CN113348071A

CN113348071A - Film

Info

Publication number: CN113348071A
Application number: CN202080011076.8A
Authority: CN
Inventors: 克里斯多佛·科恩克尔; 乔纳森·休伊特
Original assignee: Innovia Films Ltd
Current assignee: Innovia Films Ltd
Priority date: 2019-01-30
Filing date: 2020-01-30
Publication date: 2021-09-03
Also published as: EP3917767A1; BR112021014826A2; GB201901263D0; WO2020157242A1; GB2580929B; GB2580929A

Abstract

A coextruded multilayer film includes 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 of the multilayer film in the machine direction and/or the cross direction is greater than the average tear propagation strength of an identical film in which the first and second layers are replaced by a single layer having the same thickness and same composition as the first layer and second layer in combination.

Description

Film

The present invention relates to a multilayer film and a method for producing the same. The multilayer film has improved average tear propagation strength.

The polymer film can be made from a variety of different polymers and manufacturing processes, and thus has a variety of different properties that can be tailored to the requirements of the application. In general, specific polymers and film production techniques impart specific property benefits to themselves, while none of the polymers or production methods are prodigies.

The film manufacturing processes are divided into three categories: oriented film (uniaxially oriented or biaxially oriented), cast film or blown film. All of these processes include an extrusion step to form a uniform sheet or loop of polymer, which is then further processed via a stretching step to thin the film to its desired final state, where it is then wound into a continuous roll of film. The fundamental difference between the different production processes is the conditions under which the polymer is stretched and thinned. In the oriented film process, the extruded polymer sheet or ring is cooled down and then reheated to a temperature at which 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, both cast or melt blown processes involve stretching the film under a large amount of molten liquid or flow conditions, which introduces a significantly lower level of molecular orientation, and thus the films are generally less stiff than their oriented counterparts, but are tougher and have significantly higher tear propagation resistance.

Film tearing during manufacture and use is a well-known problem. While increasing the thickness of the film may help improve the average tear propagation strength, this is not always possible, depending on the end use of the film and the desired optical properties. Furthermore, the average tear propagation strength increases not directly related to the increase in thickness, and thus may be difficult to predict.

Average tear propagation strength has previously been improved by including in multilayer films components that can affect the average tear propagation strength by changing the elasticity or other properties of the film layer. For example, WO2016112256 discloses a tear resistant film comprising an elastomeric inner layer, which may comprise a styrene-butadiene-styrene (SBS) block copolymer.

WO9618678 also discloses a film comprising a component selected from the list of suitable compounds (e.g. SBS) which are said to improve tear properties. Similarly, WO2006060766 discloses a film comprising an SBS block copolymer inner layer.

The properties of the major polymeric components within the film, such as their density, are also known to play a role in improving the average tear propagation strength. For example, WO2007104513 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 comprising a multilayer film comprising an ethylene-based polymer having a density greater than or equal to 0.945 g/cc.

Heat treatment is also known to improve the tear resistance properties of the film. US2008131681 discloses a film with improved tear strength comprising a polyethylene matrix containing a polypropylene material, which is warmed during production to the point where the polyethylene material melts but the polypropylene material does not.

A similar film is disclosed in US2012321866, wherein the film comprises a polypropylene matrix comprising a polyethylene material. The film is heated to between the melting points of the two components and cooled without stretching.

JP 397689 discloses a film comprising two layers containing different polyester components, which, after orientation of the film, is heat-treated to between the melting points of the two components, so that the layer with the lower melting point melts and loses its orientation. This is said to improve the forming and workability, stability over time, solvent resistance and dimensional stability.

The optical properties of the film are important in many film applications. Thus, to improve the average tear propagation strength, any modification to the components of the film or to the method of making it should preferably not compromise the optical properties of the film, such as haze or gloss.

Accordingly, there is a need for a film that exhibits improved average tear propagation strength, preferably while also maintaining the optical properties of the 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 of the multilayer film in the machine direction and/or the cross direction is greater than the average tear propagation strength of the same film in which the first and second layers are replaced by a single layer having the same thickness and the same composition as the first layer combined with the first and second layers.

The inventors have surprisingly found that including less orientation layers 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 retains the advantageous results of orientation, such as improved mechanical, optical and barrier properties. However, it has been surprisingly found that the inclusion of a separate, less oriented second layer increases the average tear propagation strength properties of the multilayer film.

Average tear propagation strength can be measured according to ASTM D1938 using the pant tear test. Reference herein to "identical film" relates to films having the same layer structure, e.g. the same additives and the same additional layers such as skin layers.

It is therefore necessary to compare the average tear propagation strength between the film of the invention and a film having the same layer structure, but in a film having the same structure the first and second layers are replaced by a single layer having the same thickness as the combined first and second layers, said single layer 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 layer and the second layer are formed simultaneously using the melt extruded from the die. Thus, a simultaneous stretching process is inherently required during the production of the film, as the two layers are co-extruded and must therefore be formed together before the film is stretched. The simultaneous stretching process is preferably carried out at a temperature at which the first material has a higher amount of residual crystallinity than the second material, thereby creating different levels of orientation in the two layers.

This is in contrast to methods known in the prior art, where a high temperature is applied to the film after it is oriented to melt the material having the lower melting point. When co-extruded and stretched at elevated temperatures, different orientation effects are observed within the lower melting point material layer than when subsequently heat treated at said elevated temperatures.

Additionally, stretching the film of the present invention at elevated temperatures may mean that the polymeric material of the second layer has a single melting peak upon initial heating, as determined by DSC. This is not seen in films that use heat treatment to melt the lower melting point material after orientation, as in this case the lower melting point material will have two melting peaks in the initial heating step.

The different levels of orientation in the film of the present invention may result from the different physical states of the first polymeric material and the second polymeric material. In particular, under certain conditions, the second polymeric material may have a lower degree of residual crystallinity than the first material. The specific conditions may be conditions used during the manufacture of the multilayer film, in particular conditions used during orientation of the film.

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

Thus, the simultaneous stretching process for making the film may be performed under conditions where the second polymeric material has a lower degree of residual crystallinity than the first material, and thus the first layer will be more oriented than the second layer as it contains more residual crystallinity.

Thus, the present invention provides a way to create differently oriented layers within a coextruded multilayer film. This provides a single process that is cheaper, more efficient and more environmentally friendly than extrusion coating or lamination.

The first polymeric material may have a residual crystallinity of 30% to 100%, while the second polymeric material may have a residual crystallinity of 0% to 15% under the specified conditions. Preferably, the first polymeric material may have a residual crystallinity of 40% to 90%, and the second polymeric material may have a residual crystallinity of 0% to 5% under the specific conditions.

The ability to produce a significant level of orientation in the film depends on the polymer properties and/or the stretching conditions. Film-forming polymers are generally either semi-crystalline (e.g., polypropylene and its majority copolymers and polyethylene and its majority copolymers) or amorphous (e.g., G-PET, styrene butadiene rubber, ethylene-propylene rubber).

In the case of amorphous polymers, the ability to produce orientation is determined only by the temperature influence on the chain mobility, since higher temperatures lead to less orientation due to high chain mobility. Amorphous materials have little or no residual crystallinity, and the residual crystallinity does not vary greatly with conditions such as temperature and pressure.

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

The first polymeric material and the optional 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 largely unoriented. The second layer may be completely unoriented. The amount of orientation in the second layer can be measured using techniques known in the art, such as birefringence, birefringence retardation, and dichroic ratio analysis.

When the first polymeric material is semi-crystalline and the second polymeric material is amorphous, the simultaneous stretching process may occur under conditions where the first polymeric material is not completely 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 polymeric material and the second polymeric material are semi-crystalline, the second polymeric material may have a lower melting point range than the first polymeric material. This may mean that there are conditions under which the second polymeric material is more molten than the first polymeric material.

The amount of residual crystallinity is inversely proportional to the degree to which the material melts, which means that more of the molten material becomes less oriented during the stretching process. Thus, the simultaneous stretching process may be performed under conditions where the second polymeric material is more molten than the first polymeric material.

By including semi-crystalline polymers with different melting point ranges in the multilayer structure, and then carefully selecting the optimal manufacturing conditions, multilayer films with layers of different degrees of orientation can thus be produced.

The second polymeric material may be at least partially molten under certain conditions where the second polymeric material has a lower amount of residual crystallinity than the first polymeric material. Under the conditions described, the first polymeric material may also be at least partially molten, but must not be completely molten to ensure that the second polymeric material is more molten than the first polymeric material.

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 temperature range in 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 in this temperature range. Preferably, more than 50% of the second polymeric material is molten in this temperature range, more preferably, more than 75% of the second polymeric material is molten in this temperature range.

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 20 ℃ higher than the peak of the second melting point range. The peak of the first melting point range may be 50 ℃ 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 about 150 ℃, preferably from about 150 ℃ to about 200 ℃, and most preferably about 160 ℃.

The second polymeric material may have a peak in the second melting point range of less than about 200 c, preferably less than about 15 c, and most preferably from about 100 c to about 150 c.

Thus, in one embodiment, the first melting point range has a peak value of about 150 ℃ to about 200 ℃ and the second melting point range has a peak value of about 100 ℃ to about 150 ℃.

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

Thus, the features described above in relation to the first and second melting point ranges 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 be composed of two layers. The multilayer film may include a third layer on the opposite side of the second layer from 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 the same as 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 from the first layer. The third layer may comprise a third polymeric material, wherein conditions may exist where the residual crystallinity of the third polymeric material is greater than the residual crystallinity of the second polymeric material. This may be the same condition that the residual crystallinity of the first polymeric material is greater than the residual crystallinity of the second polymeric material. Thus, the second and third layers may have different orientation levels due to the simultaneous stretching process under the conditions. The third layer may be more oriented than the second layer.

The difference in residual crystallinity between the second polymeric material and the third polymeric material 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 polymeric material and the third polymeric material may be due to a difference in melting point range. The above discussion of 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 a residual crystallinity of 30% to 100% and the second polymeric material may have a residual crystallinity of 0% to 15% under the specific conditions, and preferably, the third polymeric material may have a residual crystallinity of 40% to 90% and the second polymeric material may have a residual crystallinity of 0% to 5% under the conditions. The residual crystallinity of the first polymeric material may fall within the same range as the residual crystallinity of the third polymeric material under the conditions described.

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

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

The second layer can comprise about 15% to about 85% of the total film thickness. Preferably, the second layer may comprise from about 40% to about 70% of the total film thickness.

In addition to the first layer, the second layer, and optionally also the third layer, the multilayer film may comprise one or more layers. The multilayer film structure may be ABA, ACB, ABCD, ABCBA, ABCBD or ABCDE. Additional layers may include adhesive layers (e.g., pressure sensitive adhesives), adhesive release layers (e.g., for use as backing materials in a method of making a release sheet for a label), tie layers, primer layers, print layers, barrier layers, peelable layers, active layers, voided layers, hardened layers, colored layers, and/or coatings.

The multilayer film may include a fourth layer comprising a 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 include a fourth layer comprising a fourth polymeric material, wherein the fourth polymeric material has less residual crystallinity under certain conditions than the first polymeric material. This may be the same condition where the residual crystallinity of the second polymeric material is less than the residual crystallinity of the first polymeric material. Thus, the first and fourth layers may have different orientation levels due to the simultaneous stretching process under the conditions. The first layer may be more oriented than the fourth layer.

The difference in residual crystallinity between the first polymeric material and the fourth polymeric material may be due to the first polymeric material being semi-crystalline and the fourth polymeric material being amorphous. Alternatively, the difference in residual crystallinity between the first polymeric material and the fourth polymeric material may be due to a difference in melting point ranges. The above discussion regarding 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 a residual crystallinity of 30% to 100% and the fourth polymeric material may have a residual crystallinity of 0% to 15% under the specific conditions, preferably, the first polymeric material may have a residual crystallinity of 40% to 90% and the fourth polymeric material may have a residual crystallinity of 0% to 5% under the specific conditions. The residual crystallinity of the second polymeric material may fall within the same range as the residual crystallinity of the fourth polymeric material under the conditions.

The multilayer film may comprise one or more additive materials in one or more of the layers present. The additives may include: dyes, pigments, colorants, metallized and/or pseudo-metallized coatings (e.g., aluminum), lubricants, antioxidants, surfactants, hardening aids, gloss-improving agents, degradation aids, UV attenuating materials, UV light stabilizers, sealing additives, adhesion promoters, antiblocking agents, additives that improve ink adhesion and/or printability, or crosslinking agents (e.g., melamine formaldehyde resins).

The stiffness of the multilayer film may be increased by increasing the stiffness of one of the layers, preferably the first layer. Means for increasing the stiffness of the layers are known in the art and include the use of hard resins (e.g., hydrocarbon resins such as fully hydrogenated C5 or C9 materials), or other compatible stiffness enhancing agents (e.g., COCs, fibers or minerals such as clays), or where appropriate cross-linking agents in one or more layers.

The stiffness of the multilayer film may not be significantly different from the stiffness of the same film in which the first and second layers have been replaced by layers of the same thickness having the same composition as the first layer. The stiffness of the multilayer film may be lower than the stiffness of the same film in which the first and second layers have been replaced by layers of the same thickness and composition as the first layer. Preferably, the stiffness value of the multilayer film, measured using the Gurley stiffness test or by observing the young's modulus (which is proportional to the stiffness), is within 30% of the corresponding value of the same film, preferably within 10% of the corresponding value of the same film. Thus, the multilayer film maintains suitable mechanical properties while exhibiting improved average tear propagation strength.

The stiffness of the multilayer film may be further enhanced by adding a stiffer material, such as Cyclic Olefin Copolymer (COC) or polyester, in one or more layers of the film. The multilayer film may include additional reinforcing layers comprising such materials. If the stiffness of the multilayer film is lower than the stiffness of the same film in which the first and second layers have been replaced by layers of the same composition as the first layer of the same thickness, such a material may change thickness such that it is not significantly different from the thickness of the same film in which the first and second layers have been replaced by layers of the same thickness of the same composition as the first layer.

The multilayer film may have a thickness of from about 10 microns to about 150 microns, preferably from about 15 microns to about 100 microns.

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

The multilayer film may have a 45 ° gloss of about 85 to about 110.

The wide angle haze and/or 45 ° gloss value of the multilayer film may not be significantly different from a film of the same thickness made entirely of the first polymeric material. Preferably, the wide angle haze and/or 45 ° gloss values are within 30% of the corresponding values for the same film, preferably within 10% of the corresponding values for a film of the same thickness made entirely of the first polymeric material. Thus, the multilayer films of the present invention maintain suitable optical properties while exhibiting improved average tear propagation strength.

This is particularly true in embodiments that include a first layer and a third layer formed on either side of a second layer. This embodiment exhibits improved average tear propagation strength while maintaining the desired 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 (e.g., HDPE, LDPE, LLDPE, or ULDPE), a rubber (e.g., SEBS or SBBS), or a copolyester. The rubber may be an activated rubber.

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

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

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

The film may have a tensile strength greater than about 40MPa, preferably greater than about 60 MPa. The film may have a tensile strength of about 40MPa to about 130 MPa. The film may have a tensile strength of about 60MPa to about 115 MPa.

For a 20 micron film, the average load of the film before tearing may be greater than about 0.05N. The average load of the film before tearing may be greater than about 0.09N. The applied load and/or resulting tear may be in the longitudinal or transverse direction.

For a 20 micron film, the average maximum load of the film before tearing may be greater than about 0.07N. The average maximum load of the film before tearing may be greater than about 0.1N. The applied load and/or resulting tear may be in the longitudinal or transverse direction.

The average tear propagation strength of the film can be measured using the trouser tear test. The test is summarized in ASTM D1938. Thus, this test is applied to the multilayer film of the present invention and to the same film in which the first and second layers are replaced by a single layer having the same thickness as the combined first and second layers, the single layer having the same composition as the first layer, and thus it can be determined whether the second layer is oriented less than the first layer.

The presence of a less oriented second layer may also affect the softening curve of the multilayer film as determined by thermomechanical analysis compared to the same film in which the first and second layers are replaced by a monolayer having the same thickness as the combined first and second layers (the monolayer having the same composition as the first layer).

Softening can be measured by placing a small sample of the membrane under a penetrating probe and subjecting the membrane to a constant force of 0.5N. At 5 deg.C for min^-1The probe recession (or recession) is measured as a function of temperature at a fixed heating rate. A negative dimensional change indicates softening of the probe as it is pressed down through the sample. A positive dimensional change indicates sample expansion in the z-plane.

In the films of the present invention, the negative dimensional change may increase abruptly at the melting point or glass transition temperature of the second polymeric material.

The presence of a less oriented second layer may also affect the mechanical properties of the multilayer film as determined by dynamic mechanical analysis, as compared to the same film in which the first and second layers are replaced by a single layer having the same thickness and composition as the combined first and second layers, and having the same composition as the first layer.

The storage loss of the film of the invention may be lower than the same film in which the first and second layers are replaced by a single layer having the same thickness and the same composition as the combined first and second layers. The energy storage loss may be less than 75%, less than 60%, or less than 50% of the energy storage loss of a film of the same thickness made entirely of the first polymeric material. The energy storage loss can be measured at 20 ℃.

The loss modulus of the film of the present invention may be lower than the same film in which the first and second layers are replaced by a single layer having the same thickness and composition as the combined first and second layers. The loss modulus may be less than 75%, less than 60%, or less than 50% of the loss modulus of a film comprising only the first or second polymeric material. The loss modulus can be measured at 20 ℃.

The ratio of loss modulus to storage modulus of the films of the invention may be lower than the same films in which the first and second layers are replaced by a single layer having the same thickness and composition as the combined first and second layers. The ratio may be less than 75%, less than 60%, or less than 50% of a film comprising only the first or second polymeric material. The loss modulus can be measured at 80 ℃.

According to a second aspect of the present invention, there is provided a method of making a multilayer film, the method comprising the steps of: (a) co-extruding 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 conditions wherein the first layer has a higher amount of residual crystallinity relative to the second layer; and (c) stretching the sheet or tube under said conditions to produce a film.

Step (c) may optionally be carried out thermally stable at elevated temperatures (heat setting or annealing) and/or by cooling the film to room temperature.

Thus, the film is stretched under conditions where 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 completely unoriented (e.g., cast film). This means that the second layer of the multilayer film behaves like a cast material, which increases the average tear propagation strength of the multilayer film. When co-extruded and stretched at elevated temperatures, a different orientation effect is observed within the second layer material than when subsequently heat treated at said elevated temperatures. Different DSC curves can also be obtained by the method of the invention.

Since the first polymeric material is semi-crystalline and the second polymeric material is amorphous, differences in residual crystallinity may occur during step (c).

The amount of molten polymer 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 melts more than the first material during step (c), the second layer will be given less orientation than the first layer, as discussed above. The conditions during step (c) may be such that the second polymeric material is predominantly or completely molten during stretching. Preferably, greater than 50% of the second polymeric material is molten at this temperature, and more preferably, greater than 75%.

Under the conditions at which step (c) occurs, the first polymeric material may have a residual crystallinity of from 30% to 100% and the second polymeric material may have a residual crystallinity of from 0% to 15%. Preferably, the first polymeric material may have a residual crystallinity of 40% to 90% and the second polymeric material may have a residual crystallinity of 0% to 5%.

If both the first and second polymeric materials are semi-crystalline, 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 for the first aspect of the invention applies equally to the second aspect of the invention.

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

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

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

The same conditions can 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 include optional heat stabilization, for example by heat setting or annealing. This can occur at higher temperatures than the stretching temperature and can reduce shrinkage of the film.

The film may be uniaxially stretched. Uniaxial stretching can be achieved using tenter or Machine Direction Orienter (MDO) processes.

The film may be biaxially stretched. The biaxial stretching may be performed sequentially or simultaneously. When stretching in one or two directions, sequential stretching may require higher temperatures than are required for simultaneous stretching. Biaxial stretching can be accomplished using bubble or tenter frame processes.

In the case of a continuous stretching process, the film may be biaxially stretched to a stretch ratio of greater than about 3 x 6. The film may be biaxially stretched to a stretch ratio of less than about 6 x 12. The film may be biaxially stretched to a stretch ratio of about 5x 10.

In the case of the simultaneous stretching process, the film may be biaxially stretched to a stretch ratio of greater than about 4 x 4. The film may be biaxially stretched to a stretch ratio of less than about 10 x 10. The film may be biaxially stretched to a stretch ratio of about 7 x 7.

The multilayer film may be composed of two layers. The third layer may be coextruded on the side of the second layer opposite 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 tie layer may be coextruded with the first and second layers. The tie layer is preferably between and may be in contact with the first and second layers.

The third layer may be the same as 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 from the first layer. The third layer may comprise a third polymeric material, wherein under the conditions of step (c), the residual crystallinity of the third polymeric material is greater than the residual crystallinity of the second polymeric material. Thus, the third layer may be more oriented than the second layer as a result of the simultaneous stretching process under the conditions.

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

The fourth polymeric material may be amorphous or semi-crystalline. The multilayer film can include 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). Thus, the first layer may be more oriented than the fourth layer as a result of the simultaneous stretching process under stretching conditions.

The differences in residual crystallinity between the second polymeric material and the third polymeric material and between the first polymeric material and the fourth polymeric material may be due to differences in melting point ranges. The discussion regarding the third and fourth layers in relation to the first aspect is equally applicable to the second aspect.

The extrusion of step (a) may be performed at a temperature above the melting point ranges of both the first and second polymeric materials, and optionally also at a temperature above the melting point ranges of the third and fourth polymeric materials (if present in the film). The extrusion temperature may be from about 200 ℃ to about 250 ℃, preferably about 235 ℃.

The multilayer film may further include a skin layer as the outermost layer of the film. Skin layers may be formed on the first and third layers of the film. The skin layer may be applied before or after the stretching process.

The film may be passed over heated rollers after it is stretched. This may serve to thermally stabilize the film.

The film may also be passed through a cooling zone or over a cooling roll after it has been stretched and optionally thermally stabilized. This may reduce the temperature of the film below the melting point range of the second polymeric material and the optional fourth polymeric material.

The method as described above may be used to produce a membrane as described above. Thus, the features outlined above in relation to the first aspect of the invention are equally applicable to films produced using the method of the second aspect of the invention.

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

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

The multilayer film of the third aspect of the present invention may be a film as described above. Thus, the features outlined above in relation to the first and second aspects of the invention are equally applicable to films produced using 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. The article may be a package, label, ticket or other security document. The articles exhibit improved average tear propagation strength compared to other known articles, including articles comprising the same film in which the first and second layers are replaced by a single layer having the same thickness and same composition as the combined first and second layers. The features outlined above in relation to the first and second aspects of the invention are equally applicable 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 packaged or labelled using the package or label discussed above. The package or label exhibits 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 invention are equally applicable to the package or label of the fifth aspect of the invention.

Aspects of the invention will now be exemplified in the following specific embodiments, which are included by way of example only and are not to be considered as limiting the scope of protection.

Throughout the examples, the test methods outlined in table 1a and the materials outlined in table 1b were used.

TABLE 1a

TABLE 1b

Gloss measurements were made based on ASTM D2457. Gloss results were recorded at 45 ° using a calibration unit using a Novo-gloss Lite unit calibrated to zero reference, then set on a black background of known reflectance or NovoGloss 45 ° Rhopoint meter. The unit was tested periodically against a background of provided calibration blocks and black patches. Results were taken on samples and reported as the average of 3 tests.

The testing was based on ASTM D1003. The WAH of a sample is the percentage of transmitted light that deviates from the incident beam by more than 2.5 degrees by forward scattering when passing through the sample. WAH results were recorded using pre-calibration units (Hazemeter M57 and a Spherical Hazemeter from Diffusion Systems). Each variant was tested 3 times on a sample web and the average results were recorded.

The NAH of the test specimen is parallel light, which is scattered by an arc for more than 6 minutes (0.1 °) when passing through a film or film substrate sample from an incident light beam, and is measured as a percentage of the total light transmitted through the film. The results were recorded using a pre-calibrated "Rayopp" laser hazemeter and recorded over a length of 25mm wide film strip, recording the maximum and minimum results obtained on the sample.

Melting point information for the materials used in the following examples is summarized in table 2, while crystallinity information for the materials is summarized in table 3.

TABLE 2

TABLE 3

Polymers exhibit significant cold crystallization

Comparative example

An industrially produced standard biaxially oriented polypropylene (BOPP) film comprising Moplen HP420M was produced by simultaneous biaxial stretching to a stretch between 6.5 x 6.5 and 8.5 x 8.5 using a double bubble type process. Samples of different thicknesses were prepared and tested using the standard ASTM tear test (ASTM D1938). The results of the tear test are shown in fig. 1, which shows a positive correlation between film thickness and tear strength, independent of the stretch ratio in the range.

Comparison of the multilayer films according to the present invention with these control results can be used to observe the improvement in average tear propagation strength at any thickness. In particular, for any test film, the results above the trend line indicate an improvement in average tear propagation strength.

The optical properties of two of these control films are summarized in table 4.

TABLE 4

Figure 2 compares a standard tenter film (Jindal MB666 — oriented polypropylene film with acrylic coating) to the control film discussed above. As shown in this figure, tenter (sequentially oriented) films generally have lower average tear propagation strengths than bubble (simultaneously oriented) films. The tear properties of the film were almost symmetrical in the machine and cross directions, and the results were based on the 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 obtain a lamellar structure ABCBA, where layers a and B were polypropylene (Moplen HP420M) and layer C was one of two different core layers, namely Eltex KV349 (random propylene terpolymer) and Moplen RP220 (modified propylene random copolymer).

The approximate ratio of layer thicknesses is a + B: C: B + a ═ 1:2: 1. The extrusion was carried out at 235 ℃ with a die and the temperature of the extruder was increased from the first zone of 190 ℃ to 230 ℃. The extrudate was cast onto a chill roll at 30-36 ℃ and produced cast sheets approximately 1mm thick.

The cast sheet samples were then cut into square blocks and simultaneously biaxially stretched using a Bruckner Karo IV film stretcher to produce films.

For each variant, the block was stretched to a stretch ratio of 7 × 7 at a temperature of 150 ℃ and 156 ℃. Another set of films was produced by stretching at a temperature of 156 ℃ to a stretch ratio of 5x 5. The melting point range for polypropylene (Moplen 420M) is above these temperatures, while the melting point ranges for Eltex KV349 and Moplen RP220 are below these temperatures, as summarized in table 2. Furthermore, as shown in table 3, the percentage of residual crystallinity at the stretching temperature for Moplen 420M is much higher than the Eltex KV349 and Moplen RP 220.

Thus, after stretching, the film comprises two more oriented outer layers comprising the first polymeric material and one less oriented core layer comprising the second polymeric material.

The average tear propagation strength of the film of example 1 measured using the tear test is shown in table 5 and fig. 3. As shown in this figure, there is a significant improvement in the average tear propagation strength between the film of example 1 and the control film. In all cases, the Eltex KV349 film had better average tear propagation strength than the Moplen RP220 film.

TABLE 5

The improvement in average tear propagation strength increases with increasing stretching temperature. This is believed to be due to the reduced orientation of the core layer at higher temperatures.

The films were then tested for various optical properties using the tests in table 1, the results of which are summarized in table 6 below. These results show that a less oriented core layer comprising a low melting point polymeric material has little effect on the optical properties of the film. Furthermore, the shrinkage value is low, which is considered to be due to the decreased orientation of the core layer, and thus this shows little preference for shrinkage. If the core layer is already oriented, greater shrinkage is expected.

TABLE 6

The results of example 1 show that using two more oriented outer layers comprising a first higher melting point range polymer material, either side of a less oriented core layer comprising a second lower melting point range material had little effect on the overall appearance, but the average tear propagation strength was greatly improved.

Example 2

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

For each variant, the block was stretched to a stretch ratio of 7 × 7 at a temperature of 156 ℃. The melting point range for polypropylene (Moplen HP420M) was above these temperatures, while the melting point ranges for Eltex KV349 and Moplen RP220 were below these temperatures, as summarized in table 2. Furthermore, as shown in table 3, the percentage of residual crystallinity at the stretching temperature for Moplen 420M is much higher than the Eltex KV349 and Moplen RP 220.

Thus, the film comprises one more oriented core layer comprising the first polymeric material and two less oriented outer layers comprising the second polymeric material.

The average tear propagation strength of the film of example 2 measured using the tear test is shown in fig. 4 and table 7. As shown in this figure, improved average tear propagation strength was observed when the film was stretched at 156 ℃, with none of the outer layer materials showing significant improvement over the other layer materials. As with the film of example 1, the average tear propagation strength increases with increasing stretching temperature. This is believed to be due to the reduced orientation of the outer layer at higher temperatures.

TABLE 7

The films were then tested for various optical properties using the tests in table 1, the results of which are summarized in table 8 below. These results indicate that a less oriented outer layer comprising a polymeric material having a lower melting point range has a significant impact on gloss and haze values. These results are very poor and significantly worse than the membrane of example 1. Thus, while inclusion of such an outer layer does improve the average tear propagation strength, it does not significantly reduce the optical properties of the film.

As with the film of example 1, the shrinkage values were low. This is believed to be due to the reduced orientation of the outer layer, which therefore shows little preference for shrinkage.

TABLE 8

Thus, these results indicate that the combination of a first layer comprising a first polymeric material having a higher melting point range and a second layer comprising a second polymeric material having a lower melting point range improves the average tear propagation strength within the multilayer film.

However, the order of the layers within a multilayer film has an impact on the optical properties of the film, as the presence of a less oriented layer as an outer layer is detrimental to the optical properties. As illustrated in example 1, such a reduction in optical properties is not seen when a less oriented layer is present as a core layer within the film.

Thus, embodiments in which the less oriented layer is the core layer and the more oriented layer is the outer layer provide improved average tear propagation strength while maintaining favorable 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 lamellar structure ABCBA, where layer a was polypropylene (Moplen HP420M), layer B and layer C were Eltex KV349 (random propylene terpolymer).

The throughput of extruders A, B and C was varied to obtain various layer thicknesses to obtain samples having the compositions shown in table 10 below. The extrusion was carried out at 235 ℃ with a die and the temperature of the extruder was increased from the first zone of 190 ℃ to 230 ℃. The extrudate was cast onto a chill roll at 30-36 ℃ and produced cast sheets approximately 1mm thick.

Although the overall film thickness remained constant, the ratio of layer thicknesses varied in each sample. The blocks were all stretched to a stretch ratio of 7 x 7 at a temperature of 156 ℃. The melting point range of polypropylene is above this temperature and the melting point range of Eltex KV349 is below this temperature, as shown in Table 2. Furthermore, as shown in table 3, the percentage of residual crystallinity at the stretching temperature was much higher for Moplen 420M than for Eltex KV 349.

Thus, the film comprises more oriented outer layers comprising the first polymeric material and less oriented core layers comprising the second polymeric material.

The average tear propagation strength of the film of example 3 measured using the tear test is shown in fig. 5 and table 9. As shown in this figure, the average tear propagation strength of the multilayer film generally increases with increasing thickness of the less oriented core layer, with the film of example 3 showing a significant increase in tear strength. Samples that did not meet the expected results showed slight fluctuations in cast sheet structure, which is believed to be due to pulsing and/or fluctuations of the extruder during its production.

TABLE 9

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

Watch 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 lamellar structure ABCBA, where layer a was polypropylene (Moplen HP420M), layer B and layer C were linear low density polyethylene (Dowlex 2106) or ultra low density polyethylene (Attane 4607).

The throughput of extruders A, B and C was varied to obtain samples having the composition A: B + C + B: A1: 2: 1. The extrusion was carried out at 235 ℃ with a die and the temperature of the extruder was increased from the first zone of 190 ℃ to 230 ℃. The extrudate was cast onto a chill roll at 30-36 ℃ and produced cast sheets approximately 1mm thick.

For each variant, the block was stretched to a stretch ratio of 7 x 7 or 5x5 at a temperature of 156 ℃. The melting point range of polypropylene (Moplen HP420M) is above this temperature, while the melting point range of Dowlex 2106 and Attane 4607 is below this temperature, as summarized in table 2. Furthermore, as shown in table 3, the percentage of residual crystallinity at the stretching temperature for Moplen 420M is much higher than the Eltex KV349 and Moplen RP 220.

The average tear propagation strength of the film of example 4 measured using the tear test is shown in fig. 6 and table 11. As shown in this figure, Dowlex 2106 film has significantly better tear propagation resistance than the Attane 4607 film. The film of example 4 had a lower average tear propagation strength than the film of example 1, but all were better than the control film. This difference in tear propagation strength is due to the difference in tear mechanisms, which is highlighted by a significant increase in the maximum tear load. The film of example 4 was also found to have lower stiffness than the film of example 1 due to the addition of the polyethylene material 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 summarized in table 12 below. These results show that the film of example 4 shows comparable optical properties to the corresponding polypropylene film disclosed in table 10. Thicker films exhibit slightly poorer optical properties than thinner films.

TABLE 12

Fig. 7 shows the tear propagation of the film of example 4. The film may be torn as shown in fig. 7, i.e. typically for a given length (about 5mm in fig. 7) before "sticking", forming a hole at this point. The tear is then re-torn from the random point and the results are repeated. Alternatively, the film may tear very little in sections (about 1 mm). This is in contrast to the tear observed in BOPP films, where the film tears and propagates in a single linear motion.

These failure mechanisms cause widely varying tear strengths for each sample and provide widely varying maximum tear strengths. However, those skilled in the art will appreciate that repeated experiments were conducted to obtain average tear strength values, as has been done to produce the values summarized in table 11 above.

Example 5

Cast sheet samples were produced using a Dr Collins 5 layer cast film line. The multilayer extrusion system was configured to obtain a lamellar structure ABCBA, where layers a and B were polypropylene (Moplen HP420M), layer C was amine modified styrene-ethylene-butylene-styrene (Tuftec MP10) or polyamide modified styrene-ethylene-butylene-styrene (Tuftec M1913), both amorphous.

For each variant, the block was stretched to a stretch ratio of 7 x 7 or 5x5 at a temperature of 156 ℃. The melting point range of polypropylene (Moplen HP420M) is above this temperature, while the melting point ranges of Tuftec MP10 and Tuftec M1913 are below this temperature. Thus, the film comprises more oriented outer layers comprising the first polymeric material and less oriented core layers comprising the second polymeric material.

The average tear propagation strength of the film of example 5 measured using the tear test is shown in fig. 8 and table 13. As shown in this figure, these films exhibited an average tear propagation strength that was significantly higher than the films of any of the foregoing examples. It was also found that the film of example 5 had a lower stiffness than the film of example 1 due to the addition of the rubber material in the core layer. The stiffness of the membrane of example 5 is comparable to the stiffness of the membrane of example 4. Just like the films of example 4, the tear propagation of these films was not uniform.

Watch 13

The films were then tested for various optical properties using the methods in table 1, the results of which are summarized in table 14 below. These results indicate that the film of example 5 exhibits optical properties comparable to the corresponding film disclosed in table 10. As expected, thicker films showed slightly poorer optical properties than thinner films.

TABLE 14

Example 6

Cast sheet samples were made from the monolayer structure (samples 6.1 to 6.4) using only a main core extruder on a laboratory scale Rondol multilayer casting line. Samples were prepared by blending Eltex KV349 (random propylene terpolymer) with polypropylene (Moplen HP420M) and extruding the blend to form a monolayer. Some of these blends were pre-mixed by a PRISM twin screw mixing extruder (PRISM), while others were mixed in an extrusion unit (MULTII).

The extrusion was carried out at 230 ℃ with a die and the temperature of the extruder was increased from the first zone of 190 ℃ to 230 ℃. The extrudate was cast onto a chill roll at room temperature and cast sheets of about 1.2mm thickness were produced.

These blocks were then biaxially stretched to a stretch ratio of 7 x 7 at a temperature of 156 ℃. The melting point range of Moplen HP420M is above this temperature, while the melting point range of Eltex KV349 is below this temperature, as shown in table 2. Furthermore, as shown in table 3, the percentage of residual crystallinity at the stretching temperature for Moplen 420M is much higher than the Eltex KV349 and Moplen RP 220.

Multilayer samples (samples 6.5 to 6.7) were made into cast sheet samples using a Dr Collins 5 layer cast film line. The multilayer extrusion system was configured to give a lamellar structure ABCBA, where layer a was polypropylene (Moplen HP420M), layer B was Eltex KV349 (random propylene terpolymer) and layer C was Eltex KV 349.

The extrusion was carried out at 235 ℃ with a die and the temperature of the extruder was increased from the first zone of 190 ℃ to 230 ℃. The extrudate was cast onto a chill roll at 30-36 ℃ and produced cast sheets approximately 1mm thick.

Film samples were produced from cast sheets as described. Cast sheet samples produced by Rondol and Dr Collins units were then cut into square blocks and then simultaneously biaxially stretched using a Bruckner Karo IV film stretcher under the conditions described in table 15 below to produce films.

The average tear propagation strength of the film of example 6 measured using the tear test is shown in table 15. As shown in this table, the PRISM pre-mixed material exhibited slightly better tear strength. However, the blended monolayer films have significantly poorer tear strength than the multilayer films according to the present invention.

Watch 15

The films were then tested for various optical properties using the tests in table 1, the results of which are summarized in table 16 below. These results show that there is no significant change in properties when the blend is premixed compared to mixing in an extruder. However, the optical properties of the blended films are significantly worse compared to the multilayer films according to the present invention.

TABLE 16

The results of example 6 show 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. Rather, these properties depend on the presence of separate layers containing the material, 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 obtain a lamellar structure ABCBA, where layer a was polypropylene blended with 46% hydrocarbon resin masterbatch (70% hydrocarbon resin in PP carrier) (Moplen HP420M) and was about 35% of the total structure and layer B was unmodified polypropylene (Moplen HP420M) or random propylene terpolymer (Eltex KV349) and was about 47% of the total structure. Layer C was about 18% of the total structure and contained polypropylene (Moplen HP420M) blended with 46% hydrocarbon resin masterbatch (70% hydrocarbon resin in PP carrier). Layers B and C were varied as shown in table 17.

TABLE 17

These blocks were then stretched to a stretch ratio of 7 x 7 at a temperature of 150 ℃ or 156 ℃ as outlined in table 17.

The average tear propagation strength of the film of example 7 measured using the tear test is shown in fig. 9. The figure shows that the film shows significant improvement in both average and maximum average tear propagation strength, including the soft KV349 material instead of the standard polypropylene. In fact, the inclusion of KV349 almost doubled the average tear propagation strength of the resulting material.

The average tear propagation strength properties are further summarized in table 18 below. As shown in the table, the addition of a layer of less oriented KV349 material in the structure increased the average tear propagation strength of the film by about 150%.

Watch 18

The films were then tested for various optical properties using the methods in table 1, the results of which are summarized in table 19 below. These results show that when the film is stretched at 150 ℃, the optical properties are very good and have little effect on gloss or wide angle haze. Films oriented at draw ratios greater than 5x5 showed comparable low narrow angle haze results.

Watch 19

Example 8

Monolayer structures (samples 8.1 and 8.2) were prepared by premixing an amine modified styrene-ethylene-butylene-styrene (Tuftec MP10) with polypropylene (Moplen HP420M) with a die at 230 ℃ using a PRISM twin screw mixing extruder and the extruder was raised from a temperature of the first zone of 190 ℃ to 230 ℃.

The resulting material was used to produce cast sheet samples on a laboratory scale Rondol multilayer casting line using only a main core extruder. The extrusion system is configured to provide a sheet of blended material. The extrusion was carried out at 230 ℃ with a die and the temperature of the extruder was increased from the first zone of 190 ℃ to 230 ℃. The extrudate was cast onto a chill roll at room temperature and cast sheets of about 1.2mm thickness were 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 obtain a sheet structure ABCBA, wherein layer a was polypropylene (Moplen HP 420M). A Single Layer (SL) structure is formed by combining the B layer and the C layer or using only the C layer. The same material is used in both B layers to make a Dual Layer (DL) structure, where layer C is the same material as layer a. In all multilayer cases, the non-polypropylene layer of the material was amine-modified styrene-ethylene-butylene-styrene (Tuftec MP 10).

The output of the extruders A, B and C was varied to obtain samples having the compositions shown in Table 20. The extrusion was carried out at 235 ℃ with a die and the temperature of the extruder was increased from the first zone of 190 ℃ to 230 ℃. The extrudate was cast onto a chill roll at 30-36 ℃ and produced cast sheets approximately 1mm thick.

The cast sheet samples produced above were then cut into square pieces and simultaneously biaxially stretched using a Bruckner Karo IV film stretcher to produce films.

The average tear propagation strength of the film of example 8 measured using the tear test is shown in table 20. As the table shows, when comparing the same thickness, the tear strength obtained by blending the material into polypropylene is significantly lower than that of the multi-layer variant of the film according to the invention. Splitting the second material layer appears to reduce the average tear propagation strength on comparable composition samples, as in the DL sample, when compared to a Single Layer (SL) of the same material. This may be due to an increase in the orientation of the layers caused by the surface stretching effect of the material.

Watch 20

The films were then tested for various optical properties using the methods in table 1, the results of which are summarized in table 21 below. These results show that blending materials into polypropylene has a significant impact on the optical properties of the film compared to the multilayer variant of the film. Samples in which the soft material is present as multiple layers rather than a single layer show better optical properties.

TABLE 21

TABLE 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 structure ABCBA, where layers B, C, B and D were polypropylene (Moplen HP420M) and layer a was LLDPE (Dowlex 5057).

The cast sheet samples were then cut into square blocks and simultaneously biaxially stretched at 156 ℃ using a Bruckner Karo IV film stretcher to produce films.

The average tear propagation strength of the film of example 9 measured using the tear test is shown in fig. 10. As shown in the figure, the tear strength increases with an increase in the proportion of the less oriented layer (a). The data in table 22 further supports these results.

TABLE 22

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

TABLE 23

Example 10

The softening curves of the three films were determined by thermochemical analysis. The composition of the film is shown in table 24 below. Film 1 is a bubble film, while

films

2 and 3 correspond to examples 4.2 and 8.4 above, respectively. Therefore, the orientation of the intermediate layers of film 2 and film 3 is expected to be less than the orientation of film 1.

Watch 24

A small sample of each membrane was placed under a penetrating probe and subjected to a constant force of 0.5N. At 5 deg.C for min^-1The probe recession (or recession) is measured as a function of temperature at a fixed heating rate. A negative dimensional change indicates softening of the probe as it is pressed down through the sample. A positive dimensional change indicates sample expansion in the z-plane.

As shown in fig. 11, the softening curve of the film correlates with the melting point or glass transition temperature of the core layer. For reference, the melting range of polypropylene is 160 ℃ to 166 ℃, the melting range of polyethylene is 120 ℃ to 135 ℃ (depending on density), and the abnormal thermal transition of MP10 occurs at 30 ℃ to 75 ℃.

Example 11

The mechanical properties of the three membranes were determined by dynamic mechanical analysis. The composition of the film is shown in table 25 below. CL30 and B28 are Films commercially available from Innovia Films Limited, while QE1 and Q7L10P correspond to examples 4.2 and 8.4 above, respectively. Therefore, the intermediate layer of QE1 and Q7L10P was expected to be less oriented than the other films.

TABLE 25

At an oscillation frequency of 1Hz, 0.15% strain and 2 ℃ min^-1Dynamic Mechanical Analysis (DMA) was performed at a fixed heating rate.

As shown in fig. 12, the standard BOPP film occupies a higher storage modulus range. Multilayer materials containing polyethylene exhibit a significant reduction in storage modulus, probably due to a combination of lack of orientation in the PE layer and inherent lack of elasticity in the PE raw material.

The storage modulus of the multilayer film containing MP10 rubber was further reduced, also due to its lack of orientation. The rubber is mostly amorphous and therefore does not contribute to energy storage from crystallinity. Despite being highly elastic materials, all energy dissipation occurs through chain motion in the amorphous phase. Since this chain movement is inhibited by the high crosslink density, the loss modulus of the rubber is also very low, as shown in fig. 13. Thus, this elasticity is most apparent in the tan δ curve shown in fig. 14, which is the ratio of loss modulus to storage modulus. Tan δ increases with increasing damping, so a highly elastic material will have low damping and low Tan δ.

The difference between CL30 and B28 (independent of MFI) can be explained by the difference in processing conditions for the two membrane types. B28 annealed at higher temperature and lower throughput effectively makes heat-set harder for longer while constrained in the MD, thus reducing MD orientation through stress relaxation, resulting in much lower apparent storage modulus. The same is true for the loss modulus.

Example 12

The DSC curve of the films according to the invention was measured using modified ASTM D3418, where the data was generated at 20 ℃/min instead of 10 ℃/min.

The film had the layers summarized in table 26 (in the order shown) and a thickness of about 42 microns. The layers were coextruded using the bubble process at about 156 ℃ and then blown into a film at which the LLDPE melted almost completely, resulting in a film according to the invention.

Watch 26

Layer composition	Layer% of the total film
		KV353	1.43
Polypropylene	18.83
		Polypropylene	18.83
LLDPE Dowlex 2106	6.87
		LLDPE Dowlex 2106	8.09
LLDPE Dowlex 2106	6.87
		Polypropylene	18.83
Polypropylene	18.83
		KV353	1.43

Figure 15 shows DSC curves for duplicate 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.

Films of the same construction were coextruded and stretched and then heat treated in an oven at 145 ℃ or 150 ℃ to melt the LLDPE. FIG. 16a shows the DSC curve at 150 ℃ and FIG. 16b shows the DSC curve at 145 ℃. As shown, the lower temperature peak (corresponding to the melting point of the LLDPE, i.e. the second layer material) has a bimodal peak during the initial heating step.

Claims

1. 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 oriented less than the first layer such that the average tear propagation strength of the multilayer film in the machine direction and/or the cross direction is greater than the average tear propagation strength of an identical film in which the first and second layers are replaced by a single layer having the same thickness and the same composition as the first layer combined with the first and second layers.

2. The multilayer film of claim 1, wherein the second polymeric material has a lower amount of residual crystallinity under certain conditions than the first polymeric material.

3. The multilayer film according to claim 1 or 2, wherein under certain conditions the first polymeric material has a residual crystallinity of from 30% to 100% and the second polymeric material has a residual crystallinity of from 0% to 15%, preferably wherein the first polymeric material has a residual crystallinity of from 40% to 90% and the second polymeric material has a residual crystallinity of from 0% to 5%.

4. The multilayer film of any of claims 1-3, wherein the second polymeric material has a lower melting point range than the first polymeric material.

5. The multilayer film of any of claims 1-3, wherein the first polymeric material is semi-crystalline and the second polymeric material is amorphous.

6. The multilayer film of any of claims 1-5, wherein the second layer is completely unoriented.

7. The multilayer film of any one of claims 1 to 6, wherein the film further comprises a third layer on an opposite side of the second layer from the first layer, optionally wherein the third layer is the same as the first layer.

8. The multilayer film of any of claims 1-7, comprising a fourth layer that is less oriented than the first layer.

9. The multilayer film of any of claims 1-8, wherein the second layer comprises 15% to 85% of the thickness of the multilayer film.

10. The multilayer film of any of claims 1-9, wherein the film has a wide angle haze of less than 12 and/or wherein the film has a 45 ° gloss of 85 to 110.

11. The multilayer film according to any one of claims 1 to 10, wherein the first polymeric material is polypropylene, polyethylene, PET or nylon and/or wherein the second polymeric material is a polypropylene terpolymer, polypropylene copolymer, polyethylene, rubber or copolyester.

12. The multilayer film of any of claims 1-11, wherein the film has a tensile strength of 40MPa to 130MPa, wherein the average load before tear at 20 microns thick is greater than 0.05N and/or wherein the average maximum load before tear at 20 microns thick is greater than 0.07N.

13. A method of making a multilayer film comprising the steps of:

(a) co-extruding a first layer comprising a first polymeric material and a second layer comprising a second polymeric material to form a sheet or tube;

(b) subjecting the sheet or tube to conditions in which the first layer has a higher amount of residual crystallinity relative to the second layer; and

(c) stretching the sheet or tube under the conditions to produce a film.

14. The method of claim 13, wherein the film is heated or cooled to a temperature at which the second polymeric material is more molten than the first polymeric material.

15. The method of claim 13, wherein the first polymeric material is semi-crystalline and the second polymeric material is amorphous.

16. The method of any one of claims 13 to 15, wherein the film is heated to a temperature at which, in step (b), the first polymeric material has a residual crystallinity of 30% to 100% and the second polymeric material has a residual crystallinity of 0% to 15%, preferably wherein the first polymeric material has a residual crystallinity of 40% to 90% and the second polymeric material has a residual crystallinity of 0% to 5%.

17. The method of any one of claims 13 to 16, wherein the film is stretched in a bubble process.

18. The method of any one of claims 13 to 17, wherein a third layer is coextruded on the opposite side of the second layer from the first layer, optionally wherein the third layer is the same as the first layer.

19. The method of any one of claims 13-18, wherein a fourth layer is coextruded with the first layer and the second layer, wherein the fourth layer has less residual crystallinity during step (c) than the first layer.

20. The method of any one of claims 13-19 to form the multilayer film of any one of claims 1-12.

21. Use of a second layer comprising a second polymeric material in a multilayer film, 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 cross direction of the multilayer film compared to a same film in which the first and second layers are replaced by a monolayer having the same thickness and the same composition as the first layer and the second layer in combination.

22. An article formed from the film of any of claims 1-12.

23. The article of claim 22, wherein the article is a package, label, ticket, or other security document.

24. An article packaged or labeled using the package or label of claim 23.