Patent Description:
Peelable seals between polymeric films and a variety of surfaces are well known in the packaging art, for example for the packaging of foodstuffs and medical equipment, and are for example disclosed in <CIT>; <CIT> and <CIT>.

The ability to peel such seals generally facilitates the opening of sealed packages and in addition, such seals often provide tamper evidence once they have been peeled.

The mechanism by which such seals can be peeled and the force required to do so will usually depend on the materials involved in forming the heat seal. Peeling will either involve separation at the boundary between different materials in the seal area or the internal rupture of a polymeric layer through the seal area.

The ability of the seal to peel through the seal area in general depends on both the composition of the heat-seal layer and on the compatibility at the interface between the seal layer and the surface to which it is sealed.

Thus, the incompatibility at the interface between the seal layer and the surface to which it is sealed can range from high, with the result that unacceptably low seal strengths result, to low, with the resulting heat seals being so strong that they will not peel before tearing of the film occurs.

Internal rupture through the layer thickness of the seal layer upon peeling will occur for a seal layer based on a partially miscible polymer blend having compatibility with the films to which it is heat sealed.

<CIT> discloses a wrapping film capable of forming peelable seals comprising <NUM> to <NUM> % by weight of a heat-sealable rigid polymer (A) (e.g. homopolymer of ethylene) which, in film form, has an elasticity modulus in excess of <NUM>,<NUM>/cm<NUM>, measured according to ASTM D <NUM>-<NUM>; <NUM> to <NUM> % by weight of a non heat-sealable polymer (B) (e.g. homopolymer of styrene or copolymer of styrene and butadiene or mixtures thereof), which does not form a solution with polymer (A); <NUM> to about <NUM> % by weight of a heat-sealable branched chain polymer (C) (e.g. polyethylene) which, in film form, has an elasticity modulus below <NUM>,<NUM>/cm<NUM> measured according to ASTM D <NUM>-<NUM>; and <NUM> to <NUM> % by weight of a polymer (D) (e.g. copolymer of ethylene and methyl acrylate) compatible with polymers (A), (B) and (C), the sum of the polymers (A), (B), (C) and (D) representing <NUM> % by weight of the polymeric material. The peelable seals may comprise <NUM> to <NUM> % by weight referred to the total of polymers (A), (B), (C) and (D), mineral fillers such as talc. The film may be used as the only wrapping material or it may be used to form complex films by lamination onto other films such as polyethylene, polypropylene, halogenated polymers, polyamides, polycarbonates, polystyrene, or the like.

<CIT> discloses a heat-sealable packaging film producing a peelable seal on a wide range of packaging films, the peelable seal layer comprising: (a) <NUM> to <NUM> % by weight of at least one ethylene polymer selected from polyethylene having a density of from <NUM> to <NUM>/cm<NUM> and an ethylene-vinyl acetate copolymer containing at most <NUM> % by weight of vinyl acetate, (b) <NUM> to <NUM> % by weight of polystyrene, and (c) <NUM> to <NUM> % by weight of a thermoplastic, elastomeric styrene-butadiene-styrene or styrene-isoprene-styrene block copolymer.

This type of packaging peel film does not any longer satisfy the request of modern coextruded multilayer films that require properties like inclusions of oxygen barriers or tear-free peelable film/paper combinations, tamper evidence, a broad cohesive peel-seal window, transparency, sterilisability by irradiation or ethylene oxide and hot tack.

<CIT> discloses a heat sealing film having a haze of no more than <NUM> % and having a sealant layer comprising.

The document does not address the problem of a peelable film.

<CIT> discloses a homogenous blend comprising <NUM> to <NUM> % by weight of a metallocene catalyzed ethylene polymer, and from <NUM> to <NUM> % by weight of a styrene-butadiene block copolymer having from <NUM> to <NUM> % by weight of <NUM>,<NUM>-butadiene monomer units and from <NUM> to <NUM> % by weight styrene monomer units, wherein the styrene-butadiene block copolymer has a transmittance of <NUM> % and a haze of <NUM> %, both measured according to ASTM D1003. The seal layer does not comprise polystyrene and the peeling consists in a separation of the seal layer from the substrate at the boundary i.e. in the original plane of joining the two films.

For medical applications in particular, the peel film is required to be tamper evident and the cohesive failure mechanism during opening of the packaging has to occur without zipping, fiber tear or angel hairs. This means that the peel trace should be smooth and exempt from all kind of visual disruptions. Furthermore, the film must be peelable on various substrates.

In order to meet most of these criteria, a seal layer comprising an incompatible polymer blend or the introduction of an intermediate layer between the seal layer and the (multi)layer film have been disclosed.

<CIT> discloses a heat-sealable polymeric film comprising an outer, polymeric heat-seal layer, an intermediate polymeric layer in contact with the heat-seal layer, the intermediate polymeric layer being capable of cohesively splitting when the heat-sealed film is peeled, and at least one further polymeric layer. The heat-seal layer is obtained from a blend containing <NUM> to <NUM> % by weight of polyethylene or ethylene/propylene copolymer and <NUM> to <NUM> % by weight of an additive such as an alphamethyl styrene resin, a vinyltoluene/alpha-methyl styrene resin or a modified aromatic resin. The intermediate layer comprises a blend of <NUM> to <NUM> % by weight of polyethylene or ethylene/propylene copolymer and <NUM> to <NUM> % by weight of talc, wherein polyethylene or ethylene/propylene copolymer partially may be replaced by an incompatible polymer.

<CIT> discloses a heat-sealable coextruded multilayer packaging film peelable on a variety of substrates including polyethylene, polypropylene, polyester, polyvinyl chloride, polyamide, polyacrylonitrile and paper, comprising at least one peelable layer from a pre-compounded precursor formulation containing <NUM> to <NUM> % by weight of an ethylene homo and/or copolymer; <NUM> to <NUM> % by weight of a styrene homo or/and copolymer; and <NUM> to <NUM> % by weight of a thermoplastic elastomeric styrene-butadiene-styrene block copolymer. The peel force in the peelable seal film is modified by the presence of <NUM> to <NUM> % by weight of homogeneously branched linear low density polyethylene and/or polybutene-<NUM>. D3 however does not deal with clarity or haze of the seal layer.

While a consistent peel force over a wide temperature range in combination with a clean peel is obtained, a particular problem is faced related to the optical properties.

Thin film extrusion blends of incompatible polymers in general result in hazy films, related to the differences in refractive index, thus limiting their usage in applications where optical aspects and transparency is not critical. In medical applications such as the packaging of pre-filled syringes, where clear look-through is important and where clear bottom webs, like glycol modified polyethylene terephthalate, are applied, hazy seal layers hardly can be permitted.

Therefore it is important to develop a heat-sealable coextruded multilayer packaging film comprising a transparent heat-seal layer peelable on a variety of substrates by cohesively splitting.

The present invention aims to provide heat-sealable multilayer film for the production of heat-sealable packages that do not present the drawbacks of the heat-sealable films of the state of the art.

The present invention aims to provide heat-sealable multilayer film for the production of heat-sealable packages, said heat-sealable multilayer film having a cohesively peelable seal layer with increased clarity and reduced haze relative to the state of the art peelable seal layers.

The present invention discloses a heat-sealable multilayer film (A) comprising:.

Preferred embodiments of the present invention disclose one or more of the following features:.

The present invention further discloses a heat-sealed package comprising the heat-sealable multilayer film (A) and a substrate (B), in contact with the heat-seal layer (b), said substrate (B) comprising at least one layer selected from the group consisting of polyamide, polyester, polycarbonate, polyvinyl chloride, polypropylene, polyethylene, polyacrylonitrile, polyester-glycol copolymer, ethylene vinyl acetate and high impact polystyrene.

Preferred embodiments of the heat-sealed package of the present invention disclose one or more of the following features:.

The present invention further discloses a method for producing the heat-sealed package, comprising contacting the substrate (B) with the heat-seal layer (b) of the heat-sealable multilayer film (A) and sealing at a temperature comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM> using a dwell time comprised between <NUM> and <NUM> seconds, preferably between <NUM> and <NUM> seconds and a pressure comprised between <NUM> and <NUM> kPa, preferably between <NUM> and <NUM> kPa, and cooling down the heat-sealed package.

Preferred embodiments of the method for producing the heat-sealed package of the present invention disclose:.

The present invention further discloses the use of the heat-sealed package for medical packaging application.

According to the present invention there is provided a heat-sealable multilayer film (A) comprising a support layer (a), said support layer (a) comprising at least one polymeric layer and a peelable seal layer (b) of a polymer blend comprising an ethylene polymer, a styrene polymer and a styrene-butadiene block copolymer complying with specific rheological properties, as further defined, said seal layer (b) being characterized by a cohesively splitting upon peeling and additionally by an increased clarity and a reduced haze relative to the state of the art seal layers.

The peelable seal layer (b) of the present invention comprises a polymer blend comprising:.

<NPL>, the elastic or storage compliance is given by: <MAT> wherein:.

σ<NUM>, γ<NUM> and δ vary with the oscillation frequency and are common output of dynamic rheology measurements.

In order to carry out the measurements, the styrene butadiene block copolymer crumbs are pressed into a disk with a diameter of <NUM> and a height of <NUM>.

By "ethylene polymer", the present invention means homopolymers of ethylene and copolymers made from the copolymerization of ethylene and one or more ethylenically unsaturated monomers selected from the group consisting of (meth)acrylate esters, vinyl alkanoates and alpha olefins.

Polyethylene of the peelable heat-seal layer (b) is preferably selected from the group consisting of low density polyethylene, linear low density polyethylene, high density polyethylene, poly(ethylene vinylacetate), poly(ethylene methyl acrylate) and poly(ethylene butyl acrylate). Polyethylene of the pealable heat-seal layer (b) more preferably is low density polyethylene.

By "styrene polymer", the present invention means general-purpose (also known as glass-clear) polystyrene (GPPS), high impact polystyrene (HIPS), anionically polymerized polystyrene and the copolymers made from the copolymerization of styrene and one or more monomers selected from the group consisting of alpha-methylstyrene, acrylonitrile, butadiene and (alkyl) (meth)acrylate.

By "styrene-butadiene block copolymer", the present invention means any type of block copolymer, such as a linear, a di-block, a radial or a branched block copolymer, preferably prepared by the anionic polymerization method. The styrene/butadiene block copolymers for being used in the heat-seal layer (b) of the present invention, preferably have a styrene content of <NUM> % by weight or more, more preferably between <NUM> and <NUM> % by weight, most preferably between <NUM> and <NUM> % by weight and further are characterized by a weight average molecular weight (Mw) preferably comprised between <NUM> kDa and <NUM> kDa, more preferably between <NUM> kDa and <NUM> kDa as determined by gel permeation chromatography in tetrahydrofuran against polystyrene standards.

The styrene-butadiene block copolymer of the present invention preferably is a thermoplastic elastomer copolymer, characterized in that the average of the elastic or storage compliance, describing its elastic properties, measured at a frequency of <NUM> rad/s (J'<NUM>) at a temperature of <NUM> and the elastic or storage compliance, measured at a frequency of <NUM> rad/s (J'<NUM>) at a temperature of <NUM>, is at least <NUM>-<NUM> Pa-<NUM>, preferably at least <NUM><NUM>-<NUM> Pa-<NUM>.

The inventors have surprisingly found that the average of elastic or storage compliance at a frequency of <NUM> rad/s and <NUM> rad/s is the key parameter for the seal layer (b) benefitting from an increased clarity and a reduced haze, as measured according to ASTM <NUM>-<NUM> with a Haze Gardner from Braive Instruments, wherein:.

The seal layer (b) of the heat-sealable multilayer film (A) of the present invention is characterized by a clarity percentage of at least <NUM> %, preferably of at least <NUM> %, more preferably of at least <NUM> % and a haze percentage of less than <NUM> %, preferably of less than <NUM> %, more preferably of less than <NUM> %, wherein clarity and haze are measured according to ASTM D-<NUM>.

The peelable heat-seal layer (b) of the multilayer film (A) of the present invention is cohesively splitting upon peeling. By cohesively splitting the present invention means splitting through the heat-sealed layer (b), i.e. an internal rupture of the seal layer (b), contrary to adhesively splitting involving separation at the boundary between seal layer (b) and support layer (a) or between seal layer (b) and the substrate (B) to which it is sealed.

The seal layer (b) of the heat-sealable multilayer film (A) of the present invention is characterized by an increased clarity percentage and a reduced haze percentage, measured according to ASTM <NUM>-<NUM>, relative to the clarity percentage and haze percentage of a heat-sealable layer of the prior art comprising:.

The increase in clarity percentage relative to an identical seal layer composition except for the styrene-butadiene block copolymer type, is at least <NUM> %, preferably at least <NUM> %, more preferably of at least <NUM> % and is given by:
<NUM>. (clarity % invention / clarity % prior art).

The reduction of haze percentage relative to an identical seal layer composition except for the styrene-butadiene block copolymer type, is at least <NUM> %, preferably of at least <NUM> %, more preferably of at least <NUM> % and is given by:
<NUM>. (haze % pior art - haze % invention) / haze % prior art.

A proper selection of the styrene-butadiene block copolymer allows for an increase in clarity percentage, relative to an identical seal layer composition except for the styrene-butadiene block copolymer type, of at least <NUM> % and even at least <NUM> % and a reduction in haze percentage of at least <NUM> % and even at least <NUM> %.

In general, the increase in clarity percentage and the reduction in haze percentage, as defined in the present invention, is obtained from a styrene butadiene block copolymer characterized by an average of the elastic or storage compliance at a frequency of <NUM> rad/s and the elastic or storage compliance at a frequency of <NUM> rad/s, which is at least <NUM> times, preferably at least <NUM> times, more preferably at least <NUM> times higher versus a prior art styrene butadiene block copolymer where a haze percentage of more than <NUM> % and a clarity percentage of less than <NUM> % is observed for the peelable seal layer (b) comprising said prior art styrene butadiene block copolymer.

The heat-sealable multilayer film (A) of the present invention comprises a support layer (a), said support layer (a) preferably comprising at least one polymer layer selected from the group consisting of polyamide, polyester, polycarbonate, polyvinyl chloride, polypropylene, low density polyethylene, medium density polyethylene, polyester-glycol copolymer and ethylene vinyl acetate.

The support layer (a) in general is characterized by an overall thickness comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>.

The heat-sealable multilayer film (A) in general is characterized in that the peelable seal layer (b) represents <NUM> % or less, preferably <NUM> % or less, more preferably <NUM> % or less of the total thickness of (A).

The heat-sealable multilayer film (A) is characterized in that the thickness of the peelable seal layer (b) is comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM>.

The heat-sealable multilayer film (A) of the present invention is used for the production of peelable heat-sealed packages through heat sealing said multilayer film (A) to a substrate (B).

Said substrate (B) being part of the heat-sealed package according to the present invention comprises one or more polymeric films selected from the group consisting of polyamide, polyester, polycarbonate, polyvinyl chloride, polypropylene, polyethylene, polyacrylonitrile, polyester-glycol copolymer, ethylene vinyl acetate and high impact polystyrene.

Optionally, the substrate (B) may be metallized or comprise a metallic foil, a coated paper or a fibrous substrate such as uncoated paper, paperboard, cellulosic non-woven, polymeric non-woven or woven fabric.

Optionally, the substrate (B) comprises a peelable seal layer said layer comprising:.

The substrate (B) for being used in the heat-sealable package may be the heat-sealable multilayer film (A) of the present invention.

The heat-sealable multilayer film (A) of the present invention has the particularity of being heat sealable to a substrate (B) comprising a low density polyethylene, medium density polyethylene, or glycol modified polyethylene terephthalate such as <NUM>,<NUM>-cyclohexanedimethanol modified polyethylene terephthalate (Eastar™ Copolyester EN001 from Eastman) layer as the contact layer to the peelable layer (b) of the heat-sealable multilayer film (A). For the particular case of glycol modified polyethylene terephthalate, plasma treatment of the peelable seal layer (b) is recommended.

Peelable packages produced from the multilayer films (A) and the above substrates (B) have shown peel strengths, for separating the multilayer film (A) and the substrate (B), of <NUM> N/<NUM> or less, preferably <NUM> N/<NUM> or less, more preferably <NUM> N/<NUM> or less, the rupture being internal the peelable seal layer (b). Peeling forces are measured with an Instron tensile tester at a <NUM> ° peel angle with an opening speed of <NUM>/min.

Multilayer films in accordance with the present invention can be produced by known methods. However, it is generally preferred to produce them by coextruding melts of the polymers and additives required for the various layers of the final films, followed by cooling to solidify the polymers in the form of a film.

Films in accordance with the present invention can also be produced by coextrusion coating of the heat-seal layer (b) on to a suitable support layer (a).

The heat-sealable multilayer film (A) of the present invention may be used for the production of peelable heat-sealed packages through heat sealing said multilayer film to a substrate at a temperature comprised between <NUM> and <NUM>, preferably between <NUM> and <NUM> using a dwell time comprised between <NUM> and <NUM> seconds, preferably between <NUM> and <NUM> seconds and a pressure comprised between <NUM> and <NUM> kPa, preferably between <NUM> and <NUM> kPa.

For the particular case where the substrate (B) is the heat-sealable multilayer film (A) of the present invention, the peelable seal layer (b) of the multilayer film (A) is contacted and sealed to the peelable seal layer (b) of the substrate (B).

The heat-seal layer (b) of the multilayer film (A) preferably is subjected a plasma treatment, more preferably a corona plasma treatment, at a frequency power in the range of <NUM> to <NUM>, adjusted to provide a surface energy of at least <NUM> mN/m, preferably of at least <NUM> mN/m, more of at least <NUM> mN/m, according to ASTM D2578.

Preferably the heat-seal layer (b) is subjected to a plasma treatment just before contacting and heat sealing to the substrate (B).

Plasma treatment may be omitted for the particular case where the contact layer of substrate (B) to the peelable layer (b) of the heat-sealable multilayer film (A) is a polyethylene, a polypropylene or the peelable seal layer (b).

The following illustrative examples are merely meant to exemplify the present invention but is not destined to limit or otherwise define the scope of the present invention.

Films (A) were extruded at a net width of <NUM> and with a total thickness of <NUM> by using a <NUM> <NUM>-layer Egan-Davis conical blown film die. Each layer had a separate extruder. The peelable layer (b), with composition as in Table <NUM>, was extruded with a <NUM> W&H single screw extruder at the inside of the bubble and with a layer thickness equal to <NUM> % of the total film thickness.

The heat-sealable multilayer film (A) subsequently was contacted and sealed to a substrate (B), said substrate (B) being either the heat-sealable multilayer film (A), the peelable layer (b) of the multilayer films contacting each other, or a <NUM> thick glycol modified polyethylene terephthalate sheet, the peelable layer (b) of the heat-sealable multilayer film (A) contacting the glycol modified polyethylene terephthalate sheet (B). For the latter case, the peelable layer (b) of the heat-sealable multilayer film (A) was Corona treated, providing a surface energy of about <NUM> mN/m.

The multilayer films were sealed with a Kopp sealer; the upper jaw was heated at <NUM> while the lower contained a shore 60A rubber at room temperature. Sealing was performed at a <NUM> kPa pressure with a dwell time of <NUM> second. In order to avoid elastic deformation the film was protected against the upper jaw with an oriented polyethylene/polyethylene terephthalate laminate of <NUM> thickness.

Peeling forces were measured with an Instron tensile tester at a <NUM> ° peel angle with an opening speed of the clamps equal to <NUM>/min and shown in Table <NUM>.

The clarity and haze were measured with a Haze Gardner from Braive Instruments NV, according to ASTM <NUM>-<NUM> and shown in Table <NUM>.

Small-amplitude oscillatory shear flow measurements were performed on styrene-butadiene block copolymer, in the linear visco-elastic region with the TA Instruments AR2000 dynamic plate-plate rheometer installed at <NUM>. The plate diameter is <NUM> and the applied die-gap was <NUM>. Prior to measurement, styrene-butadiene crumbs were pressed in disks with the diameter equal to the rheometer plates. The elastic or storage compliance (J') was measured at <NUM> and <NUM> rad/s; the average is shown in Table <NUM>.

In Table <NUM>, Examples <NUM>, <NUM>, <NUM> and <NUM> are comparative [(J'<NUM> + J'<NUM> )/<NUM> < <NUM>-<NUM> Pa-<NUM>]: they all are characterized by a haze percentage above <NUM> % and a clarity percentage of less than <NUM> %, contrary to Examples <NUM>, <NUM>, <NUM> and <NUM> [(J'<NUM> + J'<NUM> )/<NUM> > <NUM>-<NUM> Pa-<NUM>], characterized by a haze percentage lower than <NUM> % and a clarity percentage of more than <NUM> %.

Comparing Example <NUM> (according to the invention) to Example <NUM> (comparative) shows an increase in clarity percentage of more than <NUM> % and a haze percentage reduction of more than <NUM> %.

From columns <NUM> and <NUM>, it appears that the selection criteria for clarity and haze [i.e. (J'<NUM> + J'<NUM>)/<NUM> of the styrene-butadiene block copolymer > <NUM>-<NUM> Pa-<NUM>] has no significant influence of the peel strength.

Claim 1:
A heat-sealable multilayer film (A) comprising:
a) a support layer comprising at least one polymeric layer;
b) a peelable seal layer of polymer blend, said blend comprising:
- <NUM> to <NUM>% by weight of an ethylene polymer;
- <NUM> to <NUM>% by weight of a styrene polymer; and
- <NUM> to <NUM>% by weight of a styrene-butadiene block copolymer, characterized in that the average of the elastic or storage compliance at a frequency of <NUM> rad/s (J'<NUM>) and the elastic or storage compliance at a frequency of <NUM> rad/s (J'<NUM>) is at least <NUM>-<NUM> Pa-<NUM>, preferably at least <NUM><NUM>-<NUM> Pa-<NUM> measured at a temperature of <NUM> on a dynamic plate/plate rheometer for a plate diameter of <NUM> and a die gap of <NUM>, wherein: <MAT> with:
tan δ = G"/G' ;
G" being the viscous or loss modulus; and
G' being the elastic or storage modulus.