Toughening of aromatic epoxy via aliphatic epoxy copolymers

Polymer xxx (2014) 1e6 Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer Toughening of aromatic epoxy via aliphatic epoxy copolymers Markus A. Downey*, Lawrence T. Drzal Department of Chemical Engineering and Materials Science, Composite Materials and Structures Center, Michigan State University, 2100 Engineering Building, 428 South Shaw Lane, East Lansing, MI 48824, USA a r t i c l e i n f o a b s t r a c t Article history: Received 23 September 2014 Received in revised form 17 October 2014 Accepted 26 October 2014 Available online xxx While aromatic diglycidyl ether of bisphenol A (DGEBA) based epoxy polymer matrix systems are important for high-performance applications, their brittle nature is an issue that needs to be addressed. In this paper the authors show that small additions of a more flexible aliphatic epoxy copolymers, both di- and tri-functional, can significantly increase the notched Izod impact strength (56e77%) over the neat DGEBA, while not detrimentally affecting other mechanical properties such as glass transition temperature and flexural properties. In fact, at 1 wt% concentrations, the tri-functional epoxy shows a slight increase (~2%) in the glass transition temperature compared to neat DGEBA. The improvement in impact toughness is attributed to the more flexible backbone of the aliphatic epoxy molecules. The total miscibility of the aromatic and aliphatic epoxies within the investigated concentration range (up to 20 wt %) allows for this toughening approach to be directly applied to current composite production methods, such as resin transfer molding (RTM). © 2014 Elsevier Ltd. All rights reserved. Keywords: Toughening Aliphatic epoxy Enhanced impact strength 1. Introduction Light-weighting is a concept that replaces traditional materials of construction such as steel and aluminum with advanced metal alloys and composites to achieve the goals of weight reduction and reduced energy use. Fiber-reinforced composites play an important part in many light-weighting strategies. The aviation industry, both airframe and turbine engine, has been implementing composite materials on their products in an effort to reduce weight and increase efficiency. The modern airliners, such as the Boeing 787 and Airbus A350 XWB, are comprised of more than 50% composite materials [1]. The appeal of thermosetting epoxy resins used as the matrix material in fiber-reinforced composites for high-performance structural applications is their high strength-to-weight ratio, good stiffness and high corrosion resistance. One of the drawbacks is the brittle nature of the highly cross-linked epoxy system, i.e. their lack of toughness [2]. Once formed in an epoxy material, a crack will propagate through the material uninhibited. Unlike metals, where the crack propagation will be quickly impeded by * Corresponding author. Tel.: þ1 517 353 4708. E-mail addresses: [email protected] (M.A. Downey), [email protected] (L.T. Drzal). intersection of a grain boundary [3], no such mechanism exists in epoxies. Much research effort has already been put into the toughening of epoxy polymers as has been recently summarized by P. Mohan [4]. Two general approaches can be taken to enhance the toughening of an epoxy polymer: first, adding filler materials as a crack arresting mechanism and second enhancing the amount of energy that can be adsorbed before a fracture occurs by addition of a more flexible aliphatic co-polymer. In pursuit of the former, nano-scale filler materials have been shown to have toughening benefits at low filler loadings (<0.3 wt%) [5,6]. Many different nano-fillers have been investigated, including thermoplastic co-polymers [7,8], inorganic nano-fillers [9,10] and organic nano-fillers [11e13]. All of these filler materials have shown promise in enhancing the toughness of epoxy polymers via different toughening mechanisms, such as crack bridging, crack pinning and crack deflection which have been proposed in the literature [5]. Other approaches that have shown toughening of epoxies include the use of microscale particles filled with healing agent to impart some selfhealing properties [14]. As with any filler based toughening mechanism, the suspension of particulates in the matrix phase can pose a challenge when these toughening approaches are applied to production methods using liquid molding processing. For example, resin transfer molding (RTM) and vacuum-assisted resin transfer molding (VARTM) inject the epoxy resin matrix material under pressure into a layup of reinforcing fibers [15]. Particulates that are http://dx.doi.org/10.1016/j.polymer.2014.10.052 0032-3861/© 2014 Elsevier Ltd. All rights reserved. Please cite this article in press as: Downey MA, Drzal LT, Toughening of aromatic epoxy via aliphatic epoxy copolymers, Polymer (2014), http:// dx.doi.org/10.1016/j.polymer.2014.10.052 2 M.A. Downey, L.T. Drzal / Polymer xxx (2014) 1e6 suspended in the liquid matrix material can be filtered out by the weave of reinforcing fibers, resulting in an uneven distribution of nano-filler material. Since a uniform distribution of the nano-filler is essential, this would negate any potential toughening benefits. The second approach of adding a more flexible aliphatic epoxy copolymer would address the issue of processability, provided that the epoxy copolymers are miscible. Having the same functional epoxy groups, the copolymers will undergo the same reaction with the diamine curing agent to form a highly cross-linked network. The main difference between the molecules is the flexibility of the chain segments between the functional groups. The higher flexible aliphatic chains should be able to absorb more impact energy prior to fracturing. Misaki et al. [16] showed increases of toughness of several different aliphatic epoxies at a constant concentration of 30 wt%. The increased toughness was associated with decreases in tensile modulus and glass transition temperature. Zhang et al. [17] looked at different concentrations of a di-functional aliphatic epoxy in the range of 7e15 wt% which showed substantial increases in impact strength. However, these were also associated with substantial decreases in modulus and glass transition temperature. A survey of the current literature indicates that the toughening of aromatic epoxies with low concentrations (<5 wt%) of aliphatic epoxies has not been systematically investigated. This study includes this lower concentration range. As with any toughening approach, the optimization of a single parameter is straightforward. However, a substantial increase in toughness is mostly done to the detriment of other material properties such as modulus or glass transition temperature. Both parameters are essential for structural applications. A balanced approach of enhancing impact toughness without detrimentally affecting the other system properties is desirable and presented in this work. 2. Materials The aromatic epoxy, diglycidyl ether of bisphenol A (DGEBA, Epon 828, Fig. 1), with an epoxy equivalent weight (EEW) of 185e195, as well as the tri-functional aliphatic epoxy, trimethylolpropane triglycidyl ether (Heloxy 48, Fig. 2) with an EEW of 138 were purchased from Momentive Specialty Chemicals Inc. The difunctional aliphatic epoxy, polypropylene glycol diglycidyl ether (PDGE, Fig. 3) with an EEW of 190 (Mn 380), as well as the curing agent meta-phenylenediamine (m-PDA) were purchased from SigmaeAldrich. All chemicals were used as received. 3. Methods The DGEBA was weighed out in a glass beaker and the appropriate amount of aliphatic epoxy (either di- or tri-functional) was added. The mixture was then placed on a stir plate and heated to about 70  C under agitation with a magnetic stir bar. The heating of the epoxy mixture was primarily done in an effort to reduce the viscosity during mixing, degassing and casting of the material into the molds. A stoichiometric amount of m-PDA was melted in an oven. In the case of di-functional aliphatic, the stoichiometry was constant at 14.5 phr based on the similar EEW of the two epoxies. Fig. 2. Trimethylolpropane triglycidyl ether (tri-functional aliphatic). Fig. 3. Polypropylene glycol diglycidyl ether (di-functional aliphatic). The stoichiometry for the tri-functional aliphatic, needed to be adjusted for each tri-functional aliphatic epoxy concentration and ranged from 14.6 to 15.5 phr. After addition of the liquefied curing agent and homogenizing the DGEBA/aliphatic epoxy/mPDA mixture by hand, the mixture was degassed in a vacuum oven for about 5 min to eliminate trapped gas bubbles. After degassing, the mixture was poured into silicone molds for the appropriate sample configuration and placed in a convection oven. The samples were cured at 75  C for 2 h and post-cured at 125  C for 2 h. The cured samples were polished on a Struers Abramin polisher with 320, 600, 1200 and 4000 grit paper to yield a smooth sided sample prior to mechanical testing. Neat DGEBA/mPDA samples were made without the addition of any aliphatic epoxy copolymer to serve as a baseline. All mechanical properties were tested on the bases of at least two replicates at each aliphatic co-polymer concentration, with each replicate consisting of at least 4 samples. The flexural properties were determined on the basis of ASTM D790 using a United Testing Systems SFM-20 load frame with a 100lbf load cell. The flexural samples had dimensions of 75  12.5  3 mm. The support span width to thickness ratio was taken at 16:1 and the crosshead speed was calculated per ASTM D790-10. The notched Izod impact properties were determined using a TMI pendulum impact tester on the basis of ASTM D256 using a 1 ft-lbf hammer. The notched Izod impact samples used a molded pre-notch and had dimensions of 63.5  12.5  12.5 mm. The thermo-mechanical properties were determined using a TA Instruments Q800 dynamic mechanical analysis (DMA) instrument using a single-cantilever beam setup in air from room temperature to 250  C at a ramp rate of 3  C/min and a TA Instruments Q2000 differential scanning calorimeter (DSC) using a modulated DSC at from 25 to 250  C at 3  C/min and a modulation of ± 2  C. Fig. 1. Diglycidyl ether of bisphenol A (DGEBA). Please cite this article in press as: Downey MA, Drzal LT, Toughening of aromatic epoxy via aliphatic epoxy copolymers, Polymer (2014), http:// dx.doi.org/10.1016/j.polymer.2014.10.052 M.A. Downey, L.T. Drzal / Polymer xxx (2014) 1e6 3 Fig. 4. Tand function of DGEBA toughened with di-functional (left) and tri-functional (right) aliphatic epoxy determined by DMA. Fracture surfaces were investigated using a Zeiss EVO LS25 scanning electron microscope under high-vacuum. Samples were coated with a 3 nm thick layer of tungsten prior to observation. 4. Results and discussion The aromatic and aliphatic epoxies were found to be completely miscible in the concentration ranges investigated in this study (up to 20 wt%). Simple mechanical mixing was sufficient to achieve a homogenous mixture. Fig. 4 shows representative tand curves for the different di- and tri-functional aliphatically toughened DGEBA. Tand also referred to as the material loss factor is the ratio of the loss to the storage modulus of a material [18] and the maximum of the tand peak represents one measure of the glass transition temperature, which will be shown later in this paper. The single a transition is further indication of the homogenous structure that is formed between the two epoxies during curing. This bodes well for applying aliphatic toughening to current production methods, such as pre-preg production and resin transfer type processes. Fig. 5 shows representative curves of the storage modulus, which is related to the stiffness of the material, of the di- and trifunctionally toughened DGEBA. At lower temperature, the storage moduli are similar for all aliphatic copolymer concentrations. This behavior is consistent with that seen in the flexural modulus as shown in Fig. 6. As the temperature increases, the storage modulus decreases. The temperature at which the storage modulus decreases substantially (onset temperature), decreases with increasing aliphatic epoxy concentration. The temperature decreases are not as large for the tri-functional aliphatically toughened samples as can be seen from narrower temperature band, Fig. 6. Flexural modulus of di- and tri-functionally toughened DGEBA/mPDA. which all curves fall into. The 1wt% toughened samples, both the diand tri-functional, show very similar behavior to the neat DGEBA. The flexural modulus as shown in Fig. 6 was found to be independent of aliphatic epoxy concentration within the investigated concentration range. This is true for both the di- and tri-functional aliphatic epoxies. Since the tensile and flexural properties tend to mirror each other, the flexural behavior is consistent with previously published work by Zhang et al. [17] where the Young's modulus was found to be constant up to 5 wt% of a di-functional aliphatic epoxy concentration. The flexural test is conducted at a slow loading rate so the polymer chains have a chance to rearrange to accommodate the load and the enhanced flexibility of the Fig. 5. Storage Modulus of DGEBA toughened with di-functional (left) and tri-functional (right) aliphatic epoxy determined by DMA. Please cite this article in press as: Downey MA, Drzal LT, Toughening of aromatic epoxy via aliphatic epoxy copolymers, Polymer (2014), http:// dx.doi.org/10.1016/j.polymer.2014.10.052 4 M.A. Downey, L.T. Drzal / Polymer xxx (2014) 1e6 Fig. 7. Flexural strength of di- and tri-functionally toughened DGEBA/mPDA. Fig. 8. Notched Izod impact strength of di- and tri-functionally toughened DGEBA/ mPDA. Fig. 10. Glass transition temperature of di- and tri-functionally toughened DGEBA/ mPDA determined from maxima of tan d curve. aliphatic epoxy chains does not come into play. Hence, there are no measureable differences in these slow loading rate properties between the samples with different aliphatic epoxy concentrations. The flexural strength shown in Fig. 7 exhibited different behaviors for the di-functional and tri-functional aliphatic epoxies. For the di-functional aliphatic, the flexural strength is comparable with neat DGEBA up to a concentration of about 5 wt%. Above this concentration the flexural strength decreases. At a concentration of ~17 wt% the flexural strength drops by about 7% compared to the neat DGEBA. The tri-functional aliphatic on the other hand shows a flexural strength that is comparable to neat DGEBA up to a concentration of about 10 wt%. Above 10 wt% the flexural strength decreases with a reduction of about 6% at 20 wt% tri-functional aliphatic. The trend of flexural strength decreasing with increased addition of aliphatic epoxy concentrations is again consistent with the results from Zhang et al. [17], which found a reduction in upper yield stress with increasing aliphatic epoxy concentrations. The Fig. 9. SEM micrograph of the fracture surface of notch Izod impact samples. Neat DGEBA (top, left) and DGEBA toughened with tri-functional aliphatic epoxy: 1 wt% (top, right), 5 wt% (bottom, left), 15 wt% (bottom right). Please cite this article in press as: Downey MA, Drzal LT, Toughening of aromatic epoxy via aliphatic epoxy copolymers, Polymer (2014), http:// dx.doi.org/10.1016/j.polymer.2014.10.052 M.A. Downey, L.T. Drzal / Polymer xxx (2014) 1e6 5 Fig. 11. Reversible heat flow of di-functionally (left) and tri-functionally (right) toughened DGEBA/mPDA determined from modulated DSC measurements. reason cited for this behavior is the enhanced flexibility of the aliphatic chain segments relative to the rigid aromatic chain segments of the DGEBA. The constant flexural strength up to a higher concentration of tri-functional aliphatic when compared to the difunctional aliphatic is probably due to the formation of a network with shorter distances between crosslinks of the tri-functional aliphatic resulting in a higher crosslink density. As shown in Fig. 8, the notched Izod impact strength increases significantly with the addition of aliphatic epoxies. Even low concentrations of aliphatic epoxy increased the notched Izod impact strength considerably. The di-functional aliphatic increases the impact strength by 56% over the neat DGEBA at 1 wt%. The impact strength further increases with increasing concentration up to about 77% at an aliphatic concentration of 10 wt%. No further increase is seen above this concentration. The tri-functional aliphatic shows a greater increase of 77% at 1 wt% concentration. Above 1 wt %, the impact strength is approximately constant with increasing tri-functional aliphatic concentration. In contrast to the slow loading rate of the flexural test, the notched Izod impact test is done under a high loading rate as the pendulum strikes the sample. The higher flexibility of the aliphatic chains enables energy to be absorbed during impact. As Misaki et al. [16] postulated, the more flexible materials will fracture with crack branching, leading to a more uneven fracture surface that requires additional energy for fracture. These findings are supported by the fracture surfaces as shown in Fig. 9. The fracture surface of the neat DGEBA sample exhibits the characteristic smooth surface of a brittle fracture. The featureless mirror region covers most of the failure initiation region. With the addition of the aliphatic copolymer at concentrations of 1, 5 & 15 wt%, hackle features begin to appear in the mirror region. As the aliphatic concentration is increased, the rough hackle features increase in frequency and the smooth mirror region becomes less evident. At 15 wt% tri-functional aliphatic, the mirror region has mostly disappeared. The rougher surface is associated with a higher crack velocity and indicates crack branching [19]. With higher energy dissipation rates, the rough morphology coincides with increased impact toughness. The impact of the addition of aliphatic epoxies on the glass transition temperature (Tg) is shown in Fig. 10. At low concentrations (1 wt%) of di-functional aliphatic, the Tg is constant. Above that concentration, the Tg decreases with increasing di-functional aliphatic concentration. The maximum reduction is around 10% at ~17 wt% di-functional aliphatic. At 1 wt% tri-functional aliphatic, the Tg has an increase of about 2%. Up to a concentration of 5 wt% tri-functional aliphatic, the Tg is comparable to neat DGEBA. Above a concentration of 5 wt%, the Tg decreases with a maximum reduction of 6% at 20 wt% tri-functional aliphatic. For a given concentration the reduction in Tg for the tri-functional aliphatic epoxy is smaller when compared to the di-functional aliphatic epoxy. This can be attributed to the different network formation of the tri-functional epoxy. The lower EEW of the tri-functional aliphatic implies a shorter distance between cross-links and a higher cross-link density. Representative curves of the reversible heat flow from the modulated DSC measurements are shown in Fig. 11. The DSC measurements confirm the trends in the glass transition temperature that were seen in the measurements from the DMA. The application of the presented toughening mechanism to fiber-reinforced composites needs to be confirmed and will be presented in a follow-up publication. As has been shown in the literature, the effectiveness of toughening mechanisms can be less pronounced in a system that includes reinforcing fibers. Hunston et al., [20,21] showed that for brittle matrix materials, the increase in toughness can be more pronounced in the fiber-reinforced composite than the base matrix. This increase effect in the fiberreinforced composite is attributed to the additional energy expended for fracturing of reinforcing fibers, fiber pullout and crack deflection. With highly toughened resins however, the toughening effect was reduced by as much as one-third. The toughening mechanisms that rely on the formation of large crack-tip deformation zones can have these deformation zone constrained between reinforcing fibers, reducing the effectiveness. Since the proposed aliphatic toughening mechanism relies on the crosslinking of more flexible chains into the epoxy network, the transfer to the fiber-reinforced composite is expected to be good. 5. Conclusion The results presented here highlight an approach to enhance the impact toughness of an aromatic DGEBA/mPDA epoxy system by the addition of di- and tri-functional epoxy copolymers. At low aliphatic epoxy concentrations (1 wt%) the impact toughness can be significantly enhanced (57e77%) without reductions in the flexural properties or glass transition temperature. The excellent miscibility of the epoxy copolymers makes the application of this toughening mechanism directly implementable for current fiber-reinforced epoxy composite production methods. Acknowledgment The authors would like to thank General Electric Aviation for their financial support for this research. Please cite this article in press as: Downey MA, Drzal LT, Toughening of aromatic epoxy via aliphatic epoxy copolymers, Polymer (2014), http:// dx.doi.org/10.1016/j.polymer.2014.10.052 6 M.A. Downey, L.T. Drzal / Polymer xxx (2014) 1e6 References [1] Marsh G. 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