In recent years, a lot of research has been done on the problems of slow drying speed, low initial viscosity, poor wettability on non-polar substrates, poor water resistance, and low heat resistance, and some suggestions have been made.
First, adjust the structure
(1) Under the premise of ensuring emulsion stability, the content of hydrophilic groups is as low as possible, which can improve the water resistance of the film.
(2) The smaller the relative molecular mass of the polyether polyol, the harder the film and the better the water resistance.
Second, improve the process
Emulsifying equipment with excellent process performance. Heat treatment can improve the film strength and water resistance. For the cross-linked water-based polyurethane, heating forms a cross-linked structure, which improves water resistance and heat resistance. For thermoplastic water-based polyurethanes, the salt agent such as triethanolamine, ammonia, and acetic acid is released from the film by heat, increasing hydrophobicity. In addition, some degree of cross-linking and branching will also occur.
Third, cross-linked modification
Cross-linking is an effective method to improve the performance of waterborne polyurethane, and can improve the water resistance, heat resistance and adhesive strength of the film. Cross-linking is divided into internal cross-linking and diplomatic alliances. The so-called internal cross-linking means that the aqueous polyurethane contains a reactive functional group during the synthesis, and cross-linking can be formed after the heat treatment. Internally cross-linked aqueous polyurethanes can be produced, for example, using partially trifunctional polyols or isocyanates, incorporating amine or epoxy groups, blocked isocyanate emulsions, multifunctional crosslinking agents, and the like. The disadvantage of the internal cross-linking method is that the prepolymer has a large viscosity and is difficult to emulsify. At the same time, the aggregation between polyurethane particles is poor and the film-forming performance is not good.
The Diplomatic Union Act is to add a cross-linking agent before use. After the film formation process or film formation, the chemical reaction is heated to form a cross-linked film. Compared with internal cross-linking, the obtained emulsion has good performance, and its disadvantage is that it is a two-component system and there is no single-component type convenience. In industry, cross-linking of carboxylic acid type anionic emulsions is mostly carried out, and the diplomatic linkage method is further divided into a high temperature cross-linking method and a room temperature cross-linking method. Using formaldehyde, melamine-formaldehyde resin, epoxy compound cross-linking agent, crosslinking reaction is generally performed at 120-1800C. Aziridines, carbodiimides, polyisocyanates, and metal salt compounds are used as cross-linking agents to cross-link at room temperature, but due to rapid cross-linking reactions, gels are easily generated in a short time, and are generally used. The crosslinking agent is added when the polyurethane emulsion is used.
First, compound modification
In order to improve certain properties of waterborne polyurethanes, it can be blended with other waterborne resins such as epoxy resin emulsions, acrylate emulsions, VAE emulsions, neoprene latexes, natural latex, polyvinyl acetate emulsions, urea formaldehyde resins, etc. to make new waterborne resins. Adhesives have improved performance. Aqueous polyaniline is blended with an epoxy resin emulsion to improve adhesion, water resistance, and solvent resistance.
Aqueous polyurethanes are blended with acrylate emulsions with properties ranging between polyurethane and acrylate emulsions.
Polyurethane and acrylate copolymer emulsion (PUA) has excellent properties, good adhesion to metal, and improved solvent resistance. The process of copolymerization and compounding is firstly to synthesize a polyurethane prepolymer that is based on hydroxyethyl (or propyl) acrylate as an end group. After emulsification and dispersion, the emulsion is copolymerized with an acrylic monomer to obtain a PUA composite emulsion.
Waterborne polyurethanes can also be used as "seed polymers", and as polymer emulsifiers, monomers such as acrylates, acrylonitriles, vinyl acetate, styrene and the like are added, and persulfate-based initiators are used for soap-free emulsion polymerization to obtain them. Core-shell hybrid water-based trees have better performance than simply blending acrylate emulsions with polyurethane emulsions. PUA composite emulsion technology is another new achievement in the research of polymer materials in the world today, and it is also a new direction for the development of waterborne polyurethane.
Fifth, graft modification
An appropriate amount (about 20%) of styrene monomer was added to the polyurethane emulsion, and a polyurethane-styrene grafted emulsion was prepared by an in-situ polymerization method to improve the adhesion and water resistance. N210 polyether, TDI, DMPA (dimethylolpropionic acid) (5.9%) N-methylpyrrolidone was added to a three-neck reaction flask equipped with a stirrer and a thermometer. NCO:OH=(1.3~1.4): 1 reacts at 800C for 3 h. When the -NCO value reaches or is slightly lower than the theoretical value, it is cooled to 600C and triethanolamine is added to carry out each of them to obtain a prepolymer. Prepolymers. The prepolymer was cooled to 300C, and 50C quantitative deionized water was added under stirring. An appropriate amount of chain extender ethylenediamine was added, and the mixture was stirred at 300C for 4 hours to obtain a polyurethane (PUL). The synthesized PUL and styrene monomers were added to a four-reaction bottle equipped with a stirrer and a thermometer. The mixture was emulsified and emulsified for 30 minutes. The temperature was increased to a certain temperature and the initiator system was added (tert-butyl hydroperoxide: tetraethylene pentamine=1: 1.5) 2.8~4.2*10-3/100g, reacted at 500C for 4h, and finally obtained PUL-styrene grafted emulsion, the conversion rate was 91%, and the grafting efficiency was 62.7%.
6, water-based vinyl polyurethane adhesive
Water-based vinyl polyurethane adhesive refers to an organic solution of a polyisocyanate (or its prepolymer) dispersed in an aqueous solution of polyvinyl alcohol, carboxymethyl cellulose, or a vinyl acetate emulsion, an acrylate emulsion, a VAE emulsion, a styrene butadiene latex, etc. The adhesive made in the base emulsion is also called an aqueous polymer isocyanate adhesive (API). Waterborne vinyl urethane adhesives can be either water-soluble or emulsion-based, as well as single-component and two-component.
First, the composition of water-based adhesive
(1) Vinyl polymers are commonly used in aqueous solution of polyvinyl alcohol (PVA) or vinyl synthetic resin emulsion. The degree of polymerization of PVA is 300-2500, the alcoholysis degree is 88-99%, and the mass percentage concentration is 5%-20%. should.
(2) Polyisocyanates Polyisocyanates that can be used in the manufacture of water-based vinyl urethane adhesives include PAPI, MDI, TDI, TDI-TMP adducts, HDI, XDI, triphenylmethane triisocyanate, etc., and their end-NCOs with polyols. Prepolymers. The most commonly used is PAPI (polymethylene polyphenyl polyisocyanate).
(3) Solvents In principle, any solvent that can dissolve polyisocyanic acid and its prepolymers can be used, such as aliphatic aromatic hydrocarbons, ketones, esters, but water-insoluble organic solvents. More appropriate, because the solvent is incompatible with water, water can not quickly enter the fine droplets composed of polyisocyanate and solvent, and the solvent protects the inner NCO groups of the droplets from reacting with water, so that the adhesive has a suitable service life. The smaller the solubility of the solvent in water, the better the water resistance of the adhesive.
Normal > solvent name
Normal align=left> Boiling point
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Normal align=left> Solubility in water
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Normal align=left> Adhesion strength after water immersion
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