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
1999-12-27
2001-04-17
Sellers, Robert E. L. (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...
C525S113000, C525S486000, C525S488000, C525S528000, C525S530000, C525S533000, C525S934000
Reexamination Certificate
active
06218483
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a two-component powder coating system by which the curing of a coating occurs at a significantly faster rate and/or at a significantly lower temperature and produces an exceptionally smooth surface. This invention also relates to thermally stable powder coating whereby white coatings do not yellow during thermal curing. This invention also relates to the electrostatic coating of metal and, in particular, to the coating of heat sensitive substrates such as wood with a coating powder and the low temperature curing of that coating to achieve a pleasing smoothness. More particularly, it relates to an improvement of the humidity resistance of such a coating.
Traditionally, coating powders have been made by the extrusion of a mixture of resins and curing agents to obtain a homogeneous mixture and then grinding the extrudate and screening the comminuted product to obtain the desired particle sizes and particle size distribution. The powder is then electrostatically sprayed onto a substrate, traditionally a metal substrate, and cured at temperatures much higher than 200° F. The curing of powder coatings on heat sensitive materials such as wood, plastic, and the like has been limited by the fact that the extrusion of a mixture of a resin and a low temperature curing agent, i.e., one that is active at 200° F. or less, would cause the coating powder to gel in the extruder because the extrusion typically generates enough heat to raise the temperature to 200° F. or higher. It has been thought throughout the art that the curing agent must be mixed with the resin by extrusion to obtain a uniform cure and film appearance. It has also been commonly accepted that a low gloss film must be cured at a high temperature, e.g., about 300° F. or higher.
In this invention, a self-curing epoxy resin is first melt- mixed (e.g., extruded) with a small amount of catalyst or with a low level of a low temperature curing agent, then chilled and ground to a obtain a first powder and classified in the usual manner. Additional amounts of the low temperature curing agent in powder form are then blended with the first powder to raise the level of curing agent while avoiding the pre-gelation problem. Surprisingly, the time required to obtain a smooth cured film is lowered significantly. It is surprising, also, that a low gloss film is achieved at cure temperatures much lower than 300° F.
The blending of the first powder with the powdered curing agent to make a two component coating powder is disclosed in U.S. Pat. No. 5,714,206 which issued from an application commonly assigned herewith, and in commonly assigned, co-pending Ser. No. 09/325,873 which was filed on Jun. 4, 1999. Now, it has been found that the humidity resistance of coatings made from such two component powders is improved by inactivating polyamine domains that remain in the powder because of incomplete blending and do not come into contact with the base epoxy resin and which, otherwise, would be carried over into the cured coating.
SUMMARY OF THE INVENTION
It is an object of this invention, therefore, to provide a coating powder for heat sensitive substrates.
It is a related object of this invention to provide a method for coating such substrates without the problems associated with volatile organic solvents.
It is another object of this invention to provide a low temperature process for producing a smooth, low gloss coating on wood and other such substrates.
It is yet another object of this invention to provide a powder coating for heat-sensitive substrates that has an improved resistance to high humidity.
These and other objects of the invention which will become apparent from the following description are achieved by a thermosetting powder coating system in which the thermosetting of a melt-mixed blend of an epoxy resin and (A) a catalyst or (B) an amount of a low temperature curing agent insufficient to cause substantial curing of the resin during melt-mixing is facilitated by the separate addition of (C) a polyamine as a friable low temperature curing agent; said melt-mixed blend and said separately added low temperature curing agent being further blended with (D) a scavenger which will react secondarily with polyamine domains that will not have reacted with the base epoxy resin during curing.
For the purposes of this invention, the term base epoxy resin means the principal self-curing epoxy resin as opposed to epoxy resins functioning as adducting agents for polyamines and imidazoles. Also, for the purposes of this invention, the resin means the base epoxy resin per se plus the crosslinking agent, whether it is in the melt-mix or is added separately, but not the catalyst.
DETAILED DESCRIPTION OF THE INVENTION
A particularly favored embodiment of the system is one in which competing reactions are taking place simultaneously, said reactions being:
(A) a catalyzed self-curing of a portion of an epoxy resin present in a powdered, melt-mixed blend of the resin and a catalyst, and (B) a crosslinking reaction between another portion of the powdered, melt-mixed resin and the polyamine as a friable low temperature curing agent.
Epoxy resins which are useful as the principal self-curing resins in the practice of this invention are produced by the reaction of epichlorohydrin and a bisphenol or a novolac resin. Bisphenol A/epoxy resins are sold under the trademarks ARALDITE GT-7004, 7013, 7072, 7074, 3032, 6062, and EPON 1007F, 1009F, and 1004. The epoxy novolac resins are exemplified by epoxy phenol novolac (EPN) and epoxy cresol novolac (ECN). ARALDITE GT-7220 and GT 6259 are trademarks for an EPN and an ECN resin, respectively.
The epoxy resin is self-curing, i.e., it reacts via homopolymerization during curing of the powder coating. Generally, a catalyst is required to cause the reaction to progress at a commercially acceptable rate. A preferred catalyst for this invention is an epoxy adduct of an imidazole having the general formula:
wherein R
1
, R
2
, R
3
,and R
4
are independently hydrogen or any substituent which is not reactive with the base epoxy resin. Examples of suitable imidazoles include imidazole, 2-methyl imidazole, and 2-phenyl imidazole. Suitable adducts of such imidazoles with a bisphenol A epoxy resin are available commercially from Shell Chemical Company under its trademark EPON, e.g., EPON P-101, and also from Ciba-Geigy Corporation under its designation HT-3261. For the purposes of this invention, the term imidazole is used herein to mean both the substituted and unsubstituted imidazoles. Although applicants are not bound to any theory, it is believed that an imidazole adducts to epoxy resins by an opening of the epoxy ring that results in the epoxy oxygen bonding to the C═N bond of the imidazole ring. The adducted imidazole acts as a catalyst, moving from one epoxy group to another as it facilitates epoxy ring opening and cure reactions. The imidazoles are, in themselves, the operative catalysts but they tend to be insoluble in epoxy resins. Thus, the purpose for adducting them to an epoxy resin is to make them compatible with the epoxy system. As a catalyst, the imidazole adduct is used in the systems, methods, and powders of this invention at a level of from about 0.1 to about 8 parts per hundred parts of the extruded resin (abbreviated as phr), preferably at about 2 phr. For enhanced color stability, the 2-phenyl imidazole, which is available from the SKW Chemical Co., may be used as the catalyst for curing the epoxy resin with the low temperature curing agent.
Polyamines in general are the active curing agents in this invention. They are exemplified by ethylene diamine, isophorone diamine, cyclohexylenediamine, and a fluorinated diamine such as 4,4′-hexafluoroisopropylidene bis-aniline. They must be converted, however, from their usual liquid state into a friable solid that may be pulverized in order to serve in this invention. A friable solid low temperature curing agent may be selected from a blocked polyamine such as an adduct of an epoxy resin having an eq
Muthiah Jeno
Schelessman Jaquelyn
Tullos Gordon
Rohm and Haas Company
Sellers Robert E. L.
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