by Roderick Kabel
Many people ask us why mica powder sometimes sinks to the bottom of epoxy. The short answer is that all mica powder will sink to a degree no matter what. All mica powder brands are different and unique and with that, all mica powders are not made equally.
There are a number of issues going on here and we will try to explain and offer a remedy.
In general, epoxy cure times are relevant and depending on environmental conditions, epoxy curing can be fast or slow. Here’s why it matters.
Epoxy has an A-side and a B-side and each has its own individual viscosity, or thickness. When the two are mixed, a whole new viscosity is created and is susceptible to variations of temperature, humidity and mass in any specific environment. As epoxy moves into its exothermic reaction it begins to gel, thicken, and heat begins to increase.
As the epoxy gels the viscosity also decreases whereas an object – of any size – may flow through it at a slower rate. Imagine if you dropped a marble in a glass of water and did the same thing with a marble and a glass of honey. Which glass would allow the marble to hit the bottom fastest?
Mica powder mixed in epoxy uses the same principle as the marble example. Mica powder is made from very fine particles of mica, a natural stone mineral. Even at a tiny microscopic size, mica powder particles are subject to gravity and viscosity.
Particle size is especially important not only for color composition when it comes to pigments. Particle size dictates the luster and reflection of a mica powder. Mica powder is measured by microns (µm) and the higher particle size means more sparkle but less coverage.
| 1-25 µm
The issue with mica powders sinking is this; Because deep pour epoxy can be poured at 2 inches (or more) in a single pour, the formula must allow for a slow exotherm. A slower exotherm means the deep pour epoxy formula also has a lower viscosity which takes longer to gel. This means mica powder particles do not readily suspend in the epoxy and tend to sink.
Many epoxy users are pouring general use or marine epoxy and are accustomed to thinner pouring formulas that have a faster exotherm and gel time. This quicker exotherm and gel time makes mica suspend faster, sure, but the tradeoff is that these epoxy brands crack, shrink and often, smoke. General use and marine epoxy tinted with mica powder will never produce the beautiful billowing, swirling and patterns that naturally occur from a well-managed deep pour exotherm.
Many users mix the mica powder in immediately after mixing the A Resin and B Hardener together and have great results. When working with heavier colorant particles or for a project where maximum particle suspension is required, many professional users allow the mixed epoxy to sit in the mixing bucket for up to 2-4 hours (depending on environment) before adding mica.
Waiting after initial A&B mixing allows the epoxy to begin gelling which helps to suspend the mica powder particles in the epoxy as the viscosity continues to decrease. Once mica is added and thoroughly mixed, it’s a waiting game to find the sweet spot to pour, and get perfect mica particle suspension. Furthermore, the more mica powder that is mixed in the epoxy, the better the chances for a more impregnated final tint to the epoxy.
Since this is only a suggestion, we can't say exactly when the correct time is to mix the mica powder into the epoxy. Some users may want to mix early into the gel time and some may want to wait a little longer. With that in mind, if heat begins to rise from the epoxy, the exotherm has begun and the natural billowing may not be as robust as it should be. Always try to mix the colorant in prior to any heat buildup.
A shorter wait for gelling should allow for the exothermic reaction to produce the natural billowing. A longer wait time may allow you to make your own patterns with a stir stick.
Test, test, test.
As for bubbles introduced into the epoxy gel time during mixing, our epoxy is very good at releasing air. Plus, the mica powder will hide any internal bubbles in the epoxy. Using a torch to burst bubbles throughout the cure is always recommended for a bubble free, smooth surface.
Keep in mind, different mica powder brands have different particle sizes. This is very important as well to be sure micas suspend correctly for your project. Always check the mica powder particle size when purchasing from any brand.
Mica powders available for purchase from WiseBond™ have a particle size ranging from 10-60 µm to 10-100 µm.
NOTE: Glow-in-the-dark powder will sink much more than mica powder. We do not recommend deep pour epoxy be tinted with Glow-in-the-dark powder because these particles are larger and heavier than mica powder. Glow-in-the-dark powder works best with Table Top Epoxy projects and thin pouring.
When using Glow-in-the-dark powder in any epoxy, the same mixing is suggested as above.
What we have explained is far from an exact science and testing is ALWAYS suggested prior to adding any colorant into epoxy for a final project.