Abbreviation: polypropylene, a thermoplastic resin obtained by the polymerization of propylene. There are three configurations of isotactic, random and syndiotactic, and industrial products are mainly composed of isotactic. Polypropylene also includes copolymers of propylene with small amounts of ethylene. Usually translucent colorless solid, odorless and nontoxic. Due to the regular structure and high crystallization, the melting point is as high as 167"C, heat resistance, and the products can be sterilized by steam are its outstanding advantages. The density is 0.90g/cm3, which is the lightest general-purpose plastic. The corrosion resistance tensile strength is 30MPa, and the strength and rigidity are It has better transparency and transparency than polyethylene. The disadvantage is that it has poor low temperature impact resistance and is easy to age, but it can be overcome by modification and addition of antioxidants.
Features: non-toxic, odorless, low density, strength, stiffness, hardness and heat resistance are better than low pressure polyethylene, can be used at about 100 degrees, good electrical properties and high frequency insulation Not affected by humidity, but time-varying at low temperature Brittle, not wear-resistant, easy to age. Suitable for making general mechanical parts, corrosion-resistant parts and insulating parts. Common acid and alkali organic solvents have little effect on it and can be used for tableware.
Production methods: ① slurry method. Polymerization in a diluent such as hexane is the earliest industrialized and by far the most productive method. ② liquid phase bulk method. Polymerization in liquid propylene at 70C and 3MPa. ③ gas phase method. The polymerization is carried out under the condition that the propylene is in the gaseous state. The latter two methods do not use diluent, the process is short, and the energy consumption is low. Liquid bulk methods have now shown the advantage of coming later.
Molding characteristics: 1. Crystalline material, low hygroscopicity, prone to melt rupture, and easy to decompose after long-term contact with hot metal. 2. Good fluidity, but large shrinkage range and shrinkage value, prone to shrinkage, dents, and deformation. 3. The cooling speed is fast, the pouring system and cooling system should dissipate heat slowly, and pay attention to controlling the molding temperature. It is easy to orientate when the material temperature is low and high pressure. When the mold temperature is lower than 50 degrees, the plastic parts are not smooth, and it is easy to produce poor welding and flow marks. 4. The thickness of the plastic wall must be uniform to avoid lack of glue and sharp corners to prevent stress concentration.
Polypropylene molding process: injection molding process conditions:
Drying: Drying is not required if stored properly. Melting temperature: 220~275C, be careful not to exceed 275C. Mold temperature: 40~80C, 50C is recommended. The degree of crystallization is mainly determined by the mold temperature. Injection pressure: up to 1800bar. Injection speed: Usually, the use of high-speed injection can minimize the internal pressure. If there are defects on the surface of the product, then lower speed injection at higher temperatures should be used. Runners and gates: For cold runners, typical runner diameters range from 4 to 7 mm. It is recommended to use sprues and runners with round bodies. All types of gates can be used. Typical gate diameters range from 1 to 1.5mm, but gates as small as 0.7mm can also be used. For edge gates, the minimum gate depth should be one-half the wall thickness; the minimum gate width should be at least twice the wall thickness. PP material can use hot runner system completely.
Polypropylene is a semi-crystalline material. It is harder and has a higher melting point than PE. Since homopolymer polypropylene is very brittle at temperatures above 0°C, many commercial PP materials are random copolymers with 1 to 4% ethylene added or pinched copolymers with higher ethylene content. Polymer-type PP materials have lower thermal distortion temperature (100C), low transparency, low gloss, low rigidity, but have stronger impact strength. The strength of PP increases with increasing ethylene content. The Vicat softening temperature of PP is 150C.
Due to the high degree of crystallinity, this material has good surface stiffness and scratch resistance properties. PP does not have environmental stress cracking problems. Typically, PP is modified by adding glass fibers, metal additives or thermoplastic rubber. The flow rate MFR of PP ranges from 1 to 40. PP materials with low MFR have better impact resistance but lower ductility. For the same MFR material, the strength of the copolymer type is higher than that of the homopolymer type. Due to crystallization, the shrinkage rate of Polypropylene is quite high, generally 1.8~2.5%. And the directional uniformity of shrinkage is much better than that of PE-HD and other materials. Adding 30% glass additive can reduce the shrinkage to 0.7%. Both homopolymer and copolymer Polypropylene materials have excellent moisture absorption resistance, acid and alkali corrosion resistance, and solubility resistance. However, it is not resistant to aromatic hydrocarbon (such as benzene) solvents, chlorinated hydrocarbon (carbon tetrachloride) solvents, etc. PP is also not as resistant to oxidation at high temperatures as PE.
Application engineering, with polypropylene fibers
It is divided into polypropylene monofilament fiber and polypropylene mesh fiber. Strength bundled monofilament or mesh organic fiber has inherent strong acid resistance, strong alkali resistance, weak thermal conductivity, and extremely stable chemical properties. Adding it to concrete or mortar can effectively control the micro-cracks caused by concrete (mortar) solid-plastic shrinkage, dry shrinkage, temperature change and other factors, prevent and inhibit the formation and development of cracks, and greatly improve the crack resistance and impermeability of concrete. It can be widely used in underground I-pass waterproofing, industrial and civil building T-pass roofs, walls, floors, pools, basements, etc., as well as road and bridge projects.
What is the difference between polyethylene, polyvinyl chloride, polystyrene, polypropylene
Polyethylene PE
When unpigmented, it is milky white, translucent and waxy; the product feels smooth when touched by hand, flexible and pliable; it can stretch slightly. - Generally, low-density polyethylene is softer and has better transparency; high-density polyethylene is harder. Common products: handbags, water pipes, oil drums, beverage bottles (calcium milk bottles), daily necessities, etc.
Polypropylene PP
White translucent, waxy when unpigmented; lighter than polyethylene. The transparency is also better than that of polyethylene, and it is harder than polyethylene. Common products: basins, barrels, furniture, films, woven bags, bottle caps, car bumpers, etc.
polystyrene PS
Transparent when unpigmented. When the product is dropped or tapped, it has a metal-like crisp sound, good luster and transparency, similar to glass, brittle and easy to break, and the surface of the product can be scratched with fingernails. Modified polystyrene is opaque. Common products: stationery, cups, food containers, home appliance shells, electrical accessories, etc.
polyvinyl chloride (PVC)
The natural color is light yellow, translucent and shiny. The transparency is better than that of polyethylene and polystyrene, but worse than that of polystyrene. It is divided into soft and hard polyvinyl chloride according to the dosage of additives. Soft products are flexible and tough, and feel sticky. The hardness of hard products is higher than that of low-density polyethylene. , while lower than polypropylene, whitening occurs at the inflection point. Common products: sheets, pipes, shoe soles, toys, doors and windows, wire sheaths, stationery, etc. Polyethylene terephthalate PET has good transparency, strength and toughness better than polystyrene and polyethylene, and is not easy to break. Common products: often bottle products such as cola, mineral water bottles, etc.