AquaGlass-Flex: Biodegradable Internal PET Barriers for Water, Carbonated Drinks and Juices

AquaGlass-Flex is a proposed internal barrier-coating platform for PET beverage bottles designed around a simple principle: the beverage should contact an ultrathin, safe and flexible biodegradable barrier rather than the PET wall itself.

The primary development objective is to reduce the release of PET-derived micro- and nanoparticles into beverages by at least one order of magnitude, while also reducing migration of low-molecular-weight PET-related substances. These performance values are development targets and will require independent laboratory validation.

AquaGlass-Flex is being developed in two versions.

1. AquaGlass-Flex W-LITE – Low-Cost Still-Water Version

W-LITE is intended for mass-market still drinking water, spring water and non-carbonated mineral water.

Its objective is minimum coating cost and minimum modification of existing bottling equipment.

The initial candidate is an ultrathin, single-layer bio-based coating based principally on zein and purified shellac, plasticised with food-contact-compatible components such as triethyl citrate and a small hydrophobic fraction.

A representative starting dry formulation is:

- Zein: approximately 70 wt.%
- Purified shellac: approximately 20 wt.%
- Triethyl citrate: approximately 8 wt.%
- Oleic fraction: approximately 1.5 wt.%
- Lecithin/process aid: approximately 0.5 wt.%

The initial target dry-film thickness is approximately 0.35–0.65 micrometres.

W-LITE is intended for still waters over a practical mineral and pH range, including calcium/magnesium-rich, bicarbonate, sulfate and chloride mineral waters and waters containing naturally occurring trace mineral constituents.

It is not intended initially for highly acidic or carbonated beverages.

2. AquaGlass-Flex U-BEV – Universal Beverage Version

U-BEV is the higher-performance version intended for carbonated drinking and mineral water, fruit juices, cola-type carbonated soft drinks and other acidic non-alcoholic beverages.

The proposed architecture consists of:

- the PET bottle substrate;
- an ultrathin adhesion/primer layer;
- a flexible biodegradable PHA-based beverage-contact layer.

The principal PHA candidate is P(3HB-co-4HB), with approximately 6–10% 4HB investigated to obtain an appropriate balance between barrier performance and flexibility.

A representative contact-layer formulation is approximately 94–97 wt.% P(3HB-co-4HB) and 3–6 wt.% triethyl citrate.

The initial total dry coating thickness target is approximately 1.15–1.75 micrometres.

Unlike a brittle glass coating, U-BEV is specifically intended to remain continuous while the PET bottle is squeezed, dented, twisted, pressurised by carbonation and subjected to transportation and temperature cycles.

The qualification programme will include carbonated-water conditions, low-pH environments, citric, malic and phosphoric-acid systems, sugar-containing and sugar-free beverage formulations, fruit-juice components, flavour systems and representative cola-type beverage chemistry.

3. Proposed Inline Application Technology

AquaGlass-Flex is intended to be applied inside an already formed PET bottle shortly before filling.

The proposed industrial sequence is:

- internal surface cleaning or sterile air preparation;
- optional short plasma/corona activation where required for adhesion;
- insertion of a thin coating lance through the bottle neck;
- 360-degree internal micro-spray;
- controlled bottle rotation to achieve uniform wall, shoulder and base coverage;
- rapid film formation using filtered warm sterile air and, where beneficial, near-infrared energy;
- sterile cooling;
- immediate transfer to the filling stage.

W-LITE is designed to use one coating pass and, if adhesion permits, to avoid plasma treatment in order to minimise equipment and operating cost.

U-BEV may use a primer plus a second PHA barrier pass and short plasma activation because of its more demanding mechanical, chemical and carbonation requirements.

For large-scale production the process is envisioned as a multi-position carousel rather than a single bottle waiting through the complete drying cycle.

4. Environmental End-of-Life Requirement

A critical feature of AquaGlass-Flex is the deliberate separation between in-use stability and environmental persistence.

During the bottle's intended shelf life, the coating must remain:

- water-insoluble;
- non-tacky;
- mechanically continuous;
- resistant to normal bottle deformation;
- chemically stable in the intended beverage;
- compatible with applicable food-contact requirements.

However, after disposal, the coating is specifically designed NOT to behave as another persistent plastic contaminant.

Once the coating becomes environmentally exposed through bottle crushing, fragmentation, recycling operations or environmental weathering, the development target is relatively rapid biodegradation under combinations of moisture, oxygen, sunlight/UV exposure and naturally occurring microorganisms.

The intended end state is biological degradation without persistent coating-derived microplastics, persistent toxic residues or deliberately added environmentally persistent nanoparticles.

The biodegradation rate will be experimentally determined using appropriate recognised biodegradation and ecotoxicity methods. No specific environmental degradation time is claimed before testing.

5. Mineral-Water Compatibility

AquaGlass-Flex will be tested with waters containing representative combinations of:

- calcium;
- magnesium;
- sodium and potassium;
- bicarbonate;
- sulfate;
- chloride;
- fluoride;
- dissolved silicate species;
- iron;
- manganese;
- dissolved carbon dioxide;
- oxidised and reduced sulfur species;
- different levels of total mineralisation;
- acidic, neutral and alkaline pH conditions.

An important acceptance criterion is preservation of the characteristic mineral composition of the bottled water. The coating must not act as an uncontrolled ion exchanger or adsorbent.

Trace-metal concentrations, conductivity, pH, alkalinity and principal ion concentrations will therefore be measured before and after storage.

Radon-containing natural waters are considered a separate qualification case. Since an improved polymer barrier could potentially alter radon transfer between water, headspace and the bottle wall, coated PET must be compared with uncoated PET and glass reference containers using an appropriately equipped laboratory.

6. Mechanical Qualification

The development programme includes repeated squeezing, indentation, torsion, transportation vibration, drop-related deformation, thermal cycling and, for U-BEV, internal carbonation pressure.

The coating may elastically deform and locally change thickness, but it must not develop a connected network of through-thickness cracks or delaminate from the PET substrate.

7. Development Targets

The principal engineering targets are:

- at least 10-fold reduction in PET micro/nanoplastic release compared with equivalent uncoated PET after defined ageing;
- substantial reduction, with an initial target of at least 90%, in selected PET-related migrant flux where technically achievable;
- no unacceptable coating-derived migration;
- no unacceptable taste, odour, colour or appearance change;
- preservation of the mineral profile of natural mineral waters;
- coating integrity after realistic bottle deformation;
- compatibility with intended acidic and carbonated beverages for U-BEV;
- scalable inline application shortly before filling;
- relatively rapid environmental biodegradation after disposal and environmental exposure.

All numerical performance values stated above are development objectives, not claims of already completed laboratory validation.

8. Material-Safety Philosophy

Initial AquaGlass-Flex development deliberately avoids PFAS, BPA-based chemistry, aromatic isocyanates, formaldehyde crosslinkers, intentionally added free metallic nanoparticles, TiO2/ZnO nanoparticles and other persistent nanoparticle fillers.

Final formulations will be selected only after food-contact migration, toxicological, sensory, mechanical, biodegradation and environmental compatibility testing.

9. Development Stage

The technology is currently at concept and experimental-design stage, approximately TRL 2.

The next development steps are formulation screening, coated-bottle prototype manufacture, accelerated mechanical ageing, micro/nanoplastic analysis, chemical migration testing, beverage compatibility testing, environmental biodegradation testing and subsequent pilot-line validation.