Adhesives and Sealants Testing

Simón-Portillo, Francisco J., et al. Composite Structures 348 (2024): 118511.

Adhesive bonds are often exposed to various environmental conditions that can significantly influence their mechanical performance during service. Among these factors, temperature plays a critical role in determining adhesive strength, which directly impacts the safety and durability of bonded joints. As a result, evaluating temperature effects is essential during the design phase to ensure reliable performance under varying conditions.

This study investigates how elevated temperatures affect the mechanical behavior of joints made with highly elastic adhesives, specifically polyurethane and silicon-modified polymer formulations. Shear and tensile tests were conducted at temperatures of 23°C, 50°C, and 80°C. Dumbbell-shaped specimens were used for tensile tests, while single lap joints (SLJ) were tested to assess shear behavior.

The figure illustrates the stress–strain curves of SikaFlex 252 adhesive at different temperatures, as obtained from the dumbbell tests. The results demonstrate a clear trend: as temperature increases, both the deformation resistance and stiffness of the adhesive decrease. At 50°C, the adhesive's failure strength dropped by 44% compared to the test at 23°C, and by 59% at 80°C.

Han, Xiao, et al. Composites Part B: Engineering 182 (2020): 107609.

Structural adhesives are increasingly utilized for bonding dissimilar materials due to their versatility and performance. However, environmental degradation poses a significant challenge to their reliability, particularly due to water ingress. Despite its importance, the quantitative understanding of how water diffusion impacts the local mechanical properties of adhesives remains limited. This study adopts a meso-scale approach to evaluate the effects of water diffusion on the local mechanical properties of structural adhesives at elevated temperatures.

Nanoindentation testing was employed to quantify the degradation of adhesive mechanical properties with varying moisture concentrations. The results, summarized with linear fits in Figures a and b, reveal a monotonic decline in the adhesive's elastic modulus and hardness as the moisture concentration increased from 0.0 (dry) to 1.0 (fully saturated). This trend was observed for both deionized and saltwater exposures.

Interestingly, adhesives immersed in deionized water exhibited greater degradation in elastic modulus and hardness compared to those exposed to saltwater. Moreover, as the moisture concentration continued to rise, the disparity between the two aqueous environments became more pronounced. This indicates that higher moisture concentrations exacerbate the degradation effects, leading to a more significant distinction between deionized and saltwater-exposed specimens.

Xiong, Yangyang, et al. Industrial Crops and Products 222 (2024): 119417.

Soy-based adhesives have attracted considerable attention due to their environmentally sustainable properties and cost-effectiveness. However, their application is often limited by inadequate bonding strength and water resistance, primarily caused by the polar groups in soy flour. To address this challenge, a simple and eco-friendly dual-crosslinking strategy was developed to produce a soy-based adhesive (SF/IO₄/CA) with improved water resistance.

The water resistance of SF, SF/IO₄, SF/CA, and SF/IO₄/CA adhesives was evaluated by measuring their residual rates after hot-water and boiling-water immersion (Fig. c). SF adhesives, after undergoing the curing process, exhibited dispersion and a darker color when exposed to hot and boiling water (Figs. a and b). The residual rate of the SF adhesive was 76.3% in hot water and 74.0% in boiling water, highlighting its vulnerability to moisture due to the hydrogen bonds formed, which are easily disrupted by water.

In contrast, the residual rate of the SF/IO₄ adhesive increased to 83.7% and 79.7% in hot and boiling water, respectively. This improvement suggests that the Schiff base reaction enhances the water resistance of soy-based adhesives. Notably, the SF/IO₄/CA adhesive exhibited a lighter color compared to SF, SF/IO₄, and SF/CA adhesives (Figs. a and b). Its residual rate further improved to 91.0% in hot water and 87.3% in boiling water, demonstrating superior water resistance among the tested adhesives.