Organic compounds -- part of the class 532-570 series – Organic compounds – Carbohydrates or derivatives
Reexamination Certificate
1999-07-27
2001-12-18
Peselev, Elli (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carbohydrates or derivatives
C536S056000, C536S106000, C536S114000, C536S119000, C536S123100, C536S124000
Reexamination Certificate
active
06331619
ABSTRACT:
This application is a 371 of PCT/NL97/00708 filed Dec. 18, 1997.
The present invention relates to a superabsorbent material based on polysaccharides and to a method of producing such material. A superabsorbent material is defined as a material capable of absorbing at least 20 times its own weight of water.
Superabsorbent materials of various types are known in the art. Examples are crosslinked polyacrylates and polysaccharides and with polyacryltes. A problem related to the use of superabsorbent materials is that such materials are normally based on non-renewable and/or non-biodegradable raw materials. Consequently, there is a need for superabsorbent materials which are wholly based on renewable raw materials, such as polysaccharides, and are degraded by microrganisms or other natural agents after being disposed of.
EP-A-489424 discloses modified starch compositions useful as absorbable dusting powders, obtained by slight hypochlorite oxidation of starch resulting in the presence of 0.05-0.5 wt. % of carboxyl groups (=0.002-0.018 carboxyl groups per monosaccharide unit), followed by partial crosslinking with 0.05-1.1 wt. % of phosphorous oxychloride. The powders obtained according to this prior art are not suitable for use in absorbent articles such a diapers, sanitary napkins and the like due to the fact that they do not swell to a significant extent. Kulicke et al (Starch/Stärke, 41 (1989) 140-146; ibid. 42 (1990) 134-141) have studied the effect of crosslinking with epichlorohydrin and sodium trimetaphosphate, respectively, on the absorption capacity of starch, amylopectin and commercial oxidised starch, which contains carbonyl (aldehyde or ketone) groups and carboxylic acid groups. The degree of oxidation of such oxidised starch is presumably lower than 10%. The purpose of obtaining oxidised starch was to obtain thin boiling starch (lower molecular weight). According to these authors, the absorption capacity decreases with increased crosslinker concentration and the absorption capacity of crosslinked oxidised starch is higher than that of crosslinked native starch. Although a considerable swelling capacity in pure water was reported, the swelling capacity of the crosslinked polymers in water containing sodium chloride or other salt drops to about 15 g per g polymer. U.S. Pat. No. 2,929,811 discloses crosslinking (using epichlorohydrin) and subsequent slight oxidation (<5 mol %), using periodate and chlorite, of starch for improving the viscosity characteristics of the starch. No water-absorbing properties are reported.
EP-A-23561 discloses a water-absorbing crosslinked polycarboxycellulose which is produced by carboxymethylation of cellulose, crosslinking by acid-catalysed internal esterification, and slight oxidation (<5 mol %) using periodate/bromine, lead tetraacetate or hydrogen peroxide. A disadvantage of carboxymethyl celluloses is that they are relatively expensive.
It has now been found that a superabsorbent material having an absorption capacity of at least 20 times its own weight, and based on natural, biodegradable polysaccharides, can be obtained by introducing at least 0.2 carboxyl groups per mono-saccharide unit, at least 0.1 of which is formed by oxidising a carboxyl group of the polysaccharide, and subsequently slightly crosslinking the oxydised polysaccharide.
The polysaccharides to be used according to the present invention are in particular &agr;-glucans like starch, amylose and amylopectin, &bgr;-glucans like cellulose, galactomannans like guar gum (guaran) and locust bean gum, glucomannans including e.g. xanthan gum, fructans, (arabino)xylans, galactans including alginates and pectins, as well as non-ionic derivatives such as hydroxyethyl and hydroxypropyl derivatives of such polysaccharides. Starch and guar, and to a somewhat lesser extent, cellulose, are preferred for economic reasons. The chain of the polysaccharides is important although there is no critical minimum for the molecular weight. In general, polysaccharides having a molecular weight of more than 1,000 are preferred. A molecular weight above about 25,000 may have a positive effect on the properties of the oxidised product.
The polysaccharides to be used according to the present invention may also be carboxymethylated or carboxyethylated, especially in the case of &agr;-glucans like starch, galactomannans and glucomannans. Other carboxyalkylated polysaccharides include the half esters obtained from cyclic anhydrides such as succinic and maleic anhydride (groups having the formula —O—CO—CH
2
—CH
2
—COOH or —O—CO—CH═CH—COOH), and addition products of maleic half esters to which sulphite has been added (groups having the formula —O—CO—CH
2
—CH(SO
3
H)—COOH). The degree of carboxyalkylation is preferably between 0 and 1.5, in particular between 0.1 and 1.0 carboxyalkyl groups per monosaccharide unit. Carboxymethylation and carboxyethylation can be performed in a conventional manner, i.e. by reaction of the (oxidised or non-oxidised polysaccharide with monochloroacetate, or acrylonitrile followed by hydrolysis, respectively, or by hydroxyethylation (or hydroxypropylation) followed by oxidation of the primary hydroxy groups, e.g. using a nitroxyl catalyst as described below. Thus, a useful type of products according to the invention containing both 6-carboxyl groups and carboxymethyl groups can be obtained in several ways: oxidation of carboxymethylated polysaccharide, carboxymethylation of 6-oxidised polysaccharide, (primary hydroxyl) oxidation of hydroxyethylated polysaccharide, and hydroxyethylation and oxidation of 6-oxidised polysaccharide.
The oxidation of the polysaccharide is an essential step of the present invention. The oxidation should be performed to a substantial degree, i.e. until at least 0.1 of the carbinol group per monosaccharide unit has been oxidised to a carboxyl group. The term carbinol group covers both primary, exocyclic hydroxymethyl groups (—CH
2
OH) and secondary, usually endocyclic hydroxymethylene groups (—CHOH—) of the polysaccharide Preferably at least 0.15 or even at least 0.2 carbinol group have been oxidised to a carboxyl group per monosaccharide unit. Additional carboxyl groups may be present in the form of carboxymethyl groups introduced by substitution on a hydroxyl group.
Oxidation of polysaccharides is well documented. Oxidaton can be performed using various oxidising agents, which result in various degrees of oxidation, various degrees of polymerisation and different sites of oxidation. Oxidation of polysaccharides can be focused at primary hydroxyl group, like the 6-hydroxyl group in the anhydroglucose units of glucans, which results in carboxyl-polysaccharides with preserved ring structures. On the other hand, the oxidation can be mainly directed at a vicinal diol function present in the monosaccharide rings, such as the C2-C3 site in anhydroglucose units This results in cleavage of the monosacchide units with the production of dialdehyde and/or dicarboxyl functions.
As an example, the oxidation of starch with nitrite and nitrate in phosphoric acid, mainly resulting in 6-carboxy starch, is described in NL patent application 9301172. An improved oxidation of the 6-hydroxyl groups in starch, using a hypohalite in the presence of a di-tert-alkynitroxyl catalyst is disclosed in WO 95/07303. Examples of oxidation of glucans at the C2-C3 function include the process according to EP-A427349, using low levels of hypobromite, and the process according to WO 94/21690, which uses hydrogen peroxide in the presence of alkali metal; or transition metals. WO 95/12619 describes an improved oxidation of starch with periodic acid, resulting in dialdehyde starch with extensive regeneration of a periodic acid. The dialdehyde starch can be further oxidised to dicarboxyl starch using e.g sodium chlorite and/or hydrogen peroxide. Also, dialdehyde starch can be further oxidised with iodine or bromine or with nitrogen dioxide producing dicarboxy or up to tricarboxy starch. Other known oxidation methods include metal-catalysed oxidation, eg, using ruthenium,
Besemer Arie Cornelis
Thornton Jeffrey Wilson
Peselev Elli
SCA Hygiene Products Zeist B.V.
Young & Thompson
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