Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Patent
1990-12-28
1993-09-14
Michl, Paul R.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
5253278, 5253302, 5253306, 5253309, 5253325, 525364, 525370, C08F 800
Patent
active
052449823
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a polymer or copolymer cross-linking process, cross-linkable compositions and their use, notably for preparing shaped objects and the shaped objects thus obtained.
The cross-linking of a polymer creates a three-dimensional network, the consequence of which is to increase the molecular weight. The cross-linking bridges form intermolecular bonds which then limit the movements of the polymer chains in relation to each other and consequently greatly limit the flow behaviour of these chains. Therefore the significant consequence of the cross-linking of a thermoplastic polymer is to raise its thermoplasticity threshold to high temperatures and to improve its resistance to plastic flow over a wider temperature range and therefore to obtain greater dimensional stability of the shaped objects.
Several processes have been proposed for cross-linking thermoplastics such as polyvinyl chloride and polyolefin polychloride. They are either based on radical-like reactions, or on nucleophilic condensation or substitution reactions. For example, with regard to polyvinyl chloride, an attempt was made to cross-link it by photochemical route, by electron bombardment and by gamma irradiation by subjecting the polymer to these radiations in the presence of a multi-functional monomer such as for example tetraethyleneglycol dimethacrylate or trimethylolpropane trimethacrylate, the cross-linking reaction being provided by grafting and polymerization of these multi-functional monomers onto the polymer skeleton. The main disadvantage of these processes for polyvinyl chloride is its sensitivity to these rays which bring about its degradation by elimination of the HCl in a cascade process. Moreover, the cross-linking mechanism is difficult to control because the polymerization and grafting of the functional monomers onto the polymer network bring about transfer reactions.
Furthermore, in order to obtain homogeneous and effective cross-linking, the quantities of multi-functional monomer must be quite high, the consequence of which is to modify the intrinsic properties of the cross-linked polyvinyl chloride. Finally, the processes using gamma rays or electron bombardment require a high level of technology and costly equipment.
In order to overcome these disadvantages, processes were proposed based on radical-like grafting of functionalized monomers initiated by peroxides. These functionalized monomers can contain for example alkoxysilane groups which lead via hydrolysis and polycondensation to silanol groups and to cross-linking It is thus that the process known by the process name SIOPLAST by Dow Chemicals was developed for polyethylene cross-linking, in particular for applications relating to cable manufacturing. However, this process has the disadvantage of causing a cross-linking density gradient because the cross-linking is governed by the diffusion of water in the finished object, which water is required for the hydrolysis reaction of the alkoxysilane functions.
For polyvinyl chloride, processes were proposed which bring about a nucleophilic substitution reaction of the chlorine atoms by alkaline or alkaline-earth thiolates or mixed alkaline or alkaline-earth thiolate-carboxylate compounds. These processes are well adapted mainly for implementation by coating but they are less well adapted for extrusion or calendering processes. Their main disadvantage resides, however, in the fact that they make the cross-linked materials very sensitive to photochemical degradation caused by by-products such as disulphides. For this reason, the development of these processes has not become widespread.
European Patent No. 0,032,587 proposes the cross-linking of acrylic ester copolymers by diols or monoesters or diesters in the presence of titanates of general formula Ti(OR).sub.4. These four (OR) groups can be identical or different. Moreover, this process only applies to alkene and alkyl acrylate copolymers and does not apply to halogenated polymers such as polyvinyl chloride, nor to polymers with NH functions such
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Gondard Christian
Michel Alain
Centre National de la Recherche Scientifique
Michl Paul R.
Sotra Industries
Yoon Tae H.
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