An environmentally friendly technology that uses energy, typically from a light source, to instantaneously convert, via polymerisation reactions, a liquid resin into a cross linked solid.
The reaction is often rapid and can be easily incorporated into high throughput in-line manufacturing processes e.g. coatings and inks. Energy curing produces little to no VOCs and has reduced energy input compared to evaporative curing processes.
Energy curing systems are based around several core energy technologies; UV, LED and electron beam (EB). UV cured systems provide instantaneous reaction, high throughput, low VOC and low energy use compared to traditionally cured systems. LED cured systems provide additional benefits of; extended lamp lifetimes, instant on off lamp capability, reduced energy consumption and cold cure. EB technology uses an electron beam to directly polymerise the monomers and oligomers without the use of a photoinitiation system. EB systems operate under vacuum which contributes to the high running and initial investment costs.
In cationic curing the active species released by the photoinitiator is a long-lived acidic species which induces the curing of cycloaliphatic epoxide resins. This long-lived species allows cure to continue in the absence of light energy (dark curing). High conversions are achievable, giving rise to excellent mechanical and chemical resistance. Cationic polymerisation of epoxides gives excellent dimensional stability for thick and 3D components due to low shrinkage of the epoxide on polymerisation.
There are many important parameters to consider when selecting an optimal photoinitiator system. They include, chemistry of the resin system, thickness of the coating, irradiation type (mercury lamp, LED etc), pigment absorptions, desired final properties of the coating and substrate.
The best photoinitiators for surface cure are hydroxyacetophenones (HAPs) such as SpeedCure 73 and SpeedCure 84 or Benzophenones such as Speedcure BP and SpeedCure MBP in combination with a coinitiator.
Hydroxyacetophenones such as SpeedCure 73 and SpeedCure 84 provide very low yellowing curing. SpeedCure MBF is the lowest yellowing photoinitator.
Long wavelength absorption photoinitiators such as SpeedCure TPO, SpeedCure BPO and SpeedCure TPO-L are important in achieving depth cure in dark systems. These can be supplemented by thioxanthones (SpeedCure 2-ITX, SpeedCure DETX) and/or amino acetophenones (SpeedCure BDMB, SpeedCure 97). For very dark pigmented systems and blacks SpeedCure EMK can also be used. These long wavelength absorption photoinitiators should be combined with surface cure photoinitiators to achieve effective balanced cure.
Photoinitiators for LED curing must have an appropriate absorption matching the emission of the LED light source. Phosphine oxides such as, SpeedCure TPO, SpeedCure BPO and SpeedCure TPO-L have suitable long wavelength absorption, as do SpeedCure EMK and thioxanthones such as SpeedCure 2-ITX, SpeedCure DETX, SpeedCure CPTX and SpeedCure 7010.
All photoinitiators can be used for low migration applications provided that the classification of the photoinitiator is not prohibited by the regional regulatory body or end user and the migration is below allowed threshold limits. To reduce the possibility of migration high molecular weight polymeric photoinitiators such as SpeedCure 7005 and SpeedCure 7010 can be used.