Volume 67, Issue 6

The development of (nano)materials based on the renewable cellulose is a challenge. The present article provides a brief overview of the recent research efforts carried out at the CICECO Laboratory of the University of Aveiro on the development of novel composites based on nanofibrillated plant and bacterial cellulose embedded in natural and synthetic polymeric matrices such as poly(lactic acid), chitosan, starch, and pullulan. These materials have high potential for applications in packaging, paper coating, organic electronics, and biomedical products and devices.

Milled wood lignins (MWL) were isolated from the stem (MWL S ) and pith (MWL P ) of bamboo ( Phyllostachys pubescens ). The nonacetylated and acetylated bamboo MWLs were investigated by Fourier transform infrared, quantitative 13 C-nuclear magnetic resonance (NMR), 2D heteronuclear single quantum coherence (HSQC) NMR, and 31 P-NMR spectroscopy. The MWL consists of p -hydroxyphenyl (1–2%), guaiacyl (21–31%), and syringyl (67–78%) units associated with p -coumarates and ferulates. A modified quantitative 13 C-NMR and 2D-HSQC analysis has demonstrated that the predominant intermonomeric linkages are of the type β- O -4 (45–49 per 100 C 9 units, i.e., per C 900 ) along with small amounts of other structural units such as resinols (3.6–7.4 per C 900 ), tetrahydrofuran (2.0–2.3 per C 900 ), phenylcoumaran (2.8–4.5 per C 900 ), spirodienones (1.3–2.3 per C 900 ), and α,β-diaryl ethers (2.8–2.9 per C 900 ). MWL P contained more p -coumarates than MWL S . The various degrees of γ-acylation (17–27%) were positively associated with S/G ratios in the lignins; however, γ-acylation was inversely correlated to the ratio between β-β and β- O -4 side chains in these lignin fractions. Moreover, a flavonoid compound (tricin) was also detected in the MWL S but not in MWL P . The two MWLs are very similar in terms of molecular weights and the contents of OH phen and OH aliph .

The residual wood meal left after milled wood lignin (MWL) isolation [milled wood residue (MWR)] of 5-year-old Eucalyptus globulus was fractionated to afford a xylan-lignin fraction (X-L) in 2.9% yield (based on MWR) by the method reported previously. X-L was further fractionated with the lignin solvent 1,4-dioxane/water (9:1, v/v) to give a soluble fraction (XL-F1; 24.0%) and an insoluble fraction (XL-F1-residue; 74.6%; both yields based on X-L). XL-F1-residue was further extracted with the good xylan solvent dimethyl sulfoxide and the soluble fraction was termed XL-F2 (43.0%; based on the XL-F1-residue). XL-F1 was mainly composed of lignin with a small amount of xylan and it is similar to purified MWL, whereas XL-F2 was mainly composed of xylan with some amount of lignin and it is similar to a fraction that was prepared by the extraction of crude MWL with acetic acid [lignin-carbohydrate complex (LCC)-AcOH]. The two-dimensional heteronuclear single quantum coherence nuclear magnetic resonance spectra of XL-F1 and XL-F2 were interpreted that the former has α-ether-type lignin-carbohydrate (LC) linkages and the latter might have LC linkages of the phenyl glycoside type, which are different from those in LCC-AcOH.

The carbohydrate degradation products have been examined, which are formed during the conventional kraft pulping of a softwood, hardwoods, bamboo, and wheat straw as well as soda and soda-anthraquinone pulping of wheat straw. The focus was on “volatile” acids such as formic and acetic acids and “nonvolatile” hydroxy monocarboxylic and dicarboxylic acids. The different consumption profiles were obtained for the charged alkali required for the neutralization of these aliphatic acids depending on the feedstock and the cooking method. The relative composition of the acid fraction in the black liquors of softwood and hardwood and nonwood feedstocks showed characteristic variations. However, in the case of wood kraft pulping, the variations in cooking conditions (effective alkali 19–21% and cooking temperature 155–170°C) had no significant effect on the acid composition. The total amount of volatile and hydroxy acids formed during pulping at a typical target κ number level for each feedstock ranged from 78 to 174 kg ton -1 based on o.d. feedstock. It was highest in birch kraft pulping and lowest in wheat soda-anthraquinone pulping.

Unbleached Eucalyptus kraft pulps were treated by three methods and the bleaching effects have been evaluated. The treatments were performed by a laccase-mediator system (LMS), refining (R), and urea (U) alone and in combination of these methods. The bleached pulps were characterized in terms of κ number, hexenuronic acid content, brightness, and viscosity. In addition, the lignin-carbohydrate complexes (LCCs) were fractionated and characterized with regard to the fractionation yield, lignin content, and carbohydrate composition. Moreover, the 1 H nuclear magnetic resonance spectra were recorded and the thioacidolysis-gas chromatography and thioacidolysis-size exclusion chromatography analyses were performed. The LMS treatment resulted mainly in a slightly better delignification as manifested by a κ number (K no.) reduction of 0.8 units. The effects of the U treatment included delignification (1.4 κ units), hexenuronic acid removal (3 μmol g -1 ), and improved pulp brightness (3.4 ISO units). The R treatment darkened the pulp (3 ISO units), whereas the bleaching of the R-pulp by subsequent LMS or U treatment enhanced the brightness (0.5 unit more) or delignification (0.3 unit more). The residual lignin in the pulp samples was present mainly as xylan-lignin. The Klason lignin content from the xylan-lignin fraction decreased as the degree of delignification increased. The pulp darkening followed by the R treatment was interpreted as the result of the formation of condensed lignin structures, but these condensed lignins were substantially removed by the subsequent LMS or U treatment.

Three novel curing agents ( I , II , and III) were synthesized from epichlorohydrin and ammonium hydroxide. The combinations of soy flour (SF) with one of the curing agents (SF- I , SF- II , and SF- III ) were investigated as adhesives for making interior plywood. Water resistance tests showed that plywood panels bonded with SF- I and SF- III adhesives met the requirements of interior plywood, whereas those bonded with SF- II did not. The modulus of rupture, modulus of elasticity, and internal bond strength of particleboard panels bonded with the SF- II adhesive all exceeded the corresponding minimum industrial requirements for M-2 grade particleboard.

The so-called lotus effect has inspired the production of highly hydrophobic wood surfaces based on a two-step process consisting of growing nanofilms on the wood substrate (Chinese fir) with silica sol followed by hydrophobization with hydrolyzed hexadecyltrimethoxysilane (HDTMS). The microstructure and chemical composition of the nanofilm coating were examined by field emission scanning electron microscopy, energy dispersive X-ray and Fourier transform infrared spectroscopy. The hydrophobicity of the treated wood was evaluated by water contact angle measurements. The results show that spherical nanoparticles were deposited uniformly on the wood surface, and the long-chain hydrophobic HDMTS was covalently linked to the surface of silica particles. The elevated hydrophobicity of wood was manifested by the water CA of about 141° on the longitudinal surface and 150° on the transverse surface.

Crack-tip displacement fields have been computed based on digital image correlation for spruce ( Picea abies [L.] Karst.) and birch ( Betula pendula Roth.) wood, which were submitted to pure mode I loading in the RT-direction under both green and air-dried conditions. Moreover, crack propagation was modeled based on both linear elastic fracture mechanics (LEFM) and nonlinear fracture mechanics, relying on the fictitious crack model (FCM). The measured and modeled load versus the crack-mouth opening displacement (CMOD) curves and displacement fields were compared. In the case of spruce, the load-CMOD curves simulated by the FCM coincide well with the measured ones. On the contrary, measured near crack-tip displacement fields in both green and air-dried spruce are better comparable with the LEFM predictions than with the FCM predictions. In the case of green birch, the simulated FCM curve follows the measured curve quite well, but in air-dried birch the simulated FCM curve has a better fit than the LEFM-curve only before maximum load. In birch, the FCM predicts the displacement fields better than the LEFM. In both species, moisture content has a big effect on the softening behavior. In both spruce and birch, the FCM overestimates the displacements ahead of crack tip, whereas the LEFM model underestimates the displacements.

The flatwise Young’s modulus and the flatwise shear modulus of 3-, 5-, and 7-ply plywoods made of Lauan ( Shorea sp.) veneers have been determined by conducting flexural vibration tests with various specimen lengths and by finite element analysis. The results indicate that the flatwise Young’s modulus decreases with decreasing specimen length, whereas the opposite is true for the flatwise shear modulus.

The specific energy consumption during mechanical refining operation can be reduced by choosing the optimal process parameters in the wood chipping process such that a beneficial pretreatment is obtained. In the case of the utilization of a larger knife-edge angle, which is one such process parameter, the energy reduction is presumably due to the increased compressive loading parallel to the wood fibers. In the present article, a chip damage parameter D of spruce is in focus, which is relevant for cracking parallel to the fibers. D is defined and its dependence on the chip length and edge angle of the chipping knife is analyzed numerically by means of finite element analyses (FEA). The cutting force was measured in a pilot wood chipper for a number of knife-edge angles. There is a good correlation between the experimental results and those of FEA.

Pine sapwood was treated with various types of silica sols. Whereas alkaline sols were not able to penetrate deeper into wood, neutral and acidic sols showed good penetration. The weight percent gain of treated specimens amounted to 20–25%; bulking was negligible or even slightly negative. All silica sols in the treated specimens were stable against water leaching. A water submersion test revealed hydrophobation of the wood only after treatment with a cationic silica sol; all other silica sols increased the rate of water uptake. The addition of 2% cationic sol to a malt-agar growth medium caused growth inhibition of 40–50% of the wood decay fungi Coniophora puteana and Trametes versicolor , whereas the other silica sols did not inhibit growth. Pine sapwood and beech wood blocks treated with the cationic sol showed a strong reduction in mass loss compared to the control samples after incubation with C. puteana (pine) and T. versicolor (beech) according to EN 113 and CEN/TS 15083-1; all other silica sols did not inhibit fungal decay. The cationic silica sol reduced blue staining by Aureobasidium pullulans compared to the untreated control but did not fully prevent it; all other silica sols did not inhibit blue staining.

Previous studies of the oak wood of the 17th century warship the Vasa have shown significant changes in the chemical and mechanical properties compared with recent oak. The most important factors contributing to these changes are the incorporation of iron compounds during waterlogging and the uptake of polyethylene glycol (PEG) in the course of the 17 years of preservation treatment. To investigate the effect of iron-dependent oxidative degradation reactions, recent oak wood samples were impregnated with aqueous iron(II) chloride solution (0.1 M ) and thereafter exposed to air or pure oxygen at controlled relative humidity in long-term experiments followed by tensile strength (TS) measurements. The iron-impregnated samples exposed to oxygen displayed significant effects already after 1 week and the reduction in TS was ~50% after 1 year. The samples treated with additional PEG displayed less TS reduction, whereas the addition of cysteine had no effect. The size exclusion chromatography of treated samples showed that the average molecular weight of holocellulose had decreased. The results confirm that iron compounds have a detrimental effect in wood and indicate that PEG might act as an antioxidant for the degradation processes. Concerning the Vasa , it may be concluded that most degradation related to iron compounds and oxidative processes have taken place during the first period of conservation when the wood was exposed to oxygen in a still very humid state. Thus, the current rate of oxidative degradation under the present relatively dry museum conditions should be relatively low.

A new thermal insulation system (Mirrorpanel) based on an effective thermal insulation of the air by multiple heat reflection of the radiation heat flux was produced at the Innovation Center of the University of West Hungary. The system was created for wood frame structure buildings where the units of the framework provide the fixing points. In the panels of the new system, there are narrow air fields with high thermal resistance and the lamellae in the field have low surface emission properties, which reflect a considerable part of heat radiation. This system of heat reflection and the narrow air fields was formed with the help of stretched foils. Based on the results of tests gained from the small and medium-sized insulation panels, a life-size test building was constructed, where the performance and conduct of the Mirrorpanel can be continuously studied.