Eco-Friendly Thermal Insulation Made From Rubber and Leather Waste

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jul 11 2024

A recent article published in the Journal of Materials Research and Technology proposed using expanded styrene-butadiene rubber (SBR) with leather waste as a sustainable alternative for thermal insulation in low-cost housing.

Background

Recently, there has been increased focus on the energy efficiency and thermal comfort of low-income housing, especially in areas with extreme weather conditions. Traditional building materials such as bricks and concrete, though widely used, are not energy efficient. They can lead to higher indoor temperatures, adversely affecting the comfort of residents and increasing the need for air conditioning.

In response to these challenges, research is exploring innovative, eco-efficient composites that act as thermal insulation materials. These composites aim to mitigate the impacts of climate and reduce electricity consumption. For such applications, materials that are thermally efficient, low-density, and cost-effective are preferred.

One promising avenue involves using bovine leather waste, such as trimmings and shavings, which contain chromium III sulfate—a compound used in the leather tanning process. Integrating these waste materials into polymeric matrices not only prevents the oxidation of chromium ions into harmful carcinogens but also potentially improves the thermal insulation properties of rubber composites.

This study delved into the morphological, thermal, and acoustic properties of expanded SBR (Styrene-Butadiene Rubber) composites incorporating leather residues. This approach presents a sustainable solution to enhance energy efficiency in affordable housing, leveraging waste materials for beneficial use.

Methods

SBR with a styrene content of 23.5 % and locally sourced leather shavings were used to prepare the composites for the study. The leather waste was initially micronized to achieve a particle size under 50 mm. The composites were then created using varying amounts of leather shavings: 0, 10, 20, 30, 40, and 50 parts per hundred rubber (phr), with the 0 phr sample serving as the pure rubber benchmark.

The blending of these materials was carried out using an open two-roll mill mixer. The rheological properties of the homogenized samples were analyzed through isotherms at a temperature of 160 °C. Following this, the materials underwent hot pressing at the same temperature to form solid samples ready for further testing.

The cross-link density of each composite was determined using the swelling method, which involves immersion in toluene for five days. The structural morphology was examined using a scanning electron microscope (SEM), which captured three images per sample to assess pore sizes, areas, and anisotropy.

Chemical composition at various points was identified through energy-dispersive X-ray (EDX) spectroscopy. Additionally, the cell density of the composites was calculated using Kumar's theoretical approximation.

The mechanical and thermal properties were further investigated using dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The acoustic absorption properties were measured across a frequency range of 500-6400 Hz at 20 °C.

Finally, to demonstrate the practical application of these findings, a prototype thermal coating measuring 19x19x155 cm^³ was prepared, showcasing the potential of these composites in real-world applications.

Results and Discussion

The incorporation of micronized leather waste as a filler in SBR foams demonstrated encouraging outcomes in various evaluated properties. A noticeable effect was seen in the torque values during vulcanization, with both minimum and maximum torque values increasing as the leather content increased. This rise can be attributed to the higher viscosity that accompanies the added leather content.

Interestingly, the rate of vulcanization for these composites increased progressively with the addition of leather waste up to 30 phr, after which it reached saturation. The saturation point suggests a threshold where the added filler creates greater resistance within the rubber matrix, impacting the vulcanization process. In terms of cross-link density, composites with 0 to 20 phr leather demonstrated the lowest values. However, a significant increase in cross-link density was observed in composites containing more than 30 phr of leather waste.

SEM analysis revealed the fibrous nature of the leather, with fibers showing a maximum length of 1.4 mm and a diameter under 0.297 mm. The addition of leather reduced the porosity and increased the rubber foams' relative density, influencing their structural and functional properties. Notably, higher porosity typically correlates with a higher cellular density, but this relationship was not evident in the 20 phr leather foams. The low number of cells formed in these foams resulted in the lowest cellular density among the samples studied.

Despite the lowest cell density, the 20 phr leather foams exhibited the best pore distribution with more varied area sizes, favorable for an effective response as a thermal insulator. Alternatively, the composite with 40 phr leather exhibited a maximum proportion of pores (approximately 70%).

Acoustically, the SBR foam with 50 phr leather exhibited the best performance within the 1500-3500 Hz range. Conversely, the 20 phr composite maintained consistent acoustic performance across a broader frequency range, from 1500 to 4500 Hz. This broader effectiveness is likely due to its lower cell density and the greater heterogeneity in its pore distribution, which can positively impact sound absorption.

These findings underscore the potential of SBR foams with leather waste as effective materials for thermal insulation and acoustic applications, with the added benefit of utilizing industrial waste productively.

Conclusion

Overall, the researchers proposed an innovative and sustainable solution for thermal insulation in low-cost housing using expanded SBR with micronized leather shavings. The composite with 20 phr emerged as the most promising alternative to conventional materials, exhibiting high thermal insulation capacity with a thermal conductivity of 0.073 W/mK and temperature attenuation of about 15 °C.

The thermal insulation properties of the other compositions also surpassed traditional materials such as plywood, fiberglass, asphalt roofing, and cement tiles. Thus, leather waste can be feasibly used as a reinforcing filler in rubber-based foams to produce sustainable thermal insulation materials, mitigating urban heating and improving the quality of life in cities.

Journal Reference

Ribeiro, G. D. et al. (2024). Sustainable construction materials for low-cost housing: Thermal insulation potential of expanded SBR composites with leather waste. Journal of Materials Research and Technology, 30, 5590-5604. DOI: 10.1016/j.jmrt.2024.04.234, https://www.sciencedirect.com/science/article/pii/S2238785424010081

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