Investigating the shape stability of moulded phenol-formaldehyde modified beech veneers by means of digital image correlation

Within the present study, measurements by digital image correlation (DIC) were conducted to determine the effect of phenol-formaldehyde (PF) modification on the shape stability of moulded beech veneers (Fagus sylvatica L.) exposed to changing climatic conditions. For this purpose, thin beech veneers were impregnated with a low (lmwPF), a medium (mmwPF) and a high molecular weight PF (hmwPF). The impregnated veneers were cured for one hour at 140 °C in a moulding form to obtain a shaped veneer with an angle of 90°. Two samples were prepared for each PF type as well as two control samples. During the DIC measurements, the samples were exposed to two different sequential climatic conditions for 46 hours. In the first segment, the samples were conditioned at 90 % relative humidity at a temperature of 20 °C for 23 hours. During the second segment, the samples were exposed for further 23 hours to 70 °C without controlled relative humidity to dry the samples. At the end of the first segment, the DIC measurements of the control samples revealed the greatest changing in angle (109.6°) coincident with the lowest shape stability, followed by the hmwPF samples (ca. 107.2°), the mmwPF samples (100.8°) and the lmwPF samples (97.6°). As a result of the second segment, the samples approach the initial shape of a 90° angle due to drying. Here, the hmwPF samples are closest to the initial shape (90.8°) followed by the lmwPF samples (92.4°), the mmwPF samples 92.6° and the control samples (96.1°).