Alternativaufschluss : MDF aus Buchenholz - Einfluss von Feuchte und Verleimung

Leinfelden-Echterdingen

2005

5 Abb.

Artikel aus einer ZeitschriftUnselbständiges Werk

4355

200126815

Medium density fibre-boards (MDF) made from thermomechanical beech pulps using an weak alkaline curing tannin-formaldehyde resin (TF-resin), a strong alkaline phenol-formaldehyde resin (PF-resin) and a binder based on polymeric diphenylmethan-diisocyanate (PMDI) were conditioned for about 12 weeks at 20° C and various relative humidities (30%, 65% and 85% relative humidity) to attain different moisture contents. Thereafter, some of the physicalmechanical properties of the boards (moisture content, density, bending strength, internal bond strength and thickness swelling) were measured. The results reveal: With increasing moisture content of the boards the bending strength of all boards dropped to different extents depending on attained moisture content and the binder used. The internal bond strength of PMDI bonded MDF decreased continuously by increasing the moisture content from 4.8% to 12.4% from 1.58 N/mm2 to 1.14 N/mm2 by nearly 39%. In case of boards bonded with a TFresin the internal bond strength increased first from 1.26 N/mm2 to 1.50 N/mm2 by increasing the moisture content from 5.4% to 8.6% and declined on further increase of the moisture content from 8.6% to 14.4% by 24% from 1.50 N/mm2 to 1.14 N/mm2. Similar behaviour was found in case of PFbonded MDF. However, the change in the internal bond strength was less pregnant. It seems that a postsetting of the alkaline tanninformaldehyde and phenolformaldehyde resin takes place on increasing the moisture content in the above mentioned range.