A further problem associated with the coating of cartons is the non-biodegradability of paraffin derived materials. For example, a carton used to contain milk or soft drink cannot be marketed as being truly biodegradable if coated with such a material.

According to the present invention there is provided a wax-like material comprising a polysaccharide backbone substituted with long carbon chain residues.

The preferred substitution is by esterification. In the materials of the invention, the wax-like properties are, in effect, provided by the long carbon chains which are immobilised by substitution in the polysaccharide molecule. The molecular weight of the wax-like material (which will of course be dependent on the length of the polysaccharide backbone as well as type and degree of substitution) will determine the temperature at which the wax-like material will melt. Typically the molecular weight of the wax-like material will be at least 8500 more preferably at least 18,000.

A wax-like material in accordance with the invention and having a molecular weight 80,000-90,000 will typically melt in the range

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50°-60°.

Generally the molecular weight of the polysaccharide to be substituted will not exceed about 100,000.

The molecular weights referred to in the preceding paragraph are viscosity average molecular weights as determined by Gel Permeation Chromatography using Pullulan standard (algal- polysaccharide).

Given that the long carbon chain substituents are "immobilised" on the polysaccharide there will be no partitioning of the wax-like material into fatty products. In other words, the molecular weight of the wax-like material is too large to allow it to partition into the fatty product.

The wax-like materials are most preferably obtained by esterification of a polysaccharide material incorporating hydroxyl groups with an acid (or acid derivative, e.g. the acid chloride) providing the long carbon chain. Such waxes as obtained by esterification are also inherently biodegradable; fission of the ester bond producing a degradable fatty acid and a degradable polysaccharide. The preferred polysaccharides to be esterified are those which are soluble (e.g. to 1 or 2 %) in solvents commonly used in esterification reactions.

Preferred polysaccharides are based on hemi-cellulose, e.g. Galactomannans, xylans, and gums such as guar and acacia. If a hemi¬ cellulose is used, then it may not be necessary to modify the hydroxyl groups thereof to effect the substitution of the hemi-cellulose with the long carbon chain groups, although such modification is not precluded. It is also possible to use cellulose as the polysaccharide but In this case it will generally be necessary to use a modified form

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to allow the substitution reaction to proceed.

In many cases, it is preferred to use a hydroxy alkyl derivative (preferably one in which the alkyl group has 1-3 carbon atoms) to ensure the required solubility for the esterification reactions. Thus for example, preferred polysaccharides for use in the invention include hydroxypropyl cellulose and hydroxypropylated gums, e.g. hydroxypropyl guar. In the case of these hydroxypropylated (and other hydroxyalkylated) compounds it is preferred that all three hydroxyl groups on the "monomeric " saccharide unit are substituted. This degree of substitution in cellulose ensures that the strong hydrogen bonding in cellulose is sufficiently disrupted to allow the introduction of long carbon chain groups during the subsequent esterification reaction.

The long carbon chains preferably have at least 14 carbon atoms, more preferably 16 to 20 carbon atoms. Preferably the group has an even number of carbon atoms.

Examples of long carbon chain groups which may be introduced are thus myristyl, stearyl, palmityl, oleyl, elcosanoyl/arachridyl. These naturally occurring fatty acid derived groups are particularly preferred as they result in the production of biodegradable wax-like materials. The long carbon chain may include unsaturation which opens the possibility of providing a degree of cross-linking between the long chain substituents to modify the properties of the wax-like material.

It is preferred to use the acid chloride in the esterification reaction in view of its higher reactivity than the free acid.

If necessary a catalyst may be used for the esterification reaction, e.g. pyridine or other basic catalysts.

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The esterification is preferably effected so that the degree of substitution of each "monomeric" saccharide unit is at least 2, more preferably at least 2.3.

The invention will be further described by way of example with reference to the following non-limiting Examples.

Example 1

5g of hydroxy-propyl cellulose (M Ult. ca 80,000) was dissolved in 250 cm ³ of a 4:1 mixture of acetone and pyridine (base catalyst). The mixture was heated to 50° C in a reaction vessel, followed by the addition of a 3 times molar excess (based on available hydroxyl groups) of stearoyl chloride. The resultant mixture was refluxed for 1 hour and then allowed to cool. The esterified product was isolated by precipitation in a large volume of distilled water, then dried and redissolved in toluene or acetone before final reprecipitation.

Degrees of substitution (i.e. number of stearyl groups substituted per "monomeric" saccharide residue) obtained using this method are typically greater than 2. These polymers melt at 50-60° C, and are totally hydrophobic.

Example 2

5g of hydroxyl propyl cellulose (M Ult.ca 100,000 Molar Substitution (MS) ca 3) was dissolved in 250cm ³ of a 3:1 mixture of acetone and pyridine (base catalyst). The mixture was heated to 50° C in a reaction vessel, followed by the addition of a 2 times molar excess (based on available hydroxyl groups) of stearoyl chloride. The mixture was refluxed for 45 minutes and then allowed to cool. The esterified product was isolated by precipitation in a large volume of

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distilled water, then dried and redissolved in toluene before final reprecipitation. DS obtained using this method is usually around two. Wax-like polymers are obtained that melt at 90-100° C. These polymers can also form fibres if reprecipitated into cold, moving water.

Example 3

5g of Larch Arabinogalactan (M Ult ca 28,000) was dispersed in 150 cm ³ of a 2:1:1 mixture of acetone, pyridine and formamide. The mixture was heated to 60° C in a reaction vessel, followed by the addition of a 2.5 times molar excess of stearoyl chloride. The mixture was gently refluxed for 45 minutes and allowed to cool. The product was isolated by precipitation in a large volume of distilled water, then dried and redissolved in toluene before final reprecipitation. Resultant polymers melt at 50-60° C.

Example 4

5g of Larch Arabinogalactan (M Ult ca 28,000) was dispersed in 150 cm ³ of a 2:1:1 mixture of acetone, pyridine and formamide. The mixture was heated to 60° C in a reaction vessel, followed by the addition of a 2 times molar excess of palmitoyl chloride. The mixture was gently refluxed for 50 minutes and allowed to cool. The esterified product was isolated by precipitation into distilled water, then dried and redissolved in toluene before final reprecipitation. DS of around 2 are obtained using this method. Polymers melt at 60-70° C.

Example 5

5g of hydroxy-propyl guar gum (M Ult ca 27,000; MS ca 0.5) was dissolved in 150 cm ³ of a 1:1 mixture of acetone and pyridine. The

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mixture was heated to 60° C in a reaction vessel, followed by the addition of a 2.5 times molar excess of stearoyl chloride. The mixture was gently refluxed for 50 minutes and then allowed to cool. Isolation of the product was as above. DS of 2-2.3 is obtained using this method. Polymer products melt at 50-60° C.

Example 6

5g of guar gum (M Ult ca 28,000) was dispersed in 150 cm ³ of a 2:1:1 mixture of acetone, pyridine and formamide. The mixture was heated to 60° C in a reaction vessel, followed by the addition of a 3 times molar excess of palmitoyl chloride. The resultant mixture was gently refluxed for 1 hour and then allowed to cool. Isolation of the product was as above. The wax-like polymer obtained melts at 70-80° C.

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