Use of Glass Microspheres in Thermoplastic Polymer Matrix Composites

Developing light-weight materials is particularly important for engineering and industrial efficiency, as it facilitates ease of use and transport. Based on this premise, reinforcing materials with different modification capacities are used for metals, glass, plastics, composites, etc. to achieve weight reduction. On the other hand, providing lightness is considered a very important criterion especially in the automotive industry because it increases fuel efficiency and reduces the environmental effects of transport emissions [1], [2]. Hollow glass microspheres with an average density of 0.06 to 0.12 g/cc, while products in larger and smaller sizes are also available, have average particle sizes of 15 µm to 30 µm. Additionally, in new-generation microsphere structures with very good dispersion, thin-walled (0.5-1.5 μm) calcium oxide-mixed borosilicate glass particles have average diameters ranging from 16-65 μm.

Hollow Glass Microspheres (HGMs) are structures comprising hard glass exterior and inert gas interior, possessing certain unique properties such as structural lightness, low thermal conductivity and low dielectric constant.

Furthermore, surface modification processes conducted on glass microspheres also contribute to homogeneous distribution of this reinforcement within molten polymers and improvement of their physical properties [2-4]. Glass Beads (GBs) are a type of solid, filled glass additive material with a smooth surface, solid and spherical structure, with a density ranging from 2.20 to 2.50 g/cc. Polymers reinforced with glass spheres, due to their isotropic structure, exhibit less internal stress and better mechanical and processing properties compared to other reinforcement types added to polymer composites. Hollow glass microspheres (HGMs), on the other hand, are a type of rigid filler with smooth surface used particularly in rubber, polymer and coating materials for the production of light-weight and thermal insulation products. HGMs, when compared with GBs, have advantages such as low density and providing thermal and sound insulation. Since HGMs trap air within their interiors and volume changes with temperature are minimal, they are not particularly sensitive to processing temperature. However, due to being hollow and fragile, reductions in tensile and bending strength values may occur in their structures [5-8]. The use of glass microspheres is foreseen as a promising technology for polymer composite materials. Additionally, the reinforcement technology can provide advantages through its potential qualities in terms of sustainability of physical properties, as it enables better homogeneous distribution within the structure and lower process sensitivity compared to other light-weight technologies [1].

HGMs are more isotropic than polymer matrices containing fillers with high aspect ratios such as fibers and layered structures. Figure 1 shows a scanning electron microscope image comparing different additive types and the aspect ratio and density values of HGMs.

Due to the anisotropic nature of fibers and layered reinforcements in the polymer matrix, tension/shrinkage issues arise. Due to their isotropic structure,HGM-reinforced polymer composite structures exhibit much lower shrinkage rates compared to many other reinforced polymer composite structures [6].

Figure 1. a) hollow glass microspheres, b) talc, c) scanning electron microscopy (SEM) images of glass fibers [7]

Some polymers used for HGM addition to thermoplastic matrices can be listed as polyolefins (Polypropylene (PP), Polyethylene (PE), Polyamide (PA), Poly(acrylonitrile-butadiene-styrene) (ABS), Polylactic Acid (PLA), etc. [7], [9-11]. Beyond reinforcing thermoplastic polymers with glass microspheres through conventional polymer production methods, they can be used in many different production areas with their low densities. One example is selective laser sintering (SLS), which is a powder bed additive manufacturing method. Particularly in the field of thermoplastic polymer composite studies reinforced with glass microspheres (GBs), the commonly used thermoplastic polymers are primarily the Polyamide family, followed by Polystyrene (PS), Polypropylene (PP), Polyethylene (PE), Polyetheretherketone (PEEK), and Thermoplastic Polyurethane (TPU) [12-15]. Although there are some very new commercial products in this field involving glass sphere reinforcement along with carbon fiber reinforcement, this additive type is still at a very early stage of development in SLS additive manufacturing technology, making it an open field for detailed research. In our work on evaluating the effects on the properties of the final polymer composite structure obtained by adding hollow glass microspheres (HGM) to Polyamide 12 (PA 12) matrix in SLS additive manufacturing [16], a significant density reduction reaching 20% was achieved without excessive compromise on the mechanical properties of the final structure. It is foreseen that much more work will be conducted in this field and promising results will be obtained, and our research on this and similar types of additives continues.

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  Prof. Dr. Ersin Serhatlı Istanbul Technical University Department of Polymer Science and Technology Burçin Özbay Metallurgical and Materials Engineer/Researcher Fatih Sultan Mehmet Vakıf University ALUTEAM