Catalyst Selection, Creep, and Stress Relaxation in High-Performance

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Catalyst Selection, Creep and Stress Relaxation in High-Performance Epoxy Vitrimers Wenhao Liu, Daniel F. Schmidt, and Emmanuelle Reynaud Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03829 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on February 27, 2017

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Industrial & Engineering Chemistry Research

Catalyst Selection, Creep and Stress Relaxation in High-Performance Epoxy Vitrimers Wenhao Liu, 1 Daniel F. Schmidt, 1* Emmanuelle Reynaud2 1

Department of Plastics Engineering, University of Massachusetts Lowell, One University

Avenue, Lowell, Massachusetts 01854, United States 2

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

Avenue, Lowell, Massachusetts 01854, United States KEYWORDS: Catalyst, Creep, Stress Relaxation, Epoxy, Vitrimer.

ABSTRACT: Vitrimers were created from a commercial high-performance anhydride-cured epoxy in the presence of various metal transesterification catalysts. Compressive creep strains greater than 50% were observed in samples containing dibutyltin diacetate or dibutyltin bis(2,4pentanedionate) via a compression set experiment. Stress relaxation experiments were carried out in a parallel plate geometry, with a newly modified exponential decay model proposed to better account for the behavior of these systems. The results demonstrate that it is not only possible to realize reworkability on relatively short timescales (hundreds of seconds) without compromising the mechanical properties of these networks at elevated temperatures, but that, with proper catalyst selection, this may be accomplished with negligible activity under curing conditions. This effort also highlights differences in the behavior of different transesterification catalysts in

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this context. The approach to the selection and analysis of the materials reported here has implications for the design of new vitrimers more generally.

1. Introduction Thermosets have held a very important position as engineering materials since the earliest days of polymer science, and were among the first fully synthetic polymeric materials to see widespread use. These days, with applications ranging from aerospace and defense to wind turbines and chemical tanks, their market is valued at over $77.3 million, with a near-doubling anticipated in 2021.1 Crosslinked polymers often possess superior mechanical properties and chemical resistance compared to thermoplastics. However, thermosetting materials harden during the curing process and cannot be easily reshaped once crosslinks have been formed. Developing novel methods to make the formation of such crosslinks reversible on demand would provide new opportunities for these materials to be reshaped, reworked, and / or recycled. The concept of a covalent adaptive network (CAN) has been described by Bowman et al. and promises interesting properties such as recyclability and healability in covalently crosslinked networks, enabling a new class of thermoset materials.2,3 Likewise, thermoset composites based on covalent adaptive networks could be recycled or reworked follow failure. Dynamic behavior in CANs may be triggered through light,4-6 irradiation,7 or temperature changes,8-11 enabling the breakage and reformation of some fraction of covalent bonds in the network via disulfide exchange,12,13 siloxane equilibrium,14 radical reshuffling,15,16 and Diels-Alder reactions,

11,17

which in turn enables permanent deformation given appropriate processing conditions. However, there have been difficulties in realizing such behavior in rigid thermosets until recently, when a


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new group of materials termed vitrimers was introduced by Leibler and coworkers when they applied the well-established transesterification reaction to the hydroxyl and ester groups present in an anhydride-cured epoxy matrix to create a reworkable thermosetting network18. During the course of transesterification, the connectivity of the network is altered via exchange reactions, inducing stress relaxation and plastic flow at elevated temperatures without depolymerization. In the presence of a transesterification catalyst, it was shown that both elastomeric epoxy / acid and rigid epoxy / anhydride networks possess gradually decreasing viscosities with increasing temperature, as is characteristic of other vitreous materials such as inorganic glass. Subsequent follow-up work focused on the elastomeric epoxy / acid systems in particular has looked at the use of organocatalysts instead of zinc salts19, the ability to weld and reprocess such systems20, and the mechanism of action of the catalytic zinc species active in these materials21. These and various other thermosetting networks demonstrating such dynamic behavior6,22,23 and promising processability similar to thermoplastics have therefore been termed “vitrimers”24. In the research described here, the combination of outstanding mechanical properties at ambient temperature and reprocessability at elevated temperatures makes vitrimers an excellent class of materials for reworkable structural composites with utility in, for instance, the wind energy sector. As modified zinc carboxylates have been successfully applied as transesterification catalysts in ester-linked epoxy networks18-21, a series of readily available metal carboxylates and other organotin catalysts were screened and compared based on posited or known activity in transesterification reactions via simple compression set experiments. Among these catalysts, liquids at ambient temperature or low-melting solids were preferred for ease of mixing. The performance of various dibutyltin catalysts is evaluated via compression set experiments and parallel plate rheometry. The structure-activity relationship of these


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transesterification catalysts is studied via their incorporation into Momentive EPIKOTE MGS RIM 145, an anhydride-cured epoxy formulation used in structural composites that find applications in the wind energy sector25,26. Complementing prior reports in the literature, this work provides new evidence of the efficacy of this approach in a commercial high performance anhydride-cured epoxy for structural applications. 2. Experimental Section 2.1. Chemicals and Reagents EPIKOTE MGSTM RIM145 system, an epoxy / anhydride infusion system used in the wind energy industry, was obtained from Momentive Specialty Chemicals Inc. and chosen as the base matrix. Various transesterification catalysts (listed in Table 1) were purchased (Alfa Aesar, TCI America) or donated (Reaxis) and incorporated into the epoxy matrix for analysis of reworkability. Table 1. Nomenclature and Manufacturers of Various Transesterification catalysts studied. Transesterification Catalyst

Abbreviation

Manufacturer

Dibutyltin bis(2-ethylhexanoate)

Sn_h

Alfa Aesar

Dibutyltin diacetate

Sn_a

Reaxis

Dibutyltin dilaurate

Sn_l

Reaxis

Dibutyltin bis(2,4-pentanedionate)

Sn_p

Alfa Aesar

Titanium 2-ethylhexanoate

Ti

Alfa Aesar

Monobutyltin oxide

Sn_mo

TCI America

Zinc octoate

Zn_o

Reaxis

2.2. Sample Preparation


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All resin components, with or without transesterification catalyst, were first combined and mixed with a FlackTek DAC 600.1 VAC-P Speed Mixer for one minute at 1000 rpm under vacuum. The mixture was then degassed for three hours at 35°C and cured at 120°C overnight under vacuum, consistent with the manufacturer’s recommendations for achieving full cure26. All samples were observed to be fully cured (as confirmed via Shore D hardness testing), homogeneous and transparent (albeit with some coloration) following the aforementioned procedures, with no evidence of either cure inhibition or phase separation in any case. The samples were then cut with a scroll saw (Ryobi Tools) for compressive creep experiments or machined with a mill (Sherline Products) for stress relaxation experiments. The specimens for compressive creep experiments were cut into cubes measuring roughly 5 mm on a side, while those for stress relaxation experiments were thin disks with a diameter of 25 mm and a uniform thickness in the range of 2-4 mm. Pristine epoxy samples were denoted as MNeat, where M represents the base resin from Momentive. The abbreviations for specimens containing transesterification catalysts indicate the mole percentage of catalyst as a fraction of the epoxy content, as well as the catalyst type. An example would be M5Sn_a, which indicates 5 mol% of dibutyltin diacetate (vs. moles epoxy groups) in the Momentive RIM 145 system. 2.3. Compressive Creep Testing A compressive creep experiment was applied as a screening test for analyzing the amount of reworkability that could be achieved in the presence of different catalysts as a function of temperature. A home-made apparatus was used for these; the specimen was placed at the bottom of a hollow metal cylinder and compressed by a smaller solid metal cylinder with weights placed on top of it, as shown in Figure 1. The blue ellipse indicates the approximate location of the specimen inside the metal cylinder during the experiment.


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Figure 1. Home-made apparatus for compressive creep experiments. Different weights were used to adjust the initial compressive stress exerted on the specimen. This compression apparatus was transferred into a vacuum oven after the specimen had been loaded so that the experiment could be performed at elevated temperatures under vacuum to prevent degradation. 2.4. Stress Relaxation via Parallel Plate Rheometry A parallel plate fixture was used to clamp specimens, which were discs with diameters of 25 mm and uniform thicknesses in the range of 2-4 mm, in an ARES-G2 rheometer (TA Instruments). During the stress relaxation experiment, the specimen was quickly heated to the specified temperature and allowed to equilibrate for 4 minutes. An initial axial force of 2.5 N was used to prevent the specimen from slipping and a 1% strain was maintained at the outer edge of the disk throughout the test. 3. Results and Discussion 3.1. Characterization of Neat Epoxy Samples


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For neat epoxy specimens, compressive creep experiments were performed at various temperatures as a control. The compressive creep strain achieved after four hours was negligible even at 270°C, as shown in Figure 2.

Figure 2. Compressive Creep Strain for MNeat at Different Temperatures. Control experiments were likewise performed on the neat Momentive RIM 145 at 260°C and 270°C via parallel plate rheometry (Figure 3). The modulus was observed to fluctuate initially, then increase slightly after ~500 s, which may indicate some post-curing of the epoxy. Clearly, no stress relaxation took place in the neat formulation in the absence of the transesterification catalyst.


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Figure 3. Normalized Shear Modulus for MNeat at 260°C and 270°C. 3.2. Reworkability vs. Catalyst Type and Content The compressive creep experiment was applied to eliminate some of the less active catalysts based on the amount of creep strain achieved at various temperatures. In Figure 4, compressive creep experiments were performed on specimens containing different organotin compounds and different degrees of reworkability were observed based on the resulting levels of compressive creep strain for different transesterification catalysts. In this case, dibutyltin bis(2ethylhexanoate) was considered relatively inactive compared with other organotin compounds.


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Figure 4. Compressive Creep Results used to Screen Different Dibutyltin Catalysts at 260°C. In addition to compressive creep strain, the compressive creep experiment also provided information regarding the compressive strength of the materials at the specified temperatures. As it is undesirable to sacrifice mechanical properties with the addition of the transesterification catalyst, a compromise must be made between reworkability and the mechanical strength of the material at the reworking temperature. Figure 5 provides a comprehensive summary of the compressive strain and the compressive strength of formulations screened and found to possess considerable reworkability at different temperatures.


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Figure 5. Compressive Creep Strain and Compressive Strength vs. Catalyst Type and Loading. The diameter of the circles shown in Figure 5 represents the compressive strength of the corresponding material at elevated temperatures. From this data it is observed that the compressive strength decreases with higher catalyst loadings and varies significantly with catalyst type. For example, M10Sn_p, which contain 10 mol% dibutyltin bis(2,4pentanedionate), has a compressive strength of 2.0 MPa, as compared to M5Sn_p, which has a compressive strength of 2.6 MPa. Based on the criteria specified above, M8Sn_p is a formulation demonstrating both significant reworkability and high mechanical strength, as compared with M10Ti, which shows a very high degree of reworkability but poor compressive strength due to the selection and high loading of titanium 2-ethylhexanoate.


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Since dibutyltin diacetate and dibutyltin bis(2,4-pentanedionate) gave considerable compressive creep and consistent compressive strength at all tested concentrations, these two catalysts, together with dibutyltin dilaurate, which possesses a similar structure to Sn_a and Sn_p, were further evaluated using stress relaxation experiments in a parallel plate rheometer. An additional motivation for this comparison was the study of the effects of anion structure on catalyst behavior in this series of dibutyltin salts. In the presence of the catalyst, relaxation of shear stress was observed within the testing period as a result of the transesterification reaction. A typical plot is shown in Figure 6a with normalized stress relaxation shear modulus versus time at different temperatures for M8Sn_a. Since the plastic flow behavior of vitrimers has been reported to follow an Arrhenius relationship rather than the William-Landel-Ferry (WLF) behavior associated with a physical glass transition, the concept of a characteristic relaxation time (τ*) used by Brutman et al.22 was adopted here, and used with an additional modification in order to better describe our experimental data. In particular, we fit the stress relaxation curve using Equation 1 (below), with the additional power law term added in place of the usual pre-exponential constant to account for internal relaxation of the network independent of reworkability.27,28 An example of an Arrhenius plot of characteristic relaxation times extrapolated from curve fits via Equation 1 is given in Figure 6b.

 t − t lag G = t n exp − G0 τ* 

  

(1)


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Figure 6. (a) Normalized Stress Relaxation Modulus versus Time for M8Sn_a at Different Temperatures. (b) Arrhenius Plot of ln (τ*) versus 1000/T for M8Sn_a. In Equation 1, the leading power law term is incorporated into the simple exponential decay model raised by Rong et al.29 to better facilitate the initial internal stress relaxation for this epoxy-anhydride system at elevated temperatures. In addition, the term tlag is used to account for differences in thermal history due to small variations in specimen thickness (and thus mass) as well as differences in measurement temperature.

ln τ * = ln τ 0 +

Ea RT

(2)

Equation 2 applies the Arrhenius relationship to the characteristic stress relaxation time (τ*). The pre-exponential can be denoted as either τ0, which is the characteristic relaxation time at infinite temperature, or 1/k0, where k0 is the kinetic coefficient of the transesterification catalyst at the specified concentration. Based on the relationship shown in Equation 1, the intercept and the slope extrapolated from an Arrhenius plot of the type shown in Figure 6(b) correspond to ln (τ0) and Ea/R and provide


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insights concerning the behavior of the vitrimer formulation being tested. With this in mind, a series of parallel plate rheometry experiments were performed on networks containing different organotin compounds at a loading level of 5 mol% with their τ0 and Ea values extrapolated using Equation 2.

Table 2. Comparison of Activation Energies and Characteristic Relaxation Times between Different Organotin Compounds at a Loading Level of 5 mol%.

Catalyst

Activation Energy (kJ/mol)

τ0 (seconds)

τ∗ at 120°C

τ∗ at 270°C (seconds)

Sn_l

121 ± 15

8.9 x 10-10

120 days

499

Sn_a

170 ± 21

1.5 x 10-14

25 years

330

Sn_p

114 ± 16

3.5 x 10-9

42 days

287

In Table 2, the reported characteristic relaxation times at 270°C were determined experimentally, and represents the relaxation times of the material at the highest reprocessing temperature achievable without excessive degradation as judged by changes in sample appearance (higher temperatures result in significant color changes indicative of degradation). Based on the Arrhenius relationship, the characteristic relaxation time at 120°C was extrapolated in order to evaluate the level of activity expected at the cure temperature of the epoxy network.

Table 3. Activation Energies of Sn_l, Sn_a, and Sn_p at Different Molar Concentrations. Activation Energy (kJ/mol) Catalyst

5 mol% Catalyst

8 mol% Catalyst


10 mol% Catalyst

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Sn_l

121.7 ± 15.4

123.0 ± 12.2

113.1 ± 12.1

Sn_a

169.7 ± 21.1

117.4 ± 6.0

132.3 ± 15.4

Sn_p

114.0 ± 16.3

100.4 ± 12.7

102.4 ± 10.8

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In Table 3, the activation energies extrapolated from rheometry experiment on specimens with different molar concentration of dibutyltin dilaurate (Sn_l), dibutyltin diacetate (Sn_a), and dibutyltin bis(2,4-pentanedionate) (Sn_p) were tabulated. The activation energy was found to be constant at different catalyst loadings in the case of Sn_l and Sn_p, consistent with work by Rong et al. Indeed, the majority of the activation energies reported in Table 3 are statistically indistinguishable, pointing to some level of universality in behavior presumably linked to the presence of dibutyltin cations in all cases. One unexpected exception to this observation was the significant change in the activation energy of measured in the case of dibutyltin diacetate (Sn_a) as a function of catalyst concentration. The origins of the large increase in activation energy at the lowest concentration of Sn_a require more study, but the implication would seem to be that the catalyst is inherently less effective at low concentrations. One potential explanation for such behavior might be the presence of small quantities of species with a tendency to compete for Sn_a in particular. Such a species might partially deactivate a fixed amount of the catalyst, meaning a larger fraction of the catalyst might be affected at the lowest catalyst concentration. Water is a potential culprit in this scenario, given the fact that that Sn_a is significantly more polar than Sn_l and Sn_p and therefore more likely to interact with water than the other catalysts. At the same time, it is unclear how much water would be present in the network at the relatively high temperatures being studied. A second possibility might be a reduction in catalyst mobility due to loss of the counterion. In order for these catalysts to act on the network, the anion of the dibutyltin salt


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should temporarily exchange with a carboxylate bonded to the polymer network. In the Sn_l and Sn_p cases, the laurate and pentanedionate anions are large and unlikely to leave the system; the same may not be true for the much smaller, more mobile acetate anion of the Sn_a, however. In this scenario, it is posited that some small amount of acetate might be protonated and escape the system in the form of acetic acid vapor thanks to the elevated temperatures, effectively leaving some fraction of dibutyltin cations “stranded”, without a mobile carrier enabling them to diffuse through the network as needed. As with the previous argument, the effects would be disproportionately felt by the network containing the lowest catalyst loading level, given that it would contain the smallest reservoir of acetate anions. Future studies will attempt to elucidate whether either of these effects are active in this system. 4. Conclusions The reworkability of a commercial high performance anhydride-cured epoxy formulation with applications in structural applications in the wind energy sector was demonstrated using a compression set experiment and subsequent stress relaxation studies via parallel plate geometry. The compression set experiment served successfully as a screening technique for determining the relative efficiency of various catalysts, while parallel plate stress relaxation experiments provided a more in-depth analysis of the behavior of these materials. The latter technique was specifically applied to a series of three dibutyltin salts that gave significant reworkability in the compression set experiment so as to learn more about the behavior of this family of catalysts. The addition of a power law term to a previously described exponential decay model was shown to be both scientifically valid and useful in improved the fitting of the stress relaxation data. The characteristic stress relaxation times extrapolated from the modified exponential decay model imply that the transesterification catalysts are effectively inactive under curing conditions but


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may be used to activate reworkability at elevated temperatures. Finally, the effects of changes in catalyst structure and concentration were elucidated through a study of a series of analogous dibutyltin salts, with implications for the design of future vitrimeric systems.

Corresponding Author *Email: [email protected] Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol) ACKNOWLEDGMENT This material is based upon work supported by the National Science Foundation under Grant No. 1230884. The authors thank Prof. Meg Sobkowicz-Kline for her assistance and advice, as well as Momentive Performance Materials and Reaxis, Inc. for the donation of the epoxy resin system and several catalysts, respectively. REFERENCES (1): Thermosets (Phenolic, Epoxy Resins, Cyanate Esters, Polyester Resins, Polyurethane, Vinyl Ester Resins and Others) Market for Aerospace and Defense, Automotive,


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(23): Fortman, D. J., Brutman, J. P., Cramer, C. J., Hillmyer, M. A. and Dichtel, W. R. Mechanically Activated, Catalyst-Free Polyhydroxyurethane Vitrimers. J. Am. Chem. Soc. 2015, 137, 14019-14022. (24): Denissen, W., Winne, J. M., and Du Prez, F. E. Vitrimers: permanent organic networks with glass-like ﬂuidity. Chem. Sci. 2016, 7, 30-38. (25): EPIKOTE™

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content/uploads/2013/08/WindEnergy_Resin_productbulletin.pdf, viewed 2/2017. (27): Winter, H. H. and Mours, M. Rheology of Polymers near Liquid-solid Transitions. Adv. Polymer. Sci. 1997, 134, 165-234. (28): Gurtovenko, A. A. and Gotlib, Y. Y. Dynamics of Inhomogeneous Cross-linked Polymers Consisting of Domains of Different Sizes. J. Chem. Phys. 2001, 115, 67856793. (29): Long, R., Qia, H. J. and Dunn, M. L. Modeling the Mechanics of Covalently Adaptable Polymer Networks with Temperature-dependent Bond Exchange Reactions. Soft Matter 2013, 9, 4083-4096.


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Catalyst Selection, Creep, and Stress Relaxation in High-Performance

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