|HISTORY OF FLUORESCENT COLOURS :
In 1930′s, it has been experienced that certain dyes and resins combination produce brighter colours than normal colours and had the unique effect of “glowing” under ultra violet or black light. As the chemistry and manufacturing process improved, the areas of application expanded to advertising. Safety and promotional firms began to recognize the uniqueness of these bright colours and their special uses.
|Daylight Fluorescent Colours :
For example, fluorescent orange colour absorb the same orange band as the conventional, however it also converts the lower end of the spectrum and ultraviolet light into visible light of the pre-dominant wavelength. Conventional / normal colour can reflect only light in visible range, in case of fluorescent colours, it even converts absorbed UV light and reflects in visible range, there by colour appears brighter than normal colour.
When clean, bright conventional colour is able to reflect a maximum of 90% of a colour present in the spectrum, a fluorescent colour can reflect 200-300% of colour present in the spectrum.
Fluorescent colours are generally more fugitive than similar hue conventional colours; the relative degree of light stability would depend on formulations, film deposits, wall thickness, additives etc., items as well as the areas of the intended use.
Daylight fluorescent pigments are stable to indoor light or outdoor conditions other than direct sunlight. They are affected by exposure to direct sunlight. The degree and effect of change is dependent on the colour, intended end usage, pigment loading and other important factors including
- Type of vehicle / binder and over coat
- Type of plastic (Ex: Vinyl will normally give significantly better results than polyolefin’s)
- The thickness of the pigmented coatings or wall thickness of the plastic
- The pigment concentration/loading. (Higher the loading, better the light fastness – in general).
If prolonged outdoor exposure is intended use for the fluorescent coloured item, actual outdoor exposure tests should be conducted in order to be certain in satisfactory results. Accelerated testing, such as carbon arc/ xenon weather meter, will give comparative indications of light fastness, but there is no exact correlation between accelerated and actual outdoor weathering exposure.
Light fastness can be improved to an extent by :
- Higher pigmentation level.
- Ultraviolet absorbers in the pigmented formulations.
- Clear acrylic overcoats (or) clear overcoat with UV absorbers.
- All these systems should also be tested to assure satisfactory results.
MECHANISM OF COLOURS:
When light falls on any coloured object, it absorbs and reflects light, the reflected light when reaches eye, the perception of colour is observed.
NOTE: The reflected light is responsible for appearance of colour.
Conventional colour absorbs and re-emits that portion of the visible spectrum of its predominant wavelength; remaining colours are absorbed and dissipated as heat.
Fluorescent pigments are transparent organic resin particles containing dyes which are capable of fluorescing while in solid state solution.
The characteristics of fluorescent pigments will be decided by the surrounding resinous mixture.
Fluorescent colours are generally more fugitive than similar hue conventional colours, the relative degree of light stability would depend on formulations, film deposits, wall thickness, additives etc, as well as the areas of the intended use.