Resin system

Details Resin system Resin system

2001-02-09

2002-10-22

Dawson, Robert (Department: 1712)

Stock material or miscellaneous articles

Composite

Of epoxy ether

C029S855000, C029S887000, C174S13700R, C174S1200SR, C174S251000, C523S455000, C523S456000, C523S457000, C523S466000, C525S526000, C525S530000, C525S533000, C528S088000, C528S112000, C528S408000, C528S418000

Reexamination Certificate

active

06468659

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a resin systems, and more specifically to epoxy resin-hardener systems, and products made thereof.
BACKGROUND OF THE INVENTION
Epoxy resins are among the most versatile polymeric materials. Examples of their applications are coatings, adhesives, casting compositions, molding compositions, potting compositions for encapsulating electronic components, laminates and base material for printed circuits, and also matrix resins for fiber-reinforced plastics.
The conversion of monomeric or oligomeric epoxy resins into polymers requires reaction partners which are termed hardeners. Depending on the type of hardener, the curing reaction takes place at temperatures around room temperature, or at low temperatures (known as cold curing) or at elevated temperatures (known as hot curing).
For curing epoxy resins at low temperatures for industrial applications it is predominantly only aliphatic primary or secondary amines and polyaminoamides which are used, and less frequently polythiols or specific ionic catalysts. All unmodified amines have an alkaline to strongly alkaline reaction. liquid amines, in particular the aliphatic and cycloaliphatic amines, can cause skin damage extending to corrosive burns. Another disadvantage is the high volatility of liquid amines. A great disadvantage of cold curing of epoxy resins using the abovementioned hardeners is the low heat resistance and chemicals resistance of the resultant products.
To increase heat resistance, solvent resistance and chemicals resistance it is necessary to complete the curing of epoxy resins at elevated temperatures, by hot-curing using aromatic or cycloaliphatic amines, carboxylic anhydrides, polyphenols, novolaks or using latent hardeners.
It is known from Houben-Weyl, Methoden der Organischen Chemie [Methods in Organic Chemistry], Vol. E20, Makromolekulare Stoffe [Macromolecular materials], Georg Thieme Verlag Stuttgart, 1987, p. 1959 that, in particular in the case of bisphenol A resins, the curing of epoxy resins with cyclic dicarboxylic anhydrides or with tetracarboxylic bisanhydrides gives cured products with excellent electrical insulation properties and with good heat resistance. A further advantage is that curing with anhydrides, unlike curing with amines, is not significantly exothermic. However, a disadvantage is that curing temperatures of at least 120-150° C. are always required, and even then some hours are needed for curing. Even at these temperatures, the crosslinking reaction is still so slow that it is generally essential to use accelerators. However, experience has shown that the use of curing accelerators can lead to loss of quality in the cured resin systems.
U.S. Pat. No. 5,629,379 describes a cured epoxy resin system made from a mixture of four components which gels at a temperature between 80 and 120° C. and is cured at temperatures between 200 and 300° C. Besides the epoxy resin component and the anhydride hardener component, the mixture comprises in particular an additional hardener component, and also a curing accelerator component.
U.S. Pat. No. 4,559,272 describes a process for potting an electrical component by impregnating the electrical component with a hot-curing composition made from a polyglycidic aromatic amine, a polycarboxylated carboxylic anhydride and a curing accelerator, and then curing the composition. As disclosed in that publication, the curing accelerator is in particular added in order to achieve a low curing temperature. The gel point disclosed in the examples is about 100° C. In U.S. Pat. No. 4,595,623, the same applicant describes a fiber-reinforced, syntactic foam composite material which as matrix likewise has the abovementioned hot-curing composition.
DE 26 50 746 and U.S. Pat. No. 4,002,599 describe a room-temperature-curing epoxy resin composition which comprises a mixture of a polyglycidylarninophenol epoxy resin and a biphenyl anhydride. All of the examples described here are in particular based on the curing of triglycidyl-p-aminophenol (TGpAP) with benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BTDA) as sole hardener, or with hardener mixtures made from BTDA and maleic anhydride (MA).
However, the TGpAP-BTDA system described in DE 26 50 746 and U.S. Pat. No. 4,002,599 proves, in particular in industrial or practical use, to be disadvantageous and highly problematic, since the BTDA is extremely difficult to dissolve in TGpAP. The inventors therefore recommend an extreme degree of mixing, e.g. three hours in a ball mill. Another recommendation is that the BTDA should be very finely pulverized or treated with high shear forces on a three-roll mill. However, these lengthy procedures can in particular bring about hydrolysis of the anhydride, and in addition the dissolution of BTDA in TGpAP generally remains incomplete. Further to this, additional production steps lead to prolonged production times and thus to increased production costs.
The publication by J. E. O'Connor and J. A. Graham, the inventor in DE 26 50 746 and U.S. Pat. No. 4,002,599, in Adhesives Ages 21/7 (July 1978), pp. 20-23 entitled “Epoxy with Low-Temperature Cure and High-Temperature Properties Developed” includes a further description and commentary of the invention disclosed in DE 26 50 746 and U.S. Pat. No. 4,002,599. As can be found in lines 20 et seq. of the 3rd column on p. 20, the authors themselves in their invention expressly describe the TGpAP-BTDA system as an exception, as follows: “The TGpAP-BTDA system appears to be the ‘exception to the rule’ regarding BTDA as a hardener for epoxy resins”.
There is therefore a demand for epoxy resin-hardener systems which can cure at a low temperature and give products which have increased heat resistance, chemicals resistance and solvent resistance. Examples of potential applications would be adhesives, matrix resins for fiber composite materials and repair resins for components, in cases where the use of high temperatures is not permissible. Other applications would be casting compounds and potting compounds, specifically for encapsulating large electronic components, in cases where the curing can be completed at low temperature, with little exothermic effect, and therefore with a considerable energy saving, another advantage here being that the products produced have less internal stress.
SUMMARY OF THE INVENTION
It is an object of the present invention, therefore, to provide a resin system which can be crosslinked at low temperature, in particular at room temperature, and preferably without using a curing accelerator.
Another object of the present invention is to provide a resin system whose solubility behavior in particular, and particularly that of the hardener component, is better than that of known systems.
A further object of the present invention is to provide a process for producing an insulated electrical conductor, the insulation of which can take place at low temperatures, in particular room temperature.
The term “crosslinkability” here is to be taken as meaning the capability of a polymerizable system to assume an irreversible state.
Surprisingly, it has now been found that the system described by Graham and O'Connor is not an exception but merely one example of the generally applicable principle of the curing of epoxy resins by cyclic anhydrides at low temperatures. However, a precondition is that the individual polyfunctional epoxy resins, or the various polyfunctional epoxy resins which can be used in mixtures, comprise an epoxy resin having at least one aminoglycidyl group. According to the invention, a wide variety of aromatic, aliphatic, cycloaliphatic or heterocyclic acid anhydrides is suitable for the cold curing of epoxy resins, preferably giving products with increased heat resistance. The authors Graham and O'Connor did not recognize the generally applicable principle within their invention.
Surprisingly, it has now been found that there are numerous combinations of cyclic anhydrides and aminoglycidyl compounds which have marked

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