A polylactic acid film with high heat-sealing strength, its preparation method, and a polylactic acid composite film

Abstract

The present invention provides a high heat seal strength polylactic acid film and a preparation method thereof and a polylactic acid composite film, wherein the raw material composition of the high heat seal strength polylactic acid film includes a first heat seal polylactic acid, a second heat seal polylactic acid and a processing aid, the first heat seal polylactic acid is a polymeso-lactide obtained by polymerization of meso-lactide monomer, and the second heat seal polylactic acid is a copolymer of L-lactide and D-lactide. The high heat seal strength polylactic acid film provided by the present invention has a heat seal strength greater than 20N / 15mm, can be sealed at 70-90°C, is easier to process while maintaining good film mechanical properties, and is particularly suitable for use as a heat seal layer in flexible packaging.

Description

Technical Field

[0001] The present invention relates to the technical application field of thin film flexible packaging, and particularly relates to a polylactic acid film with high heat-sealing strength, a preparation method thereof, and a polylactic acid composite film. Background Art

[0002] The polylactic acid film is a new type of environmentally friendly film material, which has characteristics such as high gloss, heat-sealability, biodegradability, and bio-based source. In recent years, with the continuous in-depth application and development of polylactic acid films in the flexible packaging field, the requirements for the characteristics of polylactic acid films are getting higher and higher. Generally, for composite film packaging, the inner layer of the composite film is required to have a relatively high heat-sealing strength. The heat-sealing strength of conventional co-extruded films is limited. Therefore, developing a polylactic acid film with high heat-sealing strength can fill the market gap.

[0003] For example, a directly heat-sealable biaxially oriented polylactic acid film disclosed in a patent document with the application number 202010415595.2 (publication date: September 4, 2020) includes a three-layer structure of ABC arranged in sequence. The A layer is the surface heat-sealing layer, the B layer is the middle core layer, and the C layer is the anti-blocking layer. The raw materials of the A, B, and C layers are sliced and cast into PLA sheets, longitudinally stretched and cooled and shaped after preheating, then transversely stretched after preheating, shaped and crystallized, and finally cooled and shaped, and finally traction sliced, corona treated and then wound up to obtain a directly heat-sealable biaxially oriented polylactic acid film with a thickness of 20-100um, and the maximum heat-sealing strength is 9.5N.

[0004] Another example is a degradable biaxially oriented polylactic acid cigarette film disclosed in a patent document with the application number 201911138446.X (publication date: February 11, 2020), which is composed of three layers of A, B, and C co-extruded and stretched. By mass percentage, the A layer includes 80-90% heat-sealing type I PLA, 8-15% slip agent, and 2-5% anti-blocking agent. The heat-sealing type I PLA is composed of 70-80% crystalline PLA and 20-30% non-crystalline PLA; the B layer includes 90-95% crystalline PLA, 1-5% antistatic agent, 1-2% stiffening agent, and 1-5% toughening agent; the C layer includes 94-97% heat-sealing type II PLA, 2-3% slip agent, and 1-3% anti-blocking agent. The heat-sealing type II PLA is composed of 75-85% crystalline PLA and 15-25% non-crystalline PLA; the preparation method is to dry each component according to the dosage, melt and extrude it, then biaxially stretch it, then corona treat it, wind it up, and finally age and slit it to obtain.

[0005] The above-mentioned schemes all adopt co-extrusion and then biaxial stretching to obtain a heat-sealable polylactic acid film. Since the heat-sealing layer is only used as the surface layer and has a relatively thin thickness, it is difficult to provide a heat-sealing strength greater than 10N. In some application fields of composite films, the small heat-sealing strength is difficult to meet the needs of customers. Summary of the Invention

[0006] To solve the problem of insufficient heat-sealing strength of heat-sealable polylactic acid films in the existing technology, the present invention provides a polylactic acid film with high heat-sealing strength. The raw material composition of the film includes a first heat-sealing polylactic acid, a second heat-sealing polylactic acid, and a processing aid.

[0007] The first heat-sealing polylactic acid is poly(meso-lactide) obtained by polymerizing meso-lactide monomers, and the second heat-sealing polylactic acid is a copolymer of L-lactide and D-lactide.

[0008] It should be noted that both the first heat-sealing polylactic acid and the second heat-sealing polylactic acid can be prepared by the ring-opening polymerization method (two-step method) or the direct polycondensation method (one-step method) commonly known to those skilled in the art, and will not be elaborated here.

[0009] In one embodiment, the mass ratio of the first heat-sealing polylactic acid to the second heat-sealing polylactic acid is 5:95 to 50:50.

[0010] In one embodiment, the melt index of the poly(meso-lactide) at 190 °C and 2.16 kg is 2 - 5 g / 10 min.

[0011] In one embodiment, the thickness of the polylactic acid film with high heat-sealing strength is 30 - 100 μm.

[0012] In one embodiment, the molar ratio of L-lactide to D-lactide is 65 - 90:10 - 35 or 10 - 35:65 - 90.

[0013] In one embodiment, the melting point of the second heat-sealing polylactic acid is ≤130 °C or it has no melting point, and its melt index at 190 °C and 2.16 kg is 2 - 5 g / 10 min.

[0014] In one embodiment, the processing aid includes an anti-blocking agent, a slip agent, and an antistatic agent.

[0015] In some embodiments, the anti-blocking agent is one or more of talc, silica, calcium carbonate, montmorillonite, mica.

[0016] In some embodiments, the slip agent is one or more of PE wax, zinc stearate, silicone, calcium stearate, silicone, N,N'-ethylenebisstearamide, erucamide.

[0017] In some embodiments, the antistatic agent is one or more of tetraalkyl quaternary ammonium salts, trialkyl benzyl quaternary ammonium salts, alkyl betaines, alkyl sulfonates, alkyl phosphates.

[0018] In some embodiments, in the high heat-sealing strength polylactic acid film, the mass fraction of the anti-blocking agent is 0.2% - 2%, the mass fraction of the slip agent is 0.2% - 1%, and the mass fraction of the antistatic agent is 0.2% - 1%.

[0019] The present invention provides a method for preparing the high heat-sealing strength polylactic acid film as described above arbitrarily, and the high heat-sealing strength polylactic acid film is prepared by a calendering method or a casting method.

[0020] In one embodiment, the calendering method is to weigh and dry the above raw materials according to a specific ratio, then melt and plasticize them through a screw extruder or an internal mixer to obtain an extruded melt, and then use a four-roll calender to calender the extruded melt to obtain a high heat-sealing strength polylactic acid film with a thickness of 30 - 100 μm.

[0021] In one embodiment, the casting method is to weigh and dry the above raw materials according to a specific ratio, then melt and plasticize them through a screw extruder, extrude the melt from a T-shaped die, and obtain a high heat-sealing strength polylactic acid film with a thickness of 30 - 100 μm through cooling attachment, traction, and winding.

[0022] The present invention provides a polylactic acid composite film, which includes the high-strength polylactic acid film layer as described above arbitrarily.

[0023] In one embodiment, the high-strength polylactic acid film layer and other ordinary polylactic acid film layers are compounded through an adhesive layer.

[0024] In some embodiments, the ordinary polylactic acid film layer can be obtained through processes such as biaxial stretching, calendering, blown film, and casting.

[0025] In some embodiments, the ordinary polylactic acid film layer is processed from commercially available film-grade polylactic acid, and its specific commercially available model can be ESL or NTSS.

[0026] The high heat-sealing strength polylactic acid film provided by the present invention, on the premise of maintaining good film mechanical properties, has a heat-sealing strength greater than 20 N / 15 mm, can be sealed at 70 - 90 °C, is easier to process, and is particularly suitable for use as a heat-sealing layer in flexible packaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0028] Figure 1 Schematic structural diagram of a polylactic acid composite film provided by an embodiment of the present invention.

[0029] Reference numerals:

[0030] A Ordinary polylactic acid film layer B Glue layer C Polylactic acid film layer with high heat-sealing strength Specific embodiments

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0032] For a better understanding of the present invention, the present invention will be described in detail below with reference to specific embodiments and comparative examples, without limiting the present invention in any way.

[0033] Example 1

[0034] (1) Preparation of a polylactic acid film with high heat-sealing strength:

[0035] By weight, 40 parts of a first heat-sealing polylactic acid, 59 parts of a second heat-sealing polylactic acid (molar ratio of L-lactide to D-lactide is 90:10), 0.3 part of calcium carbonate, 0.2 part of zinc stearate, and 0.2 part of alkyl sodium phosphate are kneaded by a kneader at 150-170 °C and a calendered film with a thickness of 45 μm is obtained through a calendering process;

[0036] (2) Preparation of a composite film:

[0037] Use a biaxially oriented polylactic acid film with a thickness of 40 μm and a commercially available model of ESL is compounded with the film prepared in step (1) through an aqueous acrylic glue to obtain a polylactic acid composite film, and its film layer structure refers to Figure 1 .

[0038] Example 2

[0039] (1) Preparation of a polylactic acid film with high heat-sealing strength:

[0040] By weight, 20 parts of a first heat-sealing polylactic acid, 79.2 parts of a second heat-sealing polylactic acid (molar ratio of L-lactide to D-lactide is 70:30), 0.2 part of talc powder, 0.3 part of PE wax, and 0.3 part of tetraalkyl quaternary ammonium salt are plasticized by an extruder at 150-170 °C and a cast film with a thickness of 35 μm is obtained through a casting process;

[0041] (2) Preparation of composite film:

[0042] Use a biaxially oriented polylactic acid film with a thickness of 25 μm and a commercially available model ESL to be compounded with the film prepared in step (1) through an aqueous acrylic glue to obtain a polylactic acid composite film, and its film layer structure refers to Figure 1 .

[0043] Example 3

[0044] (1) Preparation of high heat-sealing strength polylactic acid film:

[0045] By weight, 20 parts of the first heat-sealing polylactic acid, 79.2 parts of the second heat-sealing polylactic acid (the molar ratio of L-lactide to D-lactide is 10:90), 0.2 part of talc powder, 0.3 part of PE wax, and 0.3 part of tetraalkyl quaternary ammonium salt are plastically processed by an extruder at 150 - 170 °C and a cast film with a thickness of 35 μm is obtained through a casting process;

[0046] (2) Preparation of composite film:

[0047] Use a biaxially oriented polylactic acid film with a thickness of 25 μm and a commercially available model ESL to be compounded with the film prepared in step (1) through an aqueous acrylic glue to obtain a polylactic acid composite film, and its film layer structure refers to Figure 1 .

[0048] Example 4

[0049] (1) Preparation of high heat-sealing strength polylactic acid film:

[0050] By weight, 20 parts of the first heat-sealing polylactic acid, 79.2 parts of the second heat-sealing polylactic acid (the molar ratio of L-lactide to D-lactide is 30:70), 0.2 part of talc powder, 0.3 part of PE wax, and 0.3 part of tetraalkyl quaternary ammonium salt are plastically processed by an extruder at 150 - 170 °C and a cast film with a thickness of 35 μm is obtained through a casting process;

[0051] (2) Preparation of composite film:

[0052] Use a biaxially oriented polylactic acid film with a thickness of 25 μm and a commercially available model ESL to be compounded with the film prepared in step (1) through an aqueous acrylic glue to obtain a polylactic acid composite film, and its film layer structure refers to Figure 1 .

[0053] Comparative Example 1

[0054] The difference between Comparative Example 1 and Example 1 is only that the first heat-sealing polylactic acid is replaced with an equal mass of the second heat-sealing polylactic acid (where the molar ratio of L-lactide to D-lactide is 90:10), and the other components, preparation method, and film thickness are the same.

[0055] Comparative Example 2

[0056] The difference between Comparative Example 2 and Example 1 is only that the second heat-sealing polylactic acid is replaced with an equal mass of the first heat-sealing polylactic acid, and the other components, preparation method, and film thickness are the same.

[0057] Comparative Example 3

[0058] The difference between Comparative Example 3 and Example 2 is only that the second heat-sealing polylactic acid in Comparative Example 2 is copolymerized from L-lactide and D-lactide in a molar ratio of 40:60, and the other components, preparation method, and film thickness are the same.

[0059] It should be noted that the specific parameters or some common reagents in the above examples are specific examples or preferred examples under the concept of the present invention, rather than limitations thereto; those skilled in the art can make adaptive adjustments within the concept and protection scope of the present invention. In addition, unless otherwise specified, the raw materials used can be conventional commercially available products in the art or prepared by conventional methods in the art.

[0060] The high heat-sealing strength polylactic acid films prepared in the above examples and comparative examples were tested for the starting heat-sealing temperature and heat-sealing strength, and at the same time, the film processability and the appearance of the heat-sealed part were evaluated, as follows:

[0061] Among them, the test method for the heat-sealing strength of the film is to test the heat-sealing strength of the film according to QB / T 2358-98, the heat-sealing pressure is 135 kPa, and the heat-sealing time is 2 s. The test results are shown in Table 1:

[0062] Table 1

[0063] Example 1 Example 2 Example 3 Example 4 Comparative Example 1 Comparative Example 2 Comparative Example 3 Film processability Normal Normal Normal Normal Normal Film not formed Film formation difficult Sealing start temperature [°C] 77 80 78 79 92 / 76 Heat seal strength [N / 15 mm] 32 28 30 27 14 / 30 Appearance of heat-sealed part Normal Normal Normal Normal Slight wrinkling / Normal

[0064] It can be seen from the test results in Table 1 that the polylactic acid composite film prepared by using the high heat-sealing strength polylactic acid film provided by the present invention, while maintaining good film mechanical properties, significantly improves the heat-sealing strength of the film > 20 N / 15 mm by introducing the first heat-sealing polylactic acid and the second heat-sealing polylactic acid.

[0065] From the comparison of Comparative Example 1, Comparative Example 2, and Example 1, it can be seen that the ratio of the first heat-sealing polylactic acid and the second heat-sealing polylactic acid not only affects the heat-sealing strength of the film. The present invention uses specific ratios of the first heat-sealing polylactic acid and the second heat-sealing polylactic acid to also improve the processability of the film, reduce the starting heat-sealing temperature to the range of 70-90 °C, and ensure that the appearance of the heat-sealed part is flat and does not wrinkle.

[0066] From the comparison between Comparative Example 3 and Example 2, it can be seen that the molar ratio of L-lactide and D-lactide in the present invention can improve the processing performance of the film within a specific range.

[0067] In summary, the polylactic acid film with high heat-sealing strength provided by the present invention has a heat-sealing strength greater than 20 N / 15 mm, can be sealed at 70-90 °C, and is easier to process on the premise of maintaining good mechanical properties of the film, and is particularly suitable for use as a heat-sealing layer in flexible packaging.

[0068] Although terms such as upper surface layer, core layer, lower surface layer, etc. are used more in this article, the possibility of using other terms is not excluded. These terms are only used to more conveniently describe and explain the essence of the present invention; interpreting them as any additional limitation is contrary to the spirit of the present invention.

[0069] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A polylactic acid film with high heat-sealing strength, which is characterized in that: the raw material composition of the polylactic acid film with high heat-sealing strength includes a first heat-sealing polylactic acid, a second heat-sealing polylactic acid and a processing aid, the first heat-sealing polylactic acid is poly(meso-lactide) obtained by polymerizing meso-lactide monomers, and the second heat-sealing polylactic acid is a copolymer of L-lactide and D-lactide; the mass ratio of the first heat-sealing polylactic acid to the second heat-sealing polylactic acid is 5:95 to 50:50; the molar ratio of L-lactide to D-lactide is 65 - 90:10 - 35 or 10 - 35:65 - 90.

2. The polylactic acid film with high heat-sealing strength according to claim 1, which is characterized in that: the melt index of the poly(meso-lactide) at 190°C and 2.16 kg is 2 - 5 g / 10 min.

3. The polylactic acid film with high heat-sealing strength according to claim 1, which is characterized in that: the melting point of the second heat-sealing polylactic acid is ≤130°C or it has no melting point, and its melt index at 190°C and 2.16 kg is 2 - 5 g / 10 min.

4. The polylactic acid film with high heat-sealing strength according to claim 1, which is characterized in that: the thickness of the polylactic acid film with high heat-sealing strength is 30 - 100 μm.

5. The polylactic acid film with high heat-sealing strength according to claim 1, which is characterized in that: the processing aid includes an anti-blocking agent, a slip agent and an antistatic agent.

6. The polylactic acid film with high heat-sealing strength according to claim 5, which is characterized in that: in the polylactic acid film with high heat-sealing strength, the mass fraction of the anti-blocking agent is 0.2% - 2%, the mass fraction of the slip agent is 0.2% - 1%, and the mass fraction of the antistatic agent is 0.2% - 1%.

7. A preparation method of the polylactic acid film with high heat-sealing strength according to any one of claims 1 - 6, which is characterized in that: the polylactic acid film with high heat-sealing strength is prepared by a calendering method or a casting method.

8. A polylactic acid composite film, which is characterized in that: it includes a polylactic acid film layer with high heat-sealing strength according to any one of claims 1 - 6.

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