TP photoinitiator

Figure 3.67. SEM image of some of the columnar structures produced by irradiating a solution comprising a TP photoinitiator and a crosslinkable monomer applying a dose-array measurement. The width of the smallest feature is 60 pm. Across the array, the pulse energy increases from left to right, and the exposure time increases from front to back. For the four visible columns, the pulse energies used are 0.8, 1.0, 1.2, and 1.5 mJ. (From Ref. [225] with permission of Elsevier.)...

The PM and TPS photoinitiators generally gave shorter tack-free times than DPI and at the lower concentration level, PM was somewhat more effective than TPS. This can be attributed to the more efficient use of the mercury arc radiation by the PM photoinitiator, which has an absorption peak at 313 nanometers (Figure 4), corresponding to a peak in the emission spectrum of the mercury arc. Absorption maxima for DPI and TPS are at the lower end of the spectrum, far removed from the peak output of the high pressure mercury arc. 

An explanation of the differences in cure rate between DPI and TPS is less obvious, as the absorption spectra of these two compounds are -similar. Depending on the method of preparation, however, the TPS photoinitiator frequently shows some absorbance in the spectral region between 290 and 340 nm, overlapping the band at 310 in the mercury lamp emission spectrum. This may be the result of a fortuitous contaminant not completely removed in synthesis and purification of the TPS photoinitiator. 

Storage Stability of Photosensitized Epoxides. Storage stability of photosensitized formulations is dependent upon many factors, such as temperature, sample size, structure and concentration of the photoinitiator, exclusion of light and the presence of impurities. Tests were run under parallel conditions using a mixture of epoxides known to give a short potlife with the PM photoinitiator to compare storage stability of the same formulation photosensitized with DPI or TPS. 

Table XI. Comparison of Photoinitiators (PM, DPI, TPS)—Influence of Temperature and Humidity on Tack-Free Time...

A number of nonsteady polymerization rate techniques can be used to measure tp [ 11 ]. The most widely used method involves pulsed-laser-induced polymerization in the low monomer conversion regime. Briefly, a mixture of monomer and photoinitiator (Section 6.5.3) is illuminated by short laser pulses of about 10 ns (1sec) duration. The radicals that are created by this burst of ligh propagate for about 1 sec before a second laser pulse produces another crop of radicals. Many of the initially formed radicals will be terminated by the short, mobile radicals created in the second illumination. Analysis of the number molecular weight distribution of the polymer produced permits the estimation of kp from the relation... 

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