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What does “crystallinity” mean in the plastic industry

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crystallinity is used to express the proportion of crystalline area in polymer. The variation range of polymer crystallinity is very wide, generally from 30% to 80%.
The determination methods are:
1. Density method: crystallinity = (va-v) /(VA VC) * 100%
VA specific volume of completely amorphous polymer;
VC specific volume of fully crystalline polymer;
V specific volume of sample (specific volume is the reciprocal of density);
2. Thermal analysis; 3. X
Radiographic testing, nuclear magnetic resonance, etc.
I. what is crystalline plastic? Crystalline plastics have an obvious melting point, and the molecules are arranged regularly when they are solid. The regular arrangement area is called crystal area, and the disorderly arrangement area is called amorphous area. The percentage of crystal area is called crystallinity. Generally, polymers with crystallinity above 80% are called crystalline plastics. Common crystalline plastics include polyethylene PE, polypropylene PP, polyoxymethylene POM, polyamide PA6, polyamide PA66, pet, PBT, etc. 2、 Effect of crystallization on plastic properties 1) mechanical properties crystallization makes plastic brittle (impact strength decreases), strong toughness and poor ductility. 2) Optical properties crystallization makes plastics opaque because light scattering occurs at the interface between crystalline and amorphous regions. Reducing the size of spherulites to a certain degree not only improves the strength of plastics (reduces intergranular defects) but also improves the transparency (scattering will not occur when the size of spherulites is less than the wavelength of light). 3) Thermal performance crystalline plastics do not appear high elastic state when the temperature rises, and show viscous flow state when the temperature rises to the melting temperature Tm. Therefore, the service temperature of crystalline plastics is increased from TG (glass transition temperature) to TM (melting temperature). 4) Solvent resistance and permeability are improved because the crystalline components are arranged more closely. 3、 What are the factors affecting crystallization? 1) Polymers with good chain structure, good symmetry, no branched chains or few branched chains, small volume of side groups and large intermolecular force are easy to be close to each other and easy to crystallize. 2) Temperature, the polymer moves from the disordered coil to the surface of the growing crystal. When the mold temperature is high, the activity of the polymer is improved, so as to speed up the crystallization. 3) Pressure, if there is an external force in the cooling process, it can also promote the crystallization of the polymer. Therefore, the injection pressure and pressure holding pressure can be adjusted to control the crystallinity of crystalline plastics in production. 4) Nucleating agent, because low temperature is conducive to rapid nucleation, but slows down the growth of grains. Therefore, in order to eliminate this contradiction, nucleating agent is added to the molding material, so that the plastic can crystallize rapidly at high mold temperature. 4、 What are the requirements of crystalline plastics for injection molding machines and molds? 1) the melting of crystalline plastics requires more energy to destroy the lattice, so more heat needs to be input when transforming solid into molten melt
Therefore, the plasticizing capacity of the injection molding machine should be large, and the maximum injection volume should be improved accordingly. 2) The melting point range of crystalline plastics is narrow. In order to prevent the rubber crystallization from blocking the nozzle when the nozzle temperature decreases, the nozzle aperture should be appropriately increased, and a heating ring that can independently control the nozzle temperature should be installed. 3) As the mold temperature has an important impact on the crystallinity, the mold water path should be as many as possible to ensure the uniform mold temperature during molding. 4) Crystallinity has a large volume shrinkage in the crystallization process, resulting in a large forming shrinkage rate. Therefore, its forming shrinkage rate should be seriously considered in the mold design. 5) due to the significant anisotropy and large internal stress, pay attention to the position and size of gate, stiffener and position and size in the mold design, otherwise it is easy to warpage and deformation, and then it is very difficult to improve it by the forming process. 6) Crystallinity is related to the wall thickness of plastic parts. The cooling of wall thickness is slow, the crystallinity is high, the shrinkage is large, and shrinkage cavities and pores are easy to occur. Therefore, attention should be paid to the control of plastic part wall thickness in mold design. V. the molding process of crystalline plastics 1) the heat released during cooling is large, and it should be fully cooled. Attention should be paid to the control of cooling time during high mold temperature molding. 2) The specific gravity difference between molten state and solid state is large, the molding shrinkage is large, and shrinkage cavities and pores are easy to occur. Pay attention to the setting of pressure holding pressure. 3) when the mold temperature is low, the cooling is fast, the crystallinity is low, the shrinkage is small, and the transparency is high. Crystallinity is related to the wall thickness of plastic parts. When the wall thickness of plastic parts is large, the cooling is slow, the crystallinity is high, the shrinkage is large, and the physical properties are good. Therefore, the mold temperature of crystalline plastics must be controlled as required. 4) The anisotropy is significant, the internal stress is large, and the uncrystallized folded molecules have the tendency to continue crystallization after demoulding. They are in an energy imbalance state and are prone to deformation and warpage. The material temperature, mold temperature, medium injection pressure and injection speed should be appropriately increased. In the market, there are many kinds of plastics, but people who make plastics generally only know that they are divided into engineering plastics and daily-use plastics. In essence, plastics can be divided into crystalline plastics and amorphous plastics. Crystalline plastics: nylon, propylene, ethylene, polyoxymethylene, etc; Amorphous Plastics: polycarbonate, ABS, benzene permeable, vinyl chloride, etc. The influencing factors of polymer crystallization can be divided into two parts: the regularity of internal structure, and the external concentration, solvent, temperature and so on. The more regular the structure is, the easier it is to crystallize. On the contrary, it is less easy to become amorphous polymer. Structural factors are the most important. In order to improve the crystalline orientation of polymer, from the perspective of structure, we can: increase the symmetry of molecular chain; Increase the stereoregularity of molecular chains; Increase the arrangement order of repeating units, i.e. random copolymerization; Increase the hydrogen bond in the molecular chain; Reduce the degree of branching or crosslinking of molecular chain; From the perspective of external factors, the methods that can be implemented in the factory: annealing and slow cooling can improve the crystallinity; Pay attention to the effect of stress. For example, rubber and fiber accelerate crystallization under stress conditions. Solvent selection. It is not easy to crystallize in good solvent.

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