Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From reactant having at least one -n=c=x group as well as...
Patent
1996-11-18
1998-08-18
Gorr, Rachel
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From reactant having at least one -n=c=x group as well as...
525124, 2521828, 540202, C08G 1874
Patent
active
057959501
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to straight-chain crystalline polyuretdione mainly used as a hardener for powder coatings and to a powder coating composition which contains the straight-chain crystalline polyuretdione and which exhibits excellent surface leveling, weatherability and mechanical properties.
BACKGROUND ART
Organic solvent type coatings have problems in safety and sanitariness since they employ a large amount of organic solvents. Further, in recent years, organic solvent type coatings have gradually been avoided for use in the coating industry because of the regulation of volatile organic compounds. On the other hand, powder coating has less danger of fire, causes less environmental pollution since it can be recovered for reuse, and can form a thick film. Accordingly, improvement in the physical properties of powder coatings can be expected. A demand for a powder form of a polyurethane coating has been increasing.
Today, as a polyurethane powder coating, there is mainly used a polyurethane powder coating which is prepared by melt mixing a main component of a polyester polyol and a hardener disclosed in U.S. Pat. No. 4,246,380, formed by blocking NCO groups of a polyisocyanate obtained from isophorone diisocyanate (hereinafter referred to as "IPDI") with a blocking agent such as .epsilon.-caprolactam, at a temperature where the blocking agent does not dissociate in an extruder, followed by grinding. When the polyurethane powder coating is used, the blocking agent is dissociated during baking of the coating to cause a cross-linking reaction with the polyol. Therefore, the dissociated blocking agent may badly affect the environment and sanitation. For the purpose of avoiding this defect, there is disclosed a polyuretdione type hardener without a blocking agent but with IPDI in U.S. Pat. Nos. 4,413,079 and 4,463,154, and with tolylene diisocyanate or hexamethylene diisocyanate (hereinafter referred to as "HDI") in U.S. Pat. No. 4,044,171. In this case, the polyuretdione type hardener can be used as a hardener for powder coatings since a uretdione ring comprising 2 mol NCO groups dissociates into 2 mol NCO groups during baking of the coating to react with the polyol. Although IPDI has better weatherability than an aromatic isocyanate because it is aliphatic, the weatherability is not sufficient enough and also it has a problem in the surface leveling property. Since the hardener without a blocking agent has asymmetrical NCO groups in its chemical structure, it has a high dissociation temperature, a gel fraction which is not elevated, and defects in mechanical properties such as impact strength.
U.S. Pat. No. 4,044,171 discloses a method for preparing polyuretdione of HDI in an example. The polyuretdione is used as a hardener for a solvent type coating. Even if the polyuretdione is used as a hardener for a powder coating, it is presumable that the gel fraction is not raised and the mechanical properties such as impact resistance are reduced due to a low content of latent NCO groups. Accordingly, the polyuretdione does not satisfy the essential conditions applicable to a hardener for a powder coating.
Of precoat materials on which a powder coating is baked, a precoat material coated with a powder coating using an epoxy type resin as a hardener to harden a polyol is considerably inferior in weatherability so that its use is disadvantageously limited. When a blocking type isocyanate is used as a hardener in order to improve the weatherability, the blocking agent is dispersed during baking and badly influences the environment during coating in the same way as the solvent type coating.
The object of the present invention is to provide a powder coating composition producing a coating layer excellent in surface leveling, weatherability and mechanical properties using a novel hardener for a powder coating.
DISCLOSURE OF THE INVENTION
The present inventors have made intensive and extensive studies to solve the above problem. As a result, they have succeeded in producing a powder coating
REFERENCES:
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patent: 4246380 (1981-01-01), Gras et al.
patent: 4413079 (1983-11-01), Disteldorf et al.
patent: 4463154 (1984-07-01), Disteldorf et al.
patent: 4614785 (1986-09-01), Richter et al.
patent: 4994541 (1991-02-01), Dell et al.
patent: 5208291 (1993-05-01), Wamprecht et al.
patent: 5237058 (1993-08-01), Laas et al.
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Ibuki Ichiro
Sugimoto Kenshi
Suzuki Shinji
Asahi Kasei Kogyo Kabushiki Kaisha
Gorr Rachel
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