Process for cross-linking chlorinated (CO)polymers, cross-linkab

Details Process for cross-linking chlorinated (CO)polymers, cross-linkab Process for cross-linking chlorinated (CO)polymers, cross-linkab

1991-10-28

1995-03-07

Carrillo, Ana L.

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

525123, 525131, 525179, 525183, 525184, 525195, 525196, 5253305, 5253311, 5253314, 5253315, 5253341, 525360, 525370, C08L 1100, C08L 2704

Patent

active

053958870

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a process for crosslinking chlorinated polymers or (co)polymers (hereafter (co)polymers), cross-linkable compositions and shaped objects.
A more particular subject of the present invention is a process for cross-linking (co)polymers, by themselves or in mixtures, which can notably be integrated into industrially viable operations such as extrusion, calendering, coating, injection molding or after types of molding.
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 create intermolecular bonds which thus limit the movement of polymer chains relative to one another and consequently strongly limit the flow behaviour of these chains. The cross-linking of a thermoplastic polymer therefore has the significant consequence of moving its thermoplastic threshold towards high temperatures and of improving its resistance to plastic flow over a greater temperature range and thus allows improved dimensional stability of formed objects to be obtained.
Several methods have been proposed for cross-linking thermoplastics such as polyvinyl chloride and polyolefines. They are either based on radical-like reactions, or on condensation reactions or nucleophilic substitution. For example concerning polyvinyl chloride, attempts have been made at cross-linking using photochemical means, by electron bombardment and by gamma irradiation by subjecting the polymer to these radiations in the presence of a multifunctional monomer such as, for example, tetraethyleneglycol dimethacrylate or trimethylolpropane trimethacrylate, the cross-linking reaction being ensured by the grafting and polymerization of these multifunctional monomers onto the polymer skeleton. The major disadvantage of these methods for polyvinyl chloride is its sensibility to these radiations which encourage its degradation by the elimination of 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 skeleton brings about transfer reactions.
Furthermore, to obtain a homogeneous and effective cross-linking, the quantities of multifunctional monomer must be quite high, which has the consequence of modifying the intrinsic properties of the crosslinked polyvinyl chloride. Finally, methods by gamma radiation or by electron bombardment require a high level of technology and expensive apparatus.
To overcome these disadvantages, methods are proposed based on radical-like grafting of functionalized monomers initiated by peroxides. These functionalized monomers can contain, for example, alkoxysilane groups which lead, by hydrolysis and polycondensation, to silanol groups and to cross-linking. It is in this way that the process known by the process name SIOPLAST from Dow Chemical was developed for the cross-linking of polyethylene, notably for cable making applications. However, this process has the disadvantage of creating a cross-linking density gradient, because this is governed by the diffusion of water in the finished object, water which is required for the hydrolysis reaction of the alkoxysilane functions.
For polyvinyl chloride, methods calling on a nucleophilic substitution reaction of the chlorine atoms by alkaline or alkaline-earth thiolates or alkaline or alkaline-earth mixed thiolate-carboxylate compounds, are proposed. These methods are principally well adapted to implementation by coating but they are clearly less so for extrusion or calendering processes. But their major disadvantage lies in the fact that they strongly encourage reticulated materials towards photochemical degradation induced by by-products such as disulphides. For this reason, the development of these processes is not widespread.
Therefore a process for cross-linking chlorinated polymers or copolymers is constantly being sought, which is simple, inexpensive and can be used in the absence of a solvent and with simple heating, in such a way as to be comp

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patent: 3752837 (1973-08-01), Okuto et al.
patent: 4028337 (1977-06-01), Starnes, Jr.
patent: 4647629 (1987-03-01), Michel et al.
patent: 5266637 (1993-11-01), Gondard et al.
Database WPIL, No. 86-263020, Derwent Publications Ltd., London, GB, & JP-A-53 139 659 (Osaka Soda K.K) Jun. 12, 1978.

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