The Mechanism and Research Progress of Rheology Additives for Waterborne Coatings

 0 The water
coating formulation system is more complex than solvent-based coatings. In formulation design, we should not only pay attention to the type and performance of water-based resins, but also the rational selection of various functional additives and their interaction. The fluidizer is a key component of the coating formulation, and the fluidity characteristic of the water-based coating is a complex relationship between viscosity and shear force, which determines the production, storage and construction performance of the coating .
commonly used in water-based coatings can be divided into organic and inorganic categories according to their chemical properties.
1 Inorger fluidizer
In the coatings industry, a class of special clays with silicate as the main component is commonly used as an inorient fluid changer. Most of these substances are supplied in the form of powders, which are better dispersed in coatings and can act as suspensions or gels. Inorristical fluid change agents tend to have high yield values and touch-change characteristics to improve the paint's anti-flow, anti-sinking, dehydration-resistant shrinkage and splash-resistant properties. Common clay-like fluid change agents are mainly montessite, sea bubbly and bumpstones, whose scanning electroscope photos and basic structure are shown in Figure 2.
Figure 2 Clay fluidizer scanning electroscope and basic structure diagram
1.1 montesite
montesite is a layered mineral composed of very fine particles of aquifer aluminum silicate, as shown in Figure 2 (a), the middle is aluminum oxygen octamen, the upper and lower silicate tectomes, forming a three-layer flaky structure. Bentographic soil is a typical clay mine with montessite as its main component. The layers of Montesser crystals contain water and cations, have a high ion exchange capacity, and therefore have a strong ability to absorb water expansion. In a water-based medium, the inter-layer cation of montessite is solventized and expands to separate the particles. At the same time, with the break of silicon and aluminum oxygen bonds to make its edges and surfaces charged, and the surface and edge of different electrostectants, the formation of a "house of cards"-like mesh structure. This unique three-dimensional structure can wrap water and pigments in it, so that the system is thickened and has good storage stability. When the system is subjected to strong shearing, the network structure dissociation, showing obvious shear thinning performance, so that the coating has a better leveling. Once the shear force disappears, the mesh structure can be restored under the effect of hydrogen bond, which plays a good anti-flow hanging performance.
in the static storage state of the coating, the column particles of this type of benthic soil are scattered among the flaky particles, which acts as a support. When affected by shear force, the hydrogen bond is broken and the column particles are arranged in the direction of flow. When the shear force disappears, the column particles can be rearranged quickly. Therefore, the introduction of this new type of bento-earth makes the coating have better storage stability and tentacles.
1.2 sea bubbly
sea bubbly is a fibrous magnesium silicate clay mineral with a huge surface and unique hole structure. As shown in Figure 2 (b), on a microstructure, the particles of sea bubbly appear like needle bundles with unequal shafts. The structural unit of sea foam contains a continuous silicate tetum layer, each silicon oxygen tetum shared 3 corner tops connected with the adjacent 3 tetums, forming an open channel of fixed size and parallel to the chain.
in the water-based system, sea foam particles disband to form an irregular fibrous mesh structure, including water, forming a high viscosity suspension and thickening the system. When affected by shear force, the network structure dissociation, viscosity decreased. Once the shearing effect disappears, the network structure is restored and the viscosity increases. This rheumving performance is affected by concentration, shear speed and pH. Because of its large hole structure, which allows it to hold more water molecules, the tentacles are very high, which is ideal for coating applications, such as in-tank anti-sinking effect and thick coating anti-flow hanging effect. The application of sea foam as a fluid thickener in water-based building coatings shows that the introduction of sea foam can not only effectively regulate the viscosity and leveling of the coating, but also improve the ratio and scrub resistance to a certain extent.
1.3 Bumpstone
Bump Rock is a crystalline hydrated magnesium aluminum silicate mineral with unique nano-bar structure characteristics as shown in Figure 2 (c). In crystallization, bump barstone and sea bubble stone crystals belong to the single oblique crystal system, but the hole width between the cell layers is different. After dispersion in the water medium, the bumpstone crystal breaks chemical bonds and hydrates, resulting in a small amount of surface charge. After the dissocation of the rod crystals through static electricity, Van der Huali, water force and a series of forces intertwined with each other, each other to form a messy three-dimensional mesh structure, so that the flow of water is bound. Moreover, because the rod crystal diameter is very small and has a strong adsorption ability, can absorb emulsion particles and pigment particles on the edge or endpoint of the rod crystal to form a moderate spatial network, a variety of particles filled in the bump stone crystal gap, so that the system viscosity increases, play a thickening role.
study found that the smaller the particle size of the bump bar stone, the shorter its crystal beam, the weaker the cross-tangle ability, and thus the smaller the strength of the mesh structure formed, resulting in lower fluidity performance. In addition, due to the low charge on the surface of the bumpstone, it is not significantly affected by electrolytes and pH. (Partly recommended for use in inorganic coatings) the study found that bumpstone and bentographic soil, gas silicon synergy, can effectively improve the water-based epoxy zinc-rich primer anti-flow hanging performance.
2 Organic fluidizers
Figure 3 contrast between the action modes of non-connective and connective fluid aids (yellow for emulsion particles)
organic fluidizers can be divided into non-connective and joint types according to their mode of action. As shown in Figure 3, non-connective fluidization aids are mainly expanded by absorbent water, the polymer chain winding and occupy a certain amount of space in the system, with volume rejection effect to play a thickening effect. The connective fluidization aid achieves thickening effect by combining its own hydrophobic group and collaborating with latex particles to form a three-dimensional mesh structure and restricting the free flow of latex particles.
2.1 Non-connective rheumatic aid
2.1.1 hydroxyethyl cellulose (HEC)
Figure 4 cellulose molecular structure schematic
hydroxyethyl cellulose (HEC) is a non-ionized soluble cellulose ether obtained by alkaline cellulose and ethylene oxide by etherization reaction, the chemical structure of which is shown in Figure 4, wherein the R-based is a hydrogen atom. In aqueous solution, the hydrophobic main chain of heC molecules and the surrounding water molecules cooperate with the surrounding water molecules through hydrogen bonds, absorbing a large amount of water causing volume expansion. At the same time, due to the high molecular weight of HEC, the molecular chain is entangled with each other, which makes the viscosity of the system increase.
, HEC is widely used as a thickener in water-based building coatings, and its thickening efficiency depends mainly on its molecular weight and the hydration capacity of polar groups. At the low shear rate, the cellulose molecular chain is in a disorderly winding state, which makes the system show high viscosity. At high shear rates, the orderly arrangement of molecules reduces the viscosity of the system. HEC thickening efficiency is high, the application of pH range is wide, anti-flow hanging performance is good. But at the same time, there are many defects, such as poor leveling, easy to splash and so on.
addition, because HEC belongs to natural polymer, the C-O-C glycoside bond between polysaccharides is susceptible to mold erosion and hydrolysis, resulting in chain breakage and loss of action. Usually a certain amount of fungicides are added to the water-based latex paint formulation to prevent corruption and metastase, but once cellulose hydrolyzed enzymes are produced, the effect of the fungicides on them is not obvious.
2.1.2 Alkaline swelling acrylic emulsion (ASE)
Figure 5 Alkaline swelling thickener protonization process
Alkaline swelling acrylic emulsion (ASE) is a water dispersion of acrylic co-polymers, usually composed of methyl acrylic and acrylic ethyl esters. As shown in Figure 5, the ASE is tightly coiled and curled under acidic conditions. When alkaline neutralizers are added, the acidic group in the ASE molecule is dissopheded, and the pyrethrination causes the polymer's chain to extend under the effect of electrostatic rejection. With the further increase of pH, more free long chain structure is produced, and the viscosity of the system is increased by winding. ASE thickener mainly improves medium and low shear viscosity, and its yield value and tentacle are higher.
ASE has good compatible with various emulsion systems, and bioenzyme degradation is not easy. However, in the process of use, it may occur that the initial chain is not fully stretched and the phenomenon of late thickening occurs, resulting in unstable viscosity of the system. Therefore, the addition of a moderate amount of alkaline neutralizer makes it more critical to maintain the pH of the system at 8 to 10, and should track the pH and viscosity changes of the system after the test coating has been stored at room temperature and heat storage for a period of time
Agent
2.2.1 Hydrophobic modified hydroxyethyl cellulose (HMHEC)
can obtain the hydrophobic modified hydroxyethyl cellulose (HMHEC) by chemical reaction between the hydrophobic modified reagent and the side chain of the HEC. The chemical structure of HMHEC is shown in Figure 4, but at this time the R-base is a hydrophobic alkyl chain. HMHEC is able to thicken water through hydrogen bonds and tangles in molecular chains, just like conventional HECs, and in addition to thickening coatings through the self-connectivity of side-chain hydrophobic groups and with other components of the coating, such as emulsion particles.
6HMHEC and HEC aqueous solution rheumving curves
Figure 6 shows the rheumving curves of HEC and HMHEC aqueous solutions. As can be seen from Figure 6, both HEC and HMHEC have the characteristics of shear thinning. The difference is that, at stationary or low shear rates, HMHEC double thickening effect makes it have a high viscosity, and with the increase of shear rate, the relatively weak hydrophobic joint formation of the mesh structure is destroyed, so the viscosity is rapidly reduced, and then as stable as HEC. In addition, when the shear rate is reduced, HMHEC molecules again form a molecular mesh structure and increase the viscosity, but there is a certain lag. This is due to the reversible balance of mesh structure bonding and dissocing has a certain time dependence, which is characterized by tact denatured fluid. Compared to HEC, HMHEC provides better fluidity and splash resistance for water-based coatings.
2.2.2 Hydrophobic modified alkali-soluble acrylic emulsion (HASE)
Hydrophobic modified alkali-soluble acrylic emulsion (HASE) is the introduction of hydrophobic acrylic monosomes in the chemical structure of ASE, which, like ASE, belong to anion-type thickener.
HASE basically retains the characteristics of ASE, such as pH dependence, easy dispersion, anti-microbial degradation and so on. At the same time, due to the introduction of hydrophobic structure, HASE can thicken the system by winding the polymer chain under alkaline conditions, and the long chain of hydrophobic groups in the water phase gathered together to form a beam structure and the molecules interconnected with each other, but also with latex particles, pigment particles and surfactants to form a three-dimensional network structure, so thickening efficiency is higher, better infringing. In water-based coating formulation, HASE is more sensitive to the use of dispersants, often with hydrophobic polyic acid copolymer dispersants there is a competitive adsorption relationship, resulting in pigment flocculation. Therefore, HASE is more suitable for use with polyic acid oxides and hydro-hydroploric acid co-dispersants.
addition, both ASE and HASE require pre-dilution or pre-neutralization during use to avoid gels or flocculation when mixing their highly acidic and water-based formulation systems. In addition, alkali-soluble thickeners are also sensitive to electrolytes and have poor alkaline resistance to water.
2.2.3 Hydrophobic modified ethylene oxide polyurethane (HEUR)
Hydrophobic modified ethylene oxide polyurethane (HEUR) is a hydrophobic-modified ethyl polyurethane water-soluble polymer, which belongs to the non-ionized combined thickener.
Figure 7 HEUR Molecular Structure Diagram
Figure 7 shows the composition of HEUR molecules, characterized by "pro-oil-hydro-pro-oil" three-inlay polymer, with hydrophobic fatty hydrocarbon base at both ends and hydrophobic polyglycol chain in the middle, which is extended by isocyanate. HEUR thickens the water-based system by combining its hydrophobic end with latex particles, pigments and surfactant hydrophobic structures to form a three-dimensional mesh structure, and the hydrophobic chain can also act with water molecules with hydrogen bonds to produce a thickening effect. Therefore, when the HYDRO molecule hydrophobic is strong, it shows the characteristics of false plastic shape, which significantly improves the medium and low shear viscosity while increasing the high shear viscosity. When the hydrophobicity is weak, it shows Newtonian characteristics, which mainly contribute to high shear viscosity.
HEUR molecules in the high shear force with latex particles to improve the surface viscosity of the water system, so that the coating film has a better fullness, and shear force reduced, the system three-dimensional mesh structure recovery makes the coating has a better leveling. Because of the low MOLECULAR weight of HEUR, the coating is not easy to produce splashes during the construction process. It is contracted with latex particles and does not produce volume-limiting flocculation with a high gloss. However, the inter-molecular winding in the water phase is limited, so the water thickening effect is insufficient, anti-flow hanging can not meet the requirements.
In addition, HEUR is also sensitive to dispersants, when HEUR is used with high acid content dispersants or small molecule dispersants, due to the dispersant water cooperation is stronger, resulting in water and these ions binding, and from the ethylene oxide skeleton, resulting in HEUR and the system's compatible difference, contracting cooperation is weakened. Therefore, HEUR should be used with polysicide dispersants with low acid content.
, given the complexity of water-based coating formulations, a single type of fluidizer often does not meet its fluid performance requirements. Therefore, the collaborative application of fluidized additives will complement each other and improve the production, storage and construction performance of water-based coatings.
3 Conclusion
the dual guidance of national consciousness and policy development promotes the rapid development of water-based coatings. With the continuous extension of water-based coating application system, as an indispensable part of water-based coatings, fluidizers also show diversified development. For different applications, it is very important to select the right fluidizer to give the coating specific fluid properties. In the future, environmentally friendly, functional fluidizers will be more popular.
Ji Xinghong
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