Waterborne Polyurethane Resin Modification Technology (I)

Introduction

Waterborne polyurethane is a new polyurethane system that uses water instead of organic solvents as a dispersion medium. It can be widely used in coatings, adhesives, leather finishing agents, and fabric finishing agents [1], due to its advantages of being non-toxic, inexpensive, safe, and non-flammable. With the continuous improvement of its own performance, it shows great market prospects and has the tendency to gradually replace solvent-based products [2]. The synthesis of water-borne polyurethane is mostly self-emulsifying, that is, introducing hydrophilic groups such as COOH, -SO 3 , -OH, -O - in the macromolecule of polyurethane. However, due to the existence of these hydrophilic groups and some deficiencies of the polyurethane resin itself, water-based polyurethane products have poor performance in water resistance, solvent resistance, weather resistance, etc., which limits the scope of use of such products to a certain extent [3] ]. Since the 1970s, polyurethane material researchers have adopted grafting and block technology to introduce acrylic resin, epoxy resin, styrene and other resins into waterborne polyurethane resin macromolecules according to the complementary nature of different resins. A composite resin with a cross-linked, core-shell and interpenetrating network structure compensates for the lack of performance between different resins. This article systematically elaborated various effective modification methods for improving the performance of waterborne polyurethanes.

1 Cross-linking Modification of Waterborne Polyurethane Resins

Cross-linking modification is to link linear polyurethane macromolecules together through chemical bonds to form a polyurethane resin with a network structure, which is a more effective way to convert thermoplastic polyurethane resins into thermosetting resins. The cross-linked water-borne polyurethane coating exhibits excellent water and solvent resistance and mechanical properties, and is one of the effective methods for improving the performance of the waterborne polyurethane resin. Water-retaining polyurethanes have basically reached the properties of solvent-based polyurethane resins. According to the different methods of cross-linking, it can be divided into internal cross-linking law and diplomatic law.

1. 1 cross-linking method

A method for synthesizing waterborne polyurethanes by internal cross-linking is one in which a monomer containing 3 or more functional groups is introduced into a polyurethane macromolecule to generate a polyurethane micelle having a partially crosslinked or branched molecular structure; the other is In the aqueous polyurethane emulsion, an internal crosslinking agent that can stably coexist with the emulsion is added, and these internal crosslinking agents only react with the polyurethane resin due to changes in pH, temperature, external energy such as ultraviolet radiation and the like of the emulsion system when used. The functional groups in the cross-linking reaction generate a thermosetting polyurethane resin having a network structure.

The incorporation of macromolecules with monomers containing 3 or more functional groups to produce partially crosslinked or branched structures of polyurethane resins is generally performed using a prepolymer dispersion method, i.e., crosslinking monomers such as HD I trimers or The trimethylol propane and the like are mixed well with a low molecular weight polyurethane prepolymer, dispersed in water, and then added with a chain extender such as ethylenediamine for chain extension reaction; this method synthesizes a waterborne polyurethane having a partially crosslinked structure The aqueous polyurethane prepared by the acetone method has the advantage of not consuming solvent (acetone) and simultaneously obtaining a high solid content. Ling Fang et al [4] used this method to synthesize a one-component self-crosslinking aqueous polyurethane dispersion emulsion with good performance; Ricard G Coogan [5] adopted Desmodur N-3300 supplied by Bayer as an internal crosslinking agent. The aqueous polyurethane emulsion was prepared by this process method, and the effects of different amounts of internal crosslinking agent on the mechanical properties and solvent resistance of the emulsion coating film were studied. The results showed that with the increase of the amount of the internal cross-linking agent, the swelling ratio of the coating film to acetone decreased, and the tensile elongation at break decreased. However, the solvent resistance test results showed that the amount of the internal cross-linking agent and the solvent resistance of the emulsion coating film were two. Sub-curve relationship, and the viscosity of the emulsion increases sharply with the increase in the amount of internal cross-linking agent.

In addition to the prepolymer dispersion method, acetone can also be used to prepare such internally cross-linked water-borne polyurethanes. That is, before the dispersion of the prepolymer, a partially functional trifunctional monomer such as trimethylolpropane is used instead of the bifunctional monolayer. The body undergoes a chain extension reaction, and a small amount of acetone is used as a solvent to solve the problem that the viscosity increases due to the increase in the relative molecular mass after the chain extension of the prepolymer. After the dispersion is formed, the low boiling point solvent such as acetone is depressurized and removed. The waterborne polyurethane prepared by this method has the advantages of narrow relative molecular mass distribution, easy control over the variable range of structure and particle size, and good reaction stability, but the biggest disadvantage is that the prepared emulsion film is resistant to solvent, especially to acetone. Poor and complicated process, not conducive to industrial production. Yan Fuan [6] This process uses a trifunctional high-gloss monomer to prepare an internally crosslinked water-based polyurethane leather brightener. The above two methods for preparing an internally crosslinked aqueous polyurethane all have the same inadequacies, that is, due to factors such as emulsion viscosity, gel, etc., a high crosslink density aqueous polyurethane emulsion cannot be synthesized, so this method is only suitable for preparation. Waterborne polyurethane products that do not require high crosslink density.

Obtaining a relatively high crosslinking density of the inner cross-linking process for the preparation of water-borne polyurethanes generally adopts a special molecular structure design, that is, a group that can be reacted on a polyurethane macromolecule to block or graft, and both groups are required. Under normal circumstances, no reaction occurs. The cross-linking reaction occurs only when the temperature, pH value of the emulsion system changes, or is affected by external energy such as ultraviolet light, to form a cross-linked system having a network structure. The conditions under which the cross-linking reaction takes place during the use of the emulsion can be roughly divided into high-temperature curing type, photo-curable type, room-temperature self-curing type (variable depending on the pH value of the system), and oxygen-curable types.

The synthesis of high-temperature-curable water-borne polyurethanes generally refers to the use of blocking agents that react with reactive groups in polyurethane macromolecules or cross-linking molecules to render them inactive. This ensures that the two components can coexist at room temperature and become stable as a single component. Emulsion. When the emulsion is subjected to a high temperature treatment, these blocked reactive groups can be decomposed again and participate in the cross-linking reaction and act as internal crosslinking. Bayer's XP - 7063 blocked isocyanate emulsion and Cytec's melamine 303 resin can be used as this type of internal cross-linking agent.

Photocurable waterborne polyurethanes are usually composed of polyurethane micelles containing 2 or more CCs, photoinitiators, and photosensitizers. When the emulsion system is irradiated with light of a certain wavelength, it is decomposed to produce primary active species. , which in turn initiates the reaction of double bonds in the molecular chain to produce a cross-linked structure. Photo-initiated free radical polymerization can occur by different pathways. One is the initiation of polymerization by a photoreactive species generated by direct excitation of monomers or polymer molecules with chromophoric groups; second, photoinitiator molecules or photoactive initiators. Agent molecules that transfer energy to monomers or other molecules that can initiate reactive species, and then initiate polymerization by these reactive species; thirdly, they are photoexcited molecular complexes (mostly charge transfer complexes), and by stimulated molecular complexes. Dissociation produces reactive species such as free radicals to initiate polymerization.

Photoinitiators include organic carbonyl groups having chromophoric groups, azo compounds, sulfides, halogenated compounds, and halogen molecules. Their photosensitive wavelength region is generally 300 to 450 nm. Commonly used photoinitiators are benzophenone and rice. Anthranone (4,4-didimethylaminobenzophenone), benzoin, benzoin methyl ether, benzoin isopropyl ether, etc. The effect of adding a photosensitizer is usually to eliminate the oxygen inhibition effect in the air. Commonly used photosensitizers include tertiary amines such as triethylamine, triethanolamine and dimethylethanolamine, and benzoquinone [7]. Considering the need to use triethylamine as a salt-forming agent in the preparation of water-borne polyurethanes, it can also be uniformly dispersed in water. When it interacts with benzophenone, it can generate triethylamine radicals, which can absorb air. Oxygen in the air and generate new free radicals, thereby eliminating the oxygen in the air polymerization inhibitor, so in the synthesis of photo-curable waterborne polyurethane, benzophenone-triethylamine is often used as a composite photoinitiator. Shen Xiaofeng and Yu Jianhua [8] compared the effects of several different photoinitiation systems on the properties of emulsions and coatings. The results show that the waterborne polyurethanes using benzophenone-triethylamine initiator systems perform well in all aspects. The principle of preparation of the room-temperature-curing water-based polyurethane is mainly to use the emulsion to form a cross-linking reaction when the pH value of the system changes. For example, early-use carbodiimides, cyclonitrocyclopropanes, and azomethines, etc., use water-based polyurethane emulsions to reduce the pH of water when they are volatilized, and the use of the acid in the system catalyzes these internal cross-linking agents. Polyurethane macromolecules undergo cross-linking reactions. These internal cross-linking agents have a common feature, that is, they are stable under alkaline conditions, but they have high activity in an acidic environment and can react with hydroxyl, carboxyl, etc. on aqueous polyurethane resins. The hydrophilic group reacts so that the water resistance of the aqueous polyurethane coating film can be significantly improved while the crosslinked structure is formed. RicardG Coogan [5] studied the performance of water-curable polyurethanes containing aziridine and carbodiimide. The results showed that the degree of cross-linking of waterborne polyurethane coatings after the use of aziridine and carbodiimide was significantly improved. And it was proved that these internal cross-linking agents reacted with hydroxyl and carboxyl groups on the polyurethane molecular chain. Chen Guannan et al [9] systematically studied the effect of cyclopropane cyclopropane as an internal cross-linking agent on the properties of aqueous polyurethane dispersions and their coating properties. The results showed that the cross-linking reaction of the system was achieved by the change of pH value, and the reaction was a ring. Oxygen propane is opened with a carboxyl group on the polyurethane molecular chain. In addition to using the above principles to prepare room-temperature curing waterborne polyurethanes, Yu Qiqing [10] and others used semi-continuous seed emulsion polymerization technology to synthesize glycidyl methacrylate, hydroxyethyl methacrylate and dimethylamino methacrylate. The ethyl ester self-crosslinking emulsion at room temperature separates the glycidyl methacrylate and the dimethylaminoethyl methacrylate contained in the epoxy and tertiary amine groups by a special particle design method to make them in the emulsion. During the storage process, they do not meet each other, and the emulsions encounter a cross-linking reaction due to the deformation of the particles during the film formation process. That is, a room-temperature-curable waterborne polyurethane is prepared by a structural design method.

Oxygen-curing water-based polyurethanes use the organometallic catalysts added in the emulsion system to generate oxygen radicals with oxygen in the air. These peroxide groups decompose in the emulsion to generate active radicals, which in turn trigger the dryness of the system. Oils, self-drying alkyds, and other components contained double bonds and cross-linking reactions occurred.

1. 2 Diplomatic Law

The water-based polyurethane resin prepared by the diplomatic method is also called two-component waterborne polyurethane. Compared with single-component products, two-component waterborne polyurethanes generally prepared using the diplomatic alliance method have excellent mechanical properties, solvent resistance, weatherability, low-temperature film forming properties, etc., and are gradually used as substitutes for solvent-based polyurethane products in recent years. One of the major technological developments. Two-component water-based polyurethanes can be roughly classified into three types of polyisocyanate curing type, amine curing type, and epoxy curing type depending on the type of the curing agent.

The polyisocyanate-curable two-component waterborne polyurethane resin consists of an aqueous polyol system and a polyisocyanate curing agent. In general, in order to improve the dispersibility of the polyisocyanate curing agent when the two components are mixed, the aqueous polyurethane polyol system uses a second-generation aqueous hydroxyl resin [11], that is, hydrophilic groups and fragments are directly introduced into the molecular chain. Depending on the chemical structure, it can be divided into acrylic dispersion polyols, aqueous polyurethane dispersion polyols, and polyester dispersion polyols. These dispersion polyols have their own advantages and disadvantages. Among them, acrylic dispersion polyols have the advantages of small relative molecular mass and high hydroxyl content. Therefore, two-component waterborne polyurethanes prepared using acrylic dispersion polyols have water resistance and solvent resistance. , Weatherability, gloss and other advantages, the disadvantage is the long drying time; Polyester dispersion polyol prepared two-component waterborne polyurethane with fluidity, compatibility with pigments, gloss and other advantages, the disadvantage is the presence of ester bonds The product has poor water resistance, so it is often used in combination with an acrylic dispersion polyol, in which polyester is used as a soft segment and acrylic acid is used as a hard segment. This method can better improve the water resistance of the product. Polyurethane

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