PP has a lot of useful properties, but it also lacks inherent toughness, especially at temperatures below its glass transition temperature. However, by adding an impact modifier, its impact resistance can be improved. The traditional improvement is elastomer, usually ethylene propylene rubber. It is generally believed that rubber particles distributed throughout the semi-crystalline polypropylene matrix can form many stress concentrators at the interface, and can be locally deformed to prevent fracture propagation.
Impact modifiers have been added at the time of blending. Recently, the on-site synthesis of elastomer components has been of commercial importance. Moreover, a new series of impact modifiers is being promoted as an alternative to ethylene propylene rubber, Flexomer polyolefins, Exact plastic elastomers, and Insite polymers. These are olefin polymers that fill the gap between very low density polyethylene and traditional ethylene propylene elastomers.
Chemistry and performance
The isotactic PP homopolymer is polymerized from propylene under the catalysis of a Ziegler-Natta catalyst system. The ethylene-propylene rubber component is synthesized in a series of reactors or purchased in advance and then blended with the PP homopolymer in the extruder. The resulting impact polypropylene is pelletized and sold. Field-produced impact-resistant PP copolymers can accurately control their important properties by selecting the appropriate catalyst composition and reactor conditions. The catalyst composition and reactor conditions determine the crystallinity of the matrix resin, the composition and amount of rubber components, and the overall molecular weight distribution.
Impact-resistant PP is one of the lightest thermoplastics, with a density of less than one and a lower price per pound than PET, PBT, high-impact polystyrene, and ABS. In terms of specific volume, the per unit volume cost of impact PP is lower than those of the above resins and polyvinyl chloride (PVC). Only HDPE is comparable in this respect. Impact-resistant PP is usually processed at a moderate temperature in the range of 350-550°F.
Impact polypropylene copolymers have a broad spectrum of melt flow rates, typically ranging from less than 1 to about 30. The resin with the highest melt flow rate is usually made by "visbreaking" a material with a lower melt flow rate. That is, a one-step reaction is performed on the material after coming out of the reactor to lower the average molecular weight, thereby obtaining a product with a higher melt flow rate.
Impact polypropylene copolymers are highly resistant to chemical and environmental stress cracking. After treatment, the material can have excellent Izod impact strength and low Gardner impact properties. Izod impact strength range back. 5 to more than 15 feet [$8222] pounds/inch; at -40°F, the Gardner impact strength ranges from 15 to 300 inches [$8222] pounds or more.
The rubber component provides impact strength to the polypropylene, but the impact resistant polypropylene reduces stiffness and heat distortion temperature relative to the homopolymer. Filled impact polypropylene copolymers can tolerate higher temperatures without deformation. Fillers are generally glass fibers. Mica, talc and calcium carbonate. The end users of these polymers should know that for each size of product, a trade-off must be made between different melt strengths, melt flow rates, stiffness, and heat distortion temperatures.
use
The main commercial use of impact-resistant polypropylene is injection molded parts used in automobiles, household goods, and appliances. Its impact resistance, low density, coloring ability and processing performance make it an ideal material. Medium impact resin grades with higher melt flow rates have higher flow properties, a feature that is particularly useful when injecting large parts such as automotive panels.
High Impact Resistance Resins with lower melt flow rates (generally less than 2) can be converted into films with excellent puncture resistance. This film's impact resistance and steam sterilization capability make it suitable for disposable medical waste bags. Extruded sheets can be processed into large, thick parts using thermoforming methods, such as: protective panels in the automotive industry and automotive trunk linings.
The mechanism of the elastomer component to improve the impact resistance of polypropylene can induce stress whitening when the material is impacted.
Most uses are based on the dispersion of the elastomeric component in the polypropylene matrix. Based on the contrary concept, new types of bumpers are being developed. As a result, a molecular complex structure is formed.
business information
It has been estimated that the demand for impact-resistant polypropylene copolymers has been increasing at a rate of about 15% per year over the past five years. The future growth will be accelerated due to the increase in the penetration of the automotive market, because in the automotive applications, environmental considerations and recycling will become even more important.
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