💡 In This Article
- What chemical changes occur inside the epoxy network, and how chromophores form
- How aromatic and cycloaliphatic epoxy resins compare in yellowing resistance and other properties
- How the choice of curing agent affects yellowing
- Why FT-IR can miss yellowing, and what CIE L*a*b* values add
In the previous article, Why Does Epoxy Adhesion Vary from One Metal to Another? — Why the Same Adhesive Gives Different Results, we saw that the same adhesive can bond differently depending on the substrate surface and the process conditions. An epoxy’s performance depends not only on the resin itself but also on its environment and conditions of use..
This time we look at a different kind of change: an epoxy that starts out clear or light-colored and gradually turns yellow. Heat and UV usually get the blame, but what is actually happening chemically inside the cured epoxy?
Yellowing is extremely common. It is easy to see in clear or light-colored formulations, where heat or UV exposure changes the color quickly. But “epoxy turns yellow under heat and UV” is too thin an explanation to be useful. The practical questions are why the color changes, and how to choose the resin and curing agent to limit it.
Yellowing Comes Down to Chromophore Formation
Yellowing is better understood not as the creation of a yellow substance, but as the formation of new chemical structures in the cured network that absorb light. Heat or UV initiates radical reactions in the polymer, and once oxygen is involved, oxidation proceeds. This can produce a range of oxidation products, including carbonyls and conjugated structures, and some of them act as chromophores that absorb visible light.
In epoxy/amine networks, thermal and photo-oxidation have been reported to increase carbonyl content and to form structures such as quinone methide that can contribute to color. So “carbonyls form, and that is why it turns yellow” is not accurate either. What matters is the electronic structure that the various oxidation products ultimately take on.
The secondary and tertiary amine structures formed in amine-cured systems are especially vulnerable to photo-oxidation. Hydrogen abstraction at the α-carbon of the amine produces an aminoalkyl radical, which reacts with oxygen and, via amine-oxide or imine intermediates, ultimately yields a strongly conjugated quinone methide structure. Quinoid units like this absorb strongly in the blue-to-violet range (about 400–450 nm). Because that is the complement of yellow, the eye sees a distinct yellow color.
Why Are Aromatic Epoxies at a Disadvantage?
Standard DGEBA-type epoxies contain aromatic rings. These rings are central to the resin’s stiffness and thermal properties, but they work against color stability under photo-oxidation. Where yellowing has to be kept low, cycloaliphatic epoxies are often considered, since reducing or eliminating the aromatic structure lowers UV-induced yellowing.
Still, “cycloaliphatic is better” is not the end of the story. Changing the resin structure also changes cure speed, viscosity, Tg, adhesion, and mechanical properties. Some cycloaliphatic systems have been reported to trade slower cure for better yellowing resistance, and the advantages that the aromatic structure provided are difficult to retain.
The Curing Agent Is Part of the Problem Too
It is easy to treat yellowing as a resin problem, but in real products it is not. An epoxy adhesive is a single network formed by the reaction of resin and curing agent, so even with the same resin, the final color stability can vary with the curing agent’s structure. What matters is both the curing agent’s own oxidative stability and the structures that form in the network after cure. When yellowing appears, it is worth checking in this order: resin, curing agent, additives, cure conditions.
| Resin / Curing Agent | Representative Chemistry | Yellowing Resistance (UV/Heat) | Cure Speed | Tg and Mechanical Strength | Notes and Main Uses |
|---|---|---|---|---|---|
| Aromatic epoxy | DGEBA, DGEBF | Low (prone to yellowing) | Fast | High | General industrial and structural adhesives |
| Cycloaliphatic epoxy | Cycloaliphatic | Very high | Slow (typically cationic cure) | Medium–high | LED encapsulants, optically clear adhesives |
| Aromatic amine | DDS, MDA | Very low (severe yellowing) | Moderate | Very high | Heat-resistant structural applications |
| Cycloaliphatic / aliphatic amine | IPDA, PACM, polyetheramine | Moderate–high | Fast–moderate | Medium | Clear coatings, room-temperature-cure adhesives |
| Anhydride | MHHPA, THPA | High | Slow (high-temperature cure) | High | Electrical and electronic insulating encapsulants |
FT-IR May Not Show Yellowing Clearly
When people think about analyzing the cause of yellowing, FT-IR is usually the first instrument that comes to mind. If oxidation increases carbonyl and similar structures, FT-IR should show a change, and some studies do use the carbonyl region to compare the degree of polymer oxidation.
In practice, though, a sample can look distinctly yellow to the eye while its FT-IR spectrum shows little difference. This is not an odd result. The amount of chromophoric structure may be very small relative to the whole polymer, and new peaks can be buried in or overlap with the existing spectrum. FT-IR alone may therefore be a limited way to judge the degree of yellowing.
Color Itself Is Measured More Directly with L*a*b*
To compare the degree of yellowing, the CIE L*a*b* color space can be used:
- L*: lightness (white ↔ black)
- a*: green ↔ red
- b*: blue ↔ yellow
As yellowing progresses, b* generally increases. Comparing the L*a*b* values of the initial specimen with those of a heat- or UV-aged specimen therefore puts the color change into numbers. Even when the yellowing is not obvious to the eye, the change in b* can reveal a subtle shift. At the same time, b* alone should not be taken as the whole color change, since L* and a* can shift together with it.
Next: Cure Shrinkage
As epoxy cures, its chemical structure changes, and so do its properties and its volume. In adhesive development, this cure shrinkage is more than a dimensional change. It can translate into internal stress and interfacial stress. So why does a liquid epoxy shrink as it turns into a three-dimensional network? In the next article, we will look at the origin of cure shrinkage and how it affects real adhesives..
