How to Maintain Gloss in Low-Viscosity Epoxy Sealants

In electronics encapsulation, construction joint filling, and automotive assembly, low-viscosity epoxy sealants are highly valued for their excellent penetration, deep-gap filling capability, and application convenience. However, many engineers encounter a frustrating issue: after lowering the viscosity and improving flowability, the cured film loses its expected gloss, appearing dull or even hazy. This is not just an aesthetic loss — it can sometimes signal potential problems with sealing effectiveness or interfacial performance. Today, we'll explore the mechanisms behind this phenomenon and identify practical approaches to balance wetting and gloss.

Low viscosity typically means a higher proportion of diluents or lower resin molecular weight, which directly alters the system's rheological behavior and surface chemistry.

Surface Tension Imbalance: Low-viscosity systems often have relatively high surface tension, making them harder to spread on complex or low-surface-energy substrates, leading to uneven wetting. During curing, localized shrinkage differences can create microscopic roughness that scatters light, causing gloss loss.

Filler and Additive Distribution: To reduce costs or impart specific functions, fillers are commonly added. In low-viscosity systems, fillers tend to settle more readily, increasing dispersion stability challenges. If dispersion is poor or anti-settling fails, filler enrichment at the surface after curing directly reduces gloss.

Bubbles and Volatiles: Low-viscosity systems are more difficult to deaerate. Residual micro-bubbles create tiny voids after curing, while improper solvent evaporation paths may leave surface marks. These defects all act as "light scattering centers."

Curing Shrinkage Internal Stress: Studies have shown that unevenly distributed internal stress from epoxy curing shrinkage can cause micro-wrinkling or micro-deformation of the film, severely affecting surface smoothness and gloss.

The core of solving gloss loss lies in precisely controlling the system's surface tension to find the optimal balance between "fully wetting the substrate" and "forming a smooth, glossy surface." A suitable leveling agent plays a critical role here.

Wetting is the Foundation: The leveling agent must effectively reduce the system's surface tension below the critical surface tension of the substrate, ensuring rapid and uniform spreading of the adhesive and displacing air from the substrate surface — laying the foundation for a smooth film.

Leveling is the Process: After wetting, the additive needs to promote flow before curing, eliminating brush marks, roller marks, and orange peel or vortex patterns caused by Benard cells. Once cured, these flow defects become permanent gloss killers.

Compatibility is the Guarantee: The leveling agent must be well-compatible with the epoxy resin system. Incompatibility can cause additive migration or exudation, which itself becomes the cause of haze or cratering.

Addressing the stringent requirements of low-viscosity epoxy sealants for wetting, leveling, and final gloss, Anjeka Technology offers a range of validated leveling additive solutions to help engineers achieve precise control in formulation design.

Anjeka-7331 / 7331A — The Balanced Choice
This silicone surface additive, which moderately reduces surface tension, is a classic choice for balancing wetting and leveling. It effectively improves substrate wetting, promotes leveling, and imparts good slip and gloss to the film. Suitable for a wide range of systems including solvent-free epoxies, with flexible addition (0.025–0.2%), it is a reliable starting point for optimizing surface properties.

Anjeka-7377 — The High-Efficiency Wetting Expert
For extremely low-surface-energy or structurally complex substrates, stronger wetting capability may be required. Anjeka-7377, a fluoromodified polymer leveling agent, significantly reduces surface tension at lower dosages (0.5%–2%), powerfully promoting substrate wetting and leveling while also providing auxiliary deaeration — improving the final film state from multiple dimensions.

Anjeka-4630 — Synergistic Enhancement, Balancing Internal and External
When formulating with thixotropic agents like fumed silica, Anjeka-4630 can be added as a thixotropic synergist. It enhances hydrogen bonding interactions, allowing good sag resistance at lower thixotrope loadings while improving leveling and deaeration. This creates conditions for a smoother, denser cured film — indirectly contributing to gloss improvement.

1. Systematic Evaluation
   The knowledge base strongly emphasizes that before scaling up any leveling agent, its performance in your specific epoxy system must be evaluated through experiments — with special attention to foam stabilization, recoatability, and cratering tendency. Start with small-scale trials and optimize step by step.
2. Addition Method
   It is recommended to pre-dilute the leveling agent with a suitable solvent (such as an active diluent already in the formulation) before adding under stirring, ensuring uniform dispersion in the system.
3. Formulation Synergy
   Consider the leveling agent selection together with dispersants (such as Anjeka 6110/6111) and defoamers (such as Anjeka 5680A). A well-coordinated additive system, with each component playing its role, is the key to solving comprehensive surface defects — including gloss loss.

The gloss issue in low-viscosity epoxy sealants is the result of surface chemistry, rheology, and curing kinetics working together. It is not an unsolvable problem, but rather a reminder that formulation design requires a more systematic perspective. By precisely selecting and applying leveling additives such as Anjeka-7331 and Anjeka-7377, we can fully control surface tension — ensuring excellent penetration and sealing performance while delivering satisfying appearance and gloss.