Sustainable Superabsorbent Polymers Beckon

Oct. 20, 2020
Products aim to rival or exceed performance of petroleum-based absorbents

Mostly used to prevent diapers from leaking by soaking up urine, superabsorbent polymers (SAPs) are proving to be one of the most successful chemicals ever developed. Research and analysis company Statista, New York City, predicts their market value to reach $70.4 billion by 2024.

However, the manufacture of SAPs is largely fossil-fuel based. So, the hunt is on for sustainable raw materials.

One option is to use wheat gluten (WG) proteins, a coproduct from wheat starch and ethanol processing composed of proteins that make it a promising material for absorbency applications.

In a recent issue of Advanced Sustainable Systems, researchers from the KTH Royal Institute of Technology, Stockholm, Sweden; the SLU Swedish University of Agricultural Sciences, Uppsala, Sweden; and Kyoto University, Kyoto, Japan, describe the synthesis of superabsorbent particles from nontoxic WG protein: a natural molecular cross-linker named genipin (a hydrogenated glycoside extracted from the fruit of gardenia jasminoides), together with a dianhydride (ethylenediaminetetraacetic or EDTAD), generate a material with a network structure capable of swelling up to 4,000% in water and 600% in saline solution.

This represents a tenfold increase compared to the already highly absorbing gluten reference material (Figure 1). The carboxylation (using EDTAD) and the cross-linking of the protein result in a hydrogel with liquid retention capacity as high as 80% of the absorbed water remaining in the WG network on extensive centrifugation, which is higher than that of commercial fossil‐based SAPs.

While the market for SAPs extends beyond disposable diapers, Antonio Capezza, a researcher at both the KTH and SLU, notes that they are the most demanding in terms of urine absorption properties: “By aiming to match or exceed the absorbency performance of petroleum-based superabsorbents used in many disposable diapers, we can meet the standards for most other applications that require them.”

These include personal care and medical products, flood water mitigation and rainwater retention for agriculture, among others, he adds.

Swedish innovation agency, Vinnova; grain and bioenergy specialist, Lantmännen; and hygiene and health products and services supplier, Essity funded the project. The next step is to scale up production of the material so the researchers’ industrial partners can test it in different applications.

Meanwhile, Ecovia Renewables, Ann Arbor, Mich., a University of Michigan-based start-up, is developing a range of superabsorbent biopolymers that could enable widespread use of compostable diapers. This, the company says, would greatly reduce the 3.5-million tons of disposable diaper waste currently entering U.S. landfills each year.

Ecovia’s fermentation-based bioprocess uses renewable feedstocks such as glucose and glycerol and a companion chemistry platform based on gamma polyglutamic acid to produce a family of what it describes as high-performing, multi-functional biopolymers for use in R&D activities.

Key among these is its range of AzuraGel superabsorbent biopolymers. These 100% bio-based, non-toxic and biodegradable materials absorb up to 300 times their weight in water and show improved absorption under load over starch-based biopolymers. They are being developed to minimize cost of production and maximize feedstock flexibility.

Ecovia believes its biopolymers could serve in other important absorption situations, including: packaging; soil moisture retention; seed germination and plant growth; as tackifiers for revegetation and erosion control; and in vegetable gum formulations.

Eyeing the same market are Archer Daniels Midland Company (ADM), Chicago, and LG Chem, Seoul, South Korea, who are working together to create biobased acrylic acid — a key chemical in the manufacture of SAPs.

Despite the growing demand for products developed from renewable materials, acrylic acid currently is produced almost exclusively from petrochemicals. LG Chem is one of the world’s leading manufacturers of acrylic acid; the two firms are working to develop economically viable production of 100% bio-based acrylic acid using ingredients from ADM’s corn processing.

To support production, LG Chem is considering building a bio-SAP plant in North America, and exploring additional bioplastic business opportunities.

“ADM currently produces about 30 different products from a kernel of corn, and we’re eager to explore the possibilities provided through bio-based acrylic acid,” says Chris Cuddy, president, carbohydrate solutions for ADM.

Seán Ottewell is Chemical Processing's editor at large. You can email him at [email protected].