Making a Family Portrait for Plastics

Written by Xu Gaodong and translated by Xin Xue

         Plastic is a synthetic polymer material which is extensively used. Plastic building materials is the fourth generation new building materials after steel, timber and cement. They have been used in almost every field of modernization construction and have made great contributions. Then how many types does plastic contain? I＇m afraid few people can answer this question quite clearly except some industry professionals. In the following text, the author from China Titanium Net will make a family portrait for plastics.

　　According to an authoritative scientific report on plastics, there are three kinds of classification for plastics.

　　Firstly, according to the operational performance, plastic is divided into general plastics, engineering plastics and specialty plastics.

　　In the first place, general plastics generally refer to those widely used cheap plastics with larger output and good moldability. There are five varieties of GE plastics, namely, polyethylene(PE), polypropylene(PP), PVC, polystyrene(PS) and acrylonitrile - butadiene - styrene(ABS). All of them are thermoplastics;

　　In the second place, engineering plastics refer to those plastics with good mechanical performance and dimensional stability which can be used for engineering structures, can bear certain external force as well as resist high and low temperatures. Polyamide and polysulfone are such kind of plastics. Engineering plastics are subdivided into general engineering plastics and special engineering plastics. General engineering plastics include polyamide, POM, polycarbonate, modified polyphenylene ether, thermoplastic polyester, ultra-high molecular weight polyethylene, methyl pentene polymers and vinyl alcohol copolymer etc. Special engineering plastics are further divided into cross-linked type of non-crosslinked type. The former type includes poly-amino-bismaleimide amide, poly-triazine, cross-linked polyimide and epoxy resin etc. While the latter type contains polysulfone, polyether sulfone, polyphenylene sulfide, polyimide and polyether ether ketone(PEEK) etc;

　　In the last place, specialty plastics refer to those plastics used in aviation, aerospace and other special application fields. They usually have some special functions. For instance, fluorine plastic and silicone have excellent high temperature resistant and self-lubricating functions. Reinforced plastics and foams have high strength and high buffer capacity. As far as the physical form is concerned, reinforced plastic materials are divided into granular (eg, filling calcium-plastic reinforced plastics), fibrous (eg, glass fiber or glass cloth reinforced plastics) and flake (eg, mica reinforced plastics). According to the material, reinforced plastics are divided into cloth reinforced plastics (such as rags reinforced plastics and asbestos-reinforced plastics), inorganic mineral-filled plastics (such as quartz or mica filled plastics) and fiber reinforced plastics (such as carbon fiber reinforced plastics); Foams include three types, namely, rigid, semi-rigid and soft foam. Among them, rigid foam lack flexibility and has a great compression hardness. It can only be deformed upon reaching a certain stress value and can not be restored after the stress is relieved. On the contrary, soft foam is flexible. It can be deformed easily and restored after lifting the stress. While the flexibility and other performance of semi-rigid foam is between rigid foam and soft foam.

　　Secondly, according to the chemical and physical properties, plastics are divided into thermosetting plastics and thermoplastics.

　　Thermoplastics will melt after being heated, take shape when cooling down and melt again when it＇s re-heated. The four major common thermoplastics are polyethylene, polyvinyl chloride, polypropylene and polystyrene. Their continuous usage temperature is below 100℃. Thermoplastics are subdivided into hydrocarbon, vinyl containing polarity genes, engineering thermoplastics, cellulose and many other types. They are soluble and fusible. Besides, they have excellent electrical insulation. Particularly, PTFE, PS, PE and PP all have extremely low dielectric constant and dielectric loss, thus they can be used as high-frequency and high-voltage insulation materials. Although thermoplastics are easy-to-forming, they are easy-to-creep due to their low heat resistance. Their creep level is decided by bearing load, ambient temperature, solvent and humidity. In order to overcome these shortcomings and meet the application need of space technology, new energy development and other fields, many countries are developing heat-resistant resin which can be molded by melting, such as PEEK, PES, PASU and PPS etc. Composite materials with them as matrix resin have many superior performance. For example, they can be hot-forming and welded. And they have higher mechanical properties and chemical corrosion resistance. Furthermore, they own better interlaminar shear strength than epoxy resin. While composite material made from PEEK and carbon fiber has better fatigue resistance than epoxy / carbon fiber. It owns good impact resistance, processability and creep resistance at room temperature. Moreover, it is a very good high-temperature insulating material as it can be used continuously at 240℃~270℃. Composite material made from PES and carbon fiber has higher strength and hardness at 200℃. And it maintains good impact resistance even at -100℃. Besides, it is non-toxic and non-combustible; It produces least smoke and has good radiation resistance. It is expected to be taken as the key component of spacecraft. And it can also be processed into radar dome. Formaldehyde cross-linking plastics include phenolics, amino-plastic materials (such as urea - formaldehyde - melamine - formaldehyde, etc.). Other cross-linking plastics include unsaturated polyester, epoxy resin and phthaloyl diallyl resins etc.

　　Thermosetting plastics refer to those plastics which can be solidified under heating or other conditions or have insoluble(infusible) properties, such as phenolics and epoxy plastics etc. Thermosetting plastics are divided into formaldehyde cross-linking types and other cross-linking types. They will become insoluble and infusible condensates after thermoforming. Their resin molecular, taking on a network structure which is cross-linked by linear structures, will be broke down when being heated by an intense heat. Typical thermosetting plastics include phenolic, epoxy, amino, unsaturated polyester, furan, polysiloxane and the newer poly propylene phthalate plastics. They have good heat resistance and do not deform easily after being heated. But they have low mechanical strength, which can be increased by adding fillers to make them into laminated materials or molding materials. Thermosetting plastics with phenolic resin as main material, for example, phenolic molded plastic (commonly known as Bakelite), are durable, dimensionally stable and resistant to chemical substances except alkali. Different fillers and additives can be added to thermosetting plastics according to different usage and requirement. For instance, mica or glass fiber can be used as filler if plastic with high insulation is needed; To heat-resistant varieties, asbestos or other heat-resistant fillers can be used; If earthquake-resistant varieties are required, we can take appropriate fiber or rubber as a filler and then add some toughening materials to produce a high toughness material. Furthermore, in order to meet the requirements of different usages, we can also choose aniline, epoxy, polyvinyl chloride, polyamide, polyvinyl acetal and other modified phenolic resin as fillers or additives. Moreover, phenolic resin can also be made into phenolic laminate, which is widely used in low-voltage electrical equipment due to its high mechanical strength, good electrical properties, corrosion resistance and easy processing character. Amino plastics contain urea-formaldehyde, melamine formaldehyde and urea-melamine formaldehyde etc. They are hard, colourless, translucent and resistant to scratches. After adding pigment, they will be made into technicoloured products which is commonly known as electric jade.

　　It can be used for a long time at 70 ℃ and be used for a short time at 110 ~ 120 ℃ because it is oil proof and free from the impact of weak base and organic solvents (but it＇s acid nonresistant). Therefore, it can be used for electrical products. Compared with urea-formaldehyde, melamine-formaldehyde plastics has higher hardness, better water resistance, heat resistance and arc resistance, so it can be used for arc-resistant insulation material. There are a great variety of thermosetting plastics whose main raw material is epoxy resin, among which, plastics taking Bisphenol A epoxy resin as substrate account for about 90%. It has excellent adhesion, electrical insulation, heat resistance and chemical stability. Besides, it has good mechanical strength, low shrinking and water absorption percentage. Both unsaturated polyester and epoxy resin can be made into glass fiber reinforced plastics(FRP) which has excellent mechanical strength. For example, unsaturated polyester FRP has a good mechanical performance and low density (only account for 1/5~1/4 of steel and 1/2 of aluminium), hence it is easily processed into a variety of electrical components. The electrical and mechanical properties of plastics made from phthalate ester resin are superior to thermoset phenolic and amino. It has low hygroscopicity and good forming performance. Its products are dimensionally stable. And it can resist acid, alkali, boiling water and some organic solvents. Molding material is suitable for making high temperature resistant parts with complex structure and high insulation. It can be long-term used at -60～180℃. And its highest thermal rating can achieve level F, higher than that of phenolic and amino plastics. Silicone plastic with polysiloxane structure is widely applied in the fields of electronics and electrical technology. Glass cloth is usually used as the reinforcing material of silicone laminated plastic. As for silicone molded plastic, glass fiber and asbestos are often taken as its fillers to produce high temperature resistant and high frequency components for submersible motor, electrical appliances and electronic equipment. Such kind of plastics have low Tgδ value, dielectric constant and frequency influence. Moreover, they can resist corona and arc when they are used in electrical and electronics industries. Even if the discharge caused it to decompose, it produced silicon dioxide instead of conductive carbon black. Furthermore, such materials have prominent heat resistance and it can be long-term used at 250℃.

　　The main disadvantages of polysiloxane are low mechanical strength, poor adhesion and oil resistance. For that reason, many modified silicone polymers have been developed, for example, polyester-modified silicone plastic has already been applied in the field of electrical technology.

　　Some plastics are both thermoplastics and thermosetting plastics. For example,

　　PVC is generally thermoplastic, but the new liquid PVC Japan developed is thermosetting, and its molding temperature of 60 ~ 140 ℃; In USA, there is a plastic which is called Lundekesi. It not only has the processing characteristic of thermoplastics but also has the physical properties of thermosetting plastics. The following four categories of plastics all have the characteristics of thermoplastic and thermosetting plastic. The first category is hydrocarbon plastics. They belong to non-polar plastics and are subdivided into crystalline and non-crystalline plastics. Crystalline hydrocarbon plastics include polyethylene and polypropylene etc. and non-crystalline hydrocarbon plastics include polystyrene B and others. The second category is vinyl plastics containing polarity genes. Except fluorine plastic, most of them are non-crystalline transparent bodies, including PVC, PTFE and polyvinyl acetate etc. Most vinyl monomers can be polymerized by using radical catalyst. The third category is engineering thermoplastics, which include POM, polyamide, polycarbonate, ABS, polyphenylene oxide, polyethylene terephthalate, polysulfone, polyether sulfone, polyimide and polyphenylene sulfide etc. Besides, PTFE and modified polypropylene are also included in this category. The last category is cellulose thermoplastics, which mainly include cellulose acetate, cellulose acetate butyrate, celluloid and cellophane etc.

　　Thirdly, according to the processing method, plastics are divided into moulded, lamination, injection, extrusion, blow molding, casting and reaction injection molded plastics etc. Among them, moulded plastics refer to plastics whose physical properties and processing performance are similar to thermosetting plastics; Laminated plastic refers to the integrated material made from fiber fabric soaked in resin through superposition and heat pressing; Injection, extrusion and blow molding plastics refer to plastics whose physical properties and processing performance are similar to thermoplastics; Casting plastic refer to the liquid resin mixture which can be hardened into products with certain shape after being poured in the mold in the absence of pressure or under a little pressure. For instance, MC Nylon is a casting plastic; With regard to reaction injection molded plastics, its production process is as following: First the liquid raw material is pressurized and injected into the membrane cavity and then it is solidified into a product with certain shape. For example, polyurethane belongs to this kind of plastics.

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