U.S. patent number RE28,554 [Application Number 05/164,901] was granted by the patent office on 1975-09-16 for flexible wrapping material.
This patent grant is currently assigned to Curwood, Inc.. Invention is credited to Howard J. Curler, Glenn E. Lineburg.
United States Patent |
RE28,554 |
Curler , et al. |
September 16, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Flexible wrapping material
Abstract
A flexible wrapping material resistant to flex cracking and
pinhole development and impervious to gas and moisture made by
bonding a sheet of biaxially oriented polypropylene to one surface
of a sheet of cellophane coated at least on one side with
vinylidene chloride copolymer (Saran) and bonding low density
polyethylene or other heat-sealable polymeric layer having a
melting point substantially below that of the polypropylene to the
other surface of the cellophane sheet.
Inventors: |
Curler; Howard J. (New London,
WI), Lineburg; Glenn E. (New London, WI) |
Assignee: |
Curwood, Inc. (New London,
WI)
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Family
ID: |
26860967 |
Appl.
No.: |
05/164,901 |
Filed: |
August 9, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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280200 |
May 14, 1963 |
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Reissue of: |
599566 |
Dec 6, 1966 |
03445324 |
May 20, 1969 |
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Current U.S.
Class: |
428/334; 428/510;
428/520; 428/910; 428/518; 428/532 |
Current CPC
Class: |
B32B
27/00 (20130101); B65D 75/26 (20130101); Y10T
428/31891 (20150401); Y10T 428/31971 (20150401); Y10T
428/263 (20150115); Y10T 428/3192 (20150401); Y10T
428/31928 (20150401) |
Current International
Class: |
B32B
27/00 (20060101); B65D 65/40 (20060101); B65D
75/26 (20060101); B32B 023/08 (); B32B 027/30 ();
B32B 027/32 () |
Field of
Search: |
;161/249,247,252,251,165
;117/138.8E ;99/171LP,174 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
J Jack, "Biaxial Stretching of Polypropylene, Part 1," British
Plastics, Vol. 34, June 1961, pp. 312-318. .
J. Jack, "Biaxial Stretching of Polypropylene Film, Part 2,"
British Plastics, Vol. 34, July 1961, pp. 391-394..
|
Primary Examiner: Leavitt; Alfred L.
Attorney, Agent or Firm: Mann, Brown, McWilliams &
Bradway
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of application Ser. No.
280,200, entitled "Flexible Wrapping Material," filed May 14, 1963,
now abandoned, and the disclosure thereof, to the extent that it is
not inconsistent herewith, is hereby incorporated by reference.
Claims
We claim:
1. A flexible, laminated, heat-sealable wrapping material
particularly suitable for use in the packaging of food products and
other products which are to be maintained in hermetically sealed
relationship to the atmosphere, and in which the package is of the
nonconforming type whereby the wrapping material may be subjected
to considerable flexing and bending which is conducive to flex
cracking of the material and the development of pinholes therein,
said material being characterized by its resistance to flex
cracking and pinhole development, and by its imperviousness to gas
and moisture, comprising a cellophane sheet, coated with a layer of
a vinylidene chloride copolymer, and sandwiched between and bonded
on one side to a polymeric layer having good heat-seal strength,
and bonded on the other side, by a layer of bonding material, to a
biaxially oriented polypropylene sheet treated on its inner face to
facilitate bonding so that said polypropylene sheet forms one
surface of said laminated material, and said polymeric layer forms
the opposite surface of said material, said polymeric layer being
heat-sealable at a temperature substantially below the melting
point of said polypropylene sheet, and said polymeric layer being
selected from a group consisting of polyethylene and ethylene-vinyl
acetate copolymers.
2. A wrapping material in accordance with claim 1 in which the
polymeric layer forming one surface of the sheet is low density
polyethylene.
3. A wrapping material in accordance with claim 1 in which the
polypropylene and cellophane sheets are bonded together by means of
polyethylene.
4. A wrapping material in accordance with claim 3 in which the
polyethylene bonding material is an extrusion-laminated layer
between the polypropylene sheet and the cellophane sheet and the
polymeric layer is a polyethylene extrustion-coated layer on the
cellophane sheet.
5. A wrapping material in accordance with claim 1 in which the
cellophane sheet has a thickness of the order of about 0.8 mil, the
polymeric layer has a thickness of the order of about 1.5 mils, and
the polypropylene has a thickness of the order of about 0.5
mil.
6. A wrapping material in accordance with claim 4 in which the
polyethylene has a density of about 0.910 to about 0.930 and a melt
index above 4.
7. A wrapping material in accordance with claim 1 in which the
polymeric layer is an ethylene-vinyl acetate copolymer bonded to
the cellophane sheet by extrusion coating said polymeric layer
containing vinyl acetate within the range of 0.5% to 26% by
weight.
8. A wrapping material in accordance with claim 1 in which the
polymeric layer is a preformed film, glue-laminated to the
cellophane sheet.
9. A wrapping material in accordance with claim 4 in which the
polypropylene sheet has a thickness of about 0.25 to 1.5 mils, the
thickness of the cellophane sheet is such as to weigh about one
pound per 25,000 square inches, the thickness of the polyethylene
lamination between the polypropylene and cellophane is 0.3 to 2
mils and the thickness of the extrusion-coated polyethylene layer
is 0.5 to 4 mils.
10. A flexible, laminated, heat-sealable wrapping material
particularly suitable for use in the packaging of food products and
other products which are to be maintained in hermetically sealed
relationship to the atmosphere, and in which the package is of the
nonconforming type whereby the wrapping material may be subjected
to considerable flexing and bending which is conducive to flex
cracking of the material and the development of pinholes therein,
said material being characterized by its resistance to flex
cracking and pinhole development, and by its imperviousness to gas
and moisture, consisting essentially of, and bonded together and in
the order specified (1) a biaxially oriented polypropylene sheet
having its inner face treated for receptivity to a polymeric
bonding material, (2) a polymeric bonding material, (3) cellophane
coated on at least one side with a vinylidene chloride copolymer,
and (4) a heat-sealable polymeric material bonded to the coated
cellophane and having a melting point substantially below the
melting point of said polypropylene sheet, said heat-sealable,
polymeric material being selected from a group consisting of
polyethylene and ethylene-vinyl acetate copolymers.
11. A wrapping material as set forth in claim 10, in which the
polymeric bonding material between the polypropylene and the
cellophane is polyethylene.
12. A wrapping material as set forth in claim 10, in which the
heat-sealable polymeric material is a polyethylene having a density
of about 0.914 to about 0.925.
13. A wrapping material as set forth in claim 10, in which the
polypropylene sheet has a film thickness between 0.25 and 1.5 mils,
the polymeric bonding material has a thickness between 0.3 and 2
mils, the cellophane layer has a thickness on the order of about
0.8 mil, and the heat-sealable polymeric material has a thickness
of from 0.5 to 4 mils. .Iadd. 14. A flexible, laminated,
heat-sealable wrapping material particularly suitable for use in
the packaging of food products and other products which are to be
maintained in hermetically sealed relationship to the atmosphere,
and in which the package is of the nonconforming type whereby the
wrapping material may be subjected to considerable flexing and
bending which is conducive to flex cracking of the material and the
development of pinholes therein, said material being characterized
by its resistance to flex-cracking and pinhole development, and by
its imperviousness to gas and moisture, comprising a cellophane
sheet, coated with a layer of vinylidene chloride copolymer, and
sandwiched between and bonded of one side to a polymeric layer
having good heat-seal strength, and bonded on the other side, by a
layer of bonding material, to a heat-set biaxially oriented
polypropylene sheet treated on its inner face to facilitate bonding
so that said polypropylene sheet forms one surface of said
laminated material, and said polymeric layer forms the opposite
surface of said material, said polymeric layer being heat-sealable
at a temperature substantially below the melting point of said
polypropylene sheet, and said polymeric layer being selected from a
group consisting of
polyethylene and ethylene-vinyl acetate copolymers..Iaddend..Iadd.
15. A wrapping material in accordance with claim 14, in which a
polymeric layer forming one surface of the sheet is low density
polyethylene..Iaddend. .Iadd.16. A wrapping material in accordance
with claim 14, in which the polypropylene and cellophane sheets are
bonded together by means of polyethylene..Iaddend..Iadd. 17. A
wrapping material in accordance with claim 16, in which the
polyethylene bonding material is an extrusion-laminated layer
between the polypropylene sheet and the cellophane sheet and the
polymeric layer is a polyethylene extrusion-coated layer on the
cellophane sheet..Iaddend..Iadd. 18. A wrapping material in
accordance with claim 14, in which the cellophane sheet has a
thickness of the order of about 0.8 mil, the polymeric layer has a
thickness of the order of about 1.5 mils, and the polypropylene has
a thickness of the order of about 0.5 mil..Iaddend..Iadd. 19. A
wrapping material in accordance with claim 17, in which the
polyethylene has a density of about 0.910 to about 0.930 and a melt
index above four..Iaddend..Iadd. 20. A wrapping material in
accordance with claim 14, in which the polymeric layer is an
ethylene-vinyl acetate copolymer bonded to the cellophane sheet by
extrusion coating said polymeric layer containing vinyl acetate
within the range of 0.5 to 26% by
weight..Iaddend..Iadd. 21. A wrapping material in accordance with
claim 14, in which the polymeric layer is a preformed film,
glue-laminated to the cellophane sheet..Iaddend..Iadd. 22. A
wrapping material in accordance with claim 17, in which the
polypropylene sheet has a thickness of about 0.25 to 1.5 mils, the
thickness of the cellophane sheet is such as to weigh about one
pound per 25,000 square inches, the thickness of the polyethylene
lamination between the polypropylene and cellophane is 0.3 to 2
mils and the thickness of the extrusion-coated polyethylene layer
is 0.5 to 4 mils..Iaddend.
Description
BRIEF SUMMARY OF THE INVENTION
This invention relates to a flexible wrapping material in laminate
form, particularly useful in wrapping food products, and to the
method of preparing the same. The wrapping material of this
invention is prepared by bonding to one surface of a cellophane
sheet, preferably coated on both sides with vinylidene chloride
copolymer, a biaxially oriented polypropylene sheet and to the
other surface a thin layer of low density polyethylene or other
heat-sealable polymeric material having a melting point below that
of the polypropylene. Bonding may be accomplished by gluing or by
polyethylene lamination.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a perspective view showing a nonconforming package
containing a food product, such as cheese, and showing the areas in
which pinhole leaks are inclined to develop with prior art wrapping
materials; and
FIG. 2 is an enlarged cross-sectional view showing a preferred form
of the laminate of this invention.
DETAILED DESCRIPTION
In the packaging of various food products, such as cheese and meat,
in a flexible wrapper, difficulty has been experienced with the
development of small pinholes in the wrapper at points of stress
resulting from flexing of the wrapper during shipment and handling.
This results in a loss of the essential barrier characteristics of
the wrapper and permits loss or gain of moisture by the package and
access of oxygen into the package, with consequent spoilage of the
product intended to be protected by the wrapper.
In the so-called nonconforming type of flexible wrapping package, a
bulk product, such as cheese or meat, is encompassed by the
wrapping material along the side, top and bottom margins of the
product, and the wrapping material extends a substantial distance
beyond the ends of the product where the package is end-sealed
after being gas-flushed with an inert gas such as nitrogen, carbon
dioxide, or the like. The resultant package, therefore, has end
portions which are subject to deformation, crinkling, wrinkling,
and other flexing action caused by pressures of any kind that are
applied against unsupported end portions of the package, and
particularly end pressures such as occasioned in shipment of the
package in a bulk shipping container. The continual flexing of the
unsupported ends sooner or later causes pinholes to develop in
material, with the result that gas and vapor can pass through the
wrapper and accelerate spoilage of the food product.
It should be understood that a "conforming" package, such as
results from vacuum packaging, is also subject to pinhole
development, although in this type of packaging, the problem is
ordinarily not as severe, because the wrapper makes contact with,
and is supported by, the product.
Coated and laminated wrapping materials have been used in an effort
to provide a proper combination of physical properties in a
wrapping material to adequately protect the food product contained
within the package, but none of the materials used for this purpose
have satisfactorily solved the pinhole problem of nonconforming
packages.
For example, a coated sheet has been used comprising a base
material consisting of polyethylene terephthalate (commonly known
as "Mylar" sold by E. I. duPont de Nemours & Co., Inc.) with
the base material being exteriorly coated with a vinylidene
chloride copolymer (commonly known as "Saran" and sold by Dow
Chemical Company of Midland, Michigan), and with the inside of the
wrapping material having a polyethylene film used for its
heat-sealing properties. Such a wrapping material of this general
type is sold, for example, under the trade designation "MKP-5200"
and is made by Curwood, Inc., of New London, Wisconsin. Experience
has taught, however, that this type of material, which is the best
available for nonconforming food packaging, is still subject to
pinhole cracking.
We have discovered a new and unique combination of sheet materials
which, in a laminated construction provides the required functional
properties, and, in addition, gives the necessary resistance to
flex cracking and pinholing to give a successful wrapper.
The present invention has, for its primary object, the solving of
the pinhole problem, and the accomplishment of this result while
still preserving in the food wrapper the desirable and necessary
physical properties of a satisfactory wrapping material.
A further object of the invention is to provide a wrapping material
of this general classification which uses relatively inexpensive
laminate materials; which is completely adaptable to present
heat-sealing machines and equpment; which has appropriate barrier
properties with respect to gases and vapors, and which, in addition
to being substantially pinhole-proof, is also
abrasion-resistant.
Further and other objects of the invention will become apparent as
the disclosure proceeds and the description is read in conjunction
with the accompanying drawings.
The novel wrapper of this invention comprises a cellophane
substrate sandwiched between layers or films of low temperature,
heat-sealable plastic material and a layer or film of polypropylene
on one outer surface of the laminate. Preferably, the cellophane
substrate is coated on both sides with vinylidene chloride
copolymer such as that marketed under the name "Saran" in order to
improve impermeability to gas. "Saran" is a well known product as
disclosed on pages 444 through 448 of Golding on "Polymers and
Resins," published by D. Van Nostrand Company, Inc., 1959 edition.
On page 444 it is stated: "The name `saran` represents a series of
vinylidene chloride copolymers with vinyl chloride or
acrylonitrile. (Saran F)".
Referring to the drawing, the numeral 10 indicates a layer or film
of low temperature, heat-sealable plastic material, i.e., a
material which is heat-sealable at temperatures below 300.degree.
Fahrenheit and preferably below 250.degree. Fahrenheit, and which
has good heat-seal strength. While we prefer polyethylene for this
purpose, other low temperature, heat-sealable materials such as
ethylene-vinyl acetate copolymer or ethylene-vinyl acetate
copolymer blended with microcrystalline wax and paraffin wax in
suitable proportions may be used. The base or substrate 11 of the
laminate is cellophane preferably coated on both sides with a thin
layer of Saran 14 and 15. Although we prefer to include a layer or
film 12 corresponding to 10, layer 12 may be a suitable bonding
material such as a curing type or rubber type polymer. The outer
layer 13 is a plastic film or layer having high temperature
resistance as compared to layers 10 and 12. Because of its superior
properties and relative low cost, we prefer to use an oriented
polypropylene for layer 13 and preferably balanced, heat-set,
biaxially oriented polypropylene film. Heat-set film is
dimensionally stable when exposed to sealing temperatures whereas
nonheat-set film shrinks to its original shape when exposed to
sealing temperatures. The degree of balance, i.e., whether the film
is equally oriented in both machine and cross-machine directions is
not critical. Films of different degrees of balance function
satisfactorily in the finished laminate. The most economical,
thinnest gauge polypropylene film of good quality available is
preferably used but the thickness is not critical so long as the
film has the requisite flexibility. Although 50-gauge polypropylene
film is generally used, we have used 60-gauge with entirely
satisfactory results. Film thickness may vary from 25 to 150-gauge
(0.25 to 1.5 mils). The Moplefane OTT polypropylene specified in
the examples hereinafter given is treated on both sides to
facilitate bonding, as indicated by the designation "OTT" meaning
"stabilized against shrinkage, and treated on both sides."
When polyethylene is used for layers 10 and/or 12, it should have a
density of from about 0.910 to about 0.930 and a melt index from
about 2 to about 22, although we prefer a density of about 0.916
and a melt index about about 4. Melt index is the amount of flow in
decigrams through a fixed orifice at a fixed temperature. At the
two extremes of melt index range it is difficult to control the
flow of the hot polyethylene. The thickness of the polyethylene
lamination between the polypropylene and cellophane layers is
important in that tests indicate that thicker film is more
resistant to flex cracking than the thinner film. From an economic
viewpoint it is not practicable to use a coating exceeding 2 to 3
mils in thickness. Thicknesses between 0.3 and 2 mils produce a
satisfactory coating.
Some variation in the thickness of the polyethylene coating which
forms the heat-sealing layer 10 is permissible and will be dictated
by manufacturing practice and economics as well as performance of
the finished product. Generally, a thickness of about 2 mils is
preferred but the thickness may range from 0.5 to 4 mils.
When polyethylene film is applied as the heat-sealing layer to the
substrate, the minimum thickness of the film is dictated to some
extent by the film making process. It is difficult to produce
polyethylene film by the blown method of thickness less than 1 mil,
which is sufficiently uniform for laminating. Hence, we prefer to
use film having a thickness of from 1 to 4 mils with 2 mils being
preferred. Since film resins generally are lower in melt index due
to the lower processing temperatures, the melt index range of the
preferred films is 0.5 to 12 and the preferred density is about
0.916.
As previously pointed out, instead of using polyethylene for
heat-sealing layer 10, other polymeric materials that seal at
relatively low temperatures and have good seal strength may be
used. The same materials may be used for the adhesive layer 12. As
examples of such materials may be mentioned ethylene-vinyl acetate
copolymer, with or without admixture with paraffin and
microcrystalline wax. Ethylene-vinyl acetate copolymers containing
from about 0.5 to 26% by weight are satisfactory. When using
ethylene-vinyl acetate copolymer mixed with wax as the sealing or
adhesive layer it is applied by roll coating instead of by
extrusion. Ethylene-vinyl acetate copolymer may be applied as a
coating or as a preformed film, glue-laminated to the cellophane
substrate.
Vinyl film such as polyvinyl chloride (PVS) and polyvinyl
chloride-polyvinyl acetate copolymers (PVCAC) may also be used in
place of the polyethylene. The vinyl films have even lower melting
points than low density polyethylene and in addition have good tear
strength, low water absorption and moderately good chemical
resistance, but are much more costly.
The wrapper of this invention may be used for the hand wrapping of
a product, but ordinarily the wrapper is used on an automatic
machine which forms and closes the package by means of heat
sealing. Use of a thermoplastic material for the inner surface of
the wrapper, and forming the package with a face-to-face
relationship of this material, permits the formation of a
heat-sealed closure which is gas-tight. With some products, such as
cheese and meat, it is found advantageous to displace the air from
the void space inside the package by flushing with an inert gas, or
by evacuation, before sealing the package. The retention of this
gas, or vacuum, and the substantial exclusion of the atmospheric
oxygen, is important to the proper preservation of the product.
With many products, the dimensions of the package are such that it
is necessary to provide an excess of wrapping material in order to
bring the end portions of the wrapper together in a flat,
face-to-face relationship, which is free of wrinkles that would
give channels through the heat-sealed seam. Forcing the flat
wrapper to enclose a rectangular product in this way gives rise to
three-way creases which meet in a sharp point in the end portions
of the package. As this excess portion of the wrapper is flexed
during handling and shipment, there is an accumulation of stresses
at these points, or at any other point where flexing has produced
intersecting creases. The strength and flexibility of previously
known and used wrappers has not been adequate to withstand these
repeated concentrated stresses. This has resulted in the
development of the flex cracks and pinholes in the end portions of
the wrapper, as shown at 9 in FIG. 1, with consequent loss of the
barrier properties of the wrapper, and failure of the package.
We prefer to extrusion laminate the composite sheet, although glue
lamination may be employed if desired. Where the layers are
glue-laminated, polyurethanes, polyesters and other curing-type
polymers which cross-link or permanently tacky glues, such as
rubber or synthetic rubber based compositions may be used as the
gluing material. The amount of glue used will generally range from
1 to 1.5 pounds per 3,000 square feet of sheet. The reasons for the
unexpected and exceptional performance of the preferred combination
of materials, and the preferred extrusion lamination and coating
process, with respect to resistance to flex cracking and pinhole
development, is not fully understood. It is believed to be due in
some way to the distribution of stresses and the interaction of the
essential component layers of the lamination so as to deliver
maximum performance of the wrapper.
As before stated, the most satisfactory wrapping material for
nonconforming packages has been Mylar used as the base sheet with
an inner polyethylene coating for sealing, but tests have shown it
to be subject to objection because of flex cracking and pinhole
development.
In our laminate, which uses relatively inexpensive materials, the
cellophane provides a nonthermoplastic base layer which gives good
performance over a broad range of sealing temperatures without
"burn-through," and has good strength and stretch resistance which
is desirable; the Saran, if used, supplies a good barrier against
the passage of gases and vapors; the inside polyethylene coating 10
also adds to the moisture barrier and, in addition, serves as a
heat-sealing medium for package closure; the outer polyethylene
coating or layer 12 again adds to the moisture barrier, but, more
importantly, serves, with the inside polyethylene layer 10, to
confine the cellophane layer 11 in sandwich fashion, and, thereby,
through the flexible properties of the polyethylene, give physical
interface support to the more brittle cellophane layer contained
therebetween; and the exterior layer 13 serves the purpose of
allowing the polyethylene-encased cellophane to be satisfactorily
used in conventional heat-sealing machines, since it has a melting
point above that of the adjacent polyethylene.
The oriented polypropylene film used for the outside layer 13,
which polypropylene is preferably biaxially oriented, has the
advantage of being tough, flexible, stretch-resistant, and
resistant to surface abrasion, while also preventing sticking of
the wrapper to the heat-sealing bars on the packaging machine.
Although the wrapping material of this invention is particularly
useful in conjuction with nonconforming packaging procedures, it
should be understood that the material is also applicable to and
useful in conjunction with vacuum packing and conforming
wrappers.
The relative thickness of the composite layers of the laminate of
this invention may, of course, vary within the limits which will be
recognized by those skilled in the art. In a typical sheet, the
composite thickness of the cellophane layer 11 with the flanking
Saran layers, when used, may be on the order of 0.8 mil, the inside
polyethylene coating 10 may be on the order of 11/2 mils, the
outside polyethylene coating 12 may be on the order of 0.5 mil, and
the polypropylene coating 13 may be on the order of 0.5 mil.
Before laminating the polyethylene or other heat-sealable film to
the cellophane, the cellophane is prime coated with polyethylene
imine or organic titanate such as tetra-isopropyl titanate in order
to afford better adhesion between the two layers.
Certain preferred embodiments of this invention and methods of
making the laminate are illustrated in the following specific
examples:
No. 1. A film of 50 -gauge, heat-set, biaxially oriented
polypropylene, Moplefane OTT from Montecatini of Italy, was
extrusion laminated to 250 RS 1-E cellophane (Saran-coated) from
American Viscose Corp., with five pounds, per 3,000 square feet, of
polyethylene, U.S.I. grade 203-2, having a nominal density of 0.916
and a melt index of 8.0. This lamination was then extrusion coated
on the cellophane side with 26 pounds, per 3,000 square feet, of
polyethylene, U.S.I. grade 203-2. The cellophane was prime-coated
on both sides with a dilute solution of polyethylene imine before
combining with the polyethylene.
No. 2. A film of 50-gauge, heat-set, biaxially oriented
polypropylene, Moplefane OTT from Montecatini of Italy, was
extrustion laminated to 250 RS 1-E cellophane (Saran-coated) from
American Viscose Corp., with seven pounds per 3,000 square feet, of
polyethylene, Monsanto grade MPE-70, having a nominal density of
0.919 and a melt index of 5.0, This lamination was then extrusion
coated on the cellophane side with 24 pounds, per 3000 square feet,
of polyethylene, Monsanto grade MPE-70. The cellophane was
prime-coated on both sides with a dilute solution of polyethylene
imine in the amount of 0.1 pound per 3000 square feet of cellophane
before combining with the polyethylene in order to promote adhesion
of the polyethylene to the cellophane.
No. 3. A film of 50-gauge, heat-set, biaxially oriented
polypropylene, Moplefane OTT from Montecatini of Italy, was
extrusion laminated to 250 XCP 6-05cellophane (Saran-coated) from
duPont, with five pounds, per 3000 square feet, of polyethylene,
U.S.I. grade 203-2, having a nominal density of 0.916 and a melt
index of 8.0. This lamination was then extrusion coated on the
cellophane side with 26 pounds, per 3,000 square feet, of
polyethylene, U.S.I. grade 203-2. The cellophane was prime-coated
on both sides with a dilute solution of polyethylene imine before
combining with the polyethylene.
In the foregoing examples the numeral 250 means that the cellophane
yields 25,000 square inches of film per pound. The other
designation letters and numbers preceding "cellophane" are the
manufacturer's identification of the product.
The laminated wrappers from Examples 1 and 2 above were used for
the packaging of blocks of natural cheese, using Hayssen Model RT
wrapping machine. Air was flushed from the packages with carbon
dioxide, using the gas flushing technique. A number of sealed
packages with each type of wrapper were assembled in a corrugated
shipping container.
For purposes of comparison, similar packages were prepared at the
same time on the same machine, using the best available commercial
wrapper consisting of 50 Saran-coated Mylar, coated on one side
with 2 mils of polyethylene.
Packages of each of the wrappers from the examples, and the above
control wrappers, assembled in the shipping container, were
subjected to flexing stresses, intended to simulate actual handling
and shipping stresses, by shaking the shipping container full of
packages on a circular, synchronous shaking table, having an
amplitude of one inch, and a frequency of 210 cycles per minute.
The presence of pinholes in each package wrapper was determined by
applying slight air pressure to the package through a hypodermic
needle, and inspecting the package for the origin of gas bubbles
when held under water.
The shaking period used, and the resulting pinhole development from
flex cracking observed in the above tests are shown in the
following table:
TABLE I. ______________________________________ FLEX CRACK FAILURE
OF WRAPPERS ON SHAKEN PACKAGES Flex Crack Failures On Number Hours
Submersion Test Wrapper Packages Shaking Number % Failures
______________________________________ Example No. 1 36 2.5 2 6
Example No. 1 16 2.5 1 6 Example No. 2 13 4 0 0 Mylar-Polyethylene
13 2.5 9 70 Mylar-Polyethylene 8 2.5 8 100 Mylar-Polyethylene 8 2.5
8 100 Mylar-Polyethylene 10 4 10 100
______________________________________
Another test which has been used to determine the relative
effectiveness of different wrapping materials is to subject the
packages to the same general shaking procedures as outlined in the
above test and then store the packages under refrigerated
conditions to determine the condition of the cheese at the end of a
given period of time, say 4 weeks. This test was run with the
wrapper of Example No. 2, comparing the results with the Mylar
wrapper, and it was found that the wrapper of this invention when
used in 23 packages which were shaken for a 4-hour period had only
one package develop mold, representing 4% failure; whereas, with
the Mylar-polyethylene wrapper, 26 packages shaken for a like
period of 4 hours and then stored under identical refrigeration
condition for the same period of time showed mold development in 20
of the 26 packages, or a failure of 77%.
The surprising superiority of our new wrapping material has also
been confirmed by an independent laboratory using other testing
techniques in which it was established that the Mylar material
failed after 15,000 cycles of flexing, whereas the wrapping
material of this invention showed no signs of failure after 150,000
cycles of flexing.
Considering the fact that the laminate of this invention uses as
base material the relatively fractile material cellophane, it is
astounding that by this unique coacting combination of cellophane
with other materials it is possible to obtain a wrapping material
which shows a ten-to-one, or even greater, superiority over the
strong and tough material Mylar with respect to flex cracking and
pinhole development.
The following Table II indicates these variations in the
preparation of the finished laminated sheet which may be made while
retaining the important characteristics and properties previously
pointed out. It will be apparent from the table that the essential
layers are the cellophane substrate, preferably Saran-coated,
oriented polypropylene as the outer layer bonded to the cellophane
with polyethylene or gluing material, and polyethylene or
equivalent heat-sealable material as the inside layer bonded to the
opposite surface of the cellophane by means of extrusion coating or
by means of suitable glue.
TABLE II
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Outside Bond (Between Heat-seal Layer (13) Bond (12) Substrate (11)
10 & 11) Layer (10)
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OPP.sup.(1) PE.sup.(2) Cellophane -- PE coating.sup.(3) OPP Glue "
-- PE coating OPP Glue " Glue PE film OPP PE " Glue PE film
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.sup.(1) oriented polypropylene .sup.(2) polyethylene .sup.(3)
polyethylene or equivalent low temperature heat-sealable
polymer
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