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  • 1.
    Cristescu, Carmen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknologi.
    Bonding of laminated veneers with heat and pressure only2006Inngår i: Proceedings of Ecowood 2006 / [ed] Fernando Caldeira Jorge, Porto: Fernando Pessoa University , 2006, s. 339-348Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A new engineered wood product was obtained by pressing several layers of veneers at high temperatures. Experiments with different pressing parameters were performed in a laboratory hot plate press on Fagus Sylvatica. Shear strengths test were run according to EN 314-1 and EN 314-2 “Plywood – Bonding qualities”. Samples were analysed with a digital camera to observe the change in colour and with a X-ray microdensitometry scanner to determine the density variation and profiles. Scanning Electron Microscopy (SEM) was used to study the modification of wood cell structure close to and inside the bonding zones. The results showed that the bonding properties of the laminates are good, better when veneers have the same grain direction. Beech veneer with red heartwood can improve its appearance if bonded with the technology presented.

  • 2.
    Cristescu, Carmen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknologi.
    Bonding veneers using only heat and pressure: focus on bending and shear strength2008Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    A compact laminated board can be obtained by pressing layers of veneers with no other material or substance placed between them. The process does not require the use of steam pretreatment, surface activation methods, a gastight press, friction or adhesives. It strictly involves the heat and the pressure induced by the press in the veneers. The levels of temperature, pressure and time leading to the highest bending-strength values when material factors are held constant are investigated. The objective of optimizing the process is reached using response surface methodology for modelling and analysis. The parameter interactions are found to be significant.Photography, scanning, X-ray densitometry, light microscopy and scanning electron microscopy (SEM) are the methods used to visually analyse the final product. Densification and darkening are two of the effects observed.

  • 3.
    Cristescu, Carmen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknologi.
    Self-bonding of beech veneers2015Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    This thesis presents studies on the self-bonding of veneers, and investigations into the properties of boards obtained by hot-pressing beech veneers without adhesives. The raw material is pressed at temperatures from 200 to 250ºC, at pressures from 4 to 6 MPa and pressing times from 240 to 360 s. The hot-pressing of five layers of veneers results in a self-bonded, compressed, densified board, darker than the raw material. The manufacturing process was optimised using a statistical analysis, the response surface experimental design method. The input factors were the pressing parameters and the output factors were characteristics of the product, the raw material properties being kept constant. The results showed correlations between the pressing parameters, especially temperature, as input data and the physical and mechanical properties as responses. Resistance to water absorption and swelling as well as shear strength, bending strength, and hardness were all enhanced by more severe pressing conditions. All the samples pressed at 250ºC became water-resistant while samples pressed at 200ºC delaminated rapidly in water. It was found that the colour of the edges can be used as an indication of the hardness. Chemical investigations were performed on samples taken from veneers (raw material), bond-lines and inner parts of pressed veneer. A High-Performance Liquid Chromatography (HPLC) analysis was used to detect and compare the levels of water-soluble monosaccharides (glucose and fructose) in the veneer before pressing and the levels of 5-hydroxymethyl-furfural (HMF) and furfural in boards. From the results obtained in this thesis it is suggested, that besides other degraded compounds of hemicelluloses, the monosaccharides were transformed into hydroxymethyl-furfural during hot-pressing that could further participate in forming new browning compounds that are related to the bonding phenomenon. The UV spectroscopic analysis to evaluate the content of water-soluble phenols showed that the bond-line had a higher amount of conjugated phenols than the inner veneer suggesting a migration of degraded lignin towards the veneer surfaces during pressing. A CP/MAS 13C NMR study on the solid material showed that the presence of β-ether structures and methoxyl groups in lignin was higher in the bond-line, supporting the hypothesis of a migration of lignin and possibly a condensation reaction occurring at higher temperatures, explaining the water-resistance property acquired by the boards.In a comparative study, oxidative activation with hydrogen peroxide and ferrous sulfate as a catalyst was performed before pressing. Boards made from beech veneer subjected to the pre-treatment step showed a greater mass loss, a lower thickness swelling but also a somewhat lower shear strength. For beech veneers such an oxidative pre-treatment step could be used to gain water-resistant boards pressed at lower pressing temperatures or shorter times than those needed to bond untreated veneers. Scots pine veneers were found to be less suitable for self-bonding than beech veneers. Boards of Scots pine hot-pressed without the oxidative pre-treatment delaminated in water, whereas, under the same pressing conditions, boards made from pre-treated veneers showed a water-resistant bond-line.

  • 4. Cristescu, Carmen
    et al.
    Karlsson, Olov
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknologi.
    Changes in content of furfurals and phenols in self-bonded laminated boards2013Inngår i: BioResources, ISSN 1930-2126, E-ISSN 1930-2126, Vol. 8, nr 3, s. 4056-4071Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Pressing beech veneers at high temperatures has been shown to be a reliable method for manufacturing laminated boards without adhesives. The reasons behind the self-bonding phenomenon as well as the causes of the waterproof character gained by the boards being pressed at 250 degrees C were investigated. Water leachates from the dried and the hot-pressed veneers were analysed by UV-spectroscopy, high-performance liquid chromatography (HPLC), and solid-state cross-polarization magic angle spinning carbon-13 nuclear magnetic resonance (CP/MAS 13C NMR). Press-plate temperatures during hot pressing were 200, 225, and 250 degrees C. After pressing, an increased content of 5-(hydroxymethyl) furfural (not at 250 degrees C) and conjugated phenols was observed in the bonding lines (interfaces) compared to the inner part of veneers of the self-bonded boards. Furfural contents were low and relatively similar, but 5-(hydroxymethyl) furfural (HMF) showed an abrupt decrease in the bonding line when the temperature increased from 200 degrees C to 225 degrees C and especially to 250 degrees C. The contribution of caramelization to browning and bonding is suggested. In studies with CP/MAS 13C NMR, a higher content of phenolic units in beech lignin was observed during hot pressing at 225 degrees C. Homolytical cleavage of beta-O-4 structures in lignin as well as the condensation reactions involved are discussed

  • 5.
    Cristescu, Carmen
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Neyses, Benedikt
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Sandberg, Dick
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Söderström, Ove
    Kungliga tekniska högskolan, KTH, KTH Byggnadsmateriallära.
    Modeling of the temperature distribution in self-bonded beech-veneer boards during hot pressing2015Inngår i: Pro Ligno, ISSN 1841-4737, E-ISSN 2069-7430, Vol. 11, nr 4, s. 97-103Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Auto-adhesion is a phenomenon that is in general related to the manufacture of wood-basedfibreboards, e.g. to the Masonite process or similar processes based on lignocellulosic raw materials.Auto-adhesion as a mechanism for the bonding of solid wood or veneer has not met with the sameindustrial success, but interest is increasing for environmental reasons and as a result of theincreasing cost of adhesives in wood products. The temperature in the laminate is crucial for the autoadhesionprocess that will result in bonding between veneers during hot-pressing. This paper presentsa model for the temperature evolution during the hot-pressing of a porous material, which wasdeveloped and verified for a five-veneer beech laminate pressed at a temperature of 250°C and apressure of 6MPa in an open system for 280 seconds. The result shows good agreement between themodel and the experimental temperature data during the hot-pressing. It can be concluded that a goodcontrol of the temperature evolution during the manufacture of adhesive-free veneer boards is of majorimportance to reach the target properties of the product.

  • 6.
    Cristescu, Carmen
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknologi.
    Sandberg, Dick
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknologi.
    Self-bonding of veneers with heat and pressure: a full scale test2014Inngår i: Final Cost Action FP0904 Conference: “Recent Advances in the Field of TH and THM Wood Treatment” : May 19-21, 2014, Skellefteå, Sweden : book of abstracts, Skellefteå: Luleå tekniska universitet, 2014, s. 76-Konferansepaper (Fagfellevurdert)
  • 7.
    Cristescu, Carmen
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Sandberg, Dick
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Ekevad, Mats
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Karlsson, Olov
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Influence of pressing parameters on mechanical and physical properties of self-bonded laminated beech boards2015Inngår i: Wood Material Science & Engineering, ISSN 1748-0272, E-ISSN 1748-0280, Vol. 10, nr 2, s. 205-214Artikkel i tidsskrift (Fagfellevurdert)
  • 8.
    Cristescu, Carmen
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Sandberg, Dick
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Hagman, Olle
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Karlsson, Olov
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Could colour predict hardness of hot-ptressed self-laminated beech boards2015Inngår i: Pro Ligno, ISSN 1841-4737, E-ISSN 2069-7430, Vol. 11, nr 4, s. 150-156Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Laminated self-bonded densified boards were obtained by pressing five veneers of beech(Fagus sylvatica L.) parallel-grain-oriented, without adhesive and without surface activation. Theboards were pressed according to an experimental design based on fifteen different combinations ofpressing parameters: temperature (200, 225, and 250°C), pressure (4, 5, and 6MPa), and time (240,300, and 360s). The image of the 40 board edges (radial sections) was analysed with ImageJ softwarein the red-green-blue (RGB) colour space. Brinell hardness tests were also performed. The resultsshow an almost linear relation between the brightness values (defined as the arithmetic mean of theRGB channels) and the Brinell hardness. It is suggested that brightness is a predictor of strength forself-bonded laminated boards.

  • 9.
    Cristescu, Carmen
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Sandberg, Dick
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Karlsson, Olov
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Markers of quality in self-bonded beech boards2017Konferansepaper (Fagfellevurdert)
    Abstract [en]

    A self-bonding phenomenon takes place when five layers of beech (Fagus SylvaticaL.) veneers are pressed at temperatures higher than 200ºC. If the pressing temperature between veneer surfaces reaches at least 225ºC during pressing and if the pressure applied is optimal, water-resistant bonds are formed between veneers. This study investigates the relation of thickness reduction (marker of compression) and mass loss (marker of heat treatment intensity) to boards quality. The effect of water and water vapour on the bondings between veneer in boards pressed at 200, 225 and 250ºC is studied. The conclusion is that pressing 5 layers of 2 mm rotary-cut beech veneer parallel-fibered at 225ºC, 5 MPa and 300 s leads to a thickness reduction of 33.4 % and mass loss of 1.23 %; in such boards bondings are not resistant to liquid water but are resistant to vapour after one adsorption-desorption cycle. When pressing at 250ºC, 5 MPa and 300 seconds, the thickness reduction is 50% and the mass loss 4%; in such boards no delamination was observed when soaked in water. Boards pressed athigher temperature show lower hygroscopicity. Their equilibrium moisture content (EMC) rangedbetween 3.6 and 7%. Based on the results of this study it ishypothesised that the decay resistance of self-bonded boards will increase when increasing the severity of the hot-pressing.

  • 10.
    Karlsson, Olov
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Cristescu, Carmen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Neyses, Benedikt
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Sandberg, Dick
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Pretreatment of veneers with hydrogen peroxide for self-bonded laminated boards.2015Inngår i: Proceedings of the Eighth European Conference on Wood Modification: ECWM8 / [ed] mark Huges; Lauri Rautkari; Tuuli Uimonen; Holger Militz; Brigitte Junge, Helsinki: Aalto University, School of Chemical Engineering , 2015, s. 305-310Konferansepaper (Fagfellevurdert)
  • 11.
    Karlsson, Olov
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Cristescu, Carmen
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Sandberg, Dick
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Träteknik.
    Autoadhesion of laminated boards from Scots pine veneers: effect of oxidative pretreatment2015Inngår i: Pro Ligno, ISSN 1841-4737, E-ISSN 2069-7430, Vol. 11, nr 4, s. 110-115Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper presents results from studies on the pre-treatment of veneers from Scots pineusing hydrogen peroxide and a ferrous catalyst followed by pressing at 220, 230, and 240°C. Thetreatment gave boards that did not delaminate when exposed to water followed by drying of roomtemperature, whereas boards without a pre-treatment delaminated. The press-plate temperature didnot influence the extent of delamination, but the thickness swelling was lower at higher presstemperature. Analysis of extracts from the oxidative pre-treated and hot-pressed surface materialusing UV-spectroscopy was compared with analysis of bondlines from water-stable laminated boardsfrom beech.

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