Thermoformable multilayer polymeric film

ES 2 289 757 T3

DESCRIPTION

Thermoformable multilayer polymeric film.

This invention falls within the field of thermoplastic polymer films that can be used for packaging products, especially food products, such as meat and cheese. More specifically, the invention falls within the field of thermoplastic polymer films that are thermoformable, that is, they can be softened by the action of heat, deformed to a desired shape and cooled again.

Polymeric films are widely used in the packaging field for product packaging, especially in the food industry. Films used for food packaging generally contain multiple layers, each of which confers on the film as a whole a certain desirable physical or chemical property. For example, an "oxygen barrier layer" serves to prevent packaged food from spoiling. The properties of the oxygen barrier are necessary to protect many packaged foods during prolonged storage. Additionally, an "anti-maltreatment layer" can serve to protect the packaged product from physical abuse or stresses caused by normal handling of the product during packaging, shipping, or marketing.

Although each layer of a polymeric film serves a specific purpose, the film itself must be stable and the layers must be held together and not laminated.

Ethylene-vinyl alcohol copolymers (EVOH), also known as hydrolyzed or saponified ethylene-vinyl acetate copolymers, are included among the polymers typically used as a barrier layer. In addition to its desirable properties as a barrier to oxygen and other gases, EVOH is also an effective barrier against odors, fragrances, solvents, and oils. EVOH also has good processing properties, meaning that it is relatively easy to process as a single or multi-layer film compared to other polymers. EVOH and EVOH copolymers are generally classified according to their ethylene content, for example in mole percent. Typically, as the percentage of ethylene increases in relatively low humidity applications, its gas barrier properties decrease, moisture barrier properties increase, and the resin exhibits greater processability. With higher levels of humidity, for example with the levels common in meat packaging, 90 to 92% humidity, a higher content of ethylene translates into an increase in the properties as a barrier against humidity.

However, in general there are some difficulties with the use of EVOH copolymer in polymeric films. For example, compared to other resins, EVOH copolymers have poor impact resistance, poor resistance to flex cracking, and poor stretchability.

In order to protect the EVOH layer, which as just mentioned offers poor resistance to impact and flex cracking, it is necessary to add additional layers to cover the EVOH layer. One of the layers that can be used to protect the EVOH layer is a layer that includes a polyamide. A polyamide is a high molecular weight polymer that has amide linkages throughout the molecular chain structure. Nylon-type polyamides, which are synthetic polyamides, exhibit favorable physical properties in terms of high strength, rigidity, and abrasion resistance.

In the manufacture of polymeric films it is known to obtain a multilayer film with a barrier layer of EVOH copolymer in the middle of two layers comprising a nylon-type polyamide. The following patents are of interest to illustrate the prior elaboration of polymeric films with a central layer of EVOH between two layers of a nylon.

Patent No. 4,284,674, owned by Sheptak, describes a polymeric thermal insulation product with a core layer of EVOH adhered to each side of a layer of nylon. The nylon layer is adhered to another layer of a polyolefin, such as an oriented polypropylene.

Patent No. 4,355,721, owned by Knotr, describes at column 5, lines 44 to 52, a multilayer polymeric film for food packaging with a central barrier layer of EVOH in the middle of two layers of nylon. On one side of one of the nylon layers, an adhesive layer and a high-density polyethylene (HDPE) sealant layer are arranged.

Patent No. 4,640,852, owned by Ossian, describes a multilayer polymeric film with a core layer of EVOH in the middle of two layers of nylon. The film may have an additional fourth and fifth layers of an adhesive and a heat sealable (heat sealable) polymer, arranged on one side of the nylon layers, and a sixth and seventh layers of an adhesive and a linear low-density polyethylene ( LLDPE) or a propylene-ethylene copolymer disposed on the other nylon layer.

Patent No. 4,695,491, owned by Kondo, describes a film for packaging a heat shrinkable compound. The film has a central layer of EVOH surrounded on one side by a first layer of a polymer with low permeability to hot water and on the opposite side an outer layer of an antiblocking polymer. The outer layer can be a nylon.

ES 2 289 757 T3

Patent No. 4,729,926, owned by Koteles, describes in column 4 a polymeric film with the following structure:

bonding / nylon / EVOH / nylon / bonding / LDPE / primer / outer coat.

The outer layer can be any of the different polymeric products capable of acting as a barrier against materials.

Patent No. 4,735,855, owned by Wofford, describes a seven-layer thermoformable polymeric laminate with the following structure:

anti-mist coating / bonding / nylon / EVOH / nylon / bonding / sealant.

The sealant layer can be any of a number of sealants, including an ionomer. The anti-maltreatment layer also serves as a moisture barrier layer.

Patent No. 4,746,562, owned by Fant, describes a seven-layer polymeric film with the following structure:

LLDPE / junction / nylon / EVOH / nylon / junction / LLDPE.

Each of the outer LLDPE layers also comprises an antiblocking agent.

Shah's Patent No. 4,755,419 describes an oxygen barrier oriented seven layer heat shrink film of the following structure:

blend / bond / nylon / EVOH / nylon / bond / blend.

The blend used for the outer layers can be a blend of LLDPE, LMDPE, and ethylene vinyl acetate (EVA). Alternatively, the outer layers may include an ethylene-propylene or polypropylene copolymer blend.

Patent No. 4,788,105, owned by Mueller, describes an oxygen barrier film that is adhered by an adhesive layer to a second film that includes nylon. The film also contains an outer layer of LLDPE.

Patent No. 4,816,304, owned by Nohara, describes a multilayer gas barrier container, with a core EVOH layer in between two nylon layers, an outer polyester layer and an inner polyester layer.

Patent No. 4,815,592, owned by Ossian, describes a core layer of EVOH in the middle of two layers of nylon. The film can also include a fourth layer of an adhesive and a fifth layer of a heat sealable polymer. In an alternative application the film may also include a sixth layer of an adhesive and a seventh layer of a heat sealable polymer disposed over the fifth layer.

Patent No. 4,833,696, owned by Iwanami, describes a laminate with improved resistance to flex cracking, stretching and molding ability, and excellent gas impermeability. The laminate includes a first layer of an EVOH copolymer and a thermoplastic polyester and a second layer of a composition consisting of a group containing nylon.

Patent No. 4,855,178, owned by Langley, describes a chemical barrier fabric in which a textile material is laminated in a multilayer film of one layer of EVOH between two layers of nylon. A heat-sealable polyethylene layer is arranged on the outside of the laminated material.

Patent No. 4,909,726, owned by Bekele, describes a multilayer polymeric film for packaging smoked fish with the following layer structure:

heat seal / anti-malt coating / bonding / nylon / EVOH / nylon / bonding.

The heat seal layer can be selected from a group consisting of ethylene-alphaolefin copolymers, low-density polyethylene (LDPE), and ethylene-ester copolymer and can include a release agent. The anti-maltreatment layer can be a very low density polyethylene or, alternatively, an ionomer.

Patent No. 4,983,431, owned by Gibbons, describes in Figure 3 a five-layer polymeric film that is laminated to a substrate such as cardboard. The film has the following layer structure:

ionomer / nylon / EVOH / nylon / ionomer.

ES 2 289 757 T3

One ionomer layer is laminated to the substrate while the second ionomer layer is coated with an LDPE layer.

Patent No. 4,937,112, owned by Schirmer, describes a multilayer extruded polymeric film for use in packaging smoked fish. The film has a first outer layer of a heat sealable polymeric resin, such as LLDPE, a first inner layer that includes a high molecular weight polymeric material such as LDPE, HDPE, or EVA, a second inner layer that includes a nylon, and a second outer layer that includes a self-weldable polymeric material.

Patent No. 4,999,229, owned by Moritani, describes a multilayer gas barrier polymeric film with an interlayer whose composition is 50 to 97 percent by weight EVOH and 45 to 3 percent by weight of nylon. As stated in Example 1, it is possible to arrange a nylon layer in contact with the intermediate layer.

Patent No. 5,068,077, owned by Negi, describes a multilayer polymeric film with a barrier layer of 70 to 95 weight percent EVOH and 5 to 30 weight percent nylon. The barrier layer can be placed in the middle of two layers of nylon.

Patent No. 5,194,306, owned by Blatz, describes a polymer blend consisting mostly of amorphous nylon and minority of EVOH for use as a gas barrier in a multilayer polymeric film. As an alternative application in column 6, lines 8 to 11, the invention may include a two-layer structure with one of them substantially EVOH and the other substantially amorphous nylon. At column 8, Example 20 shows a three-layer structure that includes a core layer of EVOH amid two layers of nylon.

DE-A-41 30 486 describes a 5-layer coextruded biaxially stretched casing, comprising at least 3 layers of polyamide.

Summary of the invention

A single structure multi-layer transparent polymeric film including a core EVOH layer in the middle of two nylon layers, or two inner nylon layers and an outer nylon layer, has been found to constitute a layered thermoformable polymeric film. Multiple improved. The films of the invention are coextruded and then blown into a tubular shape. The films are then quenched by means of water.

Films of the invention exhibit improved "shrinkage" or "memory" physical properties over prior film techniques. The improved physical properties mean that after the film has been used in packaging a product, it shrinks or wraps tightly around the product. The films of the invention form a much tighter wrapper and hold the tension for a longer period of time than that obtained with prior film techniques.

The films also have better gloss properties, greater transparency, and less haze than those obtained with prior techniques. The resulting film of the invention produces a packaging material with superior structure and appearance. The advantages of the films of the invention are especially evident in thermoformed applications.

The improved physical properties and appearance of the films of the invention are believed to be due to the water quench method used to form the films. The films are cooled by the application of air while the coextruded film exits the die nozzle. After air cooling, the film is re-cooled by applying water in direct contact with the film.

The film includes first and second nylon core layers. The nylon layers preferably contain 5 to 35 percent by weight of an amorphous nylon copolymer mixed with one or more other different nylons. Both the first and second nylon layers can also include a nucleating agent.

Disposed between the first and second nylon layers is a third layer of an adhesive or bonding resin. The adhesive resin can be an anhydride modified polyolefin, such as EVA or LLDPE based adhesives, or any other of the various polymeric adhesives commonly used in the multilayer filmmaking art. The fourth and fifth layers of the adhesive are arranged in contact with the first and second layers of nylon, respectively.

A sixth outer layer of nylon is placed in contact with the fourth layer of an adhesive. The sixth layer, similarly to the first and second nylon layers, preferably contains 5 to 35 percent by weight of an amorphous nylon copolymer that is mixed with one or more other nylons and may include a nucleating agent and an agent. anti-blocking.

ES 2 289 757 T3

The seventh layer of the film, the sealant layer, preferably contains a mixture of LLDPE and LPDE. The sealant layer can also include EVA, linear ultra-low density polyethylene (ULDPE), EMA, eAa, EMAA, an ionomer, or mixtures of any of these polymers.

This thermoformable multilayer polymeric film does not contain a core EVOH layer.

The films of the invention can be of any thickness, although preferably comprised in the range of 50.8 pm to 254 pm (2 to 10 thousandths of an inch) and more preferably still between 63.5 pm and 190 pm (2.5 to 7.5 thousandths of an inch).

Brief description of the figures

Figure 1: shows a typing of the films of the invention; Y

Figure 2: shows a method for making films of the invention.

Detailed description of the invention

Figure 1 shows a typing of the films of the invention. In this typing, there is no central layer of EVOH.

In this typification, the film can have any thickness, although preferably said thickness is from 50.8 to 254 pm (2 to 10 thousandths of an inch) and more preferably still from 63.5 to 190 pm (2.5 to 7 , 5 thousandths of an inch).

Layers 21 and 22 of the film are nylon, preferably an amorphous nylon copolymer mixed with a nucleated nylon homopolymer and, more preferably still, 5 to 35 percent amorphous nylon copolymer. The amorphous nylon copolymers of layers 21 and 22 are mixed with various other nylons.

An amorphous nylon copolymer is a particular type of nylon polymer that differs from crystalline or semi-crystalline nylons. Amorphous nylon copolymers are characterized by their lack of crystallinity, which can be manifested by the absence of an endothermic crystalline melting point in a test performed according to ASTM D-3417 with a Differential Scanning Calorimeter (DSC).

A lower degree of crystallinity indicates the ease with which a film can be further manufactured, such as by thermoforming, orientation in the solid state, lamination, or the like. The degree of crystallinity is also related to the brittleness of the film and, therefore, to the tendency of the film to break or crack when subjected to physical or thermal shocks. Physical or thermal shocks generally occur during subsequent conversion operations, such as in thermoforming and handling involved in the manufacture of packaging, filling and sealing of the same, and shipping the packages to the product distribution system. once filled and sealed.

Generally, an amorphous nylon copolymer is a semi-crystalline polymer that assumes an amorphous structure due to the sudden cooling of the mixture, which prevents the development of a crystalline structure and produces a transparent solid. Alternatively, chemical modifications can be made to the chemical structure of the polymer to significantly reduce or eliminate the ability of the polymer chains to arrange themselves into an organized crystal structure without the need for quenching the melt.

In the polymeric filmmaking art, amorphous nylon copolymers, like other nylons, are known to be ineffective as moisture barrier layers.

An example of an amorphous nylon copolymer suitable for use in the films of the invention is Grivory^®G21, manufactured by EMS - American Grillon, Inc. of Sumter, South Carolina. Grivory® G21 has a vitrification temperature of approximately 125 ° C, by the DSC method; a specific gravity of approximately 1.18, determined according to ASTM D792; 1.29% moisture absorption after 24 hours of immersion, measured according to ASTM D570; and a melt flow index of 90 ml / 10 min., measured according to DIN 53735.

Grivory® G21 physical properties include a tensile strength of 10,400 psi (72 MPa), determined according to ASTM D638; an elongation at break of 15%, measured according to ASTM D638; a flexural strength of 17,300 psi (119 MPa), according to ASTM D790; a flexural modulus of 416,000 psi (2,868 MPa), in accordance with ASTM D790; and a Shore hardness of 80 on the D scale.

In a preferred typing, the amorphous nylon copolymer from layers 21 and 22 is mixed with a nucleated nylon homopolymer.

In a preferred typing, a nucleating agent is added to the amorphous nylon copolymer, or the amorphous nylon copolymer is mixed with a nucleated nylon homopolymer. In an even more preferred typification, the

ES 2 289 757 T3 nucleated homopolymer is a high viscosity nucleated homopolymer, with a viscosity of about 120 or higher. One such nucleating agent is a high viscosity homopolymer, such as 3909 FN, manufactured by the Allied Signal Chemical Company.

Layer 20, arranged between layers 21 and 22, is a polymeric adhesive. Additionally, and in contact with layers 21 and 22, layers 23 and 24 are provided with a polymeric adhesive. Layers 20, 23 and 24 can be one of several polymeric adhesives commonly used in this art, for example an anhydride grafted polyolefin adhesive.

Layer 25 includes an outer nylon layer, which may preferably contain an amorphous nylon copolymer. Like layers 21 and 22, in a preferred typing, layer 25 contains about 5 to 35 percent of the amorphous nylon copolymer. The amorphous nylon copolymer in layer 25 is mixed with various other nylons. In a preferred typing, the amorphous nylon copolymer is mixed with a nucleated nylon homopolymer. The layer can also include an antiblocking agent that includes antiblocking agents common in the art of polymeric film making, such as inorganic spheres (especially those derived from a combination of silica and aluminum), talc, diatomaceous earth, silica, calcium carbonate or other particulate material, or combinations of any of these agents. The antiblocking agent serves to increase the roughness of the surface of the film, thereby lowering the coefficient of friction between films. The antiblocking agent in layer 25 may be contained in a nylon carrier composition. Such a nylon carrier composition is disclosed in Patent No. 5,109,049. This description is included here for reference. As described from column 2, line 61 to column 3, line 6, of the reference patent, nylon carrier compositions are especially effective when used in the formation of an outer layer of a multilayer film for packaging. . In a preferred typing, the nylon carrier composition is used in an outer layer and a second outer layer is capable of forming a heat seal.

In one type of film made with the nylon carrier composition, a polymeric material containing 20 to 85 percent nylon is combined with a nylon carrier composition in a percentage ranging from 80 to 15 percent; said composition also includes an antiblocking agent. The nylon carrier composition, which can contain any of several types of nylon, including nylon 6, nylon 6,6, or nylon 6,66, comprises 35 to 80 percent of an antiblocking agent and 65 to 30 percent. nylon polymer percent.

Alternatively, the film of the nylon carrier composition may be a blend of 95 to 99.5 percent of the nylon polymer and 5 to 0.5 percent of a concentrated additive, in which is included the antiblocking agent incorporated in a second nylon polymer composition.

A preferred specific antiblocking agent to include in the nylon carrier composition is manufactured by Zeelan Industries of St. Paul, Minnesota, under the name "Zeeospheres." Zeeospheres are ceramic inorganic spheres that contain silica and alumina. For example, Zeeospheres is incorporated as an antiblocking agent in Reed Spectrum Antiblocking Nylon Carrier Composition No. 1081274. Layer 25 is not a moisture barrier.

In a preferred version of typing, each of the nylon layers 21, 22, and 25 includes a mixture of amorphous nylon copolymer and a nucleated homopolymer of nylon 6. In a more preferred typing, only two of the layers, with greater preferably layers 22 and 25 comprise the blend of an amorphous nylon copolymer and a nucleated nylon homopolymer.

Layer 26 is a sealant layer capable of forming a heat seal with various other polymeric materials. Layer 26 can include any of various polymers used in sealant layers, such as LLDPE, LDPE, EVA, EMA, EMAA, an ionomer, or a blend of such polymers. A preferred sealant for this typing is a blend of LLDPE and LDPE.

The films of the invention can be made by any of several methods common in the art of polymeric filmmaking. Preferably, the films of the invention are mixed by joint extrusion through a multilayer coextrusion die according to each of the structures of the layers of the different typifications to then be formed in the general manner indicated in expired Patent No. 3,337,663, owned by Taga, and also in expired Patent No. 3,090,998, owned by Heisterkamp; both patents are incorporated herein by reference. The films are coextruded in the form of a tube, where the first outer layer of the finished film is the inner layer of the tubular or tube form. The tube is then inflated by air intake, cooled, deflated, and rolled to form one or more coils.

Figure 2 shows a preferred method for manufacturing the films of the invention. A multilayer film according to any of the different typifications of the invention is coextruded through the hole 72 of the die mold 71 in the form of molten thermoplastic material 75. The molten thermoplastic material 75 is drawn down through the plates of deflation or crushing 80, in the direction of arrows A to B by laminators 73 and 74. The roller 73 rotates on its axis in a clockwise direction, while the roller 74 does so in the opposite direction. Molten thermoplastic material 75 is expanded into a tubular or "bubble" shape by inflation caused by injecting a volume of gas through gas outlet 76.

ES 2 289 757 T3

The gas outlet 76 is located in the nozzle mold 72 so that the air exiting through it inflates the bubble.

Air rings 77 are located on the outside and / or inside of the bubble. As the molten thermoplastic material 75 swells to form a bubble, the application of air from the air rings 77 regulates the cooling and stabilizes the molten thermoplastic material. Air rings 77 can apply air at various temperatures and at various speeds. In addition to being cooled by the air from the rings 77, the molten thermoplastic material 75 is also cooled by the water rings or mandrels 78, which are also located on the outside and / or inside of the bubble. Like air rings, water rings 78 can apply water at different temperatures and speeds.

A liner 79 may be added to the nozzle that surrounds the bubble exiting the mold. The length and dimensions of the liner will affect the relative air pressures inside and outside the bubble.

The inventive film manufacturing method described above produces multilayer polymeric films whose physical properties are better than previously achieved with known techniques. These improvements are believed to be the consequence of the degree of control over the crystallinity of the polymeric film obtained by this method. The crystallinity is controlled first by the application of air through the air rings 77. The temperature and velocity of the air applied through the air rings 77 control the degree of crystallinity. Then, quenching caused by the water cooling method freezes the amount of crystallization obtained during air cooling in the finished polymeric film.

The water quenched films of the invention exhibit several improved physical properties relative to prior art films cooled by more conventional methods. For example, the films of the invention have improved transparency and processability.