Vegetable oils may enable stronger and more eco-friendly sound absorbing materials
Flexible polyurethane (PU) foams are used in a wide range of consumer, commercial and industrial products. Their soft but resilient structure makes them especially effective at enhancing comfort, which is why they’re so prevalent in furniture, bedding, packaging, and footwear. In vehicle interiors, PU foams have an additional role – their sound-damping properties help them to reduce noise and vibrations. But these foams also have a significant downside. Their two key ingredients (polyol and isocyanate) are predominantly derived from petroleum sources. This has prompted a number of researchers, including chemical engineers from the University of Seoul, to investigate alternative routes for the fabrication of PU foams.
Writing in Polymer Testing [DOI: 10.1016/j.polymertesting.2023.108069], Jungha Lee and Jung Hyeun Kim report on the performance of PU foams made with bio-derived materials. Specifically, they chose to use a polyol extracted from castor oil – a vegetable oil that is increasingly being used as a more sustainable precursor for industrial chemicals.
They fabricated their flexible PU foams via one-shot polymerization, mixing isocyanate with what they describe as a ‘polyol system’. This system comprised of polyol, a gelling catalyst, blowing catalyst, cross-linker, blowing agent, and surfactant. To isolate the performance of the castor oil-based polyol, seven different foams were made. Six of them contained increasing proportions of the bio-polyol (0, 10, 20, 30, 40, 50 %), with all other components kept constant. The seventh foam was a 50:50 mix of polyols, and used a different gelling agent – one based on tin rather than amine.
The resulting foams were characterised using a range of techniques. This included FTIR spectroscopy, which tracked the variations of functional groups during polymerization, and SEM imaging which was used to analyse the morphology of the foam’s pores and cavities. For an isocyanate, the reactive group is -N=C=O (NCO). FTIR analysis found that early on, all samples exhibited a rapid increase in NCO. However, at later stages of the reaction, differences could be seen in the NCO reaction rate. The larger the proportion of bio-based polyol in the foam, the lower the reaction rate. The authors attribute this to the secondary hydroxyl groups present in the bio-based foam; petroleum-based polyol has only primary hydroxyl groups, which experience faster reactions with isocyanate.
SEM imaging found that the average sizes of the cavities and pores decreased as the bio-polyol content increased. There was also a marked difference between the 50:50 foam made with an amine catalyst (E50) and the 50:50 foam made with tin (E50_T); the average cavity and pore sizes were smaller in the latter than in the former. The authors say that this is due to an increase in the number of urethane linkages at increased bio-polyol content. “As a result,” they explain, “CO2 gas pressure
generated by the blowing process could not merge the interconnecting cavities and pores due to reduced drainage flow and an increased polymer matrix modulus.” For foams made with the amine catalyst, and for an increasing proportion of bio-polyol, the ratio of open pores decreased while the ratio of closed pores increased. The open pore ratio reaches its lowest value in the E50_T foam.
Given that the size and shape of a foam’s cavities and pores strongly determine how good it is at absorbing sound, this morphology analysis also provided the authors with insights into the acoustic properties of their foams. The highest sound absorption coefficient was measured for E50_T. Averaging over all frequencies provided the acoustic activity and noise reduction coefficient of the PU foams. Again, the E50_T foam exhibited the optimal performance.
Compression strength tests – important for determining the long-term durability of the PU foams – were also carried out by the authors. They found that the compression strength improved with increasing bio-polyol content. The highest value (26.27 kPa) was measured for E50_T, with E50 foam in second place (15.80 kPa). And finally, the team measured VOC emissions of all samples. The concentrations of benzene, toluene, ethylbenzene, xylene, and styrene showed no significant differences, regardless of the bio-polyol content. However, concentrations of all aldehydes decreased significantly as bio-polyol content increased. The aldehyde emissions were 42% (formaldehyde), 18% (acetaldehyde), and 20% (acrolein) lower in E50_T foams than in foams made with 100% petroleum-based polyols. Authors conclude that “… applying the bio-derived polyol is promising for reducing environmental issues in various industrial applications.”
---
Jungha Lee, Jung Hyeun Kim. “Performance evaluations of flexible polyurethane foams manufactured with castor oil-based bio-polyol,” Polymer Testing 124 (2023) 108069. DOI: 10.1016/j.polymertesting.2023.108069