A lower pour point does not, by itself, establish whether a hydrogenated terphenyl heat transfer fluid can be pumped during a cold start. Selection requires a specific grade to meet both cold-state circulation requirements and hot-state temperature limits. Pour point, pumpability temperature and viscosity curves describe different conditions.
1. Confirm the grade and document version
“Hydrogenated terphenyl” is a classification and search term, not a uniform sales specification. Identify the exact grade and review its TDS and SDS. If documents use wording such as “modified terphenyl,” obtain the supplier's explanation rather than assuming identical formulations from terminology alone.
The March 2026 Therminol 66 TDS describes the fluid as “Modified terphenyl.”[1] A historical US SDS bearing Eastman's name—document 150000093438, version 2.6, revised September 4, 2020—lists terphenyl, hydrogenated (CAS 61788-32-7) at 74–87% by weight, alongside other components, in Section 3 on page 1.[2]
This supports a category association for that historical version. It does not describe a pure substance or confirm that the composition remained unchanged in 2026. The cited SDS is a republished historical copy, not a confirmed current applicable SDS. Obtain the current document from the manufacturer. All properties below come only from the March 2026 TDS; the newer TDS and older SDS must not be combined into a current supply specification.
2. Pour point is not pumpability temperature
The Therminol 66 literature lists the following typical values:[1]
Pour point: −32°C;
Pumpability temperature at a viscosity criterion of 300 mm²/s: 11°C;
Pumpability temperature at a viscosity criterion of 2,000 mm²/s: −3°C.
These values are not contradictory: their conditions differ. Using −32°C alone to approve a winter start overlooks allowable pump viscosity and piping resistance. A manufacturer's pumpability temperature does not mean its viscosity criterion is suitable for every pump in a project.
Submit the minimum actual fluid temperature, pump documentation, piping conditions and drive information for design review. This article specifies no startup temperature and does not treat pour point as permission to start equipment.
3. Compare viscosity across the relevant temperature range
The same Therminol 66 liquid-property table lists kinematic viscosity as 29.6 mm²/s at 40°C, 3.77 mm²/s at 100°C and 1,300 mm²/s at 0°C.[1] A value at a conventional test temperature cannot describe the entire cold-state behavior.
Compare candidate products at the same temperature, with consistent property definitions and comparable test methods. Dynamic viscosity and kinematic viscosity are different quantities: mPa·s and mm²/s cannot be ranked directly against each other. Obtain data covering both cold starts and hot operation. Missing curves require supplier clarification, not unsupported extrapolation.
4. Check hot-state limits separately
The Therminol 66 literature gives a maximum bulk temperature of 345°C and maximum film temperature of 375°C.[1] Both require project assessment. They are not interchangeable and are not supply-temperature settings for a particular installation.
The objective is to meet cold-state pumpability and hot-state requirements together, rather than rank grades by maximum temperature alone. Every numerical example in this article belongs to the cited Therminol 66 literature; it does not establish the performance of other hydrogenated terphenyl fluids or our products.
5. Preserve vapor-pressure and flash-point conditions
The manufacturer positions Therminol 66 for nonpressurized or low-pressure indirect heating applications and provides vapor-pressure data by temperature.[1] This does not mean a circulating system has no pressure or that equipment pressure checks can be omitted.
Flash points must also be compared using comparable methods. The table lists 184°C by ASTM D92 open cup and 170°C by ASTM D93 closed cup.[1] Combining results from different methods in a single ranking is misleading. The manufacturer also states that the product is not a fire-resistant heat transfer fluid: a higher flash point does not mean nonflammable.
6. Record conditions, not just a ranking
For each candidate grade, record product identity and document versions, viscosity at the minimum fluid temperature, the pumpability criterion, bulk and film limits, vapor pressure, material and seal compatibility, and the distinction between typical values and sales specifications.
A useful outcome identifies the conditions supported by evidence and the information still missing. If considering one of our candidate fluids, obtain its own current TDS/SDS and a response to the actual operating conditions. These examples do not establish equivalence, mixing compatibility or replacement suitability.
Conclusion
Pour point is not a measure of complete system pumpability. Verify grade identity and document versions, then align minimum fluid temperature, viscosity criteria and hot-state limits to build a defensible selection assessment.
References
[1] Eastman, Therminol 66 heat transfer fluid, SFE-HTF-19853, March 2026 edition. PDF page 2: applications and safety; page 3: Typical properties; page 4: SI liquid-property table and footnotes. All numbers are grade-specific examples; typical data are not guaranteed for every batch and do not guarantee other products or authorize site operation.
[2] Eastman, Therminol 66 US SDS, document 150000093438, version 2.6, revised September 4, 2020, page 1, Section 3. Republished historical copy. Used only for historical category association; the current applicable SDS must be obtained from the manufacturer.
