( Shenyang Shunfeng Group Co．，Ltd．，Shenyang 110141，China)
Abstract: The solvent－based coatings play an important role in all types of liquid coatings recent years． However，the solvent－based coatings are gradually modified or shifted into the waterborne coatings as driven by the environmental friendly concept． In this article，the preparation and application of the waterborne coatings has been described． And the emulsifying stability of the waterborne coatings and the cured film thereof having good water resistance are also discussed． The formulation design of high solid and low viscosity waterborne coatings should be underlined in order to improve the waterborne resistance of the cured film． In addition， some key points for the preparation and curing process of the waterborne coatings have been emphasized，which are useful for the formulation balance and stabilization of the waterborne coatings．
Liquid paint mainly refers to a mixture containing polymer resin that has been liquefied. After the liquid diluent is volatilized, it can form a continuous coating film with certain resistance and decoration. This article mainly discusses the preparation of one-component water-based coatings with water as the diluent. The polymer resin in liquefied coatings is the key substance to achieve the coating film performance, but most of the additives are mainly used in the process of liquefaction and deliquefaction. Effect, the auxiliary agent remaining after film formation almost no longer has a positive effect.
1 Amphiphilic balance and polyelectrolyte stability of water-based coatings The main additives used in water-based coatings are wetting and dispersing agents, defoamers, thickeners, rheological agents, substrate wetting agents, stabilizers, coupling agents , Bactericidal and antifungal agent, co-solvent. The basic structure and working principle of each type of additive are generally similar, but the formulas of different coating manufacturers are not exactly the same, which indicates that there is no fixed solution for the preparation of coatings. With the acceleration of the waterborne process of industrial coatings, the formulation and principle of waterborne coatings need to be studied in order to design reasonable formulas and processes according to different requirements. This will involve the principle of balance of overall formulation compatibility in addition to the principle of a single adjuvant.
1.1 Balance characteristics of water-based coatings
Water-based coating additives are usually classified by function. Since the focus of this article is not to discuss the functions of the additives, but to discuss the points of compatibility between the additives, the additives are classified according to their The balance characteristics of water-based coatings are discussed.
Amphiphilic balance and stability of polyelectrolyte are the key points to explore the compatibility of additives in the formulation. Water-based coatings are aqueous dispersions, mainly in the form of emulsions or neutralized salt-forming dispersions. Additives also work in the formulation based on this principle. The base material of architectural latex paint is air-permeable porous base material, and it is required that the topcoat also has a certain breathability, so its water resistance requirement is lower than that of water-based industrial coatings. The above basic principle has a certain negative impact on the water resistance of the coating film, so the resin in water-based industrial coatings plays a decisive role in the resistance of the coating film, and the additives in the formula generally do not improve the resistance of the coating film.
1.2 The amphiphilic balance of non-ionic additives mainly refers to the stability of oil-in-water in coating systems, which is usually achieved by non-ionic surface-active substances. Ethoxylated substances improve the water dispersibility of hydrophobic substances. Many auxiliaries are solubilized in the water phase by etherification.
1. 3 Anionic additives
A certain number of alkaline inorganic pigments and fillers are used in the coating formulation to improve hiding power, hardness, rigidity, reduce shrinkage, and have functions such as toning. The resin also needs to achieve stable oil-in-water stability through the micelle action of the emulsifier. The effect of the anionic emulsifier is the best. Therefore, the emulsion used in the coating is basically an anionic emulsion, and the acrylic acid or alkyd neutralizes the salt dispersion. It is also a slightly alkaline resin system. Therefore, water-based coatings usually use anionic additives to achieve compatibility with inorganic pigments and fillers.
2 Pigment wetting and dispersing agents
2.1 Pigment wetting and dispersing agents
Most of pigment wetting and dispersing agents are non-ionic surface active substances. The representative products used to be octylphenol or nonylphenol polyoxyethylene ether. At present, in order to meet the requirements of environmental protection, It has been generally replaced with fatty alcohol polyoxyethylene ether. Its HLB value is generally around 13. In order to reduce foaming and freezing point, branched instead of linear wetting agents have also begun to be used for pigment wetting and dispersion. But all rely on polyoxyethylene (EO) to achieve solubilization in the water phase. The amount of EO material will have an effect on the compatibility and balance of the formulation. Succinic sulfonate is an anionic wetting agent, and is generally not used in the wetting and dispersing phase of pigments, because it competes with the anionic dispersant on the surface of the pigment, and its relative molecular mass is too small, and its stability is not good. high.
2.2 Pigment dispersant
Pigment dispersants are mainly anionic polycarboxylates, including homopolymers and copolymers. They are divided into blocks and non-blocks. They mainly rely on the dual effects of electrostatic repulsion and steric hindrance, and some have an anchoring function on the pigment surface. . The carboxyl group formed after the ionization of the anionic dispersant is adsorbed on the surface of the pigment and filler. The function of the dispersant is reflected in the amount of adsorption, the effect of reducing viscosity and long-term stability. However, the viscosity reduction curve is drawn for a kind of titanium white or filler, and the pH is relatively fixed. Although organic amines can be added to disperse in the dispersion stage, the optimal viscosity curve of each single powder is not the same, and different powders must be used in the formulation, and the dispersant and pH must be constant. It is a quantification, so it is not necessary to achieve saturated adsorption on the surface of all inorganic particles. This is a potentially unstable factor in the dispersion slurry itself. In the formulation of water-based industrial coatings, the amount of resin is large, and the inorganic slurry stabilized with an anionic dispersant will form competitive adsorption with the resin. If a dispersion that neutralizes the salt is used, the competitive adsorption will be more serious. Because there is also a layer of non-ionic stabilizer on the periphery of the anionic emulsion particles, it protects the emulsion from forming a hydration layer. Although inorganic pigments and fillers have adsorbed anionic dispersants and there are nonionic wetting agents in the system, they do not provide good stability as emulsions. This is because the HLB value of the nonionic wetting agent is much lower than that of the emulsion stabilizer, that is, the amount of the EO substance is only between 9 and 12, but the nonionic stabilizer of the emulsion can reach 30 to 40. So in the paint adjustment stage, there will be unexpected changes when mixing the two components, which is the stability of polyelectrolyte.
3 Thickening rheological agent
Water is a continuous phase in water-based coatings, which is characterized by high surface tension and low viscosity. Because the viscosity is too low, high-density pigments and fillers may settle, so it is necessary to use a certain amount of thickener to maintain the suspension of the inorganic pigments and fillers. However, only the viscosity and the thixotropy provided by it cannot meet the coating requirements of water-based industrial coatings production lines. Therefore, different rheological requirements are imposed on the coatings. It is not only necessary to pay attention to the viscosity ratio in the static state and the highest shear state. Also need to pay attention to the viscosity curve at different shear rates.
3.1 Acrylic Alkali Swelling and Thickening Agents
Normal alkali swelling (ASE) and hydrophobically modified (HASE) thickeners have the characteristics of anionic dispersants and water-dispersible resins. They are anionic emulsified polymers before neutralization, and have a solid content It is a low viscosity emulsion of about 30%. When the pH of the system was adjusted to basic, the originally condensed macromolecular acrylic acid segment began to form salt. After solubilization and ionization, the carboxyl group electrostatically repelled and straightened the long chain. At this time, hydrogen bonds were formed with surrounding water molecules, and the viscosity increased. This process is also a neutralization process. There is an optimal pH condition for ASE or HASE solubilization. The relationship between the pH of the resin, the pH required by the dispersant, and the pH required by the thickener should be coordinated to avoid the occurrence of Uncontrollable competition.
3.2 Polyurethane or hydrophobic polyether thickening rheology agent
Latex paints are usually added with a small amount of polyurethane or hydrophobic polyether thickening rheology agent to improve leveling. In water-based industrial coatings, in order to improve the resistance of the coating film, it cannot be diluted with water in large quantities before construction, so cellulose or alkali-swelling acrylic thickeners are rarely used. Polyurethane-based rheological agents are usually used. In this way, the low shear viscosity is slightly lower, and the decrease in viscosity under high shear conditions can be controlled to meet the requirements of construction rheology. Such rheological agents are hydrophobically modified, non-ionic rheological agents with low relative molecular mass. Its two ends are hydrophobic and the middle is slightly hydrophilic. First of all, it needs to have a certain dispersibility in water. Because it mainly depends on the principle of hydrophobic association, one end is required to be adsorbed around latex particles or dispersion particles with a certain hydrophobicity, and the other end is In the water phase, they aggregate and associate with each other to form micelles. In the static state, multiple short-chain thickeners of this kind mutually associate to form a network structure. The polyether part in the middle of the short chain forms hydrogen bonds with water molecules, thereby regulating the aqueous system. The overall viscosity. This type of thickening principle is the principle of hydrophobic and hydrogen bonding of nonionic surfactants. If it is necessary to stabilize the hydrophobically associated network structure, the overall amphiphilic balance of the system must be ensured. If a large amount of hydrophilic non-ionic wetting agent or stabilizer is used in the system, the HLB value of the system will inevitably be increased. A large amount of non-ionic hydrophilic auxiliary agent will surround the resin or emulsion particles, preventing the hydrophobic end of the polyurethane rheological agent from forming a hydrophobic association network structure with the resin or emulsion particles. In the case that the two hydrophobic ends of the polyurethane rheological agent cannot be associated with the resin particles, they can only be turned inward in the aqueous phase, and the hydrophilic polyether chain in the middle is turned outward to form a large micelle, that is, Flowers flocculate. At this time, although there is a certain thickening effect, the coating system is easy to delaminate, and the workability is far from the design requirements.
If the total amount of non-ionic surfactants in the system is appropriate, but the concentration of polyelectrolyte is too high, the solubilization of non-ionic rheological agents in the water phase will be inhibited, and the low-shear conditions in the paint adjustment stage cannot be achieved. Disperse and form a translucent flake or filamentous floc to the surface of the coating. This is because the solubilization effect of low EO in the water phase is much lower than that of anionic polyelectrolytes. Once the amount of salt-soluble water soluble substances is too much, the effect of solubilization in the water phase by non-ion will decrease, which will be serious. Destroy the stability or workability of the system.
3.3 Cellulose ether HEC
Hydroxyethyl cellulose ether is mostly used in coatings. It is mainly used in the grinding and dispersion stage of pigments and fillers in industrial coatings to improve the suspension of inorganic particles in the water phase and shorten the dispersion time. Because the powder is usually added directly during refining, HEC still needs to be weakly alkaline to achieve rapid water solubility. So although HEC is non-ionic, it is still affected by the pH of the system.
4 Bactericidal preservatives
Due to the use of a certain amount of surfactants in water-based coatings, it is easy to deteriorate during storage, so it is necessary to add preservatives in the tank. Slow release formaldehyde type anion preservatives are usually used, but with the increase of environmental protection requirements, the use of such preservatives has been limited, so cationic and non-ionic composite preservatives are widely used. However, the efficacy of cationic additives is relatively poor, and its stability is not good in anionic coating systems.
5 Substrate wetting agent and leveling agent
Substrate wetting agent is mainly used for industrial coatings to spread and wet the surface of metals, plastics and certain woods in order to improve the adhesion of the coating film and reduce shrinkage and improve Leveling. The function of substrate wetting agent and pigment wetting agent are different, and the chemical structure is also very different. The substrate wetting agent is a surface active substance with a lower HLB value. Generally, it is not easy to form micelles in aqueous systems, so it is not stable. However, the water dispersibility is poor and the cloud point is low. When processing temperature is increased, the compatibility becomes worse, and it tends to function as a defoamer. Therefore, water-based coatings have higher requirements for shear rate, temperature, and relative humidity. These conditions are consistent for qualified coating production lines. Substrate wetting agents mainly include: low relative molecular weight polyether modified polysiloxane (non-ionic), also including some leveling agents, alkynyl alcohol (non-ionic), fatty acid ester hybrid polymers (non-ionic) ), Succinate (anion). The dispersibility of this type of substrate wetting agent in the water phase can rely on low-boiling alcohol solvents, and it can also depend on its own hydrophilic group. Its dispersibility in water is also affected by the amphiphilic equilibrium or ionicity in the system.
6 Defoaming agent
A large number of surface-active substances are used in water-based coatings, which have a strong foam-stabilizing effect, so antifoaming agents must be added to meet the requirements for coating preparation and construction. The antifoaming agent is composed of different degrees of hydrophobic substances and is insoluble in water. The antifoaming agent with good water dispersion is not easy to shrink, but it has poor antifoaming ability and is not durable. The defoamer with poor water dispersion has good durability and is suitable for spray equipment with high shear rate. Due to the introduction of a large amount of air with high shear force, it is easy to form bubbles, so it is necessary to use a defoamer with high durability and slightly poor water dispersibility.
6.1 Varieties of antifoaming agents
Antifoaming agents mainly include: mineral oil-based antifoaming agents (mainly used in latex paints and high PVC thick coatings), polyether modified polysiloxanes (suitable for low PVC, high volume) Solid, low water content coating), hydrophobic polyether (suitable for coating with higher water dilution), fatty acid ester polyether hybrid molecular defoamer, ethoxylated modified alkynol.
6. 2 Application of defoamer
The foam-stabilizing effect of latex paint mainly comes from the emulsifier in the emulsion. The monomers and polymers used in the polymer are basically covered by micelles, so the defoamer is generally evaluated based on the general emulsifier. The carboxyl portion of the side chain of the neutralized salted aqueous dispersion is neutralized to form a salt and solubilizes the hydrophobic alkyl backbone into the aqueous phase. At this time, the hydrophobic chain is not covered by micelles in the coating. The initial water resistance of the coating film, the neutrality of the resin is generally maintained at 80%. At this time, the alkyl group in the resin has a certain ability to adsorb hydrophobic defoaming substances, and the overall affinity for air is equivalent to that of the defoamer, even if The defoamer continued to float to the air surface, but there were still many fine bubbles adsorbed around the hydrophobic backbone of the resin. Therefore, highly resistant water-based industrial coatings must use a more hydrophobic defoaming substance than the alkyl main chain of the resin to peel off the bubbles. Due to the poor compatibility of the defoamer with water, if the dispersion is poor, it will cause shrinkage. Some are abroad The antifoaming agent with good effect often causes shrinkage when it is used in China. This is because the production efficiency of foreign coating equipment is very high, and the high-speed shearing also bleeds the antifoaming agent completely and uniformly at the same time, thereby achieving a balance. The antifoaming agent has a poor dispersion in the water phase under static conditions, so the coating must be stirred evenly before being applied. During the stirring process, the anionic and nonionic surface active substances in the formula play a certain role in the dispersion of the defoamer. Therefore, if the above conflicts occur in the formulation of the anionic and non-ionic surfactants, it will affect the degree of dispersion of the defoamer and may cause shrinkage. In addition, the defoaming substance has a slightly higher hydrophobicity than the main chain of the dispersion resin. If the hydrophobicity gap is too large, the poor compatibility of the defoaming substance with the main chain of the dispersion after dehydration and film formation will also cause shrinkage.
7 Co-solvents and main solvents (film-forming aids)
At present, water-based coatings cannot reach zero VOC. A small amount of low boiling point alcohol co-solvent must be used. The co-solvent has better water solubility and does not have swelling effect on the resin. It only improves the water dispersibility of some additives and controls. Freeze-thaw stability, adjust the drying speed when the coating film is deliquefied. When the amount of co-solvent is high in the formula, although it can still be diluted with water, the coating should still be a solvent-based coating, or a pseudo-water-based coating. As a result, many of the above-mentioned water-based additives cannot play a normal role in this system. The high-boiling-point solvent that acts on the resin is called a film-forming aid, which is incompatible with water and swells the resin particles during the dehydration process to play a role in plasticizing the film. The film-forming aid cannot be dispersed in the water phase, which will affect the storage and application stability of the liquefied coating. A stable formula does not pre-emulsify or increase the polarity of the film-forming assistant, but rather makes the film-forming assistant compatible with each other through the overall balance of the anionic and non-ionic surface active substances in the formula. The resin particles cannot be contacted during the storage period, and the polymer particles can be swelled in time during the dehydration process to soften and form a continuous coating film.
Compared with solvent-based coatings, the preparation of water-based coatings is more dependent on additives, and there are more varieties of additives. Water in water-based coatings is only a continuous phase, so water-based coatings are not true solutions. Almost everything in the coatings is dispersed in the water phase. Because the conditions of preparation, storage, and construction are variables, all kinds of coatings are used. After mixing the additives in the coating, they may not reflect the original set function, and may even have negative effects.