Viscosity silicone

Examples of medical textiles used in extracorporeal medical devices include the use of hollow fibres and membranes (made om polyester, polypropylene, silicone, viscose) for production of bioartificial organs, such as the kidneys, liver and lungs. 

The silicone oils and silicone resins find application as (i) lubricants (their change of viscosity with temperature is small), (ii) hydraulic fluids (they are unusually compressible), (iii) dielectric fluids, (iv) for the pro duction of water-repellant surfaces, and (v) in the electrical industry (because of their high insulating properties). 

Silicone Fluids. Sihcone fluids are used in a wide variety of appHcations, including damping fluids, dielectric fluids, poHshes, cosmetic and personal care additives, textile finishes, hydraiflic fluids, paint additives, and heat-transfer oils. Polydimethylsiloxane oils are manufactured by the equihbrium polymerisation of cycHc or linear dimethyl silicone precursors. Trifunctional organosilane end groups, typically trimethylsilyl (M), are used, and the ratio of end group to chain units (D), ie, M/D, controls the ultimate average molecular weight and viscosity (112). Low viscosity fluids,... 

Fig. 4. Kinematic viscosity—temperature relationship of dimethyl silicone fluids.

A variety of silicone polymers has been prepared ranging from low-viscosity fluids to rigid cross-linked resins. The bulk of such materials are based on chloromethysilanes and the gross differences in physical states depend largely on the functionality of the intermediate. 

The silicone fluids form a range of colourless liquids with viscosities from 1 to 1 000 000 centistokes. High molecular weight materials also exist but these may be more conveniently considered as gums and rubbers (see Section 29.6). It is conveinient to consider the fluids in two classes ... 

In practice, for fluids of viscosities below 1000 centistokes, the equilibration reaction will take a number of hours at 100-150°C. Residual esters and siliconates which may occur during the reaction are hydrolysed by addition of water and the oil is separated from the aqueous acid layer and neutralised as before. 

It has been shown" that branched polymers have lower melting points and viscosities than linear polymers of the same molecular weight. The viscosity of the silicone fluids is much less affected by temperature than with the corresponding paraffins (see Figure 29.2). 

The early 1980s saw considerable interest in a new form of silicone materials, namely the liquid silicone mbbers. These may be considered as a development from the addition-cured RTV silicone rubbers but with a better pot life and improved physical properties, including heat stability similar to that of conventional peroxide-cured elastomers. The ability to process such liquid raw materials leads to a number of economic benefits such as lower production costs, increased ouput and reduced capital investment compared with more conventional rubbers. Liquid silicone rubbers are low-viscosity materials which range from a flow consistency to a paste consistency. They are usually supplied as a two-pack system which requires simple blending before use. The materials cure rapidly above 110°C and when injection moulded at high temperatures (200-250°C) cure times as low as a few seconds are possible for small parts. Because of the rapid mould filling, scorch is rarely a problem and, furthermore, post-curing is usually unnecessary. 

Addition cure silicones can be delivered from solvent, waterborne emulsions, or 100% solids systems. The solvent free versions employ base polymers of intermediate molecular weight to achieve processable viscosity. These base polymers can have reactive moieties in terminal and/or pendant positions. These lower molecular weight, more functional systems result in a tighter crosslink network which feels rubbery to the hand. Low amounts of high molecular weight additives are included in some formulations to provide a more slippery feel [51,52]. 

The standard materials used are nonionic dimethylpolysiloxanes, and these are available in ranges of controlled MWs with viscosities varying from 10,000 to 50,000 centistokes (cs). These silicones follow the general formula ... 

One of the possible ways to account for the effect of roughness on the pressure drop in a micro-tube is to apply a modified-viscosity model to calculate the velocity distribution. Qu et al. (2000) performed an experimental study of the pressure drop in trapezoidal silicon micro-channels with the relative roughness and hydraulic diameter ranging from 3.5 to 5.7% and 51 to 169 pm, respectively. These experiments showed significant difference between experimental and theoretical pressure gradient. 

Qu et al. (2000) carried out experiments on heat transfer for water flow at 100 < Re < 1,450 in trapezoidal silicon micro-channels, with the hydraulic diameter ranging from 62.3 to 168.9pm. The dimensions are presented in Table 4.5. A numerical analysis was also carried out by solving a conjugate heat transfer problem involving simultaneous determination of the temperature field in both the solid and fluid regions. It was found that the experimentally determined Nusselt number in micro-channels is lower than that predicted by numerical analysis. A roughness-viscosity model was applied to interpret the experimental results. 

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