Polypropylene fibers atactic

Fig. 12.21. Diagrams showing (a) irregular atactic, (b) stereoregular isotactic, and (c) stereoregular syndiotactic configurations in polypropylene polymer. (Source Ahmed, M., "Polypropylene Fibers-Science andTechnology," Textile Science and Technology, 5, 16, Elsevier Science Publishers SV, Academic Publishing Div., New York, Amsterdam (1982).) Copyright M. Ahmed. By permission.

Polypropylene (PP) is a major thermoplastic polymer. Although polypropylene did not take its position among the large volume polymers until fairly recently, it is currently the third largest thermoplastic after PVC. The delay in polypropylene development may be attributed to technical reasons related to its polymerization. Polypropylene produced by free radical initiation is mainly the atactic form. Due to its low crystallinity, it is not suitable for thermoplastic or fiber use. The turning point in polypropylene production was the development of a Ziegler-type catalyst by Natta to produce the stereoregular form (isotactic). 

Polypropylene was not developed until the 1950s when Ziegler and Natta invented coordination catalysts. The structural difference between polyethylene and polypropylene is the methyl group in the propylene unit. Its presence makes a difference because it makes possible three different polymer structures Isotactic, with all methyl groups in the same plane makes the best plastic syndiotactic, in which the methyl groups alternate in the same plane and atactic, with the methyl groups randomly in and out of the plane is soft and rubbery. Polypropylene is used as film and in many structural forms. It is also used as fibers for carpet manufacture and for thermal clothing. 

The steric configuration is extremely important in the polymer. Only isotactic polypropylene (iPP) has the properties necessary for forming fibers. The molecules are cross-linked only by Van der Waals forces, so it is important that they pack as closely as possible. The isotactic molecules form a 3, helix, as shown in Fig. 12.21,16 and exhibit a high crystallization rate. The atactic molecules, shown in the figure, do not pack well, and although the syndiotactic molecules can pack better and crystallize, this configuration is not a normal product of commonly used catalyst systems. 

Description of polypropylene in the literature is often accompanied by terms atactic, isotactic, or syndiotactic. The origin of the terms is as follows. Side methyl groups in polypropylene chains can be all on the same side of plane (as it is shown above in a very simplified manner, as in reality carbon atoms in the chain are in zig-zag stereoconfiguration), on alternate sides, or in a random arrangement with respect to plane of carbon atom chain. These forms of PP are called isotactic (fiber-forming), syndiotactic, and atactic, respectively. All these forms are related to homopolymers of polypropylene. PP used for common applications, including WPC, is atactic and will be referred to here as polypropylene. 

Stereoregular polymers also have symmetrical structures, and the helices of isotactic polymers can be close-packed to produce highly crystalline material. Iso-tactic polypropylene is crystalline and an important fiber-forming polymer, whereas the atactic form has virtually no crystalline contort and has little value as a fiber indeed, it is considerably more elastomeric in nature. 

Like HDPE, isotactic polypropylene is highly crystalline with numerous uses, including fibers for rope and carpets. Atactic polypropylene, on the other hand, is much less crystalline and has few applications. 

Synonyms Atactic polypropylene Isotactic polypropylene Polypropene PP Propathene 1-Propene, homopolymer Propene polymer Propene polymers Propylene polymer Syndiotactic polypropylene Classification Thermoplastic polymer Definition Polymer of propylene monomers three forms isotactic (fiber-forming), syndiotactic, atactic (amorphous)... 

Polypropylene is a versatile polymer used in applications from films to fibers, with a worldwide demand of over 21 million Ib. It is similar to polyethylene in structure except for the substitution of one hydrogen group with a methyl group on every other carbon. On the surface, this change would appear trivial, but this one replacement changes the symmetry of the polymer chain. This allows for the preparation of different stereoisomers, namely, syndiotactic, isotactic, and atactic chains. These configurations are shown in the introduction. 

Pig. 1. Typical microhardness values of pol3maers compared with data for metals. LDPE, low density polyethylene HDPE, high density polyethylene iPP, isotactic polypropylene CEPE, chain-extended polyethylene POM, polyoxymethylene aPS, atactic polystyrene PET, poly(ethylene terephthalate) PEN, pol3Kethylene naphthalene-2,6-dicarboxylate) CF composite, carbon-fiber composite. Hardness data of metals and alloys markedly depend on composition, degree of work-hardening, processing conditions, etc. For this reason, the values in Figure 1 should be considered as typical values rather than as absolute values. Most of the data for metals are taken from Ref 1. 

Commercially available polypropylene, in the form of pellets, films, and fibers, exists as isotactic polypropylene, and this is produced by well-controlled stereoregular head-to-tail addition polymerization reaction with Ziegler-Natta-type catalysts. Formed in this manner, isotactic polypropylene is a crystalline polymer. Commercial samples typically contain small amount of atactic and/or syndiotactic polypropylene. Furthermore, blocks with different stereoregularities are also observed. The reference spectra (Reference Spectrum 2) in the Appendix provide the IR and Raman spectra of isotactic polypropylene. 

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