Understanding and Preventing Yellowing in Epoxy-based Clear Encapsulants

Epoxy-based clear encapsulants are thermosetting polymers produced through the crosslinking of an epoxy resin and a suitable hardener. Within this thermosetting mixture, a number of additives are added, such as accelerators, inert fillers, coupling agents, flame retardants, stress relief agents, and filter agents, to perform specific functions. Once cured, epoxy encapsulants enclose electronic devices to protect them from external factors, such as moisture, temperature, and contamination, to provide structure, support, and electrical insulation.

During use, epoxy-based clear encapsulants are exposed to different stressors, such as ultraviolet (UV) radiation, heat, moisture, and humid air. These stressors can cause physical and chemical changes in the encapsulants, affecting their performance and longevity. One notable effect of these changes is yellowing, a process wherein the encapsulant gradually acquires a faint golden hue. This phenomenon does not only affect the aesthetic quality but also the performance and reliability of the encapsulation as it may signal potential degradation. In response to these challenges, the industry has been implementing various strategies to minimize yellowing and preserve the clarity of the encapsulants. In this article, we will delve into the primary mechanisms and pathways that lead to yellowing. We will also briefly discuss the countermeasures adopted to reduce this issue

Yellowing Mechanistic Pathways

Different factors cause the development of an undesirable yellow hue over time. On a microscopic level, epoxy yellowing is primarily attributed to the photo and thermal degradation of the hardener, cured network, additives, and other accelerators, as well as the presence of contaminants.

1. Photo-degradation: UV Exposure

   Exposure to UV radiation can break down the main polymer chains in epoxy encapsulants, reducing their mechanical properties. This degradation creates free radicals within the chains, which may undergo oxidation to form color-causing molecules called chromophores. These chromophores absorb specific wavelengths of light, altering the encapsulant’s color.

2. Thermal-degradation: Heat Exposure

   Similar to UV irradiation, high temperatures give enough energy to the cured resin that allows the adjustment and breaking down of its polymer chain. This allows the original C–C atoms to reconfigure and form double bonds with other atoms, with oxygen being the most common one due to this element’s high electronegativity. The formed carbonyl (C=O) groups are actually chromophores that contribute to the yellowing of the encapsulant. Apart from that, C=O also creates a more dense crosslinked network that can lead to other issues, such as internal stress, cracking, and delamination.

   See more about ways to improve the adhesion of epoxy encapsulants on substrates here.
   
3. Hydrolysis: Moisture Infiltration

   When epoxy encapsulants are exposed to humid environments, water molecules infiltrate the resin matrix. The water molecules react  and weaken the epoxy bonds, causing them to break down into smaller molecules. At this point, the cured epoxy resin becomes more prone to oxidation. Oxygen in the air can react with the broken bonds, leading to further degradation. The reaction of water with the epoxy resin produces acidic by-products. These acidic compounds contribute to the yellowing of the encapsulant. The color change occurs because of altered optical properties caused by the presence of these by-products

4. Impurities and Contaminants

   Epoxy resins may contain trace amounts of impurities or contaminants that can catalyze yellowing during or after curing. For example, residual amines or phenols in the mixture can affect the encapsulant’s color. Oxidation of amines during the curing process can induce yellowing. Unreacted groups in the encapsulant, e.g. diglycidyl ethers in bisphenol A, can introduce a yellowish tint to the cured epoxy due to its inherent color and can readily form bonds with impurities and other present contaminants.

Mitigation Strategies Against Yellowing of Liquid Encapsulants

Understanding yellowing mechanisms is crucial for developing effective strategies to prevent or mitigate this phenomenon. The mechanism of yellowing depends on the polymer structure and the specific stressors affecting the encapsulant. However, oxidation is consistently present throughout the encapsulant’s life cycle, from manufacturing to end-user usage. Remember that all resins eventually yellow; meticulous material selection, UV protection, and proper handling can help preserve their clarity and appearance over time. 

• Incorporating UV stabilizer and anti-oxidants

During the encapsulant formulation, introduction of additive specifically chosen to inhibit UV-induced degradation and phenolic antioxidants is essential. These compounds acts as a protective layer, preventing the breakdown of the molecular bonds of the encapsulants. Some of these are hindered phenols and benzotriazoles, which deactivate the free radicals generated by UV exposure, effectively hindering the formation of chromophores. By preserving clarity, they extend the resin lifespan.

This mitigation strategy has been employed in one of CAPLINQ’s clear epoxy encapsulants, Optolinq LE-2161. The following optical images of cured Optolinq LE-2161 samples (10 and 30 g) demonstrate its anti-yellowing capability under two distinct conditions: exposure to 70 °C for 18 hours and  to 85 °C, 85% RH for 24 hours. The observed cloudiness is due to the presence of an anti-yellowing filler. This filler serves a crucial purpose without compromising the transmittance of the encapsulant. 

For applications requiring stringent compliance, Optolinq LE-2161HT serves as an ideal option due to its nature of being BPA-free epoxy. The image at the left is an optical image of cured LE-2161HT samples (1 and 2 mm) demonstrates its anti-yellowing capability under exposure to 70 °C for 24 hours at 90% RH. The optical images at the right are cured LE-2161HT samples (1 and 2 mm) demonstrates QUV performance using standard QUV test: UV-A (340 nm), Irradiance: 0.76±0.02 W/m^2 nm with specimen exposed to: 8hour drying, 0.25hour water spray, 3.75hour condensation and 50±3 °C black panel temperature.

• Controlled curing conditions

During the curing process, an environment with minimal moisture is desirable. To attain manufacturer’s guaranteed performance, curing conditions stated in the technical data sheet should be followed, these however can be subject to optimization to attain optimal performance as per product requirements and clients’ preference.

• Reliability testing to predict longevity

Accelerated aging tests are conducted to predict the lifespan of the cured epoxy resin. These tests simulate years of exposure by subjecting the resin to extreme conditions such as temperature variations, UV radiation, and humidity. Thermal cycling and humidity chambers are used to gain valuable predictions on the encapsulant’s endurance, as they imitate real-world temperature fluctuations and moisture levels.

Understanding and preventing yellowing in clear encapsulants is important, considering that they are widely used in various applications.

CAPLINQ offers a range of encapsulation solutions tailored to diverse applications, supporting various industries with reliable and innovative materials. Among these solutions, Optolinq LE-2161 and LE-5031 are clear epoxy encapsulants known for their exceptional anti-yellowing resistance. Contact us for more information.