Polypropylene crystallinity

FIGURE 3.4 X-ray diffraction diagrams of isotactic polypropylene crystalline forms. (From Turner-Jones, A. Aizlewood, Z.M. Beckett, D.R. Makromol. Chem., 1964, 75, 134-135. With permission.)... 

Studies on a similar group of materials - polymeric composites reinforced with sisal fibers - were conducted by Manchado et al. [35]. They analyzed the presence of different fibers, such as sisal, on crystallization of polypropylene. The composites were prepared in special chamber for mixing where the matrix was plastified at 190°C. Obtained materials were subjected to thermal analysis by DSC. The analysis of thermograms allowed for a similar finding like in Joseph s studies [34], The presence of sisal fibers, as well as other fibers used in the study, accelerated crystallization of polypropylene. This was explained by the nucleating effect of sisal filler. Also, the half-time crystallization (ti/2) decrease was observed for polypropylene with the addition of sisal fibers in comparison with unfilled polypropylene. The analysis of nonisothermal crystallization showed that the degree of polypropylene crystallinity is higher for the composites filled with sisal fibers than for unfilled polymer. 

Nucleating agents are typically added postreactor and are used primarily in injection molding application. However, they can also be used in other thermoplastic processing applications. They are generally incorporated into materials such as nylon, polypropylene, crystalline polyethylene terephthalate (PET), and other thermoplastic PET molding compounds at use levels typically below 1 percent. Incorporation is done in several ways including powder mixtures, suspensions, solutions, or in the form of a masterbatch. 

The thermoplastic polyolefin rubbers (TPOs) are physical blends of ethylene-propylene rubber with polypropylene. A form of cross-linking would appear to be provided by crystalline domains based on the polypropylene component. It would seem reasonable to presume, that in order to effectively link a rubbery matrix based on EPR with the crystalline domains based on polypropylene, that there should be some co-crystallization of regular polypropylene segments in the EPR with the polypropylene. Such linkages would then provide a means of obtaining a rubbery network linked by polypropylene crystalline structures. 

Polypropylene Crystalline, corrosion and fatigue resistance Fibers, pipe, wire covering... 

The products were fractionated by successive extraction with a series of solvents and the solubility behaviors were compared with that of the corresponding homopolymers prepared under the same conditions. A typical result of the fractionation of the PB sample is listed in Table 3. It can be seen from the result that the solubility of the PB sample is much different from that of a mixture of the two homopolymers. It is worthy to mention that the propylene homopolymer was completely dissolved after extracting by boiling toluene, but the fractions of xylene extract and residual of the PB sample still contain propylene units 38.0 and 45.2 mol%, respectively. Furthermore, the IR spectra of all the fractions except the ether-soluble fraction exhibit the absorption band of trans-1,4 polybutadiene crystalline at 770 cm and absorption band of polypropylene crystalline at 841 cm as shown in Fig.8, indicating the presence of long butadiene-butadiene sequences and long propylene-propylene sequences. 

A number of well known methods are available for the determination of crystallinity in semi-crystalline polymers. However, most of these methods are not amenable for in-spin line crystallinity measurements. A vibrational spectroscopic technique like Raman offers several distinct advantages for crystallinity measurements in the spinline (1). A calibration curve for propylene cyrstallinity was developed offline, using fibers spun, under different processing conditions, from several homo-polypropylene (hPP) and propylene-ethylene copolymers (with 5-15%E). This calibration curve was subsequently used to predict the polypropylene crystallinity, in the spin line, as a function of distance from the spinneret. The calibration model correlates the normalized intensity of the 809 cm Raman band with the DSC measured crystallinity and covers a wide crystallinity range (15-67%) with an R value of 0.989. 

Key Words Thermoplastic olefin scratch polypropylene crystallinity... 

Polyolefins. In these thermoplastic elastomers the hard component is a crystalline polyolefin, such as polyethylene or polypropylene, and the soft portion is composed of ethylene-propylene rubber. Attractive forces between the rubber and resin phases serve as labile cross-links. Some contain a chemically cross-linked rubber phase that imparts a higher degree of elasticity. 

Polymers of different tacticity have quite different properties, especially in the solid state. One of the requirements for polymer crystallinity is a high degree of microstructural regularity to enable the chains to pack in an orderly manner. Thus atactic polypropylene is a soft, tacky substance, whereas both isotactic and syndiotactic polypropylenes are highly crystalline. 

Crystallinity of polypropylene is usually determined by x-ray diffraction (21). Isotactic polymer consists of heHcal molecules, with three monomer units pet chain unit, resulting in a spacing between units of identical conformation of 0.65 nm (Fig. 2a). These molecules interact with others, or different... 

Polypropylene molecules repeatedly fold upon themselves to form lamellae, the sizes of which ate a function of the crystallisa tion conditions. Higher degrees of order are obtained upon formation of crystalline aggregates, or spheruHtes. The presence of a central crystallisation nucleus from which the lamellae radiate is clearly evident in these stmctures. Observations using cross-polarized light illustrates the characteristic Maltese cross model (Fig. 2b). The optical and mechanical properties ate a function of the size and number of spheruHtes and can be modified by nucleating agents. Crystallinity can also be inferred from thermal analysis (28) and density measurements (29). 

Thermodynamic Properties. The thermodynamic melting point for pure crystalline isotactic polypropylene obtained by the extrapolation of melting data for isothermally crystallized polymer is 185°C (35). Under normal thermal analysis conditions, commercial homopolymers have melting points in the range of 160—165°C. The heat of fusion of isotactic polypropylene has been reported as 88 J/g (21 cal/g) (36). The value of 165 18 J/g has been reported for a 100% crystalline sample (37). Heats of crystallization have been determined to be in the range of 87—92 J/g (38). 

Similarly, the random introduction by copolymerization of stericaHy incompatible repeating unit B into chains of crystalline A reduces the crystalline melting point and degree of crystallinity. If is reduced to T, crystals cannot form. Isotactic polypropylene and linear polyethylene homopolymers are each highly crystalline plastics. However, a random 65% ethylene—35% propylene copolymer of the two, poly(ethylene- (9-prop5lene) is a completely amorphous ethylene—propylene mbber (EPR). On the other hand, block copolymers of the two, poly(ethylene- -prop5iene) of the same overall composition, are highly crystalline. X-ray studies of these materials reveal both the polyethylene lattice and the isotactic polypropylene lattice, as the different blocks crystallize in thek own lattices. 

Carbon Cha.in Backbone Polymers. These polymers may be represented by (4) and considered derivatives of polyethylene, where n is the degree of polymeriza tion and R is (an alkyl group or) a functional group hydrogen (polyethylene), methyl (polypropylene), carboxyl (poly(acryhc acid)), chlorine (poly(vinyl chloride)), phenyl (polystyrene) hydroxyl (poly(vinyl alcohol)), ester (poly(vinyl acetate)), nitrile (polyacrylonitrile), vinyl (polybutadiene), etc. The functional groups and the molecular weight of the polymers, control thek properties which vary in hydrophobicity, solubiUty characteristics, glass-transition temperature, and crystallinity. 

Barrier Properties. VinyUdene chloride polymers are more impermeable to a wider variety of gases and Hquids than other polymers. This is a consequence of the combination of high density and high crystallinity in the polymer. An increase in either tends to reduce permeabiUty. A more subtle factor may be the symmetry of the polymer stmcture. It has been shown that both polyisobutylene and PVDC have unusually low permeabiUties to water compared to their monosubstituted counterparts, polypropylene and PVC (88). The values Hsted in Table 8 include estimates for the completely amorphous polymers. The estimated value for highly crystalline PVDC was obtained by extrapolating data for copolymers. 

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