THE PHYSICS OF COMPOSITE MATERIALS

THE PHYSICS OF COMPOSITE MATERIALS.

Composite materials were known to mankind in the Paleolithic age . The 300 ft high ziggurat or temple tower built in the city center of Babylon was made with clay mixed with finely chopped straw. In recent years, polymeric based composite materials are being used in many applications, such as automotive, sporting goods, marine, electrical, industrial, construction, household appliances, etc. Polymeric composites have high
strength and stiffness, light weight, and high corrosion resistance. In the past decade, extensive research work has been carried out on the natural fiber reinforced composite materials in many applications. Natural fibers are available in abundance in nature and can be used to reinforce polymers to obtain light and strong materials. Natural fibers from plants are beginning to find their way into commercial applications such as automotive industries, household applications, etc.

Global polymer production on the scale present today began in the mid 20th century, when low material and productions costs, new production technologies and new product categories combined to make polymer production economical. The industry finally matured in the late 1970s when world polymer production surpassed that of Steel, making polymers the ubiquitous material that it is today. Fibre reinforced plastics have been a significant aspect of this industry from the beginning. There are three important categories of fibre used in FRP, glass, carbon, and aramid. Glass fibre reinforcement was tested in military applications at the end of World War II Carbon fibre production began in the late 1950s and was used, though not widely, in British industry beginning in the early 1960s, aramid fibres were being produced around this time also, appearing first under the trade name Nomex by DuPont. Today each of these fibres is used widely in industry for many applications that require plastics with specific strength or elastic qualities. Glass fibres are the most common across all industries, although carbon fibre and carbon fibre aramid composites are widely found in aerospace, automotive and sporting good applications.

 COMPOSITES MATERIALS

Composites materials are materials that contain more than one material type for example fibre glass in which glass fibres are embedded within a polymeric material.  Fibre glasses acquire its strength from the glass and flexibility from the polymer. It has the characteristics of both materials. Composite Materials are materials that are made up of a mixture or combination of two or more micro- or macro-constituents that differ in form and chemical composition and which are essentially insoluble in each other. Composite plastics refer to those types of plastics that result from bonding two or more homogeneous materials with different material properties to derive a final product with certain desired material and mechanical properties. Fibre- reinforced plastics are a category of composite plastics that specifically use fibrous materials to mechanically enhance the strength and elasticity of Composites involve two or more component materials that are generally combined in an attempt to improve material properties such as stiffness, strength, toughness, etc.; the resulting properties are largely dependent on the distribution, relative amounts and geometries of the constituents. Composite materials exist in the following forms

1. Particle-Reinforced (Aggregates);

2. Fiber-Reinforced (Continuous Fiber or Chopped Fiber)

3. Natural Composites (Examples: Wood and Bone).

1.1  Particle-Reinforced (Aggregate) Composite.

Particle-Reinforced Composites include the most used (by weight) construction material in the world - Concrete. Sand (Fine Particles) and Gravel (Coarse Particles) compose from 60% to 75% of standard concrete. Note that the Particulates (sand and gravel) are combined with the Matrix (cement) which acts as a bonding agent between the particles. The addition of water to a particle-cement mixture results in complex hydration reaction that produces a network of strong ionic/covalent bonds. The Reinforcing Particles are generally stiff and hard and restrain deformation of the cement. As with polymers, Additives (termed Admixtures) are added to concrete to affect its reactions and properties:

       Accelerators speed-up the curing process,

       Retarders slow down the curing process,

       Surface Hardeners produce an abrasion resistant surface, etc.

1.2  Fiber-Reinforced Composites.

Fiber-Reinforced Composites often aim to improve the strength to weight and stiffness to weight ratios (i.e. desire light-weight structures that are strong and stiff!). Glass or Metal Fibers are generally embedded in polymeric matrices.

       Fibers are available in 3 basic forms:

·         Continuous Fibers are long, straight and generally layed-up parallel to each other.

·         Chopped Fibers are short and generally randomly distributed (fiberglass).

·         Woven Fibers come in cloth form and provide multidirectional strength.

1.3  Natural Composites (e.g. Wood and Bone).

Natural composites are made up of cellulose and bonded by lignin to form the cellulose fibre. Composites involving natural fibres have additional advantages such as environmental friendliness, and renewability (Sapuan et al., 2005). Wood [2] is natural three-dimensional

polymeric composite and consists primarily of cellulose, hemi-cellulose and lignin. In addition,

wood is an original and natural composite. The biological world offers other examples of composites in bone and teeth, which are essentially composed of hard inorganic crystals in a matrix  of tough organic collagen.

The idea of using cellulose fibers as reinforcement in composite materials is not new or recent. Man had used this idea for a long time, since the beginning of our civilization when grass and straw were used to reinforce mud bricks. In the past, composites, such as coconut fiber/natural rubber latex, was extensively used by the automotive industry.

The primary requirement for obtaining a satisfactory performance from short-fiber composites, including cellulose-based composites, is good fiber dispersion in the polymer matrix. Good dispersion implies that the fibers are separated from each other (i.e. there are no clumps and agglomerates), and each fiber is surrounded by the matrix. Insufficient fiber dispersion, on the other hand, results in an inhomogeneous mixture of resin-rich areas and fiber-rich areas.

 The physics of composite materials.

Research shows that composite is made up of the matrix which is the weaker materials and the invigorating parts (reinforcement) strengthen the matrix. The following are the component of a composite material:

1. Matrices

2. Reinforcing Fibers

Matrices

The role of matrix in a fiber-reinforced composite is to transfer stress between the fibers, to provide a barrier against an adverse environment and to protect the surface of the fibers from mechanical abrasion. The matrix plays a major role in the tensile load carrying capacity of a composite structure. The binding agent or matrix in the composite is of critical importance. Four major types of matrices have been reported:

 Polymeric, Metallic, Ceramic and Carbon. Most of the composites used in the industry today are based on polymer matrices.

·         Polymer Matrix Composites (PMCs)

·         Metal Matrix Composites (MMCs)

·         Ceramic Matrix Composites (CMCs)

·         Carbon-carbon composites (CCMs)

Polymer Matrix Composites (PMCs)

The most common advanced composites are polymer matrix composites. These composites consist of a polymer thermoplastic or thermosetting reinforced by fiber (natural carbon or boron). These materials can be fashioned into a variety of shapes and sizes. They provide great strength and stiffness along with resistance to corrosion. The reason for these being most common is their low cost, high strength and simple manufacturing principles. Polymers are divided broadly into two categories:

·         Thermosetting

·         Thermoplastics.

Thermosetting

Thermoset is a hard and stiff cross-linked material that does not soften or become moldable when heated. Thermosets are stiff and do not stretch the way elastomers and thermoplastics do. Several types of polymers have been used as matrices for natural fiber composites. Most commonly used thermoset polymers are epoxy resins and other resins, Vinyl Ester, Phenolic Epoxy, Novolac and Polyamide).Unsaturated polyesters are extremely versatile in properties and applications and have been a popular thermoset used as the polymer matrix in composites. They are widely produced industrially as they possess many advantages compared to other thermosetting resins including room temperature cure capability, good mechanical properties and transparency. The reinforcement of polyesters with cellulosic fibers has been widely reported.

Thermoplastics

Thermoplastics are polymers that require heat to make them processable. After cooling, such materials retain their shape. In addition, these polymers may be reheated and reformed, often without significant changes in their properties. The thermoplastics which have been used as matrix for natural fiber reinforced composites are as follows:

High density polyethene (HDPE)

Low density polyethene (LDPE)

Chlorinated polyethylene (CPE)

Polypropylene (PP)

Normal polystyrene (PS)

Poly (Vinyl chloride) PVC)

Mixtures of polymers

Recycled Thermoplastics

 The processing  temperature at which fiber is incorporated into polymer matrix does not exceed 230°C and the polymer  matrix are polyethylene and polypropylene and  for the like of  polyamides, polyesters and polycarbonates require processing temperatures  greater than 250°C and are therefore not useable for such composite processing without fiber degradation.

Metal Matrix Composites (MMCs)

Metal matrix composites, as the name implies, have a metal matrix. Examples of matrices in such composites include aluminum, magnesium and titanium. The typical fiber includes carbon and silicon carbide. Metals are mainly reinforced to suit the needs of design. For example, the elastic stiffness and strength of metals can be increased, while large co-efficient of thermal expansion, and thermal and electrical conductivities of metals can be reduced by the addition of fibers such as silicon carbide.

Ceramic Matrix Composites (CMCs)

Ceramic matrix composites have ceramic matrix such as alumina, calcium, aluminosilicate

reinforced by silicon carbide. The advantages of CMC include high strength, hardness, high service temperature limits for ceramics, chemical inertness and low density. Naturally resistant to high temperature, ceramic materials have a tendency to become brittle and to fracture. Composites successfully made with ceramic matrices are reinforced with silicon carbide fibers. These composites offer the same high temperature tolerance of super alloys but without such a high density. The brittle nature of ceramics makes composite fabrication difficult. Usually most CMC production procedures involve starting materials in powder form. There are four classes of ceramics matrices: glass (easy to fabricate because of low softening temperatures, include borosilicate and alumino-silicates), conventional ceramics (silicon carbide, silicon nitride, aluminum oxide and zirconium oxide are fully crystalline), cement and concreted carbon components.

Carbon-carbon composites (CCMs)

CCMs use carbon fibers in a carbon matrix. Carbon-carbon composites are used in very high temperature environments of up to 3316⁰C, and are twenty times stronger and thirty times lighter than graphite fibers.

Reinforcing fibers

This is the phase that imparts strength to the matrix and there are three common types of reinforcing fibers: fiberglass, carbon and Aramid.

Carbon fibers

Carbon fibers are used for reinforcing certain matrix materials to form composites. Carbon

fibers are unidirectional reinforcements and can be arranged in such a way in the composite

that it is stronger in the direction, which must bear loads. The strength of carbon fiber depends on the fiber alignment, the volume fraction of the fiber and matrix, and on the molding conditions. Several types of matrix materials such as glass and ceramics, metal and plastics have been used as matrices for reinforcement by carbon fiber.

Natural fiber-reinforced composites

Carbon fiber composites, particularly those with polymer matrices, have become the dominant advanced composite materials for aerospace, automobile and other applications due to their high strength, high modulus, low density, and reasonable cost for application requiring high temperature resistance as in the case of spacecrafts.

Glass fibers

Glass fibers are the most common of all reinforcing fibers for polymeric matrix composites (PMCs). The principal advantages of glass fiber are low cost, high tensile strength, high chemical resistance and excellent insulating properties. The two types of glass fibers commonly used in the fiber reinforced plastics industries are E-glass and S-glass. Another type known as C-glass is used in chemical applications requiring greater corrosion resistance to acids than is provided by E-glass.

Kevlar fibers

Kevlar belongs to a group of highly crystalline aramid (aromatic amide) fibers that have the lowest specific gravity and the highest tensile strength to weight ratio among the current reinforcing fibers. They are being used as reinforcement in many marine and aerospace applications.

Boron fiber

 Boron fibers offer excellent resistance to buckling, which in turn contributes to high compressive strength for boron fiber reinforced composites.

Natural Fibers

 Natural fibers have many significant advantages over synthetic fibers. Presently, many types of natural fibers are used in plastics including flax, hemp, jute straw, wood, rice husk, wheat, barley, oats, rye, cane (sugar and bamboo), grass, reeds, kenaf, ramie, oil palm empty fruit bunch, sisal, coir, water, hyacinth, pennywort, kapok, paper mulberry, raphia, banana fiber, pineapple leaf fiber and papyrus. Thermoplastics reinforced with special wood fillers are enjoying rapid growth due to their many advantages; lightweight, reasonable strength and stiffness. Some plant proteins are renewable materials due to their thermoplastic properties.  Composites based on biologically degradable polyester amide and plant fiber (flax and cottons) with good mechanical properties,

such as sufficient water resistance and biodegradability, have also been investigated. Kenaf, Hibiscus cannabinus L, a member of hibiscus family is also a biodegradable and environmentally friendly crop. As the filler loading increases, the composites made without any compatibilizing agent show decreased tensile strength and more brittleness, but greatly improved mechanical properties by incorporation of the compatibilizing agent. The poor interfacial binding between the filler and the polymer matrix causes the composites to have decreased tensile strength, but the tensile strength and modulus improve with the addition of compatibilizing agent. Wheat straw has been used for making composites, panel boards and anion exchangers where the straw is used in powder form rather than in the fibrous form. Jute is also one of the most common agro fibers used as a reinforcing component for thermoplastics and thermosetting matrices.

Method of Producing Natural Adhesives

The cassava starch and potato starch adhesives are  prepared by mixing 2.5g of ground flour with 50ml of dilute hydrochloric acid (0.01M HCl in H₂O) whilst heating to 94°C (Ozemoya et al., 2007). Euphorpbia tirucalli latex is extracted by cutting stems of the shrub and collecting the secreted latex emulsion, in a similar manner to tapping of natural rubber. To prepare the adhesive, 15ml to 25ml of latex was mixed with 2g of palm oil and stirred whilst heating to 75°C.

 Conclusion

Composite advancement and studies has actually increased diversity in materials selection. It has also helped engineer in producing parts as single entity by casting for example exhaust outlet manifold and car pedals.

REFERENCES

[1] http://i-cias.com/e.o/sumer.htm

[2] http://www.theplumber.com/history.htm

F. T. Wallenberger and N. Weston, “Natural Fibers, Plastics and Composites Natural”, Materials Source Book from C.H.I.P.S. Texas, 2004