Polypropylene polymers description

R273 V. Busico and A. L. Segre, Recent Advances in the NMR Description of Polypropylene , Polym. Prepr. (Am. Chem. Soc., Div.Polym.Chem.), [computer optical disk], 2001,42,6... 

With the discovery of crystalline polypropylene in the early 1950 s, polymer stereochemical configuration was established as a property fundamental to formulating both polymer physical characteristics and mechanical behavior. Although molecular asymmetry was well understood, polymer asymmetry presented a new type of problem. Both a description and measurement of polymer asymmetry were essential for an understanding of the polymer structure. 

The problem is further complicated for vinyl polymers with their problems of stereoisomerism. The first descriptions of the conformational state of isotactic polypropylene in solution go back 25 years (178, 179, 192, 193). Corradini, Allegra, and Ganis proposed a model, still essentially valid today, according to which macromolecules possess a local helical structure analogous to that observed in the crystalline state. The helix segments are rather short, only a few monomer units, after which an inversion of the helix sense occurs, with simultaneous alteration of its direction (Figure 15). As a whole this disordered con-... 

Description The process, with a combination of the most advanced high-yield and high-stereospecificity catalyst, is a nonsolvent, non-deashing process, eliminating atactic polymers and catalyst residue removal. The process can produce various grades of polypropylene... 

Most of the available commercial microporous membranes such as polysulfone, polyethersulfone, polyamide, cellulose, polyethylene, polypropylene, and polyvinylidene difluoride are prepared by phase inversion processes. The concept of phase inversion in membrane formation was introduced by Resting [75] and can be defined as follows a homogeneous polymer solution is transformed into a two-phase system in which a solidified polymer-rich phase forms the continuous membrane matrix and the polymer lean phase fills the pores. A detailed description of the phase inversion process is beyond the scope of this section as it was widely discussed in Chapters 1 and 2 nevertheless a short introduction of this process will be presented. 

Accurate description of barrier films and complex barrier structures, of course, requires information about the composition and partial pressure dependence of penetrant permeabilities in each of the constituent materials in the barrier structure. As illustrated in Fig. 2 (a-d), depending upon the penetrant and polymer considered, the permeability may be a function of the partial pressure of the penetrant in contact with the barrier layer (15). For gases at low and intermediate pressures, behaviors shown in Fig. 2a-c are most common. The constant permeability in Fig.2a is seen for many fixed gases in rubbery polymers, while the response in Fig. 2b is typical of a simple plasticizing response for a more soluble penetrant in a rubbery polymer. Polyethylene and polypropylene containers are expected to show upwardly inflecting permeability responses like that in Fig. 2b as the penetrant activity in a vapor or liquid phase increases for strongly interacting flavor or aroma components such as d-limonene which are present in fruit juices. 

In the following sections of this chapter, the catalytic conversion of individual plastics (polyethylene, polypropylene and polystyrene) is first reviewed, followed by a description of the processes developed for the catalytic cracking of plastic and rubber mixtures. Finally, methods based on a combination of thermal and catalytic treatments are considered. However, taking into account that the key factor in the catalytic conversion of plastic wastes is the catalyst itself, we will first describe the main properties of the most widely used catalytic systems for the degradation of polymers. 

The adhesive transfer of organic plastics has some special features of it own. Makinson and Tabor [24] observed that polytetrafluoroethylene sliding on glass left transferred material on the counter surface in the form of lumps, ribbons, sheets or very thin films, depending on the rubbing conditions. Pooley and Tabor [25], who studied the transfer process more intensively, also reported the behavior of other polymers such as fluorocarbon copolymers, polyethylene, polypropylene, polystyrene, polymethylmethacrylate and polyvinyl chloride. Descriptions of transfer in relation to wear were reported for PTFE by Tanaka tt ai. [20] and for polyethylene by Miller a.1. [21]... 

It is useful to separate the discussion into processes for polyethylene and polypropylene, as the requirements for these two polymers are different and have led to similar, but by no means identical, processes. A short discussion on the various reactor configurations will be presented first, followed by descriptions on how each reactor configuration is used in different polymerization processes throughout the world. Also listed are a few keys references at the end of the chapter for further reading [72-85]. Finally, the chapter will be concluded with a few considerations on the mathematical modeling of industrial olefin polymerization reactors. 

Presence of TCL changes the properties of crystalline matrix. Transcrystallization of isotactic polypropylene in the presence of different fibers has been thoroughly analyzed. Gray as the first one provided detailed description of isotactic polypropylene behavior in the presence of wood fibers using polarized light microscopy. He observed that when melted polymer is cooled down, it crystallizes in spherulite forms in nonisothermal and isothermal conditions, creating additionally a TCL. 

Implications of Metallocene Catalyst Structure on Polypropylene Structure. The previous section gave a brief description of the various types of metallocenes. In this section a general relationship between metallocene structure and type of polypropylene produced will be made. It is important to note that these are generalizations. While the stereochemistry of the metallocene plays an important role in mechanism of monomer insertion and ultimately the stereo- and regiospecificity of the polymer, the substituents and location of the substituents on the carhocyclic tt-ligands also effect the microstructure of the polymer. 

There can be many variations on this general description because the substituents bonded to the four carbons in the double bonds can be quite variable. For example, all four bonds may be to hydrogens (CH2 = CHj, ethene, or ethylene), there may be one methyl group attached (CH2 = CHCH3, propene, or propylene), there may be a chlorine attached (CH2 = CHCl, vinyl chloride), and so on. Polyethylene, polypropylene, polyvinyl chloride (PVC), and many other addition polymers have been manufactured in mass quantities by this approach and used for many consumer products. Figure 14.33 lists some of the addition polymers that have been manufactured. Also listed are the individual alkene units (monomers) that are in these polymers and some of the uses of each. 

Description of Polymers. A wide variety of crystalline poly-epichlorohydrins were examined in this study. Some typical examples are given in Table 1. All the polymers were prepared from racemic monomer, but some of them were prepared using initiators of the type previously reported to give partially optically active polymers (2-6). The polymers were shown to be isotactic by comparison of the observed d-spacings with those reported in the literature for crystalline isotactic polyepichloro-hydrin (8, 9). Also included in Table 1 for comparison are some data on polypropylene oxides prepared with different initiators. 

The inherent properties of polymers of the poly isobutylene family, particularly the chemical inertness, age and heat resistance, long-lasting tack, flexibility at low temperatures, and the favorable FDA position on selected grades, make these products commercially attractive in a variety of pressure-sensitive and other adhesives, in automotive and architectural sealants, and in coatings. An added dimension is achieved in the blendability of the polyisobutylene polymers with each other and with other adhesive polymers such as natural rubber, styrene-butadiene rubber, EVA, low molecular weight polyethylene, and amorphous polypropylene to achieve specific properties. They can, for example, be blended with the highly unsaturated elastomers to enhance age and chemical resistance. A description of poly isobutylene polymer family use in adhesive and sealant applications follows. 

In the polypropylene catalyst shown in Figure 14, an isotactic-atactic block copolymer can be formed by rotation of one ring relative to the Zr-centroid axis. (For descriptions of polymer stereochemistry, see Fig. 9.) The isospecific rac rotomer of the catalyst gives rise to the isotactic block, while the aspecific meso form gives rise to the atactic block (Fig. 14). Using UFF (4) and RFF (85,86) (as weU as ab initio methods and DFT), the workers were able to confirm experimental evidence (89,90) that the indenyl substituent, R in Figure 14, could influence the equilibrium between the rac and meso rotomers. Using RFF the workers were able to successfully predict the relative amount of isotactic and atactic blocks in the polymer and to correlate that with R. 

The molecular configuration indicating the spatial arrangement of atoms and the existence of substituted carbon atoms is a decisive factor for the description of material oxidation resistance. The two kinds of polypropylene, isotactic and syn-diotactic present different responses to the degrading activities of environmental aggressive factors (Fig. 16) [73]. The certain ordering degrees of molecules in our examples lead to the decrease in the susceptibility of polymer to oxidation. 

Polymers used for the manufacture of commodity fibres are also used in the production of multiconstituent fibres, sometimes in conjunction with a modified commodity polymer or a specialty polymer. These fibres consist of two or more polymers forming separate phases. There are essentially two types of such fibres. In the first type, each polymer occupies a discrete region of the fibre cross-section e.g. sheath/core-type fibres, where the sheath is a lower melting homopolymer or copolymer such fibres are used for thermal bonding). In the second type, the components are dispersed in a random way throughout the fibre, typically forming a matrix/fibril arrangement e.g. incorporation of dyeable polymer fibrils into polypropylene which forms the matrix). Detailed description of these fibres can be found elsewhere. 

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