Aliphatic vs Aromatic Coatings
Aromatic and aliphatic describe the isocyanate structure in urethanes and polyureas. Aromatics are tough and economical but darken in sunlight; aliphatics hold color and gloss outdoors.
Aliphatic and aromatic are chemical terms that describe the backbone of the isocyanate used in polyurethane coatings and polyurea coatings. Aromatic isocyanates contain benzene rings; aliphatic isocyanates do not. The difference controls how a coating responds to sunlight, which is why the two types are often used together, with an aromatic film for bulk protection and an aliphatic film on top for color and gloss.
The chemistry in brief
Both types cure by the reaction of isocyanate groups with hydroxyl or amine groups (see isocyanate chemistry).
Aromatic isocyanates
The common aromatic isocyanates are MDI (methylene diphenyl diisocyanate) and TDI (toluene diisocyanate), along with their prepolymers and polymeric forms. Their ring structures absorb ultraviolet light. Absorbed UV energy drives oxidation reactions that form colored species, so the film yellows, darkens and eventually chalks at the surface.
Aliphatic isocyanates
The main aliphatic types are based on HDI (hexamethylene diisocyanate) and IPDI (isophorone diisocyanate), usually supplied as trimers, biurets or prepolymers to reduce vapor pressure. Without aromatic rings, these molecules absorb far less UV, and films keep their color and gloss much longer outdoors. Strictly speaking IPDI is cycloaliphatic, but it is grouped with the aliphatics for practical purposes.
How aromatic coatings change in sunlight
Darkening of an aromatic film is mainly a surface phenomenon. Typical observations:
- Light colors shift toward yellow, tan or brown within weeks to months of outdoor exposure.
- Gloss falls and the surface may develop a fine chalk (see chalking and fading).
- The film below the affected surface layer usually retains its mechanical properties for much longer.
For this reason, aromatic coatings are often perfectly acceptable where appearance does not matter or where the film is not exposed to UV: tank interiors, below-grade waterproofing, buried pipe, and the inside of secondary containment below a topcoat.
Comparison
| Attribute | Aromatic | Aliphatic |
|---|---|---|
| Isocyanate examples | MDI, TDI | HDI, IPDI |
| UV color stability | Poor; darkens and chalks | Good to excellent |
| Gloss retention | Low outdoors | High |
| Relative cost | Lower | Higher |
| Typical role | Basecoat, primer, membrane, lining | Topcoat, finish, color coat |
| Reactivity (general) | Faster | Slower, often catalyzed |
Specific mechanical and chemical-resistance values depend on the formulation and must be taken from the product data sheet.
Typical systems
Aromatic base with aliphatic topcoat
This is the most common arrangement for exterior elastomeric work. A thick aromatic polyurea or polyurethane membrane provides waterproofing, abrasion resistance and crack bridging, and a thinner aliphatic polyurethane or polyaspartic topcoat provides color and UV stability. Examples include roof membranes, bridge decks, parking decks and secondary containment that sees sunlight.
Manufacturers document the approved pairing. ArmorThane’s data, for instance, notes that its aromatic ArmorShield polyurea darkens and chalks in sunlight and calls for a UV topcoat above the liquid line.
Standalone aliphatic
Some aliphatic polyureas and polyurethanes are designed to be used alone, without a separate topcoat. These are used where a single product must provide both protection and appearance, such as some roof coatings and decorative work.
Industrial maintenance systems
On steel, a common three-coat system is zinc-rich primer, epoxy intermediate and aliphatic polyurethane finish. Epoxies chalk in sunlight for related reasons, so the aliphatic urethane protects them (see anatomy of a coating system).
Related polymers
- Polyaspartic coatings are aliphatic polyureas made with secondary amines. They are popular as fast-return-to-service floor and topcoat products.
- Polysiloxane coatings offer an alternative weathering topcoat with different chemistry.
- Fluoropolymer coatings are used where very long color retention is needed.
Choosing between them
Questions that usually decide the choice:
- Is the surface exposed to sunlight or strong artificial UV? If not, an aromatic film is often adequate.
- Does appearance matter? Color-critical work generally calls for aliphatic chemistry at the surface.
- What is the substrate and service? Immersion and chemical service may favor specific aromatic products that have test data for the exposure.
- What does the manufacturer approve? Only use topcoat and basecoat pairings that the manufacturer lists. Recoat windows and primers matter for intercoat adhesion.
Common misunderstandings
“Darkening means the coating has failed”
Not necessarily. Color change in an aromatic film is expected and often cosmetic. Whether it matters depends on the application. Mechanical damage, cracking or loss of adhesion are separate issues.
“Aliphatic is always better”
Aliphatics are better for UV stability. They are not automatically better for chemical resistance, toughness or cost, and some aromatic products outperform aliphatics in non-UV service.
“A clear aliphatic topcoat will hide yellowing below it”
A clear topcoat may slow further change by filtering some UV, but it will not reverse discoloration that has already occurred. Pigmented topcoats give better hiding and protection.
Key terms
- Aliphatic. Built from straight-chain or ring structures without benzene rings in the isocyanate or amine backbone. Aliphatic urethanes and polyureas hold color and gloss in sunlight.
- Aromatic. Containing benzene rings, as in MDI and TDI. Aromatic systems are often tougher per unit cost but yellow and chalk under UV.
- HDI and IPDI. Common aliphatic isocyanates used in light-stable topcoats.
- MDI. The most common aromatic isocyanate in polyurea and spray foam.
- Chalking. A powdery surface layer left as binder breaks down in UV (see chalking and fading).
Frequently asked questions
Do aromatic polyureas lose strength in sunlight?
Surface degradation is gradual and mostly affects appearance first. Long unprotected exposure can erode the surface layer, which is why exposed aromatic films are usually topcoated.
Can I topcoat an aromatic film that has already darkened?
Usually yes, after cleaning and any abrading or priming required by the topcoat manufacturer, and within or outside the recoat window as the data sheet directs.
Are aliphatic isocyanates safer to handle?
Both types are sensitizers and require proper controls. See isocyanate safety.
Related entries
- Isocyanate chemistry
- Polyurethane coatings
- Polyaspartic coatings
- Chalking and fading
- Hybrid polyurea coatings (new)
- RV roof coatings (new)
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
