Preparation method of nano titanium dioxide smoke suppression and toxicity reduction agent and composite material

Abstract

The invention discloses a preparation method of a nano titanium dioxide smoke suppression and toxicity reduction agent, which comprises the following steps: a, preparation of titanium dioxide nanospheres: adding ammonia water and isopropyl titanate into a mixed solution of ethanol and acetonitrile, keeping vigorous stirring and mechanically stirring for 6-10 hours, washing, drying and grinding to obtain the titanium dioxide nanospheres; b, preparation of silane modified titanium dioxide suspension: adding the titanium dioxide nanospheres obtained in the step a into a mixed solvent of ethanol and deionized water, slowly dropwise adding a silane coupling agent KH550 under the condition of ultrasonic stirring, and reacting for 5-8 hours under the condition of 40-50 DEG C; and c, preparation of the silicon-cobalt modified titanium dioxide solid nanosphere hybrid material: dispersing the reaction product in the step b in deionized water, ultrasonically stirring for 5-30 minutes, adding sodium alginate with the mass fraction of 0.3-1wt% into the solution, vigorously stirring for 1-3 hours, then adding water-soluble cobalt salt, reacting for 1-3 hours, continuously stirring for 2-3 hours, centrifuging and washing the suspension, and drying to obtain the silicon-cobalt modified titanium dioxide solid nanosphere hybrid material. And drying to obtain the nano titanium dioxide smoke suppression and toxicity reduction agent.

Description

Technical Field

[0001] The invention relates to the field of material technology, and in particular to a method for preparing a nano titanium dioxide smoke suppressant and toxicity reducing agent and a composite material using the method, aiming to provide an environmentally friendly material with excellent flame retardant and smoke suppressant properties. Background Art

[0002] With the rapid development of modern industry, rigid polyurethane foam (RPUF) is widely used in many fields such as building energy conservation, oil transportation and household products due to its excellent physical properties. However, RPUF has a porous structure and a large specific surface area, which makes it easy to burn. Its molecular chain is mainly composed of carbon (C), hydrogen (H) and oxygen (O) elements, and contains a high proportion of combustible components, so it is classified as a flammable material. At the same time, the porous structure significantly increases the contact area with the air, and the oxygen index of unmodified RPUF is only 18%-19%. During high-temperature combustion, RPUF will release a large amount of heat in a short period of time. Although it is non-toxic itself, it will produce toxic gases such as carbon monoxide (CO) and hydrocyanic acid (HCN) during combustion, accompanied by the generation and diffusion of a large amount of toxic smoke. These factors pose a serious threat to human life safety and the ecological environment. Therefore, it is particularly important to effectively treat RPUF with flame retardant and smoke suppression to improve its fire safety.

[0003] Nano-titanium dioxide has gradually become an ideal smoke suppressant and toxic reducer due to its excellent photocatalytic performance, thermal stability and safety. Its unique microstructure and high specific surface area enable it to effectively adsorb and decompose toxic gases in catalytic reactions. At the same time, the free radicals generated by photocatalysis promote the degradation of organic matter. In addition, nano-titanium dioxide can still maintain its excellent catalytic performance under high temperature conditions and shows high thermal stability. At the same time, transition metal materials such as cobalt, nickel, molybdenum, etc. are often used to eliminate or reduce the release of volatile organic compounds and toxic smoke during the combustion of polymer materials, thereby further enhancing the fire safety performance of polymer materials.

[0004] By modifying the surface of nano titanium dioxide, its dispersibility and compatibility in the rigid polyurethane foam matrix are significantly improved, and its interface bonding with the polymer matrix is ​​enhanced, thereby improving the flame retardant performance and smoke suppression effect. The silicon-cobalt modified nano titanium dioxide smoke suppressant and toxic reduction agent of the present invention combines the good compatibility of sodium alginate with the excellent catalytic properties of cobalt ions to form a synergistic effect. This composite material not only improves the catalytic and toxic reduction performance, but also significantly reduces the release of toxic gases (such as carbon monoxide and volatile organic compounds) during the combustion process, while improving the flame retardant properties of polyurethane foam, thereby improving the safety and environmental friendliness of the material. However, to date, there has been no report on the study of the smoke suppression and toxic reduction performance of silicon-cobalt modified nano titanium dioxide smoke suppressant and toxic reduction agent obtained by amino modification and self-assembly and applied to rigid polyurethane foam materials. Summary of the invention

[0005] The purpose of the present invention is to solve the problems in the prior art.

[0006] To achieve the above object, the present invention provides the following technical solution: a method for preparing a nano titanium dioxide smoke suppressant and toxicity reducing agent, characterized in that it comprises the following steps:

[0007] a. Preparation of titanium dioxide nanospheres: Add ammonia water and isopropyl titanate to a mixed solution of ethanol and acetonitrile, and keep stirring vigorously and mechanically for 6-10 hours, and obtain titanium dioxide nanospheres after washing, drying and grinding;

[0008] b. Preparation of silane-modified titanium dioxide suspension: Add the titanium dioxide nanospheres obtained in step a to a mixed solvent of ethanol and deionized water, slowly dropwise add silane coupling agent KH550 under ultrasonic stirring, and react at 40-50° C. for 5-8 hours;

[0009] c. Preparation of silicon-cobalt modified titanium dioxide solid nanosphere hybrid material: disperse the product of step b in deionized water, ultrasonically stir for 5-30 minutes, add 0.3wt%-1wt% sodium alginate to the above solution, stir vigorously for 1-3 hours, then add water-soluble cobalt salt, react for 1-3 hours, continue stirring for 2-3 hours, then centrifuge the suspension, wash with water, and dry to obtain nano titanium dioxide smoke suppressant and toxic reduction agent.

[0010] Preferably, the water-soluble cobalt salt is one of cobalt chloride, cobalt sulfate, cobalt bromide and cobalt nitrate.

[0011] Preferably, the particle size of the titanium dioxide solid nanospheres is 100-300 nm.

[0012] Preferably, in step a, 600 mL of ethanol solution and 400 mL of acetonitrile solution are added to a 1L three-necked flask, and the reaction temperature is 25°C; 1.68 mL of NH3H2O28wt% and 3.64 mL of deionized water are added, and mechanical stirring is performed; then 20 mL of isopropyl titanate is quickly poured therein, accompanied by vigorous stirring, and the reaction is continued for 6 hours, and it is washed alternately with deionized water and ethanol three times. Finally, it is dried in a vacuum drying oven for 8 hours, the temperature is set to 80°C, and the solid particles are ground to obtain titanium dioxide nanospheres.

[0013] Preferably, the mixed solvent of ethanol and deionized water in step b is prepared by mixing 50 g of anhydrous ethanol and 10 g of deionized water.

[0014] Preferably, in the step c, 2.5 g of the product from the step b is dispersed in 10 g of deionized water, and after ultrasonic stirring for 5-30 min, 0.3 wt%-1 wt% of sodium alginate is added to the solution, and the solution is vigorously stirred for 1-3 h, followed by adding 4 g of 0.05 mol / L CoCl2·6H2O solution, reacting for 1-3 h, stirring for 2-3 h, and then centrifuging at 6500 rpm in a high-speed centrifuge, washing three times with deionized water and ethanol alternately, and finally drying in a vacuum drying oven for 8 h, setting the temperature to 80°C, and grinding the solid particles to obtain a nano titanium dioxide smoke suppressant and toxic reduction agent.

[0015] A modified rigid polyurethane foam composite material is modified by using the nano titanium dioxide smoke suppressant and toxicity reducing agent prepared by the method for preparing the nano titanium dioxide smoke suppressant and toxicity reducing agent as described above.

[0016] Preferably, the added amount of the smoke suppressant and toxicity reducing agent is 0.5%-4% of the mass of the rigid polyurethane foam matrix.

[0017] Preferably, the rigid polyurethane foam is prepared from polyol, a tin catalyst, an amine catalyst, deionized water, silicone oil, triethanolamine and polymethylene polyphenyl polyisocyanate.

[0018] The invention has the following beneficial effects: the invention provides a method for preparing a silicon-cobalt modified nano titanium dioxide smoke suppressant and toxicity reducing agent, which realizes surface modification of nano titanium dioxide through amino modification and self-assembly technology, thereby improving its dispersibility and compatibility in a polyurethane foam matrix. Specifically, the surface of nano titanium dioxide is functionally modified by amino reaction, so that its surface has amino functional groups, which helps to enhance the interfacial bonding force with a rigid polyurethane matrix. Subsequently, sodium alginate and cobalt ions are alternately deposited on the surface of the amino nano titanium dioxide through a self-assembly method to form a composite material with a synergistic effect. This preparation method is not only simple and easy, but also can effectively improve the catalytic performance of nano titanium dioxide, and finally realize the effective inhibition of toxic gases in the combustion process and the reduction of smoke particles. The sodium alginate used in the invention gives full play to its excellent compatibility, chemical stability and catalytic carbonization characteristics. Its unique molecular structure can form a uniform coating on the surface of nano titanium dioxide and is combined with the catalytic properties of titanium dioxide and cobalt ions. This structure not only enhances the interaction between modified titanium dioxide and rigid polyurethane foam, but also significantly improves the overall catalytic performance of the catalyst detoxifier in rigid polyurethane foam, thereby effectively reducing the release of harmful gases during combustion. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present invention and therefore should not be regarded as limiting the scope. For a technician in this field, other related drawings can be obtained based on these drawings without paying any creative work.

[0020] Figure 1 This is a scanning electron microscope image of the silicon-cobalt modified titanium dioxide solid nanosphere hybrid material prepared in Example 1 of the present invention;

[0021] Figure 2 This is a transmission electron microscopy image of the silicon-cobalt modified titanium dioxide solid nanosphere hybrid material prepared in Example 1 of the present invention;

[0022] Figure 3 This is a field emission scanning electron microscope (EDS) energy spectrum of the silicon-cobalt modified titanium dioxide solid nanosphere hybrid material prepared in Example 1 of the present invention;

[0023] Figure 4 This is an X-ray photoelectron (XPS) spectrum of the TiO2 nanospheres prepared in Example 1 of the present invention;

[0024] Figure 5This is an X-ray photoelectron (XPS) spectrum of the silicon-cobalt modified titanium dioxide solid nanosphere hybrid material prepared in Example 1 of the present invention;

[0025] Figure 6 The infrared spectra of TiO2, silane-modified titanium dioxide suspension and silicon-cobalt-modified titanium dioxide solid nanosphere hybrid material prepared in Example 1 of the present invention;

[0026] Figure 7 It is a heat release rate curve and a total heat release curve of the rigid polyurethane foam and the flame retardant rigid polyurethane foam nanocomposite material prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by the first technician in the field without making creative work belong to the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the invention claimed for protection, but merely represents the selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by the first technician in the field without making creative work belong to the scope of protection of the present invention.

[0028] Example

[0029] The following are only preferred implementations of the present invention. The protection scope of the present invention is not limited to the following embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention.

[0030] Example 1

[0031] The preparation of the smoke suppressant and toxicity reducing agent of the silicon-cobalt modified titanium dioxide solid nanosphere hybrid material and the application of the flame retardant rigid polyurethane foam composite material in this embodiment are carried out according to the following steps:

[0032] 1. A method for preparing a silicon-cobalt modified nano titanium dioxide smoke suppressant and toxicity reducing agent is as follows:

[0033] Step 1: Add 600mL of ethanol solution and 400mL of acetonitrile solution to a 1L three-necked flask and set the reaction temperature to 25°C. Next, add 1.68mL of NH3·H2O (28wt%) and 3.64mL of deionized water and stir mechanically. Subsequently, isopropyl titanate is quickly added to the reaction mixture, accompanied by vigorous stirring, and the reaction is continued for 6 hours. After that, the mixture is centrifuged in a high-speed centrifuge and washed alternately with deionized water and ethanol three times. Finally, the product is placed in a vacuum drying oven and dried at 80°C for 8 hours. The final sample is named TiO2 solid nanospheres.

[0034] Step 2: Add 10g of TiO2 solid nanospheres and 50g / L of anhydrous ethanol solution into a 1L three-necked flask, and then add 10g of deionized water. Stir vigorously at 30°C for 20 minutes, and then continue to stir and disperse for 5 minutes under ultrasonic treatment. Then, slowly add silane coupling agent KH550, and raise the temperature to 40°C to react for 5 hours. Subsequently, centrifuge using a high-speed centrifuge and wash alternately with deionized water and ethanol three times. Finally, place the sample in a vacuum drying oven for drying and name it silane-modified titanium dioxide suspension.

[0035] Step 3: Add 2.5g of silane-modified titanium dioxide suspension and 10g of deionized water into a 1L three-necked flask, stir ultrasonically at 30°C for 5 minutes, and then stir vigorously for 1 hour. Next, add 40g of sodium alginate solution to the above solution and continue to stir vigorously at 30°C for 1 hour. Subsequently, add 0.05mol / L CoCl2·6H2O aqueous solution to the solution and continue to react for 1 hour. Finally, centrifuge using a high-speed centrifuge, wash with water and ethanol, and dry to obtain the product silicon-cobalt modified titanium dioxide solid nanosphere hybrid material.

[0036] The scanning electron microscope, transmission electron microscope and EDS spectrum of the final product silicon-cobalt modified titanium dioxide solid nanosphere hybrid material prepared in Example 1 are respectively shown in the appendix of the specification. Figure 1 , Figure 2 and Figure 3 The results show that silane coupling agent, sodium alginate and cobalt ions are successfully adsorbed on the surface of titanium dioxide nanospheres through amino modification and self-assembly technology. In addition, the XPS spectrum of TiO2 nanospheres ( Figure 4 ) and the XPS spectrum of the hybrid material of silicon-cobalt modified titanium dioxide solid nanospheres ( Figure 5 ) and infrared spectrum comparison ( Figure 6), it can be observed that compared with the XPS spectrum of TiO2 solid nanospheres, the silicon-cobalt modified titanium dioxide solid nanosphere hybrid material has three new energy peaks, located at 690eV (Co2p), 400eV (N1s) and 100eV (Si2p), corresponding to the elements Co, N and Si, further verifying the success of silicon-cobalt modified nano titanium dioxide.

[0037] 2. Preparation of RPUF / SiCoModified Titanium Dioxide Solid Nanosphere Hybrid Material Nanocomposites

[0038] Rigid polyurethane foam (RPUF) and its modified products were prepared by a one-step foaming method. The specific formula is shown in Table 1. First, 50 grams of polyol LY4110 were added to a 250 ml disposable paper cup, followed by 0.5 grams of amine catalyst A33, 0.25 grams of organotin catalyst LC, 0.75 grams of deionized water, 1 gram of silicone oil, 1.5 grams of triethanolamine and 13.95 grams of ammonium polyphosphate (APP), and the mixture was stirred evenly with a stirrer to obtain a white material. At the same time, a certain amount of catalyst detoxification agent was added to 71.8 grams of polymethylene polyphenyl polyisocyanate (PAPI) and dispersed by mechanical stirring for 30 minutes. Subsequently, the dispersion was poured into the white material, and after vigorous stirring for about 10 seconds, the mixture was quickly poured into a pre-prepared stainless steel mold (size 250×150×60mm3), and after high temperature aging for 12 hours, the RPUF / silicon cobalt modified titanium dioxide solid nanosphere hybrid material nanocomposite material was finally obtained.

[0039] Table 1 Formula components of rigid polyurethane foam and its composite materials

[0040]

[0041]

[0042] The RPUF / silicon-cobalt modified titanium dioxide solid nanosphere hybrid material nanocomposite prepared in Example 1 exhibits good fire safety performance. From the total heat release (THR) curve, it can be seen that within the same burning time, the total heat release of pure RPUF is the highest, followed by RPUF / APP, RPUF / APP / TiO2 and RPUF / APP / silicon-cobalt modified titanium dioxide solid nanosphere hybrid material. This result shows that the synergistic effect of the RPUF / silicon-cobalt modified titanium dioxide solid nanosphere hybrid material nanocomposite effectively improves the fire safety performance of the rigid polyurethane foam material.

[0043] Example 2

[0044] The preparation of the smoke suppressant and toxicity reducing agent of the silicon-cobalt modified titanium dioxide solid nanosphere hybrid material and the application of the flame retardant rigid polyurethane foam composite material in this embodiment are carried out according to the following steps:

[0045] 1. A method for preparing a silicon-cobalt modified nano titanium dioxide smoke suppressant and toxicity reducing agent is as follows:

[0046] Step 1: In a 1L three-necked flask, add 700mL of ethanol and 300mL of acetonitrile and set the temperature to 25°C. Add 1.68mL of NH3·H2O (28wt%) and 3.64mL of deionized water and stir evenly. Quickly add isopropyl titanate and stir vigorously for 6 hours. After centrifugation and alternating washing, TiO2 solid nanospheres are obtained. Step 2: Mix 10g of TiO2 solid nanospheres with 50g / L anhydrous ethanol and 10g of deionized water, stir vigorously at 30°C for 20 minutes, and then ultrasonically treat for 5 minutes. Slowly add silane coupling agent KH550, heat to 40°C and react for 5 hours. After centrifugation and alternating washing, vacuum dry to obtain a silane-modified titanium dioxide suspension. Step 3: Mix 2.5g of silane-modified titanium dioxide suspension with 10g of deionized water, ultrasonically stir at 30°C for 5 minutes, and stir vigorously for 1 hour. Add 10g of sodium alginate solution and continue stirring for 1 hour. 0.15 mol / L Co(NO3)2·6H2O was added dropwise and reacted for 1 hour. After centrifugation, washing with water and ethanol, and drying, a silicon-cobalt modified titanium dioxide solid nanosphere hybrid material was obtained.

[0047] 2. Preparation of RPUF / SiCoModified Titanium Dioxide Solid Nanosphere Hybrid Material Nanocomposites

[0048] As shown in Table 2, rigid polyurethane foam (RPUF) and its modified products were prepared by a one-step foaming method. First, 50 g of polyol LY4110, 0.5 g of amine catalyst A33, 0.25 g of organotin catalyst LC, 0.75 g of deionized water, 1 g of silicone oil and 1.5 g of triethanolamine were added to a 250 ml paper cup and stirred evenly to obtain a white material. Then, the catalyst detoxifier was added to 71.8 g of polymethylene polyphenyl polyisocyanate (PAPI) and dispersed by mechanical stirring for 30 minutes. Pour the dispersion into the white material, stir vigorously for 10 seconds, and then pour into a 250×150×60mm 3 The mixture was placed in a stainless steel mold and aged at high temperature for 12 hours to finally obtain the RPUF / silicon-cobalt modified titanium dioxide solid nanosphere hybrid material nanocomposite material.

[0049] Table 2 Formula components of rigid polyurethane foam and its composite materials

[0050]

[0051] In this Example 2, flame-retardant rigid polyurethane foams of silicon-cobalt modified titanium dioxide solid nanosphere hybrid materials with different contents were prepared. The results showed that the addition of silicon-cobalt modified titanium dioxide solid nanosphere hybrid materials significantly improved the flame retardant properties of the foam, among which RPUF-5 performed best. Cone calorimeter test showed that the peak heat release rate of RPUF-5 was 32% lower than that of the pure sample, and the total smoke release was 60% lower, while showing excellent catalytic carbonization effect. It can be seen that the silicon-cobalt modified nano titanium dioxide smoke suppressant and toxic reduction agent prepared by amino modification and self-assembly technology has efficient flame retardant and smoke suppressant properties in the rigid polyurethane foam matrix. The rigid polyurethane foam is prepared from polyol (LY4110), tin catalyst (A33), amine catalyst (LC), deionized water, silicone oil (Si-Oil), triethanolamine (TEA) and polymethylene polyphenyl polyisocyanate (PM-200).

[0052] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a nano titanium dioxide smoke suppressant and toxicity reducing agent, characterized in that: The steps include: a. Preparation of titanium dioxide nanospheres: Add ammonia water and isopropyl titanate to a mixed solution of ethanol and acetonitrile, and keep stirring vigorously and mechanically for 6-10 hours, and obtain titanium dioxide nanospheres after washing, drying and grinding; b. Preparation of silane-modified titanium dioxide suspension: Add the titanium dioxide nanospheres obtained in step a to a mixed solvent of ethanol and deionized water, slowly dropwise add silane coupling agent KH550 under ultrasonic stirring, and react at 40-50° C. for 5-8 hours; c. Preparation of silicon-cobalt modified titanium dioxide solid nanosphere hybrid material: disperse the product of step b in deionized water, ultrasonically stir for 5-30 minutes, add 0.3wt%-1wt% sodium alginate to the above solution, stir vigorously for 1-3 hours, then add water-soluble cobalt salt, react for 1-3 hours, continue stirring for 2-3 hours, then centrifuge the suspension, wash with water, and dry to obtain nano titanium dioxide smoke suppressant and toxic reduction agent.

2. The method for preparing the nano titanium dioxide smoke suppressant and toxicity reducing agent according to claim 1, characterized in that: The water-soluble cobalt salt is one of cobalt chloride, cobalt sulfate, cobalt bromide and cobalt nitrate.

3. The method for preparing the nano titanium dioxide smoke suppressant and toxicity reducing agent according to claim 1, characterized in that: The particle size of the titanium dioxide solid nanospheres is 100-300nm.

4. The method for preparing the nano titanium dioxide smoke suppressant and toxicity reducing agent according to claim 1, characterized in that: In step a, 600 mL of ethanol solution and 400 mL of acetonitrile solution are added to a 1L three-necked flask, and the reaction temperature is 25°C; 1.68 mL of NH3·H2O28wt% and 3.64 mL of deionized water are added, and mechanical stirring is performed; then 20 mL of isopropyl titanate is quickly poured therein, The reaction was continued for 6 hours with vigorous stirring, and the mixture was washed three times with deionized water and ethanol alternately, and finally dried in a vacuum oven for 8 hours at a temperature of 80°C, and the solid particles were ground to obtain titanium dioxide nanospheres.

5. The method for preparing the nano titanium dioxide smoke suppressant and toxicity reducing agent according to claim 1, characterized in that: In the step b, the mixed solvent of ethanol and deionized water is prepared by mixing 50 g of anhydrous ethanol and 10 g of deionized water.

6. The method for preparing the nano titanium dioxide smoke suppressant and toxicity reducing agent according to claim 1, characterized in that: In the step c, 2.5 g of the product obtained after the reaction in step b is dispersed in 10 g of deionized water, and after ultrasonic stirring for 5-30 min, 0.3 wt%-1 wt% of sodium alginate is added to the solution, and the solution is vigorously stirred for 1-3 h, followed by adding 4 g of 0.05 mol / L CoCl2·6H2O solution, reacting for 1-3 h, stirring for 2-3 h, and then centrifuging at 6500 rpm in a high-speed centrifuge, washing three times with deionized water and ethanol alternately, and finally drying in a vacuum drying oven for 8 h, setting the temperature to 80°C, and grinding the solid particles to obtain a nano titanium dioxide smoke suppressant and toxic reduction agent.

7. A modified rigid polyurethane foam composite material, characterized in that: The nano titanium dioxide smoke suppressant and toxicity reducing agent is modified by the nano titanium dioxide smoke suppressant and toxicity reducing agent prepared by the method for preparing the nano titanium dioxide smoke suppressant and toxicity reducing agent according to any one of claims 1 to 3.

8. The modified rigid polyurethane foam composite material according to claim 7, characterized in that: The added amount of the smoke suppressant and toxicity reducing agent is 0.5%-4% of the mass of the rigid polyurethane foam matrix.

9. The modified rigid polyurethane foam composite material according to claim 7, characterized in that: The rigid polyurethane foam is prepared from polyol, tin catalyst, amine catalyst, deionized water, silicone oil, triethanolamine and polymethylene polyphenyl polyisocyanate.

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