The raw materials could hardly be more mundane. Post-consumer PVC pipes were collected from a construction site at the university’s Department of Chemical Engineering in Nakhon Nayok, washed, dried at 30 °C for 48 hours, and ground into roughly one-centimeter particles. The cling film waste, supplied by a local retailer, contained 30 weight percent of the plasticizer DINCH, a detail that proved crucial: in earlier work by the same group, blending flexible and rigid PVC at a 50:50 ratio raised elongation at break from 25.3 percent for neat recycled rigid PVC to 206 percent, while lowering the glass transition temperature by 34.9 °C. That flexibility is exactly what a brittle recycled matrix needs. Teak sawdust from Ayutthaya was oven-dried, ground, and sieved to an average particle size of 197 micrometers, with a bulk density of 0.27 g/cm³ and moisture content of just 1.46 percent.

To create the cellular structure, the team turned to azodicarbonamide, a widely used chemical blowing agent known as AC. Thermogravimetric analysis showed that AC decomposes sharply between 190 and 200 °C in a single exothermic step, releasing roughly 240 milliliters of gas per gram, composed mainly of nitrogen (65 percent), carbon monoxide (24 percent), carbon dioxide (5 percent) and ammonia (5 percent). That decomposition window sits almost perfectly inside the molding temperature of 195 ± 5 °C used in the study, and well below the composite’s major degradation stage, which occurs between 314.5 and 318.0 °C. This thermal matching meant the blowing agent could generate gas efficiently without triggering significant thermal damage to the PVC matrix or the wood filler, a prerequisite for producing sound foam.

Processing followed a melt compounding and closed-mold foaming route. The PVC pipe particles, cling film and sawdust were blended on a two-roll mill at 165–180 °C, then compression molded. For the foamed versions, AC was added at loadings of 0.5, 1, 3 and 5 weight percent, and the mold was preheated at 120 °C for five minutes before ramping to the foaming temperature. A tightly controlled ten-minute cycle—three minutes for filling and compression, five for foam expansion and cell growth, and two for cooling—kept expansion in check and ensured dimensional uniformity. The closed mold restricted runaway expansion, producing specimens with a characteristic skin–core structure: a denser surface layer surrounding a porous interior.

The foaming results revealed a classic optimum. Density fell steadily as AC content rose, reaching its lowest value at 3 weight percent, where gas generation, melt viscosity and cell stabilization appeared perfectly balanced. Cell density, the number of cells per unit volume, peaked at the same loading, while average cell size shrank from 40.38 micrometers at 0.5 percent AC to just 24.16 micrometers at 3 percent, accompanied by the narrowest, most uniform cell size distribution. Scanning electron microscopy of cryo-fractured, gold-coated specimens showed no surface openings up to 3 percent AC, indicating gas was effectively retained. At 5 percent, however, excessive gas evolution ruptured cells and opened escape pathways, causing cell coalescence, surface defects and a slight rebound in density—a vivid demonstration that more blowing agent is not always better.

Mechanically, the story was one of trade-offs and clever accounting. The unfoamed composite, with a flexural modulus of 2.71 GPa and flexural strength of 54.1 MPa, predictably outperformed the foams in absolute terms, since voids act as stress concentrators and reduce the load-bearing cross-section. Flexural properties fell to their lowest values at 3 percent AC (1.85 GPa and 32.90 MPa), and notched Izod impact strength dropped from 20.82 J/m for the unfoamed material to 13.33 J/m. But when properties were normalized by density, the picture reversed. Specific tensile modulus rose from 1.51 to 1.57 GPa·cm³/g, specific flexural modulus climbed from 2.04 to 2.23 GPa·cm³/g at 3 percent AC, and specific impact strength reached its maximum of 18.05 J·cm³/(m·g) at 5 percent AC. In stiffness- and strength-to-weight terms, the foams matched or beat the solid material—an essential criterion for lightweight construction.

Dynamic mechanical analysis added an unexpected bonus: superior vibration damping. All foamed composites showed tan δ values between 0.61 and 0.64, comfortably above the 0.3 threshold generally considered effective damping and approaching the 0.5 level classified as excellent, whereas the unfoamed composite managed only 0.43. The cellular architecture dissipates mechanical energy through cell-wall bending, friction at cell junctions, interfacial sliding and heterogeneous strain within the porous structure. The 3 percent AC formulation, with its maximum cell density and minimum density, delivered the greatest damping capacity. Glass transition temperatures of the foams, determined from tan δ peaks, ranged from 76.9 to 78.6 °C, slightly above the unfoamed material’s 70.8 °C, suggesting that cellular constraints subtly restrict segmental mobility of the PVC chains.

Thermal and moisture behavior rounded out the assessment. Differential scanning calorimetry confirmed the amorphous nature of the PVC matrix, with no melting peak and no change in transition pattern upon foaming, while the cling film’s plasticizer evidently acted as an internal plasticizing phase that eased foam formation. Thermogravimetry showed initial degradation temperatures declined modestly with AC content, from 240.2 °C for the unfoamed composite to 218.4 °C at 5 percent, but the major degradation temperatures remained essentially unchanged, and char residues of 26.5 to 27.9 percent matched the unfoamed value of 26.2 percent. Water absorption rose with the development of the cellular structure, peaking at 3 percent AC, yet thickness swelling after 60 days of immersion stayed between 3.88 and 7.07 percent—far below the 16.8 to 19.8 percent typical of particleboard and medium-density fiberboard after just 24 hours, thanks to the continuous polymer barrier around the wood particles.

The authors propose a three-stage deformation mechanism to explain how the foams fail under tension: an initial elastic stage in which spherical cells elongate into ellipses, a second stage of microcrack initiation at cell-wall junctions and wood–matrix interfaces, and a final stage of interfacial debonding, crack coalescence and cell collapse leading to fracture. Interestingly, at 5 percent AC, tensile and flexural properties partially recovered, apparently because over-foaming under the constrained mold caused localized densification and thicker cell walls that carried disproportionate load. The team is careful to note that a full life-cycle assessment, including recyclability and end-of-life management, remains future work. Still, the demonstration stands: three waste streams that would otherwise burden landfills can be merged into a single, foamed, dimensionally stable building material whose best formulation—3 weight percent AC—balances low density, fine uniform cells, strong density-normalized mechanics, excellent damping and good moisture resistance. It is a compelling case that the future of sustainable construction may be hiding in the demolition skip and the kitchen drawer.

Subject of Research: Upcycling mixed PVC waste streams and wood sawdust into foamed wood–plastic composites

Article Title: Toward sustainable wood–plastic composite foams fabricated via ternary blending of upcycled rigid poly(vinyl chloride) pipe, waste cling film and wood sawdust

Article References: Sukkaneewat, B., Sanetuntikul, J., Naknaen, P., Jansri, E., & Petchwattana, N. (2026). Toward sustainable wood–plastic composite foams fabricated via ternary blending of upcycled rigid poly(vinyl chloride) pipe, waste cling film and wood sawdust. Cleaner Engineering and Technology, 35, Article 101298. https://doi.org/10.1016/j.clet.2026.101298

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101298

Keywords: wood-plastic composites, recycled PVC, cling film waste, polymer foams, chemical blowing agent, azodicarbonamide, circular economy, upcycling, wood sawdust, plasticizers, damping, sustainable building materials

The star of the study is polyolefin elastomer, or POE, a flexible ethylene-octene copolymer that has become a favorite candidate for cushioning applications because it is soft, tough, and relatively inexpensive. POE foams, however, have a stubborn weakness: their melt strength is limited. When a molten sheet of POE is foamed, the growing gas bubbles, or cells, can merge with one another in a process called coalescence, and the resulting cellular structure can collapse or shrink after foaming. The result is an unstable foam with poor dimensional stability and disappointing mechanical performance. To combat this, the researchers turned to a two-pronged strategy: chemical crosslinking to stiffen the polymer network, and blending in a second elastomer, olefin block copolymer, or OBC, to reinforce the melt and refine the cell structure.

The chemistry behind the crosslinking step is straightforward but consequential. The team added 0.9 parts per hundred resin of bis(1-(tert-butylperoxy)-1-methylethyl)benzene, known in the trade as BIPB, a peroxide that decomposes under heat and generates free radicals. These radicals abstract hydrogen atoms from the polymer chains, creating chain radicals that couple with one another to form covalent crosslinks. The crosslinked network behaves like a molecular scaffolding: during foaming it holds the expanding melt together, preventing neighboring cells from rupturing and merging. The researchers confirmed the extent of crosslinking by measuring the xylene-insoluble fraction, the portion of the material that will not dissolve in hot solvent because it is tied into a network. As OBC content rose from zero to 50 parts per hundred resin, this insoluble fraction climbed from 82.2 percent to 90.9 percent, a clear sign that the blend was forming a progressively denser crosslinked architecture.

The OBC itself is a fascinating material. Produced by chain-shuttling polymerization, it consists of alternating blocks of crystallizable ethylene-octene sequences and amorphous octene-rich segments. The hard crystalline blocks act as physical crosslinks that melt at elevated temperature, while the soft amorphous blocks provide elasticity. In the POE/OBC blends, the OBC increased both the storage modulus and the complex viscosity of the melt, exactly the rheological changes that stabilize growing bubbles. At the same time, the overall apparent crystallinity of the composites dropped dramatically, from 18.1 percent down to 7.3 percent as OBC loading increased. This reduction in crystallinity matters for foaming because crystals can act as nucleation sites and also stiffen the matrix; less crystallinity means a softer, more deformable cell wall that can stretch further before failing.

With the materials in hand, the team employed supercritical carbon dioxide foaming, a technique that has become the gold standard for environmentally friendly polymer foam production. In this process, the polymer is saturated with carbon dioxide pressurized above its critical point, where the gas takes on liquid-like density and gas-like diffusivity, allowing it to dissolve into the polymer in large quantities. When the pressure is released or the temperature is raised, the dissolved gas comes out of solution and nucleates billions of tiny bubbles. Because carbon dioxide is nontoxic, nonflammable, and leaves no residue, the method avoids the ozone-depleting and greenhouse-active blowing agents that older foam processes relied on. The researchers selected foaming conditions of 100 degrees Celsius and 15 megapascals, a window where the crosslinked blends could expand uniformly without collapsing.

The foaming results revealed a composition-dependent balancing act. The formulation containing 20 parts per hundred resin of OBC, designated F-OBC-20, achieved the highest volume expansion ratio of the series, reaching 8.90 plus or minus 0.13, meaning the foam swelled to nearly nine times the volume of the original solid. At the other end of the spectrum, the formulation with 50 parts of OBC, F-OBC-50, produced the finest cellular architecture: an average cell size of just 54.3 plus or minus 5.1 micrometers and an extraordinary cell density of 1.66 times ten to the seventh cells per cubic centimeter. These microcellular structures are prized because smaller, more numerous cells distribute stress more evenly and trap gas more effectively, improving both mechanical performance and insulation.

The functional payoffs were substantial. Ball rebound resilience, a standard measure of how much energy a foam returns after an impact, increased from approximately 31 percent for the base formulation to over 43 percent for the OBC-rich composites. That improvement translates directly into cushioning materials that feel springier and recover their shape more completely after repeated compression, a property that matters enormously in footwear midsoles and sports protective equipment. Simultaneously, the thermal conductivity of the foams fell from 0.0819 to 0.0607 watts per meter-kelvin, a roughly 26 percent reduction. Lower thermal conductivity means better insulation, and the finer cells of the high-OBC foams contribute by suppressing convection and radiation within the cellular structure while the polymer itself conducts less heat.

Not every property moved in a favorable direction, and the authors are candid about the trade-offs. The specific compressive stress at 50 percent strain, which measures the load-bearing capability of the foam normalized by its density, decreased as OBC content rose. In other words, the bouncier, better-insulating foams were somewhat softer under sustained compression. This is the classic dilemma of foam design: expansion, cellular refinement, resilience, insulation, and compressive strength do not all improve together at a fixed crosslinker loading. The value of the study lies precisely in quantifying these trade-offs, giving manufacturers a data-driven basis for choosing a formulation that matches a specific application, whether that is a shoe midsole that prioritizes energy return or an insulation panel that prioritizes low thermal conductivity.

The broader significance of the work extends beyond the specific POE/OBC system. Supercritical carbon dioxide foaming is rapidly becoming the preferred route for thermoplastic and elastomeric foams across the industry, and studies like this one show how molecular design, crosslinking chemistry, and process conditions can be orchestrated together to overcome the intrinsic limitations of a given polymer. The Beijing team’s approach of using a block copolymer to simultaneously boost melt strength, refine cells, and enhance elasticity could inspire similar strategies in other elastomer families, from thermoplastic polyurethanes to poly(ether-block-amide)s, where shrinkage and cell instability have long frustrated foam producers.

For consumers, the research points toward a future in which the foam inside a sneaker, a yoga mat, or a building panel is lighter, springier, and more insulating than what is available today, all manufactured with a benign gas instead of legacy chemical blowing agents. For the polymer science community, the study provides a rigorous, quantitative demonstration that composition is a powerful lever: by simply adjusting the ratio of two compatible olefin elastomers while holding the peroxide crosslinker constant, the researchers swept across a wide property space, from high-expansion foams to microcellular insulators. As demand grows for sustainable, high-performance cellular materials, that kind of precise compositional control may prove to be the difference between a foam that fails in service and one that endures.

Subject of Research: Supercritical CO2 foaming of crosslinked POE/OBC elastomer composites for enhanced foam resilience and thermal insulation

Article Title: Supercritical CO2 foaming of POE/OBC composite foams with enhanced resilience and thermal insulation

Article References: Supercritical CO2 foaming of POE/OBC composite foams with enhanced resilience and thermal insulation. (n.d.). https://doi.org/10.1007/s00289-026-06700-2

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06700-2

Keywords: polyolefin elastomer, olefin block copolymer, supercritical CO2 foaming, polymer foams, crosslinking, BIPB peroxide, resilience, thermal insulation, cell morphology, melt strength, cushioning materials, Polymer Bulletin