Environmental Engineering Reference
In-Depth Information
polyester-styrene
(cross-linked)
CH
CH
H
H
H
O
O
+=
initiator
(
(
OCH
2
CH
2
OCCH=CHCO
C
C
OCH
2
CH
2
OCCHCHCO
OO
polyester
H
H
C
CH
cross-linking
agent (styrene)
H
FIGURE 5.2
Polymer structure of a thermoplastic polyester styrene matrix.
light, or contamination can be controlled by the addition of chemical inhibitors (e.g.,
hydroquinone, tertiary butyl catechol, quaternary ammonium salts) that react with
free radicals and prevent them from initiating the cross-linking process, thus extend-
ing the shelf life of resins.
10
While relatively small quantities (typically less than a total of 1 wt%) of catalyst
and promoter are required, the combination of specific catalyst and promoter, as
well as the amounts and ratios of reagents used, has a large impact on the time
required for polymerization and the successful processing of waste. For example,
too little catalyst or promoter may result in failure to polymerize, leaving a more
difficult and costly hazardous or mixed waste that must be treated. In the case of
LLW, failure to solidify creates a mixed waste. Too much of the catalyst/promoter
components may result in premature setting of the mixture, resulting in inhomoge-
neities of the waste within the final waste form or uncontrolled exotherms that can
volatilize waste constituents. In the case of sludges or other wastes containing
emulsified liquids, exotherms in excess of 100°C result in foaming that can overflow
the container and result in a higher waste volume. Typically, polymerization to a
solid monolithic final waste form occurs in 1 hour or less.
Thermosetting resins can be applied to treat dry active wastes (e.g., incinerator
ash, personnel protective equipment), ion exchange resins, and debris. Water-extend-
ible polyester resins are available that form an emulsion with aqueous solutions so
that wet wastes including sludges or evaporator concentrates can be solidified. In
systems where the emulsion is encapsulated within closed pores, small droplets of
solution are individually surrounded by polymer.
Polyester resins are not highly viscous and can be mixed by relatively simple
high-speed stirrer systems that can produce a homogeneous mixture of waste and
binder material. For liquid waste applications where an emulsion is created, the
mixer must deliver sufficient torque and shear to obtain and maintain the emulsion.
Often, polymer encapsulation systems are designed as in-drum mixers to avoid the
need to clean the process vessel and thereby saving cost and personnel exposure to
contaminants. In some applications the mixing blades are left in the drum and
sacrificed, rather than cleaning between runs, which requires solvents and would
generate additional secondary waste.
5.2.4
S
UMMARY
FOR
T
HERMOSETS
Advantages of thermosetting polymer systems include:
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