At the start of a jet dyeing cycle, the 10% active polydimethylsiloxane emulsion is pre-diluted with demineralised water at a ratio of 1:5 to 1:10 and introduced into the lowest point of the dye circulation sump before the fabric rope reaches the main pump. Machines with liquor ratios between 1:5 and 1:8—such as the Thies Luft-roto and Then Airflow-AFS—trap air at the pump suction because the turbulent rope impacts the liquor surface and the returning mixture accelerates through a venturi. The resulting foam blanket reduces the differential pressure across the main circulation pump, inhibits cloth plaiting, and can cause rope slip on the lifter reel. Production-scale start-up logs indicate that a dose of 0.05 g/L to 0.20 g/L of MY-210, based on total bath volume, maintains residual foam below the overflow guard level. A screening test in the laboratory blender method ASTM D3519 gives a foam half-life below 60 s at 25 °C for a 1000 mg/kg active dilution. When the dyeing temperature exceeds 120 °C for disperse dyestuffs on polyester, the emulsion droplet boundary is subjected to high shear in the venturi nozzle; phase separation under these conditions produces hydrophobic deposits on the fabric only when the emulsion is added concurrently with high-electrolyte auxiliaries or cationic after-treatments. To avoid destabilisation, the product is added before the disperse dyestuffs and after the anionic levelling agent has fully dissolved. The pH range of the bath is controlled between 4.5 and 5.5. The critical processing limit is not temperature alone but the combination of temperature, shear, and electrolyte concentration; at sodium sulfate concentrations above 50 g/L, the emulsion should not be pre-aged for more than 2 h.

What Limits Silicone Antifoam Persistence in Reactive Dyeing at 60°C?

Cellulosic knits dyed with bifunctional reactive dyes in soft-flow overflow machines experience foam formation during the alkali dosing ramp, when sodium carbonate or sodium hydroxide shifts the liquor pH from 6.5 to 11.0. The 10% active silicone emulsion is metered into the machine bypass loop rather than into the chemical dosing tank because concentrated alkali can split the emulsion. A production-scale dosing rate of 0.10 g/L to 0.25 g/L is maintained throughout the 45 min to 70 min reactive fixation period. In the presence of sulfate ions, the silicone antifoam is less soluble; this is an intended effect but means that the emulsion must be dosed continuously at low shear. Open-width overflow machines with low liquor ratios below 1:7 impart high turbulence through the jet nozzle and therefore require a finer emulsion droplet size. The foam control assessment follows DIN EN 12728, with the foam collapse time measured in a 100 mL graduated sparging tube under 30 L/h air flow; a collapse from 200 mL foam to 10 mL within 120 s is considered acceptable for cellulosic reactive cycles. Use above 0.4 g/L is not recommended because silicone may deposit on the fabric surface and interfere with subsequent wash fastness as rated by ISO 105-C06. The dosing pump should be a diaphragm or peristaltic type with a turndown ratio of at least 100:1; gear pumps create excessive shear at low flow and prematurely agglomerate the silicone droplets.

Continuous open-width preparation of woven cotton and cotton-blend fabrics generates foam in the pad trough of the saturator and in the J-box steamer because hydrogen peroxide decomposes rapidly in the presence of residual catalase and alkali. The 10% active silicone emulsion is applied at a rate of 0.2 g/L to 0.6 g/L of the pad liquor, adjusted according to wet pick-up and the concentration of wetting agent. Pad mangles operating at nip pressures between 2 bar and 4 bar and speeds above 80 m/min can generate foam at the nip entry due to air drawn into the film. The silicone antifoam reduces foam height at the trough sight glass, but the product must be added to the supply tank after the wetting agent has dispersed and before the addition of sodium silicate; sodium silicate at 35 °Bé to 40 °Bé can destabilise the emulsion if added as a concentrated slug. Because the pad bath operates at pH 10.5 to 11.5, pre-dilution in a separate mixing tank with softened water is required before feeding into the main trough; this limits the contact time between the silicone oil and free sodium hydroxide. Foam persistence in the pad trough is evaluated by measuring the height of the stable foam layer after a 5 min static period. Residual foam above 5 cm indicates an insufficient defoamer dose or a destabilised emulsion. At a wet pick-up of 70%, the active silicone deposition on dry fabric is approximately 0.014 g/kg to 0.042 g/kg, which does not affect absorbency as measured by the AATCC drop test with a wetting time under 3 s after subsequent washing.

Textile wastewater treatment plants receiving high-COD effluent from dyeing and finishing lines experience stable biological foam in activated sludge aeration basins. The foam is composed of filamentous bacteria, extracellular polymeric substances, and residual fat or fatty acid derivatives from scouring. The 10% silicone emulsion is dosed at 1 mg/L to 5 mg/L into the aeration basin inlet channel or over the splashing surface of the surface aerator, not into the raw sewage sump. A peristaltic pump with variable speed control is used to avoid overdosing. Foam thickness on the basin surface is measured with a calibrated dipstick at fixed grid points; a reduction from 30 cm to 5 cm within 2 h is typically observed. The defoamer does not affect the mixed liquor suspended solids concentration or the sludge volume index because the silicone remains largely at the air-water interface and is removed with the waste activated sludge. However, if the dose exceeds 10 mg/L, the silicone may accumulate in the secondary clarifier weir and reduce oxygen transfer efficiency; operators should monitor dissolved oxygen and maintain it above 2 mg/L in the aeration tank. The defoamer should not be added to the biological selector tank where high-shear mixers are used because that reduces the emulsion droplet size and increases dispersion, potentially causing solids flotation. The product is compatible with polyaluminium chloride and anionic polyacrylamide sludge dewatering aids when dosed 30 min after the defoamer. In this application, no direct standard test for foam persistence in activated sludge exists; the plant-specific foam index test is used, with a foam height after 10 min below 50 mL in a 1000 mL cylinder.

When Rotary Screen Shear Exceeds 10,000 s⁻¹ in Pigment Printing

Pigment printing pastes are formulated with aqueous dispersions of binder, thickener, fixer, and pigment; high-speed rotary screen printing machines with nickel screens and magnetic rod squeegees generate shear rates above 10,000 s⁻¹ at the print point. Foam entrained in the paste creates pinholes and uneven coverage on the fabric. The silicone antifoam is added to the paste at 0.05 wt% to 0.15 wt% of the total formulation. It must be stirred slowly with a planetary mixer at 20 rpm to 40 rpm for 15 min before use; high-speed dispersers above 500 rpm can break the emulsion and cause the silicone droplets to coalesce. The rheology of the print paste is evaluated after defoamer addition with a rotational viscometer at 20 rpm and 20 °C; a viscosity drop of more than 20% relative to the defoamer-free paste indicates an interaction between the silicone and the associative thickener. The defoamer is not mixed directly with the concentrated pigment dispersion because pigment dispersants are anionic and can neutralise the emulsion emulsifier layer. The printed fabric is then cured at 150 °C to 160 °C for 3 min; silicone on the printed surface does not interfere with crocking fastness as measured by ISO 105-X12 when the defoamer dose is below 0.20 wt%. Above that level, a reduction in wet crock fastness by at least half a grade has been observed on some pigment systems. In production trials, bubble-free paste is confirmed by drawing a 100 μm wire-wound film applicator over a glass plate and inspecting under a stereomicroscope at 20× magnification; more than 10 microfoam bubbles per 10 cm² indicates insufficient defoaming or inadequate slow mixing. The emulsion should not be stored below 5 °C or above 40 °C because freeze-thaw cycles and high temperatures can cause irreversible phase separation.

Emulsion Stability Boundaries in Strong Electrolyte Brines

During the synthesis of quaternary ammonium textile softener esters, the reaction vessel is charged with fatty acid, triethanolamine, and sulfuric acid catalyst. Foam arises during vacuum stripping of water at 90 °C to 120 °C and 10 kPa to 20 kPa. The silicone antifoam is added at 10 mg/kg to 30 mg/kg of reaction mass before vacuum is applied. The emulsion must be stable in the presence of sulfate and chloride ions at concentrations up to 5 wt%; phase separation at the high electrolyte content can occur after 12 h to 24 h, so the emulsion is metered continuously into the liquid ring pump seal water and the condensate return line rather than stored in the reactor charging tank. The vacuum pump is typically a liquid ring pump with a suction capacity of 200 m³/h to 500 m³/h; foam carryover reduces pump efficiency by increasing the water temperature and reducing the inlet vacuum. A defoamer dosage of 20 mg/kg active has been sufficient to maintain vacuum at 15 kPa absolute in production campaigns. Batch-to-batch variance in foam load is caused by free fatty acid content and residual catalyst; free fatty acid above 0.5 meq/g increases foam height and requires the upper limit of the defoamer range. The incompatibility of this emulsion with concentrated anionic surfactants is known; it should not be mixed into a tank containing a fully built anionic detergent at active concentration above 15 wt% because the silicone droplets may be displaced from the oil-water interface by surfactant micelles.