Importance of Mixing Ratio & Pot Life in Epoxy Adhesives

Adhesives have become an integral part of daily life. We use paper gum for envelopes, hot melt adhesives for book binding, and instant adhesives for quick repairs. With growing industrial and domestic needs, adhesive technology has evolved significantly to meet specialized applications such as modular furniture, footwear, handicrafts, upholstery, and more.

This article focuses on the basics of adhesives and then dives into epoxy adhesives in particular, highlighting the importance of mixing ratio and pot life for reliable performance in the field.

What Is an Adhesive?

An adhesive is any substance or chemical that bonds two substrates effectively based on the application. Adhesives can be single-component or two-component, solventless or solvent-based, and available in solid or liquid forms.

What Is an Epoxy Adhesive?

An epoxy adhesive contains an epoxy group. In its basic form, epoxy is a triangular chemical group where an oxygen atom is bonded with two carbon atoms in the resin. It reacts with the amine/amidoamine group in the hardener to form a thermosetting solid cured polymer. The reaction is permanent, so once cured, it cannot be reversed back to the original resin and hardener.

Epoxy adhesives are versatile and can be formulated as putty, viscous, or low-viscosity systems with pot life ranging from as low as 5 minutes to multiple hours. General-purpose epoxy adhesives are typically two-component, room-temperature curing systems (resin + hardener). For specialized industrial and automotive uses, single-component, heat-curing systems are also available.

Key Features of Epoxy Adhesives

Solventless high-strength adhesives
Room temperature curing
Two-component system (resin & hardener)
Negligible shrinkage
Excellent adhesion to many substrates (except some plastics)
Good water, heat, and chemical resistance
Excellent dimensional stability
Excellent insulation resistance
User-friendly mixing ratios
Available in paste, putty, and liquid forms
Long shelf life

Significance of Mixing Ratio

Since most DIY and industrial epoxy adhesives are two-component systems, the mixing ratio plays a critical role in performance. The manufacturer specifies the ratio in parts by weight and parts by volume, based on the epoxy equivalent of the resin and the amine value of the hardener. Users must follow the specified ratio.

Example for a general-purpose epoxy adhesive: Resin:Hardener = 100:80 by weight or 100:100 by volume (1:1 by volume). Volume-based mixing is common on-site where a balance is not always practical.

What happens if the mixing ratio is changed?

A common misconception is that adding extra hardener will speed up curing. In reality, excess hardener remains unreacted, resulting in a softer, sticky mass and poor bonding. Just like adding too much salt or spice can ruin a dish, or doubling a prescribed medicine can cause side effects, altering the ratio damages performance. Always follow the manufacturer’s instructions.

Understanding Pot Life

Pot life is the usable time window after mixing resin and hardener in which the adhesive can be applied effectively. Many application problems and complaints arise from misunderstanding this parameter.

After thorough mixing, the color should become uniform, indicating proper mixing and the start of pot life. In viscous systems, air bubbles may be trapped.
The reaction generates heat (exotherm). Initially, viscosity may reduce slightly, but then it steadily increases.
Pot life depends on the chemistry of the resin and hardener; some systems provide only minutes, while others last hours.

Main Factors Affecting Pot Life

Higher quantity and higher temperature reduce pot life because greater heat is generated.
In summer, room temperature is higher, leading to faster reactions.
A larger surface area (thin layer) increases pot life as heat dissipates into the air.

What Happens When Pot Life Is Over?

The mixed adhesive gradually becomes more viscous and eventually gels. Once gelled, it cannot be applied effectively for the intended bonding or coating application.

Practical Tips to Manage Pot Life

Mix only the required quantity to avoid wastage.
Use a plastic bowl or small plastic bucket for mixing.
After mixing, immediately spread in a thin layer to extend pot life.
For waterproofing, work early in the morning when ambient temperature is lower.
If a large quantity is needed, divide into smaller pots after mixing.
When manpower is limited, mix a larger quantity and spread quickly over a larger surface area.

Some Applications of Epoxy Adhesives

Paint brushes, glass bangles, imitation jewelry, kitchen platforms, hockey sticks, metallic pens, and potting applications are just a few of the practical uses of epoxy adhesives.

This article is intended to help workers, factory owners, and civil contractors avoid common mistakes and improve the reliability of epoxy applications in the field.

About the Author

Mr. Sanjay Barve

Mr. Sanjay Barve is a technocrat with more than 40 years of experience in the field of adhesives. He has worked with and been associated with Elantas Beck & Co, Pidilite Industries, Atul Industries, Nerofix (a subsidiary of Kansai Nerolac), and H.P. Adhesives. During his career, he has handled various industrial and domestic applications, with key contributions in application support, training for marketing teams, and customer complaint handling.