русский
Español
Français
Home / News / Industry News / Adhesive Layer Is Clear and Transparent — Why Does It Turn Yellow After Extended Storage?
Industry News
Transparent Adhesives · Optical Glue · Electronic Adhesive · Packaging Laminate
A transparent adhesive layer that is clear when freshly prepared can gradually turn yellow during storage — from transparent to light yellow, becoming more pronounced over time. Even minor yellowing can affect product appearance and final application results for transparent materials.
1
Transparent Does Not Mean Long-Term Colour-Stable
When the adhesive layer is first formed, the resin and other components are in a relatively stable state. But during storage, the material is continuously subject to oxygen, temperature, light, and humidity. As time increases, slow oxidation or structural changes occur inside the adhesive layer, and originally minor colour changes gradually manifest.
2
Resin Structure Is an Important Factor Affecting Yellowing
Some resins readily undergo oxidation and degradation under long-term storage or high-temperature environments, producing coloured structures. Some adhesives have excellent initial colour but gradually yellow over extended time — the root cause may lie in the resin's own ageing resistance capability.
3
Residual Low-Molecular Substances May Also Cause Yellowing
During adhesive production and curing, partially unreacted components or residues may remain. As storage time increases, these substances may continue to react with oxygen, ultimately forming coloured substances. This type of change is especially obvious in transparent adhesive layers.
4
High Temperature Accelerates the Yellowing Process
If the product is stored at relatively high temperatures for extended periods, molecular motion accelerates and oxidation reactions also accelerate accordingly. For the same adhesive, low-temperature storage may maintain transparency for a long time, while high-temperature storage makes yellowing more likely to appear earlier.
5
Light Exposure May Also Accelerate Colour Change
If the adhesive layer is exposed to sunlight or UV for extended periods, some resin structures may undergo photo-oxidation reactions. As exposure time increases, the adhesive layer may gradually develop: transparent → slightly yellow → obvious yellowing.
6
The Additive System Cannot Be Overlooked Either
The cure system, stabiliser, catalyst, plasticising components, and other functional additives used may all affect final colour stability. If certain components are themselves prone to oxidation, or have insufficient compatibility with the resin system, they may also become a source of long-term yellowing.
7
Why Transparent When First Made, but Progressively Yellowing?
Yellowing is not instantaneous — it is a gradual accumulation process. When newly produced, changes inside the material have not yet manifested significantly. As storage time increases, oxidation, light exposure, high temperature, and component changes continuously accumulate, ultimately causing the coloured structures inside the adhesive layer to gradually increase. Initial transparency only indicates good colour state at the beginning — it does not fully represent long-term yellowing resistance.
Summary
Transparent adhesive layers turning yellow after extended storage are usually related to resin structure, oxidative ageing, residual components, storage temperature, light exposure, and the additive system. For transparent adhesives with high appearance requirements, comprehensive assessment from raw material selection and formulation matching through to storage stability is the appropriate evaluation approach.
Frequently Asked Questions
Is there a standard test to evaluate long-term yellowing resistance without waiting months?
Accelerated yellowing tests are the standard approach: UV ageing chambers simulate light-induced yellowing; thermal ageing at 60–80°C simulates oxidative ageing under storage conditions. Measuring colour (CIE Lab b* value) at defined intervals during accelerated ageing gives a comparative ranking of formulations' yellowing resistance without waiting for real-time storage data.
Does using aromatic vs aliphatic isocyanate affect yellowing resistance in PU adhesives?
Yes significantly. Aromatic isocyanates (MDI, TDI) form urethane bonds that readily undergo photo-oxidation and produce coloured structures. Aliphatic isocyanates (HDI, IPDI) form much more UV-stable bonds and are the standard choice where long-term colour stability and non-yellowing performance are requirements.
