The ability to permanently set a fabric is highly desirable for designers and tailors because it increases the range of shapes and effects at their disposal. Unlike cotton, Merino is a protein and has the ability to be permanently set. It is this attribute that has allowed the production of the wash-and-wear Merino suit.

Permanently setting Merino

Merino may be permanently shaped or set by a rearrangement of the disulfide bond crosslinks that stabilise the Merino fibre. Permanent set is set that remains after relaxation in water at 70°C for approximately 15 minutes. This means that the set is permanent in conditions that a Merino garment would normally encounter during use, e.g. machine washing. Permanent setting operations are used to permanently change the thickness of a fabric, impart dimensional stability and to confer the required handle and drape to a Merino fabric.

Higher temperatures

Permanent setting of a Merino fabric requires control of both regain and temperature in a similar manner to cohesive set. However, for a given regain or moisture content, the temperature needs to be about 70°C hotter than that required for cohesive or temporary set.

Figure 2 depicts a curve above which the temperature and regain are sufficient for imparting permanent set to Merino within a few minutes. This curve is indicative only, as its actual position can vary between different Merino types, previous treatments and process conditions.

Permanent set can be obtained in water hotter than about 70°C and will occur during dyeing at 100°C. Creases and wrinkles inadvertently introduced during dyeing will be permanent and therefore difficult to remove.

Wet crabbing

Alternatively permanent set can be used to impart desired properties to Merino fabric and the most common form of wet permanent setting is continuous crabbing. This is an operation in which a wet fabric, sandwiched between a hot (up to 160°C) roller and an impermeable belt, is heated to temperatures above 100°C for up to 1 minute, before being rapidly quenched in cold water.

Pressure decatising

Pressure decatising is a dry finishing procedure that is carried out at the end of the finishing routine to permanently set the fabric. The fabric is wrapped onto a perforated cylinder with a wrapping cloth and steamed under pressure at a temperature of around 125°C. This process will only be effective if the regain of the fabric rises to around 20% regain during the steaming process. It minimises the friction between the yarns in the fabric structure and is used to develop the soft, supple, and excellent drape characteristics of Merino fabrics.

Chemical setting

In other applications, chemical reducing agents can be used to break the disulphide bonds in Merino to allow permanent setting to occur under milder conditions. The Siroset process allows permanent creases and pleats to be set into Merino fabrics in a normal pressing operation. This process has been in constant use by the world’s Merino processors for several decades.

Summary

Merino is able to achieve permanent set through the use of either water and high temperature or chemicals, further enhancing its desirability among designers and tailors. It is this attribute that has enabled the development of the wash-and-wear Merino suit.

Other reading

• Leeder, John, D. (1984) Merino - nature’s wonder fibre, Ocean Grove, Vic.: Australasian Textiles Publishers,
• Morton, W.E. and Hearle, J.W.S., [1993] Physical Properties
  of Textile Fibres, 3rd Ed., Manchester, UK.: The Textile Institute.
• Rippon, John et al, (2003) Merino, in Encyclopedia of Polymer Science and Technology, New York : Interscience Publishers.