Cross-linkable polyethylene (PEX) is typically produced commercially via either the Monosil Process or the Sioplas Process. In the Monosil process, a mixture of a vinylsilane, peroxide, cross-linking catalyst, and antioxidant is blended into a high-density or low-density polyethylene during an extrusion process. The extruded product is then moisture-cured in a water bath or steam sauna. The Monosil Process provides for large scale, cost-effective production of PEX. The cross-linking catalyst is typically an organotin-based catalyst such as dibutyltin dilaurate (DBTDL), or dioctyltin dilaurate (DOTDL).
In contrast, the Sioplas Process is a two-step process. In a first step, polyethylene is grafted with a mixture of a vinylsilane and a peroxide to produce a “cross-linkable” polyethylene. Separately, a cross-linking catalyst, an anti-oxidant, and polyethylene are mixed together in a single- or twin-screw extruder to form what is called the Master Batch. In the second step, the cross-linkable polyethylene and the Master Batch are then blended in a single- or twin screw extruder. The extrudate is then cooled with water thereby providing moisture for curing, or heat or low-pressure steam autoclaves are used to drive water into the material for curing. The catalyst of the Master Batch is typically also an organotin-based catalyst such as DBTDL. The Sioplas Process may be used in a wide variety of applications, utilize polyethylene from different sources, and requires a low capital investment.
Organotin compounds are known to be harmful to the environment, and they are toxic to humans. Organotin compounds may be extracted from PEX in water-contact applications, such as piping.