Research Article pubs.acs.org/journal/ascecg
Nanocomposites of a Cashew Nut Shell Derived Epoxy Resin and Graphene Platelets: From Flexible to Tough Osman Eksik,#,† Anthony Maiorana,#,‡ Stephen Spinella,‡ Ajay Krishnamurthy,† Sierra Weiss,† Richard A. Gross,*,‡ and Nikhil Koratkar*,† †
Department of Mechanical Engineering, Rensselaer Polytechnic Institute, 110 8th Street, Troy New York 11280, United States New York State Center for Polymer Synthesis, Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, 110 8th Street, Troy New York 11280, United States
‡
ABSTRACT: A series of nanocomposites were prepared from graphene platelets (GPL) and a flexible biobased epoxy thermoset matrix derived from cashew nut shell liquid. The loading of GPL in the biobased thermoset matrix ranged from 0.1 to 0.8 wt % and resulted in increased tensile strength and Young’s modulus (17 to 33 MPa and 474 to 1700 MPa, respectively). Increases in mode I fracture toughness for KIC and GIC ranged from 0.6 to 1.9 MPa·m1/2 and 906 to 1734 J/m2, respectively. Furthermore, dynamic mechanical analysis revealed that GPL incorporation resulted in increases in the α-transition temperature (peak of the loss modulus) from 27 to 50 °C and increases the storage modulus from 1000 to 2000 MPa. Also, introduction of GPL increased the onset of degradation (Td3%) for the biobased thermoset matrix by 30 °C. Results of this work demonstrate that incorporation of graphene platelets enhances all measured physical and thermal properties of the cashew nut shell derived epoxy resin and enables higher performance applications. KEYWORDS: Biobased epoxy resin, Cardanol, Toughening, Nanocomposites, Graphene
■
petroleum based polymer products.6 Cashew nutshell liquid has attracted a great deal of attention because it is a renewable byproduct of an existing industry that can be used to synthesize new epoxy resins or improve the properties of existing epoxy resins. Cardanol, the main constituent of cashew nutshell liquid, has a phenolic headgroup and a 15 carbon long alkyl chain with varying degrees of unsaturation at positions C8 to C15. Thus far, to use cardanol as a renewable epoxy resin building block, it is double bonds have been converted to epoxy moieties via enzymatic catalysis7,8 or glycidylation of its phenolic hydroxyl group was performed by reaction with epichlorohydrin.9 Although transformation of double bonds to epoxides via enzymatic catalysis is green, the reactivity of internal epoxides along hydrocarbon chains differs dramatically from glycidyl ethers and, consequently, requires higher curing temperatures or more reactive cross-linkers. Recently, Jaillet et al.10 characterized the thermomechanical properties of a liquid epoxy resins from the Cardolite Corporation under the trade name of Cardolite NC-514 (Scheme 1). They proposed that the high epoxide equivalent weight was due to dimeric species arising from phenolation of double bonds on the alkyl chain. The presence of diglycidyl ether functional epoxy resin allows for equal reactivity between the two epoxide groups for curing by common 2-part epoxy
INTRODUCTION Cured epoxy resins are highly versatile thermoset polymers that are used in applications ranging from structural adhesives to polymer composites to protective coatings.1 When fully cured, epoxy resins exhibit low shrinking, good creep resistance, and qualities of an engineering polymer such as an elastic modulus ranging from 2 to 3 GPa and glass transition temperatures typically greater than 120 °C.2 When epoxy resins are used in high filler content composites with continuous glass fiber or carbon fiber matrices, their properties are often competitive with metals such as aluminum or steel but often at a fraction of the weight. Furthermore, epoxy resin composites inherently resist corrosion. Because of their physical and chemical properties, the need for epoxy resins with tunable performance properties is expected to grow as the automotive industry seeks to replace metal parts with composites and seeks structural adhesives that adhere to dissimilar materials.3,4 Additionally, epoxy resins are the polymer matrix that enables wind turbine blade production at their current and projected sizes. Given current policy initiatives, the installation of wind energy is projected to grow to meet clean energy demands.5 Despite the advantages of epoxy resins, there are serious concerns over the safety and sustainability of currently used materials (e.g., bisphenol A derived materials), concurrent with a need to expand their performance attributes. The use of industrial byproducts and conversion of renewable raw materials into polymers has been receiving worldwide attention to reduce the environmental impact of © XXXX American Chemical Society
Received: December 11, 2015 Revised: January 27, 2016
A
DOI: 10.1021/acssuschemeng.5b01684 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
Research Article
ACS Sustainable Chemistry & Engineering Scheme 1. Main Components of the Nanocomposites Described Herein
utilized if the modulus and toughness of the resins was competitive or exceeded that of DGEBA. This paper reports the results of the reinforcement of a biobased epoxy resin derived from cashew nutshell liquid (NC514) with low loadings (
