A new generation of hydrogels is based on cheaper, abundant natural materials. Researchers tested their effectiveness for cleaning wineries and fighting wildfires.
Hydrogels are gelatinous amalgams of cross-linked polymers that can absorb and hold large quantities of water. They’re useful in absorbent disposable diapers, as well as soft contact lenses.
Were it not for factors including high manufacturing costs, hydrogels could find even broader commercial application. The synthetic polymers now used for their production are often expensive or difficult to make on an industrial scale, and frequently present environmental and safety concerns. But those limitations may soon vanish.
A team of researchers has created new hydrogels that incorporate two abundant and inexpensive basic ingredients: a cellulose polymer derived from natural sources such as wood chips and agricultural waste; and colloidal silica, a liquid suspension of nanoscale particles derived from sand.
“When we mix the cellulose and silica together, we get a stable gel,” says team leader Eric Appel, an assistant professor of materials science at Stanford University. “By altering the formulations, we can tune across an enormous range of mechanical properties. It’s not a knife-edge scenario—not a ‘gel’ or ‘no gel’ situation. Instead, you can get a whole continuum of gel states that can be useful for different applications.”
The simplicity of the process should enable the production of hydrogels at industrial volumes, promising to break the current cost barrier and make these materials useful in a host of new applications in food and beverage manufacturing as well as other fields.
In their paper, published in the Proceedings of the National Academy of Sciences, the authors describe testing their invention in two very different applications: cleaning pipes in commercial wineries and dispersing wildfire retardants.
Flushing out wine
Appel explains that wineries must pump their liquid product from vats to barrels to bottles. One challenge is that after each step, product remains in the transfer pipes, which must also be cleaned. Currently, water is used to flush out any wine remaining in the pipes, a matter of concern in drought-prone California. Most of this wine remnant must be discarded because it becomes diluted with the water.
“Up to 2 percent of product is lost by wineries during production, much of it during transfers,” Appel says.
His team members worked with Bronco Winery in Ceres, California, on a test. They used a special formulation of their hydrogel instead of water to flush out pipes filled with grape juice. Because the hydrogel did not mix with the juice, there was virtually no product loss. “And because the hydrogel is composed of materials that are food grade—cellulose and colloidal silica are both used in the food industry—there were no odor or taste issues,” Appel says. The hydrogel also cleaned the pipe at the same time.
“This reduction in both product losses and (use of) process water is what makes this hydrogel such an attractive material for winery operations,” adds coauthor Paul Franzia, engineering and environmental affairs manager at the Bronco Winery.
Appel’s team also worked with Jesse Acosta, a natural resource management specialist at California Polytechnic State University, San Luis Obispo, to test the new formulation in fighting fires.
Modern wildfire fighting is predicated on the use of retardants: slurries of water and chemicals dropped by aircraft to extinguish flames and slow burn rates. With wildfires increasing in frequency and size across North America due to climate change, demand for retardant is skyrocketing. But Appel says certain retardants contain chemicals that contaminate groundwater, and many are easily rinsed off by subsequent air drops of water, limiting the tactical approaches fire crews can take to fight fires.
In a bid to do better, Appel’s team mixed its hydrogel with a widely used fire retardant and found that the mixtures coated the fuels for far longer than the standard retardant and protected the retardants from being washed away with a subsequent water treatment. And their hydrogel-enhanced retardant had other advantages.
“These hydrogels can be dropped from a greater range of heights with less drift and evaporation,” Appel says. “And because it’s non-toxic there are no contamination issues with streams or aquifers.”
“This paper solves real-world problems,” writes Craig Hawker, the co-director of the Materials Research Lab at the University of California, Santa Barbara, and one of the peer reviewers of the PNAS paper.
Appel’s team is now conducting larger-scale testing of these materials and processes to improve their commercial potential. They are also developing new formulations of these materials for other applications, such as additives in cosmetic manufacturing and as lubricants for oil or water drilling operations.
Source: Glen Martin for Stanford University