Polypropylene, molecular weight reduction

The feed stream can be reactor polymer in powder form, which is then chemically modified (e.g., peroxide molecular weight reduction of polypropylene, known as... 

Clementini and Spagnoli (7) patented a method in which polypropylene was first functionalized with a hydroperoxide group before extrusion with acrylic acid. Other reactive sites could also be incorporated into polypropylene during its production. Chung (8) has several patents that discuss the incorporation of borane or joflra-methylstyrene into polypropylene. These functional sites can then be reacted with unsaturated monomers. Both methods supposedly limit both the amount of homopolymerization and the level of molecular weight reduction that occurs compared with other grafting methods. 

Baramboim and Rakityanskii [136-139] produced graft copolymers based on polyamides. The extrusion of a polypropylene-polycaprolactam mixture at 200-210°C produced radicals with a resultant change in polypropylene molecular weight and formation of block and graft copolymers. The intensity of degradation and, consequently, the extent of polypropylene reaction decreased with an increase in the amount of polyamide in the mixture as noted by a reduction of the mixture melt viscosity. Radical acceptors interfere with copolymerization by interacting with the macroradicals [139]. 

Some compounds, particularly hydrogen, are effective chain transfer agents [cf. Eq. (9.13)] that cause reduction of molecular weight. For this reason hydrogen is usually added in the commercial production of polyethylene and polypropylene. It is easy to modify Eq. (P9.3.10) to include the effect of hydrogen by adding an extra term ktr,Hi [ 2]/ [M] to the right side of Eq. (P9.3.10). 

The PPO2 radical reacts inter- or intramolecularly with polypropylene to form PP (Equation 3.5), which will react with oxygen as shown in the reaction represented by Equation 3.4, so that the oxidation reaction will repeat itself. The hydroperoxide group formed in the reaction shown by Equation 3.5 reacts with UV light or heat to yield a tertiary alkoxy radical and a hydroxy radical (Equation 3.6). The tertiary alkoxide may react as shown in the reactions represented by Equation 3.7a or Equation 3.7b and the hydroxy radical can also react with polypropylene as shown in the reaction represented by Equation 3.8, in order that, in each case, the product is another polypropylene radical that can react with oxygen as presented in Equation 3.4. Equation 3.7b leads to chain scission and reduction in the molecular weight of the polymer... 

The combination of HALS with a UV absorber is used in fihns of polypropylene and polyethylene as well as in thick sections. In films of LDPE, nickel quenchers were commonly used with a UV absorber, except in a very thin film, in which a higher concentration of nickel stabilizer is superior to the combination. The low-molecular-weight HALS are not sufficiently compatible with LDPE at the concentrations necessary, possibly as high as 2%, for the required protection. Incompatibility of HALS with LDPE has been overcome with the development of polymeric HALS. It is considerably better than either the UV absorber or nickel quencher or combinations of the two. For thicker films (100-200 ftm), the combination of a benzophenone-type UV absorber with polymeric HALS is significantly superior to an equivalent amount of polymeric HALS. The type of stabilizers used for linear low-density polyethylene (LLDPE) and ethyl vinyl acetate (EVA) copolymer are similar to those for LDPE. Since LLDPE has superior mechanical properties (elongation at break and tensile strength), thinner films can be used for most applications, and the loss of UV stabihty with reduction in thickness has to be compensated for by improving the stabilization system. 

Two experimental investigations of head-to-head, tail-to-tail polypropylene, obtained by reduction of poly(2,3-dimethylbutadiene), give discordant results [17,18]. Values of 4.5 and 6.1-6.4, respectively, are obtained for the characteristic ratio, C , of samples of high molecular weight. 

Polypropylene mainly undergoes a loss of molecular weight during processing. Table 4.7, and a reduction in its dispersion index M ,/M [38]. 

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