Abstract : The synthetic method, bonding principle and development direction of waterborne polyurethane adhesives are introduced.
Keywords :waterborne polyurethane;synthesis method;bonding principle;review
Waterborne polyurethanes were first successfully prepared by West German P. Schlack in 1943. Polyurethane emulsions were first industrialized in 1967 and were introduced in the US market. In 1972 Bayer took the lead in using polyurethane emulsions as leather finishing agents, and waterborne polyurethanes became important. commodity. Although the history of waterborne polyurethane is not long, its development is very rapid due to its advantages of being non-toxic, non-flammable, non-polluting, energy-saving, safe and scratch-resistant. According to reports [1], between 1992 and 1997, the average annual growth rate of waterborne polyurethanes was 8%, and a series of industrialized products emerged. They have been successfully applied to leather finishing, paper coating, steel rust prevention, fiber processing, and plastics. And wood coating, glass coating and other fields, but the country is still in the initial stage of development, application level is not high, there are not many manufacturers, and a single species, the scale is not.
1 Synthesis of Waterborne Polyurethane Adhesives
If the polyurethane molecular chain contains a small amount of hydrophilic segments or groups that are insufficient for self-emulsification, or does not introduce hydrophilic components at all, an emulsifier is added to obtain the emulsion. This method is called external emulsification [2] . In the emulsion prepared by this method, the emulsifier is left and the dispersion is unstable.
The method of introducing a hydrophilic component into a polyurethane segment and directly dispersing into water without an emulsifier is called an internal emulsification method [3～4]. According to the type of hydrophilic groups introduced into the molecular structure, self-emulsifying aqueous polyurethanes can be classified into cationic, anionic, amphoteric and nonionic types.
Cationic polyurethanes are chain extenders using N-alkyl diols in the prepolymers, tertiary amino groups introduced, and then quaternized or neutralized with acids to achieve self-emulsification [5-6]. The anion type uses 2,2'-dimethylolpropionic acid (DMPA) [7~8], diaminoalkyl iodide, tartaric acid [9] as the chain extender, and introduces carboxyl or iodic acid groups. Then neutralize and emulsify with triethylamine, ammonia, and the like. Non-ionic self-emulsifying polyurethanes can be obtained by introducing nonionic groups such as hydroxyl groups, ether groups, and hydroxymethyl groups on the polyurethane backbone [10], especially polyoxyethylene segments.
The self-emulsification method can be further divided into the prepolymer method [11-12], the solution method [13-14], the melt dispersion method [15], the ketimine-ketazine method and the protecting end group method [16-17]
1. 1 Prepolymer method
The NCO-containing prepolymers are emulsified directly in water with or without the addition of a solvent and chain extension is carried out to prepare a stable aqueous polyurethane (aqueous polyurethane-urea). This method is suitable for the low activity of fats and cycloaliphatic isocyanates due to viscosity limitations. The advantage of this method is that it does not require the recovery of large amounts of solvents.
The use of traditional prepolymers to prepare one-component waterborne polyurethanes often causes isocyanate groups to react with water, resulting in less diamine chain extension and affecting the solvent resistance of aqueous polyurethanes. Lienert's core-shell polymerization process for the preparation of waterborne polyurethane [15] overcomes the above problems. He prepared two kinds of prepolymers, and the two kinds of prepolymers had different hydrophilic groups. The two prepolymers are then dispersed in water and the hydrophilic prepolymer forms shells that enclose the less hydrophilic prepolymers. By protecting the isocyanate groups on the core by wrapping the core, diamine chain extension proceeds smoothly.
1. 2 solution method
In preparing the polyurethane prepolymer, an inert solvent (usually acetone) having a lower boiling point and being miscible with water is added to lower the viscosity to obtain a high molecular weight polyurethane solution, and finally the solvent is removed under reduced pressure. The method is simple and reproducible, but solvent recovery is difficult. Suitable for higher active aromatic isocyanates.
1. 3 melt dispersion method
The polyurethane or urea is reacted with the ionomer-terminated isocyanate prepolymer to form a terminal urea-based or urea-terminated polyurethane prepolymer. Under a certain external shear force, the prepolymer is dispersed in water and then the aldehyde is added. Derivatives react under certain conditions to produce macromolecular polyurethane emulsions.
1.4 ketimine-ketazine
Prior to emulsification, potential amines are added to the polyurethane ionomer prepolymer. During emulsification, ketimine and ketazine are hydrolyzed to form diamines or hydrazines, and chain extension occurs via amino groups. The process requires strong dispersion in the presence of a co-solvent.
1.5 Protected End Group Method
Before the polyurethane is emulsified, the end-NCO groups of the prepolymer are protected with a specific blocking agent such as phenol to make a closed polyurethane prepolymer, which loses its activity, and then a chain extender and a cross-linking agent are added. Dispersed in water to make an emulsion, heat deblocking when applied.