Influence of Feed Speed on the Content of Fine Dust during Cutting of Two-Side-Laminated Particleboards

This paper deals with the problems of wood dust production during the sawing of two-side-laminated particleboards. It points out the dangerous impact of wood dust particles on people ́s health and other unfavourable infl uences in the working environment. The aim of the paper was to introduce the research of dustiness, to determine the content of fi ne particles in sawdust and analyse the infl uence of feed rate on the granularity of sawdust created in the process of sawing two-side-laminated particleboards using modern circular saw. Sawing parameters were chosen for the optimal cutting speed of the circular-saw blade Pilana TFZL, vc = 84.3 m·s-1 and for three feed rates of a workpiece with the cut vf = 10, 15, 20 m·min-1. The collected sawdust particles were subjected to the particle-size analysis under exactly defi ned conditions by sieving and the basic granulometric analysis was made. Sieve analysis gives only a general particle-size distribution without any information considering the mass concentration of fi ne fraction of dust. Therefore, laser particle sizer Analysette 22 Microtec Plus was used to specify details concerning the size of dust particles smaller than 63 μm, which were collected in the bottom collector.


INTRODUCTION 1. UVOD
Together with the main product, chip sawdust is also produced during wood machining.The shape, dimension and amount of sawdust particles depend on physical and mechanical properties of sawed wood as well as on the shape, dimensions, sharpness of the cutting blade and technical and technological conditions of the sawing process (Prokeš, 1978;Goglia, 1994;Lisičan, 1996;Dzurenda, 2007).The sawdust produced during wood machining also contains small dust particles, which, when dispersed in the air, can pose a serious health risk to woodworkers (Hubbard et al., 1996;Beljo-Lučić et al., 2011;Čavlović et al., 2013).In accordance with the current legislation, wood-working and furniture companies have the problem how to remove or at least reduce harmful factors in the working environment.Dusty environment and excessive noise are among the greatest factors adversely affecting the human health.From hygienic aspects, the term dust implies small particles of solid materials, which are dispersed in the atmosphere or deposited on various places in the workplace.Serious health problems are mainly met in cases when the source of dust occurs in badly ventilated spaces, where there is deposition and accumulation of wood dust.It results in a considerable concentration of wood dust in the atmosphere, which can cause serious health complications (Kopecký and Pernica, 2004).Wood dust is a known inducer of cancer in the nasal cavity and recent reviews have focused on this issue.A summary of investigations of wood dust and the risk of cancer can be found in Nylander and Dement (1993), where the authors state that operatives in the woodworking industry face a higher risk of developing nasal cancer, especially those working with machines that generate wood dust (Palmqvist and Gustafsson, 1999).
Wood dust is also associated with a variety of respiratory diseases including asthma, chronic bronchitis, nasal symptoms and eye symptoms, as well as chronic impairment in lung function (Carosso et al. 1987;Enarson and Chan-Yeung, 1990;Jacobsen et al., 2010).
There are only several reports about the dust generated during woodworking.Most of these reports are focused on dust of hardwood and softwood.There are even less papers dedicated to the creation of dust during the cutting of wood-based materials.For example, Chung et al. (2000) investigated the quantity, particle size distribution and morphology of dust created during the machining of MDF.
Particle boards are made by gluing wood particles together with some kind of resin.Therefore, machining, sanding or excessive heating of such composite material can cause decomposition releasing formaldehyde, carbon monoxide, hydrogen cyanide and phenol.The atmosphere pollution created by machining particle boards is an effect of dispersion of wood dust, which may act as a carrier of other chemicals contained in such boards, e.g.wood preservatives and wood adhesives that may themselves cause health effects if inhaled (Hursthouse et al., 2004).Therefore, special interest should be focused on fi ne dust created during cutting wood-based materials with the aim of preventing occupational diseases in woodworking industry.
The aim of this work was to determine the content of fi ne particles in sawdust created during sawing the particleboards by the modern circular saw at three levels of feed speed.The expected results may be the base of the preliminary evaluation of health risk related to dustiness.

MATERIJALI I METODE
Experimental sawing was carried out at the Department of Forest and Timber Technology, Mendel University in Brno, on a modern experimental stand (Figure 1) intended for research of cutting by circular sawblades.This experimental equipment makes it possible to measure parameters of a cutting process, vibrations of circular sawblades and their noise.
For the research into dustiness, it was necessary to equip the stand with a sucking device (URBAN Technik, Libchavy, Czech Republic) and with a GTE (Gravimetric Techniques Emissions, TESO Praha, Czech Republic) for isokinetic sampling of dust.In researching dustiness, information is primarily obtained on the machine dust emission, which is the source of knowledge of the particle size composition of sawdust.Cutting conditions should be selected so that the machine dust emission can be expressed by a functional relationship to the removed chip thickness.Sampling of sawdust and dust is carried out isokinetically, i.e. under conditions of the total correspondence of the sampled air and air fl owing inside the sucking device air channel.The actual air sampling approaches the isokinetic sampling most when it is carried out by a probe with optimum dimensions and shape and if the air speed in the sampling probe nozzle is identical (as for size and direction) with the speed of air in the place of measurement.
Sampling was carried out by an isokinetic gravimetric set GTE.The sucking device has to guarantee controllable air fl ow through the sampling device in order to ensure that conditions for isokinetic sampling are kept in every point of sampling.The period of sampling was determined on the basis of the number of points of sampling and in the measurement cross-section and by the period of sampling in one point.The period of sampling in every point of sampling has to be the same, usually fi ve to ten minutes, but at least three minutes.The method of sampling has to be in accordance with the ČSN ISO 9096 standard.
In cutting, a standard circular sawblade Pilana TFZL (Pilana Saw Bodies s.r.o, Hulín, Czech Republic) was used for trimming boards to size, diameter 350 mm, number of teeth 108 (geometry α = 18°, β = 66°, γ = 6°).Before the experiment, the radius of edge blunting, which did not exceed the value of r 0 = 9 μm, was verifi ed.Cutting parameters were set up for the optimum cutting speed (v c = 84.3m•s -1 and three feed rates of a workpiece with the cut v f = 10, 15, 20 m•min -1 ).
Dustiness was evaluated for typical working conditions of circular saws.The measurement was made in trimming samples of two-side-laminated particleboards Duropal, which was produced by the company KRO-NOSPAN (Czech Republic).The density of rectangular prisms (800 × 350 × 39 mm) was ρ = 700 kg•m -3 .

Particle size analysis of sawdust 2.1. Analiza veličine čestica piljevine
Under exactly defi ned conditions, the basic granulometric analyses were carried out at the Department of Furniture Design, Poznan University Life of Sciences, by sieving, which means by screaning of sampled dust on a set of sieves with mesh size of 0.5 mm, 0.25 mm, 0.125 mm a 0.063 mm during the time T = 20 min on an automatic vibration sieving machine AS 200 (Retsch, Germany).The weights of fractions on sieves were subsequently weighed on the laboratory scales WPS 510/C/2 (Radwag, Poland) with a weighing accuracy of 0.001 g.
Sieve analysis gives only a general particle-size distribution without any information considering the fi ne fractions of dust (Dzurenda and Orłowski, 2011).Therefore, laser particle sizer Analysette 22 Microtec Plus (Fritsch, Germany) was used to determine the size distribution of dust particles collected in the bottom collector, which was below the sieve with the mesh size of 0.063 mm.This sizer automatically carries out a particle size measurement according to predetermined SOP (Standard Operating Procedure) and theoretical assumptions.The obtained results were processed by the MaScontrol (Fritsch, Germany) software in order to generate the particle-size distribution curves of the tested dust samples.It gives two types of quantities: • the sum of the distribution Q r • the density of the distribution q r The curve of the distribution sum Q r (x) shows a standardized total quantity of all particles with equivalent diameters less than or equal to x.Each point along the curve of the distribution sum represents the sum of the quantity components of all particles.The curve of the density distribution q r (x) is the fi rst derivative of Q r (x) by x.It frequently appears in the bell shape.
In agreement with dQ r (x) = q r (X) dx, q r (x) is the component of a quantity dQ r (X), which is contained in the interval dx for particles from x and x + dx.The result is a random quantity r, where: The fractions of dust in the range < 2.5 μm, 2.5 -4 μm and further are obtained based on particle size distribution curves generated during the measurements.The mass of the dust particles collected in the bottom collector, under the sieve with the smallest mesh, had to be taken in subsequent calculations to determine the content of particles with these dimensions in the total mass of dust created during the machining due to the fact that these fractions referred to the samples separated by the sieve analysis.Therefore, the fi nal result of the particlesize measurements of the smallest particles in the dust was the product of the fraction < 63 μm and the fractions calculated in applied ranges.

REZULTATI I RASPRAVA
According to the sieve analyses -granulometric composition of dry sawdust, histograms (Figure 2) of the distribution of particular particle fractions for the There is a relatively high percentage of particles smaller than 0.063 mm, which include potentially airborne particles and from the aspect of occupational health and safety, they can be the most dangerous for workers in the working environment.Substantial increase of the occurrence of these particles in the sawdust was noted with the decreasing feed speed.
Figure 3 shows the cumulative distribution of particle size of the analysed sawdust.It can be concluded that the dust created during cutting at feed speed of 10 m•min -1 is fi ner than during machining at feed speed of 15 or 20 m•min -1 .This result was expected due to a well-known fact that the amount of fi ne sawdust significantly increases with the decreasing feed speed -chip thickness (Barcík and Gašparík, 2014;Dzurenda et al., 2010).Hemmilä et al. (2003) stated that decreasing feed speed causes decreasing of airborne dust emission but this is quite the opposite of the statement that increasing of chip thickness decreases airborne dust emission.Palmqvist and Gustasson (1999) came to the conclusion that the most important factor for the amount of wood dust created from machining of different materials is the average chip thickness.
The effect of cutting parameters on particle distribution of chipped wood during cutting along the grain when the working table is in upper position with higher contact angle of teeth and cutting material is not quite clear because of complex and varying morphology of the processed material, which ranges from softwood through light and dense hardwoods to particleboards, and differences in fragmentation of chips generated during sawing (Beljo Lučić et al., 2007).
Figure 4, 5 and 6 show the results of the analysis of dust sampled from the bottom collector of the sieving machine, which were generated by MaScontrol software in analysing the measuring data from the particle sizer.The graphs show the cumulative and discrete particle-size distribution of analysed dust.These results indicate that the content of fi ne particles in the tested dust created during cutting at the feed speed v f = 10 m•min -1 is higher than when other speed values are used.Particle-size distribution obtained by the particle measurement method with laser diffraction gives a different range of the most numerous particles.It can be seen that the dust particles with the size below 63 μm account for less than 50 -75 % of the total material analyzed.This demonstrates the inaccuracy of both methods, which is caused due to the specifi c shape of wood dust particles.Their length is usually larger than the other dimensions, therefore the wood dust particles pass through a sieve mesh of smaller size than their length.
Figure 7 demonstrates the total contents of particles with assumed critical size limits of 2.5, 4 and 10 μm in the tested sawdust, which were calculated on the basis of the particle-size distribution obtained by the laser diffraction method and taking into account the mass share of the fraction < 63 μm determined by the sieving method.It is easy to see that the largest share of fi ne dust is generated at feed speed v f = 10 m min -1 and that the amount of this dust increases with the decreasing feed speed.The calculated rates of the smallest particles are shown in Table 1.The level of the occupational hazard on a given machining station caused by wood dust can be primitively estimated based on the comparison of the measured values.
It should be noted that the content of the particles with the size lower than 10 μm is only about 1 % in the total dust for the feed speed v f = 10 and 15 mmin -1 and about 0.5 % for the feed speed v f = 20 mmin -1 .This is generally negligible but during machining a large amount of dust can be created and it might pollute a huge volume of the air at the acceptable limit of dust concentration.These particles, when dispersed in surrounding air of the working place, are potentially re- sponsible for the risk of occupational diseases.Its value is up to 5 mgm -3 for agglomerated materials with phenol formaldehyde resin in the Czech Republic (ČSN ISO 481).

ZAKLJUČAK
There are many methods for the determination of particle-size distribution, but due to the fact that wood dust particles are irregularly shaped, the measurement of particle size done by different methods may give various results.Determination of the content of fi ne particles in the dust created during working of wood and wood composites often requires simultaneous application of measurement techniques with different measuring ranges due to the large dimensional range of analyzed particles.
On the basis of fi ndings and results obtained, it can be concluded that the rate of fi ne dust created when trimming to size of laminated particleboards signifi cantly increases with the decreasing feed speed.This is generally valid for other types of cutting operations of woodbased materials because a secondary partition of wood mater occurs.So this increase is particularly caused by the structure of agglomerated material but also by the used tool.Teeth of circular saw blades for trimming tend to have rather small cutting angle.Small angles may cause generating chips that are very bent and prone to disintegration (Očkajová and Beljaková, 2004), which increases the proportion of fi ne dust particles.
Although, based on tests, a seemingly insignificant amount of dust of 10 μm and smaller was found in the whole mass of the created sawdust, it can be assumed that in the process of sawing particleboards, there is a high risk of formation of dust particles harmful to the health of workers employed in woodworking stations.Even this relatively small amount of fi ne dust can cause the pollution of a huge volume of air in excess of the permitted level.