Shadow Curing
UV curing of coated articles that are not planar present problems of variable total exposure across the coating. For convex shapes, rotation in front of a lamp or use of multiple lamps can compensate for the fact that light travels in straight lines, allowing all points of such a nonplanar article to receive a given radiant energy. But for concave shapes, neither setup can expose the portion of the article that is parallel to the light direction. This situation requires a chemical solution often called “Shadow Curing.”
Since UV comes with some heat or because heat can be added during or after exposure, thermal initiation is taught to combine with UV. These initiators are free radical sources such as peroxides and diazo compounds. The former do not have as sharp an onset of radical production as do the latter, leading to the chance of initiation during storage of peroxide-containing acylated formulas. Our experience with AIBN diazo initiator was disappointing. Even at 110 deg C, this approach does not improve the degree of cure as measured by solvent double rubs. The result might be improved by use of a copper bromide catalyst, but that also lowers and broadens the onset temperature of diazo compound cleavage.
Most radiation curing formulas are stabilized to prolonged exposure to heat and light by use of hindered amine light stabilizers (HALS) and UV absorbers. The former work by capturing the peroxide radicals formed in air by absorption of UV light and the latter by absorbing this light. Peroxide radicals generated within the coating cause polymer chain scission lowering the molecular weight and weaking the rub, scratch, and adhesion strength of the coating. The reaction products of HALS peroxide capture are RO bonds to the hindered amine nitrogen atoms, essentially stable free radical complexes. We have learned that these free radical complexes can be sources of acrylate polymerization upon post heating of a radiation cured coated object. Typical results are shown in the following table.
| Post UV Cure Heating UV Cured 9 Micron Coatings on PC | ||||||
|---|---|---|---|---|---|---|
| Exposure, mJ/cm2 | 800 | 800 | 400 | 400 | 200 | 200 |
| Aspect to UV | Flat | Vertical | Flat | Vertical | Flat | Vertical |
| % HALS | 0.5 | 0.5 | 0.5 | 0.5 | 0.5 | 0.5 |
| PMDR* w/o post heat | 36 | 20 | 27 | 18 | 24 | 18 |
| PMDR* 110C, 1 hr post heat | 63 | 57 | 45 | 35 | 34 | 25 |
*PMDR is solvent double rubs, PM wet cotton swab, finger pressure, known to track crosslinking
The table shows the result of use of the HALS in the UV cure of a coating on a concave polycarbonate article. The initial cure of the flat versus the vertically displayed plastic surfaces after the UV cure shows a difference of the solvent double rubs of between 9 and 16 double rubs, the lower rubs on the vertical portion being indicative of lower UV exposure. But following a brief heating, both the flat and the vertical surfaces gain strength. The explanation is that the HALS complexes formed under the UV irradiation are able to diffuse throughout the “cured” coating, and the regeneration of free radicals from the post heat of these complexes initiates a second burst of molecular weight build by action on the residual acrylate functional molecules in the coating.