TREATMENT OF MEMBRANES

Abstract: The transport properties of a polymeric ultrafiltration membrane are modified by passing aqueous liquid through the membrane at a temperature less than the glass transition temperature, progressively lowering the pH of the liquid to break hydrogen bonds in the polymer, and reacting the activated polymer so produced with a reagent which is usually a reactive textile finishing product.

Description: TREATMENT OF MEMBRANES TECHNICAL FIELD The present invention relates to a method for the treatment of membranes to modify the properties thereof and to the membranes modified by such treatment. The membranes are industrially applicable in the field of ultrafiltration. BACKGROUND ART Membranes are obtained from various types of polymer. generally by phase inversion processes, and their structure is partly crystalline, partly amorphous. The cohesion between the molecular chains is generally achieved by hydrogen bonding, the porous structure, being extremely dividen or loose, is totally stable only below the first transition point, as far as temperature is concerned. Because of the manufacturing techniques involved, membranes generally present a residual local charge effect, due to the fact that during coagulation, or more generally phase inversion, some chains are blocked in a non-crystalline structure, and radicals have their charge potential non-counterbalanced by an adjoining counter radical from an adjoining chain. Due to this effect, rejection of molecules by membranes is greatly influenced by the relative isoelectriσ point of the molecule retained at the particular pH. not by size but because of molecular interaction between a negatively charged membrane and negatively charged proteins. What appears to be an advantage in terms of rejection is also a disadvantage in terms of membrane poisoning. In fact, when ultrafiltration or other membrane-related processes are used for the treatment of a complex medium (e.g. blood, molasses, sugar-cane juice, whey) or generally any natural medium which contains a large specturm of various molecules in terms of molecules which are repelled _Q..-H WL-O by a partly charged membrane are always accompanied by, or associated with molecules which are countercharged and which will fix onto the membrane and create membrane poisoning. Any membrane is a polymeric structure derived from natural, artificial or synthetic polymers. The cohesion of the structrue in the original polymer is due to bonds - a part of them being broken down before the formation of the membrane, and a part of which are not reconstituted when the membrane is formed in the non-crystalline region of the polymer which constitutes the membrane. For example, carboxyl groups ( C=0) and amine groups (-NH-) will be present locally at the surface of the polymer chains, and even for membranes which are described as being totally neutral, at molecular level they present part of the structure with local ion-exchange properties, or with local strong electro-chemical properties. It is an object of the present invention to provide a method for transforming the surface characteristics of the polymer of the membrane - both at the surface of the membrane as such and within the internal structure thereof - to block the electro-chemical interactions, to achieve at molecular level an anti-poisoning effect, and to maintain steady flux characteristics during long-term use of the membrane.. It is a futher object of this invention to provide a process whereby one type of membrance can be used as a matrix for further reticulation or modification of the porous structure to give a large variation of possible molecular weight cut-off from the same membrane structure. By the use of previously known processes, the modification of the porous structure of membranes by grafting is extremely difficult to achieve without modification of the original membrane matrix. To graft any molecule on the polymeric structure on a uniform basis, it is firstly necessary to achieve aσcessability inside the polymer for the molecule chosen as the grafting reagent. The method used was to increase the temperature well above the glass transition point of the polymer, to permit chains to have a sufficient level of flexibility to achieve diffusion of the reactive species. This process is. however, not suitable for use with membranes because the use of temperatures above the glass transition point would destabilize the very fine and delicate structure of the membrane. DISCLOSURE OF THE INVENTION The present invention provides a method for the modification of the transport properties of a polymeric ultrafiltration membrane which comprises subjecting the membrane to a flux of a polar solvent at a temperature below the glass transition temperature of the membrane polymer: progressively lowering the pH of the liquid to break hydrogen bonds within the polymer structure and produce an activated polymer having active groups thereon; and reacting the activated polymer with a reagent which is reactive therewith. The polar solvent can be water, through other polar solvents such as liquid ammonia give the same advantage. The polar solvent is a solvent for the ionic species used to lower the pH but not a solvent for the polymeric material of the membrane. The glass transition temperature is a well known parameter and may be measured by differential thermal analysis techniques. The glass transition temperatures for polyamide -6 and 6-6 are approximately 50°C and 54°C respectively depending on degree of polymesisation. Thus the present invention provides a process which, firstly, permits the opening of the molecular structure of the constitutive polymer of the membrane at a temperature - 4 - below the glass transition point. Secondly, it permits the grafting of various molecules and radicals onto the polymeric chain below the glass transition temperature. Thirdly, by modification of the dimensions of the graft chain employed it is possible to restrict the.pore dimensions of a given membrane and to adjust the molecular weight cut-off thereof to suit a particular need or application. Fourthly, it is possible to achieve either part of complete saturation of active surface groups, to avoid poisoning by components which have an electro-chemical interaction with the surface of the polymer. DESCRIPTION OF PREFERRED EMBODIMENTS The invention will be further described by way of example only with reference to specific embodiments of the invention. EXAMPLE 1 Membranes made from polyamide -6 are obtained from a polymer which is a product- of polymerization of caprolactam The link between the chains can be represented as in Formula (2) below \ / H - C - H H - C - H H - C - H H - C - H \ / H - C - H H - C - H / ' \ H - C - H H - C - H (2) \ ...... / C = 0 H;- N \ ;- ... / - - \ c =;o H. - N C = 0 H - C - H H - C - H / . \ H - C - H H - C - H \ / H - C - H H - C - H / \ \ , .-. / \ ...... / N -fH 0.= C N -.H 0)= C / - - ' \ / '--' \ N - H 0, > C H - C - H / ' • -- ' \ / According to Formula (2) the cohesion between chains in the crystalline part is achieved by means of hydrogen bonds between the oxyen atom of the C = 0 and the hydrogen atom of the N - H groups. This type of bonding is totally stable up { •" ; ■••' I to approximately 50 C in this example, and will be loosened at higher temperatures. This factor is used in the textile industry for the dyeing of polyamide -6 fibres using acid reactive dyestuffs at temperatures generally above 100°C. To give access to an acid reactive dyestuff of general formula (3) according to an embodiment of the present invention. (where R can have a variety of values well known in the dyestuff field) the polymer is subjected to a gradual variation in pH in the presence of a wetting agent to open up the molecular chains below the glass transition point. Preferably, the membrane is subjected to a distilled water flux at a trans-membrane pressure below the pressure at which the membrane will be used, with a gradual lowering of the pH from 7 to whatever value may be required e.g. 0.7. with continuous monitoring of the water flux. The water flux J . measured at 35 C, is plotted against pH in Figure 1. The mixture used for the treatment comprised 90% distilled water. 9% ethyl alcohol and 1% methyl alcohol. In this particular example, from pH7 to pH3 the water flux increases, showing the effect of the wetting agent (i.e. the alcohols} on the surface of the polymer to increase the transport properties in the vicinity of the polymer, and also the swelling effect due to the addition of HC1 to lower the pH. The water flux from pH3 to pH 0.7 decreases to show that a higher concentration of hydrogen ions break down hydrogen bonds inside the porous structure which then starts to collapse. This first operation is called activation of the polymer, and is stopped when the water flux is equal to. or not less than, the original water flux. At this stage, a sufficient number of hydrogen bonds have been cut without destruction of the porous structure. giving too large a drop in water flux. After .rinsing with double-distilled water, a reactive molecule (e.g.. a reactive dyestuff of general Formula (3)) is applied in the form of a dilute solution, to achieve complete saturation of the dyestuff in the polymer. The saturation effect is controlled by the fact that the shade or tone of the fibre reaches an equilibrium and that no more active sites are available for dyestuff fixation. In the particular case of polyamides. an acid-type dyestuff is used, but the colour given by the radical R for Formula (3) is of no importance. The treated membrane is then washed with distilled water to remove excess dyestuff. The sulphonic acid derivative dyestuffs available under the KITON trademark (Ciba-Geigy) are particularly suitable. Although the invention is described above with particular reference to the application of dyestuffs, the invention is applicable to the use of any reactive molecule with a reactive group having a tendency to fix onto one of the active groups of the molecular chain. For example it is possible to add reactive groups which lend to the polymer constituting the membrane, properties such as transport of alcohol but rejection of water. The surface properties of other membranes can be changed by the addition of reactive groups which change form with pH change i.e., to be acid or basic, for example amphoteric dyes. Such membranes could be suitable for applications requiring transport of water without the transport of salts. This provides membranes which are reverse osmosis membranes. Any reactive finishing product used in the textile industry for finishing fibres of the same polymeric composition as the membrane, can be used. For example, hydrophobic properties may be imparted by incorporation of a fluorocarbon or chloro luorocarbon chain, typically using the sulphonic acid derivative. Hydrophobic properties may be achieved using the classical finishing products of the textile industry, for example derivatives of hydrocarboxy cellulose. The change in molecular weight of the reactive finishing product determines the restriction of the pore structure of the membrane to achieve change in the molecular weight cut-off by reducing the diameter of the pore due to the graft-added molecular structure at the surface. Inversely, the reaction carried out according to Figure 1 shows that an increase of water flux associated with the dilation or expansion of the pores can be achieved, and if the reaction is stopped at point J W1 (Fig. 1), and the membrane polymer is grafted with small chain reactive compounds, then a membrane with high M.W. cut-off is obtained. EXAMPLE 2 Hollow fibre membranes, made from polyamide 6 - 6, was treated gradually according to Figure 1 at 32°C. using a step-by-step process involving treatment at pH 7 for 30 minutes. pH4 for 30 minutes. pH3 for 30 minutes and pH2 for 30 minutes. The membrane had an original molecula weight cut-off of 20,000 and a water flux of 1000 ml/minute at 32 C. The water flux J W2 at pH2 is 1200 ml/minute. The reaction is stopped at a water flux of 970 ml/minute. The dyestuff used is Anthracine yellow acid reactive dye for polyamide. The dyeing reaction is carried out over 2 hours at 30°C and 100 kPa difference. The treated membrane produced in Example 2 can be compared to the untreated membrane in the following manner. EXAMPLE 3 Ultrafiltration of cane sugar molasses diluted to 8 2 brix. The original flux was 2.3 1/hour/m . The treated membrane had an original flux of 13.5 2 litres/hour/m . The stablized flux after 8 days of 2 continuous operation was 10.8 1/hour/m . temperature 35oC, pressure (inlet) 50 kPa; (outlet) 150 kPa. EXAMPLE 4 Sugar can juice at 32 C. Untreated membrane stabilized flux: 15.5 1/hour/m 2; treated 32 1/hour/m2. Another application of the invention is the protection of polymer membranes through reticulation. Polyamide 6-6 membranes are susceptible to phenols which break down the membrane. For example, an untreated hollow fibre membrane of polyamide 6-6 will be dissolved by a 5% phenol solution within a period of about two hours. However another polyamide 6-6 hollow fibre membrane modified by a surface reticulation treatment according to the invention is protected from the phenol and is still functional after two days. According to the present invention the structure of a given membrane can be adapted to a particular need simply by treatment in accordance with the teaching of the present invention. This dramatically increases the utility of any given membrane and results in a reduction in membrane costs. Although the invention has been described with reference to preferred embodiments and examples, it will be appreciated that the scope of the invention is not limited to or by such embodiments and examples, and that various modifications are possible without departing from the scope of the invention disclosed herein.

Claims: CLAIMS 1) A method for the modification of the transport properties of a polymeric ultrafiltration membrane which comprises subjecting the membrane to a flux of a polar solvent at a temperature below the glass transition temperature of the membrane polymer; progressively lowering the pH of the liquid to break hydrogen bonds within the polymer structure and produce an activated polymer having active groups thereon; and reacting the activated polymer with a reagent which is reactive therewith. 2) A method according to claim 1 wherein the aqueous liquid comprises a wetting agent. 3) A method according to claim 1 wherein the flux passes through a maximum as the pH is decreased; and wherein the activation treatment is stopped when the flux falls again to a value which is at least equal to the initial flux at the start of the treatment. 4) A method according to claim 1 wherein the reagent provides groups which change from acid to basic with pH change, so as to provide a membrane suitable as a reverse osmosis membrane. 5) A method according to claim 1 wherein the reagent provides hydrophilic or hydrophobic groups. 6) A method according to claim 1 wherein the reagent is a reactive dyestuff. Ø ' J &.. 7) A method according to claim 1 wherein the polymeric membrane is composed of a polyamide. 8) A modified ultrafiltration membrane produced by the method of any proceeding claim.

Inventors: LEFEBVRE MICHEL SERGE MAXIME, LEFEBVRE, MICHEL, SERGE, MAXIME
Applicants: LIMIT INSTANT LTD, LIMIT INSTANT LIMITED
Application year: 1984
Main classification: B01D13
IPC codes: B01D67