In actual cooperation, in addition to requiring stabilizers to meet thermal stability needs, they often also require excellent processability, weather resistance, initial coloring ability, and light stability. There are also strict requirements for their odor and viscosity. At the same time, PVC products are also diverse, including pipes, sheets, blow-molded parts, injection-molded parts, foam products, paste resins, etc. Therefore, the choice of thermal stabilizers during PVC processing is very important, and most processing formulas need to be developed by the manufacturers themselves.
1.1 Organic Tin
(1) Excellent Transparency The greatest advantage of organic tin stabilizers is their excellent transparency. PVC formulations using organic tin stabilizers can produce crystal-like products. For this reason, organic tin can be used for bottles, containers, corrugated sheets, various types of rigid packaging containers, hoses, profiles, films, etc.
(2) Extraordinary Thermal Stability Currently, no other type of thermal stabilizer can surpass it in terms of thermal stability. Therefore, it is the preferred stabilizer for rigid PVC, and certain varieties also perform well in flexible products. It is suitable for all PVC homopolymers, such as emulsion, suspension, and bulk PVC, as well as copolymers, graft polymers, and blend polymers of vinyl chloride.
(3) Non-toxic Most organic tin stabilizers are non-toxic, and since the migration of organic tin stabilizers in rigid PVC is minimal, organic tin stabilizers are the preferred thermal stabilizers for food-contact PVC.
(4) Good Compatibility Organic tin stabilizers have good compatibility with PVC, so phenomena commonly seen with lead salt stabilizers and metal soap stabilizer systems, such as precipitation on metal surfaces, generally do not occur.
(5) Poor Lubricity Sulfur-containing tin stabilizers have slightly poorer self-lubricity, so many commercially available sulfur-containing organic tin stabilizers are combined with lubricants to prevent the hot melt from adhering to processing equipment during processing.
(6) Expensive Compared to other types of stabilizers, organic tin stabilizers have comprehensive performance that is closer to the ideal stabilizer. However, all organic tin stabilizers, regardless of their structure, have the main disadvantage of much higher manufacturing costs than lead-based stabilizers or metal soap compounds. In recent years, the use of new synthetic technologies or reducing their usage in formulations has led to a decrease in formulation costs. In the 1970s, low-cost tin products were developed abroad, reducing the tin content and also lowering the price to some extent.
1.2 Lead Salt
(1) Excellent Stability Experiments have shown that among commonly used basic lead salts, the heat resistance of sulfites is superior to that of sulfates, while sulfates have better heat resistance than phosphites. The effective lead content of tribasic lead sulfate, which is widely used in the PVC industry, is relatively high, showing more outstanding thermal stability than other products.
(2) Excellent Insulation Since lead salts are non-ionic and non-conductive, they are inert, which makes lead salt stabilizers widely used in the wire and cable industry.
(3) Excellent Weather Resistance Many salt compounds can act as white pigments, exhibiting strong coverage, thus having strong weather resistance.
(4) Poor Transparency Transparency is an issue related to weather resistance. When used in wire, cable, and record materials, there is no need to worry about transparency, as these products are mostly white or very dark.
(5) Low Price Lead salt stabilizers are the cheapest among all stabilizer varieties. Therefore, despite the continuous introduction of new stabilizers, lead salt stabilizers still dominate the stabilizer market half a century later. Composite lead salts introduced to solve dust and dispersion issues have increased in price but still maintain a competitive advantage over other types of stabilizers.
(6) Toxicity The toxicity of lead salt stabilizers limits their application in many situations with strict sanitary requirements. For example, many countries have revised the standards for lead content in drinking water, making it impossible to use lead salts in PVC water pipes.
(7) Poor Dispersion Lead salts have poor dispersion, but newly launched packaged products have incorporated lubricants, which have somewhat solved the dispersion issue. Because lead salt stabilizers have the above characteristics, they are particularly suitable for high-temperature processing and are widely used in various opaque hard and soft products as well as cable materials, such as various pipes, sheets, indoor and outdoor profiles, foam plastics, artificial leather, and wires, cables, records, welding rods, etc. The most important lead salt stabilizers are tribasic lead sulfate, dibasic lead phosphite, and dibasic lead stearate.
1.3 Organic Antimony
(1) Good Stability At the same temperature, organic antimony stabilizers have color stability and lower melt viscosity similar to organic tin. In twin-screw extrusion processes, the effect is particularly prominent when used with calcium stearate.
(2) Lower Price Organic antimony stabilizers are much cheaper than methyl tin or butyl tin. In addition, the usage of organic antimony is lower, so using organic antimony can achieve a better performance/price balance.
(3) Non-toxic In the United States, when manufacturing PVC pipes using twin-screw extruders, PVC made from formulations containing antimony-based stabilizers, calcium stearate, and other lubricants complies with the NSF (National Sanitation Foundation) regulations.
(4) Poor Transparency and Light Stability The transparency of organic antimony compounds is not as good as that of organic tin stabilizers and is lower than that of barium/zinc and calcium/zinc metal soap systems, being close to that of lead salts. Light stability is also poor, so antimony stabilizers are mostly used in indoor products with no color requirements. Antimony stabilizers themselves also need to be stored in opaque containers.
(5) Poor Lubricity Antimony stabilizers have poor lubricity, so their use invariably requires a large amount of lubricant.
1.4 Metal Soap
(1) Tin Soap Stabilizers Tin soap is the best-performing type among metal soaps, with advantages including no initial coloring, the ability to produce colorless transparent products, excellent light stability, and the effect of preventing precipitation adhesion. However, due to the toxicity of tin salts, there are strict regulations on their manufacture and use in occupational safety and health laws. In recent years, the use of tin soap has shown a downward trend.
(2) Zinc soap stabilizers have extremely poor thermal stability for PVC. Samples with added zinc soap turn black sharply when heated, resulting in the so-called "zinc burning" phenomenon. However, they have the following advantages: excellent initial coloring; good anti-fouling effect; can improve weather resistance; many zinc soaps are recognized as non-toxic stabilizers, so they can be used with calcium soaps in non-toxic formulations.
(3) Barium soap stabilizers have good thermal stability and excellent lubricity, but they produce red initial coloring during processing and are prone to roller sticking.
(4) Calcium soap stabilizers have poor stability, but they are recognized as non-toxic additives worldwide and have excellent lubricity.
(5) Others metal soap stabilizers used in industry include magnesium stearate, stearic acid, aluminum stearate, potassium stearate, etc. Among them, magnesium stearate is similar to calcium stearate and can be used for food contact materials; aluminum stearate is similar to zinc stearate and is permitted for food packaging by the US FDA and the Japan Vinyl Food Sanitation Association; stearic acid and potassium stearate are also non-toxic products and serve as alternatives to lead salts, tin soaps, and barium soaps.
(6) Composite metal soap stabilizers have diverse performance requirements in the processing industry, and single metal soaps often cannot meet usage requirements. Therefore, the use of composite stabilizers has become a trend. In the PVC industry, single metal soap compounds are rarely used; instead, composites of several metal soaps are common. This composite is not a simple addition of performance but utilizes the synergistic effects between components. Composite metal soap stabilizers generally include a stabilizer base (i.e., metal soap), solvents (organic solvents, plasticizers, liquid non-metal stabilizers, etc.), and functional additives (auxiliary stabilizers, transparency enhancers, light stabilizers, lubricants, etc.). They can be classified into solid composites and liquid composites based on form, and into calcium/zinc composite stabilizers, barium/lead composite stabilizers, barium/zinc composite stabilizers, etc., based on main components. Among them, calcium/zinc composite stabilizers play a crucial role in replacing toxic metals due to their non-toxicity.
1.5 Rare Earth Stabilizers
(1) Excellent thermal stability Rare earth stabilizers have better thermal stability than traditional lead salt systems and barium/zinc, barium/lead/zinc stabilizers. In certain applications, rare earth stabilizers can partially or completely replace organic tin.
(2) Good transparency The refractive index of rare earth stabilizers is very close to that of PVC resin, allowing them to replace traditionally used organic tin in products requiring higher transparency.
(3) Excellent weather resistance Rare earth elements can absorb ultraviolet light in the range of 230-320nm, thus rare earth stabilizers have anti-light aging properties, making them suitable for outdoor products such as PVC corrugated sheets and window materials.
(4) Excellent electrical insulation Some multifunctional rare earth stabilizers can replace lead salt stabilizers in cable formulations, with electrical insulation properties comparable to lead salts.
(5) Non-toxic, safe, and hygienic Rare earth elements are low-toxicity elements, posing no toxic harm to humans during their production, processing, transportation, and storage. Rare earth stabilizers are non-toxic products that can be used for food packaging and pharmaceutical packaging.
(6) Slightly poorer processing performance When the amount of rare earth stabilizers is large, the material's roll-off properties are not ideal, with a tendency to press out. Generally, better results can be achieved by using stearic acid or calcium stearate in combination.
In summary, rare earth stabilizers can be used in water supply and drainage pipes, injection molded fittings, window frame profiles, door panels, wall panels, wire troughs, foamed products, artificial leather, cable materials, soft and hard transparent products, food packaging materials, etc.
1.6 Auxiliary Stabilizers Auxiliary stabilizers include phosphoric acid esters, epoxy soybean oil, hindered phenols, etc., mainly relying on the synergistic effect with metal stabilizers to improve stabilization effects, generally referred to as co-stabilizers. Compounds such as amine-based ricinoleic acid esters, 2-phenyl-2-propanol, pulse derivatives, and 1,2-diketones can not only improve the effects of metal stabilizers when used together but also possess certain stabilization efficacy themselves. These compounds are usually referred to as pure organic stabilizers. The development of PVC stabilizers has reached a point where the progress of metal stabilizers is relatively slow, while research and development of auxiliary stabilizers are unprecedentedly active, forming a significant trend in the field of PVC stabilizers. Auxiliary stabilizers are rarely used alone and are often used in conjunction with main stabilizers to improve initial coloring or enhance long-term stability.