U.S. patent number RE31,825 [Application Number 06/567,809] was granted by the patent office on 1985-02-05 for method of making nonwoven fabric and product made thereby having both stick bonds and molten bonds.
This patent grant is currently assigned to Scott Paper Company. Invention is credited to Charles R. Mason, David K. Osteen, Lawrence Vaalburg.
United States Patent |
RE31,825 |
Mason , et al. |
February 5, 1985 |
Method of making nonwoven fabric and product made thereby having
both stick bonds and molten bonds
Abstract
The method of autogenously bonding a nonwoven web formed
predominantly of thermoplastic fibers is characterized by the steps
of directing heat into the web from only one surface thereof to
preheat the web, and then directing the preheated web through a
bonding nip formed between opposed rolls, one of said rolls being
hotter than the other roll, being capable of heating the web
surface it engages to a temperature above the melt point of the
thermoplastic fibers and being positioned to engage the surface of
the web opposite the one into which heat was directed during the
preheating operation; said webs being preheated by means completely
independent of the opposed rolls that form the bonding nip, and
most preferably by infrared panels. The nonwoven product formed in
accordance with this method also forms a part of the instant
invention.
Inventors: |
Mason; Charles R. (Hammonton,
NJ), Osteen; David K. (Williamstown, NJ), Vaalburg;
Lawrence (Vineland, NJ) |
Assignee: |
Scott Paper Company
(Philadelphia, PA)
|
Family
ID: |
26857678 |
Appl.
No.: |
06/567,809 |
Filed: |
January 3, 1984 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
161270 |
Jun 20, 1980 |
04315965 |
Feb 16, 1982 |
|
|
Current U.S.
Class: |
428/198; 156/181;
156/290; 156/308.4; 156/309.9; 156/324; 442/409; 604/370 |
Current CPC
Class: |
D04H
1/58 (20130101); D04H 1/54 (20130101); Y10T
442/69 (20150401); Y10T 428/24826 (20150115) |
Current International
Class: |
D04H
1/54 (20060101); D04H 001/54 (); D04H 003/14 () |
Field of
Search: |
;156/181,290,308.4,309.9,322 ;264/119,126,280 ;428/198,288,296 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Cannon; James C.
Attorney, Agent or Firm: Kane; John W. Yamaoka; Joseph
H.
Claims
What is claimed is:
1. A method of autogenously bonding a nonwoven web formed
predominantly of thermoplastic fibers, characterized by the steps
of directing heat into the web from only one surface thereof to
preheat the web, and then directing the preheated web through a
bonding nip formed between opposed rolls, one of said rolls being
hotter than the other roll, being capable of heating the web
surface it engages to a temperature above the melt point of the
thermoplastic fibers and being positioned to engage the surface of
the web opposite the one into which heat was directed during the
preheating operation; said web being preheated by means completely
independent of the opposed rolls that form the bonding nip.
2. The method of claim 1 characterized by forming the bonding nip
between an embossing roll having raised land areas on its surface
and a back-up roll having a resilient surface, said embossing roll
being the hotter roll.
3. The method of claim 2 characterized by the step of employing
infrared radiation upstream of the bonding nip to preheat the
web.
4. The method of claims 1, 2 or 3, characterized by heating the
surface of the web with the hotter roll to form autogenous bonds
that are predominately melt bonds penetrating only partially
through the web thickness, and forming autogenous bonds on the
preheated surface that are over 90% stick bonds.
5. The method of claim 4 characterized by forming autogenous bonds
on the preheated surface of the web that are substantially 100%
stick bonds.
6. The method of claim 4 characterized by forming the bonded web at
a speed in excess of 30.48 m/minute (100 ft/minute).
7. The method of claim 4 characterized by forming the bonded web at
a speed in excess of 91.44 m/minute (300 ft/minute).
8. A method of autogenously bonding a nonwoven web formed
predominantly of thermoplastic fibers and having a basis weight no
greater than about 0.0339 kg/m.sup.2 (1 oz./yd..sup.2),
characterized by the steps of directing heat into the web from only
one surface thereof to preheat the web, and then directing the
preheated web through a bonding nip formed in part by a heated roll
that is capable of heating the web surface it engages to a
temperature above the melt point of the thermoplastic fiber and
being positioned to engage the surface of the web opposite the one
into which heat was directed during the preheating operation for
creating autogenous bonds that, on the engaged surface, are
substantially melt bonds penetrating only partially through the web
thickness.
9. The method of claim 8 characterized by the step of establishing
the bonding nip between said heated roll and an opposed back-up
roll having a lower surface temperature than said heated roll, and
controlling the temperature of the opposed rolls, as well as the
time and pressure in the bonding nip to form over 90% stick bonds
on the preheated surface of the web.
10. The method of claim 9 characterized by forming substantially
100% stick bonds.
11. The method of claim 8 characterized by forming the bonded web
at a speed in excess of 30.48 m/minute (100 ft/minute).
12. The method of claim 8 characterized by forming the bonded web
at a speed in excess of 91.44 m/minute (300 ft/minute).
13. The method according to claim 8, 9, 10, 11 or 12 characterized
by providing the surface engaged by the hotter roll with autogenous
bonds that are virtually all melt bonds extending only partially
through the web thickness.
14. A nonwoven web made according to the method of claim 1.
15. A nonwoven web made according to the method of claim 1, and
having a cross-machine-direction wet tensile energy absorption
level of at least about 3.15 m-kg/m.sup.2 (80
inch-gram/in.sup.2).
16. A nonwoven web made according to the method of claim 1 and
having a cross-machine-direction wet tensile energy absorption
level of at least about 3.15 m-kg/m.sup.2 (80 inch-grams/in..sup.2)
and a cross-machine-direction wet tensile strength exceeding 9.83
kg/m (250 gms./in.).
17. A nonwoven web made according to the method of claim 9.
18. A nonwoven web made according to the method of claim 9, and
having a cross-machine-direction wet tensile energy absorption
level of at least about 3.15 m-kg/m.sup.2 (80
inch-gram/in.sup.2).
19. A nonwoven web made according to the method of claim 9, and
having a cross-machine-direction wet tensile energy absorption
level of at least about 3.15 m-kg/m.sup.2 (80 inch-grams/in..sup.2)
and a cross-machine-direction wet tensile strength exceeding 9.83
kg/m (250 gms./in.).
20. A method of making an autogenously bonded web comprising the
steps of:
(a) forming a nonwoven web consisting predominantly of
thermoplastic fibers;
(b) preheating the formed web from one surface thereof;
(c) directing the preheated web to an embossing station comprising
a heated embossing roll and a backup roll having a resilient
surface;
(d) passing the web through the nip formed by the heated embossing
roll and the backup roll, the heated embossing roll contacting the
other surface of the preheated web; and
(e) controlling the temperature of the preheating step, the
temperature of the heated embossing roll and the bonding pressure
so that predominately stick bonds are formed in said one surface of
the web and predominately melt bonds are formed in said other
surface of the web.
21. The method of claim 20 additionally comprising the step of
controlling the temperature of the backup roll.
22. An autogenously bonded web made according to the method of
claim 20.
23. The autogenously bonded web of claim 22 characterized in that
the autogenous bonds in said one surface are over 90% stick
bonds.
24. The autogenously bonded web of claims 22 or 23 characterized in
that the autogenous bonds in said other surface are over 80% molten
bonds.
25. A method of making an autogenously bonded web comprising the
steps of:
(a) forming a nonwoven web consisting predominately of
thermoplastic fibers;
(b) preheating the formed unrestrained web from one surface
thereof, while the web is unrestrained;
(c) directing the preheated web to an embossing station comprising
a heated embossing roll and a backup roll having a resilient
surface;
(d) passing the web through the nip formed by the heated embossing
roll and the backup roll, the heated embossing roll contacting the
other surface of the preheated web; and
(e) controlling the temperature of the preheating step, the
temperature of the heated embossing roll and the bonding pressure
so that predominately stick bonds are formed in said one surface of
the web and predominately melt bonds are formed in said other
surface of the web. .[.26. An autogenously bonded nonwoven web,
said web, prior to bonding being weaker in the
cross-machine-direction than in the machine-direction,
characterized in that the autogenous bonds on one surface include
substantially continuous molten bonds extending in a direction, in
the plane of the web, for reinforcing the web in the
cross-machine-direction, said molten bonds extending only partially
through the thickness of the web, said bonded web having a
cross-machine-direction wet tensile energy absorption level of at
least about 3.15 m-kg/m.sup.2 (80 inch-grams/in.sup.2) and a
cross-machine-direction wet tensile strength exceeding 9.83 kg/m
(250 gms/in.)..]. .[.27. the autogenously bonded nonwoven web of
claim 26 characterized by a basis weight no greater than about
0.0339 kg/m.sup.2..]. .[.28. The autogenously bonded nonwoven web
of claim 26 or 27, characterized in that the opposed web surface
has autogenous bonds that are over 90% stick bonds..]. .[.29. The
autogenously bonded nonwoven web of claim 28, characterized in that
the opposed web surface has autogenous bonds that are substantially
100% stick bonds..]. .[.30. The autogenously bonded nonwoven web of
claim 28, characterized in that the autogenous bonds on said one
surface are over 80% molten
bonds..]. .Iadd.31. A method of autogenously bonding a nonwoven web
formed predominately of thermoplastic fibers, characterized by the
steps of preheating the web to establish a temperature gradient
through the web thickness so that one surface is cooler than the
other and then directing the preheated web through a bonding nip
formed between opposed rolls, one of said rolls being hotter than
the other, being capable of heating the web surface it engages to a
temperature above the melt point of the thermopolastic fibers and
being positioned to engage the cooler surface of the preheated web;
said web being preheated by means completely independent of the
opposed rolls that form the bonding nip. .Iaddend. .Iadd.32. The
method of claim 31 characterized by forming the bonding nip between
an embossing roll having raised land areas on its surface and a
back-up roll having a resilient surface, said embossing roll being
the hotter roll..Iaddend.
Description
TECHNICAL FIELD
This invention relates generally to the field of nonwoven fabrics,
and in particular to a method of thermally bonding a nonwoven
fabric and to the autogenously bonded fabric produced thereby.
BACKGROUND ART
Nonwoven fabrics have become quite popular for many different end
uses wherein textile-like properties, such as softness,
drapeability, strength and abrasion resistance are desired. A very
significant market for nonwoven fabrics, and in particular nonwoven
webs including predominately textile-length fibers, is for facing
sheets in products such as disposable diapers. These sheets are
placed in direct contact with the baby's skin, and therefore, at
least the surface of the nonwoven fabric contacting the skin should
be extremely soft and nonabrasive to prevent chafing.
Of particular interest for use as facing sheets are carded
non-woven webs having a low basis weight of no more than about
0.0339 kg/m.sup.2 (1 oz./yd.sup.2). A representative method of
forming such a carded nonwoven web is disclosed in U.S. Pat. No.
3,772,107, issued to Gentile et al, and assigned to Scott Paper
Company. This type of web is characterized by highly directional
properties in view of the fact that the fibers tend to align in the
direction of web formation. Although some fibers are rearranged
into the cross-machine-direction during web formation, the fibrous
web generally is considerably weaker in the cross-machine-direction
than in the machine-direction.
Carded nonwoven webs commonly are stabilized by some type of
bonding operation, with an effort being made to improve the
cross-machine-direction wet tensile energy absorption level
(CDWTEA) without creating harsh, abrasive or stiff characteristics
that would make the webs unsuitable for use as a diaper facing
sheet, or for that matter, for other uses wherein soft, nonabrasive
surface characteristics are desired. Efforts to-date have met with
moderate success. However, for future generation diapers, higher
levels of softness, surface feel and drapeability are desired.
These desired tactile properties need to be achieved in webs having
the necessary strength and stretch characteristics to permit them
to function adequately as a facing sheet. This is an extremely
challenging objective since bonding the web to achieve the
necessary strength and stretch characteristics (i.e. TEA) generally
is accompanied by reduced, or impaired tactile properties.
Tensile energy absorption (TEA) is the area under the stress/strain
curve at web failure, and represents the energy absorbed by the
product as it is stretched to failure.
The TEA and strength levels reported in this application can be
determined on a Thwing Albert Electronic QC Tensile Tester,
"Intelect 500", with a 160 ounce load cell, and being set at 99%
sensitivity. The test is carried out by clamping a 0.0254 m (1
inch).times.0.1778 m (7 inch) rectangular test sample in opposed
jaws of the tensile tester with the jaw span being 5 inches. The
jaws are then separated at a crosshead speed of 0.127 m (5 inches)
minute until the sample fails. The digital integrator of the
tensile tester directly computes and displays tensile strength
(grams/inch), TEA (inch-grams/inch.sup.2) and stretch (%) at
failure. Wet TEA, strength and stretch values are obtained by
immersing the sample in water prior to testing.
One very desirable technique for stabilizing nonwoven webs is to
employ a predominate amount of thermoplastic fibers in the
construction, and then to autogenously bond the web structure by
the application of heat and pressure to the web. Thus, in these
webs the thermoplastic fibers actually constitute the bonding
medium, and no additional binder needs to be added.
Many different arrangements have been suggested for autogenously
bonding webs formed of thermoplastic fibers, as exemplified by U.S.
Pat. Nos. 3,542,634 (Such et al); 3,261,899 (Coates); 3,442,740
(David); 3,660,555 (Rains et al); 3,855,046 (Hansen et al)
4,005,169 (Cumbers); 4,035,219 (Cumbers); 4,128,679 (Pohland); and
4,151,023 (Platt et al).
Both the Coates' U.S. Pat. No. (3,261,899) and the Hansen et al
U.S. Pat. No. (3,855,046) suggest preheating the web prior to
actually establishing the desired bond structure in a subsequent
pressure bonding operation. Although Coates does broadly suggest
infrared heating a web prior to passing it through a heated
pressure nip (see Ex. V), there is no suggestion of controlling the
bond structure through the web to achieve any particular balance of
properties.
The Hansen et al U.S. Pat. No. (3,855,046) describes a web formed
of thermoplastic continous filaments that is preheated by the same
smooth-surfaced roll 30 that cooperates with the heated embossing
roll 32 to establish the bonding nip. Thus, control of the
preheating temperature independent of the bonding parameters cannot
be achieved, since the temperature to which the smooth-surfaced
roll 30 is heated must generally be balanced between the
requirements for preheating on the one hand, and the requirements
for establishing the desired bond structure. Even though other
parameters can be varied to regulate the amount of preheating, such
as controlling the amount of wrap of the web about the
smooth-surfaced roll 30 upstream of the bonding nip, it is believed
that the desired independent control of the preheating and bonding
operation is extremely difficult to obtain with this type of
arrangement. In fact, in forming low basis weight webs of less than
about 0.0339 kg/m.sup.2 (1 oz./yd.sup.2) the bond structure on each
side of the web is disclosed as being generally the same; having an
unfused bond area coefficient (ubac) of less than about 65%. The
high percentage of fused, or melt bonds, established in these
latter webs is not believed to provide the necessary tactile
characteristics (e.g., softness, drapeability, surface smoothness,
etc.) being sought after in products such as new generation diaper
facing structures.
DISCLOSURE OF INVENTION
The method of this invention employs a unique controlled gradient
bonding technique to establish autogenous (thermal) bonds within a
nonwoven web structure formed predominately, and most preferably
entirely of thermoplastic fibers. The method of this invention is
characterized by the steps of directing the web to a preheating
station at which heat is directed into the web from only one
surface thereof; directing the preheated web through a bonding nip
formed between opposed rolls; one of said rolls being heated to a
temperature close to or exceeding the melt point of the
thermoplastic fibers and the other roll (hereinafter referred to as
"the back-up roll") being maintained at a lower temperature below
the melt point of the thermoplastic fibers; the hotter roll being
positioned to engage the surface of the web opposite the one into
which heat was directed to preheat the web; said web being
preheated by means completely independent of the opposed rolls
providing the bonding nip.
Most preferably the most highly heated roll is an embossing roll
having raised land areas on its surface, and, for low basis weight
webs no greater than about 0.0339 kg/m.sup.2 (1 oz./yd..sup.2), the
back-up roll should be resilient to provide a more uniform
distribution of pressure then can be achieved with a non-resilient
roll. The preheating step preferably is carried out by employing
infrared radiation, which has been found to provide extremely
reliable temperature control.
The term "melt bond" or "molten bond", as used throughout this
application, refers to a bond established by melting fibers and is
characterized by an appearance wherein the identity of individual
fibers in the bond zone is substantially obliterated; taking on a
film-like appearance.
The term "stick bond" as used throughout this application, refers
to a bond established by heating the fibers to a tacky state in
which they are capable of sticking to each other, but wherein the
physical fiber form or appearance is still retained; albeit
generally in a somewhat flattened state.
It is extremely important in this invention that the preheating
operation take place from the side of the web opposite that engaged
by the most highly heated bonding roll; i.e., a heated embossing
roll in the preferred embodiment. This preheating operation is
believed to establish a temperature gradient through the web (the
preheated surface of the web being the hottest) that aids, or
provides for more efficient control of heat transfer through the
web during the bonding operation from the surface engaged by the
heated embossing roll than would otherwise be the case if the web
were not preheated, or if the web were preheated from the same
surface engaged by the heated embossing roll. The manner of
preheating in accordance with this invention permits the formation,
during the subsequent bonding operation, of autogenous bonds on the
preheated surface that are well over 90% (preferably 100%) "stick"
bonds, without the need for imparting excessive, web-damaging heat
energy into the opposite surface of the web through the heated
embossing roll.
Prior to this invention it was extremely difficult to control heat
transfer into and through the web to tie down fibers on the surface
opposite the heated embossing roll without also over-melting the
polymeric fibrous material. Over-melting can cause the polymer to
melt and separate, thereby forming strength-reducing and
stretch-reducing "pin holes" in the web structure. In the present
invention the autogenous bonds formed on the surface engaged by the
heated embossing roll are mostly (i.e., generally over 80%) melt
bonds (without over-melting) that extend only partially through the
web thickness to impart the necessary strength and stretch
characteristics to the web.
The method of determining the percentage of autogenous stick bonds
and autogenous melt bonds in the web will be described later in
this application.
The preheating operation in this invention aids in establishing the
desired temperature gradient through the web prior to the bonding
operation to permit, upon bonding, the establishment of the desired
stretch and strength properties, primarily through the formation of
melt bond extending partially through the web from the surface
engaged by the heated embossing roll, while at the same time
preventing "fuzzing" from the preheated surface of the web by
establishing autogenous bonds on the preheated surface that are
predominantly "stick" bonds.
The non-woven fabrics in accordance with this invention are
characterized by being two-sided, i.e., they have different
properties on their opposed surfaces. The high percentage of
autogenous bonds that are melt bonds extending into the fabric from
one surface creates a somewhat harsh surface feel, as compared to
the soft, smooth surface feel created by the high percentage of
autogenous bonds that are stick bonds on the opposed surface.
However, this high percentage of autogenous melt bonds extending
partially through the web thickness is needed to establish the
desired cross direction wet tensile energy absorption level
(CDWTEA) in the fabric. The high percentage of stick bonds on the
opposite surface of the web establishes the necessary abrasion
resistance to prevent fiber "fuzzing" without adversely affecting
the surface tactile properties.
In this invention the two-sided gradient bond construction
described above can be achieved, and actually is achieved in low
basis weight webs no greater than about 0.0339 kg/m.sup.2. These
low basis weight webs have been found to be most suitable for use
as facing sheets in products such as disposable diapers. When the
sheet is used as a diaper facing the surface in which the
autogenous stick bonds predominate is placed outwardly to contact
the wearer's skin, since it's the one with the best tactile
properties (i.e., it is the softest and smoothest). The opposite
surface containing the high percentage of autogenous melt bonds is
thus kept out of contact with the wearer's skin. Although the
benefits of this invention are known to be signficant in low basis
weight web construction no greater than about 0.0339 Kg/m.sup.2, it
is believed that the teachings of this invention may also be used
to control the properties of higher basis weight webs.
Many different types of thermoplastic fibers may be utilized in
this invention; the polyolefins being particularly useful. Most
preferably this invention employs polypropylene fibers having a
length in excess of 0.0254 m (1 inch). A suitable fiber usable in
this invention is a 0.0508 m (2 inch), 3 denier polypropylene fiber
having a melt point of 167.degree. C. (332.6.degree. F.).
Other objects and advantages of this invention will become apparent
by referring to the accompanying drawings, taken in conjunction
with the description of the best mode for carrying out the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic elevation view of an arrangement for carrying
out the preferred method of this invention;
FIG. 1A is a fragmentary elevation view of the embossing roll
illustrating the preferred arrangement of the land areas;
FIG. 2 is a scanning electron microscope photograph, at a
magnification of 20, showing one side of an autogenously bonded web
in accordance with this invention;
FIG. 3 is a scanning electron microscope photograph, at a
magnification of 100, showing a bond area on the side of the web
depicted in FIG. 2.
FIG. 4 is a scanning electron microscope photograph, at a
magnification of 20, showing the side of the web opposite that
shown in FIG. 2; and
FIG. 5 is a scanning electron microscope photograph, at a
magnification of 50, showing a bond area on the side of the web
depicted in FIG. 4.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring to FIG. 1, a schematic representation of equipment for
carrying out the method of this invention is illustrated. A
web-forming system 10, such as a carding system, is employed to
initially form a fibrous web 12. When a carding system is used the
fibers are aligned predominately in the machine direction of web
formation, as indicated by arrow 13. The preferred fibers employed
to form the web 12 are 100% polypropylene, 3 denier, 0.0508 m (2
inch) length sold under the trademark Marvess by Phillips Fibers
Corporation, a subsidiary of Phillips Petroleum Company. Other
thermoplastic fibers can be employed, and it is also believed that
the webs of this invention can be formed from a fiber blend wherein
some of the fibers are not thermoplastic. However, it is believed
that this invention requires that a preponderance, by weight, of
the fibers be thermoplastic textile-length fibers greater than
0.0064 meters (1/4-inch) in length, and preferably, greater than
0.0254 meters (1-inch) in length.
The web 12, as initially formed, is quite weak, since the fibers
are held together only by the entanglement that naturally occurs
when the fibers are deposited on a forming surface, and by the
cohesive, or frictional forces between contacting fibers. When the
web is formed by a carding or similar operation it is particularly
weak in the cross-machine-direction in view of the predominate
fiber alignment in the machine-direction of web formation.
After the web is formed it is directed through a preheating station
which, in the illustrated embodiment, includes a bank of infrared
panels 14. These panels are operated to direct infrared radiation
into the web 12 from only the surface 18 thereof. The infrared
panels preheat the web, and the web then is directed immediately to
the pressure nip of a bonding station provided by opposed rolls 20
and 22. Most preferably the roll 20 is a metal embossing roll, and
is heated to a temperature greater than the melting point of the
polypropylene fibers. The back-up roll 22 preferably is a resilient
roll formed with a one-inch thick polyamide (Nylon) cover 23 having
a 90 durometer-Shore A. Preferably this back-up roll is heated in a
controlled manner by a suitable surface heating means (e.g.
infrared panels) to a temperature below the melting point of the
thermoplastic fibers, and most preferably below the stick point of
such fibers.
It is extremely important in this invention to preheat the web from
the side opposite that engaged by the heated metal embossing roll
20. This preheating operation is believed to establish a
temperature gradient through the web (the preheated surface 18
being the hottest) that provides for more efficient heat transfer
control through the web in the subsequent bonding operation than
would otherwise be the case if the web were not preheated at all,
or if the web were preheated only from the same surface engaged by
the heated embossing roll 20. By preheating the web surface 18 to
establish a temperature gradient through the web thickness it is
easier to control the rate of heat transfer into and through the
web 12 from the surface 25, which is the surface engaged by the
most highly heated embossing roll 20. This permits the reliable
formation of autogenous bonds on the preheated surface 18 that are
well over 90% stick bonds, and most preferably 100% stick bonds,
without the need for imparting excessive, web-damaging heat energy
into the opposed surface 25 through the heated embossing roll
20.
Prior to this invention it was extremely difficult to control heat
transfer into and through the web to form the necessary structure
for tying down the fibers on one web surface, without, at the same
time, overbonding the polymeric fibrous material from the opposed
surface. Overbonding actually caused the polymer to melt and
separate from itself, thereby forming strength-reducing and
stretch-reducing "pinholes" in the web structure. In the present
invention the bonding operation is carried out to form the high
percentage of stick bonds on the preheated surface 18 with the
autogenous bonds formed on the opposed surface 25 being mostly
(i.e., generally over 80%) melt bonds that extend only partially
through the web thickness, and this is achieved without overbonding
the web. The partially penetrating melt bond construction is the
major contributor to the strength and stretch characteristics of
the web.
In the most preferred embodiment of this invention applicants rely
primarily upon heat transfer through the web from the heated
embossing roll 20 to establish the desired stick bond construction
on the preheated surface 18. In this regard the preferred method is
carried out with the backup roll 22 heated to a temperature below
the stick point of the thermoplastic fibers. Heating the backup
roll 22 has been found to be highly advantageous in enhancing the
control of heat transfer into and through the web, to thereby
permit better control over the ultimate bond structure than would
otherwise be the case if the backup roll 22 were not heated.
It is particularly desirable to employ a back-up roll 22 that is
resilient when forming webs 12 in the low basis weight range of no
more than about 0.0339 Kg/m.sup.2 (1 oz./yd..sup.2). This is
important since the resilience of the roll tends to provide a more
uniform pressure distribution then would otherwise be the case if
the back-up roll 22 were non-resilient. The control over pressure
distribution is quite important, since, in conjunction with the
temperature of the bonding rolls 20, 22 and the speed of travel of
the web 12 through the bonding nip, the pressure is an important
variable in controlling the bond structure of the web.
FIG. 1A shows a preferred pattern of land areas 24 extending
transversely across the embossing roll 20 to form transverse molten
bonds for enhancing the cross-machine-direction strength of the
bonded web. These land areas preferably occupy less than 50% of the
embossing roll area, and most preferably occupy approximately
20-25% of this area to thereby establish an autogenous bond area
through web surface 25 that occupies less than 50% of the web's
surface area, and most preferably approximately 20-25% of the web's
surface area. Although these land areas are shown as continuous,
some discontinuities can exist while still achieving the necessary
molten bond structure for achieving the most desired
cross-machine-direction strength and energy absorption levels for
diaper facing sheets, as will be set forth later in this
application. Reference throughout this application to molten bonds
being "substantially continuous" is intended to cover molten bonds
which are either completely continuous, or which have limited
discontinuities in them. After the web has been directed through
the bonding nip established between the rolls 20 and 22 it can then
be rolled up into a parent roll (not shown) for subsequent storage
and/or reuse.
In accordance with the best mode for carrying out this invention
the temperature of the infrared panels 14, as well as the
temperature of the heated embossing roll 20 and the back-up roll 22
are coordinated with the fiber characteristics, the basis weight of
the web 12, the line speed and the bonding pressure to form a
Z-direction bond gradient wherein the autogenous bonds on the web
surface engaged by roll 20 are predominately (preferably over 80%)
melt bonds that extend partially through the web thickness to
provide the desired strength and stretch in the web, and wherein
the autogenous bonds on the opposite surface engaged by the
resilient back-up roll 22 are well over 90% stick bonds to tie down
surface fibers without adversely affecting tactile properties. In
fact, in accordance with this invention the autogenous bonds on the
web surface 18 engaged by the resilient back-up roll 22 can be
controlled to be substantially devoid of melt bonds (they will be
almost entirely stick bonds) while at the same time achieving an
improved depth of penetration of melt bonds from the opposite
surface 25 to achieve a desired cross-machine-direction wet tensile
energy absorption level of approximately 3.15 m-kg/m.sup.2 (80
in-grams/in..sup.2) and higher for webs used as a diaper facing or
for similar applications. Most preferably these webs also have a
cross-machine-direction wet tensile strength of at least 9.83 kg/m
(250 gms./in).
Referring to FIGS. 4 and 5, a partial plan view of the resilient
roll side 18 of the nonwoven fabric 12 in accordance with this
invention is depicted. The bond areas in the surface are indicated
at 32, and the characteristics of these bond areas are most clearly
seen in FIG. 5. Note that the regions between the bond area 32, as
viewed in FIG. 4, show little, if any signs of heat exposure, and
the fibers in these regions tend to maintain their original,
nonflattened configuration. These regions are believed to enhance
the tactile properties of the surface 18.
Turning to FIG. 5, the autogenous bond areas 32 are characterized
by an extremely high degree of stick bonds. That is, the individual
fibers in the bond region, although somewhat flattened, maintain
their individual fiber integrity and form, and can be traced
throughout the web structure. Note that there are only a very few
regions in the bond area 32 (considerably less than 10% of the bond
area) wherein the fiber integrity is in anyway obliterated. This
high degree of stick bonds is believed to impart extremely
desirable tactile properties (e.g., softness and smoothness) to the
surface 18 of the web.
Turning now to FIGS. 2 and 3, the embossing roll side 25 of the web
12 is depicted. Referring specifically to FIG. 2, the web is
characterized by a series of autogenous bonded areas 42 with
substantial unbonded regions between them. The bonded areas 42 have
the general configuration of the land areas 24 on the embossing
roll 22 (i.e., they are in the form of undulating lines), and
include a high percentage of melted, or fused, bonds having a
film-like appearance, as is best seen in FIG. 3. The fibers
actually are melted in these completely fused areas to form molten
bonds that partially penetrate through the thickness of the web 12.
In this invention an improved control over the depth of melt
bonding is obtained without adversely effecting the tactile
properties on the surface of the web engaged by the resilient roll.
The improved control permits consistent formation of webs having
desired tactile properties with a cross-machine-direction wet
tensile strength of at least 9.83 kg/m (250 gms./in.), and a
cross-machine-direction wet tensile energy absorption level of at
least 3.15 m-kg/m.sup.2 (80 in-grs/in..sup.2), at speeds in excess
of 30.48 m/minute (100 ft./minute). In fact, webs having the above
balance of tactile and strength properties have been formed at
speeds in excess of 91.44 m/minute (300 ft./minute) employing the
unique method of this invention. Prior to this invention applicants
were not able to obtain the above strength and TEA values, along
with acceptable tactile properties, at a web speed as slow as 25.91
m/minute (85 feet/minute).
The method for determining the percentage of autogenous bonds that
are stick bonds, and the percentage of autogenous bonds that are
melt bonds will now be described. The percentage of stick bonds is
defined herein as "the unfused bond area coefficient" (UBAC), and
the percentage of melt bonds is calculated as (100-UBAC).
In this invention the percentage of autogenous bonds that are stick
bonds (UBAC) on the surface 18 is substantially greater than 90%,
and preferably 100%. On the opposed web surface 25 the UBAC should
be less than 20% (the percentage of autogenous bonds that are melt
bonds should exceed 80%).
The UBAC is determined in the following manner:
Ten 1-inch square samples are taken at random from different bonded
parts of the web. A square grid, 2.5 inches on a side, is divided
into ten equal segments and is then placed over a scanning electron
microscope photograph of the bond area of each sample, at
100.times.magnification. It is possible that the size of the square
grid will need to be modified slightly depending upon the overall
dimension of a bonded area in each of the photographs. However, the
grid size should be chosen so that it covers as much of the bonded
area in each photograph as is possible. It is believed that the
specific values of UBAC described and claimed herein is accurate
within the range of grid size variations that might be necessary
due to variations in the particular dimensions of the bond area
that are acceptable in the webs of this invention.
The bond area in each sample is allocated to one of the following
three categories (1) 0-33% fusion; (2) 33-66% fusion or (3) 66-100%
fusion. The percent fusion of a given bond area is determined by
first characterizing each region of the bond area underline each
segment of the grids as "fused" or "unfused". A region is
characterized as being "unfused" if the presence of individual
filaments can be identified anywhere in the region. Likewise a
region of the bond area is characterized as being "fused" if the
presence of individual fibers cannot be identified anywhere in that
region. The percent fusion of each of the bond areas under
investigation is the number of regions of the bond area
characterized as "fused" (each region underlying a grid segment
with no individual fibers being identifiable) divided by 10 (the
total number of grid segments). The UBAC is that percentage of the
total number of bond areas that are characterized as 0-33%
fused.
The above described test is very similar to that described in
column 14 of U.S. Pat. No. 3,855,046, discussed earlier in this
application.
The following table indicates one set of parameters for carrying
out the method of this invention, and the product properties
obtained. However, this example is by way of illustration only; the
scope of the invention being defined by the claims appended
hereto.
______________________________________ IR Temp. of Bank of 6 Panels
Back- Up- Down- Emb. up Stream Stream Line Roll Roll Three Three
Fiber Speed Temp. Temp. Panels Panels
______________________________________ Marvess (m/sec) (.degree.C.)
(.degree.C.) (.degree.C.) (.degree.C.) olefin staple .66 191.7
101.7 685 343.3 fiber-type CO1. (polypropylene) 2 inch, 3 denier
______________________________________ CD CDW- Weight CDWT Stretch
TEA MDWT ______________________________________ (Kg/m.sup.2) (Kg/m)
(%) m- (Kg/m) .030 15.9 49.7 kg/m.sup.2 85.2 5.37
______________________________________
* * * * *