Use of tree bark as insulation material

ABSTRACT

The general trend towards energy efficient building constructions has led to an increased need of insulation materials. Basically, organic (hydrocarbons) and inorganic insulation materials can be distinguished whereby the first group is dominated by petrol based products (polystyrene) and the second one is mainly mineral wool, which both have a rather poor ecological performance because of non-renewable resources or an energy intensive production process.

An option would be low density bark based panels which have proved to be a promising insulation material and are especially interesting because it can be made use of the natural 'tree insulation material', namely bark. Bark has advantageous properties like a low density, a relatively high resistance against microorganisms and a low thermal conductivity whilst also having a high heat storage capacity. Also from an economic point of view the material is interesting, as bark is a traditional by-product of timber manufacturing and is rarely used for products with a higher value added.

Therefore, insulation boards made out of differently fractionated bark particles and a tannin-hexamine resin were produced. These panels were manufactured with a density lower than 500 kg/[m.sup.3]. Furthermore, the mechanical and thermal properties of the bark panels have been evaluated.

Measurements showed that the mechanical properties (MOR, MOE, internal bond, tensile strength and compressive resistance) of the bark-based panels are comparable with those of commonly available insulation boards. Only water absorption and thickness swell after 24 h are marginally higher than those of conventional insulation materials.

In terms of the thermal characteristics, these bark based panels showed thermal conductivity values higher than those of light insulation boards. Nevertheless, the application as insulation material is possible. By analyzing the thermal diffusivity (material property describing the rate at which heat flows through a material) of bark based panels, their performance is clearly superior to existing insulation boards.

The bark boards' structure has been analyzed by the means of X-ray computed tomography and it has been found that the pore structure can be satisfactorily influenced in the pressing process, which is promising for a selective adjustment of the boards' thermal properties.

On the one hand a raw material source for a booming industry was found, on the other hand the study of a natural thermal insulator could lead to improved materials for future requirements.

Key words: Tree bark, insulation material, porosity

1 INTRODUCTION

The building industry requires a rising amount of insulation materials due to a general trend towards energy efficient buildings.

The insulation materials market is dominated by polystyrene and mineral wool, which both are based on non-renewable resources.

An option could be low density bark based panels which proved to be a promising insulation material and are especially interesting because one makes use of the natural 'tree insulation material', namely bark. Globally roughly 1.6 billion solid [m.sup.3] of wood are industrially used, although this accounts only for 43 % of the total cuts as the majority is directly burned (Barbu et al. 2014). Considering that the average bark content of a tree is...