Wavelength cure time

Figure 16 Phosphorescence spectra of the DGEBA/DDS epoxy system as a function of cure time at 433 K. Cure times are 0, 10, 20, 31, 46, 60, 120, 150, and 180 min (from bottom to top). Excitation wavelength is 330 nm. (From Ref. 99.)...

Figure i. Ratio of monomer fluoresence intensity at 377nm (Fm) to excimer fluorescence intensity at 488nm (Fd) for 1,3-bis-(I pyrene)propane in epoxy as a function of cure time. The excitation wavelength was 345 nm. 

The exciplex intensity showed quite different behavior as the setting proceeded. A comparison of the (monomer peak/monomer peak) ratio to the (exciplex peak/monomer peak) ratio was quite illuminating. We considered the initial maximum wavelength of the exciplex emission at 540 nm, and compared its intensity to the monomer intensity at 405 nm as the dissolution/ polymerization proceeded. A substantial decrease in exciplex intensity, compared to monomer intensity, was observed over the first 40 min of the cure. The ratio then leveled off, indicating that the local viscosity had reached a maximum after 40 min and that the dissolution/polymerization was considered to have reached completion at the ambient temperature of the laboratory. Since the working time for the cement was considerably less than the 40-min time period over which the exciplex/monomer intensity ratio was steadily decreasing, the intensity ratios served as in situ monitors of the cure. 

When special acrylates, epoxies or silicone photoinitiators are added, these adhesives are cured spontaneously by lightwaves or UV radiation. The crosslinking reaction is initiated at certain wavelengths only on the basis of the photoinitiators selected. It must be noted, however, that the curing rate is very high at the moment of exposure, so that technical cycle times of 0.3-10 s are feasible. 

In 1919, Sir Edward Mellanby (1884-1955) succeeded inducing rickets experimentally in dogs and curing the iUness with fish liver oU. A short time later, the Berlin physician Kurt Huldschinsky (1883-1941) observed that children with rickets, who play outside in the sun, rapidly become healthy again. The same effect was achieved artificially by exposing children to U V light of a wavelength between 230 and 315 nm (Fig. 7.39). 

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