Organic compounds -- part of the class 532-570 series – Organic compounds – Carbohydrates or derivatives
Reexamination Certificate
2003-07-08
2004-12-28
Wilson, James O. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carbohydrates or derivatives
C536S056000, C536S058000, C536S063000, C536S123100, C536S124000, C536S070000, C536S095000, C536S128000
Reexamination Certificate
active
06835828
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for the modification of cellulose and particularly to a process for the preparation of reactive cellulosic hardwood pulp to make cellulose acetate with superior properties.
BACKGROUND OF THE INVENTION
Considerable effort has been expended to improve the reactivity of cellulose so as to decrease the manufacturing costs of cellulose derivatives. One known approach to achieving such a goal is the treatment of cellulosic pulp in the presence of an alkali with an alkylene oxide to produce a trace substituted hydroxyethylated pulp. Such a process was disclosed, for example, by J. L. Riley in Solvent Spun Rayon, Modified Cellulose Fibers and Derivatives, edited by A. F. Turbak, ACS Symposium Series 58, ACS, Washington, D.C. page 149 (1977). While some improvement in reactivity has been achieved by this reaction, the improvement was not sufficient to economically justify its use. The process, for example, does not improve the reactivity of cellulose in “low catalyst” acetylation processes, widely used for the preparation of cellulose acetate.
The treatment of cellulose with alkali under “mercerizing conditions” is a well known procedure which converts cellulose from its native form, cellulose I, to a more thermodynamically stable, less crystalline form, cellulose II. The critical concentration of the caustic solution (weight % of the solution of sodium hydroxide or, less often, potassium hydroxide) necessary for mercerization is dependent upon both temperature and type of cellulose. However, in general, the caustic solution strength for mercerizing conditions at ambient temperatures will be more than 5%, and usually about 6-16%. For a given caustic concentration, mercerization occurs more readily at low temperatures than high. At a given temperature, such as 0° C., a greater caustic concentration is required to mercerize a prehydrolyzed kraft pulp than a sulfite pulp, for example. The conditions (temperature, caustic concentration) required to mercerize various cellulosic materials are well known.
Because it is less crystalline and more amorphous, mercerized cellulose would presumably be more readily accessible to reagents. Unfortunately, mercerized pulps are relatively inert to acetylation as a consequence of structural collapse in the interfibrillar spaces upon drying. Thus, the prior art initially taught that just enough alkali should be used to refine cellulose during alkaline bleaching stages, but that the caustic amount and temperatures used should be insufficient to produce mercerizing conditions in the production of acetate grade pulps.
Early attempts to improve cellulose reactivity by hydroxyalkylation scrupulously avoided mercerizing conditions. Such conditions are well known and well defined in the cellulose industry. See, for example, Pulping Processes, S. A. Rydholm, Interscience Publishers, 1965, pages 143-145. In the alkylene oxide treatment of cellulose disclosed in the above identified publication by J. L. Riley, only a 2% solution of sodium hydroxide, based on pulp, was used to promote the reaction at ambient temperatures. This is well below the concentration of sodium hydroxide necessary for mercerizing pulp.
A prior attempt to produce a modified cellulose pulp which would be reactive in low catalyst acetylation reactions is described in U.S. Pat. No. 4,399,275. It focused on the use of softwood cellulose pulps. According to the process of U.S. Pat. No. 4,399,275, softwood cellulose is mercerized in conventional fashion to convert cellulose I to cellulose II and then alkylene oxides are reacted with the mercerized softwood pulp fibers. The patentee discovered that the alkylene oxides appeared to “wedge open” the mercerized softwood fibers, so that they did not collapse upon drying and, as a result, maintained a high level of reactivity in subsequent low catalyst acetylation processes.
Propylene oxide was found to be the preferred alkylene oxide for making mercerized softwood fibers which are reactive in low catalyst acetylation processes with desirable properties being achieved with hydroxypropoxy group (—OC
3
H
6
OH) content range of 3.6 to 5.7%, corresponding to a degree of molar substitution (d.s.) of 0.08 to 0.13 per anhydroglucose unit in cellulose.
Although the softwood cellulosic materials produced according to U.S. Pat. No. 4,399,275 were extremely reactive in acetylation reactions, it was found that to achieve commercially acceptable haze levels for most products, mercerized softwood pulp had to be reacted with undesirably large amounts of alkylene oxide. For example, at least 15% by weight of propylene oxide based on oven-dried (o.d.) pulp weight was required to achieve acceptable cellulose acetate properties. In order to achieve substantially improved triacetate haze values, it was determined that treatment with about 20% by weight propylene oxide based on o.d. pulp weight was necessary. Further, the modified cellulose pulps produced according to this patent were so highly reactive in known low catalyst acetylation processes as to require special cooling equipment to slow down the acetylation reaction to avoid damaging conditions.
Because of the special processing equipment required to moderate the acetylation reaction for alkylene oxide modified softwood pulps, and because of the relatively large amounts of alkylene oxide required to produce softwood pulps which would enable the manufacture of cellulose acetate products having desirable low haze levels, commercial interest never developed in the process of U.S. Pat. No. 4,399,275.
Accordingly, there exists a need for a modified cellulose pulp which is reactive to acetylation using standard low catalyst acetylation processes, but which is not so reactive in such processes as to require special equipment to moderate the acetylation reaction. Further, it would be desirable if such a modified cellulose pulp could be made without consuming large amounts of expensive reagents as to make the manufacture of such pulp cost prohibitive. Finally, the acetylation of a modified cellulose pulp should yield cellulose acetate products having low haze levels, so that they will be suitable for use in commercial applications having stringent haze requirements, such as in viewing screens for flat panel monitors.
It is accordingly a primary object of the present invention to provide a process for the preparation of modified cellulose having only moderate acetylation reactivity, specifically reactivity that is similar to that expected from a standard acetate wood pulp.
It is an additional object of this invention to provide a process for producing cellulosic pulps which will significantly improve the properties of cellulose acetate derived from them.
It is a more specific object of this invention to provide a cellulosic wood pulp which might improve levels of cellulose acetate production and reduce costs to manufacture superior cellulose acetate products when compared, for example, to use of alternate cellulosic materials such as high purity cotton linters.
SUMMARY OF THE INVENTION
The foregoing and other objects of the invention are achieved by the preparation of reactive cellulose by mercerizing hardwood cellulosic material, and reacting the mercerized hardwood cellulose with an hydroxyalkylating agent to a molar degree of substitution of the cellulose ranging from about 0.04 to about 0.15 per anhydroglucose unit in cellulose, the degree of substitution being insufficient to render the cellulose appreciably soluble in water, and recovering the substantially water insoluble product. The hardwood sources which are suitable for use in the invention are woods such as gum, maple, oak, eucalyptus, poplar, beech and aspen.
The hydroxyalkylating agents useful in the invention are generally the alkylene oxides, including ethylene, propylene and butylene oxides and halogen substituted alkylene oxides such as epichlorohydrin. Benefits of the invention are achieved with about 2.0% to about 5.5% by weight hydroxyalkoxy (—OC
n
H
2n
OH) group content on the cellulose. This corres
Hendricks Wendy
Sears Karl D.
Henry Michael C.
Levin Kramer
Naftalis & Frankel LLP
Rayonier Inc.
Wilson James O.
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