The key difference (between MR MDF and HMR MDF) is that the polyurethane – based resin (MDI) (used in HMR MDF) does not break down in the presence of moisture (i.e. a chemical bond with the wood fibres).
All wood and wood-based products react to moisture and HMR MDF is no exception. Wood cells expand and shrink slightly in width and length as they absorb or lose moisture. This phenomenon is not fully reversible, as relieved stresses do not completely recover.
The specially formulated MDI resin used in the production of HMR MDF, increases the physical properties and durability of MDF and increases protection against thickness swell to create a premium composite panel for demanding conditions.
HMR MDF, as with all wood products in external applications, must be coated on all surfaces and edges with an exterior based system to control both moisture uptake and moisture loss. HMR MDF is designed for use in areas that are protected from direct exposure to the weather.
Typical Values in the table below compare MR MDF and HMR MDF. Most values align similarly until the one hour boil test result shows the stability of HMR MDF.