Compared with traditional solvent-based coatings, water-based wood coatings still have many deficiencies, such as poor water resistance, especially boiling water resistance, poor resistance to organic media, poor fullness of decorative wood coatings, and poor resistance to sticking back and scrubbing. , unsatisfactory mechanical strength, etc., which hinder the further promotion and application of water-based coatings. In view of the above problems, in addition to designing and synthesizing a water-based resin system with a suitable structure, selecting appropriate additives is also an important means to improve the performance of water-based wood coatings. Among them, the use of wax additives can not only improve the scratch resistance of the paint film. It also affects the adhesion resistance and gloss of the paint film.
Emulsion 4502 for waterborne wood coatings: Industrial grade, Wanhua Chemical; Wax Dispersion 2546, Wax Microdispersion H2000: Provided by Longhai Chemical; ( 2020 ), multifunctional additives ( AMP95 ): all are industrial grade,; wetting agent 308; dipropylene glycol methyl ether, dipropylene glycol butyl ether.
(1) The stirring speed of the high-speed disperser is 800 r/min in, and the special emulsion for water-based wood coatings, AMP95, dipropylene glycol methyl ether, and dipropylene glycol butyl ether are sequentially added into the paint tank, and the stirring time after each addition is 10 m in around.
(2) Dissolve the defoamer ( 681F) in the film-forming aid (NEXCOAT 795), then slowly add it to the above system, and stir for no less than 60 min after the addition.
(3) Add leveling aids, and control the stirring time at about 30 min.
(4) Add wax emulsion and stir for 15min.
(5) 120 mesh filter screen to filter out the material.
Dispersion of Wax Additives in Waterborne Coatings
In previous studies, some people called the action mechanism of wax additives as the floating theory, that is, after the wax additives are dispersed into the coating system, the wax particles will move to the physical drying or curing cross-linking process after the coating is applied. The surface drift of the paint film gradually migrates to the surface of the paint film, thus playing a certain protective role.
This mechanism of action was proposed during the study of solvent-based coating systems. However, for water-based wood coatings, there are few reports on the dispersion of wax additives in water-based systems due to their different characteristics from solvent-based coating systems.
The wax dispersion with larger particles (micron scale) can be seen in the second image, while the smaller particle size (nanoscale) cannot be seen under the magnification in the second image, and the mass distribution of the two can be based on The particle size distribution and number distribution are calculated according to statistical principles. The calculated result is that the mass ratio of large particles (micron scale) is 42%, and the mass ratio of small particle dispersion is 58%. Thus, the theoretical percentage of the particle area shown in Figure 3 to the total field of view should be: 2.1% (5.04% × 42%). According to the ratio measurement in the figure, the percentage of particle area is 2.4%, and the difference between the two is not large, so it can be inferred that the particles of the wax dispersion are basically not enriched on the surface of the paint film, which is different from the wax dispersion in the solvent-based system. The distribution in the coating is different.
Influence of Wax Additives on Scratch Resistance of Paint Films
The waterborne wood coatings formulated with Emulsion 4502 as the main film-forming substance have excellent hardness, yellowing resistance and water resistance. Applying the wax emulsion to the preparation system of the waterborne wood coatings can greatly improve the adhesion resistance and scratch resistance of the paint film . The picture below is a microscopic photo of the surface of the paint film after scratching.
As can be seen from the figure below, before adding wax additives, the surface of the paint film is easier to be scratched, but after adding different types of wax additives, under the same external force, the scratch resistance of the paint film surface is all big improvement. Among them, the large particles in the dispersion 2546 play a crucial role in resisting the damage of the paint film by external force. It can be seen from the figure d that the scratches on the surface of the paint film caused by the external scratch force are After reaching the large-diameter wax particles dispersed on the surface of the paint film, it is greatly weakened, preventing the further extension and aggravation of scratches, while H2000 has relatively weak resistance to external scratches due to the small particle size of the wax particles.
In general, the addition of wax additives improves the scratch resistance of the paint film itself, and the large wax particles dispersed in the paint film play a strengthening role and withstand the damage from external forces . Table 1 shows the effect of the amount of wax additives on the scratch resistance of the paint film.
With the addition of wax additives, the scratch resistance of the paint film surface gradually improved. When the amount of wax with large particle size reached 3%, the performance was the best, and there was no significant change when the amount was increased. Considering the cost factor, the wax The addition amount of additives in this system is controlled at 3%. For H2000 due to its own performance, in order to achieve the ideal anti-scratch effect, it is necessary to continue to increase the amount of additives.
Retack of paint film after adding wax additives
The high temperature tack back of water-based acrylic paint film is an important factor restricting its application in furniture production. The addition of wax additives can significantly improve the anti-stick back of the paint film. The adhesion properties of paint films with different types of wax additives were studied, and the prepared samples were tested according to the relevant test methods.
As an important coating additive, wax additive is evenly dispersed into water-based wood coatings, which can not only improve the scratch resistance and anti-adhesion resistance of the paint film, but also affect the gloss of the paint film. Different types of wax additives are added to the coating system and eventually become part of the paint film due to their different microstructures. The addition of wax dispersions to waterborne wood coatings can significantly improve the scratch resistance and anti-stick properties of the paint film. However, it will have a certain influence on the gloss of the paint film; and the selection of wax emulsion or microdispersion with smaller particle size can ensure the high gloss of the paint film and improve its anti-stickiness and scratch resistance.
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