Novel epoxy resin adhesives toughened by functionalized poly (ether ether ketone) s

Abstract

Epoxy resins are inherently brittle due to their high crosslink density. Hybridization of epoxies with a thermoplastic phase is an effective solution for reducing the brittleness and improving the toughness without impairing the thermal and mechanical properties. The present study focuses on the development of a novel room temperature curable epoxy resin by blending with different proportions of hydroxyl terminated poly (etheretherketone)s (PEEKTOH) as thermoplastic toughening agent. The systems were cured using triethylenetetramine (TETA). Isothermal rheological studies showed that, incorporation of PEEKTOH phases reduced the gelation time of the blend from 175 min to 30 min. The glass transition temperature of the blends were improved marginally (from 74 to 80 °C) as evidenced by dynamic mechanical thermal analysis studies. The toughened and the unmodified systems were examined for their adhesive properties on aluminium substrates. It was noted that, about 19% improvement in lap shear strength (LSS) was achieved for the toughened adhesive. Higher proportions of PEEKTOH in the epoxy matrix led to increased viscosity which consequently reduced the wetting characteristics, as proven by contact angle measurements. The fracture toughness of the cured blends improved up to 86%. Fracture morphology evaluation by scanning electron microscopy (SEM) showed a brittle fracture for the unmodified epoxy and a phase separated morphology for the toughened blends. Transparency was reduced in the toughened formulations due to phase separation of PEEKTOH.

Introduction

Polymeric adhesives offer many advantages compared to other traditional methods of joining such as welding, bolting, brazing, mechanical fastening etc. [1]. Compositions based on epoxy resins represent the most popular structural adhesives which can bond an array of similar or dissimilar materials such as metals, plastics, ceramics, wood etc. On curing, epoxy resins produce a highly cross-linked network structure with good mechanical, thermal and creep resistance properties but compromising the damage tolerance [[2], [3], [4], [5], [6], [7]]. Unmodified epoxy resins are brittle in nature and cause poor resistance to crack initiation and growth. This will lead to poor adhesive strength between the bonded assemblies and ultimately failure. Nevertheless, different strategies exist for modification of epoxy structural adhesives to improve their strength without significantly impairing other desirable properties.

Modification of epoxy structural adhesives by rubber particles can enhance the toughness and improve the adhesive strength [8]. Toughening of epoxy adhesives using varying proportions of carboxyl terminated poly (2-ethylhexyl acrylate) rubber has resulted in a two-fold increase in adhesive and impact strengths with incorporation of 9-13 wt % of liquid rubber [9]. The addition of core shell rubber (CSR) nanoparticles increased the fracture energy of epoxy structural adhesives more than by 10 times than the unmodified adhesives [10]. A synergistic influence of rubber particles and graphene nanoplatelets improved the bulk mechanical properties, fracture toughness and lap shear strength of the epoxy adhesive [11]. Significant improvement has been observed in the adhesive joint strength by modification using silica nanoparticles added at concentrations of 10 - 20 % in an epoxy [12].

The use of reactive or non-reactive types of thermoplastics in improving the adhesive properties of epoxy resins is seldom reported. Modification of epoxy adhesives with preformed polyamide particles have been reported to be effective in improving the T peel strength of an adhesive by a factor of three in comparison to conventional rubber particles [13]. A combination of carboxyl terminated butadiene acrylonitrile (CTBN) rubber and a linear poly (hydroxyl ether) [Phenoxy] thermoplastic in epoxy film adhesives have improved their fracture resistance [14]. The incorporation of electro spun nanofiber mats of polyvinyl alcohol in to epoxy adhesives resulted in an approximately 13.5% increase in shear strength properties [15]. The resinous polyamic acid and thermoplastic polyimide modification of epoxy adhesives have significantly improved the impact and tensile adhesive strength of metallic joints [16]. Functionalized thermoplastics such as amine and epoxy terminated poly (phthalazinone ether nitrile sulfone ketone) modified epoxy adhesives exhibited very high glass transition temperatures (>300 °C) with high adhesive strength (48.7 MPa) at room temperature and after various heat treatments [17]. Studies on the influence of functionalized thermoplastic poly (ether etherketone) s, in improving the adhesive properties of epoxy resins are limited. Epoxy-aminonovolac- phthalonitrile networks toughened by hydroxyl functionalized poly (ether ether ketones) have been reported with enhanced lap shear strength at RT and 150 °C on steel substrates [18].

The present paper discusses investigations on the processing and evaluation of a category of epoxy adhesives toughened with hydroxyl functionalized poly (ether etherketone) s, PEEKTOH, to address the dual roles as toughening agents and as adhesion strength promoters under various environmental conditions. Ambient curable epoxy adhesive formulations were designed and evaluated using aluminum alloy (AA 2014) substrates. Curing parameters were monitored using differential scanning calorimetry (DSC) and rheological experiments and interface properties were evaluated from lap shear strengths determined at 25 °C, -196 °C and at 100 °C. Wetting characteristics were established by contact angle measurements and correlated to the microstructure of the compositions. The morphologies were examined in detail using SEM and UV - Visible spectroscopy to understand the composition-property relationships.