Polyvinylchloride (PVC) Additives - Heat Stabilizers
Heat stabilizers used in manufacturing of Polyvinylchloride:
Degradation of Polyvinylchloride occurs when a labile chloride
group eliminates as HCl, resulting a chain reaction that generates
more HCl and long chain polyenes which is responsible for color
change of the product. The process is usually accelerated by HCl,
HCL salts, oxygen etc. The role of a heat stabilizer includes
removing labile chloride groups (by substituting with stable
ligands), preventing the oxidation, absorbing HCl, deactivating the
free radicals, terminating the polyene generation reaction and
eliminating long chain conjugated polyene sequences by addition
to the double bonds. Further, a good heat stabilizer is expected to
be highly reactive even with very low levels of addition, and provide
good color, long tem stability, odorless and nontoxic.
The suitability of a heat stabilizer is usually needs to be tested by
formulators, as not one heat stabilizer would work on every
Polyvinylchloride is inherently thermally unstable, which made it a less attractive
polymer until early 1900s. However, the discoveries made in field of heat stabilizers and
lubricants during the period of 1920- 1950, made polyvinylchloride being the second
largest volume plastic in the world. Thus, unlike most of the other plastics,
polyvinylchloride must have additives in order to process useful articles. Out of these
additives, the presence of a heat stabilizer is a must.
Other PVC additives: Plasticizers, lubricants, UV light stabilizers, impact modifiers,
process aid, fillers and pigments.
Currently, there are four main categories of heat stabilizers
used in PVC industry:
Lead based stabilizers:
e.g. Lead salts, tribasic lead sulfate, dibasic lead phthalate, etc.
Lead based heat stabilizers provide excellent long term heat
stability and electrical properties, and are usually the cheapest,
however, due to toxicity issues; usage is limited only to some
countries depending on their legislation. Further, their inherent
opacity limits their use in compounds where high clarity is a must.
Organo-tin based stabilizers:
e.g. mono/dialkyltin compounds. Usually highly alkyl-substitutes
organo-tin compounds are the most toxic, thus trialkyl-substituted
tin compounds exhibit the highest toxicity compared to di-alkyl
and mono-alkyl substituted ones. Decreasing toxicity is achieved
by keeping the trisubstituted fraction be minimal and attaching
long chain alkyl groups. Usually organo-tin heat stabilizers provide
accelerated fusion as well as enhance clarity. Thus, widely used in
clear products. Usually organo tin compounds are the most
expensive heat stabilizers; and, they are highly effective in heat
stability. Further, organo-tin stabilizers exhibit a very good
compatibility with Polyvinylchloride.
Mixed metal stabilizers:
derivatives of cadmium, barium and zinc: Usually mixtures of
metallic-stearates give excellent heat stability, and also provide
good light stability. Some metallic-derivatives may be toxic,
however, not in all cases. Development of more environmental
friendly mixed metal heat stabilizers is continuing, especially to
replace lead based compounds in rigid PVC applications, and to
achieve enhanced heat stability.
The heat stability effectiveness of most of the organic stabilizers is
very low compared to the inorganic derivatives. Thus, usually
organic-stabilizers are used as a co-stabilizer, where metal-derived
stabilizers play the major role.