Sponge cloth

Details Sponge cloth Sponge cloth

1999-01-20

2001-08-28

Kuhns, Allan R. (Department: 1732)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Cellular products or processes of preparing a cellular...

C015S244100

Reexamination Certificate

active

06281259

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a sponge cloth made of cellulose and a process for producing same.
2. Description of Related Art
The publication Ullmann's Encyklopädie der technischen Chemie [Encyclopedia of Industrial Chemistry], 3
rd
edition (1967) vol. 18, pp. 175-177, describes a process for producing a spongy cellulose structure where layers of pulp obtained from wood are swollen in a 15-20% NaOH solution that is present in excess. Sodium cellulose is formed after a period of time. The sodium hydroxide solution is then squeezed out; it contains dissolved hemicelluloses that would otherwise interfere with the subsequent process steps. The remaining pressed cake in ground form is treated with carbon disulfide, forming cellulose xanthogenate, which dissolves readily in a NaOH solution, in which it degrades after a period of time with a reduction in average degree of polymerization (DP). Depending on the grade of pulp used and the storage time, the DP is reduced from 800-1200 to 200-600.
The DP is defined as the average number of individual cellulose molecules of which a cellulose polymer chain is composed on the average.
The alkaline xanthogenate solution is then mixed with pore-forming Glauber's salt, cotton fibers and optionally a coloring pigment and the mixture is stirred or kneaded until all the components are distributed uniformly. Then the mixture is spread on a web by means of a flat device or a roller-like device. The web may have a three-dimensional pattern for profiling the surface of the product.
The mass on the web is guided for a few minutes in an alkaline coagulation bath enriched with Glauber's salt at 100 C. The xanthogenate is thermally cleaved and the pore-forming salt is leached out, forming a spongy structure as the sheet product on the web. The sheet product is washed in water and brought in contact with a small amount of 1% sulfuric acid solution to reduce the residual levels of organic carbon disulfide and hydrogen sulfide compounds.
SUMMARY OF THE INVENTION
The object of the invention is to provide a sponge cloth and a process for producing same, where the above-mentioned sulfur compounds, in particular carbon disulfide and hydrogen sulfide, are not present in the sponge cloth at all. Another object of this invention is to prevent a reduction in average degree of polymerization DP of the cellulose during the production of the sponge cloth, to achieve higher strength values and increase the storage stability of the cellulose derivatives.
These and other objects of the invention can be achieved when the starting material for production is decrystallized low-substituted (DS 0.2 through 1.5) acetylated cellulose that is soluble in solvents or solvent systems based on dimethyl sulfoxide or dimethylacetamide. For this reason, derivatization with carbon disulfide need no longer be performed to dissolve the cellulose.
DETAILED DESCRIPTION OF THE INVENTION
In a stirred vessel, 15 to 35 vol % aqueous lye, preferably NaOH solution, is poured over pulp, which is obtained mostly from wood and is available commercially as a layered material, and is left to rest for approx. 1 hour. The lye is preferably used in at least a four-fold excess, based on the dry weight of the starting pulp.
The period of time can be less than one hour if prior tests have ensured that swelling of the pulp is complete.
Then the lye is removed by squeezing to such an extent that the weight of the remaining pressed cake is three times the dry weight of the starting pulp. This pressed cake is ground to particle sizes less than 3 mm in diameter by means of a conventional beater mill or shredder mill and loosened in the process.
The particles are then placed in a stirred vessel, where a five-fold quantity of a liquid agent that has an acetylating action on sodium cellulose, i.e., the particulate material, is poured over the particles, and the mass is stirred for approx. 60 minutes.
Examples of agents that can be used include acetic anhydride, glacial acetic acid (anhydrous acetic acid) or acetyl chloride.
This causes esterification of free OH groups of cellulose by acetyl groups up to an average degree of substitution (DS) of 0.2 to 1.5.
The average degree of substitution DS indicates the number of OH groups that are substituted on the average by the average of three hydroxyl groups of the individual cellulose molecule in its polymer chain. This yields a maximum value of 3 and a minimum value of 0 for the DS.
Next the excess substitution agent, i.e., the liquid phase, is squeezed out of the solid mass while the liquid is removed by suction. The residue, the cellulose derivative mass which is then in substituted form, is washed with water at least three times and then dried to a white powder at an ambient temperature of 100 C.
The cellulose derivative powder is then dissolved in a solvent or solvent system based on dimethyl sulfoxide or dimethylacetamide at 20 C. to 50 C. to a concentration of 4 to 12 wt %.
The preferred solvents (systems) have proven to be dimethyl sulfoxide (DMSO), DMSO/lithium chloride (chloride content 5 wt %), DMSO/magnesium chloride (chloride content 3 wt %), DMSO/N-methylmorpholine N-oxide or dimethylacetamide/lithium chloride. The inorganic chlorides serve as solubilizers. The dimethyl sulfoxide systems in particular have a good dissolving power, can be handled with no problems with regard to occupational safety and can be recovered easily from laboratory wastes.
Next, 100 to 500 wt % anhydrous salt which serves as the pore-forming agent is added to this solution, which is defined as 100 wt %. Such salts are known per se; they are soluble in water and weak acids and do not enter into any interaction with the cellulose derivative.
In addition, the solution is mixed with 0.5 to 4 wt % fibers of natural or synthetic origin that are insoluble in the solvent used and optionally with up to 5 wt % conventional dyes, preferably pigment dyes, and foaming substances.
The term “anhydrous” for the pore-forming salts is to be understood in the sense that they contain only bound water of crystallization at room temperature or none at all. One example is NaCl, which is available very inexpensively.
The resulting mixture is stirred or kneaded at room temperature until a homogeneous distribution of all components is obtained, which can be observed by the fact that when the dye is added it is distributed uniformly in the mixture. This process is usually concluded after 10 to 30 minutes.
The mixture is then poured or spread on a movable web, where the thickness of the applied layer is equal to the desired thickness of the sponge cloth to be produced. If it should have a surface structure, the web used will have a corresponding three-dimensional structure on the surface facing the mixture. The thickness of the applied layer is preferably a few millimeters up to three centimeters.
The coated web is passed through a bath of boiling water, where the dwell time in this bath depends on the amount of time required for complete dissolution of the pore-forming salt and for a spongy cohesive sheet to be formed on the web. The sheet can easily be lifted away as a raw product and is washed with water until no residues of the solvent system and free coloring agents can be detected. This is followed by drying at 100 C. to form a finished sponge cloth in a conventional moving or stationary drying apparatus.
One variant of the process consists of the fact that, after the spongy structure is treated with boiling water on the moving web, during or following the subsequent washing it is treated for one hour in 0.1 to 2 N aqueous sodium hydroxide solution at 50 C. while stirring. The acetyl groups present in the sponge are thus cleaved back and subjected to ester saponification. This process can also take place at room temperature, but the reaction is greatly accelerated at 50 C. and thus is more economical. The reaction can be carried out in an open apparatus. The pore structure of the cloth is not altered but this measure imparts the

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