Effect of molecular weight of urea–formaldehyde resins on their cure kinetics, interphase, penetration into wood, and adhesion in bonding wood

  • Bora Jeong
  • Byung-Dae ParkEmail author


In this study, the effect of the molecular weight (MW) of urea–formaldehyde (UF) resins on their cure kinetics, interphase, penetration into wood, and adhesion strength was evaluated for the first time, to understand their contribution to cohesion and adhesion in bonding wood. UF resins with two final formaldehyde-to-urea (F/U) molar ratios (1.0 and 1.2) were prepared as low-viscosity resin (LVR) and as high-viscosity resin (HVR) through viscosity control. Five LVR/HVR blending ratios (100:0, 75:25, 50:50, 25:75, and 0:100) were used to obtain UF resins with different MW distributions and average MWs and, hence, different viscosities for the two molar ratios. As the viscosity during the condensation phase increased, the MW increased while the gelation time and the low molecular weight fraction decreased. The resins with F/U molar ratio of 1.2 had higher MW and activation energy than those with F/U molar ratio of 1.0. Isoconversional analysis showed that the 1.0 F/U molar ratio resin went through a multiple-step process in their curing mechanism, unlike the 1.2 F/U molar ratio resin, whose cohesion during bond formation was likely affected by the higher F content. As the MW increased, the resins with 1.0 and 1.2 F/U molar ratios exhibited the highest maximum storage modulus (Emax), greatest depth of resin penetration, thinnest bond-line, and highest adhesive strength at apparent weight-averaged Mw of 2000–2400 g/mol for the 1.0 F/U molar ratio resins (according to mixing ratios LVR/HVR = 50:50 and 25:75) and 3500–4500 g/mol for the 1.2 F/U molar ratio resins (again according to mixing ratios LVR/HVR = 50:50 and 25:75). These results suggest that the MW of UF resins has a big impact on cure kinetics that contributes to their cohesion behavior, while it also affects Emax, the depth of resin penetration, and the bond-line thickness, which all contribute to their adhesion behavior.



This work was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea, funded by the Ministry of Education, Science and Technology (Grant #2016R1D1A1A09917782).


  1. de Jong JI, de Jonge J (1952) The reaction of urea with formaldehyde. Rec Trav Chim Pays-Bas 71:643–661CrossRefGoogle Scholar
  2. de Jong JI, de Jonge J, Eden HAK (1953) The formation of trihydroxymethyl urea. Rec Trav Chim Pays-Bas 72:88–90CrossRefGoogle Scholar
  3. Dunky M (1998) Urea–formaldehyde (UF) adhesive resins for wood. Int J Adhes Adhes 18(2):95–107CrossRefGoogle Scholar
  4. Ellis S (1993) The performance of waferboard bonded with powdered phenol-formaldehyde resins with selected molecular weight distributions. For Prod J 43(2):66–68Google Scholar
  5. Fan D, Li J, Mao A (2006) Curing characteristics of low mole ratio urea–formaldehyde resins. J Adhes Interface 7(4):45–52Google Scholar
  6. Ferra JM, Henriques A, Mendes AM, Costa MRN, Carvalho LH, Magalhães FD (2012) Comparison of UF synthesis by alkaline-acid and strongly acid processes. J Appl Polym Sci 123(3):1764–1772CrossRefGoogle Scholar
  7. Gavrilović-Grmuša I, Dunky M, Miljković J, Djiporović-Momčilović M (2010a) Radial penetration of urea–formaldehyde adhesive resins into beech (Fagus Moesiaca). J Adhes Sci Technol 24(8–10):1753–1768CrossRefGoogle Scholar
  8. Gavrilović-Grmuša I, Miljković J, Ðiporović-Momčilović M (2010b) Influence of the degree of condensation on the radial penetration of urea–formaldehyde adhesives into silver fir (Abies alba, Mill.) Wood Tissue. J Adhes Sci Technol 24(8–10):1437–1453CrossRefGoogle Scholar
  9. Gavrilović-Grmuša I, Dunky M, Miljković J, Điporović-Momčilović M (2012) Influence of the viscosity of UF resins on the radial and tangential penetration into poplar wood and on the shear strength of adhesive joints. Holzforschung 66:849–856Google Scholar
  10. Gavrilović-Grmuša I, Dunky M, Djiporović-Momčilović M, Popović M, Popović J (2016) Influence of pressure on the radial and tangential penetration of adhesive resin into poplar wood and on the shear strength of adhesive joints. BioResources 11(1):2238–2255Google Scholar
  11. Gu JY, Higuchi M, Morita M, Hse CY (1995) Synthetic conditions and chemical structures of urea–formaldehyde resins. I. Properties of the resins synthesized by three different procedures. Mok Gakk 41(12):1115–1121Google Scholar
  12. He G, Riedl B (2003) Phenol–urea–formaldehyde cocondensed resol resins: their synthesis, curing kinetics, and network properties. J Polym Sci Part B Polym Phys 41(16):1929–1938CrossRefGoogle Scholar
  13. Hse CH (1971) Properties of phenolic adhesives as related to bond quality in southern pine plywood. For Prod J 21(1):44–52Google Scholar
  14. Hse CY, Xia ZY, Tomita B (1994) Effects of reaction pH on properties and performance of urea–formaldehyde resins. Holzforschung 48(6):527–532CrossRefGoogle Scholar
  15. Jeong B, Park BD (2016) Measurement of molecular weights of melamine-urea–formaldehyde resins and their influences to properties of medium density fiberboards. J Korean Wood Sci Technol 44(6):913–922CrossRefGoogle Scholar
  16. Jeong B, Park BD (2017) Effect of analytical parameters of gel permeation chromatography on molecular weight measurements of urea–formaldehyde resins. J Korean Wood Sci Technol 45(4):471–481Google Scholar
  17. Jeremejeff J (2012) Investigation of UF-resins-the effect of the formaldehyde/urea mole ratio during synthesis. Master of Science Thesis, KTH Royal Institute of Technology, Stockholm, SwedenGoogle Scholar
  18. Johnson SE, Kamke FA (1992) Quantitative analysis of gross adhesive penetration in wood using fluorescence microscopy. J Adhes 40:47–61CrossRefGoogle Scholar
  19. Korean Standard Association (2016) Ordinary plywood, KS F 3101, KSA SeoulGoogle Scholar
  20. Kumar RN, Han TL, Rozman HD, Daud WRW, Ibrahim MS (2007) Studies in the process optimization and characterization of low formaldehyde emission urea–formaldehyde resin by response surface methodology. J Appl Polym Sci 103(4):2709–2719CrossRefGoogle Scholar
  21. Laborie MPG, Salmén L, Frazier CE (2006) A morphological study of the wood/phenol–formaldehyde adhesive interphase. J Adhes Sci Technol 20(8):729–741CrossRefGoogle Scholar
  22. Meyer B, Johns WE, Woo JK (1980) Formaldehyde release from sulfur-modified urea–formaldehyde resin system. For Prod J 30:24–31Google Scholar
  23. Nearn WT (1974) Application of ultrastructure concept in industrial wood products research. Wood Sci 6(3):285–293Google Scholar
  24. Nuryawan A, Park BD, Singh AP (2014a) Comparison of thermal curing behavior of liquid and solid urea–formaldehyde resins with different formaldehyde/urea mole ratios. J Therm Anal Calorim 118(1):397–404CrossRefGoogle Scholar
  25. Nuryawan A, Park BD, Singh AP (2014b) Penetration of urea–formaldehyde resins with different formaldehyde/urea mole ratios into softwood tissues. Wood Sci Technol 48(5):889–902CrossRefGoogle Scholar
  26. Park BD, Causin V (2013) Crystallinity and domain size of cured urea–formaldehyde resin adhesives with different formaldehyde/urea mole ratios. Eur Polym 49:532–537CrossRefGoogle Scholar
  27. Park BD, Jeong HW (2011) Hydrolytic stability and crystallinity of cured urea–formaldehyde resin adhesives with different formaldehyde/urea mole ratios. Int J Adhes Adhes 31(6):524–529CrossRefGoogle Scholar
  28. Park BD, Kim JW (2008) Dynamic mechanical analysis of urea–formaldehyde resin adhesives with different formaldehyde-to-urea molar ratios. J Appl Polym Sci 108:2045–2051CrossRefGoogle Scholar
  29. Park BD, Riedl B, Hsu EW, Shields J (1998) Effects of weight average molecular mass of phenol–formaldehyde adhesives on medium density fiberboard performance. Holz Roh-Werkst 56(3):155–161CrossRefGoogle Scholar
  30. Park BD, Kang EC, Park JY (2006a) Differential scanning calorimetry of urea–formaldehyde adhesive resins, synthesized under different pH conditions. J Appl Polym Sci 100(1):422–427CrossRefGoogle Scholar
  31. Park BD, Kang EC, Park JY (2006b) Effects of formaldehyde to urea mole ratio on thermal curing behavior of urea–formaldehyde resin and properties of particleboard. J Appl Polym Sci 101(3):1787–1792CrossRefGoogle Scholar
  32. Samaržija-Janović S, Javanović V, Konstantinović S, Marković G, Marinović-Cincović M (2011) Thermal behavior of modified urea–formaldehyde resins. J Therm Anal Calorim 104:1159–1166CrossRefGoogle Scholar
  33. Starink MJ (2003) The determination of activation energy from linear heating rate experiments: a comparison of the accuracy of isoconversion methods. Thermochim Acta 404:163–176CrossRefGoogle Scholar
  34. Stefke B, Dunky M (2006) Catalytic influence of wood on the hardening behavior of formaldehyde-based resin adhesives used for wood-based panels. J Adhes Sci Technol 20(8):761–785CrossRefGoogle Scholar
  35. Tarkow H, Feist WC, Southerland CF (1966) Penetration versus molecular size—interaction of wood with polymeric materials. For Prod J 16(10):61–65Google Scholar
  36. Vyazovkin S (2000) Kinetic concepts of thermally stimulated reactions in solids: a view from a historical perspective. Int Rev Phys Chem 19(1):45–60CrossRefGoogle Scholar
  37. Vyazovkin S (2001) Modification of the integral isoconversional method to account for variation in the activation energy. J Comput Chem 22(2):178–183CrossRefGoogle Scholar
  38. Vyazovkin S, Burnham AK, Criado JM, Perez-Maqueda LA, Popescu C, Sbirrazzuoli N (2011) ICTAC kinetics committee recommendations for performing kinetic computations on thermal analysis data. Thermochim Acta 520:1–19CrossRefGoogle Scholar
  39. Wilson JB, Krahmer RL (1978) The wetting and penetration of phenolic and lingo-sulfonate resins as possible indicators of bond strength for board. In: Proceedings of the 12th international particle, pp 305–315Google Scholar
  40. Wisanrakkit G, Gillham JK (1990) The glass transition temperature (Tg) as an index of chemical conversion for a high-Tg amine/epoxy system: chemical and diffusion- controlled reaction kinetics. J Appl Polym Sci 41:2885–2929CrossRefGoogle Scholar

Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2019

Authors and Affiliations

  1. 1.Department of Wood and Paper SciencesKyungpook National UniversityDaeguRepublic of Korea

Personalised recommendations