Crystalline Properties of Thermoplastics

Crystalline Properties of Thermoplastics

Thermoplastics can be divided into two types: crystalline plastics and amorphous plastics. The molecular chains of crystalline plastics are neatly arranged, fixed and compact, while the molecular chains of amorphous plastics are arranged in disorder. Therefore, crystalline plastics are generally more heat-resistant, impermeable and have higher mechanical strength, while amorphous plastics are the opposite. Commonly used polyethylene polypropylene and polyamide (nylon) are crystalline plastics; commonly used polystyrene, polyvinyl chloride and ABS are amorphous plastics.

From the perspective of appearance characteristics, generally crystalline plastics are opaque or translucent, and amorphous plastics are transparent. But there are exceptions, such as poly-4-methylpentene-1 is a crystalline plastic, but it has high transparency, while ABS is an amorphous plastic, but it is opaque.

In thermoplastics, the molecular chain of the polymer has a linear or branched structure. Generally, the relative average molecular weight is used to characterize and measure the length of the polymer molecular chain. The greater the molecular weight, the higher the mechanical strength of the solid polymer. The better, the worse the flow properties of the polymer when it is in a flowing state.

(1) Characteristics of crystalline plastics

Finished plastics have a clear melting point, and the molecules are arranged regularly when they are solid. Regularly arranged regions are called crystal regions, disorderly arranged regions are called amorphous regions, and the percentage of crystal regions is called crystallinity. Usually, polymers with a crystallinity of more than 80% are called crystalline plastics. Common crystalline plastics are: polyethylene PE, polypropylene PP, polyoxymethylene POM, polyamide PA6, polyamide PA66, PET, PBT, etc.

(2) Effect of crystallization on plastic properties

①Mechanical properties. Crystallization makes the plastic brittle (reduced impact strength), stronger toughness and less ductile.

② Optical properties. Crystallization makes the plastic opaque because light is scattered at the interface between the crystalline and amorphous regions. Reducing the size of spherulites to a certain extent not only improves the strength of the plastic (reducing intergranular defects), but also improves transparency (no scattering occurs when the spherulite size is smaller than the wavelength of light).

③Thermal performance. Crystalline plastics do not appear in a high elastic state when the temperature rises, but when the temperature rises to the melting temperature Tm, they show a viscous flow state. Therefore, the use temperature of crystalline plastics is increased from T (glass transition temperature) to Tm (melting temperature).

④ Solvent resistance, permeability, etc. are improved, because the crystallization makes the arrangement more compact. (3) Factors affecting crystallization

① Polymer chain structure. Macromolecules with good symmetry, no branched chain or few branched chains, small side group volume, and large intermolecular force tend to be close to each other and crystallize easily.

② temperature. The polymer moves from the disordered coils to the surface of the growing crystal, and the higher mold temperature increases the mobility of the polymer and thus accelerates the crystallization.

③ pressure. If there is an external force during the cooling process, it can also promote the crystallization of the polymer, so the injection pressure and holding pressure can be increased to control the crystallinity of the crystalline plastic during production.

④ Nucleating agent. Since low temperature is conducive to rapid nucleation, but slows down the growth of crystal grains, in order to eliminate this contradiction, a nucleating agent is added to the molding material, so that the plastic can crystallize rapidly at high mold temperature.

(4) Special requirements for crystalline plastics on injection molding machines and molds

① Crystalline plastics require more energy to destroy the crystal lattice when melting, so more heat needs to be input when converting from a solid to a molten melt, so the plasticizing capacity of the injection molding machine must be large, and the maximum injection volume should be increased accordingly .

②Crystalline plastic has a narrow melting point range. In order to prevent the plastic material from crystallizing and blocking the nozzle when the temperature of the nozzle is lowered, the aperture of the nozzle should be appropriately enlarged, and a heating ring that can independently control the temperature of the nozzle should be installed.

③Since mold temperature has an important influence on crystallinity, there should be as many mold water channels as possible to ensure uniform mold temperature during molding.

D crystalline plastics undergo a large volume shrinkage during the crystallization process, causing a large molding shrinkage rate, so the molding shrinkage rate should be carefully considered in the mold design.

) Due to significant anisotropy and large internal stress, attention should be paid to the position and size of the gate and the position and size of the ribs in the mold design, otherwise warping and deformation are prone to occur, and it is quite difficult to improve it by the molding process.

⑥The degree of crystallinity is related to the wall thickness of the plastic part. The wall thickness is slow to cool, the crystallinity is high, the shrinkage is large, and shrinkage cavities and pores are prone to occur. In the design of this mold, attention should be paid to the control of the wall thickness of the plastic part.

(5) Molding characteristics of crystalline plastics

①The heat released during cooling is large, so it must be fully cooled. Pay attention to the control of cooling time when molding at high mold temperature

②The density difference between the molten state and the solid state is large, the molding shrinkage is large, and shrinkage cavities and pores are prone to occur. Pay attention to the setting of the 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. The crystallinity is related to the wall thickness of the plastic part. When the wall thickness of the plastic part is large, the cooling is slow, the crystallinity is high, and the shrinkage is large. Therefore, the mold temperature of the crystalline plastic should be controlled according to the requirements.

④ Significant anisotropy, large internal stress, uncrystallized molecules tend to continue to crystallize after demoulding, and are in a state of energy imbalance, prone to deformation and warping. The material temperature and mold temperature should be appropriately increased, and medium injection pressure and Injection speed.

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