CA2068558C

CA2068558C - Multi-layer high opacity film structures and process for producing same

Info

Publication number: CA2068558C
Application number: CA002068558A
Authority: CA
Inventors: Lajos Edward Keller; Maurice Petitjean; Jean-Pierre Frognet
Original assignee: ExxonMobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1991-05-14
Filing date: 1992-05-13
Publication date: 2004-12-21
Anticipated expiration: 2012-05-13
Also published as: KR920021307A; TR27208A; SG76457A1; CA2068558A1; DE69230708T2; US5091236A; EP0514098B1; ATE190007T1; JP3045600B2; ES2143469T3; DE69230708D1; AU1622892A; JPH05193069A; EP0514098A2; AU662738B2; GR3033363T3; PT514098E; EP0514098A3; DK0514098T3

Abstract

A multilayer opaque, biaxially oriented polymeric film structure. The film structure includes (a) a thermoplastic polymer matrix ire layer having a first surface and a second surface, within which is locate a strata of voids; positioned at least substantially within a substantial number of the voids is at least one spherical void-initiating particle which is phase distinct and incompatible with the matrix material, the void space occupied by the particle being substantially less than the volume of the void, with one generally cross-sectional dimension of the particle at least approximating a corresponding cross-sectional dimension of the void; the population of the voids in the core being such as to cause a significant degree of opacity; (b) at least one thermoplastic polymer intermediate layer having a first surface and a second surface, the second surface of the intermediate layer adhering to at least the first surface of the core layer, the intermediate layer including up to about 12% by weight of titanium dioxide contact pigment; and (c) a titanium dioxide-free, non-voided thermoplastic skin layer adhering to the first surface of the intermediate layer, the void-free skim layer and the intermediate layer together being of a thickness such that the outer surface of the skin code layer clues not, at least substantially, manifest the surface irregularities of the matrix core layer.

Description

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F-X211-L(SGC) MULTI-LAYER HIGH OPACTTY FT1M STRLrCI~IRES
At~lD S FOR PRODUCING SAME
'Ihi.s invention relates to the field of polymer films of enhanced opacity aril to a methyl of ~ such films. More particularly, the invention relates to a biaxially oriental c~OSite film structure having improved properties.
In the packaging of certain types of foods, such as snack foods like patato chips, cookies and the like, it is ion practice to employ a multilayer film. A desirable property in such a cka pa grog film is an opacity which protects the packaging product from deterioration caused by exposure to light. In particular, it has been found that certain wavelengths of light, up to about 450 rnn cause increased spoilage in such packaged products. Even when a degree of opacity is present in the film, spoilage occurs if the film allows passage of scsme light.
Benefit accrues from the addition of inorganic particles such as titanium dioxide to whiten the surface of the outer skin layer of the film structure. The increase in whiteness yields an excellent surface for printed graphics. A further benefit resulting fr~n increased whiteness in the outer skin layer of the film is that it permits the printing of laminated or unlaminated film structures without the need for white ink, offering a significant savings to the end user.
While films which employ titanium dioxide°whitsned outer skin layers do provide the aforementioned desirable benefits, such films can also yield certain undesirable characteristics. Ihese characteristics stern from the fact that titanium dioxide (Ti02) is quite abrasive and, when present on the surface of a film, may result in excessive wear of expensive printing and coating gravure roll surfaces, as well as any other surface which is contacted by such a film. Another problem which arises from the use of Ti.02 in the outer skin layers of such films is that fine deposits are laid on converting machir~eay, extruder die lips, treater bar exhausts, etc. Also, appearance problems caused F-6211-L ( SG~C) by streaks on the film, slippage on stretching either by roll or tentering can result.
Therefore, what is needed is a film structure of high opacity which provides in~roved processing characteristics and an improved range of process operability, while maintaining high whiteness, strength arid stiffness.
The film structure of the present invention is an opaque, biaxially 10 oriented polymeric film. The film structure irises (a) a thermoplastic polymer matrix core layer having a fixst surface arxi a second surface, within which is located a strata of opacif yi n g voids;
positioned within a substantial number of the voids is at least one rnicrosphere of void-initiating organic or inorganic particle which is phase distinct 15 ~d ~nc~~patible with the matrix material, the void space oc~pied by the particle being substantially less than the volume of the void, e:ith one geneLally cross-sectional dimension of the particle at least approximating a corresponding cross-sectional dimension of the void;
the population of the voids in the core beirig such as to cause a 20 significant degree of opacity; (b) at least one thermoplastic polymer intern~diate layer having a first surface azxi a second surface, the second surface of the int~nnediate layer adhering to at least the fixst surface of the core layer, the intermediate layer including up to about 12a by weight of titanium dioxide contact pigment; and (c) a 25 titaniuan dioxide-free, non-voided thermoplastic skin layer adhering to the first surface of the intermediate layer, the void-free skin layer and the intermediate layer together being of a thic~ess such that the outer surface of the skin core layer does not, at least substantially, manifest the surface irregularities of the matrix core layer.
Most preferred is a five-layer film structure, incorporating the above-described (a) , (b) and (c) layers, and further including (d) a second thermoplastic polymer intermediate layer having a first surface and a second surface, the second surface of the second intermadiate 35 layer adhering to the second surface of the core layer, the second intermediate layer including up to about 12% by weic~t of titanium F-6211-L(SGC) dioxide contact pigment; and (e) a secorxi titanium dioxide-free, non-voided thermoplastic skin layer adhering to the first surface of the intermediate layer, the second void-free skin layer being of a thickness such that the outer surface of the skin core layer does not, 5 at least substantially, manifest the surface irregularities of the matrix core. layer.
The skin layers (c) and/or (e) can be sitcple, eCOno~ical thin encapsulating layers or they can be more elaborate heat sealable 10 layers.
Also provided is a process for preparing an opaque, biaxially oriented polymeric film structure, cx~risimg the steps of: (a) mixing a major proportion of a first thermoplastic polymeric material with a minor 15 proportion of a void-initiating material of higher melting point or having a higher glass transition ter~erature than the first thermoplastic polymeric material to produce a core layer mixture; (b) heating the core layer mixture produced in step (a) to a t~erature of at least above the melting point of the first thermoplastic polymeric material;
20 (c) dispersing the void-initiaing material of higher melting point or higher glass transition t.~t~erature of the mixture prochzeed in step (a) uniformly throuc~out the molten first thermoplastic polymeric material iri the form of microspheres; (d) mixing a secotxi thermoplastic polymeric material with titanium dioxide contact pigment to produce an 25 in~te layer mixture; (e) heating the intezmeaiate layer mixture produced in step (d) to a t~eratvre of about the meltirig point of the second thermoplastic polymeric material; (f) producing a titanium dioxide-free thermoplastic skin layer mixture; and (g) formir~g a biaxially oriented coe}ctruded film structwce from the core layer 30 eWe titanium dioxide-containing intermediate layer mixture and the titanium dioxide-free skin layer mixture, the forming step conducts at a terperature and to a degree to form a strata of opacifying voids within the core layer; wherein the thermoplastic skin layer in combination with the titanium dioxide-containing intermediate 35 layer are of a thickness such that the outer surface of the matrix ~~~~5~
F-6211-L(SGC) core layer does not, at least sul~tantially, manifest surface irregularities of i:he matrix core layer.
Accordir~gly, it i:; an object of the present invention to provide a film structure of high opacity.
It is another object of the present invention to provide a film with improved processing characteristics.
l0 It is a further object of the present invention to provide a film structure having an improved range of process operability.
It is yet another object of the present invention to provide a film which may be bonded to a wide variety of substrates and coatings.
It is a yet furtlhex object of the present invention to provide a mufti-layer film structure of high whiteness which does not contribute to gravure roll wear.
The invention is further described with reference to the accompanying drawing in which Fig. 1 is a schematic diagram of a method for determining percent light transmission.
Fig. 2 is a schematic diagram of a method for det~xmining pexcent opacity.
In order to achieve the unique film structure of the present invention, it is important that a particular thickness relationship exist between the thiclrness dimension of the ire and the thickness of the skin layers. it is preferred that the core thickness be from about 60 to about 95% of the overall structure with about 65-90%
preferred. This in combination with the population and configuration of 'the voids in a total structure at least about 1.0 mil thick, will materially contribute to the overall degree of opacity of the ~0~8~5~
F-6211-L(SGC) structure. Likewise, by maintalnlr,_g the thiclmess of the skin layers within particular ranges in relation to the overall structure and to the thiclmess of the core layer, the overall ccanbir~tion results in unique advantages. Intermediate layer (b), adhering to the first surface of core layer (a) and, when present, intermediate layer (d) adhering to the secorxl surface of core layer (a) each have a thic~aress of fr~n about 5 to about 300 of the overall structure, with a thiclrness of about 5 to about 15~ preferred. This layer serves an important function in reducing water vapor transmission rate (WVTR) and contains Ti02 as a contact whitenixig agent. Ti02-free skin layers (c) and (e) , adhering to the surfaces of the iniate layers not in intact with the core layer, kaave thiclmesses of from about 0.10 0 to about 5.0% of the overall structure with thic~esses of from about 0. 20 % to about 3 . 0% preferred. The relative thinness of this layer adds to economy in production especially when the layer is an extensive heat-sealable material. A preferred five-layer structure might include, for example, a core layer with a thicJmess of about 79%
of the overall structure with interm~iate layer (b) and (d) having thicknesses of about 8o each and skin layers (c) and (e) having thic~messes of about 2.5o each.
The core is a thermoplastic polymer matrix material within which is located strata of voids. From this it is to be understood that the voids create the matrix configuration.
The films of the present invention have high opacity and low light transmission. A distinction should be made between opacity and licit transmission. Opacity is the opposite of transparency and is a function of the scattering and reflection of light transmitted through the film. Opacity is the ability, for example, to block out writing blow it. Light transmission is a function of light passing ire directly through the film.
Referring now to Figure 1, the pexcent light transmission through a film is determined by usiixl light source 2 to transmit light rays 3 directly through film 4 arad m~asuri.ng at light sensor 5, value T2 ~~8~5 F-6211-L(SGC) _s_ which is the at~unt of light which is transmitted through film 4. The amount of light nays 3 which can be directly transmitted, value T1, is aeterxnined by ms~asuring the light 3 directly transmitted by light source 2 with no interveniy film. The percent light transmission through the film can then be determined using the formula:

o Light Transmission = -where: T2 = light transmitted through a film; and T1 = light directly transmitted.
Referring now to Figure 2, for a measure of percent opacity of a film, light source 2 transmits light through film 4 onto a white surface 9 and the say pra:edure used to project light onto a black surface 10.
With both white and black surfaces, measurement at light sensor 5 is of all of the fo:Llaaing: light reflected off the up~r surface of the film 6; light transmitted through the film and reflected by the white or black surfaces 7 on the side of the film opposite from the light source; and, light scattered by the film 8..
'I~ze percent opa~~ity of the film can then be determined using the formula:
o t3pacity = 100 x ~~'e~ ~ _ ~fllight + scattered light + light transmitted through the film and reflected off a white surface; and RB = Reflected light + scattered light -E light transmitted through the film and reflected off a :blank surface.
A~~-Ti9ly, a highly reflective film may provide high opacity while allowing light transmission. This is because percent light trans-n~ission is not the equivalent of pei~ent opacity. Light transmission is the amount of light passing dixectly ~ the film.
To prevent food spoilage decreased light transmission is desirable.
In forming the core layer, as in U.S. Patent No. 4,377,616, a master batch technique can be employed by either in the case of forming the void initiating particles in situ or in adding preformed spheres to a molten thermoplastic matrix material. After the formation of a master batch, to appropriate dilution of the system can be made by adding additional thermoplastic matrix material until the desired proportions are obtained.
However, the. components may also be directly mixed and extruded instead of utilizing a master batch method.
..
The void-initiating particles which are added as filler to the polymer matrix material of the core layer can be any suitable organic or inorganic material whidi is inoon,patible with the core material at the temperature of biaxial orientation such as polybutylene terephthalate, nYl~. solid or hollaa preformed glass spheres,. metal beads or spheres, ceramic spheres, calcium carbonate, etc.
Zhe polyolefin contemplated as the core material includes polypropylene, polyethylene, polybutene and capolym~rs and blends '~~'~f~ Particularly preferred is an isotactic polypropylene containing at least about 80o by weight of isotactic polypropylene.
It is also preferred that the polypropylene have a melt flow index of from about 2 to 10 g/10 min.
It is preferred that the average diameter of the void initiating particles be from about 0.1 to about 10 microns. These microsphere particles may be of any desired shape although it is preferred that they be substantially spherical in shape. ~iis does not mean that every void is the same size. It means that, generally speaking, each wid tends ~ ~ of like shape when like particles are used even though they vary 206~~~~
F-6211-L(SGC) - g in dimensions. These voids may assume a shape defined by two opposed and ~ige contacting concave disks.
E~xienrz has shown that optimum characteristics of opacity and appearance are obtain~l when the two average major void dimensions are greater than about 30 microns.
We void-initiating particle material, as indicated alcove, should be incompatible with the core material, at least at the tempPxature of biaxial orientation.
The core has been described above as being a thermoplastic polymer matrix material within which is located a strata of voids. From this it is to be understood that the voids create the matrix configuration.
'tee term "strata" is intended to convey the understar~irig that there are many voids creating the matrix and the voids themselves are oriented so that the two major dimensions are aligned in rorrespondence with the direction of orientation of the polymeric film structure. After each void has been formed through the initiation of 'tee described particle, the particle generally contributes little else to the system. This is because its refractive index can be close enough to the matrix material that it makes rto contrilxztion to opacity. When this is the case, the opacii~y is principally a function of the light scattering effect which occurs because of the existence of the voids in the system.
A typical void of the core is defined as having major dimensions X and Y and minor dimension Z, where dimension X is aligned with machine direction orientation, dimension Y is aligned with transverse direction orientation and dimension Z approximately corresponds to the cross-sectional dimension of the spherical particle which initiated the void.
It is a necessary part of the present invention that orientation ~~.tions be such that the X and Y dimensions of the voids of the core be major dimensions in comparison to the Z di~nsion. Thus, F-6211-L(SGC) while the Z dimension generally approximates the cross-sectional dimension of the spherical particle initiating the void, X and Y
dimensions must be significantly greater.
By way of illustration, roam temperature biaxial orientation of a polypropylene matrix ~ntaining polybutylene terephthalate (PBT) spheres of the size arid amount contemplated herein, could not produce the claimed structure. Either void splitting will occur, or, voids of insignificant size would result. Polypropylene must be oriented at a temperature significantly higher than its glass transition temperature. Tne temperature conditions must permit X and Y to be at least several multiples of the Z dimension without void splitting at least to any significant degree. If this is aax~mplished, optimwn physical characteristics, including lcxa water vapor transmission rates ~ a high degree of light scattering are obtain~l without void splitting or film fibrillating.
As indicated above, the matrix polymer and the void initiating particle must be inoampatible and this term is used in the sense that the materials are two distinct phases. The spherical void initiating particles constitute a dispersed phase throughout the lower Wilting polymer which polymer will, ultimately, upon orientation, bee a void-filled matrix with the spherical pao-ticles positioned somewhere in the voids.
As a result of the biaxial orientation of the film structure herein, in addition to opacifying the core layer of the structure, the orientation improves other physical properties of the compc~ite layers such as flex-wrack. resistance, Elmerxlorff tear strength, elongation, file strength, inq~act strength and cold strength properties. The resulting film can have, in addition to a rich high quality appearance and excellent opacifying characteristics, low water vapor transmission rate characteristics and low oxygen transmission rate characteristics.
This makes the film ideally suited for packaging food products ~lliquids. The film also has attractive utility as a decorative wrap material.

F-6211-L(SGC) It is believed that because of comparative sphericity of the void-initiating particles, the voids are closed cells. This means that there is virtually no path open from one side of the core the other throughout which licpud or gas can transverse.
The opacity and low light transmission of the film is further enhanced by the addition to~ the core layer of frarn about to by weight and up to about 10 % by weiglht of opacifying eampounds, which are added to the melt mixture of th.e core layer before extrusion. Qpacifying.campounds which may be used include iron oxides, carbon black, aluminum, Ti02, and talc. The opacifying campc~unds do not contribute to void formation.
The polyolefin contemplated as the material for use in forming iniate layers (b) and (d) includes polypropylene, polyethylene, polybutene and copolymers and blends thereof. As was the case for the core layer, particularly preferred is an isotac~tic polypropylene containing at least about 80o by weight of isotactic polypropylene.
It is also preferred that the polypropylene have a x~lt flow index of fry about 2 to 10 g/10 m.
The opacity, whiteness and low light transmission of the film is further enhanced by the addition to intermediate layers (b) and (d) of Ti02 in amount of fr~n about 14 by weight and up to about 10% by weight, which is added to the melt mixture of the intern~iate layer before extrusion. Preferably, the in~iate layers contain from about 2% by weight to 69 by weight of Ti02. Additionally, the intermediate layers may also contain talc. The whiteness resulting from the inclusion of Ti02 provides. an excellent surface for graphics.
re, the whitxness allows printing of laminated or unlaminated structures without requiring white ink.
Layers (c) aril (e) are thin skin layers applied to the surfaces of intermediate layers (b) and (d) which are not in intact with the core layer (a). Layers (c) and (e) are preferably of a material having a low WV'fR. This layer may consist of a propylene; high density F-6211-L(SGC) polyethylene; linear low density polyethylene; block copolymer of ethylene arid propylene; random copolymer of ethylene and propylene;
other ethylene homopolymer, copoly~r, terpolymer; or blends thereof.
The homiopolymer contemplated herein is formed by polymerizing the respective monar~x. his can be accomplished by bulk or solution polymerization, as ttvose skilled in the art would plainly understand.
One of the preferred materials for layers (c) and/or and (e) is isotactic polypropylene. Skin layers (c) and (e) are of a thir7rness sufficient to encapsulate the Ti02-containing intermediate layers, thus achieving the desired objective of substantially eliminating processing machinery wear problems associated with Ti02-containing outer layers. riIoreover, the combination of intermediate layer (b) and skin layer (c) and intermediate layer (d) and skin layer (e) provide a thirJazess such that the outer surface of each skin layer does not, at least substantially, manifest the surface irregularities of the matrix core layer (a).
The copolymer contemplated herein for skin layers (c) and/or (e) can be selected frcnn those copolymers typically employed in the ~~a~e of mufti-layered films. For example, a block copolyr~x of ethylene and propylene is formed by sequential polymerization of the res~~ective monomers. The feeding of the monomers in forming a block copolymer is controlled so that the monomer employed in one stage of the sequential polymerization is not added until the monomer employed ~ ~e pry stage has been at least suhst~ntially conswned thereby insuring that the concentration of the monomer remaining from the pre~i.ng stage is sufficiently low to prevent formation of an excessive proportion of random copolymer. Also, as indicated above, a randcan copolymer of ethylene and propylene can be advantageously ~loY~ ~ form skin layers (c) and/or (e) .
The contemplated terpolymers which may be used for skin layers (c) and/or (e) are ~aratively low stexeoregular polymers. The terpolymers can have a melt flow rate at 446°F ranging from about 2 to hut l0 grams per 10 minutes and preferably from about 4 to about 6 grams per 10 minutes. The crystalline meltir~ point can range frarn F-6211-L(SGC) about less than 250°F to so~what greater than 371°F. The terpolymers will predte in propylene, and the ethylene and 1-~tene monc~ners can be present in approximately fram 0.3:1- 1:1 mole percentage in relation to earn other.

If desired, the exposed surface of skin layers (c) and/or (e) can be treated in a known and conventional manner, e.g., by corona discharge to improve its receptivity to printing inks and/or its suitability for such subsequent manufacturing operations as lamination.
The treat~l or untreated surface of layers (c) and/or (e) may have applied to ii., coating c~ositions or substrates such as another polymer film or laminate; a metal foil such as aluminum foil;
cellulosic webs, e.g. numPxous varieties of paper such as corrugated paperboard, craft paper, glassine, cartonboard; nonwoven tissue, 2.g., spur~bonded polyolefin fiber, melt-blown microfibers, etc. The application may employ a suitable adhesive, e.g., a hot melt adhesive such as low density polyethylene, ethylene-mathacrylate copolymer, water-based adhe~~ive such as polyvinylidene chloride latex, and the like.
Layers (c) and/or (e) may also include up to about 1~ by weight, with about 500 ppm to about 5000 ppm preferred and 1000 ppm most preferred, of inorganic par~~icles, such as amorphous silica or talc to provide antiblack properties.
SJc?n layers (c) and/or (e) can also be fa''..r~icated frem any of the heat sealable copolymers, blerx7s of hamo~lymexs and blends of copolymers) and harno~lymer(s;). heretofore employed for this purpose. Illustrative of heat sealable copolymers which can be used in 'the present invention are ethylexie-propylene oopolymexs containing from about 1.5 to about 10, and preferably from about 3 to about 5 weight ~rcent ethylene and ethylene- propylene-butene terpolymers containing from about 1 to about 10, and ~rreferably from about 2 to about 6 weight percent e~Yl~ ~d fro~.n about 80 to about 97, and preferably frcan about 88 to about 95 weight percent propylene. Heat sealable blends of F-6211-L(SGC) homapolymer which can be utilized in providing layers (c) and/or (e) include from about 1 to about 99 weight percent polypropylene h~polymer, e.g., one which is the same as, or different fr~n, the polypropylene hcxmpolymer constituting core layer (a) blended with from about 99 to about 1 weight percent of a linear laa density polyethylene (LLDPE). If layers (c) and/or (e) are heat-sealable, corona or flame treatment of layers (c) and/or (e) is not rern~ired.
Heat sealable blends of copolymfr(s) and homopolymer(s) suitable for providing layers (c) and/or {e) include: a blend of from about 5 to about 19 weight percent of poly~mtylene and from about 95 to about 81 weight percent of a oapolymer of propylene (80 to about 95 mole percent) and butylene (20 to about 5 mole percent) ; a blend of frcan about 10 to about 90 weight percent of polybutylene and from about 90 to about 10 weight percent of a copolymer of ethylene (2 to about: 49 mole percent) and a higher olefin having 4 or more carbon atoms (98 to about 51 mole percent); a blend of from about 10 to about 90 weight percent polybutylene and from ab~~ut 90 to about 10 weight percent of a copolymer of ~thhylene {l0 to abo,zt 97 mole percent) and propylene (90 ~ ~t 3 mole percent); and, a blend of from about 90 to about 10 weight percent of polybutylene, and from about 10 to about 90 weight percent of a copolymer of propyl ~y.ne ( 2 to about 79., mole percent) and butylene (98 to about 21 mole percent).
If skin layers (c) and/or (e) are not heat sealable, and that property is desired on one or both of those surfaces, then a heat sealable layer (f) may be applied to one or both of those surfaces. Heat sealable layer (f) may be, for example, vinylidene chloride polymer or an acrylic polymer; or it may be ooeactruded from any of the heat ~l~le materials described herein. Vinylidene chloride polymer or acrylic polymer coatings are preferred materials which may be applied to the exposed exterior surfaces of the skin layers.
It is preferred that all layers of the multilayer film structures of the present invention be coe~ctruded. Thereafter, the film is biaxially oriented. For example, when employing polypropylene for the F-6211-L(SGC) core matrix and the skin layers and employing PBT as the void initiating particles, a machine direction orientation may be from about 4 to about 8 and a transverse orientation may be from 4 to about times at a drawing temperature of about 100°C to 170°C to yield a 5 biaxially oriented film. A preferred film thic~mess is frcan about 0.5 mil to about 3.5 mils.
As ir~iicated above, films which employ titanium dioxide-whitened outer skin layers do provide certain desirable benefits, particularly from -~ app~rrstandpoint. FIowever, such films can also yield certain undesirable characteristics. It has been discovered that these undesirable characteristics stem from the fact that titanium dioxide (Ti02) is quite abrasive and, in fact, possess a hardness greater tt~.an even the rhro~ plating found on gravure rolls. This can result in 15 give wear of expensive printing and coating gravure roll surfaces, as well as any other surface which is contacted by such a film. Other problems which arise from the use of Ti02 in the outer skin layers of such films is that fine deposits are laid on converting machinery, extruder die lips, treater bar exhausts, etc. Also, 20 apl~rproblems caused by streaks on the film, slippage on stretching either by roll or tentering can result. The films produced in accordance with the present invention avoid the problems of films having titanium dioxide-whitened outer skin layers through the encapsulation of a titanium dioxide-whitened intex~diate layer with a ~~r titanium dioxide-free, non voided thermoplastic skin layer.
Films so produced, as demonstrated by the examples which follow, exhibit the highly desirable properties of films having Ti02-containixx~ skin layers, without the processing problems associat~l therewith.
The following specific examples are presented herein to illustrate particular embodiments of the present invention and hence are illustrative of this invention and not to be construed in a limiting ~0~~~~~
F-6211-~(SGC) E~Cample 1 This film of this example was produced for comparison with the films produced in accordance with the present invention.
A mixture of 92 percent, by weight, isotactic polypropylene (MP =
320oF., melt index = 3), containing 8 weight percent PBT (MP = 440oF.) as the core layer void- initiating material, is melted in an extruder with a screw of L/D ratio of 20/1 to provide the core layer mixture.
A second and third extruder, in association with the first extruder, are each supplied with the same isotactic polypropylene (without PBT) as the first extruder, but each containing titanium dioxide particles at 4 percent, by weight. The titanium dioxide particles are employed as a contact whitener for this intermediate layer mixture. A fourth extruder, in association with the first three extruders, is supplied wi'rh the same isotactic pcalypropylene/titanium dioxide as the second extruder, this extruder being used to provide the skin layer mixture.
A melt cnextrusaon is carried out while maintaining the cylinder of the core polymer material at a temperature sufficient to melt the polymer mixture, i.e., from about 450°F. to about 550oF, or higher.
~e I~lypropylene mixtures of the second and third extruders to be extruded as intermediate layers are maintained at about the say temperature as the polypropylene used in fabricating the core layer, as are the mUCtures being used to for the skin layers. The mixture of the fourth extruder is split into two streams to enable the forn~ation of skin layers on each surface of the intermediate layers. As may be appreciated by those skilled in the art, rather than splitting the output of the fourth extruder into two streams, a fifth extruder could be used to supply the second skin layer mixture. Such an arrangement would be desired when the material used to form the second skin layer is varied fr~n that of the first skin layer, when the thic)rness of the second skin layer is vari~l fram that of the first skin layer, etc.
A five-layer film laminate was coextruded with a core thir.,lmess representing about 80 percent of the overall extruded thiclrness, with the thickne<sses of the interm~liate layers representing about 16 percent and 'the skin layers representing about 4 percent of the film ~~6~~~~
F-6211-L(SGC) thiclmess. The unoriented film measured about 40 mils in thiclmess.
The resultant fil~;n sheet was subsequently oriented eic~t by five and one-half tia~s u:aing a ccmnnercially available sequential biaxially orientirx3 apparatus to provide a multilayer film structure. The machine direction (195) orientation is conducted at about 285oF. and the transverse direction (TD) orientation is conducted at about 300°F.
The resultant 1.3 mil multilayer film exhibits a lustrous appearance.
Example 2 ~ form a multilayer film in accordance with the present invention, a mixture of 92 percent, by weight, isotactic polypropylene (MP =
320oF., melt index = 3), containing 8 weight percent PBT (MP = 440oF.) as the core layer void- initiating material, is melted in an extender with a scxew of L/D ratio of 20/1 to provide the core layer mixture.
Again, a second and third extruder, in association with the first extruder, are each supplied with the sauna isotactic polypropylene (without PBT) as the first extruder, again containing titanium dioxide particles at 4 percent, by weight for this intermediate layer mixture.
A fourth extruder, in association with the first three extruders, is plied with the same isotactic polypropylene, this time without titanium dioxide, to provide the skin layer mixture. A melt coextrusion is carried out while maintaining the cylinder of the core polymer material at a t~erature sufficient to melt the polymer mixture, i.e., from about 450oF. to about 550oF. or higher. Again, 'tee I~l~pYlene mixtures to be extruded as intermediate layers are ~intained at about the s~ tempexature as the polypropylene used in fabricating the core layer, as is the mixture being used for the skin layers. As in Example 1, the mixture of the fourth extruder is split into two streams to enable the formation of skin layers on each a~ of the intez~rnediate layers. As was the case for Example 1, rather than splitting the output of the fourth extruder into two streams, a fifth extxuder could have been used to supply the second skin layer mixture so that the thickness or the material used to form the second skin layer could be varied from that of the first skin layer.

F-6211-L(SGC) A five-layer film laminate was coextxuded with a core thira~ess representing about 80 percent of the overall extruded thicimess, with the thickriesses of the intermediate layers representing about 16 percent and the skin layers representing about 4 percent of the film thic3azess. The unoriented film, one again, mea~~ured about 40 mils in thiclaiess. As in Fle 1, the resultant film sheet was oriented eight by five arxi one-half times using a co~mnercially available sequential biaxially orienting apparatus to provide a multilayer film structure. the mackaine direction (MD) orientation is cor~uctEd at about 285°F. and the transverse direction (TD) orientation is conducted at about 300°F. The resultant 1.3 mil multilayer filat~
exhibits a smooth and lustrous appearance.
ale 3 Another multilayer film, in accordance with the present invention, was produced. Again, the same mixture of 92 percent, by weight, isotactic polypropylene (MP = 320°F., melt index = 3), containing 8 weight percent PBT (MP = 440°F.) as the core: layer void-initiating material, was melted in the extruder of ~arnples 1 and 2. The second and third extnzders were supplied with the say isotactic polypropylene (without PBT), containing titanium diaxide particles at 4 percent, by weight for use as the intexmediate layer mixture. A fourth extruder, in association with the fixst three extruders, is supplied with the same isotactic polypropylene, this time with 1000 ppm of amorphous silica added (without titanium dioxide) to provide the skin layer mixture. A
melt ooeactrusion is carried out while maintaiW ng the cylp.nder of the core polymer material at a txature sufficient to alt the polymer mixture, i.e., from about 450°F. to about 550°F. or higher.
Again, the polypropylene mixtures to be extruded as irWiate layers are maintained at about the same temperature as the polypropylene used in fabricating the core layer, as are the mixtures being used to for the skin layers. As in E~ples 1 and 2, the mixture of the fourth extruder is split into two streams to enable the formation of skin layers on each surface of the intermediate layer. A five-layer film l'~ is ooextruded with a core thicW ess representing about 80 percent of the overall extruded thiclaiess, with the thicknesses of the 206~~~~
F-6211-L ( SG(:) _ 18 _ intermediate layexs representing about 16 percent and the skin layers representia~ about 4 percent of the film thic~'a~ess. The unoriented film again measur~i about 40 mils in thic3mess. This film shit was also oriented eight by five and one-half times using a rcially available sequential biaxially orienting apparatus. The maGhi.ne direction (I~) orientation is conducted at about 285oF. arxi the transverse direction (TD) orientation is conducted at about 300oF.
The resultant 1.3 mil multilayer film exhibits a smooth and lustrous appearance.
Example 4 Another multilayer film was produced in accordance with the present invention. Again, the same mixture of 92 percent, by weight, isotactic polypropylene (MP = 320oF. , melt index = 3) , containirx~ 8 weight percent PBT (MP = 440°F.) as the core layer void-initiating material, was melted in the extruder of the previous examples to provide the core layer mixture. The second and third e~ctruders, in association with the first extruder, were supplied with the same isotactic polypropylene (without PBT) as the first extruder, ~n~~~J titanium dioxide particles at ~~ percent by weight for use in forming the ini:ermediate layer. A fourth extruder, in association with the first three extruders, was provided with an ethylene, 1-butene, polypropylene terpolymer, instead of the isotactic polypropylene used in Examples 1 thraugh 3. As in Example 3, 1000 ppsn of a~ri~~s silica was added to the skin layer mixture. A alt coextrusion is carried out while maintaining the cylir~ler of the core polymer material at a trature sufficient to melt the polymer mixture, i.e., frarn about 450°F, to about 550°F, or higher.
Again, the polypropylene mixtures to be extruded as intermediate layers are ~~~at about the same t~a~erature as the polypropylene used in fabricating the core layer, as is the texpolymer mixture being used to form the skin layers. As in the previous Examples, the mixture of the fourth extruder is split into two streams each to enable the formation of skin layers on each surface of the intem~ediate layer.

2~~~5~8 F-6211-I~ (SGC) A five-layer film laminate is coea~t"ruded with a core thicJmess representing abo~;~t 80 percent of the overall ex#-xuded thiclrness, with the thicxnesses of the intermediate layers representing about 16 percent and the ,eJcin layers representing about 4 percent of the film 'thic~ess. The ~a~rient~l film once again ~asured about 40 mils in thic~a~ess. The :resultant film sheet was subsequently oriented eight by five and one-half times using a excially available sequential biaxially orienting apparatus to provide a multilayer film structure.
The machine direction (I~) orientation is conducted at about 285oF.
and the transverse direction (TD) orientation is conducted at about 300oF. The resultant 1.3 mil multilayer film exhibits a smooth and lustrous appearance.
The films so produced were tested light transmission, gloss, whiteness index and c~ffi.cient of friction (COF) , with optical thickness and film density alsa measured. Results obtained are presented in Table 1, below.
Table 1 Unit (JpticalIa.ght White-Coef.

bc. Weight ~Ihick. Transmis-Glossness of No. i~a/m21, m_ils sion~ ~ Index Frict, 1 20.62 1.26 22.8 69 93.5 0.46 20.82 1.30 20.6 82 93.9 0.80 3 20.51 1.26 21.6 79 94.5 0.60 4 19.61 1.26 20.8 74 95.0 0.80 As illustrated, films produced in accor<~ance with the present invention exhibit the highly desirable properties of films having Ti02--containing sxin layers.
Fle 5 This example demonstrates that films produced in accordance with the present invention do not possess the adverse wear charactPxistics of films which employ titanium dioxide- whitened outer skin layers.

~0~8558 F-6211-L(SGC) To illustrate ths: benefits of (films produced in accordance with the present invention, a pilot water experiment was established which sought to measure: the changes in gravure roll cell larxiing width and depth with time. As those skilled in the art recognise, cell depth decreases and lar~3ir~g width increases with increased wear.
Table 2 P~ OQATE12 FILMS E~~NT
CHANGES IN QtAWRE ROLL Ca~L LANDING WIDrIgi AND DF~f~I WITH TIME;

Film of Example Film of ale Time Depth Width Depth Width Hrs. (um) _~wn) ~ (tan) 1 135 ~0 132 28 As may be appreciated, gravure roll wear occurs in reverse coatirx~
applications where the film and the gravure roll surfaces are moving in opposite direcaions having only a sma:Ll wet contact region. For example, it has been found that when coating films having Ti0 -containing outer layers in operations using gravure rolls having a tz,~ncated pyramid cell structure, the gravure roll will last only 8 -10 days. The same gravure roll can be expected to last 3 - 4 a~nths when coating fi7.ms produced in accordance with the present invention.
Films so produced, as de~nstrated by the example..s, exhibit the highly desirable prapert:ies of films having Tio -ccmtaining skin layers, without the processing problems associated therewith. The data of F~amples 1-5 clearly show the unexpected superiority of this invention in providing a fi:Lm with exceptional properties.

Claims

1. An opaque, biaxially oriented polymeric film structure which comprises:
(a) a thermoplastic polymer matrix core layer, having a first surface and a second surface, within which is located a stratum of opacifying voids; positioned within a substantial number of the voids is at least one microsphere of void-initiating organic or inorganic particle which is phase distinct and incompatible with the matrix material, the void space occupied by the particle being substantially less than the volume of the void, with one generally cross-sectional dimension of said particle at least approximating a corresponding cross-sectional dimension of the void; the population of the voids in the core being such as to cause a significant degree of opacity;
(b) at least one thermoplastic polymer intermediate layer having a first surface and a second surface, the second surface of the intermediate layer adhering to at least the first surface of the core layer, the intermediate layer including up to 12% by weight of titanium dioxide contact pigment; and (c) a titanium dioxide-free, non-voided thermoplastic skin layer adhering to the first surface of the intermediate layer, the void-free skin, layer and the intermediate layer together being of a thickness such that the outer surface of the skin core layer does not, at least substantially, manifest the surface irregularities of the matrix core layer.

2. A film structure according to claim 1 which further crises:
(d) a second thermoplastic polymer intermediate layer having a first surface; and a second surface, the second surface of the second intermediate layer adhering to the second surface of the core layer, the second intermediate layer including up to 12% by weight of titanium dioxide contact pigment; and (e) a second titanium dioxide-free, non-voided thermoplastic skin layer adhering to the first surface of the intermediate layer, the second void-free skin layer and the second intermediate layer together being of a thickness such that the voter surface of the skin core layer does not, at least substantially, manifest the surface irregularities of the matrix core layer.

3. A film structure according to claim 1 or 2, wherein at least one void-free skin layer comprises an antiblocking agent.

4. A film structure according to any one of claims 1 to 3, wherein the core layer comprises isotactic polypropylene.

5. A film structure according to any one of claims 1 to 4, wherein the void-initiating particles of the core layer are selected from polybutylene terephthalate and calcium carbonate.

6. A film structure according to any one of claims 1 to 5, wherein at least one intermediate layer comprises isotactic polypropylene.

7. A film structure according to any one of claims 1 to 6, wherein at least one skin layer comprises isotactic polypropylene.

8. A film structure according to any one of claims 1 to 7, wherein at least one intermediate Payer contains from 2% to 6% by weight of TiO2.

9. A film structure according to any one of claims 1 to 8, wherein at least one skin layer comprises a heat sealable material.

10. A film structure according to any one of claims 1 to 9, wherein at least one skin layer comprises a homopolymer of propylene, a linear low density polyethylene, a high density polyethylene, a random copolymer of propylene and ethylene, a block copolymer of propylene and ethylene, a copolymer of propylene and butylene, a terpolymer of ethylene, propylene and butene, a terpolymer of ethylene, propylene and butylene, or a mixture thereof.

11. A film structure according to claim 10 wherein the skin layer comprises an ethylene, 1-butene, propylene terpolymer.

12. Use of a film structure according to any one of claims 1 to 11 in the packaging of comestibles.