Leslie H. Groom

Review: Current international research into cellulosic fibres and composites

The following paper summarises a number of international research projects being undertaken to understand the mechanical properties of natural cellulose fibres and composite materials. In particular the use of novel techniques, such as Raman spectroscopy, synchrotron x-ray and half-fringe photoelastic methods of measuring the physical and micromechanical properties of cellulose fibres is reported. Current single fibre testing procedures are also reviewed with emphasis on the end-use in papermaking. The techniques involved in chemically modifying fibres to improve interfacial adhesion in composites are also reviewed, and the use of novel fibre sources such as bacterial and animal cellulose. It is found that there is overlap in current international research into this area, and that there are complementary approaches and therefore further combining of these may make further progress possible. In particular a need to measure locally the adhesion properties and deformation processes of fibres in composites, with different chemical treatments, ought to be a focus of future research.

Multivariate Modelling of Density, Strength and Stiffness from near Infrared Spectra for Mature, Juvenile and Pith Wood of Longleaf Pine (Pinus Palustris)

In manufacturing, monitoring the mechanical properties of wood with near infrared spectroscopy (NIR) is an attractive alternative to more conventional methods. However, no attention has been given to see if models differ between juvenile and mature wood. Additionally, it would be convenient if multiple linear regression (MLR) could perform well in the place of more complicated multivariate models. Therefore, the purpose of this paper was to model the strength, stiffness and density of mature and juvenile longleaf pine to NIR spectra with MLR and principal component regression (PCR). MLR performed almost as well as PCR when predicting density, modulus of rupture (MOR) and modulus of elasticity (MOE). Choosing wavelengths associated with wood chemistry and developing principal components gave better predictive models (PCR2) than when all NIR wavelengths were used (PCR1). Models developed from mature wood did not predict wood properties from juvenile wood adequately, suggesting that separate models are needed. However, for density prediction, the area under the spectral curve appeared to be insensitive to mature and juvenile wood differences. Five of the six wavelengths associated with MOE were also associated with MOR, perhaps accounting for how MOE and MOR might be related. For pith wood, MOE and MOR were poorly related to NIR spectra, while density was strongly correlated. This inability to predict mechanical properties in the pith-wood zone warrants attention for those manufacturers interested in using near infrared to stress rate lumber within a mill.

Use of near infrared spectroscopy to predict the mechanical properties of six softwoods

Abstract The visible and near infrared (NIR) (500–2400 nm) spectra and mechanical properties of almost 1000 small clearwood samples from six softwood species: Pinus taeda L. (loblolly pine), Pinus palustris, Mill. (longleaf pine), Pinus elliottii Engelm. (slash pine), Pinus echinata Mill. (shortleaf pine), Pinus ponderosa Dougl. ex Laws (ponderosa pine), and Pseudotsuga menziesii (Mirb.) Franco (Douglas fir) were measured. Projection to Latent Structures (PLS) modeling showed that the NIR spectra of these softwoods could be used to predict the mechanical properties of the clear-wood samples. The correlation coefficients for most of these models were greater than 0.80. All six softwood species were combined into one data set and a PLS model was constructed that effectively predicted the strength properties of any of the individual softwoods. Reducing the spectral range to between 650 and 1050 nm only causes a slight decrease in the quality of the models. Using this narrow spectral range enables the use of smaller, faster, lighter, less expensive spectrometers that could be used either in the field or for process control applications.

WITHIN TREE VARIATION OF LIGNIN, EXTRACTIVES, AND MICROFIBRIL ANGLE COUPLED WITH THE THEORETICAL AND NEAR INFRARED MODELING OF MICROFIBRIL ANGLE

A theoretical model was built predicting the relationship between microfibril angle and lignin content at the Angstrom (A) level. Both theoretical and statistical examination of experimental data supports a square root transformation of lignin to predict microfibril angle. The experimental material used came from 10 longleaf pine (Pinus palustris) trees. Klason lignin (n=70), microfibril angle (n=70), and extractives (n=100) were measured and reported at different ring numbers and heights. All three traits were strongly influenced by ring age from pith while microfibril angle and extractives exhibited more of a height effect than lignin. As such, the multivariate response of the three traits were different in the axial direction than the radial direction supporting that care needs to be taken when defining juvenile wood within the tree. The root mean square error of calibration (RMSEC) for microfibril angle of the theoretical model (RMSEC = 9.8) was almost as low as the least squares regression model (RMSEC = 9.35). Microfibril angle calibrations were also built from NIR absorbance and showed a strong likeness to theoretical and experimental models (RMSEC = 9.0). As a result, theoretical and experimental work provided evidence that lignin content played a significant role in how NIR absorbance relates to microfibril angle. Additionally, the large variation in extractives content coupled with sampling procedure proved important when developing NIR based calibration equations for lignin and microfibril angle.

Atomic force microscopy of the intervessel pit membrane in the stem of Sapium sebiferum (Euphorbiacea)

Sapwood and juvenile wood of Sapium sebiferum (Euphorbiaceae) was collected during 2000 –2002. In air-dried vessel elements, the surface of pit membranes (PMs) in the outermost growth ring was coated with plaque-like or interstitial material that was 2–5 nm thick. This coating was phase dark and overlaid a phase bright layer of globules and reticulately arranged microfibrils (MFs) that was 25–50 nm thick. Beneath the reticulate layer there was another surface exposed during sectioning/fracturing. It had parallel MFs which appeared to be continuous with the middle lamella, and were also coated. The total thickness of the dried PM appeared to be in the range of 50–100 nm. Overwintering and heartwood PMs were encrusted with a non-microfibrillar layer that differed from the above mentioned coating. Prior to chemical treatment, specific dried, untreated PMs were located and then the sample was dismounted, treated with acidic H2O2, and observed after treatment so that before and after images could be compared. Treatment with acidic H2O2 removed some of the coating and greatly modified the fibrillar nature of the surface layer, but did not reduce its overall thickness. The native structure of sapwood PMs was observed in water. Non-dried PMs displayed two layers, each with a different type of surface. The outer layer was non-microfibrillar and covered the entire surface of the PM. The non-microfibrillar layer was extremely sensitive to mechanical perturbation by the AFM tip, and had phase characteristics similar to the coating of dried PMs. The underlying layer was thick and microfibrillar. The MFs in non-dried PMs were, like the dried MFs, phase bright but they were much more loosely intermeshed compared with those seen in dried materials. The measurable thickness (which does not represent the total thickness) of non-dried PMs frequently ranged from 90–225 nm, although a few 500 nm vertical features were measured.

Genetic improvement of fiber length and coarseness based on paper product performance and material variability - a review

Improvement of specific gravity through tree breeding was an early choice made in the mid 20th century due to its ease of measurement and impact on pulp yield and lumber strength and stiffness. This was often the first, and in many cases, the only wood quality trait selected for. However, from a product standpoint, increased specific gravity has shown to lower many paper strength and stiffness properties and has been assumed to be directly attributable to increased fiber coarseness. As a result, it is currently not clear which fiber trait would best benefit a tree improvement program for paper products. This review found coarseness to be perhaps more important to paper strength and stiffness whereas tracheid length showed better promise from a breeding point of view due to its independence from specific gravity. However, both traits possessed strong heritability and influence on product performance and thus both would be beneficial to breed for depending on organizational goals and end product mix. The objective of this paper is to review and prioritize coarseness and tracheid length from both an end use and raw material perspective. To aid in prioritization, the variation, correlation, and heritability of both traits were reviewed along with significant genetic and phenotypic correlations. Variation trends within and between families as well as within a tree were reviewed.

Characterizing the surface roughness of thermomechanical pulp fibers with Atomic Force Microscopy

Summary Loblolly pine, separated into mature and juvenile portions, was refined at various pressures (4, 8 and 12 bar). Fiber surfaces were investigated using a Scanning Electron Microscope (SEM) and an Atomic Force Microscope (AFM). Refiner pressure had a significant effect on the fiber surfaces. SEM images showed an apparent increase in surface roughness with increased refiner pressure. This was shown quantitatively with data from the AFM that was analyzed using 5, 2.5 and 1.25 μm scan sizes. A scan size of 2.5 μm was found to be the most informative in terms of quantifying the effect of the different treatments on the two fiber types. The calculated surface roughness was greatest at 8 bar for both wood types. Juvenile fibers in general had higher surface roughness values than mature fibers. The results suggest that refining pressure may influence the failure mechanism of juvenile and mature wood differently.

Prediction of wood mechanical and chemical properties in the presence and absence of blue stain using two near infrared instruments

The objective of this research was to (a) determine if blue stain in solid wood influenced calibration equations developed from a non-stained wood population, (b) assess the bias introduced when scanning was performed by the slave instrument without calibration transfer from the master instrument and (c) partition absorbance-based variation by instrument, stain and instrument × stain interaction. The results helped to determine the calibration transfer needed for this case. The dependent variables assessed from clear and stained wood were lignin, extractives, modulus of elasticity (MOE), modulus of rupture (MOR) and density When the master instrument was used for both calibration and prediction, it was found that stain-insensitive equations for the five traits could be built. However, when a slave near infrared instrument was introduced without calibration transfer, three out of five predicted traits were significantly biased by the presence of stain. Further analysis revealed an interaction between stain and instrument indicating that instrument bias was also introduced during scanning with a slave. For both multiple linear regression (MLR) and principal components regression (PCR), it was found that if a trait needed more wavelengths (or principal components) for prediction of the dependent variable, bias due to blue stain became increasingly prominent. PCR was found to perform better than MLR when stain was introduced with no calibration transfer. Such a finding alludes that PCR works better than MLR under extrapolation conditions but is not intended to support a lack of calibration transfer. Finally, the Mallows Cp diagnostic proved valuable in model selection although the well-known requirement of (Cp – p ≤ 0) appeared conservative. For MLR and PCR, a Cp – p ≤ 5 often yielded applicable models while Cp – p > 7 was about the threshold where model performance dropped.

Tracheid length prediction in Pinus palustris by means of near infrared spectroscopy: the influence of age

The prediction of tracheid length using near infrared (NIR) wavelengths can provide either useful or misleading calibrations depending on the context. This can happen since tracheid length is not directly related to the absorbance at any wavelength but is instead the result of a secondary correlation with some unknown chemical constituent. In this work, the effect of tree age and height on NIR predictability was investigated since tracheid length and chemistry may vary as a function of location within the tree. It was found that tracheid length predictability did not change with height but decreased with age. As a result, predicting tracheid length regardless of age was good (R2 = 0.72) while predictability holding age and height constant was mostly low to moderate with the exception of rings 1 and 4 which was quite strong.ZusammenfassungDas Abschätzen der Tracheidenlänge mittels NIR-Spektroskopie kann je nach Sachlage zu brauchbaren oder auch irreführenden Kalibrierungen führen. Das kommt daher, dass die Tracheidenlänge nicht unmittelbar mit der Extinktion bei irgendeiner Wellenlänge in Beziehung steht, sondern über eine indirekte Korrelation mit unbekannten Inhaltsstoffen. In dieser Arbeit wird der Einfluss des Baumalters und der Höhe auf die Verlässlichkeit der NIR-Messung untersucht, da sowohl die Tracheidenlänge als auch die chemischen Bestandteile je nach Position im Baum varieren können. Es zeigte sich, dass die Vorhersagbarkeit der Tracheidenlänge sich nicht mit der Baumhöhe ändert, jedoch mit dem Alter abnimmt. Insgesamt war die Vorhersagbarkeit der Tracheidenlänge unabhängig vom Alter recht gut (R2=0,72); bei konstantem Baumalter und -höhe war sie jedoch mäßig bis niedrig außer für die Jahrringe 1 und 4, wo die Korrelation sehr streng war.

Short communication: Response of near infrared diffuse reflectance spectra to blue stain and wood age

We analysed 320 increment cores at 10 different growth ring positions for a net total of 2691 ring samples, all taken from breast height. The objective of this research was to (a) detect which wavelengths were sensitive to blue stain and (b) determine which wavelengths were sensitive to age when subjected to chemometric interpretation. It was found that wood chemistry-associated wavelengths 1115–1195 nm were most sensitive to blue stain while those at 1335–1415 nm and 1655–1685 nm were also influenced by blue stain, but to a lesser extent. When blue stained samples are not included in the calibration set, but blue stain then occurs in the field, bias in lignin prediction may be more likely, since wavelengths associated with lignin were more sensitive to blue stain. Alternatively, lignin-associated wavelengths showed clear delineation in absorbance with age while the relationship between cellulose-associated wavelengths and age was apparent, but less clear.