Plastics are macromolecules made from monomers through addition polymerization or condensation polymerization. Microplastics can harm human health.Its deformation resistance is medium, between fiber and rubber, and it is composed of synthetic resin and additives such as fillers, plasticizers, stabilizers, lubricants, and colorants.

The main component of plastic is resin. Resin refers to a high molecular compound that has not been mixed with various additives. The term resin was originally derived from the lipids secreted by animals and plants, such as rosin and shellac. Resin accounts for about 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, with no or little additives, such as plexiglass.

In April 2023, an international research team discovered a new form of plastic pollution : plastic waste films chemically bonded to rocks.

Type Overview

Plastic raw materials are defined as a material or plastic material product made of synthetic or natural high molecular polymers that can be molded into various shapes and finally maintain the same shape.

scientific name	Abbreviation	Common name	RecycleLogo	use
Polyethylene	PE	–	–	Low temperature items
Polypropylene	PP	Folding glue, plastic	05	Microwave lunch box, can be used at around 100℃
High Density Polyethylene	HDPE	Hard Soft Rubber	02	Cleaning products, bath products
Low Density Polyethylene	LDPE	–	04	Cling film, plastic film, etc.
Linear Low Density Polyethylene	LLDPE	–	–	–
Polyvinyl Chloride	PVC	Vinyl	03	Rarely used in food packaging
General Purpose Polystyrene	GPPS	Hard plastic	–	–
Expansible Polystyrene	EPS	Styrofoam
High Impact Polystyrene	HIPS	Impact-resistant hard plastic
Styrene-Acrylonitrile Copolymers	AS, SAN	Transparent glue
Acrylonitrile-Butadiene-Styrene Copolymers	ABS	Super unbreakable glue
Polymethyl Methacrylate	PMMA	AcrylicPlexiglas
Ethylene-Vinyl AcetateCopolymers	EVA	Rubber
Polyethylene Terephthalate	PET	Polyester	01	Mineral water bottles, carbonated beverage bottles
Polybutylene Terephthalate	PBT	–	–	–
Polyamide(Nylon 6.66)	PA	nylon
Polycarbonates	PC	Bulletproof glue	07	Kettle, cup, milk bottle
Polyacetal	POM	Saigang, Dugang	–	–
Polyphenyleneoxide	PPO	Noryl
Polyphenylenesulfide	PPS	Polyphenylene sulfide
Polyurethanes	PU	Polyurethane
Polystyrene	PS	–	06	Bowl instant noodle box, fast food box
Poly tetra fluoroethylene	PTFE	Teflon, Teflon	–	–

Key Features

1. Most plastics are lightweight, chemically stable, and won’t rust;
2. Good impact resistance;
3. Has good transparency and abrasion resistance;
4. Good insulation and low thermal conductivity;
5. Generally, the formability and colorability are good, and the processing cost is low;
6. Most plastics have poor heat resistance, large thermal expansion rate, and are easy to burn;
7. Poor dimensional stability and easy to deform;
8. Most plastics have poor low temperature resistance, become brittle at low temperatures, and are prone to aging;
9. Some plastics are easily soluble in solvents.

Plastics can be divided into two categories: thermosetting and thermoplastic. The former cannot be reshaped and reused, while the latter can be repeatedly produced. Thermoplastic has a larger physical elongation, generally between 50% and 500%. The force does not change completely linearly at different elongations.

The different properties of plastics determine their uses in life and industry. With the advancement of technology, research on plastic modification has never stopped. It is hoped that in the near future, plastics can be used more widely after modification, and can even replace materials such as steel and no longer pollute the environment.

Molecular Structure

There are basically two types: the first is a linear structure, and polymer compounds with this structure are called linear polymer compounds; the second is a bulk structure, and polymer compounds with this structure are called bulk polymer compounds. Some polymers have branches, called branched polymers, and belong to linear structures. Although some polymers have cross-links between molecules, the cross-links are less, called network structures, and belong to bulk structures.

Two different structures, showing two opposite properties. Linear structure can melt when heated, and has the characteristics of low hardness and brittleness. Stereoscopic structure has high hardness and brittleness. Plastics have both types of polymers. Thermoplastic plastics are made of linear polymers, and thermosetting plastics are made of stereoscopic polymers.

Material properties

1. Chemical corrosion resistance
2. Lustrous, partially transparent or translucent
3. Most of them are good insulators
4. Lightweight and strong
5. Easy to process and can be mass-produced at a low price
6. Widely used, multi-functional, easy to color, some are resistant to high temperatures

Advantages

1. Most plastics have strong corrosion resistance and do not react with acids or alkalis.
2. Plastic manufacturing costs are low.
3. Durable, waterproof and lightweight.
4. Easy to be molded into different shapes.
5. It is a good insulator.
6. Plastics can be used to prepare fuel oil and fuel gas, which can reduce crude oil consumption.

Shortcoming

1. When recycling waste plastics, sorting is very difficult and not economically viable.
2. Plastics are easy to burn and produce toxic gases when burning. For example, polystyrene produces toluene when burning. A small amount of this substance can cause blindness and vomiting when inhaled. PVC burning also produces toxic hydrogen chloride gas. In addition to burning, high temperature environment will cause plastic to decompose toxic components, such as benzene.
3. Plastics are made from petroleum refining products, and petroleum resources are limited.
4. Plastic takes hundreds of years to decompose when buried underground.
5. Plastics have poor heat resistance and are prone to aging.
6. Due to the non-biodegradability of plastic, it has become the number one enemy of mankind and has also led to the tragic death of many animals. For example, monkeys, pelicans, dolphins and other animals in the zoo will swallow the No. 1 plastic bottles thrown by tourists by mistake, and finally die painfully due to indigestion; looking at the beautiful and pure sea surface, if you look closely, you will find that it is actually filled with various plastic garbage that cannot be accommodated by the ocean. In the intestines of many dead seabird samples, various indigestible plastics were found.

Biodegradable

” The rapid biodegradation of plastic in the mealworm’s gut reveals a new fate for plastic waste discarded in the environment,” said Professor Yang Jun from the Beihang University.

Plastics are difficult to degrade naturally in the environment, and polystyrene is the most difficult of all. Due to its high molecular weight and high stability, it is generally believed that microorganisms cannot degrade polystyrene plastics. In 2015, Professor Yang Jun’s research group at Beijing University of Aeronautics and Astronautics and Dr. Zhao Jiao from Shenzhen BGI Genomics Co., Ltd. co-published two sister research papers in Environmental Science & Technology, an authoritative journal in the field of environmental science, proving that mealworm larvae can degrade polystyrene, the most difficult plastic to degrade.

The study shows that with polystyrene foam as the only food source, mealworm larvae can survive for more than one month and eventually develop into adults. The polystyrene they gnaw on is completely degraded and mineralized into CO2 or assimilated into body fat. This discovery provides ideas for solving the global plastic pollution problem.

World Problems

Plastic waste pollution from petrochemical production is a global environmental problem. Most plastics are discarded after a single use. So far, the academic community believes that plastic products may not decompose for tens to hundreds of years in the natural environment due to their stable physical and chemical structures.

Professor Yang Jun introduced that in 2013, the world consumed 299 million tons of plastics, of which polystyrene plastics accounted for 7%, with an annual consumption of about 21 million tons. Common plastic lunch boxes, coffee cups and other materials that can withstand boiling water are polystyrene. Authoritative surveys have shown that polystyrene, a type of plastic, only degrades by 0.01%-3% in soil, sludge, rotten garbage, or manure microbial communities within 4 months.

Every year, 40 million tons of waste plastic accumulate in the environment worldwide, and about 2 million tons of waste plastic is thrown into the environment in China every year. Taking agricultural film used in farmland as an example, China’s annual production of agricultural film reaches one million tons, and it increases at a rate of 10% per year. Regardless of the crops covered, all covered soils have residual film. According to statistics, the annual residual amount of agricultural film in China is as high as 350,000 tons, and the residual film rate is as high as 42%. A large amount of residual film is left in the 0-30 cm tillage layer of farmland. In other words, nearly half of the agricultural film remains in the soil, which is a huge hidden danger in food safety.

“It may take 200-400 years for plastic to be completely assimilated by microorganisms in the soil and degraded into CO2 and water for inorganic mineralization, resulting in accumulation in the environment,” Professor Yang Jun told the Yangcheng Evening News.

Bugs help

Since 2005, Yang Jun’s team has started to study the biodegradation of plastics, focusing on the degradation of polystyrene and other plastics that are the most difficult to degrade.

Scientists have previously used several soil invertebrates, such as earthworms, millipedes, slugs, snails, etc. to see if they can eat plastic. When fed 14C-labeled plastics such as polyvinyl chloride ( PVC ), polyethylene (PE) and polypropylene (PP), the results showed that they could not be degraded.

Yang Jun believes that the idea of ​​biodegradable plastics needs to be expanded and not limited to microorganisms. Lepidoptera insects, termites, etc. can be considered. Shipworms and boring clams in the ocean can corrode polyethylene and submarine cables. It is also possible to consider isolating and cloning key enzymes and their genes that can produce active groups from these organisms.

A 2014 study by Yang Jun’s team found that waxworms ( Indian meal moth larvae) can chew and eat polyethylene PE film, and two strains that can degrade PE film were isolated from the larvae’s intestines, namely Enterobacter YT1 and Bacillus YP1. The research team then found that mealworm larvae are an animal that eats plastic more effectively. They are larger than waxworms (usually 35 mm long and 3 mm wide), and can use foam plastic as their only food. Mealworms have four life stages: eggs, larvae, pupae, and adults.

Mealworms, also known as mealworms, belong to the order Coleoptera, family Pseudocephagidae, genus Pseudocephagidae in insect taxonomy. Originally from North America, they were introduced to China from the former Soviet Union in the 1950s. Mealworms are known as ” protein feed banks”. Their dried products contain 30% fat and more than 50% protein. In addition, they also contain phosphorus, potassium, iron and other elements. Dried mealworm larvae contain about 40% protein, pupae contain 57%, and adults contain 60%.

In China, mealworms are actually similar to silkworms. They can be eaten directly, fried, or used as feed. Scorpions, centipedes, geckos, snakes, tropical fish, and goldfish fed with mealworms not only grow fast and have a high survival rate, but also have strong disease resistance and greatly improved reproductive capacity. It is very easy to breed mealworms. Farmers can use fresh oats, wheat bran, and apples to breed them.

Bugs eat plastic

Professor Yang Jun’s team purchased polystyrene plastic raw materials from Sinopec Yanshan Branch, which contained no additives or catalysts. The polystyrene plastic samples labeled with α-13C and β-13C were purchased from the United States. Yellow mealworms were purchased from insect farms in Daxing, Beijing and Qinhuangdao, Hebei, and fed with grains. These insects were in the 3rd or 4th instar (i.e., they had shed their skin 3-4 times).

The mealworms were placed in a polypropylene plastic container with a foam block. The weight of the foam block eaten by the mealworms was measured regularly. The control group was a regular wheat bran -fed mealworm. In the experiment, 500 mealworms were fed 5.8 grams of foam as the only food and raised individually in a controlled greenhouse (25 ± 1 ° C, 80 ± 2% humidity, and 16:8 light/dark cycle). During the hatching process, dead mealworms were immediately removed.

In the experiment, Yang Jun et al. fed mealworm larvae with foam plastic as the sole food source. Comparing the larvae fed normally (fed with bran) and those that were deprived of food, the results showed that during the 16-day experimental period, the dry weight of the larvae did not increase significantly (+33.6%) like the larvae fed normally, but only increased slightly by 0.2% (this is because the water content and nutritional value of foam plastic are lower than bran), but it did not decrease significantly like the larvae that were deprived of food (-24.9%). In addition, there was no significant difference in the survival rate of the larvae fed with plastic and bran.

100 mealworms can eat 34-39 mg of Styrofoam per day. During the 16-day test period, 47.7% of the Styrofoam ingested by the worms was converted into CO2. The remainder (about 49.2%) was converted into biodegradable particles similar to rabbit feces and excreted out of the body. The test used α-13C or β-13C labeled polystyrene plastic to confirm that it was mineralized into carbon 13 labeled carbon dioxide and lipids. The polystyrene foam in the larvae’s intestines did not stay for more than 24 hours before it degraded.

Larvae fed with Styrofoam as their only food were in the same health as those fed with normal food (wheat bran) after one month, and finally developed into crustacean adults. The mealworms ate holes in the foam one by one. After passing through the insects’ intestines, the chemical structure and composition of the ingested Styrofoam changed. By using gel permeation chromatography (GPC), carbon-13 nuclear magnetic resonance spectroscopy, and thermogravimetric Fourier transform infrared spectroscopy, it was confirmed that the long-chain molecules of polystyrene in the larvae’s intestines broke to form worm metabolites that were excreted in the feces.

The experiment also successfully isolated a polystyrene-degrading bacterium, Exiguobacterium sp. YT2, from the larval intestine that can use polystyrene as the sole carbon source for growth. This strain has been preserved in the General Microbiological Center of the China Microbiological Culture Collection Administration and the National Gene Bank, and is the first polystyrene- degrading bacterium reported internationally to be preserved in a culture center.

Can eat all plastic

The research team proposed the mechanism of polystyrene degradation by yellow meal larvae: in the first step, the foam plastic is chewed into small fragments by the yellow meal larvae and ingested into the intestine; in the second step, the chewing action increases the contact area between the polystyrene foam and the microorganisms and extracellular enzymes, and the ingested fragments are further depolymerized into small molecular products under the action of extracellular enzymes secreted by intestinal microorganisms; in the third step, these small molecular products are further degraded and assimilated to form the larvae’s own tissues under the action of various enzymes and bacteria and the yellow meal larvae’s own enzymes; in the fourth step, the remaining foam fragments and some degradation intermediates are mixed with some intestinal microorganisms and excreted from the body in the form of insect feces, where the foam plastic may continue to degrade further.

Element

The plastics we usually use are not a single component, but are made up of many materials. Among them, high molecular polymers (or synthetic resins ) are the main components of plastics. In addition, in order to improve the performance of plastics, various auxiliary materials such as fillers, plasticizers, lubricants, stabilizers, colorants, antistatic agents, etc. must be added to the high molecular compounds to become plastics with good performance.

Plastic additives, also known as plastic additives, are some compounds that must be added to polymers (synthetic resins) during molding to improve their processing properties or to improve the insufficient properties of the resin itself. For example, plasticizers are added to reduce the molding temperature of polyvinyl chloride resins and make the products soft; foaming agents are added to prepare lightweight, vibration-resistant, heat-insulating, and sound-insulating foam plastics; the thermal decomposition temperature of some plastics is very close to the molding temperature, and they cannot be molded without the addition of heat stabilizers. Therefore, plastic additives play a particularly important role in plastic molding.

Synthetic resin

Synthetic resin is the main component of plastics, and its content in plastics is generally 40% to 100%. Due to its large content and the fact that the properties of resin often determine the properties of plastics, people often regard resin as a synonym for plastics. For example, polyvinyl chloride resin is confused with polyvinyl chloride plastics, and phenolic resin is confused with phenolic plastics. In fact, resin and plastics are two different concepts. Resin is an unprocessed raw polymer compound, which is not only used to make plastics, but also a raw material for coatings, adhesives and synthetic fibers. Except for a very small part of plastics that contain 100% resin, most plastics, in addition to the main component resin, also need to add other substances.

Filler

Fillers are also called fillers. They 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 cheapest plastics, while significantly improving mechanical strength. Fillers can be divided into two categories: organic fillers and inorganic fillers. The former include wood powder, rags, paper and various fabric fibers, and the latter include glass fiber, diatomaceous earth, asbestos, carbon black, etc. The content of fillers in plastics is generally controlled below 40%.

Plasticizers

Plasticizers, or plasticizers, can increase the plasticity and softness of plastics, reduce brittleness, and make plastics easier to process and shape. Plasticizers (plasticizers) are generally high-boiling organic compounds that are miscible with resins, non-toxic, odorless, and stable to light and heat. The most commonly used are phthalates. For example, when producing polyvinyl chloride plastics, if more plasticizers are added, soft polyvinyl chloride plastics can be obtained. If no or less plasticizers are added, soft polyvinyl chloride plastics can be obtained.

Stabilizer

Stabilizers mainly refer to agents that keep polymer plastics, rubbers, synthetic fibers, etc. stable and prevent them from decomposing and aging. In order to prevent synthetic resins from decomposing and being damaged by light and heat during processing and use, and to extend their service life, stabilizers must be added to plastics. Commonly used ones include stearates, epoxy resins, etc. The amount of stabilizer used is generally 0.3-0.5% of the plastic.

Colorant

Colorants can give plastics a variety of bright and beautiful colors. Organic dyes and inorganic pigments are commonly used as colorants. The natural color of synthetic resins is mostly white, translucent or colorless and transparent. Colorants are often used in industrial production to increase the color of plastic products.

Lubricants

The function of lubricant is to prevent plastic from sticking to the metal mold during molding, and at the same time make the surface of the plastic smooth and beautiful. Commonly used lubricants include stearic acid and its calcium and magnesium salts.

Antioxidants

Prevent plastics from being oxidized during heat molding or high-temperature use, causing them to turn yellow or crack.

In addition to the above additives, flame retardants, foaming agents, antistatic agents, conductive agents, magnetic conductive agents, compatibilizers, etc. can also be added to plastics to meet different usage requirements.

Antistatic Agents

Plastics are poor conductors of electricity, so they are easily charged with static electricity. Antistatic agents can give plastics mild to moderate conductivity, thereby preventing the accumulation of static charge on the product.

Classification

Classified by purpose

According to the different usage characteristics of various plastics, plastics are usually divided into three types: general-purpose plastics, engineering plastics and special plastics.

General plastics

Generally refers to plastics with large output, wide application, good formability and low price. 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). These five types of plastics occupy the vast majority of plastic raw materials used, and the rest can basically be classified as special plastics, such as: PPS, PPO, PA, PC, POM, etc. They are used in daily life products in small quantities and are mainly used in high-end fields such as engineering industry, national defense science and technology, such as automobiles, aerospace, construction, communications, etc. Plastics can be divided into thermoplastics and thermosetting plastics according to their plasticity. Under normal circumstances, thermoplastic products can be recycled, while thermosetting plastics cannot. According to the optical properties of plastics, they can be divided into transparent, translucent and opaque raw materials. For example, PS, PMMA, AS, PC, etc. are transparent plastics, while most other plastics are opaque plastics.

Common plastic types, properties and uses

1. Polyethylene : Common polyethylene can be divided into low-density polyethylene (LDPE), high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE). Among the three, HDPE has better thermal, electrical and mechanical properties, while LDPE and LLDPE have better flexibility, impact resistance, film-forming properties, etc. LDPE and LLDPE are mainly used for packaging films, agricultural films, plastic modification, etc., while HDPE has a wider range of uses, including films, pipes, injection daily necessities and other fields.
2. Polypropylene: Relatively speaking, polypropylene has more varieties, and its uses are more complex and in a wide range of fields. The main varieties are homopolymer polypropylene (homoppp), block copolymer polypropylene (copp) and random copolymer polypropylene (rapp). According to different uses, homopolymer is mainly used in drawing, fiber, injection, BOPP film and other fields, copolymer polypropylene is mainly used in household appliance injection parts, modified raw materials, daily injection products, pipes, etc., and random polypropylene is mainly used for transparent products, high-performance products, high-performance pipes, etc.
3. Polyvinyl chloride: Due to its low cost and self-flaming retardant properties, it is widely used in the construction field, especially in sewer pipes, plastic steel doors and windows, panels, artificial leather, etc.
4. Polystyrene: As a transparent raw material, it has a wide range of uses when there is a demand for transparency, such as car lampshades, daily transparent parts, transparent cups, cans, etc.
5. ABS: It is a widely used engineering plastic with outstanding physical, mechanical and thermal properties. It is widely used in household appliances, panels, masks, assemblies, accessories, etc., especially household appliances such as washing machines, air conditioners, refrigerators, fans, etc., with a very large amount of use. In addition, it is also widely used in plastic modification.

Engineering plastics

Generally refers to plastics that can withstand certain external forces, have good mechanical properties and high and low temperature resistance, good dimensional stability, and can be used as engineering structures, such as polyamide, polysulfone, etc. Engineering plastics are divided into two categories: general engineering plastics and special engineering plastics. Engineering plastics can meet higher requirements in mechanical properties, durability, corrosion resistance, heat resistance, etc., and are more convenient to process and can replace metal materials. Engineering plastics are widely used in electronics, automobiles, construction, office equipment, machinery, aerospace and other industries. Replacing steel and wood with plastics has become an international trend.

General engineering plastics include: polyamide, polyoxymethylene, polycarbonate, modified polyphenylene ether, thermoplastic polyester, ultra-high molecular weight polyethylene, methylpentene polymer, vinyl alcohol copolymer, etc.

Special engineering plastics can be divided into cross-linked and non-cross-linked types. The cross-linked types include: polyaminobismaleimide, polytriazine, cross-linked polyimide, heat-resistant epoxy resin, etc. The non-cross-linked types include: polysulfone, polyethersulfone, polyphenylene sulfide, polyimide, polyetheretherketone (PEEK), etc.

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 silicones have outstanding special functions such as high temperature resistance and self-lubrication, and reinforced plastics and foam plastics have special properties such as high strength and high cushioning. These plastics all belong to the category of special plastics.

a. Reinforced plastics:

Reinforced plastic raw materials can be divided into three types in terms of appearance: granular (such as calcium plastic reinforced plastic), fibrous (such as glass fiber or glass cloth reinforced plastic), and flake (such as mica reinforced plastic). According to the material, they can be divided into three types: cloth-based reinforced plastic (such as rag reinforced or asbestos reinforced plastic), inorganic mineral filled plastic (such as quartz or mica filled plastic), and fiber reinforced plastic (such as carbon fiber reinforced plastic).

b. Foam plastics:

Foam plastics can be divided into three types: hard, semi-hard and soft foam plastics. Hard foam plastics have no flexibility and have a high compression hardness. They will only deform when a certain stress value is reached and cannot return to their original state after the stress is released. Soft foam plastics are flexible and have a low compression hardness. They are easy to deform and can return to their original state after the stress is released, with little residual deformation. The flexibility and other properties of semi-hard foam plastics are between hard and soft foam plastics.

Physical and chemical classification

According to the different physical and chemical properties of various plastics, plastics can be divided into two types: thermosetting plastics and thermoplastics.

Thermoplastics

Thermoplastics: refers to plastics that melt when heated, flow to the mold and cool to form, and melt again when heated again; that is, heating and cooling can make it reversible (liquid ←→ solid), which is the so-called physical change. The continuous use temperature of general thermoplastics is below 100°C. Polyethylene, polyvinyl chloride, polypropylene, and polystyrene are collectively known as the four general-purpose plastics. Thermoplastic plastics are divided into hydrocarbons, vinyls with polar genes, engineering, cellulose and other types. It softens when heated and hardens when cooled. It can soften and harden repeatedly and maintain a certain shape. It is soluble in certain solvents and has the properties of being fusible and soluble. Thermoplastics have excellent electrical insulation properties, especially polytetrafluoroethylene (PTFE), polystyrene (PS), polyethylene (PE), and polypropylene (PP) have extremely low dielectric constants and dielectric losses, and are suitable for high-frequency and high-voltage insulation materials. Thermoplastics are easy to mold, but have low heat resistance and are prone to creep. The degree of creep varies with load, ambient temperature, solvent, and humidity. In order to overcome these weaknesses of thermoplastics and meet the needs of applications in space technology, new energy development, and other fields, various countries are developing heat-resistant resins that can be melt-molded, such as polyetheretherketone (PEEK), polyethersulfone (PES),Polyarylsulfone (PASU), polyphenylene sulfide (PPS), etc. The composite materials with them as the matrix resin have high mechanical properties and chemical corrosion resistance, can be hot-formed and welded, and the interlaminar shear strength is better than that of epoxy resin. For example, polyetheretherketone is used as the matrix resin and carbon fiber to make a composite material, and its fatigue resistance exceeds that of epoxy/carbon fiber. It has good impact resistance, good creep resistance at room temperature, good processability, and can be used continuously at 240-270°C. It is an ideal high-temperature resistant insulating material. The composite material made of polyethersulfone as the matrix resin and carbon fiber has high strength and hardness at 200°C, and can still maintain good impact resistance at -100°C; it is non-toxic, non-flammable, has minimal smoke, and has good radiation resistance. It is expected to be used as a key component of a spacecraft, and can also be molded into a radar antenna cover, etc.

Formaldehyde cross-linked plastics include phenolic plastics and amino plastics (such as urea-formaldehyde- melamine -formaldehyde, etc.). Other cross-linked plastics include unsaturated polyesters, epoxy resins, diallyl phthalate resins, etc.

Thermosetting plastics

Thermosetting plastics refer to plastics that can solidify or have insoluble (melting) characteristics under heat or other conditions, such as phenolic plastics, epoxy plastics, etc. Thermosetting plastics are divided into two types: formaldehyde cross-linked type and other cross-linked types. After hot processing and molding, a solidified material with infusibility and insolubility is formed, and its resin molecules are cross-linked from linear structures to network structures. Further strong heat will decompose and destroy. Typical thermosetting plastics include phenolic, epoxy, amino, unsaturated polyester, furan, polysilicone and other materials, as well as newer polypropylene phthalate plastics. They have the advantages of high heat resistance and not easy to deform under heat. The disadvantage is that the mechanical strength is generally not high, but its mechanical strength can be improved by adding fillers to make laminated materials or molded materials.

Thermosetting plastics made of phenolic resin as the main raw material, such as phenolic molded plastics (commonly known as bakelite ), are durable, dimensionally stable, and resistant to other chemical substances except strong alkali. Various fillers and additives can be added according to different uses and requirements. For example, if high insulation performance is required, mica or glass fiber can be used as filler; if heat-resistant, asbestos or other heat-resistant fillers can be used; if earthquake-resistant, various appropriate fibers or rubbers can be used as fillers and some toughening agents to make high-toughness materials. In addition, phenolic resins modified by aniline, epoxy, polyvinyl chloride, polyamide, polyvinyl alcohol acetal, etc. can be used to meet the requirements of different uses. Phenolic resin can also be used to make phenolic laminates, which are characterized by high mechanical strength, good electrical properties, corrosion resistance, and easy processing. They are widely used in low-voltage electrical equipment.

Aminoplastics include urea formaldehyde, melamine formaldehyde, urea-melamine formaldehyde, etc. They have the advantages of hard texture, scratch resistance, colorlessness, translucency, etc., and can be made into colorful products by adding colorants, commonly known as electric jade. Because it is oil-resistant and not affected by weak alkali and organic solvents (but not acid-resistant), it can be used for a long time at 70°C and can withstand 110-120°C for a short time, and can be used for electrical products. Melamine formaldehyde plastics are harder than urea formaldehyde plastics, have better water resistance, heat resistance, and arc resistance, and can be used as arc-resistant insulating materials.

There are many types of thermosetting plastics made of epoxy resin as the main raw material, of which about 90% are based on bisphenol A epoxy resin. It has excellent adhesion, electrical insulation, heat resistance and chemical stability, small shrinkage and water absorption, and good mechanical strength.

Both unsaturated polyester and epoxy resin can be made into FRP with excellent mechanical strength. For example, FRP made of unsaturated polyester has good mechanical properties and low density (only 1/5 to 1/4 of steel and 1/2 of aluminum), and is easy to process into various electrical parts. The electrical and mechanical properties of plastics made of dipropylene phthalate resin are better than those of phenolic and amino thermosetting plastics. It has low hygroscopicity, stable product size, good molding performance, and is resistant to acid, alkali, boiling water and some organic solvents. Molding plastics are suitable for manufacturing parts with complex structures, both heat-resistant and highly insulating. Generally, it can be used for a long time in the temperature range of -60 to 180°C, and the heat resistance grade can reach F to H, which is higher than the heat resistance of phenolic and amino plastics.

Silicone plastics in the form of polysiloxane structures are widely used in electronics and electrical engineering. Silicone laminated plastics mostly use glass cloth as reinforcement material; silicone molded plastics mostly use glass fiber and asbestos as fillers, and are used to manufacture parts of high-temperature, high-frequency or submersible motors, electrical appliances, and electronic equipment. This type of plastic is characterized by a small dielectric constant and tg δ value, and is less affected by frequency. It is used in the electrical and electronic industries to resist corona and arc. Even if the discharge causes decomposition, the product is silicon dioxide instead of conductive carbon black. This type of material has outstanding heat resistance and can be used continuously at 250°C. The main disadvantages of polysiloxanes are low mechanical strength, low adhesiveness, and poor oil resistance. Many modified silicone polymers have been developed, such as polyester-modified silicone plastics, which are used in electrical engineering. Some plastics are both thermoplastic and thermosetting plastics. For example, polyvinyl chloride is generally a thermoplastic plastic. Japan has developed a new type of liquid polyvinyl chloride which is thermosetting with a molding temperature of 60 to 140°C. A plastic called Rendex from the United States has both the processing characteristics of thermoplastics and the physical properties of thermosetting plastics.

• Hydrocarbon plastics. They are non-polar plastics and can be divided into crystalline and non-crystalline types. Crystalline hydrocarbon plastics include polyethylene, polypropylene, etc., and non-crystalline hydrocarbon plastics include polystyrene, etc.
• Vinyl plastics containing polar genes. Except for fluoroplastics, most of them are non-crystalline transparent bodies, including polyvinyl chloride, polytetrafluoroethylene, polyvinyl acetate, etc. Most vinyl monomers can be polymerized using free radical catalysts.
• Thermoplastic engineering plastics. Mainly including polyoxymethylene, polyamide, polycarbonate, ABS, polyphenylene ether, polyethylene terephthalate, polysulfone, polyether sulfone, polyimide, polyphenylene sulfide, etc. Polytetrafluoroethylene and modified polypropylene are also included in this range.
• Thermoplastic cellulose plastics. Mainly including cellulose acetate, cellulose acetate butyrate, celluloid, cellophane, etc.

Processing method classification

According to the different molding methods of various plastics, they can be divided into many types such as film pressing, lamination, injection, extrusion, blow molding, casting plastics and reaction injection plastics.

Molded plastics are mostly plastics with physical properties and processing performances similar to those of general solid plastics; laminated plastics refer to fiber fabrics impregnated with resin, which are combined into a whole material through stacking and hot pressing; injection, extrusion and blow molding are mostly plastics with physical properties and processing performances similar to those of general thermoplastics; cast plastics refer to liquid resin mixtures that can be poured into a mold without pressure or with slight pressure and can harden into products of a certain shape, such as MC nylon; reaction injection plastics are plastics that use liquid raw materials, which are pressurized and injected into the membrane cavity to react and solidify into products of a certain shape, such as polyurethane.

Development

The first fully synthetic plastic came from Belgian-American Leo Hendrik Baekeland, who registered a patent for phenolic plastic on July 14, 1907.

Baekeland was the son of a shoemaker and a maid, born in Ghent, Belgium in 1863. In 1884, at the age of 21, Baekeland received a doctorate from the University of Ghent, and at the age of 24, he became a professor of physics and chemistry at the Higher Normal School in Bruges, Belgium. In 1889, Baekeland, who had just married the daughter of his university tutor, received another travel scholarship and went to the United States to conduct chemical research.

Encouraged by Professor Charles Chandler of Columbia University, Baekeland stayed in the United States and worked for a photographic supplier in New York. This led him to invent Velox photographic, which could be developed under light rather than sunlight. In 1893, Baekeland resigned and founded the Nepera Chemical Company.

Under the impact of the new product, Eastman Kodak, a photographic equipment manufacturer, could not bear it. In 1898, after two rounds of negotiations, Kodak purchased the patent rights for Velox photographic paper for $750,000 (equivalent to $15 million in 2013). However, Kodak soon discovered that the formula did not work. Baekeland’s answer was: This is normal. Inventors will omit one or two steps in patent documents to prevent infringement. Kodak was told: They bought the patent, but not all the knowledge. After paying another $100,000, Kodak learned that the secret was in a solution.

After earning his first pot of gold, Baekeland bought a mansion overlooking the Hudson River in Yonkers, New York, converted a barn into a fully equipped private laboratory, and worked with others to build a pilot plant in Brooklyn. The nascent power industry at the time held a huge market for insulating materials. The first temptation Baekeland smelled was the soaring price of shellac, a natural insulating material, which had been produced by cottage industries in South Asia for centuries. After investigation, Baekeland made finding a substitute for shellac his first commercial goal. At that time, chemists had begun to realize that many natural resins and fibers that could be used as coatings, adhesives, and fabrics were polymers, that is, macromolecules with repeated structures, and began to look for ingredients and methods for synthesizing polymers.

The difference is that celluloid comes from chemically treated cotton and other cellulose-containing plant materials, while phenolic plastic is the world’s first fully synthetic plastic. Baekeland named it “Bakelite” after himself. He was lucky that his British counterpart Sir James Swinburne filed a patent application only one day later than him, otherwise phenolic plastic might be called “Swinburne” in English. On February 8, 1909, Baekeland disclosed this plastic at a meeting of the New York branch of the American Chemical Society.

The emergence of counterfeit phenolic plastics also prompted Baekeland to use genuine labels such as “Intel Inside” on its products very early. In 1926, the patent protection expired, and a large number of similar products flooded the market. After negotiations, Baekeland merged with its competitors and owned a real phenolic plastic empire.

As a scientist, Baekeland enjoyed both fame and fortune. He had more than 100 patents, countless honorary positions, and was included in the Hall of Fame of both the science and business worlds after his death. He had both the business acumen that was rare among scientists and the life dullness that was too common among scientists. In addition to movies and cars, his biggest hobby was to hang out on the yacht “Ion” in a shirt and shorts. However, it is said that he only had one formal suit and always wore a pair of old sneakers. In order to let him change his clothes, his wife, an artist, picked out a $125 British blue twill serge suit in a clothing store, paid the owner $100 in advance, and asked him to display the suit in the window with a $25 tag. That night, Baekeland learned from his wife about this good thing that was cheap and good, and bought it the next day. On the way home, he met his neighbor, lawyer Samuel Untermeyer, and Baekeland’s new clothes were immediately bought by the other party for $75, which became a proud example of his showing his acumen to his wife.

In 1939, when Baekeland retired, his son George Washington Baekeland had no intention of doing business, so the company was sold to Union Carbide for $16.5 million (equivalent to $200 million today). In 1945, a year after Baekeland’s death, the annual output of plastics in the United States exceeded 400,000 tons, and in 1979 it exceeded the representative of the industrial age – steel. At the exhibition at the Science Museum in London, Baekeland’s great-grandson Hugh Clarke held a 1930s urea -formaldehyde plastic telephone in one hand and displayed a mobile phone made of biodegradable plastic in the other.

Application and development of plastics

The rapid development of the plastics industry has also brought about a series of social problems caused by waste plastics and garbage. The application of plastic products has penetrated into every corner of society, from industrial production to food, clothing, housing and transportation, plastic products are everywhere. People have begun to find that plastic waste has quietly come to us, seriously affecting our health and living environment. For example, some agricultural land has begun to reduce production due to the impact of abandoned mulch, and the “white pollution” caused by waste plastics has begun to give people headaches. Lunch boxes that do not rot and decompose cannot be effectively recycled, and there is no way to deal with plastic waste for daily use. The sharp increase in plastic waste and the social and environmental problems caused by it are in front of people and in the places where people live all over the world.

Plastics for the development of machinery

The huge market of plastics has promoted the development of the plastic machinery industry. The application scope of the plastic machinery industry is very wide, including packaging, agriculture, construction, automobiles and other fields. With the continuous development of the domestic petrochemical industry, China’s plastic machinery industry has gradually formed an independent industrial sector and has begun to take shape. China’s plastic machinery industry has developed very rapidly, basically achieving leapfrog development, and the scale of the industry is also constantly expanding.

The development potential of China’s plastic machinery industry is very huge and full of stamina, especially those models with high technological content, good performance and relatively moderate prices. In particular, super-large, precise and special injection molding machines, low-temperature and high-power single-screw extruders, multi-layer co-extrusion blow molding machines for producing high-impermeability and heat-resistant packaging materials, blow molding machines for producing industrial parts (auto parts, etc.), etc., all have good development prospects.

Nowadays, the market demand and consumption of plastic products have changed a lot, but plastic machinery is still the most concerned by major manufacturers and consumers. At present, the independent innovation ability of China’s plastic machinery industry is relatively weak, there are few high-end and personalized special products, and the industry concentration is still relatively low. However, the plastic processing industry has a good development prospect, which has played a great role in promoting the rapid development of China’s plastic machinery manufacturing industry. China’s plastic machinery industry has great development potential.

Industry Development

After a long period of struggle and opening up to the world, China’s plastics industry has formed a relatively complete industrial system and has become a basic material industry that is on par with steel, cement and wood. As a new type of material, its application field has far exceeded the above three materials. Since the 21st century, China’s plastics industry has achieved remarkable achievements and made a historic leap. As one of the pillar industries of the light industry, the plastics industry has maintained a growth rate of more than 10% in recent years. While maintaining a relatively fast development speed, the economic benefits have also been improved. The total output value of enterprises above designated size in the plastic products industry ranks third among the 19 major light industries, and the product sales rate is 97.8%, which is higher than the average level of the light industry. From the production of synthetic resins, plastic machinery and plastic products, the strong development momentum of China’s plastics industry is shown.

From January to December 2007, China’s plastic products enterprises achieved a cumulative total industrial output value of 801,815,657,000 yuan, an increase of 27.06% over the same period last year; from January to October 2008, China’s plastic products enterprises achieved a cumulative total industrial output value of 788,006,448,000 yuan, an increase of 22.16% over the same period last year.

China is a large producer of plastic raw materials and a huge consumer of plastic products. According to authoritative industry statistics, by 2013, there were 13,699 domestic plastic product enterprises of a certain scale. The national plastic product output was 61.8866 million tons, an increase of 8.02% year-on-year. Among them, the output of plastic film products was 10.893 million tons; the output of plastic daily necessities was 4.716 million tons; the output of plastic artificial leather and synthetic leather products was 3.47 million tons; the output of fiber-reinforced plastic products was 2.5986 million tons; and the output of foam plastic products was 1.465 million tons.

Greenpeace estimates that plastic production has surged 900% between 1980 and 2020, to more than 500 million tons per year. Much of this plastic ends up in landfill rather than being recycled.

Agricultural applications

Plastic consumption is growing rapidly

China is a large agricultural country. Agricultural plastic products have become indispensable materials for the development of modern agriculture. They are an irreplaceable technical measure to resist natural disasters and achieve stable, high-yield, high-quality and high-efficiency crop yields. They have been widely used in China’s agriculture, forestry, animal husbandry and fishery industries. Agriculture has become the second largest consumer of plastic products after the packaging industry.

Package

Plastic consumption tops the list

Plastic packaging materials mainly include plastic soft packaging, woven bags, hollow containers, turnover boxes, etc., which are one of the largest areas of plastic product application. In 2005, plastic packaging exceeded 7 million tons, accounting for about 1/3 of the total output of packaging materials, ranking first among various packaging materials. Plastic woven bags and heavy packaging bags have been widely used in the packaging of various mineral products, chemical products, synthetic resins, raw salt, grain, sugar, cotton and wool; beverages, washing products, cosmetics, chemical products, etc. are developing rapidly in China, and there is a great demand for indispensable plastic packaging materials such as composite films, packaging films, containers, turnover boxes, etc. Food and medicine are important materials for the national economy and people’s livelihood, and the corresponding packaging demand is very strong. The growth rate of China’s pharmaceutical packaging ranks first among the world’s eight major drug producing countries.

Entering the 21st century, China’s entry into the WTO and the development of the global economy have further promoted the development of China’s domestic demand and foreign trade, which will drive BOPP (biaxially oriented polypropylene film) and plastic flexible packaging products into a new round of high growth in market demand. According to industry estimates, China’s BOPP film market reached 1 million tons in 2005, with an average annual growth rate of more than 20%. The development of the flexible packaging industry has provided good market opportunities for the development of the BOPP industry.

The introduction of technology

The general development trend of China’s plastic products industry is: agricultural plastics (including agricultural films, water-saving agricultural equipment and geosynthetics ) still occupy an important position and will be further developed; packaging materials and plastic building materials will be the main areas of rapid growth of the plastics industry; the production and application areas of high-tech, high-value-added engineering plastic products and composite materials will continue to expand with the development of the market economy; the production of pipes, profiles, calendered products, biaxially oriented materials, films, etc. will gradually develop in the direction of economic scale; in order to protect the ozone layer, the production of foam plastics will be transformed into fluorine-free technology; in order to reduce environmental pollution, the recycling of waste plastics and the research and development of degradable plastics will be strengthened; in order to develop the variety and improve the grade of plastic products, the development and production of plastic machinery and molds will be given attention.

Processing Technology

Molding definition

Plastic molding refers to the process of making final plastic products from polymers manufactured by synthetic resin manufacturers. Processing methods (usually referred to as primary processing of plastics ) include compression molding, extrusion molding, injection molding, blow molding, calendering, etc.

Blister

After the sheet is heated to a certain temperature by a blister machine, the negative pressure generated by a vacuum pump is used to adsorb the plastic sheet onto the surface of the model, and then it is transformed into blisters or bubble shells of different shapes after cooling and shaping.

Compression Molding

Compression molding is also called compression molding or compression molding. Compression molding is mainly used for the molding of thermosetting plastics such as phenolic resin, urea-formaldehyde resin, and unsaturated polyester resin.

Extrusion

Extrusion, also known as extrusion molding, is a method of using an extruder ( extruder ) to continuously pass heated resin through a mold to extrude products of the desired shape. Extrusion is sometimes also used to mold thermosetting plastics and can be used to mold foam plastics. The advantages of extrusion are that it can extrude products of various shapes, has high production efficiency, and can be automated and continuously produced; the disadvantage is that thermosetting plastics cannot be widely processed by this method, and the product size is prone to deviation.

Injection Molding

Injection molding is also called injection molding. Injection molding is a method of using an injection molding machine (or injection machine ) to inject thermoplastic melt into a mold under high pressure to obtain a product after cooling and solidification. Injection molding can also be used to mold thermosetting plastics and foam plastics. The advantages of injection molding are fast production speed, high efficiency, automated operation, and the ability to mold parts with complex shapes, which is particularly suitable for mass production. The disadvantages are high equipment and mold costs, and difficulty in cleaning the injection molding machine.

Blow Molding

Blow molding is also called hollow blow molding or hollow forming. Blow molding is a method of using the pressure of compressed air to blow the hot resin parison closed in the mold into a hollow product. Blow molding includes two methods: blow molding film and blow molding hollow products. Blow molding can produce film products, various bottles, barrels, pots and children’s toys.

Calendering

Calendering is a molding method in which resin and various additives are processed into films or sheets through the gap between two or more calender rollers of a calender with opposite directions after the expected treatment (kneading, filtering, etc.), and then peeled off from the calender rollers and cooled to shape. Calendering is a molding method mainly used for polyvinyl chloride resins, and can be used to manufacture films, sheets, plates, artificial leather, floor tiles and other products.

Foam molding

The process of adding appropriate foaming agents to foaming materials (PVC, PE and PS, etc.) to make plastics have microporous structures. Almost all thermosetting and thermoplastic plastics can be made into foam plastics. According to the pore structure, they are divided into open-cell foam plastics (most of the pores are interconnected) and closed-cell foam plastics (most of the pores are separated from each other), which is mainly determined by the manufacturing method (divided into chemical foaming, physical foaming and mechanical foaming ).

Plastic Evolution

Plastic technology is developing rapidly. The development of new materials for new applications, the improvement of performance in existing material markets, and the improvement of performance for special applications are several important directions for new material development and application innovation.

New bioplastic with high thermal conductivity

NEC has developed a new bioplastic made from plants, which has a thermal conductivity comparable to that of stainless steel. The company mixed carbon fibers several millimeters long and 0.01 mm in diameter and a special adhesive into polylactic acid resin made from corn to produce a new type of bioplastic with high thermal conductivity. If 10% of carbon fibers are mixed in, the thermal conductivity of the bioplastic is comparable to that of stainless steel; when 30% of carbon fibers are added, the thermal conductivity of the bioplastic is twice that of stainless steel, and the density is only 1/5 of that of stainless steel.

In addition to its good thermal conductivity, this bioplastic also has the advantages of being light, easy to shape, and having little environmental pollution. It can be used to produce the frames of thin and light computers, mobile phones and other electronic products.

Color-changing plastic film

The University of Southampton in the UK and the Institute of Plastics in Darmstadt, Germany have jointly developed a color-changing plastic film. This film combines natural optical effects with artificial optical effects, and is actually a new way to accurately change the color of objects. This color-changing plastic film is a plastic opal film, which is composed of plastic balls stacked in three-dimensional space. There are also tiny carbon nanoparticles in the middle of the plastic balls, so that light is not only reflected in the edge area between the plastic balls and the surrounding materials, but also reflected on the surface of the carbon nanoparticles filled between these plastic balls. This greatly deepens the color of the film. As long as the volume of the plastic balls is controlled, light substances that only scatter certain spectral frequencies can be produced.

Plastic blood

Researchers at the University of Sheffield in the UK have developed an artificial ” plastic blood ” that looks like thick paste. Once dissolved in water, it can be transfused into patients and used as a substitute for blood in emergency situations. This new type of artificial blood is made up of plastic molecules. There are millions of plastic molecules in a piece of artificial blood. The size and shape of these molecules are similar to those of hemoglobin molecules. They can also carry iron atoms and transport oxygen throughout the body like hemoglobin. Since the raw material is plastic, this artificial blood is light and easy to carry. It does not need to be refrigerated. It has a long shelf life and is more efficient than real artificial blood. It is also cheaper.

New bulletproof plastic

In 2013, a research team in Mexico developed a new type of bulletproof plastic that can be used to make bulletproof glass and bulletproof clothing. The weight is only 1/5 to 1/7 of that of traditional materials. This is a specially processed plastic material that has super bulletproof properties compared to plastics with normal structures. Tests show that this new type of plastic can resist bullets with a diameter of 22mm. Conventional bulletproof materials will be damaged and deformed after being hit by bullets and cannot be used anymore. This new type of material will temporarily deform after being hit by bullets, but it will quickly return to its original shape and can be used again. In addition, this new material can evenly distribute the impact force of bullets, thereby reducing damage to the human body.

Plastics that reduce car noise

The American Polymer Group (PGI) uses renewable polypropylene and polyethylene terephthalate to create a new base material for moldable automotive parts that can reduce noise. This material is mainly used in body and wheel well pads to create a barrier layer that absorbs sound in the car compartment and reduces noise by 25% to 30%. PGI has developed a special one-step production process that organically combines recycled materials and untreated materials, making the two materials a whole through lamination and needle punching.

Plastic softening

The factors that affect thermoplastic molding shrinkage are as follows:

Plastic types During the molding process of thermoplastics, there are volume changes caused by crystallization, strong internal stress, large residual stress frozen in the plastic part, strong molecular orientation and other factors. Therefore, compared with thermosetting plastics, their shrinkage rate is larger, the shrinkage rate range is wider, and the directionality is obvious. In addition, the shrinkage after molding, annealing or humidity adjustment treatment is generally larger than that of thermosetting plastics.

Characteristics of plastic parts When the molten material contacts the surface of the cavity during molding, the outer layer immediately cools to form a low- density solid shell. Due to the poor thermal conductivity of plastics, the inner layer of the plastic part cools slowly to form a high-density solid layer with large shrinkage. Therefore, the thicker the wall, the slower the cooling, and the thicker the high-density layer, the greater the shrinkage. In addition, the presence or absence of inserts and the layout and number of inserts directly affect the direction of material flow, density distribution, and shrinkage resistance, so the characteristics of plastic parts have a greater impact on the size and direction of shrinkage.

The form, size and distribution of the feed port directly affect the material flow direction, density distribution, pressure holding and shrinkage compensation and molding time. Direct feed ports and feed ports with large cross-sections (especially thicker cross-sections) have small shrinkage but large directionality, while feed ports with wide widths and short lengths have small directionality. Feed ports that are close to the feed port or parallel to the material flow direction have large shrinkage.

Molding conditions When the mold temperature is high, the molten material cools slowly, has high density, and shrinks greatly. Especially for crystalline materials, the shrinkage is greater due to the high crystallinity and large volume change. The mold temperature distribution is also related to the internal and external cooling and density uniformity of the plastic part, which directly affects the shrinkage amount and directionality of each part. In addition, the holding pressure and time also have a great influence on the shrinkage. The shrinkage is small but the directionality is large when the pressure is high and the time is long. The injection pressure is high, the viscosity difference of the molten material is small, the interlayer shear stress is small, and the elastic rebound after demolding is large, so the shrinkage can also be reduced appropriately. The material temperature is high and the shrinkage is large, but the directionality is small. Therefore, adjusting the mold temperature, pressure, injection speed, cooling time and other factors during molding can also appropriately change the shrinkage of the plastic part.

When designing the mold, the shrinkage rate of each part of the plastic part is determined based on the shrinkage range of various plastics, the wall thickness and shape of the plastic part, the size and distribution of the feed port, and the cavity size is calculated. For high-precision plastic parts and when the shrinkage rate is difficult to grasp, the following method is generally used to design the mold:

• Take a smaller shrinkage rate for the outer diameter of the plastic part and a larger shrinkage rate for the inner diameter to leave room for correction after the mold trial.
• Trial mold to determine the pouring system form, size and molding conditions.
• Determine the dimensional changes of the plastic parts to be post -processed (the measurement must be made 24 hours after demolding).
• Modify the mold according to the actual shrinkage situation.
• Try the mold again and appropriately change the process conditions to slightly correct the shrinkage value to meet the requirements of the plastic part.

Plastic Type	Softening or melting range/°c	Plastic Type	Softening or melting range/°c
Polyvinyl acetate	35~ 85	Polyoxymethylene	165~185
Polystyrene	70~115	Polypropylene	160~170
Polyvinyl chloride	75~90	Nylon 12	170~180
Polyethylene (density 0.92g/cm^3)	110	Nylon 11	180~190
Polyethylene (density 0.94g/cm^3)	Approx. 120	Polychlorotrifluoroethylene	200~220
Polyethylene (density 0.96g/cm^3)	Approx. 130	Nylon 610	210~ 220
Poly- 1-butene	125~ 135	Nylon 6	215~225
Polyvinylidene chloride	115~140 (softened)	Polycarbonate	220~ 230
Plexiglas	126~160	Poly-4-methylpentene-1	240
Cellulose acetate	125~175	Nylon 66	250~260
Polyacrylonitrile	130~150 (softened)	Polyethylene terephthalate	250~260

Triangle mark

PET

Polyethylene terephthalate (Polyester)

“No. 1” PET is commonly used in: mineral water bottles, carbonated beverage bottles, etc.

Use: Heat-resistant up to 65°C, cold-resistant down to -20°C, only suitable for warm or cold drinks, easy to deform when filled with high-temperature liquids or heated, and substances harmful to the human body will melt out. In addition, scientists have found that after 10 months of use, No. 1 plastic may release the carcinogen DEHP, which is toxic to the testicles.

Therefore, throw away beverage bottles after use. Do not use them as water cups or storage containers for other items to avoid causing health problems that are not worth the cost.

Note: Do not reuse beverage bottles to hold hot water. Do not place them in the sun in a car. Do not use bottles to hold alcohol, oil, etc.

HDPE

High-density polyethylene

“No. 2” HDPE is commonly used for packaging of cleaning products and bath products.

Use: They can be reused after careful cleaning, but these containers are usually difficult to clean and will retain the original cleaning products, becoming a breeding ground for bacteria. You’d better not recycle them. Do not use them as water cups or storage containers for other items.

Note: It is difficult to clean thoroughly, and it is not recommended to recycle

PVC

Polyvinyl chloride

“No. 3” PVC is commonly used in: common raincoats, building materials, plastic films, plastic boxes, etc. It is rarely used in food packaging.

Use: This material has good plasticity and is cheap, so it is widely used. It can only withstand heat of 80 ℃. It is easy to produce harmful substances at high temperatures, and even releases toxic substances during the manufacturing process. If it enters the human body with food, it may cause breast cancer, congenital defects in newborns and other diseases. Containers made of this material are rarely used to package food. If used, do not let it heat up. It is difficult to clean and easy to leave residues, so do not recycle it. Do not buy it if it is used to hold drinks.

Note: Not for food packaging.

LDPE

Low-density polyethylene

“No. 4” LDPE is commonly used in: cling film, plastic film, etc.

Use: It is not heat-resistant. Usually, qualified PE cling film will melt when the temperature exceeds 110 ℃, leaving some plastic preparations that cannot be decomposed by the human body. The fat in the food can also easily dissolve the harmful substances in the cling film. Therefore, when putting food into the microwave, you must first remove the cling film. Harmful substances are produced at high temperatures. After the toxic substances enter the human body with the food, they may cause breast cancer, congenital defects in newborns and other diseases.

Note: When heating in the microwave, do not wrap food in plastic wrap.

PP

Polypropylene

“No. 5” PP is commonly used in: soy milk bottles, yogurt bottles, juice bottles, and microwave lunch boxes.

Usage: With a melting point of up to 167°C, it is the only plastic box that can be safely placed in a microwave oven and can be reused after careful cleaning. Special attention should be paid to the fact that some microwave oven lunch boxes are indeed made of No. 5 PP (microwave oven-specific PP is resistant to high temperatures of 120°C and low temperatures of -20°C), but due to the high cost, the lid is generally not made of special PP but of No. 1 PET. Since PET cannot withstand high temperatures, it cannot be placed in a microwave oven together with the box. To be on the safe side, remove the lid before placing the container in the microwave oven.

Note: When placing in the microwave, remove the lid.

PS

Polystyrene

“No. 6” PS is commonly used in: bowl-shaped instant noodle boxes and fast food boxes.

Don’t use the microwave to cook instant noodles

Usage: It is heat-resistant and cold-resistant, but it cannot be placed in a microwave oven to prevent the release of chemicals due to excessive temperature (it will be released at a temperature of 70°C). It cannot be used to hold strong acids (such as orange juice ) or strong alkaline substances, because polystyrene, which is harmful to the human body, will be decomposed and easily cause cancer. Therefore, you should try to avoid using fast food boxes to pack hot food.

Note: Do not use a microwave to cook instant noodles.

PC

Other plastics

Other types of “No. 7” PC are commonly used in: kettles, cups, and baby bottles.

Use: A material that is widely used, especially in baby bottles, and is controversial because it contains bisphenol A. Lin Hanhua, associate professor of the Department of Biology and Chemistry at City University of Hong Kong, said that in theory, as long as bisphenol A is 100% converted into plastic structure during the production of PC, it means that the product does not contain bisphenol A at all, let alone release. However, if a small amount of bisphenol A is not converted into the plastic structure of PC, it may be released and enter food or drinks. Therefore, it is better to be careful and pay special attention when using this plastic container.

Cleaning measures to combat bisphenol A

The higher the temperature, the more bisphenol A remaining in PC will be released, and the faster it will be released. Therefore, you should not use PC water bottles to hold hot water, so as not to increase the release rate and concentration of bisphenol A (if any). If your water bottle is numbered 7, the following methods can reduce the risk:

• Do not heat during use.
• Do not use a dishwasher or dishwasher to clean the kettle.
• Do not expose the kettle to direct sunlight.
• Before using for the first time, clean it with baking soda and warm water, and dry it naturally at room temperature, because bisphenol A will be released more during the first use and long-term use.
• If the container is dropped or damaged in any way, it is recommended to stop using it, because if there are tiny grooves on the surface of plastic products, it is easy for bacteria to hide.
• Avoid repeated use of aged plastic utensils.

Identification

Before using various plastic recycling methods to reuse waste plastics, most of them need to be sorted. Because plastic consumption channels are numerous and complex, and some post-consumer plastics are difficult to distinguish simply by appearance, it is best to mark the material type on plastic products. China has formulated GB/T16288-2008 “Plastic Packaging Product Recycling Mark ” with reference to the material type marking proposed and implemented by the American Plastics Association (SPE). Although the above marking method can be used to facilitate sorting, there are still many unmarked plastic products in China, which brings difficulties to sorting. In order to separate different types of plastics for classified recycling, we must first master the knowledge of identifying different plastics. The following introduces a simple method for identifying plastics:

Appearance Identification

By observing the appearance of plastic, one can preliminarily identify the major category to which the plastic product belongs: thermoplastics, thermosetting plastics or elastomers.

Generally, thermoplastics are divided into two categories: crystalline and amorphous. Crystalline plastics are translucent, milky or opaque in appearance, and are transparent only in the film state, with hardness ranging from soft to horny. Amorphous plastics are generally colorless, completely transparent without additives, and have a hardness ranging from harder than horny rubber ( additives such as plasticizers are often added at this time ). Thermosetting plastics usually contain fillers and are opaque, and are transparent without fillers. Elastomers have a rubbery feel and a certain degree of elongation.

Simple identification

First of all, look at it, because dark-colored pigments are generally very toxic, so generally speaking, the darker the color of the plastic, the more toxic it is. Another very important reason for dyeing plastic is that the raw material is waste plastic, in order to cover up the original color.

The second is to smell. As long as the plastic has an odor, it must not be used to hold food. The odor is generally the smell of additives, colorants, other accessories or residual monomers added to the product.

The third is touch. The plastic used to hold food usually feels smooth and shiny. If it doesn’t feel smooth, especially if it feels sticky, it must not be used to hold food, because there are too many additives in it, so many that you can’t even imagine, and it may account for more than 50%.

Next, let’s talk about some materials. Food packaging generally uses PP ( polypropylene ) and PE ( polyethylene ). These two are relatively safe. The cling film in supermarkets is generally PP, and the hot water pipes at home are also PP because they must be non-toxic. Plastic cups that can hold hot water generally use PC ( polycarbonate ), but CDs also use this, so be careful about the waste being recycled. Drain pipes generally use PVC, which is fine.

Identification methods

1. Density method : By examining the density of various plastics and using liquid as a medium to observe their sinking and floating phenomena, the major category to which the plastic belongs can be roughly identified. If the plastic can float on the water surface when placed in water, then it can be determined that the raw material is not PVC.
2. Combustion method : observe the flame color, smell and smoke of burning plastics. Usually, the flame of polyolefin materials is blue or light blue, the smell is mild and light, and the smoke is white. Most materials with benzene or chlorine tend to emit black smoke with strong smell after burning, such as ABS. In addition, PE and PP have dripping combustion, while PVC has no dripping combustion, but has self-extinguishing phenomenon.
3. Optical method: Identify by observing the transparency of the raw materials. Commonly used transparent raw materials are: PS, PC, PMMA, AS; translucent raw materials are: PE, random copolymer PP, homopolymer PP, soft PVC, transparent ABS, etc. Other raw materials are basically opaque.
4. Color identification method: Generally speaking, raw materials without additives, if they contain double bonds, will appear slightly yellow in color. For example, ABS, due to butadiene copolymerization, still contains double bonds in the polymer after polymerization, so it will appear slightly yellow.

Identification Summary

Polytetrafluoroethylene (PTFE)

Appearance: Translucent to opaque, flexible and elastic.

Flammability : Non-flammable. Has a pungent odor (HF) when hot.

Polyamide (PA)

Appearance: Translucent to opaque.

Combustibility: Hard to burn, extinguishes immediately after leaving the flame. When burning in the flame, there is blue smoke, and the upper end is orange-red; there are melting, dripping, and bubbling phenomena; you can smell the smell of burning wool.

Polycarbonate (PC)

Appearance: Transparent to opaque, hard.

Combustibility: Hard to burn. Burns in flames with lots of bright black smoke, carbonization, and bubbling; a phenol odor can be smelled.

Phenolic resin ( PF )

Appearance: (usually contains fillers ) dark in color.

Combustibility: Hard to burn. When burning in flames, a bright yellow flame can be seen, with a lot of black smoke, cracking and darkening of color.

Polyvinyl chloride (PVC)

Appearance: (same as polycarbonate)

Combustibility: Difficult to burn. It turns yellow in flame, with green edges and white smoke; it softens. A burnt smell can be smelled.

Amino resin (UF urea/ formaldehyde; MF melamine /formaldehyde)

Appearance: (including filler) hard.

Combustibility: Hard to burn. Bright yellow when burning in flames; carbonization, expansion, and cracking. Ammonia, formaldehyde, and fishy odors can be smelled.

Polyethylene (PE)

Appearance: Translucent to opaque, hard; transparent film.

Combustibility: It can burn in flames, and will slowly extinguish or continue to burn after leaving the flame. When burning, the upper end of the flame is yellow and the lower end is blue; there is melting and dripping. The smell of paraffin can be smelled.

Polypropylene (PP)

(Appearance and flammability are the same as polyethylene)

Other

Plastic Rocks

In April 2023, an international research team discovered a new form of plastic pollution: plastic waste films chemically bonded to rocks. This discovery has made scientists increasingly aware that plastic has become part of the Earth’s geology. In addition to affecting the Earth’s geology, the worrying thing about “plastic rocks” is that they can release microplastics into the environment. These plastic fragments can be transported long distances through the atmosphere and oceans, can penetrate plant tissues, and may be accidentally eaten by animals such as fish and birds.

Health hazards

Plastic packaging materials have the advantages of light weight, high strength, good impact resistance, transparency, moisture resistance, beauty, stable chemical properties, good toughness and corrosion resistance. They have widely replaced metal, wood, paper, glass, leather, etc. in the packaging field. Therefore, plastic packaging has played an irreplaceable role in alleviating China’s resource and energy pressure. However, plastic packaging materials have a fatal weakness, that is, their natural degradation time is long, some as long as more than 100 years. The non-degradability of plastics leads to the long-term existence of their waste. Moreover, they are often discarded after being consumed once, so plastic packaging waste has become an increasingly prominent environmental problem, forming the so-called ” white pollution “, which has caused great pressure on the human living environment. Therefore, the recycling of plastic packaging waste is imminent.

Health Tips

Plastic products with the code number 5 can withstand temperatures up to 130 degrees and can be filled with hot water and heated in microwaves.

The darker the straw, the less safe it is.

The use of plastic bowls and imitation porcelain bowls should be reduced.

Do not use dark-colored plastic bowls.

Melamine tableware cannot hold acidic substances.

Mineral water bottles and purified water buckets are coded “1”.

Plastic bottles can be used to store dry items.

The water bottle code 5 is heat resistant and reusable.

Qualified plastic lunch boxes are marked with “5” pp and have a small ventilation hole on the lid. Because plastics will be added with certain additives, the darker the color of the plastic, the darker the additives, or the more residual harmful substances, which is unsafe. pp stands for polypropylene, which is not suitable for making plastic lunch boxes, mainly because it is not environmentally friendly.

environmental impact

Plastic pollution is just one of the ways that people are changing the environment, and it is by far the most reported form of marine environmental pollution. Plastic is the most prevalent element of marine waste and can have harmful effects on wildlife.