Rheology and UV-curing of a star-branched polyester

Sammanfattning: The use of powder coatings has increased steadily over thelast few decades. Their popularity is due to a number of majoradvantages they offer when compared to solvent-borne systems.Near zero VOC-content, reduced energy consumption and near 100%utilisation of the material are some of the advantages. Themain components in most thermoset powder coatings are a primaryresin and a binder. The coating is applied in powder form, thenbaked and cured i.e. cross-linked. Traditionally, thecross-link reaction has been controlled by temperature. Normalcuring temperatures range between 160 and 200 °C, and thusare not suitable for applications involving heat sensitivesubstrates such as wood and plastic. One way to lower thecuring temperature is by the introduction of semi-crystallinematerial with a suitable melting temperature. The advantage ofusing a crystalline resin is its rapid melting, a first ordertransition, versus the slow softening of an amorphous resin.Low curing temperature also requires an initiating system thatis activated at low temperatures while stable at roomtemperature. This is difficult to achieve with conventionalthermal initiators. In recent yearsUV-curing of powdercoatings has obtained increased attention in industrialresearch. This technique allows fast curing at lowertemperatures than conventional powder coatings. The researchinto UV-curing was triggered by the interest in using heatsensitive substrates. Other advantages are improved storagestability, elimination of premature reaction duringmanufacturing and better levelling since viscosity does notincrease until UV-irradiated. This thesis describes thesynthesis and characterisation of a series of branchedpolyesters and investigates their possible use as powdercoatings resins. The polyesters consist of poly(e-caprolactone)(PCL) (degree of polymerisation: 5-200) initiated fromhydroxyfunctional cores with methylmethacrylate end-capping .The cores utilised are: (1) 3rd-generation Boltorn, acommercially available hyperbranched polyester withapproximately 32 hydroxyl groups, (2) a 3rd-generationdendrimer with 24 hydroxyl groups, (3) a 3rd-generation dendronwith 8 hydroxyl groups, and (4) a linear PCL for comparison.All initiators are based on 2,2-bis(methylol) propionic acid(bis-MPA). The polymers and films were characterised using1H-NMR, 13C-NMR, SEC, rheometry, DMTA, DSC and a rheometercoupled with an UV-lamp. Measurements of zero shear rateviscosity showed that h0 increased exponentially with Mw, justas expected for star-branched polymers. For Boltorn-PCL anddendrimer-PCL, h0 is independent of Mw but dependant of the armMw (Ma). However, the dendron-PCL deviated from this behaviour.The dendron-PCL Ma was higher than that of Boltorn-PCL ordendrimer-PCL at the same h0. The resins are crystalline andthe melting points range from 34-50 °C; films can beformed and cured below 80 °C. Measurement of mechanicalproperties of cured films showed that the films from the low Mwpolymer were amorphous while those with high Mw werecrystalline. The viscoelastic behaviour during the fast cureshowed that the time to reach the gel point, a few seconds,increased linearly with molecular weight. The crossover ofG?and G??was used as the gel point.