Chemistry Reference
In-Depth Information
internal pressure of about 10 lb/in.
2
and remaining monomer (about 10%) is drawn off and recycled.
The product is discharged.
Emulsion polymerizations of vinyl chloride are usually conducted with redox initiation. Such
reactions are rapid and can be carried out at 20
C in 1-2 h with a high degree of conversion.
Commercial poly(vinyl chloride)s range in molecular weights from 40,000 to 80,000. The polymers
are mostly amorphous with small amounts of crystallinity, about 5%. The crystalline areas are
syndiotactic [
317
,
318
].
Poly(vinyl chloride) is soluble at room temperature in oxygen-containing solvents, such as
ketones, esters, ethers, and others. It is also soluble in chlorinated solvents. The polymer, however,
is not soluble in aliphatic and aromatic hydrocarbons. It is unaffected by acid and alkali solutions but
has poor heat and light stability. Poly(vinyl chloride) degrades at temperatures of 70
C or higher or
when exposed to sun light, unless it is stabilized. Heating changes the material from colorless to
yellow, orange, brown, and finally black. Many compounds tend to stabilize poly(vinyl chloride).
The more important ones include lead compounds, like dibasic lead phthalate and lead carbonate.
Also effective are metal salts, like barium, calcium, and zinc octoates, stearates, and laurates.
Organotin compounds, like dibutyl tin maleate or laurate, also belong to that list. Epoxidized drying
oils are effective heat stabilizers, particularly in coatings based on poly(vinyl chloride). Some coating
materials may also include aminoplast resins, like benzoguanamine-formaldehyde condensate.
The process of degradation is complex. It involves loss of hydrochloric acid. The reactions are free
radical in nature, though some ionic reactions appear to take place as well. The process of dehydro-
chlorination results in formations of long sequences of conjugated double bonds. It is commonly
believed that formation of conjugated polyenes, which are chromophores, is responsible for the
darkening of poly(vinyl chloride). In addition, the polymer degrades faster in open air than it does in
an inert atmosphere. This shows that oxidation contributes to the degradation process. All effective
stabilizers are hydrochloric acid scavengers. This feature alone, however, can probably not account
for the stabilization process. There must be some interaction between the stabilizers and the polymers.
Such interaction might vary, depending upon a particular stabilizer.
6.17.7.1 Copolymers of Vinyl Chloride
A very common copolymer of vinyl chloride is vinyl acetate. Copolymerization with vinyl acetate
improves stability and molding characteristics. The copolymers are also used as fibers and as
coatings. Copolymers intended for use in moldings are usually prepared by suspension polymeriza-
tion. Those intended for coating purposes are prepared by solution, emulsion, and suspension
polymerizations. The copolymers used in molding typically contain about 10% of poly(vinyl acetate).
Copolymers that are prepared for coating purposes can contain from 10 to 17% of poly(vinyl acetate).
For coatings, a third comonomer may be included in some resins. This third component may, for
instance, be maleic anhydride, in small quantities, like 1%, to improve adhesion to surfaces.
Copolymers of vinyl chloride with vinylidine chloride are similar in properties to copolymers with
vinyl acetate. They contain from 5 to 12% of poly(vinylidine chloride) and are intended for use in
stabilized calendaring.
Copolymers containing 60% vinyl chloride and 40% acrylonitrile are used in fibers. The fibers are
spun from acetone solution. They are nonflammable and have good chemical resistance.
6.17.8 Poly(vinylidine chloride)
Vinylidine chloride homopolymers form readily by free-radical polymerization, but lack sufficient
thermal stability for commercial use. Copolymers, however, with small amounts of comonomers find
