The vulcanization joint bonding strength remains as the original belt, and it is one big reason to adopt this technology.
However, in the case of very old belts, it often fails or performs poorly for many technical reasons.
Why a hot vulcanized joint might not withstand the stresses of an old belt, Lets try to find answers.
Old rubber has already undergone significant “aging” due to heat, UV exposure, and oxidation.
When you apply the high heat (typically 145°C to 150°C) required for hot vulcanization, curing, and loss of elasticity occurs.
Over-curing: The heat can cause the already-brittle rubber to “over-cure” or scorch, making it even harder and more prone to cracking.
Loss of Elasticity: The high temperature can break down the remaining polymer chains in the old belt, leading to a “mushy” or charred interface that cannot hold a bond.
Over years of service of the belt, the oils and plasticizers that keep rubber flexible migrate out of the belt or dry up completely, which causes incompatibility and bond failure.
Incompatibility: When you introduce fresh uncured splice rubber (which is full of fresh oils) to a dried-out old belt, the chemical properties are too different.
Bond Failure: The new rubber may fail to cross-link with the old rubber because the chemical hooks in the old material are no longer reactive or are blocked by contaminants.
In an old belt, the internal reinforcement (textile plies or steel cords) has been subjected to millions of flex cycles as the belt remains in service for many years,s causing micro fractures and adhesion loss.
Micro-fractures: The internal structure may have microscopic damage. The high pressure of a vulcanizing press can sometimes crush or further damage these weakened internal layers.
Adhesion Loss: In steel cord belts, the galvanized coating on the cords often corrodes over time. Hot vulcanization relies on a chemical bond to that coating; if it’s gone, the cords will simply “pull out” of the new splice.
Over years of service, oils, chemicals, and fine dust penetrate deep into the belt’s pores and even into the fabric carcass and this cause out gassing.
Outgassing: During the hot vulcanizing process, these trapped contaminants or moisture can turn into gas, which creates bubbles or porosity inside the joint, acting as a weak point where the splice will eventually rip apart.
A new hot splice is usually much more flexible than the rest of an old, hardened belt, which causes stress concentration.
Stress Concentration: As the belt travels over pulleys, the transition point between the “stiff” old belt and the “flexible” new splice becomes a high-stress zone. This often leads to the old belt tearing right at the edge of the new splice (a “hinge effect”).