What is OBA agents ?
Most of the textile materials, especially the natural ones, appear slightly yellowish of off white even after they have been bleached. They are not bright white. The reason lies in slight absorbance by these substrates in blue region of visible light which leaves these materials appearing as slightly yellowish. This can be seen in the following figure.
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Across
cultures, bright white has been regarded as signifying purity and hence there
have been efforts to achieve brighter whites. Attempts have been made to tint
white goods with blue pigments since bluish whites are preferred over whites
with yellowish or greenish tinge.
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To make
whites brighter, one needs to increase the reflectance of the bleached goods
further. This is possible with certain additives which absorb UV component of
solar radiation and emit in visible range (preferably in blue violet region).
Thus, the overall reflectance of the goods becomes higher than that of
bleached goods with a blue tinge to it, making it appear much brighter.
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The first
recorded evidence of OBA application happened in 1929 when Paul Krais applied
aqueous extract of aesculetin-6-glucoside (a naturally occurring OBA) on
linen.
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| Aesculetin-6-glucoside |
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These
agents have been known as Fluorescent Brightening Agents (FBA) or Optical
Whiteners (OBA). They are distinct from blueing agents in that while blueing
agents suppress overall reflectance, the OBAs actually enhance it.
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Mechanism
of OBA action-
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OBAs
are designed to absorb light between 340 to 380 nm (UV spectrum) & emit
it in the range 425 to 450 nm (visible spectrum).
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Effective
OBAs must absorb strongly in UV region and emit a large part of the absorb
energy in the visible region. OBA molecules have an extended conjugated bond
system as it is capable of strong fluoresce.
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In
the figure (colorants & auxiliaries-39 page no.-473) given below,
the ground and excited energy levels. (S1,S2……) are
shown. So S1,S2…… represent singlet states with paired
spins & T1,T2……represents triplet states with
unpaired spins.
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The
energy levels are further subdivided in to vibrational levels.
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When
an OBA is exposed to electromagnetic radiation, the electrons are transported
to one of the excited energy levels from the ground level. The energy state
and the vibrational level to which it is transported depends on the amount of
energy absorbed (E=hν ) within an energy level
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| Absorption and fluorescence processes |
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(S1,S2……).
The electrons lose the energy very quickly (10-12s) to come down
to the lowest vibrational level in existing energy level. The return of the
electron from the lowest vibrational level of an excited energy state to one
of the vibrational level in the ground state (S0) is known as the
fluorescence, and is accompanied by release of energy in the form of EM
radiation (emission) corresponding to the frequency given by the relation
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E = hν
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Since
the energy released by the electron during fluorescence is always lower than
the energy absorbed, the wavelength of emitted radiation is always higher
than that of absorbed radiation.
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Hence, if absorption occurs in UV region, the emission may occur in violet or blue region of visible spectrum. The difference between the wave number of absorption and emission is known as stokes shift. For maximum effectiveness, an OBA should absorb around 370 nm and emit around 430 nm for a stokes’s shift of around 60 nm. Depending on the nature of OBA, it can produce violet, blue or green hued whiteness. This depends on the wavelength/wave number in which the OBA is emitting. However the hue of the OBA can also change with the concentration level. With increasing concentration, the whiteness produced by an OBA increases initially. However at higher concentration it starts aggregating & a shift in fluorescence may be observed. A blue tinged fluorescence may turn in to greenish hue which is undesirable & rather than appearing white, the material may appear coloured. This is shown in figure (page-479) below.
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| Typical absorption and emission curves for an FBA |
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Chemistry
of FBAs
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Just
like dyes, different OBAs are suitable for different fibre types. Therefore,
OBAs for cotton, wool & silk are anionic in nature, for PET hydrophobic
and for acrylic, cationic.
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In
general OBAs for cotton have poor light fastness (1-3) meaning they need to
be applied over and over again to maintain the whiteness. For the same reason
white paper turns yellowish gradually due to destruction of OBA in it.
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OBAs for
cellulosic substrates
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Brighters
for cotton are anionic in nature and are applied in a way similar to the
application of direct dyes. A range of products varying in substantivity from
high to low is available. Low substantivity OBAs are suitable for application
by continuous method while the high substantivity ones are applied by exhaust
method. OBAs can be applied during bleaching or in a separate step after
bleaching.
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Since
fading of OBAs on cellulosic substrates is faster due to poor light fastness,
they need to be fortified frequently. This is generally done by formulating
them in washing soaps and detergents.
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The
most common OBAs for cellulosic fibres are based on triazine derivatives of
diaminostilbene disulphonic acid (DAST) with following chemical structure.
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