Two Component Epoxy Adhesive Mix Ratio

What is a mix ratio?

The mix ratio of 2 part epoxy adhesive is used to determine the amount of resin and catalyst needed to achieve the listed properties of the epoxy.

2 part epoxy adhesives are often supplied in a dual cartridge with the appropriate mix nozzle to ensure the correct mix ratio and that sufficient mixing occurs simply by dispensing through the nozzle.  For many, this method of dispensing is ideal as it eliminates concerns related to the correct ratio and to adequate mixing.

In some situations, very small amounts of epoxy are required, and the nozzles are not cost-effective.  In these cases, you may prefer measuring and mixing by hand.

Mixing 2 part epoxy by hand?

1. First, determine how much total epoxy can be used within the pot life of the product. The pot life is the amount of time you have after mixing to use the epoxy before it has doubled in viscosity. Or simply how long you can leave it in the pot before use.
2. Determine the mix ratio. If the mix ratio is 10:1 that is 10 parts resin (A) to one part catalyst which is often called the hardener (B).  Sounds simple enough – but is that ratio by weight or volume?  The resin and hardener may have different densities, so the mix ratio by weight could be very different than the mix ratio by volume.  At a 10:1 ratio any error in the amounts could result in considerable difference.  For best results when mixing and measuring by hand, choose a 1:1 mix ratio epoxy.
3. Weigh or measure as needed.
4. Pick your mixing pot. Note – Large masses of epoxy will cure faster.  Curing epoxy generates heat and heat accelerates the cure – which just makes it all go faster and hotter.   Avoid mixing large quantities and steer clear of choosing foam or heat sensitive containers to mix in.
5. Mix thoroughly. Often two component epoxy adhesives have one color for the resin and another for the hardener. In these cases, it is fairly easy to visualize when you’ve mixed the adhesive thoroughly.  When both the resin and hardener are the same color, it may be more difficult.  Mix slowly in a level circular motion to avoid incorporating air into the adhesive. Scrape the sides of the container to ensure you are blending all of the material.  Mix for a full minute to be sure you’ve done a thorough job – especially if it is cold.

Choosing Meter Mix Dispense Equipment

In other applications, the dual cartridges are impractical due to large quantities used.  Dispensing equipment can be used to dispense bulk via a static mix tip.

1. Piston Pump metering and mixing dispensing equipment is ideal for higher viscosity epoxy and medium production output.
2. Gear Pump metering and mixing dispensing equipment is preferred for lower viscosity adhesives and are used in fully automated high production environments.

NOTE:  There are many different varieties of static mix nozzles.  It is important to use the one specified by the manufacturer to ensure proper mixing occurs.  The number of elements in the nozzle dictates the number of stirs the adhesive gets while passing.  Nozzles can range from, say 24 to 56 elements, so the nozzle with 24 elements only stirs the adhesive 24 times. The one with 56 has more than double the number of stirs.

How exact does this ratio need to be?

Regardless of how you get there, weighing or measuring, aim for as close to 100% accuracy as possible.  To achieve the results stated on the data sheet, you want the mix ratio to be spot on and ensure complete mixing.  Although formulations and sensitivities to specific epoxy features may vary, it’s best to control the mix ratio within no more than +/- 5%.  If you don’t maintain an accurate mix ratio and thorough mixing, bond strength, chemical resistance, and Tg can decrease whilst outgassing will increase due to unreacted epoxy ingredients.

1:1 mix ratio 2 part epoxy adhesives are less sensitive to exact ratio than 10:1 mix ratio epoxies.

When is it Ok to disregard the mix ratio?

Unless your mother insists or you are under the advice of the epoxy manufacturer, stick to the stated ratio on the technical data sheet.  (If your mother insists, I’d still ask a chemist at the manufacturer.)  Altering mix ratio will affect curing. You may have a soft cure or no cure or slower cure or a very fast cure that could create heat (exotherm), and it will affect the final properties.

The epoxy manufacturer may suggest altering the stated mix ratio for your application to achieve different results.  Your results won’t match those listed on the technical datasheet but they may be better for your application.  Manufacturers know how to dial in these ratios to affect pot life, cure speed, hardness, and adhesion.

For more tips on implementing a 2 part epoxy process, contact Permabond.