Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Patent
1985-05-24
1987-06-09
Lilling, Herbert J.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
524237, 524567, 524570, 524583, 524585, C08K 532
Patent
active
046720880
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the stabilization of thermoplastic polymer compositions.
Most thermoplastic polymers are subject to oxidative deterioration during processing or in service. This deterioration can lead to loss of properties by the polymers, e.g. embrittlement or discolouration, which can be undesirable. Most particularly thermoplastic polymers are subject to deterioration when, in service, they are in an outdoor environment or otherwise subjected to U.V. radiation. In addition deterioration can occur, e.g. as a result of oxidation, when, in service, the polymers are subjected to high temperature. Also the high shearing forces and temperatures used in polymer processing often result in deterioration.
It is well known to incorporate so-called stabilizers into thermoplastic polymer compositions to prevent or retard such deterioration processes. Such stabilizers, according to the manner in which they act, are U.V. stabilizers, thermal antioxidants or melt stabilizers. Amongst the commercially available U.V. stabilizers are the compounds of the formulae ##STR1## [bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate] and ##STR2## which are sold under the names Tinuvin 770 (ex Ciba-Geigy AG) and Cyasorb UV 531 (ex American Cyanamid) respectively. Such conventional stabilizers, in particular Tinuvin 770, are often expensive to prepare. In order to ensure that a particular polymer composition has the required stability characteristics, it is often necessary to use a mixture of stabilizers. For example there may be added to the polymer both a U.V. stabilizer and a melt stabilizer to protect the polymer from deterioration both during the processing of it and during its service. This of course tends further to add to cost.
The mechanism of the stabilization is not fully understood and clearly different stabilizers can act in different ways. For example the mechanism by which Tinuvin 770 stabilizes thermoplastic polymers is different from that by which Cyasorb UV 531 stabilizes them. Recent studies (Bagheri, Chakraborty & Scott, Polymer Degradation and Stability 4, (1982), 1-16) on the use of hindered piperidines, of which Tinuvin 770 is an example, attribute stabilizing effectiveness to the oxidation of the hindered piperidines to nitroxyl radicals during the polymer processing and subsequently when the thermoplastic polymer mixture is exposed to U.V. radiation. It is the presence of these nitroxyl radicals which is believed to give rise to the stabilization.
It has now been found that thermoplastic polymers, in particular polyolefins, e.g. polyethylene and polypropylene, and polyvinyl chloride, may be stabilized against oxidative deterioration by mixing therewith a C-nitroso-containing compound, an N-nitroso-containing compound or a nitrone compound under polymer processing conditions such that there are formed nitroxyl radicals.
According to the present invention, there is provided a process for preparing a stabilized thermoplastic polymer composition which process comprises mixing the thermoplastic polymer with a nitrogen-containing stabilizer, which is
(a) a C-nitroso-containing compound,
(b) an N-nitroso-containing compound, or
(c) a nitrone-containing compound,
It is important that the compositions prepared according to the present invention are formed under polymer processing conditions such that nitroxyl radicals are formed. The required conditions are conditions in which the polymer is subjected to high shear in which polymer chains become severed resulting in the mechanochemical formation of hydrocarbyl radicals. Mere mixing without shear, e.g. of polymer and stabilizer solutions, will not provide the U.V. stabilization properties of the present invention. While we do not wish in any way to be bound by the theory, it is believed that the macrohydrocarbyl radicals and related mechanochemical radicals formed from the polymer in the processing step react with the stabilizer compounds according to the present invention to form the nitroxyl radicals. In particular it has been observed that stabilization obtain
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Chakraborty Khirud B.
Scott Gerald
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