Influence of panel thickness on the release of volatile organic compounds from OSB made of Pinus sylvestris L.

Forest Products Journal, Jan/Feb 2008 by Ohlmeyer, Martin, Makowski, Mathias, Fried, Harald, Hasch, Joachim, Schöler, Michael

Abstract

Emissions from building products are considered to be a major source of elevated volatile organic compound (VOC) concentrations in indoor air. To investigate the effect of panel thickness on the release of VOC three oriented strandboards (OSB) of different thicknesses were characterized. The emissions deriving from the panels made of Scots pine were investigated over a period of 125 days using environmental test chambers. According to ISO 16000 parts 6 and 9, the chamber air was actively sampled on adsorbent tubes, which subsequently were analyzed in a gas chromatography (GC) and mass spectrometric (MS) system. Terpenes and aldehydes are the predominant compounds released by the OSB. Since emissions increase with the panel's thickness, it can be concluded that, aside from evaporation, diffusion from the core seems to be a major source of VOCs. This movement of compounds is probably promoted additionally by the porous and capillary structure of wood and in particular wood-based materials. These conclusions are supported by a comparison of two empirical models for estimating terpene emission rates. A double-exponential model, taking into account both evaporation and diffusion-driven emissions, achieved the best quality of fit. Thus, it is crucial to consider sample thickness when measuring and assessing the emissions from wood-based products. Furthermore, it can be concluded that the exclusive utilization of area-related factors (e.g., the area-specific emission rate) is insufficient for describing and evaluating long-term emissions from wood-based composites.

According to numerous studies, construction materials are major sources for elevated volatile organic compound (VOC) concentrations in indoor air (Brown 2002, Hodgson et al. 2002, Jarnstrom and Saarela 2003). Various adverse health effects, such as irritation or allergic diseases, are associated with high VOC concentrations. In order to evaluate the impact of building products on indoor air quality, the release of VOCs is investigated using environmental test chambers. Test specimens are stored inside such chambers in a constant climate (temperature: 23 °C, relative humidity: 50%) and at a constant air exchange rate. At specific times, generally at least after 3 and 28 days, the VOC concentration of the outgoing chamber air is determined by active sampling on an adsorbent (e.g., Tenax TA) and, subsequently, thermal desorption and gas chromatography (GC) using mass spectrometric (MS) or flame ionization (FI) detectors. All requirements of this method are described in ISO 16000 parts 6 (2004) and 9 (2006).

For improvement of indoor environmental quality and in order to promote building products with low VOC emissions, various environmental product declarations have been developed, including the European Collaborative Action (ECA 1998) and the German Committee for Health-related Evaluation of Building products (AgBB 2005). Both schemes include evaluation procedures as well as concentration limits that should not be exceeded by the tested samples. Limited VOC emissions are additionally required for some voluntary eco-labels, such as "Der Blaue Engel RAL-UZ 38" (1999) or "natureplus" (2003). Most of these schemes only consider the area-specific airflow-rate or, respectively, the area-specific emission rate for transferring the results to a model room and determining resulting model room air concentrations.

Several studies have shown that generally, a high concentration of terpenes and aldehydes characterizes the VOC emissions from pinewood-based panels (Baumannetal. 1999, 2000; Salthammer et al. 2003). Monoterpenes originate in the wood's oleoresin and are easily volatilized due to their high vapor pressures (Fengel and Wegener 1989). As a result of autoxidation, unsaturated fatty acids are degraded and several saturated and unsaturated aldehydes are formed (Back et al. 2000; Makowski et al. 2005; Makowski and Ohlmeyer 2006a, 2006b). However, since both terpenes and aldehydes derive from the wood itself, diffusion-driven emissions from the core layer of pinewood-based composites cannot be neglected. Consequently, it can be assumed that emissions would rise with the panel's thickness.

Objective

The objective of this paper is to determine whether the panel's thickness influences the VOC emissions from OSB. For this reason, three panels of different thicknesses but equal surface areas were produced and tested for their VOC emissions. Additionally, two different models for calculating emission rates were applied in order to identify the main drivers for emissions from wood-based panels.

Materials and methods

Materials

Industrially flaked and dried Scots pine (Pinus sylvestris L.) was utilized as raw material for the tested boards. One day after flaking and drying, the Strandboards were manufactured in a laboratory. The strands were blended with 4 percent pure pMDI (polymeric methylene di(phenyl isocyanate)). Subsequently, a randomized mat was formed and hot pressed with 12 seconds per one millimeter panel thickness at a temperature of 220 °C. Under these conditions, three panels of different thicknesses (16 mm, 19 mm, and 26 mm) were made. Pressing time and density of the panels are summarized in Table 1.