Composite rigid sponge structure

Details Composite rigid sponge structure Composite rigid sponge structure

2000-03-17

2001-10-23

Copenheaver, Blaine (Department: 1771)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Composite having voids in a component

C260S001000, C260S001000, C521S029000, C521S025000, C521S028000, C521S032000, C264S048000, C264S109000

Reexamination Certificate

active

06306488

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an open-celled sponge having a cellulosic component of large pore size and a polyvinylchloride component of small pore size.
2. Description of the Prior Art
In the field of pollution control, it is often sought to remove toxic organic substances from water. The organic substances may be present in dissolved or suspended form, and may be present in large amounts, or in small amounts such as in parts per million, parts per billion, or less. Exemplary toxic organic species include: polychlorinated biphenyls, dioxins, polyaromatic hydrocarbons, nitroaromatics, pesticides, herbicides, methyl t-butyl ether, and volatile halocarbons.
Current technology for removing such organic contaminants involves the use of activated carbon. However, the activated carbon, generally in granular form, must be confined within a vessel as a bed through which the water to be treated is caused to flow. The bed of carbon granules generally creates impedance to the water flow, requiring adequate pumping to achieve a sought flow rate. Also, suspended material in the in going water stream may accumulate within the bed to cause even higher flow impedance. Although activated carbon functions efficiently, its total capacity or absorbed species is low, often less than 2% of the weight of the carbon. This results in the need to handle large amounts of carbon for the removal of relatively small amounts of contaminants.
Another problem in the field of pollution control is filtration for the removal of extremely fine suspended particles. Classic filtration methods employ, for example, sand bed filters and plate and frame presses. Such equipment is of large size and considerable expense, and requires careful monitoring to assure proper performance. Filters involving a porous membrane are usually employed for removing small quantities of very fine particles that may have passed through conventional filters. The membrane filters, however, are easily clogged or “blinded” when relatively little material accumulates on the upstream surface of the membrane.
The use of a microporous form of polyvinyl chloride (PVC) in filtration applications has been disclosed in U.S. Pat. No. 3,674,722. Although the microporous PVC performs well in filtration applications, it is difficult to produce useful shaped structures from the microporous PVC because of shrinkage during molding. Also, such shaped structures have been found to be brittle. When the shaped structure involves a continuous smooth upstream filtration surface, the microporous PVC filter clogs as easily as less expensive membrane filters.
It is disclosed in said U.S. Pat. 3,674,722 at column 3, lines 41-42 that the microporous PVC can be inserted as an impregnant into porous structures such as open celled foams. It has been found however, that with most such foams, unless the PVC completely fills the pores, it tends to flake away from the foam. Such is particularly the case with rigid foams or sponges comprised of polyethylene, polyurethane, polystyrene, phenolics, metals, ceramics or glass. When the pores of the foam are completely filled with the PVC, the resultant composite structure develops a very high impedance to the passage of fluids therethrough. If the sponges become soft when immersed in water, their usefulness is diminished because of compressive effects encountered during water treatments.
It is accordingly an object of the present invention to provide a rigid sponge structure capable of absorbing organic pollutants from water.
It is another object of this invention to provide a sponge structure as in the foregoing object having relatively low impedance to the passage of fluids therethrough.
It is a further object of the present invention to provide a sponge structure of the aforesaid nature capable of filtering suspended particles out of water.
It is a still further object of this invention to provide a sponge structure of the aforesaid nature resistant to compression under hydraulic pressures encountered during water treatment.
It is yet another object of the present invention to provide a sponge structure of the aforesaid nature amenable to low cost manufacture.
These objects and other objects and advantages of the invention will be apparent from the following description.
SUMMARY OF THE INVENTION
The above and other beneficial objects and advantages are accomplished in accordance with the present invention by a composite rigid sponge structure comprising:
a) an open-pore sponge comprised of regenerated cellulose and having pores of relatively large diameter, and
b) microporous polyvinyl chloride (PVC) deposited within said pores of relatively large diameter, the weight ratio of said PVC to said regenerated cellulose being between 5/1 and 1/1,
c) the total volume of said pores being between 60% and 92% of the geometric volume of said structure, and
d) the wet compressive strength of said structure being at least ten times greater than the wet compressive strength of said open-pore cellulose sponge without said PVC.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Regenerated cellulosic sponge useful in the production of the composite sponge structure of the present invention is available from the OCelO Company of Tonawanda, N.Y. The cellulosic sponge is available in fine pore and coarse pore grades, usually in the form of slab stock of various thickness. As determined by mercury intrusion porosimeter measurement, the average pore size of the coarse pore grade is about 2 mm. and the average pore size of the fine pore grade is about 1 mm. However, there is a wide range of pore sizes within either grade of sponge, and some of the pores are too large to be measured by classic porosimeter techniques. The pore structure consists of a random array of tunnels and apertures, said array having a high degree of tortuosity.
The microporous PVC is produced substantially by the procedure of U.S. Pat. No. 3,674,722, said procedure involving forming a mixture of a colloidal dispersion of a latex of a non-film forming PVC with a water-soluble organic non-solvent for the polymer, and subjecting the mixture to thermal curing. A preferred non-film forming PVC latex useful in the practice of this invention is Vycar 351, produced by the B. F. Goodrich Company. Preferred water-soluble organic non-solvents include glycols of low volatility, a preferable species being glycerine.
The latex may be mixed with the non-solvent liquid by conventional techniques, low shear methods being preferable. It is preferred to add the non-solvent liquid slowly to the latex, rather than to add the latex to the non-solvent liquid. From 0.4 to 5 parts, by weight, of the non-solvent should be employed for each part of polymer contained in the latex. The mixture of latex and non-solvent liquid is essentially a colloidal dispersion of the polymer particles in the non-solvent liquid containing water derived from the latex. Mixtures containing the lower ranges of polymer content produce microporous PVC having higher pore volume, larger average pore size, and lower structural strength, by comparison with microporous PVC obtained from dispersions of higher polymer content.
To produce the composite rigid sponge structure of the present invention, the latex
on-solvent mixture is added to the cellulose sponge of the aforesaid nature in dry form, whereby the cellulose sponge absorbs the mixture. The amount of the mixture added is such as to be just short of producing gravity-induced run-off of the mixture from the cellulose sponge. To facilitate achievement of the proper saturation of the cellulose sponge, the impregnated sponge may be run through rollers to remove excess dispersion mixture.
The impregnated sponge is then subjected to heat treatment at temperatures in the range of 220 degrees F to 280 degrees F for a period of from 15 to 100 minutes. The effect of the heat treatment is to cause the dispersed polymer particles to cohere or sinter together, thereby forming a microporous structure within the pores of the cellulosic sponge. Grea

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