1. Field of the Invention
The present invention relates to improved microcellular and ultramicrocellular polymeric materials containing hydrofluorocarbon (HFC) inflatants. These environmentally more favorable inflatants have zero potential to deplete the earth's stratospheric ozone layer and substantially lower global warming potential than fully halogenated chlorofluorocarbons (CFCs).
2. Description of the Prior Art
The term, "microcellular", as used herein, means a synthetic organic polymeric structure having the following general properties: 1) substantially all of the polymer is present as closed polyhedral shaped cells defined by film-like cell walls having a thickness of less than 2 micrometers, preferably less than 0.5 micrometers, and 2) the cell walls have a substantially uniform thickness and density.
The term, "ultramicrocellular", as used herein, means a crystalline synthetic organic polymeric structure having the following general properties: 1) substantially all of the polymer is present as closed polyhedral shaped cells defined by film-like cell walls having a thickness of less than 2 micrometers, preferably less than 0.5 micrometers, 2) the cell walls have a substantially uniform thickness and density, 3) there is a uniform crystalline polymer orientation, and 4) there is a uniplanar crystalline polymer orientation. These ultramicrocellular structures and processes for producing such structures are further described in Blades and White, U.S. Pat. Nos. 3,227,664 and 3,227,784.
Parrish, U.S. Pat. No. 3,375,211 discloses ultramicrocellular polymeric structures containing an inflatant which has a permeability coefficient for diffusion through the cell walls less than air. The inflatant is capable of generating a vapor pressure of at least 30 mm Hg at a temperature below the softening point of the polymer. The patent discloses suitable inflatants as those selected from the group consisting of sulfur hexafluoride and saturated aliphatic and cycloaliphatic compounds having at least one fluorine to carbon covalent bond, and wherein the number of fluorine atoms exceeds the number of carbon atoms. Preferably, the saturated aliphatic and cycloaliphatic compounds are, respectively, perhaloalkanes and perhalocycloalkanes in which at least 50% of the halogens are fluorine. Specific examples of suitable inflatants include chlorotrifluoromethane, dichlorodifluoromethane, 1,1,2-trichloro-1,2,2-trifluoroethane, symmetrical - dichlorotetrafluoroethane, and perfluorocyclobutane among others.
Bonner, U.S. Pat. No. 3,375,212, discloses preparing microcellular structures composed of a high molecular weight, synthetic organic polymer having a glass transition temperature of at least 40.degree. C. These structures contain the inflatants described in the aforementioned U.S. Pat. No. 3,375,211. The patent includes an example of producing poly(vinyl chloride) microcellular fiber containing the inflatant, perfluorocyclobutane.
Although fully halogenated chlorofluorocarbon compounds have been very effective inflatants, there is concern that the release of such compounds into the atmosphere may be a factor in the depletion of earth's stratospheric ozone layer. Furthermore, these compounds may be a source of global warming. (See, P. S. Zurer, "Search Intensifies for Alternatives to Ozone-Depleting Halocarbons," Chemical & Engineering News, 8 February 1988, pp. 17-20).
The present invention is concerned with providing microcellular and ultramicrocellular polymeric materials containing environmentally more favorable inflatants which have zero potential to deplete the earth's stratospheric ozone layer and substantially lower global warming potential than fully halogenated chlorofluorocarbons.