The silicone two-tone strap's two-tone silicone curing process ensures a strong bond between silicones of varying hardness. This process relies on optimized pretreatment, coordinated process steps, and interface design. This allows the two silicones of varying hardness to form stable chemical crosslinks and physical interlocking during the curing process, preventing delamination and peeling during subsequent use. First, before the process begins, the two silicone substrates of varying hardness must undergo targeted pretreatment, which is fundamental to ensuring strong bonding. Silicones of varying hardness exhibit varying surface properties due to varying levels of reinforcing agents and crosslinkers in their formulations. Direct bonding and curing can easily hinder molecular bonding due to surface impurities or oxide layers. Therefore, the process begins by cleaning the surfaces of the two silicones to be bonded, removing impurities such as oil and dust. Plasma activation or chemical treatment is then used to break down some siloxane bonds on the silicone surfaces, increasing the number of surface active groups. These active groups then form stronger chemical attractions with the molecular chains of the silicone of the other hardness during curing, laying the molecular foundation for strong bonding.
Next, process selection and parameter control are crucial. Currently, the mainstream curing processes for silicone two-tone straps are divided into two types: secondary curing and co-curing. Each method uses different principles to ensure bond strength. During secondary curing, the first hardness silicone is first injected into the mold for a preliminary curing. During this process, the degree of curing must be strictly controlled to prevent complete curing. This process leaves a certain proportion of uncrosslinked active sites, allowing the subsequently injected second hardness silicone to fully contact these active sites. After the second silicone is injected, a further curing process is carried out. At this point, the molecular chains of the two silicones interpenetrate at the interface, and the uncrosslinked active sites react with the other's crosslinker, forming chemical crosslinks across the interface. This effectively "entangles" the two silicones at the molecular level, rather than simply physically bonding them.
Co-curing involves simultaneously injecting two hardness silicones into a mold with a zoned structure. By precisely controlling the temperature and pressure in each zone of the mold, the two silicones undergo simultaneous crosslinking reactions within the same curing cycle. The key to this method is to match the curing rates of the two silicones. By adjusting the type and content of the curing agent in each formula, this prevents gaps from forming at the interface due to one silicone curing first and the other curing later. During the synchronous cross-linking process, the molecular chains of the two silicones naturally intertwine at the interface, forming a uniform bond layer. This reduces the internal stress caused by asynchronous vulcanization and improves bond strength.
In addition, the physical structure of the interface can also help enhance the bonding effect. During the mold design stage, microscopic physical interlocking structures, such as fine grooves, bumps, or wavy patterns, are pre-designed on the bonding surface of the two silicones. As the two silicones flow during the vulcanization process, they fill these structures. After cooling and solidification, they form a physical joint—like a puzzle piece, the two silicones are "locked" together by these structures. Even under external forces, the physical interlocking structures disperse stress and prevent direct separation at the interface. Furthermore, this physical structure increases the contact area between the two silicones, making the chemical cross-links more densely distributed, further strengthening the bond.
Finally, the post-vulcanization cooling process also requires control. A slow and uniform cooling rate can reduce the internal stress caused by differential shrinkage between the two silicones. Silicones of different hardness have subtle differences in shrinkage. If cooled too quickly, microcracks can form at the interface due to uneven shrinkage. These cracks can gradually expand and lead to delamination over long-term use. Slow cooling allows the two silicones ample time to contract synchronously, releasing internal stress and ensuring a stable interface. Through the synergistic effect of these steps, the two-tone silicone vulcanization process effectively bridges the differences in properties between silicones of varying hardness, ensuring bond strength through chemical cross-linking, physical interlocking, and stress control. This ensures that the two silicone colors of the silicone two-tone strap remain tightly bonded during daily wear, bending, and pulling, preventing them from falling off or delamination.
