Polymers
7.0 Polyvinyl Chloride (PVC)
7.0 Polyvinyl Chloride (PVC)
- Its structure is
- PVC is the most important plastic currently used in industry. It is used for cable insulation, chemical plant, leather cloth, packaging and toys.
- Acetylene is manufactured by one of two processes.
- Acetylene is then treated with $HCl$ as shown below.
The reaction mixture is cooled to $100-$$130^°C$. Unreacted $HCl$ is removed by adding $NaOH$. The product is liquefied at $–40^°C$. Pure vinyl chloride is obtained on distillation. Vinyl chloride is a colourless gas with sweet odour.
- Vinyl chloride can also be prepared from ethylene.
- Suspension polymerization method of obtaining PVC follows the formulation:
Vinyl chloride, water, polyvinyl alcohol, trichloroethylene and lauroyl peroxide (initiator) are mixed and heated to $50^°C$. Polyvinyl chloride in the form of granules are obtained. It is used in pastes as filler polymer.
Emulsion polymerization method uses vinyl chloride, water, ammonium stearate and ammonium persulphate at $50^°C$ for $2$ hours. The polymer obtained is smooth spherical particles.
Head to tail structure for PVC has been proved by physical methods.
Commercial PVC is slightly branched. Branching occurs during polymerization by transfer of hydrogen from within the polymer to growing radicals.
Unsaturated terminal groups containing allylic chloride are also thought to contribute to the instability of polyvinyl chloride.
Detection of $HCHO$ after ozonolyis of PVC is evidence of unsaturated terminal.
- Commercial polyvinyl chloride has an average molecular weight in the range of $40,000 – 80,000$. Commercial polyvinyl chloride is atactic.
- PVC is a colourless rigid material. PVC has limited solubility. It is soluble in ethers such a tetrahydrofuran or ketones such as cyclohexanone, and nitro compounds such as nitro benzene.
Rigid PVC is extensively used in chemical plant because it is corrosion resistant. PVC is unaffected by acids, alkalis and strong oxidizing agents ($CrO_3$ or $HNO_3$). Exposure of PVC to UV light has adverse effect. High temperature (above $70^°C$) would change the colour of PVC to yellow, orange, brown and finally black. Lead compounds such as lead carbonate and di-basic lead phthalate stabilize PVC.
- Degradation of PVC involves liberation of hydrogen chloride.
