Waterborne polyurethane is a new type of polyurethane system which uses water instead of organic solvent as dispersion medium. Waterborne polyurethane takes water as solvent, no pollution, safe and reliable, good mechanical properties, good compatibility, easy to modify and so on.
The use of certain toxic solvents in the synthesis of polymer materials can ensure its recycling and reduce its residual rate in the product, which is also the research content of polymer green synthesis. The synthesis of waterborne polyurethane resins is a typical example of this. Waterborne polyurethane resin is a uniform emulsion formed by dispersing polyurethane in water. It has the advantages of non-combustibility, small smell, non-pollution, energy-saving, convenient operation and processing, etc. It is widely used as adhesive and coating. Compared with solvent - based polyurethane adhesives, waterborne polyurethane has the following characteristics:
(1) most waterborne polyurethane resins do not contain reactive NCO groups, so the resins are cured mainly by the cohesion and adhesion generated by polar groups within the molecule. Carboxyl groups and hydroxyl groups in waterborne polyurethane can participate in the reaction under appropriate conditions to make the adhesive produce cross-linking.
(2) viscosity is an important parameter of adhesive performance. The viscosity of waterborne polyurethane resin is generally adjusted by water-soluble thickener and water.
(3) as water is less volatile than organic solvents, water-based polyurethane adhesives dry more slowly and have poor water resistance.
(4) waterborne polyurethane resin can be mixed with a variety of waterborne resins to improve performance or reduce cost. At this point, attention should be paid to the electrical properties of water - borne resin and acid - base, otherwise it may cause the condensation of water - borne polyurethane resin.
(5) waterborne polyurethane resin smell small, easy to operate, residue easy to clean.
III. Production process
There are various classifications according to the preparation method. Examples are as follows:
(1) self-emulsification method and external emulsification method
Self-emulsifying method, also known as internal emulsifying method, refers to the method of forming stable emulsion without emulsifying agent because the polyurethane chain segment contains hydrophilic components. External emulsification method is also known as forced emulsification method, if the molecular chain only contains a small amount of hydrophilic chain segment or group not enough to self-emulsify, or completely without hydrophilic components, at this point must add emulsifier, in order to get the emulsion.
In comparison, in the emulsion prepared by external emulsification, the residue of hydrophilic small molecule emulsifing agent affects the properties of polyurethane film after curing, while the self-emulsification method eliminates this defect. Ionic self-emulsification is the main method for the preparation of waterborne polyurethane.
(2) prepolymer method, acetone method, melt dispersion method
The most common methods for self-emulsification of waterborne polyurethane are prepolymer dispersion and acetone. The prepolymer method is to conduct hydrophilic components in the prepolymer, obtain the prepolymer with a certain viscosity range, emulsify in water and conduct chain growth at the same time to prepare the stable waterborne polyurethane (waterborne polyurethane-urea).
Acetone method belongs to the solution method, which is to dilute or dissolve polyurethane (or prepolymer) with organic solvent and then emulsify. In the presence of solvent, the prepolymer and hydrophilic chain extender expand the chain reaction to produce a higher molecular weight polyurethane, the reaction process can be required to add a solvent to reduce the viscosity of polyurethane solution, so that it is easy to stir, then add water for dispersion, forming emulsion, and finally steam the solvent. Solvent with acetone, methyl ethyl ketone in the majority, so it is called acetone method.
Acetone method has the advantages of low boiling point acetone, methyl-ethyl ketone, water capacity, easy to recycle, the whole system is uniform, easy to operate, because of reducing the viscosity but also reduce the concentration, which is conducive to the preparation of high molecular weight prepolymer or polyurethane resin before emulsification, the film performance of the emulsion is better than the simple prepolymer method. However, due to the limitation of viscosity, in order to facilitate shear dispersion, the molecular weight of prepolymer is not too high, which may affect the properties of waterborne polyurethane. The small molecular weight of the prepolymer results in high content of NCO group, many urea bonds formed after emulsification, and the film is hard without softness.
The main difference between the acetone method and the prepolymer method is that in the acetone method, polyurethane is prepolymerized into the prepolymer with larger molecular weight. In the prepolymer method, solvent such as acetone can be added or not. The concepts of these two methods overlap. Some emulsification methods belong to both acetone method and prepolymer method.
Melt dispersion method is also known as melt dispersion method, prepolymer dispersion formaldehyde chain extension method. The prepolymer of NCO group containing tertiary amine group (or ionic group) was synthesized in advance, and then reacted with urea (or ammonia) in the body system to form the oligomer of urethane diureide (or urea group containing ionic group). The polyurethane biuret has enough hydrophilicity, and the dilute water solution with acid forms homogeneous solution, and then reacts with the formaldehyde water solution for hydroxymethylation. The polyurethane biuret containing hydroxymethyl can be diluted with infinite water at 50-130℃ to form stable emulsion. When the pu value of the system is reduced, polycondensation can be carried out in the dispersion phase to form high molecular weight polyurethane. After the NCO
prepolymer containing the ionic group forms the urea-terminated or diurea-based polyurethane oligomer, it is emulsified in water directly in the molten state, and then the formaldehyde solution is added for hydroxylmethylation and chain extension.
(3) direct chain extension of binary amine and ketoimine - keto nitrogen linkage method
In the prepolymer dispersion method, if the use of water soluble binary primary amine chain extender chain extension, due to -NCO and -NH2
reaction rate is fast, it is not easy to get a fine and uniform emulsion, can use ketoimine or keto nitrogen method to solve this problem. Ketimine - ketone even nitrogen method is to protect the pre polymers and ketone of diamine (ketone imide system) or hydrazine (after mixing ketone even nitrogen system), and water dispersion, dispersion process, ketimine, even nitrogen at a certain rate of ketone, hydrolysis, release free diamine or reaction, and dispersed polymer particles get a water-based polyurethane - urea with good performance.