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Acylphosphineoxides are well known and commercially accepted for their unique
curing performance in white pigmented coatings. About ten years ago, 2,4,6-
(trimethylbenzoyl)diphenylphosphineoxide (TPO)' was introduced as a first representative
of the mono-acylphosphineoxides. In the late 80's research work was focused
on the improvement of the photoinitiation efficiency based on the market need for
curing of highly pigmented wood coatings affording good hiding power. Bisacylphosphineoxides
were soon identified as the most promising candidates. From
this novel class, bis (2,6-dimenthoxybenzoyl)-2,4,4-trimenthylpentylphosphineoxide
(BAPO) was selected based on a number of criteria and recently commercialised as
a liquid 3 with the a-hydroxyketone photoinitiator 2-hydroxy-2-methyl-1 -
phenylpropan-1 -one (HK). The intention of the present study was to gain a deeper
insight into the factors controlling the cure of white pigmented acrylate coatings using
blends of BAPO and HK as photoinitiating systems and to better understand the impact
of reaction kinetics on mechanical end properties of such coatings.
The base formulation contains an epoxyacrylate oligomer and HDDA at a weight ratio
of 65:35. Rutile titanium dioxide, BAPO and HK were added at concentrations
covering the range of interest. The study was carried out using a UV processor
(conveyer belt system) equipped with two 80 W/cm medium pressure lamps and a
high-speed real-time infrared spectrometer (RTIR) for kinetic . Features of
this in-house made RTIR are a time resolution of 2 ms at an excellent signal-to-noise
ratio, optional heat filter and monochromator for the mercury emission, horizontal
sample position and optional inertisation. A high pressure "ozone-free" mercury lamp
was used, as no "deep UV" lamp with a spectrum corresponding to medium pressure
mercury lamps was available at the time of this study. Silicon wafers were used as
IA-transparent substrates for the white coatings which were applied by a spin coater
with an accuracy of 1um.
Test protocols were generated and data were analysed by an in-house developed
expert system software for experimental design and analysis. Experimental factors
used in this study were concentrations of BAPO, HK and titanium dioxide, cured film
thickness and ambient atmosphere (air or argon). Degree of double bond conversion
after 6 s exposure at 40 2 in the RTIR unit (cony. RTIR), highest conversion
rate (max. slope) and inhibition time (inh. time) from ATIR conversion curve, as well
as degree of double bond conversion (cony. proc.,) and pendulum hardness (PH
processor) upon exposure in the UV processor at 5 rn/mm were chosen as response
variables. For pendulum hardness measurements according to DIN 53157,
coating thickness had to be increased. In order to keep the optical density constant,
pigment concentration was reduced compared to thin coatings (up to 33 um dry)
used for conversion measurements. Thus, the pigment concentration range of 20-
50% for a 30 um cured coating corresponds, in terms of constant opacity, to 8-20%
for the 75 p.m cured coating.
