WO2002057073A1 - Honeycomb core for sandwich-structure materials and the preparation method thereof - Google Patents

Honeycomb core for sandwich-structure materials and the preparation method thereof Download PDF

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Publication number
WO2002057073A1
WO2002057073A1 PCT/KR2001/001115 KR0101115W WO02057073A1 WO 2002057073 A1 WO2002057073 A1 WO 2002057073A1 KR 0101115 W KR0101115 W KR 0101115W WO 02057073 A1 WO02057073 A1 WO 02057073A1
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Prior art keywords
honeycomb core
reinforcing material
core
inorganic filler
filler
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PCT/KR2001/001115
Other languages
French (fr)
Inventor
Woo-Young Jeong
Ki-Dong Kim
Kwang-Jin Chung
Choul-Mo Kang
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Sol Nanochem Ltd.
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Publication of WO2002057073A1 publication Critical patent/WO2002057073A1/en

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    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B14/00—Use of inorganic materials as fillers, e.g. pigments, for mortars, concrete or artificial stone; Treatment of inorganic materials specially adapted to enhance their filling properties in mortars, concrete or artificial stone
    • C04B14/02—Granular materials, e.g. microballoons
    • C04B14/04—Silica-rich materials; Silicates
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B3/00—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form
    • B32B3/10—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form characterised by a discontinuous layer, i.e. formed of separate pieces of material
    • B32B3/12—Layered products comprising a layer with external or internal discontinuities or unevennesses, or a layer of non-planar shape; Layered products comprising a layer having particular features of form characterised by a discontinuous layer, i.e. formed of separate pieces of material characterised by a layer of regularly- arranged cells, e.g. a honeycomb structure
    • E—FIXED CONSTRUCTIONS
    • E04—BUILDING
    • E04C—STRUCTURAL ELEMENTS; BUILDING MATERIALS
    • E04C2/00—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels
    • E04C2/30—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels characterised by the shape or structure
    • E04C2/34—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels characterised by the shape or structure composed of two or more spaced sheet-like parts
    • E04C2/36—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels characterised by the shape or structure composed of two or more spaced sheet-like parts spaced apart by transversely-placed strip material, e.g. honeycomb panels
    • E04C2/365—Building elements of relatively thin form for the construction of parts of buildings, e.g. sheet materials, slabs, or panels characterised by the shape or structure composed of two or more spaced sheet-like parts spaced apart by transversely-placed strip material, e.g. honeycomb panels by honeycomb structures
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2260/00—Layered product comprising an impregnated, embedded, or bonded layer wherein the layer comprises an impregnation, embedding, or binder material
    • B32B2260/02—Composition of the impregnated, bonded or embedded layer
    • B32B2260/021—Fibrous or filamentary layer
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2260/00—Layered product comprising an impregnated, embedded, or bonded layer wherein the layer comprises an impregnation, embedding, or binder material
    • B32B2260/02—Composition of the impregnated, bonded or embedded layer
    • B32B2260/026—Wood layer
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2260/00—Layered product comprising an impregnated, embedded, or bonded layer wherein the layer comprises an impregnation, embedding, or binder material
    • B32B2260/02—Composition of the impregnated, bonded or embedded layer
    • B32B2260/028—Paper layer
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B2260/00—Layered product comprising an impregnated, embedded, or bonded layer wherein the layer comprises an impregnation, embedding, or binder material
    • B32B2260/04—Impregnation, embedding, or binder material
    • B32B2260/044—Water-setting substance, e.g. concrete, plaster
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B29/00—Layered products comprising a layer of paper or cardboard
    • B32B29/06—Layered products comprising a layer of paper or cardboard specially treated, e.g. surfaced, parchmentised
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B14/00—Use of inorganic materials as fillers, e.g. pigments, for mortars, concrete or artificial stone; Treatment of inorganic materials specially adapted to enhance their filling properties in mortars, concrete or artificial stone
    • C04B14/02—Granular materials, e.g. microballoons
    • C04B14/30—Oxides other than silica
    • C04B14/303—Alumina
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B38/00—Porous mortars, concrete, artificial stone or ceramic ware; Preparation thereof
    • C04B38/0006—Honeycomb structures

Definitions

  • the present invention concerns a honeycomb core for sandwich type structure materials and a method for preparing the same, in which a lightweight fibrous flammable substance, such as paper, natural or synthetic fiber, or wood, is impregnated with an inorganic filler, thus the impregnated substance having an enhanced strength and flame retardancy and being further applicable as a honeycomb core for sandwich-structure materials.
  • a lightweight fibrous flammable substance such as paper, natural or synthetic fiber, or wood
  • a sandwich type structure comprises two thin face-sheets (skin layers) having high strength, a layer of a lightweight honeycomb core therebetween, and adhesive layers for adhering these layers.
  • materials used in the skin layers and the core layer should have the following properties: first, the skin layer materials should have excellent rigidity and strength and thus show high resistance to tension and compression, and second, the core layer should comprise relatively soft and evenly honeycomb form of lightweight materials having high resistance to sheer.
  • Such two separate layers are bonded with adhesives to form a single structure, which has a compression strength up to 30 times stronger than a sandwich panel comprising only skin layer.
  • honeycomb core The core layer of sandwich type structure is honeycomb-shaped and thus referred to as a honeycomb core.
  • a honeycomb means a cell having six angles, but is not limited to said form in the present invention and include lattice forms, wavy forms, convex or concave forms, etc.
  • the addition of the honeycomb core to the sandwich type structure results in light total weight of the structure and excellent compression resistance.
  • the sandwich-structure materials containing honeycomb core were used for construction of flying machines since the 1940s, and nowadays, are used for interior decorations of buildings, ships or automobiles, partition substances, and lightweight substances in the fields of leisure and sports.
  • honeycomb core of the sandwich type structure For the honeycomb core of the sandwich type structure, light metals may be used as materials. However, such light metals suffer from unsuitable weight and high price, thus not being widely used as core materials, except in special cases. Also, carbon fibers, glass fibers or polymer fibers may be used for the preparation of honeycomb cores. These substances are advantageous in greatly increasing strength and durability, but disadvantageous in poor formability and adhesiveness, and very high price.
  • a paper-comprising honeycomb core has been widely used for the preparation of doors, partitions, and lightweight panels.
  • the paper core has advantages of lightweight, low price, easy formability to honeycomb forms and easy adhesion, so that sandwich-structure materials applied to practical purposes utilize such paper core.
  • paper when paper is used as the honeycomb core of many structures of buildings, such as panels, doors, and partitions, which are made of flammable materials, rapid diffusion of flame occurs in case of fire.
  • the paper-comprising honeycomb core has poor strength, its use has been very limited.
  • inorganic or organic fibrous reinforcing materials filled with organic polymer resins have been used as the honeycomb core. When fibrous reinforcing materials are filled with organic polymer resins, the compression strength and tension strength of the core can be significantly improved. But, the fillers comprising such organic polymer resins are flammable and generate poisonous gases during fire.
  • honeycomb core for sandwich type structure materials, which has advantages of lightweight, excellent formability and adhesiveness, lowered preparation cost, superior physical and mechanical properties, such as resistance to compression, tension resistance, abrasion resistance, bending resistance, excellence in water tolerance.
  • a honeycomb core for sandwich type structure materials wherein a fibrous lightweight reinforcing material is impregnated with an inorganic filler, said inorganic filler being selected from the group consisting of alkali silicate, alkali alumina, silica sol
  • nSi0 2 -xH 2 0, colloidal type sodium composite magnesium/aluminum sulfonate (Na 2 0-Mg/Al-SC> 4 -xH 2 0) .
  • a method for preparing a honeycomb core for sandwich-structure materials comprising the steps of: impregnating a fibrous lightweight reinforcing material with an inorganic filler, said inorganic filler being selected from the group consisting of alkali silicate, alkali alumina, silica sol (nSi0 2 -xH 2 0, colloidal type) or sodium composite magnesium/aluminum sulfonate (Na 2 OMg/Al-S0 4 -xH 2 0) ; and drying the inorganic filler-impregnated core at conditions of a temperature and a period of time not foaming said inorganic filler.
  • an inorganic filler being selected from the group consisting of alkali silicate, alkali alumina, silica sol (nSi0 2 -xH 2 0, colloidal type) or sodium composite magnesium/aluminum sulfonate (Na 2 OMg/Al-S0 4 -xH 2 0) ;
  • Fig.l shows compression strength of a honeycomb core according to concentration change of a coating solution.
  • Fig. 2 shows sheer adhesion strength of a honeycomb core according to concentration change of a coating solution.
  • Fig. 3 shows a table illustrating values measured in examples of present invention.
  • Fig. 4 shows a photograph of a test piece before a flame-retardant test.
  • Fig. 5 shows a photograph in which flame is applied to a test piece.
  • Fig. 6 shows a photograph after the flame is removed.
  • Fig. 7 shows a photograph of a foamed test piece after a flame-retardant test.
  • Fig. 8 shows DSC results illustrating foamed temperature ranges of sodium silicate and heat flow.
  • a lightweight fibrous flammable material such as paper, natural or synthetic fiber, wood and so on, is used as a reinforcing material, which is impregnated with an inorganic filler, to prepare a honeycomb core for sandwich type structure materials.
  • Cores having six angles like the real honeycomb are favorable in terms of good compression strength, but lattice forms, wavy forms such as in corrugated cardboard, or convex or concave forms may be used according to requirements for the strength of the finished product.
  • Examples of the inorganic filler usable in the present invention include alkali silicates, such as sodium silicate (Na 2 OnSi0 2 -xH 2 0) and lithium polysilicate
  • alkali alumina such as sodium aluminate
  • Fig. 1 and Fig. 2 show the change of the compression strength and sheer adhesion strength of a honeycomb core of the present invention according to the change of the degree of the concentrations of aqueous inorganic filler solution (coating solution), respectively.
  • Fig. 1 and Fig. 2 also show that the propensity of the increase of such strengths becomes dull as the degree of concentration of the solutions goes higher.
  • the lightweight fibrous reinforcing material such as paper, natural or synthetic fiber, wood, etc.
  • a sprayer containing 10-70 wt% aqueous inorganic filler solution or such reinforcing material is dipped into aqueous solution bath of the filler, thereby sufficiently impregnating the reinforcing material with the aqueous solution of filler.
  • the impregnated reinforcing material is dried in a hot air dryer at the conditions of a temperature and the period of time not foaming the inorganic filler, that is, at about 60 °C to 130 °C for 1 minute to 30 minutes. If the inorganic filler is foamed, physical properties of the core are decreased, so that the temperature and time at which the filler is not foamed are used.
  • Sodium silicate used in the present invention is a concentrated aqueous alkali silicate solution obtained by dissolving silicon dioxide and alkali, which is a liquid phase substance containing 2-4 moles Si0 2 per 1 mole Na 2 0. This substance is a colorless and odorless liquid having high viscosity, and forms a solid phase glass when dried in the air.
  • Water glass comprising Na 2 0 and Si0 2 of various forms, depending on its component molar ratios, is commercially available. In this regard, when the ratio of Na 2 0:Si0 2 is 1:1, the product is called meta-sodium silicate; when the ratio is 1.5:1, ortho-sodium silicate; when the ratio is 1:2, No.
  • aqueous solutions of liquid phase sodium silicate (Na 2 0-nSiO 2 -xH 2 O) , sodium aluminate (Na 2 0-Al 2 ⁇ 3 -xH 2 0) , lithium polysilicate (Li 2 0-nSi0 2 -xH 2 0) , silica sol (nSi0 2 -xH 2 0, colloidal type) , and sodium composite magnesium-aluminum sulfonate (Na 2 0-Mg/Al-S ⁇ 4 -xH 2 0) are preferably used in a concentration of 10 wt% - 70 wt%.
  • the concentration of the aqueous solution is less than 10 wt%, the core materials prepared through spraying or dipping followed by drying do not have sufficient compression strength, and the improved mechanical and chemical characteristics of the core materials are not shown, caused by water absorption.
  • the concentration exceeds 70 wt%, the core structure is deformed and thus its basic properties cannot be maintained, and also economic burden arises because long periods of time and large amounts of energy are required to completely dry the core.
  • a drying process is conducted in a temperature range of 60-130 °C.
  • the temperature is lower than 60 °C, sufficient drying is not performed and thus weight of the core structure becomes heavier, and it is difficult to manipulate the reinforced sandwich type structure materials after drying.
  • the temperature exceeds 130 °C, the properties of the structure materials are decreased owing to boiling effects at the surface, due to the foaming phenomena in high temperature of the coated water glass.
  • a preferred period of time required for dryness ranges from 1 minute to 30 minutes.
  • the period of time is shorter than 1 minute, drying is not sufficiently performed and thus weight of the core structure becomes heavier, and it is difficult to form the reinforced sandwich type structure material after drying. Meanwhile, if the period of time is longer than 30 minutes, the honeycomb core is separated from water glass component after the coating because of thorough dryness of the surface of the core material, thus the surface being not smooth, so that the core cannot be used as the sandwich type structure materials.
  • the compression strength and the sheer adhesion strength of the honeycomb core manufactured by coating the surface of reinforcing materials with inorganic fillers are increased about 2-5 times and about 2-4 times, respectively, compared to those of a conventional not-coated core. Therefore, the ultralight honeycomb core having excellent compression resistance and flame retardancy can be prepared.
  • a paper generally useful as a honeycomb core for sandwich type structure materials which was cut to 600 mm long, 600 mm wide and 25 mm thick, and approximately 10 % solution of liquid phase sodium silicate
  • a paper as in the above example 1, and about 50 wt% aqueous sodium aluminate solution (Na 2 O-Al 2 O 3 -xH 2 0) were used as a reinforcing material and an inorganic filler, respectively.
  • the manner of coating the reinforcing material with the filler adopted a spraying technique,
  • the reinforcing material was sufficiently impregnated with the filler by spraying, dried in a hot-air dryer at about
  • honeycomb core For thusly obtained honeycomb core, the change of weight and structure before and after the filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The measured results showed that the weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the sodium aluminate-coating treatment were increased 4.5 times and 3.2 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which paper was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
  • a paper as in the above example 1, and about 30 wt% aqueous lithium polysilicate solution (Li 2 0-nSi0 2 -xH 2 0) were used as a reinforcing material and an inorganic filler, respectively.
  • the manner of coating the reinforcing material with the filler adopted a spraying technique.
  • the reinforcing material was sufficiently impregnated with the filler by spraying, dried in a hot-air dryer at 100 °C for about 10 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
  • honeycomb core For thusly obtained honeycomb core, the change of weight and structure before and after filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3.
  • the measured results showed that the weight and the external structure of the core were scarcely changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the lithium polysilicate- coating treatment were increased 3.4 times and 2.7 times, respectively, compared with those of a core before the coating treatment. From the results, it can be found that mechanical properties of the honeycomb core in which paper was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
  • a paper as in the above example 1, and about 70 wt% aqueous silica sol solution (nSi0 2 -xH 2 0, colloidal type) were used as a reinforcing material and an inorganic filler, respectively.
  • the manner of coating the reinforcing material with the filler adopted a spraying technique.
  • the reinforcing material was sufficiently impregnated with the filler by spraying, dried in a hot- air dryer at about 130 °C for about 20 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
  • honeycomb core For thusly obtained honeycomb core, the change of weight and structure before and after the filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the silica sol- coating treatment were increased 5.3 times and 3.8 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which paper was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
  • a paper as in the above example 1, and about 40 wt% aqueous sodium composite magnesium/aluminum sulfonate solution (Na 2 0-Mg/Al-S0-xH 2 0) were used as a reinforcing material and an inorganic filler, respectively.
  • the manner of coating the reinforcing material with the filler adopted a technique of dipping the reinforcing material into aqueous solution of the filler.
  • the reinforcing material was sufficiently impregnated with the filler, dried in a hot-air dryer at about 110 °C for about 30 minutes, thereby coating the surface of the reinforcing material with the filler, to yield a honeycomb core.
  • honeycomb core For thusly obtained honeycomb core, the change of weight and structure before and after the inorganic filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the sodium composite magnesium/aluminum sulfonate-coating treatment were increased 4.8 times and
  • a paper as in the above example 1, and about 60 wt% aqueous sodium silicate solution (Na 2 0-nSi0 2 -xH 2 0) were used as a reinforcing material and an inorganic filler, respectively.
  • the manner of coating the reinforcing material with the filler adopted a technique of dipping the reinforcing material into aqueous filler solution.
  • the reinforcing material was sufficiently impregnated with the filler, dried in a hot-air dryer at about 90 °C for about 15 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
  • honeycomb core For thusly obtained honeycomb core, the change of weight and structure before and after inorganic filler- coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after sodium silicate-coating treatment were increased 5.1 times and
  • a paper as in the above example 1, and about 20 wt% aqueous sodium aluminate solution (Na 2 0-Al 2 0 3 -xH 2 0) were used as a reinforcing material and an inorganic filler, respectively.
  • the manner of coating the reinforcing material with the filler adopted a technique of dipping the reinforcing material into aqueous filler solution.
  • the reinforcing material was sufficiently impregnated with the filler, dried in a hot-air dryer at about 70 °C for about 25 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
  • honeycomb core For thusly obtained honeycomb core, the change of weight and structure before and after inorganic filler- coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The measured results were shown that the weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the sodium aluminate-coating treatment were increased 3.5 times and 2.4 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which paper was used as a reinforcing material were significantly improved, with only a small change of weight and structure of .the core caused by water glass-coating treatment.
  • a general wood, and about 20 wt% aqueous sodium silicate solution (Na 2 0-nSi ⁇ 2 -xH 2 0) were used as a reinforcing material and an inorganic filler, respectively.
  • the manner of coating the reinforcing material with the filler adopted a spraying technique.
  • the reinforcing material was sufficiently impregnated with the filler by spraying, dried in a hot-air dryer at about 80 °C for about 20 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
  • honeycomb core For thusly obtained honeycomb core, the change of weight and structure before and after inorganic filler- coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the sodium silicate-coating treatment were increased 2.6 times and 1.8 times, respectively, compared with those of the core before the coating treatment. From the results, it can be confirmed that mechanical properties of the honeycomb core in which wood was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
  • EXAMPLE 9 A wood as in the above example 8, and about 40 wt% aqueous silica sol solution (nSi0 2 -xH 2 0, colloidal type) were used as a reinforcing material and an inorganic filler, respectively.
  • the manner of coating the reinforcing material with the filler adopted a technique of dipping the reinforcing material into aqueous solution of the filler.
  • the reinforcing material was sufficiently impregnated with the filler, dried in a hot-air dryer at
  • honeycomb core For thusly obtained honeycomb core, the change of weight and structure before and after the inorganic filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the silica sol-coating treatment were increased 3.4 times and 2.3 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which wood was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
  • a natural and synthetic fiber, and about 30 wt% aqueous lithium polysilicate solution (Li 2 0-nSi0 2 -xH 2 0) were used as a reinforcing material and an inorganic filler, respectively.
  • the manner of coating the reinforcing material with the filler .adopted a spraying technique.
  • the reinforcing material was sufficiently impregnated with the filler by spraying, dried in a hot-air dryer at about 70 °C for about 30 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
  • honeycomb core For thusly obtained honeycomb core, the change of weight and structure before and after the inorganic filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the lithium polysilicate-coating treatment were increased 7.8 times and 1.7 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which fiber was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
  • EXAMPLE 11 An artificial and synthetic fiber as in the above example 10, and about 50 wt% aqueous sodium composite magnesium/aluminum sulfonate solution (Na 2 0-Mg/Al-S ⁇ 4 -xH 2 0) were used as a reinforcing material and an inorganic filler, respectively.
  • the manner of coating the reinforcing material with the filler adopted a technique for dipping the reinforcing material into aqueous filler solution.
  • the reinforcing material was sufficiently impregnated with the filler, dried in a hot-air dryer at about 90 °C for about 15 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
  • honeycomb core For thusly obtained honeycomb core, the change of weight and structure before and after the inorganic filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the sodium composite magnesium/aluminum sulfonate-coating treatment were increased 3.4 times and 2.3 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which fiber was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
  • the core test-pieces prepared in the above examples were tested for their flame retardancy.
  • the used equipment was manufactured according to a standard ASTM D5025, including a burner with a length of lOO ⁇ lO mm, a tube with an inner diameter of 9.5 ⁇ 0.3 mm, a means for regulating flow of methane gas, as a feeding gas meeting a required grade (minimum purity 98 %), and a detector.
  • a hood having a minimum inner area of 0.5 m 2 was equipped with a fan for naturally discharging poisonous gas during the test. However, the fan was not allowed to operate during testing.
  • the laboratory had a temperature of 23 °C and a relative humidity of 45 %. Blue flame at a height of 20 mm was applied to the test pieces for 30 seconds, and thereafter the results were observed.
  • a burner-fitted stand and a sample-fitted stand were placed on the same support surface. The burner was inclined at an angle of 45° relative to the support surface and facing the sample. A tip of the sample was applied to the flame emitting from the end of the burner.
  • the change of external structure of the cores after the water glass-surface coating and drying treatment in the above examples 1 to 11 was observed by naked eye.
  • the compression strength and the sheer adhesion strength were measured by the method of KS F3517.
  • the shape of test pieces was of two types, that is, a corrugated cardboard type or a hexagonal honeycomb type, according to the forms of cells, each cell having a size of 10 mm.
  • the pieces were cut to 50 mm long, 50 mm wide, and 25 mm thick, and, in the case of measurement of the sheer adhesion strength, to 200 mm long, 75 mm wide, and 25 mm thick.
  • Figs. 4 to 7 show photographs of experimental procedures. From the test results, it can be confirmed that samples are suitable for 94 HB condition, and are not burned and so have excellent flame retardancy because of an average linear combustion ratio of 0 (mm/minute) .
  • the reason why samples have excellent flame retardancy is that water glass (or sodium silicates) , which is a coated inorganic ceramics, is foamed by heat and thus has excellent insulating effect between heat and paper. So, products can be safely protected from heating source.
  • Fig. 8 there is shown a graph of DSC
  • the honeycomb core for sandwich type structure materials according to the present invention is advantageous in terms of drastically improved physical and mechanical properties, such as resistance to compression, tension resistance, abrasion resistance, bending resistance, and excellence in water tolerance, while maintaining light weight through impregnation treatment of inorganic fillers. Therefore, the core prepared through surface coating treatment has greatly improved mechanical properties, including compression strength and tension strength, without changing weight and external structure of the core before and after coating, compared with conventional not-coated core. Thereby, ultra-light honeycomb core having excellent compression resistance can be manufactured.
  • the core of the present invention has the advantages of being more less expensive than and having similar mechanical properties- to, conventional expensive aramid or aluminum honeycomb core, and can be manufactured by continuous processes without requiring additional mechanical machines or equipment caused by the coating treatment, so economic benefit arising.

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Abstract

The present invention relates to the honeycomb core for sandwich-structure materials made by coating non-organic filler, such as Na2O•nSiO2•xH2O, Na2O•Al2O3•xH2O, or Li2O•nSiO2•xH2O on the surface of supporting materials, such as paper, natural or synthetic fiber, or wood, which has highly enhanced compression strength and flame retardancy with light weight. Since the present invention improves and overcomes the effects of paper, natural or synthetic fiber, or wood as honeycomb core material, the honeycomb core of the present invention can replace the conventional expensive honeycomb core made by aramid or aluminum.

Description

HONEYCOMB CORE FOR SANDWICH-STRUCTURE MATERIALS AND THE PREPARATION METHOD THEREOF
TECHNICAL FIELD The present invention concerns a honeycomb core for sandwich type structure materials and a method for preparing the same, in which a lightweight fibrous flammable substance, such as paper, natural or synthetic fiber, or wood, is impregnated with an inorganic filler, thus the impregnated substance having an enhanced strength and flame retardancy and being further applicable as a honeycomb core for sandwich-structure materials.
PRIOR ART Generally, a sandwich type structure comprises two thin face-sheets (skin layers) having high strength, a layer of a lightweight honeycomb core therebetween, and adhesive layers for adhering these layers. As such, materials used in the skin layers and the core layer should have the following properties: first, the skin layer materials should have excellent rigidity and strength and thus show high resistance to tension and compression, and second, the core layer should comprise relatively soft and evenly honeycomb form of lightweight materials having high resistance to sheer. Such two separate layers are bonded with adhesives to form a single structure, which has a compression strength up to 30 times stronger than a sandwich panel comprising only skin layer.
The core layer of sandwich type structure is honeycomb-shaped and thus referred to as a honeycomb core. Generally, a honeycomb means a cell having six angles, but is not limited to said form in the present invention and include lattice forms, wavy forms, convex or concave forms, etc. The addition of the honeycomb core to the sandwich type structure results in light total weight of the structure and excellent compression resistance. The sandwich-structure materials containing honeycomb core were used for construction of flying machines since the 1940s, and nowadays, are used for interior decorations of buildings, ships or automobiles, partition substances, and lightweight substances in the fields of leisure and sports.
For the honeycomb core of the sandwich type structure, light metals may be used as materials. However, such light metals suffer from unsuitable weight and high price, thus not being widely used as core materials, except in special cases. Also, carbon fibers, glass fibers or polymer fibers may be used for the preparation of honeycomb cores. These substances are advantageous in greatly increasing strength and durability, but disadvantageous in poor formability and adhesiveness, and very high price.
Recently, a paper-comprising honeycomb core has been widely used for the preparation of doors, partitions, and lightweight panels. The paper core has advantages of lightweight, low price, easy formability to honeycomb forms and easy adhesion, so that sandwich-structure materials applied to practical purposes utilize such paper core. However, when paper is used as the honeycomb core of many structures of buildings, such as panels, doors, and partitions, which are made of flammable materials, rapid diffusion of flame occurs in case of fire. In addition, because the paper-comprising honeycomb core has poor strength, its use has been very limited. In the meantime, for the lightweight sandwich type structure materials, inorganic or organic fibrous reinforcing materials filled with organic polymer resins have been used as the honeycomb core. When fibrous reinforcing materials are filled with organic polymer resins, the compression strength and tension strength of the core can be significantly improved. But, the fillers comprising such organic polymer resins are flammable and generate poisonous gases during fire.
DISCLOSURE OF THE INVENTION
Accordingly, it is an object of the present invention to alleviate the problems as described above and to provide a honeycomb core for sandwich type structure materials, which has advantages of lightweight, excellent formability and adhesiveness, lowered preparation cost, superior physical and mechanical properties, such as resistance to compression, tension resistance, abrasion resistance, bending resistance, excellence in water tolerance.
It is another object of the present invention to provide a flame retardant core for sandwich type structure materials that has decreased flammability and toxicity of the lightweight fibrous honeycomb core comprising paper, fiber or wood, by using inorganic materials as fillers.
It is still another object of the present invention to provide a method for preparing said honeycomb core for sandwich type structure materials.
In accordance with an embodiment of the present invention, there is provided a honeycomb core for sandwich type structure materials, wherein a fibrous lightweight reinforcing material is impregnated with an inorganic filler, said inorganic filler being selected from the group consisting of alkali silicate, alkali alumina, silica sol
(nSi02-xH20, colloidal type) or sodium composite magnesium/aluminum sulfonate (Na20-Mg/Al-SC>4-xH20) .
In accordance with another embodiment of the present invention, there is provided a method for preparing a honeycomb core for sandwich-structure materials, comprising the steps of: impregnating a fibrous lightweight reinforcing material with an inorganic filler, said inorganic filler being selected from the group consisting of alkali silicate, alkali alumina, silica sol (nSi02-xH20, colloidal type) or sodium composite magnesium/aluminum sulfonate (Na2OMg/Al-S04-xH20) ; and drying the inorganic filler-impregnated core at conditions of a temperature and a period of time not foaming said inorganic filler.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
Fig.l shows compression strength of a honeycomb core according to concentration change of a coating solution. Fig. 2 shows sheer adhesion strength of a honeycomb core according to concentration change of a coating solution. Fig. 3 shows a table illustrating values measured in examples of present invention.
Fig. 4 shows a photograph of a test piece before a flame-retardant test.
Fig. 5 shows a photograph in which flame is applied to a test piece.
Fig. 6 shows a photograph after the flame is removed. Fig. 7 shows a photograph of a foamed test piece after a flame-retardant test.
Fig. 8 shows DSC results illustrating foamed temperature ranges of sodium silicate and heat flow.
BEST MODES FOR CARRYING OUT THE INVENTION
In the present invention, a lightweight fibrous flammable material, such as paper, natural or synthetic fiber, wood and so on, is used as a reinforcing material, which is impregnated with an inorganic filler, to prepare a honeycomb core for sandwich type structure materials. Cores having six angles like the real honeycomb are favorable in terms of good compression strength, but lattice forms, wavy forms such as in corrugated cardboard, or convex or concave forms may be used according to requirements for the strength of the finished product.
Examples of the inorganic filler usable in the present invention include alkali silicates, such as sodium silicate (Na2OnSi02-xH20) and lithium polysilicate
(Li20-nSi02-xH20) ; alkali alumina, such as sodium aluminate
(Na2OAl203-xH20) ; silica sol (nSi02-xH20, colloidal type) , or sodium composite magnesium/aluminum sulfonate (Na2OMg/Al- S0-xH20) . Preferably, of these fillers, sodium silicate
(Na20-nSi02-xH20) , sodium aluminate (Na2OAl203-xH20) , and lithium polysilicate (Li20-nSi02-xH20) are used. In order to impregnate the reinforcing material with the inorganic filler, an aqueous solution of the filler is sprayed to the reinforcing material or the reinforcing material is dipped into aqueous solution of the filler. The degree of concentrations, drying times and temperatures of aqueous solution of the filler affect strength and flame retardancy of the core. Fig. 1 and Fig. 2 show the change of the compression strength and sheer adhesion strength of a honeycomb core of the present invention according to the change of the degree of the concentrations of aqueous inorganic filler solution (coating solution), respectively. Fig. 1 and Fig. 2 also show that the propensity of the increase of such strengths becomes dull as the degree of concentration of the solutions goes higher.
In the preparation method of the honeycomb core, the lightweight fibrous reinforcing material, such as paper, natural or synthetic fiber, wood, etc., is sprayed with a sprayer containing 10-70 wt% aqueous inorganic filler solution, or such reinforcing material is dipped into aqueous solution bath of the filler, thereby sufficiently impregnating the reinforcing material with the aqueous solution of filler. Thereafter, the impregnated reinforcing material is dried in a hot air dryer at the conditions of a temperature and the period of time not foaming the inorganic filler, that is, at about 60 °C to 130 °C for 1 minute to 30 minutes. If the inorganic filler is foamed, physical properties of the core are decreased, so that the temperature and time at which the filler is not foamed are used.
Sodium silicate used in the present invention, referred to as water glass, is a concentrated aqueous alkali silicate solution obtained by dissolving silicon dioxide and alkali, which is a liquid phase substance containing 2-4 moles Si02 per 1 mole Na20. This substance is a colorless and odorless liquid having high viscosity, and forms a solid phase glass when dried in the air. Water glass comprising Na20 and Si02 of various forms, depending on its component molar ratios, is commercially available. In this regard, when the ratio of Na20:Si02 is 1:1, the product is called meta-sodium silicate; when the ratio is 1.5:1, ortho-sodium silicate; when the ratio is 1:2, No. 1 sodium silicate; when the ratio is 1:2.5, No. 2 sodium silicate; when 1:3, No. 3 sodium silicate; and when 1:4, the product is called No. 4 sodium silicate. Additionally, liquid phase sodium aluminate, lithium polysilicate, silica sol and sodium composite magnesium/aluminum sulfonate are commercially available, in various forms depending on their component molar ratios. Any sodium silicate fillers can be used on the core surface,' regardless of the molar ratios of the components. According to the present invention, aqueous solutions of liquid phase sodium silicate (Na20-nSiO2-xH2O) , sodium aluminate (Na20-Al2θ3-xH20) , lithium polysilicate (Li20-nSi02-xH20) , silica sol (nSi02-xH20, colloidal type) , and sodium composite magnesium-aluminum sulfonate (Na20-Mg/Al-Sθ4-xH20) are preferably used in a concentration of 10 wt% - 70 wt%. If the concentration of the aqueous solution is less than 10 wt%, the core materials prepared through spraying or dipping followed by drying do not have sufficient compression strength, and the improved mechanical and chemical characteristics of the core materials are not shown, caused by water absorption. On the other hand, if the concentration exceeds 70 wt%, the core structure is deformed and thus its basic properties cannot be maintained, and also economic burden arises because long periods of time and large amounts of energy are required to completely dry the core.
Upon coating the surface of fibrous reinforcing materials with inorganic fillers according to the present invention, it is preferred that a drying process is conducted in a temperature range of 60-130 °C. When the temperature is lower than 60 °C, sufficient drying is not performed and thus weight of the core structure becomes heavier, and it is difficult to manipulate the reinforced sandwich type structure materials after drying. Whereas, when the temperature exceeds 130 °C, the properties of the structure materials are decreased owing to boiling effects at the surface, due to the foaming phenomena in high temperature of the coated water glass. A preferred period of time required for dryness ranges from 1 minute to 30 minutes. If the period of time is shorter than 1 minute, drying is not sufficiently performed and thus weight of the core structure becomes heavier, and it is difficult to form the reinforced sandwich type structure material after drying. Meanwhile, if the period of time is longer than 30 minutes, the honeycomb core is separated from water glass component after the coating because of thorough dryness of the surface of the core material, thus the surface being not smooth, so that the core cannot be used as the sandwich type structure materials.
In the present invention, the compression strength and the sheer adhesion strength of the honeycomb core manufactured by coating the surface of reinforcing materials with inorganic fillers are increased about 2-5 times and about 2-4 times, respectively, compared to those of a conventional not-coated core. Therefore, the ultralight honeycomb core having excellent compression resistance and flame retardancy can be prepared.
A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.
EXAMPLE 1
A paper generally useful as a honeycomb core for sandwich type structure materials which was cut to 600 mm long, 600 mm wide and 25 mm thick, and approximately 10 % solution of liquid phase sodium silicate
(Na2θ-nSi02-xH20) were used as a reinforcing material and an inorganic filler, respectively. The manner of coating the reinforcing material with the filler adopted a spraying technique. The reinforcing material was sufficiently impregnated with the filler by spraying, dried in a hot- air dryer at 60 °C for about 1 minute, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler. For thusly obtained honeycomb core, the change of weight and structure before and after the filler-coating, a compression strength and a sheer adhesion strength were easu red. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after sodium silicate- coating treatment were increased 2.2 times and 1.8 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which paper was used as the reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
EXAMPLE 2
A paper as in the above example 1, and about 50 wt% aqueous sodium aluminate solution (Na2O-Al2O3-xH20) were used as a reinforcing material and an inorganic filler, respectively. The manner of coating the reinforcing material with the filler adopted a spraying technique,
The reinforcing material was sufficiently impregnated with the filler by spraying, dried in a hot-air dryer at about
80 °C for about 5 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
For thusly obtained honeycomb core, the change of weight and structure before and after the filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The measured results showed that the weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the sodium aluminate-coating treatment were increased 4.5 times and 3.2 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which paper was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
EXAMPLE 3
A paper as in the above example 1, and about 30 wt% aqueous lithium polysilicate solution (Li20-nSi02-xH20) were used as a reinforcing material and an inorganic filler, respectively. The manner of coating the reinforcing material with the filler adopted a spraying technique. The reinforcing material was sufficiently impregnated with the filler by spraying, dried in a hot-air dryer at 100 °C for about 10 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
For thusly obtained honeycomb core, the change of weight and structure before and after filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3.
The measured results showed that the weight and the external structure of the core were scarcely changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the lithium polysilicate- coating treatment were increased 3.4 times and 2.7 times, respectively, compared with those of a core before the coating treatment. From the results, it can be found that mechanical properties of the honeycomb core in which paper was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
EXAMPLE 4
A paper as in the above example 1, and about 70 wt% aqueous silica sol solution (nSi02-xH20, colloidal type) were used as a reinforcing material and an inorganic filler, respectively. The manner of coating the reinforcing material with the filler adopted a spraying technique. The reinforcing material was sufficiently impregnated with the filler by spraying, dried in a hot- air dryer at about 130 °C for about 20 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
For thusly obtained honeycomb core, the change of weight and structure before and after the filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the silica sol- coating treatment were increased 5.3 times and 3.8 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which paper was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
EXAMPLE 5
A paper as in the above example 1, and about 40 wt% aqueous sodium composite magnesium/aluminum sulfonate solution (Na20-Mg/Al-S0-xH20) were used as a reinforcing material and an inorganic filler, respectively. The manner of coating the reinforcing material with the filler adopted a technique of dipping the reinforcing material into aqueous solution of the filler. The reinforcing material was sufficiently impregnated with the filler, dried in a hot-air dryer at about 110 °C for about 30 minutes, thereby coating the surface of the reinforcing material with the filler, to yield a honeycomb core.
For thusly obtained honeycomb core, the change of weight and structure before and after the inorganic filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the sodium composite magnesium/aluminum sulfonate-coating treatment were increased 4.8 times and
3.2 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which paper was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment .
EXAMPLE 6
A paper as in the above example 1, and about 60 wt% aqueous sodium silicate solution (Na20-nSi02-xH20) were used as a reinforcing material and an inorganic filler, respectively. The manner of coating the reinforcing material with the filler adopted a technique of dipping the reinforcing material into aqueous filler solution. The reinforcing material was sufficiently impregnated with the filler, dried in a hot-air dryer at about 90 °C for about 15 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
For thusly obtained honeycomb core, the change of weight and structure before and after inorganic filler- coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after sodium silicate-coating treatment were increased 5.1 times and
3.4 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which paper was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment .
EXAMPLE 7
A paper as in the above example 1, and about 20 wt% aqueous sodium aluminate solution (Na20-Al203-xH20) were used as a reinforcing material and an inorganic filler, respectively. The manner of coating the reinforcing material with the filler adopted a technique of dipping the reinforcing material into aqueous filler solution. The reinforcing material was sufficiently impregnated with the filler, dried in a hot-air dryer at about 70 °C for about 25 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler. For thusly obtained honeycomb core, the change of weight and structure before and after inorganic filler- coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The measured results were shown that the weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the sodium aluminate-coating treatment were increased 3.5 times and 2.4 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which paper was used as a reinforcing material were significantly improved, with only a small change of weight and structure of .the core caused by water glass-coating treatment.
EXAMPLE 8
A general wood, and about 20 wt% aqueous sodium silicate solution (Na20-nSiθ2-xH20) were used as a reinforcing material and an inorganic filler, respectively. The manner of coating the reinforcing material with the filler adopted a spraying technique. The reinforcing material was sufficiently impregnated with the filler by spraying, dried in a hot-air dryer at about 80 °C for about 20 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
For thusly obtained honeycomb core, the change of weight and structure before and after inorganic filler- coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the sodium silicate-coating treatment were increased 2.6 times and 1.8 times, respectively, compared with those of the core before the coating treatment. From the results, it can be confirmed that mechanical properties of the honeycomb core in which wood was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
EXAMPLE 9 A wood as in the above example 8, and about 40 wt% aqueous silica sol solution (nSi02-xH20, colloidal type) were used as a reinforcing material and an inorganic filler, respectively. The manner of coating the reinforcing material with the filler adopted a technique of dipping the reinforcing material into aqueous solution of the filler. The reinforcing material was sufficiently impregnated with the filler, dried in a hot-air dryer at
• about 100 °C for about 10 minutes, thereby coating the surface of the reinforcing material with the filler, to prepare a honeycomb core.
For thusly obtained honeycomb core, the change of weight and structure before and after the inorganic filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the silica sol-coating treatment were increased 3.4 times and 2.3 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which wood was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
EXAMPLE 10
A natural and synthetic fiber, and about 30 wt% aqueous lithium polysilicate solution (Li20-nSi02-xH20) were used as a reinforcing material and an inorganic filler, respectively. The manner of coating the reinforcing material with the filler .adopted a spraying technique. The reinforcing material was sufficiently impregnated with the filler by spraying, dried in a hot-air dryer at about 70 °C for about 30 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
For thusly obtained honeycomb core, the change of weight and structure before and after the inorganic filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the lithium polysilicate-coating treatment were increased 7.8 times and 1.7 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which fiber was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
EXAMPLE 11 An artificial and synthetic fiber as in the above example 10, and about 50 wt% aqueous sodium composite magnesium/aluminum sulfonate solution (Na20-Mg/Al-Sθ4-xH20) were used as a reinforcing material and an inorganic filler, respectively. The manner of coating the reinforcing material with the filler adopted a technique for dipping the reinforcing material into aqueous filler solution. The reinforcing material was sufficiently impregnated with the filler, dried in a hot-air dryer at about 90 °C for about 15 minutes, thereby preparing a honeycomb core in which the surface of the reinforcing material was coated with the filler.
For thusly obtained honeycomb core, the change of weight and structure before and after the inorganic filler-coating, a compression strength and a sheer adhesion strength were measured. The results are given in the table of Fig. 3. The weight and the external structure were hardly changed, and the compression strength and the sheer adhesion strength of the honeycomb core after the sodium composite magnesium/aluminum sulfonate-coating treatment were increased 3.4 times and 2.3 times, respectively. From the results, it can be confirmed that mechanical properties of the honeycomb core in which fiber was used as a reinforcing material were significantly improved, with only a small change of weight and structure of the core caused by water glass-coating treatment.
EXAMPLE 12
The core test-pieces prepared in the above examples were tested for their flame retardancy. As such, the used equipment was manufactured according to a standard ASTM D5025, including a burner with a length of lOO±lO mm, a tube with an inner diameter of 9.5±0.3 mm, a means for regulating flow of methane gas, as a feeding gas meeting a required grade (minimum purity 98 %), and a detector.
Useful as a harmful gas discharging apparatus, a hood having a minimum inner area of 0.5 m2, was equipped with a fan for naturally discharging poisonous gas during the test. However, the fan was not allowed to operate during testing. The laboratory had a temperature of 23 °C and a relative humidity of 45 %. Blue flame at a height of 20 mm was applied to the test pieces for 30 seconds, and thereafter the results were observed. At that time, a burner-fitted stand and a sample-fitted stand were placed on the same support surface. The burner was inclined at an angle of 45° relative to the support surface and facing the sample. A tip of the sample was applied to the flame emitting from the end of the burner. As the test results, all pieces had a combusted length of 0 mm and a linear combustion ratio (V=60L/t, mm/min.) of 0, and the flame- retardant paper core was not burned.
The change of external structure of the cores after the water glass-surface coating and drying treatment in the above examples 1 to 11 was observed by naked eye. For measurements of mechanical properties according to each example, the compression strength and the sheer adhesion strength were measured by the method of KS F3517. The shape of test pieces was of two types, that is, a corrugated cardboard type or a hexagonal honeycomb type, according to the forms of cells, each cell having a size of 10 mm. In the case of measuring the compression strength, the pieces were cut to 50 mm long, 50 mm wide, and 25 mm thick, and, in the case of measurement of the sheer adhesion strength, to 200 mm long, 75 mm wide, and 25 mm thick. In the test of the compression strength, the loaded surface of the pieces was reinforced with epoxy resin. Meanwhile, for test of sheer adhesion strength, an aluminum sheet (0.8 mm thick) having the same size as the pieces was used as surface materials and thus its both end portions were adhered with epoxy resin. Each test piece was thoroughly dried in a drying oven at 90 °C for about 12 hours, and then measured for compression strength and sheer adhesion strength, under a loading rate of 1 mm/minute. The test results of the coated pieces and the not-coated pieces were compared and presented in the table of Fig. 3, in which 5 measurements in each example were averaged.
Figs. 4 to 7 show photographs of experimental procedures. From the test results, it can be confirmed that samples are suitable for 94 HB condition, and are not burned and so have excellent flame retardancy because of an average linear combustion ratio of 0 (mm/minute) . The reason why samples have excellent flame retardancy is that water glass (or sodium silicates) , which is a coated inorganic ceramics, is foamed by heat and thus has excellent insulating effect between heat and paper. So, products can be safely protected from heating source. Referring to Fig. 8, there is shown a graph of DSC
(differential scanning calorimetry) of foamable ceramics
(water glass) . As can be seen in this drawing, for temperatures ranging from room temperature to 270 °C, the temperature range at which foamable ceramics are foamed by absorbing heat in an amount of about 116-162 J per 1 g, is about 125 °C to 200 °C.
INDUSTRIAL APPLICABILITY
As described above, the honeycomb core for sandwich type structure materials according to the present invention is advantageous in terms of drastically improved physical and mechanical properties, such as resistance to compression, tension resistance, abrasion resistance, bending resistance, and excellence in water tolerance, while maintaining light weight through impregnation treatment of inorganic fillers. Therefore, the core prepared through surface coating treatment has greatly improved mechanical properties, including compression strength and tension strength, without changing weight and external structure of the core before and after coating, compared with conventional not-coated core. Thereby, ultra-light honeycomb core having excellent compression resistance can be manufactured.
Additionally, the core of the present invention has the advantages of being more less expensive than and having similar mechanical properties- to, conventional expensive aramid or aluminum honeycomb core, and can be manufactured by continuous processes without requiring additional mechanical machines or equipment caused by the coating treatment, so economic benefit arising.
In the present invention, drawbacks of low strength and flammability of lightweight fibrous core are alleviated, whereby such core can be applied to the fields of preparing reinforcing materials for buildings or ships which require a flame-retardant standard.
The present invention has been described in an illustrative manner, and it is to be understood that the terminology used is intended to be in the nature of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims

1. A honeycomb core for sandwich type structure materials, wherein a fibrous lightweight reinforcing material is impregnated with an inorganic filler, said inorganic filler being selected from the group consisting of alkali silicate, alkali alumina, silica sol (nSi02-xH20, colloidal type) or sodium composite magnesium/aluminum sulfonate (Na20-Mg/Al-S0-xH20) .
2. The honeycomb core as defined in claim 1, wherein said fibrous lightweight reinforcing material is selected from the group consisting of paper, wood, natural fiber or synthetic fiber.
3. The honeycomb core as defined in claim 1, wherein said inorganic filler comprises sodium silicate (Na20-nSi02-xH20) , sodium aluminate (Na20-Al203-xH20) , or lithium polysilicate (Li2OnSi02-xH20) .
4. The honeycomb core as defined in any of claims 1 to 3, wherein the core shape comprises a honeycomb form, a lattice form, a wavy form, a convex or a concave form.
5. A method for preparing a honeycomb core for sandwich-structure materials, comprising the steps of: impregnating a fibrous lightweight reinforcing material with an inorganic filler, said inorganic filler being selected from the group consisting of alkali silicate, alkali alumina, silica sol (nSi02-xH20, colloidal type) or sodium composite magnesium/aluminum sulfonate (Na20-Mg/Al-S04-xH20) ; and drying the inorganic filler- impregnated core at conditions of a temperature and a period of time not foaming said inorganic filler.
6. The method as defined in claim 5, wherein said drying step is carried out at 60-130 °C for 1-30 minutes.
7. The method as defined in claim 5, wherein said fibrous reinforcing material is selected from the group consisting of paper, wood, natural fiber or synthetic fiber.
8. The method as defined in claim 5, wherein said inorganic filler comprises sodium silicate (Na0-nSiθ2-xH20) , sodium aluminate (Na20-Al203-xH20) , or lithium polysilicate (Li20-nSi02-xH20) .
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