Self - Crosslinking Isocyanates in 2-Pack Polyurethane Coatings

摘要

As a result of the increasingly restrictive low-VOC regulations, the reduction of solvents in coating formulations has become an important target for industrial research and development. Therefore, different alternatives to conventional coating systems have been developed within the last 20 years. Apart from water-based or powder-based systems another approach is the so-called high-solids coating. Due to their excellent physical and chemical properties Polyurethane (PUR) coatings find broad application, especially, in outdoor coating formulations. Therefore, the development of two component (2K)-PUR raw materials for high-solids coatings is an important research target. To reach a high solid content in 2K-PUR coatings, two different routes have been followed in the past. One possibility is to lower the viscosity either of the binder or of the polyisocyanate crosslinking agent. Disadvantages of this approach include that in most cases only a slight increase of the solids content can be obtained, and often longer curing times result from these low viscosity coating components. In the other approach, reactive thinners were used to decrease viscosity of the PUR system. Reactive thinners show the advantage that very high solid contents might be achieved, however, they usually increase the price of the coating formulations significantly because the reactive groups of the employed thinner (usually hydroxyl or amino) must be compensated by adding more expensive isocyanate crosslinkers. Self-crosslinking isocyanates (SCI) are a relatively new way of formulating (super) high-solids systems. SCI's are low-viscosity resins that contain blocked OH- (or NH-) groups as well as NCO- groups, which are in an equivalent molar ratio for our allophanate type SCI. For that reason no extra isocyanate component has to be added. After activation of the SCI, the deblocked OH- and NH- groups react with NCO-groups to form polyuretnanes and polyureas respectively. The principle and synthesis of SCI have only recently been described in the literature. In this paper the chemical composition as well as the properties of these new oligomeric materials have been investigated. Furthermore, the mechanism of crosslinking in the presence of moisture has been examined, which influences the type and concentration of chemical functionalities in the cured coating film.