Compositions – Chemically interactive reactants
Reexamination Certificate
2002-03-08
2004-06-01
Sellers, Robert (Department: 1712)
Compositions
Chemically interactive reactants
C525S486000, C525S523000, C525S526000, C528S110000
Reexamination Certificate
active
06743375
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of epoxy hardeners and in particular to new and useful groupings of epoxy hardener compositions which rapidly cure an epoxy over a range of temperatures.
There are two general types of polymers; thermoplastic and thermosetting. Thermoplastic polymers melt on heating and solidify on cooling. They can be remelted and resolidified repeatedly within limits. Thermoplastics are commonly molded by injecting a hot, molten thermoplastic into a cold steel die and removing the part from the mold as soon as the part has cooled enough to prevent distortion. The high melt viscosity of thermoplastic resins requires very high injection pressures, commonly 10,000-20,000 p.s.i. and above. These high injection pressures require enormous clamping forces to hold the two halves of the die together and so part size is limited by the clamping capacity of the molding machine. High capital investment and large production runs are characteristic of thermoplastic molding processes.
Thermosetting polymers do not melt on heating. They soften only and, if heated sufficiently will char. Thermosetting resins such as epoxies are molded by either injecting or placing by hand a prepolymer mixture consisting of epoxy resin, hardener, catalyst and various modifiers and fillers into a mold and heating for a time sufficient to complete the chemical reactions between the epoxy resin and the hardener, resulting in a thermosetting polymer having the shape and size of the mold. Molding times are considerably longer than for thermoplastic molding processes, typically five to fifteen minutes. However, injection pressures are low and so the mold clamping forces needed are also low. Thus, thermoset molding processes are characterized by low production rates and relatively low capital investment. But, large part sizes are possible.
The curing reactions of epoxies are exothermic and cure times and temperatures are determined by the heat transfer rate from the mold to the part and the scorch temperature of the epoxy. Epoxy/hardener systems which cure at low temperatures and which develop low exotherm as a result of chemical factors are very advantageous since they can be cured faster and will result in higher production rates.
Epoxies are used extensively as thermosetting adhesives for bonding wood, glass, ceramics and metals. For hand application, the epoxy resin and hardener are usually supplied in two separate syringes which have a common plunger. Pressing the plunger releases the correct proportions of epoxy and hardener. The two compounds are mixed with a spatula and applied to the bonding surfaces and then cured either at room temperature or at elevated temperature, depending on the application. Epoxy hardeners which cure rapidly at low temperature develop higher bond strength due to lower shrinkage stresses and permit higher production rates with lower energy expenditure.
Epoxy adhesives are frequently used in industrial processes in the form of “film adhesive”. A prepolymer mixture of epoxy, hardener, and other desired components is applied as a coating onto a polymer film substrate, rolled up and stored in a freezer to stop the chemical reactions between components. When needed, the film adhesive is removed from the freezer and applied to a metal or composite part, the backing is stripped off and the assembly completed and cured in an oven or autoclave.
At the point when the adhesive is removed from the freezer, the epoxy mixture begins to cure slowly at room temperature. After a certain time called the “out time”, the adhesive will become stiff and brittle and unusable. Latent mixtures having long out times are highly desirable in order to complete complex assemblies before curing. Hardeners having very long out times, or latency, but with relatively low cure temperatures and short cure times are difficult to create, further increasing their value.
Epoxies are combined with fiberglass or carbon fiber in the manufacture of composite materials. These are used extensively in military and aerospace applications, civil aircraft, sporting goods such as fishing rods, golf club shafts, tennis rackets, bows and arrows and the like. These are manufactured either by automated processes or by hand layup. Epoxies which develop excellent strength and toughness after curing at room temperature or low temperature result in composite structures having superior properties, higher production rates and lower cost. The absence of noxious vapors from the epoxy-hardener mixture is of great benefit to persons who must handle these materials.
Another application involving composites is the use of composite tooling for formed sheet metal parts. These are practical for prototyping and short production runs as a substitute for metal tools. The completed tool must be strong and hard and must cure effectively at room temperature.
Epoxies are used extensively in the “potting” of electronic circuits which are exposed to shock, vibration, and rain, for example, for protection of the circuits. The circuit is assembled and placed in a case and the liquid epoxy mixture is poured into the case, thus enclosing the circuit components and isolating them from the atmosphere as well as protecting them from vibration and shock. These are used in automobile and truck engine computers, aircraft, tanks, missiles, etc. The epoxy mixture must have a low viscosity to fill the spaces around the components before hardening. A low cure temperature is desirable to protect the electronic circuits from heat damage and to limit shrinkage which stresses components and connections. The cured epoxy must be strong and tough to resist mechanical stresses and the cure rate should be rapid to realize a high production rate.
Electronic components are “encapsulated” by dipping them into an epoxy prepolymer mixture, draining off the excess resin and curing the coating. This protects the components from atmospheric exposure. A high cure rate at low temperature is desired to prevent heat damage, keep stresses low and achieve a high production rate.
Coating systems have been developed which are used to protect metal surfaces from rust and corrosion and to enhance appearance. These are used extensively in large appliances such as washing machines, dryers, refrigerator cases, large structures such as bridge beams and architectural applications. While epoxies have enjoyed a long period of success in these applications, they have been recently partially replaced by the tougher polyurethanes. Polyurethanes have some disadvantages such as sensitivity to the resin/hardener ratio and the isocyanate resin is itself susceptible to degradation from atmospheric moisture. Nevertheless, sophisticated metering and spraying equipment has been developed for these materials. Epoxy systems having superior strength and toughness after curing at low temperatures as well as relatively low sensitivity to the resin/hardener ratio and low toxicity may permit epoxy coating systems to regain some of their lost market share.
Prior art hardeners for epoxies are disclosed in art, such as in U.S. Pat. No. 3,812,202, which teaches a two part self-hardening epoxy composition which is formed by a phenol precursor combined with a methylol acrylic polymer. The phenol precursor is made by combining bisphenol A with a polyepoxide compound to create a composition having two or more phenolic groups. The methylol acrylic polymer can be formed by polymerizing acrylamide or diacetone acrylamide with other ethylenically unsaturated monomers, followed by adding an aldehyde, such as formaldehyde, and optionally, a catalyst. The, phenol precursor and methylol acrylic polymer are mixed to a desired viscosity, applied, and heated to at least about 300° F. to cure.
U.S. Pat. No. 4,866,133 discloses a curing agent for an epoxy containing a polymeric phenol and a polyamine. The curing agent is provided as a powdered latent curing agent mixed with a liquid epoxide resin. Polyamines used in the curing agent include diethylenetriamine and triethylenetetramin
Ardes Enterprises, Inc.
Notaro & Michalos P.C.
Sellers Robert
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