CA2154322C - Heat-sealable, oriented, multilayer polyolefin film, process for the production thereof, and the use thereof - Google Patents

Abstract

The invention relates to an oriented multilayer polyolefin film which comprises a polyolefinic base layer and at least one heat-sealable top layer. The heat-sealable top layer contains at least one amorphous polymer which is in the top layer in the form of separated particles.
The invention also relates to a process for the production of the multilayer polyolefin film and to the use of the film.

Description

Hoe 94/R 038 Heat-sealable, oriented, multilayer polyolefin film, process for the production thereof, and the use thereof The invention relates to an oriented, multilayer poly-olefin film comprising a polyolefinic base layer and at least one heat-sealable top layer. The films are distinguished by low haze, high gloss and a low coeffi-cient of friction.
The prior art describes transparent films having low coefficients of friction. The demands on the processing properties of the films and their smooth running through automatic machines have constantly increased over the years. For this reason, ever-lower coefficients of friction are required, with the term "low" friction values today covering an order of magnitude of from 0.3 to 0.1, while a friction of from 0.4 to 0.5 was regarded as extremely low a few years ago.
DE-A-20 01 032 describes films made from various thermoplastics whose surface-slip characteristics have been improved by addition of carboxamides and anti-blocking agents. Since it is not possible for a suffi-cient amount of lubricant to be incorporated into the top layers alone, the additional incorporation of the amides into the base layer is recommended. These films have a coefficient of friction in the range from 0.4 to 0.8 and thus no longer meet today's quality requirements.
US-A-4,117,193 describes multilayer films comprising a polypropylene base layer containing a lubricant, an antiblocking agent and an antistatic. The top layer of these films comprises a polymer blend and additionally contains a lubricant and an antiblocking agent. The ~~~4~~z polymer blend comprises an ethylene-butylene copolymer and a polyolefinic resin such as HDPE or LDPE. It is stated that the poor surface-slip characteristics of the films cannot be sufficiently improved by the addition of lubricants and antiblocking agents alone. For this reason, the top layer is modified by addition of HDPE or LDPE in combination with a lubricant and antiblocking agent. According to the examples and comparative examples, the reduction in the coefficient of friction is essentially due to the addition of HDPE. Pure copolymeric top layers with the same additive composition have coefficients of friction of from 0.7 to 0.8. The films combine excellent coefficients of friction with good printability, but are highly unsatisfactory in haze and gloss owing to the addition of the friction-reducing polyolefinic resin.
EP-A-0 402 100 describes polypropylene films which contain from 0.01 to 0.5% by weight of a spherical Si02 and from 0.3 to 5% by weight of a hydroxy fatty acid glyceride. This invention relates to single-layer and multilayer films. Multilayer embodiments contain the combination of SiOz and glyceride both in the top layer and in the base layer. It is stated that the selected amounts of SiOZ and glyceride are essential for the advantageous properties of the films and deviations from these ranges no longer give the desired result. The films are distinguished by good transparency, surface-slip characteristics and adhesion to metal. However, they have a coating on the surface after an extended storage time which impairs the appearance of the films. This effect is also known as blooming and is caused by migration of certain additives, in particular the glycerides, to the surface of the films.
EP-A-0 182 463 describes a multilayer film which contains from 0.05 to 0.2% by weight of tertiary aliphatic amine ~:~.~4~2~

in the base layer and a combination of silicone oil and SiOz in the heat-sealable top layer. According to the description, the surprising interaction of SiOZ, silicone oil and amine in combination with a selected top layer thickness of less than 0.8 ~,m gives films having coefficients of friction of 0.3 or less. In spite of this excellent coefficient of friction, the processing proper ties of the film are poor. In particular, it is not printable and is therefore unsuitable for many applications.
EP-A-0 143 130 discloses films which contain a carboxamide in the base layer and likewise the com-bination of silicone oil and Si02 in the top layer. Like in the abovementioned EP-A-0 182 463, a synergistic action of the three selected components on the coef-ficient of friction is described. These films likewise have poor processing properties in spite of their advan-tageous surface-slip characteristics. Again, they lack the important property of printability.
EP-A-0 242 055 describes the use of an infusible organo-siloxane resin powder having a three-dimensional network structure as antiblocking agent in films. Both the silicone resin and the propylene polymer are employed in the form of a powder comprising particles having a virtually spherical shape, this particle shape being characterized by a corresponding equation for the actual degree of sphericity. The films are said to be improved over the prior art with respect to their transparency, antiblocking properties, sliding properties and appearance. The propylene/antiblocking agent mixture can also be employed as top layer material for coextruded multilayer films. However, these coextruded multilayer films are still unsatisfactory with respect to their transparency and gloss values, in particular if the top layers are applied in conventional thicknesses of greater ~~~~J~~

than 0.5 Vim. In addition, this antiblocking agent is very much more expensive than conventional antiblocking agents.
German Patent Application P 43 06 154.0 describes the use of an organically coated Sio2 as antiblocking agent in heat-sealable films. The coefficient of friction and processing behavior of the film has been improved. This specification makes no mention of the spatial shape of the antiblocking particles.
EP-A-0 353 368 describes the use of the siloxane resin powder described in EP-A-0 242 055 in combination with a hydroxyfatty acid glyceride. These films are particularly suitable for vacuum vapor deposition, but have very poor gloss and transparency.
In applying the known teaching, it has been found that some of the known antiblocking agents have adverse effects on certain film properties. The antiblocking agent impairs the transparency and the gloss of the film.
The improvement in friction is generally achieved at the expense of an increase in surface roughness. Sio2 as antiblocking agent in the production of the films results in deposits on the die lip and in abrasion on the rolls.
This means that the die lip and the rolls must be cleaned frequently, since the film otherwise runs poorly during production and the deposits on the die lip result in streaking on the film. In addition, problems occur during 3o corona treatment. The corona treatment breaks through in the areas of the roll where Si02 abrasion has occurred and results in the undesired phenomenon known as the reverse-side effect. This causes unacceptable flaws during further processing of the film, such as, for example, printing or metallization.

The present invention avoids or at least mitigates the disadvantages of the films described in the prior art. In particular, there is provided a multilayer film which is distinguished by a combination of the following properties:
~ high gloss ~ low haze ~ a low coefficient of friction ~ low surface roughness.
IO
The invention is achieved by a multilayer film of the generic type specified at the outset, wherein the heat-sealable top layer comprises at least one amorphous polymer which is in the top layer in the form of separated particles.
The base layer of the novel multilayer film essentially comprises a polyolefin, preferably a propylene polymer, and, if desired, further additives in effective amounts in each case. In general, the base layer comprises at least 50% by weight, preferably from 75 to 100% by weight, in particular from 90 to 1000 by weight, of the propylene polymer.
The propylene polymer comprises from 90 to 100% by weight, preferably from 95 to 1000 by weight, in particular from 98 to 100% by weight, of propylene and has a melting point of 120°C or above, preferably from 150 to 170°C, and generally has a melt flow index of from 0.5 to 8 g/10 min, preferably from 2 to 5 g/10 min, at 230°C and a force of 21.6 N (DIN 53 735). Isotactic propylene homopolymer having an atactic content of 15o by weight or less, copolymers of ethylene and propylene having an ethylene content of 10% by weight or less, copolymers of propylene with C4-C8-a-olefins having an a-olefin content of 10% by weight or less, terpolymers of propylene, ethylene and butylene having an ethylene ~~ 5~ 32~

content of 10% by weight or less and a butylene content of 15% by weight or less are preferred propylene polymers for the core layer, particular preference being given to isotactic propylene homopolymer. The percentages by weight given are based on the particular polymer.
Also suitable is a mixture of said propylene homopolymers and/or copolymers and/or terpolymers and/or other poly-olefins, in particular comprising monomers having 2 to 6 carbon atoms, where the mixture comprises at least 50% by weight, in particular at least 75% by weight, of propylene polymer. Other polyolefins which are suitable in the polymer mixture are polyethylenes, in particular HDPE, LDPE and LLDPE, where the proportion of these polymers does not exceed 15% by weight in each case, based on the polymer mixture.
In general, the base layer can contain lubricants, antistatics, stabilizers and/or neutralizers in effective amounts in each case, and also, if desired, hydrocarbon resin.
In a white or opaque or white/opaque embodiment, the base layer additionally contains pigments or vacuole-inducing particles or a combination thereof. Such films have a light transparency, measured in accordance with ASTM-D
1033-77, of at most 50%, preferably at most 70%.
Pigments are particles which result in essentially no vacuole formation during stretching of the film. The coloring action of the pigments is caused by the particles themselves. The term "pigment" is generally associated with a particle size of from 0.01 to a maximum of 1 um and covers both "white pigments", which give the films a white color, and "colored pigments", which give the film a colored or black color. In general, the mean particle diameter of the pigments is in the range from ~1~~322 0.01 to 1 Vim, preferably from 0.01 to 0.7 Vim, in particu-lar from 0.01 to 0.4 Vim. The base layer generally con-tains pigments in an amount of from 1 to 25% by weight, in particular from 2 to 20% by weight, preferably from 5 to 15% by weight, in each case based on the base layer.
Conventional pigments are materials such as, for example, aluminum oxide, aluminum sulfate, barium sulfate, calcium carbonate, magnesium carbonate, silicates, such as aluminum silicate (kaolin clay) and magnesium silicate (talc), silicon dioxide and titanium dioxide, preference being given to white pigments such as calcium carbonate, silicon dioxide, titanium dioxide and barium sulfate.
The titanium dioxide particles comprise at least 95% by weight of rutile and are preferably employed with a coating of inorganic oxides, as is usually used as a coating for Ti02 white pigment in papers or paints for improving the light fastness. Particularly suitable inorganic oxides include the oxides of aluminum, silicon, zinc and magnesium or mixtures of two or more of these compounds. They are precipitated from water-soluble compounds, for example alkali metal aluminates, in particular sodium aluminates, aluminum hydroxide, aluminum sulfate, aluminum nitrate, sodium silicate or salicylic acid, in the aqueous suspension. Coated TiOz particles are described, for example, in EP-A-0 078 633 and EP-A-0 044 515.
The coating may also contain organic compounds containing polar and nonpolar groups. Preferred organic compounds are alkanols and fatty acids having 8 to 30 carbon atoms in the alkyl group, in particular fatty acids and the primary n-alkanols having 12 to 24 carbon atoms, and polydiorganosiloxanes and/or polyorganohydrosiloxanes, such as polydimethylsiloxane and polymethylhydrosiloxane.

_ g _ The coating on the TiOZ particles usually comprises from 1 to 12 g, in particular from 2 to 6 g, of inorganic oxides, and if desired additionally from 0.5 to 3 g, in particular from 0.7 to 1.5 g, of organic compounds, in each case based on 100 g of TiOz particles. It has proven particularly advantageous for the TiOZ particles to be coated with A1203 or with A1Z03 and polydimethylsiloxane.
Opaque embodiments of the films contain vacuole-inducing particles, which are incompatible with the polymer matrix and result in the formation of vacuole-like cavities when the films are stretched, the size, type and number of vacuoles being dependent on the material and on the size of the solid particles and the stretching conditions, such as stretching ratio and stretching temperature. The vacuoles give the films a characteristic mother-of-pearl-like, opaque appearance caused by light scattering at the vacuole/polymer matrix interfaces. In general, the mean particle diameter of the vacuole-inducing particles is from 1 to 6 ~,m, preferably from 1.5 to 5 ~Cm. The base layer generally contains vacuole-inducing particles in an amount of from 1 to 25% by weight.
Conventional vacuole-inducing particles in the base layer are inorganic and/or organic materials which are incom-patible with polypropylene, such as aluminum oxide, aluminum sulfate, barium sulfate, calcium carbonate, magnesium carbonate, silicates, such as aluminum silicate (kaolin clay) and magnesium silicate (talc), silicon dioxide and titanium dioxide, preference being given to calcium carbonate, silicon dioxide and titanium dioxide.
Suitable organic fillers are the usual polymers which are incompatible with the polymer of the base layer, in particular those such as HDPE, polyesters, polystyrenes, polyamides and halogenated organic polymers, preference being given to polyesters, such as, for example, poly-butylene or polyethylene terephthalates. For the purposes of the present invention, "incompatible materials" or "incompatible polymers" means that the material or polymer is present in the film as separate particles or as a separate phase.
White/opaque films to which vacuole-initiating particles and pigments have been added contain the vacuole-initiat-ing particles in an amount of from 1 to 10% by weight, preferably from 1 to 5% by weight, and pigment in an amount of from 1 to 7% by weight, preferably from 1 to 5%
by weight.
The density of the opaque or white films can vary within broad limits and depends on the type and amount of the fillers. The density is generally in the range from 0.4 to 1.1 g/cm3. Pigmented films have a density in the order of 0.9 g/cm3 or above, preferably in the range from 0.9 to 1.1 g/cm3. Films containing only vacuole-initiating particles have a density of less than 0.9 g/cm3. For packaging films having a content of vacuole-initiating particles of from 2 to 5% by weight, the density is in the range from 0.6 to 0.85 g/cm3. For films having a content of vacuole-initiating particles of from 5 to 14%
by weight, the density is in the range from 0.4 to 0.8 g/cm3. Films containing pigments and vacuole-initiat-ing particles have a density in the range from 0.5 to 0.85 g/cm3, depending on the ratio between the pigment content and the content of vacuole-initiating particles.
The novel multilayer film may contain (a) further inter-layer(s) between the base layer and the top layer. This (these) interlayer(s) which may be present essentially comprises) propylene polymers or polypropylene mixtures, as described above for the base layer. In principle, the base layer and the interlayer(s) can comprise the same or different propylene polymers or mixtures. The melt flow indices of the polymers for the core layer and inter-layers) should be as close as possible in magnitude. If necessary, the MFI of the interlayer(s) can be somewhat higher, with a maximum difference of 20%. If desired, additives in effective amounts in each case can be added to the interlayers.
In a preferred embodiment of the novel film, the prop-ylene polymer of the base layer and/or interlayer is peroxidically degraded.
A measure of the degree of degradation of the polymer is the degradation factor A, which gives the relative change in melt flow index, measured in accordance with DIN 53 735, of the polypropylene; based on the starting polymer.
MFIZ
A=
A?Fh MFI~ = melt flow index of the propylene polymer before addition of the organic peroxide MFI2 = melt flow index of the peroxidically degraded propylene polymer.
In general, the degradation factor A of the propylene polymer employed is in a range from 3 to 15, preferably from 6 to 10.
Particularly preferred organic peroxides are dialkyl peroxides, where the term alkyl radical is taken to mean a conventional saturated, straight-chain or branched lower alkyl radical having up to six carbon atoms.
Particular preference is given to 2,5-dimethyl-2,5-di(t-butylperoxy)hexane and di-t-butyl peroxide.

The novel polyolefin film furthermore contains at least one heat-sealable top layer. This top layer essentially comprises heat-sealable polymers of a-olefins having 2 to carbon atoms and amorphous polymer in the form of 5 separated particles and, if desired, further additives in effective amounts in each case. In general, the top layer comprises from 75 to virtually 100% by weight, in particular from 90 to 99.5% by weight, of the heat-sealable a-olefinic polymer.
Examples of such a-olefinic polymers are a copolymer of ethylene and propylene or ethylene and 1-butylene or propylene and 1-butylene or a terpolymer of ethylene and propylene and 1-butylene or a mixture of two or more of said homopolymers, copolymers and terpolymers or a blend of two or more of said homopolymers, a copolymers and terpolymers, if desired mixed with one or more of said homopolymers, copolymers and terpolymers, particular preference being given to random ethylene-propylene copolymers having an ethylene content of from 1 to 10% by weight, preferably from 2.5 to 8% by weight, or random propylene-1-butylene copolymers having a butylene content of from 2 to 25% by weight, preferably from 4 to 20o by weight, in each case based on the total weight of the copolymer, or random ethylene-propylene-1-butylene terpolymers having an ethylene content of from 1 to 10% by weight, preferably from 2 to 6% by weight, and a 1-butylene content of from 2 to 20% by weight, preferably from 4 to 20% by weight, in each case based on the total weight of the terpolymer, or a blend of an ethylene-propylene-1-butylene ter-polymer and a propylene-1-butylene copolymer having an ethylene content of from 0.1 to 7% by weight and a propylene content of from 50 to 90% by weight and a 1-butylene content of from 10 to 40% by weight, in each case based on the total weight of the polymer blend.
The above-described copolymers generally have a melt flow index of from 1.5 to 30 g/10 min, preferably from 3 to 15 g/10 min, and a melting point in the range from 120 to 140°C. The terpolymers generally have a melt flow index in the range from 1.5 to 30 g/10 min, preferably from 3 to 15 g/10 min, and a melting point in the range from 120 to 140°C. The above-described blend of copolymers and terpolymers generally has a melt flow index of from 5 to 9 g/10 min and a melting point of from 120 to 150°C. All the abovementioned melt flow indices are measured at 230°C and a force of 21.6 N (DIN 53 735).
If desired, all the above-described top layer polymers can have been peroxidically degraded in the same way as described above for the base layer, in principle using the same peroxides. The degradation factor for the top layer polymers is generally in the range from 3 to 15, preferably from 6 to 10.
According to the invention, the top layer of the film contains at least one amorphous polymer in the form of separated particles, generally in an amount of at most 5%

z~~~~~~

by weight, preferably from 0.001 to 3% by weight, in particular from 0.01 to 2% by weight, based on the weight of the top layer. It has been found that the amorphous polymer, which is a polymeric solid per se and, as a raw material, has no particle character, is, surprisingly, in the top layer in the form of separated particles.
For the purposes of the present invention, amorphous polymers are taken to mean polymers which are solids at room temperature in spite of an irregular arrangement of the molecular chains. They are essentially non-crystal-line, and their degree of crystallinity is generally less than 5%, preferably less than 2%, or is Oo. Particularly suitable amorphous polymers are those whose glass transition temperature T~ is in. the range from 70 to 300°C, preferably from 80 to 250°C, in particular from 100 to 200°C, or whose Vicat softening temperature T~
(VST/B/120) is from 70 to 200°C, preferably from 80 to 180°C. In general, the amorphous polymer has a mean molecular weight Mw in the range from 500 to 500,000, preferably from 1000 to 250,000, in particular from 3000 to 200,000.
The refractive index of the amorphous polymer is generally in the range from 1.3 to 1.7, preferably from 1.4 to 1.6. It is particularly advantageous here if the refractive index of the amorphous polymer is in a certain ratio to the refractive index of the polyolefin of the top layer. In general, the refractive indices of the amorphous polymer and the polyolefin of the top layer differ by at most 0.1 units, preferably by at most 0.05 units.
The amorphous polymer is, surprisingly, present in the resultant film in the form of separated particles, which are clearly evident in transmitted-light photographs of the film surface. The particle size of the particles in the top layer is in the range from 0.2 to 20 ~,m, prefer-ably from 0.5 to 1.5 ~Cm.
It has been found that the particles of amorphous polymer are generally approximately spherical. For the purposes of the present invention, the term approximately spherical particles covers particles which satisfy the following condition:
i o f = AI (nI4) I o~
in which f is greater than 0.5, preferably from 0.7 to 1, and A is the cross-sectional area in mmz and D~X is the maximum diameter of the cross-sectional area in mm. The factor f is a measure of the degree of sphericity of the particles. The closer the value of f to 1, the closer the shape of the particles to the ideal spherical shape.
Suitable amorphous polymers having the property profile described above are a multiplicity of generally trans-parent polymers. Examples thereof are atactic polystyrene (T~ - 95 to 105°C, preferably 100°C), poly-a-methyl styrene (T~ - 170 to 180°C, preferably 175°C), poly-acrylates, in particular polymethyl methacrylate (T~ = 115 to 130°C, preferably 122°C), amorphous homopolymers and copolymers of polycyclic olefins (T~ = 70 to 300°C depending on composition and molecular weight), polyvinylcarbazole (T~ =180 to 220°C, prefer-ably 200°C), atactic polyvinylcyclohexane (T~ - 130 to 150°C, preferably 140°C), polyvinyl chloride (T~ = 65 to 90°C, preferably 80°C), polyacrylonitrile (T~ - 100 to 110°C, preferably 106°C), specific types of rubber, in particular cyclorubber (T~ = 70 to 120°C) , uncrosslinked, partially crosslinked and crosslinked dispersions of amorphous polymers (T~ from 70 to 200°C depending on polymerization partner and degree of polymerization).

Suitable cycloolefin copolymers are known per se and are described in EP-A-0 407 870, EP-A-0 485 893, EP-A-0 503 422 and DE-A-40 36 264.
The cycloolefin polymers employed are built up from one or more cycloolefins, where the cycloolefins employed are generally substituted or unsubstituted cycloalkenes and/or polycycloalkenes, such as, for example, bi-, tri-or tetracycloalkenes. The cycloolefin polymers may also be branched. Such products can have a comb or star structure.
Particular preference is given to cycloolefin copolymers Z5 containing at least one polycyclic olefin of the formulae I to VI below:
Rt CN
HC "~ ~ CH
R3 ~ C...._R;
HC ~~ /CH~R~
CH
CH ~ ~ CH2 He ~ CH
(it):
R3 - C! R,~ ~ CHZ
HC I CHI
1"~ CH ~ CH2 R
C H ~ C H ---.~ C H
HC ~ NCH ' (I11~.
RS--C~ Ri Ry -C~R~
~H ~ y HC ~ / CH ~ CH ~ R
1"~ C H
R~
CH H' HC ' ~H CH ~ 'CH CH
(IY).
R C R R~ C-- Ra gs -- C-R, I CH
HC ~CH~ IH ~CH\ CH~ R
~ C / 'H
R=
J y / C H -.
I H ~ CH' CH
HC
~Y).
R R
HC~ ~H ~GH_", CH ~"tH\Ri I
Rt RZ

C H -..' r..r" ~ H "_ /
CH _ CH ~ CH CH
~ R7~C R (YI).
HC-~ ~H ~CH~ C1~..-~CH~ ~H~CH~ ~
I . R
R~
The radicals R~ to R8 in the formulae I to VI may be identical or different and are H, C6-C2o-aryl, C~-C2o-alkyl, F, Cl, Br, I or a monocyclic olefin of the formula VII below in which n is a number from 2 to 10.

z~ ~~3zz Cii - CN ( Y i I ) ..
(c~z)~ .
The cycloolefin polymers are preferably prepared with the aid of transition-metal catalysts, which are described in the abovementioned specifications. Preference is given to the preparation processes of EP-A-0 407 870 and EP-A-0 485 893, since these processes give cycloolefin polymers having a narrow molecular weight distribution (M',1/M~ - 2 ) . This avoids the disadvantages such as migration, extractability or tack of the (or caused by the) low-molecular-weight constituents.
A particularly good property profile is achieved using cycloolefin polymers which have a moderate to high molecular weight in the range from 1000 to 200,000, preferably from 2000 to 180,000, in particular from 3000 to 150,000. The molecular weight is regulated during the preparation by using hydrogen and a specific choice of the catalyst and reaction conditions.
The novel multilayer film comprises the above=described base layer and at least one top layer and, if desired, further layers. Preference is given to three-layer embodiments, which have a top layer on both sides of the base layer, it being possible for these top layers to be identical or different in thickness and composition.
Preference is also given to five-layer embodiments, which contain a base layer, interlayers applied to both sides of the base layer and top layers on both sides.
The overall thickness of the novel multilayer polyolefin film can vary within broad limits and depends on the intended use. It is preferably from 5 to 70 Vim, in particular from 10 to 15 Vim, the base layer making up from about 50 to 90% of the total film thickness.
The thickness of the top layers) is greater than 0.2 ~,m and is preferably in the range from 0.3 to 2 Vim, in particular greater than from 0.5 to 1 Vim, where top layers on both sides can have identical or different thicknesses.
The thickness of any interlayer(s) present is, in each case independently of one another, from 1 to 12 Vim, preferably from 2 to 8 Vim, in particular from 3 to 6 Vim.
The values given are each based on one interlayer.
In addition to this selected top layer additive, the novel multilayer film may additionally contain neutralizers, stabilizers, lubricants, hydrocarbon resins and/or antistatics in one or more layers. The percentages by weight given below relate to the weight of the respective layer to which the additive has been added.
Neutralizers are preferably dihydrotalcite, calcium stearate and/or calcium carbonate having a mean particle size of at most 0.7 Vim, an absolute particle size of less than 10 ~,m and a specific surface area of at least 40 mz/g. In general, the neutralizer is added in an amount of from 0.02 to O.lo by weight.
Stabilizers which can be added are the conventional stabilizing compounds for polymers of ethylene, propylene and other a-olefins. The amount in which they are added is between 0.05 and 2% by weight. Particularly suitable are phenolic stabilizers, alkali/alkaline earth metal stearates and/or alkali/alkaline earth metal carbonates.
Phenolic stabilizers are preferred in an amount of from 0.1 to 0.6% by weight, in particular from 0.15 to 0.3% by weight, and having a molecular weight of greater than 500 g/mol.Pentaerythrityltetrakis[3-(3,5-di-tert-butyl 4-hydroxyphenyl)propionate] and 1,3,5-trimethyl-2,4,6 tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene are particularly advantageous.
Lubricants are higher aliphatic acid amides, higher aliphatic acid esters, waxes and metal soaps, and poly-dimethylsiloxanes. The effective amount of lubricant is in the range from 0.1 to 3% by weight. The addition of higher aliphatic acid amides in the range from 0.15 to 0.25% by weight to the base layer and/or the top layers is particularly suitable. A particularly suitable ali-phatic acid amide is erucamide.
Hydrocarbon resins are low-molecular-weight polymers whose molecular weight is generally in the range from 300 to 8000, preferably from 400 to 5000, in particular from 500 to 2000. The molecular weight of the resins is thus significantly lower than that of the propylene polymers which form the principal component of the individual film layers and generally have a molecular weight of greater than 100, 000. The hydrocarbon resins are preferably added to the base layer and/or the interlayer(s) . The effective amount of low-molecular-weight resin is from l~to 20% by weight, preferably from 2 to 10% by weight, based on the layer.
The low-molecular-weight resin recommended is a natural or synthetic resin having a softening point of from 60 to 180°C, preferably from 80 to 150°C, determined in accor-dance with ASTM E-28. Of the numerous low-molecular-weight resins, preference is given to hydrocarbon resins, specifically in the form of petroleum resins, styrene resins, cyclopentadiene resins and terpene resins (these resins are described in Ullmanns Encyklopadie der technischen Chemie [Ullmann's Encyclopedia of Industrial Chemistry], 4th Edition, Volume 12, pages 525 to 555).
Suitable petroleum resins are described in numerous specifications, such as, for example, EP-A-0 180 087.
Preferred antistatics are alkali metal alkanesulfonates, polyether-modified, i.e. ethoxylated and/or propoxylated, polydiorganosiloxanes (polydialkylsiloxanes, polyalkyl-phenylsiloxanes and the like) and/or the essentially straight-chain and saturated, aliphatic, tertiary amines containing an aliphatic radical having 10 to 20 carbon atoms which are substituted by ~-hydroxy-(C~-C4)-alkyl groups, N,N-bis(2-hydroxyethyl)alkylamines having 10 to 20 carbon atoms, preferably 12 to 18 carbon atoms, in the alkyl radical being particularly suitable. The effective amount of antistatic is in the range from 0.05 to 3% by weight. A further preferred antistatic is glycerol monostearate.
The invention furthermore relates to the production of the novel multilayer films by the coextrusion process, which is known per se.
In this process, as is customary in coextrusion, the polymer or polymer mixture of the individual layers is compressed and liquefied in an extruder, it being possible for any additives added to be present in the polymer or polymer mixture already or to be added via the masterbatch method. The melts corresponding to the individual layers of the film are then coextruded simul-taneously through a flat-film die (slot die), and the extruded multilayer film is drawn off over one or more take-off rolls, where it cbols and solidifies.
The resultant film is then stretched longitudinally and transversely to the extrusion direction, which results in orientation of the molecule chains. The stretching is 2~5~322 preferably from 4:1 to 7:1 in the longitudinal direction and from 7:1 to 11:1 in the transverse direction. The longitudinal stretching is expediently carried out with the aid of two rolls running at different speeds corresponding to the desired stretching ratio, and the transverse stretching is expediently carried out with the aid of an appropriate tenter frame.
Biaxial stretching of the film is followed by heat-setting (heat treatment), the film being kept at a temperature of from 100 to 160°C for about 0.5 to 10 seconds. The film is subsequently wound up in the conventional manner by means of a wind-up unit.
It has proven particularly favorable to keep the take-off roll or rolls, by means of which the extruded film is also cooled and solidified, at a temperature of from 20 to 90°C.
The temperatures at which longitudinal and transverse stretching are carried out can vary. In general, the longitudinal stretching is preferably carried out at from 100 to 150°C and the transverse stretching is preferably carried out at from 155 to 190°C.
If desired, one or both surfaces of the film can, as mentioned above, be corona- or flame-treated by one of the known methods after the biaxial stretching.
In the case of corona treatment, the film is expediently passed between two conductor elements serving as electrodes, such a high voltage, usually alternating voltage (about 10 to 20 kV and 20 to 40 kHz, being applied between the electrodes that spray or corona discharges can occur. The spray or corona discharge ionizes the air above the film surface and reacts with the molecules of the film surface, causing formation of ~~'5~3~2 polar inclusions in the essentially nonpolar polymer matrix.
For flame treatment with a polarized flame (cf.
US-A-4,622,237), a direct electric voltage is applied between a burner (negative pole) and a chill roll. The level of the applied voltage is between 500 and 3000 V, preferably in the range from 1500 to 2000 V. The applied voltage gives the ionized atoms increased acceleration, and they hit the polymer surface with greater kinetic energy. The chemical bonds within the polymer molecule are more easily. broken, and formation of free radicals proceeds more rapidly. Heating of the polymer here is substantially less than in the case of standard flame treatment, and films can be obtained in which the heat-sealing properties of the treated side are even better than those of the untreated side.
The amorphous polymers can be incorporated into the top layer or top layers of the film either as pure granules or as granulated concentrate (Masterbatch), by premixing the polyolefin granules or powder of the top layers) with the amorphous polymer and subsequently feeding the mixture to the extruder. In the extruder, the components are mixed further and warmed to the processing tempera-ture. It has been found that the lubricant properties and the appearance of the film also depend on the extrusion conditions (temperature and shear). Surprisingly, the lubricant properties and appearance of the film vary with the conditions in the extruder under otherwise identical conditions with respect to raw materials and stretching process. It is essential for the novel process for the production of the film that the extrusion temperature for the top layers) is above the glass transition tempera-ture/Vicat softening temperature of the amorphous polymer. In general, the extrusion temperature for the top layers) is at least 10°C, preferably from 15 to ~1~~~~2 180°C, in particular from 20 to 150°C, above the T~ or T~
of the amorphous polymer.
It is assumed that the amorphous polymer liquefies under the usual extrusion conditions for film production and then surprisingly separates during the extrusion into particulate particles of a certain size, depending on the viscosity of the polyolefin of the top layer and the viscosity of the amorphous polymer at the selected extrusion temperature, and does not agglomerate. The amorphous polymer, which is simply added as solid, is thus, after the extrusion and orientation, in the form of separated particles in the top layer of the film which act as antiblocking agent.
The novel film has better gloss and haze than known films having low coefficients of friction and is likewise distinguished by a low coefficient of friction and low surface roughness. The coefficient of sliding friction of lubricant-free embodiments of the novel films is generally in the range from 0.3 to 0.7, preferably from 0.3 to 0.5. Films additionally containing a lubricant, such as, for example, fatty acid amide, in particular erucamide, have an even further reduced coefficient of sliding friction. In the case of the novel film contain-ing erucamide in the base layer, this is generally in the range from 0.05 to 0.3, preferably from 0.1 to 0.2. The gloss of the novel film is in the range from 90 to 130, preferably from 105 to 130. The haze of transparent embodiments is in the range from 0.9 to 3.0, preferably in the range from 0.9 to 2Ø
The invention is now described in greater detail with reference to working examples.

215~~~~

Example 1 A three-layer film having an overall thickness of 20 ~Cm and an ABA layer structure, i.e. the base layer B was surrounded by two identical top layers A, was produced by coextrusion and subsequent stepwise orientation in the longitudinal and transverse directions.
The film was subjected to one-sided corona treatment on the roll side before rolling up. The roll side is the side of the film with which it is in contact with the first take-off roll. The surface tension on this side as a consequence of this treatment was from 39 to 40 mN/m.
All layers contained 0.13% by weight of pentaerythrityl tetrakis[4-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]
(~Irganox 1010) as stabilizer and 0.06% by weight of calcium stearate as neutralizer.
The base layer B essentially comprised a propylene homopolymer having an n-heptane-soluble content of 4% by weight and a melting range of from 160 to 162°C. The melt flow index of the propylenehomopolymer was 3.4 g/10 min at 230°C and a load of 21.6 N (DIN 53 735). The base layer contained 0.12% by weight of erucamide having a melting range of from 78 to 82°C and 0.12% by weight of N,N-bis-ethoxyalkylamine (~Armostat 300).
The polyolefinic top layers essentially comprised an ethylene-propylene-1-butene terpolymer containing 3.5% by weight of ethylene, 88.5% by weight of propylene and 8%
by weight of 1-butene. The top layers contained 0.05% by weight of a cyclic olefin copolymer having a T~ of 174°C
and a mean molecular weight of 34, 000. Each of the top layers was 0.8 ~Cm thick.
Example 2 Example 1 was repeated, but the top layer contained 0.15%
by weight of the same cycloolefin copolymer.

21~~322 Comparative Example 1 Example 1 was repeated, but the antiblocking agent employed was 0.15% by weight of a crosslinked silicone resin powder having a mean particle diameter of 2 ~m (~Tospearl 20 from Toshiba Silicone Co., Ltd.).
Comparative Example 2 Example 1 was repeated, but the antiblocking agent employed was 0.15% by weight of an organically coated silicon dioxide having a mean particle diameter of 2 ~m (~Sylobloc 44 from Grace).
Comparative Example 3 Comparative Example 1 was repeated, but the top layer contained 0.33% by weight of the silicone resin powder.
Comparative Example 4 Comparative Example 2 was repeated, but the top layer contained 0.33% by weight of the coated silicon dioxide.
The properties of the films of the examples and compara-tive examples are summarized in the table below.
The following measurement methods were used to characterize the raw materials and the films:
Melt flow index The melt flow index was measured in accordance with DIN 53 735 at a load of 21.6 N and at 230°C.
Meltinc~point DSC measurement, maximum of the melting curve, heating rate 20°C/min.
Determination of the minimum sealing temperature Heat-sealed samples (seal seam 20 mm x 100 mm) are produced using a Brugger HSG/ET sealing unit by sealing ~~~~3zz a film at different temperatures with the aid of two heated sealing jaws at a sealing pressure of 10 N/cm2 and a sealing time of 0.5 s. Test strips 15 mm in width are cut out of the sealed samples. The T-seal seam strength, i.e. the force necessary to separate the test strips, is determined using a tensile testing machine at a take-off rate of 200 mm/min, the seal seam plane forming a right angle with the tension direction. The minimum sealing temperature is the temperature at which a seal seam strength of at least 0.5 N/15 mm is achieved.
Seal seam strength For the measurement, two film strips 15 mm in width were laid one on top of the other and sealed for 0.5 s at 130°C and a sealing pressure of 1.5 N/mm2 (Brugger NDS
unit, sealing jaws heated on one side). The seal seam strength was determined by the T-peel method.
Friction The friction was determined in accordance with DIN 53 375. The coefficient of sliding friction was measured 14 days after production.
Surface tension The surface tension was determined by the ink method (DIN 53 364).
Roughness The roughness was determined in accordance with DIN 4768 at a cut-off of 0.25 mm.
Haze The haze of the film was measured in accordance with ASTM-D 1003-52. The Holz haze measurement was carried out in accordance with ASTM-D 1003-52, but, in order to utilize the optimum measurement range, the measurement was carried out on four film layers lying one on top of 2~~~32~

the other and using a 1° slit diaphragm instead of a 4°C
pinhole diaphragm.
Gloss The gloss was determined in accordance with DIN 67 530.
The reflector value was measured as an optical parameter for the surface of a film. In accordance with the ASTM-D 523-78 and ISO 2813 standards, the angle of incidence was set at 20 ° or 60 ° . A light beam hits the planar test surface at the set angle of incidence and is reflected or scattered thereby. The light beams incident on the photoelectronic receiver are indicated as a proportional electrical quantity. The measurement value is dimensionless and must be specified together with the angle of incidence.

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Claims (18)

1. An oriented, multilayer polyolefin film comprising a polyolefinic base layer and at least one heat-sealable top layer, wherein the heat-sealable top layer comprises an .alpha.-olefin having 2 to 10 carbon atoms and at least one amorphous polymer which is in the top layer in the form of separated particles.

2. A polyolefin film as claimed in claim 1, wherein the top layer contains a maximum of 5% by weight of amorphous polymer, based on the weight of the top layer.

3. A polyolefin film as claimed in claim 2, wherein the top layer contains from 0.001 to 3% by weight of amorphous polymer, based on the weight of the top layer.

4. A polyolefin film as claimed in any one of claims 1 to 3, wherein the separated particles of amorphous polymer have a mean particle diameter of from 0.2 to 20 µm.

5. A polyolefin film as claimed in claim 4, wherein the mean particle diameter is from 0.5 to 15 µm.

6. A polyolefin film as claimed in any one of claims 1 to 5, wherein the amorphous polymer has a glass transition temperature T G in the range from 70 to 300°C or a Vicat softening temperature T V of from 70 to 200°C.

7. A polyolefin film as claimed in any one of claims 1 to 6, wherein the amorphous polymer has a degree of crystallinity of less than 5% and a mean molecular weight M W
of from 500 to 500,000.

8. A polyolefin film as claimed in any one of claims 1 to 7, wherein the amorphous polymer has a refractive index of from 1.3 to 1.7, and said refractive index is at most 0.1 unit greater than or less than the refractive index of the polyolefin.

9. A polyolefin film as claimed in any one of claims 1 to 8, wherein the amorphous polymer is atactic polystyrene, poly-.alpha.-methylstyrene, polycarbonate, polyacrylate, an amorphous homopolymer or copolymer of a polycyclic olefin, polyvinylcarbazole, atactic polyvinylcyclohexane, polyvinylchloride, polyacrylonitrile, a natural or synthetic resin, a specific rubber type, or an uncrosslinked, partially crosslinked or crosslinked dispersion of an amorphous polymer.

10. A polyolefin film as claimed in any one of claims 1 to 9, wherein the particles of amorphous polymer are approximately spherical and satisfy the following condition:
in which f is greater than 0.5, and A is the cross-sectional area in mm2 and D max is the maximum diameter of the cross-sectional area in mm.

11. A polyolefin film as claimed in claim 10, wherein f is from 0.7 to 1.

12. A polyolefin film as claimed in any one of claims 1 to 10, wherein the top layer(s) has a thickness of from 0.2 to 2 µm.

13. A polyolefin film as claimed in any one of claims 1 to 12, wherein the white and/or opaque embodiment of the multilayer film has a light transparency, measured in accordance with ASTM-D 1039-77, of at most 50%.

14. A process for the production of a multilayer, oriented polyolefin film comprising an .alpha.-olefin having 2 to carbon atoms and an amorphous polymer in its heat-sealable top layer, in which the polymers and/or polymer mixtures forming the film are compressed and warmed in an extruder, the melt(s) is (are) subsequently extruded through a flat-film die, the resultant film is taken off over one or more rolls, the film is subsequently oriented and, optionally, heat-set and surface-treated, wherein the extrusion temperature of the top-layer polymer is above the glass transition temperature of the amorphous polymer.

15. The process as claimed in claim 14 for the production of a multilayer polyolefin film, in which the coextruded film is taken off over a take-off roll whose temperature is from 20 to 90°C, the film is biaxially stretched at a longitudinal stretching ratio of from 4:1 to 7:1 and a transverse stretching ratio of from 7:1 to 11:1, and the biaxially stretched film is heat-set, optionally corona-treated and subsequently wound up.

16. Use of a polyolefin film as claimed in any one of claims 1 to 13, as a packaging film.

17. Use of a polyolefin film as claimed in any one of claims 1 to 13, for printing and lamination.

18. A laminate comprising a multilayer polyolefin film as claimed in any one of claims 1 to 13, and paper, board or a further film made from a thermoplastic.