Development of urea-formaldehyde adhesive bond at gluing of veneer

V diplomski nalogi smo raziskovali razvoj trdnosti urea-formaldehidnega (UF) lepilnega spoja pri lepljenju bukovega furnirja. Lepilno mešanico smo pripravili iz 5 delov UF lepila v prahu in 3 delov vode. Furnir smo narezali na trakove z dolžino 117 mm in širino 20 mm ter jih uporabili za izdelavo dvoslojnega preizkušanca za strižni test. Testiranja smo opravili v laboratorijskih pogojih pri temperaturi 24,5 °C in relativni zračni vlažnosti 38,5 % ter vlažnosti furnirja 5,9 %. Naprej smo testirali preizkušance iz 0,9 mm debelih furnirjev, ki smo jih z UF lepilom lepili pri temperaturah 75 °C, 100 °C, 125 °C in 150 °C s sistemom za avtomatizirano vrednotenje zlepljenosti (ABES). Ugotovili smo, da je optimalna temperatura stiskanja bila 100 °C, saj smo pri tej temperaturi dosegli največjo povprečno strižno trdnost UF lepilnega spoja. Maksimalna strižna trdnost UF lepilnega spoja je bila najprej dosežena pri 150 °C in najkasneje pri 75 °C. Nato smo preverili vpliv treh debelin furnirja (0,5 mm, 0,9 mm in 1,4 mm) na razvoj trdnosti UF lepilnega spoja. Testiranja smo opravili z ABES napravo pri temperaturi 100 °C. Ugotovili smo, da se je utrjevanje UF lepila pri 0,5 mm debelem furnirju začelo najprej, pri 1,4 mm debelem furnirju pa najkasneje, kar je posledica daljšega časa segrevanja lepilnega spoja pri debelejših furnirjih.In this thesis we researched the development of the urea-formaldehyde (UF) adhesive bond with the gluing of cut beech veneer. The glue mixture was prepared from 5 parts of UF powder and 3 parts of water. We cut the veneer into strips with a length of 117 mm and with the width of 20 mm and used them to make double-layered test specimens for shear testing. All testing was done at controlled laboratory conditions at 24.5 °C and with a humidity of 38.5 %, with a veneer moisture content of 5.9 %. First we tested 0.9 mm thick veneer glued with UF adhesive at temperatures of 75 °C, 100 °C, 125 °C and 150 °C with an automated bonding evaluation system (ABES). The results showed that the optimal press temperature was 100 °C, because the average shear strength was the highest. The slowest achieved maximum shear strength was at 75 °C and the fastest at 150 °C. Then we tested how the thickness of the veneer affected the shear strength of the glue, using veneer thicknesses of 0.5 mm, 0.9 mm and 1.4 mm. The tests were carried out with an ABES machine at 100 °C. The results showed that the forming of the bond was the fastest with a 0.5 mm, while a thickness of 1.4 mm needed the most time, due to the longer time need for heating of the adhesive bond