Elastomer

Elastomers (sing. the elastomer also Elaste ) are dimensionally stable, but elastically deformable plastics, having a glass transition point is below the temperature of use. The plastics can deform elastically pressure load at train and but find then back to its original, undeformed shape. Elastomers are used as material for tires, rubber belts, gaskets, etc. The best known elastomers include natural rubber and silicone rubber.

Cause of elasticity

Cause of the elasticity is mainly the ability of the geknäulten polymer chains to react to a tensile stress at an elongation or disentanglement of the chains. After the tensile load drop the chains relax back to their statistically preferred coil-like state. This phenomenon manifests itself by stretching under tension and the subsequent contraction after drop in voltage. To prevent slide past each of the chains of the tensile load, the chains are interconnected by sulfur bridges in rubber. The addition of a lot of sulfur during the vulcanization of rubber thus produced, with the addition of low-sulfur soft rubber.

Entropy elasticity

Polymers are long chain molecules. Along the individual chains, the chain elements are mutually rotatable. For elastomers, these rotation is so strong that the molecules twist to a so-called polymer coils. This endeavor has no energy cause, but is merely the result of the engagement in completely random directions rotational motion along the chain. The arrangement of the individual atoms of the chain to the center of the molecule corresponds to a Gaussian distribution.

If a polymer stretched by a tensile stress, the chains align then preferably in the direction of the load off. The elastomer is thus stretched. Once the tension is omitted, the chains begin again with the random rotational motion, during which they again take the statistically justified Gaussian distribution. The chains relax and the elastomer contracts again. This effect is similar to a gas which, once compressed to expand by decompression, due to the random motion of the gas atoms again in the newly acquired space. It is an entropy effect, this type of elasticity is referred to as entropy elasticity. It usually represents the largest share of the elastic effect of an elastomer. Since the rotation is done more quickly and efficiently, the more energy it is present, this effect increases with the temperature. The increase in elasticity with temperature is a very characteristic feature of elastomers. The temperature below which the thermal energy is no longer sufficient to perform the rotations, ie glass transition temperature. Below the glass transition temperature elastomers lose their typical property.

Energy elasticity

The energy elasticity plays no role in elastomeric plastics usually. In energy- elastic bodies is hard substances for which a tensile load causes a deviation of the molecular or atomic arrangement of the energetically most favorable position. Not applicable the tensile load " slipping " the atoms or molecules back to this energetically favorable positions back. In particular the moduli of elasticity of metals and thermosets are determined by the energy of elasticity. A very low energy elastic contribution can arise through networking in elastomers.

Properties

Conventional elastomers are not fusible. The so-called thermoplastic elastomers, however, are thermoplastic, such as a substitute for natural cork in the wine bottle in certain temperature ranges.

The special feature of elastomers is that their elasticity (unlike metal springs ) is not due to attractive forces between changing atomic distances, but is a static- dynamic balance between order and entropy. Therefore, the elastomer stores no clamping energy in itself, but radiates on stretching (and other deformations) energy supplied as heat and instead increases its internal order. Like a muscle, therefore it requires for re- contraction of supply of energy, which removes the elastomer by Brownian motion of the ambient heat.

In cold elastomers lose their force and may freeze as hard as glass. A moderation increasing the temperature significantly increases the clamping force of the elastomer, as it supplies energy to it, whereby it may perform work. This phenomenon is known under the name of Gough Joule effect. ( However, excessively high temperature causes decomposition. ) The deflection is significantly stronger than about the thermal expansion of solids and is more similar to the mechanical behavior of gases in a cylinder with a movable piston. You can build a simple heat engine of a flexible wheel with spokes made ​​of rubber bands even; the wheel is heated on one side, then its rim deformed on one side by contraction of the elastomer, whereby the wheel offset by the center of gravity in rotation as relax on the cooler side of the tapes again.

Special shapes

• Thermoplastic elastomers
• Liquid crystalline elastomers
• Magnetorheological elastomers