Possibility of application of thermally modified poplar veneer in plywood production

Predmet istraživanja doktorske disertacije bio je da se ustanovi da li se kombinovanjem termiĉki modifikovanog i nemodifikovanog furnira topole može dobiti proizvod koji je dimenzionalno stabilniji u odnosu na klasiĉnu topolovu furnirsku ploĉu, uz istovremeno minimalno narušavanje mehaniĉkih svojstava. Formati topolovog furnira termiĉki su modifikovani na temperaturama od 190 °C, 200 °C, 210 °C i 215 °C u trajanju od 1 h, uz prisustvo vodene pare kao zaštitnog sredstva. Pre termiĉke modifikacije ustanovljena je znaĉajna razlika u boji izmeĊu topolovog furnira nastalog iz zone beljike i iz zone lažne srĉevine. Sa stanovišta izjednaĉavanja boje, najbolji rezultati ostvareni su pomoću tretmana na 210 °C, dok tretmani na 190 i 200 °C nisu dali željeni efekat. Od dobijenih termiĉki tretiranih i od kontrolnih (netretiranih) formata izraĊeno je 13 razliĉitih tipova petoslojnih ploĉa (za svaki tip po 6 ploĉa). Presovanje ploĉa obavljeno je po suvom vrućem postupku presovanja u industrijskim uslovima, a kao vezivno sredstvo korišćen je MUF (melamin-ureaformaldehidni) lepak. Analize su pokazale da su na ravnotežnu vlažnost i dimenzionu stabilnost dobijenih ploĉa imali uticaj i termiĉki tretmani i konstrukcije ploĉa. Promena vlažnosti i dimenzija bila je manja sa porastom temperature režima, kao i sa povećanjem uĉešća termiĉki modifikovanog furnira u konstrukciji ploĉe. Ustanovljeno je da kontrolne ploĉe imaju znaĉajno veću tvrdoću u odnosu na sve druge tipove ploĉa. Pri poreĊenju razliĉitih režima, statistiĉki znaĉajna razlika u tvrdoći ploĉa postoji, ali samo se režim na 190 °C znaĉajno razlikuje od režima na 210 i 215 °C. Na osnovu analize rezultata, zakljuĉeno je da su termiĉki tretmani manje uticali na smanjenje modula elastiĉnosti nego na smanjenje savojne ĉvrstoće. Ispitivanje smicajne ĉvrstoće u slojevima lepka, pokazalo je da svi tipovi ploĉa zadovoljavaju uslove za upotrebu u enterijeru. Sa povećanjem zahteva (testovi za uslove povećane vlažnosti i za spoljašnju upotrebu), došlo je do znaĉajnog pada smicajne ĉvrstoće u slojevima lepka. Ovaj pad smicajne ĉvrstoće najverovatnije je posledica slabljenja kvaliteta lepljene veze pod dejstvom predtretmana. Prema svim ispitivanim svojstvima, tretmani na 200 °C i 210 °C su se pokazali kao optimalni, dok se tretman na 215 °C pokazao kao preoštar i ne treba ga koristiti za termiĉku modifikaciju furnira topole.The objective of the doctoral dissertation was to establish whether the production of plywood made by combining thermally modified and unmodified poplar veneer, could yield products that would be dimensionally more stable compared to ordinary poplar plywood, with minimum decreasing of mechanical properties. Formats of poplar veneers were thermally modified at temperatures of 190 °C, 200 °C, 210 °C and 215 °C during 1 h in the presence of steam as protection agens. The data indicated that there was significant difference in color between poplar veneers made from sapwood and wet heartwood, before thermal modification. After thermal treatments, the best results in reducing of color difference between veneers gained treatment at 210 °C, while thermal treatments at 190 and 210 °C didn´t show satisfactory effect. The thermally treated and control (untreated) veneer formats were used to make 13 different types of five-layer plywood's (6 boards of each type). Plywood pressing was done in a dry-hot pressing process on industrial press, and MUF (melamine-urea-formaldehyde) glue was applied as adhesive. Analyses showed that the equilibrium moisture content and dimensional stability of plywoods were highly influenced by both thermal treatments and plywood constructions. Change of moisture content and dimensions of plywoods were lower with the increase of schedule temperature, and with the increase of percentage of thermally modified veneer in plywood construction. There was established that control plywood had significantly higher hardness then all other produced plywood types. The statistically significant difference in hardness between plywood made of thermally treated veneers at different temperatures existed, but only treatment at 190 °C was significantly different compared by treatments at 210 °C and 215 °C. Analysis of the results indicated that thermal treatments had a lower influence on the decrease of modulus of elasticity than on the decrease of bending strength. Analysis of the results of shear strength in the adhesive layers, showed that every plywood type had satisfactory properties for plywood application in indoor use. Increase of intensity of pre-treatments (tests for plywood application in conditions of high humidity and in outdoor use), caused a significant decrease in shear strength in the adhesive layers. This decrease of shear strength was a consequence of weakening of adhesive bond caused by pre-treatments. It was concluded that treatments at 200 °C and 210 °C gained optimal results, while treatment at 215 °C proved to be too intensive, and should not be used for thermal modification of poplar veneer