42    ACCELERATED AGING OF HIGH DENSITY
POLYETHYLENE GEOMEMBRANES: A NOVEL
APPROACH
Mark W. Cadwallader, Director
Research and Technical Development
Gundle Lining Systems Inc
Houston, Texas 77073
INTRODUCTION
Because of problems with natural liner materials such as clays, recent government regulations
require the use of man-made polymeric geomembrane liners for containment of hazardous waste.
Mankind is coming to the unfortunate realization that hazardous waste production is an inevitable tax
on human civilization, and that the world is not so large that waste products can be indefinitely
dumped into the environment without restricting their location and mobility. Also, the simple truth is
that disposal and containment of waste products on the earth's surface must continue for lack of a
better alternative. Incineration and deep well injection are not possible or practical for many kinds of
wastes, and they have their own peculiar environmental risks and problems. Landfills and surface
impoundments will therefore be a fact of life for some time to come.
Due to many material advantages, especially chemical resistance, high density polyethylene
(HDPE) liners have become the industry-favorite geomembranes for hazardous waste containment.
Since these liners are performing the important function of preventing the spread of hazardous waste
into the environment, and since surface containment of waste will be around a while, the following
very important question must be addressed: How long will these liners last, or retain their design
properties? Because polyethylene itself was discovered only fifty years ago, historical data required
for a definitive answer to this question is lacking. Polyethylene materials properly stabilized with
carbon black have performed well in outdoor weathering tests begun in the 1940s.1 In addition, 20
year old HDPE liner installations are still intact. But beyond these tests, very little is known about the
ultimate lifetimes of properly stabilized high density polyethylene liners. The obvious goal for a
geomembrane liner in hazardous waste containment is that it be made to last as long as is possible.
Not only is this goal good stewardship of the earth's environment, but considering the amount of
money scheduled for Superfund projects it is simply cost-effective. Since real-time data on the aging
of polyethylene is limited, however, accelerated aging techniques must be studied in order to gauge the
lifetimes of HDPE liners.
In order to design an appropriate accelerated aging test, the mechanisms behind deterioration of the
liners must first be understood. Let us therefore consider the different factors involved in the rates of
aging and deterioration of high density polyethylene liners.
THE AGING OF GEOMEMBRANES
Deterioration of a geomembrane liner is interpreted as an irreversible process in which useful
polymer properties degenerate when exposed to the environment. Both biological/chemical species
and energy sources contribute to the deterioration. They include heat, ultraviolet light, high energy
radiation, environmental stress, biological organisms, chemicals, and oxygen. The deterioration takes
place because of the rupture of primary and secondary chemical bonds in the polymer matrix. Because
HDPE liners do not contain plasticizers and other additives which act as foodstuffs for biological
organisms, they are not susceptible to decay through biological activity. 2 For practical purposes,
therefore, degradation of HDPE liners takes place via five principal means: 1) U.V. light; 2) heat;
3) environmental stress cracking; 4) oxygen; and 5) other reactive chemicals. When these degradative
modes are studied more closely, it becomes clear that their roles in polymer degradation are not the
same.
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