After more than 20 years of sharing his expertise and wisdom in the monthly Ask Dr. Dave column, Dave Dunn has retired as a columnist. Please join us in thanking him for his service and contributions to ASI and the industry as a whole.

Here we share some of the most popular Ask Dr. Dave columns over the years.

What is the difference between a hardener, a primer, an accelerator, and an activator?

This terminology can be confusing because the terms are often used interchangeably between technologies and differently by various manufacturers of the same technology. The term hardener is most often used in two-part epoxy adhesives; most manufacturers call the part that actually contains the epoxy molecule the “adhesive,” and the part that contains the curing agent the “hardener.” Some of these epoxy systems also contain additives in the hardener component to speed up the cure; these are usually called “accelerators” or “catalysts.”

In two-part reactive acrylic adhesives, the terms adhesive and initiator, or resin and catalyst, are often used interchangeably. The important thing to know is the correct mixing ratio of the two components. These days, it’s usually easier because the adhesive systems come in pre-measured syringe or cartridge containers. Some adhesive systems use so-called “primers” or “activators” that are applied to surfaces before applying the adhesive.

Sometimes the primer will serve to condition the surface and ensure maximum adhesion. A good example of this is the use of organosilane primers, which are used for ensuring good adhesion and bond durability when epoxies, acrylics, or RTV silicones are bonded to glass surfaces.

In other cases, the primer or activator actually is used to speed up the curing of the adhesive. For example, special surface primers have been designed for surface-initiated adhesives (such as cyanoacrylates) to increase the speed of cure when atmospheric humidity is very low or on so-called “inactive” surfaces or low-energy plastic surfaces.

What is the difference between a polyurethane adhesive and a polyurea adhesive?

Polyurethanes and polyureas both cure to systems that can vary from rigid to very flexible solids in their final properties. The two are quite similar, but with some obvious differences. While polyurethanes have been used for many years as adhesives and sealants, polyureas are relatively new to the industry.

From a chemical standpoint, a polyurethane is made from the reaction of an isocyanate with a polyol, whereas a polyurea is formed from an isocyanate reacting with a multifunctional amine. It is also possible to make so-called “hybrid” systems, in which the isocyanate is reacted with a mixture of hydroxyl and amino groups.

The most important difference is that the polyurea reaction is much faster than the polyurethane one, and the systems can gel within a few seconds after mixing. Polyureas have been used very successfully in the coatings industry, where the two components are mixed using plural spray equipment; polyurea adhesives, however, are relatively new.

One issue has been that the adhesives gel so quickly that the liquid does not have time to spread and wet the bonding surface. In addition, heat-sensitive substrates can be damaged by the strong exotherm generated by the fast curing. However, slowing down the curing is possible and has led to successful applications (e.g., high-speed wood bonding). In addition, both polyurea adhesives and sealants are available commercially. Polyurea adhesives also lead to new bonding possibilities where they can be used like a spot weld, and there is a technique possible where parts are preassembled and the adhesives is injected into the bond line through preformed grooves. 

What is the difference between thermoplastic and thermoset adhesives?

Adhesives can be classified in several ways, including their material origin (e.g., natural or synthetic) and their type of cure (e.g., physical curing such as drying or chemical curing). Terms such as thermoset, thermoplastic, structural, or non-structural are also used in the industry.

The term thermoset historically meant adhesives that cured or “set” on heating. The oldest type of this adhesive are the so-called PF, UF, and MF resins (phenol formaldehyde, urea formaldehyde, and melamine formaldehyde, respectively), which are commonly used to make plywood and are cured using heat and pressure.

However, many thermoset adhesives cure at room temperature (e.g., two-part epoxies, moisture-curing polyurethanes, anaerobics, and reactive acrylics). What thermosetting really means today is that the polymer chains are chemically crosslinked and do not soften on heating after curing. This makes them very suitable for structural applications where they have to support loads and be resistant to both heat and fluids.

In contrast, thermoplastic adhesives like polyvinyl acetate (white glue), cyanoacrylate, or hot melts soften on heating. Their assembled bonds tend to “creep” over time when loaded. In general, thermoset adhesives have better thermal, fluid, and environmental resistance than thermoplastics. There may be some concerns in the future that thermoset adhesives are not easily removable or recyclable, and it might be desirable to make higher performance thermoplastic systems.

How can I tell if my adhesive has fully cured?

This is a good question, as many users assume that mixing a 2-part epoxy and curing at room temperature will eventually completely cure the resin. However, this is not the case; epoxies are quite viscous to start with and, as curing proceeds, the viscosity increases dramatically to a gelled state and then to a solid. As this reaction continues, the molecular mobility decreases and reactive groups on the resin and the hardener are unable to react in order to increase the molecular weight and crosslinking. Maximum properties for epoxies are only achieved by curing at high temperatures for long periods of time to complete the reactions.

A useful way to look at a cured epoxy is to carry out differential scanning calorimetry (DSC). DSC measures the energy input or output of the solid resin as it is scanned from low to high temperatures. A typical curve for a 2-part epoxy will show two transitions: first, an endothermic (energy being absorbed) change in slope. This is the glass-transition temperature (T_g), which is important because this is the temperature at which the material changes from a rigid solid to a softer, more rubbery state, with a corresponding decrease in properties such as tensile strength and heat resistance.

As you continue to scan to higher temperatures, you will see an exothermic (energy being emitted) peak, which is the remaining epoxy being cured. If you then scan the sample a second time, you will find that the T_g is at a higher temperature (more resin has cured and crosslinked) and the exothermic peak has disappeared; that is, all the resin has cured. If you don’t have access to a DSC, university labs can often run samples for you.

That is not to say that resins cured at room temperature are not useful. They are widely used and offer tremendous convenience over heat-cured systems—as long as the properties desired are not too demanding. 

Are there adhesives and sealants that expand on curing to fill gaps?

This is a good question for two reasons: first, adhesives and sealants often have to fill large irregular gaps, and expansion on curing enables them to achieve this; and second, the products usually shrink on curing. Solvent-based or latex products shrink because of the loss of the carrier liquid on drying. Reactive adhesives that cure from a liquid to a solid can also shrink quite substantially.

This shrinkage can be mitigated in highly filled or plasticized systems, but it can become a problem where unfilled systems such as ultraviolet (UV) adhesives or low-viscosity acrylic sealants are used. The shrinkage can cause very large stresses at the bond line, which can, in extreme cases, lead to bond failure or create leak paths in sealants.

Many foamable sealants, normally polyurethanes, are used in the building or DIY industry. Well-known one-component polyurethane consumer adhesives react with moisture and generate carbon dioxide, which causes them to expand on curing up to three or four times in volume. Be careful in using them to not over apply them, however, or they will often expand outside the bond line, which can necessitate removing the excess for cosmetic reasons.

Other, possibly less well-known systems are pre-applied threadlockers and sealants that use a chemical reaction to cause expansion, as well as adhesives and sealants that contain expandable microspheres. These microspheres contain a volatile liquid or a gas, which expands them when heated. Applications involve specialty adhesives or sealants and mastics for auto body sealing.