The material at the heart of the work is what materials scientists call a highly filled sand-polymer composite: a blend in which roughly 75 percent of the mass is quartz sand, with the remaining quarter being a thermoplastic binder made from regenerated polymers. The binder itself is a 3:1 mass ratio of recycled polypropylene (PP) to virgin low-density polyethylene (LDPE), a combination chosen to retain PP’s relatively high strength and low melt viscosity while borrowing LDPE’s freeze resistance and impact toughness. Such compositions, sometimes known as “norplast” materials, are attractive for mechanical-engineering components and construction products because they consume enormous quantities of cheap filler and waste-derived polymer. But they are notoriously difficult to process: with less than 25 percent binder holding everything together, any defect in the polymer structure, any agglomeration of particles, and any phase separation between the incompatible polyolefins shows up immediately as brittleness, instability, or poor performance in the finished product.

The core challenge lies in the thermodynamics of the binder. Polypropylene and LDPE are, in their original state, thermodynamically incompatible—when melted together they tend to phase-separate during extrusion or moulding, creating weak boundaries inside the material. The team addressed this by introducing approximately 5 percent of an ethylene-vinyl acetate (EVA) copolymer, a classic compatibilizer that stabilizes the blend through active interaction at the phase boundary. On top of that, the researchers tested two nanomodifiers at extraordinarily low levels: a diamond-containing detonation carbon charge—a mixture of amorphous and crystalline carbon particles with primary sizes of 1 to 8 nanometers, produced by ZAO “Sinta” in Belarus—and pyrogenic silicon dioxide known as Aerosil-380, a white powder from Evonik Industries with 5-nanometer primary particles and a specific surface area of 380 square meters per gram. Both were introduced at just 0.01 to 0.1 percent by weight of the binder, delivered through LDPE-based masterbatches prepared by extrusion blending to ensure even dispersion.

The quartz sand itself was no ordinary filler. Washed sand with particle sizes of 180 to 260 micrometers was dried to a moisture content below 0.1 percent and then mechanically treated with a 50 percent solution of stearic acid in ethyl alcohol at 55 degrees Celsius. After the solvent evaporated, no more than 3 percent stearic acid remained on the sand surface—a deliberate chemical activation designed to increase interfacial interaction between the oxide surface of the quartz and the polymer binder. This pre-treatment, combined with the nanomodifiers, forms the technological recipe the team believes is essential for building a low-defect, highly ordered composite structure from components that would otherwise resist each other.

To quantify the effects, the researchers used the melt flow index (MFI) as their primary technological parameter—a measure of how easily the molten polymer flows under a standard 2.16-kilogram load, tested on an IIRT viscometer according to GOST 11645-2021. Unfilled binders were tested at 190 and 230 degrees Celsius; the highly filled compositions at 210 degrees Celsius, with at least five replicate measurements per composition. What they found was striking: an extreme, non-monotonic dependence of MFI on nanomodifier content in the range of 0.01 to 0.03 percent. Rather than flowing more freely with more additive, the melt actually thickened at these tiny doses—a counterintuitive result that persisted even when the processing temperature was raised from 190 to 230 degrees Celsius. The carbon charge produced the more pronounced effect, reducing MFI by roughly 30 percent at 190 degrees Celsius and about 20 percent at 230 degrees Celsius, with its viscosity minimum occurring at approximately 0.01 percent. Aerosil-380 behaved similarly but with its minimum shifted slightly higher, to around 0.03 percent.

The team interprets this thickening as evidence of physical intermolecular interactions between the charged nanoclusters and the polymer macromolecules—predominantly those of LDPE. In this picture, the nanoclusters act as anchoring points that reduce the conformational mobility of polymer chains during melting, forcing the amorphous phase into a more ordered, supramolecular arrangement. The stability of these labile intermolecular bonds is maintained up to critical temperatures; above them, the interactions are disrupted and the melt viscosity returns to values corresponding to the original polymer matrix. The difference between the two modifiers is attributed to their distinct chemical compositions and charge states. Importantly, the researchers are careful to frame these as interpretations of the rheological response rather than direct structural proofs—a distinction that matters in a field where mechanistic claims often outrun the evidence.

Adhesion told a complementary story. Because directly measuring the strength of the quartz-polymer contact inside a composite is methodologically complex, the team evaluated peel strength of polymer films on model oxide substrates—quartz-glass (silicon oxide) and aluminium (aluminium oxide)—roughened to a surface parameter of Ra = 6.3 ± 0.6 micrometers, approximately half the diameter of the filler particles, to simulate the roughness scale of the real sand-polymer interface. Films roughly 300 micrometers thick were hot-pressed at 230 degrees Celsius and bonded under no more than 2 megapascals, then peeled at 180 degrees on a Meitesi WDW-1 universal testing machine at 20 millimeters per minute, with five specimens per composition. The results showed that regardless of nanocomponent content, the peel strength of modified coatings exceeded that of the original polymer film by at least 15 percent on quartz-glass. Carbon charge additives proved more effective, delivering gains of at least 20 percent, while Aerosil-380 provided approximately 15 percent. Adhesion was consistently higher on the aluminium substrate, which the researchers attribute to the different chemical nature of the oxide surfaces: silicon oxide is covalently bonded and acidic, whereas aluminium oxide is ionic and amphoteric, facilitating chemisorbed bond formation with carboxylic acids and macroradicals generated during thermal degradation of the polymer.

Perhaps the most revealing evidence came from thermally stimulated depolarization (TSD) measurements performed on an ST1 instrument, in which samples were heated from room temperature to 200 degrees Celsius at 5 degrees Celsius per minute while the discharge current of electret charges was recorded. The TSD spectra of the pure nanomodifiers confirmed the presence of uncompensated charge sources—charge traps—either inherent to the materials or introduced during activation. The carbon charge produced a higher depolarization current above 100 degrees Celsius, consistent with its stronger ability to order the supramolecular structure of the binder. When 0.05 percent of either modifier was incorporated into the polymer matrix, the depolarization current in the 120 to 170 degrees Celsius interval dropped markedly, indicating a reduction in free charge volume and an increase in supramolecular ordering. Even more telling, the temperature range of the depolarization process corresponding to melting of the polymer matrix shifted upward by 3 to 5 degrees Celsius—direct evidence that the nanomodifiers had stabilized the binder’s internal structure against thermal disruption.

When the same nanomodifiers were introduced into the highly filled SPC-75 composite containing 75 percent quartz sand, the qualitative effects persisted but with an important shift: the MFI extremum moved to 0.05 to 0.09 percent, compared with 0.01 to 0.05 percent in the unfilled binder. The researchers attribute this shift to partial migration of nanoparticles to the polymer-quartz interface during thermal compatibility of the matrix and filler, which reduces the number of active crystallization centers available in the polymer phase at low nanomodifier concentrations. In practical terms, this means the nanoparticles are not just passive spectators—they actively participate in structuring the composite, distributing themselves between the polymer matrix and the filler boundary where they can influence both phases. The overall technological properties of the highly filled composites improved when nanomodifiers were introduced at ultra-low doses below 0.1 percent, and the results are consistent with enhanced phase stability of the PP/LDPE blend arising from the combined action of the active nanomodifier and the EVA compatibilizer.

The findings carry real weight for the circular economy. By demonstrating that a recycled PP/LDPE binder—material that would otherwise be downcycled or landfilled—can be systematically upgraded with hundredths of a percent of nanoscale carbon and silica additives, the study offers a practical formulation strategy for thermoplastic composites based on regenerated polymers, targeted at mechanical-engineering and construction products. The researchers are transparent about the limitations of their work: direct measurement of the bulk mechanical properties of the final 75-percent-filled composites as a function of nanomodifier content was outside the scope of the present study, the peel tests used planar model substrates rather than the real particle interface, no microscopic evidence for nanoparticle location was obtained, and the statistical treatment was descriptive rather than inferential. These gaps define the agenda for subsequent studies. Within the stated scope, however, the message is clear: ultra-low-level nanomodification of a recycled PP/LDPE binder is an effective route to improving the processability indicators of highly filled sand-polymer composites—turning two waste streams into a material with a structure engineered at the nanoscale.

Subject of Research: Technological factors of formation of the structure of highly filled sand-polymer composites based on regenerated thermoplastic resin

Article Title: Technological factors of formation of the structure of highly filled sand-polymer composites based on regenerated thermoplastic resin

Article References: Berg, A., Skaskevich, A., Haiduk, V., Berg, A., Zharkevich, O., Altynbaev, A., Ashimbaev, D., & Serikbayev, I. (2026). Technological factors of formation of the structure of highly filled sand-polymer composites based on regenerated thermoplastic resin. Frontiers in Mechanical Engineering, 12, Article 1860534. https://doi.org/10.3389/fmech.2026.1860534

Image Credits: AI Generated

DOI: 10.3389/fmech.2026.1860534

Keywords: Technological, factors, formation, structure, highly, filled, sand-polymer, composites, based, regenerated, thermoplastic, resin