Polyethylene Terephthalate (PET)

Polyethylene terephthalate is a linear aromatic polyester produced by polycondensation of terephthalic acid and ethylene glycol. Commercial bottle-grade resin typically exhibits a glass transition near 70–80 °C, a crystalline melting point of 245–260 °C, and a solid-state density of 1.38–1.40 g/cm³ when measured according to ISO 1183-1:2019. Intrinsic viscosity, determined by ASTM D4603-18 in a 60/40 w/w phenol/1,1,2,2-tetrachloroethane mixture at 30 °C, is the principal quality marker for bottle-to-bottle recycling because it tracks average molecular weight. Post-consumer PET bottles entering deposit return schemes therefore require a sequence of sorting, washing, melt processing, and solid-state treatments to recover bottle-grade properties after the deposit machine.

PET Bottle Recycling: What Happens After the Deposit Machine

After a bottle is accepted by a reverse vending machine, the unit reads the container barcode, compacts the container, and segregates it into PET, aluminium, or glass storage compartments. Compaction reduces storage volume by approximately 65–80%. Collection logistics then transfer baled PET to a reclaimer, where debaling and drum screening remove broken glass, sand, and fines before optical sorting. Near-infrared sorters operating in the 1,000–1,650 nm wavelength range identify PET by its carbonyl overtone absorption and separate it from HDPE, PP, and other polymers at throughputs of 3–5 t/h. Air-ejector arrays remove misclassified bottles, and colour cameras separate clear, light-blue, and green fractions before granulation. Deposit-sourced PET bales can exceed 95 wt% PET content, but caps, label fragments, residual beverage droplets, and occasional PVC shrink sleeves remain.

Polyolefin caps and labels are removed by density separation at approximately 1.00 g/cm³, because PET sinks while HDPE and PP float. PVC is the problematic exception: its density of 1.35–1.40 g/cm³ overlaps PET, so sink-float tanks cannot reliably separate it. X-ray fluorescence and laser-induced breakdown spectroscopy units are deployed to reject PVC-rich bottles and flakes. Bottle-grade rPET specifications commonly require PVC below 50 ppm; food-contact operations frequently target 10–30 ppm. Residual metal from caps or aerosol hardware is removed by eddy-current separators or magnets before granulation to protect rotating blades.

Ground flake with a nominal size of 12–16 mm is hot-washed in 1–2 wt% sodium hydroxide solution at 80–90 °C. The caustic bath saponifies polyvinyl acetate label adhesives and removes sugars, organic residues, and surface soils. Residence time of 10–20 min is required for adhesive removal from flake surfaces. Friction washers then scrub residual paper pulp, and hydrocyclones separate floating polyolefin particles. Flake moisture is reduced to below 50 ppm by thermal drying at 160 °C for 4–5 h. If wet flake enters a melt extruder, hydrolytic chain scission at 270–285 °C can reduce intrinsic viscosity by 0.04–0.10 dL/g in a single pass.

How Does Residual PVC in the Bottle Stream Degrade Melt-Phase Properties?

Residual PVC thermally dehydrochlorinates at PET melt-processing temperatures above 250 °C. The hydrogen chloride released acts as an acid catalyst for ester hydrolysis, increasing carboxylic end-group concentration and lowering molecular weight. The degradation is accelerated by residual moisture above 50 ppm because acid-catalysed hydrolysis proceeds faster than thermal chain scission alone. A reclaimer operating a co-rotating twin-screw extruder with L/D 30:1–40:1 therefore positions vacuum venting at 20–40 mbar and limits melt temperature to 270–285 °C. Screen packs of 32–50 µm remove char, gel particles, and carbonized PVC specks. If PVC is not removed before extrusion, the same screens blind rapidly and increase backpressure, forcing more frequent screen changes and creating high-shear thermal degradation zones.

After melt filtration, the extrudate is pelletized. Intrinsic viscosity drops during melt processing because the polymer is heated above its crystalline melting point. Acetaldehyde concentration rises to 3–10 ppm in the melt and must be reduced before bottle preform injection. Vacuum venting strips some acetaldehyde, but solid-state polymerization is more effective because it operates at lower temperature and provides a continuous purge gas or vacuum to remove the volatile degradation product.

Solid-State Polymerization and Intrinsic Viscosity Recovery

Solid-state polymerization raises molecular weight without melting the polymer. Amorphous rPET pellets are first crystallized at 170–180 °C to prevent sticking, then heated to 200–220 °C under vacuum below 1 mbar or dry nitrogen for 12–20 h. The polycondensation equilibrium is shifted by continuous removal of ethylene glycol and water. The process is diffusion-controlled: reaction rate depends on ethylene glycol migration from the pellet core to the surface. Pellet diameters near 2–3 mm represent a practical compromise between reaction rate and fixed-bed pressure drop. Incomplete crystallization or excessive bed temperature causes pellet agglomeration and gas-flow maldistribution in fixed-bed reactors.

Typical intrinsic viscosity progression through mechanical recycling and solid-state treatment is shown in the first table. The values are representative of bottle-grade rPET quality gates and are measured according to ASTM D4603-18.

Food-contact bottle-to-bottle operations use solid-state polymerized rPET that complies with Regulation (EU) No 10/2011 and 21 CFR 177.1630. EFSA requires a challenge test with surrogate contaminants to demonstrate that post-consumer residues are reduced to safe levels. The surrogate mixture commonly includes chloroform, toluene, benzophenone, and lindane; EFSA guidance typically uses 500 mg/kg surrogate spiking to create conservative decontamination factors. Total migration for plastic food-contact materials must not exceed 10 mg/dm² under Regulation (EU) No 10/2011. Mechanical recycling without solid-state polymerization generally lacks the residence time and diffusion conditions needed to remove deeply penetrated contaminants, so direct food-contact use is not typically achieved by sorting and melt processing alone.

When Closed-Loop rPET Confronts Hydrolytic Degradation Limits

Mechanical recycling cannot restore PET indefinitely. Each melt pass under residual moisture or acidic contamination cleaves ester bonds, increases carboxylic end groups, and reduces molecular weight. Preforms molded from rPET with intrinsic viscosity below 0.70 dL/g are prone to weld-line cracking, low burst strength, and inconsistent blow-molding behaviour. Drying before injection or extrusion must reduce moisture to below 50 ppm; storage in humid silos above 60% RH requires dehumidified hoppers because PET regains moisture quickly. Closed-loop bottle producers commonly blend solid-state polymerized rPET with virgin resin at 25–50 wt% rPET for carbonated soft-drink containers. EU Directive 2019/904 mandates 25% recycled content in PET beverage bottles by 2025 and 30% by 2030 for certain member states.

Non-food rPET applications such as strapping, staple fibre, and thermoformed trays may use lower-IV grades and bypass solid-state polymerization, but extrusion filtration and polyolefin control remain necessary. Polyolefin carryover above 2 wt% reduces tensile modulus and raises haze in sheet and preform applications. Residual adhesives produce odour, surface defects, and die-lip buildup. Deposit-sourced material, because of its relatively clean input stream, supports closed-loop recycling only when sorting, washing, melt filtration, and solid-state polycondensation are operated as a continuous chain of unit operations.