Molecular Structure and Synthesis of Foam Stabilizers

Foam stabilizers used for polyurethane foams can be categorized into two main types: non-ionic and ionic, as well as into silicone-based and non-silicone compounds. In early foam plastics, especially for polyester-based polyurethane soft foams, ionic and non-ionic surfactants without silicone were commonly used. These include sulfonated castor oil alkyds, alkyl sulfonates, and polyethylene glycol fatty acid esters (e.g., polyethylene glycol sorbitan monooleate, also known as Tween-80).  

The structure of silicone-based foam stabilizers varies widely, depending on their application in different systems for soft foams, rigid foams, and high-resilience (HR) foams. Typically, they include repeating dimethylsiloxane units, ethylene oxide (EO) units, and propylene oxide (PO) units. The linkage between siloxane and polyalkylene oxide components can take various forms, such as AB-type linear block structures, ABA-type linear block structures, and single-branch or multi-branch structures. Random copolymers of ethylene oxide and propylene oxide also differ in composition and molecular weight.  

The chemical composition of ethylene oxide-propylene oxide copolymers and their connection to organosilicon chains influence the stabilizer's surface performance and stability. During the polymerization of alkylene oxides, low-molecular-weight monohydric alcohols can be used as initiators, allowing precise control of molecular weight. Different coupling methods with polysiloxane produce a variety of foam stabilizers.  

In the foam formation process, the precipitation of insoluble polyurea disrupts foam stability. A critical function of polyether-siloxane surfactants is dispersing polyurea to enhance its compatibility with the foam matrix. This function is achieved through the polyether segments. Increasing the ethylene oxide content in the polyether chain improves polyurea solubility within the foam mixture.  

Based on the chemical nature of the linkage between the polysiloxane and polyalkylene oxide segments, silicone surfactants can be classified into Si-O-C (silicon-oxygen-carbon) and Si-C (silicon-carbon) types.  

- Si-C bonds: These are hydrolytically stable, enabling long-term storage without degradation.  

- Si-O-C bonds: These were the earliest developed silicone foam stabilizers, offering advantages like readily available raw materials, mature manufacturing processes, and effective foam stabilization. However, Si-O-C bonds are prone to hydrolysis under strong acidic or basic conditions. Stabilizers with Si-O-C bonds are unsuitable for pre-mixed formulations with tertiary amine catalysts and water, as hydrolysis compromises their stabilizing function.  

Changes in Si-O-C bond structures due to hydrolysis, although difficult to detect through conventional physical-chemical methods, significantly impact stabilizer performance. Hydrolysis results in siloxane and polyether separation, leading to system incompatibility and, in severe cases, phase separation. Consequently, Si-C-type stabilizers are predominantly used for rigid foams, semi-rigid foams, and HR foams. For block soft foams where components are typically added separately, the Si-C structure is less critical, but most current silicone stabilizers are Si-C-based.  

Synthesis of Si-O-C Silicone Surfactants

To synthesize Si-O-C-type surfactants, polyalkylene oxide oligomers and polysiloxane oligomers are first prepared, followed by the reaction of their active groups. Control over parameters such as the EO-to-PO ratio, hydroxyl value, and molecular weight is essential for tailoring the hydrophilicity and reactivity of the intermediates.  

Specialization and Optimization of Foam Stabilizers  

With the growing variety of foam plastics and advancements in foaming equipment, foam stabilizers are increasingly specialized, refined, and diversified. Both linear and branched polyether-siloxane stabilizers are generally suitable, though branched structures offer superior stability. Commercial stabilizers often consist of multiple types of organosilicon-polyether copolymers with varied molecular structures and compositions, improving formulation adaptability and foam stabilization.  

Factors Affecting Performance  

- Altering the EO/PO ratio and sequence affects surfactant solubility and hydrophilicity in polyols.  

- Increasing molecular weight and EO content in polyether chains enhances surface activity.  

- Dimethylsiloxane chains combined with pendant polyether chains maximize surface activity, commonly used in rigid polyurethane foams.  

- Soft foam formulations use multi-pendant polyether copolymers to ensure foam stability.  

Flame Retardancy Enhancements

Modifying the polysiloxane structure in stabilizers can improve flame retardancy. For example, polar groups like allyl cyanide in the siloxane chain enhance the solubility of polymethylsiloxane during combustion, reducing adverse effects on molten flow behavior and lowering flammability.