1. Types of thickeners and thickening mechanism
(1) Inorganic thickener:
Inorganic thickeners in water-based systems are mainly clays. Such as: bentonite. Kaolin and diatomaceous earth (the main component is SiO2, which has a porous structure) are sometimes used as auxiliary thickeners for thickening systems because of their suspension properties. Bentonite is more widely used because of its high water-swellability. Bentonite (Bentonite), also known as bentonite, bentonite, etc., the main mineral of bentonite is montmorillonite containing a small amount of alkali and alkaline earth metal hydrous aluminosilicate minerals, belonging to the aluminosilicate group, its general chemical formula is : (Na,Ca)(Al,Mg)6(Si4O10)3(OH)6•nH2O. The expansion performance of bentonite is expressed by expansion capacity, that is, the volume of bentonite after swelling in dilute hydrochloric acid solution is called expansion capacity, expressed in ml/gram. After the bentonite thickener absorbs water and swells, the volume can reach several times or ten times that before absorbing water, so it has good suspension, and because it is a powder with a finer particle size, it is different from other powders in the coating system. The body has good miscibility. In addition, while producing suspension, it can drive other powders to produce a certain anti-stratification effect, so it is very helpful to improve the storage stability of the system.
But many sodium-based bentonites are transformed from calcium-based bentonite through sodium conversion. At the same time of sodiumization, a large number of positive ions such as calcium ions and sodium ions will be produced. If the content of these cations in the system is too high, a large amount of charge neutralization will be generated on the negative charges on the surface of the emulsion, so to a certain extent, It may cause side effects such as swelling and flocculation of the emulsion. On the other hand, these calcium ions will also have side effects on the sodium salt dispersant (or polyphosphate dispersant), causing these dispersants to precipitate in the coating system, eventually leading to the loss of dispersion, making the coating thicker, thicker or even thicker. Severe precipitation and flocculation occurred. In addition, the thickening effect of bentonite mainly relies on the powder to absorb water and expand to produce suspension, so it will bring a strong thixotropic effect to the coating system, which is very unfavorable for coatings that require good leveling effects. Therefore, bentonite inorganic thickeners are rarely used in latex paints, and only a small amount is used as thickeners in low-grade latex paints or brushed latex paints. However, in recent years, some data have shown that Hemmings’ BENTONE®LT. organically modified and refined hectorite has good anti-sedimentation and atomization effects when applied to latex paint airless spraying systems.
(2) Cellulose ether:
Cellulose ether is a natural high polymer formed by the condensation of β-glucose. Using the characteristics of the hydroxyl group in the glucosyl ring, cellulose can undergo various reactions to produce a series of derivatives. Among them, esterification and etherification reactions are obtained. The cellulose ester or cellulose ether derivatives are the most important cellulose derivatives. Commonly used products are carboxymethyl cellulose, hydroxyethyl cellulose, methyl cellulose, hydroxypropyl methyl cellulose and so on. Because carboxymethyl cellulose contains sodium ions that are easily soluble in water, it has poor water resistance, and the number of substituents on its main chain is small, so it is easily decomposed by bacterial corrosion, reducing the viscosity of the aqueous solution and making it smelly, etc. Phenomenon, rarely used in latex paint, generally used in low-grade polyvinyl alcohol glue paint and putty. The water dissolution rate of methylcellulose is generally slightly lower than that of hydroxyethylcellulose. In addition, there may be a small amount of insoluble matter during the dissolution process, which will affect the appearance and feel of the coating film, so it is rarely used in latex paint. However, the surface tension of methyl aqueous solution is slightly lower than that of other cellulose aqueous solutions, so it is a good cellulose thickener used in putty. Hydroxypropyl methylcellulose is also a cellulose thickener widely used in the field of putty, and is now mainly used in cement-based or lime-calcium-based putty (or other inorganic binders). Hydroxyethyl cellulose is widely used in latex paint systems because of its good water solubility and water retention. Compared with other celluloses, it has less effect on the coating film performance. The advantages of hydroxyethyl cellulose include high pumping efficiency, good compatibility, good storage stability, and good pH stability of viscosity. The disadvantages are poor leveling fluidity and poor splash resistance. In order to improve these shortcomings, hydrophobic modification has appeared. Sex-associated hydroxyethylcellulose (HMHEC) such as NatrosolPlus330, 331
(3) Polycarboxylates:
In this polycarboxylate, the high molecular weight is a thickener, and the low molecular weight is a dispersant. They mainly adsorb water molecules in the main chain of the system, which increases the viscosity of the dispersed phase; in addition, they may also be adsorbed on the surface of latex particles to form a coating layer, which increases the particle size of the latex, thickens the hydration layer of the latex, and increases the viscosity of the internal phase of the latex. However, this type of thickener has a relatively low thickening efficiency, so it is gradually eliminated in coating applications. Now this kind of thickener is mainly used in the thickening of color paste, because its molecular weight is relatively large, so it is helpful to the dispersibility and storage stability of color paste.
(4) Alkali-swellable thickener:
There are two main types of alkali-swellable thickeners: ordinary alkali-swellable thickeners and associative alkali-swellable thickeners. The biggest difference between them is the difference in the associated monomers contained in the main molecular chain. Associative alkali-swellable thickeners are copolymerized with associative monomers that can adsorb each other in the main chain structure, so after ionization in aqueous solution, intra-molecular or inter-molecular adsorption can occur, causing the viscosity of the system to rise rapidly.
a. Ordinary alkali-swellable thickener:
The main product representative type of ordinary alkali-swellable thickener is ASE-60. ASE-60 mainly adopts the copolymerization of methacrylic acid and ethyl acrylate. During the copolymerization process, methacrylic acid accounts for about 1/3 of the solid content, because the presence of carboxyl groups makes the molecular chain have a certain degree of hydrophilicity, and neutralizes the salt-forming process. Due to the repulsion of charges, the molecular chains are expanded, which increases the viscosity of the system and produces a thickening effect. However, sometimes the molecular weight is too large due to the action of the cross-linking agent. During the expansion process of the molecular chain, the molecular chain is not well dispersed in a short period of time. During the long-term storage process, the molecular chain is gradually stretched, which brings Post-thickening of viscosity. In addition, because there are few hydrophobic monomers in the molecular chain of this kind of thickener, it is not easy to generate hydrophobic complexation between molecules, mainly to make intramolecular mutual adsorption, so this kind of thickener has low thickening efficiency, so it is rarely used alone. It is mainly used in combination with other thickeners.
b. Association (concord) type alkali swelling thickener:
This kind of thickener now has many varieties because of the selection of associative monomers and the design of molecular structure. Its main chain structure is also mainly composed of methacrylic acid and ethyl acrylate, and the associative monomers are like antennae in the structure, but only a small amount of distribution. It is these associative monomers like octopus tentacles that play the most important role in the thickening efficiency of the thickener. The carboxyl group in the structure is neutralized and salt-forming, and the molecular chain is also like an ordinary alkali-swellable thickener. The same charge repulsion occurs, so that the molecular chain unfolds. The associative monomer in it also expands with the molecular chain, but its structure contains both hydrophilic chains and hydrophobic chains, so a large micellar structure similar to surfactants will be generated in the molecule or between molecules. These micelles are produced by the mutual adsorption of association monomers, and some association monomers adsorb each other through the bridging effect of emulsion particles (or other particles). After the micelles are produced, they fix the emulsion particles, water molecule particles or other particles in the system in a relatively static state just like the enclosure movement, so that the mobility of these molecules (or particles) is weakened and the viscosity of the system increases. Therefore, the thickening efficiency of this type of thickener, especially in latex paint with high emulsion content, is far superior to that of ordinary alkali-swellable thickeners, so it is widely used in latex paint. The main product representative The type is TT-935.
(5) Associative polyurethane (or polyether) thickening and leveling agent:
Generally, thickeners have very high molecular weight (such as cellulose and acrylic acid), and their molecular chains are stretched in aqueous solution to increase the viscosity of the system. The molecular weight of polyurethane (or polyether) is very small, and it mainly forms an association through the interaction of the van der Waals force of the lipophilic segment between molecules, but this association force is weak, and the association may be made under certain external force. Separation, thereby reducing the viscosity, is conducive to the leveling of the coating film, so it can play the role of leveling agent. When the shear force is eliminated, it can quickly resume association, and the viscosity of the system rises. This phenomenon is beneficial to reduce viscosity and increase leveling during construction; and after the shear force is lost, the viscosity will be restored immediately to increase the thickness of the coating film. In practical applications, we are more concerned about the thickening effect of such associative thickeners on polymer emulsions. The main polymer latex particles also participate in the association of the system, so that this kind of thickening and leveling agent also has a good thickening (or leveling) effect when it is lower than its critical concentration; when the concentration of this kind of thickening and leveling agent When it is higher than its critical concentration in pure water, it can form associations by itself, and the viscosity rises rapidly. Therefore, when this kind of thickening and leveling agent is lower than its critical concentration, because the latex particles participate in partial association, the smaller the particle size of the emulsion, the stronger the association, and its viscosity will increase with the increase of the amount of emulsion. In addition, some dispersants (or acrylic thickeners) contain hydrophobic structures, and their hydrophobic groups interact with those of polyurethane, so that the system forms a large network structure, which is conducive to thickening.
2. Effects of different thickeners on water separation resistance of latex paint
In the formulation design of water-based paints, the use of thickeners is a very important link, which is related to many properties of latex paints, such as construction, color development, storage and appearance. Here we focus on the impact of the use of thickeners on the storage of latex paint. From the above introduction, we can know that bentonite and polycarboxylates: thickeners are mainly used in some special coatings, which will not be discussed here. We will mainly discuss the most commonly used cellulose, alkali swelling, and Polyurethane (or polyether) thickeners, alone and in combination, affect the water separation resistance of latex paints.
Although thickening with hydroxyethyl cellulose alone is more serious in water separation, it is easy to stir evenly. Single use of alkali swelling thickening has no water separation and precipitation but serious thickening after thickening. Single use of polyurethane thickening, although water separation and post-thickening The thickening is not serious, but the precipitate produced by it is relatively hard and difficult to stir. And it adopts hydroxyethyl cellulose and alkali swelling thickening compound, no post-thickening, no hard precipitation, easy to stir, but there is also a small amount of water. However, when hydroxyethyl cellulose and polyurethane are used to thicken, the water separation is the most serious, but there is no hard precipitation. Alkali-swellable thickening and polyurethane are used together, although the water separation is basically no water separation, but after thickening, and the sediment at the bottom is difficult to stir evenly. And the last one uses a small amount of hydroxyethyl cellulose with alkali swelling and polyurethane thickening to have a uniform state without precipitation and water separation. It can be seen that in the pure acrylic emulsion system with strong hydrophobicity, it is more serious to thicken the water phase with hydrophilic hydroxyethyl cellulose, but it can be easily stirred evenly. The single use of hydrophobic alkali swelling and polyurethane (or their compound) thickening, although the anti-water separation performance is better, but both thicken afterward, and if there is precipitation, it is called hard precipitation, which is difficult to stir evenly . The use of cellulose and polyurethane compound thickening, because of the farthest difference in hydrophilic and lipophilic values, results in the most serious water separation and precipitation, but the sediment is soft and easy to stir. The last formula has the best anti-water separation performance due to a better balance between hydrophilic and lipophilic. Of course, in the actual formula design process, the types of emulsions and wetting and dispersing agents and their hydrophilic and lipophilic values should also be considered. Only when they reach a good balance can the system be in a state of thermodynamic equilibrium and have a good Water resistance.
In the thickening system, the thickening of the water phase is sometimes accompanied by the increase of the viscosity of the oil phase. For example, we generally believe that cellulose thickeners thicken the water phase, but the cellulose is distributed in the water phase
Post time: Dec-29-2022