Developing greener composites from cellulose nanocrystals and biopolyurethane

2 June 2017
Verónica Mucci, Juan Manuel Buffa, Pablo Marcelo Stefani, Mirta Inés Aranguren, Aiga Ivdre, and Ugis Cabulis
Environmentally friendly nanocomposite materials from bio-based polyurethane and cellulose nanocrystals show significantly improved mechanical properties with only 1 nanocrystal filler.

Over the past few decades, increasingly widespread interest in sustainable development has led research efforts in the production of materials from renewable resources to grow exponentially. In particular, considerable effort is currently devoted to replacing synthetic reinforcements in composites with natural fibers or particles, a result of the renewability and low environmental impact of these natural materials.1 Following this trend, we have prepared nanocomposite materials from a waterborne polyurethane (WBPU) and cellulose nanocrystals (CNCs). The WBPU is formulated from a bio-based monomer—Agrol 3.6 from BioBased Technologies, a hydroxylated soybean oil—in water, avoiding the use of polluting solvents. In addition, the CNC filler is sourced from inexpensive forestry industry materials, and adds significant value to the composites. Here, we present our synthetic methodology, and the mechanical, physical, and thermal properties of the resulting WBPU/CNC nanocomposite films.2

We obtain the CNCs by acid hydrolysis of a commercial microcrystalline cellulose material. Solvent exchange of the aqueous CNC suspension with acetone is performed and the resulting CNC dispersion is mixed with the dried macrodiol using a homogenizer. After solvent evaporation, we prepare the nanocomposites by adding a 4,4 ′ -diphenylmethane diisocyanate pre-polymer and dibutyltin dilaurate catalyst, and pour the resulting mixture into a pre-heated mold (80°C). The mold is closed and pressed at 0.5MPa and 80°C for 2h then transferred to an oven at the same temperature for 16h.

Using this method, we prepared neat WBPU and composites with CNC concentrations of 0.5, 1, and 2% by weight (wt%). The nanocomposite films are yellow in color and transparent even at 2wt% CNC. Furthermore, we assessed the CNC dispersion in the nanocomposites under crossed polarized light and found that the CNCs are uniformly distributed in the WBPU matrix (see Figure 1).


Photographic images of waterborne polyurethane/cellulose nanocrystal (WBPU/CNC) composite materials obtained under crossed polarized light: neat WBPU (a, left) and composites containing 0.5% by weight (wt%) (b), 1wt% (a, right), and 2wt% (c) CNC filler.

Including CNCs in the matrix increases the glass transition temperature of the composites—the temperature at which a material changes from a hard ‘glassy’ state to a rubber-like state—from 31°C in neat WBPU to 35°C in the 2wt% CNC sample, a result of restriction in the movement of the polymer chains. In addition, the storage modulus (the elastic response of a material under dynamic shear) of the 2wt% nanocomposite in the rubbery plateau region is significantly higher than that of neat WBPU. We also evaluated the thermal degradation properties of the samples by thermogravimetric analysis, which showed similar characteristics regardless of the amount of CNC present in both air and nitrogen atmospheres. This indicates that the CNC filler is well coated in the matrix and hence no additional degradation occurs.

It is well known that when the CNC concentration reaches the percolation threshold—the point at which infinite connectivity of the filler occurs—the mechanical properties of the composite greatly increase. However, above an optimum point, further CNC addition leads to agglomeration and an associated reduction in the mechanical properties,3, 4 a trend observed in this study. For example, the elastic modulus (a measure of the stiffness of a material) of the 1wt% CNC sample is over four times that of neat WBPU and the tensile strength is more than doubled (see Figure 2). Above percolation, however, the 2wt% CNC composite shows significant decreases in both elastic modulus and tensile strength to below the values obtained for neat WBPU.


The elastic modulus (top) and tensile strength (bottom) of WBPU/CNC nanocomposites containing 0–2wt% filler. Agrol: Bio-based WBPU source.

In summary, we have significantly improved the mechanical properties of a bio-based WBPU using CNCs, a nanofiller that is also a natural polymer. Notably, the amount of filler required to achieve this improvement is remarkably low, and higher CNC concentrations have a negative effect. These results show the properties of bio-based PUs are easily tailored by CNC addition. Our future work in this area will include focusing on greener WBPUs, increasing the bio-generated raw material content of the WBPUs, and further investigating CNC addition to develop materials with competitive properties.


Authors

Verónica Mucci
Research Institute for Materials Science and Technology (INTEMA)

Verónica Mucci obtained her PhD from UNMdP in 2010 and became a staff researcher at INTEMA in 2013. Her research focus is nanocomposite materials, including dental applications and nanocomposites from renewable resource-based polymers and reinforcements.

Juan Manuel Buffa
Research Institute for Materials Science and Technology (INTEMA)

Juan Manuel Buffa has a BS in chemistry and is currently a materials science PhD student at UNMdP. His thesis work is focused on optically active films with high CNC content.

Pablo Marcelo Stefani
Research Institute for Materials Science and Technology (INTEMA)

Pablo Marcelo Stefani obtained his PhD from INTEMA in 1999. His research interests include composites based on bacterial cellulose, bio-based epoxy foams, and particle boards based on agroindustrial residues and biogenic adhesives.

Mirta Inés Aranguren
Research Institute for Materials Science and Technology (INTEMA)

Mirta Inés Aranguren has a PhD from University of Minnesota and is currently a professor at UNMdP. Her research interests include polymers, composites, and nanocomposites from biomass.

Aiga Ivdre
Latvian State Institute of Wood Chemistry (LSIWC)

Aiga Ivdre graduated with a degree in chemical engineering from Riga Technical University in 2012, and is currently a PhD student at the same university. She joined the Laboratory of Polymers at LSIWC as a researcher in 2011. Her research involves polyurethane foams, including bio-based foams.

Ugis Cabulis
Latvian State Institute of Wood Chemistry (LSIWC)

Ugis Cabulis received his PhD in chemical engineering from LSIWC in 1993. He has been director of LSIWC since 2012, and is also the head of the Laboratory of Polymers. His main scientific focuses are polyurethane materials from bio-based and recycled feedstocks, and cryogenic insulation for liquefied gases.


References

  1. S. H. Park, K. W. Oh and S. H. Kim, Reinforcement effect of cellulose nanowhisker on bio-based polyurethane, Compos. Sci. Technol. 86, pp. 82-88, 2013.

  2. V. L. Mucci, A. Ivdre, J. M. Buffa, U. Cabulis, P. M. Stefani and M. I. Aranguren, Composites made from a soybean oil biopolyurethane and cellulose nanocrystals, Polym. Eng. Sci., 2017.

  3. X. Cao, H. Dong and C. M. Li, New nanocomposite materials reinforced with flax cellulose nanocrystals in waterborne polyurethane, Biomacromolecules 8, pp. 889-904, 2007.

  4. M. I. Aranguren, N. E. Marcovich and M. A. Mosiewicki, 17: Mechanical performance of polyurethane (PU)-based biocomposites, Biocomposites: Design and Mechanical Performance (Woodhead Publishing, 1st edn), pp. 465-486, 2015.

DOI:  10.2417/spepro.006913