Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Patent
1997-01-03
1998-06-30
Szekely, Peter A.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
524 47, 525 5426, C08L 302
Patent
active
057734950
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the use of plasticisers for thermo-plasticising starch.
2. Description of Related Art
As a polymer starting material, starch is suitable for a plurality of applications particularly in the field of the packaging industry owing to its ecological properties. In contrast to the polymer raw materials which can be gained by the petrochemical industry, starch is a regrowable natural material which, in addition, is biologically degradable. The demand is placed on this natural polymer, as is on the synthetic polymers, to be easily processable at increased temperatures. These thermoplastic properties, however, are caused by the molecular structure. Thus it has been seen that long-chain unbranched molecules show favourable thermoplastic properties, whereas branched molecules such as the amylo pectin of starch, and in particular cross-linked molecules such as native starch, practically have no thermoplastic properties.
It has now been seen that the union of molecules of starch which are cross-linked via intermolecular and intramolecular hydrogen bridges can be loosened up by the addition of plasticisers in the presence of water.
Pursuant to U.S. Pat. No. 5,362,777, glycerol is used as plasticiser. The effect of glycerol as plasticiser consists of the fact that the small movable glycerol molecules push between the chain molecules in the presence of water quantities to be determined empirically and thus prevent the formation of hydrogen bridge-type bonds between the starch molecules, so that the largely undesirable cross-linking of the starch molecules can be prevented. As a result of this interaction between the glycerol and the starch molecules, a kind of an envelope is formed around the starch molecules, so that the coiled union of molecules of starch is loosened up and thus a molecular laminar and thus thermoplastic (i.e. with flowing capabilities) condition of the starch can be achieved. The disadvantage of such small movable plasticiser molecules such as glycerol is that they migrate from the union of molecules by diffusion and, in particular, in contact with water-containing materials, with the water acting as extraction agent.
If such thermo-plasticised starch products are used as packaging materials, the packaged goods are contaminated by the migrating plasticiser molecules.
Efforts were therefore made to use higher molecular substances, which owing to their bulky molecular structures do not tend to such migration, as plasticisers for starch also in the presence of empirically determinable water quantities. Pursuant to EP-A 304 401 thermoplastic polyvinyl alcohol is proposed as plasticiser. As a result of its high-molecular structure it does not migrate from the union of starch molecules. However, in the case of pure polyvinyl alcohol it is not sufficiently water soluble owing to its high degree of saponification, namely over 99%. Partly acetylated polyvinyl alcohol with 10 mol % of vinyl acetate units, for example, is suitable, but under increased temperatures it tends to the cleavage of acetic acid, because the acetyl group can easily be split off both thermally as well as acid- and base-catalysed. The cleavage of acetic acid is particularly undesirable with respect to foodstuff packagings.
It is therefore the object of the invention to eliminate the aforementioned disadvantages in the use of plasticisers for the thermo-plasticising of starch.
SUMMARY OF THE INVENTION
In accordance with the invention, the use of biologically degradable graft copolymers in the presence of empirically determined quantities of water as plasticiser for the thermo-plasticising of starch and/or starch mixtures is proposed which are producible from polyvinyl alcohols with a degree of saponification of 88 to 99.5 mol % by acid-catalysed acetalisation and/or transacetalisation reaction with carbohydrates containing carbonyl groups or presumptive carbonyl groups, with the degree of acetalisation, expressed in carbonyl equivalents per 100 vinyl alcohol e
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Haschke Heinz
Tomka Ivan
Szekely Peter A.
Teich Aktiengellschaft
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