The defoamer should have the following characteristics :
1. The surface tension of the defoamer is smaller than the surface tension of the liquid to be defoamed
2. The solubility in the liquid to be defoamed should be as small as possible
3. To have good dispersibility and strong dispersibility
4. Can not react with the liquid to be defoamed, and can not affect the stability of emulsion and resin
5. Good defoaming durability, so a good defoamer should have foam suppression, foam breaking and long-lasting defoaming capabilities.
The thickening effect of cellulose ether is the viscosity of HEC with different molecular weights measured with a Hepler falling ball viscometer at 20°C in a 2% aqueous solution.
Due to the entanglement of cellulose molecules, the flow of the aqueous solution is pseudoplastic. At low shear rates, the viscosity is higher, and the shear rate is increased, the molecules are oriented in the direction of flow, and the resistance against external movement is reduced, which will lead to a decrease in viscosity and shear thinning. After the shearing stops, the viscosity resumes. The higher the molecular weight of the cellulose ether, the higher the thickening effect. Low-molecular-weight cellulose ethers exhibit lower pseudoplasticity. In water, the cellulose ether shows a certain degree of thixotropy when the concentration is high, and does not show any thixotropy when the concentration is low.
In addition to the molecular weight, the degree of substitution of the cellulose ether chain also affects the rheological curve. All in all, methyl hydroxyethyl cellulose (MHEC) exhibits lower pseudoplasticity than hydroxyethyl cellulose HEC, and methyl cellulose MC is better than hydroxyethyl cellulose (HEC) and ethyl hydroxyethyl cellulose. (EHEC) has lower pseudoplasticity. At the same concentration, the thickening effect of MC is stronger than that of HEC and CMC . However, the thickening effect and rheological curve shape are mainly determined ptfe powder cost by the molecular weight, and the degree of substitution also has a small effect. The choice of cellulose ether for coatings should not only consider the shape of the rheological curve, but also other properties such as splashing, sagging, leveling, foam stabilization, water retention, and compatibility with other components in the formulation.
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