Small wooden samples of fir (Abies alba Mill.) and beech (Fagus sylvatica L.) were chemically modified by citric acid (CA) as non-formaldehyde cross-linking system reagent and cured by convection heating at three different temperature regimes. Two different CA solutions were used, one with NaH2PO2 and the other with NaH2PO4 as a catalyst. The dimensional stability of the modified wood was determined by the anti-swelling efficiency (ASE) using the water soak/oven dry method. Almost equally large improvement of dimensional stability of wood was attained using NaH2PO4 as when NaH2PO2 was applied as a catalyst.Mali drveni blokovi jelovine i bukovine kemijski su modificirani neformaldehidnim sustavom za umrežavanje – limunskom kiselinom (CA), u trima različitim temperaturnim režimima. Upotrijebljene su dvije otopine limunske kiseline. U jednoj je katalizator bio NaH2PO2, a u drugoj NaH2PO4. Stabilnost dimenzija modificiranog drva određena je tzv. učinkom smanjenja bubrenja (ASE) metodom potapanja u vodi i sušenja do apsolutno suhog stanja. Upotrebom NaH2PO4 postignuto je gotovo jednako poboljšanje stabilnosti dimenzija kao i primjenom NaH2PO2

Bogoslav Šefc

Drago Katović

Jelena Trajković

Marin Hasan

Martina Frančić

Sandra Bischof Vukušić

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Bogoslav [efc, Jelena Trajkovi}, Marin Hasan1, Drago Katovi}, Sandra Bischof Vuku{i}2, Fran~i} Martina3Dimensional stability ofwood modified by citricacid using differentcatalystsStabilnost dimenzija drva modificiranogalimunskom kiselinom uz razli~ite katalizatoreOriginal scientific paper · Izvorni znanstveni radReceived – prispjelo: 13. 1. 2009.Accepted – prihva}eno: 25. 2. 2009.UDK: 630*812.23; 630*841.61ABSTRACT • Small wooden samples of fir (Abies alba Mill.) and beech (Fagus sylvatica L.) were chemically mo-dified by citric acid (CA) as non-formaldehyde cross-linking system reagent and cured by convection heating atthree different temperature regimes. Two different CA solutions were used, one with NaH2PO2 and the other withNaH2PO4 as a catalyst.The dimensional stability of the modified wood was determined by the anti-swelling efficiency (ASE) using the wa-ter soak/oven dry method. Almost equally large improvement of dimensional stability of wood was attained usingNaH2PO4 as when NaH2PO2 was applied as a catalyst.Keywords: dimensional stability of wood, beech wood, fir wood, chemical modification, citric acid, NaH2PO2,NaH2PO4SA@ETAK • Mali drveni blokovi jelovine i bukovine kemijski su modificirani neformaldehidnim sustavom zaumre`avanje – limunskom kiselinom (CA), u trima razli~itim temperaturnim re`imima. Upotrijebljene su dvije oto-pine limunske kiseline. U jednoj je katalizator bio NaH2PO2, a u drugoj NaH2PO4.Stabilnost dimenzija modificiranog drva odre|ena je tzv. u~inkom smanjenja bubrenja (ASE) metodom potapanja uvodi i su{enja do apsolutno suhog stanja. Upotrebom NaH2PO4 postignuto je gotovo jednako pobolj{anje stabilno-sti dimenzija kao i primjenom NaH2PO2.Klju~ne rije~i: stabilnost dimenzija drva, jelovina i bukovina, kemijska modifikacija, limunska kiselina, NaH2PO2,NaH2PO41 INTRODUCTION1. UVODDimensional stability of wood is often an impor-tant factor that limits its usage. It is caused by shrinkingand swelling of wood. Shrinkage of the cell wall occurswhen water molecules escape from between long-chaincellulose and hemicellulose molecules while swelling isa reverse process. There are several options as to how toovercome this issue, and one of the most promising iswood modification. One of the main aims of wood modi-fication was to improve its dimensional stability. SinceDRVNA INDUSTRIJA 60 (1) 23-26 (2009) 23....... [efc, Trajkovi}, Hasan, Katovi}, Bischof Vuku{i}, Fran~i}: Dimensional stability...1 The authors are assistant, associated professor and assistant at the Faculty of Forestry, University of Zagreb, Croatia. 2The authors are professorand associated professor at Faculty of Textile Technology, University of Zagreb. 3The author is a bachelor of wood technology.1 Autori su asistent, izvanredna profesorica i asistent [umarskog fakulteta Sveu~ili{ta u Zagrebu, Hrvatska. 2Autori su profesor i izvanredna pro-fesorica Tekstilno-tehnolo{kog fakulteta Sveu~ili{ta u Zagrebu, Hrvatska. 3Autorica je diplomirana in`enjerka drvne tehnologije.wood consists of approximately 50% of cellulose thereare many similarities with cotton textiles, which mainlyconsist of cellulose - up to 98%. For that reason theagents that have shown good results in textile finishingapplications can be applied in chemical modification ofwood as well. Textile or wood is usually impregnatedwith emulsions or solvents of the applied agents. The re-action of chemicals and wood usually occurs at highertemperatures. Cross-linking chemicals reacting withhydroxyl groups reduce the hygroscopicity of wood andthe tendency to swell or shrink (Rowell et al, 1988; Ro-well 1991; Yasuda and Minato, 1994; Ashaari et al,1990). One of the reactants often applied is DMDHEU(1.3-dimethylol 4.5-dihidroxy ethylene urea). At highertemperature the N_methylole reactants form ether linka-ges accelerated with Lewis acid catalyst (MgCl2). Woodmodified by DMDHEU showed great improvement inwood stability, while tensile strength was reduced (Mi-litz, 1993; Xie et al, 2005). One of the problems that li-mit its usage is formaldehyde release at higher tempera-tures which can be toxic, potentially carcinogen and cau-se dermatitis (Solja~i} and Katovi}, 1988).In late 80s the research for non-formaldehyde fi-nishes in cotton textiles focused on polycarboxylicacids (PCA). Welch and Andrews (1988) reported that1,2,3,4-butanetetracarboxylic acid (BTCA) is an effec-tive cross-linking agent for cotton cellulose. NaH2PO2proved to be the best catalyst (Bischof Vukusic et al,2002; Schramm et al, 2002).The mechanism involved is a two-step esterifica-tion. In the first step anhydride is formed, while in thesecond, this cyclic anhydride reacts with hydroxyl gro-ups (Figure 1).Bischof Vukusic et al. (2006) showed that CAand BTCA crosslink with wood and reduce swellingand shrinking of wood. They also showed that the di-mensional stability of wood achieved in this way iscomparable to that achieved with DMDHEU. In thisstudy NaH2PO2 was used as a catalyst in CA and BTCAsolutions, because it already proved to be the best ca-talyst for PCA modification of cotton cellulose (Bi-schof Vukusic et al, 2002; Schramm et al, 2002).This research was aimed at establishing the effectof replacing one costly catalyst (NaH2PO2) with a lessexpensive one (NaH2PO4) on dimensional stabilisationof wood. The second aim was to shorten the time of cu-ring while achieving comparable anti swelling effi-ciency (ASE) of wood.2 MATERIALS AND METHODS2. MATERIJALI I METODEThe samples with dimensions 202010 mm(TRL) were cut from quarter-sawn air dried boardsof fir (Abies alba Mill.) and beech wood (Fagus sylvati-ca L.). They were signed in succession from one end fordifferent treatments according to Figure 2. There were10 replicates of samples for each treatment. After airdrying and conditioning at 20 °C and 65% relative humi-dity, the samples to be modified (T, S and Z) were impre-gnated with the specific CA solution, and control sam-ples (KT and KZ) were impregnated with distilled water.CA solutions were water solutions of 6.9% of CA and6.5% catalyst. One solution catalyst contained NaH2PO2and the other NaH2PO4. The impregnation cycle consi-sted of a 5-minute initial vacuum of 2 kPa. The vacuumvessel was then filled with specific treating solution(control distilled water) and maintained under the samevacuum for 3 hours, followed with an 18-hour soaking atatmospheric pressure. The samples were then drained,24 DRVNA INDUSTRIJA 60 (1) 23-26 (2009)[efc, Trajkovi}, Hasan, Katovi}, Bischof Vuku{i}, Fran~i}: Dimensional stability... ........Figure 1 Cross-linking via ester linkages of CA and BTCA with cellulosic chains (Schramm, 1999a).Slika 1. Umre`avanje CA i BTCA s lancima celuloze preko esterskih veza (Schramm, 1999a)measured and left to air dry at 20 °C and 65% relative hu-midity to constant mass. One portion of samples (T) wasthen cured at 140 °C for 5 hours, the second portion (S)at 160 °C for 5 hours, and the third portion (Z and KZ) at180 °C for 2 hours. Controls marked KT were thensimply dried at 100 °C to constant mass.For lower temperatures of treatment, as shownpreviously, the influence of temperature itself on thevolumetric swelling coefficient is insignificant compa-red to the influence of the CA modification (Katovi} etal, 2004). It explains why the same control (KT) wascompared to the sample modified by CA at 140 °C (T)and to the one modified by CA at 160 °C (S). However,samples modified by CA at 180 °C (Z) were comparedto the water impregnated controls (KZ) that were airdried at 20 °C and 65% relative humidity to constantmass and then exposed to 180 °C for 2 hours.Dimensional stability was quantified by compa-ring the total volumetric swelling of treated and controlsamples. After modification and air drying all samplesincluding controls were oven dried at 100 °C to a con-stant mass and then vacuum impregnated with distilledwater and allowed to soak for 24 hours. The total volu-metric swelling (v), reduction in water absorption (R),and anti-swelling efficiency (ASE) were calculated byEq. (1):v (%) = 100 (Vmax – Vmin) / Vmin;R (%) = 100 (wc – wt) / wc,ASE (%) = 100 (v c - v t) / v cWhere Vmax is the volume, at a moisture contentgreater than the saturation point of the cellular walls ofwood, Vmin is the volume, after drying at 103 °C, w - themoisture content, c - control, t - treated.3 RESULTS AND DISCUSSION3. REZULTATI I RASPRAVAThe improvement in dimensional stabilisation ofbeech and fir wood modified by citric acid withNaH2PO2 or NaH2PO4 as a catalyst cured at three diffe-rent regimes are presented in Table 1.Total volumetric swelling of untreated fir woodwas around 16% and total volumetric swelling of beechDRVNA INDUSTRIJA 60 (1) 23-26 (2009) 25....... [efc, Trajkovi}, Hasan, Katovi}, Bischof Vuku{i}, Fran~i}: Dimensional stability...Figure 2 Succession of wood samples in quarter sawn boardsSlika 2. Redoslijed uzimanja uzoraka iz ~etvrta~aT - samples impregnated by CA solutions and cured at tempe-rature of 140 °C / uzorci impregnirani otopinama CA i zagri-javani pri temperaturi 140 °CS - samples impregnated by CA solutions and cured at tempe-rature of 160 °C / uzorci impregnirani otopinama CA i zagri-javani pri temperaturi 160 °CZ - samples impregnated by CA solutions and cured at tempe-rature of 180 °C / uzorci impregnirani otopinama CA i zagri-javani pri temperaturi 180 °CKT - control samples cured at temperature of 100 °C / kon-trolni uzorci zagrijavani pri temperaturi 100 °CKZ - control samples cured at temperature of 180°C / kontrol-ni uzorci zagrijavani pri temperaturi 180 °CTable 1 Dimensional stability of beech and fir wood achieved using two different chemical treatments and three curing regimesTablica 1. Stabilnost dimenzija bukovine i jelovine postignuta primjenom dviju razli~itih otopina za impregnaciju i uz tri raz-li~ita re`ima zagrijavanjaSpeciesVrsta drvaImpregnation so-lution / Otopinaza impregnacijuCuring temperature, °C /time, h / Temperaturazagrijavanja, °C/ vrijeme, hWeight per-cent gain / Do-bitak mase, %R1%ASE2%CV3%ReplicatesBroj ponavljanjaBeech wood(Fagus sylvaticaL.)bukovina6.9% CA + 6.5%NaH2PO2140/5 9.6 19 39 18 10160/5 9.7 19 45 13 10180/2 7.8 15 41 10 106.9% CA + 6.5%NaH2PO4140/5 7.6 15 43 13 10160/5 6.8 16 43 14 10180/2 6 16 40 20 10Fir wood(Abies alba Mill.)jelovina6.9% CA + 6.5%NaH2PO2140/5 17.9 22 57 13 10160/5 15.1 20 51 10 10180/2 14.9 19 54 8 106.9% CA + 6.5%NaH2PO4140/5 17.1 20 54 19 10160/5 14.6 20 56 17 10180/2 14.8 20 57 18 101reduction in water absorption (smanjenje apsorpcije vode); 2anti-swelling efficiency (u~inak smanjenja bubrenja); 3coefficientof variation (koeficijent varijacije)wood was around 20%. The difference can be explai-ned mainly with lower density of fir wood (0.46 g/cm3)compared to beech wood (0.76 g/cm3).The effect of CA modification on dimensional sta-bilisation of wood was almost equal using either of ca-talysts applied. For the same treatment conditions ASEwas always greater in modified fir wood (over 50%) thanin modified beech wood (around 40%) probably due tohigher wood percent gain in fir wood (Table 1). It isknown that ASE increases directly with the amount ofchemicals remained in the wood after modification (Ro-well et al, 1976). The results of a previous experiment(Bischoff Vuku{i} et al, 2006) showed similar ASE va-lues for curing at 140 °C, but in double curing time (10hours). Curing at 160 °C for 5 hours resulted also withASE comparable to that achieved at 140°C, and even tothe ASE achieved at 180 °C for 2 hours. The applicationof both acid solutions using three different curing regi-mes revealed more favourable results in ASE improve-ment compared to DMDHEU application (BischofVuku{i} et al, 2006; Katovi} et al, 2004). These resultsfurther emphasise possible use of CA as non-formal-dehyde cross-linking system reagent.Although it was not the object of investigation co-lour change was observed particularly in samples mo-dified at 180 °C. In accordance with the ASE and we-ight percent gain values water absorption was also re-duced more in modified fir wood (20%) than in modi-fied beech wood (16%), and it does not differ much bet-ween modes of modification applied.The quantity of CA cross-linkages to wood achie-ved by application of different modification regimes isstill not known. Further research of leaching of modifiedwood is needed to prove the strength of cross linkages towood. Thus the prevailing effect of CA on dimensionalstability of wood would be tested, i.e. it would be deter-mined whether it is cross-linking or bulking.4 CONCLUSION4. ZAKLJU^AKHeating temperature, modification period, andcatalyst type and amount are important limiting factorsfor successful chemical wood modification.With the application of CA as a non-formal-dehyde agent using NaH2PO2 as a catalyst, the averageASE was about 54% in fir wood and about 40% in be-ech wood. The ASE improvement in both wood specieswas similar when NaH2PO4 was applied as a catalyst.The price of NaH2PO4 is lower compared to NaH2PO4.Comparable ASE results can be achieved eitherby shortening the curing time and/or by increasing thecuring temperature.5 REFERENCES5. LITERATURA1. Ashaari, Z., Barnes, H.M., Vasishth, R.C., Nicholas,D.D., Lyon, D.E. 1990: Effect of Aqueous polymer treat-ments on wood properties, part 1: Treatability and dimen-sional stability. Mississippi Forest Products Laboratory,USA. IRG WP 90-36102. Bischof Vuku{i}, S., Katovi}, D., Schramm, C., Trajko-vi}, J., [efc, B. 2006: Polycarboxylic acids as non-for-maldehyde antiswelling agents for wood. Holzforschung60:439-444.3. Bischof Vuku{i}, S., Katovi}, D., Solja~i}, I. 2002: DP fi-nishing with PCA and phosphono-based catalysts.AATCC Rewiev, 2:14-16.4. Katovi}, D., Trajkovi}, J., Bischof Vuku{i}, S., [efc, B.2004: Alternative agents and methods for chemical modi-fication of wood. Drvna Industrija 55:175-180.5. Militz, H. 1993: Treatment of timber with water solubledimethylol resins to improve their dimensional stabilityand durability. Wood Sci. and Techn. 27:347-355.6. Rowell, R. M.; Gutzmer, D. I.; Sachs, I. B; Kinney, R. E.1976: Effects of Alkalyine Oxyde Treatments on Dimen-sional Stability of Wood. Wood science, 9 (1), 51-54.7. Rowell, R.M., Youngquist, J.A., Montrey, H.M. 1988:Chemical modification: adding value through new FPLcomposite processing technology. Forest Products Journal.8. Rowell, R.M. 1991: Chemical modification of wood. In:Wood and Cellulosic Chemistry. Hon, D.N., Shiraishi, N.Marcel Dekker Inc., New York. Pp. 703-756.9. Schramm, C., Bischof Vuku{i}, S., Katovi}, D. 2002:Non-formaldehyde durable press finishing of dyed fa-brics: evaluation of cotton-bound polycarboxylic acids.Coloration Technology 118:244-249.10. Schramm, C., Rinderer, B. 1999a: Influence of additiveson the formation of saturated PCA produced during dura-ble-press curing with citric acid. Journal of Society ofDyers and Colourists 115:306-311.11. Solja~i}, I.; Katovic, D. 1988: The effect of heating on therelease of formaldehyde from durable-press finished fa-brics. Journal of Society of Dyers and Colourists. 104(10): 384-386.12. Welch, C.M., Andrews, K.A. 1988: Tetracarboxylicacids as Formaldehyde Free durable press finishingagents. Textile Research Journal, 58:480-486.13. Xie, Y., Krause, A., Mai, C., Militz, H., Richter, K.,Urban, K., Evans, P.D. 2005: Weathering of wood modi-fied with the N-methylol compound 1,3-dymethylol-4,5-dihydroxy-ethyleneurea. Polymer Degradation and Stabi-lity. 89:189-199.14. Yasuda, R., Minato, K. 1994: Chemical modification ofwood by non-formaldehyde cross-linking reagents.Wood Science and Technology. 28:101-110.Corresponding address:Assistant BOGOSLAV [EFC, PhDWood Science DepartmentUniversity of ZagrebFaculty of ForestrySveto{imunska 25HR-10002 ZagrebCroatiae-mail: sefcºsumfak.hr26 DRVNA INDUSTRIJA 60 (1) 23-26 (2009)[efc, Trajkovi}, Hasan, Katovi}, Bischof Vuku{i}, Fran~i}: Dimensional stability... ........

Stabilnost dimenzija drva modificiranoga limunskom kiselinom uz različite katalizatore

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