Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2001-03-16
2003-05-27
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S123000, C525S457000, C525S458000, C428S425100, C428S425800, C428S423100
Reexamination Certificate
active
06569944
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for preparing binders for powder coating compositions from urethanes that contain (meth)acryloyl groups and that are cured after application to a substrate and melting by heat by means of high-energy radiation, in particular UV radiation, and also to powder coating compositions containing these binders.
2. Description of the Prior Art
Particularly high-quality powder coatings are obtained using oligourethanes or polyurethanes. Such compounds are described, for example, in EP-A 410 242. They are prepared by reacting at least one organic polyisocyanate with an alcohol component containing (meth)acryloyl groups and additionally at least one compound free of (meth)acryloyl groups and containing isocyanate-reactive groups.
These compounds have to be prepared in an organic solvent since a solvent-free preparation is unsuccessful due to the high viscosities of the compounds at temperatures below 120° C. An increase in the preparation temperature above 100 to 120° C. involves the risk, known to the person skilled in the art, of a spontaneous, unintended polymerization of the compounds containing (meth)acryloyl groups. The disadvantage of the binders described in EP-A 410 242 is that the preparation takes place in a solvent that has to be removed again at the end of the preparation at high cost. Even small residues of solvent considerably affect the melting behavior, which is critical for the application of powder coatings.
To lower the melt viscosity of powder coating compositions containing unsaturated polyesters or acryloyl-functional polyacrylates, the powders are mixed, according to the teachings of EP-A 636 669, with a low-molecular-weight crosslinking substance having vinyl-ether, vinyl-ester or (meth)acrylate functionality after their preparation. In the application cited, the crosslinking substance is also prepared in a solvent that has to be removed afterwards. Furthermore, mixtures of vinyl ethers or vinyl esters with unsaturated (meth)acrylates involve an appreciably higher risk of an unintended spontaneous polymerization (copolymerization of electron-abundant and electron-deficient double bonds) occurring than, for example, in pure (meth)acrylate systems.
In the journal of Coatings Technology, Vol. 70, No. 884, Sep. 1998, 57-62, A. Hult et al. describes radiation-curable powder coatings containing mixtures of amorphous (meth)acrylate-functional polymers (polyacrylate) and crystalline (meth)acrylate monomers. Again, it is necessary to prepare the above-mentioned polymers and monomers separately and using solvents, which have to be removed after the preparation at high cost.
An object of the present invention is to provide a process for preparing oligo-urethanes and polyurethanes containing (meth)acryloyl groups as binders for powder coatings, which process does not have the known disadvantages of preparation in solvents.
This object may be achieved in accordance with the present invention by preparing urethane acrylates for powder coatings without solvent as described hereinafter.
SUMMARY OF THE INVENTION
The present invention relates to a process for preparing urethanes containing (meth)acryloyl groups that are suitable as binders for powder coatings by reacting a polyisocyanate component with a hydroxyl group-containing component that contains at least one compound additionally containing (meth)acryloyl groups in a melt of compounds containing (meth)acryloyl groups, which are crystalline in the solid state, and have a melting point of 30 to 100° C. and a dynamic viscosity at 111° C. of less than 10 Pa·s, at elevated temperature until the NCO content has dropped below 0.5 wt. %.
The present invention also relates to the use of these urethanes as binders for powder coating compositions for coating substrates made of wood, metal, plastics or mineral substances, which may have been primed.
DETAILED DESCRIPTION OF THE INVENTION
The urethane acrylates prepared according to the invention are valuable constituents for powder coating compositions. After melting and radiation-induced curing, the resulting powder coatings yield high-quality lacquer films that are not crystalline.
The process according to the invention is performed in two stages. The compounds containing (meth)acryloyl groups, which are crystalline in the solid state and have a melting point of 30 to 110° C., are prepared in a first stage. The urethane acrylates disclosed, for example, in EP-A 410 242 are then prepared without using solvents in the melt of said compounds at temperatures below 120° C.
Various reactions may be used to prepare the compounds containing (meth)acryloyl groups that are crystalline in the solid state and have a melting point of 30 to 110° C. Thus, it is possible to react monohydroxyalkyl (meth)acrylates containing 2 to 8 carbon atoms in the alkyl group with mono-, di- or triisocyanates. Examples of monohydroxyalkyl (meth)acrylates include 2-hydroxyethyl (meth)acrylate, 2- or 3-hydroxypropyl (meth)acrylate and 2-, 3- or 4-hydroxybutyl (meth)acrylate.
Examples of isocyanates include cyclohexyl isocyanate, butylene diisocyanate, hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI), 3(4)-(isocyanatomethyl)methyl-cyclohexyl isocyanate (IMCI), trimethylhexamethylene diisocyanate (2,2,4- and/or 2,4,4,-trimethylhexamethylene diisocyanate), the isomeric bis(4,4′-isocyanatocyclohexyl)methanes, isocyanatomethyl-1,8-octoane diisocyanate, 1,4-cyclohexylene diisocyanate, phenyl isocyanate, tolyl isocyanate, 1,4-phenylene diisocyanate, 2,4- and/or 2,6-tolylene diisocyanate (TDI), 1,5-naphthylene diisocyanate, 2,4′- and/or 4,4′-diphenylmethane diisocyanate (MDI), triphenylmethane-4,4′,4″-triisocyanate. Also suitable are derivatives of these monomers which contain urethane, isocyanurate, allophanate, biuret, uretdione and/or iminooxadiazinedione groups. HDI, IPDI, TDI, MDI and the isomeric bis(4,4′-isocyanato-cyclo-hexyl)methanes and mixtures thereof are preferred. HDI is particularly preferred.
A prerequisite for the suitability of a particular reaction product of the above-mentioned compounds is its melting range, which must be between 30 and 110° C. for the process according to the invention. Preferred are compounds that have a sharp melting point. Furthermore, the dynamic viscosity of the compounds at 111° C. must be less than 10 Pa·s, preferably less than 5 Pa·s, and more preferably less than 1 Pa·s. Particularly preferred compounds are the reaction products of 1 mole of HDI, TDI or MDI with 2 moles of 2-hydroxyethyl acrylate or 2-hydroxyethyl(meth)acrylate.
The reaction of hydroxy-functional acrylates with isocyanates is known, for example, from Chemistry & Technology of UV & EB Formulations For Coatings, Inks & Paints, Vol. 2, 1991, SITA Technology, London, pages 73-97 by P. K. T. Oldring (ed.). In this procedure the isocyanate is taken and while an oxygen-containing gas, preferably air, is passed through it, it is reacted with the hydroxy-functional acrylate at a temperature above the melting point of the product to be prepared in the temperature range 30 to 120° C. until the NCO content has dropped below 0.5, preferably below 0.2 wt. %. It is also possible to take the hydroxy-functional acrylate and react it with isocyanate under the same conditions.
This addition reaction to form the (poly)urethane can be accelerated in known manner with suitable catalysts, for example, tin octoate, dibutyltin dilaurate or tertiary amines, such as dimethylbenzylamine. The resulting urethane acrylate can be stabilized against premature and undesirable polymerization by adding suitable inhibitors and antioxidants, such as phenols and/or hydroquinones, in amounts, in each case, of 01 to 0.5 wt. %, based on the weight of the (poly)urethane. These auxiliaries can be added either at the same time as and/or after the reaction resulting in the (poly)urethane.
In addition to the isocyanate-hydroxyl reaction described above, epoxy-acid reactions or the reaction of hydroxy compounds
Fischer Wolfgang
Thometzek Peter
Weikard Jan
Zwiener Christian
Bayer Aktiengesellschaft
Bissett Melanie
Gil Joseph C.
Roy Thomas W.
Seidleck James J.
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