Advanced recycling solutions for absorbent hygiene articles

What are Super Absorbent Polymers (SAPs) ?

Super-absorbent polymers (SAPs) constitute the major and important part of diapers which are water-absorbing polymers that can absorb and retain water up to 300 times their weight. SAPs’ ability to absorb water depends on the ionic concentration of the aqueous solution. These are commonly made from the polymerization of acrylic acid blended with sodium hydroxide in the presence of an initiator to form a poly-acrylic acid sodium salt which is sodium polyacrylate, and is a non-biodegradable polymer sourced from petroleum.

How does the diaper recycling goes in general ?

Diapers contain three major elements, namely top sheet, absorbent core and back sheet. The top sheet and back sheet are made of plastic materials, and technologies for recycling of plastic are readily available in the industry. The process of diaper recycling begins with shredding of waste diapers, followed by the separation of shredded materials. The shredded wastes are then sent to a pulper, and the diaper materials are sterilized and exposed to special chemical treatments to deactivate the SAP. After several washes, the plastic components that form the back sheet of diapers are separated and then compressed into small pellets for industrial applications. Remaining parts of the diaper enter a screening process, where any traces of plastic and other organic materials are separated. The cellulose fibers are further subjected to mechanical washing, cleaning and screening processes. Knowaste, an innovative waste treatment business specializing in the recycling of AHPs, converts pulp from the used diapers into sanitized reusable paper pulp and plastic components that can be turned into building materials such as roof tiles. The company has patented technologies for the recycling of disposable baby diapers, incontinence products and disposable bed liners.^[3]

What are the contributions from major brands towards recycling ?

Major manufacturers, namely Procter & Gamble, Kimberly Clark and Unicharm, have already initiated efforts towards handling of AHP waste. These companies have partnered with recycling companies with the sole aim of reducing waste and greenhouse gas emissions. P&G, the owner of Pampers, is working with Fater, a joint venture between P&G and Angelini Industries of Italy. Fater has been producing items such as plastic bottle caps, school desks, urban playgrounds and viscose clothing from plastic materials made out of AHP waste. The patented technology uses a rotating autoclave wherein all the organic substances from used products are exposed to sterilization temperature, and plastic and cellulose fibers get separated from one another (US9156034B2, US11033907B2). The process eliminates CO₂ emissions and the savings would be equal to taking 115,000 cars off the road per year. However, P&G has no plans to use Fater’s output in its diapers, until it completes consumer research on the idea^[4].

How is P&G working to recycle

P&G has also been making efforts to recycle SAP through its partnership with the University of Michigan. The patented technology (US20210054161) uses sonication to break long polymer chains, leaving its building blocks or acid groups intact, which allows it to perform other reactions without changing its chemical properties. When these acid groups are reacted and converted into ester groups, their properties are changed from being water-soluble to organic-soluble, and become tacky like an adhesive. The extracted PAA is used in the synthesis of pressure-sensitive adhesives used in Post-it notes. Prof McNeil and Dr Takunda Chazovachii of the University of Michigan demonstrated that the adhesives made from extracted polymers are more sustainable than those made from petroleum origin, leading to 22% reduction in global warming potential and 25% reduction in energy consumption^[5].

According to P&G’s patent US20210054163 titled ”Recycling of Superabsorbent Polymer with an Extensional Flow Device”, when the feed stream in the form of a gel (from the used diaper) flows in the extensional flow device (such as Liquid Whistle) and experiences the extensional flow at short residence times, SAP degrades to Polyacrylic acid (PAA) (essentially, without decarboxylation). The said device includes an inlet chamber, an orifice, and a mixing chamber in the flow direction where a blade is located in front of the orifice. The vibration of the blade in the Liquid Whistle with its natural frequency generates extensional stresses in the SAP feed stream/gel, as it flows through the orifice, causing stretching and breaking of the cross-linker, its attachments to the backbone, and the backbone bonds. The gels generate slip at the walls of the inlet chamber and orifice, but would not generate extensional stresses as they flow as plug flow in the inlet chamber and orifice. PAA produced from the degradation of SAP with the extensional flow device has narrow Molecular Weight Distribution (MWD), i.e., low Polymer Dispersity Index (PDI). 5% of the extracted PAA is then blended with virgin polymers and their swelling properties evaluated. It is found that there is not much of a difference between an unused adsorbent article and an article containing 5% recycled PAA.

Several examples are presented in the patent on the process of degradation of used SAP into PAA. PAA is extracted from used diapers and the energy consumed in the extraction process termed as Specific energy is calculated and provided in Table 1. The total specific energy for 5 pass product is much less than the energy required for preparing PAA from fossil-derived propylene, 18.45 MJ/Kg as against 50 MJ/kg for Propylene derived AA (disclosed in the background art ).

The amount of extractables from recycled SAP and virgin SAP were tested and the results have been presented in Table. 2. The results clearly show that amount of extractables is nearly half when recycled SAP is used as compared to virgin SAP. It is evident that more studies

Envirocomp of New Zealand, which was acquired by OCS Ltd., established a nappy composting facility in Christchurch, New Zealand in 2009^[6] with sponsorship from Kimberly-Clark. The facility uses HotRot technology to prepare the compost^[7]. OCS which is also the parent company of Cannon Hygiene, leader in washroom services, opened the first composting plant in Rochester Kent, UK in December 2013. The companies have composted millions of diapers mixing them with green waste to make a compost suitable for commercial gardening^{[8] [9]}.

How is Kimberly-Clark working to recycle

Apart from using as a compost material, Kimberly-Clark has also made efforts to retrieve SAP from used diapers. US9650488, assigned to Kimberly-Clark, explains the belching method to recover SAP from used diapers. SAPs contain sodium ions as they are obtained by polymerization of acrylic acid and sodium hydroxide. When brought in contact with moisture including human urine, artificial urine, or water sodium ions in the polymer molecules readily dissociate to form a concentration gradient between the inside and outside of the polymer materials, thereby causing an inflow of moisture from outside of polymer materials. If relatively higher concentrations of sodium ions are provided outside of the polymer materials, counter-flow is created to belch moisture absorbed by the polymer materials.

When intermolecular repulsion between SAP materials is induced by negative charge generated by oxidation of sodium ions present in the polymer molecules, and if an oxidant (e.g., a divalent metal cation) that weakens the repulsion between negative charges is present, the repulsion is weakened to decrease the distance between the molecular chains of the polymers. As a result, the space for retaining moisture becomes narrower such that moisture absorbed in the polymer materials is discharged. Tests have been conducted with different sea waters to determine the recovery of cellulose and SAP raw materials using this method.

Fluff pulp (GP4860, Georgia-Pacific) is put into a beaker and 600 cc of physiological saline is added, followed by stirring. It is then allowed to sufficiently absorb moisture for 40 minutes to become a gel. After completion of the reaction, fluff pulp is isolated by using a 1.18 mm mesh. SAP is isolated therefrom by using a 300 µm mesh. Each isolated sample is dried in a drier between 105 and 110⁰C for at least 12 hours.

SAP recovery rates were determined by removing water from the absorbent in gel state after artificially swelling SAP and cellulose pulp in physiological saline. The conditions used were similar to that of the actual use of absorbent articles. The results have been presented in Table 3, which indicates that SAP recovery rates increase with increase in concentration of calcium chloride, reaction time and salinity of sea water.

How is Essity working to recycle to recycle

Essity in Sweden, previously known as SCA, through its collaboration with Renewi in Europe, is currently in the process of demonstrating the sustainable solution for a recycling of nappies and incontinence materials. Renewi has been working on a method to recover the valuable raw materials, including high-quality cellulose fiber, and is planning to reuse the cellulose recovered in the recycling process as a secondary raw material for the chemical industry. Details of the technology are not known.

How is Unicharm working to recycle

In WO2020241025A1, Unicharm has disclosed a method for regenerating SAP derived from a used absorbent article. It is possible to reactivate the SAP by inactivating it with an acidic aqueous solution (inactivation step) and neutralizing the same with an alkaline aqueous solution (reactivation step). The collected absorbent articles are placed in the solution tank, which contains an acidic aqueous solution (eg. citric acid) that inactivates the SAPs. The used absorbent articles are passed through biaxial crushers and a series of separators, at the end of which, pulp fiber is removed from the SAPs. The highly water-absorbent polymer contained in the pulp fibers is decomposed using Ozone solution. The pulp fiber removed from the SAP is neutralized and reactivated by an alkaline aqueous solution (NaOH), which substitutes the H ions in the SAP with alkali metal ions. The wet SAP is dehydrated by washing it with a hydrophilic organic solvent. A surface treatment step is performed where fine hydrophilic silica particles are added to the surface of the SAP and supplied into a room that receives hot air and is dried by the use of a paddle. The dried SAP is then crushed and graded according to the particle size. Thus, the SAP derived from the used absorbent article is regenerated to produce a recycled SAP.

Two sources of SAP raw materials were tested – Aquakeep (from Sumitomo Seika Chemical Co., Ltd.,) produced by a reverse phase suspension polymerization method and Sun Wet (from SDP Global Co., Ltd.) produced by an aqueous solution polymerization method. In the test method, 1 kg of virgin SAP was allowed to absorb 40 times as much as physiological saline, then inactivated for 30 minutes with an acidic aqueous solution (1% citric acid) and immersed in 10 times the mass of water containing 1 ppm ozone, for 2 minutes. It was then neutralized with sodium hydroxide of 0.4 mol/L for 30 minutes and reactivated, followed by immersion in methanol for 30 minutes for dehydration and then washed. Then, fine silica

The water absorption ratio, water retention ratio, and the time taken by the recycled polymer for water absorption matched that of the virgin polymer on addition of 3-5% silica, indicating that the reclaimed SAP can be reused in the body of an absorbent product. The evaluation results have been presented in Table 5.

Achieving circular economy through recycling

Increase in waste created by diapers has forced companies manufacturing them to provide sustainable solutions. Technologies are being developed to recycle almost all parts of the diaper. Diaper manufacturers seem to be collaborating with partners to overcome multiple challenges such as lack of adequate recycling facilities, educating consumers about separating diaper wastes from other wastes, addressing process complexity due to the mixing of natural films/plastics with synthetic polymers, reactivating the recycled SAPs, and finally, consumer acceptance in using a recycled diaper. Recycling of diapers will truly contribute to circular economy.

REFERENCE

1. Sustainability Report
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2. EP2596810B1Method for sterilisation of used absorbent sanitary products
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3. About Us
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4. Italy is home to the world’s first industrial diaper recycling facility
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5. Superabsorbent nappy polymers could be recycled into stationery
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6. Envirocomp and Kimberly-Clark Sign Global Agreement to Sponsor Composting Facilities for Disposable Diapers
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7. Voluntary Product Stewardship Scheme To Reduce Sanitary Hygiene Waste To Landfill Receives Accreditation
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8. Giving New Life to Old Diapers
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9. Cannon Hygiene offers Nappy Composting in South East of England
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