The Powder Coatings technology for metal substrates is a well established process consisting in the application of a powdered fusible material to a substrate, heating the powder in contact with the substrate, melting and flow of that powder, usually incurring in some chemical reaction, and finally cooling the resultant to form a solid coating.
The coating material may simply comprise a film-forming thermoplastic material or may comprise a reactive material, as an epoxy resin together with a curing agent, so that during the course of melting and reflowing, reaction occurs to cure the material in situ on the substrate. In any event, powder coating materials are generally such that they have to be melted and at relatively high temperatures, 150° C. or more. As a result, powder coating processes are generally not suitable for the coating of heat-sensitive materials such as wood or plastics material. In order to coat such materials, it would be necessary to have a product which had a relatively low melting temperature, e.g. below about 120° C., whilst at the same time being solid at ambient temperature, which material could subsequently be cured in situ on the substrate during the melting/reflowing stage or afterwards.
The resins used in coatings can have a very different origin: polyester, epoxy, isocyanates, etc. Developed binders as amorphous polyester acrylates resins, for example, can not fulfill the requirements due to their high Tg. Urethane acrylates (UA), in contrast, have a wide versatile chemistry that can be used to coat these materials.
For example, EP 783 534 discloses a powder coating composition comprising a solid ethylenically unsaturated resin having a melting point from 60 to 120° C. and obtained by reacting a polyisocyanate with at least stoechiometric amount of an alcoholic component based on an ethylenically unsaturated (acrylate or methacrylate) monoalcohol and at least one saturated monoalcohol and optionally at least a monomeric polyol. In spite of their improvements with respect to prior art, these powder compositions require still improved processability, storage stability and an improved compromise between low viscosity and good flow and impregnability from one side and antisagging resistance on the other side with high enough melting point and improved weathering and outdoor resistance and mechanical performances for better suitability for the coating at relatively lower temperatures of thermally sensitive substrates like wood and plastics. In spite of these performances, they do still need to be improved, particularly in terms of hardness and chemical resistance and weathering of the resulting coating.
EP-A-0 410 242 discloses a powder coating composition which comprises the reaction product of one or more organic polyisocyanates with one or more monovalent alcohols containing (meth)acryloyl groups, and one or more compounds which are free from (meth)acryloyl groups and contain at least two isocyanate-reactive groups. The viscosities of these products are generally not suitable for thermally sensitive substrates due to a high viscosity which does require heating for lowering such a viscosity.
Powder coating application should in fact deal with a compromise: melting, flowing and curing proceed more or less at the same time. Moreover, the high temperatures needed to cure, make standard powder coating systems difficult to coat heat-sensitive materials such as wood or plastics. Storage conditions are a determinant restriction too, because the said powder coating compositions should be stable under storage conditions.
Consequently, with respect to this closest prior art, there is a need for improvements in terms of processability, storage stability and of compromise between low viscosity, good flow and impregnability from one side and antisagging resistance on the other side. This is obtained with a lower enough melting point for a better suitability of the coating at relatively lower temperatures, for thermally sensitive substrates like wood and plastics.
It has now been found, in accordance with the present invention, that useful solid ethylenically unsaturated powder resin materials, having relatively low melting point and suitable for use in powder coating compositions, particularly for heat-sensitive substrates, can be prepared by reacting a polyisocyanate with an alcoholic component having a specific composition.
The first subject of the invention relates to a powder resin comprising the reaction product of a polyisocyanate with a specific alcoholic component comprising besides a polyol, selected from diols, triols or tetrols, a specific combination of a saturated and of an unsaturated monoalcohol.
The second subject concerns a powder coating composition comprising the said powder resin.
Another subject concerns a powder coating composition comprising at least one powder resin of the invention.
It is also part of the invention a coating resulting from the cure of the said powder coating composition.
Another subject is related with the coated substrates resulting from the said coating composition.
Finally, specific uses of the powder resin of the invention are also part of the invention.