1. What are the characteristics of plastics?

Plastic is a polymer material with resin as the main component, which has fluidity under certain temperature and pressure conditions, can be molded into a certain geometry and size, and keep its acquired shape without change after curing. Resins can be divided into natural resins and artificial resins, the latter also known as synthetic resins. Synthetic resins are made of coal, calcium carbide, petroleum, natural gas and some agricultural byproducts as the main raw materials, first to produce low molecular compounds (monomers) with certain synthetic conditions, and then synthesized by chemical and physical methods to obtain polymer compounds, chemically they are often called polymers or polymers. Although the polymer is the main component of the plastic, but the performance of the polymer alone often can not meet the requirements of the process of molding production and molding after the use of requirements, in order to overcome this defect, the polymer must be added to a certain number of additives, and through these additives to improve the performance of the polymer. For example, the addition of plasticizers can improve the flow properties and molding properties of the polymer; the addition of reinforcing agents can improve the strength of the polymer, etc. Therefore, it can be considered that the plastic is a polymer compound made of a combination of polymer and some additives. Plastics are well known for their use as everyday objects. Their use in industry has also become widespread because of their special advantages.

1) Advantages of plastics

  • Small density, light weight Plastic is generally light, its density in 0. 83 ~ 2. 3g / cm 3 range, only 1 / 7 to 1 / 4 of steel, aluminum 1 / 2. This is very beneficial to reduce the mass of machinery and equipment, especially for the requirements of reducing the weight of vehicles, ships, aircraft, rockets, missiles, artificial satellites and other sophisticated technology, has a more important significance.
  • High specific strength and specific stiffness Normally, the strength of plastics is lower than that of metals, but the mechanical properties of various reinforced plastics are comparable to those of metals. Since their density is much lower than that of metals, their specific strength (i.e. strength/density), specific stiffness (i.e., stiffness/density) is comparable to, or much higher than, that of metals. In the field of space technology, this property of plastics is of great importance.
  • Good chemical stability The chemical resistance of plastics is better than that of metals, with good corrosion resistance to acids, alkalis and other chemicals. Among them, the King of Plastics PTFE plastic has the highest chemical stability, even aqua regia can not do anything to it. Therefore, the plastic is widely used in chemical equipment and its anti-corrosion equipment. The most common rigid polyvinyl chloride pipes and containers are widely used in the field of corrosion protection and in water supply and drainage works in buildings.
  • Excellent electrical insulation, low dielectric loss Plastic is a poor conductor of electricity, its excellent electrical insulation, low dielectric constant, dielectric loss is very small, comparable with ceramics and rubber. Therefore, many plastics have become indispensable high-frequency materials, are widely used in the power, electrical and electronic industries as insulation materials and structural parts, such as wire and cable, knobs and sockets and electrical housings.
  • Friction reduction, wear resistance and good self-lubrication Most plastics have a small coefficient of friction and can be used as friction reduction and wear resistance materials, and some can even work effectively without lubricants and have good self-lubrication properties. Suitable for use in the presence of abrasive particles or impurities It can be used as a friction material under the harsh conditions of the world. In addition to high specific strength, low transmission noise, can be made into gears, cams and pulleys and other machine parts, such as textile machines in many cast iron gears have been replaced by plastic gears. Can also be glued or painted machine tool metal guide (with nylon 1010), the manufacture of brake blocks (asbestos phenolic plastic) and so on.
  • Excellent sound dissipation and heat insulation Plastics have excellent sound dissipation and heat insulation properties. The transmission friction parts made of plastic can reduce noise, reduce vibration, improve working conditions. Foam, in particular, is often used as sound insulation, thermal insulation or thermal insulation material.
  • Good molding and coloring properties Plastic has good plasticity under certain conditions, which creates the conditions for its molding process. Plastic coloring is relatively easy, and a wide range of coloring, can be dyed into a variety of colors as needed. In addition, some such as plexiglass, polystyrene, polycarbonate, etc. have good optical transparency.
  • In addition to corrosion protection, plastics also have a variety of protective properties such as water, moisture, air, vibration and radiation resistance. Especially after modification, more advantages, more widely used.

2) Disadvantages of plastics

Despite these advantages and the wide range of uses for plastics, they have some serious drawbacks that have not yet been overcome Serves.

  • Poor stiffness, not resistant to pressure. Under load, the plastic will slowly develop viscous flow or deformation, i.e. creep. Elephant.
  • Poor heat resistance. Most of them can only be used at about 100°C. Only a few varieties can be used above or below 200°C.
  • High coefficient of thermal expansion, easy to age, easy to burn.
  • The precision of the products is low. Although the molding properties of plastics are good, the shrinkage is difficult to control due to the influence of the molding process. The dimensional accuracy of the manufactured product is low.

2. What are the composition of plastics?

Plastics are generally composed of resins and additives (auxiliaries). The resin determines the main physicochemical, processing and physical-mechanical properties of the plastic. Plastics can be pure resins (e.g. Plexiglas is composed of polymethyl methacrylate), or it can be a mixture with various additives. The purpose of adding additives is mainly to improve the physical and mechanical properties of pure resin and processing properties, improve the use of efficiency or save resin, reduce costs. Additives account for a small proportion of the plastic material, but have a great impact on the quality of plastic products. Different types of plastics, due to different molding and processing methods and conditions of use, the type and amount of additives required are also different.

Resins

It is the basic and most important component of plastics, accounting for about 4 0 % to 70% of multi-component plastics, and almost all of the resin in one-component plastics. 100%. The resin not only acts as a glue for the other components, but also determines the type and main properties of the plastic, such as mechanical strength, hardness, ageing resistance, elasticity, chemical stability and photoelectricity

Here are 6 addition agents listed

1) Bulking agent

Filler, also known as filler, is another important component of plastics, accounting for about 40% to 70% of The purpose of the addition is to improve the strength, hardness, impact toughness, electrical insulation, heat resistance and shrinkage of the plastic, to expand the scope of use, or simply to reduce costs, etc. Commonly used fillers are wood powder, talcum powder, diatomaceous earth, limestone powder, aluminum powder, carbon black, mica, molybdenum disulfide, asbestos, paper, glass fiber, etc. Among them, fiber filler can improve the structural strength of plastics; asbestos filler can improve the heat resistance of plastics; mica filler can enhance the electrical insulation of plastics; mica filler can enhance the electrical insulation of plastics. Graphite and molybdenum disulfide fillers can improve the friction and wear resistance of plastics, etc.

2) Plasticizer

Plasticizers can increase the softness, extensibility and plasticity of plastics, reduce the flow temperature, embrittlement temperature and hardness of plastics, which is conducive to the molding of plastic products. Plasticizers should have good compatibility with the resin, colorless, tasteless, non-toxic, small volatility, non-flammable, good chemical stability, stable to light and heat Commonly used plasticizers are organic substances with low pressure liquid or low melting point solid, mainly esters and ketones. Such as phthalate esters, sebacate Acid esters, dioctyl phosphate, xylene phosphate, adipate, benzophenone, chlorinated paraffin and camphor, etc.

3) Stabilizer

It is used to prevent or slow down the degradation, oxidation, chain breaking, cross-linking of plastic products due to the action of heat, light, oxygen, radiation or other factors during the molding process and use, resulting in discoloration, brittleness, cracking and aging phenomena, stabilizing the quality of plastic products and extending their service life. Usually require the stabilizer can be soluble with the resin, and molding will not decompose, not chemically react with other additives, stable in the use of environmental, volatile small, colorless Common stabilizers include antioxidants (phenolic compounds, etc.), light shields (carbon black, etc.), UV absorbers (2- hydroxybenzophenone, phenyl salicylate, etc.), and heat stabilizers (aluminum stearate, lead trisodium phosphite, etc.) Today’s most high-performance plastic stabilizer is methyl tin heat stabilizer (181), very effective for rigid polyvinyl chloride (PVC) molding; and because of its high safety, so especially for food packaging and highdefinition rigid polyethylene products; because of its non-toxic and become a widespread replacement for other highly toxic plastic heat stabilizers. It has been widely used in the United States, Europe and Japan, and has also started to be used in large quantities in China in recent years.

4) Colorant

It is used to give plastics a certain color and beauty. About 80% of plastic products are colored and then made into final products. Generally, the coloring agent is required to be stable, heat-resistant, light-resistant, not easy to change color, strong coloring power, bright color, and plastic bonding firmly Colorants are divided into organic dyes and inorganic pigments according to their solubility in the coloring medium. Dyes are soluble in the resin to be colored, and the products are transparent; pigments are insoluble in the colored medium, and the products are translucent or opaque. Pigment not only has coloring property for plastic, but also has the function of filler and stabilizer. usefulness

5) Curing agent

Curing agent is also known as hardener or curing agent. Its main role is to generate transverse bonds between polymer molecules, so that the macromolecules cross-link, so that the linear structure of some synthetic resins cross-linked into a body structure, so that the resin has thermosetting properties Different varieties of resins should use different varieties of curing agents. Hexamethylenetetramine is commonly used for phenolic resins; amines, phenolic anhydrides, and Polymers: Polyester resins commonly used as peroxides, etc.

6) lubricants

Lubricants, also known as mold release agents, prevent plastics from sticking to the mold or equipment during the molding process, so that the product can be easily released and the surface is smooth. Commonly used lubricants are stearic acid and its salts, paraffin waxes and synthetic waxes, etc. T h e dosage is 0. 5 % to 1. 5%.

Other addition agents

Reinforcing materials -increase the strength and rigidity of plastic products The most commonly used reinforcement materials are glass fiber, asbestos, quartz, carbon black, silicate, calcium carbonate, metal oxides, etc. Flame retardant -increases the flame resistance of plastics, o r can make them self-extinguishing Commonly used flame retardants include antimony oxide and compounds of aluminum and boron, halides and various types of phosphate esters, tetrachlorobenzenedicarboxylic anhydride, tetra-O benzene dicarboxylic acid Antistatic agents – eliminate or reduce the static electricity generated by friction during the processing and use of plastics, ensure the safety of production operations, and make the plastic surface less susceptible to dust absorption. Most antistatic agents are electrolytes, which have limited solubility with synthetic resins, so that they can migrate to the surface of the plastic, thus absorbing moisture and eliminating static electricity Commonly used are long-chain aliphatic amines, amides, etc

Foaming agent – can be vaporized at a certain temperature or decompose gas when heated, mainly used in the preparation of foam, so that the foam structure commonly used are dichlorodifluoromethane azodicarbonamide and azobenzene, etc

Conductive agents, magnetic conductive agents, etc.

3. What are the classifications of plastics?

There are many different types of plastics and many ways to classify them.

1) Classification by characteristics when heated

Thermoplastics Thermosetting Plastic
Physical changes occur when heated, softening or melting from a solid to a viscous fluid state, but then hardening to a solid when cooled, and the process can be repeated many times without changing the molecular structure of the plastic itself Easy processing and molding, high mechanical properties, but heat resistance and rigidity are poor Wide range of applications, renewable At a certain temperature, after a certain time of heating, pressure or the addition of hardening agent, a chemical reaction occurs and hardens. After hardening, the chemical structure of the plastic changes, the texture is hard, insoluble in solvents, no longer softened by heating, and will decompose if the temperature is too high. It has the advantages of high heat resistance, not easy to deform under pressure, but the mechanical properties are not good At present, it is mainly used as low-pressure extrusion packaging electronic components and casting molding, etc. usefulness
Polyvinyl chloride (PVC), polystyrene (PS), polyvinyl chloride (PVC) Polyethylene (PE), polypropylene (PP), nylon (PA), polyformaldehyde (POM), polycarbonate (PC), ABS plastics, polyphenylene ether (PPO), polysulfone (PSF), fluoroplastics, polyesters, and plexiglass (PMMA), etc Phenolic, epoxy, amino, unsaturated polyester, furan, polyphthalic diacrylate and polysilicon ether resins, etc.

2. Classification by use and characteristics

General Purpose Plastics High temperature resistant plastic
High production volume (more than three -quarters of total plastics production ), low price, wide range of applications. Average performance, used only as a general non-structural material, mostly for household products Good heat resistance, most of t h e m can work above 150℃, some of them can also work under 200~250℃ for a long time, but generally the price is high, the output is small, and the application range is not wide
6 major general purpose plastics: polyethylene, polyvinyl chloride, polystyrene, polypropylene, phenolic and aminoplastics, etc. Polytetrafluoroethylene, polytrifluoroethylene, silicone resins, epoxy resins, polyimide, polyphenylene sulfide, polybenzimidazole, polydiphenyl ether, aromatic nylon, polysulfone, etc.

General Engineering Plastics Special Engineering Plastics
Comprehensive engineering properties (including mechanical properties, heat and cold resistance, corrosion resistance and insulation properties, etc.) are good. Generally, it can partially replace metal materials as load-bearing structural parts, heat-resistant parts and load -bearing parts in high-temperature environment, dielectric products and insulating supplies in high temperature, humidity and wide range of frequency conversion conditions. Smaller production, more expensive, and relatively narrow range of uses Special engineering plastics, also known as functional plastics, have certain special functions and are suitable for certain special applications, such as for conductive, piezoelectric, thermoelectric, magnetic conductive, photoelectric, anti-radiation, optical fiber, liquid crystal and polymer separation film, and are specialized in friction and wear applications, etc., and also include plastics modified for certain special applications
7 major engineering plastics: ABS, polycarbonate (PC), paraformaldehyde (POM) , polyamide (nylon) (PA), PET, PBT, polyphenylene ether (PPO) Polysulfone, polytetrafluoroethylene, thermoplastic polyesters, chlorinated polyethers, polyetheretherketone, ultra-high relative molecular mass polyethylene, epoxy plastics and unsaturated polyesters Polysulfone, polyimide, p o l y p h e n y l e n e sulfide, polyarylate, polyphenylene ester, polyetherketone, fluoroplastics, silicone rubber, and epoxy plastics Magnetically conductive plastics, electrically conductive plastics, photosensitive resins, etc

3) Classification according to the ordered state of resin macromolecules in plastics

Amorphous plastics Crystalline plastics
The resin macromolecular chains show a random arrangement without rules. In the pure resin state, the plastic is transparent The mechanical properties are isotropic During the process of cooling down from the molten state to the product, the molecular chains of the resin can orderly and closely stack to produce a crystalline structure. No fully crystalline plastics are semi-crystalline, showing a coexistence of amorphous and crystalline phases Crystalline structures are only found in thermoplastics
ABS, PC, PVC, PS, PMMA, EVA. AS etc. PE, PP, PA, POM, PET, PBT, etc.

4) Classification by light transmittance of plastics

transparently plastic translucent plastic non-transparent plastic
Light transmittance above 88% Light transmission but below 88% light transmission windowless
PMMA, PS, PC, Z – Polyester, etc. PP, PVC, PE, AS, PET, MBS, PSF POM, PA, ABS, HIPS, PPO, etc.

5) Classification by hardness of plastics

rigid plastic semi-hard plastic soft plastic
Tensile modulus of elasticity of 700 MPa or more Tensile modulus of elasticity between 70 and 700 MPa Tensile modulus of elasticity below 70 MPa
ABS, POM, PS, PMMA, PC, PET PBT, PPO, etc PP, PE, PA, PVC, etc. Soft PVC, Styrene – Butadiene Copolymer, Soft PVC TPE, TPR, EVA, TPU, etc

6) Classification by resin synthesis method

Polymeric plastics Polycondensation type plastics
Produced by polymeri zati on reactions. This resin is generally made from unsaturated bonds containing （The monomers (mainly double bonds) are linked to each other by double bonds to form large molecules, and the basic chemical composition remains unchanged, with no low molecular products released during the reaction Polymeric plastics are all thermoplastics It is produced by a condensation reaction. These resins are generally made by joining two or more monomers by reaction between functional groups (usually at least two functional groups) to form a chemical reaction that is completely different from the original molecule, and when combined, produces water or other simple substances
Polyolefins, polyhalogenated olefins, polystyrene, paraformaldehyde, acrylic plastics, etc. Phenolic plastics, amino plastics, silicone plastics, etc.