Silane x-linking explained
In brief this process involves the following steps:-
- making a silane grafted polymer and a catalyst masterbatch
- extruding these together in order to make the desired object, for example cable insulation or a hot water pipe
- curing or crosslinking the polymer by exposure to moisture
Polyethylene is used in this explanation as the polymer, which has long carbon chains with many repeat units, a small segment of which is shown in the diagram below. This is processed through a heated extruder with silane and a small quantity of peroxide. The peroxide dissociates and can cause the double bond in the silane to break and the removal of hydrogen atoms from the polyethylene backbone to form reactive sites.
If two of these reactive sites are in close proximity, they will link together and the silane molecule will graft onto the polymer backbone. This can happen at a number of different points along the polyethylene molecule. This new product is called the graft.
This graft is then mixed with a catalyst masterbatch, typically in the ratio 95:5. This catalyst masterbatch is usually based on a tin catalyst such as di-butyl tin dilaurate (DBTDL) in a polymer carrier. This can either be pre-mixed with the graft or dosed into the extruder hopper along with the graft. This mixture is extruded to make the pipe or cable and the curing reaction occurs after the extrusion.
The first step in the curing process is that water must be introduced to hydrolyse the graft to replace the alcohol groups on the ends of the side chains.
In ambient cure silane systems there is enough moisture entering the polymer from the atmosphere unaided to enable this to happen. With polymers that are more resistant to water, it is necessary to accelerate this process by putting the cable into a hot water bath or in a steam chamber.
Once this has occurred the side chains on adjacent molecules are then able to join together assisted by the presence of the tin catalyst.
Crosslinked Polyethylene
The important point to note is that there are three available sites on the end of each side chain and that there are many of these side chains on each polymer chain. This means that chains can connect in many ways to form a three dimensional network. The formation of this network changes the polymer from a thermoplastic material (one that can be melted and reprocessed time and time again) to a thermoset product (one that cannot be reprocessed by heating again).
The time it takes to cure the polymer is determined by the thickness of the object and the temperature it experiences. For example, a cable insulation of 0.6mm will fully crosslink in four hours at a temperature of 80oC in steam or water. An ambient cure insulation of the same thickness will take around 5 days to cure at room temperature.