1. external emulsification method
This method is the earliest method to prepare waterborne polyurethane, and external emulsification is a forced emulsification method in the presence of emulsifier and high shear force. It was first invented by Pschlack, and W.yandott of DuPont synthesized PU emulsion in 1953 by this method. The synthesis process is as follows: firstly, polyether polyol and organic isocyanate are synthesized into PU prepolymer, and then the chain is extended with small molecular diol or diamine to obtain PU organic solution; Then, under strong stirring, an appropriate amount of emulsifier aqueous solution is gradually added to form a crude emulsion, and finally, it is sent to a homogenizer to form an emulsion with a suitable particle size. However, due to the disadvantages of long reaction time, large amount of emulsifier, coarse emulsion particles, poor storage and poor physical and mechanical properties of adhesive layer, this method is basically not used in production at present. Later, a method called low temperature sealing method was developed to prepare PU emulsion, which can reduce the amount of emulsifier and prepare emulsion with good stability. In this method, the end -NCO prepolymer is capped with oxime, lactam, NaHSO3, acetoacetate and other capping agents, and then dispersed in an aqueous solution containing emulsifier with polyamine to form a stable PU emulsion.
2. Self-emulsification method
The preparation of stable polyurethane emulsion is mainly through self-emulsification, and the key is to introduce hydrophilic groups into the molecular skeleton of polyurethane. Hydrophilic groups enter the PU molecular skeleton through the chain extension of hydrophilic monomers, which are composed of salt-forming groups and salt-forming reagents. According to the types of hydrophilic groups, the waterborne polyurethane emulsion prepared by this method can be divided into four types: anionic, cationic, amphoteric and nonionic, of which anionic is dominant. There are many preparation processes of self-emulsifying polyurethane emulsion, and the preparation methods are mainly divided into acetone method, prepolymer dispersion method, hot melt method and ketimine/ketazine method. The same feature is that PU prepolymer with moderate molecular weight, NCO end-capped or blocked NCO is prepared first, and the difference is mainly in the chain extension process. At present, the main methods in industrial production are acetone method and prepolymer dispersion method (or prepolymer mixing method). The synthesis process is as follows.
2. 1 acetone method
Acetone method was successfully studied by Ddieterich of Bayer company in Germany. Firstly, polyether or polyester diol and isocyanate were used to prepare prepolymer, then proper amount of acetone was added to reduce viscosity, then N- methyldiethanolamine was used to extend chain, then acetone was added to reduce viscosity, then ionization reagent was added, and the mixture was stirred and ionized. Dispersing the ionized polyurethane in a medium containing 80% acetone and 20% water, and finally evaporating the acetone to obtain a polyurethane emulsion with a particle size of 0.03 ~100μ m. Preparing a high-viscosity prepolymer containing NCO end groups by acetone method, then adding acetone to reduce the viscosity, chain extending with hydrophilic monomers, adding into water with high-speed stirring, vigorously shearing and dispersing in water, emulsifying, and distilling under reduced pressure to recover the solvent.
Anhui University Qi et al [2] prepared WSPU by acetone method. WSPU is a special-shaped polyurethane and a new functional material. It has a series of advantages such as large deformation, easy processing, adjustable phase transition degree, biodegradability and good biocompatibility. The preparation process is as follows: Polycaprolactone diol (PCL) and 2,4-toluene diisocyanate (TDI) are added into a four-necked bottle in turn, and react at 80℃ for 3 hours under the conditions of stirring and nitrogen introduction, then a small amount of acetone solvent is added, and after stirring for 65438+/-00 minutes, the catalyst, dimethylolpropionic acid (DMPA) and crosslinking agent trimethylolpropane (TMP) are added. Triethylamine was added to the polyurethane prepolymer under rapid stirring for 5 minutes, and then the calculated amount of water was added for emulsification. After 65438 00 minutes, acetone was removed under reduced pressure to prepare 30% WSPU emulsion with 30% solid content. Sample testing of WSPU processing. Finally, when the molecular weight of PCL is 5000, the emulsion performance is stable and the shape memory recovery rate reaches 95%.
Zhang Faxing and Wei Xiaoli of Sichuan University of Science and Technology [3] first synthesized the hydrophilic chain extender DHPA, and then prepared the sulfonic acid WPU microemulsion. The synthesis process is as follows: adding a certain amount of Ng2 10 and IPDI into a four-necked flask equipped with a condensing reflux tube, an electric stirrer and a thermometer, adding a proper amount of dibutyltin dilaurate as a catalyst, heating to the required temperature for a certain time, and measuring the NCO- content in the prepolymer by di-n-butylamine titration (if the theoretical value is reached, stop the prepolymerization, if not, continue the reaction until the theoretical value is reached). Then cooling to a suitable temperature, adding a certain amount of DHPA dissolved in N- methyl -2- pyrrolidone, reacting for a certain time, adding a small amount of acetone to dilute, neutralizing with triethylamine, adding deionized water to disperse under rapid stirring, and finally distilling under reduced pressure to remove acetone to obtain stable sulfonic acid WPU microemulsion. Compared with conventional carboxylic acid WPU microemulsion, sulfonic acid WPU microemulsion has higher solid content and lower surface tension, and has better stability at low temperature, high temperature and room temperature.
Wang Cui, Wu Youshi and Wu Lili of Shandong University prepared waterborne polyurethane emulsion by acetone method. The synthesis process is as follows: in a 500ml four-necked flask equipped with an electric stirrer, reflux condenser, thermometer and nitrogen inlet and outlet, add 1 10℃ vacuum dehydrated polyester diol, add metered MDI acetone solution at 60℃ and react for 10 ~ 20min, then add DMF solution of DMPA and stir for 5 ~/kloc. Drop catalyst, keep the temperature for 50 ~ 90 minutes, and when the content of -—NCO reaches the theoretical value (determined by n-butylamine titration), add TEA to form salt. When the isocyanate content in the system is less than 0.2%, the reaction is finished, and the temperature is reduced to below 30℃. Then, a large amount of water is quickly added to the system and stirred at high speed for 65438+/-0h. If you want to extend the chain again, first add ethylenediamine and then add water. Finally, the low-boiling solvent (acetone) was removed by vacuum distillation to obtain the finished waterborne polyurethane.
The preparation of waterborne polyurethane by acetone method has the advantages of easy control of reaction, good repeatability, easy control of emulsion particle size and high emulsion quality. It is one of the most widely used methods at present, especially when the molecular weight of PU is large, it consumes a lot of organic solvents, which is difficult to recover, complicated in process and high in cost. It's dangerous.
.2 Prepolymer dispersion method
This method has been developed in recent years. Firstly, hydrophilic monomers were introduced into the polymer, and PU prepolymer containing ionic bonds was prepared by ionization, and then dispersed in water to form prepolymer emulsion. Finally, PU emulsion was prepared by chain extension with diamine in water phase. The process is simple, and the branched PU emulsion can be prepared without a lot of organic solvents, but it is only limited to the special end -NCO prepolymer with low activity aliphatic isocyanate as the main component.
A patent of Bayer Company in Germany reported that the adduct of propoxylated 2- ene-1, 4- butanediol and sodium bisulfite (Mw30 1) 15.2g was added to 429 g polyethylene glycol (MW 2 143) at 80℃.
Wu Xionghu, Yang and Ding Shaolan of Shaanxi University of Science and Technology [5] used isophorone diisocyanate (IPDI), polypropylene oxide diol (PPG), polybutylene adipate polyol (PTAd), tetrahydrofuran polyether polyol (PTMG) and dimethylol propionic acid (DMPA) as raw materials to synthesize waterborne polyurethane leather brightener by prepolymer dispersion method. The synthesis process is as follows: in a four-neck flask equipped with an electric stirrer, reflux condenser, thermometer and nitrogen device, oligomer polyol, IPDI and a small amount of catalyst are added to react at a certain temperature for about 2 hours under the protection of nitrogen, until the NCO content is close to the theoretical value, DMPA and TMP are added to continue the reaction for about 2 hours until the NCO content reaches the theoretical value, so as to obtain a prepolymer, then the temperature is lowered to 50℃, and measured TEA and appropriate amount of acetone are added to fully react. Finally, acetone is evaporated under reduced pressure.
Du Hui, Zhao Yuhua and Wang Junwei of Chinese Academy of Sciences prepared a series of polycarbonate diol (PCDL) waterborne polyurethane (WPU) adhesives by prepolymer dispersion method. The synthesis method is as follows: PCDL and polyester diol containing sulfonic acid groups are added into a four-necked flask equipped with a mechanical stirrer, a thermometer and a reflux condenser, and vacuum dehydrated at 100 ~ 120℃ until the water content is lower than 0.5%; Then adding metered isocyanate and solvent acetone at 50-60℃, keeping the temperature for a period of time, and then adding 1, 2- dimethylolpropionic acid (DMPA) and 1, 4- butanediol (BDO) to continue the reaction; When the content of -NCO in the reaction solution is basically consistent with the design value, TEA is added to neutralize carboxyl groups, then distilled water is added for emulsification and dispersion, and ethylenediamine is added for further chain extension; Finally, acetone was removed under reduced pressure to obtain PCDL·WPU emulsion adhesive.
Water-borne polyurethane adhesive for shoes was prepared by prepolymer dispersion method of Cheng Feng, Xiang Ling and Yu Jiankun of Sichuan University with dimethylolpropionic acid (DMPA) and sucrose as hydrophilic chains and crosslinking agents. The synthesis process of WBPU is as follows: dehydrated polyethylene glycol (PEG- 1000) and N- methyl -2- pyrrolidone solution of dimethylolpropionic acid (DMPA/NMP,11W/W) are added to four reactors equipped with stirrer, reflux condenser tube, mercury thermometer and nitrogen inlet and outlet. Then add isophorone diisocyanate (IPDI) and catalyst -M, under the protection of nitrogen, mix the system evenly and react for 0.5h, then raise the temperature to 80℃ and stir evenly for 2h, then lower the temperature to 60℃, then gradually add 1, 4- butanediol and sucrose, and after reacting for 1h, add the measured γ-aminopropyltriethoxy. Polyurethane prepolymer cooled to 255℃ was added to triethanolamine (TELA) aqueous solution for shear emulsification, and the whole emulsification process was carried out in ice water bath. After stirring evenly, triethylamine was added for neutralization to form salt, and the shear emulsification reaction lasted for 40 minutes. Finally, the solvent was removed by vacuum distillation, and the waterborne polyurethane emulsion with solid content of about 50% was obtained.
To sum up, the process of preparing waterborne polyurethane by prepolymer mixing method is simpler than that by acetone method, and no organic solvent is needed. The cost is reduced, but the product quality is not as good as that of acetone method, and it is only suitable for the synthesis of aliphatic waterborne polyurethane.
2.3 melt dispersion method
This is a solvent-free method for preparing waterborne polyurethane. Melt dispersion method [8] refers to the close combination of addition polymerization of isocyanate and polycondensation of amino groups. Firstly, isocyanate-terminated prepolymer containing hydrophilic groups was synthesized, and then reacted with excessive urea at high temperature (130℃) to generate biuret, and then reacted with hydroxymethylation in formaldehyde aqueous solution to obtain high molecular weight polyurethane. This method consumes a lot of energy.
2.4 Ketimine and Keto Diazo Method
Ketimine and ketodiazotization method [9] refers to mixing prepolymer with ketone-protected diamine (ketoimine system) or hydrazine (ketodiazotization system), and then dispersing with water. In the process of dispersion, ketimine and ketazine are hydrolyzed at a practical rate, releasing free diamine or hydrazine to react with dispersed polymer particles, and the obtained waterborne polyurethane-urea has good properties. The coating film prepared by this method is better.
3. Conclusion
In addition, the synthesis methods of polyurethane emulsion include direct mixing with water and solid spontaneous dispersion. Each of the above methods has its advantages and disadvantages. Comparatively speaking, the acetone method is more mature, and the prepolymer dispersion method has a better prospect because of its simple synthesis process. The development of waterborne PU is changing with each passing day, and the general development trend is to develop in the direction of high performance and low cost. Major foreign companies are vigorously developing the variety, quantity and performance of PU emulsion products. The research and development level of domestic PU emulsion is relatively low, which is mainly limited by the weak domestic chemical foundation. Close to foreign countries in the production of basic raw materials and product development, vigorously develop new varieties, and improve the production capacity and synthesis technology level of domestic PU emulsion.