From the perspective of mechanical strength, PP should be high. But flexible, impact resistance, PE is better. When making a film, the transparency PP is better than that of PE, but PE is good if the sealing temperature and heat sealing strength are used. In addition, in terms of temperature resistance, PP is also higher than PE, and generally PP can withstand pasteurization temperature. PE polyethylene is waxy and has a smooth waxy feel. When not dyed, low density polyethylene is transparent, while high density polyethylene is opaque. Polyethylene is a highly polymerized chain formed by the addition of repeating -CH2 units by addition and polymerization of ethylene (CH2=CH2).
The properties of polyethylene depend on its mode of polymerization; high density polyethylene (HDPE) is formed by Ziegler-Natta polymerization under moderate pressure (15-30 atmospheres) of organic compound catalysis. The polyethylene molecules polymerized under such conditions are linear and have a long molecular chain with a molecular weight of several hundred thousand. If it is under high pressure (100-300 MPa), high temperature (190–210C), free radical polymerization under peroxide catalysis, it produces low density polyethylene (LDPE), which is branched structure. Polyethylene is insoluble in water and has little water absorption.
For some chemical solvents, such as toluene, acetic acid, etc., it is only slightly soluble at temperatures above 70 °C. However, the particulate polyethylene can be melted or solidified with a change in temperature between 15 ° C and 40 ° C, and melts when the temperature is raised to absorb heat; when the temperature is lowered, it solidifies and releases heat. And because it has a small amount of water absorption, is not easy to be wet, and has insulation properties, it is a good building material. Typical application range of PP: Automotive industry (mainly using PP with metal additives: fenders, ventilation pipes, fans, etc.), instruments (dishwasher door liner, dryer ventilation pipe, washing machine frame and cover, refrigerator door lining Pads, etc., consumer goods (turf and garden equipment such as lawn mowers and sprinklers, etc.).
Chemical and physical properties: PP is a semi-crystalline material. It is harder and has a higher melting point than PE. Since the PP temperature of the homopolymer type is very brittle above 0 ° C, many commercial PP materials are random copolymers of 1 to 4% ethylene or a clamp copolymer of a higher ratio of ethylene. The copolymer type PP material has a low heat distortion temperature (100 ° C), low transparency, low gloss, low rigidity, but has a stronger impact strength. The strength of PP increases as the ethylene content increases. The Vicat softening temperature of PP is 150 °C. Due to the high degree of crystallinity, this material has excellent surface stiffness and scratch resistance.
There is no environmental stress cracking problem in PP. Typically, PP is modified by the addition of glass fibers, metal additives or thermoplastic rubber. The flow rate of PP has a MFR ranging from 1 to 40. Low MFR PP materials have better impact resistance but lower elongation. For materials of the same MFR, the strength of the copolymer type is higher than that of the homopolymer type. Due to crystallization, the shrinkage rate of PP is quite high, generally 1.8 to 2.5%. And the direction uniformity of shrinkage is much better than materials such as PE-HD. The addition of 30% glass additive can reduce the shrinkage to 0.7%. Both the homopolymer type and the copolymer type PP materials have excellent moisture absorption resistance, acid and alkali corrosion resistance, and solubility resistance. However, it is not resistant to aromatic hydrocarbons such as benzene solvents, chlorinated hydrocarbons (carbon tetrachloride) solvents, and the like. PP does not have oxidation resistance at high temperatures like PE. PVC PVC can be widely used in the building materials industry. There are two important reasons: First, the unique properties of PVC (rain, fire, antistatic, easy to form), and second, the low input and high output of PVC. The PVC film has high light resistance and fire resistance, ensuring high quality of the PVC film. So comprehensively, PVC is actually a low-input, high-volume product. Melting temperature: 185~205°C Mold temperature: 20~50°C