Superabsorbent polymers (SAPS) are the working components inside diapers and sanitary towels. These heavy-duty chemicals absorb and retain bodily fluids like blood and urine up to hundreds of times their original weight.
SAPs revolutionised personal care when they hit the personal hygiene market in the early 1980s. Now, the chemical industry must mount a new wave of innovation, this time targeted at improving product sustainability. Like most polymers, SAPs are not renewable or biodegradable as their basic chemicals are derived from oil. Because single-use applications are common, these non-renewable SAPs contribute massively to air and water pollution. In the US, around 12 billion sanitary pads are disposed of every year while in the UK, diapers account for 2-3 percent of household waste.
For the last two decades, companies have tried to lower the carbon footprint of SAP hygiene items with efficiency measures such as decreasing product weight. However, a wholesale shift to biobased SAPs is required to achieve fully renewable, biodegradable, and low carbon items. One 2013 study in the International Journal of Life Cycle Assessments suggested that SAPs in disposable diapers contribute most of the product’s total environmental burden, even though absorbents comprise only a fraction of its total weight. It also found that diapers made from a combination of petrochemical and biobased chemicals brought environmental benefits, including for human toxicity, freshwater eutrophication, and marine ecotoxicity.
SAPs belong to a class of materials called cross-linked hydrophilic polymers. The most common base for making these polymers is acrylic acid but polyacrylamide, polyvinyl alcohol, and polyethylene oxide are also used, depending on the application. Regardless of the specific polymer used, the chemical structure of SAPs are what underpin their unique properties. When sodium or potassium ‘crosslinking agents’ are added to a polymer, its molecules bond into larger chains to form a three-dimensional lattice. Sat within the lattice, the sodium or potassium counter ions create a space with a much lower concentration of electrolytes than in the surrounding liquid. Electrolytes are any substance containing ions, including blood and urine. The difference in electrolyte concentration between the SAP and target liquid is known as the osmotic gradient. This is what draws blood and urine into the SAP material.
Personal hygiene is one application where the physical attributes of a product’s active chemicals are non-negotiable. To capture the absorbents market, renewable SAPs must match or exceed fossil-fuel derived equivalents on performance. Apart from high absorption, the biobased SAP must retain its structure after swelling, have a low soluble content, low price, and be pH neutral upon absorption. Other requirements are that it should be odourless, colourless, and nontoxic for hygiene and comfort.
Global chemicals companies leading on biobased SAP
Some major chemicals companies have entered the biobased SAP space over the last few years. These large producers have focused on creating plant-based versions of common base materials for SAP manufacture, such as acrylic acid. One of the most established companies to turn towards biobased SAPs is Nippon Shokubai’s Europe subsidiary, a long-time specialist in conventional SAP and petrochemical acrylic acid manufacture.
In 2021, Nippon Shokubai Europe advanced its biobased product line by obtaining certification from the ISCC for a bio-based propylene, the substance that goes into making acrylic acid. Their renewable propylene is made from glycerol, commonly sourced from vegetable oils. Nippon Shokubai Europe’s move to biobased acrylic acid forms part of the company’s overarching strategy to achieve carbon neutrality by 2050. It is now developing a biodegradable and recyclable SAP.
Archer Daniels Midland is another global company involved in biobased SAP development. In the early 2000s, the company developed and patented a method for using starch to produce a high-quality SAP. In September 2019, LG Chem and Archer Daniels Midlands announced a joint development agreement to create a bio-based acrylic acid, the leading polymer ingredient in SAP manufacture. The deal will see biotech company AGM supply biobased components to LG Chem, part of LG Chem’s pivot towards the bioplastics segment.
Although diapers and sanitary care are the main applications for SAP, its water retention properties are useful in agriculture too. In 2017, Indian agrochemicals company UPL obtained a patent for Zeba, a naturally derived, starch-based, completely biodegradable, SAP designed to function as a soil amendment. Zeba increases the water holding capacity of the soil, improves the nutrient use efficiency in the crop’s root zone and has a positive effect on the soil microbiome. In 2022 UPL signed a pact with Shreenath Mkhasoka Sugar Mill which hopes to save 600 crore litres of water and 500 tonnes of urea using the technology.
A new generation of biobased SAPs hit the market
The companies above have been working with starch and glycerol but green chemicals startups are turning to more unusual feedstocks for biobased polymers requires in SAPs. These second-generation feedstocks and materials tend to offer better product biodegradability on top of renewability.
Ecovia Renewables, a University of Michigan spinoff founded 2014, is pioneering SAP fermentation. Unlike most SAPs on the market, Ecovia’s renewable product is not made from a bio-based acrylic acid. The company uses polyglutamic acid instead, an organic chemical occurs naturally in the silken threads of natto, a fermented soy-based food that is popular in Japan. Ecovia’s ferments polyglutamic acid using the company’s suite of non-GMO microbes. The SAP made from this material has a range of applications. Apart from hygiene products, it can go into making refrigerant gels for cold chain packaging and soil amendments for water retention.
Algae is another second-generation SAP feedstock just starting to enter the market. Students at the University of Utah in 2017 developed a sanitary towel containing an brown algae-based SAP. The entire product breaks down within weeks to months. SHERO’s product is a good example of how mixing biomaterials can enhance the performance capabilities of personal hygiene products. In their product, the superabsorbent interior is finished with a final outer layer made from corn that traps moisture inside and prevents leakage. The students were targeting the NGO sector and wanted a low-cost product that could be made easily by communities in developing countries. However, the group plan to sell the product in the US to eco-conscious consumers. SHERO’s biobased sanitary towel is a rare example of a high demand product equally placed to serve both social and environmental goals.
Bamboo is another candidate for biobased SAP feedstocks which is becoming increasingly popular as a sustainable material. The plant grows back so quickly that cultivated bamboo does not lose much carbon capture potential even if regularly harvested. In 2022, Daye launched an anti-bacterial pad containing a sustainable bamboo absorbent core. It is fully compostable in natural conditions. Like SHERO’s product, its pad combines a biobased SAP with a corn-based material to replace the non-recyclable plastic bodies that make up conventional sanitary items.
Many more biological substances can be turned into SAPs. Researchers in Advanced Sustainable Systems reported that superabsorbent particles from wheat gluten protein are capable of swelling 4000 % in water and 60% in saline solution, a tenfold performance increase compared to an existing gluten-based materials. It retains as much as 80 percent of the absorbed water, higher than commercial fossil-based SAPs.
Breakthrough in in scaled chitosan and cellulose-based SAPs would be a huge win for the biobased industry. Chitosan is found in shell waste from the seafood industry and cellulose is the most abundant organic compound in nature. The Indian Institute of technology in Madras has already demonstrated a biodegradable SAP using chitosan from seafood waste and citric acid.[1] In 2022, Indian researchers once again advanced the possibilities for biobased SAP by creating a superabsorbent crosslinked hydrogel using cellulose from water hyacinth.
The market
SAPS are one of the most important consumer chemical classes and are foundational to the global personal hygiene industry. As such, they offer a huge commercial opportunity for bio-enterprises. In 2020, 2.8 million tonnes of SAPs (both biobased and petrochemical) went on the market, with the US and China accounting for 45 percent of global sales in the product. By 2023, total production is expected to go above 3.1 million. The market for biodegradable SAPs is expected to grow at a compound annual growth rate of 8 percent between 2022 and 2030.
Market prospects are favourable for biobased personal care. Consumers are increasingly sensitive to the environmental impacts of personal care items while the broader biobased polymer market has also been on an upward trajectory. In 2020, for the first time in years, the compound annual growth rate for bio-based polymers was 8 percent, around four points higher than overall growth for polymers.
Bio-based SAP innovators and suppliers can capitalise on the sustainability ambitions of larger chemical and personal care companies. With big players in the SAP market like Nippon Shokubai and LG Chemicals already heading towards bio-based alternatives, bio-businesses should watch other leading chemicals companies that supply the personal hygiene market, including BASF SE, Evonik Industries, Sumitomo Seika Chemicals, and Sanyo.
