Influence of resin content and pulping temperature on the formaldehyde release from medium density fibreboards (MDF)

Urea formaldehyde resins (UF-resins) are the main binders used in the wood-based panels industry. Recently, increased awareness of ecology and clean indoor requirement boosted research activity to reduce the formaldehyde emanation from wood-based composites and resulted in the development of UF-resins of extremely low molar ratio (F:U) to be used as a bonding agent. To meet the tight specifications of different regulations enforced in several countries like F**** in Japan (JIS A 1460 2001) UF-resins of molar ratio F:U 1:1 or even much less are used today as adhesives in the industry. Decreasing the molar ratio of formaldehyde to urea (F:U) below a certain limit leads, however, to a significant deterioration in the properties of the resin itself (low reactivity, lower moisture resistance) and in the physical-mechanical properties of the boards (high thickness swelling, low internal bond strength) bonded therewith (Sundin et al. 1987). To offset the negative impact of low molar ratio resins on the board properties, the level of resin loading in the boards is generally increased.

The influence of resin content in particleboards has been studied by Petersen et al. (1973) using resins of molar ratio between F:U 1.6 and 1.4. The results clearly indicate that by using high molar ratio resins of F:U 1.6:1 increasing the binder content between 5 % and 12 % leads to a remarkably higher perforator value; by using F:U resins of 1.4:1 the influence of resin level, however, was found to be too insignificant over the range of binder content between 5 % and 12 %. Efforts were in vain to find literature systematically covering the influence of resin content on the formaldehyde release in particle- or fibreboards bonded with very low formaldehyde UF-resins of molar ratio F:U of 1:1 or lower.

The influence of resin content in medium density fibreboards (MDF) was evaluated on 16 mm-boards with a density of 0.78 g/cm³ made from thermo-mechanical pulps (TMP) prepared from beech wood at two different pulping temperatures (140 °C and 175 °C) using two resin levels (12 % and 20 % solids based on dry fibres). The boards were made using the blow-line technique with a UF-resin of molar ratio 1:1 containing about 3 % melamine. No hardener was used to cure the resin. The pressing temperature was 190 °C and the pressing time was 24 s/mm. After conditioning for three weeks at 20 °C and 65 % rel. humidity the formaldehyde release of the boards was assessed using the perforator technique (EN 120) and the flask method (EN 717-3) after a reaction time of 3 h and 24 h. The measured perforator values were also corrected to a moisture content of 6.5 % according to EN 312 using the empirical formula developed by Jann and Deppe (1990).

As can be seen from Fig. 1, the corrected perforator values decreased significantly by raising the resin level in the boards from 12 % to 20 %. This applies to boards made from thermo-mechanical fibres (TMP) generated from wood at a pulping temperature of 140 °C and 175 °C.

The decrease in the perforator value was particularly pronounced in case of boards made from fibres prepared at a pulping temperature of 175 °C. The perforator value decreased by about 28 % by increasing the binder level from 12 % to 20 % in case of MDF made from TMP prepared at 140 °C and by more than 42 % for boards made from TMP prepared at 175 °C. As there are some uncertainties regarding the correction factor in case of boards with subtle differences in the perforator values in the low emission region the uncorrected perforator values were also measured (Johnsson et al. 2011). The same trend was also evident by comparing the measured but uncorrected perforator values (Fig. 2). The measured perforator values decreased slightly with increasing pulping temperature of the fibres at the same binder level of 12 %; the drop in the perforator value due to the increase in the pulping temperature is much more significant at a resin level of 20 % in the boards. This may be due to the higher fibre acidity of thermo-mechanical pulps produced at 175 °C (pH-value ∼5.2) compared to that of fibres produced at 140 °C (pH-value ∼5.9) as lower pH-values enhance hardening of the resin.

Using the flask method (EN 717-3) for assessing the formaldehyde release after a reaction time of 3 h and 24 h leads to more or less the same results (Fig. 3). At a resin content of 20 % in the boards lower values of formaldehyde release were measured compared to the values obtained for boards bonded with 12 %. This was especially distinctive after a reaction period of 24 h. Differences in the formaldehyde release were also registered between the boards made from TMP prepared by pulping wood at 175 °C compared to those prepared from TMP obtained by wood pulping at 140 °C. However, the influence of resin content in the boards is much more pronounced than that of the pulping temperature.

The formaldehyde release of medium density fibreboards (MDF) bonded with low molar ratio (F:NH₂) urea-formaldehyde resins (UF-resins) decreases significantly by increasing the binder content from 12 % to 20 %. Increasing the pulping temperature of wood from 140 °C to 175 °C reduces also the emanation of formaldehyde from the boards. The liberation of formaldehyde was measured by the flask method (EN 717-3) and the perforator technique (EN 120).