..... Šefc, Trajković, Sinković, Hasan, Ištok: Compression Strength of Fir and Beech Wood... DRVNA INDUSTRIJA 63 (1) 45-50 (2012) 45 Bogoslav Šefc, Jelena Trajković, Tomislav Sinković, Marin Hasan, Iva Ištok 1 Compression Strength of Fir and Beech Wood Modified by Citric Acid Čvrstoća na tlak jelovine i bukovine modificirane limunskom kiselinom Original scientific paper • Izvorni znanstveni rad Received – prispjelo: 20. 7. 2011. Accepted – prihvaćeno: 9. 2. 2012. UDK: 630*812.72; 630*841.61; 674.032.475.242; 674.031.632.22 doi:10.5552/drind.2012.1123 ABSTRACT • Previous articles have shown that modification of wood by citric acid (CA) improves dimensional stability and resistance of wood against fungi attack. However, chemical modification of wood also modifies its mechanical properties in some way. The compression strength of wood is one of its representative mechanical properties. Modified wood with lower values of compression strength has limited purpose. The intention of this work is to show the effect of wood modification by citric acid on the compression strength of wood. Fir wood (Abies alba Mill.) and beech wood (Fagus sylvatica L.) were impregnated by citric acid with sodium-di- hydrogen-hypophosphite (NaH 2 PO 2 ) as a catalyst. Part of the impregnated samples together with control samples were cured at the temperature of 140 °C for 10 hours and the remaining samples were cured in microwaves for 35 minutes. The average compression strength parallel to the grain of wood modified by CA, using different re- gimes of curing, was compared to the strength of unmodified wood. The average compression strength parallel to the grain of wood was retained after modification. In the case of fir wood, the average compression strength was even improved after modification. The results indicate that wood modified by citric acid may be considered for the purposes where compression strength properties are equally important as improved durability and dimensional stability of wood. Keywords: Fir wood (Abies alba Mill.), beech wood (Fagus sylvatica L.), chemical modification, citric acid (CA), compression strength SAŽETAK • Objavljeni su radovi pokazali da se modifikacijom drva limunskom kiselinom poboljšava stabil- nost dimenzija drva i njegova otpornost na djelovanje gljiva. Međutim, kemijska modifikacija drva na neki način mijenja i mehanička svojstva drva. Tlačna je čvrstoća jedno od bitnih mehaničkih svojstava drva. Modificirano drvo sniženih vrijednosti tlačne čvrstoće ima ograničenu upotrebu. Cilj rada bio je pokazati učinak modifikacije drva limunskom kiselinom (CA) na tlačnu čvrstoću drva. Drvo jele (Abies alba Mill.) i bukve (Fagus sylvatica L.) impregnirano je limunskom kiselinom uz dodatak natrij-dihidrohen-hipofosfita (NaH 2 PO 2 ) kao katalizatora. Dio impregniranih uzoraka zagrijavan je zajedno s kontrolnim uzrocima pri temperaturi od 140 °C deset sati, a drugi je dio 35 minuta bio izložen mikrovalovima. U radu je uspoređena prosječna čvrstoća na tlak paralelno s vlakancima modificiranog drva zagrijavanoga različitim postupcima s odgovarajućom čvrstoćom nemodificiranoga drva. Prosječna čvrstoća na tlak paralelno s vlakancima drva zadržana je i nakon modifikacije. Prosječna čvrstoća na tlak modificirane jelovine čak je i povećana u usporedbi s nemodificiranom. Rezultati upućuju na mogućnost 1 The authors are assistant professor, associate professor, associate professor, assistant and assistant researcher at the Faculty of Forestry, Uni- versity of Zagreb, Zagreb, Croatia. 1 Autori su docent, izvanredna profesorica, izvanredni profesor, viši asistent i znanstvena novakinja Šumarskog fakulteta Sveučilišta u Zagre- bu, Zagreb, Hrvatska.