High ignition temperature

Merino is less likely to burn than most textiles. Compared to cotton which catches light at 255 degrees, the temperature must reach 570-600 degrees before Merino will ignite.

High Limiting Oxygen Index

In order for fire to be sustained under the standard test conditions used to measure Limiting Oxygen Index, Merino requires a very high level of oxygen - significantly higher than occurs in nature. The proportion of oxygen in the atmosphere is 21%, more than sufficient for cotton, rayon, nylon and polyester to burn. However, Merino requires an oxygen level of 25.2% - more than 4% higher than ambient levels - or the fire in the standard test won't be supported.

Low heat of combustion

When it does burn Merino releases less heat than common synthetics. Its heat of combustion is a low 4.9 Kcal/g while polyester is 5.7 Kcal/g, cotton is 3.9 Kcal/g and nylon is a very high 7.9 Kcal/g.

Low rate of heat release

Though cotton has a lower heat of combustion than Merino, its rate of heat release is much higher and the wider fire science community now recognises that it is the rate of heat release that determines the real hazard in fire situations.

Self-extinguishing

If Merino does catch alight, it tends to self-extinguish.  The Merino fibre contains naturally high levels of nitrogen – an element commonly used as a fire retardant – and when it is heated sufficiently to combust, it tends to produce an insulating foam which excludes oxygen and hence puts out the fire.

Won’t melt

While polyester melts at 252-292° and nylon succumbs at an even lower 160-260°, Merino never melts so it can’t stick to the skin like many common synthetics, sparing burn victims from significant medical complications.

Summary

A high ignition temperature, a low heat of combustion and the necessity for an oxygen level higher than occurs in nature means Merino is less likely to catch alight than most synthetics. But, its low rate of heat release, its tendency to produce a foam that self-extinguishes, and the fact that it never melts further explain why Merino has the highest natural fire resistance of all commonly encountered textile fibres.

Source

• CSIRO, “Flame resistance of wool”
• Flame Resistant Fibres and Fabrics  (2003)., Performance Apparel Markets, Issue 6, 3rd Quarter, 2003
• Benisek, L. (1976) Development of Flame Resist Treatments for Wool, Wool Science Review No. 52, pp 30 – 63.
• Horrocks, A.R., (2003) Flame-retardant finishes and finishing, in: Heywood, D. (Ed.) Textile Finishing, Bradford, Eng.: Society of Dyers and Colourists
• Horrocks, A. R.  et al. (2005) Developments in Flame Retardant Textiles-a Review.  In Polymer Degradation and Stability. Vol 88, Issue No. 1, April