In the textile industry, Polyester / Cotton (P/C) blends have a dominant market share of about 60% in the knit sector. These blends are popular due to their aesthetic value and user-friendly performance. 100% cotton knit fabric exhibits lower shape retention but higher moisture absorbency whereas 100% polyester fabric shows lower absorbency with higher strength and shape retention. Limitations of both fibers are offset by their blending which shows good strength and shape retention with an acceptable absorbency. However, processing of P/C blends poses some challenges as cotton is non-absorbent in grey stage and needs to be scoured/bleached similar to 100% cotton before dyeing. In the exhaust dyeing of P/C blends, the general practice is to first run the fabric through a preparatory process of scouring/bleaching and then dye each component separately under its optimum conditions, i.e. in a two-bath process. In high temperature (HT) dyeing method, the polyester portion is dyed at 130oC and cotton part is dyed at 60–80oC, however, the dye bath is to be drained. This normally is a long process of over 8 hours of production time to dye both components with satisfactory fastness property.
To address the issue of productivity, economy and rising environmental concerns, several attempts have been made in the past to shorten this process.
- Polyester and cotton dyeing without pre-bleaching — Shorter processing time.
- Only jet dyeing machine complete combined bleaching and dyeing step.
- No need to use jiggers for cotton bleaching thus makes jigger available for dyeing. Improves productivity of the plant.
- No need for reduction clearing.
- It bleaches cotton portion, hence most suitable for light and medium shades retaining their brightness.
- Avoids at least two drains in entire processing, thus reducing water consumption, steam for heating, process time, effluent load.
For Polyester/Cotton or Polyester/Viscose woven, knit & yarn (67:37 & 48:52 blend)
The absorbency of fabric after polyester dyeing using Dyebleach BL.
Absorbency, sec — 20–21 sec
Treatment—1.5% Caustic soda +2% H2O2
Absorbency, sec — 1–2 sec
Treatment —2% Dyebleach-BL+1.5% Caustic soda + 2% H2O2
Wash fastness test showing no need for reduction clearing: ISO 105 C10