The present invention concerns a multilayer film and a method of production thereof. The multilayer film has an improved average tear propagation strength.

Polymer films are can be made of a variety of different polymers and manufacturing processes and consequently possess a diverse range of properties which can be tailored to suit the requirements of the applications of use. Typically, specific polymers and film production techniques lend themselves to specific property benefits with no one polymer or production method being the panacea.

Film manufacturing processes fall into three categories; oriented film (either mono or biaxial orientation), cast film or blown film. All these processes involve an extrusion step to form a uniform sheet or annulus of polymer that is then further processed via a stretching step to thin the film down to its desired final state, where it is then wound into continuous reels of film. The fundamental difference between the different production methods is the conditions under which the polymer is stretched and thinned. In an oriented film process, the extruded polymer sheet or annulus is cooled down and then reheated to a temperature where it can be stretched, which causes the polymer chains to become highly oriented under the action of stretching. Orienting the polymer chains significantly enhances properties such as tensile strength, stiffness and clarity but has a negative impact on tear propagation strength. Conversely the cast or melt blown processes both involve stretching the film in a largely molten, liquid or mobile state which introduces significantly lower levels of molecular orientation and consequently the films are typically less stiff than their oriented counterparts but are tougher and have significantly higher tear propagation resistance.

Film tearing during production and use is a well-known problem. While increasing the thickness of a film may help to improve average tear propagation strength, this is not always possible depending on the end use of the film and the optical properties desired. Further, the average tear propagation strength does not increase in direct correlation to the increase in thickness and so can be difficult to predict.

Average tear propagation strength has previously been improved by including components within the multilayer film that can affect the average tear propagation strength by modifying the elasticity or other properties of the film layers. For example, WO20161 12256 discloses a tear resistant film that includes an elastic internal layer, which may comprise a styrene- butadiene-styrene (SBS) block copolymer. W09618678 also discloses a film that comprises a component selected from a list of suitable compounds, such as SBS, which are said to improve tear properties. Similarly, W02006060766 discloses a film that comprises an SBS block copolymeric internal layer.

The properties of the main polymeric components within the film, such as their density, is also known to play a role in improving the average tear propagation strength. For example, W02007104513 discloses a film comprising a multimodal high-density polyethylene composition comprising a low molecular weight polyethylene component and a high molecular weight polyethylene component. EP2310200 also discloses a film with improved tear resistance, which comprises a multilayer film including an ethylene-based polymer with a density greater than or equal to 0.945 g/cc.

It is also known that heat treatment may improve the tear resistant properties of a film. US2008131681 discloses a film with improved tear strength, which comprises a polyethylene matrix containing a polypropylene material, which is warmed during production to the point at which the polyethylene material melts, but the polypropylene material has not melted.

A similar film is disclosed in US2012321866, in which the film comprises a polypropylene matrix that includes a polyethylene material. The film is heated to between the melting point of the two components and is cooled without being stretched.

JP3976489 discloses a film comprising two layers containing different polyester components, which is heat treated to between the melting point of the two components after the film has been oriented, such that the layer with the lower melting point melts and loses its orientation. This is said to improve forming and processability, stability over time, solvent resistance and dimensional stability

The optical properties of a film are important in many film applications. Any modifications to the components of the film or the method of manufacture thereof in order to improve the average tear propagation strength should therefore preferably not detriment the optical properties of said film, such as the haze or gloss.

Thus, there is a need for a film that demonstrates improved average tear propagation strength, preferably while also maintaining the optical properties of films known in the art. According to a first aspect of the present invention, there is provided a coextruded multilayer film comprising a first layer comprising a first polymeric material and a second layer comprising a second polymeric material, wherein the second layer is less oriented than the first layer such that the average tear propagation strength in the machine direction and/or in the transverse direction of the multilayer film is greater than said average tear propagation strength of both an identical film wherein the first and second layers are replaced with a single layer of the same thickness as the first and second layers combined having the same composition as the first layer.

The inventors have surprisingly found that including a less oriented layer in a coextruded film increases the average tear propagation strength of the film. The presence of a more oriented first layer means that the multilayer film maintains the advantageous results of orientation, such as improved mechanical, optical and barrier properties. However, the inclusion of a separate, less oriented second layer has surprisingly been found to increase the average tear propagation strength properties of the multilayer film.

Average tear propagation strength can be measured using a trouser tear test according to ASTM D1938. An“identical film” as referred to herein relates to a film having the same layer structure, for example the same additives and the same additional layers such as skin layers.

Thus, the necessary comparison of average tear propagation strength occurs between a film of the present invention and a film having the same layer structure, except that the first and second layers are replaced by a single layer of the same thickness as the first and second layers combined, having the same composition as the first layer (i.e. the first polymeric material and any additives).

The multilayer film is coextruded. This means that the first and second layers are formed contemporaneously using melt that is extruded from a die. Thus, a contemporaneous stretching process is inherently required during the production of the film as the two layers are coextruded and so have necessarily been formed together before the film is stretched. The contemporaneous stretching process preferably occurs at a temperature at which the first material has a higher amount of residual crystallinity than the second material, thereby resulting in different levels of orientation in the two layers.

This contrasts with the methods known in the prior art, in which an elevated temperature is applied to the film after it has been oriented in order to melt the material with the lower melting point. Different orientation effects are seen within the lower melting point material layer when it is coextruded and stretched at an elevated temperature compared to when it is subsequently heat treated at said elevated temperature.

Additionally, stretching the film of the present invention at the elevated temperature may mean that the polymeric material of the second layer has a single melting peak on an initial heat, as determined by DSC. This would not be seen in films where heat treatment was used to melt the lower melting point material after orientation, as the lower melting point material in this instance would have two melting peaks in the initial heating step.

The different levels of orientation in the film of the present invention may arise from the different physical states of the first and second polymeric materials. Specifically, the second polymeric material may have a lower degree of residual crystallinity than the first material under a certain condition. The certain condition may be a condition used during the manufacture of the multilayer film, particularly a condition used during orientation of the film.

Thus, the term“condition” can refer to the conditions under which the material is quenched from the melt to form a solid sheet or tube for orientation (for polymers which crystallise from the melt on cooling), the conditions under which the sheet/tube is reheated for orientation (time/temperature) and finally the conditions of orientation (temperature, draw ratio and draw speed) in the case of materials that exhibit strain induced crystallisation. There may be a combination of these parameters under which the second polymeric material has a lower degree of residual crystallinity than the first polymeric material.

The contemporaneous stretching process used to manufacture the film may therefore occur under a condition in which the second polymeric material has a lower degree of residual crystallinity than the first material, and so the first layer will be more oriented than the second layer as it contains more residual crystallinity.

The present invention therefore provides a way in which to create differently oriented layers within a coextruded multilayer film. This provides a cheaper, more efficient and more environmentally friendly single process compared to extrusion coating or lamination.

The first polymeric material may have between 30 and 100% residual crystallinity, while the second polymeric material may have between 0 and 15% residual crystallinity under said certain condition. Preferably, the first polymeric material may have between 40 and 90% residual crystallinity, while the second polymeric material may have between 0 and 5% residual crystallinity under said certain condition. The ability to generate significant levels of orientation in the film is dependent upon the polymer properties and/or the stretching conditions. Film-making polymers are typically either semi-crystalline (e.g. polypropylene and most co-polymers thereof and polyethylene and most co-polymers thereof) or amorphous (e.g. G-PET, styrene butadiene rubbers, ethylene- propylene rubbers).

In the case of amorphous polymers, the ability to generate orientation is determined by the temperature impact on chain mobility alone, in that higher temperatures result in less orientation as a consequence of high chain mobility. Amorphous materials have little to no residual crystallinity and the residual crystallinity does not change greatly with conditions such as temperature and pressure.

In the case of semi-crystalline polymers, the ability to orient the molecules is aided by the presence of residual crystallinity in the sheet, which reduces the molecular mobility via entanglements sufficiently to promote chain orientation. Increasing the proportion of molten polymer reduces the residual crystallinity and therefore also reduces the degree of molecular orientation possible.

The first polymeric material and optionally also the second polymeric material may be semi crystalline. Alternatively, the first polymeric material may be semi-crystalline and the second polymeric material may be amorphous.

The second layer may be mostly unoriented. The second layer may be entirely unoriented. The amount of orientation in the second layer can be measured using techniques known in the art, such as a birefringence, birefringent retardation and dichroic ratio analysis.

When the first polymeric material is semi-crystalline and the second polymeric material is amorphous, the contemporaneous stretching process may occur under a condition at which the first polymeric material is not entirely molten, such that it has a greater residual crystallinity than the second, amorphous polymeric material. This will result in the first polymeric material being more oriented than the second polymeric material.

When both the first and second polymeric materials are semi-crystalline, the second polymeric material may have a lower melting point range than the first polymeric material. This can mean that there are conditions at which the second polymeric material is more molten than the first polymeric material. The amount of residual crystallinity is inversely correlated to the degree to which the material is molten, meaning that more molten materials become less oriented during the stretching process. The contemporaneous stretching process may therefore occur under a condition at which the second polymeric material is more molten than the first polymeric material.

By including semi-crystalline polymers with different melting point ranges in a multilayer structure and then carefully selecting the optimal manufacturing conditions, it is therefore possible to generate multilayer films with layers of differing degrees of orientation.

Under the certain conditions at which the second polymeric material has a lower amount of residual crystallinity than the first polymeric material, the second polymeric material may be at least partly molten. Under said conditions, the first polymeric material may also be at least partly molten but must not be entirely molten to ensure that the second polymeric material is more molten than the first.

The melting point range of the first polymeric material may be sufficiently higher than the melting point range of the second polymeric material such that there is a range of temperatures at which the first polymeric material is in an at least partially molten state and the second polymeric material is in a more molten state than the first material. The second polymeric material may be predominantly or completely molten at this range of temperatures. Preferably, more than 50% of the second polymeric material is molten at this range of temperatures and more preferably, more than 75% of the second polymeric material is molten at this range of temperatures.

All of the first melting point range may be higher than the second melting point range. The two ranges may not overlap. Alternatively, the ranges may overlap such that only a portion of the first melting point range is higher than the second melting point range.

The peak of the first melting point range may be higher than the peak of the second melting point range. The peak of the first melting point range may be more than 20°C higher than the peak of the second melting point range. The peak of the first melting point range may be more than 50°C higher than the peak of the second melting point range.

The peak of the first melting point range of the first polymeric material may be above around 150°C, preferably between around 150°C and around 200°C and most preferably around 160°C. The peak of the second melting point range of the second polymeric material may be below around 200°C, preferably below around 150°C and most preferably between around 100°C and around 150°C.

Thus, in one embodiment, the peak of the first melting point range is between around 150°C and around 200°C, while the peak of the second melting point range is between around 100°C and around 150°C.

The melting point range of the first layer comprising the first polymeric material may be different to the melting point range of the first polymeric material. The melting point range of the second layer comprising the second polymeric material may be different to the melting point range of the second polymeric material. This may be the case if additional components are included within the layers.

Thus, the features outlined above in relation to the first melting point range and the second melting point range may equally apply to the melting point range of the first layer and the melting point range of the second layer.

The multilayer film may consist of two layers. The multilayer film may comprise a third layer on the opposite side of the second layer to the first layer. Thus, the second layer may be sandwiched between the third layer and the first layer. The second layer may be a core layer, rather than an outer layer, which has been found to improve the optical properties of the film.

The third layer is preferably semi-crystalline. The third layer may be identical to the first layer. The third layer may comprise the first polymeric material but may include different additional components.

Alternatively, the third layer may be different to the first layer. The third layer may comprise a third polymeric material, wherein there may be a condition under which the residual crystallinity of the third polymeric material is greater than that of the second polymeric material. This may be the same condition under which the residual crystallinity of the first polymeric material is greater than that of the second polymeric material. The second and third layers may therefore have different levels of orientation as a result of a contemporaneous stretching process under said condition. The third layer may be more oriented than the second layer.

The difference in residual crystallinity between the second and third polymeric materials may be due to the third polymeric material being semi-crystalline while the second polymeric material is amorphous. Alternatively, the difference in residual crystallinity between the second and third polymeric materials may be due to the differences in melting point range. The discussion above in relation to the relationship between the first melting point range and the second melting point range applies equally to the relationship between the second melting point range and the third melting point range.

The third polymeric material may have between 30 and 100% residual crystallinity and the second polymeric material may have between 0 and 15% residual crystallinity under said certain condition, and preferably the third polymeric material may have between 40 and 90% residual crystallinity and the second polymeric material may have between 0 and 5% residual crystallinity under said condition. The residual crystallinity of the first polymeric material may fall within the same range as that of the third polymeric material under said condition.

The multilayer film may further comprise skin layers as the outermost layers of the film. The skin layers may be formed on the first and third layers of the film. Suitable skin layers include heat seal polyethylene, polypropylene copolymers or terpolymers.

The first and the third layers may each make up around 25% of the total film thickness. The second layer may make up around 50% of the total film thickness. The ratio of the thicknesses of the first, second and third layers may be 1 :2:1.

The second layer may comprise between around 15 and around 85% of the total film thickness. Preferably, the second layer may comprise between around 40 and around 70% of the total film thickness.

The multilayer film may comprise one or more layers in addition to the first, second and optionally also third layers. The multilayer film structure may be ABA, ACB, ABCD, ABCBA, ABCBD or ABCDE. The additional layers may comprise an adhesive layer (e.g. a pressure sensitive adhesive), an adhesive release layer (e.g. for use as the backing material in the peel plate method for making labels), a tie layer, a primer layer, a print layer, a barrier layer, a peelable layer, an active layer, a cavitated layer, a stiffening layer, a coloured layer and/or a coating layer.

The multilayer film may comprise a fourth layer comprising the second polymeric material. The fourth layer may be adjacent to the second layer or may be separated from the second layer by one or more other layers. The second layer may be the only layer comprising the second polymeric material in the multilayer film. The multilayer film may comprise a fourth layer comprising a fourth polymeric material, wherein the fourth polymeric material has less residual crystallinity than the first polymeric material under a certain condition. This may be the same condition under which the residual crystallinity of the second polymeric material is less than that of the first polymeric material. The first and fourth layers may therefore have different levels of orientation as a result of a contemporaneous stretching process at said condition. The first layer may be more oriented than the fourth layer.

The difference in residual crystallinity between the first and fourth polymeric materials may be due to the first polymeric material being semi-crystalline while the fourth polymeric material is amorphous. Alternatively, the difference in residual crystallinity between the first and fourth polymeric materials may be due to the differences in melting point range. The discussion above in relation to the relationship between the first melting point range and the second melting point range applies equally to the relationship between the first melting point range and the fourth melting point range.

The first polymeric material may have between 30 and 100% residual crystallinity and the fourth polymeric material may have between 0 and 15% residual crystallinity under said certain condition, preferably the first polymeric material may have between 40 and 90% residual crystallinity and the fourth polymeric material may have between 0 and 5% residual crystallinity under said certain condition. The residual crystallinity of the second polymeric material may fall within the same range as that of the fourth polymeric material under said condition.

The multilayer film may comprise one or more additive materials in one or more of the layers present. Additives may comprise: dyes, pigments, colorants, metallised and/or pseudo metallised coatings (e.g. aluminium), lubricants, anti-oxidants, surface-active agents, stiffening aids, gloss-improvers, prodegradants, UV attenuating materials, UV light stabilisers, sealability additives, tackifiers, anti-blocking agents, additives to improve ink adhesion and/or printability or cross-linking agents (such as melamine formaldehyde resin).

The stiffness of the multilayer film can be increased by increasing the stiffness of one of the layers, preferably the first layer. Means for increasing the stiffness of a layer are known in the art and include the use of a hard resin (e.g. hydrocarbon resins such as fully hydrogenated C5 or C9 materials), or other compatible stiffness enhancers (e.g. COC, fibres or minerals such as clays), or crosslinking agents where suitable in one or more of the layers. The stiffness of the multilayer film may not be substantially different to that of an identical film in which the first and second layers have been replaced by a layer of the same thickness having the same composition as the first layer. The stiffness of the multilayer film may be lower than that of an identical film in which the first and second layers have been replaced by a layer of the same thickness having the same composition as the first layer. Preferably, the stiffness values of the multilayer film, as measured using the Gurley Stiffness Test or by looking at the Young’s Modulus (which is proportional to stiffness), are within 30% of the corresponding values for said identical film, preferably within 10% of the corresponding values for said identical film. Thus, the multilayer film maintains suitable mechanical properties while demonstrating an improved average tear propagation strength.

The stiffness of the multilayer film can be further enhanced by adding a stiffer material (e.g. Cyclo Olefinic Copolymers (COCs) or polyesters) in one or more layers of the film. The multilayer film may comprise an additional stiffening layer comprising such a material. If the stiffness of the multilayer film is lower than that of an identical film in which the first and second layers have been replaced by a layer of the same thickness having the same composition as the first layer, such materials can modify the thickness so that it is not substantially different to that of an identical film in which the first and second layers have been replaced by a layer of the same thickness having the same composition as the first layer.

The multilayer film may be between around 10 and around 150 microns thick, preferably between around 15 and around 100 microns thick.

The multilayer film may have a wide angle haze of less than around 12, preferably less than around 8 and most preferably less than around 4. The multilayer film may have a wide angle haze of less than around 2.5.

The multilayer film may have a 45° gloss of between around 85 and around 110.

The wide angle haze and/or the 45° gloss values of the multilayer film may not be substantially different to a film of the same thickness made entirely from the first polymeric material. Preferably, the wide angle haze and/or the 45° gloss values are within 30% of the corresponding values for said identical film, preferably within 10% of the corresponding values for a film of the same thickness made entirely from the first polymeric material. Thus, the multilayer film of the invention maintains suitable optical properties while demonstrating an improved average tear propagation strength. This is particularly the case in the embodiment comprising first and third layers formed on either side of the second layer. This embodiment demonstrates improved average tear propagation strength while maintaining the required optical properties of the film.

The first polymeric material may be polypropylene, polyethylene such as HDPE, PET, or Nylon. Preferably, the first polymeric material comprises propylene and may be polypropylene. The first layer comprising the first polymeric material may be biaxially oriented.

The second polymeric material may be a polypropylene terpolymer, a polypropylene copolymer, a polyethylene (such as HDPE, LDPE, LLDPE or ULDPE), a rubber (such as SEBS or SBBS), or copolyester. The rubber may be an activated rubber.

Suitable tie layers may also be included in the multilayer film, optionally between the first layer and the second layer.

The film may have a shrinkage of less than around 2.8 at 120°C in the transverse direction and/or less than around 0.25 at 120°C in the machine direction. The film may have a shrinkage of less than around 1.5 at 120°C in the transverse direction and/or less than around 0.15 at 120°C in the machine direction.

The film may have a shrinkage of less than around 2.5 at 80°C in the transverse direction and/or less than around 0.25 at 80°C in the machine direction. The film may have a shrinkage of less than around 1.6 at 80°C in the transverse direction and/or less than around 0.15 at 80°C in the machine direction.

The film may have a tensile strength of greater than around 40 MPa, preferably greater than around 60 MPa. The film may have a tensile strength of between around 40 and around 130 MPa. The film may have a tensile strength of between around 60 and around 1 15 MPa.

The average load of the film before tearing may be greater than around 0.05 N for a 20 micron film. The average load of the film before tearing may be greater than around 0.09 N. The load applied and/or the tear created may be in the machine or the transverse direction.

The average maximum load of the film before tearing may be above around 0.07 N for a 20 micron film. The average maximum load of the film before tearing may be above around 0.1 N. The load applied and/or the tear created may be in the machine or the transverse direction. The average tear propagation strength of the film can be measured using the trouser tear test. This test is outlined in ASTM D1938. Applying this test to a multilayer film of the invention and to an identical film wherein the first and second layers are replaced with a single layer of the same thickness as the first and second layers combined having the same composition as the first layer can therefore identify whether the second layer is less oriented than the first layer.

The presence of a less oriented second layer can also affect the softening profile, as determined by thermomechanical analysis, of the multilayer film compared to that of an identical film wherein the first and second layers are replaced with a single layer of the same thickness as the first and second layers combined having the same composition as the first layer.

Softening can be measured by placing a small sample of film under a penetration probe and subjecting the film to 0.5N constant force. The depression (or retraction) of the probe is measured as a function of temperature at a fixed heating rate of 5°C min ¹ . A negative dimension change denotes softening as the probe depresses through the sample. A positive dimension change denotes sample expansion in the z-plane.

The negative dimension change may suddenly increase at the melting point or the glass transition temperature of the second polymeric material in films of the present invention.

The presence of a less oriented second layer can also affect the mechanical properties, as determined by dynamic mechanical analysis, of the multilayer film compared to that of an identical film wherein the first and second layers are replaced with a single layer of the same thickness as the first and second layers combined having the same composition as the first layer.

The storage loss of the films of the present invention may be lower than an identical film wherein the first and second layers are replaced with a single layer of the same thickness as the first and second layers combined having the same composition as the first layer. The storage loss may be less than 75%, less than 60% lower or less than 50% of that of film of the same thickness made entirely from the first polymeric material. The storage loss may be measured at 20°C.

The loss modulus of the films of the present invention may be lower than an identical film wherein the first and second layers are replaced with a single layer of the same thickness as the first and second layers combined having the same composition as the first layer. The loss modulus may be less than 75%, less than 60% lower or less than 50% of that of a film comprising only the first or second polymeric material. The loss modulus may be measured at 20°C.

The ratio of the loss to storage modulus of the film of the present invention may be lower than an identical film wherein the first and second layers are replaced with a single layer of the same thickness as the first and second layers combined having the same composition as the first layer. The ratio may be less than 75%, less than 60% lower or less than 50% of that of a film comprising only the first or second polymeric material. The loss modulus may be measured at 80°C.

According to a second aspect of the present invention, there is provided a method of making a multilayer film comprising the steps of (a) coextruding a first layer comprising a first polymeric material and a second layer comprising a second polymeric material to form a sheet or tube, (b) placing the sheet or tube under a condition at which the first layer has a higher amount of residual crystallinity to the second layer; and (c) stretching the sheet or tube under said condition to produce a film.

Step (c) may optionally be followed by heat stabilisation (heat setting or annealing) at an elevated temperature and/or cooling the film to room temperature.

The film is therefore stretched at a condition under which the first layer comprising the first polymeric material becomes more oriented than the second layer comprising the second polymeric material. The second layer may be entirely unoriented (like a cast film). This means that the second layer of the multilayer film behaves similarly to a cast material, which increases the average tear propagation strength of the multilayer film. Different orientation effects are seen within the second layer material when it is coextruded and stretched at an elevated temperature compared to when it is subsequently heat treated at said elevated temperature. Different DSC curves may also result from the method of the present invention.

The difference in residual crystallinity during step (c) may arise due to the first polymeric material being semi-crystalline while the second polymeric material is amorphous.

The amount of molten polymeric material during stretching also affects the amount of residual crystallinity in the layer, such that the presence of more molten material means less residual crystallinity. Thus, if the second material is more molten than the first material during step (c), less orientation will be imparted to the second layer compared to the first layer, as discussed above. The condition during step (c) may be such that the second polymeric material is predominantly or completely molten during stretching. Preferably, more than 50% of the second polymeric material is molten at this temperature and more preferably, more than 75%.

At the condition under which step (c) occurs, the first polymeric material may have between 30 and 100% residual crystallinity, while the second polymeric material may have between 0 and 15% residual crystallinity. Preferably, the first polymeric material may have between 40 and 90% residual crystallinity, while the second polymeric material may have between 0 and 5% residual crystallinity.

If both of the first and second polymeric materials are semi-crystalline, then the first polymeric material has a first melting point range and the second polymeric material has a second melting point range. The discussion of the first, second, third and fourth melting point ranges in relation to the first aspect of the invention apply equally in relation to the second aspect of the invention.

The melting point ranges of the first and second polymeric materials and/or of the first and second layers may not overlap or may overlap partially such that only a portion of the first melting point range is higher than the second melting point range. The film may be heated to a temperature at which the second polymeric material and/or the second layer is more molten than the first polymeric material and/or the first layer before stretching the film.

The peak of the first melting point range may be higher than the peak of the second melting point range. The peak of the first melting point range may be more than 20°C higher than the peak of the second melting point range. The peak of the first melting point range may be more than 50°C higher than the peak of the second melting point range. The same may be true for the melting point ranges of the first and second layers.

The film may be heated to a temperature above the peak of the melting point range of the second polymeric material and/or second layer but below the peak of the melting point range of the first polymeric material and/or first layer in step (c).

The same conditions may be maintained throughout the stretching process. The temperature of step (c) may be the same as the maximum temperature to which the film is heated in step (b). Step (c) may comprise optional heat stabilisation, for example by heat setting or annealing. This may occur at a higher temperature than the stretching temperature and may reduce the shrinkage of the film. The film may be monoaxially stretched. The monoaxial stretching may be achieved using a stenter or machine direction orienter (MDO) method.

The film may be biaxially stretched. The biaxial stretching may be done sequentially or simultaneously. Sequential stretching may require a higher temperature when drawing in one or both directions compared to the temperature required for simultaneous stretching. The biaxial stretching may be achieved using a bubble or stenter method.

In the case of a sequential draw process, the film may be biaxially stretched to a draw ratio of above around 3 x 6. The film may be biaxially stretched to a draw ratio of below around 6 x 12. The film may be biaxially stretched to a draw ratio of around 5 x 10.

In the case of simultaneous draw process, the film may be biaxially stretched to a draw ratio of above around 4 x 4. The film may be biaxially stretched to a draw ratio of below around 10 x 10. The film may be biaxially stretched to a draw ratio of around 7 x 7.

The multilayer film may consist of two layers. A third layer may be coextruded on the opposite side of the second layer to the first layer. Thus, the second layer may be sandwiched between the third layer and the first layer. The second layer may be a core layer, rather than an outer layer, which has been found to improve the optical properties of the film.

A tie layer may be coextruded with the first and second layer. The tie layer is preferably between the first and second layer and may be in contact with both the first and second layers.

The third layer may be identical to the first layer. The third layer may comprise the first polymeric material but may include different additional components.

Alternatively, the third layer may be different to the first layer. The third layer may comprise a third polymeric material, wherein the residual crystallinity of the third polymeric material is greater than that of the second polymeric material at the conditions of step (c). The third layer may therefore be more oriented than the second layer as a result of a contemporaneous stretching process under said condition.

The third layer is preferably semi-crystalline. The film may be heated to a temperature above the melting point range of the second polymeric material, but below the melting point range of the first polymeric material and the third polymeric material, before stretching the film. The film may be heated to a temperature above the melting point range of the second layer, but below the melting point ranges of the first layer and the third layer, before stretching the film.

The multilayer film may comprise a fourth layer comprising the second polymeric material. The fourth layer may be adjacent to the second layer or may be separated from the second layer by one or more other layers. The second layer may be the only layer comprising the second polymeric material in the multilayer film.

The fourth polymeric material may be amorphous or semi-crystalline. The multilayer film may comprise a fourth layer comprising a fourth polymeric material wherein the fourth polymeric material has less residual crystallinity than the first polymeric material in step (c). The first layer may therefore be more oriented than the fourth layer as a result of a contemporaneous stretching process at the stretching condition.

The difference in residual crystallinity between the second and third polymeric materials and between the first and fourth polymeric materials may be due to the differences in melting point range. The discussion regarding the third and fourth layers in relation to the first aspect applies equally to the second aspect.

The extrusion of step (a) may be conducted at a temperature above the melting point range of both the first polymeric material and the second polymeric material, and optionally also the third and fourth polymeric materials, if present in the film. The extrusion temperature may be between around 200°C and around 250°C, preferably around 235°C.

The multilayer film may further comprise skin layers as the outermost layers of the film. The skin layers may be formed on the first and third layers of the film. The skin layers may be applied before the stretching process or afterwards.

The film may be passed over heated rollers after it has been stretched. This may act to heat stabilise the film.

The film may also be passed through a cooling zone or over chilled rollers after it has been stretched and optionally heat stabilised. This may reduce the temperature of the film to below the melting point range of the second and optionally also fourth polymeric material. The method as outlined above may be used to create the film as outlined above. Thus, the features outlined above in relation to the first aspect of the present invention apply equally to the film produced using the method of the second aspect of the invention.

According to a third aspect of the present invention, there is provided a use in a multilayer film of a second layer comprising a second polymeric material, wherein the second layer is less oriented than a first layer comprising a first polymeric material, to increase the average tear propagation strength in the machine direction and/or the transverse direction of the multilayer film compared to an identical film wherein the first and second layers are replaced with a single layer of the same thickness as the first and second layers combined having the same composition as the first layer.

As outlined above, the inclusion in a multilayer film of a more oriented first layer can provide various advantageous results of orientation, such as improved mechanical, optical and barrier properties. However, the inclusion of a separate, less oriented second layer has surprisingly been found to increase the average tear propagation strength properties of the multilayer film.

The multilayer film of the third aspect of the present invention may be the film as outlined above. Thus, the features outlined above in relation to the first and second aspects of the present invention apply equally to the film produced using the use of the third aspect of the invention.

According to a fourth aspect of the present invention, there is provided an article formed from the film discussed above. Said article may be a package, a label, a banknote or another security document. Said article demonstrates an improved average tear propagation strength compared to other known articles, including articles comprising an identical film wherein the first and second layers are replaced with a single layer of the same thickness as the first and second layers combined having the same composition as the first layer. The features outlined above in relation to the first and second aspects of the present invention apply equally to the package or label of the fourth aspect of the invention.

According to a fifth aspect of the present invention, there is provided an article that is packaged or labelled using the package or label discussed above. Said package or label demonstrates an improved average tear propagation strength compared to other known packages or labels. The features outlined above in relation to the first and second aspects of the present invention apply equally to the package or label of the fifth aspect of the invention. Aspects of the present invention will now be exemplified in the following specific embodiments, which are included by way of example only and are not considered limiting to the scope of protection.

Throughout the examples, the test methods outlined in Table 1 a and the materials outlined in Table 1 b are employed.

Table 1a

Table 1 b

Gloss measurements are taken based on ASTM D2457. Gloss results are recorded at 45° using a calibrated unit either using a Novo-gloss Lite unit calibrated to a zero reference and then set on a black background of known reflectance or a NovoGloss 45° Rhopoint meter. The unit is regularly tested against both the supplied calibrated block and the background to black sheet. Results are taken over a sample and reported as an average of 3 tests. Testing is based on ASTM D1003. WAH of a specimen is the percentage of transmitted light which, in passing through the specimen, deviates from the incident beam by more than 2.5 degrees by forward scattering. WAH results are recorded using a pre-calibrated unit (Hazemeter M57 and Spherical Haze meter from Diffusion Systems). Each variant is tested 3 times across the sample web and an average result recorded.

NAH of a specimen is the parallel light which is scattered by more than 6 minutes (0.1 °) of an arc when passing through the film or film substrate sample from the incident beam and is measured as a percentage of total light transmitted through the film. Results are recorded using a pre-calibrated“Rayopp” laser haze meter and recorded over the length of a 25mm wide film strip, recording both the maximum and minimum results achieved over the sample.

The melting point information for the materials used in the examples below is outlined in Table 2, while the crystallinity information of the materials is outlined in Table 3.

Table 2

Table 3

*Polymer exhibits significant cold crystallisation

Control Example

Industrially made standard biaxially oriented polypropylene (BOPP) films comprising Moplen HP420M were produced by simultaneous biaxial stretching to a draw of between 6.5 x 6.5 and 8.5 x 8.5 using a double bubble type process. Samples of differing thicknesses were made and tested using the standard ASTM tear tests (ASTM D1938). The results of the tear tests are shown in Figure 1 , which demonstrates a positive correlation between film thickness and tear strength that is independent of draw ratio over the range described.

Comparison of the multilayer films according to the present invention to these control results can be used to visualise the improvement of average tear propagation strength at any thickness. Specifically, a result above the trend line for any tested film indicates an improvement in average tear propagation strength.

The optical properties of two of these control films are outlined in Table 4.

Table 4

Figure 2 compares a standard stenter film (Jindal MB666 - an oriented polypropylene film with an acrylic coating) with the control film discussed above. As demonstrated in this figure, stenter (sequentially oriented) films generally have a lower average tear propagation strength than bubble (simultaneously oriented) films. The tear properties of this film were almost symmetrical in the machine and transverse directions and the results are based on an average of 5 tests.

Example 1

Cast sheet samples were produced using a Dr Collins 5 layer cast film line. The multilayer extrusion system was configured to give a sheet layer structure ABCBA, in which layers A and B are polypropylene (Moplen HP420M) and layer C is one of two different core layers, namely Eltex KV349 (a random propylene terpolymer) and Moplen RP220 (a modified propylene random copolymer). Approximate ratios of layer thickness were A+B:C:B+A = 1 :2:1. Extrusion was carried out with the die at 235°C and extruders increasing in temperature from the first zone at 190°C to 230°C. The extrudate was cast onto chilled rollers at 30-36°C and an approximately 1 mm thick cast sheet was produced.

The cast sheet samples were then cut into square plaques, which were simultaneously drawn biaxially to produce a thin film using a Bruckner Karo IV film stretching machine.

For each variant, the plaques were stretched to a draw ratio of 7 x 7 at temperatures of both 150°C and 156°C. A further set of films was created by stretching to a draw ratio of 5 x 5 at a temperature of 156°C. The melting point range of polypropylene (Moplen 420M) is above these temperatures, while the melting point ranges of both Eltex KV349 and Moplen RP220 are below these temperatures, as outlined in Table 2. Additionally, the percentage of residual crystallinity is much higher for Moplen 420M at the draw temperatures than for Eltex KV349 and Moplen RP220, as shown in Table 3.

Thus, after stretching, the films comprise two more oriented outer layers comprising a first polymeric material and a less oriented core layer comprising a second polymeric material.

The average tear propagation strengths of the films of Example 1 , as measured using the tear test, are shown in Table 5 and Figure 3. As demonstrated in this figure, there is a significant improvement in average tear propagation strength between the films of Example 1 and the control films. In all cases, the Eltex KV349 films have a better average tear propagation strength than the Moplen RP220 films.

Table 5

The improvement in average tear propagation strength increases with an increased draw temperature. This is believed to be due to decreased orientation of the core layer at the higher temperature.

The films were then tested for various optical properties using the tests in Table 1 , the results of which are outlined in Table 6 below. These results show that the inclusion of a less oriented core layer comprising a low melting point polymeric material has little effect on the optical properties of the film. Further, the shrinkage values are low, which is thought to be due to the reduced orientation of the core layer, which therefore shows little preference for shrinkage. Had the core layer been oriented, larger shrinkages would be expected.

Table 6

The results of Example 1 show that the use of two more oriented outer layers comprising a first, higher melting point range polymeric material, either side of the less oriented core layer comprising a second, lower melting point range material has little effect on the overall appearance, but greatly increases the average tear propagation strength.

Example 2

Cast sheet samples were produced using a Dr Collins 5 layer cast film line. The multilayer extrusion system was configured to give a sheet layer structure ABCBA, in which layers B and C are polypropylene (Moplen HP420M) and layer A is one of two different core layers, namely Eltex KV349 (a random propylene terpolymer) and Moplen RP220 (a modified propylene random copolymer).

Approximate ratios of layer thickness were A:B+C+B:A = 1 :2:1. Extrusion was carried out with the die at 235°C and extruders increasing in temperature from the first zone at 190°C to 230°C. The extrudate was cast onto chilled rollers at 30-36°C and an approximately 1 mm thick cast sheet produced. The cast sheet samples were then cut into square plaques, which were simultaneously drawn biaxially to produce a thin film using a Bruckner Karo IV film stretching machine.

For each variant, the plaques were stretched to a draw ratio of 7 x 7 at a temperature of 156°C. The melting point range of polypropylene (Moplen HP420M) is above these temperatures, while the melting point ranges of both Eltex KV349 and Moplen RP220 are below these temperatures, as outlined in Table 2. Additionally, the percentage of residual crystallinity is much higher for Moplen 420M at the draw temperatures than for Eltex KV349 and Moplen RP220, as shown in Table 3.

Thus, the films comprise a more oriented core layer comprising a first polymeric material and two less oriented outer layers comprising a second polymeric material.

The average tear propagation strengths of the films of Example 2, as measured using the tear test, are shown in Figure 4 and Table 7. As demonstrated in this figure, improved average tear propagation strength is seen when the films are drawn at 156°C, with neither outer layer material showing a significant improvement over the other. As with the films of Example 1 , the average tear propagation strength increases with an increased draw temperature. This is believed to be due to decreased orientation of the outer layers at the higher temperature.

Table 7

The films were then tested for various optical properties using the tests in Table 1 , the results of which are outlined in Table 8 below. These results show that the inclusion of the less oriented external layers comprising a polymeric material with a lower melting point range has a dramatic effect on the gloss and haze values. These results are very poor and are significantly worse than the films of Example 1. Thus, while the inclusion of such outer layers does improve the average tear propagation strength, it does not do so without significantly reducing the optical properties of the film.

As with the films of Example 1 , the shrinkage values are low. This is thought to be due to the reduced orientation of the outer layers, which therefore show little preference for shrinkage. Table 8

These results therefore demonstrate that the combination of a first layer comprising a first polymeric material with higher melting point range and a second layer comprising a second polymeric material with a lower melting point range improves the average tear propagation strength within a multilayer film.

However, the order of the layers within the multilayer film has an impact on the optical properties of the film, as the presence of the less oriented layer as an outer layer is detrimental to the optical properties. This decrease in optical properties is not seen when the less oriented layer is present as a core layer within the film, as exemplified in Example 1.

Thus, the embodiment in which the less oriented layer is a core layer and the more oriented layer is an outer layer provides improved average tear propagation strength while maintaining the advantageous optical properties.

Example 3

Cast sheet samples were produced using a Dr Collins 5 layer cast film line. The multilayer extrusion system was configured to give a sheet layer structure ABCBA, in which layer A is polypropylene (Moplen HP420M) and layers B and C are Eltex KV349 (a random propylene terpolymer).

Output of extruders A, B and C were altered to give various layer thicknesses leading to samples with compositions shown in Table 10 below. Extrusion was carried out with the die at 235°C and extruders increasing in temperature from the first zone at 190°C to 230°C. The extrudate was cast onto chilled rollers at 30-36°C and an approximately 1 mm thick cast sheet produced.

The cast sheet samples were then cut into square plaques, which were simultaneously drawn biaxially to produce a thin film using a Bruckner Karo IV film stretching machine. The ratio of the layer thicknesses was varied in each sample, although the overall film thickness was kept constant. The plaques were all stretched to a draw ratio of 7 x 7 at a temperature of 156°C. The melting point range of polypropylene is above this temperature, while the melting point range of Eltex KV349 is below this temperature, as shown in Table 2. Additionally, the percentage of residual crystallinity is much higher for Moplen 420M at the draw temperatures than for Eltex KV349, as shown in Table 3.

Thus, the films comprise more oriented outer layers comprising a first polymeric material and a less oriented core layer comprising a second polymeric material.

The average tear propagation strengths of the films of Example 3, as measured using the tear test, are shown in Figure 5 and Table 9. As demonstrated in this figure, the average tear propagation strength of the multilayer film generally increases with the increasing thickness of the less oriented core layer, with the films of Example 3 showing a marked increase in tear strength. The samples which did not fit to the expected result showed slight waves in the cast sheet structure, which is thought to be due to pulsing and/or surging of the extruder during their production.

Table 9

The films were then tested for various optical properties using the tests in Table 1 , the results of which are outlined in Table 10 below. These results show that in general, the thickness of the less oriented core layer does not overly affect the optical properties of the multilayer film.

Table 10

Example 4

Cast sheet samples were produced using a Dr Collins 5 layer cast film line. The multilayer extrusion system was configured to give a sheet layer structure ABCBA,, in which layer A is polypropylene (Moplen HP420M) and layers B and C are either a linear low density polyethylene (Dowlex 2106) or an ultra low density polyethylene (Attane 4607).

Output of extruders A, B and C were altered to give samples with compositions A:B+C+B:A 1 :2: 1. Extrusion was carried out with the die at 235°C and extruders increasing in temperature from the first zone at 190°C to 230°C. The extrudate was cast onto chilled rollers at 30-36°C and an approximately 1 mm thick cast sheet produced.

The cast sheet samples were then cut into square plaques, which were simultaneously drawn biaxially to produce a thin film using a Bruckner Karo IV film stretching machine.

For each variant, the plaques were stretched to a draw ratio of either 7 x 7 or 5 x 5 at a temperature of 156°C. The melting point range of polypropylene (Moplen HP420M) is above this temperature, while the melting point ranges of both Dowlex 2106 and Attane 4607 are below this temperature, as outlined in Table 2. Additionally, the percentage of residual crystallinity is much higher for Moplen 420M at the draw temperatures than for Eltex KV349 and Moplen RP220, as shown in Table 3.

Thus, the films comprise more oriented outer layers comprising a first polymeric material and a less oriented core layer comprising a second polymeric material.

The average tear propagation strengths of the films of Example 4, as measured using the tear test, are shown in Figure 6 and Table 11. As demonstrated in this figure, the Dowlex 2106 film has significantly better tear propagation resistance than the Attane 4607 film. The films of Example 4 have lower average tear propagation strengths than the films of Example 1 but are all better than the control films. This difference in tear propagation strength is because the tear mechanism is different, which is highlighted by the significant increase in the maximum tear loads. The films of Example 4 were also found to have a lower stiffness than the films of Example 1 , due to the addition of the polyethylene materials in the BCB core layer. Table 11

The films were then tested for various optical properties using the tests in Table 1 , the results of which are outlined in Table 12 below. These results demonstrate that the films of Example 4 show comparative optical properties to the corresponding polypropylene films disclosed in Table 10. The thicker films show slightly worse optical properties than the thinner films.

Table 12

Figure 7 illustrates the tear propagation of the films of Example 4. The films may tear as shown in Figure 7, i.e. normally for a given length (approximately 5mm in Figure 7) before“sticking”, at which point a hole is formed. The tear then resumes from a random point, repeating the above result. Alternatively, the films may tear by very small segmental tears (approximately 1 mm). This is different to the tearing seen in BOPP films, in which the film tears and propagates in a single linear motion.

These mechanisms of failure cause widely varied tear strengths of each sample and give widely varied maximum tear strengths. However, the skilled person would know to conduct repeat experiments to arrive at an average tear strength value, as has been done to create the values outlined in Table 1 1 above.

Example 5

Cast sheet samples were produced using a Dr Collins 5 layer cast film line. The multilayer extrusion system was configured to give a sheet layer structure ABCBA, in which layers A and B are polypropylene (Moplen HP420M) and layer C is either an amine-modified styrene- ethylene-butylene-styrene (Tuftec MP10) or a polyamide-modified styrene-ethylene-butylene- styrene (Tuftec M1913), both of which are amorphous.

Approximate ratios of layer thickness were A+B:C:B+A = 1 :2: 1. Extrusion was carried out with the die at 235°C and extruders increasing in temperature from the first zone at 190°C to 230°C. The extrudate was cast onto chilled rollers at 30-36°C and an approximately 1 mm thick cast sheet produced.

The cast sheet samples were then cut into square plaques, which were simultaneously drawn biaxially to produce a thin film using a Bruckner Karo IV film stretching machine.

For each variant, the plaques were stretched to a draw ratio of either 7 x 7 or 5 x 5 at a temperature of 156°C. The melting point range of polypropylene (Moplen HP420M) is above this temperature, while the melting point ranges of both Tuftec MP10 and Tuftec M1913 are below this temperature. Thus, the films comprise more oriented outer layers comprising a first polymeric material and a less oriented core layer comprising a second polymeric material.

The average tear propagation strengths of the films of Example 5, as measured using the tear test, are shown in Figure 8 and Table 13. As illustrated in this figure, these films demonstrate average tear propagation strengths significantly higher than the films of any of the preceding examples. The films of Example 5 were also found to have a lower stiffness than the films of Example 1 , due to the addition of the rubber materials in the core layer. The stiffness of the films of Example 5 is comparable to that of the films of Example 4. As with the films of Example 4, the tear propagation of these films was non-uniform.

Table 13

The films were then tested for various optical properties using the methods in Table 1 , the results of which are outlined in Table 14 below. These results demonstrate that the films of Example 5 show comparative optical properties to the corresponding films disclosed in Table 10. The thicker films show slightly worse optical properties than the thinner films, as expected. Table 14

Example 6

Single layer structures (samples 6.1 to 6.4) were made as cast sheet samples using only the main core extruder on a laboratory scale Rondol multilayer cast line. The samples were made by blending Eltex KV349 (a random propylene terpolymer) with polypropylene (Moplen HP 420M) and extruding the blend to form a single layer. Some of these blends were pre-mixed through a PRISM twin screw mixing extruder (PRISM), while others were mixed in the extruding unit (MULTI).

Extrusion was carried out with the die at 230°C and extruders increasing in temperature from the first zone at 190°C to 230°C. The extrudate was cast onto chilled rollers at room temperature and an approximately 1 2mm thick cast sheet produced.

These plaques were then biaxially stretched to a draw ratio of 7 x 7 at a temperature of 156°C. The melting point range of Moplen HP 420M is above this temperature, while the melting point range of Eltex KV349 is below this temperature, as shown in Table 2. Additionally, the percentage of residual crystallinity is much higher for Moplen 420M at the draw temperatures than for Eltex KV349 and Moplen RP220, as shown in Table 3.

Multilayer samples (sample 6.5 to 6.7) were made as cast sheet samples using a Dr Collins 5 layer cast film line. The multilayer extrusion system was configured to give a sheet layer structure ABCBA, in which layer A is polypropylene (Moplen HP420M), layer B is Eltex KV349 and layer C is Eltex KV349.

Extrusion was carried out with the die at 235°C and extruders increasing in temperature from the first zone at 190°C to 230°C. The extrudate was cast onto chilled rollers at 30-36°C and an approximately 1 mm thick cast sheet produced. Film samples were produced from the cast sheets as described. The cast sheet samples produced from both the Rondol and the Dr Collins unit were then cut into square plaques, which were then simultaneously drawn biaxially to produce a thin film using a Bruckner Karo IV film stretching machine under the conditions mentioned below in Table 15.

The average tear propagation strengths of the films of Example 6, as measured using the tear test, are shown in Table 15. As demonstrated in this table, PRISM pre-mixed materials demonstrate a slightly better tear strength. However, the blended single-layer films have a significantly worse tear strength compared to the multi-layered films according to the present invention.

Table 15

The films were then tested for various optical properties using the tests in Table 1 , the results of which are outlined in Table 16 below. These results show that there is no significant change in properties when the blend is pre-mixed compared to being mixed in the extruder. However, the optical properties are significantly worse in the blended films compared to the multilayer films according to the present invention.

Table 16

The results of Example 6 demonstrate that the improved average tear propagation strength and optical properties are not simply due to the combination of the first and second polymeric materials in the film. Instead, these properties depend on the presence of separate layers containing said materials, the two separate layers having different levels of orientation.

Example 7

Cast sheet samples were produced using a Dr Collins 5 layer cast film line. The multilayer extrusion system was configured to give a sheet layer structure ABCBA, in which the A layers are polypropylene (Moplen HP420M) blended with 46% of a hydrocarbon resin masterbatch (70% Hydrocarbon Resin in PP Carrier) and are approximately 35% of the total structure and the B layers are either unmodified polypropylene (Moplen HP420M) or a random propylene terpolymer (Eltex KV349) and are approximately 47% of the total structure. Layer C is approximately 18% of the total structure and contains polypropylene (Moplen HP420M) blended with 46% of a hydrocarbon resin masterbatch (70% Hydrocarbon Resin in PP carrier). Layers B and C are varied as shown in Table 17.

Extrusion was carried out with the die at 235°C and extruders increasing in temperature from the first zone at 190°C to 230°C. The extrudate was cast onto chilled rollers at 30-36°C and an approximately 1 mm thick cast sheet produced.

The cast sheet samples were then cut into square plaques, which were then simultaneously drawn biaxially to produce a thin film using a Bruckner Karo IV film stretching machine.

Table 17

As outlined in Table 17, these plaques were then stretched to a draw ratio of 7 x 7 at a temperature of either 150°C or 156°C.

The average tear propagation strengths of the films of Example 7, as measured using the tear test, are shown in Figure 9. This figure demonstrates that the films showed significant improvement in both average and maximum average tear propagation strength with the inclusion of the soft KV349 material in place of a standard polypropylene. In fact, the inclusion of KV349 nearly doubled the average tear propagation strength of the resulting material

The average tear propagation strength properties are further outlined in Table 18 below. As demonstrated in this table, the addition of a layer of the less oriented KV349 material in the structure increased the average tear propagation strength of the film by approximately 150%.

Table 18

The films were then tested for various optical properties using the methods in Table 1 , the results of which are outlined in Table 19 below. These results show that when films are drawn at 150°C, the optical properties were very good and there was little effect on the gloss or wide angle haze. Films that have undergone orientation at a draw ratio of over 5 x 5 showed comparable low narrow angle haze results.

Table 19

Example 8

Single layer structures (samples 8.1 and 8.2) were made by pre-mixing amine-modified styrene-ethylene-butylene-styrene (Tuftec MP10) with polypropylene (Moplen HP420M) using a PRISM twin screw mixing extruder with the die at 230°C and extruders increasing in temperature from the first zone at 190°C to 230°C. The resultant material was used to produce cast sheet samples using only the main core extruder on a laboratory scale Rondol multilayer cast line. The extrusion system was configured to give a sheet of the blended materials. Extrusion was carried out with the die at 230°C and extruders increasing in temperature from the first zone at 190°C to 230°C. The extrudate was cast onto chilled rollers at room temperature and an approximately 1 2mm thick cast sheet produced.

Multilayer structured cast sheet samples (samples 8.3 to 8.9) were produced using a Dr Collins 5 layer cast film line. The multilayer extrusion system was configured to give a sheet layer structure ABCBA, in which layer A is polypropylene (Moplen HP420M). Single Layer (SL) structures were formed by either combining the B and C layers or using the layer C only. Double layer (DL) structures were made using the same material in the two B layers, with layer C being the same material as layer A. In all multilayer cases the non-polypropylene layer of material is an amine-modified styrene-ethylene-butylene-styrene (Tuftec MP10).

Output of extruders A, B and C were altered to give samples with compositions as shown in Table 20. Extrusion was carried out with the die at 235°C and extruders increasing in temperature from the first zone at 190°C to 230°C. The extrudate was cast onto chilled rollers at 30-36°C and an approximately 1 mm thick cast sheet produced.

The cast sheet samples produced above were then cut into square plaques, which were then simultaneously drawn biaxially to produce a thin film using a Bruckner Karo IV film stretching machine.

The average tear propagation strengths of the films of Example 8, as measured using the tear test, are shown in Table 20. As demonstrated in this table, blending the material into polypropylene gave tear strengths significantly lower than those for the multilayer variants of the film according to the invention, when comparing like for like thicknesses. Splitting the second material layer, as in the DL samples, appears to reduce the average tear propagation strength on comparable composition samples, when compared with a single layer (SL) of the same material. This is probably due to an increase in layer orientation caused by surface draw effects of the material. Table 20

The films were then tested for various optical properties using the methods of Table 1 , the results of which are outlined in Table 21 below. These results show that blending the material into polypropylene had a marked effect on the optics of the film when compared to the multilayer variants of the film. Samples where the soft material is present as multiple layers instead of a single layer showed better optical properties.

Table 21

Example 9

Cast sheet samples were produced using a Dr Collins 5 layer cast film line. The multilayer extrusion system was configured to give a sheet layer structure ABCBD, in which layers B, C, B and D are polypropylene (Moplen HP420M) and layer A is LLDPE (Dowlex 5057).

Extrusion was carried out with the die at 235°C and extruders increasing in temperature from the first zone at 190°C to 230°C. The extrudate was cast onto chilled rollers at 30-36°C and an approximately 1 mm thick cast sheet produced.

The cast sheet samples were then cut into square plaques, which were simultaneously drawn biaxially at 156°C to produce a thin film using a Bruckner Karo IV film stretching machine.

The average tear propagation strengths of the films of Example 9, as measured using the tear test, are shown in Figure 10. As demonstrated in this figure, the tear strength increases with increasing proportions of the less oriented layer (A). These results are further supported by the data in Table 22.

Table 22

The films were then tested for various optical properties using the methods of Table 1 , the results of which are outlined in Table 23 below. These results show that the side of the less oriented layer (A) of the film had a lower gloss value.

Table 23

Example 10

The softening profile of three films was determined by thermochemical analysis. The composition of the films is shown below in Table 24. Film 1 is a bubble film, while Films 2 and 3 correspond to Examples 4.2 and 8.4 above respectively. The middle layer of Films 2 and 3 is therefore expected to be less oriented than that of Film 1.

Table 24

A small sample of each of the films was placed under a penetration probe and subjected to 0.5N constant force. The depression (or retraction) of the probe was measured as a function of temperature at a fixed heating rate of 5°C min ¹ . A negative dimension change denotes softening as the probe depresses through the sample. A positive dimension change denotes sample expansion in the z-plane.

As shown in Figure 11 , the softening profile of the films correlates with either the melting point or the glass transition temperature of the core layer. For reference, the melting range of polypropylene is between 160 and 166°C, the meting range of polyethylene is between 120 and 135°C (depending on density) and the unusual thermal transitions for MP10 occur between 30 and 75°C.

Example 11

The mechanical properties of three films were determined by dynamic mechanical analysis. The composition of the films is shown below in Table 25. CL30 and B28 are commercially available films from Innovia Films Limited, while QE1 and Q7L10P correspond to Examples 4.2 and 8.4 above respectively. The middle layer of QE1 and Q7L10P is therefore expected to be less oriented than that of the other films.

Table 25

Dynamic mechanical analysis (DMA) was conducted at an oscillatory frequency of 1 Hz, at 0.15% strain and a fixed heating rate of 2°C min ¹ . As shown in Figure 12, the standard BOPP films occupy the higher storage modulus ranges. The multilayer material containing polyethylene exhibits a significant reduction in storage modulus, which is likely due to a combination of lack of orientation in the PE layer and inherent lack of elasticity in the PE raw material.

The multilayer film containing the MP10 rubber is a further step lower in storage modulus, again owing to its lack of orientation. The rubber is largely amorphous hence there is no contribution to storage from crystallinity. Although a highly elastic material, all energy dissipation occurs through chain motion in the amorphous phase. As this chain motion is inhibited by the high crosslink density, the loss modulus for rubbers is also very low, seen in Figure 13. This elasticity is therefore most obvious in the tan delta curves displayed in Figure 14, which is a ratio of the loss to storage moduli. Tan delta increases with increasing damping and so a highly elastic material will have low damping and a low tan delta.

The differences between CL30 and B28 (irrespective of MFI) may be explained by the differences in processing conditions for these two film types. B28 is annealed at a higher temperature and lower output, effectively heat setting harder for longer whilst being constrained in the MD, therefore MD orientation is reduced through stress relaxation and the apparent storage modulus is much lower as a result. The same is also true for the loss modulus.

Example 12

The DSC curves of a film according to the invention were measured using a modification of ASTM D3418, in which the data is generated at 20°C/min instead of 10°C/min.

The film had the layers outlined in Table 26 (in the order shown) and a thickness of around 42 microns. The layers were coextruded before being blown into a film using a bubble process at around 156°C, at which temperature the LLDPE is almost completely molten, thereby resulting in a film according to the present invention.

Table 26

Figure 15 illustrates the DSC curves of repeat experiments. As shown, the lower temperature peak (corresponding to the melting point of the LLDPE, i.e. the second layer material) has a single peak during the initial heating step.

A film of the same structure was coextruded and stretched, before being heat treated in an oven at either 145°C or 150°C in order to melt the LLDPE. Figure 16a illustrates the DSC curves at 150°C, while Figure 16b illustrates the DSC curves at 145°C. As shown, the lower temperature peak (corresponding to the melting point of the LLDPE, i.e. the second layer material) has a double peak during the initial heating step.