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New Toughening Agents (Micro & Nano) for Structural Adhesives

Edward M. Petrie – Mar 31, 2017

New Toughening Agents for Structural Adhesives Structural adhesives are designed for high strength, creep resistance, and resistance to harsh service environments such as high temperatures and moisture. As a result, the base resins used in structural adhesive formulations tend to be highly crosslinked and relatively brittle materials. They have high modulus and poor resistance to crack initiation and growth. Especially at high strain rates, they are characterized by:

 − Low impact
 − Peel, and
 − Fatigue strength

Thus, two qualities are often required for structural adhesives that unmodified resins lack – flexibility and toughness. Formulators have overcome these problems either by flexibilizing the molecular structure or incorporating toughening modifiersinto the adhesive.

However, it must be recognized that flexibility and toughness have different meanings in adhesive technology. Also, they operate by very different mechanisms.

So, let's understand how to flexibilize and toughen structural adhesives for improved performance.

Improving Performance of Structural Adhesives

The ability of an adhesive to absorb energy without catastrophic failure can be increased through the processes of flexibilizing or toughening. This can be done by incorporating additives into the adhesive formulation. However, flexibilizers and tougheners operate by different mechanisms and provide an entirely different set of properties as shown in the figure below.

Tougheners and flexibilizers in epoxy resin

Properties of tougheners and flexibilizers in epoxy resin systems


Flexibilizers are additives that generally result in reduced modulus and higher elongation. A common method of flexibilizing an adhesive is by blending the base resin with other, more elastic polymers to create a hybrid adhesive. However, this technique usually combines the good and bad characteristics of each resin system. It was often used with early epoxy structural adhesives (e.g., epoxy-nylon, epoxy-polysulfide, and epoxy-urethane).

Other flexibilizing techniques are:

 −  Adding flexible chemical groups to the base polymer molecule - either via the base resin or the curing agent, and
 − Adding flexible diluents (e.g., non-reactive plasticizers or reactive long chain flexible molecules similar to the base resin) to the formulation

Table below summarizes the trade-offs that the formulator must make when flexibilizing a structural adhesive formulation using the methods described above.

The addition of flexibilizing additives will generally cause:
Increase in:
 − Peel strength
 − Impact strength
 − Fatigue properties
 − Thermal shock resistance
 − Elongation
 − Coefficient of thermal expansion

Decrease in:
 − Internal stresses
 − Glass transition temperature
 − Heat resistance
 − Chemical resistance
 − Hardness
 − Modulus
 − Tensile strength
Trade-Offs in Properties that Occur Due to Flexibilizing the Adhesive

In general, flexibilizers work by reducing the crosslink density of the adhesive.

 − Flexibilizers improve peel and impact strength by allowing the adhesive to deform under the application of stress.
 − They reduce mechanical damage by lowering modulus or plasticization, and this allows distortion of the adhesive and distribution of stress.1,2

However, the added flexibility reduces the overall glass transition temperature of the system. Thus, elevated temperature performance and environmental resistance of the higher strength constituent are also reduced.


Modern structural adhesive formulations benefit from the use of tougheners. These additives operate on a completely different mechanism than flexibilizers. Where, flexibilized adhesives are molecular blends of polymers in a single-phase system, toughened structural adhesives have discrete particles embedded in the resin matrix of the adhesive.

Thus, there are two distinct phases:

 −  The larger phase constitutes the base resin, and
 − The other phase consists of small (on the order of 10-6 to 10-9 meters) distributed particles

Achieving Toughening NetworkThis toughening network can be accomplished in several ways:

 − One approach is by simply incorporating small, vulcanized rubber particles into the adhesive formulation. Surface activated rubber particles and tough thermoplastic particles have been used in this way to improve structural adhesives.

 − Another approach is to incorporate micro-voids in the cured adhesive by a foaming reaction. This can be done by dissolving CO2 or N2 in the liquid resin or by chemical blowing agents.

 − A more productive approach is to add micro- or nano-sized elastomeric or viscoelastic particles into the adhesive formulation.

These techniques have been developed over the years and will be studied in detail in this article.

Differences between Flexibilizers and Tougheners

 − Flexibilizers work by deforming whereas; tougheners improve properties because the viscoelastic inclusions absorb energy and stop a crack from propagating throughout the bondline. Toughened adhesives tolerate damage by preventing crack growth and, thus, limiting the damage area. This significantly improves the following with minimal change in the gross properties of the base resin:

 − Fracture toughness
 − Impact strength by providing crack pinning
 − Stress distribution mechanisms within the adhesive
 − Tensile-shear and peel strength

This minimal change is an important difference between flexibilizers and tougheners and the most distinct value proposition attributed to the use of toughening agents.

 − Cured, toughened structural adhesives generally exhibit two glass transition temperatures. One for the cured resin material and another for the phase separated elastomer. In comparison to flexibilized adhesive, toughened adhesives exhibit only a moderate increase in elongation. Also, there is no or very little decrease in glass transition temperature.

The properties of flexibilized, toughened, and unmodified epoxy adhesives are compared in table below:

Adhesive Type Modification Tensile-Shear Strength, psi
T-Peel Strength, pli
Glass Transition Temperature, °C
Epoxy Unmodified 2000-3500 2-8 126
Epoxy-nylon Single phase 5500-7200
8-130 80
Toughened epoxy Two phase 3700-6000 22-90 126

Properties of Flexibilized, Toughened, and Unmodified Epoxy Structural Adhesives

Until the development of toughened epoxy formulations, it was very difficult to provide an epoxy adhesive having both high impact or peel strength and high heat resistance. A variety of toughening agents have been used to modify structural adhesives without significantly affecting other properties of the base resin as flexibilizers do. So, let's understand the techniques to toughen the adhesives.

 » Continue reading to explore some of the best tougheners and achieve optimal combination of properties in your structural adhesive! 

2 Comments on "New Toughening Agents (Micro & Nano) for Structural Adhesives"
Sreeparna D Oct 26, 2017
Thanks Hartwig for sharing your comments. We've made the updates.
Hartwig L Oct 13, 2017
I recommend replacing the first picture showing the bonding of an automotive windshield. Due to the mechanical requirements epoxy adhesives are not used and are also not suitable for this application. Windshiled bonding is typically done by moisture curing polyurethane adhesives. Besodes this the article provides a good overview about the different toughening technologies for epoxy adhesives

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