INTRODUCTION TO COMPOSITES
One key advantage of polymers is the ability to easily add fillers to the base Polymer (matrix) to significantly improve performance, reduce cost, or achieve a combination of both. These fillers can be combined to tailor a specific Polymer Matrix Composite or PMC for short. A composite is defined as the macroscopic combination of two or more distinct materials, having a recognizable interface between them. Composites can be created to drastically alter mechanical, electrical, thermal, tribological, and environmental properties of the base polymer.
COMPOSITES FOR IMPROVED MECHANICAL PROPERTIES
There are four main types of polymer reinforcements used to improve mechanical properties. These are particle, discontinuous fiber, continuous fiber, and woven fabric reinforcements. In order to provide a useful increase in properties, there must be a significant volume fraction of reinforcement used, typically 10% or more.
Particle reinforcements are classified as a discontinuous reinforcement. In order to be considered a particle, the reinforcement will have dimensions that are roughly equal in all directions (Isotropic). Particles include shapes such as spheres, flakes and granules. Particle reinforcements are used to improve mechanical properties, and for cost reduction as the particle may be less costly then the base polymer.
Particle reinforcements are generally the lowest cost reinforcement, but they also have the smallest improvement in mechanical properties. Some common particle reinforcements include:
Particle reinforcements can also have secondary benefits such as decreased warp or enhanced aesthetic appeal. In the case of Mica, it is often used paints and 3D printer filaments to provide a "metallic sparkle" appearance.
DISCONTINUOUS FIBER REINFORCEMENTS
Discontinuous fiber reinforcements are commonly referred to as Short Fiber Reinforcements. The primary function of a fiber reinforcement is to increase the strength and stiffness of the matrix material. In order to be considered a fiber, the reinforcement must have a high aspect ratio. The aspect ratio is defined as the fiber length divided by the fiber diameter. Typical aspect ratios for commonly used fiber reinforcements are ~50-200. In general, fibers with higher aspect ratios translate into superior mechanical properties over an identical fiber with a lower aspect ratio.
Short fibers are generally "milled" (1/32" - 1/4") or "chopped" (1/8" to 1/2"+) length from a continuous roving with chopped fibers having a higher aspect ratio. For PMC's there are three (3) main fiber reinforcements used. Glass, carbon and aramid fibers represent top three.
Glass fibers are amorphous and primarily used to reinforce thermoplastic and thermoset PMC's. As such, glass fibers are versatile and are available in virtually unlimited supply. There are two main categories of glass fibers which are low-cost general purpose fibers an special purpose fibers. For the scope of this discussion we will focus on general purpose glass fibers known as E-glass which represent a large portion of glass fibers used in polymers. S-glass is also used due to it's lower density and higher stiffness when compared to E-glass and is a less expensive then carbon fiber.
Boron free E-glass is widely used in the injection molding industry as the glass is dry blended and extrusion compounded in a preliminary step before injection molding. The compounded pellets (now containing E-glass fibers) can be feed directly into the injection molding machine. For 3D printer filament the compounded extrusion is simply wound on a spool instead of being pelletized.
Glass fibers reinforced polymers tend to achieve higher performance when combined with a particle reinforcement. One particular example is the combination of glass fiber with Mica. The glass fiber provides the bulk of the stiffness improvement while the Mica enhances dimensional stability and reduces thermal expansion & warp. It's is common to use glass/mica in a 2:1 or 1:2 ratio depending on the matrix and application. Below is a SEM image of typical glass fibers.