved.

(iv) Shrinkage and friction is reduced.

(v) Electrical and thermal conductivities are modified.

3. Flakes

Flakes are very thin solid like materials.

Example :  Mica flakes

(i) They impart equal strength in all directions in a plane compared to fibres which reinforce unidirectionally only.

(ii) Flakes can be packed more efficiently than fibres (or) spherical particles.

(iii) Flakes especially mica can be used in electrical and thermal insulating appliances.

4. Whiskers

Whiskers are thin strong fibre like material made by growing a crystal.

Examples : Graphite, silicon carbide, silicon nitride

(i) Perfect whiskers possess much higher strength than in bulk, but irregularities decreases their strength.

(ii) They possess high elastic modulus and high degree of crystallinity.

(iii) They possess exceptionally high strength.

(iv) As it is costly and difficult to incorporate in matrix they find limited use in composite.