Note: Descriptions are shown in the official language in which they were submitted.
21~5~6
SELF-ADHERING ELASTIC COMPOSITE
Backqround of the Invention
Field of the ~nvention
The present invention relates to a self-adhering, elastic composite
which may be used to impart elastic properties to flexible,
non-elastic substrates.
Description of the Related Art
Vulcanized rubber or synthetic rubber elastic bands or threads have
typically been used to provide elastic properties to flexible
substrates by attaching the elastic to the substrate using materials
such as thread, yarn, or adhesive in a sewing, weaving, or adhesive
process. The attachment of elastic bands to the underlying flexible
substrate generally consumes additional materials and manufacturing
resources and poses problems in the industry. Natural vulcanized or
crosslinked synthetic rubbers are difficult to feed continuously and
at high speeds, in view of their tendency to stretch and relax during
mechanical processes, resulting in articles with broken elastics,
articles having an elastic with too great or too little tension, or
articles with partly attached elastics. Further, adhesives which
have typically been used in the past to bond elastic bands to a
flexible substrate generally have had poor adhesion to the elastic
bands resulting in the separation of the elastic during any
substantial flexing of the substrate.
For example, it is known to apply an adhesive, in the form of a
spray, along the length of continuous elastic bands contacting a
continuous substrate web. The elastic bands are generally in a
stretched condition while the adhesive is applied in a nonstretched
216~ ~86
- condition. Alternatively, the stretched elastic bands can be coated
with the adhesive prior to contact with the substrate web.
Typical elastic materials are generally crosslinked,
three-dimensional networks of vulcanized natural or synthetic rubber.
The crosslinked three dimensional structure comprises a reversible
energy storing network. Stress applied to the substance results in a
strain or deformation of the three dimensional network which stores
energy, applied during stress, which can be spontaneously
substantially recovered upon the removal of the stress.
Pressure sensitive adhesives, in contrast to elastic materials,
generally require a different set of properties. Upon the
application of stress or force to a pressure sensitive adhesive, in
the form of pressure, the adhesive must deform in order to come into
intimate contact through viscous flow with the surface of a substrate
in order to form adhesive bonds by van der Waals attraction. In
order to preserve the adhesive bond, upon removal of the stress or
pressure, the adhesive must not recover from the deformation.
Substances that are pressure sensitive adhesives generally exhibit
viscous flow and, therefore, inherently do not substantially recover
from such deformation.
Elastic materials, therefore, generally have minimal adhesive
properties, and pressure sensitive adhesives generally have minimal
elastic properties. Commonly available pressure sensitive adhesive
or elastic materials do not have the correct balance of properties
which would result in a truly suitable self-adhering elastic
material, since the molecular properties that result in elasticity
are those that commonly result in the absence of adhesive properties.
Attempts to prepare a single composition material that may be used as
a self-adhering elastic material has generally required a compromise
between the desired elastic and adhesive properties. Another problem
with such materials is that, upon aging, the material will suffer a
high loss of either the elastic or adhesive properties, or both.
i 8 ~
A need, therefore, exists for a self-adhering elastic material that
has a unique combination of properties combining both substantial
elastic and adhesive properties. Such a material should further be
capable of being continuously processed and applied to flexible
substrates at high speeds using automatic machines. A further need
exists for a self-adhering elastic material which, during flex, will
resist detachment from the substrate. Another need exists for a
self-adhering elastic material having adequate peel force which can
be attached with strong bonds to a flexible substrate at high machine
speed without breaking. Another need exists for a self-adhering
elastic material that substantially retains its elastic and adhesive
properties after aging.
SummarY of the Invention
The present invention concerns a self-adhering elastic composite
exhibiting both substantial elastic and adhesive properties that is
highly machine processable and which substantially retains its
elastic and adhesive properties with aging.
One aspect of the present invention concerns a self-adhering elastic
composite comprising an adhesive material and an elastic material,
wherein the elastic material is continuous along a relaxed length of
the self-adhering elastic composite, and the self-adhering elastic
composite exhibits desired elastic and adhesive properties.
One embodiment of such a self-adhering elastic composite has a
relaxed length and comprises an adhesive material attached to an
elastic material, wherein the elastic material is continuous along
the relaxed length of the self-adhering elastic composite, and
wherein the self-adhering elastic composite exhibits the following
properties:
a. the ability to be stretched at least about 50 percent of
the relaxed length;
b. an aged Creep value that is not more than 25 percent when
the self-adhering elastic composite is aged at about 72-F
for about 2 weeks when stretched at a 50 percent extension;
216~
c. an aged Creep value that is not more than 25 percent when
the self-adhering elastic composite is aged at about 110-F
for about 24 hours when stretched at a 50 percent
extension;
d. an aged Peel Force value that is not less than about 80
percent of the original Peel Force value when the
self-adhering elastic composite is aged at about 72-F for
about 2 weeks when stretched at a 50 percent extension; and
e. an aged Peel Force value that is not less than about 80
percent of the original Peel Force value when the self-
adhering elastic composite is aged at about 110-F for about
24 hours when stretched at a 50 percent extension.
Another embodiment of such a self-adhering elastic composite has a
relaxed length and comprises a first layer attached to a second
layer, wherein the first layer comprises an adhesive material, the
second layer comprises an elastic material continuous along the
relaxed length of the self-adhering elastic composite, and the
self-adhering elastic composite exhibits desired elastic and adhesive
properties.
Another embodiment of such a self-adhering elastic composite has a
relaxed length and comprises an adhesive material matrix attached to
and substantially encasing an elastic material continuous along the
relaxed length of the self-adhering elastic composite, the
self-adhering elastic composite exhibiting desired elastic and
adhesive properties.
In another aspect, the present invention concerns a gatherable
elastic laminate comprising a gatherable substrate attached to a
self-adhering elastic composite wherein the self-adhering elastic
composite exhibits desired elastic and adhesive properties.
In another aspectt the present invention concerns a disposable
absor-bent product comprising a self-adhering elastic composite
wherein the self-adhering elastic composite exhibits desired elastic
and adhesive properties.
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2 1 ~ 6
One embodiment of such a disposable absorbent product comprises a
liquid-permeable topsheet, a backsheet attached to the
liquid-permeable topsheet, an absorbent structure positioned between
the topsheet and the backsheet, and a self-adhering elastic composite
positioned between the topsheet and the backsheet wherein the
self-adhering elastic composite exhibits desired elastic and adhesive
properties.
Brief DescriDtion of the Drawinqs
Fig. 1 represents one embodiment of a self-adhering elastic composite
of the present invention.
Fig. 2 represents another embodiment of a self-adhering elastic
composite of the present invention.
Fig. 3 represents a disposable diaper according to the present
invention.
Fig. 4 represents a plot of the stress-strain force measurements of a
self-adhering elastic structure sample stretched using a tensile
tester.
Detailed Description of the Preferred Embodiments
In one aspect, the present inventlon is a self-adhering elastic
material which is a composite comprising an adhesive material and an
elastic material. It has been found that, by using two separate but
compatible materials in combination, one an adhesive material and the
other an elastic material, it is possible to prepare a self-adhering
elastic composite that exhibits improved adhesive and elastic
properties as compared to known materials.
As used herein, the terms ~self-adhering elastic material~,
~self-adherlng elastlc composite~, and other related terms are meant
to represent a material that exhibits both substantial adhesive and
elastic properties such that the self-adhering elastic material can
provide elastic properties to a flexible, non-elastic substrate
without any need for additional attachment means to attach the
21~4~
self-adhering elastic material to the flexible, non-elastic
substrate.
As used herein, the term "adhesive material~ is intended to mean a
material that is generally capable of bonding two other materials
together. Such bonding may result from the application of a pressure
force, in the case of a pressure sensitive adhesive material, or a
sufficiently high temperature, in the case of a hot-melt adhesive, to
contact and bond the adhesive material to a substrate. Specifically,
as used herein, an adhesive material is meant to be a material that
exhibits a Peel Force value, as described herein, that is greater
than about 300 grams per 25.4 millimeters of width of the adhesive
material. Suitably, the adhesive material also exhibits an Initial
Modulus value, as described herein, that is between about 1X106 to
about 4X106 dynes per square centimeter and a Stress at 50 Percent
Extension value, as described herein, that is between about 0.1x106
to about 4X106 dynes per square centimeter.
Materials suitable for use as the adhesive in the present invention
may be of any known type, such as a thermoplastic hot-melt adhesive,
a reactive adhesive, a pressure sensitive adhesive, or the like, as
long as the adhesive material exhibits the properties specified
herein. An example of a thermoplastic hot-melt adhesive includes a
synthetic rubber-based adhesive based on polystyrene-polybutadiene-
polystyrene chemistry and a tackifier based on hydrocarbon chemistry.A description of compositions of hot-melt adhesives can be found, for
example, in ~CRC Elastomer Technology Handbook~, edited by
Nicholas P. Cheremisinoff (CRC Press, 1993), Chapter 24, incorporated
herein by reference.
Examples of reactive àdhesives include crosslinked amine-epoxide
compounds or moisture-cured polyurethanes. The chemistry of such
reactive adhesives is known to those skilled in the art and may be
found, for example, in ~Contemporary Polymer Chemistry~, by
Harry Alcock and Frederick Lampe (Prentice Hall, 1990), incorporated
herein by reference.
21~54~G
The adhesive material is beneficially present in the self-adhering
elastic composite of the present invention in an amount of from
greater than 0 to less than 100 weight percent, suitably from about
1 to about 99 weight percent, and more suitably from about 5 to about
95 weight percent based on the total weight of the adhesive material
and the elastic material in the self-adhering elastic composite.
As used herein, the term "elastic material" is intended to mean a
material that is generally capable of recovering its shape after
deformation when the deforming force is removed. Specifically, as
used herein, an elastic material is meant to be a material that
exhibits a Peel Force value that is less than about 300 grams per
25.4 millimeters of width of the elastic material and is capable of
being stretchable to a stretched, biased length which is at least
about 125 percent, that is about 1.25 times, its relaxed, unbiased
length, and that will recover at least 40 percent of its elongation
upon release of the stretching, elongating force. A hypothetical
example which would satisfy this definition of an elastomeric
material would be a one (1) inch sample of a material which is
elongatable to at least 1.25 inches and which, upon being elongated
to 1.25 inches and released, will recover to a length of not more
than 1.15 inches. Many elastic materials may be stretched by much
more than 25 percent of their relaxed length and many of these will
recover to substantially their original relaxed length upon release
of the stretching, elongating force. This latter class of materials
is generally beneficial for purposes of the present invention.
Suitably, the elastic material also exhibits an Initial Modulus value
that is between about 3X106 to about 120X106 dynes per square
centimeter and a Stress at 50 Percent Extension value that is between
about 1X106 to about 20X106 dynes per square centimeter.
The term ~recover~ relates to contraction of a stretched material
upon termination of a biasing force following stretching of the
material by application of the biasing force. For example, if a
material having a relaxed, unbiased length of one (1) inch were
elongated 50 percent by stretching to a length of 1.5 inches, the
material would have been elongated 50 percent and would have a
2165 ~
stretched length that is 150 percent of its relaxed length. If this
exemplary stretched material contracted, that is, recovered to a
length of 1.1 inches after release of the biasing and stretching
force, the material would have recovered 80 percent (0.4 inch) of its
elongation.
Materials suitable for use as the elastic material herein include
diblock, triblock, or multiblock elastomeric copolymers such as
olefinic copolymers such as styrene-isoprene-styrene,
styrene-butadiene-styrene, styrene-ethylene/butylene-styrene, or
styrene-ethylene/propylene-styrene; polyurethanes, such as those
available from E. I. Du Pont de Nemours Co., under the trade name
Lycra polyurethane; polyamides, such as polyether block amides
available from Ato Chemical Company, under the trade name Pebax
- polyether block amide; or polyesters, such as those available from
E. I. Du Pont de Nemours Co., under the trade name Hytrel polyester.
The elastic material is beneficially present in the self-adhering
elastic composite of the present invention in an amount of from
greater than 0 to less than 100 weight percent, suitably from about
1 to about 99 weight percent, and more suitably from about 5 to about
95 weight percent based on the total weight of the adhesive material
and the elastic material in the self-adhering elastic composite.
A number of block copolymers can be used to prepare either the
adhesive or the elastic material useful in preparing the
self-adhering elastic composite of this invention. As will be
appreciated by one skilled in the art, the actual components used,
the relative amounts of each component used, and/or the process
conditions used to prepare the block copolymer will need to be
different so as to separately prepare an adhesive block copolymer
material or an elastic block copolymer material that each
respectively exhibit the properties desired herein.
Such block copolymers generally comprise an elastomeric midblock
portion and a thermoplastic endblock portion. The block copolymers
used in this invention generally have a three dimensional physical
21~S ~8~
crosslinked structure below the endblock portion glass transition
temperature (T9). The block copolymers are also generally
thermoplastic in the sense that they can be melted above the
endblock T~, formed, and resolidified several times with little or no
change in physical properties (assuming a minimum of oxidative
degradation).
One way of synthesizing such block copolymers is to polymerize the
thermoplastic endblock portions separately from the elastomeric
midblock portions. Once the midblock and endblock portions have been
separately formed, they can be linked. Typically, midblock portions
can be obtained by polymerizing di- and tri-unsaturated C~-C10
hydrocarbons such as, for example, dienes such as butadiene,
isoprene, and the like, and trienes such as 1,3,5-heptatriene, and
the like. When an endblock portion A is joined to a midblock
portion B, an A-B block copolymer unit is formed, which unit can be
coupled by various techniques or with various coupling agents C to
provide a structure such as A-B-A, which is believed to comprise two
A-B blocks joined together in a tail-to-tail A-B-C-B-A arrangement.
By a similar technique, a radial block copolymer can be formed having
the formula (A-B)nC, wherein C is the hub or central, polyfunctional
coupling agent and n is a number greater than 2. Using the coupling
agent technique, the functionality of C determines the number of
A-B branches.
Endblock portion A generally comprises a poly(vinylarene), such as
polystyrene, having an average molecular weight between 1,000
and 60,000. Midblock portion B generally comprises a substantially
amorphous polyolefin such as polyisoprene, ethylene/propylene
polymers, ethylene/butylene polymers, polybutadiene, and the like, or
mixtures thereof, having an average molecular weight between about
5,000 and about 450,000. The total molecular weight of the block
copolymer is suitably about 10,000 to about 500,000 and more suitably
about 200,000 to about 300,000. Any residual unsaturation in the
midb~ock portion of the block copolymer can be hydrogenated
selectively so that the content of olefinic double bonds in the block
copolymers can be reduced to a residual proportion of less than
8 G
5 percent and suitably less than about 2 percent. Such hydrogenation
tends to reduce sensitivity to oxidative degradation and may have
beneficial effects upon the desired properties of the material being
prepared.
Suitable block copolymers used in this invention comprise at least
two substantially polystyrene endblock portions and at least one
substantially ethylene/butylene midblock portion. Ethylene/butylene
typically comprises the major amount of the repeating units in such a
block copolymer and can constitute, for example, 70 percent by weight
or more of the block copolymer. The block copolymer, if radial, can
have three or more arms, and good results can be obtained with, for
example, four, five, or six arms. The midblock portion can be
hydrogenated, if desired.
Linear block copolymers, such as A-B-A, A-B-A-B-A, or the like, are
suitably selected on the basis of endblock content, large endblocks
being preferred. For polystyrene-ethylene/butylene-polystyrene block
copolymers, a styrene content in excess of about 10 weight percent is
suitable, such as between about 12 to about 30 weight percent. ~ith
higher styrene content, the polystyrene endblock portions generally
have a relatively high molecular weight. A commercially available
example of such a linear block copolymer elastic material is a
styrene-ethylene~butylene-styrene block copolymer which contains
about 13 weight percent styrene units and essentially the balance
being ethylene/butylene units, commercially available from the Shell
Chemical Company under the trade designation KRATON G1657 elastomeric
resin. Typical properties of KRATON G1657 elastomeric resin are
reported to include a tensile strength of 3400 pounds per square inch
(2 x 106 kilograms per square meter), a 300 percent modulus of
350 pounds per square inch (1.4 x 105 kilograms per square meter), an
elongation of 750 percent at break, a Shore A hardness of 65, and a
Brookfield viscosity, when at a concentration of 25 weight percent in
a toluene solution, of about 4200 centipoise at room temperature.
Both the adhesive material and the elastic material may be in the
form of a film, foam, fibrous web, threads, or the like. Suitably,
- 10 -
216S~6
both the adhesive material and the elastic material are in the form
of nonwoven materials. As used herein, the term ~nonwoven~ is
intended to mean that a material has been formed without the use of a
weavlng process.
A nonwoven film generally has the structure of a continuous sheet of
material, with no identifiable, individual fibers or the like.
Nonwoven films are known to be able to be prepared by a variety of
processes such as, for example, extrusion processes.
A nonwoven foam generally has the structure of being a dispersion of
a gas in a liquid or solid. Such foams are generally prepared by the
mechanical incorporation of air or another gas into a solution or
mixture of the material to be foamed.
A fibrous web generally has the structure of individual fibers or
threads which are interlaid, but not in an identifiable, repeatable
manner. Nonwoven webs are known to be able to be prepared by a
variety of processes such as, for example, meltblowing processes,
spunbonding processes, film aperturing processes, and staple fiber
carding processes. Nonwoven webs generally have an average basis
weight of not more than about 300 grams per square meter and suitably
have an average basis weight from about 3 to about 100 grams per
square meter.
A nonwoven thread or fiber generally has the structure wherein the
length is at least about 10 times greater than the width or radius.
Such nonwoven threads or fibers may be shaped or essentially round.
Nonwoven threads or fibers are known to be able to be prepared by a
variety of processes such as, for example, extrusion processes.
The adhesive material and the elastic material useful in the present
invention must be substantially compatible so they may be attached to
one another to form a self-adhering elastic composite. As used
herein, the term ~compatible~ is meant to represent that the adhesive
material and the elastic material can exist attached in intimate
contact with each other for long periods of time with no
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2 1 ~
substantially adverse affect of one on the other. In particular, the
attaching of the elastic material to the adhesive material does not
substantially affect the adhesive properties of the adhesive
material, and the attaching of the adhesive material to the elastic
material does not substantially affect the elastic properties of the
elastic material. Furthermore, the adhesive material and the elastic
material should be effectively attached to each other such that the
two materials may not be easily separated from each other during use
of the self-adhering elastic composite. Suitably, the Peel Force
value required to separate the adhesive material from the elastic
material, in the self-adhering elastic composite of the present
invention, will be greater than about 500 grams per linear inch.
As such, by attaching together an adhesive material and an elastic
material that are compatible, it is possible for the composite to
exhibit the desired elastic and adhesive properties as described
herein.
The self-adhering elastic composite of the present invention will be
a three dimensional structure having a length, a width, and a depth.
Since the self-adhering elastic composite will be capable of being
stretched, the self-adhering elastic composite will have a relaxed
length, width, and depth, respectively, as measured when the
self-adhering elastic composite is not under any tension or force,
such as a biasing force. The self-adhering elastic composite will
also exhibit various stretched lengths, widths, and depths,
respectively, as measured when the self-adhering elastic composite is
stretched under a tension or force.
In one embodiment of the present invention, the self-adhering elastic
composite will comprise at least two layers. At least one layer will
comprise an adhesive material. At least one layer will comprise an
elastic material. In the instance where the self-adhering elastic
structure consists of two layers, a first layer will comprise an
adhesive material and will be attached to a second layer comprising
an elastic material. Suitably, the first layer will consist
essentially of an adhesive material and the second layer will consist
essentially of an elastic material.
21~ ~8~
In such an embodiment, the self-adhering elastic composite is
suitably prepared by separately preparing or forming the adhesive
material layer and the elastic material layer and then attaching the
layers together. Alternatively, such a self-adhering elastic
composite may be prepared in a single process step such as by using a
multi-layered coextrusion process.
In conventional elastic laminating processes, the adhesive material
is typically sprayed or applied onto a pre-stretched elastic material
before lamination with a substrate. In these instances, the adhesive
material does not substantially contribute to the mechanical
properties of the laminate since the adhesive material is not a load
bearing member of the laminate. The adhesive material in these
instances generally functions solely as an attachment material.
In the present invention, however, the adhesive material, in addition
to the elastic material, also acts as a load bearing member since the
adhesive material may be substantially stretched along with the
elastic material while remaining attached to the elastic material.
Hence, in the self-adhering elastic composite of the present
invention, the elastic material essentially physically acts as a
recoverable spring and the adhesive material essentially physically
acts as a viscous dashpot in parallel with the elastic material.
Thus, both the adhesive material and the elastic material are load
bearing members with the adhesive material being viscous and the
elastic material being elastic. As such, the mechanical properties
of the self-adhering elastic composite of the present invention are
determined by both the adhesive and elastic material components, with
the adhesive material also acting as an attachment material to a
substrate.
In a beneficial embodiment of the present invention, the
self-adhering elastic composite will comprise three layers interlaid
on top of each other. The top and bottom layers will comprise an
adhesive material and the middle layer will comprise an elastic
layer.
21~ ~8~
Fig. 1 illustrates a self-adhering elastic composite according to
such an embodiment. Self-adhering elastic composite 10 includes two
adhesive material layers 11 attached to opposite sides of an elastic
material layer 12. Both of the adhesive material layers 11 and the
elastic material layer 12 are in the form of nonwoven films.
In another embodiment of the present invention, the self-adhering
elastic composite will comprise an adhesive material matrix attached
to and substantially encasing an elastic material. As used herein,
the term ~encase~ and related terms, are intended to mean that the
adhesive material substantially encloses or surrounds the elastic
material. Generally, in such an embodiment, the elastic material
will be in the form of fibers, threads, or a fibrous web which are
encased in an adhesive material matrix.
Such a self-adhering elastic composite of the present invention is
suitably prepared by first forming the elastic material and then
substantially encasing the elastic material with an adhesive material
matrix.
Fig. 2 illustrates a self-adhering elastic composite according to
such an embodiment. Self-adhering elastic composite 20 includes an
adhesive material matrix 21 and elastic material threads 22. The
adhesive material matrix 21 is seen to substantially encase the
elastic material threads 22 within the adhesive material matrix 22.
The self-adhering elastic composite is suitably extrudable such that
it can be formed into a nonwoven material. A nonwoven self-adhering
elastic composite may be in the form of a film, a web, or the like.
It is desirable that the self-adhering elastic composite of the
present invention exhibit both desirable elastic and adhesive
properties which is in contrast to known materials which generally
only exhibit either desirable elastic or desirable adhesive
properties.
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211~5 ~8~
Elastic properties desired of the self-adhering elastic composite of
the present invention include effective stretchability, aged Creep,
Initial Modulus, Stress at 50 Percent Extension, and Stress
Relaxation values.
The self-adhering elastic composite should exhibit the ability to be
stretched so as to be extended at least about 50 percent, suitably at
least about 75 percent, more suitably at least about 100 percent, and
most suitably at least about 200 percent, and up to about
10,000 percent of the relaxed length of the composite.
The Creep value is meant to represent the increase in relaxed length
exhibited by a material after having been extended about 50 percent
by stretching. As such, the Creep value is the difference between
the relaxed length after about 50 percent extension and the original
relaxed length before about 50 percent extension, divided by the
original relaxed length, and multiplied by 100 percent to give a
value in percent. An aged Creep value is meant to represent the
increase in relaxed length exhibited by a material after having been
extended about 50 percent by stretching and maintained at the about
50 percent extension for a period of time and under specific
temperature conditions.
One suitable aging condition is to maintain the 50 percent extended
material at about 72-F (about 22-C) for about 2 weeks, after which
the material exhibits an aged Creep value that is suitably not
greater than about 25 percent, more suitably not greater than about
20 percent, and most suitably not greater than about 15 percent.
Another suitable aging condition is to maintain the 50 percent
extended material at about 110-F (about 43-C) for about 24 hours,
after which the material exhibits an aged Creep value that is
suitably not greater than about 25 percent, more suitably not greater
than about 20 percent, and most suitably not greater than about
15 percent.
21 ~5~6
As used herein, all percentage extensions are expressed as a percent
of the unextended or relaxed length of a material. Thus, 100 percent
extension means that the untensioned material has been stretched to
twice its relaxed, or untensioned, length.
The Initial Modulus value of a self-adhering elastic composite is
meant to represent the amount of force initially needed to stretch
the self-adhering elastic composite and, thus, represents the
stiffness of the self-adhering elastic composite. It is desired that
the self-adhering elastic composite not exhibit an Initial Modulus
that is too low such that the self-adhering elastic composite will be
too soft and viscous. Also, it is desired that the self-adhering
elastic composite not exhibit an Initial Modulus that is too high
such that the self-adhering elastic composite causes red markings on
the body of a person wearing a disposable absorbent product including
the self-adhering elastic composite.
Thus, a self-adhering elastic composite of the present invention
generally exhibits an Initial Modulus value that is beneficially from
about 3X106 to about 120X106 dynes per square centimeter, suitably
from about 3X106 to about 80X106 dynes per square centimeter, and
more suitably from about 20X106 to about 80X106 dynes per square
centimeter, as measured according to the methods described in the
Test Procedures section herein.
The Stress at 50 Percent Extension value of a self-adhering elastic
composite is meant to represent the amount of force exerted by the
self-adhering elastic composite when it is elongated 50 percent by
stretching and, thus, generally represents the donning tension of a
disposable absorbent product including the self-adhering elastic
composite. It is desired that the sèlf-adhering elastic composite
not exhibit a Stress at 50 Percent Extension value that is too low,
since such may result in the slipping or falling, for example, of a
disposable absorbent product that includes the self-adhering elastic
composite. Also, it is desired that the nonwoven sheet not exhibit a
Stress at 50 Percent Extension value that is too high, since such may
cause the self-adhering elastic composite to exert too much force,
- 16 -
2 1~ ~ ~ 8 ~
for example, against a wearer of a disposable absorbent product
including the self-adhering elastic composite, thus causing
redmarking on the wearer.
Thus, the self-adhering elastic composite of the present invention
exhibits a Stress at 50 Percent Extension value that is beneficially
from about 3X106 to about 10x106 dynes per square centimeter,
suitably from about 3X106 to about 9x106 dynes per square centimeter,
and more suitably from about 4X106 to about 7X106 dynes per square
centimeter, as measured according to the methods described in the
Test Procedures section herein.
The Stress Relaxation value of a material is meant to represent the
decay or drop in tension exhibited by the material when it is allowed
to relax for 20 minutes in an elongated state after having been
elongated 50 percent by stretching. It is desired that the
self-adhering elastic composite of the present invention not exhibit
a Stress Relaxation value that is too high, since such will indicate
that the self-adhering elastic composite will lose too much tension
after having been subjected to a stretching force and, thus, will not
be able to provide sufficient tension to hold a disposable absorbent
product in place on a wearer.
Thus, the self-adhering elastic composite of the present invention
generally exhibits a Stress Relaxation value that is beneficially
less than about 35 percent, suitably less than about 30 percent, and
more suitably less than about 25 percent, as measured according to
the methods described in the Test Procedures section herein.
Adhesive properties desired of the self-adhering elastic composite of
the present invention include effective Peel Force values.
The Peel Force value is meant to represent the amount of force
required to detach two materials adhered together. It is desired
that the self-adhering elastic composite of the present invention not
exhibit a Peel Force value when attached to a non-elastic substrate,
such as a gatherable materlal, that is too low since such w111
~165~Q~
indicate that the self-adhering elastic composite will not
effectively adhere to the non-elastic substrate to which it is
attached and may detach during use. Also, it is desired that the
self-adhering elastic composite not exhibit a Peel Force value that
is too high, since such will generally indicate that the
self-adhering elastic composite will exhibit high viscous properties.
Thus, the self-adhering elastic composite of the present invention
generally exhibits a Peel Force value, when attached to a non-elastic
substrate, that is beneficially greater than about 350 grams per
25.4 millimeter width, suitably greater than about 400 grams per
25.4 millimeter width, and more suitably greater than about 450 grams
per 25.4 millimeter width as measured according to the methods
described in the Test Procedures section herein.
It is also desirable that the self-adhering elastic composite exhibit
desirable aged Peel Force values. An aged Peel Force value is meant
to represent the Peel Force value exhibited by a material after
having been extended 50 percent by stretching and maintained at the
50 percent extension for a period of time and under specific
temperature conditions.
One suitable aging condition is to maintain the SO percent extended
material at about 72-F (about 22-C) for about 2 weeks, after which
the material exhibits an aged Peel Force value that is suitably not
less than about 80 percent, more suitably not less than about
85 percent, and most suitably not less than about 90 percent, of the
Peel Force value of the material prior to such aging.
Another suitable aging condition is to maintain the 50 percent
extended material at about 110-F (about 43-C) for about 24 hours,
after which the material exhibits an aged Peel Force value that is
suitably not less than about 80 percent, more suitably not less than
about 85 percent, and most suitably not less than about 90 percent of
the Peel Force value of the material prior to such aging.
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~21~5~
The self-adhering elastic composite of the present invention may
generally be of any size or dimension as long as the self-adhering
elastic composite exhibits the desired elastic and adhesive
properties as described herein. When used in a disposable absorbent
product, a self-adhering elastic composite will typically have
dimensions of a width about 0.75 inch (about 1.9 centimeter), a
length of about 6 inches (about 15 centimeters), and a depth of about
0.02 inch (about 0.05 centimeter).
The self-adhering elastic composite of the present invention may also
be used or combined with other self-adhering elastic materials, with
the self-adhering elastic composite of the present invention being
used as a separate layer or as an individual zone or area within a
larger, composite self-adhering elastic material. The self-adhering
elastic composite of the present invention may be combined with other
self-adhering elastic materials by methods well known to those
skilled in the art, such as by using adhesives, or simply by layering
the different materials together and holding together the composite
materials with, for example, the self-adhering characteristics of the
different materials.
In another aspect of the present invention, it is desired to use a
self-adhering elastic composite to prepare an elastic laminate
comprising at least one gatherable material attached to at least one
self-adhering elastic composite.
Such an elastic laminate may be prepared by tensioning the
self-adhering elastic composite so as to elongate it, then attaching
the self-adhering elastic composite to at least one gatherable
material to form an elastic laminate, and then relaxing the elastic
laminate so that the gatherable material is gathered by relaxing the
self-adhering elastic composite. Typical conditions for attaching
the self-adhering elastic composite to the gatherable material
include overlaying the stretched self-adhering elastic composite and
the gatherable materials and applying heat and/or pressure to the
overlaid materials so as to create bonding sites between the overlaid
materials.
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~16~6
Various gatherable materials can be utilized in forming the elastic
laminate. Such gatherable materials can include, but are not limited
to, non-elastic fibrous webs such as carded non-elastic polyester or
non-elastic polypropylene fibrous webs, spunbonded non-elastic
polyester or polypropylene non-elastic fibrous webs, non-elastic
cellulosic fibrous webs, polyamide fibrous webs, and blends of two or
more of the foregoing. Particularly suitable is using the gatherable
material as outer cover layers with the self-adhering elastic
composite sandwiched as an intermediate layer between the gatherable
material layers. Basis weights for the elastic laminate are
beneficially between about 4 to about 100 grams per square meter and
suitably between about 6 to about 30 grams per square meter.
In another aspect of the present invention, a disposable absorbent
product is provided, which disposable absorbent product comprises a
liquid-permeable topsheet, a backsheet attached to the topsheet, an
absorbent structure positioned between the topsheet and the
backsheet, and a self-adhering elastic composite of the present
invention wherein the self-adhering elastic composite is typically
positioned between the topsheet and the backsheet.
While one embodiment of the invention will be described in terms of
the use of a self-adhering elastic composite in an infant diaper, it
is to be understood that the self-adhering elastic composite is
equally suited for use in other disposable absorbent products known
to those skilled in the art.
Fig. 3 illustrates a disposable diaper 1 according to one embodiment
of the present invention. Disposable diaper 1 includes a
backsheet 2, a topsheet 4, an absorbent structure 6 positioned
between the backsheet 2 and the topsheet 4, and a self-adhering
elastic composite 8 positioned between the backsheet 2 and the
topsheet 4. Self-adhering elastic composite 8 is a self-adhering
elastic composite according to the present invention. Specifically,
in the illustrated embodiment, self-adhering elastic composite 8 is
used as leg elastics positioned on either side of the absorbent 6 of
the diaper.
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Those skilled in the art will recognize materials suitable for use as
the topsheet and backsheet. Exemplary of miterials suitable for use
as the topsheet are liquid-permeable materials, such as spunbonded
polypropylene or polyethylene having a basis weight of from about
15 to about 25 grams per square meter. Exemplary of materials
suitable for use as the backsheet are liquid-impervious materials,
such as polyolefin films, as well as vapor-pervious materials, such
as microporous polyolefin films.
Disposable absorbent products, according to all aspects of the
present invention, are generally sub~ected during use to multiple
insults of a body liquid. Accordingly, the disposable absorbent
products are desirably capable of absorbing multiple insults of body
liquids in quantities to which the absorbent products and structures
will be exposed during use. The insults are generally separated from
one another by a period of time.
Test Procedures
A commercial tensile tester was used to stretch, at a stretch rate of
about 300 millimeters per minute and at a temperature of about 23-C,
a material sample, in the form of a film, that was about 3 inches
(about 7.6 centimeters) wide, about 100 millimeters long, and of
about 0.036 inch (0.09 centimeter) depth, to a stretched extension of
about 50 percent of original length, or about 50 millimeters, such
that the stretched film had a total stretched length of about
150 millimeters. During such stretchlng of the film sample, the
stretch force, in grams, was measured. Once the desired stretched
length was obtained, the film sample was held at the 50 percent
stretched extension for about 20 minutes. During these 20 minutes,
the stress relaxation force of the film sample was measured. A
representative plot of a stress-strain force measurement-is shown in
Fig. 4. The mechanical properties of the film sample were determined
as follows:
Initial Modulus: The Initial Modulus value, in dynes per square
centimeter, was taken to be the slope of a tangent (line A in Fig. 4)
drawn to the curve of the stress/strain measurements at the origin
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'21~S ~86
(0 percent stretch), normalized with respect to the area of the
cross section of the film sample.
Stress at 50 Percent Extension: The Stress at 50 Percent Extension
value, in dynes per square centimeter, was determined by simply
reading the force value at 50 percent extension of the film sample
(point B in Fig. 4), normalized with respect to the area of the
cross-section of the film sample.
Stress Relaxation: The Stress Relaxation value, recorded as a
percentage, was determined by measuring the difference in stress
force for the 50 percent extended film sample between when the sample
first reaches the 50 percent stretched extension (point C in Fig. 4)
and then after the 20 minute relaxation time period (point D in
Fig. 4), dividing by the initial stress for the 50 percent extended
film sample (point C in Fig. 4), and then multiplying by 100 percent.
Peel Force: The Peel Force value is a measurement of the adhesive
bond strength of a film sample and is measured according to the
standardized test method PSTC-1, revised as of August 1989,
incorporated herein by reference.
Example
Samples were prepared of elastic laminates consisting of a
self-adhering elastic material bonded to two layers of a gatherable
substrate.
As a control, films of a conventional self-adhering elastic material,
comprising a substantially homogeneous composition of a
styrene-isoprene-styrene block copolymer, oils, and tackifying resins
and commercially available from Findley Adhesives Inc. under the
trade designation H-2209, were used. For each control sample, three
layers of this film were combined by overlaying and adhering them to
each other so as to prepare a single film having a depth of about
0.036 inch (about 0.9 millimeter) deep.
2 l~S ~86
Sample 1 was prepared by sandwiching and adhering a layer, of a depth
of about 0.012 inch (about 0.3 millimeter) of an elastic material, an
experimental composition comprising styrene-ethylene/butylene-styrene
block copolymer and tackifying resins, available from Findley
Adhesives Inc. under the designation E-2, between two layers, each of
a depth of about 0.012 inch (about 0.3 millimeter) of a hot-melt
adhesive material, comprising styrene-isoprene-styrene block
copolymer, oils, and hydrogenated polycyclopentadiene and polyvinyl
toluene tackifying resins commercially available from Findley
Adhesives Inc. under the trade designation H-2096.
For each sample, several laminates were formed by stretching a sheet
of the self-adhering elastic material, having the dimensions of about
1 inch (about 2.5 centimeters) wide, about 3 inches (about
7.6 centimeters) long, and about 0.036 inch (about 0.9 millimeter)
deep, by about 300 percent of the original length of the
self-adhering elastic material to a total length of about 12 inches
(about 30.5 centimeters). The stretched self-adhering elastic
material was then sandwiched between two layers of unstretched spun
bond material, comprising spunbond polypropylene with a basis weight
of about 0.5 ounce per square yard, available from Kimberly-Clark
Corporation, and bonded together by pressing the laminate with a
5 pound (2.3 kilogram) roller. The laminate was then allowed to
relax. Both the control sample and Sample 1 laminates exhibited
stretch of about 280 percent such that the relaxed laminate had a
total length of about 3.2 inches (about 8.1 centimeters).
A 3 inch (about 7.6 centimeter) section was marked off on each
laminate to be used for measurement purposes. The laminate samples
were mounted on cardboard backing by stapling the ends of the
laminates in a fixed position on the cardboard backing while the
3 inch marked section of each laminate was stretched to a 50 percent
extension such that the stretched 3 inch section had a total length
of about 4.5 inches (about 11.4 centimeters). One set of laminate
samples was allowed to age at about 72-F (about 22-C) for about
2 weeks. A second set of laminate samples was allowed to age at
about 110-F (about 43-C) for about 24 hours.
21~S '~8~
After aging, the laminate samples were removed from the cardboard
backing and allowed to relax for about one-half hour before testing
for adhesive and elastic properties. Two inch long sections of each
laminate sample were obtained and tested on a standard tensile
tester, commercially available from Sintech Company.
The film samples were then measured for aged Creep values. The
Control sample exhibited an aged Creep value, for aging at about 72-F
for about 2 weeks when stretched at a 50 percent extension, of about
16 percent and an aged Creep value, for aging at about llO-F for
about 24 hours when stretched at a 50 percent extension, of about
40 percent.
Sample 1 exhibited an aged Creep value, for aging at about 72-F for
about 2 weeks when stretched at a 50 percent extenslon, of about
16 percent and an aged Creep value, for aging at about llO-F for
about 24 hours when stretched at a 50 percent extension, of about
10 percent.
For each of the tested laminates, the Peel Force value exceeds the
Tensile Strength of the laminates.
Those skilled in the art will recognize that the present invention is
capable of many modifications and variations without departing from
the scope thereof. Accordingly, the detailed description and
examples set forth above are meant to be illustrative only and are
not intended to limit, in any manner, the scope of the invention as
set forth in the appended claims.
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