CA1230719A - Multilayer food wrap with cling - Google Patents

Abstract

MULTILAYER FOOD WRAP WITH CLING
Abstract A multilayer film food wrap having outer layers of a polyolefin resin including a cling agent, and a core layer of a resin having a high melting point, resistance to heat, low permeability to oxygen, moisture and odor, and film toughness.
Methods of preparing the film and fabricating the film into a bag are also disclosed.

Description

~:3~7$9 MULTILAYER FOOD WRAP ~ITH CLI~G
Field of the Invention This invention relates to multilayer food wrap films having ~ood cling properties and high resistance to heat, making them ~uitable for use in direct food contact cooking. The cling propertiefi of the films enable them to cling to ~hemselves, or to ensure a tight seal when the films are used as a food wrap or a cover for containers.

Backaround of the Invention Thermoplastic ~esin films are popularly used as household wraps and bag~ to contain food and other items. A necessary characteristic of such wrap films is the ability to "cling", that is, to adhere to itfielf or to form a tight seal with other surfaces, as when the film is used as a cover for containers. The clingabili~y of the film preserves the freshness and moisture con~ent of wrapped food items and al~o prevents the migration of food odors.
Thermoplastic resin films do not generally possess inherent cling characteristics sufficient to satisfactorily adheLe to themselves and other surfaces. The cling charac~eristic of thermoplastic films is generally obtained through the use of so-called cling agent6. Cling agent6 are chosen for their ability to produce a surface on a thermoplastic film that can be sealed, opened and resealed, and are selected with due care in consideration of the use of thermoplastic films in food contact applications.

123~ 9 With the increa6ed u~e of microwave ovens, it has become desirable for food cling wraps to he used in microwave cooking. Currently, commercial food wrap~ such as Saran ~rap , Handi-Wrap , and Glad Wrap are produced as a 6ingle layer film 6tructure that i6 modified with addi~ives to provide the desired "cling" property. These commercial cling wraps are useful in microwave cooking as a cover for microwave cooking utensil~
and, as such, are not in contact with the food with which they are to be used. Functionally, it would be desirable to have a food wrap ~hich withstands higher temperatures so that it could be used in direct food contact, and in es~ence, serve the purpo6e of a microwave cooking utensil. That is, food would be wrapped in the food wrap, frozen, and when needed the food could be cooked in a microwave oven in the original food wrap.
Commercial food cling wraps may be produced from various polymers such as low density polyethylene, polyvinylidene chloride copolymers, and polyvinyl chloride copolymers. Currently, only polyvinylidene chloride and polyvinyl chloride type food cling wraps are able to with6tand ~omewhat higher microwave oven cooking temperatures, that i~, on the order of up to about 335F. Howevee, at these temperatures, the polyvinylidene chloride and polyvinyl chloride type food wraps have limited use for direct food contact m;crowave cooking since many fatty foods and foods high in sugar content require higher cooking temperatures than such food wraps can withstand without degrada~ion.

~- l40n4 ~30~7~9 In addition, in the production of plastic films for use as food wraps, a balance must be attained between the film's "cling" and "handleability" characteristics. "Handleability" is the characteristic of the film to resist the tangling that results from the film clinging to itself during use. While cling agents impart good cling properties to the film, it has been found that an increase in the amount of cling agent that is incorporated into the film corresponds to a decrease in the handleability of the film. With increased cling agent, the tendency of the film to adhere to itself and tangle increases.
It is known that handleability is improved through the use of "antiblocking" agents.
"Antiblocking" agents prevent thin plastic films from sticking to one another. Antiblocking agents suitable for use in plastic films include, but are not limited to, diatomaceous silica, such as those commercially available under the tradenames "SUPERFLOSS", "SILVERFROST"*, and "CELITE"* which are silica products of the Johns-Manville Company~
The concentration of the antiblocking agent incorporated into the film generally ranges from about 100 to about 10,000 parts per million, and usually from about 3,000 to about 7,000 parts per million. As disclosed in U.S. Patent 3,028,355, the diatomaceous silica, referred to as diatomaceous earth in the patent, should generally have an average particle size of between about 2 to about 6 microns. Other antiblocking agents selected by those skilled in the art can also be suitably used.

* Trademark ~lLZ30~

In addition to having good "cling~ and "handleability~ characteristics", as well a~ having a resi6tance to heat for use in direct food contact microwave cooking, the food wrap ~hould have good barrier properties such as oxygen, moisture, and food odor ~mpermeability. Further, the food wrap ~hould po se~s a deqree of toughness. All of these properties cannot be found in a single layer film food wrap. It iB only by providing a multilayer film that all of the aforementioned desirable properties and characteristics may be attained in a food wrap. Mul~ilayer films consist of two or more layers, each imparting specific desired characteristics to the multilayer film.
SummarY of the Invention The present invention relates to a multilayer film suitable for use in the manufacture of food wraps. In general, the multilayer film of this invention comprises at least outer layers of a polyolefin resin including a cling agent, and a core layer of a thermoplastic resin having a high melting point, resistance to heat for use in direct food contact microwave cooking, low permeability to oxygen, moisture and food odor, and film toughness.
In one embodiment of this invention, the multilayer film comprise6 a three-layer film having outer layers of a polyolefin resin containing a cling agent and a core layer having the aforedescribed charact.eristics and properties. In a preferred mode of this embodiment., the multilayer film comprises two outer layers of a polyolefin cont.aining a cling agent and a core layer of a 3~719 polyamide~ The optimum composition of the multilayer film of this embodiment co~prises two outer layers of polypropylene containinq a cling agent, and a core layer of a polyamide such as Nylon, and preferably Nylon-6.
In another embodiment of this invention, the multilayer film comprises a five-layer film having outer layers of a polyolefin resin containing a cling agent, a core layer of a resin having a high melting point, resistance to heat, low permeability to oxygen, moisture and food odor, and film toughness, and a bonding layer between the core layer and each of the outer layers. In a preferred mode of this embodiment, the multilayer film comprises two outer layer6 of a polyolefin containing a cling agent, a core layer of a polyamide, and a bonding layer between the core layer and each of the outer layers. The optimum composition of ~he five-layer film of this embodiment comprises two outer layers of polypropylene containing a cling agenl, a core layer of a polyamide such as Nylon-6, and a bonding layer between the core layer and each of the outer layers.
Any suitable thermoplastic resin or copolymer, or mixtures thereof, may be used for the outer layers of the multilayer films of this invention. Typical thermoplastic resins include polyolefins such as low and high density polyethylene, polypropylene, and polybutylene.
However, of the polyolefin resins, polypropylene is preferred for the outer film layers because of its higher melting point and better resistance to heat.

~L23~7~L9 The outer polyolefin layers, preferably comprising polypropylene, contain a cling agent which makes the film functional as a food wrap. Further, it has been found that not all polypropylene resin types are optimally suitable for the addition of cling agents thereto. More specifically, optimum results are obtained when the outer film layers comprise a polypropylene random copolymer having a melt flow index of between about 2 and about 25 decigrams per minute.
Any suitable cling agent may be present in the outer layers of the multilayer films of this invention. Typical cling agents include glycerol monooleate, sorbitan monooleate, sorbitan trioleate, glycerol dioleate, mono- and diglycerides, vegetable oils, epoxidized soybean oil, fatty esters, polyisobutylene polymers, polybutene polymers, ethoxylated nonylphenols, mineral oil, saturated aliphatic and alicyclic hydrocarbons, polyalkylene ether diols, such as polyepichlorohydrin, trihydric and higher polyhydric alcohols, the liquid esters of the higher molecular weight fatty acids, and polyethers, and surfactants. Preferably, the cling agent present in the outer layers of the multilayer film is glycerol monooleate.
A satisfactory concentration of the cling agent in each of the outer layers of the multilayer film into which it is incorporated is one in the range of from about 0.5 to about 2.0 weight percent, preferably about 0.8 weight percent, based on the weight of each outer film layer. The optimum concentration of cling agent in a thermoplastic ~23~7~9 resin layer is usually dependent on the particular resin system employed.
Any suitable thermoplastic resin or copolymer, or mixtures thereof, may be employed for ~he core layer of the multilayer films of this invention. Typical thermoplastic resins having a high melting point, resistance to heat, low permeability to oxygen, moisture and food odor, and film toughness include the polyamides such as Nylon-6, Nylon-6.6, and Nylon-12; polybutylene terephthalate, polyethylene terephthalate, ethylene-vinyl alcohol, and mixtures thereof.
However, it is preferred that the core layer comprise a polyamide, and more preferably, that the polyamide comprise Nylon-6, which is commercially available under the tradename Capron-8207* from the Allied Chemical Company.
When the multilayer film of this invention comprises a five-layer film having a bonding layer between the core layer and each of the outer layers, the bonding layer should be sufficient to provide a bonding strength between said core layer and said outer layers of at least about 200 grams~inch of film.
Any suitable bonding material, or mixtures thereof, that exhibit strong adhesion to polar resins may be employed as the bonding layer between the core layer and each of the outer layers in the multilayer film of this invention. Typical bonding materials include adhesive resins such as ionomer copolymers, chemically modified polyolefins, ethylene-vinyl acetate copolymers, ethylene-acrylic * Trademark 1~3~7~ ~

acid copolymers, polyolefins grafted with acrylic acid, and other multi-polymer compo~itions. The chemically modified polyolefin may be obtained f rom a number of polyolefin resin~, such as high, medium and low density polyethylenes, polypropylenes, ethylene ~inyl acetate copolymers, and ethylene-acrylic acid copolymers, which are modified by the provision of functional groups to the polymer, which have a strong affinity for the nylon molecule, and which will form strong bonds to nylon under the heat and pressure involved in the coextrusion proce6s. These bonding materials are generally commercially available. For example, ionomer copolymer6 may be obtained from E. I. DuPont de Nemour~ and Co. under the tradename Surlyn resin. Likewise, the modified polyolefins are available from Chemplex Company of Rolling Meadows, Illinois, uncler the tradename Plexar re~ins, such as Plexar-3, which is a modified ethylene-vinyl acetate copolymer adapted for cast film coextrusion. The preferred bonding materials are selected from modified polyolefins such as Plexar-3, and other multipolymer compositions 6uch as CXA-3101 available from E. I. DuPont de Nemours and Co. The bonding layer between the core layer and each of the outer layers may have a thickness of from about 0.03 mil to about 0.60 mil, preferably from about 0.05 mil to about 0.15 mil. When present, the thickness of the bonding layers may range from about 10 percent to about 20 percent of the total thickness of the multilayer films of this invention.

~23071g The total thickness of the multilayer films of this invention may range from about 0.3 mil to about 3 mil6, preferably from about 0.5 mil to about 1.0 mil, and more preferably, about 0.75 mil. The thickness of each of the outer layers of the multilayer films of this invention may range from about 0.1 mil to about 1.0 mil, preferably from about 0.2 mil to about 0.4 mil, and more preferably, about 0.32 mil. The thickness of the core layer of the multilayer films of this invention may range from about 0.05 mil to about 1.0 mil, preferably from about 0.08 mil to abou~ 0.5 mil, and more preferably, about 0.11 mil.
The multilayer film of this inventior. may have an outer layer to core layer to outer layer thickness ratio of from about 1:1:1 to about 4:1:~, but the preferred layer thickness ratio i8 about 3:1:3. When an adhesive layer is present, the thickness of the adhesive layer is included in the outer layer thickness ratio values.
The n~ultilayer films of this invention may be produced by any of several well-known methods.
Preferably, the film may be produced by what is commonly known as the slot cast extrusion method.
The film may also be produced by what is commonly known as the air blown film tubular exteusion method, but this latter method is le~s preferred.
The slot cast method produces a film of better clarity than the other methods known to the art.
Various cling enhancing agents and/or antiblocking agents, as the case may be, can be incorporated into .

~L2307~L9 the outer layers of the multilayer films of this invention by mixing the agents with the resin, preferably prior to extrusion thereof. For better control, it is preferred to mix the cling agent and/or the antiblocking agent with all of the resin of a particular layer to a final concentration, rather than master batching the additive to the resin, that is, adding a high concentration of the additive to a small amount of the resin and then adding that mixture to the total resin. The multilayer film may be slot cast on conventional extrusion equipment using a conventional slot cast multilayer die or a multilayer adapter for a single layer slot cast die.
The multilayer films of this invention can be slit to a preferred width, for example about 12 inches to about 18 inches, wound on cardboard coLes, and packaged in dispensing cartons having the typical metal sawtooth edge cutter bar.
The invention will become more apparent when considered together with the following examples, which are set forth as being merely illustrative of the invention and which are not intended, in any manner, to be limitative thereof.
Unless otherwise indicated, all par~s and percentages are by weight.
The Cling-to-Glass test used in the examples i6 designed to measure relative values of cling and employs an Instron testing instrument, such as an Instron Model TM or its equivalent. The apparatus consists of a glass plate, a clamp which holds one end of a film specimen, about 2.5q cm wide, a string which 3L~307~9 connects the clamp at one end and a 30 gram weigbt at the other, a metal or gla~6 rod over which the 6tring slides, a rin~6tand to hold the rod parallel ~o the Instron crosshead, and a metal plate which allows the glass plate to be attached to ehe Instron cro~shead.
The sample, about 22.86 cm long by about

2.54 cm wide, is prepared by placing the film wrap between two pieces of paper and cutting on a paper cutter.
About 20.32 cm of the 2~.B6 cm sample is placed on the gl~ss plate. The film is firmly folded flat onto the plate with a rubber roller.
The additional 2.54 cm of the sample is clamped in the small ~teel clamp. ~he glass plate is mounted horizontally on the Instron crosshead. A glass or metal rod i6 held parallel to the p~ate edge and to the crosshead, about 12.7 cm from the ed~e- of the pla~e. The string attached to the clamp holding the sample is hung over the rod. A 30 gram mass is attached to the end of the string. In this position the mass is exerting a force which tries to slide the film specimen over the glass plate. Generally, the coefficient of friction is 60 great that the film will not slide. Initially, the plate and cros6head are 6.35 cm above the rod, The cros~head is lowered at a rate of 50,8 cm per minute and as the plate drops below the level of the rod, the film begins to peel from the plate. When the film releases completely from the plate, the crosshead is stopped. The distance the plate has dropped below the rod is related to the magnitude of adhesive force or cling between the film and the plate.

~3~)~19 Cling-to-Glass is ~eported as centimeters of plate travel for the film sample.
Exam~le I
A three-iayer-film was manufactured usin~
the slot cast extrusion method. Polypropylene having a melt flow index of about 6 decigrams per minute, and a cling agent comprising glycerol monooleate, were mixed in a Banbury mixer. The cling agent was pre~ent at a concentration of about 0.8% by weight of the mixture. The resulting mixture was fed to an extruder connected to a multiple orifice slot cast extrusion die and the molten mixture was extruded through the two outer orifices of the die forming the outer layers of the three-layer film. Nylon-6 having an approximate melting point of 425F and a density of 1.13 g./cc was extruded through the center crifice of the die forming the core layer. The mult,ilayer film had a total thickness of about 0.75 mil, and a layer to layer to layer thick~ess ratio of about 3:1:3.
Example II
A three-layer film was prepared as in Example I except that the multilayer film had a total thickness of about 0.75 mil and a laye~ to layer to layer thicknes& eatio of about 2:1:2.
Exam~le III
A three-laye~ film was prepared as in Example I except that the multilayer film had a total thickness of about 0.50 mil and a layer to layer to layer thicknes6 ratio of about 3:1:3.

~23~9 Example IV
A three-layer film was prepared a6 in Example I except that the multilayer film had a total thicknes~ of about 0.~0 mil and a layer to layer to layer thickness ratio of about 2:1:2.
In Table 1, the effectiveness of the multilayer films of Examples I through IV was compared with a commercial single layer polyvinylidene chloride food wrap, and also with a commercial 6ingle layer polyethylene food wrap in a microwave oven cooking test. The microwave oven was a commercial Amana range set on high for a cooking time of about 6 minutes. The food ~amples comprised (A) about 200 grams of beef patty consisting of about 70~ lean ground beef, and (B) about 150 grams of pork sausage made into a patty.
During the microwave cooking test, one each of the food samples was individually wrapped with the multilayer film food wraps of Examples I through IV, the commercial polyvinylidene chloride food wrap, and the commercial polyethylene food wrap, and the twelve wrapped food samples were placed in the oven, thr~ee samples at one time. Failure of the food wrap was defined as any rupture of the film to permit leakage of grease or juices accumulated from the food 6amples. Table 1 summarizes these test results.

D-1400g 1~3~7~

Table 1 Miorowave Oven Test Performance Food Wrap Film Beef Pattv Pork Patty Example I no failure no failure Example II no failure no failure Example III failed failed Example IV failed failed Polyvinylidene chloride failed failed Polyethylene failed failed From the above results, it was determined that the high melting point Nylon-6 in the core layer of Examples I and II serve6 as a high temperature supportinq layer in the multilayer film structure. It was also ~ound that total film thickne6~ and layer ratios are important factors ~ince the multilayer films of Examples III and IV
permitted some leakage of food juices.
ExamPles V - VIII
Example V through Example VIII demonstrate that not all polypropylenes in combination with nylon are suitable for food wraps containing a cling agent. It was found that when employing a high melt flow polypropylene and some low melt flow polyallomers, a lo~s of cling properties was experienced after extrusion. Polyallomers are crystalline polyolefins made from two or more ,polymers such as ethylene-propylene block copolymers. Example V comprised a slot cast ~230~

extruded three-layer film consi6ting of outer layers of a polypropylene copolymer having a melt flow index of about 7 decigrams per minute. Example VI
was a ~lot cast extcudea three-layer film compri~ing outer layers of a polypropylene homopolymer havinq a melt flow index of about 12 decigram~ per minute.
Example VII and Example VIII were slot cast extruded three-layer film~ having outer layer~ of a polyallomer having a melt flow index of about 2 decigram~ per minute, and about 6 decigrams per minute, respectively. The three-layer films of Examples V through VIII each had a core layer of Nylon-6, and each of the film composition6 contained about one percent by weight of glycerol monooleate as the cling agent in each of the outer layers. The multilayer films of Examples V through VIII had a t~tal film thickness of about 0.75 mil, and a layer to layer to layer thickness ratio of about 3:1:3.
Table 2 summarizes the results of cling tests performed on these film compositions.
Table 2 Clinq ProPerties Of Various PolYProPYlene TYPes .

Clinq (inches~ Blocking ExamPle Glas~ Self Clinq (qrams) V 8.6 9.7 40 VI 5.2 4.9 10 VII 5 5 4,3 9 VIII 6.0 1.0 9 In Table 2, it should be noted that the higher the cling ~alue, the better is the clinging ~L23~

property of the film composition. It can be ~een from the results that a multilayer film containing the polypropylene copolymer of Example V provide6 the ~ost de~icable cling propertie~
ExamDle~ IX - ~IV
Example IX through Example ~IV illustrate the barrier properties of various food wrap compositions with respect to moi~ture vapor transmission, oxygen permeability, and onion odor permeability. Example I~ throuqh Example XII
comprised three-layer films manufac~ured by slot cast extrusion wherein the outer layers were made from polypeopylene having a melt flow index of about 7 decigrams per minute and a density of about 0.897 g/ml., and each outer layer contained about 0.8 percent by weight of glycerol monooleate. The core layer comprised Nylon-6. The film w~ap of Example IX h~ a total film thickness of about 0.43 mil, and a layer to layer to layer thickness ratio of about 2;1:2. The film wrap of Example X had a total film thickness of about 0.71 mil. and a layer to layer to layer thickness ratio of about 2:1:2. The film wrap of Example XI had a total film thickness of about 0.42 mil, and a layer to layer to layer thickness ratio of about 3:1:3. The film wrap of Example XII
had a total film thickness of about 0.64 mil, and a layer to layer to layer thickne~s ratio of about

3:1:3. The film wrap of Example XIII was a single layer commercial polyethylene product having a film thickness of about 0.45 mil. The film wrap of Example XIV was a single layer commercial ~23~

poiyvinylidene chloride product having a film ~hickne~s o~ abou~ 0.46 mil.
The barrier properties of the film wcap compositions of Examples IX though XIV are summarized in Table 3. In Table 3, the values shown for the moisture vapor transmission are reported as grams per 100 squace inches of film wrap per 24 hours. The oxygen permeability values are reported as cubic centimeters per 100 square inches of film wrap per 24 hour~. For the onion odor pecmeability results reported, sliced raw onions were placed in separate beakers which were covered with the film wrap of Examples IX through XIV~ respectively. A
panel of five persons rated the sample beakers for odor detection. The panel was in 100% agreement for the odor detection evaluation.
Table 3 Barrier Pro~ertles Moisture Vapor Oxygen Onion Odor Example Tran6mission PermeabilitY Permeability IX 2.5 21.6 yes X 2.3 9.~ no XI 2.9 81.1 yes ~II 2.1 17.0 no XIII 1.8 960 yes XIV 0.5 1~8 no From the above ce6ult~, it can be seen that polyvinylidene chlocide film wrap has excellent bacrier properties~ whereas polyethylene has very poor oxygen barrier properties. It can also be seen that of the three-layer film wraps, the compositions of Example X and Example XII provide the most satisfactocy barcier pcopecties.

~-14004 ~L2~7~9 The film wrap compositions of Example IX
through Example XIV were also evaluated for strength and toughness properties. Table 4 summarizes the various physical pLoperties evaluated on these film wrap compositions.
The values for the physical properties reported in Table 4 were obtained pursuant to the ASTM test methods indicated in Table 4 with the exception of the melting point values and the puncture toughness values. The melting point values given in Table 4 were obtained with a dif~erential scanning calorimeter. The composition of polymer blends wherein the parent polymers have sufficiently different polymer crystallinity melt temperatures can be determined u6ing differential scanning calocimetcy. The method i8 based on the measurements of the heat of fusion for the respective pa~ent polymers. The amount of polymer crystallinity, upon which this heat of fusion is based, i6 sensitive to thermal histo~y. Known standards upon which a calibration curve is based, and samples, are p~etreated in the same manne~ to obtain valid correlation~.
The punctu~e toughness values were obtained using an Instron Tensile-Compre~sion Tester. The tensile tester used was Instron Model TM, and the compcession teste~ was Model G-03-2 having a full scale range of 0-1 to 0-50 pounds employed with compression cell CC. The test is similar to ASTM
method D-1709 which employs f~ee falling darts and E-154 which employs a slowly moving plunger to measure the puncture resi6tance of films used as ~23~73L9 - 19 _ vapor bar~iers. The puncture resistance is a measure of the force required to rupture a ~est ~pecimen, and the energy ab~orbed by the film during rupture.

123~)71~

O O ~ D O er ~ ~ D ~ O U~
X _/ a:) 1~') 11~ N

H I ~ ~ ~0 Ct) ~D
X ~ ~ ~ O ~ ~ ~ ~ ~ ~ <~I

O~ ~
H ~ ~I Cl~ ~ CO U~ O r-- ~D
i-l I~ U~ ~D~ ~) ~D 1-` 0 0 ~`I
X
~D ~
H O ~ O 11~O ~-- O O ~`J
a)l E ~ ~ ~ o t` ~D
~: O Cl~ . ~ ~ er CO ~ ~ ~ u~ o ~
Xl -1 ~1 ~ ~ ~ ~ D ~ ~` ~ ~1 ~ er ~ .
X ~r ~ u~ 1--~ ~ o o o a q) O
~ . ~ O~
E~ ~ ~ ~ ~ o Cl~
E~ 0 co OD ~
cC G a a a a ^ C
.
C U~

C C C C
--o o o o O C v C V C J~ C J~
_I o o o ~ o o O ~
u~ 8 ~ v ~ ~ ~I v ~ v ~ ~4 J ~--I S ~ ~ O
c ' :: C .r~ a) oP ~ a~ ~~ ~ ra ~ a) E ~
o~ u, ~ a) ~ u~ ~ u) _I~ u) a) ~ u~ ~ c O ~ ~ ~ ~ ~ V ~ L~
v a) a~ c a~ GJ ~ O
~ ~ U~ O C ~ O O C
E ~ ~ u) ~ E~ o C C ~ C C S C O ~ C
O3 (~] a _I O ra ~ O 11~ V X ~
.~,v o r~ c ~ ~ c e ~ ~ o ~
rn r~ e ~ c E~ v ~ v v ~ c C O r~ u~ e ~

3~ g From the values obtained for the film compositions of Examples I~, X, XI and XII, it can be seen that these compositions possess puncture toughness and tensile strength load proper~ies which are superior to those of the single layer commercial polyethylene product of Example XIII. Further, the film compositions of Examples IX, X, XI and XII
possess puncture toughness energy properties which are superior to thofie of both the single layer 0 commercial polyethylene product of Example XIII and the single layer commercial polyvinylidene chloride product of Example XIV. In addition, the tear strength properties of the film compositions of Examples IX, X, XI and XII are substantially greater than those of the polyvinylidene chloride product of Example XIV. It can also be seen that the film compositions of Examples IX, X, XI and XII have a high melting point, i.e., substantially higher than the single layer polyethylene film of Example XIII, thus enabling their use in micro-wave cooking applications.
The multilayer film wrap compositions of this invention al60 possess clinging properties enabling the film wrap to seal to itself when used for wrapping foods, storing the wrapped food in a freezer, and cooking the food in the film wrap such as in a microwave oven. The multilayer film wraps are also well-suited for use as a cover for containers, and can also be used to protect sandwiches and other foods.
Although the present invention has been described and set forth in some detail, it should be ~2~)7~L~

further under6tood that the same is su~ceptible to change~, ~odifications and variation~ without depa~ting from the scope and spirit of the invention a~ 6et forth in the appended claims. Such changes, modifications and variations are within the scope of this in~ention.

Claims (32)

1. A multilayer film suitable for use in microwave oven cooking consisting essentially of outer layers of a polyolefin resin including a cling agent, each of said outer layers having a thickness of from about 0.1 mil to about 1.0 mil, said polyolefin resin being selected from the group consisting of polyethylene, polypropylene, and polybutylene, and a core layer of a thermoplastic resin having a melting point of at least about 335°F, resistance to heat, low permeability to oxygen, moisture and odor, and film toughness, said thermoplastic resin being selected from the group consisting of polyamides, polybutylene terephthalate, polyethylene terephthalate, ethylene-vinyl alcohol, and mixtures thereof.

2. A multilayer film in accordance with claim 1 wherein said cling agent is selected from the group consisting of glycerol monooleate, sorbitan monooleate, sorbitan trioleate, glycerol dioleate, mono- and diglycerides, vegetable oils, fatty esters, polybutene polymers, polyisobutylene polymers, ethoxylated nonylphenols, mineral oil, saturated aliphatic and alicyclic hydrocarbons, and surfactants.

3. A multilayer film in accordance with claim 1 wherein said cling agent is present in each of said outer layers at a concentration of from about 0.5 to about 2.0 percent by weight, based on the weight of said outer layers.

4. A multilayer film in accordance with claim 1 wherein said polyamides include Nylon, Nylon-6, Nylong-6.6, and Nylon-12.

5. A multilayer film in accordance with claim 1 wherein said outer layers of polyolefin resin comprise polypropylene and said core layer comprises a polyamide.

6. A multilayer film in accordance with claim 1 including a bonding layer between said core layer and said outer layers.

7. A multilayer film in accordance with claim 6 wherein said bonding layer provides a bonding strength between said core layer and said outer layers of at least about 200 grams/inch of said film.

8. A multilayer film in accordance with claim 6 wherein said bonding layer comprises an adhesive resin selected from the group consisting of ionomer copolymers, modified polyolefins, ethylene-vinyl acetate copolymers, ethylene-acrylic acid copolymers, polyolefins grafted with acrylic acid, and mixtures thereof.

9. A multilayer film in accordance with claim 1 wherein said outer layers of polyolefin resin comprise a polypropylene random copolymer having a melt flow index of between about 2 and about 25 decigrams per minute.

10. A multilayer film in accordance with claim 1 wherein the total thickness of said film is from about 0.3 mil to about 3 mils.

11. A multilayer film in accordance with claim 1 wherein the thickness of said core layer is from about 0.05 mil to about 1.0 mil.

12. A multilayer film in accordance with claim 6 wherein the thickness of said bonding layer is from about 0.03 mil to about 0.60 mil.

13. A multilayer film in accordance with claim 1 wherein said film has an outer layer to core layer to outer layer thickness ratio of from 1:1:1 to about 4:1:4.

14. A bag fabricated from the multilayer film of claim 1.

15. A method of preparing a multilayer film suitable for use in microwave oven cooking comprising slot cast extruding outer layers of a polyolefin resin including a cling agent, each of said outer layers having a thickness of from about 0.1 mil to about 1.0 mil, said polyolefin resin being selected from the group consisting of polyethylene, polypropylene, and polybutylene, and a core layer of a thermoplastic resin having a melting point of at least about 335°F, resistance to heat, low permeability to oxygen, moisture and odor, and film toughness, said thermoplastic resin being selected from the group consisting of polyamides, polybutylene terephthalate, polyethylene terephthalate, ethylene-vinyl alcohol, and mixtures thereof.

16. A method in accordance with claim 15 wherein said cling agent is selected from the group consisting of glycerol monooleate, sorbitan monooleate, sorbitan trioleate, glycerol dioleate, mono- and diglycerides, vegetable oils, fatty esters, polybutene polymers, polyisobulytene polymers, ethoxylated nonylphenols, mineral oil, saturated aliphatic and alicyclic hydrocarbons and surfactants.

17. A method in accordance with claim 15 wherein said cling agent is present in each of said outer layers at a concentration of from about 0.5 to about 2.0 percent by weight, based on the weight of said outer layer.

18. A method in accordance with claim 15 wherein said polyamides include Nylon, Nylon-6, Nylon-6.6, and Nylon-12.

19. A method in accordance with claim 15 wherein said outer layers of polyolefin resin comprise polypropylene and said core layer comprises a polyamide.

20. A method in accordance with claim 15 including applying a bonding layer between said core layer and said outer layers.

21. A method in accordance with claim 20 wherein said bonding layer provides a bonding strength between said core layer and said outer layers of at least about 200 grams/inch of said film.

22. A method in accordance with claim 20 wherein said bonding layer comprises an adhesive resin selected from the group consisting of ionomer copolymers, modified polyolefins, ethylene-vinyl acetate copolymers, ethylene-acrylic acid copolymers, polyolefins grafted with acrylic acid, and mixtures thereof.

23. A method in accordance with claim 15 wherein said outer layers of polyolefin resin comprise a polypropylene random copolymer having a melt flow index of between about 2 and about 25 decigrams per minutes.

24. A method in accordance with claim 15 wherein the total thickness of said film is from about 0.3 to about 3 mils.

25. A method in accordance with claim 15 wherein the thickness of said core layer is from about 1.0 mil.

26. A method in accordance with claim 20 wherein the thickness of said bonding layer is from about 0.03 mil to about 0.60 mil.

27. A method in accordance with claim 15 wherein said film has an outer layer to core layer to outer layer thickness ratio of from about 1:1:1 to about 4:1:4.

28. A method in accordance with claim 15 including fabricating a bag from said multilayer film.

29. A multilayer film suitable for use in microwave oven cooking comprising two outer layers of polypropylene containing a cling agent, and a core layer of a thermoplastic resin having a melting point of at least about 335°F, resistance to heat, low permeability to oxygen, moisture and odor, and film toughness, said thermoplastic resin being selected from the group consisting of polyamides, polybutylene terephthalate, polyethylene terephthalate, ethylene-vinyl alcohol, and mixtures thereof.

30. A multilayer film in accordance with claim 29 wherein said polypropylene comprises a polypropylene random copolymer having a melt flow index of between about 2 and about 25 decigrams per minute.

31. A method of preparing a multilayer film suitable for use in microwave oven cooking comprising slot cast extruding two outer layers of polypropylene containing a cling agent, and a core layer of a thermoplastic resin having a melting point of at least about 335°F, resistance to heat, low permeability to oxygen, moisture and odor, and film toughness, said thermoplastic resin being selected from the group consisting of polyamides, polybutylene terephthalate, polyethylene terephthalate, ethylene-vinyl alcohol, and mixtures thereof.

32. A method of preparing a multilayer film in accordance with claim 31 wherein said polypropylene comprises a polypropylene random copolymer having a melt flow index of between about 2 and about 25 decigrams per minute.