US20050129886A1

US20050129886A1 - Multilayer films

Info

Publication number: US20050129886A1
Application number: US10/953,666
Authority: US
Inventors: Carlos Gandolphi; Fernando Pardal
Original assignee: Henkel AG and Co KGaA
Current assignee: Henkel AG and Co KGaA
Priority date: 2002-03-30
Filing date: 2004-09-28
Publication date: 2005-06-16

Abstract

A multilayer film is disclosed in which an outer surface is coated with a first hot-seal adhesive and the opposite outer surface comprises a layer based on a polyolefin compound or polyvinyl compound, wherein the first hot-seal adhesive may A) contain cellulose nitrate and one or more acrylic polymers, and B) can be sealed to the layer based on a polyolefin or polyvinyl compound. A process for the production of this multilayer film is also disclosed. The multilayer film according to the invention is suitable for producing flexible packagings, in particular for packaging foodstuffs. The multilayer film according to the invention is characterized by low amounts of coating material and adhesive and thus by a low overall weight.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 USC Sections 365 (c) and 120 of International Application No. PCT/EP03/03020, filed 24 Mar. 2003 and published 9 Oct. 2003 as WO 03/082570, which claims priority from German Application No. 10214386.2, filed 30 Mar. 2002, each of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a multilayer film, a process for the production of this multilayer film, as well as its use in flexible packagings.

DISCUSSION OF THE RELATED ART

Flexible packagings consist in general of the base carrier materials paper, aluminum foil and/or plastics film. By combination with further materials, for example films that may consist of various thermoplastic polymers, a multilayer film can be fabricated in the form of a film composite with widely varying properties, which in addition to meeting the basic requirement of mechanical protection also exhibits barrier properties with regard to liquids, light, gases as well as aromas.
Flexible packagings are widely used in the foodstuffs sector as well as in the non-foodstuffs sector. For example, flexible packagings are used to package sterile medical articles, for example needles, surgical bandages or gloves. In the cosmetics sector flexible packagings are used for example to package soap, shower gel or shampoo. In the semi-luxuries sector multilayer films are used for example as flexible packaging material for the aroma-tight wrapping of cigarette packets. In the foodstuffs sector flexible packagings are used to package sausages, coffee, farinaceous products, soup cubes, bread and bakery items, confectionery (e.g. chocolate bars) and snack food (e.g. potato crisps). Snack food is mentioned here as an example of foodstuffs that may be sensitive to oxygen and/or light and/or rapidly dry out or tend to exhibit undesirable absorption of moisture and odors, or loss of aroma. Multilayer films that are metallized are often employed for packaging these foodstuffs. The metallisation is normally carried out by vacuum deposition of metals or metal oxides, preferably aluminum, as a thin film of ca. 30 to 80 nm on one side of the film.
The metal layers or metal oxide layers exhibit barrier properties, in particular with respect to water vapour and light, and thus protect the fat contained for example in snack foods, which are susceptible to photooxidiation, against becoming rancid.
The barrier properties are improved by adhesives that are used for bonding two or more materials, for example plastics films, to produce a composite film.
Multilayer films in the form of composite films are produced by laminating a base carrier material, hereinafter termed a carrier film, with further films to form a composite film, normally in a dry lamination process. Dry lamination is understood to be the large-area, continuous joining of flexible films by bonding, the bonding being carried out in the dry but still adhesive-active state after prior drying. Dry lamination may in this connection take place in-line after printing (one-stage process) or off-line (two-stage process).
Two-component (2C) lamination adhesives, in particular 2C polyurethane adhesives, and melt adhesives are employed for the bonding. The application amount of the lamination adhesive is in the range from ca. 1 g/m²to 5 g/m².
The disadvantage of dry lamination with lamination adhesives based on polyurethanes are the waiting times of ca. 2 hours to 48 hours before the adhesive curing process is complete and possibly further films can be added to the composite film, or before the composite film can be cut into the finished shape. Before the corresponding articles, in particular foodstuffs, can be packaged, it must be ensured that the composite film is free from monomers, in particular monomeric diisocyanates, from the lamination adhesive that are still possibly present and are capable of migration. To this end further waiting times of the order of magnitude of 5 to 7 days must as a rule be observed.
A multilayer film may also be produced by extrusion lamination. Extrusion lamination is understood to mean the application in an extrusion process of olefinic homopolymers or copolymers in liquid form, in particular polyethylene, from slit dies onto the films (strips) to be joined, which are generally pretreated with primers. Primers are preliminary coats having for example a coupling, passivating and/or corrosion-inhibiting action, which are applied in very thin layers (ca. 1 g/m²). They consist in most cases of dilute solutions of the adhesive constituents that are also to be employed for the subsequent bonding.
Extrusion lamination is normally carried out off-line, i.e. not in a continuous process after printing. The application amount of the olefinic homopolymers or copolymers in liquid form used in extrusion lamination is ca. 12 g/m²to 20 g/m², which means that the total weight of such a multilayer film is significantly higher than the weight of a comparable multilayer film that has been produced by means of dry lamination.
Whereas dry lamination is carried out in practice at normal speeds of up to 500 m/min, the speed in the case of extrusion lamination is usually up to 200 m/min.
The closure of the flexible packaging is effected by sealing. For this purpose the multilayer film in the form of a single film or a composite film must comprise at least one sealable layer, which is arranged on the outside, the inside, or on the outside and inside of the single film or composite film. The term “sealing” is generally understood to denote the bonding-type setting of a soft sealing medium. The addition of heat and/or application of a minimum sealing pressure are necessary for this purpose. A distinction is accordingly made between hot sealing and cold sealing, in which the question of which type of sealing is employed depends for example on the materials used, the object to be packaged (for example its heat sensitivity), and the type of packaging machinery.
In cold sealing, cold sealants are used that effect a bonding of two polymer layers at roughly room temperature under high pressure. Cold sealants are predominantly applied as aqueous dispersions in an amount of 1-6 g/m²to the polymer to be bonded and are synthesized based on rubber and rubber-like polymers, PVDC (polyvinylidene chloride), PVAC (polyvinyl acetate) and poly (meth) acrylates. The disadvantage of cold sealants is that they can be sealed only with one another and often have an insufficient adhesion.
Hot sealant dispersions, hot sealant lacquers, melt adhesives as well as films of thermoplastic elastomers and extrusion coatings are used for hot sealing.
Hot sealant dispersions are predominantly PVDC-, PVAC-, poly (meth) acrylate—or latex-containing dispersions which, after evaporation of water, form sealable, dry and as a rule transparent coatings in application amounts of ca. 2 to 15 g/m². The disadvantage of coatings of hot sealant dispersions is that they frequently have a tendency to bond with certain printing inks.
In contrast to hot sealant dispersions, in the case of hot sealant lacquers organic solvents are employed using the same or similar polymers. The hot sealant lacquers are employed in an amount of ca. 1 to 12 g/m². As a rule hot sealant lacquers cannot be used to seal dissimilar materials.
Hot sealant adhesives based on melt adhesives generally contain as basic constituents ethylene/vinyl acetate copolymers, which are deposited on the substrate by roller application or also by extrusion.
Films of thermoplastic polymers and extrusion coatings are also used for hot sealing, these films then often being referred to as “weld films”, and the term “welding” is used instead of “sealing”. Co-extrudates also belong to this group, the term co-extrudates being understood to mean multi-ply films whose plies are extruded fused together in one workstage, namely extrusion. All polymers, preferably elastomers, that are thermoplastic in a temperature range from ca. 50° to ca. 220° C. are sealable and weldable. In particular the extrusion coating of PC (polyethylene) on carrier films such as aluminum, polypropylene, polyester and polyamide permits a multiplicity of packaging material specifications.
Peelable sealing at low sealing temperatures of films with certain other films or thermoforming sheets is achieved by co-extrusion coatings of PE-EVA mixtures. Special polyethylene copolymers that contain carboxylate groups and in which the carboxylate groups in some cases contain zinc or sodium ions as counterion, also seal at low sealing temperatures but with high sealing seam strengths.
WO 00/39200 discloses a co-extruded BOPP film with peelable sealing properties, in which the film is printed in a screen-like manner in a sealing region left uncolored with a thermoplastic lacquer that contains at least one polyvinvyl butyral (PVB) or at least one ethylene/vinyl acetate copolymer (EVA copolymer) as feature-determining solids component and that can be sealed only weakly with co-extruded BOPP. On heating, the spacing effect of the thermoplastic lacquer becomes successively weaker due to progressive softening with rising temperature, as a result of which the co-extruded BOPP surfaces come increasingly into sealing contact (page 3, lines 1 to 11). The disadvantage of this invention is that colorant-containing sealing regions obviously cannot be coated with the thermoplastic lacquer. There is also no indication that metallized film composites can be sealed in a practicable manner according to this invention.
DE 100 02 167 discloses a highly transparent, high-gloss, not easily flammable UV-stable and thermoformable film that is provided on at least one surface side with a hot sealing lacquer or a hot sealing layer. The film can be sealed at temperatures below 150° C. with itself or with other materials (page 2, lines 56-57). The hot sealing layer consists of polyolefins, copolymers thereof, polyvinylidene chloride, special cover sealing lacquers, preferably polyethylene, in particular LDPE (claim 9). This specification does not contain any suggestions regarding the choice of the hot sealing lacquer.
DE 197 07 786 discloses a plastics film for the production of packaging containers, in which an outer surface of the film is coated with a sealing lacquer and the opposite outer surface is coated with an acrylate-based lacquer. The acrylate-based lacquer performs the function of a release coat and prevents superimposed plastics films from adhering to one another during storage (page 6, lines 1 to 7). These types of films are not particularly suitable for sealing an outer surface with the opposite outer surface.
U.S. Pat. No. 6,077,602 discloses a heat-sealable polyolefin film containing a polyolefin substrate with a first side and a second side. The first side has a polymer coating based on a nitrile monomer, and a metal or a polymer coating based on acrylic polymer is deposited on the second side. The coating of the first side can be sealed with itself or with the coating of the second side (page 4, lines 37-61).
DE-OS-29 00 291 discloses a coating composition for polyolefin products based on methyl methacrylate/butyl methacrylate copolymers, polyester resin and nitrocellulose of moderate viscosity. The Offenlegungsschrift does not disclose whether a sealing of an outer surface coated with this coating composition with the opposite outer surface not coated with this coating composition is possible and is intended.
Despite the multifarious possibilities listed here of introducing onto the market sealable films or multilayer films as a flexible packaging having widely diverse properties, there is a need for inexpensive flexible packagings with improved or at least equally good properties compared to the multilayer films or flexible packagings presently available on the market.

SUMMARY OF THE INVENTION

The object of the present invention was accordingly to provide inexpensive multilayer films for flexible packagings. A further object was to provide multilayer films for flexible packagings that permit a sealing with different materials, preferably with printed and/or metallized surfaces.
An object of the present invention was furthermore to provide films that can be used without complicated process steps and directly, in other words without fairly long intermediate storage, for packaging, in particular for packaging foodstuffs.
This object is achieved by a multilayer film in which an outer surface is coated with a first hot-seal adhesive and the opposite outer surface has a layer based on a polyolefin or polyvinyl compound, wherein the first hot-seal adhesive contains
A) cellulose nitrate and acrylic polymers and
B) can be sealed with the layer based on a polyolefin or polyvinyl compound.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows in cross-section an embodiment of a multilayer film according to the present invention.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS OF THE INVENTION

The multilayer film contains at least one carrier film (A1).
The carrier film (A1) consists of plastics material, paper or metal. In the preferred embodiment of the invention the carrier film (A1) consists of a plastics film. Depending on the requirement profile and intended use, the carrier film (A1) may be combined with further films of plastics material or metal to form a composite film, and may also comprise layers of paper, paperboard, metal or adhesive.
The carrier film (A1) has a weight of 15 to 45 g/m², preferably 25 to 42 g/m²and particularly preferably 30 to 38 g/m², and is normally produced continuously in the form of strip material according to the process known to the person skilled in the art. The carrier film (A1) contains preferably at least one polymer from the group: polyamide (PA), polyurethane (PUR), polypropylene (PP), poly (ethylene/co-vinyl alcohol) (EVOH), polyvinyl alcohol (PVOH), polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polystyrene (PS) or polymethyl methacrylate (PMMA). Preferred are PET homopolymers and copolymers that have a reduced crystallisation tendency. The crystallisation tendency of PET is reduced, for example, by incorporating by polymerization comonomers such as isophthalic acid or 1,4-cyclohexanedimethanol.
The carrier film (A1) may exist in a non-oriented (undrawn) modification or in a monoaxially as well as biaxially oriented modification. In a preferred embodiment the carrier film (A1) consists of oriented polypropylene (OPP), in particular biaxially oriented polypropylene (BOPP).
Preferably the multilayer film is a carrier film (A1) in which one outer surface is coated with the first hot-seal adhesive and the opposite outer surface comprises a sealable layer based on a polyolefin or polyvinyl compound.
The layer (B) of the first hot-seal adhesive covers directly or indirectly, preferably directly, and substantially, in other words preferably over the whole surface, an outer surface of the carrier film (A1) and is located on the opposite outer surface, which comprises a layer based on a polyolefin or polyvinyl compound. “Indirectly” means that the layer (B) of the first hot-seal adhesive is applied for example to a layer, metallic layer or surface of a further film containing printing inks that does not form the carrier film (A1) but is combined with the latter, of the multilayer film.
The amount of applied layer (B) is 0.1 to 6 g/m², preferably 0.3 to 4 g/m²and particularly preferably 0.5 to 2 g/m².
The first hot-seal adhesive forming the layer (B) may contain the following components:
I) 5 to 60 wt. %, preferably 10 to 45 wt. %, particularly preferably 15 to 35 wt. % of cellulose nitrate,
II) 0.5 to 20 wt. %, preferably 1 to 15 wt. %, particularly preferably 2 to 10 wt. % of acrylic polymer,
III) 0.5 to 20 wt. %, preferably 1 to 15 wt. %, particularly preferably 2 to 10 wt. % of polyalkylene glycol,
IV) 0.5 to 15 wt. %, preferably 1 to 10 wt. %, particularly preferably 2 to 8 wt. % of a chelate compound with titanium as central atom,
V) 0 to 10 wt. % of at least one plasticizer,
the sum of the components I) to V) totalling 100 wt. %.
The following components are preferably contained in the first hot-seal adhesive:
I) 45 to 60 wt. % of at least one cellulose nitrate (I),
II) 0.5 to 20 wt. % of at least one acrylic polymer (II),
III) 0.5 to 20 wt. % of at least one polyalkylene glycol (III),
IV) 0.5 to 15 wt. % of at least one chelate compound (IV) with titanium as central atom,
the sum of the components (I) to (IV) totalling 100 wt. %.
The acrylic polymer (II) may have an average molecular weight of 30,000 to 80,000. The acrylic polymer (II) is preferably a methyl methacrylate copolymer.
The polyalkylene glycol (III) is preferably a water-soluble polypropylene glycol.
The chelate compound with titanium as central atom (IV) is preferably a compound of ethyl acetonate, acetyl acetonate, lactic acid and its salts, as well as triethanolamine.
Plasticizers (V) that may be used include monohydric or polyhydric alcohols, preferably glycol monophenyl ether, hexamethylene glycol, glycerol and in particular polyalkylene glycols with a molecular weight of 200 to 6000. Polyethylene glycols with a molecular weight of up to about 1000, preferably up to about 600, are preferred. Polypropylene glycol and polybutylene glycol, as well as polymethylene glycol, may also be used. Esters are conveniently used as plasticizers, for example, liquid polyesters and glycerol esters such as glycerol diacetate and glycerol triacetate, as well as neopentyl glycol dibenzoate, glyceryl tribenzoate, pentaerythritol tetrabenzoate and 1,4-cyclohexanedimethanol dibenzoate. Finally, alkylmonoamines and fatty acids with preferably 8 to 36 carbon atoms may also be used. Plasticizers based on aromatic dicarboxylic acid esters are preferably used, i.e., the corresponding esters of phthalic acid, isophthalic acid or terephthalic acid. The alcohol residue in these esters used as plasticizers normally contains 1 to 8 carbon atoms. Medicinal white oil and naphthenic mineral oil are especially suitable.
The composition for the layer (B) of the first hot-seal adhesive is applied to the surface to be coated in the form of a 20 to 30 wt. % solution or dispersion in volatile organic solvents using conventional methods known to the person skilled in the art. As solvents there may, for example, be used ethanol, propanol, iso-propanol, ethyl acetate, n-propyl acetate, acetone, methyl ethyl ketone, low boiling point hydrocarbons or mixtures of these solvents.
After the evaporation of the solvent the first hot-seal adhesive remains as a dry layer (B) on the outer surface of the multilayer films. In a particular embodiment of the multilayer film according to the invention a layer of the carrier film (A1) or of the composite film is metallized or contains an aluminum foil that is joined to the underlying layer, for example a plastics film, via an adhesive layer. The layer (B) of the first hot-seal adhesive is applied to the metallized side or to the metal film and can be sealed on an opposite outer surface that comprises a polyvinyl-based layer. If the first hot-seal adhesive is applied to a metallic layer, then the following components preferably are additionally also present:
VI) 1 to 10 wt. % of an inorganic acid,
VII) 1 to 10 wt. % of an organosilicon compound, the sum of the components (I) to (VII) totalling 100 wt. %.
As inorganic acid (VI) there is preferably used an acid from the group comprising phosphorus-containing or sulfur-containing acids, for example sulfuric acid or phosphoric acid.
As organosilicon compound (VII) there are preferably used compounds of the general empirical formula (R₁O)₃Si-R₂-X.
As R₁O substituents there may be used methoxyl, ethoxyl or â-methoxylethoxyl groups.
As spacer R₂there is preferably used (CH₂)_ywhere y=1 to 10. “X” is a reactive terminal group, preferably an amino, hydroxy, vinyl, methacrylic or epoxide group.
The layer (B) of the first hot-seal adhesive protects the metallized surface against contamination and renders the metal coating resistant to abrasion. The coating operation with a protective lacquer or a removable film that is otherwise normally employed for this purpose is thus omitted.
In a particularly preferred embodiment of the invention the first hot-seal adhesive is a high-gloss sealing lacquer.
In another particularly preferred embodiment of the invention the first hot-seal adhesive covers substantially the whole area of the outer surface. Within the scope of the invention is it particularly preferred if the opposite outer surface, which contains a layer (A2) based on a polyolefin or polyvinyl compound, is securely sealed to the layer (B) of the first hot-seal adhesive. The precondition for this is a so-called “inside to outside” sealing. The layer based on a polyvinyl compound particularly preferably contains a sealable polyacrylate composition.
In a particular embodiment of the invention, in connection with the multilayer film the coating of the first hot-seal adhesive is partially replaced over the whole layer thickness by the layer of a second hot-seal adhesive in such a way that both coatings jointly substantially cover the whole area of the outer surface.
The layer (B) of the first hot-seal adhesive is partially replaced in particular by the layer (C) of the second hot-seal adhesive, in those regions that are sealed with the layer (A2) arranged on the opposite outer surface.
The layer (C) of the second hot-seal adhesive is applied to the opposite side of the layer (A2) based on a polyolefin or polyvinyl compound. The amount of the applied layer (C) is 0.1 to 5 g/m², preferably 0.2 to 3 g/m²and particularly is preferably 0.3 to 1 g/m².
The layer (C) is a sealable layer of hot-seal dispersion adhesive, hot sealing lacquer or melt adhesive.
The layer (C) of the second hot-seal adhesive may contain the following components:
i) 25 to 90 wt. %, preferably 35 to 80 wt. %, particularly preferably 45 to 70 wt. % of at least one ethylene/vinyl acetate copolymer,
ii) 1 to 50 wt. %, preferably 15 to 40 wt. %, particularly preferably 20 to 30 wt. % of at least one tackifying resin,
iii) 1 to 40 wt. %, preferably 5 to 30 wt. %, particularly preferably 10 to 20 wt. % of at least one wax-based lubricant,
the sum of the components i) to iii) totalling 100 wt. %.
The second hot-seal adhesive particularly preferably contains the following components:
i) 25 to 90 wt. % of at least one ethylene/vinyl acetate copolymer,
ii) 1 to 50 wt. % of at least one tackifying resin,
iii) 1 to 40 wt. % of at least one wax-based lubricant,
the sum of the components i) to iii) totalling 100 wt. %.
The ethylene-vinyl acetate copolymer i) is preferably characterized by its vinyl acetate content of 30 to 40 wt. % as well as by a melt index of 2 to 5 g/10 minutes (ASTM D 1238).
The tackifying resin ii) may be selected from:
a) hydroabietyl alcohol and its esters, in particular its esters with aromatic carboxylic acids such as terephthalic acid and phthalic acid,
b) preferably modified natural resins such as resin acids from gum rosin, tall oil rosin or wood rosin, for example fully saponified gum rosin or alkyl esters of optionally partially hydrogenated colophony with low melting points, such as for example methyl, diethylene glycol, glycerol and pentaerythritol esters,
c) acrylic acid copolymers, preferably styrene/acrylic acid copolymers, and
d) resins based on functional hydrocarbon resins.
It is also possible to use as tackifying resin, an alkyl ester of partially hydrogenated colophony, the alkyl group preferably containing 1 to 6 carbon atoms. It is preferred to use partially polymerized tall oil rosin, hydrogenated hydrocarbon resin and colophony glycerol esters and mixtures of these resins.
The wax iii) to be used as lubricant may be carnauba wax, paraffin wax, oxazoline wax or a mixture of two or more of these waxes.
In particular, in the second hot-seal adhesive
(i) the ethylene-vinyl acetate copolymer (i) may be characterized by its content of vinyl acetate of 30 to 40 wt. % as well as a melt index of 2 to 5 g/10 minutes (ASTM D 1238);
(ii) the tackifying resin (ii) may be at least one resin based on functional hydrocarbon resins and/or hydrogenated hydrocarbon resins;
(iii) the wax (iii) to be used as lubricant may be a carnauba wax, paraffin wax, oxazoline wax or a mixture of two or more of these waxes.
The second hot-seal adhesive for the formation of the layer (C) is applied directly or indirectly in the form of a 20 to 30 wt. % solution or dispersion in volatile organic solutions to the surface of (A1) to be coated, using conventional methods known to the person skilled in the art, and after application is located on the opposite side of the layer (A2) based on a polyolefin or polyvinyl compound. The second hot-seal adhesive can be sealed on a layer based on a polyolefin compound, located on the opposite outer surface. The layer based on a polyolefin compound particularly preferably contains ethylene-vinyl acetate copolymers and/or polypropylene copolymers.
The outer surface that comprises the layer (A2) based on a polyolefin or polyvinyl compound is defined as the “inside” of the multilayer film. The side that is provided with the sealing layer (C) is defined as the “outside”. The term indirect application means that the sealing layer (C) in a multilayer film is not applied directly to the carrier film (A1) but to the last layer of the composite, counting the layer (A2) based on a polyolefin or polyvinyl compound as the first layer.
For the application of the second hot-seal adhesive there may be used as solvent for example ethyl acetate, n-propyl acetate, acetone, methyl ethyl ketone, low boiling point hydrocarbons or mixtures of these solvents.
After evaporation of the solvent the second hot-seal adhesive remains as a dry layer (C) on the surface of the multilayer film. The sealing layer (C) may almost completely cover an outer surface of the multilayer film, but for reasons of economy is preferably applied only in regions in which sealing properties are desired. These regions may be (color) printed, and the sealing layer (C) is accordingly applied in a sealing region free of printing ink as well as in a sealing region containing printing ink.
Preferably the sealing layer (C) is applied in specific patterns or formations, for example is applied in strip form to the respective longitudinal sides of the multilayer film. Further strips of the sealing layer (C) may be arranged between the longitudinally applied sealing layer (C) at specific gaps over the width of the multilayer film. The gap between the strip-form sealing layer (C) applied over the width of the multilayer film defines the length of the individual packaging. Preferred patterns of the applied sealing layer (C) are described in WO 00/39200.
The layer (A2) based on a polyolefin or polyvinyl compound on the opposite side of the multilayer film is formed by co-extrusion, for example in a flat-film method or film blowing method, or is applied by lamination, extrusion lamination or extrusion coating. The polyolefin-based layer (A2) can be sealed to the layer (C) of the second hot-seal adhesive, and the polyvinyl-based layer can be sealed to the layer (B) of the first hot-seal adhesive.
The layer (A2) contains thermoplastic polymers that melt at temperatures between 50° C. and 200° C., preferably between 90° C. and 180° C., and can be sealed. The term sealable layer is understood to mean a layer which, after application of temperature and pressure of a predetermined magnitude over a predetermined action time, such as are well known and obvious to those skilled in the art, forms a melt hot seal with itself or with the opposite outer surface of the multilayer film.
The sealable layer (A2) contains thermoplastic polymers from the group comprising homopolymers, alternating or random copolymers, block copolymers or graft polymers, as well as mixtures thereof. In particular the sealable layer (A2) contains at least one polymer from the group: polyethylene terephthalate (PET), polyamide (PA), low density polyethylene (LDPE), low density linear polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP), polybutylene (PB), polyhexene, poly(ethylene/vinyl acetate) (EVA), poly(ethylene/butyl acrylate) (EBA), poly(/ethyl acrylate) (EAA), poly(ethylene/acrylic acid) (EMAA) or modified polyolefin copolymers with carboxyl groups or carboxylic acid anhydride groups, in particular polypropylene-polyethylene copolymers, propylene-butylene copolymers or ethylene-propylene-butylene terpolymers.
Of the modified polyolefin copolymers, polypropylene copolymers or polyethylene copolymers are preferred that have been produced by graft polymerisation with monomers from the group comprising alpha, beta, singly unsaturated dicarboxylic acids, for example maleic acid and/or singly unsaturated carboxylic acids, for example (meth)acrylic acid, and in which the carboxyl groups at least in part contain zinc or sodium ions as counterions.
For example, the multilayer film may consist of an oriented homopolymer of polypropylene (OPP) with an heat-sealable layer (A2) of polypropylene and/or polyethylene copolymer.
Within the scope of the invention a multilayer film with only one carrier film (A1) is particularly preferred, the surface of the carrier film (A1) being identical to the sealable layer (A2). (A1) and (A2) contain preferably substantially OPP, biaxially oriented polypropylene (BOPP), or biaxially oriented PET. There may also be used for example multilayer films in which the carrier film (A1) together with the sealable surface layer (A2) has a thickness of approximately 0.2 mm to 2 mm, preferably 0.3 mm to 1.8 mm. The sealable layer (A2) of the multilayer film is permanently sealable or peelable with itself or with the layer (B) or (C). Within the scope of the invention it is preferred if the sealable layer (A2) is peelable with respect to itself. This requires a so-called “inside to outside” type of sealing.
Depending on the further use, the multilayer film according to the invention may optionally be provided with functional layers, such as for example metallic or transparent inorganic or organic barrier layers. The fabrication of a metallic barrier layer is performed by means of a conventional vacuum coating process. The vacuum coating process may be used to coat or metallize the composite film according to the invention with a plurality of metals, for example zinc, nickel, silver, copper, gold, indium, tin, stainless steel, chromium, titanium or aluminum. According to the present invention an aluminum coating is preferred, which is applied directly to the surface of the carrier film (A1) or indirectly, in other words to a surface of a further layer of the multilayer film. In FIG. 1, for example, layer (F) may be a metallic barrier layer.
Depending on further processing and treatment, for example for a subsequent printing, lamination or metallizing, the surface of the multilayer film or the surface of the carrier film (A1) may be pretreated directly. By means of a corona treatment the surface is for example cleaned, oxidized and activated for the following treatment process and the surface adherence is thereby improved. The multilayer film may be metallized with a shiny, highly reflecting surface or with a satin, weakly reflecting surface, depending on whether the surface of the multilayer film or of the individual film (carrier film) that is to be metallized directly underwent a shiny or matte surface treatment.
After the optional pretreatment and metallization, the multilayer film in the form of the composite film—or of the carrier film (A1)—is preferably printed directly. The printing is carried out as a rule by feeding the produced carrier film strip or composite film horizontally in-line into the printing machine. It is also possible to wind the carrier film (A1) or the composite film on rollers in preparation for the printing.
In the printing machine the multilayer film is then printed with a repeating design and/or other impressions, for example images, decorations or text in one or more colours, directly onto the metallized or onto the non-metallized surface of the composite film, or onto the surface of the carrier film (A1). Intaglio printing machines, gravure or flexographic machines are typically used. In this connection the printing of the composite film takes place on the oppositely facing side of the sealable outer layer (A2). Transparent metal-containing and/or opaque printing inks are preferably applied by means of conventional printing methods such as rotational intaglio printing or flexographic printing methods. It is particularly preferred to use transparent printing inks, which allows the reflectivity of the metal-coated surface to appear through the printing ink and thus permits attractive and distinctive representations of the graphics as well as of the packaging materials to be achieved. Printing inks are composed of 1) colorants (pigments including fillers or dyes), 2) binders and 3) additives (e.g., drying agents, diluents, wax dispersions, catalysts and initiators for radiation drying).
On account of their adhesion and physical resistance properties, colorants based on polyester resin binders are preferred, though colorant systems based on binders from the group comprising: cellulose nitrate; polyvinyl resin such as polyvinyl chloride (PVC), polyvinyl acetate (PVAC) and copolymers based on hydroxyl-modified PVC and PVAC; resins based on polymers and copolymers of acrylic acid and methacrylic acid and their esters; polyurethane; polyamide or polyvinyl butyral (PVB) may also be used.
If the surface onto which the first hot-seal adhesive is applied is printed for example with images, decorations or text in one or more colors, then the first hot-seal adhesive layer (B) protects this surface, for example against scratching. The layer (B) of the first hot-seal adhesive and the layer (C) of the second hot-seal adhesive are characterized by a good compatibility with commercially available printing inks. In a preferred embodiment of the invention the first and/or second hot-seal adhesive is applied over a sealing region containing printing ink and can be sealed with the opposite outer surface. For example, layer (D) in FIG. 1 is a layer with printing inks. In a particularly preferred embodiment the layers (B) and (C) are clear and transparent so that the underlying printed region remains easily recognizable.
After sealing, a high seal seam strength is produced also over the regions that contain the printing inks. The application of an adhesion-accelerating substance such as polyethyleneimine (PEI) as preliminary coat or primer, as is conventionally carried out and described for example in EP 0544003 B1, is not necessary.
The layer (B) of the first hot-seal adhesive, preferably in the combination where the coating of the first hot-seal adhesive is partially replaced over the whole layer thickness by the layer of the second hot-seal adhesive, replaces a film with a sealable surface layer that is otherwise normally inserted into the film composite and is sealed as a welding film to the sealable outer layer (A2) of the multiple film. Apart from the welding film, the adhesive that is otherwise normally used in the lamination of the replaced film to crosslink the film, is at the same time also no longer necessary. This is illustrated by FIG. 1, wherein FIG. 1 represents a particular embodiment of the invention. The figure simply serves to illustrate the invention and do not reproduce the actual size relationships.
The present invention also provides a process for the production of a multilayer film, in which
a) a first hot-seal adhesive and optionally a second hot-seal adhesive is/are applied in the form of a 20 to 30 wt. % solution or dispersion of the relevant composition in volatile organic solvents to the outside of a multilayer film that comprises on the opposite outer surface a layer of a polyolefin compound or polyvinyl compound and
b) the solvent is evaporated, so that substantially the whole area of the outer surface is covered by a layer of the first hot-seal adhesive and optionally by a layer of the second hot-seal adhesive, wherein the coating of the first hot-seal adhesive is partly replaced over the whole layer thickness by the layer of the second hot-seal adhesive.
The hot-seal adhesives are applied by conventional methods known to the person skilled in the art.
The first hot-seal adhesive and second hot-seal adhesive are applied with appropriate application devices to the surface to be coated and cover substantially directly or indirectly an outer surface of the carrier film (A1), the sealable layer (A2) based on a polyolefin compound or polyvinyl compound being located on the opposite outer surface. This essentially means that preferably the whole surface is preferably uniformly coated by (B) and (C). At the same time this means that (B) and (C) have the same layer thickness.
The layer (B) of the first hot-seal adhesive is sealed to the opposite outer surface that comprises a layer based on a polyvinyl compound. The layer (C) of the second hot-seal adhesive is sealed to the opposite outer surface that comprises a layer based on a polyolefin compound.
The sealing of the layer (B) and/or of the seal layer (C) to the layer (A2) based on a polyolefin compound or polyvinyl compound takes place as an “outer to inner sealing” at the conventional sealing temperatures, as a rule from 100° C. to 160° C., and at the conventional sealing pressure, as a rule between 0.3 and 6 bar.
The sealable multilayer film produced under “inside to inside sealing” is endowed with peelable properties, in other words flexible packagings produced therefrom can be opened sufficiently easily and as far as possible in a tear-free manner without having to exert a ripping force, wherein despite technically unavoidable raw material variations as well as variations in the pressure conditions, sealing temperatures and application amounts, the sealing strength is guaranteed to remain substantially constant over a broad tolerance range.
The multilayer film according to the invention is characterized by one or more of the following advantageous properties:
(a) Smaller coating amounts and adhesive amounts are required. This results overall in a lower total weight of the composite film.
(b) Flexible packagings produced from the multilayer film according to the invention have properties that are just as good as comparable multilayer films that have an additional welding film.
(c) The curing times in the dry lamination process are significantly shortened due to the absence of a welding film.
(d) A treatment with primers is omitted.
(e) The productivity is raised:
Printing and dry lamination are carried out in-line and therefore permit a higher output per unit time. Since less film material is used, the amount of waste is reduced. Storage times, which are otherwise normally necessary in order to achieve freedom from monomers, are shorter or are omitted altogether.
The layer (B) of the first hot-seal adhesive imparts a lower coefficient of friction to the surface of the composite film and thus permits an improved processing. The multilayer film according to the invention is suitable for the production of flexible packagings by sealing. For example, the multilayer films according to the invention serve for the packaging of sterile medical articles, such as for example needles or bandages. Furthermore, the multilayer film according to the invention is suitable for packaging articles in the cosmetics sector, for example soap, shower gel or shampoo. The multilayer film according to the invention is also suitable for packaging foodstuffs, for example sausages, coffee, farinaceous products, soup cubes, bread and confectionery, sweets and snack food.
The invention accordingly also provides flexible packagings that can be produced from the multilayer film according to the invention.
The invention will now be described in more detail.

EXAMPLES

I. Packaging for Bread and Confectionery
Comparison I:
A co-extruded OPP film weighing 18 g/m²(Mobil Chemical) is printed, coated with 3 g/m²of a commercially available 2C polyurethane adhesive, for example LIOFOL UK 3640 with curing agent 6800 from Henkel KGaA, and laminated with a further co-extruded OPP film with weighing 18 g/m²(Mobil Chemical).
Sealing is performed “inside” to “outside” using a heated sealing tool.
Example I according to the invention:
A co-extruded OPP film weighing 34 g/m²(Mobil Chemical) is printed and is also coated with 1.0 g/m²of the first hot-seal adhesive and 0.6 g/m²of the second hot-seal adhesive.
The sealing is performed “inside” to “outside” using a heated sealing tool, the lacquered surface being sealed to the sealable surface of the co-extruded OPP film.
Packaging for Snack Food
Comparison II:
A co-extruded OPP film weighing 18 g/m²(Mobil Chemical) is printed, coated with 3 g/m²of a commercially available 2C polyurethane adhesive, for example from Henkel KGaA (LIOFOL UK 3640 with curing agent 6800) and laminated with a further co-extruded OPP film that is metallized on one side and weighs 18 g/m²(Mobil Chemical).
Sealing is performed “inside” to “outside” using a heated sealing tool.
Example II according to the invention:
A co-extruded OPP film weighing 34 g/m²(Mobil Chemical) is printed and is also coated with 1.0 g/m²of the first hot-seal adhesive high gloss lacquer and 0.4 g/m²of the second hot-seal adhesive.
The sealing is performed “inside” to “outside” using a heated sealing tool, the lacquered surface being sealed to the sealable surface of the co-extruded OPP film.
II. Method
The printing was carried out with printing inks from Sun Chemicals, recommended for OPP films.
A standard printing machine for reel-type goods was used for the tests (Schiavi Rotatary Gravure Printer, “Patriot” type). The hot-seal adhesives were applied by means of a segmented grid roller, the first hot-seal adhesive being applied first, followed by the second hot-seal adhesive. The region of the film outer surface to be sealed (sealing zone) was coated with the second hot-seal adhesive, while the remaining region of the film outer surface was coated with the first hot-seal adhesive, so that both hot-seal adhesives together covered an outer surface of the film.
The application was carried out at 80° C. to 100° C. and at a printing machine speed of 180 m/min.
The sealing of the outside coated with the hot-seal adhesives to the opposite outer surface of co-extruded OPP was carried out at 130° C.-140° C. and 3 bar for a sealing time of 1 second. A sealing machine from Brugger-Feinmechanik, Munich, type HSG-C1237 (sealing jaws 150×110 mm treated with polytetrafluoroethylene) was used for the sealing operation.
III. Results
The multilayer films according to the invention produced in I. and II. are characterized by a low overall weight and equally good composite adhesion and seal seam adhesion properties compared to the corresponding comparison multilayer films of the prior art.
IV. Formulations
The formulations of the first and second hot-seal adhesives used in the examples of implementation are given in the following table (Table 1).
In the case of an OPP film metallized on one side for packaging snack foods, Example II., the metallic surface was pretreated in a known manner with the primer VITEL B 2200 B (ca. 4% high molecular weight polyester, 96% ethyl acetate from Bostik Company) and was then coated with both hot-seal adhesives.

In a further variant of Example II., packaging for snack foods, a mixture of 98 wt. % formulation a) from Table 1, 1% phosphoric acid and 1% epoxysilane (SILQUEST A 187 (Cromptan (OSI)) or SILANE Z 6040 (Dow Corning) was used as first hot-seal adhesive.

TABLE 1


Wt. %	Chemical Name	Trade Name	Source

a) First hot-seal adhesive

59.8 wt. %	Ethyl acetate		various
			suppliers
3 wt. %	Propylene glycol methyl	DOWANOL	DOW
	ether	PM
3 wt. %	Methyl methacrylate/butyl	NEOCRYL	NeoResins
	methacrylate copolymer	B725
30.7 wt. %	Nitrocellulose	NC 50	Boainain,
	solids content: 50%		Brazil
1.5 wt. %	Oxydipropyl dibenzoate	BENZOFLEX	Velsicol
		9-88
2.0 wt. %	Tetraalkyl titanate	TYZOR GBA	DuPont

b) Second hot-seal adhesive

5.0 wt. %	Ethylene/vinyl acetate	ELVAX 40 L	DuPont
	polymer	08
10.0 wt. %	Ethylene/vinyl acetate	ELVAX C 11	DuPont
	polymer	3226
6.0 wt. %	Hydrocarbon resin	NEVEX 100	Neville
			Cindu
0.25 wt. %	N,N′-ethylene bis-	ADVAWAX	Morton
	oleylamide	240	International/
			USA
3.75 wt. %	Carnauba wax		Kahl & Co.
			Vertriebs
			GmbH Wax
			Refinery
37.5 wt. %	Toluene-free light gasoline,	VM & P-	Americhem,
	boiling point range 120-	Naphtha	USA
	150° C.
37.5 wt. %	N-propyl acetate		various
			suppliers

Claims

1. A multilayer film having an outer surface having a coating of a first hot-seal adhesive and an opposite outer surface having a layer based on a polyolefin or polyvinyl compound, wherein the first hot-seal adhesive:

A) comprises cellulose nitrate and one or more acrylic polymers; and

B) can be sealed with the layer based on a polyolefin or polyvinyl compound.

2. A multilayer film according to claim 1, wherein the first hot-seal adhesive is comprised of:

I) 45 to 60 wt. % of at least one cellulose nitrate (I);

II) 0.5 to 20 wt. % of at least one acrylic polymer (II);

III) 0.5 to 20 wt. % of at least one polyalkylene glycol (III);

IV) 0.5 to 15 wt. % of at least one chelate compound (IV) with titanium as central atom; and

V) 0 to 10 wt. % of at least one plasticizer; the sum of the components (I) to (V) totalling 100 wt. %.

3. A multilayer film according to claim 1, wherein the first hot-seal adhesive is comprised of:

I) at least one cellulose nitrate (I);

II) at least one methyl methacrylate copolymer having an average molecular weight of 30,000 to 80,000;

III) at least one water-soluble polypropylene glycol; and

IV) at least one chelate compound with titanium as central atom (IV) comprising a chelating compound selected from the group consisting of ethyl acetonate, acetyl acetonate, lactic acid, lactic acid salts, and triethanolamine.

4. A multilayer film according to claim 2, wherein the first hot-seal adhesive is additionally comprised of:

VI) 1 to 10 wt. % of at least one inorganic acid; and

VII) 1 to 10 wt. % of at least one organosilicon compound, the sum of the components (I) to (VII) totalling 100 wt. %.

5. A multilayer film according to claim 4, wherein at least one inorganic acid (VI) is selected from the group consisting of phosphorus-containing and sulfur-containing acids.

6. A multilayer film according to claim 1, wherein the first hot-seal adhesive comprises a compound of the general empirical formula (R₁O)₃Si-R₂-X, wherein R₁O is selected from the group consisting of methoxyl, ethoxyl and methoxylethoxyl groups;

R₂is (CH₂)_ywhere y=1 to 10; and

X is selected from the group consisting of amino, hydroxy, vinyl, methacrylic and epoxide groups.

7. A multilayer film according to claim 1, wherein the first hot-seal adhesive is a high gloss sealing lacquer.

8. A multilayer film according to claim 1, wherein the first hot-seal adhesive covers substantially the whole area of the outer surface.

9. A multilayer film according to claim 1, wherein the layer based on a polyolefin or polyvinyl compound contains a sealable polyacrylate composition.

10. A multilayer film according to claim 1, wherein the outer surface additionally has a coating of a second hot-seal adhesive in such a way that the coating of the first hot-seal adhesive and the coating of the second hot-seal adhesive together substantially cover the whole area of the outer surface.

11. A multilayer film according to claim 10, wherein the second hot-seal adhesive is comprised of:

i) 25 to 90 wt. % of at least one ethylene/vinyl acetate copolymer;

ii) 1 to 50 wt. % of at least one tackifying resin; and

iii) 1 to 40 wt. % of at least one wax-based lubricant; the sum of the components i) to iii) totalling 100 wt. %.

12. A multilayer film according to claim 10, wherein the second hot-seal adhesive is comprised of:

(i) at least one ethylene-vinyl acetate copolymer i) having a vinyl acetate content of 30 to 40 wt. % and a melt index of 2 to 5 g/10 minutes (ASTM D 1238);

(ii) at least one tackifying resin ii) selected from the group consisting of functional hydrocarbon resins and hydrogenated hydrocarbon resins;

(iii)at least one wax iii) selected from the group consisting of carnauba wax, paraffin wax, and oxazoline wax.

13. A multilayer film according to claim 10, wherein the second hot-seal adhesive can be sealed to the layer based on a polyolefin or polyvinyl compound situated on the opposite outer surface.

14. A multilayer film according to claim 13, wherein the layer based on a polyolefin or polyvinyl compound contains one or more copolymers selected from the group consisting of ethylene-vinyl acetate copolymers and polypropylene copolymers.

15. A multilayer film according to claim 1, wherein the first hot-seal adhesive is applied over a sealing region of the outer surface containing printing ink.

16. A process for producing a multilayer film according to claim 1, said process comprising:

a) applying the first hot-seal adhesive in the form of a 20 to 30 wt. % solution or dispersion in one or more volatile organic solvents to the outside surface; and

b) evaporating the one or more organic solvents;

so that at least a portion of the area of the outer surface is covered by a layer of the first hot-seal adhesive.

17. A flexible package comprising a multilayer film according to claim 1, wherein the layer of the first hot-seal adhesive on the outer surface is sealed to the layer based on the polyolefin or polyvinyl compound.

18. A flexible package according to claim 17, wherein the sealing of the first hot-seal layer to the layer based on a polyolefin or polyvinyl compound is carried out at 100° to 160° C. and between 0.3 bar and 6 bar.

19. A flexible package according to claim 17, wherein the outer surface additionally has a coating of a second hot-seal adhesive such that the coating of the first hot-seal adhesive and the coating of the second hot-seal adhesive together substantially cover the whole area of the outer surface and the layer of the second hot-seal adhesive is sealed to the layer based on the polyolefin or polyvinyl compound.

20. A method of packaging a foodstuff, said method comprising packaging said foodstuff in a multilayer film according to claim 1 and sealing the layer of the first hot-seal adhesive on the outer surface to the layer based on the polyolefin or polyvinyl compound.