Clan Prod1lCt8 and ~ ~ (JOOO) 117-1~3 C Springer-Verlag 2.000

Polystyrene/wood composites and hydrophobic wood coatings fromwater-based hydrophilic-hydrophobic block copolymers

Marja-Leena Kosonen, 80 Wang, Gerard T. Caneba, Douglas J. Gardner, Tim G. Rials

Received: 13 May 1999 / Accepted: 5 February 2000

M. L. Kosonen, B. Wang, G. T. Caneba (8)Department of Chemical Engineering,Michigan Technological University, Houghton, MI 49931, USA

D. J. GardnerDepartment of Forest Management and Advanced EngineeredWood Composites Center, University of Maine, Orono,ME 04469, USA

Abstract The combination of synthetic thermoplasticpolymers and wood is normally problematic becausewood surfaces are hydrophilic while typical thermoplasticpolymers are hydrophobic. A possible solution is to useblock copolymer coupling agents. In this work we showthe use of a potentially useful synthetic method ofproducing hydrophilic-hydrophobic block copolymers ashydrophobic coatings and coupling agents in polystyrene!wood flour composites. In particular, wood veneers arecoated with water-based emulsions of hydrophilic-hydro-phobic block copolymers &om styrene and methacrylicacid. Dried coated surfaces are shown to become hydro-phobic through dynamic contact angle measurements.When wood flour is coated with the hydrophilic-hydro-phobic block copolymer based on styrene and acrylicacid, significant improvement in the ultimate tensileproperties of composites formed &om coated wood flour!polystyrene mixtures is realized. Since no volatile organiccompounds (VOCs) are used in coating wood surfacesand subsequent composite production, improvement inmechanical properties of thermoplastic/wood flourcomposites are shown to occur in environmentallyresponsible formulations.

IntroductionBeginning with plywood and moving through to orientedstrandboard, the combination of wood and syntheticpolymers has helped to maintain the competitive positionof the forest products industry while enhancing the utili-zation of our forests in terms of both scope and effi-ciency. Today's environmental and social concerns haveplaced even greater demands on both the forest resourceand the forest products industry; and once again, thecombination of wood and synthetic polymers may help to

T. G. RialsUSDA Forest Service, Southern Research Station,2500 Shreveport Highway, PineviI1e, LA 71360-2009, USA

ease that pressure. Synthetic plastics have becomeincreasingly prevalent in traditional wood productmarkets over the years; yet through the technologicaladvances responsible for this encroachment, new materialsystems and applications for wood/polymer materialshave been presented. Such an opportunity can be foundin the area of fiber-reinforced polymers (FRPs; Gerstle1987), where some of the more dramatic technologicaladvances have been made. The development of woodfiber-reinforced plastics continues to receive high priorityas a long-term goal in wood product research.

Iri part, this priority stems from the growing resourcepresent in the form of virgin fiber from low-value hard-woods and recycled fiber (Bentley 1989) from paper orpallets that is, in many respects, ideally suited forpolymer reinforcement. Characterized by its light weightand high strength, this natural fiber offers a number ofadvantages over currently used reinforcing fibers in termsof cost and weight, as well as processing considerations(Kokta 1988). Unfortunately, the consolidation of woodfibers with thermoplastic polymers also presents anumber of unique problems that must first be addressed.The most notable shortcoming of wood fiber/thermo-plastic polymer systems is the lack of adhesion betweenthe two components (Zadorecki and Flodin 1985; Seferisand de Ruvo 1986; Kokta 1988). Consequently, an inferiorfiber/matrix interface is established that prevents thenecessary stress transfer from the polymer matrix to theload-bearing fiber, reducing the function of the fiber tothat of a poor filler or extender.

Iri spite of the widely recognized importance of thefiber/matrix interface, physico-chemical factors thatdetermine its strength and quality remain poorly definedfor the wood/thermoplastic polymer system. Certainly,the question has been extensively addressed throughnumerous reports on the effect of various compatibilizersand coupling agents on the mechanical properties of thecomposite (Woodhams et al. 1984; Dalvaget al. 1985).While these studies have focused on the problem andshed considerable insight into potential solutions, theinterpretation of the results has been complicated by theuse of secondary properties to evaluate fiber/polymerinteractions, rather than the direct observation of interfa-cial performance or quality. In spite of the widely recog-nized importance of the fiber/matrix interface, thephysio-chemical factors that determine its strength andquality remain poorly defined for the wood-thermoplasticpolymer system. One unanswered question of particularsignificance to nonpolar polymer matrices, such as poly-

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ethylene and polystyrene, is the contribution of molecularentanglements to fiber/polymer compatibility and, subse-quently, interfacial strength and quality.

Rather than using a coupling agent as a component ofa mixture that also includes the wood fiber and thermo-plastic (polystyrene), we have directly placed the couplingagent (hydrophilic-hydrophobic block copolymer) on thewood surface before being blended with the polystyrene.This should result in minimal waste of the couplingagent, inasmuch as micelle formation of the block copo-lymer can be virtually eliminated. Since the block copo-lymer has surfactant properties, it can be dispersed in anaqueous medium without the use of volatile organiccompounds (VOCs). Upon drying of wood surfacesexposed to the aqueous emulsions of the hydrophilic-hydrophobic block copolymer, hydrophobic surfaces wereobtained based on contact angle measurements. Polysty-rene composites of treated wood flour further showedthat an improvement in tensile properties is obtainedover a similar composite of untreated wood flour.

Experimental

Preparation of hydrophilic-hydrophobic block copolymers

Materials used for polymerizationMethacrylic acid, acrylic acid, and styrene monomerswere purchased from Aldrich Chemicals, and theycontain the usual amount of inhibitors. They werepurified using a double distillation technique underreduced pressure. The initiator, 2,2 '-azobisisobutyroni-true (AIBN), was obtained from Eastman Kodak. It wasused without further purification.

Size exdusion chromatographyMolecular weights of the dried polymer samples from thereactor fluid were measured using a size exclusion chro-matograph (SEC). The chromatography system hasrefractive index and multi-angle light scattering detectors(Wyatt Technologies). Thus, measured molecular weightswere absolute, and there was no need to calibrate molec-ular weights after injecting a few samples. For the anal-

Preparation and testing of coated woodand polystyrene-wood composites

ysis of intennediate polystyrene samples, we used a silicacolumn (Supelco LC-l Supelcosil) with liquid chromato-graphy-grade toluene as the carrier fluid. For the analysisof the block copolymer samples, tertrahydrofuran (THF)was used as the carrier fluid and four styrene-divinylben-zene polymer columns were used as packing material.

Nudear magnetic resonanceHigh-resolution proton and 13C pulsed NMR spectros-copy of the copolymer materials was carried out using a220-MHz Varian XL-200 and Varian 400-MHz NMR spec-trometers. To minimize interference of proton signals forthe 13C spectra. proton noise decoupling was applied.

Block copolymer formationFormation of the block copolymer was done by polymer-izing styrene in ether through the so-called free-radicalretrograde-precipitation polymerization (FRRPP) process(Caneba 1992). After four times the initiator half-life, live

1 For example, it was reported in the April-May 1990 issue ofthe Chemical Reporter that the bulk price for acrylic acid is$O.64nb, while the bulk price for methacrylic acid is $0.95-1.04/lb.

polystyrene radicals are reacted with methacryiic acid toform the hydrophilic block. which is presumably arandom copolymer of styrene and methacrylic acid. Amixture of 1.5 m1 inhibitor-free styrene, 10 m1 ethyl etherand 0.0173 grams AIBN was charged into a thick-wallglass tube (10 mm in diameter), bubbled with nitrogen,and sealed with a Teflon cap (see Fig. I). A very smallmagnetic stirrer bar was used. The tube was placed in an80 °C water bath for about 10 hours and then removedand quenched in dry-ice for half an hour. A small quan-tity of sample was taken for conversion data. Undernitrogen protection, 3.0 grams methacryiic acid wasadded to the solution and immersed in the 80 °C bath for8 hours before cooling to room temperature. The finalproduct, which contains the polystyrene-block-poly(sty-rene-co-methacrylic acid) copolymer, was purified threetimes. SEC and NMR techniques were used to evaluate itsmolecular weight and segment ratio. The SEC analysiswas done at Dow Chemical's analytical laboratory usingTHF as the solvent. Proton and 13C NMR analyses weredone using a Broker 4OO-MHz instrument with THF-ds asthe solvent.

For larger-scale work, we generated a polystyrene-block-poly(styrene-co-acryiic acid) polymer. which wethink would offer better processability and economiccharacteristics compared to the polystyrene-block-poly(s-tyrene-co-methacrylic acid) system. Since acrylic acid hasa lower boiling point of 140-141 °c than that of metha-crylic acid at 163 °c, unreacted acrylic acid is less difficultto strip from the reactor fluid and polymer product(Dean 1985). Also, the glass-transition temperature ofpoly(acrylic acid) at 106 °c is more appropriate for proc-essing with polystyrene (glass transition at 100 °C). Thisis compared to the fact that poly(methacrylic acid) has aglass transition temperature reported at 228 °c (Brandrupand Immergut 1989). Finally, it should be noted thatacrylic acid as a bulk material costs less than methacrylicacid I.

MaterialsMaple, oak, and ash veneers with 6% moisture contentwere used for contact angle measurements. Maple flour(60-mesh Acer saccharum from American Wood Fibers)was used to form the wood flour/plastic composites.Applied coatings and coupling agents, polystyrene-block-poly(styrene-co-methacrylic acid) (or PS-P(MAA-S» andpolystyrene-block- poly(styrene-co-acrylic acid) (or PS-P(AA-S», were produced using the FRRPP process(Caneba 1992). Base polystyrene material used togenerate the composites was obtained from Dow Chem-ical (Styron 685D, with number average molecular weightof 270,000 Da). The PS-P(AA-S) copolymer used in the

M.L. Kosonen et aI.: Polystyrene/wood composites and hydrophobic wood coatings from water-based hydrophilic-hydrophobic block copolymers

ug

Fig. 1. Apparatus used in thesynthesis of the PS-P(MAA-S)block copolymer

preparation of composites was brought to a pH of 6.5with ammonia, in order to obtain a stable emulsion inwater.

Preparation of the compositesWater-based emulsions of the PS-P(AA-S) copolymerwere mixed with wood flour that was dried to a moisturecontent of 0.3 wt%. The mixtures were dried in a vacuumoven for 48 h at 70 °C to a moisture content of 1 wt%.The base polystyrene material was ground into powderwith the aid of liquid nitrogen. Then it was mixed withcoated and uncoated wood (using a 20-liter mechanicalmixer) at a 90/10 polystyrene/wood wt/wt ratio. Theresulting solid mixture was then fed into a Brabendertwin-screw extruder at 210 °C and 15 rpm, and thestrands were immediately pelletized. To prepare the testsamples, the pellets were flattened in a hot press at 200 °Cfor 5 min. The composite plate was cooled in a cold pressfor another 10 min to a thickness of 1.5-3.0 mIn. Fromthe resulting composite plate, tensile bars (with a gagelength of approximately 114 mm and width of approxi-mately 13 mm) were cut and allowed to stand for at least8 h before stress-strain testing.

Aside from using uncoated wood to form the refer-ence composite, we also prepared tensile bars from thebase polystyrene using the same thermal history as thatof the composites.

Testing of compositesTensile testing measurements were done with a universaltesting machine from SATEC Systems (capacity 120 cs -120,000 Ib). A strain rate of 0.2 in/min (0.085 mm/s) was

Contad angle measurementsDynamic contact angle analysis of coated and uncoated(blank) veneer samples was performed using thefollowing procedure:1. Veneers were cut into 2-cm x 2-cm pieces.2. Cut veneer samples were sanded to reveal fresh wood

surfaces.3. Exposed surfaces were coated with four different solu-

tions (0.02, 0.1, and 0.2 wt% ammonia-water solutionsat pH = 10 of the polystyrene-block-poly(styrene-co-methacrylic acid), and 0.2 wt% of ammonia-watersolution at pH = 6 of polystyrene-block-poly(styrene-co-methacrylic acid) with 3 dip-dry cycles. Drying wasdone in air at room temperature.

4. After final air-drying, samples were allowed to air-dryfor 10 more hours and kept in a desiccator.

5. Dynamic contact angle analyses of the coated sampleswere performed with a Cahn dynamic contact angleanalyzer (model DCA-322), which is based on theWilhelmy plate technique. High-performance liquidchromatography-grade water was used as the probeliquid, and the samples were immersed at a rate of194 mls to a depth of 15 rom.

6. Blank samples were analyzed after they were sanded.

Oean Products and ProceIIeI2 (JOOO)

used in testing the samples. Stresses were obtained fromthe measured force divided by the cross-sectional area ofthe samples. Strain was based on change in length rela-tive to the sample gage length (approximately 114 rom).

Table 1. Molecular weight data for the intermediate polystyreneand polystyrene-block-poly(styrene-co-methacrylic acid) copo-lymer

Purifiedcopolymerproduct

Rawcopolymerproduct

PSintermediate

Number-average 10200molecular weight,g/molPolydispersity index 1.74

34500 228000

8.02 1.83

Table 2. Result of study of solubilities of the polystyrene-block-poiy(styrene-co-methacrylic acid) copolymer and its compo-nents (+ means totally soluble; - means insoluble)

PS Block productPMAASolvent

++...+

-...+~...

+

Partially +

Results: block copolymer fonnationFigure 2 shows a refractive-index signal from a size exclu-sion chromatography system for the polystyrene material(PS intermediate) that was taken from the reactor justbefore the addition of methacrylic acid, as well as thoseof raw and purified copolymers (see Table 1 for thequantitative molecular weight results). The purified copo-lymer product is a polystyrene-block-poly(styrene-co-methacrylic acid) material, because the PS intermediatewas at 60% conversion only. This means that unreactedstyrene was present when methacrylic acid was addedinto the reactor.

The refractive index signal from the raw product(Fig. 2) clearly shows another peak of higher molecularweight (compared to that of the PS-intermediate peak) THF

T Ith b .b d th . . f th 0 ueneat can e attri ute to e contInuation 0 e propaga- Water

tion of live radicals. In the purified product, the PS-inter- CH Clmediate peak totally disappeared. The purification of the Meili~olraw product was done with an investigation of the solu-bility of this material to various solvents. Table 2 showsthat THF is a solvent for the raw and purified products.

With THF as the solvent for the raw copolymerproduct, we added water to precipitate homopolystyrenecontaminant. The supernatant was dried and redissolvedin THF. The addition of CH2Cl2 precipitated poly(metha-crylic acid). Gravimetric analyses of solid materials invarious solutions and precipitants gave us the followingcompositions of the raw product: 78 wt% polystyrene-block-poly( styrene-co- methacrylic acid) copolymer,17 wt% polystyrene homopolymer, and 5 wt% poly(meth-acrylic acid) homopolymer. These gravimetric resultstranslate to a 1 :2.5 styrene-to-methacrylic acid weightratio. Our NMR work gave a ratio of 1: 2.4.

When the hydrophilic-hydrophobic block copolymerwas placed in an ammonia-water solution of pH = 10, wewere able to disperse it at the 0.2-wt% level. Subse-

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quently. a slice of maple veneer was dipped into thiscopolymer solution and then air-dried. After a givennumber of dip-dry cycles. scanning electron micrographsof the coated surfaces were obtained. After the first dip-dry cycle. the wood surface seemed to change from awetting to more of a nonwetting behavior. Micrographsof the dried surfaces of the wood showed that good coat-ings were obtained even after 10 dip-dry cycles. Contin-uous coats were applied onto individual fibers after4 dip-dry cycles (Fig. 3). After 10 dip-dry cycles. there isvery little indication of the polymer filling up spacesbetween wood fibers (Fig. 4).

Advancing contact angle values of water (shown inTable 3) indicate that dip-dry cycles from ammonia solu-tions of polystyrene-block-poly(styrene-co-methacrylicacid) at pH = 6 and pH = 10 resulted in hydrophobicwood surfaces. i.e.. the contact angles increased to more

I than 90°. The table also indicates that the system fromthe pH = 10 solution results in the same degree of hydro-phobicity of the coated wood surfaces as the system fromthe pH=6 solution.

Using a similar synthetic procedure. we produced thePS-P(AA-S) material to coat wood flour and generatedpolystyrene/wood flour composites from it. Figure 5shows the plot of yield and ultimate stresses for the 90/10polystyrene/wood flour wtlwt composites at O. 15. and3 wt% PS-P(AA-S) loadings on wood. For the base poly-styrene used. we obtained a mean yield stress of 32 MPaand a standard deviation of 6 MPa. The ultimate stress

8 was at 32 MPa with a standard deviation of 3 MPa.Figure 6 shows a plot of the yield and ultimate strains forthe 90/10 polystyrene/wood flour wtlwt composites at O.1.5. and 3% PS-P(AA-S) loadings on wood. The meanvalue of the yield strain for the base polystyrene was2.1% with a standard deviation of 0.4%. The mean value

Fig. 2. Refractive-index (RI) signal from a size exclusion chro-matography system for the polystyrene material that was takenfrom the reactor just before addition of methacrylic acid (PSintermediate), as well as RI from raw and purified copolymers

MoL. Kosonen et aI.: Polystyrmelwood composites and hydrophobic wood coatings from water-baled hydrophilic-hydrophobic block COpolymen

Table 3. Dynamic contact angle measurements (advancingangle) of water on various uncoated surfaces (blank) andsurfaces coated with ammonia-water solutions of polystyrene-block-poly(styrene-co-methacrylic acid) (PS-P(MAA-S» atpH=10 and pH=6. Values are averages with standard devia-tions in parentheses

Coating Maple Cherry Ash

Uncoated or Blank0.02 wt% at pH = 100.1 wt% at pH=IO0.2wt% at pH=IO0.2 wt% at pH = 6

84.092.692.493.491.1

87.795.294.094.095.9

82.592.092.791.990.0

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of its ultimate strain was 2.4% with a standard deviationof 0.9%.

Discussion of resultsWe have demonstrated the possibility of forming blockcopolymers when we added methacrylic acid to thestyrene reactor system in ether after most of the initiatormolecules had decomposed into radicals. From Fig. 2, theblock copolymer fomtation is apparently not efficientenough. In reality, the data for the raw product will havea bigger homopolystyrene peak at 10,200 g/mol becausethe block copolymer molecular weight is not as high as itlooks. In fact, from the methacrylic acid-to-styrene ratioadded of 2.0, intemtediate polystyrene conversion of 60%,and second-stage conversion of 80%, the result is a blockcopolymer molecular weight of about 32,640 g/mol. Thisis lower than the value for the raw product (34,500 g/mol) and the purified product (228,000 g/mol) as shownin Table 1. One should realize that molecular weightmeasurements for the block copolymer are not accurate

Fig. 3. Scanning electron micrographs of wood surfaces with(a) and without (b) coating from the PS-P(MAA-S) emulsion.The coated sample (a) was obtained from 4 dip-dry cycles.Magnification is 480 X

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00 1.5 3

PS-P(AA-S) on Wood, Wt. %

Fig. S. Yield and ultimate stresses of 90/10 polystyrene/woodwtiwt composites at various PS-P(AA-S) loadings on the woodsurface. Values can be compared with yield and ultimatestresses for the base polystyrene at 32 i: 6 and 32 i: 3 MPa,respeCtively

Fig. 4. Scanning electron micro~phs of coated wood samplesfrom 10 dip-drr cycles. Coating was done by cycles of dippingthe wood into the PS-P(MAA-S) emulsion, and then air-dryingfor several minutes. Magnification is 480 x

(0.8)(0.8)(1.6)(2.1)(2.1)

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PS-P(AA-S) on Wood. Wt.%

Fig. 6. Yield and ultimate strains of 90/10 polystyrene/woodwt/wt composites at various PS-P(AA-S) loadings on the woodsurface. Values can be compared with yield and ultimate strainsfor the base polystyrene at 2.1 % 0.4 and 2.4:t 0.9%, respectively

considering that molecular weights were measured usingpolystyrene standards. Since the raw product has lost agreat proportion (in mole numbers) of relatively short-chain molecules, it is no surprise that the purifiedproduct will have a higher average molecular weight thanthe stoichiometric value. Also, under the conditions usedfor block copolymerization, live polymer radicals in theinterior of the particles would not be completely acces-sible to the monomers. This could explain why there isstill some (insubstantial) amount of homopolystyrenecontaminating the block copolymer in the raw product.On the other hand. short chain live radicals could also bepresent before the addition of the second monomer(methacrylic acid). This explains the small poly(meth-acrylic acid)-rich material extracted from the rawproduct. There is also the possibility that the methacrylicacid segments of the block copolymer could segregate inthe size exclusion chromatography columns because THFis not a good solvent for poly(methacrylic acid). Thispossibility has been well documented by researchers whohave done molecular weight measurements of low molec-ular weight carboxy-terminated polystyrene with knownmolecular weight distributions (Hirao et al. 1993). Finally,it was cited that a polymer with an ionizable group(-COOH in our case) could show a higher hydrodynamicvolume for the same molecular weight. due to electro-static repulsion, if the carrier fluid has a relatively lowionic strength, such as the THF used in our analysis(Barth 1986). Thus, the molecular weight of the copo-lymer seems to be higher than it really is.

Wood veneers of maple. cherry, and ash were coatedwith PS-P(MAA-S) from ammonia-water at differentconcentrations and with pH=6 and pH=10. In Table 3 itcan be seen that the hydrophobicity of the veneer surfacewas increased after applying the polystyrene-block-poly(s-tyrene-co-methacrylic acid) material. Without the coating,

2.5

2

1.5

1

0.5

contact angles for all the species were between 82 and88°, whereas after coating they were between 90 and 95°.It is also recognizable that contact angles did not consid-erably increase with the concentration (amount) of Ps-P(MAA-S). Therefore, a good hydrophobic surface isobtained with low concentrations (0.02 wt%) of PS-P(MAA-S) at pH = 10.

In Fig. 5, yield-stress values of the composites areslightly lower than that of the base polystyrene at 32:t: 6MPa, whereby a slight improvement was observed for thecoated composite with 3 wt% copolymer on wood. On theother hand, definite improvements in ultimate stresseswere obtained from the coated composites compared tothe uncoated one. In fact, the composite with 1.5-wt%coating on wood exhibited a mean ultimate stress of33 :t: 4 MPa compared to that of the base polystyrene at32 :t: 3 MPa. For the composite coated with 3-wt% copo-lymer on wood, the ultimate stress was at 31:t: 4 MPa. Allthese above-mentioned data indicate good bondingbetween the wood and base polystyrene for the coatedwood/polystyrene composites. One particular note ofinterest is the higher values of the ultimate strains for thecoated wood-based composites at ultimate-stress valuessimilar to that of the base polystyrene. This indicates thatthese composites should exhibit better impact propertiesthan composites made from uncoated wood flour. As faras the mechanism of failure is concerned, the above dataseem to indicate substantial cohesive failure in the coatedwood-based composites. The exact sequence of events isstill subject to debate, and it can only be resolved by amore detailed study of the fracture surfaces.

The significant improvement in composite propertiesusing dilute emulsions of the coupling agent in watercould be attributed to the efficiency of the application ofthe block copolymer. To promote adhesion between thehydrophilic wood surface and the hydrophobic polysty-rene material, the coupling agent should be placed at theinterface. The effectiveness of this placement of blockcopolymer coupling agents is always in question if thewood, thermoplastic, and coupling agent are mixed in ablending machine. Another factor that contributes to theefficiency of the use of the coupling agent in this study.isthat it is introduced within a liquid phase. This almostguarantees uniform dispersion of the block copolymer.Normally, one can always use organic solvents as thecarrier liquid. However, in practical applications this isnot only uneconomical but it is a pollution hazard. Withthe use of water as the carrier fluid for the hydrophilic-hydrophobic block copolymer as hydrophobic coatingand composite coupling agent, we have achieved a signif-icant improvement in material performance in an envi-ronmentally responsible and economical manner.

Inefficiencies in the production of the hydrophilic-hydrophobic block copolymers that we have observedshould be surmountable enough for practical implemen-tation, considering that effective coupling between thewood and polystyrene is obtained at less than 1.5 wt% onwood. We also found that removal of homopolymercontamination (polystyrene) is quite convenient throughheating of the raw product in ammonia water. Thehomopolymer contamination separates out as a coagu-

MoL a:-- et II.: Poiystyrene/wood composites and hydrophobic wood coatings from water-based hydrophilic-hydrophobic block copolymers

123

lant. As far as the cost of the synthesis method, all wecan say is that it is the least costly of all the block copo-lymer formation methods around It is also becomingincreasingly efficient as we discover new solvents andprocess conditions. It is likely that process conditions willbe discovered that can easily be implemented in a closed-loop solvent use and recycle operation.

ConclusionsWe have shown that emulsions of neutralized polysty-rene-block-poly(styrene-co-methacryiic acid) can be usedto form hydrophobic coatings on wood surfaces. Throughcontact angle measurements, the emulsion was found toform a more hydrophobic surface on wood When diluteemulsions of polystyrene-block-poly(styrene-co-acrylicacid) were used to coat wood flour surfaces, compositesof polystyrene with coated wood flour exhibited slightlybetter yield stresses than composites from uncoated woodflour. Improvements that are even more significant wereobtained in ultimate stresses and yield and in ultimatestrains for coated-wood/polystyrene compositescompared to their uncoated counterparts. In fact, ulti-mate stresses and strains of the coated wood/polystyrenecomposites have been found to be similar to those of thebase polystyrene. All these indicate that the use of hydro-philic-hydrophobic block copolymers on wood surfacescan result in improvement to the mechanical propertiesof wood-thermoplastic composites in a manner that isboth economical and environmentally responsible.

Acknowledgements This work was partially sponsored by theNational Science (CTS-94041S6) and the USDA Forest Service(Agreement SRS 33-CA-98-427). We would also like toacknowledge the assistance of Dr. Stephen Hahn of the DowChemical Analytical Group for doing size exclusion chromato-graphy work on our intermediate polystyrene and polystyrene-block-poly(styrene-co-methacrylic acid) copolymer.

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