Enhancing the resistance to fatigue crack propagation of epoxy using triblock copolymers

5 September 2017
Andreas Klingler and Bernd Wetzel
Adding triblock copolymers to an epoxy resin significantly increases its resistance to fatigue crack propagation (by more than 130) without reducing its glass transition temperature or ductility.

Epoxy resins are a special class of high-performance thermosetting polymers that are used in a variety of today's automotive, aerospace, and energy applications (e.g., as adhesives and coatings, or as polymeric matrices for fiber-reinforced composites). Such polymers combine excellent mechanical properties with high adhesive strength, low shrinkage and water absorption, and high thermal and chemical stability.

Although epoxies have a number of superior properties compared with other polymers, they have the drawback of being inherently brittle. This leads to low fracture toughness and poor resistance to fatigue crack propagation (FCP). Various strategies have been investigated to overcome this problem by toughening epoxy resins, e.g., by modifying the resin with phases of a compliant nature, such as rubber. Unfortunately, the introduction of rubber modifiers usually decreases the glass transition temperature (Tg) and reduces the strength and elastic modulus of the thermoset. A fairly new approach to toughening epoxy resins while maintaining their mechanical properties and Tg is to introduce block copolymers (BCPs) as a second phase.1, 2

BCPs comprise at least two chemically different constituents (i.e., A, B, C, ...) and generate polymeric macromolecules of various compositions, such as diblock (AB), triblock (ABA, ABC), multiblock (ABCD), or alternating compositions (ABABAB ...). In contrast to classical tougheners, such as liquid rubbers, the use of BCPs enables a variety of nanosized structures to be obtained in thermosets (e.g., spherical and cylindrical micelles, vesicles, and other continuous and discontinuous structures). These phases form due to thermodynamic forces between the BCP and the epoxy molecules. The structure can thereby occur prior to curing,3 during the curing process,4 or in combinations thereof.5 Understanding and subsequently gaining control over the phase-separation mechanisms of BCPs in thermosetting polymers will therefore enable a BCP phase morphology to be tailored for a required application and the specific properties of the base thermoset to be improved.

In our work, we have evaluated the effect of modifying an epoxy with BCP on the FCP behavior of the epoxy. To this end, we blended a bisphenol-A-based epoxy resin (D.E.R. 331, DGEBA from Dow Chemical) with different concentrations of an ABA-type triblock copolymer (Nanostrength M53 from Arkema) consisting of polymethylmethacrylate-blocks (A-block) surrounding a central polybutylacrylate-block (B-block). We then stoichiometrically cured the system using 3, 3 ′ -dimethyl-4, 4 ′ -diaminodicyclohexylmethane (HY 2954 from Hunstman), a cycloaliphatic amine curing agent. After preparing compact tension specimens, we examined the materials' resistance to the propagation of an artificially introduced crack during fatigue loading.

We found that the resistance of the epoxy to FCP was significantly enhanced by the addition of the BCP.6 The fatigue crack growth rate per cycle of loading was gradually reduced with increasing BCP concentration, and at the same time the material was able to withstand much higher stresses at the crack tip (see Figure 1). The BCP-modified epoxy reached critical stress-intensity-factor ranges (ΔKIc) of more than double the reference epoxy system. Furthermore, Tg slightly increased, from 175°C (neat) to 183°C (with 2wt% BCP).


Fatigue crack propagation results (da/dN vs. ΔK) of an epoxy resin (EP)—D.E.R. 331 from Dow Chemical—with different concentrations (0, 2, 6, 8, and 10wt%) of an ABA-type triblock copolymer (Nanostrength M53 from Arkema). da/dN: Fatigue crack growth rate per cycle of loading. ΔK: Stress-intensity-factor range. EP: D.E.R. 331. BCP: Block copolymer.

We analyzed scanning electron micrographs of the composites to determine the mechanisms behind this toughening effect. In the case of epoxy with BCP concentrations of below 6wt%, we were able to ascribe the toughening effect to the composite's microstructure, which comprised microphase-separated and well-dispersed spherical BCP-rich inclusions (see Figure 2). In the epoxy composites with BCP concentrations of 6wt% and higher, on the other hand, we observed the formation of interconnected, isle-like epoxy-rich domain structures that also contained spherical BCP-rich inclusions (see Figure 3). We did not, however, detect any phase inversion.6 The spherical BCP-rich particles induced a variety of toughening mechanisms, such as crack pinning, cavitation, debonding, as well as fibrillation and bridging. These mechanisms resulted in a reduction of the crack driving force. However, we found that the BCP-rich domains plastically deform and debond from the epoxy-rich matrix, followed by a total rupture of the phases.


Fatigue fracture surface of EP with 2wt% BCP.


Fatigue fracture surface of EP with 6wt% BCP.

In summary, we have demonstrated that adding a commercially available triblock copolymer to an epoxy resin enhances its toughness and significantly increases its resistance to FCP. We have thus obtained a comprehensive and expanded understanding of the toughening effects of a block-copolymer-modified epoxy and were able to show that it is possible to significantly toughen a highly brittle thermosetting matrix against FCP without reducing the Tg. For example, we developed a low-cost epoxy system containing 6wt% BCP that provides a reduction of the fatigue crack growth rate of 37% and an increase in ΔKIc of more than 100% compared with the neat epoxy system.6 In our future work, we aim to focus on obtaining a deeper understanding of the effect of different BCP compositions on epoxy resins. Specifically, we hope to make use of the self-assembling character of these materials for fiber-reinforced structures.


Authors

Andreas Klingler
Institute for Composite Materials (IVW GmbH)

Andreas Klingler is a research associate and PhD student. His research interests include polymer nanocomposites, fiber-reinforced polymers, and fracture mechanics (with a special focus on FCP).

Bernd Wetzel
Institute for Composite Materials (IVW GmbH)

Bernd Wetzel is research director for the Materials Science Department. His research interests focus on high-performance polymers, nanocomposites, coatings, and biomimetic materials, including tribology, fracture mechanics, and processing–structure–property relationships. His research projects cover topics from the fundamentals up to industrial application.


References

  1. A. Bajpai, A. K. Alapati and B. Wetzel, Toughening and mechanical properties of epoxy modified with block co-polymers and MWCNTs, Procedia Struct. Integrity 2, pp. 104-111, 2016.

  2. E. M. Redline, C. Declet-Perez, F. S. Bates and L. F. Francis, Effect of block copolymer concentration and core composition on toughening epoxies, Polymer 55, pp. 4172-4181, 2014.

  3. S. Maiez-Tribut, J. P. Pascault, E. R. Soulé, J. Borrajo and R. J. J. Williams, Nanostructured epoxies based on the self-assembly of block copolymers: a new miscible block that can be tailored to different epoxy formulations, Macromolecules 40, pp. 1268-1273, 2007.

  4. H. E. Romeo, I. A. Zucchi, M. Rico, C. E. Hoppe and R. J. J. Williams, From spherical micelles to hexagonally packed cylinders: the cure cycle determines nanostructures generated in block copolymer/epoxy blends, Macromolecules 46, pp. 4854-4861, 2013.

  5. M. Asada, S. Oshita, Y. Morishita, Y. Nakashima, Y. Kunimitsu and H. Kishi, Effect of miscible PMMA chain length on disordered morphologies in epoxy/PMMA-b-PnBA-b-PMMA blends by in situ simultaneous SAXS/DSC, Polymer 105, pp. 172-179, 2016.

  6. A. Klingler and B. Wetzel, Fatigue crack propagation in triblock copolymer toughened epoxy nanocomposites, Polym. Eng. Sci. 57, pp. 579-587, 2017.

DOI:  10.2417/spepro.006972