High mechanical strength flame retardant waterborne polyurethane based on hydrogen bond synergism

Abstract

The application belongs to the field of materials and relates to a flame-retardant material, in particular to a corrosion-resistant waterborne polyurethane and a corrosion-resistant waterborne polyurethane modified flame-retardant material. The flame-retardant waterborne polyurethane emulsion is prepared from castor oil (CO), polycaprolactone diol (PCL), OP550, 1,4-bis(2-hydroxyethoxy)benzene (HQEE) and 2,2-dimethylol butyric acid (DMBA), and the molar ratio of CO, PCL, OP550, HQEE, DMBA and IPDI is controlled to be 48:12:(3-20):(186-352):(55-97):(328-553). The prepared WPU has excellent flame-retardant effect.

Description

Technical Field

[0001] This invention belongs to the field of materials and relates to a flame retardant material, particularly to a corrosion-resistant waterborne polyurethane and a flame retardant material modified with corrosion-resistant waterborne polyurethane. Background Technology

[0002] More than 10,000 fires occur worldwide every day, causing hundreds of deaths. In recent years, my country has experienced approximately 40,000 fires annually, resulting in over 2,000 deaths, 3,000 to 4,000 injuries, and direct property losses amounting to billions of yuan each year. Reports indicate that in 2019, a total of 233,000 fires were reported, resulting in 1,335 deaths, 837 injuries, and direct property losses of 3.612 billion yuan.

[0003] Flame retardant materials are materials that can inhibit or delay combustion while not being easily combustible themselves. They are widely used in clothing, petroleum, chemical, metallurgy, shipbuilding, fire protection, and national defense. Common types include: ① Organohalides. Primarily bromides, commonly including decabromobiphenyl acid (DBDPO), tetrabromobisphenol A (TBBPA), and brominated polystyrene (BPS). Chlorides are only used in chlorinated paraffin and chlorinated polyethylene. Halides are often used in combination with antimony trioxide or phosphides. ② Organophosphorus compounds. These can be divided into inorganic phosphorus and halogenated phosphorus. Halogen-free phosphorus is mainly composed of phosphoric acid, such as triphenylene oxide (TPP). Halogen-free phosphorus needs to be added in combination with phosphorus halides. Halogenated phosphorus molecules contain both phosphorus and halogen elements, exhibiting intramolecular synergistic effects, and therefore can be used alone. Common examples include trichloroethylene (TECP). ③ Nitrogen-based. Main types include triazine, commonly used in PA and PU, and used in combination with phosphorus-based flame retardants.

[0004] The key to the greening of flame-retardant materials lies in the selection of flame retardants. Polymerized or macromolecular flame retardants, due to their inherent low toxicity and non-bioaccumulation properties, have become a hot development direction for green and environmentally friendly flame retardants.

[0005] In my country, the flame retardant application of polymers (various plastics, including engineering plastics) primarily utilizes additive bromine-based flame retardants, which are characterized by low dosage, high flame retardant efficiency, and wide applicability. However, polybrominated diphenyl ethers (PBDPO) in the bromine-based system produce toxic and carcinogenic polybrominated benzo[a]alkane (PBDD) and polybrominated benzo[a]furan (PBDF) during combustion. Furthermore, halogenated flame retardants generate large amounts of smoke and toxic, corrosive gases during combustion, leading to serious drawbacks such as corrosion of electrical system switches and other metal objects that cannot be caused by fire alone, and harm to the human respiratory tract and other organs.

[0006] CN115141596A discloses a high-strength, high-toughness polyurethane thermally conductive structural adhesive and its preparation method. By utilizing the combined effects of multifunctional polyester polyol, polyether polyol-modified isocyanate-terminated polyurethane prepolymer, and low molecular weight isocyanate, the strength and toughness of the material can be improved simultaneously.

[0007] CN202011153945.9 provides a high-insulation, low-temperature expansion-resistant, flame-retardant polyurethane material, its preparation method, and its application. The high-insulation, low-temperature expansion-resistant, flame-retardant polyurethane material comprises, by mass percentage: 20-60% low molecular weight polyol, 0-30% chain extender, 0.1-4% catalyst, 0.1-2.5% antioxidant, 0.1-2.5% defoamer, 0.1-2.5% wetting and dispersing agent, 5-70% flame retardant, 5-40% plasticizer, 0-30% filler, and 1-30% curing agent.

[0008] CN201410130856.0 discloses a phosphated lignin-based flame-retardant reinforced polyurethane rigid foam and its preparation method. The polyurethane foam is prepared by mixing and casting with 50-90 parts of bio-based polyol, 10-50 parts of phosphated lignin, 1-3 parts of amine catalyst, 0.05-0.2 parts of tin catalyst, 0.5-2 parts of foam stabilizer, 100-140 parts of isocyanate and 4-7 parts of water.

[0009] However, there are still many difficulties in synthesizing waterborne polyurethanes that simultaneously meet the requirements of mechanical properties and flame retardant properties. Summary of the Invention

[0010] The purpose of this invention is to address the shortcomings of existing technologies by providing a polyurethane material, particularly a high-strength, biodegradable, flame-retardant waterborne polyurethane, its preparation method, and applications. We have prepared a series of biodegradable, flame-retardant waterborne polyurethanes with excellent mechanical properties. To achieve all these functions within a single design, we carefully designed the molecular structures of the soft and hard segments of the waterborne polyurethane. Biodegradable castor oil (CO) and polycaprolactone diol (PCL) were used as the soft segments. The flame-retardant properties of ExolitOP550 and trimethylolphosphine oxide (THPO) are described. The high mechanical strength is due to the multiple inter- and intra-hydrogen bond interactions within the system. Figure 1 Its tensile strength ranges from 23 to 39 MPa, and its limiting oxygen index (LOI) is as high as 28.34%. Importantly, our WPU is biodegradable, with a mass loss rate of 37% after being naturally buried in soil for 3 months. This work will provide a new approach for manufacturing environmentally friendly, high-performance waterborne polyurethane.

[0011] The present invention adopts the following technical solution:

[0012] This invention provides a flame-retardant waterborne polyurethane emulsion, prepared from castor oil (CO), polycaprolactone diol (PCL), OP550, 1,4-bis(2-hydroxyethoxy)benzene (HQEE), and 2,2-dimethylolbutyric acid (DMBA). The molar ratio of CO, PCL, OP550, HQEE, DMBA, and IPDI is controlled at 48:12:(3-20):(186-352):(55-97):(328-553).

[0013] This invention also provides a flame-retardant material modified with water-based polyurethane, comprising a substrate and a film cured from a flame-retardant water-based polyurethane emulsion coated on the surface of the substrate. The substrate may include cardboard, wood, cotton, etc.

[0014] The polyurethane-cured film has a thickness of 0.5–2 mm.

[0015] A method for preparing flame-retardant waterborne polyurethane modified flame-retardant materials includes dripping or spraying the above-mentioned WPU emulsion onto a substrate, or impregnating the substrate with the WPU emulsion. The WPU emulsion is the aforementioned flame-retardant waterborne polyurethane (WPU) emulsion.

[0016] The fire performance index (FPI) and fire growth index (FGI) of the flame-retardant waterborne polyurethane modified cardboard are 0.033 and 12.96, respectively.

[0017] This invention provides a method for synthesizing the flame-retardant waterborne polyurethane (WPU) emulsion, comprising the following steps:

[0018] 1) Place castor oil (CO), polycaprolactone diol (PCL), OP550, 1,4-bis(2-hydroxyethoxy)benzene (HQEE), and 2,2-dimethylolbutyric acid (DMBA) in a vacuum oven and dry them;

[0019] 2) Place the dried CO, PCL, and OP550 in a reaction vessel, add solvent, and place it in an oil bath at 25-40°C. Then, slowly add IPDI dropwise to the three-necked flask containing CO, PCL, and OP550, followed by the addition of catalyst.

[0020] 3) Raise the temperature of the oil bath to 70-90℃ and react for 1-3 hours.

[0021] 4) Add HQEE and DMBA dissolved in acetone, then add the catalyst. Let the reaction proceed for another 3-5 hours.

[0022] 5) First, lower the temperature of the oil bath to 30-50℃, then add triethylamine at 30-50℃ and stir for 20-40 minutes.

[0023] 6) Add a predetermined amount of distilled water containing 3 wt% THPO, and stir at 1100-1300 rpm for 0.5-2 hours. This yields a flame-retardant waterborne polyurethane (WPU) emulsion.

[0024] Preferably, the drying in step 1) is performed at 100–120°C for 1–3 hours. More preferably, it is performed in a vacuum oven at 110°C for 2 hours.

[0025] Preferably, the vacuum in step 1) has a vacuum degree of 125–140 Pa. Preferably, the vacuum in step 1) has a vacuum degree of 133 Pa.

[0026] Preferably, in step 2), the solvent is tetrahydrofuran (THF), and the amount of tetrahydrofuran used is sufficient to dissolve the reactants in step 1). The catalyst is dibutyltin dilaurate (DBTDL), and the amount of catalyst (DBTDL) is sufficient to initiate the polymerization reaction. IPDI stands for isophorone diisocyanate.

[0027] Preferably, in step 2), the container is placed in an oil bath environment at 30°C.

[0028] Preferably, in step 2), a condenser reflux device is assembled, nitrogen is introduced into the system to purge the internal air, and then IPDI is slowly added dropwise to a three-necked flask containing CO, PCL, and OP550 using a constant pressure funnel, followed by the addition of catalyst (DBTDL).

[0029] Preferably, in step 2), the molar ratio of CO, PCL, OP550 and IPDI is controlled at 48:12:3:328, 48:12:7:386, 48:12:14:473 or 48:12:20:553.

[0030] Preferably, in step 4), the molar ratio of CO, PCL, OP550, HQEE, DMBA, and IPDI is controlled at 48:12:(3-20):(186-352):(55-97):(328-553); the content of OP550 in WPU is 5-20 wt%. More preferably, the molar ratio of CO, PCL, OP550, HQEE, DMBA, and IPDI is controlled at 48:12:3:186:55:328, 48:12:7:230:65:386, 48:12:14:294:81:473, or 48:12:20:352:97:553.

[0031] Preferably, in step 5), the molar ratio of triethylamine to DMBA is (0.8–1.5):1. More preferably, in step 5), the molar ratio of triethylamine to DMBA is 1:1.

[0032] Preferably, the predetermined amount of 3 wt% THPO distilled water in step 6) refers to: the amount of THPO added is 3% of the total mass of all monomers in the reaction process, that is, the total mass of monomers is PCL + CO + IPDI + HQEE + DMBA + triethylamine, denoted as M. At this point, 0.03 M of THPO is weighed and added to 2 M of distilled water to prepare 3 wt% THPO distilled water. The final emulsion prepared in this way is considered to have a solid content of 33%.

[0033] THPO stands for trimethylolphosphine oxide. THPO is a phosphorus-containing flame retardant, which is equivalent to being blended into a polyurethane system to improve the flame retardant properties of the polyurethane material.

[0034] The obtained flame-retardant WPU exhibits a high tensile stress of 35 MPa, an elongation of 156%, and a toughness of 48.69 MJ·m. -3 The limiting oxygen index (LOI) is above 27.55%, preferably between 27.55% and 28.34%.

[0035] This invention provides applications of the flame-retardant waterborne polyurethane (WPU) emulsion, including its use in electrical insulation encapsulation materials, electrical component potting compounds, waterproof coatings, anti-corrosion coatings, cold-resistant coatings, fire-retardant coatings, fire-retardant expansion seals, and adhesives.

[0036] The present invention has the following beneficial effects:

[0037] The prepared WPU exhibits excellent flame retardant properties. The WPU achieves the highest "V0" flame retardant rating, meeting the UL-94 test standard. As the proportion of OP550 increases from 10 wt% to 20 wt%, the limiting oxygen index (LOI) of the WPU increases from 27.55 to 28.34. The peak heat release rate (PHRR) of the modified cardboard is reduced by 32.15% compared to the unmodified cardboard. The average heat release rate (HRR) decreases from 221.02 kW·m² before modification. -2 Reduced to 41.07 kW·m after modification. -2 It decreased by 81.4% ( Figure 6 b). For example Figure 6 As shown in c, the ignition time (Tig) of the cardboard increased from 13s before modification to 21s after modification, an increase of 61.5%.

[0038] The fire performance index (FPI) and fire growth index (FGI) of the cardboard before and after modification changed from 0.03 and 14.41 to 0.033 and 12.96, respectively, indicating that flame-retardant WPU has great potential in flame-retardant applications. Attached Figure Description

[0039] Figure 1Preparation of waterborne polyurethane and schematic diagram of its internal hydrogen bond network.

[0040] Figure 2 XRD analysis of flame-retardant WPU.

[0041] Figure 3 a) DSC curve of degradable WPU. b) DMA curve of degradable WPU.

[0042] Figure 4 The adhesion strength of biodegradable WPU on different substrates and actual product images.

[0043] Figure 5 Digital photographs of biodegradable WPU at different ignition times.

[0044] Figure 6 a) Heat release rate (HRR) of the base paper and modified paper. b) Average HRR of the base paper and modified paper. c) Ignition time (Tig) of the base paper and modified paper. d) Heat flux of 35 kW·m -2 At the same time, the fire resistance performance index (FPI) and fire growth index (FGI) of flame retardant WPU.

[0045] Figure 7 Polyurethane flame retardant performance testing apparatus. The top surface temperature is determined by an infrared camera. A sheet of paper with a thickness of 0.55 mm and a 0.1 mm coating on both sides is selected. The infrared thermal imager is set at 45 degrees and positioned 30 cm above the sample, taking pictures every 3 seconds until the paper burns.

[0046] Figure 8 a. The highest surface temperature of the original paper, determined by an infrared camera. b. The highest surface temperature of the modified paper, determined by an infrared camera. Detailed Implementation

[0047] Raw materials and reagents

[0048] Polycaprolactone diol (PCL), castor oil (CO), isoflurane diisocyanate (IPDI), 2,2-dimethylolbutyric acid (DMBA), triethylamine (TEA), and phosphate-buffered saline (PBS) were all analytical grade and purchased from Shanghai Maclean Biotechnology Co., Ltd. Acetone was analytical grade and purchased from Shandong Jinan Qiguang Science and Trade Co., Ltd. 1,4-bis(2-hydroxyethoxy)benzene (HQEE) and dibutyltin dilaurate (DBTDL) were analytical grade and purchased from Shanghai Aladdin Biotechnology Co., Ltd. Tetrahydrofuran (THF) was analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. Phosphoryltriethanolamine (THPO) was analytical grade and purchased from Wuhan Smex Biotechnology Co., Ltd. Amano lipase was analytical grade and purchased from Xibao Biotechnology (Shanghai) Co., Ltd. Sylgard 184 (PDMS) was analytical grade and purchased from Shanghai Enlaibao Trading Co., Ltd. Exolit OP550 was analytical grade and purchased from Clariant Chemical Co., Ltd.

[0049] Experimental instruments and equipment

[0050] Analytical balance, ZB603C, Mettler Toledo Instruments Co., Ltd. Microcomputer-controlled electronic universal testing machine, WDW-02, Jinan Hengsishengda Instrument Co., Ltd. Vacuum drying oven, DZF-6020, Gongyi Yingyu Yuhua Instrument Factory. Circulating water vacuum pump, SHZ-D(Ⅲ), Gongyi Yingyu Yuhua Instrument Factory. Pull-out adhesion tester, XH-M, Beijing Tiandi Xinghuo Instrument Co., Ltd. Electric thermostatic drying oven, DHG-9070A, Gongyi Yuhua Instrument Co., Ltd. Rotary evaporator, RE-1002, Shanghai Yarong Biochemical Instrument Factory. Digital display constant speed high-power electric stirrer, JB90-SH, Shanghai Specimen Model Factory. Precision salt spray tester, LS-UT-6, China LESTEST Company. Heat-collecting thermostatic magnetic stirrer, DF-101S, Shanghai Meiyingpu Instrument Manufacturing Co., Ltd. Ultrapure water system, GWB-1B model, Beijing Purkinje General Instrument Co., Ltd. Air compressor, KMS model, Yongkang Zhenmei Home Furnishings Co., Ltd. Emulsifying mixer, EUROSTAR 20 model, Aika (Guangzhou) Instrument Equipment Co., Ltd. Electrochemical analyzer, CHI660E, Shanghai Chenhua Instrument Co., Ltd.

[0051] High-strength biodegradable flame-retardant waterborne polyurethane testing and characterization

[0052] Mechanical performance testing

[0053] Mechanical tensile tests were performed using a WDW-02 electronic universal testing machine at room temperature with a tensile speed of 20 mm·min⁻¹ and a specimen thickness of 0.02 mm. Stress-strain curves were measured. According to national standard 36363, an INSTRON 5982 universal mechanical testing machine was used at room temperature with a puncture speed of 10 mm·min⁻¹. -1 The specimen thickness was 0.3 mm, and the penetration strength of the specimen was measured.

[0054] Differential scanning calorimetry (DSC) measurements were performed on a Mettler Toledo DSC1 STARe differential scanning calorimeter and an FRS5 sensor to determine the thermal behavior of the samples. All experiments were conducted under a dry nitrogen atmosphere. The samples were first heated to 150 °C and held for 2 minutes to remove thermal history, then cooled to -80 °C, and then heated back to 150 °C at a heating / cooling rate of 10 °C / min. The glass transition temperature (Tg) was used as the midpoint of the heat flow change for the second heating scan.

[0055] Flame retardant performance test

[0056] According to ISO 5660 standard, the paper size before modification was 10cm × 10cm × 0.55mm, and the paper size after modification was 10cm × 10cm × 2mm, with a heat flux of 35 kW·m⁻². Cone calorimetry tests were conducted using a British FTT0007 cone calorimeter. Vertical combustion tests were performed using an FTT0082 (instrument, British) according to GB / T2408-2008 standard, with a size of 15cm × 3cm × 2cm. Limiting oxygen index tests were performed using a British FTT0077 oxygen index tester according to ASTM D2863 standard. Measurements were taken on a 15cm × 3cm × 2cm sample, and 15 parallel experiments were conducted to ensure data accuracy.

[0057] Degradation performance test

[0058] Fourier transform infrared (FTIR) spectroscopy was performed using a Thermo Scientific Nicolet iS20 for characterization of films with a thickness of 0.05 mm. Since the degradation of the films was uncontrollable and could not reach the specified thickness, each test was repeated at least three times to ensure the accuracy of the experimental results.

[0059] Scanning electron microscopy (SEM) images were captured under vacuum using a Hitachi Regulus 8220 SEM. A thin layer of gold (2-3 nm) was coated onto the cross-section of the sample, and images were taken of naturally fractured cross-sectional areas.

[0060] Corrosion resistance test

[0061] Electrochemical measurements were performed on the coating under CASS (ASTM B368) conditions. A CHI 660D electrochemical workstation (Shanghai Chenhua, China) was used to conduct electrochemical tests on the coating under a stable open-circuit voltage. The test area of ​​the coating was 38.465 cm². 2 Ag / AgCl is the reference electrode, and Pt is the counter electrode. The polarization curve scan rate is 1 mV·s. -1 Electrochemical impedance spectroscopy at 10 -2 Hz~10 5 The experiment was conducted within the Hz frequency range, with a sinusoidal signal perturbation of 5mV. Each experiment was repeated at least three times to ensure the accuracy of the results.

[0062] X-ray photoelectron spectroscopy (XPS) was performed in an ESCALAB Xi+ chamber at 12 mA and 12 kV using a monochromatic aluminum source. The chamber pressure was 5 × 10⁻⁹ mbar. To detect spectra at different depths, an Ar gun was used at 1.5 × 10⁻⁹ mbar before measurement. -6 At mbar and 3kV pressure, the grating size is 2×2mm. 2 The samples were etched at different times (0, 10, 20, 30, 40, and 50 nm). Data were analyzed using CasaXPS software. All binding energies were related to the c1s peak of the surface imprecise carbon at 285 eV. Etching depth increased proportionally with etching time.

[0063] Thermogravimetric analysis (TGA) was performed using a TGAQ50 thermogravimetric analyzer. Samples were placed in a crucible under a nitrogen atmosphere with a heating rate of 10 °C / min to analyze the material's pyrolysis behavior from 0 °C to 800 °C. The results showed that the addition of flame retardant had little effect on thermal stability.

[0064] X-ray diffraction (XRD) was performed using a Rigaku D / max-2500 diffractometer equipped with a Cu Kα radiation source (λ = 0.15406 nm) (40 kV, 200 mA). The morphology and structure of SIPCs were characterized using field emission methods.

[0065] The following embodiments are further illustrations of the present invention, but the present invention is not limited thereto.

[0066] Example 1: Synthesis of Flame-Retardant Waterborne Polyurethane (WPU)

[0067] A biodegradable wetted polymer (WPU) was designed and prepared using biodegradable CO and PCL as soft segments. Rigid 1,4-bis(2-hydroxyethyl)benzene (HQEE) was used as a chain extender to improve the mechanical properties of the material's molecular chain. The introduction of hydrophilic 2,2-dihydroxymethylbutyric acid (DMBA) into the molecular chain promoted the emulsification process, ultimately resulting in a high-molecular-weight polymer network. We believe that the presence of hydrogen bonds within the system is the reason for the high mechanical strength of the entire network; that is, dense hydrogen bonds are distributed throughout the polymer network, and the strong synergistic effect of hydrogen bonds in different components gives the polymer network excellent mechanical strength. Figure 1 ).

[0068] Synthesis steps:

[0069] 1) Place castor oil (CO), polycaprolactone diol (PCL), OP550, 1,4-bis(2-hydroxyethoxy)benzene (HQEE), and 2,2-dimethylolbutyric acid (DMBA) in a vacuum oven and dry them at 110°C for 1.5 hours under vacuum.

[0070] The vacuum level is 133 Pa (gauge pressure is -90 kPa, absolute pressure is 10 kPa).

[0071] 2) Place the dried CO, PCL, and OP550 in a three-necked flask, add 5 ml of tetrahydrofuran (THF), and place in an oil bath at 30°C. Assemble a reflux condenser, purge the system with nitrogen to remove all air, and then slowly add IPDI dropwise to the three-necked flask containing CO, PCL, and OP550 using a constant pressure funnel. Next, add 50 μL of catalyst (DBTDL), and finally raise the temperature of the oil bath to 80°C and react for 2 hours.

[0072] Two solvents were used in the entire reaction. Tetrahydrofuran was used as the solvent, and acetone was also used. The role of acetone was to dissolve 2,2-dimethylolbutyric acid (DMBA), as DMBA is far more soluble in acetone than tetrahydrofuran. Because the amount of DMBA used was small, the amount of acetone used was also much smaller compared to tetrahydrofuran. The dried DMBA was first dissolved in acetone before being added to the reaction system.

[0073] The molar ratios of CO, PCL, OP550, and IPDI are controlled at 48:12:3:328, 48:12:7:386, 48:12:14:473, and 48:12:20:553.

[0074] 3) Add HQEE and DMBA dissolved in acetone, then add 50 μL of DBTDL. Let the reaction proceed for another 4 hours.

[0075] Tetrahydrofuran (THF) is added during the reaction to reduce viscosity. The purpose of using tetrahydrofuran as a solvent is to lower the viscosity of the system during the reaction, because if it's too viscous, the magnetic stirrer will be unable to rotate, ultimately leading to experimental failure. The timing and amount of addition depend entirely on whether the magnetic stirrer can rotate normally; if it can, no addition is needed, otherwise, add it.

[0076] 4) First, lower the temperature of the oil bath to 40℃, then add 0.553~0.984g of triethylamine at 40℃ and stir for 30min.

[0077] After the third preparation step is completed, the polyurethane has been formed. At this time, carboxyl groups still exist on the polyurethane molecular chain. The purpose of adding triethylamine at this time is to neutralize the carboxyl groups on the polyurethane molecular chain. The molar ratio of triethylamine to DMBA is 1:1.

[0078] The contents of OP550 in WPU were 5, 10, 15, and 20 wt%. The resulting WPUs were designated as samples of 5 wt% OP550 (denoted as S1), 10 wt% OP550 (denoted as S2), 15 wt% OP550 (denoted as S3), and 20 wt% OP550 (denoted as S4).

[0079] We investigated the effects of different components on the mechanical properties of polyurethane by adjusting the proportion of soft segment raw materials. A series of flame-retardant WPUs were synthesized by adjusting the molar ratio of PCL and CO. With increasing PCL / CO molar ratio, the tensile strength decreased significantly, while the elongation at break increased. This is because CO, as a tertiary alcohol, has more reaction sites; increasing CO leads to a significant increase in polymer crosslinking density, thus exhibiting higher mechanical strength. To verify this, we used FTIR testing for characterization. FTIR observed a value of 1700 cm⁻¹. -1 The increased peak area at the point of origin is related to the increased number and density of hydrogen bonds, demonstrating that the increase in CO content has a positive effect on tensile strength. When PCL:CO = 1:9, the tensile strength reaches as high as 49.94 MPa, while the elongation at break is only 60%.

[0080] To verify our successful synthesis of WPU, we performed FTIR testing on the prepared material to detect the 2230–2270 cm⁻¹ region. -1 The change in the isocyanate (-N=C=O) peak and the disappearance of the isocyanate peak in the FTIR spectrum prove the successful synthesis of a series of flame-retardant WPUs.

[0081] Example 2: Synthesis and Monomer Ratio Control of High-Strength Biodegradable Flame-Retardant Waterborne Polyurethane (WPU)

[0082] Synthesis steps:

[0083] 1) Place castor oil (CO), polycaprolactone diol (PCL), OP550, 1,4-bis(2-hydroxyethoxy)benzene (HQEE), and 2,2-dimethylolbutyric acid (DMBA) in a vacuum oven and dry them at 110°C for 1.5 hours under vacuum.

[0084] The vacuum level is 133 Pa (gauge pressure is -90 kPa, absolute pressure is 10 kPa).

[0085] 2) Place the dried CO, PCL, and OP550 in a three-necked flask, add 5 ml of tetrahydrofuran (THF), and place in an oil bath at 30°C. Assemble a reflux condenser, purge the system with nitrogen to remove all air, and then slowly add IPDI dropwise to the three-necked flask containing CO, PCL, and OP550 using a constant pressure funnel. Next, add 50 μL of catalyst (DBTDL), and finally raise the temperature of the oil bath to 80°C and react for 2 hours.

[0086] Two solvents were used in the entire reaction. Tetrahydrofuran was used as the solvent, and acetone was also used. The role of acetone was to dissolve 2,2-dimethylolbutyric acid (DMBA), as DMBA is far more soluble in acetone than tetrahydrofuran. Because the amount of DMBA used was small, the amount of acetone used was also much smaller compared to tetrahydrofuran. The dried DMBA was first dissolved in acetone before being added to the reaction system.

[0087] The molar ratios of CO, PCL, OP550, and IPDI are controlled at 48:12:3:328, 48:12:7:386, 48:12:14:473, and 48:12:20:553.

[0088] IPDI stands for Isophorone Diisocyanate.

[0089] The catalyst is dibutyltin dilaurate (DBTDL), and its function is to increase the reaction rate. Dibutyltin dilaurate is the most commonly used catalyst in polyurethane preparation; other catalysts can also be used in industrial production. There is no specific limitation on the proportion of catalyst added; the experimental addition amount in this invention was 50 μL each time, and those skilled in the art can adjust it according to actual needs.

[0090] 3) Add HQEE and DMBA dissolved in acetone, then add 50 μL of DBTDL. Let the reaction proceed for another 4 hours.

[0091] Tetrahydrofuran (THF) is added during the reaction to reduce viscosity. The purpose of using tetrahydrofuran as a solvent is to lower the viscosity of the system during the reaction, because if it's too viscous, the magnetic stirrer will be unable to rotate, ultimately leading to experimental failure. The timing and amount of addition depend entirely on whether the magnetic stirrer can rotate normally; if it can, no addition is needed, otherwise, add it.

[0092] 4) First, lower the temperature of the oil bath to 40℃, then add 0.553~0.984g of triethylamine at 40℃ and stir for 30min.

[0093] After the third preparation step is completed, the polyurethane has been formed. At this time, carboxyl groups still exist on the polyurethane molecular chain. The purpose of adding triethylamine at this time is to neutralize the carboxyl groups on the polyurethane molecular chain. The molar ratio of triethylamine to DMBA is 1:1.

[0094] 5) Add the predetermined amount of distilled water containing 3 wt% THPO, and stir at 1200 rpm for 1 hour. The final product is an emulsion with a solid content of 33%.

[0095] THPO stands for trimethylolphosphine oxide, a phosphorus-containing flame retardant that is blended into polyurethane systems to improve the flame retardant properties of polyurethane materials. OP550 is Exolit OP550, a medium-viscosity liquid based on a non-halogenated phosphorus polyol with a functionality of approximately 10%, primarily suitable for the production of flame-retardant polyurethane foams; the OP550 used in this invention was purchased from Clariant Chemicals Ltd.

[0096] The contents of OP550 in WPU were 5, 10, 15, and 20 wt%. The resulting WPUs were designated as samples of 5 wt% OP550 + 3% THPO (denoted as S6), 10 wt% OP550 + 3% THPO (denoted as S7), 15 wt% OP550 + 3% THPO (denoted as S8), and 20 wt% OP550 + 3% THPO (denoted as S9).

[0097] Considering the high toughness of WPU, Example 2 selected WPU with PCL:CO = 1:4, and further synthesized flame-retardant WPU by adding flame retardant OP550 and THPO.

[0098] Example 3: Study on flame retardant and mechanical properties

[0099] This invention achieves excellent flame retardant properties by using OP550 and THPO as synergistic flame retardants; neither of these two alone can achieve the same flame retardant effect. We controlled the addition amount of THPO to 3% and discussed the effects of different contents of OP550 on the mechanical and flame retardant properties of the material. The limiting oxygen index of the samples is shown in Table 1.

[0100] Table 1 Limiting oxygen index of the samples

[0101]

[0102] The mechanical properties of the flame-retardant WPU are shown in Table 2. It was found that as the specific gravity of OP550 gradually increased from 5 wt% to 20 wt%, the tensile strength gradually decreased from 39 MPa to 23 MPa. Further analysis of the synthesized flame-retardant WPU revealed that XRD testing confirmed that all the flame-retardant WPUs were amorphous structures. Figure 2 Furthermore, thermogravimetric analysis revealed that the thermal degradation behavior of the flame-retardant WPU remained almost unchanged with the increase of the flame retardant ratio, indicating that the addition of OP550 had no effect on the overall thermal stability of the material.

[0103] Table 2. Mechanical properties of flame-retardant WPU.

[0104]

[0105]

[0106] To investigate the internal properties of the materials, we further studied the thermal properties of the flame-retardant WPU samples S6-S9 synthesized in Example 2 using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The DSC curves showed that as the proportion of OP550 gradually increased from 5 wt% to 20 wt%, the glass transition temperature (Tg) gradually decreased from 57.37 °C to 39.79 °C. Figure 3 a) This indicates that increasing the proportion of OP550 and decreasing the proportion of castor oil reduces the number of hydrogen bonds formed in the system, leading to a decrease in Tg. DMA results show that the change in Tg follows the same pattern as DSC, with the storage modulus (G') decreasing continuously with increasing OP550 proportion, which is also consistent with the tensile results. Figure 3 b). We believe that the crosslinking density of the WPU internal network structure is closely related to its mechanical strength. The crosslinking density of the WPU internal network structure is directly related to the ratio of CO to OP550 content. Therefore, to understand the effect of OP550 ratio on mechanical properties, the crosslinking density (N) was calculated (Equation 1).

[0107] Ge = NRT (1)

[0108] Ge represents the plateau modulus G′ in the storage modulus curve, and R and T are the gas constant and absolute temperature, respectively. As the proportion of OP550 increases from 5, 10, and 15 wt% to 20 wt%, the N of the flame-retardant WPU decreases from 1024.94, 642.77, and 414.16 to 232.11 mol·m⁻³, indicating that increasing the proportion of OP550 weakens its mechanical properties.

[0109] Example 4: Study on the adhesion strength of flame-retardant waterborne polyurethane

[0110] Polyurethane, as a traditional adhesive, also exhibits excellent adhesion properties in the synthesized flame-retardant WPU. The S7 sample (10 wt% OP550) synthesized in Example 2 showed an adhesion strength of 4.6 MPa on a glass substrate. Figure 4 As shown, it exhibits extremely high adhesion properties on various substrates. The adhesion strength of the S7 sample synthesized in Example 2 on the substrate is shown in Table 3.

[0111] Table 3. Adhesion strength of S7 sample on different substrates

[0112]

[0113] Example 5: Study on the flame retardant properties of waterborne polyurethane

[0114] A vertical burning test was conducted to assess the flame retardancy of WPU. Figure 5 As shown, the biodegradable WPU (10wt% OP550 sample S7 in Example 2) self-extinguished rapidly after 10 seconds of ignition. After a second ignition, the cotton bed (which is commercially available cotton) was not ignited, proving that the WPU has the highest "V0" level flame retardant performance and meets the UL-94 test standard.

[0115] The Limiting Oxygen Index (LOI) is an important parameter for evaluating the flame retardancy of materials. It is the minimum concentration of oxygen required to support the combustion of a polymer, expressed as a volume percentage of oxygen. A limiting oxygen index greater than the atmospheric oxygen concentration is one of the standards for flame-retardant materials. Materials with an index greater than the atmospheric oxygen concentration (21%) can also be called flame-retardant materials; however, for safety reasons, a material with an LOI of 27 is generally considered flame-retardant. For our biodegradable WPU, as the proportion of OP550 increased from 10 wt% to 20 wt%, the LOI of the WPU increased from 27.55 to 28.34 (Table 2).

[0116] Example 6: Flame-retardant materials modified with waterborne polyurethane and their testing.

[0117] To further investigate the steady-state combustion behavior of the material, we measured the heat flux at 35 kW·m using WPU-modified cardboard. -2 Heat dissipation performance at that time.

[0118] The preparation method of WPU-modified cardboard involves completely encapsulating the cardboard with WPU. This includes the following steps:

[0119] WPU emulsion was evenly dripped onto cardboard, and after drying, a layer of cured polyurethane film formed around the cardboard. The thickness of the cured film in the experiment was 0.72 mm.

[0120] from Figure 6 As can be seen, the peak heat release rate (PHRR) of the modified cardboard decreased by 32.15% compared to the unmodified cardboard. The average heat release rate (HRR) decreased from 221.02 kW·m² before modification. -2 Reduced to 41.07 kW·m after modification. -2 It decreased by 81.4% ( Figure 6 b). A longer ignition time (Tig) means the material itself is more difficult to ignite. For example... Figure 6 As shown in c, the Tig of the cardboard increased from 13 s before modification to 21 s after modification, an increase of 61.5%. To further evaluate the real combustion behavior under fire conditions, we introduced two important parameters ( Figure 6 d), Fire Performance Index (FPI) and Fire Growth Index (FGI) (see Formulas 2 and 3);

[0121]

[0122]

[0123] tPHRR is the time to reach PHRR. Generally, the higher the FPI value and the lower the FGI value, the better the fire resistance of the material. In our work, the FPI and FGI values ​​of cardboard changed from 0.03 and 14.41 before and after modification to 0.033 and 12.96, respectively, indicating that flame-retardant WPU has great potential in flame-retardant applications.

[0124] Igniting the modified cardboard with an alcohol lamp further demonstrated the advantages of biodegradable WPU. Figure 7 ).like Figure 8 As shown, when the unmodified cardboard burned for 24 seconds, it was burned through and the highest surface temperature reached 507.7°C, while the modified paper remained intact after 24 seconds of burning, with the highest surface temperature reduced by 32%. Furthermore, the mechanical and flame-retardant properties of our WPU were compared with those of previously reported flammable and flame-retardant WPUs. We also achieved a stress of 35 MPa and an LOI of 27.55 for our WPU. These values ​​represent the highest levels reported for most flammable and flame-retardant WPUs.

[0125] Our WPU, in addition to its flame retardancy and high mechanical strength, also exhibits good degradability. The degradability of WPU plastic was studied using enzyme immersion and soil burial methods. When a degrading enzyme (0.05 g enzyme and 1 g WPU in a phosphate-buffered saline solution) was added at a rate of 0.05 g / g, and the WPU plastic was immersed at 50°C for 15 days, obvious surface breakage was observed. After 30 days of immersion, the WPU plastic cracked into small pieces. The mass loss rate of the WPU plastic after 40 days of immersion was 12.25%. Furthermore, adjusting the dosage of lipase accelerated the degradation process. When the enzyme addition was adjusted to 0.2 g / g, the mass loss rate reached 15.2% after 40 days of immersion. Degradation performance tests were also conducted by burying WPU plastic in campus soil. With increasing burial depth, the WPU plastic gradually cracked. The mass loss rate of the WPU plastic reached 37% after 3 months of burial.

Claims

1. A flame-retardant waterborne polyurethane emulsion, prepared from castor oil (CO), polycaprolactone diol (PCL), Exolit OP550, 1,4-bis(2-hydroxyethoxy)benzene (HQEE), and 2,2-dimethylolbutyric acid (DMBA), wherein the molar ratio of CO, PCL, Exolit OP550, HQEE, DMBA, and IPDI is controlled at 48:12:(3~20):(186~352):(55~97):(328~553); the synthesis method of the flame-retardant waterborne polyurethane (WPU) emulsion includes the following steps: 1) Place castor oil CO, polycaprolactone diol PCL, Exolit OP550, 1,4-bis(2-hydroxyethoxy)benzene HQEE, and 2,2-dimethylolbutyric acid DMBA in a vacuum oven and dry them; 2) Place the dried CO, PCL, and Exolit OP550 in a reaction vessel, add solvent, and place it in an oil bath environment at 25~40℃. Then slowly add IPDI dropwise to the three-necked flask containing CO, PCL, and Exolit OP550, followed by the addition of catalyst. 3) Raise the temperature of the oil bath to 70~90℃ and react for 1~3 hours; 4) Add HQEE and DMBA dissolved in acetone, then add the catalyst; continue the reaction for another 3-5 hours; 5) First, lower the temperature of the oil bath to 30~50℃, then add triethylamine at 30~50℃ and stir for 20~40 minutes; 6) Add a predetermined amount of distilled water containing 3 wt% trimethylolphosphine oxide (THPO), and stir at 1100~1300 rpm for 0.5~2 hours to obtain flame-retardant waterborne polyurethane (WPU) emulsion.

2. A flame-retardant material modified with flame-retardant waterborne polyurethane, comprising a substrate and a film cured from the flame-retardant waterborne polyurethane emulsion of claim 1 coated on the surface of the substrate.

3. The flame-retardant waterborne polyurethane-modified flame-retardant material as described in claim 2, characterized in that, The substrates include cardboard, wood, and cotton.

4. The flame-retardant waterborne polyurethane-modified flame-retardant material as described in claim 2, characterized in that, The film cured from the flame-retardant waterborne polyurethane emulsion has a thickness of 0.5~2mm.

5. The method for preparing flame-retardant materials modified with flame-retardant waterborne polyurethane as described in any one of claims 2 to 4, comprising dripping or spraying a flame-retardant waterborne polyurethane emulsion onto a substrate, or immersing the substrate in a flame-retardant waterborne polyurethane emulsion.

6. The flame-retardant waterborne polyurethane emulsion as described in claim 1, characterized in that, The drying in step 1) is performed at 100~120℃ for 1-3 hours; The vacuum mentioned in step 1) has a vacuum degree of 125~140 Pa.

7. The flame-retardant waterborne polyurethane emulsion as described in claim 1, characterized in that, In step 2), the molar ratio of CO, PCL, ExolitOP550 and IPDI is controlled at 48:12:3:328, 48:12:7:386, 48:12:14:473 or 48:12:20:

553. In step 4), the molar ratio of CO, PCL, Exolit OP550, HQEE, DMBA, and IPDI is controlled at 48:12:(3~20):(186~352):(55~97):(328~553); the content of Exolit OP550 in WPU is 5~20 wt%. In step 5), the molar ratio of triethylamine to DMBA is (0.8~1.5):

1.

8. The flame-retardant waterborne polyurethane emulsion as described in claim 7, characterized in that, In step 4), the molar ratio of CO, PCL, ExolitOP550, HQEE, DMBA, and IPDI is controlled at 48:12:3:186:55:328, 48:12:7:230:65:386, 48:12:14:294:81:473, or 48:12:20:352:97:553; in step 5), the molar ratio of triethylamine to DMBA is 1:

1.

9. The flame-retardant waterborne polyurethane emulsion according to any one of claims 1, 6-8, characterized in that, The obtained flame-retardant waterborne polyurethane exhibits a high tensile stress of 35 MPa, an elongation of 156%, and a toughness of 48.69 MJ·m. -3 The limiting oxygen index is above 27.55%.

10. The flame-retardant waterborne polyurethane emulsion as described in claim 9, characterized in that, The limiting oxygen index is 27.55~28.34%.

11. The application of the flame-retardant waterborne polyurethane (WPU) emulsion as described in any one of claims 1, 6-10, for use in electrical insulation encapsulation materials, waterproof coatings, anti-corrosion coatings, cold-resistant coatings, fire-retardant coatings, fire-retardant expansion seals, and adhesives.

12. The application of the flame-retardant waterborne polyurethane (WPU) emulsion as described in claim 11, characterized in that, The adhesive is an electrical component potting compound.

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