PP material and PP plastic sheet properties

Source:pulixin By Administrator Posted:2022-07-09
PP plastic sheet is a thermoplastic resin obtained by polymerizing propylene. There are three configurations of isotactic, random and syndiotactic, and industrial products are mainly composed of isotactic. PP polypropylene also includes copolymers of propylene with a small amount 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, which are its outstanding advantages. With a density of 0.90g/cm3, it is the lightest general-purpose plastic. Corrosion resistance, tensile strength 30MPa, strength, rigidity and transparency are better 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.
 

①Characteristics of polypropylene resin:

Polypropylene is light in weight and has a density of 0.90-0.91g/cm3, which is the lightest among general-purpose plastics. The mechanical strength, rigidity and stress cracking resistance of polypropylene are all higher than those of polyethylene, and it has outstanding elongation and bending fatigue resistance.

Polypropylene has excellent heat resistance, the long-term use temperature can be as high as 100oC ~ 120oC, and the use temperature can reach 150oC when there is no load. Polypropylene is the only general-purpose plastic that can be boiled in water and can withstand the sterilization temperature of 1350C.

The low temperature resistance of polypropylene is not as good as that of polyethylene, and the embrittlement temperature is about -10oC to -30oC (polyethylene is -60oC). The impact resistance at low temperature and even room temperature is not good, and the brittle cracking at low temperature is the main disadvantage of polypropylene.

Polypropylene is susceptible to aging due to the action of light, heat and oxygen during molding and utilization. Polypropylene will age and become brittle in 12 days in the atmosphere, and will deteriorate after 4 months of indoor storage. Usually, it is necessary to add ultraviolet absorbers, antioxidants, carbon black and zinc oxide to improve the weather resistance of polypropylene products.

Polypropylene is a non-polar plastic with excellent chemical stability, and the higher the crystallinity, the better the chemical stability. Except for the strong acid (such as fuming sulfuric acid, nitric acid), which has an erosive effect on it, there is no solvent that can dissolve polypropylene at room temperature, but low molecular weight aliphatic hydrocarbons, aromatic hydrocarbons and chlorinated hydrocarbons can soften or swell it. effect. Its water absorption is very small, and the water absorption rate is less than 0.01%.

Polypropylene has excellent electrical insulation properties, especially high-frequency insulation, high breakdown voltage strength, and low water absorption, which can be used as heat-resistant insulating materials for 120oC.
 

②Characteristics of polypropylene sheet:

Polypropylene sheets are also made of polypropylene resins of the same grades, which are usually used in vacuum forming and are copolymerized polypropylene resins and modified polypropylene materials. Modified polypropylene materials should focus on improving the molding process performance, transparency and heat resistance. The modification has the following aspects.

a. Improve the formability of polypropylene sheet
In order to improve the vacuum forming properties of polypropylene sheets, the most commonly used method is to blend and modify polypropylene with polyethylene (PE). Mixing a small amount (20% to 30%) of low density polyethylene (PE) in polypropylene can improve the shortcomings of narrow melting properties of polypropylene and easy dripping when the sheet is heated. Ordinary vacuum forming can be used. forming process. Of course, the addition of polyethylene will also bring some negative effects on performance, such as a certain degree of decline in rigidity and heat resistance, which should be considered in the application of hollow.

b. Improve the transparency of polypropylene sheets and their molded products
Adding a nucleating agent (transparency agent) to polypropylene is an effective way to improve the transparency of polypropylene sheets and their molded products. After the nucleating agent is formulated, a large number of fine crystals will be produced during the production of polypropylene sheets, reducing or even avoiding the production of large spherulites. The reduction or elimination of large spherulites will significantly improve the transparency of polypropylene. At the same time, due to the action of the nucleating agent, a large number of fine crystals are formed, which improves the cleanliness of polypropylene. Modified with nucleating agent, while improving the transparency of polypropylene, its rigidity is also improved.

c. Improve the heat resistance and rigidity of polypropylene sheet
The heat resistance and rigidity of polypropylene sheets can be significantly improved by filling and modifying methods. The most widely used fillers are talc, calcium carbonate, etc., among which the modification effect of talc is more prominent. By adding 30%~40% talc, the obtained polypropylene sheet not only has a significantly improved rigidity, but also its service temperature can be increased to above 140°C, while maintaining the excellent hygienic properties and better oil resistance of the pure polypropylene sheet , and the price is lower. Its shaped products can be suitable for microwave heating.
 

③ Vacuum forming conditions of PP plastic sheet

In the vacuum forming process, the forming performance of modified polypropylene sheet with (20% to 30%) low density polyethylene (PE) is the best.

The requirements of polypropylene for molding conditions, the mold release slope: 20-30 (optimal 5o) for punch, 10-3o (optimally 5o) for concave mold; molding temperature: sheet temperature 150-180°C, mold The temperature is 70℃~90℃; in the case of molding shrinkage, the punch is 0.02~0.03mm/mm, and the female die is 0.03~0.04mm/mm.

Because polypropylene sheet has the characteristics of rich material source, low price, non-toxic, odorless, good transparency and high mechanical strength, the polypropylene sheet is vacuum-formed and processed into various packaging products, which are widely used in food. , Medicine, machinery parts, daily necessities and other packaging.

At present, polypropylene packaging products on the market, in addition to simple lunch boxes, water boxes, spoons, bowls, pickle boxes, packaging boxes of various native products, yogurt cans, trays, etc. have also entered the market. Because polypropylene products can be microwaved and sterilized at 125°C, they are widely used in ready-to-eat food packaging.

In addition, polypropylene vacuum forming products have also been gradually applied in agricultural production in recent years, such as seedling trays, breeding cups, etc.

Bonding issues are the same as PE, which can be bonded after pretreatment; hotplates for welding and friction welding are very successful.
Post-processing: When punching by punching die and trimming die, the cutting gap needs to be 5mm, the material temperature is too high, and burrs appear in the cutting. Corresponding measures should be taken to maintain the minimum deformation during sealing. For heat sealing with a coating layer, the degree of distribution is 150~180°C, and the sealing time is 1s.

Special composite sheet PP/EVOH/PP, PP/PVDC/PP, filledPP/PP, PP/EVA, PS/PVDC/PE/PP, filled PP/PP/filled PP, EVOH/PP/PP, EVOH/PP/PP, PP/EVOH/PA, PA/PP, elastic modified PP material (PP+EPDM).