Companies in coatings technology try to lower the VOC (Volatile Organic Compound) content of their systems. The use of powder coatings, waterbased systems and UV-curable coatings has increased substantially over the past decades. Another option to reduce the emission of organic solvents is to increase the solids content of solventbased systems. This article describes a few approaches used to adjust the resin system in order to lower the viscosity so that less solvent is needed to bring the paint to application viscosity.
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A key effect causing high viscosity of resin solutions is that resin molecules of moderate to high molecular weight have long tails that stretch out. This gives chain entanglement: the tails hinder each other just like the tails of spaghetti do. An approach to reduce chain entanglement is to design resin molecules that have a more spherical shape. Such polymeric molecules are more branched.
Resin molecules that have a higher degree of branching than conventional resins are called hyperbranched resins. These polymers have lower resin viscosity and therefore less solvent is needed to obtain application viscosity of the complete system. Several innovative companies, like DSM and Perstorp, are strong in (raw materials for) hyperbranched resins.
Another phenomenon, causing high viscosity of resins that are dissolved in organic solvents, is that intermolecular interactions between different resin molecules give a physical network. Physical networks that can give a strong contribution to viscosity can be temporarily broken down when the interacting groups are blocked, for example by using suitable chainstopper solvents. The solvents evaporate during film formation and the physical network can build-up again.
As most physical networks in paints are based on hydrogen bonding, a useful approach is to select solvents that are able to form a hydrogen bond with a resin molecule, thus 'stopping' the physical resin network. Solvents that can give substantial viscosity reduction of systems based on alkyds and polyesters contain a C=O group. These solvents can act as hydrogen acceptor. MPA, 1-methoxy-2-propyl acetate, is a solvent that is suitable to lower solvent content of paints based on polyester resins and alkyd resins.
The viscosity of binder systems can be lowered by combining polymeric resins with reactive diluents. 'Reactive' means that the molecules join in the crosslink reaction during film formation. This implies that the molecules should contain reactive groups of the same sort as the resin. The word 'diluent' implies that the molecules must be miscible with, and lower the viscosity of, the resin system. Reactive diluents must have a specific chemical structure as well as a low molecular weight in order to comply with these two criteria. Of course, the reactive diluent must not contribute to VOC. This implies that a good reactive diluent does not evaporate during film formation. Reactive diluents are widely used in, for example, 2-component epoxy-amine systems. Epoxy-functional molecules of low molecular weight are mixed with conventional epoxy resins to lower the viscosity of the complete binder system.
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