Crazing is the formation of micro-cracks or voids within the adhesive bulk or near the interface when the material is subjected to mechanical stress. These internal defects scatter light, causing the material to appear white or foggy.

Causes:

• High Internal Stress: Excessive volumetric shrinkage during the curing process (a common characteristic of highly reactive acrylates) can build up significant internal stress in the bond line, particularly when bonding rigid, inflexible substrates (like glass or ceramics).
• External Stress: Applying or developing excessive mechanical stress (e.g., thermal expansion mismatch, bending, or impact) on the finished assembly can initiate crazing in a brittle adhesive.
• Brittle Formulation: Adhesives with a high cross-link density (highly rigid) are more prone to crazing than flexible formulations.

Solutions:

• Choose Flexible Adhesives: Select an adhesive with lower modulus and higher elongation. These materials can absorb stress without fracturing the polymer network.
• Minimize Cure Shrinkage: Use adhesives that are filled or formulated with higher molecular weight oligomers, as these shrink less upon polymerization.
• Optimize Cure Cycle: A slower, more complete cure (e.g., using a step-cure profile or a thermal post-cure) can relax internal stresses, making the cured adhesive less brittle.

2. Whitening from Moisture or Chemicals

Whitening due to environmental exposure is a sign of material degradation or absorption.

A. Moisture Absorption (Hydrolysis)

• Mechanism: When exposed to high humidity or immersion in water, the adhesive material absorbs moisture. This water uptake can cause two problems:
  1. Phase Separation: The absorbed water molecules interfere with the light path, causing scattering and a hazy appearance.
  2. Hydrolysis: In some adhesive types (e.g., certain polyesters or epoxies), water can chemically break down the polymer chains (hydrolytic degradation), leading to degradation products that whiten the material.
• Solution: Use hydrolytically stable adhesives, such as those based on pure polyurethanes or silicones, especially for applications exposed to steam, hot water, or high RH environments.

B. Chemical Attack (Solvent Fogging)

• Mechanism: Exposure to solvents, cleaners, or aggressive chemicals can swell the polymer network. The solvent penetrates the adhesive, causing localized disruption of the polymer structure or leaching out uncured components, which can change the refractive index and cause fogging.
• Solution: Verify the adhesive’s chemical resistance against all expected post-assembly cleaning agents (e.g., IPA, acetone) or operating environment chemicals. Switch to an adhesive that has demonstrated resistance to the specific chemical in question.

3. Fogging (Outgassing on Neighboring Surfaces)

While less common, “fogging” can also refer to outgassing where volatile residual components from the adhesive vaporize and condense on nearby surfaces, particularly optical components like lenses or mirrors.

• Solution: Ensure the adhesive is 100% fully cured (addressing the risk of incomplete cure). For sensitive electronics or optics, use low-outgassing adhesives that meet industry standards like NASA or ESA specifications.