Multilayer films having at least five film layers, wherein at least one layer is flame retardant

The present invention provides unified multilayer films having at least one layer that includes a flame retardant film layer. In preferred embodiments the flame retardant film layer is an internal layer. In particularly preferred embodiments of the present invention, multilayer films include flame retardant layers alternating with non flame retardant layers. In other preferred embodiments, multilayer films include alternating layer of different flame retardant materials.

TECHNICAL FIELD
 This invention relates to flame retardant films, and more particularly,
 multilayer films having flame-retardant layers.
 BACKGROUND OF THE INVENTION
 Flame retardant films have been used in many applications where polymeric
 properties offer unique performance advantages over properties of other
 inherently flame retardant materials such as metal sheets and foils, and
 ceramics. Typically, the polymeric articles are either inherently flame
 retardant or rendered flame retardant by the addition of flame retardant
 additives. However, these approaches are limiting.
 Polymer films made of inherently flame retardant polymers such as polyvinyl
 chloride (PVC) and polyimide (PI) usually have a limited range of
 properties. For example plasticizers are generally added to PVC to render
 it more readily processable, however plasticizer migration often adversely
 affects adhesion to subsequent surfaces. In addition, PVC generally has
 little elasticity and low to moderate tensile strength. Similarly, PI is
 difficult to process and more expensive than most common polymers.
 Polymer films made of blends of polymer materials and flame retardant
 materials also have limited performance. While the range of polymer
 materials is broad, the concentration of flame retardant material is
 generally high enough to significantly adversely affect mechanical
 properties of the polymer material. In addition, the flame retardant
 materials often migrate to the film surfaces and adversely affect adhesion
 to subsequent surfaces.
 Thus, there is a need for new flame retardant polymeric films and articles
 that have a broader range of mechanical mechanical properties and reduced
 surface fouling.
 SUMMARY OF THE INVENTION
 The present invention provides flame retardant films that not only have
 desirable mechanical properties and reduced surface fouling, but have
 improved flame retardant efficiency and/or reduced cost when compared with
 conventional flame retardant polymer films or articles. The present
 invention provides unified multilayer films of at least five film layers
 wherein at least one layer, preferably an internal layer, comprises a
 flame retardant film layer and at least one layer comprises a non-flame
 retardant film layer.
 Preferably, multilayer films include layers that include a flame-retardant
 film alternating with layers that include a film that is not a flame
 retardant. In other preferred embodiments, multilayer films have layers of
 different flame retardant films. For example, the construction can include
 alternating layers of a first flame retardant film, a second flame
 retardant film and a non-flame retardant film.
 One aspect of the present invention provides a multilayer film having a
 unified construction of at least 5, preferably 10, more preferably at
 least 13 substantially contiguous film layers wherein at least one layer
 (preferably one internal layer) comprises a flame retardant film layer and
 at least one layer comprises a non-flame retardant film layer.
 Another aspect of the present invention provides a multilayer film having a
 unified construction; wherein the construction comprises at least 5,
 preferably 10, more preferably at least 13 substantially contiguous layers
 of organic polymeric material; the construction comprising layers
 comprising a flame retardant film alternating with layers comprising a
 film that is not flame retardant.
 The present invention also provides a process of preparing a
 flame-retardant multilayer film. The process includes melt processing
 organic polymeric material to form a unified construction of at least 5
 substantially contiguous film layers of organic polymeric material,
 wherein at least one internal layer of the organic polymeric material
 comprises a flame retardant film. Preferably, all the layers are
 simultaneously melt processed, and more preferably, all the layers are
 simultaneously coextruded.
 A further aspect of the present invention provides a process of preparing a
 multilayer film, the process comprising melt processing organic polymeric
 material to form a unified construction of at least 5 substantially
 contiguous layers of organic polymeric material, the construction
 comprises film layers comprising a flame retardant film layer, alternating
 with non flame retardant film layers.
 Herein, the following definitions are used:
 "Unified" means that the layers are not designed to be separated or
 delaminated as would a tape in roll form.
 "Flame retardant" means a characteristic of basic flammability has been
 reduced by some modification as measured by one of the accepted test
 methods such as the Horizontal Burn or Hanging Strip tests.
 "Flame retardant additive" means a compound or mixture of compounds that
 when incorporated (either chemically or mechanically) into a polymer
 serves to slow or hinder the ignition or growth of fire.
 "Flame retardant films" means polymeric films which are inherently flame
 retardant, or have been rendered flame retardant by means of a flame
 retardant additive.
 "Melt processable" means polymers that are fluid or pumpable at the
 temperatures used to melt process the films (e.g., about 50.degree. C. to
 about 300.degree. C.), and do not significantly degrade or gel at the
 temperatures employed during melt processing.
 "Pressure sensitive adhesive" means an adhesive that displays permanent and
 aggressive tackiness to a wide variety of substrates after applying only
 light pressure. It has a four-fold balance of adhesion, cohesion,
 stretchiness, and elasticity, and is normally tacky at use temperatures,
 which is typically room temperature (i.e., about 20.degree. C. to about
 30.degree. C.).
 "Melt viscosity" means the viscosity of molten material at the processing
 temperatures and shear rates employed.
 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 The present invention is directed to multilayer products in the form of
 films of organic polymeric material, wherein the films have at least one
 layer, preferably at least one internal layer that includes a flame
 retardant film layer and at least one layer comprises a non-flame
 retardant film layer. Each of the other layers may include a flame
 retardant film layer, or a film layer that is not a flame retardant. In
 certain preferred embodiments there are flame retardant film layers
 alternating with film layers that are not flame retardant. In other
 preferred embodiments there are alternating layers of a first flame
 retardant film layer, a second flame retardant film layer and a non-flame
 retardant film layer. The two outermost film layers may be flame retardant
 films, non-flame retardant films, or one of the outermost layers may
 include a flame retardant film layer and the other a film layer that is
 non flame retardant. Each layer of the construction is continuous and has
 a substantially contiguous relationship to the adjacent layers.
 Preferably, each layer is substantially uniform in thickness. The multiple
 layers in any one construction are "unified" into a single multilayer film
 such that the layers do not readily separate.
 Flame retardant films used in the flame retardant film layer(s) include
 films which are inherently flame retardant, or have been rendered flame
 retardant by means of a flame retardant additive. Inherently flame
 retardant films are prepared from polymers, which due to their chemical
 structure either do not support combustion, or are self-extinguishing.
 These polymers often have increased stability at higher temperatures by
 incorporating stronger bonds (such as aromatic rings or inorganic bonds)
 in the backbone of the polymers or are highly halogenated. Examples of
 inherently flame retardant polymers include poly(vinyl chloride),
 poly(vinylidine chloride), polyimides, polybenzimidazoles, polyether
 ketones, polyphosphazenes, and polycarbonates. Useful inherently flame
 retardant films generally have a Limiting Oxygen Index (LOI) of at least
 28% as determined by ASTM D-2863-91.
 Useful flame retardant additives include halogenated organic compounds,
 organic phosphorus-containing compounds (such as organic phosphates),
 inorganic compounds and inherently flame retardant polymers. These
 additives are added to or incorporated into the polymeric matrix of the
 polymer film to render an otherwise flammable polymer flame retardant. The
 nature of the flame retardant additive is not critical and a single
 additive may be used. Optionally, it may be desirable to use a mixture of
 two or more individual flame retardant additives.
 Halogenated organic flame retardant additives are thought to function by
 chemical interaction with the flame: the additive dissociates into radical
 species that compete with chain propagating and branching steps in the
 combustion process. Useful halogenated additives are described, for
 example, in the Kirk-Othmer Encyclopedia of Technology, 4.sup.th Ed., vol.
 10, pp 954-76, John Wiley & Sons, N.Y., N.Y., 1993.
 Included within the scope of halogenated organic flame retardant additives
 are substituted benzenes exemplified by tetrabromobenzene,
 hexachlorobenzene, hexabromobenzene, and biphenyls such as
 2,2'-dichlorobiphenyl, 2,4'-dibromobiphenyl, 2,4'-dichlorobiphenyl,
 hexabromobiphenyl, octabromobiphenyl, decabromobiphenyl and halogenated
 diphenyl ethers, containing 2 to 10 halogen atoms.
 The preferred halogenated organic flame retardant additives for this
 invention are aromatic and aliphatic halogen compounds such as brominated
 benzene, brominated imides, chlorinated biphenyl, or a compound comprising
 two phenyl radicals separated by a divalent linking group (such as a
 covlaent bond and having at least two chlorine or bromine atoms per phenyl
 nucleus, chlorine containing aromatic polycarbonates, and mixtures of at
 least two of the foregoing. Especially preferred are hexabromobenzene,
 decabromodiphenyl oxide and tetrabromobisphenol A.
 Among the useful organic phosphorus additives are organic phosphorus
 compounds, phophorus-nitrogen compounds and halogenated organic phosphorus
 compounds. Often organic phosphorus compounds function as flame retardants
 by forming protective liquid or char barriers, which minimize
 transpiration of polymer degradation products to the flame and/or act as
 an insulating barrier to minimize heat transfer.
 In general, the preferred phosphate compounds are selected from organic
 phosphonic acids, phosphonates, phosphinates, phosphonites, phosphinites,
 phosphine oxides, phosphines, phosphites or phosphates. Illustrative is
 triphenyl phosphine oxide. These can be used alone or mixed with
 hexabromobenzene or a chlorinated biphenyl and, optionally, antimony
 oxide. Phosphorus-containing flame retardant additives are decribed, for
 example, in Kirk-Othmer (supra) pp. 976-98.
 Typical of the preferred phosphorus compounds to be employed in this
 invention would be those having the general formula
 ##STR1##
 and nitrogen analogs thereof where each Q represents the same or different
 radicals including hydrocarbon radicals such as alkyl, cycloalkyl, aryl,
 alkyl substituted aryl and aryl substituted alkyl; halogen, hydrogen and
 combinations thereof provided that at least one of said Q's is aryl.
 Typical examples of suitable phosphates include, phenylbisdodecyl
 phosphate, phenylbisneopentyl phosphate, phenylethylene hydrogen
 phosphate, phenyl-bis-3,5,5'-trimethylhexyl phosphate), ethyidiphenyl
 phosphate, 2-ethylhexyl di(p-tolyl) phosphate, diphenyl hydrogen
 phosphate, bis(2-ethyl-hexyl) p-tolylphosphate, tritolyl phosphate,
 bis(2-ethylhexyl)-phenyl phosphate, tri(nonylphenyl) phosphate,
 phenylmethyl hydrogen phosphate, di(dodecyl) p-tolyl phosphate, tricresyl
 phosphate, triphenyl phosphate, halogenated triphenyl phosphate,
 dibutylphenyl phosphate, 2-chloroethyldiphenyl phsophate, p-tolyl
 bis(2,5,5'-trimethylhexyl) phosphate, 2-ethylhexyldiphenyl phosphate,
 diphenyl hydrogen phosphate, and the like. The preferred phosphates are
 those where each Q is aryl. The most preferred phosphate is triphenyl
 phosphate. It is also preferred to use triphenyl phosphate in combination
 with hexabromobenzene and, optionally, antimony oxide.
 Also suitable as flame-retardant additives for this invention are compounds
 containing phosphorus-nitrogen bonds, such as phosphonitrilic chloride,
 phosphorus ester amides, phosphoric acid amides, phosphonic acid amides or
 phosphinic acid amides.
 Among the useful inorganic flame retardant additives include compounds of
 antimony, such as antimony trioxide, antimony pentoxide, and sodium
 antimonate; boron, such as barium metaborate, boric acid, sodium borate
 and zinc borate; aluminum, such as aluminum trihydroxide, magnesium, such
 as magnesium hydroxide; molybdenum, such as molybdic oxide, ammonium
 molybdate and zinc molybdate, phosphorus, such as phosphoric acid; and
 tin, such as zinc stannate. The mode of action is often varied and may
 include inert gas dilution, (by liberating water for example), and thermal
 quenching (by endothermic degradation of the additive). Useful inorganic
 additives are described for example in Kirk-Othmer (supra), pp 936-54.
 Especially useful are mixed additives of an antimony additive and a
 halogenated organic additive, describes as "antimony-halogen" additives
 which produces an especially effective flame retardant. The two additives
 react synergistically at flame temperatures to produce an antimony halide
 or oxyhalide which produce radical species (which compete with chain
 propagating and branching steps in the combustion process) as well as
 promoting char formation.
 Inherently flame retardant polymers may be used in the form of films, in
 the form of particles dispersed in a polymer matrix, or as a blend in a
 compatible polymer. Examples of inherently flame retardant polymers
 include poly(vinyl chloride), poly(vinylidine chloride), polyimides,
 polybenzimidazoles, polyether ketones, polyphosphazenes, polycarbonates
 and polysiloxanes.
 The additives are generally incorporated into the flame retardant film
 layers by addition of the additive(s) to the melt prior to film formation.
 The materials may be added neat, as a melt blend of the additive in a
 polymer, or with the use of a cosolvent or comptatibilizer to render the
 additive and polymer compatible, which may be subsequently removed prior
 to film formation. When using an inherently flame-retardant polymer as an
 additive, it may be melt blended if compatible, or a cosolvent or
 compatibilizer may be used. Alternatively the inherently flame-retardant
 polymer additive may be added as fine particles to the melt. In the case
 of halogenated organic additives and organic phosphorus additives, they
 may be added neat in the form of liquid or solids to the melt. Care should
 be exercised to choose an additive that is stable at the melt temperature
 of the polymer.
 The particle size of the inorganic additive (or organic additives which do
 not melt) should be less than the the thickness of the flame
 retardant-film layer(s) into which it is incorporated to ensure uniform
 thickness of the multilayer film. Preferably the particle size is less
 than one-half, more preferably less than one-third the thickness of the
 flame retardant film layer(s). In general, the smaller the particle, or
 the more surface area the particle presents, the more effective the flame
 retardant properties.
 Generally, when using a low viscosity liquid flame retardant additive, it
 is preferable to use a low viscosity polymer, whereby the best dispersion
 is obtained when the two viscosities of the polymer matrix and dispersed
 phases are closely matched. Alternatively, when using a solid flame
 retardant it is preferable to use a high viscosity polymer as high
 viscosities are necessary to generate the stresses necessary to produce a
 homogenous dispersion. Viscosities may also be matched by judicious
 selection of process temperature conditions. Further information on
 multiphase flow in polymer processing may be found in Han, Multiphase Flow
 in Polymer Processing, Academic Press, N.Y., 1981, pp 229-235 and in
 Elmendorpp, Dispersive Mixing in Liquid Systems, Mixing in Polymer
 Processing, C. Rauwendaal, ed., Marcel Dekker, Inc., N.Y., pp. 17-53.
 Flame retardant additives are added in sufficient amounts to render the
 multilayer film flame retardant. Generally the additives are added in
 amounts of 40 wt. % or more in each flame retardant layer. Preferably the
 additives are added in amounts of at least 50 wt. % in each flame
 retardant layer or in the amounts of 10 to 90 wt. % of the unified
 multilayer film.
 Polymeric materials used in the multilayer films of the present invention
 include one or more melt-processible organic polymers, which may include
 thermoplastic, thermoplastic elastomeric or elastomeric materials.
 Thermoplastic materials are generally materials that flow when heated
 sufficiently above their glass transition temperature, or if
 semicrystalline, above their melt temperatures, and become solid when
 cooled. They may be elastomeric or nonelastomeric.
 Thermoplastic materials useful in the present invention that are generally
 considered nonelastomeric include, for example, polyolefins such as
 isotactic polypropylene, low density polyethylene, linear low density
 polyethylene, very low density polyethylene, medium density polyethylene,
 high density polyethylene, polybutylene, nonelastomeric polyolefin
 copolymers or terpolymers such as ethylene/propylene copolymer and blends
 thereof; ethylene-vinyl acetate copolymers such as those available under
 the trade designation ELVAX from E.I. DuPont de Nemours, Inc.,
 Wilimington, Del.; ethylene acrylic acid copolymers; ethylene methacrylic
 acid copolymers such as those available under the trade designation SURLYN
 from E.I. DuPont de Nemours, Inc.; polymethylmethacrylate; polystyrene;
 ethylene vinyl alcohol; polyesters including amorphous polyester; and
 polyamides.
 Elastomers, as used herein, are distinct from thermoplastic elastomeric
 materials in that the elastomers require crosslinking via chemical
 reaction or irradiation to provide a crosslinked network which imparts
 modulus, tensile strength, and elastic recovery. Elastomers useful in the
 present invention include, for example, natural rubbers such as CV-60, a
 controlled viscosity grade of rubber, and SMR-5, a ribbed smoked sheet
 rubber; butyl rubbers, such as Exxon Butyl 268 available from Exxon
 Chemical Co., Houston, Tex.; synthetic polyisoprenes such as CARIFLEX,
 available from Shell Oil Co., Houston, Tex., and NATSYN, available from
 Goodyear Tire and Rubber Co., Akron, Ohio; ethylene-propylenes;
 polybutadienes; polybutylenes; polyisobutylenes such as VISTANEX,
 available from Exxon Chemical Co.; and styrene-butadiene random copolymer
 rubbers such as AMERIPOL SYNPOL available from American Synpol Co., Port
 Neches, Tex.
 In the present invention, preferred organic polymers and homo-and
 copolymers of polyolefins including polyethylene, polypropylene and
 polybutylene homo- and copolymers.
 Thermoplastic materials that have elastomeric properties are typically
 called thermoplastic elastomeric materials. Thermoplastic elastomeric
 materials are generally defined as materials that act as though they were
 covalently crosslinked at ambient temperatures, exhibiting high resilience
 and low creep, yet process like thermoplastic nonelastomers and flow when
 heated above their softening point. Thermoplastic elastomeric materials
 useful in the multilayer films of the present invention include, for
 example, linear, radial, star, and tapered block copolymers (e.g.,
 styrene-isoprene block copolymers, styrene-(ethylene-butylene) block
 copolymers, styrene-(ethylene-propylene) block copolymers, and
 styrene-butadiene block copolymers); polyetheresters such as that
 available under the trade designation HYTREL from E.I. DuPont de Nemours,
 Inc.; elastomeric ethylene-propylene copolymers; thermoplastic elastomeric
 polyurethanes such as that available under the trade designation MORTHANE
 URETHENE from Morton International, Inc., Chicago, Ill.; polyvinylethers;
 poly-.alpha.-olefin-based thermoplastic elastomeric materials such as
 those represented by the formula --(CH.sub.2 CHR).sub.x where R is an
 alkyl group containing 2 to 10 carbon atoms, and poly-.alpha.-olefins
 based on metallocene catalysis such as ENGAGE,
 ethylene/poly-.alpha.-olefin copolymer available from Dow Plastics Co.,
 Midland, Mich.
 The multilayer films are typically prepared by melt processing (e.g.,
 extruding). In a preferred method, the flame retardant and non-flame
 retardant layers are generally formed at the same time, joined while in a
 molten state, and cooled. That is, preferably, the layers are
 substantially simultaneously melt-processed, and more preferably, the
 layers are substantially simultaneously coextruded. Products formed in
 this way possess a unified construction and have a wide variety of useful,
 unique, and unexpected properties, which provide for a wide variety of
 useful, unique, and unexpected applications.
 Preferably, the multilayer films range in thickness from about 25 to about
 750 micrometers (.mu.m) thick (more preferably, no greater than about 150
 .mu.m, and most preferably, no greater than about 50 .mu.m). The thickness
 (or volume fraction) of the multilayer film and the individual film layers
 depend primarily on the end-use application and the desired composite
 mechanical properties of the multi-layered film. Such multilayer films
 have a construction of at least 5 layers, preferably, at least 10 layers,
 more preferably, at least 13 layers, and even more preferably, at least 29
 layers. For preferred embodiments, there are generally no more than about
 500 layers, more preferably, no more than about 200 layers, and most
 preferably, no more than about 100 layers, although it is envisioned that
 constructions having many more layers can be made using the materials and
 methods described herein.
 Depending on the polymers and additives chosen, thicknesses of the layers,
 and processing parameters used, the multilayer films will typically have
 different properties at different numbers of layers. That is, the same
 property (e.g., tensile strength, modulus, fire retardancy) may go through
 maximum at a different number of layers for two particular materials when
 compared to two other materials.
 In any one construction of the alternating layers of flame retardant film
 layers and non flame retardant film layers, each of the flame retardant
 layers typically includes the same material (flame retardant additive in a
 polymer matrix or in an inherently flame retardant polymer) or combination
 of materials, although they may include different materials or
 combinations of materials. Similarly, each of the layers that is not flame
 retardant typically includes the same material or combination of
 materials, although they may include different materials or combinations
 of materials.
 Multilayer films can include an (AB).sub.n construction, with either A
 and/or B layers as the outermost layers (e.g., (AB).sub.n A, (BA).sub.n B,
 or (AB).sub.n). In such constructions, each of the B layers has flame
 retardant properties as a result of the incorporation of a flame retardant
 additive or the use of an inherently flame retardant polymer, which may be
 the same or different in each layer, and each of the A layers does not
 have flame retardant properties, which may be the same or different in
 each layer. Multilayer films can also include A, A' B, and B' film layers,
 with any of the A, A', B or B' layers as the outermost layers. Preferably
 the A layers are the outermost layers. In such constructions, each of the
 B and B' layers may include a different flame retardant film layer and
 each of the B layers may include a different non-flame retardant film
 layer. In each of these constructions, n is preferably at least 2, and
 more preferably, at least 5, depending on the materials used and the
 application desired.
 In embodiments with alternating different flame retardant layers (B,B'),
 the multilayer films can take advantage of the properties of each of the
 flame retardant film layers. For example, a construction with alternating
 layers of an organic halogenated flame retardant and an inorganic antimony
 trioxide flame retardant has the synergistic effect of reducing the
 concentration of radical species and promoting char formation. Similarly
 the use of organic halogenated flame retardant and hydrated alumina will
 retard flames by reducing radical species and the enthalpy of combustion.
 Preferred embodiments include three or more layers of a flame-retardant
 additive in a polymer matrix and three or more layers of the same polymer
 matrix that is not a flame retardant (i.e lacking the flame-retardant
 additive). More preferred embodiments include only two types of materials,
 one inherently flame retardant polymer and one that is not flame retardant
 in alternating layers. Other preferred embodiments include only two
 different flame-retardants in alternating layers.
 The two outermost layers of multilayer films of the present invention can
 include one or more flame-retardant films, which may be the same or
 different in each of the two outermost layers. Alternatively, the two
 outermost layers can include one or more films that are not flame
 retardant, which may be the same or different in each of the two outermost
 layers. Furthermore, the inventive films include embodiments in which only
 one of the outermost layers includes one or more flame-retardant films.
 The individual layers of multilayer films of the present invention can be
 of the same or different thicknesses. Preferably, each internal layer is
 no greater than about 25 micrometers (.mu.m) thick, and more preferably,
 no greater than about 5 .mu.m thick. Each of the two outermost layers can
 be significantly thicker than any of the inner layers, however.
 Preferably, each of the two outermost layers is no greater than about 150
 .mu.m thick, more preferably no greater than 50 .mu.m thick. Typically,
 each layer, whether it be an internal layer or one of the outermost
 layers, is at least about 0.01 .mu.m thick, depending upon the materials
 used to from the layer and the desired application.
 Multilayer films wherein one or more of the layers is a flame retardant can
 be made that have many significant and unexpected properties. These can
 include, for example, good flame resistance, reduced surface fouling, good
 weatherability, relatively low material costs, good flame resistance, and
 sufficient tensile strength for handling, relatively high break elongation
 and toughness, relatively high yield and break stress, good drape and
 softness, good stretch release properties, and paper-like tensile,
 elongation and tear properties. Each multilayer film of the present
 invention will not necessarily have all of these advantageous properties.
 This will depend on the number of layers, the types of materials, the
 affinity of the materials for each other, the modulus of the different
 materials, and the like.
 Preferably one or both of the outer layers are not flame retardant, the
 multilayer films can be used as single- or double-sided flame retardant
 tapes, nonadhesive films for use as backings for tapes, or flame retardant
 films for use as adhesive layers in tapes, for example. This is because
 they have advantageous mechanical properties, tensile strength, a
 relatively high break elongation (i.e., elongation at break) and
 toughness, good yield and break stress, as well as beneficial tear
 properties, despite the incorporation of one or more flame retardant film
 layers.
 When used as a backing for an adhesive tape, the multilayer film of the
 present invention may further comprise a pressure-sensitive adhesive
 layer. Pressure sensitive adhesives useful in the present invention can be
 self tacky or require the addition of a tackifier. Such materials include,
 but are not limited to, tackified natural rubbers, tackified synthetic
 rubbers, tackified styrene block copolymers, self-tacky or tackified
 acrylate or methacrylate copolymers, self-tacky or tackified
 poly-.alpha.-olefins, and tackified silicones. Examples of suitable
 adhesives are described in U.S. Pat. No. Re 24,906 (Ulrich), U.S. Pat. No.
 4,833,179 (Young et al.), U.S. Pat. No. 5,209,971 (Babu et al.), U.S. Pat.
 No. 2,736,721 (Dexter), and U.S. Pat. No. 5,461,134 (Leir et al.), for
 example. Others are described in the Encyclopedia of Polymer Science and
 Engineering, vol. 13, Wiley-lnterscience Publishers, New York, 1988, the
 Encyclopedia of Polymer Science and Technology, vol. 1, lnterscience
 Publishers, New York, 1964 and in D. Satas, Handbook of Pressure Sensitive
 Adhesives, 2.sup.nd Edition, Van Nostrand Reinhold, New York, 1989.
 A pressure sensitive adhesive useful in the present invention typically has
 an open time tack (i.e., period of time during which the adhesive is tacky
 at room temperature) on the order of days and often months or years. An
 accepted quantitative description of a pressure sensitive adhesive is
 given by the Dahlquist criterion line (as described in Handbook of
 Pressure Sensitive Adhesive Technology, Second Edition, D. Satas, ed., Van
 Nostrand Reinhold, New York, N.Y., 1989, pages 171-176), which indicates
 that materials having a storage modulus (G') of less than about
 3.times.10.sup.5 Pascals (measured at 10 radians/second at a temperature
 of about 20.degree. C. to about 22.degree. C.) typically have pressure
 sensitive adhesive properties while materials having a G' in excess of
 this value typically do not.
 Suitable polymers for use in preparing the films of the present invention,
 whether they are inherently flame retardants or not, are melt processable.
 That is, they are fluid or pumpable at the temperatures used to melt
 process the films (e.g., about 50.degree. C. to about 300.degree. C.), and
 they are film formers. Furthermore, suitable polymers do not significantly
 degrade or gel at the temperatures employed during melt processing (e.g.,
 extruding or compounding). Preferably, such polymers have a melt viscosity
 of about 10 poise to about 1,000,000 poise, as measured by capillary melt
 rheometry at the processing temperatures and shear rates employed in
 extrusion. Typically, suitable polymers possess a melt viscosity within
 this range at a temperature of about 175.degree. C. and a shear rate of
 about 100 seconds.sup.-1.
 In melt processing multilayer films of the present invention, the polymers
 in adjacent layers need not be chemically or physically compatible or well
 matched, particularly with respect to melt viscosities, although they can
 be if so desired. That is, although materials in adjacent polymeric
 flowstreams can have relative melt viscosities (i.e., ratio of their
 viscosities) within a range of about 1:1 to about 1:2, they do not need to
 have such closely matched melt viscosities. Rather, the materials in
 adjacent polymeric fiowstreams can have relative melt viscosities of at
 least about 1:5, and often up to about 1:20. For example, the melt
 viscosity of a flowstream of polymer B (or A) can be similar or at least
 about 5 times, and even up to about 20 times, greater than the melt
 viscosity of an adjacent flowstream of polymer A (or B).
 In melt processing polymers of different flame retardants film layers
 and/or non flame retardant film layers, the differences in elastic
 stresses generated at the interface between the layers of different flame
 retardants is also important. Preferably, these elastic differences are
 minimized to reduce or eliminate flow instabilities that can lead to layer
 breakup. With knowledge of a material's elasticity, as measured by the
 storage modulus on a rotational rheometer over a range of frequencies
 (0.001 rad/sec. &lt;.omega.&lt;100 rad/sec.) at the processing temperature,
 along with its viscosity at shear rates less than 0.01 second.sup.-1, and
 the degree to which the material's viscosity decreases with shear rate,
 one of skill in the art can make judicious choices of the relative
 thicknesses of the layers, the die gap, and the flow rate to obtain a film
 with continuous, uniform layers. Generally, the elastic stresses at 100
 sec.sup.-1 by a more viscous polymer should be greater than the elastic
 stress generated by the less viscous polymer. Further, the ratio of the
 storage modulus to the viscosity at 0.01 sec.sup.-1 for the more viscous
 polymer should be greater than that of the less viscous polymer.
 Significantly, relatively incompatible materials (i.e., those that
 typically readily delaminate as in conventional two layer constructions)
 can be used in the multilayer films of the present invention. Although
 they may not be suitable for all constructions, they are suitable for the
 constructions having larger numbers of layers. That is, generally as the
 number of layers increases, relatively incompatible materials can be used
 without delamination occurring. in addition, film properties such as
 elongation at break and toughness often increase as the number of layers
 increases, depending on the materials used.
 The flame retardant layer (B or B') can include a single flame retardant, a
 mixture (e.g., blend) of several flame retardants, or a mixture (e.g.,
 blend) of a flame retardant and a material that is not a flame retardant
 (e.g., a nontacky thermoplastic material, which may or may not be
 elastomeric), as long as the layer has flame retardant properties.
 Examples of some flame retardant blends are described in Kirk-Othmer
 (supra). Similarly, the nonflame retardant layer (A or A') can include a
 single polymer that is not a flame retardant, a mixture of several such
 polymers, , as long as the layer does not have flame retardant properties.
 The materials of the non-flame retardant layer (A or A') can be modified
 with one or more processing aids, such as plasticizers and lubricants, to
 modify their properties. Plasticizers and lubricants useful with the
 polymeric materials are preferably miscible at the molecular level, i.e.,
 dispersible or soluble in the thermoplastic material. External lubricants
 that are incompatible with the polymer can also be added that act by
 migrating to the surface of the polymer melt and reducing frictions with
 the extrusion equipment (the die or extruder barrel for example). Examples
 of plasticizers and lubricants include, but are not limited to,
 polybutene, paraffinic oils and waxes, fatty acids including stearic acid
 and calcium stearate, petrolatum, liquid rubbers, and certain phthalates
 with long aliphatic side chains such as ditridecyl phthalate. When used, a
 processing aid is typically present in an amount of about 5 parts to about
 300 parts by weight, and preferably up to about 200 parts by weight, based
 on 100 parts by weight of the polymeric material in the nonflame retardant
 layer.
 Other additives, such as fillers, pigments, crosslinking agents,
 antioxidants, ultraviolet stabilizers, and the like, may be added to
 modify the properties of either the flame retardant layers (B or B) or the
 nonflame retardant layers (A or A'). Each of these additives is used in an
 amount to produce the desired result.
 Pigments and fillers can be used to modify cohesive strength and stiffness,
 cold flow, and tack, as well as chemical resistance and gas permeability.
 For example, aluminum hydrate, lithopone, whiting, and the coarser carbon
 blacks such as thermal blacks also increase tack with moderate increase in
 cohesivity, whereas clays, hydrated silicas, calcium silicates,
 silico-aluminates, and the fine furnace and thermal blacks increase
 cohesive strength and stiffness. Platy pigments and fillers, such as mica,
 graphite, and talc, are preferred for acid and chemical resistance and low
 gas permeability. Other fillers can include glass or polymeric beads or
 bubbles, metal particles, fibers, and the like. Typically, pigments and
 fillers are used in amounts of about 0.1% to about 20% by weight, based on
 the total weight of the multilayer film.
 Crosslinkers such as benzophenone, derivatives of benzophenone, and
 substituted benzophenones such as acryloyloxybenzophenone may also be
 added. Such crosslinkers are preferably not thermally activated, but are
 activated by a source of radiation such as ultraviolet or electron-beam
 radiation subsequent to forming the films. Typically, crosslinkers are
 used in amounts of about 0.1% to about 5.0% by weight, based on the total
 weight of the multilayer film.
 Antioxidants and/or ultraviolet stabilizers may be used to protect against
 severe environmental aging caused by ultraviolet light or heat. These
 include, for example, hindered phenols, amines, and sulfur and phosphorus
 hydroxide decomposers. Typically, antioxidants and/or ultraviolet
 stabilizes are used in amounts of about 0.1% to about 5.0% by weight,
 based on the total weight of the multilayer film.
 Intermediate layers may be used in a multilayered construction to adhere
 different polymeric materials having insufficient interlayer adhesion.
 Intermediate layers, or tie layers, generally have an affinity for both of
 the principal layers and typically consist of materials that will not
 significantly reduce the overall tensile properties of the multilayer
 construction. Some useful tie layers include, for example, copolymers
 containing blocks that have an affinity for each of the principal layers,
 which flow when melted and cool to a tack-free state.
 Tie layers, which are typically hot melt adhesive (i.e., tacky when in the
 melt state), can also be used to enhance the adhesion between each of the
 layers if so desired. Materials useful in the tie layers include,
 ethylene/vinyl acetate copolymer (preferably containing at least about 10%
 by weight of vinyl acetate units), carboxylated ethylene/vinyl acetate
 copolymer such as that available under the trade designation CXA, from
 E.I. DuPont de Nemours, Inc., copolymers of ethylene and methyl acrylate
 such as that commercially available under the trade designation POLY-ETH
 EMA, from Gulf Oil and Chemicals Co., ethylene/acrylic acid copolymer such
 as that available under the trade designation SURLYN (a copolymer of
 ethylene with a methacryic acid) from E.I. DuPont de Nemours, Inc., maleic
 anhydride modified polyolefins and copolymers of polyolefins such as that
 commercially available under the trade designation MODIC, from Mitsubishi
 Chemical Co., polyolefins containing homogeneously dispersed vinyl
 polymers such as those commercially available under the trade designation
 VMX from Mitsubishi Chemical Co. (e.g., FN-70, an ethylene/vinyl acetate
 based product having a total vinyl acetate content of 50% and JN-70, an
 ethylene/vinyl acetate based product containing dispersed
 polymethylmethacrylate and having a vinyl acetate content of 23% and a
 methyl methacrylate content of 23%), POLYBOND (believed to be a polyolefin
 grafted with acrylic acid) from B.P. Chemicals Inc., Cleveland, Ohio,
 PLEXAR (believed to be a polyolefin grafted with ftinctional groups) from
 Quantum Chemicals, inc., Cincinnati, Ohio, a copolymer of ethylene and
 acrylic acid such as that commercially available under the trade
 designation PRIMACOR from Dow Chemical Co., Midland, Mich., and a
 copolymer of ethylene and methacrylic acid such as that commercially
 available under the trade designation NUCREL from E.I. DuPont de Nemours,
 Inc.
 The multilayer films of the present invention can be prepared directly by
 extrusion, for example, with the outermost layers being preferably non
 flame retardant. Frequently, incorporation of a flame retardant into one
 or both of the outermost layers can degrade the surface and/or mechanical
 properties of the outermost layer. Halogenated organic flame retardants,
 for example, may tend to migrate to the surface of the film and render the
 surface non-amenable to further coating, by a pressure sensitive adhesive
 for example. Alternatively, the films can be made with one or both of the
 outermost layers being flame retardant layer(s) depending on the
 application.
 The multilayer films of the present invention can be used as the backings
 or substrates for single-sided or double-sided adhesive products, such as
 tapes. Preferably the multilayer films used as backings in tape have a non
 flame retardant layer as at least one of the outermost layers. Such films
 can be prepared using extrusion techniques, for example, to produce such
 products directly (i.e., with one or both outermost layers of the film
 being an a pressure sensitive adhesive layer). Alternatively, a multilayer
 film can be coated with an adhesive material using conventional coating
 techniques. Furthermore, such products can be coated with a low-adhesion
 backsize (LAB) material, which restricts adhesion of various types of
 surfaces to the film when it is wound as a coil or is stacked on itself A
 wide variety of known adhesive materials (e.g., any of the pressure
 sensitive materials described herein) and LAB materials (e.g.,
 polyolefins, urethanes, cured silicones, fluorochemicals) can be used as
 well as a wide variety of known coating techniques, including solvent
 coating and extrusion coating techniques.
 Multilayer films of the present invention can be made using a variety of
 equipment and a number of melt-processing techniques (typically, extrusion
 techniques) well known in the art. Such equipment and techniques are
 disclosed, for example, in U.S. Pat. No. 3,565,985 (Schrenk et al.), U.S.
 Pat. No. 5,427,842 (Bland et al.), U.S. Pat. No.5,589,122 (Leonard et
 al.), U.S. Pat. No. 5,599,602 (Leonard et al.), and U.S. Pat. No.
 5,660,922 (Herrid(e et al.). For example, single- or multi-manifold dies,
 full moon feedblocks (such as those described in U.S. Pat. No. 5,389,324
 to Lewis et al.), or other types of melt processing equipment can be used,
 depending on the number of layers desired and the types of materials
 extruded.
 For example, one technique for manufacturing multilayer films of the
 present invention can use a coextrusion technique, such as that described
 in U.S. Pat. No. 5,660,922 (Herridge et al.). In a coextrusion technique,
 various molten streams are transported to an extrusion die outlet and
 joined together in proximity of the outlet. Extruders are in effect the
 "pumps" for delivery of the molten streams to the extrusion die. The
 precise extruder is generally not critical to the process. A number of
 useful extruders are known and include single and twin screw extruders,
 batch-off extruders, and the like. Conventional extruders are commercially
 available from a variety of vendors such as Davis-Standard Extruders, Inc.
 (Pawcatuck, Conn.), Black Clawson Co. (Fulton, N.Y.), Berstorff Corp.
 (N.C.), Farrel Corp. (Conn.), and Moriyama Mfr. Works, Ltd. (Osaka,
 Japan).
 Other pumps may also be used to deliver the molten streams to the extrusion
 die. They include drum loaders, bulk melters, gear pumps, and the like,
 and are commercially available from a variety of vendors such as Graco LTI
 (Monterey, Calif.), Nordson (Westlake, Calif.), Industrial Machine
 Manufacturing (Richmond, V.A.), and Zenith Pumps Div., Parker Hannifin
 Corp. (N.C.).
 Typically, a feedblock combines the molten streams into a single flow
 channel. The distinct layers of each material are maintained because of
 the laminar flow characteristics of the streams. The molten structure then
 passes through an extrusion die, where the molten stream is reduced in
 height and increased in width so as to provide a relatively thin and wide
 construction. This type of coextrusion is typically used to manufacture
 multilayer film constructions having about 10 layers or more.
 However, the use of a feedblock is optional, as a variety of coextrusion
 die systems are known. For example, multimanifold dies may also be
 employed, such as those commercially available from The Cloeren Company
 (Orange, Tex). In multimanifold dies, each material flows in its own
 manifold to the point of confluence. In contrast, when feedblocks are
 used, the materials flow in contact through a single manifold after the
 point of confluence. In multimanifold die manufacturing, separate streams
 of material in a flowable state are each split into a predetermined number
 of smaller or sub-streams. These smaller streams are then combined in a
 predetermined pattern of layers to form an array of layers of these
 materials in a flowable state. The layers are in intimate contact with
 adjacent layers in the array. This array generally comprises a stack of
 layers which is then compressed to reduce its height. In the multimanifold
 die approach, the film width remains constant during compression of the
 stack, while the width is expanded in the feedblock approach. In either
 case, a comparatively thin, wide film results. Layer multipliers in which
 the resulting film is split into a plurality of individual subfilms which
 are then stacked one upon another to increase the number of layers in the
 ultimate film may also be used. The multimanifold die approach is
 typically used in manufacturing multilayer film constructions having less
 than about 10 layers.
 In manufacturing the films, the materials may be fed such that either a
 flame retardant layer or the non-flame retardant layer forms the outermost
 layers. The two outermost layers are often formed from the same material.
 Preferably, although not necessarily, the materials comprising the various
 layers are processable at the same temperature. Significantly, although it
 has been generally believed that the melt viscosities of the various
 layers should be similar, this is not a necessary requirement of the
 methods and products of the present invention. Accordingly, residence
 times and processing temperatures may have to be adjusted independently
 (i.e., for each type of material) depending on the characteristics of the
 materials of each layer
 The volume fraction of the A and B layers depends primarily on the ratio of
 the viscosities of the component polymers or polymer mixtures (including
 the addition of the flame retardant additive). For example, if the outer
 "A" layer has a higher viscosity than the "B" layer, process stability
 considerations suggest that the "B" layer have a greater volume fraction
 (i.e &gt;50%). Conversely, if the A layer has a lower viscosity than the B
 layer, process stability should increase if the B layer has a smaller
 (i.e. &lt;50%) volume fraction. These considerations are generally true
 regardless of the number of layers and the total flow rate of the process.
 Other manufacturing techniques, such as lamination, coating, or extrusion
 coating may be used in assembling multilayer films and products from such
 multilayer films according to the present invention. For example, in
 lamination, the various layers of the film are brought together under
 temperatures and/or pressures (e.g., using heated laminating rollers or a
 heated press) sufficient to adhere adjacent layers to each other.
 In extrusion coating, a first layer is extruded onto a cast web, a
 uniaxially oriented film, or a biaxially oriented film, and subsequent
 layers are sequentially coated onto the previously provided layers.
 Extrusion coating may be preferred over the melt coextrusion process
 described above if it is desirable to pretreat selected layers of the
 multilayer film or if the materials are not readily coextrudable.
 Continuous forming methods include drawing the multilayer film out of a
 film die and subsequently contacting the extruded multilayer film with a
 moving plastic web or other suitable substrate, After forming, the
 multilayer films are solidified by quenching using both direct methods,
 such as chill rolls or water baths, and indirect methods, such as air or
 gas impingement.
 The films of the present invention can be oriented, either uniaxially
 (i.e., substantially in one direction) or biaxially (i.e., substantially
 in two directions), if so desired. Such orientation can result in improved
 strength properties, as evidenced by higher modulus and tensile strength.
 Preferably, the films are prepared by co-extruding the individual polymers
 to form a multi layer film and then orienting the film by stretching at a
 selected temperature. For example, uniaxial orientation can be
 accomplished by stretching a multilayer film construction in the machine
 direction at a temperature of about the melting point of the film, whereas
 biaxial orientation can be accomplished by stretching a multilayer film
 construction in the machine direction and the cross direction at a
 temperature of about the melting point of the film. Optionally
 heat-setting at a selected temperature may follow the orienting step.

EXAMPLES
 This invention is further illustrated by the following examples which are
 not intended to limit the scope of the invention. In the examples, all
 parts, ratios and percentages are by weight unless otherwise indicated.
 The following test methods were used to characterize the flame retardant
 films in the examples:
 Test Methods
 Horizontal Burn
 Burning characteristics of multilayer films were evaluated according to
 ASTM D1000 except the film were first laminated to a 25 micrometer thick
 layer of pressure-sensitive adhesive (a blend of 50 parts KRATON.TM. 1107
 polystyrene/poluisoprene block coplolymer available from Shell Chemical,
 50 parts NATSYNT.TM. 2210 polyisoprene homopolymer available from Goodyear
 Tire and Rubber, 75 parts WINTACK PLUS.TM. hydrocarbon tackifier available
 from Goodyear, 30 parts ENDEX 160.TM. end-block reinforcing resin and 2
 parts IRGANOX.TM. 1010 antioxidant available from CIBA-Giegy, as described
 in U.S. Pat. No. 5,500,293) as in the vertical burn test) to permit the
 film to stick to a brass rod that was used in the test. The brass rod was
 wrapped with two overlapping layers of tape and supported in a horizontal
 position. A gas burner flame was applied for 30 seconds and immediately
 removed. The time required for the sample to self-extinguish is measured.
 This test differentiates among tapes with wide ranges of burning
 characteristics but is less precise for tapes of narrow ranges of burning
 characteristics.
 Hanging Strip
 Burning characteristics of multilayer films were evaluated according to
 ASTM 568-77 A45 cm.times.25 cm with a 38 cm gauge length sample was
 suspended from a clamp inside a protective metal shield that was located
 in a fume hood. A gas burner flame of a given height was applied until
 film ignited. Flame was removed immediately and the time needed to burn 38
 cm of sample length or for sample to self-extinguish was measured. This
 test differentiates among tapes with wide ranges of burning
 characteristics but is less precise for tapes of narrow ranges of burning
 characteristics.
 Tensile Testing
 Tensile properties of the multilayer films were evaluated using a standard
 tensile/elongation method on an Instron mechanical testing frame at 12
 inches/minute (30.5 cm/minute). Sample were of 0.5 inches width (1.27 cm)
 and gauge length of 4 inches (10.2 cm). Thickness ofthe samples depended
 on process conditions and were measured using a Mitutoyo Liner Thickness
 Gage.

Materials Used
 Material Description
 Fina .TM. 3374 Isotactic polypropylene available from Fina Oil & Chem,
 Dallas, TX.
 Rexflex .TM. Significantly atactic polypropylene available from
 W101 Huntsman Polypropylene Corp., Woodbury, NJ.
 1 Nat-2P-W A brominated imide and antimony trioxide blended into a
 polyethylene polymer at a 45:55 weight concentration
 with a 3:1 ratio of brominated imide to antimony., avail-
 able as PE Conc. 1 Nat-2P-W from M.A Hannah, Elk
 Grove Village, IL.
 LLDPE 6806 Liner low density polyethylene, available from Dow
 Chemical Co., Midland MI.
 SpaceRite .TM. Alumina trihydroxide, available from Alcoa Chemicals,
 S11 Charlotte, NC.
 Engage .TM. A metallocene polymerized olefin, containing 24%
 8100 octane comonomer available from Dow Chemical Co.,
 Midland, MI.
 LDPE 1550 Low density polyethylene, available from Eastman
 Chemical Products, Inc., Kingsport, TN.
 ATH FR Alumina trihydroxide compounded with ethylene vinyl
 acetate at a 60% by weight concentration, available from
 Mach 1 Compounding, Macedonia, Ohio.
 Elvax .TM. 410 An ethylene vinyl-acetate copolymer available from E.I.
 DuPont de Nemours, Inc., Wilmington DE.
 O521-48 FR Magnesium hydroxide compounded with polypropylene
 at a 50% by weight concentration, available from Mach
 1 Compounding, Macedonia, Ohio.
 Escorene .TM. Isotactic polypropylene available from Exxon Chemical
 3445 Co.
 Environstrand A blend of tetrabromobisphenol A with antimony oxide
 in atactic polypropylene, available as Envirostrand 5P280
 from Great Lakes Chemical, West Lafayette, IN
 PPSC 912 An ethylene-propylene copolymer with a melt index of
 65, available as Profax SC 912 from Montell North
 America, Wilmington DE
 Examples 1-3, Comparative Examples 1-4
 Examples 1-3 were multilayer films having 13 layers of a construction
 A(BA).sub.5 BA. They were prepared to illustrate the effect on overall
 flame retardant properties of using various amounts of a flame retardant
 additive in a B layer compared to using similar amounts in a blended
 composition.
 In Example 1, the non flame retardant layers were made of Rexflex.TM. W101
 and 35% Fina.TM. 4 3374, melt mixed in a weight ratio of 65:35 and
 conveyed in a BERLYN single screw extruder (BERLYN, 51 mm, having an L/D
 of 30/1, commercially available from Berlyn Corp., Worchester, Mass.,
 operating with zone temperatures increasing from 149.degree. C. to
 238.degree. C.) to A slots of a feedblock having 13 slots. The feedblock,
 made as described in U.S. Pat. No. 4,908,278 (Bland et al.), allowed two
 flow streams fed into the 13 slots in an alternating manner to come
 together in a multilayer flow stream having layers arranged as A(BA).sub.5
 BA. The temperature of both the feedblock and the die were set at
 204.degree. C. The flame retardant layers were made from 1 Nat-2P-W, fed
 by a single screw extruder (KILLION Model KTS-125, 32 mm, having an L/D of
 24/1, commercially available from Killion, Inc., Cedar Grove, N.J.)
 operating with zone temperatures increasing from 132.degree. C. to
 238.degree. C. into B slots of the feedblock. The resulting multilayered
 flow stream was passed through a single orifice film die and drop cast
 onto a chill roll set at a temperature of 15.degree. C. and collected. The
 line speed was 4.6 m/min., the individual flowrates of A and B were such
 that flame retardant material comprised a calculated 14.3 weight percent
 of the overall multilayered film and the overall thickness was measured at
 150 micrometers.
 Examples 2 and 3 were made essentially as in Example 1, except the flow
 rates of the materials were adjusted to obtain flame retardant
 concentrations of 33.3 and 47 weight percent, respectively.
 In Comparative Example 1, the same material used in the A layer of Example
 1 was fed into the Berlyn extruder of Example 1, conveyed through a
 feedblock and a single layer die and drop cast onto a chill roll. The
 temperatures of the extruder increased from 149.degree. C. to 238.degree.
 C., the feedblock and the die were set at a temperature of 204.degree. C.
 and the chill roll was set at a temperature of 15.degree. C. The overall
 thickness was 150 micrometers and the flame retardant concentration was 0
 weight percent.
 Comparative Examples 2-4 were made essentially as in Comparative Example 1
 except the flame retardant additive used in the B layers of Example 1 was
 melt blended to result in an overall flame retardant concentration in
 weight percent of 14.3, 33.3 and 47, respectively.
 Examples 1-3 and Comparative Examples 1-4 were tested for Hanging Strip
 Flammability, and Horizontal Burn. The test results, film layers and flame
 retardant concentrations are shown in Tables 1 and 2.
 TABLE 1
 FR Hanging Strip
 Ex. Layers % Flame Comments
 1 13 14.3 3 sec, SE Flaming drips
 2 13 33.3 3 sec, SE Flaming drips, hard to ignite
 3 13 47.0 &lt;1 sec, SE Very difficult to ignite
 C1 1 0.0 34 sec, 38 cm Flaming drips
 C2 1 14.3 17 sec, 38 cm Flaming drips, easy to ignite,
 C3 1 33.3 15 sec, SE Flaming drips
 C4 1 47.0 2 sec, SE No drips
 As seen, the films of the invention exhibited improved flame retardant
 performance as blends having the same overall concentration of flame
 retardant material.
 TABLE 2
 FR Horizontal Burn
 Ex. Layers % Flame Comments
 1 13 14.3 18 sec No drips, high char
 2 13 33.3 1 sec No drips, high char
 3 13 47.0 &lt;1 sec No drips, high char
 C1 1 0.0 127 sec Flame drips, all tape burned
 C2 1 14.3 11 sec Flame drips, low char
 C3 1 33.3 16 sec No drips, high char
 C4 1 47.0 4 sec No drips, high char
 As seen, the films of the invention exhibited improved flame retardant
 performance over blends having the same overall concentration of flame
 retardant material when the flame-retardant concentration was sufficient.
 Example 4 and Comparative Examples 5-6
 Example 4 illustrates the effect of two flame retardant materials in the
 flame-retardant layer on overall flame retardant performance.
 A multilayer film was made essentially as in Example 1, varying the polymer
 matrix. Flame retardant additive and process conditions as noted. The non
 flame retardant "A" layers were made of Reflex.TM. W101 and Fina.TM. 3374
 in a weight ratio of 75:25 instead of 65:35. The flame retardant "B"
 layers were made of a mixture of LLDPE 6806, 1-Nat-2P-W and Alcoa
 Spacerite.TM. S11 in a weight ratio of 25:56:19. The set temperature in
 the Berlyn extruder varied from 138.degree. C. up to 193.degree. C. The
 temperature in the Killion extruder varied from 132.degree. C. up to
 182.degree. C. Die and feedblock at 193.degree..
 Comparative Example 5 was made essentially as in Comparative Example 1
 except the non flame retardant polymer was made of Reflex.TM. W101 and
 Fina.TM. 3374 in a weight ratio of 75:25 instead of 65:35.
 Comparative Example 6 was performed essentially as in Comparative Example 2
 except as follows. The non flame retardant material was made of Reflex.TM.
 W101 and Fina.TM. 3374 in a weight ratio of 75:25 instead of 65:35. The
 flame retardant mixture used in the "B" layer of Example 4 was melt
 blended with the non flame retardant material to result in an overall
 flame retardant concentration of weight percent of 35.
 Examples 1-3 and Comparative Examples 1-4 were tested for Hanging Strip
 Flammability and for Tensile Stress. The test results, film layers and
 flame retardant concentrations are shown in Table 3.
 TABLE 3
 Tensile Stress
 FR Hanging Strip At 10% strain
 Ex. Layers % Flame Comments KPa (psi)
 4 13 35 &lt;1 sec, SE.sup.1 Drips but not flaming 535(775)
 C5 1 0 27 sec, all Flaming drips 635(920)
 C6 1 35 4 sec, SE Flaming drips 597(865)
 .sup.1 Self extinguished immediately after burner removed, could not be
 ignited.
 As seen, a film of the invention exhibited improved flame retardant
 performance of a blend having the same overall concentration of flame
 retardant. The lower tensile stress value of Example 4 was attributed to
 poor mixing.
 EXAMPLES 5-6
 These examples were prepared to illustrate the use of halogen-free flame
 retardant materials with two different non-flame retardant materials.
 Examples 5 and 6 were made in a manner similar to Example 1 except the
 materials were different and some process conditions were changed. In
 Example 5, the materials used in the not flame retardant "A" layers were
 Engage.TM. 8100 and LDPE 1550 melt blended in a weight ratio of 50:50. In
 Example 6, the materials used in the non-flame retardant "A" layers were
 Reflex.TM. W101 and Fina.TM. 3374 in a weight ratio of 75:25 instead of
 65:35. In both examples, the materials used in the flame retardant "B"
 layers were a mixture of ATH FR and Elvax.TM. 410 in a weight ratio of
 90:10. A KILLION single screw extruder (KILLION Model KTS-125, 32 mm
 single screw extruder with L/D of 24/1 and fitted with a mixing screw
 containing an Eagan mixing section) was used instead of a BERLYN single
 screw extruder to convey the non-flame retardant material to the "A" slots
 of the feedblock and a second KILLION single screw extruder, also fitted
 with a mixing screw containing an Eagan mixing section, conveyed the flame
 retardant material to the "B" slots. In examples 5 and 6, the first
 KILLION extruder was operated at temperatures in the first zone to the
 last zone ranging from 149.degree. C. to 177.degree. C. and 149.degree. C.
 to 188.degree. C., respectively. In both examples the temperatures of the
 feedblock, die and chill roll were maintained at 177.degree. C.,
 177.degree. C. and 20.degree. C., respectively. The film speeds for
 Examples 5 and 6 were 4.9 and 5.5 m/min, respectively. The film thickness
 for both was about 130 microns.
 Comparative Example 7 was made as in Example 5 except no flame retardant
 material was fed into the "B" slots and the flowrate was adjusted to
 result in a one layer film having a thickness of about 130 micrometers
 where all seven layers merged into a single indistinguishable layer.
 Examples 5-6 and Comparative Example 8 were tested for Horizontal Burn. The
 test results, film layers and flame retardant concentrations are shown in
 Table 4.
 TABLE 4
 FR Horizontal Burn
 Ex. Layers % Flame Comments
 5 13 37 13 sec No/low char, 2-3 drips, flames do not
 burn down length of rod, tape does
 not burn readily, bubbling during
 burning & no smoke.
 6 13 38 16 sec Same as Ex 5 except 1-2 drips.
 C7 1 0 120 sec No char, flaming drips, burns entire
 length of rod and light smoke.
 As seen, the films of the invention exhibited substantial flame retardant
 performance.
 EXAMPLE 7
 These examples were prepared to illustrate the use of an inorganic flame
 retardant additive with two different non flame retardant materials.
 Example 7 was made essentially as in Example 6 except as follows. The
 materials used in the non-flame retardant "A" layers were Rexflex.TM. W101
 and Fina.TM. 3374 in a weight ratio of 75:25. The flame retardant additive
 in the "B" layer was 0521-48. The temperatures for the "A" layer extruder
 and the "B" layer extruder were set to increase from between 182.degree.
 C. and 204.degree. C. and between 171.degree. C. and 227.degree. C.,
 respectively.
 Comparative Example 8 was made essentially as in Example 7 except no flame
 retardant additive was fed into the "B" slots and the flow rate was
 adjusted to result in a one layer film having a thickness of about 130
 micrometers.
 Example 7 and Comparative Example 8 were tested for Horizontal Burn. The
 test results, film layers and flame retardant concentrations are shown in
 Table 5.
 TABLE 5
 FR Horizontal Burn
 Ex. Layers % Flame Comments
 7 13 42 18 sec High char, no drips, flames do not burn
 down length of rod, tape does not burn
 readily, flakes/ash produced during
 burning & no smoke.
 C8 1 0 127 sec Flaming drips and all tape burned.
 As seen, the films of the invention exhibited substantial flame retardant
 performance.
 EXAMPLES 8-10
 These examples were prepared to illustrate the effect of flame retardant
 materials that melted a processing temperatures on layer thickness.
 Example 8 was made essentially as in Example 1 except some equipment,
 processing conditions and materials were the different. The not flame
 retardant "A" layers were made of Escorene.TM. 3445 and the flame
 retardant "B" layers were made of a mixture of 50% Great Lakes
 Environstrand and 50% PPSC 912. The "B" layer material was conveyed to the
 "B" slots with a twin screw extruder (LEISTRITZ Model LSM 34 GL, 34 mm,
 having 42/1, commercially available from Leistritz Corp., Sommerville,
 N.J.). The temperature of the extruder for the "A" layers varied from
 160.degree. C. up to 193.degree. C. The temperature of the extruder for
 the "B" layers ranged from 150.degree. C. up to 177.degree. C. The
 individual flowrates of A and B were such that flame retardant material
 comprised a calculated 25 weight percent of the overall multilayered film
 and the overall thickness was measured at 100 micrometers.
 Examples 9 and 10 were made in a similar manner to Example 8 except Example
 9 used a 29 layer feedblock and Example 10 used a 91 layer feedblock.
 Examples 8-10 were tested for both Hanging Strip and Horizontal Burn. The
 test results, film layers and flame retardant concentrations are shown in
 Table 6.
 TABLE 6
 Ex. Layers FR % Hanging Strip Horizontal Burn
 8 13 25 Melted and dripped &lt;1 sec, low char, low smoke,
 would not ignite extinguishes upon removal of
 flame
 9 29 25 Melted and dripped &lt;1 sec, low char, low smoke,
 would not ignite extinguishes upon removal of
 flame
 10 91 25 Melted and dripped &lt;1 sec, low char, low smoke,
 would not ignite extinguishes upon removal of
 flame
 As seen, the thickness of the flame retardant layer could be quite thin
 without adversely affecting the flame-retardant performance of the overall
 film by loss of layer integrity.
 Each of the patents, patent applications, and publications cited herein is
 incorporated by reference as if each were individually incorporated by
 reference. The various modifications and alterations of this invention
 will be apparent to those skilled in the art without departing from the
 scope and spirit of this invention.