Process for the preparation of ethylene/chlorotrifluoroethylene polymer membranes

Abstract

The invention relates to a process for the production of membranes based on ethylene/chlorotrifluoroethylene polymers having a melting temperature not exceeding 200? C. The process relies on the diffusion induced phase separation of the ethylene/chlorotrifluoroethylene polymer from a solution and comprises the steps of providing a solution comprising an ethylene/chlorotrifluoroethylene polymer having a melting temperature not exceeding 200? C. in a solvent; casting the polymer solution into a film; immersing the film in a non-solvent bath to precipitate the polymer. Membranes made of compositions comprising an ethylene/chlorotrifluoroethylene polymer having a melting temperature not exceeding 200? C. and at least one second polymer are also disclosed.

Claims

1. A process for the preparation of an ethylene/chlorotrifluoroethylene polymer membrane comprising the steps of: preparing a polymer solution comprising an ethylene/chlorotrifluoroethylene polymer in a solvent, wherein the polymer in the polymer solution consists essentially of ethylene/chlorotrifluoroethylene polymer that has a melting temperature not exceeding 200? C.; casting the ethylene/chlorotrifluoroethylene polymer solution into a film; immersing the film in a non-solvent bath to precipitate the polymer via diffusion induced phase separation; wherein the solvent is selected from the group consisting of N-methyl-2-pyrrolidone, dimethylformamide, acetone, dimethylacetamide, tetrahydrofuran, methyl ethyl ketone, dimethylsulfoxide, dioxane, ethyl acetate, propylene carbonate, and mixtures thereof.

2. The process of claim 1 wherein during the casting step the polymer solution has a temperature not exceeding 180? C.

3. The process of claim 2 wherein during the casting step the polymer solution has a temperature of at least 100? C. and not exceeding 150? C.

4. The process of claim 1, wherein the non-solvent is water.

5. The process of claim 1, wherein the polymer solution comprises more than one ethylene/chlorotrifluoroethylene polymer having a melting temperature not exceeding 200? C.

6. The process of claim 1 wherein the ethylene/chlorotrifluoroethylene polymer has a melting temperature of at least 150? C. and not exceeding 195? C.

7. The process of claim 1, wherein said ethylene/chlorotrifluoroethylene polymer membrane is a hollow fiber or capillary membrane; wherein said film is a tubular film, and wherein the polymer solution is cast into said tubular film around a supporting fluid through a spinneret.

8. The process of claim 1, wherein said ethylene/chlorotrifluoroethylene polymer membrane is a porous membrane.

9. The process of claim 1, wherein said ethylene/chlorotrifluoroethylene polymer membrane is an asymmetric membrane.

10. The process of claim 1, wherein the polymer solution comprises about 10% to 20% by weight of the ethylene/chlorotrifluoroethylene polymer having a melting temperature not exceeding 200? C.

11. The process of claim 1, wherein the ethylene/chlorotrifluoroethylene polymer has a melting point not exceeding 190 ? C.

12. The process of claim 1, wherein the polymer solution consists essentially of ethylene/chlorotrifluoroethylene polymer in a solvent.

13. The process of claim 1, wherein the polymer solution further comprises at least one additional component selected from the group consisting of pore forming agents, nucleating agents, and fillers.

Description

EXAMPLES

(1) Characterization

(2) Melt flow rate was measured following the procedure of ASTM 3275-81 at a temperature of 220? C. and 2.16 Kg.

(3) Melting temperature was determined by DSC at a heating rate of 10? C./min, according to ASTM D 3418.

(4) Polymer composition was determined by material balance and carbon elemental analysis.

(5) Porosity determination: the porosity was calculated from the following relationship:

(6) $\mathrm{Porosity}\%=\frac{d_i-d_{\mathrm{pm}}}{d_i}?100$
wherein d.sub.i is the density of the polymer used for manufacturing the membrane, d.sub.pm is the density of the porous membrane itself, and d.sub.i and d.sub.pm are determined by weighing specimens of the copolymer and of the membrane in air and in water at 25? C. following the procedure of ASTM D 792 (method A-1).

Example 1

Preparation of ECTFE1

(7) An ECTFE polymer (ECTFE1) having the molar composition 42% E and 58% CTFE was synthesized in an industrial reactor at a temperature of 15? C. and at a pressure of 7.2 absolute bar.

(8) ECTFE1 had the following properties: melting temperature: 185? C.; melt flow rate: 1.4 g/10 min.

Example 2

Preparation of ECTFE2

(9) In an enameled autoclave equipped with baffles and stirrer working at 450 rpm were introduced: 3 l of demineralized water, 52.5 g of chloroform, 35 ml of an aqueous solution of hydroxypropylacrylate (HPA) (40% volume) and 7 kg of chlorotrifluoroethylene. The temperature was brought to 15? C. and ethylene was fed into the autoclave at a pressure of 8.2 absolute bars. A solution of trichloroacetylperoxide (TCAP) in isooctane (0.12 g.sub.TCAP/ml) maintained at ?17? C. was continuously fed to the autoclave during the polymerization. 35 ml of the solution of hydroxypropylacrylate and water were fed at consumption of 20, 40, 60, 80, 100, 120, 140, 160, and 180 g of ethylene.

(10) The pressure was maintained constant during the polymerization by continuously feeding ethylene to the reactor up to a consumption of 200 g; the initiator solution was introduced at a rate of 30 ml/h. After 345 minutes the product was discharged from the autoclave and dried at 120? C. for about 16 hours. The amount of dry polymer obtained was 1563 g.

(11) ECTFE2 had the following properties: molar composition: E/CTFE/HPA: 40/55/5; melting point: 178? C.; melt flow rate: 0.38 g/10 min.

(12) Determination of the Solubility of ECTFE Polymers in N-methyl-2-pyrrolidone (NMP) at 120? C.

(13) The solubility of ECTFE polymers having different melting temperatures was determined by preparing mixtures of the polymer at different concentrations in NMP. The mixtures were stirred at 140? C. for 60 minutes and then allowed to cool down to 120? C.

(14) The value of the solubility is the maximum amount of polymer which affords a transparent and homogeneous solution, without the presence of any phase separation in the system.

(15) The solubility values for ECTFE1, ECTFE2 and two commercially available ECTFE polymers sold under the trade name Halar? by Solvay Solexis SpA (Bollate, Italy) are reported in Table 1 together with their corresponding melting temperature.

(16) TABLE-US-00001 TABLE 1 Solubility in NMP at 120? C. Tm (? C.) (% w/w) ECTFE1 185 23 ECTFE2 178 12 Halar? 902 ECTFE 220 <5 Halar? 901 ECTFE 240 <5

(17) General Procedure for the Preparation of Membranes

(18) A solution of the ECTFE polymer in NMP, having a concentration between 10 to 20% by weight, was prepared in a glass round bottom flask equipped with two necks. A glass condenser was put on the flask and the apparatus was held at 150? C. for 2 hours at constant magnetic stirring (300 rpm) to obtain a clear homogeneous polymer solution.

(19) A temperate glass plate (30 cm?25 cm) was put on the base of a motorized film applicator (Elcometer 4344/11) equipped with an electrical heating system and heated to maintain the temperature on the glass surface at 120? C. 20 grams of the polymer solution were poured onto the glass plate and cast into a thin film with the help of a casting-knife. Immediately after casting the polymer solution had a temperature of about 120? C. The glass plate was quickly removed from the applicator and dipped in a water bath (having a capacity of 10 l) held at a temperature of approximately 40? C. The glass plate was then extracted from the water bath, the membrane removed from the plate and dried.

Examples 3 and 4

Membranes Made of ECTFE1

(20) Two membranes made of ECTFE1 were prepared following the general procedure described above starting with polymer solutions having a concentration of 16% and 20% by weight of the polymer in NMP. The resulting membranes had the following properties:

(21) ECTFE1 16% by weight solution: membrane thickness 120 ?m, porosity 60%;

(22) ECTFE1 20% by weight solution: membrane thickness 200 ?m, porosity 64%.

(23) Both membranes were tested in an aqueous solution of NaOH (5% by weight) for 14 days at 70? C. to determine their chemical resistance. After the treatment the membranes were physically undamaged and showed only a slight yellowing.

Example 5

Membrane Made of ECTFE2

(24) A membrane was prepared starting from a 12% by weight solution of ECTFE2 in NMP following the general procedure described above. The resulting membrane had a thickness of 100 ?m and 70% porosity.

(25) The membrane was tested in an aqueous solution of NaOH (5% by weight) for 14 days at 70? C. to determine its chemical resistance. After the treatment the membrane was physically undamaged and showed only a slight yellowing.

Example 6

Membrane Made of ECTFE1 and Halar? 902 ECTFE

(26) A membrane was prepared following the general procedure described above starting from a transparent and homogeneous solution prepared by dissolving a blend of 84% by weight ECTFE1 and 16% by weight Halar? 902 in NMP at 120? C. The total concentration of the polymer in solution was 16% by weight (13.5% by weight ECTFE1, 2.5% by weight Halar? 902). The resulting membrane had a thickness of 75 ?m and 56% porosity.

Comparative Example 1

Attempt to Prepare a Membrane with Halar? 902 ECTFE

(27) Following the general membrane preparation procedure described above, a solution of Halar? 902 in NMP at 120? C. was prepared. The resulting solution contained less than 5% by weight of the polymer. The polymer solution was cast into a film on a glass plate. When the cast film was immersed in the water bath only a loose precipitate formed. No membrane could be obtained.