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

A study was conducted on the technological factors influencing the formation of a low-defect structure in highly filled sand-polymer composites based on a recycled blend of polypropylene (PP) and low-density polyethylene (LDPE) in a 3:1 mass ratio, containing approximately 75 wt% quartz filler and modified with ultra-low levels (0.01–0.1 wt%) of detonation carbon charge and highly dispersed silicon dioxide (Aerosil-380). The polymer binder was prepared by extrusion blending using LDPE-based masterbatches of the nanomodifiers. An extreme dependence of the melt flow index (MFI) on nanomodifier content was established. This behaviour is consistent with a proposed mechanism involving physical intermolecular interactions between nanoclusters and macromolecules, predominantly those of LDPE. Introduction of the nanomodifiers increased the peel strength of binder coatings on oxide substrates by 15%–20% relative to the unmodified matrix. Thermally stimulated depolarization (TSD) measurements showed that the nanomodifiers act as charge traps, reducing the intensity of the depolarization current and shifting the temperature interval associated with melting of the thermoplastic binder upward by 3–5 °C. These observations are interpreted as evidence of increased supramolecular order in the polymer matrix. Qualitatively similar effects were observed in highly filled formulations; however, the MFI extremum shifted to 0.05–0.09 wt% nanomodifier, which is attributed to partial migration of nanoparticles to the polymer–quartz interface. The technological properties of the highly filled sand-polymer composites improved when the nanomodifiers were introduced into the binder at ultra-low doses (less than 0.1 wt%). The results are consistent with an enhanced phase stability of the PP/LDPE blend arising from the combined action of the active nanomodifier and an ethylene–vinyl acetate (EVA) copolymer compatibilizer. The data can be used in the development of thermoplastic composite formulations based on recycled polymers for mechanical-engineering and construction products. Direct measurement of the bulk mechanical properties of the final 75 wt% filled composites as a function of nanomodifier content was outside the scope of the present work and is identified as a limitation.

Sand-polymer composites intended for mechanical engineering components must possess a set of performance characteristics that ensure their effective use in various assemblies and mechanisms (Zhetessova et al., 2022). The areas of application of these materials are determined not only by the mechanical and tribological properties of the polymer binder–filler system, but also by the resistance of its components to adverse environmental factors. These factors include high and low temperatures, oxidative reactions, and abrasive wear (Kaya et al., 2025; Cuberes, 2023).

The basic characteristics of commercially produced polymer-matrix materials often fail to meet the high performance requirements of engineering products. For construction applications, low-density polyethylene (LDPE) and its blends are among the binders most frequently used (Wang et al., 2024; Formela et al., 2022). However, these polyolefins typically exhibit tensile strengths of only 15–30 MPa and cannot, by themselves, provide the required level of product performance. In addition, binder compositions based on recycled household and industrial waste demand strict control of processing characteristics. This control usually requires additives for thermal stabilization and improvement of melt rheology (Formela et al., 2022; Chen et al., 2024).

It is well known that polypropylene (PP) and LDPE are thermodynamically incompatible in their original state. When processed together, this incompatibility leads to phase separation of the main components during moulding or extrusion (Amjad et al., 2024; 24; 25). The problem is commonly solved by introducing compatibilizers, such as maleic-anhydride-grafted polymers or ethylene–vinyl acetate (EVA) copolymers, which stabilize the blend through active interaction at the phase boundary (Girish et al., 2025; 24; 25). The practical application of highly filled “norplast” compositions nevertheless remains limited. Difficulties arise in ensuring compositional homogeneity, structural stability and the expected properties of the final products (Hammad et al., 2025; Malkin et al., 2024; Yurchenko et al., 2025).

Stable deformation-mechanical characteristics of polymer composites containing more than 50 wt% filler can be achieved technologically. This requires the formation of an ordered, low-defect structure in a highly filled system with a low content of the polymer binder (less than 25 wt%), provided that agglomeration of the components is prevented (Malkin et al., 2024; Yurchenko et al., 2025). The main factors that enable this condition are a sufficiently high melt flow of the polymer binder during mixing with the dispersed filler, active interfacial interaction between the components, and the formation of supramolecular order in the polymer binder (Malkin et al., 2024; Yurchenko et al., 2025; Nadeem et al., 2021). Rheology of highly filled systems is known to be extremely sensitive to interfacial phenomena and to the presence of a structured interphase layer (Malkin et al., 2024).

In the development of functional polymer composites, fillers and modifiers in the form of powders and fibres with particle sizes below 100 μm are widely used (Nadeem et al., 2021; Uglov et al., 2016). Metal particles, oxides, oxygen-free compounds and carbon-based materials with sizes ranging from 1 to 100 nm are employed as nanomodifiers (Hammad et al., 2025; Nadeem et al., 2021; Pavlets et al., 2022; Vasina et al., 2022). Of particular interest are particles of natural inorganic substances (clays, micas, graphite and others) that, under specific conditions, form nanodispersed assemblies with individual plate thicknesses of about 1 nm (Avdeychik et al., 2024; Hotta and Paul, 2004). When such fillers are introduced at 3–5 wt% into an aliphatic polyamide matrix, the strength characteristics of the composite can increase by a factor of 1.5–2 (Antonov et al., 2024). The decisive factor in this improvement is often the sequence of component introduction, which governs the formation of the required composite structure (Mashkin et al., 2020). The underlying mechanism is commonly associated with the formation of grafted polymer layers on the active surfaces of layered mineral nanoplates (Kelarakis, 2024). Nanoscale carbon and silica additives have also been shown to modify the rheology and interfacial behaviour of polyolefin melts even at very low loadings (Hammad et al., 2025; Kelarakis, 2024; Hotta and Paul, 2004).

Despite these advances, the literature contains only fragmentary information on the interaction between thermoplastic binders modified with nanocomponents and dispersed fillers at high loading levels (above 50 wt%) (Yurchenko et al., 2025; Mazitova et al., 2022; Alhalili, 2023; Hotta and Paul, 2004; Brette et al., 2024). In particular, systematic data on recycled PP/LDPE binders modified with ultra-low contents of detonation carbon charge or pyrogenic silica, and subsequently highly filled with quartz sand, remain scarce. For this reason, it is of interest to investigate the influence of active nanocomponents on the interaction between the polymer matrix and quartz filler during the formation of highly filled composite materials.

The objectives of the present work were: (i) to quantify the effect of ultra-low additions of detonation carbon charge and Aerosil-380 on the MFI of a recycled PP/LDPE (3:1) binder and of the corresponding 75 wt% quartz-filled composite; (ii) to evaluate the peel strength of the modified binder on model oxide substrates that chemically resemble the filler surface; and (iii) to examine the charge state of the nanomodifiers and of the modified binder by TSD. Direct bulk mechanical testing of the final highly filled products as a function of nanomodifier content was not performed in this study; related mechanical data for the unmodified highly filled system have been reported previously (Yurchenko et al., 2025).

Polypropylene (PP) manufactured in accordance with TY BY 500055085.016–2021 “Recycled Polymers. Technical Specifications” by OJSC “Belvtorpolymer” (Belarus) was used as the principal polymer of the binder (Figure 1). Recycled PP of this grade is characterized by a tensile strength σt ≥ 35 MPa and a melt flow rate MFR = 18 ± 1 g/10 min (230 °C, 2.16 kg).

Low-density polyethylene LDPE 10803–020 (OJSC “Naftan”, Belarus) was used both as the minor component of the binder and as the carrier of the nanomodifier masterbatches. This grade is a high-pressure (low-density) polyethylene with σb ≥ 22 MPa, MFR = 2.6 ± 0.3 g/10 min (190 °C, 2.16 kg) and a Charpy impact strength corresponding to “no fracture” under standard test conditions. Throughout this manuscript the polyethylene component is therefore identified as LDPE (not HDPE). The PP:LDPE mass ratio in the binder was 3:1. This ratio was selected to retain the relatively high strength and low melt viscosity of PP while improving freeze resistance and impact toughness through the LDPE addition.

Two nanomodifiers were used. The first was a diamond-containing detonation carbon charge (TU RB 100056180.003–2003) manufactured by ZAO “Sinta” (Belarus): a mixture of carbon particles of amorphous and crystalline structure (diamond-like and graphite-like) with primary particle sizes of 1–8 nm and stable aggregates of 20–30 nm. Hereafter this additive is termed “carbon charge”. The second was pyrogenic silicon dioxide Aerosil-380 (Evonik Industries GmbH, Germany), a white powder with an average primary-particle size of 5 nm and a specific surface area of 380 m 2 /g.

Washed quartz sand with a particle size of 180–260 μm, dried at 95 °C ± 5 °C to a moisture content of no more than 0.1 wt% in an SHS-80–01 SPU drying oven, was used as the filler. To increase interfacial interaction between the filler surface and the binder, the dust-free quartz sand was treated by mechanical mixing with a 50% solution of stearic acid in ethyl alcohol, heated to 55 °C ± 3 °C, until the solvent had completely evaporated. After evaporation of the alcohol the stearic acid content was not more than 3 wt% relative to the quartz sand.

To improve the thermodynamic compatibility of the polyolefin components in the sand-polymer mixture, approximately 5 wt% (relative to the whole formulation) of ethylene–vinyl acetate copolymer SEVA 11306–075 (TU 2211–211-00203335–2013, OJSC “NefteKhimSevilen”, Russian Federation) was introduced. The investigated formulations are summarized in Table 1.

The rheological properties of the melts were evaluated from the melt flow index (MFI) under a load of 2.16 kg. Unfilled binders were tested at 190 °C and 230 °C; highly filled SPC-75 compositions were tested at 210 °C. Measurements were performed on an IIRT viscometer (Figure 2) in accordance with GOST 11645–2021 “Plastics. Methods for determining the melt flow index of thermoplastics”, Method A, by measuring the mass of the extruded polymer melt at equal time intervals. At least five replicate tests were performed for each composition. Results are reported as the arithmetic mean ± standard deviation. The adhesion characteristics of the binder films were investigated in joints of film samples approximately 300 μm thick with quartz-glass and aluminium substrates (five specimens of each composition). These substrates were used as model oxide surfaces chemically related to the quartz filler (SiO2) and to a typical amphoteric oxide (Al2O3). They do not reproduce the particle geometry, confinement and multi-contact topology of the real sand–polymer interface; the extent to which the peel data can be extrapolated to the bulk composite is therefore limited.

To simulate, at least in terms of roughness scale, the interaction of the polymer binder with the filler surface, the quartz-glass and aluminium plates were treated with abrasive materials to create a roughness corresponding to approximately half the diameter of the filler particles (Pavlets et al., 2022). The required roughness parameter of the solid substrate, Ra = 6.3 ± 0.6 μm, was achieved by mechanical processing on a sample-preparation machine using a P80 flexible abrasive disc at 100 ± 10 rpm and was monitored with a Surftest SJ-210 (Mitutoyo) contact profilometer.

The film material was preformed by hot pressing at 230 °C ± 5 °C and 3 MPa. Bonding of the film to the rigid substrate was performed on a hot press at a pressing force of no more than 2 MPa with a holding time under pressure of 30 s. The peel strength of the polymer coating from the substrate was evaluated in accordance with GOST 32299–2013 (peel test) by peeling the coating from the rigid substrate at an angle of 180° under uniaxial tension on a Meitesi WDW-1 universal testing machine (Figure 3) at a crosshead speed of 20 mm/min. Five specimens were tested for each composition; means ± standard deviations are reported.

The charge state of the PP + LDPE blend and of the neat nanomodifiers (carbon charge, silicon dioxide) was studied by thermally stimulated depolarization (TSD) on an ST1 instrument (Figure 4). At least three replicate measurements were performed for each composition. While heating the samples in the measuring cell from room temperature (20 °C) to 200 °C at a rate of 5 °C/min, the discharge current of electret charges released during heating was recorded. The resulting spectrum is the dependence of current on sample temperature. The character of this dependence reflects the release of charges held by traps, the presence of which is indicative of an electrophysical contribution to the interaction between the components of the composite system (Vasina et al., 2022). Unless otherwise stated, experimental points in Figures 5–7 are mean values of n ≥ 5 (MFI, peel strength) or n ≥ 3 (TSD) independent measurements. Error bars represent one standard deviation. Differences discussed in the text are those whose mean values lie outside the overlapping standard-deviation intervals of the compared compositions; a formal multi-comparison hypothesis test was not applied and is noted as a limitation of the statistical treatment.

In the technological process of producing highly filled composite compositions, the quality of the final products and the level of their operational properties are determined by the sequence of component introduction and by the active participation of these components in the formation of the composite structure. The melt flow index (MFI) is used here as the primary technological parameter. It characterizes the processability of the polymers and also allows a phenomenological analysis of interactions between the components of the processed compositions.

To evaluate the efficiency of the interaction between the highly dispersed modifiers and the polymer binder, the dependence of the MFI of the PP + LDPE mixture on the nanofiller content was studied (Figure 5). The nanofillers were introduced into the PP-based binder via an LDPE-based masterbatch.

According to the literature (Avdeychik et al., 2024; Antonov et al., 2024), the effective content of highly dispersed modifiers should generally not exceed 0.2–0.5 wt%. Higher concentrations promote the formation of large agglomerates and lead to a decrease in the deformation and strength properties of the modified polymers.

Analysis of the experimental results revealed an extreme (non-monotonic) dependence of the MFI in the low-concentration range of 0.01–0.03 wt% (Figure 5). This effect was observed for both the carbon-containing and the silicon-containing modifiers. The decrease in MFI remains evident when the processing temperature is raised from 190 °C to 230 °C. The thickening effect produced by the carbon charge (Figure 5a) is more pronounced at 190 °C (approximately 30% reduction of MFI relative to the unmodified binder). At 230 °C the reduction is about 20%. Nanodispersed silicon dioxide exhibits a similar modifying effect (Figure 5b); however, the minimum of the MFI dependence occurs at a somewhat higher nanomodifier content in the polymer binder (approximately 0.03 wt% versus 0.01 wt% for the carbon charge).

The observed increase in melt viscosity is consistent with a proposed mechanism involving physical intermolecular interactions between nanoclusters and macromolecules, which would reduce the conformational mobility of the polymer chains during melting (Mashkin et al., 2020). The difference between the two modifiers is attributed to their different chemical composition and charge state (Kelarakis, 2024). These statements are interpretations of the rheological response rather than direct structural proofs.

The increase in melt viscosity of the PP + LDPE polymer blend indicates the formation of supramolecular order. This order is manifested through labile intermolecular bonds in the amorphous phase of the polymer matrix (Antonov et al., 2024; Mazitova et al., 2022). The stability of these supramolecular structures is maintained up to critical temperatures. Above these temperatures, the intermolecular physical interactions are disrupted, and the melt viscosity returns to values corresponding to the original polymer matrix.

Ordering of the polymer matrix based on the PP + LDPE blend contributes to improved deformation and strength properties of highly filled sand-polymer composites (Nadeem et al., 2021). This improvement can ensure their effective use in mechanical engineering products. However, a decrease in melt flow properties reduces the technological possibilities of achieving critically low binder contents in highly filled composites.

We hypothesized that the activity of nanomodifiers interacting with polymer matrix macromolecules would produce a similar effect during adhesive contact between the modified polymer and the sand filler.

To investigate this hypothesis, it is necessary to evaluate the adhesive interaction between quartz filler and the components of a PP + LDPE polymer matrix containing carbon-blend and silicon-oxide nanomodifiers in the range of 0.02–0.2 wt%. Objectively assessing the adhesive strength of the “quartz filler-polymer matrix” contact in the initial state of the components is methodologically complex. Therefore, the adhesive strength of polymer composite films containing nanocomponents was evaluated on solid oxide substrates - silicon oxide and aluminum oxide.

A study of the peel strength of the polymer binder film on a quartz substrate showed that the activity of the nanocomponents is reflected in the adhesive strength of the coatings. Regardless of the nanocomponent content, the peel strength of the modified coating from the quartz substrate exceeds that of the original polymer film by at least 15% (Figure 6).

It has been established that carbon-batch additives provide a more effective increase in adhesion strength on oxide substrates (at least 20%) compared with highly dispersed silicon oxide (approximately 15%). The modifying ability of the nanocomponents is associated with the charge state of their cluster particles, as indicated by the TSC spectra.

With identical coating compositions, higher adhesive strength is observed when the films are applied to an aluminum substrate, the surface of which consists of an Al 2 O 3 oxide film. This higher adhesive strength is attributed to the different chemical nature of the oxide surfaces. Silicon oxide possesses covalent bonds and is acidic, whereas aluminum oxide has an ionic character and is amphoteric.

As a result, carboxylic acids and macroradicals formed during thermal degradation of the polymer macromolecules facilitate the formation of chemisorbed bonds between the polymer film and the aluminum oxide coating. In addition, the introduction of cluster particles with an uncompensated charge into the PP + LDPE composite promotes electrophysical dipole interactions. These interactions involve polar groups (carbonyl, carboxyl, and hydroxyl) of oxidized macromolecules and active sites on the oxide film of the solid substrate.

The contribution of thermal oxidation products to the adhesive strength of polyolefin coatings depends on the temperature and duration of processing and service. However, thermal oxidation of the polymer matrix simultaneously reduces its original deformation and strength characteristics. Therefore, controlled thermal oxidation cannot be used as a deliberate method to improve adhesion between the polymer coating and the substrate or between the polymer matrix and oxide fillers.

A more effective approach to increasing interfacial adhesion in polymer composites, including highly filled systems, is the use of active or activated components. To evaluate the effectiveness of highly dispersed carbon charge and silica powders as modifiers of the polymer matrix in sand-polymer composites, we investigated the charge state of the initial nanomodifiers and of the modified thermoplastic binders. Specifically, thermally stimulated depolarization (TSD) measurements were performed on both the pure nanomodifiers and the composite materials.

This method provides information on the electric fields generated within the composite by the combination of components of different chemical natures. It allows determination of the temperature ranges of structural relaxation, as well as the presence and density of trapped charges.

The TSD spectra of the pure modifiers (Figure 7a) confirm the presence of uncompensated charge sources (traps), either inherent or introduced during activation. Comparison of the spectra shows that the carbon charge produces a higher depolarization current at temperatures above 100 °C. This observation is consistent with the stronger ability of the carbon charge to order the supramolecular structure of the polymer binder and to enhance interfacial interaction between the polymer matrix and the sand filler during thermomechanical mixing.

Introduction of the nanomodifiers into the polymer matrix reduces the conformational mobility of the macromolecules through interaction of their active centres with charged nanoclusters (Figure 7b). This effect is evidenced by a decrease in the intensity of the thermally stimulated depolarization current in the temperature range 120 °C–170 °C for binders containing 0.05 wt% of either silicon dioxide or carbon charge. The reduction in free charge volume indicates an increase in the degree of supramolecular ordering of the polymer matrix. In addition, the temperature range of the depolarization process corresponding to melting of the polymer matrix shifts upward by 3 °C–5 °C.

Further research is required to establish a more detailed correlation between these electrophysical effects and the resulting composite structure.

The rheological characteristics of highly filled composite materials containing quartz sand were studied under conditions of modification with active nanoparticles of detonation synthesis carbon charge and pyrogenic silicon dioxide (Figure 8), using the above-mentioned method at a test temperature of 210 °C. The obtained dependences indicate that the active structuring effect of nanocomponents in the polymer matrix based on a PP + LDPE blend retains its character in the presence of quartz filler under conditions of a sharp limitation of melt flowability. Moreover, the melt thickening effect in the region of small additions of nanomodifiers is more gradual and shifts slightly towards an increase in the modifier content (0.05–0.09 wt.%) when filling the polymer matrix with quartz sand compared to the original binder (0.01–0.05 wt.% - Figure 4). The shift of the viscosity peak toward higher nanomodifier concentrations is consistent with the migration of a portion of the nanoparticles to the quartz filler–polymer matrix interface between the quartz filler and polymer binder phases during the process of thermal compatibility of the matrix and filler, contributing to a decrease in the content of active crystallization centers in the region of low concentrations of nanomodifiers (Alhalili, 2023; Hotta and Paul, 2004; Brette et al., 2024).

The following experimental findings were established. Ultra-low additions (0.01–0.03 wt%) of detonation carbon charge or Aerosil-380 produce a reproducible, temperature-persistent minimum of the MFI of a recycled PP/LDPE (3:1) binder. In the 75 wt% quartz-filled composite the same qualitative effect is retained, but the MFI extremum shifts to 0.05–0.09 wt% (referred to the binder).

The same additions increase the 180° peel strength of binder films on quartz-glass and aluminium model substrates by approximately 15% (Aerosil-380) to 20% (carbon charge) relative to the unmodified matrix.

TSD spectra show that both nanomodifiers carry trapped charge. Incorporation of 0.05 wt% of either modifier into the binder reduces the depolarization-current intensity in the 120 °C–170 °C interval and shifts the melting-related TSD feature upward by 3 °C–5 °C.

The following interpretations are proposed and are not claimed as directly proven mechanisms.

The MFI minimum is attributed to physical intermolecular interactions between charged nanoclusters and macromolecules, predominantly those of LDPE, which reduce chain mobility in the melt. The shift of the extremum in the filled system is attributed to partial migration of nanoparticles to the polymer–quartz interface. The TSD changes are attributed to the action of the nanomodifiers as charge traps and to an increase in supramolecular order of the matrix. The peel-strength gain is attributed to improved electrophysical and acid–base interaction with oxide surfaces. The EVA copolymer is considered to assist compatibilization of the thermodynamically incompatible PP and LDPE phases; the nanomodifiers may contribute to this compatibilizing action, but this contribution has not been isolated experimentally.

Limitations of the work include the absence of a nanomodifier-dependent mechanical-property series for the bulk SPC-75 composite, the use of planar model substrates rather than the real particle interface, the lack of direct microscopic evidence for nanoparticle location, and the descriptive rather than inferential statistical treatment of the data. These points define the agenda for subsequent studies. Within the stated scope, the results support the technological conclusion that 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 intended for mechanical-engineering and construction products.