U.S. patent application number 12/150368 was filed with the patent office on 2009-02-12 for microwaveable vacuum skin package.
Invention is credited to Serena Della Bianca, Marzia Cominetti, Francesco Gallo.
Application Number | 20090041912 12/150368 |
Document ID | / |
Family ID | 8178491 |
Filed Date | 2009-02-12 |
United States Patent
Application |
20090041912 |
Kind Code |
A1 |
Bianca; Serena Della ; et
al. |
February 12, 2009 |
Microwaveable vacuum skin package
Abstract
The present invention relates to a self-venting microwaveable
package of food product comprising a) a rigid or semi-rigid, flat
or shaped, support member; b) a food product loaded onto the
support a), and c) a flexible thermoplastic film enclosing the food
product on the support member, said flexible film being draped over
the food product, by vacuum and/or air pressure, in a vacuum skin
packaging arrangement, substantially conforming to the shape of the
food product and being sealed to the support member outside the
periphery of the food product; said package being characterized in
that the support member a) as well as the flexible film c) are of
microwaveable thermoplastic materials and the peel strength between
the support member a) and the flexible film c) is comprised between
about 2.0 and about 4.0 N/25.4 mm. The invention also relates to a
method of thawing and/or cooking, or re-heating a food product in a
microwave oven comprising the step of packaging the food product in
a VSP package where the food product is enclosed between a
microwaveable rigid or semi-rigid support member and a
microwaveable top skin web wherein the peel strength between the
support member and the top skin web is comprised between about 2.0
and about 4.0 N/25.4 mm, and placing the package into the microwave
oven to cause the radiant energy of the microwave oven to thaw
and/or cook, or re-heat the food product as desired, without the
need of creating or setting free a venting hole in the package and
without removing or loosening the top skin film before putting the
package into the oven.
Inventors: |
Bianca; Serena Della; (RHO,
IT) ; Gallo; Francesco; (Milano, IT) ;
Cominetti; Marzia; (Lainate, IT) |
Correspondence
Address: |
Rupert B. Hurley Jr.;Sealed Air Corporation
P.O. Box 464
Duncan
SC
29334
US
|
Family ID: |
8178491 |
Appl. No.: |
12/150368 |
Filed: |
April 28, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10486534 |
Feb 10, 2004 |
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12150368 |
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Current U.S.
Class: |
426/234 |
Current CPC
Class: |
C08L 23/20 20130101;
B65D 75/305 20130101; C08L 23/20 20130101; C08L 23/08 20130101;
C08L 23/0876 20130101; C08L 2666/04 20130101; C08L 2205/02
20130101; B65D 75/5855 20130101; C08L 2666/04 20130101; C08L 23/08
20130101; C08L 23/0853 20130101; B65D 2205/00 20130101; B65D
81/3461 20130101 |
Class at
Publication: |
426/234 |
International
Class: |
B65D 81/34 20060101
B65D081/34 |
Claims
1-5. (canceled)
6. A method of thawing and/or cooking, or re-heating a food product
in a microwave oven comprising: packaging the food product in a
vacuum skin package where the food product is enclosed between a
microwaveable rigid or semi-rigid support member and a
microwaveable top skin web wherein the support member and the top
skin web are sealed together over a wide area corresponding to the
whole support surface not occupied by the product and the peel
strength between the support member and the top skin web is between
about 2.0 and about 4.0 N/25.4 mm, and placing the package into the
microwave oven to cause the radiant energy of the microwave oven to
thaw and/or cook, or re-heat the food product as desired, without
making a venting hole in the package before putting the package
into the oven and without removing or loosening the top skin web
before putting the package into the oven, with the top skin web
rising over the product by pressure from vapor released from the
product during heating, with a channel being created between the
top skin web and the support member to vent off excess vapor.
7. The method of claim 6 wherein the peel strength between the
support member and the top skin web is between about 2.0 and about
3.5 N/25.4 mm.
8. The method of claim 7 wherein the peel strength between the
support member and the top skin web is between about 2.0 and about
3.0 N/25.4 mm.
9. A blend of from about 30 to about 70 wt. % of an EVA copolymer
containing from more than 40 to about 80 wt. % vinyl acetate, from
about 15 to about 50 wt. % of a polybutylene, and from about 15 to
about 50 wt. % of an optionally neutralized ethylene-acrylic or
methacrylic acid copolymer.
10. The method according to claim 6, wherein the support member or
the top skin web has a seal layer, or a layer adjacent to a seal
layer, comprising a resin blend of low cohesive strength.
11. The self-venting microwaveable package according to claim 10,
wherein the resin blend having low cohesive strength comprises a
blend of: (a) ethylene/acrylic acid copolymer and/or
ethylene/methacrylic acid copolymer; (b) a modified ethylene/vinyl
acetate copolymer; and (c) polybutylene.
12. The method according to claim 11, wherein the resin blend of
low cohesive strength comprises a blend of from about 30 to about
70 weight percent of an EVA copolymer containing from more than 40
to about 80 weight percent vinyl acetate, from about 15 to about 50
weight percent of a polybutylene, and from about 15 to about 50
weight percent of an optionally neutralized ethylene-acrylic or
methacrylic acid copolymer.
13. The method according to claim 10, wherein the resin blend of
low cohesive strength is present in a layer adjacent the sealing
layer of the support member, and the sealing layer has a thickness
of from about 1 .mu.m to about 7 .mu.m.
14. The method according to claim 13, wherein the sealing layer of
the support member has a thickness of from about 1 .mu.m to about 6
.mu.m.
15. The method according to claim 14, wherein the sealing layer has
a thickness of from about 1 .mu.m to about 5 .mu.m.
16. The method according to claim 10, wherein the peel strength
between the support member and the flexible thermoplastic film is
obtained by providing the top skin web with a thin seal layer and a
layer of low cohesive strength adjacent the thin seal layer, with
the layer of low cohesive strength comprising the resin blend of
low cohesive strength.
17. The method according to claim 6, wherein the peel strength
between the support member and the flexible thermoplastic film is
achieved between a seal layer of the support member and a seal
layer of the flexible film, by providing these seal layers with
resins or resin blends of a sufficiently different chemical
nature.
18. The method according to claim 6, wherein the peel strength
between the support member and the flexible thermoplastic film is
achieved by using in at least one of the support member and the
flexible thermoplastic film, a thin sealing layer and an adjacent
layer of a thermoplastic material suitably selected so that a bond
between the sealing layer and the adjacent layer is low.
Description
[0001] The present invention relates to a vacuum skin package
suitable for the presentation and preservation of a food product
and suitable for use in cooking, re-heating and/or thawing the
packaged food product in a microwave oven.
[0002] More particularly the present invention refers to a
self-venting microwaveable vacuum skin package where the packaged
food product can be cooked, re-heated or thawed in the package
without prior removal of the top web or without prior manual
opening of the package.
[0003] A wide variety of products, especially food products like
meat, sausages, cheese, ready meals, and the like, are being
offered in visually attractive packages made from two thermoplastic
webs using the vacuum skin packaging process.
[0004] Vacuum skin packaging (VSP) is a process well known in the
art for using a thermoplastic packaging material to enclose a food
product. The vacuum skin packaging process is in one sense a type
of thermoforming process in which an article to be packaged serves
as the mold for the forming web. An article may be placed on a
rigid or semi-rigid support member, that can be flat or shaped,
e.g., tray-shaped, bowl-shaped or cup-shaped, and the supported
article is then passed to a chamber where a top web is drawn upward
against a heated dome and the softened top web is then draped over
the article. The movement of the web is controlled by vacuum and or
air pressure, and in a vacuum skin packaging arrangement, the
interior of the container is vacuumized before final welding of the
top web to the support web. In a vacuum skin package the upper
heated film thus forms a tight skin around the product and is
sealed to the support.
[0005] Skin packaging is described in many references, including
French Patent No. 1,258,357, French Patent No. 1,286,018,
Australian Patent No. 3,491,504, U.S. Pat. No. RE 30,009, U.S. Pat.
No. 3,574,642, U.S. Pat. No. 3,681,092, U.S. Pat. No. 3,713,849,
U.S. Pat. No. 4,055,672, and U.S. Pat. No. 5,346,735. U.S. Pat. No.
5,346,735.
[0006] The term "vacuum skin packaging" (hereinafter "VSP") as used
herein indicates that the product is packaged under vacuum and the
space containing the product is evacuated from gases. It is
therefore desirable that both the top skin film formed around the
product and that used for the support member present a barrier to
oxygen, air, and other gases detrimental to the shelf or storage
life of a food product. Another key requirement for the skin
packaging material in the VSP process, is a high degree of
formability/stretchability to avoid a common and recurrent problem
in such operations which is the occurrence of wrinkles and other
irregularities in the final packaged product.
[0007] Typically, skin packaging provides just a barrier film that
upon removal from the package leaves the product exposed to
atmosphere, which is sufficient for certain types of
applications.
[0008] When the packaged food product needs to be thawed and/or
cooked or re-heated before eating, this is generally done by using
a microwaveable support member, typically tray-shaped, and removing
the top skin web just before putting the tray in the oven. Food
treated in this way however tends to become dry in texture and
consistency rather than tender and moist. Furthermore when the
product to be thawed and/or cooked, or re-heated, contains some
sort of sauce or juice, e.g. oil, butter, wine, seasoning or
marinating liquid, etc., this will splatter on the inner surfaces
of the microwave oven. The possibility of putting the package into
the oven without removing the top skin lid would therefore be
highly appreciated. By keeping the lid on top of the product in the
microwave oven the food products will be cooked in an healthy
manner, e.g. by steaming, and the oven inner surfaces will be kept
clean. Furthermore, by removing the top skin only when the package
is removed from the microwave oven, it will be possible to use the
bottom support as a disposable dish for serving the product.
[0009] In any case, whether the food product is a raw or a cooked
product and has to be cooked or just heated, considerable water
vapour pressure is generated within the package during the heating
cycle. While cooking or heating under a slight overpressure of
vapour improves the taste and texture of the end product, a too
high overpressure within the package should be avoided to ensure
against sudden explosions of the packages within the oven. This
particularly with VSP packages as the top and bottom webs are
sealed together over a wide area (corresponding to the whole
support surface not occupied by the product) and because the skin
top web is highly formable and will therefore be overstretched by
the excess vapour creating a balloon over the product before either
bursting of the film or opening of the seal to release the excess
vapour. This is a particularly negative effect for the VSP packages
because, when the package is removed from the oven at the end of
the cooking or heating step, the overstretched film will drape down
over the cooked product giving an unpleasant appearance to the
package.
[0010] In general the control of the pressure in the ovenable
packages is achieved by creating at least one venting hole just
before cooking, either by piercing the top lid of the package
before inserting it in the oven or by removing the protective label
covering the venting hole created during the packaging step.
Alternatively the sealed area is narrowed in a limited zone to
create there a weakness in the seal so that the seal will open up
in that zone upon the generation of the overpressure.
[0011] Both alternatives would not fit easily a VSP package.
[0012] The venting hole should in fact be positioned in that part
of the top web that covers the food product as otherwise the
presence of a venting hole would be immaterial if the top web is
sealed to the bottom one. Creating such a venting hole in the
package, by piercing the top skin, may damage the packaged product,
while creating the hole in the top web before packaging may give
problems during the stretching step as the web may tear starting
from the hole.
[0013] Also reducing the area of the seal in a controlled manner is
difficult in a VSP package as the area where the top web is sealed
to the bottom one is large and varies widely depending on the size
of the support and of the product.
[0014] It has now been found that if a food product is packaged in
a VSP package made with microwaveable thermoplastic materials and
the peel strength between the top skin web and the bottom support
web is comprised between about 2.0 and about 4.0 N/25.4 mm, it is
possible to thaw and/or cook, or re-heat, the packaged product in a
microwave oven without the need of removing, loosening or
perforating the top lid before putting the package into the oven,
without any risk of explosion and avoiding any substantial
stretching of the top skin web. The top skin web in fact will raise
over the product by the pressure of the vapour released by the
product or by the sauce accompanying the product but it will not be
overstretched because a channel for the excess vapour to vent off
will then be created, between the top skin and the bottom webs,
from the packaged product to the outermost edge of the package. The
top skin will remain raised over the product until the heating is
on, allowing the steaming of the product, and will then return to
its original position as soon as the heating is over.
[0015] A first object of the present invention is therefore a
self-venting microwaveable VSP package where a food product is
vacuum skin packaged between a microwaveable rigid or semi-rigid
support member, that can be flat or shaped, e.g., tray-, cup- or
bowl-shaped, and a microwaveable top skin web which is draped over
the food product, wherein the peel strength between the support
member and the top skin web is comprised between about 2.0 and
about 4.0 N/25.4 mm.
[0016] A second object of the present invention is a method of
thawing and/or cooking, or re-heating a food product in a microwave
oven comprising the step of [0017] packaging the food product in a
VSP package where the food product is enclosed between a
microwaveable rigid or semi-rigid support member and a
microwaveable top skin web wherein the peel strength between the
support member and the top skin web is comprised between about 2.0
and about 4.0 N/25.4 mm, and [0018] placing the package into the
microwave oven to cause the radiant energy of the microwave oven to
thaw and/or cook, or re-heat the food product as desired, without
the need of creating or setting free a venting hole in the package
and without removing or loosening the top skin film before putting
the package into the oven.
[0019] It is a third object of the present invention a new blend of
polymeric materials that can suitably be employed in the
manufacture of a microwaveable VSP package providing for the
desired limited peel strength.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1a illustrates a perspective view of a conventional
vacuum skin package in a microwave oven, before cooking begins.
[0021] FIG. 1b illustrates a perspective view of the conventional
vacuum skin package of FIG. 1a in a microwave oven, during
cooking.
[0022] FIG. 2a illustrates a perspective view of a vacuum skin
package of the invention, in a microwave oven, before cooking
begins.
[0023] FIG. 2b illustrates a perspective view of a vacuum skin
package of the invention, in a microwave oven, during the cooking
cycle.
[0024] FIG. 3 illustrates a perspective view of a conventional
vacuum skin package in a microwave oven, after the cooking
cycle.
[0025] FIG. 4 illustrates a perspective view of a vacuum skin
package of the invention, in a microwave oven, after the cooking
cycle.
DETAILED DESCRIPTION OF THE INVENTION
[0026] In its first aspect the present invention relates to a
self-venting microwaveable package of food product comprising
[0027] a) a rigid or semi-rigid, flat or shaped, support member;
[0028] b) a food product loaded onto the support a), and [0029] c)
a flexible thermoplastic film enclosing the food product on the
support member, said flexible film being draped over the food
product, by vacuum and/or air pressure, in a vacuum skin packaging
arrangement, substantially conforming to the shape of the food
product and being sealed to the support member outside the
periphery of the food product;
[0030] said package being characterized in that the support member
a) as well as the flexible film c) are of microwaveable
thermoplastic materials and the peel strength between the support
member a) and the flexible film c) is comprised between about 2.0
and about 4.0 N/25.4 mm.
[0031] In a preferred embodiment said peel strength is comprised
between about 2.0 and about 3.5 N/25.4 mm, and in a more preferred
embodiment it is comprised between about 2.0 and about 3.0 N/25.4
mm.
[0032] Typically the packaging materials to be used for the bottom
support (or "lower web") and for the top skin (or "upper web" or
"forming web") would be multi-layer structures, containing at least
one layer with gas-barrier properties. While the bottom support
needs to be rigid or semi-rigid and, in case it is shaped, also
easily thermoformable, the top skin needs to be a flexible film
endowed with a sufficient stretchability for use in a VSP
process.
[0033] As used herein the term "microwaveable", as well as the term
"microwave-compatible", when referred to the structures useful for
the manufacture of the VSP package of the present invention,
include those structures that are "substantially
microwave-transparent" as well as those that are
"microwave-active". While the structures substantially
microwave-transparent are those capable of being crossed by at
least 80%, preferably at least 90% of the microwaves generated by a
microwave oven without any sort of interference therewith, the
microwave-active are those that incorporate microwave reflective
components intended to modify the energy deposition within the
adjacent foodstuff. To be "microwaveable" in both cases, under the
conditions of use, the packaging material should not be degraded or
deformed and it should not release more than 60 ppm of global
contaminants to the packaged food in contact therewith. In
practice, packaging materials that withstand a heat treatment at
121.degree. C. for 1/2 hour (conditions that are drastic enough not
to be reached normally in microwave cooking) without deforming and
releasing less than 60 ppm of contaminants, are considered to be
"microwaveable" according to most of the food laws. Examples of
resins suitable for use in the manufacture of the package of the
invention include polyolefins (such as propylene-based polymers or
preferably cross-linked polyethylene-based polymers), polyesters,
nylons and any other thermoplastic material that under the
conditions of use will not be altered by microwaves.
[0034] There are several mechanisms by which the required limited
peel strength can be achieved in the VSP package of the
invention.
[0035] In a first one, this is obtained by suitably selecting
bottom and top webs with sealing layers that upon application of a
force of between about 2.0 and about 4.0 N/25.4 mm would separate
at their interface ("peel" one from the other). This can be
obtained by suitably selecting resins or resins blends for said
sealing layers of a sufficiently different chemical nature.
[0036] In a second mechanism this is achieved by using in at least
one of the bottom and top webs a thin sealing layer and an adjacent
layer of a thermoplastic material suitably selected in such a way
that the bond between the sealing layer and said adjacent layer is
very low. A typical example is a system where either the upper or
the lower web comprises a very thin layer of polyethylene adjacent
to a polyamide surface. The vapour pressure that generates in the
package breaks through the thin polyethylene sealing layer and the
low bond between polyethylene and polyamide then permits
delamination between these two layers to take place giving the
opening up of the package. The peel strength in this case is
directly related to the bond between the sealing layer and the
adjacent layer and accordingly it depends primarily on the chemical
similarity or dissimilarity of the two materials. However, the
extrusion conditions such as pressure, temperature and time of
contact between the molten materials also have a major effect on
the final bond strength between these two layers.
[0037] In a third mechanism, the desired limited peel strength is
obtained by using in the seal layer, or in the layer adjacent to a
very thin seal layer, of one of the two webs, a resin blend that
has a low cohesive strength. In such a case the vapour generated
during the cooking cycle will internally break said layer along a
plane parallel to the layer itself, with or without prior breakage
of the thin sealing layer, giving the opening up and the venting of
the package. In this case the value of the peel strength will
depend on the type of material selected for the low cohesion
layer.
[0038] In a preferred embodiment the VSP package of the present
invention is obtained by using a bottom support web containing, as
the layer adjacent to a thin sealing layer, a low cohesion layer
combined with any type of top skin web that seals to the bottom
sealing layer.
[0039] Blends with low cohesive strength that can be used to obtain
the desired limited peel strength are for instance those described
in WO 99/54398, comprising a copolymer of ethylene and acrylic acid
or methacrylic acid, a modified EVA copolymer, and a
polybutylene.
[0040] A new blend that also provides for the desired low peel
strength is based on the high vinyl acetate content EVA copolymers,
such as those containing from more than 40 to about 80% by weight
of vinyl acetate, e.g. those currently sold by Bayer as
Levamelt.TM. grades. Said new blend would typically comprise from
about 30 to about 70 wt. % of a high vinyl acetate content EVA,
from about 15 to about 50 wt. % of a polybutene, and from about 15
to about 50 wt. % of an optionally neutralised ethylene-acrylic or
methacrylic acid copolymer. It has been found that with the use of
the new blend, that therefore represents a further specific object
of the present invention, it is possible to lower the peel strength
without impairing the manufacturing process.
[0041] For the new blend, the preferred grade of high vinyl acetate
EVA copolymer, would be the grade with the highest vinyl acetate
content still approved for the packaging of food products. The wt.
% of the EVA copolymer in the blend is at least 30%, but preferably
at least 35%, and more preferably at least 40%. Preferably the Melt
Flow Index (MFI are measured under the conditions of ASTM D 1238 at
190.degree. C. and 2.16 kg) of the EVA copolymer is higher than 10
g/10 min, more preferably higher than 15 g/10 min.
[0042] The ethylene-acrylic or methacrylic acid copolymer typically
contains from about 4% to about 18% by weight of acrylic or
methacrylic acid units. Said copolymer can also contain,
copolymerized therein, an alkyl acrylate or methacrylate, such as
n-butyl acrylate or methacrylate or isobutyl acrylate or
methacrylate. Said copolymer can be in the free acid form as well
as in the ionized or partially ionized form wherein the
neutralizing cation can be any suitable metal ion, e.g. an alkali
metal ion, a zinc ion, or other multivalent metal ions; in this
latter case the copolymer is also termed "ionomer". In the new
blend preferably the ethylene acrylic or methacrylic acid copolymer
is an ionomer. Suitable polymers are those having a low melt flow
index of less than 5 g/10 min and more preferably less than 2 g/10
min (still evaluated according to ASTM D 1238 at 190.degree. C. and
2.16 kg). The wt. % of this component in the new blend is at least
15%, preferably at least 20% and even more preferably at least
25%.
[0043] The polybutene used in the new blend can be a homopolymer
consisting essentially of a butene-1, butene-2, isobutene repeating
units as well as a co-polymer such as an ethylene-butene copolymer.
The polybutene component is contained in the new blend in an amount
of at least 15 wt. %, preferably at least 20 wt. %.
[0044] The blend is simply obtained thoroughly mixing the three
components in pellet form and then melt extruding it.
[0045] In the preferred embodiment where the bottom support web
contains a thin sealing layer and a low cohesion layer adjacent
thereto, the thickness of the low cohesion layer is preferably in
the range of from about 2 to about 30 .mu.m while the thickness of
the seal layer is in the range of from about 1 .mu.m up to about 7
.mu.m, preferably up to 6 about .mu.m and even more preferably up
to 5 about .mu.m.
[0046] In this preferred embodiment where the low cohesion layer is
not the sealing layer, this latter one preferably comprises a
polyolefin. More preferably it comprises at least one member
selected from the group consisting of ethylene-.alpha.-olefin
copolymers, low density polyethylene (LDPE), medium density
polyethylene (MDPE), high density polyethylene (HDPE),
ethylene-acrylic acid copolymers (EAA), ethylene-methacrylic acid
copolymers (EMAA), ethylene-vinyl acetate copolymers (EVA) and
ionomers. Even more preferably it comprises ionomer or EVA.
[0047] The thickness of the overall sheet used as the bottom
support is not critical at all and will depend on the type of
application foreseen. It will typically have a thickness up to
about 1,200 .mu.m, preferably up to about 1,000 .mu.m, generally
comprised between about 200 and about 750 .mu.m.
[0048] Thinner films, e.g. from about 50 to about 200 .mu.m, are
employed as top skin webs. Said films will contain a sealing layer
that would seal to the sealing layer of the bottom support web.
Typically ethylene-.alpha.-olefin copolymers, EAA, EMAA, EVA and
preferably ionomers are employed for the sealing layer.
[0049] Both the bottom support web and the top skin webs preferably
contain a gas-barrier layer. The gas-barrier layer generally
includes a polymeric material with low oxygen transmission
characteristics, such as PVDC, EVOH, polyamides, polyesters or
blends thereof.
[0050] PVDC is any vinylidene chloride copolymer wherein a major
amount of the copolymer comprises vinylidene chloride and a minor
amount of the copolymer comprises one or more unsaturated monomers
copolymerisable therewith, typically vinyl chloride, and alkyl
acrylates or methacrylates (e.g. methyl acrylate or methacrylate)
and the blends thereof in different proportions. Generally a PVDC
barrier layer will contain plasticisers and/or stabilizers as known
in the art.
[0051] EVOH is the saponified product of ethylene-vinyl ester
copolymers, generally of ethylene-vinyl acetate copolymers, wherein
the ethylene content is typically comprised between 20 and 60% by
mole and the degree of saponification is generally higher than 85%
preferably higher than 95%.
[0052] Polyamides used as gas barrier layer can be homo- or
co-polyamides. This term specifically includes those aliphatic
polyamides or copolyamides commonly referred to as e.g. polyamide 6
(homopolymer based on .epsilon.-caprolactam), polyamide 69
(homopolycondensate based on hexamethylene diamine and azelaic
acid), polyamide 610 (homopolycondensate based on hexamethylene
diamine and sebacic acid), polyamide 612 (homopolycondensate based
on hexamethylene diamine and dodecandioic acid), polyamide 11
(homopolymer based on 11-aminoundecanoic acid), polyamide 12
(homopolymer based on .omega.-aminododecanoic acid or on
laurolactam), polyamide 6/12 (polyamide copolymer based on
.epsilon.-caprolactam and laurolactam), polyamide 6/66 (polyamide
copolymer based on .epsilon.-caprolactam and hexamethylenediamine
and adipic acid), polyamide 66/610 (polyamide copolymers based on
hexamethylenediamine, adipic acid and sebacic acid), modifications
thereof and blends thereof. Said term also includes crystalline or
partially crystalline, aromatic or partially aromatic,
polyamides.
[0053] The thickness of the gas barrier layer will be suitably
selected in order to provide the overall structure with an oxygen
transmission rate (evaluated by ASTM D3985) of less than 150
cm.sup.3/m.sup.2.atm.d, at room temperature and 0% relative
humidity, preferably of less than 100, and even more preferably of
less than 50 cm.sup.3/m.sup.2.atm.d.
[0054] Other layers can be present in both the bottom support and
the top skin webs. Said additional layers should serve the purpose
of providing the necessary bulk to the sheets and improving their
mechanical properties, i.e. increased puncture resistance,
increased abuse resistance, etc., or to better tie the various
layers one to the other. Any type of microwaveable resin can be
employed for these purposes.
[0055] For the evaluation of the peel strength the following
internal standard procedure has been employed: strips of 25.4 mm in
width and 300 mm in length are cut from mock VSP packages prepared
starting from the suitably selected upper and lower webs sealed
together in a VSP packaging cycle (e.g., using a MULTIVAC.RTM. CD
6000 machine) under the setting conditions of actual use; manually
separate the upper and lower webs until the lower web may be fixed
into the lower clamp of a dynamometer, and the upper web into the
upper one, taking care that the area to be tested lies in the
middle of the two clamps and that an adequate tensioning between
the two extremities of the fixed sample is obtained; the peel
strength is then measured with a crosshead speed of 200 mm/min and
a jaw distance of 30 mm.
[0056] In another aspect the present invention relates to a method
of thawing and/or cooking, or re-heating a food product in a
microwave oven comprising the step of [0057] packaging the food
product in a VSP package where the food product is enclosed between
a microwaveable rigid or semi-rigid support member and a
microwaveable top skin web wherein the peel strength between the
support member and the top skin web is comprised between about 2.0
and about 4.0 N/25.4 mm, and [0058] placing the package into the
microwave oven to cause the radiant energy of the microwave oven to
thaw and/or cook, or re-heat the food product as desired, without
the need of creating or setting free a venting hole in the package
and without removing or loosening the top skin film before putting
the package into the oven.
[0059] The VSP package according to the present invention does not
require any particular system for releasing the vapour overpressure
during the microwave treatment. In particular no puncturing of the
top web nor any loosening of the top web are needed. The package
can just be put into the microwave oven as such and heated without
any problem. At first the vapour released by the food product
during the energy treatment will raise the top skin over the
product thus creating a sort of vapour cushion around the product
that maintains the product texture and taste and when the vapour
pressure reaches the threshold value, then the package opens up and
the excess vapour is vented off through the sort of channel that
creates between the upper and lower webs of the packaging material.
Still however during the cooking cycle a vapour cushion is
maintained around the product. At the end of the cooking cycle,
when no additional vapour is generated by the product, the top skin
film lowers on top of the product with no wrinkles or plies as it
has not been overstretched during the cooking cycle. The package
can then be withdrawn from the oven, the top skin removed, and the
food product can be served, if desired, directly on the bottom
support. No particular care should be taken while removing the top
skin film as the hot vapour has been vented off during the cooking
cycle.
[0060] FIG. 1a) illustrates a conventional VSP package, with a peel
strength of 5.3 N/25.4 mm, before the cooking cycle and FIG. 1b)
illustrates the same package during the cooking step with the top
skin ballooning clearly visible. FIG. 2a) illustrates a VSP package
of the present invention, with a peel strength of 2.4 N/25.4 mm,
before the cooking cycle (FIG. 2a) no different from FIG. 1a)) and
FIG. 2b) shows the same package during the cooking cycle. FIGS. 3
and 4 show the two types of packages after the cooking cycle where
the photograph indicated as FIG. 3 is that of the conventional VSP
package of FIGS. 1a) and 1b) (clearly visible are the wrinkles that
are generated by the overstretching of the top skin film during the
heating step) and the photograph indicated as FIG. 4 refers to the
package of the invention of FIGS. 2a) and 2b). No wrinkles or plies
can be noticed with the VSP package according to the present
invention.
[0061] More particularly the structure of the bottom support web
and that of the top skin web of the package according to the
present invention shown in FIGS. 2a), 2b), and 4, are the following
ones (the partial thickness of the various layers--in .mu.m--is
indicated between parentheses)
[0062] Bottom Support Web [0063] Ionomer (based on
ethylene-methacrylic acid copolymer)--Surlyn.RTM. 1705-1 by
DuPont--Sealant layer (4) [0064] Blend of [0065] 40% EVA with 45%
VA with MFI in the range 15-35 g/10 min (ASTM D1238--190.degree. C.
and 2.16 kg)--Levamelt.TM. L456 by Bayer, [0066] 30% of
polybutylene with MFI=20 g/10 min (ASTM D1238 190.degree. C. and
2.16 kg)--Shell polybutene 0400 by Shell, and [0067] 30% of ionomer
with MFI=1.3 g/10 min (ASTM D1238 190.degree. C. and 2.16
kg)--Surlyn.RTM. 1601 by DuPont
[0068] Cohesive Failure Layer (12) [0069] Linear low density
polyethylene modified with maleic anhydride--Tie layer (13) [0070]
Low density polyethylene--Bulk layer (140) [0071] Linear low
density polyethylene modified with maleic anhydride--Tie layer (10)
[0072] Ethylene-vinyl alcohol copolymer--barrier layer (11) [0073]
Linear low density polyethylene modified with maleic anhydride--Tie
layer (13) [0074] Polypropylene--Outer abuse resistant layer
(400)
[0075] Top Skin Web [0076] Ionomer (based on ethylene-methacrylic
acid copolymer)--Surlyn.RTM. 1702 by DuPont--Sealant layer (6)
[0077] Low density polyethylene--Bulk layer (14) [0078]
Ethylene-vinyl acetate copolymer with 19% VA--Bulk layer (19)
[0079] Linear low density polyethylene modified with maleic
anhydride--Tie layer (3) [0080] Ethylene-vinyl alcohol
copolymer--barrier layer (8) [0081] Linear low density polyethylene
modified with maleic anhydride--Tie layer (3) [0082] Ethylene-vinyl
acetate copolymer with 19% VA--Bulk layer (11) [0083] Low density
polyethylene--Bulk layer (26) [0084] High density
polyethylene--Outer abuse resistant layer (10)
* * * * *