1. Field of the Invention
This invention relates to novel foamed polymeric products which are prepared by foaming an ionic polymer in the presence of a volatile polar compound which acts as a plasticizer for the ionic groups present in said ionic polymer. The ionic polymer comprises from about 0.4 to 10 mole % pendant acid groups, especially sulfonic acid groups which have been neutralized to a degree of at least 97%, preferably 100%. In a most preferred embodiment of the instant invention, the foamed polymeric product is prepared from a sulfonated polystyrene polymer. This high strength, low density foam of the instant invention can be reprocessed by admixing with a low boiling solvent for the sulfonate groups, e.g., methanol, and repeating the above foaming process.
2. Description of the Prior Art
Foamed plastics have represented one of the fastest growing markets in the polymer industry in the past 15 years. This growth is expected to continue, and it is claimed that the potential usage of foamed plastics could far exceed the ability of the plastics industry to supply the needed materials.
There are essentially only two major flexible foam products now available in any large volume. They are polyurethane foam and plasticized poly (vinylchloride) foam. At this time, semiflexible foamed polyolefins are being commercialized for special applications; however, these do not represent large volume products.
Flexible polyurethane foams are normally prepared by the reaction of a diisocyanate, a hydroxyl terminated polyol, water, surfactant, catalysts, and possibly an external blowing agent. When these are intimately blended, a number of reactions occur very rapidly. In a matter of seconds a polymer is formed, expanded and cross-linked. The timing of polymerization and expansion is critical and is controlled by the catalysts, surfactants, and relative concentration of the diisocyanate and water.
The various chemical reactions which can occur are exceedingly complex and have been the subject of numerous publications and thirty years of intensive research. Furthermore, the diisocyanates employed in such studies are expensive and quite toxic, such that they can provide severe hazards to the personnel performing these reactions. It is also evident that after achieving a cured polyurethane foam, the process is irreversible. Therefore, if the resulting product does not meet specifications, it is of little or no value.
These problems are cited to demonstrate that despite almost overwhelming problems and major expense, these polyurethane systems have grown to their present volume due to product performance and market need.
A second flexible foam system which has achieved commercial success is that of foamed polyvinylchloride (PVC), suitably pasticized to yield a flexible cellular product. These materials are higher density (12 to 20 lbs./cu. ft.) than polyurethanes (as low as 1-2 lbs./cu.ft.). Vinyl foam is expanded by the use of chemical blowing agents near the melting point of PVC. These expand to generate the cellular structure. Deficiencies of PVC foamed systems are the difficulties of achieving low foam densities, lack of strength of the foams at high temperature (since they are not chemically cross-linked, these systems can collapse without close control of foaming temperature) and restriction of the foaming concept to a single base polymer, that of PVC.
U.S. Pat. No. 3,322,734 teaches that ionic polymers, for example, partially sulfonated polystyrene and partially carboxylated polystyrene, can be employed as plastics for molding objects and utilized to prepare foams. The present invention differs from that patent on a number of very important points. U.S. Pat. No. 3,322,734 teaches that the presence of a modest amount of carboxyl or sulfonic acid groups, if neutralized to a critical degree, permits processability by conventional plastic processes at elevated temperatures, and yet retains ionic associations at ambient temperature. The neutralization process simply involves reaction of the acid moiety with a suitable metal salt, metal oxide, metal hydroxide, etc. to a suitable extent. That art teaches that the acid form should not be completely neutralized -- preferably the neutralization should be only 80% complete (i.e., the metal hydroxide or other compound should be added in an amount corresponding to 80% of the stoichiometric amount of acid present), and in no case should exceed 90% of the stoichiometric equivalence. (Similarly that patent teaches a minimum fraction of the acid groups must be neutralized, i.e., 10%). Thus it is emphasized clearly in the prior art that incomplete neutralization of the acid moiety is essential in order that the resulting products be fabricable. The patentee also discloses that the products that are neutralized to a level of more than 90% possess no significant advantages over the products of his claimed range.
Thus, those products are conventional plastic systems in that they respond to elevated temperatures and shear such that the ionic associations are diminished or virtually eliminated. Consequently flow occurs and the products can be molded or foamed much like conventional thermoplastics, such as polystyrene or polyethylene. Similarly, if one creates a foam from these ionic polymers, and exposes it to elevated temperatures (for example, 100.degree. to 150.degree. C.) the ionic interactions are diminished and flow occurs -- that is, the foam collapses. The dimensional stability of such materials at elevated temperatures is inherently poor.
The present invention differs from the ionic polymer foams disclosed in the prior art in the following critical areas:
a. The products of the present invention are neutralized to a degree of at least 97% and are preferably neutralized completely.
b. The neutralized compositions of this invention are not readily processed by plastics processing equipment even at very high temperatures because, in the absence of a suitable additive, the ionic groups are very strongly associated.
c. The products, as described here, due to their strong associations, behave as crosslinked polymers at very high temperatures, manifesting unusual and valuable dimensional stability.
d. These associations of these ionic polymers are broken up by the addition of suitable agents which disrupt the ionic domains, permits the foaming process, and then removes itself from the vicinity of the ionic groups.
e. In the absence of these suitable plasticizing agents these products are not foamed into desirable products under practical conditions because the strong ionic associations preclude formation of a stable, desirable cell structure.
It is evident from this earlier discussion that chemically cross-linked foams possess certain advantages, especially resistance to flow at elevated temperatures. However, such advantages are achieved at a substantial cost in complexity of chemical reactions, in processing problems, inability to reuse scrap, inability to refoam defective parts, etc.
On the other hand, conventional thermoplastic foams, such as polystyrene foams, polyvinyl chloride foams, ionic polymer foams of the prior art etc. possess the virtues of easy processability, reuse of scrap, and simplicity of the foaming operation. Yet these systems all possess the failing of poor dimensional stability at elevated temperature. It is evident that for both classes of systems, chemically cross-linked foams and thermoplastic foams, each possess virtues and deficiencies which are inherent in their mode of preparation.
The present invention provides nearly all of the advantages of thermoplastic foams, and yet retains in the foamed part nearly all of the virtues of the chemically cross-linked foam.