If you’ve been to your local hardware store to look at buying epoxy you may have been shocked at how many different epoxy products there appear to be. However, the good news is that it’s not as complicated as it first appears. In fact, there aren’t very many types of epoxy at all.
Is all epoxy the same? No, not all epoxy is the same. This guide will include an introduction to epoxy, a history of epoxies, how epoxies are used, the types of epoxy and resins in common use, how to choose the right kind of epoxy, modifications that can be made to epoxies and how to make them and some tips on working safely with epoxy.
We’ll help you understand everything you need to know about epoxy and how to make the right choice of epoxy for your next project.
The Basics: What Is An Epoxy Resin?
Don’t worry this isn’t going to turn into a complicated chemistry lesson but from a scientific standpoint, an epoxy resin is from a set of reagents that have an “epoxy group” which is a particular array of two carbon atoms each bonded to a single oxygen atom.
This group of atoms likes to react with pretty much anything and this is good news for the person who wants to use epoxy because it means it reacts very well with a hardening agent to cure.
Curing is the process by which an epoxy resin goes from a soft, runny (to some extent), liquid to a hard solid.
It is possible to get epoxy to harden without using a hardening agent – you can, in fact, use a chemical catalyst (which is a chemical that takes part in a reaction but isn’t used up by it) to get epoxy to react with itself to harden. However, this isn’t done in most applications because it’s more expensive than using a hardening agent.
Certainly, any epoxy resins that you buy for use in your home or workshop are going to use a hardening agent rather than a catalyst.
One notable fact about the epoxy reaction is that it takes place easily at room temperature and though epoxy reacts in an exothermic manner (that is it gives out heat) as long as you don’t use excessive amounts of epoxy (which is a bad idea because it ruins the epoxy bond, anyway) it doesn’t give out enough heat to be a fire hazard.
A Brief History of Epoxies
It is possible that the first epoxy reaction was observed in 1891 by a German scientist by the name of Lindmann. However, it appears that his full experimental notes have not survived the years and it’s hard to be certain about this.
What is certain is that a Swiss scientist, Pierre Catan, began investigating the idea of the epoxy reaction just before World War 2 in the early 1930s and that Shlack, also of Germany, began their own investigations in 1933.
By 1939, both the US and Europe were trying to make use of the epoxy reaction in several settings but had had little success. The first patent for a commercially viable epoxy resin was filed in 1943 by Caston Greenlee of the United States but manufacturing didn’t take place until 1947 when the war was finally over.
Since then, the epoxy reaction has become a staple of woodworking and other industries worldwide and new products and compounds which use the reaction are still being patented today.
Where Are They Used?
Epoxy resins have a very wide range of uses and that’s because of the specific properties that come when an epoxy resin cures:
- They have a high degree of chemical resistivity. This is not perfect, mind you, and epoxies are vulnerable to acids but are very stable when exposed to alkaline environments.
- They bond very well with a wide variety of substrates. The ability to work with a wide variety of materials is a huge plus point for epoxies and if they bond to a surface, they generally bond with a very strong bond.
- They have superb compressive, flexural and tensile strength and that means they can be employed in situations that place higher levels of stress on the bond.
- They don’t shrink very much while curing. This means that when you are using them to bond surfaces, they won’t move the surfaces out of alignment and that they will bond securely.
- They are great electrical insulators. You can use epoxy to shield against electrical problems and it maintains this property even as it ages and in the toughest of environments.
- They don’t corrode. Well, technically they can corrode but it takes a very long period of time because they’re very resistant to becoming corroded.
- They can take high levels of physical punishment. Epoxies can be used to cushion against drop damage or falling damage because of this.
- They cure at a wide range of temperatures. The reaction itself is exothermic but you can cure epoxy in almost any temperature, though the time it takes to cure will vary depending on the heat.
- They have a high-level of fatigue strength. Epoxy won’t wear out quickly over time.
So, as you can see – epoxy resins are quite handy and if we were to put together a full list of their uses, it would take a book to do it.
Their use in building and construction, however, includes the manufacture of:
- Various other products and materials
The Various Types Of Epoxy And Resin
There is an infinite number of variations of epoxy, in theory, and certainly, there are so many different choices out on the shelves of hardware stores that it’s easy to become overwhelmed but, in fact, there are only a few types of epoxy resin in reality – the vast majority of choices you see, are slightly modified versions of epoxy resin tailored for a specific job.
So, let’s take a look at the three 3 most common types of resin on the shelf and the differences between them:
- Pure epoxy – which can come in one or two-component forms
- Polyester resin – which isn’t epoxy but is relevant to the other type of epoxy
- Epoxy acrylates – which are a sort of blend between pure epoxy and polyester resins
Pure Epoxy, What Is It?
One Component Epoxy Resin
The joy of a one-component epoxy resin is that you can use it right out of the container – you don’t need to mix it, remove any bubbles or even meter it.
The downside of this is that they require you to supply the heat to cure them with and they require high temperatures for curing with.
You will find that a one-component epoxy is usually supplied with a heating element. This is an “initiator” (e.g. it starts the curing reaction).
If not, you’ll need to supply a heating element or oven to do the curing in.
You’d expect a one-component epoxy resin to take between 30 minutes and an hour to cure. This is typically done at a temperature range of between 350 degrees Fahrenheit and 400 degrees Fahrenheit.
If the particular epoxy requires induction curing – you will need to cure it in this manner for around 5-7 seconds at between 325 degrees Fahrenheit and 450 degrees Fahrenheit. You do this prior to curing in the oven.
It is also possible to get a one-component epoxy that is fully cured by ultra-violet light which is present in sunlight.
One part epoxies tend to form a slightly weaker bond than the two-component epoxy resins and they can work out a little more expensive too.
Two Components Epoxy Resin
The two-part epoxy is the most common and the sort that most people will have in their home or woodworking workshop.
The two parts are the epoxy resin and the hardening agent. Unlike a one-component epoxy, it is important for the user to mix a two-part epoxy and this must be done so that:
- the mix is thorough, failing to mix epoxy properly leads to parts of the epoxy not curing when it is applied
- there are no bubbles introduced to the mix by stirring, bubbles are more than a little unsightly – they cause a weakening of the epoxy bond and can lead to a brittle finish which is unfit for purpose
When the two parts of a two-component epoxy resin are mixed, they lead to an exothermic (heat releasing) reaction which triggers curing. (Curing of epoxy is actually a process known as “polymerization” which is where the compounds in the epoxy change shape to form plastics or polymers).
You can cure a two-component epoxy resin at room temperature but it’s also possible to raise the temperature to speed up the curing process. It’s possible to get epoxy to cured strength within just 5 minutes and it can take up to 8 hours.
We’ve found that most off-the-shelf two-component epoxies will solidify in 2-3 hours. If you can’t raise the temperature to speed up curing, you can always add a catalyst to give the reaction a bit of a boost.
It’s important, however, to let a two-component epoxy cure for around 7 days at room temperature before stress or strength testing takes place.
When you epoxy is completely cured, the bond should have reached the same kind of strength as machine molded plastic.
How To Choose The Right Type Of Epoxy (One Vs Two)
In the main, for home use and DIY – you don’t need to put too much thought into this. Two-part epoxies will let you bond almost anything together and can be used for strengthening wood too.
Typical two-part epoxy products will include:
- Epoxy primers
- Epoxy paste fillers
- Epoxy consolidants
- Epoxy glues
- Epoxy resin
In general terms, the ability to mix the two components allows for the most intense chemical reaction, which, in turn, allows you to get the strongest bonds. Given that this is, usually, the objective of the home woodworker, two components are what people tend to use.
The other big advantage of a two-component epoxy is that it cures at room temperature which means you don’t need any special equipment to heat and cure it with.
One component epoxies, on the other hand, tend to turn up on manufacturing assembly lines a lot because they tend to bake other products to cure them as part of their finishing processes, so, it’s more convenient to bake the epoxy at the same time which reduces the need for mixing equipment and improves productivity.
One advantage of using one component epoxies is that you don’t have to keep an eye on the mixing ratio. (For most two-component epoxies you will find a 1:1 epoxy: hardener ratio is normal but it is also possible to find other ratios and you should always read the instructions on an epoxy carefully before you mix it – mistakes can be very costly).
Polyester Resin, Not Epoxy But Very Useful
There is another type of resin which is often used in woodworking and in industrial processes for similar purposes as epoxy and that is a polyester resin. It’s important to note that polyester resin is not epoxy and though the end products are similar, they are not the same.
Polyester resins have some real advantages too:
- They can resist water and a range of chemicals (they are much better at handling acid than epoxy resins)
- They won’t age quickly or weather quickly (and, in fact, tend to last even longer than epoxy)
- They are relatively low cost (though epoxy tends to be cheaper upfront, the fact it needs replacing more often can lead to the polyester resin being cheaper over the lifetime of an object)
- They are fairly heat resistant
- They don’t shrink very much during curing
However, they also have some disadvantages:
- They smell dreadful – the styrene scent that comes off this kind of resin is quite unpleasant
- They are harder to mix – a two-part epoxy is a much simpler thing to deal with
- They are more toxic and require breathing apparatus and protective equipment as standard when working with them
- They have a limited range of substrates they will bond to
- Overall, the final cured plastic will tend to be a bit weaker than an epoxy finish
So, if you see a resin that’s not epoxy – it’s probably a polyester resin.
Epoxy Acrylates – Epoxy Meets Polyester
So, if you want the properties of epoxy and you want the properties of polyester resins, then it’s possible to source an epoxy acrylate.
Epoxy acrylates are very different from epoxy resins when in use and, in particular, the vast majority of these products are UV cured rather than curing at room temperature or under a particular heat.
They have a much faster cure time than traditional epoxy resin which makes working with epoxy acrylates much easier under certain circumstances.
They have a greater degree of hardness after curing than epoxy which means that they are generally more durable than epoxy resins.
You will also find that epoxy acrylates have the highest degree of chemical resistance and, in particular, are much better at handling acids than epoxy resins are.
Just like epoxy resins, epoxy acrylates can be tailored in a huge number of different ways to promote specific properties. One very popular modification is the no volatile organic compounds variant which is completely safe to use indoors even in spaces that are not particularly well-ventilated.
Modifying Epoxy Resins
It is possible to easily modify the properties of any given epoxy resin and the majority of these modifications, when working with two-component epoxies, are made to the resin side of the mix.
The only exception to this is when you are looking to add a catalyst to the epoxy to boost the curing time – catalysts and other forms of chemical accelerators are always added to the hardening agent.
Dyeing Epoxy Resins
Possibly the most common modification made to epoxy resins is the act of adding colors to the resin in order to allow the epoxy to either better match the material it is being bonded with or, occasionally, to give the finish a new color.
It is very easy to change the color of epoxy resin and you can buy professional tints from any hardware store to add to the resin. You can even use household items to change the color of your epoxy and Paprika, in particular, is a popular choice for its red shade.
We’ve put together a complete guide on how to dye epoxy resins here, so we won’t go into the process, but the basic rule of thumb is 2-6% dye by weight to epoxy. If you use more than this, it will interfere with the curing process. Less and the color is unlikely to take properly.
Fillers And Epoxy Resins
Fillers are used to thicken epoxies for certain specific purposes (such as when they are used to replace rotted wood or for when you want to make casts, for example).
You can purchase epoxy resins with the filler already in them and this is the easiest way to use an epoxy with filler.
However, it’s also a straightforward job to add your own if you want to. You just need to mix the resin and its associated hardener in the correct ratios for about two minutes, then add the filler until you have the right consistency – make sure that you’ve completely blended the filler into the epoxy evenly at this point.
Rheological Additives (Thixotropes and Viscosity Suppressants)
Epoxy consolidants are an example of an epoxy resin which has been treated with a viscosity suppressant. This makes the epoxy thinner than it would normally be and thus, capable of soaking into wood fibers and filling even the smallest of gaps.
The easiest way to use epoxies that have these additives is to buy them in that form.
Again, you can make a thinner epoxy at home if you want – you can add paint thinners or acetone at about a ratio of 1:10 (so 1 gallon of thinner would be added to 10 gallons of epoxy).
You can also add denatured alcohol (which is poisonous and categorically not for human consumption) – you can thin epoxy much more with this that with pain thinners or acetone but you will pay a price in that the finished product will not be as strong as ordinary epoxy.
If you choose to use denatured alcohol the ratio is about 1:5 (so 1 gallon of denatured alcohol to 5 gallons of epoxy).
It is useful to note that this thinning property of denatured alcohol also makes it an ideal product to use to clean up an epoxy spill which hasn’t, yet, cured.
Property Promoting Agents (UV Resistance, Wetting Agents, Adhesion Promotes, Flame-Retardant)
You can, in theory, add your own property promoting agents to epoxy resin but we’re now straying into the area of rather more specialist chemistry and, by far, the easiest way to get properties such as UV-resistance (which is often very necessary as epoxy yellows in direct sunlight) is simply to buy them already in the epoxy.
You’ll find that you can get a wide range of such specific properties at the hardware store.
Processing Aids (De-aerating Agents, Mold-Release Agents)
The same is true of epoxies with de-aerating and mold-release properties, it is unlikely that you would be adding these agents to the epoxy yourself, though, again, in theory, it’s perfectly practical for you to do so if you have experience and the chemical understanding to do so safely.
We think that for the majority of people – this isn’t going to be the case and if you need these properties, it’s best to buy your epoxy with them.
Working With Epoxy: Health & Safety Considerations
Finally, it’s important to acknowledge that working with epoxy is a relatively safe experience and, this is particularly true if you are working on a small project. However, there are some safety considerations that can help you work with epoxy in any environment without risking your own health.
We’d recommend that you wear gloves and safety glasses whenever you work with epoxy. There is always a risk of splashing it on your skin and causing either adhesion or an allergic reaction and the same risk is true of your eyes which might lead to blindness. Given that gloves and glasses are cheap and easy to find there’s no reason to take any such risks.
You also need to ensure that the area in which you work with epoxy is fully ventilated. We’d recommend that if you’re indoors, you open all doors and windows and that you use a fan to blow air away from you and out of the door.
If this doesn’t feel sufficient, you should also consider using a respirator mask. These are again, relatively low cost, and they will ensure that your lungs don’t come to any harm and that you are not overwhelmed with fumes.
You should ensure that this respirator mask is up to the right local standard for safety and not just buy something at random online. It should come with an organic vapor cartridge for maximum protection.
Do we hope that all epoxies are the same? A Guide to epoxy types and uses has been useful for you. As you can see, it’s quite straightforward for almost all home projects you will be working with a two-part epoxy resin which has been designed for a particular task (or which you’ve modified for that task).
Working with epoxy is very easy and safe. The end products are durable and pleasant too. It may seem a little intimidating at first, but you will find that you quickly learn everything you need to know to get epoxy to work for your projects.