EP4334132A1

EP4334132A1 - Multilayer stretch film with cling properties

Info

Publication number: EP4334132A1
Application number: EP22728685.3A
Authority: EP
Inventors: Marnix Elmar BUIT; Willem Gerard VAN BUITEN
Original assignee: Trioworld Apeldoorn BV
Current assignee: Trioworld Apeldoorn BV
Priority date: 2021-05-05
Filing date: 2022-05-05
Publication date: 2024-03-13
Also published as: NL2028156B1; WO2022235161A1

Abstract

The present invention relates to a multilayer stretch film comprising at least a first stretchable layer (i) with slip properties comprising a propylene random copolymer containing in the range of from 2 up to 10 wt% ethylene, and a second stretchable layer (ii) with cling properties comprising a combination of metallocene linear low density polyethylene (mLLDPE) and in the range of from 5 up to 15 wt% polypropylene plastomer. The multilayer stretch film, when being used to wrap goods, e.g. on pallets, these goods can be placed against each other and then moved freely past each other without hindrance to the films. The multilayer stretch film has preferably an unwinding force below 5 kg and preferably an unwinding noise from a supplier roll below 85 dB. The present invention furthermore relates to the method of producing such a multilayer stretch film, using cast extrusion or blow extrusion.

Description

Multilayer stretch film with cling properties

Field of the invention

The present invention relates to a multilayer stretch film, particularly a multilayer stretch film having improved properties over existing stretch films. Such multilayer stretch films are widely used in stretch-cling applications for bundling and stretch wrapping of articles.

Background of the invention

Stretch wrap or stretch film is a highly stretchable plastic film that is wrapped around items. The elastic recovery keeps the items tightly bound. Stretch films that self-seal when portions are overlapped are known as cling films. It is frequently used to unitize pallet loads. Types of stretch film include bundling stretch film, hand stretch film, extended core stretch film, machine stretch film and static dissipative film. The most common stretch wrap material is linear low- density polyethylene or LLDPE, which is produced by copolymerization of ethylene with alpha- olefins, the most common of which are butene, hexene and octene. The use of higher alpha-olefins (hexene or octene) gives rise to enhanced stretch film characteristics, particularly in respect of elongation at break and puncture resistance. Other types of polyethylene and polypropylene can also be used. Many films have about 500% stretch at break but are only stretched to about 100 - 300% in use. Once stretched, the elastic recovery is used to keep the load tight. US 7,572,519 B2, the contents of which are incorporated herein by reference, describes a multilayer stretch film having cling properties. The multilayer stretch film comprises at least a first layer (i) with cling properties comprising an olefin (co)polymer composition, a second layer (ii) with slip properties comprising another olefin (co)polymer composition and at least one core layer (iii) comprising an olefin (co)polymer composition between the first layer (i) and the second layer (ii). The outer surface of the first layer (i) with cling properties is provided with a non-stretchable thin film of one or more slip agent(s) covering at least 50% up to 100% of the outer surface of the first layer (i), lowering the cling properties of the unstretched composite film. The thin film of slip agent(s) is provided either by direct application of the slip agent(s) onto the outer surface or by use of slip agent(s) having limited compatibility with the (co)polymer composition as additive in the (co)polymer composition of the first layer (i). The limited compatibility of the slip agent(s) with the (co)polymer composition of the first layer (i) results in migration of the slip agent(s) to the outer surface and formation of a nonstretchable thin film at the outer surface of the first layer (i). The multilayer stretch film is preferably composed substantially entirely of ethylene or propylene (co)polymer compositions or mixtures thereof. In this document slip properties are defined as ‘non- cling’ properties, i.e. a film having a low value of cling (e.g. less than 5 g/cm as measured according to ASTM D4649). Conversely, cling properties are defined in terms of higher values of cling according to ASTM D4649, e.g. 30-180 g/cm, 50-150 g/cm, 50-80 g/cm, 80-160 g/cm or 90- 130 g/cm.

A disadvantage of the above multilayer stretch film is that the outer layer of the film when stretched and in use for packaging of goods is not sufficiently smooth. This results in difficulties when more wrapped pallets are close to each other as they stick to each other and they cannot be easily removed or replaced. On the other hand, if the outer layer is too smooth very often there is insufficient cling to package goods.

Accordingly, there is a demand for stretch film with cling properties that have a smooth surface when in use but still have good cling properties so that the self seal properties are sufficient when portions are overlapped. There is furthermore a demand for stretch film with cling properties, which can be suitably used for stretch wrapping operations of articles or a group of articles whereby the unwinding force and unwinding noise from a supplier roll are suitably low. There is also a demand for simplification of the process to produce such films.

Summary of the invention

It is an object of the present invention to provide a multilayer stretch film comprising at least a cling layer and a slip layer that in use has a good smooth outside to prevent clinging to other wrapped articles. It is a further object of the present invention to provide a multilayer stretch film that has a good balance of the desired mechanical properties. It is furthermore an object of the present invention to provide a multilayer stretch film exhibiting an improved low noise level during unwinding from a supplier roll and wrapping goods but having also sufficient cling properties to be useful for wrapping. It is yet another object of the present invention to develop a practical manufacturing process that can be easily scaled up.

Accordingly, the present invention relates to a multilayer stretch film comprising at least a first stretchable layer (i) with slip properties comprising a propylene random copolymer containing in the range of from 2 up to 10 wt% ethylene, and a second stretchable layer (ii) with cling properties comprising a combination of metallocene linear low density polyethylene (mLLDPE) and in the range of from 5 up to 20 wt% polypropylene plastomer.

The present invention also relates to a method of producing the above described multilayer stretch film, using cast extrusion or blow extrusion. Detailed description of the invention

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

The term “polypropylene random copolymers” as used herein, includes thermoplastic resins produced through the polymerization of propylene, with ethylene and/or butene bonds introduced in the polymer chain. The resins provide a broad range of characteristics, and are used in a wide range of applications. According to the present invention, the polypropylene random copolymers are produced comprising besides propylene at least within the range of from 2 up to 10 wt% ethylene.

The term “metallocene linear low density polyethylene” as used herein, includes LLDPE made by the use of metallocene catalysts. Linear low density polyethylene is a polymer with significant numbers of short ramifications, commonly made by co-polymerization of ethylene with long-chain olefins. The linearity of LLDPE results from the different manufacturing processes of LLDPE and LDPE. Generally, LLDPE is produced at lower temperatures and pressures by co polymerization of ethylene and alpha-olefins such as butene, hexene or octene. The co polymerization process produces an LLDPE polymer having a narrower distribution in the molecular weight than LDPE and in combination with the linear structure, significantly other properties.

The term “smooth” as used herein, includes that the outer layers are slippery and that goods packed with the film with smooth outer layer can easily slide past each other.

Throughout the application the term “improved functional properties” may mean increased mechanical properties. Alternatively or additionally such improvements may include a reduced “neck-in”, a higher resistance to puncture, or a higher maximum stretch or elongation.

Alternatively or additionally improved functional properties may comprise other advantageous characteristics which allow the stretch film to be used for packaging goods, such as a high “force on load”, a high “load stability”, a high consistency, less breakage per 1000 pallets and or less breakage per roll of stretch film while wrapping pallets. Throughout the application the term “inner layer” and “core layer” are the same layer, being the one or more layers between the two outer layers of the multilayer stretch film.

Throughout the application the term “slip properties” denotes a relatively low value of cling according to ASTM D4649 and a relatively low coefficient of friction (COL) as measured according to ASTM D1894. Throughout the application the term “cling properties” denotes a relatively high value of cling as measured in accordance with ASTM D4649

The present invention is a novel and strong multilayer stretch film with cling properties that has a smooth surface when in use but still has good cling properties so that the self seal properties are sufficient when portions are overlapped. Furthermore, when used in wrapping operations of articles or a group of articles, the unwinding force and unwinding noise from a supplier roll with the multilayer stretch film of the invention are suitably low.

The multilayer stretch film comprises a first stretchable layer (i) with slip properties comprising a propylene random copolymer containing in the range of from 2 up to 10 wt% ethylene, preferably in the range of from 3 up to 8 wt% ethylene, more preferably in the range of from 3 up to 6 wt% ethylene. One of the advantages of this amount of ethylene in the propylene random copolymer is that this grade has controlled rheology, which makes it easier to process. Furthermore, it forms a smooth layer when being used, but still comprises sufficient cling properties in combination with the second stretchable layer (ii) with cling properties according to the invention.

Preferably, the second stretchable layer (ii) with cling properties of multilayer stretch film comprises in the range of from 6 up to 15 wt% polypropylene plastomer, more preferably in the range of from 6 up to 10 wt% polypropylene plastomer. The properties of this layer are such that in combination with the first stretchable layer (i) with slip properties the correct balance between smoothness and cling has been found.

Advantageously, the second stretchable layer (ii) with cling properties of the multilayer stretch film comprises propylene plastomer with a density (as measured according to ASTM D792) in the range of range 0.840g/cm3 up to 0.890g/cm3 and a melt mass flow rate (as measured according to ASTM D1238) in the range of from 5 up to 15g / 10 min.

Advantageously, the second stretchable layer (ii) with cling properties of the multilayer stretch film comprises a (m)LLDPE copolymer comprising butene, hexene or octene with a density in the range of from 0.900g/cm3 up to 0.922g/cm3 and a melt mass flow rate in the range of from 1.0 g/10 min up to 15 g/10 min.

The COF (Coefficient of Friction) is a value that represents the relationship between the force of friction and the normal force between two objects. It is an important value for companies that manufacture stretch films for packaging goods. Advantageously, the multilayer stretch film has the non-cling layer (or slip layer) with a static coefficient of friction (COF) of < 2.5 as measured according to ASTM D1894. The composition of the core layer of the multilayer stretch film is less relevant for the cling properties and the smoothness of the film. However, the core layer is still important for many other properties of the multilayer stretch film, such as exertion of a high force on goods to be packaged, in order to wrap them tightly together. Another example is that the stretch film provides a high “load stability”. When subjected to accelerations or decelerations, e.g. during transport, the stacked goods should not slide off or tilt off the pallet, and after such acceleration or deceleration the stack of goods should preferably return to its original position and shape.

Another important design consideration is the amount of stretch or elongation that the film allows for. This may be measured in accordance with ASTM D882. If the amount of stretch is high, a relatively long wrapping length is obtained for each unit length of film. Similarly, the stretch film should preferably have a relatively small amount of contraction in the transversal (i.e. width) direction when it is being stretched to wrap around goods to be packaged. When the film is stretched in lengthwise direction it will contract in both width direction and in thickness direction. If the contraction in width direction - the so-called “neck in” - is small, a relatively large surface may be covered by each length of film. Contraction in the thickness direction is, up to a certain point, desired. And finally, tear resistance under conditions of use is an important functional property of the stretch film. The film should still have a high tear resistance after it has been stretched during wrapping, i.e. in its stretched condition.

According to the invention, it is preferred that the multilayer stretch film further comprises at least one core layer (iii) comprising an olefin (co) polymer composition present between the first layer (i) and the second layer (ii). Preferably, the core layer comprises mLLDPE comprising butene, hexene or octene copolymer. More preferably, the core layer further comprises polypropylene random and/or homo and or block copolymer. Even more preferably, the core layer comprises 5 - 30 wt% polypropylene and 70 - 95 wt% mLLDPE comprising butene, hexene or octene copolymer.

In yet another embodiment of the multilayer stretch film according to the invention more core or inner layers are present. Preferably, at least one second core layer is present in the multilayer stretch film. The at least one second core layer preferably comprises at least 70% PP plastomer by weight, more preferably at least 80%, even more preferably at least 90%, most preferably substantially 100%. Such an amount of PP plastomer may improve performance of the stretch film.

Applicant has found that a stretch film including these materials in inner layers may show improved functional properties. In another embodiment, the at least one first core layer may comprise at least one of a polypropylene (PP) terpolymer; a polypropylene (PP) plastomer; a polypropylene-butene random copolymer. Additionally or alternatively the at least one second core layer may comprise at least one of a polypropylene (PP) terpolymer; a polypropylene (PP) plastomer; a polypropylene-butene random copolymer. A PP terpolymer may here mean any suitable polypropylene based terpolymer. A PP plastomer may here mean any suitable PP plastomer, more specifically all suitable plastomeric polypropylene polymers. A polypropylene- butene random copolymer may mean a random copolymer comprised of propylene and butene monomers.

In one embodiment of the multilayer stretch film, the at least one first core layer comprises a PP terpolymer and/or a polypropylene -butene random copolymer, and the at least second core layer comprises a PP plastomer. Although the PP terpolymer and or the polypropylene -butene random copolymer in the first core layer(s) and the PP plastomer in the second core layer(s) each provide improvements by themselves, the applicant found that the combination of these materials in the first and second core layers has a greater effect than the sum of the individual improvements.

In a further preferred embodiment of the multilayer stretch film, the PP terpolymer may comprise polypropylene (PP), polyethylene (PE) and polybutene (PB). This combination of polyolefines has been found to yield improvements in the functional properties of the film. In yet another preferred embodiment of the multilayer stretch film according to the invention the multilayer stretch film has a thickness of between 10 pm and 100 pm, preferably between 12 pm and 50 pm, more preferably between 15 pm and 30 pm, most preferably of approximately 20 pm. A film having such a thickness may have an appropriate thickness to provide sufficient strength whilst still being easy to handle. A thinner film may not provide sufficient strength, and a thicker film may involve an inefficient use of material and may not be able to be elastically extended by large enough amounts during wrapping of e.g. goods on a pallet. The thickness of all core layers may be substantially equal, or the thickness of all core layers may differ.

In yet another embodiment of the multilayer stretch film according to the invention the core layers are essentially equal in thickness. The first and second core layers being of the same thickness may balance their advantageous properties which may result in a film with improved functional properties.

Alternatively, the thickness of the core layers may be different. In particular, one of the first core layer(s) and the second core layer(s) may have a thickness that is approximately twice the thickness of the other. A thickness ratio of 1:2 between the first and second inner layers may result in the enhancement of one or more functional properties. Such a layout of a multilayer stretch film may provide an adequate balance of bulk properties provided by the core layers.

The use of at least one core layer may allow for a certain structural rigidity, whilst the additional core layers may provide additional desired characteristics, amongst which may be improved functional properties. The core layer may be a mLLDPE layer (metallocene Linear Low- Density Polyethylene). The core layer may be comprised of mLLDPE only.

In yet another embodiment of the multilayer stretch film according to the invention the first stretchable layer (i) with slip properties makes up between 2% and 20% of the thickness of the stretch film, preferably approximately 10%; the second stretchable layer (ii) with cling properties makes up between 2% and 20% of the thickness of the stretch film, preferably approximately 10%; the core layer makes up between 60% and 96% of the thickness of the stretch film, preferably approximately 80%. A multilayer stretch film with such a layout may possess improved function properties. More specifically a multilayer stretch film with such a layout may provide an advantageous balance between the properties of the different layers. Alternatively or additionally a multilayer stretch film with such a layout may provide a synergistic effect between the different layers.

In yet another embodiment of the multilayer stretch film according to the invention the multilayer stretch film further comprises at least one group of microlayers, in which group of microlayers: the at least one first inner layer makes up at least one first microlayer; and the at least one second inner layer makes up at least one second microlayer, wherein said at least one first microlayer and said at least one second microlayer are adjacent. Such an at least one group of microlayers may provide an enlarged synergistic effect of microlayers and/or may distribute the properties of different layers along the thickness of the multilayer stretch film. Accordingly or alternatively such an at least one group of microlayers may improve functional properties of the multilayer stretch film.

There may be one group of microlayers, wherein one first microlayer and one second microlayer lie adjacent, i.e. flat upon each other. Alternatively there may be more than one such groups of microlayers, for instance two groups of microlayers. The groups of microlayers do not need to be identical, however they may be identical or mirrored.

In yet another embodiment the multilayer stretch film further comprises a plurality of first microlayers and a plurality of second microlayers, wherein the first and second microlayers are arranged in an interspersed fashion, preferably in an alternating fashion. In this way the properties of the various layers are distributed or preferably uniformly distributed. There may be an equal amount of first microlayers and second microlayers, or there may be one more or one less first microlayer than second microlayers in one such alternating arrangement of microlayers.

Alternatively the number of first microlayers and the number of second microlayers may be different. There may be a plurality of first microlayers and or a plurality of second microlayers in one group of microlayers, or in more than one group of microlayers. In yet another embodiment of the multilayer stretch film according to the invention the multilayer stretch film comprises at least two groups of microlayers, separated by at least one core layer.

The use of a core layer may allow for a certain structural rigidity, whilst the two groups of microlayers adjacent the core, may provide additional desired characteristics, amongst which may be improved functional properties. The core layer may be an mLLDPE layer (metallocene Linear Low-Density Polyethylene). The core layer may be comprised of mLLDPE only. The two groups of microlayers may be substantially equal to each other, or they may substantially be each others mirror image in a plane running through the core layer. Alternatively the two groups of microlayers may not be equal. There may be no other layers between the core layer and the groups of microlayers, or there may be one or more other layers between the core layer and the groups of microlayers. There may be more than two groups of microlayers, for instance there may be 3, 4, 5 or any other number of groups of microlayers. Each group of microlayers may in that case be separated from other groups of core layers by a core layer or some other layer.

In yet another embodiment of the multilayer stretch film according to the invention the multilayer stretch film further comprises: a first sub-skin layer disposed between the first stretchable layer (i) with slip properties and at least one group of microlayers; and a second sub skin layer disposed between the second skin layer and at least one other group of microlayers. The first and second sub-skin layers may provide an interface between the at least one group of microlayers and other layers. The first and second sub-skin layers may provide an increase in functional properties. The multilayer stretch film may be built up of, from one side to the other, a first stretchable layer (i) with slip properties comprising a propylene random copolymer containing in the range of from 2 up to 10 wt% ethylene, a first sub-skin layer, a first group of microlayers, a core layer, a second group of microlayers, a second sub-skin layer, second stretchable layer (ii) with cling properties comprising a combination of metallocene linear low density polyethylene (mLLDPE) and in the range of from 5 up to 20 wt% polypropylene plastomer.

As stated above, the plurality of first microlayers and the plurality of second microlayers may be disposed in groups of microlayers, wherein these groups of microlayers may or may not be of the same thickness.

In yet another embodiment of the multilayer stretch film according to the invention the first stretchable layer (i) with slip properties comprising a propylene random copolymer containing in the range of from 2 up to 10 wt% ethylene, makes up between 2% and 20% of the thickness of the stretch film, preferably approximately 10%; the first microlayer or the plurality of first microlayers combined make(s) up between 20% and 40% of the thickness of the stretch film, preferably approximately 30%; the second microlayer or the plurality of second microlayers combined make(s) up between 20% and 40% of the thickness of the stretch film, preferably approximately 30%; the at least one core layer makes up between 10% and 30% of the thickness of the stretch film, preferably approximately 20%; and the second stretchable layer (ii) with cling properties comprising a combination of metallocene linear low density polyethylene (mLLDPE) and in the range of from 5 up to 20 wt% polypropylene plastomer makes up between 2% and 20% of the thickness of the stretch film, preferably approximately 10%.

The applicant has found that such a multilayer stretch film produces exceptionally high operating capabilities such as a lower neck-in and a higher Elmendorf tear strength both in machine direction and in transversal direction.

In yet another embodiment of the multilayer stretch film according to the invention the multilayer stretch film comprises at least 10 first and second microlayers combined, preferably at least 22 microlayers, more preferably at least 20 first and 20 second microlayers, most preferably at least 46 first and second microlayers. The applicant has noted that such an amount of microlayers may provide an advantageous balance between functional properties, which collectively may thereby be improved.

In the case of an even amount of first and second microlayers combined the first and second microlayers may be separated in two groups of equal amount of microlayers. In the case of 46 first and second microlayers combined the first and second microlayers may be divided into two groups of in total 23 microlayers. The two groups of microlayers may have one first microlayer at their center and may have two outer second microlayers. Alternatively the two groups of microlayers may have one second microlayer at their center and may have two outer first microlayers.

In the case of an even number of microlayers per group of microlayers, the two outer microlayers of that group may be one first microlayer and one second microlayer. In yet another embodiment of the multilayer stretch film according to the invention one of the, or each first microlayer and the, or each, second microlayer has a thickness that is approximately twice the thickness of the other of the or each first microlayer and the or each second microlayer.

A thickness ratio of 2: 1 between the first and second microlayers may result in the enhancement of one or more functional properties. In yet another embodiment of the multilayer stretch film according to the invention the multilayer stretch film has an Elmendorf tear strength in the machine direction according to ASTM 1922-94a of at least 80 kN/m, preferably at least 90 kN/m, more preferably at least 100 kN/m. A multilayer stretch film with such a high Elmendorf tear strength in the machine direction may be especially suitable for packaging goods on a pallet. In yet another embodiment of the multilayer stretch film according to the invention the multilayer stretch film having a stretchability or elongation according to ASTM D882 of maximum 500%, more preferably in the range of from 200 up to 400%.

In a further embodiment of the multilayer stretch film according to the invention the multilayer stretch film has an unwinding force below 5 kg and an unwinding noise from a supplier roll below 85 dB. Applicant has noted that such force and noise are acceptable for use in commercial applications.

Advantageously, the multilayer stretch film of the present invention comprises sufficient cling to prevent endings from flying loose. Advantageously, the multilayer stretch film has sufficient holding force to stabilize both heavy and light loads during transport.

According to the invention, it is preferred that the multilayer stretch film has the non-cling layer with a static coefficient of friction (COF) of <2.5. The COF (Coefficient of Friction) is a value that represents the relationship between the force of friction and the normal force between two objects, and is an important value for companies that manufacture stretch films for packaging goods. Advantageously, the multilayer stretch film, when being used to wrap goods, preferably on pallets, these goods can be placed against each other and then moved freely past each other without hindrance or damage to the films.

Advantageously, the multilayer stretch film of the present invention furthermore comprises an agent being selected from an additional pigment and a UV stabilizer. An additional pigment may be advantageous for varying the colour and/or the opacity of the stretch film further.

The invention also relates to a method of producing a multilayer stretch film according to the invention, using cast extrusion or blow extrusion. Blown films are oriented by virtue of their manufacturing process. Cast films are made by extruding melted resin horizontally through a flat die to create a sheet of material that is pinned to a highly polished chilled roller by means of an air curtain or vacuum box.

Preferably, the method of producing a multilayer stretch film according to the invention, comprises the steps of: a) providing starting materials to extruders; b) heating the starting materials; c) feeding the starting materials through a feedblock, in which the starting materials are combined into a strand; d) widening the strand by stretching the strand in a transversal, first direction using a mouthpiece; e) lengthening the widened strand by stretching the widened strand in a lengthwise, second direction using at least one roller, wherein the starting materials are laid out in layers by the feedblock, the group of layers comprising:

- at least a first stretchable layer (i) with slip properties comprising a propylene random copolymer containing in the range of from 2 up to 10 wt% ethylene, and

- a second stretchable layer (ii) with cling properties comprising a combination of metallocene linear low density polyethylene (mLLDPE) and in the range of from 5 up to 20 wt% polypropylene plastomer. Such a method is advantageous in that it provides a multilayer stretch film that may have improved functional properties. The steps of the method may be performed in any suitable order.

The strand may have transversal dimensions of approximately 1 inch (0.025 m) by approximately 4 inch (0.1 m) upon exiting the feed block. The widened strand may have transversal dimensions of approximately 1 mm (0.001 m) by 4650 mm (4.65 m). The smaller dimension of 1 mm may be called the thickness dimension, the larger dimension of 4650 mm may be called the width dimension. The widened strand may be lengthened using the at least one roller, until the thickness of the widened strand has been reduced until anywhere between 10 pm and 100 pm, preferably between 12 pm and 50 pm , more preferably between 15 pm and 30 pm, most preferably however until the thickness has been reduced to approximately 20 pm. Said at least one roller may rotate with a circumferential velocity that is larger then the velocity of the widened strand leaving the mouthpiece. Said at least one roller may be cooled in order to cool the multilayer stretch film so that it may be rolled up for storage and/or transport.

In another embodiment of the method for producing a multilayer stretch film according to the invention, at least 5 extruders are used, preferably at least 7 extruders, more preferably at least 9 extruders are used. A method involving such an amount of extruders may allow for the production of a multilayer stretch film comprising as many different materials as there are extmders. A stretch film comprising more different materials may be advantageous in that it has improved functional properties.

In yet another embodiment of the method for producing a multilayer stretch film according to the invention the starting materials are heated to a temperature between 150°C and 350°C, preferably between 200°C and 300°C, more preferably between 250°C and 270°C. Such a temperature may be especially suitable for the extrusion of the starting materials. More specifically, such a temperature may be low enough to avoid heating the starting materials past the melting point, whilst being high enough so that the starting materials soften and are still softened enough to be deformed by the mouthpiece and the at least one roller without being reheated.

The starting materials may become substantially solid at a temperature of 120°C. In yet another embodiment of the method for producing a multilayer stretch film according to the invention the method has a throughput of 2000 kg/h to 3000 kg/h, preferably of 2250 kg/h to 2750 kg/h, more preferably around 2500 kg/h. Such a throughput may be high enough for commercial production of multilayer stretch film whilst not exceeding stress limits on the extruders.

A multilayer stretch film produced following the above described method may provide an excellent smooth outside to prevent clinging to other wrapped articles. It is a further object of the present invention to provide a multilayer stretch film that has a good balance of the desired mechanical properties.

The following non-limiting figures show the present invention further.

Figure 1 illustrates an example of a potential cross section of a film.

Figure 2 illustrates a schematic top view drawing of the cling measurement.

Figure 3 illustrates the top view drawing of the smoothness test.

In Figure 1 an example of a potential cross section of a film is illustrated. Layer A represents the cling layer, layer B represents the core layer existing of for example mLLDPE C6, layer D represents a layer containing for example random polypropylene, layer E represents a layer existing of mLLDPE C6 and layer C is the slippery outer layer. To prepare this film, each layer is fed by an individual extruder. The layers D and E are split in the feed block into different polymer streams that are alternating each other.

Figure 2 illustrates a schematic top view drawing of the cling measurement. To measure the cling force of the prepared films the Highlight Film Evaluation Test Stand was used, developed by Highlight Industries, Inc. of Wyoming MI, USA (www.highlightindustries.com). In step 1 a film is wrap-stretched wounded around a steel drum with a flat side and the tail is brushed against the drum. The winding is stopped and the film is cut. In step 2 the layer is laid on the last winding with a decent touch. The brush retracts the end of the tail that is connected to the loadcell. In step 3, the arm with loadcell peels off the film of the underlying layer. The force that is required is measured in grams.

In Figure 3 the top view drawing of the smoothness test is being presented. The smoothness of the films was tested by packing goods on pallets: 3 different pallets of bagged goods were wrapped with the film that is tested. The wrapped pallets are placed softly against each other step 1 and 2). The pallet in the middle is pulled out using a forklift (step 3).

The following, non-limiting examples are provided to illustrate the invention. Example 1

As an example of the present invention multilayer stretch films were produced via cast extmsion. The resins were blended and dosed into an extruder. The melted material was fed through a feedblock and die, to cool down on a chillroll and winded on a core. The prepared films were compared with a reference stretch film. The multilayer stretch films were produced using the materials defined in the table below. The reference film was a multilayer stretch film that is commercially available and widely used. The films had a variable thickness. Extruder A comprised the second stretchable layer (ii) with cling properties and extruder C the stretchable layer (i) with slip properties. The composition of the layers was varied as shown in table 1 below. An example of the films is shown in Figure 1.

Table 1 - Composition of multilayer stretch films

Of these films the smoothness and cling force were measured. The results are in Table 2 below.

To measure the cling force of the prepared films the Highlight Film Evaluation Test Stand was used, developed by Highlight Industries, Inc. (www.highlightindustries.com). Figure 2 illustrates a schematic representation of the measurements. The film is wound on a steel drum with a flat side. The winding is stopped and the film web is cut. This layer is laid on the last winding with a decent touch. Then this layer is peeled off and the force that is needed is measured in grams.

The unwinding noise is measured at 1 meter distance of the unwinding roll, during unwinding at the Highlight Film Evaluation Test Stand with a dB measurement device.

The smoothness of the films was tested by packing goods on pallets: 3 pallets with bagged goods were wrapped on a Mateo A300 wrapping machine (manufactured by Mateo International BV of Deventer, NF - www.matco-international.com) using 270% pre-stretch and a second stretch so that the films were stretched 270% on the pallet. These settings resulted in a stable packed pallet comparable to what is used in the industry. To test the smoothness of the films, the wrapped pallets were placed softly against each other. The pallet in the middle was pulled out using a forklift. The removal was judged using the following valuation:

1 = to be placed back

2 = packable without damage 3 = side slightly broken

4 = broken side

5 = not to be caught Table 2- Results of the cling force, smoothness test and the unwinding noise of the prepared films The results show that a PP outer layer is needed to have a smooth outer layer together with sufficient cling and a low unwinding noise. If the density of the outer PE layer is increased, this will result in a smooth outside layer, but will also result in a loss of cling from the cling agent. If more cling agent is used, the amount of cling will increase, but this will also increase the unwinding noise to dangerous levels. In the above, the invention has been disclosed using examples thereof. However, the skilled person will understand that the invention is not limited to these examples and that many more examples are possible without departing from the scope of the present invention, which is defined by the appended claims and equivalents thereof.

Claims

1. A multilayer stretch film comprising at least a first stretchable layer (i) with slip properties comprising a propylene random copolymer containing in the range of from 2 up to 10 wt% ethylene, and a second stretchable layer (ii) with cling properties comprising a combination of metallocene linear low density polyethylene (mLLDPE) and in the range of from 5 up to 20 wt% polypropylene plastomer.

2. The multilayer stretch film of claim 1, further comprising at least one core layer (iii) comprising an olefin (co) polymer composition present between the first layer (i) and the second layer (ii).

3. The multilayer stretch film of claim 1 or 2, wherein the first stretchable layer (i) with slip properties comprises a polypropylene copolymer containing in the range of from 3 up to 8 wt% ethylene.

4. The multilayer stretch film of any of the previous claims, wherein the second stretchable layer (ii) with cling properties comprises in the range of from 6 up to 15 wt% polypropylene plastomer.

5. The multilayer stretch film of any of the previous claims, wherein the second stretchable layer (ii) with cling properties comprises propylene plastomer with a density of range 0.840g/cm3 up to 0.890g/cm3 and a melt mass flow rate in the range of from 1 g/10 min up to 15 g/10 min.

6. The multilayer stretch film of any of the previous claims, wherein the second stretchable layer (ii) with cling properties comprises a (m)LLDPE copolymer comprising butene, hexene or octene with a density in the range of from 0.900g/cm3 up to 0.922g/cm3 and a melt mass flow rate in the range of from 1.0 g/10 min up to 15 g/10 min.

7. The multilayer stretch film of claims 2-6, wherein the core layer comprises mLLDPE comprising butene, hexene or octene copolymer.

8. The multilayer stretch film of claim 7, wherein the core layer further comprises polypropylene random and or homo and or block copolymer and or ter-copolymer.

9. The multilayer stretch film of claim 8, wherein the core layer comprises 5-30% polypropylene and 70-95wt% mLLDPE comprising butene, hexene or octene copolymer.

10. The multilayer stretch film of any of the previous claims, wherein the non-cling layer has a COF of < 2.5.

11. The multilayer stretch film of any of the previous claims, wherein:

- the first stretchable layer (i) with slip properties makes up between 2% and 20% of the thickness of the stretch film, preferably approximately 10%;

- the second stretchable layer (ii) with cling properties makes up between 2% and 20% of the thickness of the stretch film, preferably approximately 10%;

- the core layer makes up between 60% and 96% of the thickness of the stretch film, preferably approximately 80%.

12. The multilayer stretch film of any of the previous claims, having a thickness between 8 pm and 100 pm, preferably between 10 pm and 50 pm , more preferably between 14 pm and 25pm, most preferably of approximately 17 pm.

13. The multilayer stretch film of any of the previous claims, having a stretchability of maximum 500%, more preferably in the range of from 200 up to 400%.

14. The multilayer stretch film of any of the previous claims, wherein the film has an unwinding force below 5 kg and an unwinding noise from a supplier roll below 85 dB.

15. The multilayer stretch film of any of the previous claims, wherein the film has sufficient cling to prevent endings from flying loose.

16. The multilayer stretch film of any of the previous claims, wherein the film has sufficient holding force to stabilize both heavy and light loads during transport.

17. The multilayer stretch film of any of the previous claims, wherein the film, when being used to wrap goods, preferably on pallets, these goods can be placed against each other and then moved freely past each other without hindrance or damage to the films.

18. The multilayer stretch film of any of the previous claims, wherein the film furthermore comprises an agent being selected from an additional pigment and a UV stabilizer.

19. Method of producing a multilayer stretch film according to any of the preceding claims, using cast extrusion or blow extrusion.

20. Method of producing a multilayer stretch film according to claim 19, comprising the steps of: a) providing starting materials to extruders; b) heating the starting materials; c) feeding the starting materials through a feedblock, in which the starting materials are combined into a strand; d) widening the strand by stretching the strand in a transversal, first direction using a mouthpiece; e) lengthening the widened strand by stretching the widened strand in a lengthwise, second direction using at least one roller, wherein the starting materials are laid out in layers by the feedblock, the group of layers comprising:

- at least a first stretchable layer (i) with slip properties comprising a combination of propylene random copolymer and in the range of from 2 up to 10 wt% ethylene, and

- a second stretchable layer (ii) with cling properties comprising a combination of metallocene linear low density polyethylene (mLLDPE) and in the range of from 5 up to 20 wt% polypropylene plastomer.

21. Method of producing a multilayer stretch film according to claim 20, wherein the starting materials are heated to a temperature between 150°C and 350°C, preferably between 200°C and 300°C, more preferably between 250°C and 270°C.