Clean and Green Bioplastic Composites: Comparison of Calcium Sulfate and Carbon Nanospheres in...

Margaret J. Sobkowicz1

Jonathan L. Feaver1

John R. Dorgan1

1Department of Chemical Engineering,Colorado School of Mines, CO, USA.

Research Article

Clean and Green Bioplastic Composites:Comparison of Calcium Sulfate and CarbonNanospheres in Polylactide Composites

Two environmentally friendly fillers, carbon nanospheres (CNS) derived from cellu-lose and calcium sulfate anhydrite (CaSO4), a by-product of the polylactide (PLA) pro-duction process, are compared as nucleating agents in commercial-grade PLA. CNSand CaSO4 are compounded with PLA using solution blending. Additionally, CaSO4 ismelt mixed with PLA. Crystallization kinetics are explored using differential scanningcalorimetry (DSC) and polarized optical microscopy. Mechanical properties are exam-ined using dynamic mechanical thermal analysis (DMTA) and morphology is deter-mined using field-emission scanning electron microscopy (FE-SEM). It is found thatthe filler does not increase the PLA crystallization rates significantly at loadings up to15 wt% (CNS) or 20 wt% (CaSO4). The lack of effect on crystallization kinetics is attrib-uted to the high D-lactic acid content of commercial-grade PLA, and also to poor dis-persion of the fillers in the PLA matrix. The glassy shear storage modulus of the com-posites is found to increase by 50% in the highest weight loadings tested. These cleanand green bioplastic composites may be able to offset the use of fossil resource-basedmaterials.

Keywords: Carbon; Crystallization; Green bioplastic composites; Gypsum; Polylactide;

Received: March 19, 2008; revised: May 10, 2008; accepted: May 16, 2008

DOI: 10.1002/clen.200800076

1 Introduction

As petroleum-based resources become scarcer and more costly, theinterest in – and need for – renewable resource-based materialsincreases. Polylactide (PLA) is a renewable, degradable polyestermade from fermentation of sugars, most commonly corn sugar.Large-scale production of PLA in the US was realized by Nature-Works in 1998 [1]. The fossil energy requirement for the PLA produc-tion process as implemented by NatureWorks is substantially lessthan that for other commercially produced polymers as shown bylife cycle assessment [2]. In 2002, NatureWorks earned a PresidentialGreen Chemistry Challenge award for greener reaction conditions,due to their solvent-free polymerization process. PLA is a high-mod-ulus, high-gloss, transparent homopolymer with mechanical prop-erties similar to polystyrene [3]. PLA's glass transition temperatureis around 608C, which leads to a low heat distortion temperatureand thus precludes its use in hot food packaging applications. It isalso rather brittle and slow to crystallize. PLA is currently used pre-dominantly in packaging and fiber production, and also extensivelyin biomedical applications due to its biodegradability. As part of the

fermentation process that results in lactic acid, a precursor for thelactide monomer, calcium hydroxide (slaked lime), and sulfuricacid (H2SO4) are added for neutralization and hydrolysis, respec-tively. Consequently, insoluble gypsum (CaSO4) precipitates out as aby-product of the reaction [4]. It is of great interest to find synergis-tic uses for gypsum as a filler in the PLA resin, thereby decreasingthe cost of the polymer and increasing its competitive advantageover other commodity plastics. Two of the twelve Principles ofGreen Chemistry – the use of renewable feedstocks and the preven-tion of waste – are satisfied by the use of the gypsum by-product orother renewables-based fillers in PLA [5]. In addition, the use of theby-product gypsum in PLA composites fulfills the Green Chemistryprinciple of maximizing atom efficiency [5].

Carbon nanospheres (CNS) derived from cellulose [6] are a renew-able nanomaterial with potential as a filter for bioplastics. In a pre-vious work CNS were examined as a crystal nucleation agent for PLAand polypropylene [7]. The earlier work studied crystallization in alaboratory preparation of PLA, but not in commercial grade PLAresin. It was found that CNS can increase the crystallization rate inPLA by 40%, and up to 500% in polypropylene. Nanocomposites –those incorporating particles of nanoscale dimensions – offer evengreater possibilities for modification of plastic properties thanmicrocomposites at low filler loading levels, due to their extremelyhigh surface area. The critical parameters dictating performance ofa micro- or nanocomposite are dispersion of the filler and interac-tions at the polymer/filler interface [8, 9].

Originally added as a way of controlling the cost of polymericmaterials, mineral fillers have found other significant roles as rein-

Correspondence: Prof. J. R. Dorgan, Department of Chemical Engineer-ing, Colorado School of Mines, Golden, CO 80401, USA.E-mail: [email protected]

Abbreviations: CNS, carbon nanospheres; DMTA, dynamic mechanicalthermal analysis; DSC, differential scanning calorimetry; PLA, polylac-tide; PLLA, pure poly(L-lactic acid); TGA, thermogravimetric analysis;TNPP, tris(nonylphenylphosphite)

i 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.clean-journal.com

706 Clean 2008, 36 (8), 706 –713

Clean 2008, 36 (8), 706 – 713 Clean and Green Bioplastic Composites 707

forcement in polymer composites. Talc is often used in polymers asa crystal nucleating agent, and research has shown that the smallerthe average particle size, the more effective talc can be at enhancingcrystallization kinetics [10, 11]. Filler surface treatments can alsoplay a major role in crystallization kinetics [12, 13]. The addition ofnucleating fillers does not alter spherulite growth rate, but thenucleus density increases leading to finer crystalline microstruc-ture. There is no significant increase in maximum percent crystal-linity reached [14, 15], which means that physical property enhance-ment in composites is dependent on the nature of the particle –matrix interaction and not on changes in equilibrium percent crys-tallinity. Small molecules such as sodium benzoate are used tonucleate polymers [16], and in the case of PLA, crystallites of thepoly(L-lactic acid) (PLLA)/poly(D-lactic acid) stereocomplex can alsoenhance nucleation in pure PLLA [17 – 19]. However, sections of D-lactic acid in a PLLA backbone interrupt the chain packing duringcrystallization, and PLA copolymerized with D-lactic acid groupsabove a certain percentage can no longer crystallize [20]. As men-tioned before, it would be preferable to use a renewable or waste by-product filler to nucleate crystals in PLA. A previous work examinedmechanical properties of gypsum/PLA composites up to 50% CaSO4

weight loading [21]. Murariu et al. explored the effect of usinghydrated gypsum directly (as obtained from the production of PLA),compared with predrying and crystallizing the gypsum by-productinto b-anhydrite calcium sulfate. It was found that compositemechanical properties are only significantly improved when usinggypsum heated to 5008C, forming the stable b-anhydrite calciumsulfate crystalline form which will not readily absorb water.

In addition to the obvious benefits of using a waste by-productfrom the PLA production process, adding a significant volume frac-tion of gypsum to PLA could render it cost-competitive with petro-leum-based resins. PLA, at around $1/lb, is becoming more competi-tive with other conventional polymer resins as the price of oil stead-ily rises. The high surface area of CNS is attractive from a nucleationstandpoint, but the projected price ($10/kg) is prohibitively high forthe nucleation application alone, whereas gypsum would be a real-istically priced filler. The present work compares the crystallizationproperties of calcium sulfate anhydrite and CNS composites madeusing commercial-grade PLA. The PLA used in this study has a higherdegree of D-lactide optical impurity than the laboratory materialused in the previous work; therefore crystallization is expected tobe slower than in the purer PLLA. We also examine the mechanicalproperties of PLA composites with CNS and CaSO4 filler materials.

2 Materials and Methods

2.1 Materials

Commercial-grade PLA (2002D, melt-flow index 4 – 8 g/10 min, a4%D-lactide) was supplied by NatureWorks LLC. PLA resin was crystal-lized at 1108C for 24 h prior to compounding. Calcium sulfate anhy-drite (CAS-20-4) with an average particle diameter of 4 lm was sup-plied by United States Gypsum Company. CNS were obtained fromFullerene Sciences and used as received. Chloroform and methanolwere reagent grade, obtained from Sigma – Aldrich.

2.2 Sample Preparation

Gypsum – PLA composites were prepared by two different methodsin this study: solution blending and melt mixing. Samples were pre-

pared with various weight percents of CAS-20-4 (0.0, 0.2, 1.0, 2.0, 5.0,10.0, 20.0 wt%). CNS – PLA composites were prepared using solutionblending only, with various weight percents of CNS (0.2, 1.0, 2.0,5.0, 15.0 wt%). PLA was fully dissolved in chloroform at 10 wt% PLAand mixed with 5 mg/mL solutions of CAS-20-4 in chloroform orCNS in chloroform. Additional chloroform was added to the lowerweight percent samples in order to maintain constant solution vis-cosity. These samples were magnetically stirred for 48 h and thenprecipitated dropwise into methanol. The precipitates were driedunder vacuum (25 in. Hg) overnight at 808C before testing.

For the melt mixing procedure CAS-20-4 was dried at 1108C under23 in. Hg vacuum for 24 h. PLA resin was dried for 24 h at 808Cunder 23 in. Hg vacuum. The drying step was to minimize hydroly-sis of PLA chains due to residual moisture present during melting.Samples were prepared in a Haake RheoMix 3000. PLA was fullymelted at 1808C and 0.5 wt% tris(nonylphenylphosphite) (TNPP) wasadded as a stabilizer. Finally, the required amount of CAS-20-4 wasadded and mixed at 50 rpm for 15 min.

2.3 Sample Characterization

To determine thermal properties of composites and for the isother-mal crystallization studies differential scanning calorimetry (DSC)was performed using a Perkin Elmer DSC-7 calibrated against anindium standard. Solution-blended precipitates were premelted ona hot plate for a few minutes prior to DSC analysis. Samples of 12 –20 mg were placed in aluminum pans. The testing program was asfollows: samples were heated from 5 to 2008C at 108C/min, held at2008C for 5 min, and quenched to 1108C (the optimum crystalliza-tion temperature) at 508C/min to crystallize isothermally. Sampleswere then cooled to 58C and heated from 5 to 2008C at 108C/min asecond time to determine glass transition temperature (Tg), meltingtemperature (Tm), and enthalpy of fusion (DHm). Mechanical proper-ties were determined through dynamic mechanical thermal analy-sis (DMTA) using an ARES-LS rheometer with torsional rectangularfixtures. Composite samples were vacuum/compression moldedinto rectangular bars, crystallized at 1108C for 3 h, and physicallyaged for 24 h prior to DMTA. The testing was carried out at 0.05%strain, 1 Hz, with a temperature ramp from 30 to 1608C at 58C/min.

In order to verify the weight loading of the composite samples,thermogravimetric measurements (TGA) were made on a Seiko TG/DTA 220 calibrated against an indium standard. Alumina pans wereused and analysis was carried out with a helium gas sweep on 12 to30 mg composite samples. The heating program was as follows: 30to 8008C at 108C/min, hold 30 min at 8008C, and 800 to 308C at208C/min.

Microscopy techniques were used to investigate composite mor-phology. Fracture surfaces of the composites were investigatedusing field-emission scanning electron microscopy (FE-SEM). Com-pression molded bars were cooled with liquid nitrogen and brokenwith pliers. A JEOL JSM-7000F microscope was used with an accelera-tion voltage of 1.5 keV, small probe current (2 kV), and working dis-tance of 6 mm. Polarized optical microscopy images of crystalliza-tion were obtained using a Nikon microscope with a Mettler ToledoFP82 hot stage and FP90 temperature controller (accuracy of (0.28C).Isothermal crystallizations were carried out at 1308C (rather than1108C) in order to reduce the quantity of nuclei formed and thusbetter observe spherulite growth with time.


708 M.J. Sobkowicz et al. Clean 2008, 36 (8), 706 –713

3 Results and Discussion

Figure 1 shows electron micrographs of the two filler types, CNSand CaSO4 (note the difference in magnification). The individualCNS are approximately 40 nm in diameter, but they tend to aggre-gate in large clusters. Both CaSO4 and CNS aggregates are roughlyspherical in that no dimension is significantly larger than another.

Representative DSC data for both CaSO4 and CNS composites canbe found in Fig. 2. The left-hand side presents the heating scan overthe glass transition temperature (l608C) and melting temperature(l1608C) of PLA. Final crystallinity values reported in Table 1 are cal-culated from the area under the melting peak:

%X ¼ DHm

DH100c

6100 ð1Þ

where DHm (the enthalpy of melting) is taken from the area of theDSC melting peak divided by the mass of the sample and DH100

c isthe theoretical melting enthalpy of a 100% crystalline sample (95 J/g for PLA [22]). The right-hand side of Fig. 2 shows DSC heat flowcurves over the isothermal crystallization. Crystallization kineticsdata also reported in Table 1 are calculated from the integration ofthis exotherm.


Figure 1. FE-SEM images of CNS and CaSO4 filler materials.

Figure 2. DSC scans: (left) temperature scan over Tg and Tm (right) iso-thermal crystallization at 1108C.

Figure 3. Integrated DSC curves for (top to bottom) CaSO4-PLAMM,CaSO4-PLASB, CNS-PLASB composites.


Figure 3 presents the integrated DSC data over the crystallizationexotherm for all composites. The addition of up to 10% CaSO4 doesnot increase the crystallization rate in either melt-mixed or solu-tion-blended samples. In fact, all but the 20 wt% melt-mixed sam-ples appear to crystallize slower than neat PLA. When 20% CaSO4 isadded the crystallization appears to be slightly faster for both com-pounding techniques. The CNS composites show slightly faster crys-tallization than neat PLA in all composites except at 5%, which mayhave been due to poor dispersion. Due to the small sample size inDSC, a nonrepresentative sample resulting from imperfect particledispersion may be responsible for this anomalous result.

The analysis of phase change developed by Avrami [23, 24] is avery common method for describing the crystallization kinetics ofpolymer composite systems [14, 25], and it was applied here todescribe the crystallization kinetics resulting from the DSC data.For polymers, the growth-rate expression was simplified by Mandel-

kern using the assumption that the nucleation rate instantaneouslyreaches a steady state and the crystallite growth rate is constant andlinear:

1� kðtÞ ¼ �exp ð�ktnÞ ð2Þ

Here k(t) is the fraction of unconverted, or amorphous material inthe sample at time t, k is termed the Avrami constant, and n is theAvrami exponent [26, 27]. It is difficult to assign a physical meaningto k because it takes units according to the value of n. The exponentn specifies the geometry of growth of the crystals, given homogene-ous or heterogeneous nucleation. Equation (2) can be used in con-junction with experimental data to determine the Avrami parame-ters, which provide a means of comparison for rate of crystallizationof various samples. The double logarithm of Eq. (2) provides the lin-ear relationship:


Table 1. Actual weight louding and crystallization data for all CaSO4 and CNS composites.

Sample name TGA wt% % Xm n t1/2 (min)

Melt-mixed CaSO4 composites0-PLAMM 0 – 39.7 3.53 l 0.02 6.50 l 0.01CaSO4 – 0.2 – PLAMM 1.1 l 0.6 39.4 3.59 l 0.01 6.97 l 0.01CaSO4 – 1 – PLAMM 1.8 l 0.6 41.5 3.08 l 0.01 6.97 l 0.01CaSO4 – 2 – PLAMM 2.8 l 0.6 40.4 3.74 l 0.01 6.61 l 0.01CaSO4 – 5 – PLAMM 5.8 l 0.6 39.8 4.02 l 0.02 7.39 l 0.01CaSO4 – 10 – PLAMM 10.5 l 0.6 39.5 3.50 l 0.02 7.40 l 0.01CaSO4 – 20 – PLAMM 20.5 l 0.6 43.6 3.28 l 0.01 6.30 l 0.01

Solution-blended CaSO4 composites0-PLASB 0 l 1.1 41.9 3.22 l 0.01 7.16 l 0.01CaSO4 – 0.2 – PLASB 0.1 l 1.1 40.6 2.89 l 0.02 7.91 l 0.01CaSO4 – 1 – PLASB 1.6 l 1.1 42.3 3.47 l 0.02 8.03 l 0.03CaSO4 – 2 – PLASB 3.3 l 1.1 43.2 3.44 l 0.01 6.96 l 0.01CaSO4 – 5 – PLASB 5.4 l 1.1 39.7 3.31 l 0.01 7.56 l 0.01CaSO4 – 10 – PLASB 9.9 l 1.1 41.1 2.74 l 0.01 10.86 l 0.01CaSO4 – 20 – PLASB 17.5 l 1.1 40.8 3.48 l 0.02 6.46 l 0.01

Solution-blended CNS composites0-PLASB 0 l 1.1 41.9 3.42 l 0.01 7.73 l 0.01CNS – 0.2 – PLASB 0.9 l 1.1 41.8 2.02 l 0.01 7.84 l 0.01CNS – 1 – PLASB 1.4 l 1.1 38.1 2.19 l 0.01 5.99 l 0.01CNS – 2 – PLASB 2.3 l 1.1 42.3 2.35 l 0.01 5.81 l 0.01CNS – 5 – PLASB 4.4 l 1.1 43.4 2.62 l 0.02 6.05 l 0.01CNS – 15 – PLASB 15.1 l 1.1 40.2 3.71 l 0.02 8.02 l 0.01

Figure 4. Examples of Avrami linear fits for (top) pure PLAMM and CaSO4-5-PLAMM and (bottom) pure PLA and CNS-5-PLASB.


lnf�ln½1� XðtÞ�g ¼ ln kþ n ln t ð3Þ

where X(t) is the fraction of maximum crystallinity (assumed here tobe equilibrium crystallinity). An example of an Avrami plot for purePLA and CNS-filled PLA is shown in Fig. 4. It should be noted that sec-ondary crystallization processes, as seen in the pure PLAMM sampleAvrami plot, are known to occur due to modification of the spheru-lites at late times [25]. This is seen in some of the Avrami plots of ourPLA composites; however, all linear fits were made only of datafrom after the induction time (determined to be 1 min after quench-ing) and before the onset of secondary crystallization. Finally,another useful quantity is t1/2, defined as the time to reach 50% ofmaximum crystallinity:

t1=2 ¼lnð2Þ

k

� �1=n

ð4Þ

For all of the composites prepared, Table 1 presents the trueweight loadings measured using TGA, the percent crystallinity (%X)

from DSC scans, as well as the Avrami exponent and crystallizationhalf-times derived from analyzing the DSC data. Error on theAvrami exponent and crystallization half-time was taken from thestatistics of the linear fit. It should be noted that two separate prepa-rations of 0% PLASB samples were made and the values vary slightly.Target weight percents were not met in all cases; however, thetrends follow the intended weights of filler. Error in these valueswas calculated from the standard deviation of three repeated meas-urements of one weight loading for each preparation type: solutionblending and melt mixing. The deviation from target weight frac-tions is attributable to small sample size and poor dispersion, whichwould lead to nonrepresentative sampling of the composites.

It appears that, as expected, filler additions do not have a substan-tial effect on equilibrium crystallinity of the samples. Although theAvrami model does not differentiate between heterogeneous andhomogeneous nucleation, experimental values of n between 3 and 4(heterogeneous nucleation) indicate sphere-like or three-dimen-sional objects and values between 2 and 3 indicate two-dimensional


Figure 5. Polarized optical microscopy images of composites crystallized at 1308C; a) pure PLASB, b) CNS-1-PLASB, c) CaSO4-1-PLASB.


growth. The Avrami exponents mostly fall between 2 and 4, whichindicates that crystal growth is between two- and three-dimensionalwith heterogeneous nucleation [27]. In the case of CaSO4 composites,the values for t1/2 do not show a trend toward faster nucleation, butfor the CNS composites the t1/2 time drops up to approximately2 wt% and then begins to rise again. This may be due to confine-ment of chain motion from the high filler content, or a higherdegree of aggregation in the 5 and 15% CNS composites, whichcould lead to lower overall surface area available for nucleation. Sol-ution-blended and melt-mixed CaSO4 samples show significant dif-ferences in crystallization half-time and Avrami exponent. Thiscould be attributed to the TNPP stabilizer added to the melt-mixedsamples. While the heat treatment in the DSC is brief, it could con-tribute to some degradation of molecular weight in the unstabilizedsamples.

Figure 5 shows polarized optical microscopy images of pure PLA,CNS – 1 – PLASB, and CaSO4 – 1 – PLASB during crystallization at1308C for 20 and 45 min. From these pictures it is clear that spheru-lite growth rate is the same for all composites, as was assumed inthe development of the Avrami analysis. It is also apparent from Fig.5b that the CNS are aggregated into clusters, some as large as tens ofmicrons in diameter, which supports the poor dispersion hypothe-sis. The CaSO4 particles are difficult to see in the micrographs due tolack of contrast with the polymer; however a nonpolarized image isshown in Fig. 6. It is clear that there are some larger particles on theorder of tens to hundreds of microns.

Representative FE-SEM images of fracture surfaces of pure PLAand the highest weight loadings CaSO4 and CNS are shown in Fig. 7.All images are at the same magnification, and the pure PLA surfaceappears the smoothest. Gypsum particles are visible in the CaSO4-20sample and although the CNS are not visible the surface is signifi-cantly rougher in the CNS-15 sample than in the other two samples.The rough surface may be an indication of ductile failure as the pol-ymer pulls away from the CNS particles, but without stress – straindata from the fracture it is impossible to have certainty about themechanism of failure. The surface roughness may also be due toagglomeration of CNS in the composite.

Figure 8 shows storage shear modulus (G9) versus temperatureand tan (d) = G99/G9 results from the DMTA experiment. The glassymodulus increases by 0.5 GPa in the highest weight loading compo-sites of gypsum and CNS compared with the neat PLA. This modulus

increase is not expected to result in a decrease in impact properties,based on the findings of Murariu et al. [21]. Interestingly, the solu-tion-blended CaSO4 samples have slightly lower moduli than themelt-mixed samples, which may be attributed to the TNPP stabilizeradded during melt mixing; TNPP has a molecular weight of 689 g/mol and is expected to be largely nonvolatile. The rubbery modulus


Figure 6. Non-polarized (10x) optical micrograph of CaSO4-1-PLASB.

Figure 7. FE-SEM images of composite fracture surfaces.


increases about 0.1 GPa in the composites shown. The increase inrubbery modulus does not appear to cause processing concerns inthe composite materials. Results for tan d show a slight decrease inthe peak intensity, but no shift in temperature at which the peakoccurs. The decrease in intensity reflects an increased stiffness ofthe material at the glass transition, but the glass transition temper-ature itself is unaffected by the presence of the filler materials. Thisis an indication that the polymer chains do not have preferentialaffinity for the fillers, since the onset temperature of segmentalmovement remains the same.

4 Concluding Remarks

Commercial-grade PLA is difficult to crystallize due to the higher D-lactic acid content. It is likely that the materials used as nucleatorsin this study could not overcome the slow crystallization kineticsbecause of poor dispersion in the PLA matrix. If smaller particlesizes and enhanced dispersion could be achieved, CaSO4 or CNS maymake better nucleators. A future publication will address function-alization of the filler surfaces to compatibilize them with the poly-mer matrix and encourage dispersion. While the addition of CaSO4

and CNS to commercial-grade PLA does not have a pronouncedeffect on crystallization kinetics of the polymer, it also does notappear to have a detrimental effect on the mechanical properties ofthe PLA. From the perspective of Green Chemistry as well as eco-

nomics, CaSO4 is a logical filler choice for cost reduction of PLA dueto its availability as a waste stream from lactide production. Thepresent projected cost of CNS is prohibitive for most lower-end pack-aging applications, but if significant improvements in toughness orelectronic properties of composites can be achieved these novel, bio-based renewable nanofillers may compete with some current nonre-newable filler materials.

Acknowledgments

This research was supported by National Research Initiative Com-petitive Grant 2006-35504-16618 from the USDA Cooperative StateResearch, Education, and Extension Service and by the National Sci-ence Foundation through grant CMMI-0700869.

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