Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
2000-10-11
2003-04-08
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C428S364000, C428S375000, C428S395000, C428S475500
Reexamination Certificate
active
06544622
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to aramid honeycombs and a method for producing the same. More particularly, the invention relates to a honeycomb core using aramid honeycomb sheets as base materials, and a method therefor.
2. Description of the Prior Art
Background
It has been known a honeycomb core structure comprising an assembly of a number of hollow, columnar honeycomb cells separated by cell walls adhered linearly to each other. Further, it has been also known to use aramid honeycomb sheets as base materials for the cell walls. Aramid honeycomb sheets comprising a nylon-type resin, particularly para-aramid honeycomb resin sheets, have flame-retardant, tough and other excellent properties required for a honeycomb core structure.
A honeycomb core structure comprising aramid honeycomb sheets used as the base materials for cell walls (hereinafter referred to as aramid honeycombs) is disclosed in Kokai (Jpn. unexamined patent publication) No. 4-226745. According to the publication, the aramid honeycomb sheets contain 0 to 50% by weight of a binder and 50 to 100% by weight of para-aramid fibers, wherein the para-aramid fibers represent 20% to 80% by volume of the total material.
The conventional aramid honeycombs, however, are defective in the following points.
First, the aramid sheets have a defective texture or structure. Sheets containing a large amount, i.e., 50% by weight or more, of para-aramid fibers are bulky and are inconvenient for handling. When the aramid paper sheets are produced, fibers are not uniformly stirred in a liquid such as water and, therefore, it is difficult to provide a papermaking treatment to the fibers to disperse the ingredients on a filter. Consequently, the produced aramid sheets are defective in that the para-aramid fibers are not uniformly dispersed and the sheet density is varied between portions of the sheets.
As a consequence, the sheets are wrinkled locally at portions differed in the number of para-aramid fibers, causing inconvenience in the operation of the production of the honeycombs. Further, a number of large pinholes are produced in sheets through which adhesives pass from one side surface to the other side. For these and other reasons, the production of the aramid honeycombs is often troubled.
Second, the paper sheet strength is defective. Since the sheets contain a large amount, i.e., 50% or more, of para-aramid fibers, Freeness value, i.e., a water-maintaining property and a water-filtering property, is unsatisfactory, resulting in the reduction in the bonding property of the binder mixed in a papermaking liquid, such as water. Since the binder is flowed off without being fixed to fibers and, thus, since the fibers are not bonded to each other, the paper strength of the aramid sheets decreases. Thus, when such aramid sheets are used as base materials, the produced honeycombs had an insufficient toughness.
Fibers have high restoring property. Therefore, thirdly, sheets become excessively thick when they are produced with a large amount, i.e., 50% or more, of para-aramid fibers since the fibers restore their volume after the sheets are produced and the sheets get thick, causing a reduction in sheet density. It was pointed out, therefore, that such thick sheets are inappropriate as base materials for honeycombs.
Prior Art
As has been mentioned above, the aramid honeycombs disclosed in said Kokai 4-226745 have problems caused by the sheets used as base materials. As a counter measure, meta-aramid pulps which act also as a binder were used and aramid sheets were provided with a calender treatment under a high. temperature and at a high pressure to be used as base materials for aramid honeycombs.
That is, meta-aramid pulps, which are excellent in a fixing property, have been used in addition to the main ingredient, i.e., 50% or more of para-aramid fibers, to overcome the problems of the lack in the fixing property and of the week paper strength. Further, the aramid sheets were, after provided with a paper-making treatment, calendered at a high temperature and under a high pressure to produce a thin film so as to overcome the third problems of being a thick sheet and of a reduction in density.
The meta-aramid pulp has a high softening point of over 200° C. Therefore, para-aramid fibers and meta-aramid pulps are merely entangled with each other in the sheet produced by a normal method with a paper-making liquid such as water and dried at 150° C. or lower.
That is, the meta-aramid pulps are not bonded in a liquid form to the para-aramid fibers, as is the case with a normal binder. Therefore, the sheets thus produced have not the strength sufficient for the production of aramid honeycombs.
To obtain the necessary strength, a high temperature and high pressure calendering treatment was needed according to the conventional processes. In fact, the aramid sheets after produced have been calendered under a high pressure of 29.4×10
4
N/m(300 kg/cm) applied linearly and at a high temperature of around 300° C. In this way, the meta-aramid pulps were softened, melted, fluidized and then hardened to act just as a normal binder to obtain a sheet strength necessary for the production of aramid honeycombs.
Second, aramid honeycombs were produced using the aramid sheets as base materials by a conventional enlarging process which comprises the steps of applying adhesives linearly to the sheets, piling the sheets such that each of the sheets are shifted by a half pitch of the linearly applied adhesives, applying pressure to the piled sheets under heating to bond the sheets to each other, and enlarging the sheets to the direction counter to the piling direction to obtain aramid honeycombs comprising cell walls of the aramid sheets. The aramid honeycombs thus obtained were provided with an after treatment so that the cell walls are coated by and impregnated with a reinforcing resin.
Problems to be Solved by the Invention
For the conventional aramid honeycombs and the method therefor, following problems have been pointed out.
First, no improvement has been provided for solving the problem of the defective texture or structure.
The para-aramid fibers are still not uniformly dispersed and so are unevenly present in the sheets. The meta-aramid pulps, fluidized by the high pressure and high temperature calendering treatment, are used only to fill in between para-aramid fibers. Consequently, the density in sheets is varied locally and is scattered. Thus, the heat shrinkage ratio is different between the portions containing much fibers and those containing less fibers, and the sheets at the cooling stage are not uniformly shrank under heat.
Therefore, the aramid sheets conventionally used are not smooth, and are liable to be wrinkled. Since the sheets are defective in preciseness, they are difficult in handling and are difficult to be piled precisely.
Further, there are cases where a number of large pinholes are formed in the aramid sheets. As the result, the adhesives applied linearly to the surface pass through the pinholes to the other surface under a high pressure and a high temperature, whereby each of the piled sheets is adhered to each other to form a block and the respective sheets are unable to be enlarged or expanded.
Second, a problem of the cost was pointed. For the conventional aramid sheets, expensive meta-aramid pulps are used and the honeycombs using the expensive aramid sheets are accordingly expensive.
Further, the conventional aramid sheets are provided with a calendering treatment under high temperature and high pressure. The cost for the treatment is added to the total production cost.
Third problem is related to the insufficient strength of the aramid honeycombs. The aramid sheets used as base materials for the honeycombs are calendered under a linear pressure of 29.4×10
4
N/m(300 kg/cm) and at a temperature of around 300° C., and no many pores are retained since the melted, fluidized and hardened meta-aramid pulp fill in pores. After the sheets are formed to honeycombs, a reinfo
Baker & Botts L.L.P.
Jones Deborah
Showa Aircraft Industry Co., Ltd.
Xu Ling
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