Research on the Influencing Factors on Mechanical Stability of Styrene Acrylic Emulsion

TANG Bing, LUO Yun
(BATF Industry Co., Ltd., Foshan 528322, Guangdong, China)

Abstract: Styrene acrylic emulsion formula was analyzed. The influences of monomer ratio, synthesis process, emulsifier and hydrophilic monomer on mechanical stability of styrene acrylic emulsion were investigated. Emulsifier and hydrophilic monomer had great influence on the mechanical stability. Through core-shell process, by adding hydrophilic monomer, the prepared modified styrene acrylic emulsion had good mechanical stability.

0 Introduction
The ability of a polymer emulsion to withstand damage from external factors is called the stability of the polymer emulsion. The stability of the polymer emulsion is one of the most important physical properties of products such as latex coatings and emulsion adhesives. The basis for finished application performance. Mechanical stability refers to the stability of the emulsion when subjected to mechanical operations. Because in the process of preparing coatings, pumping, stirring, and spraying during coating are required, the emulsion and its coatings must withstand mechanical operations.
Through testing, find out the relevant factors that affect the mechanical stability of the emulsion, and select methods to improve the mechanical stability.

1 Test Section
1.1 Test raw materials
Distilled water, styrene, methyl methacrylate, isooctyl acrylate, acrylamide, acrylic acid, crosslinking monomer, anionic emulsifier, reactive emulsifier, ammonium persulfate, ammonia water Selling industrial products.


1.2 Test equipment
5 000 mL four-necked flask, condenser tube, thermometer, heating rice cooker, temperature controller, peristaltic pump, hose, and several beakers with different capacities.


1.3 Polymerization process

In this test, a combination of core-shell and semi-continuous pre-emulsification dripping process was used for polymerization.


1.3.1 Preparation of the core layer pre-emulsion solution
Dissolve part of the reactive emulsifier and anionic emulsifier in a 1000 mL beaker with an appropriate amount of distilled water, and then stir with a disperser. After all the emulsifiers are completely dissolved, sequentially Add an appropriate amount of styrene, acrylamide, acrylic acid, and ammonium persulfate to disperse for 10 minutes.


1.3.2 Preparation of the shell pre-emulsion solution
Dissolve the remaining reactive emulsifier and part of the anionic emulsifier in a 2 000 mL beaker with an appropriate amount of distilled water, and stir with a disperser. After all the emulsifiers are completely dissolved, Then, methyl methacrylate, isooctyl acrylate, acrylamide, hydroxyethyl methacrylate, acrylic acid, crosslinking monomer, and ammonium persulfate were added in order to disperse uniformly.


1.3.3 Preparation of kettle bottom and specific steps of test
First dissolve the remaining anionic emulsifier with distilled water and add it to a four-necked flask. At the same time, it is equipped with a condenser tube and a thermometer, and the temperature is raised to 80 ° C while stirring. An appropriate amount of an aqueous solution of ammonium persulfate that has been dissolved is used as an initial introduction. An appropriate amount of the core layer pre-emulsion is added to a four-necked flask. After the temperature is maintained for 30 minutes, the remaining core layer is dropped within 30 to 60 minutes. Pre-emulsions. After 60 minutes of incubation, the shell pre-emulsion was added dropwise. After dripping within 2 to 3 hours, the solution was kept for 1 hour, the temperature was lowered to below 50 ℃, and the pH value was adjusted to 7.5 with ammonia water, and the material was filtered out.


1.4 Determination of experimental research route
1.4.1 Stability test method for styrene-acrylic emulsion mechanical stability The basic formula of the emulsion mechanical stability test is shown in Table 1. (1) First put 10 g of the emulsion to be tested in a dispersion tank. (2) Next, add 14 g of water and 5 g of film-forming aid C-12 to the dispersion tank. (3) Place the dispersing cylinder in the dispersing machine, start the dispersing machine, adjust the rotation speed to 100-500 r / min, disperse at the rotating speed for 2 to 3 minutes, then increase the rotating speed of the dispersing machine to 3 000 r / min, and add 90 g of 80 mesh marble and start timing at the same time. (4) Observe the system obtained by dispersion. If the system is not gelled, it means that the mechanical stability of the emulsion to be tested is good; if the system is gelled, it means that the mechanical stability of the emulsion to be tested is poor, and record its time.

Basic formula for testing emulsion mechanical stability

As can be seen from Table 1, this is a relatively basic formula for testing the mechanical stability of emulsions. This article will also test on the basis of this formula. The major influences on emulsions in the table are film-forming aids, emulsions, and Gradation of sand. So the following will also explore the mechanical stability of real stone paint from these three aspects. The emulsion was a commercially available styrene-acrylic emulsion.

1.4.1.1 Effect of Adding Amount of Film-forming Auxiliary on Mechanical Stability
Under the condition that other proportions of the formula remain unchanged, adjust the amount of film-forming auxiliary: 5 g, 4 g, 3 g, 2 g, 1 g (see Table 2). The influence of the film-forming aid on the continued stability is examined. The test results are shown in Table 3.

Reference formula for testing the influence of the amount of film-forming additives on mechanical stability

It can be seen from Table 3 that the addition amount of the film-forming aid has a great influence on the mechanical stability of the emulsion. Under the condition that other conditions are not changed, the addition amount of the film-forming aid is reduced from 5 g to 1 g, and it can be concluded that the mechanical stability of the emulsion is significantly increased.
1.4.1.2 Effect of water addition on mechanical stability

The reference formula for testing the effect of water addition on mechanical stability is shown in Table 4, and the test results are shown in Table 5.

Reference formula for testing the effect of water addition on mechanical stability

Water in this test formula mainly plays the role of dispersing the emulsion. Too much will cause the emulsion to be too thin, which will adversely affect the mechanical stability test and increase the test result.


1.4.1.3 Effect of sand grade on mechanical stability

The reference formula for testing the effect of sand addition on mechanical stability is shown in Table 6, and the test results are shown in Table 7.

Reference formula for testing the effect of sand grade on mechanical stability

After a large number of tests, it can be concluded that the effect of sand on mechanical stability is large, and the effect on mechanical stability increases with the increase of sand. Because the sand will physically damage the emulsion during dispersion, the larger the sand amount, the more severe the damage. However, when the amount of sand is added to a certain amount, because the amount of sand is too large, the emulsion cannot completely wet the sand.
In summary, according to the actual needs and other influencing factors, this article chooses the basic formula A as the test formula.


1.4.2 Emulsion formulation design
From the styrene-acrylic emulsion formulations tested in the previous experiments, the effects of different factors on the mechanical stability of the emulsion were investigated. Styrene, methyl methacrylate and isooctyl acrylate were used as the main monomers, and an appropriate amount of Hydrophilic monomer, cross-linking monomer.


1.5 Emulsion performance index (see Table 8)

Emulsion Performance Index

2 Results and discussion
2.1 Dosage and matching of emulsifier
Emulsifier is one of the components of the emulsion system. It plays a very important role in the emulsion and has a great influence on the speed of the polymerization reaction, the particle size of the emulsion and the molecular weight of the polymer. The anionic emulsifier is charged to form an electric double layer. At the same time, a certain degree of hydration is added to make the polymer more stable and increase the polymerization rate. The latex particles with narrower particle size distribution and smaller particle size are obtained. Since real stone paint needs better water resistance, some reactive emulsifiers can be added to improve the water resistance of the emulsion. The effect of the amount of emulsifier on the stability of the emulsion is shown in Table 9.

Effect of emulsifier dosage on emulsion

It can be seen from Table 9 that with the increase of the emulsifier, the mechanical stability of the emulsion also increases, but the increase in viscosity is not conducive to production. The water resistance of the emulsion coating film increases within a certain range with the increase of the emulsifier. However, too much amount will reduce water resistance. This is caused by the large particle size of the emulsion when the emulsification amount is too low, and the film formation is not very dense; when the emulsifier is too much, the migration of the emulsifier will affect the water resistance effect of the emulsion coating film, and at the same time, the excessive emulsifier will affect The stability improvement effect is also limited. From the above, it can be determined that the amount of emulsifier is 0.9% is better, and the cost can also be controlled.


2.2 Research on Synthesis

Processes Existing synthesis processes are various. Commonly used in production include one-step dropping and core-shell processes. Therefore, under the premise of ensuring the total amount of the formula and the proportion, two experiments were conducted to change the polymerization process. The influence of the mechanical stability was examined, and the results are shown in Table 10.

Effect of Polymerization Process on Watermark Recovery

It can be known from Table 10 that the one-step drop-to-core core-shell process is helpful for mechanical stability, and in theory, the hard core and soft shell process will cause the emulsifier to reduce the monomer’s encapsulation and reduce the stability of the emulsion. Therefore, the test uses a one-step dropping process.


2.3 Functional monomers
Functional monomers include hydrophilic monomers and cross-linking monomers. This paper also considers these two categories. Although the addition of hydrophilic monomer will reduce its water resistance, it also has more prominent benefits. In order to improve the mechanical stability of the emulsion, some acrylamide can be added, and the stability of calcium ions is also increased, see Table 11.

Effect of acrylamide on mechanical stability

As can be seen from Table 11, acrylamide has a significant effect on mechanical stability, but excessive acrylamide has a significant effect on reducing the water whitening resistance of the emulsion.

3 Conclusion
(1) Through the adjustment of the testing formula, it was found that changing the amount of the film-forming aid had a greater impact on the mechanical stability performance, and that with the increase of the film-forming aid, the mechanical stability performance became worse.
(2) Through the adjustment of the test formula, determine the basic emulsion mechanical stability test formula: emulsion 10 g, water 14 g, alcohol ester 12 5 g, and sand 90 g.
(3) Through the adjustment of the emulsion formula, it is determined that the dropping process is a one-step dropping process, which is more conducive to the improvement of mechanical stability than the core-shell process.
(4) In the test, the amount of emulsifier has a great influence on the mechanical stability of the emulsion. It can be obtained through tests that when the amount of emulsifier is 0.9%, the mechanical stability of the emulsion is better.
(5) Through the test, it is determined that increasing the amount of acrylamide is beneficial to improve the mechanical stability of the emulsion, but too much acrylamide will reduce the water resistance of the emulsion.

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