What is the Silane Cross-Linking (Sioplas or Monosil Process)?


Silane cross-linking is a widely used method to cross-link polyethylene and other polymers. This process involves grafting silane molecules onto a polymer chain, followed by a hydrolysis and condensation reaction that forms siloxane (Si-O-Si) cross-links.

Mechanism of Silane Cross-Linking

Step 1: Grafting Reaction

In this step, polyethylene reacts with a silane compound in the presence of a peroxide initiator. The peroxide decomposes under heat, generating free radicals that graft the silane groups onto the polymer chain.

Reaction Formula:
PE + R-Si-(OCH₃)₃ → PE-Si-(OCH₃)₃
•    Where:
- PE: Polyethylene.
- R-Si-(OCH₃)₃: Vinyltrimethoxysilane (a typical silane compound).

Step 2: Cross-Linking Reaction

This step involves two sub-processes: hydrolysis and condensation.

Hydrolysis:
The methoxy groups (-OCH₃) on the grafted silane undergo hydrolysis when exposed to moisture, forming silanol groups (-Si-OH).

Reaction Formula:
PE-Si-(OCH₃)₃ + 3H₂O → PE-Si-(OH)₃ + 3CH₃OH

Condensation:
Silanol groups condense to form siloxane bonds (Si-O-Si), creating a three-dimensional network.

Reaction Formula:
2PE-Si-(OH)₃ → PE-Si-O-Si-PE + 3H₂O

Processes: Sioplas vs. Monosil

1. Sioplas Process (Two-Step Process)
1. Grafting: Polyethylene pellets are grafted with silane in an extruder and cooled to form silane-grafted polyethylene.
2. Cross-Linking: The silane-grafted polyethylene is processed into cables and exposed to moisture (humid environment or water bath) to complete cross-linking.

Advantages:
- Flexibility in storage and transport of silane-grafted polyethylene.
- Allows for controlled production.

2. Monosil Process (One-Step Process)

In this process, silane, peroxide, and polyethylene are fed into the extruder simultaneously. The grafting and cross-linking occur in a single extrusion step, with moisture curing completing the process.

Advantages:
- Faster and more efficient than the two-step process.
- Ideal for high-throughput manufacturing.

Applications of Silane Cross-Linking
- Low-voltage (LV) and medium-voltage (MV) power cables.
- Insulation and sheathing layers.
- Plumbing pipes (PEX) for hot and cold water.
- Wire harnesses and tubing in automotive applications.

Key Features
- Low-Cost Infrastructure: Unlike continuous vulcanization (CV), silane cross-linking does not require high-pressure steam systems.
- Moisture Dependence: Consistent moisture exposure is critical for uniform cross-linking.
- Scalability: Suitable for both small-scale and large-scale production.

Advantages of Silane Cross-Linking
- Energy-efficient compared to steam curing.
- Allows for flexible production processes.
- Produces materials with excellent thermal, mechanical, and chemical resistance.

Challenges
- Requires precise control of moisture during curing.
- Slower cross-linking process compared to other methods like e-beam curing.