July 2022

Maintenance and Reliability

Developing and utilizing a novel, toughened epoxy adhesive

When identifying solutions that can offer assurance and longevity, the maintenance and reliability of asset repairs can be challenging.

When identifying solutions that can offer assurance and longevity, the maintenance and reliability of asset repairs can be challenging. This article will examine the use of structural adhesives as the first-choice solution.

Since structural adhesives provide high modulus and high strength, they can be used for affixing metal substrates or components. Even though adhesives are used in a wide range of industries (including aerospace, rail and construction), they are not presently recognized internationally like traditional methods.

Traditional practices—such as welding, riveting, nuts and bolts, and mechanical fixing—are perceived as the go-to methods. However, they all have their inherent inadequacies. Welding, for instance, can be hazardous to health. Riveting, nuts, bolts and fasteners can concentrate stress.

This article introduces a novel, two-component, solvent-free toughened epoxy adhesive material that provides high adhesion to metallic substrates, while also being able to withstand high movement or cyclic fatigue vs. general epoxy materials.

In addition to potential application areas, this article also discusses several benefits of this adhesive material, including its ease of use, and its load bearing and impact-resistance properties.

Maintenance and repair procedures

Most industrial maintenance or repair procedures can involve either welding or the use of mechanical fasteners, as these methods can be perceived as easy and quick fixes; however, while these procedures might initially seem to correct the issue, they may cause more harm than good. Depending on the repair situation, welding or drilling to connect mechanical fasteners on a storage tank containing flammable liquid is not recommended for obvious reasons. This is where a structural adhesive can offer a solution for that type of maintenance repair.

There are many structural fixings used across a range of industries that may be part of any maintenance or repair, including support brackets (e.g., cable trays, antennas, heating coils, filter pans) or any other internal fixtures in vessels that suffer from corrosion, impacts or vibration damage.

Process equipment or piping can suffer from steel thinning or even from wall defects, which will require either monitoring or repairing depending on the severity of the integrity lost. Structural fittings are generally used for fixing static members, but they may be subjected to forces unbeknown at the time of installation, including thermal cycling of the joints, cyclic loading or vibration due to the fatigue of a component. If repairs are needed, the contractor may be in a situation where a choice of solutions can be made. If so, before a decision is made on a course of action, the strengths and weaknesses of the possible solutions should be identified.

Welding is regularly used for repairs since it is widely available. Although welding is well regulated and results in a high-strength repair, it does come with inherent risks (including heat stress and other types of harm to the user), as welding can cause both acute and chronic health risks.¹ The application of welding repairs onto piping sections, storage tanks, or process systems and equipment should not be undertaken due to the high temperatures involved and the combustible nature of the process fluids or gases running through these systems or being stored in these components.

Bolted joints are a simple and low-cost repair because of their ease of disassembly/reassembly. However, bolted joints often use dissimilar metals, which contributes to galvanic corrosion, adds weight to the joint, and requires routine inspection and tensioning. Also, drilled holes in the support material create uneven stress distributions.

Structural adhesives have a high bond strength and are lightweight. In addition, adhesives that are applied to cover the entire joint result in a uniform stress distribution, reducing metal distortion under strain.

The importance of a strong bond

Adhesive bonding is the joining of similar or dissimilar members together, while creating permanent high-strength bonds that can transfer structural stress without loss of structural integrity.

Regardless of the joint type used, it is important to understand the different stresses that are imparted onto a bonded assembly. Adhesives perform the best when the stress is two-dimensional to the adhesive, allowing the force to be applied over the entire bond area.

Joints that are well designed for adhesives place most of the stress into compression or shear modes. Adhesives perform the worst when stress is one-dimensional to the adhesive, concentrating the load onto the leading edge of the bond line. Joints that are placing stress on peel or cleavage adhesion concentrate this stress onto the leading edge, which may lead to premature bond failures, especially if subjected to vibration, impact or fatigue.

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 adhesive^a 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.
• 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 adhesive^a 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 Metal-to-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 adhesive^a was applied between two identical grit-blasted 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 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 adhesive strength of a material when bonded between two ridged metallic substrates. Two samples with dimensions of 100 mm × 25.4 mm × 2 mm were overlapped lengthwise by approximately 12.7 mm and bonded to a minimal bond-line thickness with the proprietary adhesive^a (FIG. 2). The specimen was allowed to cure before being attached to a 25-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.

Fatigue resistance: ISO 9664:1993. Fatigue resistance is the highest stress that a material can withstand for a given number of cycles without breaking. A standard static tensile shear adhesion test was conducted to determine the mean breaking stress—24.17 MPa was used as the mean stress in fatigue testing. The mean breaking stress value was 35%; therefore, the 35% mean shear stress was 24.17 MPa × 35% = 8.461 MPa. At four different alternating stresses, fatigue testing was conducted at 30 Hz until failure. Results were as follows:

1. 80% = 6.8 MPa (8.461 MPa × 80%) stress amplitude cycles between
2. 60% = 5.1 MPa (8.461 MPa × 60%) stress amplitude cycles between
3. 57.5% = 4.9 MPa (8.461 MPa × 57.5%) stress amplitude cycles between
4. 55% = 4.7 MPa (8.461 MPa × 55%) stress amplitude cycles between.

The ISO 9664:1993 fatigue stress cycle is shown in FIG. 3.

Impact resistance: ASTM D256-10 (2018). Impact tests can be used to assess the toughness of a material. A material’s toughness is a factor of its ability to absorb energy during plastic deformation. Brittle adhesives have low toughness due to the small amount of plastic deformation that they can endure. Conversely, tougher materials can absorb greater energy during fracture; therefore, they have improved impact resistance.

The Izod impact test enables samples to be tested in two forms: notched or unnotched. In this case, the testing was notched, with a V-shaped notch of approximately 2.5 mm in depth, and a total defect angle of 45° in the center of the specimen sample, with dimensions of 12.7 mm × 12.7 mm × 65 mm. The notch concentrates stress and enables a crack propagation measurement.

Three-point load test. This non-standard test was used to assess the relative flexibility of adhesives when applied to a metallic substrate. In this test, mild steel panels of dissimilar dimensions (Panel 1’s thickness was 550 mm × 50 mm × 10 mm, and Panel 2’s thickness was 225 mm × 50 mm × 10 mm) were stressed to the point that the adhesive failed. The panel was held in position at two points—one at either end of the sample—and was gradually stressed at a single point in the center of the specimen via a hydraulic press (FIG. 4). The greater the displacement (i.e., the further the press travels until failure), the more flexible the adhesive. The thickness of the adhesive will influence the degree of flexibility, so the analysis should be duplicatable for repeatability purposes.

In the case of this testing (the manufacturing stage), the specimens were compressed by hand pressure only to replicate in-field applications of achieving below the maximum bond-line thickness of 2 mm.

Testing results and discussion

Results of the cleavage adhesion test are shown in TABLE 2. Tensile shear adhesion test results are detailed in TABLE 3. Tensile fatigue resistance test results are shown in TABLE 4. From a mean breaking stress of 35% (8.461 MPa), the proprietary adhesive^a will survive 10⁶ cycles at 56.6%, with an alternating stress amplitude of ± 4.791 MPa = 13 MPa–3.67 MPa. The S-N curve of the proprietary adhesive^a is shown in FIG. 5. Results of the impact-resistance tests and the three-point load tests are detailed in TABLES 5 and 6, respectively.

Takeaways

The following takeaways can be drawn from the use of the proprietary adhesive^a as a solution for the repair and/or maintenance of assets:

1. The proprietary adhesive^a offers high resistance to structures that are subjected to forces such as peel, cleavage, vibration or cyclic loading. These include, but are not limited to, support brackets for fire
   deluge systems, internal and external fixtures on process equipment, wear pads, and wind girders
   on storage tanks.
2. As the proprietary adhesive^a offers an array of additional practical features (i.e., ease of application, the ability to hold its own structure when placed in vertical applications and superior adhesion to metallic substrates), the toughened epoxy can be used on structural support reinforcements, load transfer supports, and metallic staircases and ladders.
3. Plate bonding to repair thinning walls or wall defects on areas such as pipe/piping, process equipment, storage tank floating roofs and platform decks can utilize the proprietary adhesive, as it offers high impact resistance and flexural properties.
4. The fatigue-resistant adhesive has been optimized for metal-to-metal adhesion, and it exhibits an extensive data list with more than 20 tests solely based on adhesion. This performance data can be used for FEA or other simulations to aid in bond designing and/or qualification of the adhesive in areas (such as handrails and walkways) that would normally be seen as high risk for standard epoxies. HP

NOTE

^a Belzona 7311 fatigue-resistant adhesive

LITERATURE CITED

1. Health and Safety Executive, “Health risks from welding,” UK Government, online: https://www.hse.gov.uk/welding/health-risks-welding.htm
2. International Organization for Standardization, “ISO 17212:2012: Structural adhesives—Guidelines for the surface preparation of metals and plastics prior to adhesive bonding,” February 2012.

The Author

Related Articles

From the Archive

Comments