While the core of the Merino fibre is hydrophilic and capable of absorbing up to a third of its dry weight in moisture vapour, the surface of Merino is hydrophobic. That is, it repels liquids.

Waxy coating

The Merino fibre has a very thin, waxy, lipid coating chemically bonded to the surface. This bonded layer extends over the overlapping scales on the surface of the fibres and cannot be easily removed by scouring, washing or processing. A consequence of this surface layer is that Merino fibres have a naturally low surface energy - lower than cotton, nylon or polyester and comparable with the hydrophobic surface of polypropylene. (Fig 1)

It’s this low surface energy that makes Merino water-repellent because droplets that touch lightly on the surface of Merino will bead and roll off before being absorbed into the fabric.

Summary

A waxy lipid coating on the scales of the Merino fibre lower its surface energy and make it naturally water repellent.

Sources:

• CSIRO, “Stain resist, anti-soiling and easy clean”, 2005.
  
• Holme, I., (2003) Water-repellency and water proofing, in Heywood, D. (Ed.) Textile Finishing, Bradford, UK: Society of Dyers and Colourists, pp. 135-213.

All figures and tables courtesy of CSIRO unless otherwise noted.