Aliphatic vs aromatic polyurethane is the choice that decides how a roof coating looks after a few summers. It has little to do with whether the roof stays watertight. Aromatic systems are tough, economical and fast, but they change colour in sunlight. Aliphatic systems keep their colour and gloss, which is why they are used as the top layer.
Most exposed liquid roofing systems use both: an aromatic membrane for waterproofing, and an aliphatic topcoat for UV stability. This article explains why, with values from Krypton Chemical technical data sheets (TDS).
The chemistry in one paragraph
Polyurethanes and polyureas are built from isocyanates. Aromatic isocyanates (such as MDI) contain a benzene ring. Under UV light, that ring structure forms coloured by-products, so the surface yellows or darkens. Aliphatic isocyanates have no aromatic ring. They are far more resistant to UV-driven colour change. The Wikipedia article on polyurethane gives more background on isocyanate types.
What the data sheets say about UV
The manufacturer's own wording is the clearest evidence:
| Product (TDS) | Chemistry | UV statement in TDS |
|---|---|---|
| Polyurea Rayston | Aromatic | "A colour change is to be expected under sunlight. This change does not affect its mechanical properties." |
| Impermax | Aromatic PU | "A colour change is expected… This discolouration does not affect its properties." |
| Impertrans | Aliphatic PU | "High retention of brightness, color and mechanical resistance when exposed to weather and UV radiation." |
| Colodur | Aliphatic PU | "Does not yellow on exposure to sunlight." |
| Impermax A | Aliphatic PU membrane | "Does not yellow if exposed to sunlight." |
So the aromatic layer keeps working when it discolours. The aliphatic layer is there for appearance, gloss retention, wear resistance and reflectance.
Aliphatic vs aromatic polyurethane topcoats: TDS values
| Property | Impertrans (colourless / pigmented) | Colodur (colourless / pigmented) | Impermax A |
|---|---|---|---|
| Hardness (ISO 868) | 65–70A / 85–90A | 53D / 60D | 80A |
| Elongation at break | 276% / 380% | 173% / 70% | above 250% |
| Tensile strength | 10.4 / 11.7 MPa | 27.4 / 15 MPa | above 6 MPa |
| Solar reflectance, white | 83.4% (ASTM E903) | 84.7% (ASTM E903) | 81.1% (ASTM E903) |
| Thermal emittance, white | — | 0.82 | 0.83 |
| SRI, white | 103 | 105 | 100 |
| Service temperature | -40 to 80 °C | -40 to 80 °C | up to 140 °C |
| Abrasion (Taber) | — | 11 mg (CS-10, 1 kg) | 43.5 mg (CS-17, 1000 cycles) |
A dash means the value is not stated in that TDS.
How to read this table
- Impertrans is the elastic topcoat. With 276–380% elongation, it follows a highly elastic membrane without cracking. The TDS lists it for waterproofing membranes "of high elasticity".
- Colodur is the hard topcoat. At 53–60D, with low Taber abrasion, it suits foot traffic and wear. With quartz sand broadcast, the TDS reports slip-resistance Class 3.
- Impermax A is an aliphatic membrane, not just a topcoat. Its TDS says it is normally applied as a second, exposed layer over an aromatic 1K polyurethane, which "would save a step" because no separate thin topcoat is needed.
Matching topcoat elongation to the membrane
A topcoat that is much stiffer than the membrane below can craze when the membrane stretches. That is a common cause of "cracked roof coating" complaints where the waterproofing underneath is still intact.
Practical pairing from the TDS:
- Very elastic polyurea such as Impermax Polyurea H Supreme (650% elongation): its TDS recommends "aliphatic finishes with good elasticity", such as Impertrans Pigmented or Impertrans Eco.
- Trafficked balconies and terraces: Colodur over Impermax, as shown on the roofs, balconies and terraces page.
- Two-layer liquid roof without a separate topcoat: aromatic Impermax, then Impermax A.
A waterborne, almost solvent-free aliphatic option also exists: Colodur Eco. Its TDS positions it mainly for heavily used floor coatings, including outdoor areas.
Cool roof finishes
White aliphatic finishes double as cool roof coatings. They reflect solar radiation and lower surface temperature. The relevant metrics are solar reflectance (ASTM E903), thermal emittance (ASTM C1371) and the Solar Reflectance Index (ASTM E1980). All three appear in the data sheets above.
An SRI of 100 corresponds to a standard white reference surface. Values above 100, such as Colodur's 105 and Impertrans' 103, indicate a cooler surface than that reference under the test conditions.
Independent product ratings are published by the Cool Roof Rating Council. Remember that dirt pick-up reduces reflectance over time, so the TDS cleaning and maintenance guidance matters.
See Krypton's cool roofs application page and the case note on the Impermax Aqua cool roof.
Application notes from the TDS
- Impertrans: 1–2 layers at 200–250 g/m², depending on the opacity required. Apply by brush, roller or airless.
- Pot life: colourless Impertrans and Colodur about 6 h; pigmented versions about 2 h (1 kg, 20 °C, 50% RH).
- Shelf life: colourless 12 months, pigmented 6 months.
- VOC: Impertrans 450 g/l (colourless) and 380 g/l (pigmented), below the 500 g/l limit of 2004/42/EC. Ventilate accordingly.
- Maintenance: reapply decorative layers when worn by traffic or weather, as the membrane TDSs recommend.
Key takeaways
- In aliphatic vs aromatic polyurethane, the difference is UV colour stability, not waterproofing.
- Aromatic membranes discolour without losing mechanical properties; an aliphatic topcoat protects appearance.
- Match topcoat elongation to the membrane: Impertrans (276–380%) for elastic membranes, Colodur (53–60D) for wear.
- White aliphatic finishes reach SRI 100–105 in the TDS, which supports cool roof designs.
FAQ
What is the difference between aliphatic and aromatic polyurethane?
Aromatic polyurethanes use isocyanates with a benzene ring and yellow under UV. Aliphatic polyurethanes do not have that ring and keep their colour and gloss outdoors.
Do aromatic polyurethane membranes need a topcoat?
For appearance, yes, if exposed to sunlight. Krypton's data sheets state the colour change does not affect mechanical properties, but they recommend an aliphatic finish such as Impertrans or Colodur.
Why is my polyurethane roof coating turning yellow?
It is almost certainly an aromatic layer exposed to UV. Overcoat it with an aliphatic topcoat after cleaning, following the TDS recoating guidance.
What is a good SRI for a cool roof coating?
Higher is cooler. The white aliphatic finishes here declare SRI 100–105 (ASTM E1980). Check local building-code thresholds and rated values.
Further reading
- Coatings and roof systems at Krypton Chemical Solutions, including protective systems.
- Product catalogue at kryptonchemical.com.
About the author
Krypton Chemical, S.L. Marketing Department, Nidal Malkawi
Krypton Chemical has manufactured construction chemicals in Spain since 1999. Technical resources: kryptonchemicalsolutions.com · Products: kryptonchemical.com
