PNNL-13501 - Pacific Northwest National Laboratory

Promising chemical durability, as measured by weight
gain during a 24-hour immersion in water, was shown by
thestyrene/butadiene and acrylic latex samples when
cured at 80°C. Natural latex samples have good
resistance to water absorption but exhibit poor mechanical
properties.
To test leach resistance, simulated waste (a mixture of
75% sodium nitrate [NaNO3] and 25% sodium hydroxide
[NaOH]) was mixed with the emulsion and the samples
were dried and cured at 80°C. The dried waste form was
placed in de-ionized water and the pH was monitored. As
seen in Figure 1, both the styrene/butadiene and the
acrylic latex samples effectively immobilize the salt
waste. The pH of the waste form test solutions approach
limiting values which are well below the baseline
measured for an equivalent quantity of salt dissolved in
water (the dashed line at pH = 12).
pH of Water
14
13
12
11
10
9
Styrene/Butadiene Latex
8
Silicon Latex
Acrylic Latex
7
Natural Latex (Hartex)
6
10% Waste Loading
Simulated Waste Only
0.0 0.5 1.0
Time (hrs)
1.5 2.0
Figure 1. Leach resistance of cured samples of commercially
available emulsions loaded with 10 wt% simulated salt waste
Multicomponent Stable Emulsions and Cured Materials
Multicomponent emulsions incorporating epoxy resin
were developed to enhance the mechanical properties and
chemical durability of the styrene/butadiene and the
acrylic latex materials. Commercially available epoxy
emulsions proved to be ineffective. In-house techniques
for emulsifying epoxy resin, using a variety of surfactants,
produced stable emulsions containing as much as 50%
epoxy mixed with the polystyrene/butadiene and
polyacrylic latex.
Addition of a suitable cross-linking agent caused the
epoxy rings inside the emulsion particles to open up and
react to form a tough plastic material. Materials cured at
80°C for 1 to 2 days are tough and hard and exhibit good
chemical durability. To date, the best composites absorb
only 6% water after immersion for several days.
312 FY 2000 Laboratory Directed Research and Development Annual Report
Salt Loaded Waste Forms
The two-step flowsheet used to fabricate waste forms
loaded with salt waste is shown in Figure 2. The epoxy
resin is emulsified using sorbitan monopalmitate as a
surfactant and mixed with either polystyrene/butadiene or
polyacrylic emulsions. Sodium nitrate is added to the
aqueous solution to simulate salt waste loading. Addition
of the cross-linking agent and curing at 80°C produces a
mechanically robust plastic matrix which surrounds the
salt-rich regions of the material. The high degree of
cross-linking makes the plastic matrix resistant to attack
by organic solvents, as indicated by minimal material
extraction during immersion in tetrahydrofuran.
Mechanically sturdy waste forms loaded with as much as
60 wt% NaNO3 can be made using the polyacrlic/epoxy
resin emulsions. However, optical characterization
suggests that the samples have phase separated at a coarse
level into salt-rich and polymer-rich regions.
Consequently, these materials do not display good leach
resistance because of the inhomogeneous nature of the
cured material.
PSB Latex
Epoxy resin
Surfactant
Waste s alt
Fast stirring and cross-linking
Composite
loaded
waste salt
Figure 2. Process flowsheet for encapsulating salt waste in
an aqueous-based polyceram waste form
An important result of this project was the discovery that
use of a faster cross-linking agent results in improved
homogeneity and a significant reduction in the leach rate.
Figure 3 shows a waste form made from the polystyrenebutadiene/epoxy
resin emulsion loaded with 20 wt%
NaNO3 using a fast acting cross-linking agent. This