A polylactic acid bi-stretch film material and a method of making the same

Abstract

The application discloses a kind of polylactic acid double-stretching film materials and preparation method thereof, belong to the field of biaxially oriented film.The polylactic acid double-stretching film material includes the following mass fraction of raw materials: polylactic acid 95~99 parts, melt strength improvement aid 1~5 parts, hydrolysis inhibitor 0.3~1 parts and processing aid 0.5~1 parts;Melt strength improvement aid is ultra-high molecular weight polar polymer;Ultra-high molecular weight polar polymer contains one or a combination of two of epoxy group and side chain ester group.The melt strength improvement aid improves the melt strength and melt uniformity of polylactic acid, thereby improving the yield and film thickness uniformity of polylactic acid double-stretching film;The addition of hydrolysis inhibitor improves the heat processing stability of the material, to obtain a kind of processing performance excellent, while taking into account melt strength and uniformity, thickness fluctuation small polylactic acid double-stretching film material.

Description

Technical Field

[0001] This invention relates to the field of biaxially oriented films, and particularly to a polylactic acid biaxially oriented film material and its preparation method. Background Technology

[0002] Biaxially oriented film is a type of plastic resin-based film manufactured through a specific biaxial stretching process. This involves first extruding plastic resin into a sheet-like substrate, then controlling parameters such as stretching temperature, rate, and stretch ratio to perform longitudinal and transverse stretching. Due to the stress-induced orientation, the macromolecular chains in the stretched matrix material transform from a completely random, entangled state to a more ordered arrangement parallel to the stress direction, thereby optimizing the molecular structure and significantly improving the film's physicochemical properties. Films obtained through biaxial stretching exhibit high tear resistance, impact resistance, dimensional stability, and oxygen and moisture barrier properties, making them a key material in various fields such as packaging, electronics, and biomedicine.

[0003] Compared to traditional petroleum-based biaxially oriented polypropylene (POP), biaxially oriented polyester (PEP), and biaxially oriented nylon (OY) films, biaxially oriented polylactic acid (PLA) films exhibit excellent biocompatibility and biodegradability, aligning with the trend towards green and environmentally friendly development. Furthermore, PLA is a transparent material, resulting in biaxially oriented films with high light transmittance. Compared to ordinary blown biodegradable films, PLA biaxially oriented films offer superior tensile strength and optical properties, along with better gloss and transparency, making them suitable for applications such as food and agricultural product packaging, shrink wrap, biodegradable tape, and disposable food container window film.

[0004] Polylactic acid (PLA), as a thermoplastic crystalline polymer, undergoes changes in orientation, crystallinity, crystal form, and grain size during biaxial stretching molding, thereby improving its tensile strength, barrier properties, heat resistance, and transparency. However, the PLA matrix contains a large number of linear molecular chains, few long branches, and low branching degree, resulting in insufficient melt entanglement density and low melt strength. Furthermore, it is prone to thermal oxidative degradation or hydrolysis at processing temperatures, leading to molecular chain breakage and further reducing melt viscosity and strength. Consequently, the yield of the obtained PLA biaxially stretched film is low, and the thickness fluctuates greatly. During application, uneven thickness can easily lead to problems such as film breakage.

[0005] To address this issue, a traditional method for enhancing the melt strength of polylactic acid (PLA) is to add chain extenders. Chain extenders react with the terminal groups of the PLA molecular chains, causing the chains to lengthen or become slightly branched, thereby increasing melt strength. However, adding chain extenders can easily lead to uneven reactions, resulting in excessive local cross-linking, forming gel points, and affecting the uniformity and transparency of the bistretched film. Furthermore, the viscosity of the melt increases significantly after chain extension, which, combined with the inherent high viscosity of PLA, makes processing difficult.

[0006] Therefore, it is of great significance to obtain a polylactic acid bistretched film that is easy to process, takes into account melt strength and uniformity, and has a uniform thickness. Summary of the Invention

[0007] This invention provides a polylactic acid bistretched film material and its preparation method, which can solve the problems of low melt strength and poor melt uniformity of polylactic acid bistretched films in the prior art.

[0008] In a first aspect, the present invention provides a polylactic acid biaxially stretched film material, comprising the following raw materials in parts by weight:

[0009] Polylactic acid 95-99 parts;

[0010] Melt strength improving agent 1-5 parts;

[0011] Anti-hydrolysis agent 0.3-1 part;

[0012] Processing aids: 0.5–1 part;

[0013] The melt strength improving agent is an ultra-high molecular weight polar polymer; the ultra-high molecular weight polar polymer contains one or a combination of two of the following: epoxy groups and side chain ester groups.

[0014] Preferably, the melting point of polylactic acid is 150-160℃; the melt index is 2-4 g / 10min@190℃ / 2.16kg.

[0015] Preferably, the anti-hydrolysis agent includes carbodiimide anti-hydrolysis agents.

[0016] Preferably, the processing aids include an antioxidant and a lubricant in a mass ratio of (0.2-0.4):(0.3-0.6).

[0017] Preferably, the antioxidants include hindered phenolic antioxidants and phosphite antioxidants in a mass ratio of 1:(1-2).

[0018] Preferably, the lubricant includes one or more combinations of erucamide, oleamide, polyethylene wax, ethylene bis-stearamide, and silicone oil.

[0019] By adopting the above technical solution, this invention adds ultra-high molecular weight polar polymers as melt strength improvers to polylactic acid (PLA) matrix materials. The long polymer chains of these ultra-high molecular weight polymers, after blending with the PLA matrix, can form denser physical entanglements in the melt, thereby improving the melt strength and strain hardening ability of PLA. Simultaneously, since the ultra-high molecular weight polar polymers of this invention are all flexible chains with relatively low melting points and glass transition temperatures, they can improve the melt strength and glass transition temperature of the PLA matrix while simultaneously improving processing performance without significantly increasing viscosity and affecting coating.

[0020] Furthermore, the ultra-high molecular weight polar polymer of this invention contains polar groups including epoxy groups and side chain ester groups, which can intertwine with polylactic acid molecular chains through intermolecular forces. Combined with its ultra-high molecular weight, it forms a network pseudo-crosslinked state in the melt, which can increase the melt entanglement density and thus increase the melt strength. However, unlike strongly polar groups, it will react chemically with polylactic acid molecular chains, resulting in uneven reaction and problems such as gel points.

[0021] Furthermore, the addition of ultra-high molecular weight polar polymers can increase melt elasticity. The highly elastic melt formed during melting can not only transmit shear force through a physical cross-linking network, thus ensuring uniform stress and heat distribution, but also suppress uneven flow, thereby greatly improving the yield and thickness uniformity of the polylactic acid biaxially stretched film. This achieves a balance between high melt strength and good melt uniformity in polylactic acid biaxially stretched films.

[0022] The polylactic acid matrix of this invention is selected from polylactic acid with a melting point of 150-160°C. Within this melting point range, it can ensure that the polylactic acid is fully melted and that the molecular chains are intact. This facilitates crystal formation and arrangement into an ordered structure during the stretching and orientation process, and allows for rapid crystallization within the optimal crystallization temperature range. This provides a thermodynamic basis for the crystallization ability and speed of polylactic acid during the biaxial stretching film processing, avoids the adverse effects of high-temperature degradation on crystallization performance, and prevents the formation of large spherulites due to excessively rapid crystallization during the biaxial stretching process, which can cause fogging in the biaxial stretching film. It also provides good basic conditions for the reinforcing effect of melt strength improving agents.

[0023] Meanwhile, the present invention also adds an anti-hydrolysis agent to the polylactic acid matrix. On the one hand, it can remove small molecules generated during the melt processing of the polylactic acid matrix and block the hydrolysis process. On the other hand, it can react with the end groups of the polylactic acid molecules to form stable covalent bonds, prevent degradation caused by end groups, thereby significantly improving the heat processing stability of the material and thus improving the melt uniformity of the material during the stretching process.

[0024] Preferably, the ultra-high molecular weight polar polymer has a molecular weight of 5.0 × 10⁻⁶. 6 ~10.0×106 .

[0025] Preferably, the ultra-high molecular weight polar polymer includes one or more combinations of polyvinyl acetate, ethylene-vinyl acetate copolymer, and epoxy-modified core-shell acrylate copolymer.

[0026] By adopting the above technical solution, the melt strength improving agent of the present invention has an ultra-high molecular weight, which can enhance melt strength without affecting the processing performance of polylactic acid matrix material, improve processing stability, and avoid the problem of uneven distribution. More preferably, the ultra-high molecular weight polar polymer of the present invention includes polyvinyl acetate, ethylene-vinyl acetate copolymer, and epoxy-modified core-shell acrylate copolymer.

[0027] In addition to their ultra-high molecular weight, which enables them to form physical entanglements with polylactic acid molecules and improve melt strength, polyvinyl acetate and ethylene-vinyl acetate copolymers also have a high vinyl acetate content due to their high molecular weight. The increased content of side chain ester groups in their molecular chains creates intermolecular forces between the copolymers and the ester bonds on the polylactic acid molecular chains, thus forming a network structure that can absorb stress and make the stress distribution more uniform. This improves the uniformity of the melt and results in polylactic acid bistretched films with uniform thickness and high stretch ratio obtained through stretching.

[0028] Epoxy-modified core-shell acrylate copolymers, on the one hand, contain epoxy groups that can play a similar role to side-chain ester groups, providing crosslinking entanglement sites and enhancing melt strength; on the other hand, they can enhance melt elasticity. The introduction of highly elastic melts can effectively transfer melt shear force, enhance processing performance, and improve melt uniformity.

[0029] Secondly, the present invention provides a method for preparing a polylactic acid biaxially stretched film material, comprising the following process steps:

[0030] S1. Weigh the raw materials according to the corresponding mass parts, add melt strength improver, anti-hydrolysis agent and processing aid to polylactic acid in sequence, stir and mix for 5 to 10 minutes to obtain the mixture;

[0031] S2. The mixture is melt-plasticized and extruded to obtain polylactic acid biaxially stretched film material.

[0032] Preferably, the processing temperature for melt plasticizing is 170–190°C.

[0033] The beneficial effects of this invention are:

[0034] 1. The polylactic acid biaxially stretched film material of the present invention contains a melt strength improving agent. Specifically, the melt strength improving agent is a polar polymer with ultra-high molecular weight. Its ultra-high molecular weight structure can improve the melt strength and the processing performance of the material at the same time. The long-chain molecular chains form a network pseudo-crosslinked state through physical entanglement and the intermolecular forces between the contained polar groups and polylactic acid molecules, which enhances the elasticity of the melt. The highly elastic melt can transmit shear force through the formed physical crosslinked network, so that the heat distribution is uniform and the flow unevenness can be suppressed. Thus, while enhancing the melt strength of the polylactic acid material, the melt uniformity is improved, and the yield and film thickness uniformity of the polylactic acid biaxially stretched film are improved.

[0035] 2. This invention selects polylactic acid with a melting point of 150-160℃, which provides a thermodynamic basis for the crystallization ability and crystallization rate of polylactic acid during the biaxial stretching process, avoiding the adverse effects of high-temperature degradation on crystallization performance, and preventing the formation of large spherulites due to excessively rapid crystallization during biaxial stretching, which causes fogging of the biaxial stretching film; in addition, an anti-hydrolysis agent is added, which can remove small molecules and prevent degradation caused by end groups, thereby significantly improving the heat resistance and processing stability of the material. Detailed Implementation

[0036] The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0037] Example

[0038] Example 1: A polylactic acid bistretched film material was prepared according to the following method:

[0039] S1. Weigh out 95 parts of polylactic acid (melting point 150℃, melt index 3g / 10min@190℃ / 2.16kg) and 5 parts of melt strength improving agent polyvinyl acetate (average weight-average molecular weight 6.0×10⁻⁶). 6 0.3 parts of carbodiimide and 0.5 parts of processing aids, wherein the processing aids include an antioxidant and ethylene bis-stearamide in a mass ratio of 0.3:0.5, and the antioxidant is a combination of antioxidant 1010 and antioxidant 168 in a mass ratio of 1:1; then melt strength improver, anti-hydrolysis agent and processing aids are added to polylactic acid in sequence, and the mixture is stirred for 5 minutes to obtain a mixture;

[0040] S2. The mixture is melt-plasticized and extruded to obtain polylactic acid biaxially stretched film material, wherein the melting-plasticization processing temperature is 180℃.

[0041] Example 2: A polylactic acid bistretched film material was prepared according to the following method:

[0042] S1. Weigh 97 parts of polylactic acid (melting point 150℃, melt index 3g / 10min@190℃ / 2.16kg) and 3 parts of melt strength improving agent ethylene-vinyl acetate copolymer (average weight-average molecular weight 6.0×10⁻⁶). 6 0.6 parts of carbodiimide and 0.5 parts of processing aids, wherein the processing aids include an antioxidant and ethylene bis-stearamide in a mass ratio of 0.3:0.5, and the antioxidant is a combination of antioxidant 1010 and antioxidant 168 in a mass ratio of 1:1; then melt strength improver, anti-hydrolysis agent and processing aids are added to polylactic acid in sequence, and the mixture is stirred for 5 minutes to obtain a mixture;

[0043] S2. The mixture is melt-plasticized and extruded to obtain polylactic acid biaxially stretched film material, wherein the melting-plasticization processing temperature is 180℃.

[0044] Example 3: A polylactic acid bistretched film material was prepared according to the following method:

[0045] S1. Weigh 99 parts of polylactic acid (melting point 150℃, melt index 3g / 10min@190℃ / 2.16kg) and 1 part of melt strength improving agent epoxy-modified core-shell acrylate copolymer (average weight-average molecular weight 6.0×10⁻⁶). 6 1 part carbodiimide and 0.5 parts processing aids, wherein the processing aids include an antioxidant and ethylene bis-stearamide in a mass ratio of 0.3:0.5, and the antioxidant is a combination of antioxidant 1010 and antioxidant 168 in a mass ratio of 1:1; then melt strength improver, anti-hydrolysis agent and processing aids are added to polylactic acid in sequence, and the mixture is stirred for 5 minutes to obtain a mixture;

[0046] S2. The mixture is melt-plasticized and extruded to obtain polylactic acid biaxially stretched film material, wherein the melting-plasticization processing temperature is 180℃.

[0047] Comparative Example

[0048] Comparative Example 1, a polylactic acid bistretched film material, differs from Example 1 only in that the amount of polyvinyl acetate added as a melt strength improving agent is 8 parts.

[0049] Comparative Example 2, a polylactic acid bistretched film material, differs from Example 1 only in that it uses an equal amount of material with an average weight-average molecular weight of 2.0 × 10⁻⁶. 6 Polyvinyl acetate is used to replace the melt strength improving agent polyvinyl acetate.

[0050] Comparative Example 3 is a polylactic acid bistretched film material, which differs from Example 1 only in that it does not contain the melt strength improving agent polyvinyl acetate.

[0051] Comparative Example 4 is a polylactic acid bistretched film material, which differs from Example 1 only in that the melting point of polylactic acid is 165°C.

[0052] Comparative Example 5 is a polylactic acid bistretched film material, which differs from Example 1 only in that it does not contain carbodiimide anti-hydrolysis agent.

[0053] Performance testing

[0054] Sample preparation: The polylactic acid bistretched film material obtained in the examples and comparative examples was processed to obtain a polylactic acid bistretched film with a thickness of 15 μm. A control group was set up. The raw material of the polylactic acid bistretched film in the control group was polylactic acid resin. The melting point of polylactic acid resin was 150℃ and the melt index was 3g / 10min@190℃ / 2.16kg.

[0055] The biaxial stretching process specifically includes the following process parameters:

[0056] The extruder temperature is 175-185℃; the quench roll temperature is 40℃; the longitudinal stretching roll temperature is 60℃; the setting roll temperature is 40℃; the transverse stretching roll temperature is 70℃; and the setting roll temperature is 100℃.

[0057] Performance testing:

[0058] 1. Maximum stretch ratio: Calculate the product of the stretch ratios of the bi-stretched film sample in both the MD (longitudinal) and TD (transverse) directions (MD). TD) is denoted as the maximum stretch ratio;

[0059] 2. Thickness fluctuation test: According to the relevant records in GB / T 6672-2001 "Mechanical measurement method for determination of thickness of plastic film and sheet", the sample double-stretched film was measured at multiple points, and the thickness fluctuation value 2σ compared with the standard thickness of 15μm was calculated.

[0060] 3. Surface test: Observe whether the surface of the biaxially stretched film of the sample is fogged.

[0061] The results of the above experiments are shown in Table 1:

[0062] Table 1 Performance test results

[0063]

[0064] According to Table 1, combined with Examples 1-3 and the control group, it can be seen that Examples 1-3 have higher maximum stretch ratios and lower thickness fluctuations compared to the control group. This indicates that compared to the unmodified polylactic acid matrix material, the polylactic acid biaxially stretched film material provided by this invention has higher melt strength and melt uniformity, resulting in a uniform biaxially stretched film thickness and good processing stability. This may be because Examples 1-3 contain melt strength improvers and anti-hydrolysis agents, which utilize the physical entanglement of long-chain molecules at ultra-high molecular weight and the intermolecular interactions between the contained polar groups and polylactic acid molecules to form a network pseudo-crosslinked state, enhancing melt strength, melt elasticity, suppressing uneven flow, and improving heat resistance processing stability, thereby improving the uniformity of the obtained polylactic acid biaxially stretched film thickness.

[0065] Based on Examples 1 and Comparative Examples 1-3, it can be seen that the maximum stretch ratio of Comparative Examples 1-3 decreased and the thickness fluctuation value increased compared to Example 1. This indicates that the melt strength and melt uniformity of Comparative Examples 1-3 decreased. The reason for this may be that Comparative Example 1 increased the amount of melt strength improver, which, on the one hand, leads to uneven dispersion of the excessive melt strength improver in the polylactic acid matrix, thus disrupting the melt uniformity. On the other hand, excessive polar groups lead to excessive molecular chain entanglement and poor diffusion ability, both of which result in thickness fluctuations and performance degradation in the final polylactic acid biaxially stretched film material during stretching processing. Comparative Example 3, without the addition of melt strength improver, lacks control over the polylactic acid melt entanglement density, leading to a significant performance degradation.

[0066] In Comparative Example 2, a lower molecular weight polar polymer was used to replace the ultra-high molecular weight polar polymer. Although the polar groups contained therein could have a certain interaction force with the polylactic acid molecules, the lack of a large number of long-chain molecules caused the melt entanglement density to decrease, resulting in a decrease in the melt strength and melt uniformity of polylactic acid.

[0067] Compared to Example 1, Comparative Example 4 began to show a hazy phenomenon on its surface. The reason for this may be that Comparative Example 4 used polylactic acid with a melting point of 165°C as the substrate. Polylactic acid with a melting point of 165°C is a medium-purity material, and its crystallization ability under thermal stress is much higher than that of polylactic acid with a melting point of 150°C, which is a low-purity material. Therefore, the bistretched film material obtained in Comparative Example 4 is prone to crystallization and the formation of large spherulites during the bistretching process, resulting in the hazy phenomenon of the bistretched film sample and a decrease in the stretching ratio.

[0068] The thickness fluctuation of Comparative Example 5 is larger than that of Example 1. This may be because no anti-hydrolysis agent was added in Comparative Example 5. During the processing, the probability of polylactic acid degradation at high temperature increases, the number of small molecules in the system increases, resulting in greater fluctuations in melt strength and a decrease in the uniformity of the bilayer film of the sample.

[0069] The above-disclosed embodiments are merely a few specific examples of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A polylactic acid bistretched film material, characterized in that, The raw materials include the following parts by weight: Polylactic acid 95-99 parts; Melt strength improving agent 1-5 parts; Anti-hydrolysis agent 0.3-1 part; Processing aids: 0.5–1 part; The melt strength improving agent is an ultra-high molecular weight polar polymer; the ultra-high molecular weight polar polymer contains one or a combination of two of epoxy groups and side chain ester groups; The weight-average molecular weight of the ultra-high molecular weight polar polymer is 5.0 × 10⁻⁶. 6 ~10.0×10 6 ; The ultra-high molecular weight polar polymer includes one or a combination of two of polyvinyl acetate and ethylene-vinyl acetate copolymer; The polylactic acid has a melting point of 150–160°C and a melt index of 2–4 g / 10 min at 190°C / 2.16 kg.

2. The polylactic acid bistretched film material according to claim 1, characterized in that, The anti-hydrolysis agent includes carbodiimide-based anti-hydrolysis agents.

3. The polylactic acid bistretched film material according to claim 1, characterized in that, The processing aids include an antioxidant and a lubricant in a mass ratio of (0.2-0.4):(0.3-0.6).

4. The polylactic acid bistretched film material according to claim 3, characterized in that, The antioxidants include hindered phenolic antioxidants and phosphite antioxidants in a mass ratio of 1:(1-2).

5. The polylactic acid bistretched film material according to claim 3, characterized in that, The lubricant includes one or more of the following: erucamide, oleamide, polyethylene wax, ethylene bis-stearamide, and silicone oil.

6. A method for preparing a polylactic acid bistretched film material, used to prepare the polylactic acid bistretched film material according to any one of claims 1 to 5, characterized in that, The process includes the following steps: S1. Weigh the raw materials according to the corresponding mass parts, add melt strength improver, anti-hydrolysis agent and processing aid to polylactic acid in sequence, stir and mix for 5 to 10 minutes to obtain the mixture; S2. The mixture is melt-plasticized and extruded to obtain polylactic acid biaxially stretched film material.

7. The method for preparing a polylactic acid bistretched film material according to claim 6, characterized in that, The melting and plasticizing processing temperature is 170–190°C.

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