Stock material or miscellaneous articles – Composite – Of polyamide
Patent
1998-01-13
1999-10-26
Mullis, Jeffrey C.
Stock material or miscellaneous articles
Composite
Of polyamide
428483, 428500, 428516, 428517, 524552, 525316, C08J 518, B32B 2508, C08L 5302
Patent
active
059725192
DESCRIPTION:
BRIEF SUMMARY
Transparent, self-adhering gas-permeable stretch films (cling films) are known. They are used in the food packaging sector and are described, for example, in JA-A-76 55 352 and JA-A-74 20 254. The commercially available films are generally composed of PVC, making it difficult to dispose of used films.
FR-A-2 686 887 and FR-A-2 691 436 describe the production of stretch films from a mixture of styrene-butadiene-styrene block copolymers and a styrene-butadiene rubber as soft phase, which may also contain polyisobutylene. These films have good recovery after stretching (memory effect) and can be produced so as to be self-adhering. On the other hand, however, their thermal stability is too low as a result of the high proportion of butadiene; (the entire soft phase is composed of pure polybutadiene). In addition, polybutadiene is easily crosslinked: while a low degree of crosslinking is necessary to achieve the elastomeric behavior of the soft phase, if crosslinking is too extensive the gel fractions formed are seen in practice as fish-eyes. It is therefore difficult to produce a satisfactory film having the right degree of crosslinking.
Because of these disadvantages, films made from ethylene copolymers (mostly having up to about 25% of 1-hexene or 1-octene as comonomers) are therefore increasingly recommended as packaging films. Some commercially available products of this type have excellent thermal stability and are therefore easy to process.
However, because of its molecular structure this class of polymer to a large extent lacks the crosslinking points which are typical of styrene-butadiene-styrene polymers and which make it possible for these polymers to have rubbery thermoplastic behavior, ie. a recovery effect (memory effect) after undergoing stretching.
It is therefore an object of the invention to find a polymer material suitable for the food sector and for producing transparent thin films permeable to gases and combining good cling properties with high optical quality (ie. a low portion of fish-eyes) with, at the same time, very good memory effect. A further object is to avoid the clouding which often occurs with known packaging films which is the result of droplets of water condensed during the storage of foods and probably arises as a result of their non-polar surface whenever there is a marked difference in conditions during packing and storage, especially if packing is carried out at temperatures above the subsequent storage temperature. Finally, a still further problem with known films is their severe tendency to shrink, in particular transversely to the direction of extrusion (transverse shrinkage); this tendency must be kept small so that the required tolerances can be maintained on manual or automatic packing machinery.
We have found that this object is achieved by means of a film which gives only slight clouding as a result of water condensation and which, within three months in usual storage after production, has a transverse shrinkage of less than 3%. The other properties set out above as being desirable are possessed to a high degree by films produced according to the invention, and the films can easily be disposed of after use, since they can be incinerated to give virtually entirely carbon dioxide and water.
The invention provides a transparent polymer mixture in roll form, in particular for the food packaging sector, made from, based on the total weight of P1 to P5, from at least one block S which forms a hard phase and incorporates units of a vinylaromatic monomer, and at least one elastomeric block B/S which forms a soft phase and incorporates both units of a vinylaromatic monomer (S) and of a diene (B), where S is the vinylaromatic block and B/S is the soft phase, ie. the block built up randomly from diene units and vinylaromatic units, and where the glass transition temperature T.sub.g of the block S is above 25.degree. C. and that of the block B/S is below 25.degree. C. and the phase volume ratio of block S to block B/S is selected so that the proportion of hard phase in the enti
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U.S. Serial No. 08/849,918, filed Jun. 18, 1997.
Derwent Publications, Section Ch, Week 7721, Class A12, AN 77-37302Y (Abstract of JP 52 047 849, Apr. 16, 1977).
Derwent Publications, Section Ch, Week 7443, Class A12, AN 74-74873V (Abstract of JP 49 020 254, Feb. 22, 1974).
Derwent Publications, Section Ch, Week 9324, Class A12, AN 93-191645 (Abstract of JP 05 117 492, May. 14, 1993).
Knoll Konrad
Niessner Norbert
BASF - Aktiengesellschaft
Mullis Jeffrey C.
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