Polyurethane Emulsion Technology

1 Introduction <br>Water-based polyurethane (emulsion, latex, water-soluble) is water as a dispersion medium, the system does not contain or contain a small amount of organic solvents, non-flammable, non-toxic, non-polluting environment, saving energy and easy processing Such advantages, but also has general polyurethane resin (PUR) intrinsic high strength, wear resistance and other excellent performance, making it increasingly aware of the importance of environmental protection in the world today, become the direction of PUR product development, its research in recent years active. As early as 1943, PSchlack successfully developed waterborne PU. For decades, many waterborne PU products have been successfully used in textile, printing and dyeing, leather processing, coatings, adhesives, wood processing, construction, and papermaking industries. The PU emulsion in China was first developed by the Shenyang Leather Institute in 1976. Since then, it has successively produced Beijing 5# emulsion, Tianjin Leather PU-1 emulsion and PU emulsion leather finishing materials. After entering the 1980s, PU emulsions The development of the company has become more active. For example, Chengdu Wangjiang Chemical Plant, Chenguang Research Institute of Chemical Industry, Shanxi Chemical Research Institute, and Chengdu University of Science and Technology have carried out research and development.

2 Preparation of Polyurethane Emulsions General polyurethanes are very hydrophobic, and new synthetic methods must be used to prepare PU emulsions. There are two general methods used: external emulsification and internal emulsification (also known as self-emulsification) .

2.1 External emulsification method The so-called external emulsification method is a forced emulsification method in the presence of emulsifiers and high shear forces. It was originally invented by PSchlack. In 1953, DuPont's Wyandott used this method to synthesize PU emulsions. The synthesis process is to synthesize PU prepolymer with polyether diol and organic isocyanate, and then extend the chain with small molecule diol or diamine to obtain PU organic solution, and then gradually add the aqueous solution of emulsifier under strong stirring. A coarse-grained emulsion is formed and finally fed into a homogenizer to form an emulsion of suitable particle size. The method has a long reaction time, a large amount of emulsifier, and poor emulsion storage stability. A method developed later called a low-temperature sealing method (also called thermal reaction method) to prepare a PU emulsion can reduce the amount of emulsifier and produce a stable emulsion. In this method, a terminal-NCO prepolymer is capped with a capping agent such as hydrazine, lactam, NaHSO3, acetoacetate, etc., and then dispersed together with a polyamine in an aqueous solution containing an emulsifier to form a stable PU emulsion. .

2.2 In-house emulsification The preparation of stable PU emulsions is mainly carried out by self-emulsification. The key is to introduce hydrophilic groups (mostly groups that form ionic bonds) in the molecular framework of polyurethanes. Hydrophilic groups are formed by the chain extension of hydrophilic monomers into the PU molecular skeleton, which consists of salt-forming groups and salt-forming reagents. According to the nature of salt-forming groups, self-emulsifying PU emulsions can be further classified into anionic, cationic and diionic types. There are many kinds of preparation methods of self-emulsifying PU emulsion. The two most important ones are: acetone method and prepolymer dispersion method (or prepolymer mixing method).

2.2.1 The acetone method The acetone method was successfully studied by the German Bayer company D Dieterich. In this method, a polyether or polyester diol and an isocyanate are first used to prepare a prepolymer. After adding an appropriate amount of acetone to reduce the viscosity, the chain is extended with N-methyldiethanolamine, and then acetone is added to lower the viscosity, and then the ionization reagent is added ( Quaternization reagent), stirring ionization. The ionized PU was dispersed in a medium containing 80% acetone and 20% water, and finally acetone was distilled off to prepare an emulsion-type polyurethane having a particle size of 0.03 to 100 μm. The advantages of the acetone method are that the reaction is easy to control, the reproducibility is good, and the emulsion quality is high, but more acetone is needed, especially when the PU molecular weight is large, the amount is more, the economy is uneconomic, and the danger is great.

2.2.2 Prepolymer dispersion method This method has been developed in recent years. It is first introduced into the polymer with hydrophilic monomers, ionized to produce a PU prepolymer containing ion bonds, and then dispersed in water to form a prepolymer emulsion, and finally carried out with diamine in the aqueous phase. Chain extension to make PU emulsion. The process is simple and does not require a large amount of organic solvents, can produce a PU emulsion with a degree of branching, but is limited to special end-NCO prepolymers (prepolymers mainly made from low-activity aliphatic isocyanates), And the product quality is not as good as the acetone method. In addition, PU emulsion synthesis methods include melt dispersion method, ketimine and ketene nitrogen method [2], and water direct mixing method, solid spontaneous dispersion method [3] and so on. Each of the above methods has its own advantages and disadvantages. In comparison, the acetone method is mature, while the prepolymer dispersion method has a better prospect (mainly its synthesis process is simple).

3 Relationship between properties and structure of polyurethane emulsion
3.1 Emulsion stability
Lorenz explained the stability of ionic PU emulsions using the double layer theory. Taking a carboxylic acid PU emulsion as an example, the zeta potential is related to the content of -COOH, the pH (PKa) and the degree of ionization (α). The more -COO- groups, the smaller the pKa value of -COOH, and the larger the alpha value, the higher the zeta potential and the more stable the emulsion. Chen Jiahua et al. studied the stability of anionic PU emulsions. The results showed that DMPA with a solids content of more than 0.22 mmol·g-1 can produce stable emulsions with shelf-life at room temperature greater than 1 year and acute heat-stability period greater than 60 days. If the same macromolecular chain contains ionic and non-ionic hydrophilic groups, due to their synergistic effect, a small amount of internal emulsifiers (ie, ionic hydrophilic groups) can be used to achieve the stability requirements. The hydrophilicity of the membrane can be reduced to the level of solvent-based PU.

3.2 Water Resistance of Emulsion Film Forming Agents Most PU emulsions produced by self-emulsification methods generally have a moisture absorption rate and a volume expansion ratio that increase with the increase in the amount of ionic hydrophilic monomers. In order to overcome this inherent defect of polyurethanes, an effective way to improve water resistance is cross-linking. There are roughly three types of cross-linking methods. (1) Introduction of a cross-linked structure in the PU body This method is mainly to introduce cross-linking using a trifunctional or higher-functional macromolecular polyol or a small molecule chain extender, thereby improving the water resistance and strength of the film. (2) Introduction of reactive crosslinking groups in the PU structure One of the methods is to use epoxy groups for cross-linking. There is also the use of PU structure -OH or -COOH reaction with some cross-linking agents to achieve the purpose of cross-linking. (3) Crosslinking using metal ions This method utilizes a carboxylic acid-type diol chain extender to form a terminal-NCO prepolymer, and then adds a metal ion for crosslinking, which is achieved by the salt formation reaction of a carboxyl group-COOH and a metal ion. The purpose of cross-linking. Commonly used metal compounds are Al(OH)3, Ca(OH)2, Mg(OOCCH3)2, ZrO(OOCCH3)2, and the like. The greatest weakness of this type of system cross-linking is that ionic bonds are reversible under certain conditions and do not form permanent crosslinks.

3.3 Thermal properties of emulsion film formers
Cooper, Chan and Chen have done a lot of research on the thermal properties of ionic PU elastomers, given a detailed explanation of the ionic PU elastomer DTA, DSC curve, and also discussed its thermal degradation mechanism. From a chemical point of view, urea groups have the best thermal stability, and urethane groups also have better thermal stability. The thermal properties of polyether-based PU elastomers are inferior to that of polyesters, and the thermal stability of elastomers synthesized from aliphatic diisocyanates is not as good as that of aromatic diisocyanates.

3.4 Mechanical properties of emulsion film formers
BKKim et al. studied the relationship between dynamic mechanical properties of aqueous PU film and its structure, and discussed the effects of ion content, molecular weight of prepolymer, chain extender dosage, soft segment length, and soft segment type on the dynamic mechanical properties of the film forming material. The results show that with the increase of ion content, the glass transition temperature (Tg) moves to a high temperature, and as the molecular weight of the prepolymer increases, the Tg decreases. The effect of ion content on polymer modulus and elongation at break is complex, showing a change process that decreases first and then rises.

4 Application of PU Emulsion <br> Currently, PU emulsion has been widely used in coatings industry, leather industry, printing industry, textile industry and construction industry, and has gradually penetrated into all walks of life, becoming an important component in the material field.

4.1 Adhesives
PU emulsion adhesives have been serialized with brands such as Germany's Bayer Company and Japan's Dainippon Ink Company. The research and application of water-based PUs have reached a considerable scale, while domestic development is still in progress. PUs for adhesives are mainly Still solvent-based, although there are some manufacturers of water-based PU adhesive, but the small scale, theoretical and applied research is not deep enough, the product is relatively simple. In addition, waterborne PU adhesives have made great progress in the fields of wood processing, automobile industry and food packaging.

4.2 Leather processing In the field of leather processing, PU emulsions are mainly used as finishing agents and retanning agents. According to the requirements of finishing, PU emulsions come in different categories and can be roughly divided into tonics, primers, middle finishes, and top finishes by changing the three raw materials for synthetic PUs (macrodiols, diisocyanates, and expanders). Chain agent) and its preparation, processing technology, you can get a variety of performance, in line with the various requirements of the above products.

4.3 Fabric Finishing and Printing and Dyeing Auxiliaries
PU emulsion can be used for paint padding and large-area paint printing. The printing place has soft, smooth, clear outline, uniform coloring, no seepage and low cost.

5 Conclusion <br>The development of waterborne PU is changing with each passing day, and the overall development trend is to develop towards high performance and low cost. Major foreign companies are strengthening the study of high solids content of stable PU emulsions. At the same time, the type, quantity and performance of PU emulsion products have been vigorously developed. The research and development level of domestic PU emulsions is relatively low, which is mainly limited by weak domestic chemical industry. We should move closer to foreign countries in basic raw material production and product development, such as the development of high-performance hydrophilic monomers, the synthesis of aliphatic isocyanates, and the control and monitoring of polyether production, to overcome the poor emulsion stability and light resistance of existing products. Poor quality and other defects, vigorously develop new varieties, improve the domestic PU emulsion production capacity and level.