Polypropylene processing characteristics

The elastomeric polypropylene materials studied in this chapter are from a class of thermoplastic elastomers since they possess the physical properties of elastomers along with the processing characteristics of thermoplastics. These materials are characterized by a low degree of crystallinity (23-26), where the crystalline regions dispersed in the amorphous matrix essentially provide physical cross-links to the amorphous elastomeric segments of the chain (19, 20). The size and distribution of these crystalline regions in the amorphous matrix thus have important influences on the mechanical properties. 

One of the most important developments in recent years has been the technology to extrude extremely fine filaments of less than 1.0 denier while maintaining all of the strength, uniformity, and processing characteristics expected by textile manufacturers and consumers. This product development began with the preparation of conjugated bicomponent filaments that were post-processed to split into ultrafine fibers. The process technology later matured and has been applied to polyesters, polyamides, polypropylene, and polyethylene and polyphenylene sulfide. 

Rubber matrices have commonly been used as a second phase to improve the toughness of brittle thermoplastic materials, such as polypropylene and polyethylene. These systems, commonly referred to as polyolefin thermoplastic elastomers (TPOs), are a special class of thermoplastic elastomers that combine the processing characteristic of plastics at elevated temperatures with the physical properties of conventional elastomers at service temperature, playing an increasingly important role in the polymer material industry. Polyolefin blends attract additional interest due to the possibility of recycling plastic wastes, avoiding the complex and expensive processes of separation of the different components. 

Keep in mind that the conceptual models of polymer structure illustrated in Figures 1.2 and 1.3 have been highly simplified. For example, it is possible for a copolymer to have a wide range of substructural elements giving rise to an impressive range of possibilities. In industrial practice, polyethylenes, including UHMWPE, are frequently copolymerized with other monomers (e.g., polypropylene) to achieve improved processing characteristics... 

Natural fiber reinforced polymer composites have attracted the attention of the research community [88] and extended to almost all the fields. Much work is done in the application of natural fiber as reinforcement in polymer composite [114]. Natural fibers are an attractive research area because they are eco-friendly, inexpensive, abundant and renewable, lightweight, have low density, high toughness, high specific properties, biodegradability and non-abrasive to processing characteristics, and lack of residues upon incineration [120, 119]. Natural fiber composites such as hemp fiber-epoxy, flax fiber-polypropylene (PP), and china reed fiber-PP are particularly attractive in automotive applications because of lower eost and lower density. 

Pulp-like olefin fibers are produced by a high pressure spurting process developed by Hercules Inc. and Solvay, Inc. Polypropylene or polyethylene is dissolved in volatile solvents at high temperature and pressure. After the solution is released, the solvent is volatilised, and the polymer expands into a highly fluffed, pulp-like product. Additives are included to modify the surface characteristics of the pulp. Uses include felted fabrics, substitution in whole or in part for wood pulp in papermaking, and replacement of asbestos in reinforcing appHcations (56). 

Polypropylene. PP is a versatile polymer, use of which continues to grow rapidly because of its excellent performance characteristics and improvements in its production economics, eg, through new high efficiency catalysts for gas-phase processes. New PP-blend formulations exhibit improved toughness, particularly at low temperatures. PP has been blended mechanically with various elastomers from a time early in its commercialisation to reduce low temperature brittleness. 

Foi example, nylon pile fabrics, exhibiting higher moisture regain, have different traction characteristics under wet and dry conditions than do polypropylene-based materials. Effects of artificial turf fabric constmction on shoe traction ate given in Table 2. Especially effective in aiding fabric surface uniformity is texturing of the pile ribbon, a process available for the two principal pile materials nylon and polypropylene. 

Carbon dioxide gas was used as a physical blowing agent to produce medium density polypropylene foam sheets using a single screw extruder. The mechanical properties of the foam were similar in the machine direction and in the transverse direction. Abetter surface finish and a lower density was produced by using a commercial wrapping film as a cap layer. The process conditions and the die design data are presented and an attempt made to relate them to the product characteristics. 4 refs. 

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