Polypropylene-based thin film

Abstract

A biaxially oriented film comprising a layer comprising at least one isotactic polypropylene having a degree of crystallinity greater than or equal to 80%, at least one hydrocarbon resin, and having a thickness of between 5 µm and 11 µm.

Description

Description Title: POLYPROPYLENE-BASED THIN FILM Technical field

[0001] The present disclosure relates to the field of polymer-based films for packaging, in particular for food packaging. More specifically, the present disclosure relates to polypropylene-based films. Prior art

[0002] Polymer-based films are widespread. Their proliferation represents a major environmental issue. One way to reduce the environmental impact of polymer-based films is to reduce their thickness, as discussed in document EP 0 712 714.

[0003] The choice of the polymer that essentially constitutes a film is based on the intended use of the film. Among the commonly used polymers, polypropylene occupies a significant place, particularly for food packaging. In order to make packaging recyclable, it was necessary to reduce the thickness of plastic films to reduce their share of the packaging.

[0004] However, reducing the thickness of a polymer-based film leads to losses of mechanical properties, this is particularly true for polypropylene-based films. In order to overcome the loss of mechanical properties, it has for example been proposed to modify the manufacturing process, as disclosed for example in document US 4,261,944.

[0005] A method for reducing the thickness of a polypropylene-based film has also been proposed. This method, described in JP 2003 191 324 A, requires a high number of stretches, which cannot be applied to thinner films with a thickness of less than 15 μm, since the film would break at such a low thickness.

[0006] Although the solutions proposed to date are generally satisfactory, the fact remains that further reducing the thickness of films and restoring mechanical properties that provide flexibility in the possible uses of a film is a challenge that must be met in order to reduce the impact on the environment while guaranteeing good packaging quality. Summary of the invention

[0007] Two of the objectives of the invention, closely linked to each other, are the reduction of the environmental impact of polymer-based films and the reduction of their manufacturing cost.

[0008] Another objective is to obtain a low-thickness polypropylene-based film with sufficient mechanical properties for its use in different application areas, particularly in food packaging.

[0009] Thus, the primary objective of the present invention is to obtain a polypropylene film having a maximum thickness of 11 μm and exhibiting at least one of the following properties: -P1- low shrinkage, -P2- high breaking strength, -P3- high elongation at break, -P4- moisture barrier, -P5- Twist effect

[0010] At least one of the above objectives is achieved by the invention which relates to a biaxially oriented film comprising a layer comprising at least one isotactic polypropylene having a crystallinity rate greater than or equal to 80%, at least one hydrocarbon resin, and having a thickness of between 5 pm and 11 pm.

[0011] The invention also relates to a packaging, in particular a food packaging, comprising the film described above. Brief description of the drawings Fig. 1

[0012] [Fig. 1] is a diagram of a three-layer film according to the invention. Fig. 2

[0013] [Fig. 2] is a bar diagram of elongation at break in the machine direction. Fig. 3

[0014] [Fig. 3] is a bar diagram of the elongation at break in the transverse direction. Fig. 4

[0015] [Fig. 4] is a bar diagram of Young's modulus along the machine direction. Fig. 5

[0016] [Fig. 5] is a bar diagram of Young's modulus in the transverse direction. Fig. 6

[0017] [Fig. 6] is a bar diagram of shrinkage in the machine direction and in the transverse direction. Fig. 7

[0018] [Fig. 7] is a semi-developed formula of an isotactic sequence of propylene units of a portion of a polypropylene. Fig. 8

[0019] [Fig. 8] is a set of three photos showing the difference between a twistable film, i.e. one with a twist effect, and a non-twistable film. Definitions

[0020] In the context of the present invention, the following terms are understood to mean: - "resin" a compound used in the manufacture of plastics. By use, we call resin a polymer. - "isotactic polymer" a polymer comprising an asymmetric carbon in its chain, in the case of polypropylene, if the methyl groups are all located on the same side of the plane represented by the polymer chain, then the polypropylene is isotactic, as illustrated in figure 7 (article by C. Duval on polypropylene from Techniques de l'ingénieur of 2004) - "Crystallinity rate" the percentage of recurrences of monomeric units, it is measured according to the characterization method disclosed in document WO 2022 / 00434 A1. The fraction of mesopentad [mmmm] (unit in %) is measured by NMR 13 C. The mesopentad fraction is measured according to the method described by "Zambelli et al., Macromolecules, Vol. 6, p. 925 (1973)". The NMR measurement 13 C is carried out by dissolving 200 mg of the polymer sample in an 8:2 (by volume) o-dichlorobenzene and benzene solution at 135 0 C. - "twist effect" Twist retention property applied to a film which must be greater than 80%, a film exhibits a twist effect when the film is manually rotated 360° and there is a loss of less than 20% of the twist as illustrated in figure 8. Detailed description of the invention

[0021] The film

[0022] As mentioned above, the invention relates to a biaxially oriented film comprising a layer comprising at least one isotactic polypropylene having a crystallinity rate greater than 80%, at least one hydrocarbon resin, and having a thickness of between 5 pm and 11 pm.

[0023] Film thickness is measured using a Mahr C1216 millitron comparator with a P1300 flat probe. It should be noted that film thickness is generally not exactly linear across the entire film. This is because due to manufacturing processes, particularly during stretching, the film surfaces are not perfectly flat. This results in a variation in film thickness of up to 0.6 pm standard deviation in laboratory tests and up to 0.3 pm standard deviation on an industrial scale.

[0024] Polypropylene

[0025] In the context of the invention, the polypropylene is isotactic and has a crystallinity rate greater than 80%. Preferably, the polypropylene has a crystallinity rate greater than 85%, more preferably greater than 90% and even more preferably greater than 95%.

[0026] Advantageously, polypropylene has a number average molar mass of between 230,000 and 250,000, more advantageously between 240,000 and 250,000. The molar mass of the resins can be measured by hot steric extrusion chromatography using trichlorobenzene solvent and infrared detection. The number and weight masses are then obtained, which give the macrostructure, following ISO 16014 standards.

[0027] While polypropylene may have some branching, it is preferably linear.

[0028] Hydrocarbon resin

[0029] The hydrocarbon resin is preferably selected from the group comprising, preferably consisting of, petroleum resins, terpene resins, styrene resins, cyclopentadiene resins, cyclononadiene resins and mixtures thereof.

[0030] Advantageously, the hydrocarbon resin has a number-average molar mass of between 500 and 5000, more advantageously of between 800 and 4000 and even more advantageously of between 1000 and 3000.

[0031] The molar mass of the resins can be measured by steric extrusion chromatography using tetrahydrofuran solvent, a Waters column and differential refractometer detection. The masses in number and weight are then obtained, which give the macrostructure.

[0032] The hydrocarbon resin prevents the formation of cracks in the isotactic polypropylene layer with a crystallinity rate greater than 80%.

[0033] The mass ratio

[0034] The mass ratio of polypropylene to hydrocarbon resin depends on the degree of crystallinity of the polypropylene. The higher the degree of crystallinity of the polypropylene, the lower the ratio must be. Generally, the mass ratio of isotactic polypropylene to hydrocarbon resin is between 2.9 and 19. Preferably, the mass ratio of isotactic polypropylene to hydrocarbon resin is between 2.9 and 19, preferably between 2.9 and 10, and more preferably between 2.9 and 7.

[0035] Film structure

[0036] According to the invention, the film has a thickness of between 5 μm and 10 μm. Advantageously, the film has a thickness of between 6 μm and 9 μm, more advantageously of between 6 μm and 8 μm.

[0037] The film according to the invention may further comprise a second layer. Preferably, the second layer comprises one or more polymers chosen from the group comprising, preferably consisting of, ethylenic polymers, maleic-anhydride modified polyethylenes, propylene polymers, ethylenic-propylene copolymers, ethylenic-propylene-butylenic copolymers, heterophasic copolymers, butylene homopolymers, butylene copolymers, metallocene polymers, ethyl-methyl acrylate (EMA) polymers, ethylene-vinyl acetate (EVA) polymers and mixtures thereof.

[0038] The film may further comprise a third layer, identical to or different from said second layer. Preferably, the third layer comprises one or more polymers chosen from the group comprising, preferably consisting of, ethylenic polymers, maleic-anhydride modified polyethylenes, propylene polymers, ethylenic-propylene copolymers, ethylenic-propylene-butylenic copolymers, heterophasic copolymers, butylene homopolymers, butylene copolymers, metallocene polymers, ethyl-methyl acrylate (EMA) polymers, ethylene-vinyl acetate (EVA) polymers and their blends.

[0039] Thus, according to one embodiment of the invention, the film is a three-layer film of structure A / B / C. Preferably the layer comprising the isotactic polypropylene and the hydrocarbon resin is layer B.

[0040] Advantageously, the film is free of antistatic agents such as fatty acid esters. This embodiment is advantageous because fatty acid esters can be detrimental to the film's performance, particularly in terms of oxygen barrier and moisture barrier when the film is metallized. Furthermore, films comprising fatty acid esters are difficult to recycle. This can also be extended to migrating products such as tertiary amines or fatty amides such as erucamides which are used for their migration property in order to have sliding properties.

[0041] Movie Properties

[0042] The film according to the invention presents a compromise allowing its use in different fields of application. The properties of the film can be adjusted depending on its destination.

[0043] In particular, the film according to the invention exhibits a shrinkage, when said film is maintained for 15 min at 140°C, less significant than the shrinkage of a film formed with a standard resin. The shrinkage gain, depending on the film formation method and the equipment used, is generally between 25% and 50% in the machine direction and between 40% and 60% in the transverse direction.

[0044] The film according to the invention has an elongation at break in the machine direction greater than 180% and an elongation at break in the transverse direction greater than 45%.

[0045] The film according to the invention also has excellent breaking strength greater than 12.5 daN / mm 2 in machine direction and greater than 35 daN / mm 2 in a transverse direction.

[0046] The Young's modulus of the film according to the invention is greater than 290 daN / mm 2 in machine direction and at 690 daN / mm 2in the transverse direction. The ratio M between the Young's modulus in the machine direction (MD) and the Young's modulus in the transverse direction (TD) according to the following equation 1: M = Y(MD) / (Y(MD) + Y(TD) (equationl ) is generally less than 0.35 and preferably between 0.2 and 0.35. This value of M corresponds to a process window which allows to have a thin film in a stable manner (by limiting breakages) and with satisfactory mechanical and thermal properties.

[0047] Thus, the film according to the invention has a compromise between elasticity, mechanical resistance and thermal stability, opening up great flexibility in the possible uses of said film while having the advantage of being thin. In particular, when the film according to the invention is implemented in a lamination process, or undergoes treatment by metallization or deposition of silicone or alumina, its properties, in particular barrier properties, are not, or only slightly, degraded.

[0048] Uses of the film

[0049] The film can be used for various applications, but is particularly suitable for food use. Thus, the invention also relates to a food packaging comprising the film described above.

[0050] Film preparation process

[0051] The film according to the invention is preferably prepared according to the following process: E1: (co)extrusion including a matrix of composition of 1 to 3 layers of the polymer(s) constituting each of the layers to form a sheet of film, E2: quenching said film sheet, obtained in step E1, on a cooling or casting drum, with a controlled temperature in the range of approximately 5°C to 45°C to solidify said film sheet followed by secondary cooling on another temperature-controlled cooling drum to form a sheet, E3: stretching said sheet, obtained in step E2, in the machine direction, at about 95°C to 165°C, at a ratio of about 4 to 5.5 times the original length, to obtain a uniaxially oriented sheet, E4: stretching said uniaxially oriented sheet, obtained in step E3, in the transverse direction, at about 130°C to 180°C, at a ratio of about 7 to 10 times the original length, then heat-set at about 100 and 200°C to obtain a biaxially oriented film. Examples

[0052] Materials used:

[0053] Polymer P1: isotactic homopolymer with a crystallinity rate of less than 60% according to the mesopentad method, a melt flow index of 2.2g / 10 min at 230°C under 2.16kg, a typical melting temperature of 232-274°C.

[0054] Polymer P2: 60-80% semi-crystalline isotactic homopolymer following the mesopentad method described above. The melt flow rate is 2g / 10min at 230°C and under 2.16kg, and the typical melting temperature is 232-274°C.

[0055] Polymer P3: high crystallinity polymer at 98% according to the mesopentad method, with a melt flow index of 2.0g / 10min at 230°C under 2.16kg, and a softening temperature of 163°C.

[0056] Polymer P4: C5 type hydrocarbon polymer, 60% polynorbornene with a melt flow index of 36g / 10min at 190°C under 2.16kg with a softening point at 140°C.

[0057] Example 1: Film preparation

[0058] Eight films composed of three layers with an A / B / C structure (Figure 1) were prepared according to the following protocol: - The 3 layers A, B and C are extruded, using a coextrusion process including a three-layer composition matrix. The central polymer layer (B) is sandwiched between the outer skin layer (A) and the outer protective layer (C), - The outer layer (A) of a three-layer laminated sheet is cast onto a cooling or casting drum with a controlled temperature in the range of about 15°C to 45°C to solidify the unoriented laminated sheet followed by secondary cooling on another temperature-controlled cooling drum, - The unoriented laminated sheet is stretched in the machine direction at about 95°C to 165°C at a ratio of about 4 to 5.5 times the original length and then heat-set at about 50°C to 100°C to obtain a uniaxially oriented laminated sheet with minimal thermal shrinkage, - The uniaxially oriented laminated sheet is fed into a stenter and preheated to between about 130°C and 180°C and stretched in the transverse direction at a ratio of about 7 to 10 times the original length, then heat-set to give a biaxially oriented sheet with minimal thermal shrinkage.

[0059] Each of the eight films includes the same A layer and the same C layer.

[0060] Layer A has a thickness of 0.7 μm and contains 49.4% by weight of the polymer P1, 50% by weight of a resin comprising maleic anhydride units and 0.6% by weight of a processability agent.

[0061] Layer C has a thickness of 1 pm and is made of a polyethylene-polypropylene copolymer.

[0062] Each of the eight films has a different B layer as shown in Table 1 below: [Table 1]

[0063] The thickness of each of the eight films was measured using a Mahr C1216 comparator with a P1300 flat probe. The results are listed in Table 2 below: [Table 2]

[0064] Example 2: Measurements of the mechanical properties of films

[0065] The elongation at break (AR), tensile strength (TS) and Young's modulus (MY) of each of the eight films were measured in the machine direction (MD) and transverse direction (TD) according to ASTM D882. The shrinkage (RT) of each of the eight films was also measured, in the machine direction (MD) and transverse direction (TD), on 10 cm square samples placed at 140°C for 15 minutes. The results are listed in Table 3 below: [Table 3]

[0066] The elongation at break in the machine direction (Figure 2) is similar for films E2 and E3 as for films CE1, CE2, CE3 and CE4, however, by direct comparison, the elongation at break is much higher for film E1 than for film CE2, for film E2 than for film CE4 and film E3 than for film CE5. Thus, the replacement of polymer P2 by polymer P3 allows a significant increase in the elongation at break in the machine direction.

[0067] In the transverse direction (Figure 3), the elongation at break of films E1, E2 and E3 is much better than the elongation at break of films CE1 to CE5.

[0068] Thus, the P3 polymer, presenting a high crystallinity, makes it possible to improve the elongation at break, in the machine direction and in the transverse direction, compared to the P2 polymer, of average crystallinity.

[0069] Although the breaking strength is slightly lower for E1 film (lowest amount of P3 polymer), E1 film has acceptable breaking strength.

[0070] The Young's moduli of all the films are similar, whether in the machine direction (Figure 4) or in the transverse direction (Figure 5). This means that their respective rigidity is similar. However, the increase in the crystallinity rate by using polymer P3 instead of polymer P2 should be accompanied by an increase in rigidity and therefore in the Young's modulus. The combination with polymer P4, i.e. a hydrocarbon resin, makes it possible to obtain a better elongation at break for the films according to the invention (E1 to E3) than for the films outside the invention (CE1 to CE5) while maintaining good breaking strength and adequate rigidity.

[0071] In other words, the use of the combination of a high crystallinity polymer (polymer P3) and a hydrocarbon resin (polymer P4) in the proportions in accordance with the invention makes it possible to obtain a thin film while retaining the elastic properties of the film.

[0072] Furthermore, the shrinkages (Figure 6) of the films according to the invention (E1 to E3), whether in the machine direction or in the transverse direction, are much lower than the shrinkages of the films without the invention (CE1 to CE5). This indicates that the films according to the invention are more thermally stable than the films without the invention.

[0073] Example 3: Moisture barrier

[0074] The moisture barrier effect of each of the eight films was measured according to ASTM F1249. The results are listed in Table 4 below: [Table 4]

[0075] Polypropylene films inherently have a good oxygen barrier. The hydrocarbon resin layer provides an improvement of approximately 1% over a polypropylene film without a hydrocarbon resin layer.

[0076] Example 4: Influence of the preparation process

[0077] The protocol detailed in Example 1 was reproduced with the same components as for films E1, E2 and E3, but the machine direction stretching was carried out at a ratio of approximately 6 to 8 times the original length.

[0078] All the films obtained in this example broke during stretching.

Claims

Claims

1. Biaxially oriented film comprising a layer comprising at least one isotactic polypropylene having a crystallinity rate greater than or equal to 80%, at least one hydrocarbon resin, and having a thickness of between 5 pm and 11 pm.

2. Film according to claim 1, characterized in that it has a ratio M between the Young's modulus in the machine direction (MD) and the Young's modulus in the transverse direction (TD) according to the following equation 1: M = Y(MD) / (Y(MD) + Y(TD) (equationl ) less than 0.35 and preferably between 0.2 and 0.

35.

3. Film according to any one of the preceding claims, characterized in that the isotactic polypropylene and the hydrocarbon resin are present at a mass ratio of isotactic polypropylene to hydrocarbon resin of between 2.9 and 19.

4. Film according to any one of the preceding claims, characterized in that it further contains a second layer comprising one or more polymers chosen from the group comprising, preferably consisting of, ethylenic polymers, maleic-anhydride modified polyethylenes, propylene polymers, ethylenic-propylene copolymers, ethylenic-propylene-butylenic copolymers, heterophasic copolymers, butylene homopolymers, butylene copolymers, metallocene polymers, ethyl-methyl acrylate (EMA) polymers, ethylene-vinyl acetate (EVA) polymers and mixtures thereof.

5. Film according to claim 4, characterized in that it further contains a third layer, identical to or different from said second layer, comprising one or more polymers chosen from the group comprising, preferably consisting of, ethylenic polymers, maleic-anhydride modified polyethylenes, propylene polymers, ethylenic-propylene copolymers, ethylenic-propylene-butylenic copolymers, heterophasic copolymers, butylene homopolymers, butylene copolymers, metallocene polymers, ethyl-methyl acrylate (EMA) polymers, ethylene-vinyl acetate (EVA) polymers and mixtures thereof.

6. Film according to any one of the preceding claims, characterized in that the hydrocarbon resin has a number-average molar mass of between 500 and 5000, more advantageously of between 800 and 4000 and even more advantageously of between 1000 and 3000 by steric extrusion.

7. Film according to any one of the preceding claims, characterized in that the hydrocarbon resin is chosen from the group comprising, preferably consisting of, petroleum resins, terpene resins, styrene resins, cyclopentadiene resins, cyclononadiene resins and their mixtures.

8. Film according to any one of the preceding claims, characterized in that said crystallinity level is greater than or equal to 85%, preferably greater than or equal to 90%, more preferably greater than or equal to 95%.

9. Film according to any one of the preceding claims, characterized in that it has a thickness of between 6 pm and 9 pm, preferably between 6 pm and 8 pm.

10. Film according to any one of the preceding claims, characterized in that, when said film is maintained for 15 min at 140°C, said film exhibits less shrinkage than the shrinkage of a film formed with a standard resin, in particular said film exhibits a shrinkage gain generally of between 25% and 50% in the machine direction and between 40% and 60% in the transverse direction.

11. Packaging comprising the film according to any one of claims 1 to 10.

12. A process for preparing a film according to any one of claims 1 to 10 comprising the following successive steps: E1: (co)extrusion including a matrix of composition of 1 to 3 layers of the polymer(s) constituting each of the layers to form a sheet of film, E2: quenching said film sheet, obtained in step E1, on a cooling or casting drum, with a controlled temperature in the range of approximately 5°C to 45°C to solidify said film sheet followed by secondary cooling on another temperature-controlled cooling drum to form a sheet, E3: stretching said sheet, obtained in step E2, in the machine direction, at about 95°C to 165°C, at a ratio of about 4 to 5.5 times the original length, to obtain a uniaxially oriented sheet, E4: stretching said uniaxially oriented sheet, obtained in step E3, in the transverse direction, at about 130°C to 180°C, at a ratio of about 7 to 10 times the original length, then heat-set at about 100°C to 200°C to obtain a biaxially oriented film.

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