Minerals: Properties that Rock
Minerals are used in many applications for plastics for a variety of uses. Simply put, minerals have properties that rock! This blog will discuss a small selection of these minerals, their uses, and what minerals are commonly used within the plastics industry.
Minerals, commonly called fillers, used in the plastic industry have been selected through the years based on their physical properties, both fillers themselves, and how they affect the physical properties of composites or masterbatches made from these mineral fillers. Mineral fillers can be largely divided into two classes: functional and non-functional fillers. Non-functional fillers are used to displace more-expensive resin and provide cost savings to customers.
It is important to note that even using “non-functional” fillers affects nearly every end-property of composites or masterbatches made from them: surface, color, density, shrinkage, expansion coefficient, conductivity, permeability, and mechanical and thermal properties. The presence and arrangement of fillers can affect even the intrinsic properties of the matrix, such as crystallinity or glass transition temperature. Important properties to consider when formulating with fillers include particle size and distribution, surface coating, surface structure, and size, hardness, and price.
Particle size, distribution, and surface structure will affect the reinforcing properties of the mineral filler. These can be understood via particle size distribution curves and aspect ratios of particles. An example of a particle size distribution is shown below in Figure A – the plotted line shows the percentage of particles smaller in diameter than the corresponding size on the x-axis.
Figure A: Example of Particle Size Distribution
Minerals used in the plastics industry and their uses can be classified mainly by their aspect ratio, the length to width of filler particles, which gives information about how much the shape deviates from a sphere. As the aspect ratio increases, particles go from spherical (CaCO3, silica, and nepheline syenite) to platy (talc and mica) to fibrous (glass, carbon, or nylon fibers), with the given examples being commonly used minerals in the polymer industry.
All these different types of minerals can have various surface coatings that will allow the polymer to adhere well to the surface. If an improper coating is used, then agglomeration is likely, and any beneficial property of including the filler could be compromised.
Though most who are in the know will associate minerals with their antiblock properties or use them as a resin filler, other properties can help manufacturers of plastic articles. Of note is the increased heat dissipation of filled systems that allows for faster cycle times for thermoforming or injection molding. Many minerals can dissipate heat orders of magnitude better than the polymer they are carried in. This means that residual heat from processing can be more readily taken away, reducing the cooling cycle time. In addition, these minerals will often have lower heat capacities than polymers, meaning they are more readily heated and cooled, further affecting cooling cycle time. This, combined with reduced material cost, can be a great productivity boon to producers.
Minerals are often used in low-warp formulations as well due to their low coefficients of thermal expansion. Suppose a mineral is improperly selected for inclusion in the formulation. In that case, the expansion difference due to heat can leave residual stress, but if properly selected, this can help reduce warpage as the mineral acts as an “anchor” for the polymer. This same effect can help reduce shrinkage for injection molded parts. Just these few examples show that mineral additives can have more uses than just displacing resin for plastics manufacturers, though cost reduction is always an attractive idea.