Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
2001-10-10
2002-09-17
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C521S054000, C521S089000, C521S094000, C521S095000, C521S135000, C521S140000, C521S178000
Reexamination Certificate
active
06451876
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains to two part thermosettable composition systems based on epoxy resins. Each component of the system exhibits good storage stability (e.g., no phase separation) yet reacts when combined to provide a cured material having exceptionally good compression strength and modulus. When a blowing agent is present, the foam which is produced is remarkably uniform in appearance and is essentially free of the large voids often found in conventional two part thermosettable compositions, even when a relatively large mass is utilized.
DISCUSSION OF THE RELATED ART
Methods of reinforcing hollow structural members using two part, epoxy-resin-based systems are known in the art, as illustrated by the disclosure of U.S. Pat. No. 4,995,545 (incorporated herein by reference in its entirety). One part is a mixture of thermosetting resin and expandable microspheres, preferably also including a filler such as hollow glass microspheres in an amount effective to provide a paste-like consistency. The second part includes a curing agent which is effective to cross-link and cure the thermosetting resin present in the first part when the two parts are combined, as well as a filler such as the aforementioned hollow glass microspheres. An exothermic reaction takes place upon mixing, causing the expandable microspheres to increase in size and thereby foaming the composition.
U.S. Pat. No. 4,995,545 suggests that suitable curing agents for the second part of the system are primary polyamines, secondary polyamines, and polyamides (including aliphatic amidoamines). One problem that has arisen with the two part systems described in the aforementioned patent is that although the second part has good chemical stability at ambient temperatures, the curatives tend to phase separate from the hollow glass microspheres preferred for use as the filler material.
In particular, when the curing agent side is stored in a 55-gallon drum, the hollow glass microspheres phase separate to form a hard top layer over a bottom liquid layer comprising the curatives. Additionally, the curing agent side phase separates when heated and/or when pressure is applied, even when freshly prepared. The liquid curing agents tend to drip, for example, when the curing agent side is heated at about 66° C. (150° F.) and subjected to an application pressure of about 35 kg/cm
2
(500 psi). These problems make it quite difficult to dispense or handle the curing agent side by pumping, as would be desirable in an OEM vehicle assembly operation. It would therefore be highly desirable to develop a second part which exhibits better storage and processing stability and is pumpable at elevated temperatures and pressures.
Another problem which has been encountered with known two part systems is the tendency for large voids or holes to develop in the thermosettable composition as the heat generated by the exothermic reaction of the two parts expands the expandable microspheres. This problem is especially pronounced when reactive diluents having relatively low boiling points are present in the first part of the two part system and when a comparatively large mass of the thermosettable composition is being used. The non-uniformity of the resulting foam limits the compression strength and modulus levels which can be attained with such systems. Since these properties are critical when the foam is to be used to reinforce a hollow structural member, it would be very desirable to have available two part systems exhibiting more controlled foaming and a more uniform cell structure.
Obtaining a foamed epoxy resin with an optimum cellular structure is recognized as quite challenging, as there are a number of interrelated parameters which affect the foaming/curing process. The rheology of the epoxy/curative mixture during the rise of the foam is important, for example. As the epoxy resin crosslinks and cures, the mixture becomes more viscous. This is believed to be necessary to retain the cellular structure produced by expansion of the blowing agent. Coalescence and collapse of the foam will occur if the mixture is insufficiently viscous. On the other hand, a mixture which becomes extremely viscous and gels or sets up too quickly may prematurely terminate the foam rise, thus interfering with full expansion and density reduction. Controlling the viscosity is not straightforward, however, especially since it will vary with the temperature of the mixture, which often changes significantly during the course of curing/foaming and within the mass of the reacting mixture (the core temperature will often, for example, be much higher than the temperature at the outer edges). Another process parameter related to foam rheology is the epoxy cure rate, which is dependent on the processing temperature as well as the chosen epoxy resin and curing agent. If the epoxy-curative system is fast-reacting with a large exotherm, the cure rate may be too rapid to allow the foam to rise. Further, the excessive heat from a large exotherm can lead to burning or charring of the foam interior. If the epoxy reacts too slowly, the exotherm may not be sufficient to fully activate the blowing agent. Other processing parameters which influence foam quality and cell structure include surface tension and cell nucleation.
SUMMARY OF THE INVENTION
The invention provides a two component system capable of being cured to provide a structural reinforcement adhesive. When a blowing agent is present, expansion takes place to provide a reinforcing foam. One component (Component A) comprises one or more epoxy resins. In one particularly preferred embodiment, Component A comprises at least one epoxy resin which is a glycidyl ether of a polyhydric phenol, at least one reactive diluent, at least one rubber (preferably a liquid nitrile rubber), hollow glass microspheres, at least one thixotropic agent and at least one thermally activated blowing agent such as expandable microspheres. Component B comprises a curative system comprised of at least one aliphatic polyamine, at least one amidoamine, at least one alcohol, and at least one adduct of a polyamine and an epoxide. Hollow inorganic (preferably, glass) microspheres are present in one or both of Components A and B. In one preferred embodiment of the invention, at least one rubber and at least one thixotropic agent are also present in Component B. Component B exhibits good storage stability (e.g., minimal phase separation). Combining the two components initiates exothermic reaction of the epoxy resin(s) and the curative system; the heat evolved causes the blowing agent to activate and foam the mixture.
The curing and expansion (when a blowing agent is present) proceed in a remarkably controlled fashion to provide a foam having uniform cell structure. Minimal gassing, burning or cracking takes place in the interior of the foam, even when a relatively large mass of the two part system is employed. This was quite surprising, since normally considerable problems are encountered when attempting to cure and foam a large quantity of an epoxy resin due to the greater potential for developing high internal (core) temperatures as compared to a small quantity where dissipation of the heat generated during the exothermic reaction can take place more readily. The ability to reproducibly obtain a foam of consistent quality was also unexpected in view of the difficulties generally encountered in trying to control and adjust all of the different processing parameters known to affect expansion of an epoxy resin.
When cured, foams provided by the present invention can have compression strengths in the range of from about 140 to about 280 kg/cm
2
(about 2000 to about 4000 psi) and a modulus in the range of from about 6300 to about 10,500 kg/cm
2
(about 90,000 to about 150,000 psi). The foams also may have remarkably high compression strength (e.g., about 100 kg/cm
2
or 1500 psi) at 80° C. (175° F.). Without wishing to be bound by theory, it is believed that this may be attributable to the highly crosslinked character and the resulting relative
Foelak Morton
Henkel Corporation
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