Geometrical and mechanical characterisation of hollow thermoplastic microspheres for syntactic foam applications

Matthew E Curd,Neil F Morrison,Michael J.A Smith,Parmesh Gajjar,Zeshan Yousaf,William J Parnell

doi:10.1016/j.compositesb.2021.108952

Matthew E Curd, Neil F Morrison + Show 4 more

Open Access

https://doi.org/10.1016/j.compositesb.2021.108952

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Abstract

Recently, hollow thermoplastic microspheres, have emerged as an innovative filler material for use in polymer-matrix composites. The resulting all-polymer syntactic foam takes on excellent damage tolerance properties, strong recoverability under large strains, and favourable energy dissipation characteristics. Aside from syntactic foams, thermoplastic microspheres are finding increasing usage in a variety of applications and industries. Despite this, there is an absence of statistical geometrical and mechanical data for certain classes of thermoplastic microspheres. In this work we characterise two classes of thermoplastic microsphere using X-ray computed tomography, focused ion beam and electron microscopy. We observe the spatial distribution of these microspheres within a polyurethane-matrix syntactic foam and show that the volume-weighted polydisperse shell diameters follow a normal distribution. Interestingly, polydispersity of the shell wall thickness is not observed and furthermore the wall thickness is not correlated to the shell diameter. We utilise the geometrical information obtained in analytical micromechanical techniques in the small strain regime to determine, for the first time, estimates of the Young's modulus and Poisson's ratio of the microsphere shell material itself. Our results contribute to potential future improvements in the design and fabrication of materials that employ thermoplastic microspheres, including syntactic foams.

Highlights

Syntactic foams are lightweight synthetic composites consisting of a metal, ceramic, or polymer matrix and microsphere inclusions [1,2,3,4,5]
Their presence will not affect the characterisation of the microsphere diameter distribution as they were removed from the labels prior to analysis
Radiography was conducted on 25 microspheres (920 grade) and the shell thicknesses were inferred from the radiographs

Summary

Introduction

Syntactic foams are lightweight synthetic composites consisting of a metal, ceramic, or polymer matrix and microsphere inclusions [1,2,3,4,5]. The mechanical properties of syntactic foams can be tailored by adjusting the matrix material and/or the type and volume fraction of the microspheres [2,3,4]. For these reasons, syntactic foams are suitable for applications within numerous industries including the aerospace, automotive, marine, and sports equipment sectors [3, 4, 6,7,8,9]. The material behaves linearly once again due to densification of the microspheres, i.e., the filling of the microsphere cavities with debris For this reason, glass microsphere foams are non-recoverable over a large strain regime and are inappropriate in applications involving large repetitive strains. Recently it has been suggested that a mixture of glass and plastic microsphere fillers gives rise to hybrid materials with unusually high filler fractions and associated useful material properties [26, 27]

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