Environmental Engineering Reference
In-Depth Information
significantly at a dose of 10
9
rad. For example, in tests at BNL, the total gas produced
as a result of irradiation (G value given in mol/100eV) was ten times lower than
cement for doses up to 10
8
rad (0.029 compared with 0.24 for cement), but more
than 2.5 times higher than cement at 10
9
rad (0.43 compared with 0.16 for cement).
5
5.3.2
P
OLYETHYLENE
Polyethylene is a thermoplastic polymer that melts to a viscous molten liquid, which
can be applied for either microencapsulation or macroencapsulation of waste. If
mixed with small particles of waste to form a homogeneous molten mixture and
then cooled back to a solid, the waste particles are microencapsulated within the
polymer matrix. For larger particles (> 60 mm) such as lead or other debris, poly-
ethylene can be used to form an impermeable macroencapsulation envelope around
the waste particles, thereby minimizing leaching in the disposal environment.
Polyethylene is produced by polymerization of ethylene gas, and the structure
of the plastic can be tailored to yield products with a wide variety of physical and
mechanical properties. Unlike thermosetting polymers, the polymer chains in ther-
moplastic polymers like polyethylene are not normally cross-linked. It is available
in two distinct forms, low-density polyethylene (LDPE) and high-density polyeth-
ylene (HDPE). LDPE, with densities ranging between 0.910 and 0.925 g/cm
3
, is
formed by creating a high degree of chain branching, which effectively keeps layers
of polymer apart and reduces the density. HDPE, with densities ranging between
0.941 and 0.959 g/cm
3
, is produced from long polymer chains with relatively little
branching so that layers pack more tightly and thus increase the density. Each type
is available in a wide variety of melt viscosities (specified as melt index), defined
by ASTM D-1238 as the quantity of material that can flow through a given orifice
at specified temperatures in units of g/10 min.
17
Higher melt indices reflect lower
melt viscosity plastics (i.e., flow more readily) and vice versa. LDPE has a lower
melt temperature (120°C) than HDPE (180°C) and lower melt viscosities (typically
1 to 55 g/10 min) and thus is more widely used for waste encapsulation.
Typically, polyethylene is processed by extrusion, in which the material is fed
through a heated barrel by either a single helical screw or twin screws meshed
together. For polyethylene microencapsulation, waste and binder are metered
together using volumetric or loss-in-weight feeders to maintain a constant ratio of
materials. For macroencapsulation, clear plastic is extruded around compacted waste
to form a low-permeability barrier. A production-scale single-screw extruder system
for polyethylene microencapsulation is shown in Figure 5.5. Alternatively, polyeth-
ylene can be processed by kinetic mixer, in which the materials are mixed in a
chamber by a high-speed blade, and the frictional heat that is created melts the
plastic. A kinetic mixer system for polyethylene microencapsulation processing is
shown in Figure 5.6.
Although polymers in general, and polyethylene in particular, are relatively
newly engineered materials, they can be expected to withstand harsh chemical
environments over long periods of time under anticipated disposal conditions, based
on favorable results in short-term performance testing. For example, compressive
strength for polyethylene-encapsulated waste forms can range between 7.03 MPa
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