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
2001-07-10
2003-08-05
Nutter, Nathan M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S193000, C525S194000, C525S213000, C525S233000, C525S235000, C525S238000, C525S239000, C525S240000
Reexamination Certificate
active
06602958
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
This invention relates to adhesives. More particularly, this invention relates to two-part room-temperature curing methacrylate based adhesives that are used to bond a wide variety of materials, including thermoset plastics, thermoplastics, metals, wood, ceramics and other materials and combinations of materials. This invention involves a significant improvement in the ability of adhesives to bond certain difficult-to-bond composite materials with minimum required surface preparation. Another feature of this invention is the high degree of elastic behavior of the cured adhesives and the ability of the cured adhesive materials to retain a high proportion of their elastic behavior after exposure to elevated temperatures or long-term aging.
2. Background Art
The three common classes of two-part room temperature curing reactive adhesives are epoxies, polyurethanes, and acrylics. The discussion of these prior art adhesives and the inventive adhesives emphasizes structural applications, wherein a very strong bond is achieved between two structural members of an assembly, and the bond is often strong enough to cause failure of the material before the assembly fails. However, all of these adhesive materials, can be and are used to advantage in less demanding applications, as well, in which case one or more of the advantages of the particular adhesive fulfills a specific bonding requirement.
Epoxy adhesives, which are the earliest, best known and among the most common structural adhesives in general use, consist of an epoxy resin adhesive component and an amine, polyamide, or combined amine and polyamide hardener components. Faster curing epoxies can be formulated with polymercaptan hardeners that are generally used in combination with polyamide and amine hardeners.
Polyurethane adhesives generally consist of an isocyanate-terminated polyol and a hardener or curative component that consists of a polyol or amine or a combination of polyols and amines.
The epoxy and polyurethane adhesives cure upon mixing when the hardener component reacts with the epoxy or polyurethane resin component in an addition polymerization process.
Methacrylate or acrylic adhesives that are used in the same applications as epoxies and polyurethanes generally consist of a polymer-in-monomer solution of an elastomer or thermoplastic resin or a combination thereof in a monomer such as methyl methacrylate. Hardening is achieved when a combination of a peroxide and an amine is introduced into the polymer-in-monomer mixture to initiate a free-radical curing reaction. Generally, the adhesive component contains either the amine or peroxide component and the co-reactive peroxide or amine component is mixed with the adhesive just prior to bonding.
Each of the three common reactive adhesive classes has characteristic advantages and disadvantages. For example, epoxies tend to be characterized as safe and relatively easy to mix and apply, but tend to be somewhat rigid and sensitive to cleanliness of the surface to be bonded. Polyurethanes are generally considered to be much more flexible and elastic, but also suffer from sensitivity to surface contamination, moisture and humidity. Both of these adhesive types have the limitation that fast-curing products tend to have very short open working time after mixing, and products with more acceptable open time have very long cure times. This limitation is imposed by the linear reaction mechanism that is characteristic of the addition polymerization reaction by which they cure.
In terms of the characteristics of the cured adhesive and resulting bond, epoxies are considered to be very strong because of their high modulus or rigidity and resulting high lap shear strength. They are generally recommended for bonding metals because of their affinity for metal surfaces and high shear strengths. However, their rigid nature limits their usefulness in applications that require flexibility in the adhesive bond. Epoxies also have limited ability to bond thermoplastic materials.
Polyurethanes are generally much more elastic, tough and flexible than epoxies. Elasticity, toughness and flexibility are beneficial when adhesive bonds are subjected to peeling or impact forces, and when bonds and bonded assemblies are subjected to dynamic fatigue stresses. However, polyurethanes are not as useful as epoxies for bonding metals, and are generally more suitable for bonding plastic materials in applications that are subjected to bending and impact stresses.
Two-part acrylic or methacrylate adhesives overcome two of the major drawbacks of the epoxies and polyurethanes. They are much more tolerant of unclean or unprepared surfaces, and they have a much more favorable cure profile in terms of open working time and cure rate. In addition, they exhibit equal or better affinity for metal and plastic surfaces than either epoxies or polyurethanes. However, some materials, in particular certain composite materials, are difficult to bond in the “as received” condition. Specific examples include certain gel coats, which are highly crosslinked and inert polyester compounds that form the outer or “show” surface of fiberglass reinforced polyester (FRP) composite materials used to fabricate boats and other structures exposed to outdoor weathering.
Other examples are closed molded polyester composites, which are materials formed by processes other than the open molded processes used to produce FRP composite structures. Examples of closed molding processes and materials are sheet molding compounds (SMC), resin transfer molded (RTM) composites and pultruded composites.
The essential issues with closed molded processes and products are (1) these processes produce polyester composite articles with reduced emission of and worker exposure to the styrene component in polyester resins and are rapidly replacing open molded processes, and (2) these materials are generally characterized by resistance to the solvating effect of the methacrylate monomers that normally soften or penetrate the bonding surface prior to hardening of the adhesive. In addition, many of these materials use processing aids to provide smooth surfaces for painting. These materials can also interfere with the bonding process.
Other materials are used to facilitate release from the molds used to fabricate parts from them. Such materials are often added directly to the molding compound, in which case they are referred to as “internal” mold releases. Other materials may be sprayed on to the mold surface prior to molding. These materials are referred to as “external” mold releases. All of these processing aids can interfere with the formation of strong adhesive bonds.
The problems experienced in bonding these materials with prior art methacrylate-based adhesives, as well as the additional and undesirable processing steps required to use them, including grit-blasting, sanding, solvent wiping and priming are described in detail in U.S. Pat. No. 3,838,093, which is discussed in further detail below.
Epoxy adhesives based on standard DGEBA (diglycidyl ether of bisphenol-A) resins, cured with hardeners based on combinations of amines, polyamides and other additives used to impart specific properties, have effectively been used to bond some closed molded composite materials. However these adhesives do not completely cure at room temperature, and generally require thermal post-curing to develop their full physical strength.
Recent developments in polyurethane adhesive technology have been directed toward improving adhesion to these composite materials as disclosed, for example, in U.S. Pat. Nos. 5,340,901 and 5,548,056. However, as with epoxy adhesives, these materials often require thermal post curing. Even though polyurethanes do ultimately develop their full physical strength at ambient temperatures, such post curing may be required to meet process speed requirements or to develop full, reliable or reproducible adhesion to the composite surface, or both. In some cases, solvent-based primers are used to develop adhesion at a
Briggs Paul C.
Minato Masaki
Osae Samuel B.
Cox Scott R.
IPS Corporation
Nutter Nathan M.
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