Title of Invention	A PROCESS FOR PRODUCING CELLULOSE CARMONATE
Abstract	A method for producing cellulose carbamate by reacting cellulose with urea, characterized in that urea is added in at least two steps.

Full Text

Two-stage reaction for producing cellulose carbamate The present invention relates to a method for producing cellulose carbamate. It is known that, as far as environmental protection is concerned, the conventional viscose process for producing cellulose regeneration products poses problems due to the use of large amounts of carbon disulfide. It is further known that cellulose carbamate is therefore used instead of cellulose xanthogenate for producing cellulose regeneration products. Cellulose carbamate is produced by reacting cellulose with urea. DE-A-19635707 describes a method for producing cellulose carbamate, wherein alkali-treated cellulose is first squeezed off, rinsed with alcohol and optionally substituted with an acid anhydride. Urea is then added in one step for conversion into cellulose carbamate. EP-A-57105 also describes a method for producing cellulose carbamate from alkali-treated cellulose. After the alkali treatment liquid has been squeezed off, there will be not only a washing step with ethanol, but also a neutralizing step with acetic acid to remove excessive alkali. Urea is added in one step, i.e. the total amount of urea is added once. The prior-art methods are time and cost intensive because of the necessary washing and neutralizing treatments following the alkali treatment of cellulose. The neutralizing and/or washing treatment is carried out to remove excessive lye that would be detrimental to the reaction with urea. It is therefore the object of the present invention to provide a method which permits a reaction with urea in a process that is as simple as possible, together with a high yield with respect to the urea used. This object is achieved by a method for producing cellulose carbamate by reacting cellulose with urea, the method being characterized in that urea is added in at least two steps. Preferred embodiments follow from the subclaims. Furthermore, the present invention provides a method for producing cellulose carbamate by reacting cellulose with urea, the method being characterized in that urea is added continuously during the reaction. Preferred embodiments follow from the subclaims. The cellulose used in the method according to the invention is a standard, alkali-pretreated cellulose. Pretreatment may be carried out with alkali and/or alkaline-earth lye. Both types of pretreatment are normally called alkali treatment or ripening. Preferred are here NaOH, KOH, Ca(0H)2 and Mg(0H)2. Aqueous NH3 solutions and liquid NH3 can also be used. Preferably, the alkali-treated cellulose that is used has a residual alkali content of 0.1 to 3% by wt., more preferably of 0.2 to 2.5% by wt. The molecular weight of the cellulose used is not limited in particular, and all types of cellulose may be used. It is known to the person skilled in the art that the cellulose in the ripening process can be set to a specific molecular weight range. This range is defined by the application/use of the carbamate to be synthetized. The person skilled in the art is aware of the respectively desired molecular weight ranges and also of the process measures required for achieving a desired molecular weight range. Prior to the addition of the urea, the cellulose used is preferably shredded or comminuted to permit a thorough homogeneous blending. Normally, this comminuting operation is already carried out during the alkali treatment, i.e. prior to the mixing with alkali lye or thereafter, but prior to ripening. The reaction of cellulose with urea can preferably be carried out in an inert medium, such as a solvent. These media are chosen in response to the temperature range in which the carbamate synthesis is carried out. The medium is characterized in that it boils within the range of the synthesis temperature used. It is also possible to use a medium having a lower boiling point, which will then boil under pressure at synthesis temperature. Likewise, a medium with a higher boiling point may be used under conditions of a reduced pressure, the pressure being chosen in accordance with the desired synthesis temperature (= boiling temperature). In principle, all solvents that are known to be inert may be used as solvents. Preferred are substances from the group of aliphatic and/or aromatic hydrocarbons that boil between 125°C and 200°C. Particularly preferred are o-xylene, p-xylene, m-xylene, technical mixtures of the three xylenes, butylbenzole, trimethylbenzoles of all substituent positions, propylbenzole, toluene, ethylbenzole, cyclopentane and substituted cyclopentanes, such as ethylcyclopentane, propylcydopentane, butylcydopentane, pentylcyclopentane, hexylcydopentane, the substituents being present either individually or in combination in arbitrary substituent positions, cyclohexane, ethylcyclohexane, heptane, hexane, octane, nonane, decane, undecane, isopropylbenzole, as well as ethyl butyrate and dipropylketone. Mixtures of the solvents may be used. Particularly preferred are xylenes. The ratio of cellulose to medium (solvent) is normally from 1:1 to 1:30 (ratio by weight; cellulose : solvent), preferably from 1:3 to 1:10. According to the invention urea is added in at least two steps. Urea may here be added in a pure form or in the form of a solution of the pretreated cellulose. The solution can be produced with suitable solvents, e.g. the ones mentioned above with respect to the reaction of cellulose with urea. The concentration of the urea is normally from 50 to 100% of the saturation concentration, preferably from 90 to 99.5% of the saturation concentration in the solvent used. Preferably, the urea is added in solid form. At the same time, one or more adjuvants may be added together with the urea. Examples thereof are the urea dimer, known as biuret, and the higher condensates of urea (triuret...), ammonium carbamate, ammonium cyanate, N-alkytated ureas, semicarbazide, cyanamide, dicyandiamide, biguanide, guanidine, chloroformic acid ester, aliphatic and aromatic isocyanates and diisocyanates, such as methylisocyanate, phenylisocyanate or hexamethylenediisocyanate. In the first embodiment of the invention (multistage urea addition), the total molar ratio of urea to cellulose in the method of the invention is normally 6:1 to 0.4:1. For obtaining this total molar ratio the urea is preferably added to the cellulose in equal amounts each time, preferably at a ratio of 0.2:1 to 0.5:1 per addition. The corresponding absolute amount per addition depends on the residual alkali content of the pretreated cellulose and the desired degree of substitution of the final product. The degrees of substitution range from the saturation of all hydroxy! groups of the cellulose, i.e. from 3 per monomer molecule, corresponding to a substitution degree 3 (100%), to a saturated OH group per 10 monomer molecules, i.e. a substitution degree of 0.1, or less. The respectively desired substitution degrees are chosen in dependence upon the further use of the cellulose carbamate and are known to the person skilled in the art. The desired substitution degree can be achieved through a suitable process, particularly with respect to the temperature and the duration of the method. Such measures are known to the person skilled in the art. The respectively achieved substitution degree can be easily determined by determining the nitrogen content of the carbamate. Preferably, the mixture of cellulose and urea that has been obtained after the first addition of urea is heated in the method according to the invention. Preferably, the heating operation takes place for 2 to 30 minutes, more preferably 5 to 20 minutes, to 100-180X, more preferably 130-160°C. Preferably, water is removed after the first addition of urea and prior to heating, e.g. by applying a vacuum, optionally with simultaneous heating to 60-80'*C. The water, however, is preferably removed by entrainment of the water with xylene, the latter being returned into the reaction after water separation. Subsequently, there will be cooling for a short period of time, if necessary, and the second urea addition will be performed. Subsequently, there will be heating again, preferably to 100-1 SOX, more preferably 130-160X, and the reaction will be continued for 15 to 250 minutes, more preferably 30 to 180 minutes. Optionally, urea may be added in further steps until the desired number of urea additions is achieved. In this process, heating is preferably performed between the further urea-adding steps for 2 to 30 minutes each time, preferably 5 to 20 minutes, to 100-180°C, preferably 130-160°C. Optionally, water is first removed after each addition, as described above. When the reaction is carried out under heating, this is preferably carried out at ambient pressure both after the first and the second and every further urea addition. The same is true for the addition of urea. The total reaction time is calculated as the sum of the individual reaction times after the respective urea additions. Preferably, the total reaction time is 10 to 150 minutes, more preferably 25 to 120 minutes. Preferably, the reaction is earned out after the second urea addition at reaction temperatures rising with an increasing reaction time. Solubility and storage stability of the produced cellulose carbamate can thereby be improved. Reaction methods at rising reaction temperatures are e.g. described in DE-A-19635473. The subsequent steps of the method in producing the cellulose carbamate in compliance with the method of the present invention are in accordance with the conventional and known techniques. A particularly preferred method of producing cellulose carbamate in accordance with the present invention comprises the following steps: a) activating the cellulose with sodium hydroxide solution; b) squeezing off the excessive sodium hydroxide solution; c) ripening the cellulose treated with sodium hydroxide solution; d) washing the ripened cellulose with the sodium hydroxide solution of step b) and subsequently with water; e) mixing the washed cellulose with urea in a molar ratio of cellulose : urea of 1:0.2 to 1:0.5: f) adding xylene and water removal by heating to 50-80°C at a pressure of 10 to 200 mbar; g) relieving to ambient pressure and heating to 100-160°C for 5 to 30 minutes; h) carrying out the second urea addition with the molar ratio cellulose : urea as indicated in step e); i) renewed heating to 130-160°C for 30 to 180 minutes. Cellulose carbamate is obtained. Subsequently, the cellulose carbamate obtained in this way is filtered off and washed. After drying the cellulose carbamate obtained in this way may be used in spinning processes. The method according to the invention can be carried out in standard reactor types known to the person skilled in the art. For example, a stirred-tank reactor may be used that is operated batchwise. Likewise, it is possible to use a conical reactor, a wobble reactor or a tubular reactor. Preferably, however, the method according to the invention is carried out in a cascade of stirred-tank reactors. One stirred-tank reactor may for instance be provided each urea addition. Such a process may result in a quasi continuous reaction because the stirred-tank reactors that are operated individually batchwise are arranged one after the other such that a continuous process is achieved on the whole. With such a process it is furthermore possible to carry out the individual reaction steps after the respective urea additions under individually adjustable temperature or pressure conditions. This may be advantageous with respect to the characteristics of the final product. The respectively added amounts of urea and the corresponding reaction times may be chosen for the individual reactors of the cascade of stirred-tank reactors in accordance with the above observations. Preferably, however, an identical amount of urea is again supplied in each addition and the reaction time is again 5 to 20 minutes per stirred-tank reactor. Instead of the cascade of stirred-tank reactors, it is also possible, in case of need, to use a combination of stirred-tank reactor and tower or a tower cascade and a ring-disk reactor. The respective reactors are equipped with the known devices for carrying out methods at an elevated temperature and optionally at an elevated pressure. An advantage of the method of the invention is that the reaction with urea can take place directly with alkali-containing cellulose without intermediate neutralizing or washing stages. Surprisingly, it has been found that the two-stage urea addition to non-neutralized alkali cellulose leads to a yield that is increased in comparison with neutralization and single-step addition of the urea with respect to the urea used. In a simplified process a better yield is achieved. Even if neutralized, alkali-treated cellulose is used, a higher yield will be obtained in comparison with methods which also employ neutralized, alkali-treated cellulose in a single-step urea addition. Therefore, the method of the invention turns out to be superior to conventional methods. An improved process that is above all less cost-intensive is obtained on the whole by omitting unnecessary neutralizing and washing treatments and through the higher yield with respect to the urea. In a further embodiment of the invention urea may also be added continuously during the reaction time. Preferably, the addition is carried out during two thirds of the total reaction time. This development of the method according to the invention is preferably carried out in a discontinuous stirred-tank reactor. The observations made above in connection with the multistage urea addition as to reaction time, molar ratios, solvents, reaction time, reaction temperature and the further stages of the method are also applicable to the embodiment of the method of the invention which makes use of a continuous urea addition. The continuous addition of urea achieves the same effects as described above for the multistage urea addition. The following embodiments will further explain the invention: Production of an alkali-treated cellulose: 1 kg of cellulose sheets was swollen and activated in a steeping press in 18% sodium hydroxide solution at 40°C for two hours. Subsequently, the alkali cellulose was freed in a press from excessive lye by compaction to a ratio of 2:1 and torn in a Blaschke shredder into small pieces. The flocculent alkali cellulose was put for several hours into a revolving tube operated at 40°C for ripening and for setting the DP value (indicated molecular weight, mean number of monomer units) of 320. Washing was then carried out with 30 liters of the first alkali washing liquor of a previous test. A second washing with 20 liters of fresh water yielded a residual alkali content of 2% by weight. The total mass was divided into four equal parts for the following examples. Comparative Example 1 The cellulose (see above) was washed once again with fresh water and the remaining residual alkali content was neutralized with acetic acid. After neutralization two further washing operations were carried out, as well as dehydration on a vacuum nutsche by squeezing off the residual water. In a Blaschke kneader 70 g urea was added, corresponding to a molar ratio of cellulose (AGU) to urea of 1 : 0.75, and blended for 3 hours. Subsequently, the residual water was removed in a reactor by stripping with xylene at 80°C and under a slight vacuum. After complete water removal the temperature was raised up to boiling of the xylene at ambient pressure (145°C). The cellulose/urea mass was reacted at said temperature for a period of 2. The conversion of urea to carbamate was 80%. Comparative Example 2 The second sample was filled into the Blaschke kneader without neutralization and mixed with urea. The synthesis carried out under the same conditions as in test 1 yielded a urea conversion of 76%. Comparative Example 3 The third sample was washed with fresh water once more and had a residual alkali content of 0.5% by weight prior to mixing with urea. The mixing with urea and the synthesis were carried out as in test 1. The urea conversion was at 76%. Example 4 The fourth sample was filled into a Blaschke kneader without neutralization and at a residual alkali content of 2% by weight into a Blaschke kneader and mixed with (1) 35 g urea, corresponding to a cellulose urea ratio of 1 : 0.375. The subsequent synthesis was briefly stopped after water stripping and a reaction time of 15 minutes at 145°C, and furthermore (2) 35 g urea was added. The synthesis was then continued for 1 VA hours. The urea conversion was at 91%. The above-mentioned tests show that the method according to the invention does not require a neutralizing treatment and, nevertheless, yields a higher conversion in comparison with urea additions carried out in one step. This simplifies the method considerably and increases the yield of the method at the same time, which particularly entails cost advantages as well. Patent Claims 1. A method for producing cellulose carbamate by reacting cellulose with urea, characterized in that urea is added in at least two steps. 2. The method according to claim 1, characterized in that in the at least two steps of adding urea, a molar ratio of cellulose to urea of 1 to 0.5 to 0.2 is observed each time. 3. The method according to claim 1 or 2, characterized in that heating to 120-180°C is carried out each time for 5 to 20 minutes between the at least two steps of adding urea. 4. The method according to at least one of claims 1 to 3, characterized in that two urea-adding operations are carried out and that heating to 120-180°C is performed for 30 to 180 minutes after the second urea addition. 5. The method according to at least one of the preceding claims, characterized in that water is removed between the at least two urea adding operations. 6. The method according to at least one of claims 1, 2, 3 or 5, characterized in that urea is added in 3 to 5 steps. 7. The method according to at least one of the preceding claims, characterized in that the method is performed in a cascade of stirred-tank reactors. 8. The method according to at least one of the preceding claims, characterized in that the total molar ratio of urea to cellulose is 0.4:1 to 6:1. 9. A method for producing cellulose carbamate by reacting cellulose with urea, characterized in that urea is added continuously during the reaction. 10. The method according to claim 9, characterized in that the addition is carried out continuously over two thirds of the total reaction time. 11. The method according to claim 9 or 10, characterized in that the total molar ratio of urea to cellulose is 0.4:1 to 6:1. 1 12. A method for producing cellulose carbonate substantially as herein described and exemplified

Documents:

374-chenp-2003-abstract.pdf

374-chenp-2003-claims duplicate.pdf

374-chenp-2003-claims original.pdf

374-chenp-2003-correspondnece-others.pdf

374-chenp-2003-correspondnece-po.pdf

374-chenp-2003-description(complete) duplicate.pdf

374-chenp-2003-description(complete) original.pdf

374-chenp-2003-form 1.pdf

374-chenp-2003-form 19.pdf

374-chenp-2003-form 26.pdf

374-chenp-2003-form 3.pdf

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