CN1639234A - Film suitable for food packaging - Google Patents

Abstract

Controlled permeability films are described that comprise at least on film-forming polymer and an inert, nonporous filler material having an average particle size such that the ratio of the average particle size of the filler to the film thickness is 0.67 to 0.99. Packages comprising such a controlled permeability film, and methods for making the film and using it to improve the storage life of a perishable article are also described.

Description

Films for Food Packaging

This application claims priority to provisional applications USSN 60/360,447, filed February 28,2002, and USSN 60/368,306, filed March 28,2002.

technical field

The present invention relates to plastic films having controlled permeability characteristics. The films of the invention are particularly suitable for use in packaging. The invention also relates to methods of making such films, packages comprising such films, and methods of improving the shelf life of perishable items such as fresh produce.

Background technique

In current product distribution and sales, many different packaging materials are used. One major category of packaging material is plastic film. To meet the specific performance requirements of various packaging applications and packaged products, there are many types of plastic films that differ in both composition and structure.

In the packaging of fresh-cut products such as fruits and vegetables, the preservation of freshness, aroma, texture and eating quality over time presents specific challenges. It has been found that when the rate of respiration and maturation of such products can be suitably reduced and the growth of pathogens inhibited, the shelf life of packaged products can be extended and quality degradation and undesirable phenomena such as malodorous odors can be reduced. Specific requirements and recommended conditions for long-term storage of essentially all types of fruits and vegetables are known in the art. One factor that significantly affects the shelf life of packaged products is the presence of gases, including oxygen and carbon dioxide. Depending on storage conditions, such as temperature and relative humidity, each type of product has its specific optimum range of oxygen concentration, carbon dioxide concentration, and relative concentration of carbon dioxide to oxygen, under which respiration can be suitably retarded and quality maintained to the greatest possible extent.

In order to provide improved product-specific storage conditions including favorable concentrations of oxygen and carbon dioxide, plastic films with defined controlled gas permeability properties and improved atmosphere packaging have been proposed. For example, US Patent No. 4,879,078 discloses a controlled packaging atmosphere film comprising a polymer and 36-60 wt% inert filler. The membrane is uniaxially stretched to provide a carbon dioxide and oxygen permeability of 5,000-10,000,000 cc/100 in ² -atm-day. However, the relatively high amount of filler and the required stretching step are disadvantageous. International Patent Application WO94/04655 discloses multilayer films having an oxygen permeability of about 150-450 cc/100 in ² -atm-day or greater, up to about 1000 cc/100 in ² -atm-day, comprising elastomeric First outer layer and second layer of single-site catalyst polyethylene. Disadvantages of this membrane include its limited permeability range and the requirement of a multilayer structure. For ecological and/or economical reasons, it is desirable that controlled permeability performance can be achieved in monolayer membranes. International Patent Applications WO92/02580, WO95/07949 and WO99/33658 each disclose controlled permeability membranes comprising a membrane-forming polymer and filler particles which are larger than the intrinsic membrane thickness. There is a need to further improve the properties of such films, for example, with regard to appearance including visual appearance, printability and security. Films that can be easily printed on with excellent clarity are particularly desirable in retail packaging.

There remains a need for plastic films suitable for use in packaging applications, especially such applications involving perishable items, which provide suitable controlled permeability characteristics suitable for extending the shelf life of the packaged object without compromising safety. Such controlled permeability films should also have excellent optical and tactile properties as well as good mechanical properties to provide adequate protection for the packaged items.

Summary of the invention

In one embodiment, the invention is a controlled permeability membrane comprising:

A. A polymer film with an intrinsic thickness, and

B. Fragmented, non-porous, inert fillers incorporated into the membrane and having an average particle size such that the ratio of filler average particle size to membrane thickness is 0.67-0.99.

In another embodiment, the invention is a controlled permeability membrane comprising:

A. A polymer film with an intrinsic thickness, and

B. Non-porous inert granular fillers introduced into the membrane and having an average particle size such that the ratio of filler average particle size to membrane thickness is 0.67-0.99,

The compressive force experienced by the particulate filler introduced into the membrane causes at least a portion of the particulate filler to break apart.

In another embodiment, the invention is a compressed controlled permeability membrane comprising:

A. A polymer film with an intrinsic thickness, and

B. A non-porous, inert particulate filler incorporated into the membrane and having an average particle size such that the ratio of filler average particle size to membrane thickness is 0.67-0.99, with at least a portion of the particulate filler being fragmented.

In another embodiment, the present invention is a method of preparing a controlled permeability membrane, the method comprising:

A. Blending the film-forming polymer with an inert, non-porous particulate filler having an average particle size;

B. forming a film from the blend of (A) having an inherent film thickness such that the ratio of filler average particle size to film thickness is 0.67-0.99, and

C. Subjecting the film of (B) to a compressive force such that at least a portion of the particulate filler breaks up.

In another embodiment, the present invention is a method of preparing a controlled permeability membrane, the method comprising:

A. Blending the film-forming polymer with an inert, non-porous, crushed particulate filler having an average particle size;

B. Formation of a film from the blend of (A) with an intrinsic thickness such that the ratio of filler average particle size to film thickness is 0.67-0.99.

In another embodiment, the invention is a package comprising a controlled permeability membrane, wherein the membrane has an intrinsic thickness and comprises an inert, non-porous, crushed particulate filler having an average particle size, the ratio of the average particle size of the filler to the thickness of the membrane It is 0.67-0.99.

In another embodiment, the present invention is a method of extending the shelf life of perishable items, such as fresh produce, by at least partially packaging the items in a film of controlled permeability, wherein the film has an inherent thickness and contains particles of average particle size. Inert, non-porous, crushed granular filler with an average particle size to film thickness ratio of 0.67-0.99.

Detailed description of the invention

The following terms used herein have the prescribed meanings.

The singular generally includes the plural and the plural generally includes the singular.

The term "comprising" means "comprising".

The term "copolymer" means a polymer in which two different monomers are polymerized to form a copolymer.

The term "interpolymer" means a polymer in which at least two different monomers are polymerized to produce an interpolymer. Such materials include, for example, copolymers, terpolymers, and the like.

The term "film" means a flat article and includes sheets, strips, tapes and ribbons.

The β (β) ratio is the ratio of the carbon dioxide permeability to the oxygen permeability.

The term "non-porous" is used to describe objects, such as filler particles, that do not extend from one surface of the object to the other, or from a point on the surface of the object to the surface of the object in the case of spherical or other one-surface objects A natural pore, void, groove, or similar passage extending from another point on the The channels of a porous object are large enough to allow gas or liquid small molecules, such as oxygen, nitrogen, water, benzene, etc., to pass through the object. Non-porous objects include porous objects with blocked or otherwise blocked channels, such as hydrated minerals.

The terms "broken", "broken", "broken" and similar terms are used to describe non-porous objects, such as filler particles, which contain pores, voids, grooves or similar channels extending from one surface of the object to the other surface of the object, Allowing a gas or liquid to pass through an object; pores, gaps, etc. in an object are the result of activating the object, for example, by subjecting the object to a compressive force of sufficient magnitude to create one or more such passages in the object.

The terms "activation", "activation" and similar terms mean the creation of channels in a non-porous object, such as filler particles, by any means, but typically by subjecting the object to pressure.

The term "intrinsic thickness" means the calculated thickness or gauge length of a single layer film or layers of a multilayer film. Intrinsic film thickness is the film thickness without filler. Intrinsic thickness is calculated as the product of film weight in grams times film density in grams per cubic centimeter. The density of the film is the sum of the weight percent polymer from which the film was made times the polymer density, plus the weight percent filler times the filler density.

All parts and percentages are by weight unless otherwise indicated.

Unless otherwise stated, any given range includes both endpoints for the stated range. Furthermore, all numerical values, such as ranges, include all numerical values from lower values to upper values in increments of one unit provided that there is a separation of at least two units between any lower value and any higher value. As an example, if it is stated that the number of components or values of process variables such as temperature, pressure, etc. are 10-125, preferably 30-75 and more preferably 40-60, it means values such as 20-100, 35-70 and 50-55 etc. are expressly included in this specification. For values that are less than one, one unit is considered to be 0.0001, 0.001, 0.01 or 0.1, as appropriate in the context in which it is used. These are one example of what is specifically meant and all possible combinations of values between the lowest value and the highest value specifically recited are meant to belong to the specification.

Surprisingly, it has been found that membranes comprising at least one membrane-forming polymer and an inert, non-porous filler material having an average The particle size is such that the ratio of filler average particle size to intrinsic film thickness is 0.67-0.99. The oxygen and/or carbon dioxide permeability of the film can be effectively adjusted to meet controlled atmosphere requirements or recommendations for a particular packaged product, such as to extend the shelf life of the product. The controlled permeability film of the present invention can be easily printed thereon and has excellent optical and mechanical properties. The films provided herein can be produced in a cost effective manner.

The present invention provides monolayer and multilayer films wherein the monolayer film or at least one layer of the multilayer film comprises (i) at least one film-forming polymer, and (ii) has an average particle size such that the ratio of filler average particle size to film thickness 0.67-0.99 inert, non-porous filler material. The gas permeability of such films or layers is improved during the activation step. Such membranes or membrane layers that have undergone an activation step are referred to as controlled-permeability membranes or controlled-permeability layers, which are also objects of the present invention. In a multilayer film, additional layers are selected to impart or enhance certain desired film properties, such as hot tack, seal resistance, and/or structural properties, without substantially interfering with the controlled permeability of the controlled permeability layer characteristic. The multilayer films of the invention may comprise one or more tie layers, if appropriate. Preferably, the additional layer is selected so as not to substantially interfere with, or substantially adversely affect, the controlled permeability properties of the controlled permeability layer. For ecological and/or economical reasons, the films of the invention generally contain as few layers as possible to meet the desired and/or required performance attributes. Preferred film structures of the present invention are monolayer, bilayer or trilayer films, more preferably monolayer films.

In one embodiment, the multilayer film of the present invention comprises 2 to about 7 layers, at least one layer, and preferably one layer comprising at least one film-forming polymer and having an average particle size such that the ratio of filler average particle size to layer intrinsic thickness 0.67-0.99 inert, non-porous filler material. In another embodiment, the multilayer film of the present invention comprises from 2 to about 7 layers, at least one layer, and preferably one layer comprising at least one film-forming polymer and having an average particle size such that the filler average particle size is inherent to the multilayer film. Inert, non-porous filler material with a thickness ratio of 0.67-0.99. These multilayer films are typically laminates constructed using common techniques such as thermal or adhesive lamination or extrusion coating. In embodiments where the ratio of filler material average particle size to intrinsic thickness of the layer is 0.67-0.99, the layer is typically activated prior to its incorporation into other layers of the film. In embodiments wherein the ratio of the average particle size of the filler material to the intrinsic thickness of the multilayer film is from 0.67 to 0.99, activation is after formation of the multilayer film.

The activation step is used to improve the gas permeability of a film or layer comprising (i) at least one film-forming polymer, and (ii) an inert, non-porous filler material whose average particle size of the filler is proportional to the intrinsic thickness of the film or layer The ratio is 0.67-0.99. Typical activation steps include pressure treatment, heat treatment, or a combination of these treatments. The activation step typically increases the gas permeability of such films or layers, particularly the oxygen transmission rate, compared to films or layers comprising non-activated, inert, non-porous fillers. In other words, a membrane comprising, eg, from about 0.01 to 15 wt. % of an activated, inert, non-porous filler material will exhibit increased gas permeability compared to a membrane that is similar in all respects except that it has not been subjected to an activation step. In addition, the activation step can increase the carbon dioxide transmission rate without affecting the water vapor transmission rate, at least at low filler concentrations (eg, 0.01-0.025). Advantageously, the desired controlled permeability properties are obtained without stretching the membrane.

In a preferred embodiment, the step of activating comprises subjecting a film comprising at least one layer comprising at least one film-forming polymer and a filler material having an average particle size to a compressive force, such as by contacting the film with a platen or a roller, The ratio of the average particle size of the filler material to the intrinsic thickness of the film is 0.67-0.99. For example, the film may be pressure treated by passing it between pressure rolls, which may optionally be heated. All or only a portion of the membrane may be activated, such as subjecting only a portion of the width of the membrane, or every other segment of the length of the membrane, or a designated area of the membrane area to the compressive force.

The compressive force or pressure range should exceed the compressive strength of the filler particles. The force is typically in the range of 2.5-100 kg, preferably in the range of 5-75 kg. The compressive force should be sufficient to at least partially break up the filler particles, such as sufficient to create cracks, pores or grooves in the filler particles that extend from one surface of the particle to the other surface of the particle. The contacting step can be performed at ambient temperature or at elevated temperature. Preferably, the temperature is between room temperature and less than the melting point of the polymer from which the membrane is prepared, more preferably 10-15% lower than the melting or softening point of the highest melting polymer from which the controlled permeability membrane is prepared. ℃.

Preferably, controlled permeability membranes of the invention that have undergone an activation step or include at least one layer that has undergone such a step have a beta ratio in the range of 0.8-3.5, more preferably 0.8-2.5, even more preferably 0.8-2.0, and most preferably 0.8-1.5. Methods and suitable equipment for determining oxygen and carbon dioxide permeability are known in the art, for example ASTM D3985.

The films of the present invention may be of any thickness or gauge appropriate for the intended use of the film. For economical and/or ecological reasons it may be desirable to minimize film thickness. Monolayer films according to the invention typically have a thickness of from about 10 microns (0.4 mils) to about 125 microns (5 mils), preferably from about 30 microns (1.2 mils) to about 75 microns (3 mils). The overall thickness of the multilayer films according to the present invention is typically from about 25 microns (1 mil) to about 250 microns (10 mils). Controlled permeability membranes according to the invention may be printed on the outer and/or inner surfaces. The film may be printed, preferably after activation, for example by offset or rotogravure printing equipment. If appropriate, the inks used are suitable for food packaging. Advantageously, the controlled permeability membranes according to the present invention are designed to have excellent optical clarity, be soft and tough.

The films of the invention can be prepared by any suitable manufacturing process, and their properties can be tailored to suit any particular end use. Suitable manufacturing processes include, for example, blown film extrusion, flat die extrusion, coextrusion, extrusion coating and lamination techniques. The film may be in any suitable form, such as roll stock for wholesale or retail, and use in conventional equipment. Preferably, the films according to the invention are designed so that they can be easily machined at cost effective line rates.

The film-forming polymer can be of any suitable type and generally includes, but is not limited to, homopolymers, copolymers, interpolymers such as block, graft, random and alternating copolymers or interpolymers. The term "polymer" includes any possible geometric configuration of the material, including isotactic, syndiotactic and random symmetries. Suitable types of polymers include, for example, polyolefins such as homopolymers, copolymers, interpolymers and blends thereof of ethylene and linear or branched alpha-monoolefins containing at least 3, preferably 3 to 10 carbon atoms things. Examples of homopolymers useful in the present invention are polyethylene, polypropylene and poly(1-butene). Representative examples of suitable copolymers are ethylene/propylene copolymers, ethylene/butene copolymers, ethylene/pentene copolymers, ethylene/hexene copolymers, ethylene/heptene copolymers and ethylene/octene copolymers. Suitable classes of polyethylene include, but are not limited to, high-pressure low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE). Examples of other homopolymers, copolymers and interpolymers that can be used are polyester, nylon, polystyrene, vinyl polymers including polyethylene terephthalate and polybutylene terephthalate Such as polyvinyl chloride, polyvinyl acetate, ethylene vinyl acetate copolymer and ethylene-vinyl alcohol copolymer, ethylene methacrylic acid copolymer (ionomer), ethylene/styrene copolymer, polyalkylene oxide polymer, and polycarbonate. Representative examples of blends are homopolymers, such as polyethylene or polypropylene, and copolymers, such as blends of ethylene/butylene or ethylene/octene copolymers and blends of copolymers.

The choice of film-forming polymer should be consistent with the film requirements, eg with regard to processability and physical properties. Preferably, the film-forming polymer is suitable for food packaging applications and fulfills the respective requirements imposed by the competent regulatory authority. Such preferred polymers include, for example, polyethylene polymers, polypropylene polymers, ethylene vinyl alcohol copolymers and ethylene vinyl acetate copolymers.

In compositions for forming controlled permeability membranes according to the present invention, the amount of film-forming polymer is preferably 99.99 wt% or less (based on the amount of film-forming polymer and filler in the film or layer). Preferably, the amount of film-forming polymer is 85 wt% or higher, more preferably 90 wt% or higher, and most preferably 96 wt% or higher. In a particularly preferred embodiment, the amount of film-forming polymer is from 98% to 99.975% by weight, based on the combined weight of the film and filler.

The filler materials used in the present invention are inert to the film-forming polymers, meaning that they do not chemically interfere with or otherwise affect the film. Suitable fillers are organic or, preferably inorganic, particulate materials. Fillers can be natural or synthetic materials. The filler particles are non-porous. Preferably, the particles are substantially spherical and have a length to diameter ratio of about 1 to about 2. Preferred are fillers that have a suitably low compressive strength such that they are relatively easily broken during the activation process, but are not substantially damaged during compounding or film fabrication. Suitable filler materials may have an average particle size of 30-70 microns. For monolayer films, the average particle size is typically from about 8 to about 113 microns.

Non-porous filler materials suitable for use in the present invention include, for example, pumice, tuff, rhyolite, quartz andesite, reticulated pozzolan, slag, volcanic gravel, perlite, coal, silica, metal oxides such as alumina , sulfates, such as barium sulfate, carbonates, such as calcium carbonate and clay. Particularly preferred fillers are mineral, spherical materials such as non-porous silica, such as glass beads. The use of mixtures of different filler materials is also contemplated. Advantageously, the filler material is uniformly dispersed throughout the film-forming polymer used to prepare the films and/or layers of the invention.

In one, less preferred embodiment of the invention, the inert, non-porous filler particles are broken up prior to their blending with the film-forming polymer from which the film or layer is prepared. In this embodiment, the particles are first activated by any suitable means, such as crushing between plates or rollers, and then blended with the film-forming polymer, and the blend is then made by any suitable process film or layer. This method of making controlled permeability membranes is less preferred than breaking the particles after membrane fabrication, since particle breakage outside the membrane typically results in disintegration of the particles. In other words, the particles simply break down into smaller non-porous particles, such as fragments, etc. along the channels created. When fragmented in the film matrix, the particles retain their physical integrity regardless of the desired channel formation from one surface to another. In the case of a sphere, of course, the particle has a single surface and a pathway from one point on the surface to another point on the surface.

For the present invention, what is important is the particle size of the filler material as compared to the intrinsic thickness of the membrane or layer due to activation to achieve a controlled permeability, or compared to the intrinsic thickness of the overall multilayer structure. The average particle size of the filler material is less than the intrinsic thickness of the film, layer or multilayer structure prior to activation. Advantageously, the activation step does not alter the intrinsic thickness of the controlled permeability layer or controlled permeability membrane. The intrinsic thickness before activation is substantially the same as the film thickness or gauge after activation.

The average particle size of the filler material is at least two thirds of the intrinsic thickness of the film or layer. Preferably, the ratio of average particle size to film thickness or gauge length is at least 0.70, and more preferably at least 0.80. The average particle size is smaller than the intrinsic thickness of the film or layer, ie the ratio of average particle size to film thickness or gauge is 0.99 or less, and preferably the ratio of average particle size to film thickness or gauge is 0.90 or less.

Preference is given to filler materials with a narrow particle size distribution. A narrow particle size distribution was found to be particularly suitable for achieving consistent activation results. Particularly preferred are filler materials for which the average particle size, and the size of 90% of all particles, meet the general and preferred requirements described in the preceding paragraph with respect to intrinsic film thickness. The desired particle size of the filler material depends on the intrinsic thickness. For example, for monolayer films, the preferred particle size is 14-68 microns.

The particle size of the filler material can be determined by methods known in the art, for example by the Coulter method or by microscopy.

The amount of filler in the membranes of the present invention is selected such that it is sufficient to provide the desired controlled permeability after the membrane has been subjected to an activation step. Preferably, the amount of filler in the controlled permeability layer - based on the total weight of filler and film-forming polymer present in the layer - is at least 0.01 wt% or greater, preferably 0.05 wt% or greater, more preferably 0.1 wt% or greater. The amount of filler in the controlled permeability layer should be 15wt% or less, preferably 10wt% or less, more preferably 8wt% or less, most preferably 6wt% or less, particularly preferably 4wt% or less. A particularly preferred range for the amount of filler is 0.025% to 8% by weight.

Surface modifiers can be used to modify the surface of the filler, eg to make it more hydrophobic. Surface modification can be used to improve the dispersion of the filler and/or its adhesion to the polymer matrix. Advantageously, the filler is uniformly dispersed in the polymer matrix. Suitable agents are known in the art and include, for example, polymers and fatty acids. Preferred surface modifiers are in accordance with FDA regulations and include, for example, calcium stearate.

Compositions for films of the present invention comprising film-forming polymers and filler materials may further include additives to impart or enhance certain properties of the film, including, without limitation, pigments, antioxidants, stabilizers, anti-fog agents , plasticizers, tackifiers, waxes, flow promoters, surfactants, materials added to enhance the processability of the composition, etc. Since this permeability is a result of creating porosity in the filler, these additives typically have little, if any, effect on the permeability of the membrane resulting from the activation step. These additives typically have a greater effect on the permeability of the membrane in those cases where permeability is primarily a function of the solution/diffusion transfer mechanism. The compositions can be prepared by conventional blending techniques using equipment such as two-roll mills, Banbury mixers, and single and twin screw extruders.

The films of the present invention are particularly useful in packaging applications, such as fresh cut produce packaging. Preferred are films suitable for use in food and horticultural packaging, most preferably fresh produce including eg fruit, vegetables and flowers. Fresh produce can be packaged in a controlled atmosphere, maintaining the beta ratio at an optimal level for the packaged item to improve its shelf life, preservation quality and/or reduce or prevent malodor.

A package according to the invention comprises a controlled permeability film of the invention and one or more items, preferably at least one item which benefits from controlled permeability or improved atmosphere packaging. Such items include, for example, food products, such as vegetable fields and fresh fruits, flowers or microorganisms, and especially those items that are prone to respiration or oxidation, such as cut fruits and vegetables. As used herein, "wrapped" and like terms mean wrapping or surrounding one or more items with a controlled permeability film according to the present invention in such a way that the film provides a barrier between the items and the environment. Preferred packages include fresh and/or pre-cut produce such as fruit, vegetables or flowers. Products that benefit from being packaged by the films of the present invention include, but are not limited to, carrots, cauliflower, cauliflower, cabbage, mushrooms, Brussels sprouts, beans, chicory, celery, radishes, spinach, asparagus, parsley, okra, Korean Thistles, tomatoes, vegetable blends, pears (nashi), pears, plums, berries, grapes, apricots, oranges, bananas, peaches, kiwis, vines, peaches, or mangoes. The films according to the invention enable the maintenance of an improved atmosphere in the packaging consistent with the needs for product respiration, ethylene production and maturation. The optimum atmosphere is different for each product. The controlled permeability films or packages of the present invention can be designed to provide relatively high oxygen transmission rates. Such high ORT films are particularly suitable for packaging products with high breathing requirements, avoiding or significantly reducing phenomena associated with low package oxygen levels, such as malodorous odors. Alternatively, the controlled permeability films or wrappers of the present invention can be designed to allow relatively low oxygen transmission, as would be required for relatively lower oxygen transmission products, such as romaine lettuce.

If appropriate, the package may comprise one or more films and components other than the packaged article, such as a plate, tray, or container or box-like component. The wrapper may be in any suitable form, eg in the form of a bag, wrapper, pouch or container, such as a container, wherein the controlled permeability membrane according to the invention is a panel in at least one side of the container. Bags provided by the present invention include, for example, unfilled or filled retail consumer disposable bags with zippers or other snap closures, open bags, food storage bags, household storage bags, refrigerator bags, sandwich bags, and garbage bags. A particular embodiment of the present invention relates to a food preservation bag comprising a controlled permeability membrane according to the present invention and, optionally, one or more food items. Wraps provided by the present invention include, for example, retail consumer disposable wraps for packaging various commodities, such as meat, produce, etc., prior to sale of those commodities or for home storage.

Packages according to the invention can be produced by methods known in the art, for example by sealing the film according to the invention at the periphery by heating, or by forming, filling and sealing equipment. The package may include a reseal mechanism, such as a zipper or "zipper" type mechanism, that allows the user to repeatedly reseal the package. The films and packages of the present invention are suitable for retail packaging and food service industries, eg, schools, restaurants, hospitals, etc., where appearance, long shelf life and safety are essential.

The invention also provides a package which is a controlled permeability container, wherein the permeability within the container is controlled by using a film according to the invention as a breathable panel in a window in one or more walls of the container. Alternatively the container is prepared from a substantially gas impermeable material. The permeability and area of the panels can be designed to provide a flux of oxygen and carbon dioxide approximately equal to the predicted respiration rate for the quantity of fresh product packaged.

The application of the controlled permeability membrane of the present invention is not limited to the packaging of products or organisms, but may also include other uses including further packaging applications as well as non-packaging applications, such as:

- monitoring the respiration rate, where the respiration rate can be determined from the known permeability of the membrane and the accumulation of breathing gas;

-enhancing the effectiveness of sorbent, scavenging, or indicating polymer additives where the permeation of gas or liquid through the polymer limits the additive;

- fresh red meat packaging applications;

- building and constructing home wraps;

-Medical applications, including disposable healthcare sheets and pajamas, and personal hygiene applications

- packaging of meat (other than fresh red meat), poultry, dairy or fresh produce;

- packaging of drugs, pharmaceuticals and microbial culture media;

- Packaging of living organs.

Example

The following examples are illustrative of the invention but are not intended to limit the scope of the invention in any way.

The following methods were used to determine the following parameters:

Density (g/cm ³ ): ASTM D-792

Melt index (g/10min): ASTM D-1238, at 190°C/2.16kg load (condition E);

Dart impact A (g): ASTM D-1709 (66cm (26in) drop height)

Elmendorf Tear Strength (g): ASTM D-1922, Method A.

Tensile properties including 1% and 2% secant modulus: ASTM D-882. Tensile properties and modulus were measured on 2.54 cm x 20.3 cm (1 in x 8 in) specimens with a 10 cm (4 in) gauge length. The crosshead speed was 50.8 cm/min (20 in/min) for tensile properties and 2.54 cm/min (1 in/min) for modulus.

"MD" means machine direction and "CD" means cross direction.

The film-forming polymer used in these examples was a commercially available high pressure LDPE (ethylene homopolymer) having a density of 0.923 g/cc and a melt index ( _I2 ) of 1.9 g/10 min. The packing material used in these examples was non-porous silica beads available from Potters Industries (a division of PQ Industries, USA). Monolayer films were blown on an Egan extruder using a 3 inch die and a 70 mil die gap. The maximum mesh size on the screen assembly was 120 (approximately 200 microns) to prevent buildup of silica beads before exiting the die. The melt temperature was about 220°C and the extruder profile was 150/160/171/177/182/193/204°C and the blow-up ratio was 2.5.

For activation, the film is run through sets of calender rolls stacked parallel to each other and operating at the same speed. A temperature of 23°C and a pressure of 0.41 N/mm were set to control the level of permeability achieved in the membrane. The membrane gauge length was not changed during the activation step. The oxygen transmission rate of non-activated film samples 1-3 is 900 nmol m ^-1 s ^-1 GPa ^-1 . In Table 1 the composition of the film samples is given. Sample 4 is not a sample according to the invention.

Table 1 sample filler Average particle size (μm) Filler loading (wt%) Film gauge length (μm) Particle Size to Film Gauge Length Ratio 1 Spherical glass 3000, type A 35 0.025 41 0.9 2 Air beads 602590 38 0.05 38 1.0 3 Air beads 602590 38 0.1 38 1.0 4 Spherical glass 2900, type A 42 0. 69 0.6

In Table 2 the controlled permeability properties of the activated membranes are listed.

Table 2 sample Oxygen transmission rate [nmol/m*s*Gpa(cc*mil/100in ² /day/atm)] Carbon dioxide transmission rate [nmol/m*s*Gpa(cc*mil/100in ² /day/atm)] beta ratio 1 48860(17450) 50120(17900) 1.0 2 6440(2300) 8960(3200) 1.4 3 15400(5500) 18060(6450) 1.2 4 3360 (1200—— N/M N/M

N/M means "not measured".

The mechanical properties of film samples 1-3 after activation are listed in Table 3

table 3 sample number 1 2 3 Dart Impact A 64 93 84 Elmendorf A, CD (G) 198 226 201 Elmendorf A, MD (G) 170 126 137 1% secant modulus, CD(ksi/MPa) 41.5/286 33.3/230 34.7/239 2% secant modulus, CD(ksi/MPa) 35.7/246 28.6/197 29.6/204 1% secant modulus, MD(ksi/MPa) 29.9/207 28.3/195 32.4/223 2% secant modulus, MD(ksi/MPa) 27.3/188 25.8/178 28.3/195 CD Stretch Yield (psi/MPa) 1567/10.8 1735/12.0 1757/12.1 yield strain 13 14 15 Ultimate strength (psi/MPa) 2136/14.7 2572/17.7 2284/15.8 Fracture strain 420 522 446 Toughness (ft*lb/cu.In/kJ/cm ³ ) 1225/14.9 1576/19.1 1348/16.3 MD Stretch Yield (psi/MPa) 1766/12.2 187712.9 1947/13.4 yield strain 14 15 16 Ultimate strength (psi/MPa) 3331/23.0 3063/21.2 2741/18.9 Fracture strain 176 222 168 Toughness (ft*lb/cu.In/kJ/cm ³ ) 807/9.8 975/11.8 736/8.9

Table 4 reports comparative oxygen transmission data for a number of different membranes, both in the present invention and not in the present invention. This data demonstrates the importance of non-porous glass bead size/membrane thickness ratio, activation and presence of glass beads.

Table 4 sample Bead ^±± /film ratio activated/unactivated OTR ^** 5 ^* 0.61 activated 500 6.1 ^* 0.9 activated 7500 6.2 ^* 0.9 Not activated 350 7 ^* 1.0 activated 2300 8 ^* 1.1 activated 4000 9 ± no beads --- 800 10 ^* ± no beads --- 1080

^* LDPE503A (0.923g/cc, I ₂ of 1.9)

± ELITE ^* 5400G, (0.916g/cc, I ₂ of 1.0), 1.5 mil gauge length

^* ± AFFINITY ^* PL 1850, (0.902g/cc, _I2 of 3.0), 0.8 mil gauge length

±± non-porous glass beads

^* Trademark of The Dow Chemical Company

Claims (21)

1. A membrane of controlled permeability, comprising: A. A polymer film with an intrinsic thickness, and B. Fragmented, non-porous, inert filler introduced into the membrane and having an average particle size such that the ratio of filler average particle size to membrane thickness is 0.67-0.99. 2. A membrane of controlled permeability, comprising: A. A polymer film with an intrinsic thickness, and B. Introducing into the membrane and non-porous, inert particulate filler having an average particle size such that the ratio of filler average particle size to membrane thickness is 0.67-0.99, the particulate filler being subjected to sufficient compressive force to break at least a portion of the particulate filler. 3. A controlled permeability membrane comprising: A. A polymer film with an intrinsic thickness, and B. Introducing into the membrane and non-porous/inert particulate filler having an average particle size such that the ratio of filler average particle size to membrane thickness is 0.67-0.99, subjecting the membrane to sufficient compressive force to break up at least a portion of the particulate filler. 4. A compressed controlled permeability membrane comprising: A. A polymer film with an intrinsic thickness, and B. Non-porous, inert particulate filler introduced into the membrane and having an average particle size such that the ratio of filler average particle size to membrane thickness is 0.67-0.99, at least a portion of the particulate filler being in a fragmented state. 5. A method for preparing a controlled permeability membrane, the method comprising: A. Blending the film-forming polymer with an inert, non-porous particulate filler having an average particle size; B. forming a film from the blend of (A) having an intrinsic thickness such that the ratio of filler average particle size to film thickness is 0.67-0.99, and C. Subjecting the film of (B) to a compressive force sufficient to break up at least a portion of the particulate filler. 6. A method for preparing a controlled permeability membrane, the method comprising: A. Blending the film-forming polymer with an inert, non-porous, crushed particulate filler having an average particle size; B. Formation of a film from the blend of (A) with an intrinsic thickness such that the ratio of filler average particle size to film thickness is 0.67-0.99. 7. The film of claim 1, wherein the filler constitutes 0.01-15% by weight of the film. 8. The film of claim 1 having a beta ratio of 0.5-2.5. 9. The film of claim 1 comprising a polyolefin polymer. 10. The film of claim 1 having an intrinsic film thickness of 10-125 microns. 11. The film of claim 1 having a single layer construction. 12. The film of claim 1 having a multilayer construction wherein the filler is incorporated into a single layer. 13. The film of claim 1 having a multilayer construction wherein the filler is incorporated into at least two layers. 14. The film of claim 1, wherein the filler has an average particle size of 30-70 microns. 15. The membrane of claim 1, wherein the filler is a spherical mineral. 16. The membrane of claim 1, wherein the filler is a silicon-containing material. 17. The film of claim 1 in the form of a bag or sachet. 18. A package comprising the controlled permeability film of claim 1. 19. A method of improving the shelf life of a perishable item comprising packaging the item with a controlled permeability film according to claim 1. 20. The film of claim 1, wherein the filler constitutes 0.01-5% by weight of the film. 21. The film of claim 1, wherein the filler constitutes 0.01-2 wt% of the film.