Coating processes – With post-treatment of coating or coating material – Heating or drying
Reexamination Certificate
2002-12-10
2004-08-10
Tucker, Philip (Department: 1712)
Coating processes
With post-treatment of coating or coating material
Heating or drying
C523S404000, C523S417000, C523S420000, C523S457000, C528S093000, C528S095000, C528S413000
Reexamination Certificate
active
06773754
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to epoxy gels that are used to form the exterior coating of a wide variety of products made from composite materials. More particularly, the invention involves epoxy resins that form gels at room temperature and may be used in the fabrication of gelcoats on new parts or as a repair for damaged gelcoats.
2. Description of Related Art
Epoxy resins that are used to form gelcoats are, like other epoxy resins, typically cured by a reaction between a monomer containing an epoxide end-group and a “hardener”, i.e., a multifunctional nucleophile, such as an amine or a hydroxyl-containing moiety. For many epoxy resins, the curing process is usually carried out at an elevated temperature dictated by the nature of the prepolymeric species involved. For gelcoats, however, it is also important that the epoxy resin be able to form a gel relatively rapidly at room temperature.
Gelcoat resins are typically applied to the surface of a mold that is being used to form a composite part. During application, the gelcoat resin must remain sufficiently fluid so that is can be brushed on or otherwise applied to the mold to form a uniform layer of resin. The gelcoat is then allowed to cure at room temperature to form a gelled layed. Once the gelcoat has gelled, layers of composite material (e.g., fiberglass and epoxy resin) are applied to form the main structure of the part. It is essential that the gelcoat be completely gelled prior to application of the remaining composite material. If gelling is not complete, the gelcoat could be damaged and the exterior surface of the resulting part disrupted.
Gelcoats are usually prepared by mixing the gelcoat resin with a hardener to form an activated gelcoat resin. The time that it takes for the activated gelcoat resin to go from a viscous liquid to a gelled body at room temperature is known as the “gel time”. In general, it is desirable to shorten the gel time as much as possible so that the overall fabrication time will also be reduced. Shortened gel times are also important when the gelcoat resin is being applied as a repair to the surface of a damaged part. However, it is equally important that any attempt to accelerate gel formation and shorten gel times does not adversely affect other properties of the gelcoat. For example, the glass transition temperature (Tg) of the finally cured gelcoat is an important property that may be reduced substantially when gel times are shortened.
One approach to shortening gel times involves adding various combinations of chemicals to the resin to accelerate the gelling process. One such combination of chemicals is the accelerator system that is disclosed in U.S. Pat. No. 5,243,014. This system is based on aqueous or non-aqueous mixtures of an inorganic salt with an aliphatic hydroxy compound and an aminophenol. Due to the complex chemistries involved in the gelling of epoxy resins, it is difficult to predict what, if any, correlation there is between the various chemicals that are added to the gelcoat and the gel time or resulting properties of the cured gelcoat.
In view of the above uncertainties, there is a continuing need to select and develop new combinations of chemicals that are capable of shortening the gel times of resins used in gel coatings without adversely affecting other properties of the cured gelcoat, such as the glass transition temperature.
SUMMARY OF THE INVENTION
The present invention provides gelcoat systems having a greatly reduced room temperature gel time, when compared to a non-accelerated gelcoat system, without adversely affecting the Tg of the cured gelcoat. It was discovered that aqueous solutions of inorganic nitrate salts could be used as an accelerator to substantially decrease room temperature gelling times without substantially reducing the Tg of the finally cured gelcoat.
Based on the discoveries of the present invention, a kit is provided for use in forming an epoxy gelcoat. The kit is composed of: a hardener component that includes a curing agent; and a resin component that includes a liquid epoxy resin. The two components are mixed together to form an activated gelcoat resin that is converted from a liquid to a gel over a period of time at room temperature. As a feature of the present invention, an aqueous accelerator is included as part of the kit. The aqueous accelerator contains an aqueous solution of an inorganic nitrate salt.
It was found that the use of an aqueous solution of an inorganic nitrate salt as an accelerator caused a significant reduction in the room temperature gel time observed, from about 6 hours to about 1 hour. A further, advantage of the aqueous nitrate accelerator is that the Tg of the cured gelcoat remains substantially unchanged relative to the Tg of similar non-accelerated gelcoats.
The present invention also covers methods for making gelcoats where the gel times are substantially reduced by use of an aqueous nitrate accelerator. The methods include curing of the gelled coating to form the finally cured gelcoat. The invention is applicable to methods for forming gelcoats during the initial fabrication of a part as well as methods for repairing existing gelcoats. The invention is especially useful in situations where short gel times are desired, such as in mass production of composite parts or field repair of damaged structures.
The above discussed and many other features and attendant advantages of the present invention will become better understood by reference to the following detailed description.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is useful for shortening the room temperature gel time of a variety of epoxy resins that are typically used in making gelcoats. Exemplary gel coat resins that may be accelerated include aliphatic difunctional or trifunctional epoxy polymers. Preferred resins that may be used to form the gelcoat include Araldite DY-T/CH which is available from Vantico (Duxford, United Kingdom). Heloxy 48 which is available from Resolution Polymers, Inc. and CVC Erisys GE-30 or GE-31.
Conventional additives may also be added to the epoxy resin as is well known by those skilled in gelcoat technology. Such additives include thixotropic agents, ultraviolet light (UV) absorbers and pigments. Exemplary thixotropic agents include, fumed metal oxides, glass beads, and microballons. Fumed silica oxides such as CABOSIL (Cabot Corp., Boston, Mass.) are preferred thixotropic agents. An exemplary pigment is titanium oxide. Other pigments that are normally used in gelcoats may also be used.
Suitable UV absorbers may include benzophenones and hindered amine light stabilizers, if desired. As is known, the above resins are inherently resistant to UV light so that the use of a UV absorber additive is optional.
The gel coat resin must be mixed with a hardener in order for room temperature gelling to occur. The hardener should include a curing agent. Amine curing agents are suitable with diamine curing agents being preferred. Triamine curing agents may be used if desired. Exemplary diamine curing agents include aliphatic diamine and cycloalphatic amines such as isophorone diamine and TCD-diamine. Hardeners that include aminophenol are not preferred because they may adversely affect the UV absorbing and mechanical properties of the gel coat
The gel coat resin is activated by combining it with the hardener. The gel times for the activated resin and the final properties of the gel coat will vary depending upon the particular combination of resin, additives and hardener as well as the relative amounts of each ingredient. The activated liquid gelcoat should be sufficiently fluid so that it may be applied to the mold or part surface to form a uniform layer. The activated resin should preferably have a viscosity in the range of 100 to 1000 poise at a shear rate of one per second. In addition, the final properties of the cured gel coat, such as Tg, will also vary depending upon the above parameters as well as the cure conditions. Typical gel times for non-acce
Aylward D.
Hexcel Composites, Ltd.
Shapiro & Dupont LLP
Tucker Philip
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