Silicone surfactants Surface activity

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... 

Most softeners consist of molecules with both a hydrophobic and a hydrophilic part. Therefore, they can be classified as surfactants (surface active agents) and are to be found concentrated at the fibre surfaces. Most softeners have a low water solubility. Therefore softening products are usually sold as oil in water emulsions containing 20-30 % solids. The softener molecules typically contain a long alkyl group, sometimes branched, of more than 16 and up to 22 carbon atoms, but most have 18 corresponding to the stearyl residue. Exceptions to this molecular structure are the special categories of silicones, paraffins and polyethylene softeners. About one-third of the softeners used in the textile industry are silicone based. 

Hoffmann, H. Ulbricht, W. Surface Activity and Aggregation Behavior of Siloxane Surfactants. In Silicone Surfactants Hill, R. M., Ed. Surfactant Science Series Dekker New York, 1999 Vol. 86, Chapter 4, pp 98-136. 

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... 

One useful method of aqueous defoaming is to add a nonfoam stabilizing surfactant which is more surface-active than the stabilizing substance in the foam. Thus a foam stabilized with an ionic surfactant can be broken by the addition of a very surface-active but nonstabiliz-ing silicone oil. The silicone displaces the foam stabilizer from the interface by virtue of its insolubility. However, it does not stabilize the foam because its foam films have poor elasticity and rupture easily. 

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]. 

Silicone surfactants are specialty surfactants that are primarily used in applications that demand their unique properties. Most applications are based on some combination of their (a) low surface tension, (b) surface activity in nonaqueous media, (c) wetting or spreading, (d) low friction or tactile properties, (e) ability to deliver silicone in a water-soluble (or dispersible) form, (f) polymeric nature or (g) low toxicity. The major applications will be discussed briefly in following sections. 

Certain comb-type silicone surfactants have been shown to stabilize emulsions in the presence of salts, alcohol and organic solvents that normally cause failure of emulsions stabilized using conventional hydrocarbon surfactants and a study by Wang et al. [66,67] investigated the cause of this stability. Interaction forces due to silicone surfactants at an interface were measured using AFM. Steric repulsion provided by the SPE molecules persisted up to an 80% or higher ethanol level, much higher than for conventional hydrocarbon surfactants. Nonionic hydrocarbon surfactants lose their surface activity and ability to form micelles in... 

The surface activity of silicones is often exploited by using them as additives. For this reason, aspects of the two most important additive forms, copolymers and surfactants, are also included in this discussion. These two classes come together in the relatively low molecular weight PDMS-poly(alkylene oxide) block and graft copolymers that are commonly used as polyurethane foam stabilizers. Other short-chain silicone surfactants designed for aqueous systems and other silicone-organic copolymers are also available. 

Summary Organomodified trisiloxanes show remarkable surface activity. Thus they can be used as additives in detergents, foaming agents or agrochemicals. However, this class of compounds has a limited range of applications, since it is susceptible to hydrolytic decomposition in aqueous solution. The hydrolysis occurs at the silicon-oxygen-bonds present within the molecules. Recently, a new class of silane surfactants free of Si-0-bonds has been developed. These silane surfactants proved to be hydrolytically stable even under extreme pH. Furthermore they exhibit surfactant properties comparable to those typical of trisiloxane surfactants. 

Tphe surface activity of block copolymers containing dimethylsiloxane units as one component has received considerable attention. Silicone-poly ether block copolymers (1,2,3) have found commercial application, especially as surfactants in polyurethane foam manufacture. Silicone-polycarbonate (4, 5), -polystyrene (6, 7), -polyamide (8), -polymethyl methacrylate (9), and -polyphenylene ether (10) block copolymers all have surface-modifying effects, especially as additives in other polymeric systems. The behavior of several dimethylsiloxane-bisphenol A carbonate block copolymers spread at the air—water interface was described in a previous report from this laboratory (11). Noll et al. (12) have described the characteristics of spread films of some polyether—siloxane block co-... 

Silicon-organic and other element-organic surface active substances with improved thermal stability and other unique properties, thanks to which they can be used under rough conditions (high temperatures and pressures, aggressive media), represent a special class in the modern assortment of available surfactants. Another important group of surface active substances... 

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 ... 

To prevent cissing, an additive that reduces interfacial tension is required in the paint. When interfacial tension falls, the particle is wetted by the finish and absorbed into the film. Surface-active agents Surfactants, see below) reduce interfacial tension. Alternatively, an agent can be added that will reduce the liquid surface tension so much, that the interfacial tension also becomes low. Silicone oils do this effectively. Very little silicone oil is required, because it finds its way almost entirely to the surface. Silicones are semi-organic compounds... 

The most commonly known siloxane surfactants are the so called silicone polyethers , but other nonionic, as well as ionic surface active agents have been prepared and used over the years in cosmetics, textile condihoning, foam stabilization, coatings or agriculture. 

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 effect of a silicone surfactant on the burning behaviour of the polyurethane foam can be quite significant even though they normally represent less than 1 % of the plastic material. It is due to the fact that in any case the decomposition of a polyurethane foam starts at the surface. Because of the surface activity of the foam stabilisers it is easy to rationalise their enrichment on the surface and it is the surface that is the most influential part of the polymer regarding flame spread development. 

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