Development of Biobased Unsaturated Polyester Containing

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MATERIALS AND INTERFACES Development of Biobased Unsaturated Polyester Containing Functionalized Linseed Oil Hiroaki Miyagawa,† Amar K. Mohanty,*,† Rigoberto Burguen˜ o,‡ Lawrence T. Drzal,§ and Manjusri Misra§ School of Packaging, Michigan State UniVersity, 130 Packaging Building, East Lansing, Michigan 48824-1223, Department of CiVil and EnVironmental Engineering, Michigan State UniVersity, East Lansing, Michigan 48824-1226, and Composite Materials and Structures Center, Michigan State UniVersity, 2100 Engineering Building, East Lansing, Michigan 48824-1226

Biobased neat unsaturated polyester materials containing epoxidized methyl linseedate (EML) were processed with cobalt naphthenate as a promoter and 2-butanone peroxide as an initiator. Up to 25 wt % of the unsaturated polyester resin (UPE) was replaced by EML to process biobased UPE. The combination of UPE and EML resulted in an excellent combination for a new biobased thermoset polymer with a relatively high elastic modulus and a high glass transition temperature with the replacement of UPE with EML. Izod impact strength increased with increasing amount of EML. The Izod impact strength of the biobased UPE was correlated with failure surface morphologies observed by scanning electron microscopy. Introduction The importance of natural products for industrial applications has become extremely clear in recent years with increasing emphasis on environmental issues, waste disposal, and depletion of nonrenewable resources. Renewable resource-based polymers can form a platform to replace/substitute petroleum-based polymers through innovative design for new biobased polymers that can compete or even surpass the existing petroleum-based materials on a cost-performance basis with the added advantage of ecofriendliness. As a result, the development of new biobased thermoset polymers is being accelerated.1-8 We have previously reported the thermophysical properties of developed biobased epoxy containing epoxidized linseed oil (ELO) cured with an anhydride curing agent9 and an amine curing agent.10 Petroleum-derived unsaturated polyesters (UPEs) are currently the most widely utilized thermoset polymer, because of their low cost; ease of handling; and good balance of mechanical, electrical, and chemical properties. Especially, known as generalpurpose UPEs, orthoresins are the least expensive among all UPEs. The current price of a general-purpose UPE is as low as $1.81/kg, which is less expensive than a general purpose epoxy resin.11 It is currently difficult to completely replace all petroleum-based polymer materials, in view of performance competition. As a result, it is more realistic to gradually and partially substitute petroleum-based polymer materials. In addition, the recycling of thermoset polymers has not fully been established, and the life of most thermoset polymers ends with incineration. As a result, the incineration of petroleum-based polymers results in an increase of carbon dioxide in the atmosphere. This creation of carbon dioxide from the incinera* To whom correspondence should be addressed. Tel.: 517-3553603. Fax: 517-353-8999. E-mail: [email protected]. † School of Packaging. ‡ Department of Civil and Environmental Engineering. § Composite Materials and Structures Center.

tion of the petroleum-based polymers is outside the natural cycle of oxygen and carbon dioxide generation. Thus, this industrial activity adds to the greenhouse effect, which is one of the most critical issues facing the earth’s environment. If biobased polymers were well developed and widely used, it would be possible to radically reduce the emissions of carbon dioxide from petroleum-based polymers, even if a recycling system for the polymers were not completely established. In other words, the carbon dioxide would be emitted from only biobased materials. Linseed is plentifully available all over the world. Linseed oil is one of the oldest commercial oils and has been used as a drying oil in painting and varnishing for several hundred years. Several varieties of functionalized linseed oils are currently commercially available for various applications in coatings and plasticizer additives. More value-added applications of such functionalized linseed oils will give a significant return to the agriculture sector while reducing the burden from petroleumbased products. Consequently, it is beneficial to combine some suitable functionalized linseed oils with petroleum-based UPEs to develop new biobased UPE. The modification of UPE resins with functionalized vegetable oils could be of intense research interest in the future, although the current price of functionalized linseed oil is at least $6.53/kg, which is more expensive than petroleum-based general-purpose UPEs. This study focuses on biobased UPE materials containing epoxidized methyl linseedate (EML) processed with cobalt naphthenate as a promoter and 2-butanone peroxide as an initiator. The thermophysical and impact properties of the newly developed biobased neat UPEs are discussed. The failure surface morphology was observed by scanning electron microscopy (SEM) after completion of the Izod impact testing in order to correlate failure features with the obtained Izod impact strength values.

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Experiments Materials. The main component was ortho UPE (Polylite 32570-00, Reichhold Inc., Durham, NC), which contains 33.5 wt % styrene. The building blocks of Polylite 32570-00 are diethylene glycol, propylene glycol, maleic anhydride, and phthalic anhydride. A biobased modifier, EML (Vikoflex 9010, Arkema, Booming Prairie, MN), replaced 10-25 wt % of the UPE. EML is a mixture of methyl esters of fatty acid compositions that construct the linseed oil. The detailed compositions are 40-50 wt % methyl linolenate epoxy, 2426 wt % methyl oleate epoxy, 17-22 wt % methyl linoleate epoxy, 4-7 wt % methyl palmitate, and 2-5% methyl stearate. The mixture of UPE and EML was processed with cobalt naphthenate (Sigma Aldrich, St. Louis, MO) as a promoter and 2-butanone peroxide (Sigma Aldrich) as an initiator. A constant ratio by weight of the mixture of UPE and EML to the promoter and initiator was utilized to cure all samples: 0.03 part promoter and 1.50 parts initiator were added to the mixture of UPE and EML. All samples were cured at 100 °C for 2 h and then at 160 °C for 2 h. Characterizations. (i) Dynamic Mechanical Analysis (DMA). Dynamic mechanical properties were measured with a TA Instruments DMA Q800 instrument operating in the three-point bending mode at an oscillation frequency of 1.0 Hz. It was found that the storage modulus was close to the tensile modulus when the oscillation frequency of 1 Hz was selected.12 The amplitude and static force were 75 µm and 1.0 N, respectively. DMA specimens were in the form of rectangular bars with nominal dimensions of 2.2 mm × 15 mm × 50 mm. Data were collected from ambient to 150 °C at a scanning rate of 2 °C/min. The glass transition temperature, Tg, was assigned as the temperature at which the loss factor was a maximum. The loss factor curve at the top was fit to a Gaussian curve in order to precisely determine the value of the glass transition temperature. A minimum of three specimens of each composition were tested. (ii) Izod Impact Testing. The Izod impact strength was measured for biobased neat UPE at room temperature. Izod impact specimens with the same dimensions as prescribed in ASTM D 256 standard were tested with a 453-g pendulum. The dimensions of the notched Izod impact specimens were 63.5 mm (length) × 12.7 mm (width) × 10 mm (thickness), with a notch depth of 1.5 mm and a radius of 0.25 mm. All specimens were cast in silicone molds having the same dimensions. Each specimen was held as a vertical cantilever beam and was impacted on the notched face by a single swing of the pendulum. A minimum of seven specimens for each composition were tested. (iii) Fractographic Observations. Impact failure surfaces of biobased UPE/EML specimens were observed with a JEOL 6300 SEM instrument with a field emission filament at a 10kV accelerating voltage or a JEOM 6400 SEM instrument with a LaB6 filament at a 15-kV accelerating voltage. The failure surfaces of the Izod impact specimens were coated with Au thin films having a thickness of a few nanometers.

Figure 1. Effect of EML concentration for neat UPE: (a) storage modulus, (b) loss factor.

Figure 2. Change in storage modulus of neat biobased UPE containing EML at 30 °C measured by DMA.

Results and Discussion Thermophysical Properties. Figure 1 shows the temperature dependency curves of the storage modulus and loss factor of UPE containing EML. Figure 1a shows that the storage modulus decreased with increasing amounts of EML. In Figure 1b, the peak positions of the loss factor curves were approximately 85100 °C when up to 25 wt % UPE was replaced by EML. The peak positions of the loss factor curves were shifted to lower

temperatures with increasing amounts of EML. In addition, the breadth of the loss factor peak seemed changed with different amounts of EML. These results are discussed further below. Figure 2 shows the relation between the storage modulus at 30 °C, below the glass transition temperature, and the amount of EML replacing UPE. The symbols represent experimental results, and the solid line is a least-squares fit to the data. The

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Figure 3. Change in glass transition temperature and heat distortion temperature of neat biobased UPE containing EML measured by DMA.

Figure 5. Change in storage modulus of neat biobased UPE containing EML at (Tg + 40) °C measured by DMA for estimating cross-link density.

Figure 4. Peak factor of loss factor curves of neat biobased UPE containing EML.

Figure 6. Change in cross-link density of neat biobased UPE containing EML.

storage modulus of neat UPE linearly decreased from 3.97 to 1.12 GPa with increasing amounts of EML up to 25 wt %. Judging from the least-squares fit line in Figure 2, it can be expected that the storage modulus of biobased epoxy containing 30 wt % EML will be only 356 MPa. Therefore, it became obvious that a rigid biobased UPE containing 30 wt % EML is not suitable for load-bearing applications. However, it was concluded that it was possible to replace up to 25 wt % of petroleum-based UPE with EML while maintaining a high storage modulus in the gigapascal range. Figure 3 shows the relation between the glass transition temperature, Tg, determined from the peak position of loss factor curve and the amount of EML replacing UPE. Symbols and lines show the experimental results and the average value of all neat UPEs, respectively. The glass transition temperature of the neat UPE was 101 °C. Once 5 wt % of UPE had been replaced by the same amount of EML, the glass transition temperature was significantly decreased to 88.8 °C and reached an almost constant value even when the amount of ELO was increased to 25 wt %. The decrease of Tg was due to the reduction of the cross-link density, because each fatty acid composition has only one CdO bond, whereas UPE has several CdO bonds, which provide more ability to cross-link in the presence of the styrene monomer. In other words, the smaller number of CdO bonds with the addition of EML results in a lower cross-link density. The minimum obtained value of Tg equal to 86.6 °C is still considered adequate for some engineering load-bearing applications. As seen in Figure 1b, the peak of the loss factor curve for all samples was broad. Figure 4 shows the relation between the peak factor of the loss factor curve and the amount of EML replacing UPE. The peak factor is defined as the value of fullwidth at half-maximum (FWHM) divided by the height of the

peak. This parameter can provide a qualitative assessment of the homogeneity of the polymer network and the distribution of the molecular weight. Therefore, a broader peak is reflected by a larger value for the peak factor. The peak factor of all neat UPEs increased from 113 to 216 °C with increasing amounts of EML to 25 wt %. This suggests that biobased UPEs have a broader glass transition region because of the heterogeneous EML/UPE polymer network. The heterogeneity of the biobased UPE network is due to the addition of EML, which is produced from natural resources and consists of five different fatty esters. This heterogeneous EML/UPE polymer network is also discussed with respect to FESEM micrographs in the following section. Cross-link Density. The cross-link density of the biobased neat UPE was evaluated by DMA. Figure 5 shows the relation between the storage modulus at (Tg + 40) °C and the amount of EML. As expected, the storage modulus at (Tg + 40) °C decreased with increasing amounts of EML. Considering this result together with those shown in Figure 4, it is obvious that the cross-link density decreased with increasing amounts of EML. The trace in Figure 5 showing the change of storage modulus at (Tg + 40) °C was used for cross-link density evaluations. The cross-link density can be calculated using the equation

E′ ) 3νeRT

(1)

where E′, νe, R, and T are the storage modulus at (Tg + 40) °C, the cross-link density, the gas constant (8.314 J·K-1·mol-1), and the temperature in K, respectively. Figure 6 shows the change of cross-link density calculated according to eq 1 using the storage modulus shown in the trace of Figure 5. Clearly, the cross-link density for the neat biobased UPE decreases when

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Figure 7. Change in Izod impact strength of neat biobased UPE containing EML.

the amount of EML is increased. This decrease of the crosslink density ultimately leads to the decrease in the storage modulus shown in Figures 2 and 5. Izod Impact Strength. Figure 7 shows the change of Izod impact strength of neat biobased UPE with different EML contents. As seen in Figure 7, no difference in Izod impact strength was observed when 10 wt % of the UPE was replaced by EML. After more than 10 wt % of the UPE had been replaced by the same amount of EML, the Izod impact strength increased to 16.3 J/m, approximately a 40% increase. Therefore, the addition of EML to UPE was beneficial in increasing the Izod impact strength.

To understand the mechanism behind of the increase of the Izod impact strength, the failure surface morphologies of the biobased UPEs were observed by SEM. Figure 8 shows lowmagnification SEM images of the impact failure surfaces of various neat UPE samples containing 0-25 wt % EML. In Figure 8a and b, the failure surfaces of neat UPEs containing 0-10 wt % EML were generally flat and featureless. This suggests that the behavior of neat UPEs with 0-10 wt % EML was elastic, and hence the crack propagated in a planar manner under the impact loading. In fact, the Izod impact strengths of these neat UPEs containing 0-10 wt % EML were constant. It should be noted that the neat UPE samples containing 0-10 wt % EML were colorless and transparent. Therefore, phase separation did not occur with less than 10 wt % EML. On the other hand, in Figure 8c and 8d, the failure surface morphology of biobased UPEs containing 20-25 wt % EML became rougher. It should also be noted that the neat UPE samples with 20-25 wt % EML were white and nontransparent. This suggests that all constituents of UPE and EML were not homogeneously mixed at the molecular level because of the different solubilities of UPE and EML and also explains the reason for the higher Izod impact strength upon addition of 20-25 wt % EML. Figure 9 shows high-magnification SEM images of impact failure surfaces of neat biobased UPEs containing 20-25 wt % EML. It is obvious that the failure surface roughness of the biobased UPE containing 25 wt % EML (Figure 9b) was larger than that with 20 wt % EML (Figure 9a). The larger failure

Figure 8. Low-magnification SEM images showing the impact failure surface of various biobased neat UPEs (scale bar ) 20 µm): (a) UPE without EML, (b) biobased UPE containing 10 wt % EML, (c) biobased UPE containing 20 wt % EML, and (d) biobased UPE containing 25 wt % EML.

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Conclusions This article has discussed the thermophysical and impact properties of a new biobased UPE containing up to 25 wt % EML processed with cobalt naphthenate as a promoter and 2-butanone peroxide as an initiator. The failure surface morphologies under the impact loading were observed by SEM. The storage modulus, cross-link density, and glass transition temperature decreased with increasing amounts of EML, which replaced the same amounts of UPE. The biobased UPEs containing more than 20 wt % EML were not transparent because of the creation of EML-rich rubbery phases. As a result, the Izod impact strength of the UPEs increased significantly with the addition of more than 20 wt % EML. This effect is attributed to phase separation, which was reflected in the opaque color of the samples and which was observed as particle-like morphologies on the failure surfaces by SEM. Acknowledgment The authors are grateful to National Science Foundation for the financial support to this project (NSF-CMS 2004 Award #0409666). The authors are also thankful to Arkema (Booming Prairie, MN), Kemlite Inc. (Joliet, IL), and Reichhold Inc. (Durham, NC) for supplying samples. Literature Cited

Figure 9. High-magnification SEM images showing the impact failure surface of neat biobased UPEs (scale bar ) 2 µm): (a) biobased UPE containing 20 wt % EML and (b) biobased UPE containing 25 wt % EML.

surface area is directly related to a higher failure energy dissipation. In fact, the Izod impact strength of the biobased UPE containing 25 wt % EML was larger than that of the biobased UPE containing 20 wt % EML. In this figure, a particle-like feature at the nanometer scale on the failure surfaces was observed. We have previously studied the phase separation of biobased epoxy containing epoxidized soybean oil (ESO) where it was observed that the Izod impact strength of the epoxy increased because of the second ESO-rich phase.13 Although a clear phase separation like that of the ESO-rich rubber phase was not observed in Figure 9a,b, the neat biobased UPEs containing 20-25 wt % EML was not transparent. Therefore, it is likely that some particle-like features observed on the failure surface in Figure 9a,b were also from EML-rich rubbery phases. The failure surface area after crack propagation increased, which can be observed in Figure 9b, and as a result, the existence of the nanoscale rubber phases caused greater energy dissipation during crack propagation under the impact loading. It is thus possible to expect that fracture toughness will also increase with the addition of EML.

(1) Dirlikov, S.; Frischinger, I.; Chen, Z. Phase separation of two-phase epoxy thermosets that contain epoxidized triglyceride oils. AdV. Chem. Ser. 1996, 252, 95-109. (2) Ratna, D.; Banthia, A. K. Epoxidized soybean oil toughened epoxy adhesive. J. Adhesion Sci. Eng. 2000, 14 (1), 15-25. (3) Ratna, D. Mechanical properties and morphology of epoxidized soyabean-oil-modified epoxy resin. Polym. Int. 2001, 50, 179-184. (4) Li, F.; Larock, R. C. New soybean oil-styrene-divinylbenzene thermosetting copolymers. III. Tensile stress-strain behavior. J. Polym. Sci. B: Polym. Phys. 2001, 39, 60-77. (5) Can, E.; Kusefoglu, S.; Wool, R. P. Rigid, thermosetting liquid molding resins from renewable resources. I. Synthesis and polymerization of soy oil monoglyceride maleates. J. Appl. Polym. Sci. 2001, 81, 69-77. (6) Khot, S. N.; LaScala, J. J.; Can, E.; Morye, S. S.; Williams, G. I.; Palmese, G. R.; Kusefouglu, S. H.; Wool, R. P. Development and application of triglyceride-based polymers and composites. J. Appl. Polym. Sci. 2001, 82, 703-723. (7) Belcher, L. K.; Drzal, L. T.; Misra, M.; Mohanty, A. K. Physicomechanical and morphological studies of bio-fiber reinforced bio-based epoxy resin composites for automotive exterior applications. Polym. Mater.: Sci. Eng. 2002, 87, 256-257. (8) Mehta, G.; Mohanty, A. K.; Misra, M.; Drzal, L. T. Biobased resin as a toughening agent for biocomposites. Green Chem. 2004, 6 (5), 254258. (9) Miyagawa, H.; Mohanty, A. K.; Misra, M.; Drzal, L. T. ThermoPhysical Properties of Epoxy Containing Epoxidized Linseed Oil, Part 1: Anhydride-Cured Epoxy. Macromol. Mater. Eng. 2004, 289 (7), 629-635. (10) Miyagawa, H.; Mohanty, A. K.; Misra, M.; Drzal, L. T. ThermoPhysical Properties of Epoxy Containing Epoxidized Linseed Oil, Part 2: Amine-Cured Epoxy. Macromol. Mater. Eng. 2004, 289 (7), 636-641. (11) The information was obtained from http://www.plasticsnews.com/ subscriber/resin/price4.html. (12) Miyagawa, H.; Rich, M. J.; Drzal, L. T. Thermophysical Properties of Anhydride-Cured Epoxy/Nano-Clay Composites. Polym. Compos. 2005, 26 (1), 42-51. (13) Miyagawa, H.; Misra, M.; Drzal, L. T.; Mohanty, A. K. Fracture Toughness and Impact Strength of Anhydride-Cured Biobased Epoxy. Polym. Eng. Sci. 2005, 45 (4), 487-495.

ReceiVed for reView August 2, 2005 ReVised manuscript receiVed October 26, 2005 Accepted November 28, 2005 IE050902E