Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Patent
1991-10-08
1993-03-02
Lipman, Bernard
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
524506, 5253273, 525342, C08F 800
Patent
active
051910195
DESCRIPTION:
BRIEF SUMMARY
The invention concerns a method for production of thermostable crosslinked polymer. For a long time a slightly crosslinking of halogen-containing polymer, especially PVC, has been considered as an attractive method to improve the mechanical properties of the polymer at high temperatures. Many different technical solutions have been tried. For PVC .for example crosslinking by socalled reactive plasticizer has had a certain commercial progress. These normally are di- or trifunctional or methacrylates, which are normally added in amounts of 20-50 phr (phr=parts per 100 parts polymer). These will be crosslinked when being exposed to radiation or free radicals. They are making a relatively dense network which is grafted to the PVC-chains. The drawback with this system is that a strong heterogeneous network is introduced, this resulting in a considerable brittleness. At the same time the thermal stability of the polymer will be poorer by being exposed to radiation or free radicals (see for example W. A. Salmon and L. D. Loan, J. Appl. Polum. Sci, 16, 671 (1972)).
A fundamentally different method for crosslinking of PVC is to utilize the fact that the carbon-chlorine bond in PVC is polar, which results in the possibility to substitute the chlorine by a nucleophilic attack. Especially thiolations have turned out to be suited nucleophiles. (See for example W. A. Salmon and L. D. Loan, J. Appl. Polum. Sci, 16, 671 (1972)) Mori and Nakamura have described crosslinking of PVC by different dithioltriazines in several articles and patents. These have turned out to have suitable reactivity, volatility, poisonousness and level of smell which can vary with different substituents on the dithioltriazine. The crosslinking takes place through a metal salt of dithioltriazinc, especially magnesium--and natriumsalt has been used.
This method has not been any commercial success. This may have several explanations. The crosslinking reactions are started by heat. Flexible PVC is processed at temperatures being very close to the decomposition temperature of the substance. For unplasticised PVC the decomposition - and processing temperature are in the same range. This results in a very difficult balance by first processing of the material without obtaining crosslinking, at the same time as a rapid crosslinking started by heat shall be carried out afterwards. Either too many of the reactions occur during the processing when a rapid system is used, or the thermical load will be too large when a slow system is used. The "rapid" dithioltrazines, for example 2-dibutyamino - 4,6-dithioltriazine, in addition have proven to have a detrimental effect on PVC. This results in that a good working PVC-compound for crosslinking with good heat stability, hardly could be produced according to this method.
Another very interesting method is crosslinking of PVC by organic alkoxysilanes. By this method it is possible to separate the crosslinking from the processing by using a crosslinking reaction which is catalyzed by water. The processing then occurs in relatively dry state, for example by extrusion. The crosslinking is carried out in a following step, by supply of moisture in the shape of steam, hot or make it possible to produce a thermostable, crosslinked PVC. Several patent applications are filed concerning use of organic silanes for crosslinking of halogen-containing polymers, for example the use of mercaptopropyltrimetoxysilane (I) for crosslinking of PVC and its copolymers, as in the patent applications DE 3719151 Al and JP 55151049. According to this method the difunctionality of (I) is used in such a way that the mercaptogroups are added to the PVC-chain, and the alkoxysilane-groups are used for crosslinking by hydrolysis and following condensation to --Si--)--Si-- bonds.
This method however has a weakness because the grafting to the PVC chains through the mercaptogroups not easily occurs. According to patent application DE 3719151 Al, an alkaline lead sulphate is needed to activate the mercaptogroups. Still very long period of time for cross
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patent: 4091048 (1978-05-01), Labana et al.
patent: 4314043 (1982-02-01), Kojima et al.
patent: 4446259 (1984-05-01), Vasta
patent: 4477628 (1984-10-01), Kato et al.
Dahl Roger
Pedersen Steinar
Redford Keith
Roots Jaan
Lipman Bernard
Norsk Hydro a.s.
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