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Acid Coagulation

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Coagulation and Raw Rubber
Converting liquid latex to solid rubber
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Acid Coagulation
Using formic or acetic acid to solidify latex

To turn liquid latex into solid rubber, strain it, dilute it to 15-20% dry rubber content, stir in formic acid or vinegar, and leave it 30-60 minutes until a soft white coagulum separates from clear serum. Add the acid slowly and stop as soon as the serum runs clear, because too much acid darkens the rubber and makes it stiff. The coagulum is then washed and rolled to squeeze out water.

Part of Rubber Processing

Using a weak acid to clot latex into a solid mass that can be rolled, dried and compounded.

Acid Neutralises The Charge That Keeps Rubber Particles Apart

The corpus’s Coagulation article explains the mechanism: acid neutralises the electrical charge that keeps the rubber particles apart, so they clump. Acid is the most controllable of the methods; smoke coagulation is the alternative in Smoke Curing (RSS).

What You Need

  • Strained latex from Latex Collection and Preservation.
  • Acid: formic acid, or acetic acid in the form of vinegar. For vinegar strength see Vinegar Strength. The carrier does not give the strength of the formic acid its 1-2% refers to.
  • A shallow plastic, glazed or wooden container; avoid bare iron or copper with formic acid.
  • Clean water, a stirring stick and a roller or mangle.

Method: Coagulating A Batch

Step 1: Strain the latex through fine mesh.

Step 2: Dilute to 15-20% dry rubber content with water.

Step 3: Add acid slowly, stirring gently. The carrier gives formic acid at 1-2% of the latex volume or acetic acid (vinegar) at 3-5% of the volume. The Latex Processing article gives 15-20 mL of vinegar per litre of strained latex, which is 1.5-2%. The two vinegar figures differ by about a factor of two and the corpus does not reconcile them.

Step 4: Stop when the serum runs clear, the coagulation article’s end-point. It is a better guide than a fixed dose because the strength of the vinegar varies.

Step 5: Let stand 30-60 minutes. A soft white coagulum forms.

Step 6: Remove and wash in clean water.

Step 7: Roll or press between rollers to squeeze out water and serum; the latex-processing article puts the water content of the mass at 40-60%.

Worked Example

Dilution: the tapping article gives 15-30 g of dry rubber from 40-80 mL of latex, which is 15 ÷ 40 = 0.375 g per mL, or roughly 37% rubber (taking 1 mL of latex as about 1 g, an assumption). Diluting to 20% needs a factor of 37.5 ÷ 20 ≈ 1.9; to 15% a factor of 37.5 ÷ 15 = 2.5. So 1 L of fresh latex becomes about 1.9-2.5 L, which means adding 0.9-1.5 L of water. This is derived from two other articles’ figures and should be checked against real latex.

Acid for 1 L of fresh latex: formic acid at 1-2% is 10-20 mL; vinegar at 3-5% is 30-50 mL, or 15-20 mL by the other article. The corpus does not say whether the percentage applies before or after dilution.

Common Mistakes

  • Dosing by the percentage alone. Vinegar strength varies; watch the serum.
  • Too much acid. The rubber darkens and stiffens.
  • Skipping the wash. Residual acid attacks the rubber and any equipment it later touches.

Safety

Formic acid burns

Concentrated formic acid burns skin and eyes and its vapour irritates the lungs (recalled general fact, not from the corpus). Dilute acid into water, never water into acid, work in moving air, wear eye protection and gloves, and keep a bucket of clean water within reach. Keep acid away from children and from food containers.

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