Viscosity reduction of cellulose derivatives

In the method of making low molecular weight cellulose ethers and esters by contacting such ethers or esters of higher molecular weight with HCl or other hydrogen halide gas, yellowing of the depolymerized or degraded product is inhibited by treating the dry material with SO.sub.2 at about ambient temperature. Residual acid in the product can be neutralized conveniently by treatment with a weak base such as ammonia or sodium bicarbonate. The low molecular weight-low viscosity product is useful in detergent compositions and in coatings for medical pills and capsules.

BACKGROUND OF THE INVENTION 
It has long been known that cellulose derivatives of relatively high 
molecular weight are readily depolymerized or degraded by treatment with 
strong mineral acid at moderate temperatures to produce corresponding 
polymers of much lower molecular weight. Cellulose derivatives such as 
lower alkyl and hydroxy lower alkyl ethers, for example, methyl cellulose, 
ethyl cellulose, hydroxypropyl cellulose, and methyl hydroxybutyl 
cellulose, cellulose esters such as the acetate and butyrate, and methyl 
hydroxylpropyl cellulose phthalate, carboxyalkyl cellulose such as 
carboxymethyl cellulose, and other such derivatives having two or more of 
such substituents on the cellulose molecule are representative examples of 
the class. The derivatives of relatively low molecular weight are of 
interest particularly because of their higher solubility in water which 
makes them useful as modifiers in detergent compositions and as 
water-dispersible films for coating medical tablets and like applications. 
This kind of acid-promoted degradation or depolymerization is conveniently 
done by contacting the essentially dry powdered cellulosic polymer with 
gaseous HCl or other hydrogen halide in a fluidized bed operation, for 
example, or by contacting the powder or slurry of the powder in an inert 
organic liquid with gaseous halide in a mixing device such as a rotating 
mixer, ribbon blender, or the like. For the slurry type of 
depolymerization, a relatively low boiling inert and essentially anhydrous 
organic liquid with boiling point below 100.degree. C is preferred. Since 
the cellulosic polymers are essentially insoluble in common organic 
solvents, substantially any such solvent can be used, for example, 
methylene chloride, methanol, 1,1,1-trichloroethane, carbon tetrachloride, 
acetone, hexane, and benzene. In any of these modes of depolymerization, a 
moderate temperature from ambient temperature to about 80.degree. C is 
suitable. 
Residual acid in the polymer product can be removed by purging the dry 
powder or a slurry of the powder with air or nitrogen or by washing the 
powder with an organic solvent such as described above. The last traces of 
acid can be removed by neutralization with a weak base. A dry, essentially 
pure product with no need for further processing for many applications is 
thereby provided. 
Hydrogen halide initiated depolymerizations of the kind described are 
disclosed by Reid, U.S. Pat. No. 1,864,554 and by Ouno, U.S. Pat. No. 
3,391,135. 
Unfortunately, the low molecular weight polymers made by these processes 
typically develop a yellow or brown color and so are undesirable for many 
applications. 
It is known that high molecular weight cellulose ethers and esters can be 
bleached or brightened by stirring a slurry of the cellulosic polymer in 
aqueous lower alkanol containing dissolved bisulfite ion. This process is 
described by Whitmeyer in U.S. Patent 3,549,617. Although the brightening 
treatment disclosed in that patent is effective for the higher molecular 
weight ethers and esters described therein which are insoluble in aqueous 
alkanol, the corresponding low molecular weight polymers produced by 
HCl-initiated depolymerization are much more affected by the presence of 
significant quantities of water and form unmanageable gels or actually 
dissolve when slurried in aqueous alkanol as described by Whitmeyer. 
SUMMARY OF THE INVENTION 
It has been discovered that SO.sub.2 can be used to bleach or brighten 
these low molecular weight cellulosic polymers in the absence of an 
aqueous medium. It has now been found that in the process for making a low 
molecular weight cellulosic polymer by contacting the powdered, 
essentially dry polymer of relatively high molecular weight with hydrogen 
halide at moderate temperature as described above, yellowing of the low 
molecular weight product is inhibited or substantially prevented by 
contacting the product as an essentially dry material with gaseous 
SO.sub.2 at about ambient temperature. The improvement is conveniently 
obtained by admitting SO.sub.2 gas to a depolymerization reactor as a 
second process step after the hydrogen halide-initiated depolymerization. 
Hydrogen chloride is of course the preferred halide for the 
depolymerization step. 
DETAILED DESCRIPTION 
The quantity of SO.sub.2 used in this improvement is not critical because 
any significant proportion will have an anti-yellowing effect. In most 
cases, about 0.05-2 percent SO.sub.2 based on the weight of polymer is 
sufficient and 0.08-0.2 percent SO.sub.2 is usually sufficient. The 
prevailing room or outside temperature is satisfactory for the SO.sub.2 
treatment, i.e., about 15.degree.-40.degree. C, although temperatures 
somewhat below or above this range can be used. 
The SO.sub.2 treatment of the essentially dry, HCl-depolymerized polymer is 
preferably done in the same way as described above for the 
depolymerization process, that is, by contacting the dry free-flowing 
powder with SO.sub.2 in a fluidized bed apparatus or by contacting the dry 
powder or slurry of the powder in a dry organic solvent with SO.sub.2 in a 
rotating mixer, a ribbon blender, or other such mixing device. Suitable 
organic solvents for a slurry treatment process are listed above in the 
description of the depolymerization process. Treatment of the dry, finely 
divided powder in a fluidized bed is preferred. 
The cellulosic polymers to which the improved process is applicable are 
those known to the art as cited and enumerated above, that is, the alkyl 
ethers, hydroxyalkyl ethers, carboxyalkyl ethers, and the esters of low to 
ultra-low molecular weight, e.g., those polymers which have a 2 percent 
aqueous solution viscosity below about 100 cps at 20.degree. C, 
particularly those having a 2 percent viscosity below 10 cps. Normally 
these polymers contain from trace amounts up to about 5 percent of 
moisture although their appearance is that of essentially dry solids. 
Polymers containing about 0.01-5 percent by weight water, are operable in 
the process but preferably the moisture content is limited to a maximum of 
about 3 percent. 
Residual HCl or SO.sub.2 can be removed from the treated polymer powder by 
any convenient means as previously described, that is, blowing with air or 
nitrogen, or by washing with a dry low boiling organic liquid such as 
methanol, methylene chloride, acetone, or other such common organic 
solvent previously described as suitable for treatment of the powdered 
cellulosic polymer as a slurry. In order to remove the last traces of 
acid, it is usually necessary to contact the product polymer with a weak 
base, preferably by blowing dry NH.sub.3 through the reactor, or blending 
the powder or slurry with dry sodium bicarbonate. 
By this improved process, cellulosic polymers of relatively high molecular 
weight, for example, methyl cellulose and hydroxy lower alkyl methyl 
cellulose, having a 2 percent water solution viscosity as high as several 
hundred thousand centipoises at 20.degree. C can be transformed to the 
corresponding polymers as stable low color powders having a 2 percent 
aqueous solution viscosity below 100 cps at 20.degree. C. Since the 
problem of color in the product becomes more severe as polymers are 
degraded to lower molecular weights, this improved process is of most 
value in the production of ultra-low molecular weight products. The 
SO.sub.2 -treated polymer dissolves in water at a significantly faster 
rate, a substantial advantage over the untreated material in many 
applications.

EXAMPLE 1 
Into a one-liter glass flask equipped with a ground joint and stopcock was 
charged 102 g of hydroxybutyl methyl cellulose (2 percent aqueous solution 
viscosity = 14700 cps in Ubbelohde viscometer at 20.degree. C, salt = 0.41 
percent, moisture = 2.3 percent). The flask was connected to a 
conventional vacuum line, and was evacuated to 4 mm Hg. Hydrogen chloride 
was expanded into the flask from a one-liter storage flask having 540 mm 
Hg of HCl at 22.degree. C. The gas transfer was stopped when HCl pressure 
in the storage flask reached 71 mm Hg. The amount of HCl introduced to the 
flask was calculated to be 0.9 percent by weight of the cellulose ether. 
The flask containing cellulose ether and HCl was isolated from the vacuum 
line, and rotated by an electric motor at ambient temperature. The flask 
was opened after 70 hours of degradation in vacuo at 22.degree. C. 
Titration of the cellulose ether showed that it contained 0.8 percent HCl 
and 1.84 percent salt. The cellulose ether was poured onto a 100-mesh 
screen and was purged with nitrogen for one hour at ambient temperature. 
Titration of sample showed that it contained 0.09 percent HCl, 0.7 percent 
salt and 0.4 percent moisture. About 15 g of the purged cellulose ether 
was put into an 8-ounce bottle, and was neutralized with excess ammonia 
gas. 
The bulk of the purged cellulose ether (78 g) was placed into another 
one-liter glass flask, connected to the vacuum line, and evacuated to 4 mm 
Hg. Sulfur dioxide gas was introduced to the cellulose ether in a manner 
similar to that described earlier for HCl, i.e., 107 mm Hg of SO.sub.2 gas 
was transferred from a one-liter storage flask at 22.degree. C to the 
cellulose ether. The amount of SO.sub.2 was calculated to be 0.48 percent 
(by weight). The flask was rotated for 4 hours, opened, and contacted with 
ammonia gas to neutralize residual acid. Analysis of the sample showed 0.7 
percent salt as NaCl, 0.4 percent moisture, 2 percent viscosity = 22.8 
cps, and an APHA color rating of 10. The degraded cellulose ether without 
SO.sub.2 treatment had an APHA color rating of 50. 
EXAMPLE 2 
Using the procedure of Example 1, 100 g of methyl cellulose (2 percent 
viscosity = 3260 cps, salt = 0.38 percent, moisture = 3.5 percent) was 
contacted with 2.16 percent HCl gas (calculated from pressure and volume 
data). The flask was then equilibrated with nitrogen to atmospheric 
pressure, and degraded in an oven maintained at 50.degree. C for 72 hours. 
The flask was evacuated at 6 mm Hg for 2 hours, then opened to air for 
sampling. Titration showed the cellulose ether contained 0.92 percent 
residual HCl. About 35 g of the degraded cellulose ether was neutralized 
with excess ammonia gas. The bulk of the degraded cellulose ether (58 g) 
was put into another one-liter flask, evacuated, and contacted with 0.69 
percent by weight of sulfur dioxide gas. The flask was rotated for 4 hours 
at room temperature and then residual SO.sub.2 was neutralized with excess 
ammonia gas. Analysis of the product showed: salt (as NaCl) = 1.6 percent, 
moisture = 2.6 percent, 2 percent viscosity = 1.44 cps. A 0.25 percent 
aqueous solution had an APHA color of 125. A 0.25 percent solution of the 
degraded cellulose ether without sulfur dioxide treatment showed an APHA 
color of over 200. 
EXAMPLE 3 
A quantity of 7810 g of finely ground (99 percent passed through a #40 U.S. 
standard sieve) hydroxypropyl methylcellulose containing about 2 percent 
moisture was loaded into a vertical Plexiglas plastic column (30 cm 
diameter .times. 152 cm height) with a perforated Teflon plate at the 
bottom and connected to a blower and heat exchanger for controlled 
fluidized bed operation. The column was flushed with nitrogen to remove 
air and was heated to 54.degree. C. Gaseous HCl was then introduced into 
the bottom of the column and was passed up through the fluidized bed of 
hydroxypropyl methylcellulose (HPMC) for 25 minutes at 
54.degree.-63.5.degree. C at an average rate of 6 g/minute. The 
temperature was maintained at 59.degree.-63.5.degree. C for an additional 
108 minutes to facilitate degradation. During operation, entrained solids 
were separated from effluent and both solids and gas were recycled to the 
reactor. 
A large part of the HCl was removed from the treated HPMC by blowing 
nitrogen through the fluidized bed at 60.5.degree.-68.5.degree. C for 1.5 
hours, thereby reducing the HCl content to 0.27 percent from the original 
0.93 percent. The nitrogen-flushed bed was then cooled to 32.degree. C and 
15 g of SO.sub.2 was passed through the bed at a rate of about 5 g/minute. 
In order to neutralize residual acids in the treated HPMC, 14 g of NH.sub.3 
was then passed into the bed of powder. Excess NH.sub.3 was removed by 
blowing with nitrogen and the product was removed from the column. It was 
found that the viscosity (in 2 percent aqueous solution at 20.degree. C) 
of the product had been reduced from an original 4,000 cps to 4.31 cps as 
measured by the Ubbelohde viscometer. The color of the 2 percent solution 
of treated material was below 25 APHA as compared to &gt;75 APHA color for a 
similar solution of HCl-degraded material which had not been given the 
SO.sub.2 treatment. 
EXAMPLE 4 
A two-liter glass reactor flask was charged with about 1150 g of dry 
acetone. Gaseous HCl was bubbled into the acetone until aliquot titration 
showed an HCl concentration of 3.6 percent by weight of the solution. The 
weight of the resulting HCl/acetone solution was 1135 g. The reactor was 
then equipped with a stirrer, a reflux condenser, and thermocouples. One 
hundred grams of hydroxypropyl methyl cellulose (2 percent viscosity = 
12500 cps, salt = 1.08 percent, moisture = 3.0 percent) was charged into 
the HCl/acetone solution. Concentration of HCl to cellulose ether was 
calculated to be 41 percent by weight. The reactor was heated with an 
electrical heating mantle to 56.degree. C in 30 minutes and the contents 
were stirred at that temperature for one hour and 40 minutes. The slurry 
was filtered and washed with excess acetone. A sample of 36 g of the 
filter cake was taken, stirred into about 100 cc of acetone, and 
neutralized with sodium bicarbonate. The rest of the filter cake was 
stirred into 945 g of acetone containing 0.13 percent of SO.sub.2. The 
slurry was stirred for one hour at ambient temperature. Sodium bicarbonate 
powder, 0.029 g, was charged into the slurry, and the slurry was kept 
stirred for one hour at ambient temperature. The slurry was then filtered, 
washed with excess acetone, and air-dried. Product analysis showed: 
2 percent viscosity = 2.47 cps 
volatile = 2.8 percent 
salt (as NaCl) = 2.05 percent 
Apha color of 2 percent solution = 10 
The product without SO.sub.2 treatment had an APHA color rating of between 
25 and 50. 
EXAMPLE 5 
A four cubic foot capacity, nickel clad stationary reactor was equipped 
with a water jacket, a horizontally rotated paddle agitator and other 
usual accessories. Into the reactor was loaded 22700 parts of powdered 
hydroxypropyl methyl cellulose (2 percent viscosity = 4140 cps, salt = 0.6 
percent, moisture = 3 percent). The agitator was rotated at about 20 rpm, 
and the content was heated to 60.degree. C by circulating hot water. The 
reactor was evacuated to 63 mm Hg, and 170 parts of HCl gas was added 
rapidly (0.75 percent by weight to cellulose ether). The reactor 
temperature rose to 72.degree. C briefly due to the exothermicity of gas 
absorption, but was returned to 60.degree. C after 20 minutes. The reactor 
was agitated at jacket temperature of 60.degree. C for a total of 51/2 
hours, then evacuated to 70 mm Hg for one hour to remove HCl. A small 
sample was taken from the reactor under nitrogen blanket, and was 
neutralized with excess NH.sub.3 gas. The reactor was evacuated to 70 mm 
Hg, and 30 parts of SO.sub.2 gas was added (0.13 percent by weight to 
cellulose ether). The reactor was cooled to ambient temperature in 30 
minutes, and 45 parts of NH.sub.3 gas was added (0.2 percent by weight to 
cellulose ether). After agitating the contents for 10 minutes, the excess 
NH.sub.3 gas was removed by evacuation, and the finished product packaged. 
The degraded or depolymerized cellulose ether showed: 
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2 percent viscosity (at 20.degree. C, Ubbelohde) 
3.5 cps 
Moisture 1.9 percent 
Salt (as NaCl) 1.16 percent 
APHA Color on 2 percent solution 
75 
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A 2 percent solution of product without SO.sub.2 treatment has an APHA 
color rating of 125.