PET Chemical Resistance: What Attacks It and What It Ignores

Polyethylene terephthalate (PET) is a semicrystalline polyester whose chemical stability derives from the terephthalate ester repeat unit, the aromatic ring, and the crystalline fraction developed during solid-state polymerization, injection molding, or stretch blow molding. Chemical resistance is not a single grade property. The same material can withstand a nonpolar aliphatic hydrocarbon for 1,000 h at 23 °C and yet crack within minutes under a fixed tensile strain in methylene chloride. The default evaluation methods are ISO 175:2010 and ASTM D543-21, which use mass change, dimensional change, and retained mechanical properties after immersion or contact. In oriented bottle sidewalls with crystallinity between 25% and 35%, the amorphous regions provide the principal sorption and diffusion pathways, while the crystalline domains remain comparatively inaccessible. The total solubility parameter of PET is approximately 20.2 MPa^1/2; nonpolar aliphatic hydrocarbons with solubility parameters below 15 MPa^1/2 fall outside the swelling envelope, while chlorinated solvents near 19–21 MPa^1/2 interact strongly with the amorphous phase. This morphological asymmetry explains why chemical attack is often selective: it preferentially softens or stress-cracks the amorphous phase without visibly dissolving the crystalline scaffold.

What Attacks PET More Effectively: Hot Water or Sodium Hydroxide?

Hydrolysis of the ester bond is the dominant chemical degradation mode in aqueous environments above the glass transition. Water cleaves the terephthalate ester linkage, producing carboxylic acid and hydroxyl end groups, and the molecular-weight loss can be tracked as a drop in intrinsic viscosity by ASTM D4603-18, which uses 60/40 w/w phenol/1,1,2,2-tetrachloroethane at 30 °C. At 70 °C, semicrystalline PET immersed in distilled water for 7 days can show a measurable reduction in tensile elongation when tested per ISO 527-2:2012; at retort conditions of 121 °C saturated steam, hydrolysis accelerates because the amorphous phase is above its glass transition and chain mobility increases. The acid end groups generated by hydrolysis can autocatalyze further chain scission, which makes high-temperature water exposure a self-accelerating process. This is why injection molding and extrusion require resin moisture below 50 ppm before melting. Production-scale desiccant-wheel dryers are typically specified with a dew point of -40 °C or lower, and residence times of 4 h to 6 h at 160 °C to 180 °C. In moisture-conditioned resin processed through a twin-screw extruder with L/D 27–32, hydrolysis-induced melt viscosity loss leads to barrel pressure variation and increased acetaldehyde generation. For chemical resistance testing, water exposure is not inert when the temperature exceeds 50 °C and should be ranked by retained elongation-at-break and intrinsic viscosity, not by visual appearance alone.

Sodium hydroxide is a more direct attack than neutral water. Aqueous NaOH at 5 wt% to 10 wt% and temperatures from 40 °C to 60 °C saponifies surface ester groups, producing disodium terephthalate and ethylene glycol. The failure mode is surface erosion, haze formation, and loss of burst strength rather than uniform bulk dissolution. Returnable bottle washing lines use cleaning solutions with pH above 12 and bath temperatures above 50 °C; field data from such lines show that repeated caustic washing of PET containers introduces pitting and stress cracking at the label panel and support ring. In laboratory immersion per ASTM D543-21, a 10 wt% NaOH solution at 23 °C produces visually minor attack, while the same solution at 60 °C produces measurable erosion within 24 h. Therefore, PET is not suitable for prolonged contact with hot alkaline cleaning-in-place media or strong food-processing bases. The attack is temperature-dependent and surface-specific, and chemical resistance charts that omit temperature are not sufficient for component design.

Concentrated Oxidizing Acids and Phenolic Solvents Represent a Different Attack Class

For dilute mineral acids, the response is generally resistance. Hydrochloric acid at 10 wt% and sulfuric acid at 30 wt% at ambient temperature produce high retention of tensile strength over 7-day immersion in ISO 175:2010 tests. The aromatic ring and dense crystalline packing limit proton diffusion. Concentrated sulfuric acid above 40 wt%, concentrated nitric acid, and other strong oxidizing agents attack PET by sulfonation, nitration, or oxidative chain scission; the local exotherm from acid dilution can also raise the surface temperature and shift the degradation mechanism. Phenol, cresols, and chlorophenols are aggressive because they combine solubility-parameter overlap with hydrogen-bonding capacity. Molten or concentrated phenol dissolves PET; cresol-based stripping agents attack the surface rapidly. This sensitivity to phenolic solvents is why PET is not used for solvent-based paint stripping tools or components exposed to phenolic process chemicals without specific barrier protection.

Among non-acid solvents, chlorinated species are the most dangerous class. Methylene chloride, with a solubility parameter of approximately 20.3 MPa^1/2, chloroform at approximately 19.0 MPa^1/2, and 1,1,2,2-tetrachloroethane have solubility parameters near the PET amorphous phase and produce swelling, stress cracking, and localized dissolution. The bent-strip environmental stress cracking test per ISO 22088-3:2016 is used to rank susceptibility; PET specimens under fixed strain and in contact with methylene chloride typically exhibit cracking rapidly at strain levels well below the neat yield strain. Ketones such as acetone and methyl ethyl ketone are less aggressive than chlorinated solvents but can swell amorphous regions and induce haze in low-crystallinity sheet or film. Esters such as ethyl acetate and butyl acetate behave similarly at 23 °C; swelling is moderate, and the visual change is often reversible after drying. The practical issue is not mass loss but the interaction of solvent swelling with molded-in stress. A chemical that produces no visible attack on unstressed plaque can still propagate cracks in a snap-fit or threaded boss.

When Aliphatic Hydrocarbons, Diesel, and Motor Oil Are Contacted

Because long-chain aliphatic hydrocarbons have low solubility parameters, they do not attack PET. n-Hexane has a solubility parameter near 14.9 MPa^1/2, outside the PET swelling range. Immersion in n-hexane, n-heptane, iso-octane, diesel fuel, or mineral oil at 23 °C for 7 days per ISO 175:2010 typically produces mass uptake below 0.5% and no significant tensile strength shift when tested per ISO 527-2:2012. This inertness permits PET fuel filter housings and oil-contacted connectors to be specified when lightweight, chemical-resistant thermoplastics are required. The boundary condition is fuel composition. Methanol, ethanol, and aggressive aromatic oxygenates can plasticize or stress-crack PET when the part is under molded-in tensile stress at temperatures above 60 °C. A rating for “fuel” without composition and temperature is therefore insufficient; blend-specific testing is required. For chlorinated brake cleaners, the same component may fail rapidly if the cleaner is allowed to pool in a stressed housing. Published data for complex fuel blends in long-term automotive service is limited, so component validation relies on immersed tensile bars per ASTM D543-21 combined with controlled-strain stress cracking per ISO 22088-3:2016.

At ambient temperature, aliphatic alcohols are generally not aggressive. Ethanol at 23 °C for 7 days may induce slight haze in low-crystallinity material but often leaves tensile strength above 85% of the unexposed value; methanol is more aggressive at elevated temperature or in fuel blends. Methanol at high temperature can act as a transesterification agent, producing dimethyl terephthalate and ethylene glycol from the polyester chain. Ethylene glycol and propylene glycol at ambient are largely resisted, but automotive coolant service above 90 °C introduces water/glycol hydrolysis and oxidation that can shorten service life. Long-chain fatty oils and food oils are largely ignored at ambient, which is why PET is used for edible oil bottles. Elevated frying temperatures or hot oil exposure above 100 °C may cause crystallization-induced haze and dimensional change rather than chemical dissolution.

For design qualification, chemical resistance testing protocols are sometimes misinterpreted. Mass uptake below 1% does not guarantee mechanical strength retention; a small amount of solvent in the amorphous phase can lower yield stress and accelerate creep rupture without producing visible weight change. The correct mechanical test sequence after ISO 175:2010 immersion is ISO 527-2:2012 tensile testing at 23 °C, with tensile strength and elongation retained relative to unexposed control specimens. In stress-cracking evaluations, ISO 22088-3:2016 bent-strip specimens are clamped to a fixed radius and exposed to the chemical; time to crack is recorded at different strain levels. For design purposes, the resulting strain threshold is more useful than a pass/fail rating because the same chemical may appear harmless in an unstressed cup but fail in a molded-in-stress threaded closure. Moisture conditioning before testing is also controlled by ISO 291:2008, because the amount of absorbed water in the amorphous phase can shift the glass transition and influence the mechanical baseline.

Extrusion, Drying, and the Chemical Resistance of Stretched Parts

Before any chemical exposure, the chemical resistance of a PET part is partly fixed during processing. Resin with high initial intrinsic viscosity, for example 0.82 dL/g for injection stretch blow molded carbonated-soft-drink preforms, retains a larger molecular-weight cushion when hydrolysis or ester cleavage occurs. Drying is the first processing variable: residual moisture above 50 ppm entering a reciprocating-screw injection unit with L/D 20–25 or a twin-screw extruder with L/D 27–32 produces melt-phase hydrolysis that reduces intrinsic viscosity and creates carboxylic acid end groups. These acid end groups can autocatalyze further hydrolysis. Preform injection commonly uses melt temperatures of 270 °C to 285 °C; at these conditions, wet resin produces acetaldehyde, silver streaking, and reduced mechanical ductility. The resulting preform may pass a visual inspection and still carry a localized molecular-weight deficit that lowers chemical resistance in the bottle sidewall. This is why chemical failure in caustic wash trials often correlates with injection plastication parameters and dryer dew point rather than resin brand alone. The same orientation step increases crystallinity in the sidewall from the preform state to 25%–35%, which reduces sorption but also creates local amorphous regions under high orientation stress; those regions are the preferred sites for solvent attack.

Solid-state polymerization raises molecular weight and crystallinity before melt processing. PET resin for high-pressure carbonated beverage bottles is often solid-state polymerized to an intrinsic viscosity of 0.80 dL/g to 0.84 dL/g, while strapping and industrial fiber grades may reach higher values. Higher intrinsic viscosity does not make PET chemically resistant to chlorinated solvents, but it delays the onset of brittle failure after hydrolysis because the critical molecular weight for load-bearing chain entanglement is maintained longer. This is relevant in returnable/refillable bottle applications that undergo repeated caustic washing at 60 °C. A preform with an initial intrinsic viscosity of 0.82 dL/g that loses 0.10 dL/g during drying or processing has already consumed part of the safety margin before the first chemical exposure. This explains why chemical resistance specifications often combine a minimum intrinsic viscosity requirement with a maximum residual moisture specification and a defined injection melt temperature window.

In food-contact use, PET must also satisfy extraction and migration limits, not only mechanical compatibility. Structures used in aqueous or fatty food contact are evaluated under FDA 21 CFR 177.1630 and EU Regulation No 10/2011, with migration testing according to EN 1186-1:2002. These regulatory tests impose food simulants, time, and temperature conditions that overlap with chemical resistance testing but measure migrant transfer rather than polymer property retention. In practice, an oil bottle may exhibit high retention of mechanical properties in edible oil and still require a documented migration test. Hot-fill applications above 85 °C require both hydrolysis resistance and bottle-panel design that accounts for thermal stress because the combination of heat, water, and tensile stress reduces the chemical resistance of the amorphous phase. Such parts are typically qualified through a combination of ISO 175:2010 chemical immersion, ASTM D543-21 coupon testing, and fill-line trial data at target temperature and holding time.