Hot glue is a craft and repair staple in most households. Whether it’s securing embellishments, building models, or reattaching broken items, it always comes through.
But the most crucial test for any glue is how its bond holds up to water. Water will easily loosen bonds it comes into contact with if the glue is not waterproof.
So, is hot glue waterproof? Yes, hot glue is waterproof. It is insoluble and remains impermeable even in complete immersion. However, despite hot glue being waterproof, the hot glued joint will not always withstand water in every circumstance imaginable.
Read on to learn more about hot glue and what makes it waterproof. You also find out whether it is suitable for your water-related project or not and why.
What Is Hot Glue, And Can It Withstand Water?
Hot glue is one of the most ubiquitous glues for domestic and industrial purposes; it almost needs no introduction. But for the sake of those not familiar with it, let’s go basic.
Hot glue, sometimes known as melt adhesive, normally comes as clear sticks. The hot glue sticks work with a particular device known as a hot glue gun, where the glue sticks are inserted. My favorite hot glue sticks are these ones.
When the gun is powered on, it heats up and melts the glue stick dispensing it in a liquid state through a nozzle. Once applied onto a surface, it will solidify within seconds and bond with it.
Now let’s look at what hot glue is chemically. Hot glue is a thermoplastic adhesive that liquefies with high heat but hardens when brought to room temperature.
It contains long chains of amorphous polymers as its building blocks. These chains are of a repetitive unit and intertwine to form a complex matrix.
This structure cannot be broken by water, solvents, and even weathering. What this means is that water can not penetrate a layer of hot glue, making it waterproof.
Even held below the water surface, it will not budge. There’s simply no entry or pathway for water molecules to go through.
You might quickly think that hot glue will do exceptionally well under water or even outdoors with the rain and other elements. This isn’t always the case.
It is the hot glue in itself that is indestructible by water molecules. However, a hot glue bond can succumb slowly or fast to water and outdoor conditions depending on the following couple of factors.
Type Of Bonding Surface
The material of the surface being bonded has a significant impact on how well or poorly a hot glue bond will fair in water. Despite the hot glue itself being water-resistant, if the material is not, then the bond is likely to fail.
Water will soak through the material and get in between it and the layer of hot glue. Wood is one perfect example. Unless it has been treated with a waterproof sealant, the hydrophilic material will absorb so much water.
What follows is swelling and expansion of the wood, which interferes with the bond by applying extra pressure. The shifting also disturbs the joint, which eventually fails. Wood and hot glue is an outstanding combo for dry applications but the worst to immerse in water.
Another bad water immersion option would be a plastic hot glue bonded surface. Plastic generally contracts in water when exposed for an extended period.
The plastic surface and the thermoplastic glue would both contract separating from one another. What about glass? Well, glass experiences contractions and expansion too but within a negligibly small range compared to plastic.
Nonetheless, long-term exposure of the bond to water ultimately causes it to give in. The non-porous nature of the glass doesn’t help the situation either because the bond site is already not that strong.
Like plastic, the bond is only superficially attached to the glass. The thermoplastic glue has no carriers to penetrate the non-porous glass. Consequently, water can get in between and force them apart.
Hot glue fabric bonds surprisingly score best when it comes to water exposure. Despite being a highly absorbent material, fabric has the highest survival rate.
Being very porous, the hot glue goes through and around the weave, making a seriously solid bond.
Even when immersed in water, the bond won’t budge in a very long time. The threads are surrounded into a mesh by the impermeable hot glue, creating a practically inseparable bond site.
When it comes to metals, specialized hot glues exist just for the material, which makes it difficult to compare. But typically, metal items are rarely meant to go near or underwater.
Type Of Glue
The type of hot glue is another critical determinant of how waterproof your hot glue joint will be. You may be familiar with one or two conventional brands, but there exists a wide variety of hot glues.
They have varying capabilities, and you are sure to find even much better options than you are used to. Some provide high-stress, high-strength bonds, while others lie low in the performance chart.
If you are going to expose your hot glue bond to wet conditions, you can expect much more from high-quality glue. Conventional ones like EVA-based hot glue are too weak and not built for such demanding applications.
Polyolefin (APAO) based and polyamide (PA) based hot glues are great examples of high-quality hot glues.
If you are worried about UV deterioration as well, Polyethylene (PE, PEMA, PEnBa) based hot glue is an even better option. It is not only higher in strength but UV resistant too.
If you don’t mind spending extra for an industrial-grade performance in the water, go for polyurethane reactive hot glue (PUR). It is not common thus not easily accessible, but the bond is robust, waterproof, UV resistant, and temperature resistant.
However, it will set you back extra dollars because it is expensive. It also comes in tubes, not sticks; therefore, you’ll need to purchase a specialized gun for it.
Does Hot Glue Hold Up Well Outdoors?
When crafting anything for outdoor applications, you’ll often seal it with a sealant to waterproof it. Because hot glue is waterproof, the question as to whether it can be used as a sealant often comes up a lot.
Hot glue is indeed 100% water-resistant and waterproof. Sadly it cannot be used as a waterproof sealant for outdoors or even indoors. It is practically impossible to lay down hot glue from the way it is dispensed. It cannot be spread or brushed on and just dries too quickly to make it even possible.
The more common reason you might be interested in using hot glue outdoors is as an adhesive. Perhaps you need to glue parts of an item that will spend a significant amount of time outside, like a lampshade, birdhouse, etc.
It goes back to what we discussed before, and that is the quality of the glue and the material that you are bonding. A high-quality hot glue will hold up better outdoors than a regular one.
But water is not the only element to think about. Even if hot glue is waterproof, there’s wind, temperature, and UV radiation to think about.
UV is a known enemy for hot glue and causes it to deteriorate very fast. Only Polyethylene based hot glue and Polyurethane reactive hot glues are UV resistant.
The temperature fluctuations outside can also cause hot glue to contract and expand and eventually crack. Constant gushing wind hastens the process.
Just to clear this misconception up, the sun can not melt hot glue. The lowest heat needed to melt low heat grade hot glue is about 170°F. The sun never raises any surface below it to that temperature.
Therefore hot glue could hold up outdoors but not for too long. There are better adhesive options.
In conclusion, hot glue is waterproof. Once it hardens, nothing is going through it, not water, not chemicals. On its own, it remains unaltered by water even when submerged for many years.
However, when it comes to the effectiveness of a hot glue bond subjected to water, things change because other materials are involved. It is the material of the bonding surface and the strength of the hot glue that will determine how long it stays.
You’ll have the least success with non-porous surfaces like glass. Porous surfaces like woven fabrics will buy you plenty of bond time in the water.
Typically there is no hot glue that can make an entirely waterproof bond underwater. They all eventually give in. Only the time it takes varies.
So, what are we trying to say? You can even use conventional hot glue to put together your model boat or ship. The bond will tolerate frequent splashes and occasional dunking in water.
However, if you need to glue back a broken water filter or pump that spends its entire life inside a fish tank, you should set your hot glue standards very high but bond longevity expectations low.
Up Next: How Do Hot Glue Guns Work?