Recovery and permanent deformation
1. Recovery
In the course of the creep experiment, the rubber gradually recovers after the external force is removed, as shown in Fig. 5-35.ABC is the creep curve, and the rubber will recover according to the CD curve when the load is removed at time t2. In case of elongation deformation, the amount of recovery is expressed as a percentage of the original length, i.e.:
Rubber deformed by force, when the external force is removed, the rubber molecules produce a contraction force equal in size and opposite in direction to the external force (as described in the first section of this chapter). Due to the rubber has viscosity, the internal force is not balanced, after the removal of the external force, the molecules are curled, rearrangement, it takes quite a long time to restore the equilibrium of the deformation before the situation. Therefore, recovery is also a relaxation process, the mechanism of which is the same as creep, while deformation is the opposite. The same can be illustrated with the viscous resistance model, can also be made at a certain temperature, different time recovery combination curve.
2. Permanent deformation
After removing the external force that deforms the rubber, the rubber will gradually return to its original shape, and if the part that cannot be restored, this part is called permanent deformation. Wood tested several kinds of well vulcanized rubber, proved that the deformation can be restored to the original length. The so-called permanent deformation, only the specimen is not long enough to park, the temperature is not high enough, has not been fully restored to the original length only. Often the same temperature, the same observation time in the specimen has not yet recovered part of the permanent deformation is called. Permanent deformation of the rubber material recovery is slow, indicating that the viscosity of the rubber or relaxation time is longer. Not up to the positive vulcanization time of the rubber, often have a large permanent deformation, so the permanent deformation of rubber can be used as a reference index to determine the degree of vulcanization.
Raw rubber has elastic deformation and plastic flow, after the removal of external force, the elastic deformation part can be recovered, while the plastic deformation part can not be recovered, leaving a real permanent deformation. However, due to the viscous obstruction, under general measurement conditions, a portion of the elastic deformation portion that has not yet recovered is also included. Figure 5-36 illustrates this relationship. The test was made by making a film of raw rubber solution, elongating this film 3901 TP3T, and then removing the external force and measuring the amount of recovery. At 25 degrees Celsius, after more than 20 hours after the recovery curve flattens out; if the temperature is raised to 50 degrees Celsius, there is a significant recovery; temperature is raised to 100 degrees Celsius, the recovery is even greater. Finally, the specimen will be put into the solvent, a little expansion, can be restored to point B, as shown in the figure, the remaining elongation of point B is the real permanent deformation. It can be seen that increasing the temperature helps rubber recovery, expansion reduces the intermolecular forces, so that the molecules are easy to rotate, the recovery effect is accelerated.