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Development of renewable resource-based cellulose acetate bioplastic: effect of process engineering on the performance of cellulosic plastics.

INTRODUCTION

Sustainabiity, industrial ecology eco-efficiency and green chemistry are guiding the development of the next generation of advanced materials, products and processes. Biodegradable polymers and bio-based polymer products based on annually renewable agricultural and biomass feedstock can form the basis for a portfolio of sustainable, eco-efficient products that can capture markets currently dominated by products based exclusively on petroleum feedstock (1-8). Cellulose from trees is attracting interest as a substitute for petroleum feedstock in making plastics (cellulosic plastic-cellulose esters) for the consumer market (1).

Cellulosic plastics (Fig. 1), such as cellulose acetate (CA), cellulose acetate propionate (CAP), and cellulose acetate butyrate (CAB), are thermoplastic materials produced through the esterification of cellulose. A variety of raw materials such as cotton, recycled paper, wood cellulose, and sugarcane are used in making cellulose ester biopolymers in powder form. Such powders combined with plasticizers and additives are extruded to produce various grades of commercial cellulosic plastics in pelletized form. Phthalate based plasticizers, currently used in commercial cellulose ester plastics, are under environmental scrutiny and perhaps pose a health threat, raising concerns about their long-term use. One of our objectives is to replace phthalate plasticizers with an eco-friendly plasticizer based on citrate and blends of citrate and derivatized vegetable oil when designing sustainable cellulosic plastics for bio-composite applications. Besides material selection, processing plays a vital role in the design of new bio-based polymers. Most commercial cellulose acetate products are clear, strong and stiff and have a broad range of applications. Some applications of cellulose ester biopolymers are film substrates for photography, toothbrush handles, selective filtration membranes in medicine, and automotive coatings (9). In general, those cellulose acetates with acetyl substitution numbers of 2.2 or less are biodegradable in soil and marine environments and are suitable for composting. Those with higher substitution numbers ranging from 2.2 to 3.0 are less biodegradable. The main drawback of cellulose acetate is that its melt processing temperature is very close to its decomposition temperature, as determined by the structure of its parent cellulose (10). This means that cellulose acetates should be plasticized if they are to be used in thermoplastic processing applications. The plasticization of cellulose acetate, although reported by Ghiya et al (11), has not been studied in detail in terms of its effects on process ing and properties.

Citric acid esters are nontoxic and have been approved as plasticizers for many applications such as additives in medical plastics, personal care, and food contact and are useful in a variety of 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. The effects of TEC plasticizer on the physico-mechanical properties and thermal behavior of the resulting cellulosic plastics with varied processing conditions were studied.

The development of cellulose plastic as a matrix polymer in bio-composites requires that properties such as percent elongation, flexibility and impact behavior be tailored to meet the needs of the application. Our recent research results (12-14) have demonstrated that natural fiber reinforced polypropylene (PP) composites have the potential to replace glass fiber-PP composites. With the goal to replace or substitute polypropylene with cellulosic plastic when designing more eco-friendly and sustainable bio-composite materials, our fundamental studies focus on structure-processing-property relationships. This paper gives a detailed analysis of the effect of material selection and processing conditions on the performance of cellulose ester bio-plastics.

EXPERIMENTAL

Materials

Powdered cellulose acetate (CA), free of any plasticizer or additives, was supplied by Eastman Chemical Co., (Kingsport, Tennessee) for the present study. The degree of substitution of the cellulose acetate was approximately 2.5. The triethyl citrate (TEC) plasticizer was provided by Morfiex Inc., (Greensboro, North Carolina). The CA powder was dried overnight in a vacuum oven at 800[degrees]C before processing.

Processing

Three processes commonly used for thermoplastics were used to make test specimens of plasticized cellulose acetate for studying the effect of processing conditions on the physico-mechanical properties of the corresponding cellulosic plastics.

I. Compression Molding: A specific amount of liquid plasticizer was added drop-wise onto dried cellulose acetate powder, and the mixture was stirred mechanically in a kitchen mixer for 30 minutes. The mixture was compression molded using a three-piece picture-frame mold in a Carver Press SP-F 6030 at three different temperatures: 170[degrees]C, 180[degrees]C, and 190[degrees]C. During compression molding at a specific temperature, for the first ten minutes the materials were kept at a constant pressure of 1.1 MPa and then held at 2.67 MPa for last five minutes. The samples were cooled while the pressure was maintained.

II. Extrusion Followed by Compression Molding: In this process, the cellulose acetate powder and TEC plasticizer in the desired proportions were pre-mixed mechanically and then extruded as thermoformable sheets. The resulting thermoformable sheets were then compression molded to make the test specimens. In this case, the cellulose acetate with TEC plasticizer was fed into a ZSK 30 twin-screw extruder (Werner-Pfleiderer). The temperature profile of extruder from Zone 1 through Zone 6 was kept between 170[degrees]C and 180[degrees]C. and the exit die temperature was 190[degrees]C. The extruder was operated at two different screw speeds (100 rpm and 200 rpm). The resultant thermoformable sheets were compression molded at 180[degrees]C and 1.1 MPa for 10 minutes and at 2.67 MPa for 5 minutes, and then cooled while the pressure was maintained.

III. Extrusion Followed by Injection Molding: The samples were extruded as described above (subsection II) and instead of collecting thermoformable extruded sheets, through proper die arrangement, the thin strands of plasticized cellulose acetate were pelletized and stored for future injection molding. The injection molding machine used was an 85-ton Cincinnati-Milacron press. The molding conditions were as follows: temperatures on zones 1 to 3 as well as the die were held between 180[degrees]C and 190[degrees]C with 45 seconds' cooling time; the fill, hold and pack pressures were maintained at 8.2, 4.8 and 5.5 MPa, respectively. In addition, the optimum processing temperature for injection molding was determined to be between 180[degrees]C and 190[degrees]C. The resulting dumbbell-shaped products were used appropriately to test mechanical properties based on ASTM standards.

Analysis and Testing

Differential scanning calorimetry (2920 Modulated DSC, TA Instruments) and thermogravimetric analysis (Hi-Res TGA 2950, TA instruments) were used for the thermal analysis of cellulose acetate plastics. In DSC measurements, the pure and plasticized cellulose acetate samples (10 mg) were sealed in aluminum pans and heated from 25[degrees]C to 200[degrees]C at a rate of 10[degrees]C/min (cpm), held at 200[degrees]C for 5 min, and then cooled to -60[degrees]C before a second heating scan at 10 cpm from -60[degrees]C to 300[degrees]C. A nitrogen purge (50 ml/min) was maintained throughout the test. Dimensional stability of the materials as a function of temperature was measured per ASTM D 696, using Thermo-mechanical Analyzer (TMA) 2940 from TA Instruments. The heating rate was 4 cpm over a temperature range of 25[degrees]C to 100[degrees]C. The coefficient of thermal expansion (GTE) at 80[degrees]C was determined with respect to 25[degrees]C as the reference temperature from the resulting plot in units of [micro] m/m[degrees]C.

Tensile strength and elongation at break (EB) were determined using a United Calibration Corporation test frame, model SFM-20 per ASTM D 638. Test conditions were a crosshead speed of 5.1 mm/min, 0.9 kg pre-load and a 454 kg capacity load cell. Impact resistance testing was carried out with an Impact tester from Testing Machines Inc., model 43-OA-0 1, according to the Izod method (ASTM D 256). The thickness of the samples for impact measurements was maintained at approximately 3.15 mm for compression molded test specimens and 3.3 mm for injection molded test specimens. Samples were notched using a TMI Notching Cutter, model TMI 22-05, and were left for one day at ambient temperature to equilibrate prior to testing. A pendulum capable of delivering impact energy of 6.8 J was used for these experiments.

Environmental scanning electron microscope (ESEM) images were taken using a Phillips Electroscan 2020. The specimens were fractured after cooling in liquid nitrogen and were gold sputtered before the images were taken.

RESULTS AND DISCUSSION

Cellulose acetate (CA), a renewable resource--based plastic, has the potential to replace polypropylene as a matrix polymer in eco-friendly bio-composite materials (15-17). Recently, several of world's leading chemical companies have announced major new businesses based on bio-resources instead of petrochemicals (18, 19), as biopolymers move into the mainstream (20. 21). Biopolymers were not developed by nature to be plastics, but rather to act as cellular components to build the structure of an organism to survive in the environment. Biopolymers must be modified to make them suitable matrix polymers for commercial composite applications. The main drawback of cellulose acetate plastic is that its melt processing temperature is close to its decomposition temperature. Cellulosic plastics can be developed either by blending with another suitable polymer or by being plasticized with a miscible plasticizer for specific applications. Cellulose ester polymers are blended with different polymers for film and molding applications (22-24). Plasticizers are widely used in the plastics industry to increase the processability, flexibility, and ductility of polymeric materials.

Comparison of Properties of Plasticized Cellulose Acetates--Compression Molding vs. Extrusion Followed by Compression Molding. The effect of the above processing conditions on the tensile strength, modulus and impact strength of plasticized cellulose acetates containing a fixed amount of TEC plasticizer (30 wt%) is illustrated in Figs. 2 and 3. The samples A. B and C are obtained through compression molding (CM) of the mixture of pure cellulose acetate powder and TEC plasticizer at temperatures of 190[degrees]C, 180[degrees]C, and 170[degrees]C, respectively. As can be observed, the sample processed at 170[degrees]C ("C") exhibited superior tensile and impact strengths over the samples processed at higher temperatures. The plasticized cellulose acetate obtained through extrusion followed by compression molding ("D") exhibited better tensile properties versus compression-molded samples. All the plasticized cellulose acetates had elongation at break (Fig. 3) in the range of 24% to 34%.

If we compare the tensile properties of the compression-molded sample ("B") versus the extruded followed by compression-molding sample ("D"), we find 48% and 13% enhancements of the tensile strength and modulus, respectively, of the latter material. The superior strength of the extruded material is attributed to the better mixing of cellulose acetate and TEC plasticizer during high shear extrusion processing. The lower impact strength of the extruded followed by compression molding material versus the compression molding material suggests that cross-linking of cellulose acetate with TEC may have occurred during the melt extrusion processing, which exposes the material to shear stresses during processing. This observation is supported by the environmental scanning electron microscope images in Fig. 4. Homogeneity of the blended mixture is observed, indicating that it will serve as a good polymer matrix. A similar reduction in the elongation at break for sample "D" is also observed, in Fig. 3. The plasticized c ellulose acetate obtained through compression molding exhibits superior impact properties as contrasted with samples obtained through extrusion followed by compression molding.

The main motivation of this research is to develop cellulose ester plastics to replace polypropylene in biocomposite applications. Although PP has adequate tensile and flexural properties, its low impact strength necessitated the invention of thermoplastic olefins (TPO), known as extended polypropylene, which is industrially produced through incorporation of an elastomeric component into the virgin polypropylene. The impact strength of plasticized cellulose acetate under various processing conditions ranges from 173 J/m to 419 J/m (Fig. 3), which approaches to impact strength value of TPO (25). Such high-impact plasticized cellulosic acetate can be used as a suitable matrix for biocomposites in automotive applications that require superior impact strength.

Comparison of Properties of Plasticized Cellulose Acetates--Extrusion Followed by Compression Molding vs. Extrusion Followed by Injection Molding. Plasticized cellulose acetate fabricated through extrusion followed by injection molding showed better tensile properties and lower impact properties over its counterpart obtained through extrusion followed by compression molding. As can be seen in Table 1, when using injection molding versus compression molding, we get a significant increase in tensile strength and modulus, about 40% and 60%, respectively. The additional shear force applied during injection molding may have enhanced the cross-linking between the cellulose acetate and TEC plasticizer, thereby increasing the stiffness of the injection molded material. As expected, the increased stiffness reduces the impact strength of the injection molded material (Table 1).

Different processing temperatures in the injection molding experiments were used to study the effect of temperature on the properties of the resulting cellulosic plastics (Table 2). Increasing the injection molding temperature from 180[degrees]C to 190[degrees]C enhanced the tensile strength by about 32%, whereas the impact strength decreased by about 75%. Hence, a 190[degrees]C processing temperature was utilized. To study the effect of extruder screw rpm on properties, we used the plasticized cellulosic acetates fabricated through extrusion followed by injection molding processing for comparison. We found that material processed at 100 rpm exhibited better mechanical properties than that processed at 200 rpm (Table 3). This means that the longer residence time (lower rpm) gave better mixing between the CA powder and the TEC plasticizer, which led to a stiffer product as cross-linking was enhanced. In all our previous experiments with extrusion, we processed the materials at 100 rpm.

Effect of Plasticizer Content on Performance of Cellulose Acetate Plastic. In our above-mentioned discussions on processing-property evaluations of plasticized cellulose acetate, we kept the plasticizer content at 30 wt%. In order to investigate the effect of varying the amount of TEC plasticizer on the performance of cellulosic plastics (Figs. 5-7), we prepared samples fabricated through extrusion followed by compression molding. It was not possible to process the cellulose acetate with plasticizer contents of 10 and 15 wt%. With an increase of plasticizer content from 20 to 40 wt%, the tensile properties decreased, while impact strength and percent elongation increased (Figs. 5 and 6). The plasticized cellulose acetate containing 30 wt% plasticizer shows tensile strength about 45% less and impact strength about 86% more than those of plasticized cellulose acetate with 20 wt% plasticizer.

In order to obtain cellulosic plastic with an optimum balance of strength and stiflhess, 30 wt% plasticizer content cellulose ester is quite appropriate for biocomposite applications. Again, it is easier to process cellulose ester with 30 wt% plasticizer than with 20 wt% plasticizer. Although 40 wt% plasticized cellulose ester would be processed easily, the tensile strength of such plastic would be too low for any structural applications, especially in composites. As much as 21% and 63% improvements in impact strength and elongation at break, respectively, were found in 40 wt% plasticizer content cellulose ester plastic as compared to 30 wt% plasticizer content cellulose ester plastic (Fig. 6). In stress-strain plots (Fig. 7), the 40 wt% plasticizer content cellulosic plastic sample covers a larger area under the curve, indicating that it is a ductile material, and hence it has the highest impact strength and elongation at break, as shown in Fig. 6.

Thermal and Thermo-Mechanical Behaviors of Plasticized Cellulose Acetates. The plasticized cellulose acetates fabricated through extrusion followed by compression moldings were studied via thermal analysis. The TGA curves of pure and plasticized cellulose acetates (with 20%. 30% and 40% TEC plasticizer) show overall addition of plasticizer shifts the maximum decomposition temperature toward the higher temperature range (Fig. 8). Pure cellulose acetate shows a smooth weight loss curve as contrasted with plasticized cellulose esters. We observe that as the plasticizer content increases, the maximum decomposition temperature of GA plastics also increases as can be observed in Fig. 8. Additionally, the thermal stability of the materials shows that as the plasticizer content increases, the coefficient of thermal expansion (GTE) increases as well (Fig. 91). It is well known that the GTE of the low molecular weight TEC plasticizer is greater than the cellulose acetate polymeric matrix, and thus with increase of plas ticizer content in the formulations, the GTE of the resulting plasticized cellulose acetate increases, as shown in Fig. 9. The presence of plasticizer limits the crystallinity in the matrix and increases the free volume present in the system. Thus, increasing the amount of plasticizer in the formulation gives a more flexible material. The increase of CTE with decrease of rigidity of polymeric materials also has been observed by Than et al. (26).

The sharp crystalline melting peak of pure cellulose acetate also disappears upon plasticization, as observed from the DSC curves (Fig. 10), confirming the shift of properties of the CA from rigid to ductile. Similar observations from DSC have also been noted by Ghiya et al. (11), indicating that the resulting plasticized CA is an amorphous material.

CONCLUSIONS

Cellulose acetate, a renewable resource--based bio-plastic, has excellent potential to replace petroleum--based polyolefins as matrix polymers in "green", eco-friendly bio-composites. It is encouraging to see that with control of plasticizer content, the cellulosic plastic can exhibit the stiffness and toughness properties comparable to those of either PP or TPO. Through plasticization of cellulose acetate by an environmentally friendly citrate plasticizer, the cellulose acetates are processable at 170[degrees]C-180[degrees]C, much below the melting point of cellulose acetate (233[degrees]C). Extrusion allows better mixing of cellulose acetate and plasticizer because of the high shear of extrusion processing, as shown by ESEM images. The material processed at lower extruder screw rpm (100 rpm) exhibited better mechanical properties than its counterpart processed at 200 rpm. The extruded sample showed lower elongation at break; thus it is a stiffer material and shows the least impact strength, whereas the comp ression molded sample exhibits the highest elongation at break, and thus was a tougher material and exhibited the highest impact strength. Materials processed by extrusion followed by injection molding exhibited better properties as compared to those processed by extrusion followed by compression molding, as additional shear forces applied during injection molding resulted in stiffer product. Cellulosic plastics fabricated through injection molding at a higher temperature (190[degrees]C) exhibited better tensile properties over their counterparts injected molded at a comparatively lower temperature (180[degrees]C). Thus, processing parameters played a vital role in designing cellulosic plastics with the desired properties for bio-composite applications.

ACKNOWLEDGMENTS

Financial support from the NSF/EPA (Award Number DMI-0124789) under the 2001 Technology for a Sustainable Environment (TSE) program is gratefully acknowledged. We are thankful to Eastman Chemical Company (Kingsport, TN), Johnson Controls (Milwaukee, Wisconsin) and Flaxcraft, Inc. (Cresskill, NJ] for theft collaborations. The authors also thank Dr. Brian D. Seller, Group Leader of Cellulose Ester Technology, Eastman Chemical Company, for the cellulose acetate samples and his valuable discussions during the course of this research work. The authors also thank Morflex Inc. (Greensboro, North Carolina) for supplying citrate plasticizer.

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[FIGURE 3 OMITTED]

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[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

[FIGURE 10 OMITTED]
Table 1

Effect of Optimized Processing Conditions on the Tensile Strength and
Tensile Modulus of Plasticized (30% TEC) Cellulose Acetate.

 Tensile Tensile Elongation at
Processing Strength (MPa) Modulus (GPa) Break (%)

E-CM at 180[degrees]C 26 [+ or-] 1.1 1.3 [+ or-] 0.1 24 [+ or-] 0.0
E-IM at 190[degrees]C 36 [+ or-] 0.4 2.1 [+ or-] 0.2 13 [+ or-] 1.7

 Impact Strength
Processing IZOD (J/m)

E-CM at 180[degrees]C 173 [+ or-] 35
E-IM at 190[degrees]C 68 [+ or-] 15

E-CM = Extruded followed by Compression Molding.

E-IM = Extruded followed by Injection Molding.

Note: Reported are mean values along with its standard deviation
([+ or-]).

Table 2

Effect of Temperature Variation for Extruded-Injection Molded CA
Plastics (30% TEC).

 Tensile Tensile
Processing Strength (MPa) Modulus (GPa)

E-IM at 180[degrees]C 27 [+ or -] 3.2 2.1 [+ or -] 0.0
E-IM at 190[degrees]C 36 [+ or -] 0.4 2.1 [+ or -] 0.2

 Elongation at Impact Strength
Processing Break (%) IZOD (Jim)

E-IM at 180[degrees]C 8.3 [+ or -] 3.2 241 [+ or -] 0.0
E-IM at 190[degrees]C 13 [+ or -] 1.7 68 [+ or -] 15

Table 3

Effect of Varying rpm of Extruder for Extruded-Injection Molding CA
Plastics (30% TEC)

 Tensile Tensile
Processing Strength (MPa) Modulus (GPa)

E-IM at 190[degrees]C, 100 rpm 36 [+ or -] 0.4 2.1 [+ or -] 0.2
E-IM at 190[degrees]C, 200 rpm 32 [+ or -] 3.8 2.1 [+ or -] 0.1

 Elongation at Impact Strength
Processing Break (%) IZOD (J/m)

E-IM at 190[degrees]C, 100 rpm 13 [+ or -] 1.7 68 [+ or -] 15
E-IM at 190[degrees]C, 200 rpm 14 [+ or -] 1.2 88 [+ or -] 21


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A. K. MOHANTY *

* To whom correspondence should be addressed.

E-mail: mohantya@egr.msu.edu
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Author:Mohanty, A.K.; Wibowo, A.; Misra, M.; Drzal, L.T.
Publication:Polymer Engineering and Science
Geographic Code:1USA
Date:May 1, 2003
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