Penn State University’s Sierra R. Astle, Nicholas Clark, Matthew Mancini, Shaylee Traugh and John C. Mauro* discuss a unique family of glass compositions which can be considered as a notable low-carbon alternative to conventional soda lime silicate products.

Glass is a widely manufactured material, and as such glass contributes a significant amount to total yearly carbon dioxide emissions worldwide.

The US Environmental Protection Agency (EPA) has shown that flat glass contributes anywhere from 0.465 to 0.579 metric tons CO2 equivalent (CO2e) per ton of glass manufactured.1

Kayaçetin and Tanyer asserted that glass has a global warming contribution of 2.15 kg CO2e per kg produced.2

The demand for window glass is expected to grow by 3.2% by 2027 and will only increase emissions produced by this industry.3

The catastrophic consequences of carbon dioxide emissions for the earth are significant, with existing climate changes theorised to remain ‘largely irreversible’ for up to 1000 years even after such emissions cease.

Ongoing effects of atmospheric carbon, including droughts, ice cap melting, sea level rise and other impacts, inflict harm on local ecosystems and displace individuals residing in these vulnerable climates.4

To take a step towards the solution to this problem, the glass industry can and should address the two main sources of emissions: the embodied energy used in melting the raw batch materials and the carbon dioxide released as a byproduct from the melting process.

The current industry standard for most everyday glass products is soda lime silicate, a composition having a majority silica network including sodium oxide and calcium oxide. Soda lime silicates have a melting temperature in the range of 1400-1500°C.5

Considering that carbon-based energy sources still dominate in industry, the energy required to reach such high temperatures has an implicit carbon footprint of its own.

Soda lime silicates are also made using carbonate batch materials such as Na2CO3 and CaCO3, which release CO2 as a byproduct of a heat decomposition reaction.6

Addressing the dual challenges of melting temperatures and emissions during glass product manufacturing is critical, and LionGlass may be the answer.

Developed by researchers at The Pennsylvania State University, this family of glass compositions not only meets these requirements but offers additional benefits.

Its potential as a low-carbon, high-crack resistance alternative to traditional soda lime silicate window glass provides promise for the reduction of carbon emissions within the glass industry.

Approach

Aiming to reduce both the CO2 emissions of the batch and the melting temperature required for manufacturing, the inventors of LionGlass developed a distinctive series of compositions.

This innovation belongs to a family of glasses known as zinc aluminosilicophosphates, or ZASP glass. The primary constituent of this glass is phosphorus pentoxide, supplemented by silica, alumina and zinc oxide, along with differing quantities of other oxides.

In this system, phosphorus pentoxide takes the role of primary network former, while silica and alumina contribute as additional network formers, underscoring the complex nature of this promising new glass family.

Zinc is used to alter mechanical and thermal properties such as hardness, crack initiation threshold, and melting temperature. Select compositions were optimised for efficient melt schedules and subsequently characterised for their material properties.

Results

Melting Point and Emissions

Thermal properties of a glass at all stages of manufacturing – melting, cooling, and processing – are crucial. LionGlass was created to have a lower melting point than soda lime silicates. Researchers found LionGlass compositions melt at about 1150°C to 1200°C.

This is a few hundred degrees Celsius lower than soda lime silicates, which are melted at about 1400°C to 1500°C. The energy saved by the decreased melting temperature alone would lower both emissions and manufacturing cost. Owing to its substantial phosphate content, LionGlass may be synthesised without the use of carbonate batch materials.

This further reduces the direct carbon footprint of the glass.

Viscosity

Melt viscosity is an important property to consider when manufacturing. Viscosity dictates how a molten glass flows at a given temperature. Using differential scanning calorimetry, data was collected to parametrise the MYEGA model, which describes the change in viscosity of the supercooled liquid as it approaches its glass transition temperature.7

The viscosity of LionGlass was measured to be significantly less than that of soda lime silicate glass across the full range of temperatures as seen in Fig 1.

A molten glass that reaches a target viscosity at a lower temperature is safer and more affordable to work with. For example, soda lime silicate reaches its softening point, the point where it begins to slump underneath its own weight, at about 730°C.8 LionGlass reaches this same point more than 100 degrees lower, at about 590°C.

LionGlass, then, can be processed at lower temperatures than soda lime silicate. Fig 2 shows a comparison of temperatures at critical processing points between LionGlass and soda lime silicate.

Fig 2 shows a direct comparison of different critical processing points, such as the softening point, between LionGlass and soda lime silicate, plotted against an ideal line of y=x. All data points fall below the y=x line, indicating that LionGlass reaches critical processing points at a lower temperature than soda lime silicate does.

This is beneficial for processing and makes LionGlass a candidate for glass to be used as an art medium. Viscosity becomes even more important for glassblowers and other glass artists who need a long working time before their glass cools and loses its pliability. For example, the working range of a glass is defined as the temperature range when a glass’s viscosity is between 103 to 106.6 Pa.s.8

The working range of LionGlass is about 590°C to 700°C, whereas the working range of soda lime silicate is about 730°C to 1205°C.9 Glassblowers and other artists would then need to spend less time and energy heating the glass to their desired viscosity. LionGlass has exciting opportunities as a medium for glass art.

Mechanical Properties

The mechanical properties of a glass are critical for its material lifetime. A glass jar or window is not commercially viable if it scratches, cracks or shatters too easily. One consideration of sustainable materials is the duration a product can last without replacement.

The crack initiation resistance of LionGlass was found to be superior to that of soda lime silicate glass. Developed glasses were polished and tested using Vickers indentation. The initially developed LionGlass composition, shown in Fig 3, was found to have such a high crack initiation resistance that it exceeded the measuring capabilities of the indentation instruments used.

LionGlass has a crack resistance more than ten times greater than that of soda lime silicate glass, as seen in Fig 3. A 50% crack initiation threshold – the force at which a sample cracked 50% of the time –was used to determine crack-resistance of the glass qualitatively.

The 50% initiation threshold for soda lime silicate glass was found to be roughly 0.12 kgf. The higher crack initiation threshold of LionGlass suggests that it can enable thinner glass products than conventional soda lime silicate windows without compromising its mechanical performance. This lightweighting implies a more efficient utilisation of materials, reduced manufacturing costs, optimised transportation and an overall decrease in emissions.

Optical Properties

Light transmission through glass is important in both architectural and artistic applications. Transmission microscopy within the UV-Vis range was performed using mirror-polished samples of approximately 1 mm thick.

The UV-Vis data, seen in Fig 4, shows wavelength absorption differences between LionGlass and soda lime silicate glass.

The presented UV-Vis data delineates a comparison of the absorption coefficients between LionGlass and traditional soda lime silicate, with both demonstrating similar intensity levels across a wavelength range of 250nm to 850nm.

There is also shift in the UV absorption edge of LionGlass due to the incorporation of zinc. In the realm of architectural glass applications, it is imperative to maintain a consistent emission of light across the visible spectrum. Light emission influences the ambient lighting conditions within residential and commercial structures.

Conclusion

LionGlass, a unique family of glass compositions, emerges as a notable low-carbon alternative to conventional soda lime silicate products, thanks to its lower melting temperature, enhanced crack resistance and superior thermal and optical properties.

Its lower melting point results in reduced energy consumption during manufacturing and its enhanced workability makes it an optimal candidate for the production process.

Capable of attaining a crack initiation threshold greater than ten times that of soda lime silicates, LionGlass can enable lightweighting of everyday glass products, leading to overall reduced material and energy utilisation. Its optical properties align well with those of soda lime silicate glass, while its distinctive composition, devoid of carbonate materials, lowers carbon dioxide emissions during melting.

The exploration of other compositions within the LionGlass family is underway, including various properties such as melting point, hardness, crack initiation resistance, glass transition temperature and viscosity curves.

*The Pennsylvania State University,

University Park, PA, USA,

Web: https://sites.psu.edu/glass

Email: jcm426@psu.edu

References

1. “U.S. Flat Glass Industry Carbon Intensities (2019)” (United States Environmental Protection Agency, May 21, 2022), https://www.epa.gov/system/fil....

2. N.C. Kayaçetin and A.M. Tanyer, “Embodied Carbon Assessment of Residential Housing at Urban Scale,” Renewable and Sustainable Energy Reviews 117 (January 1, 2020): 109470, https://doi.org/10.1016/j.rser....

3. “U.S. Commercial Windows Market Size, Share & Trends Analysis Report By Frame Material (Vinyl, Wood, Metal), By Mechanism (Swinging, Sliding), By End Use, By State, And Segment Forecasts, 2020 - 2027,” Market Analysis Report (Grand View Research, 2020), https://www.grandviewresearch.....

4. Susan Solomon et al., “Irreversible Climate Change Due to Carbon Dioxide Emissions,” Proceedings of the National Academy of Sciences 106, no. 6 (February 10, 2009): 1704–9, https://doi.org/10.1073/pnas.0....

5. Arun K. Varshneya and John C. Mauro, Fundamentals of Inorganic Glasses, 3rd ed. (Elsevier, 2019).

6. “Glass Is the Hidden Gem in a Carbon-Neutral Future,” Nature, Editorials, 599, no. 7–8 (2021), https://doi.org/10.1038/d41586....

7. John C. Mauro et al., “Viscosity of Glass-Forming Liquids,” Proceedings of the National Academy of Sciences of the United States of America 106, no. 47 (2009): 19780–84.

8. “Physical Properties of Glass,” Eurofins EAG Laboratories, n.d., https://www.eag.com/app-note/p....