Note: Descriptions are shown in the official language in which they were submitted.
~ma~~
2~59~~~
ADHESIVE COMPOSITION AND PROCESS THERE~'~OR
The present invention relates to an adhesive
composition in the form of a foam. More particularly the
invention relates to a hot melt adhesive which, on the
one hand has elastic properties and is in the form of a
foam in the solid state while, on the other hand, has
very low and in many cases negligible elasticity in the
molten state as shown by essentially Newtonian behaviour
at the processing (application) temperature.
In the manufacture of many different types of
article it is necessary to bond an elastic material to a
non-elastic substrate. One example is in the manufacture
of disposable diapers where elastic strips are used to
provide leg and waist elastication. Typically, the
elastic strips consist of stretchable rubber which is
fixed to the body of the diaper by layers of adhesive,
for example hot melt adhesive, affixed to both surfaces.
This arrangement involves a number of difficulties.
For example, the elastic strip is often covered with an
anti- blocking agent such as talc during manufacture to
prevent the strip bonding to itself. However, this
anti-blocking agent can, in turn, render bonding of the
elastic strip to the body of the article much more
difficult. Furthermore, since the hot melt adhesives
which are used do not themselves have real elastic
properties, application of adhesive tends to "kill" the
stretch of that portion of the elastic to which it is
219939
2
applied. Finally, the use of the two materials, rubber
and adhesive, is costly.
A composition having adhesive properties on
application from the molten state and preferably also
pressure sensitive adhesive properties, and elasticity
comparable to that, of rubber, whilst also possessing
suitable rheological properties for melt processing, is
highly desirable since it would be capable of replacing
the rubber and the adhesive used at present and would
represent a considerable advance in the manufacture of
articles such as diapers. Such a composition would also
find numerous other applications in many diverse fields.
Further advantages would follow if the composition could
be presented for use as a foam.
Many types of composition are in use commercially as
hot melt adhesives and a much wider range of compositions
have been suggested for use in this field. A hot melt
adhesive can be defined as a composition which shows
adhesive properties when applied to a substrate in the
molten state. This does not preclude the composition
also showing adhesive properties at room temperature,
e.g. pressure sensitive adhesive properties. In general,
hot melt adhesives are formulated for properties such as
adhesion to a variety of surfaces, heat stability and
processing properties rather than for elasticity and any
attempt to increase elasticity has a detrimental effect
on adhesion and other desirable properties of the
composition. As yet, none of the commercial hot melt
.._ 219939
3
adhesive compositions combine good adhesion with elastic
properties comparable to those of rubber. In addition
whilst there have been proposals to foam certain hot melt
adhesives, no foamed hot melt adhesive has come into
general use.
Natural rubber is generally too viscous to be used
as a basis for hot melt adhesives and cured rubber cannot
be melted. Many hot melt adhesives are based on
thermoplastic block elastomers since these can be melted.
Many such elastomers with a variety of properties are
available commercially. For use as an adhesive,
thermoplastic block elastomers generally have to be
blended with tackifying agents to improve adhesion.
Other additives such as plasticisers may also be
necessary depending on the application.
Certain prior art proposals have attempted to
provide hot melt adhesives which also have elastic
properties. For example US-A-4 418 123 attempts to
provide a self-adhering elastic having a combination of
elastic and adhesive properties. The composition is
defined in very broad terms as a combination of a block
copolymer comprising at least one substantially amorphous
rubbery polymeric mid block and at least two glassy
poly(vinylarene) end blocks, together with a midblock
associating resin and an endblock associating resin. All
of the specific examples (with the exception of a
comparative example with unsatisfactory properties) are
based on styrene-isoprene-styrene block copolymers.
2159939
- - 4
Despite the claims made for them, as far as the present
applicants are aware, none of the compositions
exemplified in US-A-4 418 123 show true elastic
properties comparable to those of rubber combined with
good adhesion and processability (see Comparative Example
A below) .
Similarly EP-A-0 424 295 (HB Fuller France SARL)
relates to a thermoplastic elastic intended particularly
for elastication of diapers which comprises:
a) at least one synthetic rubber which is of the
block copolymer type comprising at least one rubber
middle block and at least two glassy end blocks;
b) 20 to 150 by weight based on the block
copolymer of at least one "tackifying" resin which
associates with the middle block of the copolymer;
c) 10 to 50$ by weight based on the block
copolymer of at least one "tackifying" resin which
associates with the terminal blocks of the copolymer; and
- d) 5 to 35~ based on the weight of the composition
of a mineral oil.
It is suggested that these compositions can be
foamed. The compositions exemplified in EP-A-0 424 295
are generally based on SIS and an example of a
composition based on SBS shows unsatisfactory properties.
There are no examples of the production of foams.
It has now been found that, by careful selection of
the components, hot melt adhesives can be produced having
the desirable combination of properties referred to
215 9939
above, i.e. good adhesion from the melt (and in many embodiments also
at room temperature), elastic properties comparable to those of rubber;
and good processibility from the melt and it has also been found that
these hot melt adhesives can be foamed to provide stable foams.
Various aspects of the invention are as follows:
An elastomeric hot melt adhesive in the form of a stable foam
comprising a hot melt adhesive composition which comprises at least one
thermoplastic elastomer and at least one tackifying resin, the
thermoplastic elastomer being selected from the group consisting of
l0 styrene/butadiene/styrene (SBS) copolymers and blend of
styrene/butadiene/styrene (SBS) copolymer with
styrene/ isoprene/ styrene (SIS) in which SIS is present in an amount not
more than 50% by weight of the total block copolymer, and wherein the
said hot melt adhesive composition is further characterized in that:
15 a) it is capable of bonding, when applied from the molten state, to
materials selected from the group consisting of plastic and cellulosic
materials and mixtures thereof with a 90° peel force of not lower than
0.5
N/ cm;
b) it has a tensile strength retention after 50 cycles of at least 40%;
2 0 and
c) it has a viscosity of not more than 120,000 cps at 180°C and an
applied shear of 80 sec-1.
A process for the production of an adhesive which comprises
(i) providing a hot melt adhesive composition which comprises
2 5 at least one thermoplastic elastomer and at least one tackifying resin,
the
thermoplastic elastomer being selected from the group consisting of
styrene/butadiene/styrene (SBS) copolymers and blends of
styrene/butadiene/styrene (SBS) copolymer with
styrene/ isoprene/ styrene (SIS) in which SIS is present in an amount not
3 0 more than 50% by weight of the total block copolymer, and wherein the
said hot melt adhesive composition is further characterised in that:
a) it is capable of bonding, when applied from the molten state, to
materials selected from the group consisting of plastic and cellulosic
materials and mixtures thereof with a 90° peel force of not lower than
0.5
3 5 N/Cm;
~,.
5a 2 1 5 9 9 3 9
b) it has a tensile strength retention after 50 cycles of at least 40%;
and
c) it has a viscosity of not more than 120,000 cps at 180°C and an
applied shear of 80 sec-1;
(ii) foaming the said hot melt adhesive composition;
(iii) extruding the foam produced; and
(iv) cooling the foam.
Feature a) referred to above relates to the adhesive properties of
the composition and in all cases the composition should have the
properties of a hot melt
A'
~~59939
6
adhesive in that it is capable of bonding appropriate
substrates, typically plastic and/or cellulosic
materials, when applied from the molten state. In
particular, "capable of bonding" means that the
composition is capable of showing adhesion on plastic
and/or cellulosic materials sufficient especially for
application in the construction of hygienic absorbent
articles. When applied from the molten state between two
substrates of plastics and/or cellulosic materials the
composition gives a bond strength, measured as 90° peel
of not lower than 0.5 N/cm. The composition at a weight
of 5 g/m2 is applied in the molten state between the
substrates and 48 hours after bond formation the 90° peel
strength is measured at 23°C and at a separating speed of
300 mm/min.
As described in more detail below, many compositions
which may be foamed according to the invention also bond
appropriate substrates at room temperature and may show
tl~e properties of a pressure sensitive adhesive.
Feature b) referred to above relates to the elastic
properties of the composition. The test which is used is
described in more detail below and involves measuring the
extent to which elastic properties are retained over 50
cycles of stretching and relaxation. The range over
which the composition is stretched is related to the
modulus of the composition and the likely degree of
stretching of the composition in use. The figure of 40$
for tensile strength retention indicates that the elastic
2.59939
properties of the composition are comparable with those
of natural rubber and are preferably superior thereto.
Preferably the tensile strength retention is at least
50~, more preferably at least 60~.
Feature c) above relates to the processability of
the composition and a viscosity of 120,000 cps or less at
180°C (applied shear 80 sec-1) indicates that the
composition can be applied using conventional apparatus
for use with hot melt adhesives. Preferably the
viscosity is 60,000 cps or less, more preferably 30,000
cps or less. It is also highly desirable that the
composition which may be foamed according to the
invention show substantially Newtonian rheological
behaviour, in particular viscosity does not vary
significantly with applied shear. As discussed in more
detail below, many compositions which may be foamed
according to the invention show Newtonian behaviour at
intended processing temperatures, e.g. around 180°C.
Compositions which may be foamed according to the
invention can be formulated with any desired modulus
depending on the desired end use. In this connection,
reference can be made to the intrinsic modulus which is
the modulus of the composition prior to foaming and the
apparent modulus which is the modulus of the foamed
composition. Apparent modulus is a function of the
intrinsic modulus and percent foaming (i.e. foam
density). Apparent modulus of a foamed composition is,
of course, always lower than intrinsic modulus.
21 X9939
__
Intrinsic modulus has effects on the main properties of
the composition and it is convenient to divide the
compositions of the invention into low intrinsic modulus
and high intrinsic modulus compositions. As used herein
the terms "low modulus composition" and "high modulus
composition" refer to intrinsic modulus.
Low modulus compositions are defined as compositions
having a modulus of 0.5 MPa or less at 500 elongation
(six times the initial length of sample) measured at 23°C
under an elongation rate of 500 mm/minute. Generally low
modulus compositions have a modulus in the range 0.05 to
0.5 MPa, preferably 0.05 to 0.3 MPa. Low modulus
compositions generally show good adhesive properties at
room temperature and may also be pressure sensitive
adhesives. These compositions are usually stretched
immediately after they are formed, for example after
extrusion from the melt as a strip or thread and foaming
using a blowing agent. Stretching may take place
immediately before or during application to an article so
that the compositions are effectively applied in the
stretched state. Low modulus compositions are typically
used under an elongation of 400 to 1000.
Since low modulus compositions will usually be
stretched immediately after extrusion and foaming it is
desirable that they should have a relatively high setting
point so that they solidify quickly on extrusion.
Preferably the setting point (measured by the Dynamic
2~~9939
9
Mechanical Analysis method described in more detail
below) is at least 80°C, more preferably at least 100°C.
High modulus compositions are defined as
compositions having a modulus of greater than 0.5 MPa at
500 elongation (six times the initial length of sample)
measured at 23°C under an elongation rate of 500
mm/minute. Preferably the high modulus compositions have
a modulus of from 1 MPa to 10 MPa. Since pressure
sensitive adhesive character is generally inversely
proportional to modulus, compositions with high modulus
are often applied immediately after foaming with the
material still in a molten or semisolid state although
some may retain sufficient pressure sensitive adhesive
character to be applied at room temperature. Application
in a molten or semisolid state implies that the
composition is applied without stretching with stretching
generally taking place in use and this applies
particularly to compositions with a modulus of 1 MPa or
higher. For this reason high solidification temperature
is less critical for high modulus compositions beyond the
need to foam a stable foam but for convenience these are
also preferably formulated to have a setting point of at
least 80°C, more preferably at least 100°C. High modulus
,compositions are generally used at a lower degree of
stretching. They are capable of giving sufficient .
elastic force at low deformation (typically no higher
than 50$). However, in cases where the compositions
retain sufficient pressure sensitive adhesive character
~I~9939
0
so that they can be applied at room temperature in a
stretched state, they can be used at an elongation of up
to 400. For practical reasons, the lower limit of
apparent modulus is about 0.05 MPa although given that
the apparent modulus is a function of intrinsic modulus
and foam density, this lower limit applies to both high
and low intrinsic modulus compositions.
The essential components of the composition which
may be foamed according to the invention are a
thermoplastic elastomer and a tackifying resin and these
will now be discussed in general terms.
Thermoplastic elastomers are a very interesting
chemical and technological class of polymers which are
distinguished by their characteristic behaviour. At room
temperature they behave as cured rubbers showing high
elasticity but in contrast to cured rubbers they can be
melted and reprocessed in the same way as normal
thermoplastics.
_ This behaviour results from a particular chemical
structure. Most thermoplastic elastomers are block
copolymers, i.e. their molecules are formed by blocks of
different natures linked together. Different blocks can
alternate along the chain as relatively short blocks
(multiblock structure of the form A-B-A-B-A etc); or the
molecules can have a three block structure of the form
A-B-A where A are terminal blocks and B is a central
block of a different nature (linear three block
copolymers); or the molecules can have a "radial" or
2159939
_ n
"star" structure represented as (AB)x where all midblocks
B are chemically linked together at a central point and
terminal blocks A are radially disposed each at the end
of block B. Structures formed by only two blocks
(diblocks) of the form AB are ineffective as
thermoplastic elastomers in terms of their elastic
behaviour.
The chemical nature of the different blocks can be
varied and the resulting copolymers can be classified for
example as polyurethanes, polyesters, polyethers,
polyether- ester amides, etc. However, a common
characteristic is the following: different blocks are
physically incompatible so that they are mutually
insoluble. The material can thus be considered an
heterogeneous system in which different blocks, even if
chemically linked in the same molecule, exist as separate
entities. Blocks A of different molecules tend to
associate together in microscopic regions or "domains"
with the same happening for blocks B. The material so
formed has an heterogeneous structure of domains A and B,
each well separated, with the one present at the lower
level being dispersed microscopically in the other one
which constitutes a continuous phase. This continuous
phase is generally formed by "soft" or rubbery blocks B
which give to the material its elastic properties, while
the dispersed phase A is formed by "hard" non-elastomeric
blocks. Below the glass transition temperature or
softening point of the hard blocks each molecule of the
2159939
12
copolymer has its A blocks fixed in at least two points,
i.e. they are "confined" in the hard domains.
Accordingly, the rubbery part of the molecule can undergo
stretching but without flowing relative to other
molecules and when the external stretching force is
relaxed it returns to its initial position for reasons of
entropy.
Thus in thermoplastic elastomers, the hard blocks
work as a physical vulcanization and the advantages of
this processing are clear. The chemical linkages that
form the vulcanized structure of a standard rubber cannot
be removed by heating and at sufficiently high
temperature the rubber simply begins to decompose. On
the other hand, in thermoplastic elastomers heat can
effectively melt the hard domains: the material can thus
be melted and processed, but hard domains giving back the
pseudo-vulcanization, are formed again simply by cooling
the material. It is apparent from the above explanation
that diblocks, which contain only one hard and one soft
block, cannot contribute to elastic properties.
Diblocks can improve processibility but their
content in the material must be confined within certain
limits so that they do not reduce elasticity to an
unacceptable extent. In addition, the total amount of
hard blocks is important too low a content will give
poor elastic properties (similar to an insufficiently
cured rubber), whereas too high a content will make the
2159939
13
material behave as a very hard, super- cured rubber,
again with very poor elasticity.
Amongst thermoplastic elastomeric block copolymers,
the so called Styrenic Block Copolymers (SBC) are well
known and widely used in many applications on account of
their very good properties. Styrene block copolymers as
a class are described for example in Thermoplastic
Elastomers: A Comprehensive Review, Legge, Holder &
Schroeder (Eds), Hauser Publishers (1987), Chapters 3, 4
and 12(1). They can have the structures already
mentioned above as:
- multiblock A-B-A-B-A-B- ..etc.
- linear triblock A-B-A
- radial or "star" polymers (AB)x where x > 2.
A represents a "hard" block of a vinyl-arene polymerized
monomer, generally styrene or alpha-methyl-styrene and B
represents a "soft midblock" generally formed by a
rubbery monomer such as poly(butadiene), (isoprene),
(ethylene- butylene) or (ethylene-propylene) rubbers.
The content of diblock molecules A-B in such
products can be as high as 80~ by weight and in special
commercial products can even form the totality of the
polymer. These products are used for particular
applications because, for the reasons discussed above
they have no or very poor elastic properties. Diblocks
can help processing and improve adhesive properties but
in order to retain good elastic characteristics their
2159939
_ 14
content in the thermoplastic elastomeric block copolymer
should be kept lower than 40$ by weight.
SBC's are widely used as substitutes for vulcanized
rubbers, their hardness, modulus and general mechanical
and elastic properties being strongly related to the
content of hard blocks, formed especially by polystyrene.
They have also found use as base polymers for hot melt
adhesives because of their generally good mechanical
characteristics, easy tackification of their rubbery
midblocks and good thermal stability which make them
superior to traditional bases for hot melts such as
ethylene-vinylacetate copolymers. However the main
object of standard compositions has been to optimize
adhesive properties with retention of at least some of
the elastic properties, typical of the base polymer, not
being taken into account.
Thermoplastic elastomeric block copolymers known as
SBC's, typically have the following characteristics:
They are formed by two kinds of monomers each
polymerized in blocks of the same monomer units, the
blocks being distinct even if chemically linked inside
the copolymer molecule. Moreover the two kinds of blocks
must be mutually incompatible.
- The structure according to which the two kinds of
present blocks are linked in the molecule can be:
- alternating multiblock as .... A-B-A-B-A-B....
- triblock linear as A-B-A
- radial or star structure as (A-B)x where x > 2.
2~ 59939
- "A" represents blocks of a polymer derived from a
vinyl- arene monomer, typically styrene or
alpha-methyl-styrene. They are called hard blocks
because at room temperature these polymeric species are
hard, glassy and fragile materials being under their
glass transition temperature (Tg).
Typically useful constituents for hard blocks have Tg
well above room temperature and preferably higher than
90°C.
- "B" represents blocks of a rubbery polymer having a
Tg < 0°C and preferably < -40°C.
Typically these "soft" blocks are formed by rubbers such
as polybutadiene and polyisoprene.
In the common technological lexicon the resulting
thermoplastic elastomeric block copolymers are often
referred to by the abbreviations SBS and SIS
respectively.
As already discussed in terms of the mechanism of the
generation of elastic properties in these type of
polymers, and particularly the function of hard blocks in
giving a physical vulcanization to the polymer, useful
SBC's contain at least two hard blocks "A" per molecule
and at least one soft block "B". Molecules formed by one
block of A and one block of H (the so called diblocks)
should, for use in the present invention, be kept lower
than 40$ by weight in the base polymer.
It is widely recognised in the literature that
differences exist between SIS and SBS copolymers which
21 X9939
16
are relevant to the formulation of hot melt adhesives.
SBS copolymers generally cost less than comparable SIS
copolymers and SBS copolymers can be synthesized to
exhibit better elasticity than comparable SIS copolymers.
However it has not hitherto been possible to take
advantage of these potentially advantageous properties of
SBS copolymers as a result of the fact that SBS
copolymers have not generally shown adequate adhesive
properties and SIS copolymers are much easier to tackify.
For this reason hot melt adhesives have generally been
formulated using SIS copolymers as the predominant SBC.
Both US-A-4418123 and EP-A-0424295, which are discussed
above, clearly prefer SIS as the SHC on which the
compositions are based and neither document discloses a
composition based on SBS which has satisfactory
properties.
It has been found according to the present invention
that compositions in which SBS copolymers are the main
polymers) can be produced with satisfactory adhesive
properties. At the same time these compositions retain
the advantages of SBS copolymers with respect to
elasticity which have been mentioned above. Thus,
compared to compositions based on other SBC's,
compositions can be formulated according to the present
invention with better elastic properties, quicker elastic
return, more flat stress/strain diagrams even at
elongations > 1000 ~ and give compositions of better
processability (more Newtonian rheological behaviour).
2159933
-- m
However, it should be noted that direct comparison
between SIS and SBS based compositions is very difficult
since the compositions need to be formulated in different
ways. Accordingly, it would not generally be possible to
substitute an SBS copolymer for an SIS copolymer in a hot
melt composition and obtain satisfactory properties and
other adjustments need to be made to the formulation
depending on the nature of the SBC in order to obtain
optimum results. The way in which compositions according
to the invention should be formulated to obtain the
desired properties is discussed in more detail
hereinafter.
Thus the compositions foamed according to this
invention are based on SBS copolymers or a blend of
SBS/SIS in which SIS is present at levels equal or less
than 50~ by weight of the total block copolymer.
All thermoplastic elastomers can be processed in the
molten state using various technologies and in various
apparatus, in all cases showing in the solid state
properties similar to those of a cured rubber.
Potentially all thermoplastic elastomers can be made
adhesive. Some adhere well enough in the molten
conditions to different substrates. However it is
clearly highly desirable to obtain thermoplastic
elastomers which are capable of adhering at room
temperature or at only moderately elevated temperature to
various substrates.
219939
1g
Thus, whilst pure thermoplastic elastomers have some
adhesivity at high temperature, this adhesivity can be
conveniently enhanced both in terms of the strength of
the bonds formed with different substrates and in terms
of the range of temperatures at Which strong bonds are
formed.
This enhancement is obtained by the use of at least
one suitable tackifying resin. More particularly, much
better adhesive properties and even self adhering
properties at room temperature (pressure sensitive
behaviour) can be obtained by blending thermoplastic
elastomers with the materials known as tackifying resins
which, as a class, are well known in the literature.
When thermoplastic elastomers are assembled at room
temperature (e. g. because it is desired to pre-stretch
them in the solid state and bond under tension) it is
necessary that they exhibit the typical behaviour of true
pressure sensitive adhesives and this generally requires
a_blend of a thermoplastic block elastomer and tackifying
resin. It should be noted that in order to enhance the
adhesive properties of the thermoplastic elastomer (both
at high temperature and at room temperature), only the
soft (rubbery) blocks of its molecule should be modified
by the tackifying resin. Thus, only interactions between
the soft (rubbery) blocks and a resin, substantially
compatible with them, causes the generation of tack
while the eventual modification of hard blocks with a
21 X9939
19
resin never leads to the development of adhesive
behaviour.
Not only do the hard blocks not exhibit any adhesive
activation but their eventual modification by a
tackifying resin could "soften" their mechanical
strength. This risks impairing their ability to function
as "centers of physical vulcanization" for the elastomer,
consequently destroying elastic behaviour.
So, for the various thermoplastic block elastomers,
depending on the chemical nature of their soft and hard
blocks, suitable tackifying resins can be identified
which must be compatible (i.e. soluble and capable of
creating the appropriate physical modification of the
system) only with the soft or rubbery blocks, whilst
compatibility with the hard blocks is as low as possible
or even zero, in order to retain as much as possible of
primary elastic properties of the polymer. However, the
amount of tackifying resin must be controlled since the
addition of quantities of tackifying resin(s), which are
too large, even if the tackifying resin is compatible
only with the midblocks (soft blocks) and fully
incompatible with the hard blocks, could still impair the
elastic properties of the resulting formulation. In any
case, the addition of the resin constitutes a dilution of
the concentration of the hard block domains, weakening
their ability to function as centers of "physical
crosslinking" for the elastomer. Thus both the content
of hard domains in the base thermoplastic block elastomer
2159939
20
and the content of the elastomer in the final formulation
must be such to ensure a sufficient final concentration
of hard domains in the formulation to retain appropriate
levels of "physical vulcanization" and thus of elastic
properties.
Therefore, on the one hand, it is important to
control the final concentration of hard blocks in the
composition. On the other hand, the addition of resins)
which are compatible only with the hard blocks and their
domains, is completely ineffective in the development
and/or improvement of adhesive properties. Resins
compatible with the hard blocks will, by swelling the
hard domains, stiffen the composition, increase modulus
and (compared to similar levels of tackifying resins
compatible with the midblock) will tend to increase
viscosity. In a system already containing a tackifying
resin compatible with the soft domains, the addition of
resins compatible with the hard domains will also
decrease the adhesivity. Accordingly, in general terms,
only limited quantities of resins compatible with the
"hard blocks" can be used without too great an impairment
of the overall properties of the adhesive elastic hot
melt. Generally they will only be used in special cases,
for example, if an additional increase in modulus is
required for some applications: or (using high softening
point hard block compatible resins) if a higher setting
temperature or a better temperature resistance is
desired.
~1 ~993g
21
The compositions which are foamed according to the
invention can be used to elasticate structures in which
they are applied without the use of any glue, for example
structures where elastication is obtained conventionally
by elastic formed of vulcanized rubbers. One material
(the foamed adhesive elastic hot melt) can substitute for
the use of two materials (the rubber and the glue to fix
it) with a substantial saving in costs. Normally rubber
elastics are covered with talc to prevent sticking of the
elastics in the packaging. Talc can give rise to
problems at the stage of adhesion with glues.
Moreover the thermoplastic, adhesive elastic hot
melt can be directly extruded in varied geometrical forms
directly during the construction of product which are to
be elasticated. It can be extruded as strips and as
films, etc. Structures such as strips or films can be
also foamed before the extrusion, obtaining elasticated
structure which are particularly soft. Elastication can
be also applied according to non-linear (curved)
geometries which makes the anatomical fitting, of the
product comprising the elastication, to the wearer's body
particularly good. This is very difficult to obtain with
standard rubber yarns of ribbons. Under different
geometrical forms the foamed adhesive elastic hot melts
can be applied both in an already stretched or an
unstretched state. In the first case the extruded melt
is cooled immediately after the extrusion die and foaming
2159939
22
and stretched at the desired elongation. In this case it
is advisable that it possesses the following properties:
- a relatively high setting point so that it
solidifies immediately after the extrusion and can be
elastically stretched. An elastic stretching can be
given only to a solid material, because any force applied
to a molten or semisolid material will cause only a
plastic lengthening along the direction of force without
any elastic tension.
- good pressure sensitive properties of adhesion
because the adhesive elastic hot melt will contact the
substrates) when already cold, e.g. at room temperature.
When the material is applied without any prior
elastic stretching and directly contacted to the
substrates) to which it has to adhere at the outlet of
the extrusion die and immediately after foaming, pressure
sensitive behaviour is less important because although
foamed the material is still in a liquid or semisolid
state and bonding is made when the material is still
above room temperature.
All these features are particularly suitable for the
elastication of hygienic, absorbent articles, although
the potentialities of the foamed materials according to
the invention are clearly not limited to these
applications. The use of materials applied in an already
stretched form can substitute for all of the presently
used rubber elastics in baby and adult diapers, in
catamenials, etc. when it is desired to have parts of the
2j59939
23
product already under elastic tension offering, as a
result of their extreme versatility, the possibility of
new elasticated structure of practically infinite
variety. In this case, another advantage of the foamed
elastic adhesive hot melts over rubber elastics is worthy
of note. As will be shown in more detail below, the
preferred elastic hot melt compositions have a
stress/strain diagram that is much flatter than a rubber
elastic, i.e. even if already under tension a further
stretching (e.g. due to the movements of the wearer of
the absorbent article) causes a very low increase in
modulus and in the tensile strength that is perceived by
the wearer. This is especially true for low modulus
compositions.
Compositions foamed according to the invention that
are more conveniently applied in the unstretched state
are typically used to give elastic return to
structures/products only when the whole final
structure/product is subject to some deformation during
use. Normally in this case the typical deformations that
are given in use to an absorbent article are very
limited, e.g. of the order of 5-50~. Accordingly, it is
necessary that the adhesive, elastic hot melt contained
in these structures is able to respond with a sufficient
elastic return force to external stresses even at these
low elongations. For these applications, it will be
generally more convenient to use formulations at higher
modulus.
24 ' 2 1 5 9 9 3 9
In summary the use of thermoplastic block elastomers, in different
physical forms and with different application processes, for the
elastication of structures and particularly of hygienic articles, is very
advantageous.
The foamed formulations of the present invention show optimum
properties, typical of hot melts, ranging from compositions that can more
conveniently be strongly bonded to substrates at high temperature from
the melt to about 50°C, to compositions that retain a permanent strong
adhesivity on most substrates even at room temperature being true
pressure sensitive adhesives. Moreover the compositions are
characterized by retention of distinct elastic properties from the base
thermoplastic block copolymer, showing all of the typical behaviours that
define an elastomeric material in the technological sense.
Thus when stretched in the solid state and when the stress is
relaxed they will return quickly to their initial length with only minor
permanent (plastic) deformation. The formulations preferably having a
2 0 distinct pressure sensitive character, can be applied even at room
temperature, both in the unstretched or preferably in the stretched state
for the elasHcation,
215993
_ 25
in different geometrical forms in various items and
particularly in absorbent, hygienic products such as baby
and adult diapers or adult incontinence products
different from diapers or feminine catamenials.
Compositions having lower pressure sensitive character
will be more conveniently be applied at temperature over
50°C, in the stretched or preferably in the unstretched
state for the elastication of the same structures and
products. In particular when applied in the unstretched
state they will work at limited extension, e.g. up to 50~
(i.e. final stretched dimension = 1.5 times initial
dimension).
In order to show even at these low extensions a
distinct elastic return force, these formulations will
generally have a higher modulus than the previous ones,
the two kinds of materials being, in fact, the extremes
of one field of formulations all of which are both
excellent hot melt adhesives and retain excellent elastic
properties, the passage from one to another being
gradual.
As already indicated, the basic compositions which
are foamed according to the invention can be divided into
"low modulus" and "high modulus" formulations, this
distinction being based on their modulus value and on
their behaviour as pressure sensitive adhesives.
Thus the present invention is concerned with foams
made from a family of compositions, based on at least one
thermoplastic elastic block copolymer in which SBS
21 ~993t9
26
copolymers) are the main copolymers) and at least one
tackifying resin essentially compatible with the soft
(rubbery) blocks of the aforementioned copolymer, the
tackifying resin being used mainly to improve adhesivity,
both at high and at room temperature of the
aforementioned copolymer. The compositions are
extrudable and in the solid state retain a distinct
elastic behaviour typical of elastomers from which they
are derived. As typical examples and without any
limitation, these compositions can be extruded and
applied in the form of strips or continuous films. As
examples of applications in the field of absorbent
articles, they can be used for the leg elastication of
diapers, as elastic waistband in the same, for the
elastication of catamenials and of adult incontinent
products other than diapers.
A lower modulus generally means that adhesive
materials have a more aggressive adhesivity, so that the
low modulus formulation generally have higher tack
typical of pressure sensitive adhesives. They are able
to form very strong bonds with many substrates on simple
contact even at room temperature or in any case lower
than 50°C.
Ratios between hard and soft blocks in the base
thermoplastic elastomeric copolymer are very important in
determining the elastic behaviour and the mechanical and
adhesive properties.
2j 59939
27
Generally the higher the content of hard blocks
(that conventionally will be referred to as "styrene
content") the higher the modulus, the more evident are
elastic properties, the quicker is elastic return after
relaxation of stretching but the lower is adhesivity and
especially pressure sensitive behaviour. All this is
true provided that the level of hard blocks does not
become so high that it forms the continuous phase and the
material becomes a hard and no longer elastic material.
Useful SBC's can contain from 10 to 50~ of styrene
by weight. However, when modified with the tackifying
resin, the behaviour of the resulting composition will be
clearly governed, in terms of all of the aforementioned
properties, by the resulting content of styrene or of
hard blocks in the compositions: so that it is determined
both by the level of styrene in the base copolymers) and
by the content of copolymers) in the final composition.
Too low a level of final block styrene will give poor
elastic properties. Too high a level will increase the
modulus and decrease the adhesivity to an unacceptable
extent. Increasing final styrene level in the
composition by increasing the content of copolymers)
will increase excessively the viscosity and decrease
processability. So both the level of copolymers) in the
composition and their content of styrene should be chosen
to optimize the final styrene content and thus all the
above mentioned properties. Optimum ranges will be
indicated below.
2~~9939
28
If desired the rubbery part of the SBC can itself be
cured (in a similar manner to the curing of natural or
synthetic rubbers) by using suitable chemical or physical
means, in particular curing systems known for synthetic
rubber which are not actuated by heat. This will have
the effect of increasing the modulus of the overall
composition.
The tackifying resin is added mainly to improve
adhesive properties of the base copolymers) even to the
extent of arriving at the typical behaviour of a pressure
sensitive adhesive. Moreover it improves the
processability of the thermoplastic elastomer both by
giving to that composition lower absolute values of
viscosity (as compared to the pure block copolymer) and a
rheological behaviour that, at the indicated levels of
resin, is practically Newtonian, i.e. the viscosity is
dependent only on temperature and does not change with
applied stress, a property which is very advantageous for
easy processing. It is known that SBC's which have many
very interesting characteristics, may be difficult to
process as a result of non-Newtonian behaviour in the
molten state as pure materials. This means that not only
do they show very high viscosity but also, under the
influence only of temperature, they do not appear to melt
even at very high temperatures near 200°C. They can even
begin to thermally decompose before showing a distinct
fluid state. In order to make them flow and so to be
able to process them, it is necessary to apply
21 X9939
29
temperature and high mechanical stress. In any case
processing of pure SBC's is difficult, viscosities are
high and highly dependent on the combination of
temperature and applied stress.
The basic composition of SBC and tackifying resin
foamed according to the present invention is capable of
giving materials which, whilst retaining very good
elastic and adhesive properties, are also easily
processable because of both relatively low viscosities
and of practically Newtonian (or acceptable
Newtonian-like) rheological behaviour. This latter
property was measured as variation at constant
temperature (180°C) of the viscosity under two levels of
applied shear rate, 20 and 80 sec-1. The ratio of these
two viscosities is hereinafter called the "Newtonian
Index" (N. I . ) .
An ideal Newtonian fluid should have N.I. - 1 while
a pure SBC could have, in the same conditions an N.I.
even > 6~.i.e. the viscosity variation, only due to the
variation of applied shear rate from 20 to 80 sec-1 is
more than 6 times which can cause severe problems for
regular and easy processing. For easy processability it
is necessary that the compositions have only limited
variation of viscosity at constant temperature with
variation of applied shear rate.
More particularly, it is preferred that compositions
show a Newtonian or almost Newtonian rheological
behaviour based on the molten material at 180°C, by
2159939
comparing the viscosities under a shear rate of 20 and 80
sec-1. Preferred compositions do not show a variation in
viscosity > 50~ i.e. a ratio between viscosities (N. I.)
not higher than 1.5.
It has been found that the most preferred
compositions based on SBS's have an almost ideally
Newtonian behaviour, with an N.I. not higher than 1.05.
In order to retain sufficiently the elastic
properties of the base polymer it is necessary that the
tackifying resin is compatible mainly with and preferably
essentially only with, the soft, rubbery blocks of the
block copolymer and does not interfere to a significant
extent with hard blocks. This is governed both by the
chemical nature of the resin and by its molecular weight.
The compatibility of the resin with the rubbery blocks
and its incompatibility with the hard blocks can be
measured, for example, by determining the variation of Tg
of soft and hard blocks deriving from the addition of
resin. In particular, incompatibility with the hard
blocks is considered satisfactory if their Tg (originally
at 100°C if they are formed from polystyrene) is not
changed more than 15°C by the addition of 100 parts of
resin to 100 parts of copolymer. Measurements of the two
Tg's requires appropriate equipment. Accordingly both
the experience of formulators and the technical
literature of resin suppliers can be taken into account
to determine which tackifying resins are chemically
compatible with the soft blocks and incompatible with the
2~5993g
- - 31
hard blocks of SBC's. As used herein, the term
"compatible essentially only with the soft blocks" means
that a tackifying resin is compatible with the soft
blocks of the copolymer and is incompatible with the hard
blocks to the extent that Tg of the hard blocks is not
significantly changed and more preferably decreased by no
more than 15°C on admixture of 100 parts of tackifying
resin to 100 parts of copolymer. Preferably the Tg of
hard blocks is not decreased at all.
More specifically a suitable main tackifying resin
will be chosen from the following chemical groups which
have high compatibility with soft blocks of SBC's and low
or no compatibility with their hard blocks:
- hydrocarbon resins
- aliphatic resins
- polyterpene resins
- terpene phenolics
- synthetic CS resins
- synthetic C5/C9 resins
- rosins and rosin esters
as well as their totally or partially hydrogenated
derivatives. They can be used as the pure resin or also
in blends.
When more than one resin is used, the main
tackifying resin system, defined as the essential
resin/blend of resins present at least at a level of 50~
of the total amount of resin, are characterised by having
a softening point between 85 and 150°C and more
2159939
_ 32
preferably between 100 and 140°C (all softening points
being measured by the well known Ring & Ball (R & B)
method) .
Tackifying resins having softening point < 85°C are
considered to have a prevailing plasticizing effect which
may in any case be important for the development of good
adhesive and elastic properties but is to be
distinguished from the tackifying effect. This is due to
the fact that in the processing of the present elastic
hot melt compositions quick setting of the material after
extrusion is desirable, especially for compositions which
are to be stretched before application on the substrate,
which is clearly possible only with solid materials. For
this reason it is desirable that the setting point of
these compositions is not less than 80°C and more
preferably greater than or equal to 100°C.
Setting point is most accurately determined by using
the technique known as Dynamic Mechanical Analysis under
sinusoidal stress, which is well known in the science and
technology of polymers and adhesives. According to this
technique three main rheological parameters of the
material are determined as a function of temperature:
- elastic or storage modulus G'
- the viscous or loss modulus G "
- the angle ° (delta) and its tangent, being the phase
shift between G' and G " .
G' is higher than G " when the material is solid.
When the material is fluid G " becomes higher than G'.
2159939
33
Naturally, G' is higher at low temperature and the
reverse at high temperatures. The crossing temperature
between G' and G " is taken as the true rheological
solidification (or melting) point of the material.
For use according to the present invention, it is
preferable that the crossover temperature for the
composition is greater than or equal to 80°C and more
preferably greater than or equal to 100°C.
The position of the crossover point is dependent on
many physical parameters of the hot melt. However, it
has been found that the main influence is the softening
point of the main tackifying resin and a secondary
influence is the content and molecular weight of the
copolymer. So the tackifying resin should preferably
have a softening point from 85 to 150°C and more
preferably from 100 to 140°C provided that in any case
the overall composition has a true rheological
solidification temperature at least of 80°C and more
preferably at least of 100°C.
Besides the thermoplastic elastomeric block
copolymer and the main tackifying resin, compositions can
contain additional components which improve specific
properties. A more detailed description of the
compositions and of their principal properties is given
below.
For practical reasons of clarity of description,
further description will relate specifically to "low
modulus" and "high modulus" compositions, it being
215993
34
understood that, as already indicated, these terms refer
to intrinsic modulus. As already noted, apparent
modulus, which depends on intrinsic modulus and degree of
foaming, should for practical reasons, be higher than
0.05 MPa.
The low modulus compositions are elastic,
extrudable, adhesive compositions based on at least one
thermoplastic elastomeric block copolymer, suitably
modified by the proper addition of at least one
tackifying resin essentially compatible with its soft
blocks. The polymer, or at least the polymer present at
the highest level, is a polystyrene/ polybutadiene block
copolymer. In this embodiment the compositions foamed
according to the invention have an intrinsic modulus of
0.5 MPa or less, essentially from 0.05 MPa to 0.5 MPa and
preferably less than or equal to 0.3 MPa~ the modulus
being measured at 23°C at 500 elongation (six times the
initial length of sample) under an elongation rate of 500
mm/minute. Moreover the compositions have viscosities at
180°C and with an applied shear rate of 80 sec-1 of
120000 centipoise (cps) or less and preferably 60000 cps
or less and more preferably 30000 cps or less.
The low modulus compositions will typically contain
from 10 to 80$ by weight, and more preferably from 15 to
50$ by weight, of SBC or of a blend of SBCs having the.
following characteristics:
- a molecular structure that can be multiblock, linear or
radial (star) provided that it contains per molecule, at
2159939
least two hard-blocks formed by a vinyl-arene polymer and
preferably polystyrene or poly-alpha-methyl-styrene, and
at least one soft or rubbery block, the soft block of the
SBC, or of the SBC present at the highest level, being
polybutadiene. The diblock content in the SBC(s) should
be kept lower than 40$ by weight.
- the aromatic content (conventionally referred to
hereinafter as "block styrene content") of the SBC(s) can
vary from 10 to 50$ by weight and preferably from 20 to
50$ by weight.
However in order to retain significant elastic
properties, both the SBC(s) level in the final
composition and the styrene content thereof should be
chosen so to have a final block styrene content in the
composition from 3 to 17$ by weight and preferably from 6
to 15$ by weight.
The composition also contains tackifying resin or a
blend of the same essentially compatible with the soft
blocks of SBC.
The preferred resins belong to the chemical groups known
as:
- hydrocarbon resins
- aliphatic resins
- polyterpene resins
- terpene phenolic resins
- synthetic C5 resins
- synthetic C5/C9 resins
- rosin and rosin esters
2159939
36
as well as their totally or partially hydrogenated
derivatives thereof.
The tackifying resin/blend of resin has/have a R&B
softening point from 85 to 150°C and preferably from 100
to 140°C. The level of such resin/blend of resin in the
composition can be from 20 to 90~ by weight. However, in
a preferred embodiment the content of resin/blend of
resin described above is from 30 to 55$ by weight the
remainder being formed by the additional components
described below which enhance elastic and/or adhesive
properties.
In any case both the level and the softening points
of the tackifying resin/blend of resins as well as those
of additional components described below will be chosen
so that the final composition has a true rheological
setting temperature (measured as crossing temperature of
G' and G ") not below 80°C and preferably not below
100°C.
- It has also been found that adhesive and/or elastic
and/or mechanical properties of the binary blends
SBC/tackifying resin can be improved by using additional
components.
Adhesive properties can be enhanced by adding
limited quantities of high molecular weight rubbers such
as polyisoprene, polybutadiene, polyisobutylene, natural
rubber, butyl rubber, styrene/butadiene rubber (SBR) or
styrene/isoprene rubber (SIR) and blends thereof. These
polymers have high viscosities and, in the uncured state,
-- ' 2159939
37
have poor elastic properties. However, adding them in quantities up to
15% by weight of the formulation and using polymers with Mooney
viscosities ML (1+4) at 100°C from 30 to 70, the resulting compositions
show improved pressure sensitive adhesive properties whilst still
retaining final viscosities within a useful range and without any
detrimental effect on final elastic properties. A particularly suitable SBR
is the product sold by Enichem under the trade name EUROPRENE
SOLT"' 1205 and by Fina under the trade name FINAPRENET"' 1205. This
product is described as an SBR in which styrene is partially distributed in
blocks. Of the total styrene content of 25% by weight, from 15 to 18% has
a block structure and the remainder is randomly co-polymerised with the
butadiene.
Plasticization of the composition can have very good effects not
only on the adhesive properties and on the viscosity but can also even
improve elastic behaviour by reducing the internal (molecular) frictions
that dissipate elastic energy during stretching and subsequent relaxation.
2 0 In general, the composition may contain up to 40% by weight of
plasticiser(s). In a preferred embodiment, the compositions contain at
least one of the following plasticizers:
- up to 40% by weight of a tackifying resin with a softening point
from 50 to 85°C,
2 5 - up to 20% by weight, and preferably up to 15% by weight, of a
liquid hydrocarbon resin, rosin ester or
A
2159939
38
polyterpene resin with a softening point not higher than 30°C,
- from 3 to 30% by weight and preferably from 5 to 15% by weight
of a paraffinic or naphthenic mineral oil having an aromatic content of
less than 10% by weight in order not to interfere with the styreruc
domains,
- up to 15% by weight of a liquid polyisoprene or depolymerized
natural rubber or polyisobutylene, polybutene or polypropylene oils and
the liquid copolymers thereof, for example PARAPOLT"~ (Exxon), or LIRTM
(from KURARAY).
The amount of plasticizer should be such that the setting
temperature is not lowered beyond the limit referred to above. In a
preferred embodiment the total plasticizer content in a low modulus
formulation is not less than 10% by weight and not higher than 40% by
weight.
In low modulus compositions, the use of aromatic resins, which
have no effect on adhesive properties, which interfere with the hard
2 0 blocks of SBC's and which stiffen the composition and tend to increase
viscosity, is generally not desirable and the preferred level of aromatic
resins is zero. However, limited quantities of an aromatic resin or a
blend of aromatic resins, for example 20% by weight or less, more
preferably 10% by weight or less can be used as a reinforcement for
2 5 compositions which have low total styrene content (say up to 6% ) or
which include significant amounts of SIS, for
219939
39
example 30~ by weight or more of SIS based on the total
SBC(s). In fact SIS copolymers, especially the ones
which have a styrene content < 30~ by weight when diluted
into the composition by resin and other additives, can
show an inadequate (too low) modulus and poor
characteristics of elastic return as a result both of the
intrinsic lower modulus of SIS and the low concentration
of styrene, acting as a physical vulcanizing agent. In
this case the aromatic resin can both increase modulus to
a useful level and increase the density of hard domains,
that are swollen by the resin. Useful aromatic resins
have a softening point from 115 to 160°C and are
chemically identified as derivatives of styrene,
alpha-methyl-styrene, vinyl-toluene, coumarone- indene
and copolymers thereof alkyl-aryl resins etc.
Apart from the components discussed above the
compositions can contain the usual additives such as
antioxidants, U.V. inhibitors, pigments and colouring
materials, mineral fillers etc. Generally in a total
amount up to 20$ by weight.
Without limitation as to their most suitable
processing and use, the low modulus formulations are
typically used in the stretched state at typical
extension levels of 400-1000. In the unfoamed state,
the compositions are characterized by very high .
elongation at break (over 1100 and often over 19000 and
very good adhesive, often pressure sensitive adhesive
properties. Elongation at break in the foamed state
2159939
depends on degree of foaming as well as the nature of the
foaming, i.e., the size of individual voids as well as
total void volume.
In order to simulate the application of the
composition into an absorbent article, pressure sensitive
adhesive properties were measured as loop tack (or "Quick
Stick Tack") and 90° peel according to the standard
methods FINAT Test MEthod No 9 for the loop tack and
FINAT test Method No 2 for the 90° peel, modified as
defined herein.
- For both tests the compositions were applied on a
polyester film at a weight of 80 g/m2.
Adhesive properties, both as loop tack and 90° peel,
were measured on a polyethylene film fixed on the
standard test plate.
- Loop tack values were expressed as peak values,
ignoring the initial peak.
- The 90° peel was evaluated after compression by a
490 g roll passed back and forth, i.e. two passes, one in
each direction, and measurements were made 20 minutes
after contact of adhesive and polyethylene.
The compositions according to the invention
generally have 90° peel > 7 N/cm (separating speed = 300
mm/min). Materials which can usefully be bonded and
assembled at room temperature into a hygienic product are
considered to be those which show on polyethylene 90°
peel strength > 3 N/cm.
215 9939
41
High modulus formulations are elastic, adhesive, extrudable
compositions similar to those described previously and having the
following characteristics:
1) They have an intrinsic modulus higher than 0.5 MPa and more
preferably not lower than 1 MPa up to 10 MPa.
2) At 180°C and with an applied shear rate of 80 sec-1, they have
viscosities of 80000 cps or less preferably 50000 cps or less and more
preferably 35000 cps or less.
3) They are based on the same types of SBC's referred to previously,
but which have a final block styrene content of from 15 to 30% by weight
and preferably from 15 to 25 % by weight.
4) The SBC or blend of SBC's which is used has a diblock content not
exceeding 25% and preferably not exceeding 10%. The most preferred
polymers are those with no content of diblocks such as those marketed by
DEXCO Co under the trade name VECTORT"'.
5) The preferred SBC(s) contain from 20 to 50% by weight of styrene
2 0 and the preferred level of SBC or blend of SBC's in the composition is
from 35 to 75% by weight, provided that both the styrene content of SBC's
and their level in the composition are such as to match the requirement of
point 3) above about final styrene content.
6) The tackifying resin/blend of tackifying resins has/have the same
2 5 chemical and physical characteristics as already discussed above.
However, the preferred content is from 20 to 40% by weight.
A
2~ X9939
- - 42
7) The content of high molecular weight rubbers such as
polyisoprene, polybutadiene, polyisobutylene, natural
rubber, butyl rubber, SIR and SBR should not exceed 10~
by weight and preferably is less than 5$ by weight based
on the total composition.
8) The total content of plasticizers, as previously
described should not exceed 25~ by weight.
9) Aromatic resins or blend of the same are still
preferably avoided for their detrimental effect on
adhesive and stress/strain properties (steeper
stress/strain diagrams generation of a yield point and
consequently of an unrecoverable plastic deformation).
However, as in the previous case, these materials can be
present at levels up to 20~ by weight with acceptable
properties in the composition provided that the
non-adhesive/non elastomeric part of the composition
does not exceed 50~ by weight of the total composition.
This non-adhesive/non-elastomeric part is formed by the
sum of the total styrene content in the composition plus
the content of aromatic resin/resins.
Other requirements and other possible further
components and additives remain the same. In particular
it is still required that the true rheological setting
temperature (measured as the crossing point between G'
and G ") is not lower that 80°C and preferably not lower
that 100°C.
Again with no limitations on their processing and
use, these high intrinsic modulus compositions are often
2159939
43
applied in the unstretched state, especially the ones
having moduli > 1 MPa. This preferred use is due to the
fact that they are capable of giving sufficient elastic
return forces even at low deformations (typically not
higher than 50$) which are often met during use of
stretchable hygienic articles which can conveniently be
made elastic and resilient in this way. This is also due
in part to the fact that assembling with the composition
in the stretched state implies the need to apply the
composition at about room temperature and so requires
that it adhere strongly to substrates even in these
conditions (pressure sensitive adhesive properties). The
pressure sensitive character of adhesives tends to be
inversely proportional to their elastic modulus.
However, some of the high modulus compositions
foamed according to the invention still retain distinct
and useful pressure sensitive behaviour (90° peel on PE >
3 N/cm) and can be applied also at room temperature and
in the stretched state at typical elongations up to 400.
Adhesive properties are measured under the same
conditions as for low modulus compositions.
The unexpectedly good level of elasticity of the
foams according to the invention can be measured as
retention of tensile strength after cyclic deformation.
This is a test that simulates conditions in use on a
hygienic article where the movements of the wearer can
cause further and subsequent elongations of the
elasticated parts which, for optimum behaviour, must
215999
44
regain their initial length with only minor losses of
tensile strength. All the compositions are tested
starting from an already stretched state and are given a
further elongation of about 15~ of the initial stretched
length, in order to simulate movements of the wearer.
The compositions were cyclicly stretched and released
fifty times from the initial elongation to the further
stretched elongation.
The $ retention in tensile strength at the initial
elongation after 50 cycles of stretching at a speed 500
mm/minute, compared to the initial tensile strength, was
taken as a measure of the elasticity of the materials.
Tests were performed at room temperature on bands 2.54 cm
wide.
- Low modulus compositions were stretched at an
initial elongation typical of intended applications of
800 and then cyclicly further stretched and released 50
times between 800 and 920 (ie from 9 to 10.2 times the
initial length of the sample).
- High modulus compositions were tested in the same
manner but at an initial elongation typical of intended
applications of 300 and under 50 cycles of further
elongation and relaxation between 300 and 345.
The term "tensile strength retention after 50
cycles" as used herein refers to the test described
above. A natural rubber vulcanized elastic produced by
the company JPS Elastomerics which is used in the leg
elastication of diapers and applied at an initial
2159933
stretched deformation of 220 was taken as a reference
and was cyclicly deformed 50 times between 220 and 255
It was found that under these conditions the natural
rubber vulcanized elastic, after 50 cycles, had an
average retention of tensile strength equal to 47~ of the
initial tensile strength at 220. This level of
retention of tensile strength was considered indicative
of good elastic behaviour.
More generally it was observed that materials that
do not lose more that 60$ of their initial tensile
strength in these test conditions, show good elastic
behaviour. Retention of tensile strength less than 40$
represents unsatisfactory elasticity as indicated by slow
return to the initial length after release of stretching,
high permanent plastic deformations etc.
As will be shown in the following examples,
compositions, both low and high modulus, which can be
foamed to produce foams according to the invention show
good elastic behaviour, at the same level or better than
the natural rubber vulcanized elastic.
More specifically high modulus compositions retained
up to 67.5 of their initial tensile force and low
modulus compositions up to 59.8.
Elastic properties were also judged by the following
method: The compositions in the form of bands 2,54 cm
wide, were tested at 23°C and at a stretching speed of
1000 mm/minute, with 3 cycles of elastic hysteresis
between zero and a typical possible elongation in
215993
46
application i.e. 800 for low modulus and 300 for high
modulus compositions. The elastic energy of each cycle
evaluated as the area of the cycle was recorded and the
ratio between the elastic energy of the third and the
first cycles was determined as retention of elastic
energy after 3 hysteresis cycles. For good elastic
behaviour it is believed that under these test
conditions, a retention of elastic energy not lower than
30$ is required.
The present invention also provides a process for
the production of foams. For the production of foams,
the compositions described above can extruded in various
desired forms, for example in the form of a strip, and
foamed at the moment of extrusion by use of conventional
foaming technology to provide an adhesive foam. The
foamable composition is generally extruded at a
temperature at which it is molten, for example at from
130 to 230°C, and foaming can be effected by physical or
chemical means. Foaming by physical means involves use
of an inert gas or an inert liquid. For example an inert
gas such as nitrogen or carbon dioxide can be blown under
pressure into the molten composition. Alternatively, an
inert volatile liquid, such as methylene chloride, can be
included in the composition which then evaporates at the
extrusion temperature and acts in the same way as the
inert gas. Foaming by chemical means involves use of a
chemical blowing agent such as diazocarbonamide which
decomposes at the extrusion temperature to release
2~~9939
_ 47
sufficient amounts of gas to foam the extruded
composition. Preferably the foaming is achieved using an
inert gas.
Immediately following blowing and extrusion the
composition is cooled, generally by natural cooling to
room temperature, to stabilise the foam. At the same
time the blowing action helps cooling and stabilises the
walls of the cells of the foam and helps in setting up
the foam. The apparent modulus of the foamed composition
can be adjusted to a required value by varying the
density of the foamed mass, for example by varying the
quantity of blowing agent or the pressure where the
foaming agent is a gas blown directly into the molten
composition.
If required, the composition can be foamed and
extruded on-line in the production of an article into
which the foamed composition is to be incorporated to
yield a foamed elastic, for example in the form of a
strip, which is sufficiently adhesive to bond to the body
of an article, for example the body of a diaper. Strips
of the elastic/adhesive material foamed according to the
invention produced in this way can be used in those
applications where two materials (an elastic foam, for
example a polyurethane foam, and an adhesive) have been
used previously with consequent savings in cost,
increased assembling efficiency and better performance of
elasticated portions of the article. In addition, for
use, for example, in the waistband of a diaper a foamed
215999
48
elastic adhesive has the advantage that for a given contact area and a
defined elastic force, the weight of the foamed elastic adhesive is lower
than the weight of a non-foamed composition performing the same
function with consequent cost saving. The foamed structure is also
thicker but at the same time softer.
The invention is illustrated by the following examples which
should not be considered as in any way limiting on the invention. In the
case of proprietary products, details of their nature and composition is
that provided by the manufacturer.
The compositions of all of Examples 1 to 5 are all suitable for
foaming according to the invention by the method described in Example
1 or by other methods described herein. The compositions of all of
Examples 1 to 5, when applied from the molten state between plastics
and/or cellulosic materials at a weight of 5 g/m2 showed a bond strength
well in excess of 0.5 N/cm measured as 90° peel.
2 0 EXAMPLE 1
An SBC polymeric system based on SBS
(styrene-butadiene- styrene block copolymers) was
formulated as follows:
CARIFLEXT"' TR-4113 S 36% by weight
2 5 EUROPRENE SOL 1205 8%
DERCOLYTET"" A 115 45.8%
FORALT"' 85-E 6%
A
49 2 1 5 9 9 3 9
HERCOLYNT"" D-E 4%
IRGANOXT"" 1010 0.2%
where:
- CARIFLEX TR-4113 S is an oil-extended SBS copolymer available
from SHELL Co said to contain:
68.5% by weight of a linear triblock SBS having a styrene content of 35%
by weight and with a diblock content < 20 % by weight
31.5 % by weight of a naphthenic mineral oil, acting as a plasticizes, and
containing less than 5% of aromatics.
- EUROPRENE SOL 1205 is a styrene/butadiene rubber (SBR)
available from ENICHEM. (The product FINAPRENE 1205 available
from FINA is similar and could equally be used.) It is described as a
solution polymerised SBR having a Mooney viscosity ML (1+4) at 100°C
equal to 47 and a total styrene content of 25% by weight. This styrene as
partially (typically from 15 to 18%) distributed in blocks with the
2 0 remainder being randomly copolymerized with butadiene. The
randomly copolymerized styrene gives the rubbery part of the molecule
the chemical structure of an amorphous SBR, which contributes to the
development of particularly good pressure sensitive adhesivity.
- DERCOLYTE A 115 (the main tackifying resin)is available from
2 5 DRT. It is a polyterpene resin derived from alpha- pinene having a
softening point of 115°C.
2~ ~993g
- FORAL 85-E is a tackifying resin composed of a
hydrogenated glycerol ester of rosin available from
HERCULES Co. It has a softening point of 85°C.
- HERCOLYN D-E is a liquid methyl ester of rosin
available from HERCULES.
- IRGANOX 1010 is a phenolic antioxidant available
from CIBA-GEIGY.
The composition was found to have the following
properties:
- total styrene content = 10.6$ by weight of which
10.1$ is in blocks
- viscosity at 180°C at 80 sec-1 = 20520 cps
- intrinsic modulus at 500$ elongation - 0.182 MPa
(low modulus)
- elongation at break > 1400$ (1400$ was the maximum
elongation achievable on the machine used for this
determination)
- rheological setting temperature (crossover point of
G' and G'~') - 125°C
- 90° peel on PE = 16.3 N/cm
- tensile strength retention after 50 cycles between
800 and 920$ = 59.8$
- elastic energy retention after 3 hysteresis cycles
between zero and 800$ = 57.7$
- Newtonian Index (N. I.) - 1.05.
The composition showed extremely good elastic and
adhesive properties and was considered completely
21 ~~~3~
51
suitable for elastication of structures, particularly
hygienic absorbent articles. It was easily processable
i.e. extrudable, having quick setting (stretchable on
line) and having good pressure sensitive characteristics
allowing the formation of strong bonds on simple contact
with many substrates even at room temperature.
The composition is foamed to varying foam densities
using a commercially available foaming apparatus intended
for hot melt compositions (FOAMMELT apparatus produced by
Nordson, Germany). Foaming was carried out using
nitrogen under pressure as the blowing agent at a
temperature of 180°C and gas pressure was adjusted to
give foams of varying densities. The flow rate of the
elastomeric hot melt was about 1-2 grams per minute and
the foam produced was about 1 cm thick depending on the
foam density.
The following properties have measured on the foamed
composition:
_ Density Apparent Modulus
0.94 g/cm2 0.182 MPa
0.83 g/cm2 0.162 MPa
0.79 g/cm2 0.136 MPa
0.69 g/cm2 0.113 MPa
0.52 g/cm2 0.058 MPa
EXAMPLE 2
The formulation was:
2159939
52
CARIFLEX TR-41135 38.8$ by weight
FINAPRENE 1205 8.9$
DERCOLYTE A115 26$
FORAL 85-E 26$
IRGANOX 1010 0.3$
The following properties were measured:
- total styrene content = 11.5$ by weight of which
10.9$ is in blocks
- viscosity at 180°C at 80 sec-1 = 28180 cps
- intrinsic modulus at 500$ elongation = 0.188 MPa
(low modulus)
- elongation at break > 1400$
- rheological setting temperature = 120°C
- 90°C peel on PE = 10.4 N/cm
- tensile strength retention after 50 cycles between
800 and 920$ = 59.1 $
- elastic energy retention after 3 hysteresis cycles
between zero and 800$ = 48.3 $
- Newtonian Index (N. I.) - 1.01.
The composition was similar to that of Example 1,
the main variation being the fact that about 50$ of the
high softening point tackifying resin was substituted by
a lower softening point resin and the only plasticizer
was the oil contained in CARIFLEX TR-4113 S (12.2$ by
weight of the composition). Nevertheless it was found
....
' 215 9939
53
that the composition still retained a very high solidification temperature,
quick setting as well as optimum elastic and pressure sensitive adhesive
properties and excellent processability, so that it was easily possible to
extrude and immediately stretch it even to 800 % .
EXAMPLE 3
A different system based on radial SBS was tested, more precisely
a blend of one SBS with relatively low hard block content and one SBS
with relatively high hard block content.
The formulation was as follows:
FINAPRENE 415 17.7% by weight
FINAPRENE 401 7.0%
FINAPRENE 1205 8.0%
ZONATACT"" 115 LTTE 45.8
2 0 FORAL 85-E 6.3
HERCOLYN D-E 4.0%
SHELLFLEXT"" 4510 FC 11.0%
IRGANOX 1010 0.2%
where:
2 5 - FINAPRENE 415 and FINAPRENE 401 are radial SBS copolymers
available from FINA. Both are supposed to contain less than 20% diblock
and to be formed by a "star'' structure of four blocks of SB chemically
linked in a central point through the butadiene blocks.
A
21~993~
54
FINAPRENE 415 contains 40~ by weight of block styrene and
FINAPRENE 401 contains 22$ of block styrene.
- ZONATAC 115 LITE is a hydrocarbon modified terpene
tackifying resin with a softening point of 115°C
available from Arizona Co. It is supposed to be based on
limonene modified with styrene.
- SHELLFLEX 4510 FC is a naphthenic mineral oil,
available from SHELL, which is supposed to have an
aromatic content < 5~.
The following properties were measured:
- total styrene content = 10.6$ by weight of which
10.1 is in blocks
- viscosity at 180°C at 80 sec-1 = 12810 cps.
- intrinsic modulus at 500 elongation = 0.223 MPa
(low modulus)
- elongation at break > 1300$
- rheological setting temperature = 107°C
90° peel on PE = 14 N/cm
- tensile strength retention after 50 cycles between
800 and 920 = 50~
- elastic energy retention after 3 hysteresis cycles
between zero and 800$ = 44.9
- Newtonian index (N. I.) - 1.04.
The composition showed properties typical of a very good
and easily processable elastic, extrudable adhesive
material.
2159938
EXAMPLE 4
5 The following high modulus composition was made with the
formulation:
VECTOR 4461-D 44.8% by weight
ZONAREZT"" 7115 LTTE 37%
10 FORAL 85-E 5
ZONAREZT"' ALPHA 25 3%
PRIMOL 352 10
IRGANOX 1010 0.2%
15 where:
- VECTOR 4461-D is a linear SBS copolymer having 43% by weight
of styrene and non diblock content available from DEXCO Co.
- ZONAREZ 7115 LTTE is a polyterpene tackifying resin, having a
softening point of 115°C, derived from limonene, available from
2 0 ARIZONA Co.
- ZONAREZ ALPHA 25 is a liquid tackifying resin (S.P. = 25°C)
derived from alpha-pinene having a very good plasticizing effect. It is
available from ARIZONA Co.
- PRIMOL 352 is a plasticizing, paraffiruc mineral oil available from
2 5 EXXON, which is said to contain non aromatics.
The following properties were measured:
- total block styrene content =19.3% by weight
- viscosity at 180°C at 80 sec-1=16810 cps.
A
215 9939
56
- intrinsic modulus at 500% elongation =1.07 MPa (high modulus)
- elongation at break = 987%
- rheological setting temperature = 111°C
- 90° peel on PE = 6.5 N/cm
- tensile strength retention after 50 cycles between 300 and 345% _
67.5
- elastic energy retention after 3 hysteresis cycles between zero and
300% = 63.3%
- Newtonian Index (N.L) = 1.05.
The composition was a good elastic material useful especially at low
elongations. It showed acceptable semi-pressure sensitive characteristics
so that it can be bonded to materials also at room temperature in the
stretched state.
~~r a ~~rnr ~ ~
The formulation was:
VECTOR 4461-D 54.8% by weight
ECR 368 35%
PRIMOLT"' 352 10%
IRGANOX 1010 0.2%
where:
2159939
57
- ECR 368 is a hydrogenated hydrocarbon tackifying
resin, available from EXXON and having a softening point
of 100°C.
The following properties were measured:
- total block styrene content = 23.56 by weight
- viscosity at 180°C at 80 sec-1 = 39000 cps.
- intrinsic modulus at 500 elongation = 1.61 MPa
(high modulus)
- elongation at break = 947
- rheological setting temperature = 114°C
- 90° peel on PE = 2.6 N/cm
- tensile strength retention after 50 cycles between
300 and 345 = 62.3
- elastic energy retention after 3 hysteresis cycles
between zero and 300$ = 38.3$
- Newtonian Index = 1.05.
The composition was made so to maximize modulus
whilst still retaining acceptable elasticity and good
adhesivity at temperatures higher than room conditions.
Such a material is more conveniently applied in the
unstretched state and bonded directly at temperature >
50°C. It gives good elastic return forces even at very
low extensions, e.g. modulus at 20~ elongation = 0.236
MPa. However, it also works well in the stretched state,
e.g. 300$ elongation, and shows adhesive properties on PE
which are not negligible even at room temperature.
2159938
58
COMPARATIVE EXAMPLE A
The formulation was:
KRATONT"" D1107 40.2% by weight
WINGTACKT"' 95 32.9%
IC-145 26.5
WESTONT"~ 618 0.2%
IRGANOX 1010 0.2%
where:
- KRATON D1107 is a linear SIS block copolymer available from
SHELL containing 14% by weight of styrene.
- WINGTACK 95 is a synthetic polyterpene resin, having a softening
point of 95°C available from Goodyear Co.
- IC-145 is a coumarone-indene aromatic resin, having a softening
point of 145°C and available from the German Company VFT.
2 0 - WESTON 618 is a phosphite based antioxidant available from Borg
Warner Co.
- IRGANOX 1010 is as described in Example 1.
This formulation was made in accordance with the teaching of US
A-4 418123 (Example IV). According to the US patent the composition is
2 5 said to have completely satisfactory elastic, adhesive and processing
properties. The formulation of Comparative Example A has the
following differences from Example IV of US-A-4 418123:
59 2 1 5 9 9 3 g
1) The coumarone-indene resin CUMART"' LX-509 is not widely
available in Europe and the equivalent resin IC-145 was used. The resins
are chemically similar but IC-145 has a softening point of 145°C as
compared to the figure of 155°C reported for CUMAR LX-509 in the US
patent;
2) For practical reasons, the pigment (1.5 % titanium dioxide) was
omitted. The pigment was presumably present in the original
formulation to mask the light brown colour and would be expected to
have a negligible effect on adhesive and elastic properties.
The main properties can be summarised as follows:
- total block styrene content = 5.6% by weight
- viscosity at 180°C at 80 sec-1 =192000 cps
- intrinsic modulus at 500% elongation = 0.365 MPa (low modulus)
- elongation at break > 1400% (1400% was the maximum elongation
achievable on the machine used for this determination)
- rheological setting temperature (crossover point of G' and G") _
2 0 145°C
- 90° peel on PE = 15.7 N/cm
- tensile strength retention after 50 cycles between 800 and 920% _
34.3
- elastic energy retention after 3 hysteresis cycles between zero and
800% = 19.8%
- Newtonian Index (N.L) = 2.03.
A
2159939
_ 60
It was found that the above formulation had good
adhesive, pressure sensitive characteristics as indicated
by the US patent in that as measured on PE under the
above described conditions, the 90° peel was 15.7 N/cm.
However it was also found that both elastic and
processing properties for an extrudable material were
unsatisfactory. In particular:
- The processability, especially the application under
thin strips, was very difficult owing to the extremely
high viscosity and the highly non Newtonian rheological
behaviour.
- The modulus at 500$ elongation was found to be 0.365
MPa and the elongation at break to be > 1400$. However
elastic properties were unsatisfactory. In particular
under the cyclic deformation test between 800 and 920
the formulation was found to lose 65.7 of its initial
tensile strength after 50 cycles and to lose 80.28 of its
elastic energy after 3 hysteresis cycles between 0 and
800 making it unsuitable for the elastication of
products such as hygienic absorbent articles that are
stressed in use many times by subsequent stretchings due
to movements of wearer. It is worthy of note that 45~ of
the loss of tensile strength occurred between the first
and the second cycle, confirming an easy and
unrecoverable plastic deformation.
COMPARATIVE EXAMPLE H
215 9939
61
The formulation was:
TUFPRENET"~ A 30.0% by weight
ESCOREZT"" CR 368 55.0%
CATENEXT"' P941 10.0%
KRISTALEXT"" F100 5.0%
with the addition of 0.2 parts per 100 parts by weight of the above
composition of the antioxidant IRGANOX 1010, where:
- TUFPRENE A is a linear SBS available from Asahi Chemical CO.
and containing 40% by weight of styrene. Diblock content is not specified
by the manufacturer.
- ESCOREZ CR 368 is a hydrogenated modified hydrocarbon resin
available from Exxon having a softening point of 100°C.
- CATENEX P941 is a paraffiruc mineral oil available from Shell
which is supposed to have an aromatic content < 5% by weight.
- KRISTALEX F 100 is an aromatic resin based on °-methyl styrene
2 0 and styrene available from Hercules and having a softening point of
100°C.
This formulation was made in accordance with the teaching of EP-
A-0424295 (Example V). The formulation of comparative example B has
2 5 the following difference from Example V of EP-A-0424295:
0.2 parts per 100 parts by weight of the antioxidant IRGANOX
1010 was added to the composition as set out in
21 X9933
62
Example V of EP-A-0424295. As would be apparent to any
person skilled in the art, attempting to compound and
process the composition exactly as disclosed in
EP-A-0424295, i.e. without an antioxidant would
undoubtedly have led to thermal degradation of the
system. In order to follow the teaching of EP-A-0424295
as closely as possible the antioxidant used in the
comparative example A of that document was used.
However, the antioxidant was added at the usual level of
0.2~ (this being the level also used in the preceding
examples according to the invention) rather than the
unusually (and also unnecessary) high level used in
Comparative Example A of EP-A-0424295.
The main properties can be summarised as follows:
- total block styrene = 12~ by weight
- viscosity at 180°C at 80 sec-1 = 5620 cps
- intrinsic modulus at 500 elongation = 0.204 MPa
(low modulus)
elongation at break > 1300
- rheological setting temperature (crossover point of
G' and G" ) - 106°C
- 90° peel on PE = 0.8 N/cm
- tensile strength retention after 50 cycles between
800 and 920 = 21.3 .
- elastic energy retention after 3 hysteresis cycles
between zero and 800$ = 25.0$
- Newtonian Index (N. I.) - 1.16
2~ 5993
_ _ 63
It was found that processability, as shown by
viscosity and NI was acceptable although NI was high for
an SBS based composition. However, the formulation had
poor pressure sensitive characteristics and the value of
0.8 N/cm for 90° peel shows it to be practically unusable
as a low modulus elastic adhesive, intended to be applied
in the stretched state, in the assembly of hygienic
absorbent articles. Elastic properties were
unsatisfactory as indicated by the loss of about 80$ of
the tensile strength of the composition after 50 cycles
of subsequent stretching and of 75$ of its elastic energy
after 3 hysteresis cycles.
The unsatisfactory properties of the composition may
be related to the diblock content of TUFPRENE A. This is
not stated by the manufacturer and direct measurement is
difficult. However the value provided by the
manufacturer for Tensile Set at break measured according
to ASTM method D412 is 47$. This compares to much lower
values of around 10$ or lower quoted for SBS copolymers
with a diblock content < 20$ by weight. On the other
hand SHELL technical literature gives the following
figures for Tensile Set at break for copolymers with a
high diblock content
- KRATON D-1112 (SIS containing 40$ by weight diblock)
- 20$
- KRATON D-1118X (SBS containing 80$ by weight
diblock) -40$.
2159939
64
From this, it can be inferred that TUFPRENE A probably
has a diblock content well in excess of 40$.
It should be noted that the above results seem to be
inconsistent with the results reported in EP-A-0424295 in
at least some respects. Thus the 90° peel of 0.8 N/cm
reported above compares to 3.7 N/cm for 180° peel
derivable from EP- A-0424295. This may be explained at
least in part by both the different peel angle, and the
compression used in the test (400 g as compared to 2 kg)
since the composition is stiff and bonding is very much
influenced by compression. It should also be noted that
the weight of adhesive per square meter is not specified
in EP-A-0424295 and this is extremely important in
determining bond strength. The tests used according to
the present invention provide a realistic measure of the
suitability of the compositions for use in the proposed
applications. The poor properties of the composition of
EP-A-0424295 may also be related to the combination of a
hard SBS (40~ styrene) with an aromatic resin at low
plasticiser levels (10~).
EXAMPLE 6
This example relates to the stress/strain diagrams
of the compositions of Examples 1 to 5 and Comparative
Examples
A and B. The elastic hot melt compositions according to
the invention should desirably have a stress/strain
21 ~993~
diagram that is much flatter than a rubber elastic, i.e.
even if already under tension further stretching (e. g.
due to the movements of the wearer of the absorbent
article) cause only a very low increase in modulus and in
the tensile strength that is perceived by the wearer.
This is especially true for low modulus compositions and
can be seen by measuring the average increase in modulus
for a given extension. Low modulus compositions, which
are typically applied in the already stretched state,
were judged as the mean increase in modulus per 100$
increase in elongation, by dividing by 8 the total
increase in modulus between 0 and 800$ elongation (nine
times the initial length).
Referring to the low modulus compositions mentioned
in the above Examples the results were as follows:
EXAMPLE NO. MEAN INCREASE IN MODULUS PER
100$
STRETCHING
1 0.044 MPa/100$ stretching
0.045 MPa/100$ stretching
0.063 MPa/100$ stretching
Comparative Example A 0.099 MPa/100$ stretching
Comparative Example B 0.079 MPa/100$ stretching
The high modulus compositions of Examples 4 and 5, which
are generally used in the unstretched state or in any
case at lower elongations, were judged as mean increase
~1 X9939
_ 66
in modulus per 100 increase in elongation between zero
and 300$ final elongation:
EXAMPLE N0. MEAN INCREASE IN MODULUS PER
100$
STRETCHING
4 0.169 MPa/100$ stretching
0.284 MPa/100$ stretching
As a comparison, a natural rubber vulcanized elastic,
used for the leg elastication of diapers, even if applied
at much lower extension (typically 220$) showed, between
zero and 220, an average increase in modulus of 0.89 MPa
per 100 elongation.
It is possible to compare the behaviour of a natural
rubber elastic and of a low modulus composition according
to the invention.
In the case of rubber elastic, even limited
m9vements of the wearer that cause for instance a further
stretching of the elasticated parts as low as say 10~
elongation, will cause a mean increase in modulus of
about 0.09 MPa. By using a low modulus composition the
increase in modulus will be about 20 times lower and even
with the strongest high modulus compositions several
times lower so that the movements of the wearer of an
absorbent article elasticated by the compositions
disclosed in the present invention are much more free.
Accordingly, in these applications low modulus and low
67
variation of modulus with strain are a clear advantage.
