Role of fillers in polymer composites?

• A. They add weight
• B. They function as catalysts
• C. They modify stiffness, thermal expansion, and sometimes toughness
• D. They have no effect

Answer: (C)

Fillers in polymer composites are used to tailor how the material behaves under load and temperature, not to drive chemical reactions. They stiffen the composite by sharing the load with the polymer matrix—high‑modulus fillers and good interfacial bonding raise the overall stiffness. They also change how the material expands with heat: if the filler has a different, often lower, coefficient of thermal expansion, the composite’s overall expansion moves toward that of the filler as its content increases, reducing dimensional changes with temperature. Toughness is influenced too, though in a nuanced way: some fillers promote energy‑absorbing mechanisms like crack deflection or particle pull‑out and can improve toughness at certain loadings, while too much filler or poor bonding can create stress concentrations and make the material more brittle.

The other statements don’t fit as the main role. Fillers are not catalysts for polymerization, and while they add weight, the key point described here is how they modify stiffness, thermal expansion, and toughness. Saying they have no effect would be incorrect.