Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2000-12-05
2001-12-25
Woodward, Ana (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C524S514000
Reexamination Certificate
active
06333372
ABSTRACT:
This application is the national phase under 35 U.S.C. § 371 of PCT International Application No. PCT/JP99/02915 which has an International filing date of Jun. 1, 1999, which designated the United States of America.
FIELD OF THE INVENTION
The present invention relates to an aqueous primer composition for a fluororesin coating, that is, a primer composition which is used to adhere the fluororesin to various metal substrates when the fluororesin is coated on such substrates.
BACKGROUND ART
When fluororesins such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymers (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers (PFA), etc. are coated on various metal substrates, it is difficult to apply the fluororesins directly on the metal substrates since the fluororesins are non-tacky as their nature.
Therefore, to coat the fluororesins on the substrates, primer compositions, which have adhesion properties to the substrates and also to the fluororesin coating to be applied on the primers, have been developed and practically used.
In recent years, engineering plastics having excellent heat resistance are developed and thus new primers for a fluororesin coating are being developed by combining such plastics and the fluororesins.
For example, JP-A-61-111352 discloses a fluororesin coating composition consisting of an aqueous dispersion of polyethersulfone and a fluororesin. However, when this composition is used as a primer, the corrosion resistance, which is the drawback of polyethersulfone, tends to further deteriorate.
JP-A-59-199774 discloses a primer composition comprising polyamideimide or polyimide as a binder component, and a specific fluororesin. However, this composition is severely discolored in the course of baking (calcination) so that the color is changed to dark brawn. Thus, the color tone of the coating is limited. In addition, since the composition contains the highly water-absorbing binder resin, the coating of the fluororesin tends to blister due to low steam resistance in the case of rice cookers, in which rice is quickly heated, such as IH cooking jars, when the primer composition is used as the primer for a PFA powder coating which is increasingly used as the coating of the rice cookers.
JP-B-2702041 discloses a primer composition comprising polyethersulfone, polyamideimide and/or polyimide, a fluororesin, and metal powder, which are dissolved or dispersed in an organic solvent. However, in the primer composition disclosed in this patent, the dispersion medium is the organic solvent such as N-methylpyrrolidone, and the binder resin is dissolved in the solvent. Therefore, it is difficult to increase the solid content of the coating. In addition, the composition of the dispersion medium has to be improved from the viewpoint of the environmental protection, which is one of the big social problems.
DISCLOSURE OF THE INVENTION
One object of the present invention is to provide a primer composition for a fluororesin coating, which has good adhesion properties to substrates, and improved corrosion resistance and steam resistance, causes no environmental problems, and has good workability.
The present inventors have found that, when polyethersulfone having good steam resistance but low corrosion resistance as a plastic component contributing to the adhesion to the substrate, and polyamideimide and/or polyimide having good corrosion resistance but low steam resistance are mixed in the form of particles at a ratio, which can make full use of the characteristics of the both resins, in an aqueous medium together with a fluororesin and the dispersion is formed in the form of a film, the characteristics of the two resins having low compatibility each other can be exhibited, and the above-described problems of the conventional primer compositions for the fluororesin coating can be solved.
In addition, since the composition is in the form of an aqueous dispersion, the resin components are not dissolved in the solvent unlike the solvent base primer compositions, and the high solid content can be achieved. Thus, the composition of the present invention can greatly contribute to the resolution of the environmental problem which is one of the big social problems, and improve the easiness and safety of the coating works because of the use of the aqueous medium as the dispersion medium and the increase of the solid content.
Accordingly, the present invention provides an aqueous primer composition for a fluororesin coating comprising a fluororesin, polyethersulfone, and at least one resin selected from the group consisting of polyamideimide and polyimide, which are dispersed in water, wherein a weight ratio of polyethersulfone to polyamideimide and/or polyimide is from 85:15 to 65:35, and a weight ratio of the total amount of polyethersulfone and polyamideimide and/or polyimide to the fluororesin is from 15:85 to 35:65.
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the weight ratio of polyethersulfone to polyamideimide and/or polyimide is from 85:15 to 65:35, preferably from 80:20 to 65:35. When the amount of polyethersulfone exceeds the above range, the corrosion resistance, which is one of the important properties of the primer for the fluororesin coating, greatly deteriorates. When the amount of polyamideimide and/or polyimide exceeds the above range, the freedom of the color tone of the coating is limited because of the discoloration in the course of baking. In addition, the steam resistance decreases when the coating of meltable fluororesins such as PFA powder is applied on the primer.
In the present invention, the weight ratio of the total amount of polyethersulfone and polyamideimide and/or polyimide to the fluororesin is from 15:85 to 35:65, preferably from 20:80 to 35:65. When the amount of the fluororesin exceeds the above range, the adhesion of the primer to the substrates decreases, and thus the corrosion resistance deteriorates. When the amount of the fluororesin is smaller than the above range, the adhesion of the coating of the fluororesin as a top coating to the primer decreases so that the layers may be peeled, and the steam resistance and the corrosion resistance deteriorate to cause the blisters between the layers.
The concentration of the solids in the primer composition of the present invention is not limited. However, such a concentration is usually 50 wt. % or less, preferably 40 wt. % or less, from the viewpoint of the handling of the composition in the transportation and application of the composition.
The kind of the fluororesin contained in the primer composition of the present invention is not limited, and any fluororesin coating, that is used in the conventional primers for the fluororesin, may be used. Examples of the fluororesin include polytetrafluoroethylene (PTFE), modified PTFE, tetrafluoroethylene-hexafluoropropylene copolymers (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers (PFA), etc. In addition, fluororesins comprising resin particles with a core-shell structure may be used. The fluororesins may be used independently or in admixture of two or more.
In the present invention, polyethersulfone, polyamideimide and/or polyimide as the binders are pulverized and stabilized in the dispersed state in water. When the binder resins have too large particle sizes, the dispersion stability tends to deteriorate and the resin particles tend to precipitate and further the surface smoothness of the coated film of the primer decreases. In addition, when the particle size is large, the properties of each binder resin prior to blending remain, so that the intended properties of the coating film may not be attained. Thus, the average particle size of polyethersulfone and polyamideimide or polyimide is usually 8 &mgr;m or less, preferably 5 &mgr;m or less.
When the particle size of the binder resins is small, the thixotropy of the composition increases and also the viscosity of the composition increases, so that the handing and application of the composition become difficult, although the surfa
Ogita Koichiro
Sanemasa Hisato
Tomihashi Nobuyuki
Birch & Stewart Kolasch & Birch, LLP
Daikin Industries Ltd.
Woodward Ana
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