A high bio-based PHA-acrylate latex paint and its preparation method and application

Abstract

This invention discloses a high-bio-based PHA-acrylate latex paint, its preparation method, and its application, belonging to the field of waterborne coating technology. The film-forming material of this latex paint comprises 60% to 90% polyhydroxyalkanoates (PHA) with a moderate crystallinity of 40% to 60%, and 10% to 40% of a specific bio-based acrylate copolymer with a Tg not exceeding 5℃. This invention constructs a core-shell microstructure of a flexible acrylate phase encapsulating a rigid PHA phase through a key process including ultrasonic treatment. This structure achieves low-temperature film formation without VOC film-forming aids, resulting in a paint film with a bio-based content exceeding 70%, a VOC content below 0.5 g / L, and a wash resistance greater than 7000 cycles, solving the technical challenge of balancing high bio-based content and high performance in PHA coatings.

Description

Technical Field

[0001] This invention belongs to the field of water-based coating technology, specifically relating to a high bio-based PHA-acrylate latex paint, its preparation method, and its application. Background Technology

[0002] Among numerous bio-based polymers, polyhydroxyalkanoates (PHA), an aliphatic polyester synthesized by microbial fermentation, is 100% derived from biomass and is completely biodegradable in natural environments such as soil and water. It is widely regarded by the scientific and industrial communities as an ideal material for constructing next-generation environmentally friendly coating adhesives. However, despite PHA's unparalleled environmental advantages, its use as a primary film-forming agent in waterborne coatings faces a series of significant and insurmountable technical obstacles in current technologies.

[0003] First, PHA has a serious inherent brittleness problem. In particular, commercially available PHA prepared by conventional melt crystallization methods typically has a crystallinity higher than 40%, which results in films that are hard but have extremely poor toughness, similar to atactic polypropylene. These films are prone to cracking when subjected to impact or bending, and cannot meet the basic mechanical property requirements of coatings.

[0004] In this art, monomers with 3 to 5 carbon atoms constituting the main chain of poly(hydroxybutyrate) (PHA) are generally classified as short-chain PHAs (sc-PHA), such as poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). These PHAs have high molecular chain regularity and are easily crystallized, thus typically exhibiting high crystallinity, hardness, and brittleness, but good solvent resistance. PHAs composed of monomers with 6 to 14 carbon atoms are called medium- and long-chain PHAs (mcl-PHA), such as poly(3-hydroxyhexanoate) (PHH) and its copolymers. Due to their longer side chains, medium- and long-chain PHAs have increased steric hindrance, enhanced molecular chain flexibility, and significantly reduced crystallinity and crystallinity, thus typically exhibiting elastomer-like soft properties. Based on this general understanding, those skilled in the art generally expect that blending more flexible medium- and long-chain PHAs with acrylates can more effectively improve the toughness and mechanical properties of the system.

[0005] Secondly, PHA faces significant challenges in film formation. Due to its rigid molecular chains, PHA typically exhibits a high glass transition temperature (Tg) and minimum film-forming temperature (MFFT). This means that pure PHA aqueous dispersions cannot naturally aggregate to form a continuous, uniform film at room temperature or even higher. To address this issue, a conventional technique involves adding large amounts of small-molecule alcohol ethers or alcohol esters—usually accounting for more than 10% of the emulsion's solid content—to the emulsion. While these additives, acting as temporary plasticizers, effectively reduce MFFT, they are volatile organic compounds (VOCs) that will evaporate into the atmosphere after the film dries. This not only contradicts the trend towards low-VOC or even zero-VOC coatings but also significantly reduces the economic viability of PHA as a primary film-forming agent, negating its environmental benefits.

[0006] To overcome these shortcomings, existing technologies have made various attempts, but none have provided a perfect solution. One technical approach is to chemically modify the PHA molecule itself, for example, by introducing more flexible medium- to long-chain monomers to reduce its crystallinity and rigidity. However, while this method can improve the flexibility of the material itself, achieving effective compounding with other components in aqueous emulsion systems to obtain an overall performance improvement, especially achieving a balance between low-temperature film formation and high durability without VOC additives, remains a significant challenge.

[0007] Even more concerning, some existing technologies have gone in directions completely opposite to solving the film-forming problem of PHA. For example, European patent EP2476733A1 teaches the addition of PHA micron-sized particles as a matting agent to coatings. The purpose is precisely to utilize the hardness, high melting point, and non-participation in film formation of PHA particles to create a microscopic rough structure on the paint film surface to reduce gloss. This document indirectly confirms that those skilled in the art generally regard PHA's "difficulty in forming a film" as an inherent property to be utilized, rather than sought to overcome it, which constitutes a reverse guidance to the technical direction of this invention.

[0008] Although polymer blending is a common strategy for improving material properties, simple physical blending of PHA with conventional acrylates and other polymers usually leads to severe macroscopic phase separation due to the huge thermodynamic incompatibility between the two. This manifests as hazy paint film, delamination, poor adhesion, and a precipitous drop in mechanical properties, and is therefore not a feasible technical path.

[0009] In summary, despite extensive exploration of PHA applications in existing technologies, a key, unresolved technical challenge remains: no known solution can simultaneously achieve excellent mechanical toughness (overcoming brittleness), low-temperature film-forming ability without the addition of any VOC film-forming aids, and extremely high total bio-based content in waterborne latex paint systems with high content (e.g., over 60% of the total adhesive content) of commercially available rigid PHA as the primary film-forming agent. Existing technologies have failed to provide an effective way to achieve an ideal balance between the three mutually constraining goals of high performance, processability, and sustainability. Summary of the Invention

[0010] One object of the present invention is to provide a high bio-based PHA-acrylate latex paint, its preparation method, and its application, in order to solve the problems mentioned in the background art. The term "high bio-based" refers to the latex paint prepared according to the present invention, wherein the total biocarbon content of the coating film formed after drying is not less than 70%, as determined by the American Society for Testing and Materials (ASTM) standard D6866-24a or an equivalent current standard.

[0011] To achieve the above objectives, the present invention provides the following technical solution:

[0012] This invention provides a high-bio-based PHA-acrylate waterborne latex paint.

[0013] It comprises a composite film-forming composition and at least one component selected from pigments, fillers, and additives;

[0014] The composite film-forming composition comprises, on a dry weight basis, 60% to 90% PHA and 10% to 40% acrylate copolymer;

[0015] The acrylate copolymer has a glass transition temperature of not more than 5°C as determined according to ISO 11357-2:2020, and a biochar content of 28% to 30% as determined according to ASTM D6866-24a.

[0016] Furthermore, the weight percentage of the PHA to the acrylate copolymer is calculated based on the total dry weight of the composite film-forming composition.

[0017] In some specific embodiments, the PHA is provided in the form of a first aqueous dispersion, and the acrylate copolymer is provided in the form of a second aqueous dispersion.

[0018] In some specific embodiments, the latex paint comprises, by total weight: 15% to 44% of the composite film-forming composition on a dry weight basis, 6% to 28% of pigments and / or fillers, 0.1% to 5% of additives, and the balance being water.

[0019] In some specific embodiments, the content of PHA in the composite film-forming composition, on a dry weight basis, can be 60%, 62%, 65%, 68%, 70%, 72%, 75%, 82%, 85%, or 90%; the content of the acrylate copolymer, on a dry weight basis, can correspondingly be 10%, 15%, 18%, 20%, 25%, 28%, 30%, 32%, 35%, 38%, or 40%. The glass transition temperature of the acrylate copolymer can be -15°C, -12°C, -8°C, -5°C, 0°C, 1°C, 3°C, or 5°C, and the biochar content can be 28.0%, 28.5%, 28.8%, 29.2%, 29.6%, or 30.0%.

[0020] In some specific embodiments, in order to achieve the best balance between mechanical properties and bio-based content, the content of PHA in the composite film-forming composition is limited to 60% to 75% on a dry weight basis to achieve the best balance between rigidity and toughness.

[0021] In some specific embodiments, the dry weight content of the composite film-forming composition in the latex paint may be 15%, 20%, 25%, 30%, 35%, 40%, or 44% by total weight; the content of pigments and / or fillers may be 6%, 10%, 15%, 20%, 25%, or 28%; and the content of additives may be 0.1%, 0.5%, 1.0%, 1.5%, 2.0%, 3.0%, or 5%.

[0022] The PHA is an scl-PHA with a crystallinity of 20% to 60%, selected from one or more of PHB, PHBV, or poly-3-hydroxybutyrate-co-3-hydroxyhexanoate (PHBHHx) containing a comonomer content of no more than 16 molar percentage. This helps to obtain ideal physical properties. Specific examples may include one of PHB, PHBV, or PHBHHx, or mixtures thereof in any proportion. For example, when the PHA is PHBV, the molar content of the 3-hydroxyvalerate monomer can be 3.5%, 8%, 12%, 15%, 18%, 21%, 23%, or 25%; when the PHA is PHBHHx, the molar content of the 3-hydroxyhexanoate monomer can be 3%, 6%, 10%, 12%, 14%, or 15.2%.

[0023] To form the final coating product, in addition to the aforementioned composite film-forming composition, the waterborne latex paint of the present invention may also contain pigments, fillers, and additives. When no pigments and fillers are added, i.e., the content is 0%, a highly bio-based varnish can be obtained; when pigments and fillers are added, a colored paint can be obtained, with a content as high as 50% on a dry solids basis. Furthermore, the composition may also contain 0.1% to 5% of additives in total, which may be selected from one or more of wetting agents, dispersants, thickeners, defoamers, leveling agents, preservatives, and pH adjusters to optimize the production, storage, and application performance of the coating.

[0024] After drying and film formation, the PHA and the acrylate copolymer form a core-shell structure at the microscale, wherein the acrylate copolymer phase, as a continuous or semi-continuous phase, coats the PHA particle phase. This structure can be verified by the surface charge characteristics of the composite emulsion: as shown in subsequent examples, the Zeta potential of the composite emulsion of the present invention, for example -38 mV, measured according to ISO 13099-2:2012, is very close to the Zeta potential of pure bio-based acrylate emulsion -42 mV, but significantly different from the Zeta potential of pure PHA aqueous dispersion -25 mV. This indicates that the surface properties of the composite particles are mainly determined by the acrylate phase, thus confirming its existence as an outer shell phase. This unique microstructure is the physical basis for achieving the superior performance of the present invention. The composition, without the presence of volatile organic film-forming aids, can form a coating that simultaneously has a minimum film-forming temperature (MFFT) of not more than 5°C as determined according to ISO 2115:1996, and a washability of not less than 7000 cycles as determined according to GB / T 9755-2024.

[0025] This invention also discloses a method for preparing a high-bio-based PHA-acrylate waterborne latex paint. The method, without the presence of volatile organic film-forming aids, ensures that the latex paint has a minimum film-forming temperature not exceeding 5°C, and includes the following steps:

[0026] Step 1. Premix the PHA aqueous dispersion with the acrylate copolymer emulsion to obtain the composite emulsion base material;

[0027] Step 2. Disperse the pigments, fillers, and some additives in water to prepare a color paste;

[0028] Step 3. Under stirring, add the color paste obtained in step 2 to the composite emulsion base material obtained in step 1 for shear dispersion;

[0029] Step 4. The mixture obtained in Step 3 is subjected to ultrasonic treatment at a frequency of 20 kHz to 40 kHz and a power density of 150 W / L to 300 W / L.

[0030] Following the ultrasonic treatment step in step 4, a paint mixing step is also included, in which thickeners and / or leveling agents are added.

[0031] This method ensures the formation of the ideal microstructure in the final product through precise process control. The most crucial step is the ultrasonic treatment of the mixture, where the energy input is essential for forming a stable dispersion and achieving ideal properties.

[0032] Furthermore, this invention also discloses a water-based latex paint prepared by the above method, which, due to the specific energy input process it undergoes, possesses a stable microstructure and excellent performance that are difficult to obtain by conventional physical blending methods.

[0033] Finally, the present invention also discloses an application for providing a protective coating to the surface of a building or wood substrate, the application comprising the step of applying the aforementioned high bio-based polyhydroxy fatty acid ester-acrylate waterborne latex paint of the present invention to the surface of the substrate. Through this application, the excellent low-temperature film-forming ability and high durability of the paint itself can be utilized to provide a durable protective coating to the surface of a building or wood substrate.

[0034] Compared with the prior art, the following significant advantages can be obtained by using the present invention:

[0035] Extremely high bio-based content is achieved: The latex paint prepared by this invention, after drying, forms a coating film with a total bio-carbon content of not less than 70%, as determined by the American Society for Testing and Materials (ASTM) standard D6866-24a or equivalent current standards, which is significantly higher than the existing technology level.

[0036] The product exhibits exceptional environmental performance: tested according to relevant standards such as GB 30981.1-2025 or equivalent current standards, the VOC content is no higher than 0.5g / L. Furthermore, the content of harmful substances such as N-methylpyrrolidone (NMP), benzene compounds, free formaldehyde, and migratable heavy metals in the product is effectively controlled and remains at extremely low levels.

[0037] Excellent physical properties of the coating: The coating formed by this invention has excellent durability, and the number of scrub resistances measured according to relevant standards such as GB / T9755-2024 or equivalent current standards is not less than 7000.

[0038] Additional functionality: The coating of this invention also exhibits excellent indoor air purification capabilities, with a formaldehyde purification efficiency of no less than 90% as measured by relevant standards such as JC / T 1074-2021 or equivalent current standards. Detailed Implementation

[0039] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention. Unless otherwise stated, the raw materials used in this embodiment are commercially available industrial products or can be prepared by conventional methods. Unless otherwise specified, performance testing methods are performed according to the standards described in the invention summary section.

[0040] Main reagents and raw materials:

[0041] Table 1 lists the main reagents, their brands / specifications, and suppliers:

[0042]

[0043] Main analytical and testing instruments:

[0044] Table 2. Names, Models / Series, and Manufacturers of Major Instruments and Equipment:

[0045]

[0046] Main testing standards:

[0047] The table below lists the main test standards used in the embodiments of this invention.

[0048] Table 3. Main test items, standard numbers, and standard descriptions:

[0049]

[0050] Unless otherwise specified, the “PHA aqueous dispersion” or its specific type, such as PHBV aqueous dispersion, mentioned in the following examples is the “first aqueous dispersion” as defined in this invention; the “acrylate emulsion” or its specific grade, such as Acronal ECO 7074, is the “second aqueous dispersion” as defined in this invention.

[0051] Preparation of the first aqueous dispersion of PHA:

[0052] The PHA aqueous dispersion used in this embodiment of the invention is prepared using an environmentally friendly solvent-free high-energy homogenization method. Taking the preparation of PHBV aqueous dispersion as an example, the specific steps are as follows:

[0053] S1. Dissolve 1g PVA in 200ml of deionized water and heat to 80℃ to obtain the aqueous phase.

[0054] S2. Melt 10g of PHBV powder at 175℃.

[0055] S3. In a high-shear homogenizer, the molten PHBV obtained in step S2 is slowly added to the hot water phase of step S1 at a speed of 10,000 rpm, and shearing is continued for 10 minutes to form a crude emulsion.

[0056] S4. Immediately transfer the hot crude emulsion to a high-pressure homogenizer preheated to 80°C, and homogenize it 5-8 times under a pressure of 80-100MPa.

[0057] S5. After homogenization, the emulsion is cooled to room temperature by stirring to obtain a stable PHBV water-based emulsion with a solid content of 5 wt% and a milky white appearance. It is then sealed and stored at 4°C for later use.

[0058] The PHBV aqueous dispersion prepared by this method has a weight-average particle size of 1.5 μm. Depending on the requirements of the examples, the emulsion can be concentrated to a higher solid content, such as 40 wt%, by ultrafiltration or other methods. The preparation methods for other PHA aqueous dispersions are similar, requiring only appropriate adjustment of the melting temperature in step S2 according to their material properties.

[0059] Examples and Comparative Examples:

[0060] To verify the technical effects of the present invention, latex paints of the following examples and comparative examples were prepared.

[0061] Table 4. Overview of Main Materials in Examples and Comparative Examples:

[0062]

[0063] Taking Example 1 as an example, its detailed wet process formulation is shown in Table 5, and its preparation steps are as follows:

[0064] Table 5 Detailed wet process formulation for Example 1 (total 1000g):

[0065]

[0066] Step 1. Take 525g of PHBV aqueous dispersion prepared by the above PHA aqueous dispersion preparation method and concentrated to a solid content of 40%, and place it in a dispersion tank with 180g of Acronal ECO 7074 emulsion with a solid content of 50% at room temperature, so that the dry weight ratio is 70:30. Turn on the stirring and control the speed at 500 rpm. Premix for 15 minutes to obtain the composite emulsion base material.

[0067] Step 2. In another dispersion tank, add 30g of deionized water, wetting agent, dispersant, 200g of titanium dioxide and 50g of kaolin, and disperse at high speed for 20 minutes to prepare a uniform color paste.

[0068] Step 3. Under medium-speed stirring, slowly pump the color paste obtained in step 2 into the composite emulsion base material in step 1, adjust the pH to 8.5 with ammonia water, and then increase the speed to 2000 rpm for high-speed shear dispersion for 45 minutes.

[0069] Step 4. Transfer the mixture to an ultrasonic treatment device and ultrasonically disperse it at a frequency of 30kHz for 15 minutes. Then add thickener, defoamer and preservative (included in the total amount of additives), stir at low speed until uniform, let stand to defoam, and filter with a 200-mesh filter to obtain the finished interior wall latex paint.

[0070] The preparation methods of other examples and comparative examples are similar to those of Example 1, except that the types and proportions of each component are adjusted according to the logic of Tables 4 and 5. For example, Example 6 is a varnish without pigments or fillers. Comparative Example 1 additionally added ethylene glycol butyl ether, a film-forming aid accounting for 1.5% of the total weight of the emulsion. The petroleum-based acrylate emulsion used in Comparative Example 3 is the commercially available Dow Chemical PRIMAL AC-261 product, which is a pure acrylate emulsion with a solid content of 50% and an MFFT of 16°C. The preparation methods of Comparative Examples 4-6 are basically the same as those of Example 1, except that: Comparative Example 4 uses the commercially available low-crystallinity PHA copolymer PHBHHx instead of PHBV to prepare the aqueous dispersion; Comparative Example 5 uses petroleum-based acrylate emulsion B, namely PRIMAL SF-016, instead of Acronal ECO7074; and Comparative Example 6 completely omits the ultrasonic dispersion treatment in step 4.

[0071] Application examples, performance testing, and results analysis:

[0072] The latex paints prepared in Examples 1-3, 5 and Comparative Examples 1-6 were used for interior wall coating, and the varnish prepared in Example 4 was used for wood surface coating. The paint films were cured for 7 days under standard conditions (23±2℃, 50±5% relative humidity) before performance testing.

[0073] Table 6. Physical performance test results of the examples and comparative examples:

[0074]

[0075] Table 6 above visually demonstrates the physical performance advantages of this invention. Firstly, regarding film-forming properties, the MFFT of the pure PHA system in Comparative Example 1 is far above 60°C, making film formation impossible under normal conditions. However, all embodiments of this invention, by introducing a low-Tg acrylate emulsion, successfully reduced the MFFT to below 5°C, demonstrating a significant synergistic film-forming effect between the two. Secondly, regarding mechanical properties, the scrub resistance of the embodiments of this invention far exceeds the 3000 cycles of the pure PHA system in Comparative Example 1, achieving an excellent level of no less than 7000 cycles across the entire PHA content range of 60% to 90%. This is attributed to the effective balance between rigidity and toughness achieved by the formed core-shell or semi-interpenetrating network structure. Furthermore, the absolute values ​​of the Zeta potentials in the embodiments of this invention are all far above 30 mV, indicating that the composite emulsion has excellent storage stability, superior to the pure PHA aqueous dispersion of Comparative Example 1.

[0076] Table 7. Environmental protection and chemical component test results:

[0077]

[0078] Note: The detection limit for free formaldehyde content is 5 mg / kg.

[0079] Table 7 above highlights the significant advantages of this invention in terms of environmental protection and sustainability. Firstly, regarding VOC source control, thanks to the synergistic film-forming effect, the embodiments of this invention do not require the addition of volatile film-forming aids, resulting in a VOC content below 0.5 g / L, far superior to Comparative Example 1 (which included aids) and Comparative Example 3 (a conventional petroleum-based product). The results of Example 5 and the free formaldehyde content tests on all samples show that the system of this invention itself does not contain intentionally added formaldehyde sources, and its free formaldehyde content is below the detection limit of 5 mg / kg by HPLC, fully meeting the stringent requirements of high-level green standards such as the national standard GB / T 35602-2017 "Green Product Evaluation Coatings" for free formaldehyde not exceeding 10 mg / kg, further demonstrating the excellent environmental safety of this invention. Regarding bio-based content, by adjusting the PHA ratio, the total bio-based content of the product of this invention can easily reach over 70%, and in Example 2 it is as high as 93%, meeting the requirements of sustainable development. Furthermore, the product of this invention does not contain harmful substances such as NMP and benzene compounds, and its heavy metal content is far below the limit requirements of the national standard GB 30981.1-2025. Compared with the high-performance petroleum-based comparative example 3, it has an overwhelming advantage in key environmental protection indicators.

[0080] Creative analysis of key differentiating technical features:

[0081] To further demonstrate the non-obviousness of the present invention, comparative examples 4, 5, and 6 were designed, the results of which strongly support the inventiveness of the present invention:

[0082] Compared to the use of low-crystallinity PHA, i.e., Example 1 and Comparative Example 4:

[0083] Existing technologies generally teach that the crystallinity of PHA can be reduced by introducing flexible monomers, thereby improving its brittleness. Comparative Example 4 follows this conventional approach, using a blend of PHBHHx with significantly lower crystallinity to the bio-based acrylate of this invention. Those skilled in the art would typically expect better mechanical properties from such a more flexible, lower-crystallinity PHA. However, the experimental results were quite the opposite; its wash resistance was only 4500 cycles, far lower than Example 1, which used PHBV with 52% crystallinity and achieved over 7000 cycles.

[0084] The mechanism behind this unexpected technical effect lies in the fact that the success of this invention does not simply rely on maximizing the flexibility of the PHA material itself, but rather on constructing a "hard core-soft shell" structure with sufficient rigidity and structural integrity within the composite system. The PHBV used in this invention has a crystallinity within an "optimal range" of 40% to 60%. This crystallinity provides the paint film with the high hardness and strength required as a framework, while avoiding the extreme brittleness of pure PHB. Conversely, the PHBHHx used in Comparative Example 4, due to its excessively low crystallinity, results in a "core" that is too soft to provide sufficient mechanical support for the paint film. When subjected to external friction, the overly soft core cannot effectively resist deformation, leading to premature paint film failure. Therefore, this invention surprisingly discovers that in the PHA-acrylate composite coating system, there exists an "optimal performance range" for the crystallinity of PHA, contrary to the traditional notion that "the lower the better." This discovery is unconventional and solves a problem that existing technologies have failed to address.

[0085] In comparison with the use of petroleum-based acrylates, namely Example 1 and Comparative Example 5: To refute the potential argument that "it is obvious to replace the acrylates in the existing PHA system with commercially available bio-based acrylates," Comparative Example 5 replaced the bio-based acrylate Acronal ECO 7074 in Example 1 with the petroleum-based acrylate PRIMAL SF-016, which has similar physicochemical parameters such as Tg. The results showed that its scrub resistance decreased significantly to 4000 cycles, and the paint film exhibited slight haze, indicating poor compatibility. This strongly demonstrates the special synergistic effect between the specific bio-based acrylates selected in this invention and PHA, a synergistic effect that cannot be predicted by simple "equivalent substitution," resulting in a significant performance improvement.

[0086] In contrast to Example 1, which omits the ultrasonic treatment step, Comparative Example 6, with the exact same formulation as Example 1, omits only the ultrasonic dispersion treatment in step four. The results show that its MFFT significantly increases to 12°C, failing to achieve low-temperature film formation, and its wash resistance drops sharply to below 2000 cycles, resulting in an uneven film appearance. This result surprisingly demonstrates that ultrasonic treatment is not a conventional auxiliary means for dispersion in the art, but rather an indispensable key process step for achieving the low-temperature film formation and excellent mechanical properties described in this invention. Through high-energy input, it promotes the microscopic dispersion of the PHA and acrylate phases and the formation of an ordered structure in the subsequent film formation process; its role is decisive and not obvious.

[0087] Synergistic film formation effect and VOC source control analysis:

[0088] MFFT is a key indicator for evaluating the film-forming ability of latex paint. As shown in Table 6, the MFFT of the pure PHA system in Comparative Example 1 is much higher than 60°C, which means that without the addition of a large amount of VOC film-forming aids, it cannot form a continuous and uniform paint film at room temperature or even higher temperatures. This directly reveals the inherent defects of PHA as a single film-forming agent. In contrast, in Examples 1-7 of this invention, by introducing a low-Tg bio-based acrylic emulsion, the MFFT of the system was significantly reduced to below 5°C. This result strongly demonstrates the significant synergistic film-forming effect between PHA and this specific acrylic emulsion. The low-Tg acrylic polymer acts as an "internal plasticizer," and its soft segments can effectively promote the deformation and fusion of high-Tg PHA particles during the water evaporation process. This inherent synergistic effect enables the system of this invention to achieve low-temperature film formation without the addition of any VOC film-forming aids, thereby controlling the generation of VOCs from the source and making the VOC content of the final product less than 0.5 g / L, which is far superior to Comparative Example 1 and Comparative Example 3 of conventional petroleum-based products that have added film-forming aids.

[0089] Microstructure and mechanical property analysis of composite systems:

[0090] The core of this invention lies in constructing a microscopic composite structure of PHA and acrylate, thereby overcoming the brittleness of PHA. The pure PHA film (Comparative Example 1) exhibited poor mechanical properties with only 3000 scrub resistance cycles. However, in the embodiments of this invention, all samples with PHA content ranging from 60wt% to 90wt% (Examples 1, 2, 3, and 7) showed scrub resistance exceeding 7000 cycles, approaching or even surpassing the high-performance pure acrylate systems (Comparative Examples 2 and 3). This significant performance improvement can be attributed to the formed core-shell structure. As mentioned earlier, Zeta potential analysis confirmed that the surface of the composite emulsion particles is dominated by a flexible low-Tg acrylate polymer as the "shell" or continuous phase, encapsulating rigid PHA particles as the "core" or framework. In this structure, the PHA core provides the hardness and main support of the film, while the acrylate shell acts as a toughening agent and stress distributor. When the paint film is subjected to external friction, the soft acrylic shell can absorb and dissipate energy, preventing stress concentration in the brittle PHA phase, thereby preventing premature damage to the paint film and achieving a perfect combination of rigidity and toughness.

[0091] Analysis of the influence trend of key component parameters on system performance:

[0092] The effect of PHA content: Comparing the PHA content of Example 3 (60%), Example 1 (70%), Example 5 (75%), and Example 2 (90%), the trends of various performance indicators with PHA content are clearly visible. First, the total bio-based content increases linearly with increasing PHA content, rising from 72% (Example 3) to 93% (Example 2). This indicates that by adjusting the PHA ratio, the bio-based properties of the product can be precisely controlled to meet different sustainability requirements. Second, in terms of mechanical properties, the scrub resistance exhibits an excellent level of over 7000 cycles across the entire PHA content range of 60% to 90%. Specifically, data shows that the system maintains a high level of durability throughout the process of increasing PHA content from 60% (Example 3) to over 7500 cycles, and then to 90% (Example 2) to over 7000 cycles. This demonstrates that the synergistic toughening mechanism of the present invention remains effective across a wide range of components; the flexible acrylate phase can most effectively form a continuous or semi-continuous network to toughen the rigid PHA phase, achieving the optimal balance between rigidity and toughness.

[0093] Effect of Composite Film-Forming Material Content: Comparing Examples 4, 1, and 5, it can be seen that, while maintaining the dry weight ratio of PHA to acrylate, increasing the content of the composite film-forming material in the total formulation can effectively improve the overall performance of the paint film, such as increasing the hiding power from 0.93 to 0.96 and improving the scrub resistance to over 7000 cycles. However, the paint film in Example 5 is denser and fuller. This indicates that within the scope defined by this invention, the content of the composite film-forming material can be flexibly adjusted according to the cost and performance requirements of the application scenario.

[0094] The Influence of PHA Particle Size: Theoretically, under the same mass ratio, using smaller PHA aqueous dispersions, such as those close to 100 nanometers, will facilitate the formation of a denser and more uniform core-shell encapsulation structure. This is because smaller particle size means a larger specific surface area, allowing for more thorough contact with the acrylic emulsion, potentially resulting in a lower MFFT and superior mechanical properties of the coating film.

[0095] The Influence of Tg on Acrylic Emulsions: Theoretically, the Tg of acrylic emulsions is a key factor affecting the MFFT of the system. Using acrylic emulsions with lower Tgs, such as -10°C, can more effectively act as an "internal plasticizer" to reduce the film-forming difficulty of PHA. However, excessively low Tgs may lead to a sticky final film and reduced stain resistance. Therefore, the Tg range of no more than 5°C selected in this invention represents the optimal balance between ensuring low-temperature film formation and maintaining good film surface properties.

[0096] In summary, this invention scientifically blends a short-chain PHA aqueous dispersion with moderate crystallinity with a bio-based acrylate emulsion, and limits the content range of each component in the final latex paint product. By utilizing the synergistic effect between the two and a key ultrasonic treatment process, a high-bio-based, high-performance, ultra-low VOC environmentally friendly latex paint is successfully prepared, solving the problem of existing technologies that struggle to balance high bio-based content and excellent application performance.

[0097] The above embodiments are merely illustrative of the technical solutions of the present invention and are not intended to limit its scope. Those skilled in the art can make various modifications, equivalent substitutions, or variations to the above embodiments without departing from the principles and spirit of the present invention, and all such modifications, equivalent substitutions, or variations should be included within the protection scope defined by the present invention.

Claims

1. A method for preparing a high-bio-based polyhydroxy fatty acid ester-acrylate waterborne latex paint, characterized in that, The latex paint contains no volatile organic film-forming aids and has a minimum film-forming temperature of not more than 5°C as determined according to ISO 2115:1996, and forms a coating with a wash resistance of not less than 7000 cycles as determined according to GB / T 9755-2024, and includes the following steps: Step 1. Premix the polyhydroxy fatty acid ester aqueous dispersion with the acrylate copolymer emulsion to obtain the composite emulsion base material; Step 2. Disperse the pigments, fillers, and some additives in water to prepare a color paste; Step 3. Under stirring, add the color paste obtained in step 2 to the composite emulsion base material obtained in step 1 for shear dispersion; Step 4. The mixture obtained in Step 3 is subjected to ultrasonic treatment at a frequency of 20 kHz to 40 kHz and a power density of 150 W / L to 300 W / L. The polyhydroxyalkanoate and acrylate copolymer comprises 70% to 90% polyhydroxyalkanoate and 10% to 30% acrylate copolymer based on their total dry weight. The polyhydroxy fatty acid ester is a short-chain polyhydroxy fatty acid ester with a crystallinity of 20% to 60%. The acrylate copolymer has a glass transition temperature of not more than 5°C as rated according to ISO 11357-2:2020, and a biochar content of 28% to 30% as determined according to ASTM D6866-24a.

2. The preparation method according to claim 1, characterized in that, The latex paint comprises, by total weight: 15% to 44% of the polyhydroxy fatty acid ester and acrylate copolymer on a dry weight basis; 6% to 28% pigments and / or fillers; 0.1% to 5% of adjuvants; And the remaining water.

3. The preparation method according to claim 1, characterized in that, The polyhydroxy fatty acid ester is selected from one or more of poly-3-hydroxybutyrate, poly-3-hydroxybutyrate-co-3-hydroxyvalerate, or poly-3-hydroxybutyrate-co-3-hydroxyhexanoate containing a comonomer content of not more than 16 molar percentage.

4. The preparation method according to claim 1, characterized in that, After drying and forming a film, the polyhydroxy fatty acid ester and the acrylate copolymer form a core-shell structure at the microscale, wherein the acrylate copolymer phase, as a continuous or semi-continuous phase, coats the polyhydroxy fatty acid ester particle phase.

5. The preparation method according to claim 1, characterized in that, Following the ultrasonic treatment step in step 4, a paint mixing step is also included, in which thickeners and / or leveling agents are added.

6. An application for providing a protective coating to the surface of building or wood-based substrates, characterized in that, The method includes the step of applying a highly bio-based polyhydroxy fatty acid ester-acrylate waterborne latex paint prepared by any one of claims 1-5 to the surface of the substrate.

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