2-Part Adhesive Chemistries
2-part structural adhesives all rely on a chemical reaction to occur between a resin and a hardener, but the type of resin used in the formulation is what classifies an adhesive as either an MMA (methyl-methacrylate), an epoxy, or a urethane. 2-part adhesives are commonly referred to uniformly as epoxies, but that is not accurate. Epoxies are the most well-known chemistry, but MMA adhesives have begun to gain significant traction in the market, and urethanes will always have their place due to their unique characteristics.
Which 2-Part Adhesive Chemistry is Right For You?
Adhesives
MMA adhesives are gaining popularity because they are very versatile in terms of the ability to bond oily metals and low-surface-energy substrates. One of the main features of MMA's is that they save time. They do not require any surface preparation even if the substrate is contaminated, which saves a step in production. They also offer very fast cure times, often achieving handling strength in as little as a few minutes.
There are drawbacks to MMA's, the most notable being that they are very toxic and produce an odor that, if exposed to for long periods, can be detrimental to the user's health. As MMA technology evolves, manufacturers like 3M have begun to produce low-odor versions to increase the safety profile of their MMA's. Another drawback is that MMA's do not resist high heat and chemicals as well as epoxies, though their resistance to such elements is not poor.
Benefits
Very fast cure times.
No surface prep needed.
Drawbacks
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Toxicity is a concern.
Lower resistance to heat & chemicals than epoxies.
Adhesives
Epoxies are the kings of 2-part adhesives because they offer the highest bond strengths of all the chemistries and are extremely durable against heat and chemicals. They are versatile, and can be formulated to achieve excellent performance in just about any variable possible - for example, some formulations are made to be more flexible, or cure faster. However, when these variables are manipulated, it often comes at the expense of another variable. Therefore, an epoxy formulated to cure quickly may not have the same strength as a slower cure epoxy.
The main downside to epoxies is that they do require a clean, and sometimes even an abraded surface to perform optimally. This means steps must be taken to thoroughly process the substrates before the adhesive is applied. Another downside is their cure speed, which is generally slower than other chemistries. A slower cure may not be a disadvantage depending on your production process, but for applications where a fast time to handing strength is required, epoxies may not be the best choice.
Benefits
Extremely high bond strengths.
Very durable against heat & chemicals.
Drawbacks
Surface preparation is required.
Slower cure times than other chemistries.
Adhesives
Urethanes are often the forgotten chemistry because they don't provide as much versatility as others. But urethanes will always have their place because of their excellent flexibility. They are capable of withstanding high levels of vibration and impact that epoxies and MMA's cannot. So, if your application will be subjected to situations where a flexible glue is needed, urethanes should be your go-to product.
The downsides to urethanes is that they are not well-suited for high temperature environments, and they do not cure particularly fast. They also do not offer the same bond strength as epoxies, but are by no means a weak bonding glue. Finally, urethanes are moisture-sensitive prior to curing, so high levels of humidity can affect the cure speed or the shelf life if not stored properly.
Benefits
High impact & vibration resistance.
Drawbacks
Not suited for high temp conditions.
Moisture-sensitive prior to cure.
