Composite non-woven fabric material, method of its production and its use.

0001The present invention relates to a heat-resistant non-woven fabric composite material as defined in the preamble of claim 1, a method for its production and its use, particularly for cushions, conveyor belts, bag filters, sealing materials, etc., which can be used at elevated temperatures.

0002Heat resistant materials used for the above mentioned products are known and include both non-woven material formed of a single kind of fibers, and layered non-woven materials made of at least two kinds of fibers.

0003Non-woven materials may be formed exclusively of asbestos fibers, stainless steel fibers and the like heat resistant fibers. Asbestos material has been heretofore widely used because of its low price and its excellent heat resistivity. However, the use of asbestos is now strictly regulated in view of environmental and public health concerns because filaments of asbestos floating in air are now known to have adverse effects on human health.

0004Although the stainless steel fabrics are excellent in heat resistance, flexibility, springiness and cushioning property, etc., the high cost thereof prevents wide use in various fields.

0005JP-A- 24572/1975 discloses a layered fabric material comprising a compacted mass of metal fibrers such as stainless steel fibers or the like and a compacted mass of inorganic fibers such as rockwool fibers, glass fibers, asbestos fibers or the like. This material is produced by bonding the fiber masses with needle punching and firing the bonded fiber masses to form an interlocked layer at the interface of the fiber masses.

0006In the case of a layered material comprising rockwool fibers or asbestos fibers and metal fibers, the firing step is required to firmly hold the two layers together since the bonding force obtained between the interlocked layers by needle punching alone is too weak to keep the layers integral. However, the need to use a firing step makes this whole manufacturing process complicated, and the product obtained shows lowered springiness or cushioning property.

0007When the layered material is formed of glass fibers and metal fibers, its use is limited because the heat conductivity of glass fibers is low.

0008A heat-resistant composite non-woven fiber material comprising at least one steel fiber layer and at least one non-metallic non-woven fiber layer which are alternating, adjacent layers being interlocked to form an integrated composite material, and one end of the surface layers thereof being a steel fiber layer, has been known from GB-A-979 535. This prior art composite material may be a two-ply or a three-ply mat, one of the surface layers necessarily being a non-metallic fiber layer. The fibers of the non-metallic fiber layer are preferably made of a mixture of fibers of polyester, cotton or sheep wool or cellulose fibres. The fibrous fleece may be impregnated with water repellent substances. The composite materials according to this prior art are preferably provided for covering the calender or mangle rolls of mangles or calenders and laundry presses and ironers in general.

0009GB-A-979 808 discloses heat resistant pads comprising a plurality of layers each made of a mixture of carbon fibers and aromatic polyamide fibers which may be coated and impregnated with a silicone resin to improve wear resistance and impact resistance.

0010It is the object of the present invention to provide a composite non-woven fabric material which exhibits strong bonding between the different fabric layers, is stable at high temperatures and has good lubricating properties, and which can be easily produced at a reasonable cost, a method for its production and its use, particularly as sealing, filter and cushioning materials and for conveyor belts.

0011The above object is achieved according to the independent claims. The dependent claims relate to preferred embodiments.

0012The composite non-woven fiber material according to the present invention comprises at least one steel fiber layer and at least one non-metallic non-woven fiber layer which are alternating, adjacent layers being interlocked to form an integrated composite material, and one of the surface layers being a steel fiber layer; it is characterized in that <ul id="ul0001" list-style="dash"><li>the at least one steel fiber layer is made of stainless steel fibers, and</li><li>the at least one non-metallic fiber layer is made of carbon fibers.</li></ul>

0013The method according to the present invention for producing the above defined composite materials is based on laminating and interlocking at least one non-metallic non-woven fiber layer together with at least one steel fiber layer in such a manner that the non-metallic fiber layers and the steel fiber layers are alternating, and at least one surface layer of the laminate is a steel fiber layer; it is characterized in that <ul id="ul0002" list-style="dash"><li>stainless steel fiber layers are used as steel fiber layers and</li><li>a carbon fiber layer is used as non-metallic fiber layer.</li></ul>

0014For obtaining such the interlocking of the adjacent layers, needle punching or using a water jet is preferred.

0015Carbon fibers to be used in the invention may be selected from those produced from pitch materials and from organic polymers such as polyacrylonitrile (PAN), etc. The mass per area of a single carbon fiber layer is not critical, but it is usually from about 30 to about 2500 g/m², preferably from about 50 to about 2300 g/m². If this ratio is less than 30 g/m², the amount of the carbon fiber layer is greatly reduced, and the cost advantage of using a carbon fiber layer together with a stainless steel layer is lost. If this ratio exceeds 2500 g/m², the filaments positioned at the interface portions of the carbon fiber layer cannot be interlocked intimately with the filaments of the stainless steel fiber layer, and the composite is prone to separate at the interface of the carbon fiber layer when shearing forces are applied to the composite. In order to improve the resistance to separation of the composite material, a thin stainless steel fiber layer may be incorporated within the carbon fiber layer.

0016The mass per area of a single stainless steel layer is also not critical, but it is usually from about 50 to about 2500 g/m², preferably from about 120 to about 2000 g/m². When the ratio is less than 50 g/m², a uniform stainless steel fiber layer is difficult to prepare, and an uneven fiber layer exhibits poor heat resistance and abrasion resistance. When this ratio is over 2500 g/m², the relative amount of expensive stainless steel fibers is increased, and the cost effectiveness of the invention will be reduced. The diameter of the filaments forming the stainless steel fibers is usually from about 2 to about 50 µm. When the composite material of the invention is used as heat resistant cushioning material, the diameter of the stainless steel fibers preferably is from about 5 to about 20 µm. When the stainless steel fibers are formed of filaments with a small diameter of less than 2 µm, the fibers are prone to fracture. With a diameter of filaments of more than 50 µm, the needle-punching step becomes difficult because the needles used are easily broken.

0017The number of each of layers of the stainless steel fibers and the carbon fibers is not limited and variable as required. For example, the composite material according to the invention may have a two-layer structure of a single carbon fiber layer and a single stainless steel fiber layer, or a three-layer structure of two stainless steel layers and a carbon fiber layer interposed therebetween, or a multi-layer structure wherein at least two carbon fiber layers and at least two stainless steel layers are alternately laminated. The composite material of the invention may include an intermediate layer or layers between the carbon fiber layer and the stainless steel fiber layer in order to improve the tensile strength of the composite. The intermediate layer may be glass fiber cloth, stainless steel fiber cloth, etc.

0018It is essential for the composite material of the invention to have a stainless steel fiber layer on at least one exposed surface of the composite. The upper limit of the temperature at which the carbon fibers may be used in air is about 400 to about 450°C whereas stainless steel fibers are useful at temperatures up to about 600°C. Thus, the composite material of the invention can be in contact with hot articles or interrupt or shut off hot air with the stainless steel fiber layer exposed to the hot article or hot air. The composite material with the surface stainless steel fiber layer is also useful in cases where it is used at temperatures below 400°C but the carbon fiber layer has to be mechanically reinforced.

0019The stainless steel fiber layer and the carbon fiber layer are integrated into a composite through the interlocking of filaments of both layers. If the two layers are bonded with an inorganic adhesive which is stable at high temperatures over 400°C, the resultant product is low in flexibility, and the product application will therefore be limited. When an organic adhesive is used, the bonded product cannot be used at elevated temperatures.

0020Various methods for integrating the stainless steel fiber layer and the carbon fiber layer by interlocking filaments of both layers may be used. Preferably, both layers are combined by needle punching. The integrated product obtained by needle punching shows high resistance to sepa ration of layers due to intimate interlocking of filaments of both fiber layers and good springiness or cushioning properties. The composite material of the invention may also be produced by interlocking the filaments of both fiber layers with use of a water jet loom.

0021The composite material of the invention is not limited in thickness. Products with about 3 to about 50 mm in thickness can easily be prepared.

0022The composite non-woven fiber material of the invention has a wide variety of uses which include: heat resisting cushioning materials useful, for example, for supporting and relieving mechanical and/or thermal shock to glass products such as cathode ray tubes processed at high temperatures, conveyor belts for carrying heated articles, sealing material for shutting off extremely hot gases, filter materials for use at high temperatures, etc.

0023The total heat conductivity of the composite can be varied when required by changing the relative thicknesses of the stainless steel fiber layer and the carbon fiber layer.

0024The stainless steel fiber layer formed on at least one exposed surface of the composite material of the invention functions as a heat insulator and can protect the carbon fiber layer integrated therewith against oxidation due to a hot atmosphere over 400°C. Thus the composite material of the invention can, at high temperatures, replace expensive non-woven fabrics made of stainless steel fibers alone. Even at lower temperatures, the composite material of the invention economically replaces non-woven fabrics of stainless steel fibers alone which are used because of their excellent chemical resistance, flexibility, springiness, bending property, abrasion resistance, etc.

0025In addition, when the carbon fiber layer comes into contact with a moving article or element, it ensures a high abrasion resistance because of its good lubricating property.

0026In sum, the composite material of the invention shows an excellent durability under differing conditions resulting from the various improved properties mentioned above.

0027Given below are examples to illustrate the invention with reference to the accompanying drawings. <dl id="dl0001"><dt>Fig. 1</dt><dd>shows a cross section of the composite non-woven fabric material according to the invention.</dd><dt>Fig. 2</dt><dd>shows the use of the composite material of the invention as cushioning element.</dd><dt>Fig. 3</dt><dd>shows the use of the composite material of the invention as sealing element.</dd></dl>

Example 1

0028Fig. 1 is a cross section of a composite material of the invention.

0029The composite material 1 was prepared by placing on both sides of a carbon fiber layer 2 two stainless steel fiber layers 3, 3′ and then needle-punching (143 punches/100mm², 925 punches/inch²) the layers together to form the integrated composite material.

0030The carbon fiber layer 2 was formed by a non-woven fabric prepared from pitch carbon fibers (diameter 13 µm; density 1.65 g/cm³; tensile strength 70 kg/mm²; elastic modulus 3.5 · 10³ kg/mm²) using a carding machine. The mass per unit area was 260 g/m².

0031The stainless steel fiber layer 3 was a commercial web formed from stainless steel fibers (diameter 12 µm; density 7.9 g/cm³; elastic modulus 2 · 10⁴ kg/mm²). The mass per unit area was 500 g/m².

0032The characteristics of the composite material 1 were as follows: <tables id="tabl0001" num="0001"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">mass per unit area:</entry><entry namest="col2" nameend="col2" align="left">900 g/m²</entry></row><row><entry namest="col1" nameend="col1" align="left">thickness:</entry><entry namest="col2" nameend="col2" align="left">5 - 5.5 mm</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">bulk density:</entry><entry namest="col2" nameend="col2" align="left">0.18 g/cm³.</entry></row></tbody></tgroup></table></tables>

Comparison Example 1

0033A non-woven fabric was prepared by needle punching (119 punches/100mm², 766 punches/inch²) a fiber layer formed of stainless steel fibers alone.

0034The characteristics of the non-woven fabric obtained were as follows: <tables id="tabl0002" num="0002"><table frame="all"><tgroup cols="2" colsep="1" rowsep="0"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">mass per unit area:</entry><entry namest="col2" nameend="col2" align="left">1500 g/m²</entry></row><row><entry namest="col1" nameend="col1" align="left">thickness:</entry><entry namest="col2" nameend="col2" align="left">5 - 5.5 mm</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">bulk density:</entry><entry namest="col2" nameend="col2" align="left">0.3 g/cm³.</entry></row></tbody></tgroup></table></tables>

Evaluation of properties

(1) Flexibility

0035The products obtained in Example 1 and in Comparative Example 1 were tested for bending resistance in accordance with JIS (Japanese Industrial Standard)-L-1096 6-20-1.

0036The bending resistance of the material of Example 1 was 6220 mg while the value for the material of Comparative Example 1 was found to be 6800 mg.

0037Thus, it was confirmed that the product of Example 1 was substantially similar to the product of Comparison Example 1 in flexibility.

(2) Heat conductivity

0038The products of Example 1 and of Comparative Example 1 were placed on a plate kept at 300°C and were checked for the rise of temperature at the surface of the product after 3 minutes.

0039The temperature of the product of Example 1 was 190°C whereas the temperature of the product of Comparative Example 1 was 195°C.

0040It was confirmed from the results of these tests that both products were substantially the same in heat conductivity.

(3) Cushioning property at elevated temperature

0041The product 1 of Example 1 was used for supporting cathode ray tubes 4 at 600°C in a step of glass processing as shown in Fig. 2.

0042After 6 weeks, it was found that the product of Example 1 showed little change in thickness and shape and had similar properties as the product of Comparative Example 1 in heat resistance and was capable of effectively protecting fragile cathode ray tubes from mechanical and thermal shocks.

(4) Abrasion resistance

0043The products obtained in Example 1 and in Comparative Example 1 were tested for abrasion resistance in accordance with JIS-L-1021-612 using an abrasion ring of H38 and a load of 500 g.

0044The results after 1000 rotations were 705 for the product of Example 1 and 980 for that of Comparative Example 1. These values correspond to the amount of mass loss multiplied by 1000.

0045The results indicate that the product of Example 1 has substantially the same abrasion resistance as the product of Comparative Example 1.

0046The smaller mass loss of the product of Example 1 than that of Comparative Example 1 may be attributed to the higher number of punches/unit area employed in Example 1.

(5) Durability

0047The composite material obtained in Example 1 was used for sealing elements 7, 7′ of an annealing furnace 6 kept at 900°C into which a sheet of stainless steel 5 was continuously introduced at a speed of 200 m/min as shown in Fig. 3. Sealing elements of the non-woven fabric obtained in Comparative Example 1 were also used under the same conditions as the product of Example 1.

0048Each sealing element 7 or 7′ was 1700 mm x 300 mm in size and slightly pressed against the stainless steel sheet 5 to hermetically close the furnace.

0049The sealing elements 7, 7′ prepared from the product of Comparative Example 1 had to be replaced already after 2 months because filaments of the stainless steel fiber were gradually broken and lost due to high abrasion thereof in contact with the stainless steel sheet 5.

0050In contrast, the sealing elements 7, 7′ prepared from the product of Example 1 could be used for 4 months without any difficulty. The remarkable result may be attributed to the fact that the product of Example 1 has the carbon fiber layer 2 exposed at the end portion which exhibits a good lubricating property against the stainless steel sheet 5.