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What are the commonly used additives for plastic raw materials before injection molding?

March 14, 2024

Different types of additives need to be added to plastic raw materials during injection molding to meet different processing and application needs, such as plasticizers, flame retardants, colorants, heat stabilizers, lubricants, foaming agents, and antioxidants. Additives dispersed in the molecular structure of plastics do not seriously affect their molecular structure, but can improve their properties or reduce costs as chemical substances. The addition of additives can promote the improvement of the processability, physical and chemical properties of plastics and increase the physical and chemical properties of substrates.




The vast majority of synthetic resins have plasticity, but the magnitude of plasticity varies. In order to make the resin easy to plasticize and give the product flexibility, low molecular weight substances are generally added to the resin during processing, which are called plasticizers. Plasticizers are liquids or low melting point substances that should have good compatibility with resins. The commonly used plasticizers include phthalates, aliphatic dicarboxylic acid esters, phosphates, chlorinated paraffin, etc.


Flame retardant


Flame retardants are additives that can prevent plastic ignition or suppress flame spread. They are mostly inorganic or organic compounds containing elements such as halogens, phosphorus, antimony, boron, aluminum, etc. According to their usage, they can be divided into two categories: reactive and additive. Reactive flame retardants, as monomers, participate in the polymerization reaction of synthetic resins and have a relatively small impact on the properties of plastics. Additive flame retardants are commonly mixed into synthetic resins during the mixing process of plastics, which is convenient to use and has strong adaptability, but often affects the properties of plastics. Common varieties include antimony trioxide (antimony white), aluminum oxide trihydrate, zinc borate, zinc metaborate, tetrabromobutane, hexabromobiphenyl, tris (2,3-dichloropropyl) phosphate, etc. Most flame retardants often exert their functions through multiple mechanisms, therefore, multiple flame retardants are often used simultaneously to achieve the best synergistic effect.


Due to the increasingly widespread application of plastics in industrial fields such as construction, automobiles, and airplanes, and the increasingly strict requirements for flame retardancy, the study of the synergistic formula of flame retardants has become an important practical research topic. In addition, the physiological effects caused by the smoke and toxic gases generated by plastic combustion are increasingly being taken seriously. Therefore, developing non volatile flame retardants to increase the surface coking layer and its stability, and reduce the emission of toxic gases during combustion is also one of the key topics in contemporary flame retardant research.




Colorants, as raw materials in plastic processing, play a role in beautifying, decorating, facilitating identification, improving weather resistance, enhancing mechanical properties, and improving optical properties. The commonly used colorants include titanium dioxide (titanium dioxide), zinc powder (zinc oxide), cadmium red, iron trioxide, carbon black, chromium yellow, zinc yellow, and Hansa yellow. Among them, titanium dioxide can not only be used as a colorant, but also widely used in the plastic industry due to its reinforcement, anti-aging, and filling effects. Due to its sun resistance, non cracking, and non-toxic properties, titanium dioxide is increasingly used in cosmetics. In addition, it is far superior to lead white, and almost all types of fragrances use titanium dioxide instead of lead white and zinc white.


Heat stabilizer


A heat stabilizer is an additive that can prevent the degradation of plastics due to heating. Due to the outstanding thermal sensitivity of polyvinyl chloride, heat stabilizers are often used in the blending of polyvinyl chloride plastics. According to their chemical structure, they can be divided into four categories: lead salts, mixed metal salts, organic tin, and specific purpose heat stabilizers.


Lead salt: the earliest type of thermal stabilizer used. It has excellent long-term thermal stability, weather resistance, and electrical insulation, but it affects the transparency of the product, is toxic, has initial coloring, is prone to sulfur pollution, and has poor compatibility and dispersibility with polyvinyl chloride. Lead salts have no lubricity, so they should be used in combination with metal soap lubricants. Common varieties include tribasic lead sulfate and dibasic lead phosphite. Commonly used in the manufacturing of opaque PVC sheets, pipes, wires, and cable sheaths. Composite metal salts: the most commonly used type of heat stabilizer. It is often sold in the pre prepared form of liquid, paste, or powder. Common varieties include advanced fatty acid salts of barium cadmium, barium calcium zinc, barium zinc, calcium zinc, and calcium magnesium tin zinc. This type of heat stabilizer is often used in combination with organic auxiliary agents (such as phosphites, epoxy compounds, polyols, and phenolic antioxidants) to form a complex heat stabilizer that meets the requirements of different processing processes and product applications. Organic tin: This type of heat stabilizer is mainly used for various soft polyvinyl chloride products that require transparency. Common varieties include maleic acid esters, thiol salts, and carboxylic acid esters. Among them, di-n-octyltin maleate and S, S' - bis (isooctyl thioglycolate) di-n-octyltin can be used as non-toxic stabilizers in food and pharmaceutical packaging materials. Special purpose heat stabilizer: refers to some pure organic compounds with specific effects, such as those used in alkaline lotion polymerization of PVC α—— Phenylindole, amino crotonate ester, pentaerythritol or dicyandiamide used in asbestos filled polyvinyl chloride flooring materials.


The correct selection and combination of heat stabilizers can achieve the best synergistic effect. In order to meet the specific requirements of non-toxic and high weather resistance, the focus of research on heat stabilizers is to develop new varieties of mixed metal salts and organic tin compounds, varieties that use less heavy metals but can improve stability, and low toxicity or non-toxic composite varieties with synergistic effects.




In plastic processing, additives that can reduce friction between plastic particles, friction between plastic macromolecules, adhesion of plastic to metal surfaces of processing equipment, and improve plastic melt fluidity and processing efficiency. Especially in PVC processing, lubricants are essential additives. Its function can be divided into two categories: the action played between plastic particles before melting and the action played between plastic melt and the metal surface of processing equipment after melting, known as external lubrication; The action that occurs between plastic macromolecules after melting is called internal lubrication. Some lubricants have an intermediate effect between the two. The mode of action of almost all lubricants changes with the other components of the plastic. The chemical structure of lubricants is the primary factor determining their mode of action. Usually, the shorter the carbon chain of a lubricant, the stronger its polarity, and the greater its internal lubrication effect; The longer the carbon chain, the greater the external lubrication effect. A single lubricant is generally difficult to meet comprehensive requirements, and several lubricants are often used together in production, so the development of compound lubricants is rapid.


Foaming agent


Foaming agent is an additive that can generate a large amount of gas under specific conditions, causing plastics to form continuous or discontinuous microporous structures. The plastics with such microporous structure are called foamed plastics or microporous plastics. According to the way gas is generated, foaming agents can be divided into two categories: physical foaming agents and chemical foaming agents. Physical foaming agent: generally an odorless, non-toxic inert gas, or a non flammable liquid with good stability and low boiling point. Commonly used inert gases include nitrogen, carbon dioxide, and air, while commonly used low boiling liquids include tetrachloroethane, chloroform, and pentane. In addition, soluble solid compounds (such as table salt) are also commonly used physical foaming agents. Physical foaming agents are suitable for foaming polystyrene, polyethylene, polypropylene, polyvinyl chloride, etc. Chemical foaming agent: a compound that is stable at room temperature and can decompose and release a large amount of gas at plastic processing temperature. It is widely used in the manufacture of foam plastics. The commonly used chemical foaming agents in industry are mostly organic compounds that release nitrogen as the main gas-phase component, and ammonium bicarbonate and sodium bicarbonate that can decompose and release ammonia or carbon dioxide respectively. Chemical foaming agents are commonly used for foaming various thermoplastic materials. In order to reduce the decomposition temperature of chemical foaming agents, improve their dispersibility, and increase foaming capacity, a foaming promoter that can activate chemical foaming agents, also known as co foaming agents, such as salicylic acid, urea, etc, is often used.




Antioxidants can inhibit or delay the deterioration of the appearance and intrinsic properties of plastics and products caused by factors such as heat, light, mechanical stress, electric field, radiation, and heavy metal ions in additives during manufacturing, processing, application, and storage. It has a wide variety of types and can be divided into five categories based on its chemical structure: phenols, amines, phosphorus containing compounds, sulfur-containing compounds, and organometallic salts. According to different mechanisms of action, phenols and amines are also known as main antioxidants, while compounds containing phosphorus and sulfur are also known as co antioxidants. The function of the main antioxidant is to capture the active free radicals generated during oxidative degradation, thereby interrupting the chain degradation reaction and achieving antioxidant goals. The role of auxiliary antioxidants is to decompose intermediate products of oxidative degradation into non free radical products. Usually, the main and auxiliary antioxidants are used together to achieve the best antioxidant effect through their synergistic effect. The main direction of antioxidant research is to improve antioxidant efficiency, durability, and compatibility.


Additives play an important role in the production process and subsequent applications of plastic injection molding. In addition to the several additives mentioned above, commonly used plastic additives include light stabilizers, anti-static agents, etc.