The global production of plastics exceeds 350 million tonnes annually, with a substantial proportion ending up as waste after short-term use. Recycling is one of the most promising approaches to reduce plastic waste, lower carbon footprints, and achieve circular economy goals [1]. However, the mechanical and thermal stresses experienced during reprocessing often cause chain scission, oxidative degradation, and loss of stabilizers, which reduce the quality of recycled materials.

To address these challenges, it is important to establish simple, reproducible testing protocols that can quickly indicate the extent of degradation in recycled polymers. Advanced characterization methods such as differential scanning calorimetry (DSC), gel permeation chromatography (GPC), or tensile testing provide detailed insights, but they are time-consuming and expensive. In contrast, basic tests like density, melt flow index (MFI), and hardness offer rapid and accessible means of assessing degradation trends in recycled plastics.

This study focuses on three commonly recycled engineering polymers:

• Recycled Acrylonitrile Butadiene Styrene (rABS): Widely used in automotive and electronic housings, ABS is relatively stable but undergoes chain scission during reprocessing.
• Recycled Polyethylene Terephthalate (rPET): Used extensively in bottles and textiles, PET is prone to hydrolytic and thermal degradation during recycling.
• Recycled Polyamide 6 (rPA6): A common engineering polymer with good mechanical properties, but its amide bonds are highly sensitive to moisture and hydrolysis.

By comparing these three polymers across simple tests, the study aims to provide a baseline framework for evaluating recyclability in a laboratory environment.

Materials

Scrap samples of rABS, rPET, and rPA6 were collected from local recycling streams. Each material was washed to remove contaminants, dried at 60–80°C to minimize moisture absorption, and then shredded into flakes [2].

Reprocessing Procedure

A single-screw laboratory extruder (L/D: 24:1; screw diameter: 20 mm) was used. Temperature profiles:

• rABS: 200–220°C
• rPET: 260–275°C
• rPA6: 225–240°C

Screw speed: 60 rpm.

Each polymer underwent three complete extrusion cycles. Pellets were cooled in air and re-dried before testing.

Characterization Tests

Density (Archimedes' Principle)

• Method: Samples were weighed in air and in water using a density kit.
• Archimedes Principle (ASTM D792).
• Significance: Density decreases slightly with increased void formation and chain scission.

Melt Flow Index (MFI)

• Method: Standard ASTM D1238 procedure using a melt flow indexer at recommended temperatures (e.g., 220°C for ABS, 265°C for PET, and 230°C for PA6) [3].
• Significance: Higher MFI indicates reduced molecular weight and greater flowability due to chain scission.

Hardness (Shore D)

• Method: Shore D hardness tester on injection-moulded plaques (ASTM D792).
• Significance: Hardness reduction reflects molecular degradation and embrittlement.

All results represent mean ± standard deviation (n = 3).

Density           

Table 1 presents the density values (g/cm³) of rABS, rPET, and rPA6 over successive reprocessing cycles [4].

• rABS exhibited a negligible decrease in density of approximately 1% after three reprocessing cycles.
• rPET showed a slightly higher density reduction (~2%), attributed to hydrolytic chain scission and microvoid formation during extrusion.
• rPA6 experienced the most noticeable decrease (~2–3%), likely due to moisture-induced hydrolysis and void formation.

Overall, density decreased gradually in all polymers, with rPA6 showing the highest drop (Table 1).

Table 1: Density (g/cm³) Over Reprocessing Cycles.

Melt Flow Index

Table 2 summarises the melt flow index (MFI) values for all polymers over successive reprocessing cycles.

• rABS showed a moderate increase in MFI from approximately 2.5 g/10 min (Cycle 0) to 5.2 g/10 min after three cycles.
• rPET exhibited a significant increase from about 12 g/10 min to 22 g/10 min, highlighting its high sensitivity to thermal and hydrolytic degradation.
• rPA6 showed an increase from approximately 8 g/10 min to 15.5 g/10 min, indicating substantial chain scission due to hydrolysis.

Among all materials, rPET demonstrated the greatest rise in MFI, indicating the highest molecular weight degradation (Table 2).

Table 2: MFI (g/10 min).

Hardness

Table 3 shows the Shore D hardness values of rABS, rPET, and rPA6 after successive reprocessing cycles [5].

• rABS showed only a slight hardness reduction of approximately 5% after three cycles, indicating good retention of surface mechanical properties.
• rPET exhibited a hardness reduction of about 10%, consistent with embrittlement caused by molecular degradation.
• rPA6 experienced the highest reduction (approximately 12–15%), reflecting significant structural weakening due to hydrolytic degradation.

The hardness results further confirm that rPA6 is the most sensitive to reprocessing, while rABS retains better mechanical stability (Table 3).

Table 3: Shore D Hardness.

The results confirm that reprocessing causes molecular weight degradation in all three polymers, with the extent varying by polymer chemistry:

• ABS: The butadiene phase provides toughness, making rABS moderately resistant to degradation. Its MFI increases, but hardness and d ensity remain relatively stable. Hence, ABS retains good recyclability for injection-moulded parts.
• PET: PET exhibits the most severe increase in MFI, reflecting rapid chain scission due to ester hydrolysis and oxidative degradation. Density and hardness decrease moderately, which limits its reuse in load-bearing applications. rPET may require solid-state polymerization (SSP) or chain extenders to restore properties [6].
• PA6: PA6 suffers from hydrolytic degradation, leading to reduced density and hardness. Its sensitivity to moisture necessitates controlled drying before reprocessing. Although its mechanical properties drop significantly, blending with virgin PA6 or use in non-structural parts is possible.

Overall, MFI proved to be the most sensitive test for detecting polymer degradation, while density and hardness provided supportive insights [7].

This study demonstrated that simple, cost-effective tests can be used to evaluate the recyclability of engineering polymers. Among the three studied polymers:

• rABS showed moderate stability and better recyclability potential
• rPET suffered the greatest molecular degradation, reflected in the highest increase in MF1
• rPA6 exhibited significant hydrolytic sensitivity, with both hardness and density decreasing after reprocessing.

The results suggest that simple laboratory tests are sufficient to provide a baseline assessment of degradation trends in recycled polymers. For practical applications, rABS offers better performance consistency, while rPET and rPA6 may require stabilization or blending strategies.

Declarations

Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Competing Interests: The author declares that there are no competing financial or non-financial interests associated with this study.

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