Study on Preparation and Performance of a Solvent-free Waterborne Polyurethane for Wood Coatings

LIU Shan, LIN Zheng-chao, ZHANG Jie, SUN Wei-zu (Wanhua Chemical Group Co., Ltd., Yantai 264000, Shandong, China)

Abstract: With IPDI as hard segment, PTMEG1000 and PPG1000 as soft segment, and DMPA and BDO as micromolecule chain extender, a waterborne polyurethane resin is formulated by adjusting the proportion of the soft segment and the hard segment. As a film-forming resin for waterborne wood coatings, the resin not only has favorable mechanical strength and chemical resistance, but also offers outstanding film-forming properties and can effectively reduce the consumption of film-forming additive.

0 Preface

Solvent-based wood coatings contain a large amount of organic volatile compounds, which not only affects the health of production and construction personnel, but also pollutes the environment and wastes resources. With the restrictions on VOC use and emissions by environmental regulations and related policies, environmentally friendly water-based Coatings are one of the development trends in the coatings industry. Water-based resins represented by water-based polyurethane and water-based acrylates are playing an increasingly important role in coatings. Especially water-based polyurethane coatings are not only safe and environmentally friendly, but also the material has outstanding performance and has the high strength and wear resistance required by coatings. , Low temperature film formation and flexibility. At the same time, it can be mixed with acrylic resin and improve performance, so it is widely favored in the field of waterborne wood coatings.Waterborne polyurethanes for wood coatings are required to have good chemical resistance, high hardness, good permeability and fullness. According to the relationship between the structure and performance of waterborne polyurethanes, the waterborne polyurethane hardwood content is higher. A higher proportion of hard segments will affect the room-temperature and low-temperature film-forming properties of water-based polyurethanes. Therefore, during the application process, a certain proportion of film-forming additives will be added to improve film-forming properties, and the traditional water-based polyurethane synthesis process contains A large number of organic solvents NEP and NMP increase the VOC content of the formula. Based on the above background, this article uses IPDI as the hard segment, PPG1000 and PTMEG1000 as the soft segment, BDO and DMPA as small molecule chain extenders, and does not contain organic solvents such as NEP. An environmentally friendly solvent-free aqueous polyurethane emulsion Moreover, the water-based polyurethane has a very low film-forming temperature. In the application process, the amount of film-forming aids can be significantly reduced. By tracking the application performance of this water-based polyurethane, it is found that the comprehensive application performance of the emulsion is better than mainstream products .

1 Experimental part

1.1 The main raw materials of the experiment

Isophorone diisocyanate (IPDI), industrial grade, Covestro; polyoxypropylene glycol 1000 (PPG1000), industrial grade, Zibo Dongda; polytetrahydrofuran ether glycol (PTMEG1000), 1,4-butanediol (BDO), industrial grade, BASF; dimethylolpropionic acid, industrial grade, Perstrop; DMEA, ethylenediamine, industrial grade.

1.2 Synthesis of water-based polyurethane emulsion

In a four-necked flask equipped with a stirrer, a thermometer, and a reflux condenser, add the metered ratio of isophorone diisocyanate (IPDI), polypropylene oxide ether glycol (PPG1000), and polytetrahydrofuran ether glycol (PTMEG1000). ), After reacting at 80 ℃ for 1 h, add BDO, DMPA, quantitative acetone, and dilute the catalyst, and then react at 75 ℃ to reach the theoretical-NCO value, add a certain amount of acetone to dilute the prepolymer system, and lower it to a suitable temperature, add Neutralize DMEA, finish the neutralization, then pour the polymer into a dispersion cup, add deionized water under high-speed stirring at 1 200 r / min, and obtain an aqueous polyurethane dispersion under high-speed dispersion. After adding ethylenediamine, expand it. Chain, emulsion preparation is complete.

1.3 Preparation of water-based wood coatings

The waterborne wood coating formulation based on waterborne polyurethane emulsion is shown in Table 1.

Waterborne wood coating formulation based on waterborne polyurethane emulsion

Preparation process of water-based wood coatings: Add water-based polyurethane emulsion and defoamer 1 to the container in order, stir well, add matting powder, and quickly disperse at 800 ~ 1 200 r / min for about 15 min. Additive 2. Rheology additive, wetting and leveling agent, thickener, and H2O are mixed into the container slowly, and stirred at 600-800 r / min for 10 minutes; add defoamer 2, preservative, and stir 5 Filter out the material after min and let it stand.


1.4 Analysis and testing

1.4.1 Emulsion particle size

The particle size and particle size distribution of the polyurethane dispersion were measured by dynamic light scattering (DLS). In the experiment, the particle size and particle size distribution of the emulsion were measured using a Zetasizer Nano ZS90 particle size analyzer from Malvern. The test temperature was 25 ° C, the laser angle was 90 °, and the test laser wavelength was 633 nm.

1.4.2 Storage stability test

Aqueous polyurethane emulsion storage stability can be simulated by high temperature accelerated test. In a clean and sealed sample bottle, pour an appropriate amount of water-based polyurethane emulsion and place it in an oven at 50 ℃ for 1 month. After taking out, if the emulsion does not delaminate and precipitate, and there is no loss in application performance, it still meets the requirements for use. The emulsion is stable for storage.


1.4.3 Emulsion application performance test

According to international standards and laboratory standards, under the same test conditions, compared with mainstream products in the market, the drying time and hardness, mechanical properties, flexibility and chemical resistance of water-based polyurethanes are tested.


 2 Results and discussion

2.1 Performance of water-based polyurethane emulsion (see Table 2)

Performance of water-based polyurethane emulsion

As shown in Table 2, the performance of the aqueous polyurethane emulsion prepared in this paper is stable. At a solid content of 35%, the particle size of the emulsion is 55 nm, the viscosity of the emulsion is 200 mPa · s, and the viscosity is moderate. The minimum film-forming temperature of this water-based polyurethane is 7 ° C. It can achieve normal-temperature film formation without the addition of any film-forming aids, and it has excellent low-temperature film-forming properties when added with 1% to 2% film-forming aids. , Can effectively reduce the use of film-forming aids in the paint matching process. At the same time, it can be found from Table 2 that under the conditions of excellent film-forming properties, the film-forming breaking strength of the emulsion is 35 MPa, the 100% modulus is 20 MPa, and the elongation at break is 250%. The coating film has excellent mechanical properties; emulsion application With paint, the pencil has a hardness of HB, an impact resistance of 100 kg · cm, and a flexibility of 0.5 mm. It has excellent application mechanical properties.

2.2 Emulsion storage stability

After the emulsion was stored in an oven at 50 ℃ for 1 month, it was found that there was no layering of the emulsion in the sample bottle, no precipitation was found, and the application performance still met the requirements for use after the paint was mixed.2.3 Performance of waterborne wood coatingsAccording to the formula in Table 1, the coatings were prepared based on this emulsion and similar products on the market, and the following properties were compared and tested:2.3.1 Drying time and hardnessSubstrate: glass plate; drying conditions: 25 ℃, RH50%; wet film thickness: 150 μm, gloss and hardness curing at room temperature for 7 days, the results are shown in Table 3 and Figure 1.

Drying time and hardness, hardness establishes a trend

As shown in the data in Table 3 and Figure 1, after the water-based polyurethane prepared in this article is formulated into a coating, the coating film has high hardness, good wear resistance, and has advantages in drying speed and hardness establishment, which is beneficial to shorten the construction period and save time and cost. .2.3.2 Chemical resistanceSubstrate: black walnut, two sprays, 100 g / m2 each, after the first spray, dry at 50 ° C for 1 h, then sand with 400 # sandpaper, spray the second spray, leave at room temperature for 7 d, dry conditions : 25 ° C, RH50%.
Test method: 6 layers of filter paper are used to absorb the liquid to test the surface resistance.Chemical resistance test results are shown in Table 4.

Chemical resistance test results

Note: 5—best, 0—worst.Compared with similar products in the market, this water-based polyurethane emulsion has great advantages in terms of chemical resistance and permeability, and can be applied to furniture, home improvement and other fields requiring high performance.2.3.3 Blending performance with acrylicAcrylic emulsion coatings have good chemical resistance and high hardness, but at the same time they require higher film-forming aids and more brittle coatings, so they can be mixed together to improve their performance during use. This water-based polyurethane emulsion was mixed with a high-hardness self-crosslinking acrylic emulsion to test the performance at different mixing ratios, as shown in Table 5.
 It can be known from Table 5 that as the proportion of mixed polyurethane is larger, the system MFFT is lower, flexibility, impact resistance and fullness are improved, and chemical resistance is slightly decreased. When the ratio of polyurethane to acrylic emulsion is 1/2, the performance is the most good.


 3 Conclusion

Using IPDI as the hard segment, PPG1000 and PTMEG1000 as the soft segment, and BDO and DMPA as small molecule chain extenders, during the preparation of the emulsion, except for a certain amount of acetone-diluted prepolymerization system, without any other solvents, it was successfully prepared. Stable solvent-free aqueous polyurethane emulsion.
The application performance of this emulsion coating is tracked, and the conclusions are as follows:
(1) Compared with the mainstream products in the market, the drying speed and hardness establishment of the coating film have advantages, which is beneficial to shortening the construction period and saving time and cost.(2) This aqueous polyurethane emulsion does not contain any solvents and has good film-forming properties. It can be mixed with other emulsions to effectively reduce the amount of film-forming aids (solvents) and reduce the VOC of the formulation system.(3) The breaking strength and elongation at break are high, the coating film has excellent stretchability, and can be mixed with acrylic emulsion to improve flexibility.(4) Wood coatings based on this solvent-free water-based polyurethane have a smooth and smooth coating, high permeability, fullness, and high hardness. The coating film has excellent water resistance and chemical resistance, and comprehensive performance is better. The coating field has a wide range of application prospects.

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