Silicone surfactant Stabilisation

Industrially, silicone surfactants are used in a variety of processes including foam, textile, concrete and thermoplastic production, and applications include use as foam stabilisers, defoamers, emulsifiers, dispersants, wetters, adhesives, lubricants and release agents [1]. The ability of silicone surfactants to also function in organic media creates a unique niche for their use, such as in polyurethane foam manufacture and as additives to paints and oil-based formulations, whilst the ability to lower surface tension in aqueous solutions provides useful superwetting properties. The low biological risk associated with these compounds has also led to their use in cosmetics and personal care products [2]. 

Few studies have been carried out on nonaqueous emulsions, but these can be useful as topical vehicles or reservoirs for the delivery of hydrolytically unstable dmgs. Systems such as castor oil or propylene glycol in silicone oil can be formulated using silicone surfactants the HLB number clearly does not help in the formulation, especially if the continuous phase has low polarity. The key to stabilisation lies in the sufficient solubility of the emulsifier in the continuous phase. 

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. 

Modelling the Stabilising Behaviour of Silicone Surfactants During the Processing of Polyurethane Foam The Use of Thin Liquid Films... 

The rates of thinning of vertically-supported, thin liquid films of polyol solutions of various silicone surfactants have been measured [37]. It was found that the rate for films stabilised by a trimethylsilyl-capped polysilicate (TCP a highly branched silicone not containing polyethers), was much lower than that for the films stabilised by common silicone polyether copolymer surfactants. The retardation of drainage rate was correlated with an increase in surface viscosity. Furthermore, it was noted that PU foams prepared using TCP were significantly more stable than those containing the commercial surfactants. 

Initially, the drainage rates of PU films stabilised by various commercial silicone surfactants were measured. These surfactants are applied in the marketplace to stabilise flexible slabstock PU foam. These data are given in Table 5.1. 

Formulations for one-shot polyether systems are similar to those used for flexible foams and contain polyether, isocyanate, catalyst, surfactant and water. Trichloroethyl phosphate is also often used as a flame retardant. As with polyesters, diphenylmethane di-isocyanate is usually preferred to TDI because of its lower volatility. Tertiary amines and organo-tin catalysts are used as with the flexible foams but not necessarily in combination. Silicone oil surfactants are again found to be good foam stabilisers. Volatile liquids such as trichlorofluoro-methane have been widely used as supplementary blowing agents and give products of low density and of very low thermal conductivity. 

The most widely studied deformable systems are emulsions. These can come in many forms, with oil in water (O/W) and water in oil (W/O) the most commonly encountered. However, there are multiple emulsions where oil or water droplets become trapped inside another drop such that they are W/O/W or O/W/O. Silicone oils can become incompatible at certain molecular weights and with different chemical substitutions and this can lead to oil in oil emulsions O/O. At high concentrations, typical of some pharmaceutical creams, cosmetics and foodstuffs the droplets are in contact and deform. Volume fractions in excess of 0.90 can be achieved. The drops are separated by thin surfactant films. Selfbodied systems are multicomponent systems in which the dispersion is a mixture of droplets and precipitated organic species such as a long chain alcohol. The solids can form part of the stabilising layer - these are called Pickering emulsions. 

It has been mentioned above that foams can unfavourably affect the refining processes. Formation processes of non-aqueous foams are not well enough studied. In a vast review [264], comparison between aqueous and non-aqueous foams has been made. The stabilisation of non-aqueous foams (e.g. on a hydrocarbon basis) seems to be impossible with usual hydrocarbon surfactants because of the weak liquid-gas interfacial tension lowering gradients. Fluorinated or silicone-type surfactants can be used as eventually better stabilisers. These recommendations are, in our opinion, only applicable to the production of the so-called hardening foams (polystyrene foams, polyurethane foams etc.). 

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