Updated 2026-09-09 · 4 min read
Liquid cooling fluids: PG25, oils and the PFAS problem
Key takeaways
Direct-to-chip systems overwhelmingly use PG25 — 75% water / 25% propylene glycol — which is cheap, non-flammable (flash point ~109°C), non-hazardous to ship, and contains no PFAS. Fluorinated fluids (the Novec-style chemistries used in two-phase immersion) are PFAS "forever chemicals" facing an EU restriction and US reporting rules — and 3M, the leading producer, exited the business at the end of 2025. For buyers this means: prefer water-glycol systems for long-term regulatory safety, and check any fluid plan for local disposal rules and material compatibility (galvanic corrosion in mixed-metal loops).
The three fluid families
| Fluid | Used in | PFAS? | Notes |
|---|---|---|---|
| PG25 (water + propylene glycol) | Direct-to-chip cold plates | No | Industry default; non-hazardous transport; needs corrosion/biological inhibition and periodic checks |
| Hydrocarbon / silicone oils | Single-phase immersion | No | Not PFAS, but heavy, messy, flammability classes vary, and handling drives OPEX |
| Fluorinated fluids (HFE/PFPE) | Two-phase immersion, some single-phase | Yes | EU restriction in progress; US TSCA reporting; 3M exited end-2025 — supply and regulatory risk |
Why PFAS matters to a cooling buyer
Per- and polyfluoroalkyl substances (PFAS) are persistent environmental chemicals under active restriction: the EU's REACH restriction proposal would dramatically limit their use, and US EPA TSCA rules impose reporting. A cooling system built around a restricted fluid risks stranded chemistry — no refill supply, forced retrofits, disposal liabilities. Water-glycol loops avoid this entire class of risk, which is one reason direct-to-chip with PG25 became the mainstream architecture.
Maintaining a PG25 loop
- Concentration checks (refractometer) and top-ups on schedule.
- Inhibitor packages matter: mixed-metal loops (copper + aluminum) corrode without them.
- Biological growth is possible in poorly maintained loops — schedule sampling.
- Disposal is ordinary industrial wastewater in most jurisdictions — but verify locally.
Frequently asked questions
Is liquid cooling fluid toxic?
The dominant direct-to-chip fluid, PG25 (75% water / 25% propylene glycol), is low-toxicity and non-hazardous to transport — propylene glycol is the same base used in food-grade antifreeze. Fluorinated immersion fluids are a different matter: they are PFAS chemicals under active regulatory restriction. Oils are generally low-toxicity but messy to handle.
Why did 3M stop making liquid cooling fluids?
3M announced it would exit per- and polyfluoroalkyl substance (PFAS) manufacturing by the end of 2025, including the Novec-line fluorinated fluids used in two-phase immersion cooling. The exit follows PFAS litigation and global regulatory pressure (EU restriction proposal, US reporting rules), and it materially weakened the supply outlook for fluorinated-fluid immersion systems.
What is PG25 coolant?
PG25 is a mixture of 75% water and 25% propylene glycol, the standard thermal fluid for direct-to-chip liquid cooling. It is cheap, has a high flash point (about 109°C), ships as non-hazardous cargo, and contains no PFAS. It requires corrosion inhibitors and periodic concentration/microbial checks.