BACKGROUND OF THE INVENTION

This invention relates to the packaging of food. In one aspect, this invention relates to packaging food in a polyolefin packaging structure while in another aspect, the invention relates to packaging food in a laminated structure comprising a polyolefin film with improved organoleptic properties. In still another aspect, the invention relates to laminated food packaging structures comprising a polyolefin film containing a hindered phenolic antioxidant.

Polyolefin films, and particularly laminated structures containing one or more polyolefin films, are widely used to package various foods, including water. However, the use of these films is not without problems.

Converters and producers of such films and structures utilize processes which melt polymer pellets at high temperatures (for example 120 - 350°C) and expose the resulting hot polymer melt to air, particularly the oxygen in the air, until the melt has cooled. During this interval, some of the polymer is degraded to various oxidation products, particularly C2 - C20 aldehydes, but also ketones, acids and other oxygenated hydrocarbons. The human senses of smell and taste are especially sensitive to such products, even at very low concentrations. Several of these oxidation products are sufficiently mobile or volatile that they can migrate or bleed from the film into the package

contents where, if in sufficient concentration, they can impart an offensive smell or taste to the contents.

Converters and producers have addressed this migration problem in a variety of ways. One is to interpose a barrier film between the package contents and the polyolefin film such that the migration of oxidation products is slowed, if not stopped, at the interface of the barrier film and the polyolefin film. Another is to incorporate into the package structure or the package contents a perfume or flavor to mask the offensive smell or taste. Yet another is to incorporate into the package structure an absorbent to bind with the oxidation products and thus either inhibit the migration of the products out of the structure and into the package contents or if the binding does not prevent this migration, then at least to render the products organoleptically inoffensive. While all of these methods are effective to one degree or another, all address the symptoms of the problem rather than the source, that is all are focused on treating the oxidation products after they are formed rather than on preventing their formation during the manufacture of the polyolefin film.

Although antioxidants are commonly used in the manufacture of many polyolefin films, they are not commonly used in the manufacture of polyolefin films destined for use in packaging structures for foods because the antioxidants themselves, or their oxidation products, contribute to the poor organoleptic properties of the packaging structure. While these antioxidants are useful processing aids because they stabilize the polymer melt from thermal degradation and oxidation (which in turn imparts to the polyolefin film relatively uniform chemical and physical properties

across its physical dimensions) , they can and do experience some degradation. Moreover, they and their oxidation products can migrate into the package contents and if the contents are food, they can impart an off taste or smell to the food. Further, adhesion between the polyolefin film and other materials can be adversely affected. As such, the use of these antioxidants have been limited to packaging structures intended for use with products other than food, or incorporated into food packaging structures such that the polyolefin film is not in direct contact with the food.

SUMMARY OF THE INVENTION

According to this invention, a laminated packaging structure for food, the structure comprising opposing inner and outer surfaces, a substrate layer with opposing inner and outer surfaces, and a polyolefin layer containing a hindered phenolic antioxidant, is improved by manufacturing the structure such that the polyolefin layer is the inner surface of the structure, that is the surface of the structure in contact with the packaged food. The polyolefin layer can be bonded either directly or indirectly, that is through a tie layer, to the inner surface of the substrate layer.

The hindered phenolic antioxidants used in this invention not only inhibit the degradation of the polyolefin during the polymer melt phase of the film manufacture, but they themselves neither readily degrade under polymer melt conditions nor bleed from the film under conditions of typical use, both relative to many antioxidants presently in commercial use. As such, the packaging structures used in this invention require neither a barrier layer between the food and the polyolefin film, nor the use of an absorbent or masking agent .

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While any polyolefin that is suitable for use as a layer in a laminated package for food can be used in the practice of this invention, the polyethylene films are the preferred polyolefin films. Ethylene polymers and copolymers fall into two broad categories, those prepared with a free radical initiator at high temperature and high pressure, and those prepared with a coordination catalyst at high temperature and relatively low pressure. The former are generally known as low density polyethylenes (LDPE) and are characterized by branched chains of polymerized monomer units pendant from the polymer backbone. These polymers generally have a density between 0.910 and 0.935 g/cc.

Ethylene polymers and copolymers prepared by the use of a coordination catalyst, such as a Ziegler or Phillips catalyst, are generally known as linear polymers because of the substantial absence of long branch chains of polymerized monomer units pendant from the backbone. High density polyethylene (HDPE) , generally having a density of 0.941 to 0.965 g/cc, is typically a ho opolymer of ethylene, and it contains relatively few side branch chains relative to the various linear copolymers of ethylene and an α-olefin. HDPE is well known, commercially available in various grades, and is useful in this invention.

Linear copolymers of ethylene and at least one α- olefin of 3 to 12 carbon atoms, preferably of 4 to 8 carbon atoms, are also well know, commercially available, and useful in this invention. As is well

known in the art, the density of a linear ethylene/α- olefin copolymer is a function of both the length of the α-olefin and the amount of such monomer in the copolymer relative to the amount of ethylene, the greater the length of the α-olefin and the greater the amount of α-olefin present, the lower the density of the copolymer. Linear low density polyethylene (LLDPE) is typically a copolymer of ethylene and an α-olefin of 3 to 12 carbon atoms, preferably 4 to 8 carbon atoms (for example 1-butene, 1-octene, etc.), that has sufficient α-olefin content to reduce the density of the copolymer to that of LDPE. When the copolymer contains even more α-olefin, the density will drop below 0.910 g/cc and these copolymers are known as ultra low density polyethylene (ULDPE) or very low density polyethylene (VLDPE) . The densities of these linear polymers generally range from 0.870 to 0.910 g/cc.

Both the materials made by the free radical catalysts and the coordination catalysts are well known in the art, as are their methods of preparation. Relevant discussions of both these materials and their methods of preparation are found in USP 4,950,541 and the patents to which it refers, all of which are incorporated herein by reference.

LDPE is the preferred polyolefin for use in this invention, and preferably it has a melt index in the range of 2 to 25 g/10 min, more preferably 3 to 20 g/10 min. The film comprising LDPE can consist entirely of LDPE, or it can comprise LDPE in combination with one or more other polyolefins with which it is compatible. The nature and amount of other polyolefins can vary to the desired properties of the film, for example LDPE film with enhanced strength properties can contain

various amounts of LLDPE or ULDPE, while LDPE film with enhanced adhesive properties can contain various amounts of LLDPE grafted with maleic anhydride.

Any hindered phenolic antioxidant that is compatible with the polyolefin into which it is incorporated and which eliminates or reduces the formation of oxidation products during melt processing of the polymer can be used in the practice of this invention. These phenolic antioxidants can be used either alone or in combination with one another and/or in combination with other primary and secondary thermal and thermo-oxidative stabilizers. By "compatible" is meant that the antioxidant is readily incorporated into the polyolefin during processing, and that it will not easily leach from the film during use, at least at concentrations that would adversely affect the smell and taste characteristics of the package contents.

Hindered phenolic antioxidants are based on substituted phenols and polyphenols, and representative compounds include 1, 1, 3-tris (2-methyl-4-hydroxy-5-t- butylphenyl)butane, tetrakis [methylene-3- (3 , 5-di-t- butyl-4-hydroxyphenyl)propionate]methane, n-octadecyl- beta- (4 ' -hydroxy-3 ' -5 ' -di-t-butylphenyl)propionate, 1,3, 5-trimethyl-2, 4, 6-tris (3, 5-di-t-butyl-4- hydroxybenzyl)benzene, ethylene-bis [3, 3-bis (3-tert- buty-4-hydroxyphenyl)butyrate] , and the high molecular weight polyphenolic compounds manufactured and sold by Ciba-Geigy Corporation under the trademark Irganox. Hindered phenolic antioxidants with a molecular weight of at least about 220 g/mole are preferred, and those with a molecular weight of at least about 400 g/mole are more preferred. The maximum molecular weight of these antioxidants can vary to convenience, but typically it does not exceed about 800 g/mole.

The hindered phenolic antioxidants are used in substantially the same manner as most other antioxidants are used. Although the minimum amount of antioxidant used in the practice of this invention is the amount that will impart the desired organoleptic properties to the film, typically this amount is about 0.001 wt %, based on the weight of the film, preferably about 0.005 wt %. Practical considerations, such as economy, convenience, and adhesion, are the primary limitations upon the maximum amount of hindered phenolic antioxidant that can be incorporated into the polyolefin film, but typically the maximum amount does not exceed about 0.5 wt %, preferably it does not exceed about 0.1 wt %. The optimum amount of antioxidant will vary with the nature of the polyolefin film, the nature of the food to be packaged, the film extrusion conditions, the conditions under which the food is expected to be stored, the nature of the antioxidant itself, and similar considerations.

The polyolefin films of this invention, and the organoleptically improved laminated packaging structures made from these films, can be prepared in any conventional manner. For example, the films can be prepared by first melt blending polyolefin pellets and a hindered phenolic antioxidant material into a relatively homogeneous mass to form an antioxidant concentrate, that is a polyolefin material that contains an amount of antioxidant in excess of that desired in the final product. The concentrate is then typically extruded into pellets at a temperature of about 150°C, dry or melt blended with additional polyolefin pellets to achieve the desired concentration of antioxidant in the final product, and finally fabricated into a sheet by a cast film, extrusion

coating or blown film technique. Alternatively, the preparation of a concentrate step can be omitted, and the antioxidant simply blended directly with the polyolefin in the desired proportion. Fabrication temperatures for the film are typically in the range of about 200 - 350°C. The film can then be incorporated into a laminated packaging structure by any one of a number of conventional techniques. USP 4,747,902 and 5,059,459 are illustrative of some of these techniques.

The substrate layer of this invention can comprise any material that imparts desirable properties, for example vapor barrier, strength, flexibility, rigidity, water impermeability, etc., to the laminated packaging structure, and these materials include without limitation paperboard, metal foil, plastic films and regenerated cellulose film. Paperboard and metal foil, particularly aluminum foil, are preferred substrates.

The substrates of this invention have opposing inner and outer surfaces (the inner surface is that closest to the package contents) , and these surfaces can be activated, that is rendered more likely to bond with the adjacent layer, by flame treatment or corona discharge treatment . The inner surface of the substrate can bond directly to the polyolefin layer, or it can be bonded to the polyolefin layer through a tie layer, for example an ethylene/acrylic acid copolymer, or a barrier layer, such as foil, can be interposed between it and the polyolefin layer. While such a layer is not required for the desirable effects of this invention, preferably a foil layer is present because it imparts excellent inductive heat seal capabilities to the package, and inhibits oxidation of the package contents during storage.

Any of a wide variety of other materials, including polyolefins, can be bonded to the outer surface of the substrate. These materials are selected to impart properties desirable to the finished laminated structure.

The laminated structures of this invention are particularly well adapted for packaging foods sensitive to smell and taste degradation. These foods include water, fruit juices, fruit-flavored drinks containing water, milk, dry snack goods (for example potato chips, pretzels, peanuts, etc.) , fresh and frozen meats and fish, and other similar foods, especially those not processed between removal from the package and consumption.

The following examples are illustrative of certain specific embodiments of this invention. Unless indicated to the contrary, all parts and percentages are by weight .

SPECIFIC EMBODIMENTS

Example 1

Resin blends of polyolefin and hindered phenolic antioxidant were prepared by first dry blending a melt blended antioxidant concentrate composed of 2 wt % hindered phenolic antioxidant (Irganox 1010) and

98 wt % LDPE (melt index of 8 g/10 min, density of 0.916 g/cc) with various amounts of LDPE. A portion of the dry blended resins were then melt blended using a Werner-Pfleiderer (W-P) ZSK-53/5L twin screw, co-

rotation extruder. The average melt temperature was 141°C. The W-P was set at 250 rpms and produced

71 kg/hr. LDPE which did not contain any antioxidant was re-extruded under these same conditions and used as a reference.

The resins containing various levels of hindered phenolic antioxidant were cast into films using a cast co-extrusion film line and an average melt temperature of 316°C. The rolls of film were wrapped with aluminum foil immediately after fabrication to minimize the loss of low molecular weight oxidation products due to devolatilization.

The low molecular weight aldehyde concentrations of the film samples were monitored with a Perkin Elmer Sigma 2000 gas chromatograph (GC) . Sealed vials containing 1 gram of film were purged with nitrogen before analysis. GC conditions included a 105 minute thermostat time at 90°C and non-isothermal column temperatures of 75°C for 10 minutes, 75 - 200°C at 5°C per minute, and held at 200°C for 5 minutes. The retention times of the aldehyde peaks were determined using standards which were also used to calculate the relative concentrations of aldehydes in each film sample.

Blind taste evaluations were performed on samples one week after fabrication. Cast film samples were prepared using 20 g of film in 600 ml of Ozarka brand drinking water and heated to 60°C for 16 hours. The film/water mixtures were allowed to equilibrate to room temperature for 2 hours, and the film was then removed from the water before evaluation. The samples were ranked from least to most intense using a 0 - 3 point

scale. Each panelist ranked two sets of samples which were presented in random order. Results were analyzed using the Friedman Two-Way Layout and Analysis of Variance along with several multiple comparison tests to determine if significant differences could be detected between the samples. The amount of aldehydes produced in the cast films and the relative taste ranking as a function of antioxidant concentration were reported in Table I.

TABLE I

Cast Films: Aldehyde and Taste Performance n f An i xi n n n r i n

These results show that as the concentration of the antioxidant was increased, the amount of aldehyde formation was decreased and concomitantly, the off- taste of the water decreased.

Exampl 2

Another portion of the dry blended resins prepared in Example 1 were extrusion coated onto aluminum foil slip sheets (50# kraft paper/0.013 mm LDPE/0.000013 mm aluminum foil) at 61.6 m/min. A Black Clawson laboratory scale extrusion coater was used. The

average melt temperature was 316°C, and the coating thickness was 0.0254 mm. The film samples were delaminated from the aluminum foils, and the concentrations of low molecular weight aldehydes were monitored using a Perkin Elmer Sigma 2000 GC as described above. The concentration of low molecular weight aldehydes as a function of antioxidant in extrusion coated films was reported in Table II .

TABLE II

Extrusion Coated Films: Aldehydes as a Function of Antioxidant Concentration Antioxidant Aldehyde

Concentration Concentration (PP ) (ppm)

0 6.0 80 1.1

190 0.8

512 0.02

602 0

These results also show that as the concentration of antioxidant increases the amount of aldehyde formation decreases and in this particular example, decreases to a virtually nondetectable level.

Example 3

Resin blends were prepared in a manner similar to that of Example 1. Resins containing 1000 ppm of various antioxidants were prepared into films using a cast co-extrusion film line and an average melt temperature of 316°C. The rolls of film were wrapped

in foil to minimize loss of volatile oxidation products.

The low molecular weight aldehydes were monitored with a Perkin Elmer Sigma 2000 gas chromatograph. Blind taste evaluations were performed on the samples using the Triangle Test method. This test involves three samples, two of which were duplicate and one is odd, for example two samples of LDPE without antioxidant and one sample with antioxidant . The triangle was repeated (two stations) with the odd sample present in an alternative position in each station. Panelists were asked to indicate which sample was odd and if they found the relative taste to be better or worse than the other two samples and if so, to what degree. The number of times the odd sample was correctly identified determines if there was a statistically significant difference between the samples tested. The results were reported in Table III.

TABLE III

Cast Co-Extrusion Films: Comparative Taste Evaluations of Various Antioxidants

Aldehyde Concentration Taste

Antioxidant (ppm) Evaluation

None 3.17

Ascorbic Acid 2.67 No Difference Citric Acid 2.94 No Difference Lecithin 0.11 Offensive Taste

Significant

Irganox® 1010 0.12 Improved Taste

Significant

Irganox® 1076 0.12 Improved Taste

Significant

The results indicate that all of the antioxidants were capable of reducing the amount of aldehydes generated during film fabrication. The taste evaluations determined that IRGANOX® 1010 and 1076 significantly reduce the off taste that the films imparted to the water. Lecithin was as effective as the IRGANOX® antioxidants in reducing the amount of aldehydes generated during film fabrication, but lecithin has a strong characteristic odor which was transferred from the film to the water and was ultimately determined to be offensive in the taste evaluations .

Example 4

Resin blends were prepared using a melt blended antioxidant concentrate composed of 2% hindered phenolic antioxidant (Irganox® 1010) and 98% LDPE (0.917 g/ml density) . The 2% antioxidant concentrate was then dry blended with various amounts of antioxidant-free LDPE resin to produce LDPE resins containing 100, 300, 500, 700 and 900 ppm of antioxidant, respectively. The dry blended resins were melt blended using a Werner-Pfleiderer (WP) ZSK-53/5L twin screw, co-rotation extruder. The four heated zones were set for a temperature profile of 157/157/163/171°C, respectively. The die zone was set at 149°C, and the average melt temperature was approximately 141°C. The WP was set at 250 rpms and produced 71.3 Kg/hour. Antioxidant-free LDPE resin was re-extruded under these same conditions for use as a reference.

Table IV outlines the target and actual values of the hindered phenolic antioxidant found in each sample.

During the melt blending process there was a loss of activity in the antioxidant used, so the active levels reported for the samples is the amount of antioxidant available for stabilization during the extrusion coating process.

The LDPE samples, both with and without antioxidant, were extrusion coated onto aluminum foil slip sheets (50# kraft paper/0.0127 mm LDPE/1.27 x 10 ^-5 mm aluminum foil) at 67 m/min by a Black Clawson laboratory scale extrusion coater equipped with a primary and secondary extruder. The average melt temperature was 315°C and the coating thickness was 0.0254 mm. The slip sheets were cut out of the take-up roll and adhesion was tested within 12 hours after fabrication (initial) and again after 8 days (aged) .

Aluminum adhesion was tested according to ASTM F- 904-84 using 25 mm strips of the co-extruded structures. After initiating dela ination between the polymer layer and the foil, the ends were clamped in the jaws of an Instron® tensile tester and pulled at a 90 degree "T" peel with a crosshead speed of 25.4 cm/min. Three samples for each resin were tested and the average recorded. The aged (8 day) adhesion was

tested in the same manner. The results were reported in Table V.

TABLE V

Adhesion Strength to Aluminum Foil

Active Antioxidant Initial Adheshion Aged Adhesion in LDPE (ppm) (N/25mm) .N/25mm)

0 1.56 1.96

80 1.34 1.78

180 1.34 1.56

496 0.09 0.09 592 0.04 0.09

968 0.04 0.09

The overall adhesion strength of LDPE was affected by the incorporation of a hindered phenolic antioxidant into the polymer due to the minimized oxidation during fabrication, with the adhesion of LDPE to foil decreasing by 90% at concentrations of antioxidant above 180 pp .

Delaminated film samples were monitored for low molecular weight aldehyde emissions using a Perkin Elmer Sigma 2000 gas chromatograph (GC) equipped with an auto sampler. The column conditions were non- isothermal, cycling from 45°C after a 1 minute hold time, to 100°C at a rate of 2 deg C/minute and holding at 200°C for 5 minutes. The film samples (1 gram) were purged in the sealed GC vials with nitrogen and were heated at 90°C for 105 minutes. Standards for the aldehydes were run prior to the sample set in order to determine the concentration of low molecular weight aldehydes (acetaldehyde, propanal, and butanal)

contained in the film. The results were reported in Table VI.

TABLE VI Aldehyde Concentrations in Delaminated Films Antioxidant in Film Total Aldehyde ppm Concentration (ppm)

0 3.5 80 3.2

180 2.8

496 0.3

592 0.3

Some of the remaining LDPE samples, both with and without antioxidant, were co-extruded with ethylene/ acrylic acid copolymer (EAA) containing 6.5% acid in a layer ratio of 75% to 25%, respectively. The LDPE resins were placed into the Black Clawson primary extruder, a 8.89 cm, 30:1 L/D extruder with a typical LDPE-type screw. The LDPE layer was coated at 0.0381 mm (extruder speed - 66 rpm) using a target melt temperature of 288°C. The Black Clawson secondary extruder was a 6.35 cm 30:1 L/D extruder. The EAA tie layer was coated at 0.0127 mm (extruder speed - 41 rpm) with a target melt temperature of 271°C. The slip sheets were cut out of the take-up roll and samples for the taste comparisons were covered with Mylar® film to minimize any cross-contamination. Adhesion was tested within 12 hours after fabrication (initial) and after 8 days (aged) . Heat seal and hot tack tests were performed on the samples after 8 days .

Aluminum adhesion was tested according to ASTM F- 904-84 as described above. The results were reported in Table VII.

TABLE VII

Adhesion Strength to Aluminum Foil Active Antioxidant Initial Adhesion Aged Adhesion in LDPE (ppm) (N/25mm) (N/25mm)

0 2.8 9.3 ^'

180 4.2 9.3 496 4.0 8.9

The adhesion strengths in Table VII represent the strength between the tie layer (EAA) and the aluminum foil. In the co-extrusion process the two polymer layers came together in the molten state within the lips of the die; however, the antioxidant did not appear to migrate or interfere with the adhesive performance of the EAA. The aged adhesion strength for all samples increased by over 50% for all samples studied.

Hot tack was evaluated using the DTC Hot Tack Tester. The coated substrate (25 mm) was sealed at 275.6 kPa bar pressure, with a 0.5 sec dwell time, a

0.4 sec delay time, and a peel speed of 150 mm/sec. The substrate was evaluated in temperature increments of 10 degrees over a span of 100°F, with the starting temperature indicated by the point of initial seal. Five evaluations were performed on each sample and the average value was reported in Table VIII .

TABLE VIII Hot Tack on Aluminum Foil Substrate

Antioxidant in LDPE (ppm)

0

180

Antioxidant in LDPE (ppm)

496

Hot tack testing was used to evaluate the initial strength of the seal in which two polymer surfaces are bonded together. The two surfaces bonded together in these particular structures were the LDPE layers, with and without antioxidant. As seen by the data reported in this Table, the presence of hindered phenolic antioxidant did not have an adverse effect on the hot tack properties of the LDPE. Heat seal testing was performed on 25 mm strips of the co-extruded structures sealed at 275.6 Kpa bar pressure, with a 0.5 sec dwell time on the DTC bar sealer. Three seals were performed on each strip. After aging the sample for 24 hours, the force required to pull the seal apart was determined with the Instron tensile tester at 50.8 mm/min crosshead speed using a 90 degree "T" peel. Following the same initial sealing temperature as used in the hot tack testing, temperature increments of 10 degrees over a span of 100 degrees Fahrenheit were used. An average of the three seals at each 10 degree increment was reported in Table IX.

TABLE IX

Hot Seal with Aluminum Foil Substrate

Active Antioxidant in LDPE (ppm)

0

180

496

Heat seal testing was performed to evaluate adhesion of a heat seal after a set period of time. The results reported in this Table show that the presence of a hindered phenolic antioxidant did not have an adverse effect of the heat seal properties of the LDPE. Taste performance was evaluated on the samples one week after fabrication. The samples were formed into pouches using the co-extruded slip sheets cut into 16.5 cm squares and sealed using an impulse sealer set at the maximum setting (8) and a close time of approximately 2 sec. The pouches were sealed on three sides, filled with 250 ml of Ozarka® drinking water, and then the open side was folded down and clipped shut. The water was contained in the pouches for 21 hours at room temperature. The water was removed before testing and placed in glass bottles for a blind evaluation by a taste and odor panel. The signal detection method was used in which the panelists were asked to rank the samples from least to most intense (relative to off-taste and odor) . The results were reported in Table X.

TABLE X

Co-Extrusion Structures: Comparative Taste Evaluations Active Antioxidant in LDPE (ppm) Taste Evaluation

0 Control

180 No difference 496 Significant improvement

(10% significance)

The structure which contained 496 ppm antioxidant demonstrated a significant improvement in taste performance over the structure in which the polymer

layer in contact with the water did not contain any antioxidant.

While the invention has been described in considerable detail through the preceding examples, this detail is for the purpose of illustration only. Many variations and modifications can be made by one skilled in the art without departing from the spirit and scope of the invention as described in the following claims.