What kinds of chemical bonds attach dyes to fibers?
This isn't just a trivia question. The types of bonds
directly determine the properties of the dyes. The most
permanent, wash-fast dyes are the most tightly attached to
the fiber molecules. In addition, the requirements for
forming the chemical bonds are what determine the
ingredients and method for each recipe for dyeing.
Fiber Reactive Dyes
Fiber
reactive dyes, such as the dichlorotriazines (Procion MX
type dyes), are attached to the fiber by the same strong
covalent bonds that hold the atoms within the dye molecule
together. The dye actually becomes one molecule with the
fiber molecule to which it attaches. Covalent bonds are
formed by electron sharing between atoms.
Direct Dyes
Direct
dyes, including that portion of "all
purpose" dyes that is left in cotton that has been dyed
with them,
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.
Disperse Dyes
Disperse
dyes, used for synthetic fibers such as polyester, work by
being vaporized by the heat of an iron or heat transfer
press and then condensing onto and into the
fiber. (They can also by induced to migrate into the fiber
by boiling with a special carrier chemical.)
Unsurprisingly, such loosely associated dye may rub off,
though not visibly so. The fact that the dye can rub off on
the wearer makes this class of dye more allergenic than
other types of dye, though this is a problem for only a
minority of people. Such people are happier wearing
natural fibers that have been dyed with fiber reactive dye.
Acid Dyes
The attachment between acid
dyes and the protein fibers that they join to are the
most complex of all.
Proteins are made of up to twenty different amino acids,
each of which has a different side chain. At different pHs,
different dyes can form rather strong hydrogen bonding to
various of these side chains.
Many acid dyes contain a sulfonic group, or, in some cases,
a carboxylic group, which can form a strong 'salt linkage'
to a basic group in the wool molecule. (See Ingamells, under
"Further Reading", below.) A salt linkage is an ionic bond
between fixed ions of opposite charges, due to Coulombic interaction.
The strength of this bond is considered to be responsible for the
washfastness of good acid dyes.
Note that certain specialized fiber reactive dyes are
also used for wool, though the fiber reactive dyes commonly
used for cotton, such as Procion MX dye, is said to not react
appreciably with wool under acid conditions; instead, the
types of fiber reactive dyes used on cotton can be used
simply as acid dyes with wool, even if the reactive groups
on the dye have 'gone bad' by reacting with water. (The
alkaline conditions normally used with Procion MX dye on
cotton will damage wool, though they work fine on silk.)
Vat Dyes
Vat
dyes, such as indigo, are trapped within the fiber by having
been converted to a form that is insoluble in water. The vat
dye is insinuated into the fiber while in a water-soluble
form. This is a purely physical form of attachment, and not
a chemical bond at all. Vat dyes are very wash-fast, but
have the opposite reputation due to ring-dyeing. Ring dyeing
is what happens when the dyer fails to make sure that the
dye thoroughly penetrates the fiber shaft, but leaves only
the surface of each fiber coated, so that even a little bit
of wearing removes the dyed layer. Normally this is a sign
of bad dye procedure, but the fading of blue jeans due to
abrasion is generally appreciated, as it implies greater age
for a garment than is actually the case.
Naphthol Dyes
Like Vat dyes, Naphthol dyes are held in the fiber in a
water insoluble form. The technique is quite interesting:
any of a range of diazo salts are combined on the
fabric with any of a
range of naphthols (azoic coupling agents). Their reaction results in the
formation of a water insoluble dye, typically of a very
different color than either of the two components. Various
combinations result in different colors. These dyes are
widely used in batik producting in southeast
Asia; the use of wax in batik requires that dye reactions
not involve heat, so both naphthol dyes and cool water fiber reactive
dyes are suitable for this purpose. Naphtol dyes are less
expensive than many fiber reactive dyes, and peculiarly
suited to painting different colors immediately adjacent to
one another on fabric. Unfortunately, naphthol
dyes are quite toxic, and probably significantly
carcinogenic, and are thus not considered suitable for use
at home or in the artist's studio.
Natural Dyes
Natural dyes are, in most cases, acid dyes, which is why
they will bond to wool and other animal fibers, but not to
cotton. A way to improve attachment to wool, and to make
some weak attachment to cotton possible, is to use metal
ions, sometimes including toxic heavy metal ions, to form a bridge
between the dye and the fiber; this is called mordanting. Cotton is typically mordanted with tannins in addition to metal ions
Indigo is a natural dye, but it is quite different from
other natural dyes, in that it is a member (the very first
member) of the class of dye known as vat dyes, see above.
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Last updated: November 5, 2007
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"All About Hand Dyeing" site first created: July 10, 1998