How supercritical water decomposes plastic

at the very beginning of the 21st century.

The critical point of water (or any other liquid) is a state in which water can simultaneously be in a solid, liquid and gaseous state. Supercritical solvents are compounds that, having overcome their own critical point, approach the same physical properties as liquids and gases. The density, viscosity and strength of the solvent (eluent) are approximately the same as that of a liquid, and the compressibility and diffusivity coefficient are similar to those of a gas. This physical plasticity has amazing uses. Water goes into a supercritical state at temperatures above 375 ºC and pressure above 217 atmospheres, and in this form is completely mixed with non-polar gasesand also turns into a strong oxidizing agent that decomposes plastic waste. Further research showedthat subcritical water is suitable for thermochemical recycling of plastic.

Among various polymers, CFRP, a material composed of carbon fiber and a plastic matrix, is particularly difficult to degrade. Thanks to its low density, carbon fiber is very light, but strong, not inferior in strength to metal. For example, at the Polytechnic University of St. Petersburg they designed Quattro racing carthe body of which consists of carbon fiber/ The car reaches a speed of 100 km/h in 4 seconds, and without a pilot (but when fueled) weighs only 180 kg.

The scope of application of carbon fiber is aviation and astronautics, automobile and maritime transport, as well as sports. In all these industries, parts wear out quickly, and the issue of recycling carbon fiber waste becomes acute.

Polymer processing

In 2024, scientists led by Young-Chae Jung from the Korea Institute of Science and Technology proposed an innovative method for processing carbon fiber plastic using supercritical water. Currently, carbon fiber plastic is disposed of mainly by conventional combustion under controlled conditions or by the radiation method, when under the influence of hard ionizing radiation. In the latter case, the plastic matrix is ​​destroyed, but the carbon fiber is preserved. A team from KIST, in search of a more environmentally friendly alternative, discovered that simply immersing carbon fiber in supercritical water for tens of minutes – and it breaks down 99% into thin fibers:

For such processing, only supercritical water was needed – without catalysts, additional oxidizing agents or organic solvents. Shown above is a form in which carbon fiber is doped with nitrogen atoms, which requires adding glycine to supercritical water.

Carbon fibers produced by pyrolysis are heterogeneous, so for now they are mainly used as a filler in composite materials. In contrast, the carbon fiber samples produced at KIST can be used as electrodes in electric vehicle batteries. In March 2024 article This research was published in the journal Carbon.

When plastic decomposes in subcritical and supercritical water, the problem with the low thermal conductivity of such polymers is also solved. The plastic matrix can be cleaned of additives, in particular, combustion inhibitors and stabilizers, so the output can be obtained not only high-quality recyclable materials, but also fuel or hydrocarbon derivatives. In fact, we are talking about the reuse of petroleum products and the controlled conversion of polymers into monomers. There are already companies trying to use this technology on an industrial scale, for example, the British RenerELP.

Supercritical water also facilitates hydrothermal liquefaction (HTL) technology for turning food waste into fuel. Hydrothermal liquefaction is a thermochemical depolymerization process carried out in a sealed reactor, where supercritical water can act as a solvent, catalyst, and reagent. In this case, organic waste is practically converted into crude oil, a process that is faster, more efficient, and also more selective than pyrolysis or gasification. The new technological process, in particular, does not imply washing and drying the resulting material. Supercritical water is a proton donor, and therefore is used in the processing of not only plastic, but also crude oil, helping to purify it of sulfur impurities.

In Europe, the most interesting center for studying the properties of supercritical water is University of Birmingham. IN School of Chemical Engineering This university, under the leadership of Dr. Bushra al-Douri, developed its own method of processing plastic in supercritical water. This technology was then licensed to the company Stopford for industrial use. Stopford organized hydrothermal processing of plastic waste using technology CircuPlastwhere the output is precursors for the production of new plastic, there is no insoluble residue left and greenhouse emissions are significantly reduced

Returning to South Korean developments, I would like to mention the company LG Chem Ltd from Seoul, announced at the beginning of 2022 about the intention to build a plant for hydrothermal processing of plastic using supercritical water (construction started spring 2023).

LG Chem has entered into a collaboration with the British company Mura Technology (Mura), which has its own patented HydroPRT (Hydrothermal Plastic Recycling Technology) technology. It is expected that the LG Chem plant will annually produce up to 20,000 liquid product similar to pyrolysis products, but the processing process will not generate soot and other solid waste. Up to 80% of plastic waste can be converted into petroleum substitute, which will serve as an alternative to fossil hydrocarbons. The remaining 20% ​​will go into gaseous form and will be similar to associated gas. If such products are captured, they can also be reused as industrial fuel.

Fiberglass recycling and other Japanese developments

Since the early 2010s, similar developments have been underway in Japan, where Tokyo-based Toray Industries is working with Honda Motor to recycle fiberglass and recover raw materials from old car bodies. We are talking about depolymerization in supercritical water and obtaining caprolactam, which is a monomer of polyamide plastics, in particular nylon-6, used in the automotive industry. This is what it looks like installation for such processing.

We are talking about the processing of reinforced composite materials (carbon fiber and fiberglass), which usually consist of fiberglass and epoxy resins. In supercritical water at a temperature of about 380 °C, such a substance decomposes in about five minutes. This process also allows you to clean fiberglass from epoxy resins at a water temperature of about 400 °C and a pressure of 28 MPa, the purification efficiency reached 79.3% by weight, and when using potassium hydroxide as a catalyst – up to 95.3% by weight. The ultimate tensile strength of such fibers was 90-98% of that of freshly produced glass fiber. The by-products of this reaction are glycols and formic acid, which can then also be taken out of solution and used to produce polyester.

Conclusion

Since in Japan there is a particularly acute problem with the proliferation of automobile landfills with a shortage of territories for new landfills, the Ministry of Environment of this country expects to launch a state depolymerization program by 2027 using the described technology. Toray and Honda plan to commission a pilot plant that would make it possible to obtain 500 tons of epoxy resin from such processing annually, and reuse the purified plastic for the production of automobile bodies. Perhaps, with the development of such technologies and the search for new catalysts, it will be possible to obtain increasingly valuable raw materials and not only get rid of plastic, but also separate it during processing into ever finer fractions. Finally, the described processing is practically hermetically sealed and occurs without access to oxygen, so it looks much more promising than conventional combustion.