Low-Cost Qualitative Screening of Melting Ranges in Recycled Fishing Net Plastics: Comparison of Portable Handheld Device with Kofler Hot Bench and DSC

Reliable polymer identification is a prerequisite for high-quality recycling of post-consumer fishing nets and ropes. Differential Scanning Calorimetry (DSC) provides precise thermal characterization but requires stationary laboratory infrastructure and is not applicable at coastal collection points or small-scale recycling operations. This study describes and validates Melting Box 1.0 (MB 1.0), a low-cost, field-deployable device for qualitative screening of softening and melting ranges in small polymer samples. MB 1.0 repurposes a commercial vaporizer’s existing temperature-controlled heating and display functionality into a purpose-built screening tool, rather than introducing a new analytical measurement principle. Representative materials from Vietnamese fishing net recycling projects—polyethylene (PE), polypropylene (PP), polyamide (PA), and PE/PP blends—were assessed with a standardized three-tier visual scale (0 = no change; * = softening; x = melting) across five replicate measurements per sample. Agreement with the Kofler hot bench is good (mean bias for Tmelt,start: +2.9 °C; SD = 9.1 °C; n = 7), and a systematic offset of +20.6 °C (range +14.1 to +27.6 °C) was observed for PE relative to DSC first-heating peak-end temperatures. The correct material ranking (PE < PP < PA) is preserved, and all four polymer groups are reliably distinguished. At an estimated cost of 150–200 EUR—one to three orders of magnitude below the Kofler hot bench, portable NIR, or DSC—MB 1.0 is intended as a field-deployable screening complement to, not a replacement for, laboratory-grade analytics in resource-constrained recycling contexts.