European standard EN 204 sorts thermoplastic wood adhesives, mainly PVAc dispersions, into four classes by water and moisture resistance. The test method is defined in EN 205: beech specimens are conditioned in different ways, then their tensile shear strength is measured.
Classes and uses
| Class | Service conditions | Typical applications |
|---|---|---|
| D1 | Dry interior, wood moisture ≤ 15% | General assembly, hobby |
| D2 | Interior with occasional short exposure to water, wood moisture ≤ 18% | Furniture, interior doors, panels |
| D3 | Interior with frequent short exposure to water or condensation; sheltered exterior | Kitchen and bathroom furniture, sheltered exterior doors, windows |
| D4 | Interior with frequent long exposure to water; exterior exposed to weather (with suitable surface protection) | Windows, exterior doors, garden furniture, stairs |
How to reach D4
Some one-component reactive PVAc dispersions reach D3. D4 is usually achieved by adding a hardener (crosslinker). Once mixed, the blend has a limited pot life and must be used within it.
Heat resistance: WATT 91
D classes describe water resistance only. Heat resistance is tested separately under EN 14257 (WATT 91), which matters for saunas, windows and sun-exposed uses.
Points to watch
- The class describes the adhesive itself; without correct wood moisture, press time and surface protection, performance cannot be guaranteed.
- Reactive D3/D4 dispersions may contain more formaldehyde than classic PVAc; check the data sheet if low emissions are required.
- For structural applications needing formaldehyde-free performance beyond D4, consider 1C PU prepolymers.
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