Durable Water Repellency (DWR): PFAS-Free Chemistry and Durability
Water repellency is a surface-energy game PFAS-free chemistry can win — but oil repellency is still fluorine's territory.
Durable water repellency (DWR) does not make a fabric waterproof; it coats the outer surface of the fibre with a layer of such low energy that water beads up and rolls off instead of spreading and soaking in. This is why DWR and waterproofness must not be conflated: DWR delays surface wetting, while waterproofness blocks water from passing through the pores under pressure. They are different physics, measured on different test planes.
The physics: surface energy, contact angle and Cassie-Baxter
Whether a droplet spreads is decided by the energy balance between the liquid and the solid. Water has a surface tension of ~72 mN/m; as the solid's critical surface tension drops below that, water cannot spread and the contact angle rises above 90°. DWR chemistry does exactly this — it lowers the surface energy. A flat film gives roughly 90-120°, but once the micro-roughness of the fibre and the knit/woven structure enters the picture, the system shifts into the Cassie-Baxter regime: the droplet sits on fibre peaks over trapped air pockets, the true solid contact area shrinks, and the apparent contact angle climbs to 140-160° and beyond. In the lab, an octylsilane-modified amino-functional silicone finish has been reported to give static contact angles of ~145° on polyester and ~130° on cotton.
Why oil does not follow water: fluorine's privilege
This is where PFAS-free chemistry hits its fundamental ceiling. Common oils have surface tensions of only 18-33 mN/m; for a surface to repel oil as well, its critical surface tension must fall below even that, down to a few mN/m. A close-packed array of -CF3 groups delivers the lowest known solid surface tension at ~6 mN/m, whereas -CH3 groups bottom out at only 22-24 mN/m. So while hydrocarbon and silicone chemistries comfortably defeat water (72 mN/m), oil repellency (oleophobicity) is in practice a zone reachable only with fluorine. That single fact explains why a PFAS-free fabric stays weak against oils, sunscreen and skin oils.
C8 → C6 → C0: the shrinking chain
The 'C' number denotes the carbon count of the fluorocarbon side chain. C8 (perfluorooctane derivatives), the industry standard for years, delivered the highest performance but was bioaccumulative and persistent. C6 chemistry arrived as a shorter-chain interim solution; it largely preserved oleophobicity but is still a PFAS and is being phased out by both regulation and industry programmes (for example, bluesign retires C6 in January 2026). C0 (zero-fluorine / fluorine-free) is the new generation that abandons the fluorocarbon chain entirely — this is the commercial face of the term PFAS-free.
PFAS-free chemistry families
Fluorine-free DWRs are not a single substance but several chemistry families, each with distinct strengths and weaknesses.
- Dendrimer / hyperbranched polymers: 'star-like' structures whose multifunctional branches self-organise and co-crystallise into an ordered, dense array of hydrophobic side chains. They give near-C6 water repellency on polyester with relatively good wash durability; associated in the market with examples such as Bionic-Finish Eco.
- Silicone (polydimethylsiloxane) based: high dry-cleaning durability and a soft hand; needs catalyst/heat to crosslink and can be slightly weaker than hydrocarbons under dynamic wetting.
- Paraffin / plant waxes: excellent initial beading, but poor durability without crosslinking — they fade quickly with washing.
- Polyurethane / polyacrylate (including dendritic PU): high water repellency and wash durability; acrylates are cheaper but tire faster through hydrolysis. PU options with high bio-based content (70%+) exist.
Application and durability: the role of heat
DWR is usually applied by pad (immersion), squeezed, dried and cured on a stenter; depending on the resin, curing typically sits in the ~150-170°C band (in one example silicone recipe, 135°C drying plus a 170°C cure). Durability is tested against two enemies: washing and abrasion. Washing removes external contaminants and loosened molecules; abrasion physically wears the surface layer. A critical mechanism is heat 're-activation' — repellent chains that lie flat with use stand back upright with a warm tumble or iron, and beading returns. In durability testing, finishes are run through ISO 105-C10-type wash cycles; a good PFAS-free silicone finish can hold its full score (ISO-5) on polyester after 5 washes, dropping one step on cotton.
How performance is measured: tests and standards
The primary way to measure water repellency is the spray test (AATCC TM22 / ISO 4920): 250 mL of distilled water is discharged over roughly 25-30 seconds through a standard spray nozzle onto fabric tensioned on a 150 mm hoop tilted at 45°, the excess is tapped off, and wetting is graded on a visual scale. AATCC's 0-100 score converts to ISO's 0-5 grade (score 100 = grade 5, 'lotus effect'; score 90 = grade 4; score 80 = grade 3). Water passage under pressure is a separate test — hydrostatic head per ISO 811 — and is the indicator of waterproofness, not repellency. Oil repellency is graded 0-8 with AATCC TM118 (grade 0 being the lowest), using a series of hydrocarbon droplets of increasing surface tension; PFAS-free finishes score low here, which is an expected result.
| Chemistry family | Water repellency | Oil repellency | Wash/abrasion durability | Typical note |
|---|---|---|---|---|
| Dendrimer (hyperbranched) | High (near C6) | None/very low | Relatively good | Self-organising side chains; most common PFAS-free option |
| Silicone (PDMS) | High | None | Good (esp. dry-clean) | Soft hand; needs crosslink + heat cure |
| Paraffin / plant wax | Very high initially | None | Low (no crosslink) | Cheap; fades fast with washing |
| Polyurethane / acrylate | High | None/limited | PU good · acrylate moderate | Bio-based PU content can exceed 70% |
| Reference: C6 (fluorinated, PFAS) | High | Moderate-high | Good | Provides oleophobicity but subject to restriction |
Regulation: PFAS-free is no longer a choice but a timeline
The transition is now not only ethical but a legal obligation. On the EU side, the REACH restriction on PFHxA and related substances enters into force in stages; for consumer textiles, clothing and accessories the ban applies from 10 October 2026 (the April 2026 date concerns firefighting foams). At national level, France restricts PFAS in consumer textiles from January 2026, and Denmark restricts clothing and footwear containing fluorine above 50 mg F/kg from 1 July 2026. In the US, California AB 1817 bans intentionally added PFAS and total organic fluorine (TOF) content at or above 100 ppm from 1 January 2025, with that threshold stepping down to 50 ppm on 1 January 2027. On the industry side, bluesign applies a full PFAS restriction in approved materials from January 2026. The practical upshot: where oleophobicity is not critical (garment outer shells, everyday water repellency), PFAS-free chemistry is today both compliant and mature enough.