Advancements in Powder Coating Processing and in Real-time Film Formation Analysis of Thermoset Coatings

First, pigment dispersion of thermoset polyester powder coatings processed in an ultrasonic twin screw extruder (UTSE) was analyzed. A high Tg carboxylic functionalized polyester coating was extruded at several barrel temperatures and ultrasonic amplitudes. Two different crosslinkers, Triglycidylisocyanurate (TGIC) and β-hydroxyalkylamide (HAA), and pigments, TiO2 (PW6) and Phthalocyanine Blue (PB15:3), were studied. The extrudate was analyzed to quantify the amount of premature curing that occurred during extrusion via DSC. Powder coatings were created and applied to aluminum panels to study dispersion through gloss and color. It was determined that TGIC coatings had a higher risk of premature crosslinking when processed at temperatures above the crosslinker’s melting point. Ultrasonic treatment was shown to improve the hue (h) of PB15 pigmented coatings and improve the lightness (L*) of TiO2 pigmented coatings. Processing was improved by ultrasonic treatment, evident by a significant decrease in torque.

Next, a low Tg polyester resin was extruded in the UTSE. The combination of using a lower Tg resin and a smaller die enabled higher ultrasonic amplitudes to be studied. HAA formulations processed at 100 and 130°C, with and without a dispersion agent, were compared to each other. Scanning electron microscopy (SEM) and particle image analysis was used in addition to gloss and color change to visualize and quantify pigment dispersion. Ultrasound was shown to decrease UTSE torque, improve the hue of PB15 pigmented coatings, and improve the lightness (L*) of TiO2 pigmented coatings. SEM particle analysis revealed that ultrasound aided dispersion.

Also, a model formulation of a methyl methacrylate (MMA) thermoset acrylic maintenance coating was developed to study the real-time film formation process in a solution drying platform (SDP). Paraffin and soy wax additives were used to suppress monomer evaporation and oxygen inhibition. The SDP was used to track coating thickness, weight, and surface temperature. The real-time data was converted to shrinkage from evaporation, shrinkage from curing, and reaction enthalpy. This allowed the film formation process’ dependencies on ambient temperature and with wax additives to be further analyzed. Cured samples were analyzed via FTIR to determine the presence of wax and unreacted monomer at multiple interfaces. FTIR confirmed paraffin wax at the air interface in tack-free coatings, and a lack of wax in uncured surfaces. Samples with soy wax performed poorly like those without wax. The soy wax had oleic unsaturation, confirmed by NMR, that could slow free radical propagation. Lastly, FTIR indicated that unreacted poly(propylene glycol)diacrylate was present under the wax layer.