Antimicrobial and Odor-Control Chemistry

First the problem: why does polyester hold odor so stubbornly?

Sweat itself is essentially odorless. Odor appears when skin bacteria — chiefly Corynebacterium and Staphylococcus species — break down the fatty acids, branched-chain amino acids and thioalcohol precursors in apocrine sweat. Leucine, for example, is cleaved into isovaleric acid (sharp, cheesy note); S. hominis produces onion-like thioalcohols such as 3-methyl-3-sulfanylhexan-1-ol (3M3SH).

Fiber chemistry is decisive here. Cotton and viscose are hydrophilic — they absorb water (and water-soluble odorants) and release them in the wash. Polyester is hydrophobic and oleophilic: it repels water but attracts and tightly holds sebum and oil-based odor molecules. That is why synthetic sportswear tends to stay malodorous longer after the same wear. An antimicrobial finish does not mask odor; it targets the microbial activity that produces it.

Two core strategies: leaching and bound

Antimicrobial technologies split into two families by mode of action. Release/leaching chemistries slowly hand off active substance from the fiber into the surroundings; they give strong, broad initial efficacy and act on nearby bacteria, but deplete over time and through laundering. Bound (non-leaching/contact-kill) chemistries are covalently fixed to the fiber, release nothing, and kill only the microorganisms that touch them; they are more durable but can lose performance as the surface abrades.

This distinction also dictates the test method: diffusion (zone) tests are only meaningful for leaching chemistries, whereas bound chemistries must be measured by a contact/absorption-based quantitative method.

Mechanisms: silver, zinc, quaternary ammonium and bio-based

Silver (Ag+ ion): In the presence of moisture, Ag+ ions released from a silver salt or nanoparticle bind to the bacterial surface, react with thiol (–SH) groups to denature proteins, condense DNA and disrupt the cell membrane. It is a broad-spectrum, effective leaching system; in the industry, for example, silver-chloride-based technologies (such as Polygiene, made from recycled silver) use this principle. The concern is silver release during laundering and aquatic toxicity; some reports cite release on the order of a few ppb per gram after the first wash, though values vary with the chemistry and the method.

Zinc pyrithione: A mainly bacteriostatic/fungistatic active familiar from anti-dandruff shampoos; in textiles it targets fungi and odor-causing bacteria. Silane-bound quaternary ammonium (Si-QAC): the positively charged ammonium head attaches to the negatively charged cell membrane while the hydrophobic tail physically pierces it — a true contact-kill mechanism. The classic example is 3-(trimethoxysilyl)propyldimethyloctadecylammonium chloride (e.g. the 'AEM 5700' type); the silane end covalently bonds to the fiber during drying, forming a durable, non-releasing surface on cotton, nylon and polyester.

Other chemistries: PHMB (polyhexamethylene biguanide) disrupts membrane integrity; N-halamines kill broad-spectrum via oxidative active chlorine (N–Cl) and can be recharged with bleach (but degrade under UV). On the bio-based side chitosan stands out: protonated amine groups attach to the microbial surface, raising cell permeability and leaking intracellular contents. Triclosan was historically common but has been abandoned in textiles in several markets, including the EU, because of regulatory concerns and because it can break down into dioxin-related compounds.

Test methods: qualitative screen or quantitative proof?

The validity of an antibacterial claim depends on the method. Qualitative methods are for rapid screening: AATCC 147 (Parallel Streak) and ISO 20645 (agar diffusion plate) look for a zone of inhibition around the specimen — meaningful only for diffusible (leaching) actives, and not proof that the organism died. Quantitative methods are the real evidence: AATCC 100 reports percent reduction; ISO 20743 and JIS L 1902 compute log reduction (antibacterial activity value) via absorption/transfer methods; ASTM E2149 (dynamic shake-flask) is used for bound/non-releasing actives.

Threshold values: in ISO 20743 incubation is typically 18–24 hours; if the difference between control and specimen is ≥2.0 log the effect is deemed 'significant/effective', and ≥3.0 log is 'strong' (roughly 99% and 99.9% reduction). A mature specification therefore demands not the initial value but the log reduction retained after a defined number of washes (e.g. 20–50 launderings).

Measuring odor directly: ISO 17299

Antibacterial effect and perceived odor reduction are not the same thing, so a separate deodorant measurement is needed. The ISO 17299 family does exactly that: the specimen is sealed in a container holding an odorant at a controlled concentration, the remaining odor gas concentration is measured after a set time, and a reduction rate is computed against the control. Part 2 uses a detector-tube method for ammonia, acetic acid, methyl mercaptan and hydrogen sulfide; Part 3 uses gas chromatography to evaluate real body-odor molecules such as indole, isovaleric acid and nonenal.

Regulation and responsible chemistry

A fabric containing an antimicrobial substance counts as a 'treated article' under the EU Biocidal Products Regulation (BPR 528/2012): the active substance must be approved for the relevant use and the required declarations must be made. ZDHC MRSL further requires biocides to be used only in authorized application areas and below the approved concentration limit. For responsible sourcing, systems such as bluesign and OEKO-TEX independently verify the human and environmental safety of the chemistry used. The engineering decision is this: choose not the strongest initial kill, but a sustainable, measurable and proven effect over the intended service life, using approved chemistry.