Overvulcanized

Overvulcanized is a term used in elastomer processing to describe rubber that has been vulcanized beyond the optimal level of cross-link density. It occurs when cure time, temperature, or curative content is excessive, causing the polymer network to become too highly cross-linked.

The condition results in a material that is typically harder and more rigid. Overvulcanized rubber loses elasticity and resilience, and its elongation at break decreases. Dynamic properties such as damping and fatigue resistance may deteriorate, and the compound can become crack-prone under bending or impact. In some cases, prolonged exposure to heat can lead to reversion, where some cross-links cleave and elasticity further declines.

Detection and assessment often involve cure monitoring with rheometry to track cure curve and final cure state, swelling tests to estimate cross-link density, and mechanical testing (tensile and elongation performance). Visual cues are limited and not reliable; careful testing is necessary to confirm overvulcanization.

Causes include excessive cure time, too-high cure temperatures, improper formulation or processing, and inadequate quality control. Prevention centers on adhering to recommended cure schedules and temperatures, optimizing curative package, and employing cure-monitoring tools to stop curing at the appropriate point. Proper handling and storage to avoid post-cure changes can also help maintain the intended material properties.

Overvulcanization is generally undesirable for most elastomer products, where elasticity and toughness are valued, though certain applications requiring extreme hardness may employ differently cured materials under controlled conditions.