Causes of Brittle & Cracked Instant Adhesive Joints
General-purpose instant adhesives adopt pure cyanoacrylate systems. After curing, the molecules form dense crosslinks, resulting in rigid, inelastic adhesive layers with extremely low elongation at break. When bonded parts are subjected to vibration, impact, bending, or thermal cycling, the adhesive layer cannot buffer deformation stress, leading directly to microcracks and complete joint cracking.
Process-Related Causes of Cracking
- Excessive adhesive application If the bondline thickness exceeds 0.1 mm, the inner adhesive cannot fully cure, creating a massive internal-external stress difference that easily triggers cracking. The optimal bondline thickness is around 0.05 mm; thin coating delivers better toughness and higher bonding strength.
- Poor substrate surface preparation Oil stains, dust, and release agents hinder adhesive wetting, causing delamination at the bonding interface. Cracks will propagate along the gaps under load.
- Insufficient curing resting time Bending or vibrating the assembly after only a few minutes of initial cure prevents internal stress in the adhesive layer from releasing, which will lead to spontaneous cracking later.
Solutions
- Switch to modified adhesive grades Vibration-resistant toughened instant adhesives (SUITIT SG438 and SUITIT SG480) are formulated with rubber and other elastomeric components. The cured film features moderate toughness, outstanding impact and peel resistance, which drastically alleviates brittleness and cracking problems.
- Standardize operating procedures Apply only an ultra-thin, uniform coat of adhesive. Ensure substrate surfaces are free of grease, dust and moisture, and perform bonding in a well-ventilated environment with moderate humidity.
