Silane cross-linked polyethylene pipe INTERSOL ... - Watts Industries

SILANE CROSS-LINKED POLYETHYLENE PIPE
2 - CROSS-LINKING METHOD
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High density polyethylene is a thermoplastic macromolecular component, obtained from the polymerization of the
ethylene monomer (CH2 = CH2).
Its chemical formula can be represented as: - (CH2 - CH2) - n
where n defines the length of the macromolecular chain (the average value of n can also lie between
10,000 – 16,000). From now onwards we shall represent such chain as:
Hence polyethylene consists of various macromolecular (polymer) chains, whose cohesion forces cannot strictly
be considered to be true chemical bonds, rather they are electrical in nature and are commonly known as “Van
der Waals” forces. Although such cohesion forces are low, the high number of intramolecular bonds favours
obtaining of certain properties for the product.
However the low energy of the cohesion forces makes the thermoplastic materials highly sensitive to temperature,
which causes considerable decay of the properties.
Suppose in addition to the “Van der Waals” forces, we introduce intramolecular chemical bonds (the so-called
cross-linking bonds), the thermal properties of the product will be considerably improved.
Cross-linking is a process which modifies the chemical structure of the material, by creating a three dimensional
“network” structure thanks to links between the polymer chains. The new structure determines certain special
characteristics, namely:
• an increase in the maximum operating temperature
• a reduction in creep deformation (creep)
• improved chemical resistance
• improved resistance to UV rays
• improved abrasion resistance
• greater impact strength
• less notch sensitivity and abrasion
• thermal memory characteristics are conferred to the material (“thermoelastic polymer”)
2.1 - Classification of the cross-linked polyethylene
Cross-linked polyethylene is classified according to the methods used to perform the cross-linking as summed up
in the following table:
Table 2
Type of cross-linking Cross-linking agent Product symbol
a Chemical Peroxides PEX-a
b Chemical Silanes PEX-b
c Physical Electronic rays (beta) PEX-c
d Chemical Azo compounds PEX-d
Processes a, b and c are the most frequent ones and they will be described in the following pages.
2.2 - Chemical cross-linking with peroxides (PEX-a)
In the Engel method, the peroxide (chemical formula ROOR) is added to polyethylene during the extrusion phase.
The process consists of two steps, namely:
• formation of free radicals
• cross-linking
+ ROOR + ROH
+
Special machines that allow reaching of pressures up to 2000 bar are required to complete such process.