Plastic bottles used for beverage packaging are generally made of non-toxic polypropylene plastic.
Polypropylene
Molecular formulae...........CH3.
│..........│.....│
┼CH2-CH-┼
└...................┘n
Polypropylene (PP) is obtained by the catalytic polymerization of propylene using a Ziegler-Natta catalyst. It is a crystalline isotactic polymer with very regular molecular chains. The polymerization process includes four methods: solvent polymerization (slurry method), liquid phase bulk polymerization, gas bulk polymerization, and solution polymerization. However, it is mainly based on solvent (slurry) polymerization. Its isotacticity is above 95% and its molecular weight is about 8 to 150,000.
1. performance
(1) Physical properties Polypropylene is a non-toxic, odorless, odorless, milky white, highly crystalline polymer with a density of only 0.90 to 0.91 g/cm3. It is one of the lightest brands in all plastics. It is particularly stable to water, the water absorption rate in water at 24h is only 0.01%, and the molecular weight is about 8 to 150,000. Good moldability, but due to high shrinkage (1% to 2.5%), thick-walled products are easily sag, and for some parts with high dimensional accuracy, it is difficult to meet the requirements. The surface of the product is glossy and easy to color.
(2) Mechanical properties Polypropylene has high crystallinity and regular structure, so it has excellent mechanical properties, and its yield, tensile, compressive strength, hardness, elasticity, etc. are all higher than HDPE, but at room temperature and low temperature, due to its own Since the molecular structure has a high degree of regularity, the impact strength is poor, and when the molecular weight increases, the impact strength also increases, but the molding process performance deteriorates. Polypropylene has outstanding bending fatigue resistance, such as PP injection-body living hinge, can withstand 70 million folds of opening and closing with no damage marks, it's friction resistance is also good, dry friction coefficient and nylon Similar, but in oil lubrication, the friction performance is obviously inferior to nylon, PP can only be used to make gears and bearings with lower PV value and no impact load. Surface effects, such as embossing, engraving, etc. on the surfaces of their products, are easier than any other thermoplastic. Polypropylene products are particularly sensitive to gaps. Therefore, when designing the mold, care must be taken to avoid the presence of sharp corners, otherwise stress concentration will easily occur and the service life of the product will be affected.
(3) Thermal properties Polypropylene has good heat resistance. Its melting point is 164~170°C, and the product can be disinfected and sterilized at temperatures above 100°C. In the absence of external force, 150 °C does not deform, at 90 °C stress relaxation performance is good, its embrittlement temperature is -35 °C, in the temperature below -35 °C will occur brittle fracture, cold resistance is not as good Polyethylene, if reinforced with asbestos fibers and glass fibers, has high heat distortion temperature, dimensional stability, and low temperature impact properties.
(4) Chemical stability The chemical stability of polypropylene is very good. Apart from being attacked by concentrated sulfuric acid and concentrated nitric acid, it is stable to other chemical reagents, but it has low molecular weight aliphatic, aromatic and chlorinated hydrocarbons. Polypropylene can be softened and swelled, while its chemical stability increases with the increase of crystallinity. Therefore, it is suitable for various kinds of chemical pipelines and accessories, and its anti-corrosion effect is good.
(5) Electrical properties Polypropylene has good high-frequency insulation properties because it hardly absorbs water and the insulating properties are not affected by humidity. It has a high dielectric coefficient, and with the rise of temperature, it can be used to make heated electrical insulation products, and its breakdown voltage is also high, suitable for use as electrical accessories. The resistance to voltage and arc resistance is good, but the degree of static electricity is high and it is easy to age with contact with copper.
(6) Weather-resistant Polypropylene is very sensitive to ultraviolet rays. Adding zinc oxide, dilauric thiodipropionate, carbon black, or a similar cream filler can improve its aging resistance.
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