Package making
Reexamination Certificate
1998-12-01
2001-07-17
Geist, Gary (Department: 1623)
Package making
C536S088000, C536S090000, C536S091000, C536S095000, C536S096000, C536S099000, C536S100000
Reexamination Certificate
active
06261218
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process and apparatus for making low molecular weight cellulose ethers having diminished presence of carbonacious tars.
BACKGROUND OF THE INVENTION
Color contamination can occur in the manufacture of low molecular weight cellulose ethers. The contamination takes the form of carbonacious tars which are brown or black in color. The tars take the form of dark particulates in powdered cellulose ether end products. The presence of the dark particulates is problematic if the cellulose ether products are to be used in appearance-sensitive applications such as tablet coatings for pharmaceuticals. Particulate contamination in tablet coatings is unacceptable to consumers and manufacturers.
Carbonacious tars form as a result of excessive degradation during depolymerization of high molecular weight cellulose ethers to low molecular weight cellulose ethers. Depolymerization is typically effected by addition of a strong acid, i.e. hydrogen chloride, to high molecular weight cellulose ethers to cleave a fraction of the ether linkages.
Excessive degradation of cellulose ethers during depolymerization can result from the following: non-uniform moisture distribution within the cellulose ether; static cling between the cellulose ether and the internal surfaces of the depolymerization reactor; non-uniform application or adsorption of the strong acid; and catalyzation of the depolymerization reaction due to contact with catalytically active surfaces or substances.
Excessive degradation due to non-uniform moisture distribution occurs when water absorbed within the cellulose ether vaporizes and condenses on colder spots on the internal surfaces of the depolymerization reactor. Dry cellulose ether powder absorbs the condensed water and sticks to the internal surfaces. The strong acid, i.e. hydrogen chloride, is also attracted to the high moisture resulting in elevated acid content at points on the internal surfaces. The elevated acid content causes the depolymerization rate to increase severalfold compared to the bulk of the cellulose ether powder. The elevated reaction rate causes tar particulate formation at points on the internal surfaces of the depolymerization reactor.
Excessive degradation due to static cling occurs when a film of cellulose ether powder clings to stagnant regions of the internal surfaces of the depolymerization reactor. When the reactor is emptied, the powder film clings to the surfaces and is subject to overreaction from exposure to subsequent batch reactions. This overreaction can result in tar particulate formation at regions on the internal surfaces of the depolymerization reactor.
Excessive degradation can occur from the non-uniform application or absorption of the depolymerizing acid when it is introduced into the reactor in a non-gaseous form such as an aqueous liquid. The non-uniform application or absorption results in elevated acid content in portions of the cellulose ether powder. The elevated acid content causes the depolymerization rate to increase severalfold compared to the bulk of the cellulose ether powder. The elevated reaction rate can cause tar particulate formation within the cellulose ether powder.
Excessive degradation can occur from catalyzation of the depolymerization reaction by contact with catalytically active surfaces or substances. If internal surfaces of the reactor are catalytically active, the elevated reaction rate can result in tar particulate formation at those surfaces.
It would be desirable to have a process and apparatus for making low molecular weight cellulose ethers with a substantially reduced incidence of tar formation.
SUMMARY OF THE INVENTION
According to the present invention, there is a process for making a cellulose ether. The process comprises the following: a) alkalyzing a cellulose pulp; b) etherifying the alkalyzed cellulose pulp to form a cellulose ether; c) washing the cellulose ether; d) drying the cellulose ether to a reduced moisture content; e) milling the cellulose ether to a particulate form; f) adjusting the temperature of the particulate cellulose ether to about 50° C. to about 130° C.; g) continuously tumbling the particulate cellulose ether while simultaneously contacting it with an acid to partially depolymerize it to reduce molecular weight such that a two percent aqueous solution of it has a viscosity of 200 centipoise (cP) or less at 20° C.; and h) partially or substantially neutralizing the acid by contacting it with a basic compound capable of neutralizing it. The cellulose pulp preferably is alkalyzed by contacting it with sodium hydroxide. The alkalyzed cellulose pulp preferably is etherified by contacting it with methyl chloride or a mixture of methyl chloride and propylene oxide. The cellulose ether is washed preferably by contacting it with water. The cellulose ether is dried preferably by heating it. The cellulose ether is milled preferably by grinding or impacting it. Preferably, the cellulose ether is kept substantially free of contact with surfaces containing iron during depolymerization. With the present process, low molecular weight cellulose ethers which are substantially free of visible tar formation can be produced.
Further according to the present invention, there is an apparatus for making cellulose ethers. The apparatus comprises the following: a) a first reactor for alkalyzing and etherifying a cellulose pulp to form a cellulose ether; c) a washer for washing the cellulose ether; d) a dryer for drying the cellulose ether to a reduced moisture content; e) a mill for milling the cellulose ether to a particulate form; f) a second reactor which continuously tumbles the particulate cellulose ether while simultaneously contacting it with an acid and subsequently with a basic compound. Preferably, alkalyzing and etherifying are carried out in a single reactor. Preferably, the interior surfaces of the reactor in which contact between the particulate cellulose ether and the substantially anhydrous acid is occurring are substantially free of iron.
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Geist Gary
The Dow Chemical Company
White Everett
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