Reactively Compatibilized Cellulosic Polylactide Microcomposites

Reactively Compatibilized Cellulosic PolylactideMicrocomposites

Birgit Braun,1 John R. Dorgan,1,3 and Daniel M. Knauss2

Poly(lactic acid) (PLA) possesses a suite of favorable material properties that are enabling itspenetration into diverse markets (e.g., as packaging material or textile fibers). In order toincrease the range of applications for this material, it is necessary to modify its properties and

for certain applications, reduce its cost. The introduction of fibers into a polymeric matrix isan established route towards property enhancement provided good dispersion and intimateinterfacial adhesion can be achieved. In addition, cellulosic microfibers are obtainable at low

to moderate cost. In this study, reactive compatibilization of cellulosic fibers with PLA ispursued. Hydroxyl groups available on the surface of cellulosic fibers are used to initiatelactide polymerization. Various processing strategies are investigated: (1) blending preformedPLA with the fiber material, (2) through a one-step process in which lactide is polymerized in

the presence of the fibers alone, or (3) reactive compatibilization in the presence of preformedhigh molecular weight polymer. The results show that materials prepared by simultaneousintroduction of lactide and preformed high molecular PLA at the beginning of the reaction

possess superior mechanical properties compared to composites made by either purelymechanical mixing or solely polymerization of lactide in the presence of fibers. The modulus ofmaterials containing 25% fibers which are prepared by reactive compatibilization of 30%preformed PLA and 70% lactide (30/70 P/L) improves by 53% compared to the homopolymer,whereas 36% reinforcement can be achieved upon purely mechanical mixing. A further in-crease to 35% fiber loading leads to a reduction in modulus due to an excess in initiatinggroups. The same trend was observed in systems containing 65% preformed PLA and 35%lactide (65/35 P/L) with an overall achievable reinforcement that was slightly lower.

KEY WORDS: Reactive compatibilization; PLA microcomposites; cellulose fibers.

INTRODUCTION

Polylactide or poly(lactic acid) (PLA) is analiphatic polyester derived from 100% renewableresources. It is also ultimately degradable undercomposting conditions, so that all of it eventually is

recycled as it decomposes back to carbon dioxide inthe atmosphere (where the carbon originated). Theadvantages of these polyesters are numerous andinclude: (1) improved Homeland Security throughreduction of petroleum dependence through the useof renewable resources and significant energy sav-ings, (2) environmental protection because of thefixation of gas carbon dioxide and the reduction ofmunicipal landfill volumes, and (3) rural economicgrowth as the market for corn is expanded. Besidespossessing a unique combination of energy, envi-ronmental, and sustainability advantages, PLA is

1 Department for Chemical Engineering, Colorado School of

Mines, Golden, CO 80401, USA.2 Department for Chemistry and Geochemistry, Colorado School

of Mines, Golden, CO 80401, USA.3 To whom all correspondence should be addressed. E-mail:

[email protected]

Journal of Polymers and the Environment, Vol. 14, No. 1, January 2006 (� 2006)DOI: 10.1007/s10924-005-8706-y

491566-2543/06/0100-0049/0 � 2006 Springer Science+Business Media, Inc.

also of economical interest. Commercially, PLA ispolymerized from lactide; the latter is producedfrom corn sugars or cheese whey by fermentationfollowed by reactive distillation. Examples ofcompanies producing PLA on a large scale includeNatureWorks (former Cargill Dow Polymers) andat least two companies in Japan (Shimadzu andMitsui) as well as Hycail in the Netherlands. PLAnow has obvious commercial significance and isbound to be the most successful biomass basedindustrial product launched to date. However, forthis wide scale replacement of fossil based plasticsby the greener PLA to occur, concerted researchand development work on PLA property modifica-tion is needed.

The major advantage of PLA is certainly itsunique combination of economic, environmental,and sustainability benefits. Vink et al. performed acomprehensive Life Cycle Inventory (LCI) for PLAusing methodology enabling ‘‘apples-to-apples’’comparisons with petrochemical-based thermoplas-tics [1]. Among the most notable benefits of PLAare reductions in both fossil fuel use and globalwarming potential. For example, compared to PETand Nylon, PLA uses 30–50% less fossil resourcesthat results in 50–70% less CO2 emissions. Less car-bon dioxide is released into the atmosphere in theproduction of PLA polymers than in the productionof most traditional hydrocarbon based polymers.The carbon in PLA recycles in the earth’s carboncycle, much in the same manner as forest plants.Reduced fossil energy input is needed because thestructural material in PLA comes from renewablematerials captured from photosynthesis. Conven-tional thermoplastic polymers rely solely on oil re-serves for their monomer source. The study by Vinket al. also showed that PLA has the lowest non-renewable energy content compared to a variety ofcommon plastic materials [1].

PLA has sufficient physical properties for avariety of applications (primarily textile fibers andthermoformed plastic trays as well as some filmapplications), but it is a relatively brittle plastic withpoor impact properties. This precludes its use inmany applications. Another drawback of these bio-plastics for lower end applications is that they arerelatively expensive, selling for $1.00–$1.50 perpound depending on the order volume. In compari-son, a good price for a chemically modified starchor for microcomposite wood fibers is around$0.10–$0.60 per pound. The present undertakingdemonstrates a novel approach to reactively

compatibalize cellulose fibers with PLA in order toproduce materials that have improved physical andcost properties and can be sold in large volumesinto a variety of applications.

MATERIALS AND METHODS

L-lactide and preformed polylactide (PLA Poly-mer 2000D of a molecular weight of about 108,000 g/mol as determined by dilute solution viscosity mea-surement) used in this study were purchased fromCargill Dow Polymers, now NatureWorks, (Minne-tonka, MN) and used without further purification.The purity of the lactide was estimated to be96.5–97 mol% using differential scanning calorimetry(does not account for D-lactide impurities) [2].

Prior to use, lactide was dried under vacuum(22 inch Hg) at 50�C for at least 8 h. PLA wasdried under vacuum (25 inch Hg) at 80�C for atleast 14 h before being processed.

The ring-opening polymerization of lactide wascatalyzed by stannous octoate, Sn(Oct)2. It wasobtained from Sigma Aldrich and used as received.The molar ratio, R, of lactide to stannous octoatefor all reactions was R=2500. The co-catalyst tri-phenylphospine, P(/)3, has a beneficial effect on thepolymerization kinetics of L-lactide in reactions withSn(Oct)2 as catalyst [3]. P(/)3 was purchased fromSigma Aldrich and added without further purifica-tion in an equimolar amount to Sn(Oct)2. Tita-nium(IV)isopropoxide (TIP) is a catalyst fortransesterification reactions [4], and was used astransesterificiation agent in samples containing pre-formed polymer (at a level of 0.1 wt% of PLA). Forhandling purposes, solutions of the catalyst and co-catalyst as well as the transesterificiation agent wereprepared in dry, distilled toluene. The catalyst wasdeactivated using poly(acrylic acid), PAA, of amolecular weight of 2000 g/mol, (at a level of0.25 wt% of lactide) purchased from Sigma Aldrichand dissolved in dioxane for transfer purposes [5].

The cellulose fibers used in this study (CreaTechTC 2500) were supplied by CreaFill Fibers Corp(Chestertown, MD). These fibers have an averagelength of 900 lm and an average width of 20 lm(L:D ratio=45). In order to remove surface impuri-ties and increase surface reproducibility the fiberswere pretreated according to the following proce-dure: (1) 10 g fibers were suspended in 500 ml aque-ous solution of 8 wt% sodium hydroxide (NaOH),(2) the solution was placed in a sonicator for 6 h atslightly elevated temperature (about 37�C), (3) the

50 Braun, Dorgan, and Knauss

fibers were then filtered and washed with distilledwater until neutrality, (4) the washed and filtered fi-bers were dried in a convection oven at 70�C for24 h, (5) before being used in the preparation ofcomposites, the fibers were again dried under vac-uum (25 inch Hg) at 80�C for about 8 h. Repeatedwashing with distilled water was found to benecessary in order to remove any degradation prod-ucts of cellulose during the base treatment, sincethese by-products would cause the treated fibers tostick together and yellow upon drying. Figure 1shows the fibers before and after the pretreatment.

Samples were mixed and polymerized in aHaake Rheomix 3000. Polymerizations wereperformed at 200�C for 20 min at 100 rpm. Aftermelting and premixing lactide with the preformedPLA, the fibers were fed into the mixer. Once areaction temperature of 200�C was reached, therequired amount of solution containing the catalystand co-catalyst was added. At the end of the reac-tion time, the required amount of PAA solutionwas added and mixed with the material for at leasta minute prior to extraction from the mixer.

After the preparation the material was storedin a freezer for at least 3 days prior to grinding in aForemost 2A-4 grinder to a maximum grain size ofabout 5 mm with the vast majority of the pelletsbeing about 2–3 mm in diameter. Samples for test-ing were prepared by a combination of vacuum andcompression molding. The material was first meltedunder vacuum (about 25 inch Hg) at 190�C untilthe amount of gas released decreased significantly.Afterwards the material was molded in a compres-sion molding machine at 180�C for about 5 min un-der a load of 5,000 psi before being quenchedbetween water cooled plates.

Samples prepared in the manner describedabove were analyzed for glass transition tempera-ture (Tg) and amount of crystallinity in a PerkinElmer DSC-7. The machine was calibrated againstan Indium standard twice, and a baseline

established on a daily basis. The DSC testing proto-col was as follows: (1) heat from 5 to 200�C at10�C/min, (2) hold at 200�C for 5 min, (3) coolfrom 200 to 5�C at 5�C/min, (4) heat from 5 to200�C at 10�C/min. Tg and amount of crystallinitywere determined from data obtained on the secondheating cycle by inflection point method.

Mechanical properties were determined byDynamic Mechanical Thermal Analysis (DTMA)using an ARES Rheometer with torsional rectangu-lar fixtures. Before testing, the machine wascalibrated for normal force as well as torque. Testconditions were 0.1% strain and 1 Hz. The thermalscanning was performed as follows: (1) heat from30 to 110�C at 20�C/min, (2) hold for 5 min at110�C, (3) cool from 110 to 30�C at 20�C/min, (4)heat from 30 to 110�C at 10�C/min (end condition:30�C). Moduli were determined from data on thefirst heating run.

The molecular weight of materials withoutfillers was determined by dilute solution viscositymeasurements. Solutions of the material were pre-pared in tetrahydrofuran (THF) at concentrationsof 0.005 g/ml and the viscosity of the polymer solu-tion measured in an Ubbelohde viscometer at 30�C.The Schultz–Blaschke relationship allows a single-point determination of the intrinsic viscosity [g]from viscosity measurements of the pure solventand the sample solution, given the Schultz–Blaschkecoefficient kSB is known for the given polymer–solvent system at a constant temperature.

½g� ¼gsp

c�ð1þ kSB�gspÞð1Þ

Equation 1 shows the Schultz–Blaschke equation,where c is the solution concentration in g/ml, kSBthe Schultz–Blaschke coefficient, and gsp is thespecific viscosity defined by

gsp ¼gg0� 1 ¼ t

t0� 1 ð2Þ

Fig. 1. SEM micrograph showing the untreated fibers (left) and the fibers after alkali treatment (right).

51Reactively Compatibilized Cellulosic Polylactide Microcomposites

where t and t0 is the time necessary for the solutionand the solvent, respectively, to pass through a de-fined part of the viscometer. The Schultz–Blaschkecoefficient for PLA in THF at 30�C was previouslydetermined by Dorgan et al. [6] and found to be0.298 ± 0.005.

Knowing the intrinsic viscosity, the viscosityaverage molecular weight can be calculated by rear-ranging the Mark–Houwink equation

MV ¼½g�K

� �1a

ð3Þ

The parameters in this equation, K and a, are alsospecific for the polymer–solvent system at a giventemperature.

The Stockmayer–Fixman equation allows thecalculation of the viscosity-average molecularweight and appears as

½g�M

1=2

V

¼ KH þ b�M1=2

V ð4Þ

The factor KH (Mark–Houwink constant for theta-conditions) is an experimental quantity often usedto quantify unperturbed polymer chain dimensionsin dilute solutions. Values reported in the literatureshowed a large variability until recently, whenDorgan et al. performed careful experiments inorder to determine this constant [6]. KH was foundto be 0.107 ± 0.022 mL/g.

RESULTS AND DISCUSSION

Previous studies show successful improvementsin mechanical properties by the formation ofmicrocomposites through purely physical mixing ofnatural fibers into PLA compared to the base homo-polymer. Oksman et al. [7] embedded flax fibers intoa PLA matrix and compared the resulting compositeproperties to polypropylene (PP) filled with the samefibers. It was found that the mechanical properties ofthe flax-PLA composites are promising, since thecomposite strength was about 50% better comparedto similar flax-PP composites which are used in manyindustrial applications. However, microscopy studiessuggested a lack of interfacial adhesion between thepolymer matrix and the fiber surface. Huda et al. [8,9] showed improvement of the tensile strength, tensilemodulus and impact strength upon reinforcing PLAwith cellulose fibers. However, the introduction ofcellulose fibers did not affect the glass transitiontemperature significantly as measured by DSC.

The quality of composites are influenced byvarious factors, namely (1) the fiber aspect ratio, (2)the fiber orientation, (3) the fiber volume fraction,(4) the dispersion of fibers in the polymeric matrix,and (5) the interfacial adhesion between the fibersurface and the surrounding matrix [10]. Especially,the interaction between the filler surface and thepolymer matrix is a critical factor for the reinforce-ment potential of the filler. In general, macroscopicreinforcing elements always contain imperfections.Structural perfection is greater as the filler becomessmaller and smaller. However, the competing effectof increasing specific surface area with decreasingsize has to be considered [11].

Considering the fact that cellulose fibers arehydrophilic in nature suggests a lack of dispersionand interfacial adhesion when incorporated in ahydrophobic polymer matrix. In order to improvethese factors, reactive compatibilization is attractive.

Commercially, PLA is polymerized from lactidewhich is produced from corn sugars by fermentationfollowed by reactive distillation. Various catalystscan be employed in the polymerization reaction.However, the most common catalyst used is the tin-compound stannous octoate Sn(Oct)2. This catalystrequires hydroxyl groups as initiators; however,there is always a competition between natural initia-tors (residual water, alcohols) and added initiators(compounds added to the system containing hydro-xyl groups in order to initiate the reaction).

Various examples in the literature [12–14]show that the addition of multifunctional initiatorsallows the polymerization of PLA into comb-,star- and hyperbranched structures. Figure 2depicts the use of a multifunctional initiator tocreate a branched structure. The chemical struc-ture of cellulose, as shown in Fig. 3, exhibits 6hydroxyl groups per repeat unit, and therefore thesurface of a cellulose fiber also contains a signifi-cant number of OH-groups which are availablefor initiating the polymerization reaction of PLA.When these surface groups are employed as initia-tors for the polymerization, there are two majorcompeting factors which have to be considered:(1) in polymerization reactions, a high conversionrate of monomer is desired and therefore a highnumber of initiating groups is advantageous, (2)to obtain good mechanical properties, a highmolecular weight of the polymer is required. Highmolecular weight implies a low number ofpolymer chains, corresponding to a low numberof initiating groups. These competing effects


necessitate investigations into novel strategies forreactive compatibilization.

The simultaneous introduction of lactide andhigh molecular-weight PLA is able to address theabove competing effects. The lactide polymerizationis initiated from by the OH-groups on the fibersurface. The presence of preformed high molecular-weight PLA with transesterificiation agent (TIP)ensures a higher average molecular weight of PLAchains grafted to the fiber surface. The resultingmaterial consists of high-molecular weight PLAchains grafted to the fiber surface in a PLA matrix.The fibers therefore show improved interfacialadhesion and better dispersion properties.

Post-reaction deactivation of the catalystensures a controlled termination of the reaction andavoids further molecular weight changes during

molding and characterization measurements. Theeffectiveness of PAA was tested by taking samplesduring PLA synthesis and measuring molecularweight as a function of time. As shown in Fig. 4,PAA is very effective in deactivating Sn(Oct)2 com-pared to a control sample. There is no furtherincrease in molecular weight after the addition ofPAA to the system, while the final molecular weightwithout PAA addition is not reached within anhour under the present experimental conditions.

The weight average molecular weight of sam-ples which are completely polymerized in the Haakereaches about 77,600 g/mol, whereas the preformedmaterial is of a molecular weight of 108,100 g/molafter processing for 20 minutes at 200�C in thepresence of TIP. As expected, the introduction ofpreformed polymer to the PLA synthesis leads to

Fig. 2. Example for a multifunctional initiator and the resulting polymer structure.

Fig. 3. Molecular structure of cellulose, exhibiting 6 hydroxyl groups available for initiating PLA polymerization.


an increase in average molecular weight as depictedin Fig. 5.

The glass transition temperature (Tg) of sam-ples containing no cellulose fibers and polymerizedfrom L-lactide corresponds to a previous study byDorgan et al. [15] which evaluated the influence ofproportions of L- and D-lactide on the Tg. The Fox-Flory plot for PLA shown in Fig. 6 demonstratesthat the glass transition temperature increases withincreasing molecular weight before reaching a pla-teau at about 59�C at a molecular weight of about50,000 g/mol. The materials prepared in this study

reach a molecular weight within the range of con-stant Tg for 100% L-lactide.

The introduction of micro-sized cellulose fibersinto a matrix of preformed PLA does not signifi-cantly affect the glass transition temperature as mea-sured by DSC; this is displayed in Fig. 7. This hasalready been observed in a previous study by Hudaet al. [8, 9]. If no preformed PLA is present and thematerial is completely polymerized in the presence ofcellulose fibers, the glass transition temperature de-creases with increasing fiber loading level as shownin Fig. 7. According to the Fox-Flory equation

Fig. 4. Molecular weight development as a function of reaction time with and without addition of PAA (Experimental conditions: 180�C,addition of PAA after 25 min reaction time).

Fig. 5. The introduction of preformed polymer increases the molecular weight (Experimental conditions: 200�C, 20 min reaction time).


(Equation 5), a decrease in the glass transition tem-perature is an indication of lower molecular weight.

Tg Mn

� �¼ A� B

Mn

ð5Þ

A and B in the above equation are constants,with

A ¼ limMn!1 TgðMnÞ ð6Þ

For 100% L-lactide PLA, values were determined tobe: A=60.2�C, B=)71.1�C g/mol [15]. Therefore, itis reasonable to conclude that with increasing fiber

loading level, lower molecular weight PLA is pro-duced due to an excess of initiating groups.

Achievable crystallinity was also determined byDSC and is shown versus the fiber loading level inFig. 8. For samples containing only preformed PLA(100/0 P/L) no variation of the achievable crystal-linity was observed, although an increase would beexpected if the fibers can serve as nucleation agent.The reason for this observation is the presence oftalc in the preformed commercial material thatalready serves as nucleation agent. Samplescompletely polymerized from lactide (0/100 P/L)show a significantly lower percentage of achievable

Fig. 6. Tg versus molecular weight for samples with various composition of L- and D-lactide [15].

Fig. 7. Tg versus fiber loading level for samples containing only preformed PLA (100/0 P/L) and for samples completely polymerized in

the presence of cellulose fibers (0/100 P/L).


crystallinity for unfilled systems. As the fiber load-ing level increases, the achievable crystallinity in-creases for 0/100 P/L samples because the fibers canserve as nucleating agent.

The measured values of the storage moduli G¢are summarized in Table I and plots of the resultsare shown in Fig. 9. The modulus increases when

preformed PLA is physically mixed with cellulosefibers (Fig. 9a); these results are consistent with theprevious study performed by Huda et al. [8,9]. Inthe system containing 35% lactide at the beginningof the reaction the modulus increases with the fiberloading level up to 25% by weight of the polymermass; this is followed by a decrease in modulus fora further increase in fiber loading to 35% (Fig. 9b).This results indicates a drop in molecular weight atthe highest fiber loading level due to excess of initi-ating groups. However, the highest achievable mod-ulus in this system exceeds G¢ achievable by purelymechanical mixing – there is a 43% increase with35% fibers by physical mixing but a 50% increasewith 25% fibers by reactive compatibilization. Forsystems containing 70% lactide at the beginning ofthe reaction (Fig. 9c), a similar trend was observed;an initial increase in G¢ with fiber loading level wasfollowed by a decrease beyond 25% fibers due to anexcess amount of initiating groups resulting in lowmolecular weight PLA. The reinforcement achiev-able in this system is better than in systems onlyrelying on physical mixing or systems containing35% lactide at the start of the reaction.

If the polymer is completely polymerized in thepresence of the fibers, the modulus slightly increasesfor a fiber loading level of 15% followed by a sharpdecrease at higher loadings (Fig. 9d). The materialobtained with 35% fibers did not allow mechanicaltesting due to very low molecular weight. Figure 10shows a photograph of the material obtained with a

Fig. 8. Achievable crystallinity as measured by DSC for samples containing only preformed PLA (100/0 P/L) and samples completely

polymerized in the presence of cellulose fibers (0/100 P/L).

Table I. Summary of Storage Modulus G¢ for Materials Tested

Fiber loading level

(wt%)

Modulus G¢(GPa)

Reinforcement

(%)

100% Preformed PLA, 0% lactide (100/0 P/L)

0 1.4 –

15 1.9 36

25 1.9 36

35 2.0 43


0 1.4 –

15 1.7 21

25 2.1 50

35 1.5 7


0 1.5 –

15 1.8 20

25 2.3 53

35 2.0 33


0 1.6 –

15 1.9 19

25 1.4 )2235 – –


fiber loading level of 35% and 100% lactide at thebeginning of the reaction. Overall, comparison ofthe moduli measured at 40�C shows that the bestreinforcement is achieved at 25% fiber loading levelin the presence of 30% preformed PLA.

Figure 11 shows photographs of the best per-forming materials for each of the different lactide/PLA systems studied. The achievable reinforcementby reactive compatibilization is superior to theimprovement by purely mechanical mixing. Accord-ingly, the new approach demonstrates considerablepromise.

CONCLUSIONS

The experimental results obtained show thatthe novel reactive compatibilization strategy iseffective in increasing the reinforcement of the

Fig. 9. Storage modulus versus fiber loading level for (a) samples containing 100% preformed PLA, (b) samples containing 65% pre-

formed PLA and 35% lactide at the beginning of the reaction, (c) samples containing 30% preformed PLA and 70% lactide at the begin-

ning of the reaction, (d) samples containing 100% lactide at the beginning of the reaction.

Fig. 10. Photograph of material obtained with 35% fiber loading

level and 100% lactide at the beginning of the reaction.

Fig. 11. Photograph showing 100/0 P/L-35 (0% lactide, 100% preformed PLA, 35% fiber loading level), 65/35 P/L-25, 30/70 P/L-25, 0/100

P/L-15 (from left to right).


material compared to purely mechanical mixing forthe same mixing conditions. The properties of reac-tively compatibilized materials become limited by adecrease in molecular weight as the fiber loading le-vel increases due to an excess of initiating groups.Therefore, both the fiber loading level and the ratioof preformed material to lactide at the beginning ofthe reaction are important factors in designing acommercial process based on this new and innova-tive approach.

ACKNOWLEDGMENTS

The authors gratefully acknowledge fundingsupport from the EPA and NSF under the Technol-ogies for a Sustainable Environment Program. EdSchut (CreaFill) and Pat Gruber are acknowledgedfor the supply of the cellulose fibers and the accessto lactide, respectively.

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Reactively Compatibilized Cellulosic Polylactide Microcomposites