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| Home > Articles > Revolution In Polymers |
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Revolution In Polymers |
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Two basic polymerisation processes are involved in manufacturing two different classes of polymers, which are also processed by different techniques: Suspension and Dispersion polymerisation.
Suspension polymerisation results in polymers that are suitable for moulding or ram extrusion after appropriate treatment.
Dispersion polymerisation results in aqueous dispersion of polymers. After concentration and formulation, this can be used for impregnation, casting films, dedusting and coating. Coagulated powders are obtained from aqueous dispersion to be finally used in paste extrusion technology.
Processing techniques
PTFE requires sophisticated processing techniques for its conversion into various products. PTFE has an unusually high melting point and, unlike conventional thermoplastics, the resin doesnt flow above this melting point. Due to this high melt viscosity, PTFE is not melt processable by conven-tional techniques used for other thermoplastics. Other thermoplastics can be heated to a point at which, the melt viscosity is reduced to a state when the material can be transferred in a fluid condition from a processing chamber into a mould or die. It is here that the fluid is cooled to enable a replica of the mould or die to be reproduced. Whilst PTFE has a defined crystalline melting point, the viscosity above this point remains very high and cannot be reduced sufficiently for the material to flow to fill a cavity.
In some respects PTFE is a typical thermoplastic polymer, while in others it is far from typical. Like other thermoplastics, the properties of PTFE vary with change in time, temperature and crystallinity. However, fabrication methods used for PTFE can have a very large influence on the end product properties. It never really melts and above 327°C and becomes a translucent gel which cannot be moulded or extruded as a melt by conventional equipment but needs to be processed with techniques similar to those used in powder metallurgy, by cold compression, sintering and controlled cooling.
The techniques used with PTFE consist essentially of a cold shaping operation, followed by sintering during which the polymer particles fuse and coalesce, and finally the fused polymer is cooled.
PTFE as a raw material is available in two basic forms:
· Suspension Polymers as Granular Powder
· Emulsion Polymer as Paste Powders and Coating Dispersions.
Suspension polymers
These take the form of granular powder of various particle size and shape. Processing is achieved by the compaction of the powder at around ambient temperature under high pressure followed by sintering at a temperature exceeding the crystalline melting point which causes the individual groups of particles to fuse together. Sintering is then followed by a cooling cycle to return the material to a stable ambient condition. Processing granular powders can be subdivided into:
Compression moulding The simple compression process is used to produce a significant proportion of semi-finished rods, tubes, sheets and blocks that are later converted by machining and forming into a miscellany of end products.
Isostatic compression moulding : Isostatic moulding process is suitable for the moulding of intricate mouldings, particularly for larger sizes. Though the isostatic process can produce PTFE of the highest quality even in complex shapes, it is an expensive process and only really suitable for repetitive components. It is used to make containers, beakers and bottles, large pipes, rollers and cylinders, branch pipe-fittings with integral metal parts, valve and pump components and electrical high voltage insulators.
Ram extrusion : The ram extrusion process provides the feedstock for the automatic machining of a multiplicity of small annular parts in both virgin and filled compounds. The ram extrusion process can be best described as a repetitive compression process where regular powder charges are compressed one above the other whilst the compresses material is forced down the die tube. The extrusion process is continuous. The process is far more capital intensive than conventional compression moulding, but labour utilisation can be very low with one operator servicing up to six extruders simultaneously. A wide range of ancillary equipment is available to eliminate the manual process. At the downstream end, extrusions can be automatically cut to length and cartridge stored without attendance. Ram extruders are available in a variety of sizes and are used to provide solid rods or medium to thick wall tubes. Its applications are numerous, for instance, small valve and pump sets, seals, plain bearings, bushes, insulators and connectors. This is the commodity end of the PTFE processing industry, which can convert large tonnage of raw material into the feedstock for a multitude of machinists supplying almost every sector of industrial life.
Emulsion polymers : These also take the form of fine powders, which can be readily mixed with a lubricant to form pastes, or with water or solvent to provide dispersions. Processing of paste polymer cover includes extrusion of thin wall profiles and extrusion of unsintered tape and cord.
Fine powder paste extrusion : The paste extrusion process is admirably suitable to produce PTFE thin wall tubing, wire coating and in the manufacture of unsintered PTFE products such as sealing tapes and ropes. Paste extrusion is a batch process. The mixture of PTFE fine powder, lubricant, a variety of pigments and fillers (if necessary) is first moulded into a simple annular pre-form, then placed into the chamber of the extruder prior to the transfer of the extrudite through the sizing die.
Unsintered tape and profile: The manufacture of pipe thread sealing tapes follows broadly the paste extrusion process. The paste polymer is extruded into a circular section cord and is passed through a pair of calendar rolls to spread the cord into an even thin tape. Paste polymer sealing cord is also produced by the extrusion process and is used for sealing duties on flange faces and valve stems.
Dispersion coatings : The coating of metallic and other substrates with PTFE for non-stick applications has brought PTFE and some of its more prominent trade names into almost every household. Most dispersion polymers contain fine emulsion powders suspended in aqueous solutions with wetting agents. Coating dispersions are usually applied at ambient temperature by spray application onto prepared and thoroughly cleaned substrates. Domestic applications for PTFE dispersion coatings include cookware, grill pans, electric cooker parts, toasters, clothes irons. Industrial applications include, coating of valve parts, heating and welding tools moulds for rubber, plastics and food processing. PTFE dispersion coatings are also used for the impregnation of porous and fibrous substrates. Fibrous materials such as asbestos and glass cloth can be impregnated to provide good low- friction and non-stick properties.
The PTFE industry in India
Processing of PTFE was started for the first time in India in early 1960s by an entrepreneur in Ahmedabad and since then PTFE consumption has shown increasing trend. The demand of PTFE in India was around 20 to 25 tons in the sixties. In the seventies it shot up to 125 tons, and in the eighties to 250 tons. The present demand for PTFE is around 750 ton p.a., and is likely to increase in future. Today, the consumption rate of PTFE has become an indicator of economic growth of a country. India too is catching up in this race. PTFE is largely used in the field of nuclear energy, defence, railways, aircraft and the computer industry. The above figures indicate a steady growth in the PTFE consumption. A 50% of PTFE consumed in rods & tubes, sleeves, value sheets, blocks, lining sheet etc. made from ram extrusion process. The unique properties of PTFE have made it an indispensable material for the modern industry. Apart from its usual field of application, PTFE is steadily accessing new fields of application through its characteristic qualities, which unite an optimum combination of many valuable properties in terms of application purposes. So far, India has just made a beginning in the use of this wonder plastic, PTFE. But, indigenous production of PTFE coupled with liberalised policy of the Government has thrown open the doors for ventures into new application fields by the PTFE resin manufacturers, processors and young entrepreneurs.
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| Posted : 10/27/2005 |
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