Rob van der Linde

Effect of physical aging and thermal stress on the behavior of polyester/TGIC powder coatings

Physical aging, thermal stress, dynamic mechanical characteristics and thermal expansion of powder coatings from model terephthalic acid (TPA) and isophthalic acid (IPA)-based polyesters (PEs) cross-linked with triglycidyl isocyanurate (TGIC) were studied. Enthalpy relaxation and maximum compressive thermal stress were the properties used to follow the physical aging. The better mechanical properties of TPA-based PE/TGIC powder coatings seem to be due to a combined effect of several properties, i.e., cross-link density (νe), thermal expansion coefficient (αFT) and glass transition temperature (Tg). The higher the values of νe, (αFT) and Tg, the tighter is the coating network, the greater is the coating ability to expand thermally and the slower the physical aging process, respectively, all factors favoring the TPA polyester-based powder coatings.

P[CF3(CF2)5CH2MA‐co‐MMA] and P[CF3(CF2)5CH2MA‐co‐BA] copolymers: Reactivity ratios and surface properties

This study aimed at reducing the surface energy of coatings by copolymerization of commonly used monomers with fluorine-containing monomers. Copolymers of 1,1-dihydroperfluoroheptyl methacrylate (FHMA) and methyl methacrylate (MMA) or butyl acrylate (BA) are prepared by low-conversion polymerization in solution. Using 1H-NMR data and nonlinear least-squares data fitting, reactivity ratios of these systems at 80°C are determined to be rFHMA = 1.31, rMMA = 0.76, and rFHMA = 3.15, rBA = 0.38, respectively. We assume that the penultimate unit effect plays an important role in these systems. Introduction of the perfluoroalkyl side chain lowers the polymer surface energy significantly; copolymers of MMA and FHMA show a reduction in total surface energy of about 50 % at a content of 15 mol % FHMA as compared with pure PMMA. The attainable reduction in surface energy is much larger than with, for example, Teflon. This is due to the preferential adsorption of the —CF3 groups of the fluoroalkyl side chain, if compared to that of the —CF2— groups of Teflon.

Film formation from latex dispersions

The dominant, capillary forces operative in film formation of latex dispersions, and how theoretical models have dealt with them over the years are addressed. The force by the receding water surface is the dominant driving force. Capillary liquid bridge forces are less effective. Environmental relative humidity (RH) virtually does not influence these forces. With a light transmission technique, the effect of temperature and humidity on the film formation of a surfactant-free butyl acrylate latex was studied. Film formation is successful at temperatures higher than a value slightly below Tg. RH only slightly promotes film formation, presumably because water acts as a solvent. Also, the film promoting action of normal cosolvents as a function of concentration was clearly demonstrated.

Novel block copolymers of 2-vinylpyridine and ε-caprolactone: Synthesis and characterization

Novel block copolymers based on 2-vinylpyridine (2VP) and e-caprolactone (CL) have been synthesized via sequential anionic polymerization in tetrahydrofuran (THF). These block copolymers are expected to be promising pigment dispersing agents for TiO2 in e. g. polyester powder coatings. Initiation of CL by the living P2VP polymer occurred instantaneously and without side reactions. Intramolecular transesterification reactions were not observed either. However, part of the living chains was deactivated immediately after the addition of CL, which yielded bimodal molecular weight distributions. This could be attributed to strong aggregation of the alkoxide chain ends. Addition of LiCl to the polymerization mixture prevented aggregation and resulted in well-defined block copolymers. The block copolymers have been characterized with SEC, NMR and IR.

Improving pigment dispersing in powder coatings with block copolymer dispersants

The effect of three block copolymer dispersants with a poly(2-vinylpyridine) anchor block and a poly(ɛ-caprolactone) buoy block on the dispersing of TiO2 pigments in a polyester powder coating has been studied with digital image analysis on scanning electron micrographs. The mixing in a lab-scale extruder resulted in a virtually complete break-up of the pigment agglomerates. The average TiO2 particle size was only slightly reduced after further treatment of the TiO2 pigments with any dispersant. However, a significant change in the colloidal stability of the dispersions was observed. Without dispersants, the pigments reagglomerated when heated to 200°C and 235°C. This reagglomeration was completely suppressed by the use of dispersants, as a result of a steric layer around the TiO2 particles.

Diblock copolymer dispersants in polyester powder coatings

Poly(2-vinylpyridine)-b-poly(e-caprolactone) copolymers have been used as dispersants for TiO2 pigments in a polyester/TGIC powder coating. In coatings without TGIC, the block copolymer dispersants prevented the flocculation of the TiO 2 pigments at typical curing temperatures, by the formation of a steric barrier around the pigment particles. Consequently, non-crosslinked powder coatings with high gloss and excellent flow were obtained. In the presence of TGIC, the dispersants were found to catalyse the crosslinking reaction, leading to powder coatings with lower gloss and poor levelling. By replacing the basic poly(2-vinylpyridine) anchor block of the dispersants with an acidic or neutral anchor group, this undesired effect may be avoided. Alternatively, the poly(2-vinylpyridine)-b-poly( e-caprolactone) dispersants are expected to be of special benefit in powder coatings with alternative crosslinking mechanisms, such as UV curable powder coatings.

Dual‐cure processes: towards deformable crosslinked coatings

Two dual-cure processes consisting of a UV-initiated radical polymerization followed by either a UV-induced cationic polymerisation, or a thermal addition reaction, were investigated. The feasibility of the processes was studied using an acrylate-oxetane monomer for the UV/UV combination, and an acrylated oligoester for the UV/Heat combination. It was shown by FTIR and Tg measurements, that both steps of each process could be performed efficiently and separately. This allowed the production of a deformable partially cured coating, whose cure can then be completed, leading to the required final properties. Furthermore, it was demonstrated that the increase of the functionality of the reactive diluent led to a decrease of the thermal crosslinking extent. This is probably due to the reduced mobility of the reactive species that is caused by an enhanced UV crosslinking taking place during the first step.