Cold tack of urea formaldehyde resins as an important factor in plywood production

Abstract

The ability of urea formaldehyde (UF) resins to develop cold tack is needed in plywood production during pre-pressing in a cold press to ensure that the veneers stick together, can be transported, and fit into a multi-daylight hot press. The influence of defined factors on the cold tack was analyzed by determining the tensile shear strength of uncured birch veneers bonded with UF resin, and the statistically significant impact factors were determined. The factors tested were lay-up time, resin amount, resin age, moisture content, veneer temperature and pre-press time. Moisture content and veneer temperature had the highest impact on the cold tack of UF resins. A negative impact of high moisture content on the tensile shear strength and therefore on the cold tack of UF resins was proven as well as a positive impact of a comparably high (30 °C) veneer temperature. Lay-up time and pre-press time showed a minor impact on the tensile shear strength. Models were created using Design-Expert software to calculate the optimum operation conditions for cold tack.

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

“…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 layup time with 150 g/m 2 resin amount, these parameters were kept constant in the present study.…”

“…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).…”

“…Thus, the experimental setup was reduced. A detailed sample preparation procedure is available in Hogger et al (2018).…”

“…The effect of several factors (lay-up time, veneer moisture content, veneer temperature, resin age, resin amount and pre-press time) on the cold tack of urea-formaldehyde (UF) resin was investigated in a previous study (Hogger et al 2018). Veneer moisture content (MC), veneer temperature, pre-press time, lay-up time and two-factorial interactions showed a significant impact on the measured tensile shear strength of uncured UF as an indicator of cold tack.…”

“…The aim of the present study was to close this gap by analyzing the effect of selected factors on the properties of cured glue joints. MC, veneer temperature and pre-press time showed the highest impact on cold tack, therefore the minimum and maximum values of these factors from Hogger et al (2018) were used for this work.…”