Abstract
The work focused on new solutions for the production of low formaldehyde emission wood-based panels. Wood is a renewable material of excellence, contributing positively to the life cycle of carbon. In Portugal the forest-related economy is responsible for 9 % of the industrial jobs and contributes 12 % of the total exports. Portugal is also the headquarters of one of the largest global wood-based panels producers, Sonae Indústria SA.
In recent years, the wood-based panel industry has faced a big challenge: formaldehyde emission. Since the publication by the International Agency for Research on Cancer (IARC) from World Health Organization (WHO) in 2006, of the reclassification of formaldehyde as ‘carcinogenic to humans (Group 1)’, panel producers and, consequently, wood adhesive producers were compelled to reduce formaldehyde emission significantly, to avoid losing market share to other products from non-natural sources, such as plastic or metal.
The thesis begins with a general introduction followed by five parts reporting the various aspects of the developed work. The second part concerns the study the synthesis of urea-formaldehyde (UF) resins. The effect of the most important parameters (pH and temperature) is studied and the polymeric structure formed characterised. Adhesive performance of the resins applied to particleboard manufacture is assessed. In order to improve the adhesive properties of these resins, the addition of compounds containing hydroxyl groups, such as sucrose, is studied. Advanced characterisation techniques, namely Nuclear Magnetic Resonance (NMR), Raman spectroscopy and Size-Exclusion Chromatography (SEC) are essential to assess the chemical reactions involved.
The third part of the thesis focuses on the curing of UF resins. Conventional hardeners - so called latent catalysts, namely ammonium sulphate and ammonium nitrate, react at high temperature with existing free formaldehyde in the resin, forming a strong acid that promotes resin cure. Currently, these hardeners present low performance due to the lower free formaldehyde concentration in resin. On the other hand, this reaction produces hexamine as by-product, which decomposes when exposed to heat and moisture conditions during the lifetime of the panel. The performance of phosphoric, citric and oxalic acids as UF resin cure catalysts is evaluated. In addition, the contribution of hexamine to the formaldehyde emission is demonstrated.
The fourth part of the thesis deals with methods for determination of formaldehyde emission and the use of formaldehyde scavengers. The most used methods for determining formaldehyde emissions were compared: perforator (EN 120), gas analysis (EN 717-2) and desiccator (JIS A 1460). The performance of difference formaldehyde scavengers, their application methods, and the relation between data from different emission methods are discussed.
In the fifth part, the emission Volatile Organic Compounds (VOCs) of two different wood species is studied: a softwood (Pinus pinaster) and a hardwood (Populus spp.). In addition, the performance of the most promising formaldehyde scavengers in terms of total VOCs emission reduction is assessed.
Finally, in the sixth part, the general conclusions were formulated from the knowledge acquired during the preparation of the thesis, which made it possible to propose future investigation trends.