Epoxy and cyanoacrylate glue, more commonly known as super glue, instant glue or CA glue, are two of the strongest glues available to consumers – and even for professionals for that matter. Both glues have wide applicability in the workshop and around the house, and are popular choices from small repairs and assemblies.
But you may have wondered how do epoxy and super glue actually compare? When you should prefer one over the other?
In this article, I am going to compare epoxy and super glue in common household and workshop use, and answer these questions. Put very short, the key message is that epoxy is a two-component glue and more time-consuming to use, but is usually stronger and more versatile as it can fill gaps, has wider material compatibility, does not require compression, and is suited to both large and small gluing tasks. Super glue, on the other hand, is a one-component glue and easy and fast use, but cannot fill gaps, requires a good mate and compression in the glue joint, and is not suited to large glue-ups.
If you are interested in learning more, below we will go through how epoxy and super glue compare in the most important glue properties: strength, gap filling, material compatibility, work life and cure time, shelf life ease of use and safety.
|material compatibility||very good||good|
|work life||5min…12h||10…30 s|
|time to handling strength||15min…24h||< 60 s|
|time to full cure (typ.)||24 h||24 h|
|shelf life||2…3 y||1 y|
|ease of use||laborious||easy|
|safety||more toxic||less toxic|
Under optimal conditions, both epoxy and super glue can produce a very strong joint, but in practice epoxy will usually be stronger.
Epoxy is known as a very strong glue, and for a good reason: the strength of an epoxy glue joint can be up to 6000 psi (40 MPa) both in tension (pulling joint apart) and in shear (sliding the joint). To put this number into perspective, this means that, under optimal conditions, an epoxy joint is stronger than most plastics, and stronger than any wood across the grain!
However, the gluing conditions are rarely optimal in practice, and the real strength of an epoxy joint depends on the materials to be joined, the surface condition and the type of epoxy used. Because of this, the actual strength of the joint is usually only a fraction of the maximum values quoted in glue datasheets.
Compared to glues in general, epoxy is still relatively tolerant of varying gluing conditions, and will usually achieve a strength sufficient for most applications. An epoxy joint can usually handle well many different types of loading, such as tension, shear, compression and peeling of the joint and both static and dynamic or impact-like loads.
As a material, the cyanoacrylate of super glue is also quite strong. Unlike with epoxy, however, the achievable joint strengths for super glue are rarely specified by manufacturers. Since some cyanoacrylates are listed as “structural glues” by some manufacturers, we can assume that they exceed a minimum joint strength of 1,000 psi (6.9 MPa) under optimal conditions.
However, super glue is more sensitive than epoxy to gluing conditions, materials and the types of loading on the joint. Most notably, it requires a thin joint, cannot fill gaps, and is very brittle and cannot resist uneven and impact-type loading very well. Because of these factors, the useful strength of a super glue joint will in most cases be less than that of epoxy.
2 Gap filling
Gap filling means a glue’s capability to bridge wide open areas in the glue joint, and is particularly important when the surfaces to be glued do not fit well together.
Epoxies are excellent in filling gaps. With epoxy, a thick glue joint will be almost as strong as a thin joint, and will cure equally fast thanks to the chemical two-component reaction. The gap-filling capacity of epoxy is limited mainly by the tendency of the uncured liquid to run off the joint. Special thick, hihg-viscosity epoxies exist and are recommended for very wide gaps.
Super glues, by contrast, are usually poor gap fillers: they have low viscosity, i.e. they are very fluid, and are therefore prone to running off from anywhere where they are not supported by a nearby surface. Further, because of the air and surface contact nature of the hardening, super glues do not cure well in thick bonds. Although these behaviors can be improved by using high-viscosity super glues and additives, these are specialty products not often available to consumers.
3 Material compatibility
Both epoxy and super glue can be used to bond a wide range of different materials: wood, metals, plastics, leather, rubber, glass and ceramics.
With both glues, the achievable bond strength varies substantially with material and surface condition, though, so that the proper question is how well do the two glues bond different materials.
Sadly, little data is available on performance of consumer epoxies and super glues on different materials, and giving quantitative answers to this question is hard. Hard materials with roughened or porous surfaces will bond the best with both glues, and smooth non-porous and flexible materials the worst. For example, etched or sand-blasted aluminum bonds well, and low surface energy plastics such as polyethylene (PE) poorly.
Different materials require different glue properties, and it is here where epoxies may have a slight edge on super glues: epoxies are available in a wider variety of formulations than super glues, so that matching the glue version to the material at hand may be a bit easier.
Gluing flexible materials such as rubber or thin sheets subjected to peeling-type load calls for flexible or toughened glue formulations. Low surface energy plastics, on the other hand, usually require special additives to improve the surface adhesion. While both super glue and epoxy have special formulations or additives for such challenging gluing tasks, the special epoxies may be easier to find.
4 Work life
Work life or open time means the time a glue is still fluid after application and the joint can be worked on. A short working time is desirable for fast gluing of small joints, and a long working time for larger assemblies, where glue application and joint positioning takes a long time.
Epoxies are available with a wide range of work lifes from around 5 minutes to 12 hours. This means that you can usually find an epoxy glue with the right work life for each glue joint size. None of the common epoxy grades are very fast, however, so they are not well suited for hand-compressed instant joints. You should also note that a part of the work life is taken up already in mixing of the two components.
Super glues, sometimes called instant glues, have an exceptionally short work life measured in tens of seconds. This allows you to finish small gluing jobs quickly and efficiently, but it also means that you must be very fast in applying the glue, positioning the joint and clamping it together. While additives are available to modify the work life of super glues, the short work life restricts its utility in large assemblies.
5 Cure time
Cure time means the time it takes for the glue from application to harden. Since the curing or hardening of glues is often a gradual process, cure time may be reported as either time to handling strength or time to final strength, sometimes called “full cure”.
Epoxy glues take between 15 min and 24 hours to cure to handling strength, and usually 24 hours or more to reach their final strength. These times depend on the epoxy type and are correlated with the work life: epoxies with a short work life will reach handling strength sooner than those with a longer one. Although the process speed depends on the specific formulation, the time to handling strength is commonly 3–5 times the work life.
Super glues, by contrast,cure very fast, and will reach handling strength in less than 60 seconds in standard joints. Their curing continues after the initial set, though, and they will usually take up to 24 hours to reach their final strength, similar to epoxies.
6 Shelf life
Epoxy and super glue also have a different shelf life.
Most epoxies will keep for at least few years after manufacture. Opening the packaging and closing it again does shorten the life, but not dramatically. Further, unlike many other glues, epoxy does not usually block or clog the packaging caps or openings and make itself useless in this way. All of this is thanks to the two-component nature of the glue: its primary reaction is not with contact to air or moisture, but with the other component.
Super glues, even if unopened, have a shelf life of only a year or so. Moreover, as most hobbyists know, an opened bottle or tube of super glue will not stay in a usable form for all that long. As air moisture gets into contact with super glue, it will cure any glue left in the cap threads, the applicator nozzle or brush, and eventually in the package itself. Despite the manufacturer’s best efforts in package design, and yours in closing it tightly after use, in practice super glue is all too often a one-shot product.
7 Ease of use
Epoxy and super glue differ significantly in their application processes and ease of use.
Super glue is very quick and easy to use in undemanding applications. You open the bottle, spread the glue with the included nozzle or brush (see image for applicators), press and hold the surfaces together for some tens of seconds, and you are done. However, due to the short work life, poor gap fill and low toughness or the joint, gluing pieces with less than perfect fit as well as multi-piece or large assemblies may require considerable skill and dexterity.
Epoxy, on the other hand, takes some more work in application, but is less time-critical and forgiving for the user. The gluing process is longer than with super glue, since the two components of epoxy – resin and hardener – have to be accurately dosed and thoroughly mixed before application onto the workpieces. The dosing and mixing can be automated by using more advanced applicators such as dual syringes and static mixer nozzles (see image below), but only at an added cost.
However, thanks to the longer working life and good gap filling, applying epoxy requires less speed and skill, and will in most cases allow a stronger joint to be formed. This is particularly true of large and multi-piece assemblies, joints with poor fit, misalignment or inadequate compression during curing.
Both epoxies and super glues are toxic, but their risk profiles are somewhat different.
An exposure to epoxy glues may have a wide range of adverse health effects, including irritation, allergic reactions, asthma, hormonal disruption and even cancer. For some people, contact with epoxy may also cause sensitization, a chronic condition in which an allergic reaction is triggered by very mild exposure to epoxy. Epoxy is considerably more hazardous in liquid, uncured form and is relatively inert after curing, so that some epoxies are considered non-hazardous in solid form. In addition to health risks in people, epoxy is toxic to marine wildlife.
By comparison, most cyanoacrylates in super glues are only mildly toxic. Exposure to super glue may cause irritation and other allergic reactions and lead a sensitization similar to epoxy. Due to the rapid setting, super glue also carries a risk of skin and eye injuries.
To avoid the health hazards associated with both super glue and epoxy, it is important to avoid direct contact with uncured glue and ensure adequate ventilation of the working space. When working with these glues, you should wear appropriate personal protective equipment such as safety glasses and gloves.
To avoid the environmental hazards associated with these glues, you should dispose of unused glue and packaging following the instructions provided by the glue producer or the local authorities. As epoxy is most toxic in liquid form and in water, you should never pour any amount of either component into the drain.
As we have seen, epoxy and super glue, although both excellent glues, have quite different characteristics.
In a nutshell, epoxy is a high-performance all-around solution, while super glue is a quick and easy fix that can also perform quite well, given the right conditions.
So which one should you pick for a certain task? Let us sum up the comparison by answering when you should prefer epoxy and when super glue, as well as addressing some common material-specific questions.
When to use epoxy
Choose epoxy over super glue if:
- If you want the strongest possible joint and do not mind taking some time mixing the glue and waiting for it to cure
- Always if the parts do not fit together perfectly
- If the glue joints are large
- If the assembly has more than two parts
When to use super glue
Choose super glue over epoxy if:
- You have only little time to work on the glue-up
- You need to get the object into use right away
- You actually need a weak joint which you can tear open later
- If you are unsure whether you will be able to mix and handle epoxy safely
Epoxy or super glue for metal?
Both epoxy and super glue can be used for gluing metal parts together. Metals actually allow the strongest glue joints with both glues, provided the surfaces are clean and sufficiently rough. You can choose whether to use epoxy or super glue on the other properties of the glues, see above.
You should note that neither epoxy nor super glue will ever be even nearly as strong as the metal you are joining. This means that the glue joint will always remain a weak spot in the part.
Epoxy or super glue for plastic?
Both epoxy and super glue can be used for gluing most plastics. However, plastics are often trickier to glue effectively than metals, and you will have to decide which glue to use by the type of plastic you are dealing with.
Low surface energy (LSE) plastics, such as PTFE (Teflon), PP, PE, PS are often difficult to bond with standard epoxies and super glues. For these plastics, you should get special formulations of either glue that are specified for use with LSE plastics. Super glues may be somewhat easier to use here, since their bonding to LSE plastics can be improved with commonly available primers. Some super glues indended also for LSE plastics are illustrated in the image below.
However, neither epoxies nor super glues are optimal for LSE plastics. For best results, you should instead use 2-component acrylic adhesives particularly designed for bonding LSE plastics.
The glue bond strength on LSE plastics can also be improved by surface treatments using UV light, flame, solvents or acids. Nevertheless, the improvements are not very dramatic and the methods are not well suited for hobby use.
Medium and high surface energy (MSE and HSE) plastics, on the other hand, bond well with most epoxies and super glues. These plastics include ABS, acrylic, PC, polyester and PVC, and do not require any special formulations or surface treatment to form a decent bond. You should note that the bond strength will be still lower than that achieved with metals, for example.