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PEX is the resulting product from chemical joining of individual
polyethylene molecules to alter the performance of the original
base resin. The primary reason for crosslinking polyethylene
(PE) is to raise the thermal stability of the material under
load. Thus,environmental stress crack resistance (ESCR) and
resistance to slow crack growth are substantially improved.
Below
is a table summarizing the property change from PE to PEX:
|
Property |
From
PE to PEX |
| Melt
Index |
Decreases |
| Density |
Unchanged
or Decreases |
| Tensile
Strength |
Unchanged
|
| Elongation
at Break |
Decreases |
| Environmental
Stress Crack Resistance |
Increases |
| Resistance
to Slow Crack Growth |
Increases |
| Creep
Resistance |
Increases |
Hydrostatic
Design Basis (HDB)
¡¤ HDB @ 73¡ãF (23¡ãC)
¡¤ HDB @ 180¡ãF (82.2¡ãC) |
Unchanged
or Decreases |
In general, continuous temperature ratings exceeding 60¡ãC
are rare for polyethylene pipe. However, once the material
is crosslinked, the in-service temperature can be raised to
at least 100¡ãC and sometimes as high as 120¡ãC, depending
on the starting density, degree, and type of crosslinking.
PEX tubing and PEX fittings come in sizes ranging from 1/4¡±
to 2¡± in diameter. PEX tubing is manufactured to copper tubing
size OD controlled (CTS-OD) dimensions and is sold in coils
and straight lengths. The wall thickness is based upon SDR
9 values,which yield pressure ratings of 160 psi at 73¡ãF
and 100 psi at 180¡ãF. Additionally, some manufacturers have
had their tubing rated to 200¡ãF with an 80-psi maximum use
pressure. Consult the specific PEX manufacturer¡¯s literature
for appropriate pressure
ratings. Some PEX fittings shall only be joined to specific
PEX tubing as recommended by the pipe manufacturer.
Polyethylene can be crosslinked using
several technologies. All methods induce links between the
single strands of PE to form a dense network through radical
reactions. The number of links between the strands determines
the crosslink density and is an important factor in determining
the physical properties of the material. The minimum % crosslinking
for each method is specified in the ASTM F876 standard. The
three most
common methods of crosslinking polyethylene are as follows:
Peroxide - Peroxides are heat-activated
chemicals that generate freeradicals for crosslinking. This
is called the Engel Process.
Moisture-cured Vinylsilane - This
method involves grafting a reactivesilane molecule to the
backbone of the polyethylene. This iscalled the Silane Process.
Beta Irradiation - This method involves
subjecting a dose of high-energyelectrons to the PE. This
is called the Radiation Process.
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