Tropical Wood Dusts—Granulometry, Morfology and Ignition Temperature

The article considers the granulometric analysis of selected samples of tropical wood dust from cumaru (Dipteryx odorata), padauk (Pterocarpus soyauxii), ebony (Diospyros crassiflora), and marblewood (Marmaroxylon racemosum) using a Makita 9556CR 1400 W grinder and K36 sandpaper, for the purpose of selecting the percentages of the various fractions (<63; 63; 71; 200; 315; 500 μm) of wood dust samples. Tropical wood dust samples were made using a hand orbital sander Makita 9556CR 1400 W, and sized using the automatic mesh vibratory sieve machine Retsch AS 200 control. Most dust particles (between 50–79%) from all wood samples were under 100 μm in size. This higher percentage is associated with the risk of inhaling the dust, causing damage to the respiratory system, and the risk of a dust-air explosive mixture. Results of granulometric fractions contribution of tropical woods sanding dust were similar. Ignition temperature was changed by particle sizes, and decreased with a decrease in particle sizes. We found that marblewood has the highest minimum ignition temperature (400–420 ◦C), and padauk has the lowest (370–390 ◦C).

The damaging effect of wood dust is determined according to the particle size. Larger size fractions tend to settle [17,18], whereas if the particle is smaller (such as below 100 µm), the dust becomes airborne. In the production process, dust is formed that contain particles of various sizes [5]. A granulometric analysis determines the degree of crushing of the base material-which is one of the characteristic abilities of form airborne dust mixture [8].
Reinprecht et al. [19] classify tropical woods as wood that demonstrates significant resistance to biological agents and machine wear and tear, together with solid dimensional stability and pretty aesthetics. These types of wood are commonly used for exterior constructions, tiles, garden furniture, or special plywood [20][21][22]. The expansion of their processing brings the creation of their wood dust.
Padauk (Pterocarpus soyauxii) is moderately heavy, strong, and stiff, with exceptional stability. It is a popular hardwood among hobbyist woodworkers because of its unique color and low cost. It has a unique reddish-orange coloration, and the wood is sometimes referred to by the name 'Vermillion'. It is commonly used in flooring, musical instruments/objects, tool handles, furniture, other small particular wood objects, and as veneer [33].
Ebony woods have many common names, such as Gabon Ebony, African Ebony, Nigerian Ebony, and Cameroon Ebony, and originate from the equator part of Western Africa. Heartwood (955 kg·m −3 ) is usually jet-black, with little to no variation or visible grain. Occasionally, dark brown or grayish-brown streaks may be present. Ebony's common uses small items, such as piano keys, musical instrument parts, pool cues, carvings, and other small specialty items [34]. Ebony is highly valued in the Hindu religion as a building material [35], base material [36,37], and for use in other items [38].
Marblewood, also known as Angelim Rajado, is distributed from Northeastern South America. This heartwood (1005 kg·m −3 ) is yellow to golden brown, with irregular brown, purple, or black streaks [39]. It is commonly used for tuned instruments/objects, flooring, carpentry, sliced veneer, and delicate furniture. Vivek et al. [40] introduced the possibility of using marblewood dust as a partial replacement for cement and sand in concrete.
The basic samples (Figure 1a)

Preparation of Tropical Wood Dust Samples
Technical equipment was used to prepare the homogeneous dust particles dust samples (Figure 1b) was Makita 9556CR 1400 W disc sander (Makita Numazu Corp., Branesti Ilfov, Romania) and K36 sandpaper (Topex, Kinekus, Žilina, Slovakia). The grinding was carried out by the Experimental Laboratory at the Faculty of Security Engineering, University of Žilina (Slovakia). Tropical wood dust samples were prepared by a specialist in grinding. We aimed to ensure the grinding process was as close to reality as it is possible (in terms of pressure of the grinding surface of the component, grinding speed, and grinding direction (cross)). The prepared dust particles were amassed in the hopper with a hermetically sealed glass container, because it needed to stop the dust from absorbing any moisture. After the samples were ground three times, the hopper always was cleaned (in between samples). Overall, 300 g of dust was amassed from each board (three boards collection), and served for the granulometric examination. Detailed information about the preparation of dust samples can be found in Reference [29].

Experimental Methods Utilised for Characteristics of Tropical Wood Dusts and Sieve Analysis
The moisture of the tropical wood samples determined according to Reference [41], and the bulk density determined according to Reference [42], were selected characteristics of samples before sieve analysis ( Table 2). The Retsch AS 200 sieve shaker vibration machine (Retsch AS 200 control, Retsch GmbH, Haan, Germany), using the seven fraction sizes (500, 315, 200, 100, 71, 63, and <63 µm), was used for sieve analysis [43]. The sieves, in addition to the tropical wood dust, weighed 30 g (using laboratory scales with precision readings of 0.001 g). The measurement procedures were conducted five times lasting 10 min. The wood dust moisture testing was carried out according to Reference [44]-in a heated oven, at a temperature of 103 ± 2 • C for 24 h ( Table 2).

Experimental Methods for Shape of Wood Dust Particles
The size, shape, and form of wood dust particles were studied by microscopic analysis, through a wide-field microscopy system (Nikon Eclipse Ni (Nikon Corp., Tokyo, Japan)) with a Nikon DS-Fi2 camera (Nikon Instruments Inc., Melville, NY, USA) [45]. This microscope possesses two c-mount camera ports, and an electric XY stage. This is the normal configuration for the bright field observation of this microscope.
A Nikon DS-Fi2 full-HD color camera (Nikon Instruments Inc., Melville, NY, USA), that uses one port, 2560 × 1920 pixels, was also used. This camera was used to establish the image by connecting it to the computer using a USB cable. Microscopic analyses of tropical wood dusts were performed using 100 µm fraction precision.
The Institute of Research in Banská Bystrica, Slovakia, has a Nikon SMZ 1270 stereomicroscope (Nikon Corp., Beijing, China), which was used to analyze the shape of the dust particles. Equipment for the research of dust particles shape has a range with a 12.7:1 zoom head, and a magnification range of 0.63× to 8×. The samples of the 500 µm, 315 µm, 200 µm, and 100 µm fractions were measured in this stereomicroscope.

Experimentation So as to Determine the Ignition Temperature Measurement of Airborne Dusts
Specific test equipment (the detail of which can be found in Reference [29]) (VVUÚ, a.s., Ostrava, Czech Republic), with automatic weighing machines (Steinberg Systems, Łódź, Poland), an air compressor HL 100 ZU EINHELL (Einhell Corp., Landau an der Isar, Germany), and ALMEMO equipment (Ahlborn, Berlin, Germany) was used to the measurement of minimum ignition temperatures of airborne dusts (Figure 1a) according to the standard instructions [28]. The details of this are described in Vandličková et al. [29].

Results and Discussion
The average bulk density (kg·m −3 ) and dust moisture (%) were physical parameters connected with the preparation of dust samples (Table 2). Poorter et al. [46] consider of the density of tropical woods and the influence of climatic conditions on the growth and quality of wood mass. Tropical tree cumaro is considered a hardwood [47][48][49][50]. Soriano et al. [51] determined the density of cumoro in the range of 1060-1070 kg·m −3 . Marblewood is also ranked among the hardwoods. Everything about working with marblewood revolves around its incredible hardness and density [52].
Ebony is different-it has a lower density (960 kg·m −3 ), and the highest value of average bulk density (206.5 kg·m −3 ).
King et al. [53] performed research on the growth of tropical trees in the Amazon and investigated their physical properties. The research samples included padauk trees, namely, Dipterocarpus cornutus Dyer. With a density of (680 ± 0.013) kg·m −3 and Dipterocarpus globosus Vesque with a density of (690 ± 0.017) kg·m −3 .

Results of Particle Sizes (Fractions) of Dust Samples
Sanding dust particulates are minute and of complex composition, and are more hazardous to humans than the cutting dust particulates [54]. Wood dust particles ( Figure 2) are normally generated in different sizes [12,55]. The percentages of particle sizes start from the size of 500 µm (Table 3). Larger fractions occurred only at a minimal rate (at 1%). Očkajová and Marková [56]; Očkajová et al. [5,57] presented the same result, which was presented for the chosen domestic tree samples.  The dust particles prepared from African Padauk had similar composition % particle number, as well as spruce, oak, and beechwood [29].
Očkajová et al. [5] studied the size of fraction particles and share of wood dust particles (beech, oak, and spruce). Their solution formulated that the percentage share of dust particles is also very different, depending on the kind of this tree species. Our results showed similar size of particles (Table 3). Očkajová et al. [6] found a connection between the shares and densities of the wood dust. Their analyses regarding the wood dust in the sanding process showed that more dust is produced as the density of wood increases [54].
The results of dust sieving are presented by cumulative curves (Figure 2). The results are presented in passed weight percent of the individual fractions collected on a sieve with the appropriate mesh size. The results of the sieve analysis should be presented according to the appropriate standards [58,59]. Prepared continuous cumulative curves are presented in Figure 2 with complete mesh size.

Shape Analysis of Dust Particles
The characterization of wood dust samples from a morphological (Figures 3-6), and a dimensional point of view, yields information that can help epidemiologists and toxicologists to understand the causes of respiratory illnesses [60].
Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 15 Očkajová et al. [5] studied the size of fraction particles and share of wood dust particles (beech, oak, and spruce). Their solution formulated that the percentage share of dust particles is also very different, depending on the kind of this tree species. Our results showed similar size of particles (Table 3). Očkajová et al. [6] found a connection between the shares and densities of the wood dust. Their analyses regarding the wood dust in the sanding process showed that more dust is produced as the density of wood increases [54].
The results of dust sieving are presented by cumulative curves (Figure 2). The results are presented in passed weight percent of the individual fractions collected on a sieve with the appropriate mesh size. The results of the sieve analysis should be presented according to the appropriate standards [58,59]. Prepared continuous cumulative curves are presented in Figure 2 with complete mesh size.

Shape Analysis of Dust Particles
The characterization of wood dust samples from a morphological (Figures 3-6), and a dimensional point of view, yields information that can help epidemiologists and toxicologists to understand the causes of respiratory illnesses [60].       The particles in our sample show a range of shapes and sizes of particles [61]. The size 100 μm is the boundary value of a particle size, where dust is expected to becomes airborne and is potentially explosive [62,63]. Within the basic range of size particles, dust can be classified as coarse (particle diameter of > 100 μm) or fine (particle diameter of < 100 μm) [64].
The anatomical structure is preserved in analyzed particles [65]. When magnified, the fractions of 500, 315, 200, and 100 μm (Figures 3-6) appeared differently-they had their own specific shapes. Figure 3 shows the fractions of cumaru, and Figure 6 fractions of marblewood.
Selected tropical woods produce dust particles within the whole spectrum. The differences in morphology are shown in Figures 3-6. Cumaru and marblewood are hardwoods, with a density of 1000 gm-m-3, and it is possible to state the similarity of the formed particles. The relationship between the particle shape and the initiation temperature can be shown in marblewood dust, which has the highest ignition temperature. Ebony offers a limited or bounded particle shape, and its ignition temperature is lower.

Microscopic Dust Analysis
The particles of tropical dust samples (Figure 7) have the size of <100 μm. From the safety and occupational hygiene perspective, particles below 100 μm are the most dangerous in the working environment [2]. The scans clearly show that the fibers of the dust particles maintain their anatomical structure with the fibrous character of particles [66]. The two-dimensional pictures of particles can be used for determining the smallest particle sizes. The particles in our sample show a range of shapes and sizes of particles [61]. The size 100 µm is the boundary value of a particle size, where dust is expected to becomes airborne and is potentially explosive [62,63]. Within the basic range of size particles, dust can be classified as coarse (particle diameter of >100 µm) or fine (particle diameter of <100 µm) [64].
The anatomical structure is preserved in analyzed particles [65]. When magnified, the fractions of 500, 315, 200, and 100 µm ( Figures 3-6) appeared differently-they had their own specific shapes. Figure 3 shows the fractions of cumaru, and Figure 6 fractions of marblewood.
Selected tropical woods produce dust particles within the whole spectrum. The differences in morphology are shown in Figures 3-6. Cumaru and marblewood are hardwoods, with a density of 1000 gm-m-3, and it is possible to state the similarity of the formed particles. The relationship between the particle shape and the initiation temperature can be shown in marblewood dust, which has the highest ignition temperature. Ebony offers a limited or bounded particle shape, and its ignition temperature is lower.

Microscopic Dust Analysis
The particles of tropical dust samples (Figure 7) have the size of <100 µm. From the safety and occupational hygiene perspective, particles below 100 µm are the most dangerous in the working environment [2]. The scans clearly show that the fibers of the dust particles maintain their anatomical structure with the fibrous character of particles [66]. The two-dimensional pictures of particles can be used for determining the smallest particle sizes. Picture of wood samples, after sanding, taken by an electron microscope, show different and complex shapes of particles of wood dust ( Figure 7). As described in the Particle Atlas [67], the diverse geometric expression could be observed, such as cylinders, cones, rectangular prisms, and spheres [60].
Gómez Yepes and Cremades [61] analyzed the particle characteristics in Quindio (size distributions, aerodynamic equivalent diameter (Da), elemental composition, and shape factors), and particles were then characterized via scanning electron microscopy (SEM) in conjunction with energy dispersive X-ray analysis. Results from their analysis of particulate matter showed that the coneshaped particle ranged from 2.09 to 48.79 μm Da; the rectangular prism-shaped particle from 2.47 to 72.9 μm Da; the cylindrically-shaped particle from 2.5 to 48.79 μm Da; and the spherically-shaped particle from 2.61 to 51.93 μm Da.
Oak dust provides a similar comparison with marblewood dust, as oak is the hardest Slovak wood. In all investigated oak fractions, the isometric shape of particles with sharp edges and rounded corners, which is more typical for fraction <100 μm [3]. Picture of wood samples, after sanding, taken by an electron microscope, show different and complex shapes of particles of wood dust ( Figure 7). As described in the Particle Atlas [67], the diverse geometric expression could be observed, such as cylinders, cones, rectangular prisms, and spheres [60].
Gómez Yepes and Cremades [61] analyzed the particle characteristics in Quindio (size distributions, aerodynamic equivalent diameter (Da), elemental composition, and shape factors), and particles were then characterized via scanning electron microscopy (SEM) in conjunction with energy dispersive X-ray analysis. Results from their analysis of particulate matter showed that the cone-shaped particle ranged from 2.09 to 48.79 µm Da; the rectangular prism-shaped particle from 2.47 to 72.9 µm Da; the cylindrically-shaped particle from 2.5 to 48.79 µm Da; and the spherically-shaped particle from 2.61 to 51.93 µm Da.
Oak dust provides a similar comparison with marblewood dust, as oak is the hardest Slovak wood. In all investigated oak fractions, the isometric shape of particles with sharp edges and rounded corners, which is more typical for fraction <100 µm [3].

Ignition Temperature of Airborne Dust
Vandličkova et al. [29] studied the processes of ignition within airborne wood dusts. They [29] studied the specificities of the dust ignition process in different stages. The first stage is the beginning of an explosion: The dust was sprinkled into a ceramic tube furnace, with dust carbon residue. In the second stage, the unburned wood dust was ignited with the most incredibly intense flame. Subsequently, in the third state, flame slowly diminished with side effects.
The ebony dust of 500 µm, 315 µm, and 71 µm fractions were obtained the maximum flame ( Figure 8). It was noted that when using larger particles, the flame had a lower intensity. These results determined that the ignition temperature of airborne dust ( Figure 9) showed a reduction in values relating to the smaller size fraction [68].
The minimum ignition temperature of cumaru for the 500 µm fraction was 410 • C, and this temperature decreased with changing particle size.
Particles of 100 µm or less had the most significant effect on the change in the minimum temperature. The minimum ignition temperature of the ebony dust particles 500 µm, and 200 µm was identical (400 • C). The 100 µm particle fraction had a minimum ignition temperature of 380 • C. The last three size fractions had minimum ignition temperatures, due to strong dispersion in the heating furnace. An assumption was made applied to a larger dispersion of particles in a space for larger particles (of 500 µm), compared to 100 µm with the same weight of the batch.
Marblewood had the highest tested minimum ignition temperatures of all samples. The minimum ignition temperatures started from 400 • C at a particle size of 500 µm to 100 µm. Subsequently, the minimum ignition temperatures decreased with the particle size change to the level of 400 • C, at a particle size of 63 µm.
Flame propagation behaviors and temperature characteristics of four types of biomass (poplar, pine, peanut, and corn sawdust particles) with two different particle sizes (50-70 µm and 100-200 µm) distributions were studied experimentally by Jiang et al. [69]. The average flame propagation velocity and the amplitude of the velocity fluctuation are functions of the mass density of the biomass particles and depend on the particle size distributions [69].
Tropical woods came to a global market for their use in various products. Information about tropical wood fire parameters is poor. Fire parameters are not described in Safety Data Sheets [70,71]. Carrasco et al. [72] studied the heat transfer in Brazilian woods, using a sample that was thermally loaded to examine the potential for fire. Their results showed that there are curves of temperature on the time experiments that create the thickness f chair layer corresponding with Carrasco's numerical models.

Conclusions
From the perspective of this study, we can claim that we have obtained original results within the field of morphology of dust particles regarding the wood sanding process of tropical four species, as well as results concerning the determination of minimal ignition temperature airborne dust.
Our results show that fraction sizes of tropical wood dust are an important factor for fire ignition. However, the shape and morphology of tropical wood dust particles may also exert influence ignition process. All these aspects deserve to be studied further.
Marblewood has the highest minimum ignition temperature. Thermal stability can be found in its hardness and density.
Cumaru has specific behavior. It is a hardwood with a density comparable to marblewood, but the minimum initiation temperature decreases significantly with decreasing particle size (fraction <63 µm has a temperature of 370 • C).
The monitored dust particles retain their anatomical structure; the shapes of individual samples are different. These results offer different particle shapes such as: Rectangular prism-shaped particle, cylindrically-shaped particle and spherically-shaped particle.  Acknowledgments: This article was supported by the Project KEGA 0014UKF-4/2020 Innovative learning e-modules for safety in dual education.

Conflicts of Interest:
The founding sponsors had no role in the design of the study, in the collection, analyses, or interpretation of data; in the writing of the manuscript and in the decision to publish the results.