Wood and other lignocellulosics have been used as “engineering materials” because they are economical, low in processing energy, renewable, and strong. In some schools of thought, however, lignocellulosics are not considered “materials” in that they do not have consistent, predictable, reproducible, continuous, and uniform properties. This is true for solid lignocellulosics such as wood, but not necessarily true for composites made from lignocellulosics. The chemistry of the components of lignocellulosics can be modified to produce a material with consistent, predictable, and uniform properties. Properties such as dimensional instability, biodegradability, flammability, and degradation due to ultraviolet light, acids, and bases can be altered to produce property-enhanced composites.

Lignin is the stuff that makes trees “woody.” Usually constituting from one-fifth to one-third of wood, lignin strongly influences its chemical and physical properties. A major use of wood is for the production of pulp for paper and paperboard products. The residual lignin in pulp fibers greatly affects paper properties and, therefore, the uses of these pulps.

Lignin is a rather unusual substance. Polymerized through radical coupling of propenylphenols, it forms several types of interunit bonds and a three-dimensional network may result. The lignin of gymnosperms (softwoods) is made up mostly of a single monomer type. As a result, the lignins of gymnosperms do not differ much from species to species. However, the lignins of angiosperms (hardwoods) do vary considerably among species because these lignins are derived from two monomer types which are often present in differing proportions. Furthermore, the ratio of monomer types may vary among different cell types and between cell regions within a species.

This review, intended mainly for those unfamiliar with the details of lignin chemistry, will provide an overview of the formation, structure and distribution of lignins in wood and of the distribution of lignin in pulp fibers.

Kraft lignins are the lignin degradation products from kraft pulping. They are complex, heterogeneous polymers with some polar character. The molecular weight of kraft lignins greatly affect the physical properties of black liquors, and are of primary importance in separation from black liquor and in evaluating potential uses.

Several purified kraft lignins from slash pine were analyzed for number average molecular weight by vapor pressure osmometry (VPO), for weight average molecular weight by low angle laser light scattering (LALLS), and for the molecular weight distribution by high temperature size exclusion chromatography (SEC). The lignins were run in Tetrahydrofuran (THF), N, N-Dimethyl Formamide (DMF), DMF with 0.1M LiBr, and pyridine at conditions above the Theta temperature. Experimental methods are discussed.

The results show that VPO may be used to determine Mn for kraft lignins if the purity of the lignins and the identity of the impurities are known. LALLS can be used to determine Mw for kraft lignins if measurements are made at or above the Theta temperature of the lignin-solvent pair. SEC should be used at temperatures at, or above, the Theta temperature of the lignin-solvent pair. Size separation is highly dependent on the solvent used, and DMF is a much better solvent than THF for high temperature SEC. Future work using moment resolution procedures to derive an accurate calibration curve are also discussed.

Lignins, a truly abundant group of biopolymers exhibiting some significant diversity, are usually thought to be constituted by a random proportionate distribution of ten different linkages between p-hydroxphenylpropane units. Over 20 million tons of kraft lignin derivatives are produced annually in the United States by the pulping industry, but 99.9% of these aromatic polymeric materials are consumed as fuel. Such industrial byproducts are generally viewed as being almost hopelessly complicated mixtures of partially degraded and condensed chemical species. However, a very different picture has begun to emerge from a more coherent understanding of the physicochemical behavior exhibited by kraft lignin preparations. Noncovalent interactions between the individual molecular components under a variety of solution conditions orchestrate pronounced associative processes that are characterized by a remarkable degree of specificity. Their consequences may be readily observed both size-exclusion chromatographically and electron microscopically, and are reflected in an anomalous variation of glass transition temperature, Tg, with molecular weight of paucidisperse kraft lignin fractions. How these effects may influence the mechanical properties of lignin-based polymeric materials is presently being scrutinized at the University of Minnesota.

Significant aspects of fiber reinforcement of wood are introduced. The effectiveness of three different silane coupling agents; one monomeric 3-aminopropyltrimethoxy silane, two polymeric aminosilanes (one dimethoxy- and another trimethoxy substituted), was compared by measurement of interfacial shear strength in silane treated single filament composite (SFC) specimens, under dry and wet conditions. The trimethoxypolymeric silane was found to be inferior to the other two, which warrant further investigation in strength tests with basalt fiber reinforced wood specimens. Acoustic emission measurement during the SFC tests has provided evidence for the sizing effect of polymeric silane coatings, as well as for moisture attack at severe surface flaws on the fiber during silane treatment.

Incompatible polymer blends are known to have inadequate mechanical properties due to poor adhesion and high interfacial tension between the phases. One such blend that displays this type of behavior is the cellulose acetate-polystyrene blend and is typical of lignocellulosic-synthetic polymer blend systems. Tailor-made cellulose acetate-polystyrene graft copolymers have been synthesized and used as compatibilizers (emulsifiers) to reduce the interfacial tension and improve interfacial mixing. Thermal analyses and morphological studies of the blends with and without the graft copolymer demonstrated that this was indeed the case.

Full factorial studies were conducted to determine the effects of a coupling agent (a low molecular weight maleated polypropylene (MAPP)) and other composition and processing variables on the mechanical properties of a wood-flour-filled polypropylene (PP) composite. Effects of MAPP on the bonding between PP and wood veneer were also examined. At less than 1 percent by weight, MAPP produced useful increases in strength and modulus properties of the composite, and this effect was somewhat enhanced by small-particle-size wood flour and multiple extrusions. However, MAPP caused small losses in notched impact energy. High extrusion temperature (190°C to 250°C) had little influence on strength, but it decreased notched impact energy. Peel force between PP and wood veneer was increased by pretreatment with MAPP for aspen, but not for birch, aspen being more porous than birch. The effectiveness of MAPP may therefore be related to its ability to penetrate the wood and form a strongly held hydrophobic layer that is attractive to the PP, thereby increasing both the effective bonding area and mechanical interlocking.

UV-visible light absorption is an analytical method that is commonly used to determine the concentration of lignin in solution; however, there are uncertainties with this method, particularly for use in determining lignin concentration in black liquors. To try to resolve these uncertainties, a detailed study of UV-visible absorption by lignin in solution has been made.

A dual beam UV-visible instrument capable of repetitive scan was used. The effects of lignin concentration, lignin molecular weight, molecular weight dispersion, and solvent were examined as well as the effect of the reference solvent. Measurements were made for lignin dissolved in dilute caustic, DMF, THF, and DMSO. Measurements for soft wood black liquors were also made. A refined method for determining lignin concentration in soft wood black liquors resulted that is now being used in our research to evaluate lignin concentration in soft wood black liquor.

The results of the current work have led to improvements in this analytical method so that it appears to be less affected by lignin characteristics and by solution complexities.

This report presents an overview of studies on the structures of cellulose. After presenting a brief historical perspective, the report reviews diffractometrically based structural models and then describes recent developments based on models that are consistent with both diffractometric and spectroscopic observations. The primary impetus for development of these models was provided by Raman and 13C NMR (CP-MAS) spectral results that could not be rationalized on the basis of classical structural models, which are constrained by diffractometric data alone. The structures derived from integrating the spectral information into the data base, which constrain the models, represent relatively small but very significant departures from those structures derived on the basis of diffractometry alone. In addition to rationalizing all the structurally sensitive information, the new models provide a basis for complementary use of spectroscopic and diffractometric methods to monitor variations of the states of aggregation of celluloses with source and history. Thus, it is now possible to investigate the effects of different processing variables, which are important in industrial practice, on both secondary and tertiary levels of structure in celluloses. These levels of structure have a major influence on the material properties of commercial celluloses, yet adequate characterization of these levels has been quite elusive.

This paper presents a review of micromechanics models that have been used to predict the elastic and strength behavior of paper materials. The models discussed are based on a fiber network model that takes into account the properties of the fibers and the properties of the fiber-to-fiber bond.

Elastic models are described for a range of paper density and basis weight ranging from tissue papers to paperboard materials. The effects of fiber curl for low density systems is discussed. Models for predicting both the initial modulus of the paper and the nonlinear elastic behavior for small finite strains is presented. Differences between tensile and compressive straining regimes are addressed.

Limit load theory is discussed for predicting the strength of the network. Damage mechanics theory is employed to take into account hypothesized processes of fiber fracture and bond delaminations, and to relate the inelastic network micromechanics to the prediction of the strength of the material.

Wood strip surfaces (Aspen and Birch) and cellulose fibers (Birch, pulped paper fibers) were treated with azidosilane coupling agent, via immersion in methylene chloride and methanol solutions respectively, and then subsequently dried. Polypropylene films were compression molded to the wood surfaces and peel forces were measured. The treated cellulose fibers were suspended and mixed with polypropylene fibers in water, formed into thin sheets by wet forming, dried, and compression molded to melt the polypropylene.

Treated wood surfaces gave two to six times the dry peel force versus untreated wood surfaces. However, the wet peel force of treated wood degraded to that of untreated wood. The elongation, fail stress, and tensile energy absorption of treated molded sheets were all increased over the untreated fibers for both dry and wet testing; the wet-tested properties were especially enhanced by the silane treatment.

Independent effects of fibre length and strength on the properties of pulp, and on the physical properties of the dry sheets have been demonstrated. Apart from improving formation, which indirectly influences many sheet properties, reducing fibre length has little direct effect on the sheet structural and optical properties, but does reduce mechanical properties. The loss in fibre strength also has little direct effect on the sheet structural and optical properties, but severely reduces those mechanical properties which are controlled by the breaking of fibres. Implications for market pulps are discussed.

The effects of fibre coarseness on the papermaking properties of softwood pulps have been demonstrated. While reducing fibre length or strength reduces mainly sheet strength, changes in fibre coarseness influence virtually all pulp properties — drainage, wet-web strength, and the structural, strength, and optical properties of the dry sheets. The effects are large, and are explained on the basis that coarser fibres have thicker walls, are fewer per unit pulp mass, and have smaller specific surface area. Implications for the quality of market chemical pulps are discussed.

This paper examines the differences between sonically- and mechanically-determined paper elastic moduli. Four factors are examined for their contributions to observed differences. Two factors, thermodynamics and the low frequency approximation, relate to the sonic method; the other two factors, load cell stiffness and paper viscoelasticity, relate to the mechanical method. When all four factors are given proper consideration, the differences between the two methods are reduced to the order of experimental error. Quantitative relationships between the two moduli are given.

The bond breaking load and bonded area of individual fiber/fiber bonds were measured using a new fiber load elongation recorder (FLER2) and Page's polarized light scattering technique, respectively. Lightly (570 mL CSF) and moderately (345 mL CSF) refined fibers with fines subsequently removed produced bonds of equal strength. Bonds between latewood fibers were substantially stronger than those between earlywood fibers. Polymeric strength aids interacting with the fibers by either covalent or ionic bonding increased the bond strength by a factor of two.

The properties of water near surfaces are known to differ from those of the bulk. For instance, in 140 A diameter silica pores the density has been found to be 3% lower than that of the bulk while the heat capacity is 25% greater than that the bulk. Etzler [15] has proposed a statistical thermodynamic model for vicinal water. This model has been able to correlate a number of thermodynamic properties of water in silica pores. Furthermore, some of the microscopic implications of the model have been found to be consistent with molecular dynamics simulations. Here we discuss the results of a statistical geometric analysis of a molecular dynamics simulation which, as first suggested by Roentgen (1892), indicates that water indeed exists in “bulky” and “dense” states. Furthermore, recent results concerning the temperature dependence of the heat capacity of water in silica pores is discussed.

The effect of water on wood fibre materials is described from the standpoint of how the water is absorbed by the individual wood polymers. The main influence on physical properties are thus associated with water acting as a plasticizing agent for lignin and carbohydrates. Effects of drying restraints on paper are discussed in relation to a molecular orientation of the constituent polymers, made possible in the vicinity of the water-induced softening points.

Experiments on laboratory paper sheets have indicated that under certain conditions the elastic modulus is inversely proportional to both the drying shrinkage and dimensional stability. Thus it appears that the elastic response of paper to mechanically and hygroscopically induced stresses is determined by the drying strain or shrinkage of the sheet. It is not clear, however, what effect the geometry of paper network and the properties of papermaking fibers have on the relationship between paper elasticity and drying restraints.

We have developed a model for the study of the elastic properties of paper as a function of the drying shrinkage. The model is based on the assumption that there exists a unique relationship between the elastic properties and dried-in compression of fiber. As a consequence, the elastic properties of a fiber depend on its orientation relative to the principal axes of the paper sheet.

The qualitative features obtained from the model calculations are consistent with experimental observations and independent of details of the elastic properties of fiber. It is clearly shown that no unique relationship exists between the drying strain and elastic modulus of the paper sheet. However, to a reasonable accuracy certain simple relatioships do exist that appear to be of general validity.

Experimental results now available are not adequate to define the mechanism of cyclic creep response of paperboard or to test plausible hypotheses. The present study examines the effect of rate-of-change of humidity on tensile creep response while monitoring the quantities of water adsorbed and desorbed by the specimen. Relative humidity is cycled between 30 and 90 percent relative humidity. Rate of humidity change in each cycle is held constant at values from 0.01 to 6 percent relative humidity per minute. Moisture sorbed by the web in each cycle is resolved to an accuracy better than ±0.05 percent moisture content. Results suggest that cyclic creep response is independent of the rate at which humidity changes. The results appear to reject the hypothesis that cyclic creep behavior is the result of a progressive increase in equilibrium moisture content.

The investigation is concerned with strategies to reduce losses in medium compressive strength during corrugating.

Medium handsheets with a non-random fiber orientation were made using a NSSC pulp and different levels of synthetic fiber addition, including glass and Kevlar pulp, at two levels of wet pressing. Corrugating was performed using a concora fluter, and compressive strength measurements were made in the tip and flank regions of the corrugated medium.

Forming losses are reduced by increased wet pressing with a greater improvement in the flank than the tip region of the flute. A small reduction in tip forming losses is found, for a given furnish when a film forming latex is used. However, a greater reduction is evident when a non-film forming latex is employed.

Forming loss measurements were also made on non-bonded and bonded laminates. The non-bonded laminates resulted in much lower forming losses, while the heat-activated laminates resulted in large forming losses similar to those found with conventional medium.

These experiments suggest that forming losses can be reduced if cost-effective systems are designed which do not activate until the fluting process is complete.