Water-soluble biodegradable hydroxyalkyl cellulose-2-hydroxycarboxylic acid esters which can flocculate

The present patent application claims the right of priority under 35 U.S.C. §119 (a)·(d) and 35 U.S.C. §365 of International Application No. PCT/EP98/04301, filed Jul. 10, 1998, which was published in German as International Patent Publication No. WO 99/05178 on Feb. 4, 1999, which is entitled to the right of priority of German Patent Application Number 197 31 575.5, filed Jul. 23, 1997.

The present invention relates to novel water-soluble, thickening, thermoplastic, film-forming and rapidly biodegradable cellulose ether lactates.

The biodegradability of a polysaccharide derivative is dependent on the degree of substitution of each saccharide unit [c.f. j. G. Batelaan in The Handbook of Environmental Chemistry, Volume 3, Part F, Ed. O. Hutzinger, Springer Verlag; 1992, 229-336; M. G. Wirick, Journal of Polymer Science, Part A-1, 6 (1968), 1705-1718]. Thus, all industrially available cellulose derivatives are rapidly biodegradable only if they have average degrees of substitution of less than 1.0. However, non-ionic cellulose ethers only become water-soluble with average degrees of substitution >>1. The known consistency regulators for aqueous systems based on non-ionic cellulose ethers such as, for example, methyl cellulose and hydroxyethyl cellulose, thus have degrees of substitution >1.5. Thickeners for organic solvents such as, for example, hydroxypropyl cellulose, even have degrees of substitution >2. Such compounds are only sparingly biodegradable (severe chemical oxygen demand).

The object of the present invention is to develop synthesis, performable on a fully industrial scale, of water- and/or organo-soluble cellulose derivatives which biodegrade quickly and completely.

According to the invention, this is achieved by reacting hydroxyalkyl cellulose ethers with a molecular degree of substitution with hydroxyalkyl groups of less than 1.5 (MS hydroxyalkyl <1.5) to form hydroxyalkyl cellulose-2-hydroxycarboxylic acid esters with a molecular degree of substitution with 2-hydroxycarboxylic acid groups greater than 0.4 and less than 3 (0.4<MS 2-hydroxycarboxylic acid <3).

Caellulose lactates are already known. In DE 33 22 118, cellulose esters are described, which are produced by reacting cellulose with lactide or glycolide in cellulose-specific solvent systems such as dimethylacetamide/LiCl. With this method, the synthesis is achieved of water-soluble cellulose-2-hydroxycarboxylic acid esters having a low degree of substitution, which may be used as coating agents or consistency regulators. However, use of the solvent system dimethylacetamide/lithium chloride is complex and not practicable on an industrial scale.

It has been established that hydroxyalkyl celluloses with degrees of substitution of less than 1 may be reacted in conventional organic solvents, such as for example dioxan, dimethylacetamide, N-methylpyrrolidone or tert-butyl alcohol, with glycolide or lactide, the cyclic dimer of lactic acid, to form water-soluble hydroxyalkyl cellulose-2-hydroxycarboxylic acid esters. Specific solvent systems for cellulose, such as for example N-methylmorpholine oxide or dimethylacetamide/lithium chloride are not necessary therefor. The underlying hydroxyalkylcellulose ethers are, according to the prior art, produced by alkalising a cellulose with aqueous sodium hydroxide solution and subsequent etherification with alkylene oxides.

Despite their high total degree of substitution, the hydroxyalkyl cellulose-2-hydroxycarboxylic acid esters biodegrade in aqueous solution markedly more quickly than alkyl and hydroxyalkyl cellulose ethers with comparable total degrees of substitution. The hydroxyalkyl cellulose-2-hydroxycarboxylic acid esters to be produced according to the invention may be described by the general structure (I)

wherein Cell-O represents the substituted residue of a hydroxyl group on the cellulose chain and the groups A constitute a monomeric or oligomeric 2-hydroxycarboxylic acid of the structure (II)

in which D stands for hydrogen (—H) or methyl (—CH 3 ) and n is a whole number between 0 and 10. B is a polymeric ether group of the general structure (III).

—(E—O) n —  (III)

in which E stands for a branched or unbranched carbon chain with 2 to 6 C atoms and n is a whole number between 0 and 10. For all the groups A which do not correspond to the structure (II), A is hydrogen (—H).

To synthesise this hydroxyalkyl cellulose-2-hydroxycarboxylic acid ester, the hydroxyalkyl cellulose ether is suspended in an organic solvent and a reactive 2-hydroxycarboxylic acid derivative is added thereto. The reaction time is from 1 to 10 hours at reaction temperatures of between 50° C. and 150° C., preferably between 80° C. and 130° C.

Suitable cellulose ethers are, for example, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxybutyl cellulose and mixed ethers thereof with degrees of substitution (MS hydroxyalkyl ) of between 0.1 and 1.5, preferably between 0.5 and 1.0.

Suitable 2-hydroxycarboxylic acid derivatives are the cyclic dimers glycolide, L-, D- and meso-lactide as well as lactic acid esters and lactic acid oligomers with from 2 to 10 repeat units.

Suitable suspending agents or solvents are polar aprotic compounds such as, for example, dimethylformamide, dimethylacetamide, dioxan, tetrahydrofuran, N-methylmorpholine, N-methylpyrrolidone, dimethoxymethane, dimethyl ether, diethylene glycol dimethyl ether and, in addition, protic solvents such as, for example, tert-butyl alcohol. The reaction may also be performed without solvent in a lactide melt. The cellulose ether esters according to the invention are water-soluble and make good film formers. As the degree of substitution with hydroxyalkyl and -hydroxycarboxylic acid groups increases, the products additionally become organo-soluble and melt processable.

Hydroxyethyl and hydroxypropyl cellulose lactates, the sum of the degrees of substitution of which with hydroxyalkyl and lactic acid groups exceeds 1.5 (ΣMS lactate and MS hydroxyalkyl >1.5) are only water-soluble below a certain temperature. Above this temperature (flocculation temperature), the hydroxyalkyl cellulose lactates precipitate out of an aqueous solution. This flocculation temperature is dependent on the degree of substitution of the cellulose ether esters. By varying the degrees of substitution with lactate and hydroxyalkyl groups, a thermal flocculation point of the hydroxyalkyl cellulose lactates of between 30° C. and 75° C. may be established. At degrees of substitution with lactate groups greater than 3 (MS lactate >3), the flocculation point drops below room temperature. The products are then insoluble in both cold and hot water.

The products may be used for all applications for which water-soluble biodegradable products are suitable. Thus, for example, they may be used as consistency regulators for foodstuffs, cosmetics, building materials, paints and strippers or for producing microcapsules for the foodstuffs, crop protection and pharmaceutical sectors.

The degrees of substitution may be determined with conventional methods of cellulose derivative analysis, such as, for example, Zeisel cleavage, elementary analysis, gas chromatography and 13 C NMR spectroscopy. The property of biodegradability claimed according to the invention is investigated using the Zahn-Wellens test, DIN EN 29 888.

The subject matter of the present invention is explained in still more detail with the aid of the present Examples.

hydroxyethyl cellulose

MS hydroxyethyl : 2.5

Biodegradation: 13%

Hydroxypropyl cellulose

MS hydroxypropyl : 4.0

Biodegradation: 22%