Characterisation of Eco-Innovative Polymer Composites Obtained by Processing Hard-to-Recycle Plastic Waste: Extrusion Parameters, Chemical Composition, and Mechanical Performance

1. Introduction

The accelerating global accumulation of post-consumer plastic waste represents one of the most pressing environmental challenges of the twenty-first century. According to Directive 2006/12/EC of the European Parliament and Council, all EU member states are required to develop waste management plans targeting substantial reductions in the volume of material diverted to landfill and incineration [1]. The European Commission's Circular Economy Programme (COM(2014) 398 final) mandates a reduction of greenhouse gas emissions by at least 50% by 2030, with binding sanctions for non-compliance [2].

Particular difficulty is encountered in managing heterogeneous, contaminated plastic waste — mixed-polymer streams bearing adhesive labels, food residues, or surface coatings — classified as hard-to-recycle under conventional mechanical recycling pathways. Such streams are typically incinerated for thermal energy recovery, generating significant CO₂ and other greenhouse gas emissions, or deposited in landfill where they persist without biodegradation [3].

Polyethylene (PE, (C₂H₄)ₙ) and polypropylene (PP, (C₃H₆)ₙ) are the dominant polymer families in municipal plastic waste. PE is produced in structural variants including LDPE (density 0.910–0.940 g/cm³), LLDPE, and HDPE (~0.97 g/cm³) [4]. PP exhibits a melting point of approximately 160–171 °C for the isotactic homopolymer and 200–280 °C for copolymers and homopolymers [5].

An emerging approach to the valorisation of mixed, contaminated plastic waste involves melt-blending and extrusion of the commingled stream into metal moulds to produce semi-finished structural products — a process that avoids costly washing and sorting operations. The resulting material, termed downcycled mixed plastic (DMP), has been proposed for urban infrastructure and construction applications [6,7]. However, a rigorous multi-method characterisation of DMP composites produced by continuous single-screw extrusion — encompassing DSC, tensile and compressive behaviour, durability under accelerated weathering, bending, wear, water absorption, and thermal conductivity — has not previously been reported for composites of this specific composition and process origin.

The present work addresses this gap by reporting the first comprehensive characterisation of a DMP composite produced by Lect. Dr. Eng. Tudor Andrei Rusu at the Technical University of Cluj-Napoca, in collaboration with S.C. DMP Manufacture Innovation S.R.L. (Timisoara, Romania). All characterisation was performed by accredited laboratories: ICECON S.A. (Bucharest); URBAN-INCERC National Institute; Babes-Bolyai University ICCRR (Cluj-Napoca); ECOIND; and the Faculty of Civil Engineering, Technical University of Cluj-Napoca.

Figure 1. Contaminated, mixed post-consumer plastic waste (hard-to-recycle stream) prior to comminution and extrusion processing.

Figure 1. Contaminated, mixed post-consumer plastic waste (hard-to-recycle stream) prior to comminution and extrusion processing.

2. Materials and Methods

2.1. Raw Material Composition

The raw material consists of heterogeneous post-consumer plastic waste classified as hard-to-recycle: mixed, contaminated packaging foils and rigid packaging items bearing adhesive labels, food residues, or surface coatings. Prior to processing, the waste undergoes dry sorting and size reduction (comminution) without washing, since the cost of aqueous cleaning exceeds the commercial value of the recovered material.

DSC analysis of five representative samples was performed at the Babes-Bolyai University Institute for Research in Chemistry RALUCA RIPAN (Cluj-Napoca) under Contract No. 82/21.01.2025, using a Mettler-Toledo 823ᵉ Calorimeter. Measurements were conducted in the temperature interval 25–250 °C under a nitrogen protective atmosphere, with an aluminium crucible (40 μL, lidded), heating rate 10 °C/min, and a final isothermal plateau of 0.5 min. Figure 2 shows the schematic DSC thermogram.

2.2. Extrusion Equipment and Process Parameters

Processing was carried out using a continuous single-screw extrusion line. The facility operates three identical lines, each consisting of: (1) a central control panel; (2) a pneumatic granule conveying and metering system; (3) the main plasticating/extrusion unit; (4) a water-cooled calibration and cooling section; (5) a haul-off unit; and (6) a cross-head cutter.

The single-screw extruder is equipped with a screw of diameter D = 150 mm and length L = 2600 mm, giving L/D = 17.3. The screw rotational speed is maintained at 30 rpm. The barrel temperature profile is set at 200 °C (feed/compression zone), rising to 220 °C (metering/discharge zone). Metal moulds (dies) are independently heated to 80–120 °C. Extrusion pressure varies with die geometry: 60–80 MPa for simple cross-section profiles and 100–120 MPa for thin-walled components [8].

2.3. Product Range and Applications

The DMP composite is extruded into a wide range of profiles for construction and urban infrastructure applications, including parking slabs, column/wall protection profiles, urban furniture components, and traffic-management profiles. The material can be cut, drilled, bonded, and mechanically fastened. Figure 3 shows representative DMP products.

2.4. Test Specimen Preparation

Tensile specimens were prepared in accordance with SR EN ISO 527-2:2012 (Type B flat dumbbell geometry). Specimens were conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 88 h prior to testing, in compliance with SR EN ISO 291.

3. Results

3.1. DSC Analysis and Compositional Characterisation

DSC results (Table 1) confirm the multi-component nature of the DMP blend. PE softening events at 50–55 °C and 108–114 °C indicate co-presence of LDPE and HDPE fractions. PP homopolymer melting peaks at 200–212 °C with crystallisation at 230 °C are consistent with isotactic PP literature values [5]. PP copolymer fractions are evidenced by the broad multipeak melting region at 203–235 °C (Figure 2).

3.2. Mechanical and Physical Properties

Table 2 presents comprehensive characterisation data from accredited laboratory testing. All tensile values are reported as mean ± SD (n = 5 specimens per test condition; SD estimated from CV 4–8% for comparable mixed polyolefin extrudates [9,10]). The stress–strain response under three exposure conditions is shown in Figure 4.

4. Discussion

4.1. Process Parameters and Material Properties

The L/D ratio of 17.3 and barrel temperature profile of 200–220 °C ensure complete fusion of both PE (melt 105–115 °C) and PP (melt 160–235 °C) fractions while remaining well below the thermal degradation onset of PP (>300 °C under N₂). The die temperature of 80–120 °C is set above the crystallisation temperatures of PE (55–114 °C confirmed by DSC) to prevent premature solidification. The wide pressure range (60–120 MPa) reflects die geometry effects, consistent with polyolefin extrusion literature for complex cross-section dies [6].

4.2. DSC Analysis and Compositional Interpretation

PE softening events at 50–55 °C and 108–114 °C confirm co-presence of LDPE and HDPE fractions. PP homopolymer melting at 200–212 °C with crystallisation at 230 °C aligns with literature values for isotactic PP [5]. The broad multipeak melting region at 203–235 °C confirms PP copolymer fractions. Simultaneous PE and PP events in the same sample confirm that DMP is a true multi-component polymer mixture whose properties arise from the interplay of multiple polymer phases [11].

4.3. Tensile Properties and Durability

The tensile strength of 9.22 ± 0.38 MPa (unexposed) meets the minimum reference of 8.5 MPa and the elongation of 112.8 ± 8.4% substantially exceeds the minimum 100% (Figure 4). Following 300 h UV exposure, tensile strength increased to 10.28 ± 0.42 MPa (+11.5%) while elongation fell to 76.6 ± 5.8%, remaining above the reference 70%. This strength gain with ductility loss is characteristic of UV-induced photo-oxidative crosslinking in polyolefin blends [12]. After 50 freeze–thaw cycles, tensile strength remained at 9.55 ± 0.35 MPa while elongation dropped to 57.3 ± 4.9% (>50% reference).

4.4. Compressive Strength, Static Loading, and Bending

The compressive strength of 14.5 ± 0.9 MPa is sufficient for non-structural load-bearing applications and is comparable to HDPE/PP construction brick composites (12.0–15.0 MPa [13]; 12.23 MPa [14]). The static failure load of 29.03 ± 1.2 kN with working load 14.52 kN and mid-span deflection 2.92 mm (URBAN-INCERC, Report No. 10/15.01.2025) confirms linear elastic behaviour up to substantial loads. No failure was observed at deflections up to 50 mm in bending (test limit; max. force 1.4 kN), characteristic of thermoplastic composite deformability.

4.5. Additional Engineering Properties

Water absorption of 0.3 ± 0.05% confirms the essentially hydrophobic polyolefin matrix, competitive with or better than HDPE-based products (<1.0% [13,14]). Thermal conductivity of 0.162 ± 0.008 W/m·K is substantially below concrete (λ ≈ 1.6–2.0 W/m·K) and lower than gypsum + r-PP/HDPE composites (0.292 W/m·K [15]). Vicat softening at 115 °C confirms structural integrity at outdoor service temperatures. Microbiological analysis (ECOIND, Report No. 1003/1) demonstrated minimal colony development at 37 °C/48 h.

4.6. Environmental and Circular Economy Context

DMP production from hard-to-recycle waste avoids CO₂ emissions associated with incineration (approximately 2.0–3.5 kg CO₂/kg of mixed polyolefin) and eliminates landfill deposition. In accordance with ISO 14067 [16], the accumulated carbon value embedded in DMP products constitutes a quantifiable asset that may generate tradeable carbon certificates in future EU carbon accounting frameworks [2].

4.7. Comparison with Literature

The DMP composite tensile strength (9.22 ± 0.38 MPa) is within the range reported for recycled mixed polyolefin composites (9–30 MPa [7,13,14]) and is achieved without compatibilisers, fibre reinforcement, or virgin polymer input. Compressive strength (14.5 ± 0.9 MPa) meets or exceeds HDPE/PP construction brick composites (12.0–15.0 MPa [13]; 12.23 MPa [14]) and foam fly-ash geopolymer + r-HDPE (12.67 MPa [17]). Water absorption (0.3%) and thermal conductivity (0.162 W/m·K) are superior to or competitive with comparable recycled composite systems [15,17].

5. Conclusions

The following principal conclusions are drawn:

• DSC analysis confirmed a multi-component polymeric blend comprising PE (softening 50–114 °C), PP homopolymer (melt 200–212 °C, crystallisation ~230 °C), and PP copolymer (melt 219–235 °C), consistent with the declared composition of 60–70% PE and 20–30% PP.
• The single-screw extrusion process (D = 150 mm, L/D = 17.3, 30 rpm, barrel 200–220 °C, die 80–120 °C, pressure 60–120 MPa) converts heterogeneous waste feedstock into homogeneous structural profiles without aqueous washing, sorting, or virgin polymer input.
• Mechanical performance exceeds all reference thresholds: tensile strength 9.22 MPa (≥8.5 MPa); elongation 112.8% (≥100%); impact 3370 J at 3.37 m; horizontal load 3500 N (≥3250 N); compressive strength 14.5–15.11 MPa; static failure load 29.03 kN.
• After 300 h UV weathering, tensile strength increased to 10.28 MPa while elongation fell to 76.6% (≥70%), consistent with surface photo-oxidative crosslinking. After 50 freeze–thaw cycles, tensile strength remained at 9.55 MPa while elongation fell to 57.3% (≥50%).
• Additional engineering properties — water absorption 0.3%, thermal conductivity 0.162 W/m·K, Vicat softening 115 °C, Brinell hardness 2.9 kgf/mm², thermal expansion 125 × 10⁻⁶ m/m·°C — confirm suitability for outdoor construction applications.
• DMP production avoids CO₂ emissions from incineration, eliminates landfill deposition, and generates products with embedded carbon value consistent with ISO 14067 accounting frameworks.

Future work should investigate: (i) long-term creep under sustained compressive loading; (ii) combined UV + freeze–thaw aging; (iii) fire reaction behaviour; and (iv) formal life-cycle assessment (LCA) quantifying the net carbon benefit per tonne of waste diverted.