Effect of moisture content on the air permeability of oriented strand boards

Abstract

Changes in the air permeability and density profiles of 12-mm-thick oriented strand board (OSB) specimens were evaluated in relation to changes in their moisture content. The test methodology consisted of the simulation of real conditions that may occur during construction. Using a water bath, the OSB moisture content was increased from 10% to 17%, and the consequent changes in the air permeability and vertical density profile (VDP) were analyzed. The air permeability and VDP were then reanalyzed after acclimatization of the OSB to a balanced moisture content at 60% relative air humidity and 11.4 °C. After wetting the boards with an initial moisture content of 10% for 2 h and naturally re-drying them in laboratory conditions, an average increase of 11.7% in air permeability was observed. The increase in air permeability was 5.6% with a pressure difference of 50 Pa. After redrying, the boards showed a 1.1% lower average density and 14.5% lower maximum density in the surface layers. From the results, it followed that even the short-term effects of water and the related increase in moisture content of the OSB had a negative impact on the air permeability and VDP.

Full Article

Effect of Moisture Content on the Air Permeability of Oriented Strand Boards

Matěj Hodoušek,^a,* Martin Böhm,^a Anna Součková,^b and Štěpán Hýsek ^a

Changes in the air permeability and density profiles of 12-mm-thick oriented strand board (OSB) specimens were evaluated in relation to changes in their moisture content. The test methodology consisted of the simulation of real conditions that may occur during construction. Using a water bath, the OSB moisture content was increased from 10% to 17%, and the consequent changes in the air permeability and vertical density profile (VDP) were analyzed. The air permeability and VDP were then reanalyzed after acclimatization of the OSB to a balanced moisture content at 60% relative air humidity and 11.4 °C. After wetting the boards with an initial moisture content of 10% for 2 h and naturally re-drying them in laboratory conditions, an average increase of 11.7% in air permeability was observed. The increase in air permeability was 5.6% with a pressure difference of 50 Pa. After redrying, the boards showed a 1.1% lower average density and 14.5% lower maximum density in the surface layers. From the results, it followed that even the short-term effects of water and the related increase in moisture content of the OSB had a negative impact on the air permeability and VDP.

Keywords: Air permeability; Moisture content; Oriented strand board (OSB); Vertical density profile

Contact information: a: Department of Wood Products and Wood Constructions, Czech University of Life Sciences Prague, Kamýcká 129, 165 21 Praha-Suchdol, Czech Republic; b: Timber Research and Development Institute, Prague, s.e., Na Florenci 7-9, 111 71, Prague 1;

* Corresponding author: hodousek@fld.czu.cz

INTRODUCTION

Oriented strand board (OSB) is a material made from large, flat strands of wood, where the outer layers strands are oriented parallel to the long edge of the board or to the production line, and the core layer is often formed by smaller strands oriented at right angles to the outer layers (Irle et al. 2012). The outer layers, mostly compressed to higher density, ensure the major mechanical and physical properties of the boards. Manufacturing is optimized by the correct setting of production factors such as pressing time, pressure, and step-closing time. OSB is made from wooden flakes that are not so easy to compress as medium density fiberboard (MDF). Compared to MDF manufacturing, the step-closing schedules are commonly used for better densification of OSB, while their use for MDF manufaturing could be useful for MDF density profile manipulation (Wang et al. 2001). The plasticization and further densification of the flakes are the factors that require the step-closing schedules (Wang et al. 2004). The geometry and arrangement of the strands in the surface layers are also important factors. By correctly setting these production factors, a horizontal density profile (HDP) and vertical density profile (VDP) are created (Strickler 1959; Suchsland 1962; Winistorfer and Wang 1999; Wolcott et al. 2007). The VDP is mainly the result of the press closing process, where the surface layers show a higher density than the middle layers. It was experimentally ascertained that boards with M-shaped VDPs have suitable mechanical and physical properties. The moisture content (MC) and the orientation of the strands in the surface layers have a notable impact on the formation of the VDP. A higher moisture content in surface layers results in the achievement of a higher maximum density (Andrews et al. 2001). By increasing the moisture content of the core layer, it is possible to reverse the VDP (Heebink et al. 1972). García et al. (2008) stated that strand mats with a lower strand arrangement level exhibit a more balanced VDP. Another significant factor that influences the VDP is press speed and its combination with the MC of the layer. Higher press speed combined with high MC of the layer result in higher thicknes swelling of the layer (Candan et al. 2012).

Thickness swelling occurs during the reaction of moisture on a finished OSB. Higher percentages of thickness swell occur in surface layers that have a higher density. Wang and Winistorfer (2002, 2003) tested 12-mm-thick OSBs and revealed that swelling occurs to a greater extent (74%, 64%, and 57%) in the surface layers that comprise only 39% of the total sectional thickness of the board. The air permeability of the samples of particleboard was tested by Langmans et al. (2010a) with the application of a known quantity of water for a short time period in order to simulate rain. The air permeability measured after the application of the water was approximately half compared to the measurement of the dry state. After 30 min, the air permeability had 91% to 96% of the air permeability of the dry state. This experiment only studied very short and mild exposure to moisture, which could not penetrate the paraffin layer, resulting in zero swelling or change in the VDP. During longer exposure to moisture, the paraffin impregnation fails and the board swells, which is the main assumption of this paper. According to Adcock and Irle (1997), wood cells with a higher density have a greater swelling potential. Thickness swell reduces density, and this phenomenon is more significant in higher density areas. As such, the reduction of mechanical and physical properties on the basis of swelling is notably influenced by changes in the VDP (River 2003; Xu and Winistorfer 2007). Thus, swelling occurs more in the surface layers, which ensure the air impermeability of the board. Thickness swell is associated with the loss of the mechanical properties of composite materials (Suchsland 1973; Alexopoulos 1992; Wu and Piao 1999; River 2003), which is associated with increased inherent stress in the board that arises from the impact of the various swelling rates of the materials from which it is made. For strand boards, this phenomenon has been described as the straightening of wood parts (Halligan 1970; Fan et al. 2009). The strands, which are compressed and subsequently exposed to higher humidity, swell and naturally regain their initial shape. This changes their location inside the board, and these changes cause stress in the board that disrupts the bond between the glue and wooden components (Medved et al. 2006).

Air permeability, as a physical property of OSB, is strongly dependent on its manufacturing process. Taking into account that OSB is made by pressing strands of wood together, there are still some gaps allowing air to flow. This determining factor of air permeability is porosity (Al-Hussainy et al. 2013). The porosity is influenced more by thichness of the strands than by other geometric factors (Dai et al. 2005), but in general, the smaller the strands, the smaller the gaps between them (Kruse et al. 2000), which opens the possibility to decrease the air permeability by adding a small fraction into the carpet. This leads to significant decrease of carpet permeability during production (Fakhri et al. 2006). Other factors that affect the air permeability are pressure difference (Kumaran et al.2003) or different manufacturers (Langmans et al. 2010b).

The OSB/3-type board is intended for use in humid conditions where the humidity of the ambient air exceeds 85% for only a few weeks per year, according to standard EN 300 (2006). Companies that offer the construction of wooden buildings in the Czech Republic claim a construction time ranging from three weeks to several months, and the building materials are exposed to natural elements for a certain part of this period. Due to the fact that the individual parts of the building are not yet duly integrated in the building and insulated from the exterior environment, they may absorb moisture. It often occurs that the building material is exposed to a step increase in moisture content from rain. Due to the hysteresis of wood absorption, it is necessary to consider the fact that these step changes in moisture may fundamentally change the properties of the wooden materials used in the building.

The goal of this paper is to determine the differences in the air permeability of the boards after an increase in moisture content and re-drying, and to determine the changes in the VDP.

EXPERIMENTAL

Materials

The board most commonly used for building purposes, the 12-mm-thick OSB/3, was selected for air permeability testing and VDP determination. The boards were obtained from the local manufacturer of this material. The characteristics of the boards are given in Table 1.

The wood strands used to make the boards were graded using a sieve with eye dimensions of 3.5 mm × 30 mm and the ratio of the centre wood strands/surface wood strands was 50/50.

Methods

All of the test samples had a format of 1250 mm × 2500 mm, which was also the production format. The test samples were acclimatized in the environment in which the test was performed (11.4 °C, 60% relative air humidity). The air conditions were chosen according to the air conditions during the early winter, which is the most common period for construction in Central Europe. The air permeability test was conducted on five samples.

The first test was performed on the sample acclimatized to ambient temperature, after which it was exposed to higher humidity in a dip basin. Wetting was performed via submersion of all of the test samples in water for 2 h and subsequent conditioning. One additional board was placed with the rest of the boards to ascertain the VDP in order to verify the assumption that VDP changes with a change in OSB humidity, subsequently impacting air permeability. The VDP was determined using the compact X-ray density profile analyzer – DPX300-LTE (Imal, Modena, Italy), which works on the principle of weakening the energy passing through layers of material with different densities. This method is suitable for the determination of the density profile of the material (Winistorfer et al. 1986).

Six test samples of 50 mm × 50 mm were taken from one board according to EN 326-1 (1997), and the average behaviour of the VDP was determined. During wetting, the average adsorption was 10.4 g of water. The VDP was then measured again on a wet sample.

Table 1. Production Characteristics of the OSB/3 Test Sample

The test samples were exposed to many graduated pressure differences (positive and negative) and the air-flow volume achieved for each pressure difference was measured. The maximum pressure achieved was 600 Pa and the minimum pressure was 50 Pa. After clamping the sample in the test chamber with a clamping jig, overpressure differences of the following values were achieved in the chamber: 50 Pa, 100 Pa, 150 Pa, 200 Pa, 250 Pa, 300 Pa, 450 Pa, and 600 Pa (∆p_max). The pressure differences were applied according to the test method of EN 12114. At each pressure difference value, the air-flow volume in m³/h was measured. The same procedure was used for negative pressure. For this purpose, the airtight chamber commonly used for commercial tests and for the purposes of previous studies (Hodousek et al. 2015) was used. The chamber scheme is shown in Fig. 1.

This was followed by increasing the moisture content by submerging the boards in water for 2 h. The water was subsequently drained and the boards were loosely covered with polyvinyl chloride (PVC) film for 48 h to balance the moisture content in the sample. This process was intended to simulate the building construction process where the opened package of boards is exposed to rain for a short time, after which it is left to rest for a while, and only later are the boards removed and integrated in the building. The average water absorption was 1.4 kg per board with dimensions of 1250 mm × 2500 mm (0.45 kg/m², average MC 17.4%) and 10.4 g of water per test sample with dimensions of 50 mm × 50 mm (average MC 67.9%). The difference between absorption of the big and small specimens during the same time period is caused by the edge effect when water enters into the board through the edges where the water sorption is higher (Gu et al. 2005). The MC was determined using the oven dry method (EN 13183-1). The air permeability and VDP of the wet samples were measured.