Silly-Putty

The most frequently quoted example to illustrate this behaviour is the children s toy Silly Putty , which is a poly(dimethyl siloxane) polymer. Pulled rapidly it shows brittle fracture like any solid but if pulled slowly it flows as a liquid. The relaxation time for this material is 1 s. After t = 5t the stress will have fallen to 0.7% of its initial value so the material will have effectively forgotten its original shape. That is, one could describe it as having a memory of around 5 s (about that of a mackerel ). Many other materials in common use have relaxation times within an order of magnitude or so of 1 s. Examples are thickened detergents, personal care products and latex paints. This is of course no coincidence, and this timescale is frequently deliberately chosen by formulation adjustments. The reason is that it is in the middle of our,... 

Psychedelic visions expose spatial relationships and glistening shapes that span dimensions. They re the Silly Putty of reality. in—1 r ... 

Deformation is the relative displacement of points of a body. It can be divided into two types flow and elasticity. Flow is irreversible deformation when the stress is removed, the material does not revert to its original form. This means that work is converted to heat. Elasticity is reversible deformation the deformed body recovers its original shape, and the applied work is largely recoverable. Viscoelastic materials show both flow and elasticity. A good example is Silly Putty, which bounces like a mbber ball when dropped, but slowly flows when allowed to stand. Viscoelastic materials provide special challenges in terms of modeling behavior and devising measurement techniques. 

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. 

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. 

Exercise 29-9 The material popularly known as Silly Putty" is a polymer having an —O—Si(R)2—O—Si(R)2—O— backbone. It is elastic in that it bounces and snaps back when given a quick jerk, but it rapidly loses any shape it is given when allowed to stand. Which of the polymers listed in Table 29-1 is likely to be the best candidate to have anything like similar properties Explain, What changes would you expect to take place in the properties of Silly Putty as a function of time if it were irradiated with x rays (see Exercise 29-7) ... 

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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). 

Fig. 12.4, the melt is forced into a converging flow pattern and undergoes a large axial acceleration, that is, it stretches. As the flow rate is increased, the axial acceleration also increases, and as a result the polymer melt exhibits stronger elastic response, with the possibility of rupturing, much like silly putty would, when stretched fast. Barring any such instability phenomena, a fully developed velocity profile is reached a few diameters after the geometrical entrance to the capillary. 

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... 

Walker, J. 1978. The amateur scientist Serious fun with Polyox, Silly Putty, slime, and other non-Newtonian fluids. Scientific American 239 (5) 186,188,191-94, 196. 

Non-Newtonian fluid—A substance whose viscosity changes depending upon the force applied to it. Examples include mayonnaise, latex paint, and Silly Putty. 

Viscoelastic—The property describing a material that behaves either like an elastic solid or a viscous liquid, depending upon the force placed upon it (e.g., Silly Putty). 

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 ... 

General Electric scientists and engineers could find no practical applications for it Academics, of course, loved the material, as it was a beautiful teaching tool with which to demonstrate the fundamentals of viscoelasticity. It wasn t until 1949 that it was marketed as a children s toy. And as they say, "the rest is history." Silly Putty is still being produced in Pennsylvania and remains a favorite plaything for children and adults alike. 

But why has silly putty such unusual characteristics It is thought that the relatively weak inter-molecular bonds formed by the transfer of electron pairs from oxygen to boron are largely responsible. These bonds act like temporary or labile cross-links, which are continually making and breaking, and the physical and mechanical properties of the material then depend upon the time scale of the experiment... 

Silly Putty is a registered trademark of Binney Smith (used with permission). 

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. 

The low frequency range is where viscous or liquidlike behavior predominates. If a material is stressed over long enough times, some flow occurs. As time is the inverse of frequency, this means materials are expected to flow more at low frequency. As the frequency increases, the material will act in a more and more elastic fashion. Silly Putty , the children s toy, shows this clearly. At low frequency. Silly Putty flows like a liquid while at high frequency, it bounces like a rubber ball. This behavior is also similar to what happens with temperature changes. 

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