Coatings and Radiation
By Clifford K. Schoff, Schoff Associates
In June/July of 2019, I was irradiated with Bremsstrahlung X-rays to knock out prostate cancer. The radiation did its job, and the cancer appears to be gone. Being fair-skinned, I also have experienced skin cancer. My experiences have made me think about radiation effects on coatings for good or ill, as well as on me. We use UV and IR radiation for curing coatings and in analytical testing. Electron-beam curing uses high energy electrons to crosslink polymeric materials including coatings. IR and EB tend to be lumped together under the label “radiation cure.” Coatings that are used outdoors are exposed to UV radiation from the sun, and real or simulated solar exposure is used to test such coatings. We include additives in some formulations, particularly for automotive topcoats, to provide UV resistance. Coatings for military, nuclear, and space applications may encounter ionizing radiation, which causes serious problems for electronic devices. Radiation shielding is necessary—a possible job for coatings.
Many coatings are designed to dry and cure in ambient air, but even these coatings can benefit from raised temperatures via sunlamp radiation or other heating devices. Waterborne automotive basecoats are partially dehydrated by IR lamps or other relatively low heating. Some baking ovens include an IR section to “set” the surface of the coating and make it more resistant to dirt pick-up in the main thermal sections of the oven. Most ovens are dirty, and convection air flow blows that dirt around.
Radiation cure is used on a range of products, including coatings for plastics and wood flat stock: doors, paneling, flooring, etc. In most cases, the wet coatings contain no solvent (100% solids). They cure rapidly, and the cured films have outstanding toughness and scratch resistance. Because of these properties, there long has been interest in UV-cure coatings for automobile applications. Car bodies are not like flat stock, but complicated shapes can be cured by attaching IR lamps to robots. So far, UV-cured coatings are only used for certain parts, not for auto bodies.
IR analysis is a valuable technique for analyzing resins, wet paints, and coatings to see whether the expected product has been produced, to identify contaminants, to see how competitors’ products compare to yours, etc. Cure may be measured, even followed, from the appearance or disappearance and intensity of specific bands in the IR spectra. UV analysis is less commonly used for coatings, but is a valuable tool to test the effect of UV on resins and coatings, the effectiveness of UV-resistance packages, whether any of the UV package is left after baking, how long it lasts in the field, etc. In addition, UV fluorescent dyes can be added to solvents and used to test the surface uptake of solvent by primers to explain why solvent “bite” by topcoats is necessary for good topcoat–primer adhesion and to explain why primer over-baking causes topcoat adhesion failures.
Exposure to sunlight causes coatings to degrade over time and where topcoats are clear, underlying coatings can be damaged as well. Automotive coating failures have occurred when UV radiation has reached all the way through the clearcoat and the basecoat to an electrodeposition primer underneath. Outdoor exposure of coated panels in South Florida for multiple years is a common test procedure to evaluate resistance of coatings to the combination of UV, high temperatures, and high times of wetness that occurs there. Accelerated weathering can be used to speed up degradation, but the question always is: how does it compare to natural weathering? See the recent article by Mark Nichols in CoatingsTech, 17 (1), January 2020, pp. 18–25 for an excellent discussion of accelerated weathering and comparisons with putting panels on fences outdoors.
Because of the degradation of coatings by sunlight, protective additives such as UV absorbers and light stabilizers have been developed. UV absorbers do just what their name indicates. They absorb UV radiation, which they change to heat, which then dissipates. However, with time and exposure, absorbers are used up. Hindered amine light stabilizers (HALS) work by scavenging free radicals initiated by the degradation process, thereby inhibiting degradation of the polymers in the coating. Unlike UV absorbers, HALS are regenerated rather than consumed during the stabilization process. Unfortunately, baking of the coating, especially over-baking, has been seen to drive off a significant portion of the additives and loss may also occur during natural weathering. Pigments can protect coatings as well. TiO2 and carbon black are UV absorbers, but other pigments are blockers rather than absorbers. However, highly pigmented coatings provide protection for the polymers in them and for underlying coatings. The thicker the coating, the better the protection. Most automotive basecoats have high-pigment loadings that protect the primer below from UV that passes through the clear.
One form of radiation that rarely is faced by coatings is ionizing radiation, such as gamma rays, X-rays, radioactive decay particles, and the higher UV part of the electromagnetic spectrum. Electronics used in space exploration, military equipment, and at nuclear sites must be shielded from ionizing radiation. This normally is done by putting aluminum boxes around sensitive equipment. However, research at North Carolina State University has shown that coatings containing metal oxide powder provide effect shielding from gamma rays and neutron radiation at lower weight and volume than conventional shielding (M. Devanzo and R.B. Hayes., J. Rad Phys Chem, 2020: 171: 108685 DOI/j.radphyschem.2020.108685).
CoatingsTech | Vol. 17, No. 3 | March 2020