Rheological Studies of High-Performance Bioepoxies for Use in

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Rheological Studies of High-performance Bioepoxies for use in Fiber Reinforced Composite Resin Infusion Johannes Möller, Christopher N. Kuncho, Daniel F. Schmidt, and Emmanuelle Reynaud Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03825 • Publication Date (Web): 11 Jan 2017 Downloaded from http://pubs.acs.org on January 17, 2017

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Industrial & Engineering Chemistry Research

Rheological Studies of High-performance Bioepoxies for use in Fiber Reinforced Composite Resin Infusion Johannes Möller1, Christopher Kuncho1, Daniel F. Schmidt1*, Emmanuelle Reynaud2 1

Department of Plastics Engineering, University of Massachusetts Lowell, Lowell, MA, USA

2

Department of Mechanical Engineering, University of Massachusetts Lowell, Lowell, MA,

USA

KEYWORDS bioepoxy, rheology, fiber reinforced composite, epoxidized linseed oil processability

ABSTRACT

A new approach is presented to quantify the processability of high-performance bio-based epoxy formulations via vacuum assisted resin transfer molding (VARTM). Epoxidized linseed oil (ELO)

is

cured

with

methyltetrahydrophthalic

two

anhydride

anhydride

hardeners,

(MTHPA,

nadic

NMA)

ACS Paragon Plus Environment

and

methyl two

anhydride

and

catalysts,

1,8-

1


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diazabicyclo[5.4.0]undec-7-ene and 2-ethyl-4-methylimidazole (DBU, NMA). As neither gel times nor the viscosity-time curves reported vs. temperature are sufficient to assess processability, we integrate fluidity-time curves to generate a useful numerical metric for the infusibility of a given resin formulation, and report the variation of integrated fluidity with temperature. A conventional high performance anhydride-cured epoxy serves as a control. ELONMA-DBU & ELO-MTHPA-2E4MI show significantly greater infusability vs. the control at T ≤ 120°C; ELO-MTHPA-DBU shows greater infusability at T ≤ 80°C, and ELO-NMA-2E4MI shows greater infusability in general. These attractive characteristics highlight the potential of bioepoxies as the basis for more sustainable fiber composites.

INTRODUCTION Fiber reinforced composites (FRC) offer a unique combination of modulus and strength. In addition, their low density results in a strength-to-weight ratio which is notably larger than traditional metallic materials1. Most FRCs exhibit significant anisotropy, since their properties depend largely on the orientation of the reinforcing fibers. This adds more complexity to part design, but also increases design freedom and allows for improved lightweight construction. Additional advantages of FRCs are markedly lower coefficients of thermal expansion (CTE) compared to metallic materials, significant mechanical damping and excellent corrosion resistance.1 This qualifies FRCs for an abundance of application in the aerospace, marine, automotive, transportation, infrastructure, construction and wind energy industries.1-3 For the majority of these applications thermosets are used as matrix material. The presented work focuses on epoxy resins, 90% of which are based on the reaction product of 2,2-bis(4’hydroxyphenyl)propane (bisphenol A) and epichlorohydrin, yielding the diglycidyl ether of


2

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bisphenol A (DGEBA).4 While these resins display excellent performance, one disadvantage of their use is that they are typically derived from petroleum feedstocks. The environmental challenges this implies, as well as the finite nature of fossil fuel resources5 and the volatility in oil prices6 have encouraged recent research efforts on the development of bio-derived resins as a pathway to more sustainable matrix materials. An increasing amount of research has been conducted on bio-derived thermoplastic materials like starch plastics, cellulosic polymers, polylactic acid (PLA), polyhydroxyalkanoates (PHAs) and their composites7-12, with successful commercialization following13. There are parallel research efforts in the field of thermosetting matrix materials4, 14-17, including most recently reports concerning epoxies from lignin18 and tree bark extract19, but market penetration is more limited compared to bio-based thermoplastics. In this report, the focus is on the use of unsaturated vegetable oils such as linseed oil, which may be epoxidized in a clean single step process20 and used as additives in or replacements for conventional epoxy resins. Such materials have a number of advantages. They are readily available, relatively inexpensive and minimally toxic (epoxidized linseed oil, ELO, is FDA approved for food contact, for instance21). As an additional advantage, ELO is derived from a non-food crop, and has the highest functionality of any commonly available epoxidized vegetable oil (albeit in the form of secondary epoxies with low reactivity). Likewise, our group’s work with ELO-based networks indicates that they are capable of displaying combinations of modulus, strength and glass transition temperature comparable to conventional epoxies and appropriate for structural applications.22 While it is possible to find studies of the preparation and properties of fiber-reinforced bio-epoxy (nano)composites in the literature involving for example nano-cellulose23 or carbon fibers24, relatively little attention has been paid to the question of processing in this context. Rheological behavior and viscoelastic properties of conventional


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epoxy resins25-28 have been studied by various groups in the past, but the number of rheological studies of bio-based resin systems29, 30 is significantly less. While work has been carried out on bio-composite manufacturing, examples of successful processing via wet layup or compression molding depend less critically on an understanding of process rheology. Liquid composite molding (LCM) processes, such as vacuum assisted resin transfer molding (VARTM), have been identified as a promising and more cost effective approach for the manufacture of bio-based composite parts.31 For these processes, in contrast, rheological properties and their change over time and temperature are of much greater importance, as emphasized by the existence of recent efforts aimed at modeling such processes in the context of bio-based systems in particular32. Likewise, none of the process-focused reports in the literature use the approach introduced here; instead, they either focus on permeability of the fiber mats themselves33 or measure gel times34 and / or viscosity measurements35, 36 as a means to judge processability. This study addresses these issues and introduces a novel way of evaluating processability. Four high performance anhydride cured bio-based epoxy resin formulations were studied alongside a conventional epoxy / anhydride control, with both viscosity evolution and gel time measurements determined via parallel plate rheology. What follows is a report on the outcome of these efforts, the trends observed in the resultant data, and the implications for both the test method and the materials systems being studied.

MATERIALS Epoxidized linseed oil (Epoxol) with an oxirane content of 9.57% was provided by American Chemical Service. Nadic methyl anhydride (NMA) was purchased from Polysciences Inc., methyltetrahydrophthalic anhydride (MTHPA) was donated by Huntsman (Aradur 917), 1,8-


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diazabicyclo[5.4.0]undec-7-ene (DBU) was purchased from Alfa Aesar and 2-ethyl-4methylimidazole (2E4MI) was purchased from TCI Chemicals. The Momentive® EPIKOTE™ MGS® 145 system (consisting of RIMR 145 resin, RIMH 145 hardener and RIMC 145 catalyst), donated by Momentive, was used as a control system and is a high performance conventional epoxy / anhydride formulation used in structural composites. Figure 1 presents the structures of all components used in the bio-based resin formulations.

Figure 1. Structures of ELO (one of a number of potential components), MTHPA, NMA, 2E4MI and DBU (from left to right) Table 1 provides an overview of all five resin formulations investigated. The biobased resins were formulated using a 1:1 reaction stoichiometry, whereas the mix ratios of the conventional resin system were selected according to the manufacturer’s product bulletin.

Table 1. Overview of different resin formulations tested. ELO ELO ELO ELO RIMR 100 phr 100 phr 100 phr 100 phr 100 phr NMA MTHPA NMA MTHPA RIMH Curative 31.15 phr 49.71 phr 31.15 phr 49.71 phr 82 phr DBU DBU 2E4MI 2E4MI RIMC Catalyst 6.7 phr 4.5 phr 7.5 phr 6 phr 0.5 phr Resin

The catalyst loadings of the bioepoxies shown in Table 1 were selected following screening studies examining hardness as a function of catalyst loading for each formulation. The hardness of the cured resin (cast in an open-face mold) was measured at five points on the top and five


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points on the bottom of each sample produced22, and served as a proxy for the elastic modulus.37 Optimal catalyst content was defined as the minimum catalyst level necessary to maximize the recorded hardness values while minimizing the differences in hardness between the top and bottom surfaces of the sample.

EXPERIMENTAL METHODS The desired quantities of resin, curative and catalyst were weighed into a disposable poylpropylene Max 20 cup to give a total sample mass of 15 g. They were then mixed under 0.04 bar vacuum for 2 min at 800 rpm, then for 8 min at 1200 rpm in a FlackTek DAC 600.1 VAC-P speed mixer. A single mixing operation provided enough material for an entire rheological test series, thus reducing variability. In between runs, the remaining material was purged with UHP argon (Airgas), sealed inside the mixing cup with Parafilm and stored in a freezer at -12°C. Before opening the sealed container, care was taken to let the material warm up to room temperature, thus preventing condensation and contamination of the system with water. Rheological analyses were performed on a TA Instruments ARES-G2 rotational rheometer. 25 mm diameter flat parallel disposable aluminum plates with a 1 mm gap setting were used for oscillation measurements. Isothermal runs carried out between 70°C and 150°C in steps of 10°C were performed at an oscillating frequency of 4 Hz with 20% initial strain. The amount of material loaded was adjusted to fill a 1 mm gap without forming a meniscus. Before data recording started, the material was given 60 s to equilibrate at the setpoint temperature. The lengthy nature of these experiments especially at the lowest testing temperatures limited this work to one test per formulation and temperature.


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Storage modulus G’ and loss modulus G” values were plotted versus time and their crossover point was used to estimate the gel point of the material as introduced by Tung and Dynes38. As is common practice when considering the problem of composite infusion, Darcy’s law was invoked to assess the potential infusibility of a given system. Darcy’s law describes flow of a fluid through a porous medium according to Equation 1.

Q=

− κA (∆p )L µ

(1)

…where Q is the total discharge (units of volume per time), κ is the intrinsic permeability of the medium, A is the cross-sectional area of flow, (∆p) is the total pressure drop, µ is the viscosity and L is the the total length in the flow direction over which the pressure drop takes place. It can be seen that infusion rate is proportional to 1/µ, the inverse of the dynamic viscosity, also known as fluidity. At any given instant during an infusion process, higher fluidity should translate into more rapid infusion. Furthermore, if both sides of Equation 1 are multiplied by time, it becomes evident that the total volume of infused material will be proportional to the product of fluidity and time. Therefore, for each formulation, fluidity was plotted versus time, and the resultant curve was mathematically integrated up to the gel point to produce a quantitative performance metric of the degree of infusion expected from a given formulation at a given temperature.

RESULTS AND DISCUSSION


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Figure 2 shows the development of the dynamic viscosity obtained during the isothermal measurements. It can be seen that for all ELO / NMA formulations, both DBU and 2E4MI catalyzed, there is a significant delay in viscosity increase compared to the conventional control, with the DBU catalyzed system cured at 150°C representing the sole exception. Likewise, these curves show that 2E4MI is clearly the less efficient catalyst as compared to DBU. In Figure 3 the same data is displayed for the MTHPA formulations. It should be noted that the MTHPA-DBU formulation showed excessive voiding at T > 130°C, precluding the collection of high quality data at these temperatures. Here a wider range of behavior is observed. DBU and 2E4MI give faster increases in viscosity vs. the conventional control at temperatures higher than 90°C and 100°C, respectively, whereas at lower temperatures the conventional control shows a more rapid viscosity rise. Based on the overall spread of these families of curves, the data also shows that the NMA-DBU system and both MTHPA-2E4MI systems show more temperature sensitivity than the other systems; in contrast, the NMA-2E4MI system shows viscosity curves that more closely resemble those of the RIM145 in terms of evolution, albeit shifted to longer times.


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Figure 2. Development of dynamic viscosity vs. time at different temperatures for NMA cured ELO formulations with two different catalysts, as well as a conventional control

Figure 3. Development of dynamic viscosity vs. time at different temperatures for MTHPA cured ELO formul

ations with two different catalysts, as well as a conventional control


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Keeping the composite resin infusion process in mind, a slower increase in viscosity may be beneficial, since it broadens the processing window and increases the flow length, which allows for proper wetting out of the fiber layup as well as the realization of more complex geometries. That said, the point at which gelation occurs is also important in determining the limits of a resin infusion process. With that in mind, Figure 4 and Figure 5 show representative plots of moduli versus time for the four bioepoxy formulations at 120°C. The existence of a distinct time at which crossover occurs represents a convenient means of determining the gel point, one which has been applied to all materials tested.

Figure 4. Representative storage (G’) and loss (G”) modulus curves for NMA cured formulations at 120°C


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Figure 5. Representative storage (G’) and loss (G”) modulus curves for MTHPA cured formulations at 120°C Table 2 presents the gel times of all formulations studied, as determined by the G’ / G” crossover behavior. Table 2. Gel times as a function of temperature for all formulations studied

Temperature (°C) 70 80 90 100 110 120 130 140 150

Gel time (seconds) DBU-catalyzed 2E4MI-catalyzed RIM145 ELO-NMA ELO-MTHPA ELO-NMA ELO-MTHPA 58000 28000 72000 39000 57000 25000 13000 37000 17000 31000 11000 5700 18000 7500 8600 5700 2700 9300 4300 4400 2800 1300 5000 1700 2200 1400 630 3300 1000 1300 820 330 2100 550 770 460 1400 320 460 260 910 180 350


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To facilitate comparisons and determine activation energies this data is represented in the form of an Arrhenius plot in Figure 6.

Figure 6. Arrhenius plot of the effect of temperature on gel time in seconds in various ELObased bioepoxy formulations as well as in a conventional control (added curves are linear fits) Fitting parameters obtained from the linear fits shown in the Arrhenius plot are displayed in Table 3.

Table 3. Fitting parameters and standard errors obtained from the Arrhenius fits of gel times in various ELO-based bioepoxy formulations as well as a conventional control

Formulation Activation Energy (kJ/mol) Pre-exponential Factor (s-1)

NMA

MTHPA

RIM145

DBU

2E4MI

DBU

2E4MI

81.0±0.6

65.9±1.6

85.4±0.8

81.1±1.1

79.1±3.4

-17.5±0.2

-12.0±0.5

-19.7±0.2

-17.9±0.4

-16.9±1.1


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The high quality of the Arrhenius fits (r2 > 0.98 in all cases) shows that the gel times of all formulations studied here follow Arrhenius behavior over the range of temperatures studied. When comparing the two anhydrides in the context of ELO curing, NMA exhibits lower activation energies and longer gel times compared to MTHPA. Looking at the catalysts, 2E4MI lowers the activation energies and gel times more effectively vs. DBU regardless of the anhydride used. This is distinct from what is observed in Figure 2 and Figure 3. Despite the fact that RIM145, NMA-DBU and MTHPA-2E4MI display equivalent activation energies for gelation, the increase in dynamic viscosity in the MTHPA-2E4MI system is significantly delayed vs. the conventional control at all but the highest temperatures; for NMA-DBU this effect is even more pronounced. While the plots in Figure 2 and Figure 3 provide clear indications of rheological behavior, a simple metric describing the viscosity trends in these systems is not immediately obvious. Figure 6 yields simple metrics for each system in the form of gel times and activation energies, but these metrics do not account for the aforementioned viscosity trends. At this point it is worthwhile to consider these observations in the context of the chemical structure of the resins being studied. The functionality of the ELO molecules is high compared to that of conventional resins, with each ELO molecule containing an average of 5-6 epoxy groups based on the reported oxirane oxygen content. This has definite consequences as far as process rheology is concerned, given the well-known Flory-Stockmayer criterion for gelation; specifically, the greater the functionality of the species involved, the lower the extent of reaction required for network formation. At the same time, the epoxy groups in the ELO molecules are secondary epoxides, whereas in the conventional system they are much more reactive primary epoxides. Furthermore, in the case of the ELO molecules multiple secondary epoxides exist in


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close proximity along the same fatty acid chain. Once one of these groups has reacted, the others experience greater steric hindrance, further reducing their reactivity.39 This is not so in the case of the conventional control, where the primary epoxy groups are isolated from one another and the reaction of one has little impact on the reactivity of others in the same molecule. This combination of higher functionality and lower reactivity in the bio-based systems helps to explain why the gel times in the bio-based systems are overall rather similar to those in the conventional control; the higher functionality of the ELO, which favors earlier gelation, is canceled out by the lower reactivity of the epoxides it contains, which favors later gelation. This scenario also helps to explain why many of the bio-based formulations are able to “catch up” to the conventional controls, as far as the viscosity-time curves are concerned, as the temperature is increased. Once enough thermal energy is available to overcome the inherently low reactivity of the secondary epoxides in the ELO, the higher functionality of the ELO molecules drives a more rapid rise in viscosity. While admittedly this is further complicated by formulation-specific differences in the activity of the catalysts in use here – a situation that would require additional studies outside of the scope of this report – these structural arguments help to explain much of what is observed as far as gel times and viscosity-time curves are concerned. While the aforementioned explanations are informative, returning to our focus on process rheology, neither the gel times nor the viscosity-time curves alone provide sufficient information to assess and evaluate the processability of a given resin system in the context of a composite infusion process. In order to address this issue, we have applied a novel approach involving plots of inverse viscosity, also known as fluidity. Similar to Figure 2 and Figure 3, fluidity has been plotted versus time to give curves that are more amenable to numerical integration than viscosity curves that asymptotically approach infinity. These curves capture both viscosity trends and


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gelation behavior and are therefore able to fully capture the potential degree of infusion of a particular formulation at a given temperature. As an example, Figure 7 shows fluidity plots for all 5 formulations at 80°C.

Figure 7. Representative curves of fluidity versus time for each formulation at 80°C The fluidity curves were then integrated to provide a single quantitative metric describing infusibility and processability in these systems. These integrated fluidity values were then plotted vs. temperature for each resin formulation tested. The results are presented in Figure 8 and Figure 9, along with three-parameter exponential fits to guide the eye, and provide a quantitative indication of the degree of infusion that may be expected from each resin system as a function of temperature. The NMA cured, 2E4MI catalyzed bioepoxy outperforms the conventional control over the entire temperature range; the MTHPA cured equivalent does so at temperatures below 120°C. Likewise, the results for the NMA and MTHPA cured, DBU catalyzed bioepoxies imply


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a higher degree of infusion than the conventional control at temperatures below 120°C and below 80°C, respectively.

Figure 8. Integrated fluidity vs. temperature for NMA cured ELO formulations with two different catalysts, as well as a conventional control


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Figure 9. Integrated fluidity vs. temperature for MTHPA cured ELO formulations with two different catalysts, as well as a conventional control While more data is needed to confirm the exact nature of the trends observed here, this data offers clear guidance concerning the overall degree of infusion to be expected from these systems as a function of temperature, and demonstrates the potential utility of the bioepoxy formulations described here as far as achieving more rapid and complete infusion is concerned.

CONCLUSIONS Sustainable, high performance, easily processed resins are needed for composite manufacturing. As ELO-based formulations cured with NMA or MTHPA and catalyzed with DBU or 2E4MI appear to be promising candidates in this regard, their rheological properties have been studied in detail. It has been shown that neither viscosity curves nor gel time data alone are able to fully capture the degree to which these materials may be infused, emphasizing


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the need for a new approach to quantify the potential degree of infusion; this approach has been presented in terms of Darcy’s Law and utilizes a quantity referred to as integrated fluidity. This data is comparably easy to obtain using a minimal amount of material, and should facilitate the selection of materials and process parameters in the context of resin infusion processes. Integrated fluidity data can help to determine the maximum achievable flow length and optimal infusion temperature, and inform attempts to accelerate the infusion process by heating the material just enough to reduce viscosity while at the same time avoiding premature gelation. The presented approach is able to capture all these effects. Two of the four bioepoxy formulations studied (NMA-DBU, MTHPA-2E4MI) display a significantly greater potential for infusion than the conventional system at T ≤ 120°C, one (MTHPA-DBU) exhibits a greater potential for infusion at T ≤ 80°C, and one (NMA-2E4MI) shows superior performance at all temperatures tested. Finally, given the focus on processing, a word should be said concerning times to full cure for the materials studied here. While the lower reactivity of the secondary epoxy groups in the bio-based resins is advantageous in the context of infusion, this need not translate into significantly longer cure times. In particular, the recommended curing time for RIM145 of 8 hours may be matched by the bio-based systems reported here so long as an appropriate curing temperature is selected (180°C for the bio-based systems vs. 120°C for the RIM145).22 These results highlight the promise of biobased epoxy resins in the context of composite manufacture; indeed, preliminary results concerning the successful production of composites from some of these resin systems have already been reported,40 with ongoing efforts along these lines to be described in a future report.


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AUTHOR INFORMATION

Corresponding Authors Daniel F. Schmidt. E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources This material is based upon work supported by the National Science Foundation under Grant No. 1230884 ACKNOWLEDGMENT This material is based upon work supported by the National Science Foundation under Grant No. 1230884. The authors thank Prof. Meg Sobkowicz-Kline, Prof. Stephen Burke Driscoll, Dr. Akshay Kokil, Mr. Xun Chen and Mr. David Rondeau for their assistance and advice. ABBREVIATIONS 2E4MI, 2-ethyl-4-methylimidazole; CTE, coefficient of thermal expansion; DBU, 1,8diazabicyclo[5.4.0]undec-7-ene; ELO, epoxidized linseed oil; FRC, fiber reinforced composite; MTHPA, methyltetrahydrophthalic anhydride; NMA, nadic methyl anhydride; phr, parts per hundred (parts) resin; UHP, ultra high purity; VARTM, vacuum assisted resin transfer molding; LCM, liquid composite molding.

REFERENCES


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(40) Möller, J.; Kuncho, C. N.; Schmidt, D. F.; Reynaud, E. In Bioepoxy/Glass Fiber Composites, SPE ANTEC, Las Vegas, Nevada, USA, 2014 Society of Plastics Engineers: Las Vegas, Nevada, USA, 2014


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Rheological Studies of High-Performance Bioepoxies for Use in

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