Spherical glass beads are being used in many industries, due to certain advantages such as strong filling ability, smooth spherical surface, small and well distributed internal stress in the products and good processibility of the filled materials. Studies on the structure-property correlation of glass micro- sphere filled polymers have been made by several researchers over the past 20 years [88-94]. Even then, thermal, mechanical and tribological behaviour of such composites has remained a relatively less studied area. Glass micro-spheres are being used commercially with various resin matrices, both thermoplastics and thermosets, to improve the physical and mechanical properties of the resins [95]. Among the mechanical properties, hardness, modulus of elasticity and fracture toughness are known to be improved with the incorporation of glass micro- spheres in brittle resin matrices [96-98]. The tensile strength and ductility (elongation to break), on the other hand, decreases with the increase in filler volume fraction [99, 100]. Glass micro-spheres are preferred as fillers especially when composite properties such as isotropy or low melt viscosity are important. They are considered to be a reinforcement which does not create a source of stress concentration in the matrix unlike other long fibers which have sharp edges leading to stress generation in the matrix and in turn early failure of the composites.
Sahu and Broutman [89] have studied in considerable detail the mechanical and fracture properties of glass sphere filled epoxy and polyester resins with various particle-matrix interface conditions. Mallick and Broutman [93] presented a possible explanation for the strength behaviour of glass micro-sphere filled brittle resin composites and described the flexural, compressive and fracture properties of brittle epoxy matrix composite containing glass beads of 15 micron average particle size.
Few research works have also been reported recently on glass micro-sphere filled thermoplastics and concept of linear elastic fracture mechanics (LEFM)
Ph.D. Thesis
2014
Department of Mechanical Engineering, N.I.T. Rourkela Page 18
has been widely employed to study the fracture toughness of such thermoplastic composites [94-98]. Sanchez-Soto et al. [99] analyzed the fracture behaviour of a material model composed of polysterene and solid glass beads and found out that small quantities of glass beads are enough to modify polystyrene fracture behaviour changing the propagation mode, which tends to stabilize as more quantity of beads are added to the matrix. Faulkner and Schmidt [101] studied the rheological and mechanical properties of glass bead filled polypropylene (PP) composites and noted that the relative tensile modulus and relative flexural modulus were both linear functions of bead fraction. Lepez et al. [102] carried out thermo-rheological analysis of glass micro-sphere filled high density polyethylene (HDPE) and polystyrene (PS) melts and proposed a new empirical model that allowed the prediction of complex viscosity of the composite melts. Ou and Yu [103] investigated the effects of the interfacial adhesion on the micro-damage and the rheological behaviour of glass bead filled nylon. Li et al. [104] analyzed the dynamic and mechanical properties of glass micro-sphere filled low density polyethylene composites using a dynamic mechanical analyser. Lee and Yee [105] investigated the major energy dissipation mechanisms of glass micro-sphere filled epoxies based on the previously established knowledge about the micro-mechanical deformations occurring during the fracture.
Liang [106, 107] gave an insight about the tensile and flexural properties of hollow glass micro-sphere (HGM) filled acrylonitrile-butadiene-styrene (ABS) and polyvinyl-chloride (PVC) composites. Gupta et al. [80, 108] compared the tensile and compressive characteristics of vinyl-ester/glass-microballoon syntactic foams and also did a microscopic examination of their compressive fracture features. They also analyzed the flexural and compressive properties of hollow-particle filled composites and found a similar kind of observation [109]. Hollow glass micro-spheres have low density and so they reduce the weight of the composites to a great extent, but according to Kim and Khamis [110], their addition tends to reduce the Young’s modulus and ultimate strength of the
Ph.D. Thesis
2014
Department of Mechanical Engineering, N.I.T. Rourkela Page 19
composites. Even specific values of flexural stiffness are only marginally increased for high volume fractions of spheres but this difficulty can be overcome by using rigid inorganic particles; so solid glass micro-sphere might serve as a good alternative. Ferreira et al. [111] investigated the effects of hollow glass micro-sphere filled hybrid composites and studied on the addition of short fiber reinforcements on the mechanical behaviour of epoxy composites. In a recent study, Kushvaha and Tippur [112] investigated the effects of filler shape (characterized by the aspect ratio), filler volume fraction and loading rate on fracture toughness of glass-epoxy composites.
Few works have also been reported in the past on the thermal and electrical behaviour of glass micro-sphere filled polymers. Yung et al. [113] and Zhu et al. [114] investigated the thermal, mechanical as well as the dielectric properties of such HGM filled composites and concluded that the properties of composites are mainly dependent on the characteristics of HGMs. Liang [115] estimated the thermal conductivity for polypropylene/hollow glass bead composites and found that the estimated and measured thermal conductivity decreased roughly linearly with increasing the HGM volume fraction. Recently Mishra and Satapathy [116] have developed a theoretical model and proposed a correlation to estimate the effective thermal conductivity of micro-sphere filled polymers. They have also reported extensively on thermal properties of glass micro-spheres filled epoxy composites [117].