Classification of waterborne polyurethane adhesives, preparation method

Aqueous polyurethane adhesives can be classified into different categories according to different classification methods. According to their appearance and particle size, can be divided into 3 categories: transparent aqueous polyurethane solution, its particle size is less than 0.05um, viscosity depends on the molecular weight of the polymer; translucent polyurethane dispersion, particle size is between 0.02 Between um and 0.1 um, the viscosity is sometimes affected by the molecular weight; Polyurethane Cool Emulsion, particle size greater than 0.1 um, has a low viscosity and is independent of the molecular weight of the polyurethane. However, it is customary to refer to the latter two classes as polyurethane emulsifiers or dispersions. Actual applications are mostly polyurethane emulsifiers or dispersions.

According to the type of light base components can be divided into polyether type, polyester type and other types. Polycool has higher strength and hardness, better heat resistance and oxidation resistance, but polythermocycling products are easy to be hydrolyzed and not as cheap as polyether type: polyether type raw materials are readily available, inexpensive, and resistant Low temperature performance, good toughness and extensibility, but no night strength and hardness than poly-polyurethane products.

According to the cool type of isocyanate can be divided into aromatic and aliphatic. The commonly used aromatic isocyanates are such as toluene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI). However, such products have poor yellowing resistance, which limits their application. This is because the isocyanic group is directly linked to the benzene ring, and it is easy to form a wake-up structure and turn yellow. In recent years, there have been a number of varieties that are resistant to yellowing, such as xylene diphenyl diisocyanate (XDI) and hexadiisocyanuric acid (HDI). Using aliphatic diisocyanate as a cross-linker, the resulting polyurethane adhesive is also superior in hydrolysis resistance to aromatics. In order to improve the flame-retardant properties, the use of phosphorus and halogenated organic isocyanic acid is also preferred; silicone isocyanate is used to improve surface water resistance and heat resistance.

According to the number of components can be divided into single-component type and two-component type. Compared with two-component adhesives, one-component adhesives have the advantages of no need to be blended, can be used after being poured out from the container, and are easy to operate. The two-component adhesives have the properties of adjustable performance, high bonding strength, and bonding range. Wide and excellent physical properties.

The hydrophilic groups can be classified into anionic, cationic, nonionic and amphoteric based on the charge properties of the hydrophilic groups, wherein the anionic forms include sulfonic acid type and acid type. Cationic polyurethane emulsion with N-alkyl diol as chain extender, and anionic polyurethane cold emulsion with 2,2'-bis-methyl propionic acid (DMPA), diamino alkyl iodate, etc. for chain extension Agent, non-ionic polyurethane cool emulsion is introduced in the backbone of light, ether, methyl and other non-ionic groups. Ionic and non-ionic polyurethane dispersions have their own advantages and can be complementary.

The preparation of aqueous polyurethane cool can be divided into open emulsification method and external emulsification method. External emulsification method is also called forced emulsification method. That is, a small amount of hydrophilic segments or groups that are insufficient for self-emulsification are introduced into the molecular chain, or no hydrophilic component is introduced at all, and an emulsifier is added to obtain an emulsion. The emulsion is rough and unstable, and its performance is not good. Self-emulsification method, also known as internal emulsification method, refers to the introduction of hydrophilic components in the polyurethane chain, and the resulting polymer does not need an emulsifier to directly disperse in water and become aqueous polyurethane. The most studied at present is the ionic self-emulsification method. According to different synthetic processes, self-emulsification method can be divided into prepolymer solution method, melt dispersion method, ketimine-ketene nitrogen method and protective end group method.

1. Prepolymer method. The NCO-containing prepolymers are emulsified directly in water with or without the addition of a solvent, while chain extension is performed to prepare a stable aqueous polyurethane (water-soluble polyurethane). Due to the limitation of viscosity, this method is suitable for the low activity of fats and cycloaliphatic isocyanates. The molecular weight of prepolymers should not be too high. This will inevitably lead to a high content of NCO groups in the prepolymer and the resulting product after emulsification. With many keys, hard film, and lack of flexibility, the advantage of this method is that it does not require the recovery of large amounts of solvents.

2. Solution method. In a low boiling, inert solvent (usually acetone) that is miscible with water, a high molecular weight polyurethane solution containing hydrophilic groups is prepared. Diluting the solution with water, first forming a water-in-oil emulsion with a continuous phase, then adding a large amount of water, inverting the phase, turning the water into a continuous phase and forming a dispersion, and finally desolvating the solvent to give a high molecular weight. Polyurethane cool pulse dispersion. The method is simple and reproducible, but solvent recovery is difficult. Suitable for higher activity of aromatic isocyanate.

3. Melt dispersion method. The ammonia- or pulse- and ion-body-terminated isocyanate-cooled prepolymer reacts to generate a terminal vein or a terminating two-vessel polyurethane premold, and the prepolymer is dispersed under a certain external shear force. In water, aldehyde derivatives are then added and the reaction proceeds under certain conditions to produce a macromolecular polyurethane-based aqueous emulsion. -

4. Ketimine-ketene nitrogen method. Prior to emulsification, potential amines are added to the polyurethane quencher prepolymer. In emulsification, ketimine and ketazine are hydrolyzed to form diamines or umbilicals, and chain extension occurs via amino groups. In the process, the presence of cosolvents must be strongly dispersed.

5. Protect the end group method. First, choose a suitable blocking agent, protect the water-sensitive isocyanic acid NCO groups and make them inactive. Then add the chain extender and cross-linker to make the emulsion. When applied, the NCO group is deblocked by heating, and the NCO group reacts with the cross-linking agent to form a network-like polyacrylamide film. Commonly used end-capping agents are phenols, alcohols, guanamines, and intestines. This method is easy to use, and it is possible to perform the most comprehensive characteristics of polyhydric materials. The key technologies to be solved are to reduce the temperature of deblocking, reduce the toxicity of the sealant, and improve the effect of the sealant on product performance.

Source: 21st Century Fine Chemicals Network