Polyether trisiloxane (commonly referred to as trisiloxane polyether, such as trisiloxane ethoxylate or trisiloxane polyalkyleneoxide) is a highly important silicone surfactant. Its core structure consists of a hydrophobic trisiloxane group (typically heptamethyltrisiloxanyl, `(CH₃)₃Si-O-Si(CH₃)(OSi(CH₃)₃)-O-Si(CH₃)₃`) linked via a spacer group (usually propyl) to a hydrophilic polyether chain (typically polyethylene oxide `-EO-` or polyethylene oxide-polypropylene oxide `-EO-PO-` copolymer).

Simplified Core Structure: `(Me₃SiO)₂Si(Me)-R-(EO)_m(PO)_nH`

`Me`: Methyl `CH₃-`

`R`: Spacer group, usually `-(CH₂)₃-`(propyl)

`EO`: Ethylene oxide unit `-CH₂CH₂O-`

`PO`: Propylene oxide unit `-CH₂CH(CH₃)O-`

`m, n`: Degree of polymerization

Synthesis Method

The synthesis of polyether trisiloxane primarily relies on the hydrosilylation reaction, one of the most widely used and mature methods in silicone chemistry.

1.Key Raw Materials:

Hydrosiloxane: The most commonly used is 1,1,1,3,5,5,5-heptamethyltrisiloxane (`(CH₃)₃SiOSiH(CH₃)OSi(CH₃)₃`, abbreviated as MD'HM or trisiloxane hydride), which provides the reactive Si-H bond.

Unsaturated Polyether: The most common is allyl polyether (`CH₂=CH-CH₂-(OCH₂CH₂)_m-(OCH₂CH(CH₃))_n-OH` or `CH₂=CH-CH₂-(OCH₂CH₂)_m-OH`). The terminal allyl group (`CH₂=CH-CH₂-`) provides the double bond. The molecular weight of the polyether chain (EO/PO ratio and degree of polymerization) determines the hydrophilicity, water solubility, and surface activity of the final product.

2.Core Reaction (Hydrosilylation):

(Me₃SiO)₂Si(Me)H + CH₂=CH-CH₂-(EO)_m(PO)_nH → (Me₃SiO)₂Si(Me)-CH₂CH₂CH₂-(EO)_m(PO)_nH

The reaction occurs between the Si-H bond of MD'HM and the terminal double bond of the allyl polyether.

The reaction follows Markovnikov's rule, with the main product being the isomer where the Si-C bond connects to the terminal carbon of the allyl group (i.e., `-CH₂CH₂CH₂-` spacer).

3.Catalyst:

The most commonly used catalysts are platinum-based, such as:

Chloroplatinic acid (H₂PtCl₆) in isopropanol solution.

Karstedt catalyst (platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex): High activity, good selectivity, low dosage, is the first choice for industry.

Other platinum complexes (e.g., Speier catalyst).

4.Reaction Conditions:

(1).Temperature: Typically conducted at 80°C–120°C. Lower temperatures slow the reaction, while higher temperatures may cause side reactions (e.g., isomerization, siloxane rearrangement, polyether dehydration).

(2).Pressure: Atmospheric pressure is sufficient.

(3).Atmosphere: Usually performed under nitrogen or argon protection to prevent catalyst deactivation (oxygen, water, or sulfur/nitrogen/phosphorus compounds can poison platinum catalysts) and oxidation of raw materials.

(4).Solvent: If raw material viscosity is low, no solvent is needed. Otherwise, inert solvents like toluene or isopropanol may be used.

5.Reaction Process & post-treatment:

(1).MD'HM, allyl polyether, and catalyst are added to the reactor.

(2).Under inert gas protection, the mixture is heated to the reaction temperature with stirring.

(3).The reaction progress is monitored by the disappearance of the Si-H bond's infrared absorption peak (~2100–2200 cm⁻¹) or by measuring the Si-H content in the reaction mixture. Reaction time ranges from a few hours to over ten hours.

(4).After completion, the mixture is cooled.

(5).Post-treatment:

Neutralization/Adsorption: Adsorbents (e.g., activated carbon) or weak bases may be added to remove or neutralize trace catalyst residues, preventing side reactions (e.g., polyether dehydration and yellowing) during storage or application.

Filtration: Removes adsorbents or solid impurities.

Solvent Removal: If a solvent was used, it is distilled off under reduced pressure.

Low-Boiling Component Removal: Excess MD'HM (if used in excess) or low-boiling impurities may be removed under vacuum.

(6).The final product, polyether trisiloxane, is obtained as a colorless to pale yellow viscous liquid.

Key Properties & Applications

Polyether trisiloxane exhibits exceptional surface activity due to its unique "T-shaped trisiloxane hydrophobic core + flexible polyether hydrophilic chain" structure, making it indispensable in various fields, particularly agriculture.

1.Ultra-Low Surface Tension:

Its most notable feature. It can dramatically reduce the surface tension of water to extremely low levels (~20–25 mN/m or lower), far below conventional hydrocarbon surfactants (typically 30–40 mN/m).

Applications: This forms the basis for its use as a high-performance wetting and spreading agent, especially in agricultural sprays.

2.Superior Wetting & Spreading Properties:

Its ultra-low surface tension allows solutions to rapidly and completely spread on low-energy hydrophobic surfaces (e.g., waxy plant leaves, insect cuticles, plastics, metals, oily surfaces), forming an almost invisible film rather than droplets.

Applications:

(1).Agricultural Adjuvants: This is its largest and most critical application. It is added as a tank-mix adjuvant to pesticide (insecticides, fungicides, herbicides, plant growth regulators, foliar fertilizers) solutions.

Enhancing Efficacy: Promotes wetting, spreading, and penetration of spray solutions on hard-to-wet crops (e.g., rice, wheat, cabbage with thick waxy layers) or pest cuticles, reducing runoff and increasing coverage, thereby significantly improving pesticide efficiency (reducing dosage) and control efficacy.

Promoting Absorption: Helps active ingredients penetrate plant or insect cuticles faster.

Anti-Evaporation/Anti-Drift: Rapid spreading forms a thin film that reduces evaporation and wind drift.

(2).Industrial Cleaners: Used for hard-surface cleaning (e.g., vehicles, building facades, metals, glass, plastics), electronics cleaning, and degreasing, leveraging its ultra-wetting and penetrating power. Often formulated with solvents, alkalis, or other surfactants.

(3).Coatings & Inks: Functions as a leveling agent, wetting agent, or defoamer/anti-foam component, improving substrate wetting, reducing surface defects (e.g., craters, orange peel), and aiding pigment dispersion.

3.Excellent Dynamic Surface Activity & Penetration:

Rapid adsorption and alignment at interfaces, lowering dynamic surface tension quickly.

Applications: Effective in fast-moving or forming interfaces (e.g., spraying, high-speed coating, foaming).

4."Superspreading" Effect:

Under specific conditions (e.g., optimal concentration, certain polyether structures), its aqueous solutions exhibit unusually rapid and extensive spreading on hydrophobic surfaces, exceeding theoretical predictions based on surface tension.

Applications: Critical in agricultural sprays, enabling near-instantaneous full coverage on leaves.

5.Low Foaming:

Compared to many traditional surfactants, polyether trisiloxanes typically generate minimal foam.

Applications: Suitable for low-foam or foam-free applications, such as industrial cleaning or defoamer formulations.

6.Other Uses:

Personal Care: Occasionally used in high-end skincare or sunscreen products as wetting agents, emulsion stabilizers, or to impart a silky feel. Also used in defoamers.

Textile Auxiliaries: Functions as wetting agents, penetrants, or softener components.

Defoamers/Anti-foams: A key component in industrial defoamers (e.g., fermentation, wastewater treatment, papermaking, coatings), leveraging its low surface tension and rapid spreading/foam-breaking ability. Often formulated with hydrophobic particles (e.g., hydrophobic silica) and carrier oils.

Important Considerations

Stability: Under strongly acidic or alkaline conditions, the siloxane backbone (especially Si-O-Si bonds) may undergo hydrolysis or rearrangement degradation. The pH range should be controlled (typically stable under neutral or mildly acidic/alkaline conditions).

Biodegradability: The trisiloxane group is relatively stable but eventually degrades slowly into non-toxic silanols and silica. The polyether chain is more readily biodegradable. Overall, it is more biodegradable than many silicone polymers, but specifics depend on the structure.

Structural Diversity: By varying polyether chain length (molecular weight), EO/PO ratio, EO/PO arrangement (block or random), and end-capping, properties like HLB value, water solubility, surface activity, and cloud point can be fine-tuned for different applications. For example, increasing EO content enhances water solubility, while increasing PO content improves oil affinity and reduces foaming.

Summary

Polyether trisiloxane is synthesized via hydrosilylation, where heptamethyltrisiloxane (MD'HM) reacts with allyl polyether under platinum catalysis. Its core value lies in its ultra-low surface tension and the resulting exceptional wetting, spreading, and penetration capabilities. These properties make it indispensable in agricultural spray adjuvants, significantly enhancing pesticide efficacy and utilization. Additionally, it finds important applications in industrial cleaning, coatings, inks, defoamers, personal care, and textiles. By modifying the polyether chain structure, its performance can be tailored for specific needs. Users should be mindful of its stability limitations under strongly acidic or alkaline conditions.