Advantages of high performance waterborne polyurethane adhesive

According to the type of charge carried by the particles, the aqueous polyurethane can be classified into three types: anionic, cationic and nonionic. The preparation of the aqueous polyurethane is generally carried out by first synthesizing a polyurethane prepolymer of a certain molecular weight and then dispersing the prepolymer in water under the action of shearing force. At present, the preparation method is mainly a hydrophilic monomer chain extension and self-emulsification method (so-called hydrophilic monomer chain extension method), that is, a hydrophilic chain extender is introduced into the molecular structure of the polyurethane prepolymer, and the obtained polymer does not need to be obtained. An emulsifier can be directly dispersed in water to form an aqueous polyurethane. The synthesis process of the hydrophilic monomer chain extension method includes a solvent method, a prepolymer dispersed water chain extension method, a melt dispersion polycondensation method, and the like.
As early as the 1940s and 1950s, there was a small amount of research on waterborne polyurethanes. However, due to poor storage stability, the research work was not progressing until 1972, when Bayer Company of Germany officially used polyurethane water dispersion as leather coating. Started to develop rapidly. According to reports, the average annual growth rate of waterborne polyurethanes between 1992 and 1997 was 8%.
China has been studying waterborne polyurethane since 70 years, and research work has been very active in recent years. The development work has the following characteristics:
(1) From the perspective of product structure, it is mainly emulsion type, water soluble, and latex type is not common.
(2) From the perspective of raw materials, polyhydric alcohols are mainly used in polyethers, followed by polyesters, and polycarbonates are rare. There are fewer varieties of isocyanates, only TDI, HDI, MDI are only reported. Chain extenders use alcohols and amines are less used.
(3) From the perspective of preparation methods and types, it is generally self-emulsified, carboxylic acid type, anionic system; external emulsification, sulfonic acid type, quaternary ammonium salt type emulsification system; melt dispersion, solid spontaneous dispersion method, etc. .
(4) From the perspective of theory and application, focusing on application development, theoretical research is rare.
To formulate high performance waterborne polyurethane adhesives, high performance waterborne polyurethanes must be prepared. Most of the waterborne polyurethanes are mainly linear thermoplastic polyurethanes. Because the coating film is not crosslinked, the molecular weight is low, and thus the water resistance, solvent resistance, film strength and the like are still poor. In order to improve the performance of waterborne polyurethanes, it must be modified.

Modification of waterborne polyurethane
From the preparation process of waterborne polyurethane, the molecular structure of polyurethane prepolymer has a decisive influence on the final properties of waterborne polyurethane. Therefore, the modification should mainly be made from the prepolymer, and the modification purpose can be achieved by changing the molecular structure of the prepolymer, the molecular weight and the like to obtain a high performance waterborne polyurethane.

Raw material selection for synthetic prepolymer
The synthetic raw materials for the polyurethane prepolymer are mainly oligomer polyols and diisocyanates. Oligomer polyols are generally classified into two types: polyether polyols and polyester polyols. Prepolymers made from polyether polyols have good hydrolytic stability, good flexibility and extensibility, and low temperature resistance. Good; polyester polyol type prepolymerization has large cohesion and high bonding strength. The mainstream products of foreign waterborne polyurethanes are generally polyester type, while the domestic ones are mainly polyether type. It has been reported in the literature that a prepolymer having a certain ratio of a mixture of a polyether polyol and a polyester polyol is preferred. In addition, the molecular weight and functionality of the oligomer polyol also have a great influence on the properties of the prepolymer. The introduction of a part of the molecular weight of the trifunctional polyether in the molecule of the prepolymer can increase the molecular crosslink of the prepolymer. Degree, thereby improving the water resistance of the aqueous polyurethane. Diisocyanates are classified into aromatic and aliphatic (or alicyclic) types. The prepolymer synthesized from the aromatic diisocyanate has higher strength, but is not resistant to yellowing, and the aliphatic diisocyanate has yellowing resistance of the prepolymer prepared therefrom due to the absence of unsaturated double bonds in the molecular structure. excellent. Due to the limitation of raw material types, the domestic di-cyanate raw materials used for waterborne polyurethanes are mainly TDI, while the foreign countries are mainly aliphatic (or alicyclic).
There are not many brand names of polyester polyols in China, especially the grades suitable for waterborne polyurethanes are more rare, and isocyanates are mainly dependent on imports. However, with China's accession to the WTO, I believe this will definitely create good conditions for us to improve the performance of waterborne polyurethanes from raw materials.
In addition to oligomer polyols and diisocyanates, small molecule chain extenders are often used in synthetic prepolymer materials, generally amines and alcohols. The amine chain extender has high reactivity with isocyanate, and the product is polyurethane urea. The presence of the urea bond makes the film hard and the modulus increases. Commonly used amines are ethylenediamine, diethylenediamine, triethylenetetramine and the like. The amine chain extender is generally added at the same time as the prepolymer is emulsified, and is emulsified and chain extended. It has been reported that the aqueous polyurethane urea dispersion obtained by using a mixed amine as a chain extender has excellent adhesion and high glass transition temperature. Alcohol chain extenders include 1,4-butanediol, ethylene glycol, diethylene glycol, etc. The effect of alcohol chain extenders on the properties of waterborne polyurethanes has been studied in China. Trifunctional alcohols have been found. As a chain extender, it can improve the water resistance of the film.
The introduction of a hydrophilic chain extender into the prepolymer molecule is one of the key steps in the preparation of aqueous polyurethane. At present, waterborne polyurethanes are mainly anionic and self-emulsified. Anionic waterborne polyurethanes are classified into two types: sulfonate type and carboxylate type. Hydrophilic chain extenders containing a carboxyl group - typically having a dihydroxy half ester (abbreviated as a half ester) and 2,2-bishydroxymethylpropionic acid (DMPA). DMPA has a small molecular weight and a small amount provides sufficient carboxyl equivalent weight. In the study, it was found that DMPA is solid and has a high melting point, which causes unevenness during the reaction; while half-fat has good compatibility with polyol, but the amount is large, and the obtained aqueous polyurethane film is sticky. The use of DMPA as a hydrophilic chain extender is very popular abroad. The domestic DMPA source was once dependent on imports. In recent years, domestic manufacturers have been able to produce.

White emulsification process
The white emulsification process is also an important factor in the preparation of high performance waterborne polyurethane. The author talks about the practical work:
(1) The emulsification temperature should be low and not high, and should not exceed 45 °C.
(2) The salt-forming agent is more suitable for the selection of organic bases, and the amount thereof should be strictly controlled.
(3) Selecting high-efficiency emulsification equipment can achieve twice the result with half the effort.

Other modification pathways
The above-mentioned selection of raw materials and processes can produce partially branched and crosslinked aqueous polyurethanes, which generally do not undergo chemical reaction during drying and curing. This method is called internal crosslinking method and is mostly used in one-component systems. There is also a so-called diplomatic joint method, that is, when a crosslinking agent is added to an aqueous polyurethane, the crosslinking agent chemically reacts with the polyurethane molecule to crosslink, and the diplomatic joint method is equivalent to a two-component system. At present, waterborne polyurethanes produced at home and abroad are almost single-component. In the 1990s, two-component waterborne polyurethanes were valued. The research focus was on reducing side reactions, increasing the selectivity of the reaction and forming films at low temperatures. The research of two-component waterborne polyurethane is mainly concentrated in Bayer, OECE, Miles and so on. The use of acrylate to modify waterborne polyurethanes has resulted in the so-called "third generation waterborne polyurethanes", which has become a hot research topic at home and abroad in recent years.
The third generation of waterborne polyurethane has the advantages of both acrylate and polyurethane, and is increasingly becoming one of the important ways to modify waterborne polyurethane.

Matching aid
High performance waterborne polyurethane adhesives should have the following characteristics:
(1) Good resistance to water and medium.
(2) High bonding strength and high initial adhesion.
(3) Good storage stability.
(4) Resistance to freezing and thawing, high temperature resistance.
(5) The drying speed is fast, and the film forming property is good at a low ambient temperature.
(6) Good construction technology.
In order to achieve the above points, in addition to the synthesis of high performance waterborne polyurethane, the choice of filming auxiliaries is also particularly important.
The water-based polyurethane compounding agent mainly has a thickener, a leveling agent (wetting agent) and the like. Many excellent performance additives are produced from abroad, such as Borchergel L75 thickener from Borchers, Voncoat HV and leveling agent Megafacf-812 from DIC, and Safinol, a surfactant from American Gas.