Application of wax additives in waterborne wood coatings
ZHANG Shengwen, HE Lifan, LI Xiaoyu
(School of Materials Science and Engineering, Beijing University of Chemical Technology, Key Laboratory of Fundamentals of Nanomaterials Preparation Science and Technology, Ministry of Education, Beijing 100029, China)
0 Introduction
With the increasing awareness of environmental protection, water-based wood coatings are gaining more and more attention and popularity [ 1] . However, compared with traditional solvent-based coatings, water-based wood coatings still have many shortcomings[ 2] , such as poor water resistance, especially boiling water resistance, poor resistance to organic media, poor plumpness of wood decorative coatings, poor resistance to re-stick and scrubbing, and unsatisfactory mechanical strength, all of which hinder the further promotion and application of water-based coatings. In addition to the design and synthesis of suitable resin systems, the selection of suitable additives is also an important means to improve the performance of waterborne wood coatings, among which the use of wax additives can not only improve the scratch resistance of the film, but also have an impact on the adhesion resistance and gloss of the film. This paper investigates the effect of different wax additives on the final film properties when used in waterborne wood coatings.
1 Experimental section
1. 1 Raw materials
Wax dispersion (D-272), wax microdispersion (MD-2000): supplied by Winkel; wax emulsion (A-C392): supplied by H oneywell; film-forming additives (NEXCOAT 795), defoamer (681F), levelling agent (2020), multifunctional additives (AMP95): all industrial grade, supplied by Beijing Xingmeiya Chemical Co. Ltd.; wetting agent (Tween80): product of Tianjin Pharmaceutical Company; 1, 3-propanediol: product of Beijing Yili Fine Chemicals Co.
1. 2 Formulation process
( 1) Stirring speed of the high-speed disperser is 800 r /m in. Add ACIC-2602, AMP95, Tween80 aqueous solution and 1, 3 - propylene glycol into the paint mixing tank in turn, and the stirring time after each addition is about 10 m in.
( 2) Dissolve the defoamer (681F) in the film forming aid (NEXCOAT 795) and add it slowly to the above system with a stirring time of at least 60 min after addition.
( 3) Add the levelling aid, stirring time is about 30m in.
( 4) Add the wax emulsion and stir for 15m in.
( 5) Filter the material through a 120 mesh filter.
1. 3 Performance test
The particle size and distribution of the wax emulsion were determined by Nano ZS laser particle size analyzer from MALVERN. The surface morphology of the paint film was observed with an orthomorphic metallographic microscope of Chongqing Optical Instrument Factory; the recoatability of the paint film was determined according to GBIT1762- 80; the microscopic morphology of the wax particles was observed with a scanning electron microscope of Cambridge S250MK3.
2 Results and discussion
2. 1 Types of wax additives
According to the origin of waxes wax additives can be simply divided into natural and synthetic ones, where natural waxes mainly come from plants and animals and fossils; synthetic waxes are mainly chemically synthesised polyolefin waxes, such as polyethylene wax, polypropylene wax, PTFE wax, etc. Although micronised waxes are relatively simple to prepare and easy to store and transport, micronised waxes are not as easy to add and evenly disperse as other waxes in aqueous coatings, so they are less commonly used in practice. In order to facilitate the incorporation of wax additives into coating systems, wax emulsions, wax microdispersions and wax dispersions are made by direct emulsification or powdering and then emulsification.
In this paper, one type of wax additive was selected for the study, the main component of which was polyethylene wax. The wax additive D-272 is an aqueous wax dispersion based on polyethylene wax. Figure 1 shows the microscopic morphology of D-272 at different magnifications in a and b respectively. Figure 1C shows a microscopic photograph of the wax microdispersion MD-2000, from which it can be seen that the particle size of the wax microdispersions is generally below 1 Lm, basically spherical and relatively narrowly distributed; Figure 1d shows the wax emulsion A-C392, which has a small particle size in the range of 10 ~ 30 nm and a uniform distribution.
2. 2 Dispersion of wax additives in aqueous coatings
The mechanism of action of wax additives has been described in previous studies [3] as the /floating theory0 , which means that after the wax additives have been dispersed in the coating system, the wax particles will drift towards the surface of the coating film during the physical drying or curing crosslinking process after application, and will gradually migrate to the surface of the coating film, thus providing some protection, as shown in Figure 2.
This mechanism of action has been proposed in the study of solvent-borne coating systems, but for waterborne wood coatings, which have many different properties from solvent-borne coating systems, the dispersion of wax additives in waterborne systems has rarely been reported.
Figure 3 shows a microscopic photograph of the wax dispersion D-272 on the surface of a paint film.
As can be seen in Fig. 3, the wax dispersion particles seen in the field of view are mostly in the micron range, and the mass fraction of wax dispersion in the system after drying can be calculated from the amount of wax additive added to the system:
where: w is the mass fraction of wax dispersion in the film;
w1 is the solid content of the wax additive;
w2 is the solid content of the paint before the wax additive is added;
m1 is the amount of wax additive added;
m2 is the mass of the waterborne paint before the wax additive is added.
According to equation ( 1), the mass fraction of wax dispersion in the paint film is 51 04%. However, the particle size of the wax additive itself has a certain distribution (as shown in Figure 4).
The larger (micron-sized) wax dispersions in Figure 4 can be seen in Figure 3, while the smaller (nano-sized) ones cannot be seen at the magnification in Figure 3. The calculation results in a mass ratio of 42% for the larger particles (micron scale) and 58% for the smaller dispersions. This gives a theoretical percentage of the total field of view of 2.1% (5.04% × 42%) of the particle area shown in Figure 3. The percentage of particle area is 2.4%, which is not very different from the percentage measured in the figure, so it can be inferred that the particles of the wax dispersion are not basically enriched on the surface of the paint film, which is different from the distribution of the wax dispersion in the coating in solvent-based systems.
2. 3 Influence of wax additives on the scratch resistance of the paint film
Waterborne wood coatings formulated with emulsion 2602 as the main film-forming substance have excellent hardness, yellowing resistance and water resistance. The application of wax emulsions to the preparation of waterborne wood coatings can largely improve the resistance to re-stick and scratching of the paint film[5] . Figure 5 shows a microscopic photograph of the surface of the paint film after scratching.
As can be seen in Fig. 5, the surface of the paint film is easily scratched before the addition of the wax additives, but after the addition of different types of wax additives, the scratch resistance of the paint film surface is greatly improved under the same external force. The large particles in the dispersion D-272 play an important role in the resistance to external forces on the paint film. The resistance of MD2000 and A-C392 to external scratches is also relatively weak due to the small particle size.
In general, the addition of wax additives improved the scratch resistance of the film itself, which was enhanced by the large wax particles dispersed in the film to withstand the external damage[5] . Table 1 shows the effect of the addition of wax additives on the scratch resistance of the paint film.
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