The swelling of cotton with an aqueous solution of sodium hydroxide is
an important commercial treatment. Mercerization is utilized to improve
properties as dye affinity, chemical reactivity, dimensional stability,
tensile strength, luster and smoothness of the cotton fabrics (Wakida
et al., 2000, 2002a; Kim et al., 2006; Haga et al.,
2001). The treatment is normally applied either to yarn or to the fabric
itself either in the slack state to obtain stretch products, or under
tension to improve such properties as strength and luster (Bisanda, 2000;
Wakida et al., 2002b). One of the changes that occur to the treated
cotton is that its crystal structure can be converted from cellulose I
to II. In addition to change in crystallinity, accessibility and unit
cell structure, the orientation of cellulosic fibers, i.e., the fibrils
along the fiber axis, increases with swelling in alkali due to the restraining
influences of the primary fiber wall. The extent of changes that occurs
depends on the processing time, caustic concentration, temperature, degree
of polymerization and source of the cellulose, the physical state of the
cellulose, slack or tension treatment and the degree of tension (Wakida
et al., 2002b).
Different alkali systems have been used for efficient mercerization where
caustic soda solution is still one of the bests (Tóth et al.,
2003; Yuichi et al., 2006). There are two methods for mercerizing
cotton yarns and fabrics; normal mercerization and hot mercerization.
In normal mercerization, the goods are treated in caustic soda solution
at temperature 15-20 °C following by a washing and neutralization
There are difficulties in carrying out normal mercerization. Cotton exhibits
hydrophobic behavior in the loom state because of its greasy impurities
hardly allowing high viscosity caustic soda solution at low temperature
to penetrate into the fibers internal structure (Haga et al., 2000;
Vincent et al., 2006).
Increasing the mercerization temperature can decrease the viscosity of
caustic solution and improve its diffusion by swelling the fiber and decreasing
the hydrophobic effect of oily impurities. In concentrated caustic soda
solution at elevated temperature, the fabric becomes highly plastic with
every low elasticity capable of being readily stretched, leading to higher
degree of improvement in fabric properties. The other advantage of hot
mercerizing process is that no wetting agent is needed even for mercerization
of grey fabrics.
The increasing public awareness and sense of social responsibility related
to environmental issues have led the textile industry to manufacture products
with improved environmental profiles. In hot mercerization, the desizing
stage can be eliminated whether the sizing material is starch-based, a
modified starch, carboxy methyl cellulose or a synthetic size such as
polyvinyl alcohol to comply more with environmental efficiency regulations
(Qin et al., 2008; Metaxiotis, 2004). With rapid recent increase
in energy costs it can be considered useful to study the possibility of
eliminating the desizing and scouring stages of the cotton finishing process
using hot mercerizing process. In this research, the desizing and scouring
stages were eliminated as the greige cotton fabrics made up of open-end
yarns were mercerized. We have determined the effect of the temperature
of the caustic soda solution on the degree of mercerization. We measured
the degree of mercerization with barium activity number, infrared crystallinity
index and dye sorption techniques. Infrared ratio, α1372cm-1/α2900cm-1,
is proposed for measuring crystallinity index in mercerized cellulosic
materials (Chen et al., 2002).
MATERIALS AND METHODS
Grade-one type (Gorgan, Iran) cotton fiber with average length of 28
mm was used in an open-end spinning line to produce 100% cotton yarns.
Cotton yarns (20 Ne) were weaved on a weaving machine after sizing the
warps using a starch rich sizing formulation. The woven greige fabric
(120 g m-2) was then used in mercerization experiments. Analytical
grade caustic soda (99.5% purity) was purchased locally. Remazol Red RB
(C.I. Reactive Red 198) a vinyl sulphone reactive dye was used in dye
uptake measuring experiments.
Mercerization: Fabric samples in greige form were immersed
in a caustic solution (300 g L-1) in slack and stretched conditions
kept at temperatures between 15-90 °C for 5 min. A square shaped frame
with pins on its perimeter was used for applying tension on fabrics. The
greige cotton fabric was slightly stretched before being fixed on the
pins in order to eliminate the fabric slippage during alkali treatment.
The fabric was then stretched to a certain amount using a screw type stretching
device before immersing the framed fabric into the alkali solution. After
finishing the mercerization time, the immersed fabric was washed with
hot and cold water consequently to remove excess caustic soda. Any remaining
alkali was finally neutralized with dilute acetic acid solution followed
by cold rinsing.
Bleaching and dyeing: Mercerized fabrics were bleached in alkali
Hydrogen Peroxide solution in an exhaustion procedure and dyed in a laboratory
jigger dyeing machine at a liquor ratio of 40:1. Dyeing solution containing
3% (o.w.f) Remazol Red RB (Reactive dye), 10 g L-1 Na2CO3
and 20 g L-1 NaCl was adjusted at 25 °C and then raised
to 60 °C at a rate of 2 °C min and maintained at this temperature
for 90 min. Dyed samples were washed in a soap solution for removal of
hydrolyzed dyes and were dried at 50 °C in an oven.
Measurements: The adsorptivity ratio of α1372cm-1/α2900cm-1
was obtained as the crystallinity index using an infrared (IR) spectrophotometer
(IR-470, Shimadzu Co.). Measurement was performed by means of a diffuse
reflection method using a compacted pill form mixture of cut fiber segments
and potassium bromide. The infrared ratio was estimated according to the
literature (Chen et al., 2002; Esfandiari, 2008). The methods of
drawing the baselines are indicated in Fig. 1. For the
bond at 2900 cm-1 the intensity at the adjacent shoulder near
3000 cm-1 was chosen as the base. For the bond at 1372 cm-1
a line was drawn between the maxima at approximately 1290 and 1410 cm-1
giving a common baseline for the group of three bands which occur close
together in this region and which are changing simultaneously.
Barium activity number was measured according to AATCC 89-1998 test method.
The mercerized and un-mercerized cotton fabrics were cut into small lengths,
weighing 1 g were treated with 30 mL of 0.25 N barium hydroxide solutions
in 100 mL flasks. After 2 h, 10 mL of the solution was titrated with 0.1
N hydrochloric acid. A blank was also run in without any fabric sample.
If A, B and C are the titration reading for the blank, mercerized sample
and unmercerized sample respectively, then the barium activity number
is given by Eq. 1.
||IR spectrum of fabrics with different treatment temperature:
(A) untreated fabric, (B) slack mercerized (15 °C), (C) slack
mercerized (25 °C), (D) tension mercerized (15 °C)
Dye uptake was measured using K/S values of dyed samples which were originally
determined from Kubelka-Munk equation (Eq. 2) where K
and S are the absorption and scattering coefficients respectively and
R is the reflectance. A Tex-flash reflectance spectrophotometer set for
illuminant D65 and CIE 1964 standard observer was used in colour measurements.
The tensile strength and elongation were measured on a Tensolab tester
in an environment set at 65% relative humidity and 21 °C. The gauge
length in tensile tests was 150 mm and fabric samples were tested at a
rate of extension of 125 mm min-1 (breakage time 20 ±
2 sec). For strength and elongation at break, 10 specimens were tested
for each sample.
The efficiency of mercerization process in removal of sizing materials
was calculated using Eq. 3 where D0 and D
are the weight percentage of starch on greige and mercerized fabric respectively.
To calculate these individual values, greige un-mercerized fabric was
desized in an enzymatic process and its weight loss was considered as
D0. A piece of mercerized sample was also desized in the same
desizing procedure which gives the percentage of remaining sizing materials
after mercerization process (D).
To measure the shrinkage happened in slack mercerization process, four
lines, each 10 cm long, were marked on each sample in warp and weft directions
and the changes in their length were subsequently used for calculating
the shrinkage percentage according to Eq. 4:
where, L and L` are the average markers` length on the samples, before
and after the slack mercerization.
RESULTS AND DISCUSSION
Change in crystallinity index: These infrared curves reflect differences
in the OH group content in regions at 700 and 3300 cm-1. Differences
were also noted at 2900, 1372, 1429 and 893 cm-1. The strong
band at 1590 cm-1 appeared to be related to the formation of
hydrogen bonding in amorphous cellulose. Chen et al. (2002) and
Esfandiari (2007) developed an empirical infrared crystallinity index
for the native cellulose from the ratio of the absorptivities at 1429
and 893 cm-1. They proposed a new infrared ratio α1372
cm-1/α2900 cm-1 for estimating crystallinity
of cellulose samples with mixed lattices. The infrared curves suggested
that cellulose II is formed after sodium hydroxide treatments. A comparison
of the effect of temperature treatment on crystallinity index as identified
by infrared crystallinity ratio is shown in Table 1.
Generally, all caustic mercerization treatments caused a decrease in the
crystallinity index of cellulose. The decrease of crystallinity index
varied with mercerization conditions. Infrared ratio of the fabrics tension
mercerized is higher than that of the fabrics slack mercerized that this
may related to the greater degree of orientation in tension mercerization.
Table 1 showed that the crystallinity index of the sodium
hydroxide treated fabrics first decreased, reached a minimum and then
increased with an increasing treatment temperature. The initial decrease
of the crystallinity index can be related to formation cellulose II and
again increase of the crystallinity index may be attributed to the fact
that increasing treatment temperature leads to the formation a product
similar to cellulose I. The initial decrease in crystallinity index and
its further increase usually happened faster in slack mercerized fabrics
compared to tension mercerized ones which can be related to the easier
diffusion of caustic solution in former fabrics.
||The effect of mercerizing temperature on crystallinity
index of infrared ratio
Barium activity number: The barium activity number is widely used
to express the degree of mercerization. Table 2 shows
the barium activity number according to the various temperature treatments.
Increasing the mercerizing temperature could cause the barium activity
number of the sodium hydroxide treated fabrics to first increased, reached
a maximum and then decreased. There is an inverse correlation between
crystallinity index and barium activity number of mercerized fabrics.
The barium activity number of the slack mercerized fabric was higher than
that of the mercerized fabric under tension. This may be due to the easier
diffusion of chemicals following the reduction in crystallinity level
which is in agreement with the crystallinity index data.
|| The effect of mercerizing temperature on barium activity
|| The effect of mercerizing temperature on dye uptake
Dye uptake: The mercerized fabrics were bleached in hydrogen peroxide
and caustic soda and then dyed by a reactive dye. Increase in dye uptake
of mercerized samples in comparison with un-mercerized fabrics at the
various mercerizing temperatures is shown in Table 3.
The results clearly show that the dye uptakes of mercerized samples are
higher than that of the un-mercerized fabric. This related to the destruction
of crystalline regions during the swelling and structure change in mercerized
cotton. The maximum dye uptake was observed in the tension mercerized
fabrics at 65 °C whilst in slack state at 35 °C the highest amount
of dye could be exhausted by the fabric.
The penetration of sodium hydroxide into fabrics is easier in the slack
mercerized than that of the tension mercerized that this caused maximum
dye uptake of slack mercerized fabric happened at lower temperature. Saapan,
Kandidand Habib reported (Saapan, 1984) that cellulose II is formed after
sodium hydroxide treatment and the extent of conversion depended on the
experimental conditions, i.e., caustic mercerization at 20 °C caused
more conversion from cellulose I to cellulose II than is obtained by swelling
in NaOH at 90 °C. In other words, cellulose II formation diminishes
by increasing treatment temperature while cellulose I formation could
Tensile strength and elongation: Tensile strength and elongation
of un-mercerized fabric were 31.9 kgf and 18.1%, respectively in warp
direction (Table 4). The tensile strength of the slack
and under tension mercerized fabric was showed a clear increase in all
conditions. The major reason for the increased tensile strength can be
an alleviation of internal stresses and the deconvoluting of the fibres
in the fabric during swelling process. The tensile strength of mercerized
samples in slack state is higher than that of the mercerized samples under
tension. This may be related to the easier penetration of caustic soda
into fiber structure in slack mercerized samples and thus increase in
the degree of mercerizing and swelling in fibers. Since the slack mercerized
fabrics showed higher barium activity number compared to those which were
mercerized under tension, it is also expected that the tensile strength
can be higher in the slack mercerized fabrics. Based on observations,
increasing the mercerization temperature could increase the tensile strength
where maximum strength was observed by mercerization at 65 °C that
related to maximum degree of mercerizing and swelling at this point. At
temperatures higher than 65 °C, the tensile strength of under tension
mercerized fabrics is higher than that of the slack mercerized fabrics
that this may be related to the greater degree of orientation in the fabrics
caused by mercerization under tension. Increasing temperature could not
linearly raise the strength which is in agreement with the results of
the barium activity number. Elongation in warp and weft directions of
the mercerized fabrics is shown in Table 5.
||The effect of mercerizing temperature on tensile strength
in warp direction
Shrinkage: Results is showing that for a fabric made up of open-end
spun yarns, increasing the mercerization temperature can raise the shrinkage
in warp and weft directions. Maximum shrinkage was observed at 65 °C.
Shrinkage in the warp direction was usually higher than that of in the
weft direction mainly because of the higher weave density in the warp
direction (Table 6).
Size removal: The greige cotton fabrics were mercerized in slack
state and under tension within temperature range of 15-90 °C. Table
7 shows the removal size percent of the mercerized greige cotton fabric
at various temperatures where increasing the mercerization temperature
have had a significant effect on size removal percentage especially above
|| The effect of mercerizing temperature on Elongation
|| The effect of mercerizing temperature on the fabric
|| The effect of mercerizing temperature on the fabric
size removal (%)
There is a correlation between treatment temperature and size removal
percentage, as well as with degree of stretching and size removal percentage.
Raising the mercerization temperature could lower the viscosity of caustic
soda solution which facilitates penetration of alkali into the fabric.
As it is also appear, the size removal percent of the mercerized samples
in slack condition was higher than that of the mercerized samples under
tension. The greater removal size percent in the slack mercerized fabrics
may be related to the easier penetration of caustic soda into the relaxed
fabric whilst in stretched fabric the porosity of the fabric was decrease
because of the level of the applied tension which increases the compactness
of yarn and fabric structure.
The greige cotton fabrics made up of open-end spun yarns was treated
with caustic soda solutions at different temperatures, under tension and
also in relaxed conditions. With increasing in treatment temperature the
dye uptake, barium activity number and tensile strength of the mercerized
fabrics first increased and then decreased. The maximum dye uptake of
the slack mercerized fabric was observed at 35 °C, while for tension
mercerized fabric was observed at 65 °C. The maximum barium activity
number and tensile strength and minimum crystallinity index of the tension
mercerized fabrics was observed at 65 °C, while for slack mercerized
fabric was observed at 35 °C. On the other hand, results showed that
all caustic mercerization treatments caused a decrease in the crystallinity
index of cellulose. The crystallinity index of the sodium hydroxide treated
fabrics first decreased, reached a minimum and then increased with an
increasing treatment temperature.