Magdalena Sterley

Magnetic resonance imaging of moisture distribution in Pinus sylvestris L. exposed to daily indoor relative humidity fluctuations

Abstract In contact with indoor air, wood materials have a high potential passively to reduce the indoor humidity fluctuations resulting from internal moisture loads and outdoor humidity fluctuations. This ability, characterized by the moisture buffering capacity of building materials, has attracted increasing attention within building physics, but a suitable method to appraise and quantify this phenomenon is still sought. In this study, a non-invasive spectroscopic method of accessing information about the interaction between indoor air and Scots pine was investigated. A comprehensive account is given of spatially resolved moisture absorption (desorption) into (from) Scots pine by proton magnetic resonance imaging ([1H]MRI) based on an effective single-point imaging (SPI) sequence. SPI images of bound-water distribution in Scots pine with a spatial resolution on a sub-millimetre scale were acquired when one of the orthotropic directions of the wood material was exposed to typical indoor day-to-day moisture fluctuations. The nuclear magnetic resonance imaging measurements presented in this study clearly show the potential of the method to provide accurate spatial information about the wood–water interaction below the fibre saturation point and hence to characterize the moisture buffering capacity of wood materials.

Finger Jointing of Freshly Sawn Norway Spruce Side Boards – A Comparative Study of Fracture Properties of Joints Glued with Phenol-Resorcinol and One-Component Polyurethane Adhesive

Finger jointing of unseasoned Norway Spruce was studied with respect to tensile strength, adhesive penetration and durability. Finger joints were manufactured with 1) unseasoned wood and one component polyurethane (PUR) adhesive, 2) dried wood and PUR adhesive and 3) dried wood and phenol resorcinol formaldehyde (PRF) adhesive. Two levels of wood density were used. The tensile strength of the finger joints was determined and the deformations within the joint were studied with an optical measurement system (ARAMIS). The penetration of the adhesive was studied with x-ray microtomography. The durability of the joints was determined according to the standard ASTM D 4688. The results show that the tensile strength and the durability of green glued finger joints are on the same level as that of dry glued PUR joints. The penetration of the PUR adhesive is high in the unseasoned wood and cavities within the bonds seem to be smaller than in dry glued PUR joints. The tensile strength of the finger joints is dependent on density, independent on the adhesive system used. The strength of the green glued PUR adhesive bonds in finger joints measured with small scale specimens did not differ from the strength of the dry glued PUR bonds.

Compreg-type products by furfurylation during hot-pressing.

Abstract Compreg, phenolic resin impregnated veneers cured under high pressure, was developed in the USA during the early 1940s and immediately found use as a replacement for stainless steel in specific niche applications, e.g. pitch propellers and connector plates in the World War II allied air fleet. Similar material is still produced here in Europe under the tradename Panzerholz, which is used in, for example, bulletproof joinery products. This paper presents two materials, similar to compreg, developed within an EU project (Ecobinders). Veneers were impregnated either with furfuryl alcohol (FA) or waterborne furan prepolymer resin with dissolved catalyst and thereafter dried/precured in an oven. The impregnated veneers were glued with FA/lignin adhesive between the plies, and cured in a hot-press under high pressure. The result was in both cases highly densified modified wood products. The products had hardness higher than any known wood species, high dimensional stability and decay resistance. The properties were generally better for the material modified with FA than for the material modified with the prepolymer which, however, had lower weight percentage gains. Kitchen counter-top prototypes were manufactured by laminating medium-density fibreboard with the FA-compregs. The prototypes were tested for surface resistance and the results fulfilled the criteria for the application.

Green-Glued Products for Structural Applications

The results from bending tests on 107 laminated, green-glued, beams manufactured from Norway spruce side boards are presented. The beams were made by face gluing 21-25 mm thick boards using a commercial one-component moisture curing polyurethane adhesive. In addition to the bending test results, results from shape stability measurements after climatic cycling and bond line strength and durability test results are also presented. The results from the bending tests show that, by applying very simple grading rules, it is possible to obtain beams with high bending strength (with a 5%-percentile characteristic value of 40,1 MPa) and substantial stiffness (mean value of 14360 MPa). Also the shape stability of the beams and the strength and the durability of the interlaminar bonds were found to be satisfactory.

Shear fracture characterization of green-glued polyurethane wood adhesive bonds at various moisture and gluing conditions

Abstract The shear fracture properties of green-glued one-component polyurethane (PUR) wood adhesive bonds subjected to kiln drying were investigated. The local shear strength and fracture energy of the wood adhesive bonds were determined from experimentally recorded complete shear stress versus deformation curves of the bond line. A stable test set-up and small specimens that were anti-symmetrically loaded were used in order to get a uniform and pure state of shear stress. Different moisture contents (MCs) and pressing times were investigated. The fracture properties of conventionally dry-glued wood adhesive bonds and of solid wood were used as reference. The results show that the fracture energy of green-glued bonds with PUR adhesive is dependent on the MC of wood and on the pressing time. The same fracture energy and strength can be obtained by green gluing as by dry gluing, but there seems to exist a maximum MC of sapwood, in the range between 78% and 160%, and a minimum pressing time, in the range between 3 h and 48 h, for which it can be achieved. Both dry- and green-glued polyurethane adhesive bonds were more ductile than solid wood.