Plastic: Composition, Classification, Moulding, Properties, Uses and Biodegradable Plastic!
Brief History of Plastics:
The plastic is one of the recent engineering material which has appeared in the market all over the world. Some varieties of naturally occurring thermo-plastics were known to Egyptians and Romans who extracted and used these plastics for various purposes.
A number of scientists carried out experiments for plastics and as a result of long research, the birth of plastic industry took place in the nineteenth century.
The developments of plastic industry may broadly be grouped into the following three stages:
(i) The aim of first stage of development was mainly to imitate or copy the natural plastics. The main contribution during this phase was by a Scottish chemist – Alexander Parkes. He prepared a hard material in 1865 by mixing camphor and alcohol with nitro-cellulose. This material was known as the Parkesite and it was used for gear wheels, door knobs, etc.
(ii) The second stage is comprised in first twenty years or so of this century. During this stage, the work done previously in plastic industry was scientifically scrutinized and this stage laid the foundation for further scientific development of the plastics.
The notable contribution during this stage was by a distinguished Belgian scientist – Dr. L. Bakeland. He prepared a product, known as the Bakelite, in 1909. It is thermosetting substance. It was found to be strong and hard material with good insulating property.
(iii) The last stage includes the present trend and it aims at improving the old plastics and producing new varieties of plastics. Such development is mainly responsible for two world wars during which intensive research was carried out to get plastics with desired properties. The beginning of this stage was made by an Austrian scientist – Pollak. He prepared a substance from urea and formaldehyde in 1924. This substance was transparent like glass. But it was unbreakable.
It was also possible to produce it in many attractive colours and shades. A strong revolution in plastic industry came during the period of Second World War (1939-1944). Within such a short period, the plastic has proved to be a very important engineering material.
The plastics because of their wide acceptance as engineering materials are replacing the metals.
Composition of Plastic:
The plastic is an organic substance and it consists of natural or synthetic binders or resins with or without moulding compounds. In general, it may be stated that the plastics are compounds of carbon with other elements such as hydrogen, nitrogen and oxygen. The carbon combines with itself and other elements and it forms more complicated compounds.
The finished product of plastic is rigid and stable at normal temperature. It is therefore improper to associate the term plastic to such substances in the ordinary sense of the term.
It is however to be noted that the plastics are the organic substances and these are capable of flow when necessary heat and pressure are applied at some stage of their manufacture. The term plastic is therefore accepted to be the best one to describe the various diverse products of the plastic industry.
The simplest substances consisting of one primary chemical are known as the monomers or monoliths. They are to be combined or synthesised to form polymers by the process known as the polymerization.
A polymer thus consists of thousands of monomers joined together. The polymer molecule is also called a macromolecule. A polymeric material consists of a large number of these long-chain molecules as shown in fig. 16-1.
The properties such as strength, rigidity and elasticity are considerably improved by the polymerization and it further leads to the manufacture of plastics in an economic way.
Classification of Polymerization:
Polymer structures can be classified according to the shape of macromolecules as under:
(i) Branched Chain Structures:
As shown in fig. 16-2, branches of monomers are added on to the linear chain of macromolecule. The ratio of length of main chain to that of side chains is not constant. The number of branches may also vary.
(ii) Cross-Linked Structures:
As shown in fig. 16-3, chains adjacent to each other are linked together. Hence it restricts the movement between chains and therefore, some mechanical properties of material are altered.
(iii) Linear Structures:
As shown in fig. 16-4, long-chain molecules are being separated from each other. Polymers having this type of structure get strength from the inter-winning of the molecules.
(iv) Networking Structures:
As shown in fig. 16-5, this type of structure is being formed by tri-functional and tetra-functional monomers. In networking structure, it is difficult to distinguish individual molecular chains.
Methods of Polymerization:
Following are the three methods of polymerization which, as stated above, is the process of growing large molecules from small ones:
(i) Addition polymerization
(ii) Condensation polymerization
(i) Addition Polymerization:
In this method, similar or different molecules join together due to opening of double bonds and the molecular weight of the resulting polymer is equal to the sum of the molecular weights of the reacting molecules. There is no loss of any substance in this process.
The process involves three distinct stages:
(a) Beginning of the process
(b) Expansion of chain
(c) End of process.
The polymers obtained by this method are polyethylene, poly-propylene, polyvinylchloride, polystyrene, polyacrylates, etc.
(ii) Condensation Polymerization:
In this method, the low-molecular substances are removed from the high-molecular substances formed from a large number of identical or different molecules. The reaction proceeds with an evolution of ammonia, hydrogen chloride and similar other low-molecular substances. The polymers obtained by this method are phenol formaldehyde, carbamide, melamine-formaldehyde, etc.
As compared to addition polymerization, the condensation polymerization yields industrially important by-products such as HgO, HCl, etc. It is not a kinetic chain reaction by an intermolecular reaction. The addition polymerization takes a few seconds while the condensation polymerization takes even days to complete.
The co-polymerization is the addition polymerization of two or more different monomers. There are many monomers which will not polymerize with themselves but will co-polymerize with other compounds.
A co-polymer may have properties quite different from those of either component member. Thus a wide variety of plastics may be obtained by this process. The examples of this type of polymerization are vinyl chloride acetate and butadiene-styrene co-polymers.
4. Classification of Plastics:
The classification of plastics can be made by considering various aspects and for the purpose of discussion, they can be classified according to their:
(1) Behaviour with respect to heating,
(2) Structure, and
(3) Physical and mechanical properties.
(1) Behaviour with Respect to Heating:
According to this classification, the plastics are divided into two groups:
The above classification is based on the inherent characteristics of each group. These two groups can further be divided into several distinct sub-divisions. These sub-divisions are based on the raw materials from which plastics are prepared. It is interesting to note that each of above group contains several hundred different products and with the advance of plastic industry, the number of sub-divisions under each category is constantly increasing.
The thermo-plastic or heat non-convertible group is the general term applied to the plastics which become soft when heated and hard when cooled. The process of softening and hardening may be repeated for an indefinite time, provided the temperature during heat is not so high as to cause chemical decomposition. It is thus possible to shape and reshape these plastics by means of heat and pressure. One important advantage of this variety of plastics is that the scrap obtained from old and warn-out articles can be effectively used again.
The thermo-setting or heat convertible group is the general term applied to the plastics which become rigid when moulded at suitable pressure and temperature. This type of plastic passes originally through thermo-plastic stage.
When they are heated in temperature range of 127°C to 177°C, they set permanently and further application of heat does not alter their form or soften them. But at temperature of about 343°C, the charring occurs. This charring is a peculiar characteristic of the organic substances.
The thermo-setting plastics are soluble in alcohol and certain organic solvents, when they are in thermo-plastic stage. This property is utilized for making paints and varnishes from these plastics.
The thermo-setting plastics are durable, strong and hard. They are available in a variety of beautiful colours. They are mainly used in engineering application of plastics.
According to this classification, the plastics are divided into two groups:
(i) Homogeneous plastic
(ii) Heterogeneous plastic.
(i) Homogeneous Plastic:
This variety of plastic contains carbon chain i.e. the plastics of this group are composed only of carbon atoms and they exhibit homogeneous structure.
(ii) Heterogeneous Plastic:
This variety of plastic is composed of the chain containing carbon and oxygen, the nitrogen and other elements and they exhibit heterogeneous structure.
(3) Physical and Mechanical Properties:
According to this classification, the plastics are divided into four groups:
(i) Rigid plastics
(ii) Semi-rigid plastics
(iii) Soft plastics
(i) Rigid Plastics:
These plastics have a high modulus of elasticity and they retain their shape under exterior stresses applied at normal or moderately increased temperatures.
(ii) Semi-Rigid Plastics:
These plastics have a medium modulus of elasticity and the elongation under pressure completely disappears, when pressure is removed.
(iii) Soft Plastics:
These plastics have a low modulus of elasticity and the elongation under pressure disappears slowly, when pressure is removed.
These plastics are soft and elastic materials with a low modulus of elasticity. They deform considerably under load at room temperature and return to their original shape, when the load is released. The extensions can range upto ten times their original dimensions.
5. Moulding Compounds for Plastics:
To give desired properties to the finished plastic articles, certain moulding compounds are to be added to the plastics.
Following are such moulding compounds:
These compounds are added to assist and accelerate the hardening of resin. For instance, the ester acts as catalyst for urea formaldehyde. They are used for quick and complete polymerization.
The fillers are inert materials and they impart strength, hardness and other properties to the plastics. The choice of filler should be carefully made. It should be confirmed that the addition of filler does not have detrimental effect on other properties of plastics.
The fillers may be used in the following forms:
(i) Fibrous Fillers:
They increase thermal resistance and impact strength of plastics. They also increase strength and reduce brittleness of plastics. They are widely used in the manufacture of plastics. The commonly used fibrous fillers are asbestos, wood and glass fibres.
(ii) Laminated Fillers:
They make the plastics very strong. The commonly used laminated fillers are papers, wood veneers, asbestos cardboards, cotton, etc.
(iii) Powder Fillers:
They provide valuable properties to plastics such as acid- resistance, water-resistance, etc. They also lead to increase in durability, improvement in hardness and reduction in cost. The commonly used powder fillers are quartz powder, chalk, wood flour, etc.
These compounds are added to increase the hardness of resin. For instance, the hexamethylene tetramine acts as hardener for phenol formaldehyde.
The lubricants are applied on the surface of moulds so that the articles of plastics do not stick to the moulds. The application of lubricants on surface of moulds allows easy removal of articles of plastics from the moulds. The commonly used lubricants are graphite, paraffin, wax, etc.
The addition of dyes and pigments helps in two ways, namely, they act as fillers and they impart desired colour to the plastics. They should be durable and adequately fast to light. The commonly used pigments are zinc oxides, barytes, etc. The selection of pigments should be done in such a way that their addition does not alter or affect the other properties of plastics.
The plasticizers are the organic compounds which are oily in nature and of low molecular weight. They are used to separate the polymer chain by a greater distance to make the crystallization difficult. A non-crystalline solid is thereby produced from a polymer that normally crystallizes.
These compounds are added to improve plasticity and to impart softness to the plastics. They give flexibility to the material and act as lubricants. They should be chemically inert, poorly volatile and non-toxic.
The addition of plasticizers facilitates the moulding process of plastic articles. The commonly used plasticizers are camphor, triacetin, tributyl phosphate, etc. The proportion of plasticizers in plastics should not exceed 10 per cent. Otherwise the strength of plastics will decrease.
These compounds are added to dissolve the plasticizer. For instance, the alcohol is added in cellulose nitrate plastics to dissolve camphor.
6. Fabrication of Articles of Plastics:
Following are the processes involved in the fabrication of articles of plastics:
Each of these processes will now be briefly described.
This method of lubrication of articles of plastics is more or less the same as that one employed in the glass industry. A lump of plastic material is taken and by blowing, it is converted into hollow plastic articles such as jars, bottles, toys, etc.
In this process, the plastic material is allowed to pass between the cylindrical rollers. The process is used to prepare plain flat sheets of plastics.
The process consists of closely placed four revolving cylinders. The first three cylinders are heated and the last one is kept cold. The plastic material passes between first three cylinders and it is converted into thin sheets.
It is cooled while passing through the surface of cold cylinder. If cloth is to be provided with plastic coating, the cloth is inserted along with plastic material between second and third heated rollers. The roller may be provided with artistic designs which will appear on the finished product.
This process is similar in principle to that of metal casting. The resin is heated and when it is in plastic form, it is poured into the mould. The curing of articles is then done either with or without the application of heat. During curing, the low pressure may be applied, if necessary.
This process is used to prepare plastics of beautiful colours and it is most suitable for cellulose plastics. The optical properties of transparent plastics are much better, if they are cast. Apart from moulding the useful products, the casting is also widely used for potting and encapsulation, particularly in the electrical industry.
In this process, the thermo-setting resins are just applied on sheets of paper, asbestos, cloth, wood, glass, fibre, etc., and they are subjected to heavy pressure by allowing them to pass through rollers to form plastic laminates. The thickness of sheets varies from 0.12mm to 15mm. They possess excellent mechanical and electrical properties. Due to the pleasing finish surface, they are used for ornamental and decorative purposes.
This is the most commonly adopted process for the fabrication of plastic articles. The general process consists in placing the raw materials in a mould and then heating it. The moulding can be done by various methods such as compression moulding, extrusion moulding, injection moulding, jet moulding and transfer moulding. The choice of moulding method will depend on the article to be prepared.
These methods are briefly described as follows:
(i) Compression Moulding:
In this method, the moulds to receive the plastic material are prepared. The moulds are usually heated and then the plastic material is placed in the moulds. The moulds are closed and they are heated to a temperature of 100°C to 200°C under a pressure of 10 to 50 N/mm2. The plastic material gets the shape of moulds on account of heat and pressure.
In case of thermo-plastic, the moulds are cooled before the articles are taken out. Thus the moulds are to be heated and cooled alternatively in the preparation of thermo-plastic articles. Thus, for production of thermo-plastics, this method proves to be uneconomical as considerable time is lost in cooling the moulds. In case of thermo-setting plastics, it is not necessary to cool the moulds as articles of such plastics get the shape due to chemical action.
(ii) Extrusion Moulding:
In this method, the resin powder is fed through hopper at the inlet end of the revolving screw. At the outlet end, the material is heated and Extrusion moulding it is extruded or forced through a nozzle as shown in fig. 16-6. The plastic material as it comes out from nozzle is received in moulds and it is cooled with air jets or water bath.
The method of extrusion moulding is adopted for thermo-plastic resins to form continuous lengths of narrow ribbons, sheets, pipes, rod, etc. The method is also sufficiently versatile to be adopted for the production of single items such as bottles.
One of the recent developments in extrusion moulding is the co-extrusion moulding in which different materials or various combinations of the same material are extruded simultaneously to produce a laminar composite.
It is used for structural reasons to strengthen an inexpensive but weak plastic with a thin coating of stiff but expensive plastic. It can also be adopted to develop decorative effect by coextruding plastics of different colours.
The principle of extrusion moulding is simple and hence it forms a large percentage of the plastic processed throughout the world.
(iii) Injection Moulding:
This is comparatively a modern method of moulding. The plastic material is loaded, heated and then injected into the mould. It is then allowed to cool before being taken out from the mould.
As shown in fig. 16-7, the resin powder is allowed to fall through a hopper and it is then pushed by a piston into a hot cylinder. The plastic material is melted and it is then forced to fall in the cool mould under a pressure of about 160 N/mm2 through nozzle. The article gets the shape of mould and becomes solid.
The process of injection moulding is very much suitable for thermo-plastic resins. The complete process is automatic and the articles can be prepared within 10 seconds to one minute. It is thus adopted to prepare the plastic articles of small size on a large scale.
The injection moulding is thus a cyclic process and a high dimensional accuracy can be achieved in most materials although account must be taken of factors such as moisture absorption, post-moulding shrinkage, etc.
The injection moulding machines are usually rated according to their shot size and clamping force. The quality of mould to be used also plays an important role for the success of the moulding operation.
(iv) Jet Moulding:
In this method, the plastic material is moderately heated. It is then allowed to pass through nozzle, which is preheated to a high temperature. This method of moulding may be adopted for thermoplastics as well as for thermo-setting materials.
(v) Transfer Moulding:
When the process of injection moulding is applied to the thermo-setting resins, it is known as the transfer moulding. In this process, the moulds are also heated before the plastic material is injected through the nozzle. Thus the thermo-setting resins are heated in this process in two chambers, namely, cylinder and mould.
The pressure on mould is maintained till the chemical action to prepare the plastic article is completed. The plastic materials are removed from the moulds either mechanically or manually.
7. Properties of Plastics:
To appreciate the importance of plastics as an engineering material, it will be interesting to study its some of the outstanding general properties. It may however be remembered that each plastic material has its own peculiar properties to suit its particular uses. The success of plastic as an engineering material for a particular purpose will depend upon the correct choice of the variety of plastic.
Following are the properties of plastics:
Some plastics are completely transparent in appearance. With the addition of suitable pigments, the plastics can be made to have appearance of variety of attractive, opaque, stable and translucent colours.
(2) Chemical Resistance:
The plastics offer great resistance to moisture, chemicals and solvents. The degree of chemical resistance depends on the chemical composition of plastics. Many plastics are found to possess excellent corrosion resistance. Hence they are used to convey chemicals.
(3) Dimensional Stability:
This property of plastic favours quite satisfactorily with that of other common engineering materials.
The plastic lacks ductility. Hence its members may fail without warning.
The plastics are quite durable, if they possess sufficient surface hardness. The plastics, especially thermo-plastic varieties, are likely to be attacked by termites and rodents. But the danger of such an attack is not very serious due to the fact that the plastics have no nutritional value.
(6) Electric Insulation:
The plastics possess excellent electric insulating property. They are far superior to ordinary electric insulators.
Any surface treatment may be given to the plastics. It is also easy to have technical control during its manufacture. It results in mass production of plastic articles with uniformity of surface finish.
The plastics are organic in nature and hence all plastics are combustible. But, depending upon the structure, the resistance to high temperature and fire varies considerably among various varieties of plastics. The cellulose acetate plastics burn slowly. The polyvinyl chloride plastics are non-inflammable. The phenol formaldehyde and urea formaldehyde resist fire and they are used as fire-proofing materials.
The plastics can be easily fixed in position. They can be bolted, damped, drilled, glued, screw-threaded or simply push-fitted in position.
The properties of plastics are governed to some extent by humidity. The strength of plastics containing water attracting groups such as cellulosic plastics is considerably affected by the presence of moisture. On the other hand, the plastics which do not contain water attracting groups such as polyvinyl chloride plastics, offer great resistance to the moisture.
It is easy to maintain plastic surfaces. They do not require any protective coat of paints.
(12) Melting Point:
Most of the plastics have low melting point and the melting point of some plastics is only about 50° C. They cannot therefore be used in positions having high temperatures or to convey boiling water. In general, it can be said that the coefficient of thermal expansion of plastics is about ten times than that of steel.
The thermo-setting varieties of plastics are less susceptible to heat than thermo-plastic varieties of plastics. To improve the resistance of plastics to heat, the glass fibre reinforcement may be added in the structure of plastics.
(13) Optical Property:
Several types of plastics are transparent and translucent.
The most environmentally aware people condemn the use of plastics for the amount of pollution caused by them in disposal. However this is not a serious problem in comparison to the waste and pollution generated by a host of ether industries.
The plastics used for soft-drink bottles, milk and juice bottles, bread bags, syrup bottles, coffee cups, plastic utensils, etc. can be conveniently recycled into carpets, detergent bottles, drainage pipes, fencing, handrails, grocery bags, car battery cases, pencil holders, benches, picnic tables, roadside posts, etc.
(15) Sound Absorption:
The acoustical boards are prepared by impregnating fibre-glass with phenolic resins. This material has absorption coefficient of about 0.67.
An ideal section of plastics for structural member has yet not been designed. The plastics are reasonably strong. The strength of plastics may be increased by reinforcing with various fibrous materials. The plastic members can be used as tensile members as their strength to weight ratio in tension very nearly approaches to that of metals.
But the following considerations are responsible to discourage the use of plastics as the structural material:
(i) The plastics are costly.
(ii) The plastics are subject to creep under constant heavy loads.
(iii) The behaviour of plastics is very sensitive to the changes in temperature.
(iv) The stiffness of plastics is very poor.
(17) Thermal Property:
The thermal conductivity of plastics is low and it can be compared with that of wood. The foamed or expanded plastics are among the leading thermal insulators.
(18) Weather Resistance:
Only limited varieties of plastics can be exposed to weather. The important group of plastics which can resist weather effects is one prepared from phenolic resins. The certain plastics are seriously affected by ultraviolet light in the presence of sunlight.
The resistance to sunlight of such plastics can be improved by incorporating fillers and pigments which absorb or reflect the ultraviolet light at the surface. Thus the interior of plastics is protected.
The plastics, whether thermo-plastic or thermo-setting, have low specific gravity, the average being 1.30 to 1.40. The light weight of plastics reduces the transport costs and facilitates fixing.
The Indian Petrochemicals Corporation Ltd. (IPCL), near Baroda, has put a commercial brand of plastics, known as the ‘Koylene’. It is the lightest of all commercial known plastics and it is available in a wide range of grades.
It is tailor-made to suit various applications such as automotive and scooter parts, box strappings, industrial woven fabrics, ball pen refills, drinking straws, etc. This material possesses rigidity, good gloss, ability to withstand temperatures upto 100°C and easy processibility.
8. Uses of Plastics:
The uses of particular plastics are mentioned while discussing thermo-plastic resins and thermo-setting resins. There are more than 10000 different kinds of plastics available in the market and their performance abilities span those of every other known material from soft rubber to steel.
The typical uses of plastics in building are summerised below:
(i) Bath and sink units,
(ii) Cistern ball floats,
(iii) Corrugated and plain sheets,
(iv) Decorative laminates and mouldings,
(v) Electrical conduits,
(vi) Electrical insulators,
(vii) Films for water-proofing, damp-proofing and concrete curing,
(viii) Floor tiles,
(ix) Foams for thermal insulation,
(x) Joint less flooring,
(xi) Lighting fixtures,
(xii) Overhead water tanks,
(xiii) Paints and varnishes,
(xiv) Pipes to carry cold water,
(xv) Roof lights,
(xvi) Safety glass,
(xvii) Wall tiles,
(xviii) Water-resistant adhesives, etc.
The plastic sanitary fittings like taps, showers, basins, float balls, flushing cisterns, gratings, etc. are now available and various BIS specifications have been formulated for these products. The user of these products helps in conserving cement, steel and non-ferrous metals. These products are economical, resistant to corrosion, easy in installation and light in weight.
9. PVC Pipes in Buildings:
The PVC or polyvinyl chloride is the most versatile plastic and the use of PVC pipes in buildings is becoming popular day by day. It is possible to substantially change or modify the properties of PVC resin by the technique of compounding i.e., addition of other additives to PVC.
It is thus possible to prepare a PVC rigid pipe. The other applications to which PVC can be put up include footwear, bottles, gramophone records, water-stops, cables, making toys, bags, tubes, floor coverings, etc.
The use of PVC for doors and windows is also going to be popular because of the following advantages offered by PVC doors and windows:
(i) They are totally rust-proof, rot-proof, termite-proof and water-proof.
(ii) They are unaffected by coastal saline air, dry heat, sub-zero temperatures or tropical rains.
(iii) They do not fade, corrode, flake or warp and consequently, require no maintenance. All that is needed is an occasional cleaning with ordinary- soap and water.
(iv) They provide an alternative to wood.
(v) They provide better thermal insulation and may be considered as ideal for air-conditioned and heated rooms.
(vi) They restrict dust penetration through openings and hence prove ideal for operation theatres, computer rooms, food processing plants, electronics factories, pharmaceutical plants, etc.
The advantages of PVC pipes over conventional pipes of asbestos cement, cast-iron and galvanized iron can be summarised in a tabular form as shown in table 16-1.
The advantages of PVC pipes can be summarised as follows:
(i) They have good insulating properties and hence the temperature of water passing through such pipes is not affected by the outside temperatures.
(ii) They permit high, smooth and undiminished flow of water.
(iii) They have no problems of incrustation.
(iv) They possess high Hazen Williams Constant and it results into adoption of smaller size of PVC pipes as compared to the sizes of pipes of other conventional materials under similar conditions.
(v) They prove to be economical as compared to other pipes of conventional materials such as asbestos cement, cast-iron and galvanized iron.
(vi) They provide resistance to a variety of chemicals.
Following are some of the disadvantages of PVC pipes:
(i) They are liable for creep phenomena requiring closer spacings when installed above ground level.
(ii) They cannot be used at high temperatures as they are basically thermo-plastic. The recommended range of temperature for pressure applications is – 1°C to + 49°C and for non-pressure applications, the higher temperature upto 80°C can be used.
(iii) They do not have the same strength as cast-iron or galvanized iron pipes.
(iv) They possess higher coefficient of expansion as compared to the cast- iron or galvanized iron pipes.
It may however be noted that the above drawbacks of PVC pipes are not very serious and they are now widely used as pressure pipes for rural water supply, electrical conduits, telephone ducts, tube well castings, etc.
Following precautions should be taken in the design and installation of PVC pipes for their better performance:
(i) The design of PVC pipes should accommodate adequate provisions in respect of appropriate air vents, etc.
(ii) The fittings such as tees, elbows, caps, etc. used in PVC piping system should fit well with the pipes. If such fittings are too tight, they will develop stress in the system and if they are loose, they will cause leakage of water through pipe system.
(iii) The PVC pipe system should be suitably tested after installation and before putting the system in service.
(iv) The trenches for laying PVC pipes should be as narrow as possible and at the same time, they should be wide enough to allow convenient installations of the pipes.
(v) The turbulent flow of water through PVC pipes should be avoided.
(vi) They are available in different colours. But it is advisable to avoid red and black colours on external exposed walls.
(vii) They should be provided with sufficient supports and additional supports in the form of hangers, anchors, etc. should be freely used to eliminate external stresses.
(viii) They should not be used at places likely to be subjected to heavy loading.
(ix) They should not be bent too much to avoid any stresses in them.
10. Biodegradable Plastic:
Biodegradable plastic decomposes in the natural environment. It is produced from biopolymer called polyhydroxyalkanoate (PHA). This material is completely biodegradable. Biodegradation of plastics can be achieved by enabling microorganisms in the environment to metabolize the molecular structure of plastic films to produce inert humus like material that is less harmful to the environment.
They may be composed of either bio-plastics, which are plastics whose components are derived from renewable raw materials, or petroleum-based plastics. The use of bio-active compounds compounded with swelling agents ensures that, when combined with heat and moisture, they expand the plastic’s molecular structure and allow the bio-active compounds to metabolise and neutralize the plastic.
Advantages and disadvantages of biodegradable plastic: Under proper conditions biodegradable plastics can degrade to the point where microorganisms can metabolise them. This reduces problems with litter and reduces harmful effects on wildlife. However degradation of biodegradable plastic occurs very slowly. Proper composting methods are required to degrade the plastic, which may actually contribute to carbon dioxide emissions.
Degradation of oil-based biodegradable plastics may contribute to global warming through the release of previously stored carbon as carbon dioxide. Starch-based bio-plastics produced from sustainable farming methods can be almost carbon neutral.
Biodegradable plastics cannot be mixed with other plastics when sent for recycling. This damages the recycled plastic and reduces its value.
Fully biodegradable plastics are more expensive. Semi-biodegradable plastics are also available. These plastics can be manufactured to be clear or opaque, and in any colour. A disadvantage of this approach is that the products of degradation of the conventional material will remain in the environment for years.
11. Shortcomings of Plastics:
The shortcomings of plastics can be summarized as follows:
(i) Most of the plastics possess low heat resistance.
(ii) The plastics are not very hard.
(iii) The plastics disintegrate gradually and because of the effects of light, air and temperature, they lose strength, become soft and get dull, as time passes.
(iv) The plastics exhibit high creep.
(v) The plastics have a high coefficient of thermal expansion. It varies from 25 x 10-6 to 120 x 10-6 as compared to 11 x 10-6 of steel.
The plastic has proved to be a versatile building material of recent times and the properties of various types of plastics make them suitable for wide range of engineering applications. It embraces a very wide and extending range of hard and horny products which are fast replacing wood, glass, porcelain, steel and even aluminium.
The plastics in the building industry were first introduced in the early fifties in West Germany in the form of door and window profiles. There has been a steep rise in the production of plastics. From a mere 30 million kN in 1955, it has touched 1000 million kN at present.
It is estimated that on an average 25 per cent of the total plastic production in the world is used by the building industry. The per capita consumption of plastics in the developed countries ranges from 500 N to 1000 N while in our country, it is only about 2 N. There is however now increasing awareness regarding the utilization of plastic as a useful building material in our country.