Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ
Patent
1992-12-08
1996-02-06
Tantoni, Leo B.
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
2642091, 264211, 26421125, 26421116, 264558, 264561, 264562, D01D 5247
Patent
active
054894069
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
This invention relates to porous polymeric membranes and more particularly to such membranes that are prepared from polyvinylidene fluoride.
Polyvinylidene fluoride is a well known polymer of general formula (C.sub.2 H.sub.2 F.sub.2).sub.n. It has the advantages of strength and oxidation resistance.
BACKGROUND ART
Polymeric membranes may be prepared by the phase inversion technique which commences with the formation of a molecularly homogeneous, single phase solution of a polymer in a solvent. The solution is then allowed to undergo transition into a heterogeneous, metastable mixture of two interspersed liquid phases one of which subsequently forms a gel. Phase inversion can be achieved by solvent evaporation, non-solvent precipitation and thermal precipitation.
The quickest procedure for forming a microporous system is thermal precipitation of a two component mixture, in which the solution is formed by dissolving a thermoplastic polymer in a solvent which will dissolve the polymer at an elevated temperature but will not do so at lower temperatures. Such a solvent is often called a latent solvent for the polymer. The solution is cooled and, at a specific temperature which depends upon the rate of cooling, phase separation occurs and the liquid polymer separates from the solvent.
All practical thermal precipitation methods follow this general process which is reviewed by Smolders et al in Kolloid Z.u.Z Polymer, 43, 14-20 (1971). The article distinguishes between spinodal and binodal decomposition of a polymer solution.
The equilibrium condition for liquid-liquid phase separation is defined by the binodal curve for the polymer/solvent system. For btnoda/decomposition to occur, the solution of a polymer in a solvent is cooled at an extremely slow rate until a temperature is reached below which phase separation occurs and the liquid polymer separates from the solvent.
It is more usual for the phases not to be pure solvent and pure polymer since there is still some solubility of the polymer in the solvent and solvent in the polymer, there is a polymer rich phase and a polymer poor phase. For the purposes of this discussion, the polymer rich phase will be referred to as the polymer phase and the polymer poor phase will be referred to as the solvent phase.
When the rate of cooling is comparatively fast, the temperature at which the phase separation occurs is generally lower than in the binodal case and the resulting phase separation is called spinodal decomposition.
According to the process disclosed in U.S. Specification No. 4,247,498, the relative polymer and solvent concentrations are such that phase separation results in fine droplets of solvent forming in a continuous polymer phase. These fine droplets form the cells of the membrane. As cooling continues, the polymer freezes around the solvent droplets.
As the temperature is lowered, these solubilities decrease and more and more solvent droplets appear in the polymer matrix. Syneresis of the solvent from the polymer results in shrinkage and cracking, thus forming interconnections or pores between the cells. Further cooling sets the polymer. Finally, the solvent is removed from the structure.
Known thermal precipitation methods of porous membrane formation depend on the liquid polymer separating from the solvent followed by cooling so that the solidified polymer can then be separated from the solvent. Whether the solvent is liquid or solid when it is removed from the polymer depends on the temperature at which the operation is conducted and the melting temperature of the solvent.
True solutions require that there be a solvent and a solute. The solvent constitutes a continuous phase and the solute is uniformly distributed in the solvent with no solute-solute interation. Such a situation is almost unknown with the polymer solutions. Long polymer chains tend to form temporary interactions or bonds with other polymer chains with which they come into contact. Polymer solutions are thus rarely true solutions but lie somewhere between true sol
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Beck Thomas W.
Grant Richard D.
Lee Matthew B.
Streeton Robert J. W.
Memtec Limited
Tantoni Leo B.
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