Some Aspects of Chemistry Teaching and Research
6. Riedl and P. D. Kamdem
Departement des Sciences du Bois, Centre de Recherche en Sciences et en lngenierie des Macromolecules. Faculte de Foresterie et de Godbsie, Universite Laval, Quebec, PQ, Canada G1K 7P4 Apart from the more traditional pulp and paper chemistry, there is an increasing place for chemistry applied to wood composites. The composite concept is taken in its widest meaning and includes wood-thermoset plastic composites such as plywood, particleboard, waferhoard and fiberboard, as well as wood-thermoplastic composites and wood-cement composites. Wood-thermoset plastic composites are used especially in the construction and furniture industry, while wood-thermoplastic composites may be used in automobiles. Wood-cement composites are found mostly in Europe, in the construction industry. Such composites are cost effective for replacement of solid wood or plywood, especially in a country like Canada, where trees are slow to attain maturity; it enables the industry to use trees of smaller diameter t o a full extent. Since, at least for some time, large-diameter trees will be increasingly unavailable, the economic advantage of such composites is expected t o increase. A composite is a material made up of distinct, phaseseparated (on a macroscopic scale) ingredients. Often, one phase, which can be a reinforcing agent, is dispersed in another phase, made up of a less expensive and performant material that can be molded or worked. For instance, in carbon-fiber-reinforced epoxy, the epoxy is the continuous moldable phase, while the carbon fiber is the reinforcing agent. In waferboard (Fig. I), the wood wafers are pressed together t o make a nearly continuous phase, while the thermoset polymer "spot welds" the wafers together: one could then think of the semicrystalline cellulose as the reinforcing agent, with lignin and phenol-formaldehyde being the continuous phase. Thus the role of the thermoset polymer is similar to that of lignin in the wood, and, indeed, its structure is similar. One can also think of a composite as made up of two phases and an interface between the two phases: the interaction of both components a t the interphase is critical and determines the performance of the composite, through the adhesion: it is then obvious that a good knowledge of the surface and interfacial properties is important in the engineering of such materials. Chernlstry of Wood Surlaoas The bulk composition of dry extractive-free hardwood is approximately 43% cellulose, 38% hemicelluloses, and 20% lignin. Cellulose is a polymer made up of linear chains of 1,4P-bonded anhydroglucose units. The degree of polymerization is between 9,000 and 15,000. Hemicelluloses are copolymers of several different sugars, with a branched structure and a low degree of polymerization (100-200). Lignin is a heavily branched polymer made up of anetwork of hydroxyand methyl-substituted phenyl propane units. The surface of wood, a t least immediately after sawing, is made up of the same proportion of these polymers as in bulk wood, and also contains sap components such as sugar6 and resins (collectively known as "extractables"), as well as 5-20% water, depending on the relative humidity of the surroundings.
All wood surfaces as used in furniture and construction are artificial surfaces, i.e., made by sawing, chipping, defihration, or otherwise. When another natural product such as an apple or a banana is cut in two, oxidaGon of the surface occurs: this can be visually confirmed as browning of the surface. The same is true of wood, although the effect is slower and less extensive. Several changes occur a t the wood-air interface on different time scales: oxidation of the surface, especially during exposure to high temperatures durinesawine or drvine. of woodextractives at the . -. mieration .. surface, modification (increase) of the celluloseilignin ratio, acidification of the surface. and attack bv bioloeical aeents such as fungi. Most of these processes occur onthe top few micrometers yet are much detrimental to adhesion and performance of the final composite. Several traditional techniaues, such as critical surface tension measurements and soivent extraction, can characterize these changes in one way or another. For instance, the critical surface tension of a freshly cut wood surface can change from 70 to 40 erg/cm2 in a few weeks of exposure outdoors. This low surface energy is typical of a plasticlike, hydrophobic surface, rather than an hydrophilic, hydroxyl-group-covered surface, such as the fresh wood surface. I t is still verv difficult to auantifv the relative proportions of hydroxyl (.&ohol or hydroxyphenol), ketonic, acid and ester, aliphatic and aromatic groups at the surface. Some new analytical techniques such as electron spectroscopy for chemical analysis (ESCA), inverse phase gas chromatography (IPGC), attenuated total infrared reflectance (ATIR), and solid-state NMR can give molecular information on processes a t the wood-air interface. We will review rapidly the first two techniques mentioned. The ESCA technique consists of irradiating a surface, in a high vacuum, with X-ray photons. Secondary photons are emitted by the inner shell electrons of the surface atoms, and their energy is characteristic of the nature of these atoms and their &vironments. Only atoms very near the surface (5 nm or less) can emit. The spectra obtained can be integrated to "eive the oonulation of each kind of atom. The snectra of carbon atoms is particularly informative, as shown in Figure 2. the ESCA sDectra of m a ~ l wood e in the reeion of hindine ehergies of caibon atoms. he energy of the peaks is dete; mined relative t o a reference peak, in this case the carbon atom not bonded to an oxygen atom, that is, an aliphatic carbon. One then obtains achemicalshift. as in NMR, but in electronvolts (eV), not ppm's. In addition to the peakdue to ali~haticcarbon, two additional Deaks can be detected, one at2.0 eV, due t o carbon singli bonded t o oxygen, as in alcohols, and another smaller peak, a t 3.8 eV, due to the carbon of a ketonic, carboxylic, or carboxylic ester moiety. As the wood surface age8 or undergoes chemical treatment, the relative population of these &ace species evolves; the relative height of the peaks can then be related to changes in properties i f the surface. It is also especially practical to follow such evolution with the oxygenlcarbon ratio, since there is also an ESCA peak for oxygen. As the wood surface
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ages, its oxygenlcarbon ratio increases, which is evidence of an apparent oxidation of the surface (mostly conversion of alcohol tocarhoxylicgroups, although it is nit possible, with ESCA, to identify the chemical changes occuring on oxygencarrying moieties a t the surface) i d loss ofthe lignin, which is amorphous and more prone to degradation than the semicrystalline cellulose. Indeed, weathered wood has a gray color, which is nothing else than "dirty cellulose", while the lignin has been eliminated. The adhesive properties of such weathered wood are much inferior to that of
fresh wood, although it is not clear whether this is due to the poor adhesive properties of partially oxidized cellulose or lignin or the absence of lignin itself. IPGC is a technique that has been used to characterize thermoplastic polymer and cellulose. It is termed "reverse" since the user is interested in the properties of nonmobile phase in the column rather than the eluate. The principle of the technique is simple: an inert carrier gas ferries a small quantity of a small "probe" molecule (hexane or decane) through a column packed with smallparticles of the polymer
Flgure la. Woodccmposlte panel bask materlala: UF(whhe 1lquld)andPF (dark llquM)glues w h , beglnnlngtop lefl and clockwise, flbsrs, particles, veneer, and wafers
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radiative, or mechanical means. The chemical method is eenerallv used for initiatingarafting bv a radical chain transFer reacGon. It is based on redox system made up of persulfate ion, hydrogen peroxide, and ferrous ion, in aqueous . .. solution at room temperature. Persulfate ions decompose when heated in water solution creating sulfate radicals: while decomposition of hydrogen peroxide in presence of ferrous ion leads to hydroxyl radiof cellulose or cals: both react with the hvdroxvl " erouns lignin and transfer the unpaiied electron onto these polymer chains. It is also oossible to eenerate free radicals from cellu" lose or lignin by irradiation using intense ultraviolet light, high-energy radiation such as gamma rays from radioactive isotopes as from cobalt-60, or highly energetic electrons from accelerators. However, irradiation causes several undesirable effeds such as cellulose and lignin degradation by splitting glucosidic linkages. Complications as to government regulations, training personnel for manipulations, and high investment for radioactive sources and environmental protection discourages its use. Mechanical means such as milling and electric discharges produce radicals, but they are seldom used and then only for timber surface activation. Radical polymerization can he broken down in four steps. The first is to activate wood fiber or cellulose bv a soda treatment that increases accessibility of reactive hydroxyl sites (mercerization). The second s t e is ~ xanthation or thiocarbonation of the wood fiber, which consists of an introduction of a small amount of xantbate groups (Fig. 3) onto the substrate backbone. The third step is formation of the macroradical on the wood fiber by the action of a redox system such as ferrous ion (Fezf) in the presence of hydrogen peroxide. Finally, the fourth step is the reaction of the wood macroradical with monomer to form a conolvmer (Fie. 3) and subsequent polymerization of the monomer to form a grafted polymer chain. It is also possible for the radical to be
a
~
-
Figure 2. ESCA spectra of a w w d surlace (adapted from ref 1): aecandary electron emission inlenslty as a function of chemical shifl, in electron volts. relative to an aliphatlc carbon. or fiber. The extent of the accessible surface, A, and the chemical nature of the material (e.g., its enthalpy of interaction with the probe) can be calculated from simple thermodynamics: where N, the number of molecules at the surface and a, the surface area occupied by a probe molecule adsorbed on the surface (2). For very small gas probe samples, e.g., in the Henry's law regiorl of the adsorption isotherm, enthalpies of at infinite dilution can be obtained from an adsorption, AHS, Arrhenius relation:
where V$ is the volume of retention of the probe (which is the volume of carrier gas necessary to elute the vapor of the probe from a column containing 1g of stationary phase at temperature T ) and R, the gas constant. The value of AHS obtained depends on the molecular composition of the surface. This method has the advantage of being inexpensive in equipment; however, it is time consuming, and several factors can introduce artifacts in the measurements such as nonequilibrium or kinetics of adsorption effects, degradation or modification of the nonmobile phase, and, for precise work, necessity to extrapolate resulrs to zero coverage and zero flow rate. Nevertheless, good results have been obtained with cellulose and wood fibers (21. The technioue is now being applied to native wood fibeisand chemical& modified wood fibers to evaluate chances of the extent and the nature of the surface.
Radical polymerization
on
/ Cell,
o-CH,-CHX-(cn, -cHx)n Hd
Wood Grafting
Grafting has a potential for tailoring materials properties to specific end uses. The natural abundance and attractive properties of cellulose and wood fiber appear to make it an ideal grafting substrate. Grafting polymer onto wood fiber or cellulose is a process that consists of linking polymer and cellulose to nroduce material with imnroved selected Dronerties without altering others. Several methods have deendeveloped for grafting cellulose and wood fiber: three of these are deemed "best" and will be reviewed shortly. The three techniques are grafting initiated hv radical ~olvmerization or xanthation, Ky anionic polymerization, A d by eerie ion polymerization. Xanthae-Initiated Radical CopoIymerlzation
Radical polymerization is by far the most widely used technique. The radicals used are generated by chemical,
L
(CHZ-CHX]
"z02
Cell-0-C-s
Il
S
Hd
+
M
4
Cell-0-c-S
+
no'
+
tic
I1
S
HOiMI,
Figure 3. Reactions for grafting pobmers to woad by free radical copolymerikation. Volume 67 Number 7 July 1990
545
transferred from the cellulose or lignin molecule to a monomer or a growing polymer chain: this results in homopolymerization of the monomer and is undesirable (Fig. 3). Several polymers such as polystyrene, polyacrylonitde, polymethylmethacrylate, and polyacrylamide have been grafted to wood using this technique. Anionic Polymerization This method ~roduceshieh-vield eraftine throueh a contge &bst& is c i l l ~ l o s ~celluor trolled reaction.*~owever, lose acetate. not wood, and the reaction conditions (nonaqueous sol"ents, temperature and pressure different from normal and inert atmosphere) are very demanding, relative to the other techniques mentioned here. One method (3)for grafting polystyrene involves the production of a polystyrylcarboxylate anion, which displaces the mesylate group of mesylated cellulose acetate with formation of an ester linkage. Ceric ion-initiated Copolymerization When mixine tetravalent ceric ion (Ce4+)with the cellulose or wood fibers,a ceric-ion cellulose c6mplex is initially created and, as a result of electron transfer, ceric ion is reduced to cerousion (Ce3+), and a free radical is generated on the cellulose or the polymer backbone, especially a t the primary alcohol group (methylol). These radical sites then initiate graft copolymerization of monomer present in the reaction mixture. Some homopolymer is produced due to free radical transfer from the graft copolymer to unreacted monomer. The resultine cooolvmer molecule is made uo of a linear cellulose backbkenebr yignin containing branch& or grafts of synthetic polymer. These are linked to cellulose or lignin through covalent bonds so that any mild treatment, short of hvdrolvsis, cannot s e ~ a r a t ethe maft from the main chain. . ofsynthetic polymers such as The mechanical polyethylene, poly(methylmethacrylate), and polystyrene reinforced with grafted wood fihers are much improved: increases of 50% in elastic moduli and 100% of breaking length have been cited (4). Adheslve Chernlslry Urea-formaldehyde (UF) and phenol-formaldehyde (PF) thermoset polymers are used to bond together most of the wood composites produced in the United States and in Canada. The oroduction of UF and P F in the United States (1983) is 1:17 and 1.32 billion lb (dry weight), 75%and 45% of which..resoectivelv. . ".are used in the com~ositewood industrv (5). For comparison, the production of the largest volume thermoolastic. low-densitv oolvethvlene. was 7.8 billion lb in the unites states in 19f9-(6j. urea-fo&aldehyde (about 10% of the dry mass of the composite) is used mostly for composites destined for furniture use, since this thermoset polymer is sensitive to hydrolysis by water and cannot be used outdoors. Phenol-formaldehyde (2-5% of the dry mass of the composite) is used for exterior-grade waferboard and plywood. The students in forest engineering and wood science learn how to synthetize prepolymers for each kind of thermoset. This training is important since an increasing number of waferboard and particleboard manufacturers produce their own prepolymers. This permits optimization of processes through tailor-made prepolymers, since wood furnish. that is. the wood ~articlesused, varies with eeographic'dy determined variables such aswood speciesand dimensions. The amount of thermoset ~olvmerin the composite must be minimized, since it coniributes in a disproort ti on ate manner to the cost of the board: eenerally, the amount of polymer added is in proportion with the specific surface of the wood material to be bonded together, that is, particleboard needs more adhesive polymer than waferboard. The prepolymers consist of concentrated (-50% solids) water solutions, for example, syrups of low molecular weight 546
Journal of Chemical Education
(degree of polymerization 1-loo), partly cross-linked macromolecules. These syrups are prepared by following simple empirically determined industrial recipes: for instance, to prepare a low-molecular-weight, low-viscosity phenol formaldehyde, otherwise known as a type A resol, phenol, and formaldehvde are mixed toeether in a reaction vessel. the solution isihen brought to basic pH and thecontents heated to 6.i90 O C durine several hours: a reddish viscous liouid is obtained. The reactions taking place in such a preparation are given in Figure 4, for phenol-formaldehyde and urea-formaldehvde: The first ster, is addition, known as methvlolation. of formaldehyde on the urea or phenol. This may happen onup to four different sites on the urea molecule and three different sites on the phenol molecule. The methylolated species, in the second step, react with urea or phenol to create methylene bridged ureaor phenolmolecules. The result isa threedimensional network characteristic of such thermosets (Fig. 5). A mean substitution of a little over two methylene bridges on each urea or phenol is sufficient to create the network. However, in the prepolymer mixture, the reaction is s t o o ~ e dwhen a limited amount of cross-linkine has taken pl&. (If is not, then the reaction vessel has to be cleaned of the thermoset ~ o l v m emass r w i t h ~ i c kand hammer. much to the chagrin of the-student!) The degree of polymerization is controlled by continuously monitoring the viscosity of the mixture: the polymerization or cookis stopped a t apredetermined value of the viscosity. The prepolymer thus prepared has the appearance of a heavy syrup and consists of a water solution of a mixture of low-molecular-weight branched polymers. The syrups, after being characterized for pH, % solids, viscosity, and polymerization or set time (known as gel time), are comhined with the wood furnish in arotary blend-
Phenol-Formaldehyde
Figure 4. Chemical reactions leading to the cure (polymerlzatlon) of hnmset polymers (a) phenol-formaldehyde, (b) urea-formaldehyde.
er. A wood-particle (wafers or particles) mat, of the dimension of the board to heformed, is then weighed and prepared. This mat is introduced in a hydraulic press and pressed for several minutes a t 150-200 "C to obtain a final density of about 0.65 gIcm3. After cooling, random samples are then tested for standardized values of strength - and resistance to water hydrolysis. On a more fundamental basis, the degree of polymerization of the syrups can he followed by size exclusion chromatography (SEC), formerly known as gel permeation cbromatograpy. Figure 6 shows the SEC recording of a relatively high molecular weight phenol-formaldehyde syrup. The polydispersity of these samples is of course high and characteristic of such prepolymers. I t is also nossible to follow the kinetics of the nolvmerization and cross-linking of the prepolymer thr~;~h-thermal analvsis. Figure 7 shows a tvoical differential scannine calorimetry trace of a thermos& polymer: as temperatire increases in the samnle of the PF resol nrenolvmer or svrun, an exothermic reactron takes places a t abo& 120 OC, corresponding (small Peak a) to addition of formaldehyde on phenol, which was almost complete in the resol prepolymer. The second, much larger, endothermic peak corresponds to the cure of the thermoset polymer. This peak can be represented with the following simple equation
where a is the extent of the nolvmerization reaction, T,the temperature, Z, a pre-exponenhal (or scaling) fac
