From Pipes and Cling Film to Foam: Recycled PVC Gets a Lightweight Makeover

Every year, mountains of construction debris and kitchen waste end up in landfills, including rigid poly(vinyl chloride) pipes pulled from demolished buildings and the flexible cling film that wraps countless products before being discarded almost immediately. A research team in Thailand has now shown that these two very different PVC waste streams, together with ordinary teak sawdust from a local sawmill, can be transformed into lightweight foamed wood–plastic composites with properties suitable for building applications. Writing in Cleaner Engineering and Technology, Benjatham Sukkaneewat of Srinakharinwirot University and colleagues describe a ternary blending strategy in which ground recycled PVC pipe provides the rigid structural matrix, plasticized cling film waste acts as an internal toughening phase, and wood sawdust supplies a lignocellulosic filler. The key innovation is a carefully controlled chemical foaming step that inflates the composite with millions of microscopic gas cells, cutting density while preserving, and in some density-normalized measures actually improving, mechanical performance.

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