Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED COMPOSITE ELASTIC MATERIAL AND
PROCESS FOR PRODUCING THE SAME
FIELD OF INVENT10N
The present invention relates to a composite elastic material having
improved stress relaxation and improved dimensional stability. This invention
also relates to a method of manufacturing composite elastic materials that are
dimensionally stable and latent and that have improved stress relaxation.
BACKGROUND OF THE INVENTION
The present invention relates to a dimensionally stable and/or latent
composite elastic material having improved stress relaxation and creep
resistance and laminates thereof. The present invention also relates to a
method of manufacturing the same.
As used herein, the term "composite elastic material" refers to a
multicomponent or multiiayer elastic material in which one layer is elastic. A
composite elastic material that is "dimensionally stable" is one that retains
its
dimensions, i.e., length and width, under actual use conditions. Use
conditions generally involve body temperature, humidity and heat. A "latent
elastic laminate material" refers to a laminate material that has an
efasticizable component that is dormant but that can be activated at will,
normally using a stimulus such as heat. In other words, a latent elastic
laminate material will become elastic when activated.
The term "stress relaxation" is defined as the load required to hold a
constant elongation over a period of time. The term "creep" is defined as the
loss of shape or dimension of an article due to some irreversible flow or
structural breakdown under a constant load or force. There are two kinds of
creep: (1 ) time-independent, in which the shape changes because of the
irreversible flow or structural breakdown under a constant load or force and
does not recover when the force is removed; and (2) time-dependent,
wherein some of the shape recovers when the force is removed.
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Composite elastic materials and laminates thereof have a wide variety
of uses, especially in the areas of absorbent articles and disposable items.
As used herein, the term "absorbent articles" refers to devices which absorb
and contain body exudates and, more specifically, refers to devices that are
placed against or in proximity to the body of the wearer to absorb and contain
the various exudates discharged from the body. The term "absorbent
articles" is intended to include diapers, training pants, absorbent
underpants,
incontinence products and the like. The term "disposable" is used herein to
describe absorbent articles not intended to be laundered or otherwise
restored or reused as an absorbent article.
Generally, composite elastic material is a continuous filament-type
structure in which a layer of continuous, generally parallel, elastic
filaments
are bonded to at least one facing layer using a heated calender roll and an
anvil roll. Continuous filament laminates are disclosed in U.S. Patent No.
5,366,793 to Fitts, Jr. et al. and U.S. Patent No. 5,385,775 to Wright, both
of
which are incorporated herein by reference in their entirety.
Typically, the calender roll is patterned in some way so that the
resulting laminate material is not bonded across its entire surface. The anvil
roll may also be patterned if it is desired. The maximum bond point surface
area for a given area of surface on one side of the laminate generally will
not
exceed about 50% of the total surface area. Typically, the percent bonding
area varies from about 10% to about 30% of the area of the laminate
material. Such a process is disclosed, for example, in U.S. Patent No.
5,385,775 to Wright and U.S. Patent No. 4,041,203 to Brock et al., both of
which are incorporated herein by reference in their entirety.
One disadvantage to this method of lamination is that the patterned
rolls severely damage the elastic filaments. The damage to the elastic
filaments affects the elastic properties and, thus, the performance of the
composite elastic material and laminates thereof by causing the fibers to
break during use, at body temperature) and under stretched conditions.
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A need, therefore, exists for a method of manufacturing a composite
elastic material that is dimensionally stable. Additionally, there is a need
for
a method of manufacturing a composite elastic material without damaging the
polymer strands during the manufacturing process.
SUMMARY OF THE INVENTION
The present invention provides a process for producing a
dimensionally stable and/or latent elastic laminate material. The process
includes the steps of providing a polymeric material having a first length,
stretching the polymeric material to a second length and bonding at least one
non-woven facing to the polymeric material in a calender having two smooth-
surtaced rolls in order to reduce damage to the structure of the elastic
material. The present invention also provides a process wherein 100% of the
surtace area of the roll contacts the elastic material. The present invention,
therefore, produces a single composite elastic material having minimal to
negligible damage to the polymeric material by calendering a polymeric
material and at least one non-woven facing between smooth-surfaced rolls.
The process of the present invention not only overcomes the problems
of the prior art, but also provides several advantages. These include: (1 ) a
substantial improvement in the pertormance of the resulting laminate via
dimensional stability; (2) a reduced load loss over time under actual use
conditions; (3) a possible cost reduction through the use of a lower amount of
elastic material in the final laminate; (4) the production of latent and/or
heat
shrinkable materials; and (5) the use in the resulting laminate of the full
potential of the elastic polymer. The present invention also provides a
method of manufacturing a low cost elastic non-woven material.
Composite elastic laminate materials produced according to the
present invention may be used as elastic components of personal care
absorbent articles such as, for example, in the side panels of diapers and
training pants. They may also be used in the leg elastic and gasketing of
diapers, training pants, incontinence devices and the like.
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The foregoing and other features and advantages of the present
invention will become apparent from the following detailed description of the
presently preferred embodiments, when read in conjunction with the
accompanying examples.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of the process for forming a composite
elastic material according to the present invention.
FIG. 2 is a perspective view of an exemplary disposable garment) in
this case training pants, that utilizes the laminate material made according
to
the present invention.
FIG. 3 is a graph of stress relaxation modulus versus time determined
during stress relaxation testing of a composite elastic laminate material
produced using a pattern roll and testing of a composite elastic laminate
material produced using two smooth rolls.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is directed to a dimensionally stable composite
elastic material and laminates thereof having improved stress relaxation and
improved creep resistance. The present invention also relates to a method
for forming composite elastic laminate materials that are dimensionally stable
and latent and that have improved stress relaxation properties.
Referring now to the drawings wherein like reference numerals
represent the same or equivalent structure and, in particular, to FIG. 1 of
the
drawings, there is illustrated at 10 a process for forming a composite elastic
material using smooth-roll calendering.
According to the present invention, an elastic web 12 is unwound from
a supply roll 14 and travels in the direction indicated by the arrow
associated
therewith as the supply roll 14 rotates in the direction of the arrows
associated therewith. The physical structure of the elastic web 12 can be a
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film, a non-woven or strands. The elastic web 12 passes through the nip 16
of the S-roll arrangement formed by the stack rollers 20 and 22.
The elastic web 12 may also be formed in a continuous process such
as, for example, the process described below, and passed through the nip 16
5 without being stored on a supply roll.
A first gatherable layer 24 is unwound from a supply roll 26 and travels
in the direction indicated by the arrow associated therewith as the supply
roll
26 rotates in the direction of the arrows associated therewith. A second
gatherable layer 28 is unwound from a second supply roll 30 and travels in
the direction indicated by the arrow associated therewith as the supply roll
30
rotates in the direction of the arrows associated therewith. The first
gatherable layer 24 and the second gatherable layer 28 pass through the nip
32 of the calender roll arrangement 34 formed by the caiender roll 36 and 38.
The first gatherable layer 24 and/or the second gatherable layer 28 may be
formed by extrusion processes such as, for example, meltblowing processes,
spunbonding processes or film extrusion processes and passed directly
through the nip 32 without first being stored on a supply roll.
The elastic web 12 passes through the nip 16 of the S-roll
arrangement 18 in a reverse-S path as indicated by the rotation direction
arrows associated with the stack rollers 20 and 22. From the S-roll
arrangement 18, the elastic web 12 passes through the pressure nip 32
formed by the calender roll arrangement 34. Additional S-roll arrangements
(not shown) may be introduced between the S-roll arrangement 18 and the
calender roll arrangement 34 to stabilize the stretched material and to
control
the amount of stretching. Because the peripheral linear speed of the rollers
of the S-roll arrangement 18 is controlled so as to be less than the
peripheral
linear speed of the rollers of the caiender roll arrangement 34, the elastic
web
12 is tensioned between the S-roll arrangement 18 and the pressure nip of
the calender roll arrangement 34. Importantly, filaments of the elastic web 12
in strand form should run along in the direction that the film is stretched so
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that the filaments can provide the desired stretch properties in the finished
composite material. By adjusting the difference in the speeds of the rollers,
the elastic web 12 is tensioned so that it stretches a desired amount and is
maintained in such stretched condition while the first gatherable layer 24 and
the second gatherable layer 28 are joined to. the elastic web 12 during their
passage through calender rolls 36 and 38 to form a composite elastic
material 40. The surfaces of calender rolls 36 and 38 are smooth and
patternless. Thus, the bond area of the composite elastic material 40 is
100% because both of the calender rolls are smooth-surfaced.
Preferably, the stretched length of the polymeric material is at least
about 50% of the original length. Preferably, the stretched length is up to
about 95% of the ultimate elongation of the elastomer. It should be noted
that different elastomers have different elongations. Even more preferably,
the stretched length should be in the range of about 200% to about 600%
where percent elongation is defined according to the following formula:
final length - initial length °
x 100 /o
initial length
Referring again to FIG. 1, the composite elastic material 40
immediately relaxes upon release of the tensioning force provided by the S-
roll arrangement 18 and the bonder roll arrangement 34, whereby the first
gatherable layer 24 and the second gatherable layer 28 are gathered in the
composite elastic material 40. Preferably) a permanent elongation length of
from about 1.5 times the original length is retained after the stretched
filri~ is
allowed to relax.
The composite elastic material 40 is then wound up on a winder 42.
The composite elastic material 40 may be wound under tension or without
tension. if wound without tension, the composite elastic material will be
stored in the roll in its unstretched state such that the material may be
stretched at any time. If wound with tension) the composite elastic material
is
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stored in a stretched condition. The elastic properties of the material stored
in
the stretched condition can be reactivated using heat or other conditions.
Alternatively, the composite elastic material 40 may continue in line for
further
processing or conversion (not shown).
Polymeric materials that are useful in forming the elastic web 12 are
generally known as "elastomers." An eiastomer is a rubber elastic material
capable of stretching to several times its original, relaxed length and which
tends to recover completely its elongation upon release of the stretching,
biasing force. As used herein, the term "recover" refers to a contraction of a
stretched material upon termination of a biasing force following the
stretching
of the material by application of the biasing force. Examples of these
materials are indexed as "elastomers" in Bradley et al., Materials Handbook,
284-290 (M'Graw-Hill) Inc. 1991 ), incorporated herein by reference.
The elastomers useful in the present invention may be selected from
the group consisting of elastomeric thermoplastic polymers. The physical
structure of the elastomer can be strands, cast or blown film, crimped, any
non-woven web of fiber of a desired thermoplastic polymer or a combination
thereof.
Suitable elastomeric thermoplastic polymers include styrene block
copolymers such as, for example, those available under the trademark
KRATON~ from Shell Chemical Company of Houston, Texas. KRATON~
block copolymers are available in several different formulations, a number of
which are identified in U.S. Patents 4,663,220; 4,323,534; 4,834,738;
5,093,422; and 5,304,599 which are incorporated herein by reference.
Other exemplary elastomeric materials that may be used include
polyurethane elastomeric materials such as, for example, those available
under the trademark PELLATHANE~ from Dow Chemical Company of
Midland, Michigan or under the trademark ESTANE~ from B.F. Goodrich &
Company of Akron, Ohio or under the trademark MORTHANE~ from Morton
Thiokol Corporation; polyester efastomeric materials such as, for example,
those available under the trade designation HYTREL from E.I Dupont de
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Nemours & Company of Wilmington, Delaware and those known as
ARNITEL~ which were formerly available from Akzo Plastics of Arnhem,
Holland and now available from DSM of Sittard, Holland; and polymers from
metallocene-based catalysis which are available under the name ENGAGE~
from Dow Chemical Company of Midland) Michigan for polyethylene-based
polymers and from Exxon Chemical Company of Baytown, Texas under the
trade name ACHIEVE~ for polypropylene-based polymers and EXACT~ and
EXCEED~ for polyethylene-based polymers.
The gatherable layers, or facings, 24 and 28 may be fibrous non-
woven materials such as, for example, spunbonded webs or meltblown webs.
A non-woven web, as described herein, means a web having a structure of
individual fibers or threads that are interlaid, but not in an identifiable,
repeating manner. A plurality of fibrous non-woven facings 24 and 28, as
shown in FIG. 1, may also be used depending on the basis weight of the non-
woven material used.
The gatherable layers 24 and 28 may be formed by a variety of
processes including, but not limited to, meltbiowing and spunbonding
processes. Meltblown fibers are fibers formed by extruding a molten
thermoplastic material through a plurality of fine, usually circular,
capillaries
of a meltblowing die as molten threads or filaments into converging high-
velocity, usually hot, gas (e.g., air) streams which are flowing in the same
direction as the extruded filaments or threads of the molten thermoplastic
material so that the extruded filaments or threads are attenuated, i.e., drawn
or extended, to reduce their diameter. The threads or filaments may be
attenuated to microfiber diameter which means the threads or filaments have
an average diameter not greater than about 75 microns) generally from about
0.5 microns to about 50 microns, and more particularly from about 2 microns
to about 40 microns. Thereafter) the meltblown fibers are carried by the high-
veiocity gas stream and are deposited on a collecting surface to form a web
of randomly disbursed meltblown fibers. The meltblown process is well-
known and is described in various patents and publications) including NRL
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Report 4364, °Manufacture of Super-Fine Organic Fibers" by B.A.
Wendt,
E.L. Boone and D.D. Fluharty; NRL Report 5265, "An Improved Device for the
Formation of Super-Fine Thermoplastic Fibers" by K. D. Lawrence, R.T. Lukas
and J.A. Young; U.S. Patent No. 3,676,242 to Prentice; and U.S. Patent No.
3,849,241 to Buntin et al. The foregoing references are incorporated herein
in by reference in their entirety. Meltbiown fibers are microfibers which may
be continuous or discontinuous, are generally smaller than 10 microns in
average diameter and are generally tacky when deposited onto a collecting
surface.
Spunbonded fibers are small diameter fibers that are formed by
extruding a molten thermoplastic material as filaments from a plurality of
fine,
usually circular, capillaries of a spinneret with the diameter of the extruded
filaments then being rapidly reduced as by, for example, non-eductive or
eductive fluid-drawing or other well-known spunbonding mechanisms. The
production of spunbonded non-woven webs is illustrated in patents such as,
for example, U.S. Patent No. 4,340,563 to Appel et al.; U.S. Patent No.
3,802,817 to Matsuki et al.; U.S. Patent No. 3,692,618 to Dorschner et al;
U.S. Patent No. 3,542,615 to Dobo; U.S. Patent No. 3,502,763 to Hartman;
U.S. Patent No. 3,502,538 to Peterson; U.S. Patent Nos. 3,341,394 and
3,338,992 to Kinney; U.S. Patent No. 3,276,944 to Levy; and Canadian
Patent No. 803,714 to Harmon. The disclosures of these patents are herein
incorporated by reference in their entirety. Spunbonded fibers generally are
not tacky when deposited onto a collecting surface. Spunbonded fibers
generally are continuous and have average diameters (from a sample of at
feast 10) larger than 7 microns and, more particularly, from about 10 microns
to about 20 microns.
Preferably) the non-woven gatherable layers comprise spunbonded
fibers. For most uses, the total weight of the spunbonded facing material is
' about 0.4 ounces per square yard.
Various techniques may be employed to secure the elastic web 12
onto the non-woven facings 24 and 28 such as, for example, adhesive
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bonding. In adhesive bonding, an adhesive such as a hot melt, pressure
sensitive adhesive is applied between the polymeric material and the facing
to bind the polymeric material and facing together. The adhesive can be
applied by, for example, melt spraying, printing or meltblowing. Various types
5 of adhesives are available, including those produced from amorphous
polyalphaolefins, ethylene vinyl acetate-based hot melts and KRATON~
brand adhesives available from Shell Chemical Company of Houston, Texas.
The resulting composite elastic material 40 has elastic properties
identical to those of the pure polymeric material. The resulting laminate
10 material suffers no loss of elasticity and provides better body conformance
than laminate materials produced using one or more patterned calender rolls.
The filaments in the laminate materials of the present invention are also less
likely to break.
The stress relaxation of the resulting composite elastic material 40 is
at least about 1 psi less than the stress relaxation of a thermal, pattern-
bonded material of similar composition at a time of about 8 hours. Preferably,
the stress relaxation is at least about 2 psi less than that of a pattern-
bonded
material at a time of about 8 hours.
Referring now to FIG. 2, there is illustrated a disposable garment 50
incorporating an elastic laminate made according to the present invention.
Although training pants are shown in FIG. 2, it will be understood that use of
the elastic laminate produced according to the present invention is not
limited
to such articles and may also be used in a wide variety of applications
including, but not limited to, diapers, incontinence devices and the like.
Referring again to FIG. 2, the disposable garment 50 includes waste
containment section 52 and two side panels 54 and 56 defining a waist
opening 58 and a pair of legs openings 60 and 62. FIG. 2 illustrates the
disposable garment 50 fitted on a wearer's torso 64 in dashed lines. Side
panel 54 includes stretchable side member 66 and stretchable side member
68 connecting intermediate member 70 which is made of a non-stretchable
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material. Similarly, side panel 5fi includes stretchable side member 72 and
stretchable side member 74 connecting intermediate member 76 which is
made of a non-stretchable material. Disposable garment 50 also includes
front waist elastic member 78 and rear waist elastic member 80 for providing
additional elasticity along waist opening 58. Leg elastics 82 are provided
with waist containment section 52 between side panels 54 and 56.
The composite elastic material of the present invention may be used to
form various portions of the disposable garment 50 and particularly, the side
panels 54 and 5fi. The laminate material may also be used in the leg elastics
82 of the disposable garment 50.
EXAMPLES
A composite elastic material was made using two smooth-surfaced
calender rolls according to the present invention. A control material was
made using pattern rolls according to the prior art process. Both materials
were made from the same polymer blend.
Stress Relaxation
Stress relaxation of the control and inventive samples was measured
on a Sintech 1/S tensile test frame available from Sintech, Inc. of Stoughton,
Massachusetts. Sample size was about 3 inches wide and 7 inches long.
Each speciman was clamped between the jaws of the grip at a 3-inch grip-to-
grip distance. Each sample and the grip fixtures were enclosed in an
environmental chamber and equilibrated at 100°F for 3 minutes. Each
sample was then stretched to a final constant elongation of 4.5 inches (50%
elongation) at a cross-head displacement of 20 inches per minute. The load
required to maintain the 50% elongation as a function of time was monitored
for 8 hours for each sample.
Data from the Sintech 1/S system was reduced by calculating the
engineering stress (pounds per square inch, or psi) from a knowledge of the
initial cross-sectional area of each sample. Strain, or elongation, was
calculated from the initial grip-to-grip distance and the constant elongation.
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The ratio of the stress and strain gives the stress relaxation modules (psi).
This data was used to generate stress relaxation modules versus time curves
for the control and inventive composite elastic laminate materials. FIG. 3 is
a
graph showing the stress relaxation modules versus time curves of the
composite elastic Laminate material produced using a pattern roll (A) and the
composite elastic laminate material produced using two smooth rolls (B).
The resulting data can be fitted to the following power-law
model to obtain the exponent) m:
m
(~t=0.1 min.)( )'
wherein a is stress, t is time and m represents how fast the material loses
its
load, or elastic properties. Table I shows the rate of actual load loss, or
slope, as calculated using the above-described formula and the actual load
loss in 8 hours at 100°F. As can be seen in the Table, the use of the
smooth
calender rolls decreases the magnitude of the slope and the percent load
loss favorably.
TABLE I
Sample ID Slope % Load Loss
Pattern Roll -0.13 68%
Smooth Rolls -0.10 55%
Of course, it should be understood that a wide range of changes and
modifications can be made to the embodiments described above. It is,
therefore, intended that the foregoing description illustrate rather than
limit
this invention and that it is the following claims, including all equivalents,
that
define this invention.
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