Lignins for Phenol Replacement in Novolac-Type Phenolic Formulations. II. Flexural and Compressive Mechanical Properties A. Tejado, 1 G. Kortaberria, 1 C. Pen ˜ a, 1 M. Blanco, 1 J. Labidi, 1 J. M. Echeverrı ´a, 2 I. Mondragon 1 1 Materials and Technologies Group, Chemical and Environmental Engineering Department, University of the Basque Country, Plaza Europa 1, 20018 Donostia-San Sebastia ´n, Spain 2 Hexion Ibe ´rica, Epele 39, Carretera a Navarra, 20120 Hernani, Spain Received 2 April 2007; accepted 17 June 2007 DOI 10.1002/app.27003 Published online 12 September 2007 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Several hexamethylenetetraamine-cured novolac-type lignophenolic resins have been subjected to me- chanical analysis to check the influence of the addition of dif- ferent types of lignins (kraft pine lignin, soda/anthraquinone flax lignin, and sulfonated kraft lignin from mixed softwoods) and the influence of different phenol substitution levels. Both flexural and compression tests have been performed to evalu- ate the influence of the curing temperature and pressure. The results have been compared with those from a commercial novolac system. Flexural tests show that lignin incorporation leads to systems with increased rigidity, whereas up to 82% of the maximum strength can be retained with respect to a commercial phenolic (phenol–formaldehyde) at a 45 wt % substitution degree. Compression analyses show that modi- fied prepolymers are not significantly affected by the presence of lignin at a medium curing pressure (150 bar), but their deformability results considerably increase at high curing pressures (400 or 600 bar). Ó 2007 Wiley Periodicals, Inc. J Appl Polym Sci 107: 159–165, 2008 Key words: mechanical properties; polyaromatics; renew- able resources; resins; structure-property relations INTRODUCTION Phenol–formaldehyde (PF) resins were the first com- pletely synthetic resins employed in the production of molding compounds. They are well known to possess excellent properties, but their inherent brit- tleness makes necessary, in most cases, the use of additives in formulations. By means of mixtures with other compounds (reinforcement agents, accel- erators, pigments, etc.), PF resins can be employed for the production of a wide variety of products that have to be subjected to different mechanical require- ments during their lifetime. In this way, organic (cotton, cellulose, etc.) and inorganic (glass) fibers can be incorporated to enhance mechanical and impact resistance, wood flour can be incorporated for modifying the hardness and final appearance, sil- icate minerals can be incorporated for enhancing thermal resistance and decreasing postcuring shrink- age, mica can be incorporated for increasing insulat- ing properties, and so forth. 1 The use of lignin as a partial substitute for phenol in the formulation of lignin–phenol–formaldehyde (LPF) resins assumes the introduction of bulky struc- tures with limited reactivity, which results in an increase in the resins’ brittleness. 2–5 A mechanical analysis of LPF resins seeks to minimize this adverse effect, trying to maintain the mechanical properties with little variation with respect to conventional resins. The best results have been obtained when lignin is added at the beginning of the synthesis procedure. 4,6 In this work, several novolac-type LPF resins, in which 25 or 45 wt % phenol has been substituted by lignin, have been subjected to mechanical analysis after being hexamethylenetetraamine (HMTA)-cured. Both flexural and compression tests have been per- formed to evaluate the effects derived from the incorporation of lignin into polymeric formulations. EXPERIMENTAL Materials Three different types of lignins [kraft pine lignin (L1), soda/anthraquinone flax lignin (L2), and sulfo- nated kraft lignin from mixed softwoods (L3)] were used for lignophenolic resin synthesis. The physico- chemical characterization of L1 and L2, which were isolated from industrial black liquors, has been Correspondence to: I. Mondragon (inaki.mondragon@ ehu.es). Contract grant sponsor: Hexion Ibe ´rica. Contract grant sponsor: Gobierno Vasco. Journal of Applied Polymer Science, Vol. 107, 159–165 (2008) V V C 2007 Wiley Periodicals, Inc.