What is the role of “water”, an important part of water-based coatings?

2022-06-05   Pageview:26

Water, colorless and transparent, has no taste, is non-toxic and harmless, and is ubiquitous. The freezing point of water is 0 °C and the boiling point is 100 °C, so water-based paints are required to be stored above 0 °C. Water has stable chemical properties, non-flammable and non-explosive, so water-based coatings are non-dangerous goods, and the production, storage and transportation processes are very safe. Water is a polar solvent. According to the principle of similar compatibility, the binders and pigments and fillers as the basic components of coatings must be emulsified and dispersed in water with the help of emulsifiers and dispersants, respectively. Emulsifiers are also called surfactants. One end of the molecular chain is polar and the other end is non-polar. The film-forming substances are often organic polymer materials that are emulsified and exist in the micelles formed by surfactants. , isolated from each other by means of electric double layer protection or steric hindrance, thus stably suspended in water. The pigments and fillers in the coating are adsorbed on the surface of the pigments and fillers by the negatively charged dispersants, and exist in the water in the form of pigments and fillers dispersed slurry. Such dispersants can be inorganic salts, such as some metaphosphates. Modern coating production often uses polymer dispersants, such as sodium or ammonium salts of polyacrylic acid.

As water in the coating industry, it is not only used as a dispersion medium, but also participates in the production and manufacturing process of water-based coatings. During the production process, it helps to adjust the production process and improve the quality of coatings. After production, it is used to clean tools and equipment, and water is used during coating construction. Also acts as a diluent. This article focuses on the relationship between water and waterborne coatings.

1 Use reasonable water to improve the production technology level of water-based coatings
1.1 Pre-dispersion of thickener
Many thickeners, such as alkali-swellable thickeners or hydrophobically modified alkali-swellable thickeners, often require pre-dilution before they can be added to the dispersion tank during the production process. Since these thickeners are all acidic, their molecular chains will agglomerate when they are not added to the paint. Once added to the paint, the carboxyl groups on the molecular chains will repel each other when they encounter an alkaline environment. Expanded, on the one hand, it adsorbs the surrounding water molecules and reduces the free flow of solvent water. On the other hand, due to its huge size, it limits the free movement of other coating components. The hydrophobically modified thickener can also adsorb to different coatings. The surface of the latex particles restrains each other, so as to achieve the purpose of thickening. In an environment with a higher pH value, the effect of such thickeners is more obvious, but concentrated addition often leads to excessive local thickening, resulting in a greater risk of coating flocculation. The solution is to dilute this type of thickener with water in advance. The ratio of thickener and water is generally 1:1 or 1:2. When adding, slowly add it dropwise and speed up the dispersion speed, which can effectively avoid the viscosity difference. The phenomenon of uniform, flocculation and thickening.

1.2 Pre-slurry of cellulose
Hydroxyethyl cellulose is generally used in the production of water-based coatings. This type of cellulose is rigid, and many hydrophilic groups are connected to the molecular chain skeleton. For cellulose to function in waterborne coatings, it must first be well dispersed. Due to the relatively large molecular weight, it is often difficult to dissolve well in the production process, and it can neither play a thickening role in the coating, but also form small particles that affect the fineness of the coating. The solution is to make a pre-dispersed slurry of cellulose, first add cellulose to high-speed stirring water, and then add some organic amines to adjust the pH value. Under alkaline conditions, cellulose can be better dissolved, generally obtained. The concentration of the cellulose slurry is around 2%, and this process can also be done directly in the dispersing tank before the coating grinding stage. The same method is also applicable to the pre-dispersion of matting powder, the difference is that cellulose pulp needs to add organic amine to help dissolve, and matt pulp is to help disperse in water by adding a small amount of dispersant.

1.3 Pre-dilution of coalescents
Coalescents are essential components for most waterborne coatings. It can help water-based coatings lower the minimum film forming temperature and form better performance films. Film-forming aids have different drying speeds and film-forming effects according to molecular weight and molecular polarity. In water-based coatings, film-forming aids exist in the interior of latex particles and on the interface between latex particles and water, respectively, according to the strength of non-polarity. Solvents that are completely dissolved in water can only be solvents rather than film-forming aids. Such as alcohol and glycol. Coalescents commonly used in water-based coatings include dodecyl alcohol ester (alcohol ester 12), dipropylene glycol butyl ether, etc. For those solvents with large molecular weight and strong non-polarity, such as alcohol ester 12, it is recommended to use water at a ratio of 1:1 or 1:2 to pre-dilute and then add it during the stirring process of the paint. The alcohol ester 12 is insoluble in water, and the purpose of stirring is to form a short-lived dispersion of small oil droplets. If the alcohol ester 12 is added directly without dilution, a large amount of film-forming aids will be incorporated into the local latex particles, which may cause rapid volume expansion, break through the protection of the emulsifier’s electric double layer, and eventually lead to gel demulsification. For non-polar and weak coalescents, pre-dilution with water is not necessary.

1.4 Pre-dilution of preservatives
Water-based coatings are often anionic protection systems, that is, the hydrophilic end of the emulsifier is negatively charged. The preservatives used in this water-based paint are often isothiazolinone preservatives, and there are often copper ions in the preservatives, so these preservatives are often transparent blue-green. Since copper ions are divalent cations, if the local concentration is too high, it is easy to importer of PE wax break the anion-protected latex. The sodium nitrite anti-flash rust agent used in water-based metal anti-rust coatings is often prepared as a 15% solution with water, and is added slowly during the stirring process of the coating to avoid demulsification. During the coating construction, water is often added to adjust the construction viscosity. If there are more calcium and magnesium particles in the water, that is, the hardness of the water is usually higher, and adding it to the coating will easily lead to the demulsification of the coating. The author has encountered such an incident. The interior wall paint of a well-known paint company sells very well in Shanghai, but after it was sold to Sichuan, local customers found that every bucket would flocculate after adding water to the paint. The reason is that Sichuan Groundwater is mostly used in the area, and the water quality is relatively hard, which destroys the protection of the anionic electric double layer of the latex particles, resulting in the demulsification of the paint. We cannot change the usage habits of consumers and require them to use deionized water to dilute the paint. Only companies can increase the calcium ion stability of the paint by changing the formula, such as adding a certain emulsifier to enhance the stability of latex particles. Therefore, for the addition of cationic components, it must be pre-diluted with water. According to the above requirements, in the production process of coatings, water is needed to adjust the process in many places. In addition, some raw material barrels have residual raw materials, especially some emulsions with high viscosity, which need to be washed with water and added to the tank. You can first calculate the total water in the formula, weigh it separately, and deduct this part of the water if it is added through the prefabricated slurry. In actual production, do not add all the water at one time, but combine the process requirements with step-by-step pre-dilution and pre-mixing to ensure that the components of the coating are fully and evenly dispersed and mixed. It is best to add an appropriate amount or reduce a certain amount of water. water to meet the factory viscosity requirements. The same formula, adding the same amount of components, the resulting viscosity may not be the same, and finally need to be adjusted with water.

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