WO1993002860A1

WO1993002860A1 - Improved self-sealing stretch film for laboratory use

Info

Publication number: WO1993002860A1
Application number: PCT/US1992/006618
Authority: WO
Inventors: Daniel Perlman
Original assignee: Brandeis University
Priority date: 1991-08-07
Filing date: 1992-08-07
Publication date: 1993-02-18
Also published as: AU2444792A

Abstract

A method for covering and sealing an article. The method includes: (i) providing a sized portion of a waterproof, adhesive, self-sealing, and heat-resistant polyolefinic stretch film having an upper and a lower surface, and a softening temperature in excess of 100 °C, wherein at least one surface of the film includes an adhesive agent able to bond the film together during stretch wrapping, and at least one surface of the film is free from constituents which can dissolve in or react with an organic solvent or a caustic agent, wherein the sized portion of the film is sufficient to cover a portion of the article, (ii) placing the film over that portion of the article, and (iii) pressing the film toward the article to adhere the film to the article.

Description

DESCRIPTION

Improved Self-Sealinσ Stretch Film for Laboratory Use

Background of the Invention

This invention concerns self-sealing thermoplastic stretch films and their use in the laboratory.

A stretchable self-sealing plastic wrap has been used for many years in the chemical laboratory to provide a convenient water-tight closure on a test tube or other container. This stretch wrap known as PARAFILM® is manu¬ factured by the American National Can Corporation and is described in the manufacturer's brochure titled "Parafilm M, the all purpose laboratory, film and dispenser". The film is permanently self-adherent when stretched over and around an object or a container opening. Irreversible elongation and thinning of PARAFILM® occurs during the stretch wrapping process. Reversibly adherent stretch-wrap films which are composed of one or more thermoplastic layers and which include the polyethylene-based cling-type stretch wraps, are also known. For example, Kishi et al., U.S. Patent 4,073,782, describe a wrapping film containing either polyethylene or a copolymer of vinyl acetate and ethylene (or a mixture of these) , plus sorbitan onooleate and liquid paraffin. The monooleate serves as an antihazing agent while the viscosity and quantity of paraffin oil are selected to control the film cling. Shadle, U.S. Patent 4,082,877, describes a composite laminar film containing an elastomeric layer and a seal- able layer of a polymeric composition containing an interpolymer of ethylene and alkenoic acid.

Climenhage et al., U.S. Patent 4,337,188, describe a polyolefin composition for manufacture of film having cling properties. The mixed composition includes a poly¬ ethylene blend, an elastomer copolymer including ethylene and propylene, and an agent selected from a class of

SUBSTITUTE SHEET organic compounds (certain organic liquids or waxy solids such as mineral oil and liquid polyolefin) .

Cipriani et al. , U.S. Patent 4,379,197, describe a stretch wrapping film consisting of linear low density polyethylene (LLDPE) and sorbitan monooleate. The LLDPE provides strength and toughness to the film while the monooleate provides the appropriate degree of cling.

Briggs et al., U.S. Patent 4,399,180, describe a triple-layered coextruded polyolefin stretch wrap having a relatively thick core layer of linear low density poly¬ ethylene co-polymers such as ethylene co-polymerized with at least one C₄ to C₁₀ alpha-olefin, and one or two rela¬ tively thin skin layers of highly branched low density polyethylene. The resulting multilaminate structure has a reduced tendency to tear compared to other fins.

Cooper, U.S. Patent 4,425,268, and 4,456,788, describes a composition for stretch wrap film including a high molecular weight copolymer of ethylene and vinyl acetate, a linear copolymer of ethylene and higher alkene, and a tackifier to impart cling. Cooper also describes a multilayer composite stretch film composition including a first layer of a high molecular weight ethylene and vinyl acetate copolymer and a tackifier. A second layer which is adhered to the first, is principally linear low density polyethylene.

Breck, U.S. Patent 4,657,982, describes polymer blends and films including up to 99% of a linear copolymer of ethylene and C₄ to C₈ α-olefin or a mixture of this linear copolymer and high density polyethylene or ethylene-vinyl acetate copolymer, and 0.5% - 10% each of low molecular weight and higher molecular weight poly- butene. The mixture of polybutenes imparts higher cling strength than that achieved with single polybutenes.

Knott, U.S. Patent 4,671,987, describes a thin printable multilaminate stretch wrap which exhibits strong cling. The film includes at least two layers, one layer containing a blend of ethylene-vinyl acetate (EVA)

SUBSTITUTESHEET copolymer and a tackifier, and a second layer free of tackifier (thereby allowing printing) , including at least 50% linear very low density polyethylene (VLDPE) . The film may also include a layer of LLDPE positioned between the tackified EVA layer and the VLDPE layer of the film. Japanese Patent 53,034,845, describes the use of chlorinated paraffin as a tackifier in an ethylene-vinyl acetate self-adhesive stretch film.

Summary of the Invention This invention relates to the laboratory use of a self-sealing stretch-wrap film which is liquid-tight, insoluble in certain organic solvents, and tolerant to a temperature of 100°C. The film is designed to function as a removable protective covering and sealing film for bottles, test tubes, flasks, beakers and the like. It is also designed to overcome the limitations of the prior art laboratory film (PARAFILM®) .

In the field of commercial packaging, a large number of single and multilayer thermoplastic stretch-wrap films have been developed for over-wrap packaging of goods, particularly for wrapping pallet loads, and the stretch- packaging of foodstuffs. Physical properties which are usually sought in such films include strength, stretch- ability, optical clarity and cling. The property of cling is functionally defined as the tendency of a film to bind to itself without the use of a surface adhesive or heat treatment following contact between two surfaces of the film. Generally cling films are not sticky to the touch and do not adhere to metal, wood and many other foreign materials. Cling, which is a reversibly and weakly adhesive attachment may be quantified as the force (in grams per centimeter of film width) required to pull apart a pair of strips of the filming shearing mode (antiparallel pulling on the opposing ends of the paired strips) after the strips have been placed in face to face contact with 2.54 cm (one inch) of their lengths

SUBSTITUTESHEET overlapping. Generally a stretch wrap peels and unrolls readily, having been engineered to exhibit a maximum of cling with little adhesive strength perpendicular to the film. The latter type of adhesion is undesirable since it can prevent storage of the film in roll form and causes adjacent stretch-wrapped packages in storage to adhere together and become damaged. If permanent self-adhesion of a stretch-wrap is desired (such as sealing a pallet wrap to itself, the film is generally cut and attached to the previous layer of film using heat-sealing, tape or surface-adhesive application. The latter adhesive bonding of surfaces, whether of a permanent or a removable nature, may be quantified by the "peel strength", i.e., the force (in grams per centimeter of film width) required to pull apart the adjacent ends of a pair of adhered strips of film in a peeling mode (180° bending over of one strip followed by pulling the strips in opposite directions) . The preferred pressure-sensitive adhesives utilized in this invention and described below provide peel strengths exceeding those found in cling wraps.

Applicant has discovered that the addition of a pressure-sensitive type adhesive agent providing firm yet reversible self-adhesion to stretch wrap films is advan¬ tageous and can be accomplished in a commercially useful manner. Using one strategy, solvent resistant adhesives are blended with polyethylene and rubber to form a single layer, blended film. Using a second strategy, adhesives are externally applied to polyolefinic stretch ilms used in commercial packaging, e.g., polyethylene stretch films used for pallet wrapping. This second strategy is more productive and cost-effective, and is described below.

The presently invented self-adherent laboratory stretch ilm differs markedly in structure from PARAFILM® in that the adhesive component of the film is solvent resistant, heat resistant, and has been segregated (in the form of a separate layer or layers) , from the structural and elastic layer or layers of the film. All of the components of the new film including both the structural and the adhesive components are substantially resistant to common organic solvents and are resistant to temperatures of at least 100°C. The adhesives utilized in the new film provide a strong and immediate pressure-sensitive bonding of the film to itself. Unlike PARAFILM®, this bonding does not depend upon first placing the film in a stretched configuration (PARAFILM® must be stretched to approxi¬ mately twice its original length for its waxy and lubri- cious surface to become dull and adequately cohesive) . Also, unlike PARAFILM®, whose self-adhesive layers tend to permanently "weld" together, layers of the present film which are stretch-adhered together can be unwrapped and rewrapped. In further distinction to PARAFILM® (whose blended paraffin-containing composition necessitates a non-sticking interleaf paper for the film to be stored in roll form) , the stretch films of this invention employ an adhesive which remains bound to that side of the film to which it was originally applied. Thus the new film can be stored in roll form without an interleaf paper, then unrolled, and the adhesive remains selectively bound as described. By eliminating the awkward interleaf paper (which must be peeled away before using a sheet of PARAFILM®) , the new film can be utilized immediately from the storage roll. In terms of material consumed in the stretch-wraps, PARAFILM® which is approximately 0.005 inches thick (5 mils) contains the equivalent of about 1 mil (25% by weight) rubberized polyethylene and about 4 mils paraffin. By contrast, a typical sheet of the new stretch-wrap film requires only about a I mil thickness of material. The elimination of paraffin allows this economy of material.

Thus, in a first aspect, the present invention features a waterproof, adhesive, self-sealing, and heat- resistant polyolefin-based stretch film, and its use in laboratory wrapping applications. The term laboratory, is meant to include scientific and hospital facilities for conducting research experiments, analyses, tests, and other such technical procedures. The term self-sealing is used to describe the ability of an adhesive film to adequately bond to itself to establish and maintain a liquid-tight seal over an inverted standard 500 ml glass water-filled erlenmeyer flask opening. The term stretch film is used to describe a film which tolerates at least 200% and preferably 500% elongation without breaking. In a related matter, stretch-wrapping is a process of cover- ing an article or a portion of an article with such a stretch film. This process involves stretching the film over the article and pressing the film toward the article in a manner which secures the film to itself. These laboratory applications for the film include, but are not limited to the covering and sealing of laboratory devices, samples which are of animal, vegetable or mineral origin, vessels containing chemicals, and other like articles. The film has an upper and lower surface. At least one side or surface of the film contains an adhesive agent able to bond the film together in the course of the stretch-wrapping process. This bonding capacity includes both bonding of the adhesive surface to itself as well as to the side of a film which lacks adhesive. At least one side or surface of the film is free from constituents which can dissolve in or react with the following common organic solvents and caustics: ethyl ether, carbon tetra- chloride, chloroform and hydrochloric acid, sulfuric acid and sodium hydroxide. The film surface may thus be used as an effective barrier against such organic solvents and caustics. The softening temperature of the film is above 100°C, thereby allowing its use on objects exposed to boiling water. The method for using the stretch-film involves providing a sized portion of the above-described film sufficient to cover a portion of the article being wrapped, placing the film over at least a portion of this article, and pressing the film toward the article to adhere the film to the article. Alternatively, the film is placed and also stretched over at least a portion of the article being wrapped. Another method of use includes placing the film over the article being wrapped and overlapping the film on itself, such that at least one overlapping surface of the film which carries the adhesive agent contacts the other over¬ lapping surface of the film, and pressing the overlapping surfaces of the film toward the article to adhere the film to itself. In a second aspect the invention features flasks, test tubes, containers and other laboratory articles in¬ cluding samples of animal, vegetable or mineral materials covered with a waterproof, adhesive, heat-resistant and solvent resistant self-sealing polyolefin-based stretch- wrap.

In preferred embodiments of the above aspects, the stretch film has a thickness between 0.0005 Inches (0.5 mils) and 0.0025 inches (2.5 mils), the film is a multi- layered sheet comprising at least one elastic structural layer containing a polyethylene resin such as LLDPE. The polyethylene may be blended with a rubber such as ethylene-propylene rubber (EPDM) in a ratio of between approximately 50 - 80% by weight polyethylene and 20 - 50% by weight rubber. The stretch film is constituted and configured to provide a degree of mechanical resistance to stretching compatible with and convenient for the manual stretch-wrapping of laboratory flasks, test tubes and the like. More specifically, when a stretching force of between approximately 0.5 lb. and 5 lb. is applied (at room temperature) to a one inch wide strip of the presently invented film, it commences elongation. The film is also constituted and configured to allow at least 200% and preferably 500% elongation before breakage where % elongation equals: (final length - original length) x 100 ÷ original length, to provide a film which is useful for manual stretch wrapping of laboratory devices. At least one self-adhesive agent which is preferably pressure-sensitive, has a softening temperature in excess of 100°C and is applied by extrusion or solvent coating onto the elastic structural layer. The self-adhesive resin provides the self-sealing feature of the stretch wrap and is preferably resistant to organic solvents including alcohols, ethers, petroleum derived oils and ketones. The adhesive is also unreactive with caustic chemicals such as strong acids and alkaline agents. The degree of self-adhesion measured by the force of resist- ance to 180° peeling of two self-adhered layers (either two adhesive-coated surfaces, or one adhesive-coated and one uncoated film surface) is equal to between approxi¬ mately 5 and 200 grams per centimeter of film width (see Table 1, for example) . In other preferred embodiments, the self-adhesive layer comprises one or more pressure sensitive amorphous polyolefin resins which are chemically inert and possess suitable adhesive properties for binding to a polyethylene-based stretch film. The useful amorphous polyolefin resins must be both adhesive and cohesive to provide the requisite self-sealing property which must be sustained over the temperature range of about 0° to 100^βC and preferably -20°C to 100°C. Useful amorphous poly¬ olefin resins include the amorphous propylene-ethylene copolymers, the amorphous propylene-hexene copolymers and the amorphous terpolymers.

In still other preferred embodiments, the self- adhesive plastic stretch wrap comprises multiple elastic structural layers. For example, the film may comprise three coextruded polyethylene-containing layers, e.g., LLDPE layers, in which one highly elastic inner core layer is symmetrically sandwiched between two outer layers. Such a symmetrically-layered structure provides strength and minimizes curling of the film following stretching. A self-adhesive layer such as an amorphous polyolefin described above is subsequently applied to such a multi- layered film, using a standard solvent or extrusion coating process described in the prior art. The quantity of such adhesive applied by a solvent coating process is between approximately 0.2 and 5.0 grams per square meter of film surface. For maximizing resistance of the film to certain organic solvents including chloroform and benzene, only one surface of the film is coated with the adhesive agent while the second surface of the film is free from constituents which can dissolve in or react with an organic solvent or a caustic agent. Further features and advantages of the invention will become apparent from the following specification including a description of the preferred embodiments of the inven¬ tion and from the claims.

Description of the Preferred Embodiments

Film

The structure of the films of this invention are generally described above. Each component of such films will now be described in detail and one or more specific and non-limiting examples provided.

Elastic Structural Layer(s) of the Film

For the purpose of this invention, the thermoplastic structural layer(s) of the film are preferably transparent or semi-transparent, must provide a chemically inert barrier to common organic solvents and caustic agents, and must liberate no leachable or particulate contaminants when incubated in aqueous and organic solutions at room temperature. The film surface was tested for solubility and reaction with these chemical agents by contacting the surface for 12 hr at 20°C with each agent (held in a glass petri dish) and subsequently drying and monitoring the film for weight loss and haze formation. Results from these tests were negative. The testing sensitivity for solubility (measured by weight loss) was 0.1 mg per 100 mg of film or 0.1%. The film must withstand freezing down to -20°C without cracking, must possess a softening tempera¬ ture above 100°C, and must exhibit appropriate elastic properties including tolerating at least 200% stretch- elongation at room temperature without breaking. In general, the polyolefin thermoplastics, and in particular, polyethylene-based stretch films provide an appropriate chemical barrier, as well as stretch and thermal resist¬ ance characteristics described above. Addition of a suitable inert rubber such as EPDM to the polyethylene improves the elastic properties of the film.

Example 1:

Single layer blended stretch film compositions comprising polyethylene and rubber were extruded without an adhesive to form the elastic structural layer of the present film. Film blend #1 contained approximately 75% by weight linear low density polyethylene (LLDPE) and approximately 25% by weight EPDM rubber (Polysar #306, obtained from Polysar Canada Corp.) Blend #2 contained approximately 60% by weight LLDPE and 40% by weight EPDM rubber. The resulting 2 mil thick extruded films when stretch-tested, commenced elongation with forces of 3 and 2.5 pounds respectively for 1 inch wide sample test strips of the materials. The total elongation prior to breakage was approximately 450% for blend #1 and 600% for blend ≠2. One mil thick extruded ilms of the above blends were also formed and shown to commence elongation with about 1.5 pounds force being applied to 1 inch wide test strips. Extruded film samples were then tested for resistance to organic solvents, by incubation in the solvents overnight at 20°C. The samples were found to be resistant to all of the solvents tested including ethanol, acetone, ethyl ether, carbon tetrachloride, toluene, hexane and mineral oil. The samples were also resistant to strong acids and alkalis including concentrated sulfuric acid, hydrochloric acid and sodium hydroxide. Multi-layered industrial stretch films comprising low density polyethylene have also been used to form the foundation of the present wrap. These stretch films are manufactured for the commercial packaging industry (e.g., for pallet wrapping and food packaging applications) and are available from the Exxon Chemical Company, Dow Chemical Company and E.I. DuPont De Nemours and Company for example. A particularly useful multilayered pallet stretch wrap is available from the Borden Chemical Company, Resinite Division (Andover, MA) and is known as LOADMASTER® film. This film is described by the manufac¬ turer as a slot-cast extruded LLDPE film consisting of three coextruded layers of modified LLDPE resins. The LOADMASTER® film was obtained in 1.0 and 2.0 mil thick- nesses and is observed to be transparent, resists tearing, exhibits good elastic memory when stretched in both the machine and transverse direction, and tolerates a high degree of elongation before breakage (700% in the machine direction and 900% in the transverse direction). The 1.0 mil thick film possesses a resistance to stretching comparable to that of PARAFILM® commencing elongation when a force of approximately 600 g is applied to a one inch wide rectangular test strip of the film. A similar strip of the 5 mil thick PARAFILM® material commences stretching when a force of 450 g is applied. Regarding temperature stability, the LOADMASTER® film has a softening tempera¬ ture of 125^βC consistent with its principal constituent, LLDPE. The chemical stability of this film is excellent. No solubility in ethanol, acetone, ethyl ether, carbon tetrachloride, toluene, hexane or mineral oil, could be detected following a two week incubation at room tempera¬ ture in these solvents. Solubility was monitored by testing film samples for any weight loss following solvent incubation and drying of the film samples. The film was also unreactive with strong acids and alkalis. Self-Adhesive Surface of the Film

At least one surface of the stretch film contains an adhesive agent which is preferably pressure-sensitive and capable of bonding the film to itself during stretch wrap- ping. The adhesive can be co-extruded during manufacture of the stretch film. Alternatively the adhesive can be applied to the film after its manufacture by a solvent or extrusion coating process. Typically the adhesive is applied to one side of the stretch film rather than throughout the film (paraffin is throughout PARAFILM®) .

Example 2:

Using a solvent coating process, several amorphous polyolefins have been applied in toluene at a concentra¬ tion of approximately 10% (w/v) to polyethylene based stretch wraps. Specifically, an amorphous terpolymer, an amorphous propylene-hexene copolymer and an amorphous propylene-ethylene copolymer were each applied from 5 - 10% (w/v) toluene solutions onto LLDPE stretch wrap film (1 mil LOADMASTER® film from Borden Inc.) using a standard commercial gravure roller process for applying liquid adhesive. This roller coating process has provided uniform and reproducible coatings containing between 0.2 and 5.0 g adhesive per square meter of film surface. The toluene in the coating is rapidly evaporated from the film by passing the film through a warming oven immediately after application of the coating. The toluene vapor liberated from the coating is incinerated to prevent environmental air pollution.

The amorphous polyolefins used in the present film coatings (obtained from the Eastman Chemical Company) are generally soft and provide moderate adhesive strength. By contrast, acrylic and rubber-based adhesives are commonly used in the manufacture of tape products to provide stronger adhesion. The amorphous polyolefins are pre- ferred in the present invention because they are less chemically reactive than rubber or acrylic adhesive and are also insoluble in most organic solvents at room temperature (20 - 25°C) . Furthermore, a very strong anchoring bond is formed between the polyolefin adhesive coatings and the polyolefin stretch wraps due to their chemical similarities. The adhesive properties of two of the amorphous polyolefins which were applied in toluene by the gravure process to 1 mil thick LOADMASTER® LLDPE film were evaluated by quantitating the peel strength (force in g/cm of film width) required to separate adhered strips of the film. In Table 1 the adhesion between pairs of the adhesive-coated film surfaces is compared with the adhe¬ sion between one coated and one uncoated film surface. The "base-line" levels of adhesion between two surfaces of the uncoated film are also provided. It is noted that the uncoated LLDPE film chosen for this study is manufactured with one side having cling and the other not. Since the adhesive was always applied to the cling side of the film, the surfaces tested for adhesive strength were the exposed surfaces existing after the coating step, i.e., the adhesive-coated surface and the uncoated non-cling surface of the film. Data are presented for the adhesive coatings produced using D-147, an amorphous terpolymer polyolefin, and D-117, an amorphous propylene-ethylene copolymer, both of which can be obtained from Eastman Chemical Company or obtained as generic equivalents manufactured by standard procedures well known to those skilled in the art. Useful amounts of the adhesives applied by the solvent coating process were found to range from approximately 0.2 g/m to approximately 5 g/m of film surface. It is evident in Table 1, that for the D-117 adhesive the cohesive peel strength or force required to separate two film surfaces coated with the adhesive is approximately 3 - 5 fold stronger than the adhesive peel strength measured by the force required to separate the coated and uncoated sur- faces. The adhesive and cohesive peel strengths shown in Table 1, for the D-117 adhesive are nevertheless both suf¬ ficiently high to provide secure film bonding and liquid- tight stretch-closures for the purposes of the present invention. To be useful in this invention, the adhesive and cohesive peel strength as described above and in Table 1 must each measure at least 5 g/cm of film width.

Example 3:

Laminate-Modified PARAFILM® Stretch Film

As described above, one preferred embodiment of the present invention incorporates a waterproof and substan¬ tially solvent-resistant adhesive placed on one side of a polyolefin stretch film (such as linear low density poly¬ ethylene or polyolefin plus rubber) . The polyolefinic surface is inherently resistant to organic solvents and is furthermore free of organic solvent-extractible film addi¬ tives including tackifiers, lubricants, plasticizers and the like.

Some of the benefits of the above film structure may also be obtained by heat-laminating PARAFILM® together with a thin layer of polyethylene stretch film (e.g., 0.5 mil LLDPE film) . The resulting film structure can be used to provide organic solvent-resistant closures for con¬ tainers, provided that the PARAFILM® surface is oriented away from the solvent. The PARAFILM® layer provides its expected self-adhesive and elastic properties while the LLDPE layer provides the chemical barrier property. It is noted however, that the resulting laminated film structure has the temperature susceptibility of PARAFILM®, is rela¬ tively expensive to manufacture, ad film storage (in roll form) requires the presence of a non-adherent interleaf sheet.

Method of Use of the Film

The presently invented laboratory stretch film may be used for many of the same applications as PARAFILM®. Thus, it may be used to cover, wrap and stretch-seal test tubes, flasks, bottles, and other vessels made from glass. plastic or metal and containing any one of a variety of materials. The film is used in laboratories including scientific laboratories, environmental, agricultural and industrial testing laboratories, and hospital facilities which conduct research experiments, analytical procedures, diagnostic tests and the like. The film is flexible, self-sealing and essentially impermeable to moisture. Thus, the film is beneficially used to reduce water loss due to evaporation from culture flasks, petri dishes and other vessels holding aqueous solutions, aqueous frozen solids, or naturally hydrated materials such as plant and animal specimens. Since one of the preferred structural materials for the stretch-film is LDPE which is imperme¬ able to both the liquid and vapor phases of most organic solvents, the film is also beneficially used to seal ves¬ sels containing such solvents to reduce their evaporation. The film is also used as a securing and restraining film placed over solid closures to prevent their accidental opening and reduce passage of water and chemical vapors either around or through such closures. Thus, screw-cap, snap-cap and plug-type closures are beneficially secured by the presently invented stretch wrap during sample incu¬ bation, long-term sample storage and shipping, for example. Also, used as a total covering and enclosure sheet for glass containers and other fragile items, the present wrap is useful in reducing the frequency of breakage. In the event of container breakage, any liquid spillage and resulting damage to the surrounding.materials is reduced by the presence of this sealing film around the container.

The film of the present invention may be applied to an article using several alternative methods. Unlike PARAFILM®, the film possesses a pressure-sensitive adhesive on at least one of its surfaces so that it adheres to itself and to other objects regardless of whether the film has been stretched. A sized portion of the film which is sufficient to cover either a portion or the whole of the article, is placed over or around the article. For example, to seal a container opening, a square of film is taken from the roll and placed over the opening, such as a beaker, flask or test tube opening. The film perimeter which extends outward on all sides of the opening is pressed downward and inward toward the article to adhere the film to the article.

Introducing the additional step of stretching the film while placing it over the article is useful in form- ing a tight seal over or around the article. For example, liquid-tight seals are conveniently formed over a variety of glass, plastic and metal container openings by stretch¬ ing the film over the opening and then wrapping the overhanging film perimeter around the circumference of the opening while the film is still under the tension. Sealing of a covered petri dish, containing a gelled nutrient medium is accomplished in a somewhat different manner. An elongated rectangular segment of the film is stretched around the circumference of the dish (including the joint) and is pressed inward to secure the adhesive film to itself. The same stretch-wrapping technique is used to secure closure caps and stoppers on containers such as test tubes being incubated, stored or shipped.

Introducing the additional step of overlapping a portion of the film on itself such that at least one over¬ lapping surface of the film which carries the adhesive agent contacts another surface of the film, is a useful and often important part of placing the film over the article being sealed. This overlapping step enables the user, by subsequently pressing these overlapping surfaces of the film toward the article, to adhere the film to itself as well as to the article such as a petri dish, flask, or test tube.

A one side adhesive-coated LDPE-based stretch film of the present invention is used in several different ways to seal a variety of articles. With the adhesive surface oriented inward, a petri dish for example, is sealed as described above. The adhesive surface is first stretched around and pressed onto the circumference of the petri dish after which the film, having already circled the dish, overlaps itself thereby permitting bonding of the film to itself. The same stretch film (one side adhesive- coated, the other side flee from constituents which can dissolve in or react with an organic solvent or caustic agent) is used to cover containers holding organic solvents such as benzene and chloroform which may slowly dissolve some of the adhesives of the present invention. In this case, the adhesive surface of the film is prefer¬ ably oriented away from the container article and its solvent. The film is first placed over the container opening, then stretched down and around the opening, and finally pressed inward toward the container to secure the film. The adhesive coating bonds the film to itself, maintaining stretch-tension in the film including that portion of the film providing the liquid-tight seal over the container opening. There are many uses for the present film, many of which do not require film stretching to achieve sealing. For use in which the pressure-sensitive adhesive surface of the film is oriented toward an article, the seal may be achieved by contacting the film to itself and pressing the film together to obtain a "sealed envelope" or pouch type of structure. Alternatively the adhesive film surface may be used to contact and seal a smooth-lipped or smooth- flanged opening such as found on glass and plastic flask and bottle openings and weighing dishes for example. When the film is pressed firmly onto these articles, an essentially water vapor-impermeable seal is obtained.

The range of temperatures which can be tolerated by the new stretch film are much greater than with PARAFILM®. As previously indicated, solvents such as carbon tetra- chloride, chloroform, and ethyl ether (which dissolve PARAFILM®) do not affect the new film. Temperatures between 60° and 100°C (which produce in failure of PARAFILM®) are also well tolerated by the new film. Thus, stretch-film covered test tubes may now be safely incubated in a boiling water bath.

For other applications requiring visual inspection or optical measurements through a film, where the lack of transparency and substantial thickness of PARAFILM® (5 mils) may be problematic, the presently invented films have a glass-like clarity and are only about 1 mil in thickness. In certain working environments, the presence of an interleaf sheet (which prevents irreversible self-adhesion of PARAFILM® during storage) is inconvenient or problem¬ atic. For example, in the darkroom environment it may be difficult to see and remove an interleaf sheet. Further- more, when samples must be handled and sealed rapidly (for example, sequential stretch-sealing of test tube samples during the time course of an experiment) , the removal of an interleaf sheet is inconvenient. Therefore the elimi¬ nation of the interleaf sheet with the new stretch film is a significant improvement over PARAFILM®.

One of the preferred structures for the film of the present invention utilizes a LLDPE stretch wrapping film which has been coated on one side with a solvent-resistant adhesive. In contrast to PARAFILM® which is a homogeneous film (without "sidedness") , the new film can be used with its adhesive surface oriented either inward toward the article being covered or outward. Applicant has discov¬ ered that a one side adhesive-coated LLDPE stretch film (e.g. Borden LOADMASTER® film) provides a liquid-tight seal when the film is stretched and self-adhered over a test tube or flask opening according to the method of the present invention regardless of whether the adhesive is oriented outward or inward. This result allows the user of the new film to choose the optimal film orientation to meet the needs of a particular physical or chemical appli¬ cation. For example, when covering a sample containing an adhesive-solubilizing organic solvent such as chloroform, the adhesive surface of the film can be oriented outward. If however maximum adhesion of the film to the container is required (e.g. , for long term storage of liquids with minimal evaporation, or for shipping of samples) then the adhesive surface would be oriented inward towards the container.

Table l

Adhesive Strength for Adhesive-Coated

1 mil LLDPE Stretch Wrap

Δ Measured as the force required to pull apart the adjacent ends of a pair of adherent strips of film in a peeling mode, i.e., 180° bending over of one strip followed by pulling the strips in opposite directions. + l mil thick LLDPE film with "cling" on one side only * adhesion between uncoated non-cling surface of film and coated surface Other embodiments are within the following claims.

Claims

1. A method for covering and sealing an article, comprising the steps of: providing a sized portion of a waterproof, adhesive, self-sealing, and heat-resistant stretch film comprising a polyolefin, said film having an upper and a lower surface, and a softening temperature in excess of 100°C, wherein at least one said surface of said film comprises an adhesive agent able to bond the film together during stretch-wrapping of the film and wherein at least one surface of said film is free from constituents which can dissolve in or react with an organic solvent or a caustic agent, wherein said sized portion of said film is sufficient to cover a portion of said article, placing said film over at least said portion of said article, and pressing said film toward said article to adhere said film to said article.

2. The method of claim 1, wherein said polyolefin is selected from polyethylene and rubberized polyethylene- based stretch films.

3. The method of claim 2, wherein said polyethylene comprises linear low density polyethylene.

4. The method of claim 2, wherein said rubberized polyethylene comprises ethylene-propylene rubber.

5. The method of claim 1, wherein said stretch film has a thickness of between 0.0005 inches and 0.0025 inches.

6. The method of claim 1, wherein a one-inch wide strip of said film is caused to elongate when a stretching force of at least 0.5 lb. is applied to said film.

7. The method of claim 1, wherein said stretch film is constituted and configured to allow at least 200% elongation before breakage of said film.

8. The method of claim 1, wherein said providing step comprises providing at least one surface of said film with a coating of an adhesive agent by application of a solvent-borne adhesive or by extrusion.

9. The method of claim 8, wherein said adhesive agent has a softening temperature in excess of 100°C.

10. The method of claim 8, wherein said adhesive agent is substantially resistant to an organic solvent and unreactive with a caustic chemical.

11. The method of claim 8, wherein said adhesive agent is a pressure-sensitive amorphous polyolefin..

12. The method of claim 11 wherein said amorphous polyolefin is selected from an amorphous terpolymer, an amorphous propylene-ethylene copolymer, and an amorphous propylene-hexene copolymer.

13. The method of claim 1, wherein said adhesive agent acts to resist peeling separation of two adhered surfaces of said film by a force of resistance equal to between 5 and 200 grams per centimeter of film width.

14. The method of claim 8, wherein said coating is applied by a solvent-borne adhesive application method, and comprises between 0.2 and 5.0 grams of adhesive agent per square meter of one surface of said film.

15. The method of claim 9, wherein said coating is applied by a solvent-borne adhesive application method, and comprises between 0.2 and 5.0 grams of adhesive agent per square meter of one surface of said film.

16. The method of claim 10 wherein said coating is applied by a solvent-borne adhesive application method, and comprises between 0.2 and 5.0 grams of adhesive agent per square meter of one surface of said film.

17. The method of claim 11, wherein said coating is applied by a solvent-borne adhesive application method, and comprises between 0.2 and 5.0 grams of adhesive agent per square meter of one surface of said film.

18. The method of claim 12, wherein said coating is applied by a solvent-borne adhesive application method, and comprises between 0.2 and 5.0 grams of adhesive agent per square meter of one surface of said film.

19. The method of claim 1, wherein said placing step comprises stretching said film over at least a portion of said article.

20. The method of claim 1, wherein only one surface of said film comprises said adhesive agent and the second surface of said film is free from said constituents.

21. The method of claim 1, wherein prior to said placing step, said surface comprising said adhesive agent is oriented facing away from said article.

22. The method of claim 19, wherein prior to said placing step, said surface comprising said adhesive agent is oriented facing away from said article.

23. The method of claim 20, wherein prior to said placing step, said surface comprising said adhesive agent is oriented facing away from said article.

24. The method of claim 1, wherein prior to said placing step, said surface comprising said adhesive agent is oriented facing toward said article.

25. The method of claim 19, wherein prior to said placing step, said surface comprising said adhesive agent is oriented facing toward said article.

26. The method of claim 20, wherein prior to said placing step, said surface comprising said adhesive agent is oriented facing toward said article.

27. The method of claim 1 wherein said placing step comprises overlapping the film on itself, such that at least one overlapping surface of said film which comprises said adhesive agent contacts the other overlapping surface of said film and, wherein said pressing step comprises pressing the overlapping surfaces of said film toward said article to adhere said film to itself.

28. An article covered with a waterproof, adhesive, self-sealing, and heat-resistant stretch film comprising a polyolefin, said film having an upper and a lower surface, and a softening temperature in excess of 100°C, wherein at least one said surface of said film comprises an adhesive agent able to bond the film together during stretch-wrapping of the film, and wherein at least one surface of said film is free from constituents which can dissolve in or react with an organic solvent or a caustic agent.

29. A waterproof, adhesive, self-sealing, and heat- resistant stretch film comprising a polyolefin, said film having an upper and a lower surface, and a softening temperature in excess of 100°C, wherein at least one said surface of said film comprises an adhesive agent able to bond the film together during stretch-wrapping of the film, and wherein at least one surface of said film is free from constituents which can dissolve in or react with an organic solvent or a caustic agent.