PRIOR ART Conventionally, a batch method has been used for converting cellulose acetate fibers to cellulose fibers with alkali treatment in a liquid dyeing machine.

However, such a method has disadvantages in that costs are increased 10 because the reaction time for converting cellulose acetate fibers into cellulose fibers is long and the amount of fibers able to be added per batch is limited.

Another alkali treatment method is achieved by use of a continuous weight reduction machine in which alkali can be continuously added. Such a method is usually used to reduce the weight of fabrics at high temperatures and 15 concentrations of alkali. However, such a method suffers from disadvantages in that cellulose fibers of excellent quality cannot be obtained because a conversion rate of cellulose acetate into cellulose is not constant.

DISCLOSURE OF THE INVENTION Therefore, it is an object of the present invention to avoid the disadvantages 2 0 occurring in the prior art, and provide a novel method for preparing cellulose fibers from cellulose diacetate fibers at low temperatures in a cold-pad-batch type.

To accomplish the above object, the present invention provides a method of producing cellulose fibers by use of a cold-pad-batch process, which comprises the step of adding alkali into fibers or fiber goods at least partially composed of 25 cellulose diacetate fibers with a substitution of 2.0 to 2.75 (acetification of 45 to

59.5%) at low temperature in order to convert cellulose acetate fibers into cellulose fibers.

BEST MODES FOR CARRYING OUT THE INVENTION Upon saponification of cellulose diacetate with alkali, its acetyl groups are converted to hydroxyl groups with concomitant rearrangement of molecular chains from an amorphous form to a crystalline form by folding or packing. Natural and regenerated cellulose fibers have the crystalline structures of cellulose I and II, respectively, while a mixed crystalline structure of cellulose 11 and IV is found in the cellulose fibers produced by saponification of cellulose diacetate, which are measured to have a crystallinity of 14 to 34% (by a gravimetric method) and a birefringence of 0. 012 to 0. 024.

Cellulose fibers obtained by the saponification of cellulose diacetate fibers are a kind of rayon fibers. The cellulose fibers prepared from cellulose diacetate range from 1.48 to 1.51 gm/cm3 in specific gravity, 1.2 to 2. 5gf/de in tensile strength, 20 to 50% in ductility, and 12 to 13 % in moisture regain under standard conditions. These physical properties are similar to those of general rayon fibers.

In accordance with the present invention, the saponification of cellulose diacetate/cellulose triacetate fabrics is done with strong alkali, or with strong alkali and weak alkali in one bath or two baths.

Examples of alkali compounds useful in the saponification of the present invention include alkali metal hydroxides, such as sodium hydroxide, alkali earth metal hydroxides, such as calcium hydroxide, and alkali metal carbonates, such as sodium carbonate. Such alkali compounds may be used independently or in combination with a reaction rate controlling agent. Well known are those which are based on phosphonium or quaternary ammonium. Examples of commercially available reaction rate controlling agents include NEORATE NCB of Korea Fine Products, which is a phosphonium based reaction rate controlling agent; and KF NEORATE NA-40 of Korea Fine Products, DYK-1125 of IPPOSHA Co. , Japan, caserine PES of MEISEI CHEMICAL WORKS, LTD., Japan, caserine PEL of

MEISEI CHEMICAL WORKS, LTD. , Japan, caserine caserinePEF of MEISEI<BR> CHEMICAL WORKS, LTD. , Japan, and SNOGEN PDS PDSof Dae Young Chemical,<BR> Co. , Korea, all being quaternary ammonium-based reaction rate controlling agents.

According to the present invention, fibers or fiber goods may be used, which at least partially comprise cellulose diacetate fibers with a substitution of 2.0 to 2.75 in order to convert cellulose acetate fibers into cellulose fibers.

Furthermore, the present invention is characterized in that such a method comprises the step of adding alkali into the fibers or fiber goods at least partially comprising cellulose diacetate fibers at low temperature with the use of a cold-pad- batch process in order to convert cellulose acetate fibers into cellulose fibers.

According to an embodiment of the present invention, an alkali solution of 20 to 53% is charged into a padding cistern of a cold-pad-batch weight loss device.

Scoured and dried fibers or fiber goods at least partially comprising cellulose diacetate fibers are dipped into the alkali solution. Wet pick-up of the alkali solution, which is penetrated into the cellulose diacetate fibers, may be 50 to 100%.

The fibers are taken down on a storage roll, and then maintained at 20 to 70°C for 1 to 48 hours, followed by washing the fibers with water.

Preferably, the alkali aqueous solution ranges, in concentration, from 20 to 53%. For example, a reaction is not fully accomplished when the concentration is less than 20%. The upper limit is the saturation concentration of the alkali aqueous solution.

Furthermore, alkali is preferably added to fibers at 20 to 70°C, because a reaction is not fully accomplished when the temperature is less than 20°C, while a heterogeneous reaction occurs and molecular chains are cut when the temperature is more than 70°C.

A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be meant to limit the present invention.

Throughout this specification, weight loss, degree of deacetylation of cellulose acetate fibers, and breaking strength, breaking elongation and dyeing property of the resulting fibers are defined as follows:

*degree of deacetylation: the resulting fibers were analyzed for deacetylation with the use of IR spectroscopic analyzer (MAGNA 750, Nicolet, USA), and degrees of deacetylation were obtained by checking the existence of a carbonyl band of an acetyl group at 1760cm- *weight loss was calculated from the measurements of sample weights before and after alkali treatment as shown in the following equation: 1 t 1 (%) (pre-alkali treatment weight-post-alkali treatment weight) 100 pre-alkali treatment weight *breaking strength and breaking elongation: breaking strength and breaking elongation were measured by elongating a 50 mm long sample at a rate of 200mm/min using Universal Testing Machine (Zwick 1425, Germany).

* dyeing property: the resulting fibers were dyed at 0.5 % o. w. f. of C. I. <BR> <BR> <P>Direct Blue 200 (Kayarus Supra Blue 4BL, manufactured by Nippon Kayaku Co. , Japan), which is a direct dye, at a temperature of 100°C for 30minutes, and then soaped and rinsed at 70°C. A reflectance (R) of the dyed fibers was measured by a spectrophotometer (Color-Eye 7000A, Machbeth, USA), and a K/S value was calculated with the use of a measured R value and an equation of K/S = (1-R) 2/2R.

Dyeing property of the dyed fibers was evaluated by the K/S value.

EXAMPLE 1 Plain clothes (warp 75d/20f, weft 120d/33f, weft density 75ply/inch, <BR> <BR> manufactured by SK Chemicals Co. , Korea) comprising cellulose diacetate fibers with a degree of acetyl substitution of 2.55 (acetification of 56.9%) were scoured and dried. Into a padding cistern of a cold-pad-batch weight loss device were charged a 53% NaOH solution, and a reaction rate controlling agent of lg/L.

Scoured cellulose diacetate fibers were dipped into the alkali solution. Wet pick- up of the alkali solution, which was penetrated into the cellulose diacetate fibers, was 70%. The cellulose diacetate fibers were rolled on a storage roll, and then saponified at 20°C for 24 hours. Subsequently, the saponified fibers were washed with water to remove the remaining alkali. Next, the fibers were dried.

Cellulose fibers were obtained from such saponification step, with a weight loss of 40% based on the weight of the initial cellulose diacetate fibers.

5 Analysis of the resulting fabrics by IR spectroscopic analyzer confirmed that a carbonyl band of an acetyl group at 1760cm-1 was apparently observed in a region which was initially cellulose diacetate fibers, while a carbonyl band of an acetyl group at 1760cm-1 completely disappeared and a hydroxy stretching band at 3400cm-1 was greatly increased in cellulose fibers with weight loss of 40%.

10 Therefore, it may be apparent from the above analysis that all acetyl groups were completely converted to hydroxyl groups.

Physical properties of the resulting fibers were described in Tables 1.

EXAMPLE 2 The procedure of example 1 was repeated except that an 25% NaOH 15 solution was used, and the cellulose diacetate fibers were saponified at 70°C.

Cellulose fibers were obtained, with weight loss of 39.5% based on a weight of initial cellulose diacetate fibers through such saponification.

As described in example 1, analysis of the resulting fabrics by IR spectroscopic analyzer confirmed that acetyl groups were completely converted to 2 0 hydroxyl groups.

Physical properties of the resulting fibers were described in Tables 1.

COMPARATIVE EXAMPLE 1 Plain clothes (warp 75d/20f, weft 120d/33f, weft density 75ply/inch, manufactured by SK Chemicals Co. , Korea) comprising cellulose diacetate fibers 25 with a degree of acetyl substitution of 2.55 (acetification of 56.9%) were scoured and dried. Into a liquid dyeing machine were charged a 2.7% alkali aqueous solution of a weight 15 times greater than a weight of the cellulose diacetate fibers,

containing a reaction rate controlling agent at lg/L, along with scoured cellulose diacetate fibers, followed by temperature elevation from 30°C to 98°C at a rate of 2°C/min. After the maximum temperature was maintained for 30min, the temperature was allowed to decrease to 30°C at a rate of 2°C/min. Following the drainage of the liquid, the remaining alkali was removed by washing with water.

Next, the fibers drawn out of the machine were dried.

Cellulose fibers were obtained from the saponification step, with a weight loss of 40% based on a weight of initial cellulose diacetate fibers.

As described in example 1, analysis of the resulting fabrics by IR spectroscopic analyzer confirmed that acetyl groups were completely converted to hydroxyl groups, and that physical properties and dyeing property of cellulose fibers prepared at low temperature by use of cold-pad-batch process were similar to those of cellulose fibers prepared by use of a liquid dyeing machine according to comparative example 1.

TABLE 1 Example 1 Example 2 Comp. Ex. 1 Concentration of NaOH (%) 53 25 2. 7 Temperature (°C) 20 70 98 Weight loss of acetate (%) 40 39. 5 40 Denier (De) 168. 9 170. 4 164. 4 Breaking strength (gf/de) 1. 35 1. 33 1. 34 Breaking elongation (%) 42. 5 43. 0 42. 0 Dyeing property (K/S) 8. 51 8. 42 8. 47

INDUSTRIAL APPLICABILITY As described above, cellulose fibers resulting from cellulose diacetate fibers by use of a cold-pad-batch process at low temperature according to the present invention, have similar chemical structures and physical properties in comparison with conventional viscose rayon fibers and cellulose fibers obtained with the use of a liquid dyeing machine at high temperature, and can be applied in the same

manner as that of conventional viscose rayon fibers and cellulose fibers with the use of a liquid dyeing machine. Also, the cellulose fibers can be produced in simple, economical, and safe manner, because the cellulose fibers are produced by use of a cold-pad-batch process at low temperature.