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Role of Leveling agents in Textile Wet-ProcessingLevel dyeing is
achieved by i) controlled exhaustion of the dye so that it is taken
up evenly and ii) migration of
dye after initially unlevel sorption on the fiber. In some cases
level dyeing can be influenced by dye-bath pH
and/or presence of electrolytes. However, to produce better
yields with correct depth, leveling agent is a
must.
Mechanism of Leveling agents :
Since leveling agents are surfactants, they may be anionic,
cationic, non-ionic or amphoteric in nature. The
main mechanism of non-ionic agents is to form water soluble
complexes with the dye. Two main groups of
ionic agents can be identified. They are either dye-substantive
(products which have an affinity for the dye) or
fiber-substantive (products which have an affinity for the
fiber). In case of dye-substantive leveling agent,
there is competition between the leveling agent and the fiber
for the dye. Products which have an affinity for
the fibre compete with the dye for dye-sites on the fiber. The
attractive forces between agent and dye, create
a counter balancing mechanism against dye-fiber attractive
forces, restraining the uptake of dye by the fibre.
As the temperature increases, the complex gradually breaks down
releasing the dye for uniform sorption by
the fiber.
Truly speaking, for an effective leveling agent that functions
by this mechanism, the stability of dye-agent
complex is crucial. If these forces create relatively unstable
complex, then leveling action may be ineffective.
On the other hand if these forces create too stable complex,
then it will not be broken down with the rise in
temperature and will not release the dye for uniform sorption by
the fiber. Thus the object is to make a
leveling agent such that it forms an optimum complex, rather
than maximum complex. This is done by
adjusting HLB (Hydrophilic-Lipophilic Balance) of the
surfactant.
The dye-agent complex varies with the nature of dyes, nature of
the substrate and nature of the leveling
agent. Hence there is a need of different leveling agents for
different dyeing processes. Fibre-substantive
leveling agents must be of the same ionic nature as that of the
dye. i.e. anionic agents are used with anionic
dyes and cationic agents are used with cationic dyes. While
selection of appropriate leveling agent, care must
be taken that the ionic charge is not too weak. Otherwise it
will not function as an effective leveling agent. If
the ionic charge is too strong then, it may exert blocking
effects, preventing sorption of the dye. Thus an ideal
leveling agent is the one who is adsorbed by the fibre quickly
than the dye, but the agent-fibre bond must be
weak enough to allow displacement with dye ions.
Leveling for Cellulosic dyeing :
During exhaust dyeing of cellulosics, pad batch or continuous
dyeing with direct, vat, sulphur, naphthol and
reactive dyes as well as indigo types, it is necessary to
prevent the disturbing effect of inactivated ions or any
prematurely oxidized dye in order to prevent stains, markings
and unlevelness.
Matlevel DLR liquid is designed to satisfy the needs which
effectively combines dispersing and leveling
properties to uniformly dye the fabric. It allows the dyer to
simplify the process to shorten the dyeing time byi) proper
handling and introduction of the totality of the electrolyte
previously to the dyes and
ii) linear addition of alkali, without a dispensing system at a
selected temperature in max. 30 mins.
Leveling for Polyester dyeing :
Polyester dyeing at 130C in beam and package dyeing machines has
placed especially great demands on
aspects of both, initial dispersion quality and subsequent
leveling under adverse conditions. The crux of the
problem lies in the inherent thermodynamic instability of all
dye dispersions, there being an overall tendency
of fine particles to undergo Ostwald ripening with consequent
formation of large particles.
Matlevel DLP is such a dispersing and leveling agent, which
disperses the dye molecules evenly and maintains
an equilibrium state in water.
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Leveling for Texturised Polyester dyeing :
In the dyeing of 100% texturised polyester material, barriness
is observed due to dye-ability differences in the
material. The difference arises due to variation of temperature
and tension during texturisation. The
temperature can be controlled to avoid variation, but it is
difficult to control the variation in tension. This is
revealed only after dyeing particularly when dyes of lower
migration properties are used.
Matlevel DLTP is the most suitable dispersing & leveling
agent which not only covers barriness effectively but
also promotes the migration thereby achieving level dyeing.
Leveling for Polyamide dyeing :
Polyamides refer to various natural (polypeptides) and synthetic
materials containing free amino groups.
Examples of polyamides include nylons, wool, and silk. Nylon
fiber is commonly dyed with acid dyes which are
anionic in character, including premetallized acid dyes, in a
batch process referred to as exhaust dyeing. For
example, nylon fiber which has been made into fabric may be dyed
in a jet-dyeing machine, whereby a
continuous loop of the fabric is circulated throughout the dye
bath by impinging the dye bath liquor against
the fabric in a venturi nozzle. Care must be taken during the
dyeing process to obtain a uniform distribution of
dye on the fabric, referred to as leveling.
Since acid dyes are negatively charged, the dyes are attracted
to positive dye sites appearing in the targeted
substrate. With respect to nylon, positive dye sites can be
created by exposing the free amino groups
contained within the polymer matrix to an acid. In particular,
when exposed to acidic conditions, the amino
groups are activated by protonation and become positively
charged and cationic. Once positively charged, the
acid dyes are strongly attracted to the cationic sites.
In general, acid dyes have a high affinity for protonated
polyamide materials, meaning that the dyes have a
strong tendency to quickly bind to the polymer. Unfortunately,
however, once in contact with the cationic
polymer surface, acid dyes have a tendency to poorly diffuse
into the polyamide. In other words, acid dyes
exhibit such a high rate of strike that they do not diffuse
evenly into polyamides. Thus, if the dye is absorbed
by the polymer too quickly, the polyamide material can absorb
the dye unevenly and not exhibit a constant
shade or color. In such cases, anionic leveling agents act by
competing for the dye sites and are mainly used to
counter-act fiber-oriented unlevelness due to physical and
chemical irregularities in the fibre.
Matlevel DLN liquid due to its substantivity for nylon fibre,
enforces a slow and even rate of absorption which
results in excellent penetration and level dyeing.
Anionic leveling agents act by competing for the dye sites and
are mainly used to counter-act fibre-oriented
unlevelness due to physical and chemical irregularities in the
fibre. As acid dyes are negatively charged,
cationic leveling agents form complexes with acid dyes and may
precipitate when used alone. Hence in order
to counteract both the types of unlevelness it is necessary to
use amphoteric leveling agent.
Matlevel DLMC is an amphoteric leveling agent which, due to its
substantivity for 1:2 metal complex dyes
enforces a slow and even rate of absorption which results in
excellent penetration and level dyeing.
Leveling for acrylic dyeing :
For acrylic dyeing, basic dyes, also called as cationic dyes are
used by exhaustion techniques. Basic dyes being
cationic in nature, it is advisable to use a leveling agent that
will compete with it. i.e it must be a cationic
retarder rather than a leveling agent. This is because, in
addition to having an effect on the rate of dyeing,
cationic retarders also assist migration to an extent that
depends on the fiber and the substantivity of the
dyes. They tend to have higher diffusion rates than dyes and to
be adsorbed at lower temperatures (65-70C)
Acrylic fibres vary significantly in the number of anionic sites
available for sorption of cations, but level dyeing
accrues when the number of cations in the system (retarder as
well as dyes) is just enough to saturate the
anionic sites in the fibre. This means more retarder will be
needed for fibres of high saturation value and for
paler shades.
Matlevel CAR liquid due to its substantivity for acrylic fibres
is most suitable for dyeing of all types of acrylic
fibres by any of the normal methods. While Matlevel CAR liquid
retards the build up of cationic dyestuffs it
does not block the fibre to any appreciable extent, so the bath
is well exhausted during the boiling process.
