Plastic additives are compounds that must be added in order to improve the processing performance of the polymer (synthetic resin) during molding or to improve the performance of the resin itself. For example, plasticizers are added to lower the molding temperature of polyvinyl chloride resins and soften products. Another example is to add a foaming agent in order to prepare a light-weight, anti-vibration, thermal insulation, and sound insulation foam. The thermal decomposition temperature of some plastics is very close to the molding processing temperature, and it cannot be molded without adding a thermal stabilizer. Therefore, plastic additives occupy a particularly important position in the plastic molding process. A large class of additives are used in plastic molded products, including plasticizers, heat stabilizers, antioxidants, light stabilizers, flame retardants, foaming agents, antistatic agents, mildew inhibitors, colorants and additives whitener, filler, coupling agent, lubricant, releases agent.

Figure 1. Plastic additives and their application.

Applications:

• Plasticizer: Plasticizers are a class of additives that increase the plasticity of polymer resins and impart flexibility to products. They are by far the largest plastic additives category in terms of production and consumption. Plasticizers are mainly used for PVC soft products and are also widely used in polar plastics such as cellulose. The types of compounds involved in plasticizers generally include phthalates, fatty dicarboxylates, trimellitates, polyesters, epoxy esters, phenyl alkyl sulfonates, phosphates, and chlorinated paraffin. In particular, phthalates are the most important.

Figure 2. Phthalate plasticizer.

• Thermal stabilizers: Thermal stabilizers generally refer to stabilizers used in heat-sensitive resins like polyvinyl chloride and vinyl chloride copolymers. They are very easy to release hydrogen chloride when they are heated, which in turn leads to thermal aging degradation reactions. Thermal stabilizers generally achieve thermal stabilization by absorbing hydrogen chloride, replacing active chlorine, and double bond addition. The wide variety of heat stabilizers widely used in the industry includes basic lead salts, metal soaps, organic tins, organic antimony, and epoxy compounds, phosphites, polyols, and other main or organic auxiliary stabilizers.
• Antioxidant: The auxiliaries with the main function of inhibiting the thermal-oxidative degradation of polymer resins belong to antioxidants that are the most important plastic stabilization auxiliaries, and almost all polymer resins are related to the application of antioxidants. According to the action mechanism, the traditional antioxidant system generally includes the main antioxidant, auxiliary antioxidant, and heavy metal ion passivator. The main function of the main antioxidant is to capture polymer peroxy free radicals, also known as "peroxy radical scavengers" and "chain termination antioxidants". Auxiliary antioxidants, also known as "peroxide decomposers", can decompose polymer peroxides, including thio-dicarboxylic esters and phosphite compounds, and are usually used in conjunction with main antioxidants. Heavy metal ion passivators, commonly known as "anti-copper agents", can complex transition metal ions and prevent them from catalyzing the oxidative degradation of polymer resins, such as acyl hydrazides. The main function of the main antioxidant is to capture polymer peroxy free radicals, also known as "peroxy radical scavengers" and "chain termination antioxidants".
• Foaming agent: The additive used in polymer coordination system is called foaming agent, which obtains polymer products with microcellular structure by releasing gas to reduce the apparent density of the products. Inorganic foaming agents are mainly heat-sensitive carbonates (such as sodium carbonate, and ammonium bicarbonate), nitrites, and borohydrocarbons, which are characterized by endothermic foaming process, also known as endothermic foaming agents. The varieties of organic foaming include azo compounds, N-nitroso compounds, and sulfonylhydrazides. The foaming process of organic foaming agents is often accompanied by the exothermic reaction, which is also known as an exothermic foaming agent.

Figure 3. Benzenesulfonylhydrazide blowing agent.

References:

1. S. Wagner, M. Schlumme. (2020) “Additives in a circular economy of plastics: current dilemma, policy analysis, and emerging countermeasures.” Res. Conser. And Recycle. 158:104-110.
2. H. Nakata, A. Nishidome, N. Shikata. (2013) “Benzoriazole UV stabilizers in road dust and estimation for their amounts on the surface of the road.”J. Environ. Chem. 23:37-42.