Polymers are produced from intermediates like these parafins:
These intermediate monomers are polymerised by addition. Thus if 500 or more ethylene monomers polymerise, the result is Polyethylene.
Thermoplastics are molecules that are chains of 500 or more carbon atoms, The distance between adjacent carbon atoms being 1.5 x 10 -8 cm. All the molecules may not contain same number of monomers. However, some control over the length of polymer chain is ensured during polymerisation. The polymer is classified or specified by its average molecular weight. This also gives rise to molecular weight distribution of particular grade.
Most monomers are gases. A short polymer chain of low molecular weight will be liquid. Large molecular chain gives solid material. Higher molecular weight results in increased strength and stiffness. Higher molecular weight also increased viscosity of melt. Hence higher molecular weight melt requires higher power ( pressure) to fill the mould. Higher the molecular weight are difficult to process. Ultra high molecular materials are not mouldable.
High molecular weight material is accompanied by a very high melt viscosity. To reduce the viscosity the processing temperature must be increased, but this results in degradation of polymer. Therfore while processing the thermal stability of the polymer dictates the processing temperature and residence time of melt in the plasticising unit of the machine. Limitation of residence time dictates the speed of cooling to avoid the degradation of polymer.
Many thermoplastics are deteriorated by prolonged exposure to oxygen and ultraviolet radiation in sunlight. Resistance to such deterioration is improved by higher molecular weight. It means few molecules with fewer terminal monomers (which are reactive) in the carbon chain. Longer molecules are less mobile. Even if high molecular weight degrades it results in medium molecular weight with acceptable properties instead of a low molecular weight.
Inter-molecular order refers to the geometric arrangements of adjacent polymer molecules in the solid mass. There are three types of inter-molecular order.
Polymers are classified
Thermosets are cross-linking polymers in which the final macro-molecules are formed by chemical reaction under the influence of heat and pressure. Once this reaction is complete, thermosets can not be altered from this state by further application of heat and pressure.
Phonel (PF), Urea (UF), Melamine (MF) formaldehyde resins, Polyester (UP) resines and epoxy (EP) resins are typical Thermosets.
Elastomers are plastics or modified natural substances with a limited degree of cross linking capability. They deform readily under stress but recover their original shape as soon as this stress is removed.
Thermoplastics consist of long chain macromolecules which are not interlinked. Their characteristics property is that they may be moulded when the temperature is increased beyond their softening range, and on cooling revert to the solid state in its new moulded shape. This process may be repeated indefinitely, but it is in fact limited by the ageing stability of the particular material. This means that after undergoing a certain number of processing operations, the original properties of the material are altered as a result of excess thermal stress. HDPE, LDPE, PP, PS, ABS, NYLON, PVC, PMMA, PBT, etc are thermoplastics.
By alloying of two polymers it is possible to get in one material advantages of two or more polymers. At present, the following alloys are available.
|
PVC / Acrylic |
Tough with good flame and chemical resistance. |
|
PVC / ABS |
Easily processed with good impact and flame resistance. |
|
PC / ABS |
Hard with high heat distortion temperature and good notch impact strength. |
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ABS / Polysulphane |
Less expensive than unmodified Polysulhone. |
|
PPO / HIPS |
Improved processability and reduced cost. |
|
SAN / Olefin |
Good weatherability. |
|
Nylone / Elastomer |
Improved notch impact strength. |
Reinforced Plastics
Reinforcement in plastics enhances the mechanical strength and reduces the shrinkage. There are two types of reinforcement used in plastics.
Higher content of fibrous reinforcement would impart higher mechanical properties to the material. It may not necessarily confer high rigidity.
Higher content of mineral reinforcement may give higher rigidity but poor tensile strength. Combination of fiber as well as mineral reinforcement can be possible. To enhance the bonding between various components, processing aid will be required in the compound.
In reinforced polymer, the stresses in the weaker polymer matrix is transferred to stronger fibers. Stress transfer between polymer matrix and fiber can be improved by incorporating binders or coupling agents in the compound. Smaller diameter and longer fiber length provides larger surface area per unit weight of fiber. This would help in enhancing the stress transfer in the compounded material.
Glass fiber is most popular reinforcement in polymers in thermoplastic as well thermoset polymers. It increases the mechanical properties of the compound by 1.5 to 2 times.
Carbon/graphite fiber reinforcement improves the mechanical strength by 3 times. It imparts conductivity to the material.
THINGS TO KNOW ABOUT POLYMER CHARACTERISITICS
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