As previous stated, silicone rubber has become one of the most important materials in the polymer family due to its thermal stability, environmental resistance, transparency, and biocompatibility. But sometimes, the inherent good properties of the silicone rubber are not enough, hence it is necessary that the material be filled with hard particulates or fibers, with silica being the most commonly used reinforcing filler. The reinforcement depends on several factors, such as particle size, structure, and filler content . The combination of a silicon rubber matrix and the filler results in an increase in properties, mainly mechanical, but additional synergies are created which affect the physical properties of the material. If the interaction between silicone rubber and filler is not well understood, some processing problems can occur, and changes in the behavior of the silicone rubber compound can seem like a mystery. It could be thought that conventional rubbers (based on carbon and hydrogen) and silicone rubber (based on silicon and oxygen) were completely different. However, a well-known phenomenon in the former can be also found in silicone rubber, and it is called bound rubber.
Bound rubber is formed by interactions between the silicone rubber chains and the filler particles. It is the macroscopic effect of the physical absorption, chemical sorption, and mechanical interaction between them  and can be compared with a rubber gel structure. Bound rubber appears during the mixing of the silicone rubber and the additives, creating a complex structure , which means bound rubber appears when the compound is uncured. These chains cannot be extracted from the unvulcanized silicone rubber through use of the appropriate solvent – toluene, in the case of silicone rubber. The formation of bound rubber is related to the silicone rubber and filler chemical structure (polarity) and microstructure (configuration and molecular weight) . Several models for bound rubber are proposed, and the most accepted  stated that a shell of rubber is formed around the filler aggregates, although silicone rubber segments also join to the filler particles . Figure 1 shows in detail how bound rubber is formed. A first rubber layer is formed around the filler particle, and the rubber is closely in contact with the particle, which is also called tightly bound rubber. The second rubber layer is bound to the first layer and is called loosely bound rubber. There is also rubber trapped in the filler aggregates which is called occluded rubber.
Figure 1. Scheme of the bound rubber and occluded rubber present in a silicone rubber compound .
The presence of bound rubber also affects the resistance, viscosity and vulcanization behavior of the compound. The concentration of the filler affects the presence of the bound rubber. The higher the concentration, the higher the content of bound rubber. It can be result of the decrease in the distance between filler particles, which favors the formation of bridging chains, which means more active sites and more chains in the absorption sites of the filler .
The shape of filler also influences the presence of bound rubber. First, the bound rubber that connects the filler particles can be direct or indirect. A direct connection is a direct contact between the individual bound layers, and an indirect contact is the formation of a bridge between the particles. Figure 2 shows the scheme of the difference between both contacts. Related to the particle shape, amorphous aggregates generate a higher amount of bound rubber and form a homogeneous continuous structure (Figure 3a)). If the aggregate has a better shape (more spherical), the uniformity of the bound rubber is not good (Figure 3b)) and rubber filaments form. If the aggregate shape is perfect (spherical), only a small amount of bound rubber exists, and a continuous structure is not formed (Figure 3c)) . Figure 1 shows the schematic representation of bound rubber and filler network depending on the shape of the filler.
Figure 2. Indirect and direct contact .
Figure 3. Bound rubber formed around different shapes of fillers and its effect on the chemical reaction path .
With regards to the material flow, the viscosity of the silicone rubber compound increases with the filler content, and it was already seen that an increase in fillers creates a stronger interaction at the silicone rubber-filler interface. The stronger the interaction, the more the tendency to form bound rubber .
Related to the vulcanization behavior, the effect of the bound rubber is noteable, that is, in the last stages of the reaction, the higher the content of bound rubber, the higher the inhibitory effect, so a longer time will be required to completely vulcanize the compound . The continuous structure of the bound rubber limits the diffusion and reduces the availability of reaction sites. This blocks the chemical reaction and decreases the vulcanization rate. Figure 3 also shows the effect of the bound rubber structure on the chemical reaction paths.
The mechanical properties of the final compound, after vulcanization, also change in the presence of bound rubber. For example, the stiffness, which is proportional to the modulus, increases with bound rubber content due to the formation of a pseudo-vulcanized structure . The disadvantage of this is the possibility that the unreacted silicon bonds are activated with mechanical stress, which can cause the tightening of the rubber gel, change in the mechanical behavior of the component, and lead to permanent deformation of the final product. The tensile strength is also affected. The higher the content of bound rubber, the higher the strength obtained in the vulcanized silicone rubber compound. When tensile stress is applied, the bound rubber controls the behavior just before the rupture of the silicone rubber compound .
In conclusion, the effect of different fillers on the presence of bound rubber in silicone rubber compounds is related to the changes in the physical, chemical, and mechanical properties of the base silicone rubber. The higher the content of bound rubber, the higher the final mechanical properties will be, but longer the processing times are needed for the compound. Depending on the final application of the compound, bound rubber can be an advantage or a disadvantage. In the latter case, two options are possible. First, avoid longer storage times, and if this is not possible, an ammonia treatment is used. Here, the ammonia penetrates the two bound rubber layers and an alcohol is used to cover the filler particles. A coupling agent is also employed to improve the interaction between the filler and the silicone rubber. This inhibits the formation of bound rubber.
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