Advances in wool technology
166
to accelerate combustion. The limiting concentration of oxygen (LOI) required
to support combustion of wool in standard tests is about 25% which is
higher than the ambient oxygen concentration in air (21%) (
Table 7.3).
When
wool is heated to the point of combustion, it forms an intumescent char
which provides an insulating layer of pyrolysed material separating heat and
oxygen from the fuel. The fact that wool does not melt or drip prevents
further flame spread, and releases only low heat,
therefore the flame is easy
to extinguish.
Horrocks and Davies (2000) describe the pyrolytic reactions that occur
during the heating of wool. Wool pyrolyses by a complex series of reactions
to yield a number of products at increasing temperatures. Initially at 230–
240
°C, rupture of the helical structure occurs and the major ordered part of
the wool protein undergoes a solid to liquid phase change. At 250–295
°C an
endothermic reaction occurs associated with the release of sulphur compounds
due to the breaking of the cystine disulphide bonds and simultaneous release
of hydrogen sulphide. Above 250
°C general pyrolytic decomposition occurs,
including char-forming reactions with dehydration and loss of other volatiles.
In the presence of air, formation of sulphur dioxide occurs between 270 and
320
°C. Cleavage of the cystine disulphide bond is seen to play a very important
role in the thermal degradation and combustion of keratin. It has been suggested
that the oxidation of cystine may be the initial
exothermic reaction in the
burning of wool.
The sensitivity to oxidation of the cystine disulphide bond between adjacent
protein polymeric chains supports any burning mechanisms and so pre-
oxidation of cystine to cysteic acid residues can improve flame retardancy,
especially if 60% or more of the disulphide bonds are oxidised. However
such a high degree of crosslink rupture significantly reduces the wet strength
retention of the fibre.
Despite the fact that wool fibres are inherently less flammable than most
other fibres, the inherent flame-retardant properties of wool need to be enhanced
in order to meet specific flammability tests or specific end uses such as
Table 7.3 Flammability properties of wool fibre
LOI
25.3 %
Ignition
temperature
570–600
°C
Heat of combustion
4.9 kcal/g
Flame temperature
680
°C
Melting point
No melting
Burning behaviour
Difficult to ignite, burns slowly with
formation of char, supports combustion
with difficulty
© 2009 Woodhead Publishing Limited
Wool finishing and the development
of novel finishes
167
aircraft carpets and seat covers. Flame-retardant finishes for wool fibres are
mainly focused on enhancement of char-formation in the condensed phase,
although bromine-containing, vapour phase-active surface treatments are
effective for most textile materials. Currently there is interest in the use of
intumescents, and research and development in this area has been undertaken
by Horrocks and Davies (2000).
Ammonium phosphates and organophosphorus species with Lewis acidic
properties are effective flame retardants for wool, and each enhances char
formation.
One well-known process, Zirpro (developed by the former
International Wool Secretariat), based on the reaction of zirconium and titanium
salts with wool, also enhances char formation. Zirpro treatments are based
on the exhaustion of negative charged zirconium or titanium salts, under
acid conditions, onto positively charged wool. This results in the deposition
of only about 3% of flame retardant inside the fibre with negligible effect on
properties such as handle. These treatments stabilise and further crosslink
the protein structure. K
2
ZrF
6
and K
2
TiF
6
are
the current commercial flame
retardants used. Their application along with zirconium acetate solution can
achieve low smoke emission for wool. Another flame-retardant treatment for
wool is the use of tetrabromophthalic anhydride (TBPA). TBPA can be
incorporated with the Zirpro treatment to reduce after-flaming times and
minimise heat release. The current commercial flame retardant finishes for
wool are summarised in
Table 7.4.
Flame-retardant finishes for textiles, including wool-based materials, have
been comprehensively reviewed by Horrocks (1986, 2003, 2005), Schindler
and Hauser (2004b) and Pure Strategies Inc. (2005). Environmental issues
associated
with flame-retardant finishes, especially brominated flame retardants,
have been extensively discussed during the past decade (Barton, 2000b;
Holme, 2001, 2006b;
International Dyer, 2003; Dombrowski, 2006). A
comprehensive assessment of the impact of chemicals on the environment
and human health has been conducted (Barton, 2000b; Holme, 2001;
International Dyer, 2002; Dombrowski, 2006). The requirements of
flammability legislation and regulation have changed significantly. With
increasing environmental demands as well as demands for higher technical
performance, much effort is being expended by manufacturers and scientists
to improve existing products and develop new eco-friendly products for the
flame-retardant finishing of textiles.
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