Decomposing polymers using NOR-HALS compounds

Details Decomposing polymers using NOR-HALS compounds Decomposing polymers using NOR-HALS compounds

1998-12-14

2000-10-17

Michl, Paul R.

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

Synthetic resins

Treating polymer containing material or treating a solid...

521 47, 528503, C08J 1110, C08J 1122

Patent

active

061334146

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a method of reducing the molecular weight of polymers, for example waste plastics and used plastics, in which at least one of the so-called "NOR--HALS" compounds described below is added and heating is carried out at temperatures of 280.degree. C. or more.
In connection with the disposal of plastics, chemical recycling, i.e. the decomposing of polymers to give oligomers and low molecular mass products, is increasing in importance in comparison to the conventional thermomechanical reforming processes.
Chemical recycling can take place in a variety of ways, for example by hydrogenation, visbreaking, gasification or pyrolysis (U. Hofmann, M. Gebauer, Kunststoffe 83, 259 (1993); D. E. Vesper, U. Guhr Kunststoffe 83, 905 (1993), H. Wanjek, U. Stabel, Kunststoffe 84, 109 (1994); G. Menges, J. Bandrup, Kunststoffe 84, 114 (1994); P. Mapleston, Mod. Plast. Int. 1993, 32), or else by an extrusion process. Oligomers in particular are produced in this case [W. Hasberg, D. Vesper, M. Gebauer, Kunststoffe 84, 103 (1994), W. Micheli, V. Lackner, paper given at Int. Conf. on Advances in the Stabilization and Degradation of Polymers, Lucern, CH, 1994,177]. The use of catalysts has also been proposed (DE 4 224 990).
To adapt the molecular weight distribution in the case of plastics such as polypropylene use is made, during the preparation of compound formulations, primarily of peroxides, which bring about the decomposition of excessively long chains. These peroxide compounds break down at comparatively low temperatures and are therefore of only limited suitability for the targeted decomposing of polymers at higher temperatures (i.e. above customary processing temperatures). The use of these compounds in relatively high concentrations also carries with it a safety risk, which necessitates appropriate protective measures.
The objective is therefore to provide additives which accelerate decomposition and become active only at high temperatures. For these purposes, suitable additives have now been found within the class of the "NOR--HALS" compounds described below.
The invention accordingly relates to a method of reducing molecular weight in polymers at temperatures of 280.degree. C. or more, in particular from 280 to 400.degree. C., for which NOR--HALS compounds comprising at least one group ##STR2## (I), in which G is hydrogen or methyl and G.sub.1 and G.sub.2 are hydrogen, methyl or together are .dbd.O, are added as decomposition catalysts. It is judicious to operate by an extrusion process.
It is self evident that the polymers must be exposed to the elevated temperature for a sufficient period of time for the desired decomposition to occur. This period of time is normally longer than the time during which elevated temperatures are employed in the processing of the polymers. The decomposition times may vary greatly depending on the temperature, the amount of material to be decomposed and the nature of the extruder if used. They are usually from about 2 to 120 minutes, in particular from 5 to 90 minutes. As is common knowledge, so-called sterically hindered amines (Hindered Amine Light Stabilizers, HALS) are frequently added as light stabilizers to the polymers at the processing stage.
The NOR--HALS compounds described are, however, also suitable for limiting the molecular weight and/or controlling the molecular weight distribution of polymers, especially polypropylene, in the course of compounding, and in this context, as described earlier for peroxides, they bring about decomposition of the chains.
In the method of the invention the NOR--HALS compounds are present judiciously to the extent of from 0.01 to 10.0 for example to the extent of from 0.05 to 5.0 preferably to the extent of from 0.05 to 3.0, but in particular to the extent of from 0.1 to 2.0% by weight. One or more of these compounds may be at hand in connection with this method. The weight percentages are based on the total amount of these compounds. The basis for calculation in this context is the total weight of the polymer excluding th

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Patent Abstracts of Japan No. 01101337.
Derwent Abstr. 94-043698/06 for DE 4224990.

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