Frank Stöckel

Micromechanical properties of the interphase in pMDI and UF bond lines

Micromechanical properties of cured polymeric diphenylmethane diisocyanate (pMDI) and urea formaldehyde (UF) adhesive and wood cell walls (beech) in adhesive contact compared with cell walls without adhesive contact were measured in situ by means of nanoindentation. Using UV-microphotometry obtained absorbance spectra of micromechanical investigated cell wall regions gave a strong indicator for the presence of pMDI compounds in wood cell walls. Nanoindentation results reveal that both pure UF and UF-penetrated cell walls show a very brittle character. In contrast, pMDI adhesive is very tough and soft at the same time, and when diffused in cell walls, it does not mechanically embrittle the cell structure.

Tensile shear strength of UF- and MUF-bonded veneer related to data of adhesives and cell walls measured by nanoindentation.

Abstract The tensile shear strength of veneer lap joints was characterised. The joints were produced with an Automated Bonding Evaluation System (ABES) using urea-formaldehyde (UF) as well as melamine-urea-formaldehyde (MUF) adhesive formulated for particleboard production. At a fixed heating temperature of 110°C, a systematic increase in bond strength was observed for both adhesives with increasing cure time. The absolute bond strength was significantly higher for MUF compared to UF. Nanoindentation experiments with the same specimens used for ABES revealed a very hard, stiff and brittle character of the UF resin, whereas the MUF proved significantly less hard and stiff, and less brit-tle. Wood cell walls in contact with adhesive, i.e., where adhesive penetration into the cell wall was assumed, showed significantly altered mechanical properties. Such cell walls were harder, stiffer and more brittle than unaffected reference cell walls. These effects were slightly more pronounced for UF than for MUF. Comparing UF and MUF, the micro-mechanical properties of cured adhesive and interphase cell walls confirm earlier observations that tougher adhesives can lead to higher macroscopic bond strength. In strong contrast to that, no obvious correlation was found between micromechanical properties and the strong cure time dependence of macroscopic bond strength.

Macro- and micromechanical characterization of wood-adhesive bonds exposed to alternating climate conditions

Abstract Three-part specimens were produced from Norway spruce wood (Picea abies Karst.) and bonded with the following adhesives: melamine-urea-formaldehyde (MUF), phenol-resorcinol-formaldehyde (PRF), and a two-component emulsion polymer isocyanate (EPI). The effect of alternating climate conditions on bond strength was studied by tensile tests. The specimens were exposed to a three-step ageing cycle lasting for 7 days [50°C/95% relative humidity (RH), -20°C/65– 70% RH and 75°C/15% RH] which was repeated 24 times. In general, a decrease in internal bond strength of all exposed specimens was observed but it was highest in the case of MUF-bonded joints. Furthermore, a significant decrease of the tensile strength of the wood adherend perpendicular to the grain in the tangential direction was determined after the cyclic climatic changes. The mechanical performance of the different adhesives in the bond line was tested by means of nanoindentation. Reduced values of elastic modulus, hardness, and total indentation were observed after climatic treatment, particularly for the rigid MUF adhesive, whereas the flexible adhesive EPI did not show such changes.

Light microscopic detection of UF adhesive in industrial particle board

Abstract A new procedure to detect urea–formaldehyde adhesive in industrial particle board is presented. The method uses thin sections stained with a visible dye (gentian violet) and a fluorescent dye (brilliant sulphaflavine), respectively, in a two-step procedure. Microscope images of a selected area of interest acquired in visible and fluorescence modes are combined to obtain sufficient contrast, enabling semi-automated detection of adhesive by means of image analysis. No addition of dye prior to particle board production is necessary.

Low-Velocity Transverse Impact of Small, Clear Spruce and Pine Specimens with Additional Energy Absorbing Treatments

AbstractSingle-blow impact testing of spruce and pine specimens were done to assess the effects of additional E-glass reinforcing on the tension face of a beam along with a rubber lamina adhered to...

State-of-the-art: intermediate and high strain rate testing of solid wood

The following state-of-the-art report summarizes important work done in wood science concerning intermediate and high strain rate testing. Intermediate testing may be done with hydraulic machines, which are generally classified as rapid loading. Intermediate testing may also be done using a transverse impact of a beam specimen with a pendulum, drop mass, or toughness tester, whereas high strain rate testing is generally done with the Kolsky bar. The article analyzes the different experimental testing apparatuses, the conclusions past researchers have made about them, and the toughness and strength measurements which were usually done. The transverse impact test is examined in detail because of its adjustability for specimen sizes. The value of this research is it delves into certain impact mechanics principles which are missing from the analysis of impact testing in wood science, which must be included to validate previously held assumptions. The physical response of wood to a dynamic load is not any different from other materials such as metals or rigid foams and is governed by the same principles. Nevertheless, over the years the application of impact mechanics principles to wood testing has been scarce and “black box” experimental, where empirical approaches such as the duration-of-load were often preferred. An example of these principles is the influence of higher modes of vibration plays a greater role on the stress state in testing than its influence on deflections. This principle has been thoroughly investigated for small strain rate vibrations, however, not applied to impact testing. Presently, there is no consensus on a constitutive strain rate model for wood under intermediate and high strain rates. This article provides direction for obtaining dynamic Young’s modulus and yield strength, which can be normally expected in the design of structures subjected to dynamical loadings, for the future creation of applicable constitutive strain rate models.