Mechanical constraints on lignin deposition during lignification

Summary The ultrastructure of lignifying cell walls in Pinus radiata D.Don was investigated using potassium permanganate staining and transmission electron microscopy. Lignin deposition occurred at numerous discrete sites within various cell wall regions, suggesting the presence of some initiating agent at these sites. In the middle lamella region, lignin deposition occurred by addition of protolignin monomers to spherical particles of lignin. Lignification was completed by expansion of these spherical particles, initially forming irregular patterns of lignification followed by infilling of adjacent areas. In contrast, lignification in the secondary wall occurred by deposition of protolignin monomers onto the ends of expanding lignin lamellae between cellulose microfibrils leading to greatly elongated patches of lignin due to the greater rate of deposition along the microfibril axis compared to that across it. It is concluded that the cellulose matrix in which lignin deposition occurs, in the secondary wall, can exert a mechanical influence which limits the rate of lignin deposition in the direction perpendicular to the microfibril axis.     

Nanostructural assembly of cellulose,hemicellulose, and lignin in the middle layer of secondary wall of ginkgo tracheid

Physical, chemical, and biological properties of wood depend largely on the properties of cellulose, noncellulosic polysaccharides, and lignin, and their assembly mode in the cell wall. Information on the assembly mode in the main part of the ginkgo tracheid wall (middle layer of secondary wall, S2) was drawn from the combined results obtained by physical and chemical analyses of the mechanically isolated S2 and by observation under scanning electron microscopy. A schematic model was tentatively proposed as a basic assembly mode of cell wall polymers in the softwood tracheid as follows: a bundle of cellulose microfibrils (CMFs) consisting of about 430 cellulose chains is surrounded by bead-like tubular hemicellulose-lignin modules (HLM), which keep the CMF bundles equidistant from each other. The length of one tubular module along the CMF bundle is about 16 ± 2 nm, and the thickness at its side is about 3–4 nm. In S2, hemicelluloses are distributed in a longitudinal direction along the CMF bundle and in tangential and radial directions perpendicular to the CMF bundle so that they are aligned in the lamellae of tangential and radial directions with regard to the cell wall. One HLM contains about 7000 C₆-C₃ units of lignin, and 4000 hexose and 2000 pentose units of hemicellulose.     

On the fine structure of bamboo fibres

Summary The thick-walled bamboo fibres exhibit a polylamellate structure with alternating broad and narrow lamellae. Characteristically the cellulose fibrils in the broad lamellae are oriented almost parallel to the longitudinal axis of the fibre (2–20°), whereby there is a gradual but only slight increase in the angle from middle lamella to lumen. The narrow lamellae consist of fibrils oriented almost perpendicular to the cell axis (85–90°); this angle remains constant in all the successive narrow lamellae. The concentration of lignin is higher in the narrow lamellae than in the broad ones. Xylan seems to occur in a higher concentration in the narrow lamellae. The pits between the fibres are bordered. The results are discussed in relation to earlier data on wall structure and development. A model for the thick-walled bamboo fibre is presented with a new terminology for the various lamellae.We are thankful to Mrs. R. Schultze and Miss B. Schröder for their technical assistance. Grateful acknowledgement is also due to Dr. D. Keyser and Mrs. K. Hoffmann for the SEM facilities and to Prof. Dr. J. Bauch and Mrs. R. Endeward for the UV-microspectrophotometry. Prof. T. E. Timell (Syracuse) and Dr. O. Faix were helpful with the preparation of hydrofluoric acid concentrations.     

Preparation and characterization of electroless nickel-coated cellulose fibres

The aim of this project was to fabricate electroless nickel-coated cellulose fibres. Ultimately, these nickel-coated cellulose fibres will be used to develop a cost-effective polymer composite for electromagnetic interference (EMI) shielding and/or electrostatic discharge (ESD) applications. An attempt has been made to impart electrical conductivity onto cellulose fibres via an electroless nickel plating process. The aim was to achieve a homogeneous, continuous layer of Ni on the cellulose fibres, and the plating conditions were optimized to achieve this. The relationship between the coating morphology and the performance of the coated fibre was also of interest. Testing was carried out using cellulose filter paper and then loose cellulose fibres. Scanning electron microscopic (SEM) images of nickel-coated filter paper showed that nickel particles are bonded to the cellulose fibres. The coating appeared more compacted and continuous as plating time increased, corresponding to lower surface resistivity. This observation suggested a correlation between morphology and electrical conductivity of the coating. For nickel-coated cellulose fibres, after optimization of plating conditions, a uniform deposition of nickel particles around the cellulose surfaces was confirmed by SEM images. EDS and XRD results confirmed similar coating could be produced on filter paper and cellulose fibres. SEM images of cross-sectioned nickel-coated cellulose fibres illustrated nickel particles had penetrated into the inner walls of the cellulose fibres. This impregnation of the fibres should act to increase the bonding between nickel coating and cellulose fibres. These nickel-impregnated cellulose fibres could now be incorporated into a polymer matrix to create an electrically conducting composite with the right processing parameters.     

Heterogeneity of lignin concentration in cell corner middle lamella of white birch and black spruce

Summary Raman microprobe spectroscopy was used to study the concentration of lignocellulosics in the cell corner middle lamella. Spectra obtained from 1.6 m regions, from 30 cell corner middle lamellae of both birch and spruce, showed the presence of lignin. However, the relative concentration of lignin to cellulose varied considerably. These results corroborate the view expressed in previous reports of the need for caution in using the lignin concentration values of cell corner middle lamella as a internal reference for studying the variation of lignin concentration in other morphological regions of the cell wall, such as secondary cell wall layers.The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service     

纳米结晶纤维素改善脲醛树脂胶黏剂游离甲醛的研究

为了降低脲醛树脂胶黏剂的游离甲醛释放量,以俄罗斯落叶松浆粕为原料,利用酸水解,硅烷偶联剂改性等制备获得表面改性纳米结晶纤维素(M-NCC),并将M-NCC作为吸附材料用于脲醛树脂基材制备胶合板,研究M-NCC对脲醛树脂胶黏剂游离甲醛的吸附效果和机理。结果表明:透射电镜(TEM)检测可知,制备的NCC颗粒长度约为100~200 nm、直径约为20~30 nm,结晶度为57.3%,相比原始纤维素提高了41.1%;硅烷偶联剂3-(2-氨乙基)-氨丙基甲基二甲氧基硅烷(MCPS)使NCC的浸润性显著增强,当MCPS用量为8%时M-NCC接枝率达到27.2%,M-NCC对脲醛树脂的接触角为65.7°,下降32.7%;由扫描电镜(SEM)照片可观测到,表面改性显著改善了低浓度NCC颗粒在脲醛树脂基材中的分散状态,当M-NCC质量分数2%时,纳米颗粒的分散性能明显下降;M-NCC可有效吸附脲醛树脂基材中的游离甲醛,当M-NCC质量分数1.5%时可致游离甲醛比对照组(0.51 mg/L)下降50.9%。     

Characterization of cellulose fibril aggregates isolated by the combination of ultrasonication and homogenizer

Cellulose fibril aggregates (CFAs) in micro/nano scales are made by the process of delaminating cellulosic fibers in high-pressure homogenizers. However, blockage of the slot during processing by cellulose fibers or particles is possible and so high-intensity ultrasonication was probed as a pretreatment method. This study was unique because of the combination of ultrasonication and homogenizer to produce high quality cellulose fibrils while preventing blockage that typically occurs using conventional homogenizer methods. The CFAs were characterized by the water-retention value (WRV), dynamic light scattering, scanning electron microscopy, atomic force microscopy, and two-dimensional (2-D) correlation spectroscopy. It was found that the ultrasonic treatment was helpful for CFA isolation before homogenizer processing. Less than 10 minutes of ultrasonic processing eliminated most of the blockage at the beginning of the homogenizer process. The results indicate that increasing the ultrasonic treatment time or homogenizer pressure significantly increased the WRVs of the treated cellulose while the CFAs were a mixture of different sized particles in the range of micro to nano scales. 2-D correlation spectroscopy between the near- and mid-infrared region helped to further explain the mechanism of size reduction which was attributable to the breakage of β-1,4-glucosidic bonds along the molecule axis due to shear forces.     

Accessibility, reactivity and supramolecular structure of E. globulus pulps with reduced xylan content

The correlation of structural assembly on a molecular level with macroscale properties such as accessibility and reactivity was investigated. A series of TCF-bleached E. globulus kraft dissolving pulps was prepared aiming at a specification suitable for viscose application. The removal of xylan to a comparable level was achieved by different pre- and post-treatments. Solid-state CP-MAS ¹³C NMR was used to determine the degree of order and the lateral fibril dimensions of cellulose fibrils. The results of the NMR measurements were related to the processability of these pulps during viscose manufacture, expressed in terms of filterability of the viscose dope and its amount of undissolved particles. The cellulose crystallinity did not affect the pulp reactivity. It was noticed that the cold caustic extracted (CCE) pulps revealed both large fibril aggregate width as determined from NMR data and low reactivity toward xanthation at the same time. These pulps exhibited significantly higher amounts of alkali-resistant xylan than those prepared by prehydrolysis kraft cooking.     

The Delignification Pattern of Ailanthus excelsa Wood by Inonotus hispidus (Bull.: Fr.) P. Karst.

In vitro laboratory decay tests on Ailanthus excelsa Roxb. wood revealed that I. hispidus exhibits a combination of both white-rot and soft-rot patterns of wood decay. Early stages of wood decay showed dissolution of the middle lamella as well as defibration and localized delignification of fiber walls; vessels, axial, and ray parenchyma remained unaltered. Delignification commenced from the middle lamellae at the cell corners without any marked effect on the primary and secondary wall layers. In later stages of growth, the species produces typical soft-rot decay pattern by forming erosion channels through the S₂ layers of fiber walls, transverse bore holes in the cell walls, and erosion channels alongside/following the orientation of cellulose microfibrils. The rays showed signs of cell wall alterations only after the extensive damage to the fiber walls. After 120 days of incubation, the vessels also showed localized delignification, the erosion of pits, and separation from associated xylem elements. The extensive weight losses under natural and in vitro decayed wood as well as the very soft nature of severely degraded wood indicate that I. hispidus alters wood strength and stiffness.     

Studies on opposite wood in conifers Part I: Chemical composition

Summary Opposite wood, normal side wood, and compression wood were isolated from leaning stems of Abies balsamea, Larix laricina, Picea mariana, Pinus resinosa, and Tsuga canadensis and were subjected to analyses for lignin and relative carbohydrate composition. There were no statistically significant differences between the data obtained for opposite wood and side wood. Contrary to some earlier reports, opposite wood has exactly the same content of lignin, cellulose, and hemicelluloses as has corresponding normal wood.This paper is dedicated to Dean Edwin C. Jahn in honor of his 70th birthday.     

Nanocomposites from natural cellulose fibers incorporated with sucrose

The present work shows for the first time worldwide that sucrose can be easily placed by simple techniques within the micropores or nanostructure of the mercerized non-dried cotton linter fibers to create a low-cost cellulose substitute. Such sucrose-containing nanocomposites find suitable use as specialty absorbent paper. Relative to the sucrose-free paper, the sucrose-containing counterparts exhibit greater breaking length and remarkably high water uptake (WRV) up to a sucrose content of 8–15% w/w. Mercerization of cotton linters before incorporating them with sucrose greatly enhanced the retention of sucrose in the prepared paper nanocomposites as compared to the case of unmercerized cotton linters. We assume that regions of the cell wall lamellae, on both sides of the sucrose spacers, are stressed during drying because the sucrose spacers hinder them to relax. This leads to a strain, which makes some microfibrils partially released and protrude out of the fiber. Thus, a sort of fiber beating takes place. We called this phenomenon incorporation beating or encapsulation beating to differentiate it from chemical and mechanical beatings, and it explains the great increase in breaking length of the paper nanocomposites prepared from the mercerized non-dried sucrose-loaded linters. This article is dedicated to Professor Dr. Dr. h.c. mult. Walter Liese on the occasion of his 80th birthday.     

A study of the bonding forces between the epithelial cells surrounding the resin canals of slash pine (Pinus elliottii Engelm.) holocellulose

Summary Although the bonds between the cellular components of pine epithelia (isolated from holocellulose) can be strengthened by the addition of multivalent ions such as those of calcium, iron, uranium, etc., it was observed that these bonds are only weakened and not destroyed by the removal of the ions from the tissues. Reagents, such as KOH and K₂SO₄, which do not yield soluble calcium salts, do not cause a discernible weakening of the bonds between the cellulose components of epithelia pretreated with Ca ion. Washing the weakened epithelia with water causes them to swell. The separation of epithelial cells occurs only after the application of mechanical forces to the tissues. Electron microscopic examination of the swollen and unswollen tissues shows that fibrils and lamellae extend between the individual cells which must be destroyed by mechanical action before cell separation can occur. The swelling of the tissues by washing after removal of multivalent ions was attributed to the expansion of electrical double layers and to the increasing osmotic pressure from the increasing Donnan potential, while the difference in swelling as a result of HCl treatment vs. KCl and K₄Fe(CN)₆ was attributed to differences in charge development and to hydrogen bonding between acidic components. The restrengthening of the tissue as a result of the readdition of multivalent ion was attributed to the collapse of the electrical double layers and the reduction of the Donnan potential, with the re-forming of bonds between the components of the epithelia. Evidence suggesting that multivalent ions actually participate in bonding was obtained and although the nature of the bonding cannot be determined, both salt formation and complex formation between the ions and the components of the tissues is suggested.     

Tracheid differentiation in Pinus radiata

Summary Differentiating tracheids in Pinus radiata D. Don have been examined with the electron microscope. Despite the fact that one of the major differentiation processes is cellulose formation, little ultrastructural evidence has been found to indicate how this occurs. On the other hand, there is ample evidence of the incorporation of non-cellulosic material into both the expanding primary wall and the developing secondary wall. The only structure which could possibly be related to cellulose formation is a system of osmiophilic particles which have been found in the space between the plasmalemma and the developing wall, or attached to the most recently formed wall layer. The absence of microtubules during the main phase of secondary wall formation supports the view that these structures are not involved directly in the biosynthesis of cellulose.The author wishes to thank Mrs B. A. Hodgkiss for technical assistance.     

Theoretical water sorption energies by conformational analysis

Summary Values of the sorption energies of single molecules of water on all available sorption sites of amorphous cellulose I have been obtained by conformational analysis. The sorption energies are equated to the total energy (Etot) of interaction between the water molecule and all the atomic groups of the cellulose. Van der Waal, H-bond and electrostatic energies comprised the E_tot. The interference of water molecules on two vicinal sorption sites were also obtained and sites in which such interference can occur were identified for amorphous cellulose. Curves relating E_tot to percentage equilibrium moisture content are reported as well as exact sorption isotherms constructed as E_tot as a function of humidity for the amorphous and crystalline cellulose I. These isotherms were constructed for different relative proportions of amorphous to crystalline cellulose and can be constructed for any of their relative proportions from the isotherms for amorphous-only and crystalline-only cellulose. The sorption energies of the monolayer were all calculated. Curves of energy of sorption of bound water were also obtained by introducing the calculated energy values in equations from already established sorption theories.Sorption capacities of amorphous and crystalline Cellulose I were calculated and the respective isotherms constructed and discussed from the point of view of existing sorption theories.     

纳米增强木塑复合材料研究现状

木塑复合材料(WPC)是利用植物纤维和塑料加工制备而成的一种环境友好型的新型绿色材料,具有力学性能好、吸水性低及可循环利用等优点,被广泛应用于家具、户外设施、建筑装饰和园林景观等领域.文中从纳米材料对WPC的物理力学、阻燃和耐老化等方面性能的影响出发,总结了近年来利用刚性粒子、碳纳米管、纳米蒙脱土等纳米无机化合物,以及...     

NaClO添加量对TEMPO氧化纳米微纤丝结构与性能的影响

以毛竹粉为原料,采用TEMPO催化氧化联合超声处理制备纳米微纤丝,通过改变Na Cl O与纤维素的质量比,研究了Na Cl O添加量对纳米纤维素的长径比、纤维形态以及羧基含量等特性的影响。结果表明:随着Na Cl O添加量的增加,纳米纤维素羧基含量逐渐提高,长径比增大;当Na Cl O添加量为15 mmol/g时,纳米纤维素羧基含量可高达1.646 mmol/g,横截面直径可达6～10 nm,长径比为273～455;随着纳米纤维素羧基含量的增加,纳米纤维素悬浮液的透光率和剪切黏度不断增大。虽然TEMPO催化氧化程度不断加深,但纳米纤维素的晶型并未遭到破坏,仍然呈现出典型的纤维素I晶体结构,而随着羧基含量的增加,纳米纤维素的结晶度和热稳定性有一定程度的下降。     

Influence of carboxyl group on the acid hydrolysis of cellulose

Cellulose isolated from wood is more susceptible than cotton cellulose to homogeneous hydrolysis in phosphoric acid. The influence of carboxyl group introduction at the C6 position on the hydrolysis rate of cellulose in 82.5% phosphoric acid was studied as a model of the oxidation of cellulose during pulping. The rate constant of hydrolysis for dissolving pulp was larger than that of cotton cellulose at temperatures of 25°–35°C. Mercerized cotton cellulose was partially oxidized regioselectively at the C6 position by a free radical system using 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO). The oxidized cellulose was obtained at a range of 1.7–12.7 mEq carboxyl content per 100 g of cellulose. The hydrolysis rate of the oxidized cellulose samples accelerated with increasing carboxyl content in the samples.Part of this work was presented at the 5th Annual Meeting of the Cellulose Society of Japan, Kyoto, July 1998     

Micromorphology,moisture sorption and mechanical properties of a biocomposite based on acetylated wood particles and cellulose ester

Abstract

One of the major issues in a long-term perspective for the use of wood–plastic composites (WPCs) in outdoor applications is the moisture sensitivity of the wood component and the consequent dimensional instability and susceptibility to biological degradation of the composite. In this work, the effects of using an acetylated wood component and a cellulose ester as matrix on the micromorphology, mechanical performance and moisture uptake of injection-moulded WPCs have been studied. Composites based on unmodified and acetylated wood particles, specially designed with a length-to-width ratio of about 5–7, combined with both cellulose acetate propionate (CAP) and polypropylene (PP) matrices were studied. The size and shape of the wood particles were studied before and after the processing using light microscopy, and the micromorphology of the composites was studied using a newly developed surface preparation technique based on ultraviolet laser irradiation combined with low-vacuum scanning electron microscopy (LV-SEM). The water vapour sorption in the composites and the effect of accelerated weathering were measured using thin samples which were allowed to reach equilibrium moisture content (EMC). The length-to-diameter ratio was only slightly decreased for the acetylated particles after compounding and injection moulding, although both the unmodified and the acetylated particles were smaller in size after the processing steps. The tensile strength was about 40% higher for the composite based on acetylated wood than for the composite with unmodified wood using either CAP or PP as matrix, whereas the notched impact strength of the composite based on acetylated wood was about 20% lower than those of the corresponding unmodified composites. The sorption experiments showed that the EMC was 50% lower in the composites with an acetylated wood component than in the composites with an unmodified wood component. The choice of matrix material strongly affected the moisture absorptivity of the WPC. The composites with CAP as matrix gained moisture more rapidly than the composites with PP as matrix. It was also found that accelerated ageing in a Weather-Ometer® significantly increased the moisture sensitivity of the PP-based composites.     

稀酸预处理改善玉米秸秆酶水解性能的机制探讨

为了探讨在稀酸预处理提高玉米秸秆在纤维素酶酶解阶段提高纤维素转化率的机制,利用一系列的检测方法:FT-IR、XRD、SEM和比表面积分析仪分析了预处理前后玉米秸秆在形态学和物理化学性质方面的变化.在经过稀酸预处理后的玉米秸秆在纤维素酶酶解阶段其纤维素转化率有较大的提高,经过170℃,60 min,固液比1∶15(g∶mL),1.00 g/mL酸质量浓度的条件预处理后,从31.88％提高到95.74％.XRD结果显示预处理后玉米秸秆的结晶度有所增加,从原料的37.8％增加到58.7％,但是当预处理强度增加到一定程度后,结晶度没有较大的变化,基本维持在58％.玉米秸秆的表面结构在稀酸预处理后,原来的光滑表面变得粗糙、多孔,这样的表面有利于纤维素酶与玉米秸秆的接触,预处理后玉米秸秆的比表面积有很大程度的增加,经过170℃,60 min,固液比1∶15,1.00g/mL酸质量浓度的条件预处理后,玉米秸秆的比表面积从0.329 m2/g增加到2.878 m2/g,这都有利于改善纤维素酶对纤维素的作用,增加纤维素转化率.