The hardness and elongation of embossed silicone have a significant impact on the bonding strength. The two need to be optimized in coordination to achieve the best bonding effect. The specific analysis is as follows:
The impact of hardness on bonding strength
Tear resistance
Low hardness silicone (such as 20-30A): Excellent tear resistance, suitable for complex patterns or fine molds. It has high elasticity, can buffer external impact, and reduce cracking caused by stress concentration at the bonding interface. For example, in leather embossing, low hardness silicone can fit the curved surface tightly to avoid degumming.
High hardness silicone (such as 50-80A): The tear resistance is weakened, especially at right angles. However, high hardness silicone can withstand greater pressure and is suitable for large products (such as split molds). It is necessary to improve the cohesive strength by optimizing the vulcanization process (such as secondary vulcanization) to indirectly enhance the bonding strength.
Deformation and stress distribution
Hardness is related to the tensile stress. Too high hardness will increase the brittleness of the material and it is easy to fatigue and break after long-term stress; too low hardness may cause separation of the bonding interface due to excessive deformation. For example, in the embossing of fine fleece fabrics of sweatshirts, if the hardness of the silicone is insufficient, the fabric may turn upside down during molding, resulting in poor adhesion.
Principles of hardness selection
Small/fine products: low hardness (20-30A) is preferred to balance elasticity and tear resistance.
Large/high-strength products: high hardness (50-60A) can be selected, but the lack of tear resistance needs to be compensated by additives (such as nanofillers) or process adjustments (such as segmented vulcanization).
The effect of elongation on bonding strength
Interface adaptability
High elongation (such as more than 900%): silicone can stretch with the deformation of the substrate to maintain close contact between the bonding interface. For example, in the embossing of elastic fabrics (such as sportswear), high elongation silicone can adapt to the expansion and contraction of the fabric to avoid degumming.
Low elongation: the material is more rigid and suitable for scenes with small deformation of the substrate (such as metal skeleton bonding), but it is necessary to ensure that the vulcanization is complete to avoid degumming caused by internal stress.
Fatigue and wear resistance
Elongation is related to crystallinity. High elongation silicone usually has good wear resistance, but long-term repeated stretching may cause fatigue fracture. For example, in shoe embossing, it is necessary to choose silicone with moderate elongation (500-700%) to balance flexibility and durability.
Elongation optimization direction
Adjust the formula: balance elongation and hardness by changing the proportion of polymer substrate or adding plasticizer. For example, addition-type silicone can control elongation by adjusting the vinyl content.
Process control: Prolonging the vulcanization time or increasing the temperature can increase the cross-linking density, appropriately reduce the elongation but enhance the cohesive strength, and need to be weighed according to product requirements.
Cooperative optimization of hardness and elongation
Application scenario matching
Complex patterns/fine molds: Choose low hardness (20-30A) + high elongation (more than 700%) silicone to ensure fit and tear resistance.
Large/high-strength products: Choose high hardness (50-60A) + medium elongation (400-600%) silicone, and improve cohesive strength through secondary vulcanization.
Experimental verification
Test the peel strength of silicones with different hardness/elongation (such as ≥5N/cm) with a tensile testing machine, and select the best combination in combination with weather resistance tests (such as high temperature and high humidity cycles). For example, an automotive parts manufacturer increased the bonding strength by 25% by adjusting the hardness of the silicone to 40A and the elongation to 600%.
Process compensation
For high-hardness silicone, sandblast the substrate surface or apply a primer (such as a silane coupling agent) to enhance chemical bonding; for high-elongation silicone, optimize the mold exhaust system to avoid bubbles causing bonding defects.