Alfred J. Stamm
North Caroiina State of the University of North Carolina at Raleigh
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Fundamental Physical Approach to WOO^ and Cellulose Science
The author has been giving a one year course in Wood and Cellulose Chemistry in the School of Forestry a t Xorth Carolina State for the past four years that differs considerably from related courses given a t other institutions. It is a three credit per semester course consisting of two lectures and one three hour laboratory period per week, given to seniors and graduate students majoring in Wood Technology and in Pulp and Paper Technology. I'rerequisites for the course are one year courses in general college chemistry and in physics, mathematics through introductory calculus, at least a one semester course in quantitative analysis, a year of organic chemistry and either a year of physical chemistry or a concurrent course in physical chemistry. The novel feature of the course is the emphasis placed on the physical approach. About one-third of the time in both lecture and laboratory is devoted to what might be called "conventional wood chemistry" consisting of considerations of the chemical structure and reactions of wood compouents and their analysis together with the manner in which they are combined in wood. The remaining two-thirds of the time is devoted to the various physical properties of wood and wood comporrents and their interrelationship and how these properties affect and place restrictions on wood and cellulose processing methods. The author feels that this strong emphasis on the physical aspects of the subject is justified in that almost all present uses of wood are based upon utilizing its physical attributes rather tharr its chemical constitution. Further, all processing metliods other than pulping are predominantly physical in nature. Reading assignments for the conventional organic and analytical part of the, course are made in "Chemical Processing of Wood" by Stamm and Harris, Chemical Publishing Co., N.Y., 19.53 and in "Wood Chemistry" edited by Wise and Jahn, Reinhold, Publishing Corp., N. Y., 1952. Unfortunately both of these books are out of print, so the students have to read their assignments in the library. Students are also encouraged to read parts of "Chemistry of Wood" by Hagglund, Academic Press, X. Y., 19.51. KO present book covers more than a small fractiou of the fundamental material included in the physical properties of wood and cellulose portion of the course. Because of this, the author took on the task of writing a combined text and reference book covering the fundamental physical aspects of the subject. A large part of this 27-chapter book entitled "Wood and Presented as part of the Symposium on Education in the Field of Cellulose, Wood, and Fiber Chemistry before the Division of Cellulose, Wood, and Fiber Chemistry and the Division of Chemical Education at the 145th Meeting of the American Chemical Soriety, New York, N. Y., September, 1963.
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Journol o f Chemicol Education
Cellulose Science" has been used in mi~ueogwphform ill the course during the past two years. The book is expected to be pnblished by Ilouald I'r~ss of Ken. 1-ork early in 1964. Course Content
Five different types of physical properties are cow sidered: structural, interfacial (solid-fluid relatiouships), thermal, mechanical, and el~ctrical. T h e s ~ are further broken dowu into specificproperties and thr techniques for studying them. A great deal of crossreferencing is given in the bcok and iu the course to show the interrelationships of the val ious properties. The discussion of properties and methods of measuring them is followed by a presentatiou of the fulldamental principles goveruing four different basic types of wood and cellulose processing procedures: drying, impregnatiug, adhesiou, and fiber-fiber hondiug. The gross structure of wood has been previously c o n sidered chiefly from two standpoints: the plaut physiology approach which covels the structure from a growth and a reproduction standpoint and the dendrological approach which places chief emphasis upou the structural differences for identification and classification purposes. The gross structure of mood is considered here largely from the standpoiut of numbers and dimensions of the structural components, such as the determination of the mean double cell wall thickness of softwoods from specific gravity measurements, and the numher of pits per fiber by a simplified count technique on gross-sectional photomicrographs. This necessitates considerable cross-referencing to methods of measurement covered later iu the course. The presentatiou of the firre layer structure of the cell walls of wood is preceeded by brief descriptions of the measuring tools namely, polarization, electron microscopy, and X-ray diffraction. Emphasis is placed upon the different types of structural information 01)tained from X-ray measurements on cellulose: the dimensions of its unit cell, the number and relative positions of the anhydroglucose groups in the unit cell. the effect of strong swelling agents upon the unit cell, the degree of crystallinity of the different types of cellulose, the dimensions of the ciystallites, orientation of the crystallites both in the microfibrils and in thr fibers, and the rate of development of crystallinity in the various celluloses. The molecular properties of cellulose obtained by seven different types of measurements using dilute solutions are compared. It is shown how they collectively indicate that cellulose is dispersed as single molecular chains of varying length that act like stiff rods iu the lower molecular weight range and more l i e random coils as the molecular weight and corresponding
degree of polymerization increase. The different types of average molecular weight and degree of polymerization are covered. Both the effects of physical and chemical treatments upon the degree of polymerization (Dl') and the reduction in D P upon t h ~ strength and other properties are included. The section on interfacial properties of wood and wood products is introduced hy first considering the bulk and substance specific gravities since these make it possible to calculate void volumes, void cross sections, and approximate surface areas of the permanent voids needed in interfacial considerations. This is followed by a general discussion of the phenomenoi~ of absorptioii as it applied to all types of adsorbents and adsorbates, and how they fall into five different types including both monomolecular and polymolecular adsorption. Theories and equations for these different types of adsorption are considered. The adsorption of water vapor by various cellulosic materials is treated in considerable detail. The polymolecular nature of the adsorption, the small extent to whicti adsorption occurs on pre-existing surfaces compared to the formation of solid solutions, the nature of and reasons for adsorption hysteresis, the effect of temperature on adsorption; the meaning of fiber-saturation point, how it can be estimated from adsorption isotherms, and how the portion of the adsorption that is monomolecular can be estimated are included. Limited available data on the adsorption of gases and nonaqueous vapors are considered as well as adsorption from solution. Methods for estimating the extent of microscopically visible internal surface and the transient intra-fiber contact area from various adsorption measurements are also considered. Considerable emphasis is placed on the phenomenon of shrinking and swelling. Such factors as the conditions under which swelling occurs, and how the nature of the cellulosic material, its structural orientation and specific gravity, and the polarity and the size of the molecules of the swelling medium effect the extent of swelling are all included. The swelling of paper is considered separately from that of wood as dispersion of the fibers as d l as actual swelling of the fibers is involved. Shrinking and swelling has such a detrimental effect upon many of the uses of wood that the five known fundamental approaches to dimensional stabilization are covered in considerable detail. Drying wood, treating it with preservatives, fire retardants, and dimension stabilizing chemicals, and impregnation of chips with pulping liquors all involve the movement of liquids in wood. The nature of the movement under both pressure permeability and diffusion conditions, the controlling structure and the laws governing the movement are hence given considerable attention. It is also shown how flow and diffusion measurements are helpful in determining specific structural dimensions and relationships. Heat has a profound effect upon a number of the properties of wood. The heat of wetting of wood is quite large. Methods for its measurement and calculation from adsorption isotherms are covered, including the calculation of such thermodynamic properties as the total integral heat, the free energy and the entropy changes, and the free surface energy.
The kinetics of thermal degridation and of hydrolysis are covered, the former over time periods of minutes to years. Activation energies are calculated from the data. Thermal properties such as specific heat, heat conductivity, thermal expansion, and exothermic reaction temperature are covered. Effects of heat on mechanical and electrical properties are discussed 1111der these other headings. The mechanical properties are covered only from a theoretical standpoint, with chief emphasis on the effects of moisture content, chemical treatments, and temperature upon the strength. The effects of specific gravity and artificially densifying mood upon its mechanical properties is also considered. The large effect of chauges iu moisture content below the fiber-saturated point and the relatively small effects of specific gravity and ash content upon the dc electrical conductivity of wood are shown to make possible the nondestructive determination of the moisture content and the fiber-saturation point from the dc conductivity. I t is also shown how such measurements can be used to estimate the relative diffusion coefficients through liquid filled wood arid through mood substance below the fiber saturation point in the three structural directions. High frequency ac conductivity, dielectric, and power factor measurements are also shown to be suitable under certaiu conditions for determining moisture contents. I t is shown how high frequency currents generated in wood can be used for internally heating vood thus making possible the use of this technique for drying and gluing wood. The electrokinetic phenomenon of electroosmosis and streaming potential are considered and how they can be used for determining such structural dimensions as minimum, average, and maximum fiber lengths, and average size of the small openings in the pit membranes. Nually the application of the physical principles to four important fields of wood processing are considered. Drying is shown to be largely diffusion controlled. The limitations imposed upon tlie system due to this fact are considered. Proposed new approaches are examined on the basis of their theoretical soundness. Impregnation of wood is considered from a theoretical standpoint. The pits of softwoods and the pits and tyloses of hardwoods are shown to control penetration. How these facts affectwood preservation are considered. Adhesion is approached from a fundamental specific adhesion standpoint. The work of cohesion of wood and work of adhesion of water to wood are estimated from free surface energy data. Theoretical bonding forces only a few-fold greater than observed values are obtained. Reasons for the deviation are considered. Properties of fibers leading to self bonding are considered. Extents of fiber-fiber bonding estimated by optical and nitrogen adsorption methods are compared with theoretically calculated values. The importance of long, well-hydrated fibers that have been beaten sufficiently to remove the primary wall and increase their flexibility is pointed out. Efforts are made to tie this mass of observations and deductions drawn therefrom into as unified a state as possible. The subject matter is presented and taught with the hope that it will lead to better future uses for the world's most important reproducible raw material, wood. Volume 41, Number 4, April 1964
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