Theory of Dyeing-F. Jones

8/17/2019 Theory of Dyeing-F. Jones

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THE THEORY OF DYEING

15

The Theory

of

Dyeing

F.

JONES

Department of Colorir Chemistry and Dyeing, University of Leeds, Leeds LS2

9JT

Introduction

Previous reviews of research work directed toward

a

greater

understanding

of

the way in which dye molecules are transferred

from th e dyeing medium to the polymer

or

substrate phase have

stressed that

a

unified fundamental theory, applicable to all

dyeing processes, is still

far

f rom

a

reality and may never be

attained. The main reason for this is that in any dyeing process

there are many variable parameters, some

or

all of which are

mutually dependent. T o achieve any progress in such studies, it is

necessary to control these parameters

so

that the effect

of

each

on

the dyeing system can be determined. This may not always be

possible. Thus, altering the dye concentration within an aq ueous

dyebath in order

to

study the concentration changes of dye

within a substrate may, even where

all

other conditions can be

maintained constant, produce

a

change in the structure

of

the

solvent and

a

possible alteration in th e nature of the dye species

partaking in dyeing.

Some

of

these variable parameters in the molecular dyeing

theory for ionic and non-ionic systems have been discussed

recently by McGregor a nd Peters

1).

Structural features of both

dye and polymer which may influence the thermodynamics and

the kinetics of dyeing includ e:

I ) the nature, conce ntration, distribution and degree of ionisation

of

ionisable groups in the dye in the solvent and subs trate phases

(2)

the molecular and ionic interaction s of all the species present in

both phases

(3)

the volume fraction, configuration and distribution

of

both

the crystalline and th e amorphous regions, and the degree

of

ionisa-

tion

of

ionisable groups, in the s ubstr ate

4)

the existence of reversible and non-reversible stresses within

the polymer before

and

during coloration

5 )

structural changes

in

the solvent distributed betwaen the

two

phases.

Since these changes are not independent, the researcher has to

adopt several simplifications and use model experiments where

variables can be contro lled.

The

results obtained in this manner can be compared with

calculated results obtained from theoretical models utilising th e

same number of variables. Model systems can therefore be used

only

to

illustrate specific poin ts in dyeing theory. One su ch model

(2) ,

used to illustrate the equilibrium values and kinetics

of

dyeing of non-ionic dyes, is based on simple mixing theory in

which dye molecules may be treated as occupying mean positions

in a quasi-crystalline

or

liquid (substrate) lattice. The funda-

mental concept

is

identical with that

(3)

describing the thermo-

dynamic behaviour of non-ionic solutes in which the solute

structure in the solvent is com parab le with that

of

a supercooled

liquid. It can thus be shown that the partial molar enthalpy of

solution,

AE,

of disperse dyes in the fibre is directly related t o th e

melting point of the dye according to the expression

where

4

and

4

represent the site fraction

of

the dye at tempera-

tures T and

Tm,

the dyeing temperature and melting point,

respectively. The value 0,,, may be obtained by extrapolation.

Eqn

1

is equally valid for th e solution

of

disperse dyes in water.

As certain non-ionic dyes can exist as metastable liquids

at

temperatures as low as 20°C

4),

the possibility

of

interpretation

inherent within this model m ay fruitfully bear fu rther examina-

tion. The model

is,

however, subject to serious limitations. It

applies only to ideal systems, i.e. those

of

low equilibrium dye

concentration, and can

be

applied only when it is known that

water plays

no

part in determining the equilibrium saturation

value

of

the dye in the substrate. More recent work

5 )

on the

adsorption

of

azobenzene and p-nitroaniline vapo urs by subs-

trate films in the presence and in the absence

of

unsaturated

water vapour has shown that the equilibrium saturation values

of

these compounds in second ary cellulose acetate are inversely

related to the amount

of

water vapour absorbed.

The

simple

binary mixing theory

of

dyeing may be more successfully

applied to hydrophobic polymers such as polypropylene, where

water plays an insignificant role in the abs orpti on

of

dye.

By using the same basic assumptions

( 2 ) .

with regard to the

state and distribution of dye within the polymer, and applying

phenomenological equations, it can also be shown from this

model that the kinetics

of

dyeing of hydrophobic fibres can be

expressed by

C =

S[I -exp(-8r)l

. 2)

where

Cr

s the total amoun t

of

dye absorbed at time

t ,

S

is the

solubility

of

the dye within the fibre and

8

is

a

rate constant.

Comparison

of

experimental rates of dyeing with Eqn 2 for

assumed values

of

shows good agreement, although this

expression does not ta ke into account

a n y

localised variation

of

activities existing at surfaces an d phase boundaries.

Much published research

on

dyeing and coloration is directed

towards the further understanding of practical commercial

processes, and comparatively little

is

published on what is

sometimes regarded

as

academic dyeing theory. Many experi-

mental results from the former group would be

of

more value to

the theoretician

if

the chemical structures and purity

of

additives

in the dyeing process, dyes and polymers could be disclosed and

the conditions during dyeing systematically defined. Although

much is published, this review has therefore had to be limited

to

salient pape rs in which these restriction s have been met.

Dyeing theory is concerned with the thermodynamics and

kinetics

of

processes occurring within th e dyebath, the interaction

between dyeing species and the internal an d external surfaces of

fibres, transfer from these surfaces by diffusion processes to

equilibrium positions within the substrate and the behaviour of

dyes once this type

of

equilibrium has been achieved. It is

proposed therefore to discuss recent advances in dyeing theory

under these headings and to include recent work carried out in

less conventional systems, such as heat-fixation methods. Since

coloration with reactive dyes entails simultaneous physical

absorption and chemical reactions within the dyebath an d within

the substrate, this topic will be considered separately, although

this division is somewhat unreal. By this means it is hoped t hat

the reader will

be

given a more comprehensive view

of

dyeing

theory without any artificial division based on s ubst rate types.

State

of

Dyes

in Solutions

and

Dispersions

Concepts concerning the structural nature

of

water

(6),

the

presence

of

cavities a nd ice-like clusters

of

water molecules that

are not ‘inter-cluster’ hydrogen-bonded, and the influence of

solutes in changing these structures have meant tha t ideas

on

the

structure

of

aque ous dye solutions h ave been modified in recent

years. This has been particularly relevant to studies of the

phenomenon of association of dye species in the solution.

Coates

7)

has surveyed the forces of interaction between like

ionic species which can lead to d imerisation an d higher degrees of

association and has discussed this process from its thermo-

dynamic and kinetic aspects. Although the standa.rd free-energy


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16

REVIEW

OF

PROGRESS

IN COLORATION; JONES

change involving both enthalpy change and entropy change in the

dimerisation for any dye can be determined from the equilibrium

constant governing the reversible formation of dimer from two

monomeric species and its temperature dependence, the values

obtained in practice depend on the accuracy of the means by

which the equilibrium constant is obtained.

Thus,

to

a first approximation the dimerisation of a number of

ionised monosulphonated dyes and of some positively charged

basic dye cations is accompanied by standard entropy changes of

from - 0 to -20 cal deg-1 mole-1. This decrease is made up in

part by a

loss

in translational entropy of the monomeric ion and in

part by a gain in entropy of the water. This gain of entropy by the

solvent is due to the probable decrease in the structural order of

the water promoted more effectively in the vicinity of a mono-

meric ion than in the region of

a

dimeric species. Conversely,

entropy values obtained for non-ionic dye vapours, which exist

mainly

in

the form of dimers

a),

show that, under anhydrous

conditions, the decrease in entropy related to dimer formation is

only of the order of -1.0 to

-2.0

cal deg-1 mole-1. This

decrease is therefore much less than that generally observed for

dimerisation in solution. It must, however, be pointed out that

association in the vapour is between molecules that, although

polarisable, do not possess a fully developed charge. It appears

then that the structural nature of water promotes association and

explains the large equilibrium shift towards monomer formation

at high temperatures, since the structure of the solvent is strongly

dependent on temperature. If association of dye molecules were

dependent only on the forces of interaction contributing to

hydrophobic bonding, then the phenomenon would also be

observed in other solvents. Since association is much less marked

in organic solvents of low dielectric constant, it can be concluded,

at least for basic dyes possessing a non-localised positive charge

9), that increased water-water interactions overcome the

repulsion forces acting between dye ions of similar charge.

The possibility that dye ions associate in solution is very real

and, if this is not taken into account, errors in determining such

parameters as ionisation constants can be considerable. The

problem can be overcome to some extent by using very dilute dye

solutions, where the probability of collision may reasonably be

expected to be low. Even so, self-associationoccurs at concentra-

tions as low as one milligram of dye anion per litre of solution

(10). In determining ionisation constants for a number of

monosulphonated 00'-dihydroxyazo mordant dyes, it was

necessary to use mixtures of dioxan and water to overcome

association effects and extrapolate the ionisation constants to

values for pure water. It was then possible to compare the effect

of association on the ionisation constant related to the ionisation

of the first hydroxyl group in these dyes. This is increased when

association occurs. Under more alkaline conditions both

hydroxyl groups are ionised and association is minimised or

eliminated since the repulsion forces between the trivalent fully

ionised entities are much larger.

The influence of additives such as urea

I I ) ,

formamide

(12),

N-alkylacylamides

(13)

and alcohols

(9)

on the structure and

properties of dye solutions has recently been studied. It is

generally accepted that additives of this type induce disorientation

of the water structure in the vicinity of the dye ion, thereby

reducing aggregation attributed to

loss

of hydrophobic interaction.

It has been pointed

ou t

14) that this mechanism has not been

conclusively proved, although no doubt a decrease in dye-dye

interaction by the action of urea on dye solutions does occur, and

leads to increased rates of dyeing. The swelling action on protein

substrates and reduction of hydrophobic interaction in the

substrates by urea also contribute to an increase in the rates of

dyeing, thus illustrating the interdependence of parameters

mentioned earlier.

Non-ionic disperse dyes possess very low solubilities n aqueous

dispersions at the dyeing temperature, and association, which

may occur in.the absence of formally charged structures, may be

very difficult to detect by conventional means. Anomalies in

rates of dyeing found by McDowell and Weingarten

15)

n

applying four pure disperse dyes to polyester material have, on

the other hand, been interpreted in terms of an increase in

particle size of the dye, leading to lower aqueous solubility. This

reasoning assumes that the rate of dyeing

is

directly related to the

concentration of dissolved dye molecules. When the pure dye is

pretreated in boiling water, the rate of dyeing in some cases

decreases and in others increases. These authors had earlier

16)

drawn attention to the classical equation relating solubility to

particle size and molecular weight, viz.

3)

where S, is the mean solubility of a particle of radius r,

y

is the

free surface energy,

p

is the density of the solid and

S

is the

minimum solubilitywhen the particle size

is

increased. There must

be a maximum value of r for the condition Sr =S nd this can be

shown 17) to be approximately

10-2

pm, which is much less than

the mean radius of disperse dye particles. Further, the anomalies

in rates of dyeing were inconsistent, and it is now suggested that

the inconsistency may be explained by the formation of different

structural modifications

of

the solid dye during dyeing. These

modifications could be verifiable from X-ray diffraction data.

That this possibility has not been put forward by the authors is

surprising, since in another context 18) it is stated that X-ray

diffraction data are obtained as a matter of routine.

Solid-state transitions and polymorphic changes occurring

through solution and recrystallisation mechanisms are well

established in non-ionic dyes and pigments. In azo pigments

transitions occur on heating the pigment in an aqueous environ-

ment 19), and Apperley (20)has recently studied the influence of

surface-active agents on the morphology of

C.I.

Disperse Yellow

3

at the coupling stage.If such changes are occurring

in

dyebaths,

then further research on specific systems from this aspect may

throw considerable light on anomalous results obtained in

dyeing research.

J h e t i c s of

Dyeing

Diffusion processes in dyeing are essentially those describing

the mass transfer of dye from the external aqueous phase to the

interior of the substrate, and the distribution of dye within this

substrate up to its saturation equilibrium value. A complete

description of transfer mechanisms should therefore include

possible formation of a boundary layer in proximity to the fibre

within the aqueous phase, the boundary conditions at the inter-

face between this layer and the fibre, a possible diffusion layer

between the interface and the fibre interior, and the mechan-

ism by which the dye is transferred to the centre of the fibre.

When possible changes with respect to concentration and the

effect

of

additives other than dyes in the above phases are con-

sidered, in relation to structural changes occurring within the

substrate, it can be seen that the overall dyeing mechanism is very

complex. Fundamental studies of rates of dyeing are therefore,

where possible, limited to conditions that will allow assessment of

transfer processes with a minimum of dependent parameters.

Weisz and Zollinger

(21)

have considered the transfer of dye in

solution within substrate capillaries in which partial sorption or

immobilisation of the diffusant occurs. In this model, where

sorption of this type is reversible, he apparent diffusion coefficient

DA s defined by

. 4)

where

P

is the fraction of the substrate accessible to diffusion

processes,b is a tortuosity constant, which is less than

4 3 ,

Cf/C,

is the ratio of the total concentration

of

dye,

Cj;

per unit

volume

of fibre to an external dyebath concentration

C,,,

at equilibrium.


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17

n

is the equilibrium partition coefficient between the mobile

portion of the internal dye concentration and the external dye

concentration, and

D

is the true diffusion coefficient

of

the

mobile

dye molecules within th at region of the su bstra te accessible

to diffusive motion. T he co nstant

a

depends on the particular

form

of

the absorption isotherm and falls within the range

1

* l a6 depending on the affinity of th e dye in th e system.

Eqn 4 is of general applicability and has been applied to an

earlier model for diffusion in cellulose

(22)

when the dye is

considered to be migrating along water-filled pores with simul-

taneous adsorption. By assuming that t he accessibility parameter,

P,

an

be

equated with a fractional uptake

of

water by th e fibre

and that the dye concentration is the same in the internal and

external aqueous phases, i.e.

n

=1, for dyes

of

high affinity where

a

approaches

1

‘6,

the eq uation ha s been successfully applied t o

previously published results on diffusion. Although there must be

some reservation on the assumed values

of

n and

a,

t is interest-

ing to observe that, for a direct dye, the value of th e true diffusion

coefficient, D,

of

the m obile dye molecules within the fibre is very

similar in magnitude to the value fo r th e diffusion coefficient of

dye molecules within the bulk aqu eous phase. It i s concluded that

for this dye the pore model with partial dye sorptio n is adeq uate

to account for the kinetics of dye transpo rt. Agreement is not as

good for acid dyes on nylon, but, with further assumptions abo ut

the aqueous solubility of disperse dyes, values for D obtained

from the general Eqn

4

for disperse dyes applied to polyesters and

cellulose acetates closely follow the

D

values for diffusion in

water alone.

Th e general applicability of this equa tion ca nno t, however, be

substantiated until further quantitative dat a on dye solubility and

free aqueous diffusion coefficients have been obtained. Very

little recent information is available on t he latter. Murfet

(23)

has

determined the relative diffusivity

of

C.I. Acid Red

1

using

a

vertical diffusion cell with

a

sintered-glass membrane. A lthough

the true aqueous diffusivity of the dye cannot

be

determined,

since the tortuosity and effective area of the sinter are not

amenable to absolute measurement, it is interesting to n ote tha t

relative diffusivity decreases in t he presence of urea, in co ntras t

with the fact that ur ea causes an increase in the rate of dyeing of

this dye on wool

14). One possible interpretation is that urea

accelerates dyeing by influencing other parameters such as the

structure

of

the boundary layer and the substrate, while having an

opposing effect on th e migration

of

dye in the internal aqueous

phase.

Although Fick‘s equations are often used as

a

basis for

diffusion studies, it is now accepted that theoretically determined

rate curves are comparatively insensitive to initial and boundary

conditions and to differential equations used in their derivation.

To

gain more detailed information on t he diffusion mechanisms,

it is necessary to obta in concentration-distance profiles, usually

by cross-sectioning fibres or films and determining dye distribu-

tion by microdensitometry. This has been done

( 2 4 )

for the

dyeing of various polymer films with disperse dyes in the presence

and in the absence

of

carriers. That in conventional dyeing

processes there are at least three components-dye, sub strate and

solvent-leads

to

a complex situation. Th e addition of carrier is

a

further complication. Even with the simplest systems rates

of

dyeing cannot be adequately dealt with in Fickian terms, since

the latter are expressly concerned with a binary system.

According to McGregor

et al. (24) ,

each component will

occupy

a

definite fraction

of

the fibre phase,

so

that, when

restrictions are imposed on this total volume, we might expect

that a n inward flow or volume flux of dye would

be

accompanied

by an outward flow

of

dyebath medium and that these opposing

flows might interact. This relative motion

of

molecules in

a

multi-component system can cause

a

change in volume or a

hydrodynamic bulk flow

of

the system. It becomes necessary

then, particularly at high diffusant concentrations, t o correct the

measured diffusion

flow

to take account of any hydrodynamic

transfer of the component under investigation. This could be

achieved by taking into account

a

frame

of

reference within

which the diffusion process is measured. T he reference frame may

be delineated in several ways. By conducting diffusion experi-

men ts in pre-swollen fibres

or films,

t can be assumed that no

furth er change in volume occurs o n dyeing, and he re the reference

frame is fixed with respect to the surface of the fibre

or

film.

Under these conditions Fick‘s laws do appear to apply and

reference-frame effects may become important only in carrier

dyeing and in dyeing fibres that have not been pre-swollen.

Irreversible changes in polyester structures have recently been

observed

(25)

n carrier dyeing with disperse dyes.

A method f or determining concentration-distance profiles

without cross-sectioning an d thereby standardising the frame-of-

reference effects still furth er ha s been developed by Blacker and

Patterson (26). The m ethod utilises the continuou s changes in

transmitted monochromatic light when

a

dyed filament, of

circular cross-section, is scanned across its longitudinal axis by

moving th e filament across

a

narrow slit. A microspectrophoto-

meter is used for this purpose, the results being suitable for

com pute r calculation to determine the dye distribution across the

fibre. Changes in profile shap e for

a

number of disperse dyeings

on polyester and nylon 6.6 filaments over

a

range

of

dyeing times

were obtained. The profile shape and the observation that in all

cases

a

time-dependent increasing surface concentration

of

dye

occurs showed that th e rate of transpo rt of dye to a boundary

just within the surface

of

the substrate

is

no higher than that at

which dye is transferred

to

the interior. It is also interesting to

observe that fo r nylon

6.6

this latter rate

is

extremely high, even

durin g the initial stages of dyeing, since horizo ntal profiles were

obtained. This behaviour is difficult

to

interpret unless it is

assumed either that th e subs trate is behaving like

a

liquid or th at

the driving force

for

diffusion is

a

variation in activity and not

concentration

of

the diffusing species.

When the dye and the substrate possess charges

o f

opposite

sign, which usually applies to t he nylon-acid dye system und er

acid conditions, the charge on the fibre becomes increasingly

negative during dyeing. This happens in the dyeing of nylon

with Orange I (C.I. Acid Orange 7), the surface potential

becoming increasingly more negative

as

dye concentrations both

within

( 2 7 )

and on the surface

( 2 8 )

increase. The initial surface

potential depends on the history

of

the substrate. Bell

( 2 9 )

has

found that rates of dyeing

of

acid dyes on nylon 6.6 are directly

proportional to the surface area and the saturation equilibrium

value

of

the dye on th e substrate. Th e latter values ar e considered

to be directly related to ami ne end-group con tent, bu t, in contrast

with Suzawa a nd Saito’s results

(28) ,

it is assumed that there is

no possibility of adsorption

of

dye on the surface. Bell’s con-

clusions must therefore be taken with som e reservation, p articu-

larly when it is noted th at dye concentrations in the fibre phase

were estimated solely on the basis of changes in dye concentration

occurring within the dyebath.

When the ion and the substrate are oppositely charged,

interaction between species can lead t o additional factors in the

interpretation

of

molecular diffusion processes. Some attention

has been paid to this problem by Mayer

(30)

in the dyeing

of

acrylic fibres (negative sites) with cationic basic dyes under

commercial conditions. Diffusion is satisfactorily achieved only

below a certain temperature and strict temperature control is

necessary t o achieve

a

reasonable rate of dyeing consistent with

levelness. Above this maximum temperature, bond formation,

represented by salt linkage, inhibits the attainment of adequate

rates

of

diffusion. Information to investigate this problem in

a

fundamental way is, however, lacking a t present.

A generalised treatment for explaining non -Fickian behaviour

has been given recently

(31) .

This type of diffusion, usually

attributed to substrate changes, can also occur when a second


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18

REVIEW OF PROGRESS IN COLORATION; JONES

independent process such as a n immobilising chemical reaction

is superimposed with a comparable time scale. The presence of

blind pores acting as diffusant sinks in a porous subs trate can

also have this effect. Anomalous diffusion may also arise in

systems in which thermody namic diffusion coefficients, theo-

retically determined from practical diffusion coefficients an d

diffusant solubility, depend both on activity and on penetration

distance

in

such a way that activity and distance variables canno t

be separately assessed. Solutions to this general problem require

a large amount

of

computation and are wcrthwhile only

in

particular circumstances.

The molecular interpretation of diffusion of dyes in polymeric

systems is based on the strong dependence of the apparent

diffusion coefficients D A on temperature, which often follows a

simple Arrhenius eq uatio n, viz.

D A -

Do exp - E / R T )

. . 5 )

where

D,,

s a pre-exponential

or

proportionality constant and E

is the activation energy of diffusion. This energy is required by

the diffusing molecule to enable it

to

jump from one absorption

site

to

a vacant neighbour. The pre-exponential factor is related

to

the jum p distance and entropy

of

diffusion. For more hydro-

phobic fibres, the activation energy

of

diffusion is related to the

energy

of

hole formation

within

the polymer which allows the

dye molecule to diffuse.

I n

this respect it has been observed

(32)

that the activation energy

of

diffusion for disperse dyes

in

unmodified polypropylene

is

higher (by approximately

14

kcal/

mole) than that observed when the same dyes are applied

to

cellulose acetate. T he difference may be du e t o the temperature

dependence of interchain bonding

in

polypropylene and to the

swelling of cellulose acetate caused by absorbed water.

In

confirmation,

it

has previously been found that deso rption from

vapour-dyed polypropylene film occurs simply on cooling,

whereas desorption from vapour-dyed cellulose acetate occurs

only in

the presence

of

water vapour.

I t

is therefore possible that

at high temperatures diffusion into polypropylene is a process

simply of mixing.

Thermodynamics of Dyeing Processes

When diffusion is allowcd to contitwe

unt i l

n o further dye is

absorbed, the dye-substrate-solvent system can be considered

to be

i n

a state of dynamic equilibrium p rovided th at the physicril

and chemical forces appertaining to the processes of adsorption

are completely reversible, there being, for instance, n o perm anent

change in the structure of the substrate during dyeing. The

variation

with

temperature of the amount

of

dye absorbed at

equilibrium (the equilibrium sorption value) allows thermo-

dynamic quantities such as the standard affinity or free-energy

change, and heat and entropy of dyeing to be determined.

Application of these concepts to dyeing systems, however,

requires certain assumptions about the ideal behaviour

of

the dye

species in the solvent, the internal solution and substrate phases.

Assumptions have also

to

be made abo ut the internal o r available

volume in the substrate within which absorp tiono ccurs .

The free-energy change,

lc n

the transfer

of

one mole of dye

from

its standard state

in

solution

to

its standard state

in

the

fibre is given by

. . 6)

where lF is the standard enthalpy or heat-content change

in

the

process and

15

s the corresponding entrop y change. Th e stan-

dar d stat e of the dye may be arbitrarily defined for both phases.

Energies

of

bonding between dye and substrate for different dyes

may

be

compared only on this basis within the limits

of

the above

assumptions. High heats o f bonding between dye and substrate

molecules indicate a large change

in

entropy or decrease

in

randomness

of

dye molecules on absorption, but

it

has been

pointed out

( 3 3 )

that such correlations may be spurious when

LIZ'

nd

.IF'

re obtained from the same set

of

data, and con-

firmation is required by mathematical transformation.

i c

~

lF

- - T A T

lyer

el a/ ( 3 4 )

have applied Eqn

6

to show that a relation

between

A H '

and

4 3

does exist in the dyeing

of

cellulose with

Chlorazol Sky Blue

FF C.1.

Direct Blue

1

and that

4Ec

alues

increase with increasing size of alkali-metal cations present

during absorption. Calculated values

of

activity

of

the dye in

solution were used together with a variable substrate-volume

parameter defined previously

35)

as the product of the surface

area available for dye sorption and the thickness

of

the diffuse

double layer existing between the bulk dyebath phase and the

oute r surface of cellulose.

As A i r

ncreases in this manner there

is a corresponding increase

in

the value

of

the entrop y change

uhic h is considered t o be due t o different packing arrangements

of dye molecules at the surface of the substrate. The interpreta-

tion of these results, however, in terms of a breakdown in water

structure

in

the vicinity of the cellulose in the presence

of

large

cations (as is done by the authors) must be treated cautiously,

since thermodynamic data are concerned only with differences

in initial and final states and can give no information on the

mechanism by which the final sta te is appro ached . It is interesting

in this connection to note th at, in the dyeing

of

cellulose with the

leuco ani on of a non-sym metrical vat dye-a process similar to

the application

of

direct dyes-stacking or associ ation

of

the dye

anion occurs on the fibre, and this leads to an oxidised dyeing

in which associates are present before oxidation

(36).

More attention has been paid recently to research on the

dyeing

of

wool an d nylon with acid dyes. Interaction has generally

been considered as electrostatic bonding between dye anion

and positively charged sites such as protonated amine groups

existing

in

the substrate under acid conditions. This conclusion

has been deduced from studies of sorption isotherms where no

further increase in dye sorption beyond the value equivalent

to

the number

of

charged sites has been obtained. T he possibility

that van der Waals forces arising from dipole-induced dipole

interaction an d dispersion forces also operate must not be exclu-

ded. Possible substrate changes, particularly when the attainment

of equilibrium is prolonged, leading to the exposure of sites at

which interaction with dye anions may occur, must also be con-

s

dered.

One approech to reduce the number of these parameters

(37)

has been to study the absorption of dye anions that normally

have no substantivity for cellulose, by a cellulose substrate

modified by conversion of some of the hydroxyl groups to

8-aminoethyl groups. Any dye bonding occurring should there-

fcre be specific

to

the amino groups. Thermodynamic affinities

and heats of dyeing show that t he dye anion has interacted with

the protonated amin o group. Changes in accessibility compared

with unmodified cellulose are shown to be negligible, but the

modification reaction in which polyethyleneimines having

substantivity for the substrate

(38)

are formed may lead

to

a

substrate

in

which more than one type

of

adsorption site

is

present. The assumption of lack of substantivity of these acid

dyes for cellulose and their precise mode

of

interaction with

amin o groups have been called into question 39), but in a simpler

system

( 4 0 ) ,

when the same dyes have been applied to amino-

polypropylene under acid conditions, the correlation between

protonated amino groups and equilibrium sorption values has

been explained on the basis

of

a simple ion-exchange model.

Although Langmuir-type adsorption isotherms indicate

a

probable stoichiometric relation, amounts of dye absorbed over

and above th e limiting value (overdyeing) give rise to Freundlich-

type isotherms. This phenomen on is attributed t o th e presence. of

associated dye species within the dyebath. Theoretical expressions

describing differences

in

the two types of isotherm for acid dyes

on nylon depend not only on the degree

of

association of dye in

the dyebath but also

on

the equilibrium established between

sorbed and mobile dye anions within the polymer phase 41).

This latter type

of

equilibrium behaviour has been considered


5/8


19

by Marshall

42),

particularly for Orange I1 applied to a wool

substrate at the isoelectric point. By using

a

model based on

DoMan equilibrium partition and applying the condition

of

electrical neutrality in internal and external solutions, variations

in dye-sorption isotherms can be attributed to the influence

of

dye Concentration

on

the equilibrium between absorbed dye and

mobile dye in the internal solution. In approximating estimated

values of this equilibrium constant over a range of concentra-

tions, it was necessary to vary th e equilibrium constant to give the

best fit to the experimentally determined isotherms. This variation

may be used to determine the activity coefficient and hence th e

degree of association

of

the dyes within the internal aqueous

phase, and, when this w as calculated, the values of the activity

coefficient were similar to thos e calculated for the external dye-

bath phase.

In the thermodynamics

of

dyeing hydrophobic fibres with

disperse dyes, isotherms that are linear up to th e point of satura-

tion with respect to the dyebath phase are usually obtained.

Since most disperse dyes have very limited solubility in water,

even at the dyeing temperature, experimental difficulties arise in

determining whether such solutions a re monomolecularly dis-

persed or contain associated species. Recent research by

McDowell and Weingarten

18)

has no t revealed any conclusive

results on this point, but it was shown that at

120°C

non-linear

isotherms could

be

obtained with 1 amino-4-hydroxyanthra-

quinone on polyester film. The isotherms were linear up to the

saturation solubility of the dye in the aqueo us phase, the gradual

approach to

a

maximum value for up take within th e film beyond

this point being attributed to changes in the substrate. Th e heats

and entropies of dyeing

for

a

number of dyes, when determined

from absorption isotherms, were higher than the values deter-

mined from the temperature coefficient of the ratio of solubilities

of the dye in polyester to the solubilities in water. In thermo-

dynar+ c terms this difference is explained

43)

by the Fact that

the heat of dyeing determined from sorption isotherms is an

integral heat of dyeing where the total heat evolved is due not

only to the interaction between dye and subs trate molecules but

also, in the later stages, to the interaction between entering dye

molecules and substrate that already contains dye molecules.

The heat of dyeing obtained f rom solubility ratios approaches the

value for a heat of interaction between dye molecules and

a

substrate containing dye. In the limit th e difference between the

two is equal to

RT,

which at 150°C (the maximum dyeing tem-

perature used) is 0.84 kcal/mole. T he experimentally determined

difference is greater tha n this value an d lies in th e range 1.78-4.45

kcal/mole

for

the dyes considered. Two contributory causes are

possible. The first is that association of dye could

occur

in the

substrate and the second that contributions to the experimentally

determined heats of solution of the dyes in water may arise fro m

heats of solid-state transitions occurring in the dye-solid suspen-

sion. Whether disperse dyes are associated in the substrate is

still an open question

(see

below) and the possibility of solid-

state transitions occurring in disperse dye suspensions needs to

be more fully investigated.

Chemical Reaction with Substrates and Fibres

Although use is made of conventional physico-chemical

concepts in elucidating the mechanisms

of

reaction

of

coloured

compounds with substrates, discussion of the subject has been

arbitrarily divorced from normal dyeing theory in the past

since reactive-dyeing mechanisms involve no t only interaction by

physical ionic and dipole-induced dipole and dispersion forces

but also formation of covalent bonds with the substrate. With

reactive dyes, simultaneous hydrolysis

of

the dye by water in

both external and internal phases can occur. The resultant

changes in chemical structure

of

both dye and substrate during

dyeing therefore lead

to

more complex mechanisms of abso rptio n

and fixation.

Rattee has recently reviewed

44)

the chemistry

of

these

reactions from

a

kinetic stan dpoint. I n view of the heterogeneous

nature of the dyeing process, the initial model system used has

been one in which a homogeneous reaction phase-soluble

reactive dye, soluble alcohol acting a s the sub strate and water-

is considered. Reaction of dye with alcohol is analogous with

fixation

of

dye o n cellulose. The fixation a nd hydrolysis reactions

are bimolecular, although the total reaction norm ally behaves as

a

pseudo-unimolecular reaction because water and alcohol are

present in excess. It can be shown that, under conditions where

the alcohol is very slightly ionised, th e ratio

of

the rate constants

of fixation and hydrolysis, i.e. the reactivity ratio, Z, must

be

cons tant at any temperature an d be independent of pH co nditions.

At higher pH values, the ionisation of the alcohol may become

significant and th e reactivity rat io in this case is no longer con-

stant but depend s on pH. W hen the model is extended

to

include

cellulose, which contains ionisable primary and secondary

alcoholic groups, the situation becomes more complex, since

reactions may proceed at different rates in the fibre and

in

the

aqueous phase. The distribution of dye between the two phases

assumes

a

greater significance und er these conditions and the rate

of

diffusion into the fibre also plays a part. Hydrolysis occurs

both in th e dyebath and in the internal water phase, but for the

purpose of this discussion the latter effect can be shown to be

minimal. Simultaneous reaction

of

the dye with the fibre and

diffusion

of

the dye within the fibre can be accommodated by

applying

a

simplified Danckwerts’ equation

44) i,n

which the

efficiency of fixation E, defmed as the ratio of th e rate of reaction,

dfldr, to the rate of hydrolysis, dhldt, is given by

where

[ D ] F

s the con stant surface dye concentration,

[Ills

is the

dye concentration in the dyebath,

Z

is the previously mentioned

reactivity ratio, [C-] is the concentration

of

ionised hydroxyl

groups in the cellulose and KH

is

the bimolecular hydrolysis

constant. The apparent argument therefore is that the prime

factor determining the efficiency

of

fixation is the ratio

[ D ] F / [ D ] , ,

the substantivity ratio. The diffusion coefficient, D, and the

reactivity ratio have only minor effects,

as

d o

pH

conditions.

The last-named play

a

secondary role since the hydroxyl-ion

concentration i n th e external phase

([OH -I,,)

influences the value

of [C-1.

Strictly, this hydroxyl-ion concentration is related to the

hydroxyl-ion concentration within the internal aqueous phase

and the situation is in practice more difficult to interpret.

Changing the pH

of

the dyebath will cause

a

more

or

less

negative potential to develop at the substrate surface and,

where the surface potential becomes more negative, mutual

anionic repulsion between the surface and dye anion will cause

the value of

[D]F

to decrease.

Assuming that the dyebath

concentration is constant, the substantivity ratio will also

decrease. This effect, however, can

be

mitigated by increasing

the pH, which causes an increase in the diffusion coefficient.

Fro m a practical point of view, therefore, any process that leads

to a high degree of exhaustion

of

the dye will tend to give

a

high

substantivity ratio and consequently improve the overall efficiency

of the dyeing operation. It is interesting to observe 46) that the

influence

of

urea in improving the efficiency

of

reactive dyeing

cannot be due solely to an increase in dye solubility, since this

effect would reduce the substantivity ratio where the amount

of

bonded dye

[ D ] F

s unchanged. Improved efficiency is therefore

attrib uted to a n increase in the diffusion coefficient on increasing

the concentration

of

dye in the bath.

Practical confirmation of the validity of th e modified Danck-

werts’ equation

7 )

cannot be obtained under conditions where

dye hydrolysis and fixation occur simultaneously. Sumner

and

Taylor

47)

have attempted to overcome this problem in a

serni-quantitative way by considering the dyeing behnviour of


6/8

20

REVIEW OF PROGRESS IN COLORATION; JONES

reactive dyes when hydrolysis is at a m inimum. This occurs under

slightly acid conditions for reactive dyes on cellulose. If, now , the

dyeing behaviour of dyes containing non-reactive residues but

similar in structure t o reactive dyes is observed over a ra nge of

pH values, it can be assumed that the changes in affinity and

rates of diffusion of these inert dyes will be paralleled by similar

but theoretical changes occurring with the reactive dyes. The

affinity and diffusion coefficients of

t he

latter can therefore be

determined over a p H range by extrapolation.

For

two out of the

three dyes examined in this way, calculated values of D increased

and values of

[ D ] F

decreased with increasing alkalinity. In

contrast, the third dye exhibited a minimum in its D values and

a ma xi mu m f or [ D l ~ a tH

I 1 *5 .

Hydrolysis in the dyebath of dichlorotriazine reactive dyes in

which

the

chromogen is linked to the reactive residue through an

imino group may no t be a simple second-order reaction b ut may

be complicated by the presence

of

a deprotonated imino form

(-i;j:)Bxisting as a result of acid-base equilibrium between the

latter and the imino form (-NH-) itself

(48).

The products of

hydrolysis of each form

will

be identical, although the rate

constants for hydrolysis for each fo rm will differ. These constants

and the acid-base equilibrium constant cannot be determined

without a knowledge of the activity coefficients

of

each species.

Products

of

activitycoefficientsand rateconstants can, however, be

approximated by computer processes. When this approximation

is carried out, the derived activation energies of hydrolysis are

found

to

depend on temperature with a maximum curvature at

30°C. This illustrates a comm on feature either in the dye struc-

tures

or

in the system as

a

whole. Th e observation

of

a minimum

in the mean activity coefficient in solutions

of

Orange

11

at this

temperature (49) determined by differential vapour-pressure

manometry is relevant in this context and may indicate structural

changes in the aqueous solvent in the vicinity

of

t he

dye anion

at this temperature.

More recently, the emphasis in reactive-dyeing theory has

shifted towards protein substrates and the investigation of sub-

structures within the protein that can react with the dye. The

elucidation

of

reaction mechanisms is more difficult than with

cellulose, since reactions are possible with a greater number

of sites of different types, the possibility of fibre degradation

is higher and fibre morphology is more complex. Very little

quantitative information was available until S hore, in an a dm ir-

able series of papers 50) and adopting the approach previously

taken for the reaction of dyes with cellulose, examined the r ate

of

reaction of a monoch lorotriazine dye with a number of model

compound s related

in

structure

to

the amino-acid residues

in

pro-

teins. If it is assumed t ha t the reactivities or dissociation constants

of

the groups

in

the protein are unaffected by their neighbours

then their reaction rates and activation energies will be com-

parable with those of model compo und s in aqueous solutions.

By such comparison the order of relative reactivity in water-

soluble proteins is cysteine thiol

>

N-terminal amino > histidine

> imidazolyl, etc., down to lysine amino and serine alcoholic

groups. This assumes that the availability of

t he

groups in the

protein to the reactive dye is equally as great as their availability

as model com pou nds in a homogeneous solution, but this

is

not

very likely. By extending the s tudy

51)

o include water-insoluble

proteins of known composition the most important groups to

react

were again shown to be the cysteine thiol, the primary amin o

groups

of

lysine and N-terminal amino-acid residues.

It

is

important to note that the thiol groups are reactive over the

whole pH range, whereas primary amino groups exert an influ-

ence only under alkaline conditions. Conditions with respect to

p H

in

kinetic a nd therm odynamic studies will differ from those

adop ted in t he reactive dyeing of cellulose. It is not possible then

to

adopt comparative techniques such as those

of

Sumner and

Taylor 47),

since the dye reacts readily un der n eutral

or

slightly

acid conditions.

By using

a

mixture of hydrolysed and reactive dye and applying

an equation resulting from a combination of Danckwerts

equa tion an d Hill s equation for diffusion into an infinite cylinder

with the condition of a satur ated fibre surface, Shore has shown

52)

that under acid conditions the diffusion coefficient of both

dye species increases with pH . Abov e p H 4.0 eaction of the dye

with wool predom inates a nd below this p H reactive dye preferen-

tially hydrolyses. Under neutral conditions the reactivity of the

dye for a wool substrate was less than the theoretical level,

suggesting that there m ay be specific chemical hindrance to the

reaction in the solid phase. A similar observation had been made

previously in comparing the rate constants for reaction of

a

dye

with a w ater-soluble alcohol an d with cellulose

(53).

Independent confirmation that hydrolysis of reactive dyes is

minimised i n

the

pH range

4-6

had been given by Lewis and

Seltzer

54) .

In the pad-batch dyeing

of wool

at roo m temperature

with dichlorotriazines under weakly acid conditions, the degree

of fixation app roach ed unity, particularly in the presence of

additives such as sodium metabisulphite. It is considered that th e

high rate

of

reaction may be due to th e formation

of

thiol groups

in the wool arising from the reduction of disulphide bonds. It

may also be ad ded that in the absen ce of reducing agents, under

acid conditions, protonation of the triazine nitrogen atoms may

increase the activity

of

the dichlorotriazine dye and allow

increased reaction with un-ionised thiol groups. Simultaneously,

the hydrolysis reaction would be minimal since a very low con-

centration of hydroxyl ion would be present.

The

specific

reaction with thiol groups in model compounds and substrates

requires furthe r investigation.

CHROME MORDANTING

Although not usually regarded as reactive dyeing in any sense,

the chrome mordanting of wool also entails direct chemical

reaction of the chromium with the fibre. Chromium is usually

applied as a hexavalent cation, in which form it is able

to

diffuse

into the wool. During application, the effect of heat accelerates

the reduction to trivalent chromium, probably by the action of

the substrate. The

Cr(1II)

can then react with the wool at

a

comparatively low rate. Hartley

55)

has considered the possible

reaction sites, reaction being excluded at amin o, thiol or phenolic

groups

56),

and concludes from pH changes during absorption

and from spectral data that the ionised carboxyl groups

in the

wool

take part in

the

formation of metal complexes.

This reaction is interpreted in terms

of a

prob able first-order

nucleophilic reaction whereby the presence of at least one

strongly negative ligand, e.g. sulphate, in a water-saturated

6-co-ordinated chromium cation facilitates the loss of a water

ligand. Thi s

loss

occurs as the rate-controlling step in the reaction

by the displacement

of

charge from the strongest negative ligand

to the central metal cation. The resultant penta-co-ordinated

transition-state entity is then in a sterically favourable orientation

to react with an ionised carboxyl group in the substrate. In

support

of

this reasoning, the more negative ligand remains

co-ordinated with the chromium after reaction. Although this

proposed mechanism still leaves room for 1

I

dye-chromium

complexes to be formed by ligand replacement, the existence

of

this type

of

complex in wool has never been proved. Th e forma-

tion of 2:l dyeechromium complexes requires displacement of

a

wool-carboxyl-ligand, and furthe r research is necessary.

High-temperature Dyeing under Anhydrous Conditions

The process of padding a fibrous material with a dispersion

of

a

non-ionic dye, drying and then heating the treated fibre to

a

high (190-220°C) temperature

for

a

short period to produce a

dyeing is by now familiar. The greatest use of such a process,

e.g. the Thermosol method, is in the coloration of polyester-

cellulose mixtures to dye the polyester component. Dyeing occurs


7/8

THE

THEORY

OF

DYEING

21

under high-temperature anhydrous conditions. The mechanisms

by w hich dye molecules

are

transferred from the solid particle

to

the substrate have been variously described as particle dissolution

in the substrate 57),

a

partial co ntact mechanism 58)

or

an

evaporation and dye-vapour absorption process

(59).

By similar

but independent experiments Datye 60) nd Sumner

et al

61)

have shown that, on padding and drying, the dye particles and

solution are preferentially ta ken u p by the cellulose component,

whereas in the fixation stage the dye

is

preferentially absorbed

by the polyester component. It follows, therefore, that the

vaporisation

of

the dye plays an imp ortant pa rt in the transfer

mechanism. Contribution s

to

the dyeing mechanism by transfer

of dye across dyed and undyed fibres throu gh sites of contact o r

by transfer through t he medium

of

additives may or may not be

relatively important.

Both Sumner

et

al

and Datye have found that dispersing

agents or migration inhibitors have no effect on the rate of

transfer. By determining the amount

of

dye absorbed when the

polyester subst rate is placed at various fixed distances from the

source of dye, an approximately linear relation between the

amoun t absorbed an d distance is found, from which the amoun t

absorbed a t zero distance, i.e. w hen th e dye sou rce and polyester

are in contact, can be extrapolated. Since there is good agreement

between these extrapolated values and those experimentally

determined, for a number of disperse dyes over

a

range

of

temperatures, Sumner

et al.

conclude that a single transfer

mechanism, viz. by vaporisation and absorption of dye vapour,

is adequate t o explain the amo unt

of

dye absorbed. Confirmation

is given from linear plots

of

ln[D]/, where [D]/ is the amount

of dye absorbed in unit time, against reciprocal temperature,

It is assumed that the rate of abso rptio n is controlled specifically

by the rate

of

diffusion d[D],/dt of the dye vap our in air. The

rate is directly related to the diffusion coefficient in air,

D

nd

to the vapour pressure

of

the dye

p

and inversely related to the

distance x between source and substrate, i.e.

. . . 8)

Sincc In

D

is proportional to reciprocal temperature and by

applying the Clausius-Clapeyron relation between

p

and

T,

Ean 8 mav be exmessed as

In(d[D],/dr)

In

D

+

Inp

- n x

In(d[D],/dr)

In K +

C

n x --

where K,

C

and

N

are constants and

L

is the latent heat of

sublimation

of

the dye at

a

total pressure of one atmosphere.

Plots

of

In[rate of transfer] or ln[D]+ at unit time against

1/T

should therefore be linear and of negative slope independent of

x .

This has been fou nd experimentally to

be

the

case

even when the

value

of x

is zero. On the other hand, the relation between [D]/

and

x

determined by Daty e

60)

as mor e curvilinear and extra-

polation to

zero

distance could lead to results indicative of an

additional small contribution by a direct contact mechanism.

Dyeing by application

of

unsaturated dye vapour

to

both

polyester and nylon substrates at high temperature has been

adequately dem onstrated 62). t is of interest t o observe

5 )

hat,

when model comp oun ds such as p-nitroaniline an d azobenzene

are applied to cellulose acetate by vapour-absorption methods

in the presence of unsaturated water vapour, the sorption

equilibrium values decrease as t he con centration

of

water vapour

within the substrate increases. Since sorption occurs under

equilibrium conditions, the decrease cannot be attributed to a

reduced rate of dye transfer by th e presence of water vapour, but

is possibly due to increased competition fo r sites between sorb ate

and water or

to

changes in subst rate structu re influenced by the

presence

of

water. In conventional dyeing, the amount

of

water

present in the substrate cannot be controlled in this way. The

comparatively high saturation values and their rapid attainment

obtained in high-temperature fixation or solvent-dyeing processes

may therefore

be

explained by the absence

of

water in addition

to

the high thermal energy

of

both dye and substrate molecules.

It h as been noted recently 63) hat the total amount

of

a

dye

in

a

mixture absorbed by the substrate under heat-fixation

conditions can be less tha n tha t absorbed when the dye is applied

separately and tha t long er fixation times a re required. In view of

these observations, it is possible that the vapour pressure of the

dye solid is reduced to that of a more stable polymorph, the

solid-solid transition taking place more readily

in

the presence

of

a second vapour component. Th e question of the heat stability

and the possibility of structural transitions

of

disperse dye

particles under conditions

of

high-temperature fixation requires

furthe r investigation, since n o research has been published in this

field.

The Physical State

of

Dyes

in

Polymers

U p o this point we have considered t he state

of

dyes in solution

and the kinetics and thermodynamics of dye transfer from the

dyebath to the substrate. Since this transfer process is dynamic

in natu re, it is possible that fur ther changes in the physical state

of the dye molecules in the polymer can take place outside the

environment

of

the transfer medium and more particularly when

the dyed material is heated, washed or exposed

to

light. It is

generally accepted tha t changes

in

colour occurring in the soaping

of vat and azoic dyeings can be attributed to the formation of

aggregates within the cellulose and some evidence has already

been given 36) hat association of vat-dye anions takes place

within the substrate during dyeing.

At present, it is not known with any real certainty whether

disperse dyes in hydrophobic sub strates exist as monomolecular

dispersions or whether they associate to fcrm large

or

small

aggregates. Giles

et

al.

have drawn attention

to

this probleni and

consider tha t associates

of

non-ionic dye molecules are present in

dyed nylon, polyester and cellulose acetates. Thcy base their

conclusions on two main arguments. Firstly (64,

65)

by measur-

ing the rate

of

fading,

as

expressed by changes in optical density

of

the dyed subs trate with time when exposed to light, the ord er

of

the fading reaction can be classified according to whether the

rate

of

fading changes approximately exponentially or linearly

with time. Some dyes may fade according to a combination

of

these orders of reaction, w hereas other (ano malo us) dyes exhibit

an initial increase in the optical density

of

the dyed film.

These

differences in reaction order

or

ra te

of

fiiding are attributed to

the presence and growth

of

aggregates

o f

dyc molecules within

the

film

on exposure to light. Secondly

6 6 ) ,

or those dyes con-

taining two absorpticn bands

in

th e visible region

of

their spectra,

the ratio of the molar extinction coefficientsof each band increa-

ses with increase in dye concentration

in

solution. Changes

in

the

ratio are related to changes

in

the degree of association as the

concentration

of

dye is varied. F rom similar changes in the spec-

tra of dyed hydrophobic films arid dyed films exposed to light

for different periods, it is concluded by analogy that such films

contain both monomolecularly dispersed and aggregated dye

molecules and that the average aggregation number increases as

fading proceeds.

It has been pointed ou t

67)

hat, since the spectrophotometric

technique used to examine dyed films utilises monochromatic

light that is partially plane-polarised, changes

in

extinction ratios

may be due to dichroic effects within the

film.

It has also been

observed 26) hat the dichroic orientation factor

of

dyed

filaments increases as the amount of dye present increases, since

the dye molecules first occupy the least oriented parts of the

substrate and th e zones

of

higher orientation are occupied only

at higher concentrations

of

dye.

If

t he

aggregation number

of

the dye increases during fading,

then there must b e

a

contribution to rates of fading from a dye-

migration mechanism. By studying the change in the dichroic

orientation factor of dyed polyester films with temp erature, any

reversible dye migration taking place as

a

consequence

of

heat

treatment would

be

indicated by

a

reversible change in the


8/8

22

REVIEW

OF

PROGRESS

IN COLORATION;

JONES

dichroic orientation factor. Nakayama e t

al (68)

have shown

that, provided the amorphous polymer structure does not change

irreversibly, the orientation factor is in fact reversible. It is

concluded that, even

if

dye migration occurs at higher tempera-

tures, the dye molecule reverts to the same type

of

absorption

site on cooling.

The view that disperse dyes are at least initially monomolecu-

larly dispersed

in

hydrophobic fibres is favoured by Husy e t al.

69).

In

their more recent experiments they qualitatively show that

exposure to light of cellulose acetate dyed with an azo disperse

dye

in

which a trans+ rearrangement can take place is

accompanied by contractions in the dimensions of the substrate.

With dyes that

do

not undergo this phototropic change, the

dimensions

of

the substrate remain unchanged. These results

indicate a close interaction between dye and substrate molecules

which is favoured more by

a

molecular dispersion than by an

associated state.

Conclusions

A

reading of this review will show that no new theories of

dyeing have been postulated and that a comprehensive theory of

dyeing is still far from reality. In

a

recent survey, Valko (70)

suggests that, whereas thermodynamic studies of dyeing can

make useful contr ibutions to the general theories ofintermolecu-

lar forces, diffusion processes and the influenceof parameters such

as concentration, temperature, electrolyte concentration and

polymer structure

on

these processes remain largely uninvesti-

gated and would be

of

greater relevance to application methods.

As is shown in this review, however, when equilibrium studies are

carried out and assessed

in

conjunction with the growing amount

of information

o n

the structure of water and aqueous solutions,

their relevance to dyeing theory should not be underestimated.

Dyeing theory has been previously retarded by lack of knowledge

about non-ideal behaviour, but it is now possible, e.g. by differen-

tial manometry or vapour-pressure osmometry, to determine the

mean activity coefficients

of

dyes in solution. This may be the

first

step

in

determining activity coefficients of dyes in the

internal aqueous phase and inthesubstrate. Finally,muchresearch

is still needed on the heat stability

of

dye dispersions and solids

and on the thermodynamics and kinetics of heat-fixation

processes.

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Theory of Dyeing-F. Jones

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