NO864707L - SEALABLE MULTILAYER FILMS. - Google Patents

Description

The present invention relates to multi-layer foils which are particularly useful

as packaging foils as they exhibit a sufficient sealing strength at relatively low sealing temperatures.

It is known that multilayer foils based on a polypropylene foil and sealing layer of polymers such as ionomer resins (e.g. "Surlyn") are sealed with sufficient firmness at relatively low temperatures. This also applies to multi-layer foils that have a sealing layer of ethylene/vinyl acetate copolymers.

However, both known multilayer foils have the disadvantage that they

an electrical pretreatment required for printing on the foils no longer seals with sufficient firmness. Even when only one sealing

layer of a two-sided sealable multi-layer foil is electrically pre-treated, then the pre-treated multi-layer foil can no longer be used for a number of wrapping purposes; i.e. for packages where a seal must take place between a pre-treated versus a pre-treated or non-pre-treated foil surface.

Surprisingly, by providing the multi-layer foils according to the invention with an asymmetric structure, it succeeded in overcoming these disadvantages and in providing sealable multi-layer foils which can be applied without any problem to pressure.

The present invention therefore relates to heat-sealable, at least mono-axially oriented multilayer foils, which can optionally be applied pressure,

with an asymmetric structure of a foil of a propylene polymer as a base layer and two external sealing layers, of which one sealing layer (1) contains, respectively consists of, an ionomer resin or an ethyl en/vinyl acetate copolymer and possibly acrylic acid derivatives., and the other sealing layer (2) contains, or consists of, a copolymer of ethylene/acrylic acid and possibly acrylic acid derivatives,

whereby these sealing layers can possibly be printed.

Asymmetric structure means that the chemical composition of

the two outermost sealing layers are different. Due to this asymmetric structure, these multilayer foils provide

the invention also seals seams with sufficient sealing strength at relatively low temperatures when they contain a pressure on one surface.

The base layer of the sealable multilayer foils according to the invention consists of

the substance of a propylene polymer containing up to 10% by weight of an extra-rich α-olefin with no more than 8 carbon atoms, preferably ethylene.

An isotactic polypropylene with a density of 0.9-

0.91 g/cm^ and a melt index of 1-4 g/10 min. at 230°C/21.6 N (according to DIN 53735). The base layer can contain 3-15% by weight, preferably 8-12% by weight, of an additive which is incompatible with the propylene polymer, preferably an inorganic additive, such as calcium carbonate, silicon dioxide, sodium aluminum silicate and/or titanium dioxide, whereby an opalization of the foil is achieved .

As ionomer resins for the production of the sealing layer (1) they can

the known copolymers of oc-olefins, a,p-unsaturated monocarboxylic acids and possibly derivatives of these a,p-unsaturated monocarboxylic acids, preferably anhydrides, amides or esters of lower aliphatic alcohols are used, whereby the carboxyl groups are neutralized to the extent of at least 10 mol-% with metal ions. For salt formation, septic one-, two-

and trivalent ions of metals from groups I to III, IVa and VIIII from there Periodic Table of the Elements (see "Handbook of Chemistry and Physics", Chemical Rubber Publishing Co., 37th edition, page 392).

Suitable monovalent metal ions are sodium, potassium, lithium, cesium, silver, mercury and copper. Suitable divalent metal ions are beryllium, magnesium, calcium, strontium, barium, copper, cadmium, mercury, tin,

lead, iron, cobalt, nickel and whitefish. Suitable trivalent metal ions are aluminum and iron. Preferably, cations and/or sodium ions are used for neutralization. The ionomer resin should contain at least 50 mol% of an α-olefin, preferably ethylene. Preferred ionomer resins contain 1 to 25 mol% of the α,β-unsaturated monocarboxylic acid, whereby at least 10%, preferably 30% of the carboxyl groups are neutralized by metal

the ions. Particularly suitable for the production of the multilayer foils according to the invention are copolymers of more than 80 mol-% ethylene and less

than 20 mol% acrylic acid and possibly acrylic acid derivatives, which to an extent of at least 10%, preferably at least 30%, are neutralized with zinc ions ("Surlyn 1705").

As sealing layer material for the sealing layer (1), ethylene-vinyl acetate copolymers can also be used whose vinyl acetate content is 2-10 ect-%, preferably 6-9 wt-%, and whose melt index is in the range 0.5-3 g/10 min. at 190°C/21.6 N (DIN 53725), preferably in the range 1-

2 g/10 min.

To produce the sealing layer (2), which can also be pre-treated electrically without losing its sealing ability, copolymers of ethylene/acrylic acid and possibly acrylic acid derivatives, such as acrylonitrile or acrylic acid esters of lower aliphatic alcohols (C1-C4), are used.

By the copolymers in this sealing layer. it is not about, or not to a significant extent about, ionomer resins. The carboxyl groups are thus essentially not, or respectively not, neutralized.

The copolymers preferably contain 1-20% by weight of acrylic acid and possibly acrylic acid derivatives, especially 3-10% by weight of acrylic acid and possibly acrylic acid derivatives. Particularly preferred is the use of copolymers with 6-9% by weight of acrylic acid. The melt index of the sealing layer materials should be in the range 1-20, preferably 3-15, particularly preferred 5-

10 g/10 min. at 190° C/21.6 N, measured according to DIN 53735.

The multilayer foils according to the invention can next to the base layer and those

the outer sealing layers also contain an additional layer, such as a gas barrier layer and possibly a primer layer. For the production of the gas barrier layer, the known polymers can be used, preferably ethylene/vinyl acetate copolymers consisting of 40 to 85 mol%, preferably 60 to 75 mol%, vinyl acetate units, which are saponified to an extent of at least 90%, preferably more than 95 mol%. The intrinsic viscosity numbers [n] of the polymers, measured in a solvent mixture of 85% by weight toluene and 15% by weight water, should preferably lie between 0.07

and 0.17 l/g, particularly preferred between 0.09 and 0.15 l/g.

For the production of the gas-barrier layer, especially an oxygen and/or aroma barrier layer, not only the above-mentioned ethylene/vinyl alcohol copolymers can be used, but also other polymers with gas-barrier properties, such as e.g. polyvinyl alcohol and polyamides. Particularly suitable are mixtures of ethylene/vinyl alcohol copolymers as described above and 10-60% by weight of the aforementioned polymers. As polyamides, polyamide-6, -12 are particularly suitable.

Suitable mixtures are also those described in EP 00 64 330 A 1 and EP 00 63 006 A 1, where mixtures of ethylene/vinyl alcohol copolymers and polyamides which additionally contain suitable plasticizers are used for the production of gas barrier layers.

If the multi-layer foils according to the invention have a gas barrier layer, then as a rule a sufficient cohesive strength is only achieved by using a primer layer. For the production of such primer layers, modified polyolefins are preferably used. These are particularly homo- or copolyolefins with carboxyl groups such as e.g. polypropylene or polyethylene, which contains grafted on at least one monomer from the group of oc,3-monounsaturated dicarboxylic acids, such as e.g. maleic acid, fumaric acid, itaconic acid or their acid anhydrides, acid esters, acid amides or acid imides. Also suitable are copolymers of ethylene with oc,p-unsaturated carboxylic acids such as acrylic acid, methacrylic acid and/or their metal salts (zinc, sodium) and/or their alkyl-(Ci~C4) esters or corresponding graft polymers of the mentioned monomers on polyolefins such as polyethylene or polypropylene or partially saponified ethylene/vinyl acetate mixed polymers, which are optionally graft polymerized with a monomer of the above-mentioned acids and exhibit a low degree of saponification, or mixtures thereof.

Polyolefins such as polypropylene or copolymers of propylene and ethylene with a maximum of 1% by weight grafted with oc.p-monounsaturated dicateboxylic acid anhydrides, such as maleic anhydride, or their saponified mixed polymers, possibly mixed with polyolefins, are particularly preferably used for the production of the primer layer.

The primer polymers can also be incorporated in the sealable layer or in the gas barrier layer. The layers of the multi-layer foil according to the invention can also be equipped with common additives and aids such as e.g. lubricants, anti-blocking agents and anti-static agents in normal amounts.

Consequently, polydiorganosiloxanes or mixtures thereof can be used which at 25°C exhibit a kinematic viscosity of at least 100 mm^/sec. Suitable polydiorganosiloxanes are polydialkylsiloxanes, polyalkylvinyl siloxanes, olefin-modified siloxane oils, polyether-modified siloxane oils and olefin/polyether-modified silicone oils, ethoxy-modified silicone oils and alcohol-modified silicone oils, polydialkylsiloxanes with preferably 1-4 C atoms in the alkyl group.

Polydimethylsiloxane is particularly suitable. Preferably, the polydiorganosiloxane should exhibit a kinematic viscosity of at least 10^ mm^/sec. at 25°C, particularly preferably at least 10° mm^/sec. Furthermore, known unsaturated fatty acid amides, saturated fatty acid amides, such as e.g. stearic acid amide, erucic acid amide, further the thermoplastic polymers incompatible with the polymers in the layer materials, such as e.g. polyamide-12 or inorganic spacers such as SiO 2 , CaCO 3 and/or antistatic compounds such as long-chain, aliphatic, tertiary amines which may optionally be monounsaturated, such as ethoxylated tertiary amines, particularly preferred N,N-bis(hydroxyethyl)-(<C> i2<-C>i6)-alkylamide in the layers.

The multi-layer foils according to the invention can be produced by usual methods such as lamination, coating and/or melt (co)extrusion.

The multilayer foils according to the invention are at least monoaxially oriented; preferably subjected to biaxial stretching. Hereby, the longitudinal stretching is preferably carried out in a ratio of 5:1 to 7:1, and the transverse stretching in a ratio of 7:1 to 10:1.

The multi-layer foils according to the invention preferably have a total thickness of 10-100 pm, particularly preferably 20-50 pm, whereby the base layer has a thickness of 20-50 pm and the heat-sealable layers preferably a thickness of 0.8-2 pm, particularly preferred on 1.2-1.5 pm.

The multi-layer foils according to the invention are particularly suitable as packaging material, especially as wrapping foils, for example for foodstuffs, and for this purpose can be printed on and equipped with tear-off strips.

In the following examples, the following examination methods are used to determine the values: The opacity was determined according to ASTM D 1003 - 52. To determine the gloss of the foil, a gloss meter was used according to Dr. B. Lange, Berlin, consisting of a low-voltage incandescent lamp and a condenser, which illuminates the sample under 45° and a second arm under an angle of 45° with the measuring plane with pinhole aperture and photocell. Tilbublet is a finely adjustable indicating instrument (microammeter) which also contains the power supply and a device to keep the voltage to the incandescent lamp constant. The gloss is defined as the below 45° regularly reflected light proportion in percent, calculated on the basis of a surface silvered mirror as 100% standard. When determining the gloss, a polished black glass plate is used as an intermediate standard on the glass meter, because the silver mirror standard is not resistant to tarnishing caused by air pollution. This exhibits exactly 5% of the gloss of a surface silvered mirror glass plate.

The galvanometer reading of the photocell on the gloss meter is adjusted by means of the black glass intermediate standard to obtain an increase in the reading accuracy of the scale division by means of the adjustable potentiometer. The foil to be examined is placed across the direction of travel, on the basis of the optical axis of the gloss meter, over matt black photographic paper flat under the gloss meter and measured in five flawless places. The galvanometer is divided into 10 and the mean value is indicated.

To determine the strength of the sealing seams, the sealing strength is first examined at a sealing pressure of 50 N/cm^. By sealing firmness is meant the force required under defined conditions (50 N/cm^ pressure and a sealing duration of 0.5 seconds) to separate the sealing seams produced at the temperatures indicated in Table 1. The sealing strength is given in N and refers to a 15 mm wide test strip.

To determine the scratch resistance, the haze determined using the Ulbricht sphere is determined. Below, the opacity of a foil is measured before and after scratching. Scratching takes place with the help of 40 g of dust-free silicon carbide which, during a defined period of time, is applied at an angle of 45° to the horizontally aligned foil.

Example 1<-1>Using normal methods, apply to a base film of polypropylene with

density 0.905 g/cm<3>, melt index 3.3 g/10 min. at 230 • C/21.6 N and melting range of 160-166° C on one surface (A) a sealing layer of an ethylene/acrylic acid copolymer whose carboxyl groups are neutralized with zinc ions, and on the other surface (B) an ethylene is applied /acrylic acid copolymer with 3 mol-% acrylic acid, and the composite material is stretched so that a composite foil is obtained with a longitudinal stretch in a ratio of 5:1 and a transverse stretch in a ratio of 10:1.

Hereby, the base foil has a thickness of 17.4 pm, and the sealing layers

both a thickness of 1.3 pm.

Before determining the sealing strength, gloss and scratch resistance, the sealing layer, which consists of ethylene/acrylic acid copolymer, is electrically pretreated (Corona treatment). The measurement results are listed in table 1.

Example 2 (comparison)

A multi-layer foil is produced as stated in example 1, but both surfaces of the polypropylene base foil are equipped with an ethylene/acrylic acid copolymer whose carboxyl groups are neutralized with zinc ions. Corresponding to example 1, a sealing layer is again electrically pretreated. The measurement results are listed in table 1.

Claims (7)

1. Heat-sealable, at least monoaxially oriented, optionally printable multilayer foils with an asymmetric structure, characterized in that they consist of a foil of a propylene polymer as a base layer and two outer sealing layers, of which 1) the one sealing layer (1) contains an ionomer resin or an ethylene/vinyl acetate copolymer, and 2) the second sealing layer (2) contains a copolymer of ethylene/acrylic acid and possibly acrylic acid derivatives. 2. Multi-layer foil according to claim 1, characterized in that the sealing layer (2) can be applied with pressure. 3. Multi-layer film according to at least one of the preceding claims, characterized in that the base layer essentially consists of a propylene polymer containing up to 10% by weight at most of a further oc-olefin with at most 8 carbon atoms. 4. Multi-layer foil according to at least one of the preceding claims, characterized in that it exhibits a sealing layer (1) of an ethylene/acrylic acid copolymer whose carboxyl groups are at least partially neutralized with Zn ions, and a sealing layer (2) of an ethylene/ acrylic acid copolymer. 5. Multi-layer film according to at least one of the preceding claims, characterized in that it contains an additional layer. 6. Multi-layer foil according to at least one of the preceding claims, characterized in that it is stretched biaxially. 7. Process for wrapping foodstuffs with a foil, characterized by . that at least one foil according to at least one of the preceding claims is used.