The tear strength of liquid silicone fresh-keeping boxes is a key indicator of their durability. The vulcanization process, as a key step in determining the crosslink density of the molecular chains, directly affects the material's resistance to damage from stretching, bending, or impact. By optimizing the crosslinker type, catalyst selection, and process parameter control in the vulcanization system, the tear resistance of liquid silicone fresh-keeping boxes can be significantly improved, extending their service life.
The core of the vulcanization process lies in chemically converting the linear molecular chains of liquid silicone into a three-dimensional network structure. Traditional vulcanization systems often use organic peroxides or sulfur-containing compounds as crosslinkers, but these substances may leave small molecular byproducts, reducing the purity and tear strength of the material. In modern processes, platinum vulcanization systems have become mainstream due to their high efficiency and environmental friendliness. Platinum catalysts precisely control the crosslinking reaction rate, preventing molecular chain breakage caused by over-vulcanization. They also generate byproduct-free silicon-oxygen crosslinks, resulting in a more uniform and dense network structure. This structure effectively disperses stress concentrations under external forces, reduces the starting points for crack propagation, and thus improves tear strength.
The choice of crosslinker directly affects the mechanical properties of the vulcanized material. For liquid silicone fresh-keeping boxes, multifunctional crosslinkers such as hydrogenated silicone oils are preferred. The multiple Si-H bonds within their molecular chains react with vinyl groups in the silicone matrix, forming multi-branched crosslinking points and increasing the density of interchain entanglements. Compared to monofunctional crosslinkers, multifunctional systems can construct more complex three-dimensional networks, allowing stress to be transferred through multiple pathways when the material is subjected to stress, avoiding localized overload. Furthermore, the molecular weight distribution of the crosslinker must be strictly controlled. Products with a narrow molecular weight distribution can reduce uneven crosslinking and ensure consistent overall material performance.
Cure process parameters are also crucial in controlling crosslink density. Temperature is a key factor in controlling reaction rate: too low a temperature will result in incomplete crosslinking and loose connections between molecular chains; too high a temperature may trigger thermal degradation of the molecular chains, destroying the established crosslinked structure. Therefore, a gradient temperature ramp profile should be designed based on the silicone formulation and equipment capabilities to ensure the crosslinking reaction is completed gradually under mild conditions. Curing time control requires a balance between production efficiency and cure depth: extending the cure time can increase crosslink density, but excessively long cure times may lead to material embrittlement. In practice, the optimal cure window must be determined through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).
Pressure and demolding timing significantly affect material density. Applying moderate pressure during the cure process can expel air from the mold cavity, reduce porosity, and tighten the cross-linking structure. Furthermore, the timing of demolding must be tailored to the material's characteristics: premature demolding may cause deformation in incompletely cured areas, disrupting molecular chain alignment; while late demolding may lead to surface damage due to adhesion between the material and the mold. Modern processes often utilize intelligent temperature-controlled molds and automatic demolding devices, enabling precise control by real-time monitoring of the cure degree.
Post-cure treatment is an effective means of further enhancing tear strength. Residual stress or incompletely reacted functional groups may remain in cured liquid silicone fresh-keeping boxes. A secondary heat treatment (e.g., at 150°C for two hours) can promote residual cross-linking and improve the three-dimensional network structure. Post-cure also eliminates internal stress within the material, reducing deformation accumulation during use, thereby indirectly improving tear resistance.
Formula optimization and process synergy are the ultimate path to improving tear strength. By adjusting the ratio of the silicone matrix to fillers, a reinforcing phase can be introduced while maintaining flexibility. Adding rigid particles such as nano-silica creates an "island structure" that dissipates energy through particle debonding when subjected to stress, slowing crack propagation. Injection molding technology allows for more precise filler dispersion, avoiding stress concentration caused by localized accumulation. This integrated material-process design enables the tear strength of liquid silicone fresh-keeping boxes to surpass the limits of traditional materials.
