Under the action of vulcanization pressure and temperature, the root cause of bubbling in vulcanized conveyor belt joints lies in trapped air, residual volatiles, or poor interlayer bonding. Specifically, the main causes can be divided into the following four categories:
1. Rubber Compound Quality Issues
Performance defects of the rubber compound itself are the fundamental factors leading to bubbling, mainly reflected in two aspects:On one hand, excessive volatiles: Plasticizers, solvents, or small-molecule substances generated by aging that remain in unvulcanized rubber will volatilize rapidly at the vulcanization temperature of 145-160℃. Once these gases cannot be discharged in time, they will accumulate between the rubber and the base material (canvas/steel cords), eventually forming bubbles.On the other hand, insufficient adhesion: Improper adhesive ratio, expired rubber compounds, or low vulcanization activity will directly reduce the bonding force between the rubber and the base material. When gas pressure is generated internally, this weak adhesion cannot resist the pressure, which in turn causes delamination and is ultimately accompanied by bubbling.
2. Non-Standard Joint Preparation
Joint preparation is a key link before vulcanization; non-standard operations can easily lay hidden risks for bubbling. There are three specific problems include: Firstly, poor cleanliness of the base material surface: If oil stains, dust, or moisture remain on the surface of canvas or steel cords, an "isolation layer" will form between them and the rubber. When heated during vulcanization, the air or pollutants in this layer expand, leading to joint bubbling.Secondly, inadequate base material treatment: If the canvas is not ground to expose fresh fibers, or rust and oxide layers on steel cords are not completely removed, the contact area between the rubber and the base material will be significantly reduced. The smooth surface lacks sufficient mechanical interlocking force, making it easy for gas to accumulate at the interface.Thirdly, splicing operation errors: Such as uneven cutting of joint bevels, misalignment of layers, or problems like gaps and overlapping layers. Among these, uneven rubber thickness hinders the uniform transmission of vulcanization pressure, leading to air trapping; while gaps not filled with rubber compound will directly turn into bubbles when heated.
3. Out-of-Control Vulcanization Parameters
The "three key factors" (temperature, pressure, time) in the vulcanization process are crucial to joint quality. If any parameter is abnormal, it may directly cause bubbling:
Specifically, temperature issues have the most direct impact: When the temperature is too high, a "hard shell" will form rapidly on the rubber surface, trapping unvolatilized substances inside; when the temperature is too low, the vulcanization reaction rate slows down, resulting in insufficient time for gas discharge; and uneven temperature (e.g., overheated edges and cool centers) can also damage the stability of interlayer bonding, indirectly causing bubbling.
In terms of pressure, if the pressure is lower than the standard value (0.8-1.2MPa for canvas belts, 1.5-2.0MPa for steel cord belts), it will be unable to effectively squeeze out air and volatiles from the rubber layer; if pressure unevenness occurs due to deformed vulcanizing plates or leaking seals, it will also cause local air trapping and form regional bubbles.
As for the time parameter, insufficient time will lead to incomplete vulcanization, resulting in loose rubber structure and residual volatiles; excessive time will trigger "reversion" (molecular chain breakage), and additives in the rubber compound may decompose to produce new gases, which also causes bubbling.
4. Environmental Factors and Operational Errors
In addition to the above process factors, environmental conditions and human operations may also induce bubbling. Additionally, specific factors can be divided into:
From the environmental perspective: When the environmental humidity is >80%, the rubber or base material is prone to absorbing moisture. This moisture evaporates into water vapor during vulcanization heating, thereby forming bubbles; if the environmental temperature is too low, it will reduce the fluidity of the rubber, which not only easily leads to air trapping but also slows down the vulcanization reaction, affecting gas discharge efficiency.
At the operational level, common mistakes include: Failure to compact the rubber gradually from the center to the edges when laying it, or improper cutting size of the rubber compound, leading to air being trapped in the rubber layer; poor equipment sealing (e.g., leaking gaskets), allowing external air to enter or internal pressure to escape; releasing pressure prematurely before the joint cools to below 80℃—at this point, the unshaped rubber cannot contain internal gas, and the gas will expand to form bubbles.
In summary, the essence of bubbling in vulcanized conveyor belt joints is either failed gas discharge (trapped air, volatiles, or moisture) or weak interlayer bonding (inferior rubber compound, improper preparation). Therefore, during practical troubleshooting, priority should be given to checking the rubber compound quality, then inspecting the joint preparation, followed by verifying whether the vulcanization parameters meet the standards, and finally investigating environmental and operational issues. Key attention should be paid to three critical points: base material cleanliness, vulcanization pressure uniformity, and temperature stability.
