Polypropylene (PP) as a thermoplastic polymer began commercial production in 1957 and is the first of a stereoregular polymer. Its historical significance is even more evident in that it has been the fastest-growing major thermoplastic, with its total world output reaching 24 billion pounds in 1991. It has a very wide range of applications in the field of thermoplastics, especially in fiber and filament, film extrusion, injection molding and other aspects.
Chemistry and nature
PP is a metal-organic stereoregular catalyst (Ziegler-Natta type), and the propylene monomer is synthesized under controlled temperature and pressure conditions. Due to different catalysts and polymerization processes used, the molecular structure of the resulting polymer has three different types of stereochemical structures, and the number is also different. These three structures refer to isotactic polymers, syndiotactic polymers and atactic polymers. In isotactic polypropylene (the most common commercial form), the methyl radicals are on the same side of the polymer backbone, and this structure is easily crystallized. The isotactic form of crystallinity gives it good resistance to solvents and heat. The catalyst technology used during the first decade has minimized the generation of non-isochronous isomers, eliminating the need for separation of non-valued random components and simplifying production steps.
There are two main processes for producing polypropylene: one is a gas phase process; the other is a liquid propylene slurry process. In addition, there are old-fashioned slurry process installations that use a liquid saturated hydrocarbon as the reaction medium. The properties of a typical isotactic polypropylene homopolymer are shown in Table 1.
In comparison, high density and low density polyethylene have higher densities, lower melting points and lower flexural modulus, ie, stiffness. These performance differences lead to different end uses. Stiffness and easy orientation make polypropylene homopolymers suitable for making various fibers and for extending tapes, and their higher heat resistance makes them useful for making hard high-pressure containers and appliances and molded parts of automobiles.
The main factors affecting the processing and physical properties of polypropylene homopolymers include: molecular weight (usually expressed in terms of flow rate); molecular weight distribution (abbreviated MWO); stereoregularity and auxiliaries. The average molecular weight of polypropylene ranges from about 200,000 to 600,000. The molecular weight distribution is generally expressed as the ratio of the weight average molecular weight (Mww) of the polymer to the number average molecular weight (Mnn), Mww/Mnn. This formula is also called polydispersity index.
The molecular weight distribution of a polymer has a significant influence on its processability and end-use performance. This is because the molten polypropylene is sensitive to shear, that is, its apparent viscosity decreases when the applied pressure increases. Polypropylenes with a wide range of molecular weight distribution are more shear sensitive than narrower distributions, so materials with a wide range of molecular weight distributions are easier to process during injection molding. Certain specific uses, especially fibers, require a narrow range of molecular weight distributions. The molecular weight distribution is related to the catalyst system and the polymerization process.
Common peroxides are chemically cracked in the extrusion process behind the reactor, narrowing the molecular weight distribution. This process is called controlling the rheology (CR) process.
Compared with polyethylene, isotactic polypropylene is more susceptible to oxidative degradation by light and heat. Under normal processing and end-use conditions, polypropylene undergoes random chain scission, resulting in reduced molecular weight and increased flow rates.
All commercial polypropylenes contain stabilizers to protect the material during processing and provide satisfactory end-use performance. For special applications, other additives must be added in addition to antioxidants and UV inhibitors. For example, lubricants and antiblocking agents are added to the film formulation to reduce the coefficient of friction and prevent the film from sticking to itself. Antistatic is added to the packaging material to eliminate static charges. In order to increase transparency or shorten the model cycle, nucleating agents are needed.
Homopolymer resins are usually classified by flow rate and end use. The flow rate depends on both the average molecular weight and the molecular weight distribution.
Some special applications require flow rates up to 400 dg/min, while common commodity homopolymers have flow rates in the range of 0.5 to 50 dg/min. The flow rate is usually the most important factor in determining the processing characteristics.
Processing and application
The excellent flow properties of polypropylene, its wide range of flow rates, and other unique polymer properties combine to give it excellent processing properties. The lower flow rate can meet the processing requirements of the extrusion tape, ribbon filaments and monofilaments, as well as the tensile strength and low elongation of the finished product, while maintaining sufficient transverse integrity, leading to the winder guide Cracking and dust flying conditions are minimal. To counteract their low transverse strength and fracture propensity (fibrillation), more oriented films to fiber products,
Table 1 Typical properties of isotactic polypropylene homopolymers
Density (D-792)a 0.90 to 0.91
Flexural modulus, MPa (D-790) 1241ï½ž1516
Melting temperature, Â°F 320~340
Izod impact strength with notches, feet [$8222] pounds/inch (D-256) 0.5-1.0
Note a: ASTM test method
Source: Eastman Chemical Company
Such as: coarse denier textiles, string and rope, usually require a flow rate in the range of 7 to 20. The decorative strip product containing blowing agent is extruded from polypropylene with a flow rate close to 10 so that the melt strength and directional ability can be properly balanced. This polymer is moderately oriented and produces a smooth, satin-like surface effect with sufficient lateral strength.
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