In an article recently published in the journal Polymers, researchers discussed the processing of polyol/cellulose nanofibre dispersions for polyurethanes and their rheological characteristics.
Study: Processing and rheological properties of polyol/cellulose nanofibre dispersions for polyurethanes. Image Credit: DmyTo/Shutterstock.com
The vast variety of polyhydric alcohols with different molecular weights and architectural styles are together referred to as polyol. A sustainable nanomaterial with hydroxyl groups on the surface, nanocellulose is often generated as a high aspect ratio nanofibre (CNF) or relatively lower aspect ratio nanocrystal (CNC).
The "quality" of the nanomaterial's dispersion in the polyol, which is reliant on the polyol's loading levels, chemical makeup, and dispersion technique, determines the demonstrated improvements in the attributes of nanocomposites. Because of this, it's important to look at the rheological behavior of polyols inserted into nanocellulose and comprehend how loading levels affect the interaction processes that cause viscosity to rise, as well as gelation.
The only rheological research conducted so far has been on CNC in high molecular weight diols. The interactions between CNF, which is long and flexible, and the high functionality polyol, which is liquid at room temperature and employed in a variety of applications like polyurethane foams, are still not fully understood. Nor are the rheological characteristics or the effect of loading levels.
About the Study
In this study, the authors examined a polyether polyol packed with long, thin cellulose nanofibres for its mechanical and viscoelastic properties. It shed light on how different loading levels affected the way that interaction, dispersion, and gelation mechanisms behave. The steady-shear rheological experiments identified four distinct flow behavior regimes connected to rising yield stress and viscosity with rising CNF loading in the polyol. The comprehension of interactions between CNF scattered in the polyol and the determination of the threshold concentration of CNF for percolation network formation were made possible by oscillatory-shear rheology. The relationship between the mechanical characteristics of polyol nanocomposites pointed to the production of fractal dimensions in two intrafloc networks at larger CNF loadings.
The team investigated the rheological characteristics of a high functionality polyether polyol loaded with CNF at various loading levels. Spinifex grass, a native Australian abundant arid biomass, was used to create the CNF through a mild alkaline treatment and scalable agitator bead milling (ABM). Microscopical examination and rheological tests were performed on the polyol dispersions that the CNF had integrated. The Herschel-Bulkley model was used to assess how CNF affected polyol dispersions' viscosity and yield stress. Two popular nanocomposite models were utilized to estimate the percolation threshold.
The researchers examined the linear viscoelastic characteristics of the CNF/polyol dispersions to determine the critical volume fraction of the percolation network of CNF. By analyzing the fractal dimensions and strength of the links present in the dispersions of CNF/polyol, the scaling law model was used to describe the characteristics of the percolation network. The findings from this work were utilized to improve nanofibre-filled polyols used in mining, resin, paint, and cosmetics applications.
Fluidic behavior was shown by plain polyol and CNF/polyol dispersions at loading amount less than or equal to 0.28% weight-averaged. CNF/polyol dispersions with loading levels less than 0.56% by weight showed G′′ > G′ at low frequencies and G' > G" at higher frequencies, both of which indicated dominant elastic behavior.
The complicated viscosity of the polyol dispersions exhibited no effect of lower shear viscosity for polyol dispersions below loading levels below 0.14% w/w. The CNF/polyol/water dispersion underwent transmission electron microscopy (TEM) investigation, which revealed the presence of nanofibrils with an average width of 23.7 ± 3 nm and a minimally expanded range of width up to 170 nm.
According to a power-law exponent value of 2.07, which was comparable to the exponent values of 1.8 and 2.6 reported for CNC/polyethylene glycol and CNC/PLA nanocomposites, respectively, the apparent yield stress values of CNF/polyol dispersions increased as a function of volume fraction. However, compared to the nanocomposites made of CNC/polyethylene glycol of 0.15% vol and CNC/PLA, the percolation threshold of CNF/polyol dispersions was substantially lower. By increasing G', the integration of 0.07% w/w of CNF demonstrated a distinct change in rheological characteristics. The observed value of the percolation exponential constant, t, was 2.2 and falls within the predicted range for particle networks that fill space.
The CNF/polyol dispersions were shown to have predominate fiber-polyol interactions below the percolation threshold and predominate fiber-fiber interactions above the percolation threshold. A modified model was used to successfully describe the important rheological characteristics. CNF/polyol dispersions were discovered to form networks with transition linkages at an intraflocs fractal dimension two beyond the percolation due to fiber-fiber and fiber polyol interactions, and the consistency of the scaling laws were assessed for the acquired rheological parameters.
In conclusion, this study discussed the development of the CNF/polyol dispersions made by combining CNF suspension with polyol for best dispersion and vacuum drying. With the use of rheological and microscopic characterization, the type of CNF dispersion, interaction, and gelation in polyol with regard to the loading levels was successfully determined. With an increase in CNF content, the CNF/polyol dispersions showed a pronounced shear-thinning and unique shear viscosity behavior. When the CNF loading was increased, the storage modulus and complex viscosity at low frequencies increased logarithmically.
0.56% w/w CNF were predicted for the G'-G" cross-over sites. The percolation threshold was then calculated using two methods and was discovered to be within the anticipated range of transition to percolation.
The authors mentioned that it may be possible to use gels formed at modest CNF loadings in polyols as a long-lasting rheological modifier in a variety of applications. They stated that this study sheds light on the significance of the morphological and rheological characteristics of polymer matrices using CNF, as well as the possibility of enhancing their ultimate qualities.
Haridevan, H., Chaleat, C., Pooley, L., et al. Processing and rheological properties of polyol/cellulose nanofibre dispersions for polyurethanes. Polymer 125130 (2022). https://www.sciencedirect.com/science/article/abs/pii/S0032386122006188