The "One helmet and one belt" security operation launched by Bureau of Traffic Management of Ministry of Public Security has led to a sharp increase in demand for helmets. But in fact, penalty for not wearing a helmet is limited to motorcycles, and there is no penalty for e-bikes yet, so enthusiasm of "scalpers" for helmet hype has plummeted, and price of ordinary helmets has fallen. fell from peak to regular price. This is a series of dramatic changes in just a few weeks. Market conditions are constantly changing, and today we will talk about another material that can also be used as a helmet shell material - polypropylene + carbon fiber.
Figure 1. Carbon fiber material
1. Carbon fiber material
Carbon fiber (carbon fiber) is made from organic fibers such as rayon, pitch or polyacrylonitrile, which are carbonized or graphitized with an inert gas at 1500°C. It is divided into polyacrylonitrile carbon fiber, pitch-based carbon fiber, viscose-based carbon fiber, etc., in which carbon content reaches 90%. In order to achieve best results in practical applications, composite materials (polypropylene, polyamide, polycarbonate, etc.) made using carbon fibers as reinforcing materials have become subject of research. In this "helmet" storm, some people have also suggested using carbon fiber reinforced polypropylene as helmet shell material. It can be seen from Figure 2 that carbon fiber starts to decompose at 500°C, indicating that heat resistance is very good.
Figure 2. Carbon fiber TG diagram
The structure of a carbon fiber can be viewed as consisting of two-dimensional ordered crystals and cavities. The content, size and distribution of cavities have a great influence on characteristics of carbon fibers. Carbon fiber is a "soft on outside, hard on inside" type of material. Its weight is lighter than that of aluminum, but its strength is higher than that of steel, and it has characteristics of corrosion resistance and high modulus. It is widely used in civil and military industries. Its main properties are as follows: Table 1-1 shows.
Table 1-1 Key Performance Characteristics of Carbon Fiber
2. Preparation method of carbon fiber reinforced polypropylene composite material
In order to prepare composite materials that meet requirements of practical applications, carbon fiber and polypropylene must be realized through a specific molding process to reflect superior performance of composite materials. Once raw material required for experiment is determined, process parameters play a decisive role in performance of PP composite material.
2.1 Shaping method
The process of compression molding is to put prepreg and polypropylene into a mold of a certain specification and use a thermocompression molding machine to mold at required temperature and pressure.
2.2 Extrusion molding method
The extrusion molding process consists of curing continuous fiber bundles, belts or fabrics impregnated with polypropylene adhesive through extrusion molds under action of traction, somewhat similar to glass fiber reinforced materials.
2.3 Injection molding
Injection molding is also an important method for producing thermoplastic composites. The thermoplastic resin and crushed fibers are melted in barrel of injection molding machine, by pressure of plunger or screw, and material is injected into closed mold at a very high speed from nozzle, and product is obtained after cooling and shaping.
3. Influence of a Compatibilizer on Properties of Polypropylene Composites Reinforced with Carbon Fiber
Typically, a layer of epoxy resin sizing agent is applied to surface of commercially available carbon fibers, which is used to improve interfacial bond strength between fibers and matrix resin. However, polypropylene, as a non-polar material, has a weak binding force between them without action of a compatibilizer. Thus, when obtaining polypropylene composites reinforced with carbon fiber, mechanical properties of composites can be improved by adding various mass fractions of polypropylene grafted with maleic anhydride.
Figure 3. Effect of compatibilizer content on tensile and flexural strength of carbon fiber reinforced polypropylene composites
 Yuan Haibing. Properties of polypropylene composites reinforced with chopped carbon fiber[J]. Synthetic resins and plastics, 2018.35(02):24-28.
As amount of compatibilizer is increased, tensile strength and flexural strength of composites first increase and then become stable. When amount of compatibilizer is more than 30%, compatibilizer does not improve tensile strength and bending strength of composite material, but reduces tensile strength and bending strength of composite material.
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