Putty, bouncing

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. 

It must be emphasized that the type behavior shown in Figure 3.5 depends not only on the material but also very definitely on conditions under which the test is made. For example, the bouncing putty silicone is putty-like under slow rates of loading (type curve a) but behaves as an elastic solid under high rates of impact (type curve b or d). 

Bouncing putty is somewhat similar in that the Si-O-B bond occurs occasionally along the chain. It is based on a polydimethylsiloxane polymer modified with boric acid, additives, fillers, and plasticizers to give a material that shows a high elastic rebound when small pieces are dropped on a hard surface but flows like a viscous fluid on storage or slow application of pressure. 

In many cases, a material may exhibit the characteristics of both a liquid and a solid, and neither of the hunting laws will adequately describe its behavior. The system is then said to be in a viscoelastic state. A particularly good illustration of a viscoelastic material is provided by a silicone polymer known as bouncing putty. If a sample is rolled into the shape of a sphere, it can be bounced like a rubber ball, i.e., the rapid apphcation and removal of a stress causes the material to behave like an elastic body. If, on the other hand, a stress is applied slowly over a longer period the material flows like a viscous liquid, and the spherical shape is soon lost if left to stand for some time. Pitch behaves in a similar, if less spectacular, manner. 

The inherent lack of strength of a crossllnked polydlmethylsiloxane system is directly related to lack of interchain interactions at ambient temperatures. Several approaches have been explored to increase this interaction. Incorporation of small amounts of boron into the chain gives a material with extremely high resilience under shock loading which flows under slow deformation, the so called bouncing putty. 

The inclusion of a small number of B—0—Si bonds in the polydimethylsiloxane structure leads to significant effects. Two products are of some commercial interest, namely fusible elastomers and bouncing putty . 

Bouncing putty is prepared by heating a dimethyl silicone with ferric chloride and boric oxide or other boron compounds and adding suitable fillers and softeners to the resultant gum. The material is readily shaped by kneading and may be drawn into threads on application of moderate tension. When dropped on a hard surface, the material shows high elastic rebound it shatters, however, when given a sharp blow. 

A good example of this is provided by the children s toy called bouncing putty . This is, in fact, liquid polydimethylsiloxane (PDMS), in which the polymer chain ends have hydroxyl groups, and the liquid is filled with solid boric oxide powder (Figure 8.7). 

Other examples of viscoelastic materials are synovial fluid, molten polymers (with thread-forming properties used in fiber spinning or film blowing), and "bouncing putty" or "nutty putty," which will flow if stretched slowly, but bounces if struck hard against a hard surface. 

We see from eq. 3.2.18 and from the spring and dashpot representation that for slow motions the dashpot or Newtonian behavior dominates. For rapidly changing stresses, the derivative term dominates, and thus at short times the model approaches elastic behavior (recall Figure 3.1.2 and the bouncing putty in Figure I.l). 

Finally, though it has so far found little industrial use, bouncing putty is worth a mention if only on account of its entertainment value. It is made by heating to ca.200° a methyl silicone oil with about 5% of its weight of boric oxide and an inert filler. It slowly flows under its own weight, like a liquid, but bounces very well when formed into a ball and thrown at a hard surface. A very sharp blow can fracture a ball of bouncing putty. 

On reacting this product with boric acid, there is an end-capping of the chain, yielding the self-adhesive polymer. On the other hand, bouncing putty is obtained when —Si—0—B— bonds are distributed on the backbone of the chain. 

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