Fluorescence was first observed as long ago as 1565 and gets its name from a type of naturally occurring mineral, Fluorspar (Fluorite), which glows under ultra violet (UV) irradiation.
Fluorescence is a photo-luminescence phenomenon, meaning a molecule can emit energy in the form of visible light after it has been irradiated with a radiation source that is not visible. When a material that is capable of fluorescing is irradiated with UV radiation, the absorbed energy forms an unstable state in the molecule that stabilizes itself by spontaneously emitting excess energy at a different wave-length, which we see as fluorescence.
This can be observed as various colors depending on the emission wave-length – from deep blue, violet, green and even shades of yellow (see Figure 1). As soon as the UV radiation source ceases, fluorescence also ceases.
Figure 1: Fluorescence Emission Colors and Relative Intensities of Regalite Partially Hydrogenated Hydrocarbon Resins with Varying Degrees of Hydrogenation
Fluorescence: Comments from the Industry
The attitude of adhesive manufacturers varies. Unquestionably, Asian markets require non-fluorescent hot melt adhesives, but there are two distinct paths to producing them. Some manufacturers maximize freedom of formulation by using fluorescent tackifying resins and eliminate the fluorescence by adding a suitable UV absorber. Others insist that all raw materials should inherently not fluoresce. Even so, most hot melt nonwoven assembly adhesives are based on styrene block copolymers (SBC) that can only be suitably compounded with fluorescent hydrocarbon tackifier resins. Because there are no non-fluorescent tackifiers available, the use of some UV absorber additive is inevitable.
Hot melt adhesives based on styrene-isoprene-styrene (SIS) generally use highly hydrogenated hydrocarbon resins that tend to have low fluorescence which reduces the amount of UV absorbing additives required. Adhesives using styrene-butadiene-styrene (SBS) use either partially hydrogenated or aromatic modified aliphatic hydrocarbon resins with high fluorescence. While using SBS-based adhesives will result in lower cost, they will require relatively high levels of UV absorbing additives to meet the non-fluorescence requirements.