Biodegradable modified waste polystyrene film and preparation method thereof

Abstract

The invention relates to the technical field of waste polystyrene treatment, in particular to a biodegradable modified waste polystyrene film and a preparation method thereof. The problem existing in the prior art is that waste styrene is modified into a biodegradable polystyrene material through conventional chemical grafting, an initiator or a catalyst needs to be additionally added, and the recycling cost is high. In order to solve the problems, the invention provides the biodegradable modified waste polystyrene film, through ester exchange reaction of methyl lactate and a PS molecular chain under specific conditions, a lactate group is directly grafted to a PS main chain, and the problem of non-degradability of PS is solved from the chemical structure. The whole modification process is carried out under heating and sealing conditions, the transesterification reaction is driven by utilizing the properties of reactants, an initiator or a metal catalyst commonly used in traditional grafting modification is omitted, the post-treatment step is simplified, and the production cost is reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of waste polystyrene treatment, and particularly relates to a biodegradable modified waste polystyrene film and a preparation method thereof. BACKGROUND

[0002] Polystyrene (PS) is a kind of general-purpose plastic, which is widely used in packaging, cushioning, catering and many other fields due to its light weight, heat insulation and low cost. However, its chemical structure has a stable C-C main chain and benzene ring side group, which makes PS extremely difficult to degrade in the natural environment.

[0003] A large amount of waste PS foams and products, such as meal boxes, packaging materials, etc., not only form microplastic pollution due to their brittle and fragile nature, posing a serious threat to marine and terrestrial ecosystems, but also mean that valuable non-renewable resources are wasted, as the raw materials are petroleum and natural gas. At present, the output of waste PS waste is huge, which has formed a lasting pressure on soil, water and ecological system, and the problem of waste PS pollution needs to be solved as soon as possible.

[0004] At present, the main treatment methods for waste PS are landfill and incineration. Landfill treatment will occupy a large amount of land resources, and because PS degrades slowly, it will occupy land space for a long time. Incineration treatment may produce toxic and harmful gases, posing a risk of secondary pollution, and also emitting CO2, which will exacerbate the greenhouse effect.

[0005] The following problems exist in recycling waste PS through resource utilization: Physical recycling: The common method for physical recycling is melt regranulation, but this method often leads to a decrease in material performance. More importantly, physical recycling does not change the nature of PS being non-degradable, and the regenerated material will still have a long-term impact on the environment, and is mostly used for low-value products such as picture frames and fillers, which is economically poor.

[0006] Chemical modification by adding degradable components: Blending and adding degradable components (such as starch, polylactic acid) is a common approach in chemical modification. However, this method usually faces the problems of poor compatibility and weak interfacial bonding, resulting in a significant deterioration of the mechanical properties of the modified material, which cannot meet the requirements of material performance in actual applications.

[0007] Chemical grafting of degradable units: Some studies have attempted to introduce degradable units on the PS chain through chemical grafting, but most of the processes require the introduction of additional initiators or catalysts, which are complex and costly. Moreover, the reaction process and the microstructure control of the final material are difficult to coordinate, making it difficult to give the material degradability while maintaining or even improving its mechanical properties.

[0008] Resource utilization of waste PS is a key breakthrough in solving the plastic pollution problem, but it faces many challenges in practice. On the one hand, PS is prone to thermo-oxidative degradation and molecular chain breakage during melt recycling, which reduces the performance of recycled materials and limits their application range. On the other hand, it is extremely difficult to directly "decompose" PS into small molecules that can be utilized by microorganisms, and its transformation path tends to be "upgrading and remanufacturing". Summary of the Invention

[0009] The existing technology has the problem that conventional chemical grafting modification of waste styrene into biodegradable polystyrene materials requires the addition of additional initiators or catalysts, resulting in high recycling costs. To address these issues, this invention provides a biodegradable modified waste polystyrene film, the preparation method of which includes the following steps: (1) Clean, dry waste PS fragments are added to ethyl acetate and heated until fully swollen to obtain a swollen solution; (2) In a sealed environment and at the swelling temperature in step (1), methyl lactate is added to the swelling solution and stirred for at least 30 min. The lactate groups are grafted onto the PS molecular chain through transesterification. Then, dichloromethane is added to the reaction system and stirred for at least 5 h. After the reaction is completed, a viscous liquid is obtained. (3) Add the above viscous liquid into the film forming mold, and after drying, you will get a biodegradable modified waste polystyrene film.

[0010] Preferably, the mass ratio of waste PS to methyl lactate is 1.8:8.8 to 1.8:8.7.

[0011] Preferably, the molar ratio of methyl lactate to dichloromethane is 8.3:1.6 to 8.4:1.5.

[0012] Preferably, the swelling temperature does not exceed 70°C.

[0013] Preferably, the drying in step (3) is natural drying at room temperature in a fume hood.

[0014] Preferably, the size of the waste PS fragments is 3-10cm. 3 .

[0015] Beneficial effects: (1) This invention directly uses waste PS as raw material and transforms it into a new type of film material with use value through chemical modification, which effectively reduces "white pollution", conforms to the concept of circular economy and sustainable development, and turns waste into treasure; (2) This invention directly grafts lactate groups onto the PS backbone through transesterification reaction of methyl lactate and PS molecular chains under specific conditions. The introduced ester bonds are more easily broken by microorganisms or hydrolysis in natural environments (such as soil and composting conditions), thereby fundamentally improving the non-degradability of PS from a chemical structure perspective, and giving the modified film biodegradable properties; (3) The entire modification process of the present invention is carried out under heating and sealing conditions. The transesterification reaction is driven by the properties of the reactants themselves, eliminating the initiators or metal catalysts commonly used in traditional grafting modification, simplifying the post-processing steps, reducing production costs, and avoiding environmental and toxic problems that may be caused by catalyst residues. (4) This invention creatively utilizes the synergistic effect of dichloromethane and methyl lactate. Dichloromethane not only serves as a reaction medium to ensure the homogeneity of the system, but its unique volatility also induces the formation of a uniform nanoscale microphase separation structure during the subsequent drying and molding stage. This fine microstructure is the key to achieving both good strength and toughness in the material, overcoming the performance degradation caused by conventional blending modification, and realizing high-performance modification; (5) This method organically combines the chemical grafting reaction of methyl lactate with the physical structure forming process dominated by dichloromethane in the same system. Methyl lactate is responsible for functionalization (introducing degradability), while dichloromethane is responsible for microstructure construction (improving mechanical properties). The two work together to complete the transformation from waste PS to high-performance biodegradable film material in one step. The process route is short and efficient. Attached Figure Description

[0016] Figure 1 The test results of the thin film stress-strain curves obtained in Example 1 and Comparative Examples 1-4 of this invention.

[0017] Figure 2 Test results of the biodegradability of the film obtained in Example 1 of this invention.

[0018] Figure 3 XRD patterns of the thin films obtained in Example 1 and Comparative Examples 1-4 of this invention. Detailed Implementation

[0019] The present invention will be described in detail below with reference to embodiments. However, it should be understood that the following embodiments are merely illustrative examples of implementation of the present invention and are not intended to limit the scope of the present invention.

[0020] Example 1

[0021] A method for preparing a biodegradable modified waste polystyrene film is as follows: (1) Wash, dry and cut the PS waste into 3-10cm³ pieces; (2) Under closed reaction conditions, 30 mL of ethyl acetate was heated to 60 °C in a water bath. Then, 1.8 g of PS fragments were added to ethyl acetate and stirred at a constant temperature for 30 min. Then, 8 mL of methyl lactate was added to the reaction system and stirred at a constant temperature for 30 min. Then, 1 mL of dichloromethane was added to the reaction system and stirred at a constant temperature for 5 h. After the reaction was completed, the resulting viscous liquid was transferred to a special mold and dried at room temperature in a fume hood for 24 h to obtain a biodegradable modified waste polystyrene film with a thickness of 0.13 mm.

[0022] Comparative Example 1 is the same as Example 1, except that the amount of methyl lactate added in Comparative Example 1 is 4 mL and the amount of dichloromethane added is 4 mL.

[0023] Comparative Example 2 is the same as Example 1, except that the amount of dichloromethane added in Comparative Example 2 is 2 mL.

[0024] Comparative Example 3 is the same as Example 1, except that dichloromethane was not added in Comparative Example 3.

[0025] Comparative Example 4 is the same as Example 1, except that methyl lactate was not added in Comparative Example 4.

[0026] Comparative Example 5 is the same as Example 1, except that in Comparative Example 5, 8 mL of methyl lactate and 1 mL of dichloromethane were added to the reaction system simultaneously. The specific method is as follows: (1) Wash, dry and cut the PS waste into 3-10cm³ pieces; (2) Under closed reaction conditions, 30 mL of ethyl acetate was heated to 60 °C in a water bath. Then, 1.8 g of PS fragments were added to ethyl acetate and stirred at a constant temperature for 30 min. Then, 8 mL of methyl lactate and 1 mL of dichloromethane were added to the reaction system at the same time. The reaction was stirred at a constant temperature for 5 h at 60 °C. After the reaction was completed, the resulting viscous liquid was transferred to a special mold and dried at room temperature in a fume hood for 24 h to obtain a biodegradable modified waste polystyrene film with a thickness of 0.13 mm.

[0027] The XRD test results of the thin films obtained in Example 1 and Comparative Examples 1-4 of this invention are shown in the appendix to the specification. Figure 3 As shown in the image, methyl lactate-modified PS exhibits a coexistence of an amorphous matrix and a nano-ordered phase. While retaining the inherent amorphous structure of polystyrene (~19°), new sharp diffraction peaks appear at 21.38°, confirming the locally generated ordered structure induced by methyl lactate. This unique microstructure of the ordered phase dispersed within the amorphous matrix is ​​the fundamental reason for the material's synergistic improvement in high strength and toughness, and its two-stage controllable degradation behavior.

[0028] In Example 1, the XRD pattern showed a sharp diffraction peak at approximately 21.38°, while exhibiting a series of weaker but identifiable diffraction signals in the 40-50° range. This indicates that a locally ordered nanophase structure was successfully constructed within the originally amorphous polystyrene matrix through chemical grafting of methyl lactate and physical structural induction by dichloromethane. The diffraction peaks in the 40-50° range may correspond to the diffraction of higher-order crystal planes of the ordered phase or the structural features of microphase separation interfaces, further confirming the formation of periodically arranged microregion structures within the material.

[0029] In contrast, the XRD patterns of each pair of examples showed varying degrees of structural weakening or disappearance in the 40-50° range, for the following reasons: Comparative Example 1 (imbalanced ratio of methyl lactate to dichloromethane): The diffraction signal was significantly weakened, indicating that the integrity of the ordered structure was disrupted; Comparative Example 2 (excess dichloromethane): The structural order was further reduced, and the diffraction peaks tended to be diffused; Comparative Example 3 (without dichloromethane): There was basically no significant diffraction in the 40-50° range, indicating that effective microphase separation could not be formed in the absence of structure-directing agents; Comparative Example 4 (without added methyl lactate): The spectrum shows typical amorphous diffuse packets with no ordered diffraction peaks, confirming that the introduction of lactate groups is the key to inducing an ordered structure.

[0030] These results collectively demonstrate that the chemical modification of methyl lactate endows the material with degradability and structural enhancement potential, while dichloromethane induces uniform nanophase separation through volatilization during the drying process. The synergistic effect of the two achieves the integration of "chemical grafting-physical shaping", ultimately forming a composite thin film material with both degradability and excellent mechanical properties.

[0031] Performance testing

[0032] The films obtained in Example 1 and Comparative Examples 1-5 of this invention were subjected to relevant performance tests, including mechanical property tests (test standard is GB / T 1040.3-2006). The test results are shown in Table 1 and the appendix to the specification. Figure 1 As shown in the figure. Test results indicate that Example 1 exhibits the best mechanical properties, with the highest tensile strength (>16 MPa) and continuous strain hardening capability; Comparative Example 2 shows a typical balance between strength and toughness. The comparative test results demonstrate that methyl lactate is the key reactive monomer for improving the material's strength and toughness, while dichloromethane plays an irreplaceable role in forming a uniform microstructure and preventing early stress softening. When the volume ratio of methyl lactate to dichloromethane is between 8:1 and 4:1, the material forms a stable two-phase structure with a balance of rigidity and flexibility at the microscopic level, achieving an ideal combination of high strength, high elongation, and deformation stability at the macroscopic level. Example 1 represents the optimal performance solution within the system.

[0033] The biodegradability of the film obtained in Example 1 was tested according to the following method: After completing accelerated UV aging (2 hours of UV irradiation), the thin film samples were immediately subjected to damp heat aging tests. They were suspended above a sealed desiccator containing a saturated potassium chloride solution (prepared from 9.1 g potassium chloride and 18.8 mL deionized water), ensuring no contact with the liquid surface. The entire apparatus was then placed in an 80°C oven, maintaining a relative humidity of 97%. Test results are shown in the attached instruction manual. Figure 2 As shown in the figure. Figure a shows the surface state of the film before degradation, and Figure b shows the surface state of the film after degradation. The test results show that before degradation, the film is light white and sheet-like, with a dense and slightly rough surface, a slight luster, uniform color, and regular edges. After degradation, the film is grayish-white and sheet-like, with a loose and rough surface, no luster, increased impurities, rough edges, and decreased regularity. The film exhibits a "fluctuating degradation in the early stage and a significantly slower rate in the later stage" characteristic within a 60-day degradation cycle. The cumulative mass loss rate reaches 24.82% in the first 50 days, and the degradation rate drops to the lowest level between 50 and 60 days, with a final cumulative mass loss rate of 27.01%.

[0034] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A biodegradable modified waste polystyrene film, characterized in that, The preparation method includes the following steps: (1) Clean, dry waste PS fragments are added to ethyl acetate and heated until fully swollen to obtain a swollen solution; (2) In a sealed environment and at the swelling temperature in step (1), methyl lactate is added to the swelling solution and stirred for at least 30 min. The lactate groups are grafted onto the PS molecular chain through transesterification. Then, dichloromethane is added to the reaction system and stirred for at least 5 h. After the reaction is completed, a viscous liquid is obtained. (3) Add the above viscous liquid into the film forming mold, and after drying, you will get a biodegradable modified waste polystyrene film.

2. The biodegradable modified waste polystyrene film according to claim 1, characterized in that, The mass ratio of waste PS to methyl lactate is 1.8:8.8 to 1.8:8.

7.

3. The biodegradable modified waste polystyrene film according to claim 2, characterized in that, The molar ratio of methyl lactate to dichloromethane is 8.3:1.6 to 8.4:1.

5.

4. The biodegradable modified waste polystyrene film according to claim 1, characterized in that, The swelling temperature does not exceed 70°C.

5. The biodegradable modified waste polystyrene film according to claim 1, characterized in that, The drying in step (3) is natural drying at room temperature in a fume hood.

6. The biodegradable modified waste polystyrene film according to claim 1, characterized in that, The size of discarded PS fragments is 3-10cm. 3 .

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