In this article we will discuss about:- 1. Classification of Plastics 2. Moulding Processes of Plastics 3. Other Processes 4. Fiber Glass Reinforced Plastics 5. Compounding Materials 6. Polymer/Plastic Coatings 7. Testing of Plastics.
Contents:
- Classification of Plastics
- Moulding Processes of Plastics
- Other Processes of Transforming Plastics
- Fiber Glass Reinforced Plastics
- Compounding Materials
- Polymer/Plastic Coatings
- Testing of Plastics
1. Classification of Plastics:
A plastic in broadest sense is defined as any non-metallic material that can be moulded to shape. The most common definition for plastics is that they are natural or synthetic resins, or their compounds, which can be moulded, extruded, cast or used as films or coatings. Most of the plastics are of organic nature composed of hydrogen, oxygen, carbon and nitrogen. The synthetic plastic development dates from 1900 when Dr. Beekland announced the production of phenol-formaldehyde. Since then several new plastics have been developed.
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Plastics, most commonly, are classified as:
1. Thermoplastic, and
2. Thermosetting.
1. Thermoplastic:
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Thermoplastic materials are those which soften on the application of heat with or without pressure, but they require cooling to set them to shape.
They can be heated and cooled any number of times, only they should not be heated above their decomposition temperatures.
They are main long chain straight or slightly branched molecules and the chains are held close to each other by secondary weak forces of type van der Waal’s forces. During heating, as the temperature increases the secondary forces are reduced and the sliding of these long chain molecules can easily occur one over the other at a reduced stress level.
They are highly plastic and are easy for moulding or shaping.
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They have low melting temperatures and are not so strong as the thermosetting plastics.
Since they can be repeatedly used, they have a resale value.
Some commercial thermoplastics are- Polythene, Polyvinyl chloride (PVC), Polystre Polytetrafluroethylene (PTFE) etc.
2. Thermosetting:
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Thermosetting materials are those plastics which require heat and pressure to mould them tin shape.
They cannot be resoftened once they have set and hardened.
They are ideal for moulding into components which require rigidity, strength and some resistance to heat.
In general, resins formed by condensation are thermosetting.
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Thermosetting resins have three-dimensional molecular structure and have very high molecular weights.
Due to cross-linking thermosetting resins are hard, tough, non-swelling and brittle. Hence they cannot be softened or remoulded as in the case of thermoplastic resins. Moulding and casting are the processes often used with such materials.
Some important commercial examples of this type are- Phenolics, Polyesters, Epoxies, Silicones etc.
The difference between thermoplastic and thermosetting materials may be explained in terms of molecular structure. The thermoplastics are essentially long chain macromolecules with a limited number of cross links. When heated and compressed, the chains glide over each other and fluid materials take the shape of any mould in which they are placed.
The thermosetting plastics are characterised by strong cross links between the chains; once these are formed by heat and pressure, the plastics set to a rigid infusible solid. Figure below represents the two structures where R stands for monomer unit.
2. Moulding Processes of Plastics:
The main moulding processes are described below:
1. Shaping:
In this process of moulding, the hot plastic mass is spread over the shape and then allowed to cool and harden.
2. Compression Moulding:
The mould is filled with the required amount of moulding powder and then heat and pressure are applied until cure is complete.
This process is employed mainly for thermosetting resins.
The process can be speeded up by the use of transfer moulds in which the moulding powder is heated to the plastic stage and then forced into mould proper when the curing is completed. This has the advantage that only the exact amount of resin is fed to the mould and therefore, no flash is formed, thus eliminating the necessity of trimming the article. Also as the contents of the mould are in plastic condition when pressure is applied there is no danger of uneven distribution of pressure and development of strains.
3. Injection Moulding:
The pellets of thermoplastics are first compressed in the pressure chamber and then pushed into a heating chamber.
The softened material in flowing state is then forced with pressure (about 960 N/mm2) through a nozzle into a cold mould having cavity of the desired shape. Plastic articles of intricate shapes can be formed in these cavity moulds. The articles can be removed by opening the moulds.
The plastic mass is forced through a die, thus producing continuous lengths of tube or rod of the desired cross-section which can ultimately be cut upon machines as required.
In this method, the articles are moulded in cold condition and are then baked in an oven or cured by steam.
3. Other Processes of Transforming Plastics:
It is the process in which under the action of heat and pressure, layers of plastics or layers of other materials such as wood, fabric, metal etc. can be made into a composite, sandwiching a layer of plastics.
It is a process similar to glass blowing where various articles of plastics are made by blowing viscous mass of plastic material.
It is used for fast and cheap production.
This can be done manually or by automatic and semi-automatic machines. In automatic machines the temperature and blowing of plastics can be controlled for better quality products.
It is the production of sheet of materials by rolling the plastics between multiple rollers.
Plastic sheets, rods and tubes can be machined with ordinary lathe machines to obtain the desired shapes and sizes. These can also be cemented or riveted with suitable glues and riveting materials. Thermoplastics can also be welded by using certain solvents.
These are also called plastic laminates and are formed by impregnating sheets of fibrous materials such as paper, linen, canvas or silk with a synthetic resin and then compressing the sheets together with application of heat. The synthetic resins may be phenolic resin, urea formaldehyde or a vinyl resin. The resin is usually, dissolved in alcohol.
The material in roll form is immersed in the resin solution at atmospheric pressure at room temperature and then run through a drier at 150°C. The rolls are next cut into sheets of given size, which are arranged into stacks. These stacks finally are compressed in a hydraulic press at about 170°C under a pressure of 200 bar. The sheets are thus bonded to one another.
Properties:
The laminated plastics have the following properties:
(i) Light and strong.
(ii) Machinable.
(iii) Resistance to wear, acids and alkalis.
(iv) Impervious to water and oil.
(v) Have a high dielectric constant.
Uses:
The laminated plastics are used for:
(i) Electric insulation.
(ii) Making silent gears,
(iii) Water lubricated bearings.
(iv) Pulley wheels.
(v) Pump parts
(vi) Press tools.
(vii) Decorative purposes in wall paneling, translucent paneling and table and counterparts.
4. Fiber Glass Reinforced Plastics
:
The Fiber glass reinforced plastic (or FRP) is formed by using two materials in conjunction with each other to form a composite material of altogether different properties.
In FRP, the glass fibers provide stiffness and strength while resin provides a matrix to transfer load to the fibers.
Properties of FRP:
Following are the properties which have made the FRP the most commercially successful composite material of construction:
(i) Aesthetic appeal.
(ii) Dimensional stability.
(iii) Light weight.
(iv) Easy to repair.
(v) Durable.
(vi) Corrosion resistant.
(vii) Requires less energy for production.
(viii) Least maintenance required.
(ix) The tooling is inexpensive and fast.
(x) FRP products transmit a great deal of light.
Applications:
Following are the applications of FRP:
(i) Water storage tanks.
(ii) Roof sheets.
(iii) Domes.
(iv) Structural sections.
(v) Doors and window frames.
(vi) Concrete shuttering.
(vii) Internal partitions and wall paneling,
(viii) Temporary shutters.
5. Compounding Materials
:
In order to obtain the desired plastics certain materials are added with polymers (resins) to reduce the cost or to enhance mechanical properties.
These materials are described below:
The main purpose of binders is to hold other constituents of the plastic together, as the name implies.
A binder may compose of 30-100 percent of the plastic.
The binders may be synthetic or natural resins. Thus, resins are the basic binding materials in the plastic.
On the basis of the type of resin used in preparation, the plastic itself is called thermoplastic or thermosetting plastic.
Fillers are added to reduce the cost and enhance the strength and hardness of plastics.
These may be fibrous type like asbestos, glass fiber, mica etc., or non-firbous type, e.g., china clay, carbon black, talc, zinc oxide, calcium carbonate. Quartz and mica are used to improve hardness.
Inorganic fillers like asbestos are added to improve heat and corrosion resistance, shredded textiles are used to increase impact strength and so on.
The proportion of the filler added can be as high as 50 percent of the plastic,
Low molecular weight (approx. 300) materials blended with polymers are called plasticisers.
These are added to improve flexibility and processing of plastic articles, and to reduce the temperature and pressure required for moulding. However, a plasticiser may reduce the tensile strength and chemical resistance of plastics.
Some of the plastics are- Polyesters, epoxies, nitrile rubbers. These are normally chemically inert, non-volatile and nontoxic.
The percentage of plasticisers can be as high as 60 percent of the plastic.
If the plasticiser is added in excessive quantity, the bond will be mainly between the small molecules and we will have a liquid. In a common paint, excess plasticiser is added to make the paint liquid and allow brushing. After the paint is applied the plasticiser evaporates and the paint dries.
This evaporation is usually accompanied by some polymerisation and cross-linking with oxygen. The residual plasticiser makes the film of paint tough and flexible with time, however, the combination of further oxidation and loss of plasticiser can result in brittleness and flaking of the paint.
“Blending” or “alloying” is combining of two or more distinct polymer molecules to form a new product with different characteristics.
The stabilisers are added to minimise the effect of heat, sunlight, ozone.
White lead, barium, cadmium laurate are examples of stabilisers.
6. Colouring Agents (Pigments):
These are added to give desired colour to plastics.
Some of the colouring agents are- Zinc oxide, white lead, titanium dioxide, dyes.
6. Polymer/Plastic Coatings
:
The miscellaneous applications of polymers/plastics are:
1. Coatings
2. Adhesives
3. Films
4. Foams.
We shall discuss here only “Coatings’.
The surface of materials are applied coatings to serve one or more of the following purposes:
(i) To protect the object/item from corrosive or deteriorative reactions that the environment may produce.
(ii) To improve the appearance of the object/item.
(iii) To provide electrical insulation.
In coating materials, many of the ingredients are polymers and majority of which are of organic origin.
The organic coatings may be classified as:
(i) Paints
(ii) Varnishes
(iii) Enamels
(v) Lacquers.
Paint is a mechanical mixture of “vehicle” and a “pigment”. The vehicle binds together the pigment particles in the paint. It consists of drying oil and a solvent thinner. The “pigments” provide both colour and binding power.
The thin film of paint when applied over the surfaces gets dried up due to oxidation, polymerisation or evaporation of portions of its components and gives a film which has a considerable binding power.
Varnishes are colloidal dispersions or solution of synthetic or natural resins in oil or in thinner or in oil and thinner both and contain no pigments. When varnish is applied on a material surface, the thinner evaporates and the oil resin mixture partially polymerises to form a clear dry film.
Enamel is a mixture of both paint and a varnish. The enamel dries at room temperature through oxidation and polymerisation or at elevated temperature where the film gets baked.
Lacquer is a finishing material that dries quickly by evaporation of solvents and forms a film from its non-volatile constituents. It is solution or dispersion of cellulose nitrate or cellulose derivatives, resins and plasticizers in solvents and thinners.
The other organic coatings are:
(i) Dispersion coatings (finally divided insoluble resins dispersed in organic media);
(ii) Emulstion coatings (synthetic resins latex dispersed in water by means of dispersing agent); and
(iii) Latex coatings.
The organic protective coatings should have the following characteristics:
1. High hiding power.
2. Resist corrosion when applied on metals.
3. Film produced should be washable.
4. Should be able to resist the atmospheric conditions to which these are exposed.
5. Should be able to give the necessary gloss whenever required.
Organic coatings can be easily applied and are flexible and cheap. They can be applied by brush, spray or roller coasters. They can also be applied by hot spraying, by dipping articles in resin powders.
7. Testing of Plastics:
Plastics are tested for electrical and mechanical properties.
The various characteristics measured are defined below:
1. Insulation Resistance after Immersion in Water:
A specimen immersed in distilled water for 24 hours is cleaned and dried.
The specimen is then placed between pairs of electrodes and its resistance is measured at a potential difference of 500 V D.C., the temperature and humidity being under control.
This property measures the electrical breakdown through the material.
Dielectric strength is measured as voltage gradient as volts I mm to cause electrical breakdown.
It is the temperature at which a penetrator of circular section of 1 mm diameter will penetrate a specimen of thermoplastic to a depth of 1 mm under specified load and uniform rate of temperature rise.
Plastic is extruded through a die by a loaded piston under controlled conditions of temperature. Number of grams extruded per 10 minutes is determined as melt flow index.
Refer to Fig. 9.14.
A plastic specimen of rectangular section (B = width, D = thickness) is supported like a simply supported beam over a span L and a load (W) in the centre of the span applied to cause fracture.
The cross breaking strength is measured as 1.5WL / BD2
6. Impact Strength:
Refer to Fig. 9.15.
Izod impact test in which a matched specimen of specified dimension (1/88 ” thick x 1/2″ wid x 2 1/2″ long) held like a cantilever is struck by a surging hammer at free end.
The energy absorbed in fracture is the measure of impact strength.
The impact strength is measured in kgf-cm or J/m. This figure is calculated on the basis of 1 m match length.