The best approach in most cases is to check heat stability and color retention of the formulation with and without these additives. It helps to pre-determine the extent of any potential problem.
Vinyl ingredients which can have a potential impact on PVC heat stability are discussed below:
Vinyl Resins
The wide variety of PVC resins is probably the single greatest factor which explains the large stabilizers offering confronting compounders.
PVC homopolymer is made by suspension, bulk (or mass), and emulsion polymerization methods. The amount and type of residual components on the resin shipped to users (catalyst residues, suspension agents, emulsifying agents, etc.) can differ a lot. Two vinyl resins made via the same method by two different producers can have a different response to the same stabilizer system.
PVC copolymers and PVC with other co-monomers (propylene, cetyl vinyl ether, vinylidene chloride) also have a different response to a given stabilizer system.
One of the most striking differences in stabilizer response occurs with mixed metal Ba/Zn and Ca/Zn. We call this phenomenon [Zinc Sensitivity". It is a drastic discoloration- even burning- in PVC compounds subjected to progressive exposure to heat.
The degradation which occurs with zinc (sudden blackening & burning) is typical. Indeed, zinc chloride forms when the zinc salt`s carboxylic acid displaces labile chlorine on the polymer chain. It is a strong Lewis Acid and a degradation catalyst for PVC. The phosphite component provides a measure of long-term stability by itself.
Ba/Cd and Ba/Cd/Zn systems benefit upon addition of the phosphite. It is believed to chelate or [tie up" zinc and cadmium chlorides formed during the process of stabilization. For this reason, phosphites are known to retard the [zinc burning" effects described above. The further addition of the epoxy component to a mixed metal system results in dramatic improvement in heat stability. This is truly synergistic (recall epoxy performance alone).
Vinyl acetate - vinyl chloride copolymers are quite zinc sensitive (depending on acetate content). The use of zinc in the stabilizer system should be avoided.
Other types of copolymers generally respond to stabilizers like PVC suspension homopolymers. They exhibit varying degrees of zinc sensitivity. Most of these copolymers (propylene, cetyl vinyl ether modified PVC) have greater inherent heat stability than the acetate copolymers.
Emulsion polymerized PVC resins of today are mainly plastisol dispersion resins. They have very small, smooth surfaced particles. Most dispersion resins respond well to mixed metal and organotin stabilizer systems.
