scholarly journals Measurement of moisture-related strain in bonded ash depending on adhesive type and glueline thickness

Holzforschung ◽  
2016 ◽  
Vol 70 (2) ◽  
pp. 145-155 ◽  
Author(s):  
Markus Knorz ◽  
Peter Niemz ◽  
Jan-Willem van de Kuilen
Keyword(s):  
High Strain ◽  
Urea Formaldehyde ◽  
Wood Adhesive ◽  
Image Correlation ◽  
Adhesive Bonds ◽  
Mechanical Loads ◽  
Resorcinol Formaldehyde ◽  
Emulsion Polymer ◽  
Distinct Strain ◽  
Adhesive Type

Abstract Structural wood-adhesive bonds (WAB) have to be durable while subjected to considerable stresses caused by mechanical loads and moisture content changes. To better understand the moisture-related durability of WABs, knowledge is important of how moisture changes generate strain in the bond. In this paper, strain on end-grain surfaces of bonded ash specimens was analyzed by means of digital image correlation. Strains were generated by wood shrinkage, and the evaluation was focused on shear strain (SStr). The bond lines were studied depending on the adhesive type – phenol resorcinol formaldehyde (PRF), melamine urea formaldehyde (MUF), polyurethane (PUR), and emulsion polymer isocyanates (EPI). Moreover, three different glueline (GL) thicknesses of MUF were taken into consideration. Comparing the adhesive types, SStr distributions (SStrD) were strongly influenced by adhesive elasticity. MUF and PRF bonds were quite rigid and were associated with pronounced strain amplitudes in and close to the GL together with strain dissipation reaching deep in the wood. PUR and EPI adhesives were more elastic and therefore allowed for smoother strain transition showing less distinct strain peaks. GL thickness had significant impact on SStrD. A high strain level and direct strain transition between adherends was found for the 0.01 mm GL, whereas a pronounced strain decrease was observed in the 0.1 and 0.2 mm GLs. This indicates different stress levels in the wood-adhesive interface dependent on GL thickness.

Mechanical characterisation of wood-adhesive interphase cell walls by nanoindentation

Holzforschung ◽  
2006 ◽  
Vol 60 (4) ◽  
pp. 429-433 ◽  
Author(s):  
Johannes Konnerth ◽  
Wolfgang Gindl
Keyword(s):  
Elastic Modulus ◽  
Cell Walls ◽  
Urea Formaldehyde ◽  
Wood Adhesive ◽  
Interphase Cell ◽  
Adhesive Bonds ◽  
Resorcinol Formaldehyde ◽  
Interphase Region ◽  
Mechanical Characterisation ◽  
Penetration Behaviour

Abstract The elastic modulus, hardness, and creep factor of wood cell walls in the interphase region of four different adhesive bonds were determined by nanoindentation. In comparison with reference cell walls unaffected by adhesive, interphase cell walls from melamine-urea-formaldehyde (MUF) and phenol-resorcinol-formaldehyde (PRF) adhesive bonds showed improved hardness and reduced creep, as well as improved elastic modulus in the case of MUF. In contrast, cell walls from the interphase region in polyvinylacetate (PVAc) and one-component polyurethane (PUR) bonds showed more creep, but lower elastic modulus and hardness than the reference. Considering the different cell-wall penetration behaviour of the adhesive polymers studied here, it is concluded that damage and loss of elastic modulus to surface cells occurring during the machining of wood is recovered in MUF and PRF bond lines, whereas damage of cell walls persists in PVAc and PUR bond lines.

scholarly journals Adhesive-related warping of thin wooden bi-layers

2019 ◽  
Vol 53 (5) ◽  
pp. 1015-1033
Author(s):  
Axel Rindler ◽  
Oliver Vay ◽  
Christian Hansmann ◽  
Johannes Konnerth
Keyword(s):  
Mechanical Properties ◽  
Urea Formaldehyde ◽  
Wood Adhesive ◽  
Wood Products ◽  
Adhesive Systems ◽  
Emulsion Polymer ◽  
Curing Characteristics ◽  
Engineered Wood Products ◽  
Hydrophilic Material ◽  
Identical Material

Abstract Warping of layered wood-based panels is still a challenging problem in the development of thin engineered wood products. Wood as an anisotropic and hydrophilic material tends to change its volume and mechanical properties with changing moisture content. Besides the wood components, also the mechanical properties of certain adhesives are sensitive to moisture changes. A moisture load onto the adhered wood is resulting in different stress and strain states between the adherends. It is expected that adhesives with different moisture-related properties participate differently to this interaction. To observe an adhesive-related warping, thin spruce/HDF (Picea abies and high-density fibreboard) bi-layers with identical material geometries were manufactured under laboratory conditions, using different wood adhesive systems, which are currently used in furniture and flooring industry [polyurethane (PUR), emulsion polymer isocyanate (EPI), polyvinyl acetate (PVAc), urea formaldehyde (UF) and ultra-low emitting formaldehyde amino adhesive (ULEF)]. The bi-layers were exposed to certain relative humidity conditions, and the resulting deformation was measured with a high-precision laser distance detector. Moisture-dependent warping of the bi-layers was obtained in relation to the used adhesive systems. As a result of the study, it can be shown that initial warping after panel manufacturing strongly depends on the adhesive curing characteristics and, especially, on the amount of water that is released into the wood adherend. For the post-setting panel warping, a differentiation into two adhesive groups became visible: rigid and flexible adhesives. Rigid adhesives (UF and ULEF) showed a higher degree of warping compared to the group of flexible adhesives (PUR, EPI and PVAc).

Contact-free measurement and non-linear finite element analyses of strain distribution along wood adhesive bonds

Holzforschung ◽  
2005 ◽  
Vol 59 (6) ◽  
pp. 641-646 ◽  
Author(s):  
Erik Serrano ◽  
Bertil Enquist
Keyword(s):  
Finite Element ◽  
Measurement Technique ◽  
Strain Distribution ◽  
Wood Adhesive ◽  
Load Level ◽  
Image Correlation ◽  
Large Strains ◽  
Adhesive Bonds ◽  
Non Linear ◽  
Contact Free

Abstract The strain distribution along wood adhesive bonds was studied using a contact-free measurement system based on a white-light digital image correlation (DIC) technique. Two different specimen geometries and three different adhesives were investigated. The specimen geometries were according to the standards EN302-1 and ASTM D905. The adhesives tested were a phenolic resorcinol (PRF), a one-component polyurethane (PUR) and an epoxy (EPX). In addition to the experimental investigation, a finite element study using a non-linear fracture mechanics model for the adhesive bond line was carried out, aimed at investigating whether deformation measurements could predict differences in the mechanical behaviour of the adhesives. The measurement technique was found to be capable of distinguishing, in terms of their strain distributions at a given load, adhesives that differed markedly from one another. For example, the brittle PRF adhesive showed more localised strains than the more ductile EPX and PUR adhesives did at the same load level. Another conclusion from this study is that the measurement technique used is applicable to situations in which large strains occur. Thus, the technique used here is of great interest for use in the calibration of finite element models and constitutive theories and for the design of test set-ups.

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

Holzforschung ◽  
2010 ◽  
Vol 64 (6) ◽  
Author(s):  
Jürgen Follrich ◽  
Frank Stöckel ◽  
Johannes Konnerth
Keyword(s):  
Bond Strength ◽  
Mechanical Performance ◽  
Urea Formaldehyde ◽  
Tensile Tests ◽  
Tangential Direction ◽  
Wood Adhesive ◽  
Bond Line ◽  
Adhesive Bonds ◽  
Climate Conditions ◽  
Micromechanical Characterization

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.

Microstructure and Quantitative Micromechanical Analysis of Wood Cell–Emulsion Polymer Isocyanate and Urea–Formaldehyde Interphases

2017 ◽  
Vol 23 (3) ◽  
pp. 687-695 ◽  
Author(s):  
Lizhe Qin ◽  
Lanying Lin ◽  
Feng Fu ◽  
Mizi Fan
Keyword(s):  
Cell Wall ◽  
Cell Walls ◽  
Laser Scanning ◽  
Urea Formaldehyde ◽  
Wood Adhesive ◽  
Laser Scanning Microscopy ◽  
Confocal Laser ◽  
Micromechanical Analysis ◽  
Emulsion Polymer ◽  
Uf Resin

AbstractEmulsion polymer isocyanate (EPI) and urea-formaldehyde (UF) were selected as typical resin systems to investigate the microstructure of wood–adhesive interphases by fluorescence microscopy (FM) and confocal laser scanning microscopy (CLSM). Further, a quantitative micromechanical analysis of the interphases was conducted using nanoindentation. The FM results showed that the UF resin could penetrate the wood to a greater extent than the EPI resin, and that the average penetration depth for these two resin systems was higher in the case of latewood. CLSM allowed visualization of the resin distribution with contrasting colors, showing that the EPI resin could not penetrate the cell wall, whereas UF resin could enter the cell walls. The micromechanical properties of the cell walls were almost unaffected by EPI penetration but were significantly affected by UF penetration, especially in the first cell wall from the glueline. This further confirmed that only cell walls with resin penetration can improve the mechanical properties of the interphase regions.

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