Depth Profiling of Radiation Curable Coatings Investigated by Confocal Raman Spectroscopy

Increasing amounts of radiation curable coating raw materials are used for the
surface refinement of furniture, wooden floor coverings, paper etc. Instead of
dissolved resins BASF offers polymerizable monomers, oligomers and photoinitiators
under the trade names Laromer and lucirin, which will be cured after a
simple deposition process by UV light or electron beam to become a protective
coating 1 - 4]. The drying of the coating is finished within fractions of a second,
due to the high reactivity of the compounds, which results in a 3-dimensionally
crosslinked polymer molecule nearly without any extractables.

This technological advantage, however, simultaneously bears the disadvantage,
that the reacted polymer as a crosslinked structure withstands every classical
polymer characterization and classification. As a consequence there is an urgent
need for new and easily applicable measurement techniques to characterize
these solid 3-dimensionally crosslinked films.

In this context the method of real-time infrared spectroscopy (RT-IR) has to be
mentioned which was mainly developed by Prof. Decker, Mulhouse 4 -6]. Thus
the temporal reaction yield could be followed in the millisecond range by the
decreasing absorption of the acrylester double bonds while simultaneouly curing
with UV light. Important informations on the polymerization rate, content of
unreacted double bonds, inhibition by ogygen, polymerization quantum yield, etc.
will be gained by evaluating the reaction yield curves. The experimental refinement
of RT-IR, rapid scanning FT-IR, used at BASF AG allows the fast recording
of a series of spectra which gives insight into the reaction kinetics of different
reacting compounds. The method of RT-IR faces difficulties in the investigation of
the depth profile of curing due to the integral transmission measurement. Extensive
measurements at different sample thicknesses have to be performed to estimate
the depth profile of curing in a relative unprecise way.