Improvement of thermal insulation capacity on wood shaving insulation mats by the addition of carbon black (Part 1): The effects of addition rate and properties of carbon black

In order to decrease the thermal conductivity of wood shavings mats, a simple way of adding carbon black to wood shavings was tried in this study and the effects of both the additive rate and the properties of carbon black on the mats' thermal conductivity were investigated. The wood shavings used in this study were laboratory-made, with a relatively uniform shape, and they were prepared from lumber with three different levels of density. The comparison method specified in Japanese Industrial Standard A1412-2 was applied for measuring thermal conductivity. At first, black writing fluid (BF) was used as carbon black. The additive rate was 2, 4, 8, and 12 % on an oven-dry weight base. The mats with a density of three levels ranging from 40 to 75 kg/cubic m, corresponding to three levels of raw material density, were fabricated, and their thermal conductivity was measured as a function of mat moisture contents from 3 to 15 %. The results showed that the addition of BF reduced the mat thermal conductivity by 6 % on average and 14 % at most when the mats were in low moisture conditions corresponding to a relative humidity of 40 %, and that the smallest addition rate of 2 % was enough. However, the effects of BF addition disappeared when the mats had a moisture content of 14 to 17 % corresponding to a relative humidity of 80 %. This is supposedly because the glue in BF has hygroscopicity higher than wood. The first trial abovementioned clarified that a small addition rate of 2 % was enough and that the additive required a lower hygroscopicity than wood. Therefore, three types of carbon black (CB) that are widely used in printing and have low hygroscopic properties were employed. The mats with a density of 50, 57 and 65 kg/cubic m were fabricated with a CB additive rate of 0.25, 0.5 and 2 % on oven-dry weight basis, and the mats were measured for their thermal conductivity as a function of mat moisture contents from 3 to 15 %. The results showed that the addition of CB reduced the mat thermal conductivity by 6 % on average when the mats were in a low moisture condition corresponding to a relative humidity of 40 % and that the smallest addition rate of 0.25 % was enough. Furthermore, almost the same effects as this were observed even when the mats had a moisture content of 14 to 15 % corresponding to a relative humidity of 80 %. The lowest thermal conductivity (0.042 W/mK) was achieved at a combination of mat density of 65 kg/cubic m and a CB addition rate of 0.25 %. The effect of the types of CB was not significant except that the CB with a smaller particle diameter tended to show a smaller effect on reducing the mat thermal conductivity at a higher moisture condition. In order to investigate a reason why a thermal conductivity of mats was reduced by the addition of a small amount of CB, we calculated the apparent thermal conductivity of coarse pores in the mats, which includes the effects of convective and radiant heat transfer. It was found that the apparent thermal conductivity of coarse pores decreased from 0.045 W/mK to 0.042 W/mK by adding CB by only 0.25 %, which suggests that physical surface modification by adding CB may play a role of reducing convective and/or radiant heat transfer in coarse pores in the mats.