Primary heat stabilizers

Rigid Applications. The use of the lead stabilizers is very limited in the United States but, they are stiU used in several rigid PVC appHcations in Europe and Asia. The highest use of lead stabilizers in rigid PVC is for pipe and conduit appHcations. Tribasic lead sulfate is the primary heat stabilizer with lead stearates included to provide lubrication. The lead products are typically fully formulated, usually including lubricants and pigments for pipe extmsion appHcations. These lead one-packs, when used at about 1.8—2.5 phr, provide all of the stabilizer and lubrication needed to process the polymer. A lead one-pack contains tribasic lead sulfate, dibasic lead stearate calcium stearate, polyethylene wax, paraffin wax, ester wax, and pigments. 

Heat stabilizers may be classified as either primary or secondary. Primary heat stabilizers inhibit dehydrochlorination in chlorine-containing polymers and react with any liberated hydrogen chloride to delay further degradation. Mixed metal salts are primary heat stabilizers and form metal chlorides with hydrogen chloride. However, mixed metal salts have a destabilizing effect which results in discolouration of the polymer. This effect is counteracted by the introduction of secondary heat stabilizers such as organophosphites and epoxy compounds. Organotin and lead primary heat stabilizers can be used alone. 

The phosphite esters function in this formulation as primary heat stabilizers by replacing labile chlorides on PVC via the Arbuzov rearrangement mechanism. When phosphites are used at optimized levels in combination with catalytic amounts of zinc and normal levels of epoxidized soybean oil, they provide excellent early color and adequate long-term stability to flexible PVC compounds. Their use also helps in lowering stabilization cost, elimination of toxic metals, elimination of plate out, poor printability, viscosity drift in plastisols, haze, bloom, water blush, and staining. 

There are three major types of primary heat stabilizers, which include ... 

Heat stabilizers belong to one of the two major classes primary heat stabihzers and secondary heat stabilizers. When heated, chlorinated vinyl resins liberate HCl which causes further polymer degradation and discoloration. Primary heat stabilizers fimction both by retarding this dehydrochlorination and by reacting with liberated HCl to delay progressive degradation. 

When mixed metal salts are used as primary heat stabilizers, metallic chlorides are formed by the reaction with labile Cl. These materials have a destabilizing effect that sometimes result in color formation in... 

Of less importance are antimony mercaptides, which find occasional use as low-cost replacements for organotins. The organotin and lead stabilizers are usually present as the only heat stabilizers in the resin formulation. However, the mixed metal stabilizers are used in combination with secondary heat stabilizers. The secondary heat stabilizers are usually organophosphites and epoxy compounds, but polyols and beta diketones are also used. The major types of primary heat stabilizers, along with their end uses are summarized in Table 4.13. 

Antimony. Antimony compounds are effective at low concentrations as primary heat stabilizers in rigid PVC applications. They have National Sanitation Foundation (NSF) acceptance for use in potable PVC water pipe. A disadvantage of antimony compounds is their poor light stabihty. 

The selection of which heat stabilizer or combination of heat stabilizers to use will depend on the processing system, the end nse application, and the effect on processing properties of the resin. There are over 100 suppliers of primary heat stabilizers. Among the major suppliers are Witco Corp., Akzo Chemical Inc., and Ferro Corp. 

Organotin compounds are the most widespread primary heat stabilizers. Since the first patent taken out by Ingve in this domain [56], a great number of organotin compounds have been found to stabilize PVC. Only tetravalent tin derivatives, mainly di- or tri-substituted, are efficient stabilizers. Divalent tin derivatives are not efficient [57] they function as weak stabilizers [58]. The same holds true for tetrasubstituted derivatives such as tetrabutyltin [59]. The most efficient are dibutyltin or dioctyltin, but there are many possibilities for the substitution of the two free valencies. 

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