Denim Finishing – Fabric Processing Principles For Higher Quality And Profits

This is a guest post by Harry Mercer. This is a part of the  presentation that he made in the denim seminar held recently at Mexico  . It gives various tips about getting better quality denim fabrics by taking care of parameters at finishing stage.

Why Is Finishing Quality Critical?

Denim finishing is critical to profits in that customers who pay the highest prices are very demanding with regard to shrinkage differences between and within fabric rolls. These customers will accept fabric that is with 0-4% shrinkage, but with no more than 0.5% difference in one shipment.

Challenges In Denim Fabric Finishing

• Control of shrinkage between fabric rolls and within rolls is more difficult than with other fabrics.
• The heavy weight, twill denim construction is dimensionally unstable after weaving.
• Denim has high shrinkage after weaving, which requires very high compression(12-15%) at Sanforizing which can be difficult .

Why Fabric Shrinks ?

• Tension that is applied to cotton yarns in weaving results in yarns being stretched beyond their “natural length”.
• When the denim garment is washed, water and agitation relax the yarns and they are returned to their minimal length.
• Variations in tension at weaving produce variations in fabric shrinkage.

Denim Finishing Process

• In basic denim finishing, fabric arrives from weaving directly, without de-sizing, and is brushed to remove contamination, singed with flame to make the fabric smoother by reducing hairiness, padded with a simple recipe, passed over a pair of skew rolls to reduce fabric torque which causes skew movement and then dried.

Finishing Recipes

The basic objectives for a denim recipe are:

• Provide lubrication for quality Sanforizing – Improve stability in garment cutting by the application of a stiffening agent.
• Including a sewing lubricant to save the customer money with sewing needles and to reduce damage from needle burn.

1. Sanforizing Lubricant

• The Sanforizer mechanically pushes weft yarns together, which reduces shrinkage.
• This mechanical shrinkage requires a high degree of friction between the rubber belt  and the denim.
• A Sanforizing lubricant is necessary for the shrinkage to be consistent.

2.   Sewing Lubricants

• During the garment forming, the needles used for sewing become hot because of friction.
• This heat damages the needle and can also produce holes in the garments.
• A sewing lubricant reduces needle costs and results in better garment quality.
• The garment cutting is also improved.

Stiffening Agents

• Also known as “hand-builders”, these are necessary to stabilize the fabric during fabric cutting to avoid “snapback”.
• Snapback occurs when the weft yarns are stretched during cutting, then as the cutting blade moves up, the garment panel shrinks to a width smaller than planned, which produces a distorted garment.

Finishing Chemicals

• A non-ionic, fatty-acid softener based on stearic acid produces the best Sanforizing (8 grams of 100% softener per kilogram of fabric)
• Polyethylene-based sewing lubricants are best. ( 2 grams of 100% per kilogram)
• 40 or 90 fluidity tapioca starch is the best hand-builder used for denim.

Finishing Procedure

• Basic denim finishing chemicals should provide improved fabric surface effects for Sanforizing, cutting and sewing.
• The finishing chemicals should not penetrate into the fabric, so no heat is required and no wetting agent is necessary.
• For uniformity of chemical application, a wet pick-up of at least 70% should be used.

Sanforizing Denim Fabrics

There are 4 variables that are critical for shrinkage, fabric defects and elongation:

1. Temperature

2. Moisture

3. Quantity of Pressure

4. Time of Pressure

1.Critical Temperature

There are 3 critical temperatures:

1. The temperature of the steam-heated cylinder which heats the rubber-belt.

• If this temperature is too high or too low, the surface of the rubber-belt is not able to provide consistent compression.
• Generally, cotton fabrics are processed easily between 105 and 125 degrees C.
• Heavyweight denims which arrive from weaving with 12-15% potential shrinkage require about 140 degrees.

2. Temperature of the Palmer Unit for drying the denim.

3. Temperature of the fabric as it enters the rubber-belt.

Fabric that is both hot and moist is easier to compress.

• For heavyweight denim, 14% moisture and 80 degrees centrigrade provides adequate fabric elasticity.
• For the best control, fabric moisture should be controlled by spray and cooling water at the Sanforizer, followed by heating before the rubber belt.

2.Sanforizing Moisture Control

• A general rule is that for each oz/square yard, 1% moisture should be applied. For example, for a 10 ounce fabric, 10% moisture.
• Most denim finishing is now on the “integrated range. Moisture is controlled by the drying cylinders on the finishing machine, then fabric passes directly to the Sanforizer.
• A final moisture of 4-5% is necessary in order to stabilize the fabric compression.
• If the moisture is higher, the fabric will elongate which increases final shrinkage.
• If the moisture is lower, the denim will elongate after absorbing moisture from the air.
• Sanforizers require consistent steam pressure and should be located close to the steam source.

Separate Sanforizing

• In separate Sanforizing, the denim is dried to about 6% moisture by the dyeing cylinders on the finishing range.
• Then the fabric is transported to the Sanforizer where the correct amount of moisture is applied by water sprays and by the cooling water for the rubber belt.
• This system usually provides better control.

3.Pressure (% of Compression )

• Sanforizing is a form of “mechanical shrinkage”.
• If a fabric has 12% shrinkage with 20 weft yarns/cm and 3% at 22/cm, then 10% compression by the rubber belt will result in the weft being pushed together which increases the weft yarns from 20-22/cm and reduces the shrinkage by 10 points.

4.Time Of Compression

• Denim has a very high % of shrinkage as it arrives from weaving up to 15%.
• The heat, moisture and time of pressure determine the control of shrinkage. – The time of pressure is determined by the speed of Sanforizing.
• Heavy denims should not be Sanforized at more than 35 meters/min. Slower speeds result in better control.

Conclusion

• Denim finishing is the most difficult of all apparel fabrics.
• Control of fabric quality requires a higher level of control which begins at weaving
• The moisture and temperature must be monitored and controlled at each step.
• Sanforizing is more complicated than it appears and requires the most attention.

For those who are interested in having deeper knowledge on denim finishing could perhaps check a new Denim Finishing Manual from Bozetto  called “Bozzetto Denim Finishing Handbook" – which provides indepth information and tips on denim finishing. Harry Mercer has been an important contributory to this manual .  This manual can be requested from Mr. Giacomo Mussetti  at giacomo.mussetti@bozzetto.it

This is a guest post by Harry Mercer. Mr. Mercer has 30 years experience in the denim business including 3 prominent U.S. denim companies. He is an expert colorist for measurement and color matching as well as textile testing.