Glass Powder: Application in Fibreglass Manufacture

Fibreglass is a widely used material in house building, as a primary insulator, and as a structural material in its own right, for example in the manufacture of small boat hulls. Its applications depend on how it is made and the composition. Strictly speaking, however, fibreglass is the structural material. The material resembling foam and used as an insulation is glass wool, commonly referred to as fibreglass, however most of the principles are the same and the outcome derives from differences in the manufacturing process rather than chemical composition. Structural material fibreglass will not be covered in this article. ‘Fibreglass’ and ‘glass wool’ are used interchangeably as the insulation material.

Fibreglass as an Insulator (glass wool)

The process of making fibreglass insulation bears many similarities to making candy floss. A liquid glass is produced by heating sand to ca. 1,500 °C and then forced through a fine mesh by centripetal force, where the molten glass cools and solidifies upon contact with the air. Binders are then used to adhere the fibre strands together and add mechanical strength. The greater rate of spinning, the finer the fibre diameter(1). The resultant mass is heated to ca. 200 °C to induce polymerisation, and then undergoes a finishing protocol known as calendaring. The overall procedure traps pockets of air between the fibres, thus making it an excellent insulator, as air is a poor conductor of heat. Glass wool is particularly suited to long term applications owing to its long term stability and resistance to fungal growth(2).

glass wool insulation

Typical Compositions

Fibreglass/glass wool is primarily made from sand, itself mostly silica and/or glass cullet. Glass is produced in the traditional method and subjected to the process above when in the liquid form. Melting down glass/ground glass to manufacture glass wool has been a utilised process since the late 1970s. Compositions of glass wool are typically formed in the region of 70% glass, 0.5 to 0.7% phenolic resin binder, 0.5% mineral oil with the balance being quartz and limestone(3). Research from that time showed that glass powder waste (from plate or container glass production) could easily be melted down and made into a fibreglass insulation with the same performance as had it been made from fresh sand/silica(4) with wide ranges of sources of glass can be used, including coloured glass.

Effects of Adding Glass Powder

The performance difference between glass wool made from ‘virgin’ sand and that made with ground glass is subtle - this itself is a strong reason to use glass powder. In general, standard glass wool is rated with a thermal conductivity not exceeding 0.05 W/mK(5).

Performance is generally regarded as superior when the glass wool is formed into thicker sections, as opposed to multiple layers of fibre covering one area(6), with maximum performance noticed when glass fibre is sandwiched between plates of solid materials such as wood or sheet metal. Such layered panel’s report effective conductivity of as low as 0.001 W/mK(7), testament to the performance of the ground glass-produced fibreglass. When used as an evacuated thermal insulation, ground glass fibreglass outperforms microporous powder insulation by almost 1.5 times the insulation capability(8). When exposed to atmospheric loads, glass wool compresses to densities in excess of 176 kg/m3.

The wider glass industry has long used cullet (and powdered glass) as it is less expensive than the raw material silica and it requires less energy from furnaces to melt it(9). When combined alongside fresh raw material, the addition of cullet/powdered glass can decrease the energy requirement for melting - i.e. it behaves akin to a flux(10).

reinforced plastic fiberglass

Experimental data shows that the performance of the glass wool is closely related to the diameter and length of the fibres produced(11). These parameters are dependent on many aspects, but chief amongst them is temperature, where a 1,250 °C melt has been found to produce idealised 3.9 μm diameter fibres using the centripetal spinning process, from solely ground glass cullet, boric acid and no new silica(12). This value is approximately 500 °C less than the melting point of pure silica. Glass fibres produced using ground glass/cullet tend to be stronger in mechanical terms, which whilst not important as glass wool is not used structurally, strength will lead to a longer lifetime of a material(13), with tensile strength also being notable.

Compared to other insulation materials, glass wool derived from powdered glass produces fibres that are isotropic, that is where thermal expansion coefficients and Young’s modulus are the same in both the direction of the fibres and perpendicular to them(14), with mechanical properties being largely independent of temperature. A uniform thermal expansion is vastly preferred. Carbon and Kevlar fibre-type insulation materials do not share such behaviours and are relatively mechanically weak in comparison.

These advantages are highly valuable to the fibreglass producer, who is able to produce more high-quality fibreglass insulation at a noticeably lower price. Furthermore, the utilisation of powdered glass in insulation diverts thousands of tons of glass waste from landfill, thereby making glass production as a whole more environmentally sound.

Summary

  • Glass powder is a viable source of material for the production of all types of fibreglass
  • Due to its inexpensive nature, manufacturers find significant cost savings when replacing all or part of the molten glass mix with ground glass powder/cullet
  • Ground glass powder can act as a de facto flux, reducing the temperature required for the melt compared to using new silica, and thus ensuring a lower demand on resources such as electricity, and thus gives rise to cost savings and operational efficiencies
  • The use of glass powder is not only economically attractive, but environmentally also
glass_powder

References

1          B. B. Li et al., Adv. Mater. Res., 2012, 457-458, 1573

2          M. Klamer et al., Int. Biodeterior. Biodegrad. J., 2004, 54, 277

3          R. Gellert, Inorganic mineral materials for insulation in buildings, in: M. R. Hall (ed.) Materials for energy efficiency and thermal comfort in buildings, CRC Press, Boston, 2010

4          A. H. Goode et al., Glass Wool From Waste Glass, Bureau of Mines, United States Department of the Interior, Washington DC, 1972

5          F. P. Incropera and D. P. De Witt, Fundamentals of heat and mass transfer, Wiley, New York, 1990

6          W. Wu, Adv. Mater. Res., 2012, 446-449, 3753

7          H.P. Ebert et al., Vacuum, 2008, 82, 680

8          R. Michael, J. Thermal Insulation, 1991, 14, 195

9          Remade-Scotland, Glass recycling handbook: Assessment of available technologies, Remade-Scotland, Glasgow, 2003

10        R. Farel et al., Resources Conserv. Recycl., 2013, 74, 54

11        T. Advocat et al., J. Non-Crystall. Solids, 2008, 354, 4917

12        J. Zhou et al., Adv. Mater. Res., 2012, 415-417, 1996

13        K. Sonsakul and W. Boongsood, IOP Conf. Ser.: Mater. Sci. Eng. 2017, 273, 12006

14        G. Hartwig, Cryogenics, 1988, 28, 4