Lignins for Phenol Replacement in Novolac-Type Phenolic Formulations, Part I: Lignophenolic Resins Synthesis and Characterization A. Tejado, 1 G. Kortaberria, 1 C. Pen ˜ a, 1 J. Labidi, 1 J. M. Echeverrı ´a, 2 Inaki Mondragon 1 1 Materials and Technologies Group, Chemical and Environmental Engineering Department, University of the Basque Country (UPV/EHU), 20018 Donostia-San Sebastia ´n, Spain 2 Hexion Ibe ´rica S.A., Epele 39, Ctra a Navarra, 20120 Hernani, Spain Received 2 April 2007; accepted 17 June 2007 DOI 10.1002/app.26941 Published online 25 July 2007 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Several lignin (L)-based novolac-type pheno- lic prepolymers with two phenol (P) substitution degrees have been synthesized using three L from different origin (kraft pine L, soda/anthraquinone (AQ) flax L, and sulfonated kraft softwood L). These lignophenolic (LPF) resins have been characterized by means of free P and formaldehyde (F) analy- sis, viscosity, chemical structure (FTIR), and thermal behavior (DSC). Temperature at which curing reaction begins (DSC), hardening time at several curing temperatures (rheology) and volumetric changes (PVT) have also been determined for the curing process with 10 wt % hexamethylenetetraamine (HMTA). All the results have been compared to those obtained for a commercial novolac. Depending on the L type employed a homogeneous material with lower curing onset temperature and gelation time than reference PF can be obtained even at high P substitution level (45 wt %). Ó 2007 Wiley Periodicals, Inc. J Appl Polym Sci 106: 2313–2319, 2007 Key words: lignophenolic resins; synthesis; differential scanning calorimetry; glass transition; gelation INTRODUCTION Phenol-formaldehyde (PF) resins, also called pheno- lic resins, were first synthesized by Leo H. Baeke- land in 1908, becoming the first completely synthetic resins to be created. Because of their excellent behav- ior and relatively low production costs, PF resins are currently one of the most used thermosetting resins, and their use is fundamental in diverse areas such as wood composites, isolating materials, adhesives, and many others. In fact, they differ from the rest of polymeric materials (either thermosets or thermo- plastics) in terms of the wide variety of applications they can be used in. 1 The main drawback associated to the production of PF resins derives from their environmental impact, as P is a volatile aromatic compound and F has been classified as high-risk product, 2 which restricts more and more of their uti- lization. For this reason, there is an urgent necessity of designing new materials where the whole process (raw materials extraction, production, and degrada- tion) are sustainable and respectful with the environ- ment. Lignin (L) has been found to be a promising natural alternative to phenolic resins to be used as a partial replacement of P in the formulations. In the last decades of the 20th century, the utilization of L as a copolymer in the area of materials engineering began to be considered as possible. The structural simi- larity existing with PF resins led to several attempts of incorporating L into phenolic formulations as a partial substitute of P, 3–5 with the double objective of lowering production costs and reducing toxicity problems. Since then, much research work has been done trying to opti- mize L incorporation in PF formulations, being most of the works dedicated to resole-type resins 6–21 and only a few of them to novolac-type lignophenolic resins. 22–26 In both cases, L from different origins have been tested either nonmodified 12–18,23–25 or after several modifica- tions such as fractionation, 3,8,9,27–31 hydroxymethyla- tion, 10,11,20,22,26,32,33 or phenolation. 6,7,34,35 These previ- ous treatments are applied with the aim of improving L reactivity, so that L-based systems fulfil all the requirements of commercial PF, but it seems that there is a great inconsistency among existing data and the utilization of both modified and nonmodified L leads to dissimilar results. This fact, which is undoubtedly a consequence of the huge variability of L structure derived from its natural origin, suggests that much work must still be done to completely understand the real possibilities of L as a component in PF resins. With the aim of contributing to this search, three types of unmodified L, namely kraft pine L, soda/ arthraquinone (AQ) flax L, and sulfonated kraft Correspondence to: I. Mondragon (inaki.mondragon@ ehu.es). Contract grant sponsors: Hexion Ibe ´rica, Gobierno Vasco (Spain). Journal of Applied Polymer Science, Vol. 106, 2313–2319 (2007) V V C 2007 Wiley Periodicals, Inc.