Secondary heat stabilizers

When used in PVC organophosphites act as secondary heat stabilizers. In other resins such as PP, PS, HOPE and ABS plastics they function as antioxidants. p-Diketones are well-known co-stabilizers more particularly for zinc carboxylates. The synergistic action of epoxidized fatty acid esters like ESO or butylepoxy stearate as plasticizer, HCl scavenger and stabilizer for labile chlorine atoms in the catalytic presence of Zn and Cd salts is shown in Fig. 7 (see also 5.2.3)... 

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. 

Food wrap. [Data from Drapex 6.8. PVC secondary heat stabilizer/secondary plasticizer. 

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. 

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. 

Beta diketones. Beta diketones are secondary heat stabilizers used in combination with Ca/Zn and Ba/Zn metallic heat stabilizers to improve initial color. Beta diketones usually require the presence of other secondary heat stabilizers such as epoxidized oils and organophosphites. 

Polyfunctional alcohols. Polyfunctional alcohols are secondary heat stabilizers used in combination with mixed metal products. They function by forming complexes that deactivate the metallic chloride by-products of the primary stabilizers. 

Epoxy ester. Epoxy ester plasticizers have limited compatibility with PVC. Therefore, they are used at low levels. Epoxidized soybean oil (ESO), the most widely used epoxy plasticizer, is also used as a secondary heat stabilizer. As a plasticizer, it provides excellent resistance to extraction by soapy water and low migration into adjoining materials that tend to absorb plasticizers. Other epoxy plasticizers include epoxidized linseed oil and epoxidized tall oils. Tall oils are prepared from tall oil fatty acids and Cs-Cs alcohols. 

The secondary heat stabilizers are usually organophosphites and epoxy compounds, but polyols and -diketones are also used. Alky/aryl organophosphites are often used with liquid mixed-metal stabilizers in the stabilization of PVC resin. Epoxy compounds function both as plasticizers and stabilizers in flexible and semirigid PVC. (3-Diketones are secondary heat stabilizers used in combination with Ca/Zn and Ba/Zn metallic heat stabilizers to improve initial color. Polyfunctional alcohols are secondary heat stabilizers used in combination with mixed-metal products. 

Polymer compositions containing pentaerythritol are also used as secondary heat-, light-, and weather-resistant stabilizers with calcium, zinc, or barium salts, usually as the stearate, as the prime stabilizer. The polymers may be in plastic or fiber form (87—89). 

The effects of release additives on bulk properties must also be carefully considered, particularly with integral additives to plastics. For example, partial solubility usually confers some plasticizing effect. This may improve impact strength but could reduce the heat distortion temperature. Some release additives such as metallic soaps have secondary antioxidant and heat-stabilizer benefits. Such effects are exploited in multipurpose formulations. 

The inclusion of heat stabilizers is essential to protect the system against thermal decomposition at elevated temperatures during processing. For this purpose, tin carboxylate esters or liquid calcium-zinc stabilizers are preferred. Thio-tin compounds are very effective as heat stabilizers but must be regarded with caution, bearing in mind that they can lead to unpleasant and unacceptable residual odours. Secondary stabilizers that can be used include epox-idized soya bean oil. 

The hindered phenolics are primary antioxidants and are normally used in conjunction with the thiodipropionate esters. These combinations are synergistic. They are of only limited value as heat stabilizers when used alone the thiodipropionate esters are secondary antioxidants, also of only limited value when used alone. 

There always has been an interest in additives that perform both a plasticising and heat stabilising effect. The most obvious of these is epoxidised soya bean oil (ESBO), or epoxidised esters, which can be incorporated at 2-6 phr (particularly with mixed metal heat stabilisers), to improve heat stability by HC1 absorption and also give a secondary plasticisation... 

An additive system was developed for poly(vinyl chloride) for medical applications. The additives include primary stabilisers (Ca-Zn stearate and Zn stearate), secondary stabilisers (epoxides) and lubricants (ethylene bisamide and high density polyethylene), to improve melt processing and heat stability. The use of the stabilisers resulted in reduced equipment down-time, increased the level of recycled material which could be incorporated, and enhanced the product characteristics, including colour, clarity, blush, aqueous extractables and particle generation. 5 refs. 

ZDEC is used in the following applications as a fast primary or secondary vulcanization accelerator in NR, SBR, IIR, EPDM and for natural and synthetic latex a stabilizer in butyl, butadiene, and urethane rubbers an antioxidant in rubber-based adhesive systems a stabilizer in cement a heat stabilizer for polyethylene. 

Despite the fact that APRT is coded for by a gene on chromosome 16, there seemed to be differences between APRT from normal and I.esch-Nyhan subjects in crude lysates. As the enzymes are purified, these differences lessen both in terms of heat stability, chromatographic and antigenic activity (R3, R4). Any differences must be reflective of secondary changes in the cell milieu and could be lost on purification. 

In films for food packaging, only FDA-approved plasticizers can be used. Most often, an adipate plasticizer, usually di(2-ethylhexyl) adipate (DEA), combined with epoxidized soybean oil is used. The oil epoxide is frequently termed a secondary plasticizer, because it is not used alone, and has heat-stabilizer properties in addition to plasticization effects. 

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