NBR vs FKM vs PU: Main U-Seal Materials Comparison

Engineers faced with the choice among NBR, FKM, and PU for a U‑seal application often ask: “Which one is best?” The truthful answer is: it depends. This article provides a three‑dimensional comparison—chemical, physical, economic—and concludes with a rapid‑decision tree.

1. Chemical Resistance Comparison

Fluid NBR PU FKM

Mineral hydraulic oil ★★★ ★★★ ★★★

Water‑glycol ★ ★★ ★★★

Phosphate ester ✗ ✗ ★★★

Gasoline / diesel ★★ ★★ ★★★

Hot water / steam ✗ ✗ ★★

Ozone / UV ★ ★★ ★★★

Interpretation: PU outperforms NBR in water‑glycol but is inferior to FKM. The search term “PU vs TPU U‑type” (520 monthly, USA) reminds us: TPU is a thermoplastic polyurethane with slightly higher temperature capability but marginally lower abrasion resistance.

2. Physical Properties Comparison

Property NBR PU FKM

Hardness range (Shore A) 40–95 70–98 60–95

Tensile strength (MPa) 10–25 30–55 10–20

Abrasion resistance Medium Excellent Good

Low‑temp limit (°C) –30/–55 –20/–50 –15/–45

High‑temp limit (°C) 100 80 200

Compression set Medium Good Excellent

Conclusion: PU is the mechanical properties king; FKM is the thermal/chemical king; NBR is the balanced all‑rounder.

3. Economics & Availability

· NBR: Lowest raw material cost; shared moulds; shortest lead time.

· PU: Raw material cost 1.5–2× that of NBR, but service life in abrasive conditions can be 3–5× longer—lower total cost.

· FKM: Raw material cost 5–8× that of NBR; specify only when temperature or aggressive fluids mandate it.

Search Validation:

India’s 617.7K monthly searches for “PU” dwarf those for “FKM.” This confirms that in cost‑sensitive markets with moderate operating conditions, PU is the preferred NBR replacement.

Decision Tree:

1. Temperature >120 °C? → FKM

2. Temperature < –30 °C? → Low‑temp NBR

3. Water or fire‑resistant fluid? → FKM (first choice) or PU

4. Severe wear, high pressure, abrasive particles? → PU

5. None of the above? → NBR