polypropylene (polypropylene)

Used to produce extruded film and composite film plastic products

① slurry method. Polymerization in diluents (such as hexane) is the earliest industrialized method and the largest production so far. ② liquid phase bulk method. Polymerization in liquid propylene at 70℃ and 3MPa. ③ gas phase method. Polymerization under the condition that propylene is gaseous. The latter two methods do not use thinner, and the process is short and the energy consumption is low. Liquid phase bulk method has now shown the advantage of coming from behind. Molding characteristics (1) physical properties: PP is a non-toxic, tasteless milky white and highly crystalline polymer, which is one of the lightest varieties of all plastics at present and is particularly stable to water, the water absorption rate of 14h in water is only 0.01%. The molecular weight is between 8 and 150,000, and the formability is good. However, due to the large shrinkage rate, the original wall products are easy to sag, the surface gloss of the products is good, and it is easy to color. (2) mechanical properties: PP has high crystallization degree and regular structure, so it has excellent mechanical properties. Its strength, hardness and elasticity are higher than that of high density PE(HDPE). The outstanding feature is that the bending fatigue resistance (7 × 10 ^ 7) is selected for bending without damage marks when it is opened and closed. The dry friction coefficient is similar to nylon, but it is not as good as nylon when it is oily and slippery. (3) thermal performance: PP has good heat resistance, melting point is 164 ~ 170℃, and products can be sterilized at temperatures above 100℃. Under the action of no external force, 150℃ will not deform. Embrittlement is-35℃, and embrittlement will occur below-35℃, and the heat resistance is not as good as PE. (4) chemical stability: PP has good chemical stability. Besides being eroded by concentrated sulfuric acid and concentrated nitric acid, it is relatively stable to other chemical reagents, but low molecular weight aliphatic hydrocarbons, aromatic hydrocarbons can soften and swell PP, and the chemical stability increases with the increase of crystallinity. Therefore, PP is suitable for making Russian-Chinese chemical pipelines and fittings with good anti-corrosion effect. (5) electrical properties: polypropylene has excellent high-frequency insulation properties. Because it hardly absorbs water, its insulation properties are not affected by humidity and have higher dielectric coefficient. With the increase of temperature, it can be used to make heated electrical insulation products with high breakdown voltage, and is suitable for electrical accessories, etc. The resistance to voltage and arc is good, but the static electricity is high, and it is easy to age when contacting with copper. (6) weather resistance: polypropylene is very sensitive to ultraviolet rays. Adding zinc oxide thiopropionic acid dilaurel grease and carbon black-like milky white filler can improve its aging resistance. Molding Process Injection molding machine selection: there is no special requirement for injection molding machine selection. Because PP has high crystallization, it needs to adopt computer injection molding machine with high injection pressure and multi-stage control. The clamping force is generally determined according to 3800T/㎡, and the injection volume is 20%-85%. Drying treatment: if stored properly, drying treatment is not required. Melting temperature: the melting point of PP is 160-175℃, and the decomposition temperature is 350℃, but the temperature setting cannot exceed 275℃ during injection processing. The temperature of the melting section is preferably 240 °c. Mold temperature: Mold temperature 50-90 ℃, high mold temperature for higher size requirements, core temperature is more than 5 ℃ lower than cavity temperature. Injection pressure: higher injection pressure (1500-1800bar) and hold pressure (about 80% of injection pressure) are adopted. About 95% of the full stroke is transferred to maintain the pressure, which takes a long time to maintain the pressure. Injection speed: in order to reduce internal stress and deformation, high-speed injection should be selected, but some grades of PP and moulds are not applicable (bubbles and gas lines appear). If there are light and dark stripes diffused by the gate on the surface of the engraved pattern, low-speed injection and higher mold temperature should be used. Runner and gate: flow diameter 4-7mm, needle gate length 1-1.5mm, diameter can be as small as 0.7mm. The shorter the length of the edge Gate, the better, about 0.7mm, the depth is half of the wall thickness, the width is twice the wall thickness, and increases with the length of the melt flow in the mold cavity. The mold must have good exhaust property. The exhaust hole depth is 0.025mm-0.038mm and the thickness is 1.5mm. To avoid shrinkage marks, large and round injection ports and circular flow channels must be used, the thickness of the stiffener should be small (for example, 50-60% of the wall thickness). The thickness of products made of homopolymerized PP cannot exceed 3mm, otherwise there will be bubbles (thick wall products can only use copolyzed PP). Melt glue back pressure: 5bar melt glue back pressure can be used, and the back pressure of color powder can be adjusted appropriately. Post-treatment of products: in order to prevent shrinkage and deformation caused by post-crystallization, products generally need to be soaked in hot water.