What PET Plastic Actually Is: The Code-1 Resin Behind Every Bottle

Polyethylene terephthalate is a condensation polymer of purified terephthalic acid and monoethylene glycol. The repeating unit is —[O–CH₂CH₂–O–CO–C₆H₄–CO]—, pairing the para-substituted terephthalate ring with an aliphatic glycol segment. Melt-phase polymerization is carried out at 270–290°C under vacuum below 2 mbar, followed by pelletizing and, for bottle grades, solid-state polymerization. The resin identification code 1 assigned by ASTM D7611-20 identifies the polymer class, but it does not distinguish bottle resin from fiber, film, or engineering grades. Melt density is typically 1.27–1.28 g/cm³ at 270°C, and solid-state density spans 1.33–1.40 g/cm³ depending on crystallinity as measured by ISO 1183-1:2019. Bottle grades often contain 1–3 mol% isophthalic acid or 1–4 mol% cyclohexanedimethanol to slow quiescent crystallization and widen the injection stretch blow molding window.

What Distinguishes Bottle Grade from Fiber Grade and Film Grade?

Intrinsic viscosity is the primary release parameter separating PET conversion routes. Bottle-grade resin for injection stretch blow molding carries 0.72–0.84 dL/g as measured under ASTM D4603-18 in a 60/40 phenol/1,1,2,2-tetrachloroethane solvent at 30°C. Fiber grades for high-speed winding are produced at 0.55–0.68 dL/g, while biaxially oriented film resins commonly fall between 0.60–0.70 dL/g. Solid-state polymerization raises amorphous precursor pellets from 0.30–0.50 dL/g after melt-phase polycondensation to the bottle range. In continuous SSP reactors, nitrogen purge gas maintains residual oxygen below 20 ppmv, and pellet temperature is held at 200–220°C. The pellets must first be surface-crystallized at 140–160°C to prevent agglomeration before the main SSP step. Field observations from vertical SSP columns indicate that pellet surface moisture above 500 ppm produces wall sticking and non-uniform molecular weight distribution across the finished batch.

Drying Is Not Optional Before Extrusion or Injection Molding

Hydrolytic chain scission during melt processing is controlled by aggressive pre-drying. The practical moisture ceiling for bottle preform injection is 50 ppm, with high-clarity specifications requiring 30 ppm or lower. Desiccant wheel dryers with process air dew point below −40°C are operated at 160–170°C for 4–6 h. Wet pellets pushed into the melt create additional acetaldehyde and carboxylic end groups, producing preform haze, gate-stringing, and an intrinsic viscosity loss of 0.02–0.05 dL/g across a single heat history. Over-drying above 180°C initiates thermal oxidative yellowing and surface crystallinity, so the dryer must hold the specified temperature band rather than simply maximize drying rate. In plants where ambient relative humidity exceeds 60%, opened gaylords should not remain exposed for longer than 30 min before matrix loading.

During preform injection, bottle-grade PET is melted with nozzle temperatures of 270–290°C and barrel zones between 260°C and 285°C. The melt is injected through hot runner valve gates into tooling that may range from 48 to 96 cavities, with clamp force requirements from 250–600 t depending on projected area and melt viscosity. Fill pressures are generally 80–140 MPa. Mold water temperatures of 8–12°C quench the preform rapidly enough to keep crystalline content below 5%. Reheat stretch blow molding then brings the preform surface to 95–105°C before axial stretching and hoop stretching with blowing air at 20–40 bar. The resulting biaxial orientation raises hoop-direction yield stress to 120–160 MPa at 23°C under ISO 527-3, compared with roughly 50–60 MPa for unoriented amorphous sheet. The critical process conflict is that excessive reheat raises acetaldehyde and spherulitic haze, while insufficient reheat increases blow pressure and creates wall-thickness scatter. Rotary blow molders therefore specify preform surface temperature uniformity within ±1°C.

Thermal Crystallization Versus Orientation-Induced Crystallization

PET crystallizes by two distinct mechanisms. Quiescent spherulitic crystallization produces spherulites that scatter visible light when their radii exceed approximately half the wavelength of visible light, causing optical cloud. Amorphous PET quenched below the glass transition at 78–80°C remains transparent. The maximum quiescent crystallization rate occurs near 170–180°C, so preform cooling must pass through that window rapidly. Opaque CPET trays are intentionally crystallized in molds held at 140–160°C with inert mineral nucleation, reaching crystallinity above 35%. Oriented bottle sidewalls achieve 20–30% crystallinity after stretching, but the crystals are small and oriented, limiting visible haze. Hot-fill containers are heat-set in the blow mold at 120–140°C to reduce free shrinkage. Heat-set sidewalls exhibit linear shrinkage below 1.5% after 30 min at 85°C in internal volume displacement testing. Published data for this specific configuration is limited to production validation studies rather than open laboratory publications.

Barrier Thresholds and Carbonated Soft Drink Shelf Life

Carbonated soft drink bottles rely on carbon dioxide retention and oxygen ingress control. Oxygen permeability of oriented bottle wall measured by ASTM D3985 is approximately 0.03–0.06 cm³·mm/(m²·day·atm) at 23°C and 0% RH. Carbon dioxide permeability is 0.12–0.25 cm³·mm/(m²·day·atm) under similar conditions. Water vapor transmission rate of biaxially oriented PET film measured by ASTM F1249 is typically 8–15 g/m²·day at 38°C and 90% RH for 25 µm film. These values allow a monolayer 500 mL bottle with 0.30 mm average sidewall thickness to retain acceptable carbonation for 8–12 weeks at 20°C. Oxygen-sensitive beer and juice products require additional scavenging chemistry or multilayer structures because the barrier is not absolute. At 85% RH, oxygen transmission increases as water plasticizes the amorphous phase. Barrier performance also depends on hoop stretch ratio. Stretch ratios below approximately 8:1 to 10:1 reduce sidewall crystallinity and produce an abrupt rather than linear loss of carbon dioxide retention, an effect observed on commercial blow molders when preform wall thickness is reduced without a proportional increase in stretch-rod speed.

When Post-Consumer Recyclate Is Introduced at Rates Above 25%

Mechanical recycling of bottle-grade PET returns flake with lower molecular weight and higher carboxylic end-group concentration than virgin resin. After hot alkaline washing and solid-state re-polycondensation, food-grade rPET pellets may meet 0.70–0.80 dL/g, but the melt is more sensitive to hydrolysis and thermal degradation. At addition rates above 25%, injection molders observe reduced melt consistency and an increase in black specks from degraded polymer gel. Twin-screw extruders used for rPET pelletizing are specified with L/D ratios of 32:1 to 44:1, vacuum venting, and melt filtration screen packs of 20–40 µm absolute rating. The melt-phase intrinsic viscosity loss during a standard preform molding cycle may be 0.03–0.07 dL/g, higher than for virgin bottle resin. Chain extenders based on pyromellitic dianhydride or multifunctional epoxidized styrene-acrylic oligomers are used at 0.2–0.5 wt%, but they raise back-pressure and require precise liquid dosing at the feed throat. Reheat behavior also changes because residual dyes and degradation products reduce infrared absorption. Antimony or titanium based reheat agents are sometimes added at 2–5 ppm metal content to restore preform surface heating. Food-contact compliance for PET in the United States is governed by FDA 21 CFR 177.1630, while European recycled PET food-contact authorization falls under Regulation (EU) 2022/1616, which replaced Regulation (EC) No 282/2008.

Acetaldehyde Generation and Preform Haze

Acetaldehyde is generated by thermal degradation of PET ester linkages and is the dominant flavor-active volatile in bottle walls. The beverage industry applies a maximum acetaldehyde specification commonly between 3 ppm and 5 ppm in bottle wall material for carbonated soft drinks, depending on package size and fill sensitivity. Melt-phase production controls include maintaining polycondensation temperature below 290°C, minimizing residence time in hot runners, and holding melt moisture below 50 ppm. Solid-state polymerization reduces residual acetaldehyde by nitrogen stripping, but subsequent injection molding and reheat blowing regenerate a measurable fraction. Low-acetaldehyde grades are produced with additional modifier packages and are processed at nozzle temperatures near 270°C rather than 290°C. The trade-off is higher melt viscosity and increased fill pressure. Preform haze from acetaldehyde is compounded by spherulitic crystallization when cooling is too slow. Processors monitoring high-cavitation preform systems report that valve-gate sticking and silver streaking correlate more strongly with barrel residence time above 285°C than with absolute throughput, a field observation that guides startup and color-change procedures.