Plastic additives (or additives) are of various categories and have different functions. They can be divided into five categories according to functions and effects of additives: synthetic additives, cross-linking additives, functional additives, stability additives, and process additives.

Various additives

 

Definition: A plastic additive that improves interface properties between synthetic resin and inorganic filler or reinforcing material in plastic compounding. Also known as surface modifier. It can reduce viscosity of synthetic resin melt during plastic processing, improve dispersion of fillers to improve processing performance, thus enable product to obtain good surface quality and mechanical, thermal and electrical properties. Its dosage is generally 0.5-2% of filler dosage. Coupling agents generally consist of two parts: one part is an inorganic-friendly group that can react with inorganic fillers or reinforcing materials; the other part is an organic-friendly group that can react with synthetic resins.
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Definition: Compatibilizer, also known as compatibilizer, refers to an auxiliary agent that uses bond force between molecules to promote combination of two incompatible polymers to obtain a stable blend. Here it refers to a polymer compatibilizer. PE-g-ST, PP-g-ST, ABS-g-MAH, PE-g-MAH, PP-g-MAH, etc. are used in plastic modification to obtain a blending material with good performance.
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Definition: Dispersant is a surfactant with two opposite properties of lipophilicity and hydrophilicity in molecule. It can evenly disperse solid particles of inorganic and organic pigments that are difficult to dissolve in liquids, at the same time prevent sedimentation and agglomeration of solid particles to form agent required for stable suspension.
Dispersant is an auxiliary agent that promotes uniform dispersion of various auxiliary materials in resin, is mostly used in masterbatch, coloring products and highly filled products. Including hydrocarbons (paraffin oil, polyethylene wax, oxidized polyethylene wax, etc.), fatty acid soaps, fatty esters and fatty amides, etc.
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Definition: Lubricants are added to improve fluidity and demolding properties of plastics, especially thermoplastics, during processing and molding process. Their main function is to reduce friction between plastic material and processing machinery, between molecules inside plastic material during processing, thereby improving processing performance of plastic and improving performance of product.
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Mechanism: Surface tension of lubricant molecule is relatively small. During molding process, it migrates to surface of sample to form a thin layer of lubricant molecules, which reduces surface scratch coefficient and scratch visibility.
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An auxiliary agent whose main function is to inhibit thermal oxidation degradation of polymer resin belongs to category of antioxidant. Antioxidants are main type of plastic stabilization aids, and almost all polymer resins involve application of antioxidants. According to mechanism of action, traditional antioxidant systems generally include primary antioxidants, auxiliary antioxidants, and heavy metal ion passivators.
Primary antioxidant has main function of capturing polymer peroxyl radicals, is also known as "peroxyl radical capture agent" and "chain termination antioxidant", involving two major series of products: aromatic amine compounds and hindered phenol compounds. Auxiliary antioxidants have function of decomposing polymer peroxy compounds, also known as "peroxide decomposers", including thiodicarboxylic acid esters and phosphite compounds, and are usually used in combination with primary antioxidants. Heavy metal ion passivators are commonly known as "anti-copper agents", which can complex transition metal ions to prevent them from catalyzing oxidative degradation reaction of polymer resins. Typical structures include hydrazide compounds.
In recent years, with in-depth study of polymer antioxidant theory, classification of antioxidants has also undergone certain changes. The most prominent feature is introduction of concept of "carbon radical capture agent". This free radical scavenger is different from traditional main antioxidant. They can capture polymer alkyl free radicals, which is equivalent to adding a line of defense in traditional antioxidant system. Such stabilizing agents reported today mainly include aromatic benzofuranone compounds, bisphenol monoacrylate compounds, hindered amine compounds and hydroxylamine compounds. Ternary antioxidant system composed of them and main antioxidant and auxiliary antioxidant can significantly improve antioxidant stabilization effect of plastic products.
It should be pointed out that amine antioxidants have coloring pollution and are mostly used in rubber products, while phenolic antioxidants and their composite antioxidant systems composed of auxiliary antioxidants and carbon free radical scavengers are mainly used in plastics and bright-colored rubber products.

 

Broadly speaking, all additives that can improve impact resistance of hard polymer products are collectively referred to as impact modifiers. Impact modifier in traditional sense is basically based on theory of elastic toughening, compounds involved are almost without exception various copolymers and other polymers with elastic toughening effects.
Taking rigid PVC products as an example, varieties widely used in application market today mainly include chlorinated polyethylene (CPE), acrylate copolymer (ACR), methacrylate-butadiene-styrene copolymer (MBS), ethylene-vinyl acetate copolymer (EVA) and acrylonitrile-butadiene-styrene copolymer (ABS). Ethylene propylene diene monomer rubber (EPDM) used in toughening modification of polypropylene also belongs to scope of rubber toughening.

 

Most plastic products are flammable, which brings many hidden dangers to application safety of their products. To be precise, it is more appropriate to call flame retardants flame retardants, because "flame retardant" contains two meanings of flame retardancy and smoke suppression, which is more extensive than concept of flame retardants. However, people have been accustomed to using concept of flame retardants for a long time, so flame retardants referred to in literature are actually general term for flame retardant and smoke suppression additives.
Flame retardants can be divided into additive flame retardants and reactive flame retardants according to their usage. Additive flame retardants are usually added to base resin, and they are simply physically mixed with resin; reactive flame retardants are generally monomers containing flame retardant elements and reactive groups in molecule, such as halogenated acid anhydrides, halogenated bisphenols and phosphorus-containing polyols. Due to their reactivity, they can be chemically bonded to molecular chain of resin and become part of plastic resin. Most reactive flame retardant structures are still monomers for synthesizing additive flame retardants.
According to different chemical compositions, flame retardants can also be divided into inorganic flame retardants and organic flame retardants. Inorganic flame retardants include aluminum hydroxide, magnesium hydroxide, antimony oxide, zinc borate and red phosphorus, etc. Organic flame retardants are mostly halogenated hydrocarbons, organic bromides, organic chlorides, phosphates, halogenated phosphates, nitrogen flame retardants and nitrogen-phosphorus intumescent flame retardants. Function of smoke suppressants is to reduce smoke emission and toxic and harmful gas release of flame retardant materials. Most of them are molybdenum compounds, tin compounds and iron compounds. Although antimony oxide and zinc borate also have smoke suppression properties, they are often used as flame retardant synergists and are therefore classified as flame retardant systems.

 

Plasticizers are a type of additive that increases plasticity of polymer resins and gives products flexibility. They are also the largest category of plastic additives produced and consumed to date. Plasticizers are mainly used in PVC soft products, and are also widely used in polar plastics such as cellulose.
Types of compounds involved in plasticizers generally include phthalates, fatty dicarboxylates, trimellitic acid esters, polyesters, epoxy esters, alkyl sulfonate phenyl esters, phosphate esters and chlorinated paraffins, especially phthalates.

 

If not specified, heat stabilizers refer specifically to stabilizers used in processing of polyvinyl chloride and vinyl chloride copolymers. Polyvinyl chloride and vinyl chloride copolymers are heat-sensitive resins. They are very easy to release hydrogen chloride during heat processing, which in turn triggers thermal aging degradation reactions. Heat stabilizers generally achieve purpose of heat stabilization by absorbing hydrogen chloride, replacing active chlorine and double bond addition.
Types of heat stabilizers widely used in industry generally include primary stabilizers such as basic lead salts, metal soaps, organic tin, organic antimony, and organic auxiliary stabilizers such as epoxy compounds, phosphites, polyols, and diketones. Composite stabilizer varieties composed of primary stabilizers, auxiliary stabilizers and other additives play an important role in heat stabilizer market.

 

Light stabilizers, also known as ultraviolet stabilizers, are a type of stabilizing additives used to inhibit photo-oxidative degradation of polymer resins and improve weather resistance of plastic products. According to different stabilization mechanisms, light stabilizers can be divided into light shielding agents, ultraviolet absorbers, excited state quenchers and free radical scavengers.
Light shielding agents are mostly carbon black, zinc oxide and some inorganic pigments or fillers, and their function is achieved by shielding ultraviolet rays. Ultraviolet absorbers have a strong absorption effect on ultraviolet rays, and convert harmful light energy into harmless heat energy through intramolecular energy transfer, thereby preventing polymer resins from absorbing ultraviolet energy and inducing photooxidation reactions.
There are many types of compounds involved in ultraviolet absorbers, mainly including benzophenone compounds, benzotriazole compounds, salicylate compounds, substituted acrylonitrile compounds and triazine compounds. Excited state quenchers are intended to quench the energy on excited polymer molecules, restore them to ground state, and prevent them from further causing breakage of polymer chains. Excited state quenchers are mostly some nickel complexes. Free radical scavengers use hindered amines as functional groups, and their corresponding nitroxide free radicals are basis for capturing polymer free radicals. Moreover, since this nitroxide free radical is regenerative during stabilization process, light stabilization effect is very outstanding. So far, it has developed into light stabilizer category with the most varieties and the largest production and consumption.
Of course, role of hindered amine light stabilizers is not limited to capturing free radicals. Studies have shown that hindered amine light stabilizers often have functions of decomposing hydroperoxides and quenching singlet oxygen.

 

Auxiliary agent used in polymer compounding systems to obtain polymer products with microporous structures by releasing gas to reduce apparent density of products is called a blowing agent. According to different ways of generating gas during foaming process, foaming agents can be divided into two main types: physical foaming agents and chemical foaming agents.
Physical foaming agents generally release gas by changes in their own physical state, and are mostly volatile liquid substances. Fluorochlorocarbons (such as Freon), low alkanes (such as pentane) and compressed gases are representatives of physical foaming agents.
Chemical foaming agents foam based on gas released by chemical decomposition. According to different structures, they are divided into inorganic chemical foaming agents and organic chemical foaming agents. Inorganic foaming agents are mainly some heat-sensitive carbonates (such as sodium carbonate, ammonium bicarbonate, etc.), nitrites and boron hydride compounds, etc., which are characterized by endothermic foaming process, also known as endothermic foaming agents. Organic foaming agents have a very prominent position in plastic foaming agent market. Representative varieties include azo compounds, N-nitroso compounds and sulfonyl hydrazide compounds. Foaming process of organic foaming agents is often accompanied by exothermic reactions, and they are also known as exothermic foaming agents. In addition, some additives that can adjust decomposition temperature of foaming agent, that is, foaming additives, are also included in category of foaming agents.

 

Function of antistatic agent is to reduce surface resistance of polymer products and eliminate electrostatic hazards that may be caused by static electricity accumulation. According to different usage methods, antistatic agents can be divided into two types: internal addition type and coating type.
Internal addition type antistatic agent is added to plastic formula by adding or blending. After molding, it migrates from inside of product to surface or forms a conductive network, thereby achieving purpose of reducing surface resistance and discharging charge.
Coating type antistatic agent is attached to surface of plastic products by coating or infiltration, thereby absorbing moisture in environment and forming an electrolyte layer that can discharge charge.
From perspective of chemical composition, traditional antistatic agents are almost without exception surfactant compounds, including quaternary ammonium salt cationic surfactants, alkyl sulfonate anionic surfactants, non-ionic surfactants such as alkanolamines, alkanolamides and polyol fatty acid esters, etc. However, newly emerged "high molecular weight permanent antistatic agents" broke this convention. They are generally hydrophilic block copolymers, which are combined with base resins in the form of alloys to conduct charges by forming conductive channels. Compared with surfactant antistatic agents, this high molecular weight permanent antistatic agent will not be lost due to migration, volatilization and extraction, so antistatic property is long-lasting and stable, and is rarely affected by environmental humidity.

 

Mold inhibitors, also known as microbial inhibitors, are a type of stabilizing agent that inhibits growth of microorganisms such as mold and prevents polymer resins from being eroded and degraded by microorganisms. Most polymer materials are not sensitive to mold, but their products are susceptible to mold because plasticizers, lubricants, fatty acid soaps and other substances that can breed mold are added during processing. Plastic mold inhibitors contain many chemical substances, and the more common varieties include organic metal compounds (such as organic mercury, organic tin, organic copper, organic arsenic, etc.), nitrogen-containing organic compounds, sulfur-containing organic compounds, halogen-containing organic compounds and phenol derivatives.

 

Fluorescent brightener is a fluorescent dye, or white dye, and is also a complex organic compound. Its characteristic is that it can excite incident light to produce fluorescence, so that dyed material obtains a glittering effect similar to fluorite, making material seen by naked eye very white.

 

Filling and reinforcement are important ways to improve physical and mechanical properties of plastic products and reduce cost of matching. Reinforcing materials involved in plastics industry generally include fibrous materials such as glass fiber, carbon fiber, and metal whiskers. Fillers are an incremental material with a lower matching cost, including calcium carbonate, talcum powder, clay, mica powder, silica, fly ash, red mud, and natural minerals such as wood powder and cellulose, synthetic inorganic substances and industrial by-products.
In fact, it is difficult to distinguish between reinforcing agents and fillers, because almost all fillers have a reinforcing effect. Due to large amount of fillers and reinforcing agents used in plastics, some have formed their own industry system, and are usually not discussed in category of processing aids. It should be noted that modification effect of nano-filled reinforcement materials on plastics, which are widely studied today, has far exceeded significance of filling and reinforcement. Their application will bring a new revolution to plastics industry.
Coupling agents are surface modifiers for inorganic and natural filling and reinforcement materials. Since reinforcement and filling materials in plastics industry are mostly inorganic materials and amount of compounding is large, direct compounding with organic resins often leads to a decrease in processing and application performance of plastic compounds. As a surface modifier, coupling agents can organicize surface of inorganic materials through chemical or physical effects, thereby increasing amount of compounding, improving processing and application performance of compound. Coupling agents reported generally include long-chain fatty acids, silane compounds, organic chromium compounds, titanate compounds, aluminate compounds, zirconate compounds, and anhydride-grafted polyolefins.

 

Crosslinking of plastics is essentially not much different from vulcanization of rubber, but use of crosslinking aids is not exactly same. There are two main ways to crosslink resins: radiation crosslinking and chemical crosslinking. Organic peroxides are the most widely used crosslinking agent in industry. Sometimes, in order to increase degree and speed of crosslinking, it is often necessary to use some co-crosslinking agents and crosslinking accelerators. Co-crosslinking agents are used to inhibit free radical breakage reaction that may be produced by organic peroxide crosslinkers on main chain of polymer resins during crosslinking process, improve crosslinking effect, and improve performance of crosslinked products. Their role is to stabilize polymer free radicals.
Main function of crosslinking accelerators is to accelerate crosslinking speed and shorten crosslinking time. Curing agents for thermosetting plastics such as unsaturated polyesters and epoxy resins also belong to category of crosslinking agents. Common types include organic amines and organic anhydride compounds. In addition, photosensitizers used in ultraviolet radiation crosslinking processes can also be regarded as crosslinking aids.

 

Mold release agent can be applied to surface of molds or processing machinery, or added to base resin to make molded products easier to demold and improve their surface finish. The former is called a coating-type mold release agent, which is main body of mold release agent, and the latter is an internal mold release agent, which has characteristics of easy operation. Silicone oil substances are the most widely used type of mold release agent in industry.