Preparation of Polyether-Modified Polysiloxane Defoamers

　　Ordinary polysiloxane defoamers, due to their low surface energy and strong hydrophobicity, exhibit poor miscibility with other organic substances, leading to particularly ineffective performance in aqueous systems. For instance, under high-shear and high-temperature conditions—such as those encountered in jet dyeing processes—conventional polysiloxane defoamers tend to form film-like precipitates, which can cause spotting on the dyed materials. In contrast, polyether-modified polysiloxanes, thanks to the incorporation of hydrophilic polyether segments, can disperse and emulsify effectively in water while maintaining excellent emulsion stability. Additionally, these polymers possess inverse solubility properties, making them ideal for high-temperature, high-pressure liquid-flow dyeing processes. As a result, polyether-modified polysiloxanes have increasingly gained attention in recent years for their applications in defoaming agents. In this experiment, we used highly hydrogenated silicone oil as the raw material and synthesized low-hydrogenated silicone oil via a regulating polymerization method. Meanwhile, propargyl alcohol was employed as the initiator, and NaOH served as the catalyst to carry out ring-opening copolymerization of ethylene oxide and propylene oxide, yielding an allyl-terminated polyoxyalkylene ether. Finally, this allyl-terminated polyoxyalkylene ether was used to graft-modify the low-hydrogenated silicone oil, resulting in the desired polyether-modified polysiloxane, which was then formulated into a defoamer.
　　1 Experiment
　　1.1 Main Ingredients
　　Dimethylcyclosiloxane Mixture (DMC): High-Hydrogen Silicone Oil: Active hydrogen mass fraction is 1.60%.
　　Hexamethyl disiloxane (MM): Purity ≥99%; Ethylene oxide and propylene oxide: Industrial grade.
　　Methyl Glucoside Sesquisterate (SS), Polyethylene Glycol Ether (SSE-20): Industrial grade; Gas-phase Silica: Industrial grade.
　　1.2 The preparation of low-hydrogen silicone oil was carried out using the oligomerization method [3]. Specifically, 111.5 g of DMC, 6.5 g of high-hydrogen silicone oil, and 2.2 g of MM were added to a reaction vessel. Concentrated sulfuric acid was used as the catalyst, and the mixture was reacted at 60–65°C for 3–5 hours. Afterward, the reaction mixture was cooled to room temperature, neutralized with sodium bicarbonate, and then filtered. Finally, the low-boiling components were removed by vacuum distillation at 110°C, yielding low-hydrogen silicone oil with an active hydrogen mass fraction of 0.09%.
　　1.3 Synthesis of Allyl-Polyoxyalkylene Ether
　　In a high-pressure reactor, add a measured amount of allyl alcohol and an alkaline catalyst, securely seal the reactor lid, and then purge the system three times with N₂. Next, evacuate the reactor and gradually heat it up; while stirring continuously, introduce ethylene oxide and propylene oxide at temperatures between 90°C and 110°C. Once the feeding is complete, allow the reaction to mature until the pressure inside the reactor drops to negative values. Finally, cool the mixture, discharge the product, and subject it to neutralization, bleaching, and decolorization. Afterward, filter and dehydrate the solution to obtain the terminal allyl-terminated polyoxyalkylene ether.
　　1.4 Synthesis of Polyether-Modified Polysiloxane
　　Under the catalytic action, the Si-H groups in hydrogen-containing silicone oil can undergo a hydrosilylation reaction with the double bonds in the terminal allyl polyoxyalkylene ether, yielding Si-C-type polyether-modified polysiloxanes. The addition reaction is as follows:
　　In a four-neck flask, first co-boil the reaction raw materials (low-hydrogen silicone oil and allyl-terminated polyoxyalkylene ether) with toluene containing 25% by mass to remove water. Then, using an isopropyl alcohol solution of chloroplatinic acid containing 30 × 10⁻⁶ platinum by mass as the catalyst, heat the mixture under a nitrogen atmosphere to 100°C for 4 to 4.5 hours. After evaporating the solvent, the resulting product is a polyether-modified polysiloxane.
　　1.5 Formulation of Polyether-Modified Polysiloxane Defoamer
　　Mix a specific ratio of hydrophobic gas-phase silica and polyether-modified polysiloxane, and stir the mixture at 160–180°C for 3 hours. Once cooled to room temperature, a silicone paste is obtained. Next, add a measured amount of emulsifier and thickener, then heat while stirring until the emulsifier is fully dissolved. Continue stirring for about 2 more hours to form a crude emulsion, which is subsequently homogenized using a high-speed homogenizer for approximately 10 minutes, resulting in a stable emulsion product.
　　1.6 Performance Evaluation of Defoamers
　　1.6.1 Stability
　　Centrifugal Stability: Centrifuge the emulsion at 4000 r/min for 30 minutes in a high-speed centrifuge, then observe whether layering or oil floating occurs. Thermal Stability: Store the emulsion at (85±2)℃ for 48 hours, and check whether phase separation (demulsification) takes place.
　　1.6.2 Particle Size of the Defoamer Emulsion
　　The particle size and its distribution were measured using a Zetasizer 3000 dynamic light scattering instrument.
　　1.6.3 Defoaming Performance of the Defoamer
　　Defoaming involves two key functions: eliminating already formed foam—this is called defoaming—and preventing foam from forming in the first place—known as antifoaming.
　　In a 600 mL graduated cylinder, add 100 mL of a sodium dodecylbenzenesulfonate aqueous solution with a mass fraction of 2.5% as the foaming liquid. Then, continuously bubble N₂ gas into the inverted nozzle submerged in the foaming liquid at a flow rate of 3 L/min. Once the foam volume reaches 500 mL, carefully introduce a specific amount of defoamer (0.05% by mass of the foaming liquid) using a rubber-tipped dropper, and simultaneously start timing. Record the exact time it takes for the foam to completely disappear—shorter times indicate better defoaming performance of the additive. Next, note the time required for the foam volume to once again reach 500 mL; longer durations suggest superior foam-inhibiting properties of the defoamer.
　　1.6.4 Water Dispersibility of the Defoamer
　　Take 5g of the defoamer into a beaker, add 25g of distilled water, and shake vigorously to observe the dispersion result. - If the solution appears milky white with no visible white oil phase and no oil spots on the surface, the defoamer exhibits excellent water dispersibility. - If the solution remains milky white but small white oil-phase particles are present, the dispersibility is good. - If the solution turns milky white and transparent with noticeable larger white oil-phase particles, the dispersibility is moderate. - If the defoamer barely disperses in water at all, its water dispersibility is poor.
　　2 Results and Discussion
　　2.1 Structural Design of Polyether-Modified Polysiloxanes
　　Substances that are completely soluble in water exhibit poor defoaming and foam-inhibiting properties in aqueous media. Therefore, poly(oxyalkylene)-modified polysiloxanes must possess a certain degree of hydrophobicity—specifically, the hydrophobic siloxane and poly(propylene oxide)ether segments within the copolymer need to be sufficiently long, while the hydrophilic poly(ethylene oxide)ether segment should remain relatively short. When the molecular weight of the polyether-modified polysiloxane is lower, the defoaming performance is stronger, but the foam-inhibiting ability tends to be weaker. Conversely, at higher molecular weights, the foam-inhibiting effect becomes more pronounced, though the defoaming capacity diminishes. Depending on the specific application requirements, an optimal ratio of poly(oxyalkylene) to polysiloxane can be tailored to produce defoamers with varying cloud points. Based on this principle, the hydrogen-terminated silicone oil selected for this experiment has a molecular weight ranging from 3,000 to 10,000 g/mol, with an active hydrogen mass fraction between 0.03% and 0.12%, while the polyether segment has a molecular weight of 400 to 1,500 g/mol.
　　2.2 Hydrophobic Treatment of White Carbon Black
　　Combining gas-phase silica with polyether-modified polysiloxane can enhance the defoaming performance of the polyether-modified polysiloxane. In this experiment, the silica was hydrophobically treated using hexamethyldisilazane at a feed ratio of 100:20, followed by a treatment at 240°C for 6 hours. After incorporating the hydrophobic gas-phase silica, the defoaming agent’s defoaming time was reduced by 5 to 7 seconds, while its foam-suppressing duration increased by 7 to 12 minutes compared to the original formulation.
　　2.3 Selection of Emulsifiers
　　It is well known that polysiloxanes are difficult to emulsify, making the selection of an appropriate emulsifier crucial. While ionic emulsifiers offer excellent stability, they tend to generate excessive foam themselves. Therefore, non-ionic emulsifiers are typically chosen for formulating defoamers. Additionally, composite emulsifiers often deliver better emulsification performance compared to single emulsifiers. In this experiment, we evaluated the emulsifying effectiveness of four emulsifiers: Tween-60 (sorbitan monostearate polyoxyethylene (20) ether), Span-60 (sorbitan monostearate), methyl glucoside sesquistearate (SS), and polyoxyethylene ether (SSE-20). Based on the principles of combining emulsifiers, these four were paired into two distinct systems: Span-60/Tween-60 and SS/SSE-20. The results revealed that when the Span-60/Tween-60 system was used at concentrations exceeding 5.0%, it not only provided stable emulsions but also exhibited outstanding defoaming and foam-inhibiting properties. On the other hand, even a small amount of the SS/SSE-20 combination proved highly effective in emulsifying silicone oil, allowing for lower overall emulsifier content in the defoamer while maintaining excellent stability of the emulsion system. Based on these findings, SS/SSE-20 was selected as the optimal composite emulsifier, used at a concentration of approximately 1.5%. This emulsifier blend was then employed in the phase-inversion emulsification process to prepare the final defoamer emulsion.
　　2.4 Selection of Thickening Agents
　　To enhance the stability of the defoamer emulsion, a thickening agent must be added to the formulation as an emulsion stabilizer. Commonly used thickeners include hydroxyethyl cellulose, carboxymethyl cellulose, alginate derivatives, and polyvinyl alcohol. In this experiment, polyethylene glycol 6000 distearate (with an HLB value of 18.4, indicating its strong affinity for both water and oil) was selected as the thickening agent. The addition of polyethylene glycol 6000 distearate will alter the overall HLB value of the defoaming system. To ensure optimal performance of the defoamer emulsion, the combined HLB value of the emulsifier and thickener should closely match that of the silicone paste. Experimental results indicate that when the composite emulsifier is used at a concentration of 1.5%, the ideal dosage range for the thickener is 0.5% to 0.8%.
　　2.5 Performance of the Defoamer
　　Comparing the defoaming performance of a pure polyether-modified polysiloxane defoamer emulsion with that of a polyether-modified polysiloxane defoamer containing fumed silica, it was found that the addition of fumed silica significantly enhanced both the defoaming and foam-controlling abilities of the defoamer.
　　The two main types of defoamers commonly used in industry are polyether defoamers and silicone emulsion defoamers. When the defoamer prepared in this experiment was compared with several commercially available ones, the results showed that the homemade defoamer outperformed the commercial products in both defoaming time and foam-inhibiting duration.
　　3. Conclusion
　　Using high-hydrogen silicone oil as the raw material, a low-hydrogen silicone oil is produced via a copolymerization method. Meanwhile, propargyl alcohol is used as the initiator, with NaOH as the catalyst, to carry out ring-opening copolymerization of ethylene oxide and propylene oxide, yielding an allyl-terminated polyoxyalkylene ether. This allyl-terminated polyoxyalkylene ether is then grafted onto the low-hydrogen silicone oil, resulting in a polyether-modified polysiloxane. The resulting polyether-modified polysiloxane is subsequently formulated into a highly effective defoamer by blending it with gas-phase silica treated with hexamethyldisilazane for hydrophobicity, along with SS/SSE-20 and polyethylene glycol 6000 distearate (added at 1.5% and 0.5–0.8%, respectively), combined with a thickening agent. This defoamer exhibits excellent emulsification stability, superior defoaming and foam-inhibiting properties, and dissolves rapidly in water. It can be used independently or in combination with other treatment agents, making it versatile for both aqueous and non-aqueous systems. Overall, this defoamer stands out for its outstanding performance and broad application potential.