Organic compounds -- part of the class 532-570 series – Organic compounds – Carbohydrates or derivatives
Reexamination Certificate
2000-09-11
2003-02-04
Wilson, James O. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carbohydrates or derivatives
C536S056000, C536S057000, C536S058000, C536S059000, C536S060000, C536S061000
Reexamination Certificate
active
06515121
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods for the manufacture and collection of cellulosic particles of the kind known as fibrids.
Cellulosic fibrids can be made by mixing together under turbulent conditions a spinning solution of cellulose and a coagulating liquor. One example of such a solution is viscose, which contains sodium cellulose xanthate. Examples of coagulating liquors for viscose include aqueous salt solutions and aqueous acid solutions. The fibrids so produced comprise, often predominantly, fine fibrous particles a few microns in diameter; of comparable size to the fibres in woodpulp. These fibrids may also comprise platelike or globular particles of similar size to those fibres and of more or less irregular shape. For example, in the familiar salt figure test for viscose ripeness, viscose is mixed with aqueous sodium chloride. If the sodium chloride concentration is too low, a solution is formed; if it is too high, a more or less coherent precipitated lump is formed; but at the correct concentration, fibrids are formed. The fibrids formed in the salt figure test contain residual xanthate groups. If such fibrids are acidified, or if an acidic coagulating liquor is used, then the xanthate groups are destroyed and cellulose is regenerated.
BACKGROUND ART
Modified cellulose particles have been proposed as additives in papermaking, in particular to assist in formation of the paper web (sheet) and in sludge dewatering. In a series of papers in Das Papier (1980, vol. 34, pp. 575-579; 1981, vol. 35, pp. V33-V38 and pp. 555-562; and 1983, vol. 37, pp. 181-185), Käufer et al. disclose cationically-modified cellulose particles prepared by the reaction of woodpulp with reagents such as 3-chloro-2-hydroxypropyltrimethylammonium chloride and the use of such particles as additives in papermaking.
Philipp and Lang (Faserforschung und Textiltechnik, 1966, vol. 17, pp. 299-304) disclose the addition of poly(ethyleneimine) to diluted viscose and titration of the resulting mixture with aqueous acid. A precipitate formed at mildly or moderately alkaline pH (8-11), depending upon the exact experimental conditions. The precipitate was described as a floc, indicating that it was composed of fibrids. The first-formed precipitate was believed to comprise a salt of a polymeric cation (protonated poly(ethyleneimine)) and a polymeric anion (cellulose xanthate). The authors refer to such salts generically as “symplexes”. Titration was continued until the mixture became acid (pH 3), and at least partial regeneration of cellulose xanthate to cellulose occurred, thus yielding cationically-modified cellulosic particles by a viscose process. Philipp and co-workers elsewhere propose the use of cationically-modified cellulose particles and symplexes as additives for use in papermaking (Dawydoff et al., Acta Polymerica, 1987, vol. 38, pp. 307-313, and Philipp et al., Progress in Polymer Science, 1989, vol. 14, pp. 91-172).
WO-A-96/26220 discloses a process in which a cationic polymer is added to diluted viscose and the resulting mixture is mixed with a coagulating and regenerating liquor such as dilute sulphuric acid to form cellulosic fibrids useful as additives in papermaking. The viscose was added to the coagulating liquor, or vice versa. The resulting slurry contained about 0.3, 0.5 or 1 percent of fibrids, and the fibrids were collected from it by filtration.
Cellulosic fibrids may be collected from the aqueous slurry in which they are prepared by conventional methods such as settling, filtration and centrifugation. The cost of such collection varies inversely with the concentration of fibrids in the slurry. It is accordingly desirable on economic grounds to prepare slurries which contain high proportions of fibrids. Simple mixing, of the kind disclosed by Philipp and co-workers or in WO-A-96/26220, is not well-suited to the manufacture of slurries containing more than about
1
percent by weight of fibrids. These slurries are thick, semi-fluid, porridge-like materials which are difficult to mix. In consequence, coagulation of viscose in a concentrated fibrid slurry carries the risk of coagulation occurring unevenly, resulting in the production of hard oversized particles rather than the desired fibrids. It is an object of the invention to provide a convenient method of manufacturing concentrated fibrid slurries.
DISCLOSURE OF THE INVENTION
According to the invention, there is provided a method for the manufacture of cellulosic fibrids by the viscose process, including the steps of:
(1) mixing together (a) viscose and (b) a coagulating and regenerating liquor under turbulent conditions, thereby forming a suspension of cellulosic fibrids in a spent liquor; and
(2) collecting the fibrids from the spent liquor,
characterised in that the coagulating and regenerating liquor supplied into step (1) contains cellulosic fibrids withdrawn from step (1). The expression “spent liquor” is used as a convenient name for the byproduct liquor produced in the method of the invention; it will be appreciated that this liquor often retains some coagulating and regenerating powers.
We have found it convenient to employ a conventional viscose suitable for fibre manufacture and, prior to its introduction into mixing step (1), to dilute it with water to reduce its cellulose content to a value in the range from 1 to 4 percent by weight. If desired, such a conventional viscose may be diluted with a slightly acidic liquor, for example with a liquor which includes a proportion of spent bath; provided that this does not result in undue precipitation. This may offer advantages in terms of process economy.
The coagulating and regenerating liquor may be any of those known for viscose processes, particularly aqueous acid. Conveniently, it is dilute aqueous sulphuric acid, which is the most usual acidic component in such liquors, preferably at a concentration in the range from 0.5 to 5, more preferably from 1 to 3.5, percent by weight. If desired, the liquor may contain conventional amounts of sodium sulphate (a byproduct of the process), for example up to 25 percent by weight. This is advantageous, because the liquor can accordingly be recovered for reintroduction into the process using conventional measures such as evaporation and crystallisation. It will be appreciated that processes such as these are preferably carried out on spent liquor from which essentially all fibrids have been removed. We have found that the coagulating and regenerating liquor does not need to contain, and preferably contains little or no, auxiliary coagulating substances such as the zinc sulphate used in many processes for the manufacture of conventional viscose fibres. Furthermore, the presence of such substances in the fibrids produced by the method of the invention may be undesirable in some end-uses, for environmental and/or technical reasons. If desired, the liquor may contain conventional surface-active agents as employed in other viscose processes.
The temperature of the coagulating and regenerating liquor is preferably in the range from 60 to 100° C., more preferably in the range from 80 to 95° C. Use of high temperatures makes for rapid coagulation and regeneration and assists removal of byproduct carbon disulphide and hydrogen sulphide by degassing. Degassing may be assisted by injecting steam into the slurry resulting from mixing step (1). Such sulphur-containing byproducts may be collected or disposed of in conventional manner.
The mixing step (1) is conveniently carried out by injecting both the viscose and the coagulating and regenerating liquor into a high-shear mixing chamber, for example a Y-shaped chamber, or mixing head, although any method of mixing which generates sufficient turbulence and shear to produce the desired fibrids may be employed. We have found that, in contrast to known methods of forming fibrids, the method of the invention permits the formation of fibrids of the desired morphology even when the viscose is mixed with a coagulating and regenerating liquor which contains a high proportion of preformed f
Mackiewicz Andrzej Marek
Scudder Guy Edward
Seddon Sabine
Acordis Kelheim GmbH
Howson and Howson
Lewis Patrick
Wilson James O.
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