Study on defoaming properties of water-based wood coatings with high viscosity varnish
In the construction process of water-based wood coatings, it is generally recommended to use the "thin coating multiple times" construction method. This is because when water-based coatings are thick coated, the drying speed is slow, the hardness rises slowly, and the defoaming is difficult to achieve good construction effects. However, in practical production applications, in order to improve production efficiency, water-based wood coatings are also required to be thick coated at once, especially in some areas where the board surface requirements are not very high, such as window frames, frame and other surface coatings. This application is quite common.
The characteristic of this field is the use of medium pressure mixed air spraying, which has a simple process and high production efficiency. The construction feature is high construction viscosity, with a one-time thick coating of up to 250 μ m. One of the most difficult problems to solve when using this process for production is defoaming.
A large number of bubbles during spraying cannot be eliminated during the drying process of the coating, resulting in a large number of pinholes on the surface of the board.
This has almost become an important obstacle for the expansion of water-based wood coatings into other professional fields such as furniture, especially for varnishes, of which 70% -90% are resins, making defoaming much more difficult than solid paints (containing titanium dioxide and other powders). In door and window coatings, about 2/3 are matte varnish coatings. Therefore, it is necessary to conduct in-depth research on the defoaming performance of water-based matte varnish during medium pressure mixed air spraying.
The way of producing foam
Foam during production
In the production process of coatings, high-speed dispersion is often required to ensure uniform mixing of various components, resulting in the generation of a large number of bubbles. If these bubbles cannot be broken after being placed, it will inevitably affect the elimination of foam during construction. In the production process of water-based wood varnish with high resin content, it is difficult to eliminate foam when the viscosity is high. In order to meet the needs of construction and storage, the viscosity must not be too low. When using a medium pressure mixed air spray gun for construction, the construction viscosity should generally be above 80KU. In order to meet this construction requirement, the initial viscosity of the product must be at least 90KU (30 ℃) to meet the normal construction or other construction needs even when the viscosity of the coating decreases in high temperature environments in summer. At a viscosity of 90KU (30 ℃), it is not difficult to simply solve the problem of defoaming. However, in the actual production process, due to the need to consider the issue of thick coating and sagging during construction, it is necessary to improve the thixotropy of the coating, enhance its thixotropy, and make defoaming even more difficult. Therefore, in the production process of coatings, a good balance should be struck between rheology and defoaming.
On the other hand, developing appropriate production processes can greatly reduce the generation of bubbles during coating production. When producing matte varnish, the matte powder is first made into a paste to avoid the large number of bubbles generated by high-speed dispersion of the matte powder in the resin; Add an appropriate amount of defoamer with good anti foaming properties before starting the mixing process; Before high-speed dispersion, adjust the coating to the appropriate viscosity to avoid introducing a large amount of air into the coating when the dispersion liquid level oscillates, which can reduce the generation of bubbles during coating production.
Foam during construction
1. Substrate
When applying wood coatings, due to the presence of numerous capillaries in the wood itself, it is often necessary to first seal the substrate. If the substrate is not properly sealed, the air in the capillary tube of the substrate will escape during coating application and enter the wet film of the coating, forming bubbles. If the bubbles cannot be broken, pinholes or volcanic craters will form. In the construction process of water-based coatings, especially in fully enclosed coating, due to their high construction viscosity and thick one-time spraying, it becomes more difficult to remove the wet film of the coating during construction. Therefore, in the fully enclosed coating of water-based coatings, generally 1-2 layers of sealing primer should be applied to expel the air in the capillary tubes of the substrate as much as possible. Otherwise, once the air enters the thick coating of the subsequent process, the bubbles formed will be difficult to eliminate.
2. Spray gun
Construction tools are also a factor in the generation of bubbles in coatings. In the construction of water-based furniture coatings, both the primer and topcoat are sprayed. Generally speaking, the better the atomization, the less likely bubbles are generated during construction. So when using an air cup gun for spraying, the effect is better than that of medium pressure mixed air spraying. There are two main reasons for this: first, the air cup gun has a higher atomization air pressure during spraying, usually 0.6-0.8 MPa. After the paint is atomized, it reaches the surface of the construction object at a higher speed, so even a small amount of bubbles can be broken when the paint particles "hit" the surface of the object. However, the atomization air of the medium pressure mixed air gun is only 0.1-0.2 MPa, and its atomization effect is worse than that of the air gun. The speed at which the paint particles reach the surface of the object is also lower than that of the air gun, so the bubble breaking ability during the construction process is poor. The second reason is that the viscosity of air gun spraying is lower than that of medium pressure mixed air gun spraying, which makes it easier for the paint to atomize, break bubbles, and eliminate bubbles during the later surface drying process of air cup gun spraying.
On the other hand, adjusting the oil and air volume of the spray gun during construction is also an important factor affecting the generation of bubbles. Whether it is an air spray gun or a medium pressure mixed air spray gun, the oil and air volume can be flexibly adjusted, and the mixing ratio should be determined according to the viscosity of the coating or the construction effect to be achieved by the coated object. In semi open coating construction, it is sometimes required to spray it "dry" (with a small amount of oil and a large amount of air), while in fully enclosed coating, it is generally necessary to spray it "wet" (with a slightly larger amount of oil and a slightly smaller amount of air). Generally, I should be applied before construction by adjusting the oil and gas volume to achieve the best atomization state of the coating, in order to achieve the best construction effect.
Factors affecting defoaming of water-based wood coatings
1. Resin
In the defoaming experiment of water-based high viscosity matte varnish, resin is the most critical factor affecting the difficulty of defoaming. At present, there are three main types of water-based wood coatings: water-based polyurethane (PU), water-based acrylic (AC), water-based polyurethane, and water-based acrylic composite (PU A). Due to different synthesis mechanisms, the difficulty of defoaming varies greatly among the three types of resins.
Water based PU and PUA are relatively easier to defoam, while water-based AC is relatively more difficult. This is because the former does not require or rarely uses emulsifiers in the synthesis process, as emulsifiers themselves are surfactants that are prone to foaming when stirred. However, in the domestic water-based furniture coating market, water-based AC has been widely used due to its significant advantages in price, drying speed, water resistance, and other aspects. In particular, the core-shell polymerization lotion has a lower film forming temperature, greatly reduces the VOC content, and has good hardness, elasticity and adhesion resistance after film forming. At present, it has been widely used in outdoor wood products surface coating.
In AC lotion, the difficulty of defoaming is different due to different types of emulsifiers and synthesis methods when lotion is synthesized. For example, anionic lotion system has small particle size and is easy to form foam. The aqueous AC lotion with soap free polymerization and core-shell polymerization is better than the traditional aqueous AC lotion in defoaming. The experiment shows that the core shell polymerized lotion can greatly reduce the difficulty of defoaming in the production process when designing the formulation of high viscosity matt varnish.
2. Viscosity
Whether stored in the paint tank or during the surface drying process after construction, the higher the viscosity of the paint, the more difficult it is to defoam. In actual production, in order to prevent sedimentation, sagging, etc., the coating must always be controlled within a certain viscosity range. In the medium pressure mixed air spraying of water-based wood coatings, it is necessary to ensure that the construction viscosity of the coating is 80KU or above, and the factory viscosity of the finished product is often above 90KU, in order to meet the viscosity changes caused by temperature rise and the need to add water to adjust viscosity during construction. According to the different rheological properties of coatings, the viscosity of coatings should generally be controlled between 90-120KU (25 ℃)
3. Defoamer
In water-based high viscosity matte varnish, defoaming is difficult due to its high resin content and viscosity. The amount and types of defoamers used during production are relatively large. Generally, 2-3 defoamers need to be used in combination to suppress, defoam, and defoam before, during, and after high-speed dispersion. In order for defoamers and other additives to mix well with the resin, they must be dispersed at high speed, which will generate a large number of bubbles. Therefore, it is necessary to add a certain amount of defoamer with good anti foaming properties before high-speed dispersion.
The commonly used defoamers for water-based wood coatings mainly include mineral oil and organosilicon. The former has lower cost and weaker defoaming ability, mainly composed of 85% carrier oil and 15% hydrophobic particles. Hydrophobic particles are generally made of gas-phase silica, stearic acid metal salts, etc. These defoamers are prone to blackening of the coating and can be used for putty and primer with low gloss; The latter is mainly composed of lotion of strongly hydrophobic organosilicon and polyether modified polydimethylsiloxane, which has no effect on gloss, transparency, etc., and is the main defoamer for water-based wood coatings at present. But some articles also point out that some mineral oil modified defoamers are more effective than organic silicon defoamers. Therefore, the selection of defoamers should be based on different resins and experimental results.
4. Thickener
In the production of water-based high viscosity matte varnish, the choice of thickener is very important, which is the key to the defoaming performance of the product during storage or application. At present, there are two main types of commonly used thickeners: associative and alkali swelling. The former can provide better leveling and shear viscosity, while the latter can improve storage stability and anti sagging properties. In production, the former is beneficial for defoaming and defoaming, while the latter is easier to stabilize. However, due to the good fluidity of the former, its anti pollution ability during construction is greatly reduced. Therefore, in order to maintain the stability of the coating storage and have good anti sagging and anti pollution ability during construction, water-based matte varnish sometimes needs to be combined with a certain amount of alkali swelling thickener even at high viscosity. There are many types of these two types of thickeners, and how to mix them depends on the experimental results after selecting the resin.
5. Matte powder and wetting dispersant
In water-based matte varnish sprayed with medium pressure mixed air, matte powder is generally chosen as the matting agent instead of matting wax slurry. This is mainly due to the fact that medium pressure mixed air spraying does not have very high requirements for the tactile feel of the board surface, and also takes into account factors such as defoaming, price, and system thickening. The price of using matting wax slurry (powder) is relatively high, and it is easy to produce foam when dispersing, and it is difficult to eliminate it.
In high viscosity varnish, the anti settling effect of matting powder is almost negligible, while its impact on defoaming properties becomes very important. Generally, surfaces treated with hydration are easier to defoam than those without treatment, and those with lower oil absorption are easier to defoam than those with higher oil absorption. Experiments have shown that using different wetting and dispersing agents has a significant impact on the defoaming of the system. The better the wetting effect of the dispersant on the surface of the matting powder, the more beneficial it is for the defoaming of the system.
6. Design and control of production processes
In the production of coatings, process design and control are also very important. In the production of water-based coatings, in order to achieve uniform mixing of resin and other components while minimizing the generation of bubbles, it is necessary to develop appropriate production processes and strictly control them.
For example, in the production of matt varnish, additives and matting powder are usually added into the resin to disperse at high speed to the desired fineness. In this process, a large number of bubbles will be generated. When the viscosity is low, these bubbles will generally be eliminated after standing for 24 hours, but when the viscosity is high and the system thixotropy is strong, these foam will be difficult to be eliminated after a long time. At this time, the production of foam can be effectively reduced by adding the matting powder after it is made into slurry. On the other hand, when adding defoamers or other difficult to disperse additives, it is best to dilute them and slowly add them in a dispersed state to minimize the speed required for dispersion and reduce the time required for uniform dispersion; Some foaming agents such as wax slurry should be added as much as possible during the low-speed dispersion process in the later stage.
epilogue
The high viscosity construction of water-based wood coatings can increase the thickness of one-time construction and improve production efficiency, while also reducing the possibility of sagging and bubble generation during construction. However, it increases the difficulty of defoaming during construction, greatly affecting the surface effect after construction and becoming the main obstacle to the expansion of water-based wood coatings to other application fields. In the study of defoaming properties of water-based high viscosity varnish, the selection of resin and thickener is the most important, followed by the selection of defoamer, matting powder and other additives, and finally the optimization of production process and adjustment of viscosity. A good product not only needs to have good physical and chemical properties, but also good construction performance. Only by combining the two well can it demonstrate excellent coating effects, meet consumer requirements, and promote the rapid development of water-based wood coatings.
