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Contents
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GENERAL
What are FWAs?
Fluorescent Whitening Agents (FWA) are molecules designed
specifically to increase the perceived whiteness of a given substrate. They are also known as Fluorescent Brightening Agents (FBA), Fluorescent Brighteners (FB), Optical Brighteners (OB), Optical Bleachers or
Fluorescent Bleachers.
What is the mechanism of action of a FWA?
A FWA is a fluorescent molecule i.e. it
absorbs light in certain region of the wavelength region (typically 340-360 nm) and emits it in a different one (mainly 430 to 460 nm). Fluorescence is thus blue light that is added on a (normally yellow) substrate
increasing the perceived whiteness. FWA are characterized by its absorption and fluorescence maximum, its quantum yield and its affinity for a substrate.
How many types of FWA do exist?
Optical brightening started by observing that
washing bleached cotton with certain types of chestnut imparted washed goods a whiter appearance; the active compound is a natural coumarin that shows a blue fluorescence. Many other fluorescent molecules (also
coumarin derivatives) are applied as fluorophores in FWA, doubtless stilbene derivatives are the ones with highest application volumes. One distinguishes FWA according to affinity, solubility, quantum yield, and
fluorescence shade (reddish, neutral or greenish).
Is there a classification for FWAs?
Molecules used as Fluorescent Whitening Agents are registered and classified with the Colour Index, since they are regarded as a particular form of
dyestuff. There exist more than 400 molecules registered as FWA, their code can be found with the Colour Index as C.I. Fluorescent Brightener.
Do all FWAs show the same fluorescence?
One must bear in mind that the perceived
whiteness achieved by using fluorescence is based on an additive color mixing principle, where the reflectance from the substrate is mixed with the fluorescence of the FWA: the observed whiteness will depend on the
nature of these two light sources. For this reason there are FWAs that show fluorescence leading to neutral whites (dominant wavelength at 470 nm), reddish whites (dominant wavelength below 470 nm) and greenish
whites (dominant wavelength above 470 nm). The selection of a particular FWA is made considering the target whiteness and the reflectance of the substrate (that includes the shading agent).
What are the advantages of shading?
Shading should be strictly used to obtained
whites with specific and predetermined tint, in this respect shading can be successfully applied to production advantage. In practice however shading is used as a cheap and easy way to increase whiteness. While this
is true during assessment, it introduces a large amount of metamerism, sometimes to such extent, that production may be limited to narrow specifications and small number of recipes.
How can different materials be brought to match the same white?
Matching white
colors is already a difficult task, this is increased by materials of different nature. The first step is to decompose the perceived whiteness into its components in spectral terms. Next the different types of
illuminant must be specified in order to assess the degree of metamerism of the selected substrate. After these steps have been completed the final match in terms of shading agent and fluorescence can be carried
out; in numerous cases the match is quite limited, in others it is possible for just one illuminant, in worst case it cannot be done at all. Matching substrates of different nature imposes an additional limit, which
sometimes is impossible to comply with.
What is the greening effect?
FWA molecules fluoresce fully only under isolation
conditions. As a result of overloading, poor application or wrong conditions it may occur that FWA molecules build agglomerates, where fluorescence is quenched and/or agglomeration induces a shift in fluorescence
maximum resulting in less, green fluorescence. Depending on the nature of the origin, production conditions can be changed to eliminate greening; already produced material can be normally reverted to original
whiteness.
What is the level of the light-fastness of FWA?
Light-fastness properties of FWA
will depend on the particular chemical structure of the family under consideration. While some molecules show very high light fastness (for example for plastic applications), other families (notably stilbene-based
ones) show only fair values. Unfortunately there is no established norm method for judging light-fastness of FWA; since perceived whiteness is the sum of different contributions, where fluorescence is just one of
them, one should consider measuring light fastness of FWA under isolated conditions. In general it must be said, that substrates have in general similar, in some cases poorer light fastness as the FWA itself.
What is indoor whiteness?
While traditionally whiteness has been assessed instrumentally for D65 conditions (outdoor daylight) samples are normally viewed under conditions with much less amount of UV, although the spectral distribution of the incoming light may be similar to daylight (for example glass windows filtrate out a large part of the the UV radiation from the sun). This fact limits considerably the correlation of instrumental and sensorial assessments, also because current visual assessment booths do not have a device for adjustment and control of the UV amount. To overcome this difficulty the term indoor whiteness has been coined that is the instrumental whiteness determined under illuminant C conditions, in other words, under lesser amount of UV while the visible part resembles daylight.
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Instrumental Measurement
Have instruments for whiteness measurements special features?
In general yes,
instruments for whiteness must have a Xenon lamp or at very least a light source with a spectral profile close to D65 distribution not only in the visible but also in the UV region; as a general rule the light source must present a higher irradiation UV level (than D65) such that this amount can be regulated down through a movable filter to D65 conditions. Recently with the appearance of the concept of indoor whiteness a spectral distribution close to illuminant C is targeted, the amount fo UV required is lower than D65 conditions, such that it can be reached by a QHT source.
How should instruments for whiteness be calibrated?
Instruments for whiteness
require additionally to black and white calibration a further step that sets up or control (during daily calibration) the amount of UV of the light source; the latter is done with a plastic fluorescent tile provided
with the instrument and it functions as a working standard of the UV level, its values are determined during the set-up step of the instrument. Summarizing the whole calibration procedure has three steps: black,
white and UV calibration.
What standard should be used for calibrating fluorescence?
There are three groups of
institution offering fluorescent reference calibration samples for setting up the scale for proper assessment:
- Paper samples from STFI and TOL
- Textile samples from Hohensteiner Institute
- Textile and (newly) polyester samples form TITV
Much effort has been (and is being) invested in achieving a good correlation between all standards (for details see chapter on Calibration)
How often should instruments be calibrated?
Modern instruments are in general quite
stable such that a calibration on a daily basis can be advised, however some points must be mentioned:
i) instruments must be calibrated after a warm-up period of at least 30 minutes; electronic materials are
sensitive to temperature variations, for this reason instruments must be in a (nearly) thermal equilibrium as a condition for the calibration to be valid over a period of some hours. While some instruments are in
air-conditioned environments still the rule of 30 minutes warm-up must be observed. Those instruments equiped with flash-lamps can stay turned on for long periods of time with no detriments on the operative
lifetime; while this is a good idea to maintain a long term thermal equilibrium, instruments must be calibrated at least once a day.
ii) electronic components are sensitive to voltage variations; many
instruments are in laboratories close or whithin industrial complexes, where voltage experiences large variations during the day as result of industrial activity. It is advisable (also in general) to connect
instruments to stabilized power supply otherwise calculated numbers may present large variations as the calivbration looses validity.
iii) the white tile for white calibration is a working secondary standard, its
validity is time limited and depends on the use and storage conditions. Working standards should be regularly refreshed to assure stabilkity of measured results, it is convinient to have a reference secondary
standard to check and refresh the working standard.
iv) lifetime of set up of the UV level is limited normally to at most one year; at least once a year the UV level should be fully recalibrated, or earlier if
the lamp has to be changed. Check of the UV level must be conducted at least once a week.
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PAPER
What is brightness?
Brightness is a weighted integration of reflectance values
around a narrow spectral region peaking at 460 nm. Originally developed for assessing the progress of bleaching process, it has been used for characterization of appearance of pulps. Since its value has been related
to the base white, its value is used often (and falsely) as a measure of whiteness. The introduction of fluorescence as a whitening component of paper outdates the use of brightness as characterization of paper; in
general brightness values are false when fluorescence is present, and they are useless if fluorescent, shaded paper is assessed. There have been many attempts to redefine brightness into a no non-sense number,
however without much success; some of the redefinition include:
i) Brightness (+UV): is brightness measured without any filter between light source and sample. Since the intensity of UV is a function of the lamp age and manufacturing, UV levels are not controlled and the number means nothing.
ii) Brightness with controlled UV intensity (for example Brightness D65): is brightness measured with regulated UV intensity corresponding to daylight or more recently to C-illuminant. Since the number is a mixture of true reflectance and fluorescence, it has no information and depends strongly on the calibration of UV level.
iii) Brightness (-UV): is brightness measured with a 400 nm cut-off filter. Since the fluorescence is not totally suppressed by this filter, measured brightness is still a mixture of true reflectance and fluorescence and thus inadequate to characterize samples.
iv) Brightness difference (with and w/o UV): is an attempt to estimate the amount of FWA present in the sample. If brightness with controlled UV intensity is used, brightness can be used in closed system for assessing FWA levels of same paper production. As stated before, fluorescence is not totally suppressed by (-UV) conditions and cannot be used in absolute manner. Moreover, since the amount of fluorescence is strongly dependent of UV absorption by substrate, correlation is possible only within certain ranges of uncertainty.
Bear in mind that brightness must be measured according conditions stated in norms (for example T-542, T-465, ISO 2469, etc.), and never use a cut-off
filter higher than 400 nm.
Does brightness express perceived whiteness?
Brightness is not a color-related
parameter and as such it is not related in any way to the appearance of a sample. If applied in a closed-circuit system, it can be used to estimate the level of bleach of a pulp; it should never be used for
expressing or quantifying appearance in an absolute way.
What types of FWA are used in paper?
In general there are two families for paper
applications: stilbene and di-stiryl-biphenyl types. Traditionally the first family is better known and is divided according to the number of sulphonic acid groups present in the molecule. One distinguishes di-,
tetra- and hexa-sulpho FWA and their field of application is related to their solubility and affinity. The larger the number of sulphonic acid groups the higher the solubility and the less the affinity. Di-sulphonic
types are better for applications in furnish, while hexa-sulphonic types are better suited for size press applications.
Why do I need a co-binder in coating applications?
FWA develop fluorescence only
when their molecules are fully isolated; co-binder bonds FWA molecules to itself avoiding the formation of crystals and thus developing fluorescence. For this reason fluorescence level is dependent on co-binder
concentration.
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TEXTILES
How are FWA applied?
FWA are similar to any dyestuff applied in textile industry,
and they are applied using the same techniques. There are specific FWA for different substrates; one distinguishes cellulose and plastic applications. Cotton applications are dependent on the affinity of FWA for the
fiber, but also environment conditions (pH, temperature, etc.) must be observed. Since whiteness of textiles are found at the high end, especial attention must be paid to the degree of development of the FWA i.e. to
the particular build-up curve (whiteness gain vs. concentration of FWA) in order to estimate the limit concentration for the application sought. As textile applications are at the limit of performance of FWA,
special attention must be paid to the level of base white, in case of cotton materials to the degree and extension of bleaching; very small losses in base white result in large losses of final perceived whiteness.
Can whiteness be increased by shading?
In general this is always possible, the
limits of shading are uncertain and not very well defined because the taste of the consumer will be indicative (and mandatory) for the final whiteness. There is not such as best whiteness, here again beauty is in
the eye of the beholder. When shading consider to use light- and wash-fast dyes.
How application conditions affect whiteness?
By careful selection of FWA for a given
application most of possible problems can be ruled out, this includes resistance to particular bleaching agents. While FWA are quite stable to temperature, special attention must be paid to pH ranges, since chemical
structures are sensitive to pH variations. A look into specification sheet will help to avoid glitches.
What is the effect of additives on whiteness?
In general additives are developed
avoiding incompatibilities, as a general rule however, everything that is yellow or may develop (by natural means or decomposition) into yellow colored compounds will affect greatly the action and performance of the
FWA.
Can plastic fibers be optically brightened?
There is a variety of FWA that diffuse
into plastic under certain conditions imparting fluorescence and increasing perceived whiteness. One must bear in mind however that its action can be limited by optical properties of substrate in the UV region; in
many cases plastic fibers contain Titanium dioxide and/or UV absorbers that will limit the performance of the FWA.
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DETERGENTS, COSMETICS AND HOUSEHOLD ARTICLES
Do FWAs improve whiteness of washed goods?
FWAs used in detergents are water-soluble
compounds that show affinity only for cellulose materials (for example cotton). Low bleached cotton clothes or those appearing natural (ecological look) will be certainly affected by washing, since every time fibers
will absorb FWA thus gaining fluorescence and appearing whiter. In general however, textiles contain already high levels of FWA such that no much improvement can be expected. The main reason for applying FWA in
detergents lies on the fact that through abrasion, light exposure, bleaching, etc. FWA molecules are attacked chemically and destroyed or just washed out, this results in an irreversible loss of whiteness that is
restored through adsorption of FWA from detergent.
Are colored fabrics affected by FWA in detergents?
Light shades will be affected by
the blue fluorescence of the FWA, especially yellow ones, where a noticeable shade shift will result. Blue shades will be reinforced and will gain lightness giving them a more brilliant appearance, this is also true
for some other medium and darker tones.
How are FWA integrated in detergent powders?
There are many ways to introduce the
FWA into detergent powders, the particular one depending on manufacturing conditions and also if FWA is used to improve powder appearance. If powder is just a vehicle for FWA it can be directly applies as powder,
liquid or slurry, as a compound during dry mixing or sprayed together with final components (there some specific incompatibilities with certain perfumes). If used also to improve appearance it should be incorporated
before drying by extensive mixing with powder/liquid components; especial attention must be paid to moisture levels of final powder and to drying conditions, otherwise poor fluorescence or even greening of final
powder may result.
Why does a greenish appearance result after repeated washings?
Overload of the FWA
results in agglomeration of the molecules and in a loss and shift of maximum of fluorescence. In general this appear as a consequence of repeated washing without exposing the fabric to stress i.e. light exposure,
soiling, mechanical use, etc. In general levels of FWA in detergents are too low to expect an overload under normal conditions.
Are there other reasons for observing greening?
Some detergents (notably in sun-belt
countries) contain small amounts of photo-bleaching agents, which are destroyed during sun exposition. Photo-bleaching agents are normally (deep blue) colored and impart shading to certain degree. If after repeated
wash cycles fabrics are not exposed (wet) to sunlight, photo-bleaching agent starts accumulation to the point that it is clearly visible. Exposing the fabrics (wet) to sunlight will return them to their original
white color.
Can FWAs be used to increase the perceived whiteness of soap bars, candles and other articles?
All principles for increasing perceived whiteness (as explained for cellulosic products) are fully valid for all objects regardless its nature and composition. In case of
soaps or wax candles due attention must be paid to the nature of the FWA for proper application: FWA molecules must be dissolved and distributed homogeneously within the mass in order to achieve the desired effect.
It must beared in mind that most of the materials used in soaps (or candles) are quite transparent and present thus a low base white; the base white must be built first by applying an appropiate pigment (however
other than Titanium dioxide due to its strong UV light absorption) that is able to increase appreciably the opacity.
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PLASTICS
How are FWA applied to plastic materials?
Plastics are in general more or less
colorless and transparent materials. In order to make them to appear solid pigment charges are used to increase opacity and give them a particular color. Using white pigments (normally mineral ones) one is limited
to the whiteness intrinsic to the pigment itself, there is no manner to break this upper limit of reflectance unless one applies the techniques of shading and optical brightening. FWA are added directly to the
plastic mass (for example during manufacture of master batch) and it develops quite easy. Special attention must be paid to the highest concentration attainable without getting into greening; posterior dilution
however corrects most of greening problems.
Are there incompatibilities with plastic additives?
While in general there are no
chemical incompatibilities, there are many optical incompatibilities one should bear in mind when applying FWA. Titanium dioxide is a preferred material for augmenting opacity, but it has the property to absorb
light in the UV region to a large extent; since FWAs need UV light to develop fluorescence its performance deteriorates heavily in presence of Titanium dioxide; just 1 to 2 % of Titanium dioxide are enough to reduce
the performance of FWA to almost the level of disappearance. A further additive with similar effect is the use of UV absorbers as preservers against aging of the plastic; one must consider here using the FWA as a UV
absorber. Every additive that is yellow colored (plasticizers, enhancers, etc.) or results into yellow colored compounds will have a detrimental action.
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