Curing principle of acidic silicone sealant

The curing mechanism of acidic silicone sealants (commonly known as acetate-type silicone sealants) is based on a condensation reaction, with moisture curing as its core. The following is a detailed description of the curing process:

1. Core Reaction: Condensation Reaction (Deacetic Acid Reaction)
Main Components:

Base Polymer: Hydroxyl-terminated polydimethylsiloxane (α,ω-dihydroxypolydimethylsiloxane).

Crosslinker: Methyltriacetoxysilane (most commonly used), provides crosslinking sites.

Catalyst: Organotin compounds (such as dibutyltin dilaurate), accelerate the reaction.

Reaction Mechanism:

The acetoxy groups (CH₃COO⁻) in the crosslinker (such as methyltriacetoxysilane) react with the silanol groups (Si-OH) of the base polymer, releasing acetic acid (CH₃COOH).

At the same time, the base polymers are crosslinked through newly formed Si-O-Si bonds, forming a three-dimensional network structure.

2. The Key Role of Moisture
Cure Trigger:
Water vapor (humidity) in the environment penetrates the colloid and reacts with the acetoxy groups in the crosslinker to form acetic acid and release hydroxyl groups (OH⁻), further promoting crosslinking.

Deep Cure Direction:
Cure progresses from the surface of the colloid toward the interior, as moisture gradually diffuses inward from the air-contact surface.
Thickness Limit: A single application thickness is typically no more than 10mm. Excessive thickness will result in incomplete internal cure.

3. Step-by-Step Analysis of the Curing Process
Surface Dry (Dry to Touch):
An elastic film forms on the surface (approximately 5-30 minutes). The acetic acid evaporates, producing a noticeable sour odor, but the interior remains uncured.

Deep Cure:
Moisture continues to penetrate, extending the crosslinking reaction inward, and the hardness gradually increases (curing depth is approximately 3mm in 24 hours).

Full Cure:
Usually takes 7-14 days (depending on humidity and thickness), ultimately forming a tough silicone rubber elastomer.

4. Key Influencing Factors
Factor Influencing Mechanism
Humidity: Higher humidity (>50% RH) results in faster curing; dry environments significantly slow curing.
Temperature: Higher temperatures accelerate the reaction (optimal range: 15-30°C); curing is extremely slow at low temperatures.
Adhesive Thickness: Thicknesses ≤ 10mm will fully cure; thicker layers require layering or a neutral adhesive.
Substrate Porosity: Porous substrates (such as concrete) will absorb released acetic acid, potentially affecting crosslinking efficiency.
5. Characteristics and Limitations of Acidic Adhesives
Advantages:

Fast curing and strong adhesion (especially to glass and ceramics).

Low cost and strong versatility.

Disadvantages:

Corrosiveness: Released acetic acid can corrode metals (copper, iron), cement substrates, and carbonate stone (marble, limestone).

Odor: A noticeable sour odor is produced during the curing process.

Limited Temperature Resistance: Long-term heat resistance is approximately 150°C, lower than that of neutral weathering adhesives (above 200°C).

6. Application Precautions
Avoid contact with sensitive materials: Neutral adhesive is preferred for metals, electronic components, and natural stone.

Apply in a ventilated environment to minimize acetic acid gas accumulation.

Control adhesive application thickness: If the thickness exceeds 10mm, apply the adhesive in batches or use a different adhesive type.

Humidity Management: In dry environments, spray with water mist to accelerate curing (avoid direct contact with the adhesive surface).