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
1998-01-20
2003-09-02
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...
C525S118000, C525S502000, C525S508000, C525S523000, C525S530000, C525S533000, C528S088000, C528S089000, C528S090000, C528S091000, C528S093000, C528S094000, C528S112000
Reexamination Certificate
active
06613839
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to epoxy resin compositions containing compounds which inhibit the cure of the epoxy resins at lower temperatures and to latent catalyst compositions for curing epoxy resins. The invention further relates to compositions useful for curing epoxy resins which comprise cross-linkers for the epoxy resins and the inhibitor described above. The invention further relates to adhesives and coatings prepared from such inhibited resin compositions, and to materials coated with or encapsulated by such inhibited resin compositions. The invention further relates to laminates, prepregs, dielectric films, insulated and/or encapsulated materials for electrical devices such as motors and transformers and composites prepared from such inhibited resin compositions.
Epoxy resins are used in a variety of applications, such as powder coatings, in encapsulation of electrical components, in composites, in solution coatings, in prepregs and in laminates. Also related to electronics, is the use of epoxy as adhesives to adhere copper foil to circuit boards which may, or may not, be manufactured for an epoxy matrix. In many of these uses, it is desirable to partially cure the epoxy resin in a controllable manner, such that at a later time, when desired, the epoxy resin can be fully cured. Such partial curing proportionately reduces the gel time of the resin. In many uses it is desirable to contact a cross-linker with an epoxy resin in the presence of a catalyst for the reaction and have the formulation remain stable, that is uncured for a period of time. At present it is quite difficult to control the curing of an epoxy resin and retain the stability of a composition comprising an epoxy resin, a cross-linker and catalyst for the reaction of the epoxy resin with the cross-linker.
In some uses the epoxy resin, cross-linker, and catalyst are contacted in a solution and then applied as a coating on a substrate such as reinforcing fibers. In some cases, one or more solvents are used to reduce the viscosity of the formulations for better coating and impregnation performance. Often it is desirable to remove this solvent before significant curing takes place, otherwise the solvent may be entrapped in the cured epoxy resin. Entrapped solvent adversely affects the final properties of the cured resin. Solvent removal is facilitated by use of substrates having a thickness dimension in the order of less than 5 mm to provide solvent a relatively short migration path to the surface of the formulation from which the solvent may escape. Often, the solvent is removed by exposing the coated article to elevated temperatures. However, at such elevated temperatures the epoxy resin may begin to cure resulting in increased molecular weight and viscosity. Thus, the method chosen for removal of solvents may cause the solvent to be trapped.
What is also needed is a resin system having a sufficiently wide processing window in order that solvent may be removed by heating the resin to remove the solvent without trapping solvent within cured polymer. The inhibitor feature permits the preparation of prepregs and laminates of consistent good quality.
After the solvent has been removed, a glass cloth coated with the resin, cross-linker and catalyst may be partially cured by exposing it to temperatures at which curing occurs. Such a product is referred to as a prepreg. The prepregs may then be stacked to build-up thickness, or formed into a shape and exposed to subsequent elevated temperature conditions under which the cross-linker and resin completes the cure of the thermosetting resin. In general, this involves contacting the various components of a laminate at elevated temperatures under pressure, for a period of time sufficient for the epoxy resin to further cure. In this curing process the resin coating on the glass cloth flows under pressure and mixes with the coating on adjacent glass cloths thereby resulting in a fusing of the glass layers together by means of a matrix of the cured epoxy resin.
It is desirable that prepregs have predictable qualities which are less sensitive to variations of temperature and residence time of the oven. The thickness of the laminate can be better controlled with prepregs having consistent properties. High concentrations of multifunctional components such as styrene-maleic anhydride copolymers, multifunctional phenolic cross-linkers such as tetraphenol ethane, multifunctional phenolic novalac epoxy resins, and high viscosity of the resin system components may contribute to erratic behavior of the resin system and corresponding unpredictable prepreg production. Undesired physical qualities, or poor prepreg appearance may also result from solvent entrapment in the resin of high viscosity resin systems.
It is also desirable that the resin demonstrate sufficient thermal decomposition temperature in order that laminate properties will not be adversely affected by subsequent processing steps, for example, immersion in molten solder, and to provide consistent properties in high temperature operating environments as occur in automobile ‘under the hood’ applications. The temperature of the onset of thermal decomposition is a measurable property which is believed to reflect performance properties of a laminate in high temperature environments.
Manufacturers of prepregs and electrical laminates desire to increase production rates without capital investment for example, by increasing production rates on existing equipment. In order to facilitate faster processing the epoxy resin coated substrates must be exposed to higher temperatures, the epoxy resin compositions must contain higher levels of catalysts, or both. Unfortunately, both measures taken to increase production result in less control over the curing reaction. Thus the need exists to control the reaction of resin systems at higher temperatures and catalyst levels.
Laminates may be manufactured in a continuous process. In the continuous process the prepregs are contacted at much higher temperatures than in conventional laminate processing for shorter periods, for example, at temperatures in a range from 200 C to 230 C for 1 to 4 minutes. It is very difficult to achieve complete cure under such conditions. One potential solution to this is to add a larger amount of catalyst or accelerator to the epoxy resin. Unfortunately increasing amounts of catalyst may limit solvent removal without sufficient curing of the epoxy resin.
Several references disclose curable epoxy resin compositions which include as a catalyst a salt of fluoroboric acid, (tetrafluoroborate, fluoroboric acid, and hydrofluoroboric acid), and fluoroborates. GB 963,058 discloses amine salts of hydrofluoroboric acid. U.S. Pat. No. 4,438,254 discloses phosphonium salts of tetrafluoroborate. U.S. Pat. No. 3,048,552 discloses a quaternary ammonium fluoroborate salt. “Proposed Mechanism for the Curing of Epoxy Resins with Amine-Lewis Acid Complexes or Salts” by James J. Harris and Samuel C. Temin, J. Ap. Pol. Sc., Vol. 10, pp. 523-534 (1966) discloses fluoroborate salts of aliphatic and aromatic amines and pyridine. Several patents disclose that onium salts of tetrafluoroborate (fluoroboric acid) are useful as accelerators in epoxy resin curing reactions. U.S. Pat. Nos. 4,318,766; 4,322,456; 4,410,596; 4,544,732; 4,554,342; 4,581,436; 4,766,196;, and CA-A-893,191.
Japanese 58/138729 discloses thermosetting molding compositions whose essential components are a resin component consisting of (a) epoxy resin, (b) an alkenylphenol polymer, and (c) a latent curing accelerator together with (d) fibers and/or granular fillers, wherein the alkenylphenol polymer is dispersed and mixed as a powder in the epoxy resin along with the fillers. It discloses as the latent curing accelerator tetra-substituted boron salts of ammonium compounds, phosphonium compounds, arsonium compounds, imidazolium compounds, pyridinium compounds, or morpholinium compounds. Only tetraphenyl or tetrabutyl borate salts are believed to be disclosed.
U.S. Pat. No. 3,947,395 discloses sur
Everett John P.
Gan Joseph
Sellers Robert E. L.
The Dow Chemical Company
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