Polymer polypropylene 1-20 INDEX

Doi et al. studied the effect of different aluminium alkyls on the polydispersity of syndiotactic polypropylene obtained with the V(acac)3-alkyl aluminium halide soluble catalytic system, at temperatures below —65 °C. These authors found that, by varying the type of aluminum alkyl not only the propagation and transfer rates are changed, but also the polymer polydispersity index decreases in the following order ... 

Artificial surfaces must be resistant to cigarette bums, vandaUsm, and other harm. Fire resistance is most critically evaluated by the NBS flooring radiant panel test (10). In this test, a gas-fired panel maintains a heat flux, impinging on the sample to be tested, between 1.1 W/cm at one end and 0.1 W/cm at the other. The result of the bum is reported as the flux needed to sustain flame propagation in the sample. Higher values denote greater resistance to burning results depend on material and surface constmction. Polypropylene turf materials are characterized by critical radiant flux indexes which are considerably lower than those for nylon and acryflc polymers (qv) (11). 

The properties of commercial polypropylene vary widely according to the percentage of crystalline isotactic polymer and the degree of polymerization. Polypropylenes with a 99% isotactic index are currently produced. 

Polypropylene is characterised by isotactic index which is percentage of polymer not dissolving in boiling, n-haptane Commercial polypropylenes are having isotactic index of 95-98 per cent. 

Oilfields in the North Sea provide some of the harshest environments for polymers, coupled with a requirement for reliability. Many environmental tests have therefore been performed to demonstrate the fitness-for-purpose of the materials and the products before they are put into service. Of recent examples [33-35], a complete test rig has been set up to test 250-300 mm diameter pipes, made of steel with a polypropylene jacket for thermal insulation and corrosion protection, with a design temperature of 140 °C, internal pressures of up to 50 MPa (500 bar) and a water depth of 350 m (external pressure 3.5 MPa or 35 bar). In the test rig the oil filled pipes are maintained at 140 °C in constantly renewed sea water at a pressure of 30 bar. Tests last for 3 years and after 2 years there have been no significant changes in melt flow index or mechanical properties. A separate programme was established for the selection of materials for the internal sheath of pipelines, whose purpose is to contain the oil and protect the main steel armour windings. Environmental ageing was performed first (immersion in oil, sea water and acid) and followed by mechanical tests as well as specialised tests (rapid gas decompression, methane permeability) related to the application. Creep was measured separately. 

FIGURE 4.2 Oxygen and nitrous oxide index of polypropylene-chloroparaffin mixtures, 95-5 wt%, with added antimony trioxide. (From Costa, L. et al., Polym. Degrad. Stabil., 14(2), 116, 1986. With permission.)... 

Improvements of clarity in transparent materials can be obtained in a variety of ways. If unplasticized poly(vinyl chloride) includes an impact modifier with refractive index matching that of the polymer it is possible to use thinner sections and so increase clarity, rates of output, and gloss. With polyolefins, similar results may be achieved by including a nucleating agent to accelerate crystallization even polypropylene, which normally is translucent, thus can be made in an almost clear form. 

Polyolefins have a central position in the marketplace of synthetic polymers, in terms of annual production volume . In the 1960s, Natta and coworkers reported that syndio-enriched polypropylene could be prepared by polymerization of propylene at —78°C in the presence of a mixture of vanadium tetrachloride and Et2AlCl . The molecular weight increased steadily for 25 h, and the polydispersity index (1.4 < My /M < 1.9) was moderately low °. This was the first hint of a possible control on this type of coordinative polymerization. 

The kinetic curve would then be the result of two curves, one representing the 1st order decay attributed to isospecific polymerization centers, and the other representing a stationary state attributed to the less stereospecific centers. This expression can be credited with taking into consideration a stationary state and, furthermore, it is in agreement with the inverse correlation between productivity and isotacticity of the polymer found experimentally. In fact, assuming Is to be the isotacticity of propylene produced by the isospecific centers, unstable with time, and IA the isotacticity of polypropylene produced by the less specific centers, stable with time, the total isotactic index IIt is given by the expression ... 

The polymers produced by Ziegler-Natta polymerization normally have very wide molecular weight distributions. The polydispersity index PDI (= Myj/Mn) is 5-20 for polyethylene and 5-15 for polypropylene. The cause of the wide dispersity is not precisely known. Some workers believe that the propagation reaction becomes diffusion controlled after a few percent conversion and it is this which is responsible for the large dispersity. Some other workers believe that the rate constants are dependent upon the molecular size. 

Polyethylene and polypropylene blended with iron carboxylate complexes, for example, acetylacetonate (FeAcAc) and stearates (FeSt), and irradiated by UV light under accelerated aging conditions were shown to act as effective phtoactivators giving rise to rapid photoxidation as shown from the rapid rate of carbonyl formation without any induction period (see Fig. 16.4a for FeAcAc in HDPE) and with a reduction in molar mass (see Fig. 16.2a for FeSt in LDPE). However, these complexes have been shown to cause considerable oxidation to both PE and PP during processing reflected in a sharp increase in the polymer s melt flow index (reflecting chain scission and drop in molar mass) (Fig 16.4b) and act, therefore, as thermal prooxidants and cannot be used without the use of additional antioxidants in the system [2,3,17-19,48,49]. 

It should be mentioned that the relationships between average molecular weights, MWD, and the power-law index of the respective polymer melts are not clear and completely unexplored in case of wood-filled composites. For example, increasing viscosity does not always improve physical properties of products. It was found that the increase of MFI of polypropylene from 3 to 30 g/10 min did not alter the efficiency of wood fiber dispersion and did not result in an improvement of any measured property of WPC. On the contrary, a change of MFI for HDPE from 0.15 to 7.0 led to better wetting of wood fiber and superior mechanical properties of the WPC. 

High density (HDPE), 52 Irregularities, 52 Linear low density (LLDPE), 52 Low density (LDPE), 52 Molecular weight, 52 Melt flow index, 53 Melting temperature, 51 Moisture absorption, 51 Polymeric forms, 52 Resistance to chemicals, 52 Resistance to oxidation, 52 Shrinkage, 54 Unsaturations, 54 a-transition, 51 P-transition, 51 y-transition, 51 Polyisocyanate, 79 Polylactic acid, 79, 91 Polymer alloys, 48 Polymer processing additives, 646 Polymer rheology, 619 Polymeric forms, 52 Polyphase PlOO, 451 polypropylene (PP), 2, 11 Polypropylene homopolymer, 70... 

Several polymers were evaluated in the form of a surface coating on glass beads packed in columns to determine their ability to retain platelets when whole human blood passes over the surface. This ability was measured as the platelet retention index p, the fraction of platelets retained on the column. Lowest values of p were found for poly(ethylene oxide), polypropylene oxide), poly(tetramethylene oxide) (in the form of polyurethanes), and polydimethylsiloxane. Highest values (around 0.8) were found for cross-linked poly(vinyl alcohol) and the copolymers of ethylenediamine with diisocyanates. Intermediate values were found for polystyrene and its copolymers with methyl acrylate, for polyacrylate, and for poly(methyl methacrylate). The results are interpreted in terms of possible hydrophobic and hydrogen bonding interactions with plasma proteins. 

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