Influence of cold tack properties on bond performance of cured urea–formaldehyde resin

Birch veneers (Betula) with dimensions of 500 × 500 mm² and a thickness of 1.5 mm were stored in standard climate (20 °C, 65% r.h.) prior to specimen preparation. An industrial UF plywood resin, meeting the requirements for formaldehyde emission class E1, with a solid content of 63% (± 3%), viscosity between 300 and 500 mPa*s and density of 1.3 g/cm³ was provided by Metadynea Austria GmbH (Krems, Austria). 1.75% (liquid hardener on liquid resin) of a 20% aqueous ammonium chloride solution was used as hardener (provided by Metadynea Austria GmbH, Krems, Austria).

Three relevant parameters (veneer moisture content, veneer temperature, pre-press time) were selected at two levels (high and low) for this work. As described by Hogger et al. (2018), these three parameters had the most severe influence on the cold tack. All samples were stored in standard climate (20 °C and 65% RH) before they were adjusted to the required temperature and MC level (± 1%) using two climate chambers (WTB-Kühlbrutschrank, Binder, Tuttlingen, Germany; Temperature and Climatic Test Chamber WK3 180/40, Weiss Technik AG, Altendorf, Switzerland).

In a previous study (Hogger et al. 2018), increasing lay-up time, MC and the two-factorial interaction of resin amount with moisture content decreased cold tack, while higher veneer temperature, longer pre-press time and the two-factorial interaction between lay-up time and MC enlarged the corresponding strength value. Since in previous experiments, both the minimum and the maximum tensile shear strength could be obtained by combining 10 min lay-up time with 150 g/m² resin amount, these parameters were kept constant in the present study. Thus, the experimental setup was reduced. A detailed sample preparation procedure is available in Hogger et al. (2018).

In a full factorial test design, 20 samples were tested for each combination as presented in Table 1. After pre-conditioning and pre-pressing in the respective climates, the veneers were pressed to cure in a hot press (Langzauner GmbH, Lambrechten, Austria) at a press temperature of 110 °C, a specific pressure of 1.5 MPa and a press time of 10 min. Half of the samples were mechanically tested within 30 min after hot pressing (unconditioned) and the other half after 14 days of conditioning in standard climate (20 °C, 65% RH) using a universal testing machine (Zwick/Roell Z020, Ulm, Germany) in tensile shear mode. A load cell with a load capacity of 5 kN and a cross head speed of 3 mm/min was used to induce and measure the applied force. The tensile shear strength (TSS) was calculated by dividing the maximum force by the measured overlap area. The wood failure percentage (WFP) was evaluated visually on the basis of EN314-1 (2005).

Experimental results were visualized by boxplots, using SPSS software (Version 21, Statistical Package for Social Science, IBM®, Armonk, USA). The responses and corresponding parameters were analyzed using independent samples Student´s t-tests (TSS) and a Mann–Whitney U Test, as a non-parametric method for the not normally distributed WFP, for p ≤ 0.05.

In order to evaluate the influence of the selected factors on the tensile shear strength, a multiple regression analysis with a linear variable selection, based on a t-test, was performed with the statistical software Design-Expert® (version 10, Stat-Ease, Inc., Minneapolis, USA), which led to a poor coefficient of determination (R²). The results are not displayed in this work.