Direct dye
A dye can generally be described as a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fiber.
Direct Dyes are commonly used on
cotton fibers. These Dyes are mixed in all purpose dyes along with the Acid
Dyes. The color of Direct dyes on cotton fibers is not bright in respect to
other dyes. The wash fastness of these dyes is not very good. The only
advantage of these dyes is that the light fastness is little more better. Light
fastness means the resistant capacity against fading in light. But, this is
also in few cases only. One more benefit of using Direct
Dyes is that these can
be used in the same dye
bath with the Acid
Dyes.
If anyone wants to get desired result by mixing Direct Dyes with unmixed
cellulose then he/she will have to take these direct dyes in their pure form
unless the use of these dyes become worthless. It is preferable to use direct
dyes on the cellulose fibers. But if these dyes are accompanied with 'reserving
agents' then these can be also used on stain silk and wool. But, unfortunately
the 'reserving agent' is not available to home dyers.
Few Direct Dyes, like Direct Orange 39 and Direct Blue 86 are considered as
having very high Light fastness capacity. These two have a rating of 6 out of
8.
Direct dye, also called Substantive Dye, any of a class of colored, water-soluble compounds that have an affinity for fibre and are taken up directly, such as the benzidine derivatives. Direct dyes are usually cheap and easily applied, and they can yield bright colours. Washfastness is poor but may be improved by aftertreatment. Most packaged dyes sold for home use are direct dyes
Major
types of Direct Dyes : There
are two major types of Direct Dyes:
CATIONIC DIRECT DYES
The main structural features of cationic direct dyes are:
(a) the structural single and double bondings are extended.
(b) the molecular structure is planar, and
(c) positive charges are more than the negative charges.
Composition: Some common features of cationic direct dyes are
(a) extended conjugation, (b) planar molecular structure, and (c) an excess of positively
charged over negatively charged surface groups. In the Color Index scheme,
cationic direct dyes are classified along with Basic Dyes. (Note: When
papermakers refer to "basic dyes" they usually are referring to the
products of somewhat lower molecular size that are commonly used in coloring
mechanical fibers and other high-yield fibers. Such molecules are not
considered to be "direct," since they don't have strong affinity to
bleached kraft fibers.) The word "conjugation" implies an alternating
series of double and single bonds, e.g. -CH=CH-CH=CH-CH=. In a direct dye molecule the chain of conjugation
almost always contains two or more aromatic groups. Diazo groups, -N=N-, are
also present in many dyes. The function of the extended conjugation is to
achieve a suitably low energy for the promotion of the outermost electrons
between the bonding and antibonding states, p to p*. Depending on its
wavelength (or color), the energy supplied by a photon of visible light when it
is absorbed may be able to promote an electron from the bonding to the
antibonding state in the dye molecule. The color also can depend on substituent
groups on the molecule that tend to supply or withdraw electron density. For
this reason, ionic groups on a dye molecule can have a big effect on its color,
and shifts in electrolyte concentrations can affect the hue. The word
"direct" implies that the dye has a sufficient molecular size so that
it has a strong preference to remain on surfaces. In other words, the user can
add it "directly," without a fixative or mordant, and it will stick
to the surface. Cationic direct dyes have particularly high affinity for
anionic surfaces due to their content of cationic amine groups.
Function: Coloration of paper, adjustment of the tint of white paper, and matching of a paper's appearance to a standard agreed upon by the customer. Cationic direct dyes can be very valuable in cases where (a) the effluent water has to be colorless, or (b) where a high frequency of production changes between different colored products provides an incentive for keeping the white water free of residual dye.
Strategies for Use: The most notable issue in the use of cationic direct dyes is their tendency to "granite." In other words, they tend to irreversible stain the first surfaces with which they come into contact. This can lead to a speckled appearance of the paper product. To avoid this, users must use adequate dilution and add the dye at a point of good agitation. Cationic direct dye never should be added to white water, since it will stain the unretained fines. Likewise, it should never be added in a chest, since it will stain the fibers that happen to be near to the point of addition. The second critical rule is to avoid direct contact between a cationic direct dye solution and streams of highly anionic additives. For example, a mixture of cationic direct dye and conventional (anionic) direct dye produces an insoluble tar. All direct dyes usually are delivered to a paper mill in the form of concentrates. These may be batch-diluted, though it is becoming more common to meter the concentrated dyes continuously with gear pumps or reciprocating positive displacement pumps. Rather than adding the concentrate directly to the process, the stream is combined with a relatively large amount of pure water (no fibers or fines). The dilution ratio is not critical. When producing deep shades of paper (highly colored) it is common to use a combination of cationic direct and (anionic) direct dyes, added at different points in the process. In such a situation the cationic dye can serve as a fixative for the anionic dyes, usually added later.
Cautions: See MSDS. Cationic direct dyes can cause objectionable and persistent stains of clothing (both yours and the paper machine's
ANIONIC DIRECT DYES
Anionic Direct Dyes are used for coloring papers, controlling tint and shade and correction of two sided paper color. The main structural features of the Anionic Direct Dyes are: the planar vary from each other, the structural single and double bonds are very much extended, and contains one or more than one sulfonate groups. These sulfonate groups make them soluble in the water.
Composition: The "direct dye" classification in the Color Index system refers to various planar, highly conjugated molecular structures that also contain one or more anionic sulfonate group. It is because of these sulfonate groups that the molecules are soluble in water. Though most direct dyes still can be obtained in powder form, it is increasingly popular to receive them as liquid concentrates. The advantage of concentrates is that they are easy to handle and meter. The disadvantage is that the surfactants and co-solvents needed to keep the dye concentrates stable may interfere with retention and sizing in the case of very deeply colored grades.
Functions: Coloration of paper, tint and shade control, correction of two-sided paper color.
Strategies for Use: Where you add the dye depends on such factors as the tintural strength of the desired paper color, the dye fixation strategy, and the color control strategy. Those responsible for process control will always advocate adding either all or some of the dye to the thin stock, within seconds upstream of where the stock comes out of the headbox. This practice can eliminate most of the time delay that is always a headache for online control strategies. But thin-stock addition might not give as high retention as addition to the thick stock. In some cases, especially in the past, it has been common to produce very deeply dyed products by adding the dye in a chest, giving the dye a long time to penetrate into the fiber cell walls. Other ways to achieve high retention of dyes include the use of highly cationic polyelectrolytes, alum, or polyaluminum chloride. The best efficiency of the colorant usually is achieved by adding the fixative ahead of the dye. However, the reverse order may give better rub-fastness or water-fastness. Another promising strategy is to add some cationic direct dye at a separate addition point upstream of the addition point from the (anionic) direct dye.
Cautions: The main hazards in handling of dyes are aesthetic; it is easy to stain hands and clothing. Consult the MSDS for other hazards.
How
direct dyes bond to the fiber
Direct dyes are only loosely associated with the fiber molecule through the property called substantivity, which is the tendency of the dye to associate with the dye without strong bonds. This substantivity is increased by increasing the size of the dye molecule, so direct dyes tend to be large. Small dye molecules tend to be bright, while large dye molecules tend to be duller (as there are more parts that can absorb additional wavelengths of light), so direct dyes are generally much less bright in color than fiber reactive dyes. Substantivity is said to result from a combination of the relatively weak Van der Waals forces and some hydrogen bonding.
Single dye bath
Direct dyes are applied in hot water, typically between 175°F and 200°F. They can be applied in the same boiling-water dyebath with acid dyes (whether for same-color effects, as in all-purpose dyes, or contrasting effects, as in the case of AlterEgo brand dyes)
Light fastness
Light-fastness means the ability of any fibers or plastic materials to resist the fading or degradation when exposed to sunlight or to ultraviolet light. In general term, light-fastness is to be measured by the time duration for which the printed digital image lasts without fading.
How to solve the wash fastness problem for Direct Dyes In most Direct Dyes, the wash fastness quality is poor. After every dyeing, these dyes bleed a little with every washing. This results in losing brightness and also damage other clothes which are washed in the same load. So, there is a process i.e. 'special after treatment' can be used to solve this problem. For accomplishing this process, a cationic bulking agent i.e. RETAYNE is used. This agent glues the dyes into the fibers. This process is used for dyeing fabric with Direct Dyes and these dyed fabrics are free from bleeding. |
Safety
Issues with Direct Dye
Most direct dyes are safe to use. As with all dyes, it is very important not to breathe the dye powder (or indeed any powdered substance), and direct exposure of the dyebath to the skin should be avoided by the use of gloves.
The hazardous carcinogenic direct dyes that are based on benzidine are no longer allowed to be used in direct dyes or dye mixtures sold in the United States. They were, however, sold in the US in all-purpose dyes before and throughout the 1970s. Benzidine-based direct dyes are still used industrially in some countries, but their use should be strictly avoided. The specific dyes to avoid are direct black 1, direct red 28, direct black 38, direct blue 6, direct green 6, direct brown 95, direct brown 2, direct blue 2, and direct black 4. More information on this issue is included near the bottom of my page on all-purpose dyes. See the US government document, Health Hazard Alert--Benzidine-, o-Tolidine-, and o-Dianisidine- Based Dyes.
Some o-dianisidine-based direct dyes are still being sold for household use in the US, in the form of dye-impregnated tie-dye color cords, and may be sold in other forms in other countries. They are still available for industrial use in the US and elsewhere. Be sure to request an MSDS from your dye supplier for each direct dye that you use, and avoid all dyes that are based on benzidine or o-dianisidine. Dyes that are made from these chemicals may break down to form them again, after they are absorbed into the body, so they are not safe to be exposed to. Both benzidine and o-dianisidine are suspected cancer-causing chemicals. A careful adult who understands safety precautions can probably use these dyes safely, but they should never be given to children to use, since children are less likely to follow all safety rules.
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Disadvantages of Direct Dyes |
- Few Direct Dyes have low light fastness.
- Many Direct Dyes are bland and dull in color.
- Direct Dyes provide duller color than the color provided by the fiber reactive dyes.
- The wash fastness quality is also low.
