Artificial weathering of surfaces from laminated phenol-formaldehyde resin impregnated compressed wood: impact of top veneer type and overlay application

Björn Weiß,Tino Schulz,Jürgen Schubert,Lars Passauer,Henry Burkhardt,Philipp Flade

doi:10.1007/s00107-020-01648-0

Abstract

Panels from laminated phenol-formaldehyde resin (PF) impregnated and compressed wood (Compreg) with top veneers from European species of beech, maple and white birch were prepared, each without or coated with resin pre-impregnated overlay papers. To clarify the outdoor resistance of Compreg surfaces, long-term artificial weathering experiments were performed. Weather-related color changes, gloss retention and microscopic and surface topographical surface alterations were examined. It has been observed that the application of PF impregnated paper overlays and combinations of them with melamine formaldehyde resin (MF) treated films significantly improved the weather resistance of Compreg surfaces. The extent of UV and moisture-induced surface damages of Compreg has been shown to also be dependent on the veneer types used as surface top layer. Highest surface and color stabilization against weather was achieved providing Compreg with top veneers from maple combined with PF overlays or applying both MF and PF impregnated overlay films as surface finish.

Highlights

Impregnation with thermosetting resins and subsequent hot compression of pre-impregnated wood veneers reaching raw densities up to 1400 kg m−3 is an attractive and promising technology to improve strength properties, dimensional stability and durability of laminated wood against decay and weather (Hill 2006; Stamm and Seborg 1955)
The manufacture of the respective products is a proven technology based on the following sub-processes. (I) Impregnation of wood veneers, in Europe commonly rotarycut veneers from European beech, with water-soluble lowmolecular phenol-(PF) and rather more rarely melamineformaldehyde (MF) condensation resins. (II) Pre-drying of impregnated veneers at 100–150 °C. (III) Subsequent hot pressing and deformation/densification of pre-treated veneers with pre-crosslinked reactive resins at specific pressures of 10–25 MPa and temperatures of 100–150 °C to highly compressed laminated wood (Niemz and Wagenführ 2012; Dunky and Niemz 2002; Stamm and Seborg 1955)
The resins applied act as plasticizer, soften the wood cell wall and enable a non-destructive cell wall collapse and fiber deformation while wood is compressed in transverse direction (Shams et al 2004)

Summary

Introduction

Impregnation with thermosetting resins and subsequent hot compression of pre-impregnated wood veneers reaching raw densities up to 1400 kg m−3 is an attractive and promising technology to improve strength properties, dimensional stability and durability of laminated wood against decay and weather (Hill 2006; Stamm and Seborg 1955). Similar products called “Compreg” were developed in the US during World War II at the Forest Products Laboratory (Stamm and Seborg 1941). (III) Subsequent hot pressing and deformation/densification of pre-treated veneers with pre-crosslinked reactive resins (prepregs) at specific pressures of 10–25 MPa and temperatures of 100–150 °C to highly compressed laminated wood (Niemz and Wagenführ 2012; Dunky and Niemz 2002; Stamm and Seborg 1955).

Methods

Results

Conclusion