RADIO FREQUENCY SEALABLE EVOH-BASED PACKAGING STRUCTURE

Abstract: A combination of laminated materials which provides a non-foil packaging material (12) having enhanced sealability employing RF electric energy. In this combination, the relatively low, and otherwise inadequate, RF susceptibility of one or more layers of the laminate is enhanced to a value which provides fast and efficient heat sealing of the laminate to itself. This enhancement is accomplished through the incorporation within the laminated (12), in a selected order of lamination, of one or more layers (40) which tend to direct the heat generated by one or more inner layers (28) to that outer layer (14) of the laminate which is to be heated to an effective heat sealing temperature. Ethylene vinyl alcohol (EVOH) has been found to be particularly suitable as a primary RF susceptor in the combination of the present invention. A method for RF electric energy heat sealing of the packaging material is disclosed.

Description: RADIO FREQUENCY SEALABLE EVOH-BASED PACKAGING STRUCTURE BACKGROUND OF INVENTION This invention relates to structural materials used in the formation of packaging for consumer products, particularly food products, and more particularly liquid food products such as juices, and the methods for heat sealing these structural materials. Commonly, the products are packaged under aseptic conditions. Shelf life of a packaged product is of importance in the many consumer products. Much effort has been expended in the development of packaging materials and packaging methods and apparatus to achieve extended shelf lives for the products contained within the package. Packaging the product under aseptic conditions has been employed to extend shelf life of certain products, particularly food products, and more particularly liquid food products. Choice of the packaging material for the package also has been extensively examined. Currently, form, fill and sealing technology (at times referred to as FFS) is widely employed for forming a package, filling the package with a product, and thereafter sealing the package to complete the packaged product. FFS machines are widely used, particularly in the food packaging industry. FFS machines of the prior art perform heat sealing by means of (a) induction heat generation using low frequency, e.g. 1000 kilohertz, energy, which requires the inclusion in the packaging material of a conductive material such as aluminum foil, carbon impregnated polymers, or the like, (b) impulse sealing utilizing thermal conduction generated by establishing an external temperature gradient across the material being sealed (a typical design utilizes a heated element applied under intimate contact pressure to one or both sides the material layers) , and (c) ultrasonic sealing utilizing the generation of heat through high frequency mechanical vibration and friction at the seal interface to melt the adjoining surfaces. FFS presents at least three major concerns with respect the suitability of the packaging material which can be used in the FFS machine. First, the packaging material used in FFS machines (a) must provide barrier protection of the packaged product as will permit the product to withstand the normal conditions to which it is exposed between the time of its packaging and its consumption by a consumer, (b) must be compatible with the mechanical forces imposed on the packaging material in the course of its being folded and sealed in the course of its processing through the FFS machine, and (c) must be capable of repeatedly forming effective sealing of the formed and filled packages at high production rates such as up to about 15,000 packages per hour. Since FFS machines are designed to provide automatic and substantially continuous operation, the speed with which the folding, and particularly the sealing operation, is carried out is of critical importance . Packaging materials of the prior art which provide extended shelf life for their packaged product, particularly liquid food products, commonly are laminates. Most commonly, the laminate includes a substrate and at least an outer heat sealable layer. For various purposes other layers of differing materials may be added to the laminate. For example, it has been common heretofore to include within the laminate a layer of aluminum foil which serves the dual functions of a barrier to the transmission of oxygen into the package and as a susceptor for high frequency (HF) or induction heat sealing of the filled package. Foil layers in packaging material laminates are undesirable due to the cost of the foil, but more importantly, because foil-containing laminates, when folded, tend to develop breaks or ruptures at the locations of the folds due to the unyielding nature of the foil, with consequential loss of the protective nature of the foil layer. Ethylene vinyl alcohol (EVOH) has also been used heretofore as an effective oxygen barrier, particularly for providing extended shelf life for packaged food products. EVOH as used in the prior art as an oxygen barrier layer, has not been heretofore employed as an RF susceptor for heat sealing purposes. But rather, in the prior art, when using EVOH as the oxygen barrier layer, it has been necessary to incorporate into the laminate a different material as the RF or HF susceptor for heat sealing. It is desirable therefore to provide a packaging material which provides for enhanced protection for products packaged using the packaging material, often in the form of extended shelf life for the packaged product, which is compatible with existing FFS machines, which is relatively inexpensive and which is amenable to RF heat sealing. It is particularly desirable to provide a packaging material for forming a parallelepiped container for liquid food products. It is an object of the present invention to provide a packaging material which is heat sealable using RF energy. It is another object to provide a packaging material which is amenable to the formation of packages employing FFS machines . Other objects and advantages of the present invention will be recognized from the description contained herein, including the claims and the figures in which: Figure 1 is a schematic representation, taken in cross-section, of a fragment of a laminated packaging material embodying various of the features of the present invention; and Figure 2 is a schematic representation, taken in cross-section, of a fragment of two portions of a laminated packaging material disposed in facing relationship to define a seal interface therebetween and depicting the application of RF electric energy to the two portions. SUMMARY OF INVENTION The present inventors have discovered a combination of laminated materials which provide a non-foil packaging material having enhanced sealability employing RF energy. In this combination, the relatively low, and otherwise inadequate, RF susceptibility of one or more layers of the laminate is enhanced to a value which provides fast and efficient heat sealing of the laminate to itself. This enhancement is accomplished through the incorporation within the laminate, in a selected order of lamination, of one or more layers which tend to direct the heat generated by one or more inner layers to that outer layer of the laminate which is to be heated to an effective heat sealing temperature. By this means, the inventors have found that polymeric materials which have useful barrier and/or structural properties can be combined in a manner that provides an overall adequate RF susceptibility that permits the materials of the laminate to generate heat suitable to effect heat sealing of the packaging material. Ethylene vinyl alcohol (EVOH) has been found to particularly suitable as a primary RF susceptor in the combination of the present invention. In particular, it has been found that the interfacial sealing temperatures of contiguous layers of the present laminated packaging material is a function of the internal heat generation of the polymeric layer or layers and dissipation of this generated heat by conductive heat transfer. RF active polymers possess polar molecules that respond to time varying electric energy field excitation by the dissipation of the energy as heat via molecular rotations. The degree to which the polymer responds to the RF electric energy field is dependent upon material properties such as the dissipation factor (i.e., the product of the dielectric constant and the loss tangent) , as well as the strength and frequency of the electric field. The present inventors have observed that the heat sealing properties of the laminated material also vary with temperature as heat is generated. DETAILED DESCRIPTION OF INVENTION According to the present invention, two layers of a non-foil, RF susceptible composite material are brought into intimate contact (sealing surface to sealing surface facing relationship) , as in the FFS process of liquid packaging, held together under adequate contact pressure and then exposed to a RF electric energy field for a specified time period (with the electric field perpendicular to the plane of the composite material) . In this situation, heat is generated through energy losses due to the thermal motions of molecular dipoles in the polar components of the composite structure. Further, a temperature gradient is established across the thickness of each of the composites (of particular interest in the direction of the RF field) . Due to the plane of symmetry of the seal interface created by the twinned composite layers, the temperature of the interface increases as heat generated in the RF susceptor layer or layers flows toward both the interface and the outer surfaces of the composite material . Employing these newly discovered concepts, in the present invention there is provided a substrate layer which exhibits a first thermal conductivity and which is laminated with at least one polymeric layer which exhibits a thermal ■ conductivity which is higher than the thermal conductivity of the substrate layer, thereby providing that heat generated within the polymeric layer when it is subjected to RF electric energy tends to flow away from the substrate layer and toward the area of desired heat sealing. When the temperature of the polymeric layer reaches the point where the polymer transitions through the melt phase, a partial or complete fusing of the molecular chains at the seal interface occurs . In accordance with one aspect of the present invention, the degree to which fusion occurs, is dependent upon the clamping pressure within the seal area and the duration of the RF electric energy application. In the instance where a plurality of polymeric layers is provided between the substrate and the outermost sealing layer, it has been found that each of these polymeric layers undergoes some degree of melting and/or fusion during the course of the sealing operation. A time period must be included in the sealing cycle during which the clamping pressure is held after fusion of the interface to allow both the interface and the inner composite layers to cool below the melt phase to ensure adequate seal strength for the specific packaging application. As noted, during the sealing process, its has been found that the individual laminate polymer layers between the substrate layer and the seal interface may also transition through the melt phase. This discovery permits the selection of these intermediate polymeric layers to enhance the transfer of heat to the sealing interface. More specifically, the generation of heat in each individual layer (j) of the polymer structure by the RF electric energy field is described by the equation : P,-2- ■ f-k 0 • c-(T,f)-tan.(δ( ,f)) Eq. 1 where: j = subscript denoting each layer in the laminate material P = power input into each layer, j f = frequency of the applied electric field k 0 = constant A p = area of each layer perpendicular to the applied electric field thk 1 = thickness of each layer in plane to the applied electric field R = volume resistivity of each layer T = temperature of material V 0 = voltage drop across the laminate structure induced by the applied electric field ε ' = dielectric constant tan(δ)= dissipation factor and where each laminate layer's contribution c absorb , is equal to the properties described by: 2 ,... absorb/ T.O-RjCT.f) • ε j (T,f)-tanδ j (T,f) Eq. and the the ability to transfer heat, thermal diffusivity, (T) , is: Ec *' where: - Thermal diffusivity K = Thermal conductivity C p = Speci ic heat p = Density The product of (T)» c abβorb (T,f) describes the ability for a polymer to absorb the RF electric energy and transfer the heat to the seal inter ace. The higher the value, the more desirable the material is for the incorporation in the present packaging material structure. The product of α(T)» c Λbsoι;b (T,f) may also be treated in differential forms and integrated over appropriate temperature & frequency ranges to obtain an overall maximum value for a given layer. Selection of the present laminated packaging material structure involves determining the dielectric and thermal properties of each RF susceptible layer of the laminate as a function of temperature and frequency. The conduction heat transfer, Q, occurring within the present structure that enables sealing to occur is described by the following equation (convection and radiation heat transfer are considered negligible) : p(T)- c D (T)- [i-dτ ] Eq . 4 where: p = Density C p = Specific heat where : and the subscript (j) denotes the corresponding layer in the composite structure. Appropriate boundary condition temperatures are prescribed for the adjacent layers and the describing equations are solved for by maximizing the interfacial seal temperatures as a function of time, frequency, laminate thickness and position within the laminate. The present invention exploits the combined electrical and barrier characteristics of EVOH in particular to provide a simple, inexpensive, easily manufactured composite packaging material while providing functional RF electric heat sealability and barrier characteristics necessary for extended shelf life product containment. In the embodiment of the laminated packaging material 12 of the present invention as depicted in Figure 1, there is provided a sealant layer 14 which defines a first outermost layer of the depicted laminate and includes first and second planar surfaces 16 and 18. This sealant layer is disposed in facing relationship to, and bonded to a first planar surface 20 of a tie layer 22 , which, in turn, has its second planar surface 24 disposed in facing relationship to, and bonded to a first planar surface 26 of a RF susceptor layer 28. The opposite and second planar surface 30 of the susceptor layer is disposed in facing relationship to, and bonded to, a first planar surface 32 of a second tie layer 34. The opposite and second planar surface 36 of the second tie layer 34 is disposed in facing relationship to, and bonded to, a first planar surface 38 of a substrate layer 40. The opposite and second planar surface 42 of the substrate layer is disposed in facing relationship to, and bonded to, a first planar surface 44 of a gloss layer 46 (i.e., print surface). This gloss layer defines a second outermost layer of the depicted laminate. The laminate depicted in Figure 1 may be manufactured employing techniques well known in the industry such as extrusion or common lamination employing various bonding enhancing practices such as ozone treatment or corona discharge treatment of the facing surfaces of one or more of the layers of the laminate. In a preferred embodiment of the laminated packaging material of the present invention as depicted in Figure 1, that outermost layer 46 comprises gloss low density polyethylene (LDPE) and is intended to be that surface of the laminate which forms the outside exposed surface of the finished package. This surface preferably is printable and sufficiently tough to withstand normal treatment occurring during manufacture, boxing, transport and storage of a packaged product. A suitable gloss LDPE layer may be within the weight range of between about 5 and about 10 lbs/3000 ft 2 . The LDPE layer is disposed in facing relationship and bonded to a substrate layer 40 comprising paperboard. The substrate layer provides structural integrity to the packaging material, hence to the finished package. In the present invention, the material of construction of the substrate layer is also chosen for its low heat transfer characteristics so that it tends to retard the transfer of heat therethrough. Other materials of construction of the substrate layer may include like-functioning materials or similar materials, or combinations of these and/or other like-functioning materials. A suitable paperboard for the substrate layer may have a weight of between about 70 and about 350 lbs/3000 ft 2 . Employing a resin tie layer 34, a layer of ethylene vinyl alcohol (EVOH) 28 is bonded to the substrate 40. This tie layer 34 preferably is relatively thin and serves primarily to bond the EVOH layer to the substrate. One suitable resin for use as a tie layer between the paperboard substrate and the EVOH layer is Tymor 1220 E, available from Morton Chemicals. A suitable weight range of a resin tie layer is between about 0.25 and about 15 lbs/3000 ft 2 - The EVOH layer 28 of the depicted laminate functions as an oxygen barrier, and as discovered by the present inventors, as a suitable primary RF susceptor for heat sealing purposes . The actual EVOH coating weight necessary for barrier protection is shelf-life dependent. Typically about 5 lbs/3000 ft 2 of EVOH (at 29% by mole % ethylene content) is adequate for most long shelf-life applications. The maximum coat weight is dependent upon, but not limited to, RF electric energy heat generation requirements as dictated by the selection equations set forth hereinabove . Importantly, it has been found that the ethylene content of the EVOH may be chosen to provide adequate heat generation response to an RF field. In accordance with this aspect of the present invention, the ethylene content of the EVOH may range from about 10 to about 70 mole %. In the preferred embodiment, the ethylene is a homogeneous component of the copolymer. The lower mole percentages of ethylene provide greater barrier characteristics and enhanced RF susceptibility. Because processibility of the copolymer through an extruder degrades as the ethylene content decreases, ethylene mole percentages below about 27 mole % are not currently available in commercial quantities, so that at the present time 29 mole % ethylene is preferred for commercial application of the present invention. Employing a further resin tie layer 22, a layer of low density polyethylene (LDPE) or linear low density polyethylene (LLDPE) 14 is bonded to the surface 26 of the EVOH layer. This tie layer may be substantially identical to the tie layer 34. This layer of LDPE or LLDPE is the outermost layer of the laminate opposite the gloss LDPE layer 46 and defines the heat sealable layer of the laminate. Further this layer 14 functions as a moisture barrier, which function, in part, dictates the desired minimum thickness of this layer. Typically, about 10 lbs/3000 ft 2 of either LDPE or LLDPE will provide an adequate moisture barrier. The moisture barrier property of this layer is to be taken in conjunction with the combined laminate tensile properties necessary to obtain pinhole prevention, resistance to distribution damage, and adequate seal strength, typically a maximum of about 35 lbs/3000 ft 2 . Normal manufacturing extrusion coat weight tolerances typically are up to about ± 10%. With reference to Figure 2 in which like components of the depicted laminated packaging material are identified by primed numerals, in a typical RF electric energy heat sealing activity, first and second portions 60 and 62, respectively, of laminated packaging material of the present invention are disposed contiguously with the outermost heat sealing layers 14' and 14" thereof in facing relationship to form a seal interface 64. Pressure from a pair of sealing jaws 66 and 68 is applied to the facing portions 60 and 62 to urge these portions into intimate contact with one another. RF electric energy from a source thereof (not shown) is imposed across the contiguous layers including the seal interface. The lines 70 of the RF electric energy field are directed substantially perpendicular to the plane of the seal interface. In this configuration, heat is generated within each of the individual layers of each portion of the packaging material which contains polar molecules. In accordance with the present invention, the EVOH layers 28' and 28" are the primary RF susceptor layers of the two portions 60 and 62. However, the resin tie layers 22" and 22", 34' and 34" and the polyethylene-based layers 14' and 14" of each portion of the packaging material may contribute to the generation of heat when in the presence of the RF field. Because each of the substrate layers 40' and 40" has a low coefficient of heat conductivity relative to the EVOH layer, the tie layers and the heat sealing layers, the heat generated in these layers 40' and 40" tends to be transferred by conduction to the heat sealing layers 14' and 14". The sealing frequency range of the RF energy employed to effect a heat seal as depicted in Figure 2 typically will range between about 1 and about 300 MHZ depending upon the particular material structure of the laminated packaging material. The sealing range may be determined through use of the optimization equations provided hereinabove and will account for dielectric relaxation and thermal property changes of the materials of the laminated packaging material in the course of their exposure to the RF energy. About 65 MHZ has been found to be particularly suitable. A typical sealing cycle includes a time period to generate adequate heat to increase the temperature of the seal interface up to and above the melt point of the sealing layer and a time period for cooling of the sealing layer (and any other layers which have entered the melt phase) below the freeze points of the composite layers while they remain fixed under the clamping pressure of the sealing jaws. Sealing cycles typically are on the order of 50 to 500 milliseconds, employing the present invention. Whereas it is of importance in the present invention that the order of lamination, that is, relative positions of the several layers of the laminated packaging material, be chosen to enhance the transfer of heat from the primary RF electric energy susceptor layer (EVOH layer) to the heat sealing layer, it is to be recognized that various alternatives to the depicted embodiment exist. For example, paperboard is preferred as the construction materials for the substrate layer, but other material having the required structural integrity and relatively low heat conductivity may be employed. Consequently, the thickness of the substrate layer may be varied. RF heat sealing tests were conducted on laminates constructed as depicted in Figures 1 and 2 and which incorporated various levels of ethylene content in the EVOH layer of the laminate. These tests were conducted at 65 MHZ RF electric energy and at a pressure of about 300 psi. At ethylene contents of 29 and 44 mole % in the EVOH layer, the seals formed were strong and exhibited good poly-stretch. When tested to destruction, the seals most usually failed in the interlaminate bonds .

Claims: WHAT IS CLAIMED: Claim 1. A non-foil laminate packaging material sealable by radio frequency electric energy comprising a layer of ethylene vinyl alcohol which exhibits radio frequency electric energy susceptibility within a radio frequency range of between about 1 and about 300 MHZ to generate heat suitable for effecting sealing of the packaging material. Claim 2. The laminate packaging material of Claim 1 and including a layer of heat sealable material of either linear low density polyethylene or low density polyethylene. Claim 3. The laminate packaging material of Claim 1 wherein the radio frequency electric energy susceptibility of said layer of ethylene vinyl alcohol is sufficient to heat a polyethylene-based polymer to its fusion point within a time of less than about 500 milliseconds. Claim 4. The laminate packaging material of Claim 1 wherein the ethylene content of said ethylene vinyl alcohol is between about 27 and about 44 mole % . Claim 5. The laminate packaging material of Claim 3 wherein the ethylene content of said ethylene vinyl alcohol is about 29 mole %. Claim 6. A non-foil laminate packaging material sealable by radio frequency electric energy comprising a layer of ethylene vinyl alcohol which exhibits radio frequency electric energy susceptibility within a radio frequency range of between about 1 and about 300 MHZ sufficient to generate heat suitable for effecting sealing of the packaging material including a substrate layer and a layer of a polyethylene- based polymer, the layer of ethylene vinyl alcohol being disposed between and bonded to the substrate layer and the layer of a polyethylene-based polymer. Claim 7. The laminate packaging material of Claim 6 and including a further layer of polyethylene-based polymer disposed on and bonded to that surface of said substrate layer opposite the layer of ethylene vinyl alcohol. Claim 8. The laminate packaging material of Claim 6 wherein said layer of ethylene vinyl alcohol comprises between about 27 and about 44 mole % ethylene. Claim 9. The laminate packaging material of Claim 6 wherein the layer of ethylene vinyl alcohol comprises about 29 mole % ethylene. Claim 10. The laminate packaging material of Claim 6 and including first and second tie layers, said first tie layer being disposed between and bonded to facing surfaces of the substrate layer and the layer of ethylene vinyl alcohol and said second tie layer being disposed between and bonded to the facing surfaces of the polyethylene-based layer and the ethylene vinyl alcohol layer. Claim 11. A method for the sealing of a packaging material, especially suitable for food products and providing an extended shelf life for the food product comprising the steps of laminating a layer of ethylene vinyl alcohol with a layer of a polyethylene-based polymer to define a laminate in sheet form, said layer of ethylene vinyl alcohol having a susceptibility to radio frequency electric energy to generate heat that is sufficient to effect heat sealing of said polyethylene-based polymer, disposing, in facing relationship with their respective layers of polyethylene-based polymer contiguous, at least two portions of said laminated sheet intended to be heat sealed together, applying pressure to said two portions of said laminated sheet at least in the area thereof intended to be heat sealed, while applying said pressure, subjecting at least said two portions of said laminated sheet to radio frequency electric energy within the range of between about 1 and about 300 MHZ for a time sufficient to effect heating of said two portions of said laminated material to the fusion temperature of said layers of polyethylene-based polymer, ceasing the subjection of said at least two portions of said laminated sheet to radio frequency electric energy and, maintaining said pressure until said at least two portions of said laminated sheet cool to the solidification temperature of said layers of polyethylene-based polymer. Claim 12. The method of Claim 11 and including the step of limiting the time of application of said radio frequency electric energy to said at least two portions of said laminated sheet to less than about 500 milliseconds. Claim 13. The method of Claim 11 and including the step of selecting the ethylene content of said ethylene vinyl alcohol to between about 27 and about 44 mole %. Claim 14. The method of Claim 13 wherein the ethylene content of said ethylene vinyl alcohol is selected to be about 29 mole %. Claim 15. The method of Claim 11 wherein the weight of said layer of ethylene vinyl alcohol is selected to be about 5 lbs/3000 ft 2 . Claim 16. The method of Claim 11 and including the step of selecting the weight of said layer of polyethylene- based polymer to between about 5 and about 10 lbs/ft 2 . Claim 17. The method of Claim 11 and including the step of laminating said layer of ethylene vinyl alcohol to a paperboard substrate layer. Claim 18. The method of Claim 17 and including the step of selecting the weight of said paperboard substrate layer to between about 70 and about 350 lbs/ft 2 . Claim 19. The method of Claim 17 and including the step of laminating a tie layer between said layer of ethylene vinyl alcohol and said paperboard substrate layer. Claim 20. The method of Claim 19 and including the step of laminating a tie layer between said layer of ethylene vinyl alcohol and said layer of polyethylene-based polymer. Claim 21. The method of Claim 19 or Claim 20 wherein said tie layer comprises a resin. Claim 22. A non-foil laminate packaging material sealable by radio frequency electric energy comprising a substrate having a relatively low thermal conductance, at least one layer of ethylene vinyl alcohol laminated to said substrate, and at least one layer of a thermoplastic polymeric material laminated to said layer of ethylene vinyl alcohol . Claim 23. The laminate of Claim 22 wherein said substrate comprises paperboard. Claim 24. The laminate of Claim 23 wherein said paperboard has a basis weight of between about 70 and about 350 lbs/3000 ft 2 - Claim 25. The laminate of Claim 22 wherein said layer of thermoplastic polymeric material is selected from the group consisting of low density polyethylene and linear low density polyethylene. Claim 26. The laminate of Claim 22 wherein said layer of ethylene vinyl alcohol includes between about 27 and about 44 mole % ethylene. Claim 27. The laminate of Claim 22 wherein said layer of ethylene vinyl alcohol has a weight of about 5 lbs/3000 ft 2 .

Inventors: DAVIS RICHARD F, GALLOWAY ALESA R, ZUBER JAMES D, DAVIS, RICHARD, F., GALLOWAY, ALESA, R., ZUBER, JAMES, D.
Applicants: INT PAPER CO, INTERNATIONAL PAPER COMPANY
Application year: 1997
Main classification: B32B31
IPC codes: B29C65, B32B27