What Is Storage Hardening in Natural Rubber and What Role Does Viscosity-Stabilizing Treatment Play?

Phenomenon and Impact

After storage, conventional natural rubber may exhibit increased Mooney viscosity and initial plasticity, often requiring premastication to a target Mooney before internal mixing. Storage hardening causes batches of the same grade to behave inconsistently depending on arrival times.

Role of Viscosity-Stabilizing Treatment

Constant-viscosity natural rubber inhibits storage hardening, keeping Mooney viscosity more stable during storage. If the raw rubber arrives close to the customer’s target processing viscosity and the internal-mixing process can achieve dispersion directly, separate premastication may be shortened or even eliminated.

But it cannot be promised that using constant-viscosity rubber eliminates mastication entirely; A/B verification with the same formulation is required to determine the actual time reduction. Mechanical shear and a certain degree of molecular-chain degradation still occur during direct mixing, so some plasticizing effect is still present.

Verification Methods

The direct way to evaluate storage stability is to record Mooney changes after accelerated storage or different inventory durations, and compare the mean, standard deviation, and drift across multiple batches, rather than relying solely on single-batch ex-factory data.