THE PROPERnES OF THE WOOD-POLYSTYRENE INTERPHASEDETERMINED BY INVERSE GAS CHROMATOGRAPHY

John SimonsenAssistant Professor

Zhenqiu Hong.

and

Timothy G. RialsResearch Physical Scientist

USDA Forest ServiceSouthern Experiment Station

Pineville. LA 71360

(Received April 1996)

ABSTRACT

The propenies of the interphase in wood-polymer composites ~ important determinaou of thepropenies of the final composite. This study used inverse ps chromatOll'Bphy (IGC) to measureinterpbasal propenies of composites of polystyrene and two types of wood fiber fillers and an inOllaDicsubstrate (CW) with varying amounts of surface cove,. of polystyrene. Glass transition temperatures,thennodynamic parameters. and the London component of the surface ~ energy (Y .L) were deter-mined. Values for Y.L became constant at higher coverages, allowing the thickness of the interphaseto be estimated.

Ke:1'words: Inverse ps chromatOll'Bphy, wood, plastic, polystyrene, interphase. interface, composite.

INTRODUCTION

Wood-plastic composites have been inten-sively studied in recent years (Woodhams etal. 1984: Takase and Shiraishi 1989: Yam etal. 1990: Maldas and Kokta 1991), panicularlythe development of composites made fromwood and polypropylene (PP), polyethylene(PE), .polystyrene (PS), poly(vinyl chloride)(PVC), and polyester (Yam et al. 1990). Wood-plastic composites manufactured from thesematerials offer many potential benefits, suchas low cost. increased stiffness. recyclability.ease of processing. flexible design ofpropenies.and higher strengths than unreinforced plas-tics. These composites also provide the op-ponunity to use recycled plastics and recycledwood as raw materials (Rowell et al. 1993)."'-"'olld Fibrr~. 29(11.1997. pp. 7s..84J: 1119 7 by !be 5«-iet.v of W God ScieDce aDd T ecbIIOIoIY

A fiber-reinforced plastic can be consideredto consist of three phases: the bulk plastic ma-trix, the fiber filler, and the interphase betweenthe two. The strength of the interphase de-pends on the extent of bonding between theplastic and the filler. That bonding may in-clude mechanical, covalent, ionic, and hydro-gen bonds and van der Waals forces. The vander Waals force includes dipole-dipole inter-actions, induced dipole interactions, and Lon-don dispersion forces.

Although many technological advances haveimproved the material properties of thesecomposites. problems still remain, primarilythe lack of adhesion betWeen the wood and theplastic (Kishi et al. 1988). Wood fiber is a hy-drophilic material that contains cellulose.

Graduate StudentDepanment of Fo~t ProduCts

Oreaon State UniversityCorvallis. OR 97331

76 WOOD AND FIBER SCIENCE. JANUARY 1997. V. 29(1)

hemicellulose, lignin, and other extractivecompounds. Most plastics, such as PS. PE, andPVC. however, are hydrOphobic materials. Thedifferent polarities of the fibers and the plasticslead to incompatibility and. therefore. poorbonding in the interphase region between them.This prevents the necessary stress transfer fromthe plastic matrix to the load-bearing fiber.relegating the function of the fiber to that of anonreinforcing. filler (Hon and Chao 1993).Typically, little of the strength and stiffness ofthe fiber is translated to the final material prop-erties of the composite when the wood andplastic are poorly bonded. Under stress. thepoorly bonded composite will fail when thefibers pull out from the plastic matrix. Thus.the properties of the composite. especially theultimate properties, are largely determined bythe interphase.

The problem of limited adhesion has beenextensively addressed in numerous reports onthe effects of compatibilizers and couplingagents on the mechanical propenies of com-posites (Woodhams et al. 1984; Maldas et al.1989; Liang et al. 1994; Sanadi et al. 1994;Youngquist 1995). Although those studies haveprovided considerable insight into potentialsolutions. their evaluation of composite per-formance has been primarily focused on sec-ondary properties rather than on the specificcharacteristics of the interphasal region withinthe material system. Therefore. very little un-derstanding has been gained on the funda-mental physicochemical parameters that con-trol the structure and properties of the woodfibers/polymer matrix interphase. In this study.we used inverse gas chromatography (IGC) toinvestigate the interphasal properties of twotypes of wood-plastic composites.

(2)

= F J (tR - t..,)

where F IS a corrected flow rate of the ~er

gas and j is a pressure-gradient correction fac-

tor for gas compressibility given by (Braun andGuillet 1975: Conder and Young 1979)-

J = ~ [ (PI/P 0)2 - 1

2 (PI/PoP - 1

where P j and Po are the inlet and outlet pres-

sures of the GC column (Braun and Guillet

1975).At small concentrations of the probe, the

retention volume obeys Henry's law at zero

coverage (infinite dilution) (Dorris and Gray

1980)

INVERSE GAS CHROMATOGRAPHY

There are few experimental techniques whichyield data that can be mathematically relatedto the properties of the interphase. One suchtechnique is IGC (Braun and Guillet 1976:Schreiber and Lloyd 1989), which was devel-oped on the basis of conventional gas chro-matography (GC) in the late 1970s (Smidsrod

273.15 .yo=. Tc

where W = the weight of the adsorbent in thecolumn and T c = the column temperature.

~VN (4)

and Guillet 1969). The instrumentation ofGCand IGC is similar. In IGC, a nonvolatile ma-terial to be investigated is packed in a GCcolumn. This stationary phase is then char-aCterized by injecting known volatile chemicalcompounds, called probes, into the column.The time required for the probe to pass throughthe column is called the retention time, ta,which can be translated into a number of im-portant thennodynamic parameters. The ob-tained thennodynamic data can be related tothe properties of the stationary phase (Bolvariet al. 1989).

Retention times can be nonnalized by in-jecting a marker, for example methane, whichgives a marker retention time. tm. Net reten-tion volume (V N) is then calculated from thefollowing equation (Demenzis and Kontomi-bas 1989):

VN (1)A

I~ ~-~ -, .

. rI,. =-

VN = KsA (3)

where Ks = the surface panition coefficient forthe adsorbate (m), and A = the total surfacearea of adsorbent in the column (m2).

It is nonnal practice to express V N as a spe-cific net retention volume at OOC and per gramof adsorbent;

Simofl#1l If a/. -lac OF WOOD-POLYSTYRENE INTERPHASE 77

The retention behavior of probes changes asthe temperature rises through T. (Braun andGuillet 1975, 1976). Below the T., the reten-tion time of the probe arises mainly from ad-sorption of the probe onto the surface of thestationary phase, and In(V .O) decreases linearlywith increasing temperature. When the tem-perature reaches T., probe molecules begin ab-sorption into the PS because of the increasedmobility in the PS molecular backbone. re-sulting in an inflection point on the In(V .O) vs.liT plot. Thus IGC data can be used to de-termine the T. in our simulated interphase,which may then be tracked as a function ofPSloading, or equivalently, interphase depth.

Gray and coworkers (Mohlin and Gray 1974)extended the IGC technique to the study offibers. Further development of the theory al-lowed thermodynamic functions of adsorptionto be derived from retention volume data(Kamden and ReidI199l):

for an homologous series of alkanes. They as-sumed VV A to be eq~ to the free energy ofadsorption per unit area of the surface. or

AGi(CH~N 801]

W,,= - (9)

where N - Avogadro's number and acH2 = thesurface area occupied by an incremental CH2group on an alkane. I1Go A(CH2), the surfacefree energy of adsorption of an incrementalmethylene group, is obtained by measurementof the free energy of adsorption at infmite di-lution for an homologous series of alkanes.Thus the value for the surface free energy maybe obtained from IGC data.

Using a value of 0.06 nm2 for a CH2, and theknown value for the surface tension of a meth-ylene unit, the London component of the sur-face free energy may be calculated from:

I I 'I1G~(C~»)2 (10)

(5)

.1H~ = -R~~

d(t)(6)

(7)

where ~ :, the differential heat of adsorptionof the injected probe (kJ/mol); at zero cover-age, qd = .lHA °; T = column temperature (K);p... - adsorbate vapor pressure at the standardstate; n. = the reference two-dimensional sur-face pressure, defined by De Boer (Dorris andGray 1981).

In the case of a nonpolar liquid adhered toa solid surface, the work of adhesion can beseparated into two terms (Kamden and Reid11991 ):

",L =-, . .4 i'CH2 \ N 8cH1 I

Kamdem and Riedl (1991) used this tech-nique to study the surface of chemithermo-mechanical pulp (CTMP) fiber onto whichmethyl methacrylate (MMA) had been graftedthrough the xanthate procedure of Maldas etal. (1989).

Garnier and Glasser (data presented at theACS National Meeting, August 1992) dem-onstrated that IGC may be useful for charac-terizing the surface of model cellulosic beadsthat were modified by grafting. Determiningthe acid-base components of the surface energyallowed the estimation of an adhesion param-eter for various combinations of polymer ma-trix and modified filler. From that approach,combined with dynamic mechanical analysis,they were able to identify an. interphasallayerof immobilized polymer chains and relate itsinfluence to composite propenies.

Inverse gas chromatography has been usefulin characterizing the surface of modified lig-nocellulosic material as it affects adhesion tothermoplastic polymer matrices. In our study,IGC was used to monitor the development andstructure of the interphase region by charac-

WA = 2("Y "Yk)'" (8)where "Y ~ the surface tension of the nonpolarliquid. and "YSL = the London component ofthe solid surface free energy.

Dorris and Gray (1980) used this concept

79Simol&sell n aJ. -IGC OF WOOD-POLYSTYRENE INTERPHASE

Aldrich Chemical Company, Milwaukee. Wis-consin, and were used without further purifi-cation.

Sample surface areas were measured with N2BET adsorption (Table 1). Due to limited re-sources and aquipment difficulties. only a lim-ited number of samples were measured.

0.45-

~0.35-

ESCA surface analysis

Selected samples were analyzed with a Sur-face Science model SSX-l 00 ESCA spectrom-eter. An aluminum X-ray anode was used witha quartz monochromator and a 60011. spot size.Charge neu~tion was conduc~d on wIsamples with an electron flood gun and an Niscreen stabilized at ground potential 0.5 mmabove each sample. Results were ploned astotal oxygen to total carbon (O/Ctow) and. ad-ditionwly for CW. total silicon to total carbon(Si/Clou. ).

0.25-

5~

1.8

4

~1.2

3

.0.8

,,"0.4-2 0 2 4 S 8 10 12 14

PS Loading, or.

FIO. I. Silic:on/carbon~ aDd oxyaen/~ fromESCA scans ofCW and TI4 columns.

RESULTS AND DISCUSSION

Inverse gas chromarograph.v

The retention times of the probes were ob-tained directly from IGC chromatograms. Thecoefficient of variability was typically less than3%.

WOOD AND FIBER SC1ENCE. JANUARY 1997. V. 29(1)

TAKa 3 . ESI;1IIDUIl T. /rom 1 GC .

c-.. .

1S-9'1S-9'60S-"60S-"60S-"

TMP-l'PS-TMP-4~PS.TMP-8~PS.TMP-l ~PS-TMP-2~PS.

70

60

50

0; 4C~I

30

2Q

0 5 10 15 20 2570

60

tersection of the plotted data line and the re-gression (Table 3). The T. transition observedhere was broader than that observed on othersubstrates (Lipatov and Nestorov 197's). Thismay be due to the variability of the wood solidphase, which resulted in a variety of ps. mor-phologies on the surface with a correspond-ingly broad distribution of T .'s. While thisbroad transition did not allow for precise com-parisons between different ps. loadings, the T.appeared to be > SQ8C for 2 and 4% ps. load-ing and - 700c for S, 12, and 20% loadings.On other substrates, T. has appeared higher atlow surface loadings (Lipatov 1967). This ispresumably caused by the restricted mobilityof the initially adsorbed polymer layers. As thethickness of the adsorbed polymer layer in-creases. surface effects should diminish. andthe T. would be expected to approach that ofthe bulk matrix. The lOOC temperature inter-val for determination of T. limited the accu-racy of the measurement.

Values for thermod.vnamic parameters

Enthalpies were obtained from Van't Hoffplots (Eq. 6) for 0% PS-CW and 12% PS- T14.Enthalpies of adsorption. which include theenthalpy of vaporization of the probe at 60-(:',were derived forCW, TI4, and TMP from thelinear regressions of plots of In(V.O) vs. 1000/Tusing Eq. (6) for the temperature range ,SO-to 7Q8C (Fig. 3). The enthalpies for CW de-creased from 0 to 4% PS loading and thenbecame relatively constant. This may have beencaused by the increasing coverage of the sub-strate surface by the PS. If the only surfaceavailable to the probe is PS. the enthalpy shouldbe constant. Further additions of PS will addto the depth of the PS layer on the CW, butwill not alter the enthalpy.

50

40

30

205 10 15 20 250

% PS Loading (w/w)

FIG. 3. EDtbalpy of probe 8dsorption 00 CW. TMP. aDdT 14 substrates as a funCtion of PS 1oadin1o

81Si-" aI.-IGC OF WOOD-POLYSTYRENE INTBPHASE

~"'0.e2

°c

~

The results for the wood solid phases weremore variable: the TMP results showed a max-imum and the Tl4 showed a minimum en-thalpy. The reasons for this are unclear. butmay reflect the formation of energy sites dif-ferent from either the uncoated wood substrateor the bulk matrix PS (or ps. for TMP) phase.Another possibility is that adsorption was notthe only interaction betWeen the probe and thesolid phase. If the probes were being absorbedto some extent. and that adsorption was a func-'lion of probe size. then we would expect to seeirTegular results. Even so. comparison with themonotonically decreasing shape of the CWgraph suaests that there was some interaction,as determined by enthalpy measurements, be-tween the wood substrate and the PS thatchanged the behavior of the probes.

The enthalpies of the CW and TMP columnstended toward the same value at higher (12and 20%. respectively) PS loadings (Table 4).We would expect the enthalpy to become con-stant at higher loadinas because the solid phaseshould be completely covered with PS~ there-fore, the probe would be interacting only withPS (or ps.). The enthalpy of the Tl4 columndid not level off with increasing PS loading, asdid the CW and TMP columns. Since the sur-face areas were similar (see Table 1). we ex-pected the depth of the PS layer to be similaralso. There are several possible explanationsfor this behavior, such as absorption of theprobe into the PS, migration of DouaIas-firextractives from the Tl4 into the PS, or PS-T 14 interactions that gave rise to an alteredPS morphology on the surface. Additional re-search is required to explain this behavior.

Both of the wood solid phase columns ap-peared to show more variability in the values

Carbon Number (n)Fla. 4. Free enersy u a function of probe carbon numberfor ~ted CW and T 14 substrates.

of the measured thennodynamic parametersthan did the CW column, probably reflectingthe greater variability of the wood substrate.The free energy of adsorption was calculatedfrom the retention volume via Eq. (5). Limiteddata were available because this function re-quires a value for the surface area. The coatedand uncoated CW columns showed decreasingfree eneraY with increasing carbon number (n)(Fig. 4); the values seemed to be independentof the PS loading. The T 14 columns alsoshowed decreasing AGAo values with increas-ing n. The PS-coated T 14 showed a greater

WOOD AND FIBER SCIENCE. JANUARY 1997. V. 29(1

TABLE S Depth qf th~ intrrphm~.100

80

60

did the uncoated column with respect to car-bon number. At this point. we lack sufficientdata to draw any conclusions about the causeof the nonlinearity in the entropy-carbon num-ber relationship of the 12% PS-TI4.

The slope ofplo1S of ~GAo vs. n yielded thefree energy of adsorption of a single methyleneunit (~Go A(CH2». That free eneray may beconverted to the London component of thesurface free eneraY t -;SL. via Eq. (10). This pa-rameter was compared among different sub-strates as a measure of their surface energy(Fig. 6). For the CW columns. -;SL rose initially.then leveled off after about 8% PS loading.indicatin& that. from the penpective of surfaceeneray t additional increments of PS beyond8% added to the bulk matrix phase and inter-actions with the CW substrate decreased. TheTMP showed different behavior. as -;SL de-creased with increased pse loading. then lev-eled off at about 12% PS*. The final -;SL valuesfor CW and TMP were similar. again indicat-ing that the PS coating was not influenced bythe underlying surface at higher loadings. -;SL

for T 14 showed very different behavior fromthat of either CW or TMP: -;SL increased to12% PS loading. then decreased at 20%. Lev-eling of the curve was not observed. indicatingthat the TI4 surface continued to affect the PScoating even at the hi&her loadings.

40

~"0E-

2

°:Ccn~

20

100

80

60

40

20

7 10 12 138 9 11

Carbon Number

FIG. S. Entropy as a fuDctiOD of probe carbon Dumberfor PS-coated CW and T14 substrates.

slope than the uncoated column, reflecting thedifferences in adsorption energy for the twodifferent surfaces.

Entropy values were calculated from free en-erg)' and enthalpy values via Eq. (7) (Fig. 5).The coated and uncoated CW columns showedsimilar slopes of entropy vs. carbon number.with the PS-coated columns showing less neg-ative entropy values. Reftecting the enthalpyresults, the CW columns appeared to presentonly a PS surface to the probes at PS loadingsgreater than 4%. The 12% PS- T14 column,however, showed a more varied response, than

Depth of the interphase

The depth of the interphase was estimatedby selecting the PS loading that defined theextent of the interphase (.8). assuming that thePS density in the interphase was the same asthe bulk density of 1.04 g/cc. and modelingthe surface as a fiat sheet. If the surface is as-

Simonsr" rt aI. -IGC OF WOOD-POLYSTYRENE INTERPHASE 83

sumed to be approximated by a flat sheet ofthe same area as the measured surface area,then the thickness of the interphase will be thevolume of the interphase divided by the sur-face area (Table 5).

(11)surface area . density

65

N

E .

zE

-J(/)~

The interphase depth appeared to be similarfor CW and TMP, but noticeably larger forT14. Although our anificially prepared se-quences of surfaces may not accurately reflectreality in filled polystyrene composites, theydo provide an approximation of the interphasedepth. These data suppon a model of polymerdeposition on a substrate surface of con-strained polymer backbones in the initial lay-ers, with a gradual transition to bulk polymerbackbone morphologies. In this study, the Tl4appeared to promote longer range interactionsthan did either the CW or the TMP.

CONCLUSIONS

The T. for PS on the various substrates wasshown to be a function of the PS loading, i.e.,the depth of the interphase. The use of IGCallowed the determination of T. under con-ditions difficult or impossible to measure withother techniques. Enthalpy values showed aleveling for CW, a maximum for TMP, and aminimum for T14. Entropy values indicateddifferent interactions between PS and CW orT14. On the basis ofYsL values, the depth ofthe interphase was shown to be approximatelyO.l~. This study demonstrates that inverse gaschromatography can be a powerful tool for thestudy of interphases in filled polymer com-posites.

5 10 15 20 25035

34

33

32

31

30

29

28

27

26ACKNOWLEDGMENTS

This project was supported by U.s. Depart-ment of Agriculture grant number 19-94-017.The E5CA analyses were performed by the Bu-reau of Mines Albany, Oregon, Research Cen-ter, whose able assistance is gratefully ac-

knowledged.

20 250 5 10 15

% Polystyrene (w/w)

FIG. 6. London component of the surface free eneray forCW. T 14. and TMP substrates as a function ofPS loading.

60

55

50

45

40

35

30

25

WOOD AND FIBER SCIENCE. JANUARY 1997. V. 29(IJ84

REFERENCES

BoLVARJ. A. E.. T. C. WARD. P. A. KoNING. AND D. P.

SHEEHY. 1989. Experimental techniques for inversegas chromatography. Paaes 12-19 in D. R. Uoyd. T. C.Ward, and H. P. Schreiber. eds. Inverse gas chroma-tograpby. ACS Symposium Series No. 391. American

Chemical Society. Washington. DC.BRAUN.J. B.. ANDJ. E. GUU.LET. 1975. Studies of poly-

styrene in the reaion of the glass transition temperatureby inverse gas chromotography. Macromolecules 8(6):

882-888.-. AND -. 1976. Study of polymers by inverse

gas chromatOlfaphy. Adv. Polym. Sci. 21:107-145.CoNDER.J.R..ANDC.L. YOUNG. 1979. Physicochemical

measurement by gas chromatOlfaphy. John Wiley It

Sons, New York, NY.DEMDTZIS. P. G.. AND M. G. KONTOMJNAS. 1989. Ther-

modynamic study of _ter sorption and _ter vapor

diffusion in poly(vinylidene chloride) copolymers. Pages77-86 in D. R. Uoyd. T. C. Ward, and H. P. Scmiber.eds. Inverse gas chromatography. ACS Symposium Se-

ries No. 391, Ammcan Chemical Society. WashiDitoD.

DC.DoUJS. G. M., AND D. G. GRAY. 1980. Adsorption of

n-alkanes at zero surface coveraae on celluose paper and

wood fibers. J. Colloid Interface Sci. 77(2):353-362.-. AND -. 1981. Adsorption of hydrocarbons

on silica-supported water surfaces. J. Phy~. Chem. 85:3628.

HOH.D.N.-S..ANDW.Y.CHAO. 1993. Composites frombenzylated wood and polystyrenes: Their processabilityand viscoelastic properties. J. Appl. Polym. Sci. 50:7-II.

KAMD£N. D. P., AND B. RElDL. 1991. IOC characteriza-tion ofPMMA grafted onto CTMP fiber. J. Wood Cbem.

Technol. 11(1):57-91.KIsHI. H.. M. YOSHIOKA. A. YAMANOI. AND N. SHIaAISHl.

1988. Composites of wood and polypropylenes 1. Mo-

kuzai Gakkaishi 34(2):133-139.LlANG. B-H., MO1T. S. M. SHALD.. AND G. T. CANE8A.

1994. Propenies of transfer-molded wood-fiber/poly-st)Tene composites. Wood Fiber Sci. 26(3):382-389.

LIPATOV. Yu. S. 1967. Physical chemistry of filled poly-mers. International Polymer Science and TechnologyMonograph *2. Rubber and Plastics Research Associ-ation of Great Britain. Boston Spa. Wetherby, West

Yorks. England.

-. AND A. E. NfSTDOV. 1975. The influence ofthickness of polymeric stationary phase on its propenies

detennined by ps chromatography. Macromolecules

8(6):889-894.MALDAS. L.. AND B. V. KoKTA. 1991. Inftuenceofmaleic

anhydride as a coupling aaent on the performance ofwood fiber-polyStyrene composites. Polym. Eng. Sci.

31(18):1351-1357.-. -. AND C. DANEAULT. 1989. Inftuence of

coupliq 8,eDU and treatmenu on the mechanical prop-erties of cellulose fiber-polystyrene composites. J. Appl.

Polym. Sci. 37:751-775.MOHUN. U-B.. AND D. G. GRAY. 1974. Gas chroma-

topaphy on polymer surfaces: adsorption on cellulose.

J. Colloid Interface Sci. 47(3):747-754:ROWEU.. R. M.. H. SPELTD. R. A. AAoLA, P. DAV1S, T.

FRDDG. R. W. HDONOWAY. T. RIALS. D. LUNBE, R.

NARAYAN. J. SIMO~. AND D. WIUTL 1993. Op-ponunities for composites from recycled wasteWood-based resources: A problem analysis and retearch plan.

Forest Prod. J. 43(1):5S-63.SANADI. A. R.. R. A. YOUNG. C. CLEMONS. AND R. M.

R~ 1994. Recycled newspaper fibers as rein-fon:iD& fillers in thermoplastics: Pan 1- Analysis of ten-sile and impact propenies in polypropylene. J. Reinf.

Plastics Compos. 13:54-67.

SC1IREIBEa. H. P.. AND D. R. UDYD. 1989. Overview ofinverseaaschromatopaphy. Pqes 1-10 in D. R. Uoyd,T. C. Ward. and H. P. Schreiber. eds.Inverse ps cbro-matopapby. ACS Symposium Series No. 391. Ameri-

can Chemical Society. WashingtOn. DC.

SMlmaOD. 0.. AND J. E. GUn.1ET. 1969. Study of poly-mer-solute interactions by ps chromatOl1"8pby. Mac-

romolecules 2:272-275.

TAXASE. S.. AND N. StuaAISIU. 1989. Studies on com-posites from wood and polypropylene. II. J. Appl. Po-

Iym. Sci. 37:64S-659.

WOODHA.a, R. T.. G. THOMAS. AND D. K. RODOEaS. 1984.

Wood fibers as reinfon:ing filleR for polyoleftns. Polym.

En&o Sci. 24( I 5): 1166-1171.

YAM. K.. B. GoooI. C. LAI. AND S. SElXE. 1990. Com-posites from compoundina wood fibers with recycledhiIh density polyethylene. Polym. Eng. Sci. 30( II ):693-

699.

y OUNOQUIST. J. A. 1995. The marriaae of wood and

nonwood materials. Forest Prod. J. 45(10):25-30.