Browsing by Author "Vuorinen, Tapani, Prof., Aalto University, Department of Forest Products Technology, Finland"

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  • Chemical characteristics of surface colour of birch veneer (Betula pendula)
    (2015) Yamamoto, Akio
     School of Chemical Technology | Doctoral dissertation (article-based)
    This thesis investigates the discoloration of veneer produced from silver birch (Betula pendula) logs by means of biochemical and chemical analyses on the veneer and sap obtained from wood which was soaked at varying temperatures (20-70 ˚C). Drastic colour development was confirmed from the sap squeezed from green birch wood. The results from biochemical tests on the sap by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) showed the presence of polyphenol oxidases (PPO) which might form quinonoid intermediates in sap. Further chemical analysis by gas chromatography coupled with mass spectrometry revealed that the major phenol, which acts as a substrate for PPO, was epicatechin (EC). Control of the discoloration by either sodium sulphite (Na2SO3), or ethylenediaminetetraacetic acid (EDTA) was demonstrated. This thesis also investigates the chemical impact of elevated soaking temperature on the birch log being soaked. The results from spectrophotometric analysis on the veneer showed a significant increase in lightness (L*) and a decrease in yellowness (b*) in the veneer produced from logs soaked at 70 ˚C. Conversely, the effect of long-term storage resulted in a significant reduction in L* and an increase in b*. The results from the chemical analysis of squeezed sap revealed that the physical interaction between the core part of log and soaking water is limited when soaking at elevated temperatures. Assessment of the soaking water quality showed an increase of organic loading in water as soaking temperature increases. Bark seemed to be the major source of the organic substances. The results from the analysis of the squeezed sap from the soaked log showed that glucose and fructose were dominant free monosaccharides and their concentration was increased during soaking. The concentration of carboxylates, sulphite and sulphate had a positive correlation with soaking time at 70 ˚C. EC in the sap was diminished under soaking. In addition, the proanthocyanidins (PAs) contents had a moderate negative correlation between the contents and the soaking temperature. It was observed that phenolic compounds including EC and PAs played a major role in the colour change of the veneer, and their concentration was altered by elevated temperature.
  • Mechanochemical reactions in lignocellulosic materials
    (2015) Solala, Iina
     School of Chemical Technology | Doctoral dissertation (article-based)
    In this work, industrially relevant mechanochemical processes of lignocellulosic materials were investigated. To this day, mechanochemistry remains unknown even to most chemists, and reports on its effects in the forest products industry are scarce. As many processes of the pulp and paper sector include mechanical treatments, the effect of such treatments on the chemical properties of the materials should be known.  The treatments that were studied herein were: preparation of nanofibrillated cellulose (NFC) by Masuko refining, thermomechanical pulping at high temperatures (HT-TMP) and ball milling in the presence of a reactive vinyl monomer, namely, styrene. The HT-TMP was also studied as a potential reinforcing agent in biocomposites. Material properties were characterized with techniques such as electron paramagnetic resonance (EPR) and ultraviolet resonance Raman (UVRR) spectroscopies, optical analysis of fiber dimensions, tensile testing, and scanning electron microscopy (SEM).  The main hypothesis of the work was that mechanoradicals are formed upon various mechanical treatments. This was confirmed for kraft pulp homogenization into NFC and reactive ball milling of cotton. With mechanical pulping, the situation is more complex: thermal and mechanical effects are simultaneous and therefore difficult to distinguish. The same holds true for composite preparation by extrusion, where both heating and shear forces are present.  Mechanoradicals were shown to be able to act as starting points for cellulose copolymerization but the efficacy of these reactions was greatly affected by the presence of oxygen and water. HT-TMP fibers were shown to have potential in industrial composite reinforcement due to their hydrophobicity, ease of dispersion in the PLA matrix, and low specific energy consumption.  In conclusion, acknowledging the formation of mechanoradicals in industrial processes is crucial. These radicals may participate in unwanted side reactions, such as oxidation or polymer degradation. On the other hand, mechanical processing may yield new avenues for fiber modification.
  • Pretreatment categories, process alternatives and material characteristics in enzymatic hydrolysis of lignocellulose
    (2016) Pihlajaniemi, Ville
     School of Chemical Technology | Doctoral dissertation (article-based)
    Fractionation of lignocellulose materials to sugars is a major strategy for the production of renewable fuels and chemicals. This study compares the potential of two major pretreatment categories, hydrothermal treatment and delignification, and contributes to scientific understanding of the phenomena behind enzymatic hydrolysability of wheat straw. Delignification was found to allow higher sugar yields. Since enzyme consumption is a key cost of the fractionation process, the optimal yield target depends on enzyme price. To allow yield optimization, a novel empirical model was developed for the process sugar yield as a function of enzyme consumption and hydrolysis time. The usability of the model was demonstrated by comparing the feasibility of different process alternatives for fractionation. The changes in the material properties of lignocellulose by pretreatments were correlated to cellulose hydrolysability, and for the first time, the importance of the different properties was determined statistically. In the order of importance, the hydrolysis yield depended on cellulose surface area, pore accessibility, lignin content, lignin surface chemistry, cellulose crystallinity and hemicellulose content. During enzymatic hydrolysis, the surface area of cellulose correlated linearly with the total cellulose content, but contrary to expectations, hydrolysis did not reveal fresh lignin surfaces. Different rate constraining mechanisms were incorporated in a Michaelis-Menten type kinetic model, and it was found that permanent hydrolysis-dependent enzyme inactivation should be included with the previously well-established effects of product inhibition and reduction of hydrolysability. For improving fractionation processes, different technological solutions were studied. A flow through process was found to improve fractionation by delignification, but no additional improvement was achieved by counter-current operation. By studying and simulating the packing density and flow properties of a packed straw bed, a flow-through process was found to be possible without clogging the straw bed by compaction. The height of an industrial scale column is restricted by the applicable flow rate. With the simulation model, it was possible to determine the maximum volumetric throughput as a function of column height. Recycling of the solid residue during enzymatic hydrolysis was found to be inefficient for enzyme recycling, but efficient for product removal, with similar benefits as sequential hydrolysis. Both processes significantly improved the volumetric productivity of hydrolysis by increasing the solids concentration without reducing yield. Alternatively, this benefit could be redirected into increasing the yield by maintaining reaction volume with additional water, leading to dilution of the hydrolysis conditions.
  • Study of Norway spruce cell wall structure with microscopy tools
    (2016) Reza, Md. Mehedi
     School of Science | Doctoral dissertation (article-based)
    The distribution and orientation of wood cell wall polymers play an important role in its physical, chemical and mechanical properties, and thus in the transformation into final products. Specifically, the orientation of cellulose elementary fibrils (EF) controls the performance of wood in almost every end use. Moreover, lignin is covalently linked to many of the cell wall polysaccharides, which imposes a serious technical challenge during the degradation of cellulose into value added products. Therefore, the deep understanding of the organization of the cell wall materials is imperative. In this study, normal Norway spruce wood was studied with a high-resolution cryo-transmission electron microscope (cryo-TEM). Both, two- (2D) and three-dimensional (3D) imaging techniques within TEM were applied on ultrathin wood sections to understand the wood structure. The defibration mechanisms in high-temperature thermomechanical pulping (HT-TMP) was also studied with a conventional TEM. Furthermore, the accessibility of cell wall lignin was studied with TEM and Raman microspectroscopy by analysing fresh and solvent extracted ultrathin sections of Norway spruce branch wood. The results showed that the organization of EFs varies from layer to layer and also within a single layer. In addition to the well-adopted concept of longitudinal EF angle in tangential plane, this study showed the presence of an out-of-plane EF angle relative to the cell wall plane. The S1 layer had a transverse EF orientation with a predominant radial lamellar structure of EF bundles. Both crossed and parallel EF orientations were detected in the S1-2 transition layer, which was supported by the defibration mechanisms in HT-TMP. EFs in the outer-S2 layer had a relatively high longitudinal EF angle and a large out-of-plane angle with respect to the tangential plane, which continued to decline inward and became almost axial in the inner-S2 layer. A transverse, out-of-plane EF orientation in the S3 transverse sections was observed. The models of the wood cell wall summarize most of the findings regarding the wood ultrastructure. Study of the lignin extracted ultrathin sections showed the change of lignin concentration in all cell wall layers during the extraction process. However, lignin obtained after extraction consists mainly of secondary wall lignin as this area contains most of the total cell wall lignin in conifer tracheids. The new observations on the wood cell wall structure may lead to a better understanding of the reactivity of cellulosic fibers in biochemical, chemical and mechanical treatments.
  • Thermal stability of high-density paper made of refined softwood kraft pulp
    (2016) Vänskä, Emilia
     School of Chemical Technology | Doctoral dissertation (article-based)
    The further development of softwood-based products will help to increase the competitiveness of the Finnish forest industry and could generate new opportunities and revenue for the northern hemisphere forest industry in general. This thesis examined the thermal stability of refined softwood kraft pulp as raw material for high-density paper for food packaging applications. A thermal treatment device that allows for the accurate combination of high temperature (175–300 °C) with a variable water vapor content (1 – 98 v/v-%) in the atmosphere is introduced for thermal stability studies of kraft pulp. The performance of the device was evaluated with different paper sheets by monitoring the changes in their chemical, optical and strength properties such as intrinsic viscosity, brightness, and burst strength, respectively. The results demonstrated that the innovative thermal treatment device offered excellent performance within the targeted ranges of temperature and water vapor atmospheres. The effects of enzyme pretreatment and refining on the thermal stability of paper were also studied. The enzymatic pre-treatment impaired the chemical and physical properties of the paper, such as the degree of polymerization of cellulose and tensile strength, which in turn also resulted in a reduction of the thermal strength stability of the paper. Residual lignin in pulp was found to counteract the hydrolytic thermal degradation of paper. This was demonstrated by the systematic analysis of ultraviolet resonance Raman spectra and by the determination of the light absorption coefficient. Furthermore, the role of residual lignin in pulp on the thermal stability of refined paper was studied with unbleached and oxygen delignified pulp sheets. Results showed that the residual lignin in pulp is able to attenuate the thermal degradation of pulp polysaccharides as demonstrated by the degree of polymerization of cellulose results. In order to understand the effect of lignin in pulp refining on the thermal stability of paper, bleached pulp was pretreated with kraft lignin - separated from black liquor - prior to refining. When compared to the reference pulp and pulp treated with antioxidant (ascorbic acid), the kraft lignin impregnated pulp improved the burst strength stability of the refined pulp sheets after the hot humid thermal treatment (225 °C and water vapor atmosphere of 75 v/v-%). This work is a significant step towards the optimization of paper making processes that influence the thermal stability of papers and provides important insights into how the thermal stability of kraft pulp in papers can be improved.
  • Wood biomass characterization by Raman spectroscopy
    (2013) Lähdetie, Anni
     School of Chemical Technology | Doctoral dissertation (article-based)
    Lignin is nature's second most abundant polymer after cellulose. However, many uncertainties still remain about the structure, formation, and reactions of lignin as well as its distribution in plant cells. The analysis of the lignin structure is challenging, and the isolation of lignin from biomass introduces changes, regardless of the isolation method applied. In order to be able to utilize lignin efficiently and sustainably, potent analytical techniques are needed. Raman spectroscopy is a nondestructive technique that is applicable to characterize wood biomass in situ. The objective of this research was to utilize Raman spectroscopy in wood biomass research, especially in the analysis of the structure and reactions of lignin. To obtain a more detailed understanding of applicability of Raman spectroscopy in bleaching studies, unbleached and bleached kraft pulps were analyzed. Furthermore, chemical changes in thermomechanical pulp (TMP) during enzymatic treatment were monitored. UV resonance Raman (UVRR) spectroscopy was shown to be a valuable technique in lignin analysis in situ, especially for pulp samples with a low lignin content. Monitoring small changes in TMP samples with a high lignin content using Raman spectroscopy proved to be challenging, as the resonance enhancement of the aromatic band became very strong. To develop the quantitative Raman spectroscopic analysis method for phenolic hydroxyl group determination, the effect of pH on lignin analysis was investigated using lignin model compounds and wood pulps. UVRR spectroscopy was shown to be equally applicable to samples with a high lignin content and those with a very low lignin content. Raman spectroscopy of lignin-containing samples may produce laser induced fluorescence which overlaps with Raman bands. The fluorescence properties of the lignin model compounds were studied. Lignin model compounds containing a biphenyl structure exhibited strong laser induced fluorescence with visible excitation in Raman spectroscopy. Wood did not emit laser induced fluorescence, but when it was chemically treated under alkaline conditions, considerable amount of fluorescence was detected in the Raman spectrum. The effect of excitation wavelength on the Raman spectra of wood-based samples was investigated. Several lasers were used to obtain excitation ranging from ultraviolet to near infrared radiation to see how the different laser frequencies affected the Raman spectra of wood biomass samples. The intensities of the characteristic vibration bands of different structural features in the Raman spectra of wood-based samples were shown to depend on the excitation wavelength.