Rubber Reinforcement and its Classification

Principles of rubber reinforcement are discussed and new nomenclature is proposed to classify reinforced rubbers. The engineering tensile strength σb of amorphous, gum, non-crystallizable rubbers is only about σb≈3 MPa. These have no commercial use. However, if such rubbers contain enough stiff (hard or rigid) “entities” of density ρ and specific surface area S, such that 600/ρ m2/mL>S>60/ρ m2/mL, then σb>20 20 MPa. These rubbers are considered highly reinforced. The value of S for a stiff “entity” is largely dependent on its characteristic smallest dimension, d. For spheres or rods, d is diameter, and for plates, d is thickness. For all three shapes, the range of d corresponding to the limits of S given previously is approximately 100 nm>d>1 nm. Sometimes, the prefix nano- has been used to designate materials that contain stiff “entities” in this range of d. Other times, the prefix nano- has denoted a smaller range of d, 10 nm>d>1 nm. Thus, the term nano- is ambiguous in the literature. It is proposed to divide the size range, 1 nm<d<100 nm, of stiff “entities” into two ranges with different designations. When 10 nm>d>1 nm, the prefix proposed is nano-, and when 100 nm>d>10 nm, meso- is proposed as the prefix. This division may seem superfluous and arbitrary, but there is scientific merit for it. The way that the stiff “entities” come about provides criteria for further categorization. If the “entities” are (insoluble) particles that have similar d before and after incorporation (i.e., d is largely pre-determined) into an elastomer, then “composite” is the suffix proposed. But, if the stiff “entities” evolve and d depends on the kinetics and thermodynamics of phase separation, then the suffix is “structured.” Nomenclature for four types of rubber are then proposed: meso-composite, meso-structured, nano-structured and nano-composite. Commercial examples of the first three are known: black-filled SBR vulcanizates (meso-composite, σb≈20–30 MPa), SBS triblock copolymers (mesostructured, σb≈20–30 MPa), and elastomeric ionomers (nano-structured, σb≈50–60 MPa). Both meso-rubbers need about 20–25% (by volume) stiff “entity” to attain high reinforcement, whereas, only 2% stiff “entity” (ionic domains) imparts high reinforcement to nano-structured ionomers. Highly reinforced meso-rubber contains a higher concentration of stiff “entity” than that present in highly reinforced nano-rubber. However, in both meso- and nano-rubbers, the average distance between stiff “entities” is about 5 nm. This distance is similar to the spacing of crosslinks in vulcanizates that are optimally crosslinked. To the author's knowledge, no nano-composite rubber that is highly reinforced has been made. Nonetheless, the existence of high strength nano-structured ionomers suggests that highly reinforced nano-composites may be possible.