Hydrocarbon Processing - February 2021 - 63

Maintenance and Reliability
expectations for the system. Upon completing this exercise and
formulating a sample batch of the determined formula, the next
steps were performed to confirm that the required gel time, Tg
and lap shear values were achieved with the formula.
Gel time determination. Gel time is the length of time that the
two-part epoxy takes to gel after full mixing at a specified temperature. This value can be useful to determine a working time of
the system so that installers know the amount of time they need
to install the product correctly. The test was performed with an
80-gram mass at 24°C (75°F), using a standard laboratory gel
timer for four different batches to achieve and confirm repeatability in the results. The average result for the measured gel time
was 149 min, demonstrating that the goal was achieved. TABLE 1
shows the results for the gel time for the different batches.
Lap shear strength determination. Adhesion testing, using a lap shear test, is used for a specific bond strength for the
epoxy system to the metallic substrate. The testing was performed according to ASTM D5868-08. Composite plates of
the resin and fiber system were bonded to carbon steel plates
with the epoxy of the system and, using a universal testing machine, pulled in shear until failure of the adhesion occurred.
The type of failure observed was cohesive. TABLE 2 shows the
results of lap shear strength for samples cured at 24°C (75°F).
The average value of all eight samples tested was 1,158 psi
(8 MPa), doubling the required goal.

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Tg determination. The Tg is the temperature range where a
thermosetting polymer changes from a hard, rigid (or glasslike) state to a softer, more rubbery state. This is a critical value
to know and understand for a composite repair system, as the
system must be able to maintain its mechanical properties-
consequently, upper temperature limitations should be known
to ensure proper function.
The Tg of the polymer being considered was measured
using differential scanning calorimetry (DSC) per ASTM
E1356. FIG. 2 shows the Tg of the samples that were subjected
to a heat-cool-reheat cycle from 20˚C to 260˚C, with a ramp
of 10˚C/min. The Tg obtained was 134°C (273°F), which
demonstrated that the goal for this property was achieved [i.e.,
130°C (266°F)].
In addition to these physical properties that were evaluated,
the " practical " requirement for ease of use was also considered
from the beginning. The requirement was for the components
to be easy to mix, which meant that the viscosities should not
be such that they would be too high, creating a more difficult
mixture and causing difficulty in saturating the fiber. There was
not a specific target set, but an upper limit of 50,000 centipoise
(cps) was the expected maximum value. Upon mixing of the
epoxy, the final viscosity was measured to be approximately
25,000 cps, indicating a viscosity that could be successfully
implemented within the system.
Once each of these primary goals was achieved, the next step
was to test for compatibility with the 98% sulfuric acid solution.

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Hydrocarbon Processing - February 2021

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