Xiaoning Lu

Wheat straw particleboard bonding improvements by enzyme pretreatment

Three kinds of enzyme were used to treat wheat straw. Particleboards bonded with a UF resin were made with the treated wheat straw as a raw material. The results obtained show that the enzyme action is not affected under certain conditions when separated from the living cell producing it. The free-radical content of wheat straw was observed by electron spin resonance (ESR) to clearly increase after it is treated with cellulases from Trichoderma sp. and Aspergillus niger. The wheat straw benzene-ethanol or ether extracts content decreases after wheat straw treatment with lipases from Candida rugosa. This indirectly indicates that the surface wax of wheat straw is reduced if wheat straw is treated with lipases. The pH-values of wheat straw are somewhat changed by the enzyme treatment indicating that its buffering capacity is decreased. Wheat straws surface wax is one of the main adhesion inhibitors in its use as a raw material for particleboard. The better properties of UF-bonded particleboard made using enzyme treated wheat straw can be explained on this basis. In this regard the effect of cellulases from Aspergillus niger yields the best result.

Lignocellulosic substrates influence on TTT and CHT curing diagrams of polycondensation resins

Lignocellulosic substrates such as wood were found to have a marked modifying influence on a well-defined region of CHT diagrams during hardening of phenol–formaldehyde (PF) and urea–formaldehyde (UF) polycondensates. This was ascribed to more complex resin phase transitions due to resin/substrate interactions peculiar to these substrates. The chemical and physical mechanisms of the interactions of the resin and substrate causing such CHT diagram modifications are presented and discussed. The Di Benedetto equation describing the glass transition temperature Tg of the system as a function of the resin degree of conversion p has been slightly modified to take into account the modified CHT diagram. The modified CHT diagram can be used to good effect to describe the behavior of polycondensation resins when used as wood adhesives during their curing directly into the wood joint.

Curing conditions effects on the characteristics of thermosetting adhesives-bonded wood joints — Part 1: Substrate influence on TTT and CHT curing diagrams of wood adhesives

Lignocellulosic substrates such as wood were found to have a marked modifying influence on a well-definite region of CHT diagrams during hardening of phenol-formaldehyde (PF) and urea-formaldehyde (UF) polycondensates. This was ascribed to more complex resin phase transitions due to resin/substrate interactions peculiar to these substrates. The chemical and physical mechanisms of the interactions of resin and substrate causing such CHT diagram modifications are presented and discussed? The Di Benedetto equation describing the glass transition temperature Tg of the system as a function of the resins degree of conversion p has been slightly modified to take into account the modified CHT diagram. The modified CHT diagram can be used to good effect to describe the behaviour of polycondensation resins when used as wood adhesives during their curing directly in the wood joint.

Curing conditions effects on the characteristics of thermosetting adhesives-bonded wood joints

Thermomechanical analysis (TMA) results indicate that simple post-treatment (such as some postcuring) conditions capable of improving the mechanical performance of aminoplastic resins-bonded particleboard without any degradation do indeed exist. A model to describe the decrease in temperature under different conditions of a particleboard after hot pressing has been developed and shown to correlate well with experimental results of board temperature variation after pressing, both on cooling and during postcuring under different conditions. From this, conditions of temperature and time favourable to improve panel performance by postcuring treatments were also determined. The validity of the improvements forecasted under such conditions was then confirmed at molecular level for urea-formaldehyde (UF) adhesive/wood joints by TMA testing, and finally confirmed by testing the mechanical performance of laboratory UF-bonded boards prepared under the postcuring treatment conditions identified. Noticeable improvements in several UF-bonded board properties such as internal bond (IB) strength, thickness swelling, MOE and MOR were observed by implementing short postcuring periods at temperatures much lower than pressing temperature. The panels performance improvements observed were explained on the basis of the shifts in relative importance of already described and well-known molecular level rearrangements of the cured adhesive network in modern, lower formaldehyde content UF adhesives. The conclusion was that modern, lower formaldehyde content UF adhesives can considerably benefit from short, hot postcuring periods as regards board performance, a trend in clear contrast with the degradation and loss of performance this practice was known to induce in the older, very much higher formaldehyde content aminoplastic resins of the past. The coupling of the simple and very rapid TMA technique with the developed model allows the rapid scan of many other post-treatment schedules and thus to forecast other still possible improvements in the performance of UF-bonded and other-adhesives-bonded particleboard.ZusammenfassungErgebnisse der TMA zeigen, daß es tatsächlich einfache Bedingungen zum Nachhärten von Spanplatten gibt, womit die mechanischen Eigenschaften von Aminoplastgebundenen Platten ohne Abbauerscheinungen verbessert werden können. Es wurde ein Modell entwickelt, das den Temperaturverlauf in Spanplatten unter verschiedenen Bedingungen nach dem Heißpressen beschreibt. Das Modell korreliert gut mit experimentellen Temperaturmessungen nach dem Pressen, und zwar sowohl während des Abkühlens als auch beim Nachhärten unter verschiedenen Bedingungen. Daraus wurden günstige Temperatur- und Zeitbedingungen abgeleitet. Die Gültigkeit der vorhergesagten Verbesserungen wurde zunächst auf molekularem Niveau an Holz/Leim-Verbindungen (UF-Harz) überprüft, und schließlich durch mechanische Tests an Laborspanplatten, die unter den entsprechenden Bedingungen hergestellt wurden, bestätigt. Merkliche Verbesserungen einiger Eigenschaften von UF-gebundenen Spanplatten (wie Querzugfestigkeit, Dickenquellung, MOE und MOR) ergaben sich bei kurzen Nachhärtzeiten und Temperaturen weit unterhalb der Preßtemperatur. Die Verbesserung der Leistungsfähigkeit der Platten wird erklärt durch bereits beschriebene und wohlbekannte molekulare Umlagerungen des Netzwerks der ausgehärteten modernen UF-Harze mit niedrigem Formaldehydgehalt. Die Schlußfolgerung ist, daß die Leistung moderner UF-Harze mit niedrigem Formaldehydgehalt beträchtlich gesteigert werden kann durch kurze Nachhärtezeiten bei hohen Temperaturen. Dies steht im deutlichen Gegensatz zu früheren Konzepten bei Verwendung älterer Aminoplaste mit hohem Formaldehydgehalt; denn dabei mußte mit Abbau und Qualitätsverlusten gerechnet werden. Die Kombination der einfachen und rasch durchzuführenden TMA mit einem Simulationsmodell ermöglicht eine rasche Beurteilung verschiedener Nachbehandlungs-Fahrpläne und somit die Voraussage weiterer möglicher Qualitätssteigerungen von UF-gebundenen Spanplatten.

Green wood gluing by traditional honeymoon PRF adhesives

1 Materials and methods Small glulam laboratory beams of dimensions 500×80×60 mm (length by width by depth) of Norway spruce wood of 450 kg/m3 density were bonded with a traditional (Pizzi and Cameron 1984) PRF honeymoon, fast set separate application wood adhesive approved for structural exterior grade applications, at 20 ◦C, at a wood moisture content of 55%. The PRF honeymoon adhesive was composed of (i) part A, a commercial PRF adhesive at pH 8 to which had been added as hardener 20% formaldehyde powder on resin solids content and some coconut shell flour wood filler; (ii) part B, the same commercial PRF resin to which no hardener was added, but a 33% NaOH water solution had been used to correct the pH to 12. The two components were applied each to a face of Norway spruce (Picea abies) wood slats of 500×80×30 mm that had previously been conditioned to 55% equilibrium moisture content. The beams had then been assembled and then clamped and left in the clamp at 20 ◦C for 2 hours only. The shear strength increase as a function of time of a set of specimens was initially tested 2 hours after assembly (hence immediately out of the clamp), 4 hours, 24 hours and 7 days after assembly to build the curve of strength increase as a function of time shown in Fig. 1. The percentage moisture content (MC) was of 55% at the 2 hours, 4 hours and 24 hours tests. For the 7 days test the moisture content was allowed to either (i) slowly decrease towards the equilibrium moisture content (emc) reaching at 7 days a value of 17.6% MC, or (ii) it was dryed at 30 ◦C to 9.1% MC. The shear strength and percentage wood failure results obtained after testing the 7 days old sample dry, after 24 hours cold water soaking and after 2 hours boiling are shown in Table 1.

Interior MDI/pine tannin plywood adhesives without formaldehyde

Three-ply beech-wood laboratory plywood panels were prepared by using glue mixes obtained by adding to a 40% radiata pine tannin water solutions polymeric diisocyanate p-MDI and fine powder paraformaldehyde.

The study of linear vibrational welding of moso bamboo

Abstract Moso bamboo (Phyllostachys pubescens) linear-vibration welded joints with three different combinations between the inner and outer surfaces were studied in this paper. The tensile-shear strength, the microstructures and welded layer thickness observed by SEM and the density profiles measured by X-ray densitometer of the welded samples have been considered and compared. The maximum average tensile-shear strength of outer-outer, inner-inner and outer-inner welded joints were 5.91, 7.15 and 6.24 MPa, respectively. And only the inner-inner welded joints have the bamboo failure after the shear test. The welded joint is composed of a lot of bamboo fibers entangled in the molten and flown intercellular materials which decide the final mechanical performance of the welded joints. The thickness of the outer-outer welded layer is the smallest and the inner-inner welded layer is the highest. However, the maximum density reached in the inner-inner welded joint is smaller than in the outer-outer welded joint owing to the different vascular distribution in the cross section of the moso bamboo. The relative density can be used to reflect the tensile-shear strength of the welded joints. The natural structure of bamboo really do have significant influences on the welded interface.

The effect of high voltage electrostatic field (HVEF) treatment on bonding interphase characteristics among different wood sections of Masson pine (Pinus massoniana Lamb.)

Abstract High voltage electrostatic field (HVEF) treatment has been investigated as an optimization method for enhancing the bonding performance of wood via increasing its polarization degree and improvement of the penetration of phenol formaldehyde (PF) adhesive. As the wood surfaces from cross cut (C), radial cut (R) and tangential cut (T) behave differently, five cut combinations formed the samples to be tested, namely C-C, R-R, R-T, T-T (always parallel to grain) and T-T⊥, where the grains were perpendicular to each other. The gluing and HVEF treatments were performed simultaneously. The sample surfaces were characterized by electron spin resonance (ESR) spectroscopy, dynamic contact angle (CAdyn) measurements, X-ray densitometry, fluorescence microscopy, Fourier-transform infrared (FTIR) spectroscopy and measurements of compression shear bonding strength (CSBS). An increased surface energy led to decreased CAdynS in the following order: cross section<tangential section<radial section. Obviously, the triggered free electrons of the HVEF treatments changed the wood surfaces. The penetration depth of PF into wood cell decreased significantly and the maximal density increased after the HVEF treatment. The lower CAdyns also contributed to the better reaction of the wood surface with the PF resin. The CSBS of the five sample combinations was enhanced owing to a better performance of adhesive aggregation, which was increased by 18% (C-C), 24% (T-T), 26% (T-T⊥), 31% (R-T) and 42% (R-R), respectively. Pore size and pore size distribution contributed a lot to the bonding properties of HVEF-treated wood sections.

Comparison of linear vibration welded joints in three different directions of wood tauari (Couratari oblongifilia)

Abstract Without any adhesive and other chemical additives, the mechanically induced welding can make good wood joints satisfying the relevant requirements for application. The bonding performances of the wood Tauari (Couratari oblongifilia) welded joints in three different directions were studied here: parallel to the wood grain, perpendicular to the wood grain and end grain to end grain. The results show that the compression shear bonding strength of the perpendicularly welded joints was the best, parallel welded joints took the second place while the strength of the end grain butt joints was so small that it could not resist the sewing machine. The figures of scanning electron microscopy and the density profiles show the different structures and density distributions of the welded joints which indicate the reasons behind the big differences of the compression shear bonding strength.

Palmyra palm bonding by vibrational welding

Palmyra palm is a very valuable commercial crop in coastal areas of India, northern Sri Lanka, and mainland southeastern Asia. Friction welding has several advantages compared to traditional mechanical fasteners or gluing. In this study, mechanically induced linear vibration friction welding was applied to palmyra palm. The high levels of carbohydrates and lignin contribute to the bonding joint a lot, but the lack of entangled and mixed fiber makes the bonding strength not that high.