Ding et al. J Wood Sci
(2020) 66:55
Page 7 of 9
Fig. 7 Shape comparison of big ( a ) and small ( b ) MDF sanding particles
fibers. Some of sieve apertures were even clogged by the partially passed fibers in the vibration process, which was the main reason for the 2% mass loss in the SA experi- ments. That’s why fibers could still be found in the sample retained on the 40 μm sieve, but no single one was pre- sent in the sample passing through (Fig. 3). Besides shape characteristics, factors like the impact forces between the particles during sieve vibration and the cohesion of small particles all contribute to the retaining of particles on the sieves. Compared to SA, IA obviously provided a better insight into the morphological characteristics of MDF sanding dust by providing detailed information down to the sub- micrometer level. SA did not present enough information on the fine particles due to the limit of mesh size, which makes it more suitable for the analysis of coarse parti- cles from sawing or milling processes. Particle irregular- ity, especially aspect ratio, was another factor limiting the application of SA for wood particles, which was the main contributor to the under-evaluation of fine particles smaller than 40 μm in the SA study. Conclusions The morphology of MDF sanding dust was investigated by sieve analysis, scanning electron microscopy and flat- bed scanning image analysis. The great majority of the MDF sanding dust was found to be inhalable particles smaller than 100 μm. Moreover, other chemical compo- nents were found in the dust samples, which influences not only the size distribution, but also physiochemical properties of MDF sanding dust. The relative span of particle size was wide. Bigger particles showed a wider distribution of aspect ratio, while smaller ones exhibited homogeneous appearance. Only the surface texture was
its handling properties. On the contrary, when the par- ticle size is similar to the fiber cell wall thickness, almost no anatomical characteristics of wood can be found, indi- cating a homogeneous handling behavior totally different from that of big particles. Contrary to AR, solidity analysis revealed a very nar- row distribution with a RS value of only 0.26. The mean and 50th percentile solidity values were 0.89 and 0.93, respectively (Table 1). The high solidity means less con- cave positions on MDF sanding dust surfaces, which can thereby be characterized as flat or smooth surface. The AR and solidity values demonstrated that full breakage and surface erosion of wood fibers occurred during the sanding of the MDF panels. The wood fibers were subject to the interactions with the sanding belt grits, the panel surface and other particles, which broke the fibers and eroded small irregularities on particle sur- faces, resulting in smaller, shorter and smoother particles discovered in this study. Comparison between SA and IA Both SA and IA revealed that more than 95% of the MDF sanding dust particles was suspended particles, most of which were smaller than 40 μm and capable of enter- ing in the human respiratory tract. However, difference existed between SA and IA in the mass percentage of those smaller than 40 μm. According to IA, more than 90% of the particles fell in this range while it was 79.6% in SA. Several factors contribute to the statistical differ- ence between IA and SA. For the fibrous particles, it is possible to fall through millimeter-wide apertures after sufficient vibration. But the penetration of apertures as small as 40 μm wide can hardly be achieved as they are much smaller than the longitudinal dimension of the
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