Figure 1. Plastic Composite in Sleepers
The emergence of Plastic industries has become a boon for various engineering applications. Plastics are durable, light weight materials, which are easy to transport. The major advantage of plastic is it is recyclable in nature, which helps to conserve landfill space, natural resources and reduce pollution. Earlier in railways, wooden, concrete or steel sleepers are used. Due to their high cost and limited life span, nowadays plastics sleepers are used all over the world at large scale. An attempt has been made to study various engineering plastics used in railways and the advantages of plastics over other construction materials. Moreover, special attention is given to Fiber reinforced plastics (FRP) i.e. plastics composites. The study concluded that plastics composites are more reliable, more economically viable and have longer life span.
Plastic is a substance that can be easily formed or molded into desired shape. Plastics are formed by polymerization, condensation and co-polymerization reactions. These reactions give high molecular weight materials which soften on heating and become rigid on cooling [1] .
In the manufacture of plastics, raw materials viz. coal, petroleum, cellulose (prepared from wood and cotton), salt, sulphur, limestone, air, water etc. are used. These are subjected to several processes of chemical decomposition and recombination giving intermediates, which are then made into plastics. Plastics when mixed with tar have wide applications in the construction of roads. This not only enhances the strength of road but also increases the durability of road. Roads constructed with this composition do not get easily spoiled even during rainy days. These may move for at least 10 years and will save public money. In Indian plastic industry, the production of phenol formaldehyde started in 1946, polystyrene and polyethylene in 1957, polyvinyl chloride in 1961 and cellulose acetate in 1963.
The important engineering plastics used in railways are,
Apart from these, LDPE (Low Density Polyethylene) sheet is widely used for roofing and covering of goods in the wagon [2]. Some important plastics polymers which are used in Indian railways with their, applications are given below:
Polyethylene is formed by polymerization of ethane. They are wax like thermoplastics which can become soften at 80o -100o C. They have excellent electrical insulation and chemical resistance. Its density is less than water. Carbon black is generally added to increase its efficiency. Its thin films are transparent. Its uses in our daily life are house wares, bottles, trays, toys, food stuffs carrying containers, ink pen refilling tubes,wires and cables.
When tetrafluoroethylene is subjected to polymerization in the presence of acetyl peroxide, the product obtained is highly heat resistant and possess high thermal stability having melting point around 327o C. Teflon is tough, flexible and possess excellent resistance to heat, chemical and electric current. It has very low co-efficient of friction and good weathering resistance. It is widely used in variety of seals, gaskets, packing, valve and pump parts and lab equipments. It is also used in transformers, condensers, as laminates for PCBs, and in coating for reducing the friction.
Nylon-6 is a polymer of cyclohexane which get polymerized in the water. Nylon-66 is combination of hexamethylene diamine and adipic acid. It has properties such as toughness, rigidity, abrasion resistance, heat resistance which make it for special purposes. It has variety of applications in mechanical engineering parts such as gears, bearings, bushes, valves and seals. Different types of Nylon like Nylon films, Nylon filaments are being used. Nylon films have high clarity and low odour transmission. Nylon filaments are being used in brush, surgical items, sports and fishing nets. Its flexible nature is used for tubing and pipe for chemicals and petrol. Its powder form is used in chemical resistant coating.
Polycarbonates are special plastics having melting point 225-250oC.It is quite rigid and tough which is stable up to 140oC.These plastics are transparent and possess high electrical insulation. Its properties such as low water absorption, high dimensional stability are used in electrical and electronics engineering, for time switches, relays, batteries etc. This polymer has been widely used in domestic purposes such as food processors bowl, power tool, hair drier, electric razor and microwave cookware.
Polypropylene is widely used in fibers, films and injection moldings'. It provides excellent resistance to continued faxing. It is also used in lunch boxes and car accelerators pedals. In car, it is used in battery cases, bumpers, radiator grills, chairs and flower pots. Its monofilaments are used in rope, nets and packaging.
Urea-formaldehyde has physical properties such as hardness and toughness which makes it suitable for strong knock resistant electrical fittings. It is also scratch resistant and has good electrical insulation. These properties make it suitable for engineering applications [3].
Bakellite is the result of polymerization of Phenolformaldehyde in basic medium. It is widely used in Adhesive, electrical goods, combs, fountain pen and mica table tops.
Polymethyl(meth)acrylate (PMMA) is hard, rigid and transparent material with good weather resistance. It has good electrical, insulation and optical properties.
The advantages of plastics over the construction materials are listed below.
A fiber reinforced resin is a composite material consisting of a network of reinforcing fibers embedded in a mixture of thermo setting resins [4].The resins used consists of a syrupy liquid which when combined with a suitable catalyst or hardener, lead to formation of cross linked hard infusible solid.
By impregnating the fibrous material with the catalyzed resin, a composite material or laminated material is formed. When glass fiber is used, it form laminations and when carbon fiber is used, it forms composites [5] .
Resins used are:
FRP has wide variety of applications in terms of technical properties summarized as follows [6].
Some more highlighted properties are described as:
One of the important application of FRP material is to increase the strength of structural building especially its columns, beams or slabs.
There is a trend to change railway sleeper from traditional timber to ultra-modern composites. No doubt, wooden sleepers are still used in many parts of the world where lighter, less busy lines providing a resilient track, make vibration and noise. Wooden sleepers are cheap, easy to carry and easy to maintain. Wood is more susceptible to wear and tear than any other modern materials. Now, the modern material used is plastics composites which have all natural counterpart. Life span of plastics composites is more than 50 years. These plastics composites can be placed alongside older wooden sleepers. One advantage is that it is generally made from recycled material [7] . Figure 1 shows plastics composites in railway sleepers. Recent study reveals that one mile of wooden sleeper requires 810 mature oak trees but using plastics composites requires two million plastics bottles, 8.9 million plastics bags and 10,800 post consumer tyres.
Figure 1. Plastic Composite in Sleepers
So, plastic composites emerge like a new and advanced technology which is environmentally friendly. As far as economic analysis, the cost of manufacturing is very less as compared to concrete and timber market. The use of plastics composites in railway sleeper has the following abilities.
Plastic composites consists of waste or recycled polyolefin's (Polyethylene), Propylene polymer, Propylene-ethylene copolymer (or) combination of all these polymers [8] .
Their properties are:
No doubt, plastics have been great discovery in modern world. Plastics and polymers have become almost a part of our life. Recycled plastics are alternative promising technology to save our environmental pollution. There is a need to generate awareness among scientific community and industries to change their mind sets and develop advanced techniques to prepare such composites having high durability, low cost and higher life span. The ultimate goal of research is to convert plastics into recyclable one and prepare composites, which is environmental friendly. Research is being conducted to promote the use of plastics composites in railway sleepers. This not only reduces the cracks in railway sleepers, but also reduces the noise pollution [9]. There is need of industrial research to develop innovation for making melt-blended composites from old waste materials. This is also a need to stress on improved methods for melt-blended fiber and plastics on commercial scale.