Methods for preparing alkali cellulose and water-soluble cellulose ether

This application claims priority from Japanese Patent Application No. 2007-337084; filed Dec. 27, 2007, the disclosure of which is incorporated herein by reference in its entirety.

1. Field of the Invention

The present invention relates to methods for preparing alkali cellulose and water-soluble cellulose ether having the low insoluble fiber content.

2. Description of the Related Art

Water-soluble cellulose ether is produced by reacting cellulose having, in the molecule thereof, both a crystalline portion and a non-crystalline portion with an etherifying agent to convert the crystalline portion into the non-crystalline portion and thereby making the cellulose ether soluble in water. It is said that crystallinity of cellulose owes to a hydrogen bond between intramolecular hydroxyl groups attributable to the skeleton structure of the cellulose molecule. This hydrogen bond, which is firm, disturbs hydration with a water molecule in water and becomes a cause for making the cellulose water-insoluble. Cellulose ether is prepared by converting the cellulose into alkali cellulose with an aqueous solution of an alkali such as NaOH, thereby breaking its crystallinity; and reacting the alkali cellulose with an etherifying agent to substitute the hydroxyl group of the cellulose by the etherifying agent. The resulting alkali cellulose however does not completely lose crystallinity. It is industrially difficult to substitute all the hydroxyl groups of the cellulose by raising the degree of substitution for ether so that commercially available cellulose ethers are water-soluble but have a water-insoluble portion. The water-insoluble portion sometimes has a fiber scale of pulp, which is a raw material cellulose, exceeding even 1000 μm.

Water-soluble cellulose ether becomes tacky when dissolved in water so that it is used for a thickener for transparent shampoos and rinses, hair styling agents, eye drops, detergents for contact lens and the like. For example, methyl cellulose or hydroxypropyl cellulose, which is water-soluble cellulose ether, has a hydrophilic group and a hydrophobic group in the molecule thereof and thereby shows interfacial activity. It is therefore used as a suspension stabilizer in suspension polymerization of vinyl chloride or vinylidene chloride and becomes useful also as a raw material for transparent plastic wrap for domestic use. Products in such applications are desirably transparent. Unless water-soluble cellulose ether is water-soluble and transparent at the molecular level with regard to an aqueous solution of the water-soluble cellulose ether, defective portions appear in the products and they may lead to inferior transparency or inferior function. An aqueous solution of cellulose ether desirably has a high viscosity. Cellulose ether having a high viscosity has the higher insoluble fiber content than cellulose ether having a low viscosity so that it is thought to be difficult to obtain a transparent product.

With a view to overcoming the above-described problems, Japanese Patent Application Examined Publication 53-12954/1978 proposes a method comprising a step of allowing a raw material pulp to adsorb an aqueous alkali solution having a concentration of 15 to 75% by weight at 5 to 80° C., and then pressing the resulting pulp within 10 seconds to remove an extra portion of the aqueous alkali solution, a step of repeating the above step to obtain the corresponding alkali cellulose, and a step of reacting the alkali cellulose with an etherifying agent.

Japanese Patent Application Unexamined Publication No. 10-259201/1998 proposes a method comprising steps of impregnating a pulp having dichloromethane extract content of 0.07% by weight or less with sodium hydroxide, pressing the resulting pulp to obtain the corresponding alkali cellulose, and then etherifying the alkali cellulose.

According to Japanese Patent Application Unexamined Publication 2001-354701, cellulose ether is produced by a method comprising steps of pulverizing a pulp sheet having a sheet density of 0.4 to 1.0 g/ml into powders having an average particle size of 1000 μm or less, adding an alkali to the powders to yield the corresponding alkali cellulose, and then reacting the alkali cellulose with methyl chloride, propylene oxide and the like.

According to A. W. Anderson and R. W. Swinehart, Tappi, Vol. 39, No. 8, 548-553, August, 1956, presented is a method for producing alkali cellulose, comprising a step of impregnating a pulp sheet having a sheet density of 0.47 to 1.17 g/ml in a bath containing an alkali solution for 0.5 to 4.5 seconds.

According to U.S. Pat. No. 2,102,205, pulp is impregnated in an aqueous solution of sodium hydroxide for 2 hours, and then pressed.

The present inventors have found that in the method according to Japanese Patent Application Examined Publication No. 53-12954/1978, adsorption/removal of an aqueous alkali solution is performed twice so that the pulp swells with the solution and becomes fragile during the second adsorption/removal and as a result, troubles tend to occur and the cellulose ether produced by this method does not have a satisfactory quality; in the methods according to Japanese Patent Application Unexamined Publication Nos. 10-259201/1998 and 2001-354701, distribution of the alkali tends to be uneven because of use of powdery pulp so that cellulose ether having sufficiently low insoluble fiber content cannot be obtained; and in the method according to A. W. Anderson and R. W. Swinehart, Tappi, Vol. 39, No. 8, 548-553, August, 1956, the distribution of the alkali becomes uneven due to too short impregnation time so that satisfactory cellulose ether cannot be produced. The inventors have also found that the alkali cellulose produced by the method according to U.S. Pat. No. 2,102,205 has an extremely high sodium hydroxide/cellulose weight ratio of 3.0 so that an amount of side reaction increases, and as a result such alkali cellulose is not suited for the preparation of cellulose ether.

Considering the above findings, the present invention aims at providing a method for preparing cellulose ether which is transparent as dissolved in water and has low water-insoluble content.

The present inventors have carried out an extensive investigation with a view to overcoming the above-described problem. As a result, it has been found that cellulose ether which is transparent as dissolved in water and has low water-insoluble content can be prepared by using, as a raw material, alkali cellulose prepared by a method comprising steps of bringing a pulp sheet having a certain sheet density or being formed from pine, or chips into which the pulp sheet has been converted, into contact with an excess alkali metal hydroxide at a certain temperature for a certain period of time and then removing an extra portion of the alkali metal hydroxide, leading to the completion of the invention.

More specifically, the present invention provides a method for preparing alkali cellulose, comprising a contact step of bringing a pulp sheet having a sheet density of 0.60 g/ml or less or being formed from pine, or chips into which the pulp sheet has been converted, into contact with an alkali metal hydroxide solution at 5 to 70° C. for 10 to 600 seconds to obtain an alkali cellulose reaction mixture, and a drain step of draining the reaction mixture, wherein an amount of the alkali metal hydroxide solution to be used for the contact step is selected so that the alkali cellulose obtained by the drain step has a ratio of a weight of alkali metal hydroxide component determined by neutralization titration of the alkali cellulose to a weight of solid component in the pulp {(alkali metal hydroxide component)/(solid component in the pulp)} of 0.3 to 1.5. The invention also provides a method for preparing water-soluble cellulose ether, comprising a step of reacting the alkali cellulose with an etherifying agent.

According to the present invention, cellulose ether which is transparent as dissolved in water and has low water-insoluble content can be prepared.

The present invention now will be described more fully hereinafter in which embodiments of the invention are provided with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.