Isotactic polypropylene temperature

The value of the glass-transition temperature, T, is dependent on the stereoregularity of the polymer, its molecular weight, and the measurement techniques used. Transition temperatures from —13 to 0°C ate reported for isotactic polypropylene, and —18 to 5°C for atactic (39,40). 

Eastman Chemical has utilized a unique, high temperature solution process for propylene polymerization. Polymerization temperatures are maintained above 150°C to prevent precipitation of the isotactic polypropylene product in the hydrocarbon solvent. At these temperatures, the high rate of polymerization decreases rapidly, requiring low residence times (127). Stereoregularity is also adversely affected by high temperatures. Consequentiy, the... 

In the early 1950s, Ziegler observed that certain heterogeneous catalysts based on transition metals polymerized ethylene to a linear, high density material at modest pressures and temperatures. Natta showed that these catalysts also could produce highly stereospecific poly-a-olefins, notably isotactic polypropylene, and polydienes. They shared the 1963 Nobel Prize in chemistry for their work. 

In appearance and on handling the material is somewhat intermediate between a wax and a rubber. It is also semi-tacky. Like isotactic polypropylene it is attacked by oxygen but unlike the isotactic material it swells extensively in aliphatic and aromatic hydrocarbons at room temperature. It is also compatible with mineral fillers, bitumens and many resins. 

Specialty waxes include polar waxes for more polar adhesive systems. Examples would be castor wax (triglyceride of 12-hydroxy stearic acid) or Paracin wax N- 2 hydroxy ethyl)-12-hydroxy stearamide) which are used in polyester, polyamide, or with high VA EVA copolymer-based systems. Other common polar waxes are maleated polyethylenes, which are used to improve the specific adhesion of polyethylene-based adhesives, and low molecular weight ethylene copolymers with vinyl acetate or acrylic acid, which are used to improve low temperature adhesion. High melting point isotactic polypropylene wax (7 155°C) and highly refined paraffin wax (7,n 83°C) are used where maximum heat resistance is critical. Needless to say, these specialty waxes also command a premium price, ranging from 2 to 5 times that of conventional paraffin wax. 

The effect of thermal aging on polyethylene and isotactic polypropylene have been studied by Konar et al. [49]. They used contact angle, contact angle hysteresis, and XPS to characterize the modified surfaces of the polymers. Hysteresis increased with aging temperature. In the case of polyethylene, thermal aging led to a significant increase in adhesion strength of polyethylene with aluminium, but the increase in the case of polypropylene was much less marked. 

The term tactidty refers to the configuration of polymer chains when their constituent monomer residues contain a steric center. Figure 1.8 illustrates the three principal classes of tacticity as exemplified by polypropylene. In isotactic polypropylene, the methyl groups are all positioned on the same side of the chain, as shown in Fig. 1.8 a). In syndiotactic polypropylene, the methyl groups alternate from one side to the other, as shown in Fig. 1.8 b). Random placement of the methyl groups results in atactic polypropylene, which is shown in Fig. 1.8 c). We can readily observe the effects of tacticity on the properties of polypropylene isotactic polypropylene is hard and stiff at room temperature, syndiotactic polypropylene is soft and flexible, and atactic polypropylene is soft and rubbery. 

Figure 8. Relaxation strength versus crystallinity in isotactic polypropylenes of Figure 7. Unrelaxed low temperature modulus Q)> relaxed y modulus (A), relaxed 0 modulus (0), relaxed a modulus ( >). Filled symbols are for the isothermally crystallized (68%) specimen. 

A number of systems which in polymer literature are normally referred to as mesophases are obtained under kinetic control. Examples are the smectic phase of isotactic polypropylene [18,19], mesomorphic syndiotac-tic polypropylene [20-22], mesomorphic PET [23,24], and other instances where intermediate degrees of order result after quenching polymers from the melt to temperatures often close to Tg. In these cases disorder is plausibly more static than in bundles close to T0 and these phases usually crystallize upon heating to an appropriate temperature in the stable crystal phases. 

The mass fraction crystallinity of molded PHB samples is typically around 60%. As shown in Table 3, PHB resembles isotactic polypropylene (iPP) with respect to melting temperature (175-180°C), Young s modulus (3.5-4 GPa) and the tensile strength (40 MPa). In addition, the crystallinity of iPP is approximately 65% [18]. Accordingly, the fracture behavior of PHB may be anticipated to be tough at room temperature. Molded PHB samples do indeed show ductile behavior, but over a period of several days at ambient conditions, they slowly become more brittle [82, 85, 86]. Consequently, the elongation to break of the ultimate PHB (3-8%) is markedly lower than that of iPP (400%). 

The isotacticities and activities achieved with nonbridged metallocene catalyst precursors were low. Partially isotactic polypropylene has been obtained by using a catalyst system of unbridged (non-ansa type) metallocenes at low temperatures [65]. A chiral zirconocene complex such as rac-ZrCl2(C5H4 CHMePh)2 (125) is the catalyst component for the isospecific polymerization of propylene (mmmm 0.60, 35% of type 1 and 65% of type 2 in Scheme Y) [161]. More bulky metallocene such as bis(l-methylfluorenyl)zirconium dichloride (126) together with MAO polymerized propylene to isotactic polypropylene in a temperature range between 40 and 70°C [162]. 

Fig. 2.22. Dependence of the elastic modulus E and the mechanical loss factor 6 on temperature for various polymers. Curves 1 elastomer (statistical copolymer of ethylene and propylene) curves 2 isotactic polypropylene (semicrystalline)...

In contrast, the curve E2 (isotactic polypropylene) is characteristic for partially crystalline polymers. The modulus is three decades higher than in an elastomer. At the glass transition temperature [T (2) 0 °C] the decay of the E modulus is small it does not drop to the lower level of the molten state before the melting point. 

Zannetti, R., Celotti, G. C., Fichera, A. and Francesconi, R. The structural effects of annealing time and temperature on the paracrystal-crystal transition in isotactic polypropylene. Makromol. Chemie 128, 137 (1969)... 

Recently, Ewen 1291 has found that the soluble Cp2Ti(Ph)2 (Ph = phenyl)/methyl-alumoxane catalyst produces isotactic polypropylene ([m] = 0.83-0.85) at temperatures below —30 °C. The polymerization was carried out in the temperature range of —85 to 50 °C. The highest activity was achieved at —45 °C, and the isotactic (meso) dyad fraction [m] of the produced polypropylene decreased from 0.85 to 0.50 with an increase in the polymerization temperature. Figure 28 shows the time dependence of polymer yield, Mn, number of polymer chain produced per titanium atom, [N], and mjmn, obtained at —60 °C. The yield of polymer is almost proportional to time, but Rln increases to a constant value. The number of polymer chains [N] increases with time, and the value of Mw/Mn increases toward 2.0, indicating that chain transfer... 

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