Process for preparing cellulose ethers

Non-gelling cellulose ethers are purified by mixing same with a dialdehyde under acidic conditions and under high agitation, forming a slurry of the treated cellulose ether, rapidly transferring the slurry to a filter and then dewatering said slurry.

BACKGROUND OF THE INVENTION 
This invention relates to a process of purifying cellulose ethers and, in 
particular, to a process for purifying cellulose ethers which do not 
exhibit a gel point in liquid water. 
Water-soluble cellulose ethers such as methyl cellulose, 
hydroxypropylmethyl cellulose, hydroxyethyl cellulose, hydroxyethylmethyl 
cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, and the like 
are widely used as thickeners, binders, drug additives and for many other 
uses. Such cellulose ethers are commonly prepared by reacting cellulose 
pulp with sodium hydroxide and an etherifying agent such as 
methylchloride, ethyleneoxide, propyleneoxide, chloroacetic acid and the 
like. Because many side reactions occur during the etherification 
reaction, the crude cellulose ether contains substantial amounts of 
impurities such as salt and various glycol ethers. It is usually necessary 
to remove these impurities before the cellulose ether is ready for use. 
Since these impurities are generally water-soluble, it would be desirable 
simply to wash the crude cellulose ether to dissolve the impurities 
therefrom. Unfortunately, however, the cellulose ether is itself 
water-soluble and accordingly, water washing to remove impurities results 
in substantial loss of product as well. 
Certain cellulose ethers such as methyl cellulose and hydroxypropylmethyl 
cellulose exhibit an inverse solubility in water with rising temperature. 
That is, as the temperature of the water increases, the cellulose ether 
becomes less and less soluble therein. For such cellulose ethers, there is 
typically a characteristic gel temperature above which the cellulose ether 
is not water-soluble. Such gelling cellulose ethers may be water washed by 
conducting the washing step above the gel temperature of the cellulose 
ether. 
Unfortunately, other cellulose ethers such as hydroxyethyl cellulose and 
hydroxyethylmethyl cellulose do not exhibit such a gel point at any 
temperature below the boiling point of water. Accordingly, one cannot 
solve the washing problem simply by using hot water to conduct the 
washing. It is therefore necessary to employ some other means to purify 
these non-gelling cellulose ethers. 
It is known to render such non-gelling cellulose ethers temporarily 
insoluble in water by crosslinking them with a dialdehyde such as glyoxal. 
For example, in U.S. Pat. Nos. 3,347,847 and 3,527,751 hydroxyalkyl 
cellulose is prepared as a suspension in isopropanol or an 
isopropanol/water azeotrope. The crude hydroxyalkyl cellulose is, while in 
the presence in said azeotrope, treated with acid and glyoxal to crosslink 
it for water washing. 
Similarly, in U.S. Pat. No. 3,709,876, hydroxyethylmethyl cellulose is 
prepared in a dry process (i.e., without liquid diluent) and is 
subsequently treated with glyoxal and acid to render it temporarily 
water-insoluble for washing. 
Unfortunately, however, these prior art processes have distinct 
disadvantages. For example, the process described in U.S. Pat. Nos. 
3,347,847 and 3,527,751 both require the use of an organic solvent to 
prepare and wash the cellulose ether. Thus, this organic solvent must be 
later removed from the cellulose ether, which presents the common problems 
normally associated with solvent recovery. 
The process of U.S. Pat. No. 3,709,876, while free of the disadvantages 
associated with the use of organic solvents, requires treating the 
cellulose ether with a relatively high (5 to 15 percent treatment level) 
amount of glyoxal to sufficiently crosslink the cellulose ether for 
washing. In addition, despite the glyoxal treatment, large amounts of the 
crude cellulose ether remain water-soluble and are subsequently lost 
during the water washing step. For these reasons, the efficiency of the 
process of U.S. Pat. No. 3,709,876 is far lower than desired. 
Accordingly, it would be desirable to provide a process for purifying crude 
non-gelling cellulose ethers in which the use of organic solvents is not 
required and low amounts of the crude cellulose ether are dissolved or 
otherwise lost during the purification process. 
SUMMARY OF THE INVENTION 
This invention is such a process for purifying a crude non-gelling 
water-soluble cellulose ether. The process comprises first mixing, under 
high agitation, a crude non-gelling water-soluble cellulose ether with 
from about 1 to about 5 percent by weight of said cellulose ether of a 
dialdehyde under acidic conditions whereby the cellulose ether is rendered 
water-insoluble. Then, the water-insoluble cellulose ether is formed into 
an aqueous slurry which is transferred to a filter by means of a slurry 
pump such that substantially none of the cellulose ether becomes dissolved 
in the water. Said slurry is subsequently dewatered on a filter to remove 
water-soluble impurities therefrom. 
The process of this invention provides several distinct advantages over 
prior art processes. First, the amount of dialdehyde necessary to 
satisfactorily render the cellulose ether water-insoluble is substantially 
reduced in this process. Additionally, the time required to obtain a 
satisfactorily crosslinked crude cellulose ether is substantially reduced 
from the time required in the prior art process. Additionally, the 
cellulose ether is more completely rendered water-insoluble using the 
process of this invention as compared to prior art processes. Because of 
these advantages, a faster, less expensive, more effective means of 
washing the crude cellulose ether is provided. 
DETAILED DESCRIPTION OF THE INVENTION 
The cellulose ether employed herein is one which is soluble in water at all 
temperatures below the boiling point of water at atmospheric pressure 
(i.e., below about 100.degree. C.). Such cellulose ethers are referred to 
herein as "non-gelling" cellulose ethers because they do not form 
gelatinous precipitates as an aqueous solution thereof is heated to its 
boiling point. 
Suitable such non-gelling cellulose ethers include, for example, 
hydroxyethyl cellulose, hydroxyethylmethyl cellulose, 
carboxymethylhydroxyethyl cellulose, carboxymethylhydroxyethylmethyl 
cellulose, hydroxyethylhydroxypropyl cellulose, dihydroxypropyl cellulose, 
dihydroxypropylhydroxyethyl cellulose and like cellulose ethers. 
The crude cellulose ether employed herein contains impurities which are 
capable of being removed from the cellulose ether by water washing. 
Preferably, said impurities comprise materials that are water-soluble. 
Said impurities are generally by-products of the etherification reaction 
of cellulose (i.e., salt and diverse water-soluble glycols and glycol 
ethers and the like). 
Advantageously, the crude cellulose ether is prepared in a dry process as 
described, for example, in U.S. Pat. No. 3,709,876. Alternatively, the 
crude cellulose ether is prepared in a slurry in a liquid organic diluent. 
However, if such a diluent is employed it is preferred to remove 
substantially all of said diluent before purifying the cellulose ether 
according to this process. 
In the first step of the process of this invention, the crude cellulose 
ether is mixed with from about 1 to about 5 percent by weight of said 
cellulose ether of a dialdehyde under acidic conditions. While many 
relatively low molecular weight dialdehydes are useful herein, glyoxal is 
preferred. Said mixing is advantageously performed at a pH from about 1 to 
about 6, preferably from about 2 to about 4. Adjustment of the pH of the 
crude cellulose ether is advantageously made by adding a weak organic 
acid, such as acetic, citric or, especially, formic acid. Most preferably, 
the crude cellulose ether is mixed with a treating solution comprising 
water, formic acid and glyoxal containing sufficient amounts of formic 
acid to adjust the pH into the aforementioned ranges and from about 1 to 
about 5 percent by weight of the cellulose ether of glyoxal. 
The aforementioned treatment is made under conditions of high agitation. In 
general, the agitation is such that the dialdehyde becomes thoroughly 
mixed with the cellulose ether within about 30 seconds of contact 
therewith. 
Said mixing may be performed in any suitable mixing apparatus which 
provides an adequate shear rate as described hereinbefore. Especially 
suitable are high intensity mixers such as are available commercially from 
Welex, Incorporated, Blue Bell, Pa. 
Upon mixing the crude cellulose ether and the dialdehyde, the crude 
cellulose ether becomes temporarily crosslinked within about 1 to 15, 
preferably about 1 to 5 minutes after said mixing is begun. After said 
crosslinking has progressed to a point where the cellulose ether is 
rendered insoluble in water, the crude crosslinked cellulose ether is 
formed into an aqueous slurry by the addition of water. Sufficient water 
is employed to form a relatively dilute slurry. Generally, from about 10 
to about 100, preferably about 20 to about 40 parts by weight of water are 
employed per part of crosslinked cellulose ether. The temperature of the 
water in said slurry may range from ambient to 100.degree. C. but is 
preferably from about 15.degree. to about 30.degree. C. 
The resulting slurry is then transferred to a suitable filter by means of a 
slurry pump such that dewatering of the slurry is begun before significant 
amounts of the cellulose ether can dissolve in the water. In this 
invention, it is desirable to minimize the time in which the cellulose 
ether is in contact with the wash water. Accordingly, the particular 
slurry pump employed is chosen in order to minimize the contact time. 
Preferred slurry pumps include screw feeders, volumetric screw feeders, 
and especially, progressive cavity pumps such as are sold under the name 
Moyno Quick Disassembly Pumps, by Moyno Pump Division of Robbins and 
Myers, Springfield, Ohio. In many of such slurry pumps, both the contact 
of the cellulose ethers into the wash water and transfer of the resulting 
slurry are accomplished in the pump in one operation. 
Water may be removed on the filter by gravitation or by the use of vacuum 
pulling equipment. Any suitable filter may be employed herein, but one 
which permits the continuous dewatering of the slurry, such as a straight 
line belt filter is preferred. Sufficient water is removed such that the 
resulting purified cellulose ether has a moisture content from about 10 to 
about 80 percent, preferably 40-60 percent based on the weight of the 
cellulose ether. 
If desired, the resulting purified cellulose ether may be further washed 
while on the filter to remove additional impurities therefrom. Each such 
washing is, of course, followed by dewatering to a moisture content as 
described hereinbefore. 
The resulting purified cellulose ether can be again rendered water-soluble 
by the addition of an aqueous base such as a dilute sodium hydroxide 
solution. The glyoxal crosslinks in said cellulose ether are hydrolyzed 
under basic conditions thereby destroying the crosslinks and restoring the 
solubility of the cellulose ether. If desired, however, the cellulose 
ether may be employed in the crosslinked form. Such crosslinks will slowly 
hydrolyze even under neutral conditions to form a water-soluble product. 
A major advantage of the process of this invention is that the entire 
treating and washing time is reduced from a typical 2 to 3 hours when 
using conventional processes, to about 1 to 1.5 hours. In addition, the 
amount of dialdehyde crosslinking agent necessary to render the cellulose 
ether water-insoluble is also significantly reduced. Still another 
advantage is that using this process, the crosslinking of the crude 
cellulose ether is much more uniform and complete than the crosslinking 
afforded by conventional processes. Accordingly, losses of product during 
the washing step are greatly reduced. Typically, less than 10, preferably 
less than about 7, more preferably less than about 5 percent of the crude 
cellulose ether is lost during the washing process. Using conventional 
processes, the said filtered losses are often as high as 40 percent of the 
weight of the crude cellulose ether. 
The cellulose ether purified according to this process may be used for the 
same purposes and in like manner as similar cellulose ethers which are 
purified using conventional processes. In particular, said cellulose 
ethers are useful as thickeners, rheology modifiers, surfactants, binders, 
adhesives, protective coatings and like applications.

The following examples are provided to illustrate the invention, but not to 
limit the scope thereof. All parts and percentages are by weight unless 
otherwise indicated. 
EXAMPLE 1 
Into a 6 liter steam-jacketed laboratory Welex blender, are added 674.6 
grams (g) of a crude hydroxyethylmethyl cellulose containing about 76.3 
percent by weight HEMC, 12 percent by weight salt and 11.7 percent by 
weight organic impurities. The Welex blender is turned on to 1500 rpm. The 
jacket is heated with steam and water so that the temperature of the crude 
HEMC is 75.degree. C. To the heated crude HEMC is added 33.7 g of a 32 
percent glyoxal, 18 percent formic acid and 50 percent water solution. The 
amount of this solution added is 5 percent by weight of the cellulose 
ether (on a crude basis). After the addition of the glyoxal solution, the 
temperature of the HEMC is cooled to 40.degree. C. The product is then 
slurried in 5,000 ml cold water for 10 minutes and then filtered. Of the 
original HEMC, 94.6 percent is recovered from the filter. Only 3.0 percent 
of the original HEMC (on a purified basis) is found to be dissolved in the 
wash water. 
EXAMPLE 2 
Into a Welex blender operating at 500 rpm are added 70.6 lbs of a crude 
HEMC. Said crude HEMC contains 35 lbs of a hydroxyethylmethyl cellulose 
having a methoxy degree of substitution of 0.94 and hydroxyethyl molar 
substitution of 2.23. To the blender is then added 1.4 lbs of a treating 
solution containing 50 percent water, 32 percent glyoxal and 18 percent 
formic acid. This corresponds to the addition of 2 percent glyoxal by 
weight of the HEMC (on a purified basis). The treating solution and crude 
HEMC are mixed in the Welex blender for 30 minutes and then transferred to 
a Moyno progressive cavity pump. To the pump are gradually added 2585 lbs 
of cold water to form a slurry of the crude HEMC. The slurry is then 
pumped onto a straight line belt filter where it is dewatered. From the 
belt filter are recovered 110 lbs of product of which 31.9 lbs are HEMC 
(dry). Total recovery of the product is 93 percent. The purified product 
contains 2.6 percent salt and trace amounts of other organic impurities. 
Total time required for the treating slurry formation and dewatering steps 
is 30 minutes. 
COMATIVE EXAMPLE NO. C-1 
A 27.6 lb portion of crude HEMC (14.5 lbs HEMC on a purified basis) are 
heated to 80.degree. C. and sprayed with a treating solution containing 50 
percent water, 32 percent glyoxal and 18 percent formic acid. Sufficient 
treating solution is added so that 7 percent by weight of the HEMC 
(purified basis) of glyoxal is added to the crude HEMC. After the addition 
of treating solution, the HEMC is agitated at 500 rpm for 15 minutes and 
then cooled to 40.degree. C. The resulting product is then 
water-insoluble. The insolubilized product is washed by filling the 
reactor with water and then pressuring the mixture to a slurry tank. From 
a slurry tank the mixture is dumped into a belt line filter. A vacuum of 
10 mm mercury is pulled on the filter. The product is then manually taken 
from the filter, weighed and analyzed. The resulting product contains 72 
percent moisture. Of the HEMC treated in this manner, only 91 percent is 
recovered. The total time required for purification is about 150 minutes.