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BACKGROUND OF THE INVENTION
This invention relates to new and improved network structures and methods
for making such network structures, and particularly to network structures
and methods for making them by embossing or forming ribs in both sides of
a thermoplastic polymeric sheet in a particular manner so as to permit
spontaneous fibrillation or opening of the network structure upon drawing
in one direction or in two preferably perpendicular directions and to
provide a uniform open network structure having desirable strength
characteristics.
In the manufacture of networks, it has previously been proposed to form
continuous diagonal grooves in one direction in one side of a sheet of
plastic material and continuous diagonal grooves in the opposite direction
on the other side of the sheet so that upon subjecting the sheet to
biaxial stretching the thin parts of the sheet, at the crossing points of
the grooves, split and form perforations thereby opening the material into
a network. For example, see U.S. Pat. No. 3,488,415 to A. G. Patchell et
al. The networks therein disclosed are formed in such a manner as to have
thicker masses at the points where the ridges cross, which behave as
discrete areas of reinforcement, since on biaxial stretching or drawing of
the embossed sheet the thick areas where the ridges cross orient only to a
limited extent if at all. The tensile strength and tear characteristics of
such a network are relatively poor because the presence of the unoriented
thick areas weakens the tensile strength and tear resistance of the
network so prepared, and such a network is not uniform in appearance. U.S.
Pat. No. 3,500,627 to Charles W. Kim discloses making yarn by forming on
one side of a ribbon of plastic material a plurality of parallel filament
forming ribs and on the other side a plurality of fibril forming
cross-ribs arranged at an acute angle to the filament forming ribs. The
ribbon is then uniaxially oriented and mechanically fibrillated by means
of a toothed fibrillating device to break the fibril forming ribs and form
a yarn having fibrils extending laterally therefrom. Use of mechanical
fibrillation makes reproducing uniform network structures very difficult.
SUMMARY OF THE INVENTION
The subject invention relates to network structures and methods of making
network structures by forming on one side of a sheet of thermoplastic
polymeric material a plurality of parallel continuous main ribs
interconnected by webs of reduced thickness, and forming on the other side
of the sheet a plurality of parallel continuous tie ribs interconnected by
webs of reduced thickness. The tie ribs are formed at any desired angle to
the main ribs. The main ribs preferably have a cross-sectional area which
is at least 1.5 times as great as the cross-sectional area of the tie
ribs, and the main ribs have a height which is at least three times as
great as the thickness of the webs between the main ribs. By forming the
main ribs and tie ribs with a cross-sectional area ratio of at least 1.5:1
and a main rib height to web thickness ratio of at least 3:1, it is
possible, among other things, to spontaneously open or fibrillate the
ribbed sheet into a network by drawing, and to orient the main ribs
continuously and uniformly, thereby making the main ribs very strong. It
is this feature which provides a network structure having high tensile
strength in the direction parallel to the main ribs. Additionally, by
having continuous main ribs which are uniformly oriented, the tear
strength in the direction across the main ribs is greatly enhanced.
After the main and tie ribs are formed in the plastic sheet the sheet is
drawn in a direction to effect orientation of the main ribs continuously
and uniformly, and may be drawn in two different, preferably
perpendicular, directions to orient both the main and the tie ribs. For
example, when the main ribs are formed in the machine direction and the
tie ribs are formed in the cross-machine direction a network structure may
be formed with only one draw, in this instance in the machine direction.
Alternatively, a more open network structure can be formed by sequential
or simultaneous drawing in both the machine and cross-machine directions.
In sequential drawing of a sheet having main ribs in the machine direction
the first draw is customarily in the cross-machine direction. Upon
drawing, the thinnest areas in the sheet namely the areas where the webs
between the main ribs cross the webs between the tie ribs, become oriented
and normally open up spontaneously, leaving a uniform pattern of holes or
voids in the sheet. Under some conditions and levels of draw the web
openings may not occur during the initial draw but may occur only during
the subsequent perpendicular draw. In any event, the web openings occur
spontaneously and thus there is no need for mechanical fibrillation. This
spontaneous fibrillation or opening of the webs converts or forms the tie
ribs into tie filaments and the main ribs into main filaments.
Hereinafter, the term tie ribs shall be used to refer to the tie ribs
embossed on the sheet which are ordinarily interconnected by webs. After
the webs split or open up, the tie ribs are separated and will be referred
to as tie filaments. Likewise, the main ribs are referred to as main ribs
while interconnected by webs, but after the webs split or open, the main
ribs are separated and will be referred to as main filaments. These main
filaments are continuous if in the machine direction, or if at an angle to
the machine direction, are continuous from one edge of the sheet to the
other.
It has been found that highly desirable strength characteristics are
obtainable in a network structure having main filaments in one direction
crossed by tie filaments in another direction wherein the main filaments
are dominant in size so that all, or substantially all, of the orientation
at the cross-over points of the main and tie filaments is applicable to
the main filaments. The tie filaments are normally smaller and are usually
oriented to provide sufficient structural integrity for the network
structure, tending to keep it flat and prevent folding, thus maintaining
the main filaments in parallel and uniformly spaced relation. The single
layer plastic network structures thus formed are dimensionally stable,
self-supporting, easy to handle, and have high tensile strength in the
direction of the main filaments and high tear resistance in the other
direction. Such nets are particularly useful for reinforcing paper
products and nonwoven fabrics based on staple fibers and for covering
absorbent pads.
Additionally, the network structures so formed may be made into multi-layer
fabrics by bonding together two or more layers of network structures
having the same or different configurations so that the main filaments
cross in various directions to provide a multi-layered product having
certain desired strength characteristics. For example, orthogonal
constructions can be made wherein the main filaments of one layer cross at
90.degree. to the main filaments of another layer to simulate the
appearance and physical properties of woven fabrics and to provide high
strength and tear resistance in two directions. Diagonal constructions,
wherein the main filaments of the two layers cross preferably at about
90.degree. to each other with the main filaments of both layers being at
an angle to the machine direction of the fabric, possess stretch and
recovery properties in the machine direction similar to those of knitted
fabrics: Fabrics made from three or more layers of networks each having
the main filaments in different directions have excellent dimensional
stability, high strength and tear resistance in all directions and high
burst strength. For example, triaxial constructions, wherein a diagonal
construction is utilized having interposed between the two layers a
network having main filaments formed in the machine direction, provide
high bursting strength with minimum weight. Isometric constructions,
wherein the main filaments of at least four layers are positioned at about
45.degree. angles to each other, provide strength in all directions with
dimensional stability heretofore unattainable in woven, knit or other
nonwoven fabric structures with equivalent unit weight.
Additionally, the subject network structures which have main filaments in
the machine direction can be made into monofilaments, tapes or yarns by
separating the network structure into strips which may be subsequently
fibrillated and twisted or bulked to entangle the main filaments of the
strips. If desired, the strips may also be crimped or false twisted.
Other advantages of the present invention will be apparent from the
following detailed description of the invention when considered in
conjunction with the following detailed drawings, which drawings form a
part of the specification. It is to be noted that the drawings illustrate
only typical embodiments of the invention and are therefore not to be
considered limiting of its scope, for the invention may admit to other
equally effective embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective schematic view illustrating apparatus for embossing
ribs on both sides of an advancing sheet of plastic material in accordance
with the principles of this invention.
FIG. 2 is an enlarged perspective view of a portion of the embossed sheet
shown in FIG. 1.
FIG. 3 is an enlarged perspective view of a portion of an embossed sheet
having main ribs which are spaced relatively far apart and have relatively
deep grooves therebetween and tie ribs which are spaced close together and
have relatively shallow grooves therebetween.
FIG. 4 is an enlarged perspective view of a portion of another embossed
sheet having main ribs which are spaced relatively close together and have
shallow grooves therebetween and tie ribs which are spaced relatively far
apart and have relatively deep grooves therebetween.
FIG. 5 is an enlarged perspective view of a portion of the top of a network
structure obtained after drawing and orienting the embossed sheet shown in
FIG. 2 in two directions.
FIG. 6 is an enlarged perspective view of the bottom of the network
structure shown in FIG. 5.
FIG. 7 is a plan view illustrating a portion of a network structure having
main filaments in the machine direction and tie filaments in the
cross-machine direction.
FIG. 8 is a plan view illustrating a portion of a network structure having
main filaments in the cross-machine direction and tie filaments in the
machine direction.
FIG. 9 is a plan view illustrating a portion of a network structure having
main filaments formed at an angle to the machine direction with tie
filaments formed in the machine direction.
FIG. 10 is a plan view illustrating a portion of a network structure having
main filaments formed at an angle to the machine direction with tie
filaments formed perpendicular to the main filaments.
FIG. 11 is a perspective schematic view illustrating apparatus for making
multi-layer fabric structures in accordance with the principles of the
subject invention.
FIG. 12 is a perspective schematic view illustrating other apparatus for
making multi-layer fabric structures in accordance with the principles of
this invention.
FIG. 13 is a plan view illustrating a portion of a three-layer triaxial
fabric with one layer having main filaments formed in the cross-machine
direction and the other two layers having their main filaments formed at
equal opposite angles to the cross-machine direction main filaments.
FIG. 14 is a plan view illustrating a portion of two-layer diagonal fabric
formed by bonding together two network structures having their main
filaments formed at equal opposite angles to the machine direction and
desirably, but not necessarily, perpendicular to each other.
FIG. 15 is a plan view illustrating a portion of a four-layer isometric
fabric made by bonding together in any desired order the two layers shown
in FIG. 11 and the two layers shown in FIG. 14.
FIG. 16 is a perspective view illustrating apparatus for reinforcing paper,
foil, non-woven fabrics or films by utilizing a central network structure
made in accordance with the principles of this invention.
FIG. 17 is a view illustrating apparatus for making network structures into
yarns.
FIG. 18 is an enlarged view of the leasing rods of FIG. 17 used to separate
or tear the network structure into strips.
FIG. 19 is an enlarged plan view of a portion of a strip before
fibrillation.
FIG. 20 is an enlarged plan view of the strip of FIG. 19 after fibrillation
illustrating the broken tie filaments.
FIG. 21 is a view of a portion of an air jet interlaced multi-filament yarn
having protruding side fibers.
FIG. 22 is a view of a portion of a bulked entangled multi-filament yarn.
DESCRIPTION OF THE INVENTION
Referring now to FIGS. 1 and 2, there is shown an embossing roll 21 having
a plurality of grooves 22 formed therein for forming a plurality of
transverse main ribs 23 on an advancing sheet of thermoplastic polymer
material 24 with the ribs 23 being interconnected by webs 25 of reduced
thickness. Another embossing roll 26 having a plurality of annular or
helical grooves 27 formed therein is positioned opposite roll 21 for
forming a plurality of longitudinal tie ribs 28 on the other side of the
sheet 24 with the tie ribs being interconnected by webs 30 of reduced
thickness. The embossing rolls 21 and 26 rotate in the direction shown by
the arrows. There are a variety of different ways to effect the double
embossing described herein. One method is to feed a molten plastic sheet,
such as 24, coming directly from an extrusion die into the nip of two
counter-rotating embossing rolls, such as 21 and 26, which are urged
toward each other by facilities which are not shown. The desired
separation between the rolls and ultimately the thickness of the embossed
sheet is readily controlled by regulating thickness of the extruded sheet
entering the embossing rolls and the pressure between the two embossing
rolls. The roll temperatures typically are internally controlled and serve
to quench and solidify the molten plastic forming the desired embossed
patterns on each side.
Alternatively, a previously cast flat sheet or film may be re-heated to its
softening temperature and then advanced through a pair of embossing rolls,
such as 21 and 26. Another method may utilize a polymer which is in powder
form and which is introduced into the nip between two heated rolls, not
shown, to permit the heated rolls to melt and soften the polymer and form
it into a sheet which is then advanced between two embossing rolls such as
21 and 26. An additional method is to pass a previously cast flat sheet or
film between two embossing rolls pressed together under a sufficiently
high pressure that the embossed patterns are pressed into the sheet
without having to melt or soften the sheet. It is evident that many
embossing techniques may be utilized to carry out the principles of this
invention. Alternatively, instead of using embossing rolls to form the
desired ribbed configuration on both sides of a sheet, such a
configuration may also be accomplished by using a pair of relatively
movable concentric dies such as shown and described in the aforementioned
U.S. Pat. No. 3,488,415.
It has been found that the most advantageous range of the ratio of the
cross-sectional area of the main ribs, to the cross-sectional area of the
tie ribs is between 1.5:1 to 100:1 with the ratio of the height of the
main ribs to the thickness of the webs between the main ribs being at
least 3:1 or greater. This relationship permits subsequent drawing and
orientation steps to form the ribbed sheet into a network structure having
uniformly spaced main filaments oriented uniformly and continuously along
their lengths and being quite uniform in cross-section. With continuous
tie ribs and cross-sectional area ratios less than 1.5, uniform continuous
orientation of the main filaments is not obtained except with special
polymers, or special embossing conditions or methods as described for
example, in U.S. Pat. No. 3,922,329, because of a tendency for there to be
thick areas where the main filaments and tie filaments cross and for those
areas to remain either unoriented or only slightly oriented on drawing. As
shown in FIG. 2, the cross-sectional area A.sub.1 of the main ribs and the
cross-sectional area A.sub.2 of the tie ribs each includes the web area
adjacent to the base of each respective rib. Also identified in FIG. 2 is
the height T.sub.1 of the main ribs and the thickness T.sub.2 of the webs
interconnecting the main ribs.
The cross-sectional shape of the ribs formed may vary. They may be
semi-circular, rectangular, triangular, truncated, or any other desired
shape. Furthermore, the shapes of the main and tie ribs may be the same or
different. Likewise, the shape and size of the grooves separating the main
or tie ribs is not critical. The grooves may be narrow so that the ribs
are close together, or wide so that the ribs are more widely separated.
Furthermore, the tie ribs may be spaced farther apart than the main ribs
or vice versa. The size of the openings in the network structure may be
controlled to some degree by controlling the spacing of the main and tie
ribs.
Referring to FIG. 3, there is shown a portion of an embossed sheet
identified generally as 36 having a plurality of main ribs 37 formed on
one side of the sheet, and a plurality of tie ribs 38 formed on the other
side of the sheet in a direction perpendicular to the direction of the
main ribs 37. The main ribs 37 are spaced farther apart than the tie ribs
and have relatively wide webs 39 of reduced thickness therebetween. The
tie ribs 38, however, have almost no web therebetween, but there is an
area or line of reduced thickness at 40 between each pair of adjacent tie
ribs. Note that the height 41 of the main rib 37, which is measured from
the web 39 to the top of the main rib 37, is much greater than the height
of the tie rib 38 which is measured from the bottom of 40 to the top of
tie rib 38. However, referring now to FIG. 4 there is shown an embossed
sheet generally designated as 43 having a plurality of closely spaced main
ribs 44 formed in one direction on one side of the sheet, and a plurality
of spaced apart tie ribs 46 formed on the other side of the sheet in
another direction. The web which is the line or area of reduced thickness
47 between the main ribs 44 is now very small, while the web 48 between
the tie ribs 46 is relatively larger. Thus, it can be seen that the
invention is relatively independent of the spacing between the ribs and
the height of the ribs.
Additionally, the direction of the main ribs is not critical. The main ribs
may be formed in the machine direction of the sheet, or transverse to the
machine direction, i.e., 90.degree. thereto, or at any angle in between.
With the main ribs formed in either the machine direction or the
transverse direction, orienting the main ribs along their longitudinal
axes is easily accomplished by use of either a conventional linear
differential speed draw roll device or a conventional tenter. Likewise, if
the embossed ribs are diagonal to the machine direction, orientation of
the ribs and net formation may be achieved using the same type of
equipment. In orienting main ribs which are formed at an angle to the
machine direction along their longitudinal axes, it is sometimes
advantageous to utilize a long draw gap linear drawing unit so that upon
drawing in the machine direction the sheet is permitted to neck down and
cause orientation of the main ribs principally along their longitudinal
axes. In drawing in such a manner, it is usually desirable that the linear
draw be preceded by a cross-machine direction orientation by passing the
sheet through a tenter.
The direction of the tie ribs on the reverse side of the sheet should be at
an angle to that of the main ribs, which in many cases is desirably
90.degree., but can also be other angles. Any angle between about
15.degree. and 90.degree. between the directions of the main ribs to the
tie ribs is acceptable.
When the embossed sheet having a first pattern of continuous main ribs on
one side and a second pattern of continuous tie ribs on the other side is
drawn, the thin areas of the sheet, namely the areas where webs 25 and 30
cross, spontaneously split, forming openings. After the second draw is
completed, a network structure such as or similar to that shown in FIGS. 5
and 6 is achieved. The main ribs 23 of the embossed sheet shown in FIGS. 1
and 2 have been separated into main filaments 53 which are oriented
continuously and uniformly. The tie ribs 28 have also been separated and
oriented into tie filaments 54 which interconnect the main filaments 53
and keep them uniformly spaced apart. FIG. 6 shows the back side of the
network shown in FIG. 5 wherein it can be seen that the tie filaments 54
may extend continuously and without interruption across the main filaments
53.
In drawing the embossed sheet, the preferred amount of draw would depend on
such factors as the polymer employed, the embossing pattern employed, and
the degree of separation of the main filaments desired in the final
network structure. Customarily, the first drawing or orientation step
involves drawing the embossed sheet in a direction generally transverse to
the direction of the main ribs to cause orientation of the thinner areas
of plastic material between the main ribs. Referring, for example, to the
embossed sheet shown in FIG. 2, since the main ribs 23 are formed in the
cross-machine direction, the first draw would normally be, but is not
necessarily, in the machine direction. This draw could be effected by
using conventional linear differential speed draw rolls. This orientation,
which is usually 1.5.times. or greater, generally results in incipient or
actual voids or openings being formed between the main ribs with the
formation of small tie filaments spanning the openings between the main,
as yet unoriented, ribs or filaments. Drawing to an extent greater than
five times its original length (5.times.) at this stage is usually
undesirable since cross-orientation of the polymer at the cross-over
points of the main ribs and tie ribs may occur. This may interfere with
the desired uniform orientation of the main filaments in the subsequent
drawing steps.
As an alternative, it may be desirable to carry out an initial draw such
as, for example, up to 2.times., in the direction of the main ribs prior
to the drawing step transverse to this direction. This initially orients
and strengthens the main ribs and tends to prevent any possible distortion
or development of cross-orientation of the polymer in the cross-over areas
during the transverse orientation.
The second orientation step is normally carried out in a direction
generally parallel to the main ribs. Thus, referring again to the embossed
sheet shown in FIG. 2; the second orientation would be in the
cross-machine direction. This transverse drawing step, could be carried
out on a conventional tenter. The transverse draw causes orientation of
the main ribs along their longitudinal axes and separation of the small,
connecting tie filaments. The amount of draw will determine the strength
and size of the resulting main filaments. It can vary from as low as
1.5.times. to 10.times. or greater. The maximum draw will depend on the
orientation characteristics of the polymer employed, among other things.
Temperatures for drawing will vary depending upon the polymer employed but
generally will be slightly lower than those employed for orienting flat
sheets of the same polymer. While reference has been made to first and
second sequential drawing steps, both draws may be carried out
simultaneously, if desired.
The network structures produced by the foregoing methods contain as desired
longitudinal, transverse or oblique oriented main filaments interconnected
by normally lower denier, oriented tie filaments, with the main filaments
having orientation continuously over their lengths. Examples of the
different configurations of network structures that can be made are shown
in FIGS. 7, 8, 9 and 10. In FIG. 7, a network structure is shown having
main filaments 73 extending in the machine direction, the direction of the
arrow, and tie filaments 74 being formed in the cross-machine direction
90.degree. to the machine direction. In FIG. 8, the main filaments 75 are
formed transverse to the machine direction, indicated by the arrow, and
the tie filaments 76 are formed parallel to the machine direction. In FIG.
9, the main filaments 77 are formed at an angle to the machine direction,
shown by the arrow, and the tie filaments 78 are formed parallel to the
machine direction. Alternatively, the tie filaments 78 may be formed in
the cross-machine direction or so they are perpendicular to the main
filaments such as shown in FIG. 10. When the main filaments 77 are formed
at an angle of 75.degree. or less to the machine direction, in order to
orient such filaments, it is sometimes desirable to draw in the machine
direction while permitting necking down of the network structure.
Ordinarily, in making this configuration, the cross-machine draw in a
tenter, if desired, comes first, followed by the machine direction draw
allowing neck-down. It is apparent that many other configurations of
network structures may be made in accordance with the principles of this
invention, having the main filaments at any desired angle wherein maximum
tensile strength is desired and the tie filaments formed at an angle
relative to the main filaments.
The network structures described herein have good tensile strength in the
direction of the main filaments which reflects the degree and uniformity
of orientation along the length of these filaments. This strength is lower
in the opposite direction because of the smaller size of the
interconnecting tie filaments. The tear strength is high in the direction
transverse to the main filaments because of the strength of the main
filaments. It is to be noted that the network structures such as shown in
FIGS. 5 and 6 have tie filaments which are continuous and cross over the
main filaments or integrally join the main filaments, without there being
notches at the junctures as is characteristic of many network structures
prepared by previous methods. Such notches at the junctions or cross-overs
enable a network to tear easily in either direction.
The subject network structures, while useful as single layer network
structures, may also be employed to prepare very useful multi-layer fabric
structures. Referring to FIG. 11, there is shown one network structure,
generally designated as 81, having main filaments 82 formed in the machine
direction and tie filaments, not shown, formed in the cross-machine
direction being laminated or bonded to a second network structure,
generally designated as 83, having main filaments 84 formed in the
cross-machine direction. Tie filaments are not shown in any of the network
structures shown in FIGS. 11-16 to facilitate illustration and description
of the fabric structures. Nevertheless, the tie filaments are present in
each network and may be assumed to be as shown in FIGS. 7-10 or as
previously described. One way of bonding the two network structures 81 and
83 together is to pass them through rolls 79 and 80 into a preheater 84 to
heat the networks under tension without adversely affecting the
orientation thereof and then advance them into the nip of two heated
calender rolls 86 and 87 to bond the plastic materials to each other.
Rolls 79 and 80 rotate very slightly slower than rolls 86 and 87 to
maintain the networks 81 and 83 under tension during heating to avoid loss
of orientation. Likewise, it may be desirable to use a tenter, a series of
closely spaced rolls or other means to prevent lateral shrinkage of the
networks in this area. This bonding or lamination process forms a
two-layer fabric which has the appearance and physical properties of a
woven fabric having high strength and good tear resistance in both the
machine and cross-machine directions. Such a fabric has substantially no
stretch, in the machine and cross-machine directions, but does stretch on
the bias.
Three or more layer fabrics can also be prepared with the main filaments of
each being formed in different directions to provide fabrics having
excellent dimensional sta | | |
