(12) United States Patent
`Wu et al.
`
`US006383425B1
`(10) Patent N0.:
`US 6,383,425 B1
`(45) Date of Patent:
`May 7, 2002
`
`(54) METHOD FOR EXTRUDING FOAMED
`POLYPROPYLENE SHEET HAVING
`IMPROVED SURFACE APPEARANCE
`
`9/1980 Eslinger et a1. ........... .. 156/243
`4,221,624 A
`4,267,146 A * 5/1981 KurtZ et a1. . . . . . . . .
`. . . .. 425/461
`4,372,739 A
`2/1983 Vetter et a1.
`425/466
`
`4,596,833 A
`
`6/1986 Endo et a1. . . . . .
`
`. . . . . .. 521/60
`
`.
`
`.
`
`4,650,816 A
`
`3/1987 Bertrand . . . . . .
`
`. . . . .. 521/85
`
`(75) Inventors‘ JAelsshealvttlé lggilgvg?gg{gnsl)es H‘ Han’
`9
`’
`
`5,134,174 A * 7/1992 Xu et al. . . . . .
`5,149,579 A
`9/1992 Park et al.
`
`. . . .. 521/143
`428/213
`
`.
`
`,
`
`,
`
`(73) AsslgneeZ BP_C°rP°rat1°n North Amenca Inc"
`Ch1Cag0>IL(US)
`
`(*) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(1)) by 0 days.
`
`(21) APP1- NO-I 09/386,759
`(22) P116011
`Aug- 31, 1999
`
`Related US. Application Data
`(60) Provisional application No. 60/098,958, ?led on Sep. 3,
`1998, provisional application No. 60/122,129, ?led on Mar.
`1, 1999, and provisional application No. 60/128,173, ?led
`on Apr- 6, 1999-
`(51) Int. c1.7 .............................................. .. 1329c 44/20
`(52) US. Cl. ........................... .. 264/51; 264/53; 264/54;
`425/461
`(58) Field of Search ............................ .. 264/51, 53, 54;
`425/461
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`5,180,751 A
`
`1/1993 Park et al. . . . . . .
`
`. . . . . .. 521/51
`
`1/1993 Morita et al. ............... .. 521/85
`5,180,754 A
`7/1993 KretZschmann et al.
`252/350
`5,225,107 A
`5,462,974 A 10/1995 Lee ........................... .. 521/79
`5,494,429 A
`2/1996 Wilson et al. ........ .. 425/192 R
`5,688,457 A 11/1997 Buckmaster et al. ...... .. 264/211
`5,753,155 A * 5/1998 Hanusa ...... ..
`264/46.5
`5,929,129 A * 7/1999 Feichtinger ............... .. 521/134
`
`EP
`JP
`
`FOREIGN PATENT DOCUMENTS
`0611793
`8/1994
`4732585
`8/1972
`_
`_
`* cued by examlner
`_
`_
`P rlmary EX?mln€V—A11aI1 R. KllhIlS
`(74) Attorney, Agent, or Firm—Wallace L. Oliver; Richard
`1561110“
`(57)
`
`ABSTRACT
`
`An improved Coat hanger @Xtrusion die wherein the aperture
`of the die land Widens along the extrusion axis of said die to
`the die exit opening, thereby forming an expansion Zone,
`and a method for extruding foamed polypropylene sheet
`having an improved surface appearance.
`
`3,920,365 A * 11/1975 Mules ...................... .. 425/461
`
`13 Claims, 2 Drawing Sheets
`
`///
`
`Page 1 of 12
`
`BOREALIS EXHIBIT 1057
`
`

`
`U.S. Patent
`
`May 7, 2002
`
`Sheet 1 0f 2
`
`US 6,383,425 B1
`
`FIG. 1
`
`Page 2 of 12
`
`

`
`U.S. Patent
`
`May 7, 2002
`
`Sheet 2 0f 2
`
`US 6,383,425 B1
`
`Page 3 of 12
`
`

`
`US 6,383,425 B1
`
`1
`METHOD FOR EXTRUDING FOAMED
`POLYPROPYLENE SHEET HAVING
`IMPROVED SURFACE APPEARANCE
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`This application claims the bene?t of US. Provisional
`Application No. 60/098,958, ?led Sep. 3, 1998, US. Pro
`visional Application No. 60/122,129, ?led Mar. 1, 1999, and
`US. Provisional Application No. 60/128,173, ?led Apr. 6,
`1999.
`
`10
`
`BACKGROUND OF THE INVENTION
`
`2
`sheet With a smooth surface skin and a uniform cell struc
`ture. According to Park et al, high melt strength, high melt
`elasticity polypropylene With a particulariZed combination
`of molecular and rheological characteristics including bimo
`dal molecular Weight distribution and a minor component
`that is highly branched is necessary to provide acceptable
`foam sheet. Patentees provide comparisons shoWing that
`loW density foam sheet extruded using conventional or
`generic polypropylene resins, further characteriZed as
`polypropylene resins With monomodal molecular Weight
`distributions and an absence of signi?cant branching, gen
`erally have roughened sheet surfaces and non-uniform
`microcellular structure and are unacceptable for commercial
`use.
`The surfaces of extruded polyole?n foam sheet generally
`lack the smooth, shiny, uniform and substantially unblem
`ished surfaces observed With extruded styrenic foam sheet,
`particularly including higher density ABS foam sheet. For
`example, surface roughness is commonly encountered When
`extruding polyethylene foam sheet, and lack of uniformity in
`cell structure and distribution at the surface is visually more
`readily apparent because of the transparent nature of un?lled
`polyethylene. Sensible surface roughness, that is, roughness
`that can be sensed tactilely, may be reduced by contacting
`the loWer melt temperature polyethylene sheet With a pol
`ishing roll during the extrusion process to give a smooth,
`more even surface. The surface imperfections that remain
`are mainly visible density variations and are generally
`uniformly distributed, providing a textured or marbleiZed
`surface appearance that is pleasing and generally acceptable.
`Rigid polypropylene foam sheet obtainable from conven
`tional resins by the processes currently knoWn and practiced
`in the art continues to be someWhat lacking in surface
`appearance characteristics. Characteristically, polypropy
`lene foam sheet extruded With conventional processes and
`using conventional or generic polypropylene resins Will
`have regularly-spaced markings in the form of alternating
`bands or corrugation-like markings extending the length of
`the sheet in the machine direction. In light, loW-density
`foams obtained from conventional polypropylene resins,
`particularly soft, ?exible foams having densities of 20 lb/ft3
`(0.3 g/cm3) and loWer, these bands may have the form of a
`regularly spaced, Wave-like or sinusoidal distortion, forming
`a corrugated sheet. The bands or corrugations become less
`pronounced for rigid foam sheet and, particularly at higher
`foam densities, are seen as surface ?aWs or appearance
`defects that take the form of linear, valley-like surface
`depressions along the machine direction.
`The surface roughness of sheet extruded using these
`higher melting resins is more difficult to smooth adequately
`using a polishing roll. Moreover, the imperfections and
`visible density variations found in the surfaces of extruded
`polypropylene foam sheet are often not uniformly distrib
`uted over the surface, and are generally quite visible, even
`for sheet that otherWise is tactilely smooth. In rigid, higher
`density foams, such as are sought for the production of food
`service articles, the defects more often appear as a pattern of
`alternating linear bands of high and loW foam density,
`characteriZed by readily visible variations in translucence
`and surface gloss, possibly including surface voids, bubbles,
`streaks and uneven color. Such ?aWs may be Without sig
`ni?cant effect on the mechanical properties of the foam, and
`generally do not affect the performance of ?nished goods
`fabricated from such foam. HoWever, in consumer goods,
`food packaging, and the like, these visible surface defects
`and related cosmetic ?aWs are highly undesirable, thus
`limiting acceptance of polypropylene foam sheet by the
`industry.
`
`15
`
`25
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`30
`
`35
`
`This invention relates to polyole?ns, and more particu
`larly to improved expanded or foamed compositions com
`prising propylene polymers. Still more particularly, the
`invention relates to an apparatus including an improved
`extrusion die and to a method for the extrusion of rigid or
`semi-rigid polypropylene foam sheet having improved sur
`face appearance. Polypropylene foam sheet according to the
`invention is readily thermoformable into shaped articles that
`are particularly useful in rigid and semi-rigid packaging and
`in fabricating trays, plates, containers and other articles used
`in food service applications.
`Polystyrene has found Wide acceptance for use in food
`service applications because of its good rigidity and shape
`retention and, as foam sheet, it is readily molded and
`thermoformed. HoWever, polystyrene articles suffer from
`loW service temperature, and generally are fragile and lack
`chemical resistance. The food service and packaging arts
`have long sought alternative materials that do not have these
`undesirable characteristics.
`Polyole?n resins are Widely knoWn for their ease of
`fabrication and are found in a great variety of applications.
`Propylene polymers, or polypropylene resins, are particu
`larly noted for their good heat resistance and mechanical
`properties, and resin formulations based on polypropylene
`are supplied to meet the demands imposed by a variety of
`structural and decorative uses in the production of molded
`parts for appliances, household goods and autos. Impact
`modi?ed polypropylene and elastomeric ethylene-propylene
`copolymers have found application in automotive applica
`tions including interior trim as Well as in exterior parts such
`as bumper facia, grill components, rocker panels and the
`like. Polypropylene resins have the thermal and chemical
`resistance to Withstand exposure to the Wide variety of
`environments encountered in automotive uses, and are easily
`molded at a cost far beloW that of metal stamping to provide
`parts that resist rust and corrosion and are impact resistant,
`even at loW temperature. Considerable effort has been
`expended in recent years to develop rigid expanded or
`foamed polyole?n sheet as a replacement for polystyrene
`foams, particularly for use in food service applications. A
`number of processes for producing polypropylene foam
`55
`have been disclosed and are Well described in the art,
`including for example the methods disclosed in US. Pat.
`No. 5,180,571 to J. J. Park, et al and those set forth in the
`references cited and summariZed therein.
`At the surface of extruded foam sheet there generally may
`be found a layer consisting substantially of crystalline
`polypropylene (PP). This surface layer or skin is important
`to part appearance and surface hardness. The thickness and
`crystallinity of the PP surface layer that forms depends in
`part upon extrusion conditions including die temperatures
`and cooling rates, and upon annealing. The Park et al patent
`is directed to the extrusion of polypropylene to provide foam
`
`40
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`Page 4 of 12
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`

`
`US 6,383,425 B1
`
`3
`Coextrusion of multilayer sheets having solid outer skins
`and a foamed core has been disclosed in the art and is Widely
`used to overcome surface appearance problems encountered
`in the production of a variety of prior art foam sheet
`materials including those made from polystyrene and ABS.
`Foam core sheet, provided With a shiny or glossy unfoamed
`surface layer formed of the same or another resin, may be
`improved in resistance to surface abrasion and cuts and have
`a superior appearance. The more rigid skin serves to stiffen
`the foam structure, alloWing a lighter and thinner structure
`While attaining maximum bending stiffness. Foam sheet
`coextrusion processes are Well described in the art for use
`With a variety of resins such as polystyrene and ABS, and
`methods have been recently disclosed for use in the coex
`trusion of multilayered foam sheet comprising polyole?ns
`including polypropylene. Coextrusion processes suffer the
`disadvantage of generally requiring more costly feedblocks,
`dies and related machinery having a more complicated
`design, thereby increasing the complexity of the operation
`and raising cost of producing such foam sheet.
`Thus, there continues to be a need for a reliable method
`for the manufacture of rigid, high density foamed polypro
`pylene sheet comprising conventional generic polypropy
`lene resins With reduced visible surface defects and related
`cosmetic ?aWs and having the attractive, defect-free surface
`appearance necessary for acceptance in the food service and
`packaging industries.
`SUMMARY OF THE INVENTION
`The invention pertains to apparatus and method for the
`production of foamed polypropylene sheet With improved
`appearance characteristics. More particularly, the invention
`relates to an improved extrusion die for use in extruding
`foamed polypropylene sheet having excellent surface
`appearance and to a method for extruding improved foamed
`polypropylene sheet.
`Rigid, foamed polypropylene sheet according to the
`invention has a loW average surface roughness uniformly
`distributed over the surface of the foam sheet, and a sub
`stantial absence of corrugation and surface banding. When
`molded or otherWise thermoformed, the invented foam sheet
`Will afford rigid or semi-rigid articles having improved
`surface appearance While retaining a good balance of
`mechanical properties including stiffness and toughness.
`The invention thus may also be characteriZed as a method
`for providing molded articles having improved appearance
`comprising expanded or foamed polypropylene.
`BRIEF DESCRIPTION OF THE DRAWING
`FIG. 1 is a diagrammatic, perspective vieW, partially in
`phantom, of a typical coathanger-type sheet extrusion die.
`FIG. 2 is a sectional vieW of a prior art polyole?n sheet
`extrusion die, taken along line 2—2 of FIG. 1.
`FIG. 3 is a sectional vieW, taken along line 2—2 of FIG.
`1, shoWing an improved sheet extrusion die according to the
`invention.
`FIG. 4 is a sectional vieW, taken along line 2—2 of FIG.
`1, shoWing an alternative embodiment of the improved sheet
`extrusion die according to the invention.
`FIG. 5 is a fragmentary, enlarged sectional vieW shoWing
`the detail of the die land portion of the embodiment of FIG.
`4.
`
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`
`DESCRIPTION OF THE PREFERRED
`EMBODIMENT(S)
`Thermoformable polyole?n foam sheet having improved
`surface appearance together With high modulus, service
`
`65
`
`4
`temperature and thermal insulation properties Will be
`obtained by extruding a foamable polyole?n composition
`employing an improved, coathanger-type foam sheet extru
`sion die according to the invention.
`Processes for extruding foam sheet generally employ an
`extrusion apparatus having single or multiple extruders,
`Which may be single or tWin screW extruders, to conduct the
`mixture of polypropylene resin and additives through the
`plasticating and mixing steps, and provide a molten, foam
`ing or foamable resin mass to the inlet of a sheet extrusion
`die. Preferably the extrusion die Will be a coathanger type
`sheet extrusion die Wherein the inlet extends to a coathanger
`shaped plenum in the form of a relatively Wide and vertically
`narroW cavity, elongated in the horiZontal or transverse
`direction (Y axis) and relatively narroW in the vertical
`direction (Z axis). The resin ?oW direction or machine
`direction may also be referred to as the extrusion axis (X
`axis). The plenum is in liquid communication With an exit
`port or mouth extending along the Width or Y axis of the die,
`forming a slit de?ned by die lips. Molten, foaming resin
`enters the die through the inlet, is spread across the Width of
`the die by Way of the plenum, passes betWeen die lips and
`exits through the exit port or die exit in a molten or
`semi-molten state as a continuous sheet. The extruded sheet
`Will then be cooled to become solidi?ed, for example by
`being passed through a roll stack to cool the foam and ?nish
`the sheet. Differential roll speeds and take-up speeds may be
`employed to draW the foam sheet, orienting the crystalline
`polypropylene and achieving a ?nal form and thickness for
`the sheet.
`Turning noW to the draWings, it may be seen in FIG. 1 that
`a typical coathanger die, generally designated by the refer
`ence numeral 10, for extruding thermoplastic sheet Will
`comprise a ?rst or loWer half 1 and a second or upper half
`2, indicated in phantom. Assembled in opposed relationship,
`the halves form cavity 3. Molten foaming resin enters the die
`through inlet 4 and ?oWs into coathanger-shaped plenum 5.
`Plenum 5 spreads the molten resin uniformly across the
`Width of the die as it ?oWs through the preland passage 6 to
`melt Well 7. Adjustable choke means (not shoWn) may be
`included to provide control of resin ?oW, and any differences
`in pressure still remaining may be evened out by melt Well
`7. The molten resin continues through planar extrusion
`passageWay or die land 9 de?ned by the opposing, spaced
`apart faces of loWer die lip 11 and upper die lip 12, exiting
`the die through exit opening 8 forming a sheet.
`It Will be understood that the die body may include
`passages for heating and cooling, and further that clamping
`and fastening means and means for assembling the die to the
`extruder, also required, have been omitted from the draWings
`for clarity.
`In FIG. 2 it Will be seen that Prior Art sheet extrusion die
`20 includes assembled upper and loWer halves 21 and 22,
`together de?ning cavity 23, and upper and loWer die lips 14
`and 15. As described above, molten foaming resin Will be
`supplied under pressure by extruder means (not shoWn) to
`die cavity 23 through inlet 24, in ?uid communication With
`exit opening 28. FloWing into the coat hanger plenum 25,
`and dammed by the narroWing of the cavity at preland
`passage 26, the melt stream is spread across the Width of the
`die by plenum 25 and ?lls melt Well 27. The molten resin,
`further regulated by adjustable choke means 61, ?oWs from
`melt Well 27 and passes through extrusion passageWay or die
`land 29, exiting the die through exit opening 28 as continu
`ous foam sheet.
`Either or both of die lips 14 and 15 may be made ?xed or
`removable as desired. As shoWn in FIG. 2, loWer die lip 14
`
`Page 5 of 12
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`

`
`US 6,383,425 B1
`
`5
`is made removable, secured to the loWer body portion 12 by
`a plurality of bolts or other suitable fastening means, While
`upper die lip 15 is ?xed. Upper die lip 15 may be provided
`With adjusting means 62 as shoWn, for the purpose of
`adjusting the gap betWeen the die lips at exit opening 28.
`In prior art sheet extrusion dies such as shoWn in FIG. 2,
`the opposing faces of the upper and loWer die lips 14 and 15
`that make contact With the molten resin stream are planar
`and substantially parallel, de?ning a smooth extrusion pas
`sageWay or die land 29 having a substantially uniform height
`or thickness. The continuous foam sheet exiting the die lips
`in a molten or semi-molten state Will have suf?cient internal
`pressure to undergo further expansion on exiting, reaching a
`?nal thickness and surface condition When the temperature
`of at least the outer skin falls beloW the crystalliZation
`temperature and solidi?es.
`To complete the ?nishing of the extruded foam sheet,
`conventional sheet extrusion processes generally employ a
`?nishing roll stack, not shoWn, Which may contain chilled
`rolls to further cool the sheet. Optionally, cooling means
`such as an air stream may be provided at the exit port to
`quickly cool the surfaces of the emerging foam sheet.
`Though initially shaped by the exit opening 28, the ?nal
`thickness of the foam sheet Will thus depend in part on the
`cooling means employed, the roll gap Within the ?nishing
`roll stack, and the ratio of the extrusion rate to the take-up
`rate, Which may be selected to draW the sheet as desired and
`orient the crystalline polypropylene component of the foam
`sheet.
`In FIG. 3, shoWing an embodiment of an improved sheet
`extrusion die according to the invention, the improvement
`over the prior art die of FIG. 2 Will be seen to reside in the
`modi?cation of die lip 16 Which together With upper die lip
`17 Will thus provide a die land 39 With Widening aperture
`doWnstream along the extrusion axis from near melt Well 37
`to exit opening 38, thereby providing expansion Zone 65.
`Molten foaming resin ?oWing from melt Well 37 of die 30
`enters expansion Zone 65, expands under the in?uence of the
`pressure drop and exits the die port or exit opening as a
`continuous foam sheet. Foam expansion to a given thickness
`is substantially completed Within the die land. By avoiding
`any signi?cant further expansion after exiting the die port,
`surface banding, corrugation and similar surface markings
`Will be signi?cantly reduced and may be entirely eliminated.
`Such defects are commonly seen in foam sheet extruded
`using prior art extrusion dies Without such expansion Zone
`such as, for example, the die shoWn in FIG. 2.
`It may be found desirable to aid the cooling of the foam
`sheet While in the expansion Zone 65, thereby speeding
`crystalliZation, and provision may be made for including
`cooling means in the upper and loWer die lips 16 and 17 near
`exit opening 38 for this purpose. Finishing rolls, draW rolls
`and the like may be employed as desired to complete the
`process and ?nish the foam sheet to a ?nal thickness and
`form.
`In FIG. 4, shoWing an alternative embodiment according
`to the invention, die land 49 is provided With Widening
`aperture doWnstream along the extrusion axis by modi?ca
`tion of loWer die lip 18 and upper die lip 19 thus providing
`an expansion Zone 65 Whereby foam expansion to a given
`thickness may be substantially completed Within the die
`land.
`The geometry of extrusion dies Will be designed for
`particular conditions characteriZed from the rheology of the
`melt. It Will be readily understood that placing particular
`numerical values on the variation of die gap geometry in the
`
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`65
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`6
`manner set forth above Would be unduly limiting in that the
`die opening or die gap is a function of the desired ?nal foam
`product thickness as Well as many other factors. If one
`skilled in the art Were to determine the proper die geometry
`and siZe and the process throughput rate to produce a given
`product, the appearance and uniformity of the sheet product
`are subject to improvement as described as a function of the
`modi?cations of the die lip according to this invention set
`forth herein.
`In de?ning a complete improved process of producing a
`foamed polypropylene sheet according to the invention, one
`must take into account the die gap or opening and the length
`of the die land or extrusion passage, as Well as the operating
`parameters including throughput rate, product siZe and bulk
`density. The geometry parameters are not determined inde
`pendent of the operating parameters.
`Turning to FIG. 5, Wherein the die land of FIG. 4 is shoWn
`in enlarged, fragmentary vieW, it Will be seen that the
`Widening aperture of die land 49 may be further character
`iZed by the height or thickness t1 of the passage at die exit
`opening 48 and the height or thickness t2 at the narroW point,
`ie the minimum height Within the die land. Generally, the
`initial thickness of the extruded foam sheet Will be deter
`mined substantially by the height of the exit opening 48.
`Hence, thickness t1 may be as small as 0.01 inch (0.025 cm)
`to as great as 0.2 inch (0.5 cm) or greater, depending upon
`Whether the die is intended for the production of thin, dense
`expanded ?lm-like sheet or thicker foam sheet, and may be
`even greater Where the die Will be used in producing foam
`board or the like. To be suitable for the production of foamed
`sheet having acceptable surface appearance according to the
`invention, the geometry may be characteriZed in part by the
`difference (t1—t2), ie the difference betWeen the height or
`thickness t1 of the passage at exit opening 48 and the height
`or thickness t2 at the minimum height Within the die land,
`Which Will lie in the range of from about 0.004 to about 0.10
`inch (0.01—0.25 cm). Alternatively described, the geometry
`of the die land for producing suitable foam sheet over the
`range of interest may be characteriZed by the ratio tl/t2
`betWeen the height or thickness t1 of the passage at exit
`opening 48 and the minimum height Within the die land t2,
`Which Will be in the range of from about 2 to about 5.
`The geometry of the die land suitable for producing foam
`sheet according to the invention may be further character
`iZed by length 11 of die land or passageWay 49, determined
`from the doWnstream edge of melt Well 47 to said exit
`opening, and length 12 of the Widened portion forming
`expansion Zone 65. Generally, the length of initial, narroW
`portion of the die land, de?ned by the difference (ll-l2), ie
`by the difference betWeen the length of die land 49 and the
`length of the expansion Zone 65, Will lie in the range of from
`about 0.125 inch to about 0.75 inch (0.3—1.9 cm).
`These parameters Will be found to serve generally for the
`design of improved extrusion dies according to the invention
`to meet the requirements for producing extruded foam sheet
`over a Wide range of production volumes, including the
`embodiment shoWn in FIG. 3.
`In the operation of the foam sheet extrusion apparatus,
`upper die lip adjustment means may be employed to modify
`the die gap or opening Within a narroW range to further
`control sheet thickness. The improved sheet extrusion die
`according to the embodiment of FIG. 4 may be operated
`using a take-up rate selected to remove the foam sheet from
`the die prior to expanding to the full height t1 of the die exit
`opening 48, alloWing the operator to adjust and maintain
`sheet thickness over a range limited by the minimum and
`
`Page 6 of 12
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`
`US 6,383,425 B1
`
`7
`maximum heights t1 and t2 Within the aperture of the
`expansion Zone 65.
`Foamed polypropylene sheet extruded employing the
`improved extrusion die according to the invention Will have
`improved surface appearance, With little or no surface
`banding, corrugation or similar surface markings, and may
`be further characteriZed as having a loW level of surface
`roughness that is uniformly distributed, giving the foam
`sheet a pleasing and acceptable cosmetic appearance.
`Surface roughness and uniformity in surface roughness
`may be determined by image analysis and thus quanti?ed,
`thereby providing a numerical basis for distinguishing
`acceptable from unacceptable foam sheet. More particularly,
`rough surfaces re?ect light nonuniformly; areas of a surface
`that are smooth are more re?ective, and in a gray scale
`photomicrograph of a surface, appear White or light gray.
`Areas containing defects such as the voids and indentations
`that form corrugation bands Will scatter light and thus appear
`in a gray scale photomicrograph as dark gray or black. When
`digitiZed using computeriZed image analysis methods and
`translated to a binary, black/White image, the average sur
`face roughness of the foam sheet Will be related to the
`amount of black in the binary image, determined as a
`fraction of the total surface area.
`The uniformity of the distribution of roughness over the
`surface may be also determined from the photomicrograph,
`again by image analysis. The uniformity of surface rough
`ness Will be related to the variation in roughness over the
`area of the photomicrograph, determined by examining
`equal areas of the image, averaging the roughness for each
`area, and then obtaining the standard deviation. The surface
`having greatest uniformity Will be the surface having the
`loWest standard deviation.
`Foam sheet With a loW average surface roughness, uni
`formly distributed over the surface as re?ected by a loW
`standard deviation in roughness over the examined area
`generally Will be considered to have an acceptable appear
`ance.
`It Will be understood that improved sheet extrusion dies
`according to the invention may also be found useful for the
`extrusion of unfoamed polypropylene sheet, or of multilay
`ered foam sheet or foam core sheet, and further that foam
`extrusion dies having a die land including an expansion Zone
`according to the teachings hereof may be made in alternative
`con?gurations including improved annular dies and in con
`?gurations suitable for the extrusion of foam board and
`plank as Well as in pro?le extrusion of shaped foam struc
`tures With improved surface appearance.
`The particular advantage of the apparatus and method of
`this invention as disclosed herein is that high density foam
`sheet having excellent surface appearance may be provided
`using any of the great variety of commercially available
`polypropylene resins, and Without resorting to specialty
`resin grades and blends. The polyole?n compositions useful
`in the extrusion of thermoformable, rigid, foamed polypro
`pylene sheet employing the improved extrusion die accord
`ing to the method of the present invention thus may com
`prise a substantially linear polypropylene homopolymer, or
`a copolymer of propylene and a minor amount, up to about
`30 Wt. %, more preferably up to about 20 Wt. % of an
`alpha-ole?n, including ethylene, and Which preferably may
`have up to 6 carbon atoms. The polymer may be syndiotactic
`or isotactic, hoWever it is preferable to use an isotactic
`polypropylene homopolymer having an isotactic index of
`greater than 0.85, more preferably greater than 0.92, the
`articles obtained from said homopolymer having better
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`physical properties. The melt ?oW index of the polymer Will
`be from about 0.3 to about 10, preferably from about 1.0 to
`about 4.0 g/ 10 min., determined according to ASTM D1238,
`Condition L. Such polymers are readily prepared by a
`variety of catalyZed polymeriZation processes Well knoWn in
`the art, including processes employing Ziegler-Natta cata
`lysts and those based on metallocene catalysts.
`A Wide variety of extrusion grade, ?lm-forming polypro
`pylene resins Without signi?cant branching having an essen
`tially monomodal molecular Weight distribution and the
`requisite MFR are readily available in the trade and most
`Will be found useful for providing foam sheet having
`improved surface appearance according to the teachings of
`this invention.
`According to the teachings of the prior art, it has been
`generally believed that high melt strength resin formulations
`are required in order to successfully extrude foamed
`polypropylene having good cell structure and acceptable
`surface appearance, and the art has developed specialty
`formulations comprising particular grades of propylene res
`ins having particularly de?ned molecular Weight and rheo
`logical properties including a bimodal molecular Weight
`distribution comprising a highly branched minor component
`for these uses. Blend compositions having the requisite melt
`strength have also been formulated using polypropylene that
`has been modi?ed, for example through crosslinking, or
`With particular polymeric additives, highly branched ole?n
`polymers or the like. Although these specialty resins and
`resin formulations may also be found suitable for use in the
`practice of this invention, foam sheet having improved
`surface appearance may be readily produced from readily
`available generic grades of polypropylene, i.e. propylene
`resins With monomodal molecular Weight distributions and
`Without a signi?cant level of branching, by employing the
`improved apparatus and methods of the present invention as
`set forth herein.
`As disclosed and described in the art, foamable polypro
`pylene compositions Will generally further comprise a bloW
`ing agent and a crystalliZation nucleating agent. The bloWing
`agent may be of the type Well knoWn and Widely used for the
`production of expanded polystyrene and polyole?ns includ
`ing polypropylene, including organic bloWing agents such
`as, for example, aZodicarbonamide, diaZoaminobenZene,
`aZo-bis-isobutyronitrile and analogs thereof, and inorganic
`bloWing agents such as, for example, ammonium carbonate,
`sodium bicarbonate and the like. Physical bloWing agents
`such as nitrogen, carbon dioxide and other inert gases and
`agents that undergo phase change from liquid to gas during
`the foaming process such as chloro?uorocarbons (CFC),
`HCFC, loW boiling alcohols, ketones and hydrocarbons, are
`also knoWn for these uses and may also be found useful in
`the practice of this invention. The bloWing agent may further
`comprise one or more additives to reduce its decomposition
`temperature.
`The amount of bloWing agent to be used depends on its
`nature and on the desired density for the expanded polypro
`pylene and Will be selected according to practices Well
`understood by those skilled in the resin formulating art.
`Generally, bloWing agents are available to the trade in the
`form of concentrates; the concentrates Will be added to the
`formulation at levels that Will provide from about 0.2 to
`about 10 Wt. % active foaming agent, preferably from about
`0.4 to about 5 Wt. % active foaming agent, based on total
`Weight of the formulation. The amounts of physical bloWing
`agents such as liquid bloWing agents and inert gases needed
`to provide the desired foam densities may readily be deter
`mined according to common commercial practice.
`
`Page 7 of 12
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`

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`US 6,383,425 B1
`