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A novel biobased resin-epoxidized soybean oil modified cyanate ester.

INTRODUCTION

Vegetable oil represents one of the cheapest and most abundant biological feedstocks available in large quantities, and its use as starting material offers numerous advantages, such as low toxicity and inherent biodegrad-ability (1-3). The double bonds in these vegetable oils are used as reactive sites in coatings, and they can also be functionalized by epoxidation. Epoxidized vegetable oils show excellent potential as inexpensive, renewable materials for industrial applications (4).

In recent years, extensive work has been done to develop polymers from epoxidized triglycerides or fatty acids (5-11). These renewable resource-based polymers can form a platform to replace/substitute petroleum-based polymers through innovative design for new biobased polymers that can compete with 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 thermosetting polymers is being accelerated (12-17).

The current difficulties with some of the biobased polymers in commercial application are mainly due to their inferior mechanical and thermophysical properties in comparison with the conventional petroleum-based polymers that are intended to replace. Therefore, it is currently difficult to completely replace petroleum-based polymer materials for nothing more than the necessary mechanical and thermophysical properties.

However, it is not necessary to completely replace petroleum-based polymer materials immediately. As a result, a good solution combines different features and benefits of both synthetic and biobased materials to reduce the dependence on petroleum. Consequently, biobased materials were used to partly replace thermosetting resins, such as epoxy resin and unsaturated polyester (18-22).

At present, of all the available plant oils, soybean oil appears to be the most attractive alternative resource for biobased materials because of its low-price and abundant supply. The largest category of industrial soybean oil used includes plastics and resins. Epoxidized soybean oil (ESO) is currently mainly used as a plasticizer or stabilizer to modify the properties of plastic resins such as PVC, and ESO can also be used as a reactive modifier or diluent of other systems. For example, Park et al. (23) have studied the ESO and epoxy resins blends to obtain modified networks. The thermal stability and glass transition temperature of the blends were slightly decreased with increasing the ESO content, which might be caused by the reduction of crosslinking density of the epoxy network. Although the critical stress intensity factor and flexural strength were increased up to 10 wt % ESO content, Ratna et al. (24) prepolymerized ESO with the amine hardener to obtain the liquid epoxidized soybean rubber, which was used to modify epoxy networks. They found that phase separation took place and the optimum properties could be obtained at a suitable concentration of the modifier.

More recently, researchers have begun to explore the feasibility of manufacturing polymer composites from ESO. Crivello et al. (25) reported the fabrication and mechanical characterization of glass-fiher-reinforced and ultraviolet-cured composites from epoxidized vegetable oils. Up to now, the widespread structural applications of ESO have been limited because of its low crosslinking density and mechanical performance. The development of soybean oil-based resins for structural applications is still a challenge for the polymer and composite industries.

This study focused on biobased cyanate ester (CE) resin materials containing epoxidized soybean oil processed with copper (II) acetylacetonate as a catalyst and nonylphenol as a co-catalyst. The curing conversion, water absorption behaviors, thermophysical and mechanical properties of the newly developed biobased CE were discussed.

EXPERIMENTAL

Materials and Sample Preparation

The cyanate ester resin l,l'-bis(4-cyanatophenyl) ethane with a cyanate equivalent of 132 g/equiv was obtained from Shanghai Huifeng Technical and Business (Shanghai, China). A hiohased modifier, epoxidized soybean oil (oxirane contains as 6.8%), was obtained from Shanghai Tongxin Chemical Auxiliary Plant (Shanghai, China), replaced 5-30 wt % of the CE. The chemical structures of ESO and CE resin were shown in Scheme 1.

The blend of CE and ESO was cured with copper (II) acetylacetonate (Sigma-Aldrich) as a catalyst and nonylphenol (Sigma-Aldrich) as a co-catalyst. A constant ratio of the catalyst and co-catalyst to the blend of CE and ESO by weight was used to cure all samples. The mixing ratio was 100 parts by weight of the blend of the CE and ESO to 0.05 parts of the catalyst and 2 parts of the co-catalyst. The homogeneous mixture of ESO modified CE was prepared by dissolving ESO to the CE monomer in a sealed mixer, stirred vigorously at 120[degrees] C under nitrogen gas for at least 10 min till CE was completely dissolved, after the mixture had cooled to 80 [degrees]C, and then copper acetoacetate and nonylphenol catalyst were added. The samples were degassed under vacuum for a few minutes, and then cured isothermally at 177[degrees]C for 2 h, and post-cured at 210[degrees]C for 2 h.

[ILLUSTRATION OMITTED]

Curing Conversion Measurement

The curing conversion was determined by Perkin-Elmer differential scanning calorimeter (DSC, Pyris 1). The DSC was calibrated with high-purity indium. The samples around 10 mg were weighed and loaded into small aluminum lids, then, they were isothermally cured at 177[degrees]C under a nitrogen flow of 20 ml/min, the heat flow curves were obtained. Curing conversion was calculated from the contrasting residual exotherm, which was observed in scans over the temperature range from 50 to 350[degrees] C, with the total exotherms of uncured samples.

Fourier Transform Infrared Spectroscopy Measurement

Fourier transform infrared spectroscopy (FTIR) was performed using a Thermo Nicolet Nexus 440 Spectrometer with spectral range coverage from 4000 to 400 c[m.sup.-1]. CE and ESO were dissolved in C[H.sub.2]C[1.sub.2] and measured on KBr plates while the modified systems were cured on NaCl slice for FTIR measurement.

Morphology Observation

The morphologies of the samples cured isothermally at 177[degrees]C for 2 h and postcured at 210 [degrees] C for 2 h were observed by scanning electron microscope (SEM, Tescan TS5163MM). The cured samples were fractured in liquid nitrogen and coated with a fine gold layer before observation.

Dynamic Mechanical Analysis

The dynamic mechanical properties were collected with a Netzsch DMA 242 operating in the three-pointbending mode at an oscillation frequency of 1.0 Hz. The specimens for DMA were prepared in the form of cuboid bars with nominal dimension of 1 X 15 X 50 m[m.sup.3]. The data were collected from ambient temperature to 265 [degrees]C at a scanning rate of 3[degrees]C/min. The glass transition temperature (T.sub.g]) was assigned as the temperature at which the loss factor was the maximum. There were at least three specimens of each composition were tested.

Mechanical Properties

The mechanical properties of cured samples were recorded on an Instron-5565 universal tester at a constant temperature according to China State Standard GB 1040-79.

Water Absorption Testing

Five square samples (1.5 X 1.5 X 1.0 [cm.sup.3]) were cut from corresponding cured panel and dried at 80[degrees]C in full vacuum until no residual water could be detected in the samples. Then the dried samples were dipped in a jar full of boiling deionized water for various times and, after being wiped with a dry cloth, they were weighed on a balance to obtain the percentage of absorbed water. The average weight obtained from the five samples was plotted against time to compare the water absorption behaviors of different systems.

RESULTS AND DISCUSSION

Curing Conversion of the Modified Systems

Our proposal of ESO modified CE systems was based on the hypothesis of reactive sites of epoxy groups in ESO could co-reacted with CE resin during curing process. The copolymerization of epoxy groups and CE resin or phase separated structure would enhance some of the thermal and mechanical properties according to the previous report on reactive rubber modified thermosetting systems (26). Therefore, the co-reaction of ESO with CE resin was studied. Figure 1 showed the curing conversion of neat cyanate ester resin and modified systems cured isothermally at 177[degrees]C. As one can see, the neat CE had a curing conversion of about 80% at 10 min, while the systems with ESO showed an increase in curing conversion with the enhancement of ESO content, for example, the 20 wt % ESO modified blend had a conversion of 90% at the same time scale as neat CE resin.

[FIGURE 1 OMITTED]

The total heat of the reaction of neat CE resin was found to be - 102 kJ/mol (-388 J/g) of cyanate groups, which was also reported by other authors in the copper (II) naphthenate and nonylphenol catalyzed systems (27). With the addition of ESO, the total heat of reaction are -383, -378, -391, -372, and -367 J/g for 5, 10, 15, 20, and 30 wt % ESO modified systems, respectively, which showed a decline due to the co-reaction of cyanate with epoxy group and phase separation.

[ILLUSTRATION OMITTED]

It is well-known that epoxy group could co-react with cyanate, and the hydroxyl in commercial epoxy could catalyze the reaction of cyanate and enhance the curing rate (27). Similarly, in our studied systems, the curing rate of ESO modified systems showed a slight increase when compared with the neat CE resin due to the existence of hydroxyl groups in ESO.

Curing Co-reaction of the Modified Systems

FTIR was used to monitor the curing co-reaction of the modified systems. The consumption of cyanate ester group was clearly demonstrated by the decrease of the characteristic epoxy peak at 2230 c[m.sup.-1] together with the epoxy group at 830 c[m.sup.-1]. The appearance of cyanurates and Oxazolidinone group at 1565 cm.sup.-1 and 1760 c[m.sup.-1] attributed to the homopolycyclotrimerization of cyanate ester and its co-reaction with epoxy group. The hydroxyl group as a result of co-reaction of epoxy group showed an increase of peak area at 3400 c[m.sup.-1]

As shown in Fig. 2, the oxazolidones were produced by the co-reaction of cyanate and epoxy group of ESO in the modified systems. The band at 1760 c[m.sup.-1] increased with the increment of ESO weight content due to the co-reaction of epoxy group with cyanurates.

[FIGURE 2 OMITTED]

According to the FTIR results and literatures, a schematic illusion of the co-reaction between cyanate and epoxy group was shown in Scheme 2. Trimerization of cyanates were formed and copper acetylacetonate was used as a catalyst, there were isomerization process after oxiranes were inserted to cyanurate and the oxazolidones were produced from isocyanurates (27).

Morphologies of the Modified Systems

For polymer blend, the morphology is one of the most important factors that control the properties of the modified systems. The morphologies of fractured surfaces of blends were observed by SEM. As shown in Fig. 3, homogeneous structures were observed for the neat CE, and low weight percent ESO (5 and 10 wt %) modified CE systems, while ESO-rich particles (correspond to the holes in the images) with diameters of about 0.2-1.2 [micro]m were observed in the modified systems with above 15 wt % ESO. The diameter of ESO-rich particles by measuring them in a randomly selected square of 10 [micro]m X 10 [micro]m from the SEM images and calculated the average diameter and standard errors. In the 15 wt % ESO modified system, the ESO-rich particles were dual-distribution and their average diameters were 0.2 and 0.5 [micro]m, respectively, the average diameter of total particles is about 0.4 [micro]m with a standard error of 0.2 [micro]m. The average diameter of the ESO-rich particles in the 20 wt % ESO modified system was about 1.2 [micro]m with a standard error of 0.15 [micro]m. And in the system with 30 wt % ESO, the average diameter of the ESO-rich particles was about 0.7 [micro]m with a standard error of 0.1 [micro]m. Compared with 15 wt % ESO modified system, the average diameter of ESO-rich particles in the 20 wt % ESO modified system increased because of the high ESO content, however, in the system modified with 30 wt % ESO, the average diameter of ESO-rich particles decreased, which may be caused mainly by the decrease of phase separation time because of acceleration of curing reaction.

[FIGURE 3 OMITTED]

Mechanical Properties of the CEIESO Blends

Generally, two-phase structures are more effective for the toughening purpose rather than the homogenous structure in blend systems. As the morphologies of ESO modified CE systems showed great difference at various ESO contents, it would be expected to obtain an apparent difference between the ESO/CE blends in the mechanical properties.

As shown in Fig. 4 and Table I, a series of tensile and flexural tests were carried out to study the effect of ESO content on the mechanical properties of the blends. Compared with neat CE resin, all specimens with ESO showed larger yields before breaking. The higher proportion of epoxidized soybean oil resulted in a more ductile behavior with one-step curing process. High elongation (up to nearly 8.5%) was observed with 20 wt % ESO. Figure 4 compared the ductility behavior of the ESO modified systems and the neat CE resin. Obviously, the flexibility properties in the blends came from the two phase structure. As shown in Table I, the improvement in the ductility was accompanied by corresponding reduction of modulus while the tensile strength changed a little.

[FIGURE 4 OMITTED]

Previous work of toughening by ESO in epoxy systems can result in a dramatic decay of the tensile strength with a higher content of vegetable oil (28). However, in this work, the reduction of the tensile strength was less pronounced with enhanced ESO formulation. This might be caused by the difference of reactivity of ESO with epoxy resin and cyanate ester resin. The epoxy groups of ESO have much lower reactivity than the most common type of epoxy resin (such as DGEBA) due to the stereo effect in ESO; therefore, it can only cured partially with epoxy resin. However, cyanate ester resin with catalysts could react quickly with ESO, and the mechanical properties indicated that there was part of the ESO participated in the crosslinking reaction, rather than acted as a plasticizer or toughening agent only.

Table 1 presented the tensile and flexural properties of various compositions. The highest modulus was observed for the neat CE formulation. With 5 wt % content of ESO, the tensile strength and elongation at break were even improved somewhat over those of the neat CE system. However, the modulus degraded dramatically with increasing in the ESO content up to 30 wt %. For the one-step curing process, the direct mixing of the ESO with the base CE prevented the ESO from fully participating in the crosslinking because of the phase separation. Therefore, the resulting crosslinked thermosets were plasticized by only partially reacted ESO at high contents. For the one-step curing process, the use of ESO was limited to 20 wt % without much degradation of the mechanical strength.

At higher ESO content (30 wt % as an example), both the tensile strength and elongation at break began to decrease but the extent was much less pronounced. As a matter of fact, the flexural properties did not show any degradation at 20 wt % ESO modified system. The result showed that the potential exist of adding more ESO as supplement for cyanate ester resin.
TABLE1. Tensile properties of ESO modified CE systems (error, 8%).

Sample Neat CE ESO (5 wt %) ESO (10 wt %)

Modulus (GPa) 2.4 2.2 1.6
Tensile strength (MPa) 64 68 63
Elongation at break (% ) 3.9 4.7 4.8

Sample ESO (15 wt %) ESO (20 wt %) ESO (30 wt %)

Modulus (GPa) 1.3 1.3 1.3
Tensile strength (MPa) 61 59 51
Elongation at break (% ) 6.8 8.3 5.5


Water Absorption Behaviors of CE/ESO Systems

Figure 5 showed the water absorption curves of neat CE and CE/ESO blends. When the content of ESO was lower than 5 wt %, water absorption of modified CE casting dipped in boiling water descended slightly with increasing of ESO content. However, the water absorption of modified systems was much higher than that of the neat CE when ESO content was more than 15 wt %, which could be caused by the integrated effect of phase separation, the difference of free volume, and polarity of triazine ring and oxazolidinone ring. Water absorption of CE/ESO blends dropped due to the increase of hydrophobic property when low content of ESO is introduced. However, it rose with the increasing of ESO content because of the change from nonpolar hydrophobic triazine ring group to hydrophilic oxazolidinone. Thus, the dominant effect on the water absorption was the decrease of hydrophobic property when ESO content was low; but with high percentiage of ESO, the hydrophobic property of CE matrix might be kept to a constant value, thus the water absorption increased obviously when ESO concentration was over 15 wt %.

[FIGURE 5 OMITTED]

Thermal Properties

The ability of a polymeric material to withstand load at elevated temperatures is one of the key aspects of engineering performance to be studied. Dynamic mechanical analysis (DMA) is a method that measures the stiffness and mechanical damping of a cyclically deformed material as a function of temperature. The loss factor (tan [delta]) is a sensitive indicator of crosslinking and the glass transition temperature ([T.sub.g]) value is assumed as the temperature corresponding the maximum of tan [delta] versus temperature curve, therefore, the [T.sub.g] value is assumed as the temperature corresponding the maximum of the tan [delta] versus temperature curve, therefore, the [T.sub.g] of sample could indicate the cross-linking density of a polymeric material.

The representative storage moduli (G') and glass transition temperatures ([T.sub.g]) of the six samples were shown in Fig. 6, the G' and [T.sub.g] of the ESO modified CE blends were much lower than that of neat CE network. This might be attributed to the reduced cross-linking density of the CE network because of the plasticizing effect of ESO. In addition, the morphlogy of the blend may be another contributor, with increasing of ESO content, the ESO particles separated from CE matrix, partly reacted ESO formed the seconded phase as in the blends with 15, 20, and 30 wt% ESO, which would decrease G' and [T.sub.g] notably at the same time. To sum up, the presence of ESO had a negative effect on the thermal properties of cyanate ester network.

[FIGURE 6 OMITTED]

CONCLUSIONS

The morphologies of the CE/ESO blends changed from homogeneous below 10 wt % of ESO content to separated particles of ESO when its content was above 15 wt %, especially, the uniformly separated ESO-rich particles with diameter of about 0.7-1.2 [micro]m were observed in the modified systems with 20 wt % and 30 wt % of ESO. The co-reaction of ESO and CE resin increased the curing conversion and curing rate of the modified systems. Enhanced elongations at break were observed for the modified systems with increase of ESO content, while the tensile strengths keep about the same level at the same time. The glass transition temperatures and storage moduli of the networks in the glassy state and rubber plateau were observed to be lower than those of neat CE with the increase of ESO weight percent. At low ESO content, the water absorption of blends would drop down compared to that of neat cyanate ester resin. From the results and discussion, the novel biobased resin-epoxidized soybean oil would be an expecting replacer at suitable content and modifier for cyanate ester resin.

Guozhu Zhan, (1) Lin Zhao, (1) Sheng Hu, (1) Wenjun Gan, (2) Yingfeng Yu, (1) Xiaolin Tang (1)

(1) The Key Laboratory of Molecular Engineering of Polymers, Ministry of Education, Department of Macromolecular Science, Fudan University, Shanghai, 200433, China

(2) Department of Macromolecular Materials and Engineering, College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, China

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Correspondence to: Yingfeng Yu: e-mail: yfyu@fudan.edu.cn or Xiaolin Tang; e-mail: txl@fundan.edu.cn

Contract grant sponsor: National Natural Science Foundation of China (NNSFC); contract grant number: 20704008; contract grant sponsor: The Specialized Research Fund for the Doctoral Program of Higher Education of China (SRFDP); contract grant number: 20070246001; contract grant sponsor: Key Project Science Foundation of Shanghai Municipal Commission of Education; contract grant number: 06zz78; contract grant sponsor: Shanghai Leading Academic Discipline Project; contract grant number: P1402.

DOI 10. 1002/pen.21096

Published online in Wiley InterScience (www.interscience.wiley.com) [C] 2008 Society of Plastics Engineers
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Article Details
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Author:Zhan, Guozhu; Zhao, Lin; Hu, Sheng; Gan, Wenjun; Yu, Yingfeng; Tang, Xiaolin
Publication:Polymer Engineering and Science
Article Type:Technical report
Geographic Code:1USA
Date:Jul 1, 2008
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