Glass Powder In Rock/Mineral Wool Manufacture

Mineral wool is a broad term that encompasses any fibrous material that is formed from the spinning or drawing out of molten rock-like materials, such as ceramics and glasses. Typical applications for mineral wool include soundproofing (1) and, most prominently, thermal insulation (2). One type of mineral wool is rock wool, also referred to by the name of leading manufacturer Rockwool. Mineral wool first appeared in the 1840’s yet it wasn’t until some thirty years later that commercial production began in Germany. The first mineral wool suitable for high temperature insulation was made commercially available in the 1950s.

1-Rockwool-Insulation-Sheet
rockwool pipe insulation

Typical Compositions

Broadly speaking, mineral/rock wool is made up of natural materials (for example, igneous rocks such as basalt) and typically on the order of 30% by weight recycled materials such as ground glass, sand or other similar aggregates. The balance is composed of phenolic resins, which act as binders, and mineral oil, which prevents sticking. Commercial mineral wools are subject to stringent EU regulations EN 13162 and EN 13500(3).

Mineral wools are made by melting together the constituent parts into a molten glass-like material and the subsequent spinning and drying of this molten material into a wool-like structure. This structure traps air and it is this that provides thermal insulation, as air is a poor conductor of heat.

Effects Of The Addition Of Glass Powder

As with other fibrous materials manufactured from melted glassy or glass-like substances, rock wool often has a variety of materials added to it to either enhance properties, reduce cost, increase production efficiency - and often a combination of all three. The most common additive is ground glass (glass powder)(4). Oftentimes in mineral wool manufacture, ground glass is added to replace some of the other materials, frequently being responsible for a lowering of the melting temperature required.

When manufacturing rock/mineral wool using ground glass, the operation temperature is around 1,500 °C, this is much lower than the melting point of fresh silica. Overall, the use of ground glass behaves akin to a flux in ensuring a lower overall melting temperature, saving the time it takes to heat, and the amount of energy required(5).

close up of rockwool

One of the advantages of using glass powders in the spun wool fibre-type insulation production is that the spreading of the melt decreases, thereby making for a more efficient spinning process. Furthermore, additional ground glass will have no detrimental effects with regard to alkali resistance(6).

Glass powder is regarded as an inexpensive material, and whilst typical raw materials for rock wool are relatively cheap, they must be mined. Ground glass is either sourced as a by-product or from recycling/waste streams, as such, the use of this reduces waste and lowers cost(7,8). A common theme amongst fibrous materials that have been at least partly from ground glass is an added dimension of strength, which itself can lead to ongoing durability over time(9).

If the glass powder added to a mineral wool composition is of the borosilicate variety (i.e. during its own manufacture, some of the silica was replaced by boron oxides) then the resulting mineral wool may enjoy even higher levels of durability(10). Ground glass contamination levels may not exceed 10 ppm of ferrous metals and 20 ppm non-ferrous metals for use in rock/mineral wool applications(11).

Further to the aforementioned lowering of melting temperature, the diversion of glass waste/cullet from landfill is a significant environmental benefit. As per the US Department of Energy, mineral wool contains on average 75% recycled materials(12). The source of the major aggregate material (basalt) can also be from waste streams, for example mine tailings(13).

rockwool insulation being installed

Considerations/Applications

The most common use for rock wool is in building insulation, where it is regarded as being superior in performance to fibreglass (glass wool) insulation on the order of around 30%, meaning less needs to be used per unit area of insulation required.

Resin bonded panels are becoming a common use for fibrous insulation materials. A pad of rock wool is sandwiched between pieces of particleboard, sheet metal or other boards and set using a resin. These structures can, depending on the choice of board, be structural. Notably, however, they have far superior insulation properties compared to plain rock wool(14). Additionally, the use of rock wool decreased flammability - although reduced ignitability is associated with a decrease in mechanical strength performance(15).

Highly valued amongst its fibrous material peers for its resistance to heat, rock wool is frequently used as a fire resistant material(16), and also as a furnace insulation/lining owing to its low combustibility(17).

Rock wool has found a nice but growing use in horticulture as a medium for growing plants. The porous nature of the overall material is stated as useful for efficient watering and drainage of various flowering and fruit-producing plants(18).

Rock wool is a fibrous material and under certain conditions, fibres can become detached from the bulk and will pass through the air, and can enter the lungs. During manufacture and installation of rock wool, it is thus imperative to ensure no fibres are allowed to enter airways of workers in anything greater than a trace amount.

foundry using coal dust

Summary

  • Mineral wool (rock wool) is a widely used material for insulation
  • Rock wool is composed of ground minerals (basalt) and glass that have been melted down and spun to form a wool-type material
  • Adding glass to the mixture not only decreases costs, but lowers the overall melting temperature, reduces material sent to landfill and adds durability
  • Ground glass is added to rock wool production mixtures typically around 30% by weight
  • Rock wool outperforms fibreglass insulation; up to 75% of rock wool can be from recycled sources
glass_powder

References

1          P. H. Parkin et al., Acoustics, Noise and Buildings, Faber and Faber, London, 1979

2          J. A. Schaeffer, Ind. Eng. Chem., 1935, 27, 1298

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          T. K. Pavulshkina and N. G. Kisilenko, Glass and Ceramics, 2011, 68, 5

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

6          V. I. Onishchuk et al., Glass and Ceramics, 1999, 56, 5

7          P. W. Johnson and J. A. Barclay, Economic Studies of Uses of the Glass’ Fractions from Municipal Incinerator Residues, Bureau of Mines, US Department of the Interior, Washington DC, 1973

8          R. K. Collings et al., Mineral waste utilisation studies, in: Fourth Mineral Waste Utilisation Symposium, Chicago, 1974

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

10        H. Seven et al., End-of-waste criteria for glass cullet: technical proposals, JRC-IPTS, European Commission, Seville, 2011

11        Glass for Europe, Recycling of glass from construction and demolition waste, European Commission, Luxembourg, 2005

12        US Department of Energy (online), Insulation Materials, accessed 16 Mar 2020, https://www.energy.gov/energysaver/weatherize/insulation/insulation-materials

13        M. Pelino et al,. Int. J. Mineral Process., 1998, 53, 121

14        M. Vacula et al., Adv. Mater. Res., 2014, 923, 195

15        P. Borysiuk et al., Eur. J. Wood and Wood Prod., 2011, 69, 337

16        A. Tugnoli et al., Int. J. Thermal Sci., 2019, 136, 107

17        A. K. Bose and M. R. K. Rao, Fibre Insulation Refractories in Reheating Furnaces, in: Proc. Indian Ceramic Soc, Jamshedpur, India, 1978

18        C. Sonneveld and G. W. H. Welles, Plant and Soil, 1988, 111, 37