Indi an Journal of Fibre & Textile Research Vol. 27, December 2002, pp. 422-428

Kinetics of dyeing acrylic acid-grafted cotton fabric with basic dyes

Eg la l H K EI-Ge ndy"

National Centre for Radi ation Research and Technology. Atomic Energy Authority, POBox 29, Nas r City. Cairo, Egypt

Received 29 JalluGlY 2001; revised received alld accepted 4 JlIly 2001

The e ffec t of gra ft yield , dye concentration and dycing temperature on the colour strength of acry lic acid-gra fted cotton fabric has been studied for Sandocryl Blue (SB) and Rhodamine Red (RR) basic dyes. It is observed that the colour strength increases wi th the increase in graft yie ld and leve ls off at higher degrecs of graft ing. The bcst dyeing is achi eved at pH 3.9 and 2.72 for SB and RR aqueous solutions respectivcly. Thc colour strength o f gra fted fabri cs increases rapidly with the increase in dyeing time and levels o ff within few minutes, depending on the dye concentration (Co) and te mperature of the dye bat h. Thc initi al dyeing rates (R~) for SB dye are hi gher than those for RR dye. The dye ing process follows a 0.62 order kinetics and is almost indepcndent of the dye ing tempcrature or the type of dye. Dye ing rate and rate constant increase considerabl y with the increase in dye ing temperature . Activat ion energ ies of 11.2 and 14.36 kJ/mole were calculated for SB and RR dyes respective ly. The mechan ism of the dyei ng process for the two dyes is diffusion controlled and their dye ing rates depend on the type of basic dye. Considerably hi gh dyeing rates are found even at te mperatures as low as 282 K and the completion o f the dyeing process takes place in a matter o f minutes. About 70-99% of the colour strength of the samples dyed at 90°C for I h is obtained at dye bath temperatures ranging from 9"C to 55°C and a dye ing time of 80s.

Keywords: Activation energy, Cotton fabric , Dyeing rate, Radi at ion grafting

1 Introduction Extensive investigations on the improvement in the

phys ical and chemical properties of natural and synthetic polymeric substrates by graft copoly­merization with vinyl monomers have been reported. Considerable literature on the subject is given by Chapiro l and Hebeish and Guthrie2

. Grafting of cotton with polyacrylonitrile, polyacrylamide and poly(methyl methacrylate) affected its mechanical properties3

. Moisture regai n, water sorbency and electrical conductivity of the grafted polymeric materials were found to depend on the type of polymer-monomer combinations. Increase or decrease in the physical properties were reported for grafting natural and synthetic materials with different vinyl monomers4

.12

. Dyeing properties of cotton, polyester and polyamide fabrics showed substantial improvements on radiation grafting of these fabrics with the proper monomer. Grafting the fabrics with dimethylaminoethyl methacrylate increased consider­ab ly the colour strength of reactive Drimalan Red dye 13

. Nylon-6 fabrics grafted with N-vinyl pyrrolidone (NVP) showed increased dyeability towards the Solar Orange direct dye7

. Radiation grafting of cotton with poly(acrylic ac id) improved

, Phone: 6342328: Fax: 002-02-2749298; E-mail : Eg lal-Elgendy l @hotmail.com

s ignificantly the affinity of Sandocry l Blue basic dye to the fabrics6

.

Basic dyes are known to have no affinity towards cotton fabrics. Thi s drawback was overcame by the introduction of ac idic groups into the polymer macromolecules via radiation grafti ng of cotton fabrics in acrylic ac id solution6

. In this work, the effect of pH of the dye bath and the degree of grafting on the colour strength of the dyed fabrics has been studied. It is found that the grafted cotton fabrics do not show any dyeing affinity towards Sandocryl Blue dye in neutral or alkaline media and that the best results are obtained at pH 3.9. Increase of graft yield results in substantial increase in the colour strength of the fabrics. The kinetics of dyeing acry lic ac id-grafted cotton fabrics with Sandocryl Blue and Rhodamine B basic dyes has also been studied . The effect of graft yie ld, dye concentration and dyeing temperature on the colour strength of the dyed fabrics has been investigated and the reaction order and the activation energy of the dyeing process determined.

2 Materials and Methods 2.1 Materials

Mill-scoured and bleached cotton fabr ics, obtained fro m El-Beida Co., Kafr El-Dawar, Egypt, were treated with sodium carbonate (5 giL) and non-ionic

EL-GENDY: KIN ETICS OF DYEING ACRYLIC ACID-GRAFTED COTTON FABRIC 423

detergent (Sandozin NIT liquid) at boil for 4h, thoroughly washed with cold water, dried at ambient temperature and then used for grafting.

Acrylic acid (AAc) monomer and other chemical reagents were used as-received. Sandocryl Blue B-3G (SB) and Rhodamine B (RR)basic dyes, produced by Sandoz, Switzarland, were used . The structures of the dyes are as follows:

(H,c,),O~O N(C,HJ, +

CI

Sandocryl Hlue 1:\-30 (SB dye)

Rhodamine B (RR dye)

2.2 Methods 2.2.1 Radiation Grafting

Grafting was carried out by the direct irradiation method in a 60Co gamma source of 2.25 Gy/s dose rate for different doses and AAc concentrations to achieve a wide range o f graft yield. Dry weighed cotton fabrics were introduced into a wide mouth tube provided with ground-joint stopper. The so l vent, monomer and Mohr's sa lt were added so that the fabrics were completely immersed in the monomer solution. The samples were then de-aerated by bubbling nitrogen for 5min and left overnight before being placed in the gamma cell. Optimum graftin g conditions were achieved by impregnating cotton samples in aqueous AAc solutions containing 0.75 % owf Mohr's salt as inhibitor and 1:40 fabric-to-liquor ratio6

. Different degrees of grafting were obtained by using proper AAc concentrations.

2.2.2 Dyeing Procedure and Colour Strength Measurement

Stock dye solution (I %) was prepared by pasting the dye in acetic acid be fore the addition of the required distill ed water. Aqueous dye solutions containing 2% owf dye were prepared from the dyestuffs at 1 :50 fabric-to-liquor ratio . The pH of the dye bath was adjusted and the dyeing carried out in the presence of 10% sodium sulphate and O. L giL

Sandozin NIT liquid as a wetting agent. The temperature of the dye bath was then ra ised to 90°C and kept constant for I h. After dyeing, the samples were rinsed in hot water containing non-ionic detergent, followed by tap water rinse and a ll owed to dry . Thi s dyeing procedure was applied to dye samples at different pH and graft yields. Samples dyed at di fferent dye concentrations and temperatu res for kinetic studies were rinsed in water befOlt heating at 50°C for 15min.

The colour strength of dyed fabrics, expressed as KIS values, was measured using a Texicom-lCS Spectrophotomete r at maximum wavelengths of 660 nm and 528 nm for Sandocryl Blue and Rhodamine B dyes respective ly. KIS was calculated using the Kubelka-Munk equation '4:

KIS = [ (I-R) 2 I 2R ] - [ (l_Ro)2 I 2Ro ]

where R and Ro are the decimal frac tions of the reflectance of dyed and as-grafted samples; and K and S, the absorption and scattering coefficients respecti vely .

3 Results and Discussion 3.1 Effect of pH

The effect of pH of SB and RR dye so lutio ns on the colour strength (KIS value) of grafted cotton fabrics is shown in Fig. I. The results indicate that both dyes g ive high colour strength values in ac id ic so lutions. The affinity of grafted fabric s (4.5 % graft yield) towards SB dye solution increases from KIS 0.2 to 5 .6 as the pH decreases from 11.3 to 5.6. Further decrease in pH to 3.9 increased the co lour strength to 18.76. The colour s trength of grafted fabrics (8.6% graft yield) dyed in RR soluti ons increases from 2.53

20.-----------------------------~ 18

~ 16

~ 14

~ J2 i:i 10 !i] .... 8

'" .3 6 o u 4

-+-SB dye

...... RR dye

2

O+-----,-----r-----r-----r-~~

2 4 6 8 10 J2 pH

Fig. I- EfTeet of pH on the colour strength of grafted cotton fab­rics dyed with SB and RR basic dyes. The degree o f grafting for SB and RR dyes is 4.5 % and 8.6% respecti vely

424 INDIAN J. FIBRE TEXT. RES., DECEMBER 2002

to 3.43 as the pH decreases from 10 to 2.88 . A sudden increase in colour strength to 11.43 is obta ined as the pH is decreased to 2.72. These results show the higher affinity of SB dye towards grafted fabri cs, spec ial ly in the pH range 5.6-3 .9, when compared with that of RR dye although the degree of grafting of the laller is almost twice that of SB dye. The increase in KIS per decrease in pH in the range 5-3.9 amoun ted to 10.7 for SB dye while the corresponding va lue for RR dye is only 0.11. The las t va lue increased to 6.9 for further decrease in pH from 2.9 to 2.72 . Consequentl y, all dyeing ex periments were carri ed out at pH as close as possiblc to 3.9 and 2.72 for SB and RR dyes respecti ve ly.

3.2 Effect of Degree of Graft ing

The effect of the degree of grafting (graft y ield) on the co lour strength of cotton fabri cs dyed in S Band RR dye solutions is shown in Fig. 2. The results show l inear increase in co lour strength w ith the increase in graft y ield (CY) up to KIS va lue 10. The per cent increase in C Y amounts to 4.5 and I .n fo r SB and RR dyes respecti ve ly. Further increase in CY shows a decrease in the rate of change of co lour strength \ovi th the change in the degree of grafting w ith a tendency to leve ling oil at graft y ields higher than 16%. T hese rcsult s indi cate that dyeing of grafted co tton fabri cs in SB dye solutions gi ves KIS va lucs higher th an those for RR so lu tions hav ing the same degree of graft ing. This supports the results presented in Fig. I and emphasizes the hig h~r affinity of grafted cotton fabr ics towards SB dye compared to that towards RR dye. Moreover, the best dyei ng condition is achieved at graft yields lower than 10%. Conseq uentl y, the stud ics on the dyeing k ineti cs of grafted corton fabri cs in SB and RR dye soluti ons have been carri ed out at pH 3.9 and 2.75 and on collon samples w ith graft y ields less th an 10%.

3.3 Dye ing Kinetics

In the kinet ic study, the colour strength of dyed colton fabrics is measured by the KIS va lue per unit graft y ield to avo id the contribu tion of the degree of grafting in KIS measurements as shown in Fig. 2. Prelim inary experimcnts were carri ed out to study the dependence of the co lour stren gth on the dyei ng t ime. Dye i ng ti mc interva ls rang i ng from I min to I Omi n were tri ed and the corresponding KIS va lues per unit gran y ield were measured . All sampl es gave almost the samc co lour streng th value. Conseq uentl y, the intervals in the order of 10 s to 20 s were used to

measure th ~ dye ing time. In each experi ment, the grafted and ungrafted sampl es were dyed together in the same dye bath and the net increase in co lour strength per unit graft y ield was ca lcu lated.

3.3. 1 Sandoeryl Blue Dye

The dependence of the colour strength of grafted cotton fabrics on the dyeing ti me for dye concentrat ions of 1.25x I 0 .. 1, 2.Sx I 0"1 and 5.0x I 0.4

moles/L is shown in Fig. 3 for SB dye so lutions

3 0 .---------------------------------~

25

~ 20 -.c

§l

~ 15

~

::l 10 0

a u

5

0

0 10 15

Degree ufgra ning, %

-+- SI3 dye '

...... RR dye

20 25

Fig. 2- EJTcc l of degree of gl'aJ'iing on thc colou r st rength oj cot­ton fabri cs dyed in SB and RR dyt.: solutions dd justcd to pH 3.9 and '2.72 rcspect ivc ly

'0 Q)

';;" ~ ro ... Oil ~

'2 :l 1-0 Q)

...f?<

~ .~ c <l)

::: VJ ,... :l 0 0

U

3.5 .

3.0

2.5

2.0

1.5

1.0

0.5

0

4.5

4.0

3.5

3.0

2.5 2.0

1.5

1.0

0.5

n 6

Temp., 297 K

Temp .. 313K

~----

-'-O.OOU1:!5 mole/I. __ 0.0002 ' mulelL

-- (l.OOOS mold!.

--0.0(10 12 5 molell ___ (),OO()25 mold L

-'-0.0005 nl(llt:JL

__ U.OOO 125 moleJl ........ 0.OU025 molOJ I. __ 0.0005 molo'1.

O ~----~-----T------~----~----~ 20 40 60 80 100

Dyei ng time, s

Fi g. 3-Dcpcndenct.: of co lour strcngth of graftcd (Ollon fabric s on dyeing tilllt.: for diffcrcnt conccntrations (I f SB dye <,olutions adjusted to pll 3.9 at 297. 3 13 and 332 K

EL-GENDY: KIN ETICS OF DYEING ACRYLIC ACID-GRAFTED COTTON FABRIC 425

adjusted to pH 3.9 and dyed at 297, 313 and 332 K. The general feature of the curves is nearly the same except that the co lour strength increases with the increase in dye concentration and dyeing temperature. The colour strength increases linearly with the increase in dyeing time up to about 12s fo ll owed by slower increase with a tendency to leveling off at 60s. The initial rate of dye ing (R,,) is calcu lated from the slope of the I i near part of the colour strength-dyeing time relationship. Table I shows the dependence of R" on dye concentration (Co) and dyeing temperature (7) for S8 dye solutions. It is observed that as Cr, increases the Rei also increases. The relative rates of dyeing at concentrati ons of 2.5x lOA and 1.25x I O·~ moles/L for the temperatures 297, 313 and 332 K are nearly independent of dyeing temperature with an average value of 1.64. The ratio increases with the increase in dye concentration and becomes 2.505 for Co of 5.0x 10-4 and 2.5x I OA moles/L. Thi s indicates that doubling the dye concentrat ion increases R" ratios in a non-linear dependence as given in Fig. 4. Consequently, the order of the dyeing process (n) has to be calcu lated.

Fig. 5 shows the logarithmic piots of R" versus Co at different values of T as given in Table I . The relationship is linear and its slope n is almost independent of T wi~h an average value of 0.61. Therefore, the KI dependence on Co can be written as fo llows:

(R"hn dye = kd c.,06 1

where k" is rate constant of the dyei ng process. The va lue of kd is obtained from the above equation by I ' R C 0 6 1 d'fe I t' 7' P ottlIlg " versus o · at I .erent va ues 0 as

shown in Fig. 6. A linear relationship is obtained , the slope of which gives the va lue of k" at the corresponding dyeing temperature T as g iven in Table 2. The table shows that k" increases with the increase in T.

Tablc I- Dcpendence of ini :ial dye ing ratc (Rd) on dye conccnlralion (Co) and dyc ing tcmpcrature (7) for

S8 dyc , o llili ons adju stcdto pH 3.9

Co. mo lcs/L 1.25x IO·4 2 .5x I 0-4 5 .0x I0·4

R" (S· I) at 297 K 0 .067 0. 10 0 . 16 Rd(s·l)at 3 13 K 0 .08 0 . 13 0.2 1

Rd (S' I) at 332 K 0. 10 0 . 18 0.25

Tabl e 2- Valucs of thc dyc ing r31e constant (kd ) 3t d iffc rcnt dyc ing tcmpcratures (7) for S8 dye so ilitions adjustcdto pH 3.9

T, K

k.t . (Llmo1c)/s

297

20.5

3 13

28.7

332

33. 1

It is of interest to compare the KIS per unit graft yield va lues obtained from their dependence on dyei ng time at differe nt dyeing conditi ons, as given in Fig. 3, with those obtained from the initi al slopes of

3.0 -r--------------------,

<)) 2.5 ~ 00 2.0 c: .;:; -6' 1.5

<)) ;> .~ 1.0 -;:; ~ 0.5

1.0 2.0 3.0 4.0 5.0

Relative dye concentration

Fi g. 4- EITcct of doubling the dye concentrat ion on the rclati vc dyeing rates of S8 dye

o ,---------------------------------,

-0.2

-0.4

....... T~ 297 K • T~ 3\3 K -A-T~ 3J2 K

cie -0.6

U' 53 -0.8

-1.0

-1.2

-1 .4

-4.0 -3.9 -3.8 -3.7 -3.6 -3.5 -3.4 -3 .3 -3.2 log Co

Fi g. 5--Logarithmic plot s of thc dyeing rale (I?J) vs dye conce n­tration (Co) at d ifferent dyeing te mperatures fo r S8 dye

0.30 y------------------,

';'

0.25

~ 0.20

r£ ~ 0.15 OJ c '0; 6 0. 10

0.05

• T= 297 K • T= 313 K A T= 332 K

O ~--r----r_--~--~----~--~

o 0.0002 0.0004 0.0006 0.0008 0.00 I 0.00 12

Dye conccnlmtion 10 the power 0.61 , mol elL

Fig. 6- Linear de pende nce of S8 dye ing r3Le o n dye concent ra­ti on 10 the power 0 .6 1 for solu tions adj usted to pH 3.9 and dyed al 297,3 13 and 332 K

426 IND IAN J. FIBRE TEXT. RES .. DECEMBER 2002

KIS versus g raft y ie ld presented in Fig. 2. The values considered are those obtained at the dye concentration

of 5x lO-4 moles/L, which is almost equal to the dye concentration corresponding to 2% owf in 1:50 fabric­to- liquor rati o. The KlS per unit g raft y ie ld obtained at different dyeing conditions (Fig. 3) indicates the increase in KIS from 3.3 to 5.1 as the dyei ng temperature increases from 297 K (24°C) to 332 K (59nC) for samples dyed for 80s. The corresponding va lue for sample dyed at 363 K (90°C) for I h, as obtained from Fig. 2, is 5.17 indicati ng that 64-98.6% of the colour strength is obtained at such low dye ing temperatures and ex tremely short dyeing time.

3.3.2 Rhodamine 13 Red Dye

The dependence of the colour strength of grafted cotton fab rics on dye ing time for dye concentrations o f 1.25x lO-4, 2.Sx I0-4 and 5.0xI0-4 moles/L is shown in Fig. 7 for RR dye solutions adjusted to pH 2.75 and dyed at 282, 305 and 327 K. The general features of the curves are the same as those for SB dye except that the corresponding values of colour strength per unit graft yield are much lower. The initial dyeing rates (Rei) calculated at the corresponding dyeing temperature are g iven in Table 3. The results indi cate that as the dye concentration (Co) increases , the dyeing rate increases. The re lative dyeing rate at the correspondi ng dye concentration and dyei ng temperature increases from an average va lue of J .505 to 1.7 as C, is doubl ed (Table 3 and Fig . 8). The results indicate that the relati ve dyeing rates are independent of dyei ng te mperature and are similar to those for SB dye.

Logarithmic plots of Rei versus CIl at different va lu es of T are show n in Fig. 9. The rel at ionship is linear and its slope n is independent of T and g ives an order of 0.63. Consequently, the equation relating Rd to Co can be presented as follows:

(R ) k C O(i~ d RR dye = d ° The values of the rate constant k" at different T for

the RR dye are obtained from the plot of Rd versus CoO.

63 shown in Fig. 10. Linear relationship is ob ta ined, the slope of which g ives the va lue of kd at

Table 3- Dcpcndcncc of the initial dyeing ratc (Rei) on dye concentration (Co) and dycing temperaturc (T) for RR dyc

solutions adjustcd to pH 2.75

Co. llloles/L 1.25x I0-4 2.5x I0-4 5.0x I0-4

Rei (s-') at 282 K 0.0 18 0.026 0.048

Rd (s- ') at 305 K 0.030 0.055 0.090

Rd (s-') at 327 K 0.050 0.075 0.1 20

the corresponding dyeing temperature T as given in Table 4. It is observed from Table 4 that kd increases with the inc rease in 1'.

Colour strength per unit graft yield values, similar to those reported for the SB dye, are calculated from

Figs 2 and 7 at 5x J 0-4 moles/L RR dye concentration and different dyeing conditi o ns. The results indicate

1.0 -r------------------,

0.8

0.6

::9 0.4 aJ .~ 0.2 ¢::

(1j 0 I-<

bI) ..... '2 1.2 ;:\

I-< 1.0 aJ

0.,

~ 0.8 ~ ~ 0.6

-5 Q.4 on t:: 0 .2 aJ .P 0 (/)

I-< 1.4 ;:\ 0 1.2 '0 1.0 U

U.8

0.6

0.4

0.2

0

0

Temp_, 282 K

--0.O()(11 25 moldL --O.O{)()25 IIlok/1.. ~0.OOO5 mo!e/L

Temp., 305 K .--------'-0.000125 mole!L __ 0.00025 ",ok/L ~OJH '0 5 molelL

Temp., 327 K __ -------

• • ~-------..

--O.OOO I2~ IOo l<ll. --0.0002:; moldL ~0.(I(l05 mole/l .

20 40 60 80

Dyeing time, S

100

Fig. 7- Dcpenclcncc uf colour strcngth of graficd cotton fabri cs on dyeing timc for dilTcrcnt concentrations of RR dyc solutions adjusted (0 pH 2.75 at 282, ~05 and 327 K

I.X 1.6

2 1.4 ~ eo 1.2 c .~ 1.0

-;; 0.8 ::: ;;; 0.6

~ 0.4 0.2

.....

O e---r--'r--'---.---.---r--'---'-~

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4_0 4.5

Relative dye concentration Fi g. 8- EITcct of doubling thc dye concc nlrati on on thc relati vc dycing rales of RR dye

EL-GENDY: KINETICS OF DYEING ACRY LIC ACID-GRAFTED COTTON FABRIC 427

o

-0.4

0::" -0.8

OJ

.Q -1.2

-1.6

-2.0

--+-T- 282 K ---T= 305 K -*"'" T= 327 K

-4.0 -3.9 -3.8 -3.7 -3.6 -3.5 -3.4 -3.3 -3.2

log Co

Fig. 9- Logarithmic plots of thc dyeing rate (Rd) Vol" dye concen­trat ion (Co) at diffe relll dyei ng temperatures for RR dye

0. t4 --,------------------..,

0. 12

';"", 0. 10 --'-

r? 0.08 ~ !" 0.06 0> <:

.~ 0.04 o

0.02

• T- 282 K • T~ 305 K ~ T~ 327 K

O .. ~---r------r_----_r----~r_--~

o 0.002 0.004 0.006 0.008 0.010

Dye concentration to the power 0.63 . mole/l

Fig. I O- Linear dependence of RR dye ing ra te on dye concentra­tion to the power 0.63 for so lutions adj usted to pH 2.75 at 282 , 305 and 327 K

Table 4- Values of the dyeing rate constalll (A:d) at el i ffe re III dyeing temperalUres en for RR elye solutions adjusteelto pH 2.75

T. K A:d. (Umole)/s

282

0. 19

305

11 .34

327

14.44

that 71-99% of the co lour strength of the samples dyed at 90"C fo r 3600s is obtained for samples dyed at temperatures ranging from 9"C to 54°C for 80s.

3.3.3 Activation Energy for Dyeing

The dependence of k" on T for SB and RR dyes indicate that kd increases with the increase in T. However, the va lues of kd for SB dye are twice of those for RR dye for the same T. The dependence of kd on T is presented by the Arrhenius type plot of the natural logarithm of kd versus the in verse of T as shown in Fi g. 11 . The relat ionship is linear and the slope of it gives the value of Q/R, where Q is the act ivation energy of the dyeing process and R is the uni versal gas constant. Activation energies of I 1.2 and 14.36 kJ/mole are ca lcul ated for SB and RR dyes respectively. The general equat ions relating the rate

4.0 ..------------------------------------,

3.5

"';J 3.0 "< 01)

.2 2.5

2.0 • 5B dye

• RR dye

I.S +-----,------,----r----r---~r_-_.-~

2.9 3.0 3. 1 3.2 3.3 3.4 3.5 3.6 1000 I T

Fig. II - Arrhenius-type plot of the natu ral logarithm of the ra te constant (kd) vs the in verse of absol ute temperature (7) for SB and RR dyes

constan t kd to T for SB and RR dyes are given as foll ows:

(k ) . - 2 05x I 03 - I 1.2 (kJ /lllo le )/RT " SH dye - . e

(k ) - 3 06x I 03 - 14.3 6 (kJ/ rno lc )mT d RR dye - . e

The results obtained for SB and RR dyes indicate that the order of the dyeing process is the sa me for both dyes and is independent of the dyeing temperature. This shows that the dyeing process for grafted cotton fabrics with basic dyes is of the same nature although the dyeing rate constant for SB dye is hi gher than that for RR dye. The differences in the dyeing rates for both dyes can be attributed to the different molecular structure and molecular weight of the dyes. The molecular weight of RR dye is 478.5 whil e that for SB dye is 359.5. Moreover, the calcul ated values for the acti vation energy of both dyes ( 11.2 and 14.36 kJ/mole) are hi gh enough to exp lain the reaction bet"':een grafted cotton fab ri cs and basic dyes. This reaction takes place at an extremely hi gh rate and requires very small acti vat ion energy. This suggests that the ca lcul ated rates and act ivati on energies are those for the diffusion of the dye molecules through grafted and dyed cotton fabri cs to reach grafted areas in the bulk of the fabri c matrix. Since the diffusion of the dye and its reac tion with grafted cotton molecules are dependent processes, the slowest process (d iffusion) is the rate controlling. The small val ues of the rate constant for RR dye,if compared with th at for SB dye, and its hi gh activation energy can thus be attributed to the bigger structure and molecu lar weight of RR dye molecule that are responsible for retarding its diffusion through the dyed and grafted cotton fabrics. Once the surface

428 INDIAN J. FIBRE TEXT. RES .• DECEMBER 2002

layer of grafted fabric reacts with the dye, the newly formed F-COO-Dye structure starts to oppose the dye diffusion and slows down the dyeing process. This explains the leveling off of the KIS values after about ISs dyeing time. Increase in dye concentration increases the concentration gradient and hence increases the driving force for the diffusion process which results in higher KIS va lues at constant dyeing time. The di f fu sion process is temperature dependent in a way similar to that given by A rrhenius equation. The ex ponential dependence o/" the dyeing rate constant on absolute temperature emphasizes th at the rate controlling process is that of di f fusion. The resul ts also indicate th at dyeing AAc-grafted colton fabr ics with basic dyes can be ca rried out at temperatures lower th an those usuall y appli ed in industry and at ex tremely short dyeing ti me. Thi s reduces considerab ly the cos t of energy consumption in heating the dye bath as wcll as cutti ng down the timc necessary to dye cotton fabri cs.

4 Conclusions

4.1 T he decrease in the pH of the dye solution in­creases considerab ly the colour strength of acry li c acid-grafted cotton fabrics dyed w ith Sandocry l Blue (SB) and Rhodamine Red (RR) bas ic dyes. T he best dyeing is achieved at pH 3.9 and 2.72 for SB and RR aqueous solutions respcc ti ve ly .

.t.2 T he co lour strength o f grafted fabri cs increases rapidly with the increase in graft y ield and dyeing time and levels off as the graft ing exceeds 8% and dyeing time passes l Os.

" .3 T he initi al dyeing rate (Rei) increases w ith the ill­crease in dye concentrati on an d temperature of (he dye bath . T he values of Rei for SB dye are higher than those for RR dye.

..... T he dyeing process fo llows a 0.62 order kineti cs and is al most independent of the dyeing temperature and type of dye.

4.S The dyeing rate constan t kd (the slope of Rd-C" re lationship) increases exponentiall y w ith the increase in dyeing temperature and follows the A rrhenius-type re lationship. Acti vati on energies of 11.2 and 14.36 kJ/mo/e are calculated for SB and RR dyes respec­tive ly .

4.6 The mechanism of the dyci ng process for the two dyes is diffusion controlled and the dyeing rate de­pends on the type of basic dye. Considerably high dyeing rates are found even at temperatures as low as 282 K and the completion o f the dyeing process takes place in a matter of minutes. A bout 70-99% o/" the colour strength of the samples dyed at 90"C for I h is obtained at dye bath temperatures ranging from 9° to SSoC and a dyeing time o f 80s. 4.7 The relatively low dyeing temperature and the extremely low dyeing time required to dye acrylic acid-grafted cotton fabrics with basic dyes cut the dyeing cost to a minimum and show the importance of study ing the dyeing kinetics on the economy of the process.

Acknowledgement T he author is thankful to Dr S T awf ik fo r help in

the measurement o f KIS values and to Prof I A El­Shanshoury for va luable assistance during the course of thi s work .

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