Curing principle of acrylic sealant

The curing principle of acrylic sealants (especially the most common water-based acrylic sealants) is primarily a physical drying process, rather than a chemical reaction. Its core principle lies in the evaporation of water and the subsequent fusion and entanglement of polymer particles, ultimately forming a continuous, solid film with elasticity and adhesion.

The following is a detailed description of the curing process:

Application Extrusion:

The sealant is extruded into the joint as a milky white, paste-like emulsion. At this point, it primarily consists of tiny acrylic polymer particles suspended in an aqueous medium (water and additives).

Initial Water Evaporation (Surface Drying):

Once exposed to air, the water on the surface of the sealant begins to evaporate rapidly.

As the surface water content decreases, the polymer particles on the surface become closer and accumulate.

This stage typically takes from a few minutes to tens of minutes (depending on ambient temperature, humidity, joint thickness, and ventilation conditions). A non-sticky “skin” (surface drying) forms on the surface of the sealant, losing its fluidity but not yet achieving its final properties.

Continued Water Evaporation and Particle Fusion (Through Drying/Curing):

This is the most critical process. Water continuously migrates from the interior of the sealant to the surface and evaporates.

As the moisture content continues to decrease:

Polymer particles become densely packed: The distance between particles becomes increasingly smaller.

Capillary pressure: The tiny meniscus formed between the particles generates strong capillary pressure, pressing the particles tightly together.

Polymer particle deformation and fusion: Under the action of capillary pressure and gravity, the polymer particles (typically composed of elastic acrylic copolymers with a low glass transition temperature (Tg)) begin to deform, squeeze each other, and fuse (coalesce), gradually eliminating the interfaces between the particles.

Molecular chain diffusion and entanglement: Polymer molecular chains cross the original particle boundaries, diffuse, penetrate, and entangle with each other.

Forming a continuous elastic film:

When the moisture is almost completely evaporated (reaching equilibrium), the polymer particles are completely fused and the molecular chains are fully entangled, ultimately forming a continuous, uniform, elastic, and adhesive solid film. This film resists seam displacement and provides a waterproof seal.

Key factors affecting curing speed:

Ambient humidity: This is the most important factor. High humidity significantly slows moisture evaporation, significantly prolonging curing time (slowing both surface and through-drying). Low humidity accelerates curing.

Ambient Temperature: High temperatures accelerate moisture evaporation and polymer chain motion, accelerating curing. Low temperatures slow evaporation and molecular motion, delaying cure and potentially leading to incomplete cure or reduced performance.

Joint Thickness/Depth: Thicker joints have a longer path for moisture to escape to the surface, significantly increasing curing time (following the square law: doubling the thickness can quadruple the curing time).

Ventilation: Good air circulation helps remove saturated, moist air from the adhesive surface, accelerating moisture evaporation and curing.

Substrate Moisture Absorption: Porous substrates (such as concrete, wood, and gypsum board) absorb some moisture from the adhesive, potentially accelerating surface cure. However, be aware that this can result in overcure at the interface and undercure deeper down (especially in thick joints), sometimes even compromising adhesion.

Key Features and Considerations:

Physical Process, Not Chemical Crosslinking: Unlike silicone sealants (condensation-cure or addition-cure), polyurethane sealants (moisture-cure), or MS sealants (moisture-cure), acrylic sealants do not undergo a chemical reaction to form new bonds (crosslinks) during their curing process. Curing relies entirely on the physical evaporation of water and the physical fusion/entanglement of polymer particles.

Reversibility (theoretical): Theoretically, a fully cured acrylic film may swell or even partially redisperse under prolonged immersion in sufficient water or solvent (this is one of the main reasons why it is not suitable for long-term water immersion). However, under normal operating conditions (especially in a dry state), it is stable.

Deep Cure Depth Depends on Moisture Diffusion: Curing occurs from the surface to the inside. Curing within a thick layer requires time for the internal moisture to diffuse to the surface and evaporate. Therefore, deep cures take a long time (days to weeks) to achieve final properties (such as maximum elasticity and bond strength).

“False Cure” Phenomenon: A dry and hard surface does not necessarily mean the interior is fully cured. Before the internal cure is complete, the adhesive cannot withstand large displacements or loads, and its adhesion strength will not be optimal. Painting too early can also cause problems.

Temperature Sensitivity: At low temperatures, polymer particles become harder and more difficult to deform and fuse. This can prevent the formation of a continuous elastic film, affecting final performance. Therefore, a minimum application temperature requirement (such as 5°C or 10°C) is often set.

Summary:

The curing of acrylic sealants is a physical drying process. Essentially, water evaporates → polymer particles pack tightly → deform and fuse under capillary pressure and gravity → molecular chains diffuse and entangle → forming a continuous elastic solid film. This process is highly dependent on environmental conditions (humidity, temperature) and joint thickness. Understanding the curing mechanism can help ensure proper application (e.g., controlling joint shape and thickness) and provide reasonable expectations of cure time and final performance.