How does the defoaming property of high-viscosity varnishes for water-based wood coatings compare?
During the application of water-based wood coatings, it is generally recommended to apply the coating in thin layers and multiple times. This is because when water-based coatings are applied in thick layers, they dry slowly, the hardness increases slowly, and it is difficult to defoam, making it difficult to achieve a good application result. However, in actual production applications, in order to improve production efficiency, water-based wood coatings are required to be applied in thick layers at one time, especially in fields where the requirements for the surface are not very high, such as the coating of window frames, frames, and other surfaces. This application is already quite common.
This field is characterised by the use of medium-pressure airless spraying, which is simple in process and highly productive. The construction features a high viscosity and a single thick coat of up to 250 μm. One of the most difficult problems to solve when using this process is defoaming.
When spraying, a large number of air bubbles cannot be eliminated during the drying process of the coating film, resulting in a large number of pinholes on the surface.
This has almost become an important obstacle to the expansion of water-based wood coatings into other professional fields such as furniture, especially varnishes, 70%-90% of which are resins, which makes defoaming much more difficult than solid-coloured paints (containing titanium dioxide and other powders). In the painting of doors and windows, about 2/3 is matte varnish coating. Therefore, it is necessary to conduct in-depth research on the defoaming performance of water-based matte varnishes when sprayed at medium pressure.
Causes of foam
Foam in the production process
In the production of coatings, high-speed dispersion is often required to mix the ingredients evenly, resulting in a large number of bubbles. If these bubbles cannot be broken after standing, they will inevitably affect the elimination of foam during application. In the production process of water-based wood varnishes with high resin content, foam is more difficult to eliminate when the viscosity is high, and the viscosity must not be too low in order to meet the needs of application and storage. If the product is applied using a medium-pressure airless spray gun, the viscosity should generally be above 80 KU. To meet this requirement, the initial viscosity of the product should be at least 90 KU (30°C), so that the product can still be applied normally even when the viscosity decreases in hot summer environments, or for other application needs. At a viscosity of 90 KU (30°C), it is not difficult to simply solve the problem of defoaming. However, in the actual production process, since the problem of sagging during thick coating on vertical surfaces must also be considered, it is necessary to improve the thixotropy of the paint. Therefore, a good balance must be struck between rheology and defoaming during the production process.
On the other hand, if a suitable production process is developed, it is also possible to reduce the bubbles generated during paint production. For example, in the production of matte varnish, the matting powder is first made into a slurry to avoid the large number of air bubbles generated during high-speed dispersion when dispersing the matting powder in the resin. An appropriate amount of defoamer with good foam suppression properties is added before starting the mixing; the coating is adjusted to the appropriate viscosity before high-speed dispersion to avoid bringing a large amount of air into the coating when the dispersion surface is shaken, etc. All of these can reduce the generation of air bubbles during coating production.
Foam during application
1. Substrate
When applying wood coatings, the wood itself often has a lot of capillaries, so the substrate often needs to be sealed first. If the substrate is not sealed properly, the air in the capillaries of the substrate will escape during the coating process, enter the wet film of the coating and form bubbles. If the bubbles cannot be broken, they will form pinholes or craters. During the application of water-based coatings, especially when fully enclosed, the high application viscosity and one-time thick spraying make it more difficult to defoam the wet film of the coating during application. Therefore, when applying water-based paint in a fully enclosed manner, it is generally necessary to apply one to two coats of sealing primer to try to expel as much air as possible from the capillaries of the substrate. Otherwise, once the air enters the thick coating in the subsequent process, the resulting bubbles will be difficult to eliminate.
2. Spray gun
The construction tool is also a factor in the generation of bubbles in the paint. In the application of water-based furniture paints, both the primer and the topcoat are sprayed on. Generally speaking, the better the atomisation, the less likely it is that bubbles will be created during application. This is why spraying with an air cap gun is better than medium-pressure airless spraying for two main reasons: firstly, the atomisation air pressure when spraying with an air cap gun is higher, generally 0.6–0.8 MPa. After the paint is atomised, it reaches the surface of the object being painted at a higher speed, so that even if there are a small number of bubbles, they can be broken when the paint particles ‘hit’ the surface of the object. The atomising air of the medium-pressure air-mix spray gun is only 0.1 to 0.2 MPa, and its atomisation 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 effect during construction is worse. Second, the viscosity of the sprayed paint is lower when using an air spray gun than when using a medium-pressure airless spray gun. This makes it easier to atomise the paint, break the bubbles and defoam the surface during the drying process.
On the other hand, the amount of oil and air in the spray gun during application is also an important factor affecting bubble formation. Whether it is an air spray gun or a medium-pressure air-mix spray gun, the amount of oil and air can be flexibly adjusted, and the mixing ratio should be determined according to the viscosity of the paint or the desired application effect on the object being painted. For example, when semi-open coating is used, sometimes it is required to spray ‘drier’ (small oil volume and large air volume), while when fully enclosed coating is used, it is generally required to spray ‘wet’ (slightly larger oil volume and slightly smaller air volume). Generally, before application, the atomisation of the coating should be optimised by adjusting the oil and air volume, in order to achieve the best application results.
Factors affecting defoaming of water-based wood coatings
1. Resin
In defoaming experiments with water-based high-viscosity matte varnishes, the resin is the most critical factor affecting the ease of defoaming. Currently, there are three main types of water-based wood coatings: water-based polyurethane (PU), water-based acrylic (AC), and water-based polyurethane and water-based acrylic composite (PU A). Due to differences in the synthesis mechanism, the ease of defoaming of the three types of resins also differs greatly.
Defoaming waterborne PU and PUA is relatively easy, while defoaming waterborne AC is relatively difficult. This is because emulsifiers, which are a type of surfactant, tend to cause foaming when agitated. However, in the domestic waterborne furniture coating market, waterborne AC has been widely used due to its advantages in terms of price, drying speed, water resistance, etc. In particular, core-shell polymerisation emulsions have a lower film-forming temperature, greatly reduce the VOC content, and have good hardness, elasticity and anti-blocking properties after film formation. They are currently used in large quantities for the surface coating of outdoor wood products.
Among AC emulsions, the difficulty of defoaming varies depending on the type of emulsifier and the synthesis method used during emulsion synthesis. For example, anionic emulsion systems have small particle sizes and are prone to foaming. Water-based AC emulsions produced using soap-free polymerisation or core-shell polymerisation are easier to defoam than traditional water-based AC emulsions. Experiments have shown that when formulating high-viscosity matte varnishes, the use of core-shell polymerisation emulsions can greatly reduce the difficulty of defoaming during production.
2. Viscosity
Whether it is stored in the paint can or during the drying process after application, defoaming is more difficult when the paint has a higher viscosity. In actual production, the viscosity of the paint must be controlled within a certain range to prevent settling and sagging. When medium-pressure airless spraying water-based wood coatings, the working viscosity of the coating must be kept at 80 KU or above, while the finished product’s ex-factory viscosity must often be kept above 90 KU to compensate for viscosity changes caused by temperature increases and the need to adjust the viscosity with water during application. Depending on the rheological properties of the coating, the viscosity is generally controlled between 90 and 120 KU (25°C).
3. Defoamers
In water-based high-viscosity matte varnishes, defoaming is more difficult due to the high resin content and high viscosity. The amount and type of defoamer used during production is relatively high, and generally two to three defoamers are used in combination, which are used before, during and after high-speed dispersion to suppress, defoam and deaerate. In order for the defoamer and other additives to mix well with the resin, high-speed dispersion is required, which will generate a large number of bubbles. Therefore, it is necessary to add a certain amount of defoamer with better defoaming properties before high-speed dispersion.
The main defoamers commonly used in water-based wood coatings are mineral oil and silicone. The former is low in cost and has weak defoaming ability. It is mainly composed of 85% carrier oil and 15% hydrophobic particles, and the hydrophobic particles are generally made of fumed silica, metal stearate, etc. This type of defoamer is prone to blackening of the coating film and can be used in putty and primers with low gloss. The latter is mainly composed of emulsions of strongly water-repellent organosilicon and polyether-modified polydimethylsiloxane, which have little effect on gloss and transparency. It is currently the main defoamer for water-based wood coatings. However, some articles have pointed out that some defoamers modified with mineral oil are more effective than organosilicon defoamers. Therefore, the selection of defoamers should be based on different resins and determined based on experimental results.
4. Thickeners
The choice of thickeners is very important in the production of water-based high-viscosity matte varnishes. This is the key to the defoaming properties of the product during storage or application. Currently, there are two main types of commonly used thickeners: associative and alkali-swellable. The former can provide better flow and shear viscosity, while the latter can improve storage stability and anti-sagging properties. In production, the former is conducive to defoaming and degassing, while the latter is easier to stabilise foam. However, because the former has better fluidity, its resistance to contamination during application is greatly reduced. Therefore, in order to maintain the storage stability of the coating and provide better sag resistance and anti-contamination during application, water-based matte varnishes sometimes need to be used in combination with a certain alkali-swellable thickener even when the viscosity is high. There are many types of these two types of thickeners, and how to match them needs to be determined based on the experimental results after the resin is selected.
5. Matting agents and wetting and dispersing agents
In medium-pressure airless sprayed water-based matting varnishes, matting agents are generally selected as matting agents, rather than matting wax pastes. This is mainly because the feel of the surface is not very important in medium-pressure airless spraying, and also because of defoaming, price and system thickening considerations. Matting wax pastes (powders) are relatively expensive to use, and are also prone to foam during dispersion, which is difficult to eliminate.
In high-viscosity varnishes, the anti-settling effect of matting agents is almost irrelevant, while the effect on defoaming properties becomes very important. In general, surfaces that have been hydrated are easier to defoam than those that have not, and surfaces with low oil absorption are easier to defoam than those with high oil absorption. Experiments have shown that the choice of wetting and dispersing agent has a significant effect on defoaming. The better the wetting of the matting agent surface by the dispersing agent, the more conducive it is to defoaming.
6. Design and control of production processes
The design and control of production processes is also very important in the production of coatings. In the production of water-based coatings, a suitable production process needs to be formulated and strictly controlled in order to achieve uniform mixing of the resin and other ingredients with minimal bubble formation.
For example, in the production of matte varnish, the resin is usually mixed with additives and matting agents at high speed until the required fineness is reached. During this process, a large number of bubbles will be generated. When the viscosity is low, these bubbles will usually disappear after standing for 24 hours. However, when the viscosity is high and the system is thixotropic, these bubbles will be difficult to eliminate even after a long period of time. At this time, adding the matting agent after making it into a slurry can effectively reduce the generation of bubbles. On the other hand, when adding defoamers or other additives that are not easy to disperse, it is best to dilute them and slowly add them in a dispersed state to minimise the speed required for dispersion and reduce the time required for uniform dispersion. Some additives that are prone to foaming, such as some wax pastes, should be added during the late low-speed dispersion process as much as possible.
Conclusion
The high viscosity of water-based wood coatings can increase the thickness of a single application, improve production efficiency, and reduce the possibility of sagging and bubble formation during application. However, it also makes defoaming more difficult during application, which greatly affects the surface effect after application and is a major obstacle preventing the expansion of water-based wood coatings into other application fields. In the research on defoaming properties of water-based high-viscosity varnishes, the selection of resins and thickeners is the most important, followed by the selection of defoamers, matting agents and other additives, and finally the optimisation of the production process and adjustment of viscosity. A good product must not only have good physical and chemical properties, but also good application properties. Only when the two are well combined can it achieve excellent coating results and meet consumer requirements, and the development of water-based wood coatings will accelerate.
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