Starch-Thickened Reactive Print Paste Breaking Down
A block-printing workshop prints 100 cotton stoles with a reactive dye paste. The paste is thickened with maize starch at 8% and contains 5% urea and 2% sodium bicarbonate. It is mixed at 9 a.m. By noon the paste has become thin, and prints taken after that time show blurred edges. The stoles are steamed at 102°C for 8 minutes, washed, and soaped. The colour yield is weak, wash fastness is 2–3, and the fabric feels stiff. The workshop also has sodium alginate, but the printer has been using starch because it is cheaper and gives a thick paste. The print room is hot and humid (35°C, 80% RH). The buyer requires sharp outlines, a soft handle and wash fastness of at least 4.
1. Explain why maize starch is chemically unsuitable as a thickener for reactive dye printing, and link each fault - thin paste, blurred edges, weak colour, stiffness and poor fastness - to that choice.
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Why starch fails
Maize starch is a polysaccharide with many hydroxyl groups. In the alkaline paste (sodium bicarbonate), those hydroxyls react with the reactive dye's reactive groups. The thickener competes with cotton for the dye, so less dye is available to fix to the fabric - hence weak colour yield and poor wash fastness. Starch also hydrolyses and breaks down under alkali, heat and shear; in a hot print room (35°C) the 8% paste loses viscosity within hours, becomes thin and spreads under the block, causing blurred edges. The starch film dries hard and is difficult to wash out; trapped unfixed dye and starch residue make the fabric stiff and reduce wash fastness. The 2% bicarbonate is enough to drive both dye fixation and starch degradation.
2. Recommend a replacement thickener and a revised print paste/steaming procedure. Include approximate starting amounts and the reason for each change.
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Recommended replacement
Use sodium alginate 4–6% as the thickener. Alginate is ionic and does not react with reactive dyes; it gives a clear flexible film and washes out easily. A starting paste: sodium alginate 5%, urea 5–8%, sodium bicarbonate 1.5–2%, plus the dye, made up with cold water. Add bicarbonate just before printing; keep the paste cool and use it within a few hours.
Procedure: print, dry gently, steam at 102–105°C for 8–10 minutes, wash at 60–80°C with 1 g/L nonionic detergent, then soap and rinse. Reason: alginate preserves colour yield and handle; the controlled steaming fixes the dye without over-degrading the thickener; thorough washing removes unfixed dye and thickener.
3. Describe how you would check print sharpness and wash fastness before the batch is shipped, and what result would justify rejecting the paste.
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Checks
- Print sharpness: print a test strip with the same block at the start, middle and end of the run. Inspect the edges under a magnifier or against a standard; measure line widening if a microscope/scale is available. Reject paste that shows feathering beyond the agreed tolerance.
- Wash fastness: ISO 105-C06 at 40°C, assess colour change and staining on the grey scale. Also check rubbing fastness (ISO 105-X12) and fabric handle after washing.
- A result below 4 on wash fastness, visible edge blurring, or a stiff handle after washing would justify rejecting the paste/batch and reprinting with fresh alginate paste.