Heat stabilizer

Heat stabilizers are used to prevent degradation of polymers when they are exposed to heat, especially during processing. For most polymers, the primary mode of reaction is oxidation, so antioxidants function effectively as heat stabilizers. For a few sensitive polymers, however, other forms of reaction are more important. In particular, a major disadvantage of PVC is its poor thermal stability. Degradation takes the form primarily of dehydrochlorination, yielding HCl and resulting in formation of a double bond in the main polymer chain. 

The additives in PVC bottles for cooking oil and other food products must have FDA clearance. For flexible packaging materials, the most common stabilizers are mixed metals such as barium-zinc and calcium-zinc, which replaced the older cadmium-zinc formulations. For rigid blow molded containers and calendered sheets, or- 

Certain polymers require heat stabilizers because of degradation at high processing and performance temperatures. This is notably the case for polyvinyl chloride (PVC), which undergoes dehydrochlorination [43,44]. 

HCl damages steel in processing equipment and discolors the polymer [45] (e.g., coloring it green). 

Typical thermal stabilizers for PVC are organic metal salts and soaps, phosphite, esters, and epoxy compounds. Examples are barium/zinc carboxylates [46], barium/cadmium carboxy-lates [47], calcium/zinc stearate [48], lead stearate [49],organotinmercaptides[50],organotin sulfides, organotin carboxylates[51], among others. Costabilizers such as 1,3-diketones, dihydropyridines, epoxy plasticizers, P-ketocarboxylic acid esters, phenolic antioxidants, a-phenylindols, and phosphates are used to improve the stabilizer s elfectiveness. 

When polymers are overheated, many polymer chains, notably a olefins such as polypropylene, break down to form radicals, which propagate as shown below. 

Heat stabilizers, which are essentially antioxidants, interfere with the chain propagation of a polymer radical in two ways. The addition of stabilizer/inhibitor (In-H) stabilizes the peroxy radicals 

According to their major components, common heat stabilizers can be divided into base heat stabilizers, fatty acids, organic tin compounds, composite heat stabilizers, and pure organic compounds. 

Base heat stabilizers are inorganic and organic acid lead salts combined with a base. They have excellent heat resistance, weathering resistance, and electrical insulating properties, as well as low cost, poor transparency, and a certain toxicity the concentration generally is 0.5%-5.0%. 

Fatty acid heat stabilizers are compounds consisting of fatty acids and metal ions and are also known as metal soap heat stabilizers. Their performances depend on the types of metal ions and acid radical groups. The general concentration is 0.1%-3.0%. 

Composite heat stabilizers are liquid or solid compounds based on salts or metal soaps and compounds based on organic tin. Metal salts include Ca-Mg-Zn, Ba-Ca-Zn, Ba-Zn, and Ba-Cd common organic acids include organic fatty acids, naphthenic acid, oleic acid, benzoic acid, and salicylic acid. 

Organic compound heat stabilizers include not only primary stabilizers that can be used alone (mainly organic nitrogen compounds), but also polyalcohols and phosphite esters with a high boiling point. Phosphite esters are often used together with metal stabilizers to enhance the weather resistance and transparency of composite materials and improve surface color and luster of products. 

The incorporation of UV stabilizer is necessary for protection against UV degradation. Enhanced UV light stability can affect the color stability and the mechanical properties of plastics. UV stabilizers protect PE from damage by sunlight in outdoor environments. Similarly, carbon black in PE mulch film protects and blocks the penetration of radiation. 

Heat stabilizers are targeted exclusively at PVC. Plasticizers are almost as dependent on PVC as heat stabilizers. Lead stabilizers have served the PVC industry as heat stabilizers. The European Commission has highlighted concerns about the dangerous nature of lead stabilizers which should be phased out, but did not indicate a specific timetable. The commission also highlighted that lead stabilizer would not be approved for use in PVC pipes after 2003. In America, unlike in Europe, window and door profiles were stabilized with tin stabilizers. However, in Asia, lead stabilizers still are in use along with PVC. 

Stabilizers of fairly low thermal stabilizing performance include combinations of zinc, cadmium, lead, potassium, and barium which give better thermal stability. 

Antimony oxide can also be prepared from the hydrolysis of antimony trichloride. Antimony trichloride is prepared by the reaction of calcium chloride with stibnite and oxygen. 

PVC is an important and widely used polymer but it has poor thermal stability. With heat or light, it can undergo dehydrochlorination reactions, forming alkenes. 

This reaction is sometimes called unzippering and is thought to originate at certain structural defects within the polymer [23]. 

Nine structural defects have been shown to occur in all samples of commercial PVC and they include some structures which contain tertiary chlorines as seen in two of the identified defect structures. 

When the PVC unzippers, it discolors and becomes brittle. To prevent this dehydrochlorination reaction, heat stabilizers are added. Lead stabilizers are effective and a major type of heat stabilizer. Data from 2003 [24] shows lead stabilizers to represent more than 50% of the world PVC stabilizers, but less than 20% in the U.S. where tin stabilizers predominate. The use of lead stabilizers is on the decline and being phased out due to concerns about the health effects of lead. Nonetheless, the news occasionally reports that lead has been found in a toy or other PVC article imported from a country such as China. The source of lead is likely due to the stabilizer. Another type of heat stabilizers is the mixed metal packages which are based upon mixtures of different 

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