CA1186929A - Low density nonwoven sheets - Google Patents

Abstract

TITLE
LOW DENSITY NONWOVEN SHEETS
ABSTRACT
Coherent expanded nonwoven sheets having a density of less than 0.16 g/mL are comprised of fibrids of a wholly synthetic polymer, preferably not melting below 130°C, and optionally containing up to 80% by weight floc. The sheet is comprised of a plurality of paper-like membranous layers which join and separate at random throughout the thickness of the sheet to form expanded cells. The expanded nonwoven sheets are prepared by rapid vaporization of water in a wet, never-dried sheet of fibrids containing at least 60% by weight water. Rapid vaporization of the water preferably is induced by dielectric heating. Embossing may be used before or during expansion to produce a sheet which is expanded only in selected areas.

Description

~6~29 TITLE
LOW ~ENSITY NONWOVEN S~IEEI~S
DESCRIPTION
Technical Field 5This invention relates ~o coherent, expanded, wet~laid nonwoven sheets comprised of wholly synthetic polymer fibrids and op~ionally short length fibers. The sheets are sui~able ~or use as thermal and acoustical insulation having a low den5itY-B
Wet-laid nonwoven sheets comprised of wholly synthetic pol~meric fibrids and short length staple fib~rs are known from U.S. Patent 2~9~g,7~8.
lS Incr~ased bonding of these shee~s can be obtained by application of heat and/or pressure. Pressure can be applied with engraved rolls which produce a pattern on the sheets. Dielectric heating may be used to increase bonding in the sheets. The sheets are 20 paper-like or clo~h-like, depending on the ma~erials used~ Typical densi~ies are of the order of 0.4-0.6 g/mL. While these sheets are said to be useful in acou~tical insula~ion, a lower density material could provide better acoustical and thermal insulation properties at a lower co~tO Somewhat lower density sheets can be obtained in uw alendered form. For instance, an uncalendered aramid paper sheet is available co~mercially having a density of about 0~3 g/mL; but even lower density sheets of such materials would be more useful in many applications if they could be made economically and ~ree of undesirable contaminants.
Low density nonwoven sheets are prepared according to U~SO Patent 3,759,775 (Re: 30,061) by impregnating a nonwoven web with an aqueous liquid '~

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containing a binder and rapidly vaporizing the water, e.g~, by dielectric heating, to expand the structure while simultaneou.sly setting the binderO The nonwoven webs are preferably air~laid.
Expanded fibrous ma~erial may be prepared according to British Patent 1,4G8,262 by confining the fibrous material under pressure with a puffing agent, preferably with heating, followed by release o~ the pressure and expansion of the material. The heat may be provided by dielectric means.
The solvent, heat and flame-resistant propertie~ of synthetic aramids, such as poly[m-phenylene isophthalamide], which make them very u~eful under certain conditions, also make it e~pecially di~ficult to fabricate the polymers into a useful. expanded low-densi~y form. This invention proviçles among other things a novel proce~s for making such polymers into lightweight structures.
One object of this invention is a coherent shee~ comprised of wholly synthetic polymer fibrids and having a density of less than 0.16 g/mL (10 lbs/~t3). Another object i~ a paper-making process for providing such a low density ~heet fr4m wholly synthetic polymer fibrids by wet-laying, without 25 needing any adhesive binder, and especially when the fibrids are of an aramid.

This invention provides a coherent expanded nonwoven sheet comprised of fibrids of a wholly synthetic polymer and optionally up to 80% by weight of short fibers or floc, the sheet having an apparent density (as defined herein) of less than 0.16 g/mL, and being comprised o~ a plurality of paper-like layers lying substantially horizontally in the plane of the sheet which join and separate at random throughout the thickness of the sheet to form expanded macroscopic cells. The paper-like layers are comprised of membranous elements which are comprised of the fibrids.
This invention further provides a coherent expanded nonwoven sheet comprised o~ fibrids of a wholly synthetic polymer which fibrids form a multiplicity of layered membranous elements which join with ana separate from one another at random to form a three-dimensional, highly irregular network of numerous interleaved macroscopic cells with tapered edges positioned substantially throughout the thickness of the expanded sheet.
In a typical cell cross section the maximum width of the cell, usually running in a direction substantially parallel to the plane of the sheet, is considerably greater than its maximum height at right angles thereto, usually, for example, by a factor of at least 2. Because of the manner in which the cells are formed, by separation and joining of the layered membranous elements, frequently the edges of a cell as shown by cross section are substantially tapered with respect to a thicker inner portion of -the cell.
In addition to the fibrids, the sheet can contain up to 80% by weight of short fibers, i.e., floc, based on the total weight of fibrids and fibers; preferably less than 70% fi~ers, and more preferably 20~50% fibers. The type and ~uantity of fi~ers to be used depend upon the strength and other physical properties desired in the sheet as taught, for example, in U.S. Patent 3,756,908.
The sheet can be expanded completely, i.e., entirely throughout its length and width as well as -~ 3 thickness; but as a preferred embodiment cliscrete portions of the sheet remain unexpanded~ e.g., portions arranged in a random or, more preferably, patterned manner about its surface area. Most S preferably since embossing limits expansion even in unembossed regions, no more than 50~ of the sheet, based on total surface area~ is not expanded according to this invention.
The fibrids are cc>mprised of a synthetic fiber-forming polymer having sufficient heat resistance to survive the sheet expanding process, as described hereinafter~ i~e~, without substantial loss of shape and integrity through fusing ox decomposition. The floc must be heat resistant likewise. Since flashing steam from rapidly evaporating water is the preferred expanding medium, the fibrids and the floc preferably should not melt or decompose below about 130C for best results.
The expanded sheets of the invention do not requi~e any, and preferably do not contain, adhesive binder material for structural integrity; but small quantities of such ma~erials are not necessarily excluded provided they do not interfere with the fibrid membrane formation, which forms the expanded cell structure, or with other desired properties~
Preferably no adhesive binder material is used and the sheet then is considered as "adhesive-free" and consists essentially of the fibrids and of any short fibers as described above, allowing, of course, for minor amounts of conventional nonstructural additives such as pigments, dyes, chemical stabilizers and so forth.
This invention provides expanded nonwoven sheets of "apparent" densities of less than 0.16 g/mL
and preferably less than 0 r 10 g/mLO When the sheet 6~

is completely expanded throughout to a u~iform thickness, its actual and "apparent" densities are theoretically the same.
Thicknesses of the expanded regions more than five times greater than those of any unexpanded region~ e.g., due to embossing, can be achieved. For even lighter weight material, expansion to a thickness ten times that of the embossed thickness is achievable.
Preferably the fibrids are comprised of an aromatic polyamide, more specifically an aramid~ and most preferably poly (m-phenylene isophthalamide) .
Preferably the floc also i comprised of an aromatic polyamide, most preferably poly~m-phenylene isophthalamide) or polyl~-phenylene t~rephthalamide). In another preferred embvdiment, the floc is comprised of glass fibers.
This invention also provides a process for preparing a coherent expanded nonwoven sheet

2~ comprising preparing a wet mixture with water of 20-100% by weight fibrids of. a wholly synthetic polymer and 0-80~ by weight floc, complementally to total 100% and both as described hereinbefore, forming a wet nonwoven sheet: of the mixture on paper~forming equipment, maintaining water in the formed ~heet and preferably at at least 40% by weight, adding additional water, if needed, to increase the water content of the sheet to at l~ast 60~ by weiyht, and heatin~ the wet sheet to vaporize the water rapidly and to expand the sheet to provide a product having an apparent density of less than 0.16 g~mL. The expansion is accompanied by the formation of macroscopic cells as described hereinbefore.

Up to 50% of the total area of the sheet can be e~b~ssed before or during expansion to provide a sheet which is expanded only in selected areas.
Preferably the wet nonwoven sheet contains 20-80% by weight fibrids and complementally 20-80% by weight floc; more preferably less than 70% fibers; and most preferably 50-80% by weight fibrids and 20-50~ by weight floc to a total of 100%, all on a dry basis.
As used herein percentages are by weight unless otherwise specified. Preferably the fibrids are comprised of an aromatic polyamide, most preferably poly(m-phenylene isophthalamide). Preferably the floc is comprised of an aromatic polyamide, mos~
pre~erably poly(m-phenylene isophthalamide~ or lS poly(~-phenylene terephthalamide). Fibers of the latter most preferably are pulped, i.e., macerat4d as d~scribed hereinafter. Most preferably the rapid vapori~ation of wat~r is induced by dielectric heating.
The low-density sheets of this invention axe especially useful for providing thermal and/or acoustical insulation. When they are composed of materials known for yood flame retardance ~e.g~, poly[m-phenylene isophthalamide], poly~-phenylene terephthalamide], poly[vinylidene fluoride], or glass) they are particularly useful in aircraft and so forth where flame retardance and lightness of weight are important. Such flame-retardant sheets are also useful as inner liners of textile goods such as protective clothing~ and they may be impregnated with resins to form low-density composite rigid structures useful~ for example, in floors and walls of aircraft.

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BRIEF DESCRIPTION OF THE DRA~INGS
Figure 1 is an enlarged cross-sectional view of an expanded nonwoven sheet of the invention.
Figllre 2 is an enlarged cross-sectional view of a nonwoven sheet of the invention which is expan~led only in discrete areas.
Figure 3 is a diagrammatic illustration of one rneans for carrying out the process of the invention.
~
Nonwoven sheets suitable for use in the process of the present invention are conveniently prepar-ed as taught in U.S. Patent 2~9g9f788 from ~iynthetic polylTeric fibrids prepared by ~hear 15 precipitation of solutiorls of the polymer, preferably into an aqueous medium. The fibrids should not b i~olated but rather are directly converted into sheet structures by the usual paper-forming techniques.
Preferably, the aqueous mix used to prepare the nonwoven sheets by paper-makiny methods will contain short staple fibers (floc) in addition to the fibrids. Other materials may be added if desired.
The as-formed wet s~.eets must not be fully dried before expansion. P~eferably for better and more uniform expansion they should never be dried to a water content of less than 40% and more preferahly no less than 60% before expansion, particularly for fibrids of MoeD-I. Good expansion requires a water content of at least 60~; therefore, if the water corltent i5 less than 60%, additional water should be added to the sheet before expansionO If desired, the sheet ~.ay b~ saturated with water before expansion.
Additional water may be added either before or af ter any embossing step~

The term "floc" is used to describe short length fibers as ~ustomaril~ used in the preparation of wet-laid sheets. ~loc suitable for use in this invention will nor~lally have lengths less than 2.5 cm, most preferably about 0~68 cm~ Linear density is from 0.55 to 11.1 or more dtex, more preferably in the range 1.0 to 3.5 dtex. In the examples, unless indicated otherwisep the floc employed was of poly(m-phenylene isophthalamide~
fibers with a linear density of 2.2 dtex and a cut length of about 0.68 cm. While .suitable floc can be prepared from ~ilaments which have no~ been fully drawn and/or hea~-stabilized (crys~allized), i~ is preferred that the floc be cut from highly drawn and heat-stabilized filaments. 8uch ~loc provides maximum ~rength and resistance ~o shrinkage of resultant sheets. Both synthetic pol~meric and inorganic flocs may be used.
Floc whi~h ha.s bee.n "pulped" is also suitable either alone or a~ any part of the total floc. Pulping results upon maceration of floc to shatter the fiber and generate fibrous elements of irregular shape comprising numerous fine fibrils.
Pulping is conveniently ach:ieved using a well-known double~disc wet-refiner.
Fibrids are very small, nongranularO
flexible, fibrous or film-like particles. At least one of their three dimensions is of minor magnitude relative to the largest dimen~ion. They are prepared by precipitation of a solution of the fibrid-material using a non-solvent under very high shear, as is known. Suitable fibrids and methods for their preparation are described in U.S. Patent 2,999,788 issued September 12, 1961, to P. W. Morgan~ Fibrids are always prepared as dispersions in liquid. They can be converted to aqueous slurries by suitabie washing techniques. For use according to the teachings of this invention, fibrids must not be dried or heated above their glass-transition temperature before being fed to a paper-making mach1ne. If dried, redispersion is difficult and effectiveness in this invention is greatly reduced if not destroyed. If heat-set, the flexibility required for good performance is severely diminished. Fibrids characteristically have a high absorptive capacity for water and when deposited OQ a screen have sufficient strength even when wet to permit processing on a paper machine.
~here fibrids of poly(m-phenyl~ne 1~ isophkhalamide) (MPD-I) are invslved in the examples, they are prep~red essenti~lly as in the following specific method. A solution at about 120C
containing about 14% by weight of MPD-~ and having a viscosity between 5 and 7.5 Pascal-second~ ~an inherent viscosity of about 1~6) is passed to a fibridator of the type disclosed in U~S.
Patent 3,018,091. The solution contains 77O5~
dimethylacetamide, 2% water, and 6.5% CaC12 (all percentages by weight). Thle polymer solution is fed to the fibridator at approximately 550 kg of solids per hour. The precipitant liquid is fed at 15-20C
to the fibridator and controlled to contain from 30-40~ dimethylacetamide, 58-68% water/ and about 2 CaC12 all to total 100% (all percentages by weight)~ Flow~rate of precipitant to the fibridator is about 28.4 kg per kg of polymer solution~ A rotor ~peed of about 7000 rpm generates the shear requir~d to produce fibrids of good papermaking quality. The fibrids are washed with water until the residual ~s contents of dimethylacetamide and chloride are each about 0.5% by weigh~ or less, based on the polymer.
The fibrids are then reflned to imprsve their filmy characteristics using a disc-refiner at 0~8%
consi~tency to provide a Schopper Riegler Freeness of 300-400 mL. Using the Clark Fiber Classification (TAPPI Standard T-233 su-64), a typical fibrid size characterization is:
Screen size mesh ~ Retained __ _ __.__ 14 1<0

3~.0 100 34.0 Total 81.0 Suitable shee~s for use herein can be made 15 by uniformly depositing an aqueous ~lurry of the paper-makin~ f 7 brous material onto a ~oraminous surEace (e.g., a fine-mesh screen or fabric) through which much of the water quickly drains to form an initial sheet. Sheets prepared one at a time on 2Q laboratory ~cale paper-forming equipment are designated ~ andsheetsl~. The laborator~scale paper-forming machine used to make handsheets as described in some of the examples was provided with a headbox for receiviny fibrous slurry, a 20x2~ cm (8.0 x 8.0 in~ drainage screen, and a vacuum provision below ~he scre~n to assist in removing water.
rhe detailed procedure for preparing a specific handsheet comprising 57 wt% fibrid and 43 wt~ floc is gi~en belowO ~andsheets of different 3~ fibrid/floc ratios and of different fibrid and/or floc materials were prepared analogously/ specific forming yuantitie~ being provided in the Examples.
An aqueous slurry composed of 40 9 of the MPD-I fibxids and 460 g of water was added to a blender containing 3000 g of waterO Then 30 g of dry ~69~

MPD-I floc was added and the mixture blended for 30 min. To 1000 mL of this mixture was added 2000 mL of water, and the ~econd mixture was blended for 15 min. A 1200 mL aliquot of the second mixture was poured into the headbox of a laboratory-scale paper-forming machine containing a 1.3 cm head of water. After removal of water by application of water-jet vacuum for 25 secr the handsheet was pulled from the screen. It retained about 88 wgt~ ~ water and weighed 205 g/m ~ Repetition of the procedure except that only a 600 mL aliquot of the second mixture was used yielded a hand~heet weighing 119 g/m .
The wet fibrid/floc papers, before or during expansion, may be embossedO ~y "embossing" is mean~
the appli~ation of pressure in a patterned array of pressure-points such that, upon expansion, the pressed areas do not expand significan~ly. Such embos~ing limi~s the expansion obtainable in the unembossed areas because of the continuity of fibrous materials ~rom embossed to unembossed areas.
Embossing, however, imparts a de~ree of rigidity and durability in use which exceeds tha~ of expanded sheet products not embossed prior to or during expansion~ Because of the limited expansion for embo~sed sheet products, no more than about 50% of the surface area of the sheet ~hould be embo~sed~
Embossing surfaces may be in a variety of known forms, g~nerally either flat plates or preferably paired driven rolls with a pattern of protrusions. Tests have revealed that on~-sid~d embossiRg against a plain surface as well as two-sided embossin~ between mated patterns can be used. It is obvious that similar results are obtainable with two-sided embossing between ~36~2~

mismatched patterns whereby compressed areas result only when protuberances on both sides mate at the embossing nip. While the pressure re~uired to produce enough compaction at embossed areas to S prevent expansion on heating depends to a minor extent on the fibrid-material involved/ it has been found that about 10 MPa (1500 psi) on the embossed area, and prefera~ly about 24 MPa (3500 psi), is sui~able~
Dielectric expansion of bo~h embossed and unembos~ed sheets is shown in the examples. Unless otherwise designated, the "diamond-embo~sedl' sheets were pressed between mated sheets of expanded metal webbing having a pattern of diamond~shaped openi~gs defined by two sets of linear strips of metal parallel to one another in each set and intersecting with no increase in thickness s~t-to-set. Also, unless other~ise designated, the li~ear metal strips were 2.5 mm (0.10 in) wide and defined di~nond shaped openings with 2.54 cm (1.0 in) and 0.76 cm (0.30 in) major and minor axes. "Plain~embossed" designates patterned arrays of embossing by di~crete pr~tusions spaced in square array. ~n ~he examples, unless otherwise stated, "plain em~)ossed" indicates square protrusions 0.13 cm (0.05 in) on each side spaced so their centers are in square ar~ay 0O445 cm (0.175 in~
on each side . I t i~ apparen~ that any geometr ic array of embossing elements may be employed.
When ~he never-dried fibrid/floc ~heet i5 heated rapidly enough, water-vapor is generated at ~uch a high rate that the ~heet expands in thickness, except at suitably embossed areas. Preferably the sheet i5 heated by passing through a di.electric field of sufficient intensity. The available frequencies of dielectric energy generally vary from about 13 MHz 6~

up to about 2450 M~æ but only certain discrete fre~uencies in this range are generally permitted by the various countries. The selection of a frequency depends most significantly on the width of the sheet and on power coupling. If the sheet width exceeds one-half the wave-length of the frequency used, a node (or series of nodes) of a standing wave results. Since there is no energy dissipation at a nvde, uneven heating results. Thus, the sheet width .i~ preferably le5s than one-half of the wave length of the frequency used; and typically no wider than one quarter wavelengthO Maximum frequencies prefer~ed for ~everal sheet widths a-re:
Width M~z .102 4.0 738 ~.~03 ~0 363 0.305 12 246 0.610 24 123 1c219 48 ~1.5 2.438 96 30.8

4.877 1~2 15.4 In the follow~ng examples, the dielectric heater u~ed was a "Thermall 1ll Model CCH/8.5 heater produced by W. D. LaRo5e & Associates, Inc., of Troy~
N.Y~ and rated at 8.5 kW operating at 84.2 MHz~
Fixed ele~trodes wider than the samples treated were located beneath a variable-speed conveyor b~lt with a sheet of polytetrafluoroethylene between the belt and the electrodes. Each electrode (the first ground and the second "hot") extended transversely to the direction of belt movement and was separated from the other along the direction of belt~movement by a variable amount. Unless specified otherwise, ~he latter spacing was approximately 7 0 6 ~m. Such an arrangement of electrodes relative to the object to be heated produces what is called a fringing field.
As is well-known, polar dipoles within a material Lry to align with an applied electric field which~ when oscillated at high frequency, produces internal heat due to rotation of the polar dipoles, One form of the classical equation for power developed in an oscillating electric field is P/v = 2 f o r"Erms2 where P/v is power developed in the material (~/cm3) f is f re~uency (Hz~
Erm8 is electric field strPngth in the material (Vrms/cm~
r" is relative dielectric loss factor of the mater i al ( "/ O ) " is absolute dielectric 105s factor of the material O is free-space permittivity (F/cm).
Thus, the power density dev~eloped in the material depends on the fre~uency~ the relative dielectric loss factor of the materialv and the square of the electric field strength produced in the material.
Electrical e~fects other than dipole oscillation may also contribute to heating.
- The water in the sample being heated couples more or less effectively depending on the identities and concentrations of impurities. Very poor coupling 30 results at lower f requencies as in the Examples when distilled water is used. Good coupling results when ordinary tap water or industrial water is used.
Extraordinary coupling is known to and does resul z~

when detergents and/or wetting agents are added to the water.
Figure 1 is a scanning electron micrograph ~taken at 20X magnifica~ion) of a sross-section of an unembossed expanded sheet of the invention showing multiple interleaved expanded macroscopi~ cells 10 throughout its thickness formed by membranous elements of fibrids 12 arranged in paper-like layers and containing numerous short fibers 14 (seen as straight whi~e lines).
Figure 2 is ~ canning elec~ron mi.crograph (~aken at lOX magnification) of a c.ross-section of an expanded, embossed sheet of the invention. Expanded portions 8 contain many interleaved macroscopi~ cells 10 ormed ~y a networ k of membranous elements of fibrids 12 arranged in paper-like layers. The expanded portions 8 are separated by thinner portions 16 caused by embossing of the sheet prior to expansion.
Figure 3 illustrates one embodiment of the process of the present invention wherein a wet-f ormed nonwoven sheet comprised of wholly synthetic polymer ~ibrids and short length fibers containing at least 40% water at all times since its formation is taken f rom rol.l 2, passed around rollers 3 into wetting tank ~ where additional water is added to the sheet, .
. the moisture content of the sheet is monitored with - moisture meter 5, the sheet is embossed between matching patterned steel rolls 6 and passed between 30 electrodes of dielectric heater 7 wherein the sheet is expanded. The sheet may be further dried and/or heat set in infrared oven 18, passed through an inspection stand 9 around additional rollers 20 onto wind-up roll 11. The expanded sheet may be 3S simultaneously sli~ while being wound up.

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Heat treatment of the expanded sheets ~or stabilization against shrinkage at elevated temperatures of use is often desirabie. The floc norm~lly employed will have been heat-set by heating at or above its polymer glass-transition temperature be~ore being combined w' th fibrids and wet-laid; so it will not shrink appreciably~ The fibrid polymer, however, must be essentially unori~nted and uncrystallized before wet-laying, which can result in shrinkage of the expanded sheets at elevated use temperatu~esr ~specially when the fibrids comprise more than B5% by weight of the total fibrid~floc contellt., Below 85 wgt ~ of MPD-I fibrids, linear shrinkage is us1lally less than or about 5% decreasing 15 to essentially 0% at and below 20 wgt % ~ibrids at temperatures at about the glass transition temperature of the fibrids. At subsequent use-temperatures below the glass-~ransition temperature, shrinkage is substantially zero. For the poly(m-phenylene isophthalamide) fibrids, heat-setting temperatures are usually 265-270~C.
Tests Basls weiqht is determined ,by weighing a dry sheet sample of known area and converting the result mathematically to the appropriate units of weight per unit areaO
Thickness of a sheet is measured using a caliper ~0 load on sample~ and converting the result mathematically, if necessaryf to the appropriate 0 units for calculating density.
y is computed as the basis weight divided by the thickness of a sheet, with appropriate conversion o units to provide the units g/nL. For sheets which ha~e embossed unexpanded areas, the thickness of the most highly expanded portions of the sheet is used in 1~

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computing ap~arent density, i.e., the density the sheet would have if no areas had been embossed and all areas had been allowed to expand uniformly to the same maximum degree~ Whether embos~ed or unembossed, sheet thickness is measured perpendicularly to the plane of the sheet; thus, pleating or folding of the sheet to further increase its space~filling capability has no effect on the calculated apparent density~ Likewise, basis weight is the weight per unit area of the planar sheet which, within the limitation of the art of wet-laid paper-~ormation; is unif~rm. In order to define a density specification inclusive o~ all sheets herein, the term l'apparent density" is applied ~o all calculated densities as lS described above.
Tensi~ E~g~ is measured on 2.54 cm wide samples __ clamped between 5.08 cm ~spase~ jaws o~ an Instron tensile tester according to ASTM-D-828-60 with elongation at 50%/min~ The sample is conditioned at ~o least B hours at 21C (70F). and 65% Relative Humidity before testing~
EXAMPLE I
Poly(m~phenylene isophthalamide) ~MPD-I) fibrid/floc handsheets were prepared at varyirlg fibridjfloc weight ratios. All ratios and percentag s repor~ed are based on weight~ Table I
characterizes preparative conditions and the handsheets obtained. Column A designates- composition of a volume of never dried fibridæ in tap water.
Column B does the same for a slurry of floc in tap water. Volumes A and B were added to a blender and a~ter blending, a portion, C, was taken and blended with an addi tlonal volume of tap water, D . A 1200 mL
aliquot o~ the resultant blend was formed into a 3~ handsheet which, as collected, contained the ~36~

indicated ~ water. The last column indicates the fibrid/floc weight ratio.
Table II reports the procedures involved first in diamond-embosslng and then in dielectric heating 20 cm x 10 cm ~8.0 in x 4.0 in) sections cut from the above handsheets. Two similarly prepared items identified as I-C'and I-D'are al30 incorporated~ In Table II, r~l, W2, and W3 are, respecti~ely, the sample weights before embossing, after embossing, and after dielPctric heatingO "Time in dielectric ~ield" denotes the time required for each increment o~ sample to pass from one ~o ~he other electrode on the conveyor b~lt operated at the indicated speed. The expanded samples were dried at 150C after which thickn~s~es at essentially zero contact pressure were measured botb at crests (t~ickes~ expanded portions) and nodes (thinnest embo~sed portions). ~ry~weight of each ~ample is the last column. Pressures utilized in embossing were not measured, but were adequate and at least 2S qreat as subsequently determined I:o be workableO As can be seen from examination of nocle thicknesses, ~ome expansion occurred at ~he nodes at fibrid percentages of about 85 or greatex; and this minor expansion was visible as tiny bubbles. The integrity of the nodes was not, however~ impaired~ Test sheet I-J contained no f~brids. While it was possible to form and treat the handsheet, it becamer during dielectric heating, only a loose mat of f ibers without structural integrity and without embossed nodes.
Table III provides additional sheet properties. The thicknesses are of handsheets before embossing and dielectric heating and are useful in comparing with the crest and node thicknesses of ~able ~I. The basis weights, tensile strengths, and ~36~

elongations were all measured on the embo~sed and expanded sheets dried at room temperature. Maximum tensile properties are seen to result at fibrid/floc weight ratios in the range 95/5 to 50/50. "Apparent density" is computed as space occupied by the expanded sheet between flat plates; i.e~, it is computed from basis weight (Table ~ and crest thickness (Table II):
= (~W) x 10 tc where = apparent density (g/mL) BW - basis weight ~g/m2) tc = crest thickness (mm)~
Scanning electron microsraphs of expanded portions of a sheet cross section of Item I-A showed a macroscopic cell tL ucture of membranous elements substantially as shown in FIG. 1 bu~ without the short fibers. Cross sections of Items I-F and I-G
show a layered structure of fibers, membranous elements and macroscopic cells somewhat like FIG. 1 but with many more ~ibers and a less complete network of the cellsO A cross sec~ion of I~H shows a paper-like layered structure of fibers and fragmented membranous elemen~s with substantially no membranous cell structure as in FIG. 1.

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U ~ y 6~2 EXAMPL II
This example tests the effect of belt speed in the dielec~ric heater on expansion achieved.
Belt-speed determines the time during which a sample is exposed to heating~
The sheets for these tests were all prepared using a commercial E'ourdrinier papermaking machine.
Two sheets were made, one for each b~lt-speed series~
differing only in percentage of water retained.
Fibrids of ~D~I at about 0.5 weight percent in tap wat~r were fed to one inlet port of a mixing "tee".
A slurry o MæD-I floc at about 0.35 weight percent in tap wa~er was ed to the o~her inlet por~ of the mixing "tee~. Fibrid-to-floc weight ratio was 60/40. ~fflu2nt was fed to the headbox and then to the forming wire. The resultant sheet was passed over normal drying cans at a temperature reduced to result in a collected sheet of desired moisture content. The high-pressure calender rolls normally used in papermaking were completely by-passed.
In Table IV ar~ presented d~ta relevant to expansion by dielectric hea~ing~ The "% water" is of the shee~ ~s prepared. Nl, W~, and W3 (as d~fined in Example I) are for the actual 10 cm x 10 cm ~4.0 x 4~0 in) specim,ens heated. "% water removed" is based on weights before and after dielectric heating ~W2 and W3) and on dry welght. Diamond-embossi~g was performed at an unmeasured but ample pressure~ l'Dry weight" is weight measured after drying the embossed and expanded sp~cimen at 150C. Where two crest thicknesses are given, they represent a measured rangeO
On examination of Tahle IV it is apparent that good expansion occurred in each test. Ionger 9Z~
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The expanded por~ions of the sheets contained many expanded macroscopic cells of membranous elements similar to FIG. 2.

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EXAMPLE III
This example investigates the effect of different degrees of drying of the sheet as wet-laid. The sheets were prepared as described in Example II except that more intensive drying on the drying cans was used. All specimens cut to 20 x 10 cm. (8.0 x 4.0 in~ were diamond-embossed before dielectric heating. They were alss immersed in tap water to increase their water contents before 10 dielectric heating.
Table V presen~s the relevant processing and thickness details. Headlngs have ~he same meanings a~ in Table IV except that~ under "% water", the fir~t number refers to the sheet as removed from the 15 papermaklng machine, and 'che second number applies to the re-we~ted ~heet, and (2) the "crest Shickness"
mea~urements were all on the dried expanded sheets, double entries indicating ranges.
Specimens III-A, III-13, and IIï-E all 20 expanded excellently and uni.formly. Specimens IIï-C
and III-D expanded very irregularly with some portions expanded little, if at all. This confirms the need for at least 40% by weight water retained in the we~-laid sheet as prepared for best results.
Sample III-F (dried and re-wet before dielectric heating) showed very little expansion and considerable delamination along the embossed lines.
The 0.38 mm ~15 mil~ thick uncalendered Nome~ T-411 aramid paper did not expand at all even though soaked in tap water for 64 hours.

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EXA~LE IV
This example documents the relative effectiveness of tap wa~er (IV-A) and distilled water (IV B). Handsheets were prepared as in Example I.

5 The IV~A handsheet contained 87~ by weight water as prepared. The IV-B handsheet contained 89% by weight water. The fibrid/floc weight ratio was 60/40.
Specimens of each 20 x 20 cm (B.0 x 8.0 in) were cut, diamond embossed, and ~ubjected to dielectric heating.
Sample IV-A (tap water) weighed 61 g after emhossing and 15 g after dielectric heating. About 88% of the water vapori~ed~ Residence time in the heater was 30 second~. Crest/node thicknesses were 3.6 mm/0.28 mm. An excellent and uniformly expanded sheet re~ulted.
5ample IV~B (distilled water) weighed 62 g after embossing and 43 g af~er dielectric heating.
Ab~ut 35% of the wa~er vaporizedO Residence time in thP oven was 150 s. Crest/node thickness~s were 1.3 to 3.3 mm/0.28 mm. Very irregular and incomplete 2 xpansion result~d.
The water supplies used were characterized as to mineral content and results are shown below in parts per million. The notation "ND'I designates "none detectable".
Na K Ca M~ Al Cu Fe Si P S Cl-IV-A 12 3.4 29 12 ND 002 1 ~ ND 5 35 EXAMPL~ V
This example shows the effect on expansion of added surfactant. Handsheets were prepared as described in Example I except that the fibrid/floc ~eight ratio was 57~43 and that, about 6 min after adding the 2000 mL of tap water to the blender, a 10 mL volume of a 33 weight percent a~ueous solution ~36 of Product BCO (Du Pont tradename for its cetyl betaine surfactant) and a 5 mL volume of antifoaming agent (Dow Antifoam B) were added. After each nevex-dried sheet (% water shown in Table VI) was ~ade, it was cut to 10 x 10 cm (4.0 x 4.0 in) size before diamond embossing and dielectric heating.
In Table V, tests V-A to V-D are for sheets as just described. Tests V-E to V~I are for sheets equivalent except that no Product BCO and no antifoaming agent were added. By comparing the belt speed~ an~ crest thicknesses, i~ is apparent that the additives enabled full expansion at the highest belt-speeds available, but that control tests V-H and V-I reached less than full expansion at a bel~-speed somewhat less than the maximum available~

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EXAMPLE VI
This example describes the preparation and dielectric heating of handsheets wherein some or all of the MPD-I floc is replaced by poly(p-phenyleneterephthalamide) (PPD-T) floc. The handsheets were prepared using the procedure of Example I.
PPD-T pulped floc was used as a 37 percent by weight suspension in waterS Three handsheets were made having MPD-I fibrids/MPD-I floc/PPD-T pulped floc in parts by weight (dry weight basi~)o 57/3Bo4/4~6t 57/33.6/9.4, and 57/28.6/14.4, respectively.
lS Water content of ~he wet sheets was 86-87% by weight. All were diamond~e~bossed and passed through the dielectric heater at 0.4 m/minD All three expanded sheets were essentially identical with crest thicknesses in the range 4.2 to 4.8 mm and node thicknesses o~ about 0.25 .
A handsheet of ~IPD~ I fibrids/MPD-I
floc/PPD-T floc (60/35~5) WclS 1.1 mm thick as prepared and contained 85% by weight wa~er. Af ter diamond~embossing, it was pcassed through the ~ielectric heater at 0.4 m/nnin. It expanded immediately to crest/node.thicknesses of 4O6 mm/0.25 mm. Dry basis weight was 193 g~m2.
The PPD-T floc of this and the next sheet was cut from tow of Kevla~ 29 aramid yarns ~Du Pont).
A handsheet of MPD-I fibrids/PPD-T floc (50/50) was prepared using only PPD-T floc; i.e., all the MPD-I floc was substituted with PPD~T flo~
Preparation, embossing, and dielectric heating were as described for the previous test. The crest/node thicknesses were 3 . O mm/O . 25 mm, and the dry basis weight was 166 g/m . The expanded portions of the sheets coniained many expanded macroscopic cells of membranous elements similar to FIG~ 2.
E XAMP LE V I I
This example shows expanded sheets prepared from 2~D-I ~ibrids and glass floc in a 57/43 weight rati.o. The glass floc was 3 . 2 mm (O .125 in) long and 8 l~m diameter glass sta~le obtained from Pittsburgh Plate Glass. The handsheet was made following the general procedure of Example I. It was diamond-~mbossed and then passed through the dielectric heater at 0.4 m/min. Expansion was immediate providing crest/node thicknesses of 3~0-3O3 mm/0~23-0~25 mm. The expanded portions of ~he sheets contained many expanded macroscopic cells of mem~ranous elements similar to FIG. 2. When the dried expanded sheet was held in the flame of a laborat~ry burner, very little shrinkage occurred.
ELI~NPLE VIII
This example illus~rates the use of a thermoplastic polymer for the f.ibrld and,/or the floc components. The thermoplas~ic polymer employed was poly (ethylene terephthalate) or which the abbreviated name 2G-T is used hereaf ter .
2G-T Fibrids The 2G-T polymer u~3~d in preparing fibrids had a relative viscosity (LR~) vf 22 where: (1) LRV
is the ratio at 25C of the flow time~ in a capillary viscometer for solution and solvent, (2~ the solution is 4.75 weight percent polymer in solvent, and (3) the solvent is hexafluoroisopropanol containing 100 ppm of H2SO4.
Fibrids were prepared by trickling 200 mL of a 10% (w/w) solution in trifluoroacetic acid of the 35 above polymer into 300 mL of water while stirring 69~9 rapidly in a blender. The fibrids obtained were washed in tap water until ~he effluent had a pH of 4. The ~inal aqueous slurry was 29% by weight fibrids.
2G-T Floc The 2G T floc employed was of Dacro~ Type 54 polyester staple with a cut length of ~.35 mm (0.25 in) and a linear density per filament of 1.67 dtex (1.5 denier).
2G T Flbrid/2G~T floc (S0~4~1 In~o ~ blender containing 3.5 L of tap wa~er were added 148 g of the a~ove 2G T fibrid slurry and 30 g of 2G-T fLoc, After blending for 15 min, 1100 mL of the mixture was added to 2 L o~ tap water, and the new slurry was blended for 10 min~ A 1200 mL
aliquot o~ the ~inal mixture was added to ~he headbox of a 20 cm x 20 cm (800 X 8~0 in) labora~ory sheet former, The wet sheet removed after pulling vacuum for about 25 seconds comprised about 89% water.
The above sheet ~74.4 9) was diamond-embossed, re~ulting in loss of weight to 65.8 g~ The embossed sheet was dipped into water containing 0.83% cetyl b~aine tProduct BCO -Du Pont) whereupon its weight increased to 74.1 g.
Upon passage of the wet, embossed sheet through the dielectric h~ater at 0.4 m/min, expansion of the unembossed areas was rapid. Weight of the sheet after expansion was 10.1 g. Another pass through the heater removed ~h~ ~emaining wa~cer, reducing the sheet weight to 7.8 g (184 g/m )~ Crest~node thicknesses were 6.4 mm/0.25-0.37 mm.

In a blender originally containing 3.5 L of water were blended for 15 min 30 g of MPD-I floc and 225 g of an aqueous slurry of 2G-T fibrids prepared llB69Z5~

as described above at 19.5~ solids. An 1100 mL
aliquot of the resulting mixture was added to 2 L of tap water and blended or 10 min. A 1200 mL aliquot of the final mixture was converted to a 20 cm x 20 cm (8.0 x 8.0 in) handsheet, as above, to form a wet handsheet of 87% water.
The wet handsheet, after diamond-embossing, was passed through the dielectric heater at 0~4 m/minO Resultant crest/node thicknesses were ~.3 mm/0~25 mm.
The expanded por~ions of this sheet, as well as of the above all 2G-T fibrid/floc sheet, contained paper-like layer~ of membranous elements and scatter~d expanded macroscopic cells.
EX~PLE IX
This example describes the preparation o MPD-I fibrid/MPD-I floc sheets which were dielec~rically heated without any embossing to provide very low densities~
W~t sheets containing about 83% water were prepared using a commercial Fourdrinier machine. The wet sheet was about 1.14 mm (0. 045 in) thick, and the fibrid/floc weight ratio wa's 60/40. Unlike the customary papermakiny proce'ss on this machine, the 2S dryer rolls were operated at sufficiently low temperature~ to prevent complete removal of water, and the we~-laid sheet was not calendered.
From the above product, specimens 20 cm x 10 cm (8.0 in x 4.0 in) were cut. Before exposure to dielectric heating, each was dipped in an aqueous deter~ent solution for a given time, and then wiped dry. Two dips used contained cetyl betaine (Product BCO-Du Pont) at 0.83 and i.65~, respectively. These were prepared by diluting ~.5 and 5~0 g, respectively, of 33 weight percent Product BCO with water until the solution weighed 100 g. The other two dips were of 2.5 and 7.5 weight percent LPS~
Lotion Soap (Calgon) in water. Columns headed "%
BCO" and ~l% LP~" in Table VII identify these dips, and the column labeled "Soak Time" identifies the length of time each specimen xemained in the specified dip~ The expanded specimens were quite irregular in thickness. "Expanded thickness" in Table VII is an average value; so the calculated ~Volume~' and "Density" are approximate~
While the dried, expanded sheets of Table VII could rela~ively easily be separated into thinner layers, they had sufficient structural integrity ~o permit handling, cutting, shaping, e~c. without layer lS separations. They are well-suited for use as flame retardant thermal or acoust.ic insulation~
A scanning electron microyraph at 20X
magnification o~ a cross section through a thickness of Item X-A (see FIG. 1) shows a multiplicity of layered membranous elements which join with and separate rom one another al: random forming a highly irregular~ three-dimensional network of numerous interleaved macroscopic cells with tapered edges throughout the thickness. The elements form a plurality of paper-like layers lying substantially horizontally in the plane of the sheet.

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6~;29 EXAMPL~ X
This example illustrates plain embossing as described hereinbefore and the effectiveness of the expanded sheet for thermal insu]ation.
Using a slurry in water of 60 wgt % MPD-I
fibrids and 40 wgt % ~P~-I floc, a sheet was prepared using a paper-making machineO It had 17 wgt % solids (83 wgt % water~ and had a dry basis weight of 208 g/m . A 20 x 20 cm (8.0 x 8.0 in) sample of the we~ sheet was plain-embossed and ~hen expanded by passage through the dielectric heater at 0.4 m/min.
It expanded immediat~ly ~.o provide crest/node thicknesses of 2,5 mm/0.25 mm. The apparent density is calculated to be 0O083 g/mL. Fig. 2 is a scanning electron micrograph at lOx of a thic~nes~
cross-section of the product on a line maximizing the appearance of unexpanded embossed portions and showing the cell structure of the invention in the expanded portions. About 90~ of the face area was expanded.
Seven of the expanded sheets were stacked to give a ~o~al thickness of 19 mm under 0.0138 kPa (0.002 lb/in ) pressure (total area basis).
Thermal conductivi~y at 25C was measured to be 0.035 W/moK using the method described by J. 1.
Cooper and M. S. F~ankosky in Journal of Coated Fabrics, Vol. 10, 107 (1980).
The expanded sheet of this example was heat-set unrestrained in a nitrogen atmosphere.
~eating from ambient to 255C occurred over a 90 minu~e interval, and 265C was maintained for an additional 15 minutes. Linear shrinkage as a result of this treatment was about 5%, and the crests diminished in thickness by about 33%. From these shrinkages the apparent density after shrinking is calculated to be 0.137 g/mL. At subsequent exposures to temperatures of 240C or less, there was essentially no shrinkageO

~5 ~5

Claims (20)

39 What I claim is:

1. A coherent expanded nonwoven sheet comprised of fibrids of a wholly synthetic polymer and optionally up to 80% by weight floc, the sheet having an apparent density of less than 0.16 g/mL and being comprised of a plurality of paper-like layers of membranous elements which join and separate at random throughout the thickness of the sheet to form expanded macroscopic cells.

2. An expanded nonwoven sheet of claim 1 containing 20-80% by weight fibrids and complementally 20-80% by weight floc, each not melting below 130°C.

3. An expanded nonwoven sheet of claim 1 containing 50-80% by weight fibrids and 20-50% by weight floc.

4. An expanded nonwoven sheet of claim 1 wherein discrete areas comprising up to 50% of the total area of the sheet are not expanded.

5. An expanded nonwoven sheet of claim 1 wherein the fibrids are comprised of an aromatic polyamide.

6. An expanded nonwoven sheet of claim 5 wherein the fibrids are of poly(m-phenylene isophthalamide).

7, An expanded nonwoven sheet of claim 5 containing floc which is comprised of an aromatic polyamide.

8. An expanded nonwoven sheet of claim 7 wherein at least some of the floc has been pulped.

9. An expanded nonwoven sheet of claim 1 wherein the membranous elements form a three-dimensional, highly irregular network of numerous interleaved macroscopic cells with tapered edges.

10. Process for preparing a coherent expanded nonwoven sheet comprising preparing a wet mixture of fibrids of a wholly synthetic polymer, and optionally up to 80% by weight floc, forming a wet-laid nonwoven sheet of the fibrids on paper-forming equipment, maintaining water in the sheet, adding additional water if needed to increase the water content of the sheet to at least 60% by weight, and heating the wet sheet to vaporize the water rapidly and to expand the sheet to provide a product having an apparent density of less than 0.16 g/mL.

11. Process of claim 10 wherein the water content is maintained at at least 40% by weight.

12. The process of claim 11 wherein up to 50% of the total area of the sheet is embossed before expansion to provide a sheet which is expanded only in unembossed areas.

13. The process of claim 11 wherein the wet nonwoven sheet contains 20-80% by weight fibrids and complementally 20-80% by weight floc on a dry basis.

14. Process of claim 10 wherein the wet nonwoven sheet contains 50-80% by weight fibrids and 20-50% by weight floc on a dry basis.

15. Process of claim 10 wherein the fibrids are comprised of an aromatic polyamide.

16. Process of claim 15 wherein the fibrid5 are poly(m-phenylene isophthalamide).

17. Process of claim 16 wherein aromatic polyamide floc is used.

18. Process of claim 17 wherein poly(p-phenylene terephthalamide) floc is used.

19. Process of claim 17 or 18 wherein the floc is pulped before mixing with the fibrids.

20. Process of claim 10 wherein the rapid vaporization of water is induced by dielectric heating.