Mercerizing Range

Overview

A mercerizing range treats woven or knitted cotton with strong caustic soda to raise its luster, tensile strength and dye affinity. The process is named after John Mercer, who found in the 1840s that cotton soaked in caustic swells and changes character. Holding the swollen fabric stretched to width under tension is what produces the bright sheen, so the modern continuous range is built around a Tension Stabilizing Zone that grips the selvedges and resists the shrinkage caustic would otherwise cause.

The line runs open width and in line: fabric enters at one end, passes through caustic, is held stretched while the caustic acts, then has the caustic washed and neutralized out before it leaves. A Caustic Recovery Section section collects the weak lye washed off the cloth and sends it to an evaporator so the caustic is reused rather than dumped, which is both an effluent and a cost concern.

How it works

Cloth comes off a batch or J-box into the In-Feed Section. An Entry Scray stores 8-20 m of slack so a new roll can be sewn on without stopping the line. The Weft Straightener reads weft angle with its Weft Scanner Head heads and drives a Skew Correction Roller and Bow Correction Roller to pull the weft straight, because once the caustic locks the structure any skew is permanent. Spreader Roller bow rollers open creases toward the selvedges.

The fabric then enters the Impregnation Section section. It dips into the Saturator Trough, which holds cold caustic at 270-330 g/L NaOH, equivalent to 28-32 Be. Several Immersion Roller rollers route the cloth under the surface so it floods with lye. Cold caustic mercerizes better than hot, so a Caustic Cooling Coil holds the bath at 18-25 C and a Caustic Density Sensor reads concentration in Baume. The Squeeze Padder then squeezes the cloth between two Padder Bowl bowls loaded by a Nip Loading Cylinder, leaving an even 70-90 percent wet pickup. A row of Timing Roller dwell rollers gives the caustic 40-60 s to penetrate the fiber and swell it.

In the Tension Stabilizing Zone the swollen fabric is gripped at both edges by spring Edge Clip jaws riding on the Clip Chain chains. The chains run on a Chain Guide Rail and one of them sits on a motorized Width Adjust Rail so the operator can set the held width, which is what stops the cloth shrinking and is the reason mercerized cotton stays bright. While the fabric is stretched, a Hot Caustic Shower Pipe floods it with caustic at 60-70 C to drive the reaction, then a Stabilizing Spray Pipe wets it with weak caustic so the cloth stays at width as the concentration falls and the swelling relaxes. A Clip Opener Cam cam releases the selvedges at the exit.

Washing and neutralizing

The relaxed fabric passes into the Wash and Stabilizing Section. A cascade of Wash Box compartments runs counter-current, with each Counter-Current Weir sending water back against the cloth so the cleanest rinse meets the cleanest fabric. Wash Roller rollers carry the web through each box and a Vacuum Suction Slot fed by the Vacuum Pump pulls residual liquor out. A Neutralizing Box doses dilute acetic acid to bring the cloth to pH 6-7 so no caustic carries into later processing.

Caustic recovery

The wash water leaving the boxes carries dilute caustic, around 5-7 percent NaOH. Rather than discharge it, the Weak Lye Tank collects it and the Evaporator Feed Pump sends it through a Recovery Heat Exchanger into a multi-effect evaporator. The evaporator boils off water and reconcentrates the lye back to 28-32 Be, checked by the Recovery Density Sensor, and the Strong Lye Tank returns it to the saturator. A Lint Filter Screen removes fiber lint upstream so it does not foul the evaporator.

Heating, drive and control

The Steam Heating System system uses Steam Coil Heater coils to bring the hot caustic showers and the wash boxes to temperature, with Temperature Probe RTDs closing the loop. The Line Drive System uses several Section Drive Motor section motors tied through a Line Shaft and Chain Drive Gear gearing so tension is held constant from entry to the Delivery Batcher. The Control System PLC reads concentration and tension and trims fresh caustic through the Dosing Controller, with the operator working from the Operator HMI touchscreen.

Materials, variants and use

The wetted parts are stainless steel 316L because hot caustic is aggressive, and the whole line sits on a Frame and Tanks with bunded trays against spills. Two main variants exist: the chain mercerizer described here, which holds the fabric on a clip chain and gives the best luster, and a roller or chainless mercerizer that controls width between rollers for lower-luster work at higher speed. Knit goods often run on tubular versions of the same idea. Mercerizing is used on shirting, sheeting, sewing thread and yarn destined for reactive dyeing, where the improved dye uptake means deeper shades from less dye.