Yarn Mercerisation -Part-1

What is mercerisation?
Mercerisation is a process of treating cotton yarn or fabric in concentrated solution of caustic soda under tension for a specified time duration and washing off of the caustic solution under the same conditions. This process bestows the following important properties to cotton.

1) High dye affinity
2) Lusture
3) Improved tensile strength

In this issue we are going to discuss about the machinery required for mercerising the yarn and the process and recipe details of mercerisation.

Machinery Required:
The yarn mercerising machine should have the following facilities to treat the yarn in high concentration of caustic lye.
1) A rotating hank holding device viz hank holding arm
2) A squeezing bowl
3) A shallow trough to hold the required volume of concentrated caustic dye
4) A device to make and keep the hank yarn under suitable tension.
5) A washing arrangement to remove the casutic lye from the yarn after the impregnating time is over.
6) A timer arrangement - either mechanically or electronically controlled device to hold the yarn iside the caustic bath for the required number of minutes and to wash it after exactly after the same number of minutes after impregnation.

Mercerisation Process Sequence:
1) Yarn hanks are evenly placed on the pair of rollers that hold the yarn hanks.
2) The hanks are properly positioned on the rollers. The rollers may be alligned either horizontally or vertically according to the machine design, rotating the rollers, with a view to confirm that the yarn hank does not get ruffled and the yarn strands remain perfectly parallel. Please note that the rollers must be capable of rolling on both directions.
3) Application of tension to the hank yarn, while rotating and raising the merceirising lye tray, till the lye covers the lower part of the rotating hanks. The yarn tension is measured in terms of increase in length between the rollers.
4) At this stage of the operation, the sqeezing roller should commnce sqeezing with a light sqeeze, so that the mercerising lye gets uniformly soaked in the yarn, in turn, an equal overall mercerising effect.
5) The light sqeeze is maintained. The yarn tension is increased to the maximum pre-determinded level and in maintained at this level. The time for which the maximum tension should be retained, depends on the yarn count, twist and the folded state of the yarn.
For each quality of yarn, this dwell time under tension should be worked out before-hand, so that the instructions can be rigourously followed by the operators.
6) At the end of the dwell time, the mercerising-lye-tray should be lowered. Excess lye taken up by the yarn is squeezed-out by applying higher pressure on squeezing rollers.After the squeeze out the lye tray is moved away.
7) The wash tray is brought in to position. Hot water is sprayed over the yarn hanks while it is still under the light sqeeze . The yarn tension should be maintained during the hot wash.
8) While still maintaining the yarn tension, the following measures should be taken:
(a) The squeezing pressure should ne raised in order to squeeze out the still remainling lye in the yarn.
(b) The squeezing pressure should be reduced to keep only a light squeeze.
(c) Cold water should be sprayed over the hank so as to wash out the yarn hank to an alkali free stage.
9) After completing the mercerising of the yarn, as indicated in the above stages, the squeeze and the tension on the hank are released. The hanks are then unloaded from the pair pf rollers for the next process, which may be bleaching, dyeing or plain drying.

If all the above stages are maintianed as said above the degree of mercerisation would be excellant.

Recipe Details:

1) The mercerising lye concentration is maintained at 250 to 300 grams/liter.
2) A powerful wetting agent that does not get affected by the high concentration of caustic lye is used between to 1 to 2 grams/liter.
3) The impregnation time depends on the quality of yarn and the lye strength. Ranges from 2 to 3 minutes.