Plywood made from plasma-treated veneers: Shear strength after shrinkage-swelling stress

Abstract

Thermally modified and unmodified beech veneers in untreated and plasma-treated state were immersed in melamine solution at different concentrations. The plasma pre-treated veneers exhibited significantly higher melamine loads than the untreated veneers at equal impregnation duration. Subsequently the veneers were manufactured into 5-layer plywood boards; the plywood samples then underwent an extreme testing procedure based on DIN-EN 314-1/2 in order to proof the bonding quality by means of shear strength. The plywood boards made of plasma pre-treated veneers exhibited up to 2.7-fold improvement in shear strength compared to plywood made of untreated veneers. Results differed markedly based on material (unmodified/thermally modified veneer) and concentration of melamine solution

Plywood Made from Plasma-Treated Veneers: Investigation of Performance Differences between Plasma-Pretreated and Untreated Beech Veneers at Comparable Melamine Resin Load

“…In order to provide a concrete reference to the previously discussed and published studies [9,13], native and thermally modified beech veneers and melamine resin were used in this study. In the context of this study, the term "reference" is used exclusively for the plywood specimens made from non-plasma-pretreated beech veneers, where these veneers remained in the immersion bath until a liquid uptake or WPG (WPG = Weight Percentage Gain) of plasma-pretreated veneers (1 s impregnation time) was achieved.…”

“…In fact, only comparisons of the plasma-pretreated samples and the reference with the same load will be treated in this study, without including in the tests the references that have the same impregnation time (1 s, with a correspondingly lower load) as the plasma-pretreated samples. The comparisons of the plasma-treated samples with the "regular" references (in the proper sense), where the relevant process parameters (impregnation time of the veneers) remained the same as the parameters of the plasma-treated samples, have already been discussed in detail [9,13].…”

“…The veneer sheets were transported centrally by a roller conveyor through the gap (5 mm) between two electrode arrays. Each electrode array consisted of five individual electrodes made of rectangular Al 2 O 3 tubes (15 × 15 × 400 mm 3 ; wall thickness = 2.5 mm) filled with bronze powder, with a gap of 5 mm between the individual electrodes of an array [9,13,16]. Two radial fans above and below the electrode arrays generated an ambient air flow towards the electrode arrays (cooling and discharge homogenization).…”

“…The authors derived this hypothesis from the observation that the typical plasma effects (decrease in surface energy and pH) could be detected not only in the near-surface areas, but also in the core material of plasma-treated wood veneers [12]. As a consequence, plywood samples made from beech veneers treated with plasma and subsequently impregnated with melamine resin showed higher shear strength than the reference [13]. Recent studies [9,13,14] on the comparability of plywood samples made from plasma-pretreated and non-pretreated beech veneers raise the following question: how do plywood specimens perform with respect to dimensional stability and mechanical properties when the individual (non-plasma-pretreated) veneers of these specimens have the same melamine resin uptake-with a correspondingly longer immersion time-as the plywood made from plasma-pretreated veneers (impregnation time 1 s)?…”

“…Recent studies [9,13,14] on the comparability of plywood samples made from plasma-pretreated and non-pretreated beech veneers raise the following question: how do plywood specimens perform with respect to dimensional stability and mechanical properties when the individual (non-plasma-pretreated) veneers of these specimens have the same melamine resin uptake-with a correspondingly longer immersion time-as the plywood made from plasma-pretreated veneers (impregnation time 1 s)? Although this question has already been briefly touched upon in Reference [13], there is a lack of systematic investigation and discussion on this issue. The present study aims to close this gap of knowledge.…”