Selecting the right adhesive is critical for any project. Adhesives are not the same and making the wrong selection can mar the appearance of the project. Worse, the wrong adhesive or the wrong application of that adhesive may make the joint fail, causing the loss or destruction of the project.
The “right adhesive” or “best adhesive” depends a lot on the exact application. Material, temperature, moist, stresses and many other factors can play a part in how well a particular adhesive works for a given project. While one adhesive may work in a large number of different applications, just misunderstanding one of those factors can turn that success into a failure.
Keep in mind that there is a lot of advertising hype about different adhesives, just like there is about any other product on the market. Gorilla glue is an excellent product; but like any other product, it sells largely based upon its reputation and the marketing image. People buy it because they have been convinced that it has to be strong.
Often, the bigger issue isn’t how strong the glue is, but the materials that the glue is being used on. The strongest glue in the world won’t make Styrofoam strong. While the bond may hold, the material itself will break. Likewise, the strongest glue may not work well on glass, because the slick surface of the glass doesn’t give the glue much to hold onto.
How strong the material that is being bonded together is probably the single most important factor in making a good glue joint. I was reminded of this the other day, when my granddaughter knocked over a ceramic figurine that belonged to my wife’s grandmother. It had been broken before and whatever glue someone had used to glue it back together had held, but the ceramic broke in another spot.
Both epoxy and Gorilla glue are stronger than the wood they might be used to attach together. So regardless of which one is used on a project, the project may fail, if the wood itself is put under too much stress. Cross-bracing may be needed, to add additional strength to the project, which the adhesive can’t supply.
Porosity of the material being glued is a major consideration. Most adhesives will bond better with wood, than they will with plastic, metal or glass, because wood is a naturally porous material. That porosity is important, because the adhesive seeps into those pores, giving it a greater grip on the material. Without that, all the adhesive has is it’s cohesion with the smooth surface. That’s why it’s a good idea to sand metal and plastic surfaces, when gluing them, giving the glue something more than a smooth surface to stick to.
Surface texture of the material being bonded makes a huge difference in the work that the adhesive has to do. Two pieces of wood, which have smooth mating surface will bond together much better than two which have saw marks on the surface. While there are adhesives which have gap-filling properties, any gap means that the adhesive is carrying the entire structural load. Using the wrong adhesives to make up for poor surface texture or lazy building practices can be disastrous.
What Makes One Glue Better than Another?
Because of all the variables, it’s a bit difficult to say that one adhesive is better than another. One adhesive might work better than another in once circumstance, while the other might be better when used for something else. Nevertheless, the following characteristics of the adhesive are all important when making a decision.
Tensile strength refers to the strength of a material when pulling on it, as if one is trying to pull it apart. This is why advertisements for Super glue (by whatever brand), always show it holding up large amounts of weight. Super glue is extremely strong in tensile strength, but not so strong in shear strength.
In a sense, shear strength is the opposite of tensile strength. While tensile is about pulling on the material, shear is about trying to break across the material. This aspect becomes extremely important in cases where the project might be subject to bending or something might be pressing against the side of the joint.
Obviously all adhesives need the ability to adhere to the material that is being bonded together. But not all adhesives bond to all surfaces, making this one of the most important characteristics that people look for. This makes adhesion a gateway in the decision process. As long as that’s all it is, there’s no problem. On the other hand, if that’s all that’s looked at, then the best adhesive might not be selected.
Some applications require that the adhesive fill gaps between two pieces being joined together, because the pieces don’t have a smooth, fitted finish. This is common when bonding wood, as the time it takes to smooth wood surfaces out for gluing adds to the total project time and cost. Construction adhesive is designed to be gap filling, so that carpenters don’t have to take the time to smooth out dimensional lumber before attaching it together.
Epoxy putty offers probably the best gap filling properties of any adhesive on the market, because it is a putty, rather than a liquid. As an epoxy it has the same high shear and tensile strength of any other epoxy product. But the putty does have a bit of a disadvantage, as it doesn’t have the adhesion of regular epoxy.
Marine applications obviously require moist resistance, but those aren’t the only ones. Projects which are built to be outdoors or for use in a damp basement might require adhesives which are moisture resistant, so that the moisture in the environment doesn’t soften the adhesive, causing a failure.
Back to the Original Question
Going back to the original question, about whether epoxy or gorilla glue is stronger, we need to understand exactly what these two adhesives are, including the strengths of each.
Epoxy is a two-part reactive polyepoxide; a class of reactive polymers which contain epoxide molecule groups. In other words, it’s actually a thermoset plastic. That means that the two parts of the epoxy react with each other, generating heat and creating a chemical reaction that mixes them together, forming a very strong material, with excellent tensile and shear strengths. Of all adhesives, epoxy has the strongest shear strength.
Gorilla glue is a water-activated polyurethane; a different type of plastic. Polyurethanes are used in a variety of adhesives and finishes. The water activating of Gorilla Glue helps to ensure that a bond is not made, where it is not desired.
There are a number of different Gorilla glues (all made by the same company), just as there are a number of different epoxies (not all made by the same company). Some Gorilla glues are stronger than others, having fibrous fillers added to increase the shear strength of the glue. But the biggest thing that Gorilla Glue has going for it, is that it is easy to work with.
Many people don’t like working with epoxy, because of having to mix the adhesive. There is no such thing as an epoxy that doesn’t have to be mixed. This also means that there is always some material which is wasted, increasing the cost of working with this adhesive.
As I already mentioned, both Gorilla glue and epoxy are stronger than the wood they hold together, making both of them excellent choices. However, epoxy bonds well with more materials than Gorilla Glue does, as well as having higher shear strength. So, epoxy is the better of the two.
What About Other Glues?
With that basic question out of the way, it’s only natural to question comparisons between other potential adhesives. So let’s look at a few:
Epoxy vs Super Glue
Both epoxy and super glue have excellent adhesion characteristics, bonding to a wide variety of materials. The big plus for super glue is its rapid dry time, which by far beats even the fastest curing epoxy. However, super glue (by any brand) has low shear strength, while epoxy has the highest shear strength of any adhesive currently on the market.
The other big difference is that the heavier-bodied epoxy is better a gap filling, than super glue is. Some gel super glues have been developed to try and make up for this. While they do, to some extent, they still don’t do so as well as epoxy. For gap filling with super glue, it’s best to add some powdered substance, like baking soda.
Epoxy vs JB Weld
This one is almost a misnomer, as JB Weld is an epoxy; specifically, an epoxy putty (although the company also makes liquid JB Weld epoxy). As an epoxy, JB Weld enjoys the same high shear strength of epoxy, which has actually been enhanced by the fillers used to make it into a putty.
JB Weld is actually at its best, when used as a gap filling adhesive for metals. It can be used to replace a broken flange on a metal casting in a car’s engine. Once hardened, it can be drilled and shaped as if it were metal and it can handle the heat that engines generate. However, the adhesion of JB Weld isn’t as strong as epoxy, especially on porous surfaces, because the putty doesn’t soak into the pores.
Epoxy vs Construction Adhesive
Construction adhesive comes in caulking tubes, designed to make for quick and easy application of the adhesive over a large or long area, such as gluing two pieces of dimensional lumber together, making a beam out of them. It’s strengths are that it is easy to apply and provides excellent gap-filling properties.
In reality, the two adhesives are not comparable, as they can’t really be used in the same applications. It would be extremely difficult, if not impossible, to use epoxy for the types of applications where construction adhesive is needed, especially where a lot of gap filling is needed. Likewise, construction adhesive can really only be used where a gap exists, as it will only crush down so far. This makes it impossible to use in some cases where epoxy would be.
Epoxy vs Wood Glue
Epoxy is both stronger than wood glue and is waterproof, whereas wood glue is usually polyvinyl acetate. This material has been developed with the ability to penetrate wood fibers, creating a bond which is stronger than the wood itself. It is much more convenient to use, and since it forms a bond stronger than the wood itself, there really isn’t any need for the greater shear strength of epoxy.
However, epoxy has much better gap filling properties than wood glue does, so in cases where it is important to fill those gaps, epoxy might be used. For this reason, epoxy has found a place in woodworking, especially when working with live edge slabs or when turning wood where gaps in tee wood need to be filled.
Wood Glue vs Construction Adhesive
Because of its ability to penetrate into the wood’s fibers, wood glue forms a stronger bond than construction adhesive does. However, just as with epoxy, the wood glue doesn’t have the gap filling ability that construction adhesive does.
Construction adhesive dries somewhat rubbery, allowing for some flexibility in the joint. This can be important in home construction, where structural members may need to be able to move slightly due to changes in the live load. wood glue couldn’t perform as well in these instances.
Epoxy vs Silicone
Silicone adhesives or silicone caulking used as an adhesive, are much more flexible than epoxy, making them ideal for use in cases where that flexibility is needed, such as the bonding of a bathtub to the shower walls or to the substrate. While there are flexible epoxies, all silicone adhesives are flexible.
Epoxy has a much higher adhesion than silicone, even on non-porous surfaces. It is possible to peel off dried and cured silicone much easier than trying to peel off epoxy. Silicone is also much more resistant to high temperatures, being able to withstand temperatures up to 200°F without problem. By comparison, most epoxy is limited to 120°F.
Epoxy vs Brand X
Many people raise questions about whether epoxy is better than a particular glue product, like the Gorilla Glue mentioned in the title of this article. The reality is, each of those brands have multiple products, some of which might also be epoxies. Asking if Loctite or Gorilla Glue is better than epoxy raises the question of which glue product is being chosen. So it’s important to understand what exact product in that product line is being referred to.
In most cases, the other product is going to be easier to work with than epoxy is, simply because epoxy is a two-part adhesive and there aren’t all that many other two-part adhesives we customarily use. The tradeoff is that in just about any case, epoxy is stronger, as well as being able to bond with so many different materials. That makes it an excellent go-to adhesive, when one is unsure what they should use.
Which is Stronger, Adhesive or Nails?
By and large, most adhesives are stronger than screws or nails, when used properly. This difference comes primarily from the fact that the glue is covering the entire surface, while screws and nails are going to be localized to a few discrete locations.
Many carpenters and woodworkers use a combination of adhesives and fasteners for most projects. The fasteners give them quick holding power, holding the project together, while the adhesive is drying or curing. Once the adhesive cures, it becomes the main thing holding the project together, with the nails or screw being the backup.
It is always important to keep in mind the grain of the wood being fastened. Neither glue or screw work well in the end grain of wood. While the end grain provides a lot of porosity for the glue to soak into, that glue can also pull right back out. Screws can pull out easily as well, because the threads break up the wood fibers, meaning that there is little holding them in place. Both work well though for side grain.
Nails are usually the best fastener for end grain, because they work by compression; the wood fibers pressing against the sides of the nails. This can happen equally well with end grain, as it can with side grain. Even so, using glue in conjunction with those nails, regardless of it being side grain, helps to seal the end grain, preventing rotting and thereby helping to stabilize the nails in the wood.