Effect of Eco-Friendly Triethyl Citrate Plasticizer - American Chemical

Biomacromolecules 2004, 5, 2281-2288

2281

“Green” Nanocomposites from Cellulose Acetate Bioplastic and Clay: Effect of Eco-Friendly Triethyl Citrate Plasticizer Hwan-Man Park,† Manjusri Misra,† Lawrence T. Drzal,† and Amar K. Mohanty*,‡ Composite Materials and Structures Center, 2100 Engineering Building, Michigan State University, East Lansing, Michigan 48824, and The School of Packaging, 130 Packaging Building, Michigan State University, East Lansing, Michigan 48824 Received May 25, 2004; Revised Manuscript Received July 1, 2004

“Green” nanocomposites have been successfully fabricated from cellulose acetate (CA) powder, eco-friendly triethyl citrate (TEC) plasticizer and organically modified clay. The effect of the amount of plasticizer varying from 15 to 40 wt % on the performance of the nanocomposites has been evaluated. The morphologies of these nanocomposites were evaluated through X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM) studies. The mechanical properties of nanocomposites are correlated with the XRD and TEM observations. Cellulosic plastic-based nanocomposites with 20 wt % TEC plasticizer and 5 wt % organoclay showed better intercalation and an exfoliated structure than the counterpart having 30/40 wt % plasticizers. The tensile strength, modulus and thermal stability of cellulosic plastic reinforced with organoclay showed a decreasing trend with an increase of plasticizer content from 20 to 40 wt %. The nano-reinforcement at the lower volume fractions (φ e 0.02) reduced the water vapor permeability of cellulosic plastic by 2 times and the relative permeability better fits with larger platelet aspect ratios (R ) 150). 1. Introduction The environmental impact of plastic waste is an area of growing concern. Significant amounts and varieties of plastics, notably polyolefins, polystyrene, and poly(vinyl chloride) are currently produced from fossil fuels, consumed and discarded into the environment, ending up as nondegradable wastes. Their disposal by incineration produces a net increase in carbon dioxide and, in some cases, toxic gases, which contribute to global pollution. As a result, there is considerable interest in biodegradable polymers, which can be used as alternatives to traditional plastics, thus reducing the amount of waste. A noteworthy development of polymer technology is the discovery of polymer/clay nanocomposites (PCN)s. Nanocomposites offer the potential for the diversification and application of polymers due to their excellent properties such as high heat distortion temperature, dimensional stability, improved barrier properties, flame retardancy, and enhanced physico/thermo-mechanical properties. Layered silicate nanocomposites have been studied for nearly 50 years,1 although the polymer-clay nanocomposite concept was first introduced by researchers from Toyota,2 who made nanocomposites from polyamide 6 and organophillic clay. An extensive amount of literature is now available on nanocomposites with matrixes of epoxy,3,4 polyamide,5 polystyrene,6 polyurethane,7 poly(ethylene terephthate)8 and polypropylene,9 poly(buthylene succinate-co-adipate),10 and styrene-acrylonitrile copolymer.11 Opportunities for utiliza* To whom correspondence should be addressed. E-mail: mohantya@ msu.edu. † Composite Materials and Structures Center. ‡ The School of Packaging.

tion of polymer clay nanocomposites especially with polypropylene (PP), thermoplastic polyolefin (TPO) and nylon based nanocomposites12 are particularly attractive for transportation applications. Bio-based polymers are moving into the mainstream and the polymers that are biodegradable or based on renewable “feedstock” may soon be competing with commodity plastics. Performance limitations and high cost, however, have limited these biopolymers to niche markets. Nanoreinforcement of bio-based polymers with organoclay can create new valueadded applications of “green” polymers in the 21st century materials world. Renewable resource-based biodegradable polymers including cellulosic plastic (plastic made from wood), polylactic acid, PLA (corn-derived plastic), polyhydroxyalkanoate, PHA (bacterial polyesters), and thermoplastic starch (TPS) are some the potential biopolymers which with effective reinforcement with nanoclay can generate “green” nanocomposites.13-18 However, these “green” nanocomposites that mean a bio-based materials, renewable resource, and environmentaly friendly need to be sustainable to compete with existing petroleum-based polymer clay nanocomposites. Cellulose from trees is attracting interest as a substitute for petroleum feedstock in making plastic (cellulosic plastics from cellulose esters) in the commercial market.19 Cellulosic plastics such as cellulose acetate (CA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB) are thermoplastic materials produced through esterification of cellulose. Different raw materials such as cotton, recycled paper, wood cellulose, and sugarcane are being used in making the cellulose ester biopolymers in powder form. Such

10.1021/bm049690f CCC: $27.50 © 2004 American Chemical Society Published on Web 09/10/2004

2282

Biomacromolecules, Vol. 5, No. 6, 2004

cellulose ester powders in the presence of different plasticizers and additives are extruded to produce various grades of commercial cellulosic plastics in pelletized form. The main drawback of cellulose acetate plastic is that its melt processing temperature exceeds its decomposition temperature. This requires cellulose acetates to be plasticized. Phthalate plasticizers, used in commercial cellulose ester plastic, are now under environmental scrutiny as a health threat and thus there is some concern about their long-term use. One of our main purposes is to replace phthalate plasticizer by ecofriendly plasticizer like citrate20 and blends of citrate and derivatized vegetable oil in designing more eco-friendly cellulosic plastic formulations. So far there are no reports on the preparation of biodegradable plasticized CA/clay hybrid nanocomposites. In our current investigations on cellulose acetate plastic-clay based nanocomposites, we have chosen triethyl citrate as the plasticizer. Melt processing through extrusion-injection molding is adopted in fabricating the nanocomposites. By compounding organically modified montmorillonite clays with a plasticized CA matrix via melt extrusion, we hope to produce exfoliated and/or intercalated clays inside the continuous matrix phase. This paper reports the results of optimized processing conditions and varying the amount of TEC plasticizer. Thermal (heat deflection temperature, HDT), morphological (X-ray diffraction, XRD, atomic force microscopy, AFM and transmission electron microscope, TEM), and mechanical (tensile and impact strength) properties of the resulting nanocomposites were used to evaluate this research. 2. Experimental Details 2.1. Materials. Cellulose acetate, CA (CAB- 398-30) without additives in powder form and triethyl citrate (TEC, Citroflex 2) were supplied by Eastman Chemicals Co., Kingsport, TN and Morflex, Inc. North Carolina, respectively. The degree of substitution of cellulose acetate (CA) is 2.45. An organically modified montmorillonite (organoclay), e.g., Cloisite 30B, density 1.98 g/cm3, was purchased from Southern Clay Co. and used as the reinforcement. The ammonium cation of Cloisite 30B is reported to be methyl tallow bis-2-hydroxyethyl quaternary ammonium. 2.2. Melt Compounding. The CA and clay were dried under vacuum at 80 °C for at least 24 h before use. The CA powder and TEC plasticizer with varying ratios (CA/TEC ) 80:20, 70/30, 60/40 by wt %) were mixed mechanically with a high speed mixer for about 5 min and then stored in sealed polyethylene bags for specific time periods prior to further processing. The pre-plasticized mixture was then mixed with the desired quantity of organoclay followed by mixing with a high-speed mixer. The mixtures (CA + TEC + organoclay) were then melt compounded at 160-220 °C for 2-20 min at 100 rpm with micro-compounding molding equipment, DSM Micro 15 cm3 compounder (DSM Research, Netherlands-mini extruder with injection molder). The mini extruder is equipped with a screw having a length of 150 mm, L/D 18, and a net capacity of 15 cm3. After extrusion, the melted hybrid samples were transferred through

Park et al.

a preheated cylinder to the mini injection molder (pre-set with desired temperature and cooling system) to obtain the desired specimen samples for various measurements and analysis. The injection-molded samples (virgin plasticized cellulose acetate polymers and their nanocomposites) were placed in tightly sealed polyethylene bags to prevent any moisture absorption prior to physical and mechanical property measurements. 2.3. Characterization of Nanocomposite. XRD studies of the samples were carried out using a Rigaku 200B X-ray diffractometer (45 kV, 100 mA) equipped with Cu KR radiation (λ ) 0.1546 nm) and a curved graphite crystal monochromator at a scanning rate of 0.5°/min. A transmission electron microscope (TEM) (JEOL 100CX) was used to determine the morphology of nanocomposites at an acceleration voltage of 100 kV. Microtomed ultrathin film specimens with thicknesses of 70 nm were used for TEM observation. A dynamic mechanical analyzer (2980 DMA, TA instruments, U.S.A.) was used to measure the heat deflection temperature (HDT) of the nanocomposites with a load of 66 psi according to ASTM D648. The dynamic storage modulus (E′), loss modulus (E′′), and mechanical loss tangent (tan δ ) E′′/E′) were also measured with this DMA instrument. The injection-molded samples were cut to the desired shape for the DMA measurements. During the DMA measurements the single cantilever mode at frequency of 1 Hz, amplitude of 30 µm, scan rate of 4 °C/ min, and the temperature range of 25-170 °C were used. Tensile properties and flexural properties of injection mold specimens were measured with a United Testing System SFM-20 according to ASTM D638 and ASTM D790, respectively. System control and data analysis were performed using Datum software. Notched Izod impact strength was measured with a Testing Machines Inc. 43-02-01 Monitor/Impact machine according to ASTM D256 with a 1 ft-lb pendulum. Water vapor permeability experiments of virgin cellulosic plastic and their nanocomposites were conducted with Mocon Permatran-W model 3/33 (Mocon, Minneapolis, MN). The thin films (thickness 250 µm) of samples were made by compression moldings. Such samples were kept in a controlled humidity and temperature chamber (temperature: 37.8 °C at 100% and 0% of relative humidity, RH) for 24 h. The water vapor permeability was calculated21 water vapor permeability ) film thickness * permeation (1) permeation ) transmission rate/pressure difference (∆P) (2) ∆P ) S(R1 - R2)

(3)

where, ∆P ) vapor pressure difference, R1) relative humidity at the source expressed as a fraction (R1 ) 1.00 for a 100% RH chamber), R2 ) relative humidity of the vapor sink expressed as a fraction (R2 ) 0 for the 0% RH chamber (dry side)), and S ) vapor pressure of water at the test temperature (S ) 1.931 in Hg at 100% RH at 37.8 °C. AFM imaging was conducted using a Nanoscope IV atomic force microscope (AFM) from Digital Instruments

Effect of Eco-Friendly Triethyl Citrate Plasticizer

Biomacromolecules, Vol. 5, No. 6, 2004 2283

(Santa Barbara, CA) equipped with an E scanner. Samples were mounted onto a stainless steel disk using a sticky tab (Latham, NY). The microscope was allowed to thermally equilibrate for thirty minutes before imaging. Scanning rates less than 1 Hz were used. Room temperature was maintained at 22 ( 1 °C. Images were recorded in tapping mode using etched silicon probes (Digital Instruments). The samples are cross or plane-sectioned with by diamond knife at room temperature. The sectioned surface was polished with 4000# grit until a smooth surface was obtained. 3. Results and Discussion In this study, we used powder cellulose acetate powder unlike the pellet form of plastics. The commercial cellulose acetate plastics being plasticized with phthalate plasticizers are sold in pellet forms. Our research is not only targeted to take the advantage of powder form of cellulose ester in our nanocomposite fabrications processing techniques but also is targeted to find suitable eco-friendly plasticizer in formulating the cellulosic plastics. Phthalates plasticizer used in commercial cellulosic plastic is under criticism. The citric acid has a long history of use in food and beverage flavoring. The citric acid esters have a similar history as plasticizers. They are nontoxic and have been approved for many applications such as additives in medical plastics, personal care, and food contact and are used as plasticizers with a variety of different polymers. Since citrate esters are a derivative of natural compounds, it is of interest to determine their effect on plasticization with special reference to processing parameters. An environmentally friendly citrate plasticizer (triethyl citrate, TEC) is used under the present investigation to improve the processibility of cellulose acetate plastic. We have mentioned under experimental section that in this study initially the cellulose acetate is pre-plasticized with the liquid citrate plasticizer through mechanical mixing followed by powder nano-clay addition and mixing again. The powder-powder mixing of polymer and clay allows homogeneous mixing. The TEC plasticizer content was varied from 15 to 40 wt %, but the content of organoclay (Cloisite 30B) used in this study was optimized at 5 wt %. The effect of varying clay contents on performance of such nanocomposites will be reported in our future communication. In the plasticization study, the time required for the TEC plasticizer to be in contact with the CA powder in order to the preswell CA/TEC mixture was found to play a key role in controlling the properties of plasticized cellulose esters as well as their nanocomposites. The “preswelling” time, i.e., the time for the plasticizer to be imbibed by the plastic, to yield a freeflowing powder that will feed properly into extruders has been reported in the literature.22 The melt compounding temperatures of plasticized cellulose acetates and their nanocomposites were fixed to 220, 180, and 165 °C respectively for 20, 30, and 40% TEC plasticized cellulose acetates (cellulosic plastics). Generally, with an increase of plasticizer content, the melt processing temperature is reduced. Microstructure of Hybrid. Figure 1 shows the XRD patterns of pure Cloisite 30B clay, Cloisite 30B clay swelled with TEC plasticizer, and CA/TEC/Cloisite 30B nanocomposites with different TEC plasticizer content but with a fixed

Figure 1. XRD diffraction patterns: (a) CA/TEC (80/20 wt %) 95 wt %/Cloisite 30 B 5wt %, (b) CA/TEC (70/30 wt %) 95 wt %/Cloisite 30 B 5 wt %, (c) CA/TEC (60/40 wt %) 95 wt %/Cloisite 30 B 5 wt %, (d) TEC/Cloisite 30B (20/80 wt %) mixture, and (e) Cloisite 30B.

amount (5 wt %) of Closite 30 B. The XRD peak shifted from 4.9° (Figure 1e) for pure Cloisite 30B to 2.4° for CA/ TEC (60/40 wt %) 95 wt %/Cloisite 30B 5 wt % nanocomposite (Figure 1c). This indicates significant intercalation and slight exfoliation in the hybrid structure. For CA/TEC/ Cloisite 30B nanocomposite of 20 and 30 wt % TEC plasticizer content, no clear peak was observed at 2.4°, indicating partial exfoliation of organoclays in the CA/TEC matrix. It may be due to interaction between carbonyl group of CA and free hydroxyl group of organic modifier part (methyl tallow bis-2-hydroxyethyl quaternary ammonium) in the organoclay Cloisite 30B. Figure 1d shows that TEC plasticizer is able to swell the organoclay in the TEC/ organoclay (20/80 wt %) mixture at 25 °C for 24 h. This intercalation (swelling) of the TEC/organoclay system may be due to the compatibility or interaction through hydrogen bonding between the -OH group of the plasticizer triethyl citrate (TEC) and -OH group of organic modifier part present in the organoclay Cloisite 30B. However, in our experiment, at first the pre-plasticized CA/TEC matrix was prepared. Therefore, most of the TEC was physically or chemically absorbed into CA powder before nanocomposites preparation. Then nanocomposites were made with this preplasticized CA and organoclay powder in the melt compounding system. Therefore, if some unabsorbed or excess of TEC plasticizer remains in the hybrids system, the amount of this should be a little, and it may be able to swell the organoclay. The extent of swelling of clay increases with increase of plasticizer. Thus in higher plasticizer content system (30-40 wt % TEC) of the hybrid; more swelled organoclay particles would be generated. Such swelled organoclays are more difficult to exfoliate by external shear forces during extrusion processing as compared to CA polymer intercalated organoclay system having optimum plasticizer content (here 20 wt % of TEC plasticizer). This is because the long polymer chain intercalated organoclay (in optimized 20 wt % plasticized hybrid system) is easier to break down by the applied shear forces during processing thus making further exfoliation (Figure 1a) rather than low molecular plasticizer swelled orgnoclay which exhibited intercalated morphologies (Figure 1, parts b and c). We have investigated this excess TEC swelling content effect in hybrid nanocomposites through comparing the CA/

2284


Figure 2. AFM phase image of the CA/TEC/Cloisite 30 hybrid nanocomposites and swelling of TEC/Cloisite 30 mixtures: (a) Planeview of the CA//TEC(60/40 wt %)/Cloisite 30B hybrid, scan size 5 um; (b) Lateral-view of the CA//TEC(60/40 wt %)/Cloisite 30B hybrid, scan size 500 nm; (c) Cloisite 30B/TEC mixture, scan size 600 nm: all tapping mode.

TEC (60/40 wt %)/organoclay hybrid nanocomposites and an swelled TEC/organoclay (20/80 wt %) mixture morphology in AFM tapping mode. The AFM phase images of plane or cross sectioned nanocomposite sample were used. The swelled TEC/organoclay (20/80 wt %) mixture was dispersed in acetone and then deposited on mica surface by spin coater. In the Figure 2a, the AFM image of CA polymer intercalated clay show mostly intercalated sandwich structure and clearly reveals the difference topology from swelling of low molecular TEC/organoclay (see Figure 2, parts b and c). Original d001 spacing of the organoclay is 1.8 nm by XRD. Figure 2b image shows intercalation with different thickness (4∼ 30 nm) and some exfoliated platelets of thickness ∼20 nm.

Park et al.

These large intercalated clays (about 30 nm) are not detected by XRD (2θ > 1°). The intercalated platelet size of swelled system with low molecular weight TEC plasticizer (triethyl citrate, Mw 267) is less than 4 nm. Therefore, in the XRD curve (Figure 1c) of CA/TEC (60/40 wt %)/clay hybrid nanocomposites was resulted mainly from CA polymer intercalated to organoclay rather than TEC plasticizer intercalation. To further explain the morphology of the CA/TEC/organoclay hybrid nanocomposite structures, TEM micrographs are presented in Figure 3. The TEM images show that the CA/ TEC (80/20 wt %)/Cloisite 30B hybrid shows better exfoliation than the CA/TEC (70/30 wt %) or (60/40 wt %)/ Cloisite 30B hybrids. It can also be seen that the intercalation of clay increases with an increase in plasticizer content. It can also be observed from the XRD and TEM results that for exfoliation of clay in the CA polymer, the optimum loading (about 20∼30 wt %) of TEC plasticizer is corresponding to commercial petroleum based CA plastic/phthalate plasticizer system. The TEM observations of nanocomposites have been are correlated with the XRD results. Mechanical Properties. Tables 1 and 2 show the flexural, tensile, and impact properties of the CA/TEC/and/or Cloisite 30B hybrids with different 20, 30, and 40 wt % of TEC plasticizer contents. The melt compounding temperature for preparing nanocomposites with 20, 30, and 40 wt % TEC contents is optimum at 220, 180, and 165 °C, respectively, from the mechanical and micro structural aspect. CA degrades at higher temperature. Therefore, the above melt temperatures are the optimum processing temperatures with respect to the percentage of TEC contents. The melt temperature of hybrids with a composition of e15 wt % of plasticizer contents to the CA matrix exceeds the thermal degradation temperature of CA. A commercial CA plastic available contains 20-30 wt % of phthalate type plasticizer. Therefore, we investigated effect 20∼ 40 wt % of TEC contents on the nanocomposites properties. The notched izod impact strength and the tensile elongation at break of the CA/TEC/Cloisite 30B hybrid sharply increased with increasing TEC content. On the other hand, the strength and modulus (flexural and tensile) of the CA/TEC matrix and CA/TEC/organoclay hybrids decreased with increasing TEC plasticizer content. Apparently the nonsolvent plasticizer TEC acts as an internal lubricant and migrates into the rigid CA molecular chain making it flexible, softening the CA plastic and clay hybrid composites. Mohanty et al. reported that the plasticized CA with an increase of plasticizer content from 20 to 40 wt % and the tensile properties decreased, whereas impact strength and percent elongation increased.20 The addition of the clay nanoplatelets increased the tensile modulus of CA/TEC/clay hybrids with varying TEC content (20%, 30%, and 40% by wt) increased by 51%, 33%, and 110%, respectively, compared to their CA/TEC matrix counterparts. The maximum strength and modulus (flexural and tensile) were attained in the 15 wt % TEC plasticized cellulose acetate plastic based nanocomposites with flexural strength of 118MPa and modulus of 5.8 GPa. Our studies indicate that the use of a higher content of TEC plasticizer in CA



Figure 3. TEM micrographs of the CA/TEC/Cloisite 30B hybrid nanocomposites with deferent the TEC contents at clay 5 wt %: (a) CA/TEC (80/20 wt %), (b) CA/TEC (70/30 wt %), (c) CA/TEC (60/40 wt %). Table 1. Mechanical Strength and Modulus of the Extruded CA/TEC/Clay Hybrid Composites with Different TEC Contents: Clay (Cloisite 30B), Melt Compounding Time: 6min property/ CA/TEC (wt %)

clay adding (wt %)

compounding temp. (°C)

flexural strength (MPa)

flexural modulus (GPa)

tensile strength (MPa)

tensile modulus (GPa)

tensile elongation at break (%)

85/15 80/20 75/25 70/30 60/40 (85/15)95 (80/20)95 (75/25)95 (70/30)95 (60/40)95

0 0 0 0 0 5 5 5 5 5

220 210 195 180 165 220 215 195 180 165

109 ( 12.0 84 ( 0.9 55 ( 1.0 48 ( 1.0 20 ( 0.5 118 ( 1.2 98 ( 3.0 73 ( 2.5 52 ( 0.7 25 ( 0.6

4.3 ( 0.3 3.5 ( 0.1 2.2 ( 0.1 2.0 ( 0.2 0.8 ( 0.3 5.8 ( 0.5 4.1 ( 0.3 3.8 ( 0.1 2.8 ( 0.1 1.6 ( 0.8

81 ( 21 70 ( 4 64 ( 9 61 ( 4 24 ( 3 120 ( 7 105 ( 4 77 ( 3 70 ( 4 39 ( 2

3.5 ( 0.6 4.1 ( 0.1 2.2 ( 0.6 2.1 ( 0.4 0.8 ( 0.3 6.0 ( 0.2 4.8 ( 0.7 2.8 ( 0.1 2.6 ( 0.4 1.7 ( 0.4

5.3 ( 1.2 8.5 ( 2.4 8.8 ( 1.1 11.6 ( 0.1 14.7 ( 0.1 5.0 ( 1.0 7.2 ( 3.0 8.1 ( 0.5 10.0 ( 0.8 14.0 ( 3.0

Table 2. Izod Impact Strength and Energy Absorption of the CA/TEC/Cloisite 30B Hybrid Composites with Different Plasticizer TEC Contents izod impact strength (J/M)

energy of absorption (MPa)

CA/TEC (wt %)

clay 0 wt %

clay 5 wt %

clay 0 wt %

clay 5 wt %

80/20 70/30 60/40

39 ( 21 129 ( 45 184 ( 30

26 ( 5 43 ( 16 38 ( 8

410 ( 13 367 ( 20 288 ( 25

458 ( 20 432 ( 17 334 ( 21

enhances the impact strength but is detrimental to stiffness characteristics of the resulting bioplastic. Thus, maintaining a balance between processability, optimum impact strength,

and stiffness is an important aspect for nanocomposites for their possible application in the transportation industry. It can be seen from the stress-strain and energy absorption plots (Figure 4 and Table 2) that, with the addition of clay, the energy absorption of hybrid nanocomposites with 20, 30, and 40 wt % plasticizer content increased by 12%, 18%, and 16%, respectively, compared to CA/TEC matrix. In general, if the modulus of a composite material increases, the energy absorption decreases. In our nanocomposite system, the energy absorption of hybrid nanocomposites increased with modulus. Mulhaupt et al. reported on similar experiments with cellulose acetate plasticized with a cyclic ester mixture

2286


Figure 4. Tensile strength and elongation curve of the CA/TEC/clay hybrid composites with different TEC plasticizer contents at clay 0 or 5 wt %: (a) clay 0 wt %, TEC 20 wt %; (b) clay 0 wt %, TEC 30 wt %; (c) clay 0 wt %, TEC 40 wt %; (a′) clay 5 wt %, TEC 20 wt %; (b′) clay 5 wt %, TEC 30 wt %; (c′) clay 5 wt %, TEC 40 wt %.

Figure 5. Storage modulus and tan delta curve of the CA/TEC matrix with different the TEC plasticizer contents at clay 0 wt %: (a) CA/TEC (80/20 wt %), (b) CA/TEC (70/30 wt %), (c) CA/TEC (60/40 wt %).

plasticizer.23 They reported that increase in the cellulose acetate percentage leads to an increase of the viscosity during the processing. The softening and melting temperature decreased with increasing component of plasticizer. Our results were similar to that obtained by Mulhaupt et al., i.e., with increase TEC component the softness of the sample increased. Tensile strength, modulus, and elongation at break decreased. Increased cellulose acetate component at the cost of plasticizer shifts the properties in the direction toward the properties of pure cellulose acetate. Dynamic Mechanical Properties and Heat Deflection Temperature (HDT). Figures 5 and 6 show the temperature dependence of the storage modulus, tan δ, and glass transition temperature (Tg) for the pristine CA/TEC and the nanocomposites intercalated with 20∼40 wt % of TEC contents, respectively. Table 3 reveals DMA results and the heat distortion temperature (HDT) of these hybrids. The storage modulus and Tg of the CA/TEC/clay hybrid composites increased with decreasing plasticizer content. The storage modulus of the nanocomposite at 30 °C with 20 wt % plasticizer was found to be 5.71 GPa, twice as high compared to the nanocomposite with 40 wt % plasticizer (2.25 GPa). A possible explanation for improvement of storage modulus with reinforcement of clay might be attributed to the creation of a three-dimensional interconnecting network of silicate layers, thereby strengthening the material through mechanical percolation.24,25 The reason for the storage modulus decrease

Park et al.

Figure 6. Storage modulus and Tg of the CA/TEC/clay hybrid composites with different TEC plasticizer contents at clay 5 wt %: (a) CA/TEC (80/20 wt %), (b) CA/TEC (70/30 wt %), (c) CA/TEC (60/ 40 wt %).

with increase of plasticizer content in CA based plastic is based on the fact that such increased content increases the segmental motion in the CA backbone thereby decreasing the storage modulus. A similar observation, e.g., the decrease of stiffness (modulus) and increase of toughness (impact strength) with increase of plasticizer content in CA based plastic, has been reported in our earlier report.20 As the plasticizer content increase, the Tg of CA matrix decreases. In Table 3, the Tg of CA/TEC 80/20 wt % and 60/40 wt % hybrid is 130 and 86 °C, respectively. The Tg of CA/TEC 60/40 wt % hybrid decreased about 30% compared to the CA/TEC 80/20 wt %. The increase in plasticizer content increases the segmental motion in the CA backbone thereby decreasing the Tg. The lubricity theory views the resistance to deformation as arising from intermolecular friction. According to this view, the plasticizer acts as a lubricant to facilitate movement of the resin macromolecules over each other, thus reducing the internal resistance to deformation.22 On the other hand, the shift and broadening of tan δ peak to higher temperatures indicates an increase in Tg and broadening of the glass transition temperature of the nanocomposites with clay content (Figures 5 and 6). The shift in Tg as measured by the tan δ peak was 8 °C for the hybrids containing 20 wt % TEC hybrid (Tg ) 138 °C) in contrast to CA/TEC plastic (Tg ) 130 °C). The broadening and increase of Tg of the hybrids after adding Cloisite 30B is attributed to the fact that the presence of the clay surface restricted segmental motion of the cellulosic plastic. The other possible reason is that chemical bonding between the hydroxyl groups at the surface of the silicate with the groups on the cellulosic plastic takes place in the interface.26 The HDT is closely related to the Tg of thermoplastics, therefore, the HDT behavior of CA/TEC matrixes and hybrids are similar to the Tg behavior from the DMA results (Table 3). Water Vapor Permeability (WVP). Cellulosic plastics are hydrophilic in nature. Effective nano-reinforcement is expected to reduce the water vapor permeability (WVP) of cellulosic plastic. WVP of nanocomposites with different TEC plasticizer and organoclay content was examined in a controlled temperature and relative humidity chamber (i.e., 37.8 °C, 100% and 0% RH), and the results are presented in Figure 7 in terms of PC/P0, i.e., the permeability of the nanocomposite (PC) relative to that of the neat plasticized CA matrix (P0). In Figure 7, parts c-e, there is a strong



Table 3. Storage Modulus, Glass Transition Temperature (Tg), and Heat Distortion Temperature (HDT) of the CA/TEC/Cloisite 30B Hybrid Composites with Different Plasticizer TEC Contents

Tg (°C) by tan delta peak

storage modulus (GPa) at 30 °C

HDT (°C)

CA/TEC (wt %)

clay 0 wt %

clay 5 wt %

clay 0 wt %

clay 5 wt %

clay 0 wt.%

clay 5 wt %

80/20 70/30 60/40

130 110 86

138 114 95

95 67 45

107 75 60

4.07 2.95 1.74

5.71 3.57 2.25

100 and 150, respectively. The theoretical curve and experimental value show that at the lower volume fractions (φ e 0.02) the relative permeabilities are better fit with larger platelet aspect ratios R ) 150, whereas at the higher volume fractions (φ g 0.05), the relative permeabilities are better fit with smaller platelet aspect ratios (R ) 100). This is due to greater layer aggregation at higher organoclay loadings. Ruijian Xu et al. reported a similar WVP behavior in the poly (urethane urea)/organoclay nanocomposites.32 4. Conclusions

Figure 7. Water vapor relative permeability curve of the CA/TEC hybrid composites with different TEC plasticizer and organoclay contents: (a) Aspect ratio R) 150, theoretical relative permeability, (b) aspect ratio R) 100, theoretical relative permeability, (c) CA/TEC (80/20 wt %), (d) CA/TEC (70/30 wt %), (e) CA/TEC (70/40 wt %): c, d, and e; experimental relative permeability value of nanocomposites; clay contents 0∼10 wt %.

reduction in permeability, reaching 2-fold at the highest organoclay content. The lowering of WVP in the nanocomposites is due to the presence of ordered dispersed silicate layers having a large aspect ratio in the polymer matrix. Similar observations were also noted by other researchers.27,28 This is a consequence of the more tortuous path required for gas molecules to penetrate the membrane, and the magnitude of the reduction is considerably larger than what is observed upon typical chemical modification.29 The WVP of the CA/TEC/clay hybrid was increased with increasing hydrophilic TEC plasticizer content. This may be due to the following reasons; the larger amount of plasticizer in the nanocomposites reduces the film rigidity and extent of clay exfoliation, resulting in higher water vapor transmission rate (WVTR). The solid lines in Figure 7, parts a and b, represent predictions for the permeability through the thickness of a nanocomposite film that has dispersed and completely oriented filler layers. In the dilute and semidilute regime30-32 PC/P0 ) (1 + µ R2 φ2)-1

(4)

where R is the platelet aspect ratio, φ the organoclay volume fraction, and µ a “geometric factor”,31 µ ) π2/(16 ln2 R). We consider here CA/TEC/organoclay nanocomposites, with value of length of clay; Lclay) ∼400 and ∼600 nm (average value from TEM and AFM), and the value of clay platelet thickness, Dclay ) 4 nm from XRD results. Therefore, the platelet aspect ratios as calculated from TEM and AFM are

We have successfully developed injection moldable biobased “green” nanocomposite formulations from cellulose acetate powder, triethyl citrate plasticizer, and organically modified clay. The addition of TEC plasticizer at 20 wt % showed the best intercalation and exfoliation of clay as well as the best physical and mechanical properties of the resulting nanocomposites. The mechanical properties of nanocomposites have been are correlated with the XRD, AFM, and TEM observations. The tensile strength and modulus of cellulosic plastic reinforced with organoclay were improved by decreasing the plasticizer content, the heat deflection temperature was improved, the water vapor permeability was 2 times reduced, but the impact strength was decreased. Cellulosic plastic-clay based nanocomposites demonstrate the potential for replacing/substituting polypropylene-clay nanocomposites for utilization in the automotive and transportation industries. Acknowledgment. The authors are thankful for NSFNER 2002 Award no. BES-0210681 under “Nanoscale Science and Engineering (NSE); Nanoscale Exploratory Research (NER) program” for providing financial support for this research as well as NSF 2002 Award no. DMR0216865, under “Instrumentation for Materials Research (IMR) program” for providing financial support for the DSM micro compounding molding system to carry out this investigation. The collaboration with Ford Motor Company and Eastman Chemical Co. is gratefully acknowledged. Authors are thankful to Dr. Brian D. Seiler of Eastman Chemical Co. and Dr. Deborah F. Mielewski of Ford Motor Co for their encouragement and supports. Authors are also thankful to Eastman Chemical Company Kingsport, TN, Morflex, Inc. North Carolina, Nanocor Co., IL, and Southern Clay Co., TX for providing cellulose ester, plasticizer, and various clay samples, respectively. References and Notes (1) Carter, L. W.; Hendricks, J. G.; Bolley, D. S. U.S. Patent 2,531,396, 1950.

2288


(2) Okada, O.; Kawasumi, M.; Usuki, A.; Kurauchi, T.; Kamigaito, O. Mater. Res. Soc. Symp. Proc. 1990, 171, 45. (3) Pinnavaia, T. J.; Lan, T.; Wang, Z.; Shi, H.; Kavaratna, P. D. ACS Symp. Sep. 1996, 622, 250. (4) Messersmith, P. B.; Giannelis, E. P. Chem. Mater. 1994, 6, 1719. (5) Yano, K.; Usuki, A.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci. Part A: Polym. Chem. 1993, 31, 2493. (6) Vaia, R. A.; Ishii, H.; Giannelis, E. P. Chem. Mater. 1992, 5, 1694. (7) Wang, Z.; Pinnavaia, T. J. Chem. Mater. 1998, 10, 3769. (8) Ke, Y.; Long, C.; Qi, Z. J. Appl. Polym. Sci. 1999, 71, 1139. (9) Hasegawa, N.; Kawasumi, M.; Kato, M.; Usuki, A.; Okada, A. J. Appl. Polym. Sci. 1997, 63, 137. (10) Lee, S. R.; Park, H. M.; Lim, H. T.; Kang, T. K.; Li, X.; Cho, W.-J.; Ha, C.-S. Polymer 2002, 43, 2495. (11) Kim, J. W.; Jang, L. W.; Choi, H. J.; Jhon, M. S. J. Appl. Polym. Sci. 2003, 89, 821. (12) Mohanty, A. K.; Misra, M.; Drzal, L. T. Proceedings of 9th Annual Global Plastics EnVironmental Conference (GPEC 2002), February 26 & 27, 69-78, Detroit, MI, Society of Plastics Engineers: East Sussex, U.K., 2003. (13) Mohanty, A. K.; Drzal, L. T.; Misra, M. Polym. Mater. Sci. Eng. 2003, 88, 60-61. (14) Ray, S. S.; Yamada, K.; Okamoto, M.; Ueda, K. Polymer 2003, 44, 857. (15) Maiti, P.; Batt, C. A.; Giannelis, E. P. Polym. Mater. Sci. Eng. 2003, 88, 58-59. (16) Park, H.-M.; Lee, W.-K.; Cho, W.-J.; Ha, C.-S. J. Mater. Sci. 2003, 38, 909.

Park et al. (17) Park, H. M.; Li, X.; Jin, C.-J.; Cho, W.-J.; Ha, C.-S. Macromol. Mater. Eng. 2002, 287, 553. (18) Ray, S. S.; Okamoto, M. Macromol. Rapid Commun. 2003, 24, 815. (19) Wilkinson, S. L. Chem. Eng. News 2001, 22, 61. (20) Mohanty, A. K.; Wibowo, A.; Misra, M.; Drzal, L. T. Polym. Eng. Sci. 2003, 43 (5), 1151-1161. (21) Dhoot, S. N.; Freeman, B. D.; Stewart, M. E. Barrier Polymers. Encyclopedia of Polymer Science and Technology; John Wiley & Sons: New York, 2002. (22) Bruins, P. F. In Plastic Technology; Reinhold Publishing Co. Chapman &Hall, Ltd.: London, 1965; Vol. 1, pp 1-7, 193-199. (23) Mulhaupt; R.; Schatzle; J.; Warth, H. U.S. Patent 5,480,922, 1996. (24) Ward, W. J.; Gaines, G. L.; Alger, M. M.; Stanley, T. J. J. Membr. Sci. 1991, 55, 173. (25) Lan, T.; Kaviratna, P. D.; Pinnavaia, T. J. Chem. Mater. 1994, 6, 573. (26) Giannelis, E. P.; Messersmith, P. B. U.S. Patent 5,554,670, 1996. (27) Okada, A.; Usuki, A. Mater. Sci. Eng. 1995, C3, 109. (28) Jimenez, G.; Ogata, N.; Kawai, H.; Ogihara, T. J. Appl. Polym. Sci. 1997, 64, 2211. (29) Weisberg, D. M.; Gordon, B.; Rosenberg, G.; Snyder, A. J.; Benesi, A.; Runt, J. Macromolecules 2000, 33, 4380. (30) Cussler, E. L.; Hughes, S. E.; Ward, W. J.; Aria, R. J. Membr. Sci. 1988, 38, 161. (31) Fredrickson, G. H.; Bicerano, J. J. Chem. Phys. 1999, 110, 2181. (32) Xu, R.; Manias, E.; Snyder, A. J.; Runt, J. Macromolecules 2001, 34, 337.

BM049690F

Effect of Eco-Friendly Triethyl Citrate Plasticizer - American Chemical

Recommend Documents