(193f) Depth Profiling of the Degree of Crosslink for a UV Cured Coating

Since their introduction in the early 1980's, basecoat/clearcoat paint systems have been used extensively in the automotive industry. The advantages of these systems include superior appearance and high gloss retention. These coatings must provide long-term resistance to chemical, mechanical, and physical damage. In particular, the clearcoat must resist chemical agents, scratching, chipping, thermal cycles, and UV radiations while maintaining a high gloss.

There are several curing/crosslinking methodologies used for basecoat/clearcoat systems including UV curing. The crosslink density or degree of crosslink can affect both mechanical (i.e. tensile strength, fracture toughness, and durability) and physical (i.e. glass transition temperature, solubility, and adhesion) properties of the final product. Therefore, measuring the crosslink density is important for both product consistency and developing new curing systems.

In this study, the crosslink density of a basecoat/topcoat, UV-cured system was studied as a function of coating depth using IGC. The depth profile was established by microtoming the coating in 5 micron increments.

METHOD/THEORY

IGC is a well-known tool for the characterization of particulates, fibers and films. It involves the sorption of a vapor (probe molecule) with known properties onto an adsorbent stationary phase (solid paint sample) with unknown physico-chemical properties. This approach inverts the conventional relationship between mobile and stationary phase found in analytical chromatography. The stronger the interaction, the more energetic the surface and the longer the retention time. For this reason a range of thermodynamic parameters can be derived from the retention behavior.

IGC has been used previously to determine crosslink densities. In short, a finite concentration of solvent (probe molecule) is injected through a column containing the material of interest. The Flory-Huggins interaction parameter is then determined from the probe molecule retention time. Then, the Flory-Rehner equation is used to determine the crosslink density which is defined as the number of moles of subchain units (chains between crosslinkages) per gram of sample.

The clearcoat used in this study was a urethane acrylate with a starting (uncrosslinked) molecular weight around 1000 amu. The basecoat was a standard thermally cured automotive polyester basecoat. After curing the basecoat, the clearcoat was crosslinked by passing the sample under a medium pressure mercury arc lamp. The total dose was approximately 1-2 J/m². The resulting film was microtomed into slices measuring 2.5 cm x 3.0 cm x ~5 micron. Slices were made through the clearcoat and basecoat layers. The reported density for the samples was 1.2 g/cm³.

For the IGC experiments, the microtomed samples were cut into small strips and packed into silanized glass columns. IGC measurements were carried out using the SMS-IGC 2000 system (Surface Measurement Systems, UK). The samples were measured at 368 K. Decane (HPLC grade, Aldrich) was used as the probe molecule.

RESULTS AND DISCUSSION

First, decane isotherms were collected on films taken from different coating depths. The isotherms extended beyond the linear (or Henry) portion of the isotherm, confirming the solvent concentration was in the finite concentration regime. Next, the decane isotherm was used to calculate the Flory-Huggins interaction parameter as a function of volume fraction of decane. The volume fraction of decane is defined as the volume of decane in the solid phase divided by the total volume (decane + film). At low decane volume fractions the Flory-Huggins interaction parameter increases steadily. At higher decane volume fractions, the interaction parameter approaches a steady value. This would coincide with the polymer network being completely swollen with decane.

Finally, the degree of crosslink is calculated using the Flory-Rehner equation. As expected, the degree of crosslink approaches a plateau value at higher decane volume fractions. The degree of crosslink values at the plateau represent the true crosslink density for the sample. The results clearly indicate the degree of crosslink increases the further away from the surface. To illustrate, for the 0-5 micron sample a crosslink density of 2899 mol/g was measured, but for the 45-50 micron sample a crosslink density of 4065 mol/g was obtained. The error margins measured indicate the differences between the samples are much greater than the measurement errors. The higher degree of crosslink with increasing depth profile is likely due to the effects of oxygen inhibition during curing of the coating. This effect is common in UV cured coatings, where the free radical nature of the curing process can be severely retarded by atmospheric oxygen scavenging the free radicals. This leads to a reduction in cure (crosslink density) in many UV cured coatings when cured in the presence of oxygen.

CONCLUSIONS

The crosslink density for a UV-cured clearcoat at different coating depths was investigated by IGC. IGC allowed quick and reproducible determination for crosslink densities for this system. The degree of crosslink increased with profile depth. The increase in crosslink density as a function of depth into the clearcoat is likely due to the effects of oxygen inhibition during curing of the clearcoat. Also, studies using different UV-cure conditions or alternative materials may yield additional information about crosslinking mechanisms.