Note: Descriptions are shown in the official language in which they were submitted.
1 336672
This invention relates to multiple layer sheet materials,
especially multiple layer packaging sheet materials which are
used to fabricate packages through the formation of heat
seals about the periphery of the package. The invention is
particularly related to the ability of the packages to
tolerate flexure of the package and to tolerate shock
stresses generated by impact forces, as when the package is
dropped. The preferred sheet materials of the invention
include a functional capability to provide barrier to
transmission of one or more gases such as oxygen or water
vapor.
In forming heat seals in such sheet materials, the
amount of heat which is driven through the sheet material to
the sealant layer to soften that layer sufficiently for
formation of the heat seals, as a secondary and undesirable
function, is sufficiently intense to significantly soften the
vinylidene chloride copolymer layer during the formation of
the heat seals.
Packages made with conventional ones of such sheet
materials, to the extent their overall thicknesses is less
than about 0.38 mm, tend to be deficient in impact
- resistance; especially packages where such sheet materials
are used as closure lids on formed trays. In such
applications, the lid is the weakest member of the packages
such that any failure of the package typically occurs in the
lid sheet material adjacent the heat seal.
'~
.. -- 1
1 336~72
"Softening temperature" as used herein means any
determinant and measurable temperature which identifies a
condition at which the polymer experiences a change which
tends to make it more fluid, and subject to flowing at normal
conditions used in forming heat seals. While the DSC melting
point is generally intended herein, other tests could equally
well be used so long as they are applied equally to all the
layers being compared.
This invention provides improved multiple layer sheet
materials wherein there is provided an impact layer having
elastomeric properties and an improved impact tolerance in
the resulting sheet material, such that the impact tolerance
of the package is improved.
More particularly, the invention provides increased
impact tolerance at the heat seal locus while maintaining the
ability, in the sheet materials, to form heat seals
sufficiently strong to maintain the integrity of the seal
through out the intended use life of the package until it is
intentionally opened.
The invention provides a multiple layer sheet material
comprising a first sealant sheet structure comprising
polypropylene, a second substrate structure comprising at
least one polymeric layer, and an impact layer disposed
between the first sealant sheet structure and the second
substrate structure. The composition of the impact layer
comprises a first component including a polypropylene polymer
and, as a second component, a polymeric composition providing
. - 2 -
1 33667~
elastomeric properties to the impact layer and increased
shock tolerance to the multiple layer sheet material. The at
least one polymeric layer in the second substrate structure
is disposed toward the impact layer.
In preferred embodiments of the invention, the second
component of the impact layer comprises either an ethylene
butene-l copolymer containing 85% to 95% ethylene, or a
polyisobutylene. Specially preferred embodiments include, as
a third component of the impact layer, a second polymeric
component providing elastomeric properties to the impact
layer and shock tolerance to the multiple layer sheet
material. Typically that component itself is generally
recognized as possessing elastomeric properties.
In certain embodiments, and especially where a peelable
seal property is desired, the first sealant sheet structure
comprises a combination of polypropylene and a polypropylene
having a density of at least about 0.930, preferably at least
about 0.940, most preferably at least about 0.950, referred
to herein as high density polyethylene (HDPE). The HDPE
serves as a modifier to the polypropylene to control the
strength of heat seals formed with the sheet material.
In certain embodiments of the invention, and especially
(but not only) those embodiments where the primary polymer in
the sealant sheet structure is modified to reduce the seal
strength, the sheet material exhibits improved shock/impact
tolerance when the sealant sheet structure comprises, as a
third component, a polymer selected from the group consisting
- 3 -
1 336~7~
of polyisobutylene and ethylene butene-l copolymer containing
85% to 95% ethylene.
In certain embodiments of the invention, the substrate
structure comprises a layer which provides for the overall
sheet material an oxygen transmission rate of no more than
about 5 cm/m2 day at 23C, 0% relative humidity.
In certain embodiments, the substrate structure includes
a layer which provides for the overall sheet material a
moisture vapor transmission rate of no more then about 2cm/m2
day at 100% relative humidity.
In some embodiments, the sheet material is comprised
entirely of extensible materials, such as unoriented
polymers. In other embodiments, the sheet material,
preferably in the substrate structure, includes one or more
layers of a less extensible, and more dimensionally stable,
material. Such layers may comprise, for example, paper or a
molecularly oriented polymer layer such as a biaxially
oriented layer of nylon, polypropylene, or polyethylene
terephthalate. Other materials which are highly suitable for
use in the substrate structure are polycarbonates and
vinylidene chloride copolymers.
In certain embodiments, and especially those
incorporating a more dimensionally stable layer, the sheet
material includes, between the sealant sheet structure and
the impact layer, a layer which provides for the overall
sheet material an oxygen transmission rate of no more than
about 2 cc/m2 day.
,~
; 7 - 4 -
1 ~3~6 12
The sheet materials of the invention are suitable for
use in fabrication of packages.
FIGURE 1 is a cross-section of a sheet material of the
invention, showing generally the substrate structure, the
sealant structure, and the intermediate impact layer.
FIGURE 2 shows a pictorial view partially cut away of a
tray package which is closed with a lid made with the sheet
material of this invention.
FIGURE 3 shows a pictorial view of a pouch made with
sheet material of this invention.
FIGURES 4 - 15 show cross-sections of various exemplary
sheet materials of the invention.
The applicants herein have disclosed that an impact
layer characterized by certain base polymer selection and
modified by elastomer-providing polymeric materials, can
increase the shock/impact tolerance, and to some extent, the
flexural properties, of a sheet material having a sealant
structure on one side of the impact layer and a substrate
structure on the other side of the impact layer.
Referring to FIGURE 1, the sheet material lO has an
impact layer 12 disposed between sealant structure 14 and
substrate structure 16.
FIGURE 2 shows generally how the flexible sheet
materials of the invention are used as lid material in
closure of preformed semi-rigid trays 2 as illustrated in
FIGURE 2. The sheet materials of the invention can also be
used as the entire packaging structure, such as in formation
b 1 2
of a pouch as at 4 in FIGURE 3, by bringing portions of the
sheet material into facing relationship about a common space
and forming heat seals as at 6 in facing portions of sealant
structure 14. After a product has been placed in the pouch
through opening 8, the remaining open side is sealed closed
in complete fabrication of the filled, closed, and sealed
packages.
The primary problem being addressed by the inventors
herein in provision of a sheet material for pre-formed, semi-
rigid trays such as the tray 2 seen in FIGURE 2, and
especially trays which are to be subjected to high thermal
stresses, such as processing temperatures in the range up to
about 121C. Such conditions are commonly encountered in the
retort processing of such products as food and medical
supplies. This invention is directed at providing lid sheet
material for such uses.
It is known to use a sheet material of the order of
/PET/Saran*/PP sealant/
as a lid material. But packages made with such lid material
are deficient in impact resistance. In U.S. patent no.
4,894,266 there is disclosed the use of a second (interior)
layer of PET, whereby the basic structure of the sheet
material is
/PET/Saran /PET/PP sealant/,
and wherein the sum of the thicknesses of the two layers
of PET in the 4-layer sheet material is the same as the
thickness of PET used in the foregoing 3-layer sheet
Trade mark *
- 6 -
! ~6612
material. And while improvement in impact resistance of the
completed package is achieved thereby, and as taught therein,
still further improvement, especially in impact resistance is
desirable.
Referring to the above 3-layer and 4-layer structures,
it is seen that those sheet materials generally comprise a
substrate substructure and a sealant layer. The sealant
layer may, of course, comprise a plurality of layers as
disclosed in U.S. patent no. 4,937,139.
In this invention, an impact layer 12 is interposed into
the sheet material structure between the sealant substructure
14 and the substrate substructure 16. Both the sealant
substructure 14 and the substrate substructure 16 are shown
in FIGURE 1 as single layers for simplicity of comprehending
the structural concept of the invention. As will be seen
hereinafter by illustration and discussion, either
substructure 14 or 16, or both, may be either a single layer
or a plurality of layers
FIGURES 4 - 15 illustrate specific examples of sheet
structures of the invention. Referring to FIGURE 4, layer 12
is the impact layer and layer 14 is a single layer sealant
structure comprising polypropylene. The substrate
substructure 16 comprises an oxygen barrier layer 18 of
ethylene vinyl alcohol copolymer (EVOH), or polyvinyl alcohol
(PVOH), between two layers 20 of nylon, and an adhesive layer
24. The vinyl alcohol of layer 18 is of the type generally
capable of providing good oxygen barrier. Other oxygen
-- 7
1 ~66/2
barrier materials can, of course, now be designed into the
substrate 16. Layers 20 may be any of the nylons generally
processible with the selected vinyl alcohol, but are
preferably nylon 6. The composition of adhesive layer 24 is
selected with respect to the specific composi~ions chosen for
layers 12 and 20, such that it will serve satisfactorily its
bonding function. Generally representative of extrudable
resins acceptable for layer 24, and which will usually
accommodate coextrusion of the entire sheet material, are the
anhydride modified extrudable adhesive resins. Preferred
extrudable resins will have strong adhesion to both layer 12
which contains a major component of polypropylene -- at least
40% -- and nylon layer 20. Exemplary of acceptable such
resins is the polypropylene-based Admer series of resins
from Mitsui Petrochemical, such as QF 500 ; QF 550 ; and QF
551 . Alternatively, layer 24 may represent an adhesive
laminant such as a urethane adhesive. Sheet materials
illustrated by FIGURE 4 generally have an oxygen transmission
rate of no more than 5 cm3/m2 day at 23C, 0% relative
humidity (RH).
Referring now to FIGURE 5, layer 12 is the impact layer.
Layer 14 is the sealant layer comprising polypropylene.
Layer 26 is a vinylidene chloride copolymer. Layer 28 is
polyethylene terephthalate (PET). The impact tolerance of
such a structure can be improved by incorporating a second
layer of PET in the position illustrated as layer 12 in
FIGURE 5, as disclosed in U.S. patent no. 4,894,266. In this
Trade mark *
- 8 -
1 336~/2
invention, an improved impact tolerance is obtained by the
use of the elastomer-modified polypropylene-based layer 12.
Sheet materials illustrated by FIGURE 5 generally have an
oxygen transmission rate of no more than 3 cm3/m2 day.
Referring to FIGURE 6, layers 12 and 14 are the impact
and sealant layers respectively. Substratè substructure 16
comprises layers 24, 30, 32 and 34. Layer 30 is a
polyetheramide copolymer or a blend of polyetheramide and
EVOH, in enhancement of oxygen barrier properties. Layer 32
is oriented PET, providing an abuse resistant outer layer
which is tolerant of high temperature. Layers 24 and 34 are
adhesives suitable for bonding together the respective layers
at their corresponding interfaces. Layers 34 may be, for
example, a curing type urethane adhesive such as Adcote*
76T198 from Morton Chemical Company. Layer 24 may be, for
example, Admer 550.
FIGURE 7 illustrates a rather uncomplicated example of
the invention. Layers 12 and 14 are the impact and sealant
layers respectively. Layer 38 is high density polyethylene
(HDPE). Layer 34 is an adhesive such as Adcote 76T198. The
substrate 16 comprises layers 34 and 38. The HDPE in layer
38 provides a low moisture vapor transmission rate, leas then
2 cm3/m2 day at 100% RH.
It is seen that layers illustrated as having common, or
substantially similar compositions and structures, are
numbered herein similarly throughout the several
illustrations.
Trade mark *
. 1, _ 9
~ 33S67~
Turning now to FIGURE 8, layers 12 and 14 are the impact
and sealant layers respectively. Layer 28 is PET in
provision of an abuse resistant, high temperature tolerance
outer layer. Layer 38 is HDPE which provides a low moisture
vapor transmission rate. Layers 34 are adhesives generally
of the curing type urethane such as Adcote 76T198.
Substrate substructure 16 includes layers 28, 38, and both
adhesive layers 34.
FIGURE 9 illustrates a tough, abrasion resistant
structure. Layers 12 and 14 are the impact and sealant
layers respectively. Layer 28, is PET. Layer 20 is a
polyamide. Layers 34 are adhesives such as Adcote 76T198.
Layer 20 may include an EVOH in blend composition. Substrate
16 includes layers 20, 28, and both adhesive layers 34.
FI&URE 10 illustrates another simple structure of the
invention in which substrate 16 comprises a layer 28 of PET
and adhesive layer 34. Layers 12 and 14 respectively
represent the impact and sealant layers.
In FIGURE 11, layers 12 and 14 are the impact and
sealant layers, respectively. Layer 20 is nylon, especially
a barrier nylon such as Selar PA from DuPont or MXD6 from
Mitsubishi Gas Chemical Company; or a polyacrylonitrile.
Layer 34 is an adhesive such as Adcote 76T198. Substrate 16
comprises layers 20 and 34.
In FIGURE 12, layers 12 and 14 are the impact and
sealant layers, respectively. Layer 40 is polycarbonate.
Trade mark *
,, -- 10 --
1 ~36672
Layer 34 is an adhesive such as Adcote 76TI98. Substrate 16
includes layers 34 and 40.
In FIGURE 13, layers 12 and 14 are the impact and
sealant layers respectively. Layer 42 of paper provides a
readily printable surface, for presenting a graphic message.
Two primer layers 44 of ethylene acrylic acid copolymer (EAA)
bond the fibrous paper layer 42 to two respective layers 46
of low density polyethylene (LDPE). Layer 34 is an adhesive
such as Adcote 76TI98. Layer 47 is oriented polypropylene
(OPP). Substrate 16 includes layer 42, two layers 44, two
layers 46, adhesive layer 48 which bonds one LDPE layer 46 to
the impact layer 12, adhesive layer 34 and OPP layer 47.
Adhesive layer 48 should bond well to both the LDPE of layer
46 and the composition, especially polypropylene, of layer
12. An example of a commercially available adhesive suitable
for layer 48 is AP220L , an anhydride modified LDPE-based
adhesive from Mitsubishi Chemical Industries.
In FIGURE 14, layers 12 and 14 are the impact and
sealant layers and are separated from each other by a layer
18 of EVOH, which is bonded to layers 12 and 14 by two
extrudable adhesive layers 24 of, for example, Admer* 550.
In this structure, layer 18 EVOH and adhesive layers 24 are
not functionally part of either impact layer 12, sealant
layer 14, or substrate 16. Substrate 16 comprises layer 42
of paper, two layers 44 of EAA, two layers 46 of LDPE
adhesive layer 48 as of AP220L , adhesive layer 34 and OPP
layer 47.
Trade mark *
.- -- 11 --
1 3766/2
Thus it is seen that the sheet materials of the
invention may include, in addition to the three primary
functional groupings 12, 14, 16, further layer structuring.
In this case, the EVOH provides a gas permeable barrier
between the sealant layer 14 and the impact layer 12.
In FIGURES 4-14, the sealant structure 14 has been
illustrated as a single layer 14. In any of the sheet
materials of this invention, however, it is contemplated that
the sealant substructure can be a plurality of layers.
Thus FIGURE 15 shows the sheet material of FIGURE 4, but
wherein the sealant substructure 14 consists of a 3-layer
substructure, comprising two layers 50A and 50B of
polypropylene, optionally elastomer-modified such as with a
polyisobutylene or an ethylene butene-l copolymer containing
85% to 95% (mole) ethylene, and a layer 52 of a blend of
polypropylene (40% to 70% by weight, preferably 50% to 60%)
and HDPE, the layer 52 composition being optionally
elastomer-modified (2% by weight up to 40%) such as with a
polyisobutylene or an ethylene butene-l copolymer containing
85% to 95% ethylene. The outer polypropylene layer 50A is
subject to contact with other surfaces and objects and so it
elastomer-modification content is generally limited to about
10% by weight to avoid tackiness and associated blocking,
although up to about 20% can be used with certain elastomers.
Interior polypropylene layer 50B is not so subject to
external contacts, and so may be more highly modified, i.e.
up to 30% or 40% modifier, and indeed may take on many of the
-i - 12 -
1 ~J6672
properties of impact layer 12, such that it may assist layer
12 in achieved impact/shock tolerance properties for the
sheet material.
So layer 50A is specially adapted to creating a strong
bond with a facing surface such as the flange 3 of tray 2
(FIGURE 2). Layer 52 is especially adapted for cohesive
failure when a strong opening force is applied. Layer 50B is
a bridging, compatibilizing or adhesion-promoting layer, or a
combination of those, to enhance the interfacing and joining
of sealant structure 14 into the balance of the sheet
material. And the composition of layer 50B may include an
elastomer-providing component which cooperates with layer 12
in providing impact/shock tolerance to the sheet material.
Any of the elastomers which are acceptable for use in layer
12 will be acceptable for use in layer 50B, albeit usually in
lesser fraction.
Reflecting, now, on the overall composite of the
disclosure with respect to FIGURES 4-15, it is seen that the
substrate 16 can take an a wide variety of configurations:
and that it is limited functionally with respect to
compatibility, for use in the sheet materials of the
invention only by its ability to be flexible, or semi-
flexible, with thicknesses of .025 mm to 2 mm, preferably
.05 mm to 0.50 mm, and its ability to be bonded to the
adjacent layer in the sheet material as at impact layer 12.
To the extent the sheet material is intended for high
temperature use, such as retort conditions of 121C, then the
- 13 -
l JJ ~6 72
composite of the materials used in substrate 16 desirably
have tolerance for exposure to such temperatures.
The impact layer 12 generally comprises, as a base
polymer, polypropylene. in some respects, the polypropylene
of layer 12 may be considered as a portion of the material
used in fabrication of the heat seal, or in holding
cohesively together the materials used in forming the heat
seal, especially where sealant structure 14 is a single
layer. Since layer 12 is intended generally to serve the
properties of lending impact tolerance and flexural
capabilities to the sheet material, preferably the
polypropylene selected for use in layer 12 is of a less
crystalline nature for a polypropylene, has a lower softening
temperature, and compositionally includes at least a
fractional amount of ethylene as a copolymer, preferably on
the order of about 2% to about 5~ ethylene. In any event,
the polypropylene used in layer 12 composition should have
the capability to withstand the temperatures to which the
sheet material is to be subjected without excessively
thinning, especially during the fabrication of heat seals.
Thus the ethylene comonomer in the propylene used for layer
12 is usually of a minor nature.
Thus any of the polypropylenes may be used as the first
component of the combination in the composition of layer 12.
Polypropylenes which are copolymers having about 2 to about 5
mole percent ethylene are preferred, as the copolymer
provides some minimum level of additional resilience to
- 14 -
1 336672
polypropylene as compared to a homopolymer. Nonetheless, the
term "polypropylene" as used herein with respect to layer 12
is intended to include homopolymers and copolymers except
where specified otherwise, since the primary impact tolerance
of the sheet material is not so much dependent on the
polypropylene as it is on the elastomer-modifying component.
Whether the polypropylene is a homopolymer or copolymer, its
resilience is enhanced substantially by the incorporation of
the elastomeric component. Without the incorporation of the
elastomeric component, layer 12 is incapable of performing
its intended function of providing flexural properties and
improved impact/shock tolerance to the sheet material.
For example, a blend, for layer 12, of 60% by weight of
polypropylene and 40% polyisobutylene is superior to the same
polypropylene in an unblended composition. Similarly, a blend
of 65% by weight polypropylene and 35% by weight ethylene
butene-l copolymer containing about 90% ethylene is superior
to the same polypropylene in an unblended composition.
Further improved performance of layer 12 in the sheet
materials of this invention, and especially as respects
impact tolerance of the sheet material when fabricated into a
package as illustrated in FIGURES 2 and 3, is sometimes seen
when the polypropylene is combined with two materials, as
disclosed herein, which provide elastomeric properties.
Generally the polypropylene is present in an amount of about
40% to about 70%, preferably about 50% to about 60% by weight
of layer 12. A lower level of about 40% is generally
15 -
1 3 ~` 6672
preferred in order to maintain at least a minimum degree of
the high temperature tolerance of the composition which is
provided by the polypropylene. An upper level of about 70%
is generally desired in that a minimum amount of about 30% of
the material providing the elastomeric properties is
generally preferred in order to attain the impact tolerance
desired of the sheet materials of the invention. In some
cases, however, greater than 70% of the composition may be
polypropylene, where the properties of the polypropylene
enhance a particular need of the sheet material for the end
use intended and/or where other layers assist in provision of
impact/shock tolerance, such as, for example, layer 50B of
~IGURE 15. It should be noted, however, that where the
polypropylene is greater than 70% of the composition of layer
12, less of the shock tolerance and flex properties may be
evident in the sheet material unless compensated by
properties of other layers. In some cases, the layer 12
composition may contain up to as high as 90% polypropylene,
with only the remaining 10% of the composition of layer 12
comprising the material providing elastomeric properties.
While the incorporation of virtually any amount of the second
component (namely the material providing elastomeric
properties) into the composition will provide some benefit,
generally improvement in the impact tolerance and the
flexural properties of the sheet material are first evident
at a level of about 10% by weight elastomer. And while up to
about 60% of the second component providing elastomeric
- 16 -
~ 336672
properties may be used, the most desired balance of
properties is generally achieved when the material providing
elastomeric properties is present in amount of about 30% to
about 60% by weight.
Where the third component, which is the second material
providing elastomeric properties, is used in layer 12, the
third component is generally present in an amount which
represents a ratio of about 1/6 to about 2/3 of the second
component. While it is entirely possible that the
elastomeric components be present in up to equal amounts,
generally the greatest improvements in the impact/shock
tolerance and flexural properties are seen where one
component is present in a greater amount than the other.
While the use of only one modifier providing elastomeric
properties in layer 12 does provide a significant level of
improvement in the impact tolerance of the sheet material and
its flexural properties, and its simplicity is advantageous,
further improvements are observed in some embodiments when
the impact layer includes two elastomer modifiers. The
composition of layer 12 thus may include at least three
components; the base polypropylene polymer, the first
elastomer modifier, and the second elastomer modifier. The
relationship among the three components is a mystery, in that
the use of the second and third components as defined herein
may provide a blend having superior properties as compared to
a blend comprised of only two components, but having the same
total amount of elastomer modifier. Thus, the most preferred
- 17 -
1 ~3~672
family of some of the compositions of layer 20 is that which
includes three components, within the ranges specified.
In addressing the technical issues surrounding the
composition of sealant layer 14, it is seen that heat
S tolerance and heat stability are important to the functioning
of the sheet material in high temperature environments. For
those applications requiring these parameters, propylene
polymers are seen to be excellent for tolerating the severe
processing conditions. Also, the sealability of propylene
polymers to, for example, formed trays having a propylene
based surface, is excellent. Unfortunately, to the extent
propylene homopolymer or propylene copolymer is used by
itself as the sealant substructure 14 for sheet material 10,
the adhesion between sealant substructure 14 and flange 3 of
the tray as seen in FIGURE 2, and the cohesive strength of
sealant substructure 14, are so strong that the ability to
open the package may be impeded.
The applicants herein have found a particularly
advantageous capability to control the peel strength of seals
made with the sheet material, while providing an acceptably
strong seal for protecting the contents on the interior of
the package. This capability is achieved by providing a
special family of blend compositions of propylene polymers
for at least one layer of sealant substructure 14. In
general, this special family of blend compositions is a
combination of about 65% to about 95% by weight of a
propylene homopolymer or copolymer and conversely about 35%
~S ~
~ - 18 -
1 336672
to about 5% of an ethylene polymer, or copolymer, having a
density of at least about .930, preferably at least about
0.940, most preferably at least about 0.950, and referred to
herein as a high density polyethylene (HDPE) as seen at layer
52 in FIGURE 15. The special blend compositions can also be
used in any of the single layer sealant substructures 14, as
in the embodiments illustrated in FIGURES 4-14. To the
extent the propylene polymer is a homopolymer or copolymer
contA; n ing up to about 5 percent ethylene, the larger
fractions of HDPE are preferred in the blend composition, up
to about 40% HDPE. To the extent the propylene polymer
contains more ethylene, such as containing 20% ethylene and
80% propylene, then smaller fractions of HDPE are preferred
in the blend composition, such as 10%.
In general, as increasing amounts of propylene are used
in the composition of layer 14, the force required to peel
the package open becomes commensurately greater. To the
extent that the sheet material 10 has excellent interlayer
adhesion and appropriate layer cohesive strengths, these
greater peel strengths are acceptable, and thus up to about
90% propylene polymer may be used. To the extent the
interlayer adhesion within sheet material 10 is of a lesser
degree, using high amounts of propylene (for example over 80%
of a copolymer having 95% or more propylene) can result in
delamination within the sheet material 10 when an attempt is
made to peel sheet material 10 from the closed and sealed
package. Thus where interlayer adhesions are more moderate,
,~ -- 19 --
1 336672
it is desirable to use less propylene polymer in the blend,
and respectively more ethylene. Preferred compositions range
between about 65% and about 75% by weight propylene and about
35% to about 25% HDPE. As the fraction of propylene polymer
decreases below about 65%, the strength of the seal
fabricated with layer 14 may be reduced to the point where
shock/impact resistance of the sealed package is reduced and
the preferred seal strength is not achieved.
The adhesion between the layers in sheet material 10 is
effected by the tendency of the sheet material to elongate
under elongation stresses. It can also be effected by
compression of one or more of the layers during the heat
sealing process. To the extent the sheet material can be
elongated, the elongation puts stresses on the intersurfaces
between the several layers, as each of the differing
individual layers responds to the stresses according to the
properties of its composition. This tends to weaken the
adhesion at those respective interfaces. Layer compression
has a similar effect, in applying lateral and longitudinal
stresses to layer interfaces.
Thus those sheet structures 10 which can be elongated,
or undergo significant compression during heat sealing,
generally work best when they are combined with a sealing
layer 14 which comprises a blend of propylene and ethylene
polymers in the lower end of the range of propylene polymer,
wherein the lid can be peeled off with more modest forces.
On the other hand, since higher fractions of propylene
- 20 -
1 3~6672
polymer do yield packages having stronger seals and stronger
cohesion in layer 14, higher fractions of propylene polymer
are preferred where their use can be tolerated. Thus the
amount of propylene polymer in the blend of layer 14 is
generally in the higher end of the range for those sheet
materials which have less extensibility, such as those
materials containing a layer of paper.
The amount of HDPE used in the composition of layer 14
is preferably selected with reference to the nature of the
propylene polymer which is contemplated for use in the
composition and the adhesive and cohesive strengths in and
between the several layers in the sheet material. A
relatively larger amount of HDPE is used in the blend where
the propylene content of the propylene polymer is in the
upper portion of its range. To the extent the amount of
propylene in the propylene polymer is reduced, lesser amounts
of HDPE are used in the composition. In most cases, the
propylene polymer is a copolymer having at least a small
amount of ethylene, i.e. 2%, in its composition.
With respect to the composition of layer 14 comprising
the base polymer of, for example, polypropylene and a
modifying material, such as high density polyethylene, for
reducing the strength of the seal formed by layer 14, the
percentages used herein are used only with respect to those
two components. To the extent a third component, such as,
for example, a component providing elastomeric properties is
incorporated into layer 14, the fractional compositions
- 21 -
t 33667~
recited for the base polymer and the modifying polymer are
not altered according to the incorporation of the elastomer.
Rather, the fraction of the elastomer is considered with
respect to the overall composition of layer 14. To that end,
the composition of, for example, 65% propylene polymer and
35% HDPE may be blended with an elastomeric material such as
a polyisobutylene, or an ethylene butene-1 copolymer as
earlier recited. A preferred blend for the composition of
layer 14 comprises, for example 20%, of an elastomeric
component and 80% of the composition of the base polymer and
the modifying polymer, wherein the base polymer is about 65%
and the modifying polymer about 35% of that subcombination of
80%.
FIGURE 2 illustrates generally the functioning of the
sheet material of the invention as a lid on a tray 2. In
sealing of the sheet material 10 to the tray 2, sealant
substructure 14 is joined to flange 3, and thus comprises the
sealant layer as previously discussed. As the sheet material
10 is peeled from the tray flange in opening the tray, a
tearing occurs in sealant substructure 14 where the sealant
substructure is sealed, as at 4, to the flange 3, to thus
provide access to the interior 5 of the container. The
peeling of the sheet material 10 away from flange 3 is
usually accompanied by a tearing in sealant substructure 14
which comprises a cohesive failure in sealant substructure 14
in the area of the seal 4 at flange 3, especially where a
modifying material such as HDPE is used as a component of at
.
r 22
1 336672
least one of the layers in sealant substructure 14. In some
cases, however, the sealant substructure 14 peels cleanly
away from the sealed area 4 at flange 3 without leaving any
significant portion of layer 14 on the flange. Typically,
though, the removal of sheet material 10 from the package by
way of peeling it from flange 3 results in a separation of
substructure 14 in the seal area 4 such that a first portion
of substructure 14 remains on the flange and a second portion
is removed with the sheet material 10.
The invention has been described herein with respect to
the required components of the sheet material 10 being impact
layer 12, sealant substructure 14, and substrate 16.
However, as illustrated in FIGURE 14, the invention is
entirely functional if additional layers of material are used
between layers 12 and 14, so long as the advantageous
properties of impact tolerance and flexural properties
provided by layer 12, and appropriate interlayer adhesions,
are maintained. Likewise, additional layers may be used
elsewhere in the sheet material.
EXAMPLE 1
A six layer sheet material as shown in FIGURE 5 is made
as follows. Polyethylene terephthalate film .024 mm thick is
adhesively laminated using Adcote 76TI98 adhering to a film
of vinylidene chloride vinyl chloride copolymer which is .025
mm thick. The vinylidene chloride copolymer film is obtained
from Dow Chemical Company as HB-100 . The polyethylene
terephthalate is Mylar from DuPont. An impact/sealant
Trade msrk *
-~ - 23 -
1 336672
composite structure is then fabricated by coextruding impact
layer 12 and a sealant layer 14 in an air cooled blown
tubular coextrusion process. The composition of layer 14 is
70% of a polypropylene polymer, containing about 2.7%
ethylene and 30% of an HDPE having a density of 0.958. The
composition of layer 12 is 56% polypropylene, about 33%
polyisobutylene, and about 11% of an ethylene butene-l
copolymer. Layer 14 is approximately 0.025 millimeter thick.
Layer 12 is approximately .08 mm thick. Layer 28 is
adhesively laminated to layer 26 using Adcote* 76T198. The
resulting sheet material is thus about 0.16 mm thick.
COMPARATIVE EXAMPLE 1
A sheet material is made as in EXAMPLE 1 except that the
impact layer is replaced by a 0.025 mm thick layer having the
same composition as the sealant layer. The resulting sheet
material is thus about 0.16 mm thick.
EXAMPLE 2
A sheet material is made as in EXAMPLE 1 except that the
substrate 16 of FIGURE 4 is used in place of the 4-layer
substrate 16 (layers 26 and 28) of FIGURE 5. In this
example, the entire 6-layer structure is obtained by a single
coextrusion process. Layer 18, about 0.01 mm. thick, is EVOH
containing 29% ethylene. Layers 20 are nylon 6, about .025
mm thick each. Extrudable adhesive layer 24 is Admer QF-
550 and is just thick enough to provide good adhesion, at
about 0.005 mm.
Trade mark *
,~
- - 24 -
1 ~ ~6612
EXAMPLE 3
A sheet material is made as in EXAMPLE 1 except that the
substrate 16 of FIGURE 6 is used in place of the 4-layer
substrate of FIGURE 5. In this example, the layers 12, 14,
30 and 36 are coextruded. Layer 30 is a blend of 50~ PEBAX
5512 from ATO CHEMIE and 50% Nippon Gohsei EVOH Soarnol ET
containing 38% (mole) ethylene: layer 30 being 0.012 mm
thick. Extrudable adhesive layer 24 is Admer QF-550 and is
just thick enough to provide good adhesion, at about .004 mm.
EXAMPLE 4
A sheet material is made as in EXAMPLE 1 except that the
substrate 16 of FIGURE 7 is used in place of the 4-layer
substrate of FIGURE 5. In this example, a single layer of
HDPE .025 mm thick is laminated to the impact layer using
Adcote 76T198 adhesive.
EXAMPLE 5
A sheet material is made as in EXAMPLE 4 except that a
layer of oriented PET .012 mm thick, obtained as Mylar from
DuPont Company, is adhesively laminated to the HDPE layer
using the 76TI98 adhesive, to thus make a sheet material as
illustrated in FIGURE 8.
EXAMPLE 6
A sheet material is made as in EXAMPLE 5 except that a
layer of polyamide .025 mm thick is used in place of the HDPE
layer to make a sheet material as illustrated in FIGURE 9.
The polyamide used to make the samples are Selar PA from
DuPont and MXD6 from Mitsubishi Gas Chemical.
Trade mark *
- 25 -
1 336672
EXAMPLE 7
A sheet material is made as in EXAMPLE 4 except that a
layer of oriented PET .024 mm thick, obtained as Mylar from
DuPont Company, is used in place of the HDPE layer, to make a
sheet material as illustrated in FIGURE 10.
EXAMPLE 8
A sheet material is made as in EXAMPLE 7 except that a
layer of polyamide .024 mm thick is used in place of the HDPE
layer to make a sheet material as illustrated in FIGURE 11.
Polyamide materials used to make the samples are Selar PA
from DuPont and MXD6 from Mitsubishi Gas Chemical. Samples
are also made using polyacrylonitrile.
EXAMPLE 9
A sheet material is made as in EXAMPLE 8 except that the
lS entire sheet material is made as a simple coextrusion using
Admer QF-SS0 extrudable adhesive in place of 76T198 -
EXAMPLE 10
A sheet material is made as in EXAMPLE 8 except that a
layer of polycarbonate 0.012 mm thick is used in place of the
polyamide layer, to make a sheet material as illustrated in
FIGURE 12.
EXAMPLE 11
A sheet material is made as in Example 1 except that the
substrate 16 is that illustrated in FIGURE 13. Thus a layer
of paper 42 at 65 gm/m2 is extrusion laminated on both
surfaces to layers of LDPE .02S mm thick using, as the
extrusion laminate, primer coatings of EAA on each side of
Trade mark *
1 336672
the paper, to thus make a 5-layer substructure of
LDPE/EAA/Paper/EAA/LDPE. This 5-layer substructure was then
- extrusion laminated to the impact layer using a Mitsubishi
Chemical Company Industries extrudable adhesive AP 220L ,
which is an anhydride modified low density polyethylene, to
thus make a sheet material as illustrated in FIGURE 13. A
layer of OPP, 0.012 mm thick was laminated to the outer layer
of LDPE using Adcote 76T198.
EXAMPLE 12
A sheet material is made as in EXAMPLE 11 except that
the impact layer and the sealant layer were separately
fabricated and a .012 mm layer of EVOH containing 31 mole
percent ethylene was interposed between the sealant layer and
the impact layer. Extrudable adhesive Admer QF-550 was used
on both sides of the EVOH to bond the EVOH to the respective
impact and sealant layers.
EXAMPLE 13
A sheet material is made as in EXAMPLE 2 except that a
3-layer sealant substrate is used in place of the single
sealant layer used in EXAMPLE 2. The 3-layer sealant
substructure is formed by coextrusion, along with the impact
layer. The interior layer is compounded by mixing together
pellets of 2 parts polypropylene containing 2.7% ethylene as
random copolymer and 1 part HDPE, density .958 to make a base
polymer masterbatch. To 72 parts by weight of the
masterbatch was added 23 parts by weight of a polyisobutylene
concentrate containing 65% polyisobutylene and 35%
Trade mark *
- 27 -
1 336672
polypropylene; and 5 parts by weight of ethylene butene-l
copolymer containing 90% ethylene. The resulting mixture of
pellets is 56% by weight polypropylene, 24% high density
polyethylene, 15% polyisobutylene, and 5~ of an ethylene
butene-l copolymer containing 90~ ethylene. The two outer
layers of the 3-layer structure are polypropylene copolymer
containing 2.7% ethylene. The interior, 4-component mixture
layer is .025 mm thick. The two outer layers are each about
.012 mm thick.
The sheet materials so made in the above EXAMPLES all
have improved impact tolerance as compared to similar sheet
materials, but lacking the modifying materials providing the
elastomeric properties in the impact layer, especially where
the modifying materials themselves exhibit elastomeric
properties.
The sheet materials of EXAMPLE 2 are used to fabricate
lids for preformed trays as seen in FIGURE 2, wherein the
trays have a sealing surface comprising polypropylene.
Closed and sealed packages are formed by fabricating heat
seals wherein the sheet material of the lid is sealed to the
flanges of the trays as illustrated in FIGURE 2.
The films of Example 1 and Comparative Example 1 are
used to provide a closure lid for sealing to semi-rigid trays
such as is illustrated as tray 2 in FIGURE 2. The tray
flange 3, to which the subject lid material is sealed, is
about 1.3 mm thick, with a sealing surface layer about 0.25
mm thick and having a composition comprising a blend of 60%
- 28 -
1 336hl2
polypropylene and 40% high density polyethylene. At the
outer periphery, the trays are about 159 mm long by about
114 mm wide.
Before formation of the heat seal, to close the package,
as seen in FIGURE 4, 340 milliliters of water are inserted
into the tray as content. Finally the lid is applied, and
sealed to the tray to form the closed and sealed package.
The closed and sealed packages are retort processed at 121C
with about 1.75 Kg/cm2 of pressure for 30 minutes of cook
time followed by 20 minutes of cooling. Come-up time is 15
minutes.
The thus processed sealed packages are subjected to the
standard USDA Immediate Container Abuse Test, without
overwrap, using a standard drop chute apparatus. This test
is intended to simulate dropping of individual packages on a
controlled, reproducible basis.
The chute is inclined at an angle of 15 from the
vertical and has a rigid base plate at 90 angle to the
chute, also the direction of fall. The chute itself has
guide rods or iron corner guides, continuous from the top
release point of contact to the base. The chute is fitted
with a package release mechanism.
Each package is dropped twice, first on its longer side,
and then on its shorter end. The packages impacted the rigid
base with an impact of about 35 cm kg.
- 29 -
1 3356~2
Packages representing Comparative Example 1 and Example
1 are drop tested in the manner described above. Overall, 0%
out of 35 packages of Comparative Example 1 so tested survive
both drops intact, whereas 23% out 53 packages of Example 1
so tested survive both drops intact. Failures typically
occur by splitting of the lid at the inner sealant junction.
The test results show that the sheet materials of the
invention have better survival rates than sheet materials
without herein recited impact layer between the sealant
structure and the substrate structure. Thus, the results
show that the film structure results in a closed package
having an improved impact tolerance.
The sheet materials of this invention are highly
desirable for use as closure members on packages having other
sheet structures such as that seen in FIGURE 2, wherein the
films 10 of the invention are used to provide lids on
otherwise formed receptacles 2. Thus are the sheet materials
of the invention highly adapted and highly satisfactory for
use as lid stock in preformed trays as seen in FIGURE 2.
The sheet materials of the invention are also highly
satisfactory for use in fabrication of pouches, wherein the
films of the invention comprise essentially all of the wall
area of the package structure as seen in FIGURE 3. The
FIGURE 3 illustration, of course, shows the package opened on
one end for the insertion of the product after which a
corresponding heat seal is used to close that open end to
complete closure of the package.
- 30 -
1 3366/~`
For the sheet materials of the invention as illustrated
in FIGURES 4 - 15, the overall sheet material thickness is
typically of the order 0.04-0.50 mm, preferably about 0.06-
0.40 mm thick, most preferably about 0.10 - 0.30 mm.
The temperature of seal equipment applied to the outer
layer of the sheet material varies, depending on, among other
things, the compositions of especially the outer layer of
substrate 16 which contacts the seal equipment, and the seal
layer as at 14 which forms the seal. Conventional amounts of
seal pressure usually are in the range of 2.8 kg/cm2 to 6.3
kg/cm2, and commonly 2.8 Kg/cm2 to 4.2 Kg/cm2. Typical dwell
time is .25 to 2.0 seconds, more commonly 1 - 1.5 seconds.
The seal conditions, of course, depend on the specific
substructure 16 selected for a given embodiment.
Thus it is seen that the invention provides an improved
multiple layer polymeric sheet material having a substrate
substructure in combination with a sealant substructure, and
an elastomer-modified impact layer between the sealant
substructure and the substrate structure. The improved
multiple layer sheet materials have increased impact
tolerance as compared to those sheet materials which do not
include the impact component which provides the elastomeric
properties. The improved multiple layer sheet materials
provide increased impact tolerance in a package while
maintaining the ability in the sheet materials to form strong
heat seals.
~,
~ - 31
