Additives for isotactic polypropylene

Commercial grades of polypropylene may be blended with a number of other additives. Of these the most important are  

In the early stages of development of polypropylene rubbers, particularly butyl rubber, were used to reduce the brittleness of polypropylene. Their use declined for some years with the development of the polypropylene copolymers but interest was greatly renewed in the 1970s. This interest has been centred largely around the ethylene-propylene rubbers which are reasonably compatible in all proportions with polypropylene. At first the main interest was with blends in which the rubber content exceeded 50% of the blend and such materials have been designated as thermoplastic polyolefin elastomers (discussed in Section 11.9.1). There is also increasing interest in compounds with less than 50% rubber, often referred to as elastomer-modified thermoplastics. It is of interest to note 

In general the selection of pigments for polypropylene follows the same considerations as for polyethylene. Because of the higher processing temperatures and the lesser resistance to oxidation, selection does, however, require rather more care. 

To improve the resistance to ultraviolet light carbon black is often useful as a light screen. Its use in fibres and films is clearly very restricted and in these instances ultraviolet absorbers and/or quenching agents are used. Recent developments include the greater use of hindered amine and nickel compounds. 

As with most polyolefins and polydienes the presence of copper has a strong adverse effect and most antioxidants are relatively ineffective. In these instances quite good results may be achieved by the use of 1% of a 50 50 phenol alkane-dilauryl thiodiproprionate blend instead of the 0.1-0.2% of antioxidants more commonly used in polypropylene. 

260 Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers 11.1.4 Additives for isotactic polypropylene 

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Ethylene-propylene rubbers are suitable additives for isotactic polypropylene. They can be added to the matrix by melt-mixing or by block copolymerization, using Ziegler-Natta catalysts (the copolymer is made by adding ethylene, as a second monomer, during the final stages of PP polymerization). 

C. Marco, M.A. G6mez, G. Ellis, and J.M. Arribas, Highly efficient nucleating additive for isotactic polypropylene studied by differential scanning calorimetry. Journal of Applied Polymer Science, 84(9) 1669-1679, March 2002. 

The thermal and nncleation behavior of a-PVDF depends on its microstmcture. Samples with higher head-to-head inversions display a significarrt memory effect." Their initial spherulitic morphology is maintained in snccessive recrystaUizatiorrs." The nucleation density can be increased by addition of nncleating agents PTFE or flavanthrone." The nuclei densities in PVDF is nearly two orders of magnitude lower than that for isotactic polypropylene." ... 

Consequently, the Phillips Petroleum scientists isolated crystalline polypropylene between October 9,1951 and April 16,1952. Although, initially, the United States Patent and Trademark Office awarded the composition of matter patent for isotactic polypropylene (prepared with a Ti-based catalyst composition) to Montecatini on February 6, 1973 (U.S. Patent 3,715,344), the Federal District Court of Delaware reversed the United States Patent and Trademark Office decision on January 11, 1980, and awarded the composition of matter patent to Phillips Petroleum based on the earlier research carried out by Hogan and Banks (See Chapter 3 of this book for additional details on the historical origins of polyethylene and polypropylene.)... 

One additional aspect of the Sailor and Hogan paper is important to highlight as it pertains to the litigation process that took place in the Federal District Court (Delaware) in order to resolve the dispute in awarding the composition of matter patent for isotactic polypropylene to Phillips Petroleum on January 11, 1980. 

Voyiatzis and Andrikopoulos discuss adding an orientation-sensitive, resonant Raman additive to a polymer mixture prior to drawing in order to calculate molecular orientation continuously. The approach was tested with poly(vinyl chloride) (PVC), isotactic polypropylene (iPP), and poly(vinylidene fluoride) (PVF2). The ratio of a band from the additive to the orientationally insensitive CH2 bending mode at 1435 cm-1 in the polymer was computed. While the addition of such an additive is unreasonable for many industrial processes, the authors note a favorable alternative for industrial PVC samples. 

The chemical structure of a polymer determines whether it will be crystalline or amorphous in the solid state. Both tacticity (i.e., syndio-tactic or isotactic) and geometric isomerism (i.e., trans configuration) favor crystallinity. In general, tactic polymers with their more stereoregular chain structure are more likely to be crystalline than their atactic counterparts. For example, isotactic polypropylene is crystalline, whereas commercial-grade atactic polypropylene is amorphous. Also, cis-pol3nsoprene is amorphous, whereas the more easily packed rans-poly-isoprene is crystalline. In addition to symmetrical chain structures that allow close packing of polymer molecules into crystalline lamellae, specific interactions between chains that favor molecular orientation, favor crystallinity. For example, crystallinity in nylon is enhanced because of... 

Fig. 3.3.5 H decoupled C spectra of isotactic polypropylene for different spinning frequencies o>r =l7tv and orientation angles i/r of the rotation axis, (a) Static sample. The wideline resonances of the different carbons overlap, (b) MAS spectrum with fast sample spinning. Narrow signals are observed at the isotropic chemical shifts only, (c) MAS spectrum with slow sample spinning. In addition to the centre line, sideband signals are observed at seperations naiR from centre lines, (d) OMAS spectrum with fast sample spinning. The orientation of the axis deviates from the magic angle. Each resonance forms a powder spectrum with reduced width, which can serve as a protractor (cf Fig. 3.1.3). Adapted from [Blu4] with permission from Wiley-VCH.

A number of block copolymers prepared with Ziegler-Natta catalysts have been reported however, in most cases the compositions may include significant amounts of homopolymer. The Ziegler-Natta method appears to be inferior to anionic polymerization for synthesizing carefully tailored block copolymers. Nevertheless, bock copolymers of ethylene and propylene (Eastman Kodak s Pofyallomers) have been commercialized. Unlike the elastomeric random copolymers of ethylene and propylene, these are high-impact plastics exhibiting crystallinity characteristics of both isotactic polypropylene and linear polyethylene. They also contain homopolymers in addition to block copolymers. 

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