Evaluation of surface roughness of laminated veneer lumber (LVL) made from beech veneers treated with various fire retardants and dried at different temperatures

Forest Products Journal, Jan/Feb 2008 by Dundar, Turker, Ayrilmis, Nadir, Candan, Zeki

Abstract

In this study the surface roughness of laminated veneer lumber (LVL) treated with various fire-retardants and dried at different temperatures was investigated. Commercially manufactured veneer of beech wood (Fagus orientalis L.) was treated with borax-boric acid, monoammonium phosphate and diammonium phosphate using a full-cell pressure process. Concentration of the chemical solutions was adjusted to provide average retention of 56 kg/m^sup 3^. The veneers were than dried at different temperatures (120, 140, 160, and 180 °C), and experimental LVL panels were made from these veneer sheets. A stylus method was employed to measure the surface roughness and evaluate the surface characteristics of the samples. According to the results of this study, surface roughness of the LVLs made from untreated veneers was not influenced by different drying temperatures. When the drying temperature was above 140 °C, surface roughness of LVLs made from fire-retardant treated veneers was significantly increased as compared to untreated control specimens. Although surface roughness values of the specimens made from treated veneers dried at less than 140 °C were also higher than that of untreated specimens, these increases were not statistically significant. No significant difference relative to surface roughness was found among the fire-retardant chemicals used in this study. When each treatment group was individually evaluated, low veneer drying temperatures (120 to 140 °C) did not affect surface roughness of the LVLs, while a significant increase was found at high temperatures (160 to 180 °C).

Laminated veneer lumber (LVL), as the name implies, is an alternative to solid-sawn or glue-laminated lumber (Haygreen and Bowyer 1996). It is a solid, highly predictable, uniform lumber product because natural defects such as knots, slope of grain, and splits have been dispersed throughout the material or have been removed altogether. LVL is typically made from dried and graded veneer that is coated with waterproof adhesives, assembled in an arranged pattern, and formed into billets by curing in a heated press (Anonymous 2006).

Surface roughness of veneer plays an important role in plywood manufacture. Because of its effect on gluing and bonding characteristics, veneer surface roughness is an important factor in the production of LVL. Faust and Rice (1986) found that the use of rough veneers in LVL decreased bonding quality approximately 33 percent compared to LVL made from smooth surface veneers. Rough veneers reduce contact between the layers resulting in a weak glueline and low strength properties of the plywood (Kantay et al. 2001 ). Veneer with a rough surface can also cause excessive resin use and may result in resin bleeding through the face veneer. Roughness of face veneer can be improved to a certain extent by sanding; however, this increases overall production costs (Lebow and Winandy 1998, Taylor et al. 1999).

Aydin and Colakoglu (2002) determined the effects of veneer drying temperature on wettability, surface roughness, and some properties of plywood. They concluded that surface smoothness of both alder and spruce veneers increased with increasing drying temperature. The smoothest surfaces were obtained at 180 °C drying temperature for alder veneers and at 150 °C for spruce veneers. It has been observed in several studies that drying and heat treatment of solid wood at high temperatures cause changes in the properties of wood. But it has not been widely studied whether similar changes occur in thin sections such as veneers when drying temperature is higher but the exposure time is shorter than those applied to solid wood (Lehtinen 1998). Unsal et al. (2005) reported that drying temperatures normally used on veneers (100,115, and 130 0C for 3 min) did not have a marked effect on surface roughness of sliced beech (Fagus orientalis Lipsky) veneers. They recommend that it will be more beneficial to use higher drying temperatures, such as 150 0C and higher, in further studies.

Since LVL is commonly used as a component in furniture manufacture, surface roughness also plays an important role in various thin overlays or finishes, which are sometimes directly applied to the sanded surface of LVL treated with fire-retardant.

Markets for structural LVL are expanding, and the products are being used in more challenging conditions. Fire-retardanttreated (FRT) LVL is being used increasingly in exterior exposures, for both roofing and siding. Now, FRT-molded wall panels are being produced such that the composite becomes both skin and structural elements. The fire-retardant chemicals mostly used for wood products contain boron, phosphorus, phosphoric acid (PA) and its salts, including monoammonium phosphate (MAP) and diammonium phosphate (DAP). These phosphates are among the oldest known fire-retardant compounds, and they are usually included in most proprietary systems used for wood (Lebow and Winandy 1998). Boron compounds are relatively safe and effective for increasing the resistance of composites to fire and biodégradation. The influence of these chemical treatments on the surface roughness of experimental plywood was quantified based on three roughness parameters obtained from the stylus-type profilometer. Three main roughness parameters, average roughness (R^sub a^), mean peak-to-valley height (R^sub z^), and root mean square (RMS) roughness (Rq) obtained from the surface of LVL were used to evaluate the effect of chemical treatments on the surface characteristics of the specimens.