Optically active additive

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Optically active additive (OAA) is an organic or inorganic material which, when added to a coating, makes that coating react to ultraviolet light. This effect enables quick, non-invasive inspection of very large coated areas during the application process allowing the coating inspector to identify and concentrate on defective areas, thus reducing inspection time while assuring the probability of good application and coverage. It works by highlighting holidays and pin-holes, areas of over and under application as well as giving the opportunity for crack detection and identification of early coating deterioration through life. The use of optically active additives or fluorescing additives is specified in US Military Specification MIL-SPEC-23236C.^[1] The use of OAAs and the inspection technique is described in the SSPC document Technology Up-date 11.

Inorganic versus organic

There are two common types of optically active additives available commercially: inorganic and organic. Inorganic OAAs exhibit large particle sizes of 5 to 10μm (no mobility), are light-stable, can have a choice of colours as shown in image above, are useful in a wide range of coating systems, and are more expensive. Some inorganic OAAs can exhibit some degree of afterglow aiding inspection.

Organic OAAs require low addition levels, are soluble in solvents and organic liquids (mobile), are blue under UV (emitting the same colour as lint, oil, grease etc.), can fade quickly, have limited use in a range of coating systems and are less expensive. They are also indistinguishable from old tar epoxy-type coatings still seen on some structures and vessels. Organic OAAs have no afterglow.

Physics of optically active technology

If a single

, the energy of each photon is given by multiplying its frequency in cycles per second by a constant (Planck's constant, 6.626 x 10⁻²⁷ erg seconds). It follows that the wavelength of a photon emitted from a luminescent system is directly related to the difference between the energy of the two atomic levels involved.

In terms of

to produce a specific colour.

References

Sources

• Buckhurst and Bowry. "An optically-active coating system for coating ballast tanks." Paper T-44, presented at the Paint and Coatings Expo 2005, SSPC, Pittsburgh, 2005
• Department of Defense Single Stocking Point for Specifications and Standards (DoDSSP), Standardisation Document Order Desk, 700 Robbins Avenue, Bldg 4D, Philadelphia, PA 19111–5094
• Technology Update 11 – Inspection of Fluorescent Coating Systems, SSPC, Pittsburgh October 2006 [1]
• Planck, M. "On the law of distribution of energy in the normal spectrum", Annalen der Physik, 4, 553, 1901
• Paint&Coatings.com; 28 November 2000. "Scottish company develops additive to revolutionize coating inspection process" [2]