PROPERTIES OF PLYWOOD WITH LOW CONTENT OF FORMALDEHYDE .2. CREEP-BEHAVIOR

Tests were carried out on both industrial plywood with low content of formaldehyde and laboratory plywood. There were six series plywood from different industrial manufacturers including blockboard and a blockboard with particleboard decks. Twelve lab-series (5 x 1.5 BU) were produced of different formulae by the technical department of WKI. In addition to testing creep behaviour, bending and bonding properties as well as formaldehyde emissions were determined (part 1). Testing of creep behaviour was carried out in accordance with the FESYP method as is common practice when testing particleboard. Creep behaviour of plywood under permanent load in alternate climates was primarily influenced by the board structure. This can be seen in particular with regard to industrial plywood. So, for example, a blockboard with particleboard deck (vertical to block core) had a creep factor two to three times higher compared with that of a nine fold beech plywood. The influence of humidity became apparent at climatic changes. Values measured in humid climate had a factor of 0.5 higher than those measured in dry climate. Laboratory plywood of the same structure as industrial plywood had a creep factor of 2 to 3 times higher than the latter. This is mainly because of the changes in humidity of the samples experienced in alternating climates. Whereas there were hardly any differences of humidity in all plywood in the dry period with values ranging from 7.1% to 7.7%, differences became significant in the humid period, humidity of industrial plywood rose to a max. of 11.7%, laboratory plywood, however, came up to 14.3%. No connection could be detected between formaldehyde emissions and creep behaviour. Plywood with low content of formaldehyde did not show significantly higher long-term bending behaviour.