Process for preparing homogeneous cellulose solution by...

Details Process for preparing homogeneous cellulose solution by... Process for preparing homogeneous cellulose solution by...

2000-11-07

2004-01-13

Wilson, James O. (Department: 1623)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carbohydrates or derivatives

C536S124000

Reexamination Certificate

active

06677447

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for preparing a homogeneous cellulose solution and more particularly, to a process for preparing a homogeneous cellulose solution by forming a mixture where powdered cellulose pulp is mixed and swollen with a liquid N-methylmorpholine N-oxide (hereinafter, referred to as NMMO) hydrate solvent supercooled under its melting point, and by subjecting the mixture to a dissolution treatment, thereby producing a homogeneous cellulose solution having a substantially low decomposition characteristic which is used in a field manufacturing cellulose fibers or films, by an easy and fast method.
2. Description of the Related Art
An NMMO hydrate functions as a cellulose non-solvent (hereinafter, referred to as NMMO non-solvent), when it has a water content of about 18% or more by weight, thereby swelling cellulose, whereas as a cellulose solvent (hereinafter, referred to as NMMO solvent), when it has a water content of about 18% or less by weight, thereby dissolving cellulose.
Conventionally known processes for preparing cellulose solution by using NMMO solvent are as follows:
First, sheets of cellulose pulp are mixed and swollen with NMMO non-solvent having a water content of 22% by weight and then, an excess of water contained in the mixture is distilled and removed, thereby cellulose dissolves (For example, see U.S. Pat. No. 4,142,913, U.S. Pat. No. 4,144,080, U.S. Pat. No. 4,196,282 and U.S. Pat. No. 4,246,221). However, the above-mentioned processes should require distillation of the excess water under a reduced pressure in the presence of cellulose, thereby result in a large amount of energy consumption and a long time preparing solution. As a result, the processes suffer from problems that the decomposition of cellulose and the discoloration of NMMO are serious.
As another example, European Patent No. 356,419 discloses cellulose solution preparation process in which cellulose is swollen in a NMMO non-solvent having a water content of 40% by weight and then, an excess of water is distilled under a reduced pressure in a screw extruder on which a fan type flight is mounted. As yet another example, International Patent Application Publication No. WO 94/06530 discloses cellulose solution preparation process using a thin film evaporator as the pressure-reducing distiller. However, the above-referenced processes suffer from problems that the productivity thereof is low and the process and apparatus are complicated.
Second, sheets of pulp are swollen in a liquid NMMO solvent (having a water content of 15% by weight) at a temperature in the range of 85° C. to 95° C. over its melting point and the resulting mixture swirls and heats without any concentration process, thereby a cellulose solution was produced. This process is disclosed in U.S. Pat. No. 4,211,574. In this way, a gel film is undesirably formed on the surface of pulp sheet, which serves to inhibit the NMMO solvent from diffusing into the interior of pulp sheet, thereby the production of a homogeneous solution results in failure.
Third, cellulose powder and powdered NMMO solvent are simply mixed and dissolved by means of an extruder, thereby producing cellulose solution (For example, see SU 1645308 A1). In case the two components are mixed in large quantities, however, a part of cellulose powder remains at non-dissolved state in the produced solution, thereby the production of a homogeneous solution results in failure.
On the other hand, a solid NMMO solvent (whose melting point is 78° C.) at a room temperature and cellulose pulp pieces are thrown into a disintegration mixing mill and disintegrated to mix at a temperature in the range of 40° C. to 100° C., thereby pellet type mixing granules are produced. Next, the granules are supplied into an extruder to produce a cellulose solution. For example, this process is disclosed in U.S. Pat. No. 5,584,919. In this case, however, there occur some problems that it is difficult to manage solid NMMO solvent and thus to carry out continuous processes.
Fourth, International Patent Application No. PCT/KR97/00104 discloses a process preparing cellulose solution by a twin-screw extruder. A liquid NMMO solvent (whose melting point is 78° C.) at a temperature in the range of 90° C. to 100° C. is supplied into the first block of the twin-screw extruder, cooled in the second block at a temperature of 75° C. Then powdered pulp having an apparent diameter 180 mm is supplied into either the second block or third block at a temperature of 75° C., and mixed. A mixture produced is heated to a temperature of 120° C., while passing through the sections in order of transfer, agitation, melting, solution homogenization, defoaming and constant discharging. It is, however, in this process, found that the swelling and dissolution of pulp powder occur simultaneously during the transfer and agitation processes. In addition, the block receiving solvent and the block receiving pulp powder are different, so that the twin-screw extruder exhibits low efficiency to arrange at least 9-14 blocks (having a L/D 36-48). Furthermore, since pulp powder has an apparent specific gravity in the range of 0.04 to 0.08, it is conveyed only in small amount, thereby resulting in a low productivity.
As stated above, the conventionally known processes have disadvantages such as low homogeneity of produced cellulose solution, complicated processes and the like. To solve the disadvantages, it is desirable that a liquid NMMO solvent is diffused into the interior of pulp at a fast rate and thus swells the pulp, before cellulose is dissolved in the liquid NMMO solvent. At this time, it is preferable to minimize the dissolution of cellulose by the liquid NMMO solvent and to maximize swelling thereof by the liquid NMMO solvent.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a process for preparing a homogeneous cellulose solution which is capable of producing a cellulose-NMMO mixture where powdered cellulose pulp is first swollen in a liquid NMMO solvent supercooled under its melting point by minimizing the dissolution of cellulose by the liquid NMMO solvent and maximizing the swelling thereof by the liquid NMMO solvent. And thus produced powder of the mixture is subjected to a dissolution treatment, thereby producing a high homogeneous cellulose solution.
The liquid NMMO solvent is produced, for example, in such a method that a large amount of water is evaporated out of a NMMO water solution with a water content of 50% or more by weight under a reduced pressure and the resulting NMMO solution is concentrated. The production principle of the liquid NMMO solvent supercooled under its melting point from the concentrated NMMO solvent as cellulose solvent has close relation to thermal behavior of the NMMO hydrate solvent, that is, the melting behavior, and isothermal and non-isothermal crystallization behavior by cooling of NMMO hydrate solvent.
TABLE 1
Thermal Behavior of NMMO Hydrate
Water
Heating and Cooling Rate
Content (% by
Heating and Cooling Rate (5° C./min)
(10° C./min)
weight)
Tm (° C.)
Tc (° C.)
Tm (° C.)
Tc (° C.)
22
42
46
—
18
42
60
 5
45
63
5 below
zero
15
39
77
21
41
78
 7
13.3
78
25
80
20
12
78
—
95
105
30
—
72
—
85
102
—
33
73
—
11
76
89
97
113
31
—
82
—
87
103
125
30
77
 85
8
75
90
97
130
30
79
97
78
90
103
135
30
78
 99
6
75
90
95
141
27
80
113 
77
92
103
148
27
79
118
3
74
90
99
156
—
80
127 
77
93
104
177
—
80
139
0
[74]
—
103 
180
—
80
140 
[75]
—
105
183
—
81
143
Tm: melting point (observed by heating from 0° C.)
Tc: crystallization temperature (observed by cooling from a maximum melting temperature to 20° C. below zero).
[ ]: slight thermogram
Table 1 shows thermal behavior of NMMO hydrate produced by subjecting 50% by weight of NMMO water solution made by BASF Co. to pressure reduction, distillation and concentration (An accurate water content in NMMO hydrate was measured by using Karl-Fischer method. Every sample

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