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Essay on Plastics
Essay Contents:
- Essay on the Introduction to Plastics
- Essay on the Classification of Plastics
- Essay on the Raw Materials of plastics
- Essay on Important Plastics
- Essay on Laminating Plastics
- Essay on the Moulding of Plastics
- Essay on the Important Properties of Plastics
- Essay on the Joining of Plastics
- Essay on the Applications of Plastics
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Essay # 1. Introduction to Plastics:
Today plastics play a dominant role both for industrial and domestic applications because of their excellent properties and merits. These are very popular because of their high specific strength and stiffness (as a result of low relative density), corrosion resistance, good electrical and thermal insulating properties, low coefficient of friction, toughness and resilience with good vibration damping capacities, ease of fabrication and inexpensiveness compared to other metals on volume basis.
However, they have some limitations like low operating temperatures, high thermal expansion, low strength in comparison to metals, and in-flammability. The whole range of plastics is based on various ways of combining carbon, hydrogen and oxygen atoms to produce molecules which have characteristics quite different from other elements.
Plastics may be defined as organic materials [containing a synthetic high polymer as the major constituent; polymers being materials of high molecular weight formed by joining together many (poly) small molecules] that can be easily moulded or shaped by mechanical or chemical action to give non-crystalline substances that are solid at ordinary temperatures. Plastics may simply be defined as materials made up of long-chain molecules based on carbon and hydrogen. The basic unit of a long-chain molecule is a mer.
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The structure of one type of mer is:
It has 2 carbon bonds unused. If we add 2 more hydrogen atoms we produce methane, but by joining it to other mers we create a chain.
These chains can be very long, containing 103 to 105 mers. If the first and last mers are rendered stable by adding a hydrogen atom to the unused bond, a plastic material known as polyethylene is formed. Thus polyethylene can be written as CnH2n+2 (n being 104 to 105).
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There are many other possible ways of building up a long-chain molecule. Each different combination gives rise to a characteristic set of properties. Plastics lead themselves to wide applications because of their toughness, water resistance, ease of fabrication and remarkable colour range.
The plastics are derived from two sources, i.e. natural and synthetic. The plastics which are available as such are called natural resins. The important natural resins are lac, resin, casein etc. The plastics which do not occur in nature but are prepared artificially are called synthetic resins. Urea and phenol formaldehydes, polystyrenes, acrylic plastic etc. are examples of synthetic plastics.
The plastics are formed either by the process of polymerisation or condensation, or both, Polymerisation is defined as the formation of high molecular weight hydrocarbon from low hydrocarbons under the excessive temperature, pressure and catalytic action. Cross linking of two or more polymers is known as co-polymerisation.
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Condensation is the formation of high molecular weight compounds from the lower ones by combination and elimination of a molecule, usually of water.
C6H5OH + CH2O + C6H5OH — C6H4OH — CH2 — C6H4OH + H2O
Essay # 2. Classification of Plastics
:
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Plastics are usually synthetic organic materials, resins in solid form. However there are some inorganic plastics also like phosphorus polymers, ceramo-plastics, silicones, glass resins etc.
The plastics may be classified into various ways depending upon their basic raw materials, their distinctive properties and the broad applications. From the engineering point of view they can be divided into two branches, i.e. thermosetting plastics and thermoplastic plastics.
i. Thermosetting Plastics:
They are formed from the intermediate products which under the influence of heat, pressure, etc. undergo chemical changes of condensation and polymerisation to form a rigid final shape which is unaffected by heat or solvents. The thermosetting resins owe their characteristic hardness, rigidity, and heat resisting properties due to cross-linked net two structure—three dimensional molecules.
The long molecular chains are not only interlinked but also bonded together by additional covalent bonds (cross- linked). Phenol formaldehyde, urea formaldehyde and melamine formaldehyde are important thermosetting plastics. Other examples are polyesters, silicons, epoxy resins, phenolic, epoxides, urethanes, elastomers, etc.
The molecular structures can be modified and alloyed by forming copolymers to give desirable characteristics. Properties can be further modified by other chemical additives and reinforcing materials.
ii. Thermoplastic Plastics:
These contain linear to branched long-chain molecules which are not interconnected. They are obtained from the substituted derivatives of ethylene which can be made to polymerise under the influence of heat and catalyst. These consist of long molecular chains entangled with one another, but not actually bonded together.
They have high inherent plasticity which increases as the temperature is raised. These materials are softened by heat and affected by certain solvents. They do not undergo chemical changes when heated and on cooling their plasticity is retained because the structure is unchanged. The plastic behaviour is dependent on composition and therefore the structure.
A notable feature of these resins is the ability of their scrap or rejects to be reworked along with the new material. Some common thermoplastics are polyethylene, P.V.C., polystyrene, polypropylene, nylon. These can be further divided into cellulose plastics and non-cellulose plastics.
Important cellulose plastics are:
Polyethylene, polycarbonates, polyolifins, fluoro- carbons, acetals, acrylic and asphalt are other examples.
These recover visco-elastic strains during a period of time after unloading and are thus best suited for shrink fit packaging.
Essay # 3. Raw Materials
of Plastics:
The common basic raw materials are coal, petroleum, cotton, wood, gas, air, salt and water.
Plastics are made from the moulding composition which is prepared from two or more of the following materials:
i. Binder:
These are natural or synthetic resins: cellulose derivatives are good binders.
ii. Filler:
Fillers are added to reduce cost, shrinkage, brittleness and for improving finish and tensile strength. The fillers commonly used are wood flour, cotton fibre, paper, pulp, graphite, carbon black, mica and some metallic oxides like zinc oxide, lead oxide. Percentage used is generally 50%.
iii. Plasticizers:
Plasticizers are added to improve plasticity, toughness, flexibility and resistance to temperature and moisture. Commonly used plasticizers and camphors, esters of pethalic, oleic acids. Tetrabutyl phosphate and triphenyl phosphate are fire retardants. Percentage of plasticizers varies from 10 to 60%.
iv. Colour:
Dyes and pigments are used to provide colour to the plastics. The colouring matter should be resistant to sunlight.
v. Catalyst:
They accelerate the condensation and polymerisation and hence formation of thermosetting resins. Resins of vinyl group use peroxide to initiate polymerisation.
vi. Lubricants:
These are employed to make the moulding of plastic easier and to impart flawless finish Stearates, oleates and soaps are utilised as lubricants.
Essay # 4. Important Plastics
:
i. Polymers:
In finished form, plastics consist of long chain molecules called polymers.
Some commercially, readily available polymers are:
Acrylics (Polymethyl methacrylate), Acetals (Polyformaldehyde), Nylons (Polyamides), Phenolics (Polyphenolformaldehyde), PVC (Polyvinyl Chloride), PVDC (Polyvinylidene-chloride), Fluorocarbons (Polytetrafluoro- ethylene), Polyethylene, Polyurethane, Silicones, Epoxy, Polyesters, Polycarbonate, ABS (Acrylonitrile Butadiene Styrene), Polyimide.
The important characteristics of polymers are:
Compared to metals, plastics expand with temperature at a rate many times greater than a metal.
The stiffness of plastics is much lower than the stiffness of metals. They permanently deform under load at room temperature and at stresses below their yield strength. Mechanical properties are usually significantly lower than those of metals.
The mechanical properties of plastics are significantly reduced at moderate temperature.
Plastics can be very notch sensitive.
The properties of plastics can be highly anisotropic.
Some plastics can absorb significant amounts of moisture and change size.
ii. Thermoplastics:
(i) Natural Resins:
Naturally occurring shellac, asphaltic and bituminous material when mixed with some suitable fillers (mica, asbestos), are used as plastics. They have low dielectric strength, are hard and have low thermal conductivity. They are used as binding materials for grinding wheels, phonographic discs etc.
(ii) Cellulose:
Cellulose are converted into ester nitrates in the presence of catalyst.
(a) Cellulose Nitrate:
These are prepared by the process of nitration of cellulose.
Concentrated HNO and little H2SO4 are used:
The cellulose nitrate is mixed with camphor and colouring matter and pressed in moulds. It is tough but brittle and water resistant. It is transparent. On exposure to air, it gets decolourised and becomes brittle. It is inflammable and affected by acids and alkalies. It is used in toilet materials, pens, tooth brushes, radio dials, drawing instruments, etc.
(b) Cellulose Acetate:
It is prepared by the action of acetic acid, acetic anhydride on cellulose in the presence of H2SO4 (catalyst)
Cellulose triacetate is then hydrolysed to give mono and diacetate which are commonly used after mixing with plasticizers, and dyes. The cellulose are more stable to heat and light and are less inflammable.
Other properties include mechanical strength, impact resistance, transparency, colorability, fabricating versatility, mouldability and high dielectric strength. Some of its uses are automobile steering wheels, handles, windows, goggles, musical instruments and radio appliances, control boards, etc.
(c) Non-Cellulose:
These are tough, non-inflammable, chemically inert and water and electric resistant. These are light and transparent.
These esters are polymerised to give acrylic plastics in the presence of Na2O2. Applications are cockpit canopies and wall partitions.
(d) Polyethylene:
These are converted into sheets, tubes, tapes and foils. These are tough, translucent, light, colourless, have low tensile strength and conduct electricity partially. These are resistant to attack of water, acid and alkalies. These are used for packing materials.
(iii) Polystyrene:
These are obtained by the polymerisation of styrene prepared from benzene and ethylene.
Polystyrene are transparent, brittle and have high refractive index and insulating capacity. They are unaffected by water, acids and alkalies. They are mainly used for the manufacture of lenses, television and refrigerator insulation, vats for chemical and hydraulic equipment’s.
(iv) Vinyl plastics:
Polyvinyl resins are synthetic materials made from the compounds having (—CH = CH2) group.
A resin of more importance is copolymer of vinly acetate and vinly chloride.
Polyvinyl Acetate:
It is colourless, transparent, resistant to water, chemicals and atmospheric oxygen. It is used for adhesives etc.
Polyvinyl Chloride:
It is tough, resistant to water and chemicals, light. Mainly used for cable coverings, coated fabrics, tank linings etc.
Copolymerised Aetate Chloride:
i. It is non-warping, non- shrinking, tough, strong, rigid, resistant to chemicals.
ii. It is used for floor tiles, cable coverings, belts, raincoats, watch straps, radio dials, sound records etc.
Effect of Time and Temperature on Properties of Thermoplastics:
In general, thermoplastic exhibit high strength and brittle behaviour at low temperatures, and lower strength and ductile performance at higher temperatures.
Fig. 4.17 shows a curve between modulus or tensile strength versus temperature. It would be seen that at low temperatures between A and B Hooke’s law is followed, the material is glassy and has low ductility.
As the temperature is increased, the bonds between molecules are overcome by thermal agitation and the flow becomes viscous. At point B, the side chains loosen; some main chain movement occurs; material shows mixed elastic and viscous behaviour. At point C main chain loosens and material becomes rubbery. At point D, thermal excitation overcomes bonds and material flows.
It would be seen that viscoelasticity (combination of elastic and viscous flow characteristics) is associated with polymers, i.e. their interdependence between time, strain and temperature. When polymers are strained, their molecules move and such movement takes time.
iii. Thermosetting Plastics:
i. Phenol Formaldehyde:
These are most commonly used plastics. Phenol and formaldehyde are made to condense in the presence of acid catalyst to produce hydroxyl benzyl alcohol.
Phenol then again combines with alcohol at a temperature of 150°C. The reaction is completed in 3 hours.
Water is removed by vacuum
It is mixed with fillers, dyes and (CH2)6 N4 catalyst and heated by steam pipes to be dried to a powdery mass. These can be worked by compression moulding. Tubes and rods are made by extrusions. The plastics produced are cheap, insulating and are immune to atmospheric oxidation. It is mainly used for radio and television cabinets, switch boxes and other electrical instruments.
The following reactions take place in the presence of basic catalyst:
The cast plastics are hard, heat resisting and can be worked on machines. These are mainly used for the manufacture of scientific instruments, decorative articles, gears and bearings, etc.
ii. Amino-Aldehyde:
Urea formaldehyde and melamine-formaldehyde are commercially important amino- resins.
Essay # 5. Laminating Plastics
:
Laminating plastics comprise sheets of paper, fabric, asbestos, wood or similar materials, which are first impregnated or coated with resin and bonded together by heat and pressure to form commercial materials.
These materials are hard, strong, impact resisting, unaffected by heat or water and have good machining characteristics which permit is fabrication into gears, handles, bushings, furniture and many other articles. Laminations are classified into three categories depending upon the pressure required to cure the resin in manufacture.
Laminations cured at 0 to 2 kgf/cm2 are called contact pressure laminations. Those cured at pressures below 27 kgf/cm2 are called low pressure laminations and those cured between 80 to 140 kgf/cm2 are referred to as high pressure laminations. Commercially laminates are available in sheets, rods, tubes and special shapes. Among these, sheet form is more common.
i. High Pressure Laminates:
Phenolics, melamines, ureas and silicons are the most commonly used resins in high pressure laminates. Commercially they are available in sheet, rod, tube and some simple moulded parts, such as refrigerator’s inner doors etc.
Firstly, the resinoid material is dissolved by a solvent to convert it into liquid varnish. Rods of fabric paper are then passed through a bath for impregnation. They are then passed through a drier which evaporates the solvent, leaving a fairly stiff sheet impregnated with the plastic material.
The whole of operation is a continuous process and is shown in Fig. 4.8. The sheets are cut into convenient sizes and staked together by hydraulic press, sufficient to make up the desired thickness of the final sheet. Under action of heat and pressure, a hard rigid plate having desirable properties for many industrial applications is formed.
For the production of tubes, the paper or the fabric is rolled on a steel mandrel of desired diameter and then placed within the steel mould for the final processing as shown in Fig. 4.9. In the production of rods, the paper or fabric is rolled over the mandrel of very small diameter, which is withdrawn before the material is placed in the mould.
High pressure laminations are chiefly composed of cotton-cloth, paper, asbestos and glass fabrics. Heavy cloth laminates are used for gear blanks, cams and other industrial purposes. Electrical insulation and punch stock materials are made from paper laminates.
ii. Low Pressure Laminates:
Commercially available low pressure laminates are plywood and glass reinforced polyester parts. Now-a-days, its application is also extended to sandwich-type construction such as foam, resin bonded paper or balsa wood which gives superior strength and weight.
The following points highlight the four main methods used for moulding of plastics in industries. The methods are: 1. Compression Moulding 2. Transfer Moulding 3. Injection Moulding 4. Extrusion Moulding.
Essay # 6. Moulding of Plastics:
i. Compression Moulding:
In compression moulding and transfer moulding the monomers are partially polymerised in a separate operation and the polymerisation reaction is completed in the mould. The partially polymerised material is prepared as pallets for compression moulding.
It is placed in a heated mould. After the compound is softened and becomes plastic, the upper part of the die moves downwards, compressing the materials to the required shape and density; continuous heat and pressure produce the chemical reaction, leading to cross linking between the molecule chains, that hardens the thermosetting material.
The mould remains closed until the curing is complete. Compression moulding is suitable for large, bulk parts of both thermosetting and thermoplastic materials and for small parts of thermosetting materials. Thick sections can be a problem as centre may not be properly heated and the curing action may be incomplete. The moulding compound may be in granular form or in a performed slug.
The moulding temperature of thermosetting materials ranges from 150°C to 180°C. The moulding pressure ranges from 135—535 kgf/cm2.
The temperature and pressure for thermoplastic materials depend upon type of material and percentage of plasticizer. The time required to harden the mould piece ranges from 1 to 15 minutes depending upon the maximum thickness of the moulded article and the cooling facilities in the die.
ii. Transfer Moulding:
The thermosetting moulding power is prepared by mixing filler with resin which is only partly condensed and makes it capable of being softened on re-heating. The resin and filler are mixed together on heated rolls until the desired characteristics of the moulding powder are attained.
The most commonly used fillers are wood, flour, nylon and glass. For greater resistance to impact, filler used may be chopped fabric and for resistance against heat, asbestos may be used. The filler can also be impregnated with resin syrup, drying and partial cure being effected in an oven.
In transfer moulding the following points are to be borne in mind while designing:
(j) Material should flow easily in the mould to take the required shape.
(ii) All parts of the charge must be heated so as to harden, i.e. none of the materials should harden before other parts of the charge had an opportunity to flow into the correct position.
When sections are thick, it becomes necessary to preheat the charge of material before placing it in the mould.
(iii) Facilities for removing the product from the mould without damage.
Transfer moulding is generally employed for thick sections. In this process charge of moulding powder is forced from a cylinder through a relatively restricted opening into the main mould. Thus due to flow through restricted passage, the moulding powder becomes heated much more rapidly and uniformly than by conduction from the walls of mould.
A high pressure of the order of 700 kg/cm2 is applied in order to counteract the pressure exerted by steam and other volatile substances and thus avoiding the cavities formation within the product.
In conventional transfer moulding, on auxiliary plunger is used and the material is displaced from the well into cavities when the upper part of the mould presses the lower one. It is generally preferable for delicate mould sections.
However, transfer equipment i.e., heating of mould or separate container for heating can be eliminated by utilising high frequency heating of the performs.
In order to prevent the damage to the moulding in removing it from the mould, and to minimise frictional forces, usually a chamfer (about 1%) is desirable. Usually ejector pins are provided at suitable sections in order to facilitate the removal of product from mould.
Transfer moulding is useful for incorporating metal parts in the moulding.
iii. Injection Moulding:
It makes use of heat softening characteristics of thermoplastic materials. These materials soften when heated and reharden when cooled. No chemical change takes place when the material is heated or cooled, the change being entirely physical. For this reason, the softening and rehardening cycle can be repeated any number of times.
The granular moulding material is loaded into a hopper from where it is metered out in a heating cylinder by a feeding device. The exact amount of material is delivered to a cylinder which is required to fill the mould completely. The injection ram pushes the material into the heating cylinder and in doing so pushes a small amount of heated material out of the other end of cylinder through the nozzle and screw bushing and into the cavities of the closed mould.
The material is cooled to a rigid state in the mould. The mould is then opened and piece is ejected out. The temperature to which the material is raised in the heating cylinder is usually between 180—280°C. The higher the temperature the lower the viscosity and more readily it can be pushed into the die.
Every type of material has a characteristic moulding temperature, the softer formulations require lower temperature, and the harder formulations require higher temperature. Intricate pieces, large pieces, several cavities in the die and long runners all tend to increase the temperature requirements. When the plastic material is pushed from nozzle end of the cylinder, it enters through channels into the closed mould.
In the majority of cases, the mould is kept cold, in order to cool the moulded articles ready to the point at which the mould can be opened and pieces ejected without distortion. This is done by circulating water through the mould frame. Sometimes, it is necessary to use a warm mould, and mould temperature as high as 150°C is used for very special jobs. However material sets faster in a cold die and the cycles are shorter.
The cooling of plastics under pressure is desirable to avoid “shrink” marks on the surface. Automatic devices are commercially available to maintain mould temperature at required level.
Advantages and Limitations:
Since speed is one main advantage in injection moulding, complicated moulds with inserts should be avoided, if possible. Injection moulding need not be single cavity, but the higher rate of production reduces the need for large number of cavities. Moulding normally requires no further machining.
The savings resulting from higher production rates are partially offset by higher capital expenditure for machines and higher operating costs. Injection moulding is generally limited to forming thermoplastic materials, but equipment is available for converting the machines for moulding thermosetting plastics and compounds of rubber.
In the case of thermosetting plastics the sprue and runner material is wasted as recycling of this product is not possible. Further it is difficult to mould parts with large variations in wall thickness.
Fig. 4.5 (b) shows in schematic form an injection moulding machine with its modular parts in detail and Fig. 4.5 (c) shows a completely automated injection moulding shop.
To be able to produce economically, it is essential to cut down set-up times during change-overs.
In a fully automated injection moulding shop, the entire injection moulding process operates completely automatically and includes:
i. CNC closed loop controlled injection moulding machines
ii. Automatic raw material feed
iii. Robots for removal of finished products
iv. High speed mould clamping mechanism
v. High speed mould change over system
vi. Mould store and conditioning device for preheating of moulds
vii. High speed changing of the injection unit
viii. Production control by central computer
ix. Production data acquisition.
iv. Extrusion Moulding:
Extrusion process is a continuous operation in which powdered polymer or monomer, is fed by a screw along a cylindrical chamber. As the powder moves toward the die, it is heated and melts. The molten plastic is forced through a die opening of the desired shape. The material in granular form together with necessary additives is placed into a feed hopper which feeds the cylinder of the extruding machine.
The hopper portion is kept cool by circulating water in order to avoid pre-mature softening of the feed and a blockage in the supply system. The cylinder is so heated by electricity, oil or steam, that closely controlled temperature zones are set up along its length. A rotating screw is used for carrying and mixing of material through cylinder and forcing it through a die of required shape.
The screws are designed to suit each application. These vary in size and form.
Generally a screw has three zones, viz.,
(i) feed zone in which case screw core is of smaller size and fins (blades) are bigger to carry more low density stock,
(ii) compression zones in which screw core diameter gradually increases or the screw pitch decreases, and
(iii) metering stage.
The extruded shape (which may be in the form of rod, tube, section, or sheet) coming from the die is carried through a cooling medium by a conveyor and when it has been cooled sufficiently to retain shape, it is cut into lengths or coils.
In some instances, the material must be held to a shape during its cooling. Cooling is done by exposure to air at room temperature, by passing through a liquid bath at a controlled temperature, or by jets of compressed air. Too rapid cooling must be prevented as it causes warpage and sets some internal strains in the finished pieces.
The raw material must have a uniform particle size and a controlled moisture content to maintain close dimensional tolerances and smooth finished surface on the extruded product. The temperature of each heat zone of cylinder must be held constant to ensure good extrusion.
Depending on die design and opening, forms like meter wide sheets, solid and hollow sections of many forms, blown film to about 2 m diameter, wire covering and blow mouldings can be produced. Several take off and post extrusion operations are carried out to produce some modifications.
Advantages of extrusion are low initial cost, continuous production, introduction of anisotropy to provide high uniaxial strength. However some further work may be required in assembling the components. Extrusion die can also be used to produce a tube into which compressed air or nitrogen is blown to give a film bubble from which sheet of upto 0.7 mm thickness can be produced (Fig. 4.7).
Essay # 7. Important Properties of Plastics:
i. Use of Laminated Sheet:
Laminated sheet whether reinforced with fabric or paper is not an isotropic material. The properties in the plane of the sheet generally very because of preferred orientation of paper fibres, and the difference in warp and weft of fabric. This effect can however, be minimised by arranging the individual sheet alternately at right angles before laminating.
A more serious departure from isotropy is the lack of strength of the bond between laminations. This has to be taken into consideration in machining and designing. In the design of parts made from laminated sheets, it should be borne in mind that no shearing forces act along the plane of laminations.
The shear strength must act perpendicular to laminations. Further the tensile strength must act in the plane of lamination and tensile forces should not be acting perpendicular to laminations.
The laminated material does not possess ductility; therefore, the stress concentration should be avoided as far as possible. Its use becomes expensive where high laminating pressure is to be applied and is limited by the size of the part. However low pressure laminating is possible by moulding and the large parts can also be produced economically in that case.
ii. Reinforced Plastics:
A big disadvantage of thermoplastic and thermosetting plastics is that they lack in mechanical strength. This is overcome by the use of reinforced plastics which are particularly suitable where elastic stability is the limiting factor. At such places, reinforced plastics have an added advantage of less weight, thereby making parts light and resulting in a net saving.
Its use will be appreciated very much at places where very thin sheets of steel are required from strength point of view to support low loads. But the steel sheet will buckle due to very small thickness, though it may be supported by ribs even.
In such cases, thick sheet of reinforced plastic may be used due to its low density and thus the moment of inertia which is proportional to cube of thickness is increased considerably. In buckling, it is product of El which matters and it is more for reinforced plastics due to high value of I (moment of inertia).
iii. Gears:
Steel gears possess the disadvantage of noisy operation and less resistance to shock loading. For silent operation of gears and where high resistance to shock loading is required, reinforced plastic gears are used extensively.
Plastic gears are usually machined from fabric reinforced laminated blanks. Nylon gears are also very common in use and possess sufficient inherent toughness and are also self-lubricating. The plastic gears, however, can’t be stressed as much as steel gears.
iv. Bearings:
At various places plastic bearings result in huge savings in power and avoid lubrication problem also, because water may be used as lubricant. It has been shown that low viscosity of water coupled with its efficiency as a cooling agent render it a very efficient lubricant and also the seizure between metal to metal contact is avoided at low speeds and heavy loads.
The non-metallic bearings made of laminated phenolic with canvas or paper base are much stronger than metallic bearings. Graphite impregnation for lubrication is frequently used. The lubrication can be effected by either grease or water.
Water lubrication has made plastic bearings of particular value in food industries where the contamination from oil lubricated metallic bearings cannot be tolerated. Water lubricated bearings are also extensively used in mills where they have been found much suitable than metallic bearings.
To avoid corrosion of shaft when stationary for long time, some soluble oil may be passed through such a bearing.
v. Brake and Clutch Linings:
For brake and clutch linings such materials are required which have high coefficient of friction and minimum wear and resistance to high temperatures. The introduction of alcohol soluble phenol formaldehyde has resulted in stronger and more consistent product fulfilling the above requirements.
But with it, the coefficient of friction decreases considerably at high speeds. Modern practice is to embody a combination of resins to have the best resistance to wear, consistent with the maintenance of good braking performance at high temperature.
vi. Plastics in Foundry:
(i) It is used as a core binder in the form of linseed oil.
(ii) In shell moulding process.
(iii) In pattern making, the phenol formaldehyde acts as best metal to metal adhesive.
vii. Plastics for Low-Friction Applications:
Plastics find wide applications in mechanical components like bearings, bushings, slides etc.
The advantages of such plastics are:
i. Self-Lubrication even at very low temperatures,
ii. Vibration absorption and impact resistance,
iii. Less tendency for contamination,
iv. Anti-Welding characteristics,
v. Free from corrosive and abrasive action,
vi. Compatibility with oils and greases,
vii. Minimum galling and scoring,
viii. Design flexibility and maintenance free operation,
ix. High load-low speed operation is possible.
Plastics have the following disadvantages:
a. Less thermal conductivity,
b. High rate of thermal expansion,
c. Low temperature resistance,
d. Lower structural strength and rigidity,
e. Lower radiation stability.
Some low friction and wear plastics and their typical applications are given below:
viii. Plastics for Heavily-Stressed Mechanical Components:
Plastics also find application involving efficient and smooth operations like gears, cams, racks, coupling and rollers etc., and offer the following advantages viz., Quietness of operation, lightness and lower inertia, less lubrication needed, ease of manufacture, cheapness in quantity, less critical tolerances, operation in gritty, abrasive or corrosive conditions.
These have limitations of lower strength, lower resistance to deformation and creep, lower effective operating temperatures and water absorption.
Some typical plastics for high stress applications are given below:
ix. High Temperature Engineering Plastics:
Plastics find applications for high temperature uses as in brake linings, gaskets, hot air ducts, high temperature insulation flexible joints etc. The important characteristics to be considered are flexural and torsional creep, deflection temperature, heat resistance and glass transition temperature.
Polymers for high temperature applications must be pure to acquire heat stability, high molecular weight to give a high melting point, regular structure of chain, high Van-der-Waals forces to bind the molecules together, raising the melting points achieved mainly by the presence of polar groups.
Some typical plastics for high temperature applications are given below:
Essay # 8. Joining of Plastics:
i. Mechanical Fastening:
This is the simplest way to join plastic parts. In this method a fastening element (hinge, latch or detent) is formed into the parts to be joined and thus costs the least. Only the stronger, tougher plastics are suitable for this method since the joint must survive the strain of assembly, service load and possible repeated use. This type of fastening is suitable only for lightly loaded, non- rigid assemblies where precision is not critical.
Mechanical fasteners (screws, rivets, pins, sheet- metal nuts) are the commonly used joining methods. They require a plastic that is strong enough to withstand the strain of fastener insertion and subsequent high stress around the fastener. Threaded fasteners work best on thick sections.
Thread-forming screws are preferred for softer materials, while thread-cutting screws work best on the harder plastics. Push-on lock nuts and clips may be better for thinner sections. If a fastener is to be removed a number of times, metal inserts are recommended. They may be moulded in place, forced, glued or expanded into moulded or drilled holes, or inserted ultrasonically.
ii. Spin Welding:
Plastic parts can be joined by a technique similar to inertia welding used for joining metal parts. In the spin welding of plastics, one part is held stationary, while the other is attached to a spindle which is brought upto predetermined speed and then forced against the stationary part.
Parts thus fuse together under the heat generated by friction. One limitation of this method is that the rotating parts must be symmetrical. For joining a few pieces, arrangements can be made in workshop. For mass joining, more sophisticated equipment should be used.
iii. Solvent Bonding:
In this method of joining, plastics (thermoplastics only) are joined by softening them by solvent, and then clamping or pressing together. In this way plastic molecules intermingle and the parts bond together when the solvent evaporates.
The fusion time is a direct function of the solvent’s evaporation rate and may be shortened by heating. Pressure to be applied is also critical as too much pressure may distort the parts. This is of course a slow process.
iv. Ultrasonic Welding:
In this method two parts to be joined are placed together and the pulses are transmitted from a generator to the parts by a resonant vibrating tool (called horn) causing them to vibrate against each other at frequencies around 20 kHz. The parts are heated and fused together. This process is suitable only for thermoplastics with the exception of thermosetting resins and Teflon.
The pressure contact between the two parts is critical and it should be just sufficient to cause heating by friction. It is very fast process. The process requires fairly rigid materials. It is possible to join dissimilar metals also, provided both have same melting temperatures. It is best used to spot weld plastic sections.
v. Induction Welding:
In this method, two pieces of same thermoplastics to be joined together are pressed together with a metal wire or insert in the joint area (as shown in Fig. 4.15) and the high frequency (about 450 kHz) magnetic field switched on around it, which causes the encased metal to be heated up thereby melting the plastic, and the compression produces a good fusion weld. The metal remains inside the part.
In case it is desired not to add the metal piece into the joint, then metal powder may be added to the original plastic moulding, but a much higher frequency of about 3 to 5 MHz is needed to effect the weld. It is a high cost technique and is suitable for difficult to weld plastics such as polypropylene, and for shapes that can’t be fitted into an ultrasonic welding machine.
vi. Dielectric Welding:
This method finds applications in welding films and thin sheets in packaging process. It utilises the technique of breaking down the plastic under high voltages and frequencies (10 to 100 MHz) to produce dielectric heating and fuse the plastic. Welding speed is a function of dielectric loss factor, material thickness, and the area subjected to the impressed voltage.
vii. Hot-Platen Welding:
In this method, the thermoplastic is first softened contacting it with a heated tool and then pressing together. With films or sheet, the material is passed under a hot roller. The sticking between the hot tool and the plastic material is prevented by coating the tool with flourocarbon. Temperature control is critical so that the bond on part does not deteriorate.
A variety of this process is the hot- platen welding which can be used for welding large, irregularly shaped moulded or extruded parts. The parts to be joined are placed into fixtures. A heated platen is positioned between the parts and the edges to be joined are pressed against it (Refer Fig. 4.16). The platen is removed and the parts, with their edges now plasticised are pushed together. After a brief holding and cooling period, the fixture is opened and the completed assembly removed.
viii. Hot Gas Welding:
In this method the welding rod comprises a thermoplastic rod which is heated along with the parts to be joined by an inert gas until the parts soften and can be pushed together. This is a low speed process for fabricating large structural parts from sheet. In this method, the operator skill is very critical for both weld strength and appearance.
ix. Vibration Welding:
This is a new technique for joining plastics and produces pressure-tight joints in circular, rectangular, or irregularly shaped parts made from almost any thermoplastic material even in dissimilar materials having a melting temperature spread as great as 35°C. The process is particularly suited for hollow container type components having the weld joint in a single plane.
In this method, the friction heat is developed by pressing two plastic parts together and vibrating the parts at 120 cycles per second in the plane of the joint. After 2-3 seconds, vibration is stopped at the exact required relative position of the two pieces. Pressure is maintained briefly while the softened plastic cools. Joint strength is very nearly that of the parent material. The process is adaptable to fully automated systems.
Essay # 9. Applications of Plastics:
From 1920 to 1950, commodity thermosets had the maximum demand as well as production whereas the period 1950-1975 was mainly dominated by commodity thermoplastics. The scene today has engineering plastics, advanced composites and specialty thermoplastics, all vying for a place in the market. New polymers and refinements of the existing polymers are far ever increasing (Tables 4.1 and 4.2).
The overall percentage of tonnage consumption of plastics round the world today stands at about 36 per cent in packaging. 20 per cent in building, 10 per cent in electrical and electronics, 5 per cent in transport, 5 per cent in furniture, 4 per cent in toys and leisure, and others about 19 per cent.
The rapid growth in the usage of plastics has largely occurred because traditional materials have been replaced by newer ones. The cost of fabricating the item has been reduced as plastics lead to mass production techniques and this has stimulated consumption. The list of traditional materials and applications that have switched to plastics is enormous, a selection is shown in Table 4.3.
