Evidence grade: A1 — First-hand testing (TDS/spec sheets, ASTM/ISO, lab tests)
Answer: Because Shore hardness and impact toughness measure two different things. A D85 epoxy can be excellent at resisting a dent and poor at absorbing a blow — West System's own datasheet shows two hardeners with nearly identical Shore D values differing by more than 2× in Izod impact energy [1].
Effort: N/A — debunk/reference, no procedure
The mechanism
Hardness asks: how hard is it to push a small indentor into the surface? Shore D (per BS EN ISO 868, the method named in the datasheet below) answers that, and only that. Impact strength asks a different question: how much energy does the material absorb before it fractures? The notched Izod test (ASTM D-256) answers that one. A material can score high on the first and low on the second. That is exactly what "hard but brittle" means.
The chemistry behind it: epoxy gains hardness from dense crosslinking — a tight three-dimensional mesh of polymer chains. That tight mesh resists indentation beautifully. But it also gives cracks a straight, easy path to run through. More crosslinking, same chemistry family, often means harder and more chip-prone at once.
The proof, from one datasheet
West System's physical-properties sheet lists six of its own systems side by side. Compare two:
| System | Shore D (14 days) | Izod impact, J/m (ASTM D-256) |
|---|---|---|
| 105 resin + 206 slow hardener | 83 | 28.84 |
| 105 resin + 207 special hardener | 82 | 67.82 |
One Shore-D point of difference — and the "softer" one absorbs 2.3× the impact energy. Drop something on the 206-cured repair and the 207-cured repair, and the harder-feeling one is the one that chips first [1].
The same table shows the other end of the spectrum: G/flex at Shore D 75 — noticeably "softer" — posts 68.34 J/m impact and 32.7% elongation. It dents where the rigid systems chip [1]. None of this is a defect; it is a design choice the formulator made.
What this means when you choose
- Shop by impact values, not by Shore D, when the repair will take knocks: dropped tools, kicked brackets, vibration. Hardness tells you about scratch and dent resistance — nothing about chipping.
- The most crosslinked system is not the toughest. If a TDS trumpets only hardness, it is telling you half the story.
- Match the failure mode, not the bragging number. A repair that chips needs toughness; a repair that dents needs hardness. They are not the same repair.
Honest limits
- The Izod and Shore values above are measured on cast resin specimens, not on bonded joints — they compare materials, not your specific repair. Bond geometry, thickness, and substrate all move the real numbers.
- The datasheet itself notes these are typical properties, not a specification — end users should test for their application [1].
- "Toughness" in formal materials science has several definitions (fracture toughness, impact energy, area under the stress–strain curve). This article uses the workshop sense: resistance to chipping and cracking under a blow, which is what Izod measures.
- Cross-references: the heat that changes these numbers lives in Repair Lab B8 (exotherm); the cure that builds the crosslinks in Repair Lab B1.
Sources
- West System — Typical Physical Properties of West System Epoxy (Wessex Resins, Version B 08/08). Shore D per BS EN ISO 868; Izod impact per ASTM D-256. 105/206: Shore D 83 (14d) / Izod 28.84 J/m; 105/207: Shore D 82 (14d) / Izod 67.82 J/m; G/flex: Shore D 75 (14d) / Izod 68.34 J/m / elongation 32.7%. URL: https://www.ecfibreglasssupplies.co.uk/user/TechnicalDataSheet/618.pdf
A Shore D 85–90 epoxy is hard — and hardness alone is exactly what this article says not to shop by.
Repair Lab · Release 1 · Evidence last checked September 30, 2026 · A project of AOJEL