Environmental Stress Crack Resistance
of Polyethylene
this test method is shown in Figure 3. Specimens are machined directly from pipe or from a molded plaque. Typical
samples measure 10 x 25 x 100 mm. A single notch cut into the test specimen acts as a crack initiation point. Side
notches are made to facilitate the cracking in the main notch. Notch depth is dependent on sample thickness.
However, typical notch depths are 138 microns on the front and 40 microns on each side. The applied load expedites
the cracking mechanism, leading to sample failure. Failure is classified as a complete separation at the notch,
indicating a brittle failure. Time to failure is recorded for each sample.
The Full Notch Creep Test (FNCT)
This method is accepted throughout Europe as the standard
method to test PE Pipe grade materials exhibiting very high
ESCR values. The FNCT test is preferred to the PENT test in
Europe, as it leads to shorter failure times. This is due to its
particular specimen design and to the presence of a surface-
active environment, such as IGEPAL®. No generally accepted
test conditions have yet been established for the FNCT test,
contrary to the PENT test. Nevertheless, the FNCT test is being
increasingly discussed in the European PE pipe industry, to the
point of establishing material specifications prior to offering a
detailed description of the test method. This method is
diagrammed in Figure 4. Typical test specimens measure 10 x
10 x 100 mm and are machined directly from pipe or from a
molded plaque. Specimens are notched on all four sides,
ensuring that notches are coplanar, with typical notch depth of
1,500 microns. The specimen is inserted in the grips of the
tensile creep machine and dipped in a temperature controlled
and circulated testing bath at 80° or 95°C. Typical loads are set between 4 and 5 MPa. Time to failure is recorded for
each sample.
Principle Variables Affecting ESCR in Polyethylene
The major variables that affect ESCR in polyethylenes include Molecular Weight, Molecular Weight Distribution,
Chain Branching (measured indirectly by density), and ESCR testing conditions (i.e., reagent concentration,
temperature, stress). In general, resistance to slow crack growth (ESCR) decreases as the amount of crystallinity
increases in a material.
Molecular Weight
Fracture is concerned with the premature failure of the strength properties of the affected materials. In polymers,
strength is strongly dependent on molecular weight. As the melt index of a polymer decreases, the average molecular
weight increases. This means that the polymer’s chains on average contain more molecules. It also means that
higher proportions of the chains are long compared to total number of chains present. All else being equal, stress
crack resistance of polyethylene improves as molecular weight increases (melt index decreases).
Molecular Weight Distribution (MWD)
In general, “narrow” molecular weight distribution polyethylenes have poorer ESCR values than do “broader”
molecular weight distribution polymers, all else being equal. However, this generalization should be viewed with
caution because a large number of other factors, including catalyst type and co-monomer distribution, have larger
degrees of influence on ESCR than does MWD.
Chain Branching and/or Density
ESCR is directly influenced by the type, length and complexity of chain branching. For polyethylenes, density is a
convenient, if not wholly accurate, measure of short chain branching. As a general rule of thumb, as branching
increases, so does ESCR. Thus, as density decreases, ESCR generally increases. ESCR appears to be particularly
sensitive to subtle variations in crystal structure and thus to differences in short chain branching. For example, it has
been shown experimentally that, all else being equal, hexene copolymers generally have higher ESCR values than
do butene copolymers.