Europaisches Patentamt
`
`European Patent Office
`
`Office europeen des brevets
`
`111111111111111111111111111111111111111111111111111111111111111111111111111
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`@ Publication number:
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`0 520 028 81
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`EUROPEAN PATENT SPECIFICATION
`
`@ Date of publication of patent specification: 17.05.95 @ Int. Cl.6: C08J 9/12, C08J 5/18,
`C08L 23/12
`
`@ Application number: 91906914.6
`
`@ Date of filing: 13.03.91
`
`@ International application number:
`PCT/US91/01678
`
`@ International publication number:
`wo 91/13933 (19.09.91 91/22)
`
`@ POLYPROPYLENE FOAM SHEETS.
`
`® Priority: 14.03.90 US 493541
`
`@) Date of publication of application:
`30.12.92 Bulletin 92/53
`
`@ Publication of the grant of the patent:
`17.05.95 Bulletin 95/20
`
`@ Designated Contracting States:
`AT BE CH DE DK ES FR GB GRIT Ll LU NL SE
`
`@ References cited:
`EP·A· 178 282
`EP·A· 359 517
`DE·A· 2 633 307
`GB·A· 2 099 434
`
`EP·A· 178 283
`EP·A· 0 001 791
`GB·A· 1 262 889
`
`@ Proprietor: THE JAMES RIVER CORPORATION
`Tredgar Street,
`P.O. Box 2218
`Richmond,
`Virginia 23217 (US)
`
`@ Inventor: GAYLORD, Norman, G.
`28 Newcomb Drive
`New Providence, NJ 07974 (US)
`Inventor: KATZ, Leon
`195 Dogwood Court
`Stamford, CT 06903 (US)
`Inventor: PARK, John, P.
`952 Manchester Road
`Neenah, WI 54956 (US)
`
`@ Representative: van Gennip, Johannes Sim·
`eon w. (NL)
`Vereenigde Octrooibureaux
`Nieuwe Parklaan 97
`NL-2587 BN The Hague (NL)
`
`Note: Within nine months from the publication of the mention of the grant of the European patent, any person
`may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition
`shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee
`has been paid (Art. 99(1) European patent convention).
`
`Rank Xerox (UK) Business Services
`13.10/3.09/3.3.31
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`BOREALIS EXHIBIT 1048
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`EP 0 520 028 81
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`Description
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`1. Field of Invention
`
`This invention relates to polypropylene foam sheets and a process for their manufacture. Specifically,
`this invention relates to polypropylene foam sheets which are rigid or semi-rigid and thermoformable into
`shaped articles for use in packaging and service applications.
`
`2. Description of the Prior Art
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`A foamed plastic or cellular plastic has an apparent density which is decreased by the presence of
`numerous voids or cells dispersed throughout its mass (ASTM D883-80C). The cells may be interconnected
`(open-celled) and/or discrete and independent (closed-celled).
`The prior art discloses various methods for the preparation of foamed plastics. These include leaching
`15 out a solid or liquid which is dispersed in a plastic, sintering small particles of a plastic and dispersing
`cellular particles in a plastic. However, the most widely used method involves the dispersion of a gaseous
`phase throughout a fluid polymer phase and the retention of the resultant expanded form.
`The theory of the expansion process and the properties of various foamed plastics are reviewed in
`"Cellular Plastics", in Encyclopedia of Polymer Science and Engineering, vol. 3, pp. 1-59 (1985), which is
`incorporated herein by reference. As disclosed therein, the expansion process consists of three steps:
`creation of small discontinuities or cells in a fluid or plastic phase, growth of these cells to a desired volume
`and stabilization of the resultant cellular structure by physical or chemical means.
`The formation of discontinuities or bubbles within the fluid polymer, may arise from gases that are
`injected into the fluid polymer, low boiling liquids that are incorporated into the system as blowing agents
`25 and volatilize due to increased temperature or decreased pressure, gases that are produced as a result of a
`chemical reaction within the fluid polymer and chemical blowing agents which undergo thermal decomposi(cid:173)
`tion to form a gas.
`The rate of growth of the bubbles or cells depends upon the viscoelastic nature of the polymer phase,
`the blowing agent pressure, the external pressure on the foam, the cell size and the permeation rate of the
`30 blowing agent through the polymer phase.
`Cell or bubble stabilization relates to cell wall stability and the drainage of material from the membrane
`or wall which separates cells. Increasing the viscosity of the fluid reduces the drainage effect. The viscosity
`increase may be caused by chemical reactions which increase molecular weight through polymerization or
`crosslinking, or by temperature reduction, ultimately below the second order transition or crystallization
`temperature to prevent polymer flow.
`The present invention relates to rigid or semi-rigid foam sheets for use in food service applications. The
`prior art has utilized polystyrene for the manufacture of foam sheets for these applications. However,
`polystyrene articles suffer from low service temperature, and little or no photochemical or biological
`degradability and are relatively expensive.
`Polypropylene does not have these undesirable characteristics. Various processes have been reported
`in the prior art for the preparation of flexible or rigid polypropylene foams. The processes are designed to
`promote the three-step process described hereinbefore, i.e. creation of cells in a fluid or plastic phase,
`growth of the cells and stabilization of the resultant cellular structure.
`Blowing agents used in the preparation of polypropylene foam include azodicarbonamide (Lee et al, J.
`45 Appl. Polym. Sci. 32, 4639 (1986); EPO Pat. Appl. EP 190,021 ), chlorofluorocarbons (EPO Pat. Appl. EP
`1791, EP 71 ,981, EP 181 ,637; U.K. Pat. 1 ,400,494; U.K. Pat. Appl. GB 2,099,434 A), carbon dioxide (EPO
`Pat. Appl. EP 291 ,764), hydrocarbons, e.g. propane, butane, pentane (U.K. Pat. 1 ,400,494; U.K. Pat. Appl.
`GB 2,099,434 A) and water (EPO Pat. Appl. EP 122,460).
`Crystallization rate accelerators and/or nucleating agents used in the preparation of Polypropylene foam
`include titanium dioxide (EPO Pat. Appl. EP 122,460; U.K. Pat. Appl. GB 2,099,434 A) talc (U.K. Pat.
`1 ,400,494; U.K. Pat. Appl. GB 2,099,434 A), silica and silicates (EPO Pat. Appl. EP 1791; U.S. Pat.
`4,467,052), zeolite 4A (EPO Pat. Appl. EP 178,282, EP 178,283), sodium benzoate (Colton, Plast. Eng. 44-
`(8), 53 (1988) and dibenzylidene sorbitol (EPO Pat. Appl. EP 178,282).
`Citric acid-sodium bicarbonate combinations are considered as blowing agents in some patents and as
`55 nucleating agents in other patents (EPO Pat. Appl. EP 178,283; U.K. Pat. 1 ,400,494; U.K Pat. Appl. GB
`2,099,434 A; U.S. Pat. 4,467,052).
`The use of crosslinking agents during the preparation of a polypropylene foam has been reported in the
`prior art and include peroxides (Nojiri et al, Furukawa Review ~. 34 (1982) through Chem. Abstracts 97,
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`21725u (1982); EPO Pat. Appl. EP 181 ,637, 190,021) in the absence or presence of multifunctional vinyl
`monomers, azido functional silanes (EPO Pat. Appl. EP 181 ,637), vinyltrimethoxysilane (Lee et al, J. Appl.
`Polym. Sci. 32, 4639 (1986) and ionizing radiation in the presence of polyacrylic monomers (Nojiri et al,
`Furukawa Review g, 34 (1982); U.S. Pat. 4,424,293).
`Low density polypropylene foams "free from creases on the surface" have been prepared by
`incorporating high molecular weight fatty amides, amines or esters in the molten polyolefin (EPO Pat. Appl.
`EP 1791).
`The prior art teaches that polypropylene is not a unique material, i.e. processes that are applicable to
`the preparation of foam or microcellular structures from other polymers are applicable to the preparation of
`10 polypropylene foams.
`EPO Pat. Appl. EP 1791 describes "a process for the preparation of expanded thermoplastic synthetic
`resins" and discloses polyethylene, ethylene-vinyl acetate copolymer and isotactic polypropylene as the
`applicable thermoplastic resins.
`EPO Pat. Appl. EP 71 ,981 describes "foamed polypropylene resin molded articles" and discloses the
`use of ethylene-propylene copolymer as well as polypropylene.
`EPO Pat. Appl. EP 122,460 describes "resin foam produced by an aqueous medium" and discloses
`polymer foams from polypropylene, polyethylene and polystyrene.
`EPO Pat. Appl. EP 291 ,764 describes the "extrusion of propylene polymer foam sheets" and discloses
`a process for extruding blends of ethylene-propylene block copolymers containing less than 20% ethylene
`20 with block copolymers containing less than 5% ethylene or random ethylene-propylene copolymers or
`polypropylene.
`U.K. Pat. 1,400,494 describes "foamed polymeric sheet material and process therefor" and discloses
`polypropylene, high density polyethylene and nylon-12 as the preferred operable polymers while indicating
`that copolymers of ethylene with vinyl acetate or vinyl chloride can be conveniently used.
`U.K. Pat. Appl. GB 2,099,434 A describes an "extrusion process for propylene resin foams' and states
`that the resin may be isotactic polypropylene, an ethylene-propylene block or random copolymer or blends
`of polypropylene with numerous olefin homopolymers and copolymers.
`U.S. Pat. 3,637,458 describes "microcellular foam sheet" from a linear, thermoplastic crystalline
`polymer and claims isotactic polypropylene and linear polyethylene foam sheet.
`U.S. Pat. 3,819,784 describes " a process for preparing molded polyolefin foam" and discloses that
`suitable polyolefins used in the process include low density polyethylene, high density polyethylene,
`isotactic polypropylene, poly-1-butene and copolymers of ethylene with propylene or vinyl acetate.
`U.S. Pat. 3,830,900 describes "method of forming foamed plastic sheets" and discloses that the
`method is applicable to polyvinyl chloride, polystyrene, polyethylene, polypropylene and acrylonitrile-
`35 butadiene-styrene copolymers.
`U.S. Pat. 4,467,052 describes a "tray for packaging food products" and discloses an injection molding
`process for the preparation of foam trays from blends of polypropylene and styrene-butadiene rubber.
`Colton (Piast. Eng. 44(8), 53 (1988) describes "making microcellular foams from crystalline polymers"
`and discloses microcellular polypropylene and ethylene-propylene copolymer foams.
`EPO Pat. Appl. 181 ,637 describes "lightly crosslinked linear olefinic polymer foams" prepared from
`melt blends of one or more polymers selected from high density polyethylene, linear low density
`polyethylene, polypropylene and polystyrene.
`EPO Pat. Appl. EP 190,021 describes "heat-foamable crosslinked propylene resin compositions" and
`discloses blends of propylene-a-olefin copolymers or 1-butene-o:-olefin copolymers with polypropylene.
`U.S. Pat. 4,424,293 describes "crosslinkable polypropylene composition" and discloses foams from
`isotactic polypropylene and ethylene-propylene copolymer.
`The prior art uses "polypropylene" as a self-explanatory term for a polymer prepared from propylene
`monomer. In some cases the terms "isotactic polypropylene" and "crystalline polypropylene" are used. In
`only a few patents is the polypropylene characterized to any further extent.
`EPO Pat. Appl. EP 71 ,981 discloses polypropylene foams prepared from resins having a latent heat of
`crystallization of (9-28 cal/g) 3.77 - 11.7(x1 04 J/kg). U.S. Pat. 3,637,458 discloses polypropylene foams
`prepared from polymers of "at least film foaming molecular weight, substantially free from crosslinking, and
`having a work-to-break (WTB) value of at least (1 0,000 inch-lbs/inch3) 6.90 x 107 N/m2 ". U.K. Pat. Appl. GB
`2,099,434 A discloses polypropylene foams prepared from resins having a melt tension of at least 3 grams
`55 at 190 • C and a maximum/minimum melt tension ratio of not more than 2.5/1.
`Application of the processes of the prior art to generic or commercial polypropylene resins, described
`as polypropylene, isotactic polypropylene or crystalline polypropylene, fails to yield the polypropylene foam
`sheet of the present invention.
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`SUMMARY OF THE INVENTION
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`An object of the present invention is to provide a foam sheet material having a high impact strength, a
`modulus suitable for rigid or semi-rigid packaging applications and convertible into trays, plates, cups,
`bowls, dishes, containers and other articles used in food service.
`Another object of the instant invention is to provide a thermoformable foam sheet which has a high
`service temperature.
`A further object of the present invention is to provide a foam sheet having high insulation properties and
`cost advantages over existing foam sheets.
`Yet another object of the present invention is to provide a foam sheet prepared from a polypropylene
`resin and having a density ranging from (2.5 to 25 lbs/ft3) 40.1 to 401 kg/m3 and a modulus of at least
`(10,000 psi) 6.90 x 107 N/m2 .
`Still another object of the present invention is to provide a polypropylene foam sheet having a uniform
`cell structure and smooth surfaces.
`It has now been found that these improvements in a foam sheet can be achieved by extruding high
`melt strength, high melt elasticity polypropylene, characterized by at least (a) either high M2 or high M21Mw
`ratio, and (b) either high equilibrium compliance Jeo obtained from creep measurements or high recoverable
`strain per unit stress Sr/S obtained from steady shear measurements.
`In one embodiment, the present invention provides a rigid or semi-rigid polypropylene foam sheet
`20 having a density ranging from (2.5 to 25 lbs/ft3) 40.1 to 401 kg/m3, tensile and flexural moduli of at least
`(10,000 psi) 6.90 to 107 N/m2 , a cell size of (5-18 mils) 0.127-0.457 mm and a thickness ranging from about
`(0.02 to 0.20 inches) 0.508 to 5.08 mm. The polypropylene foam sheet is thermoformable and has uniform
`cell structure and smooth surfaces.
`In another embodiment of this invention, a process is provided for producing the polypropylene foam
`sheet of the invention. The process may be conducted using a single or tandem extrusion line. The latter is
`preferred and, by the use of primary and secondary extruders in series, a continuous foam sheet is
`produced. The process comprises mixing polypropylene resin, having a high melt strength and a high melt
`elasticity, with a nucleating agent in the primary extruder, plasticating the mixture, injecting a physical
`blowing agent into the plasticated mixture to form a foaming mixture, which is transferred to a secondary
`30 extruder, mixing and cooling the foaming mixture and extruding the foaming mixture through an annular or
`flat die into a continuous foam sheet.
`In another embodiment of this invention, a method is provided for forming rigid or semi-rigid articles
`from the polypropylene foam sheet of the invention. The process comprises heating the foam sheet to a
`temperature which permits deformation under vacuum or pressure, supplying the softened foam sheet to a
`forming mold and cooling the foam sheet to form a rigid or semi-rigid article having the shape of the mold.
`It has been found that preferred polypropylene foamed sheets of the invention possess an advantageous
`degree of drawability, making it possible to form such products having a finished total area of more than 3,
`often up to 6, times the area of the blank from which the product is formed.
`In another embodiment of the invention, a rigid or semi-rigid multilayer structure is provided. The
`40 multilayer structure comprises at least one layer of the polypropylene foam sheet of the invention and at
`least one layer having functional properties, e.g barrier properties.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`Fig. 1 is a schematic drawing of a tandem foam extrusion line.
`Fig. 2 is a scanning electron microscope (SEM) micrograph of a cross section of a polypropylene foam
`sheet prepared from polypropylene resin A-6.
`Fig. 3 is an SEM micrograph of a cross section of a polypropylene foam sheet prepared from
`polypropylene resin A-7.
`Fig. 4 is an SEM micrograph of a cross section of a polypropylene foam sheet prepared from
`polypropylene resin A-2.
`Fig. 5 is an SEM micrograph of a cross section of a polypropylene foam sheet prepared from
`polypropylene resin A-17.
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`55 DETAILED DESCRIPTION OF THE INVENTION
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`In accordance with the present invention, it has been found that a thermoformable polypropylene foam
`insulation properties, may be
`sheet having high modulus impact strength, service temperature and
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`produced by a process which comprises the steps of m1xmg a polypropylene resin having specific
`molecular and rheological characteristics with a nucleating agent, plasticating the mixture, introducing a
`physical blowing agent into the plasticated mixture to form a foaming mixture, mixing, cooling and extruding
`the foaming mixture through an extruder die into a foamed extrudate which is formed into a continuous
`foamed sheet.
`Acceptable foam sheets produced from the operable polypropylene resins by this process, have a
`density between (2.5 and 25.a lb./ft3) 4a.1 and 4a1 kg/m3 , suitable for semi-rigid and rigid packaging and
`food service applications and have a substantially uniform cell structure and smooth surfaces. Foams which
`are unacceptable and unsatisfactory for thermoforming into objects for the indicated applications, have non-
`10 uniform cell structures, rough surfaces and densities outside of this range.
`The base resin plays the major role in determining the foamability and the properties of the foam
`products made therefrom. The polypropylene resins which yield acceptable foams, particularly when
`processed by the method disclosed herein, may be distinguished from the polypropylene resins which yield
`unsatisfactory foams, by their molecular and rheological characteristics.
`The melt strength of a polymer is important in processes such as foaming where deformation is
`primarily elongational and tensile stresses are present. High molecular weight polypropylene resins are
`frequently characterized as "high melt strength" (HMS) resins. However, unexpectedly, it has been found
`that this characterization is inadequate and that numerous high molecular weight polypropylene resins,
`designated and marketed as "high melt strength" resins fail to yield acceptable foam sheets.
`The molecular weight distribution in a sample of polypropylene may be determined by high tempera-
`ture gel permeation chromatography (GPC). The Waters 15a CV GPC chromatograph may be used at
`135 • C with trichlorobenzene as carrier solvent and a set of Waters u-Styragel HT, (1 a3, 1 a4 , 1 as and 1 a6
`A) 1 a2, 1 a3, 1 a4 and 1 as nm columns. The solution concentration is a.2% (w/v) and the flow rate is 1
`ml/min.
`GPC provides information about (a) the number average molecular weight Mn which is the arithmetical
`mean value obtained by dividing the sum of the molecular weights by the number of molecules and thus is
`dependent simply upon the total number of molecules, (b) the weight average molecular weight (Mw) which
`is the second-power average of molecular weights and is more dependent on the number of heavier
`molecules than is Mn, and (c) the z-average molecular weight (Mz) which is the third-power average of
`30 molecular weights.
`Colligative properties are related to Mn, bulk properties associated with large deformations such as
`viscosity and toughness are affected by Mw values and melt elasticity is more closely dependent on Mz
`values.
`The polypropylene resins which are effective in yielding acceptable foams by the process of the
`35 present invention, are of high molecular weight with an Mz value above 1.a x 1 a6 and an M21Mw ratio above
`3.a. The polydispersity index Mw!Mn
`is of less significance since it does not differentiate between
`polypropylene resins which give acceptable foams and those which give unsatisfactory foams. Resins
`having Mz and M21Mw values below the indicated values yield foam sheets which are unacceptable.
`GPC chromatograms of resins which yield unacceptable foam sheets, using a viscometer detector,
`show a unimodal molecular weight distribution and plots of the branching factor g' versus log molecular
`weight (Mw), where
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`show the absence of significant branching, i.e. the chains are essentially linear. In contrast, resins which
`yield acceptable foam sheets show a bimodal molecular weight distribution, wherein the major component
`is largely linear while the higher molecular weight minor component is highly branched.
`Melt flow rates of resins which may be utilized in the process of the present invention, range from a.2 to
`12 g/1 a min, measured in a melt flow instrument at 23a • C under a load of 2.16 kg.
`The importance of melt elasticity in the conversion of polypropylene resins to acceptable foam sheets,
`indicated by Mz values is confirmed by rheological characterization of polymer melts in a shear field.
`The rheological characterization of the polypropylene resins was conducted with a programmed
`Rheometries Mechanical Spectrometer (RMS-saa). Resin pellets were compression molded into sheets
`from which samples were stamped out with a 25 mm diameter circular die. Tests were conducted at 21 a±
`55 1 • C using 25 mm parallel plate geometry with a 1.4 mm gap. Creep data were obtained under a constant
`stress of (1 aaa dyn/sq.cm) 1 aa N/m2 for a period of a-3aa sec. The creep compliance J(t) is given by
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`t,(t)
`=-=
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`J(t)
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`t
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`+
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`where
`t = strain
`a 0 = stress
`Jeo = equilibrium compliance
`'llo = zero shear viscosity
`The equilibrium compliance Jeo is a measure of melt elasticity and is determined by first plotting strain
`against time at constant stress. The strain as a function of time is divided by the stress to give J(t). Jeo is
`the intercept of the J(t) against time plot.
`Polypropylene resins which yield acceptable foam sheets, by the process of the present invention, have
`equilibrium compliance Jeo values above (12 X 1 o-s cm2/dyne) 1.2 X 1 o-3 m2/N. Resins having Jeo values
`below this value yield unacceptable foam sheets with non-uniform cell structure and uneven surfaces.
`The recoverable shear strain per unit stress Sr/S also distinguishes polypropylene resins which yield
`acceptable foam sheets from those which yield unacceptable foam sheets. This quantity is a fundamental
`20 measure of melt elasticity. Using the programmed Rheometries Mechanical Spectrometer, the polymer melt
`is subjected to clockwise rotational shear strain by the driver and the resulting shear stress S and first
`normal stress N, are measured by a transducer. The shear rate range is 0.01-10 s- 1, the time before
`measurement is 2.2 min and the time of the measurement is 0.3 min. Normal stress measurements are
`obtained at each shear rate. The recoverable shear strain Sr is obtained from the first normal stress
`25 difference.
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`The normalized quantity Sr/S, i.e. recoverable shear strain per unit stress is a measure of melt elasticity.
`Polypropylene resins which are extrudable to acceptable foam sheets have Sr/S values at 1 sec- 1 above (5
`X 1 o-s cm2/dyne) 5.0 X 1 o-4 m2/N, generally above (7 X 1 o-s cm 2/dyne) 7.0 X 1 o-4 m2/N, while
`35 unacceptable foam sheets are obtained from resins with lower values.
`In addition to the polypropylene resins which have the molecular and rheological characteristics
`indicated above, blends of polypropylene resins with other polymers are also of use in the practice of the
`present invention, provided said blends have the necessary molecular and rheological characteristics. Thus,
`blends of the polypropylene resins which are useful in the preparation of the foam sheets of this invention,
`40 with other polymers such as linear polypropylene or polyethylene resins may be used if such blends haved
`the required characteristics. Further, blends of linear polypropylene resins with branched polyethylene or
`polypropylene resins may be used when such blends have the necessary molecular and rheological
`characteristics. The branched polyolefins may be prepared by exposure of the linear polyolefins to low level
`radiation, in accordance with U.S. Pat. 4,525,257 or by other appropriate methods.
`In accordance with the present invention, an extrusion process is provided for converting the poly-
`propylene resins having the necessary molecular and rheological characteristics to acceptable foam sheets.
`Twin or single screw extruders may be used. Single extruders or, preferably, primary and secondary
`extruders, generally called tandem extruders, are effective in conducting the mixture of polypropylene resin
`and additives through the necessary plasticating, mixing and cooling steps which are followed by extrusion
`to foam sheet having uniform cell structure and smooth surfaces.
`A tandem extrusion line is schematically represented in Figure 1. The base resin and the nucleating
`agent are added from separate feeders through a single port to the unheated zone 1 of the primary extruder
`7. The mixture is moved by the plasticating and mixing screw trough the heated zones 2-6. The blowing
`agent is added to the plasticating mixture in zone ~- The resultant "foaming mixture" is transferred to the
`secondary extruder ~ through the heated crossover ~- Mixing and cooling occur as the screw carries the
`mixture through heated zones 1 0-13. The melt pump 14 moves the "foaming mixture" into the heated die
`15 and through zones 16-18. The die may be either a circular (annular) or flat die. The extruded foaming
`mixture forms a foamed extrudate which is sheet-like when coming through a flat die, or tubular when
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`coming through an annular die. In either case, the extrudate is cooled by an air ring ~ attached to the die
`lip. If an annular die is used, the extruded foam tube is pulled over a cooling/sizing drum 20 where it is
`further cooled by an air ring ~- The extruded foam tube or sleeve is split while passing over the drum 20.
`The extrudate from either a flat die or an annular die, the latter after splitting and spreading, is then
`flattened into a foam sheet by passage over a series of rolls, e.g. an S-wrap, and then taken up on a winder.
`The continuous foam sheet is then aged for a period of time to allow for diffusion of the blowing agent and
`air through the cell walls to bring it to equilibrium, prior to further fabrication, if any.
`The extrusion process disclosed above is representative and not limiting as to equipment and
`procedural details. Alternative equipment and variations in the procedure will be obvious to those skilled in
`the art.
`A polypropylene foam sheet may be provided with a substantially non-cellular outer layer or skin. Such
`a skin may give the foam a superior outer appearance in that the foam structure with skin may have a shiny
`or glossy appearance, and is also resistant to surface abrasion and cutting. The skin also acts as a stiffener
`to enable a lighter and/or thinner structure having a maximum bending stiffness to be obtained. A skin may
`be formed on a single layer structure by changing the flow rate and/or the temperature of the air which is
`applied to the surface of the tubular or flat extrudate coming out of a die. Alternatively, a skin layer may be
`formed by using a multimanifold die or combining feedblock to co-extrude a non-foamed polypropylene or
`other layer on the outside of a polypropylene foam layer.
`In accordance with another embodiment of the present invention, a multilayer foam sheet is provided.
`20 This foam sheet comprises at least one layer of the polypropylene foam sheet of this invention and at least
`one functional layer. The presence of functional layers in the multilayered foam sheet of the invention can
`effectively act as a water vapor or gas barrier. The use of functional layers in combination with poly(cid:173)
`propylene foam layers thus can enable a product to be produced that is effective for use as a container
`which not only has the advantageous properties of the polypropylene foam, but in addition can act as a
`25 barrier to air or water vapor and thus can be useful in packaging application where an extended shelf-life is
`desirable.
`The functional layer which can be utilized in accordance with the present invention include ethylene(cid:173)
`vinyl alcohol and vinylidene chloride copolymers and polyamides. A typical multilayered foam sheet
`configuration, in accordance with the invention, might include one or more functional layers sandwiched
`30 between two layers of polypropylene foam sheet. Typically, the thickness of the functional layer or layers
`will constitute less than about 5% of the total thickness of the multilayer construction. In cases where the
`materials utilized as the functional layer are not compatible with or adherent to the polypropylene foam
`layers, it may be desirable, in accordance with the invention, to utilize "tie" layers between the functional
`and polypropylene foam layers. These tie layers may function to hold the functional and polypropylene
`foam layers together and thus act as adhesives.
`Typical tie layers are based on olefin copolymers containing polar functionality, e.g. ester, carboxyl and
`amide groups, generally prepared by copolymerization of an olefin monomer of graft copolymerization of an
`olefin polymer with one or more monomers containing the poly functionality. Thus, polypropylene-maleic
`anhydride and polypropylene-acrylic acid graft copolymers and the like are effective tie layers.
`In accordance with the invention, a process is provided for producing a multilayered polypropylene
`foam sheet. The process comprises the steps of mixing pelletized polypropylene resin with a nucleating
`agent, plasticating the mixture, introducing a physical blowing agent into the substantially plasticated
`mixture to form a foaming mixture, mixing and cooling the foaming mixture, supplying the foaming mixture
`and a separately plasticated functional resin to a combining feedblock or multimanifold die of an extruder,
`45 and co-extruding the foaming mixture and plasticated functional resin into a continuous multilayered foam
`sheet. The latter may then be aged for a period of time prior to further processing, if any.
`The process and materials utilized to form the foaming mixture are the same as those described earlier
`herein in the process for producing the single layer foam sheet. The polypropylene foam layers may be
`formed by use of tandem extruders, as in the single layer process. The functional layers utilized in the
`50 process for producing multilayered polypropylene foam sheets are preferably plasticated in separate,
`additional extruders. In addition, tie layers utilized between functional and polypropylene foam layers also
`may be fed from separate extruders.
`In accordance with the invention, the polypropylene foaming mixture and the functional resin are
`combined by means of a multimanifold die, which has multiple inlet ports,or a combining feedblock, each of
`55 which is well known in the art. In addition, if the use of tie layers is desired, these materials may also be fed
`to the multimanifold die or combining feedblock. By means of the die or feedback, three or more layer
`structures can be co-extruded by utilizing either an annular die with multiple inlet ports or a flat die utilizing
`multiple inlet ports or a combining feedblock. By the use of such equipment, it is possible to produce a
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`multi-layered foam structure having, for example, an outside polypropylene foam layer and an inside
`polypropylene foam layer with a functional layer sandwiched therebetween. In addition, tie layers may be
`utilized between the functional and polypropylene foam layers.
`A skin may be formed on an outside polypropylene foam layer which is part of a multilayer structure in
`a similar manner to that used with a single layer structure.
`The foaming mixture for the preparation of the polypropylene foam sheet of the present invention
`consists of the polypropylene resin, blowing agent and nucleating agent.
`The molecular and rheological characteristics of the polypropylene used in the process of the present
`invention, have been described hereinbefore and include at least
`(a) either M2 <;:; 1.0 x 106
`or M21Mw <;:; 3.0
`and (b) either Jeo ;;;; (12 x 1 o-s cm2/dyne)
`1.2 x 10-3 m2/N
`or Sr/S ;;;; (5 x 1 o-s cm2/dyne)
`5.0 x 1 o-4 m2/N at