Whether polyester contains metal is complex, reflecting its manufacturing process and functional demands. Pure polyester (PET) is a synthetic organic polymer that is chemically metal-free. However, the industrial process relies on metal compounds, which leave behind trace residues. Furthermore, specialized modern fabrics intentionally incorporate metallic elements for advanced properties.
The Chemical Composition of Polyester
Polyester is a synthetic fiber derived from petroleum, built primarily from carbon, hydrogen, and oxygen atoms. The most common form used in textiles is polyethylene terephthalate, or PET, the same polymer found in plastic bottles. This polymer is formed by a long chain of repeating molecular units called esters.
The chemical reaction that produces PET involves combining two organic compounds: ethylene glycol and terephthalic acid. These molecules link together in a process called polymerization, forming the long, stable polymer chains that make up the polyester fiber. Since the bulk of the final fiber is composed of these carbon-based units, the material is inherently non-metallic.
The Role of Metal Catalysts in Manufacturing
While the final polyester chain is metal-free, the reaction that forms it requires the help of metallic compounds known as catalysts. Catalysts speed up a chemical reaction without being consumed, making industrial production economically viable. In the case of PET, the polycondensation reaction that forms the long polymer chains is too slow without this acceleration.
For decades, the standard compound used as a catalyst is antimony trioxide, which contains the metalloid antimony. Antimony compounds are used in the vast majority of virgin PET production worldwide due to their high activity and low cost. The catalyst compound does not completely exit the polymer matrix; its residue becomes chemically embedded and trapped within the fiber structure. Alternative catalysts, such as compounds of titanium or germanium, are sometimes used but are not as common as antimony.
Health and Environmental Safety of Trace Residues
The presence of embedded antimony residue, typically measured in parts per million (ppm), raises questions about the safety of the final textile product. The consensus is that the antimony is largely bound within the solid polymer structure, making it non-bioavailable under normal wearing conditions. However, the residue can migrate, or leach, under specific circumstances, such as high heat or exposure to certain liquids.
Testing standards regulate this trace metal content for consumer safety. The OEKO-TEX Standard 100 certification, for example, limits the amount of antimony that can be extracted from a textile sample to less than 30 micrograms per gram of fabric. Studies using artificial sweat solutions show that only a very small fraction, often less than two percent of the total antimony present, is mobilized from the fabric under conditions simulating human perspiration.
Specialized Polyesters with Intentional Metallic Additives
Separate from manufacturing residues, some modern polyester fabrics contain intentionally added metal for specific performance functions. These are deliberate design choices that result in a significantly higher metal content than the trace catalyst residue. The most common functional application is the introduction of silver or copper nanoparticles.
These metallic particles are embedded or coated onto the polyester fibers to impart antimicrobial properties, preventing the growth of odor-causing bacteria on sportswear and medical textiles. Another common use is in anti-static and conductive fabrics, where metal-plated fibers or fine stainless steel wires are blended with the yarn. These conductive elements, which can include silver, nickel, or carbon materials, dissipate static electricity or provide electromagnetic shielding.