Vulcanisation, discovered by Charles Goodyear in 1839, is the chemical process of heating rubber with sulphur to improve its durability, elasticity, and resistance to temperature and wear. Pre-vulcanisation, rubber is soft and unstable, with a consistency just like chewing gum (which is also made with a type of uncured rubber). It lacks the mechanical properties needed for industrial applications and deforms easily under stress when compared to say, steel.

In polymer science they often use the bowl of spaghetti analogy. When uncured, the polymer chains on a molecular level are independent and usually random, like a bowl of noodles. The chains can slide past each other which produces the stretchy chewing gum effect.

When vulcanised, the introduction of the crosslinks (usually sulphur, but not always) bridges the chains together restricting their individual movement past each other and creating the effect we think of with rubber’s ability to deform and return to its original shape. If you go with a higher crosslink density or shorter type of crosslink (peroxide vs sulphur), the resulting material can end up more rigid, making the exact makeup of your vulcanisation material an important consideration.

Through vulcanisation, the rubber is permanently transformed into a tougher, more resilient material. This change is what allows conveyor belts to perform reliably under heavy loads, constant movement, and harsh environmental conditions.

Belt splicing is used to join conveyor belts for installation, extension, or repair. In steel cord belts, this involves removing the majority of the rubber surrounding the cords.

Once the cords are aligned, the Splice is assembled atop a belt press using the Steel Cords and sections of uncured rubber. The Cover Panels are replaced, and the cords are assembled in a specific arrangement determined by the Splice Designer, with fatigue resistant Bonder rubber placed between the cords and filling all voids. The top of the press is assembled atop the splice and heat and pressure are applied to complete the vulcanisation process.

During the Curing process the uncured rubber flows, displacing any remaining air pockets, and the individual component surfaces come into contact, allowing bonds to form. As the process proceeds, the rubber begins to vulcanise forming strong bonds across the individual materials and developing the required mechanical properties. Without proper vulcanisation, the splice would not have the strength or integrity required to handle operational loads, making it one of the most critical steps in conveyor belt splicing and installation.

At a molecular level, raw rubber consists of long polymer chains that are not connected. During vulcanisation, sulphur and other curatives create cross-links between these chains. These bonds significantly improve the material’s strength and elasticity.

As a result, vulcanised rubber behaves more like a spring. It can absorb and release energy, returning to its original shape after being deformed. This property is essential for conveyor belts, which are constantly subjected to tension, flexing, and impact.

Fenner Conveyors splice kits are specifically engineered to optimise this chemical reaction for our conveyor belts, ensuring consistent and reliable vulcanisation outcomes. This works, however, when the splice kit is matched to the correct belt. Using the wrong kit for the belt construction means the chemical reaction cannot do its job, regardless of how well the process is executed.

The vulcanisation process during belt splicing occurs in two key phases. The first phase involves a controlled and gradual increase in temperature and pressure. This allows the rubber components to soften and flow, filling the spaces between the steel cords, ensuring proper material distribution and surface contact between the individual splice components. It also provides time for trapped air and gases to escape before the rubber begins to cure.

The second stage is curing, where the onset of vulcanisation is realised and completed. This begins as the heat rises in the press, which accelerates the vulcanisation reaction as the temperature approaches the target, typically around 145°C.

At this point, the chemical cross-linking process takes place, and the rubber sets into its final, durable form. The duration of each phase depends on factors such as belt thickness and the composition of the splice materials.

A useful way to think about this process is to compare it to cooking a steak. Applying too much heat too quickly can overcook the outside while leaving the centre underdone. Similarly, a splice must be heated gradually to ensure the entire section reaches the optimal vulcanising temperature and is then held there for the required time and pressure to ensure the splice is fully vulcanised.

While heat is essential for vulcanisation, pressure plays an equally important role. As the rubber heats, gases are released, and the material expands. Consistent pressure ensures two things:

1. Trapped air is removed, promoting surface contact between the individual components of the belt splice and prepared belt ends.
2. Gas bubble formation is prevented. Gas is created as part of the vulcanisation process; the application of consistent pressure prevents this gas from forming bubbles that become voids within the splice.

Maintaining pressure also helps the belt retain its correct shape throughout the process, noting the Press, and Edge Bars, act as a mould forming the shape of the splice. Without it, internal defects such as voids or weak bonds can develop, reducing the overall strength and durability of the splice.

When vulcanisation is not carried out correctly, a range of issues can arise that compromise performance.

One of the most significant risks is an uncured centre within the splice. In this case, the bonder rubber between the Steel Cords remains a gum like material with little adhesion to the steel cords and virtually no strength, which will likely lead to splice failure.  

Another issue is the formation of blisters, bubbles, or voids within the belt. These are typically caused by trapped gases, inconsistent pressure, or uneven heating. Such defects weaken the splice structure and can lead to premature cover wear or failure, as well as visible deformation of the belt surface.

Pressure can be seen as having a minimum threshold for completing a successful splice. If you have enough pressure to be above the vapor pressure of the formed gasses, you will displace them and generally prevent gas pockets forming. Pressures beyond the minimum are useful for improving material flow and ensuring release of trapped air but are not as critical to overall quality of the rubber as it cures. Another way to look at it is that the minimum pressure is to ensure proper rubber vulcanisation and higher pressures are for flow, contact, and air release.

Overheating can also cause damage. In some cases, excessive heat can lead to the rubber hardening beyond the target figure and results in cracking in service. In others, it can seemingly reverse the vulcanisation process, returning the rubber to a soft, unstable state. This is actually a continuation of the vulcanisation process, breaking existing polysulfide links and reforming them less-efficiently, losing crosslink density. Both outcomes significantly reduce the lifespan of the splice seen as rapid wear or damage at the surface of the splice.

The issues above are all process failures. They happen during vulcanisation itself, in the press. However, there is also material compatibility to consider. This is a different problem, and in some ways a harder one to catch, because it begins before the work even starts.

Not every splice kit works with every belt. Different belt constructions are built around specific rubber compounds, and the splice kit used during vulcanisation must be chemically matched to that construction. This goes beyond picking the right size or belt rating. The curing system within the splice rubber needs to be compatible with the belt's own compound. If it is not, the crosslinks formed during vulcanisation will not bond correctly to the belt material.

What makes this particularly difficult to spot is that the splice can appear complete when the press is removed. The rubber flows, the surface looks clean, and nothing obvious flags a problem. But without proper chemical bonding between the splice rubber and the belt, the structure lacks integrity. Under operational load, that splice will not hold. In some cases, it does not just underperform, it pulls apart.

Highly controlled vulcanisation is the foundation of effective conveyor belt splicing. It is the process that ensures a repaired or joined section performs as a seamless part of the belt.

Achieving consistent vulcanisation outcomes requires the right combination of materials, equipment, and expertise. Careful control of heat, pressure, and timing is essential, as even small variations can lead to significant issues. Equally critical is material selection, the splice kit must be matched to the belt. No amount of correct technique will compensate for the wrong rubber compound.

By following proven splicing practices and using high-quality, chemically optimised splice kits, operators can maximise conveyor reliability, extend belt life, and reduce the risk of costly downtime.