Method of Making a Cellulose Acetate Low Density Microcellular Foam

The present invention is directed to a low density, microcellular foam and method of making such foam having a density of 0.065 to 0.6.times.10.sup.3 kg/m.sup.3 and a cell size of less than about 0.3.mu.m to 2 .mu.m. The foam is produced from a polymer solution composed of a cellulose acetate as the polymer, magnesium perchlorate as a swelling agent, water as a non-solvent, and acetone and ethyl lactate as solvents.

As pointed out above, inasmuch as the known low density fine cell size (microcellular) foam had a cell size of about 25.mu.m, applications such as encapsulation of laser targets requiring a foam layer of 10 to 60.mu.m thickness, for example, could not be accomplished with existing foams due to the relatively large cell size. To achieve both low density and micrometer size cells in the same foam to meet requirements, such as those for laser targets, a phase-inversion technique was used to produce the cellular structure of this invention.

Phase-inversion processes to make low-density microcellular foams have been used in making asymmetric, reverse-osmosis membranes for water desalination, as exemplified in articles by R. E. Kesting, J. Appl. Polm. Sci. 9, 663(1965); R. E. Kesting et al, J. Appl. Polym. Sci. 9, 1873 (1965); and B. Kunst et al, J. Appl. Polym. Sci. 14, 1983 (1970). However, the method and solution formulations of this invention differ substantially from those used for making the membranes.

Basically, the procedure utilized for making the cellulose acetate foam having both low density and small cell size is to dissolve the polymer in a solvent system consisting of a good solvent, a swelling agent, and a non-solvent. This solution may be cast on a flat plate to form a thin sheet or into a mold to form a desired configuration and then dipped into a water bath, maintained at 0.degree.-10.degree. C., to gel the polymer; or it may be extruded through an orifice, having a desired cross-sectional configuration, directly into the water bath. The polymer gels in the ice water, which is a non-solvent for cellulose acetate, and the solvent and swelling agent diffuse out of the polymer gel. After soaking in water, the polymer is frozen in liquid pentane and transferred to a freeze-drying chamber. Water and solvents sublime during freeze-drying, and the gel structure solidifies into a low-density microcellular foam.

In the phase-inversion process the initially homogeneous solution separates into two interspersed liquid phases prior to the actual transition from solution to gel. These interspersed phases can be considered as consisting of droplets of one liquid phase embedded in a matrix of the other. These droplets represent the incipient voids in the finished gel and they must be stabilized in the solution-to-gel transition. Three principle steps occur in the phase-inversion process during the extrusion of the cellulose acetate solutions into cold water: (1) loss of solvents, (2) gelation, and (3) loss of residual solvent and swelling agent.

From the above brief description of the general process for making polymer gels, it is clear what the cellulose acetate polymer system has definite requirements that must be met to achieve polymer gel with the desired properties. Since the variables affecting this process are many and very specific, the particular formulation used so that care must be taken in low density, small cell size foams are produced.

The following two solutions are examples of formulations that give good foams. However, formulations using other solvents and swelling agents can possibly be used though not as effectively:

Solution No. 1 produced foam of 0.08 to 0.1 g/cc density and No. 2 produced foam in a density 0.065 g/cc.

The cellulose acetate E-4655 is made by Eastman. Cellulose acetate is a high viscosity polymer of about 17 Pa's, which has been substituted to 39.4% acetyl and contains 3.8% hydroxyl. This is one of several cellulose esters that could be used, the other esters vary in viscosity from 1 to 25 Pa's (10-25 poises) and also in acetyl and hydroxyl content. This specific polymer is used herein to maintain a high viscosity in dilute solutions. The polymer concentration is a primary variable in controlling the density of the final foam. Foams produced from the above formulation with 6% cellulose acetate generally have densities from 0.08 to 0.15.times.10.sup.3 kg/m.sup.3 depending on other variables. Formulations used with as little as 3% cellulose acetate achieved foam densities of 0.065 .times. 10.sup.3 kg/m.sup.3. The highest density foam produced by this formulation thus far is 0.6.times.10.sup.3 kg/m.sup.3 using 22% cellulose acetate; however, higher densities are possible, but not preferable for laser target encapsulation.

The next two components in the solution formulation, magnesium perchlorate [Mg(CLO.sub.4).sub.2 ] and water, make up the swelling agent and non-solvent, respectively. The exact mechanisms of these components' interactions with the polymer and each other is not clearly understood. However, they are needed to make porous polymer gels. If they are not used, only full-density films can be fabricated. As the magnesium perchlorate concentration in the swelling agent increases, the amount of water incorporated into the polymer gel increases (to a limit) and thus reduces the ultimate foam density. This is due to the Mg.sup.++ ions, which hydrate the water present. Studies have shown that magnesium ions from magnesium perchlorate can have up to 80 waters of hydration aggregated about each Mg.sup.++ ion. This action is demonstrated by the fact that water is a non-solvent for cellulose acetate, while a concentrated solution of magnesium perchlorate in water will dissolve cellulose acetate. The magnesium ions also probably form complexes with the OH and CO groups on the cellulose acetate. However, it has been shown that the Mg.sup.++ ions are not strongly bound to the cellulose acetate gel since soaking the polymer gel in water for 24 hours will reduce the Mg.sup.++ ion concentration to 50 ppm.

The acetone and ethyl lactate in the above formulation are solvents for cellulose acetate. They were chosen to meet requirements of the phase-inversion and extrusion processes. For phase inversion to take place and yield microcellular gels, the solvents must be miscible with each other and soluble in water. Also, the solvents must diffuse out of the polymer gel readily into water, and any residual solvents must come out on freeze-drying without changing the size or shape of the frozen gel. This is important because in the planned procedure of the encapsulation process, for example, the amount of time the gel can soak in water to allow the solvents to diffuse out is 24 hours or less. This means that some residual solvent may remain in the gel and must be removed at low temperatures during the freeze-drying process. Therefore, the solvents must have high vapor pressures at low temperatures.

Ethyl lactate was chosen as a solvent due to the extrusion applications desired therefor. During extrusion, the cellulose acetate solution is forced from a small cross-section orifice, such as a 100-450.mu.m diameter needle, the outer end of which is located below the surface of the water bath and is pointed down. The polymer solution will fall away from the orifice as a smooth cylinder, square, or other orifice configuration if its density is about 1.0.times.10.sup.3 kg/m.sup.3. The addition of ethyl lactate increases the solution density. Although a cellulose acetate solution with an all-acetone solvent would be better for the freeze-drying process because of the higher vapor pressure of acetone, this solution will not extrude smoothly in the apparatus.

Another very important variable in achieving foams with the desired properties is the temperature of the water bath into which the solution is extruded. To get foams with cell sizes in the micrometer range requires that the polymer solution be gelled in water of 0.degree.-10.degree. C. Gels produced at 0.degree. C. are opalescent while gels produced from the same solution at 25.degree. C. are opaque. The opalescent gels have submicroscopic pore structure and are called ultragels. The opaque gels have pore structures that are discernible under a microscope and are called microgels. Foams produced from ultragels are preferable because they are translucent to light. Foams produced from microgels are opaque to transmitted light which, for example, in encapsulation applications makes locating microspheres therein more difficult. However, some microgels have pores in the 1 to 2.mu.m range and would make acceptable foams.

To produce the low-density, microcellular foam having cell sizes ranging from 0.3.mu.m to 2.mu.m, the following operational sequence is carried out for an exemplary solution.

1. Dissolve 6.0 grams of Eastman E-4655 cellulose acetate in solution composed of 10.0 grams magnesium perchlorate, anhydrous, 30.0 grams water, 15.0 grams ethyl D-L lactate, and 39.0 grams acetone at a temperature of 25.degree. to 50.degree. C.

2. Dip, if used in a configured mold or container, or extrude, through a configured orifice, the thus formed polymer solution into a water bath maintained at 0.degree.-10.degree. C., causing gelling of the solution. Polymer solution temperature may be from 25.degree. C. or cooled to -10.degree. or -20.degree. C. Water temperature ranging from 0.degree. to 10.degree. C.

3. Soak the resulting cellulose acetate gel in the water bath of a temperature of 0.degree.-30.degree. C. for time periods of 16 to 24 hours to provide sufficient time for the solvents and swelling agents to diffuse out of the gel.

4. Freeze the thus soaked polymer gel in -70.degree. to -80.degree. C. liquid pentane. This step requiring a time period of a few minutes to less than 5 min.

5. Freeze-dry (4 to 8 hours) depending on thickness, wherein water and solvents sublime and the gel structure solidifies into low density microcellular foam.

6. Cutting the foam to required shape, where necessary, or removing the formed foam from the mold or container. The cutting may be accomplished by a vibrationally driven razor or other appropriate means.

Table I sets forth the results of a chemical analysis of cellulose acetate foam produced by the above-described phase-inversion, freeze-dry process, showing the principle components.

Table II gives the trace elements present in the produced cellulose acetate foam as found by chemical analysis.

Table III sets forth the heat capacity of the thus produced cellulose acetate polymer determined by a Perkin Elmer DSC-1 scanning calorimeter.

While the above-described low density, small cell size foam has various applications, it has particular application in the encapsulation of a quality of fusion fuel for laser targets, wherein a cell size not greater than 2.mu.m, a density of about 0.1.times.10.sup.3 kg/m.sup.3, and a chemical composition of low average atomic number are required. Further, the fuel can be suspended in the polymer solution and encapsulated therein by the process of making the foam, either by positioning a fuel capsule in a mold and filling the mold with the solution, or by injecting the fuel capsule into an extrusion orifice configured to position the capsule while the solution flows therearound during the extruding process, such that the fuel capsule is properly located with respect to the foam thickness in the end product.

It has thus been shown that the present invention provides a low density foam having a cell size no greater than 2.mu.m, and as small as about 0.3.mu.m, and method for making the foam based upon a phase-inversion technique of a specified formulation followed by a freeze-drying operation.

While particular parameters have been described for illustrating the invention, modifications will become apparent to those skilled in the art, and it is intended to cover in the appended claims all such modifications as come within the spirit and scope of the invention.