Polypropylene PP

Polypropylene PP is a thermoplastic synthetic resin with excellent properties. It is a colorless and translucent thermoplastic lightweight general-purpose plastic in appearance. It has the characteristics of chemical resistance, heat resistance, electrical insulation, high strength mechanical, as well as good high wear resistance processing properties. So polypropylene is widely used in the manufacture of many kind of products. PP granules has been widely applied in many fields, such as, machinery, automobiles, electronic appliances, construction, textiles, packaging, agriculture, forestry, fisheries and food industries.

According to the different spatial positions of methyl groups in the molecule, it can be divided into Isotactic polypropylene(IPP), Syndiotactic polypropylene(SPP) and Atactic polypropylene(APP). In the case of APP, methyl groups are arranged randomly, while methyl groups of SPP is arranged alternately and IPP is arranged uniformly. The arrangement of methyl groups has an effect on crystallinity. Commercial virgin polypropylene for sale in the market is generally isotactic, with an isotactic index typically between 85% and 95%. Stereoregularity affects the physical properties of polymers.

Based on the type of monomer, it can be divided into polypropylene homopolymer and polypropylene copolymer. PP homopolymer means that there is only one kind of chain link in the main chain. For PP copolymer, in addition to the propylene chain, the main chain is also distributed with the chains formed by the reaction of other monomers. Copolymer PP can change the properties of the PP resin to a great extent.

For the manufacture of machinery and auto parts. Polypropylene PP has good mechanical properties. So it can be directly manufactured or modified to manufacture parts of mechanical equipment, such as, industrial pipes, agricultural water pipes, motor fans, infrastructure templates, etc. Modified polypropylene resin can be molded into bumpers, anti-scratch strips, automobile steering wheels, instrument panels and interior trim parts, etc.

Electronic and electrical industrial devices. Modified polypropylene PP can be used to make the insulating shell of household appliances and the inner tank of washing machine. Meanwhile, it can be used as the insulating material of wires, cables and other electrical appliances. 

Used in construction industry. Polypropylene fiber is the lightest of all chemical fibers, and has the advantages of high strength, good toughness, good chemical resistance, antimicrobial resistance, and low price. PP resin modified with glass fiber reinforcement or modified with rubber and SBS is widely used to make building engineering templates. The foamed polypropylene PP can be used to make decorative materials.

Used in agriculture, fishing and food industry. Polypropylene resin can be used to make greenhouse canopies, plastic films, culture bottles, agricultural tools, fishing nets, etc. Meanwhile, it can also be used to make food turnover boxes, food bags, beverage packaging bottles, etc.

Used in textile and printing industry. Polypropylene PP is the raw material of synthetic fibers. PP fibers are widely sued to make lightweight and beautiful durable textile products. 

Other industrial fields. In the chemical industry, PP polypropylene can be used to prepare various corrosion-resistant pipelines, storage tanks, valves, special-shaped packings in packed towers, filter cloths, corrosion-resistant pumps and linings of corrosion-resistant containers. In medicine industry, it can be used to make medical devices. Polypropylene resin can also be developed and applied in the energy field through grafting, compounding and blending processes.

There are many molding methods of virgin PP resin, such as, injection molding, extrusion molding, blow molding, vacuum thermoforming, coating, rotational molding, welding, machining, electroplating and foaming, etc. In addition, it can be sprayed on the metal surface.

PP film is transparent and shiny, and has low permeability to water vapor and air. It can be divided into blow molding film, cast film, biaxially stretched film, etc.

Injection products can be used as engineering accessories for automobiles, machinery, instruments, radios, textiles, national defense, etc. Also suitable for the producing of daily necessities, turnover boxes, medical and health equipment, building materials, etc.

PP extrusion products include pipes, profiles, monofilaments, fishing ropes, packing belts, binding ropes, woven bags, fibers, composite coatings, sheets, plates, etc.

• Good chemical resistance. Diluted bases and acids don’t readily react with polypropylene, making them ideal for containers of such liquids, like cleaners, first aid products, etc.
• High flexural strength and good fatigue resistance due to its semi-crystalline nature. Polypropylene retains its shape after extensive twisting and bending. PP polypropylene acts elastically within a certain range of deformation, but it also undergoes plastic deformation early in the deformation process. It can be manufactured as a living hinge by CNC, injection molding, thermoforming or crimping. Living hinges are extremely thin pieces of plastic that can repeatedly bend without breaking.
• Good insulativity. PP resin has a very high resistance to electricity and is very useful for electronic components. It is a good electrical insulator.
• It can easily be copolymerized with other polymers, such as, polyethylene, to form composite plastics.
• In addition, it has a relatively smooth surface, is very resistant to moisture absorption, and has good impact strength.

The process methods used to produce polypropylene PP can be divided into the following major categories, solvent polymerization, solution polymerization, liquid-phase bulk method and gas-phase bulk method.

Solvent polymerization

1. Propylene monomer is dissolved in an inert liquid-phase solvent, and the solvent is polymerized under the action of a catalyst. The polymer is suspended in the solvent in the state of solid pellets ad a tank-type stirring reactor is used.
2. There are deashing and solvent recovery processes, the process is long and complicated, the equipment investment is large, and the energy consumption is high. But the production is easy to control and the products quality is good.
3. Polypropylene granules are separated by centrifugal filtration and then boiled and dried by air flow and extruded for granulation.

Solution polymerization

1. Using high-boiling straight-chain hydrocarbons as solvent and operating at a temperature higher than the melting point of polypropylene PP. All the obtained polymers are dissolved in the solvent and are homogeneously distributed.
2. Evaporate and remove the solvent by high temperature stripping method to obtain molten polypropylene, and then extrude and granulate to obtain pellets products.

Liquid-phase Bulk method

This method doesn’t need to add any other solvent in the reaction system, and the catalyst is directly dispersed in the liquid phase propylene to carry out the liquid-phase bulk polymerization of propylene. The polymer is continuously precipitated from the liquid phase propylene in the form of the fine particles. As the reaction time increase, concentration of polymer particles in the liquid phase propylene increases. When the propylene conversion rate reaches a certain level, unpolymerized propylene monomer is recovered by flash evaporation to obtain a powdered PP granule. The technological process has the advantages of simple, less equipment, less investment, low power consumption and production cost.

Gas-phase bulk method

1. The process can produce a full range of polypropylene pp products including homopolymers, random copolymers and impact copolymers with a small amount of equipment.
2. This method allows for the adjustment of operating conditions over a wide operating range while maintaining uniform product properties.
3. Due to the small number of equipment used, the maintenance workload is small, and the reliability of the device is improved. Due to the limitation of the reaction kinetics of the fluidized bed itself, and the low operating pressure reduces the storage of materials in the system, the process is safer to operate than other processes. And there is no danger of equipment overpressure in the event of an accident out of control.
4. The process has no liquid waste discharge and only little hydrocarbon emissions to the atmosphere, so the impact on the environment is very small. Compared with other processes, this process is easier to meet various strict specifications for environmental protection, health and safety.