Mass transfer resistances particleboard

For bare particleboards in suitable test chambers, mass transfer resistances are usually found to lie between 1,500 and 10,000 s/m. When there is no internal circulation or when there is insufficient turbulence, it is not uncommon to find an extra mass transfer resistance for the gas phase of 12,000 s/m at a ventilation rate of 0.75 per hour. A more detailed estimation is given in the summary. 

Figure 9 illustrates the effect of veneering on formaldehyde emission of particleboard. For the veneering the same type of resin was used as in the production of the particleboard. Pressing conditions are not comparable. Veneering has increased the equilibrium value a little, from 0.48 to 0.56 mg/m. The mass transfer coefficient however, decreased very much. The mass transfer resistance shows an increase from 2,400 sec/m to 11,000 sec/m. In the case at issue, the formaldehyde concentration, at a loading factor of 1 m /m of the veneered particleboard, is below that of the bare particleboard, only at a ventilation rate in excess of 0.2 per hour. 

To explain this, it can be argued that a not inconsiderable increase in resistance to mass transfer has been set up in the gas phase, which in fact may vary from situation to situation. Such situations are indeed normal in everyday practice. This explains why in practice, especially at low ventilation rates, much lower concentrations are found, than would follow from measurements done in climate chambers with good circulation. Such intensive circulations remain absolutely necessary if determination of the characteristic particleboard parameters is wanted, independant of the test environment. 

In this presentation the term "diffusion barrier will be used for finishes or overlays for particleboard that increase the diffusion resistance of the particleboard surface, thus retarding the rate of mass transfer (formaldehyde emission) from the board to the surrounding air. 

The concentration of formaldehyde in the air of a room containing particleboards, will depend on the content of formaldehyde in the boards and on the rate of its release. The formaldehyde content of a particleboard is determined by the binder used to manufacture the board and a number of production parameters. The release rate is affected by the temperature and the relative humidity of the surrounding air, but also by some of the physical properties of the board. The most important one probably is the diffusion resistance of the surface layer, which may be expressed by means of a mass transfer coefficient. 

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See also in sourсe #XX -- [ ]

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