Cotton is an abundant natural fibre which consists of practically
pure cellulose (about 88e96%) [1]. Cotton may be
described chemically as poly(1,4-b-D-anhydroglucopyranose)
(Fig. 1) [2]. Natural cellulose fibres carry a small negative
charge (zplateauZ11 mV) due to the presence of some carboxylic
acid groups from oxidation at the primary hydroxylic
sites [3]. At a pH higher than 8, some of the hydroxyl groups
on the hydroxymethyl side chains may also be ionized increasing
the negative charge significantly [4]. The negative charges
on the surface of cellulose repel anionic dyes and hence the
efficiency of dye fixation on cellulosic fibres is generally
low. To counter this problem, a number of studies on cotton
dyeing have been carried out to improve the dye uptake and
fastness properties. Most research focus is on introducing
cationic sites into the cotton fabrics for interactions with
anionic dyes [5e8].
Lac dye is used extensively as a natural food additive
[9,10], and in cosmetics [9], as well as a colourant for silk
and cotton dyeings [11]. In the north and northeast of Thailand,
lac dye is a natural red dyestuff extracted for cotton and silk
dyeings [11]. However, lac dye has a low affinity for cotton
because cotton does not have any cationic sites for the attachment
[12]. An alternative way to overcome this problem is the
pretreatment of cotton with cationic agents. Rastogi et al. [13]
created affinity in cotton for lac dye by introducing cationic
sites in the fibre. Cotton fabrics were treated with a cationic
agent, Discofix DBA, and then they were dyed with lac
dye. It was found that the cationised cotton which was dyed
with lac dye exhibited a good colour yield and wet fastness
properties even without mordanting. In addition, poly(ethyleneimine)
(PEI) has been used as a cationic agent in cotton
dyeing with lac dye [14]. It was found that PEI increased
the dye adsorbed on cotton and also decreased the dye desorption
from the fibre