NO157943B - PROCEDURE FOR THE PREPARATION OF AN EXPANDABLE, NON-WOVEN SINGLE OR MULTILAYER SHEETS EXISTING MENTALLY OF A MIXTURE OF ARAMIDE FIBRIDES AND SHORT ARAMIDE FIBERS. - Google Patents

Abstract

A low density abrasion-resistant non woven aramid sheet made by expanding a dried, wet-laid sheet of fibrids and fibres, which has self-bonded regions made with heat and pressure. The re-wet sheet is heated dielectrically to expand the interior of the non densified portions without substantially roughening or disrupting their surface skin. The sheet may be used in upholstered furniture, especially in aircraft seat cushions and similar applications.

Description

This invention relates to a method for producing an expandable, non-woven single-layer or multi-layer sheet consisting essentially of a mixture of aramid fibrids and short aramid fibers, which sheets have a low density and have a smooth, less porous, wear-resistant surface.

Non-woven sheet materials with low density (less than 0.16 g/ml) consisting of aramid fibers and fibrids, which are known from European patent application no. 73 668, are useful, among other things, for heat insulation and acoustic insulation. These low-density materials are produced from wet-deposited sheets of a fibrid-fiber mixture which, without being dried, are expanded by rapid heating to form a continuous low-density sheet with several paper-like layers of membrane-shaped elements forming expanded macroscopic whip cells practically through the entire thickness of the sheet. Although the tensile strength and the surface integrity and structure of these known sheets are sufficient for many applications, other applications require less porous sheets with better strength properties and surface wear resistance than those which have been produced by this wet deposition process where no drying is carried out. Although drying these known wet-deposited sheets, e.g.

by passing them over smooth heated drums or rollers,

provides denser sheets with a smooth surface and can result in a stronger and tougher sheet material, previous attempts to expand such dried sheets to the much lower densities required for certain applications have not given satisfactory results.

It is the aim of the invention to provide a method by which such wet-laid, non-woven sheets of aramid fibrids and fibers are expanded after they have first been dried.

It has thus now been shown that, under suitably adapted conditions, wet-laid paper-like, non-woven sheets of aramid fibride/fibre mixtures are allowed to dry, rewet and expand into new sheet structures with low density,

which exhibits uniformly expanded portions with a smooth, dense,

skin-like outer surface, which expanded portions, if desired, can have an internal structure varying from a sponge-like structure to an open and balloon-like (air-filled) structure.

According to the invention, a method is thus provided for the production of an expandable non-woven single-layer or multi-layer sheet consisting essentially of a tangled mixture of 30-90% by weight aramid fibrids and complementary 70-10% by weight short aramid fibres, which single-layer or multilayer sheets have expanded portions delimited by self-bonded, densified areas and consisting of a chamber formed by two opposing, dense, smooth, skin-like surface layers of said fibrids and fibers, which two surface layers surround a much less dense interior, by which method one or more smooth, wet-deposited sheets of the aforementioned aramid materials and a pattern of non-expandable densified areas is impressed into such a sheet or an assembly of such sheets using heat and pressure, so that the fibers/fibrids in the sheet or sheet assembly are bonded together by self-bonding through the entire thickness in the densified areas and expandable parts are delimited between these, and the patterned single-layer or multi-layer sheet is heated dielectrically in a water-moistened state, so that the water quickly evaporates and expanded parts are formed in the single-layer or multi-layer sheet. The new method is characterized by the fact that the wet-deposited sheet or sheets - before the pattern is impressed into the sheet or during the assembly of sheets - are dried by heating to remove practically all water and to form sheets with a dry, smooth surface, that the pattern is impressed into the dry sheet, respectively in an assembly of such dry sheets, that the resulting pattern-embossed single-layer or multi-layer sheet is stretch-bent with subsequent saturation with water, and that the stretch-bent and wet single-layer or multi-layer sheet is subjected to said dielectric heating.

When the sheet material for expansion consists of only one layer of the dried sheet, the expansion proceeds more easily if the thickness of the dried sheet is greater than 0.381, preferably greater than 0.508 mm.

The sheet obtained by the method has sufficient

sufficiently small porosity to give a time in the drip porosity test of at least 10 seconds. The expanded, inflated portions are preferably bounded by densified areas arranged in a segmented, linear pattern. Segmentation of such densified linear areas has been shown to improve both the uniformity of and under certain conditions also the degree of expansion in the expanded portions of the sheet material.

The sheet has significantly improved wear resistance compared to the previously known sheet materials produced by a wet-deposition-expansion process without an added drying step. The improved sheet may have an abrasion resistance, as measured by Taber's abrasion test, of at least 1000 cycles, preferably at least 2000 cycles, before failure occurs. It has sufficient flexibility and fire resistance for use in fire-stopping sheets in upholstered furniture and similar applications, where a thin, flexible, light and fire-resistant material is required.

The term "short fibres" means, as in the art, fibers of short length which are commonly used in the production of such wet-deposited, paper-like sheets. Fiber lengths suitable for this application are generally less than 2.5 cm and most preferably less than 0.68 cm. Suitable linear densities for the fibers are from 0.55 to 11.1 dtex, preferably from 1.0 to 3.5 dtex. In order to achieve maximum strength and shrinkage resistance, it is preferred that the short fibers have been cut from highly stretched and heat stabilized filaments. The "fibrids" used here are the fully synthetic, small, non-granular, flexible, fibrous or film-like particles which are known in the art, and which are described e.g. in the above-mentioned European patent application and in US Patent No. 2,999,788.

Although the fibrids and fibers may be of any aramid polymer, it is preferred that the fibrids and at least some of the short fibers consist of poly-(m-phenylene-isophthalamide), i.e. MPD-I. In order to achieve better fire resistance, and especially to achieve protection against "open breakage" when the material is exposed to flames, some of the short fibers preferably consist of poly-(p-phenylene-terephthalamide), i.e. PPD-T.

A good balance between durability and fire protection is achieved with an approximately equal mixture of fibers of PPD-T and MPD-I, e.g. a mixture of 60% fibrides with 20% of each fiber type. To achieve improved properties and compatibility in the manufacture of sheet materials, the PPD-T fibers can be transferred to a pulp to increase their fibrillar character, as is known in the art.

It is appropriate to dry the wet-deposited sheets without calendering, while stretching over heated drums or rollers with a smooth surface, as is known in the papermaking art.

To avoid expansion of the sheet material in the densified areas, they must be formed using both heat and high pressure, and preferably using ultrasonic devices which serve as their own heat source through ultrasonic vibration of the sheet material.

Because of the increased difficulty in expanding such sheets after they have once been dried, it is preferred that the water contain a dielectric coupling agent for rapid heating.

To more effectively and easily saturate the layered sheet material with water before expansion, it is also preferred that the sheet is mechanically processed or stretch-bent dry or in the presence of water to facilitate the uptake and penetration of water into the interior of the sheet. This can be achieved e.g. in that the sheet is guided in a sinusoidal path over a series of 90 degree edges, e.g. edges with a radius of less than 1.6 mm, in a water bath. If the dry sheet is subjected to tensile bending, the sheet must then be soaked. Such tensile bending will not only be able to reduce the time required for the penetration of water into the sheet, but also increase water absorption and provide a more cellular internal structure in the expanded parts.

A very surprising aspect of the invention is the possibility of expanding parts of the sheets after they have once been dried, without causing any significant breakage of the sheet's surface. Thus, sheets can be produced with a much smoother, less porous surface than the sheets that were previously produced by expansion of wet-deposited, non-dried sheets.

A further surprising aspect is the possibility of regulating the nature of the interior of the expanded parts as mentioned above. The nature of these inner parts depends on a number of factors, including the field between the densified areas (the larger the field, the more open and less sponge-like the interior becomes), tensile bending of the dried sheet before expansion, which tends to give a more cellular, sponge-like interior, the dielectric coupling ability of the water in the sheet (which increases, for example, in the presence of a surfactant or a dissolved ionized salt), the strength of the electric field during the dielectric expansion, the speed with which the sheet is passed through the heating zone (residence time), the thickness of the sheet that is expanded, and the number of layers of individual sheets that are used to build up the sheet that is produced. Other possibilities include interposing a non-dried, non-woven sheet between two dried sheets prior to the formation of the densified areas, which, when expanded, can provide a filled, cellular internal structure with a dense outer skin.

Of course, the tensile strength of the resulting expanded sheet will depend, among other things, on the thickness or basis weight of the sheet being expanded. For example, sheets with a base weight of approx. 200 g/m 2 and a thickness of approx. 0.58 mm in typical cases have a tensile strength of at least 11.53 kg/cm. The best tensile strength and wear resistance can be obtained with sheets that have been thermoset at a temperature sufficiently high to crystallize the polymer materials in the sheet.

Wetting the sheets with water from the tap before the expansion is carried out, gives a water absorption of at least 75% by weight, calculated on the dry sheet. However, a water absorption of approx. 140%. Water containing the electrical coupling agents can reduce

the percentage uptake that is necessary for good expansion. In typical cases, a sheet with a basis weight of approx. 200 g/m<2 >soak in tap water for approx. 50 seconds, so that approx. 140% water and good expansion is achieved. If, however, it is desired to reduce water absorption, water containing

up to 5% by weight of the electrical coupling agent is sprayed onto the surface of the sheet until a water content of as little as 50% is achieved, whereby good expansion will nevertheless be achieved.

It is also to be noted that tensile bending as above

mentioned can reduce the time it takes for the water to penetrate the sheet, and reduce the percentage of water content required for good expansion. Furthermore, stretch bending can be used to improve the expansion of sheets with a low water concentration.

Adequate patterning of the sheet with the densified areas allows for control and uniformity of the expansion along and across the sheet during the expansion process. The distance between and the distribution of the areas can be varied to achieve the desired degree of expansion.

It will be clear that the invention enables a wide variety of stylistic designs, depending on such factors as the pattern of the densified areas and the nature of the sheet to be expanded. When they do not have an adverse effect on the properties of the sheet or on the desired application, other materials such as mica can also be incorporated into the sheet.

The formation of the densified areas can be carried out with the help of any suitable, heated embossing rollers, plates etc., but an ultrasonic embossing or binding device is preferred. The block in an ultrasonic device can be designed with suitably raised parts which give the desired pattern when the sheet is fed through the device. Ultrasonic bonding devices can easily and with uniform results provide bonding conditions comparable to pressure/temperature conditions of 281 kg/cm 2 and 275°C, which have been shown to be effective. Suitable patterns include diamond patterns, square patterns, rectangular patterns, circular patterns and patterns of other geometric shapes. When patterns with small individual expanded portions are used, ultrasonic bonding appears to facilitate the expansion of the small portions. Preferably, the densified areas constitute only a fraction, e.g. 20% or less, of the total surface area of the sheet.

The improved toughness and integrity of the surface of the sheets produced according to the invention is evident from their resistance to loss of material when an adhesive tape is applied to the surface and peeled off. This can be measured quantitatively using the adhesive tape test described here, in which sheets produced according to the invention give a loss of material that is less than 4 mm/cm o. In this test, the densified areas preferably give essentially no loss of material . Generally, in the same way as for wear resistance, it will be the case that the smaller the surface area of each expanded part, the better the performance will be in the test. Consequently, preferred sheet materials produced according to the invention have separate expanded portions which each occupy a surface area in the range from 0.1 to 25 cm 2 .

In the sheets containing mixtures of short fibers of

MPD-I and PPD-T increase the fire resistance of the sheet with an increasing amount of PPD-T fibers, while the wear resistance tends to decrease.

Preferred sheets produced according to the invention have expanded portions with a mainly smooth, two-dimensional surface (practically free of loose filaments and visible surface irregularities; somewhat similar to letter paper) and may even have a somewhat glazed or glossy appearance, which distinguishes clearly stand out from the rather irregular textured, furry and more porous surfaces of sheets produced according to the previously known process where no drying step is used.

The sheets produced according to the invention, especially preferred sheets containing a mixture of fibers of poly-(m-phenylene-isophthalamide) and poly-r(p-phenylene-terephthalamide), have sufficiently increased strength and wear resistance compared to non-dry ready expanded sheets to have a significantly improved lifetime in use when they are used as a fire-stopping layer in aircraft, e.g. as carpet underlay and especially in airplane seat cushions.

Generally, it will be the case that the larger the inflated part's surface area, the more open the central parts of the interior are. The wear resistance and uniformity from batch to batch tend to deteriorate with increasing area and especially when inflated batches have a surface area on each side of the sheet of more than 25.8 cm <2>.

Other uses for the sheets produced according to the invention include insulation against fire, heat and sound and insulating fur in protective clothing. Other applications will be self-explanatory based on the physical and chemical properties of these light sheets.

Known ultrasonic bonding devices can be used to provide almost any desired densified pattern, with rectilinear geometric patterns such as diamond patterns or square patterns being preferred because they are simple and effective. Particularly preferred are such patterns which are formed by two groups of essentially parallel lines with a distance between the lines of from 1 cm to 2.5 cm in each group. Ultrasonic bonding to form the pattern on the dry sheets enables not only great diversity with regard to the design of the pattern but also more effective self-bonding which prevents blow-off or delamination of the densified areas under the conditions required by the expansion process.

The densified areas should be at least 0.5 mm wide and

1 mm long and should be separated from each other by spaced parts of approximately the same length in the longitudinal direction. Also round, dense parts of approx. 1 mm can be used. Such division or segmentation improves the control of the expansion from batch to batch along and across the expanded sheet.

With the help of the invention, expanded aramid sheets are produced which can have an abrasion resistance which is from 3 to 10 times greater than the abrasion resistance of the comparable sheets which are produced by the known method where no drying step is used.

Another advantage of the method according to the invention compared to the known method where no drying step is used, is the improved productivity achieved by carrying out the expansion with less water (e.g. up to 5 times less than the amount of water used in the wet sheet process) . The best results require the use of a dielectric coupling agent such as the ionic surfactant Woolite®, the surfactant cetyl betaine, or an ionic salt such as sodium sulfate.

The thermal insulation properties can be improved by tensile bending of the samples before or during the wetting process to facilitate a better development of the internal cellular structure, although with a certain loss of tensile strength.

The dried sheets for use in the method according to the invention for producing the improved sheets can be produced using the known papermaking apparatus and techniques described e.g. in US Patent No. 3,756,908 and in European Patent No. 73,668.

Test methods

The drip porosity test

This test measures the time it takes for a given solution of sodium chloride in water to penetrate the expanded sheet material. The time this takes is a measure of the product's surface density and porosity. A product with a denser, less porous surface will resist penetration and retain the solution for a longer time.

A 0.95 liter jar with a wide opening ("Mason" pickle jar), 12.4 cm high and with an opening of diameter 6.4 cm is used for this test. An air hole of diameter approx. 0.16 cm is drilled in the bottom of the jar. The jar is supplied with a conventional, ring-shaped screw lid (a ring) with a central opening of 6.4 cm. When the test begins, the vent is plugged and the jar is filled with 600 ml of salt solution (0.9 wt% NaCl). A round test piece of the expanded material to be tested is cut out so that it fits exactly in the annular screw cap and completely closes the central opening. Annular seals, e.g. of rubber, which fits the ring-shaped lid and has a central opening of the same dimensions as the lid, is placed above and below the round test piece, so that a watertight seal is achieved. The test piece and the seals are placed in the lid,

and the lid is screwed tightly onto the jar, so that the upper side of the jar is completely closed and the test piece covers the central opening in the lid. With the air hole blocked, the jar is then turned over and placed on a glass plate that is mounted approx. 20.3 cm above a mirror. A stopwatch is started at the same moment that the plug is removed from the vent. The test piece is observed in the mirror with regard to penetration of the solution into the test piece. Penetration is easily observed when the solution penetrates the test piece and wets the glass plate. Occasionally, some condensation (a light "fog") will be observed on the surface of the glass.

However, this condensation is not considered as penetration of the solution into the test piece. The time that elapses between the opening of. the air hole and the time when the glass plate is moistened are noted. If the solution penetrates the test piece as soon as the jar is turned, the time is recorded as zero seconds. The elapsed time for three randomly selected samples of each material taken for testing is noted, and the average of the three times is noted as the result for the material.

The adhesive tape pull-off test

The delamination weight in the adhesive tape pull-off test is

a measure of the amount of material that adheres to an adhesive tape after it has been applied, pressed and then removed from the surface of the fully dried expanded product. The amount of material that adheres to the adhesive tape is a direct measure of the integrity and toughness of the surface. A tough and strong structure will emit smaller amounts of material that adhere to the tape than a softer, less dense structure, which emits larger amounts that adhere to the adhesive tape.

For the purpose of the test, one side of the expanded material to be tested is designated as side A, while the other side is designated as side B. On side A, a line designated as the MD line is drawn in the machine direction, if the machine direction is known or can be demonstrated . If it cannot be detected, the line is drawn in an arbitrarily chosen direction. By machine direction is meant the production direction in a commercial papermaking machine. Sides A and B are different as a result of the fiber deposition, as the underside, which faces the forming wire, is different from the exposed side. A line designated as the TD line (in the transverse direction) is drawn perpendicular to the MD line on the A side. Eight test strips of width 2.5cm and length 15.2cm are cut from the expanded material, namely one set of four strips parallel to the MD line and another set of four strips parallel to the TD line, testing as much as possible avoiding including the raised fields in the sample strip and using the same cutout pattern for the four strips cut in each direction so that all strips cut in a given direction are similar.

The adhesive tape used in the test is an essentially transparent, 2.5 cm wide adhesive tape with adhesive on one side only (Scotch©810 Magic Transparent Tape). In the tear-off test according to ASTM test D-3330-76 82°C Peel Adhesion test, the adhesive tape gives a value of 279 g/cm for adhesion to steel. The adhesive tape is applied to each test strip so that it covers the test strip evenly across its entire width, from one end to approx. 0.6cm from the other end, and the tape is folded back over itself with the adhesive flaps towards each other, so that it has an approx. 0.6cm long hem of double thickness near the end of the strip that the adhesive tape does not reach all the way to. The adhesive tape is applied to the A side of two of the four test strips

in each of the MD and TD sets, with the hemp arranged at each end of the two test strips, and on the B side of two of the test strips in each set, also in this case with the hemp arranged at each end for these in the case of test strips. The test strips, with the adhesive tape applied, are then pressed between plates at 11.5 MPa.

The hemp end of a test strip (the end that is not completely covered with adhesive tape is then firmly fixed across its width in the lower jaw of an apparatus for determining the tensile strength ("Instron" Model 1130 with a 500 gram load cell), while the hemp end of the adhesive tape is secured in a clamp attached to the upper jaw of said apparatus. The apparatus for determining the tensile strength is then started and the jaws are moved apart at a rate of 12 inches per minute. When the adhesive tape has been pulled completely off the test strip , the device is stopped.

From each strip from which the adhesive tape has been torn, an exact 12.7cm long piece is cut out, which is then weighed on a scale to the nearest 0.01g. The average weight of a clean 12.7 cm strip is then determined and subtracted from the weight of the 12.7 cm long strips from which adhesive tape has been peeled to determine the weight of adherent surface material removed from each test strip.

The eight test strips give eight measurements per sample sheet,

and the mean value for the eight results is given in mg per cm.

Tabers are worn j€ >trial

This test is carried out in accordance with ASTM Test Method D-1175-64T page 283 (Rotary Platform, Double Head Method) using grinding wheels with grain size CS-10 at a load of 50()g per wheel. Failure is considered to have taken place when a hole of any size can be observed across the sheet. The results are given in the number of cycles before failure occurs. Preferred materials of the invention withstand 1,000 or more cycles before failure, although for some applications materials that are less wear resistant will be satisfactory.

Setes1itas j eprove

(Boeing's "Squirmin Herman" seat wear test).

This test is carried out by producing conventional aircraft seat cushions with an inner polyurethane foam material surrounded by an inner cover of the expanded sheet material to be tested, and an outer cover of a conventional seat cushion textile material, e.g. a seat cover material of wool/nylon (90/10) with a basis weight of ° 441 g/m 2. The expanded sheet material is sewn to the inside of the outer seat cover, and the seat cover is designed so that it can be easily removed for inspection of the expanded sheet material flex, by that a zip is sewn on to enable the seat cushion to be opened.

The seat cushion is then fixed in the apparatus for testing seat wear shown in fig. 2 of the article of "Textiles is Ready When You Are" by Sally A. Hasselbrack in Textile World, May, 1982, page 100. The seat wear test apparatus includes a seat weight made of soft rubber, weighing 64 kg, and which is designed as a human's seat area and lined with a trouser-like. The covers are made from a 2-post knitted tricot fabric of 100% polyester. During a 2 minute cycle, the seat tester is in contact with the seat cushion for 1 minute and 40 seconds, while it is lifted off the cushion for 20 seconds. While in contact with the cushion, the seat tester is rocked through a 25 degree arc at 13.5 cycles per second. minute, while turning the pad through a 35 degree arc at a rate of 18 cycles per minute. minute. The test is stopped and the seat cushion fabric with the sewn-on inner cover is removed at certain intervals for inspection of the cover. The inner cover is considered to have failed when a hole of any size can be observed across the inner cover. If the inner cover is intact after 50 hours of testing, the expanded sheet material is considered to have passed the test.

Example 1

This example illustrates the production according to the invention of expanded sheets and the production of fireproof airplane seat cushions using the expanded sheets.

The aramid papers for making these expanded

sheets were all produced conventionally in a commercial Fourdrinier papermaking machine. Fibrids of poly (m-fe-nylene-isophthalamide) (MPD-I) in an amount of approx. 0.5% by weight in tap water was added to one inlet of a mixing T-tube. A 50/50 slurry of 0.64 cm long 2.2 decitex (2-denier) MPD-I short fibers and a poly(p-phe-nylene-terephthalamide (PPD-T) short fiber pulp of length 4 mm (mean length of 0 .5-8mm) with a degree of freedom of 450-475 (Canadian Standard Freeness) in an amount of about 0.35% by weight in water from the line was added to the second inlet of the mixing tee. , short fibers and pulp were 60/20/20. The off-run was fed to the headbox and then to the forming wire. The resulting sheet was fed over steam-heated drying drums maintained at a surface temperature of 140°C

for 2 minutes and wound up as a completely dried sheet on a cylindrical paper roll. The procedure was carried out at a setting of the papermaking machine which was intended to produce 0.58 mm thick dried sheets with a basis weight of approx. 200g per m.

The dried sheets were then embossed ultrasonically by the sheets being unwound from the cardboard rolls and each sheet taken to an ultrasonic embossing station where each sheet was embossed between an ultrasonic horn and a block. The horn used (manufactured by Branson Co., Eagle Road, Danbury, Conn) had an effective surface of length 15.2 cm and width 1.3 cm. The horn was pressed against a 15.2 cm long pattern, rotating block (drum) with the sheet placed between them. The drum rotated at a surface speed of 3.05-3.96 m/min. and had a diameter of 7.6 cm, with perimeter lines consisting of rectangular, projecting, 1.9 cm long and 0.64 mm wide edges spaced 1.9 mm apart and lying in a plane perpendicular to the axis of the drum. The horn vibrated with a frequency of 20,000 cycles per second. second and

an air pressure set at 1.41-2.11 kg/cm 2 was used to achieve a pressure between the drum and the horn. In this case, two different drums were used, one with projecting edges lying in a plane arranged at a distance of 1.3cm from each other and the other with projecting edges lying in a plane arranged at a distance of 2.5cm from each other. Two sheets were embossed separately on each drum, being passed between the horn and the drum a sufficient number of times to cover the sheet in its entire width (each pass parallel to the previous pass and with an adjusted distance between them) in one direction and then sufficiently many times in the transverse direction to produce two pairs of sheets, where each pair of sheets has a square pattern with a square of 1.3 cm and 2.5 cm respectively.

The four embossed sheets were then moistened by passing them through a container of tap water to which was added a weight % ionic surfactant ("Woolite"). The sheets were passed through the container at a speed of 61 cm per minute. minute, the contact time being 2 3 seconds. The wetted sheets containing at least 120% water were then dielectrically expanded by passing them from the container through a 20 KW dielectric heater operated at 27 MHz. The sheets were passed between a single set of 122'cm electrodes spaced 5-8'cm apart.

The sheets have separate expanded portions defined by the densified divided grid pattern, each expanded portion consisting of a chamber formed by two opposing, smooth, dense skin-like surface layers surrounding an interior where the material density increases outwards from a less dense central area re-

nom a denser, cellular sponge-like or laminar area to the two opposing dense/skin-like surface layers. The sheets with the wider pattern have expanded sections with a more open interior.

Two of the dielectrically expanded sheets with different sizes of the raised pattern were then heat treated, while the other

two were not heat treated. The heat treatment was carried out on

a frame (Bruckner frame) at a minimum of tension and at 260°C, for 3 minutes. The four resulting sheets are designated as follows: Sample A - raised squares with side edge 1.3 cm; not heat treated.

Test piece B - raised squares with side edge 1.3 cm; heat treated.

Test piece C - raised squares with side edge 2.5 cm; not heat treated.

Test piece D - raised squares with side edge 2.5 cm; heat treated.

The four sheets produced as described above were then sewn as a front to the inside of a seat cover fabric of wool/nylon (90/10) with a basis weight of 441 g/m 2. The lined fabrics were then used for production of conventional airline seat cushions, with the raised pattern expanded sheets as an inner cover around the polyurethane foam material from which the seat cushions were made. When seat cushions made using the four sample sheets were tested with the seat wear test, cushions made using the test pieces were found to pass the test (no breakage of the protective expanded sheet after 50 hours of testing). Specimen B was tested even longer and still passed the test after 100 hours. Seat cushions manufactured using each of the test pieces also passed Boeing's heat release test, the OSU Heat Release Test, (no involvement of the polyurethane foam in the flame for at least 30 seconds) at watts cm 2. Other characteristics for the four expanded test pieces are listed in the table:

Three comparable expanded sheets, which were not made in accordance with the invention, but had the same 60/20/20 composition (except that 2 mm long (average for lengths from 0.5 to 4 mm) PPD had been used -T fibers instead of 4 mm long fibers and that they had a CSF (Canadian Standard Freeness) of 300-425), were prepared in substantially the same manner, except that the expansion was done with non-dried sheets and that a raised diamond pattern (4.45 cm x 1.91 cm and 2.5 mm wide, continuous, condensed lines) was mechanically embossed at room temperature in the wet sheet. The expanded sheets had a very coarse-textured, three-dimensional surface that was visible to the naked eye. A non-heat treated expanded sheet gave instant wetting (zero seconds) in the drip porosity test and 500 cycles before failure in the Taber's abrasion test. Two heat-treated, expanded sheets (30 seconds and 3 minutes at 260°C) gave respectively 1 second and 650 cycles and 0 seconds and 700 cycles in the same tests, which showed that they were all markedly worse in these tests than the sheets produced according to the invention . The results of the adhesive tape pull-off test for the three test pieces were respectively 5.79, 5.79 and 6.3 mg/cm 2. Tested as an inner cover in the conventional seat wear test, the first test piece failed, while the second passed the test marginally and the third passed the test. Although the third test piece passed the test, its bulkiness and drapeability were somewhat degraded as a result of heat curing.

Example 2

This example illustrates the production according to the invention of sheets from two separate sheets of aramid paper which are bound together and then expanded.

The aramid papers for making these expanded products were all produced in a conventional manner using a commercial Fourdrinier papermaking machine from approx. 55% MPD-I fibrids and approx. 45% MPD-I short fibers of 2.2 decitex (2 denier) with a cut length of 0.64 cm. After the wet sheets were formed in the machine, they were passed over a series of dryers maintained at temperatures ranging from 85 to 115°C for lower basis weight papers and at temperatures ranging from 110 to 140°C for papers with higher basis weight, as contact times that were suitable for drying the papers were used. The papers were not subjected to any subsequent calendering.

In one embodiment, two fully dried 0.3 m

wide and 0.6 m long sheets of aramid paper, each with a basic

weight of 4 0.7 g/m<2> and of thickness 0.10 mm, brought together by the ultrasonic embossing station described in example 1. The sheets were superimposed and were ultrasonically embossed and bound together in the densified areas with a segmented square

ter with side edge 1.3 cm.

The raised pattern sheets were then wetted by passing them through a container of tap water to which 2.5% by weight of ionic surfactant ("Woolite") had been added. The wetted sheets were then expanded dielectrically. The resulting material retained good bond integrity in the raised, densified regions that bounded expanded portions with open, balloon-like chambers formed by two dense, smooth, tough, and strong skins. The operating conditions used for the wetting and the dielectric expansion were as described in example 1, except that the residence time and field strength were increased.

In another embodiment, two completely dried sheets of aramid paper of different thicknesses were ultrasonically bonded together. One sheet was 0.25 mm thick and had a basis weight of 81.4 g/m<2>.

The second sheet was 0.38 mm thick and had a basis weight of 12 9.9 g/m 2 . The two sheets were ultrasonically embossed, bonded, wetted and expanded dielectric as described above. The resulting material was similar to the material prepared above, but some development of an internal cellular or laminar structure in the expanded paria on the inner surface of the skin layers was observed for this thicker sheet.

The resulting materials from the two designs denote

nes as follows:

Specimen II-A: 0.10 mm sheet bonded to 0.10 mm sheet. Specimen II-B: 0.25 mm sheet bonded to 0.38 mm sheet.

The properties of the expanded sheets are:

Example 3

A 0.584 mm sheet of practically the same MPD-I composition as in Example 2 was ultrasonically embossed in the square pattern (12.7 mm and 25.4 mm). The sheet was then "stretched" by pulling it over a 90° edge of a brass block held in the hand while immersed in a liquid consisting of 2.5% "Woolite" in tap water. After several such stretches (8 times), twice each way in the machine direction for each side of the sheet, the sheet was kept in the liquid for 1 minute.

The sheet was then dielectrically heated in an 85 MHz RF heater with a single set of electrodes spaced 76.2 mm apart at a belt speed of 0.914 m/min.

The resulting material expanded readily to form discrete, uniform expanded portions with dense, smooth, skin-like surface layers and a much less dense interior.

Claims (1)

Process for the production of an expandable, non-woven single-layer or multi-layer sheet consisting essentially of consisting of an entangled mixture of 30-90% by weight aramid fibrids and complementary 70-10% by weight short aramid fibres, which single-layer or multi-layer sheet has expanded sections delimited by self-bonded, densified areas and consisting of a chamber formed by two opposite, dense, smooth, skin-like surface layer of said fibrids and fibres, which two surface layers surround a much less dense interior, by which method one or more smooth, wet-deposited sheets of the said aramid materials are formed and such a sheet or assembly of such sheets is impressed with a pattern of non-expandable densified areas under the application of heat and pressure, so that the fibres/fibrids in the sheet or sheet assembly are joined by self-bonding through the entire thickness in the densified areas and expandable parts are delimited between these, and the patterned single-layer or multi-layer sheet is heated dielectrically in a water-moistened state, so that the water evaporates quickly and expanded parts are formed in the ettla gs or multilayer sheet, characterized in that: the wet-deposited sheet or sheets - before the pattern is impressed into the sheet or in the assembly of sheets - are dried by heating to remove practically all water and to form sheets with a dry, smooth surface, the pattern is impressed into the dry sheet, respectively in an assembly of such dry sheets, the resulting patterned single-layer or multi-layer sheet is stretch-bent with subsequent saturation with water, and the stretch-bent and wet single-layer or multi-layer sheet is subjected to said dielectric heating.