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Why Do Polyethylene Tanks Crack?
Guest article by Marshall Lampson, Dr. A. Brent Strong and Dr.
Materials crack when the forces
(internal and external) at any particular point exceed the
strength of the material at that same point. The forces are
measured over an area so that we can distinguish between a force
applied over a small area (very damaging) versus the same force
that is spread over a large area (generally less destructive).
Therefore, the forces are expressed in terms of force per area,
the units we normally call pressure or, more commonly in
mechanical systems, stress.
In many plastic parts, just the
molding process will induce internal stresses. These internal
stresses can be caused by several factors such as: orientations of
the molecules as they are pushed into a mold cavity in a process
such as injection molding, the pulling of the molecules as they
move through a die in a process such as extrusion, pushing of the
molecules when they are pressed between rollers or dies in
processes such as calendaring or compression molding.
molding allows the molecules to tumble freely inside the mold,
which results in parts that are relatively, stress free. However,
even rotational molding can induce stresses. For instance, when
there are differences in thickness between two areas within a
part, the thicker area will shrink more than the adjacent thin
area as the part cools. Because the thin area has already
solidified as the thick area is still shrinking, the molecules
that are along the boundary of the thick and thin areas will be
stressed. (They have one end fixed in the cool thin area, but are
being pulled into the thick area as it shrinks.) This is like
stretching springs, which is obviously an internal stress.
Failure or cracking will normally
not occur just from the internal stresses in a plastic part.
However, these stresses remain in the parts for very long periods
of time and, when added to an external stress, the sum can exceed
the local strength of the material and a crack can occur. Note
that the local strength of the material at any point is changing
with time. The action of UV light and oxygen can break the
molecular chains, thus decreasing their strength. (This process is
called degradation.) Therefore, the secret to avoiding cracks is
to have low molding stresses, to avoid or limit the amount of
degradation, and also to avoid external stresses.
Tanks fail primarily from two major
types of external stresses: chemical stress (microlevel) and
mechanical stress (macrolevel). In most tank loading applications
the total stress is a combination of the two.
Most polyethylene tanks develop
external stresses over a lifetime of use. Regardless of the
polyethylene material that the tank is manufactured from or the
way it was molded, sometime during the service life of the tank,
additional stresses will occur. Once this happens, or after it has
happened repeatedly over considerable time, it is appropriate to
think about replacing the tank. The nice advantage of polyethylene
tanks is that they are very robust, easily spreading forces over
wide areas (a property called toughness) and also resisting the
chemical forces that cause degradation and mechanical forces.
Therefore, with polyethylene tanks, you don't have to experience a
premature failure due to unnecessary stress that causes cracks and
Stress is really a combination of stress and degradation and
happens when the stored chemical oxidizes or plasticizes the
polyethylene. In other words, when a chemical leeches electrons
from the molecular chain of polyethylene, the chain becomes
susceptible to oxygen attack. This in turn will lead, over time,
to the embrittlement of the polyethylene.
Chemicals can also have a
mechanical effect on the tank because some chemical can act as
plasticizers. A plasticizing chemical modifies the molecular
structure of a polyethylene tank by making the wall soft and
causing swelling and expansion. Fortunately, very few chemicals
have a plasticizing effect on polyethylene. (This is not true of
polyesters and vinylesters, which are far more susceptible to
plasticizing effects.) Note also that if the molecules are tied
together, as when crosslinked, the swelling is less, thus reducing
the effect of chemical plasticizing.
We have found over the past 30
years that crosslinked polyethylene offers a tremendous advantage
over linear polyethylene and over fiberglass reinforced polyester
and vinylester tanks when talking about chemical stress and
attack. Let's look at some ways to anticipate chemical stress in a
polyethylene tank, and then we will look at mechanical stress and
ways ways to alleviate it.
- Check the ESCR (Environmental
Stress Crack Resistance) of the polyethylene tank you intend
to put into service. The ESCR provides the tank manufacturer
and end use customer with a measurable time frame for how long
a polyethylene tank can withstand the corrosive effects of
certain chemicals. Most linear polyethylene used for tank
manufacturing is rated between 50 and 400 hours of continuous
exposure to harsh chemicals.
- Make sure the tank(s) you are
ordering are compatible with the chemical you will be storing.
- Buy your tank from a reputable
manufacturer, one that is able to minimize the internal
stresses, which might result from improper molding. All
rotational molding operations are not the same.
Now that we have talked a little
about chemical stress, we can focus on mechanical stress and how
to control it.
the name implies, "mechanical stress" is caused when
stress is applied by physical means. This stress can be caused by
impacts, but can also be caused by stresses imparted from changes
made to the tank structure.
The most common place for
mechanical stress in a polyethylene tank is where a hole is cut to
attach a fitting. Many times, when fixtures such as pipes, pumps,
level indicators, or valves are hung onto a fitting, a binding
effect is created causing a cantilevering action, this causes the
polyethylene tank wall to expand and contract under stress and
causes premature failure instead of expanding and contracting
It is interesting to note that if a
crack develops due to this stress, the behavior of the crack is
different depending on the polyethylene material the tank is made
of. If the tank is manufactured from HDPE, the crack has a
tendency to unzip or grow quickly which can lead to catastrophic
failure. Due to the 3-D nature of the molecules in XLPE tanks,
this unzipping effect is greatly reduced or eliminated and
provides a much safer atmosphere for the end user. The failure in
a xlpe tank will be leakage through a small crack, not
Another cause of mechanical stress
is the improper placement of a fitting. By nature, all plastic
tanks are in a state of flux or movement due to temperature
variance or liquid level in the tank at any given time. It is
important that the fitting be placed at the right location. The
fitting should be situated above the knuckle where the wall
thickness is most even. This allows the tank to expand and
contract without causing the fitting bolts to bind in the area of
the cut or drilling holes.
Since plastic tanks expand and
contract, it is important to use some sort of flexible connector
or expansion joint with the correct flexibility to allow the tank
to move naturally and alleviate binding in the fitting area.
Expansion joints are commonly used and work well based on using
the correct size and material. Poly Processing Company or your
local distributor can help you make the correct choice.
One of the other products that work
well with polyethylene tanks is flexible hose. A flexible hose is
adequate to control vibration as well as moderate levels of
expansion. We recommend using care when choosing flexible hose for
your application. It is important that the application does not
have opportunity for compression. Flexible hose is really only
good for expansion of the polyethylene tank. Plumbing may also be
used to allow expansion through the use of extra elbows and / or
extra lengths of pipe.
- Marshall Lampson, Vice
President, Innovation and Technology, Poly
- Dr. A. Brent Strong, Lorin Farr
Professor of Entrepreneurial Technology Professor of
Manufacturing Engineering Technology, Brigham Young University
- Dr. Raed Al-Zubi, National
Innovation Specialist, Poly Processing Company
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