Tubular container with a heat seal having non-symmetrical inner and outer beads

A sealed composite container for products is provided having a paperboard body ply with a liner ply adhered on the inner surface thereof. One end of the container is rolled outwardly to form a rim and exposing the inner surface of the liner ply, which comprises a heat sealable composition. A lid for the container, also having a layer of heat sealable composition, is placed adjacent to the exposed heat sealable composition of the container liner and heat sealed thereto. The heat seal has both an inner and an outer bead formed on either side of the heat sealed area, wherein the inner bead comprises a larger amount of the heat seal compositions than the outer bead. A method and apparatus for forming the heat seal is also provided.

FIELD OF THE INVENTION
 The present invention relates to food containers and methods and apparatus
 for making food containers, and more particularly relates to heat seals
 used to seal such containers.
 BACKGROUND OF THE INVENTION
 Food and drink products and other perishable items are often packaged in
 tubular containers, which are sealed at both ends. These tubular
 containers typically include at least one structural body ply and are
 formed by wrapping a continuous strip of body ply material around a
 mandrel of a desired shape to create a tubular structure. The body ply
 strip may be spirally wound around the mandrel or passed through a series
 of forming elements so as to be wrapped in a convolute shape around the
 mandrel. At the downstream end of the mandrel, the tube is cut into
 discrete lengths and is then fitted with end closures to form the
 container.
 Tubular containers of this type typically include a liner ply on the inner
 surface of the paperboard body ply. The liner ply prevents liquids, such
 as juice, from leaking out of the container and also prevents liquids from
 entering the container and possibly contaminating the food product
 contained therein. Preferably, the liner ply is also resistant to the
 passage of gasses, such as oxygen and nitrogen, so as to prevent odors of
 the food product in the container from escaping and to prevent atmospheric
 air from entering the container and spoiling the food product. Thus, the
 liner ply provides barrier properties and the body ply provides structural
 properties.
 In addition, current commercial containers often have membrane-type lids or
 end closures heat sealed to a curled or bead-shaped rim of the composite
 container wall to form a peelable seal. The rim is formed by turning
 outwardly the end of the container to position the inner layer of the
 liner material on the outwardly curved surface.
 A major difficulty in developing a usable heat seal between the container
 lid and the rim of the container wall is balancing bond strength with ease
 of opening for the end user. During transport, the sealed containers
 experience temperature and pressure extremes that stress the heat seal and
 can lead to rupturing of the container. The bond strength must be
 sufficient to withstand the rigors of transportation. In particular, when
 containers packaged and sealed at one elevation are then subjected to
 lower ambient air pressure, such as during air transportation or when
 transported to consumers at higher elevations, a relative positive
 pressure is created within the container which could cause the seal
 between the lid and the container to rupture. This ability of the
 container to avoid rupturing under such conditions is known as burst
 strength. However, as the burst strength increases, there is generally a
 concomitant increase in difficulty of opening of the container, which is
 exhibited by the peel strength or peel resistance of the container. The
 higher burst strength indiscriminately prevents both rupturing during
 transport and opening by the end user.
 Certain types of heat sealable coatings have been used in both the lidding
 and liners of conventional containers. For example, SURLYN.RTM. polymer, a
 product of Dupont, is a material known in the art and is commonly used as
 a heat seal coating. SURLYN.RTM. polymer is an ionically cross-linked
 polymer with limited flow characteristics when heated. Typically, the
 layer of the container and the layer of the membrane which contact each
 other are constructed of SURLYN.RTM. polymer, and may be coated with a
 wax. These two layers of SURLYN.RTM. polymer are heat sealed along the top
 surface of the container bead. The two SURLYN.RTM. polymer layers create
 an extremely strong bond layer that remains relatively uniform in
 thickness across the seal area. Due to the strong cross-linked bond
 created by SURLYN.RTM. polymer, however, opening the container can require
 a peel force which is too high for some consumers and usually results in
 tearing and exposure of the other layers of the container wall, such as
 the paperboard body wall, as is illustrated in U.S. Pat. No. 4,280,653 to
 Elias. This gives the top of the container a ragged, undesirable
 appearance.
 In the parent application, Ser. No. 09/065,783, the formation of two heat
 seal beads is described. The two beads comprise an inner heat seal bead
 and an outer heat seal bead, each heat seal bead being formed of the heat
 sealable polymers of the seal layers of the membrane and the liner. The
 two beads are formed by using heat and pressure to force the heat sealable
 polymers to flow away from the central portion of the heat seal area and
 towards the interior and exterior of the container. The reduction in the
 amount of heat seal material in the central heat seal area reduces the
 bond strength in the central heat seal area and allows opening of the
 container without unsightly tearing of the liner and exposure of the
 paperboard layer of the container wall. However, formation of the beads
 according to the parent application does not entirely erase the difficulty
 of balancing burst strength and ease of opening.
 It would be advantageous to provide a sealed container and method for
 sealing a container that combine improved ease of opening and an
 attractive appearance after opening with the seal strength and barrier
 properties required for protection of the products within the container.
 SUMMARY OF THE INVENTION
 The composite container of the present invention successfully balances the
 need for ease of opening with the burst strength necessary to maintain a
 hermetic seal despite changes in pressure routinely experienced during
 transportation of the container. The present invention provides a sealed
 container having a larger heat seal bead on the interior side of the heat
 seal area than on the exterior side of the heat seal. The inner heat seal
 bead contains a greater amount of heat seal material and has a greater
 width than the outer heat seal bead. Since it had been discovered that the
 inner bead is primarily responsible for maintaining burst strength and the
 outer bead is primarily responsible for peel strength, a larger inner bead
 and smaller outer bead result in a good balance between burst strength and
 ease of opening.
 In one embodiment, the present invention provides a sealed composite
 container for products having a tubular body member that includes at least
 one paperboard body ply. A liner ply is adhered to the inner surface of
 the tubular body member and includes a barrier layer and a seal layer. The
 seal layer defines the innermost surface of the liner ply and comprises a
 heat sealable composition. At least one end of the body member and liner
 ply is rolled outwardly to form a curled or bead-shaped rim exposing the
 seal layer. A lid for closing the end of the tubular body member is also
 provided. The lid has a barrier layer and a seal layer adjacent to the
 seal layer of the liner ply on the rim. The seal layer of the lid also
 comprises a heat sealable composition. The two seal layers are adhered
 together to form a heat seal between the lid and the liner ply. The heat
 seal comprises an inner bead formed of the heat sealable composition of
 the seal layers and facing the interior of the container and an outer bead
 also formed of the heat sealable compositions on the opposite side of the
 heat seal. The inner bead is larger than the outer bead, meaning that the
 inner bead comprises a larger amount of the heat seal compositions than
 the outer bead.
 Specifically, the width of the inner bead is about 90 to about 190 microns,
 preferably about 130 to about 180 microns, and the width of the outer bead
 is about 60 to about 140 microns, preferably about 80 to about 120
 microns. In terms of percentage, the width of the inner bead is about 10
 to about 40% greater than the width of the outer bead, preferably about 20
 to 30% greater. The heat seal further comprises an intermediate region
 between the two beads having a width of about 0 to about 30 microns.
 The seal layer of the liner ply is preferably selected from the group
 consisting of high-density polyethylene, low-density polyethylene,
 metallocene catalyzed polyolefins, and mixtures thereof. The seal layer of
 the lid is preferably selected from the group consisting of ethylene vinyl
 acetate, high-density polyethylene, low-density polyethylene, ethylene
 methyl acrylate, metallocene catalyzed polyolefins, and mixtures thereof.
 In one embodiment, the seal layer of the lid comprises about 10 lbs./3000
 ft.sup.2 to about 50 lbs./3000 ft.sup.2 of the heat sealable composition,
 preferably at least about 20 lbs./3000 ft.sup.2. Additionally, the seal
 layer of the lid has a thickness of about 0.6 to about 3.3 mils.
 In the embodiment described above, both the liner ply and the lid include
 separate seal layers. However, the present invention does not require the
 use of two separate seal layers. At least one of said lid and said liner
 ply must have a seal layer comprising a heat sealable composition that
 forms a heat seal between the lid and the liner ply, but it is not
 necessary for both the liner ply and the lid to have seal layers.
 In one embodiment, the rim of the container defines a heat seal surface,
 wherein the heat seal surface has an apex portion, an inner portion
 sloping away from the apex portion and toward the interior of the
 container, and an outer portion sloping away from the apex portion and
 toward the exterior of the container, the inner portion sloping away form
 the apex portion at a greater rate than the outer portion. This rim design
 causes the inner bead to extend further along the heat seal surface toward
 the interior of the container than the outer bead extends along said heat
 seal surface toward the exterior of the container.
 A method of manufacturing tubular containers is also provided. The method
 includes providing a tubular member comprising at least one paperboard
 body ply having an inner surface and a liner ply adhered to the inner
 surface of the body ply, wherein the liner ply comprises a barrier layer
 and a seal layer. The seal layer defines the innermost surface of the
 liner ply and comprises a heat sealable composition. At least one end of
 the tubular container is rolled outwardly to form a rim exposing the seal
 layer of the liner ply. A lid is also provided closing the end of the
 tubular container, wherein the lid comprises a barrier layer and a seal
 layer. The seal layer of the lid comprises a heat sealable composition.
 The two seal layers are contacted together and heated under conditions
 that are sufficient to render the heat sealable compositions of the seal
 layers flowable. The seal layers are also pressed together to
 preferentially encourage flow of the heat sealable compositions in the
 direction of the interior of the container to form an inner bead and an
 outer bead of the heat sealable compositions, wherein the inner bead
 comprises a larger amount of the heat sealable compositions than the outer
 bead. In this manner, the lid is hermetically sealed to the liner ply.
 Preferably, the pressing step comprises pressing the seal layers together
 with an inclined surface to preferentially encourage flow in the direction
 of the interior container. The heating step preferably comprises heating
 the seal layers to about 175.degree. C. to about 275.degree. C. The
 pressing step preferably comprises pressing the seal layers together for
 about 0.5 to about 1.75 seconds at a sealing pressure of about 30 to about
 60 psi.
 As described above, only one seal layer is required. If only one seal layer
 is used, the rim and lid are placed into contact and the seal layer is
 heated in order to render the heat sealable composition of the seal layer
 flowable. The rim and lid are pressed together to preferentially encourage
 flow in the direction of the interior of the container as described above.
 An apparatus for sealing a lid to a container is also provided. The
 apparatus includes a sealing head having a central axis and a sealing
 surface, wherein at least a portion of the sealing surface is inclined
 towards the central axis of the sealing head. The sealing head has an
 engaged sealing position and a disengaged position. An actuator
 operatively connected to the sealing head moves the sealing head between
 the two positions. Preferably, the sealing surface is inclined at an angle
 from about 2.degree. to about 20.degree., and most preferably at an angle
 of about 7.degree. to about 12.degree..

DETAILED DESCRIPTION OF THE INVENTION
 The present invention now will be described more fully hereinafter with
 reference to the accompanying drawings, in which preferred embodiments of
 the invention are shown. This invention may, however, be embodied in many
 different forms and should not be construed as limited to the embodiments
 set forth herein; rather, these embodiments are provided so that this
 disclosure will be thorough and complete, and will fully convey the scope
 of the invention to those skilled in the art. Like numbers refer to like
 elements throughout.
 A tubular container 10 according to the present invention is illustrated in
 FIG. 1. Although illustrated as having a circular cross section, the tube
 may have any cross sectional shape, which can be formed by wrapping the
 tube around an appropriately shaped mandrel. One example is a generally
 rectangular shaped tube having rounded corners.
 The embodiment illustrated in FIG. 1 is particularly advantageous for
 packaging potato crisps and includes a flexible closure or lid 11, also
 referred to as a membrane-type closure or lid, and a reusable plastic end
 cap 12 over the seal. Various other end closures may be used; however,
 depending upon the type of food product that is to be packaged such as,
 for example, dough.
 As illustrated in more detail in FIG. 2, the tubular container 10 includes
 a wall having a body ply 13 which is preferably formed of paperboard and a
 liner ply 14 which is preferably formed of a polymeric material adhered to
 the inner surface of the body ply 13. The upper end of the tubular
 container 10 is rolled over so as to form a bead-shaped rim 15. The lid 11
 is hermetically sealed to the top of the rim 15 as discussed below. The
 end cap 12 is then snapped over the rim 15 and may be reused after the lid
 11 has been removed. A closure (not illustrated), for example a metal
 closure, can be secured to the opposite end of the container 10.
 The lid 11 is constructed of multiple layers. Optionally, the layer
 disposed on the outermost surface of the lid 11 away from the inside of
 the tubular container 10 is a paper or paperboard layer 18, such as a
 kraft paper layer. A barrier layer 20 is also provided that serves as a
 barrier to the passage of liquids and/or gasses such as oxygen. If a
 barrier is required for both liquids and gasses, the barrier material is
 preferably selected from the group consisting of polyethylene
 terephthalate, modified polyethylene terephthalate, polyethylene
 napthalate, metallized polyester or metallized polypropylene and mixtures
 thereof. The barrier layer 20 may also be constructed of metal oxide and
 silicate coated polyester, metal oxide and silicate coated polypropylene,
 ethylene vinyl alcohol and mixtures thereof. Alternatively, the barrier
 layer 20 comprises an aluminum foil layer.
 Advantageously, the lid 11 further includes a seal layer 22 comprising a
 heat sealable composition and positioned such that the seal layer 22 of
 the lid 11 is adjacent to the seal layer 26 of the liner ply 14. The seal
 layer 22 of the lid 11 is preferably constructed of a material selected
 from the group consisting of ethylene vinyl acetate, high density
 polyethylene, low density polyethylene, ethylene methyl acrylate,
 metallocene catalyzed polyolefins and mixtures or blends thereof. The seal
 layer 22 of the lid 11 preferably has a melting point within the range of
 about 70.degree. C. and 130.degree. C. Most preferably, the melting point
 of the seal layer 22 is between about 80.degree. C. and 110.degree. C. and
 in particular is 83.degree. C.
 In one embodiment, the lid 11 is formed as a laminate having a paperboard
 layer 18 adhered to the barrier layer 20 using a coextruded adhesive layer
 (not shown). The adhesive layer is constructed of materials selected from
 the group consisting of low density polyethylene, ethylene methyl acrylate
 (EMA), ethylene-methacrylic acid copolymers (EMAA) and mixtures thereof.
 The seal layer 22 is coated on the opposing surface of the barrier layer
 20. The seal layer 22 may be formed by extrusion coating, as a blown film
 laminated by extrusion or as a blown film laminated with a thermoset
 adhesive. In one embodiment, the seal layer 22 is formed as a dual layer
 coextrusion of high density polyethylene and ethylene methacrylate
 copolymer.
 The seal layer 22 of the lid 11 is preferably between about 0.6 and about
 3.0 mils in thickness, most preferably at least about 1.5 mils in
 thickness. The seal layer 22 comprises a heat sealable composition weight
 between about 10 to about 50 lbs./3000 ft.sup.2 and preferably about 20 to
 about 40 lbs./3000 ft.sup.2. Most preferably, the seal layer 22 has a heat
 sealable composition weight of about 25 lbs./3000 ft.sup.2 or more. The
 relatively thicker seal layer 22 prevents natural variations in the
 container manufacturing process from affecting the consistency of the heat
 seal. For example, imperfections in the rim 15 and variations in the
 container height have a significant effect on the sealing process. The
 additional heat seal material fills any cracks and fissures created in the
 rim 15 and is also able to create a continuous seal around seams in the
 container wall, such as the seams created by anaconda folds or overlap
 seams in the liner. The additional seal material also contributes to
 better sealing by compensating for slight differences in container height
 that might otherwise lead to a reduction in seal strength. Further, the
 increased amounts of seal layer 22 material allow creation of a heat seal
 despite the presence of contaminants introduced into the heat seal area
 during the manufacturing process, such as wax. By using a thicker seal
 layer, the heat seal may be formed at lower sealing temperatures. A
 preferred construction of the seal layer is disclosed in U.S. patent
 application Ser. No. 09/416,194, filed concurrently herewith and entitled
 "Sealant Layer for Container Lid." This application is assigned to the
 assignee of the present application and is expressly incorporated herein
 by reference.
 The liner ply 14 is also typically constructed of multiple layers. With the
 exception of the outermost seal layer 26, the composition of the liner ply
 14 is not critical to the present invention. Preferably, one of the layers
 forms a barrier to moisture and/or gasses, depending on the application.
 It will be understood that various barrier materials and liner plies could
 be employed depending upon the item being packaged. For example,
 conventional liners include a layer of foil backed with kraft paper.
 However, in a preferred embodiment, the liner ply 14 is substantially
 entirely formed of polymeric material. In particular, liner plies such as
 described in U.S. Pat. No. 5,829,669 to Drummond et al. or U.S. Pat. No.
 5,846,619 to Cahill et al. both of which are assigned to the assignee of
 the present invention and are hereby incorporated by reference, may be
 used.
 In the embodiment illustrated in FIG. 2, the liner ply 14 includes a seal
 layer 26, a moisture barrier layer 28 and an adhesive layer 30. The
 barrier layer 28 is resistant to the passage of liquids and gasses such as
 oxygen. If a high barrier is required for both liquids and gasses,
 preferred barrier materials are metallized polyester or metallized
 polypropylene. Some food products, such as juices, do not require a gas
 barrier and other barrier materials may be used (although the barrier may
 also be generally resistant to the passage of gasses). It will be
 understood that various barrier materials could be employed depending upon
 the item being packaged. Alternative barrier materials include nylon, EVOH
 (ethylene vinyl alcohol polymer and copolymer), polyvinylidene chloride,
 polyethylene, polypropylene, metallized polypropylene, metal oxide and
 silicate coated polyester, metal oxide and silicate coated polypropylene
 and the like as will be apparent to the skilled artisan.
 One surface of the barrier layer 28 may include a thin metallized coating
 32 to provide a metallic appearance and also to enhance the barrier
 properties. The metallized coating 32, which may be formed of aluminum, is
 significantly thinner than a foil layer, however, and is not necessary for
 strength or barrier properties in certain applications.
 An adhesive layer 30 is preferably below the metallized coating 32 and
 defines the radially outermost surface of the liner ply 14. The adhesive
 layer 30 may have multiple layers coextruded together. The adhesive layer
 30 may be selected from the group consisting of metallocene catalyzed
 polyolefins, ethylene-methacrylic acid, ethylene methyl acrylate, ethylene
 butyl acrylate, ethylene acrylic acid, ethylene vinyl acetate, and blends,
 mixtures and copolymers thereof. The adhesive layer 30 may also be a
 thermoset adhesive layer.
 A seal layer 26 defines the radially innermost surface of the liner ply 14.
 The seal layer 26 provides a surface against which the adhesive layer 30
 is adhered when a first marginal edge portion 41 of the liner ply 14 is
 brought into an overlapping relationship with a second marginal edge
 portion 42, as shown in FIG. 6. The seal layer 26 also forms the heat seal
 between the lid 11 and the liner 14 in conjunction with the seal layer 22
 of the lid.
 The seal layer 26 of the liner ply 14 is preferably constructed of a
 material selected from the group consisting of high density polyethylene,
 low density polyethylene, metallocene catalyzed polyolefins and mixtures
 or blends thereof. In embodiments of the seal layer 26 including a
 polyolefin polymer, the polyolefin is preferably high density polyethylene
 or a high density polyethylene blend containing up to 30% low density
 polyethylene. The seal layer 26 of the liner ply 14 preferably has a
 melting point within the range of about 110.degree. C. and about
 140.degree. C. Most preferably, the seal layer 26 has a melting point
 between about 120.degree. C. and 130.degree. C.
 FIG. 2 illustrates the sealed end of the tubular container of a preferred
 embodiment of the present invention wherein the two seal layers, 22, 26
 are heat sealed together. As shown in greater detail in FIG. 3A, a sealed
 composite container for products is provided having a heat seal between
 the liner ply 14 and the lid 11 in the form of an inner heat seal bead 36
 and an outer heat seal bead 38. The inner heat seal bead 36 and the outer
 heat seal bead 38 are formed of the heat sealable compositions of the seal
 layer 26 of the liner ply 14 and the seal layer 22 of the lid 11. The heat
 sealable compositions of both seal layers 22, 26 are displaced outwardly
 from the intermediate region during the heat sealing operation and are
 cooled to form the beads 36, 38. The inner heat seal bead 36 faces the
 interior of the tubular container 10 and the outer heat seal bead 38 is
 disposed on the opposite side of the heat seal area from the inner heat
 seal bead 36. When cooled, the heat seal comprises a thin intermediate
 region 40 between the inner heat seal bead 36 and the outer heat seal bead
 38. In certain places, the heat sealable compositions may be completely
 displaced from between the barrier layers 20 and 28 such that the barrier
 layers are in abutting contact. However, the inner and outer beads 36, 38
 maintain double barriers against the passage of liquids and gasses so that
 a hermetic seal is maintained. The intermediate region 40 preferably has a
 lower bond strength than the inner heat seal bead 36 and the outer heat
 seal bead 38. The width of the intermediate region 40 is about 0 to about
 30 microns. The term "bead" as used herein is intended to be distinguished
 from prior containers having relatively flat heat seal where very little,
 if any, flowing of the heat seal compositions occurs. In addition, this
 embodiment is not limited to use with only liners having a straight
 overlapping seam, but the heat seal beads 36, 38 could also be used with
 an anaconda fold seam.
 In effect, the inner heat seal bead 36 and the outer heat seal bead 38
 provide a double seal having a high tensile or burst strength. The burst
 strength of the bead seals gives the container 10 a strong seal against
 forces acting upon the container in a direction normal to the heat seal
 (i.e., normal to the plane defined by the end of the tubular container
 10). Since most forces acting upon a container during storage and transit
 will occur normal to the heat seal area, the high burst strength of the
 inner heat seal bead 36 and outer heat seal bead 38 of the present
 invention is especially advantageous for use with product containers.
 Burst strength may be tested using an altitude chamber. Typically, the
 sealed container 10 is placed in the altitude chamber and then subjected
 to an external partial vacuum for a predetermined period of time to
 determine whether the heat seal is capable of withstanding differences
 between interior container pressure and external air pressure. Suitable
 testing conditions include subjecting the container to a vacuum of 10 in.
 of Hg for 30 minutes at room temperature. The containers 10 of the present
 invention are capable of maintaining a hermetic seal during a thirty
 minute exposure to a vacuum of 10 in. of Hg at room temperature.
 Notwithstanding the high burst strength, the peel strength of the heat seal
 formed according to the present invention is relatively low, resulting in
 a container that exhibits relative ease of opening. This is in contrast to
 conventional containers where two SURLYN.RTM. polymers are ionically
 crosslinked together in a relatively flat heat seal, resulting in a bond,
 which is sufficient to tear the liner 14 when removing the lid 11 (rather
 than tearing through the SURLYN.RTM. polymer). The preferred range for
 peel strength is about 5 to about 10 lbs./linear inch. In one embodiment,
 the heat seal has a peel strength of about 7 to about 10 lbs./linear inch.
 Thus, the heat seal of the present invention combines the shear strength
 and tensile strength necessary to prevent unwanted breaches of the tubular
 container 10 with relatively low peel strength for ease of opening by the
 consumer.
 FIGS. 3A-3C illustrate the opening mechanism for container 10 of the
 present invention. As shown by FIGS. 3B and 3C, the shearing force
 generated during opening of the container 10 causes a tear to propagate
 through the seal layer 26 of the liner ply 14 and/or the seal layer 22 of
 the lid 11. The seal layer 26 of the liner ply 14 and the seal layer 22 of
 the lid 11 provide a bond strength between the barrier layer 20 of the lid
 11 and the barrier layer 28 of the liner ply 14 that is lower than the
 bond strength between the barrier layer 28 of the liner ply 14 and the
 paperboard body ply 13 (or any other intermediate layers such as the
 metallized coating 32 of the liner 14). As a result, shearing that takes
 place during the opening of the tubular container 10 occurs only between,
 and not through, the barrier layers of the liner ply 14 and lid 11. When
 the tubular container 10 of the present invention is opened, unsightly
 tears through the barrier layer 28 of the liner ply 14 do not occur.
 It has been discovered that the inner bead 36 of the double bead seal
 provides the primary resistance to tensile forces acting upon the
 container, such as those burst forces generated by changes in internal
 pressure during transport. However, the outer bead 38 provides the primary
 resistance to opening by peeling of the peelable heat seal formed between
 the lid 11 and the liner ply 14. As a result, it has been discovered that
 the heat seal is advantageously formed having a larger inner bead 36 and a
 smaller outer bead 38. The resulting container exhibits both improved ease
 of opening by virtue of the smaller outer bead 38 and improved burst
 strength for withstanding the rigors of transportation by virtue of the
 larger inner bead 36.
 The heat seal of the present invention has an inner bead 36 having a
 greater width than the outer bead 38. Bead width is defined as the
 distance between the barrier layer 20 of the lid 11 and the barrier layer
 28 of the liner ply 14 measured at the longest point across the bead in
 the vertical plane as shown by reference symbol A of FIG. 3A. Note that
 the width measurement is generically defined as the distance between the
 next adjacent layer to the seal layer 22 of the lid 11 in the construction
 of the lid and the next adjacent layer to the seal layer 26 of the liner
 14 in the construction of the liner. In the preferred embodiment, the next
 adjacent layers are the barrier layers. However, in other embodiments, the
 next adjacent layers may be layers of other types. Specifically, the width
 of the inner bead 36 is about 90 to about 190 microns, preferably about
 130 to about 180 microns, and most preferably about 140 to about 160
 microns. The width of the outer bead 38 is about 60 to about 140 microns,
 preferably about 80 to about 120 microns, and most preferably about 95 to
 about 120 microns. As will be understood, the width of the beads will
 depend on a number of factors, including the heat sealable materials used
 to form the seal layers 22, 26, the heat sealing conditions and the like.
 In terms of relative widths of the inner bead 36 and outer bead 38, the
 width of the inner bead is about 10 to about 40% greater than the width of
 the outer bead, preferably about 20 to 30% greater, and most preferably
 about 22 to about 26% greater.
 A preferred shape of the rim 15 of the container 10 is illustrated in FIG.
 4. As shown, the rim 15 of the container 10 is rolled outwardly to expose
 the heat seal layer 26 of the liner 14. The rim 15 creates a heat seal
 surface that contacts the seal layer 22 of the lid 11. The heat seal
 surface includes an apex portion 84, an inner portion 86 sloping away from
 the apex portion and toward the interior of the container 10, and an outer
 portion 88 sloping away from the apex portion and toward the exterior of
 the container. Preferably, the inner portion 86 slopes away from the apex
 portion 84 at a greater rate than the outer portion 88. This rim 15 design
 encourages the inner bead 36 to extend further down the heat seal surface
 toward the interior of the container 10 and discourages the outer bead 38
 from extending down the heat seal surface toward the exterior of the
 container. In this manner, the shape of the rim 15 affects the direction
 of flow of the heat sealable compositions and the relative shapes of the
 inner bead 36 and outer bead 38. By encouraging flow of the heat sealable
 compositions further down the heat seal surface toward the interior of the
 container, better burst strength is obtained due to the anchoring effect
 of the inner bead 36. It is believed that the shape of the inner bead 36
 creates a shear component of resistance which counters the forces caused
 by internal container pressure more effectively than the more tensile
 resistance of prior art designs. Additionally, peel strength is maintained
 at a reasonable level because the incline or slope of the heat seal
 surface toward the exterior of the can is lesser in degree and does not
 encourage flow of the heat sealable compositions in that direction. As
 desired, the above-described shape of the rim 15 maintains the outer bead
 38 at a relatively smaller size as compared to the inner bead 36. The
 desired rim 15 shape can be formed during the initial formation of the
 container rim or by a secondary forming process. Alternatively, the
 desired rim 15 shape may be formed in conjunction with the heat sealing
 operation. The rim can also have a substantially planar portion as shown
 in FIG. 4 and disclosed in U.S. patent application Ser. No. 09/416,169,
 filed concurrently herewith and entitled "Container With Heat Seal Surface
 Having a Substantially Planar Portion." This application is assigned to
 the assignee of the present application and is expressly incorporated
 herein by reference.
 A method and apparatus for sealing a container for products is also
 provided. The sealing method and apparatus are illustrated in FIGS. 5A-5C.
 The heat seal may be created using any suitable apparatus known in the
 art. In some sealing systems, wax is applied to the rim 15 to hold the lid
 11 in place prior to formation of the heat seal. As noted above, the
 relatively thick seal layer 22 of the lid 11 allows the formation of a
 heat seal having sufficient bond strength despite the presence of wax in
 the heat seal area. The present invention is also compatible with sealing
 systems that utilize a vacuum system to initially hold the lid 11 in place
 prior to the heat sealing operation.
 Using either apparatus, the heat sealing method of the present invention
 includes providing a tubular member having a paperboard layer 13 and liner
 ply 14 adhered to the inner surface of the paperboard layer. As described
 above, the liner ply 14 includes a barrier layer 28 and a seal layer 26,
 the seal layer defining the innermost surface of the liner ply and
 comprising a heat sealable composition. Once an end of the tubular member
 is rolled outwardly to form a rim 15, a lid 11 may be contacted with the
 liner ply 14 for forming the heat seal. The lid 11 includes a barrier
 layer 20 and a seal layer 22, wherein the seal layer comprises a heat
 sealable composition. The seal layer 22 of the lid 11 is contacted with
 the seal layer 26 of the liner ply 14. The two seal layers 22, 26 are then
 heated under conditions sufficient to render the heat sealable
 compositions flowable and pressed together so as to preferentially
 encourage more flow of the heat sealable compositions in the direction of
 the interior of the container to form an inner bead 36 and an outer bead
 38, wherein the inner bead contains a larger amount of heat sealable
 compositions than the outer bead.
 In a preferred embodiment, the pressing step is accomplished by pressing
 the seal layers 22, 26 together using an inclined surface, such as
 inclined heat sealing head 44. The heat sealing head 44 is preferably
 constructed of metal, such as aluminum, coated copper or other heat
 conductive material. The heat sealing head 44 is heated by heat source 46.
 The heat source may be any suitable type of heat source known in the art.
 The heat sealing head 44 does not have to be heated. The heat seal layers
 22, 26 could be heated independently using a separate heat source. The
 heat sealing head 44 has an engaged sealing position in contact with the
 lid 11 and a disengaged position. The heat sealing head 44 is moved
 between the two positions by an actuator 48. The actuator 48 may be any
 type of actuator known in the art, including mechanical, pneumatic, and
 the like.
 The angle of the inclined surface of the heat sealing head 44 affects the
 amount of material that flows to form the beads as well as the relative
 size of the beads. The angle of the inclined surface of the heat sealing
 head 44 is about 2 to about 20 degrees, preferably about 7 to about 12
 degrees. In one embodiment, the angle of the inclined surface is about 10
 degrees. In another embodiment, the angle is about 3.degree.. The inclined
 surface of the head 44 causes molten polymer from the seal layers to move
 towards the interior of the container to form the inner bead. As this
 movement occurs, the molten polymer advantageously "fills in" any
 irregularities in the liner and lid surfaces, thus improving the integrity
 of the seal.
 The heat sealing conditions, such as temperature, pressure, and time,
 depend on a number of factors, including the heat sealable compositions
 used and the thickness of the heat seal layers. In one embodiment, the
 heat seal layers are heated to between about 175.degree. C. to about
 275.degree. C., preferably about 205.degree. C. to about 230.degree. C.,
 and most preferably about 210.degree. C. to about 225.degree. C. In one
 embodiment, the heat sealing temperature is about 218.degree. C. The heat
 sealing pressure is about 30 to about 60 psi, preferably about 40 to about
 50 psi. In one embodiment, the heat sealing pressure is about 45 psi. The
 heat sealing time, meaning the period of time during which heat sealing
 pressure is applied, is about 0.5 to about 1.75 seconds, preferably about
 0.9 to about 1.5 seconds, and most preferably about 1.15 to about 1.35
 seconds. In one embodiment, the heat sealing time is about 1.25 seconds.
 Although the container embodiments discussed above include two seal layers,
 22 and 26, the present invention does not require the use of two seal
 layers. At least one of the liner and lid must include a seal layer in
 order to provide the necessary heat seal beads, 36 and 38, as described
 above. However, two seal layers are not necessary to practice the present
 invention. If a single heat seal layer is used, the heat seal layer may be
 constructed of high density polyethylene, low density polyethylene,
 ethylene vinyl acetate, ethylene methyl acrylate, metallocene catalyzed
 polyolefins and mixtures thereof.
 The containers 10 of the present invention may be manufactured by the
 process illustrated in FIG. 6. As shown, a continuous strip of paperboard
 body ply material 13 is supplied to the apparatus and is first passed
 through a pair of opposed edge skivers 50. The edge skivers remove part of
 the square edge of the body ply 13 to create first 52 and second 54 edges
 having a beveled configuration. The body ply 13 is then advanced through
 an adhesive applicator 56, which applies an adhesive 21 to the upper
 surface of the body ply 13. The adhesive 21 is advantageously an aqueous
 adhesive, which overcomes the many problems associated with solvent based
 adhesives. No special equipment is needed to capture solvents, which
 evaporate from the adhesive in order to comply with environmental
 regulations. Preferred adhesives are aqueous low glass transition
 temperature ethylene vinyl acetate (&gt;18%) materials. One preferred
 adhesive is No. 72-4172, which is available from the National Starch and
 Chemical Company. Another adhesive that may be used is No. 33-4060, which
 is also available from the National Starch and Chemical Company. The
 adhesive 21, as well as other adhesive layers used to construct the
 container 10, may be applied in the form of a foam as described in
 copending U.S. patent application Ser. No. 09/197,275 entitled, "Composite
 Container Having Foamed Adhesive," which is assigned to the assignee of
 the present invention and hereby incorporated by reference.
 The body ply 13 and wet adhesive 21 applied thereto are then passed
 underneath a heater 58 which evaporates at least part of the water content
 of the aqueous adhesive 21 to render the adhesive substantially tacky. It
 is important that the correct amount of heat is supplied to the adhesive.
 Insufficient heat will not evaporate enough water in a sufficiently short
 period of time with the result that the adhesive will not be rendered
 sufficiently tacky. Conversely, too much heat will overdry the adhesive
 and cause the adhesive to lose tackiness. A preferred type of heat source
 is an infrared heater although various other heat sources, e.g., forced
 air heating or the like can be used. After heating the adhesive 21 on the
 body ply 13, the body ply 13 and the liner ply 14 are fed to the shaping
 mandrel from opposite directions. The body ply 13 is passed under skive
 adhesive applicator 60 which applies the skive adhesive 24 to the beveled
 surface of the skived second edge 54 of the body ply 13. The skive
 adhesive 24 is preferably a hot melt adhesive of the type which is
 conventional in the art, although it could also be a water based adhesive
 including one or more polymers. Polyvinyl acetate and ethylene vinyl
 acetate are the preferred liquid adhesives. The skive adhesive 24 helps
 provide a stronger body ply bond especially for single body ply
 containers.
 The surface of the liner ply 14 that contacts the body ply 13 is subjected
 to a corona treatment station 62. The opposite surface of liner ply 14 is
 coated with lubricant from a roller 64, which allows the liner ply to
 slide smoothly during the winding operation.
 The liner ply 14 is then passed under an infrared heater 66, which heats
 the second marginal edge portion 42 of the liner ply. After the infrared
 heater 66, the second marginal edge portion 42 of the liner ply 14 is then
 passed under at least one forced air heater 68.
 The body ply 13 and the liner ply 14 are then wrapped around a shaping
 mandrel 70 from opposite sides of the mandrel. Each ply is first wrapped
 under the mandrel 70 and then back over the top in a helical fashion with
 the liner ply 14 wound against the surface of the mandrel. The first
 marginal edge portion 41 of the liner ply 14 is exposed on the mandrel 70
 and is subjected to heat from a second forced air heater 72.
 As the body ply 13 is further wrapped and the first edge 52 of the body ply
 13 advances back under the mandrel 70 after one complete revolution, it is
 brought into contact with the second edge 54 of the ensuing portion of the
 body ply 13 which is first coming into contact with the mandrel. The
 skived edges 52, 54 become abutted together and the skive adhesive 24
 adheres the edges together to form a spirally wound tube which advances
 along the mandrel 70.
 With regard to the liner ply 14, the first marginal edge portion 41 is
 brought into an overlapping relationship with the second marginal edge
 portion 42 to create a sealed straight lap seam. The seal is formed by a
 polymeric adhesive layer 30 of the first marginal edge 41 becoming bonded
 to the second marginal edge 42. However, a strip of hot melt adhesive
 could alternatively be used for securing and sealing the liner overlap.
 The tube is then advanced down the mandrel 70 by a conventional winding
 belt 74, which extends around a pair of opposed pulleys 76. The winding
 belt 74 not only rotates and advances the tube, but also applies pressure
 to the overlapping edges of the body ply 13 and liner ply 14 to ensure a
 secure bond between the respective ply edges.
 An outer label ply 16 is then preferably passed over an adhesive applicator
 78 and wrapped around the body ply 13. The label ply 16 could be applied
 before the winding belt 74. At a cutting station 80, the continuous tube
 is cut into discrete lengths and removed from the mandrel 70.
 The ends of the containers 10 are then rolled outwardly to form the rim 15
 and the lid 11 is subsequently heat sealed thereto as described above. An
 end closure, such as a metal closure, is attached to the other end of the
 container 10. Typically, the lid 11 and end closure 12 are applied to one
 end of the container 10 prior to filling of the container. After filling,
 an end closure is applied to the opposing end.
 Many modifications and other embodiments of the invention will come to mind
 to one skilled in the art to which this invention pertains having the
 benefit of the teachings presented in the foregoing descriptions and the
 associated drawings. Therefore, it is to be understood that the invention
 is not to be limited to the specific embodiments disclosed and that
 modifications and other embodiments are intended to be included within the
 scope of the appended claims. For example, the tubular containers
 according to the present invention are not necessarily helically wound but
 may instead be longitudinally wrapped to create a "convolute" tube having
 an axially extending seam. In addition, although the tubular containers
 according to the present invention have been described primarily in
 connection with food products, it is to be understood that the containers
 could be used in connection with other products where the liner ply is
 advantageous such as, for example, ink or caulk. Although specific terms
 are employed herein, they are used in a generic and descriptive sense only
 and not for purposes of limitation.