Christopher A. Lukey

Theoretical study of adhesion between graphite, polyester and silica surfaces

This study examines the interaction between graphite and polyester-based polymers and silica in order to compare the adhesive properties of these surfaces. Surface interaction energies were calculated at different interfacial separations, and the resultant adhesion energy curves were used to determine the Work of Separation (W sep) and equilibrium interfacial separation (d 0). Adhesion between graphite and polyester was calculated to be significantly greater than between graphite and silica. Our calculations indicate that Van der Waals Forces lead to significant adhesion between graphite and polyester. However, the Van der Waals attraction is approximately 30% less between graphite and silica.

An atomic force microscopy study of weathering of polyester/melamine paint surfaces

The use of an atomic force microscope (AFM) for rapid assessment of the durability of exterior polyester/melamine paints has been investigated. The study established an AFM imaging technique that produces data representative of weathering rates of paint films under a range of weathering regimes of varying severity. The effect of scan size on the average roughness parameter was investigated, leading to the adoption of two specified scan sizes. It was further found that both TappingMode™ and contact mode imaging gave equivalent detail of surface topography for these samples. However, TappingMode™ gave a clearer representation of the pigment/binder composite structure. Importantly, a technique was developed which reduces sample to sample variability by allowing a specified area to be repeatedly imaged as a function of weathering time. The images collected were found to show important detail regarding the mechanism of weathering, and the role of binder stability and pigment loss.

The use of chemiluminescence analysis to measure hydroperoxide levels in photo-oxidised polyester/melamine surface coatings

Abstract Chemiluminescence (CL) analysis has been used to quantify hydroperoxide concentrations as a function of UV exposure time (UVA), for a number of polyester/melamine surface coatings on aluminium substrates. Kinetic analysis of the results allows the evaluation of three parameters: k 1 , the hydroperoxide formation rate constant; k 2 , the hydroperoxide decomposition rate constant and C 0 , the initial chromophore concentration. It was found that k 1 correlates with rate of film ablation, observed when clearcoat formulations were subjected to exterior exposure in a tropical environment.

Factors affecting the photolysis of polyester-melamine surface coatings

Abstract A series of polyester/melamine clearcoat films with varying amounts of melamine crosslinker, and prepared under a range of cure conditions, has been exposed to high intensity UV radiation at cryogenic temperatures in nitrogen, under stress in nitrogen, and in an oxygen-containing atmosphere. The free radicals produced were detected, analysed and quantified by ESR spectroscopy. The results show that the type and concentration of radicals produced is critically dependent on film composition, cure conditions (oven temperature and dwell time), the atmosphere under which photolysis is conducted (N2 or N2+O2), and the extent of mechanical deformation. At the optimum crosslinker concentration and optimum cure conditions, free radical yields were minimised, suggesting that free chain ends are the main photolabile species. New radical species, and significantly increased radical yields, were found in photolysis of samples with a greater than optimum concentration of crosslinker subjected to mechanical strain. In an oxygen-containing atmosphere, peroxy radicals were produced, the direct detection of which is reported here for the first time.

THE EFFECT OF COMPOSITION ON SURFACE PROPERTIES OF POLYESTER–MELAMINE THERMOSET COATINGS

Surface properties of polyester-melamine thermoset coatings have been investigated using XPS, thermal analysis methods, surface energy measurement, and small particle adhesion using a dedicated force rig. Bulk glass transition temperatures and surface energies changed little over a wide composition range (5–50 wt% melamine). At low melamine content (<20 wt%), particle adhesion behaviour was similar to that of pressure-sensitive adhesives; however, at high melamine contents particle adhesion was uniformly negligible. XPS and thermal analysis showed that at high melamine concentrations the surface melamine content was higher than expected, suggesting the formation of a hard, highly crosslinked, self-condensed, melamine-rich surface layer. This was not observed at lower melamine concentrations. The presence of this glassy melamine-rich surface layer is believed to be responsible for the low particle adhesion at high melamine concentrations.

Adhesion-Induced Interactions between Micron-Sized Zirconia or Carbon Spheres and Melamine-Cross-Linked Polyester Surfaces

The thermodynamic work of adhesion between micron-sized zirconia or carbon spheres and polyester–melamine surfaces was determined using the Johnson-Kendall-Roberts (JKR) relationship, after first validating the JKR approach for this system. The calculated works of adhesion for both zirconia and carbon were similar for any given substrate composition and were found to be approximately 35 mJ/m2 at low melamine concentrations, close to theoretical predictions. The apparent work of adhesion decreased with increasing melamine concentration, most likely due to the presence of a glassy melamine-rich surface layer, which is not representative of the bulk. The value found for low melamine concentration was assumed to be a true value for cocondensed polyester–melamine, and this was used to estimate surface modulus and the amount of excess melamine in the surface as a function of bulk composition.