Fluorine-free bio-based waterborne polyurethane with high strength and water resistance and preparation method of fluorine-free bio-based waterborne polyurethane

Abstract

The invention relates to fluoride-free bio-based waterborne polyurethane with high strength and water resistance and a preparation method of the fluoride-free bio-based waterborne polyurethane, and belongs to the technical field of high polymer materials. According to the method, a silane coupling agent is introduced into a waterborne polyurethane system through a composite strategy of combining bio-based raw material design and terminal silane coupling agent modification, so that the high-strength bio-based waterborne polyurethane film can be formed. The preparation method comprises the specific steps of prepolymer preparation, hydrophilic chain extension, neutralization reaction, emulsification dispersion, solvent removal and curing molding. By adjusting the content of the silane coupling agent (the X value is 5-20%), the balance of the mechanical property and the water resistance of the film can be regulated and controlled, and the obtained waterborne polyurethane does not contain fluorine compounds and volatile organic solvents (VOCs) and has excellent mechanical property (the tensile strength is 35.30-45.99 MPa), hydrophobicity (the water contact angle is 87.3-107.4 degrees), adhesive force (level 0) and the like. The research provides an effective modification strategy for environment-friendly waterborne polyurethane with excellent mechanical properties and water resistance, and has remarkable environment-friendly advantages and application prospects.

Description

Technical Field

[0001] The invention belongs to the technical field of polymer materials, and in particular relates to a fluorine-free bio-based waterborne polyurethane with high strength and water resistance and a preparation method thereof. Background Art

[0002] With the increasingly stringent global environmental regulations and the deepening of the concept of sustainable development, the demand for bio-based, non-toxic and degradable products in the field of polymer materials continues to rise. As an environmentally friendly material, waterborne polyurethane (WPU) has attracted much attention in the fields of coatings, adhesives, etc. due to its low volatile organic compound (VOC) emissions and excellent film-forming properties. However, traditional waterborne polyurethanes mostly rely on petroleum-based polyols (such as polyethers and polyesters). Their raw materials are non-renewable and the carbon footprint of the production process is high, which is in significant contradiction with the development goals of green chemistry. In addition, although existing bio-based waterborne polyurethanes have partially replaced petroleum-based raw materials, they generally face bottlenecks such as insufficient mechanical strength and poor water resistance, making it difficult to meet the performance requirements of high-end application scenarios. How to break through the dual limitations of mechanical properties and water resistance while retaining the environmental advantages of bio-based materials has become a core issue that the industry needs to solve urgently.

[0003] In the prior art, in order to improve the water resistance and mechanical properties of waterborne polyurethane, it is often achieved by introducing fluorinated compounds or crosslinking agents. For example, fluorinated segments can significantly enhance the hydrophobicity and chemical resistance of materials, but their bioaccumulation, environmental toxicity and high cost issues have been criticized, and they run counter to the environmental protection trend of "fluorination-free". On the other hand, although chemical crosslinking can improve the strength of the material, excessive crosslinking can easily lead to a decrease in emulsion stability, an increase in film brittleness, and may destroy the inherent microphase separation structure of polyurethane, weakening its flexibility and elongation at break. More importantly, in the existing synthesis process, the introduction of hydrophilic groups often sacrifices water resistance, resulting in a trade-off in "hydrophilic-hydrophobic" properties. This contradiction makes the development of bio-based waterborne polyurethanes that have both high strength, high water resistance and are completely fluorine-free face huge technical challenges.

[0004] Further research found that the performance defects of existing bio-based waterborne polyurethanes are closely related to their molecular design strategies. For example, although bio-based polyols (such as polylactic acid) have renewable properties, their molecular chains are highly rigid and crystalline, which can easily lead to poor compatibility between the soft and hard segments of polyurethanes, affecting the regularity of the microphase separation structure, thereby reducing the mechanical strength and fatigue resistance of the material. In addition, traditional modification methods (such as physical blending or simple chain extension) are difficult to accurately control interfacial interactions at the molecular level, resulting in the material being prone to performance degradation in complex environments (such as high humidity and high temperature). How to achieve efficient integration of bio-based components in the molecular structure and break the performance shackles through innovative modification strategies has become a long-standing technical problem that has not been overcome in this field. Summary of the invention

[0005] In view of the above situation, to overcome the defects of the prior art, the object of the present invention is to provide a high-strength, water-resistant fluorine-free bio-based aqueous polyurethane and its preparation method.

[0006] A high-strength, water-resistant fluorine-free bio-based aqueous polyurethane and its preparation method, characterized by including the following preparation steps:

[0007] (1) Put polylactic acid diol (PLA) into a four-necked flask equipped with a mechanical stirrer, heat it to 100-120 °C, evacuate to above -0.095 MPa, and remove water for 2 hours; cool to 75 °C - 85 °C under nitrogen protection, add isocyanate and an appropriate amount of catalyst, and install a condensation device. During the reaction, monitor the -NCO content by back-titration with dibutylamine - anhydrous toluene / hydrochloric acid; when the -NCO content is close to the theoretical value, sequentially add a hydrophilic chain extender and a crosslinking agent, and let the reaction continue until the -NCO content reaches the required target value; add acetone according to the system viscosity, then cool to 50 °C and add a silane coupling agent with an X value (the molar ratio of the silane coupling agent to the residual -NCO groups in the prepolymer before capping) and react for 1 hour; after cooling to 35 °C, add triethylamine (TEA) and stir for 30 minutes to neutralize the carboxyl group.

[0008] (2) Transfer the prepolymer to a dispersion tank and emulsify it with water under high-speed stirring for 30 minutes. Finally, remove acetone by rotary evaporation to obtain a silane coupling agent-modified aqueous polyurethane emulsion (SWPU).

[0009] Among them, the diisocyanate is a benzene-free isocyanate, including cyclohexane-1,4-diisocyanate (CHDI), isophorone diisocyanate (IPDI), 4,4'-dicyclohexylmethane diisocyanate (HMDI), and one or several of them are used in combination.

[0010] Among them, the catalyst is one or a mixture of two of dibutyltin dilaurate and dibutyltin dichloride.

[0011] Among them, the silane coupling agent is one or a mixture of two of KH-550, KH-560, KH-570, and KH-792.

[0012] Among them, the X value is 5% - 20%.

[0013] The present invention relates to a high-strength, water-resistant fluorine-free bio-based aqueous polyurethane and its preparation method. Through a composite strategy that combines the design of bio-based raw materials with the modification of terminal silane coupling agents, the silane coupling agent is introduced into the aqueous polyurethane system, improving the water resistance, heat resistance, adhesion, etc. of the aqueous polyurethane. After curing, a high-strength, bio-based aqueous polyurethane film can be formed.

[0014] Method for forming a film by curing a high-strength, water-resistant, fluorine-free bio-based aqueous polyurethane: Brush the aqueous polyurethane emulsion onto a polytetrafluoroethylene mold plate (thickness = 2 mm) and a tinplate (thickness ≈ 100 μm) respectively, cure at room temperature (23 ± 2 °C) and relative humidity 50% for 5 days, and then cure at 60 °C in an oven for 24 h before testing.

[0015] For the film formed by curing the high-strength, water-resistant, fluorine-free bio-based aqueous polyurethane of the present invention, with the increase of the X value, its hardness is 78D, 71D, 74D, and 82D respectively.

[0016] For the film formed by curing the high-strength, water-resistant, fluorine-free bio-based aqueous polyurethane of the present invention, with the increase of the X value, the tensile strength decreases from 45.99 MPa to 35.30 MPa, and the elongation at break decreases from 769.64% to 623.03%. This is because the incorporation of KH-550 causes the formation of a silicon-oxygen-silicon 3D network during the surface migration of silicon atoms. This disrupts the ordered arrangement of the microphase separation between the hard segments and the soft segments, reduces the crosslinking density of the system, and thus reduces the tensile strength and the elongation at break. Although the mechanical properties decrease, these results still indicate that the coating has high mechanical properties.

[0017] For the film formed by curing the high-strength, water-resistant, fluorine-free bio-based aqueous polyurethane of the present invention, with the increase of the X value, the adhesion grade gradually increases. When the X value is 5%, the adhesion is grade 1. When the X value is 10%, the adhesion is grade 0. When the X value is greater than 10%, the adhesion is grade 0.

[0018] For the film formed by curing the high-strength, water-resistant, fluorine-free bio-based aqueous polyurethane of the present invention, with the increase of the X value, the heat resistance is enhanced, such as the thermal decomposition temperature at different mass loss percentages (mass losses of 5%, 10%, 50%, and 90% are named T5, T10, T50, and T90 respectively). Here, T5 represents the temperature at a mass loss of 5%. When the X value is 5%, T5 is 246.5 °C. When the X value is 10%, T5 is 250.3 °C. When the X value is 20%, T5 is 253.1 °C.

[0019] The film formed after curing the high-strength and water-resistant fluorine-free bio-based aqueous polyurethane of the present invention shows the wettability of water to the coating with the change of the water contact angle as the X value increases. When the X value is 5%, the water contact angle is 94.6°. When the X value is 10%, the water contact angle is 107.4°. When the X value is 20%, the water contact angle is 87.3°. This is because the hydrophobic groups of the silane coupling agent migrate to the film surface during the curing process, forming a protective layer and enhancing the hydrophobicity. However, further increasing the X value will cause the contact angle to decrease, which is due to the excessive silane coupling agent promoting the cross-linking of the Si-O-Si network and hindering the migration of silicon atoms to the surface, resulting in a decrease in the contact angle.

[0020] The present invention is scientific and reasonable, with rich raw materials, green and environmentally friendly, and easy to produce and prepare. The whole preparation process is easy to operate, fully reflecting the environmental friendliness, and providing a solid and powerful technical support for the sustainable development of modified aqueous polyurethane. It has good application prospects. Explanation of the drawings in the specification

[0021] Figure 1 It is the synthesis route diagram of the high-strength and water-resistant fluorine-free bio-based aqueous polyurethane of the present invention;

[0022] Figure 2 It is the stress-strain curve diagram of the film formed by curing the high-strength and water-resistant fluorine-free bio-based aqueous polyurethane of the present invention;

[0023] Figure 3 It is the schematic diagram of the contact angle of the film formed by curing the high-strength and water-resistant fluorine-free bio-based aqueous polyurethane of the present invention;

[0024] Figure 4 It is the schematic diagram of the heat resistance of the film formed by curing the high-strength and water-resistant fluorine-free bio-based aqueous polyurethane of the present invention.

[0025] Figure 5 It is the schematic diagram of the adhesion of the high-strength and water-resistant fluorine-free bio-based aqueous polyurethane of the present invention. Detailed description of the specific implementation mode

[0026] The following will describe the specific implementation mode of the present invention in detail in combination with examples and specific situations. Example 1

[0027] (1) 0.02 mol PLA was placed in a 250 mL four-necked flask equipped with a mechanical stirrer, heated to 120°C, and dehydrated using a vacuum oil pump for 2 h. (2) 0.0626 mol IPDI was added to the four-necked flask and stirred at 80°C for 1 h under nitrogen protection, then dibutyltin dilaurate (DBTDL) was added as a catalyst, and the reaction was continued at 80°C for 3 h under nitrogen atmosphere until all -NCO groups reached the theoretical value (di-n-butylamine-anhydrous toluene / hydrochloric acid standard titration solution method) to obtain an NCO-terminated prepolymer; when the -NCO content was close to the theoretical value, 0.0188 mol DMPA and 0.0040 mol CO were added in sequence, and the reaction was allowed to continue until the -NCO content reached the desired target value (NCO% = 2.5 wt%). Acetone was added according to the viscosity of the system (total volume < 30 mL), then cooled to 50°C and 0.0018 mol KH-550 was added for reaction for 1 hour. After cooling to 35°C, 0.0188 mol of TEA was added and stirred for 30 minutes to neutralize the carboxyl groups.

[0028] (3) The prepolymer was transferred to a dispersion barrel and emulsified with water under high-speed stirring for 30 minutes. Finally, the acetone was removed by rotary evaporation to obtain a waterborne polyurethane SWPU modified by a silane coupling agent. The waterborne polyurethane emulsion was poured onto a tetrafluoroethylene mold plate (thickness = 2 mm) and a tinplate sheet (thickness ≈ 100 μm), respectively, and cured at room temperature (23 ± 2 ° C) at a temperature of 25 ° C and a relative humidity of 50% for 5 days, and then cured in a drying oven at 60 ° C for 48 hours before testing. Embodiments 2 to 4

[0029] The difference from Example 1 is that X in Examples 2-4 represents that the molar ratio of KH-550 to the residual -NCO group in the prepolymer before termination is 10%, 15% and 20% respectively. Table 1 Synthesis formula of SWPU

[0030] The performance test results of the film formed by curing the fluorine-free bio-based waterborne polyurethane with high strength and water resistance prepared by the above embodiment are shown in the following table: Table 2 Performance indicators of the films formed by curing of fluorine-free bio-based waterborne polyurethanes with high strength and water resistance in Examples 1-4

Claims

1. A fluorine-free bio-based waterborne polyurethane with high strength and water resistance, the synthesis route and structure of which are:

2. A fluorine-free bio-based waterborne polyurethane with high strength and water resistance as claimed in claim 1, wherein the preparation method thereof is characterized by comprising the following steps: Preparation steps: (1) Put polylactic acid diol (PLA) into a four-necked flask equipped with a mechanical stirrer, heat to 100-120°C, evacuate to above -0.095MPa, and remove water for 2 hours; cool down to 75-85°C under nitrogen protection, add isocyanate and an appropriate amount of catalyst, and install a condensing device. During the reaction, monitor the isocyanate (-NCO) content by back titration of di-n-butylamine-anhydrous toluene / hydrochloric acid; when the -NCO content is close to the theoretical value, add a hydrophilic chain extender and a cross-linking agent in sequence, and allow the reaction to continue. until the -NCO content reaches the target value; add acetone according to the viscosity of the system, then cool to 50°C and add silane coupling agent with a value of X (the molar ratio of silane coupling agent to the residual -NCO group in the prepolymer before end-capping) to react for 1 hour; after cooling to 35°C, add triethylamine (TEA) and stir for 30 minutes to neutralize the carboxyl group; (2) transfer the prepolymer to a dispersion barrel and emulsify with water under high-speed stirring for 30 minutes; finally, remove acetone by rotary evaporation to obtain a silane coupling agent-modified waterborne polyurethane emulsion (SWPU).

3. The method for preparing the high-strength, water-resistant, fluorine-free bio-based waterborne polyurethane according to claim 2, characterized in that: The diisocyanate is an isocyanate that does not contain benzene series, including cyclohexane-1,4-diisocyanate (CHDI), isophorone diisocyanate (IPDI), 4,4'-dicyclohexylmethane diisocyanate (HMDI), one or a mixture of them.

4. The method for preparing the high-strength, water-resistant, fluorine-free bio-based waterborne polyurethane according to claim 2, characterized in that: The catalyst is one of dibutyltin dilaurate and dibutyltin dichloride, or a mixture of the two.

5. The method for preparing the high-strength, water-resistant, fluorine-free bio-based waterborne polyurethane according to claim 2, characterized in that: The silane coupling agent is one of KH-550, KH-560, KH-570 and KH-792 or a mixture of two of them.

6. The method for preparing the high-strength, water-resistant, fluorine-free bio-based waterborne polyurethane according to claim 2, wherein: The X value is 5% to 20%.

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