Why use Fluorescent Whitening Agents?
Why use Fluorescent Whitening Agents?
Fluorescent Whitening agents are also called Optical brightener. These additives are used in order to:
- Brighten colors
- Mask natural yellowing of plastics
- Improve initial color
- Get brilliancy of colored or black pigmented articles
These Fluorescent whitening agents work via a fluorescent mechanism which absorbs light in the UV spectrum
and emits light in the blue region of visible spectrum to yield a brighter and fresher
appearance.
Some of the suggested applications of fluorescent whitening agents include:
- Molded thermoplastics
- Film and Sheets
- Fibers
- Adhesives
- Synthetic Leather
Let's learn more about Fluorescent whitening agents, mode of action and benefits in detail...
Mode of Action
Mode of Action
Optical brighteners or fluorescent whitening agents (FWA) are colorless to weakly
colored organic compounds that in solution or applied to a substrate absorb ultraviolet
light and re-emit most of the absorbed energy as blue fluorescent light between
400-500 nm.
Material that evenly reflect most of the light at all wavelengths striking their
surface appear white to the human eye.
-
Natural fibers, for example, generally
absorb more light in the blue region of the visible spectrum ('blue defect') than
in others because of impurities (natural pigments) they contain. As a result,
natural fibers also have this yellowish cast.
- Synthetic fibers also have this
yellowish cast although not as pronounced.
The whiteness in substrate can be improved by :
- Increasing reflection
- Compensating the blue defect.
Before the advent of FWA, the common practice was to apply small amounts of blue or
violet dyes to boost the visual impression of whiteness. These dyes absorb light
in the green-yellow region of the spectrum, thereby reducing lightness.
But, since at the same time they shift the shade of the yellowish material towards
blue, the human eye perceives an increase of whiteness. Unlike dyes, FWAs offset
the yellowish cast and at the same time improve lightness because their bluing
effect is not based on subtracting yellow-green light, but rather on adding blue light.
FWAs are virtually colorless compounds which, when present on a substrate, absorb primarily invisible ultraviolet light in the 300-400 nanometer (nm) range and re-emit in the visible violet-to-blue fluorescent light.
This ability of FWAs to absorb invisible short wavelength radiation and re-emit in the visible blue light which imparts a brilliant whiteness to the light reflected by a substrate, is the key to FWAs effectiveness.
Absorption (A) of light quanta by the brightener molecules induces transitions
from the singlet ground state S0 to vibrational levels of the electronically
excited singlet states (S1).
Brighteners in the S1 state are deactivated by several routes. Fluorescence
results from radiative transitions to vibrational levels of the ground state
(F).
Deactivation processes competing with fluorescence are mainly non-radiative
deactivation to the S0 state (IC) and non-radiative transition to the triplet
state (intersystemcrossing, ISC).
The efficiency of fluorescence is measured by the quantum yield:
The quantum yield (Φ) = Number of quanta emitted / Number of quanta absorbed
It is determined by the relative rates of fluorescence emission and the competing
processes. When fixed in solid substrates, brighteners fluoresce with high quantum
yields.
Energy Diagram of Optical Brighteners and Transitions
Polymer Substrates for FWAs
Polymer Substrates for FWAs
FWAs are effective in a variety of polymer substrates such as engineering plastics
(e.g. polyesters, polycarbonate, polyamides and acrylics) thermoplastic polyurethane,
polyvinylchloride, styrene homo- and copolymers, polyolefins, adhesives, and other
organic substrates. Main applications include:
- Fibers
- Molded articles
- Films and sheets
The effectiveness of a fluorescent whitener is dependent upon the type of substrate,
processing conditions and possible interactions with other components in the
formulation such as white pigments or UV absorbers. In general, fluorescent whiteners
are effective at very low concentrations.
Titanium dioxide pigments (TiO2) absorb light in the same UV wavelength range as
fluorescent whiteners and thus generate lower whiteness degrees.
Fluorescent Whitening Agents in Flexible PVC
Anatase type titanium dioxide pigments absorb approximately 40% of the incident radiation
at 380 nm, while rutile type titanium dioxide pigments absorb about 90%.
In the flexible PVC sample, a brilliant white is obtained using only small concentrations
of FWA with anatase titanium dioxide. When rutile types are used, a slightly
reduced whiteness is noted at equal concentrations. The two figures bellow illustrate the
benefits of FWA.
Concentration dependency of whitening effect in Flexible PVC
Light Fastness of whitening effect in Flexible PVC
Fluorescent Whitening Agents in PET Fibers
An essential criterion for the technical suitability of Fluorescent Whitening Agents is its light fastness
in the substrate. The stability of brightening agents in PET fibers is
shown in the figure below:
Light Fastness of whitening effect in PET fibers
Find Suitable Optical Brighteners / Whitening Agents
View a wide range of optical brighteners / whitening agents available today, analyze technical data of each product, get technical assistance or request samples.