Mechanisms of Sunlight Degradation

Sunlight causes degradation through a process called photo-oxidation, a chain reaction accelerated by heat and oxygen.

UV Photons: The high-energy UV radiation (especially UV-A and UV-B wavelengths) in sunlight is absorbed by the polymer chains. This energy breaks the chemical bonds in the adhesive’s backbone, a process called photolysisor chain scission.
Free Radical Formation: The broken bonds create new, highly reactive free radicals.
Oxidation: These radicals react with oxygen in the atmosphere, leading to the formation of unstable peroxide and hydroperoxide groups. These groups subsequently decompose to form stable, but often colored, compounds like carbonyl groups (C=O).
Resulting Defects: This process manifests as:
- Yellowing/Discoloration: The formation of C=O groups creates chromophores, which absorb blue light and make the adhesive appear yellow or brown.
- Loss of Strength: The chain scission weakens the overall polymer network, leading to reduced tensile strength, lower flexibility, and increased brittleness.
- Surface Chalking/Cracking: Continued degradation can cause a powdery layer on the surface or visible micro-cracks.

2. Prevention and Mitigation Strategies

Preventing environmental degradation requires selecting the correct adhesive chemistry and using protective measures.

Aliphatic Formulations: Use adhesives based on aliphatic (non-aromatic) monomers and oligomers. Aliphaticstructures are significantly more stable and resistant to UV and oxidation than aromatic structures, offering superior non-yellowing performance.
UV Stabilizers and Absorbers: Select adhesives that contain integrated UV absorbers (UVA) and Hindered Amine Light Stabilizers (HALS):
- UVA absorbs the incoming UV energy and dissipates it harmlessly as heat.
- HALS scavenge the free radicals created by initial UV damage, stopping the oxidative degradation chain reaction before it can cause widespread damage.

Physical Shielding: If possible, design the assembly so that the adhesive joint is physically shaded or positioned away from direct sunlight.
Coatings: Apply a UV-blocking overcoat or clear lacquer to the cured adhesive bond. This top layer absorbs the sunlight’s UV rays, protecting the structural adhesive underneath.
Substrate Protection: When bonding through a transparent substrate (like glass), ensure the substrate itself has an inherent UV filter (e.g., laminated safety glass).