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Water-based two-component polyurethane coatings use water as the dispersion medium to minimize the emission of VOCs, while maintaining the excellent construction process, decorative appearance, physical and mechanical properties and chemical resistance of solvent-based two-component polyurethane coatings.
The hot spots and development direction of coating research.
1 Components of water-based two-component polyurethane coatings Two-component cross-linked water-based polyurethane coatings, that is, the coating is divided into two components: one is a water-based polyol component; the other is a water-dispersed isocyanate component.
1 Water-based polyol components can be divided into two categories according to the colloidal structure of water-based polymers: one is water-dispersible polyol, and the other is emulsion polyol.
The preparation of water-based polyurethane polyols can use traditional water-based polyurethane dispersion preparation methods, but there should be more hydroxyl groups in the hydrophilic groups of the polymer, which can meet the reaction requirements of two-component coatings in terms of chemical structure.
The commonly used methods are the prepolymer method and the acetone method.
Compared with two-component solvent-based polyurethane coatings, two-component waterborne polyurethane coatings have good dispersibility in polyols, which can promote the stable dispersion of unmodified, hydrophobic polyisocyanate curing agents in water.
According to different preparation methods, water-based polyol systems can be divided into emulsion polyols and dispersion polyols.
2 Emulsion polyols The earliest polyols used are emulsion polyols, which are acrylic polyol latexes with multiple structures obtained by emulsion polymerization.
The advantage of its use in a two-component system is that the relative molecular weight of the polymer is large, and the coating film dries quickly at room temperature; the disadvantage is that it has poor dispersibility to the unmodified polyisocyanate curing agent, resulting in poor appearance of the coating film.
And the application period is short.
3 Dispersion polyols In order to improve the water dispersibility of the polyol system to the polyisocyanate curing agent and improve the performance of two-component waterborne polyurethane coatings, dispersion polyols, also known as second-generation water-based hydroxy resins, can be used.
More research.
The polyol resin is first synthesized in an organic solvent, and its molecular structure contains a hydrophilic ionizable group or a hydrophilic nonionic segment, and then the resin or solution is dispersed in water to obtain a dispersion polyol.
It is characterized by low relative molecular weight, small dispersion particle size (Dn=10～200nm), superior dispersibility to the curing agent, good appearance of the formed coating film, and excellent overall performance.
According to the different chemical structure, it can be divided into acrylic polyol dispersion, polyurethane polyol dispersion and polyester polyol dispersion.
4 Compared with acrylic polyol latex, acrylic polyol dispersion has a lower relative molecular mass (Mw=2000～10000), higher hydroxyl functionality, smaller particle size, and is very effective for curing agents.
It has good dispersibility, high degree of cross-linking of the coating film, and excellent overall performance, but the drying speed of the coating film is slower than the former.
5 Polyurethane Polyol Dispersion Polyurethane dispersion itself is a class of high-performance adhesives.
It is used to formulate two-component water-based coatings, with good film appearance, excellent mechanical properties, chemical resistance and abrasion resistance.
And according to actual needs, the coating film performance can be determined by adjusting the concentration of carbamate.
The preparation of polyurethane dispersions mostly use aliphatic isocyanates, which have better color retention and weather resistance than aromatic isocyanates, and their reactivity with water is much lower than aromatic isocyanates.
1,6-Hexane diisocyanate (HDI), isophordione diisocyanate (IPDI) and hydrogenated diphenylmethane diisocyanate (H12MDI) are currently the three most aliphatic isocyanates used by researchers.
6 Polyester Polyol Dispersion The two-component coating prepared by polyester polyol dispersion has the advantages of good fluidity, strong wetting to the pigment, and high gloss of the coating film.
However, the biggest drawback of the coating film prepared by this type of dispersion is poor hydrolysis resistance.
This is because the ester bonds of polyester polyols are easily hydrolyzed, which will cause the polymer chain to break, which in turn affects the performance of the coating film.
Consider grafting acrylic polymer onto the polyester molecular chain to prepare a polyester-acrylic composite polyol dispersion to improve the hydrolysis resistance of the polyester chain.
The two-component coating prepared by the polyol composite system organically combines the soft segment of the polyester and the hard segment of the acrylic resin, which is beneficial to the improvement of the hardness and flexibility of the coating film.
2 The latest development direction of water-based two-component polyurethane coatings 1 Nano-hybrid technology introduces nanoparticles (such as graphene, silver, phosphorus, sodium, silicon, zinc, titanium, etc.
) into the water-based two-component polyurethane coating system, which can significantly improve Certain functions of the coating film, such as heat resistance, corrosion resistance, antibacterial properties, combustion-supporting properties, chemical resistance, corrosion resistance, mechanical strength, etc.
2 Resin hyperbranched technology Hyperbranched resin presents a three-dimensional molecular structure.
Compared with traditional linear structure resins, it has the characteristics of ultra-high functionality, compact molecular structure, and less chain segment entanglement.
It has extremely high reactivity and good Rheology is very suitable for low-temperature baking or room temperature air-drying water-based two-component polyurethane coating system.
3 Resin hybridization technology Resin hybridization technology is to hybridize different types of resins to generate new resins with the advantages of the original resins to meet the needs of a wider range of applications.
Waterborne two-component polyurethane coatings have both environmental protection and high performance characteristics.
As the most important environmental protection coatings, in the past ten years, no matter it is the update of raw materials and formulations, or the production and construction of coatings, they have achieved great results.
The progress has been widely recognized by the market and has become the development trend of environmentally friendly coatings.
Due to the complex formulation design of water-based coatings, the difficulty of water-based coating construction, the immature scale of the water-based market, and the industry standards that have yet to be standardized, it is determined that water-based two-component polyurethane coatings still have a long way to go.
The hot spots and development direction of coating research.
1 Components of water-based two-component polyurethane coatings Two-component cross-linked water-based polyurethane coatings, that is, the coating is divided into two components: one is a water-based polyol component; the other is a water-dispersed isocyanate component.
1 Water-based polyol components can be divided into two categories according to the colloidal structure of water-based polymers: one is water-dispersible polyol, and the other is emulsion polyol.
The preparation of water-based polyurethane polyols can use traditional water-based polyurethane dispersion preparation methods, but there should be more hydroxyl groups in the hydrophilic groups of the polymer, which can meet the reaction requirements of two-component coatings in terms of chemical structure.
The commonly used methods are the prepolymer method and the acetone method.
Compared with two-component solvent-based polyurethane coatings, two-component waterborne polyurethane coatings have good dispersibility in polyols, which can promote the stable dispersion of unmodified, hydrophobic polyisocyanate curing agents in water.
According to different preparation methods, water-based polyol systems can be divided into emulsion polyols and dispersion polyols.
2 Emulsion polyols The earliest polyols used are emulsion polyols, which are acrylic polyol latexes with multiple structures obtained by emulsion polymerization.
The advantage of its use in a two-component system is that the relative molecular weight of the polymer is large, and the coating film dries quickly at room temperature; the disadvantage is that it has poor dispersibility to the unmodified polyisocyanate curing agent, resulting in poor appearance of the coating film.
And the application period is short.
3 Dispersion polyols In order to improve the water dispersibility of the polyol system to the polyisocyanate curing agent and improve the performance of two-component waterborne polyurethane coatings, dispersion polyols, also known as second-generation water-based hydroxy resins, can be used.
More research.
The polyol resin is first synthesized in an organic solvent, and its molecular structure contains a hydrophilic ionizable group or a hydrophilic nonionic segment, and then the resin or solution is dispersed in water to obtain a dispersion polyol.
It is characterized by low relative molecular weight, small dispersion particle size (Dn=10～200nm), superior dispersibility to the curing agent, good appearance of the formed coating film, and excellent overall performance.
According to the different chemical structure, it can be divided into acrylic polyol dispersion, polyurethane polyol dispersion and polyester polyol dispersion.
4 Compared with acrylic polyol latex, acrylic polyol dispersion has a lower relative molecular mass (Mw=2000～10000), higher hydroxyl functionality, smaller particle size, and is very effective for curing agents.
It has good dispersibility, high degree of cross-linking of the coating film, and excellent overall performance, but the drying speed of the coating film is slower than the former.
5 Polyurethane Polyol Dispersion Polyurethane dispersion itself is a class of high-performance adhesives.
It is used to formulate two-component water-based coatings, with good film appearance, excellent mechanical properties, chemical resistance and abrasion resistance.
And according to actual needs, the coating film performance can be determined by adjusting the concentration of carbamate.
The preparation of polyurethane dispersions mostly use aliphatic isocyanates, which have better color retention and weather resistance than aromatic isocyanates, and their reactivity with water is much lower than aromatic isocyanates.
1,6-Hexane diisocyanate (HDI), isophordione diisocyanate (IPDI) and hydrogenated diphenylmethane diisocyanate (H12MDI) are currently the three most aliphatic isocyanates used by researchers.
6 Polyester Polyol Dispersion The two-component coating prepared by polyester polyol dispersion has the advantages of good fluidity, strong wetting to the pigment, and high gloss of the coating film.
However, the biggest drawback of the coating film prepared by this type of dispersion is poor hydrolysis resistance.
This is because the ester bonds of polyester polyols are easily hydrolyzed, which will cause the polymer chain to break, which in turn affects the performance of the coating film.
Consider grafting acrylic polymer onto the polyester molecular chain to prepare a polyester-acrylic composite polyol dispersion to improve the hydrolysis resistance of the polyester chain.
The two-component coating prepared by the polyol composite system organically combines the soft segment of the polyester and the hard segment of the acrylic resin, which is beneficial to the improvement of the hardness and flexibility of the coating film.
2 The latest development direction of water-based two-component polyurethane coatings 1 Nano-hybrid technology introduces nanoparticles (such as graphene, silver, phosphorus, sodium, silicon, zinc, titanium, etc.
) into the water-based two-component polyurethane coating system, which can significantly improve Certain functions of the coating film, such as heat resistance, corrosion resistance, antibacterial properties, combustion-supporting properties, chemical resistance, corrosion resistance, mechanical strength, etc.
2 Resin hyperbranched technology Hyperbranched resin presents a three-dimensional molecular structure.
Compared with traditional linear structure resins, it has the characteristics of ultra-high functionality, compact molecular structure, and less chain segment entanglement.
It has extremely high reactivity and good Rheology is very suitable for low-temperature baking or room temperature air-drying water-based two-component polyurethane coating system.
3 Resin hybridization technology Resin hybridization technology is to hybridize different types of resins to generate new resins with the advantages of the original resins to meet the needs of a wider range of applications.
Waterborne two-component polyurethane coatings have both environmental protection and high performance characteristics.
As the most important environmental protection coatings, in the past ten years, no matter it is the update of raw materials and formulations, or the production and construction of coatings, they have achieved great results.
The progress has been widely recognized by the market and has become the development trend of environmentally friendly coatings.
Due to the complex formulation design of water-based coatings, the difficulty of water-based coating construction, the immature scale of the water-based market, and the industry standards that have yet to be standardized, it is determined that water-based two-component polyurethane coatings still have a long way to go.
