Polypropylene
The most prominent property of polypropylene is its versatility, which can be adapted to many processing methods and uses. Its value and versatility come mainly from its excellent chemical resistance, its lowest density and highest melting point in bulk thermoplastics, and its modest cost.
Chemistry and performance
Polypropylene (PP for short) is different from polyethylene (PE) in that the former has a methyl group on every other carbon atom, which serves to make the chain harden. Unless these methyl groups are on the same side of the chain, the polymer will not crystallize. Prior to the development of stereotactic catalysts for Natta and Ziegler (independently of each other), only soft, adherent atactic polypropylene could be produced. Commercial plastic hardness and resistance to small wins originate from crystallinity. The PP chain is harder than PE, so PP has higher melting temperature and tensile strength, but lower crystallinity. The PP homopolymer has a melting point of about 330°F, depending on the heating rate and thermal history.
Inserting ethylene (random copolymerization) at intervals on the PP chain will make the chain less regular and softer, thereby reducing the crystallinity, modulus, melting point, and melting point sharpness of the polymer. Typical random copolymers are relatively transparent with melting points in the range of 293-305F. As the ethylene content increases, the crystallinity of the polymer becomes lower and lower, eventually becoming ethylene-propylene rubber (EPR).
Another important class of copolymers are impact-resistant, heterogeneous copolymers. These products are made by polymerizing rubber (sometimes PE) in a homopolymer matrix. The rubber used is usually EPR, which generates a phase that separates from the homopolymer matrix and forms a haze. Translucent appearance. These materials are not true block copolymers because the rubber phase can be extracted by solvents. A similar product can be obtained by blending EPR with PP. The impact copolymer has a similar melting point as the homopolymer.
The molecular weight and molecular weight distribution are important in PP processing. Melt flow at 446T and 4.75 pounds load is an index of melt viscosity, which is related to the weight average molecular weight. Commercial polypropylene has a melt flow as low as 0.25 g/10 minutes up to 800 g/10 minutes. The molecular weight distribution is represented by the ratio of the weight average molecular weight to the number average molecular weight. This ratio of high crystallinity PP can be as high as 11; and the PP used as the meltblown fabric can be as low as 2. l. This ratio is extremely important in the fiber spinning process, and it affects the extrusion, extrudate swell, molding internal stress and orientation process.
Like most polymers, polypropylene oxidizes, especially during melt processing. As far as PP is concerned, the free radicals attacking the tertiary hydrogen are used to protect the polymer. For long-term use of PP at high temperatures, a complex multi-component stabilizer system is used; for the limited odor or taste, the stabilization system must be very simple. If used for protection against sunlight (ultraviolet), carbon black can be added or special stabilization methods can be used.
The normal PP has a tensile strength of 34.5 MPa and a flexural modulus of about 1723 MPa. A glass-filled PP having a tensile strength of 100 MPa and a flexural modulus of 9650 MPa. The flexural modulus of mineral-filled PP can be as high as about 4480 MPa, but the tensile strength does not increase much. Maintain ductility below -75°F. Impact copolymers with tensile strengths as low as 18 6 MPa and flexural moduli as low as 689 MPa are not the most recent varieties. Modern polymerization processes can produce materials that fill the gap between polypropylene and olefin rubber.
In addition to strong oxidants and non-polar solvents, PP is very resistant to chemical attack. For example, fuming nitric acid or hot concentrated sulfuric acid can degrade PP, but lower concentrations of the solution are not harmful to PP. Liquids such as gasoline, xylene, and chlorinated hydrocarbons can swell PP and become soft. Copolymers swell higher than homopolymers. After PP is removed from this kind of solvent, its size will be restored to its original state. Due to the extreme surface inertness of PP, it is difficult to print, paint, and bond on PP without flame treatment or similar techniques.
Polypropylene has a high heat of combustion and it is difficult to make flame-retardant products. However, there are several brands of flame-retardant PP sold on the market. PP is also an excellent electrical insulator with low dielectric constant and dissipation factor. Its moisture resistance is good, but it is not a good oxygen barrier material.
use
Fiber is a major market for PP. Tensile strength can be increased by as much as 15 times through stretching or orientation. Snag products include clothing, diapers, non-fabrics. Furniture leather, agricultural bags, ropes, floor covering fabrics, belts. Rugs and carpets back the village. PP can also be cast or oriented to a film. The oriented film can be used as a cigarette. Confectionery and many items of packaging materials; non-directional film for capacitors or packaging materials.
PP sheets are used for thermoformed food containers, which must comply with FDA regulations in conjunction with moisture barrier and air transfer and flavor transfer properties. The new ultra-low modulus grades can be calendered with calenders and compete with the soft vinyl resin for the market.
PP can be processed into hollow products by injection blow molding, extrusion blow molding or stretch blow molding. In order to improve blow molding and thermoforming properties, high melt strength grade PP has been developed.
On the market there is a rapid processing extrusion-coated grade PP; the excellent electrical properties of polypropylene make it suitable for use as an insulation material for telephone lines and data transmission cables.
Many different types of injection molded parts are made of PP or its impact copolymers. In the car market. Copolymers are used for interior fittings and panels, external components and accumulators, etc.; homopolymers and filling grades are used for engine enclosures or instrument panels.
Glass filler grade PP is used as a component of decorations or appliances. All unfilled PP resins can be used as writer supplies, medical devices, including disposable radiobacteric products, and low cost homopolymers and random copolymers are used. Thin-wall molded containers expand the scope of the PP traditional packaging market, such as tamper-proof sealers and dispensers.
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