Veneer densifi cation was performed at 105±3 ºC under pressure of 1,8 N/mm2 for 30 s. The approach allowed to reduce 20-mm thick plywood pressing time by 12-25 % when compared to the non-densified controls. It was found that densifi cation did not affect shear strength of the plywood which met the requirements of the respective standards.Stlačivanje furnira provedeno je pri temperaturi 105±3 ºC i tlaku 1.8 N/mm2 tijekom 30 s. Na taj je način omogućeno skraćenje vremena prešanja furnirske ploče debljine 20 mm za 12-25 % u usporedbi s kontrolnim uzorcima od nestlačenih furnira. Istraživanje je pokazalo da stlačivanje furnira ne utječe na čvrstoću smicanja furnirske ploče koja zadovoljava zahtjeve odgovarajućih normi

Agnieszka Kurowska

Marcin Zbieć

Mariusz Mamiński

Piotr Borysiuk

Stlačivanje furnira provedeno je pri temperaturi 105±3 ºC i tlaku 1.8 N/mm2 tijekom 30 s. Na taj je način omogućeno skraćenje vremena prešanja furnirske ploče debljine 20 mm za 12-25 % u usporedbi s kontrolnim uzorcima od nestlačenih furnira. Istraživanje je pokazalo da stlačivanje furnira ne utječe na čvrstoću smicanja furnirske ploče koja zadovoljava zahtjeve odgovarajućih normi.Veneer densifi cation was performed at 105±3 ºC under pressure of 1,8 N/mm2 for 30 s. The approach allowed to reduce 20-mm thick plywood pressing time by 12-25 % when compared to the non-densified controls. It was found that densifi cation did not affect shear strength of the plywood which met the requirements of the respective standards

Kurowska, Agnieszka

Borysiuk, Piotr

Mamiński, Mariusz

Zbieć, Marcin

Hrčak - Portal of scientific journals of Croatia

..... Kurowska, Borysiuk, Mamiński, Zbieć: Veneer Densiﬁ cation as a Tool for Shortening...DRVNA INDUSTRIJA  61 (3) 193-196 (2010) 193Agnieszka Kurowska, Piotr Borysiuk, Mariusz Mamiński, Marcin Zbieć1Veneer Densification as a Tool for Shortening of Plywood Pressing TimeStlačivanje furnira kao metoda skraćenja vremena prešanja furnirskih pločaPreliminary paper • Prethodno priopćenjeReceived – prispjelo: 25. 5. 2010.Accepted – prihvaćeno: 14. 7. 2010.UDK: 630*832.282ABSTRACT • Veneer densiﬁ cation was performed at 105±3 ºC under pressure of 1,8 N/mm2 for 30 s. The ap-proach allowed to reduce 20-mm thick plywood pressing time by 12-25 % when compared to the non-densiﬁ ed controls. It was found that densiﬁ cation did not affect shear strength of the plywood which met the requirements of the respective standards.Key words: plywood, veneer densiﬁ cation, pressing time, shear strengths SAŽETAK • Stlačivanje furnira provedeno je pri temperaturi 105±3 ºC i tlaku 1.8 N/mm2 tijekom 30 s. Na taj je način omogućeno skraćenje vremena prešanja furnirske ploče debljine 20 mm za 12-25 % u usporedbi s kon-trolnim uzorcima od nestlačenih furnira. Istraživanje je pokazalo da stlačivanje furnira ne utječe na čvrstoću smicanja furnirske ploče koja zadovoljava zahtjeve odgovarajućih normi.Ključne riječi: furnirska ploča, stlačivanje furnira, vrijeme prešanja, čvrstoća smicanja1 Authors are assistant, professor, assistant and assistant at Faculty of Wood Technology, Warsaw University of Life Sciences – SGGW, Poland.1 Autori su asistentica, profesor, asistent i asistent Fakulteta drvne tehnologije Varšavskog sveučilišta – SGGW, Poljska.1 INTRODUCTION1. UVODThe pressing time is a crucial parameter of plywood processing, since it strongly affects total num-ber of cycles during a single shift and, in consequence, the total capacity of a production line. On one hand, in order to maximize the efﬁ cacy of the process, pressing time should be as short as possible. On the other hand, it should be long enough to allow the adhesive to be cured.Wood, as a porous body, is good insulator and therefore weak heat conductor (heat conductivity coef-ﬁ cient ranges from 0.12 up to 0.35 W/mK). Heat con-ductivity of wood is dependent mainly on the species, density, grain direction, moisture content and tempera-ture. Dependence of heat conductivity and density of wood is practically linear (Kollmann, 1955). Increase of density causes proportional gain of wood heat tran-sfer. Heat conductivity along the wood grain is around 2 times higher than in perpendicular direction. Low cross the grain heat conductivity is especial-ly visible during overheating veneer loads for thick board production. As a result, long heating (pre-pres-sing) times and in effect long total pressing times are needed for proper plywood production. As it is reported in literature, the shortening of plywood pressing time can be realized by steam injec-tion (Jokerst and Geimer, 1994) or by combining steam injection and veneer incising. Veneer incising followed by steam injection provided reduction of pressing time by 27% for 21-mm thick 7-ply plywood and by 32% Kurowska, Borysiuk, Mamiński, Zbieć: Veneer Densiﬁ cation as a Tool for Shortening... ......194  DRVNA INDUSTRIJA  61 (3) 193-196 (2010)for 40-mm thick 13-ply LVL when compared to stan-dard technology (Troughton and Lum, 2000). Dai et al (2003) stated that veneer incisions in 13-ply LVL ma-nufacturing did not signiﬁ cantly affect either pressing times or mechanical performance of the product.Other approach to shortening of the overheating time and total pressing time is preliminary veneer den-siﬁ cation. Bekhta and Marutzky (2007) stated that ve-neer densiﬁ cation increases the possibilities of design and control of physical and mechanical properties of the material. Densiﬁ ed beech veneers allow production of plywood with better shear strength properties with lowered glue consumption by 25 % and with pressure limited by 22 %. Densiﬁ cation process includes veneer pre-pressing by 33 % and ﬁ nal manufactured plywood by 0.1 %. Plywood properties are mainly dependent on the stage of the technological process and conditions in which the source material undergoes densiﬁ cation pro-cess (Bekhta and Marutzky, 2007; Bekhta et al, 2009). In contemporary plywood technologies this processing is not used at all.The objective of the study was to investigate the possibility of shortening the overheating time and total pressing time of plywood by preliminary veneer densi-ﬁ cation.2 MATERIALS AND METHODS2. MATERIJAL I METODEPine (Pinus sylvestris) veneers of dimensions 350 x 350 x 1.4 mm3 and 4 % moisture content were used in the experiments. The densiﬁ cation was perfor-med at 105±3 ºC in a laboratory press under pressure of 1.2, 1.5 or 1.8 N/mm2 for 15 or 30 s. The most effective plasticization of wood, densiﬁ cation and retention of the reduced thickness was obtained at 1.8 N/mm2 pres-sure and 30 s pressing time. 20-mm thick, 15-ply plywood was made using densiﬁ ed or non-densiﬁ ed veneers. A urea-formaldehyde adhesive (UF - 100 p.b.w., ﬁ ller - 16 p.b.w., hardener – 16 p.b.w., water – 23 p.b.w.) was used for bonding. Gelling time was measu-red according to the following procedure: a glass test tube with 10 g of glue was immersed in boiling water. The glue was mixed with a glass stick until gelation occurred. Measurement was made in triplicate. Total pressing time (tp) was calculated from the relation (1):   tp = tg+t0 (1)where: tg - gelling time at a given temperature, to - time of heating the most internal glueline (stack core) to a given temperature. Three temperatures were examined: 80 ºC, 90 ºC and 100 ºC. Plywood with glue loads 160 g/m2 and 120 g/m2 were pressed under pressure of 1.2 N/mm2 and at 130±2 ºC. Plywood variants and results are tabulated in Table 2.For the plywood prepared at the shortest pressing times, the shear strength of the glue lines and wood failure percentage were determined according to EN 314-1 and EN 314-2. Twenty specimens were tested in each batch. Both the veneers prior to densiﬁ cation and resultant plywood were conditioned at 20±2 ºC and 65±5 % relative humidity for 7 days.3  RESULTS AND DISCUSSION3.  REZULTATI I DISKUSIJAThe applied densiﬁ cation conditions allowed for sufﬁ cient plasticizing of the veneers and retention of their reduced thickness. Due to the process, the initial 1.45-1.55-mm thickness of the veneers (4 % moisture content) was reduced to 1.35-1.40 mm (2 % moisture content). The average densiﬁ cation ratio was 8 %. Ge-lation time (tg) of the glue is presented in Table 1.Veneer densiﬁ cation clearly reduces overheating time in comparison to unprocessed samples (Fig. 1). The shortest heating time was obtained with 120 g/m2 glue load variant. This phenomenon is caused by lower total moisture content in the veneer stack. Furthermo-re, faster stack internal temperature gain is caused by more dense wood substance. Data presented in Table 2 clearly show that 20 mm plywood made of densiﬁ ed veneers had 24-54 % shorter overheating time (to) in comparison to control sample (heating up to 100 °C). Again, taking into account glue gelation times (tg), it was concluded that veneer densiﬁ -cation shortened total pressing time (tp) by 12-25 % in 0204060801001200 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750Time of heating the most internal glueline (stack core) to a given temperature, svrijeme zagrijavanja unutrašnjeg sloja do određene temperature, sTemperatureinthestackcore, °Ctemperatura u unutrašnjem sloju, °Ccontrol plywood glue loads 160 g/m2 glue loads 120 g/m23 2 1321Figure 1 Overheating times determined for 20 mm plywood made of densiﬁ ed pine veneersSlika 1. Vrijeme zagrijavanja furnirske ploče debljine 20 mm izrađene od stlačenih furnira..... Kurowska, Borysiuk, Mamiński, Zbieć: Veneer Densiﬁ cation as a Tool for Shortening...DRVNA INDUSTRIJA  61 (3) 193-196 (2010) 195comparison to control samples (total pressing time is a sum of gelation and overheating times). It was found that (Table 3) modiﬁ cation of ve-neers by densiﬁ cation showed no signiﬁ cant effect on plywood shear strength in dry and wet state. Wood shear percentage ranged from 60 up to 100 % for dry samples, and after soaking in water it dropped to 30 – 60 %. The percentage of shear in wood in dry state is signiﬁ cantly higher for plywood made of densiﬁ ed ve-neers than in control samples (ranging between 60 and 70 %). Strength properties characterized by lower glue bonds (in dry and wet state) of plywood made of den-siﬁ ed veneers are caused by lower glue saturation pos-sibilities in modiﬁ ed veneers. This result conﬁ rms lite-rature results. Pocius (2002) concludes that one of the conditions of good glue bond formation is solubility of glue in the base (or intrinsic wettability) and surface Table 1 Glue gelation time of urea-formaldehyde adhesive at different temperaturesTablica 1. Vrijeme stvrdnjavanja urea-formaldehidnog ljepila pri različitim temperaturamaStack core temperature, °CTemperatura u središtu, °CGelation time (tg), sVrijeme stvrdnjavanja, s80 31590 186100 80Table 2 Pressing conditions and reduction of pressing timeTablica 2. Uvjeti prešanja i smanjenje vremena prešanja VariantVarijantaGlue loadsKoličina ljepilaStack core temperatureTemperatura u središnjem slojuOverheating time(to)Vrijeme zagrijavanjaShortening of lay up overheating timeSkraćenje vre mena zagrijavanjaPressing time(tg+ to)Vrijeme prešanjaShortening of pressing timeSkraćenje vremena prešanjag/m2 °C s % s %control plywoodkontrolni uzorak furnirske ploče160 100 656 0 736 0plywood from densiﬁ ed veneersfurnirska ploča od stlačenih furnira 160 100 497 24 577 2290 396 40 582 2180 333 49 648 12120 100 472 28 552 2590 365 44 551 2580 301 54 616 16development, substantially lower in densiﬁ ed veneers. The shear strength values as well as the percentage of wood failure met the requirements of the EN 314-2 standard. Basing on the obtained results it may be con-cluded that veneer densiﬁ cation may allow glue load reduction without negative impact on the bond strength properties. This is also conﬁ rmed by other researchers (Bekhta and Marutzky, 2007).4 CONCLUSIONS4. ZAKLJUČCIThis paper deals with the approach to shortening of the total possible plywood pressing time with veneer densiﬁ cation. The obtained results show overheating time shortening possibilities, and thus ﬁ nal total pres-sing time shortening of the 20 mm pine plywood with strength properties conforming to PN-EN 314-2:2001 standard. The shortest total pressing time of densiﬁ ed ve-neer plywood reached 551s (variant with 120 g/m2 glue load). Veneer densiﬁ cation allows shortening of overhea-ting time by 24-54 % and pressing time by 12-25 %, at 80-100 °C while upkeeping internal stack temperature. Densiﬁ ed veneers, except 25 % pressing time shortening, allow 25 % glue load reduction without af-fecting glue bonds strength properties. Table 3 Shear strengths and wood failure percentage of plywood pressed at the shortest pressing times (values in parentheses are standard deviations)Tablica 3. Čvrstoća smicanja i postotak loma po drvu furnirske ploče prešane pri najkraćem vremenu (vrijednosti u zagrada-ma standarne su devijacije)VariantVarijantaGlue loadsKoličina ljepilag/m2Glue line numberLinija ljepilaDry shear strengthSuha smicajna čvrstoćaN/mm2Percentage of wood failurePostotak loma po drvu%Wet shear strengthVlažna smicajna čvrstoćaN/mm2Percentage of wood failurePostotak loma po drvu%control plywoodkontrolna furnirska ploča160 1 3.44 (0.27) 60 2.73 (0.18) 407 3.59 (0.50) 70 2.81 (0.30) 50plywood from densiﬁ ed veneersfurnirska ploča od stlačenih furnira160 1 2.69 (0.19) 60 2.01 (0.19) 407 3.11 (0.39) 70 1.92 (0.15) 30120 1 2.59 (0.31) 90 1.88 (0.20) 307 3.63 (0.35) 100 2.52 (0.19) 60Kurowska, Borysiuk, Mamiński, Zbieć: Veneer Densiﬁ cation as a Tool for Shortening... ......196  DRVNA INDUSTRIJA  61 (3) 193-196 (2010)5 REFERENCES5. LITERATURA1. Bekhta, P.; Marutzky, R., 2007: Reduction of glue con-sumption in the plywood production by using previously compressed veneer. Holz als Roh-und Werkstoff 65: 87-88.2. Bekhta, P.; Hiziroglu, S.; Shepelyuk, O., 2009: Properties of plywood manufactured from compressed veneer as building material. Materials and Design 30: 947-953.3. Dai, C.; Troughton, G.E.; Wang, B., 2003: Development of a new incising technology for plywood/LVL produc-tion. Part 2. Effect of incising on LVL strength properties. Forest Products Journal 53 (11/12): 99-102.4. EN 314-1 (2007) Plywood – Bonding quality – Part 1: Test methods5. EN 314-2 (2001) Plywood – Bonding quality – Part 2: Requirements6. Jokerst, R.; Geimer, R.. 1994: Steam-assisted hot-pres-sing of construction plywood. Forest Products Journal 11-12: 34-36.7. Kollmann, F., 1955: Technologie des Holzes und der Holzwerkstoffe. Springer - Verlag, Berlin.8. Pocius, A.V., 2002: Adhesion and Adhesives Technology. Carl Hanser Verlag GmbH & Co, Munich.9. Troughton, G.E.; Lum, C., 2000: Pilot plant evaluation of steam - injection pressing for LVL and plywood prod-ucts. Forest Products Journal 50: 25-28.Corresponding address:Assistant MARCIN ZBIEĆ, Ph.D.Faculty of Wood TechnologyWarsaw University of Life Sciences – SGGWUl. Nowoursynowska 15902-776 Warsaw,  POLANDe-mail: marcin_zbiec@sggw.pl

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Veneer Densification as a Tool for Shortening of Plywood Pressing Time

Stlačivanje furnira kao metoda skraćenja vremena prešanja furnirskih ploča

Stlačivanje furnira kao metoda skraćenja vremena prešanja furnirskih ploča

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