Preparation of Waterborne Soap-free Acrylic Emulsion with High Transparency and Excellent Warm Wood Effect

MA Ji-quan, LIN Zheng-chao, LI Ning, SUN Wei-zu, QIAO Yi-tao, DENG Jun-ying
(Wanhua Chemical Group Co., Ltd., National Polyurethane Engineering and Technology Research Center, Yantai 264002, Shandong,
China)

Abstract: Methyl methacrylate (MMA), butyl acrylate (BA), methacrylic acid (MAA) and other monomers were adopted first as the raw material to synthesize macromolecular stabilizer through solution polymerization. Then the above macromolecular stabilizer synthesized was adopted for emulsion polymerization with MMA and BA as the raw material, preparing non-soap waterborne acrylic emulsion. The influence of the macromolecular stabilizer dose, glass transition temperature (Tg), acid value and other factors on the stability of emulsion polymerization were discussed. Through the research, it was found that the stable nonsoap waterborne acrylic emulsion with particle size between 50~70 nm could be synthesized if the macromolecular stabilizer dose was no less than 5%, Tg between 10~50 ℃ and acid value between 100~400 mg KOH/g. This emulsion had extremely high in-tank clarity, excellent warm wood effect and high hardness as well as fast rubbing property, and it had great application value in high-end wood coating such as American antique furniture coating.

0 Preface

American painting refers to furniture painting suitable for use and popular in Europe and the United States, with a strong European and American style. The coating features mainly reflect the retro and return to nature, fully showing the nature of the wood. It meets the psychological needs of people yearning for nature and returning to nature, and is a trend for the development of furniture in the future. At present, most American coatings are solvent-based nitro paints. With the inevitable trend of environmental protection and the progress of water-based processes, a water-based paint is required to replace the solvent-based nitro paints.Nowadays, conventional water-based acrylic emulsions have been widely used in various fields of water-based wood coatings because of their high cost performance. However, high-end coatings such as American antique coatings have high requirements on the appearance of coating films. The appearance of base coatings, such as high transparency and excellent warm wood effect.
This study describes an aqueous soap-free acrylate emulsion with high transparency and excellent warm wood effect. First, a macromolecular stabilizer is synthesized by solution polymerization, and then emulsion polymerization is performed to prepare a soap-free acrylic emulsion. The effects of the amount of macromolecular stabilizer, glass transition temperature (Tg) and acid value on the stability of emulsion polymerization were discussed. The transparency and warm wood effects of conventional emulsifier-stabilized emulsions, polymerizable emulsifier-stabilized emulsions, and soap-free emulsions were compared.


 1 Emulsion synthesis

1.1 Experimental materials and equipment

The main raw materials for emulsion synthesis are shown in Table 1.

The main raw materials for emulsion synthesis

Experimental equipment: IKA stirrer, condenser, thermometer, peristaltic pump, four-necked flask, magnetic stirrer, rotary evaporator, water bath, pH meter.

1.2 Preparation of macromolecular stabilizers

In a four-necked flask equipped with a stirrer, a condenser, and a thermometer, 100 g of ethanol was added, and the temperature was raised to 75 ° C. Mix 30 g of MMA, 40 gBA, 30 g of MAA, and 3 g of dodecanethiol, dissolve 1 g of AIBN in 10 g of ethanol, add the monomer mixture and the initiator solution dropwise at the same time, and control at 6 h. Incubate for 2 h after completion. Cool and cool to room temperature and filter the material. Finally, ethanol was removed to obtain a macromolecular stabilizer, whose properties are shown in Table 2.

Macromolecular stabilizer properties

1.3 Preparation of soap-free acrylic emulsion

In a four-necked flask equipped with a stirrer, a condenser, and a thermometer, add 10 g of the synthesized macromolecular stabilizer and 240 g of water, and heat up to 80 ° C [2]. Mix 60g of monomer MMA and 40g of BA uniformly, dissolve 0.3g of APS in 10g of water, add monomer mixture and initiator solution dropwise at the same time, control to 2h, and keep warming for 2h after the dropwise addition (as shown in Figure 1) ). Cool, cool to room temperature and neutralize after filtering. Emulsion technical indicators are shown in Table 3.

Preparation of soap-free acrylic emulsion
Emulsion technical index

2 Study on the stability of macromolecular stabilizers to soap-free emulsion polymerization

The effects of acid value, amount and Tg of macromolecular stabilizer and the amount of chain-transfer dodecyl mercaptan on the stability of the emulsion polymerization process were discussed. It can be seen from Table 4: (1) With the increase of the acid value of the macromolecular stabilizer, the synthesis process becomes stable, but when the acid value is too high as 400 mg KOH / g, the stability of the emulsion decreases, so the The acid value should be appropriate, too low or too high will not provide good stabilization; (2) as the amount of macromolecular stabilizer is increased, the stability of emulsion polymerization is improved; (3) as the macromolecular stabilizer Tg is increased High, the emulsion polymerization stability is improved, because the Tg of the macromolecular stabilizer is too low, and the latex particles collide with each other more easily during the synthesis of the emulsion and cause gelation; (4) with the increase of the chain transfer dose, the emulsion particles The larger the diameter, the more floc is due to the increase in the amount of chain transfer agent and the decrease in the molecular weight of the macromolecular stabilizer, which is not enough to stabilize the latex particles, resulting in partial coalescence.

Effect of Macromolecular Stabilizers on the Stability of Emulsion Polymerization

3 Research on transparency

The particle size of the soap-free acrylic emulsion prepared in this study is extremely small and has excellent tank transparency, similar to the appearance of a solvent-based resin, as shown in Figure 2 (a).This study compared the dry film transparency of conventional emulsifier emulsions, polymerizable emulsifier emulsions, and homemade soap-free emulsions on the market (see Table 5). It can be found that due to the presence of emulsifiers, conventional emulsifier emulsions accompany the film formation process. Due to the precipitation, migration, aggregation and crystallization of the emulsifier, the coating film is heterogeneous, light scattering occurs, and the transparency is weakened. Since the polymerizable emulsifier polymerizes to the latex particles to participate in film formation, the degree of precipitation and crystallization of the emulsifier Low, so transparency is improved; soap-free emulsions have a homogeneous coating film due to the absence of surfactants, and have the best transparency and high gloss.

Comparison of particle sizes of different acrylic emulsions
Comparison of particle sizes of different acrylic emulsions

4 Research on the effect of warming wood

The researchers tried to coat the varnish of common acrylic emulsion on the wood, and found that the effect of warming wood was poor after drying [Figure 3 (b)]. This is due to the lack of permeability of ordinary acrylic emulsions to the wood, which cannot completely fill the surface and deep cavities of the wood. When the water volatilizes , the emulsion forms a film, and there are still cavities deep in the ducts of the wood, and the light scattering effect is obvious. , Can not fully represent the original appearance of wood. Due to its extremely small particle size, the soap-free acrylic emulsion prepared in this study has excellent permeability, can fully fill the cavity of wood, has an excellent warming effect, and fully displays the original appearance of wood, as shown in Figure 3 (a).

Comparison of warming effect of different acrylic emulsions

At the same time, it was found that this soap-free acrylic emulsion has a special core-shell structure, which can impart high hardness (pencil hardness F) and fast sandability to the coating film.


 5 Conclusion

In this study, a synthetic macromolecular stabilizer was used for emulsion polymerization. The effects of the acid value, amount, Tg and chain transfer amount of the macromolecular stabilizer on the stability of the emulsion polymerization process were discussed. A stable soap-free aqueous acrylic emulsion was successfully synthesized. It does not contain surfactants, which gives extremely high transparency to the coating film, while the ultra-small particle size gives the coating film an excellent warm wood effect, and its special core-shell structure gives the coating film high hardness and fast sandability.

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