Fabric (woven – knitted – or nonwoven textile or cloth – etc.) – Woven fabric – Including a foamed layer or component
Reexamination Certificate
1999-03-03
2002-08-06
Morris, Terrel (Department: 1771)
Fabric (woven, knitted, or nonwoven textile or cloth, etc.)
Woven fabric
Including a foamed layer or component
C442S203000, C442S286000, C428S116000
Reexamination Certificate
active
06429157
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a woven fabric prepreg and a honeycomb sandwich panel. In more detail, it relates to a woven fabric prepreg little changing in tackiness with the lapse of time, having moderate drapability, and excellent in the self adhesiveness to a honeycomb core when used as skin panels of a honeycomb sandwich panel, and also relates to a honeycomb sandwich panel small in the internal porosity of the skin panels fabricated by a cured prepreg and excellent in surface smoothness with few holes and depressions on the surfaces of the skin panels.
BACKGROUND ARTS
Fiber reinforced composite materials respectively consisting of reinforcing fibers and a matrix resin have been being widely used for aircraft, automobiles and other industrial applications because of their excellent mechanical properties, etc. In recent years, with accumulated achievements of use, the fiber reinforced composite materials are required to satisfy more and more severe properties. To sufficiently manifest the mechanical properties and durability of fiber reinforced composite materials, it is important to decrease the defects contributing to the decline of strength. Especially for structural materials and interior materials of aircraft, reinforced fiber composite materials are increasingly used as skin panels in honeycomb sandwich panels for reduction of weight. Honeycomb cores used include aramid honeycomb cores and aluminum honeycomb cores. It is generally practiced to produce a honeycomb sandwich panel by laminating a honeycomb core made of aramid paper with prepreg laminates on both sides and curing the prepreg laminates while bonding the prepreg laminates to the honeycomb core as so-called co-curing.
In this case, the adhesive strength between the honeycomb core and the prepreg laminates as skin panels is important, and a method of keeping adhesive films between the honeycomb core and the prepreg laminates and curing the prepreg laminates together with the adhesive films for fabricating a sandwich panel has been popularly used. However, for further weight reduction and fabrication cost reduction of the honeycomb sandwich panel, it is demanded in recent years to directly bond the honeycomb core and the prepreg laminates (hereinafter called self adhesiveness) without using any adhesive film.
However, in the bonding without using any adhesive film, since there is no resin supplied as the resin in the adhesive films, the resin existing in the prepreg laminates must migrate onto the honeycomb core to sufficiently wet the honeycomb core walls instead of the adhesive films during fabrication, and it is a difficult problem to achieve a high adhesive strength. The portions where the resin runs down or rises from the prepreg laminates in the normal direction of the honeycomb core into the honeycomb core wall and is cured are called fillets, and it is difficult to sufficiently form the fillets between the honeycomb core and the top and bottom skin panels. If the viscosity of the resin is too low, the resin from the top skin panel into the honeycomb core wall tends to run down too much, and as a result, the adhesive strength between the top skin panel and the honeycomb core becomes insufficient. On the other hand, if the viscosity of the resin is too high, the resin cannot sufficiently wet the honeycomb core wall, and especially the adhesive strength between the bottom skin panel and the honeycomb core becomes insufficient.
On the other hand, there is also a problem that since the resin in the prepreg laminates must be distributed toward the honeycomb core wall, the absolute amount of the resin in the laminates becomes so insufficient as to make the skin panels likely to be porous. In the case of honeycomb sandwich panel, since no pressure acts on the top or bottom prepreg laminates at the portions corresponding to the hexagonal cavities during fabrication, porosity is likely to take place compared to the fabrication of ordinary prepreg laminates.
Furthermore, to decrease such defects as pits and resin blurs on the surfaces of skin panels, it has been often practiced to stick adhesive films on the surfaces of prepreg laminates, for curing them together with the prepreg laminates. However, for further weight reduction, material cost reduction and fabrication cost reduction of honeycomb sandwich panel, it is desirable to form smooth skin panels free from surface defects without using any adhesive film.
However, since there is no resin supplied as the resin in the adhesive films if no adhesive films are used, the resin remaining on the surfaces of the skin panels become less, and it is a difficult problem to achieve a high grade surface state.
Prior arts concerning the prepregs reinforced by carbon fibers and matrix resins intended for use in honeycomb sandwich panel include the following.
U.S. Pat. No. 4,500,660 discloses an epoxy resin composition obtained by adding dicyanediamide to a specific epoxy resin and the reaction product of butadiene acrylonitrile copolymer having functional groups at both the ends and an epoxy resin. The object is to improve the self adhesiveness of prepreg laminates to a honeycomb core and the interlayer shear strength of skin panels. However, the U.S. patent is not intended for improving the surface smoothness of skin panels, and judging from the curing agent used, the composition is not so sufficient in heat resistance as the composition of the present invention. Furthermore, the resin composition does not contain fine particles of a resin. So, the U.S. patent cannot achieve the object of the present invention.
Japanese Patent Laid-Open (Kokai) No. 58-82755 describes that if a composition obtained by adding dicyanediamide and diaminodiphenylsulfone as curing agents to an epoxy resin and the reaction product of a liquid butadiene acrylonitrile copolymer having carboxyl groups at both the ends and an epoxy resin is used, the self adhesiveness of the prepreg laminates to the honeycomb core, especially the adhesive strength at high temperature becomes high, while the honeycomb sandwich panel does not have defects on the surfaces. However, the patent gazette does not state anything about inhibiting the temporally caused change in the tackiness of the prepreg, and cannot achieve the object of the present invention since the resin composition does not contain fine particles of a resin.
U.S. Pat. No. 5,557,831 describes that if a highly thixotropic resin is used in a woven fabric prepreg to be co-cured for producing a honeycomb sandwich panel, the porosity in the skin panels can be effectively decreased. However, since the resin composition of the US patent does not contain fine particles of a resin, the self adhesiveness of the prepreg laminates to the honeycomb core is poor.
On the other hand, the tackiness and drapability of a prepreg often come into question when a prepreg is used. These properties greatly affect the working convenience when a prepreg is handled.
If the tackiness of a prepreg is too small, the prepreg sheets overlaid and pressed for lamination are soon delaminated. In such a case, the working ambient temperature must be raised until moderate tackiness can be obtained. On the contrary, if the tackiness of a prepreg is too large, any prepreg sheets overlaid by mistake cannot be delaminated for correction since they are strongly bonded by their own weights.
If the drapability of a prepreg is poor, the prepreg is so hard as to remarkably inconvenience lamination work, and the prepreg laminate does not accurately follow the curved surfaces of a mold or the form of a mandrel, being wrinkled or having reinforcing fibers broken, to cause defects in the form obtained. In such a case, the working ambient temperature must be raised, but it is difficult to balance drapability with tackiness. This is a very large problem in the work of fabrication.
The tackiness and drapability of a prepreg are mainly decided by the viscoelasticity of the matrix resin. In general, the viscoelasticity of an epoxy resin greatly depends on temperature, and if the working
Hayashi Masahiko
Higashi Toshiaki
Kishi Hajime
Morris Terrel
Schnader Harrison Segal & Lewis LLP
Toray Industries Inc.
Torres Norca L.
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