Polypropylene , isotactic

Isotactic polypropylene is a stereoregular polymer with all methyl groups pointing in the same direction when the backbone is stretched. In syndiotactic polypropylene the methyl groups alternate along the chain. The atactic polymer lacks regularity. All three forms retain a perfect head-to-tail structure. 

Nine million tons (1990) of polypropylene are produced annually. In 1973 this was only 3 million tons. 

The similar, older slurry process uses a less active catalyst. The monomer is dissolved in isooctane, the titanium catalyst and aluminium cocatalyst are added and this mixture is fed to the reactor which is maintained at 70°C. The inorganic corrosive (Cl) residues are removed in a washing step with alcohols. The atactic material is removed by extraction. A third process employs propene as the liquid in combination with a high activity catalyst. The Himont Spheripol process, which uses spherical catalyst particles, gives spherical polymer beads of millimetre size that need no extrusion for certain purposes. A more recent development is the gas-phase polymerization using an agitated bed. All processes are continuous processes, where the product is continuously removed from the reactor. Over the years we have seen a reduction of the number of process steps. The process costs are very low nowadays, propene feed costs amounting to more than 60% of the total cost. 

The productivity of the catalyst has been the topic of continuous research. Table 2.2 gives a review of development of the catalyst in the last three decades. 

In addition to the molecular properties, the macroscopic growth of the polymer particles is important they should be sufficiently large and dense, 

In one of the first attempts to apply rapid-scanning FTIR spectroscopy to the dynamic study of deformation phenomena in polymers isotactic polypropylene has been investigated by this technique 

Principally, in the IR spectrum of crystalline, isotactic polypropylene the absmption bands can be assigned to A- or E-mode vibrations with transition moment directions parallel and perpendicular to the 3i-helix axis, respectively 

The IR spectroscopic determination of the state of order and regularity in semicrystalline isotactic polypropylene has been discussed by numerous authors 28- 33) 

In Fig. 19a the FTIR polarization spectra taken during uniaxial elcm tion under the specified conditions are shown separately for the parallel and perpendicular polarization directions as a function of strain. With increasing elongation several absorption bands exhibit significant parallel and perpendicular dichrrasm. The 

Tdfc 1. Orientation parameters of isotactic polypropylene derived from FTIR polarization spectra recorded during uniaxial deformation 

A higher nucleation density also tends to enhance not only clarity but, in addition, generally improves mechanical properties - another desired side effect of higher nucleation. 

Among other plastics, i-PP is a commodity polymer produced and used in large quantities in packaging. The main reasons for the success of i-PP are its quite good price/performance ratio, its excellent mechanical properties and suitable optical characteristics [16]. The huge consumption of this polymer makes its recycling strategically crucial for the environmental policy of industry [4]. 

Since the invention of polypropylene (PP) (1954), which was rewarded with a Nobel Prize to Natta and Ziegler, the nse of polymers in everyday life has steadily increased. It still is, dne to snccessful research aimed at prodncing products with desired mechanical and chemical properties, tailored for a specific nse (food containers, parts of car bodies, parts of prostheses for the hnman body, components of composite material and so on). i-PP has grown to a commodity polymer with nnmerous grades for specific end uses. 

At present, the physical properties of PP shonld be tailored to specific requirements with respect to processing and strnctnre. 

Moreover, when i-PP is quenched from the melt to low temperature, a mesomorphic (also known as smectic) phase, a structural order which is intermediate between the amorphous and crystalline phase, is obtained. Although the mesomorphic phase [22] has been mentioned in the literature for a long time (sometimes denoted with different names such as smectic [23], paracrystal [24] or glass [25]) and is usually obtained dnring i-PP processing, the related crystallisation kinetics are rarely investigated with tailored experiments. 

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Snetivy D and Vancso G J 1994 Atomic force microscopy of polymer crystals 7. Chain packing, disorder and imaging of methyl groups in oriented isotactic polypropylene Po/yme/ 35 461... 

Term N J, Fairclough P A, Towns-Andrews E, Komanshek B U, Young R J and Ryan A J 1998 Density fluctuations the nucleation event in isotactic polypropylene crystallization Polymer 29 2381- 5... 

FIGURE 7 16 Poly mers of propene The mam chain IS shown in a zigzag conformation Every other carbon bears a methyl sub stituent and is a chirality center (a) All the methyl groups are on the same side of the carbon chain in isotactic polypropylene (b) Methyl groups alternate from one side to the other in syndiotactic polypropy lene (c) The spatial orienta tion of the methyl groups IS random in atactic polypropylene... 

Construct such a model for isotactic polypropylene. Estimate the volume of an isobutyl group on the scale of your model and examine whether interference between successive substituents would occur if this were the R group present. 

The majority of spunbonded fabrics are based on isotactic polypropylene and polyester (Table 1). Small quantities are made from nylon-6,6 and a growing percentage from high density polyethylene. Table 3 illustrates the basic characteristics of fibers made from different base polymers. Although some interest has been seen in the use of linear low density polyethylene (LLDPE) as a base polymer, largely because of potential increases in the softness of the final fabric (9), economic factors continue to favor polypropylene (see OlefinPOLYMERS, POLYPROPYLENE). 

Fig. 2. (a) Chain conformation of isotactic polypropylene, and (b) model of a polypropylene spheruHte. 

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). 

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... 

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. 

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. 

Modem catalysts produce a much higher percentage of isotactic polypropylene than ia the past, eliminating the need for a cosdy extraction step to remove an atactic fraction. Yields ate high enough (>10,000 g polymer/g catalyst) so that a catalyst removal (de-ashing) step is no longer requited. 

This conceptual link extends to surfaces that are not so obviously similar in stmcture to molecular species. For example, the early Ziegler catalysts for polymerization of propylene were a-TiCl. Today, supported Ti complexes are used instead (26,57). These catalysts are selective for stereospecific polymerization, giving high yields of isotactic polypropylene from propylene. The catalytic sites are beheved to be located at the edges of TiCl crystals. The surface stmctures have been inferred to incorporate anion vacancies that is, sites where CL ions are not present and where TL" ions are exposed (66). These cations exist in octahedral surroundings, The polymerization has been explained by a mechanism whereby the growing polymer chain and an adsorbed propylene bonded cis to it on the surface undergo an insertion reaction (67). In this respect, there is no essential difference between the explanation of the surface catalyzed polymerization and that catalyzed in solution. 

Polyolefins. The most common polyolefin used to prepare composites is polypropylene [9003-07-0] a commodity polymer that has been in commercial production for almost 40 years following its controlled polymerisation by Natta in 1954 (5). Natta used a Ziegler catalyst (6) consisting of titanium tetrachloride and an aluminum alkyl to produce isotactic polypropylene directly from propylene ... 

Blends of isobutylene polymers with thermoplastic resins are used for toughening these compounds. High density polyethylene and isotactic polypropylene are often modified with 5 to 30 wt % polyisobutylene. At higher elastomer concentration the blends of butyl-type polymers with polyolefins become more mbbery in nature, and these compositions are used as thermoplastic elastomers (98). In some cases, a halobutyl phase is cross-linked as it is dispersed in the polyolefin to produce a highly elastic compound that is processible in thermoplastic mol ding equipment (99) (see Elastomers, synthetic-thermoplastic). ... 

The conformation adopted by a molecule in the crystalline structure will also affect the density. Whereas polyethylene adopts a planar zigzag conformation, because of steric factors a polypropylene molecule adopts a helical conformation in the crystalline zone. This requires somewhat more space and isotactic polypropylene has a lower density than polyethylene. 

Another technical development is that of high impact isotactic polypropylene in which rubber droplets are produced in situ during the polymerisation stage. After propylene homopolymerisation ethylene is added to the reacting mass in a... 

Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilyleneto (2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

Although very similar to high-density polyethylene, isotactic polypropylene differs from the former in a number of respects of which the following are among the most important ... 

Isotactic polypropylenes produced by metallocene catalysis are now being produced by a number of different manufacturers and because different systems are used there is some variation in properties. Typically however such materials have similar density, hardness and tensile strength to conventional homopolymers but differ in having... 

Table 11.6 Comparison of some properties of syndiotactic and isotactic polypropylenes...

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

Atactic polypropylene may be obtained either as a by-produet of the manufacture of isotactic polypropylene or by specific processes designed for its direct production. 

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. 

High molecular weight atactic polyropylene is now available (Rexene-Huntsman). This is miscible with isotactic polypropylene in any proportion to give transparent blends of interest in packaging applications. 

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