White opaque opp film for tamper evident package

A tamper-evident film includes a hydrocarbon polymer core layer (a) having an upper face and a lower face; an oriented tamper-evident layer applied to at least the lower face of core layer (a), layer (b) being fabricated from a mixture of a polyolefin and a filler of particles which induce cavitation upon orientation, layer (b) having an internal cohesiveness which is less than the internal cohesiveness and bonding strength of packaging adhesives and heat seals; and, an oriented skin layer (c) applied to the upper face of core layer (a) if layer (b) has not been applied thereon, layer (c) being hydrocarbon polymer. A tamper-evident package seal and package are also provided.

BACKGROUND OF THE INVENTION 
This invention relates to tamper-evident multilayer plastic packaging films 
and to a tamper-evident package seal made therefrom using an adhesive cold 
seal. 
In the past, in the packaging of certain types of food, for example, candy 
bars, which require packaging sealed with a cold adhesive, a sealed 
package is capable of being opened and subsequently put back together 
without evidence of tampering. The present invention remedies this 
problem. A tamper-evident packaging film and seal such as that of the 
present invention can not be resealed once the seal has been opened. 
U.S. Pat. No. 4,429,015 to Sheptak discloses a multiply laminate for 
identification cards. Attempts to delaminate the card results in fibers 
being torn from a uniaxially oriented polyethylene or polypropylene layer 
of the laminate. 
U.S. Pat. No. 4,121,003 to Williams discloses switch-proof labels using a 
laminate having a pattern printed on the label inner surface and a coating 
of pressure sensitive adhesive film on the inner surface. Attempts to 
transfer the label result in disruption of the printed pattern. 
Neither the Williams nor Sheptak laminate is suitable for food packaging. 
U.S. Pat. No. 4,632,869, the contents of which are incorporated by 
reference herein, discloses an opaque, biaxially oriented polymeric film 
structure with a thermoplastic polymer matrix core layer possessing a 
stratum of voids a substantial number of which contain at least one 
spherical void-initiating solid particle of polybutylene terephthalate, 
and a void-free thermoplastic skin layer on at least one surface thereof. 
U.S. Pat. No. 4,377,616, the contents of which are incorporated by 
reference herein, discloses a lustrous, opaque biaxially oriented polymer 
laminate film structure comprising a thermoplastic polymer matrix core 
layer possessing numerous voids, a substantial number of which contain at 
least one void-initiating article, and transparent skin layers adhering to 
the surfaces of the core layer. The skin layers are of a thickness to 
cover surface irregularities inherent in a cavitated core layer. The 
structure of the core layer imparts a much higher degree of opacity than 
that imparted by the use of opacifying pigment alone, due to the effects 
of light scattering. 
These opaque biaxially oriented polymer laminate structures have been used 
to provide films with low water vapor and gas transmission rate and 
pleasing appearance. 
A plastic film having all these desirable characteristics and also having a 
tamper-evident property is much desired. 
It is an object of the present invention to provide a tamper-evident 
packaging seal utilizing a film structure which provides a tamper-evident 
property to a package seal. 
This tamper-evident packaging film can be sealed with heat or cold seals. 
Opening of the seal will result in irreversible tearing of the film so 
that the package cannot be resealed without evidence of tampering. 
SUMMARY OF THE INVENTION 
A tamper-evident film is provided which consists of three layers: a core 
layer (a) and two layers (b) and (c) on the lower and upper surfaces of 
layer (a) respectively. The core layer (a) can be fabricated from a 
polymer material of clear polypropylene, polyethylene or ethylene 
copolymerized with alpha olefins or blends thereof. The core layer (a) 
optionally may be an opaque layer which has a stratum of voids therein 
which may provide other desirable film properties. The skin layer (b) is 
fabricated from a mixture of a polyolefin and a filler which induces 
cavitation upon orientation. Skin layer (c) can be either identical or 
different from the layer (b). If different from layer (b), layer (c) can 
be fabricated from a member of the group consisting of hydrocarbon 
homopolymers, copolymers or blends thereof. 
The tamper-evident film may be used to provide a tamper-evident packaging 
seal which includes at least one tamper-evident film. In forming a package 
seal, the tamper-evident film may be bound to either an identical or a 
different film with an adhesive, heat sealant, or heat seal. The adhesive, 
heat sealant, or heat seals have a seal internal cohesion and an affinity 
for bonded surfaces greater than the internal cohesive force of the 
tamper-evident film so that if a force is applied to separate the films, 
rupture is initiated by the cavitated skin, and tamper evidence occurs. 
As a result of the present invention, a tamper-evident packaging seal is 
provided utilizing a tamper-evident packaging film with excellent barrier 
properties, a pleasing appearance, and a unique tamper-evident property 
which adds substantially to the security of the product packaged within 
it. 
For a better understanding of the present invention, together with other 
and further objects, reference is made to the following description, and 
its scope will be pointed out in the appended claims.

DETAILED DESCRIPTION OF THE INVENTION 
According to FIG. 1, there can be seen a tamper-evident packaging film 
which can be prepared in accordance with the present invention such that a 
tamper-evident seal can be provided which is effective against undetected 
opening of the seal and which can be produced on a mass production scale 
and converted for end use quite readily for use, for example, in food 
packaging and other consumer products. In particular, FIG. 1 shows in 
cross section a three-layer extruded film which includes core layer (a), 
and skin layers (b) and (c). Furthermore, an adhesive material 2 has been 
shown on skin layer (b), which in one preferred embodiment, is a cold seal 
adhesive which can be used to package food products which will suffer in 
the presence of heat required to bond a heat seal packaging film. 
In order to provide the present invention, the overall structure of the 
film shown in FIG. 1 is provided with a weakened layer (b) such that upon 
exertion of a force in a direction which tends to rupture the seal, the 
integrity of the weakened layer will be disrupted. 
Referring to FIG. 2, the structure of the present tamper-evident seal is 
shown schematically with 2 films of the present invention sealed together, 
layer (b) to layer (b). A force (f) is represented in vector format to 
show how a force might conceivably be applied to the structure. In 
particular, the composite tamper-evident seal is shown with the film being 
adhered to itself in an assembly such as that shown in FIG. 3. In such an 
assembly, the film will be joined together by the adhesive 2 which will, 
upon contact with itself, form a continuum between the outer layers (b) of 
the tamper-evident film. Accordingly, the adhesive layer 2 is shown as a 
single continuum in FIG. 2. 
FIG. 3 illustrates a package according to the present invention. The 
package may be formed from a single sheet of the film of the present 
invention with layer (b) on the inner surface. The film may be joined to 
itself with a fin seal 4. The sides of the package 5 may be sealed with a 
crimp and a cold seal adhesive (not shown). Depending on the product to be 
packaged, seals at 4 and 5 may also be heat seals. If the package is 
opened in the areas of the seals, the packaging film will delaminate so 
that the package cannot be resealed without evidence of tampering. 
In accordance with the present invention, the strength of the adhesive 
layer 2 as well as the adhesive to surface adhesion strength between the 
adhesive layer 2 and the surface of film layer (b) is stronger than the 
internal cohesive force of layer (b). Thus, when a separating force (f) is 
applied to the tamper-evident seal, the integrity of the composite 
tamper-evident seal is destroyed by separation of layer (b). As a result 
of destruction in this manner, the seal is incapable of being resealed 
thus showing evidence of tampering. 
For the packaging film used in the tamper-evident seal, the various 
materials are selected so that they possess interacting relative 
affinities (adhesion and/or cohesion). Relative affinities to be 
considered are the internal cohesion of the bonded polymer films and both 
the internal cohesion of the adhesive, heat sealant or heat seal and their 
affinities for the film surfaces which are bonded together, at least one 
of which film surfaces is comprised of layer (b). The cohesive strength of 
the adhesive, heat sealant or heat seal is greater than the internal 
cohesion of the layer (b). In addition, the adhesive, heat sealant, or 
heat seal possess an affinity for the surface of film layer (b) which is 
greater than the internal cohesive force of layer (b), so that when two 
film surfaces are sealed together and an attempt is made to separate the 
two surfaces, the adhesive layer, heat sealant or heat seal remains 
internally cohesive and adherent to the layer (b) surface and layer (b) 
will tear while the adhesive remains in place. Ideally, part or all of the 
skin layer (b) remains adherent to the adhesive or heat sealant. 
The presence of layer (b) on the adjacent inside surfaces of two adhesively 
adhered opposing films which comprise a package seal prevents the package 
from being readily and undetectably resealed after being opened. 
In packaging technology, a packaging film can be sealed with a coating that 
seals to itself. These coatings are called cold-seal coatings and are 
usually modified rubber-based material. Cohesives of this type are often 
used to package products which would be damaged by heat such as ice cream, 
candy bars, and confections. 
When a cold seal adhesive is used to seal a packaging film, the two film 
surfaces which are to be sealed together are each coated with the cohesive 
and the coated surfaces are pressed together, adhesive to adhesive. 
Alternatively, two packaging film surfaces may be sealed together by 
applying an adhesive coating on one side and the surfaces are then pressed 
together, adhesive coated film to uncoated film. 
In both of these methods, when using packaging films of the prior art, the 
seals can be separated with the packaging film intact, either by pulling 
apart two cold seal coated film surfaces, or by separating an adhesive 
coated film surface from an uncoated film surface. 
In the present invention, however, when the layer (b) surface is cold 
sealed or heat sealed to another film, if attempts are made to separate 
the seal, layer (b) separates, but the adhesive or sealant does not. 
Furthermore, the adherent film surfaces will not separate but will split 
in the skin layer (b) so that the adhesive itself remains sealed and 
covered by the remainder of the skin layer (b), and the adhesive is not 
exposed. Therefore, the seal cannot be readhered. 
The film of the present invention comprises at least one cavitated layer, 
layer (b) which is applied to at least the lower face of core (a). The 
film may also comprise two or three cavitated layers, core layer (a), with 
layer (b) applied to the lower face of core (a) or to both the upper and 
lower faces of core (a). Core layer (a) may be the same or different than 
layer (b). If layer (b) is applied only to the lower face of core (a) and 
not to the upper face, then layer (c) may be applied to the upper face of 
core layer (a). 
A tamper-evident package can be made using the film structure of the 
present invention with layer (b) inside the package, that is, layer (b) is 
the surface which is heat or cold sealed. 
The polyolefins contemplated for the substrate or core layer material and 
the tamper-evident surface layer(s) of the subject film structure include 
polyethylene, polypropylene, polybutylene and copolymers and blends 
thereof. A crystalline polypropylene containing at least 80% by weight of 
isotactic polypropylene and having a melt flow index of from about 2 to 8 
g/10 minutes is advantageously used for both the core layer and the skin 
layers. Also, titanium dioxide and particulates such as talc and syloid 
can be added to both the core and skin layers to impart brightness and 
antiblocking. 
The polymer matrix material of core layer (a) can be a clear polypropylene 
or polyethylene or ethylene copolymerized with alpha olefins or blends 
thereof. The core layer can also be an opaque layer. The opacity of the 
opaque layer is achieved by introducing to this layer a stratum of voids 
which scatter light. The voids are created by mixing the matrix resin with 
an incompatible material followed by orientation. By incompatible it is 
meant that distinct phases of the two materials will result when an 
inter-blending of the two is attempted. Examples of such incompatible 
materials include polybutylene terephthalate and polyalphamethylstyrene. 
The incompatible material can be present in up to about 20% by weight of 
the matrix film. A preferred range is from about 2 to about 10 percent by 
weight. 
A master batch technique can be employed either in the case of forming the 
spherical particles in situ or in adding preformed spheres to a molten 
thermoplastic matrix material. After the formation of a master batch, 
appropriate dilution of the system can be made by adding additional 
thermoplastic matrix material until the desired proportions are obtained. 
Tamper-evident layer (b) which is to be applied to the lower face of core 
layer (a) and optionally also to the upper face of core layer (a), also 
has a polyolefin thermoplastic polymer matrix preferably of polypropylene 
and a filler which induces cavitation resulting in a stratum of voids 
located within the polymer matrix. Secondary skin layers may also be 
present adhering to the surface of layer (b), these secondary skin layers 
being of a thickness such that they do not substantially change the 
surface characteristics of skin layer (b). 
The skin layer (b) thickness can be from about 2 to about 90% of the 
overall structure. It is preferred that the thickness of the skin layer 
(b) be from about 5 to about 20% of the overall structure. 
The void-initiating particles dispersed as a filler phase into the 
polymeric matrix of layer (b) can be organic or inorganic, and are rigid 
particles which may or may not be identical in shape. The particles have a 
melting temperature higher than that of the polymeric matrix material. 
Some materials which may be used for the dispersed filler phase include 
calcium carbonate, zinc oxide, zeospheres, polyalphamethylstyrene 
(P.alpha.MS), and polybutylene terethalate PBT). The void-initiating 
particles can be present in amounts from about 1 to about 20 weight 
percent of layer (b) prior to orientation, with about 10 to about 15 
weight percent preferred. The void-initiating particles can be present in 
tamper-evident layer (b) in an amount which is below 10% by weight of the 
total film structure, preferably about 0.05% to about 4%. 
It is preferred that the average diameter of the voidinitiating particles 
be from about 0.1 to about 10 microns. These particles initiate voids 
throughout the matrix material during biaxial orientation. 
The void-initiating particle material must be incompatible with the skin 
material, at least at the temperature of biaxial orientation. 
The tamper-evident skin layer (b) is described as being a thermoplastic 
polymer matrix material within which is located a stratum of voids 
initiated by irregularly shaped organic or inorganic particles. The term 
"stratum" is intended to convey the understanding that there are a great 
many voids within the matrix and the voids themselves are oriented so that 
the two major dimensions are aligned in correspondence with the direction 
of orientation of the polymeric film structure. After each void has been 
formed through the initiation of an inorganic or organic particle, the 
particle may contribute little else to the system. Pigments such as 
TiO.sub.2 can be present or dispersed throughout the skin matrix. The 
pigment material is present in such a particle size and shape that it does 
not, at least in any material sense, contribute any void initiation by 
itself. 
A typical void of the tamper-evident skin layer is defined as having major 
dimensions X and Y and minor dimension Z, where dimension X is aligned 
with machine direction orientation, dimension Y is aligned with transverse 
direction orientation and dimension Z approximately corresponds to the 
cross-sectional dimension of the inorganic/organic particle which 
initiated the void. 
The orientation conditions are such that the X and Y dimensions of the 
voids of the tamper-evident skin layer (b) be major dimensions in 
comparison to the Z dimension. Thus while the Z dimension generally 
approximates the cross-sectional dimension of the inorganic/organic 
particle initiating the void, the X and Y dimensions must be significantly 
greater. In addition, the orientation conditions must be such that the 
general integrity of the voids is maintained. By this, it is meant that 
during the orientation which produces the X and Y dimensions, that is, 
either by simultaneous or sequential machine direction and transverse 
direction stretching, the temperature conditions must be such as to permit 
these major dimensions to form without any destruction of the voids in any 
of its dimensions. The voids are particularly vulnerable to destruction 
during sequential orientation if the stretching temperature is too low. 
Even in simultaneous orientation if the temperature is too low, the 
stretching forces will tend to cause internal shredding and void 
splitting. This leads to a complete loss of control over the integrity of 
the individual closed voids, and the consequent integrity of the matrix 
polymer. Thus, one skilled in the art, following the present general 
guidelines, can orient at a temperature and to a degree which will yield X 
and Y dimensions approaching a maximum without causing any substantial 
splitting, shredding or overall lack of void and matrix integrity. 
Layer (c) which is optionally applied to the upper face of layer (a) may be 
fabricated from hydrocarbon copolymers, homopolymers and blends of 
homopolymers, and blends of copolymer(s) and homopolymer(s) heretofore 
employed for this purpose. For example, layer (c) may be polypropylene. 
Layers (a), (b) and (c) may be coextruded. 
The films may be biaxially oriented by conventional means. In general, this 
includes forming the film in sheet form and machine direction orienting 
(MDO) or stretching the same at the appropriate or optimum temperature, 
using transport rollers operating at different speeds. After the desired 
degree of MDO, the film is transverse direction oriented (TDO), for 
example in a tentering apparatus, to impart an orientation or stretching 
which is at right angles to the MDO. The extent 10 of orientation can be 
from about 3 to about 10 times its original dimension for the MDO and from 
about 3 to 10 times in the TDO. 
The surface of the film may be treated to insure that an adhesive will be 
strongly adherent to the film. This treatment may be accomplished by 
employing known prior art techniques such as, for example, film 
chlorination, i.e., exposure of the film to gaseous chlorine, treatment 
with oxidizing agents such as chromic acid, hot air or steam treatment, 
flame treatment and the like. Although any of these techniques may be 
effectively employed to pretreat the film surface, a particularly 
desirable method of treatment has been found to be the so-called 
electronic treatment method which comprises exposing the film surface to a 
high voltage corona discharge while passing the film between a pair of 
spaced electrodes. After electronic treatment of the substrate film 
surface it may be coated with the adhesive by conventional techniques. 
Pretreatment of the surface is desirable in cold seal/coating 
applications. Optionally, a primer and a PVDC or acrylic type coating can 
be applied to the film surface prior to the application of the adhesive to 
achieve other desirable film properties. 
A package fabricated from the film of the present invention may be sealed 
using a cold seal material such as Findley 211, 207 or with a heat seal. 
The following examples illustrate the present invention. 
In all examples, the tamper-evident films made according to the present 
invention were compared with standard packaging films as controls. 
EXAMPLE 1 
A mixture of isotactic polypropylene (94 parts, MP 160.degree. C. and melt 
index of 3.0) and polybutylene terephthalate (PBT) (6 parts, MP 
228.degree. C.) was melted in an extruder provided with a screw of L/D 
ratio of approximately 20/1. A second extruder was in association with the 
first mentioned extruder and supplied with the same polypropylene but 
without the PBT present. A third extruder was also in association with 
these two mentioned extruders and was supplied with the same mentioned 
polypropylene and calcium carbonate (15 parts). A melt coextrusion was 
carried out while maintaining the cylinder of the core material ranging 
from 200.degree. C. to 230.degree. C. The polypropylene to be extruded by 
the second mentioned extruder as a skin layer (c) was maintained at a 
temperature of 230.degree. C. The polypropylene and calcium carbonate 
blend to be extruded by the third mentioned extruder as skin layer (b) was 
also maintained at a temperature of 230.degree. C. A film structure was 
coextruded with a core thickness of 70% of the total extruded thickness. 
The skin layers (b) and (c) were each 15% of the total extruded thickness. 
Calcium carbonate was present in an amount of 3% by weight of the total 
structure. The unoriented film measured approximately 38 mils in 
thickness. This sheet was subsequently oriented five and one half by eight 
times respectively in the machine and transverse direction. The machine 
direction, or MD, orientation temperature used was about 135.degree. C. 
and the transverse, or TD, orientation temperature was about 155.degree. 
C. This resulted in a 1.5 mil film with a density of about 0.60 gm/cc. A 
modified rubber based cohesive was then applied to the skin layer (b) at a 
thickness weight of about 3 lbs per ream to simulate an overwrap type 
package. In this case, the cohesive used was a commercially available 
Findley #211 cold seal. Two of the skin layers (b) which were coated with 
cohesive, were then sealed together using a standard packaging crimp 
sealer. All seals were produced at 80 PSI with a dwell time of 1/2 second. 
These seals were then separated using a Suter Tester at a pulling speed of 
12" per minute in opposing directions. This film produced an unexpected 
complete delamination of the skin layer (b) from the core when the seal 
was pulled apart. This rendered the seal totally incapable of being 
rejoined together as there was no cohesive left exposed creating a 
tamper-evident type seal. The results are shown in Table 1. 
TABLE 1 
______________________________________ 
FILM SEAL DESTRUC- 
STRUC- SEAL STRENGTH TION RESEAL- 
TURE TYPE (gms) MODE ABILITY 
______________________________________ 
TAMPER EVIDENT FILM: 
Cavitated 
Cold 140 Delami- No 
skin seal nation 
Core (F-211) 
Poly- 
propylene 
CONTROL: 
Poly- Cold 160 Cohesive Yes 
propylene 
seal Failure 
Core (F-211) 
Poly- 
propylene 
______________________________________ 
EXAMPLE 2 
The process of Example 1 was repeated, except in this case, the cold seal 
designated was F-207 and this was applied to a polyvinylidene chloride 
(PVdC) coated version of Example 1. Similar physical properties resulted, 
as shown in Example 1, but the PVdC coating imparted, in this case, a 
lower WVTR with a higher seal strength along with the tamper-evident 
quality. The results are shown in Table 2. 
TABLE 2 
______________________________________ 
FILM MAX. SEAL DESTRUC- 
STRUC- SEAL STRENGTH TION RESEAL- 
TURE TYPE (gms) MODE ABILITY 
______________________________________ 
PVdC COATED TAMPER EVIDENT FILM: 
PVdC Cold 470 Delami- No 
Cavitated 
seal nation 
skin (F-207) 
Core 
Poly- 
propylene 
CONTROL: 
PVdC Cold 608 Cohesive Yes 
Poly- seal Failure 
propylene 
(F-207) 
Core 
Poly- 
propylene 
______________________________________ 
EXAMPLE 3 
The process of Example 2 was repeated, except in this case, the seal was a 
heat seal at 260.degree. F. and 20 psi 3/4 sec. This tamper-evident film 
again had a PVdC coating as discussed in Example 2, but for certain 
packaging applications, a heat seal is required. The results are shown in 
Table 3. 
TABLE 3 
______________________________________ 
FILM MAX. SEAL DESTRUC- 
STRUC- SEAL STRENGTH TION RESEAL- 
TURE TYPE (gms) MODE ABILITY 
______________________________________ 
PVdC COATED TAMPER EVIDENT FILM: 
PVdC 260.degree. F. 
255 Delami- No 
Cavitated 
Heat nation 
skin Seal 
Core 
Poly- 
propylene 
CONTROL: 
PVdC 260.degree. F. 
403 Cohesive Yes 
Poly- Heat Failure 
propylene 
seal 
Core 
Poly- 
propylene 
______________________________________ 
EXAMPLE 4 
The process of Example 1 was repeated, except in this case, the cold seal 
designated was F-207 and this was applied to an acrylic coated version of 
Example 1. Similar physical properties resulted, but the acrylic coating 
provided good machinability along with a high seal strength and 
tamper-evident quality. The results are shown in Table 4. 
TABLE 4 
______________________________________ 
FILM SEAL DESTRUC- 
STRUC- SEAL STRENGTH TION RESEAL- 
TURE TYPE (gms) MODE ABILITY 
______________________________________ 
ACRYLIC COATED TAMPER EVIDENT FILM: 
ACRYLIC Cold 330 Delami- No 
Cavitated 
seal nation 
skin (F207) 
Core 
Poly- 
propylene 
CONTROL: 
ACRYLIC Cold 400 Cohesive Yes 
Poly- seal Failure 
propylene 
(F207) 
Core 
Poly- 
propylene 
______________________________________ 
EXAMPLE 5 
The process of Example 4 was repeated except in this case, the seal was a 
heat seal. This tamper-evident film had an acrylic coating as discussed in 
Example 4, but for certain applications, a heat seal is required. The 
results are shown in Table 5. 
TABLE 5 
______________________________________ 
FILM SEAL DESTRUC- 
STRUC- SEAL STRENGTH TION RESEAL- 
TURE TYPE (gms) MODE ABILITY 
______________________________________ 
ACRYLIC COATED TAMPER EVIDENT FILM: 
ACRYLIC 260.degree. F. 
200 Delami- No 
Cavitated 
Heat nation 
skin seal 
Core 
Poly- 
propylene 
CONTROL: 
ACRYLIC 260.degree. F. 
260 Cohesive Yes 
Poly- Heat Failure 
propylene 
seal 
Core 
Poly- 
propylene 
______________________________________ 
The data disclosed hereinabove reveal the unique properties of the 
tamper-evident film of the present invention. It should be noted that the 
destruction mode appears only when one attempts to tamper with the 
tamper-evident seal disclosed herein. The film remains intact, retaining 
its excellent packaging properties under normal handling conditions. 
While there have been described what are presently believed to be the 
preferred embodiments of the invention, those skilled in the art will 
realize that changes and modifications may be made thereto without 
departing from the spirit of the invention, and it is intended to claim 
all such changes and modifications as fall within the true scope of the 
invention.