Aromatic Binders: The Benzene Ring Weakness
Aromatic resins (often derived from MDI – Methylene Diphenyl Diisocyanate) are the “standard” or “budget” resins found in the UK market.
At a molecular level, Aromatic isocyanates contain Benzene rings. These rings are highly efficient at absorbing Ultraviolet (UV) light. When sunlight hits an aromatic binder, the energy from the UV rays is absorbed by these rings, causing a chemical transition.
1. The “Ambering” Effect (Chromophore Formation)
As the Benzene rings absorb UV light, they transform into chromophores. This is a visible chemical change that turns the clear resin a deep yellow or brownish hue. This is why “cheap” resin driveways look patchy or “burnt” where a car has been parked—the areas under the car were protected from UV, while the rest of the drive “ambered.”
2. Chain Scission and Embrittlement
More concerning than the color change is a process called Chain Scission. The UV energy literally “clips” the polymer chains. This breaks the cross-linking that gives the resin its strength.
The Result: The resin becomes brittle. What was once a flexible, monolithic surface becomes a hard, glass-like shell that cracks under the thermal expansion of the stones or the weight of a vehicle.
Aliphatic Binders: Stability by Design
Aliphatic binders (often based on HDI – Hexamethylene Diisocyanate) are engineered specifically for exterior use.
Unlike their Aromatic counterparts, Aliphatic molecular chains are linear. They do not contain the Benzene rings that act as “magnets” for UV radiation. Because the UV light is not absorbed by the chemical bond, it passes through or is reflected without triggering a molecular breakdown.
[Image showing resin driveway ambering vs uv stable resin]
Why Professionals Insist on Aliphatic
Color Neutrality: Because no chromophores are formed, the resin remains clear. This is vital for light-colored aggregates like Silver Grey or White Quartz.
Elasticity Retention: By avoiding chain scission, the binder maintains its flexural strength. It can expand and contract with the seasons without snapping.
Long-term Adhesion: The bond between the resin and the stone remains intact, preventing “ravelling” (where stones begin to pop out of the surface).
The Cost of the “Aromatic” Shortcut
Students of civil engineering often look at the Life Cycle Cost (LCC) of materials. While an Aromatic system may be 20–30% cheaper at the point of installation, its functional lifespan is often less than 5 years before significant aesthetic or structural repairs are required.
In contrast, an Aliphatic system—installed according to the comprehensive resin engineering guide—provides a service life exceeding two decades, making it the only viable choice for professional specifications.
Summary for the Specifier
When reviewing a technical data sheet (TDS) for a resin binder, look for the isocyanate base. If it mentions MDI, it is Aromatic and will degrade. If it mentions HDI or IPDI, it is Aliphatic and UV-stable.
