Polymeric Materials

Polymeric Materials

Introduction
Joining together thousands of small molecular units known as monomers makes synthetic large molecules called Polymer. The process of joining the molecules is called polymerization and the number of these units in the long molecule is known as the degree of olymerization.

The words polymers and plastics are often taken as synonymous but in fact there is a distinction. The polymer is the pure material, which results from the process of polymerization and is usually taken as the family name for materials, which have long chain-like molecules (and this includes rubbers). Pure polymers are seldom used on and it is when additives are present that the term plastic is applied.

Classification of Polymeric Material
Polymers are classified as
• Thermosets
• Thermoplastics
Thermosets are cross-linking polymers in which the final macromolecules are formed by chemical reaction under the influence of heat and pressure. Once this reaction is complete, thermosets cannot be altered from this state by further application of heat and pressure. Phenol (PF), Urea (UF), Melamine (MF) formaldehyde resins, Polyester (UP) resins and epoxy (EP) resins are typical Thermosets.

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.

Polymer Blends

By alloying of two polymers it is possible to get in one material advantage 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.
ABS / Polysulphane	Less expensive than unmodified Polysulhone.
PPO / HIPS	Improved processability and reduced cost.
SAN / Olefin	Good weatherability.
Nylon / Elastomer	Improved notch impact strength.

General properties of plastic materials

Important characteristics of plastic materials to be considered while designing and manufacturing are:
1. Dimensional Stability: Dimensional stability is defined as the ability of a material to maintain its size and shape under various temperatures and stresses, which is necessary for satisfactory part performance in many applications. The complexity, size of the mould cavity and the tendency of the material being moulded to shrink as it cools in the mould determine the final dimensions of a moulded part.
2. Drying: Under adverse high humidity conditions or large temperature fluctuations from cold temperatures to hot temperatures, moisture pickup may occur and cause splay marks or bubble formation in formed parts. Drying the resin for about two hours at 71 - 82°C will eliminate condensed moisture on the granules and assure introduction of constant temperature granules to the fabrication equipment.
3. Compatibility: Equipment should be thoroughly purged with respective resins while fabricating. Few resins are physically compatible with themselves. Delaminating, streaking or haze will occur if incompatible resins are mixed with the material to be fabricated.
4. Outdoor Weatherability: Most of Plastics are not considered to be weather resistance plastics. Continuous long-term outdoor exposure results in both discoloration and reduction in strength and toughness properties. Weatherability can be improved by the addition of certain pigments or additives. Best results are obtained with finely dispersed carbon black or UV stabilizers.
5. Use of Regrind: Many thermoplastic resins can be reground for use as 100% regrind or blends of regrind resins with virgin resin. When you are fabricating with regrind resins, you need to use experienced judgment and screening. The use of degraded or contaminated regrind product may result in lower quality parts and performance.

Application of Plastics

Polymers will continue to replace other materials on an increasing scale. The polymers already perform satisfactorily in many applications previously employing metal, wood, paper, glass etc. Usage of polymers is already well established in Automobile, electronics, Telecommunication, Computer, Toys, Medical application, clock, house ware, plumbing, footwear, electrical switch gears, luggage, etc. New polymers with specific properties and applications are being developed. With the result, number of polymers with properties suitable for specific applications is now available.
Reasons for replacement of traditional materials:
• Availability of stronger, stiffer polymers.
• Development of processing techniques to exploit the properties,
• Design possibilities of plastics,
• Availability of accurate, meaningful data on the mechanical properties of polymers.
• Greater willingness on the part of engineers to consider plastics as raw materials in their own right, rather than substitute,
• Increasing awareness of the cost saving, energy saving, labor saving and ease of manufacturing technique.
It is not correct to say that PLASTICS will generally and universally replace all materials. Need for replacement of any part should arise from functional requirement, ease of fabrication, cost with out compromising functional needs, lower weight, lower energy requirement. New polymers with specific properties and applications are being developed.
The part should be designed with plastics material, by considering the
• Functional needs
• Service condition
• Mechanical loading and
• Duration of loading
• Polymer melt behavior ( flow, shrinkage, response to shearing )
• Strength of material Properties of plastics and
• Processing (conversion- fabrication) technique.