Formaldehyde emissions from wood-based panels: Testing methods and industrial perspectives

Abstract

Formaldehyde is an important chemical feedstock for the production of phenoplastand aminoplast thermosetting resins, by reaction with other monomers (mostly urea, butalso melamine, phenol and resorcinol). These adhesives are mainly used in themanufacture of wood-based panels: plywood, particleboard, hardboard, medium densityfiberboard (MDF) and oriented strand board (OSB). These products have a wide range ofapplications, from non-structural to structural, outdoor or indoor, mostly in constructionand furniture, but also in decoration and packaging. The WBP industry plays animportant role in the global economy and contributes for forest sustainability and carbonsequestration. In 2009, FAO (Food and Agriculture Organization) reported that a total of260 million m3 WBPs were produced in the world (Europe 29.7%, Asia 43.9%, NorthAmerica 18.3% and others 2.5%). Being economically competitive and highly performing, a major drawback offormaldehyde-based resins, mostly urea-formaldehyde, is the formaldehyde emissionduring panel manufacturing and service life. There are two sources of emission: releaseof unreacted monomer, during or after panel production, and long-term resin degradation(hydrolysis). The formaldehyde content and chemical stability of the resin will thereforeaffect emission levels. In addition, external factors like temperature, humidity or airrenewal rate will also play a role. It must be noted that wood itself contributes toformaldehyde emission, since it is a product of metabolism and decomposition processes.The actual emission level depends strongly on the type(s) of wood used in panelproduction.Due to information considering formaldehyde as potentially carcinogenic to humans,the implementation of international regulations and requirements for emissions fromWBPs has led to establishment of standard testing methods. Two main groups areconsidered: chamber methods (emulating indoor living environments, mentioned inASTM, ISO and European standards), and small scale methods, also called derived tests,oriented to industrial quality control and development. This second group includescommonly used methods, mentioned in different international standards, like the socalled:perforator (actually a test of potential formaldehyde emission), flask, desiccator,and gas analysis methods. Correlation between results from different methods has been amatter of debate, not yet completely elucidated.Based on different test methods, emission limit standards for WBPs have been issuedby several governmental organizations in Europe, Japan and United States, allowing forproduct classification according to emission level. Additionally, limits drawn by majorindustrial consumers, like IKEA, have been a defining guideline for WBP producers.In order to comply with increasingly stringent requirements, the industry has beendeveloping strategies to minimize formaldehyde emissions from WBPs. Four majorapproaches can be found: 1) reduction of formaldehyde content in resin formulation,while attempting to maintain adhesive performance, 2) addition of formaldehydescavengers to resin or wood particles, having the negative effect of consumingformaldehyde prior to resin cure, 3) implementation of surface treatments after boardproduction, and 4) use of alternative adhesive systems with reduced or no emissions, withan impact on product cost and/or performance.