I.V. levels in the recycled PET (rPET)

In the specification of the Polyethylene Terephthalate (PET), Intrinsic Viscosity (I.V.) is a measure of the polymer’s molecular weight and thus, an indicator of its physical properties and processing characteristics. The I.V. value of PET can affect its strength, toughness, clarity, and how it processes in various applications such as fiber, bottle, or film production and typically it is expressed in deciliters per gram (dl/g).

High I.V. values indicate longer polymer chains, which are desirable for applications requiring material strength and durability, such as bottles and fibers. Conversely, lower I.V. values might be suitable for less demanding applications but could indicate degradation of the polymer, which is a critical consideration for recycling processes.

The intrinsic viscosity range of PET:
Fiber grade:
0.40–0.70 Textile
0.72–0.98 Technical, tire cord
Film grade:
0.60–0.70 BoPET (biaxially oriented PET film)
0.70–1.00 Sheet grade for thermoforming
Bottle grade:
0.70–0.78 Water bottles (flat)
0.78–0.85 Carbonated soft drink grade
1.00 and above Monofilament, engineering plastic

Here’s a general guideline for categorizing I.V. levels in PET:

Low I.V. Levels

• 0.60 – 0.72 dl/g: These are considered low I.V. levels for PET. Materials in this range are typically used for applications that do not require high strength or durability, such as some packaging films and fibers. PET with low I.V. is easier to process due to its lower viscosity, making it suitable for processes where easier flow is beneficial.

Medium to High I.V. Levels

• 0.72 – 0.84 dl/g: This range is more common for bottle-grade PET, where higher strength and toughness are needed for carbonated beverage bottles and other packaging applications that must withstand pressure or provide a robust physical barrier.
• 0.85 dl/g and above: High I.V. levels are used for applications requiring the highest strength and durability, such as engineering-grade PET, certain specialty bottles, or materials subjected to high mechanical or thermal stress.

In the recycled PET offers, granulates usually have 0.74 to 0.80 dl/g for a good quality materials.

Intrinsic Viscosity (I.V.) of Polyethylene Terephthalate (PET) can be tested using various international and industry standards with ASTM, ISO and GB/T frameworks leading the way.

All three standards rely on the principle of measuring the flow time of a polymer solution through a viscometer, but the specifics of the solvent used, sample preparation, and calculation methods can vary. GBT 14190-2008 and ASTM D4603 are more specifically focused on PET, whereas ISO 1628 covers a wider range of polymers. GBT 14190-2008 is primarily used within China, ISO 1628 has broad international application across various polymers, and ASTM D4603 is widely used in the U.S. and by industries and researchers needing to adhere to ASTM standards.

These standards provide methodologies for accurately measuring the I.V., which reflects the polymer’s molecular weight and related properties.

1. ASTM D4603: This standard specifies the method for determining the I.V. of PET using a solution viscometer. It is widely used in the plastics industry, especially for materials intended for packaging and bottle manufacturing.
2. ISO 1628: This series of standards, particularly parts relevant to PET, detail procedures for determining the viscosity of polymers in dilute solution using capillary viscometers. It’s applicable to a broad range of polymers and includes specific conditions and solvents for PET.
   • DIN 53728 (old, withdrewn): This standard was used primarily in Europe and covered the measurement of the viscosity number of plastics by dilute solution viscometry, similar to the methods outlined in ASTM D4603 and ISO 1628. It has been replaced by DIN-EN-ISO-1628-5 and was officially withdrawn in 2015​​. DIN-EN-ISO-1628-5 is the part of the ISO 1628
3. GB/T 14190-2008: It’s a national Chinese standard that outlines methods for determining the I.V. of PET, which is crucial for assessing the material’s quality for fiber, bottle, and film applications. GB/T 14190-2008 is specifically tailored for PET and is widely recognized within the Chinese plastics industry, particularly for applications related to PET manufacturing, recycling, and quality control. However, for PET and related applications within China, GB/T 14190-2008 would be the go-to standard due to its direct relevance and the specificity of its methodology for PET’s intrinsic viscosity measurement.

What are the most common issues caused by unsuitable I.V. levels?

Issues Caused by Low I.V.

• Poor Mechanical Strength: Products made from low I.V. PET may not have sufficient strength for their intended use, leading to issues like breakage or deformation under normal use conditions.
  • Reduced Barrier Properties: Lower I.V. can result in a decreased barrier against gases (e.g., carbon dioxide, oxygen) and moisture, which is particularly problematic for packaging applications requiring tight control over contents’ shelf-life.
  • Processing – Difficulties: Although lower I.V. PET is easier to process due to its lower viscosity, it can result in products with inconsistent properties, especially if the material’s flow characteristics are not well-matched to the processing equipment and parameters.

Issues Caused by High I.V.

• Processing Challenges: High I.V. PET has higher viscosity, making it more difficult to process. It requires higher temperatures and/or longer times to achieve proper melt flow, increasing energy consumption and the risk of degradation.
  • Increased Costs: Materials with a higher I.V. are often more expensive, not just due to the raw material costs but also because of the increased processing requirements.
  • Processing – Risk of Degradation: Processing high I.V. PET at the required conditions to achieve good flow can lead to thermal degradation, resulting in discoloration and a reduction in physical properties.

General Issues

• Variability in Product Performance: Variations in I.V. can lead to inconsistencies in product performance, affecting everything from tensile strength and elongation to clarity and color. This can result in a product that fails to meet specifications or customer expectations.
• Recycling Challenges: PET recycling processes can be sensitive to the I.V. of the input material. Variability in I.V. can affect the quality of the recycled PET, potentially requiring additional processing steps to adjust the I.V. to suitable levels for specific applications.

Maintaining the appropriate I.V. level is crucial for ensuring that PET and other polymers meet the required specifications for their intended application, both in terms of processing efficiency and the quality of the final product.

Processing and Recycling

• Processing Consistency: The I.V. affects the processing behavior of PET, including extrusion and molding conditions. Consistent I.V. levels are necessary to produce food contact materials with uniform properties and to meet stringent quality and safety standards.
• Recycling and Sustainability: PET’s suitability for recycling is also influenced by its I.V., as variations can affect the quality of recycled material. For food contact applications, maintaining the I.V. within specific ranges during recycling processes is crucial to ensure the recycled PET meets safety standards for direct food contact.

Barrier Properties

• Enhanced Barrier Performance: Higher I.V. PET typically exhibits better barrier properties against gases (e.g., oxygen, carbon dioxide) and moisture. This is crucial for maintaining the freshness, taste, and shelf-life of food and beverages. Packaging with inadequate barrier properties can lead to product spoilage or quality degradation.

Gas Barrier

• Oxygen and Carbon Dioxide Transmission: Effective gas barriers prevent oxygen from entering the package, which can lead to food spoilage, oxidation, and loss of flavor. They also retain carbon dioxide in carbonated beverages, maintaining their fizziness and preventing them from going flat.
• Selective Permeability: Some packaging applications may require selective permeability, allowing specific gases to pass through while blocking others. This can be important for packaging of fresh produce, where the exchange of gases is necessary for respiration.

Moisture Barrier

• Moisture Transmission Rate (MVTR): The barrier against moisture is critical to prevent the ingress of water vapor that can lead to spoilage, changes in texture, or microbial growth in food products. Conversely, it also prevents the drying out of moist products.
• Hydrophobic Materials: Materials like PET have hydrophobic properties that contribute to their effectiveness as moisture barriers, essential for maintaining the intended moisture content of food products.

Chemical Barrier

• Contaminant Prevention: A strong chemical barrier is necessary to prevent the migration of harmful substances into the food and the loss of food additives or flavors out of the package. This includes preventing the transfer of low molecular weight compounds, solvents, or residues from the packaging material itself.
• Regulatory Compliance: Materials used for food contact must comply with regulations that limit the migration of specific substances to safe levels, ensuring that the packaging does not adversely affect food safety or quality.

Enhancing Barrier Properties

• Material Selection and Blending: The intrinsic properties of PET can be enhanced through copolymerization or blending with other materials to improve its barrier characteristics against gases and moisture.
• Coatings and Layers: Applying thin coatings or creating multi-layer structures with materials having complementary barrier properties can significantly enhance the overall barrier performance of PET packaging.
• Nanocomposites and Additives: Incorporating nanocomposites or specific additives into PET can improve its gas and moisture barrier properties, making it suitable for a wider range of food packaging applications.

Improving I.V. for the recycled rPET

Improving the Intrinsic Viscosity (I.V.) of recycled PET (rPET) is crucial for enhancing its performance and expanding its applicability in various products, including fibers, bottles, and packaging. Several strategies can be employed to increase the I.V. of rPET, addressing the degradation that often occurs during the recycling process:

The five strategies for improving the Intrinsic Viscosity of recycled PET:

1. Solid State Polymerization (SSP)

• Process: SSP is a post-extrusion process where the rPET, in the form of pellets or flakes, is heated under vacuum or in a nitrogen atmosphere at temperatures below its melting point. This process promotes polymer chain lengthening through condensation reactions, effectively increasing the I.V.
• Benefits: SSP can significantly enhance the I.V. without substantial degradation of the polymer, improving the mechanical and thermal properties of the rPET.

2. Chemical Modification

• Additives and Chain Extenders: The addition of chain extenders or branching agents during the extrusion process can help rebuild molecular weight through reactions with functional groups on the polymer chains. Compounds such as multifunctional epoxies, anhydrides, or isocyanates are commonly used.
• Glycolysis/Alcoholysis: In this chemical recycling process, rPET is depolymerized in the presence of glycols or alcohols, then repolymerized to achieve higher I.V. values. This approach allows for more precise control over polymer chain length.

3. Blending

• Mixing with Virgin PET: Blending rPET with virgin PET with a higher I.V. can increase the overall I.V. of the blend, making it suitable for applications requiring higher performance. This method is straightforward but depends on the availability and cost of virgin PET.

4. Optimization of Recycling Processes

• Minimizing Thermal Degradation: Optimizing the mechanical and chemical recycling processes to minimize exposure to high temperatures and mechanical shear can reduce the degree of polymer degradation, preserving the I.V. of rPET.
• Decontamination and Pre-Treatment: Effective cleaning and pre-treatment of PET waste before processing can remove impurities and contaminants that catalyze degradation, helping to maintain higher I.V. levels.

5. Use of Catalysts

• Transesterification Catalysts: In chemical recycling processes, catalysts can be used to promote the repolymerization of PET, leading to higher I.V. products. These catalysts can increase the efficiency of the polymerization reaction, reducing the time and energy required.

Improving the I.V. of rPET enhances its physical properties, making it more suitable for high-value applications. Each of these strategies has its advantages and limitations, and the choice of method depends on factors such as the intended application of the rPET, the available processing infrastructure, and cost considerations.

https://www.aquafileng.com/en/polymer-plants/polyester/recycling/

https://www.gneuss.com/en/polymer-technologies/polyreaction/polyreactor/

https://ngr-world.com/applications/pet-improvement/

General Process for Measuring I.V. in PET Flakes:

1. Sample Preparation: PET flakes are thoroughly cleaned and dried to remove any contaminants that might affect the measurement. The flakes are then dissolved in a chemical solvent, such as dichloroacetic acid or a mixture of phenol and 1,1,2,2-tetrachloroethane, at a controlled temperature to create a homogeneous solution.
2. Viscosity Measurement: The solution’s viscosity is measured using a viscometer. There are various types of viscometers, but a capillary viscometer is commonly used for I.V. measurements. The time it takes for the solution to flow through a capillary tube of known dimensions is recorded.
3. Calculation: The I.V. is calculated based on the flow time of the polymer solution relative to the flow time of the pure solvent, along with the concentration of the polymer in the solution. The calculations adjust for the effects of temperature and concentration to yield the I.V. value, typically expressed in deciliters per gram (dl/g).

Flakes, especially those obtained from post-consumer recycled material, may present more variability in terms of contamination and polymer degradation. This variability can influence the I.V. measurement, necessitating a more rigorous cleaning and selection process compared to granules.

Granules (or pellets): Are usually cleaner than recycled flakes, as they might be produced directly from (1) virgin PET resin or (2) from a more controlled recycling process. They are more uniform in size and shape, which can lead to a more consistent dissolution process, which can lead to more consistent I.V. readings.

rPET based fibres

The Intrinsic Viscosity (I.V.) of recycled Polyethylene Terephthalate (rPET) plays a crucial role in the production of rPET fibers, impacting both the processing behavior and the properties of the final fiber product. Here’s how I.V. influences rPET fiber production:

1. Processing Performance

• Spinning Efficiency: Higher I.V. rPET typically exhibits better spinning performance, allowing for smoother processing and higher production speeds. Lower I.V. materials may require modifications in processing parameters or even result in increased breakage rates during spinning.
• Melt Stability: The molecular weight indicated by I.V. affects the melt stability of the rPET during the fiber extrusion process. A higher I.V. suggests a more stable melt, which is less prone to degradation under the conditions required for fiber spinning.

2. Fiber Properties

• Strength and Durability: The mechanical properties of the fibers, such as tensile strength and elongation at break, are directly influenced by the I.V. of the rPET. Higher I.V. materials typically produce stronger, more durable fibers that are suitable for applications requiring high performance.
• Thermal Properties: The thermal stability and shrinkage behavior of rPET fibers are also affected by I.V. High I.V. rPET tends to result in fibers with better thermal properties, which are important for applications involving heat exposure.
• Dyeability and Other Functional Properties: The I.V. can influence the fiber’s surface characteristics and porosity, affecting its ability to be dyed and finished with various functional treatments. Higher I.V. fibers may offer better dye uptake and more consistent coloration.

3. Application Suitability

• Textile Quality: The quality of textiles made from rPET fibers, including aspects like hand feel, drape, and appearance, can be influenced by the I.V. High I.V. rPET is often preferred for premium textile applications where these quality aspects are critical.
• Versatility: The range of applications for which rPET fibers can be used is broadened with optimal I.V. levels. While high I.V. rPET is sought after for applications requiring robust mechanical properties, lower I.V. rPET may still be suitable for nonwoven or other applications where such demands are less critical.

The variation in I.V. can result from the recycling process itself, as degradation can occur during mechanical and chemical recycling steps. Therefore, controlling the I.V. is a critical aspect of producing high-quality rPET fibers.

Sources:

1. “Effect of recycled content and rPET quality on the properties of PET bottles” – This paper discusses the impact of incorporating recycled PET content on the intrinsic viscosity and overall properties of PET bottles, providing valuable insights into how rPET quality affects product performance. This research could be particularly useful for understanding the balance between sustainability and material properties in packaging applications.
   • Link to paper
2. “Improving the Rheological and Mechanical Properties of Recycled PET” – This study explores methods for enhancing the intrinsic viscosity of rPET, which is crucial for its mechanical performance and processability. The paper likely covers chemical modifications, additives, or processing techniques designed to improve the quality of rPET for various applications.
   • Link to paper
3. “Rheology of Recycled PET – PMC – National Center for Biotechnology Information” – Focusing on the rheological properties of rPET, including intrinsic viscosity measurements, this research provides a comprehensive look at how recycling processes affect PET’s molecular weight and viscosity, and by extension, its processing and application potential.
   • Link to paper
   • “Improving the Rheological and Mechanical Properties of Recycled PET”
4. “Rheology of Recycled PET” MDPI (Polymers) – This study explores the rheological behavior of recycled PET, providing insights into how recycling processes affect the material’s viscosity and flow properties. Understanding the rheological properties is essential for optimizing processing conditions and improving the quality of products made from rPET. Rheology of Recycled PET
5. “Recent Studies on Recycled PET Fibers: Production and Applications” SpringerLink – This paper reviews recent advancements in the production and application of fibers made from recycled PET. It covers the impact of recycling on the I.V. of PET fibers and discusses technologies and treatments to maintain or enhance the I.V. during recycling. Recent Studies on Recycled PET Fibers: Production and Applications