Pipes/piping peroxide crosslinking

Peroxide crosslinked ionomer mixtures made from the abovementioned raw materials may be used in contact with fatty foods under certain limited conditions, according to their raw materials and additives. Vinyl chloride and vinylidene chloride should not be used in materials having either ionic (ionomer) or physical (e.g. through electron beam irradiation) crosslinks. Crosslinked polyethylene can be used for the manufacture of food contact materials, e.g. for drinking water pipes and fittings. The cross-linking can be done using either peroxide or electron beam irradiation. 

FT-IR spectroscopy has been used in the investigation of welding crosslinked polyethylene pipes [23]. Three types of crosslinking systems were used namely, peroxide (PEXa), silane (PEXb) and electron beam (PEXc). Scholten and co-workers [23] observed that only PEXa pipes have a satisfactory electrofusion quality. The strength of electrofusion welds of PEXb and PEXc pipes is not acceptable. The most likely explanation for the differences in weld quality is related to the adhesion theory and more specifically to differences in composition. Figure 5.6 shows the infrared spectra of medium density polyethylene (MDPE), PEXa, PEXb and PEXc. 

Peroxides for crosslinking Foams, cables, heating pipes... 

Following the extrusion process, insulated wire and cable can be heated to dissociate the peroxide into free radicals and crosslink the polyethylene. Pipe initially at its melting point and with no wire support will need special supporting facilities (which are normally proprietary) for post extrusion crosslinking. 

A one step process was developed by BICC with Maillefer for power cables which eliminated the compounding stage (the Monosil process) [15], in which all the additives were mixed with the polymer and grafting performed in the cable extruder. Direct injection has been used to inject a liquid mixture of silane, peroxide and tin catalyst into an add-on mixer to make cable [4] and hot water pipe [16]. The water pipes were steam autoclaved for four hours at 110 °C, which was well below the softening point of the HDPE used and the resulting crosslinked pipes withstood 1000 hours in water at 95 °C with a wall stress of 4.4 N/mm [ ] As with peroxides there are health and safety issues. Tri-methoxy vinyl silane is both very flammable and toxic. 

Stamicarbon BV CRT technology - high-pressure clean tubular reactor, nonfouling reactor, constant pressure and temperature profile, peroxides as initiators, low-cost production, short residence times. Total licensed capacity 1.8 MMT/y. LDPE (density 918-930 kg/m MFI = 0.2-70 g/10 min) and EVA (10 wt% VA) copolymers for injection moulding, sheets, flexible pipes, cable sheathings, blow moulding, crosslinking, and foam, etc. 

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