Waste PVC Transformed Into Flame-Retardant Microcapsules That Keep Things Cold and Fight Ice

Every winter, roads ice over, cold chains wobble, and cities dump millions of tonnes of salt and chemical deicers onto pavements that slowly corrode bridges, contaminate groundwater, and damage vehicles. Meanwhile, mountains of discarded poly(vinyl chloride), one of the world’s most problematic plastics, pile up in landfills where its chlorine content poses a persistent environmental hazard. A new study published in the Journal of Materials Science: Polymers by Cemil Alkan of the Department of Chemistry and Erdinç Halis Alakara of the Department of Civil Engineering at Tokat Gaziosmanpaşa University in Türkiye offers a strikingly elegant way to attack both problems at once: turning waste PVC into the protective shell of microscopic thermal energy storage capsules that could keep food cold, stabilize refrigerated transport, and even delay ice formation on roads.

The research centers on microencapsulated phase change materials, or mPCMs, tiny core-shell particles in which a substance that melts and freezes at a useful temperature is locked inside a polymer wall. Phase change materials absorb large amounts of latent heat as they melt and release that heat as they solidify, acting as thermal batteries that buffer temperature swings. The catch is that the most common PCMs are solid-liquid paraffins that leak when they melt, which is why encapsulation is essential. Microencapsulation solves the leakage problem, dramatically increases the surface area available for heat transfer, and allows the particles to be blended directly into concrete, coatings, textiles, or packaging without any special handling.

What makes the Turkish team’s work novel is the shell material. Most commercial and laboratory mPCMs rely on formaldehyde-based resins such as melamine-formaldehyde or urea-formaldehyde, which can release harmful substances during synthesis and use, or on polystyrene and PMMA, which are flammable. The researchers instead dissolved waste PVC, sourced from a commercial leather company, together with 1-tetradecane, a paraffin that melts at around 5 degrees Celsius, in tetrahydrofuran and used a solvent evaporation technique to precipitate PVC shells around droplets of the paraffin. It is the first time waste PVC has been used this way for a low-temperature, cold-storage PCM, and the choice is doubly clever because PVC is inherently flame retardant, with a limiting oxygen index between 40 and 45, far above the threshold of 26 that separates flame-retardant polymers from ordinary combustible ones.

The synthesis itself is a piece of practical process chemistry. Solution A contained 26 grams of sodium chloride and 9 grams of gelatin dissolved in deionized water, with the salt raising the ionic strength to keep the water from dissolving the THF and the gelatin acting as a natural, biodegradable surfactant that stabilizes the emulsion. Solution B held the waste PVC powder and 1-tetradecane in THF. When B was dripped into the vigorously stirred A, droplets formed whose size depended on how they were added: a Pasteur pipette produced large particles, a micropipette produced small ones. The emulsion was then heated to 56 degrees Celsius so the THF evaporated, leaving behind hardened capsules that were filtered, washed with water and ethanol, and dried. Remarkably, the final particle diameter tracked the initial droplet size, giving the team a simple dial for tuning capsule dimensions, something conventional in-situ polymerization cannot easily achieve.

Two particle populations emerged. The small-particle capsules, designated mPCM/SP, averaged 145 micrometers, while the large-particle capsules, mPCM/LP, averaged 612 micrometers. Both were unimodally distributed, a sign of a well-controlled process. Fourier transform infrared spectroscopy confirmed that the characteristic C-Cl stretching peak of PVC at 1750 per centimeter persisted in the capsules and that the CH2 stretching bands of the paraffin core remained intact, indicating that core and shell coexist without strong chemical interaction, exactly what a good encapsulation should deliver. Differential scanning calorimetry showed that the capsules behave isothermally like the pure paraffin, with melting temperatures of 5.0 and 4.7 degrees Celsius and latent heats of 126.7 and 136.9 joules per gram for the large and small particles respectively. Encapsulation ratios reached 66.5 percent for the large particles and 71.9 percent for the small ones, squarely within the 60 to 90 percent range typical of the best polymer-shelled paraffin microcapsules in the literature.

One subtlety the authors confront head-on is supercooling. The encapsulated paraffin froze at slightly lower temperatures than the bulk material, a well-documented consequence of the shell adding thermal resistance and constraining nucleation within the confined core. Crucially, however, the capsules still solidified above 1 degree Celsius, comfortably inside the operating window for cold-chain and anti-icing service, and the shift is partly an artifact of the relatively fast 5 degrees per minute DSC scanning rate, which is known to exaggerate apparent supercooling. Repeated cycling showed deviations of less than 1 degree Celsius in transition temperatures and about 1.2 percent in latent heat, confirming that the phase change process remains fully reversible and that encapsulation has not degraded the PCM’s intrinsic properties.

Durability testing was equally convincing. The capsules endured 1,000 accelerated thermal cycles between -20 and 30 degrees Celsius, the equivalent of roughly 15 years of daily freeze-thaw service, with DSC signals reproduced almost perfectly after every 100 cycles and FT-IR spectra after 1,000 cycles showing no chemical change. Leak tests were brutal in their simplicity: samples were frozen at -18 degrees Celsius for 12 hours, then baked at 50 degrees Celsius for another 12 hours on filter paper. No visible PCM leakage appeared, and weight losses were a negligible 0.05 to 0.08 percent. Thermogravimetric analysis showed the capsules degrade at higher temperatures than the free paraffin, meaning the PVC shell genuinely protects the core.

The flame retardancy results may be the study’s most distinctive contribution. Because standard limiting oxygen index tests require rectangular samples that microparticles cannot form, the team built a calibration curve by burning reference plastics of known LOI, including expanded polystyrene, polyacrylonitrile, PET, nylon 66, and ABS, and correlating burning times with literature values. Using the resulting equation, they estimated LOI values of 27.6 percent for mPCM/SP and 28 percent for mPCM/LP, above the flame-retardancy threshold and a dramatic improvement over pure 1-tetradecane, whose LOI of 17 means it burns in ordinary air. The mechanism is intrinsic: when PVC decomposes it releases hydrogen chloride gas, which suppresses combustion and promotes a protective char layer. Unlike conventional flame-retardant PCM systems that rely on added ammonium polyphosphate or expandable graphite, this fire resistance comes free with the recycled shell material itself.

The application vision spans two very different worlds. In the sub-5-degree melting range, the capsules are natural candidates for cold-chain packaging, refrigerated transport containers, and insulated panels, where they would absorb heat whenever temperatures rise and release it back as things cool, smoothing out the fluctuations that spoil food and vaccines. In civil infrastructure, mixed into concrete pavements, bitumen binders, or surface coatings, the latent heat released as the capsules freeze could slow the drop of road surface temperature, delay ice nucleation, and reduce ice adhesion, potentially cutting dependence on corrosive chloride deicers. Earlier work by Farnam and colleagues demonstrated the pavement concept with other PCMs, and the Turkish team’s flame-retardant, waste-derived capsules would bring an added safety margin to exactly that use case.

Beyond the immediate applications, the study is a template for what materials scientists call waste valorization: converting an environmental liability into a high-value functional material. PVC is notoriously difficult to recycle through conventional plastic streams because its chlorine content and rheology make it incompatible with aliphatic polyesters and polyolefins, so it is usually landfilled or incinerated. Here, that same chlorine chemistry becomes an asset, delivering flame retardancy that other shell polymers must buy with additives. The process also avoids the nastier solvents common in encapsulation chemistry, using halogen-free THF whose high vapor pressure makes recovery straightforward. As the authors conclude, the work validates a route from a persistent waste problem to thermally reliable, fire-safe, size-tunable microcapsules, and it broadens the reach of phase change material technology into the low-temperature regime where cold chains, winter roads, and a warming world’s cold-storage demands increasingly intersect.

Subject of Research: Flame-retardant microencapsulated phase change materials synthesized from waste poly(vinyl chloride) for low-temperature thermal energy storage and anti-icing applications

Article Title: Novel flame retardant microencapsulated phase change materials from waste poly(vinyl chloride) for maintaining cold and anti-icing applications

Article References: Alkan, C., & Alakara, E. H. (2026). Novel flame retardant microencapsulated phase change materials from waste poly(vinyl chloride) for maintaining cold and anti-icing applications. Journal of Materials Science: Polymers, 1(1), Article 16. https://doi.org/10.1007/s44493-026-00017-2

Image Credits: AI Generated

DOI: 10.1007/s44493-026-00017-2

Keywords: phase change materials, waste PVC recycling, microencapsulation, thermal energy storage, flame retardancy, 1-tetradecane, anti-icing, cold chain, latent heat, solvent evaporation, supercooling, sustainable materials