What is a photoinitiator?
Photoinitiators are chemical compounds that that are sensitive to light. By absorbing light photoinitiators produce areactive species (either free radicals or a Bronsted or Lewis acid) that will interact with the active components in formulations to form a hardened (or cured) material. Photoinitiators take the energy from light and induce a chemical reaction by converting it into chemical energy. Photopolymerization (also known as radiation curing) is the transformation of a liquid formulation into a cross-linked polymer induced by a photoinitiator and its reaction to light.
There are 2 classes of photoinitiators: Type I and Type II
Type I Photoinitiator ssustain a uni-molecular bond cleavage after absorption of light to render the reactive species. No other species are necessary in order for these photoinitiators to work.
Type II Photoinitiators sustain a bi-molecular reaction. After absorption of light, the photoinitiator reaches excited state which reacts with another molecule (co-initiator or synergist) to create the reactive species.
The different nature of the reactive species created in each process makes it important to choose the right photoinitiator. Free radical photoinitiators are commonly used in the photopolymerization of styrene based resins. Curing can be accomplished using UV/Visible light, and stops as soon as the exposure to light is terminated.
Cationic photoinitiators are used to initiate the photopolymerization of epoxy resins. As with free radicals, the curing process can be accomplished with UV or visible light. However, in cationic systems the process continues even when the irradiation of light has stopped, and as a result, cationic process requiresa longer time to complete the curing.
As demonstrated, it is crucial to select the proper photoinitiator to obtain the perfectly cured photopolymer.
For over 30 years, Hampford Research Inc. has manufactured free radical photoinitiators based on the hexaaryl-bisimidazolyl (HABI) molecule. Through careful modification of the substituents on the aryl groups, HRI has been able to significantly improve the performance at 365, 395 and 405 nm, the wavelengths most commonly emitted by LED lights. These innovative new compounds will still produce stable lophyl radicals and offer resistance to oxygen inhibition that HABI’s are known for.
All HABI photoinitiators operate via a Norrish II type reaction mechanism, meaning they must be combined with a suitable co-initiator in order attain complete photo-polymerization. Hampford Research experts can help you choose the ideal combination for your application.
The high molecular weight as well as the low polarity of the HABI molecule makes it ideal for applications where low migration characteristics are paramount.
HABI, for LED applications is sold exclusively under the LEDCUR tradename.