Silicone putty

Seal the ends of the pipette using silicon putty, as shown in Figure A, step 2. This is the anode. 

We mentioned in Section 4.1 that whether a material deforms under applied stress is a matter of the magnitudes of the shear force exerted and the time of observation. It is common to use silicone putty (known as Silly Putty) to illustrate the above statement. If you have enough patience, you will notice that silly putty is a highly viscous material (although you may not think of it that way) that will find its own level when placed in a container. In this sense, it behaves like a liquid. On the other hand, as its name is meant to suggest, a ball of Silly Putty will also bounce when dropped to the floor. That is, under severe and sudden deformation, it behaves like a solid. The behavior of the Silly Putty thus brings to our attention the importance of time scales and deformation rates in classifying the flow behavior of materials. 

Water-soluble metal salts of alkyllrisilanols are efficient in the reduction of surface tension. A silicone putty is made by compounding a benzene-soluble silicone polymer with silica powder and an inorganic tiller. 

Finally, some fluids that undergo viscosity changes on shearing are elastic as well. These are termed viscoelastic fluids. These materials have properties of both a liquid and a solid. An excellent example is silicone putty (e.g., Silly Putty), which shows three different types of behavior depending upon the shear rate. If a piece of this material is suspended (gravity, a low shear force), it will slowly flow downward like a very viscous fluid. If it is sheared fasten it has rubbery behavior. You can observe this by rolling some of it into a ball... 

We have already discussed cross-linking during polymerization but cross-linking is often carried out after the initial polymer is made. You saw earlier how poly(dimethylsiloxane) can be cross-linked by co-polymerization with MeSiC. An alternative way of cross-linking the linear polymer uses radical reactions to convert silicone oil into silicone putty. Peroxides are used in this process. 

Materials considered. We shall give most of our attention to materials that are in the process of deforming. The material one imagines depends on the time scale envisaged the ice of a glacier deforms appreciably in a year, whereas silicone putty deforms appreciably in a few minutes water deforms... 

The ability of a material to flow is usually associated with liquids but whether a stressed body behaves as a solid or a liquid can depend on the period of time over which the stress is applied. With some materials this distinction is obvious. For example, silicone putty can be poured slowly from a container but also be bounced as a rubber ball. In other cases, the flow process can be very slow and the difference is less obvious. The physical distinction between a solid and a liquid is therefore arbitrary but a viscosity of 10 Pa s (100 TPa s) is sometimes used. Figure 5.1 shows schematically the fluidity (=l/viscosity) behavior with changes in temperature of simple molecular materials. The viscosity increases by approximately 30 orders of magnitude as one moves from a gas to a solid. The viscosity of a gas is usually about 10" Pa s (10 xPa s) and changes to 10 Pa s (1 mPa s) on condensing as a liquid. On further cooling, the viscosity rises. 

The material known as bouncing putty is also a silicone polymer with the occasional Si—O—B group in the chain, in this case with 1 boron atom to about every 3-100 silicon atoms. The material flows on storage, and on slow extension shows viscous flow. However, small pieces dropped onto a hard surface show a high elastic rebound, whilst on sudden striking they may shatter. The material had some use in electrical equipment, as a children s novelty and as a useful teaching aid, but is now difficult to obtain. 

Rubber-like materials now superseding the traditional mastics and putties used in the building industry. Such sealants (also termed mastics) are based on butyl rubber, liquid polysulphides, silicone rubbers, polybutylene, nitrile rubbers and plasticised vinyl polymers. SEBS... 

XTX-8003 (eXTrudable explosive). Los Alamos National Scientific Laboratory (LASL) designation for an extrudable expl contg PETN (80 wt% 69.9 vol %) coated with a low-temp vulcanizing silicone resin, Sylgard 182 (20 wt % 30.1 vol %) atomic compn Ci. 8<>H3.64N , 0103 3 jSi0.27 white putty curable to rubbery solid d, TMD 1.556g/cc, nominal 1.50-1.53g/cc mp 129-135°. XTX-8003 is used in special applications that require expls with small detonation failure diameters... 

Within recent years, chemists have developed a whole line of new silicon compounds called silicones. Some of them are oil-like. Some look like putty ( Silly Putty ). Still others are rubber-like. Paper and cloth can be made water-repellent by being treated with suitable silicones. 

Sealants can be. broadly divided into classes according to the. amount of movement they can successfully handle. High performance sealants such as silicones, urethanes, and polvsulfides can typically handle movements of 25% or higher. Medium performance sealants such as some acrylics can handle movements of 10-25%. Low performance sealants such as butyls, putties, and caulks accommodate movements under 10%. 

As the shear rate increases, the viscosity of some dispersions actually increases. This is called dilatancy, or shear-thickening. Dilatancy can be due to the dense packing of particles in very concentrated dispersions for which at low shear, the particles can just move past each other but at high shear they become wedged together such that the fluid cannot fill (lubricate) the increased void volume, and the viscosity increases. An example of this effect is the apparent drying of wet beach sand when walked on, the sand in the footprint initially appears very dry and then moistens a few seconds later. Other examples include concentrated suspensions (plastisols) of polyvinyl chloride (PVC) particles in plasticizer liquid and the commercial novelty product Silly Putty (which is a silicone material). 

Silicones, 5,113-114,135-136,261 Silk, 33-34,47, 61, 66,171 Silly Putty, 125-126,262 Silver Spencer 129 Silver halide, 122 Single-site catalysts, 106-107 Smart materials, 206-209, 218 for food packaging, 206-207 for military applications, 208 sensors for, 207,208 shape-memory polymers, 207-208 Society of Automotive Engineers (SAE), 121 Sodium acrylate, 122 ... 

One very impressive example of a viscoelastic material is Silly Putty ( bouncy putty ), which is a silicon-based polymer. If dropped onto a surface, it boimces back higher than a rubber ball, yet under light pressure, the material can be flattened with ease and remains in its new shape. If placed on the edge of, for example, a table, the material will slowly creep over the edge like liquid due to the influence of the force of gravity. Most working materials, however, do not behave in these extremes. 

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