Ding et al. J Wood Sci
(2020) 66:55
Page 3 of 9
a particle projection area and provides the information of particle surface structure. The aspect ratio (AR) of the dust particle was calcu- lated by Eq. 1:
gold using a sputter coater (JFC 1600, JEOL Ltd, Tokyo, Japan) and placed in a SEM (JSM 7600F, JEOL Ltd, Tokyo, Japan) for photographing. Flatbed scanning image analysis For the flatbed scanning, 20 mg sample particles were dispersed by a vacuum dispersion device (VDD270, Occhio s.a., Angleur, Belgium) where they were placed on a plastic membrane covering the top of a cylindri- cal chamber as the air inside was pumped out. Once the vacuum level in the chamber was low enough to destroy the plastic membrane, the particles fell into the chamber and gently settled on the glass plate of the image ana- lyzer (500 Nano, Occhio s.a., Angleur, Belgium) for image analysis. The images of the scanned samples were instan- taneously analyzed by the built-in software CallistoEX- PERT for calculating particle size and shape distribution. The number-weighted statistical results were converted to the mass-weighted ones by assuming that all particles have identical flatness ratios. The inner diameter ( D in ), i.e., the biggest circle inscribed into the projection area of a particle (Fig. 1) was chosen as the representative size parameter because it showed a good correlation with sieving diameter [26]. It has been suggested that one or two key shape factors can well describe the shape characteristics of a certain kind of particle [27]. Since the basic constituent of MDF is wood fiber, the length-to-width ratio was chosen as a macro-shape descriptor to describe particle geomet- ric proportion. Particle solidity was chosen as a meso- shape descriptor, which reflects the overall concavity of
W
(1)
,
AR = 1 −
L
where W is the width of the smallest box that contains the projection of a particle with the principal directions the same as the projection of the particle, and L is the length of the box (Fig. 1). Solidity was calculated by Eq. 2:
S
(2)
S =
,
S A
where S is the projection area of the particle, and S A is the area of the convex hull bounding the projection (Fig. 1). The relative extent of size or shape distribution was evaluated by relative span (RS):
P 90 − P 10 P 50
(3)
RS =
,
where P90, P50 and P10 are the 90th, 50th and 10th per- centiles of the size and shape distribution, respectively.
Results and discussion Morphological characteristics determined by SA and SEM The SA showed that the great majority (96%) of the MDF sanding dust particles was smaller than 100 μm (Fig. 2). They belong to inhalable particles tending to stay longer and travel wider in the air, which are unsuitable for the living and working environments [8, 28]. Notably, the particles smaller than 40 μm accounted for 79.6%, which are capable of penetrating into the upper respiratory tract and pose health risk to humans [5, 29]. The size characteristic distinguishes sanding dust from wood dust emitted during other machining processes, like sawing, planning and milling. There are orders of magnitude differences in size between them. The SA of pine sawdust by Chaloupkova et al. showed that only 11.93% of the particles were smaller than 630 μm [22]. A similar study on timber sawdust performed by Benthien et al. also indicated a 20% portion in the same range [25]. As mentioned above, smaller size means lower flowabil- ity due to the increase of cohesive force between parti- cles. For food particles, the influence of cohesive force could still be significant when the particle size was up to 200 μm [15, 30]. The size distribution of MDF sanding dust clearly indicates an even lower flowability. Particles retained on each sieve were observed by SEM. Fibrous particles were found in the sample retained on the 40-μm mesh-size sieve (Fig. 3a). Most of them were
Fig. 1 Inner diameter, convex hull, length and width of a particle projection
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