What are the differences in thermal expansion between filled and unfilled plastics?

The thermal expansion behavior of plastics significantly differs between filled and unfilled variants, primarily due to the presence of additive materials that alter the polymer's fundamental properties. Unfilled plastics, composed solely of the base polymer resin, typically exhibit relatively high coefficients of thermal expansion (CTE). This occurs because polymer chains have greater freedom to move and expand when heated. Common unfilled thermoplastics like polyethylene or polypropylene can have CTE values ranging from 100-200 μm/m·°C, making them dimensionally unstable under temperature fluctuations.

In contrast, filled plastics incorporate reinforcing materials—such as glass fibers, mineral fillers, or carbon fibers—which drastically reduce the composite's CTE. These fillers, often having much lower intrinsic thermal expansion coefficients (e.g., 5-10 μm/m·°C for glass), constrain the movement of polymer chains. The resulting CTE of a filled plastic can be 50-70% lower than its unfilled counterpart. A 30% glass-filled nylon, for instance, might display a CTE of 20-40 μm/m·°C, much closer to metals like aluminum.

The reduction in CTE depends on filler type, content, aspect ratio, and orientation. High-aspect-ratio fillers like fibers create more restraint through mechanical interlocking, while particle fillers provide less reduction. Additionally, well-distributed fillers ensure homogeneous expansion, whereas poor dispersion can cause anisotropic behavior. This controlled thermal expansion makes filled plastics essential for applications requiring tight tolerances across temperature ranges, such as automotive components and electronic housings, where they minimize thermal stress and improve dimensional stability.