Development Of Plastic Valves And Plastic Lined Valves

Plastic is one of the four major materials used by humans (the four major materials: wood, steel, nitrates, i.e. cement and ceramics, plastics). Plastic is a new type of material that was developed in the 20th century. The 1930s to 1950s was the period when polymer materials were fully laid, and the vigorous development of the petrochemical industry provided abundant raw materials for the production of plastics. After the 1960s, polymer materials were modified by copolymerization, blending, and compounding. By controlling the polymerization and processing process, the microstructure of polymers was improved to improve their performance and various new processes, new technologies, and new materials continued to emerge. Various engineering plastics have entered the market one after another, becoming the second largest category of non-metallic materials with an annual output volume second only to steel.
Introduction As early as 50 years ago, shortly after the advent of plastics, some people predicted that the demand for Lined Valve would increase significantly. At present, plastic valves are not only widely used in the building materials industry, automobile industry, aerospace industry, water supply and drainage industry, but also in the petrochemical industry.
According to the starting raw materials, they can be divided into natural polymer modified plastics (such as celluloid chrome, acetate fiber, casein plastic, etc.) and synthetic materials. According to the thermal behavior, it can be divided into: thermosetting (such as: phenolic plastics, amino, alkyd resin, unsaturated polyester, silicone resin, thermosetting polyimide, polybenzoxazole, polyphthalic acid, diallyl resin, polyurethane resin, epoxy resin, etc.) and thermal (such as: polyethylene, polypropylene, polyvinyl chloride, polystyrene, polyester, etc.).
According to the use, it can be divided into: general and engineering. General plastics such as: polyethylene, polypropylene, polyvinyl chloride, polystyrene, etc. Engineering plastics can be divided into general engineering plastics (such as: polycarbonate, ABS, polyamide, polyoxymethylene, polyphenylene ether, thermoplastic polyester, etc.) and special engineering plastics (such as: polyether sulfone, fluorine, polyimide, polyaryletherketone, polyarylate, liquid polymer, etc.).
Performance characteristics of several commonly used plastic materials in the valve industry
Polyvinyl chloride (PVC) plastic
Physical and mechanical properties: Polyvinyl chloride is divided into hard polyvinyl chloride and soft polyvinyl chloride. The petrochemical industry often uses hard polyvinyl chloride plastics. PVC has a low density and is easy to install; it has a low thermal conductivity and does not require an insulation layer when the medium temperature is not too high; it has a large linear expansion coefficient and expansion issues should be considered when installing the pipeline; and it has a low operating temperature, generally below 70°C.
Corrosion resistance: The main chain of PVC is made up of non-polar covalent bonds C-C, does not contain highly active groups, and has relatively stable chemical properties. Rigid PVC can resist corrosion from most acids, alkalis, salts, hydrocarbons, organic solvents and other media, except for strong oxidants (such as concentrated nitric acid, fuming sulfuric acid, etc.), aromatic hydrocarbons and ketones.
Application: Rigid PVC can resist corrosion from most acids, alkalis, salts and hydrocarbons and is widely used in oil refining and chemical corrosion protection. However, the operating temperature of rigid PVC is relatively low, generally below 70°C, and because the linear expansion coefficient of PVC is large, 5 to 6 times that of ordinary carbon steel, it cannot be used under production process conditions with relatively high temperatures and large temperature changes.
Polyethylene (PE) plastic
Physical and mechanical properties: Polyethylene is a high molecular polymer of ethylene, which can be divided into high-density polyethylene and low-density polyethylene. Low-density polyethylene has more side chains in its molecular structure and lower crystallinity; high-density polyethylene has fewer side chains and higher crystallinity. Polyethylene is a crystalline polymer, and its physical and mechanical properties are greatly affected by its high crystallinity.
Corrosion resistance: The corrosion resistance of polyethylene is better than that of polyvinyl chloride. It is mainly reflected in that in addition to corrosion resistance, it is almost insoluble in any organic solvent. Its corrosion resistance in formic acid and hydrofluoric acid media is also better than that of polyvinyl chloride. However, the environmental stress cracking phenomenon of polyethylene is more serious. Under lower stress or strain, it will suddenly crack when immersed in some media. In a corrosive environment, the use temperature of polyethylene can be slightly higher than that of rigid polyvinyl chloride when it is not under stress, but because the strength of polyethylene is lower, the allowable use temperature is lower than that of rigid polyvinyl chloride under the same stress conditions.
Application: Due to the serious environmental stress cracking of polyethylene, it will suddenly crack when immersed in some media under low stress or strain. In addition, its mechanical properties are not as good as those of polyvinyl chloride, so its comprehensive performance in the corrosive environment of petrochemical industry is not as good as polyvinyl chloride, and its application is not as extensive as that of polyvinyl chloride.
Polypropylene (PP) plastic
Physical and mechanical properties: Polypropylene is a high molecular polymer of propylene and is currently the lightest type of engineering plastics. Compared with polyvinyl chloride, its disadvantages are large linear expansion coefficient, small elastic modulus, large molding shrinkage, and high brittleness at low temperatures; its advantages are high operating temperature and high softening point (180℃). Below 70℃, the strength of rigid polyvinyl chloride is much higher than that of polypropylene, but as the temperature rises, the strength of polypropylene decreases less. When the glass transition temperature of rigid polyvinyl chloride (80℃) is exceeded, polyvinyl chloride completely loses its strength, while polypropylene still maintains its strength (120-150kg/cm2), so good high temperature performance is the biggest advantage of polypropylene.
Corrosion resistance: In addition to strong oxidants such as concentrated nitric acid, fuming sulfuric acid, and green sulfonic acid, polypropylene can resist corrosion from most inorganic acids, alkalis, and salts, and almost all organic solvents cannot dissolve polypropylene at room temperature. Due to its good high temperature resistance and corrosion resistance, polypropylene is widely used in corrosion-resistant pipelines, valve storage tanks and other components in petrochemicals.
Application: Polypropylene has the characteristics of a large linear expansion coefficient. When heated, the pipeline is heated and elongated, and when the temperature drops, it shrinks. During the production process, under the action of long-term temperature change tension and compression stress loads, the pipeline is prone to plastic bending deformation. Due to the large linear expansion coefficient, small elastic modulus, and high brittleness at low temperatures of polypropylene pipes, it is not suitable for production process environments with very high temperatures and large temperature changes. Chlorinated polyvinyl chloride (CPVC) plastic
Performance of chlorinated polyvinyl chloride: The chlorine mass fraction of chlorinated polyvinyl chloride is increased from 56.7% in PVC to 63% to 69%. The increase in chlorine content increases the irregularity of the molecules in the structure of chlorinated polyvinyl chloride, decreases the crystallinity, and increases the polarity of the molecular chain, thereby significantly increasing its heat deformation temperature. The use temperature of CPVC products can reach up to 93-100℃, which is 30-40℃ higher than PVC. The increase in chlorine content will also significantly increase the hard brittle temperature Tg of chlorinated polyvinyl chloride. Due to the increase in the molecular weight of monomer PVC, Tg will also increase at a small rate at the same chlorine content level. These characteristics of chlorinated polyvinyl chloride improve the chemical resistance and corrosion resistance of PVC, and can resist chemical corrosion such as acid, alkali, salt, fatty acid salt, oxidant and halogen; at the same time, the tensile strength and bending strength of CPVC are also improved compared with PVC. CPVC has excellent high temperature resistance, aging resistance, chemical corrosion resistance, high flame retardancy and high mechanical strength.
Application of chlorinated polyvinyl chloride: Chlorinated polyvinyl chloride has the characteristics of low cost, high hard brittle temperature, high heat bending temperature, stable chemical properties, excellent mechanical properties, etc. It is widely used in developed countries with harsh process conditions and corrosive environments. In recent years, China has also begun to introduce this non-metallic material and applied it in some petrochemical production equipment with severe corrosion, and has achieved good application results.

Due to the special properties of the above-mentioned plastics, they are widely used to produce petrochemical pipeline valves. The production process of plastic valves is mainly injection molding.
Because the main components of plastic valves are made by injection molds. In recent years, the rapid development and continuous updating of CNC machine tools have promoted the development of the mold manufacturing industry. The emergence of precision molds, large molds, and precision injection molding equipment has enabled the rapid development of plastic valve production. Another reason for the development of plastic valves is market demand. A large number of plastic valves are used in the water supply and drainage industry. This is due to certain characteristics of plastic materials:
First, the use temperature of valves for water supply and drainage pipelines is not high (normal temperature), the use pressure is not high (0.4~0.6MPa), and it is not easy to scale in the pipeline valves, and there is no pollution. Plastic valves are very suitable.
Second, it is easy to connect. In addition to flange connection, there are also threaded connection, ferrule connection and socket welding connection. Some plastic materials can be simply connected by socket bonding, while some plastics require fusion welding. Even fusion welding is much simpler than metal-to-metal welding, so it is popular with users.
Third, plastics are cheap and the cost of producing valves is low. Due to the above reasons, the development of plastic valves is very rapid. With the emergence of new types of engineering plastics, the temperature resistance, pressure resistance and corrosion resistance are constantly improving, and the application range of plastic valves will be further expanded.

We know that the use temperature of plastics is limited, usually around 70℃, and the applicable pressure is mostly around 0.6MPa.
Valves made of plastic alone are difficult to work under high temperature and high pressure conditions, and another way must be found. In continuous experiments and practice, people use the plastic processing principle of plastics, use steel as the outer shell, and line the plastic inside the shell to isolate the direct contact between steel and highly corrosive media. This not only solves the problem that the strength of plastics is low and cannot withstand high pressure, but also solves the problem that steel materials are not resistant to corrosion. Plastic lined valves increase the applicable pressure from 0.6MPa to PNl.6MPa, expanding the scope of use.
The design and production process of plastic lined valves are basically the same as those of fluoroplastic lined valves. In addition to the production of valves by compression molding and injection molding, rotational molding is a good method for the production of some large-caliber valves that is both practical and economical. The rotomolding process was first developed in developed countries such as Europe and the United States in the 1940s. It is a new plastic processing technology that emerged after plastic processing, extrusion, blow molding, injection molding, stretching and other processes. Its characteristics are: first, large products, second, strange shapes and no internal stress in the products. As a new process, rotomolding technology has been applied to valve production in recent years. It was first used for large chemical pipelines and industrial elbows, as well as storage tanks.
The production process of plastic lined valves is easier than that of fluoroplastic lined valves, and the scope of application is wider. Most fluoroplastics cannot be used for rotomolding to manufacture corrosion-resistant linings for valves and storage tanks. However, most plastics can be lined by rotomolding. Rotomolding is particularly suitable for lining large-diameter valves. It does not require bulky lining molds and large heating furnace equipment, and can be directly rotomolded in the rotomolding machine. In this way, not only the equipment and mold costs are reduced, but also the production costs are reduced, which opens up a new way to save energy and reduce consumption for the production of plastic lined valves.
Plastic valves have been widely used in petrochemical industry and water supply and drainage industry due to their excellent corrosion resistance, non-stick properties, heat preservation, electrical insulation, low production cost and easy installation. With the continuous emergence of new plastic varieties, the continuous improvement and perfection of the production process of plastic valves, the continuous increase in the variety and specifications of plastic valves and the continuous improvement of quality, the application prospects of plastic valves are extremely broad.