Overview
PET has two commercial personalities that share a name and chemistry but serve completely different markets:
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Bottle and film PET — the soda bottle plastic, the food packaging film, the magnetic tape substrate. Lower molecular weight, optimized for stretch-blow molding and film orientation. RIC #1, the most- recycled plastic globally. This is what most non-engineers mean by “PET.”
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Machined PET-P (Ertalyte) — semicrystalline extruded rod and plate stock for precision mechanical parts. A direct alternative to acetal (POM) in many applications, with better acid resistance, lower moisture absorption, and similar machinability at modestly higher cost.
This file focuses primarily on machined PET-P as the engineering material, with bottle-grade properties documented for completeness.
The selection criteria for PET-P versus other engineering plastics:
- PET-P vs. POM — PET-P has better acid resistance, lower CTE, similar machinability, lower friction (especially Ertalyte TX), and slightly better dimensional stability. POM has better impact and is slightly less expensive. PET-P wins for water-contact precision parts; POM wins for general-purpose mechanical.
- PET-P vs. PA66 — PET-P has dramatically lower moisture absorption (0.3% vs. nylon’s 2.5–3%), so dimensional stability is far better in humidity. PA66 is tougher and more impact-resistant.
- PET-P vs. PEEK — PEEK wins on every metric except cost; PET-P is 10–20% the price, often adequate for moderate-temperature applications.
Machining notes
PET-P is among the cleanest-machining engineering plastics — comparable to POM. Short to medium continuous chips with sharp tooling, very low gumming tendency, and 16 Ra surface finish readily achievable.
Practical recipe:
- Sharp carbide, positive rake, polished cutting edges
- Speed: 500–1500 SFM
- Feed: 0.005–0.015 in/rev typical
- Coolant: optional for light cuts, recommended for heavy
- Tool wear is moderate on unfilled PET-P; glass-filled grades require PCD for production runs
Stress-relief annealing at 110–140°C for 1 hr/inch is recommended after heavy machining for parts requiring tight long-term dimensional stability. PET-P has lower CTE than POM (~75 vs. ~110 × 10⁻⁶/°C) so thermal dimensional drift is less of an issue than with acetal.
Bonding is the weak point — solvent welding doesn’t work. Surface activation (plasma, flame) enables epoxy and cyanoacrylate bonding. For permanent joints, mechanical fasteners or threaded inserts are the standard methods.
Variant selection guidance
- Ertalyte / PET-P — the default machined engineering grade. Bearings, gears, precision fittings, FDA food contact parts, water- meter components.
- Ertalyte TX (PTFE-modified) — self-lubricating bearings and bushings under dry or marginal lubrication. Better than Delrin AF for many dry-running applications.
- Bottle-grade PET — stretch-blow molded bottles, food containers. Not a machining material.
- CPET trays — ovenable frozen food packaging. Crystallized for 220°C oven service.
- Rynite GF30 — injection-molded structural automotive and electrical components requiring stiffness and HDT >200°C.
- Mylar / BoPET film — capacitor dielectric, magnetic media, packaging laminates, electrical insulation. Premium film performance.
Failure modes worth designing around
Hydrolytic degradation in hot water is the defining PET weakness. PET hydrolyzes (ester bond cleavage) in continuous contact with water above ~70°C, with the rate accelerating sharply above 90°C. Continuous steam service is out of the question. The mechanism: water molecules attack the ester linkages, lowering molecular weight and embrittling the polymer. This is why bottle-grade PET is fine for cold drinks but not for hot-fill above 70°C without crystallization.
Strong base attack — PET is hydrolyzed by NaOH, KOH, and similar strong alkalis, especially at elevated temperature. This matters in two contexts: alkaline cleaning chemistries (industrial dishwashers, caustic CIP cycles) and saline-alkaline conditions in oil-and-gas service. For caustic exposure, switch to POM-C, PP, or PEEK.
Notch sensitivity — PET-P is notch-sensitive in impact loading. Sharp internal corners initiate cracks under shock load. Generous radii (R ≥ 1 mm minimum, R ≥ 3 mm for impact-critical features) reduce this significantly.
UV degradation — natural PET yellows and embrittles within months of continuous outdoor exposure. UV-stabilized grades extend service to several years but PET is not a primary outdoor material.
Crystallization-induced shrinkage in injection molding — PET shrinks 0.2–0.6% if molded amorphous (cold mold), but 1.5–2.5% if molded crystalline (hot mold). Mold designer must commit to one or the other. Bottle preforms are amorphous (clear); CPET ovenable trays are crystalline (opaque white).
Drying is non-negotiable for processing — moisture content above ~50 ppm causes hydrolysis during melt processing, producing brittle parts with lower molecular weight. Standard regime is 150–175°C for 4+ hours before processing. This is a frequent processor error.
Applications by industry
- Food and beverage packaging — the dominant PET application. Soda bottles, water bottles, food containers, salad clamshells, ovenable CPET trays for frozen meals. Global volume ~30 million tonnes/year.
- Industrial machinery (machined PET-P) — bearings, bushings, wear pads, slide rails, gears, food-equipment fittings, water-meter components, conveyor parts. Ertalyte and Ertalyte TX are dominant.
- Automotive electrical (Rynite GF30) — ignition coils, fuel system connectors, sensor housings, structural underhood components. HDT 220°C+ is the enabling property.
- Electronics (glass-filled PET) — high-density electrical connectors, relay housings, motor end caps. Competes with PBT in many of these applications.
- Capacitors and electrical (Mylar / BoPET) — capacitor dielectric film is the largest single use of BoPET. Also magnetic media substrates, high-temperature electrical insulation, slot liners in motors.
- Pharmaceutical and medical — non-implant fluid handling, FDA- compliant fittings, lab equipment, diagnostic device components.
- Water and wastewater — NSF 61 Ertalyte for water-meter components and fittings; pipe-grade not common (HDPE/CPVC dominate).
- Textile — polyester fiber is the largest single PET volume but is a different commercial form than machining or packaging stock.
- Strapping — industrial PET banding strap for cargo securement. Replaces steel strap in many applications.