Unlted States Patent [19]
`Nahmias et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,604,324
`Aug. 5, 1986
`
`[54] MULTI-LAYER POLYPROPYLENE FILM
`STRUCTURE AND METHOD OF FORMING
`THE SAME
`
`[56]
`
`.
`
`[75] Inventors: A. Michael Nahmias, V1ctor; Hee C.
`Park, Falrport, both of NY
`
`'
`
`_
`
`~
`.
`~
`-
`-
`[73] Asslgnee' 11:14?“ 0“ Cmpommn’ New York’
`'
`'
`
`References Cited
`U.S. PATENT DOCUMENTS
`3,399,156 8/1968 Bell .................................... .. 524/583
`3,783,088 5 1971 Y 11'
`t 1. .
`3,841,943 1141971 T211338; :1‘?
`4,118,438 10/1978 Matsui et a1. .
`4,348,457 9/1982 Rosenthal et a1. ................ .. 428/349
`4,377,616 3/1983 Ashcraft et a]. .
`4,419,410 12/1983 Weiner .............................. .. 428/349
`4,419,411 12/1983 Park .................................. .. 428/516
`
`_
`[21] Appl' NO" 6559614
`_
`[22] Flled:
`
`Sep.28, 1984
`
`Primary Examiner—Edith Buffalow
`Attorney, Agent, or Firm-A. J. McKillop; M. G.
`Gilman; J. P. O’Sullivan, Sr.
`[57]
`ABSTRACT
`[51] Int. Cl! ............................................ .. B32B 27/08 A multi-layer polypropylene structure of low COF also
`[52] US. Cl. .................................. .. 428/349; 428/516;
`having antistick characteristics over a wide temperature
`428/910; 525/240; 156/244.11; 156/244.24
`range and a method of forming the same
`[58] Field of Search ............. .. 428/516, 349, 910, 347;
`525/240; 156/244.11, 244.24
`
`15 Claims, No Drawings
`
`BOREALIS EXHIBIT 1019
`
`PAGE 1 OF 5
`
`

`
`MULTI-LAYER POLYPROPYLENE FILM
`STRUCTURE AND METHOD OF FORMING THE
`SAME
`
`20
`
`25
`
`30
`
`35
`
`BACKGROUND OF THE INVENTION
`The invention relates to a mult-layer polypropylene
`structure which has exceptionally low ?lm to ?lm coef
`?cient of friction over a wide temperature range and
`which, when used to wrap packages secured by a heat
`seal layer or layers, avoids package; to package sticking
`in any heat seal region
`Highly crystalline polypropylene ?lm is an excellent
`packaging material, but it has a high ?lm to ?lm coef?
`cient of friction which makes it difficult to utilize in
`automatic packaging equipment. This material in ?lm
`form has a tendency to stick at heat seal surfaces when
`adjacent heat seal regions contact shortly after sealing.
`Some years ago cigarettes packaging machines had a
`top speed of approximately 140 packs per minute. Pres
`ent day machines run at from 300-400 packs per minute,
`which is equivalent to approximately 160 ft. of ?lm per
`minute. This speed makes the handleability or machine
`ability of ?lm, i.e. the ability of the ?lm to be machine
`utilized, very critical. Film breakage or partial tearing
`and package to package sticking causes extremely ex
`pensive machine down time which cannot be tolerated.
`Certain innovations have improved on the surface
`friction characteristics of highly crystalline polypropyl
`ene in ?lm form. For example, in U.S. Pat. No.
`3,176,021, incorporated herein by reference in its en
`tirety, it is taught to include minor quantities of fatty
`acid amides into the polypropylene. In order to obtain
`the bene?ts taught by this patent, certain limitations
`must be observed. The ?lm must be formed from a melt,
`extruded at a temperature between about 400°~550° F.
`In addition, the amide must be present in from 0.005 to
`about 2.0 weight percent of the polypropylene and it
`must be present along with from about 0.1 to about 4.0
`weight percent polyethylene. Under these conditions
`and limitations the resulting polypropylene ?lm will
`have a static coef?cient of friction no higher than 0.6
`which is sign?cantly higher than present day require
`ments. In addition, such a ?lm does not have the high
`stereoregularity required by present day packaging
`45
`demands. Further, it has been found that once said ?lm
`has been subjected to the high temperatures which will
`be experienced in practice, particularly during heat
`sealing, i.e., approaching 270° F., the coef?cient of
`friction increases signi?cantly and is nowhere near the
`present day requirements of 0.25.
`In US. Pat. No. 3,399,156, incorporated herein by
`reference in its entirety, it is taught that the inclusion of
`the combination of silicone oil and ?nely divided silica
`into a polypropylene ?lm produces a ?lm which will
`have a coef?cient of friction of from 0.45 to 0.46 and be
`suitable for use in bread-wrapping machines. This is not
`a low enough co-ef?cient of friction for use in present
`day, high speed automatic packaging machinery.
`It has been found that to a certain extent high crystal
`linity in polypropylene impedes facile migration of ad
`ditives to the surface of such a polymer in ?lm form.
`This problem has been addressed in US. Pat. No.
`4,419,410, incorporated herein by reference in its en
`tirety. This application teaches that by the coextrusion
`of a skin layer of polypropylene of comparatively low
`stereoregularity with a core layer of polypropylene of
`comparatively high stereoregularity, where this core
`
`55
`
`50
`
`60
`
`65
`
`1
`
`4,604,324
`
`2
`layer contains a surface modifying agent such as an
`amide of a fatty acid, the amide will bloom to the sur
`face of the comparatively low stereoregularity polymer
`with comparative ease.
`In spite of these teachings, it remained until US Pat.
`No. 4,419,411, incorporated herein by reference in its
`entirety, to signi?cantly advance the art of making mul
`til-layer polypropylene ?lms suitable for most high
`speed packaging operations. This patent teaches a mul
`ti-layer polypropylene ?lm structure having surface
`modifying agents comprising ?nely divided silica, sili
`cone oil and an amide of a water insoluble mono-car
`boxylic acid having from about 8 to about 24 carbon
`atoms on a surface thereof.
`Notwithstanding the general excellence of this ?lm, it
`is found that when used in high speed cigarette pack
`wrapping machines, there is a tendency for the ?lm
`surfaces of contacting packages to stick together, par
`ticularly in the areas where heat sealing has occurred.
`It is an object of the present invention to provide a
`multi-layer polypropylene ?lm structure having an ex
`tremely low coef?cent of friction over a- wide tempera
`ture range which also contains a combination of surface
`modifying agents which eliminates or minimizes the
`tendency of the ?lm surfaces to stick together when
`used in high speed packaging machinery,
`It is another object to provide a process for preparing
`this multi-layer structure. A
`-
`SUMMARY OF THE INVENTION
`It has been discovered that by the inclusion of a sim
`ple polymeric material, in combination with silica and a
`silicone oil, into the skin layer or layers of the subject
`structure, a multi-layer ?lm of non-sticking characteris
`tics is obtained.
`The present invention is directed to an oriented multi
`layer polypropylene ?lm structure comprising coex
`truded layers of:
`(a) a base layer comprising polypropylene of compar
`atively high stereo-regularity;
`'
`(b) a skin layer comprising a polyole?n of compara
`tively low stereo-regularity on at least one surface
`of (a), said skin layer containing a surface-modify-~
`ing proportion of a combination of ?nely divided
`silica, a silicone oil, and an anti-stick promoting
`proportion of a polymer incompatible with said
`skin layer polyole?n, said incompatible polymer
`having a melting point of greater than about 150°
`F. above the melting point of the skin layer poly
`ole?n.
`The present invention also is directed to a process for
`preparing the above-identi?ed oriented multi-layer
`polypropylene structure comprising melt coextruding a
`combination of:
`(a) a base layer comprising polypropylene of compar
`atively high stereoregularity; and
`(b) a skin layer comprising a polyolefm of compara
`tively low stereoregularity on at least one surface
`of (a), said skin layer containing a surface-modify
`ing proportion of a combination of ?nely divided
`silica and a silicone oil, and an anti-stick promoting
`proportion of a polymer incompatible with said
`skin layer polyole?n, said incompatible polymer
`having a melting point of greater than about 150°
`F. above the melting point of said skin layer poly
`ole?n; and orienting the struture. '
`
`PAGE 2 OF 5
`
`

`
`3
`DESCRIPTION OF THE INVENTION
`The present invention is broadly directed to the im
`provement of the surface characteristics of a multi-layer
`structure, the core layer of which is highly crystalline
`or highly stereoregular polypropylene in oriented ?lm
`- form. The preferred polypropylenes of the core layer
`are well known in the art and are formed by polymeriz
`ing polypropylene in the presence of stereospeci?c
`catalyst systems. They can have a melt ?ow rate at 446°
`F. ranging from about 1 to about 25, preferably 2 to 4.
`The crystalline melting point is about 321°-325° F. The
`number average molecular weight ranges from about
`25,000 to about 100,000. The density ranges from about >
`0.90 to 0.91.
`15
`The comparatively low stereoregular polyole?n pol
`ymers contemplated as the skin material of the multi
`layer system include, for example, random co-polymers
`of ethylene and propylene and ethylene-propylene
`butylene terpolymers. Particularly preferred are the
`ethylene-propylene copolymers which contain from
`about 2 to about 7 wt. % and more preferably from 3-5
`wt. % ethylene. They can have a melt ?ow rate at 446°
`F. ranging from about 2 to 15 and preferably 3 to 8. The
`crystalline melting point is about 257° to 302° F. The
`25
`number average molecular weight range is from about
`25,000 to about 100,000. The density ranges from about
`0.89 to 0.90. These copolymers have excellent heat seal
`characteristics. They do not, however, have the excel
`lent physical characteristics inherent in the polypropyl
`ene of comparatively high stereoregularity. Thus, the
`core material of polypropylene of high stereoregularity
`possesses the excellent characteristics of good moisture
`barrier, stiffness, high strength, good optical properties.
`The thin co-extruded surface layer of the comparatively
`low stereoregular ethylene-propylene random copoly=
`mer provides excellent heat seal characteristics. In
`order to take advantage of the excellent heat seal char
`acteristics of this skin layer, the skin ?lm must be modi
`?ed so as to exhibit the characteristics of good slip and
`the tendency not to stick to an adjacent ?lm region after
`being exposed to heat-seal temperatures.
`The overall thickness of the contemplated multilayer
`structure is primarily of the highly stereoregular poly=
`propylene. The highly isotactic polypropylene portion
`of the structure should be about 93 to about 98% of the
`overall ?lm structure. The co-extruded layer of com
`paratively low stereoregularity may be present on one
`or both surfaces of the core layer. It is preferred that the
`skin layer is on both sides of the core layer and that the
`thickness of each skin can range from about 1 to 6% of
`the overall ?lm thickness. Preferably the skin layer has
`a thickness of about 0.3-0.7 microns.
`The present invention is based upon the discovery
`that a combination of three surface modifying agents
`are unexpectedly superior, to a signi?cant extent, than
`the individual agents or any combination less than all of
`the agents when the ?lm is used in high speed packaging
`machines and the ?lm undergoes heat-sealing.
`One class of surface modifying agents is ?nely di=
`vided silica. The silica can be any commercially avail
`able, ?nely divided silica, preferably that having a mean
`particle size ranging from about 0.5 to 5 microns. One
`commercially available silica has a mean particle size of
`0.75 microns and another has a mean particle size of 4.5
`65
`microns. Either material can be employed. The silica
`can be present in from about 0.05 to 0.5 wt. % prefera
`bly about 0.1 to 0.3 wt. % of the skin layer.
`
`35
`
`40
`
`55
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`60
`
`4, 604, 324
`
`5
`
`30
`
`4
`Another surface modifying agent is a silicone oil,
`preferably a dimethylsiloxane, having a viscosity of
`about 20,000 to 3,000,000, preferably 20,000 to 30,000
`centistokes. This siloxane can be present in from about
`0.3 to 0.8 wt. % of said skin layer.
`The third necessary surface modifying agent is a
`polymer incompatible with the skin layer polymer and
`it must have a melting point of at least about 150° F.
`greater than that of the skin polymer. Examples of suit
`able polymers are polytetrafluoroethylene, the nylons,
`e.g. nylon 6, nylon 66, polycarbonate and poly(methyl
`pentene), with the latter being preferred. The poly(
`methylpentene) is a '4-methylpentene-l-based polyole
`?n. In its preparation, propylene ?rst goes through the
`dimerization stage to yield 4-methylpentene-l, which is
`then polymerized to poly(methylpentene). Its melting
`point is 455° F. and its density is 0.83. On melt blending
`with the skin polymer, which is predominantly polypro
`pylene in character, two distinct phases result. On rapid
`mixing, micron size particles of poly(methylpentene)
`are dispersed throughout the skin copolymer. The
`poly(methylpentene) material can be melt blended with
`the skin ?lm polymer in any manner. Any additive
`proportion, which will promote the antistick character
`istics of heat sealed packages employing the ?nished
`?lm of the type described, is contemplated. It is pre
`ferred that from about 1,000 to about 6,000 ppm of the
`poly(methylpentene) be present in the skin layer.
`An optional and preferred surface modifying agent
`for use herein is an amide of a water-insoluble monocar
`boxylic acid having about 8 to about 24 carbon atoms
`and mixtures of said amides. Speci?c examples of this
`class of amides are erucamide, oleamide, stearamide,
`behenamide, etc. It is preferred that this additive be
`included in the highly stereoregular core polypropylene
`in an amount of about 300 to 400 ppm of the core layer
`and/or in the skin layer in an amount up to about 2000
`p.p.m. It is to be understood that the amide slip agent
`may be provided in the comparatively low stereoregu
`lar copolymer by dispersing the amide in the resin pre
`cursor of the comparatively high stereoregular polypro=
`pylene alone, the amide then exuding from the base or
`core ?lm through the copolymer ?lm to its surface.
`Thus, amide additive in the outer ?lm resin starting
`material is not necessary, but may be desired.
`The percentage of the surface modifying additives in
`the multi-layer structure should be such as to yield a
`coef?cient of friction of the surface of the skin layers of
`0.25. More particularly, the proportions should be such
`that the coef?cient of friction of the surface of the skin
`layer or layers is about 0.25 at room temperature and is
`maintainable less than 0.8 up to about 270° F.
`To further aid the heat seal characteristics and im
`prove the optical properties of the ?lm, the skin layer
`can also contain up to 10 wt. % of a natural or synthetic
`terpene resin, a wax, or a low molecular weight (e.g.
`10,000) polyethylene.
`The multi-layer ?lms of the present invention can be
`prepared employing commercially available systems for
`coextruding resins. The polypropylene homopolyrner
`of comparatively high stereoregularity containing, if
`desired, amide slip agent, can be coextruded with an
`ethylene-propylene random copolymer of compara
`tively low stereoregularity containing the appropriate
`percentage of the combination of silica, dimethylsilox
`ane and incompatible polymer dispersed therein. The
`polymers can be brought to the molten state and coex
`truded from a conventional extruder through a ?at
`
`PAGE 3 OF 5
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`

`
`4,604,324
`
`5 .
`sheet die, the melt streams being combined in an adapter
`prior to being extruded from the die. After leaving the
`die ori?ce, the multi-layer ?lm structure is chilled and
`the quenched sheet then reheated and stretched, e.g., 4
`to 6 times in the machine direction (MD), and subse
`quently, for example, 4 to 10 times in the transverse
`direction (TD). The edges of the ?lm can be trimmed
`and the ?lm wound onto a core. It is preferred, particu
`larly where the ?lm contains an amide slip agent, that
`the thus formed structure be conditioned or equilibrated
`by holding the same for a period of about 6 hours to 1
`week at a temperature from about 80° to about 125° F.,
`especially preferred is a time from about 6 hrs. to about
`72 hrs. at between 100°—125° F. This ?lm will have on
`the surface thereof a combination of the four additives,
`amide, silica, silicon oil and the incompatible polymer.
`The resulting ?lm will have a coef?cient of friction of
`0.25 or lower and will not block under conditions of
`tight winding after being held at temperatures up to
`20
`140° F.
`‘
`Some commercially available core polymers, skin
`polymers and incompatible polymers and their at least
`approximate melting points are tabulated below:
`TABLE
`
`25
`
`Polymeric Material
`
`All Homopolypropylene
`ARCO 9670 (ethylene-propylene random
`copolymer, 3 wt. % ethylene)
`ARCO W756 (ethylene-propylene random
`copolymer 3.3-3.6 wt. % ethylene)
`Chisso XF 7500 (ethylene-propylene-
`butene-l terpolymer,
`3.5 wt. % ethylene, 4.5 wt. %
`butene-l)
`'
`Chisso XF 7700 (ethylene-propylene-
`butene-l terpolymer,
`5 wt. % ethylene, 4.5 wt. %
`butene-l)
`Solvay KS 400 (ethylene-propylene random
`copolymer)
`v
`Sumitomo FL 6711 (ethylene-propylene random
`copolymer, 4-6% ethylene)
`Polycarbonate
`
`nylon-6
`PTFE
`poly(methylpentene)
`
`Melting
`Point Range
`
`' 321°-32s° F.
`284° F.
`
`273° F.
`
`266° F.
`
`250° F.
`
`273° F.
`
`255° F.
`
`35
`
`40
`
`518° F.
`(process temp.)
`446° F.
`621° F.
`455° F.
`
`Coef?cients of friction (COF) orslip may be deter
`mined by the “moving slide—stationary ?lm” method of
`U.S. Pat. No. 3,399,156, incorporated herein by refer»
`ence in its entirety.
`The contemplated ?lms can be tested by feeding a
`roll of ?lm into a Scandia-Model 712 Cigarette Pack
`Wrapping system with the capability of wrapping up to
`200 packs of cigarettes per minute. This system is
`equipped with high percision controllers and a turn
`cage discharge. During transport through the system
`each pack is subjected to three heat seal regions which
`can cause package to package sticking together. The
`heat seal time is a fraction of a second. The ?lm must
`maintain a COF at room temperature of between about
`02-03 in order to avoid unacceptable drag which
`would lead to pack jamming and machine down-time.
`Success of the operation depends on the virtual absence
`of pack-to-pack sticking, a moderate to low force neces
`sary to move the packs through the system and a seal
`range of at least 20° F. on all seal surfaces.
`
`50
`
`55
`
`60
`
`65
`
`6
`EXAMPLES
`EXAMPLE 1
`A polypropylene homopolymer of comparatively
`high stereoregularity, i.e. ARCO W472, is melt coex
`truded with skin layers of ARCO W756, an ethylene
`propylene copolymer having from 3.3-3.6 wt. % ethyl
`ene, said copolymer containing 4,000 ppm of a dimeth
`ylpolysiloxane, having a viscosity of 30,000 centistokes,
`2,000 ppm silica of a mean particle size of 0.75 micron
`and 4,000 ppm of poly(methylpentene) having a density
`of 0.83 and a melting point of 455° F. Poly(methylpen
`tene) is a product identi?ed as TPX and is obtainable
`from Mitsui Petrochemical Industries Ltd., Tokyo,
`Japan. The resulting ?lm has an overall thickness of 21
`microns with the core layer being 19.8 microns and the
`skin layers being 0.6 microns each. Utilizing the above
`described multi-layer ?lm, cigarette packages, can be
`wrapped per the above-described process and apparatus
`at a rate of approximately 172 packs per minute. This
`wrapping can be successfully accomplished with a vir
`tual absence of package-to-package sticking. A moder
`ate to low force will be necessary to move the packages
`through the machine system. There will be a seal range
`or latitude of at least 20° C. on all seal surfaces.
`
`EXAMPLE 2
`Example 1 is repeated except the base layer contains
`400 parts per million erucamide slip agent and'the skin
`layers contain 700 parts per million of the same.
`This ?lm when employed in the same cigarette pack
`age wrapping system described above will show the
`same degree of success, except that the drag force
`through the system would be reduced by approximately
`30%, from a moderate to a low level.
`
`EXAMPLE 3
`Example 1 is repeated except that the poly(methyl
`pentene) is excluded. Employing the resulting ?lm in
`the same cigarette wrapping system, the system will jam
`within one to two minutes due to package to package
`sticking and increased drag.
`EXAMPLE 4
`_ Example 1 is repeated except the skin layer consists of
`Chisso XF 7700, a terpolymer of ethylene-propylene
`butylene and the poly(methylpentene) is replaced with
`polytetra?uoroethylene having a particle size of about
`5-10 microns. The same weight proportion of PTFE as
`poly(methylpentene) is employed. The resulting ?lm,
`when employed in the same cigarette wrapping appara
`tus will have the same degree of success as the ?lm of
`Example 1.
`
`EXAMPLE 5
`Example 1 is repeated replacing the ethylene-propy
`lene copolymer with Sumitomo FL6711, which is an
`ethylene-propylene copolymer containing approxi
`mately 4-7 wt. % ethylene. Instead of dispersing the
`poly(methylpentene) in the skin ?lm material, 4000
`ppm, subdivided polycarbonate is employed. During
`preparation, the polycarbonate is melt dispersed in the
`ethylene-propylene copolymer to yield a dispersed
`phase having an average particle size of from 1-2 mi
`crons. The resulting ?lm can be employed in the same
`cigarette wrapping system and the same degree of suc
`cess as Example 1 will be experienced.
`
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`

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`4,604,324
`
`8
`6. The structure of claim 3 wherein said incompatible
`polymer is present in from about 1,000-6,000 ppm. of
`said skin layer.
`7. The structure of claim 3 wherein said silicon oil is
`dimethylpolysiloxane having a viscosity of about 20,000
`to 3,000,000 centistokes.
`8. The structure of claim 7 wherein said siloxane is
`present in from about 0.3 to 0.8 wt. % of said skin layer.
`9. The structure of claim 3 wherein said silica has a
`mean particle size of from about 0.5 to 5 microns.
`10. The structure of claim 9 wherein said silica is
`present from about 0.05 to 0.5 wt. % of said skin layer.
`11. The structure of claim 3 also containing an antislip
`proportion of an amide of a water-insoluble monocar
`boxylic acid having from about 8 to 24 carbon atoms
`wherein said amide is present in the precursor resin of
`said base layer in from about 300 to 400 ppm, and pres
`ent in said skin layer in up to about 2000 ppm.
`12. The structure of claim 11 wherein said silicone is
`dimethylsilicone having a viscosity
`of from
`20,000—30,000 centistokes and present in from about 0.3
`to 0.8 wt. % of said skin layer, said silica has a mean
`particle size of about 0.5-5 microns and is present in
`from about 0.05 to 0.5 wt. % of said skin layer and said
`incompatible polymer is poly(rnethylpentene) present in
`from about 1000-6000 ppm of said skin layer.
`13. The structure of claim 12 wherein the skin layer is
`an ethylene-propylene random copolymer.
`14. The structure of claim 13 wherein the coef?cient
`of friction of the surface of (b) is at least as low as 0.25.
`15. The process for preparing an oriented multi-layer
`polypropylene structure comprising melt coextruding a
`combination of:
`(a) a base layer comprising polypropylene of compara
`tively high stereoregularity; and
`(b) a skin layer comprising a polyole?n of compara
`tively low stereoregularity selected from the group
`consisting of an ethylene-propylene copolymer and
`an ethylene-propylene-butylene terpolymer; on at
`least one surface of (a), said skin layer containing a
`combination of ?nely divided silica and a silicon oil;
`and also containing a polymer incompatible with said
`skin layer polymer, said incompatible polymer having
`a melting point of greater than about 150° F. above
`the melting point of said skin layer polymer; and
`orienting the structure.
`5*
`* * ?t
`
`8*
`
`7
`EXAMPLE 6
`Example 1 is repeated except the silicone oil is ex
`cluded. When the resulting ?lm is used in the above
`described system, jamming will occur almost immedi
`ately because the COP will not be low enough, particu
`larly after heat sealing.
`
`EXAMPLE 7
`Example 1 is repeated with only poly(methylpentene)
`dispersed .in the skin layer. This ?lm will cause wrap
`ping system jamming almost immediately.
`
`15
`
`EXAMPLE 8
`Example 1 is repeated with nylon-6 replacing the
`poly(methylpentene) in an equal wt. %. Equally suc=
`cessful results will be obtained in using the ?lm in the
`described wrapping system.
`What is claimed is:
`1. An oriented multi-layer polypropylene ?lm struc
`ture comprising coextruded layers of:
`(a) a base layer comprising polypropylene of compara
`tively high stereoregularity; and
`(b) a skin layer comprising a polyolefin of comparau
`25
`tively low stereoregularity on at least one surface of
`(a), said skin layer containing a combination of ?nely
`divided silica, a silicon oil and a polymer incompati
`ble with said skin layer polyole?n, said incompatible
`polymer having a melting point of greater than 150°
`F. above the melting point of the skin layer polyole
`
`20
`
`2. The structure of claim 1 wherein said incompatible
`polymer is a member selected from the group consisting
`of poly(methylpentene), polycarbonate, polytetra?uo=
`roethylene and nylon.
`3. The structure of claim 2 wherein said skin layer is
`a member selected from the group consisting of an
`ethylene-propylene random copolymer and ethylene=
`propylene-butylene terpolymer.
`4. The structure of claim 3 wherein said skin layer is
`an ethylene-propylene random copolymer with an eth=
`ylene content of about 2 to about 7 wt. %.
`5. The structure of claim 4 wherein said ethylene-pro
`45
`pylene random copolymer has an ethylene content of
`about 3-5 wt. %.
`
`50
`
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`
`60
`
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`PAGE 5 OF 5