Plastic is a synthetic polymer compound, also commonly known as plastics or resin, which can freely change its shape and style. It is a material synthesized or condensed from monomer raw materials, composed of synthetic resins and additives such as fillers, plasticizers, stabilizers, lubricants, pigments, etc.

Composition of plastics

The plastic we usually use is not a pure substance, it is made up of many materials. Among them, polymer (or synthetic resin) is the main component of plastics. In addition, in order to improve the performance of plastics, various auxiliary materials such as fillers, plasticizers, lubricants, stabilizers, colorants, etc. need to be added to the polymer to become high-performance plastics.

① Synthetic resin

Synthetic resin is the main component of plastics, and its content in plastics is generally between 40% and 100%. Due to its high content and the fact that the properties of resin often determine the properties of plastic, people often consider resin as synonymous with plastic. For example, mixing polyvinyl chloride resin with polyvinyl chloride plastic and phenolic resin with phenolic plastic. In fact, resin and plastic are two different concepts. Resin is an unprocessed raw polymer used not only in the manufacture of plastics, but also as a raw material for coatings, adhesives, and synthetic fibers. Except for a very small portion containing 100% resin, the vast majority of plastics require the addition of other substances in addition to the main component resin.

② Packing material

Fillers, also known as fillers, can improve the strength and heat resistance of plastics and reduce costs. For example, adding wood powder to phenolic resin can greatly reduce costs, making phenolic plastic one of the inexpensive plastics, while also significantly improving mechanical strength. Fillers can be divided into two types: organic fillers and inorganic fillers. The former, such as wood powder, shredded cloth, paper, and various textile fibers, while the latter, such as glass fiber, diatomaceous earth, asbestos, carbon black, etc.

③ Plasticizer

Plasticizers can increase the plasticity and flexibility of plastics, reduce brittleness, and make plastics easier to process and shape. Plasticizers are generally high boiling organic compounds that are miscible with resins, non-toxic, odorless, and stable to light and heat. Phthalates are commonly used. For example, in the production of polyvinyl chloride plastic, if a large amount of plasticizer is added, soft polyvinyl chloride plastic can be obtained. If no or less plasticizer is added (dosage<10%), hard polyvinyl chloride plastic can be obtained.

④ Stabilizer

In order to prevent the decomposition and damage of synthetic resins due to light and heat during processing and use, and to extend their service life, stabilizers should be added to plastics. Commonly used ones include stearic acid salts, epoxy resins, etc.

⑤ Coloring agent

Colorants can give plastics various bright and beautiful colors. Organic dyes and inorganic pigments are commonly used as coloring agents.

⑥ Lubricant

The function of lubricants is to prevent plastics from sticking to metal molds during molding, while also making the surface of plastics smooth and aesthetically pleasing. Common lubricants include stearic acid and its calcium magnesium salts.

⑦ Antioxidant

Prevent plastic from turning yellow, cracking, etc. due to thermal oxidation during heating molding or high-temperature use

In addition to the above-mentioned additives, flame retardants, foaming agents, anti-static agents, etc. can also be added to plastics to meet different usage requirements.

Classification of plastics

According to the different usage characteristics of different types of plastics, they are usually divided into three types: general plastics, engineering plastics, and specialty plastics.

① General plastic

Generally refers to plastics with high production volume, wide applications, good formability, and low prices. There are five major types of general-purpose plastics, namely polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), and acrylonitrile butadiene styrene copolymer (ABS). They are all thermoplastic materials.

② Engineering plastics

Generally refers to plastics that can withstand certain external forces, have good mechanical properties and resistance to high and low temperatures, have good dimensional stability, and can be used as engineering structures, such as polyamide, polysulfone, etc.

③ Special plastics

Generally refers to plastics with special functions that can be used in special application fields such as aviation and aerospace. For example, fluoroplastics and organosilicon have outstanding special functions such as high temperature resistance and self-lubricating, and reinforced plastics and foam plastics have special properties such as high strength and high cushioning. These plastics belong to the category of special plastics.

Screening of plastics

① Screening of raw materials: for example, screening of materials such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), and acrylonitrile butadiene styrene copolymer (ABS), because their particles are qualitatively uniform in production, their screening purpose is to remove impurities and clumps.

② Recycling and screening of waste plastics: The formed plastic products are recycled, such as waste plastic pipes, waste plastic supports, waste plastic pipes, etc., through crushing and screening.

③ Screening of plastic particles: The formed plastic particles are classified, and the purpose of screening is to achieve fine grading;

④ Screening of resin slices: Screening is difficult because resin slices are prone to sticking and there may be excessively long single body unqualified materials. It is recommended to test the machine to clarify the screening requirements;

⑤ Screening of fillers: screening of wood powder, glass fiber, diatomaceous earth, natural mineral powder, carbon black, etc;