Hans Zweifel

Degradation of polyolefins during melt processing

The thermo-oxidative and thermo-mechanical stability of polypropylene and high density polyethylene (Cr-catalyst technology) was studied using multiple extrusions. In the case of PP, chain scission of the macromolecules, caused by β-scission, the breakdown of peroxy radicals and shear, contributed to a decrease in the molecular weight of the polymer. The PE-HD showed a tendency towards crosslinking. The molecular weight distribution and polydispersity of the polymers changed during processing. Functional group concentrations were measured using FT-IR spectroscopy and are related to rheological changes. The measurements obtained using melt flow and low shear melt viscosity techniques were only in good agreement with the results obtained by gel permeation chromatography in the case of PP. The reactions of alkyl radicals, R•, are primarily responsible for molecular weight increase or molecular weight reduction. Upon processing of PP, terminal CH2C(CH3)groups are formed by disproportionation reactions of alkyl radicals, R•. The methyl substituent does not allow an alkyl radical addition to this double bond. The increase in molecular weight upon processing of the PE-HD is caused by the addition of an alkyl radical, R•, to CH2CHgroups. The documented chemistry and reactivity of the radicals formed upon polyolefin processing are in good agreement with the experimental results.

Degradation and stabilization of high density polyethylene during multiple extrusions

Abstract The thermo-oxidative stability of high density polyethylene was studied using multiple extrusions. Unstabilized Phillips HDPE (Cr-catalyst) showed a tendency towards crosslinking whereas unstabilized Ziegler HDPE (Ticatalyst) favoured chain scission. The molecular weight distribution and polydispersity of the polymers changed during processing. Functional group concentral ions were measured by FT1R spectroscopy and related to rheological changes (melt flow, polydispersity, MW). The reactions of the vinyl group are primarily responsible for the molecular weight increase. The increase correlated with the content of the vinyl groups originally present. Trans -vinylene and vinylidene groups did not participate in radical addition reactions. The formation of trans-vinylene groups is a characteristic result of processing HDPE. Phenolic antioxidants and phosphite stabilizers are effective in preventing the reactions of vinyl groups. Blends of phenolic antioxidants and phosphites exhibit synergism.

The role of quinone methides in thermostabilization of hydrocarbon polymers-I. Formation and reactivity of quinone methides

Abstract The structures and properties of quinone methides(QM) formed by monoelectron oxidation from two commercially important classes of phenolic antioxidants are reviewed. Dimerisation of QM derived from the benzyl-type phenols is controlled by the steric hindrance on the exocyclic methylene group. Isomerisation of QM derived from the propionate-type phenols results in regeneration of the phenolic function.

The role of quinone methides in thermostabilization of hydrocarbon polymers —II. Properties and activity mechanisms

Retardation of thermal oxidation by quinone methides (QM) was observed in model experiments with liquid hydrocarbons. The retardative effect decreases with increasing temperature. Long-term heat ageing of polypropylene is more efficiently reduced by phenolic antioxidants prone to form the corresponding QM in situ. Potential mechanistic features involving reactivity of QM with free radicals, hydroperoxide decomposing antioxidants and hindered amines are reviewed from the point of view of thermal processes in polyolefins.

Thioxanthone ester derivatives: efficient triplet sensitizers for photopolymer applications

Abstract Photochemical investigations and sensitivity measurements is sensitized reactions in solution and polymer matrices have been carried out with a number of new thioxanthone derivatives. These esters are efficient sensitizers having absorption maxima ranging from 333 to 416 nm and triplet energies ranging from 230 to 270 kJ mol −1 . Use of the thioxanthone esters is especially valid in photopolymer applications, as tested in the [2 + 2]-cycloaddition of homopolymers with pendent dimethylmaleimide groups and in the radical-initiated polymerization of acrylic monomers in the presence of donors.