Radical reactions on polypropylene in the solid state

• Published in: Progress in polymer science 2002-09, Vol.27 (7), p.1195-1282
• Main Authors: Rätzsch, Manfred, Arnold, Manfred, Borsig, Eberhard, Bucka, Hartmut, Reichelt, Norbert
• Format: Article
• Language: English
• Subjects: Applied sciences; Branching; Chemical modifications; Chemical reactions and properties; Cross-linking; Exact sciences and technology; Grafting; Organic polymers; Physicochemistry of polymers; Polypropylene; Radical reactions; Solid state
• ISSN: 0079-6700; 1873-1619
• DOI: 10.1016/S0079-6700(02)00006-0

The chemical modification initiated by radicals and the grafting of isotactic polypropylene (i-PP) in the solid state are reviewed. The attack of a radical onto the polypropylene mainchain is led by abstraction of a hydrogen atom to a tert-carbon radical. The tert-carbon radical of the polypropylene mainchain is unstable and overcomes with the so-called β-scission-reaction. During the β-scission-reaction, the mainchain is broken into two parts with a double bound on the one and a primary radical on the other chain end. The resulting degradation of the molecular weight is one limitation of the radical modification of i-PP in the melt and is the main reason for the development of radical reactions in the solid state. It is well known that the β-scission-reaction depends strongly on the temperature. Below 60 °C, the recombination reactions of the i-PP-radicals predominate so that a cross-linking results. Above 60 °C, the β-scission-reaction increases and the molecular weight of the i-PP decreases in a logarithmic scale. The first part of our paper deals with the mechanism of the grafting reactions of different monomers onto i-PP below the melting point of i-PP and the special processing conditions related to it. To start the grafting at low reaction temperatures, γ or electron-ray scattering or special peroxides are used. As monomers different methacrylates, acrylates, acrylonitrile, styrene, divinylbenzene, maleic anhydride, butadiene, dimethylbutadiene, vinyltrimethoxysilane and other silanes to PP-graft copolymers are investigated. The influence of the resonance stability ( Q-value of the Qe-schema) of the monomer radicals in the process will be discussed. From this knowledge, we will further discuss a special process for the long chain branching of the melting temperature of i-PP in an extruder. The properties of the resulting materials are an important part of our review.