Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Patent
1985-06-13
1986-09-23
Foelak, Morton
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
2105002, 210654, 21050038, 521 64, 521184, 521189, C08V 936
Patent
active
046136258
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
This invention relates to porous membranes made from aliphatic thermoplastic polyamide materials.
BACKGROUND ART
Synthetic polymeric membranes are used for separation of species by dialysis, electrodialysis, ultrafiltration, cross flow filtration, reverse osmosis and other similar techniques. One such synthetic polymeric membrane is disclosed in Australian Patent Specification No. 505,494 of Unisearch Limited.
The membrane forming technique disclosed in the Unisearch Patent is broadly described as being the controlled uni-directional coagulation of the polymeric material from a solution which is coated onto a suitable inert surface. The first step in the process is the preparation of a "dope" by dissolution of a polymer. This is said to be achieved by cutting the hydrogen bonds (which link the molecular chains of the polymer together) with a solvent. After a period of maturation, the dope is then cast onto a glass plate and coagulated by immersion in a coagulation bath which is capable of diluting the solvent and annealing the depolymerised polymer which has been used. According to the one example given in this specification, the "dope" consisted of a polyamide dissolved in a solvent which comprised hydrochloric acid and ethanol.
In another membrane forming technique, the liquid material out of which the membrane is cast is a colloidal suspension which gives a surface pore density that is significantly increased over the surface pore density of prior membranes.
According to that technique, a thermoplastic material having both relatively non-crystalline and relatively crystalline portions is dissolved in a suitable solvent under conditions of temperature and time which cause the relatively non-crystalline portions of the thermoplastic material to dissolve whilst at least a portion of the relatively crystalline portion does not dissolve but forms a colloidal dispersion in the solvent. The colloidal dispersion and solvent (i.e. the "dope") is then coated onto a surface as a film and thereafter precipitation of the dissolved thermoplastic portion is effected to form a porous membrane.
Such aliphatic polyamide membranes suffer from disadvantages which limit their commercial usefulness and applicability. For example, they exhibit dimensional instability when drying and may shrink by up to 7%. Thus, it is essential that they be kept moist prior to and after use. Furthermore, it has not been possible to generate chemical derivatives of the membrane matrix which restricts the situations to which the membrane may be applied.
Another disadvantage is that such polyamide membranes are fundamentally unstable and eventually become brittle on storage. The instability has been carefully investigated by I. R. Susantor of the Faculty of Science, Universitas Andalas, Padang, Indonesia with his colleague Bjulia. Their investigations were reported at the "Second A.S.E.A.N. Food Waste Project Conference", Bangkok, Thailand (1982) and included the following comments regarding brittleness:
"To anneal a membrane, the thus prepared membrane (according to Australian Patent No. 505,494 using Nylon 6 yarn) is immersed in water at a given temperature, known as the annealing temperature, T in degrees Kelvin. It is allowed to stay in the water a certain length of time, called the annealing time. For a given annealing temperature, there is a maximum annealing time, t(b) in minutes, beyond which further annealing makes the membrane brittle. Plotting ln 1/t(b) versus 1/T gives a straight line. From the slope of this line it can be concluded that becoming brittle on prolonged annealing is a process requiring an activation energy of approximately 10.4 kilocalories/mole. From the magnitude of this activation energy, which is of the order of van der Waals forces, the various polymer fragments are probably held together by rather strong van der Waals forces or hydrogen bond(s)."
We have confirmed that the brittleness is due to a recyrstallization of water-solvated amorphous polyamide. In some cases (such as polyamide 6) bri
REFERENCES:
patent: 3904519 (1975-09-01), McKinney, Jr. et al.
Foelak Morton
Memtec Limited
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