So-called engineering plastics are industrial plastics that can replace metals and be used as industrial parts or shell materials. Engineering plastics need to have excellent comprehensive performance. Compared with general plastics, they can meet higher requirements in mechanical properties, heat resistance, durability, corrosion resistance, etc. In addition, they are easier to process and can replace metal materials.
Engineering plastics are divided into general-purpose engineering plastics and special engineering plastics. Among them, the five general-purpose engineering plastics refer to a type of engineering plastics with excellent comprehensive performance and a long-term use temperature of more than 100℃. They mainly include PA, PC, POM, PBT/PET, and PPO. Special engineering plastics refer to a type of engineering plastics with high comprehensive performance and long-term use temperature above 150℃.

 

 

If we compare "tall, rich and handsome" to high performance, high output and good appearance, by comparison:

PA: Outstanding output---a typical "rich family with only money left", so PA family needs desperate lackeys such as glass fiber/carbon fiber to protect itself.
PC: outstanding performance, output, and appearance --- can be called "tall, rich, and handsome"; but tall, rich, and handsome also has disadvantages, can't bear hardships (poor wear resistance and fatigue resistance), and easily lose temper when encountering some pressure (stress cracking, high temperature hydrolysis)
POM: high performance, but output and appearance are not outstanding --- it is a "poor boy with only height", playing basketball is his final destination, because POM is strong, tall, and durable;
PBT: high performance, output, and appearance are all OK --- average, ordinary people, go wherever they need, strong fluidity and adaptability, very cost-effective (good melt fluidity, fast crystallization, low injection molding cost)
PPO: super high performance, good appearance, but low production---typically, it has everything except money, which is opposite of PA. Therefore, PPO can only realize its actual value (improving moldability of PPO) by cooperating with richer families of PA or PS.
PC and PPO are non-crystalline (amorphous) plastics, while PBT, PA, and POM are crystalline plastics. Some properties of non-crystalline plastics and crystalline plastics are opposite, such as fluidity, crystalline plastics are usually better than non-crystalline plastics; shrinkage, non-crystalline plastics are smaller than crystalline plastics; light transmittance, non-crystalline plastics are better than crystalline plastics, etc.
As we all know, PC has high melt viscosity and poor fluidity. However, PPO has worse melt fluidity than PC, extremely poor processing and molding properties. Pure PPO resin cannot be molded by injection, which greatly limits its application. Products sold on the market are all its improved products MPPO (modified materials made by blending PPO and HIPS)
Therefore, for pure resin, among the five general engineering plastics, PC can be said to be a well-deserved "tall, rich and handsome". It was first produced by Bayer in 1959 and is now the second largest engineering plastic in terms of consumption.

Polycarbonate (commonly abbreviated as PC) is a colorless, transparent, amorphous thermoplastic material. Its name comes from carbonate group (-CO3) inside it. PC has outstanding impact resistance, good creep resistance and dimensional stability, heat resistance, low water absorption, non-toxicity, and excellent dielectric properties. It is also the most widely used transparent material among the five major engineering plastics.
Most engineers should know what kind of material PC is. It has many excellent properties. However, few people analyze in depth why it has these properties and what determines these properties. For most structural engineers, they prefer to rely on rote memorization of performance parameters to understand each material. Of course, they have their reasons. After all, I am not a material engineer. Why should I understand it so deeply? I can just check information when I need it. It seems that there is no problem.
Until one time when I interviewed a structural engineer, I asked him what material of most barreled water bottles on the market is and why this material is used?
His answer was PC, strictly speaking, food-grade PC, non-toxic, transparent, high-strength, and excellent impact resistance. Barreled water bottles made of PC are not easily damaged during transportation and are durable.
Engineer answered well, and then I asked him again, can PC also be used to make thermos bottles for drinking hot water? He said uncertainly that it should be possible, after all, it is often seen on supermarket shelves, and temperature resistance of PC materials can reach more than 100℃.
It sounds like answer is also correct, but it ignores a knowledge point, which needs to be explained from the chemical structure of PC and its synthesis process.
Polycarbonate (PC) is a general term for polymer compounds containing carbonate in main chain of molecule. For linear polycarbonates of dihydroxy compounds, following general formula is generally used:

 

Among them, depending on ester group R, it can be divided into aliphatic, aromatic, aliphatic-aromatic and other types. Among them, due to low mechanical properties of aliphatic and aliphatic-aromatic polycarbonates, their application in engineering plastics is limited. Only aromatic polycarbonates have been industrially produced, and their chemical structure is shown in figure below.

 

Synthesis process of PC is as follows: industry mainly adopts interfacial polycondensation method, adding a certain ratio of bisphenol A, sodium hydroxide solution, catalyst, molecular weight regulator and solvent (dichloromethane or dichloroethane) into reactor, passing phosgene at room temperature, pressure for phosgenation and polycondensation reaction, finally obtaining polycarbonate.

 

From above synthesis process, it can be seen that bisphenol A is main raw material for synthesizing PC. Conversely, if PC polymer is cracked for some reason, bisphenol A will naturally be released. Process of bisphenol A breaking and releasing from polymer chain is a chemical reaction process. For chemical reactions, temperature is the most important factor. On the one hand, as temperature rises, kinetic energy of molecules increases, collisions between them increase, and chemical reaction rate increases; on the other hand, as temperature rises, internal energy of molecules increases, making it easier to cross energy barrier of chemical reactions, causing polymer bonds to break from weak places, thereby dissolving bisphenol A.
Bisphenol A, also known as bisphenol propane (BPA), is often used to synthesize materials such as polycarbonate (PC) and epoxy resin. In manufacturing process of plastic products, adding bisphenol A can make them colorless, transparent, durable, lightweight and outstanding impact resistance, but excessive intake of BPA is believed to cause endocrine disorders, affect health of fetuses and children. Cancer and obesity caused by metabolic disorders are also believed to be related to this. European Union believes that baby bottles containing bisphenol A can induce precocious puberty, and from March 2, 2011, production of baby bottles containing BPA is prohibited.
Although PC bottles can withstand 100℃ hot water without deformation, high temperatures rapidly increase amount of bisphenol A dissolved from PC, so PC drinking bottles that often contain hot water should be avoided for long periods of time.
So-called second-generation rich are people who are born with huge wealth. PC has all excellent properties, just like second-generation rich.

Figure: PC performance data (for reference only)
PC has excellent comprehensive properties, especially its impact strength is very outstanding in thermoplastic resins. On the basis of maintaining excellent performance, its light transmittance (89%) is also the best in engineering plastics. Looking at all commonly used engineering plastics, without modification methods such as blending and additives, PC can be said to be the most ideal engineering plastic at present.

Reason why PC has many excellent properties is inseparable from its molecular chain structure.

 

1) Phenyl: Large conjugated benzene ring is a rigid part that is difficult to bend. It improves rigidity of molecular chain, gives polymer mechanical strength, heat resistance, chemical resistance, weather resistance and dimensional stability, reduces solubility and water absorption of polymer in organic solvents.
2) Oxygen group: also called ether bond, its role is opposite to that of phenyl group, increasing flexibility of molecular chain, making polymer have certain toughness, but increasing solubility and water absorption of polymer in organic solvent.
3) Carbonyl group: increasing mutual force between molecules, making macromolecular chains closer together, and increasing rigidity of polymer.
4) Ester group: a group with greater polarity, increasing mutual force between molecular chains, so that molecular chains are more difficult to be stretched and broken. At the same time, it is also weaker part of molecular chain, which is easy to hydrolyze and break, making polymer very soluble in polar organic solvents. It is also reason why its electrical insulation is not as good as non-polar or even weakly polar polymers, but its polarity can make polymer more compatible with other materials and have better adhesion ability.
In summary, performance shown by PC is a comprehensive reflection of various groups.

Effect of phenyl plus carbonyl group exceeds opposite effect of oxygen group, resulting in a relatively rigid molecular chain and a large attraction between each other, which makes it difficult to disentangle each other, so molecular chains are difficult to slide against each other, which is manifested as
1) Mechanical properties: polycarbonate is an organic combination of rigidity and toughness. Generally speaking, if a material has good rigidity, it will be very brittle and will break if it falls to ground. However, although polycarbonate has good rigidity and is difficult to bend, its toughness is also quite good.
2) Temperature resistance: Glass transition temperature and melting temperature are high, and temperature resistance is high. Its decomposition temperature is above 300℃, and long-term working temperature can be as high as 120℃; at the same time, it has good cold resistance, and brittle temperature is as low as -100℃; its long-term use temperature range is -60~120℃.
3) Fluidity: Molecular chain is difficult to slide, melt viscosity is high, fluidity is poor, and injection molding processability is not good. Melt viscosity of PC increases with increase of relative molecular mass. During molding, it is often more effective to improve its flow state by adjusting temperature than changing shear rate.
4) Good dimensional stability: PC's creep resistance is quite good among thermoplastic engineering plastics, better than PA and POM. Dimensional change and cold flow deformation caused by water absorption are very small, and its shrinkage rate is small, so it has good dimensional stability.
5) Optical properties: PC macromolecular chains are difficult to orient and not easy to crystallize, making polymer in an amorphous state, thus having good transparency.
6) Electrical properties: Polycarbonate has low molecular polarity, high glass transition temperature, and low water absorption, so it has excellent electrical insulation properties, close to or relative to PET, which has always been considered to have excellent electrical insulation properties.
7) Flame retardancy: PC has certain flame retardancy and has UL94 V-2 flame retardancy without need for flame retardants. If supplemented with a small amount of flame retardants, PC can reach a higher level of fire protection standards, and at the same time, it will not lose its excellent optical and mechanical properties, which is something other plastic products cannot do at all.
PC material itself can reach UL94 V-2 flame retardancy, but it still cannot meet higher requirements for flame retardancy in special fields, such as electrical and electronic and automotive fields, which require PC flame retardancy to reach V-0.

 

Why does PC have a certain flame retardancy?
Limiting Oxygen Index
Minimum volume fraction of oxygen in mixed gas that can maintain combustion of polymers is called limiting oxygen index (LOI) of polymer, or oxygen index (OI) for short. Oxygen index is an important indicator to measure whether polymer materials burn. Since volume fraction of oxygen in air is 20.9%, polymers with an oxygen index of less than 21% can generally be ignited in air.
LOI < 21% is flammable, LOI of 22%~25% is self-extinguishing, 26%~27% is difficult to burn, and above 28% is extremely difficult to burn.

Limiting oxygen index (LOI) of PC can reach 21%-25%, which is self-extinguishing. Why?
Oxygen index LOI of polymers containing a large number of aromatic groups on main chain of plastic resins, such as phenolic resin PF, polyphenylene ether PPO, polycarbonate PC, polyaramide, polyimide PA, polymaple PSU, etc., is higher than that of aliphatic hydrocarbon polymers. This is mainly because this type of polymer can condense into aromatic carbon when burning, and produces less gaseous combustible products. LOI of carbon is as high as 65%, and carbon layer formed can cover surface of burning polymer and suffocate flame. Carbonization rate of PC materials is high when burning, and they can extinguish themselves.

Although PC materials have many advantages, they also have many disadvantages:
1) Poor fatigue resistance: polycarbonate has poor ability to resist reciprocating action of cyclic stress.
2) Poor wear resistance: Molecular chain of PC materials has a high degree of freedom, and there is a large space between molecular chains. It is easily affected by external forces, easily scratches and wear on the surface. Molecular crystallinity of PC materials is relatively low, and molecular chain arrangement is relatively disordered, which makes it easy for microscopic pits to form on the surface of PC materials, thereby reducing its scratch resistance. PC materials can improve their self-lubricity by adding powdered polytetrafluoroethylene, and their wear resistance can be improved by adding glass fiber to increase hardness.
3) Poor stress cracking resistance: However, when external forces force orientation, macromolecular chain is not easy to relax, resulting in difficulty of eliminating internal stress of product after molding.
4) Poor hydrolysis resistance: Because molecular chain contains -COO (ester group), ester can undergo hydrolysis reaction to generate corresponding acid or alcohol in presence of acid or alkali. Therefore, conventional PC has low hydrolysis stability and cannot be used for products that are repeatedly subjected to high-pressure steam.
5) Chemical resistance: Polycarbonate can withstand chemical substances such as inorganic and organic dilute acids, but is not resistant to strong alkali. It can be dissolved in chlorine-containing organic solvents, and stress cracking will occur when encountering ketone solvents such as acetone.
6) Sensitivity to notches: So-called notch sensitivity, in layman's terms, is that when edge of product has a notch (collapse, burr), product will crack along notch under action of force. Since internal stress of PC products is difficult to eliminate, when product has a notch, it is easy to crack under action of force.
7) Weather resistance: Long-term exposure to ultraviolet rays will turn yellow, so it is usually necessary to add ultraviolet absorbers to improve anti-aging performance of PC.

In order to improve shortcomings of PC in performance, PC can be modified to achieve higher performance and expand its application areas.
Common polycarbonate modifications include fiber-reinforced PC, flame-retardant PC and PC alloy. The former can improve tensile strength, bending strength and heat resistance of PC, while the latter can improve impact strength, electrical insulation and processing fluidity, but heat resistance will be reduced.
PC alloy
That is, PC is blended with other polymers. The main varieties of PC blending modification are: PC/ABS, PC/PS, PC/PBT, PC/PA, PC/polyolefin, PC/LCP, etc.
1) PC/ABS alloy
It is main variety of PC, with good processing fluidity, low impact notch sensitivity and good low-temperature toughness.
2) PC/PS alloy
Good processing fluidity, good optical properties, low cost;
3) PC/polyolefin alloy
Good processing fluidity, high elastic modulus, good stress cracking resistance;
4) PC/P0M alloy
PC and POM can be mixed in any proportion. It not only has excellent mechanical properties and heat resistance of PC, but also has outstanding solvent resistance and stress cracking resistance.
5) PC/(PBT, PET) alloy
Improved PC's solvent resistance, abrasion resistance, stress cracking resistance and processing fluidity.
6) PC/PA alloy
Good impact toughness and chemical resistance.
7) PC/PMMA alloy
It has good heat resistance, UV light resistance, high notch impact strength, and its injection molded products have high weld strength and pearlescent color, and can be used in production of decorations.
8) PC/PTFE alloy
High heat resistance, dimensional stability and impact toughness, the most important feature is high wear resistance. PTFE can play an internal lubricating role in PC. Adding a small amount of PTFE can increase wear resistance of PC by 5 times.
Glass fiber reinforced
Addition of glass fiber can significantly improve fatigue strength of PC, reduce stress cracking, and greatly improve tensile, compressive strength and elastic modulus, while electrical properties and chemical corrosion resistance are still comparable to pure PC.
Flame retardant modification
Flame retardant performance of PC itself can reach UL-94 V-2 level, due to ordinary polymers such as polyethylene, etc. However, the flame retardant performance of PC is still insufficient for use in TV, computer, printer casing and components, transformer coils, automotive parts, building materials and other fields with high flame retardancy requirements, so it is necessary to perform flame retardant modification on PC.