Muscovite & Phlogopite Mica

If expertly engineered in the milling process, mica particles have aspect ratios so high as to impart outstanding stiffness, strength, HDT, CLTE and barrier properties. This is well-recognized in Europe and Japan for example but less so in the US where the full commercial and technical potential of mica has yet to be realized.

Sources and Types

The vast majority of mica, whether the more common muscovite or the phlogopite variety, is mined. Deposits occur all around the world. Mica includes a long list of silicates that occur naturally in all three major types of rocks — igneous rocks (granite), sedimentary rocks (e.g. shale and sandstone), and metamorphic rocks (slate and schist). 

As well as natural mined mica it is also made commercially using synthetic routes. Synthetic fluorophlogopite is very pure, colorless and therefore used as the substrate for special effects pearlescent pigments, for example those from Kuncai Pigments.

Processing

The key to mica is the processing because performance depends on attaining the desired particle size while retaining a high aspect ratio. Wet grinding is ideal but expensive. Specialized dry milling allows aspect ratios approaching those from wet milling but at a substantially more attractive price point. Arctic Minerals specialize in optimized mica milling, classification and surface treatment of mica on a commercial scale. The company has many years of experience and supplies every grade imaginable.

 Chemistry

Like talc and kaolin clay, mica is a silicate type mineral that naturally forms in sheets that create platy particles upon grinding. Silicates are safe and inert substances with high temperature resistance. Under the right conditions, phlogopite can slowly release nutrients that aid plant growth.

Effect on Mechanical Properties

Reinforcements are widely used to elevate the mechanical stiffness and strength of plastics and other polymer-based materials like elastomers and thermosets. Low aspect ratio fillers like calcium carbonate and dolomite do not reinforce at all. Medium aspect ratio fillers like talc, clay and wollastonite provide moderate levels of reinforcement (a simultaneous improvement in stiffness and strength). Very high aspect ratio ingredients like glass fiber and mica provide superior levels of reinforcement. In fact, the two are often blended together because glass fiber when used alone leads to warpage. Adding high aspect ratio mica retains excellent mechanical properties while solving the warpage issue.

Heat distortion temperature and Vicat softening point may also be significantly raised by adding mica because of the increase in stiffness at any given temperature. However, minerals only work well for increasing HDT / Vicat for semi-crystalline plastics like PE, PP and nylons but are not effective in non-crystalline (amorphous) plastics like PC, PS, ABS and so on. The reason is that when heated, amorphous polymers get so soft that it doesn’t matter that stiff mineral was added, the whole material sags anyway. Semi-crystalline plastics on the other hand do retain significant stiffness right up until the temperature where the crystals melt. When it comes to HDT, adhesion to between the mineral is important, so adhesion promoters such as silane coupling agents are used.

When it comes to strength, mica is a superior reinforcing filler because the particles of mica when expertly milled have very high aspect ratio.

The above-mentioned properties change in relatively predictable ways and the volumetric linear rule of mixtures is a useful tool for estimating the behavior of such systems. In contrast, some properties do not depend on the average properties of the blend but rather on local flaws. One example is impact resistance. Large particles act as flaws where a crack initiates upon impact, then spreads rapidly such that the whole material fails. It is not the average particle size that matters but the size of the largest particle or agglomerate of particles. Also, for anisotropic particles like mica, it is the largest particle dimension (the width of the platelets) that matters. If high impact resistance is desired, then small, well-dispersed particles with coarse particles removed is recommended.

Elongation to break is a term that refers to the maximum amount one can stretch the material before it breaks. This property mirrors impact resistance in that it depends on flaws. Smaller, potentially lower aspect ratio variants perform better in this regard.

Effect on Thermal Properties

Mica is extremely effective for reducing CLTE, again because of its high aspect ratio (see separate page on modification of CLTE).

Mica has much higher thermal conductivity than polymer materials so it helps heat up and cool down the melt, allowing for increased throughput and productivity.

Specific heat capacity is another of the thermal properties and it is one that is much misunderstood. Even text books get the facts wrong in many cases. Apart from rare exceptions, minerals have the same ability to store heat as other solids do, meaning that a given volume of almost any solid takes the same amount of energy to heat up releases the same amount upon cooling. So, mica is neither beneficial nor detrimental.

Density

Adding mica increased the density of the material, however, it is so strongly reinforcing that much less material is required to reach the target strength or stiffness. Therefore, mica is a top pick for automotive plastics where it allows for lighter weight and thus higher fuel efficiency. Luxury brands use mica because of that and because it is effective for sound damping, especially in the middle of the audible frequency range.

Sound

As previously mentioned, mica is one of the most effective minerals for damping sound and vibration at intermediate frequencies. It has been shown to be more effective even than other platy minerals like vermiculite or clay. The reason is that mica plates with polymer in between act like a so-called “constrained layer” sound damper where the sound reflects between the plates and is absorbed by the polymer in between.

Usually the constrained layer requires two sheets of rigid material like metal with a softer polymer or elastomer in between. By using mica, one can achieve a similar effect using just one material, thus reducing weight and cost. Here is a comparison showing how mica outperforms for sound damping. The higher the tan delta value, the more sound is absorbed (40 % filler by weight and 200Hz frequency).

Optical

Muscovite is lighter in color and is therefore the mica type of choice for most paints and coatings. Phlogopite can give higher mechanical performance and is used where its tan color is acceptable, for example in black automotive compounds used under the hood.

Abrasion

Mica is a soft mineral that cannot scratch or abrade metal. Much of the USA mined mica is contaminated with abrasive impurities, so in the US it has made a bad impression with some compounders who mistakenly blame mica when in fact, they just tried the wrong kind. The Kish Company subsidiary Arctic Minerals specialize in high purity, high aspect ratio phlogopite mica that provides an opportunity for wider adoption of this outstanding reinforcement in the North American market.

To test for abrasive behavior, the Einlehner test is useful whereby a slurry of e.g. mica in water is passed through a brass mesh and the loss in weight of the mesh recorded. Similarly, one can extrude filled polymer through a die of softer metal like brass and record the weight lost due to abrasion. These methods can be used to reassure compounders that the mineral will not cause abrasion.

Surface Treatment

Silane coupling agents are very effective on mica and other silicate minerals. Application of the correct organosilane brings benefits to strength and HDT, especially when high temperature and or moisture is expected in the application. Often silane treatments are cost prohibitive but a new breakthrough process at Arctic Minerals allows them to be applied cost-effectively with very high quality and consistency. More details about dispersants and coupling agents can be found here in this free training video.