Background of the Invention Cellulose derivatives have been commercially available since the advent of camphor plasticized nitrocellulose as the first synthetic plastic material in 1868.

About the time of the first World War cellulose acetate began to be used as a nonflam- mable aircraft lacquer. It was not until the decade of the 1940s that cellulose ethers be- came commercially important. Water soluble methyl cellulose was introduced in 1939 and ethyl cellulose became important during World War II. Sodium carboxymethyl cellulose (NaCMC) became available shortly after the war and, today is one of the most widely used cellulose derivatives. Many other cellulose ethers are produced as specialty chemicals.

For many years purified cotton linters was the major raw material for production of cellulose esters and ethers. However, for some products high alpha cellu- lose sulfite or prehydrolyzed kraft wood pulps have also been used. If NaCMC may be used as an example, it is available in various viscosity grades. The term"viscosity"here refers to viscosity of a water solution. In turn, solution viscosity is closely related to degree of polymerization (D. P.) of the cellulose raw material. High viscosity grades have traditionally been produced from cotton linters cellulose while medium and low viscosity grades are made using lower cost wood pulps. The D. P. of most wood pulps does not exceed about 1500 although some specialty dissolving pulps have considerably higher D. P. Cotton linters cellulose can be readily produced with a D. P. of 2400 or greater.

Because of their cost advantage, it would be advantageous to the manu- facturers of various cellulose derivatives if wood pulps could be used more extensively.

Although not limited to wood derived cellulose sources, the present invention serves that end by increasing the effective D. P. of celluloses to make them suitable raw materials for derivatives requiring a high D. P starting material.

Summary of the Invention The present invention relates to preparation of cellulose derivatives using crosslinked cellulose as a basic raw material. By introducing a controlled amount of crosslinking, the effective D. P. of the cellulose may be significantly raised without destroying cellulose reactivity. In turn, the crosslinked product may be used for prepara- tion of derivatives that formerly required very high D. P., and usually very high priced, cellulose sources as a starting raw material.

The preferred cellulose is a wood pulp cellulose of the type normally referred to as a"dissolving pulp"or"high alpha chemical pulp". These are cellulosic materials normally prepared by the sulfite or prehydrolyzed kraft processes. Other cellulose sources having sufficient reactivity with etherification or esterification chemi- cals after crosslinking are also considered suitable. Among these are paper grade sul- fite pulps, the paper or fluff grades of kraft pulps nominally called"kraft market pulps", cotton linters cellulose, and cellulose obtained from other plant or bacterial sources.

The term cellulose derivatives should be read broadly and is intended to include both esters, such as nitrocellulose and cellulose acetate, and ethers such as methyl cellulose, ethyl cellulose, carboxymethyl cellulose, carboxyethyl cellulose, car- boxypropyl cellulose, hydroxyethyl cellulose, carboxymethylhydroxyethyl cellulose, and many others.

Crosslinked celluloses of various types are well known; e. g., see G. C.

Tesoro, Cross-linking of cellulosics, In Handbook of Fiber Science and Technology, Vol. 11, M. Lewin and S. B. Sello eds., pp. 1-46, Marcel Decker, New York (1993), as exemplary. Crosslinking forms chemical or ionic bridges between hydroxyl groups on adjacent cellulose molecules. Commercially, crosslinking is important in cotton fabric finishes to provide wrinkle resistance. Crosslinked wood pulp fibers are also used in other consumer articles such as disposable diapers or paper towels where their relative stiffness serves to increase bulk and compression resistance. However, to the present inventors'knowledge, crosslinked cellulose has never before been used as a raw mate- rial for production of cellulose derivatives.

While many crosslinking agents are believed to be suitable in the present process, those that produce compounds resistant to D. P. loss during derivitization are preferred. In particular, those that form ether linkages with the cellulose are particu- larly preferred. Agents that produce hemiacetal linkages have also been found to be suitable.

It is an object of the present invention to provide a method for produc- tion of various cellulose derivatives using crosslinked cellulose.

It is a further object to provide high effective D. P. wood pulps as a raw material for derivitization.

It is another object to provide cellulose derivatives made using wood pulps which have the D. P. characteristics of cotton celluloses.

It is yet an object to provide cellulose derivatives having entirely new properties heretofore unattainable.

It is also an object to produce a high viscosity grade of NaCMC from a modified low cost wood pulp cellulose.

These and many other objects will become readily apparent upon read- ing the following detailed description taken in conjunction with the drawings.

Brief Description of the Drawings FIG 1. shows viscosities of CMC made from wood cellulose having varying degrees of crosslinking.

FIG. 2 shows rheological properties of four CMC samples Detailed Description of the Preferred Embodiments Certain crosslinked cellulose derivatives that have been crosskinked af- ter their formation are reported in the literature. U. Anbergen and W. Opperman, Polymer 31: 1854-1858 (1990), report swelling properties of carboxymethyl cellulose and hydroxyethyl cellulose crosslinked with divinylsulfone in alkaline solution. Their compounds were investigated as possible superabsorbent polymers. S. Suto and K. Su- zuki report swelling behavior of hydroxypropyl cellulose crosslinked with either gly- oxal or glutaraldehyde, Polymer 38: 391-396 (1997). However, the polymers were originally prepared conventionally and crosslinking was done after etherification of the cellulose.

The following examples will serve to disclose the best mode presently known of practicing the present invention.

Example 1 Preparation of DCP Crosslinked Cellulose 1,3-dichloro-2-propanol (DCP) is a known cellulose crosslinking agent.

A sample of never dried T758 wood pulp was obtained as an 18.2% solids wet lap from a Weyerhaeuser Company mill at Cosmopolis, Washington. This is a sulfite etherifica- tion grade pulp with a D. P. of 2150 and an a-cellulose content of 95.3%. A 120 g (dry weight basis) sample was added to water to produce 1200 g of a 10% consistency slurry. To this slurry was added 24 g of 10% NaOH solution. Stirring was continued for 0.5 hr at room temperature. At that time 24 g of a 10% DCP solution in water was added. Stirring was continued for 15 minutes at room temperature and then for 2 hr at 60°C. The resulting crosslinked pulp was thoroughly water washed and then dried.

D. P. of the crosslinked material was determined by its cupriethylenediamine (cuene) viscosity using ASTM method D 1975-62 to be about 2873.

Example 2 Preparation of CMC from Crosslinked Pulp Within the context of the present description the terms CMC and NaCMC should be considered synonymous. A 5 g sample of the crosslinked pulp of Example 1 was slurried in 133 mL isopropanol. To this was added 13.3 mL of 30% NaOH solution over a 30 minute period. This suspension was then stirred for 1 hr at 25°C. At that time 6 g of monochloroacetic acid was added (as 25.4 mL of a solution of 23.6g MCA/100 mL isopropanol) over a 30 minute period. The temperature was raised to 55°C and stirring continued for 3.5 hr. The resulting fibrous NaCMC was drained and washed with 70% methanol. The sample was brought to neutrality (pH 7.0) with acetic acid and again washed, first with 70% and then 100% methanol at 60°C until essentially salt free.

Example 3 In the following experiments two additional pulp grades from the above pulp mill were used. MAC is a sulfite pulp intended for cellulose acetate production and PH is a paper grade sulfite pulp. These were crosslinked in similar fashion to the sample of Example 1 although the amount of crosslinker and NaOH concentration were varied as is seen in the following table.

Table 1 Pulp DCP, % NaOH, % D. P. MAC------1585 MAC 10 1867 MAC 10 10 Insoluble MAC 10 10 Insoluble PH------1461 PH 0.1 0. 1 1461 PH 0 3. 0 1474 PH 1. 0 1. 0 1547 PH 1. 0 1. 0 1551 PH 1. 5 1. 5 1571 PH 2. 0 2. 0 1703 PH 3. 0 3. 0 1874 PH 4. 0 4. 0 1562*

*Sample not completely soluble in cuene. Indicated D. P. questionable.

It might be noted that the NaOH solution used above also extracted some residual hemicellulose and short chain cellulose and increased D. P for both grades. The extraction effect was significant at 10% NaOH concentration but of minor consequence at 3-4% NaOH usage.

Example 4 The crosslinked PH pulps of Example 3 made with 0%, 3%, and 4% DCP were used to make CMC according to the procedure of Example 2. The resulting NaCMC products were dissolved in water at percentages of 0.2 to 1.0. The resulting solution viscosities measured and are shown in FIG. 1. It is noteworthy that the NaCMC made with 4% DCP had a solution viscosity about 15 times greater than that of CMC made with uncrosslinked pulp.

Example 5 Two samples of crosslinked grade T758 sulfite pulp were made in which 1.5% epichlorohydrin, based on dry pulp, was used as the crosslinking agent. The pro- cedure was generally that of Example 1 with the following variations. One sample was prepared without any NaOH addition while the other had 1.5% NaOH. Since epichlo- rohydrin is essentially insoluble in water, a water suspension was made using high shear agitation prior to addition to the pulp slurry in water. In each case a 10% epichlo- rohydrin suspension was prepared and 18 g of the epichlorohydrin suspension was added.

NaCMC was prepared from the above samples according to the proce- dure of Example 2. A comparison sample was also made using grade T758 pulp crosslinked with 1.5% DCP, and 1.5% NaOH based on cellulose. A fourth NaCMC preparation was made as a control sample using an uncrosslinked etherification grade pulp from Borregaard, Sweden. This pulp has a D. P. of 2360 and a-cellulose content of 91.6%.

It is well known that CMC exhibits more or less thixotropy in aqueous solution. Samples of the four CMC products made above were made as 0.25% aqueous solutions. Viscosity was measured using a Brookfield viscometer with a #2 spindle at various spindle speeds. Rheological properties of these samples are shown as curves plotted on FIG. 2.

Example 6 A sample of grade T758 cellulose was crosslinked with glyoxal gener- ally according to the procedure of Example 1 with the following differences. Instead of DCP, 24 g of a 10% solution of glyoxal in water was used. In addition, 2.4 g of A12 (SO4) 3 was added as a catalyst. The crosslinking reaction was carried out for 3 hr at 85°C.

NaCMC was made from the glyoxal crosslinked cellulose according to the procedure outlined in Example 2. A control sample of CMC was also made from the uncrosslinked T758 cellulose. Both CMC samples were dissolved in water to form a 1% solution and viscosities were measured at varying shear rates as in Example 3.

Results are shown in the following Table.

Table 2 Shear Rate, rpm 50 20 10 5 2.5 1.0 0.5 Glyoxal Crosslinked, 488 592 628 648 656 8780 880 Viscosity,cp Untreated, Viscosity, cp 64 64 64 64 70 80 80

In addition to carboxymethyl cellulose, the crosslinked cellulose inter- mediate is also advantageous for preparation of hydroxyethyl or hydroxypropyl cellu- lose by reaction respectively with ethylene oxide in an alkaline environment under con- ditions extensively reported in the literature. Similarly, methyl cellulose may be pre- pared by reaction of the crosslinked intermediate with methyl chloride and ethyl cellu- lose by reaction with ethyl chloride, again under alkaline conditions.

Crosslinked cellulose is also useful for preparation of cellulose esters, especially cellulose acetate, by reacting the crosslinked intermediate with acetic anhy- dride under known conditions. In this case, it is preferable to use relatively a lower D. P. cellulose; e. g. D. P.-1400, as a starting material to reduce tendency to gel forma- tion that might interfere with filterability.

In addition to the cellulose crosslinking agents noted above in the exam- ples, many others are believed to be suitable for making crosslinked cellulose suitable for derivatizing. Among these are urea formaldehyde and melamine formaldehyde condensates, dimethyloldihydroxyethylene urea (DMDHEU), dimethyldihydroxyethyl- ene urea (DMeDHEU), citric acid, methylene-bis-acrylamide, and dicarboxylic acids such as succinic acid, and polyamide-epichlorohydrin (PAE) resins, to name several exemplary compounds While the inventors have exemplified the best mode known at the time of filing, it will be apparent to those skilled in the art that many variations in the meth- ods described and products produced can be made without departing from the spirit of the invention. It is the intent of the inventors that these methods and products should be included within the scope of the invention if encompassed within the following claims.