Build Guide

Read the Safety page first. THF and MEK are flammable and harmful to breathe. Resin dust is an irritant. Work ventilated with gloves and eye protection.

Video walkthroughs:

• How to make a cation exchange membrane in under 2 minutes

Materials

• Commercial ion exchange resin beads. Water softener resin (strong acid cation) for a cation membrane; anion exchange resin for an anion membrane. Specialized chelating resins can be used for target-ion selectivity.
• PVC or CPVC. CPVC gives better chemical resistance at elevated temperature.
• Solvent: tetrahydrofuran (THF), cyclohexanone, or methyl ethyl ketone (MEK).
• Casting surface: polypropylene sheet (solvent resistant, non-stick). Optional: polypropylene felt or mesh for reinforcement.
• Optional structural additives: fiberglass, fumed silica, sand.

Process (best mode)

1. Pulverize the resin beads into a fine powder. A high-speed blender does it in under 120 seconds. Ball milling or grinding also works, dry or wet. Target particle size under 200 microns.
2. If you pulverized wet, remove the water: settle and decant then air dry, spray dry, or sieve the slurry. You need dry powder or a controlled slurry.
3. Dissolve the PVC or CPVC in solvent at roughly 3:7 polymer to solvent by volume. Adjust for viscosity.
4. Mix in the resin powder. Typical loading is about 50% resin by volume; the workable range is 10 to 70%. More resin means better conductivity, less resin means better mechanical strength.
5. Apply the glue-like mixture to the casting surface by spreading, pouring, extruding, or spraying. Thin it with extra solvent if spraying. You can also coat it directly onto a cell frame, electrode, or reinforcing felt.
6. Dry slowly and evenly. An enclosed environment prevents uneven drying, which causes cracking and warping. Peel while partially dry if you want a free-standing film, or leave it as a permanent coating.

Tuning

• Resin type sets what the membrane passes: cation resin passes cations, anion resin passes anions, specialty resins bias selectivity toward targets like rare earths or platinum group metals.
• Resin loading trades conductivity against strength.
• Thickness is set by your application method. Thin sprayed films can come in under $1 per square foot.
• The membrane solvent-welds to PVC and CPVC cell bodies with standard pipe cement, which makes sealed cell assembly trivial.

Full Documentation

The complete patent text, process flowchart, and technical volume are in the repository. The patent is dedicated CERN-OHL-S, so every detail is public.