In last lecture, we talked about material of inner shell of helmet - expanded polypropylene, which has a shock-absorbing effect when meeting with a strong impact, usually has ability to absorb impact energy, is non-toxic, harmless, sweat-absorbing and breathable. Today I will introduce you to material of helmet shell. The shell is outermost layer of helmet that absorbs and dissipates impact during impact, so choice of shell material is very important. The following table 1-1 is most commonly used helmet shell material. ABS and PC materials have excellent performance and low cost, so this kind of material is often used as a material for helmets.
Table 1-1 Commonly used helmet shell materials
1. Solve mystery of ABS
As we all know, ABS, PE, PP, PVC and PS are among five general purpose plastics. Among them, ABS is a thermoplastic rubber hardened polymer material. Due to its special structure, it is easy to process. and has good impact resistance, corrosion resistance, high mechanical strength and other excellent properties. Therefore, ABS resin is widely used in fields such as automobiles, household appliances and daily necessities.
A in ABS resin is Acrylonitrile, B is Butadiene, and S is Styrene.
Figure 1. Schematic diagram of ABS structure
Among them, acrylonitrile (A) has a strong polar group -CN, which can improve overall high temperature resistance, chemical resistance and tensile strength; butadiene (B) contains double bonds and does not contain other functional groups, can provide excellent toughness, low temperature impact resistance and impact resistance; styrene (S) provides excellent processing characteristics, improves material stiffness and has high gloss characteristics.
The physical and chemical properties of ABS resin materials are very excellent, and their toughness is also very good. They can be used in conditions of relatively low temperatures without damage; they are resistant to water, inorganic salts, and various acids. It can exist stably, which is an important characteristic of ABS materials that can be used as helmet materials. ABS grades that can be used as raw materials for helmet shells in market include 121H, 0215A, 417, 757K, 15A1, etc.
Second, how much do you know about PC
Polycarbonate (PC) refers to a linear polymer containing -OROCO units in molecular chain. According to different repeat unit R, it is divided into aliphatic, aromatic, aliphatic-aromatic, etc. types. Among them, aliphatic and aliphatic-aromatic have poor mechanical properties, which limits their widespread use in everyday life. Therefore, at present, only bisphenol A-PC can be obtained industrially. The chemical structural formula of PC is shown in Figure 2.
Figure 2. Schematic diagram of PC structure
PC is an amorphous polymer, a transparent, low linear expansion thermoplastic. The light transmittance is 1.58 and relative density is 1.2g/cm3, in addition, it has good high and low temperature resistance, and heat resistance at 120 ° C is not a problem, thermal distortion temperature is 135-140 ° . C, and thermal decomposition temperature is 340°C.
Engineering plastics have better heat resistance than polyoxymethylene POM, aliphatic polyamide, PET and PBT, but are slightly inferior to other engineering plastics. Polycarbonate grades that can be used as raw materials for helmet shells in market include Saber 1000R, 1100, Lotte 1100, 1100U with UV protection and so on.
The types of materials used in helmets are complex, but most are made directly from raw materials. The technical threshold is relatively low and competition will be more intense. The production of helmets requires purchase of injection molding machines, raw materials, drying barrels, hair dryers, etc. The subsequent processes include painting, printing, assembly, packaging and shipping. The above raw materials, equipment and auxiliary products will undoubtedly usher in a small development peak, but all manufacturers must strictly adhere to national production regulations and launch products that reassure consumers.
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