Stock material or miscellaneous articles – Composite – Of epoxy ether
Reexamination Certificate
2000-06-29
2002-04-30
Dawson, Robert (Department: 1712)
Stock material or miscellaneous articles
Composite
Of epoxy ether
C428S297400, C428S417000, C523S427000, C523S428000, C523S429000, C525S523000, C525S524000
Reexamination Certificate
active
06379799
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is directed to epoxy resin systems, particularly low moisture absorption epoxy resin systems. The epoxy resin systems according to the invention have utility in aerospace manufacture, or other applications requiring resin systems having low moisture absorption and good retention of dimensional properties under hot and wet conditions. Prepregs, composites and resin transfer molding applications incorporating the epoxy resin systems of the invention are also disclosed.
2. Description of the Related Art
Advanced composites are high strength, high modulus materials which are finding increasing use as structural components in aerospace, automotive, and sporting goods applications. Typically, these composites comprise structural fibers such as carbon fibers in the form of woven cloth or continuous filaments embedded in a cured thermosetting resin matrix.
Most advanced composites are fabricated from prepregs, ready-to-mold sheets of fibrous reinforcement impregnated with uncured or partially cured resin. In order to be useful in commercial fabrication operations, prepreg matrix resin needs to have a long “outlife,” typically defined as the period of time the prepreg can remain at room temperature and still be useful for making cured composites; that is, the prepreg must remain pliable and retain appropriate tack (stickiness). Pliability is conferred by the resin matrix, which should remain relatively soft and deformable without cracking. Outlife is sometimes referred to herein as “tack and drape outlife.”
Resin systems containing an epoxide resin and aromatic amine hardener are often used in prepregs since they possess a balance of properties generally required for such applications. An early resin system extensively used in space applications was based on tetraglycidyl-methylenedianiline [TGMDA] epoxy resin and 4,4′-diaminodiphenylsulfone [4,4′-DDS]. This system has been used extensively in aerospace primary and secondary structures.
As a hardener, DDS has a low level of reactivity with epoxy resins at room temperature, and prepregs made using DDS-based systems have good out-life. The resulting fiber composites have high compressive strength, good fatigue characteristics, and low shrinkage during cure. Most epoxy formulations, including TGMDA, tend to absorb moisture (hygroscopic) which reduces their high temperature properties. Accordingly, there continues to be a need for resin systems and composites having reduced moisture absorption.
Other disadvantages associated with prior art epoxy/carbon fiber prepregs are a tendency toward brittleness and microcracking, and high cure temperature requirements, typically in the neighborhood of 350° F.
Typical state-of-the-art resin systems for aerospace applications include polycyanate-based resin systems. These resin systems exhibit relatively low moisture absorption, moderate to high toughness, low microcracking, and low dielectric constant. However, the high cost of polycyanate resins relative to epoxies is a disadvantage. Moreover, polycyanates are sensitive to moisture before cure, which makes special precautions necessary, such as the need to predry core materials to prevent blistering and delaminating during cure or postcure. Further, the laminate surfaces of polycyanate-based resin systems resist bonding, exhibit poor tack and drape and exhibit reduced mechanical outlife and storage life. In general, polycyanate-based resin systems require a 350° F. cure temperature. Those polycyanate systems having reduced cure temperatures exhibit especially poor tack and drape, and substantially reduced mechanical life and storage life. It has also been observed that, although initial moisture absorption is low for such systems, the moisture absorption in many cases continues to rise during long term moisture exposure and does not reach equilibrium.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an epoxy resin system having low moisture absorption that avoids the disadvantages and drawbacks associated with prior art resin systems. These resin systems are especially suitable for aerospace applications.
The resin system according to the invention has a first component which is generally formed by reacting dicyclopentadiene, epichlorohydrin and phenol to form a polyglycidyl derivative of a phenol-dicyclopentadiene epoxy polymer, and a second component which is an ortho-alkylated diamine hardener.
The epoxy resin of the instant invention has the following structural formula (I):
wherein R is hydrogen or halogen, and n is from 0 to about 0.5.
In a more preferred embodiment, R is hydrogen such that the phenol moiety is unsubstituted, and n is equal to about 0.2. An epoxy resin of this embodiment is available from Ciba-Geigy under the tradename TACTIX 556.
In general, the useful hardeners for the epoxy resin of the invention are aromatic hardeners preferably having a benzene skeleton in which substituted alkyl groups(s) are ortho to substituted amine groups(s).
In a more preferred embodiment, the ortho-alkylated aromatic compound has the following formula (II):
wherein Y is a direct bond, sulfur, oxygen, methyl, substituted methyl, or sulfoxy; R
1
and R
2
are each C
1
-C
4
straight chain or branched alkyl groups and X is hydrogen, chlorine or bromine.
In a most preferred embodiment, Y is —CH
2
—, and R
1
and R
2
are each ethyl. This diamine hardener, 4,4′-methylenebis(2,6-diethylaniline), is available from Lonza Group under the tradename Lonzacure® M-DEA.
In another preferred embodiment Y is —CH
2
—, R
1
is isopropyl and R
2
is methyl. This diamine hardener, 4,4′-methylenebis(2-isopropyl-6-methylaniline), is available from Lonza Group under the tradename Lonzacure® M-MIPA.
In still another preferred embodiment, Y is —CH
2
— and R
1
and each R
2
are isopropyl. This hardener, 4,4′-methylenebis(2,6-diisopropylaniline), is available from Lonza Group under the tradename Lonzacure® M-DIPA.
Another group of preferred ortho-alkylated aromatic diamine hardeners is represented by structural formula (III):
wherein the amine groups are meta- or para- to each other, R
3
is C
1
-C
4
branched or straight chain alkyl, R
4
and R
5
are independently hydrogen, branched or straight-chain alkyl or methylthio.
It has been discovered that epoxy resin systems having the above-described dicyclopentadiene-phenolic skeleton and ortho-alkylated diamine hardeners provide low moisture absorption similar to or better than most polycyanate and other state-of-the-art matrix materials, while at the same time providing a desirable combination of other important properties including: low microcracking after thermal cycling, good tack, drape, mechanical outlife, and storage life. The instant resin system is compatible with ancillary materials used in prepreg production, such as metal-containing catalysts which may be present in the release paper. The resin system forms a low, cured-resin density which enables lower weight structures to be utilized. This feature is especially desirable for weight-critical applications, such as aerospace applications. The resin systems are amenable to standard epoxy processing, yielding high Tg and good mechanical properties and very little change in mechanical properties under hot and wet conditions.
In many instances it is possible to provide the instant resin systems at lower cost than state of the art polycyanate resins.
A further surprising aspect of the resin systems according to the invention is their utility in resin transfer molding (RTM) processes. RTM processes generally require lower viscosity resin systems. As described at greater length hereafter, the low viscosity of the epoxidized dicyclopentadiene-phenol/ortho-alkylated diamine hardener resin system of the invention is an unexpected feature, which makes the resin system particularly suitable for RTM processes.
Therefore, in another aspect, the invention is directed to a resin transfer molding process comprising the steps of (a) tra
Cytec Technology Corp.
Dawson Robert
Feely Michael J
Fitzpatrick ,Cella, Harper & Scinto
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