Polypropylene chain scission

Gonzales, V. A., Velazquez, G. N., Sanchez, J. L. A. (1998). Polypropylene chain scissions and molecular weight changes in multiple extrusions. Polym. Degrad. Stab., 60, 33-42... 

The two polymer substrates investigated as part of the study of DBDPO mixtures were polypropylene (PP) and linear high density polyethylene (HDPE). while both PP and HDPE decompose by similar random chain scission, radical mechanisms, chain transfer occurs much more teadily during the pyrolysis of PP because of the presence of the tertiary hydrogens. In addition, only primary chain end radicals are formed when the HDPE chain cleaves homolytically. Therefore, a comparison of the PP/DBDPO and the HDPE/DBDPO mixtures volatile product distributions was undertaken. 

Electron beam irradiation is one of the methods of cross-linking in fhis process. The other methods use peroxide, multifunctional azide, or an organofunctional silane. Polyethylene resins respond to electron beam irradiation well since the rate of cross-linking exceeds significanfly fhe chain scission. Polypropylene (PP) is prone to P-cleavage, which makes if difficult to cross-link by a free radical process. For fhaf reason, PP resins... 

Polypropylene (PP), on the other hand, undergoes predominantly chain scission under all processing conditions [7, 8, 9, 10, 11] with associated reduction in the molar mass and melt viscosity (see Scheme 3). The propagation reaction (Scheme 1, reaction 3) in PP is particularly facilitated by intramolecular hydrogen abstraction leading to the formation of adjacent hydroperoxides along the polymer chain that are less stable than isolated hydroperoxides and lead to an increased rate of initiation. 

Oxidation during processing of polypropylene is principally accompanied by chain scission made evident by a reduction in melt viscosity. The oxidation during processing of polyethylene on the other hand is accompanied mainly by crosslinking. The following two tests are, therefore, used ... 

Polypropylene - Melt flow rate after the first, third and fifth extrusion, at 500°F to determine the degree of chain scission (5). Y. I, index is also determined (6.),... 

For both polyethylene and its many copolymeric variants and polypropylene, the main thermal degradative routes follow initial random chain scission. These reactions are only slightly affected by the differences in the physical structure such as crystallinity, but are influenced by the presence of impurities. However, it is largely true that while these may influence the proces-sibility and long-term stability of respective polyolefins, they may have little or no effect on the flammability. 

The curves of isothermal weight loss at 260°C for chloroparaffin/polymer mixtures show that the polymers are thermally destabilized by the presence of chloroparaffin, the destabilization increases in the sequence polyethylene, polypropylene, and polystyrene chain scission is induced in polypropylene and polystyrene whereas cross-linking is promoted in polyethylene, as shown in Table 4.3.35 At the same time, the rate of elimination of HCI from chloroparaffin is decreased by the presence of the three polymers. 

When the substituent R stabilizes radicals as in (A) and (C), chain scission is more likely than termination by coupling. Radicals (C) then propagate the depolymerization process with volatilization of polypropylene and polystyrene at a temperature at which these polymers would not give significant amounts of volatile products when heated alone. Moreover, unsaturated chain ends such as (B) would also initiate the volatilization process because of the thermal instability of carbon-carbon bonds in P position to a double bond (Equation 4.23). 

The condensed phase mechanism was explained taking into account the decrease of the pyrolysis rate of polypropylene BiCl3 could catalyze the condensation between chloroparaffin and polypropylene by addition to chain end double bonds (Equation 4.25) formed either in reaction (Equation 4.22) or in chain scission occurring during volatilization of polypropylene 31... 

Plasma vs. Corona Treatment of Polypropylene (PP1. Corona treatments of polyolefins to modify their surfaces are very common in the polymer industry. The chemistry at such surfaces has been widely studied by XPS (4). It is generally assumed that corona treatments create abundant amounts of radicals which react with oxygen to form a hydroperoxide. This reacts further to eventually form crosslinks, oxidized products (ranging from hydroxyls to esters) with and without chain scission. The latter process is believed to lead to low-molecular weight material. There is some controversy over this material. Its role in determining the surface properties of the modified polymer is not completely understood. Its formation cannot be demonstrated directly by XPS, but only by comparing spectra before and after washing. 

A method of incorporating between 5% to 45% maleic anhydride into polypropylene without chain scission or viscosity increase is described. The method entails an initial thermally induced ene reaction followed by the free radical addition of the anhydride to the polymer backbone. 

Random-chain scission the polymer is broken up randomly into smaller molecules of varying chain lengths, producing a volatile with or without a double bonds. (Polystyrene, polyisobutene, polyethylene, polypropylene, polybutadiene)... 

Polypropylene (PP). Pyrolysis of PP is favored by the branched structure of the polymer the thermal degradation also proceeds in this case via a random-chain scission, but the influence of the temperature on the product spectrum is more pronounced than in the case of PE [43, 31], At temperatures as low as 515°C, Predel and Kaminsky [26] found that PP pyrolysis leads to the production of 6.8% of gases, 36.7% of oils, 21.6% of hght waxes and 34.6% of heavy waxes. At these low temperatures the main compounds in the gas fraction are propene and butenes (about 51 and 17% in [26]), but at higher temperatures these products are converted into others [43]. Ponte et al. [31] found a remarkable... 

Hexahydropyrene sensitized chain scission of polypropylene and polyisobutylene during light irradiation Q20).Polycyclio hydrocarbons have a important role in sensitized photooxidation of polyisoprene (122).polys tyrene (123) poly(methyl methacrylate) (123-126). It is quite probable that these reactions can also occur with participation of singlet oxygen. 

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