Processing Guide for Polymer Membranes Over the last thirty years, the use of porous polymer The DIPS process is highly sensitive to spinning process membranes has achieved a significant position as a cost- variables and dope solution variability, making it essential effective means of non-destructively separating components to closely control both the membrane casting and dope from fluid mixtures. Membrane technology possesses a preparation processes. This can only be accomplished number of attractive features in that it can be carried out by using high-quality polymers with minimal lot-to-lot continuously under mild operating conditions and requires variability. little space. It is also environmentally friendly in that energy Thermally Induced Phase Separation (TIPS) technology consumption is generally very low and few, if any, additives is also becoming an attractive manufacturing method are required. It is both a cleaning and clean technology. for porous membranes. Temperature processing allows Solvay offers the industry’s broadest selection of high- the preparation of a defect-free membrane with excellent performance polymers for membranes, including: mechanical performance. Semi-crystalline polymers, such ® ® • Solef® PVDF (polyvinylidenefluoride) as Solef PVDF and Halar ECTFE, can be processed by TIPS due to their solubility in a range of solvents at high • Halar ® ECTFE (ethylene chlorotrifluoroethylene) temperature. • Udel® PSU (polysulfone) • Veradel® PESU (polyethersulfone) This document provides useful information about polymer properties as well as guidelines for preparing dope • Radel® PPSU (polyphenylsulfone) solutions in order to improve the stability and efficiency ® • Algoflon PTFE (Polytetrafluoroethylene) of your membrane preparation process. • Fluorolink® PFPE (perfluoropolyether) ® These materials are used extensively to manufacture isotropic Solef PVDF Fluoropolymers and anisotropic porous hollow fiber and flat sheet membranes PVDF is a semi-crystalline fluoropolymer obtained by for the entire range of the filtration spectrum from microfiltration the polymerization of vinylidene fluoride. Solvay Specialty (MF) to reverse osmosis (RO). Dense-film gas purification Polymers offers a wide range of PVDF polymers that are membranes are also readily prepared with these materials. marketed under the Solef® tradename. Excellent chemical resistance over a large pH range, Solef® PVDF homopolymers are used for making durable, hydrolytic stability, high strength and broad agency long-lasting membranes. Their toughness, chemical certifications make these polymers well-suited for resistance and oxidative stability allow membranes to membranes used in demanding end-use environments. tolerate a wide variety of feed streams and cleaning Typical application areas include water purification, methods. The high purity of these materials is widely wastewater treatment, pharmaceutical production, and recognized in other specialty market segments, such as blood purification along with a variety of industrial process the semiconductor industry, where released contaminants separations, such as food and beverage processing, are not tolerated. electropaint recovery and gas separation. In addition to PVDF homopolymers, which are recognized Many of the processes used to make separation worldwide for their excellent performance in membrane membranes require dissolving the polymer in a solvent. applications, Solvay has developed a wide range of Because Solef® PVDF, Udel ® PSU, Veradel® PESU and vinylidenefluoride-hexafluoropropylene (VF2-HFP) Radel® PPSU are soluble in conventional solvents, they copolymers and vinylidenefluoride-chlorotrifluoroethylene can be solvent cast by the Diffusion Induced Phase (VF2-CTFE) copolymers to respond to specific needs Separation (DIPS) process. This process is also referred of various market segments. Because copolymers are to as a Non-solvent Induced Phase Separation (NIPS). characterized by lower crystallinity than homopolymers, Technical Bulletin SPECIALTY POLYMERS they have higher flexibility than homopolymers and are Solef® PVDF homopolymers encompass a broad range of soluble in a wider range of solvents, such as ketones and molecular weights; select grades are shown in Table 1. Each tetrahydrofuran. grade has a narrow molecular weight distribution, which makes it possible to control viscosities and easier to fine-tune Solef® PVDF polymers are readily soluble in typical dope solutions as well as maximize process stability. solvents that are used to manufacture membranes, and they offer a wide range of desirable properties. In Solubility particular, Solef® PVDF suspension homopolymers are Solef® PVDF polymers are soluble in aprotic polar widely used in membrane filtration applications due to solvents, such as dimethylformamide (DMF), a number of desirable features: dimethylacetamide (DMAC), N-methyl-2-pyrrolidone • Excellent toughness and durability (NMP), triethyl phosphate (TEP) and dimethyl sulfoxide • Outstanding chlorine and UV resistance (DMSO). Solvent polarity, temperature and polymer • Stability at pH levels from 1 to 11 molecular weight affect the solubility of PVDF. Examples of binary phase diagrams of Solef® PVDF in NMP, TEP and • High-purity grades DSMO obtained according to ASTM D6038 are reported • Global agency approvals in Figures 1 to 3. Additional data showing solubility in • Easy to form MF and UF membranes other solvents are available upon request. Table 1: Typical properties of Solef® PVDF homopolymers Molecular Weight Powder Grades Melting Point [°C] Delta H [J/g] [Mw, Da] (1) Mw/Mn (1) MFI Solef® 6010 (2) 170 – 175 58 – 66 300,000 – 320,000 2.1 – 2.6 4.0 – 8.0 (5.0 kg) Solef® 6012 (2) 170 – 175 55 – 65 380,000 – 400,000 2.1 – 2.6 4.0 – 6.0 (10.0 kg) Solef® 1015 170 – 175 57 – 66 570,000 – 600,000 2.1 – 2.6 2.8 – 4.6 (21.6 kg) Solef® 6015 170 – 175 55 – 65 570,000 – 600,000 2.1 – 2.6 2.8 – 4.6 (21.6 kg) Solef® 6020 170 – 175 55 – 65 670,000 – 700,000 2.1 – 2.6 ≤ 2.0 (21.6 kg) (1) Data obtained by gel permeation chromatography in dimethylacetamide (DMAC) and calibrated using a polystyrene standard. Results should be used for a relative comparison among samples only. (2) Available in pellet form Figure 1: Phase diagram of Solef® 1015 in NMP Figure 2: Phase diagram of Solef® 1015 in TEP Solution Gel Solution Unstable Solution Gel 80 80 70 70 60 60 [°C] [°C] 50 50 40 40 Temperature 30 Temperature 30 20 20 10 10 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Concentration w/w [%] Concentration w/w [%] 2 \ Processing Guide for Polymer Membranes Figure 3: Phase diagram of Solef ® 1015 in DMSO Figure 5: Effect of concentration on solution viscosity ® Solution Unstable Solution Gel – Solef 1015 in NMP at 25 °C 80 1,E+02 70 1,E+01 60 [Pa*S] [°C] 50 1,E+00 40 Viscosity 20 % in NMP 15 % in NMP Temperature 30 10 % in NMP 1,E-01 20 1,E-02 1,E-01 1,E+00 1,E+01 1,E+02 1,E+03 Rate [1/s] 10 0 5 10 15 20 25 30 30 Concentration w/w [%] Figure 6: Effect of molecular weight on solution viscosity – Solef® PVDF in NMP at 25 °C Solution Viscosity 1,E+04 Solution viscosity depends on several parameters, Solef ® 6020 1,E+03 Solef ® 1015 including temperature, polymer concentration, polymer [Pa*S] ® Solef 6010 molecular weight and the nature of the solvent. Figures 1,E+02 4 through 10 show curves of Solef ® PVDF solutions 1,E+01 measured with Rheometer Rheometric Scientific RSFIII in steady rate sweep mode using concentric cylinders 1,E+00 geometry. Concentrations are expressed as the weight 1,E-01 of the polymer over the total weight of the solution. Viscosity at 0.01 1/s 1,E-02 PVDF solutions generally show newtonian behavior in a 0 5 10 15 20 25 30 wide range of shear rates. Very high viscosity solutions Concentration [%] tend to deviate from standard behavior at high shear Figure 7: Effect of temperature and concentration rates (shear thinning effect). Besides, the chemical nature on solution viscosity – Solef® 1015 in NMP at 25 °C and quantity of additives has a strong influence on the rheological behavior of the dope solutions. 1,E+02 Figure 4: Effect of concentration on solution viscosity – Solef® 6010 in NMP at 25 °C 1,E+02 [Pa*S] 25 % in NMP 1,E+01 20 % in NMP 15 % in NMP 1,E+01 10 % in NMP Viscosity 20 % 25 °C 20 % 50 °C [Pa*S] 15 % 25 °C 15 % 50 °C 1,E+00 1,E+00 1,E-02 1,E-01 1,E+00 1,E+01 1,E+02 1,E+03 Viscosity Rate [1/s] 1,E-01 1,E-02 1,E-01 1,E+00 1,E+01 1,E+02 1,E+03 Figure 8: Effect of solvent on solution viscosity ® Rate [1/s] – Solef 1015 at 25 °C 1,E+02 20 % in NMP 15 % in NMP 20 % in DMAC 15 % in DMAC [Pa*S] 1,E+01 Viscosity 1,E+00 1,E-02 1,E-01 1,E+00 1,E+01 1,E+02 1,E+03 Rate [1/s] 3 \ Processing Guide for Polymer Membranes Figure 9: Effect of concentration on solution viscosity Figure 12: Solef® 1015 in PC (Propylene Carbonate) – Solef® 1015 in DMAc at 25 °C Solution Unstable Solution Gel 1,E+02 120 100 1,E+01 [°C] [Pa*S] 80 1,E+00 60 20 % in DMAc Viscosity 1,E-01 15 % in DMAc Temperature 10 % in DMAc 40 5 % in DMAc 1,E-02 1,E-02 1,E-01 1,E+00 1,E+01 1,E+02 1,E+03 20 0 5 10 15 20 25 30 35 40 Rate [1/s] Concentration w/w [%] Figure 10: Effect of temperature and concentration on solution viscosity – Solef® 1015 in DMAc at 25 °C Figure 13: Solef® 1015 in Cytroflex A4 1,E+02 20 % 25°C Solution Unstable Solution Gel 20 % 50°C 210 15 % 25°C 15 % 50°C 200 [Pa*S] 190 1,E+01 [°C] 180 170 Viscosity 160 150 1,E+00 Temperature 140 1,E-02 1,E-01 1,E+00 1,E+01 1,E+02 1,E+03 Rate [1/s] 130 120 15 Figure 11: Solef® 1015 in TGDA 10 20 25 30 (Triethylene Glycol Diacetate) Concentration w/w [%] Solution