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Water based epoxy coatings are widely used in steel structure anti-corrosion, industrial equipment coating, floor protection, building waterproofing and other fields due to their advantages of low VOC, environmental safety, excellent adhesion, outstanding chemical resistance, and wide substrate adaptability. However, pure bisphenol A-type waterborne epoxy has high cross-linking density, strong molecular chain rigidity, and a brittle cross-linking network. The cured paint film generally has defects such as low elongation at break, poor impact resistance, easy bending and cracking, and obvious low-temperature brittleness. Under conditions of temperature difference alternation, external impact, and substrate micro deformation, the paint film is prone to cracking, peeling, and detachment, greatly reducing the long-term protection life of the coating. Traditional toughening methods such as nitrile rubber, ordinary polyurethane lotion and small molecule flexible additives generally have problems such as poor compatibility, easy phase separation, migration and precipitation of additives, reduction of film hardness and water resistance, and severe attenuation of heat resistance, which make it difficult to achieve high-performance balance modification.

Epoxy terminated polyurethane (EPU) toughening agent is a new type of reactive covalent crosslinking toughening material, with a molecular structure that combines flexible polyurethane segments and epoxy active end groups, distinguishing it from traditional physical blending toughening agents. Its epoxy end group can undergo synchronous ring opening crosslinking reaction with the amino group of the water-based epoxy system curing agent, covalently connecting to the three-dimensional network of the paint film. The polyurethane flexible chain segment is embedded in situ inside the crosslinked structure, achieving efficient toughening without damaging the overall density and stability of the paint film, while avoiding the disadvantages of small molecule additive migration, precipitation, and weather resistance decline. This article systematically elaborates on the molecular structure characteristics and toughening mechanism of end epoxy based polyurethane toughening agents, comprehensively explores their effects on the mechanical properties, microstructure, adhesion, water resistance, corrosion resistance, thermal stability, and construction and storage performance of waterborne epoxy coatings, clarifies the balance relationship between addition amount and performance, and the optimal application process, providing theoretical support for the formulation development and engineering application of high-performance toughening waterborne epoxy coatings.

Structure and toughening mechanism of epoxy based polyurethane toughening agent

The end epoxy based polyurethane toughening agent uses polyether/polyester polyols as flexible soft segments, isocyanates as hard segment skeleton, and epoxy groups as capping active functional groups to form a block structure of “flexible main chain+active end groups”. The soft segment polyurethane chain has high elasticity and elongation characteristics, which can provide excellent deformation buffering ability; The hard segment structure ensures the cohesive strength of molecules, avoiding the softening and significant decrease in modulus of the toughened paint film; The terminal epoxy group has completely homogeneous reactivity with the matrix resin and can participate in the curing and crosslinking of waterborne epoxy, achieving chemical bonding toughening rather than physical doping toughening.

During the curing process of waterborne epoxy, EPU toughening agent synchronously grafts, crosslinks, and entangles with the matrix epoxy resin, forming a uniform nanoscale microphase separation structure in situ inside the paint film, constructing a composite network of “rigid epoxy matrix+flexible polyurethane micro area”. The core toughening mechanism mainly includes four types of synergistic energy dissipation mechanisms: firstly, flexible polyurethane micro areas undergo elastic deformation under external force, absorbing a large amount of impact energy; Secondly, the nano microphase region can effectively pin cracks, deflect crack propagation paths, and delay crack propagation; Thirdly, during the stress process, the flexible chain segment undergoes voiding, shear yielding, and dissipates fracture energy; Fourthly, covalently bonded flexible networks can effectively disperse stress and avoid brittle fracture caused by stress concentration. Compared with the problems of macroscopic phase separation and multiple interface defects in traditional rubber toughening, EPU can achieve micro uniform toughening, while enhancing toughness and maximizing the retention of film strength, hardness, and medium resistance.

Influence on the mechanical properties of waterborne epoxy coatings

Mechanical properties are the most intuitive and core dimension for EPU toughening modification, which can significantly improve the brittleness, weak impact resistance, and easy cracking of waterborne epoxy paint films, achieving synergistic optimization of toughness and strength. Unmodified pure water-based epoxy paint film has high cross-linking rigidity, typical brittle fracture mode, low elongation at break, poor impact resistance, and is prone to direct cracking and peeling under external forces. After introducing an appropriate amount of epoxy based polyurethane toughening agent, the flexible segments are evenly distributed inside the cross-linked network, which can effectively buffer external impact and release internal stress in the coating.

With a moderate increase in the amount of EPU added, the positive and negative impact strength, elongation at break, and bending performance of the paint film are significantly improved. Related experiments have shown that moderate EPU modification can increase the elongation at break of waterborne epoxy paint films by more than 190%, improve impact resistance by 150%, and eliminate cracks during low-temperature bending, completely solving the problems of low-temperature brittleness and external cracking of pure epoxy paint films. At the same time, due to the covalent bonding of EPU into the paint film network, there are no interface peeling defects, and the tensile strength and bending strength of the paint film do not show significant attenuation, even achieving a slight improvement due to stress dispersion uniformity, which is different from the common weakness of traditional toughening agents that “toughening must reduce strength”.

However, there is a strict optimal range for the amount of EPU added: when the amount is too low, the number of flexible micro zones is insufficient, the toughening effect is weak, and the brittleness of the paint film cannot be improved; When the addition amount is too high, the proportion of flexible chain segments in the system is too large, resulting in a decrease in the overall cross-linking density and rigidity of the paint film. This can lead to a decrease in the hardness of the paint film, a deterioration in scratch resistance, and a decrease in modulus, causing an imbalance in mechanical properties. The optimal mass fraction of EPU added in the industrial formula system is 5% to 15%, which can achieve a balance between strength and toughness.

The influence on the microstructure and interface structure of paint film

Pure water-based epoxy curing has a smooth and flat cross-section with a single texture, presenting a typical brittle fracture microstructure. The crack propagation is straight and there is no energy dissipation structure, and the fracture fails instantly. After adding end epoxy based polyurethane toughening agent, a large number of wrinkles, tough dimples, concave convex tearing textures appeared on the surface of the paint film, and the fracture mode changed from brittle fracture to ductile fracture. The uniformly distributed nano polyurethane microphase region can effectively block crack propagation, making the crack propagation path convoluted and complex, greatly increasing the energy required for fracture.

At the same time, EPU has excellent compatibility with water-based epoxy matrix, with no macroscopic phase separation or agglomeration precipitation. The microstructure of the paint film is uniform and dense, and there are no interface pores, defects, or gaps that traditional rubber toughening may cause. This uniform nanocomposite structure not only ensures the overall density of the paint film, but also endows the coating with elastic deformation ability, which is the micro basis for synchronously optimizing the mechanical and anti-corrosion properties of the paint film. Excessive addition of EPU can lead to the aggregation of microphase regions, a decrease in interfacial bonding strength, and the appearance of defect pores in the cross-section, which in turn reduces the overall performance of the paint film.

The influence of paint film adhesion and substrate compatibility

The end epoxy based polyurethane toughening agent can significantly optimize the interfacial adhesion performance of waterborne epoxy coatings to metal substrates. On the one hand, polar amino ester groups in EPU molecules can form hydrogen bonds with hydroxyl and oxidation sites on the surface of metal substrates, enhancing interfacial adhesion; On the other hand, flexible chain segments can effectively release the curing shrinkage stress of the paint film, significantly reduce the volume shrinkage defects during the curing process of the epoxy system, and reduce the problems of interface stress concentration, paint film warping, and edge peeling.

The modified paint film significantly improves the adhesion level to conventional industrial substrates such as carbon steel, galvanized sheet, and aluminum alloy, especially suitable for coating steel structural equipment that is prone to deformation and vibration. When the substrate undergoes slight elastic deformation, the flexible paint film can synchronously buffer the deformation, avoiding cracking and delamination of the paint film due to substrate deformation, greatly improving the service stability of the coating under dynamic working conditions.

The impact on water resistance, salt spray resistance, and anti-corrosion performance

Traditional flexible toughening modification generally has the shortcoming of “toughening must reduce water resistance and corrosion resistance”. The migration and enrichment of flexible additives, the increase of interface defects, and the decrease in film density can lead to the easy penetration of corrosive media and the deterioration of salt spray resistance. And the end epoxy based polyurethane toughening agent, with its reactive crosslinking properties, can effectively avoid this problem. EPU fully participates in curing crosslinking, without the migration and precipitation of free small molecules, and will not form hydrophilic channels or permeation defects inside the paint film; At the same time, the uniform nano microphase structure can fill the tiny pores of epoxy curing, improving the density and shielding performance of the paint film.

The water absorption rate of waterborne epoxy paint film modified with appropriate EPU is significantly reduced, and its resistance to boiling and moisture is improved. It is not easy to whiten, bubble, or peel off in long-term humid environments. The salt spray test results show that the foaming rate and corrosion rate of the anti-corrosion coating with reasonable addition of EPU are much lower than those of the unmodified system, and the long-term anti-corrosion stability is excellent. However, excessive addition of EPU will reduce the cross-linking density of the system, and the hydrophilicity of polyurethane soft segments is relatively high, which will lead to a slight decrease in the water resistance and chemical resistance of the paint film. Therefore, the upper limit of EPU addition needs to be strictly controlled for anti-corrosion water-based epoxy coatings.

The impact on thermal stability performance

The glass transition temperature (Tg) and thermal decomposition stability are the core indicators of the service temperature range of waterborne epoxy coatings. Pure rigid epoxy paint film has a high Tg, but it is brittle, sensitive to thermal stress, and prone to cracking under cold and hot cycles. After introducing an appropriate amount of EPU, the flexibility of the cross-linked network of the paint film is improved, the internal stress is significantly reduced, and the anti cracking performance under cold and hot alternating working conditions is significantly optimized. At the same time, due to the covalent bonding between EPU and the matrix, there was no macroscopic phase separation, and the overall thermal stability of the system did not significantly decrease. The Tg only fluctuated slightly, which was much better than ordinary polyurethane and nitrile rubber toughening systems.

Excessive addition of EPU can lead to a decrease in the proportion of rigid structures in the system, enrichment of flexible segments, significant reduction in Tg of the paint film, and deterioration of high-temperature heat resistance. Under high-temperature conditions, problems such as softening, dust adhesion, and decreased scratch resistance of the paint film are prone to occur. Therefore, high temperature service industrial coatings need to strictly control the amount of EPU added to achieve a balance between toughness and heat resistance.

The impact on the construction and storage performance of coatings

The end epoxy based polyurethane toughening agent is a reactive modified component with excellent storage stability at room temperature. It does not react prematurely with the water-based epoxy A component, and there are no abnormalities in the storage period and open state of the coating. There are no issues of delamination, flocculation, thickening, or deterioration. After adding EPU, the rheological properties of the coating system are mild, the construction viscosity is moderate, and the adaptability of spraying, brushing, and roller coating is good. The paint film has excellent leveling, high flatness, and no orange peel defects.

Compared with toughening systems such as hyperbranched polymers and liquid rubber, EPU modified waterborne epoxy coatings have higher construction fault tolerance, are less prone to cracking when applied thick, have stable surface drying rates, and are suitable for industrial assembly line batch construction, making them more practical for engineering.

The relationship between existing shortcomings and performance balance in application

Although the comprehensive modification advantages of epoxy based polyurethane toughening agents are significant, there are still clear performance balancing rules in actual formulation applications. Firstly, there is a balance between the improvement of toughness, hardness, and heat resistance. The higher the amount added, the better the toughness, but the rigidity, modulus, and heat resistance of the paint film gradually decrease; Secondly, the water resistance shows a trend of first stabilizing and then decreasing with the increase of the proportion of flexible components, and excessive addition can easily cause high water absorption; Thirdly, the cost of EPU is higher than that of ordinary toughening agents, and there is a certain cost pressure for large-scale application; Fourthly, uneven dispersion and local agglomeration can easily lead to uneven microstructure of the paint film, resulting in differences in local mechanical properties.

Optimize application strategy

Based on the performance balancing characteristics of EPU modified waterborne epoxy, the optimal comprehensive performance can be achieved through precise regulation: firstly, the addition range is strictly controlled, and the anti-corrosion industrial waterborne epoxy is preferably added at a dosage of 5% to 12%, taking into account toughness, hardness, and corrosion resistance; The second is to optimize the feeding process, using low-speed stirring and uniform dilution addition to ensure that EPU is evenly dispersed in the system; Thirdly, it can be co modified with a small amount of nano fillers to enhance the rigidity and density of the paint film while toughening, achieving a three in one combination of strength, toughness, and corrosion resistance; Fourthly, according to the differentiated selection of service conditions, the EPU dosage should be moderately increased for low temperature and vibration conditions, and the upper limit should be strictly controlled for high temperature and high water resistance conditions.

Summary

The end epoxy based polyurethane toughening agent, with the unique structural advantages of epoxy active end group crosslinking and polyurethane flexible block, can achieve reactive, non migratory, nano microphase separation and efficient toughening in water-based epoxy systems, effectively solving the industry pain points of traditional water-based epoxy paint films such as high brittleness, poor impact resistance, easy cracking, and low-temperature failure. Moderate EPU modification can significantly improve the fracture elongation, impact strength, flexural toughness, and interfacial adhesion of the paint film, while maintaining excellent density, water resistance, salt spray resistance, and thermal stability of the paint film, avoiding the defects of performance degradation of traditional toughening agents. By precisely controlling the addition amount, optimizing the formula and process, the synergistic optimization of the mechanical properties, anti-corrosion properties, and construction performance of water-based epoxy coatings can be achieved. It has broad application prospects in high-performance water-based epoxy systems such as industrial anti-corrosion, equipment coating, and floor protection.

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