D references of adhesives are commonly used to identify the water resistance of wood bonding adhesives, but strictly speaking they relate to the durability of the adhesive, not just the water resistance. The standard used to determine the durability class is EN 204: 2001 – “Classification of thermoplastic wood adhesives for non-structural applications”.
Practically D1 and D2 are considered to be “interior grade” adhesives and should not be used anywhere where a high level of durability is required.
D3 adhesives are often referred to as “water resistant” and D4 adhesives are referred to as “waterproof”. Whilst this gives a simple way of identifying the difference, it is not strictly true to say that D4 is “waterproof”. The official description of the durability classes from the EN204 standard is shown below and this gives context around what you can expect the bond performance to achieve.
If you require a fully “waterproof” adhesive for bonding wood, you will need to use a phenol resorcinol resin system, e.g. Prefere 4050, or a melamine urea resin system e.g. Prefere 4535.
DESCRIPTION OF DURABILITY CLASSES
D1 = Interior, in which the moisture content of the wood does not exceed 15%.
D2 = Interior with occasional short-term exposure to running or condensed water and/or to occasional high humidity provided the moisture content of the wood does not exceed 18%.
D3 = Interior with frequent short-term exposure to running or condensed water and/or to heavy exposure to high humidity. Exterior not exposed to weather.
D4 = Interior with frequent long-term exposure to running or condensed water. Exterior exposed to weather but with protection by an adequate surface coating.