Forming web for lining a rigid container

Abstract: A forming web suitable for lining the interior of a rigid or semirigid container, such as a box made from corrugated board, includes an outer sealable layer; an interior barrier layer comprising a polymeric material with low oxygen transmission; a polymeric layer which adds bulk to the web; and an inner adherent layer. The web can also include polymeric adhesive layers for bonding the outer sealable layer to the interior layer, and the bulk layer to the interior and inner adherent layers respectively. More than one bulk or barrier layer can be included in the present invention.

Description: The present invention relates to the packaging of products, especially food products, and more particularly to forming webs useful in lining rigid or semirigid containers. Rigid and semi-rigid containers made from corrugated paperboard and thermoplastic materials are available in the market place in a variety of sizes. These containers, with or without interior thermoplastic liners or exterior coatings or laminates, offer the packager a container which can withstand the weight of many products, accommodate the large volume of many products, and often provide stackability where necessary for shipping or storage. Examples of patents directed to containers include U. S. Patent No. 3,927,821 (Dunning) which discloses a paper board carton having a bottom panel, and side and end walls, with a lining film of plastic adhered to the inner surface of the paper board. British Patent Specification 1,136,885 (Andersson) discloses a container having an outer casing of comparatively stiff sheet material such as cardboard and an unbroken protective layer of thermoplastic material disposed in the casing. The casing comprises a bottom and a number of upright side walls. Some containers include horizontal flanges extending either towards the interior of the container or towards the exterior of the container. Examples of patents directed to such containers include U. S. Patent No. 2,660,362 (Schilling) disclosing a carton with horizontal flaps at each end thereof, and a cover sheet heat sealed to the flaps. U. S. Patent No. 3,866,816 (Bemiss) discloses a self supporting outer paper board tray and a plastic liner, as well as laterally extending flanges around the upper edge of the paper board or plastic liner. U. S. Patent No. 4,533,065 (Chazal et al) discloses a container having a cardboard portion, portions of synthetic material, and an externally extending flange around the upper edge of the container. Corrugated board containers internally lined with a thermoplastic liner are useful in many packaging applications, including food packaging. Conventional corrugated paperboard includes two outer liner boards, and an intermediate fluted board located between the two outer liner boards. Beneficial features of the board (rigidity and strength) are combined with the oxygen and moisture barrier properties of polymeric film. The thermoplastic liner should adhere to the inside surface of the corrugated board container with sufficient adhesion to prevent separation of the liner from the board surface during packaging, storage distribution and display. When making a container such as that disclosed in U. S. Patent No. 4, 722, 474 ( Dropsy ), using equipment such as that disclosed in U. S. Patent No. 4,862,671 (Lanoiselee et al) adhesion of the liner to the board can be affected by many factors such as temperature settings on the machine, the physical and chemical composition of the liner, the nature of the product packaged inside the container, the manner of distribution and storage of the filled container, ambient conditions, etc. The dimensions of the container are also important, since a container such as that disclosed in the Dropsy patent requires a thermoformable liner material which is required to form a deep pocket in the corrugated board shell. When using the containers just described for packaging some meat and cheese items, it is often advantageous to use modified atmosphere packaging (MAP) techniques well known in the art. MAP can extend the shelf life of some food items. The effectiveness of MAP packaging depends in part on being able to maintain the physical integrity of the package, and avoid the occurrence of pinholes or other breaks in the package materials. EP-A-175,451 discloses a flexible, multiple layer packaging film which comprises a first layer of a heat sealable polymer, a second layer of an olefin-based adhesive polymer, a third layer of ethylene vinyl alcohol copolymer or a polyamide-based polymer, a fourth layer of an olefin-based adhesive polymer and a fifth layer of an olefinic polymer. The inventor has discovered certain forming webs of the present invention offer particularly good adhesion to corrugated board, and also provide effective gas barrier for gases such as carbon dioxide, oxygen, or nitrogen used in modified atmosphere packaging methods. The present invention provides a forming web comprising: a) an outer sealable layer; b) an interior barrier layer comprising a polymeric material with a low oxygen transmission of less than 150cm³/m²/at/24 hours (ASTM 3985) at 1 mil (25.4 µm) thickness and at room temperature; c) a polymeric layer which adds bulk to the web which is selected from polystyrene, high impact polystyrene, a styrene copolymer, a polyolefin, a polyamide or a copolyamide; and d) an inner adherent layer capable of bonding to a paperboard or other container, said inner adherent layer comprising a polymeric material selected from ethylene acrylic acid copolymer, ethylene methacrylic acid copolymer, tackified ethylene vinyl acetate copolymer, chemically modified ethylene ethyl acrylate copolymer, chemically modified ethylene methyl acrylate copolymer, chemically modified ethylene vinyl acetate copolymer, chemically modified linear low density polyethylene, chemically modified high density polyethylene, chemically modified low density polyethylene, and chemically modified polypropylene. The present invention also provides a method of preparing a web as defined above comprising: a) coextruding an outer sealable layer, an interior barrier layer comprising a polymeric material with low oxygen transmission, a polymeric bulk layer and an inner adherent layer; and b) cooling the coextruded web. The present invention additionally provides the use of a forming web as defined above for lining rigid and semi-rigid containers. The present invention further provides a rigid or semi-rigid container which comprises a lining of a forming web as defined above. The forming web of the present invention may be used as a liner in a semirigid or substantially rigid container, and exhibits good adhesion to the interior surfaces of the container. The forming web exhibits good barrier properties with respect to gases typically used in modified atmosphere packaging, as well as gases, especially oxygen, in the ambient environment on the outside of the finished container. The forming web also exhibits resistance to flex or stress cracks and very low shrink and/or memory to prevent delamination of the forming web from the box. The forming web furthermore exhibits deep drawability, which is the property of the web which allows it to be theroformed into a relatively deep container. The forming web may, for example, further comprise a polymeric adhesive layer between the outer sealable layer (a) and the interior barrier layer (b). The forming web may, for example, also further comprise a polymeric adhesive layer between the interior barrier layer (b) and the bulk layer (c), and between the bulk layer (c) and the inner adherent layer (d). The forming web maybe prepared by a method comprising coextruding an outer sealable layer, an interior barrier layer of a polymeric material with low oxygen transmission, a bulk layer and an inner adherent layer, and adhesive layers between the outer sealable layer and the interior layer, between the interior layer and the bulk layer, and between the bulk layer and the inner adherent layer; cooling the coextruded web; and winding up the cooled web. The invention may be further understood by reference to the drawings in which: FIG. 1 is a cross sectional view of a forming web in accordance with the present invention; FIG. 2 is a cross sectional view of an alternative embodiment of a forming web in accordance with the present invention; FIG. 3 is a cross sectional view of an alternative embodiment of a forming web in accordance with the present invention; FIG. 4 is a cross sectional view of an alternative embodiment of a forming web in accordance with the present invention; and FIG. 5 is a cross sectional view of an alternative embodiment of a forming web in accordance with the present invention. Referring to Figure 1, a forming web 10 includes an outer sealable layer 11, an interior barrier layer 12 comprising a polymeric material with low oxygen transmission; a polymeric layer 13 which adds bulk to the web; and an inner adherent layer 14. The outer sealable layer is preferably a heat sealable polymeric material such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE), very low density polyethylene (VLDPE), ethylene vinyl acetate copolymer (EVA), and blends of these materials. Ethylene alkyl acrylate such as ethylene n-butyl acrylate copolymer (EnBA), ethylene ethyl acrylate copolymer (EEA), and ethylene methyl acrylate copolymer (EMA); as well as ethylene acrylic acid copolymer (EAA), ethylene methacrylic acid copolymer (EMAA), and ionomer are also suitable, though less preferred resins for sealable layer 11. The interior barrier layer 12 includes a polymeric material with low oxygen transmission characteristics, such as ethylene vinyl alcohol copolymer (EVOH); amorphous nylon; polyamides such as nylon 6, nylon 11, nylon 12, and nylon 610; copolyamides such as nylon 6,12, nylon 6,66 and nylon 6,69; and blends of these materials. By "low oxygen transmission" is meant a transmission, at 0% relative humidity, of less than 150 cm³/m² atmosphere/24 hours (ASTM D 3985) at one mil (25.4 µm) thickness and at room temperature; preferably less than 100 cm³/m²; more preferably less than 50 cm³/m². The bulk layer 13 includes as a polymeric material polystyrene, high impact polystyrene, and styrene copolymers (all of these collectively herein as "PS"); polyamide and copolyamide (collectively herein as PA); and polyolefins such as propylene homopolymer and propylene copolymer (collectively herein as PP), linear low density polyethylene (LLDPE), very low density polyethylene (VLDPE), high density polyethylene (HDPE), and low density polyethylene (LDPE). For thermoforming applications, very high molecular weight versions of the HDPE and LDPE are preferred over lower molecular weight versions. By "bulk" is meant a material that contributes to the tensile strength, bulkiness, abuse resistance, machinability, burn through resistance or other property of the final web. Preferred styrene copolymers are styrene acrylonitrile copolymer, styrene methyl methacrylate copolymer; acrylonitrile butadiene styrene, styrene butadiene copolymer (e.g. E6760, E6780, EC 2100, and EB 6755 available from Chevron, KR-10 and KK-36 from Phillips, and and Styron 484 available from Dow), styrene maleic anhydride copolymer, and styrene alkyl styrene copolymer. It has been found that styrene rubber materials such as styrene butadiene copolymer contribute to the abuse resistance of the web. Materials with higher butadiene content (e.g. KR-10 with 25% by weight butadiene, EC 2100 with 18% butadiene) are therefore preferred for increased abuse resistance. When the web of the present invention is used as a forming web for lining the cavity of a container, layer 14 acts as an adherent for holding the web to the container during the filling, processing, distribution, storage and display of the container. "Adherent" means a material which bonds to a paperboard or other container; or which can be sealed, such as by heat sealing, to other thermoplastic materials. "Tackified" means the blending, compounding, or copolymerization of a material to EVA to enhance its adhesive properties. In Figure 2, a polymeric adhesive layer 21 bonds outer sealable layer 11 to barrier layer 12. Adhesive layers are often desirable, and sometimes necessary in coextruded versions of the present forming web, in order to insure adequate interlaminar adhesion. Preferred adhesives are chemically modified: low density polyethylene, linear low density polyethylene, very low density polyethylene, ethylene vinyl acetate copolymer ethylene methyl acrylate copolymer, and polypropylene. In Figure 3, a polymeric adhesive layer 22 bonds interior barrier layer 12 to bulk layer 13. Polymeric adhesive layer 22a bonds bulk layer 13 to inner adherent layer 14. Layers 22 and 22a are preferably made from the same material, although different resins can be used. Preferred adhesives are ethylene vinyl acetate copolymer and ethylene methyl acrylate copolymer, which are chemically modified to enhance their adhesive properties. This chemical modification is achieved e.g. by the use of acid or acid anhydride graft copolymers blended with a polyolefin base polymer or copolymer. In Figure 4, a forming web is shown like that of Figure 3, but also including an additional bulk layer 13a. In Figure 5, a forming web is shown having an outer sealable layer 11, non-adjacent bulk layers 13 and 13a, an interior barrier layer 12, and an inner adherent layer 14. Polymeric adhesive layers 21, 22, 22a and 23 bond the first mentioned layers as shown in the drawing. The adhesive of layer 23 preferably includes one or more of the materials described above for layers 22 and 22a. The relative position of interior barrier layer or layers 12, and bulk layer or layers 13 and/or 13a can be reversed, and this may be desirable depending on the nature of the materials used for these layers, the intended end use of the web, and other factors. Thus, the bulk layer can be closer to the inner sealable layer than the interior barrier layer, or vice versa. Multiple layers of 12, 13 or 13a can be used, either as adjacent layers or as layers separated by one or more intervening layers. As an example, a bulk layer can be disposed on each side of the interior barrier layer (with or without intervening layers) as shown by example in Figure 5. The examples described below were made by coextrusion processes well known in the art, and specifically by single step coextrusion using a round or flat die in a cast process. Alternative processes such as extrusion coating or conventional lamination can also be used to make webs of this invention. Webs of this invention can be cross-linked by irradiation or chemical means, although this is not preferred for webs for thermoforming end uses. Table 1 identifies the resins used in the examples of Table 2. In Tables 1 and 2, "Tie" refers in its conventional sense to the polymeric adhesive layers, especially chemically modified polymers, which bond other layers of the web structure together. The term is also used herein to identify and describe the preferred resins for the inner adhering layer 14. In the preferred embodiment, these resins (generally not chemically modified) bond the forming web of the invention to a rigid or semirigid container, especially one made from corrugated paperboard. "EAO" refers to linear ethylene alpha olefin copolymers, especially linear low density polyethylene and very low density polyethylene. "EA" with respect to the E327 material means an ethylene acrylate-based material. In Examples 13 through 41, the bulk layer closest to the inner adherent layer contained about 10%, by weight of the layer, of a masterbatch which included the primary resin of that bulk layer (e.g. polypropylene in Example 32) and a white pigment concentrate. In Examples 24 through 41, the outer sealable layer contained about 5%, by weight of the layer, of an antiblocking agent having LLDPE or LDPE with diatomaceous silica. Although the exact thickness of the forming web can vary, it is preferably between 10 and 20 mils (254 and 508 µm) thick. The invention has been described herein as a forming web, but the term "forming web" is used herein to include webs of thermoplastic material, as structurally described herein, even when these materials are used in end uses not involving thermoforming or other forming steps.

Claims: A forming web comprising: a) an outer sealable layer; b) an interior barrier layer comprising a polymeric material with a low oxygen transmission of less than 150cm³/m²/at/24 hours (ASTM 3985) at 1 mil (25.4 µm) thickness and at room temperature; c) a polymeric layer which adds bulk to the web which is selected from polystyrene, high impact polystyrene, a styrene copolymer, a polyolefin, a polyamide or a copolyamide; and d) an inner adherent layer capable of bonding to a paperboard or other container, said inner adherent layer, comprising a polymeric material selected from ethylene acrylic acid copolymer, ethylene methacrylic acid copolymer, tackified ethylene vinyl acetate copolymer, chemically modified ethylene ethyl acrylate copolymer, chemically modified ethylene methyl acrylate copolymer, chemically modified ethylene vinyl acetate copolymer, chemically modified linear low density polyethylene, chemically modified high density polyethylene, chemically modified low density polyethylene, and chemically modified polypropylene. A web according to claim 1 wherein the outer sealable layer (a) comprises a heat sealable polymeric material selected from: a) a low density polyethylene; b) linear low density polyethylene; c) very low density polyethylene; d) ethylene vinyl acetate copolymer; e) ethylene alkyl acrylate copolymer; f) ethylene acrylic acid copolymer; g) ethylene methacrylic acid copolymer; h) ionomer; and i) blends of these materials. A web according to claim 2 wherein the ethylene alkyl acrylate copolymer is selected from: a) ethylene n-butyl acrylate copolymer; b) ethylene ethyl acrylate copolymer; and c) ethylene methyl acrylate copolymer. A web according to any one of the preceding claims wherein the interior barrier layer (b) comprises a polymeric material selected from: a) ethylene vinyl alcohol copolymer; b) polyamide; c) copolyamide; and d) blends of these materials. A web according to claim 4 wherein the polyamide is selected from: a) amorphous nylon; b) nylon 6; c) nylon 11; d) nylon 12; and e) nylon 610. A web according to claim 4 wherein the copolyamide is selected from: a) amorphous nylon; b) nylon 6,12; c) nylon 6,66; and d) nylon 6,69 A web according to any one of the preceding claims wherein the styrene copolymer of layer (c) is selected from: a) styrene acrylonitrile copolymer; b) styrene methyl methacrylate copolymer; c) acrylonitrile butadiene styrene; d) styrene butadiene copolymer; e) styrene maleic anhydride copolymer; and f) styrene alkyl styrene copolymer. A web according to any one of claims 1 to 6 wherein the polyolefin of layer (c) is selected from a) propylene homopolymer; b) propylene copolymer; c) liner low density polyethylene; d) very low density polyethylene; e) high density polyethylene; and f) low density polyethylene. A web according to any one of the preceding claims which further comprises a polymeric adhesive layer between the outer sealable layer (a) and the interior barrier layer (b). A web according to claim 9 wherein the polymeric adhesive layer comprises: a) low density polyethylene; b) linear low density polyethylene; c) very low density polyethylene; d) ethylene vinyl acetate copolymer; e) ethylene methyl acrylate copolymer; or f) polypropylene; each of which is chemically modified to enhance its adhesive properties. A web according to any one of the preceding claims which further comprises a polymeric adhesive layer between the interior barrier layer (b) and the bulk layer (c), and between the bulk layer (c) and the inner adherent layer (d). A web according to claim 11 wherein the polymeric adhesive layer comprises: a) ethylene vinyl acetate copolymer; or b) ethylene methyl acrylate copolymer; each of which is chemically modified to enhance its adhesive properties. A web according to any one of the preceding claims wherein the web is cross-linked. A web according to claim 13 wherein the web is crosslinked by irradiation. A method of preparing a web as claimed in any one of the preceding claims comprising: a) coextruding an outer sealable layer, an interior barrier layer comprising a polymeric material with low oxygen transmission, a polymeric bulk layer and an inner adherent layer; and b) cooling the coextruded web. A method according to claim 15 which further comprises coextruding adhesive layers between the outer sealable layer and the interior layer, between the interior layer and the bulk layer, and between the bulk layer and the inner adherent layer. Use of a forming web as defined in any one of claims 1 to 14 for lining rigid and semi-rigid containers. A rigid or semi-rigid container which comprises a lining of a forming web as defined in any one of claims 1 to 14.

Inventors: BEKELE SOLOMON, BEKELE, SOLOMON
Applicants: GRACE W R & CO, W.R. GRACE & CO.-CONN.
Application year: 1991
Main classification: B32B27
IPC codes: C09J123, B32B27, C09J7, B65D5