Thickener is an important auxiliary agent in textile printing process. Sodium alginate in natural thickener is widely available, but it is sensitive to pH value. At the same time, its structural viscosity is low and its rheology is poor, which limits its application. Polyacrylic acid in synthetic thickener has obvious thickening effect, convenient use, high production efficiency and wide application, but it is sensitive to electrolyte in the system and its electrolyte resistance needs to be improved.
The introduction of silicone monomer into polyacrylic thickener can improve its weather resistance, high and low temperature resistance, and at the same time, its structural viscosity and electrolyte resistance can be improved with the help of the formed cross-linked network structure. Therefore, more and more attention has been paid to the theory and application technology of silicone modified acrylic acid polymerization.
When hydrophobic polysiloxane segments are introduced into polyacrylic acid system, a certain length of ethoxy groups are needed as bridging groups. Ethoxy group can enhance the flexibility of the associated monomer and increase the viscosity and elastic modulus of the system. Polysiloxane segments will agglomerate in the water phase, similar to the bundling process of surfactants, and these hydrophobic segments will associate with each other to form a network structure, which plays a thickening role. It should be noted that when the concentration of hydrophobic groups is low, intramolecular association mainly occurs, which has little effect on the three-dimensional network structure of the polymer and is not enough to increase the viscosity of the system rapidly. Only when the concentration of hydrophobic groups increases, intermolecular association occurs and cluster-like associations are formed, will a solid three-dimensional network structure be formed and the viscosity of the system be enhanced.
Therefore, how to control the number of ethoxy segment units and the length of polysiloxane hydrophobic units, and adjust the number of clusters and the number of hydrophobic unit aggregates in clusters are important factors that determine the thickening effect of silicone modified polyacrylic thickener.
The results show that the viscosity of thickener solution with a mass fraction of 3% can be increased from 0.06 Pa·s to 300 Pa·s when the number of polyoxyethylene units is increased from 0 to 30, and then the viscosity of the solution decreases with the increase of polyoxyethylene units. The researchers also found that the size of hydrophobic units has a certain influence on the rheological properties of the system. With the increase of carbon chain length, the hydrodynamic volume of the polymer increases and the apparent viscosity of the system increases. In addition, the longer carbon chain length is very sensitive to shear rate, and it is easy to appear shear thinning phenomenon.
The researchers developed a hydrophobic and hydrophilic polyoxyethylene ether grafted polysiloxane, which was polymerized with acrylic acid/acrylate monomer in miniemulsion to form hydrophobic modified alkali-soluble associative thickener emulsion. The thickening effect and shear resistance of the original paste were improved by adjusting the ratio of polyethoxy unit to polysiloxane hydrophobic segment unit. It was found that when its mass fraction was 0.5%, the viscosity of the solution reached 1394 mPa·s, while the viscosity of sodium alginate solution was only 1007 mPa·s at the same concentration, which had a good thickening effect. Adding 10 drops of 15% NaCl solution to 100 g thickener solution, its viscosity retention rate is above 95%, and its electrolyte resistance is good. When the thickener solution with 2% mass fraction was left for 7 days, the viscosity of the paste increased, indicating that its molecular network structure was fully swollen and expanded, resulting in higher viscosity. When the thickener is used in reactive dye printing of cotton fabric, the printing effect is better than that of sodium alginate thickener, no matter the clarity and uniformity of fine lines or the color depth of patterns.
At present, the research on silicone modified polyacrylic thickener mainly stays in the laboratory stage, and has not been industrialized on a large scale. The following points should be paid attention to in its research and development:
(1) Selecting polysiloxane segments with a certain length can form a good intermolecular hydrophobic association structure, which is convenient for adjusting the viscosity of the system. In addition, the selection of comonomers should take into account the changes of hydrophobicity, hydrophilicity and glass transition temperature;
(2) The miniemulsion polymerization adopts the monomer droplet nucleation mode, which avoids the water phase migration of monomers and is suitable for the copolymerization of hydrophobic monomers. At the same time, the amount of surfactant should be controlled in the reaction;
(3) The control of hydrophobic associating monomer is the key to the preparation of alkali swelling associative polyacrylic acid thickener. By controlling the proportion of polyethoxy unit and polysiloxane hydrophobic segment unit in hydrophobic association monomer, silicone modified polyacrylic thickener with better performance can be designed, which can enhance the rheological property of the system and improve its electrolyte resistance.
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