Shadow Curing
UV Coating Technology for Non-Planar Thermoplastic Surfaces
The Challenge of Non-Planar UV 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."
Thermal Initiation Approaches
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°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.
The HALS Solution
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 weakening 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.
Key Discovery
We have learned that these free radical complexes can be sources of acrylate polymerization upon post heating of a radiation cured coated object. This enables effective curing of shadow areas on concave thermoplastic surfaces that cannot receive direct UV exposure.
Test Results
Post UV Cure Heating of UV Cured 9 Micron Coatings on Polycarbonate
| Parameter | Test 1 | Test 2 | Test 3 | Test 4 | Test 5 | Test 6 |
|---|---|---|---|---|---|---|
| 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* 110°C, 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
Ready to Get Started?
Contact a PCI Labs Sales Representative to discuss your shadow curing requirements and discover the right solution for your application.