Linking of Polyester Resins

Amino Resins. Formaldehyde-modified amino resins (i.e., melamine, benzogua-namine, and urea resins) are the most important resins for the heat curing of hydroxyfunctional polyester resins. These resins are readily available with a low molecular mass (very good polyester compatibility, but less reactive) or in precondensed form (limited compatibility, but very reactive). In order to prevent premature reaction in the wet paint, the amino resins are blocked by etherification with, for example, methanol or butanol. Sulfonic acids (e.g., p-toluenesulfonic acid, dodecyl-benzenesulfonic acid) have proved suitable for accelerating the deblocking of amino resins during heat curing. These acids must also be used in blocked form (ammonium salts, thermolabile adducts). 

Poiyisocyanate Resins. Hydroxy-functional polyester resins play a significant role in two-pack polyurethane paints. Combinations with nonblocked poiyisocyanate resins are discussed in Section 2.9. 

Storage-stable one-pack polyester stoving enamels can be formulated with poiyisocyanate resins that are thermoreversibly blocked. Tin catalysts (e.g., dibutyl tin dilaurate) are particularly suitable for accelerating hardening with hydroxypolyester resins. 

Although linear polyester resins do not readily undergo heat curing, they form extremely flexible paint films with excellent adhesion to metal substrates. Poiyisocyanate resins are more resistant to hydrolysis than amino resins. Polyester-polyurethane paints therefore exhibit improved resistance to moisture and weathering but are more expensive. 

Radiation Curing. The cross-linking of acrylate-modified polyester resins with U V radiation or an electron beam is an energy-saving alternative to the heat-curing systems. Reactive diluents (e.g., polyfunctional acrylates) are required to adjust the viscosity for application although they increase reactivity, they reduce the flexibility and substrate adhesion of the paint film. 

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