YANXIN WANG,' JOHN SIMONSEN,"- CARLOS PASCOAL NETO,z IOAO ROCHA,z TIMOTHY G. RIALS;and ERIC HART.

'Oregon State University. Corvallis. Oregon; 2University of Aveiro, Aveiro, Portugal; JUSDA Forest Service,Southern Experiment Station, Pineville, Louisiana; . ASA, Inc., Woodburn. Kentucky

SYNOPSIS

The reaction of boric acid with wood fibers in a polymer melt was examined using 13C-

nuclear magnetic resonance (NMR). 11B-NMR. differential scanning calorimetry. dynamicmechanical analysis. and component extraction and by the determination of material prop-erties. Samples were blended at 350 and 380°F in a roll mill. The use of a plasticizer in themelt to facilitate the reaction of the acid with the wood fiber was studied. NMR data showedthat no significant reaction occurred between the boric acid and the polystyrene. Experi-mental evidence supports the reaction of boric acid with wood components. The ultimatestrength of the composites was either reduced or not significantly altered by the reaction.depending upon conditions. However. the stiffness increased significantly with boric acidadditions for the 350°F reactions. but behaved differently for the same additions at 380°F.The glass transition temperature of the polystyrene was lowered by the addition of plasticizer.as expected. while boric acid addition had little effect. Extracted samples showed that someboric acid remained with the wood fraction. These preliminary data suggest that boric aciddoes react with wood fiber under the conditions of this study. These investigations illustratethe feasibility of performing chemical reactions on the wood phase of wood/polymer com-posites during the extrusion process. Further research is recommended. ~ 1996 John Wiley

& Sons, Inc.

INTRODUCTION

The use of wood as a reinforcing filler in thermo-plastics continues to receive much attention fromthe research community. This attention is justifiedby the theoretical potential of wood fiber as a rein-forcing filler and the gap between theory and reality.The dominant theory guiding current research isthat composite performance can be improved by im-proving the interfacial shear strength between thethermoplastic matrix and the wood fiber by creatingchemical bonds across the interface between the two.Also recognized as important in the process of de-veloping useful composites from these materials is

an effective means, either chemical or mechanical,of dispersing the fibers in the matrix.\

Various compatibilization systems have been de-veloped by many labs: academic, government, andindustrial. Previous work by one of the authors in-corporated boric acid as one component of a com-patibilization system. The nature of the wood-boricacid interaction was of interest in elucidating themechanism of the overall compatibilization. Theobjective of this study was to investigate the pos-sibility of a reaction between boric acid and woodflour in a polystyrene melt.

This example of reactive extrusion opens up thepossibility of analogous, or subsequent, reactionsthat might be more effective at improving the prop-erties of the final composite. The reaction studiedhere suggests that it is practical to chemically modifythe wood phase during the extrusion of wood-poly-mer composites.

. To whom correspondence should be addre88ed.

Journal of Applied Polymer Seience. Vol. 62. 5O1-~ (19961~ 1996 John Wiley &. Sons. Inc. CCC 0021-8896/96/030601.{»

501

302 WANG ET AL.

Composites formed from polystyrene, wood flour,and boric acid, with and without a plasticizer, werestudied utilizing the determination of mechanicalproperties and NMR. In addition, glass transitiontemperatures (T g) were determined using differentialscanning calorimetry and dynamic mechanicalanalysis (DMA).

Material Properties

The modulus of elasticity (MOE) was determinedin flexure using a three-point bending apparatus inaccordance with ASTM Standard D790-86. Fivesamples were tested for each determination of MOEwherever possible. In some cases, only three sampleswere used due to material limitations. The samesamples and test procedure were used to determinethe ultimate strength, which was defined in this caseas the modulus of rupture (MOR).EXPERIMENTAL

MaterialsExtraction Procedure

A small sample (about 5 g) was dissolved in refluxingtoluene for 24 h. The wood flour was then separatedby filtration and dried in a vacuum oven at 60°C for24 h. The wood flour was then placed in boiling waterfor 30 min. then filtered. The aqueous solution wastested for boron using X-ray fluorescence spectros-copy with a Milton Roy Spectronic 301. The re-maining wood flour was digested using AmericanWood Preservers Standard A 7 -93. The resulting so-lution was analyzed for boron using a Jarrel-Ashinductively coupled argon plasma spectrometer(ICAP 9<XX».

Polystyrene (PS) was contributed by Dow ChemicalCo.. Midland. MI. as the product Styron 6850. Themelt flow rate was 1.6.

Wood flour from Douglas fir [Pseudotsuga men-ziesii (Mirb.) Franco] ground to 80-100 mesh wascontributed by the Menasha Wood Corp.. Olympia.WA. as the product T-14. The T-14 was dried invacuo at 60°C overnight and stored over a silica geldesiccant. This filler was analyzed optically andfound to have an average length of 0.9 mm with astandard deviation of 0.6 mm. The average aspectratio was 9 with a standard deviation of 5.

The plasticizer I-methyl-2-pyrrolidinone wascontributed by ISP Technologies as the product m-pyrol. It was used as received. Reagent-grade boricacid was supplied by J. T. Baker Chemical Co.. Phil-lipsburg. NJ.

Dynamic Mechanical Analysis (DMA)

Dynamic mechanical properties were determined ona Rheometrics, Inc., RSA-II solids analyzer. Thesamples were tested at 20 intervals over the tem-perature range from 30-130°C using a three-pointbending geometry and a constant frequency of 1 Hz.Typical sample dimensions for this analysis were2.8 X 12 X 48 mm.

Differential Scanning Calorimetry (DSC)

Selected samples were analyzed with a Perkin-ElmerDSC- 7 calorimeter with a System 1020 Controllerunder a nitrogen environment. The typical samplesize was about 25 mg. The analysis regime includedan initial scan from 50 to 150°C at 20° /min to re-move thermal history effects, then cooled to 50°Cand the data collected at 10° /mm to 225°C.

Sample Preparation

The wood flour was initially dried for 24 h in a vac-uum oven at 60°C. The plastic, filler, and additiveswere blended in a Brabender Plasticorder fitted witha mixing bowl and roller blades set at 30 rpm andtemperatures of 350, 380, or 400°F. The PS wasadded first and blended for 2 min to ensure completemelting. Boric acid (if required) was added andblended for 4 min. The filler was then added andblended for 8 min. Finally, plasticizer was added (ifrequired) and the sample blended for an additional8 min. The melt was then removed from the Plas-ticorder, cooled, and stored for grinding.

The blended samples were ground in a Wiley millto a particle size of approximately 3 mm (0.1 in.).These powders were then compression-molded in athermostatted Carver laboratory press. The pressconditions were 180°C (360°F) and 6.9 MPa (1000psi) for 10 min. The sample size was approximately2 X 13 X 55 mm (0.08 X .5 X 2 in.). The compressionmold held five samples.

NMR

Solid-state 13C- and llB-NMR spectra were recordedon a Broker MSL 400P (9.4 T) multinuclear spec-trometer at 100.6 and 128.3 Mhz. respectively. The1 H - 13C cross-polarization magic-angle spinning

BORIC ACID/WOOD REACTION IN POL YSTVRENE MIX 503

5.0

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.~ri..i

4.~

,f<-'

~40

4.Q

~2

B«Ic Acid. %

3 43.5

0 2

BorIc Acid. %

3 4Fipre 1 Strength or 65% polyatyene/35% wood floura8 a function or boric acid addition ror (e) 3% IOlvent at350°F, (8) 0% solvent at 350°F. and (.) 3% solvent at380°F.

Ficure 2 Elutic: modulus of 65% potystyene/35% woodflour as a function of boric acid addition for (e) 3% solventat 350°F. (8) 0% solvent at 350°F. and (.) 3% solvent at380°F.(CP/MAS) NMR spectra were recorded using a

'contact time of 1.5 ma, a lH 90° pulse of 4.5 ms, arecycle delay of 10 s, and spinning rates of 10-15kHz. Since the 4 mm doubl~-bearing Bruker probeused has a strong and rather.broad llB backgroundwhich overlaps with the faint signals given by thesamples studied here, all the llB-MAS spectra wererecorded with a recycle delay of 2 s and spinningrates of 13-15 kHz, using the following echo pulsesequence: [.../15 - tl- .../15 - t2, acquisition], withtl ~ 60 ID8, and ~ ~ 62 ID8. Preliminary experiments

carried out with boric acid showed that this pulsesequence introduces only a small line-shape distor-tion in the second-order quadrupole powder pattern.II B is a quadrupolar nucleus with spin 1 ~ ~. Thequadrupolar interaction is a tensor interaction whichmay be defined by two parameters, eq ~ V zz and '7~ (Vu - ~Y),)/VUf where V%%, Vyyo Vu are the prin-cipal elementa of a tensor describing the electric fieldgradient (EFG). For a polycrystalline sample, MASyields an NMR spectrum which is a powder patternwith a characteristic shape determined by the quad-rupole coupling constant CQ ~ eQV uIh and by ".2Hence. the spectrum provides valuable informationon the local symmetry around the 11B nucleus.

samples were studied with l~-CP /MAS NMR. The13C-NMR spectra are shown in Figures 1-4. Figure1 depicts the spectra of the starting materials ( wood.PS. m-pyrol) and their different combinations.without boric acid. The bands given by cellulose andhemicellulose carbons dominate the spectrum ofwood3 [Fig. l(a)]: Cl (105.0 ppm). C4 (88.4 and83.2 ppm). C2, C3. and C5 (74.6 and 72.3 ppm). andC6 (64.8 and 62.2 ppm). The spectral region cor.responding to aromatic carbona of lignin (110-160

50

t2

§

40

0 1 2 3 4 5

SoIv81t ~~. ~ %

Fipre 3 Strength of 65% poIYStyene/35% wood flourwith 2% boric acid 88 a function of solvent addition.

RESUL IS AND DISCUSSION

To investigate the nature of the reaction betweenboric acid and wood particles in this system, several

504 WANG ET At..

of wood/PS materials treated with boric acid [Fig.4 (c) and (d)] since these spectra are dominated bythe peaks assigned to PS. .

5.0

4.5m

Go~

W0~

liB-MAS NMR

The lIB-MAS NMR spectra are shown in Figure 5.The HPDEC spectrum of boric acid (see inset) dis-plays a second-order quadrupole powder patterncharacteristic of boron in trigonal environment. TheMAS spectrum of boric acid [Fig. 5 ( a ) ] still showsthe quadrupole doublet, although the line is broad-ened by dipolar coupling with B - 0 H protons. The

spectra of boric acid samples treated at 350 and380°F (not shown) are similar to the room temper-ature spectrum [Fig. 5 (a) J. The spectrum given byPS treated with boric acid [Fig. 5 ( b ) ] shows thatboron remains in trigonal coordination, hence pro-viding no evidence for chemical bonding. The spectraof wood + boric acid [Fig. 5 ( c ) ] and wood + PS

4.0

3.50 1 2 3 4 5

Solvent Addition. %

Figure 4 Elastic modulus of 65% polystyene/35% woodflour with 2C:jj boric acid as a function of solvent addition.

ppm) is not well resolved and the peaks are weak.However, the resonance at 56 pp.m, ascribed to thelignin methoxy carbons, is clear:ly seen.

Figure 1 (b) shows the spectrum of PS. The bandat 40.5 pp~ and the shoulder at ca. 45 ppm are as-signed to -CH2- and CH- carbons of the ali-phatic backbone of the polymer, respectively. Thestrong peak at 127.9 ppm is assigned to the tertiarycarbons of the aromatic ring, while the band at 146.2ppm is given by the quaternary carbon of the aro-matic ring.2

The spectra corresponding to the wood/ PS melt,with and without m-pyrol [Fig. 1 ( d) and (c), re-spectively) , do not present significant changes com-pared to the spectra of wood and PS alone. Thisindicates that no chemical interaction takes placebetween the two polymer systems.

The spectrum of wood ( without PS ) treated withboric acid [Fig. 2 (b)] shows a slight broadening ofthe bands assigned to C6 (64.8 and 62.2 ppm) andthe bands assigned to C3, C5, and C6 (74.6 and 72.3ppm), when compared to the spectrum of untreatedwood [Fig. 2 (a)]. This could be an indication of achange of the chemical environment surroundingthese carbon atoms, with the treatment of wood withboric acid. The spectrum of PS (without wood)treated with boric acid [Fig. 3(b)] is not signifi-cantly different from the spectrum of nontreated PSalone [Fig. 3 (a) ], indicating that no chemical in-teraction has occurred between boric acid and PS.The results obtained for wood and PS alone, treatedwith boric acid, are not easily visible in the spectra

150 100 50

PPM

Figure 5 1:'C-CP/MAS NMR spectra of (a) wood (10kHz), (b) PS (10 kHz), (c) 65/35 PS/wood flour, 0% boricacid, and 0% solvent (15 kHz), and (d) 65/35 PS/woodflour, 0% boric acid, and 3% solvent (15 kHz). The asteriskdenotes spinning side bands.

BORIC ACID/WOOD REACTION IN POL VSTYRENE MIX 505

150 100 50

PPU

Flpre 6 "C-CP/MAS NMR spectra of (a) wood flour(10 kHz) and Ib) wood flour. 4% boric: Kid, and 3% solvent(15kHz).

+ boric acid [Fig. 5 (d) ] contain both a second-orderquadrupole powder pattern due to trigonal boronand a sharp signal at ca. -} ppm, characteristic ofboron in tetrahedral coordination. The latter reso-nance indicates that the coordination of some boronatoms has changed from trigonal to tetrahedral and,therefore, suggests that boron bonds to wood. Al-though the coordination of boron to lignin cannotbe excluded,4 the coordination of boron to wood

,~ 11» 50

PPM

Fipre 8 I"C-CP/MAS NMR spectra or (a) wood (10kHz), (b) PS (10 kHz), (c) 65/35 PS/wood flour, 4% boricacid, and 3% solvent at 350°F (15 kHz), and (d) 65/35PS/wood flour, 4% boric acid, and 3% solvent at 380°F(15 kHz). The aaterisk denotes spinning side bands.

20'. 1. t. I~ II»

PPM

. ~ 40

Figure 7 13C-CP /MAS NMR spectra of (a) PS (10 kHz)and (b) PS, 4% boric acid, and 3% solvent (13.5 kHz).The asterisk denotes spinning side bands.

polysaccharides is more likely to occur as suggestedby extensive solution studies of the coordination ofboron to carlJohydrates in tetrahedral arrangements 5

(Fig. 6, structures D, Ill, and IV). However, wecannot exclude the possibility that some trigonalboron might be coordinated to wood polysac-charides ~ (Fig. 6, structure I). Such a site would

probably give a second-order quadrupole powderpattern with an isotropic chemical shift and quad-rupole parameters slightly different from those ofboric acid. It is clear from Figure 5 that the shapeof the quadrupole doublet changes from sample tosample. This would support the presence of suchtrigonal environments. However, since the NMRspectra were recorded with an echo pulse sequencewhich may introduce distortions in the line shapes,we stress that this remains a speculation and thatthis area needs much work.

Mechanical properties testing indicated that thestrength of the composite was reduced by the pres-

506 WANG Err AL.

-~ /OH""I6

-A OH>-OH

~

~ 0

01'<;;III

IVFigure 10 Potential structures of complexes betweenboric acid/borate! and carbohydrates.5

ence of boric acid (Fig. 7). While the presence of 3 %m-pyrol slightly elevated the MOR over that of noplasticizer, mixing the components at 380°F clearlyreduced composite strength.

The stiffness was markedly improved with boricacid concentrations up to 2% (Fig. 8). The improve-ment in stiffness was less pronounced for the sam-ples blended at 380°F. At the higher boric acid con-centrations of 3 and 4%, the stiffness of the 380°Fsamples decreased. This may be a result of reactionbetween the boric acid and the wood fibers, assumingthat the reaction rate is dependent upon boric acidconcentration and temperature. The differences instiffness between samples with and without m-pyrolas the plasticizer were insignificant.

The influence of m-pyrol on strength was insig-nificant up to 5% additions (Fig. 9). Stiffness showeda slight improvement at 1 % addition, but furtheradditions reduced the overall stiffness (Fig. 10). Theeffect of m-pyrol was not as great as was the effect

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of boric acid on composite mechanical properties.Addition of 2% boric acid improved the stiffnesswith only about a 5% reduction in strength (Figs.9 and 10).

The reaction between boric acid and wood wasfurther elucidated by extracting the formed com-posite and analyzing the extractants. After removingthe PS by toluene extraction, the remaining fiberwas extracted with boiling water. The boric acidcontent of the water solution was determined by X-ray fluorescence spectroscopy. The solution con-tained 89.5% of the original boric acid. This extremecondition should have removed any un reacted boricacid. It may also have saponified some of the borateesters and therefore may be underreporting the ac-tual content of the boric acid reacted with wood par-ticles. The wood remaining after extraction was di-gested and the resulting solution analyzed for boricacid via ICP. The analysis showed that the boilingwater-extracted wood particles still contained 2.1 g/kg as boric acid compared to a blank sample rununder the same conditions. These data support theNMR results, suggesting that, while the extent ofreaction was not large, some of the boric acid reactedchemically with the wood filler.

Additional evidence was observed optically. Thesamples which contained boric acid showed a redcoloration under microscopic examination. A colorchange can be indicative of a chemical reaction.

The glass transition temperature (T,) of the PSwas lowered 2.0°C, from 105.2 to 103.2°C. by theaddition of wood flour to the system (Fig. 11). Theaddition of 3% plasticizer resulted in a large dropin the T, from 103.2 to 96.1°C. Boric acid additionhad only a small effect on the T" lowering it from96.1 to 95.5°C. These data support our hypothesis

~ 50 . » . 10 0

PPM

.10 -20 .. ..

Figure 9 lIB-MAS NMR spectra of (a) boric acid, (b)PS, 4% boric acid, and 3% solvent (15 kHz), (c) woodflour, 4% boric acid, and 3% solvent (15 kHz), and (d) 65/35 PS/wood flour, 4% boric acid, and 3% solvent (spinningrates: 13-15 kHz). The inset depicts the spectrum of boricacid recorded with a Bolch decay and high-power protondecoupling.

BORIC ACID/WOOD REACTION IN POL VSTVRENE MIX 507

106 firmed or denied without additional study. In Figure12(b), the E curves show that the presence of theplasticizer reduces the E value at equivalent tem-peratures. The addition of 1 % boric acid appears toincrease the E value at low temperatures, but lowersthe onset temperature of the phase transition.

The increase in stiffness and E of the compos-ites might be accounted for if the boric acid is em-brittling the wood flour. The temperature of theblending operation (350°F) is certainly highenough to dehydrate the wood. Heat treatment is

104

102

100(,)0

~ 98

96

94

92

900 4 42 3

Boric Acid Content. % s~iI-

Figure 11 Glass transition temperatures from DSCmeasurements for (.) PS, no wood. no additives, (e) 65/35 PS/wood flour, no solvent, (8) 65/35 PS/wood flour,3% solvent, and (A) PS, no wood, 3% solvent.

that PS does not react with boric acid under theseconditions. The decrease in the PS T g with plasti-,cizer addition was expected. The small change in T..upon boric acid addition also indicates that whilethere appears to be reaction between the wood andthe boric acid it does not translate into changes inthe interaction between the wood and the PS thatwould affect the T go

This hypothesis is supported by the DMA datapresented in Fig. 12. Figure 12(a) indicates that theaddition of 6% plasticizer lowers the T g' obtainedfrom tan 0, approximately 13°C, from 115 to 102°C.Addition of 1 % boric acid lowers the T 6 an additional4° to approximately 98°C. At 4% boric acid, the Tgis once again 102°C. The cause of this rise in T 6 withboric acid addition is unknown. The samples con-taining the plasticizer also exhibit a shoulder at ap-proximately SO-90°C. This feature of the tan 0 spec-tra was observed in earlier work.s It may indicatesome sort of interaction between the wood ~d PSphases. This interaction may be of the form of anadditional or altered interphase between the PS andthe wood surface. If the plasticizer were preferen-tially located close to the wood surface, the PS as-sociated with that region might conceivably presenta lowered T 6' thus accounting for the low-temper-ature shoulder. This speculation cannot be con-

Temperature, °C

Figure 12 DMA of samples with (. . . . . ,),0% solventand 0% boric acid, (- - .) 6",'0 solvent and 0% boric acid,(- ._) 6% solvent and 1% boric acid. and (-) 6%solvent and 4% boric acid. (a) tan delta; (b) E.

508 WANG ET AL.

known to dimensionally stabilize and simulta-neously embrittle wood.7.11 If the high temperaturesand acidic environment experienced by the woodin these experiments embrittled the wood, it mightexplain the increase in stiffness. Also, if embrit-tlement is the only reaction facilitated by boricacid addition, we would expect no increase in ul-timate strength and little change in the T, andDMA spectra, which is consistent with the data.Clearly, these limited data do not allow firm con-clusions regarding wood embrittlement. Muchmore work is needed in this area.

plasticizer seemed to have little effect on compositeproperties or the boric acid-wood reaction otherthan plasticization.

While these studies do not reveal large improve-ments in the properties of the composite under in-vestigation, they do illustrate the potential for re-active extrusion in the development of advancedwood-polymer composites. Chemical reactions maybe performed on the wood substrate in the moltenpolymer matrix without significant alteration of theproperties of the matrix. Hopefully, these resultssupport the development of new techniques to im-prove the material properties of wood-polymercomposites.

CONCLUSIONS

REFERENCESSolid-state NMR has been shown to be a powerfultechnique to study the interaction of boron with thepolymer constituents of a wood/polystyrene com-posite material. liB-MAS NMR and l:IC-CP /MASNMR evidence suggest that boron chemically bondsto wood components during the preparation of acomposite material of wood + PS + boric acid. Theformation of tetrahedral complexes of boron and( probably) wood polysaccharides has been put inevidence. However, the formation of trigonal com-plexes between boron and w9Orl components cannotbe excluded. No evidence has been found for chem-ical interaction between boron and PS.

The stiffness of wood-PS composites was in-creased for certain addition levels of boric acid. Thestrength was marginally decreased by boric acid ad-dition under certain conditions. This may be due toembrittlement of the wood phase at the high tem-peratures of the experiments, accelerated by the re-action of boric acid. The slight changes in ultimateproperties and DMA spectra suggest that the inter-phasal properties of the comp()sites are little changedby the reaction of boric acid with wood. The T. ofPS decreased substantially with the addition of theplasticizer to the wood/PS system, as expected. The

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2. G. Engelhardt and D. Michel, High-Resolution Solid-State NMR 0{ Silicates and Zeolites.. Wiley, New York,1987.

3. R. T. Woodhams, G. Thomas, and D. K. Rodgers,Polym. Eng. Sci., 24(15), 1166-1171 (1984).

4. C. Pascoal Neto, J. Rocha, I. Bole, A. Gaspar, and J.Pedrosa de Jesus, in 2nd Pacific Rim Bio-Based Com-posites Symposium, Vancouver, Nov. 6-9, 1994, PosterSession.

5. P. A. J. Gorin and M. Mazurek, Carbohydr. Res., 27,325 ( 1973) .

6. J. Simonsen and T. Rials, in Materials InteractionsRelevant to Recycling of Wood-Based Materials. Ma-terials Research Society Symposium Proceedings, April1992, Vol. 266.

7. J. A. Meyer, in The Chemistry of Solid Wood, AdvancesIn Chemistry Series 207, R. Rowell, Ed., AmericanChemical Society, Washington, DC, 1984, p. 258.

8. R. M. Seborg, H. Tarkow, and A. Stamm, Forest ProdJ., 3(3),59-67 (1953).

Received AU6ust 24. 1995Accepted February 26. 1996