The individual factors a to g for each wood-based material type are listed in Table 1. The raw density should always be entered in kg/m³ and the level of adhesive content as a percentage. The proportion of hydrophobic agents should be added to the adhesive content. As wood-based materials can possess different particle sizes, raw densities and adhesive contents in the individual layers, the models are differential in practice, i.e. have to be applied specific to each layer.
Tabelle 2: Factor Values
||Factor values for panels made up of
||Rough chips / fine strands
||Rough strands / veneer
The models are not precise mathematical physical models. They have been empirically calculated on the basis of test results. Error estimation has shown that the results have an error level of around +/-10% and that measurements show a similar error range. As the adhesive content in a diffusion sample and within a layer cannot be precisely ascertained and the particle sizes can differ greatly even within a layer, the difference between the measurement and the calculation can be even greater than that stated above. However, a µ value as shown above can be expected on average, meaning that the µ value can be predicted with sufficient precision for a production batch.
Comparing the results of the diffusion measurements with small samples with a diffusion face of around 54cm² to those with a 250cm² diffusion face, it can be seen that the smaller test specimens produce greater deviations from the average (especially when there are inhomogeneous materials in the face) than their larger counterparts. ISO 12572 specifies that the diffusion resistance value has to be the (arithmetic) average from 5 samples. This approach, however, does not take into account that the process involves parallel-“connected” resistances from a physical point of view. Face (Ages) of a wood-based material consists of many small aligned faces (Ai). Thus, the face
Ages = ∑Ai .
Each partial face has its own diffusion resistance. As these partial faces lie next to each other, the diffusion resistance of the overall face (Rges) is ascertained as the total resistance of several “parallel-connected” resistances. The equation is
This equation shows that the partial faces with a very low resistance can significantly impair the total resistance. In the case of fibre panels, these (microscopic) partial faces with very low diffusion resistance are very evenly distributed across the face due to the particle structure. The bigger a particle’s face (fibre – chip – strand – veneer), the more likely it is that partial faces occur which have a very low diffusion resistance.
Whilst the results of the investigations do not enable us to forego measuring the diffusion resistance, they do allow considerably improved interpretation of the measurement results. Due to the relatively large raw density deviations in wood-based materials, it would appear to be advisable to determine the raw density of each individual test specimen and to standardise the measured values against the nominal raw densities of the measured wood-based material. The requirement to determine the raw density of each individual test specimen should be embedded in the ISO 12572 standard. Relevant parameters for standardisation of the measured values against the nominal raw densities will be compiled in the near future for each of the wood-based material forms fibreboards, particle boards, OSB and plywood boards. These should not, however, be embedded in the aforementioned standard. These parameters are, presumably, better suited to placement in the EN 13986 standard.
The ISO 12572 should, however, stipulate that the diffusion resistance of the respective material is no longer formed from the average values of the measurement results, but instead through observation of parallel-connected resistances.
In addition to the prospect of uniform, realistic values for diffusion resistance, the results of these investigations also provide the wood-based material industry with a tool with which the effects of alterations to the production process on the diffusion resistance can be realistically estimated.