Keywords: EVA; Foamed Shoe Materials; TAIC; ZnO; SEBS The formation of large bubbles (blowholes) in EVA foamed shoe soles is a common quality defect in production. The core cause lies in the imbalance between foaming and crosslinking rates — specifically, the gas generation rate from foaming agent decomposition exceeds the strength of the crosslinked network formed by EVA melt. As a result, the gas cannot be confined, leading to excessive expansion, coalescence and rupture of the cells. After eliminating process-related issues such as excessively high temperature, rapid heating, premature pressure release and uneven material mixing, the formulation ratio becomes the key factor determining the foaming effect. Targeted optimization can be implemented efficiently as follows: A single type of EVA with fixed vinyl acetate (VA) content cannot simultaneously meet the requirements for foaming fluidity and melt strength. Blending EVA grades with different VA contents is the core method to balance these properties. It is recommended to blend high VA content EVA (28%–33%) with low VA content EVA (14%–18%) at a ratio of 3:7 or 4:6: · High VA content EVA has stronger polarity and higher reactivity with crosslinking agents, enabling it to quickly form a dense three-dimensional crosslinked network, improve melt strength and effectively trap the gas generated during foaming. · Low VA content EVA offers excellent processing fluidity, ensuring uniform mixing of materials and smooth foaming molding. The synergistic effect of the two grades refines cell nucleation sites, prevents gas from accumulating in a small number of cells to form large bubbles, enhances the molding stability of shoe soles and reduces shrinkage deformation. An imbalanced dosage of accelerators is one of the direct causes of large bubbles. It is necessary to adjust the ratio of crosslinking accelerators to foaming accelerators to achieve the optimal state where crosslinking slightly precedes foaming. Triallyl isocyanurate (TAIC) is a preferred crosslinking accelerator for EVA foaming, exhibiting excellent synergism with dicumyl peroxide (DCP). It can significantly improve crosslinking efficiency, lower the crosslinking temperature by 5–10℃ and rapidly build a high-strength melt network. It is recommended to add 0.8–1.5 phr (parts per hundred parts of resin) TAIC to the EVA base material, which is particularly effective in solving large bubble problems caused by excessive foaming rate. If cell uniformity is poor, 0.3–0.5 phr trimethylolpropane triacrylate (TMPTA) can be added to further optimize the distribution of crosslinking density. Zinc oxide (ZnO) is a core accelerator for azodicarbonamide (AC) foaming agent. Excessive dosage will accelerate foaming agent decomposition, leading to a sharp increase in gas generation rate. Properly reduce the dosage of ZnO in the formulation (conventional dosage: 2–4 phr) to slow down the decomposition rate of foaming agent, allowing sufficient time for the crosslinking network to form. If crosslinking is too fast and cells are too small, the dosage can be slightly increased to 5–6 phr, or 0.5–1 phr zinc stearate can be added as an auxiliary regulator to avoid extreme imbalance. If EVA foamed shoe soles also suffer from poor resilience, insufficient shrinkage resistance or weak puncture resistance, incorporating styrene-ethylene-butylene-styrene block copolymer (SEBS) into the formulation is an effective solution. It is recommended to add 5–10 phr high-resilience SEBS: · Its soft segments can enhance the resilience and flexibility of shoe soles. · Its hard segments can improve structural stability and reduce post-molding shrinkage. Meanwhile, SEBS has excellent compatibility with EVA, which can further optimize the distribution of melt strength and help inhibit the formation of large bubbles. Note that after adding SEBS, the dosage of crosslinking agent should be appropriately adjusted (increase DCP to 1.0–1.2 phr) to ensure the strength of the crosslinking network and prevent cell collapse caused by system dilution. 1. Precise Control of Crosslinking Agent Dosage: As the core crosslinking agent, DCP dosage should be stably maintained at 0.8–1.2 phr relative to 100 phr EVA base material. Insufficient dosage results in low melt strength, while excessive dosage causes over-crosslinking, preventing cell expansion. The dosage needs to be adjusted in synergy with TAIC. 2. Adding Nucleating Agent to Refine Cells: Incorporate 0.5–1 phr ultra-fine talcum powder or calcium carbonate with particle size ≥ 2000 mesh to provide a large number of micro nucleation sites, achieving uniform gas dispersion and fundamentally avoiding large bubble formation. 3. Controlling Lubricants and Softeners: Limit stearic acid dosage to ≤ 1 phr; it is advisable to replace it with 0.5–1 phr high molecular weight polyethylene wax to reduce the weakening effect on melt strength. Control oil-based softener dosage to ≤ 5 phr to avoid reducing crosslinking efficiency. 1. EVA Base Material Blending: Laying the Foundation for Crosslinking-Foaming Balance
2. Adjusting Accelerator Ratio: Ensuring Synchronization of Foaming and Crosslinking
Increasing Crosslinking Accelerator TAIC
Reducing Foaming Accelerator ZnO
3. Adding SEBS to Improve Comprehensive Mechanical Properties While Meeting Functional Requirements
4. Supporting Precautions for Formula Optimization