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Optical Brighteners

The tendency of many organic materials to yellow is due to degradation by UV or visible light. The breakdown products absorb blue light, giving a yellow appearance in daylight.

Products that absorb UV light and then transmit it in the blue range can be used to counter this effect. They also make whites appear ‘brighter’ to the human eye.

Optical brighteners are designed to brighten colors or mask yellowing in lacquers, paints, inks, plastics, photo-processing solutions and fibers. They work via a fluorescent mechanism, absorbing light in the UV spectrum and emitting it in the blue range of the visible spectrum, resulting in a brighter, fresher appearance.

Tinopal® optical brighteners will actually appear brighter than the light which strikes them! With such qualities, it is not surprising that optical brighteners are found in numerous applications. These include:

  • Clear and pigmented lacquers
  • Paints
  • Molded thermoplastics
  • Films and sheets
  • Printing inks
  • Synthetic leather
  • Ultraviolet tracer
  • Photo processing solutions
  • Adhesives
  • Fibers

Xymara™ Markers are BASF's first range of inorganic UV fluorescent pigments. They provide high-quality, high value-added effects for applications such as :

  • Thin layer chromatography (TLC)
  • Overprint varnishes
  • Printing inks

Principles of optical brightening

Optical brighteners are colorless or slightly colored organic compounds that, in solution or applied to a substrate, absorb ultraviolet light and re-emit most of it at between 400 and 500 nm as blue fluorescent light (Figure 1).

Figure 1: Example absorption and fluorescence emission curves

Solvent DMF
Concentration

4.4 mg/l

Layer thickness 1 cm
Absorption maximum 375 nm
Fluorescence maximum 437 nm
Quantum yield 0.81

Figure 2 illustrates the processes involved in light absorption and fluorescence by optical brighteners. Absorption (A) of light quanta by the brightener molecules induces transition from the singlet ground state, S 0, to vibrational levels of the electronically excited singlet state, S 1.
Brighteners in the S 1 state are deactivated by several routes. Fluorescence results from radiative transition to vibrational levels of the ground state (F).
Deactivation processes competing with fluorescence are mainly non-radiative to the S 0 state (IC) and non-radiative to the triplet state (intersystem crossing, ISC).

The efficiency of fluorescence is measured by the quantum yield Φ:

F = 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 (ca. 0.9).

Figure 2: Energy of optical brighteners and transitions
A = absorption                   ISC = intersystem crossing
F = fluorescence               S = singlet state
IC = internal conversion     T = triplet state

Materials 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 range of the visible spectrum (‘blue defect’) than in others because of the impurities (natural pigments) they contain. As a result, natural fibers take on an unwanted, yellowish cast. Synthetic fibers also tend to yellow, although not as much (Figure 3).

Whiteness in substrates can be improved by (1) increasing reflection (reflectance) or (2) offsetting the blue defect. Bleaching does both of these to some extent, but invariably leaves behind part of the yellowish cast. Even the most thorough bleach cannot remove all traces of yellowing.

Before the advent of optical brighteners, it was common practice to apply small amounts of blue or violet dyes (called ‘bluing’) to boost the visual impression of whiteness. These dyes absorb light in the green-yellow range of the spectrum, thereby reducing lightness. However, since they shift the shade of the yellowish material toward blue at the same time, the human eye perceives increased whiteness.

Unlike dyes, optical brighteners 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. Optical brighteners are virtually colorless compounds that, when present on a substrate, primarily absorb invisible ultraviolet light in the 300-400 nanometer (nm) range and re-emit it as visibleviolet-to-blue fluorescent light.

This ability of optical brighteners to absorb invisible short-wave radiation and re-emit it in the visible blue light range, imparting brilliant whiteness to the light reflected by a substrate, is the key to their effectiveness.

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