Study on the Factors Influencing the Performance of Stain-resistant Elastic Latex Paint

LIN Tao, CHEN Xiao-bo, ZHANG Cheng-ping
(BATF Industry Co., Ltd., Foshan 528322, Guangdong, China)

Abstract: The influence of dispersing agent and wetting agent on the stain resistance of latex paint is discussed in the paper; the influence of the addition of photoinitiator, cross-linking monomer and different glass transition temperature of elastic emulsion on the stain resistance of latex paint is discussed.

0 Introduction
Due to the inherent characteristics of cement substrates (belonging to brittle materials and low tensile strength), cracks often occur when environmental temperature, dry humidity and other factors change, and ordinary exterior wall coating decoration will also be affected by the substrate. Cracking. With the extension of time, cracks will further increase and deepen, which will adversely affect and even harm buildings. The elastic latex paint can solve the above problems to a certain extent. Due to the high elasticity of the elastic outer wall coating film, when the wall cracks and cracks appear, the coating film is stretched and deformed without cracking and destruction, and it is still intact The coating film maintains the original various barrier effects and prevents the cracked wall from being directly exposed to the atmosphere. The elastic coating film has barrier properties to certain substances (water and other liquids, etc.), and can prevent some corrosive substances from invading into the wall, thereby effectively protecting the wall from harm and giving full play to its protective function. Elastic coatings are becoming more and more popular in the market. However, one of the main components of elastic latex paint is an elastic emulsion. The glass transition temperature of elastic emulsion is generally lower than 0 ℃, which makes the elastic latex paint film soft and elastic, but under the hot weather in summer, the surface of the coating film is easy It is sticky, so that the dust in the air can easily adhere to the surface of the coating film, the stain resistance is greatly reduced, and the decorative effect is poor. Poor stain resistance is a common problem of elastic latex paints. How to improve the stain resistance of elastic latex paints has become one of the problems faced by developers of elastic latex paints. In this experiment, the effects of different types of elastomeric emulsions on stain resistance were explored. For the detection method, refer to GB / T9780-2013.

1 Experimental
1.1 Experimental materials and equipment

The materials and equipment used in the experiment are listed in Tables 1 and 2.

Experimental materials
laboratory apparatus

1.2 Coating Reference Formula

The experiment uses a kind of elastic coating formula commonly used in the laboratory as the reference formula. As shown in Table 3, the effect of using different wetting agents and dispersants in the formula on the stain resistance of the elastic latex paint is discussed.

Coating formula

1.3 Preparation of Different Elastic Emulsions
1.3.1 Emulsion Reference Formula
Table 4 is the reference formula for experimental emulsion polymerization.

Emulsion Reference Formula

1.3.2 Emulsion polymerization process

In this experimental emulsion polymerization process, a semi-continuous pre-emulsified monomer drop method is used. The initiation system is a redox system. The oxidant and reducing agent are ammonium persulfate and sodium bisulfite, respectively. Emulsion polymerization process: first prepare a pre-emulsion, take a certain amount of distilled water to dissolve part of the emulsifier in a clean beaker, then slowly add monomers in sequence while stirring, and stir vigorously to form a stable emulsion for use; Then add part of the emulsifier to a four-necked flask equipped with a thermometer, a condenser, and a stirrer, add deionized water and stir to completely dissolve, and heat up; when the temperature reaches 65 ℃, a certain amount of pre-emulsions previously prepared The solution was added to the reaction flask, the initial initiator was added, and the temperature was kept at 65-70 ° C for 10 minutes. Then, the remaining pre-emulsion was added dropwise to the reaction flask at 65-70 ° C at the same time, the initiator was added dropwise, and 2.5 was added dropwise. h, keep the temperature at 65 ~ 70 ℃ for 1.0 h, then drop the post-treatment agent to 60 ~ 65 ℃, add 60min dropwise, keep it for 30 minutes, and finally reduce the temperature to below 45 ℃.
1.3.3 Preparation of elastic emulsion
(1) Preparation of different Tg elastic emulsion: the main monomer styrene (Tg = 100 ℃), butyl acrylate (Tg = -55 ℃) and functional monomer methacrylic acid (Tg = 130 ° C) to prepare an emulsion. Emulsions with different Tg were designed by different proportions of styrene and butyl acrylate. This experiment designed elastic emulsions with Tg of -30.0 ℃ (Emulsion 1-1), -27 ℃ (Emulsion 1-2), -24 ℃ (Emulsion 1-3), and -21 ℃ (Emulsion 1-4). The monomers used are shown in Table 5.

Emulsion monomer ratio of different Tg

(2) Preparation of elastic emulsions with different amounts of photoinitiator: The photoinitiator 1-hydroxy-cyclohexyl-phenylmethyl ketone used in the experiment is one of the most commonly used photoinitiators in China, which has a high photoinitiating activity , Excellent thermal stability and no yellowing. In this experiment, 0.5% (emulsion 2-1), 1% (emulsion 2-2), and 1.5% (emulsion 2-3) were added on the basis of emulsion 1-1 (Tg = 30 ° C), respectively, based on the total mass of the monomer. ) Photoinitiator, monomer formulation is shown in Table 6.

Emulsion monomer ratio of different photoinitiators

(3) Preparation of elastic emulsions with different amounts of cross-linking monomer: The cross-linking monomer used in the experiment was silicon coupling agent A-171. A-171 is a common crosslinking monomer which is widely used in emulsion polymerization. It has the characteristics of weather resistance and dust resistance. In this experiment, based on emulsion 2-2, cross-linking monomers that accounted for 0.2% (emulsion 3-1), 0.4% (emulsion 3-2), and 0.6% (emulsion 3-3) were added. The monomer allocation is shown in Table 7.

Emulsion monomer ratio of different crosslinking monomers

1.4 Testing method
The emulsion prepared above was formulated into elastic latex paint samples according to the coating formula in Table 5 for later use.
For the experimental test method, refer to GB / T 9780-2013. On the asbestos board (150 mm × 70 mm × 4 mm) coated with a transparent primer, use 120 μm and then 80 μm wire rod applicator to apply the paint sample. The interval between two coatings was 2 h, and two samples were prepared for each sample. After 7 days of curing, one sample was irradiated with UV light for 4 h, and one was not illuminated, and then 5 points were taken at the top and bottom of the sample to test the initial reflectance, the average value was taken, and the prepared coal ash [m (Fly ash): m (water) = 1: 1] Evenly spread on the sample plate (0.7 ± 0.1) g, leave it at 60 ℃ for 0.5 h and leave it at room temperature for 2 h, and then rinse the coating in a flushing device. The sample was placed at room temperature for 2 d. This is a cycle. The entire cycle is about 24 h. Repeat the two cycles in this way. Finally, test the reflectivity of the sample after the cycle. This is the final emissivity. Calculate the change in reflectivity before and after (%). The change in reflectivity (%) = (initial reflectance-final reflectance) / initial reflectance.

2 Results and discussion
2.1 Selection of dispersant and wetting agent in coating formulations
Due to experimental conditions, this experiment first selected a common elastic emulsion on the market, RS-9699 produced by Bardford, as a standard emulsion. Then the selection of dispersant and wetting agent in the coating formulation of Table 3 is discussed. In this experiment, several different dispersants and wetting agents were selected, and the same standard emulsion was used to formulate latex paints according to the coating formulations in Table 3. Test their stain resistance, and select the one with the best stain resistance A wetting and dispersing agent is used in coating formulations.
2.1.1 Dispersant
Table 8 shows the effect of stain resistance on latex paints when two different dispersants are used.

Stain resistance of different dispersants

Dispersants Sn-5027 and APC are polymer dispersants of hydrophobically modified polyacrylic acid ammonium salts, and dispersants Sn-5040 and A4100 are polycarboxylate sodium salt type dispersants. Sn-5040 and A4100 are more hydrophilic than Sn-5027 and APC. As can be seen from Table 8, under the same conditions, the use of hydrophilic dispersants Sn-5040 and A4100 significantly improved the stain resistance compared with the use of hydrophobic dispersants Sn-5027 and APC; and the use of Sn-5040 was more resistant to stains than A4100. Slightly better.
2.1.2 Wetting agent
Table 9 shows the results of stain resistance of latex paints when wetting agents with different HLB values ​​are used.

Stain resistance of different wetting agents

LCN407 and LCN088 are both alkyl polyoxyethylene ethers, which are nonionic wetting agents. ED3060 and PE20 are EO / PO-block copolymers, which are also nonionic wetting agents. The larger the HLB value, the more hydrophilic the wetting agent. From Table 9, it can be seen that the stain resistance of the latex paint gradually increases with the increase of the HLB value of the wetting agent, but the increase is not large.
Based on the above results, it is speculated that the hydrophilicity of the additives in the coating formulation will help to improve the stain resistance of the latex paint. Therefore, the more dispersant used in the coating formulation in this experiment is Sn-5040. The aerosol was selected from the more hydrophilic LCN407. The coating formulation is shown in Table 10.

Coating formula

2.2 Effect on Different elastic latex paint stain resistance properties
2.2.1 emulsion polymerization of monomers selected
choice of monomers is one of the key factors that determine the performance of the emulsion. Currently monomer species can be used in the emulsion polymerization there are many, there are 600 kinds, where the widely used mainly three categories: (1) vinyl monomers, such as styrene, ethylene, vinyl acetate, acrylonitrile, acrylamide, vinyl chloride and the like; (2) a conjugated diene monomer such as butadiene, chloroprene, isoprene and the like; (3) acrylic and methacrylic monomers such as methyl acrylate , ethyl acrylate, butyl acrylate, methyl methacrylate, butyl methacrylate and the like. Depending on the nature of the monomers, the present experiment roughly divided into hard monomer and soft monomer and the functional monomer 3 classes. Glass transition temperature of the soft monomer (Tg) generally below 0 ℃, it gives an emulsion good flexibility, elasticity is the main monomer emulsion prepared. Butyl acrylate (BA) is a commonly used soft monomer whose homopolymer has a Tg of -55 deg.] C, have good elongation properties, but used alone makes the coating surface soft, sticky, dust in the air is easy adhesion, stain resistance difference. To make an emulsion having a certain elastic strength is necessary to add a certain hard monomers, hard monomers generally have a higher Tg, styrene as conventional hard monomer whose homopolymer has a Tg of 100 deg.] C, and styrene can be soft BA copolymerized monomers, and has a certain rigidity, a certain degree of strength can give an emulsion, the emulsion stain resistant elastic some help. The main function of the monomer emulsion to provide certain special functions, such as stain resistance, solvent resistance, water resistance and the like. Therefore, in this experimental emulsion polymerization, styrene and butyl acrylate were used as the main monomers, the cross-linking monomer A-171 (vinyltrimethoxysilane), the photoinitiator 1-hydroxy-cyclohexyl-phenylmethyl ketone, and methyl formaldehyde. Acrylic acid as a functional monomer.

2.2.2 Different Tg elastic latex stain resistance performance test results
according to the above detection method, different Tg elastic latex formulated as shown in Table 11 as an elastic latex stain resistance results.

Emulsion Tg of different elastic resistance to stains

It can be seen from the test results in Table 11 that as the Tg of the emulsion increases, the change in reflectance of the latex paint sample gradually becomes smaller, that is, the better the stain resistance of the latex paint. Therefore, from this set of experiments, it can be seen that increasing the Tg of the emulsion can improve the stain resistance of the latex paint. This is because the higher the Tg of the emulsion, the higher the hardness of the coating film after the latex paint is formed, and the pollutants are less likely to adhere. The better the stain resistance. However, for elastic latex paints, the higher the emulsion Tg, the greater the strength of the coating film, and the worse the flexibility, which leads to a decrease in the elongation of the coating film, especially the low temperature elongation. Therefore, when increasing the Tg of the emulsion to improve the stain resistance of the latex paint, it is also necessary to consider the mechanical properties of the latex paint. It is best to find a balance point, which will not be discussed in this experiment. Considering the low elongation of the latex, the Tg of this emulsion preferably experiment -30.0 ℃.

2.2.3 different elastic resistance to the emulsion an amount of a photoinitiator soiling test results
according to the above detection method, different photoinitiators elastomeric emulsion is formulated dosage flexible latex stain resistance results are shown in Table 12.

Different photoinitiators elastic latex stain-resistant properties of the dosage

As can be seen from the test results, the addition of a photoinitiator, reflectance values do not change as a UV coating on substantially unchanged, substantially no change in stain resistance; 4 hUV reflectance after irradiation of the coating film becomes significantly small, but with The more the photoinitiator is added, the smaller the decrease in the reflectance change value, and the photoinitiator is more expensive than the main monomer. Therefore, this experiment believes that it is reasonable to control the amount of photoinitiator to 1% of the total monomer. 1-Hydroxy-cyclohexyl-phenyl ketone is a cleavable photoinitiator. Its mechanism of action is that when the molecule receives light to absorb light energy, it transitions to the excited singlet state, and then jumps to the excited triplet state through intersystems. The molecular structure It is in an unstable state, in which weak bonds will evenly split to generate active radicals, increase the degree of cross-linking on the surface of the coating film, increase the hardness of the coating film, and improve the stain resistance of the latex paint.

2.2.4 resistant flexible latex different amounts of crosslinking monomers soiling test results
according to the detection method described above, the amounts of different monomers elastic latex crosslinked elastomeric latex formulated stain resistance results are shown in Table 13.

Resistant flexible latex crosslinkable monomer different amount of contamination

Can also be seen, with the addition of a crosslinking monomer A-171, the reflectance variation value becomes gradually smaller template paint, latex paint stain resistance becomes better. When the cross-linking monomer is added to a certain amount, the improvement of the stain resistance of the latex paint becomes smaller, and the influence of the cross-linking monomer A-171 on the mechanics of the latex paint and the reactivity during the emulsion polymerization process are taken into account. A-171 crosslinking monomer expensive prices. This experiment therefore believed that the addition amount of the crosslinking monomer A-171 is reasonable when 0.2% of the total amount of the monomers. A-171 mechanism of the crosslinking agent is a silicon-bonded hydrolyzable alkoxy generating silanols, dehydration condensation between the silicon-containing low molecular silicone synthetic hydroxy silanols, and oligomer silanols can be a hydroxyl group forms a hydrogen bond on the substrate surface, enhance adhesion, may be formed with the hydroxyl groups in the latex paint when forming a hydrogen bond or a covalent bond, to improve the degree of crosslinking of the coating, the coating hardens, thereby improved stain resistance of paint.

3 Conclusion
From the results of this experiment, the use of hydrophilic additives than good stain resistance using a hydrophobic paint auxiliaries in formulations of paint formulation. Because most of the pollutants in the domestic air are lipophilic, it is easy to adhere to hydrophobic latex paints. In addition, hydrophobic latex paints are difficult to be wetted by rainwater, and rainwater rolls over the surface of lipophilic pollutants and cannot. Taking away the contaminants makes the hydrophobic elastic latex paint more resistant to staining.
In terms of emulsions, the higher the emulsion Tg, the better the stain resistance of the latex paint, but the emulsion Tg has a greater effect on the elongation of the latex paint, and the mechanical properties of the latex paint must be taken into account while increasing the emulsion Tg. Adding cross-linking monomers and photoinitiators to the emulsion can make the latex paint film denser, increase the hardness of the coating film surface, and reduce the tackiness of the coating film surface, which can increase the adhesion of pollutants on the surface of the latex paint. Difficulty, thereby improving the stain resistance of the elastic latex paint.

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