Production of Melamine Fortified Urea-Formaldehyde Resins with Low Formaldehyde Emission Na ´dia T. Paiva, 1,2,3 Ana Henriques, 1 Paulo Cruz, 2 Joa ˜o M. Ferra, 2 Luisa H. Carvalho, 1,4 Ferna ˜o D. Magalha ˜es 1 1 LEPAE—Departamento de Engenharia Quı ´mica, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal 2 Euroresinas—Industrias Quı ´micas, S.A., 7520-195 Sines, Portugal 3 ARCP—Associac ¸a ˜o Rede de Compete ˆncias em Polı ´meros, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal 4 DEMad—Departamento de Engenharia de Madeiras, Instituto Polite ´cnico de Viseu, Campus Polite ´cnico de Repeses, 3504-510 Viseu, Portugal Received 7 April 2011; accepted 17 July 2011 DOI 10.1002/app.35282 Published online 26 October 2011 in Wiley Online Library (wileyonlinelibrary.com). ABSTRACT: Melamine can be incorporated in the synthesis of urea-formaldehyde (UF) resins to improve performance in particleboards (PB), mostly in terms of hydrolysis resistance and formaldehyde emission. In this work, melamine-fortified UF resins were synthesized using a strong acid process. The best step for melamine addition and the effect of the reaction pH on the resin characteristics and performance were evaluated. Results showed that mela- mine incorporation is more effective when added on the ini- tial acidic stage. The condensation reaction pH has a significant effect on the synthesis process. A pH below 3.0 results on a very fast reaction that is difficult to control. On the other hand, with pH values above 5.0, the condensation reaction becomes excessively slow. PBs panels produced with resins synthesized with a condensation pH between 4.5 and 4.7 showed good overall performance, both in terms of internal bond strength and formaldehyde emissions. V C 2011 Wiley Periodicals, Inc. J Appl Polym Sci 124: 2311–2317, 2012 Key words: urea-formaldehyde resins; melamine; strongly acid process; GPC/SEC; formaldehyde emissions; internal bond strength INTRODUCTION Urea-formaldehyde (UF) resins are the most used type of adhesives used in wood-based panels (WBP) industry. 1 Worldwide, these resins represent 80% of the total production in the aminoresins class. 2 The remaining 20% correspond mainly to melamine-form- aldehyde (MF) resins, with a small percentage of res- ins synthesized from other aldehydes and/or other amino compounds. 3 According to SRI Consulting, 4 the global production of UF resins in 2008 was ~ 14 mT. Their consumption increased 2.8% in 2008, and is expected to grow an average 3.2% per year from 2008 to 2013, and just under 2% per year from 2013 to 2018. UF resins are thermosetting polymers that, before cure, consist of an aqueous solution/dispersion of unreacted monomers, linear or branched oligomeric, and polymeric molecules. 1 They are condensation products of aldehydes with compounds containing amino groups. Their success on the market is mostly because of high reactivity, good performance, and low cost. However, the hydrolytic degradation of UF polymers causes a significant weakening of resin bonds and is a source of formaldehyde emissions. 5 Another cause of formaldehyde emission from WBP produced with these resins is the presence of free (unreacted) formaldehyde. 1 In 2006 the International Agency for Research on Cancer (IARC), has classified formaldehyde as ‘‘carci- nogenic to humans.’’ 6 The new classification has led the industry to find strategies to limit formaldehyde emission levels from WPB. The most common approaches consist in: decreasing the F/U (formalde- hyde/urea) molar ratio, incorporation of other como- nomers in the synthesis and adding formaldehyde scavengers to the particles/fibers before or after resin blending. 7 The F/U molar ratio has been slowly decreased over the years from its initially high value, but causes the manufacturing processes to be more sensitive to perturbations in operating conditions and leads to particleboards (PBs) with lower physical and mechanical properties. 8 Current commercial resins have typical F/U values between 0.9 and 1.1. The incorporation of a small percentage of melamine to UF resins improve moisture/water resistance and therefore decrease formaldehyde emissions. The ami- nomethylene bonds in UF resins are quite susceptible Correspondence to: F. D. Magalha ˜es (fdmagalh@fe.up.pt). Contract grant sponsor: FCT—Foundation for Science and Technology and Euroresinas (Sonae Indu ´ stria); contract grant number: SFRH/BDE/51294/2010. Journal of Applied Polymer Science, Vol. 124, 2311–2317 (2012) V C 2011 Wiley Periodicals, Inc.