Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Noninterengaged fiber-containing paper-free web or sheet...
Reexamination Certificate
1999-07-20
2001-06-05
Sellers, Robert E. L. (Department: 1712)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Noninterengaged fiber-containing paper-free web or sheet...
C428S300700, C428S413000, C428S417000, C525S113000, C525S119000
Reexamination Certificate
active
06242083
ABSTRACT:
This invention relates to curable compositions.
The use of curable compositions such as epoxy, cyanate, phenolic and like resins, both reinforced and unreinforced, has been known for a long time in a wide variety of commercial and military applications. Of particular importance is the use of such resins reinforced with continuous fibres, both unidirectional and woven, for structural applications such as aerospace vehicle parts such as aircraft tail assemblies and wing structures. Many such applications involve the use of multifunctional epoxy resins such as tetraglycidyl compounds cured using diamine hardeners.
However, whilst composite materials made from these resins have a relatively high modulus and Tg, they are usually brittle. A number of approaches have been used to improve the mechanical properties of the resin systems both in the bulk resin system and in the interlaminar regions in laminated fibre-reinforced composite materials. Typically, to improve toughness, for example, these include the addition of rubbers, thermoplastics, particulate fillers, interlaminar tougheners etc. often in combinations.
Examples of toughened resin systems are to be found in U.S. Pat. No. 4,482,660, U.S. Pat. No. 4,500,660, U.S. Pat. No. 4,680,076, U.S. Pat. No. 4,783,506, U.S. Pat. No. 4,863,787, U.S. Pat. No. 4,977,215, U.S. Pat. No. 4,977,218, EP-A-71197 and EP-A-559437. Brief details of the disclosures of these documents is given below.
U.S. Pat. No. 4,482,660 and U.S. Pat. No. 4,500,660 both disclose epoxy resin systems which are derived from epoxy resins, a reaction product of an epoxy resin and a butadiene/acrylonitrile copolymer having carboxylic groups and a curing agent such as dicyandiamide of 4,4′-diaminodiphenylsulphone. Whilst there are no specific details disclosed, both documents suggest that using a reaction product of an epoxy resin and a solid rubber does not give rise to useful properties either in the curable composition or in the cured articles made therefrom. It is suggested that the viscosity of the curable composition can be adjusted by the addition of solid rubber particles, eg nitrile rubbers having carboxylic groups, thereto.
U.S. Pat. No. 4,680,076 discloses a tough cured resin system having a phase-inverted morphology derived from a polyepoxy resin, an aromatic oligomer (ie thermoplastic), an aromatic diamine hardener and a reactive liquid rubber.
U.S. Pat. No. 4,783,506 and U.S. Pat. No. 4,863,787 disclose a curable composition based on a polyepoxy resin, an aromatic oligomer, a diamine hardener for the epoxy resin and a reactive (eg carboxyl functionality) solid rubber, the latter being present as infusible particles having a median size between 10&mgr; and 75&mgr;. The conversion of the rubber into infusible particles is preferably achieved in situ by removing solvent from the epoxy/oligomer/rubber mixture whilst heating it. In using the composition to form fibre-reinforced composite materials, a large proportion of the particles are filtered by the fibres and remain on the prepreg surface to act as interlaminar toughening agents. Alternatively, the particles, presumably preformed, may be applied directly to the surface of the prepregs.
U.S. Pat. No. 4,977,215 and U.S. Pat. No. 4,977,218 disclose similar resin systems to U.S. Pat. No. 4,783,506 and U.S. Pat. No. 4,862,787 but in which the infusible rubber particles are preformed, have a Tg above 15° C., a size in the range 1&mgr; to 75&mgr; and comprise cross-linked carboxylated diene rubbers or carboxylated acrylic rubbers. Again, in using the composition to form fibre-reinforced composite materials, a large proportion, if not all, of the particles remain on the prepreg surface to act as interlaminar toughening agents.
EP-A-71197 is primarily concerned with novel diamine hardeners for curable compositions but again discloses the use of what is termed flexibilizing agents or elastomeric compounds such as rubbers and thermoplastics.
EP-A-559437 is concerned with a particular problem arising when fabricating honeycomb reinforcing structures which are covered with resin-impregnated fabrics; that is porosity in the fabrics which it solves by crushing the impregnated fabric to achieve a certain cover factor K, as defined therein. As EP-A-559437 discloses, the liquid rubber systems, whether present as added or as a reaction product or alone or together with a solid rubber, or solid rubber systems do not adequately overcome the problem of porosity. EP-A-559437 discloses crushing fabrics impregnated with a composition of epoxy resin, pre-curing agent and solid rubber either without reactive groups or with reactive groups. The preferred composition adds the solid rubber in a lightly cross-linked state.
In these known resin systems, the toughening effect is achieved by the generation of relatively large, infusible rubber particles which phase separated from the epoxy resin during curing. When curable compositions which have pre-formed rubber particles, eg U.S. Pat. No. 4,977,215 and U.S. Pat. No. 4,977,218, are used to make prepreg materials, the fibres have a filtering effect on the particles which effectively limits the toughening effect of the rubber to the interlaminar regions of the composite materials. Consequently, the intralaminar properties of such composite materials are enhanced, as a result of the presence of the rubber toughening agent, to a considerably lesser extent.
Furthermore, as discussed in relation to EP-A-559437, the fabrication of honeycomb and like load-bearing structures present particular difficulties. One such proposed application of honeycomb structures involves aerospace vehicle components, eg aircraft tail units, in which prepregs will be used to form a continuous skin over a honeycomb reinforcing member. Such structural components are required inter alia to have high low temperature tensile strength. As discussed above, EP-A-559437 proposed to overcome the problem of porosity by crushing the fabric. However, in all instances, it is necessary to use relatively low moulding pressures, eg approximately 0.3 MPa to avoid crushing the honeycomb material.
Consequently, the rheological properties of the curable compositions considered for such use are critical since too high a viscosity will prevent resin flow into air pockets under the applied pressure, and thus will entrap volatile vapour which will nucleate to form voids, and too low a viscosity will cause the resin to flow out of the fibres even under the modest pressure applied.
The known proposed curable compositions discussed above have serious disadvantages when considered for such applications. For example, the use of thermoplastic tougheners results in a significant increase in the viscosity, and hence the elasticity, of the composition. The use of liquid rubbers alone does not significantly affect the viscosity, and hence the elasticity, of the compositions including them. The use of infusible particles alone has no significant effect on the viscosity of the composition and thus the elasticity is too low. The use of solid rubbers which are capable of cross-linking, ie when the infusible particles are formed in situ, results again in relatively high viscosity as the composition is heated sufficiently for the rubber to react. The use of lightly cross-linked solid rubbers, eg as in EP-A-559437, also gives rise to high viscosity compositions, hence the need to crush the fabric under high pressure to achieve high coverage. Additionally, in EP-A-559437, the use of non-reactive solid rubbers requires the use of special closely-woven fabrics and, again, hence the need to crush the fabric to achieve impregnation. Furthermore, in the crushed fabrics of EP-A-559437, the low pressures may result in poor bonding of the fabrics to the honeycomb structures by preventing sufficient flow of the composition to form adhesive fillets with the honeycomb walls.
It is a primary object of the invention to provide curable compositions utilising a solid rubber wherein said compositions give rise to cured polymer matrices throughout which the rubber is
Carter Jeffrey T.
Ciriscioli Peter R.
Jenkins Stephen D.
Lucas Scott D.
Maskell Robin K.
Cytec Industries Inc.
Fitzpatrick ,Cella, Harper & Scinto
Sellers Robert E. L.
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