What factors affect the curing time of MS sealants?

What factors affect the curing time of MS sealants?
The curing time of MS sealants (the time from application to full elastomer formation) is not a fixed value. Instead, it is influenced by the material formulation and environmental conditions. These factors can be categorized into the following key categories:
1. Material Formulation Control (Core Determinant)
The sealant’s base formulation directly determines its curing potential and is an intrinsic factor influencing curing time:

The ratio of initiators to accelerators
Initiators (such as benzoyl peroxide) are the “source” of free radicals, while accelerators (such as N,N-dimethylaniline) are the “catalysts” that accelerate free radical generation.

The higher the ratio of initiators (within a reasonable range), the faster the free radical concentration increases and the shorter the curing time. (For example, increasing the initiator content in a certain formulation from 1% to 1.5% can reduce the full cure time from 48 hours to 24 hours.)

An imbalance (e.g., excessive accelerator) can lead to “implosion” (rapid localized curing but decreased overall performance), which in turn affects the curing quality. Molecular Weight and Content of Active Ingredients

A higher proportion of low-molecular-weight active monomers (such as methyl methacrylate) results in better fluidity, easier diffusion of the polymerization reaction, and faster cure.

A higher proportion of high-molecular-weight prepolymers (such as modified acrylic elastomers) results in higher system viscosity, slower reaction rates, and longer cure times, but higher final strength.

Type and Content of Inhibitors

Inhibitors (such as hydroquinone) prevent premature curing during storage. A higher inhibitor content results in a longer time required to “burn” the inhibitor after application, extending the cure induction period (for example, increasing the inhibitor content from 0.01% to 0.03% may increase the surface cure time from 1 hour to 2 hours).

II. Ambient Temperature (the Most Significant External Influencing Factor)

Temperature directly regulates the cure process by affecting the rate of free radical generation:

High-temperature accelerated cure: As the temperature increases (e.g., 25°C to 40°C), the initiator decomposition rate increases exponentially, resulting in a rapid increase in free radical concentration and a significant reduction in cure time. For example, a certain MS adhesive takes 72 hours to fully cure at 25°C, but this time can be shortened to 48 hours at 35°C.

Low-temperature curing delays: When temperatures fall below 10°C, initiator decomposition is slow, resulting in insufficient free radical generation and a significant slowdown in the curing process. For example, at 5°C, an adhesive that typically surface-cures in 24 hours may take over 48 hours, with full cure potentially extending to 7-10 days.

Risk of extreme low temperatures: When temperatures fall below 0°C, some formulations may “stop curing” due to reduced accelerator activity, causing the adhesive to remain viscous for extended periods and lose its sealing properties.

III. Adhesive Layer Thickness and Application Method

The physical form of the adhesive layer affects heat transfer and oxygen exposure (oxygen has a slight inhibitory effect on free radicals):

Thin adhesive layers cure faster: When the adhesive layer is less than 3mm thick, heat dissipation is rapid and oxygen exposure is sufficient (surface curing is unimpeded), resulting in a shorter overall curing time. For example, a 1mm thick adhesive layer may fully cure in 24 hours, while a 5mm thick layer may require over 48 hours.

Delayed curing within thick adhesive layers: When the adhesive layer is too thick (>10mm), the heat released by the internal reaction is difficult to dissipate, which may accelerate localized curing. However, the surface layer may cure more slowly due to oxygen inhibition, resulting in an imbalanced “slow outside, fast inside” state, which increases the overall curing time.

Influence of application method: Sealing strips formed by continuous application cure slightly slower than those formed by intermittent application (due to the reduced oxygen content in the enclosed environment). The larger the exposed surface area of ​​the strip (such as a wide, thin strip), the faster it cures.

IV. Ambient Humidity and Substrate Properties
Compared to temperature, these two factors have a smaller impact, but they require consideration in specific scenarios:

Minor effect of humidity: MS adhesives do not rely on moisture for curing (unlike silicone and polyurethane adhesives), so humidity (30%-80%) has little effect on curing time. However, high humidity (>90%) may cause moisture to adsorb on the adhesive layer’s surface, slightly delaying the surface drying time (extending 1-2 hours), but it does not affect the internal cure.

Indirect effects of the substrate: Smooth substrates such as metal and glass have minimal effect on heat dissipation from the adhesive layer, resulting in normal cure speed. However, porous substrates (such as stone and concrete) may absorb small amounts of monomer from the adhesive layer, resulting in a localized decrease in reaction rate and a slight increase in cure time (usually <10%).

Summary: Control Strategies in Practical Applications
If you need to shorten the cure time, you can prioritize the following optimization methods:

Select a “fast-cure” formulation (high initiator/accelerator ratio);

Control the ambient temperature between 15-30°C (optimal cure range);

Control the adhesive layer thickness between 1-5mm, avoiding excessive thickness;

Ensure the application surface is dry (to minimize the effects of high humidity).

Conversely, if you need to delay cure (e.g., adjusting the bead shape for large projects), you can choose a low-reactivity formulation or apply at a lower temperature (ensuring the temperature is ≥5°C to avoid curing stagnation).