(12) Ulllted States Patent
`Rolle et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,413,625 B2
`Jul. 2, 2002
`
`US006413625B2
`
`(54) THERMOFORMABLE FOAM SHEETING
`FOR PRODUCING OPEN CONTAINERS
`
`4,680,317 A
`5,149,579 A
`
`7/1987 Kiinel et al.
`9/1992 Park et al.
`
`(76)
`
`Inventors: Jean-Claude Rolle, 15, rue Pierre-Alex,
`CH-1630 Bulle; Michel Pittet, Route
`de l’Ancien-Stand, CH-1680 Romont,
`both of (CH)
`
`EP
`EP
`
`FOREIGN PATENT DOCUMENTS
`
`0 438 874 A2
`0 458 731 A1
`
`7/1991
`11/1991
`
`*
`
`otice:
`N '
`
`isc aimer,t e term o t is
`u ect to an
`yd'
`S bj
`l
`'
`h
`f h'
`patent is extended or adjusted under 35
`USC. 154(b) by 0 days.
`
`(21) Appl.No.: 09/844,166
`(22)
`Filed:
`Apr‘ 27, 2001
`
`Related U-S- App1iC3ti0I1 Data
`
`.
`.
`Primary Examl”e”—Heiei1.L~ PCZZMO
`(74) Attorney, Agent, or FLrm—Oppedahl & Larson LLP
`57
`ABSTRACT
`
`)
`(
`The invention relates to a thermoformable sheeting consist-
`ing of a foam material or containing at least one layer of a
`foam material. The inventive sheeting is particularly appli-
`
`(63) Continuation of application No 09/951 268 filed on Jun 9
`2000, nowabandoned, which isacontinuation ofapplication
`No. 09/117,817, filed on Oct. 26, 1998, now abandoned,
`which is a continuation of application No. PCT/EP97/
`00420, filed on Jan. 31, 1997.
`
`Cable for thermoforming Cups or trays for packaging and/or
`C0HS11H1iHg f00d Stuffs; by drawing Or deep drawing the
`-
`-
`-
`_
`Siheetmg am? expaindmg the. foam m.atena1' The foam mate
`rial of the inventive sheeting consists of a polypropylene
`
`7
`
`Foreign Application priority Data
`(30)
`Feb. 7, 1996
`(CH) ............................................... 318/96
`
`Int. Cl.
`(51)
`(52) U.S. Cl.
`
`................................................ .. B32B 3/26
`.............................. .. 428/316.6; 428/304.4;
`428/314.4; 428/314.8
`(58) Field of Search ......................... .. 428/304.4, 314.4,
`428/314.8, 316.6
`
`blend containing between 50 and 100% of high melt strength
`polypropylene’ has a densityff 0'1 to 0.7 g/CH13’ preferably
`between 0.25 and 0.5 g/cm , and a cell number of from
`greater than 3oo to 3ooo eeiis per mm3’ preferabiy between
`3
`.
`.
`.
`800 and 2500 cells per mm . The inventive sheeting can be
`diaWn With a high diawing iaiinn (6% 15) and expanded in
`be formed into containers even with difficult shapes,
`whereby no drawing problems arise and the containers
`produced show a homogeneous opaqueness.
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`3,871,897 A
`
`3/1975 Ealding
`
`10 Claims, N0 Drawings
`
`Page 1 of 3
`
`BOREALIS EXHIBIT 1037
`
`Page 1 of 3
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`BOREALIS EXHIBIT 1037
`
`

`
`US 6,413,625 B2
`
`1
`THERMOFORMABLE FOAM SHEETING
`FOR PRODUCING OPEN CONTAINERS
`
`This application is a continuation of application Ser. No.
`09/951,268 (filed Jun. 9, 2000) abandoned, which is a
`continuation of application Ser. No. 09/117,817 (filed Oct.
`26, 1998, abandoned) which is a 371 of PCT/EP97/00420
`(filed Jan. 31, 1997,
`the application being published in
`language other than English under PCT Article 21(2)).
`The invention is in the field of the packaging industry and
`relates to a thermoformable foam sheeting according to the
`generic part of the first independent claim, which sheeting
`consists of a thermoformable and expandable foam material
`or contains at least one layer of such a material.
`Open containers, such as e.g. trays or cups often used for
`packaging and/or for consuming food stuffs are usually
`produced by drawing or deep drawing a piece of flat sheeting
`made of a thermoformable plastic material. Such open
`containers have a bottom wall and a side wall or side walls.
`
`The sheeting is usually thermoformed between a pair of
`forming tools, i.e. between a female tool with a recess over
`which the preheated sheeting is positioned and a male tool
`movable into the recess of the female tool thereby drawing
`the sheeting.
`The use of a sheeting consisting of a foamed material or
`containing at least one layer of a foamed material is advan-
`tageous for making such containers because a foam sheet or
`a container made of such a sheet unit is usually stiffer than
`the corresponding solid material with the same weight per
`area. Sheeting and containers consisting at least partly of
`foamed material are opaque. The containers need a mini-
`mum mechanical strength according to their application and
`they are the better accepted, the more homogeneous their
`opaqueness and the smoother and shinier their surface is.
`A further advantage of producing the open containers of
`a sheeting which consists of a foamed material or contains
`at least one layer of such a material, is the possibility of
`expanding specific parts of the container walls by applying
`a reduced pressure to the foam when it is in a thermoform-
`able state, whereby the gas trapped in the foam expands, and
`by cooling down the foam when still under reduced pressure.
`Such processes are described e.g. in the publications U.S.
`Pat. No. 3,846,526 or JP-60192615. Furthermore, a method
`and an apparatus for drawing and expanding foam sheeting
`to form open containers is described in a co-pending appli-
`cation (same filing date as the present application).
`According to the co-pending application the containers
`are formed with the help of a male tool moving into the
`recess of a female tool and thereby drawing the preheated
`sheeting positioned over the recess of the female tool.
`During the movement of the male tool the pressure on at
`least one side of the sheeting is reduced such that it reaches
`a value (expansion pressure) below ambient pressure which
`is low enough for expanding the foam material when the
`male tool reaches its end position or immediately after-
`wards. The tools are then kept in the end position and
`expansion pressure is maintained until
`the sheeting has
`cooled down to a temperature below its plastication tem-
`perature. Then the formed container is demolded. Before the
`moving male tool contacts the preheated sheeting, the sheet-
`ing may be prestretched by applying a pressure difference to
`it (lower pressure on the female tool side). Such prestretch-
`ing is preferable in particular when producing containers by
`deep drawing (drawing ratio>1) and/or containers with a
`difficult shape (not round bottom wall, steep side walls).
`With the method described in the last paragraph and
`using tools which have, for removing air from between the
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`2
`sheeting and the tool surface, a pattern of openings on at
`least those parts of their surface which are to form the side
`walls of the container (as described in a further co-pending
`application with the same application date as the present
`application), it is possible to produce in a minimum cycle
`time from a sheeting at least partly consisting of foamed
`material, open containers with a wall thickness and wall
`density which are primarily determined by the mechanical
`properties the container is to have. This means that the
`container wall may have, according to the degree of drawing
`and of expansion a varying thickness and in particular a
`varying density.
`Using known sheetings consisting of or containing e.g.
`polysterene or polypropylene foam for producing open
`containers according to the above described method, may
`create problems in that on the, produced containers areas or
`varying degree of drawing and/or expansion show optically
`or even lead to mechanical faults which happens in, par-
`ticular when the cycle time is short and/or the drawing ratio
`is high or the shape of the container is difficult, as are e.g.
`container shapes with a non-round bottom wall and/or with
`very steep side walls.
`It is therefore the object of the invention to create a
`sheeting, consisting of a foamed material or containing at
`least one layer of a foamed material which sheeting has good
`drawing, characteristics and an opaqueness which varies
`little with density such that the sheeting can be processed
`using the method of the co-pending application in particular
`with very short cycle times and with high drawing, ratios
`and difficult container, shapes resulting in containers which
`are mechanically and estheticly of good quality. In addition,
`the sheeting as well as the containers made from the sheeting
`are to be easily recyclable and compatible with food stuffs
`of a variety as wide as possible (solid and liquid, hot and
`cold etc.).
`This object is achieved by the sheeting as defined by the
`claims.
`
`For fulfilling the condition of the recyclability and the
`food compatibility, the inventive sheeting consists basically
`of polypropylene. This means it consists fully of a polypro-
`pylene foam or contains at least one layer of polypropylene
`foam and contains a further layer or further layers e.g. of
`solid polypropylene or of other thermoformable materials,
`e.g. barrier materials.
`The opaqueness of a plastic foam material is due to light
`scattering on the cell walls. The finer the arrangement of
`these cell walls is,
`the more times light falling into the
`material is scattered and the more opaque does the material
`appear. It is found that the finer the arrangement of the cell
`walls, the less does the appearance of the material change
`when drawn and/or expanded to quite a high degree. It is
`found also that the finer the arrangement of the cell walls the
`better are the drawing characteristics of the foam. This is due
`to the fact,
`that the occurrence of cells which are large
`enough for locally changing the drawability of the foam to
`a considerable degree and for being torn open upon drawing,
`gets more probable as the cell size increases, i.e. as the
`fineness of the cell wall structure decreases.
`The measure for the fineness of the cell wall structure in
`
`a plastic foam material is the cell number (cells per mm3).
`It is found that sheetings with polypropylene foams having
`a cell number which is higher than 300 cells per mm3 and
`processed in the method according to the co-pending appli-
`cation give good quality containers for a relatively low
`drawing ratio (container depth divided by diameter of open-
`ing: between 0.5 and 1, depending on container shape and
`expansion ratio). Sheeting with polypropylene foam with a
`
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`US 6,413,625 B2
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`cell number of up to 3000 cells per mm3 gives good quality
`containers for drawing ratios of 0.8 to 1.5 (depending on
`container shape and expansion ratio). For the production of
`containers such as cups and trays for packaging and/or
`consuming food stuffs in a drawing process as described in
`the co-pending application, sheeting with polypropylene
`foam having a cell number of 800 to 2500 cells per mm3,
`preferably of 1000 to 2000 cells per mm3, is very suitable.
`The polypropylene foam of the inventive sheeting, has a
`density of 0.1 to 0.7 gr/cm3, preferably of 0.25 to 0.5 gr/cm3,
`even more preferably of 0.36 to 0.50 gr/cm3. The sheeting
`has a thickness of 0.5 to 3 mm, preferably of 1 to 2 mm and
`even more preferably of 1.2 to 1.6 mm.
`If, depending on the density of the polypropylene foam
`and on the thickness of the sheeting to be made, a polypro-
`pylene blend containing between 50 and 100% of high melt
`strength polypropylene with a melt strength of ca. 24 cN is
`used, extrusion of a sheeting with the above mentioned
`characteristics is possible without problem. High melt
`strength polypropylene (polypropylene with long chain
`branching) is available on the market as homopolymer or as
`copolymer.
`The higher the melt strength of polypropylene blend (i.e.
`the higher its content of high melt strength polypropylene),
`the easier it is to produce good quality sheeting with the
`required cell numbers. However, because a high percentage
`of high melt strength polypropylene is not only an economic
`drawback but also results in containers with rather brittle
`
`walls, it is advantageous to lower the percentage of high
`melt strength polypropylene as much as possible towards
`50%, by adding other types of polypropylene (homo- or
`copolymers) thereby lowering the melt strength of the blend
`to as low as 15 cN, preferably to between 18 and 20 cN.
`A sheeting applicable in the thermoforming method
`according to the co-pending application for producing cups
`and trays for packaging and/or consuming food which cups
`and trays have a mechanical stability suitable for
`this
`purpose, preferably consists of or contains a polypropylene
`foam containing 60 to 90%, preferably 60 to 80% of high
`melt strength polypropylene.
`For producing by extrusion the sheeting consisting of or
`containing the polypropylene foam, a solid or gaseous
`blowing agent is added to the polymer blend in a known
`manner. Furthermore other known additives for improving
`the extrusion process may be added in known concentrations
`as well as e.g. pigments for coloring the foam material.
`The cell number of an extruded foam material is not only
`dependent on the composition of the extruded polymer and
`on the additives used but also on the extrusion parameters.
`Therefore, for producing a sheeting with a polypropylene
`foam of the composition given above and having a cell
`number within a predetermined range, the appropriate extru-
`sion parameters are to be determined by experiment.
`
`EXAMPLE
`
`density: 0.43 g/cm3
`cell number: 1450 cells per mm3
`thermoforming process with prestretching:
`width of cavity: 1.2 mm
`sheet temperature: §160° C.
`expansion pressure: §0.2 bar (absolute)
`forming time (prestretching, drawing and expansion): 1.8
`sec
`
`container made from the sheeting:
`round cup: diameter of opening: 75 mm
`depth: 80 mm
`wall thickness: 1-1.2 mm
`We claim:
`
`1. Thermoformable sheeting which sheeting consists of a
`foamed propylene homopolymer or copolymer or contains at
`least one layer of such a foam and which sheeting is
`applicable for producing open containers by drawing or deep
`drawing the sheeting and expanding under reduced pressure
`the gas trapped in the foam when the foam is in a thermo-
`formable state, characterized in that the foamed propylene
`homopolymer or copolymer contains between 60 and 90%
`high melt strength propylene homopolymer or copolymer
`and at
`least one further propylene homopolymer or
`copolymer, has a density of 0.1 to 0.7 g/cm3 and a cell
`number of more than 300 cells per mm3.
`2. Thermoformable sheeting according to claim 1, char-
`acterized in that the foam has a cell number of 800 to 2500
`
`cells per mm3.
`3. Thermoformable sheeting according to claim 1, char-
`acterized in that the foam has a cell number of 1000 to 2000
`
`cells per mm3.
`4. Thermoformable sheeting according to claim 1, char-
`acterized in that the foamed propylene homopolymer or
`copolymer consists of 60 to 80% of high melt strength
`propylene homopolymer or copolymer and of at least one
`further propylene homopolymer or copolymer.
`5. Thermoformable sheeting according to claim 1, char-
`acterized in that the foamed propylene homopolymer or
`copolymer has a density of 0.25 to 0.5 gr/cm3.
`6. Thermoformable sheeting according to claim 1, char-
`acterized in that the foamed propylene homopolymer or
`copolymer has a density of 0.36 to 0.50 gr/cm3.
`7. Thermoformable sheeting according to claim 1, char-
`acterized in that the sheeting, or the at least one foam layer
`has a thickness of between 0.5 and 3 mm.
`
`8. Thermoformable sheeting according to claim 1, char-
`acterized in that the sheeting or the at least one foam layer
`has a thickness of between 1 to 2 mm.
`
`9. Thermoformable sheeting according to claim 1, char-
`acterized in that the sheeting or at least one foam layer has
`a thickness of between 1.2 to 1.6 mm.
`
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`10. Thermoformable sheeting according to claim 1, char-
`acterized in that it has a thickness of 1.4 mm and consists of
`
`55
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`sheeting:
`polypropylene blend: 80% of high melt strength
`polypropylene, 20% of polypropylene copolymer
`additives: 1,1% blowing agent
`thickness of foam: 1.4 mm
`
`60
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`a foamed propylene homopolymer or copolymer containing
`80% of high melt strength propylene homopolymer or
`copolymer and has a density of 0.43 g/cm3 and a cell number
`of 1450 cells per mm3.
`
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