Saminathan Ratnapandian

Feature	Continuous dyeing
Batch dyeing
Shade uniformity	High and reproducible	Batch-to-batch variation possible
Quality of dyeing	Better	Good
Crease defects	Can be controlled	Difficult to control
Production	High due to short throughput time	Low due to long dyeing time
Labour requirements	Less	More
Process cost	Less	More

During continuous dyeing, the dye is applied by continuous immersion of fabric in a dye bath contained in a trough and removing excess liquor using squeeze rollers as shown in Figure 1.5 [11]. The position and number of squeeze rollers differ based on the model and manufacturer of the machine. The process in its entirety is termed padding and the padding process is repeated to ensure that the fabric is thoroughly wetted to obtain level dyeing. The amount of dye liquor retained by the fabric after squeezing is termed wet pick-up and is reported as a percentage of the dry weight of the fabric. Wet pick-up is primarily controlled by the pressure applied by the squeeze rollers. However, fabric construction and composition also influence wet pick-up. For example, the minimum wet pick-up of 60 – 70% for cotton cannot be reduced by increasing the pressure [11]. In general, lower pick-up minimises migration of dye liquor in the fabric and saves energy during drying. Alternate application methods of dye liquor are spraying and printing.

Squeeze rollers


Fabric

Trough with liquor Liquor inlet

Figure 1.5 Schematic diagram of padding
With the exception of the pad-batch process, it is preferable to dry the fabric immediately after padding to obtain uniform dye fixation. Non-contact driers using a hot-air stream or infra-red radiation are employed to avoid soiling of equipment and marks on the fabric. At this stage the dye is present on the fibre surface and needs to be integrated into the material during fixation by chemical reaction, aggregation, ionic interaction or solid solution. A variety of methods and conditions are available for fixation depending on the dye and substrate. Saturated steam at 100^○C is employed for a majority of the dyes [11]. Roller type steamers are usually used for this steam fixation process. They are designed to provide an air-free environment with a cold water exit seal (usually called water-lock). Each commercial range of dyes requires an appropriate duration of steaming. The major padding-fixing combinations are described below.