1、 The definition of plastic is a polymer compound formed from monomers through addition or condensation polymerization. Its resistance to deformation is moderate, intermediate between fibers and rubber, and mainly composed of resin, fillers, plasticizers, stabilizers, lubricants, pigments, and other additives. The main component of plastic is resin, which accounts for approximately 40% to 100% of the total weight of plastic. The basic properties of plastic are mainly determined by the nature of the resin, but additives also play an important role. Some plastics are basically composed of synthetic resins without or with little additives, such as organic glass, polystyrene, etc.

2、 Plastic classification: Plastic is a very important organic synthetic polymer material with a wide range of applications. If we can have a detailed understanding of the composition and classification of plastics, it will not only help us use plastic products scientifically, but also facilitate the classification and recycling of plastics, and effectively control and reduce "white pollution".

The classification of plastics is very complicated. According to the use, they can be divided into general plastics, engineering plastics and special plastics; According to physical and chemical classification, it can be divided into two types: thermosetting plastics and thermoplastic plastics; According to the classification of molding methods, it can be divided into various types, such as molding, lamination, injection, blow molding, extrusion, casting plastics, and reaction injection plastics. Plastics are classified according to their molecular structure and mechanical properties.

According to the molecular structure of plastics, they can be roughly divided into:

1. Polyolefin plastics, such as LDPE, HDPE, LLDPE, PP, EEAEVA, PB-1, TPX;

2. Polystyrene plastics (also known as polyolefin plastics, which are important due to their variety and are listed separately), such as PS, HIPS, ABSAAS, ACS, MBS, AS;

3 Vinyl plastics, such as PVC;

4 acrylic plastics, such as PMMA;

5 nylon (polyamide), such as transparent nylon, MC nylon, PA (666610), etc;

6 polyphenyl ether esters, such as POM, PPO, NORYL, PPS, PSF, PC, PET, PBT, polyaryl ester, and polyarylsulfone;

7 cellulose plastics, such as CN, CA, CAP, CAB, EC, CEC, HEC;

8 polyurethane, such as TPU.

In polymers, the molecular structure can greatly affect the properties of materials, so materials with the same molecular structure have many commonalities. Classification according to the same molecular structure can help readers better understand the properties of plastics.

According to the mechanical properties of plastics, they can be roughly divided into:

1. Materials with lower comprehensive mechanical properties - general plastics PE, PP, EEA, EVA, PVC;

2. Materials with moderate comprehensive mechanical properties - engineering plastics PS, HIPS, ABS, AAS, ACSMBS, AS, BS, PMMA;

3 Materials with high comprehensive mechanical properties - Structural engineering plastics PA, POM, NORYL, PC, PET, PBT;

The characteristics of a material itself determine its suitability for any purpose. In fact, there is no strict distinction between general plastics and engineering plastics. General engineering plastics can also be used for structural purposes as long as they meet the requirements of mechanical structures.

3、 Synthesis process of common plastic materials

1. Polyethylene terephthalate is a polymer with the chemical formula - [OCH2-CH2OCOC6H4CO] n -. Polyethylene terephthalate can be obtained by esterification, dehydration, and condensation of terephthalic acid and ethylene glycol. At present, there are three main Technology roadmap for PET production in the world. They are: ester exchange polycondensation method (DMT method), direct esterification polycondensation method (PTA method), and ethylene oxide method (EO method).

(1) In DMT method, dimethyl terephthalate (DMT) and ethylene glycol (EG) are Transesterification, and then condensed into PET. This method mainly includes two steps: first, dimethyl terephthalate (DMT) and ethylene glycol or 1,4-butanediol undergo Transesterification in the presence of a catalyst to generate BHET or dibutyl terephthalate. The commonly used catalysts are zinc, cobalt and manganese acetate, or their mixture with Antimony trioxide, with the amount of 0.01% to 0.05% of the mass of DMT. During the reaction process, the by-product methanol is continuously discharged. The second step is to generate BHET or dihydroxybutyl ester, which is subjected to condensation reaction in the front and rear polycondensation reactors. The reaction temperature in the front polycondensation reactor is 270 ℃, and the reaction temperature in the rear polycondensation reactor is 270-280 ℃. A small amount of stabilizer is added to improve the thermal stability of the melt. The condensation reaction is carried out under high vacuum (residual pressure not exceeding 266Pa) and strong stirring to obtain high molecular weight polyester.

(2) PTA method: This method uses high-purity terephthalic acid (TPA) to directly esterification with ethylene glycol or 1,4-butanediol to generate dihydroxyethyl terephthalate or butyl ester, and then undergoes condensation polymerization reaction. The key to this method is to solve the uniform mixing of TPA with ethylene glycol or 1,4-butanediol, improve the reaction speed, and prevent etherification reactions. Compared with the ester exchange condensation polymerization method, this method can eliminate the steps of DMT manufacturing, refining, and methanol recovery, making it easier to produce polymers with high molecular weight and good thermal stability, which can be used to produce high-quality products such as tire cords. However, this method requires a high purity of the raw material TPA, and the cost of TPA purification and refinement is high.

(3) EO method: This method directly reacts with ethylene oxide and PTA to generate dihydroxyethyl terephthalate, and then undergoes condensation polymerization reaction. Its advantages are that the production process of synthesizing ethylene glycol from ethylene oxide can be eliminated, the equipment utilization rate is high, there are few auxiliary equipment, and the product is also easy to refine. The disadvantage is that the Addition reaction of ethylene oxide and TPA needs to be carried out under the pressure of 2~3MPa, which requires strict equipment, thus affecting the widespread use of this method. Japan has used this method for production in the past, but it has been phased out due to its explosive, flammable, and toxic characteristics.

2. PVC and PVC are made from vinyl chloride monomer through free radical polymerization, and the polymerization degree n is generally in the range of 500~20000. From the perspective of product classification, PVC belongs to the synthetic resin category of three major synthetic materials (synthetic resin, Synthetic fiber, and synthetic rubber). At present, PVC can be divided into four categories according to polymerization methods: suspension PVC, lotion PVC, bulk PVC and solution PVC.

(1) The main characteristic of the bulk polymerization production process is that it does not require the addition of water or dispersants during the reaction process. The polymerization is carried out in two steps. In the first step, a quantitative amount of VCM monomers, initiators, and additives are added to the pre polymerization kettle. After heating, the kettle maintains a constant pressure and temperature under strong stirring (relative to the polymerization process in the second step) for pre polymerization. When the conversion rate of VCM reaches 8% -12%, stop the reaction and send the generated "seeds" into the polymerization kettle for the second step of reaction. After receiving the pre polymerized "seeds" in the polymerization kettle, a certain amount of VCM monomers, additives, and initiators are added to continue polymerization on the basis of these "seeds", so that the "seeds" gradually grow to a certain extent. Under the action of low-speed stirring, the polymerization reaction is carried out at a constant pressure. When the conversion rate of the reaction reaches 60%~85% (depending on the formula), the reaction is terminated, and the unreacted monomers are degassed and recovered in the polymerization kettle. Then, the residual VCM in the PVC powder is further removed by steam stripping in the kettle. Finally, the PVC powder in the kettle is sent to the grading, homogenization, and packaging processes through the air conveying system.

(2) The final product of the lotion lotion polymerization method is the polyvinyl chloride paste resin (E-PVC) used to manufacture the polyvinyl chloride plastisol. The industrial production is divided into two stages: the first stage is the polyvinyl chloride latex generated by the vinyl chloride monomer through lotion polymerization, which is the suspension emulsion of the polyvinyl chloride primary particles with a diameter of 0.1~3 microns in water. In the second stage, the PVC latex is dried by spray to obtain the PVC paste resin, which is the aggregation of primary particles to obtain PVC secondary particles with a diameter of 1~100 microns, mainly 20~40 microns. After mixing this secondary particle with a plasticizer, it undergoes shear action and disintegrates into smaller particles, forming a non settling polyvinyl chloride plasticizer paste, which is called polyvinyl chloride paste in industry.

(3) The general technological process of PVC resin production by suspension polymerization is to add water, suspension agent and antioxidant in the cleaned polymerization kettle, then add vinyl chloride monomer, stir in deionized water, disperse the monomer into small droplets, which are stabilized by protective glue, and add initiator or initiator lotion that can be dissolved in the monomer to keep the reaction speed stable during the reaction process, and then heat up the polymerization, The general polymerization temperature is between 45~70 ℃. When using low-temperature polymerization (such as 42-45 ℃), high molecular weight polyvinyl chloride resin can be produced; Low molecular weight (or ultra-low molecular weight) polyvinyl chloride resin can be produced using high-temperature polymerization (usually at 62-71 ℃).

In recent years, in order to improve polymerization speed and production efficiency, two-step suspension polymerization processes have been successfully studied abroad. Generally, the polymerization degree in the first step is controlled at around 600, and some new monomers are added before the second step of polymerization to continue polymerization. The advantage of two-step polymerization is that the polymerization cycle is significantly shortened, and the resin produced has good gel properties, molding properties and mechanical strength.

Compared to other methods, the suspension method PVC production technology is easy to adjust the variety, the production process is easy to control, the equipment and operating costs are low, and it is easy to organize large-scale production and has been widely used, becoming the most important production method among many production processes.

In addition, in order to further improve the quality of general and specialized resins produced by suspension method, and improve the level of product specialization and marketization, foreign manufacturers have conducted a lot of research on the process conditions and ingredient systems of polymerization processes, further improving the polymerization conversion rate, shortening the polymerization cycle, improving production efficiency, and also developing a series of high-performance Easy to process PVC specialized resins such as ultra-high (or ultra-low) degree of polymerization resin, high apparent density resin, skin free resin, radiation resistant resin, medical resin, heat-resistant resin, etc. It can be seen that the development of various specialized materials is a symbol of the development of suspension polymerization resins, and an important means to improve product performance and develop new application fields.

3. There are five main types of polypropylene production processes: solution method, slurry method, bulk method, gas-phase method, and bulk gas-phase combination process. At present, the most advanced production processes in the world are gas phase process and bulk gas phase combination process. These process technologies use bulk method, gas phase method or a combination of bulk method and gas phase method to produce homopolymers and random copolymers, and then connect gas phase reactor systems (one or two) to produce impact resistant copolymers. These process technologies have adapted to the requirements of large-scale equipment (over 200000 tons/year) and operational economy, product diversity, and high performance, and have been widely applied. The industrial production of polypropylene in China began in the 1970s. After more than thirty years of development, a production pattern has basically formed, where multiple production processes such as intermittent bulk method, bulk method, bulk gas phase method, and gas phase method are simultaneously developed, and large, medium, and small production scales coexist.

4、 Plastic chemical process defects

1. Shrinkage marks often occur in the production process of plastic chemical engineering. The formation of this problem is mainly due to the excessive use of lubricants during production. Due to the excessive use of lubricants, the temperature of the mold during plastic molding has undergone significant changes, resulting in shrinkage marks in the finished plastic product. In the production process of plastic products, it is usually heated to melt the plastic and drain it into various grinding tools. At the same time, it is necessary to use the pressure generated around the mold to plastic the plastic and form the finished product. Because the temperature of the same mold is different due to the excessive amount of lubricant, during the plastic process, there is a problem in the cooling of the plastic, resulting in uneven solidification of the plastic products, and shrink marks.

2. Underinjection problem is a common problem in plastic chemical processes. The occurrence of underinjection problem is mainly due to the fact that plastic does not fill the entire mold or the gap between the plastic and the mold is large during the plastic chemical production process, causing serious problems on the surface of plastic products during the cooling and solidification process. This further leads to an unsmooth surface of plastic products, and even leads to gaps on the surface of plastic products. Strict supervision should be carried out on the plastic injection molding process, and the types of raw materials should be analyzed to reduce the occurrence of underinjection problems in chemical production processes and further improve the quality of plastic products.

3. The flash problem, like the underfill problem, is also a common problem in the production of plastic chemical processes. The main cause of this problem is caused by excessive injection of plastic into the mold, which mainly occurs at the edge between the plastic product and the grinding tool. At the same time, the small clamping capacity of the mold and issues such as exhaust holes may both cause flash problems.

5、 Grade 7 of plastic

In daily life, attentive friends may find small triangular markings engraved on the bottom of plastic containers, with numbers ranging from 1 to 7 inside the triangular frame. Do you know what specifications of plastic containers each number represents? Below we will explain one by one for you.

01- Common uses of PET (polyethylene terephthalate) polyethylene terephthalate: packaging of mineral water bottles, carbonated beverage bottles, etc. Polyethylene terephthalate is a common resin in daily life, with good mechanical properties, impact strength 3-5 times that of other films, good bending resistance, high transparency, UV resistance, and good gloss. The transparent plastic bottle made of it is often referred to as the "Baote bottle". This type of plastic usually has a maximum heat resistance temperature of 65 ℃ and a minimum cold resistance of -20 ℃. Therefore, it is generally only suitable for holding liquids at room temperature or at lower temperatures. If high-temperature liquids are contained or directly heated, they are prone to thermal deformation and produce harmful substances to the human body. Moreover, scientists have found that long-term use of this plastic product for more than 10 months may release carcinogens that are toxic to the human body. Therefore, this kind of plastic bottle should be immediately discarded by Waste sorting when it is used up. It should not be used as a water cup, and should not be used as a storage container to contain other items as far as possible to avoid health problems.

02- HDPE (High Density Polyethylene) Common uses of high-density polyethylene include cleaning products, shower products, white medicine bottles, plastic containers for skincare products, and plastic bags currently used in supermarkets. High density polyethylene is a thermoplastic resin with high crystallinity and non polarity. Its melting temperature is between 120~160 ℃, and the general usage temperature should be controlled within 100 ℃. In the following industrial products, it can withstand a maximum high temperature of 110 ℃. If indicated, plastic bags for food can be used to hold food. Plastic containers containing cleaning products and shower products can be reused after small self cleaning, but these containers are usually difficult to clean, leaving behind existing cleaning products and becoming a breeding ground for bacteria, resulting in incomplete cleaning. Therefore, in daily life, it is best not to recycle products made of high-density polyethylene material. They should be replaced on time and in a timely manner. Waste products should be properly recycled and classified to prevent environmental pollution and secondary pollution.

03- Common uses of PVC: raincoats, plastic films, and building materials. Polyvinyl chloride is a white powder with an amorphous structure, with a relatively low branching degree and a relative density of around 1.4. It begins to decompose at a glass transition temperature of 77~90 ℃, around 170 ℃, and has poor stability to light and heat. When it is above 100 ℃ or exposed to long-term sunlight, it decomposes to produce hydrogen chloride, which further catalyzes decomposition, causing discoloration and rapid decline in physical and mechanical properties. This type of plastic has good plasticity and is cheap, but plastic products made from this material are prone to producing two types of toxic and harmful substances: first, single molecule vinyl chloride that has not been fully polymerized during the production process, and second, harmful substances in plasticizers. These two substances are prone to release when exposed to high temperatures and oils, and toxic substances can easily cause cancer when they enter the human body with food. Therefore, do not let it be heated during use.

04- LDPE (Low Density Polyethylene) Common uses of low density polyethylene: cling film, plastic film, etc. Low density polyethylene, also known as high-pressure polyethylene, is a plastic material that uses ethylene as the raw material and is fed into a reactor. Under the action of an initiator, it is compressed under high pressure for polymerization reaction. The material from the reactor is separated by a separator to remove unreacted ethylene, and then melted, extruded, granulated, dried, and mixed. It is suitable for various forming processes of thermoplastic molding, with good molding processability. The main use of low-density polyethylene is for film products, as well as for injection molded products, medical devices, pharmaceutical and food packaging materials, blow molded hollow formed products, etc. The heat resistance of this plastic film is not strong. Qualified PE film will melt when the temperature exceeds 110 ℃, leaving some plastic preparations that cannot be decomposed by the human body. If food is wrapped in plastic film and heated directly, the oil in the food can easily dissolve the harmful substances in the film at high temperatures, posing a threat to human health. Therefore, before placing food in the microwave, it is best to take the wrapped plastic wrap first.

Common uses of 05-PP (polypropylene) polypropylene include packaging bags, microwave oven special lunch boxes, etc. Polypropylene is a semi crystalline thermoplastic. It has high impact resistance, strong mechanical properties, and resistance to various organic solvents and acid alkali corrosion. It has a wide range of applications in the industry and is one of the common polymer materials. Australian coins are also made of polypropylene. The microwave lunch box is made of this material, which is resistant to high temperatures of 130 ℃ and has poor transparency. It is the only plastic box that can be directly placed in the microwave for heating. After cleaning, it can be reused. It should be noted that some microwave lunch boxes are made of PP 05, but the box cover is made of polystyrene 06. PS has good transparency, but it is not heat-resistant, so it cannot be put into the microwave together with the box body. The appearance of PS polystyrene No.06 is easily distinguishable from PP No.05. Therefore, for safety reasons, before placing the container in the microwave, remove the lid first.

06-PS (Polystyrene) Common uses of polystyrene: bowl filled instant noodle boxes, fast food boxes, etc. Polystyrene is a product obtained by addition polymerization of styrene monomers, which was first obtained from a volatile oil in natural resin balm. Polystyrene has good corrosion resistance to acids, alkalis, salts, mineral oils, organic acids, lower alcohols, etc., but it softens or dissolves in acetone, esters, Aromatic hydrocarbon and other organic solvents. Good insulation, transparency, high refractive index, water resistance, and good coloring properties, can be made into various brightly colored plastic products. When melting, the fluidity is very good, easy to process, and can be molded and Extrusion moulding. Polystyrene can be used as a material for making bowl filled instant noodle boxes and foam fast food boxes. It is both heat-resistant and cold-resistant, but should not be placed in a microwave to avoid the release of chemicals due to excessive temperature. And it cannot be used to contain strong acids such as orange juice or strong alkaline substances, as it will decompose into polystyrene that is not good for the human body.

07- Common uses of PCs and other types of PCs: water bottles, space cups, baby bottles, etc. It is a widely used material, especially used in the manufacturing of baby bottles, space cups, etc., and is highly controversial due to its presence of bisphenol 8. Experts point out that in theory, as long as bisphenol is 100% converted into a plastic structure during the production process, it means that the product is completely free of bisphenol, let alone released. However, if a small amount of bisphenol is not frozen into a specialized plastic structure, it may be released and enter food or beverages. Therefore, when using this plastic container, it is necessary to strictly follow the instructions to store and disinfect food using the correct method to avoid repeated use of aged or damaged products. The higher the temperature, the more and faster the release of residual bisphenol. Therefore, it is not advisable to use a water bottle to hold hot water. If your kettle is numbered, the following methods can reduce the risk of use: do not heat it and do not expose it to direct sunlight. Do not use a dishwasher or dryer to clean the kettle. Before the first use, clean with baking soda and warm water, and dry at room temperature. If there are any falls or damages to the container, it is recommended to stop using it, as plastic products with small pits can easily harbor bacteria. Avoid repeated use of aged plastic utensils.