Wednesday, 3 June 2009
Polyvinyl Chloride- Manufacturing Process and Properties
Polyvinyl Chloride (Vinyon)
Vinyl Chloride is the principal raw material from which polyvinyl chloride is made by addition polymerisation. There are two methods commonly used for the production of vinyl chloride:
1. Ethylene+ chlorine--> Ethylene Dichloride--600 deg C--> Vinyl chloride +HCl
Cl-CH2-CH2-Cl--300deg C +Charcoal--> Vinyl Chloride + HCl
Cl-CH2-CH2-Cl--CH3OH+NaOH (60 deg C)--> vinyl Chloride + NaCl+ H2
2. Acetylene +HCL--150 deg C, HgCl--> CH2=CHCl (Vinyl Chloride)
the vinyl chloride monomer is polymerised in the emulsion form in an autoclave at a pressure of 50 Atm and at a temperature of 65 deg C. A suspension of the polymer is obtained which is then spray dried.
This may be done by dry spinning or wet spinning.
1. Dry Spinning: In the dry spinning process the polymer is dissolved in a mixture of CS2 and acetone, filtered and pumped at 70 deg to 100 deg through spinnerettes into a chamber, provided with heated walls, and into which air is introduced. The solvent evaporating from the extruded filaments is carried away by the air. At the bottom of the chamber the solvent free filaments are removed through a fine orifice and wound on a bobbin. The solvent is recovered and used again. the filaments are stretched to ensure that the molecular chains get oriented and the fibres become stronger and attain less extension at break, increased brightness, transparency etc.
2. Wet Spinning: In the wet spinning process, PVC is dissolved in THF (Tetra Hydro Furon) to give a highly concentrated solution, which is spun into water, through a stretch spinning funnel. The filaments ar stretched and cut into staple fibres.
1. Tenacity: Wet or Dry: 2.7-3 gpd
2. Elongation at BreaK: Wet or Dry: 12-20 %
3. Moisture Content: 0
4. Specific Gravity: 1.4
v. Effect of Heat: It contracts at temperatures above 78 deg C and shrinks to half its original length at 100 deg C.
vi. It has an excellent resistance to sunlight. It is completely resistant to insects and microorganisms. It is inherently non-flammable.
vii. It is exceptionally resistant to caustic soda, nitric acid and sulphuric acid. It has outstanding resistance to many chemicals including bleaching agents, reducing agents.