Enhanced UV Curing at a glance  [ Return towards  Enhanced radiation curing  ]

The UV curing process is a polymerisation reaction, initiated by UV radiations which transforms, within a fraction of a second, a liquid coating film applied to a substrate into a solid cross-linked polymer material.

The typical formulation of UV-curable resins is made of three basic components: a photo-initiator, a monomer and an oligomer, the two last containing reactive functions. Additives and pigments/dyes are also added in order to fit printability requirements and afford the products the desired final physical properties.

   

The UV curing process is based on a free radical polymerisation initiated by UV radiations. Exposed to a specific wavelength, the photo-initiator (PA) absorbs the UV light and forms free radicals (R^•). These radicals combine with the reactive groups (M) from the oligomer and monomer to start the polymerisation reactions (RM^•) and to yield a highly cross-linked polymer network with high molecular weight.

However, the UV curing process is slowed down by the oxygen present in ambient air. The free-radical polymerisation reaction is inhibited due to th ehigh reactivity of the molecular oxygen toward radical species (R^•). Oxygen reacts with the radicals generated from the photo-initiator to yield peroxide species (RO₂^•) and hydro-peroxides (RO₂H), which are inefficient for the polymerisation. As a result of these reactions, the surface is either poorly cured or not cured at all.

 

 

 

Several methods have been employed to overcome the detrimental effect of the oxygen inhibition:  

• The most commonly used method is to add an excess of photo-initiators in the UV formulation in order to consume the oxygen present on the surface of the web or dissolved inside the coating. However, most photo-initiators show an increased tendency to yellowing and to release odour.

 

• In the case of UV curing, a considerable increase of the UV radiation is used to increase the number of radical species to scavenge the molecular oxygen. As a consequence, the consumption of energy becomes very important and the life of the UV lamps is drastically reduced.

 

• Since UV curing processes are continuously operating processes, not entirely enclosed in specific containments, the curing reactions take place in atmospheric air. Therefore the most interesting method is to remove the oxygen present in the UV reaction zone and replace it by an inert gas such as nitrogen.

 

Air Liquide has developed a special Gas Seal Kit (GSKit) that enables to cure under reduced flow rates of nitrogen atmosphere. Our Gas Seal Kit is a cost effective, reliable and safe inerting device which can be retrofitted to existing UV chambers. A fully automated Process Control System will allow you to easily regulate the nitrogen flow rate and afford you full control of the cost and quality of your production processes.

The UV curing method, under inert conditions, is used in very different sectors that benefit from its numerous advantages.