How to Specify an Epoxy for Your Application

Choosing the right epoxy for bonding, sealing, or coating.

(Image courtesy of Master Bond.)

(Image courtesy of Master Bond.)

Epoxies have come a long way since the days of brittle, rigid compounds that were challenging to use. Choosing the right epoxy for your application is a matter of considering its specific properties, such as temperature resistance, bond strength, cure times, and toughness, to name a few.

From structural epoxies to potting compounds, coatings and sealants, Master Bond, a major manufacturer of industrial-grade epoxies, has targeted toughness as a key factor in improving performance. In epoxy systems, toughness can be loosely defined as the flexibility/compliance of the material and resistance to cracking and chipping. By zeroing in on this key characteristic, Master Bond has engineered better high-performance epoxy resins for every application.

Specifying Epoxy by Application

Of course, the first consideration in selecting the right epoxy system depends on the use. Many formulations are designed specifically for unique applications; Master Bond can work with customers to tailor a custom solution to the problem.

Structural Bonding

Epoxy is an excellent option for structural bonding, such as fixing automotive body panels to a frame. It is especially ideal for lightweighting, as a bonding contact patch of a few millimeters (less than 1/16”) of epoxy can be much lighter than metal fasteners or a weld bead.

In many structural bonding applications, thermal cycling is a major consideration. Nearly all materials have a non-negligible coefficient of thermal expansion, which can affect bonds, especially between two different substrate materials. For good thermal cycling resistance, the epoxy needs to be compliant enough to allow the substrates to flex and move without cracking the bond. Interestingly, this is the reason thermal cycling resistance is a separate consideration from simple temperature resistance.

According to Rohit Ramnath, senior product engineer at Master Bond, “Even if you choose an adhesive which gives you excellent temperature resistance, if it doesn’t cycle well thermally, you might observe cracking over time. That is where toughness comes in—to introduce a little bit of flexibility to withstand thermal cycling, while maintaining the strength that you typically need in many structural applications.”

There are several options to introduce toughness into an epoxy compound.

“To improve a formulation’s resistance to thermal cycling, we can work with the same epoxy base and add some sort of a flexibilizer or rubberized ingredient, depending on the specific chemistry of the base. The result is a more toughened formulation, but there may be a slight compromise on the strength and/or temperature resistance. So, it all depends on what the end use is, and on the work conditions the adhesive or sealant will have to withstand,” said Ramnath.

(Image courtesy of Master Bond.)

(Image courtesy of Master Bond.)

Electronics Epoxies: Potting, Conformal Coating, Heat Dissipation and Bonding

Epoxies are also widely used in electronics applications, as they can protect components from hazards like impact, electrical contact, moisture and vibration. When working with electronics, arguably the most important properties to look for in an epoxy are thermal: thermal cycling resistance, temperature resistance and how the epoxy deals with heat.

For heat dissipation, such as for heat sink bonding, it’s best to find an advanced thermally conductive adhesive specifically designed for electronics.

Historically, early epoxy formulations were not really designed for the rigors of thermal cycling. This is because the cured products were brittle and rigid. Today, the formulation can be engineered to be tougher, whether by tweaking the formula to create flexible domains within the matrix, or by adding a toughening agent to one of the epoxy components.

According to Venkat Nandivada, manager of technical support at Master Bond, an ingredient like urethane could be added to an epoxy base to create a hybrid epoxy which would exhibit good thermal cycling resistance and toughness.

Applying Epoxy in Production

Consider how the epoxy process will fit into production. Is the epoxy a two-part system which must be mixed in batches? Does the epoxy need to be vacuum degassed to remove air bubbles? Is a heating step needed for curing?

(Image courtesy of Master Bond.)

(Image courtesy of Master Bond.)

For high-volume production, consider automated or semi-automated mixing and dispensing. A gun applicator is a simple example. A gun applicator with a double-barrelled cartridge mixes and dispenses the two parts in the correct ratio. For automated mixing and dispensing, Master Bond can provide the epoxy components in containers designed for meter mix equipment.

“In a smaller production application, or if it’s for bonding, you can even go with a premixed and frozen solution where the material can be packaged in a syringe. It’s already measured, mixed, degassed, and ready for the customer to dispense the material. The only requirement for premixed and frozen syringes is that they would have to be stored at minus 40C,” said Nandivada. “For one-part epoxy systems, it’s easy. Most of the systems can be packaged in syringes, so it’s easier to deal with the one part in such cases.”

Proper Curing of Epoxy Resins

One of the challenges of engineering an epoxy formula is ensuring that it is fully cured after applying it on the components. One solution is to keep the epoxy resin and the curing agent separate until curing is desired, as with a two-part system. In a one-component epoxy, the curing agent and the resin are present within the same system.

Some epoxies require added heat to initiate curing. Curing temperatures range from room temperature to 180°F, and as high as 250°F.

To ensure that an epoxy cures properly and that the production process is properly equipped, you need to consult with the instructions for your product and the experts at Master Bond.

Surface Preparation

In sealing and adhesive bonding, epoxy forms a physical bond with the substrate. To facilitate this bond, it’s key to prepare a clean, dry substrate with appropriate surface roughness. According to Ramnath, surface finish of approximately 300 micro-inches RMS or higher is ideal for the best bond.

“For example, a simple 60 grit sandpaper could be used with a rotary tool to get the surface rough enough for a bonding application,” said Ramnath. “Where roughening is not possible, chemical etching or even plasma treatment could prepare the surface. There are various techniques or methods, but this is definitely a very important step.”

Besides finish, cleanliness is the second part of proper surface preparation.

“Cleanliness is critical. We would probably even put it above roughness, because any residue or any type of oil or grease present on the surface will prevent the epoxy from bonding or wetting the surfaces enough to get the level of adhesion that is needed. So, cleanliness of the surface is critical in these types of applications especially where structural strength is required,” added Nandivada.

To learn more about Master Bond’s epoxy products formulated for superior toughness, visit the Master Bond website.

Master Bond has sponsored this article.  All opinions are mine. –Isaac Maw