Direct Answer
Repeated thermal cycles increase PET yellowing because cumulative thermo-oxidative degradation and catalyst-state changes (surface redox, local segregation, and depolymerisation activity) produce chromophoric degradation products even when bulk antimony concentration remains unchanged.
Key Takeaways
- Repeated thermal cycles increase PET yellowing.
Does not claim: This page does not claim performance superiority or regulatory suitability; it explains mechanistic behavior under defined processing conditions.
Introduction
Thermal cycles cause cumulative PET chemical changes that create visible chromophores. Antimony species can shift local catalytic behavior (oxidation, depolymerisation) through changes in speciation or surface state under heat. Physically, repeated high‑temperature residence produces radicals and irreversible bond scission that form carbonyls, aldehydes and conjugated fragments which absorb visible light. This explanation applies when PET experiences repeated processing/reprocessing temperatures near or above Tg/melt in the presence of oxygen or residual moisture. If oxygen is rigorously excluded or processing temperatures remain low enough to avoid significant bond scission, the cumulative yellowing pathway is curtailed. Once chromophores and low‑molecular oxygenates form or Sb redistributes/precipitates, the optical change is effectively persistent under normal service conditions.
Read an overview of the material: https://www.greatkela.com/en/product/Functional_Oxide_Ceramics/227.html
Read the application details (PET Catalyst & Plastics): https://www.greatkela.com/en/use/catalysts/258.html
Common Failure Modes
- Observed failure: Gradual increase in yellowness with successive reprocessing despite constant bulk Sb. Mechanism mismatch: Bulk elemental analysis misses local Sb speciation/redistribution, therefore catalytic microenvironments change while total Sb remains unchanged.
- Observed failure: Sudden jump in discoloration after a reheat step. Mechanism mismatch: A threshold thermal or oxygen exposure event triggers rapid chain scission and conjugated chromophore formation or causes Sb precipitation/segregation, therefore optical change appears abrupt.
- Observed failure: Increased VOCs (e.g., acetaldehyde) concurrent with yellowing. Mechanism mismatch: Depolymerisation producing oxygenated volatiles accompanies chromophore formation, therefore VOC spikes correlate with chemical degradation pathways that cause color.
- Observed failure: Similar bulk Mn/Mw but worse color. Mechanism mismatch: Averaged molecular weight metrics mask localized scission and formation of conjugated end groups that change optical absorption without large bulk Mw change, therefore color can degrade independently of average Mw.
- Observed failure: Batch variability in color with identical Sb ppm. Mechanism mismatch: Differences in moisture, impurities, processing thermal history, or additive packages alter local oxidation kinetics; therefore bulk Sb ppm alone does not predict color outcome.
Conditions That Change the Outcome
- Integrated thermal exposure (time × peak temperature): Behavior changes because reaction rates for chain scission and oxidation increase with temperature and cumulative time; higher peaks and longer integrated exposure create more irreversible chromophores.
- Oxygen availability (air vs inert): Behavior changes because oxidative pathways producing carbonyl/quinone-type chromophores require O2; therefore yellowing rises in oxidative atmospheres and is suppressed in inert conditions.
- Moisture / hydrolytic species: Behavior changes because water accelerates hydrolytic chain scission and carboxyl end‑group formation, therefore residual moisture increases the rate of chromophore-generating reactions.
- Impurities and additive chemistry (metals, antioxidants, scavengers): Behavior changes because trace metals or residual monomer provide alternate catalytic or charge‑transfer pathways while antioxidants/scavengers intercept radicals, therefore impurity/additive composition shifts chromophore formation rates.
- Catalyst state (local speciation, dispersion, surface chemistry): Behavior changes because Sb activity depends on oxidation state, clustering, or surface availability; therefore redistribution or surface transformation of Sb under heat alters local catalytic selectivity even if bulk Sb is constant.
How This Differs From Other Approaches
- Thermo‑oxidative degradation: Coloration arises because heat plus O2 generates radicals, chain scission and oxidised conjugated species; this is an oxidation-driven chemical mechanism.
- Catalyst‑state mediated depolymerisation: Coloration arises because antimony species (soluble or as oxide surfaces) alter depolymerisation/oxidation selectivity under heat, producing low‑molecular oxygenates and unsaturated fragments; this is a catalysis-sensitive mechanism dependent on local speciation.
- Impurity/charge‑transfer complex formation: Coloration arises because trace transition‑metal contaminants or residual chromophoric impurities form localized oxidation centers or charge‑transfer complexes that absorb visible light; this is a heterogeneous site-limited mechanism controlled by trace species.
Scope and Limitations
- Applies to PET undergoing repeated thermal exposure at or above Tg/melt in oxidative or partially oxidative atmospheres because oxygen and thermal energy are required for the dominant oxidation/chromophore pathways.
- Does not apply to ambient storage without thermal excursions, to purely photochemical UV‑only yellowing in the absence of heat, or when extrinsic pigments/contaminants are the dominant chromophores because those are different causal routes.
- May not transfer to systems with rigorous oxygen exclusion, industrial scavenger/additive packages tailored to neutralize PET oxidation, or where Sb is locked in an insoluble, non‑accessible phase (e.g., fully encapsulated inert ATO particles), because catalyst accessibility determines catalytic impact.
- Causal pathway summary: Heat and O2 produce radicals in susceptible PET sites, radicals react to form carbonyls/unsaturated conjugates and volatiles (e.g., acetaldehyde), and these chemical products (and any Sb surface transformations that accelerate local depolymerisation) accumulate in the matrix, therefore visible yellowing increases cumulatively.
Engineer Questions
Q: Why does PET show more yellowing after five reheat cycles even though measured antimony (Sb) ppm is unchanged?
A: Because bulk Sb ppm does not report local speciation or surface state; repeated heating can change Sb oxidation state, cause local segregation or precipitation, and create microenvironments that catalyse oxidation and depolymerisation, therefore chromophore formation can increase without a change in total Sb ppm.
Q: Can measuring acetaldehyde or other VOCs predict yellowing after reprocessing?
A: Yes — acetaldehyde and other low‑molecular oxygenated degradation products are mechanistically linked to PET bond scission and often correlate with yellowing, therefore VOC monitoring during thermal cycles is a practical indicator of ongoing chromophore formation.
Q: Would replacing soluble Sb catalysts with ATO particles eliminate yellowing?
A: Not necessarily — ATO particles may reduce soluble Sb availability but surface redox activity or particle-mediated catalysis can still promote degradation if surfaces are accessible, therefore catalyst form and accessibility must be controlled to prevent catalytic pathways.
Q: Which processing controls most effectively reduce cumulative yellowing?
A: Reduce oxygen and moisture exposure, lower peak temperatures or time above critical temperatures, and use effective antioxidants or radical scavengers because these measures limit oxidation and hydrolytic scission pathways; additionally control contamination and Sb state to limit catalytic depolymerisation.
Q: Does adding more antioxidant always prevent yellowing in reprocessed PET containing Sb?
A: Not always — antioxidants intercept radicals and slow oxidation but can be overwhelmed if catalytic depolymerisation dominates or if antioxidants are depleted, therefore antioxidant selection/dose must match the degradation chemistry and processing history.
Q: How should a plant monitor for impending yellowing during reprocessing?
A: Monitor integrated thermal exposure (time × temperature), track VOCs (acetaldehyde), sample color/yellow index periodically, and where feasible analyze Sb surface speciation or leaching, because combined metrics detect chemical precursors and catalytic-state changes that precede visible discoloration.