Transport of Small Molecules in Loaded Natural Rubber: Experimental Measurement and Numerical Simulation

Small organic molecules such as plasticizers, antioxidants, and some active molecules such as scent/fragrance compounds and drug molecules are incorporated into natural rubber materials to modify their properties and processability or utilize them as carriers in delivery systems. The transport of those small molecules within the rubber can greatly alter the properties and performances of the loaded rubber materials. Thus, it is important to study the transport properties of those molecules in natural rubber matrices and the factors influencing how fast the molecules diffuse. In this work, vulcanized rubber sheets with three different cross-link densities loaded with eight small molecules with different boiling points, molecular weights, and chemical moieties were prepared. Desorption experiments were performed to determine the mass transfer coefficients and diffusion coefficients of those molecules, and the corresponding numerical models were built. Good agreements between experiments and numerical simulation were observed. It was found that the mass transfer coefficients of those small molecules decreased with increasing boiling points but remained practically constant among rubber sheets with different cross-link densities. The diffusion coefficients of those small molecules did not show evident correlation with their molecular weights, but their relationships with cross-link densities indicated that for some molecules there might be an optimal cross-link density where a small molecule reached its maximum diffusion coefficient. The resulting findings could be applied to a wide range of fields.