IE55469B1 - Improved low density nonwoven aramid sheets - 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

1 a a ‘i y This invention relates to improved low-density nonwoven sheets comprised o£ aramid fibrids and short aramid fibers, having a smooth, less porous, abrasion resistant surface and to a 5. process for making such sheets.

Low density (less than 0.16 g/mL) nonwoven sheet structures comprised of aramid fibers and fibrids, as known from European Patent Application 10. Publication 73,668, are useful in thermal and acoustical insulating applications, among other things. These low density materials are prepared from wet-laid sheets of a fibrid-fiber mixture which, without ever being dried, are expanded by rapid 15. heating to form a coherent low density sheet having a plurality of paper-like layers of membranous elements which form expanded macroscopic cells substantially throughout the thickness of the sheet. Although the tensile strength and surface integrity and 2o. configuration of such known sheets are sufficient for many uses, other uses require less porous sheets with greater strength and surface abrasion resistance, than have been obtained with the sheets made by such a wet-laid never-dried process. Whereas drying of 25. these known wet-laid sheets, such as by passing over smooth heated cans or rolls, provides denser sheets with smooth surfaces and can result in a stronger tougher sheet material previous attempts to expand such dried sheets, to the much lower densities needed 30. for some applications were unsuccessful. 2 Consequently one object of this invention is a process for expanding such fibrid-fiber wet-laid nonwoven aramid sheets after once being dried.

Another object of this invention is an improved low 5 density sheet structure comprised of aramid fiber and aramid fibrids having improved tensile strength, surface continuity and integrity and abrasion resistance. Still another object is such sheet structures having sufficient flexibility and 10 fire-resistance for use in fire-blocking sheets in upholstered furniture and similar applications where a thin, flexible, light-weight fire-resistant material iB needed.

Under properly selected conditions, wet-laid 15 paper-like nonwoven sheets of aramid fibrid/fiber mixtures can be dried, re-wet and expanded to provide novel sheet structures of low density which have uniformly expanded portions with a smooth, dense, skin-like, outer surface; which expanded portions can 20. have, as desired, interior structures ranging from ones sponge-like in nature to ones open and balloon-like (air-filled); and which sheet structures have a low porosity which resists penetration by water.

. A product of this invention is a low density, nonwoven sheet structure consisting essentially of a commingled mixture of from 30 to 90% by weight of aramid fibrids and complementally from 70 to 10% by weight of short aramid fibers, the sheet 25, having expanded portions defined by self-bonded, densified regions with expanded portions being comprised of a chamber formed by two opposed, dense, smooth, skin-like surface strata of said fibrids and fibers, which two strata enclose a much less dense 2 3 inter ior, the sheet havinq a porosity to provide a Drip Porosity Test time of at least 10 seconds. The expanded, puffed portions are preferably discretely defined by 5 densified regions arranged in a segmented, lineal pattern. Segmentation of such densified lineal regions has been found to improve both the uniformity of and under certain conditions the degree of expansion in the expanded portions of the sheet 10 structure.

The products of this invention have substantially improved abrasion resistance over products of the prior art prepared by a wet-laid never-dried expanding process. The improved products 15 can have an abrasion resistance as measured in the Taber Abrasion Test of at least 1000, and preferably at least 2000 cycles to failure.

As known in the art and as used herein the terms "short fibers" and "floe" are used 20 interchangeably in reference to fibers of short length customarily used in the preparation of such wet-laid paper-like sheets. Fiber lengths suitable for this use normally are less than 2.5 cm, and most preferably less than 0·68 cm. Suitable 25 linear densities of the fibers are from 0.55 to 11.1 dtex, and preferably in the range of 1.0 to 3.5 dtex. For a maximum strength and resistance to shrinkage it is preferred that the ehort fibers be cut from highly drawn and heat-stabilized filaments.

The "fibrids" used herein are the all synthetic, small, nongranular, flexible, fibrous or film-like particles as known in the art as taught for example in the above European patent application and as described in U.S. Patent Specification No. 2,999,788. 3 4 Although the fibrids and fibers nay be of any aramid polymer, it is preferred that the fibrids and at least some of the short fibers be comprised of poly(m-phenylene ieophthalamide), i.e, HPD-I. For 5 better fire resistance, particularly for protection against "break-open" upon exposure to flame, some of the short fibers preferably are comprised of Poly(β-phenylene terephthalamide), i.e., PPD-T. A good balance of abrasion resistance and fire 10 protection is provided with about an equal mixture by weight of fibers of PPD-T and HPD-I, such as 60% fibrids with 20% of each fiber type. For improved performance and compatibility in preparation of the sheet structures, the PPD-T fibers can be pulped to 15 increase their fibrillar character as known in the art.

The invention also concerns an improved, process for preparing an expanded nonwoven sheet comprised of aramid fibrids and short aramid fibers 20 including the steps of forming a smooth, wet-laid sheet of said aramid materials, impressing a pattern of non-expandable, densified regions into the sheet to define expandable portions between said regions, and dielectrically heating the patterned sheet while 25 wet with water to rapidly vaporize the water to create highly expanded portions in the sheet, wherein the improvement comprises: drying the wet-laid sheet by heating to remove substantially all water and provide a dry, smooth-surfaced sheet; preparing a 30 layered sheet structure for expansion comprised of at least one layer of said dried sheet by forming a pattern of non-expandable densified regions with heat and pressure to self-bond the materials together in said densified regions throughout the thickness of 35 the layered sheet structure to define expandable 4 5 portions between then; stress-flexing the dry sheet with subsequent soaking or saturating the patterned sheet structure by stress-flexing in the presence of water; and dielectrically heating the wet sheet to 5 rapidly vaporize water and expand the interior of the expandable portions of the patterned sheet substantially without disrupting their surfaces.

When the sheet structure prepared for expanding consists of only one layer of the dried sheet, 10 expansion occurs more readily if the thickness of the dried sheet is greater than 15, and preferably greater than 20, mils (508 μπ>).

The wet-laid sheets are suitably dried, without calendering, under tension over 15 smooth-surfaced heated cans or rolls as known in the paper-making art.

To avoid the expansion of the sheet structure in the densified regions they must be formed by means of both heat and high pressure, and 20 preferably by the use of ultrasonic means which operates as its 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 25 it is preferred that the water contain a dielectric coupling agent for more rapid heating.

To more effectively and readily saturate the layered sheet material with water prior to expansion, also it is preferred that the sheet be mechanically 30 worked or stress-flexed dry or in the presence of water in order to facilitate pickup and penetration of water into its interior. This can be accomplished for example by passing the sheet in a sinusoidal path over a series of 90° edges, e.g., less than 1/16 in (1.6 mm) 35 radius, in a water bath. If stress-flexing is 5 6 performed on the dry sheet, the sheet must be subsequently soaked. Such stress-flexing not only can reduce the time required for water to penetrate into the sheet, but also increase the water pickup 5 and provide a more cellular interior structure within the expanded portions.

A very Burprising aspect of this invention is the ability to expand portions of the subject sheets after once being dried without substantial 10 disruption of the sheet surface. Thus sheets can be prepared having a much smoother, less porous surface than ones prepared previously by expansion of wet-laid never-dried sheets. A further surprising aspect is the ability to control the nature of the 15 interior of the expanded portions as mentioned above. The nature of these interiors is dependent upon a variety of factors including the area between the densified regions (the larger the area the more open and less sponge-like the interior), tension 20 flexing of the dried sheet prior to expansion which tends to provide a more cellular sponge-like interior, the dielectric coupling capability of the water in the sheet (as increased for example by the presence of a surfactant or dissolved ionized salt), 25 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 being expanded, and the number of separate sheet layers used to make up the 30 prepared sheet. Other possibilities include the layering of a never-dried nonwoven sheet between two dried sheets before forming the densified regions which when expanded can then provide a filled cellular internal structure with dense outer skins. 6· 7 Of course the tensile strength of the resulting expanded sheet will depend, among other things, upon the thickness or basis weight of the sheet being expanded. For instance, sheets having a 5 basis weight of about 6 ounces per square yard (200 2 g/m ) and a thickness of about 23 mils (0.58 mm) typically can have a tensile strength of at least 10 inch-pounds (11.53 kg-cm). Best tensile strength and abrasion resistance can be provided by 10 sheets which have been heat set at a temperature sufficient to crystallize the polymer materials in the sheet.

Wetting of the sheets with tap water prior to expansion involves a water pickup of at least 75% 15 by weight of the dry sheet? but a pickup of about 140% is preferred. Water containing dielectric coupling agents can reduce the percentage of pickup necessary for good expansion. Typically, a sheet 2 2 with basis weight of about 6 oz/yd (200 g/m ) 20 can be soaked in tap water for a period of about 50 seconds to obtain a water pickup of about 140% and provide good expansion. However, if it is desired to reduce water pickup, water containing up to 5% by weight of dielectric coupling agent can be sprayed on 25 the surface of the sheet to a water content of as little as 50% and still produce good expansion.

Also to be noted, stress flexing as explained above can reduce the period of time required for water to penetrate the sheet, and can 30 reduce the percentage of water needed for good expansion. Further, stress flexing can be used to enhance the expansion for sheets with low level water concentration.

Proper patterning of the sheet with the 35 densified regions provides control and uniformity of 7 8 the expansion along, as well as across, the sheet during the expanding process. The spacing and patterning of the regions can be varied to achieve the desired degree of expansion.

It should be apparent that the invention offers a wide variety of styling possibilities depending upon such factors as the design or pattern of the densified regions and the nature of the sheet being expanded. Where they do not otherwise 10 interfere with the performance of the sheet or the desired use, other materials such as mica may be incorporated into the sheet.

The formation of the densified regions may be accomplished by the use of any suitable heated 15 embossing rolls, plates and the like, but an ultrasonic embossing or bonding apparatus is preferred. The anvil in an ultrasonic apparatus can be designed with appropriately raised portions which provide the desired pattern as the sheet is passed 20 through the apparatus. Ultrasonic bonders can easily and uniformly provide bonding conditions comparable to greater than 4000 -(27.6MPa) psi at 275°C which are found to be effective. Suitable patterns include diamond, square, rectangular, circular and other geometrical 25 shapes. With patterns having small individually expanded portions, ultrasonic bonding appears to facilitate expansion of the small portions.

Preferably the densified regions comprise only a fraction of, and for example 20% or less of, the 30 total surface area of the sheet.

The improved toughness and integrity of the surfaces of the sheets of this invention are apparent from their resistance to loss of material when an adhesive tape is applied to the surface and pulled 35 away. This can be measured quantitatively with the 8 9 Tape Pull Test as described herein in which sheets of the invention provide a loss of material of less than 2 4 mg/cm . Preferably in such a test the densified regions show substantially no loss of material in 5 this test. In general, as in abrasion resistance, the smaller the surface area of each expanded portion, the better the performance in the test. Accordingly, preferred sheet structures of the invention have discrete expanded portions which 10 individually occupy a surface area within the range of from 0.1 to 25 cm2 each.

In sheets containing mixtures of short fibers of MPD-I and PPD-T the fire resistance of the sheet increases as the quantity of the PPD-T fibers 15 increases, but the abrasion resistance tends to decrease.

Preferred sheets of the invention have expanded portions with substantially smooth, two-dimensional surface {substantially free of loose 20 filaments and visual surface irregularities, somewhat comparable to stationery paper) and can even have a somewhat glazed or glossy appearance, which is quite distinct from the rather irregular, textured, fuzzy and more porous surfaces of sheets prepared by the 25 prior known never-dried process.

The products of thiB invention, particularly the preferred product containing a mixture of fibers of poly(m-phenylene isophthalamide) and poly(£-phenylene terephthalamide), have sufficiently 30 increased strength and abrasion resistance over never-dried expanded sheets to provide significantly improved wear life when used as fire blocking layers in aircraft, for instance as a carpet underlay and especially in aircraft seat cushions. 9 10 In general the larger the surface area of the puffed portion the more open is its central interior. Abrasion resistance and portion-to-portion uniformity tend to deteriorate with increasing area 5 and especially with puffed portions having a surface area on each side of the sheet of greater than 4 square inches (25.8 cm^).

Other uses for the products of this invention include insulation against fire, heat and 10 sound and insulation linings in protective garments. Other uses are readily apparent from the physical and chemical properties of these light-weight sheets.

Known ultrasonic bonder apparatuses can be used to provide almost any desired densified pattern, 15 with straight-lined geometric forms such as diamond or square shapes being preferred because of their simplicity and effectiveness. Particularly preferred are such patterns created by two groups of substantially parallel lines having a distance 20 between lines of at least 1 cm (3/8 inch) and no greater than 2.5 cm (1 inch) in each group. Ultrasonic bonding to create the pattern on the dry sheets provides not only a high degree of pattern versatility but also more effective self-bonding 25 which prevents blow-apart or delamination of the densified regions under conditions needed for the expansion process.

The densified regions suitably should be at least 0.5 mm wide and 1 mm long and be 30 segmented by spaced interruptions of about equal length along the linear direction. About 1 mm round dense regions also may be used. Such segmentation improves control of expansion from portion to portion along and across the expanded sheet. 35 11 This invention provides expanded aramid sheet products which can have an abrasion resistance of from 3 to 10 X or more of that of the comparable sheets made by the known never-dried process.

Another advantage for the procese of this invention versus the never-dried process of the prior art is improved productivity resulting from achieving expansion with less water (e.g., up to 5 X less than that for the wet sheet process). Best results do 10 require the use of a dielectric coupling agent such as Woolite® ionic surfactant, cetyl betaine surfactant, or ionic salts such as sodium sulfate.

Thermal insulating performance in this regard can be improved by tension-flexing of the 15 samples before or during the wetting process to facilitate greater development of the inner cellular structure, but with some loss in tensile strength.

The dried sheets for use in the process of this invention for making the improved product can be 20 prepared using known paper-making apparatus and techniques as taught for example in U.S. Patent 3,756,908 and in EP 73,668.

TEST METHODS Drip Porosity Test 25 This test is a measure of time elapsed for a specific sodium chloride-water solution to penetrate the expanded sheet product. The amount of time elapsed is a measure of the product's surface density and porosity. A product with denser, less porous 30 surfaces will resist penetration and retain solution for a longer period of time.

A 0.95 1 (1 qt) wide-mouthed jar ("Mason" home-canning jar), 12.4 cm (4 7/8 in) high with a 6.4 cm (2.5 in) diameter mouth*is employed for the test. 35 An approximately 0.16 cm (0.0625 in) diameter vent 11 12 hole is drilled into the bottom of the jar. The jar is provided with a conventional screw-top annular cap (ring) with a central opening 6.4 cm (2.5 in wide).

To begin the test, the vent hole is plugged and the 5 jar is filled with 600 ml of saline solution (0.9 wt * NaCl). A circular sample of the specimen of expanded product is cut so that it fits neatly within the screw-top annular cap, completely closing the central opening. Annular gaskets, such as of rubber, 10 fitting within the annular cap and having central openings of the same dimension as the cap are placed above and below the circular sample to make a water-tight seal; the sample and gaskets are placed within the cap; and the cap is screwed tightly onto 15 the jar so that the top of the jar is completely closed with the sample covering the central opening of the cap. The jar, with the vent hole plugged, is inverted onto a glass plate which is mounted approximately 20.3 cm (8 in) above a mirror. A 20 stopwatch is started at the same moment as the vent hole is unplugged. The sample is observed in the mirror for penetration of the sample by the solution. Penetration is quickly and easily observed when solution penetrates the sample and wets the 25 glass plate. Occasionally some condensation (a light "fog*) will be observed on the surface of the glass; however, the appearance of the condensation is not considered as penetration of the sample by the solution. The time elapsed between the unplugging of 30 the vent hole and wetting of the glass plate is recorded. If the solution penetrates the sample instantly when the jar is inverted, the time is recorded as zero seconds. The elapsed time for three randomly selected samples of each specimen tested is 35 12 13 recorded and the average of the three elapsed times is reported as the result for the specimen.

Tape Pull Test Tape pull delamination weight is a measure 5 of the amount of material adhering to an adhesive tape after it has been applied, pressed and removed from the surface of fully dried, expanded product.

The amount of material adhering to the tape is a direct measure of surface integrity and toughness, λ 10 tough structure will have a smaller amount of material adhering to the tape as opposed to a softer, less dense structure which gives larger amounts adhering to the tape.

For the test, one side of the expanded 15 product to be tested is designated as the λ side and the other as the B side. On the λ side a line designated as the HD line is drawn in the machine direction if the machine direction is known or can be deduced, otherwise in an arbitrary direction.

Machine direction refers to the "as made" direction from a commercial paper-making machine. Differences in the sides Λ and B are the result of the fiber laydown; the side laid down on forming wire differing from the exposed side. Λ line designated as the TD 25 (transverse direction) line is drawn perpendicular to the HD line on the A side. Eight sample strips 2.5 cm (1 in) wide X 15.2 cm (6 in) long are cut from the expanded product, one set of four strips parallel to the MD line and another set of four strips parallel 30 to the TD line, minimizing to the extent feasible the amount of embossed areas included within the sample strip and employing the same cutting pattern for the four strips cut in each direction so that all the strips cut in a given direction resemble one another. 13 14 The tape used for the test is a substantially transparent tape, 2.5 cm (1 in) wide and having adhesive on one side only (Scotch (Trade Mark) brand 810 Magic Transparent Tape made by the 3M Co.). In 5 ASTM test D-3330-76 (180° Peel Adhesion test), the tape tests 279 g/cm (25 oz/in) for adhesion to steel. Tape is applied to each sample strip, evenly covering the entire width of the sample strip, from one end to about 0.6 cm (0.25 in) short of the other 10 end, folding the tape back on itself to provide a tab of double thickness about 0.6 cm (0.25 in) long with the adhesive surfaces inside and adhered to one another near the end of the strip not quite reached by the tape. Of the four samples in each of the MD 15 and TD sets, tape is applied to the A side in two of the samples in each set, with the tabs being at opposite ends of these two samples, and to the B sides in two of the samples in each set, again with the tabs being at opposite ends of these two 20 samples. The samples, each with tape already applied to it, are then pressed between platens at 11.5 MPa (1667 psi).

The full width of the tab end of a sample etrip (the end not completely covered by tape) is 25 then firmly grasped in the lower jaw of a tensile tester ("Instron" Model 1130 with a 500 g load cell) while the full width of the tab end of the tape is firmly grasped by a clamp attached to the upper jaw of the tensile tester. The tensile tester is then 30 started and the jaws are moved apart at the rate of 30.5 cm (12 in) per min. When the tape has been completely pulled away from the sample, the machine is stopped.

Prom each strip of pulled-off tape a 12.7 cm 35 (5 in) long piece is precisely cut and weighed on a 14 15 balance to the nearest 0.01 g. The average weight of a clean 12.7 cm strip is then determined and subtracted from the weight of the 12.7 cm pulled-off strips to determine the weight of adhered surface 5 material removed from each test strip.

The eight sample strips yield eight measurements per test sheet and the average of the eight results is reported in milligrams per square centimeter.

Taber Abrasion Test This test is carried out in accordance with ASTM Test Method D-1175-64T, page 283 (Rotary Platform, Double Head Method), using CS-10 grit size abrasive wheels applied against the specimen with a 15 load of 500 g per wheel. Failure is judged to occur when a hole of any size passing completely through the sheet can be observed. Results are reported as cycles to failure. Preferred products of the present invention survive 1000 or more cycles to failure, 20 although for some end uses products having lower resistance to abrasion are satisfactory.

Seat Wear Test (Boeing's "Squirmin Herman" Seat Wear Life Test) This test is carried out by preparing 25 conventional airplane seat cushions having a polyurethane foam composition interior surrounded by an inner lining formed of the expanded sheet product to be tested and an exterior lining of conventional seat cushion fabric, e.g., wool/nylon (90/10) 2 30 seat-cover fabric having a basis weight of 441 g/m 2 (13 oz/yd ). The expanded sheet product is sewn to the inside of the seat-cover fabric, and the seat cover is fashioned for ready removal for inspection of the expanded sheet product, e.g., by including a 35 zipper for opening up the seat cushion when desired. 16 The seat cushion is then tested on the seat wear-tester apparatus shown in Figure 2 of the article "Textiles is Ready When You Are" by Sally A, Hasselbrack in Textile World. May, 1982, page 100.

The wear-test device includes a seat weight made of soft rubber, weighing 64 kilograms (140 lbs) and fashioned in the form of a seated human posterior, enclosed by a pants-like cover made of 100% polyester 2-bar tricot knit fabric. In a 2-minute cycle, the 10 seat tester is in contact with the seat cushion for 1 minute and 40 seconds and lifted off the cushion for 20 seconds. While in contact with the cushion, the seat teeter is rocked through a 25 degree arc at 13.5 cycles per minute while the cushion rotates through a 15 35 degree arc at 18 cycles per minute. The test is stopped and the seat cushion fabric with attached inner lining is removed periodically to inspect the lining. Failure of the inner lining is judged to occur when a hole of any size passing completely 20 through the lining can be observed. If the lining is intact after 50 hours of testing, the expanded sheet product is rated as having passed the test.

Example 1 This example illustrates the preparation of 25 expanded sheets of this invention and the fabrication of flame-resistant airplane seat cushions from the expanded sheets.

The aramid papers for making these expanded sheets were all prepared conventionally using a 30 commercial Fourdrinier paper-making machine. Fibrids of poly(m-phenylene isophthalamide) (MPD-I) at about 0.5 weight percent in tap water were fed to one inlet port of a mixing "tee". A 50/50 slurry of 0.64 cm (0.25 in) long, 2.2 decitex (2-denier) MPD-I 35 floc/poly(£-phenylene terephthalamide) (PPD-T) 4 mm 16 17 long (average of 0.5-8 mm lengths), pulped floe of 450-575 Canadian Standard Freeness at about 0.35 weight percent in tap water was fed to the other inlet port of the mixing "tee".

Fibrid-to-floc-to-pulp ratio by weight was 60/20/20. Effluent was fed to the headbox and then to the forming wire. The resultant sheet was passed over the steam-heated drying cans maintained at a surface temperature of 140eC for an exposure time of 2 10 minutes and wound up as a fully dried sheet on a cylindrical cardboard roll. The process was operated with paper-making machine settings calculated to provide 0.58 mm (23 mil) thick dried sheets having a o basis weight of about 200 g per m (about 6 02 per 15 yd2).

The dried sheets Were then ultrasonically embossed by unwinding the sheets from the cardboard rolls and passing each sheet to an ultrasonic embossing station wherein each sheet was embossed 20 between an ultrasonic horn and an anvil. The horn employed (a product of Branson Co., Eagle Road, Danbury, Conn.) had an impact surface measuring 15.2 cm (6 in) long by 1.3 cm (0.5 in) wide. The horn, with the sheet in between, was pressed up against a 25 15.2 cm (6 in) long patterned rotating anvil (drum) having a surface speed of about 10-13 ft/min and a diameter of 7.6 cm (3 in) with peripheral lines of rectangular protrusions measuring 1.9 mm (0.075 in) long by 0.64 mm (0.025 in) wide, spaced 1.9 mm (0.075 30 in) apart, lying in planes normal to the axis of the anvil. The horn vibrated at a frequency of 20,000 cycles per second and an air pressure setting of 20-30 psi on the machine was used to obtain pressure between the anvil and horn. In this example two 35 different anvils were employed, one having lines of 17 18 protrusions lying in planes spaced 1.3 cm (0.5 in) apart and the other having lines of protrusions lying in planes spaced 2,5 cm (1 in) apart. Two sheets were separately embossed on each anvil, passing them 5 between the horn and anvil sufficient times as needed to cover the full sheet width (each pass parallel to the previous pass at the appropriate spacing) in one direction and then sufficient times in the cross direction to produce two pairs of sheets each pair 10 having segmented square pattern arrays of squares measuring 1.3 cm (1/2 in) on a side or 2.5 cm (1-in) on a side, respectively.

In turn, the four embossed sheets were then each wetted by passing them through a tank of tap 15 water to which 1 wt. % ionic surfactant CWoolite") had been added. The sheets were passed through the tank at the rate of 61 cm per min (2 ft per min) for a contact time of 23 seconds. The wetted sheets having at least about 120% water were then 20 dielectrically expanded by passing them from the tank through a 20 RW dielectric heater operating at 27 MHz. The sheets were passed between a single set of 122 cm (48 in) electrodes, spaced 5-8 cm (2-3 in) apart.

The Bheets have discrete expanded portions defined by the densified segmented lineal pattern of squares, with each expanded portion being comprised of a chamber formed by two, opposed, smooth, dense, skin-like surface strata which enclose an interior in 30 which the material density increases outwardly from a less dense central region through a denser cellular sponge-like or laminar region to two opposed dense skin-like surface strata. The sheets with the larger pattern have expanded portions with more open 35 interiors. 18 19 Two of the dielectrically expanded sheets having different sizes of embossed pattern arrays were then heat treated, while the other two were not heat treated. The heat setting was carried out on a 5 frame (Bruckner frame) at minimum tension at 260°C for 3 min. The four resulting sheets are designated as follows: Test Item A - embossed squares 1.3 cm on each side; not heat set.

Test Item B - embossed squares 1.3 cm on each side; heat set.

Test Item C - embossed squares 2.5 cm on each side; not heat set.

Test Item D - embossed squares 2.5 cm on 15 each side; heat set.

The four sheets prepared as described above were then sewn as a liner to the inside of woven wool/nylon (90/10) seat cover fabric having a basis weight of 441 g/m^ (13 oz/yd^). The lined 20 fabrics were then used to prepare conventional airplane seat cushions, with the embossed, expanded sheets as an inner lining surrounding the polyurethane foam composition from which the seat cushions were made. When seat cushions made from the 25 four test item sheets were tested by the Seat Hear Test, cushions made of each of the test items passed the test (no break in the protective expanded sheet after 50 hours of testing). Item B was tested longer and still passed after 100 hrs. Mock seat cushions 30 made of each of the test items also pass Boeing's OSU Heat Release Test (no involvement of the polyurethane foam by the flame for at least 30 seconds) at 5 2 watts/cm . Other properties of the four expanded sheet test items are listed in the table: 19 20 10 15 20 25 30 Taber Item Drip Porosity (seconds) Abrasion Resistance (cycles to failure) Tape Pull Test (mg /cm2 A 13 4300 0.81 B 18 6500 1.36 C 16 1800 1.3 D 19 2500 1.18 Three comparable expanded sheets, not of the invention, but o£ the same 60/20/20 composition [except for 2 mm (average of 0.5-4 mm lengths) long PPD-T fibers instead of 4 mm with a Canadian Standard Freeness of 300-425], were made in substantially the same way except for expanding never-dried sheet, and for a room temperature, mechanically-embossed diamond pattern (4.45 cm x 1.91 cm and 2.5 mm wide continuous densified lines) pressured into the wet sheet. The expanded sheets have a very rough textured, three dimensional surface to the naked eye. A non-heat-set expanded sheet gave instant wetting (zero seconds) in the drip porosity test and 500 cycles to failure in the Taber abrasion test. Two heat-set expanded sheets (30 seconds and 3 minutes at 260"C) gave, respectively, one second and 650 cycles and zero seconds and 700 cycles in the same tests showing them all to be quite inferior in these tests to the sheets of the invention. Tape Pull results for the three items are, respectively, 5.79, 5.79 and 6.3 2 mg/cm . Tested as linings in the conventional seat wear tester, the first one failed, the second one passed marginally and the third passed. However, although the third one passed, its bulk and drapability were somewhat impaired because of heat-setting. 35 21 Example 2 This example illustrates the preparation of sheets of this invention from two separate sheets of aramid papers which are bonded together and 5 subsequently expanded.

The aramid papers for making these expanded products were all prepared conventionally using a commercial Fourdrinier paper-making machine from about 55% MPD-I fibrids and about 45% MPD-I 2.2 10 decitex (2 denier) floe having a cut length of 0.64 cm (0.25 in). After the wet sheets were formed on the machine, they were passed over a series of drying cans maintained at temperatures ranging from 85*C to 115®C for papers of lower basis weight to 110o-140°C 15 for higher basis weight papers, using contact times suitable to dry the papers. The papers were not subsequently calendered.

In one embodiment two fully dried 0.3 m (1 ft) wide, 0.6 m (2 ft) long sheets of aramid paper, 7 20 each having a basis weight of 40.7 g/ra (1.2 2 oz/yd ) and actually measuring 0.10 mm (4 mil) thick (commercially available as nominally 5 mil(0.127 mm) thick paper) were brought together at the ultrasonic embossing station described in Example 1. The sheets 25 were superimposed, one upon the other, and were ultrasonically embossed and bonded together in the densified regions with segmented square pattern arrays of 1.3 cm (0.5 in) on a side.

The embossed sheets were then wetted by 30 passing them through a tank of tap water to which 2 1/2 wt % ionic surfactant ("Woolite") had been added. The wetted sheets were then dielectrically expanded. The resultant product maintained good bonding integrity in the embossed densified regions 35 defining expanded portions with open balloon-like 21 22 chambers formed by two dense, smooth, tough skins.

The operating conditions for wetting and dielectric expansion were similar to those described in Example 1 except that residence time and field 5 intensity were increased.

In another embodiment, two fully dried sheets of aramid paper of different thicknesses were ultrasonically bonded together. One Bheet was 0.25 mm (10 mil) thick having a basis weight of 81.4 g per 10 m2 (2.4 oz/yd2). The other sheet was 0.38 mm (15 2 mil) thick having a basis weight of 129.9 g/m (3.8 2 oz/yd ). The two sheets were ultrasonically embossed, bonded, wetted, and dielectrically expanded as described above. The resultant product was 15 similar to that prepared above; however, some development of an inner cellular or laminar structure in the expanded portions on the inside surface of the skin strata was observed in this thicker sheet.

The resultant products from the two 20 embodiments are designated as follows: Test Item II-A: 4 mil (Θ.1 mm) sheet bonded to 4 mil (0.1 mm) sheet.

Test Item II-B: 10 mil (0.25 mm) sheet bonded to 15 mil (0.38 mm).

Properties of the expanded sheets are: Drip Porosity Abrasion Resistance Item (seconds) (cycles to failure) II-A 24 339 II-B 35 4453 30 Example 3 Dried 23 mil sheet (0.58 mml of substantially the same MPD-I composition of Example 2 was ultrasonically embossed in square patterns (1/2 in and 1.0 in). The sheet was then "stressed" by pulling over a 90° edge 22 23 of a hand held brass block while immersed in a liquid of 2 1/2* "Woolite" and tap water, After multiple stresses (8 times), 2 times each way in the machine direction for both sides the sheet remained in the 5 liquid for a total of 1.0 minute. The sheet was then dielectrically heated in an 85 MHz RF heater with one single set of electrodes spaced 3.0 in apart at a belt speed of 3.0 ft/min. The resulting product readily expanded to form discrete uniform expanded 10 portions with dense, smooth skin-like surface strata and much less dense interiors. 23

Claims (21)

1. A low density/ nonwoven sheet structure consisting essentially of a commingled mixture of from 30 to 90% by weight of aramid fibrids and complementally from 70 to 10% by weight of short aramid fibers, the sheet having expanded portions defined by self-bonded, densified regions with expanded portions being comprised of a chamber formed by two opposed, dense, smooth, skin-like surface strata of said fibrids and fibers which two strata enclose a much less dense interior, the sheet having a porosity to provide a Drip Porosity Test time of at least 10 seconds.

2. A sheet structure of Claim 1 having a basis 2 2 weight of less than 7 oz/yd (237g/m ) and an abrasion resistance in the Taber abrasion test of at least 1000 cycles to failure.

3. A sheet structure of Claim 1 or Claim 2 in which the densified regions have a surface integrity resulting in substantially no loss of material by visual examination to the naked eye in the Tape Pull test.

4. A sheet structure of any one of claims 1 to 3 in which the densified regions define discrete expanded portions and are arranged in a segmented lineal pattern.

5. A sheet structure of claim 4 wherein discrete expanded portions individually occupy a surface area of the sheet within the range of from 0.1 to 25 cm2 each. 5· 5. A sheet structure of claim 5 having a surface integrity sufficient to provide a loss of material in 2 the Tape Pull test of less than 4mg/cm .

6. 7. A sheet structure of any one of claims 1 to 6 in which the fibrids and at least some of the short 10, fibers are comprised of poly(m-phenylene isophthalamide).

7. 8. A sheet structure of claim 7 in which from 10 to 30% of the sheet by weight consists of short fibers of poly(£-phenylene terephthalamide).

8. 9. A sheet structure of claim 7 consisting essentially 15. of fibrids and short fibers of MPD-I and short fibers of PPD-T in a ratio by weight of about 60/20/20 respectively, and the PPD-T fibers have been pulped.

9. 10. A sheet structure of any one of claims 1 to 9 in which the material density in the interior of the 2®· . expanded portions increase with distance from a less dense centre towards the surface strata through an increasingly dense sponge-like cellular region and culminating in the more dense skin-like surface strata.

10. 11. A sheet structure of any one of claims 1 to 10 25. wherein the expanded portions consist essentially of the dense skin-like surface strata and an open interior substantially free of fibrid/fiber matter in a balloon-like configuration.

11. 12. A sheet structure of claim 11 in which the surface 5. strata are derived from separate sheets of said fibrids and fibers which sheets are self-bonded together in said densi'fied regions.

12. 13. A sheet structure of claim 12 having a basis weight 2 2 of less than 3 oz/yd (102 g/m J. 10, 14. A process for preparing an expanded nonwoven sheet comprised of aramid fibrids and short aramid fibers including the steps of forming a smooth, wet-laid sheet of said aramid materials, impressing a pattern of non-expandable, densified regions into the 15. sheet to define expandable portions between said regions and dielectrically heating the patterned sheet while wet with water to rapidly vaporize the water to create highly expanded portions in the sheet, wherein the process comprises 20. drying the wet laid sheet by heating to remove substantially all water and provide a dry, smooth-surfaced sheet; preparing a layered sheet structure for expansion, comprised of at least one layer of said dried sheet, by forming a pattern of non-expandable, densified regions 25. with heat and pressure to self-bond the materials together in said densified regions throughout the thickness of the layered sheet structure which define expandable portions; stress-flexing and saturating the patterned 5, sheet structure with water; and dielectrically heating the stressed and wet sheet to expand the interior of the expandable portions of the patterned sheet substantially without disrupting their surfaces. 10. 15. A process of claim 14 wherein the fibrids and at least some of the short fibers are comprised of poly(m-phenylene isophthalamidei.

13. 16. A process of claim 14 or 15 wherein expandable portions defined by said densified regions are 15. discrete and individually occupy a surface area of from 0.1 cm^ to 25 cm^.

14. 17. A process of any one of claims 14 to 16 wherein the densified regions are arranged in a segmented lineal pattern which occupies less than 50% 20. of the sheet surface.

15. 18. A process of any one of claims 14 to 17 wherein the densified regions are formed by ultrasonic means.

16. 19. A process of any one of claims 14 to 18 wherein a single dry sheet is used having a thickness greater than 20 mils. (0.5mm). 25

17. 20. A process of any one of claims 14 to 18 wherein the layered sheet structure consists of two of said wet-laid/ dried sheets with each having a thickness of at least 4 mils (0.1 mm). 5. 21. A process of any one of claims 14 to 20 wherein the sheet structure is stressed while dry.

18. 22. A process of any one of claims 14 to 20 wherein the sheet structure is stressed while wet.

19. 23. A process according to claim 14, for preparing 10. a nonwoven sheet of aramid fibres, substantially as herein described with reference to the Examples.

20. 24. A sheet structure whenever prepared by a process according to any of claims 14 to 23. MACLACHLAN & DONALDSON Applicants' Agents,

21. 47 Merrion Square, Dublin 2.