Silanes as Cross-linking Agents for Polyethylenes: Techno Brief

Crosslinking is a type of polymerization reaction that branches out from the main molecular chain to form a network of chemical links and cross-linking agents are added to resins to enable this process. This Cross-linking Agents guide is designed to help you understand more about the use of silanes as cross-linking agents in polyethylenes, focusing on the benefits and main applications.

Trigger Crosslinking at Desired Time with Silane

One unique benefit offered by Silane crosslinking technology is its ability to trigger the crosslinking at the desired time and particularly after the extrusion of the product.

With Silane technology, crosslinking can be triggered at the desired time.

Crosslinking of polyethylene is done by grafting a trialkoxysilyl group onto the PE polymer chain. Once this is done, the combination of a tin catalyst and moisture will cause the alkoxylsilyl groups to react together to form a crosslink between the polymer chains.

See Crosslinking mechanism

There are 2 main processes used to make PEX:

  • A one step process, called Monosil
  • A two step process called Sioplas

Monosil process (One step process)

The monosil technique introduces in a single step a mixture of Vinylsilane-peroxide-crosslinking catalyst antioxidant into polyethylene during a conventional extrusion process (such as pipe or cable). The finished product is moisture-cured (water bath or steam sauna).

Advantages of Monosil process

  • Cost effective on a larger scale
  • Single step- high speed
  • Lowest variable cost
  • Wide formulation latitude and wide customization
  • No additional heat history to the PE

Sioplas process (Two steps process)

In the Sioplas process, Polyethylene is first grafted in the presence of a mixture of Vinylsilane and peroxide to make a crosslinkable polyethylene. The material can be either processed directly or stored in dry conditions for up to several months.

In a Separate step, the crosslinking catalyst, typically a tin derivative such as dibultinlaurate (DBTDL), and an anti-oxidabt are mixed with polyethylene in a single or twin-screw extruder. This is the catalyst masterbatch, part B, to be used with the silane polyethylene, part A.

In a second step, grafted polyethylene is dry blended with a catalyst masterbach (a concentrate of Tin derivative in PE), in a traditional single screw extrusion process.
The extrudate is most of the time cooled down into a water bath which provides the moisture necessary for crosslinking. The reaction is fast but diffusion of moisture in the material is a limiting factor. For this reason, hot water bath or low pressure steam autoclave are often used to speed up crosslinking.

Advantages of Sioplas process

  • Wide range of applications
  • Multiple suppliers/sourcing options
  • Cost effective significant
  • No investment needed (conventional equipment usable)
  • Can use reinforcements

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