Over the years, we've often heard our UltraClear Table Top Epoxy resin being referred to as "liquid glass".
The most obvious reason is in reference to its appearance, which is very similar to clear glass in the way it lets light pass through it with low distortion.
Additionally, because of its initial liquid state, it can conform and flow into many shapes before hardening into a resilient glasslike solid.
Some people might be wondering why and how this works, so for this article, we'll be explaining the causes behind transparency in plastics, including why epoxy resin is clear and what makes many other plastics translucent or opaque.
To start, not all epoxy finishes are clear.
Although our clear epoxy resins are transparent, there's another essential factor involved that determines the transparency of the cured epoxy finish: the hardener.
Hardener is combined with resin to activate the curing process that transforms it into a tough solid. There are different types of hardener meant for different purposes.
Some hardeners have an amber hue to them. When mixed with the corresponding resin component, they tint the end result slightly. These amber hardeners are meant for non-aesthetic purposes, and you'll often find them used with marine grade epoxy resin, since watercrafts are generally painted and at least partially submerged in water.
Don't worry, though. For the most part, this isn't something you'd accidentally end up with.
Now, then—hardener aside—what makes epoxy behave and look like liquid glass?
Two traits determine whether something is transparent or not.
Trait one: the material's refractive index
A material's refractive index is a measure of how much slower light moves through it compared to a vacuum (like outer space). If light passes into a material at an angle, the refractive index comes into play to determine where it will be when it leaves the material (if it isn't absorbed while in there). A lower refractive index means it doesn't slow light as much.
Our epoxy resins have a low refractive index.
This can get very complicated, but we'll try to keep it simple.
When a wave of light moves from one material to another (e.g., moving from air into glass), it slows down or speeds up depending on the refractive index of both materials.
If the wave hits a higher index material at a non-perpendicular angle, the part of the light outside of the material will bend toward the part of it that has already entered.
When exiting the material, the opposite tends to occur. The part of the light wave exiting will turn in the direction of the part that is still making its way through, until the wave of light has fully left the material.
For example, if you shine a light through a very thick, flat pane of transparent glass at a steep angle, it will refract. When it finally exits (all happening at a speed humans can't perceive), it will have a new trajectory, and it will be moving from a different point than it would have if the glass hadn't been in its original path.
When you see the light after its path has changed, it won't seem to be coming from its actual origin point. In fact, it's even possible to refract so heavily that the light curves back, almost in the direction it came from.
To give an analogy: Imagine a wave of light like a shopping cart at a grocery store. Normally when you push a cart, it moves forward in a straight line. Sometimes one of the wheels meets resistance and begins moving more slowly.
If you push casually, you may find that the cart veers in the direction of the wheel meeting resistance, which may also slow down your speed. If all four wheels then meet the same resistance (perhaps a sticky floor), then you could push it straight without issue as long as you applied enough force; it wouldn't veer.
Refraction is when part of the light wave catches that resistance, and the rest of the wave veers in that direction to compensate until eventually the entire wave is moving at the same speed with the same effort. It's still one singular form, much like the shopping cart, so the entire thing moves together by any means.
Why is a low refractive index important for epoxy?
A low refractive index is valuable because it means that whatever the epoxy is coating—including objects, art, photos, and the substrate itself—will be visible to human eyes with little or no distortion.
It's an ideal characteristic, since epoxy allows you to display something beautiful (like a wooden substrate) while also protecting it (due to epoxy's heavy resilience).
Trait two: the material's transparency
Many tough polymers are not visibly transparent, but epoxy is one of the few that are.
When visible light passes into and through epoxy, very little of it is absorbed (or reflected) by the material. This is what allows you to see through it and is why we call it "liquid glass".
Why does it not get absorbed?
This aspect can easily go beyond the scope of this article. However, to give a rough idea, you may recall from your school science classes that every physical material is made of atoms.
Atoms have a nucleus and electrons. The electrons are moving about outside the nucleus (center point) of the atom at incredible speeds. They have a set distance (labeled an energy level) that they occupy until something interacts with them in very specific ways to allow them to change distance (to a different energy level).
Light is made of particles called photons. When light attempts to pass through the space between the nucleus and electrons of these atoms, the nearby electrons will have a chance to absorb the photon, taking its energy.
However, it can only take the energy if it is exactly the amount needed to change position from the electron's current energy level to the next greater energy level. Any photon with less or more energy than that exact requirement will be ignored and allowed to pass by.
If most or all visible light that passes by is ultimately ignored because of this strict rule, then the material will appear transparent (which can be represented as a percentage, e.g., 97% 98%, 100%). The light will exit the materials (e.g., glass or epoxy) and can reach your eyes, allowing you to see what was on the other side.
Epoxy has a low refractive index and high transparency.
We hope you now understand why many people call it "liquid glass" epoxy, even if most don't fully grasp the incredibly complicated physical process at work.
There are many other polymers that don't have these features, making epoxy rather special in what it can be used for.
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