What Is A PVCdina
INTRO TO PVC
Poly(vinyl chloride) (PVC) has a chemistry and a physical structure that makes it broadly unique in the polymer world. PVC (often referred to vinyls or vinyl resins) ismade commercially at several molecular weights, depending on the intended applications: from Mw = 39000 g/mol, to Mw = 168000 g/mol.
1.1 Raw material
The chemical process for making PVC involves three steps: first, production of the monomer, vinyl chloride; then the linking of these monomer units in a polymerization process; and finally the blending of the polymer with additives, Figure 2. In this section, the production of raw materials will be considered.
Ethylene comes from oil or natural gas which is refined and ‘cracked’ by heating ethane, propane or butane or naptha from oil. Ethylene reacts with chlorine to form ethylene dichloride (EDC), which is finally cracked to the monomer. Chlorine is produced from electrolysis of salt (NaCl). In this process, the dissolved salt is chemically decomposed by passing an electric current through it. This produces chlorine and sodium.
Sodium reacts with water to form caustic soda (sodium hydroxide) and hydrogen gas.
The manufacture of PVC accounts for 35% of the chlorine produced industrially and 0.3% of the worlds supply of gas and oil, Figure 3. Most other polymers are largely hydrocarbons and thus are highly dependent on supplies of oil and gas; whereas PVC is much less dependent on the supply of oil and gas.
1.2 Chlorine content
Vinyl compounds often contain nearly 50% chlorine. Because of this, in a fire, vinyl provides only about half the fuel compared to other polymers. Halogens in flame retardants, including chlorine in PVC, additionally provide condensed phase and gas phase combustion resistance by a radical- trapping, flame-poisoning mechanism. This is a
reaction of the neutral halogen with hydrogen on the fuel to remove hydrogen as a fuel element and to form hydrogen halide. Thus, vinyl is unique in its resistance to combustion because it has lower intrinsic fuel content than other polymers, it can form hydrogen chloride in the condensed phase to reduce available fuel, and it can inactivate
hydrogen from fuel in the gas phase.
Additionally, the hazards in a fire are mainly carbon monoxide and heat; these are especially a problem with materials other than vinyl that readily burn. When forced to burn, vinyl produces carbon monoxide, carbon dioxide, and hydrogen chloride. Of these, the most hazardous is carbon monoxide. Hydrogen chloride is an irritant gas that can be lethal at extremely high levels, which are never reached or even approached in real fires.