The advantages of metallocene-catalysed technology for silane grafting and crosslinking of polyethylene

Session 6A
2:20 pm

Daniel Libert, Total Petrochemicals

Silane method is a preferred technique of crosslinking polyethylene to modify its properties. Higher ESCR as well as thermal resistance should no longer be demonstrated.

Conventional HDPE, LLDPE or LDPE are known to offer different sensitivity to level of silane, curing time and temperature; rate of crosslinking and gel content may differ as well.

Many parameters of the base polymer molecular structure have to be taken into account when developing a PEXb compound like vinyl unsaturation, vinylidene groups, tertiary carbons, (short and long chain) branching number and distribution, polydispersity index, molecular weight, anti-oxydants, crystallinity, … The literature shows that HDPE usually exhibits:
• lower silane grafting efficiency vs LLDPE or LDPE
• less increase in zero shear viscosity
• longer time of crosslinking
These observations are constraints for new applications although products offered today show a broad range of characteristics in terms of density (product stiffness) and Melt Index (processability). Base resins can be mapped the same way although the molecular structure may be altered by the reaction pathways with peroxide and silane.

Metallocene-catalysed HDPE or MDPE offer an alternative to conventional base resins for PEXb as their unique molecular structure will react a different way during the silane grafting and the crosslinking processes allowing to target never achieved before performance in terms of processing behaviour and mechanical properties. High Melt Index Metallocene-catalysed HDPE or MDPE offer indeed a unique alternative to Ziegler-Natta equivalent resins thanks to their more favourable molecular structure.

Daniel Libert
Polyolefins, TS&D Pipe & Blow Moulding

Pierre Belloir
Polyolefins, TS&D Pipe & Blow Moulding Technical Manager