Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1998-12-17
2001-05-08
Niland, Patrick D. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C523S220000, C523S221000, C523S223000, C524S099000, C524S210000, C524S430000, C524S432000, C524S492000, C524S493000, C524S500000, C524S544000
Reexamination Certificate
active
06228932
ABSTRACT:
FIELD OF INVENTION
The present invention relates to fluororesin powder liquid dispersion which is suitable for coating articles, specifically to a fluororesin powder liquid dispersion which does not undergo irreversible coagulation of the liquid dispersion and can form extremely thick film coatings.
BACKGROUND OF THE INVENTION
Heat-flowable fluororesins having properties such as chemical resistance, nonstickiness, heat resistance, a low friction of coefficient and electrical insulation properties, are also capable of forming pinhole-free films and are thus useful as coating materials. Heat-flowable fluororesins used in such coating applications include, for example, copolymers of tetrafluoroethylene and perfluoro(alkyl vinyl ether) (hereinafter referred to as PFA), copolymers of tetrafluoroethylene and hexafluoropropylene (FEP), and copolymers of tetrafluoroethylene and ethylene (ETFE). These resins are insoluble in water and organic solvents, and so cannot be used as solution-type coatings. Thus, these resins are applied onto substrates by various other means, including electrostatic application of powder coating compositions or used as liquid dispersions. Liquid dispersions can be water-based and stabilized with a surfactant and or can be based on organic liquids. Liquid dispersions are applied to substrates by such means as spray-coating, dipping, or curtain-coating followed by heating and fusing to generate film coatings.
The film coatings obtained from electrostatic application of powder coating compositions generally have a thickness of about 50 to 100 &mgr;m, whereas dispersion coating compositions generally produce coated films having a thickness of 20 &mgr;m. However, this level of thickness is not sufficient for applications which require corrosion prevention, which leads to the demand for developing coating compositions capable of forming thicker film coatings.
Japanese Patent Application 57-15607 discloses a fluororesin liquid dispersion for thick-film coating application comprising a heat-flowable fluororesin powder having an average particle size of 2 to 300 &mgr;m and a void fraction of not more than 0.74 in a liquid dispersing medium having a surface tension of not more than 45 dyne/cm. This dispersion makes it possible to form thick,
500-&mgr;
m coats when the surface being coated is horizontal or level, but when the surface being coated is vertical or inclined at an angle, the fluororesin powder falls off the coated surface prior to baking and film formation.
In order to form thick coats on vertical surfaces as well, U.S. Pat. No. 5,502,097 discloses a fluororesin powder liquid dispersion for thick-film coating application comprising a heat-flowable fluororesin powder having an average particle size of 5 to 300 &mgr;m and a porosity of not more than 0.74 and a total surface area of not more than 10 m
2
/cm
3
in a liquid dispersion medium having a surface tension of not more than 45 dyne/cm, and also containing an organic liquid having a boiling point of 150 to 340° C. This invention enables a thick coat to be formed even when the surface being coated is vertical. However, because the flammability of the organic liquid in the dispersion presents a danger of explosion, this has imposed limits on the working environment.
Thus, there still remains a need, especially in applications requiring corrosion resistance, such as at chemical plants where the performance of a coating is approximately dependent on the thickness of a film, for coating compositions capable of forming thick coated films on vertical surfaces and which can be applied safely.
SUMMARY OF THE INVENTION
The present invention is related to a discovery that when colloidal particles having an average particle size of not more than 1 &mgr;m are included in a heat-flowable fluororesin powder liquid dispersion, a thick powder coating can be formed on a vertical surface which coating, prior to baking and film formation, does not fall off the substrate. These effects are further enhanced by including in the dispersion a fibrous heat-resistant filler.
Consequently, this invention provides fluororesin powder coating compositions which are capable of forming a thick coating having a thickness of 100 to 1,000 &mgr;m with excellent corrosion resistance. The coatings can form a film in a single step using a simple and convenient spray gun or the like, free of any explosion hazard.
Specifically, the fluororesin powder liquid dispersion comprises 5-50% by volume based on the liquid dispersion of heat-flowable fluororesin powder having an average particle size of 5-300 &mgr;m, a porosity of not greater than 0.74 and a total surface area of not more than 10 m
2
/cm
3
, liquid dispersion medium having a surface tension of not more than 45 dyne/cm, and colloidal particles having an average particle size of not more than 1 &mgr;m. Preferably, the colloidal particles are selected from a group consisting of inorganic oxides and organic heat resistant polymer. More preferably, the fluororesin powder liquid dispersion further comprises fibrous heat resistant filler having a length of at least 20 &mgr;m and an aspect ratio of at least 2.
Preferably, the colloidal particles in the fluororesin powder liquid dispersion are particles of polyether sulfone which are added as an aqueous dispersion comprising an organic solvent and a water-miscible solvent. An aqueous dispersion of PES colloidal particles having an average particle size of not more than
1
&mgr;m is preferably prepared by (1) dissolving PES in an organic solvent capable of solubilizing PES; (2) combining the dissolved PES with a water-miscible solvent which is capable of being dissolved in the organic solvent; and (3) combining water in which there is a dissolved surfactant with the dissolved PES and the water-miscible solvent.
DETAILED DESCRIPTION
Heat-Flowable Fluororesin
Heat-flowable fluororesins which can be used in this invention are copolymers of tetrafluoroethylene and other comonomers which melt and liquify to flow at temperatures beyond their melting points. Examples of such copolymers include copolymers of tetrafluoroethylene and perfluoro(alkyl vinyl ether) (PFA) such as perfluoro(propyl vinyl ether), copolymers of tetrafluoroethylene and hexafluoropropylene (FEP), and copolymers of tetrafluoroethylene and ethylene (ETFE). “Heat-flowable” as used herein means that the fluororesin is melt fabricable, and as such, the fluororesin particles will flow and fuse together upon heating above their melting temperature.
The average particle size of the heat-flowable fluororesin powder used in this invention is preferably in the range of 5-300 &mgr;m. If the average particle size of the heat-flowable fluororesin powder exceeds 300 &mgr;m, the coating formed is prone to pinholes. On the other hand, if the average particle size is smaller than 5 &mgr;m, when a thick film is formed, the surface of the film is prone to cracks. In addition, the heat-flowable fluororesin powder may also contain a filler, or it may be a heat-flowable powder processed in which the particles have been surface-modified.
The heat-flowable fluororesin powder used in this invention has an average porosity of not more than 0.74, preferably in the range of 0.34-0.65. If the porosity exceeds 0.74, the coating formed is prone to cracks, and the surface smoothness of the coating is prone to degradation. In this invention, by the term “porosity” it is meant the volume of the space in a powder layer, expressed by the following formula:
Porosity
=
1
-
Apparent
Specific
Gravity
of
the
Powder
True
Specific
Gravity
of
the
Material
Constituting
the
Powder
Also, the heat-flowable fluororesin powder of this invention has a total the total surface area of not more than 10 m
2
/cm
3
. The term “total surface area” as used herein refers to the total surface area of the fluororesin powder p
Dupont Mitsui Fluorochemicals
Niland Patrick D.
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