Vinyl chloride exercises

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

Copolymers are comprised of chains containing two or more different types of monomers. The composition of copolymers may be quantitatively determined by using infrared spectroscopy [3, 8]. Distinctive representative modes for the polymers may be identified. For example, in the case of vinyl chloride-vinyl acetate copolymers, the ratio of the absorbance of the acetate mode at 1740 cm to that of the vinyl chloride methylene bending mode at 1430 cm can be used for quantitative analysis. Copolymers of known composition may be used for calibration. The multivariate methods described earlier in Chapter 3 may also be applied. Care must be exercised because the position and shapes of the infrared bands of the components of copolymers may be affected by the sequencing of the constituent monomers. 

Even with a semicrystalline polymer like HDPE, where the density could vary from 0.940 to 0.965, the glass-transition temperature would be different for different grades. The range of this variation in semicrystalline polymers is generally narrower than that in amorphous polymers. In the case of poly(vinyl chloride) (PVC) formulations, extra care must be exercised, as the Tg is known to vary [6] considerably depending on the type and amount of plasticizers [7, 8] (see Tables 6.2 and 6.3) and to a certain extent on the type and amount of... 

EXERCISE 8.4 Polyvinyl chloride (called PVC), which is widely used for floor coverings ("vinyl") and for plastic pipes in plumbing systems, is made from a molecule with the formula C2H3CI. Calculate the molar mass of this substance. 

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