1. Raw materials for preparing waterborne polyurethane

1. Oligomeric polyols polyether diol and polyester diol are usually used to prepare waterborne polyurethane adhesives, and sometimes a small amount of oligomeric polyols, such as polyether triol, low-branched polyester polyol and polycarbonate diol, are also used. Polyether polyurethane has good flexibility and water resistance at low temperature, and the price of polypropylene glycol (PPG) is lower than that of polyester glycol. Therefore, polypropylene glycol is the main raw material of oligomer polyol in the research and development of waterborne polyurethane in China. Polyurethane prepared from polytetrahydrofuran ether glycol has good mechanical strength and hydrolysis resistance, but its high price limits its wide application. Polyester polyurethane has high strength and good adhesion, but the hydrolysis resistance of polyester itself is worse than that of polyether, so the storage stability of polyester waterborne polyurethane prepared from general raw materials is short.

2. Diisocyanates commonly used for preparing polyurethane emulsions include aromatic diisocyanates such as TDI and MDI, and aliphatic and cycloaliphatic diisocyanates such as IPDI, HDI and H 12MDI. Polyurethane made of aliphatic or cycloaliphatic diisocyanate has better hydrolysis resistance than polyurethane made of aromatic diisocyanate, so the storage stability of waterborne polyurethane products is good. Foreign high-quality polyester waterborne polyurethane is generally made of aliphatic or cycloaliphatic isocyanate, but in China, due to the variety and price of raw materials, TDI is mostly used as diisocyanate. Polymethylene polyphenyl polyisocyanate is generally used to prepare vinyl polyurethane emulsion and isocyanate emulsion.

3. Chain extenders Chain extenders are often used to prepare waterborne polyurethane. There are many hydrophilic chain extenders that can introduce ionic groups. In addition to special chain extenders, 1, 4- butanediol, ethylene glycol, diethylene glycol, hexanediol, ethylenediamine, diethylenetriamine and other chain extenders are often used. Because the reactivity of amine with isocyanate is higher than that of water, diamine chain extender can be mixed into water or made into ketoimine, and chain extension can be carried out while emulsion is dispersed.

4. Water is the main medium of waterborne polyurethane adhesive. In order to prevent impurities such as Ca2+ and Mg2+ in tap water from affecting the stability of anionic waterborne polyurethane, distilled water or deionized water is usually used to prepare waterborne polyurethane. Water is not only used as a solvent or dispersion medium for polyurethane, but also an important raw material for reaction. At present, prepolymer method is the main method to synthesize waterborne polyurethane. When polyurethane prepolymer is dispersed with water, water also participates in chain extension. In fact, most waterborne polyurethanes are polyurethane-urea emulsions (dispersions) due to the chain extension of water or diamine. Polyurethane-urea has greater cohesion and adhesion than pure polyurethane, and urea bond has better water resistance than urethane bond.

5. Hydrophilic chain extenders Hydrophilic chain extenders are chain extenders that can introduce hydrophilic groups. This chain extender is a special raw material for preparing waterborne polyurethane. Such chain extenders often contain carboxyl, sulfonic or secondary amine groups. When it is combined with polyurethane molecules, polyurethane segments have ionizable functional groups.

(1) Carboxylic acid chain extender dimethylolpropionic acid is abbreviated as DMPA, and its full name is 2,2-dimethylolpropionic acid, also known as α, α-dimethylolpropionic acid. It is a hydrophilic chain extender commonly used in polyurethane emulsion at home and abroad. As early as 1960s and 1970s, Germany, the United States and other countries were used to prepare polyurethane emulsion. In the early 1990s, a factory in Chengdu, China was able to produce in small batches. The chain extender is a white crystal with high melting point and stable storage. Because of its small molecular weight (Mw 134), a small amount can provide enough carboxyl groups. One preparation method of DMPA is to synthesize dimethylol propionaldehyde from formaldehyde and propionaldehyde, and then oxidize it into dimethylol propionic acid with hydrogen peroxide. Dihydroxy half ester is the product of the reaction of alcohol and dianhydride. Generally, the molar ratio of alcohol to dianhydride is 1: 1, and one carboxyl group of dianhydride is esterified and the other carboxyl group remains. When preparing half ester chain extender for polyurethane emulsion, the alcohol compounds used are generally small molecular triol or oligomer triol, such as glycerol and low molecular weight polyether triol. In this way, dihydroxy compounds containing carboxyl groups can be generated. The molecular weight of triol is usually between about 100 and 2000. Anhydrides that can be used to prepare half esters include maleic anhydride (maleic anhydride), phthalic anhydride (phthalic anhydride), succinic anhydride, glutaric anhydride, etc. Half esters are generally homemade.

2. The preparation method of waterborne polyurethane is as follows:

1. Solution method (also known as acetone method) is to dissolve isocyanate-terminated prepolymer in a low-boiling solvent that is miscible with water (acetone is the most commonly used method, so this method is also called acetone method), react with hydrophilic functional compounds to generate polyurethane ionomer, stir with water to realize phase transfer, and distill and recover acetone to obtain waterborne polyurethane emulsion. The process is as follows: this method is the most commonly used and important method at present, with simple steps and good reproducibility. Almost all linear polyurethane can be implanted into plasma by this method, and then dispersed in water phase to become aqueous dispersion. The particle size of its dispersion is 0.03~0.5μm, and the particle size changes widely, and it can be opaque or translucent or milky thermoplastic polyurethane emulsion.

2. Prepolymer dispersion method This process does not need a lot of solvents, which avoids the trouble of solvent recovery and also conforms to the trend of low VOC and VOC-free future environmental protection requirements. The first step of this method is to synthesize a terminal -NCO-based prepolymer, then implant ionic groups to make it an ionic oligomer, add water and stir vigorously, during which the polyurethane prepolymer forms an aqueous dispersion, and at the same time, the terminal -NCO-based chain extension reaction with water occurs. Adding diamine as chain extender can reduce the probability of reaction between -NCO and water, and finally produce polyurethane-urea aqueous dispersion. The process is as follows: this method is simpler than acetone method, without solvent recovery process and saving energy, but the product performance is slightly worse than acetone method. Generally, the prepolymer is dispersed by adding water under strong stirring. If the -NCO activity is low or a high shear mixing and dispersing device is used, the prepolymer can be dispersed by adding water. The particle size of the emulsion prepared by this method is 0. 1~0.5μm, and polyurethane emulsions with different crosslinking degrees can be prepared.

3. Prepolymer is prepared from polyester or polyether polyol, tertiary amine and isocyanate in molten state by melt dispersion method, and hydrophilic biuret ionomer is generated by capping with excessive urea, and then dispersed in formaldehyde aqueous solution to generate hydroxymethyl biuret polyurethane oligomer, which is diluted with water to obtain polyurethane biuret emulsion. In fact, it is a polycondensation reaction between dispersed phases at low pH value to achieve the purpose of chain extension and crosslinking. The method is easy to control, and does not need solvents or effective mixing devices. Dispersed colloidal particles with a particle size of 0.03~ 10μm can be prepared, and the dispersion is stable, which is suitable for large-scale industrial production and can be used to prepare polyurethane emulsion with medium crosslinking degree.

In addition, the preparation methods of waterborne polyurethane include ketimine method and ketazine method. In these two methods, ketimine (condensation product of ketone and diamine) and ketazine (condensation product of ketone and hydrazine) are used as potential chain extenders respectively. Under normal circumstances, they will not react with isocyanate, and once they encounter water, they will start hydrolysis reaction. The hydrolysis products are diamine and hydrazine except ketone, and the hydrolysis speed is much faster than isocyanate and water. The hydrolysis process is shown in the following formula: in this process, the hydrolysis reaction products are diamine and hydrazine. This method requires the use of solvent and dispersion device, and can prepare dispersion with particle size of 0.03 ~1μ m. Under the action of emulsifier, isocyanate-terminated prepolymer was forcibly dispersed by high shear stirring device, and polyurethane emulsion could also be prepared by using diamine as chain extender. In order to reduce the viscosity of prepolymer, some solvents can be added appropriately. The emulsion obtained by this method has large particle size, about 0.7~3μm, poor stability, easy phase separation and poor film-forming performance. The film needs to be heated above 100℃ to obtain a certain strength.