Hydrophobic aerosil

M. I. Khoma. Composition of foam-extinguishing agent for drilling solutions—contains waste from production of hydrophilic and hydrophobic aerosil, modified with bifunctional silico-organic compound and diesel oil. Patent SU 1795977-A, 1993. 

Data in Table 9.1 as well as results from other tests indicate that most effective antifoams are the silicon organic compounds activated by hydrophobic Aerosil, clay KEL, processed with siloxane and KAV-1. The ethylsilicates 32 and 40 have rather a good efficiency and low price. 

Summary Solid state NMR studies of molecular motions and network structure in poly(dimethylsiloxane) (PDMS) filled with hydrophilic and hydrophobic Aerosil are reviewed and compared with the results provided by other methods. It is shown that two microphases with significantly different local chain mobility are observed in filled PDMS above the glass transition, namely immobilized chain units adsorbed at the filler surface and mobile chain units outside this adsorption layer. The thickness of the adsorption layer is in the range of one to two diameters of the monomer unit ( 1 nm). Chain units in the adsorption layer are not rigidly linked to the surface of Aerosil. The chain motion in the adsorption layer depends significantly on temperature and on type of the filler surface. With increasing temperature, both the fiaction of less mobile adsorbed chain units and the lifetime of the chain units in the adsorbed state decrease. The lifetime of chain units in the adsorbed state approaches zero at approximately 200 K and 500 K for PDMS chains at the surface of hydrophobic and hydrophilic Aerosil, respectively. 

This paper is devoted to the study of a part of the complex phenomena of reinforcement, namely the behavior of the host elastomer in the presence of filler particles. The results of solid state NMR experiments and some other methods for filled PDMS are reviewed. The short-range dynamic phenomena that occur near the filler surface are discussed for PDMS samples filled with hydrophilic and hydrophobic Aerosils. This information is used for the characterization of adsorption interactions between siloxane chains and the Aerosil surface. Possible relations between mechanical properties of filled silicon rubbers on the one hand and the network structure and molecular motions at flie PDMS-Aerosil interface on the other hand are discussed as well. 

Two types of Aerosil, i.e., hydrophilic and hydrophobic Aerosils, with different surface activities, were used for the preparation of mixtures with PDMS. Two procedures were used to mix PDMS with Aerosil, namely mechanically mixed blends and blends obtained from solution. In the first case, PDMS was mixed on a laboratory null with Aerosil. In the second case, Aerosil was added to a 0.5 wt% solution of PDMS in npentane, and the suspension was kept for 2 days. Then, while stirring, the solvent was removed, and the resulting powder samples were dried to a constant weight. 

Two types of Aerosil with different surface activity have been studied hydrophilic and hydrophobic Aerosil. Hydrophilic Aerosil contains on its siirface hydroxyl groups which are the sites of adsorption. The surface groups of hydrophilic Aerosil are able to interact with the PDMS chain both through permanent dipoles in the partially ionic siloxane bond via permanent dipole-dipole interactions and through even weaker van der Waals forces. In contrast to hydrophilic Aerosil, non-polar trimethylsilyl groups on the surface of hydrophobic Aerosil effectively decrease the dipole-dipole interaction and mainly weak van der Waals forces are formed between methyl groups of PDMS and trimethylsilyl surface groups of Aerosil. 

In the present study, the surface activity of Aerosil is characterized by molecular motions of adsorbed chain units. Highly fille PDMS has been studied by H Ti NMR relaxation experiments and H NMR spectra [8, 10, 21]. The H NMR spectra are compared in Fig. 12 for highly filled samples containing hydrophilic and hydrophobic Aerosil [21]. 

According to the discussion in Section 1.1.1.1, a broad Pake spectrum is observed for low mobile PDMS chain units, whereas a narrow line is recorded if the frequency of chain motions exceeds 10 kHz-1 MHz. The effect of hydrophilic and hydrophobic Aerosil on the chain motion is remarkably different. For PDMS filled with hydrophilic Aerosil, a broad H NMR line is observed over the whole temperature range studied. The motional narrowing is not complete, even at 433 K, nearly 300 K above the fg of PDMS. This means that mobility of PDMS chain units at the surface of hydrophilic Aerosil is hindered by adsorption interactions even at 433 K, although the strength of adsorption interactions decreases with increasing temperature. On the other hand, PDMS chains at the surface of hydrc hobic Aerosil are already desorbed at about 200 K, since the NMR line is completely narrowed at this temperature as shown in Fig. 12. 

The chain mobility at the surface of the Aerosil is also characterized by h T2 relaxation experiments [8, 10]. The relaxation rate, for the chain units adsorbed by hydrophilic and hydrophobic Aerosil... 

Thus, the H T2 relaxation experiments and the analysis of the solid-echo spectra show that the strength of adsorption bonds depends strongly both on type of Aerosil surface and temperature. The chain adsorption at the surface of hydrophilic Aerosil significantly restricts motions of chain units adjacent to the filler surface. The local motion of chain units at the surface of hydrophobic Aerosil is not hindered by adsorption interactions at temperatures above 200-250 K. The lifetime of chain units in the adsorbed state approaches to zero at approximately 250 K and 500 K for hydrophobic and hydrophilic Aerosil, respectively [9]. 

The presence of filler in the rubber as well as the increase of the surface ability of the Aerosil surface causes an increase in the modulus. The temperature dependence of the modulus is often used to analyze the network density in cured elastomers. According to the simple statistical theory of rubber elasticity, the modulus should increase twice for the double increase of the absolute temperature [35]. This behavior is observed for a cured xmfilled sample as shown in Fig. 15. However, for rubber filled with hydrophilic and hydrophobic Aerosil, the modulus increases by a factor of 1.3 and 1.6, respectively, as a function of temperature in the range of 225-450 K. It appears that less mobile chain units in the adsorption layer do not contribute directly to the rubber modulus, since the fraction of this layer is only a few percent [7, 8, 12, 21]. Since the influence of the secondary structure of fillers and filler-filler interaction is of importance only at moderate strain [43, 47], it is assumed that the change of the modulus with temperature is mainly caused by the properties of the elastomer matrix and the adsorption layer which cause the filler particles to share deformation. Therefore, the moderate decrease of the rubber modulus with increasing temperature, as compared to the value expected from the statistical theory, can be explained by the following reasons a decrease of the density of adsorption junctions as well as their strength, and a decrease of the ability of filler particles to share deformation due to a decrease of elastomer-filler interactions. 

This conclusion is supported by the analysis of the temperature dependence of the deformation energy. With increasing temperature from 203 K to about 300-400 K, a slight decrease of the deformation energy is observed for samples filled with both hydrophilic and hydrophobic Aerosil. Above 400 K, the deformation energy starts to increase, as shown in Table 1. 

Fig. 16. Temperature dependence of the tensile strength at break referred to the actual cross sectional area (o,) for cured phenyl containing silicon rubber [21, 46] ( ) without filler, filled by (O) hydrophilic Aerosil and ( ) hydrophobic Aerosil the sample composition is described in the caption to Fig. 15...

A random siloxane copolymer containing 90 mol% dimethyl-, 10 mol% methylphenyl-, and 0.3 mol% methylvinyl-chain units is used for the sample preparation. The copolymer is filled with hydrophilic Aerosil (300 m g ) (A300) and hydrophobic Aerosil (60 m g ) (AM60). The weight ratio filler/elastomer is 30 100. 

A number of sorbents have been proposed to clean water surfaces from oil [318]. The use of hydrophobic aerosil was proposed for this purpose, which, however, can hardly be accomplished for economic reasons. More promising seems to be the proposal to use natural materials for oil absorption, such as turf, diatomite, vermiculite, swelled perlite. A method has been proposed for the modification of perlite by a consequent treatment with cationic surfactants and higher carboxylic acid salts. Such modification of swelled perlite increases its oil capacity up to 600%, the water absorption decreases 10 -100-fold, and the sinkability decreases considerably. The degree of oil removal from the water surface is, according to in vitro tests data, 98 - 99%. Methods have been found to use oil-saturated sorbents. 

FIG. 33 The Bingham yield values of hydrophobic Aerosil (2% m/v) suspensions in benzene (l)-n-heptane (2) mixtures in the entire composition range. 

FIG. 34 Rheological flow curves of hydrophobic Aerosil (2%m/v) suspensions in methanol (l)-benzene (2) mixtures at different compositions. 

Let us now examine the relationship between rheological data characterizing interparticle interactions and those obtained by adsorption measurements and calorimetry. Figure 37 displays characteristic data measured in hydrophobic Aerosil in benzene-n-heptane mixtures yielding U-shaped excess isotherms and the Cal-... 

In another series of experiments, hydrophobic surfaces were used the free energy of interaction between the hydrophobic macroscopic surfaces, ViAoj, with a radius 1 mm was compared to the results obtained in the coagulation measurements of a hydrophobic Aerosil sol with particles 10 nm in size. However, this time the experiments were conducted in a hydrocarbon medium, so that the system changed from lyophilic to lyophobic. This transition was caused by the addition of ethanol or propanol, which increased the polarity of the medium. The ViAOf value is low in pure hydrocarbon (see Section 1.3) and increases with increases in the alcohol concentration. At a particular critical value, ViAoff,.), which is on the order of several mJ/m, the turbidity in the system increases due to coagulation, in complete agreement with the previous series of experiments conducted in surfactant solutions. 

Figure 12.4 Dewatering efificiency ( ) and viscosity ratio (O) in 50% water-in-crude oil A emulsions for different amounts of hydrophobic Aerosil 972 particles dispersed in the samples.

---