Seguros de daños e incendios de los inmuebles hipotecados

The thermal conductivity of wood is rather low (0.055–0.17 W/mK depending on the direction of wood texture, in comparison to 429 in silver). In a study on MDF, thermal conductivity coefficient of control MDF boards was measured to be 0.099 (W/mK). Addition of 10 % of wollastonite nanofiber (NW) to the MDF matrix

Effects of Nanotechnology on Fluid Flow in Agricultural … 83

(based on the dry wood fibers) increased thermal conductivity coefficient to 0.110 (W/mK); that is an increase of about 11.5 % (Taghiyari et al. 2013a). Addition of NW was further reported to decrease standard deviation in thermal conductivity among the replications of NW-treated MDF panels. This means that NW resulted in more homogeneity in the composite matrix.

Thermal conductivity was measured based on Fourier’s law for heat conduc- tion (Fig. 6). Circular specimens were prepared, 30 mm in diameter and 16 mm in length; all around, the specimens were covered with silicone adhesive for better in- sulation. Thermal conductivity was calculated using Eqs. (3) and (4). Temperatures were measured with 0.1 °C precision:

Q= KAT L (3) K= Q× L A× T, (4) where K thermal conductivity (W/mK) Q heat transfer (W) L specimen thickness (m)

A cross-sectional area of specimens (m2₎

∆T temperature difference ( T₁−T₂; °K)

Fig. 6  Schematic drawing of the thermal conductivity measurement apparatus in this study.

H. R. Taghiyari et al. 84

The results of the cited authors clearly proved the better cure of the resin in the core section of the MDF mat, resulting in an increase in the physical and mechanical properties (Taghiyari et al. 2013c).

Moreover, hot-press time is dependent on many factors, including the thickness of the composite mat, press temperature, closing rate, and, most importantly, mois- ture distribution throughout the mat (Taghiyari et al. 2011). Moisture of the mat cannot always be increased as it in turn increases the hot-press time; hot presses are usually considered the bottleneck in wood-composite manufacturing factories. It is therefore necessary to try to decrease the time of hot pressing of wood-composite mat; however, higher MCs increase the time significantly. Increase in the MC also causes blows in wood-composite panels; high volume of vapor should be with- drawn from the mat to the surrounding atmosphere. If the volume is too high, per- meability in the composite matrix would not be enough to have a timely withdrawal of vapor. Accumulated vapor within the composite matrix would eventually blow, once the hot-press plates open. Furthermore, for UF resin, there is a limitation of MC level (Papadopoulos 2006); that is, higher MCs than standard level for UF resin would eventually weaken the strength of resin. Finding new ways to increase the heat-transferring rate to the core section of the composite mat has always been a challenge before the wood-composite manufacturing industry. Silver nanoparticles were reported to decrease hot-press time by 10.9 % when 100 mL of NS suspension was used for each kilogram of wood particles. Copper nanoparticles were also re- ported to decrease hot-press time. It was reported that NC decreased hot-press time by 5.7 % when 100 mL of NC suspension was used. With due consideration to the lower thermal conductivity coefficient of copper in comparison to silver, this was reasoned.

The heat-transferring property of metal (Khojier et al. 2012; Sadeghi and Rastgo

2012) and mineral nanomaterials (Haghighi Poshtiri et al. 2013) was reported to improve some properties in solid woods as well as wood-composite materials. In a recent study, effects of a 400-ppm aqueous suspension of silver nanoparticles on the heat-transferring rate from the hot-press plates to the core section of MDF was studied (Taghiyari et al. 2013d). NS suspension was sprayed on the mat at three consumption levels of 100, 150, and 200 mL/kg based on the dry weight of wood fibers. Scanning electron microscope (SEM) micrographs showed uniform spread of silver nanoparticles over wood fibers (Fig. 7). A digital thermometer with tem- perature sensor probe was used to measure the temperature at the core section of the mat at 5-s intervals (Fig. 8). The probe of the thermometer was directly inserted for about 50 mm into the core of the mat (from the edge boarder of the mat), in the hori- zontal direction. Temperature measurement was started immediately after the two hot plates reached the stop bars. Measurement of temperature at the core section of the mat (immediately after the upper plate of the hot press reached the stop-bars) indicated significant difference between the temperatures of the four treatments of control, NS100, NS150, and NS200 (Fig. 9). The cited authors reported that temperatures at the core section of NS150 and NS200 were both higher than both NS100 and control treatments. The depolymerization of the surface resin bonds in the surface layers of panels with high metal nanoparticle content can be related

Effects of Nanotechnology on Fluid Flow in Agricultural … 85

to the increasing trend in the final minutes of the hot pressing; that is, in the final minutes when all MC was nearly evaporated in the surface layers, the heat resulted in the depolymerization and breaking down of resin bonds. The depolymerization increased the fluid flow in the composite matrix. As to the fact that rapid transfer of heat to the surface layers of the mat would eventually result in the depolymerization of resin, ending up in decrease in some of the physical and mechanical properties, further studies should be carried out on possible spread of metal nanoparticles or mineral nanofibers in only the core section of composite mats to facilitate the heat transfer to this part; this would also prevent overheating of the surface layers and the consequent resin breakdown.

It may, therefore, be concluded that addition of metal nanoparticles to increase the heat-transferring rate to the core section of composite mats should not neces- sarily improve all physical and mechanical properties. Furthermore, the optimum

Fig. 8  Temperature mea-

surement using a digital thermometer with its sensor probe inserted into the core section of the composite- board mat. (Source: Taghi- yari et al. 2013d)

Fig. 7  Scanning electron microscope (SEM) micrograph showing silver nanoparticles (↓) scat-

H. R. Taghiyari et al. 86

consumption level for metal nanoparticles is dependent on many factors, including the hot-press temperature, hot-press duration, thermal conductivity coefficient of metal nanoparticles, and the type and density of composite panels.

Conclusions

Wood-composite materials offer the advantage of providing a homogeneous struc- ture to be used in many applications. However, its biological nature causes it to suffer from several shortcomings, such as susceptibility to biological wood-deteriorating agents, water absorption and thickness swelling, fire, etc. A brief overview of the research project carried out on utilization of nanomaterials in the wood-composite manufacturing industry proved numerous potential applications of nanotechnology in this industry. The use of metal and mineral nanomaterials with high thermal con- ductivity coefficient helps in improved thermal conductivity and better cure of the resin, resulting in a significant decrease in gas and liquid permeability. The water- repellent property of some mineral nanoparticles (NZ) can also hinder penetration of water and vapor into wood-composite matrix; ultimately, the service life of the parts used in the furniture or structure would significantly increase.

Fig. 9  Temperature at the core section of the medium-density fiberboard (MDF) mat after the third

minute of hot pressing with 5-s intervals. ( NS = Nanosilver content mL/kg). (Source: Taghiyari et al. 2013d)

Effects of Nanotechnology on Fluid Flow in Agricultural … 87

Acknowledgment The authors appreciate the scientific support of Prof. Olaf Schmidt, Depart-

ment of Wood Biology, University of Hamburg.

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