Toughening of Epoxy Systems with Interpenetrating Polymer Network (IPN): A Review

Abstract

Epoxy resins are widely used for different commercial applications, particularly in the aerospace industry as matrix carbon fibre reinforced polymers composite. This is due to their excellent properties, i.e., ease of processing, low cost, superior mechanical, thermal and electrical properties. However, a pure epoxy system possesses some inherent shortcomings, such as brittleness and low elongation after cure, limiting performance of the composite. Several approaches to toughen epoxy systems have been explored, of which formation of the interpenetrating polymer network (IPN) has gained increasing attention. This methodology usually results in better mechanical properties (e.g., fracture toughness) of the modified epoxy system. Ideally, IPNs result in a synergistic combination of desirable properties of two different polymers, i.e., improved toughness comes from the toughener while thermosets are responsible for high service temperature. Three main parameters influence the mechanical response of IPN toughened systems: (i) the chemical structure of the constituents, (ii) the toughener content and finally and (iii) the type and scale of the resulting morphology. Various synthesis routes exist for the creation of IPN giving different means of control of the IPN structure and also offering different processing routes for making composites. The aim of this review is to provide an overview of the current state-of-the-art on toughening of epoxy matrix system through formation of IPN structure, either by using thermoplastics or thermosets. Moreover, the potential of IPN based epoxy systems is explored for the formation of composites particularly for aerospace applications.

Keywords: IPN; epoxy; mechanical properties; morphology; semi-IPN; toughening.

Figure 6

(a) Influence of different curing agents (H-957, DADPE and DADPS) on the stress intensity factor, K_C of modified epoxy systems. Reprinted with permission from ref. [11], Copyright [2003], John Wiley and Sons. (b) Fracture energy of PU modified epoxy systems obtained with various molecular weights (600–1500 g·mol⁻¹) of polyethylene glycol. Reprinted with permission from ref. [57], Copyright [2014], John Wiley and Sons.

Figure 1

Schematics of the formation of (a) full interpenetrating polymer network (IPN) and (b) semi-IPN structures. Reprinted with permission from ref. [28], Copyright [2018], American Chemical Society.

Figure 2

Optical micrographs of epoxy system modified with 16.6 wt % of methacrylate acrylonitrile butadiene styrene (MABS): (a) homogenous morphology (b) development of droplets of epoxy phase at 400 s, (c) bicontinuous structure at 550 s, and (d) elongated structures of the epoxy at 800 s. Reprinted with permission from ref. [39], Copyright [2019], The Royal Society of Chemistry.

Figure 3

(a) Schematics of the synthesis procedure and structure of PI/EP semi-IPN. Reprinted with permission from ref. [40], Copyright [2019], John Wiley and Sons; SEM images of fracture surfaces of (b) Diels–Alder (DA) polymer, (c) semi-IPN sample (DA/EP 50:50), and (d) the epoxy sample. Reprinted with permission from ref. [41], Copyright [2019], John Wiley and Sons.

Figure 4

(a) Fracture toughness of neat epoxy and epoxy system modified with different content of polyphenylene ether (PPO). Reprinted with permission from ref. [44], Copyright [2018], American Chemical Society, (b) fracture toughness of epoxy blends with PEEK-PR or PEEK-TOH. Reprinted with permission from ref. [38], Copyright [2019], John Wiley and Sons.

Figure 5

Schematic representation of structural development in epoxy/PU systems with (a) 30 wt % PU, (b) 50 wt % PU and (c) 70 wt % PU. Reaction 1: grafting reaction between -NCO groups of PU and side -OH groups of epoxy (before adding curing agent); Reaction 2: reaction between -NCO groups of PU network and -OH groups of epoxy network (after adding curing agent). Reprinted with permission from ref. [50], Copyright [2019], Elsevier.

Figure 7

Schematics of the formation of modified epoxy system using HBPs; Impact strength, flexural strength and flexural modulus of pure epoxy, linear polyol base epoxy and modified epoxy samples having different generations of HBPs (HBP-G1 to HBP-G4); SEM images of pure epoxy, linear polyol base epoxy and modified epoxy samples having different generations of HBPs; Reprinted with permission from ref. [13], Copyright [2013], Elsevier.

Figure 8

Schematics of the synthesis procedure and semi-IPN structure of neat epoxy and modified epoxy system having 5 wt % of poly(p-BAB/PBP). Reprinted with permission from ref. [60], Copyright [2019], John Wiley and Sons.

Figure 9

(a) Schematic representation of synthesis procedure of sequential acrylate/epoxy IPN, (b) elastic modulus as a function of acrylate content for sequential acrylate/epoxy IPN. Reprinted with permission from ref. [65], Copyright [2019], John Wiley and Sons.

Figure 10

(a) Phase morphologies as a function of PEI content in epoxy/PEI system (particulate morphology at 10 wt % PEI, co-continuous morphology at 15 wt % PEI and phase-inverted morphology at 20 wt % PEI). Reprinted with permission from ref. [28], Copyright [2018], American Chemical Society. (b) Fracture energy of epoxy/PEI systems as a function of PEI content at different curing temperatures. Reprinted with permission from ref. [33], Copyright [2001], John Wiley and Sons.

Figure 11

Elongation at break and G_IC values as a function of toughener content for BADCy/PEI modified system taken from refs. [32,33,82].