The compressive properties of expandable microspheres/epoxy foams
Introduction
Epoxy foams exhibit excellent properties, including light weight, low moisture absorption, low shrinkage, good mechanical properties, high thermal and chemical stability [1]. So they have been applied to fabrication of naval vessels, military vehicles, aircraft, buildings, and offshore structures.
In recent years, hollow spherical particles called microspheres have been employed as fillers in thermoset matrix materials such as epoxy resins [2]. The low-density composites were called syntactic foams. Most of the studies have been performed on the mechanical and fracture properties of the syntactic foams under compressive [2], [3], [4], [5], [6], [7], flexural [6], [8], [9], [10], [11], tensile [3], [12], [13], and thermal degradation [10], [14]. Syntactic foams exhibit high values of compressive strength, which derive from the resistance of microspheres to compressive loads. Periasamy et al. [4] observed a compressive strength of 32 MPa and modulus of 1.26 GPa for a syntactic foam (ρ = 0.696 g/cm3) with 40 vol.% glass microspheres. Swetha et al. [5] reported a compressive strength of 50 MPa and modulus of 1.42 GPa for a syntactic foam (ρ = 0.76 g/cm3) consisting of 60 vol.% microspheres. However, it still remains challenges to obtain low density (ρ < 0.6 g/cm3) materials by using rigid hollow microspheres due to the limit of their density in nature and the limit of their volume fraction added. Compared with the syntactic foams, it is easy to prepare lower density materials using a chemical foaming agent. However, the compressive properties of epoxy foams produced by using the chemical foaming agent are low [15], [16], [17]. One of the main reasons is their relatively large cell size. For example, Alonso et al. [17] made the epoxy foam by using hydrogen gas generated in reaction of siloxane with amine. And the cell size is more than 200 μm. They observed a compressive strength of 3.37 MPa and a compressive modulus of 99.1 MPa for a typical fiber-reinforced epoxy foam (ρ = 0.3 g/cm3) with 2.5 wt.% aramid fibers added.
Expandable microsphere is a non-traditional physical blowing agent. And the cell size of the expandable microsphere is relatively small at the initial stage. The microsphere can expand upon heating due to vaporization of the volatile hydrocarbon liquid as shown in Fig. 1. It has been widely used in fabrication of foamed sole [18], car protection production [19] and so on due to the excellent foaming property, good mechanical properties, and lower price [20], [21]. In this case, epoxy foams were prepared by using expandable microsphere as the blowing agent in this study. Epoxy foams with relatively low density and high compressive properties were expected.
Osamu Takiguchi et al. [22] produced epoxy foams by using azodicarbonamide as the chemical blowing agent. And they found that the average diameter of the bubble became small as the precuring time before foaming became long. Zhang XY et al. [23] reported that the mechanical properties of the epoxy foam increased after precuring process. Similar to using chemical foaming agent, many factors may have an influence on the quality of expandable microspheres/epoxy foams. To ensure high-quality foams production, the epoxy resin as a matrix in this study was precured before foaming. The precuring process helped to promote the viscosity and elasticity of epoxy mixture which was needed to withstand the high blowing pressure and to stabilize the expanding bubbles at foaming temperatures.
In this study, the epoxy foams with fixed components were prepared through two processes. In the first process, epoxy mixture was precured at a low temperature (90 °C) to obtain the appropriate viscosity for foaming. Then these samples were further foamed at a high temperature to obtain epoxy foams with different densities and microstructures. We investigated the foaming behavior of expandable microspheres/epoxy foams under different conditions of foaming temperatures and the precuring extent variables, and attempt to correlate the foam structure with compressive behavior.
Section snippets
Materials
Diglycidyl ether of bisphenol-A epoxy resin (E-51), curing agent METHPA (methyl-5,6-dihydro-4H-isobenzofuran-1,3-dione), and DMP-30 (2,4,6-Tris(dimethy-laminomethyl)phenol) used as curing aids were supplied by Shanghai Resin Company, China. Montmorillonite (DK-4) was supplied by Zhejiang Fenghong New Material Company, China. Nano-SiO2 (average diameter: 50 nm) was supplied by Xiuhan Lonfee New Material Company, China. Montmorillonite and nano-SiO2 were dried under vacuum at 70 °C for 8 h before
Gel time
The viscosity change could not be easily measured due to the effect of glass fiber added to the epoxy mixture. Therefore, gel time (tgel′) at 160 °C, instead of viscosity, is used to describe the precuring degree. When curing reaction of the epoxy resin reaches gel point, the epoxy mixture changes from sol to gel-state, in which the three-dimensional network is formed. In the expandable microspheres/epoxy system, the foaming process basically finishes when curing reaction reaches gel point.
The
Conclusions
Expandable microspheres/epoxy foams with different densities and microstructures were prepared. The effect of process parameters including the foaming temperature and the precuring extent on density, microstructure and compressive property of composites has been studied. The precuring extent and foaming temperature are two important factors for expandable microspheres/epoxy foams and affect the cell size, the cell size distribution and the density of foam. The microstructure of foams reveals a
Acknowledgement
This work was supported by China’s National “973” Special Preliminary Study Program (Grant No. 2011CB12313), major Special Project of Guizhou (Grant no. 6023), and Science and technology plan projects of Guiyang (Grant No. [2012101]4-28).
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