Polymer composition for sheet and thermoformed objects

Abstract

The present invention relates to a polymer composition comprising: a. A propylene-based homopolymer matrix; and b. Ethylene-based copolymer domains dispersed in the matrix, wherein the composition comprises > = 80.0 and < = 98.0% by weight of a propylene-based homopolymer and > = 2.0 and < = 20.0% by weight of an ethylene-based copolymer, relative to the total weight of the polymer composition. Such a polymer composition allows the manufacture of sheets that can be used in a thermoforming process to make objects having good optical properties reflected by low haze, with reduced sag occurring during processing.

Description

Technical Field

[0001] The present invention relates to a polymer composition for producing a sheet. The present invention further relates to a sheet comprising the polymer composition and to thermoformed objects thereof. Background Art

[0002] Thermoforming is one of the most commonly used thermoplastic forming technologies, especially in the field of production of packaging applications. Thermoforming allows the production of shaped objects, such as certain packaging containers, by a simple, low-cost and high-speed manufacturing method, resulting in shaped objects with desired high performance.

[0003] Typically, the thermoforming process is carried out by subjecting a film or sheet of a thermoformable material composition to a temperature at which it becomes sufficiently softened or pliable that the pliable sheet is forced into a mold of the desired shape by applying a vacuum to pull the pliable sheet into the mold, by a counterpart pushing the sheet into the mold, or by a combination of both.

[0004] In this way, one can make shapes for a variety of applications, such as containers and cups. Ongoing developments related to the requirements for shaped objects continue to drive the need for the development of materials used in sheets that are suitable for being used in such newly developed shapes and the forming methods for making them. In particular, there is an ongoing drive to reduce the weight of objects, that is, the amount of material used to make such objects. There are a variety of reasons why such weight reduction is considered desirable. For example, when the sheet used for a thermoforming process is thinner, less energy is required to bring the sheet into a thermoformable state, which is beneficial both to the energy consumption of the process and to the speed at which the process can be operated. In addition, a reduction in the amount of material used is also beneficial because it means that less waste is generated when the object reaches the end of its useful life.

[0005] Another driving force in the field of thermoforming is the increased use of polyolefin materials. Polyolefin materials constitute the majority of thermoplastic materials currently used worldwide, and therefore also constitute the majority of plastic materials that end up in recycling streams. Since recycling streams are easier to handle when the material composition therein is as uniform as possible, it is desirable to increase the fraction of polyolefin materials therein. In view of this, it is desirable that thermoformed packaging objects be made from polyolefin material formulations.

[0006] A specific type of polyolefin material that can be used in thermoforming processes is polypropylene. In particular, homopolymers of propylene are suitable for use in thermoforming processes to make thin-walled packaging objects. In the context of the present invention, such propylene homopolymers are also referred to as hPP. Sheets made of hPP provide the desired stiffness, which is desirable both during the manufacture of the object and in the application, just like the sheet.

[0007] In addition to the unsatisfactory physical properties of hPP sheets, their processability is also a problem in processes involving uniaxial extensional flow, such as thermoforming. During thermoforming, the sheet must be heated to a point where the material can undergo plastic deformation without becoming significantly thinner in one section and not thinner in another, resulting in production discontinuities due to sheet breakage and a high rate of rejects with unacceptable thickness variations. This process property in thermoforming is called sag resistance and pure hPP sheets typically have poor sag resistance.

[0008] However, there is also a need to mitigate other properties of pure hPP in order to optimize the manufacturing process of thin-walled thermoformed objects and to ensure or improve the quality of such objects. In particular, for a given application, the toughness of hPP typically needs to be improved.

[0009] According to the prior art, attempts have been made to improve hPP compositions by adding a portion of propylene-based elastomeric materials to them. Although this may lead to a limited improvement in the toughness of the material, it negatively affects other properties, in particular the optical properties.

[0010] Therefore, there continues to be a need for hPP material formulations with improved toughness and optical properties for thermoforming applications. Summary of the invention

[0011] According to the present invention, this is now achieved by a polymer composition comprising:

[0012] a. a propylene-based homopolymer matrix; and

[0013] b. ethylene-based copolymer domains dispersed in the matrix,

[0014] wherein the composition comprises ≥ 80.0 and ≤ 98.0 wt. % of a propylene-based homopolymer and ≥ 2.0 and ≤ 20.0 wt. % of an ethylene-based copolymer, relative to the total weight of the polymer composition.

[0015] Such polymer compositions allow the production of sheets that can be used in thermoforming processes to produce objects with good optical properties reflected by low haze, wherein reduced sagging occurs during processing.

[0016] For example, the composition may include the sum of the weight of propylene-based homopolymers and ethylene-based copolymers of ≥95.0 wt %, preferably ≥98.0 wt %, relative to the total weight of the polymer composition. Alternatively, the composition may be composed of propylene-based homopolymers and ethylene-based copolymers. For the avoidance of doubt, "essentially composed of..." herein means that the composition does not include other thermoplastic polymer materials other than propylene-based homopolymers and ethylene-based copolymers, except for a small amount of thermoplastic polymers that can be present as carriers in the masterbatch formulation of additives that may be present in the composition. For example, the composition may be composed of propylene-based homopolymers, ethylene-based copolymers and additives. For example, the composition may be composed of propylene-based homopolymers, ethylene-based copolymers and additives of ≤1.0 wt % relative to the total weight of the polymer composition. Preferably, the composition does not include other thermoplastic polymer materials other than propylene-based homopolymers and ethylene-based copolymers.

[0017] It is preferred that in the polymer composition the ethylene-based copolymer is present in the matrix in the shape of domains having an average aspect ratio of ≥ 4.0.

[0018] The average aspect ratio can be determined via atomic force microscopy (AFM) using a method as set forth below in the section describing the Examples in this specification.

[0019] The copolymer based on ethylene can for example comprise polymer units derived from ethylene of ≥70.0 wt % relative to the total weight of the copolymer based on ethylene, and polymer units derived from one or more α-olefins, the α-olefins being selected from 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene and 1-octene, preferably 1-octene. Preferably, the copolymer based on ethylene comprises polymer units derived from ethylene and polymer units derived from 1-butene, 1-hexene or 1-octene, preferably 1-octene. More preferably, the copolymer based on ethylene comprises polymer units derived from ethylene of ≥70.0 wt % relative to the total weight of the copolymer based on ethylene, and polymer units derived from 1-octene.

[0020] The ethylene-based copolymer may, for example, have a value of ≥855 and ≤925 kg / m 3 , preferably ≥870 and ≤910kg / m 3 , more preferably ≥890 and ≤910 kg / m 3 The density of the composite material is as follows: wherein the density is determined according to ASTM D1505 (2010).

[0021] The ethylene-based copolymer may, for example, have a melt mass flow rate measured according to ASTM D1238 (2013) at 190° C. under a load of 2.16 kg of ≧0.5 and ≦10.0 g / 10 min, preferably ≧2.0 and ≦5.0 g / 10 min.

[0022] The ethylene-based copolymer may, for example, have a molecular weight distribution M of ≥2.0 and ≤4.0, preferably ≥2.5 and ≤3.5. w / M n , where M w is the weight average molecular weight and M n is the number average molecular weight, both of which are determined according to ASTM D6474 (2012).

[0023] The ethylene-based copolymer may, for example, have ≤10.0 wt%, preferably ≤5.0 wt%, of a fraction eluting in an a-TREF analysis at ≤30°C, relative to the total weight of the ethylene-based copolymer.

[0024] The ethylene-based copolymer may, for example, have ≥ 90.0 wt%, preferably ≥ 95.0 wt%, of a fraction eluting in an a-TREF analysis between 30 and 94°C, relative to the total weight of the ethylene-based copolymer.

[0025] The ethylene-based copolymer may, for example, have a comonomer content of ≥ 15.0 wt%, preferably ≥ 15.0 wt% and ≤ 30.0 wt%, relative to the total weight of the ethylene-based polymer, preferably wherein the comonomer is 1-octene.

[0026] The ethylene-based copolymer may, for example, have ≤10.0 wt%, preferably ≤5.0 wt%, of a fraction eluting in an a-TREF analysis at ≤30°C, relative to the total weight of the ethylene-based copolymer, and ≥90.0 wt%, preferably ≥95.0 wt%, of a fraction eluting in an a-TREF analysis between 30 and 94°C, relative to the total weight of the ethylene-based copolymer.

[0027] Ethylene-based copolymers may, for example, have:

[0028] a. Molecular weight distribution M ≥ 2.0 and ≤ 4.0, preferably ≥ 2.5 and ≤ 3.5 w / M n , where M w is the weight average molecular weight and M n is the number average molecular weight, both of which are determined according to ASTM D6474 (2012);

[0029] b. ≤10.0 wt. %, preferably ≤5.0 wt. % of the fraction eluted in the a-TREF analysis at ≤30 ° C, relative to the total weight of the ethylene-based copolymer;

[0030] c. ≥ 90.0 wt. %, preferably ≥ 95.0 wt. %, of the fraction eluting in the a-TREF analysis between 30 and 94 ° C, relative to the total weight of the ethylene-based copolymer; and

[0031] d. A comonomer content of ≥ 15.0 wt%, preferably ≥ 15.0 wt% and ≤ 30.0 wt%, relative to the total weight of the ethylene-based polymer, preferably wherein the comonomer is 1-octene.

[0032] The propylene-based homopolymer may, for example, have a mass of ≥880 and ≤920 kg / m 3 , preferably ≥890 and ≤910kg / m 3 , more preferably ≥900 and ≤910 kg / m 3 The density of the composite material is as follows: wherein the density is determined according to ASTM D793 (2013).

[0033] The propylene-based homopolymer may, for example, have a melt mass flow rate of ≧0.5 and ≦10.0 g / 10 min, preferably ≧2.0 and ≦5.0 g / 10 min, as determined according to ASTM D1238 (2013) at 230° C. under a load of 2.16 kg.

[0034] The invention also relates to a sheet comprising the polymer composition according to the invention, preferably having a thickness of ≥100 and ≤2000 μm. The sheet can, for example, be a multilayer sheet comprising at least one layer, preferably a multilayer sheet comprising at least three layers, wherein the multilayer sheet comprises or consists of the polymer composition in its core layer.

[0035] The invention also relates to a process for producing a thermoformed object, comprising providing a sheet according to the invention, heating the sheet to a temperature of ≥140°C and ≤165°C, preferably ≥145°C and ≤160°C, and subjecting the sheet to a thermoforming process.

[0036] The present invention also relates to thermoformed objects comprising the polymer composition of the present invention.

[0037] The present invention also relates to the use of a polymer composition for reducing sagging in a thermoforming process, the polymer composition comprising:

[0038] A propylene-based homopolymer matrix; and

[0039] ● ethylene-based copolymer domains dispersed in the matrix,

[0040] wherein the composition comprises ≥ 80.0 and ≤ 98.0 wt. % of a propylene-based homopolymer and ≥ 2.0 and ≤ 20.0 wt. % of an ethylene-based copolymer, relative to the total weight of the polymer composition.

[0041] Analytical temperature rising elution fractionation, also known as a-TREF, can be performed using a Polymer Char Crystaf-TREF 300 equipped with a stainless steel column of 15 cm in length and 7.8 mm in internal diameter, using a solution containing 4 mg / ml of sample prepared in 1,2-dichlorobenzene, stabilized at a temperature of 150°C for 1 hour with 1 g / l Topanol CA (1,1,3-tris(3-tert-butyl-4-hydroxy-6-methylphenyl)butane) and 1 g / l Irgafos 168 (tris(2,4-di-tert-butylphenyl)phosphite). The solution can be stabilized for an additional 45 minutes at 95°C under continuous stirring at 200 rpm before analysis. For the analysis, the solution is crystallized from 95°C to 30°C using a cooling rate of 0.1°C / min. Elution can be performed from 30°C to 140°C using a heating rate of 1°C / min. The apparatus can be cleaned at 150°C. The injection volume may be 300 μl and the pump flow rate during elution may be 0.5 ml / min. The volume between the column and the detector may be 313 μl. In the context of the present invention, the fraction eluting at a temperature of ≤ 30.0°C may be calculated by subtracting the sum of the fractions eluting at > 30.0°C from 100%, so that the fractions eluting at ≤ 30.0°C and the fractions eluting at > 30.0°C add up to 100.0% by weight.

[0042] Specifically, a-TREF can be performed using Polymer Char Crystaf-TREF 300, using a solution containing 4 mg / ml polymer in 1,2-dichlorobenzene, wherein the solution is stabilized with 1 g / l 1,1,3-tris(3-tert-butyl-4-hydroxy-6-methylphenyl)butane and 1 g / l tris(2,4-di-tert-butylphenyl)phosphite at a temperature of 150°C for 1 hour, and further stabilized at 95°C under continuous stirring at 200 rpm for 45 minutes, wherein the solution is crystallized from 95°C to 30°C using a cooling rate of 0.1°C / min, and elution is performed from 30°C to 140°C at a heating rate of 1°C / min, and wherein the equipment has been cleaned at 150°C.

[0043] The invention will now be illustrated by the following non-limiting examples.

[0044] Material

[0045] The following materials are used in the examples to illustrate the invention.

[0046]

[0047] Preparation of sheets

[0048] Using the materials listed above, several sheets were made. The sheets consisted of the polymer compositions listed in the table below. The blended formulations as in Examples 2-7 were prepared by dry blending the polymer pellets and then feeding the composition into the hopper of an extruder.

[0049] Example Polymer composition 1 100% hPP 2 95% hPP, 5% PP3 3 90% hPP, 10% PP3 4 95% hPP, 5% PE1 5 90% hPP, 10% PE1 6 95% hPP, 5% PE2 7 90% hPP, 10% PE2

[0050] The sheet was prepared using a 3-layer Randcastle calendaring extrusion line equipped with three extruders, however only one extruder was used. The extruder was equipped with a screw having a diameter of 14.6 mm and an L / D ratio of 36. The calendaring line was equipped with a flat die with a die width of 160 mm and a die gap of 0.2-1 mm. The melt temperature of the extruder was 230°C and the temperature of the primary and secondary cooling rolls was 50°C. Both the primary and secondary cooling rolls had polished surfaces and water was circulated through them to provide temperature control. For sheet extrusion, the extruder throughput was 10 kg / h and the traction speed was 3 m / min. The size of the sheet obtained was 0.4 mm thick and 140 mm wide.

[0051] For each sheet, the cross-sectional morphology is determined by atomic force microscopy (AFM). The sample sheet is cut into a certain size, and the sheet cross section in the flow direction of the sheet during extrusion is cryo-microtomed at -120 ° C using a Leica EMUC7 microtoming device to obtain a flat cross-sectional surface. The sample is microtomed using a Diotome diamond knife mounted in a stainless steel holder. After cryo-microtoming, the microtomed sample is directly subjected to AFM measurement without additional treatment. AFM experiments are performed using the Dimension FastScan AFM system (Dimension FastScan, Bruker, Santa Barbara, the United States). Nanoscope Analysis 9.4 software from Bruker is used as a computer interface for operation, and Nanoscope Analysis 2.0 from Bruker is used to analyze AFM measurement results. All AFM measurements are performed at ambient conditions. For morphological characterization, AFM fastscan tapping mode was used at a frequency of 4 Hz using an AFM fastscan tip (model Fastscan-A, k: 18 N / m, f: 1400 kHz).The following morphological data concerning copolymer particles in a homopolymer matrix were obtained from the analysis of the samples.

[0052] Example 3 5 7 Aspect Ratio 2.86 6.14 6.04 Number of domains 1131 544 605 Equivalent diameter (μm) 0.46 0.58 0.58 Main axis length (μm) 0.88 0.30 0.30 Maximum free distance between particles (μm) 0.67 0.95 0.93

[0053] The sheets obtained from these examples were characterized to identify various parameters of the sheets as shown in the following table.

[0054] Example Total Energy Tensile modulus Haze Droop 1 0.56 1732 16.5 20 2 0.62 1494 17.7 3 0.71 1233 22.9 15 4 0.61 1725 16.2 5 1.14 1614 16.0 6 0.70 1666 14.8 7 0.92 1581 14.4 13

[0055] in

[0056] The total energy, expressed in J, is obtained by integrating the load-displacement curve obtained by subjecting the sample to the test according to ASTM D3763 (2018);

[0057] ● The tensile modulus was determined as the secant modulus according to ASTM D882 (2009) and is expressed in MPa;

[0058] Haze measured according to ASTM D1003 (2013), expressed in %; and

[0059] • Determine the sag by a method comprising providing a sheet sample of 220 mm length and 3.1 mm width which is clamped at either end so that the effective distance between the clamps is 200 mm. Place the sample in an oven at 160°C for 20 min, after which the sag distance at the midpoint of the sample is measured.

Claims

1. A polymer composition comprising: A propylene-based homopolymer matrix; and ethylene-based copolymer domains dispersed in the matrix, wherein the composition comprises ≥ 80.0 and ≤ 98.0 wt% of a propylene-based homopolymer and ≥ 2.0 and ≤ 20.0 wt% of an ethylene-based copolymer, relative to the total weight of the polymer composition.

2. The polymer composition according to claim 1, wherein the composition comprises ≥ 95.0 wt.-%, preferably ≥ 98.0 wt.-% of the sum of the weight of the propylene-based homopolymer and the ethylene-based copolymer, relative to the total weight of the polymer composition.

3. The polymer composition according to any one of claims 1-2, wherein the composition does not contain other thermoplastic polymer materials other than the propylene-based homopolymer and the ethylene-based copolymer, preferably wherein the composition consists of the propylene-based homopolymer and the ethylene-based copolymer.

4. The polymer composition of any one of claims 1-3, wherein the ethylene-based copolymer is present in the matrix in the shape of domains having an average aspect ratio of ≥ 4.

0.

5. The polymer composition according to any one of claims 1 to 4, wherein the ethylene-based copolymer is a copolymer comprising ≥ 70.0 wt.-% of polymer units derived from ethylene and polymer units derived from one or more α-olefins, relative to the total weight of the ethylene-based copolymer, the α-olefin being selected from 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene and 1-octene, preferably 1-octene.

6. The polymer composition according to any one of claims 1 to 5, wherein the ethylene-based copolymer has a molecular weight of ≥855 and ≤925 kg / m 3 , preferably ≥870 and ≤910kg / m 3 , more preferably ≥890 and ≤910 kg / m 3 The density of the composite material is as follows: wherein the density is determined according to ASTM D1505 (2010).

7. The polymer composition according to any one of claims 1 to 6, wherein the ethylene-based copolymer has a melt mass flow rate measured according to ASTM D1238 (2013) at 190°C under a load of 2.16 kg of ≥0.5 and ≤10.0 g / 10 min, preferably ≥2.0 and ≤5.0 g / 10 min.

8. The polymer composition of any one of claims 1-7, wherein the ethylene-based copolymer has one or more of the following: ● Molecular weight distribution M ≥ 2.0 and ≤ 4.0, preferably ≥ 2.5 and ≤ 3.5 w / M n , where M w is the weight average molecular weight and M n is the number average molecular weight, both of which are determined according to ASTM D6474 (2012); ≤ 10.0 wt. %, preferably ≤ 5.0 wt. %, of the fraction eluting in the a-TREF analysis at ≤ 30° C., relative to the total weight of the ethylene-based copolymer; ≥ 90.0 wt. %, preferably ≥ 95.0 wt. %, of the fraction eluting in the a-TREF analysis between 30 and 94° C., relative to the total weight of the ethylene-based copolymer; and • A comonomer content of ≥ 15.0 wt%, preferably ≥ 15.0 wt% and ≤ 30.0 wt%, relative to the total weight of the ethylene-based polymer, preferably wherein the comonomer is 1-octene.

9. The polymer composition according to any one of claims 1 to 8, wherein the propylene-based homopolymer has a molecular weight of ≥880 and ≤920 kg / m 3 The density of the composite material is as follows: wherein the density is determined according to ASTM D793 (2013).

10. The polymer composition according to any one of claims 1 to 9, wherein the propylene-based homopolymer has a melt mass flow rate of ≥0.5 and ≤10.0 g / 10 min, preferably ≥2.0 and ≤5.0 g / 10 min, measured according to ASTM D1238 (2013) at 230°C under a load of 2.16 kg.

11. Sheet comprising the polymer composition according to any one of claims 1 to 10, preferably having a thickness of ≥ 100 and ≤ 2000 μm.

12. The sheet according to claim 11, wherein the sheet is a multilayer sheet comprising at least one layer, preferably a multilayer sheet comprising at least three layers, wherein the multilayer sheet comprises or consists of the polymer composition in its core layer.

13. A method for preparing a thermoformed object, comprising providing a sheet according to any one of claims 11 to 12, heating the sheet to a temperature of ≥ 140°C and ≤ 165°C, preferably ≥ 145°C and ≤ 160°C, and subjecting the sheet to a thermoforming process.

14. A thermoformed object comprising the polymer composition according to any one of claims 1 to 10.

15. Use of a polymer composition for reducing sagging in a thermoforming process, the polymer composition comprising: A propylene-based homopolymer matrix; and ethylene-based copolymer domains dispersed in the matrix, wherein the composition comprises ≥80.0 and ≤98.0 wt% of the propylene-based homopolymer and ≥2.0 and ≤20.0 wt% of the ethylene-based copolymer, relative to the total weight of the polymer composition.

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