Technical Analysis of Waterborne Polyurethane Products in China

1. Characteristics of technical changes in waterborne polyurethane products

At present, most water-based PU is mainly prepared by self-emulsification method, with PU containing hydrophilic groups as the main curing component. The coating film is easy to swell when exposed to water. In addition, it lacks the cross-linking density and high relative molecular weight that can be obtained from two-component solvent-based PU coating films, so the water resistance, solvent resistance, heat resistance and gloss of these aqueous dispersion coating films are poor, seriously limited the scope of its use. Therefore, it is often used to increase the crosslinking density of the coating film to improve the water resistance of the emulsion coating film. There are two commonly used crosslinking methods: one is to add a polyol with a functionality greater than 2 when synthesizing the PU prepolymer to directly generate a crosslinked PU prepolymer, and disperse the above prepolymer in water , and extend the chain to form macromolecules, and finally form emulsions.

This method is also called the pre-crosslinking method. The disadvantage is that it is easy to increase the viscosity of the prepolymer, and it is difficult to disperse in water, which affects the stability of the emulsion. In recent years, the research on the screening and synthesis of new cross-linking agents and multifunctional chain extenders has been quite active, which has become one of the main ways to improve the physical and mechanical properties and water resistance of water-based PU. Another method is the external linking method, which uses carboxyl anionic PU emulsion for cross-linking. The cross-linking reaction occurs on the carboxyl group of the PU molecule. There are aziridine, carboimide and metal salt compounds, which are carried out at room temperature. crosslinking. This kind of crosslinking agent is generally added when using PU emulsion, because of its fast crosslinking reaction rate, it will produce gel in a short time and break the emulsion. The external linking method can successfully solve the hydrophilic problem of the PU emulsion coating film, but because of the inconvenience of construction due to the addition of a cross-linking agent to form a two-component coating agent, this method is rarely used.

2. New technology in waterborne polyurethane product market

At present, domestic and foreign research on water-based polyurethane is focused on its modification to make it functional, and the performance indicators such as water resistance and solvent resistance of the material can be increased through modification. Modification is mainly carried out by physical and chemical methods, such as grafting, block, internal and external linking with other polymer materials, blending or forming interpenetrating polymer networks.

Commonly used modifications are as follows:

1. Acrylate-modified polyacrylate products have the advantages of better weather resistance, water resistance, solvent resistance, and gloss retention than polyurethane resins, and are inferior to polyurethane resins in terms of physical and mechanical properties, elasticity, and adhesive properties. Therefore, the two are very complementary. The use of acrylates in the modification of water-based polyurethane emulsions is one of the current trends in the development of polyurethanes. Currently more popular are blending crosslinking reaction method, emulsion copolymerization method and composite emulsion polymerization method.

There are two processes in the composite emulsion polymerization method:

(1) Interpentrating Polymer Network. At least one part of the system is a cross-linked structure, which acts at the molecular level, such as using acrylate monomers as organic solvents for synthesizing polyurethane prepolymers, and then polymerizing in polyurethane emulsions to obtain acrylate-modified polyurethanes interpenetrating network emulsion.

(2) Add acrylate unsaturated monomers to the water-based polyurethane emulsion for free radical polymerization to form a so-called core-shell type acrylate-modified water-based polyurethane composite emulsion. Chen Yifang used acrylate monomer as polyurethane solvent to prepare acrylate-modified polyurethane emulsion with IPN structure. The research showed that the coating film had good water resistance and pollution resistance. Yang Jianwen et al. grafted acrylic resins with hydroxyl side groups with urethane acrylates containing residual isocyanate groups. After being neutralized by amines, they were dispersed with water to form a self-emulsifying aqueous system. Studies have shown that when the content of polyurethane in the grafted resin is 30% to 50%, the light-cured coating has better hardness, solvent resistance and water resistance.

2. Organosilicon-modified organosilicon compounds are semi-organic and semi-inorganic polymer compounds with heat resistance, water resistance, weather resistance and air permeability, and the two most notable features are oxidation resistance and low surface area. Yes, silicone polymers can also give the coating excellent flexibility and smooth silky feel; Si-O-Si molecular chains with microphase separation due to surface energy differences will migrate to the surface of the film to improve the overall performance of the coating film .

There are two main methods for modifying silicones containing amino groups:

(1) In the process of synthesizing the prepolymer, the silicone containing amino group is introduced into the polyurethane segment. Due to the outstanding reactivity of the amino group and the difference in solubility between silicone and polyurethane, the polymerization reaction needs to be carried out in a solvent , which not only makes solvent extraction difficult, but also causes environmental pollution, which limits their application.

(2) During the emulsification process of the prepolymer, the chain extension is introduced into the organic silicon containing amino groups. Studies have shown that siloxane is enriched on the surface of the film, which has an obvious surface modification effect on polyurethane materials, and the water resistance of the film is improved. Qing Ning et al. used organosilicon compounds to modify water-based polyurethane, and proved that the organosilicon segment was successfully connected to the water-based polyurethane segment by means of infrared and NMR; the amount of organosilicon compound increased, the water absorption rate of the latex film decreased, and the surface contact angle Increased, the water resistance, stability, flexibility, and aging resistance of the film have been significantly improved.

3. The structure of epoxy resin modified epoxy resin contains hydroxyl groups. This compound has strong bonding ability and moldIt has the characteristics of high weight and strength and good thermal stability. Synthetic reaction can occur directly with water-based polyurethane. Epoxy resin modification can improve the water resistance, solvent resistance, thermal creep resistance and tensile strength of polyurethane, and at the same time, it can increase the peel strength of the resin to the substrate. In the modification reaction, the branch point is introduced into the polyurethane main chain, so that the main chain part forms a network structure. In this reaction, both epoxy groups and hydroxyl groups participate in the reaction, and there is also a ring-opening reaction between carbamate and epoxy groups. . The appearance of the modified polyurethane emulsion changes from translucent to opaque as the epoxy value of the epoxy resin decreases, the film hardness and tensile strength of the modified polyurethane emulsion increase, the storage stability and elongation at break decrease, and the water resistance of the latex film enhanced. Because the epoxy value decreases, the molecular weight increases, and the carboxyl content increases, resulting in an increase in the crosslinked structure of water-based polyurethane and the content of rigid benzene rings in the molecular chain of water-based polyurethane, and the hardness, tensile strength and water resistance of the latex film are improved. At the same time, the elasticity and elongation at break of the latex film are reduced. After the molecular weight of the epoxy resin increases, the mass increases, and the hydrophilicity of the polyurethane, the transparency and the storage stability of the water-based polyurethane emulsion all decrease under the same circumstances. Guo Junjie et al. synthesized an epoxy resin-modified water-based polyurethane adhesive for bonding composite films. The modified adhesive showed strong bonding performance to various composite films, further improved peel strength, appearance, and storage stability. good. And it still has strong bonding performance after the solid mass fraction drops by 30%.

4. Cross-linking modification Cross-linking modification is to join the linear polyurethane macromolecules together through chemical bonds to obtain a polyurethane resin with a network structure. The waterborne polyurethane coating film after cross-linking modification has good water resistance, solvent resistance and mechanical properties. At present, the water-based polyurethane prepared by the mature cross-linking modification technology can reach or even surpass the solvent-based polyurethane resin in many properties. Cross-linking modification can be divided into internal cross-linking method and external cross-linking method according to different cross-linking methods. The polyurethane emulsion prepared by the internal crosslinking method is a one-component system, and the polyurethane emulsion prepared by the external crosslinking method is a two-component system. In the internal crosslinking reaction system, other components in the internal crosslinking agent emulsion system can coexist with the internal crosslinking agent and remain stable. Regardless of the cross-linking method used during cross-linking, the amount of cross-linking agent must be strictly controlled. Although the tensile strength, water resistance, and solvent resistance of the film increase with the increase of the amount of crosslinking agent, but if the amount is too large, the elongation of the film will decrease too much, and the particle size of the emulsion particles will become smaller. Large, poor fusion when forming a film, but reduces the strength of the film.

5. Nano-modified nanomaterials refer to materials with at least one dimension in the constituent phase or grain structure below 100 nm. Due to the very large interface area between nanomaterials and polymer molecules, coupled with the above-mentioned related properties of nanomaterials, there is a large interaction between the two interfaces, which has good bonding performance and better eliminates the interaction between inorganic materials and organic materials. The mismatch of thermal expansion coefficients between polymers enables them to be easily combined to form composite materials with excellent properties, such as: strong surface binding energy; strong adhesion after compounding with polymers ; Improve fluidity, increase surface hardness and wear resistance.

6. Other modification methods use natural polymers (such as lignin, starch, bark, etc.) The new type of polyurethane material prepared by the compound system has the properties of high stress, high hardness and low strain, and its physical and mechanical properties are better than that of polyurethane material synthesized by polyether triol as hydroxyl component.