Waterborne polyurethane room temperature crosslinking technology

Waterborne polyurethane is a kind of dispersion system that replaces organic solvent with water. The hydrophilic group in the molecular segment and the presence of a small amount of emulsifier reduce the water resistance, weather resistance, gloss, etc. of the paint film; The process determines that the molecular weight of the waterborne polyurethane should not be too large, which also makes the solvating property, heat resistance, mechanical strength and other physical and chemical properties of the film less than ideal, thus limiting its application range.

      Crosslinking modification is one of the most direct and effective methods to improve the performance of waterborne coatings. By cross-linking, the degree of cross-linking of the system can be improved, and the cross-linking of the body shape can be partially realized, which can effectively increase the molecular mass of the polymer, and the physical and chemical properties of the polymer are significantly improved. With the development of waterborne coating technology, room temperature crosslinking technology has received great attention. The room temperature cross-linking technology is simple in process, convenient to use than high-temperature cross-linking and radiation cross-linking technology; the process does not require heating or irradiation, which not only saves the cost of auxiliary equipment, but also saves energy; more importantly, this type Emulsions tend to react slowly or even unreacted between functional groups before film formation, so many room temperature crosslinking systems can be used to prepare one-component aqueous coatings. In recent years, research on room temperature cross-linking and curing waterborne polyurethanes has been extremely active, including the study of room temperature crosslinking mechanism and the development of some new room temperature crosslinking reaction raw materials. With the development of research, the performance of such waterborne polyurethane products has been continuously improved, and the performance of solvent-based polyurethanes has been approached, and it has been widely used in more and more fields. However, there are not many literature reports on this technology. This paper mainly introduces some room temperature crosslinking systems and their crosslinking mechanism which can be applied in waterborne polyurethane.

1 room temperature crosslinking of carbonyl and hydrazide groups

      The reaction of a carbonyl group with a hydrazide group is generally considered to be a water inhibiting crosslink. Under neutral or weak acid and weak base conditions, the presence of water in the emulsion inhibits the progress of the reaction. During the film formation process, as the moisture in the coating film and the neutralizing agent volatilize, when the pH of the emulsion reaches about 4, the carbonyl group and the hydrazide group begin to undergo an irreversible dehydration reaction and form a thiol group. This cross-linking system has been widely used in the preparation of one-component room temperature cross-linking cured aqueous polyurethane. The most commonly used carbonyl-providing monomers are diacetone acrylamide (DAAM), acetoacetoxyethyl methacrylate (AAEM), etc., wherein the latter is more active than the former and can chelate with some metal ions. For cooperative use, the stability of the emulsion preparation process is poor, and the requirements for the process conditions are high. The hydrazide group-containing compound is adipic acid dihydrazide (ADH), dihydric acid dihydrazide, oxalic acid dihydrazide, etc., but in terms of cost, performance and the like, adipic dihydrazide is generally used mainly. Jiang Shuojian made a detailed study on the room temperature crosslinking of DAAM and ADH. It is believed that the introduction of DAAM in the late stage or shell layer in core-shell emulsion polymerization can avoid a large amount of carbonyl groups being embedded in the latex particles, which is beneficial to make full use of DAAM. The ketone carbonyl group; and the hydrazide is generally used in an amount of from 0.8 to 1.0 in terms of the ketone carbonyl equivalent; the study also indicates that it is more advantageous to react the carbonyl group with the hydrazide group if a hydrophilic group is attached around the ketone carbonyl group. Cui Yuezhi et al. prepared a room temperature cross-linked acrylic modified waterborne polyurethane by synthesizing a ketone carbonyl-containing acrylate emulsion with a hydrazide group-containing aqueous polyurethane. The cross-linking was confirmed by infrared spectrum analysis. The occurrence of the reaction, the prepared waterborne polyurethane emulsion has significant improvement in water resistance, solvent resistance, breaking strength and elongation at break.

2 room temperature cross-linking based on Michael addition reaction

      The use of Michael addition reaction for cross-linking and solidification of new coatings has been carried out abroad, but it has been developing slowly. In recent years, with the progress of water-based coatings, this cross-linking system has received extensive attention. This reaction requires an alkali metal alkoxide or a strong organic base such as tetrabutylammonium hydroxide as a catalyst. The catalyst developed by the company synthesized from a tertiary amine and an epoxy compound also has a good catalytic effect. Low molecular weight trimethylolpropane triacrylate, acryl-containing acrylic polymer, and unsaturated polyester synthesized from maleic acid, methylene succinic acid, maleimide and its derivatives It can be used as a receptor for active hydrogen; acrylates containing acetoacetyl groups, malonic acid-based polyesters, polythiols, polyfluorenes and polyamines protected in the form of ketimine can be used as donors for active hydrogen. The system can be quickly crosslinked at room temperature, and the resulting coating film has good strength, water resistance and scrub resistance. The ketone carbonyl-containing bishydroxy compound was synthesized by Michael addition reaction with diethanolamine (DEA) and diacetone acrylamide (DAAM), and the compound was used as a crosslinking agent for the aqueous polyurethane dispersion, and the amino group in the polyurethane structure or The hydrazide group is reacted to prepare a room temperature crosslinked aqueous polyurethane dispersion. A composite emulsion of acetoacetyl-containing polyacrylate and terminal alkenyl polyurethane was synthesized, and 1,8-diazahetero-bicyclo(5,4,0)undec-7-ene (DBU) was used as a catalyst. . The experiment shows that the crosslinking degree of the paint film is very low without the addition of catalyst, and the hardness and impact strength are very poor. With the increase of catalyst concentration, the storage time and drying time are shortened, and the crosslinking degree and hardness of the paint film are corresponding. Improve, and the impact strength and adhesion can be best.

3 room temperature cross-linking of isocyanate groups

      Generally, the polyisocyanate compound cannot be dispersed in the aqueous phase, and its reaction with water is fast, and it will bring many performance defects in the aqueous system. However, in recent years, with the development of waterborne polyurethanes, researchers have developed polyisocyanate crosslinkers suitable for aqueous systems and have been introduced to the market. Such polyisocyanates are hydrophilically modified, and the -NCO groups are blocked by steric hindrance to reduce their reactivity with water; with the evaporation of water and the deformation of molecular structure, NCO is released and coated. The reactive group undergoes a crosslinking reaction. At present, such products include Bayer's Desmodur XP -7007, Desmodur DA, Japan DIC's CR -60N, and US Cytec's Cythane 3174.

4 room temperature cross-linking of aziridine and carboxyl group

      The three-membered ring in the aziridine compound is an unstable structure which is susceptible to attack by protons and opens to form a cross-linking reaction. The aziridine compound can react with the carboxyl group of the carboxylic acid type polyurethane emulsion at room temperature, and can be applied to the modification of the aqueous polyurethane emulsion. Since aziridine has a strong mutagenic activity and is a highly toxic chemical, most of the derivatives currently circulating on the market are derivatives of polyaziridine. Companies such as Bayer of Germany and Stahl of the Netherlands have such products. It is 3% to 5% of the polyurethane solids. An aziridine crosslinker was synthesized and its cross-linking properties and curing kinetics were studied. The results showed that the addition of an appropriate amount of aziridine cross-linking agent to the carboxyl group-containing emulsion can greatly improve the mechanical properties, water-resistant alkalinity and solvent resistance of the film; however, the cross-linking temperature to the glass transition temperature Less affected. Chen et al. used an epoxy/aziridine compound as a crosslinking agent for a terminal amino group-containing carboxyl group-containing aqueous polyurethane dispersion to crosslink the polyurethane. The results show that the epoxy compound enters the interior of the polyurethane and reacts with the amino group, and the aziridine compound acts as a potential curing agent to react with the carboxyl group in the polyurethane chain as the pH decreases during the drying process of the coating film, resulting in cross-linking. Coating film. Lai et al. first prepared a urea-butanone derivative containing aziridine group, in which the urea-butanone reacted with an amino group on the polyurethane end group to introduce an aziridine group. An aqueous polyurethane having a potential room temperature crosslinking function was prepared.

5 room temperature cross-linking of cyclic carbonate and amine group

      Since the birth of cyclic carbonate chemistry in the 1930s, it has grown into a discipline that has attracted the attention of chemists, and has grown considerably over the past 30 years. The crosslinking reaction of the cyclic carbonate with the amine group can produce a compound having a carbamate structure. Because it does not contain highly toxic components such as isocyanate in the preparation process, and the reaction process can be controlled to achieve a higher degree of crosslinking, it is considered to be an ideal direction for the preparation of environmentally friendly new polyurethane materials, attracting the attention of many researchers. It has been reported in foreign patents that a two-component aqueous coating prepared by a room temperature crosslinking reaction between a compound containing a pendant carbonate group and an amino group-containing compound has excellent coating properties. Its reaction formula is as follows:

Waterborne polyurethane

6 room temperature auto-oxidation cross-linking of double bonds in unsaturated fatty acids

      Oils containing unsaturated fatty acids are widely used in the preparation of self-crosslinkable coatings. The mechanism of this type of crosslinking reaction is that the unsaturated compound in the emulsion system absorbs oxygen to produce a peroxy group by using an added organometallic catalyst (which may or may not be added), and these peroxy groups decompose in the emulsion to generate free activity. The base, which in turn initiates a cross-linking reaction of the double bonds in the components of the dry oil or the self-drying alkyd resin in the system. Bayer's Christoph Irle et al. have conducted in-depth research in this area. They have successfully prepared cross-linked aqueous polyurethane dispersion floor coatings by utilizing the self-oxidative crosslinking properties of dry oils; due to the presence of fatty acids in the soft segments of polyurethanes, The joint reaction will inevitably lead to the loss of elasticity and wear resistance of the coating film. Through the control of the amount of oil, the coating film has good wear resistance and meets the requirements of floor coating under the premise of ensuring cross-linking. A one-component self-crosslinking polyurethane aqueous dispersion was prepared and the effects of different oils were investigated. The results showed that the linseed oil had a fast drying speed and high hardness, but the yellowing was also obvious; the soybean oil and the dehydrated castor oil yellow became lighter, but the hardness formed slowly. The smell of fish oil is very serious when it is produced and cured, and it is difficult to be accepted. The combination of linseed oil and dehydrated castor oil has the best combination.

7 room temperature crosslinking of alicyclic epoxy groups with carboxyl groups

      The epoxy group of the alicyclic epoxy compound and the glycidyl type or bisphenol type epoxy compound has a remarkable difference in stereostructure. The six carbon atoms on the alicyclic ring are not in the same plane, and the two carbon atoms connecting the oxygen atoms are subjected to large steric hindrance, so they are not easily attacked by nucleophiles such as amines, and the ring-opening reaction occurs. The reactivity of the terminal epoxy group is much lower; however, the ring-opening reaction of the electrophilic reagent such as a Lewis acid to attack the oxygen atom has no such steric effect due to the oxygen atom on the alicyclic epoxy group. The activity is not affected; and because the molecular structure does not have a group having a large electronegativity such as a benzene ring, the reactivity of the terminal epoxy group is greater than that of the electrophile. The order of activity is as follows:

RCO2H >ROH >>RNH2

      M.D. Soucek et al. prepared a room temperature cross-linking curing acrylic emulsion with core-shell structure by emulsion polymerization. The initiating system, reaction conditions, emulsion morphology and emulsion stability of emulsion polymerization were also studied. This research can also be applied to aqueous polyurethane composite emulsions. At present, such products include Dow Ell-4221, ERL-4299, UVR-6105, etc., and some alicyclic epoxy silanes developed by Compton Co., Ltd., such as Wetlink 90 and A-186, have this. Cross-linking function.

8 Metal ions collide with carboxyl groups, sulfonic acid groups, etc. at room temperature to utilize metal ions and some functional groups on the polymer molecular chain.

      The group reaction realizes the cross-linking of the coating film, and has the advantages of low cost, one-component packaging, and room temperature crosslinking; however, the compatibility of the metal salt with the emulsion is poor, and the emulsion is easily broken, so there is also storage stability. Good defect. Therefore, a stable room temperature cross-linking emulsion can be obtained only by rationally selecting the kind and content of the functional group and the kind and amount of the metal salt cross-linking agent matched thereto. The transition metal ions often exist in the form of ammonia complex ions. In the process of emulsion film formation, as the ammonia volatilizes, the metal ions gradually liberate from the complex, and the emulsion gradually changes from weakly alkaline to weakly acidic. Subsequently, the free metal ions interact with a carboxyl group or a sulfonic acid group suspended on the emulsion polymer chain to form an insoluble salt or a complex, thereby achieving crosslinking curing of the coating film. In foreign patents, an aqueous polyurethane emulsion which can be crosslinked at room temperature is prepared by using a cerium salt (usually cerium acetate) as a crosslinking agent. The patent recommends that the amount of cerium salt is 0.05 to 2 mmol of hydrazine salt per kilogram of polyurethane solids, dissolved in an aprotic solvent, and the addition of the prepolymer in the polymerization stage does not affect the progress of the reaction; The emulsion has good film properties.

9 room temperature hydrolysis polycondensation crosslinking of siloxane groups

      Organosiloxanes are a class of organic functional silane compounds that can be used in emulsion synthesis and aqueous coating systems. The structural formula can be represented by Y-R-Si(X)3, where X is an alkoxy group and Y is an organic functional group. . The organosiloxane structure contains both "organic groups" and "inorganic structures". This special composition and molecular structure make it combine the properties of organic matter with the function of inorganic substances, and has excellent water resistance and resistance. Chemical, weathering, breathable, physiologically inert and low surface tension. In the preparation of the aqueous polyurethane, a reactive siloxane group is introduced, which can be hydrolyzed to form silanol at room temperature, and the silanol condenses with the active group inside or on the surface of the polymer to form a stereo network (Si-O-Si). Cross-linking structure, improve the cross-linking density of the system, enhance the density of the film surface layer, and finally improve the mechanical properties and resistance of the film. Use TDI, PEG, DM PA, hydrophobic water and silane coupling agent KH560, etc. The raw material has prepared a polyurethane aqueous dispersion which can be crosslinked and cured at room temperature. The research shows that the addition of silane coupling agent can significantly improve the water resistance, heat resistance and other mechanical properties of the paint film. In addition, KH560 has a paint film. For plasticization, the hardness first increases with the increase of the amount, but the excessive hardness of KH560 reduces the hardness of the paint film. Subramani et al. simultaneously add amino-containing silicone to the polyurethane prepolymer emulsification stage