Silicone surfactants Structures

Inverse structures have been published for use in polyurethane foam formation [43]. Monofunctional siloxane chains are attached to a multifunctional polyether backbone. However, significant advantages over classical silicone surfactant structures could not demonstrated. A major drawback of this approach is the difficult synthesis of monofunctional siloxanes. 

The unique surface characteristics of polysiloxanes mean that they are extensively used as surfactants. Silicone surfactants have been thoroughly studied and described in numerous articles. For an extensive, in-depth discussion of this subject, a recent chapter by Hill,476 and his introductory chapter in the monograph he later edited,477 are excellent references. In the latter monograph, many aspects of silicone surfactants are described in 12 chapters. In the introduction, Hill discusses the chemistry of silicone surfactants, surface activity, aggregation behavior of silicone surfactants in various media, and their key applications in polyurethane foam manufacture, in textile and fiber industry, in personal care, and in paint and coating industries. All this information (with 200 cited references) provides a broad background for the discussion of more specific issues covered in other chapters. Thus, surfactants based on silicone polyether co-polymers are surveyed.478 Novel siloxane surfactant structures,479 surface activity and aggregation phenomena,480 silicone surfactants application in the formation of polyurethane foam,481 foam control and... 

Schmaucks, G. Novel Siloxane Surfactant Structures. In Silicone Surfactants Hill, R. M., Ed. Surfactant Science Series Dekker New York,... 

A broad range of silicone surfactants are commercially available, representing all of the structural classes—anionic, non-ionic, cationic, and amphoteric. The silicone moiety is lyophobic, i.e. lacking an affinity for a medium, and surfactant properties are achieved by substitution of lyophilic groups to this backbone. The most common functionalities used are polyethylene glycols however, a broad range exist, as shown in Table 2.8.1 [2,3]. 

The polyether-modified structures are the most important silicone surfactants currently in use, examples of which are shown in Fig. 2.8.1. These are known by many names, including silicone polyethers, polyethermethylsiloxanes (PEMSs), dimethicone copolyols,... 

An industrial blend of ethylene oxide (EO) PEMS marketed as a personal care product was examined by positive ion FIA-APCI-MS and LC-APCI-MS-MS (Fig. 2.8.8) [41]. The FIA-APCI-MS spectrum without LC separation (Fig. 2.8.8(a)) is dominated by ions corresponding to unreacted PEG (m/z 520, 564, 608, 652,...), whilst the ions corresponding to the PEMS (m/z 516, 560, 604, 648,...) could only be clearly observed following LC separation (Fig. 2.8.8(b)). Comparison of the TIC chromatograms of PEMS and PEG (Fig. 2.8.8(c) and (h)) demonstrates the dominance of the PEG by-products in the commercial formulation. It is unclear whether the observed relative intensities are representative of the actual amounts or of the different ionisation efficiencies, due to the confidential nature of the product composition. However, the spectra indicate a trisiloxane surfactant structure of that shown in Fig. 2.8.2 (R = Ac) and FIA-MS analysis of another commercial formulation of this product showed good spectra dominated by the silicone surfactants [48], indicating that the PEG by-product composition can vary significantly in commercially available PEMS formulations. 

It is important to note that the ion series observed by the API-MS method may not be representative of all the products present, not the quantities thereof. Cleavage of the ethoxylate chain removes the capacity of silicone surfactants to be ionised and therefore detected by these methods. As such, for example, the cleaved silicone head group [M(D/CH2CH2CH2OH)M, 3] was never observed by API-MS. The nature of the API-MS process is such that competition between analytes for ionisation occurs, and as such compounds with higher surface activity and EO content can be expected to dominate in the resulting spectra. Suppression effects may thus preclude observation rather than confirm absence. As a consequence, the use of additional techniques such as FTICR-MS, GC-MS, HPLC and NMR to provide complementary data was also necessary. Furthermore, the high number of possible structures for each ion series observed, rendered it difficult to assign structures with confidence. Consequently simplified M2D-C3-0-(E0)n-R... 

The most common polymeric silicone surfactants are based on polyoxyalkylene groups. The structures of graft-type (rake-type) and ABA structures are illustrated in Figures 6.17 and 6.18. It should be noted that there are many possible variants of these basic structures. The actual structure of graft-type silicone copolymers is a random copolymer of m and n rather than the blocky structure suggested by the diagram. 

Silicones readily form cyclic structures such as octamethylcyclosiloxane. Small cyclic silicones have also been used to make small-molecule silicone surfactants such as that shown in Figure 6.20 [7]. 

Silicone surfactants in aqueous solutions show the same general behavior as conventional hydrocarbon surfactants - the surface tension decreases with increasing concentration until a densely packed film is formed at the surface. Above this concentration, the surface tension becomes constant. The concentration at the transition is called the critical micelle concentration (CMC) or critical aggregation concentration (CAC). The surface and interfacial activity of silicone surfactants was reviewed by Hoffmann and Ulbricht [27]. Useful discussions of the dependence of the surface activity of polymeric silicone surfactants on molecular weight and structure are given by Vick [28] and for the trisiloxane surfactants by Gentle and Snow [29]. 

Few studies exist for ionic silicone surfactants. Several trisiloxane anionic, cationic and zwitterionic surfactants have been found to form micelles, vesicles and lamellar liquid crystals. As would be expected, salt shifts the aggregates toward smaller curvature structures [40]. 

Stuermer, A., Thunig, C., Hoffmann, H. and Gruening, B. (1994) Phase behavior of silicone surfactants with a comblike structure in aqueous solution. Tenside, Surfactants, Detergents, 31, 90-8. 

Schmaucks, G. (1999) Silicon surfactants. In R.M. Hill (ed.), Novel Siloxane Surfactants Structures. Dekker, New York. 

The high hydrophobicity of silicones can complicate their use in some applications. For example, proteins can undergo denaturation in contact with silicones [1]. In such cases, the siloxane can be modified to include a hydrophilic domain. This is typically accomplished by functionalizing the silicone with a hydrophilic polymer such as poly(ethylene oxide)(PEO). Silicone surfactants of this type have found widespread use as stabilizers for polyurethane foams, and have been investigated as a structurant to prepare siloxane elastomers for biomaterials... 

Several other experimental findings support the existence of a microceliular structure in oligomeric foams. Thus, Oween and Denis ) observed an anomalous pattern (in the expression of the authors) for certain types of silicone surfactants the liquid foam system consists of gas bubbles the sizes of which differ by several orders of magnitude. The possibility of formation of very small gas bubbles after a marked reduction of the surface tension coefficient in poly-lurethane formulations has been reported by Dubyaga and Tarakanov ). 

A premix was prepared by blending the bisphenol A-epichloro-hydrin adduct, silicone surfactant and CFC-llA. Into the premix, polymeric isocyanate and the complex catalyst were added and the mixture was immediately agitated vigorously to produce a foamed material. The resulting foam had a density of 40 kg/m with fine cell structure. 

Surfactants. Silicone surfactants, which are used in rigid urethane foams, can be used as surfactants for pyranyl foam jn-eparation. The silicone surfactants are block copolymers of polydimethylsiloxane-polyoxyalkylene ether in either linear or pendant structures. 

Of the various surface active chemistries currently available, this paper will mainly concentrate on a class of materials called Silicone Polyethers. This family of copolymers is used to provide multifunctional benefits in water borne systems. The main uses of silicone polyethers in inks and coatings include de-foaming, de-aerating, improved substrate wetting, levelling and enhanced slip properties (1,2). The three most common molecular structures for silicone surfactants are rake type copolymers, ABA copolymers and trisiloxane surfactants. These are illustrated in Figs 1,2 and 3 respectively and the performance of these structures will be described in two types of coatings ... 

Figure 1 Molecular Structure of a Rake type silicone surfactant...

Depending on their structure, silicone surfactants are surface-active not only in water but also in organic solvents. They are structurally derived from polydimethylsiloxanes (14) in which the methyl groups are partly substituted by anionic groups. 

Silicone surfactants show outstanding surface activity, e.g. the surface tensions of their aqueous solutions can be lowered to the level of 21-22 mN/m. They are in most cases oligomeric or polymeric substances. Silicone sulfonates show thermal stability and will not crystallize at low temperatures due to their highly branched structures. They show a great variety of molecular weights and structures (linear, branched, comb-like, etc). The variability of the synthetic routes also leads to a large number of products with different properties. 

The silicone surfactants can be viewed as PDMS-polyether-copolymers which are mainly based on a combination of just three structural units the methyl substituted siloxane backbone as well as a sophisticated ratio and arrangement of ethylene oxide and/or propylene oxide forming the attached polyethers and, in some cases, additional modifications. 

A typical structure of a silicone surfactant is shown in Figure 2.9. 

Examples of how this variety of structural parameters affects the development of silicone surfactants especially for liquid carbon dioxide blown foams, are many [39,40,41, 42] and this latest drive in surfactant development provides good examples of the important performance issues and how they can be addressed. 

Not taking cyclic molecules into account, the general structures of industrial silicone surfactants for flexible slabstock foam production can be seen in Figure 2.13. The main building blocks of these materials are a PDMS backbone and attached polyethers based on ethylene oxide and propylene oxide addition products. The siloxane backbones can either be linear or branched and can have their polyether substituents attached in an either pendant or terminal location. These four general structures are outlined in Figure 2.13). 

Figure 2.13 General structures of silicone surfactants (schematic depiction)...

In order to maximise surface viscosity, and therefore minimise the drainage rate, the surfactant concentration, the intermolecular cohesion and adhesion should all be high. Later in the chapter evidence is presented correlating silicone surfactant concentration and structure (which influences the intermolecular cohesion and adhesion) to surface viscosity and film drainage rate. 

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