Theory 150/Rubber elasticity/Ratio lens
Toughness Is Filler Against Network
The claim
Rubber toughness is set by the ratio of filler to crosslink density, so adding carbon black to an over-cured rubber makes it brittle.
01Mechanism
Filler reinforces by hindering chain motion while crosslinks limit extensibility; the ratio determines the balance.
02Prediction
Toughness should peak at an intermediate ratio.
03Refutation
Toughness increasing with filler would remove the network term.