1. Field of the Invention
The present invention relates to an improved microporous membrane and method of manufacture. Microporous as used herein refers to pore sizes in the range from 1.times.10.sup.-2 to 10 microns in diameter. Specifically, the present invention relates to a membrane formed by a process of ultra violet irradiation to form microsphereulites, followed by thermally-induced phase separation, yielding microporous membranes that have improved flow and mechanical properties.
2. Prior Art
Microporous membranes have an open-cell, sponge-like structure, as shown in FIG. 1. Microporous membranes are used in a variety of applications, including filtration of aqueous and organic solutions, and ionic diffusion and separation in batteries. Hydrophobic microporous membranes may be used as barriers, for example in rainwear, in salt water desalinators, air filtration for clean rooms and in medical applications such as drapes, gowns, and breathable dressings for wounds. Hydrophobicity prevents wetting of the membrane by water in its liquid form, while the microporous structure provides permeability to water vapors and other gasses.
In the production of microporous membranes, different pore sizes may be obtained by changing membrane formulation and processing parameters. Prior methods for micropore formation in membranes include the formation of microcracks on specific crystallizable polymer films, as described for example in U.S. Pat. Nos. 4,187,390 and 4,194,041. These patents disclose that controlled biaxial stretching of a crystallizable polymer, polytetrafluoroethylene, during film formation results in a mesh of modules interconnected by fine fibrils. U.S. Pat. No. 3,679,540 describes the formation of cracks on a film by cold stretching of the elastic polymer film. A subsequent series of hot and cold stretches form the micropores, which are then heat set.
Other methods of micropore formation include the leaching of microparticles from a heterogenous blend, and phase separation. Several types of different phase separation techniques have been developed. For example, U.S. Pat. No. 4,247,498 describes a process in which micropores are formed by first blending polymers and compatible liquid solvents at high temperature then cooling the resulting solution under non-equilibrium thermodynamic conditions. In the first stage of cooling, a liquid-liquid phase separation first occurs, where compatible liquid droplets are surrounded by liquid polymer. Upon further cooling, a solid-liquid phase separation occurs. The solvent is then washed out of the polymer network and dried, yielding a microporous membrane.
A radiation curing process is described in U.S. Pat. No. 4,466,931. A mixture of reactive acrylated oligomers and monomers are dissolved in a suitable solvent/non-solvent mixture. Exposure of the homogenous solution to ultra violet or electron beam irradiation initiates a polymerization process. This leads to a phase separation between insoluble, growing polymer chains and the solvent/non-solvent mixture, resulting in the formation of micropores after solvent removal. While these membranes, and their methods of formation are suitable for some uses, there remains a need for new microporous membranes with good flow rates and advantageous mechanical properties.