Bound rubber is also called carbon black gel

Bound rubber is also called carbon black gel (bound- rubber), which refers to the part of rubber in the carbon black compound that cannot be dissolved by its good solvent. Bound rubber is essentially the rubber adsorbed on the surface of the filler, that is, the rubber in the interface layer between the filler and the rubber. Bound rubber is usually used to measure the size of the interaction between carbon black and rubber. The more bound rubber, the higher the reinforcement, so bound rubber is a measure of the reinforcement ability of carbon black. Content from Rubber Garden

Nuclear magnetic resonance research has confirmed that the thickness of the carbon black bound rubber layer is about 5.0nm, and the thickness of the layer close to the carbon black surface is about 0.5nm, which is in a glassy state. The side close to the rubber matrix is ​​in a sub-glassy state with a thickness of about 4.5nm.