Polyester chips are a ubiquitous material in beverage and food packaging. From mineral water bottles and carbonated beverage bottles to kitchen oil jugs, PET products, with their lightweight, transparent, durable, and recyclable properties, are used in every aspect of modern life.
However, while these bottles are all made from bottle grade polyester chips, each one is significantly different. Polyester resin used to make these bottles is strictly categorized into different “bottle grades” based on their end use. The differences between these grades reflect a complex set of chemical characteristics, physical properties, and production processes that directly determine the bottles’ safety, functionality, and cost.
First, we need to understand the production process of polyester Chips
Polyethylene terephthalate (PET) chips are produced through the esterification and polycondensation of purified terephthalic acid (PTA) and ethylene glycol (MEG). The production process for different bottle-grade chips varies.
The production of bottle-grade polyester chips consists of two key steps:
Basic chip production. Melt polycondensation produces chips with an intrinsic viscosity (IV) of approximately 0.60-0.65 dl/g. At this point, the chips have a relatively low molecular weight and do not yet fully meet the requirements for blow molding.
Solid-phase polycondensation (SSP). It is the core process in bottle-grade PET chip production. The base chips are heated below their melting point in a vacuum or inert atmosphere to allow the molecular chain ends to react, releasing small molecules, further increasing the molecular weight and intrinsic viscosity (IV). After SSP, the IV value of bottle-grade chips typically rises to 0.70-0.88 dl/g. This significantly enhances melt strength, enabling them to withstand high pressures during blow molding without breaking, and significantly improves the material’s mechanical properties and gas barrier properties.
Introduction of Intrinsic viscosity Value
Intrinsic viscosity (IV) is a key indicator of the molecular weight and chain length of polyester chips. It directly affects the strength, toughness, and processing properties of the final product, and is the primary parameter used to distinguish different bottle grades. If you would like to know more about IV value, maybe this blog can help you.
What are the main differences between different bottle grade polyester chips?
Intrinsic Viscosity (IV) and Molecular Structure
Water bottle grade (IV: 0.70-0.78 dl/g). This grade is primarily used to make bottles for still beverages such as mineral water and purified water. These bottles don’t require high internal pressure, so the IV value of the polyester chips is relatively low. The shorter molecular chains provide excellent processing fluidity and lower energy consumption.
Carbonated beverage bottle grade (IV: 0.78-0.85 dl/g). Bottles for carbonated beverages such as cola and soda need to withstand the pressure generated by internal carbon dioxide (typically 4-6 atmospheres). A higher IV value indicates longer molecular chains and stronger intermolecular forces, resulting in superior creep resistance and burst pressure resistance, ensuring the bottles remain leak-proof and deformable throughout their shelf life.
Hot-fill bottle grade (IV: 0.80-0.88 dl/g or higher). Used for filling products such as tea beverages and juices that require high temperatures above 85℃. High temperatures can intensify the movement of ordinary PET molecular chains, causing bottles to shrink and deform severely. High IV chips not only provide increased strength and rigidity, allowing them to withstand negative pressure during filling without deformation, but more importantly, they are often closely linked to the introduction of comonomers.
Copolymerization ModificationInfluence Main Performance
To meet specific performance requirements, a third comonomer is often introduced during the PET polymerization process to alter the regularity of the molecular chain, thereby adjusting its crystallization behavior and properties.
Homopolymer PET. Synthesized from pure PTA and MEG. It exhibits rapid crystallization and high crystallinity, resulting in bottles with high transparency and sufficient rigidity, but also exhibits high brittleness and poor heat resistance.
Copolymer PET. Isophthalic acid IPA copolymerization. This is the most common and important modification method. Replacing PTA with a small amount of IPA is introduced into the PET backbone, disrupting the regularity of the molecular chain and significantly reducing the crystallization rate and crystallinity.
What Are Core Advantages of IPA Copolymerization
High transparency. During the blow molding process, slow crystallization prevents the material from forming large crystals as the preform cools in the mold, resulting in exceptional transparency and a glass-clear appearance.
Heat resistance. Low crystallinity means the material’s tendency to shrink due to crystallization when heated is greatly reduced. Therefore, hot-fill bottle grade PET chips are almost all IPA-modified co-PET, which can achieve a heat distortion temperature exceeding 80℃, meeting hot-fill requirements.
Cyclohexanedimethanol copolymer (PETG). This type of material is a non-crystalline polyester with exceptional transparency and toughness, making it easy to process and mold. However, it is expensive and is primarily used in high-end cosmetic bottles and medical devices, with limited application in beverage bottles.
Acetaldehyde (AA) Content
Acetaldehyde is a small molecule byproduct produced by the degradation of PET during high-temperature processing. It migrates into the contents, and even very low levels can affect the taste of water and beverages. Therefore, strict control of acetaldehyde content is a key characteristic of bottle-grade polyester chips, especially those intended for water and flavor-sensitive beverages.
Standard grade Acetaldehyde content: Relatively high and may be used for oil bottles or other packaging that is not sensitive to odor.
Low-AA: Through optimized polymerization processes, additives, and SSP conditions, acetaldehyde content is reduced to extremely low levels (typically less than 1 ppm). This is the preferred choice for mineral water, high-end juices, and tea beverages.
Super-Low-AA: Using more advanced technologies, such as post-solid-state polymerization purification, acetaldehyde content is reduced to 0.5 ppm or even lower. It is designed for high-end waters that are extremely sensitive to odor.
Color and Clarity
Ordinary: May have a slight yellow or gray tint due to trace amounts of thermal oxidative degradation or catalyst residue during production.
High-Clarity or Blue-Based Color: Through the use of special catalysts and precise process control, the chips appear watery white or slightly blue. This base color offsets the slight yellowing, giving the final bottle a clearer, purer appearance.
Processing Performance and Stability
Ordinary: The processing window is narrow and it is sensitive to fluctuations in blow molding process parameters, which may result in reduced yield.
High-Performance Grade: Features excellent melt strength and thermal stability, ensuring stable performance during injection molding and blow molding, resisting sag or cracking. It also offers a wide processing window, helping to improve production efficiency and yield.
Additives and Functional Masterbatches
UV absorbers. Used to protect light-sensitive beverages (such as juice and beer) from light degradation, preserving flavor and nutritional value.
Antioxidants. Prevent oxidation and aging during processing and use.
Flame retardants. Used in specialized applications such as electronic product packaging.
What Requirements do Different Bottles have for Polyester Chips
Mineral Water and Purified Water Bottles: Choose standard water bottle grades (homopolymers or low-density copolymers). Low acetaldehyde content and good clarity are key requirements, with less stringent IV and heat resistance requirements.
Carbonated Beverage Bottles: Choose carbonated beverage grades with high IV values (mostly homopolymers or low-density copolymers). High strength and compressive strength are key requirements, with acetaldehyde control being a secondary concern.
Hot-fill bottles: Choose heat-resistant bottle grade PET chips with a high IV value and IPA copolymerization modification. High heat resistance and transparency are key requirements. Bottles are typically designed with claw or spherical shapes to better withstand negative pressure.
Edible oil bottles: High oxygen barrier properties are required to prevent oil oxidation and deterioration. Copolymer PET with improved barrier properties may be used, or a barrier layer may be co-extruded.
Beer bottles: Beer is extremely sensitive to oxygen and carbon dioxide, as well as light. Therefore, specialized chips for beer bottles are typically made from copolymer PET with ultra-high IV values, ultra-low acetaldehyde, and UV-absorbing modifications. Multi-layer lamination or plasma coating is often required to achieve these extremely high barrier requirements.
High-end cosmetic or pharmaceutical bottles: Choose PETG or copolymer PET, pursuing extreme transparency and gloss, as well as the ability to create complex shapes.
What Are Costs of Different Polyester Chip Grades
Higher grades and more specialized properties naturally lead to higher costs.
Raw Material Costs: Comonomers like IPA are typically more expensive than PTA.
Production Process: Longer SSP times (to achieve high IV values), more precise temperature control, and a purer inert gas environment (to reduce acetaldehyde) all result in higher energy consumption and equipment investment.
Technology Costs: Producing stable, uniform, high-end bottle-grade chips requires deep technical expertise and process know-how, which constitutes the core intellectual property cost.
Testing and Quality Control: Rigorous testing for indicators such as acetaldehyde, IV values, and hue requires expensive equipment and manpower.