UV LED Curing is a recently developed technology designed to improve the speed, performance, and capabilities of a wide variety of curing applications. Below is a short introduction to UV LED curing. For a more complete list, see the Resources section in the sidebar.
A light-emitting-diode (LED) is a semiconductor diode that emits light when an electric current is applied in the forward direction of the device, as in the simple LED circuit. The effect is a form of electroluminescence where incoherent and narrow-spectrum light is emitted from the p-n junction in a solid state material. An LED is usually a small area (less than 1mm -2mm) light source, often with optics added directly on top of the chip to shape its radiation pattern and assist in reflection. The color of the emitted light depends on the composition and condition of the semiconducting material used, and can be Infrared, visible or Ultraviolet.
The first known report of a light emitting solid state diode was made in 1907 by a British experimenter, H.J. Round, when he noticed electroluminescence produced from a crystal of silicon carbide while using a cats-whisker detector, a thin piece of wire that touches a semiconductive crystal to make an imperfect contact junction. Other notables who created LEDs in the early to mid 1900's, Oleg Vladimirovich Losev, Rubin Braunstein, Bob Biard, Gary Pittman and Nick Holonyak Jr.
As the LED materials technology became more advanced, the light output was increased, while maintaining the efficiency and the reliability to an acceptable level. Most LEDs were made in the common 5mm T13/4 and 3 mm T1 packages, but with increasing power output, it has become increasingly necessary to shed excess heat in order to maintain reliability, so more complex packages have been adapted for efficient heat dissipation. Packages for the state-of-the-art high power LEDs bear little resemblance to early LEDs.
The development of LED technology has caused their efficiency and light output to increase greatly, with a doubling occurring about every 36 months since the 1960's, in a similar way to Moore's Law. The advances are generally attributed to the parallel development of other semiconductor technologies and advances in optics and material science. This trend is normally called Haitz Law.
Like a normal diode the LED consists of a chip of
semiconducting material impregnated with impurities to create a p-n junction.
As in other diodes current flows easily from the p-side (anode) to the n-side
(cathode) but not in the reverse direction. Charge carriers (electrons and
holes) flow into the p-n junction from electrodes with different voltages. When
an electron meets a hole it falls into a lower energy level and releases energy
in the form of a photon. The wavelength of light emitted (light color) depends
on the band width energy of the materials forming the p-n junction. The
materials used for the LED have a direct band width with energies corresponding
to infrared, visible or Ultraviolet light.
LED curing occurs when polymerization happens; the process where monomer molecules react together to form three-dimensional networks also known as polymer chains. The complexity of polymerization is due to various functional groups present in the reacting compounds. Because of the extraordinary range of properties of polymeric materials they play an essential role in everyday life. For example adhesives, inks, coatings, sealants used by mankind in daily lives undergo polymerization that requires a hardening process to be more useful. The complex chemical reaction can be photo-induced or light cured or UV cured. Curing can occur either by free radical initiators or cationic initiators. For light cure adhesives, coatings and inks to interact with UV light, a chemical called photo-initiator must be present in the formulation. These photo-initiators fragment into reactive species when exposed to light source. These fragments initiate a rapid polymerization process with monomers and oligomers to form a cross linked and a durable polymer.
Much of that chemistry relies heavily on tuned photoinitiators tuned. As a result, not all previously formulated broadband UV ink chemistry will work with monochromatic LEDs. In many cases, the chemistry must be reformulated to react and accomplish the same or similar cure results within the more restrictive but also incredibly more intense band of LED output. While this no doubt presents challenges, it also yields the positive aspect of eliminating the infrared and UVC components. As a result, when compared to conventional curing, there is less heat transfer to the substrate (no IR) and no harmful UVC rays or resulting ozone to address. The UV from current LEDs is all UVA with a slight visible component in the violet wavelength range.