Kevlar®: cross-linking strengthens polymers

Cross-linking strengthens polymers by restricting the relative movement between polymer chains. Examples of cross-linked polymers include cross-linked low- and high-density polyethylene (XLPE and PEX), epoxy adhesives (polyacrylate), and Kevlar®.

Kevlar® is a condensation polymer formed by condensation polymerization of two monomers containing benzene rings. The result is long chains of linked benzene rings. The chains of Kevlar® cross-link with hydrogen bonds provide a tensile strength ten times greater than steel, in addition to high toughness and thermal and chemical stability. Despite this, it is still lightweight and thus useful for aerospace engineering, lightweight body armor, car breaks, and boats.

Figure 1: Illustration of a bulletproof vest made of Kevlar® and its structure.