Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~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
'~
6~3~
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.
6~Z~
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
80 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~
12
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~
13
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
14
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
14
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.
69;2~
16
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 appropriate0 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~
~6~
17
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~
18
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
18
~36~
19
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.
19
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6~2
23
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~
2~
times in the dielectric heater removed ~ore water,
but did not fuxther expand the specimensO In fact,
full expansion occurred in each case only a short
distance past the first electrode, relative to total
distance (7.6 cm) separating the electrodesO
The expanded por~ions of the sheets
contained many expanded macroscopic cells of
membranous elements similar to FIG. 2.
24
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j~ Z k ~
v~ J ~ 0~ ~ C~ ~ ~ r~
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H I ~ O 00 r~ ~ r-l
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26
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.
26
27
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o~
U~
E ~ c~ C`
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W ~5~ O o o o o
O I
C~ O C`~
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28
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
IV-B 1 1 1 ND ND ND ND 1 ND ND ND
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
28
~36
29
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~
29
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I co a~ co, , o~ , ol , ol
~ ~ ~1 o O O u~
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~ ~ ! ~ ~ _, O .~ $ o o
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a a~ ~ O
~ ~ ~ o ~ o o ~
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E~ I D D D D I D D D D D
3U
31
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
32
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
33
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~
34
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
34
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.
36
C`l o~
e o o o o~ g g g
~o
o O ~ O O o o o
a~
~_ 1~ 00 ~ CO o O o o
P ~ ~ ~ ~ ~ ~ 4~
E~ P~ ~ o o
1;~ h ~ C
p:~ ~ ~ ~ ~ ~ ~ ~ ~;t ~;Ir
tJ~ ~3:
~ M
~ ~3 ~ ~
Z
l ~.~ ~ ~ ~ ~ ~ ~ c~ a~
x ~ e 5 0 o C; O O ,I O
O O O C~ C~ O O C~
P~ u~ u~
c~
Pe
0
c~
o o o
¢ ~ c,
X PC X S~ PC
36
6~;29
37
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
38
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
