Synthetic resin is a type of artificially synthesized high molecular weight compound that has or exceeds the inherent properties of natural resins. ASTM D883-65T defines synthetic resin as an organic substance with an unrestricted molecular weight but often a high molecular weight solid, semi-solid or pseudo (quasi) solid, which has a tendency to flow when stressed, often has a softening or melting range and is shell-shaped when broken.
In practical applications, it is often used synonymously with polymer or even plastic, especially referring to the basic material produced by the polymerization reaction of monomers without any additives or with only a very small amount of additives. In addition, it is sometimes used to represent uncured flowable thermosetting polymer materials.
The world’s three major synthetic materials include synthetic resins, synthetic rubber and synthetic fibers. Synthetic resin is the synthetic material with the highest output and consumption.
Application Areas
The most important application of synthetic resin is the manufacture of plastics. In order to facilitate processing and improve performance, additives are often added, and sometimes they are directly used for processing and forming, so they are often synonymous with plastics. The content of synthetic resin in plastics is generally 40~100%. Due to the large content and the properties of the resin often determine the properties of the plastic, people often regard resin as a synonym for plastic. For example, polyvinyl chloride resin is confused with polyvinyl chloride plastic, and phenolic resin is confused with phenolic plastic. In fact, resin and plastic are two different concepts. Resin is an unprocessed raw polymer, 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.
Synthetic resins are also the basic raw materials for manufacturing synthetic fibers, coatings, adhesives, insulating materials, etc. The widely used resin concrete also uses synthetic resins as the cementing material. Since synthetic resins have obvious performance and cost advantages over other competing materials, their applications have penetrated into all aspects of the national economy. Packaging is the largest market for synthetic resins, followed by building products. Electronics, electrical and automotive are also important application areas for synthetic resins. Other markets include furniture, toys, entertainment products, household appliances and medical supplies.
Basic types
There are many types of synthetic resins.
Synthetic resin industry products can be divided into general-purpose resins and special-purpose resins. General-purpose resins have large output and low cost, and are generally used in general consumer goods or durable goods. Representative varieties include five major types of synthetic resins: polyethylene, polypropylene, polyvinyl chloride, polystyrene and ABS. Special-purpose resins generally refer to resins produced for special purposes, with smaller output and higher production costs. For example, they can replace metals in machinery, electronics, automobiles and other sectors. Engineering plastics fall into the category of special-purpose resins. Important engineering plastics include polyamide, polycarbonate, polyoxymethylene, polybutylene terephthalate, modified polyphenylene ether and polytetrafluoroethylene. Another type of special-purpose resin is thermoplastic elastomer, which has elasticity similar to rubber and can be repeatedly molded when heated.
According to the chemical composition, synthetic resins can be roughly divided into two categories: one type of synthetic resin has a main chain composed only of aliphatic carbon atoms, and general-purpose resins basically belong to this category; the other type of synthetic resin contains oxygen, nitrogen, sulfur, etc. in addition to carbon atoms in the main chain, and most engineering plastics are composed of heterochain polymers.
According to engineering properties, synthetic resins can be divided into thermoplastic resins and thermosetting resins. The difference mainly comes from the chemical composition and molecular structure of the polymer. The molecular chain structure of thermoplastic resins is linear or branched. It can be plasticized (or softened, melted) and flowed after heating, and can be repeatedly plasticized and molded. Typical thermoplastic resins include polyethylene, polypropylene, poly-1-butene, polyvinyl chloride, polystyrene, etc. Thermoplastic resins can be quickly molded and can be repeatedly molded. Thermosetting resins are high molecular polymers with three-dimensional structures. They contain multi-functional macromolecules in the molecular chain. In the presence of a curing agent and under the action of heat and pressure, they can soften (or melt) and solidify (or mature) at the same time to become insoluble and infusible polymers. Common typical thermosetting resins include phenol-formaldehyde resin (commonly known as phenolic resin), urea-formaldehyde resin (commonly known as urea-formaldehyde resin), melamine formaldehyde resin (commonly known as melamine formaldehyde resin), epoxy resin, unsaturated polyester resin, polyurethane, etc.
Preparation method
Synthetic resin is a high molecular compound, which is produced by combining low molecular raw materials – monomers (such as ethylene, propylene, vinyl chloride, etc.) into macromolecules through polymerization reaction. Commonly used polymerization methods in industry include bulk polymerization, suspension polymerization, emulsion polymerization, solution polymerization, slurry polymerization, gas phase polymerization, etc. The raw materials for producing synthetic resins are rich in sources. In the early days, they were mainly coal tar products and calcium carbide, and now they are mainly petroleum and natural gas products, such as ethylene, propylene, benzene, formaldehyde and urea.
Bulk polymerization
Bulk polymerization is a polymerization process in which monomers are polymerized without the addition of other media under the action of initiators or heat, light, and radiation. The characteristics are pure products, no need for complex separation and purification, simple operation, and high utilization rate of production equipment. It can directly produce pipes, plates and other high-quality products, so it is also called block polymerization. The disadvantage is that the viscosity of the material increases continuously as the polymerization reaction proceeds, mixing and heat transfer are difficult, and the reactor temperature is not easy to control. Bulk polymerization is often used in the production of resins such as polymethyl acrylate (commonly known as plexiglass), polystyrene, low-density polyethylene, polypropylene, polyester and polyamide.
Suspension polymerization
Suspension polymerization refers to the polymerization process in which monomers are dispersed into droplets under the action of mechanical stirring or oscillation and dispersants, usually suspended in water, so it is also called bead polymerization. The characteristics are: there is a large amount of water in the reactor, the material viscosity is low, and heat transfer and control are easy; after polymerization, only simple separation, washing, drying and other processes are required to obtain resin products, which can be directly used for molding processing; the products are relatively pure and uniform. The disadvantage is that the reactor production capacity and product purity are not as good as bulk polymerization, and continuous production cannot be used. Suspension polymerization is widely used in industry.
Emulsion polymerization
Emulsion polymerization refers to the polymerization in which monomers form an emulsion in water under mechanical stirring or oscillation with the help of an emulsifier. The product of emulsion polymerization is latex, which can be used directly, or the latex can be destroyed and subjected to post-treatment procedures such as washing and drying to obtain a powder or needle-shaped polymer. Emulsion polymerization can obtain polymers with higher molecular weight at a higher reaction rate. The material has low viscosity, is easy to transfer heat and mix, is easy to control production, and is easy to remove residual monomers. The disadvantage of emulsion polymerization is that the emulsifier added during the polymerization process affects the performance of the product. In order to obtain a solid polymer, it is consumed through processes such as coagulation, separation, and washing. The production capacity of the reactor is lower than that of the bulk polymerization method.
Solution polymerization
Solution polymerization is carried out in the presence of a solvent. The selected solvent must be able to dissolve both monomers and polymers. During the polymerization process, the system is a uniform viscous solution, the polymerization system is always homogeneous, the continuous operation cycle is long, and it is easy to operate. However, the system viscosity is relatively large. Its advantage is that the homogeneous reaction is easier to control, and the molecular weight and its distribution can also be properly controlled, but the solution polymerization system is viscous, which makes heat and mass transfer difficult and uneven.
Slurry polymerization
During slurry polymerization, a solvent or the monomer itself is used as a dispersion medium. The resulting polymer is insoluble in the dispersion medium, but is dispersed in the dispersion medium in the form of particles, forming a slurry. Some earlier literature has classified it as a heterogeneous solution polymerization. This type of polymerization is characterized by low system viscosity, easy stirring, easy heat dissipation, and a higher monomer concentration, which can increase the unit equipment productivity. At present, this method can be used in the production of high-density polyethylene, polypropylene, etc.
Gas Phase Polymerization
In gas phase polymerization, gas phase monomers and catalysts are introduced into the reactor in a specified amount for one-step synthesis to obtain a dry polymer. The premise of gas phase polymerization is that the catalyst selectivity and yield must be high enough, and the product does not need to remove the residual catalyst, which can greatly shorten the process. With the emergence of highly active supported Ziegler catalysts, gas phase polymerization has become the mainstream in the manufacture of polyethylene or polypropylene. In addition, it can also be widely used in polymerization based on free radical mechanism.
Processing methods
The curing of thermoplastic resins is generally achieved by cooling the product to below the glass transition temperature or melting point, while thermosetting resins are cured by heating to produce a chemical reaction that forms a network structure. The main processing methods include extrusion, compression molding, injection molding, blow molding, rotational molding, reaction injection molding, thermoforming, foaming, etc.
Historical Development
The secretions of some trees often form resins, but amber is a fossil of resin. Although shellac is also considered a resin, it is the sediment secreted by lac insects on trees. Shellac paint made from shellac was originally used only as a wood preservative, but with the invention of the motor, it became the earliest insulating paint used. However, after entering the 20th century, natural products could no longer meet the needs of electrification, forcing people to look for new cheap substitutes.
As early as 1872, German chemist A. Bayer first discovered that phenol and formaldehyde could quickly form reddish-brown lumps or viscous substances when heated under acidic conditions, but he stopped the experiment because they could not be purified by classical methods. After the 20th century, phenol could be obtained in large quantities from coal tar, and formaldehyde was also produced in large quantities as a preservative. Therefore, the reaction products of the two attracted more attention, and people hoped to develop useful products. Although many people have spent a lot of effort on this, they have not achieved the expected results.
In 1904, Baekeland and his assistants also started this research. The initial purpose was to make an insulating varnish to replace natural resin. After three years of hard work, in the summer of 1907, they not only made an insulating varnish, but also made a real synthetic plastic material – Bakelite, which is the well-known “Bakelite”, “Bakelite” or phenolic resin. Once Bakelite came out, manufacturers soon discovered that it could not only make a variety of electrical insulation products, but also daily necessities. Edison used it to make records, and soon announced in an advertisement that he had made thousands of products with Bakelite. For a time, Baekeland’s invention was praised as the “alchemy” of the 20th century.
Before 1940, phenolic resins made from coal tar had always been at the top of the production of various synthetic resins, reaching more than 200,000 tons per year. However, with the development of petrochemical industry, the production of polymer synthetic resins such as polyethylene, polypropylene, polyvinyl chloride and polystyrene has also continued to expand. With the establishment of many large-scale factories with an annual output of more than 100,000 tons of such products, they have become the four types of synthetic resins with the largest output today.
Today, synthetic resins are combined with additives and produced into plastic products through various molding methods. There are dozens of varieties of plastics, with an annual output of about 120 million tons worldwide and more than 5 million tons in china. They have become basic materials for production, life and national defense construction.
Current situation in China
China’s synthetic resin industry has made remarkable achievements in the development of domestically produced catalysts, processes, and equipment. Compared with advanced levels abroad, there is still a certain gap in China’s synthetic resin industry. Many newly built large-scale synthetic resin plants still rely on imported technology, and some plants must purchase foreign catalysts to produce high-end products; Some high-end products in China cannot meet market demand in terms of quantity, such as the shortage of resin grades for greenhouse film, low production volume, and far from meeting the production needs of functional greenhouse film. Imported raw materials account for about 50%; There is still a certain gap between domestically produced PP-R pipe materials and imported materials, and the quality needs to be improved and enhanced.
The global synthetic resin industry is facing competition from low-cost products in the Middle East. In response to this competition, major resin production companies around the world are allocating their assets to the Middle East region for low-cost raw materials; Constructing world-class large-scale production facilities and fully utilizing economies of scale; Adopting more advanced catalysts, process technologies, and computer control, optimization, and management solutions. The resin industry is fiercely competitive. In this situation, the Chinese resin industry should further strengthen its independent innovation capabilities, better digest and absorb imported technologies, produce more high-end products that cannot be produced by Middle Eastern facilities, and make every effort to reduce production costs to cope with competition in synthetic resin products from the Middle East, neighboring countries, and major multinational corporations.
Development prospects
With the changes in the world economic landscape, the substantial adjustment of the U.S. energy structure and the change in china’s economic growth model, in the context of energy conservation and environmental protection, changes in the competitive landscape of the chemical market and upgrading of demand will bring about tremendous changes in the industry.
From the perspective of raw materials for upstream synthetic resin devices, they tend to be more diversified and lightweight to improve the competitiveness of products. From the demand side, green, functional and differentiated requirements are put forward for synthetic resin products. From the perspective of trade, due to the low-cost advantage brought by US shale gas to its chemical industry, exports to China will increase in the future. From the perspective of the china’s competition landscape, coal chemical industry, propane dehydrogenation to propylene, and methanol to olefins will all bring huge challenges to the traditional petrochemical industry. In the short term, the china’s synthetic resin production capacity has increased significantly, while demand continues to be sluggish, and synthetic resins will still be in a period of small profits in the next 2 to 3 years.
Facing a severe market, the china’s synthetic resins must take the road of technological innovation. China’s synthetic resin enterprises must first improve the technical content of their products, break through high-tech barriers, and lock in users; second, they must strengthen the technical services and after-sales services of their products, so that users can get the best price-performance ratio under the premise of appropriate purchase costs; third, they must improve product quality. Enterprises can visit competitive users, tailor-make products, strengthen quality management according to the other party’s requirements, increase certification, etc., and bind high-end customers.
