Page 329 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

PLASTICS ADDITIVES 5.9

FIGURE 5.2 Thermal degradation of PVC.

5.1.3.2 Ba/Ca Soap + Cd/Zn Soap + Epoxidized Fatty Ester + Organic Phosphite. This
synergistic combination has always been unnecessarily secretive, sold under vague names
such as “mixed metal,” “synergistic,” and so on. It is universally used for plasticized PVC,
because it is soluble, economical, and effective. The metal soap may be phenate, octoate,
neodecanoate, naphthenate, benzoate, laurate, myristate, palmitate, or stearate.
The Group IIB metal soap (Cd or Zn) is the primary stabilizer. It replaces an unstable
Cl atom by a stable ester group,
Polymer-Cl + M(O CR) → Polymer-O CR + MCl 2
2
2
2
Cd is more reliable, but worries about toxicity have practically eliminated its use. Zn is
more powerful but tricky, so compounders have had to learn how to handle it very care-
fully.
The Group IIA metal soap (Ba or Ca) is a reservoir to regenerate the essential Group
IIB metal soap:
Ba(O CR) + ZnCl → BaCl + Zn(O CR) 2
2
2
2
2
2
Ba works best, but there is some worry about toxicity. Ca is less effective but completely
nontoxic, so it is used when there is worry about toxicity.
The epoxidized fatty ester may be epoxidized soybean oil for compatibility and non-
toxicity, or epoxidized tall oil esters for low cost and low-temperature flexibility. It is gen-
erally believed to function by neutralizing HCl. It may also replace unstable Cl on the
polymer or complex ZnCl to keep it from degrading the PVC.
2
The organic phosphite is generally believed to function by complexing ZnCl to keep it
2
from degrading the PVC.
The synergistic effect is clearly seen by comparing the individual ingredients with the
total system (Table 5.7). Typical concentrations are about 2 percent metal soap, 5 percent
epoxidized fatty ester, and 1 percent organic phosphite.
5.1.3.3 Organotin Salts. The most powerful and expensive stabilizers for PVC are orga-
notin compounds, most generally of the type R SnX . The R group is most often butyl, but
2
2

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