Process for preparing a homogeneous cellulose solution using N-methylmorpholine-N-oxide

A simple method of preparing a homogeneous cellulose solution is disclosed, which comprises the steps of (a) preparing fibrillar cellulose powder; (b) injecting a molten liquid tertiary amine oxide solvent into a twin screw extruder; (c) feeding the cellulose powder of step (a) into a section of the twin screw extruder where the molten liquid tertiary amine oxide solvent fed in step (b) produces a well swollen paste with the cellulose powder fed in step (c); (d) dissolving the well swollen cellulose paste in the following melting sections in the twin screw extruder; and (e) stabilizing the solution obtained in step (d) in a storage tank.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for preparing a highly 
homogeneous cellulose solution which can be used in the production of 
cellulosic fibers or films. More particularly, the present invention 
relates to a process for preparing a highly homogeneous cellulose solution 
by directly dissolving cellulose powder in a liquid 
N-methylmorpholine-N-oxide solvent in an extruder. 
2. Description of the Prior Art 
Graenacher et al. first suggested a process for preparing a cellulose 
solution in a tertiary amine oxide in U.S. Pat. No. 2,179,181. 
Subsequently, more efficient and economical processes have been proposed. 
U.S. Pat. Nos. 4,142,913, 4,144,080, 4,196,262 and 4,246,221 disclose 
processes which comprise swelling cellulose in an aqueous solution of a 
tertiary amine oxide having a water content of 22% or more, which will not 
dissolve the cellulose due to its relatively high water content, and 
subsequently distilling off the excess amount of water in the resulting 
slurry to dissolve the cellulose therein. The processes require an 
extended period of time to perform the distillation under reduced 
pressure. However, the distillation tends to cause discoloration of the 
solution due to the thermal degradation of the solvent and cellulose. As a 
whole, these processes are complicated as well as time and energy 
consuming. 
Similarly to the above U.S. Patents, European Patent No. 356,419 to Zikeli 
et al. discloses a continuous process of producing 72 kg of cellulose 
solution per hour, wherein distillation/concentration is a still essential 
step. More specifically, the process comprises swelling cellulose in an 
aqueous N-methylmorpholine-N-oxide (hereinbelow, referred to as "NMMO") 
solution having a water content of 40%, and distilling the swollen slurry 
under reduced pressure in a screw extruder with fan-shaped flights. 
Quigley also uses a thin film evaporator as a reduced-pressure distillater 
as disclosed in WO 94/06530. However, not only is the highly viscous 
slurry not efficiently concentrated, but the reduced pressure distillation 
inevitably requires complicated apparatus. Thus, disadvantages of these 
processes are their low productivity and complexity. 
U.S. Pat. No. 4,211,574 suggests another method of producing a cellulose 
solution, wherein a cellulose pulp sheet is soaked and swollen at a 
temperature of 85 to 95.degree. C. in a tertiary amine oxide solution 
having a water content of 5 to 15%, sufficiently low to dissolve the 
cellulose therein, and is subjected to stirring and heat treatment without 
the concentration step. In this method, however, since the amine oxide 
solvent can only partially swell the surface of the cellulose pulp sheet 
and consequently will form a coating thereon, the cellulose in the 
solution tends to remain undissolved because the penetration of the amine 
oxide solvent is prohibited by the coating. Once coatings are formed on 
the surface, a homogeneous solution cannot be obtained even by vigorous 
stirring or heating. Thus, a disadvantage of the process is that it fails 
to produce a homogeneous cellulose solution. 
U.S. Pat. No. 4,416,698 proposes still another process for producing a 
cellulose solution. In accordance with the process, cellulose powder and a 
solid NMMO are simply mixed and transferred to an extrusion apparatus 
wherein the mixture is dissolved to give a cellulose solution. The two 
powder components, especially when used in a large amount, will not make a 
sufficiently uniform mixture to leave a part of the cellulose powder 
undissolved in the resulting solution. In combination with a lower 
fluidity, this makes the process unsuitable for industrialization and mass 
production. 
The various processes for producing a cellulose solution in a tertiary 
amine oxide suggested in the prior art patents as mentioned above may be 
categorized into three groups as follows: 
Group I: Processes as disclosed in U.S. Pat. Nos. 4,142,913, 4,144,080, 
4,196,262, 4,246,221, 4,290,855 and 4,324,539, and European Pat. No. 
356,419. 
##STR1## 
Group II: Process as disclosed in U.S. Pat. No. 4,211,574. 
##STR2## 
Group III: Process as disclosed in U.S. Pat. No. 4,416,698. 
##STR3## 
As set forth above, the prior art processes have the problems of being 
complicated processes and/or requiring apparatus due mainly to the 
reduced-pressure distillation, and yielding unsatisfactory homogeneity 
resulting from a high NMMO concentration in the solution. 
As a result of extensive investigation, the present inventors discovered 
that the problems of the prior art mentioned above can be eliminated by 
using fibrillar cellulose powder or fluff, instead of a cellulose pulp 
sheet as conventionally used, in a highly concentrated aqueous solution of 
NMMO having a water content of 5 to 20% and maintained at the temperature 
range of from 50 to 130.degree. C. This process can provide a homogeneous 
solution in a simpler manner, while not forming a coating on the surface 
of cellulose as in a process using a cellulose pulp sheet. 
Contrary to the conventional processes which require swelling in a tertiary 
amine oxide and subsequent distillation, the process of the present 
invention can directly produce a cellulose solution without swelling by 
using fibrillar cellulose powder. Because the solvent can easily penetrate 
through the fibrillar cellulose, the process of the invention is a simpler 
process that is able to obtain complete dissolution of cellulose in a 
shorter period of time. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a short and 
simple process for producing a highly homogeneous cellulose solution. 
Further objects and advantages of the invention will become apparent by 
referring to the remainder of the specification.

DETAILED DESCRIPTION OF THE INVENTION 
According to one embodiment of the invention, a process for preparing a 
homogeneous cellulose solution is provided which comprises the steps of: 
(a) preparing fibrillar cellulose powder with a particle size distribution 
required for dissolution in a tertiary amine oxide solvent; 
(b) injecting a molten liquid tertiary amine oxide solvent into a twin 
screw extruder; 
(c) feeding the cellulose powder of step (a) into a section of the twin 
screw extruder where the molten liquid tertiary amine oxide solvent fed in 
step (b) produces a well swollen paste with the cellulose powder fed in 
step (c); 
(d) dissolving the well swollen cellulose paste in the following melting 
sections in the twin screw extruder; and 
(e) stabilizing the solution obtained in step (d) in a storage tank. 
As a source of cellulose, fluff-like, or in consideration of quantitative 
aspect, cellulose powder of 1,000 .mu.m or less in particle size can be 
typically used to obtain a homogeneous solution. More preferably, 
cellulose powder having a particle size of 600 .mu.m or less may be used 
to obtain a higher homogeneous solution. A larger particle size, as in 
cellulose sheets, can result in the formation of coatings on the surface 
of particles or particle gels which inhibits the formation of a 
homogeneous solution. Fibrillar cellulose powder can be obtained using a 
mill fitted with ring sieves or a knife-edged blade mill, preferably with 
a knife-edged blade mill with ring sieves. 
The solvent which can be used to dissolve the cellulose powder is 
preferably a liquid aqueous NMMO solution containing 5 to 20% by weight of 
water. The temperature of the solvent is maintained in the range of 50 to 
130.degree. C. during the transportation in the extruder. The ratio of 
cellulose to NMMO solvent is such that the final cellulose solution has a 
concentration of 5 to 20% by weight, more preferably 8 to 15% by weight of 
cellulose according to the molecular weight of the polymer. 
The invention will be described in greater detail with reference to the 
accompanying drawings. 
As schematically illustrated in FIG. 1, the NMMO solvent is introduced into 
the extruder at a predetermined rate through the first hopper fitted with 
a gear pump. Pulverized cellulose powder is passed through a closed 
transporting conduit (not shown), preferably anti-static electricity 
treated, to a force feeder which introduces the powder to the second 
hopper at a predetermined rate. The NMMO solvent during the transportation 
in the extruder dissolves the cellulose being transported into the 
extruder. 
FIG. 2 schematically illustrates the structure of a co-rotating twin screw 
extruder. The extruder includes nine blocks. Each block has an LID ratio 
of 4, in which D is 30 mm. The first two blocks comprise a feeding section 
of molten liquid NMMO. The following three blocks 3, 4 and 5 comprise an 
intensive mixing and kneading section of cellulose powder and molten 
liquid NMMO to produce well swollen paste state. The blocks 6 and 7 
comprise an intensive kneading section to homogenize and melt the paste. 
The final two blocks 8 and 9 comprise a conveying section for completely 
melting and discharging the cellulose solution in NMMO. The defoaming is 
accomplished in block 8 through vacuum venting. The extruder further 
includes an injection feeder for the molten liquid NMMO and a side force 
feeder for the cellulose powder. 
The molten liquid NMMO is fed into the first block by the gear pump at a 
given rate at 80 to 130.degree. C. The temperature of the solvent is 
maintained in the range of 50 to 130.degree. C. during the transportation 
in the extruder. In the third block, the cellulose powder is fed by force 
feeding using a side feeder so that the cellulose solution having a 
concentration of 5 to 20% by weight is obtained. In blocks 3 and 4, the 
mixture of cellulose and NMMO is intensively homogenized to give a uniform 
and well swollen paste without any unswollen part. In blocks 6 and 7, the 
mixture is further homogenized and melted, and then discharged after 
defoaming in the eighth block. The cellulose solution thus obtained is 
homogeneous enough to be spun. 
The solution is then transferred to a storage tank with a pressure 
controller, wherein a highly homogeneous solution can be obtained after 
several minutes of stabilization. The highly homogeneous solution thus 
obtained is to be spun into cellulosic fibers, films and separation 
membranes. 
PREFERRED EMBODIMENT OF THE INVENTION 
The present invention will be described in greater detail by way of the 
following examples, which are not intended to limit the invention. In the 
examples, several characteristics of the resulting solution were evaluated 
as follows: 
1) Weight average degree of polymerization (DP.sub.w) 
The change of the molecular weight of cellulose caused by decomposition 
during the production of the solution was obtained as follows. 
The intrinsic viscosity of cellulose was determined in the concentration 
range of 0.1 to 0.6 g/dl at the temperature of 25.+-.0.01.degree. C. using 
Ubbelohde Viscometer No. 1 (Fisher Corporation) and 0.5M Cuene solution 
prepared according to ASTM D539-51T. The intrinsic viscosity of a sample 
[IV] was calculated from the concentration (c) and the specific viscosity 
.eta..sub.sp according to the following equation (1). DP.sub.w was 
calculated from the intrinsic viscosity [IV] as obtained above according 
to the Mark-Houwink equation as described in M. Marx, Makromol. Chem., 16, 
157 (1955); and J. Brandrup, E. H. Immergut, Polymer Handbook, 3rd ed., 
Vol. 144, Wiley-Interscience, New York, 1989. 
EQU [IV]=lim[.eta..sub.sp /C]c.fwdarw.0 (1) 
EQU [IV]=0.98.times.10.sup.-2 DP.sup.0.9 (2) 
b) Concentration of colored impurities 
The color change of the solvent during the production of the cellulose 
solution was determined with a 30% aqueous NMMO solution. 10 g of 
cellulose solution was taken and soaked in 16.17 ml of distilled water for 
60 minutes, i.e., until equilibrium was obtained. The colored impurities 
contained in the NMMO solution was determined using UV-Visible 
Spectrophotometer (Hewlett Packard Model HP8453). The absorbency of the 
solution was recorded at 450 nm. The results were presented as an amine 
oxide optical density (AOOD), which is an absorbency value of 1% by weight 
of NMMO solution at 450 nm. The AOOD of 50% aqueous NMMO (BASF Co.) was 
0.0006. 
c) Homogeneity of solution 
The homogeneity of the solution which has been transported through the 
extruder was evaluated by the presence or absence of cellulose particles. 
The undissolved particles were observed on Zeiss Polarization Microscope. 
EXAMPLE 1 
Cellulose sheets having DP.sub.w of 660 (Cellunier-F, ITT Corporation) were 
milled in a knife mill (Pallmann, PS 5-10) with a ring sieve (opening 350 
.mu.m) to obtain cellulose powder of 180 .mu.m or less in apparent 
diameter at the rate of 500 kg/hr. 
A molten liquid NMMO containing 12% by weight of water at 100.degree. C. 
was injected into the first block of a twin screw extruder (Theysohn 
ZSK-30), whose structure is illustrated in FIG. 2, at the rate of 9,000 
g/hr by a gear pump. Then, the cellulose powder was fed into the third 
block of the twin screw extruder by the side feeder at the rate of 1,000 
g/hr. The temperature profile of the twin screw extruder from the first 
block to the ninth block was: 90.degree. C., 75.degree. C., 75.degree. C., 
120.degree. C., 120.degree. C., 120.degree. C., 120.degree. C., 
120.degree. C. and 120.degree. C. 
The screws were operated at 180 rpm to discharge 10,000 g of cellulose 
solution per hour. The concentration of the solution was 10% by weight of 
cellulose. The solution was very homogeneous without including any 
undissolved cellulose particle. The DP.sub.w of cellulose of the solution 
was 600, and the AOOD of the solvent was 0.011. 
EXAMPLE 2 
The same extruder as in Example 1 was used in this example. 
The operation conditions were exactly the same as in Example 1, except that 
the temperature profile of the twin screw extruder from the first block to 
the ninth block was: 100.degree. C., 100.degree. C., 100.degree. C., 
120.degree. C., 120.degree. C., 120.degree. C., 120.degree. C., 
120.degree. C. and 120.degree. C. 
The cellulose solution obtained at this temperature profile was not 
completely homogeneous and contained some insoluble cellulose particles. 
EXAMPLE 3 
The same extruder as in Example 1 was used in this example. 
The operation conditions were exactly the same as in Example 1, except that 
the feeding rate of cellulose powder was 1,227 g/hour. The screws were 
operated at 180 rpm to discharge 10,227 g of cellulose solution per hour. 
The concentration of the solution was 12% by weight of cellulose. The 
solution was very homogeneous without including any undissolved cellulose 
particle. 
EXAMPLE 4 
The same extruder as in Example 1 was used in this example. 
Cellulose sheets having DP.sub.w of 1,000 (Rayon-EXP, ITT Corporation) was 
milled in a knife mill (Pallmann, PS5-10) with a ring sieve (opening 300 
.mu.m) to obtain cellulose powder. 
The operation conditions were exactly the same as in Example 1, except the 
molecular weight of the cellulose and the feeding rate of cellulose powder 
(780 g/hr). The screws were operated at 180 rpm to discharge 9,780 g of 8% 
by weight of cellulose solution. 
The solution thus obtained was very homogeneous and contained little 
undissolved cellulose particles. The DP.sub.w of cellulose of the solution 
was 890, and the AOOD of the solvent was 0.0013.