Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From phenol – phenol ether – or inorganic phenolate
Reexamination Certificate
2002-07-03
2003-11-18
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From phenol, phenol ether, or inorganic phenolate
C502S155000, C502S164000, C525S438000, C525S533000, C528S089000, C528S091000
Reexamination Certificate
active
06649728
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to catalysts for curing epoxy resins and particularly a phosphonium salt catalyst for curing epoxy resins. This catalyst can dramatically improve storage stability, i.e., pot lime, of epoxy resin powder coating compositions. The present invention also relates to a method for making the catalyst, an epoxy resin composition containing the catalyst, and a powder coating composition containing the epoxy resin composition.
2. Description of the Related Art
Thermosetting epoxy resins are extensively used as coating materials in civil engineering, electric, and vehicle fields because of their excellent properties.
The epoxy resins are compounded with a hardener and a curing catalyst and are cured in use. Recently, the epoxy resins have attracted attention in the form of environmentally harmonious powder coatings containing no organic solvents. In particular, a powder coating that has carboxyl groups as primary end groups and is composed of a polyester resin and an epoxy resin has a satisfactory appearance and is relatively inexpensive; hence, this powder coating is extensively used in a variety of industrial fields, such as household electric appliances and building materials. Such types of powder coating require a baking process at high temperatures for a long time. In order to reduce energy costs, hardening catalysts are added to allow low-temperature baking to be carried out.
Examples of hardening catalysts that have been used for such types of powder coating include triphenylphosphine, imidazoline, and imidazole. Powder coatings containing any of these hardening catalysts, however, have poor pot life and exhibit an increase in viscosity during blending and poor fluidity in a molding process.
Countermeasures for solving such problems are, for example, use of a metal salt complex of an adduct of polyepoxide and an imidazole compound as disclosed in Japanese Unexamined Patent Application Publication No. 10-30015 and use of an imidazole derivative having a melting point of 170° C. or more as disclosed in Japanese Unexamined Patent Application Publication No. 10-204329.
Phosphonium salts such as phosphonium borates have been known as excellent curing catalysts for epoxy resins. Tetraalkylphosphonium tetrafluoroborate has high solubility in organic solvents, a high degree of dissociation, and high electrochemical stability and is used as an electrolyte for electric double-layer capacitors.
Tetraalkylphosphonium tetrafluoroborate is commercially produced using tetraalkylphosphonium bromide (Formula (3) and sodium borofluoride (Formula (4) according to Reaction Formula (I):
In this reaction, sodium bromide (Formula (5)), in addition to the target product, is formed as a byproduct.
Even if the resulting tetraalkylphosphonium tetrafluoroborate or a known phosphonium salt is used as a curing catalyst for epoxy resin powder coatings, the pot life of the epoxy resin powder coatings is not improved.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a catalyst for curing epoxy resins that can dramatically improve the pot life of an epoxy resin powder coating composition.
Another object of the present invention is to provide an epoxy resin composition and a powder coating composition containing the catalyst and exhibiting improved pot life.
The inventors of the present invention have intensively studied phosphonium salt curing catalysts that can preferably be used in epoxy resin powder coatings and have discovered that a tetraalkylphosphonium tetrafluoroborate curing catalyst having a specific structure that is produced by the reaction of a tetraalkylphosphonium hydroxide with fluoroboric acid and has a drastically reduced amount of halogen ion content dramatically improves the pot life of the epoxy resin and the epoxy resin powder coating. The present invention has been thereby completed.
According to a first aspect of the present invention, a catalyst for curing epoxy resins comprises a tetraalkylphosphonium tetrafluoroborate represented by Formula (1):
wherein R
1
, R
2
, R
3
, and R
4
are each C1-C5 linear or branched alkyl and may be the same or different, and the halogen ion content in the tetraalkylphosphonium tetrafluoroborate is 20 ppm or less.
According to a second aspect of the present invention, a method for making the catalyst according to the first aspect includes allowing a tetraalkylphosphonium hydroxide represented by Formula (2) to react with fluoroboric acid:
wherein R
1
, R
2
, R
3
, and R
4
are defined in the first aspect.
According to a third aspect of the present invention, an epoxy resin composition comprises an epoxy resin, a hardener, and the catalyst according to the first aspect.
According to a fourth aspect of the present invention, a powder coating composition comprises the epoxy resin composition according to the third aspect.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be described in detail.
The catalyst for curing epoxy resins according to the present invention is a tetraalkylphosphonium tetrafluoroborate represented by Formula (1):
wherein R
1
, R
2
, R
3
, and R
4
are each C1-C5 linear or branched alkyl. Examples of alkyls include methyl, ethyl, propyl, butyl, and pentyl. R
1
, R
2
, R
3
, and R
4
may be the same or different.
Examples of tetraalkylphosphonium tetrafluoroborates include tetramethylphosphonium tetrafluoroborate, tetraethylphosphonium tetrafluoroborate, tetra-n-propylphosphonium tetrafluoroborate, tetra-iso-propylphosphonium tetrafluoroborate, tetra-n-butylphosphonium tetrafluoroborate, tetra-iso-butylphosphonium tetrafluoroborate, tetra-sec-butylphosphonium tetrafluoroborate, tetra-tert-butylphosphonium tetrafluoroborate, tetra-n-pentylphosphonium tetrafluoroborate, tetra-iso-pentylphosphonium tetrafluoroborate, tetra-tert-pentylphosphonium tetrafluoroborate, tetraneopentylphosphonium tetrafluoroborate, trimethylethylphosphonium tetrafluoroborate, trimethyl-n-propylphosphonium tetrafluoroborate, trimethyl-iso-propylphosphonium tetrafluoroborate, trimethyl-n-butylphosphonium tetrafluoroborate, trimethyl-iso-butylphosphonium tetrafluoroborate, trimethyl-tert-butylphosphonium tetrafluoroborate, trimethyl-sec-butylphosphonium tetrafluoroborate, trimethyl-n-pentylphosphonium tetrafluoroborate, trimethyl-tert-pentylphosphonium tetrafluoroborate, trimethylneopentylphosphonium tetrafluoroborate, triethylmethylphosphonium tetrafluoroborate, triethyl-n-propylphosphonium tetrafluoroborate, triethyl-iso-propylphosphonium tetrafluoroborate, triethyl-n-butylphosphonium tetrafluoroborate, triethyl-iso-butylphosphonium tetrafluoroborate, triethyl-tert-butylphosphonium tetrafluoroborate, triethyl-sec-butylphosphonium tetrafluoroborate, triethyl-n-pentylphosphonium tetrafluoroborate, triethyl-tert-pentylphosphonium tetrafluoroborate, triethylneopentylphosphonium tetrafluoroborate, tri-n-butyl-methylphosphonium tetrafluoroborate, tri-n-butyl-n-propylphosphonium tetrafluoroborate, tri-n-butyl-iso-propylphosphonium tetrafluoroborate, tri-n-butyl-iso-butylphosphonium tetrafluoroborate, tri-n-butyl-tert-butylphosphonium tetrafluoroborate, tri-n-butyl-sec-butylphosphonium tetrafluoroborate, tri-n-butyl-n-pentylphosphonium tetrafluoroborate, tri-n-butyl-tert-pentylphosphonium tetrafluoroborate, tri-n-butylneopentylphosphonium tetrafluoroborate, tri-n-pentylmethylphosphonium tetrafluoroborate, tri-n-pentylethylphosphonium tetrafluoroborate, and tri-n-pentyl-n-butylphosphonium tetrafluoroborate. These may be used alone or in combination.
In the catalyst for curing epoxy resins according to the present invention, the halogen ion content in the tetraalkylphosphonium tetrafluoroborate is 20 ppm or less and preferably 10 ppm or less. The pot life of the epoxy resin composition is improved at such a halogen ion content.
The phosphonium salt represented by Formula (1) and substantially not containing halogen ions can be readily formed by the reaction of a tetraalkylphosphonium hydroxide (Formula (2)) with tetrafluoroboric acid (Formul
Hara Yoshifusa
Kawakabe Hiroshi
Sugiya Masashi
Aylward D.
Dawson Robert
Nippon Chemical Industrial Co. Ltd.
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