Adhesive not sticking? Potting compound not filling? You might be missing this key ingredient.

 

Epoxy diluents are often seen as minor players in a formulation, but in reality, they have a decisive impact on overall flowability, processability, and crosslink structure stability. Given that diluents of varying functionalities offer different properties, selecting a suitable diluent is essential to ensure both ease of application and optimal performance in adhesive formulation design.

 

 

In adhesive or potting compound formulation design, we often find ourselves scratching our heads over problems like "too sticky to handle" or struggling with the dilemma of "wanting stability without making it too rigid." At moments like these, a key yet often underestimated player quietly steps onto the stage

 

—the epoxy diluent.

 

Epoxy diluents may not be the primary material, but they have a profound impact on flowability, crosslink structure, glass transition temperature (Tg), and even overall reliability. They can make an adhesive easier to apply, yet they can also be the very factor that causes brittleness or insufficient strength. The way they are selected and blended determines whether the result is a "just right" formulation or a disaster that cures too quickly or becomes excessively rigid.

 

 

If developing an epoxy resin formulation were like managing a construction project, then mono-functional and multi-functional diluents would actually play very different roles.

 

 

▲ Mono-functional diluent (left) and multi-functional diluent (right)

 

 

A mono-functional diluent is like a temporary worker on the construction site.

 

 

They work fast and adapt well, quickly moving materials on-site, helping with grouting, or handling immediate small issues. For the construction crew, these temporary workers are convenient and highly flexible, effectively improving the overall workflow's fluidity. But here's the catch: they usually don't take part in building the main structure. Relying on them too much might make the whole building go up quickly on the outside, but leave the internal structure somewhat “hollow.”

 

 

On the other hand, multi-functional diluents are like the rebar crew.

 

 

They may not move as lightly, and their materials cost more and are heavier to handle, but they do solid work—laying the foundation, tying rebar, and setting up supports. These steps determine whether a building can withstand wind and rain, endure the test of time, and cope with temperature changes. In the world of adhesives, multi-functional diluents play a similar role: they take part in forming the overall crosslinked structure, enhancing strength, Tg, and dimensional stability.

 

 

Shifting from roles back to molecules: Let's discuss how do they work

 

 

Earlier, we used a construction site analogy to describe the personalities and roles of mono-functional and multi-functional diluents. One is agile and quick, coming and going like a temporary worker. The other is stable and reliable, contributing to the structure like a rebar crew. This comparison can give you an intuitive sense of their "presence" within a formulation.

 

 

But back on the formulation "construction site," we still have to deal with the realities of molecular-level reactions. Which molecules actually participate? To what extent? What gets left behind, and what changes do they bring about?

 

Next, let's bring our two protagonists back into the reaction vessel and take a closer look at how the mono-functional diluent contributes—and where caution is needed when using it.

 

 

Mono-Functional Diluent: Easy to Thin, But Handle with Care

 

 

 

The advantages of mono-functional diluents are clear. They are small molecules with low viscosity and reasonable cost, providing a soon and effective thinning effect. For formulation engineers, when a resin is too sticky or difficult to handle, adding a bit of a mono-functional diluent can immediately make the system "flow smoothly." Whether it's brushing, potting, or mixing, both the feel and the overall fluidity are noticeably improved.

 

But its "mono-functionality" is a double-edged sword.

 

It engages in just one reaction during curing, after which its role concludes, leaving it as a chain terminator in the final crosslinked structure. This means it cannot contribute to reinforcing the crosslinked structure; in fact, it can lower the overall crosslink density. From a polymer perspective, these chain ends act like weak links in the structure, potentially reducing Tg, strength, dimensional stability, and solvent resistance.

 

 

This doesn't mean mono-functional diluents shouldn't be used; rather, you need to be clear about your goals and the associated risks:

 

• Are you willing to compromise a bit of structural integrity for better processability? That's fine.
• Are you looking for an inexpensive, quick way to produce moderate-performance adhesives? It can handle that.
• But if you're aiming for high structural strength, high-temperature resistance, electronic encapsulation, or optical applications, it may fall short.

 

In short, it's a useful tool, but not meant for building the main structure. When stability and high performance are required, di-functional or multi-functional diluents are needed to reinforce the structural network.

 

 

Multi-Functional Diluents: Not for "Thinning," but for "Reinforcing"

 

 

 

Unlike mono-functional diluent, multi-functional diluent does more than reducing the viscosity. During curing, it continues to participate in crosslinking, further extending and reinforcing the network structure. In other words, it doesn’t just show up for a single reaction and leave—it becomes an integral part of the molecular framework, like the rebar in your structural blueprint.

 

 

The characteristics of these diluents make them well-suited for applications such as:

• High-strength structural adhesives (e.g., metal-to-metal bonding)
• Encapsulation materials requiring dimensional stability (e.g., electronic potting)
• Composite resin systems that demand high crosslink density and heat resistance

 

 

Unlike mono-functional diluents, they don’t form chain ends; instead, they increase crosslinking points, raising overall Tg, modulus, and impact resistance. While these molecules typically have slightly higher viscosity and cost, in demanding formulations they are practically a guarantee of structural stability.

 

 

Chart 1: Comparison of Mono-Functional vs. Multi-Functional Diluents

Category	Characteristics	Applicable Areas	Key Technical Features
Mono-Functional Epoxy Diluents	Reduces viscosity, improves flexibility, suitable for adhesives and coatings	Low-VOC coatings, construction adhesives, flexible sealants	Low viscosity, participates in curing, enhances flexibility
Di-Functional Epoxy Diluents	Increases structural strength, participates in crosslinking	Structural adhesives, epoxy repair adhesives	Di-functional structure, improves mechanical properties, enhances processability
Multi-Functional Epoxy Diluents (Tri-, Tetra-)	Multi-functional design, increases heat resistance and modulus	Electronic encapsulation, heat-resistant composites, potting compounds	High crosslink density, good dimensional stability, suitable for thermal conductive adhesives
Weather-Resistant Mono-Functional Epoxy Diluents	Ester-based, used to adjust corrosion and weather resistance	Anti-corrosion coatings, sealants	Compact molecular structure, good chemical resistance
Low-Chlorine Epoxy Diluents	Low-chlorine design, reduces risk of electronic corrosion	PCB encapsulation, sensor potting, thin-film coatings	Total Cl < 0.3%, high insulation, prevents electrochemical corrosion
Bio-Based Epoxy Diluents	Derived from renewable sources, compliant with environmental regulations	Eco-friendly potting compounds, green building adhesives	BMT ≥ 80%, low toxicity, low VOC

 

 

When Should Mono-Functional and Multi-Functional Diluents Be Used Together?

 

 

In practice, formulation design is rarely an either-or decision; it often requires finding a balance between performance, processability, and cost. In such cases, combining mono-functional and multi-functional diluents is a highly practical strategy.

 

 

Scenario 1: Poor Processability Without Sacrificing Strength

 

Problem: Using highly crosslinked multi-functional diluents provides excellent performance, but the viscosity remains too high, making handling difficult.

Solution: Adding a small amount of mono-functional diluent, typically 5 to 10 phr of the total formulation, can improve flowability without significantly compromising performance.

 

 

Scenario 2: Balancing Flexibility and Structural Integrity

 

Problem: The final product is too hard or brittle, making it prone to cracking or delamination.

Solution: Use mono-functional diluents to introduce flexible chain segments while multi-functional diluents maintain the main structural strength. Combining both allows you to create an adhesive that offers both toughness and structural integrity.

 

 

Scenario 3: Considering Cost and Raw Material Availability

 

Problem: Multi-functional diluents can be expensive, have unstable supply, or be too viscous.

Recommendation: Mono-functional diluents can effectively reduce viscosity and lower the unit cost, provided that the usage is carefully controlled.

 

What other types of diluents are there?

 

Beyond epoxy diluents, many other types are commonly used in different resin systems, each with its own structure and applications.

 

For example:

 

Acrylic diluents are often used in UV-curable inks and coatings. They react quickly and can adjust flexibility.

 

Polyurethane-based diluents are widely applied in elastomeric materials and help improve impact resistance and softness.

 

Phenolic diluents are suitable for systems that require high Tg and heat resistance.

 

Traditional unsaturated polyester diluents such as styrene, or non-reactive diluents like alcohol, ester, or ketone types, do not participate in crosslinking but can still improve processing in some systems.

 

Each type of diluent has its own selection logic, based on factors such as flowability, reactivity, cost, regulatory compliance, and performance. Every choice can significantly influence the final behavior of the formulation.

 

 

Chart 2: Comparison of Diluents

 

Type	Application Areas	Participates in Reaction	Main Examples	Key Features
Non-reactive, viscosity-reducing, volatile	Alcohols, ketones, esters, ethers	No	Solvent-based coatings, cleaning agents, processing aids	Non-reactive diluents
Unsaturated polyester diluents	Composites, SMC/BMC, construction reinforcement	Yes	Styrene, MMA (volatile)	Highly reactive but with VOC concerns
Phenolic diluents	Electronic encapsulation, heat-resistant adhesives, chemical-resistant adhesives	Yes	Resorcinol diglycidyl ether, phenol epoxy ether	Good heat resistance, multi-functional, strengthens structure
Polyurethane diluents	Elastomeric coatings, wear-resistant materials, flexible adhesives	Yes	Urethane acrylate oligomer, polyether diol	Imparts toughness and elasticity, suitable for flexible applications
Acrylic diluents	UV-curable coatings, inks, 3D printing materials	Yes	IBOA, TPGDA, TMPTA (mono-/di-/tri-functional)	Fast curing, adjustable viscosity and flexibility, highly reactive

 

 

The problem isn't a fundamental mistake, but rather a missing detail

 

Sometimes a formulation doesn't run smoothly; it's not a mistake, just a small, helpful detail that's missing.

 

Diluents may seem inconspicuous, but they can make a big difference in viscosity, flow, curing behavior, and even the long-term reliability of the final product. They don't steal the spotlight like the main resin, yet they are often the key to making the system easier to process, apply, and maintain over time.

 

If you're dealing with common issues like inconsistent adhesion, difficulty filling molds, or brittle cured material, it may be worth looking at the diluents. You don't need to overhaul the main structure

 

—just a slight, well-timed adjustment can be exactly what's needed.