US3745057A

US3745057A - Sterilizable packages and desiccant and fumigant packages of open cell microporous film

Info

Publication number: US3745057A
Application number: US00134429A
Authority: US
Inventors: S Plovan; J Loft; J Mctaggart
Original assignee: Celanese Corp
Current assignee: Celanese Corp
Priority date: 1971-04-15
Filing date: 1971-04-15
Publication date: 1973-07-10
Anticipated expiration: 1990-07-10
Also published as: IT952648B; FR2136406A5; CA1003769A; DE2217794A1; JPS5539363B1; GB1384941A

Abstract

DESICCANT OR FUMIGANT PACKAGING PROVIDES A HIGHLY DESIRABLE END USE FOR CERTAIN OPEN-CELLED MICRROPOROUS FILMS BECAUSE OF THE PORE SIZE AND DISTRIBUTION WHICH IN THE DESICCANT PACKAGING APPLICATION ALLOWS RAPID TRANSPORT OF MOISTURE VAPOR INTO THE PACKAGE AND IN FUMIGANT PACKAGING SITUATIONS, PERMITS A SLOW RELEASE (EVAPORATION) OF A SOLID OR LIQUID FUMIGANT THESE FLEXIBLE HIGH STRENGTH FILMS NOT ONLY PREVENT GROSS CONTAMINATION OF SURROUNDINGS BUT ALSO POSSESS GOOD RESISTANCE TO HEAT, WATER AND MANY SOLVENTS STERILE PACKAGING IS A HIGHLY DESIRABLE END USE FOR CERTAIN OPEN-CELL MICROPOROUS FILMS BECAUSE OF THEIR HIGH LEVEL OF GASEOUS STERILANT TRANSMISSION; MOISTURE INSENSITIVITY AND BACTERIA IMPREMEABILITY.

Description

United States Patent 6 U5. Cl. 161l59 19 Claims ABSTRACT F THE DHSCLGSURE Desiccant or fumigant packaging provides a highly desirable end use for certain open-celled microporous films because of the pore size and distribution which, in the desiccant packaging application, allows rapid transport of moisture vapor into the package and in fumigant packaging situations, permits a slow release (evaporation) of a solid or liquid fumigant.

These fiexible, high strength films not only prevent gross contamination of surroundings but also possess good resistance to heat, water, and many solvents.

Sterile packaging is a highly desirable end use for certain open-cell microporous films because of their high level of gaseous sterilant transmission; moisture insensitivity and bacteria impermeability.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to improved desiccant or fumigant packages by the use of certain open-celled microporous films.

The present invention relates to improved sterile packaging products through the use of certain micro-porous films.

Description of the prior art It is often advantageous to package and store certain materials with a dehydrating and/or fumigating agent to prevent deleterious changes which could occur as a result of moisture and/or insect contamination. Often it is merely highly desirable to be able to obtain a moisture free product be it liquid, gas, or solid.

In dehydration operations, desiccating agents are rarely added directly to the material or atmosphere from which water is to be extracted for the agent is usually in the form or" very small granules or a fine powder to increase the surface area and thereby the elfectiveness of the agent. Direct addition of the agent would, of course, result in gross contamination of the moisture containing region; usually a highly undesirable situation. Moreover, most drying agents act vigorously when first ex posed to the atmosphere but soon become spent or saturated so that their effectiveness is short-lived and varies greatly in intensity.

In order to obviate this problem, it has been quite common in the art to enclose the granulated or powdered dehydrating agent Within a container, capsule, or coating of material which is preferably inert chemically but which is capable of absorbing and transmitting moisture. These devices are often constructed to be hollow, elongated members and can be formed of some perforated non-corrosive metal but are more often non-perforated capsules composed of pliant, gas-permeable sheet material such as paper, casein, cellophane, cellulose acetate, or laminates of paper and plastic film. The dehydrating action of the paper and/or plastic film. capsules is of course extremely slow for although the desiccating material is successfully shielded from direct contact with the material or atmosphere to be dried; the moisture from the material must be absorbed by the capsule film; diffuse through the capsule Wall; and through desorption, be absorbed internally by the desiccant, i.e. the drying agent serves to extract the moisture from the capsule covering rather than from the atmosphere or material being treated.

A disadvantage to the use of perforated containers to enclose desiccating agents lies in the inevitable pulverizing of even an initially granulated agent due to movement within the container. This in turn leads to a sitting of the agent out of the container through the perforations resulting in, if not an undesirable chemical reaction, a contaminated albeit moisture-free product.

Paper desiccant containers not only possess the sifting disadvantage associated with the perforated containers as elucidated above, but also lose their integrity quite quickly when exposed to liquids or even excessively moist, solid substances.

Soild, volatile repellants and insecticides are usually stored or shipped Within metal containers or films of regenerated cellulose; materials the organic vapors are incapable of permeating. When the fumigating effect is desired, the articles are either brought into direct physical contact with the solid fumigant, usually a most undesirable situation which can result in particulate fumigant contamination, or contained within a package having on at least one side an organic thermoplastic film through which the fumigant vapors can diffuse. Examples of such films are polyethylene, polyvinyl chloride, vinyl chloride-vinyl acetate copolymers, vinylidene chloride copolymers with vinyl chloride or acrylonitrile or ethyl acrylate, ethyl cellulose, cellulose acetate, and the like. Inherent in the use of these films is the extremely slow vapor release realized as a result of the multiphased vapor release process, i.e., vapor sublimation; film absorption; film diffusion; and finally vapor desorption.

Sterilization of medical, surgical and dental instruments, dressings, precision tools and the like is usually accomplished by one or three basic methods.

The first involves placing all of the items in a sterilization chamber; introducing the sterilant; and thence, individually removing and wrapping the items for storage as illustrated in US. Pat. No. 3,163,494. This process necessitates handling the items after they have been sterilized, thereby assuming the undesirable risk of introducing contaminates.

The second sterilization technique utilizes an instrument and/or appliance container such as described in US. Pat. No. 3,437,423 in which the items are placed during the sterilization process and in which they are later stored. These containers are usually constructed from a metal or a dimensionally stable, heat resistant, thermoplastic material and are open-top, tray-like structures almost always provided With drainage holes to eliminate condensate. As a result of this open construction, the sterilized items maintain this aseptic condition for only a short period of time.

The third method, a gas sterilization process, involves sealing each individual item in a plastic film or paper package; evacuating the air from this sealed package; and introducing an ethylene oxide mixture; e.g., a 12% ethylene oxide-88% trichlorofiuouromethane mixture, and a small amount of water vapor. A relative humidity of 3540% should exist in the sterilizing chamber at 25 C. for effective kill. A relative humidity of 35-40% should exist in the sterilizer charge (as measured at room temperature) for effective :kill.

The sterilization is accomplished at approximately to F. Subsequent ethylene oxide removal (evacuation) and air purge is required. These evacuations require considerable time for gas transport and must be slow for minimum package ruptures when used with films with only diffusion capabilities. With films, original equipment packagers will often use an 8, 12, or more hour sterilization cycle followed by an aeration period that could last from 48 hours to many days depending upon the type of product sterilized.

Ethylene oxide is a very toxic, flammable gas which forms an explosive mixture with air and has become a standard sterilant in hospitals only because of the impracticability of steam sterilizing soft goods such as gauze, bandages, etc. A problem inherent in ethylene oxide use is the fact that certain materials such as rubber and plastics, upon long exposure to the gas such as is necessitated when diffusion controlled films are utilized in the sterilizing packages, will absorb the gas, thereby requiring extremely long degassing periods to rid the item of any residual ethylene oxide before use.

Also to be noted is the fact that though the diffusion rate of ethylene oxide gas is slow through the plastic packaging films, the necessity of introducing a small amount of water vapor for effective kill, which has an even slower diffusion rate than the gas, seriously hinders production in commercial sterilization processes.

Film packages, as opposed to paper or coated paper packages, may also be steam sterilized in autoclaves at temperatures of from 230 F. to 270 F., usually from 230 to about 240 F., but still retain the inherent disadvantage of being vapor-diffusion controlled.

Commercial sterile film packages present storage problems for the gas/vapor pressure trapped within the package can only be relieved by gas-vapor difiusion through the plastic film, a slow process at best. Attempts to compress the packages for more efficient storage, only result in film or seal ruptures.

Highly porous papers are undesirable in many sterile packaging situations for, although they do permit rapid gas sterilization, they are neither completely moisture insensitive nor bacteria impermeable. These factors, of course, severely limit post sterilization shelf life for aseptic condition of packaged contents.

In an attempt to improve the barrier characteristics of paper, a polymeric coating is often applied. These coatings, while successfully decreasing the papers sensitivity to moisture, are, at best, an imperfect solution for although they can prevent bacteria from penetrating directly, they do not deter the bacteria from growing through the coating, thereby destroying the aseptic condition within the package. Extremely thick coatings will only destroy the advantage of rapid gas transport sought for in the use of the paper initially.

Another disadvantage to the use of paper as a sterile packaging material lies in the tearing of fibers and paperdust which accompanies the opening of such a package. The cellulose fibers may contaminate the sterilized item thereby defeating the purpose of using aseptic technique is dispensing sterile products.

Heretofore, porous films have been produced which possess a microporous, open-celled structure, and which are also characterized by a reduced bulk density. Films possessing this microporous structure are described, for example, in US. Pat. No. 3,426,754, which patent is assigned to the assignee of the present invention. The preferred method of preparation described therein involves drawing or stretching at ambient temperatures, i.e., cold drawing, a crystalline, elastic starting film in an amount of about to 300 percent of its original length, with subsequent stabilization by heat setting of the drawn film under a tension such that the film is not free to shrink or can shrink only to a limited extent.

While the above-described microporous or void-containing film of the prior art is useful in this invention the search has continued for new processes able to produce open-celled microporous films having a greater number of pores, a more uniform pore concentration or distribution, a larger total pore area, and better thermal stability of the porous or voidy film, e.g., for use in applications that call for repeated high temperature sterilizations.

4 SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an improved desiccant and/or fumigant packaging system by utilizing on at least one portion of the surface of the desiccant/fumigant enclosures an opencelled microporous plastic film containing interconnected pores of from ten to three thousand angstroms which will permit rapid gas/vapor transport while maintaining an impermeable solids/water barrier.

Another object is to provide for desiccant packaging an open-cell microporous film which will maintain its integrity in non-solvent liquid media.

An additional object of the present invention is to provide a desiccant container which, with a film either by itself or in combination with other materials, constituting a completely sealed, desiccant-containing envelope, will realize a controlled drying action.

An object of the present invention is to provide for improved sterile packaging systems, a breathable waterproof film containing the interconnected pores of from 10 to 3,000 A. which, by exceeding industrial standards for sterile packaging films, will yield significantly shortened gas sterilization cycles; i.e., permit rapid gas/vapor passage while excluding bacteria and other organisms when used by itself or in combination with other materials will form a sterilized, shrink-fit package when used in a steam autoclave.

Other and further objects of the present invention will be apparent to those skilled in the art from the following:

DETAILED DESCRIPTION OF THE INVENTION Porous or cellular films can be classified into two general types: one type in which the pores are not interconnected, i.e., a closed-cell film, and the other type in which the pores are essentially interconnected through tortuous paths which may extend from one exterior surface or surface region to another, i.e., an open-cell film. The porous films of the present invention are of the latter type.

The microporous films useful in the present invention are also characterized by a reduced bulk density, sometimes hereinafter referred to simply as a low density. That is, these microporous films have a bulk or overall density lower than the bulk density of corresponding films composed of identical polymeric material but having no open-celled or other voidy structure. The term bulk density as used herein means the weight per unit of gross or geometric volume of the film, where gross volume is determined by immersing a known weight of the film in a vessel partly filled with mercury at 25 C. and atmospheric pressure. The volumetric rise in the level of mercury is a direct measure of the gross volume. This method is known as the mercury volumenometer method, and is described in the Encyclopedia of Chemical Technology, vol. 4, page 892 (Interscience 1949).

Porous films have been produced which possess a microporous, open-celled structure, and which are also characterized by a reduced bulk density. Films possessing this microporous structure are described, for example, in US. Pat. 3,426,754 which patent is assigned to the assignee of the present invention. The preferred method of preparation described therein involves drawing or stretching at ambient temperatures, i.e., cold drawing, a crystalline, elastic starting film in an amount of about 10 to 300 percent of its original length, with subsequent stabilization by heat setting of the drawn film under a tension such that the film is not free to shrink or can shrink only to a limited extent.

While the above described microporous or void-containing film of the prior art is useful in this invention the search has continued for new processes able to produce open-celled microporous films having a greater number of pores, a more uniform pore concentration or distribution, a larger total pore area, and better thermal stability of the porous or voidy film. These properties are significant in applications such as filter media where a large number of uniformly distributed pores are necessary or highly desirable; and in applications such as breathable medical dressings subject to high temperatures, e.g. sterilization temperatures, where thermal stability is necessary or highly desirable.

An improved process for preparing open-celled microporous polymer films from non-porous, crystalline, elastic polymer starting films, includes 1) cold stretching, i.e., cold drawing the elastic film until porous surface regions or areas which are elongated normal or perpendicular to the stretch direction are formed, (2) hot stretching, i.e., hot drawing, the cold stretched film until fibrils and pores or open cells which are elongated parallel to the stretch direction are formed, and thereafter (3) heating or heat setting the resulting porous film under tension, i.e., at substantially constant length, to impart stability to the film. Yet another process is similar to this process but consolidates steps (2) and (3) into a continuous simultaneous, hot stretching-heat setting step, said step being carried out for a time sufficient to render the resulting microporous film substantially (less than about percent) shrink resistant.

The elastic starting film or precursor film is preferably prepared from crystalline polymers such as polypropylene by melt extruding the polymer into a film, taking up the eXtrudate at a drawdown ratio giving an oriented film, and thereafter heating or annealing the oriented film if necessary to improve or enhance the initial crystallinity.

The essence of the improved processes is the discovery that the sequential cold stretching and hot stretching steps impart to the elastic film a unique open-celled structure which results in advantageous properties, including improved porosity, improved thermal stability and a gain or enhancement of porosity when treated with certain organic liquids such as perchloroethylene.

As determined by various morphological techniques or tests such as electron microscopy, the microporous films of the improved process are characterized by a plurality of elongated, non-porous, interconnecting surface regions or areas which have their axes of elongation substantially parallel. Substantially alternating with and defined by these non-porous surface regions are a plurality of elongated, porous surface regions which contain a plurality of parallel fibrils or fibrous threads. These fibrils are connected at each of their ends to the non-porous regions, and are substantially perpendicular to them. Between the fibrils are the pores or open cells of the films utilized by the present invention. These surface pores or open cells are substantially interconnected through tortuous paths or passageways which extend from one surface region to another surface area or region.

With such a defined or organized morphological structure, the films which are treated according to the instant process may have a greater proportion of surface area that the pores cover, a greater number of pores, and a more uniform distribution of pores, than previous microporous films. Further, the fibrils present in the films of the present invention are more drawn or oriented with respect to the rest of the polymer material in the film, and thus contribute to the higher thermal stability of the film.

In fact the total surface area per cubic centimeter of material of the films of this invention have a range of from 2 to about 200 square meters per cc. Preferably the range is from about 5 to about 100 square meters per cc. and most preferably from about 10 to about 80 square meters per cc. These values can be compared with normal pin-holed film which has a total surface area per gram of about 0.1 square meter; paper and fabric which have values per gram of about 1.0 square meter and leather which has a value of about 1.6 square meters per cc. Additionally, the volume of space per volume of material ranges from about 0.05 to about 1.5 cubic centimeters per gram, preferably from about 0.1 to about 1.0 cubic centimeters per gram and most preferably from 0.2 to about 0.85 cubic centimeters per gram. Ad-

ditional data to define the films of this invention relates to nitrogen flux measurements, whereby the microporous films have Q (or nitrogen) flux values in the range of from about 5 to 400 preferably about 50 to 300. These values give an indication of porosity, with higher nitrogen flux values indicating higher levels of porosity.

Nitrogen flux may be calculated by mounting a film having a standard surface area of 6.5 square centimeters in a standard membrane cell having a standard volume of 63 cubic centimeters. The cell is pressurized to a standard differential pressure (the pressure drop across the film) of 200 pounds per square inch with nitrogen. The supply of nitrogen is then closed off and the time required for the pressure to drop to a final differential pressure of 150 pounds per square inch as the nitrogen permeates through the film is measured with a stop watch. The nitrogen flux, Q, in gram moles per square centimeter minute, is then determined from the equation:

where At is the change in time measured in second and T is the temperature of nitrogen in degrees Kelvin. The above equation is derived from the gas law, PV=Z RT.

The microporous films used in the present invention are formed from a starting elastic film of crystalline, film-forming, polymers. These elastic films have an elastic recovery at zero recovery time (hereinafter defined) when subjected to a standard strain (extension) of 50 percent at 25 C. and 65 percent relative humidity of at least about 40 percent, preferably at least about 50 percent, and most preferably at least about percent.

Elastic recovery as used herein is a measure of the ability of a structure or shaped article such as a film to return to its original size after being stretched, and may be calculated as follows:

Although a standard strain of 50 percent is used to identify the elastic properties of the starting films, such strain is merely exemplary. In general, such starting films will have elastic recoveries higher at strains less than 50 percent, and somewhat lower at strains substantially higher than 50 percent, as compared to their elastic recovery at a 50 percent strain.

The starting elastic films will also have a percent crystallinity of at least 20 percent, preferably at least 30 percent and most preferably at least 50 percent, e.g., about 50 to percent, or more. Percent crystallinity is determined by the X-ray method described by R. G. Quynn et al. in the Journal of Applied Polymer Science, vol. 2 No. 5 pp. 166473 (1959). For a detailed discussion of crystallinity and its significance in polymers, see Polymers and Resins, Golding (D. Van Nostrand, 1959).

Preferred suitable starting elastic films, as well as the preparation thereof, are further defined in British Pat. No. 1,198,695, published July 15, 1970. Other elastic films which may be suitable for the practice of the present invention are described in British Pat. No. 1,052,550, published Dec. 21, 1966 and are well known in the art.

The starting elastic films utilized in the preparation of the microporous films of the present invention should be differentiated from films formed from classical elastomers such as the natural and synthetic rubbers. With such classical elastomers the stress-stain behavior, and particularly the stress-temperature relationship, is governed by entropy-mechanism of deformation (rubber elasticity). The positive temperature coefiicient of the retractive force, i.e., decreasing stress with decreasing temperature and complete loss of elastic properties at the glass transition temperatures, are particularly consequences of entropy-elasticity. The elasticity of the starting elastic films utilized herein, on the other hand, is of a different nature. In qualitative thermodynamic experiments with these elastic starting films, increasing stress with decreasing temperature (negative temperature coefiicient) may be interpreted to mean that the elasticity of these materials is not governed by entropy effects but dependent upon an energy term. More significantly, the starting elastic films have been found to retain their stretch properties at temperatures where normal entropyelasticity could no longer be operative. Thus, the stretch mechanism of the starting elastic films is though to be based on energy-elasticity relationships, and these elastic films may then be referred to as non-classical elastomers.

As stated, the starting elastic films employed in this invention are made from a polymer of a type capable of developing a significant degree of crystallinity, as contrasted with more conventional or classica elastic materials such as the natural and synthetic rubbers which are substantially amorphous in their unstretched or tensionless state.

A significant group of polymers, i.e., synthetic resinous materials, to which this invention may be applied are the olefin polymers, e. g., polyethylene, polypropylene, poly-3-methyl butene-l, poly-4-methyl pentene-l, as well as copolymers of propylene, 3-methyl butene-l, 4-methyl pentene-l, or ethylene with each other or with minor amounts of other olefins, e.g., copolymers of propylene and ethylene, copolymers of a major amount of B-methyl butene-l and a minor amount of a straight chain n-alkene such as n-octene-l, n-hexadecene-l, n-octadecene-l or other relatively long chain alkenes, as well as copolymers of 3-methyl pentene1 and any of the same n-alkenes mentioned previously in connection with 3-methyl butene-l. These polymers in the form of film should generally have a percent crystallinity of at least 20 percent, preferably at least 30 percent, and most preferably about 50 percent to 90 percent or higher.

For example, a film-forming homopolymer of polypropylene may be employed. When propylene homopolymers are contemplated, it is preferred to employ an isotactic polypropylene having a percent crystallinity as indicated above, a weight average molecular Weight ranging from about 100,000 to 750,000 preferably about 200,000 to 500,000 and a melt index (ASTM-1958D- 1238-57T, Part 9, page 38) from about 0.1 to about 75, preferably about 0.5 to 30, so as to give a final film product having the requisite physical properties.

While the present disclosure and examples are directed primarily to the aforesaid olefin polymers, the invention also contemplates the high molecular weight acetal, e.g., oxymethylene, polymers. While both acetal homopolymers and copolymers are contemplated, the preferred acetal polymer is a random oxymethylene copolymer, one which contains recurring oxymethylene, i.e.,

units interspersed with OR-- groups in the main polymer chain where R is a divalent radical containing at least two carbon atoms directly linked to each other and positioned in the chain between the two valences, with any substituents on said R radical being inert, that is, those which do not include interfering functional groups and which will not induce undesirable reactions, and wherein a major amount of the OR units exist as single units attached to oxymethylene groups on each side. Examples of preferred polymers include copolymers of trioxane and cyclic ethers containing at least two adjacent carbon atoms such as the copolymers disclosed in US. Pat. 3,027,352 of Walling et al. These polymers in film form may also have a crystallinity of at least 20 percent, preferably at least 30 percent, and most preferably at least 50 percent, e.g. 50 to 60 percent or higher. Further, these polymers have a melting point of at least C., and a number average molecular weight of at least 10,000. For a more detailed discussion of acetal and oxymethylene polymers, see Formaldehyde, Walker, pp. -191, (Reinhold 1964).

Other relatively crystalline polymers to which the invention may be applied are the polyalkylene sulfides such as polymethylene sulfide and polyethylene sulfide, the polylarylene oxides such as polyphenylene oxide, the polyamides such as polyhexamethylene adipamide (nylon 66) and polycaprolactam (nylon 6), and polyesters such as polyethylene terephthalate, all of which are well known in the art and need not be described further herein for sake of brevity.

The types of apparatus suitable for forming the starting elastic films of this invention are well known in the art.

For example, a conventional film extruder equipped with a shallow channel metering screw and coat hanger die, is satisfactory. Generally, the resin is introduced into a hopper of the extruder which contains a screw and a jacket fitted with heating elements. The resin is melted and transferred by the screw to the die from which it is extruded through a slot in the form of a film from which it is drawn by a take-up or casting roll. More than one take-up roll in various combinations or stages may be used. The die opening or slot width may be in the range, for example, of about 10 to 200 mils.

Using this type of apparatus, film may be extruded at a drawdown ratio of about 20:1 to 200:1, preferably 50:1 to 150:1.

The terms drawdown ratio or, more simply, draw ratio, as used herein is the ratio of the film wind-up or take-up speed to the speed of the film issuing at the extrusion die.

The melt temperature for film extrusion, is in general, no higher than about 100 C. above the melting point of the polymer and no lower than about 10 C. above the melting point of the polymer.

For example, polypropylene may be extruded at a melt temperature of about C. to 270 C., preferably 200 C. to 240 C. Polyethylene may be extruded at a melt temperature of about 175 C. to 225 C., while acetal polymers, e.g., those of the type disclosed in US. Pat. 3,027,352 may be extruded at a melt temperature of about C. to 235 0., preferably C. to 215 C.

The extrusion operation is preferably carried out with rapid cooling and rapid drawdown in order to obtain maximum elasticity. This may be accomplished by having the take-up roll relatively close to the extrusion slot, e.g., within two inches and, preferably, within one inch. An air knife operating at temperatures between, for example 0 C. and 40 C., may be employed within one inch of the slot to quench, i.e., quickly cool and solidify the film. The take-up roll may be rotated, for example, at a speed of 10 to 100 ft./min. preferably to 500 ft./min.

While the above description has been directed to slit die extrusion methods, an alternative method of forming the starting elastic films contemplated by this invention is the blown film extrusion method wherein a hopper and an extruder are employed which are substantially the same as in the slot extruder described above. From the extruder, the melt enters a die from which it is extruded through a circular slot to form a tubular film having an initial diameter D Air enters the system through an inlet into the interior of said tubular film and has the effect of blowing up the diameter of the tubular film to a diameter D Means such as air rings may also be provided for directing the air about the exterior of extruded tubular film so as to provide quick and effective cooling. Means such as cooling mandrel may be used to cool the interior of the tubular film. After a short distance during which the film is allowed to completely cool and harden, it is wound up on a take-up roll.

Using the blown film method, the drawdown ratio is preferably 20:1 to 200:1, the slot opening 10 to 200 mils,

the D /D ratio, for example, 0.5 to 6.0 and preferably about 1.0 to about 2.5, and the take-up speed, for example, 30 to 700 ft./ min. The melt temperature may be within the ranges given previously for straight slot extrusion.

The extruded fiim may then be initially heat treated or annealed in order to improve crystal structure, e.g., by increasing the size of the crystallites and removing imperfections therein.

In order to render the precursor or starting film microporous, it is subject to a process generally comprising the steps of stretching and heat setting the starting films. Preferably the process comprises either the consecutive steps of cold stretching, hot stretching and heat setting or the steps of cold stretching and simultaneously hot stretching and heat setting the precursor film. Other variations on this process (such as elimination of the hot stretching step) can be carried out resulting in microporous films which, although slightly inferior to those films made by the cold stretch-hot stretch-heat set process, still find utility as the microporous films of this invention.

The term cold stretching as used herein is defined as stretching or drawing a film to greater than its original length and at a stretching temperature, i.e., the temperature of the film being stretched, less than the temperature at which the melting of the film begins when the film is uniformly heated from a temperature of 25 C. at a rate of C. per minute. The terms hot stretching or hot stretching-heat setting as used herein is defined as stretching above the temperature at which melting begins when the film is heated from a temperature of C. at a rate of 20 C. per minute, but below the normal melting point of the polymer, i.e., below the temperature at which fusion occurs. For example, using polypropylene elastic film, cold stretching is carried out preferably below about 120 C. while hot stretching or hot stretching-heat setting is carried out above this temperature.

When a separate heat setting step is employed it follows the cold stretching-heat stretching steps and is carried out at from about 125 C. up to less than the fusion temperature of the film in question. For polypropylene the range preferably is about 130 C. to 160 C.

The total amount of stretching or drawing which should occur when either a single stretching or consecutive stretching steps occur is in the range of about 10 to about 300 percent of the original length of the film prior to stretching.

The resulting microporous film exhibits a final crystallinity of preferably at least percent, more preferably about to 100 percent as determined by the aforementioned X-ray method and as previously defined an elastic recovery from a 50% strain of at least 50% preferably to Furthermore, this film exhibits an average pore size of about to 12,000 angstroms more usually 150 to 5,000 angstroms, the values being determined by mercury porosimetry as described in an article by R. G. Quynn et al. on pages 21-34 of Textile Research Journal, January 1963.

As hereinabove indicated, the subject invention relates to the use of a unique open-celled microporous film as described above and in US. Ser. No. 876,511 filed Nov. 13, 1969, now abandoned and incorporated by reference, which application is assigned to the assignees of the instant invention, as an improved, highly efiicient desiccant/fumigant packaging material. The following examples are illustrative of the desirable characteristics possessed by this film and are not intended to limit the present invention in any manner.

EXAMPLE I Crystalline polypropylene having a melt index of 0.7 and a density of 0.92 is melt extruded at 230 C. through an 8 inch slit die of the coat hanger type using a 1 inch extruder with a shallow metering screw. The length to diameter ratio of the extruded barrel is 24/ 1. The extrudate is drawn down very rapidly to a melt drawdown i0 ratio of 150, and contacted with a rotating casting roll maintained at 50 C. and 0.75 inch from the lip of the die. The film produced in this fashion is found to have the following properties: thickness, 0.001 inch, recovery from 50 percent elongation at 25 C., 50.3 percent, crystallinity, 59.6 percent.

A sample of this film is oven annealed in air with a slight tension at C. for about 30 minutes, removed from the oven and allowed to cool. It is then found to have the following properties: recovery from a 50 percent elongation at 25 C., 90.5 percent; crystallinity 68.8 percent.

The anneaied elastic film is first cold stretched at 25 C. and thereafter hot stretched at C. Total stretch is 100 percent, based on the original length of the film, and the extension ratio is 0.90:1. The film is then heat set under tension, i.e. at constant length at 145 C. on 10 minutes in air.

Porosity, tear, and tensile data of the above prepared film is as follows:

Tear, gins! Break, p.s.i.

N flux L B W 4 mils L W P 0 (3 grams) is placed between two 2" x 2" sheets of the above-prepared microporous polypropylene and the edges heat sealed. The pouch (total weight 4 gms.) is then placed upon a tray in a desiccator containing 50 cc. of water and the desiccator sealed. For comparison, 2 grams of P 0 is placed in a small open beaker within a desiccator also containing 50 cc. of water and the desiccator sealed.

After four hours the P 0 in the open beaker realizes a weight gain of 3 gms. H O/ gm. P 0 and the P 0 in the completely sealed pouch gains 1 gm. H O/ gm. P 0

EXAMPLE H A small open dish containing 7.3 gms. P 0 is placed in a room which has a relative humidity of about 65 percent. Also placed in the room is a 2" x 2" pouch, prepared as in Example I containing 4.8 gms. P 0

After four hours the P 0 in the open beaker realizes a weight gain of 2 gm. H O/gm. P 0 and the P 0 in the completely sealed pouch gains 0.225 gm. moisture/ gm. P 0

EXAMPLE III Three pint bottles are /3 filled with water-saturated benzene (0.05%). Into each bottle is placed a 2" x 2" pouch containing 2 gms. of P 0 prepared as in Example I.

At the end of the first hour, the pouch is removed from bottle #1. At the end of the third hour, the pouch is removed from #2 and at the sixth hour, from bottle #3. The water content in bottle #1 is reduced to 0.03%; in bottle #2 to 0.02%; and in bottle #3 to 0.02%.

EXAMPLE IV The film forming polymer of this example, crystalline polyethylene having a density of 0.96 and a melt index of 0.7, is melt extruded at C. through a 4 inch diameter annular die having an opening of 0.04 inch. The hot tube thus formed is expanded 1.5 times by internal air pressure and cooied by an air stream impinging on the film from an air ring located around and above the die. The extrusion is accomplished with an extruder of 24:1 length to diameter ratio and a shallow channel metering screw. The extrudate is drawn down to a drawdown ratio of 10021 and passed through a series of rollers which collapses the tube. After wind-up the film is oven annealed in a tensionless state at 115 C. for 16 hours.

After removal from the oven, the film is allowed to cool, and stretched at an extension ratio 015 0.80, by 50 percent of its original length with cold stretching being conducted at 25 C. and hot stretching being conducted at 12 EXAMPLE II The open-cell microporous film of the above Example I was mechanically cross-laminated to itself, i.e., the machine direction axes were 90 apart. The film laminate 115 C., and heat set in the oven at constant len h for minutes at 120 C., after which it is found to hi e the 5 yielded the followmg date pagan-celled microporous structure of the present inven- Teangms. Thick Three bottles as in Example III are filled with ben- Ngfilx L W mils L w Z6116 containing 0.06% water and a heat sealed pouch Of 30-60 120g 1 203 2 0 23 000 3 000 the above prepared microporous film which is mechani- 1 cally cross-laminated to itself, i.e., the machine direction mmum Va axes are 90 apart, containing 3 grns. P 0 is placed in EXAMPLE In each- W the pouches are removed from bottles 3 Microporous polypropylene film having a total surface and 4 :dfter hours pf f y Karl P1861161 area per gram of 40 square meters, a volume of space per l/ of the {emalnmg benzene W111 Yleld the followlng volume of material of 0.5 cubic centimeters per gram 2 concentfatlollsl and a N flux value of 113 was contacted with poly- T 320 propylene film which had been pin punched to yield a 36 mm filtration percent open area with 225 holes per square inch. The

Home number (hrs') (percent) microporous polypropylene film and the pin punched g 2? polypropylene film were then put through an embossing 6 M roll revolving at 8 revolutions per minute at 80 C. and

24 (1005 a pressure of 475 pounds per square inch. The physical A L V properties of the resultant product is compared with A four gram charge of paradichlorobenzene is placed 25 the apovc descnbed nficrqporous polylimpylene film with between two X sheets of the micropomus film no microporous backing 1n the followlng table: duced in Example IV and the edges completely heat sealed. Four Black Carpet Beetles and the above-identified Film gfi fig z luv/13R; Tear s lgfflf pouch are placed in a metal test chamber of 25 liter catpacity. Within 100 hours there is a 100% mortality i ff gg gj j 2g ,3 ra e.

The following examples are illustrative of the desirable fggt ggigggg for 10 pass 1 area at Pressure equal sterility-barrier characteristics possessed by this film and gMeasuredingramsper24hwrspersquaremetersper ASTM 15-96-60. are not intended to limit the present invention in any 4%22253 lfififi fiiifig tt 83833;; manner.

EXAMPLE I Th f ll l Crystalline polypropylene having a melt index of 0.7 e o owmg Pat ogemc organisms. were p acted and a density of 0.92 is melt extruded at 230 C. through tweeln layers of mlcroporous lammate film m Exan 8 inch slit die of the coat hanger type using a 1 inch 40 amp e extruder with a shallow metering screw. The length to (1) Sta h [000cm re 0 g fg diameter ratio of the extruded barrel is 24/1. The ex- (2) E s s 0 5 6 1' trudate is drawn down very rapidly to a melt drawdown 3 "K"? ratio of 150, and contacted with a rotating casting roll Omen as erugmosa maintained at 50 C. and 0.75 inch from the lip of the PMGIOCOCCEIS die. The film produced in this fashion is found to have (5) rate, guns X15 the following properties: thickness, 0.001 inch, recovery A totally enclosed pouch was formed by impulse sealing from percent elongation at 25 C., 50.3 percent, the edges of the layers. The package was placed within crystallinity, 59.6 percent. a Castle Ethylene-oxide Sterilizer and a 23" vacuum ap- A sample of this film is oven annealed in air with a 50 plied. This was immediately followed by a 8-10 pound slight tension at 140 C. for about 30 minutes, removed pressurization at 130 F. with a 12% ethylene oxide/ 88% from the oven and allowed to cool. It is then found to trichlorofiuoromethane gas mixture for 4 hours. When the have the following properties: recovery from a 50 percent bag was removed from the sterilizer, small amounts of elongation at 25 C., 90.5 percent; crystallinity 68.8 pergas residual evaporated quickly. The organisms inside cent. were completely destroyed. Sterile conditions existed in- The annealed elastic film is first cold stretched at 25 side of this bag for a period exceeding 90 days (end of C. and thereafter hot stretched at 145 C. Total stretch test period) during which time the sterile pouch was is 100 percent, based on the original length of the film, repeatedly submerged in water. and the extension ratio is 0.90:1. Nitrogen flux (at X M C.) of the resulting open-celled microporous film is 60 E PLE v 125.5 10- gram moles per cmF-min. A steile pouch was formed as in Example IV an the Porosity, tear and tensile data of the above prepared outside was coated with the pathogenic organisms of that film are compared with data from commercially availexample in a. thioglycolate medium in an attempt to deable sterile packaging films in the following table: termine if bacteria could grow through the microporous Tear, grns. Break, p.s.i. Thick, Sterile packaging material N2 flux 1 L 4 W 15 mils. L W

MPF -130 1-2 1.0 20,000 3,400 Polyethylene film (Bard-Parker) 0 121 96 3. 2 950 544 Polyester-polyethylene film (Pharmaseal) 0 5 5 1.6 8, 82010,400 Nylon film (Tower Pks.) 0 153 10 1. 5 8, 45040, 000

l Microporous film of the present invention. 1 NXIO- g. moles Nz/cm. min. at 200 p.s.i.g. difierential pressure.

3 Notched Elmendorf Tear ASTM 13-1922-67 constant radius specimen:

laminate. The bag was maintained at 37 C. and 90100% humidity for 3 days. When the pouch was removed from this controlled climate, the organisms had not penetrated the pouch, i.e., sterile conditions continued to exis wihin the pouch.

EXAMPLE VI A totally enclosed pouch was formed about a polyethylene-rubber catheter by impulse sealing two layers of the open-cell microporous polypropylene laminate film of Example III. This package was placed within a Castle Ethylene-oxide Sterilizer and a 23" vacuum applied. This was immediately followed by an 8 10 pound pressurization at 130 F. with a 12% ethyleneoxide/ 88% trichlorofiuoromethane gas mixture for 4 hours. The package was then aerated for 12 hours. At the end of this time, the residual ethylene-oxide in the polyethylene-rubber catheter was below the level which would prove to be an irritant to human body tissue.

For comparative purposes, a similar test was conducted using a non-porous polypropylene film package. To effect sterilization, a sterilant exposure time of 12 hours was necessary. To then reduce the absorbed ethylene-oxide concentration in the polyethylene-rubber cathether to a level below that which would be an irritant to human body tissue required a combined aeration and storage time of 2 days.

EXAMPLE VII The open-cell microporous laminate of Example III was tested for resistance to pressure up to 50 lbs. with dry and moist air. No penetration was noticed with a fog the size of about 1 micron.

EXAMPLE VIII A package such as that in Example IV containing identical pathogenic organisms was placed in a steam autoclave at 250-270 F. for 4 hours. The organisms were completely destroyed and a small amount of residual moisture inside of the bag after removal from the autoclave evaporated within minutes. The package experienced a small dimensional shrinkage of from about 5 to with a not inconsiderable reduction in porosity but the interior of the package remained sterile for a period exceeding 90 days during which time the package was repeatedly submerged in water.

EXAMPLE IX Crystalline 4-methyl-1-pentene having a melt index of 1.5 was melt extruded at 250270 C. through an 8 inch slit die of the coat hanger type using a 1 inch extruder with a shallow metering screw. The length to diameter ratio of the extruded barrel is 24/1. The extrudate was drawn down very rapidly to a melt drawdown ratio of 150, and contacted with a rotating casting roll maintained at 140 C. and 0.75 inch from the tip of the die. The film produced in this fashion is found to have the following properties: thickness, 0.001 inch; recovery from 50 percent elongation at 25 C., 87 percent.

A sample of this film is oven annealed in air with a slight tension at 160 C. for about minutes, removed from the oven and allowed to cool. It is then found to have the following properties: recovery from a 50 percent elongation at C., 92 percent.

The annealed elastic film is first cold stretched at 25 C. and thereafter stretched at 145 C. Total stretch is 100 percent, based on the original length of the film, and the extension ratio is 0.9. Nitrogen flux (at 65 C.) of the resulting open-celled microporous film is 25x10- gram moles per cm. min.

This film was then contacted with 4-methyl-l-pentene film which had been pin punched to yield at 36 percent open area with 225 holes per square inch. The microporous 4-methyl-1-pentene film and the pin punched 4- methyl1-pentene film were then put through an embossing roll revolving at 8 revolutions per minute at 120 C. and a pressure of 250 pounds per square inch. A pack age containing the pathogenic organisms listed in Example IV was formed as described in that example utilizing the above-prepared laminated film. The package was placed in a steam autoclave at 250270 F. for 4 hours. The organisms were completely destroyed; residual moisture within the package after removal from the autoclave evaporated within minutes; and there was less than 10% shrinkage of the package.

Illustrated above have been but a few of the many advantages inherent in this unique sterile packaging material. As a result of the extremely rapid gas transport through this film, it shows no tendency to rupture even under repeated sterilization to allow compacting and therefore efficient utilization of storage space.

An additional advantage to the use of this open-cell microporous film in sterile packaging lies in the fact that this anisotropic film with a small tug (-1-2 pounds) perpendicular to the machine production direction will snap apart leaving a clean, lint-free split, i.e., no shredding occurs to contaminate the aseptic conditions within the package.

The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the present invention.

This four gram load corresponds to the dosage of 10 pounds per 1000 cubic feet recommended by the US. Department of Agriculture.

What is claimed is:

l. In a package which allows the passage of chemical and sterilant vapors the improvement which comprises forming at least a portion of the package from a film consisting essentially of a polymer selected from the group consisting of polyolefins, polyacetals, polyamides, polyesters, polyalkylene sulfides and polyarylene oxides, wherein the film is further characterized by having a reduced bulk density as compared to the bulk density of the corresponding polymer films having no open celled structure, a crystallinity of above about 20 percent, a pore size of about 100 to 12,000 angstroms, a nitrogen flux of about 5 to 400, and an elastic recovery at 50 percent extension of greater than 40 percent and wherein the film is further characterized by a plurality of inter-connecting nonporous surface regions,

the non-porous surface regions being elongated and having their axes of elongation substantially parallel; and a plurality of porous surface regions which include a plurality of fibrils, with the porous surface regions being defined by the non-porous surface regions,

the porous surface regions and the nonporous surface regions substantially alternating, and the fibrils being connected at each of their ends to the non-porous surface regions,

the fibrils being substantially parallel to each other and being substantially perpendicular to said axes of elongation; and,

the fibrils defining pore spaces in the porous surface regions of the film.

2. The improved package according to claim 1 wherein the film has a bulk density of about 50 to percent of the bulk density of corresponding polymer films having no open-celled structure.

3. The improved package according to claim 1 wherein the film has a nitrogen flux of at least 40.

4. The improved package according to claim 1 wherein the film has a surface area of at least '30 sq. m./cc.

5. Th improved package according to claim 1 wherein the film has a perchloroethylene reaction value of zero or greater.

6. The improved package according to claim 1 wherein the film has a nitrogen flux of 50 to 200.

7. The improved package according to claim 1 wherein the film has a bulk density of about 59 to 66 percent of the bulk density of a corresponding polymer film having no open-celled structure.

8. The improved package according to claim 1 wherein the film has a surface area of 30 to 35 sq. m./cc.

9. The improved package according to claim 1 wherein the film has a bulk density of about 50 to 75 percent of the bulk density of corresponding polymer films having no open-celled structure, a nitrogen flux of at least 40, a surface area of at least 30 sq. m./cc., and a perchloroethylene reaction value of or greater.

10. The improved package according to claim 1 wherein the film has a perchloroethylene reaction value of 0 or greater, a nitrogen flux of 50 to 200, a bulk density of 59 to 66 percent of the bulk density of the corresponding polymer film having no open-celled structure, and a surface area of 30 to 35 sq. m./cc.

11. In a package which allows the passage of chemical vapors, the improvement which comprises forming at least a portion of the package from a film wherein said film consists essentially of a polymer selected from the group consisting of polyolefins, polyacetals, polyamides, polyesters, polyalkylene sulfides and polyarylene oxides, wherein the film is further characterized by having a reduced bulk density as compared to the bulk density of the corresponding polymer films having no open-celled structure, a crystallinity of above about 30 percent, a pore size of less than 5,000 angstroms, a nitrogen flux of greater than 35.4, and a breaking elongation of 50 to 150 percent characterized by a plurality of interconnecting non-porous surface regions,

the fibrils being substantially parallel to each other and being substantially perpendicular to said. axes of elongation; and,

the fibrils defining pore spaces in the porous surface regions of the film.

12. The improved package according to claim 11, wherein the film has a bulk density of about 58 to 85 percent of the bulk density of the corresponding polypropylene polymer films having no open-celled structure.

13. The improved package according to claim 1,2 wherein the film has a nitrogen flux of at least 40.

14. The improved package according to claim 12 wherein the film has a surface area of about 30 to 110 sq. m./cc.

15. The improved package according to claim 12 wherein the film has a bulk density of about 62, a nitrogen fiux of about 100 and a surface area of about 60 sq. m./cc.

16. The improved package, according to claim 12 wherein the film has a perchloroethylene reaction value of 0 or greater.

17. The improved package according to claim 11 wherein the film has a bulk density of about 58 to 85 percent of the bulk density of corresponding polypropylene films having no open-celled structure, a nitrogen flux of at least 40, and a surface area of about 20 to 110 sq. m./cc.

18. In a sterilizable package the improvement which comprises forming at least a portion of the package from a film consisting essentially of a polymer selected from the group consisting of polyolefins, polyacetals, polyamides, polyesters, polyalkylene sulfides and polyarylene oxides, wherein the film is further characterized by having a reduced bulk density as compared to the bulk density of the corresponding polymer films having no open celled structure, a crystallinity of above about 20 percent, a pore size of about to 12,000 angstroms, a nitrogen flux of about 5 to 400, and an elastic recovery at 50 percent extension of greater than 40 percent and wherein the film is further characterized by a plurality of inter-connecting non-porous surface regions,

the non-porous surface regions being elongated and having their axes of elongation substantially parallel; and a plurality of porous surface regions which include a plurality of fibrils, with the porous surface regions being definedby the non-porous surface regions,

the fibrils defining pore spaces in the porous surface regions of the film.

19. In a package for enclosing chemical compounds selected from the group consisting of fumigants and desiccants, the improvement which comprises forming at least a portion of the package from a film consisting essentially of a polymer selected from the group consisting of polyolefins, polyacetals, polyamides, polyesters, polyalkylene sulfides and polyarylene oxides, wherein the film is further characterized by having a reduced bulk density as compared to the bulk density of the corresponding polymer films having no open celled structure, a crystallinity of above about 20 percent, a pore size of about 100 to 12,000 angstroms, a nitrogen flux of about 5 to 400, and an elastic recovery at 50 percent extension of greater than 40 percent and wherein the film is further characterized by a plurality of inter-connecting non-porous surface regions,

the porous surface regions and the non-porous surface regions substantially alternating, and the fibrils being connected at each of their ends to the non-porous surface regions,

the fibrils defining pore spaces in the porous surface regions of the film.

References Cited UNITED STATES PATENTS 3,100,733 8/1963 Bundy 161-459 3,122,141 2/1964 Crowe 161-459 3,378,507 4/1968- Sargent et al 161-159 3,426,754 2/1969 Bierenbaum et a1. 161159 3,578,544 5/1971 Thorsrud 161-159 3,578,545 5/1971 Carson et a1 161--159 MORRIS SUSSMAN, Primary Examiner US. Cl. X.R.