24
(TMM)). These treatments are carried out via a ‘pad-bake’ process, where the substrates are passed through aqueous formulations containing the crosslinker, a catalyst and other components, squeezed between pressure rolls to remove excess liquor,
then dried and baked (i.e. subjected to dry heat) at about 150 °C for 0.5 to 3 min.
The catalysts used are Lewis acid salts, e.g. MgCl 2 , which generate acidic conditions during the baking process, and the crosslinking reaction is carried out through
the protonation of oxygen of N-methylol groups that lead to carbonium ions on
dehydration, which react in turn with cellulosic hydroxyls forming ether bonds. A
scheme of the crosslinking mechanism is available in Schindler and Hauser (2004).
A disadvantage of N-methylol reactants of the type described above is the reaction
of unsubstituted amine and amide groups which react with chlorine from household
and laundry detergents and bleaches to form chloramines which are hydrolyzed to
hypochlorous acid, thus leading to cellulose degradation. Another disadvantage is
the potential of formaldehyde release during the crosslinking treatments or during
wear from unreacted formaldehyde residues.
One option to reduce such release is to add formaldehyde scavengers such as
urea, diethylene glycol, ethylene urea to urea-formaldehyde products, and the other
is not to use formaldehyde in the synthesis of crosslinkers. An example of a nonformaldehyde crosslinking is N,N′-dimethyl-4,5-dihydroxyethylene urea
(DMeDHEU), which is synthesized from N,N′-dimethyl urea and gloxal. Others are
dialdehydes, such as glutaraldehyde and glyoxal, and polycarboxylic acids, such as
butanetetracarboxylic acid, citric acid, poly(itaconic) acid and poly(maleic acid)
(Harifi and Montazer 2012). The polycarboxylic acids used as cellulose crosslinkers
are characterized by a minimum of three side carboxylic groups attached to adjacent
carbons of an aliphatic chain. Esterification reactions are produced by the formation
of an intermediate cyclic anhydride in the dehydration catalyzed by agents such as
sodium hypophosphite. Polyamino carboxylic acid (synthesized by carboxylation
of linear polyamines) has also been investigated as potential non-formaldehyde
crosslinking agents (Dehabadi et al. 2012). Another option is ‘ionic crosslinking’,
where cellulose is partially carboxymethylated first to introduce anionic groups and
then treated with a cationic agent such as 3-chloro-2-hydroxypropyl trimethyl
ammonium chloride, where crosslinks are caused by ionic interactions between the
two polyions (Hashem et al. 2003; Hashem et al. 2005; Harifi and Montazer 2012).
3.2.2.2 Treatments to Impart Mechanical and Chemical Stability
Among the regenerated cellulosic materials, the fibers obtained from the lyocell
process exhibit a propensity for fibrillation, i.e. the peeling-off of thin strands
(‘fibrils’) from the surface, when subjected to mechanical forces in the wet state,
such as when textiles are washed. The phenomenon is undesirable since it to leads
to a blurry appearance of the articles. Crosslinking is employed as a means to prevent fibrillation, and the agents used include 1,3,5-triacryloyl-hexahydro-s-triazine,
dichlorohydroxytriazine
and
2,4-dichloro-6-(p-β-sulphatoethylsulphonyl)anilino-1,3,5-triazine (White 2001; Bates et al. 2006). The bifunctional colorant CI
A. P. Manian et al.
(TMM)). These treatments are carried out via a ‘pad-bake’ process, where the substrates are passed through aqueous formulations containing the crosslinker, a catalyst and other components, squeezed between pressure rolls to remove excess liquor,
then dried and baked (i.e. subjected to dry heat) at about 150 °C for 0.5 to 3 min.
The catalysts used are Lewis acid salts, e.g. MgCl 2 , which generate acidic conditions during the baking process, and the crosslinking reaction is carried out through
the protonation of oxygen of N-methylol groups that lead to carbonium ions on
dehydration, which react in turn with cellulosic hydroxyls forming ether bonds. A
scheme of the crosslinking mechanism is available in Schindler and Hauser (2004).
A disadvantage of N-methylol reactants of the type described above is the reaction
of unsubstituted amine and amide groups which react with chlorine from household
and laundry detergents and bleaches to form chloramines which are hydrolyzed to
hypochlorous acid, thus leading to cellulose degradation. Another disadvantage is
the potential of formaldehyde release during the crosslinking treatments or during
wear from unreacted formaldehyde residues.
One option to reduce such release is to add formaldehyde scavengers such as
urea, diethylene glycol, ethylene urea to urea-formaldehyde products, and the other
is not to use formaldehyde in the synthesis of crosslinkers. An example of a nonformaldehyde crosslinking is N,N′-dimethyl-4,5-dihydroxyethylene urea
(DMeDHEU), which is synthesized from N,N′-dimethyl urea and gloxal. Others are
dialdehydes, such as glutaraldehyde and glyoxal, and polycarboxylic acids, such as
butanetetracarboxylic acid, citric acid, poly(itaconic) acid and poly(maleic acid)
(Harifi and Montazer 2012). The polycarboxylic acids used as cellulose crosslinkers
are characterized by a minimum of three side carboxylic groups attached to adjacent
carbons of an aliphatic chain. Esterification reactions are produced by the formation
of an intermediate cyclic anhydride in the dehydration catalyzed by agents such as
sodium hypophosphite. Polyamino carboxylic acid (synthesized by carboxylation
of linear polyamines) has also been investigated as potential non-formaldehyde
crosslinking agents (Dehabadi et al. 2012). Another option is ‘ionic crosslinking’,
where cellulose is partially carboxymethylated first to introduce anionic groups and
then treated with a cationic agent such as 3-chloro-2-hydroxypropyl trimethyl
ammonium chloride, where crosslinks are caused by ionic interactions between the
two polyions (Hashem et al. 2003; Hashem et al. 2005; Harifi and Montazer 2012).
3.2.2.2 Treatments to Impart Mechanical and Chemical Stability
Among the regenerated cellulosic materials, the fibers obtained from the lyocell
process exhibit a propensity for fibrillation, i.e. the peeling-off of thin strands
(‘fibrils’) from the surface, when subjected to mechanical forces in the wet state,
such as when textiles are washed. The phenomenon is undesirable since it to leads
to a blurry appearance of the articles. Crosslinking is employed as a means to prevent fibrillation, and the agents used include 1,3,5-triacryloyl-hexahydro-s-triazine,
dichlorohydroxytriazine
and
2,4-dichloro-6-(p-β-sulphatoethylsulphonyl)anilino-1,3,5-triazine (White 2001; Bates et al. 2006). The bifunctional colorant CI
A. P. Manian et al.