Plastic Additives: Polymer Stabilizers, Plasticizers and Lubricants
During the manufacturing of polymeric materials, such as plastics and rubbers, a number of chemical additives are added to inhibit or retard their degradation or to achieve better product quality. Heat stabilizers, antioxidant, UV absorber, plasticizers and lubricants are the most commonly seen and widely used additives.
Heat stabilizers are mainly used for polyvinyl chloride and its copolymers. During the thermal processing of polyvinyl chloride (PVC), a small amount of molecular chain breaks before reaching the melt flow and releases hydrogen chloride, and hydrogen chloride is a kind of catalyst that will accelerate the molecular chain breaking reaction. Therefore, if the newly decomposed hydrogen chloride is not eliminated in time, the polymer chain will be continuously cracked into low molecular compounds, so that plastics such as polyvinyl chloride cannot be well-processed and well-formed. Adding appropriate alkaline substances to PVC can immediately neutralize the decomposed hydrogen chloride to achieve the purpose of stabilizing PVC. Such substances are called stabilizers.
In general, the commonly used heat stabilizers can be divided into main stabilizers and auxiliary stabilizers. Main stabilizer are mainly salts and soaps containing heavy metal cations such as lead, calcium, cadmium, zinc, barium, aluminum, lithium, and strontium. Among them, lead sulfate and lead stearate are the most widely used. Auxiliary stabilizationmainly refers to epoxidized oils and esters, which also have certain plasticizer functions.
Antioxidants are a class of chemical substances that, when present in a small amount in the polymer system, can delay or inhibit the oxidation process of the polymer, thereby increasing the service life of the polymer. Commonly used plastic antioxidants are generally divided into five categories according to molecular structure and mechanism of action, that is, hindered phenols, phosphites, thios, hindered amines (HALS) and composites.
Polymers are subject to photodegradation when exposed to UV light. UV absorbers are a class of chemical substances that can absorb ultraviolet light or reduce ultraviolet transmission. It can convert high-energy ultraviolet light into heat energy or non-destructive longer light waves to release energy, thereby protecting polymers from UV damage.
UV absorbers can be divided into the following categories according to their chemical structures: salicylates, benzophenones, benzotriazoles, substituted acrylonitriles, triazines and hindered amines.
Light shielding agent
Light shielding agent is a kind of substance that can absorb light waves harmful to polymers, and then convert light energy into heat energy to scatter or reflect light waves, thereby shielding polymers. Light shielding agents mainly include carbon black, zinc oxide, titanium dioxide, zinc barium white and other black or white substances that can absorb or reflect light waves.
In addition to the above-mentioned three polymer stabilizers, many other types of additives, such as plasticizers and lubricants, are also added in plastic processing to improve rheology and molding properties, usually with low viscosity or surface energy, and with a certain lubricating effect.
Plasticizers can increase the flexibility, extensibility, and plasticity of plastics because of their ability to weaken the van der Waals forces between polymer molecules, thereby increasing the mobility of polymer molecular chains and reducing the crystallinity of polymer molecular chains. Temperature and hardness are conducive to the molding of plastic products. Commonly used plasticizers include phthalates, sebacates, and chlorinated paraffins.
Plasticizers can be divided into two categories: primary plasticizers and secondary plasticizers. The former is characterized by good compatibility with resin, high plasticizing efficiency, migration resistance, low volatility, low oil (water) extraction, and low temperature flexibility, while the latter has poor compatibility with resin, and is mainly used in conjunction with primary plasticizers to reduce costs.
Lubricant can decrease the friction and adhesion between the particles of each layer of plastic, increase the fluidity of the resin, and can control the plasticizing time of the resin to maintain continuous production. Lubricants can be divided into external lubricants and internal lubricants. The main function of the external lubricant is to enable the polymer melt to smoothly leave the hot metal surface of the processing equipment. The compatibility between the external lubricant and the polymer is poor, and only a thin lubricant layer is formed at the interface between the polymer and the metal. The most commonly used external lubricant is stearic acid and its metal salts. The internal lubricant has good compatibility with the polymer, which can reduce the cohesion between the polymer molecules, thereby helping the polymer flow and reducing the temperature rise caused by internal friction heat.
A release agent is a functional substance between the mold and the finished product. The release agent is chemically resistant and does not dissolve in contact with different resin. The release agent also has heat and stress resistant properties, and is not easy to decompose or wear. Due to the rapid development of injection molding, extrusion, calendering, molding, lamination and other processes, the demand for mold release agent has also been greatly increased.
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