Crosslinking is the formation of a three-dimensional polymer
network (ASTM D-883, “Standard Definitions of Terms Relating to Plastics) [107]. Such
networks are insoluble, forming a gel in solvents. The linking of molecules without gel
formation is considered to be chain extension which results in increases in molecular weight
(Mw) and shifts in molecular weight distribution (MWD). While the morphology or structure
of any given material governs many mechanical properties and physical attributes of a
crosslinked product, in many applications it is important to understand the significance of the
crosslinked density, which reflects the molecular weight between crosslinks, Mc, as illustrated
in Figure 42 [12].
Figure 42. Illustration of Mc in Crosslinked Materials
In using radiation processing in the production of grafted polyethylene films for battery
separators, a very high crosslinked density is desired to control ion flow. In tire manufacture,
low crosslinked density and modest gel content (~30 to 60%) is desired in order to permit the
tire components to knit or flow together in subsequent molding operations which rely upon
thermal curing. In heat-shrinkable products, films or tubings, Mc governs the elastic recovery
force when a crosslinked product is taken above its melt transition to shrink around an object.
In coatings, controlled Mc is needed in order to balance surface hardness and impact
Mc
Loose chain ends