Hydrocarbon Processing - July 2022 - 78

Maintenance and Reliability
High-strength bonds are obtained after cleaning the substrate
by removing all contaminants, followed by roughing
the substrate-usually in the form of grit blasting-according
to internationally recognized standards.2
This is why surface
preparation is critical to success, regardless of what type of adhesive
is used.
There are three types of bonding that are important to ensure
good adhesion: adhesive, chemical and mechanical. Adhesive
bonding relies on surface energy to generate adhesion to
the substrate, while chemical bonding relies on a chemical bond
formation and on electronic bonding to produce adhesion. Mechanical
adhesion is due to the creation of an irregular profile
that allows a deeper profile to be produced. The types of structural
adhesives available are summarized in TABLE 1.
The following are two types of failure mechanisms associated
with structural adhesives:
1. Cohesive failure occurs in the bulk layer of the adhesive
material. This failure mode is limited by the strength of
the adhesive material and can be caused by insufficient
curing of the adhesive and/or by applications at a
greater thickness than what is recommended.
2. Adhesive failure occurs when the mechanical adhesion
between the adhesive and the parts being joined is
overcome by the loading. This failure mode is associated
with inadequate surface preparation, the presence of
contaminants, or insufficient curing of the adhesive,
among other factors.
The creation of a novel adhesive. Design considerations
for a proprietary fatigue-resistant adhesivea
were based on both
technical target requirements and a practicality approach. Technical
design considerations included the following:
* Excellent adhesion to metallic substrates: The
adhesive was optimized for metal-to-metal adhesion.
Type
TABLE 1. Types of structural adhesives
Strengths
Standard epoxies
Toughen epoxies
Lap-shear adhesion,
temperature resistance
Highest bond strength
(metal-to-metal)
Weaknesses
Peel and cleavage
forces
Temperature
resistance
* Suitable service temperatures: The adhesive had to
withstand service temperatures between -30°C and 60°C.
* High resistance to peel/cleavage forces: Most standard
epoxies do not perform well when subjected to peel or
cleavage forces; therefore, an epoxy that could withstand
both forces would be advantageous.
* Superior resistance to cyclic fatigue: The new adhesive
should show no signs of deterioration or cracking when
subjected to cyclic loading.
* Data package for engineering design: Performance
data collection was required for the adhesive to be
modelled through finite element analysis (FEA).
* Solvent-free formulation: Solvent-free epoxy systems
show greater mechanical properties vs. formulations
with solvents. Experiments have shown that solvents
remaining within the epoxy can hinder the cross-linking
process, resulting in lower exotherm, and affecting the
initial curing rate reaction order and glass transition
temperatures.
The practical design considerations included the following:
* High build paste structure: The adhesive should
be able to hold its own structure and not slump in
vertical applications.
* Ease of application: The system should be able
to be conveniently applied by brush, applicator
or cartridge injection.
* Fast return to service: The structural adhesive
should have shorter curing times to provide appeal
for asset owners.
The proprietary adhesivea
was subjected to the following tests
and evaluation protocols to ensure that it met the design criteria
previously discussed. Where possible, internationally recognized
standards were used. These tests/evaluations included:
1. Cleavage adhesion-ASTM D1062-08 (2015):
Standard Test Method for Cleavage Strength of Metalto-Metal
Adhesive Bonds
2. Tensile shear adhesion-ASTM D1002 (2019):
Standard Test Method for Apparent Shear Strength of
Single-Lap-Joint Adhesively Bonded Metal Specimens
by Tension Loading (Metal-to-Metal)
3. Tensile fatigue resistance-ISO 9664:1993:
Adhesives-Test Methods for Fatigue Properties of
Structural Adhesives in Tensile Shear
4. Impact resistance-ASTM D256-10 (2018): Standard
Test Methods for Determining the Izod Pendulum
Impact Resistance of Plastics.
Cleavage adhesion: ASTM D1062-08 (2015). Cleavage
adhesion is used to assess the strength of an adhesive bond
between two substrates when exposed to cleavage stress. The
proprietary adhesivea
was applied between two identical gritblasted
metallic cleavage test pieces to create a fixed bond area
of 125 mm2 of minimal bond-line thickness (FIG. 1).
The specimen was allowed to cure, and then it was attached
FIG. 1. Cleavage adhesion test.
FIG. 2. The proprietary adhesivea
was applied to two samples (A) and
then tested (B) using the tensile
adhesion test.
78 JULY 2022 | HydrocarbonProcessing.com
to a 25-kilonewton (kN) tensometer, using suitable grips. The
tensometer then applied a load at a fixed rate of 1.3 mm/min,
exerting a cleavage force on the specimen until bond failure.
This test was repeated five times to calculate an average force.
Tensile shear adhesion: ASTM D1002 (2019). Tensile
shear adhesion (or lap-shear adhesion) is used to determine the
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