Factors influencing the efficiency of photoinitiation in radiation curable ink formations.
In an effort to be able to use photoinitiators to their maximum potential, the sequence of
events that occurs in an ink formulation during the UV curing process has have been
studied and information presented to allow more effective formulation. Emphasis has been
placed on highlighting the variables that have the greatest impact both on photoinitiator
efficiency and on the suitability of individual photoinitiators and synergists for use in
particular applications. These variables were found to be photoinitiator thermal stability,
UV light utilisation, reaction mechanisms and cure reactivity.
A wide range of photoinitiators and synergists were investigated using thermogravimetric
analysis (TGA) and thermogravimetric analysis-mass spectroscopy (TGA-MS) to define
both their thermal stability and whether under heating they thermally decompose or merely
Differential photocalorimetry (DPC) was used to determine which wavelengths from a
typical medium pressure mercury curing lamp are the most important for providing cure,
with both theoretical and practical methods being used to define the extent to which these
wavelengths penetrate into pigmented and non-pigmented coatings. A procedure was
devised and validated for this purpose.
The reaction mechanism and photodecomposition products of a range of photoinitiators
were investigated using gas chromatography-mass spectroscopy (GC-MS) and radical
trapping experiments. The reaction mechanisms are discussed in terms of available
literature knowledge. Evidence is also presented suggesting that, with only particular
exceptions, cleavage photoinitiators can also react by a hydrogen abstraction mechanism in
the presence of an amine synergist.
A real time infrared spectrometer (RTIR) was set up and a method validated for following
the UV curing reaction through changes in the acrylate double bond concentration. The
advantages and disadvantages of this instrument are discussed in terms of other similar
instruments reported in the literature, and the technique subsequently used to measure the
reaction rates of a wide range of photoinitiators. Other factors such as photoinitiator
concentration, amine synergist type I level and formulation viscosity were also investigated
to determine their influence on the cure process.