1. Optimum processing is obtained with sequential internal and external lubrication rather than through both simultaneously. The effects of using lubricant blends versus sequential use of individual lubricants may, in some cases, be very marked [32].
2. Variation is encountered, for example, from one production line to another, from lot to lot of polymer (or other ingredient), or seasonally, which is most readily compenstions in which attempts to eliminate all variation become cost-ineffective. A case in point is the processing of compounds during periods of PVC shortage, with reliance on a number rather than a single grade of resin and onuse of regrind of marginal consistency.
B. Antimony Analogs of Tin Stabilizers
Although antimony carboxylates are not sufficiently stable for practical use, it has been long known that antimony mercaptides provide many of the properties of the analogous tin compounds[33]. A typical example is antimony tris(isooctylmercaptoacetae):
S-CH2COO-i-C8H17
Sb-S-CH2COO-i-C8H17
S-CH2COO-i-C8H17
In comparison with a typical dialkyltin stabilizer, the major difference is that the +3 oxidation state of antimony accommodates three active ligands. In place of an alkyl group, trivalent antimony, in a tetrahedral configuration, has an unshared electron pair. It is not, therefore, an organometallic compound. (Structures such as RSbX2 are not sufficiently stable for practical use.) An important consideration is that antimony is a less expensive metal than tin and may be converted to a stabilizer from antimony oxide, rather than a synthesized intermediate economically. Further, the presence of three active ligands leads to a lower equivalent weight (as much as 30%) than the corresponding dibutyltin derivative. This effect, in some cases, offsets the observation that, in high-demand applications, antimony analogs are inferior in efficiency compared to tin [34]. A factor involved in this is that antimony trichloride is a stronger Lewis acid than any of the dialkyltin dichlorides [35]. Antimony mercaptides, like those of tin, have solubility parameters very close to that of PVC and are not strongly dipolar. They are (in the United States) used at very low levels in rigid pipe and conduit to minimize cost. If process and service requirements can be met with a low level of stabilizer, then the lower equivalent weight and basic low cost of antimony mercaptides become attractive.
Similar in action to lead stabilizers, organotin and antimony stabilizers are effective because they complex with and deactivate labile choride sites, increasing the activation energy for the initiation of degradation. They scavenge HCl at the reaction site before elimination occurs, usually by acting as a charge transfer catalyst and conducting the acid to a secondary stabilizer (e.g., calcium stearate) but in some cases, such as with sulfur-bridged stabilizers, react with HCl themselves. Labile chloride is replaced with mercaptide or carboxylate ligands. The mercaptides are active on-the-scene antioxidants. Almost all organotin and antimony stabilizers contain antioxidants, most often butylated hydroxytoluene (BHT), to protect the stabilizers themselves.
Except for the polymeric maleates, most members of this class of stabilizers are mobile liquids. Exposure to damp air results in a mixture of this class of hydrolysis and oxidation, leading to the formation of a solid phase, appearing as a white sludge. With heating, this can often be mixed in, with little loss of stabilizer efficiency. Nevertheless, once shipped, few customers are equipped (or motivated) to combat such problems. As a result, organotin and antimony stabilizers typically contain antioxidants and proprietary additives to inhibit or delay sludge formation.
With organotin mercaptides, a high monoalkyl level favors bright early color at the expense of long processing safety. The reverse is true with high dialkyl levels. (It should be noted that this conclusion does not apply to estertins.) Thus, the ratio of monoalkyltin to diakylltin is adjusted in actual stabilizers toprovide a specific balance for a given application.