_
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`United States Patent
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`Park et al.
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`[191
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`HIIIIlllllllll|||||||||||l|l|||||||||||||Illlllllllllillllllll||l|||ll||||
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`USO0511688lA
`5,116,881
`[11] Patent Number:
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`[45] Date of Patent: May 26, 1992
`
`[54] POLYPROPYLENE FOAM SHEETS
`
`
`
`,
`
`John 1]. Park, Neenah, W1s.; Leon
`
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`
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`V Katz, Stamford, Conn.; Norman G.
`
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`
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`Gaylord, New Providence, NJ.
`'
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`
`
`R’
`Co
`f V‘
`'
`J
`'
`73 A ‘
`'
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`
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`[211 Am>1- N0-= 493541
`
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`[22] Filed;
`Mn 14, 1990
`[51]
`Int. c1.s ............................. 1203.1 9/as; C08J 9/14
`
`
`52 U.S. CI. ................................. .. 52
`43; 264 45.1;
`264/455; 264/51; 264/53; 213/54; 264//321;
`[
`]
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`_
`_
`_
`_
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`428/3l6'6’ 428/3183’ 521/79’ 552211//113:4’
`.......................... 521 142, 143, 79
`11
`
`
`
`
`
`em
`/
`References Cited
`
`
`U-5- PATENT DOCUMENTS
`
`
`1/1972
`'
`3,637,458
`
`
`
`1/1974
`3,727,443
`'
`
`
`
`3,787,543
`1/1974
`'
`
`
`
`-
`‘
`3319-784 6/1974
`
`
`
`
`,
`N _,
`3723322
`
`
`
`
`
`3/1984 B;’1f:‘:;,e"l‘1"’e'£‘a|'
`4:467:052
`
`
`
`
`gzé/3482
`_
`4,525,257 6/1985
`.................. 521/79
`4,940,736 7/1990 Altcepping et al.
`
`
`
`
`
`FOREIGN PATENT DOCUMENTS
`
`
`5/1979 European Pat. orr.
`.
`0001791
`
`
`
`
`
`[75]
`
`
`
`Inventors:
`
`
`[
`
`:
`
`
`
`
`
`F‘ 111 is
`
`
`'°
`°
`
`
`58
`
`[
`1
`
`
`[56]
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`.
`
`
`
`
`
`0071981
`.
`2/1983 European Pat. Off.
`
`
`
`
`
`.
`0122460 10/ 1984 European Pat. Off.
`
`
`
`
`
`4/1936 European pm Off_ _
`0173282
`
`
`
`0173223 4/1986 European Pm. orr.
`.
`
`
`
`
`
`OTHER PUBLICATIONS
`
`
`F. W. Billmeyer, Textbook of Polymer Science, 3d Ed.,
`
`
`
`
`
`
`
`186-219 (1984).
`
`
`
`
`
`
`
`
`
`
`A. Nojiri, T. Sawazaki, T. Konishi, s. Kudo, s.
`
`
`
`
`Onobori, Furukawa Review 2, 34-42 (1982) through
`1982.
`17250
`Ch
`.
`11.97,
`Q“; and 3 D_ uvfiebb, )..Cenu1ar Matefiaw.’ in
`K_
`
`
`
`
`
`
`
`
`
`
`~
`-
`~
`~
`Encyclopedia Polymer Science & Engineering 3, 1-59
`
`
`
`
`
`(1985).
`
`
`
`
`
`
`
`Y. D. Lee and L. F. Wang, J. Applied Polymer Science
`32(4)v4°39“‘7(‘986)-
`‘
`
`
`
`
`
`
`
`J. s. Colton, Plastics Engineering 44(8), 53-55 (1988).
`
`
`
`
`
`
`
`
`
`Primary Examiner—Morton Foelak
`
`
`
`Attorney, Agent, or Firm—Sixbey, Friedman, Leedom &
`
`
`
`
`
`Ferguson
`
`
`
`
`ABSTRACT
`[57]
`
`
`
`
`
`
`
`"A thermoformable, rigid or semi-rigid polypropylene
`foam sheet having a smooth surface and a uniform cell
`
`
`
`
`
`
`
`
`
`
`
`
`
`structure and a density of at least 2.5 lbs/ft3 is prepared
`
`
`
`
`
`
`by extruding a mixture of a nucleating agent, a physical
`blowing agent and 8 Polypropylenfl min having 3 high
`
`
`
`
`
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`
`
`
`
`
`
`melt strength and high melt elasticity.
`5 Claims, 3 Drawing Sheets
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`PAGE 1 OF 15
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`BOREALIS EXHIBIT 1007
`
`

`
`U.S. Patent
`
`29916,2V.aM
`
`Sheet 1 of 3
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`5,116,881
`
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`<09_>@O0UmU_UUIOOUIQI
`
`PAGE 2 OF 15
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`

`
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`U.S. Patent
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`692V.aM
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`2991.
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`Sheet 2 of 3
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`5,116,881
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`PAGE 3 OF 15
`
`

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`
`
`U.S. Patent
`
`2991692V.aM
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`Sheet 3 of 3
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`5,116,881
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`PAGE 4 OF 15
`
`

`
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`2
`
`. the three-step process described hereinbefore, i.e. cre-
`
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`
`
`
`
`
`ation 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 polypro-
`
`
`
`
`
`
`
`pylene foam include azodicarbonamide (Lee et al, J.
`
`
`
`
`
`
`
`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
`
`
`
`
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`
`
`2,099,434 A) and water (EPO Pat. Appl. EP 122,460).
`
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`
`
`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 tale (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),
`
`
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`
`
`sodium benzoate (Co1ton, Plast. Eng. 44(8), 53 (1988)
`
`
`
`
`
`
`
`
`and dibenzylidene sorbitol
`(EPO Pat. Appl. EP
`
`
`
`
`
`
`
`178,282).
`
`Citric acid-sodium bicarbonate combinations are con-
`
`
`
`
`
`
`sidered as blowing agents in some patents and as nucle-
`
`
`
`
`
`
`
`ating agents in other patents (EPO Pat. Appl. EP
`
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`
`
`
`
`
`
`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 Re-
`
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`
`
`
`view 2, 34 (l982) through Chem. Abstracts 97, 2l725ou
`
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`
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`(1982); EPO Pat. Appl. EP 181,637, 190,021) in the
`
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`
`
`
`
`
`absence or presence of multifunctional vinyl monomers,
`
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`
`
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`azido functional silanes (EPO Pat. Appl. EP 181,637),
`
`
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`
`
`vinyltrimethoxysilane (Lee et al, J. Appl. Polym. Sci.
`
`
`
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`32, 4639 (1986) and ionizing radiation in the presence of
`
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`polyacrylic monomers (Nojiri et al, Furukawa Review
`
`
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`
`
`
`2, 34 (1982); U.S. Pat. No. 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
`
`
`
`
`
`
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`the molten polyolefm (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
`
`
`
`
`
`
`
`
`polypropylene foams.
`
`
`EPO Pat. Appl. EP 1791 describes “a process for the
`
`
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`
`
`
`
`
`
`
`
`preparation of expanded thermoplastic synthetic resins”
`and discloses polyethylene, ethylene-vinyl acetate co-
`
`
`
`
`
`
`polymer and isotactic polypropylene as the applicable
`
`
`
`
`
`
`thermoplastic resins.
`
`
`EPO Pat. Appl. EP 71,981 describes “foamed poly-
`
`
`
`
`
`
`propylene resin molded articles” and discloses the use
`
`
`
`
`
`
`
`
`of ethylene-propylene copolymer as well as polypropyl-
`
`
`
`
`
`ene.
`
`EPO Pat. Appl. EP 122,460 describes “resin foam
`
`
`
`
`
`
`
`produced by an aqueous medium" and discloses poly-
`
`
`
`
`
`
`mer foams from polypropylene, polyethylene and poly-
`
`
`
`
`
`
`
`styrene.
`EPO Pat. Appl. EP 291,764 describes the “extrusion
`
`
`
`
`
`
`of propylene polymer foam sheets” and discloses a pro-
`
`
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`
`
`cess for extruding blends of ethylene-propylene block
`
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`
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`copolymers containing less than 20% ethylene with
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`block copolymers containing less than 5% ethylene or
`
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`random ethylene-propylene copolymers or polypropyl-
`ene.
`
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`
`
`5,116,881
`
`' 1
`
`
`
`
`POLYPROPYLENE FOAM SHEETS
`
`
`
`BACKGROUND OF THE INVENTION
`
`
`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
`
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`
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`
`
`are rigid or semi-rigid and thermoforrnable into shaped
`
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`
`
`
`articles for use in packaging and service applications.
`
`
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`
`
`2. Description of the Prior Art
`
`
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`
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`A foamed plastic or cellular plastic has an apparent
`
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`density which is decreased by the presence of numerous
`
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`
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`voids or cells dispersed throughout its mass (ASTM
`
`
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`
`
`D883-80C). The cells may be interconnected (open-
`
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`
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`celled) and/or discrete and independent (closed-celled).
`
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`
`
`The prior art discloses various methods for the prepa-
`
`
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`
`
`ration of foamed plastics. These include leaching out a
`
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`
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`solid or liquid which is dispersed in a plastic, sintering
`
`
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`
`
`small particles of a plastic and dispersing cellular parti-
`
`
`
`
`
`
`
`cles in a plastic. However, the most widely used method
`
`
`
`
`
`
`
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`involves the dispersion of a gaseous phase throughout a
`
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`
`
`
`fluid polymer phase and the retention of the resultant
`
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`
`
`expanded form.
`
`
`The theory of the expansion process and the proper-
`
`
`
`
`
`
`
`
`ties of various foamed plastics are reviewed in “Cellular
`
`
`
`
`
`
`
`Plastics”,
`in Encyclopedia of Polymer Science and
`
`
`
`
`
`
`
`Engineering, vol. 3, pp. 1-59 (1985), which is incorpo-
`
`
`
`
`
`
`
`rated herein by reference. As disclosed therein, the
`
`
`
`
`
`
`
`
`expansion process consists of three steps: creation of 30
`
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`
`
`small discontinuities or cells in a fluid or plastic phase,
`
`
`
`
`
`
`
`growth of these cells to a desired volume and stabiliza-
`
`
`
`
`
`
`
`tion 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 in-
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`
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`corporated into the system as blowing agents and vola-
`
`
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`
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`
`
`tilize due to increased temperature or decreased pres-
`
`
`
`
`
`
`sure, gases that are produced as a result of a chemical
`
`
`
`
`
`
`
`
`reaction within the fluid polymer and chemical blowing
`
`
`
`
`
`
`
`
`agents which undergo thermal decomposition to form a
`
`
`
`
`
`
`gas.
`
`The rate of growth of the bubbles or cells depends
`
`
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`
`
`
`
`upon the viscoelastic nature of the polymer phase, the
`
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`
`
`
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`
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`blowing agent pressure, the external pressure on the
`
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`
`
`foam, the cell size and the permeation rate of the blow-
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`
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`ing agent through the polymer phase.
`
`
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`
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`Cell or bubble stabilization relates to cell wall stabil-
`
`
`
`
`
`
`
`ity and the drainage of material from the membrane or 50
`
`
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`
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`
`
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`wall which separates cells. Increasing the viscosity of
`
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`
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`the fluid reduces the drainage effect. The viscosity in-
`
`
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`
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`crease may be caused by chemical reactions which
`
`
`
`
`
`
`
`increase molecular weight through polymerization or
`
`
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`
`
`crosslinking, or by temperature reduction, ultimately
`
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`below the second order transition or crystallization
`
`
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`temperature to prevent polymer flow.
`
`
`
`
`The present invention relates to rigid or semi-rigid
`
`
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`foam sheets for use in food service applications. The
`
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`prior art has utilized polystyrene for the manufacture of 60
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`foam sheets for these applications. However, polysty-
`
`
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`rene articles suffer from low service temperature, and
`
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`little or no photochemical or biological degradability
`
`
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`and are relatively expensive.
`
`
`
`
`Polypropylene does not have these undesirable char-
`
`
`
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`
`
`
`acteristics. Various processes have been reported in the
`
`
`
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`
`
`
`prior art for the preparation of flexible or rigid polypro-
`
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`pylene foams. The processes are designed to promote
`
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`PAGE 5 OF 15
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`

`
`3
`
`UK. Pat. 1,400,494 describes “foamed polymeric
`
`
`
`
`
`
`sheet material and process therefor” and discloses poly-
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`propylene, high density polyethylene and nylon-12 as
`
`
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`
`
`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 "extru-
`
`
`
`
`
`sion process for propylene resin foams" and states that
`
`
`
`
`
`
`
`
`
`the resin may be isotactic polypropylene, an ethylene- ‘
`
`
`
`
`
`
`propylene block or random copolymer or blends of 10
`
`
`
`
`
`
`
`
`
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`
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`polypropylene with numerous olefin homopolymers
`and copolymers.
`
`
`U.S. Pat. No. 3,637,458 describes “microcellular
`
`
`
`
`
`
`foam sheet" from a linear,
`thermoplastic crystalline
`
`
`
`
`
`
`
`
`
`
`
`
`
`polymer and claims isotactic polypropylene and linear I5
`polyethylene foam sheet.
`
`
`
`U.S. Pat. No. 3,819,784 describes “a process for pre-
`
`
`
`
`
`
`
`
`paring molded polyolefin foam" and discloses that suit-
`
`
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`
`
`
`
`
`able polyolefinsused in the process include low density
`
`
`
`
`
`
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`polyethylene, high density polyethylene, isotactic poly-
`propylene, poly-l-butene and copolymers of ethylene '
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`with propylene or vinyl acetate.
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`U.S. Pat. No. 3,830,900 describes “method offonning
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`foamed plastic sheets" and discloses that the method is
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`applicable to polyvinyl chloride, polystyrene, polyeth-
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`ylene, polypropylene and acrylonitrile-butadiene-sty-
`rene copolymers.
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`U.S. Pat. No. 4,467,052 describes a “tray for packag-
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`ing food products” and discloses an injection molding
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`process for the preparation of foam trays from blends of 30
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`polypropylene and styrene-butadiene rubber.
`Colton (Plast. Eng. 44(8), 53 (1988) describes “mak-
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`ing micro cellular foams from crystalline polymers” and
`discloses microcellular polypropylene and ethylene-
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`propylene copolymer foams.
`EPO Pat. Appl. 181,637 describes “lightly cross-
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`linked linear olefinic polymer foams" prepared from
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`melt blends of one or more polymers selected from high
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`density polyethylene, linear low density polyethylene,
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`polypropylene and polystyrene.
`EPO Pat. Appl. EP 190,021 describes "heat-foamable
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`cross-linked propylene resin compositions” and dis-
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`closes blends of propylene-a-olefin copolymers or 1-
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`butene-a-olefin copolymers with polypropylene.
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`U.S. Pat. No. 4,424,293 describes “crosslinkable poly-
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`propylene composition” and discloses foams from iso-
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`tactic polypropylene and ethylene-propylene copoly-
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`mer.
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`The prior art uses “polypropylene" as a self-explana-
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`tory term for a polymer prepared from propylene mon-
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`omer. In some cases the terms “isotactic polypropyl-
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`ene" and “crystalline polypropylene” are used. In only
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`a few patents is the polypropylene characterized to any
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`_
`further extent.
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`EPO Pat. App]. El’ 71,981 discloses polypropylene
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`foams prepared from resins having a latent heat of crys-
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`tallization of 9-28 cal/g. U.S. Pat. No. 3,637,458 dis-
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`closes polypropylene foams prepared from polymers of
`“at least film forming molecular" weight, substantially
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`free from crosslinlting, and having a work-to-break 60
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`(WTB) value of at least
`l0,(XX) inch-lbs/inch3". U.I(.
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`Pat. Appl. GB 2,099,434 A discloses polypropylene
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`foams prepared from resins having a melt tension of at
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`least 3 grants at 190' C. and a maximum/minimum melt
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`tension ratio of not more than 2.5/1.
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`Application of the processes of the prior art to ge-
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`neric or commercial polypropylene resins, described as
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`polypropylene,
`isotactic polypropylene or crystalline
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`5,116,881
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`4
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`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
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`foam sheet material having a high impact strength, a
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`modulus suitable for rigid or semi-rigid packaging appli-
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`cations and convertible into trays, plates, containers and
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`other articles used in food service.
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`Another object of the instant invention is to provide
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`a thermoforrnable foam sheet which has a high service
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`temperature.
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`A further object of the present invention is to provide
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`a foam sheet having high insulation properties and cost
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`advantages over existing foam sheets.
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`V Yet another object of the present invention is to pro-
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`vide a foam sheet prepared from a polypropylene resin
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`and having a density ranging from 2.5 to 25 lbs/ft3 and
`a modulus of at least 10,000 psi.
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`Still another object of the present invention is to
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`provide a polypropylene foam sheet having a uniform
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`cell structure and smooth surfaces.
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`It has now been found that these improvements in a
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`foam sheet can be achieved by extruding high melt
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`strength, high melt elasticity polypropylene, character-
`ized by at least (a) either high M, or high M,/Mwratio,
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`and (b) either high equilibrium compliance 1,0 obtained
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`from creep measurements or high recoverable strain per
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`unit stress Sr/S obtained from steady shear measure-
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`ments.
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`In one embodiment, the present invention provides a
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`rigid or semi-rigid polypropylene foam sheet having a
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`density ranging from 2.5 to 25 lbs/ft’, tensile and flex-
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`ural moduli of at least l0,()00 psi, a cell size of 5-18 mils
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`and a thickness ranging from about 0.02 to 0.20 inches.
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`The polypropylene foam sheet is therrnoforrnable and
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`has uniform cell structure and smooth surfaces.
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`In another embodiment of this invention, a process is
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`provided for producing the polypropylene foam sheet
`of the invention. The process may be conducted using a
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`single or tandem extrusion line. The latter is preferred
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`and, by the use of primary and secondary extruders in
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`series, a continuous foam sheet is produced. The process
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`comprises mixing polypropylene resin, having a high
`melt strength and a high melt elasticity, with a nucleat-
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`ing agent in the primary extruder, plasticating the mix-
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`ture, injecting a physical blowing agent into the plas-
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`ticated mixture to form a foaming mixture, which is
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`transferred to a secondary extruder, mixing and cooling
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`the foaming mixture and extruding the foaming mixture
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`through an annular or flat die into a continuous foam
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`sheet.
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`In another embodiment of this invention, a method is
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`provided for forming rigid or semi-rigid articles from
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`the polypropylene foam sheet of the invention. The
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`process comprises heating the foam sheet to a tempera-
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`ture which pennits deformation under vacuum or pres-
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`sure, supplying the softened foam sheet to a forming
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`mold and cooling the foam sheet to form a rigid or
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`semi-rigid article having the shape of the mold.
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`In another embodiment of the invention, a rigid or
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`semi-rigid multilayer structure is provided. The multi-
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`layer structure comprises at least one layer of the poly-
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`propylene foam sheet of the invention and at least one
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`layer having functional properties, e.g. barrier proper-
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`ties.
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`PAGE 6 OF 15
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`5,116,881
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`6
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`lar weights by the number of molecules and thus is
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`dependent simply upon the total number of molecules,
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`(b) the weight average molecular weight (M...) which is
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`the second-power average of molecular weights and is
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`more dependent on the number of heavier molecules
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`than is M,., and (c) the z-average molecular weight (M2)
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`which is the third-power average of molecular weights.
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`Colligative properties are related to M.,, bulk proper-
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`ties associated with large deformations such as viscosity
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`and toughness are affected by M“-values and melt elas-
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`ticity is more closely dependent on M, values.
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`The polypropylene resins which are effective in
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`yielding acceptable foams by the process of the present
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`invention, are of high molecular weight with an M2
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`value above 1.0X10" and an M;/MW ratio above 3.0.
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`The polydispersity index Mp/M,', is of less significance
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`since it does not differentiate between polypropylene
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`resins which give acceptable foams and those which
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`give unsatisfactory foams. Resins having M, and
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`M,/MW values below the indicated values yield foam
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`sheets which are unacceptable.
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`GPC chromatograms of resins which yield unaccept-
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`able foam sheets, using a viscometer detector, show a
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`unimodal molecular weight distribution and plots of the
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`branching factor g’ versus log molecular weight (M,.,),
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`where
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`g'= [nl/KM"
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`5
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`FIG. 1 is a schematic drawing of a tandem foam
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`extrusion line.
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`FIG. 2 is a scanning electron microscope (SEM)
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`micrograph of a cross section of a polypropylene foam
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`sheet prepared from polypropylene resin A-6.
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`FIG. 3 is an SEM micrograph of a cross section of a
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`polypropylene foam sheet prepared from polypropyl-
`ene resin A-7.
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`FIG. 4 is an SEM micrograph of a cross section of a
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`polypropylene foam sheet prepared from polypropyl-
`ene resin A-2.
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`FIG. 5 is an SEM micrograph of a cross section of a
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`polypropylene foam sheet prepared from polypropyl-
`ene resin A-17.
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`
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`DETAILED DESCRIPTION OF THE
`
`
`
`INVENTION
`
`In accordance with the present invention, it has been
`
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`found that a thermoforrnable polypropylene foam sheet
`
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`having high modulus impact strength, service tempera-
`ture and insulation properties, may be produced by a
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`process which comprises the steps of mixing a polypro-
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`pylene resin having specific molecular and rheological
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`characteristics with a nucleating agent, plasticating the
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`mixture, introducing a physical blowing agent into the
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`plasticated mixture to form a foaming mixture, mixing,
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`cooling and extruding the foaming mixture through an
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`extruder die into a foamed extrudate which is formed
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`into a continuous foamed sheet.
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`Acceptable foam sheets produced from the operable
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`
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`’ polypropylene resins by this process, have a density
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`between 2.5 and 25.0 lb/ft3, suitable for semirigid and
`
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`rigid packaging and food service applications and have
`a substantially uniform cell structure and smooth sur-
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`faces. Foams which are unacceptable and unsatisfactory
`
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`for thermoforrning into objects for the indicated appli-
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`cations, have non-uniform cell structures, rough sur-
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`faces and densities outside of this range.
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`The base resin plays the major role in determining the
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`foamability and the properties of the foam products
`
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`made therefrom. The polypropylene resins which yield
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`acceptable foams, particularly when processed by the
`
`
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`
`
`method disclosed herein, may be distinguished from the
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`polypropylene resins which yield unsatisfactory foams,
`
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`by their molecular and rheoloqical characteristics.
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`The melt strength of a polymer is important in pro-
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`cesses such as foaming where deformation is primarily
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`elongational and tensile stresses are present. High mo-
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`lecular weight polypropylene resins are frequently
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`characterized as “high melt strength” (HMS) resins.
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`However, unexpectedly,
`it has been found that this
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`characterization is inadequate and that numerous high
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`molecular weight polypropylene resins, designated and
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`marketed as “high melt strength” resins fail to yield
`
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`acceptable foam sheets.
`
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`The molecular weight distribution in a sample of
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`polypropylene may be determined by high temperature
`
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`
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`gel permeation chromatography (GPC). The Waters 60
`
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`
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`150 CV GPC chromatograph may be used at 135° C.
`
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`
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`with trichlorobenzene as carrier solvent and a set of
`
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`
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`Waters p.-Styragel HT, 103, 104, I05 and 105A columns.
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`The solution concentration is 0.2(w/v) and the flow
`
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`-
`rate is 1 ml/min.
`
`
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`GPC provides information about (a) the number av-
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`erage molecular weight M,. which is the arithmetical
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`mean value obtained by dividing the sum of the molecu-
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`the
`i.e.
`show the absence of significant branching,
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`chains are essentially linear. In contrast, resins which
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`yield acceptable foam sheets show a bimodal molecular
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`weight distribution, wherein the major component is
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`largely linear while the higher molecular weight minor
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`component is highly branched.
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`
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`Melt flow rates of resins which may be utilized in the
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`
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`process of the present invention, range from 0.2 to 12
`
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`g/ 10 min, measured in a melt flow instrument at 230° C.
`
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`under a load of 2.16 kg.
`
`
`
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`The importance of melt elasticity in the conversion of
`
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`
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`polypropylene resins to acceptable foam sheets, indi-
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`
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`cated by M2 values, is confirmed by rheological charac-
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`terization of polymer melts in a shear field.
`
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`
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`The rheological characterization of the polypropyl-
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`ene resins was conducted with a programmed Rheomet-
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`rics Mechanical Spectrometer (RMS-800). Resin pellets
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`were compression molded into sheets from which sam-
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`ples were stamped out with a 25 mm diameter circular
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`die. Tests were conducted at 210¥l° C. using 25 mm
`
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`parallel plate geometry with a 1.4 mm gap. Creep data
`
`
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`were obtained under a constant
`stress of 1000
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`dyn/sq.cm for a period of 0-300 sec. The creep compli-
`
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`
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`ance J(t) is given by
`
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`J =
`em __ J ——1
`
`
`
`(0
`‘U + "la
`To
`
`
`
`
`
`
`
`
`where
`
`e=strain
`
`r0=stress
`
`J.,,,= equilibrium compliance
`
`
`'ry.,=zero shear viscosity
`
`
`
`The equilibrium compliance Jen is a measure of melt
`
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`
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`elasticity and is determined by first plotting strain
`
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`against time at constant stress. The strain as a function
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`of time is divided by the stress to give J(t). J“, is the
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`intercept of the J(t) against time plot.
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`PAGE 7 OF 15
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`Polypropylene resins which yield acceptable foam
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`sheets, by the process of the present invention, have
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`equilibrium compliance 1“, values above 12X 10-5
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`cm’/dyne. Resins having J,,, values below this value
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`yield unacceptable foam sheets with non—uniforrn cell
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`structure and uneven surfaces.
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`The recoverable shear strain per unit stress Sr/S also
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`distinguishes polypropylene resins which yield accept-
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`able foam sheets from those which yield unacceptable
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`foam sheets. This quantity is a fundamental measure of to
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`melt elasticity. Using the programmed Rheometrics
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`Mechanical Spectrometer,
`the polymer melt
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`jected to clockwise rotational shear strain by the driver
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`and the resulting shear stress S and first normal stress
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`N; are measured by a transducer. The shear rate range is
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`0.0l—l0 5‘ 1, the time before measurement is 2.2 min and
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`the time of the measurement is 0.3 min. Normal stress
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`measurements areobtained at each shear rate. The re-
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`coverable shear strain Sr is obtained from the first nor-
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`mal stress difference
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`8
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`through the heated crossover 9. Mixing and cooling
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`occur as the screw carries the mixture through heated
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`zones 10-13. The melt pump 14 moves the “foaming
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`mixture" into the heated die 15 and through zones
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`16-18. The die may be either a circular (annular) or flat
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`die. The extruded foaming mixture forms a foamed
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`extrudate which is sheet-like when coming through a
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`flat die, or tubular when coming through an annular die.
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`In either case, the extrudate is cooled by an air ring 19
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`attached to the die lip. If an annular die is used, the
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`extruded foam tube is pulled over a cooling/sizing drum
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`20 where it is further cooled by an air ring 21. The
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`extruded foam tube or sleeve is split while passing over
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`the drum 20. The extrudate from either a flat die or an
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`annular die, the latter after splitting and spreading, is
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`then flattened into a foam sheet by passage over a series
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`of rolls, e.g. an S-wrap, and then taken up on a winder.
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`The continuous foam sheet is then aged for a period of
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`time to allow for diffusion of the blowing agent and air
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`through the cell walls to bring it to equilibrium, prior to
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`further fabrication, if any.
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`The extrusion process disclosed above is representa-
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`tive and not limiting as to equipment and procedural
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`details. Alternative equipment and variations in the
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`procedure will be obvious to those skilled in the art.
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`A polypropylene foam sheet may be provided with a
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`substantially non-cellular outer layer or skin. Such a
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`skin may give the foam a superior outer appearance in
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`that the foam structure with skin may have a shiny or
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`glossy appearance, and is also resistant to surface abra-
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`sion and cutting. The skin also acts as a stiffener to
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`enable a lighter and/or thinner structure having a maxi-
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`mum bending stiffness to be obtained. A skin may be
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`formed on a single layer structure by changing the flow
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`rate and/or the temperature of the air which is applied
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`to the surface of the tubular or flat extrudate coming out
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`of a die. Alternatively, a skin layer may be formed by
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`using a multimanifold die or combining feedblock to
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`coextrude a non-foamed polypropylene or other layer
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`on the outside of a polypropylene foam layer.
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`In accordance with another embodiment of the pres-
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`ent invention,