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
2000-09-05
2002-07-23
Short, Patricia A. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S124000, C525S440030, C525S458000, C528S045000, C528S060000, C528S065000, C528S066000, C528S093000
Reexamination Certificate
active
06423777
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to high-functional polyaddition compounds which contain uretdione groups, a process for the preparation thereof and their use as starting components for the preparation of polyurethane plastics, in particular as cross-linking agents for heat-curable powder coatings.
2. Description of the Prior Art
Polyaddition compounds, which contain uretdione groups, are being used increasingly as blocking agent-free cross-linking agents for highly weather-resistant polyurethane (PUR) powder coatings. Their cross-linking principle is the thermal splitting of the uretdione structure to give free isocyanate groups again followed by reaction with a hydroxy-functional binder.
Although uretdione powder coating cross-linking agents have been known for a long time, they have not penetrated the market in a decisive manner. Reasons are, inter alia, the relatively low isocyanate content of the currently available commercial products and, their low average functionality which is frequently less than 2.0. For a high durability of lacquers and coatings, however, cross-linking components with a high NCO content and as high as possible a functionality, at least 2.0, are required.
EP-A 45 994 describes reaction products of a linear, difunctional IPDI uretdione and excess amounts of diols as blocking agent-free powder coating cross-linking agents.
These polyaddition products may have up to 8 wt. % of unblocked free isocyanate groups in terminal positions and a high total isocyanate content (sum of the isocyanate groups which are present as free groups and in the form of uretdione internally blocked groups). But they only have a functionality of 2.0 and it is difficult to prepare them, in particular on an industrial scale.
Since the cleaving of thermally labile uretdione structures in the presence of hydroxy-functional reaction partners takes place to a marked extent at 100° C., this temperature must not be exceeded, if possible, during the synthesis of uretdione group-containing polyaddition compounds. However, in this temperature range, the products described in EP-A 45 994 have such a high melt viscosity that the ability to produce them in the molten state is limited due to the inability to stir them on an industrial scale. These uretdione cross-linking agents can only be prepared on a large scale dissolved in solvents, which are inert towards isocyanates. This solvent has to be removed in an additional process step. A solvent-free method of preparation for the polyaddition compounds of EP-A 669 354 requires special reaction screws or extruders under the effect of high shear forces and is costly.
The main disadvantage of the polyaddition products described in EP-A 45 994, however, is the relatively large number of unblocked, free isocyanate groups which can react with the hydroxy-functional binder during the production of powder coatings, normally performed by melt extrusion at about 100° C. These types of pre-cross-linked powder coatings exhibit such unfavorable rheological behavior during the stoving process that they cure to give coatings with only moderate optical properties, in particular completely inadequate flow properties.
The problem of undesired pre-cross-linking exists in principal for all PUR powder coating hardeners, which have a high content of free isocyanate groups. For example, in the case of the blocking agent-free powder coating cross-linking agents made from IPDI uretdione and simple diols, described in EP-A 45 996 and EP-A 45 998, at least some of the terminal free isocyanate groups are reacted with monoamines or monoalcohols. Although the problematic preliminary reaction in the extruder can be suppressed in this way, chain termination with a monofunctional compound clearly reduces the total isocyanate content, simultaneously lowering the average functionality to a value below 2.0. In addition, the products in these two publications also still have such a high fluid viscosity in the region of the temperature at which uretdione groups break apart that they,can be prepared reliably and reproducibly only in solution or by the costly process described in EP-A 669 354.
The preparation of uretdione powder coating cross-linking agents using ester or carbonate group-containing chain lengtheners is the object of EP-A 639 598. The products obtainable by this process have such low fluid viscosities at temperatures only slightly above their melting point that solvent-free production in simple stirred apparatus is possible for the first time. The uretdione cross-linking agents described in EP-A 720 994, in which dimeric diols are used for lengthening the chain, also have a sufficiently low fluid viscosity for solvent-free preparation. However, due to the relatively high molecular weights of the chain lengtheners used, the products in both publications still have lower total isocyanate contents than those in EP-A 45 996.
According to the disclosure in EP-A 669 353, uretdione-containing polyurethanes with terminal hydroxyl groups and an average functionality of 2.0 can be prepared by the solvent-free reaction of linear IPDI dimers and molar excess amounts of diols and/or polyesterdiols in special intensive compounders. The co-use of at least trifunctional polyols and/or branched polyesterpolyols, as demonstrated in EP-A 825 214 and EP-A 780 417, enables the preparation of OH-terminated uretdione cross-linking agents with functionalities between 2 and 3 or ≧3. The advantage of the high functionality of these polyaddition compounds, however, is counteracted by the disadvantage that, under stoving conditions, the hydroxyl groups present in the cross-linking molecules also react and form urethane groups, thus greatly reducing the internally blocked NCO content available for cross-linking the binder.
EP-A 760 380 discloses uretdione powder coating cross-linking agents with high latent NCO contents which are based on optionally isocyanurate group-containing HDI dimers. As described on page 5, lines 28 to 30 and page 7, lines 7 to 9 of this publication, however, products with too high an isocyanurate content or degree of branching lead to coatings with poor optical and mechanical properties. When preparing polyuretdione hardeners according to EP-A 760 380, therefore, branched, isocyanurate-containing starting polyisocyanates are generally extensively “linearized” by the co-use of monofunctional structural components.
Uretdione and isocyanurate group-containing polyaddition products of an IPDI dimer prepared by trialkylphosphine catalysis, diols and/or di-secondary amines as chain lengtheners and optionally monoalcohols or monoamines are the object of EP-A 790 266. These products can have relatively high proportions of free isocyanate groups, and thus a high total isocyanate content, in addition to high functionality, but they are prepared exclusively in organic solution due to the highly branched starting polyisocyanate.
So far no uretdione powder coating cross-linking agent has been proposed which satisfies all the practical requirements, in particular the high total content of isocyanate groups along with as high as possible a cross-linking functionality, and is also readily obtainable in a simple process.
One object of the present invention, therefore, is to provide uretdione group-containing PUR powder coating cross-linking agents, which do not have the disadvantages of the prior art described above.
This objects can be achieved by the polyaddition compounds according to the invention and the process for preparing these described in detail below. The invention is based on the surprising observation that, by reacting branched uretdione and isocyanurate group-containing polyisocyanate mixtures with a molar excess amount of ester and/or carbonate group-containing polyols in a solvent-free process in simple equipment, compounds suitable for use as powder coating cross-linking agents can be prepared. In combination with commercially available powder coating binders, despite a high content of free isocyanate groups and despite a
Freudenberg Ulrich
Grahl Michael
Halpaap Reinhard
Laas Hans-Josef
Bayer Aktiengesellschaft
Gil Joseph C.
Roy Thomas W.
Short Patricia A.
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