Intrinsic Viscosity of PET: The Quality Control Parameter

2023-10-08

PET(polyethylene terephthalate) is favored for packaging due to its aesthetic appeal, durability, recyclability, sustainability, and barrier properties. And it's a versatile thermoplastic, commonly used in fibers, textiles, and packaging. A key determinant of PET's performance and processability in extrusion and thermoforming is its Intrinsic Viscosity (IV). IV is directly linked to PET's molar mass, influencing its melting point, crystallinity, and tensile strength. Essentially, longer polymer chains result in more entanglements and a higher viscosity value. Depending on its intended use, PET's desired IV is categorized into various grades.

Understanding the significance of IV requires a brief overview of PET's production. Though many recognize the final form of rigid PET packaging, its creation might be less known.

The need for accurate Intrinsic Viscosity measurements is paramount, given the importance of characterizing PET across multiple sectors:

In Research & Development, where emerging polymers are created and defined.
Among PET suppliers, ensuring top-tier product quality.
In process plants, aiming for consistent production optimization.
And in recycling centers, where monitoring the attributes of recycled PET is essential.

PET is crafted by copolymerizing monomers like Ethylene Glycol and Terephthalic Acid, facilitated by metallic catalysts and under specific conditions. The polymer chain grows in length and viscosity during this polymerization. The process halts at the desired chain length, suited for its final application, and this is gauged using the IV testing method.

IV Range in Different PET Materials and Its Importance

PET Grade	Intrinsic Viscosity [dL/g]
Fiber grade	0.40 – 0.70
Film grade	0.70 – 1.00
Bottle grade	0.70 – 0.78
Water and soft drink bottle grade	0.78 – 0.85

You might've come across the term IV in relation to PET materials. So, what is it, and why does it matter? IV, or Intrinsic Viscosity, is a measure of a polymer's molecular weight. It provides insights into the material’s melting point, crystallinity, and tensile strength.

When the polymerization process is halted, the polymer chain length settles at a level dictated by its IV value. This value is crucial because it's influenced by the polymer chain's length. Longer chains lead to more interactions between these chains, resulting in a higher viscosity and a firmer material. Different applications require PET with specific IV values, as shown in the table above. Thus, IV serves as a pivotal quality control specification and is a prime characteristic of PET.

Dilute Solution Viscometry with Lovis 2000 M

Dilute solution viscometry is a crucial technique in quality control. By diluting polymers in select solvents, one can determine the relative viscosity, which offers insight into numerous parameters, including the Intrinsic Viscosity.

The Lovis 2000 M stands out for polymer property determination using this technique. Key benefits include:

Built-in polymer software for seamless parameter calculations
Minimal sample and solvent usage, reducing exposure to harmful chemicals
Compatibility with sample changers for increased efficiency
Exceptional chemical resistance

This document delves into the Intrinsic Viscosity measurements of PET, commonly used in soft drink bottle production. Details on measurement settings and sample preparation follow.

Intrinsic Viscosity Measurement of PET Raw Materials

The intrinsic viscosity [mL/g] of three distinct PET forms used for plastic bottle production was assessed. The samples and solvents can be found in the following.

Samples and Chemicals Employed

Sample: Polyethylene terephthalate (PET) - three batches.
Solvent 1: Dichloroacetic acid (DCA) - used for polymer dissolution and cleaning.
Solvent 2: Ethanol - used to cleanse the acid before drying.

Batch 1 (PET 1) and 2 (PET 2) were clear granulates measuring around 4 mm, while batch 3 (PET 3) was a white granulate approximately 2 mm in size.

Sample Preparation

Samples were prepared following the ISO 1628-5 standard, which is widely adopted to gauge the viscosity of thermoplastics.

To be specific:

0.250 g of the sample was placed in a 50 mL flask, with its weight precisely recorded (to the nearest 0.1 mg, as per Table 1).
A magnetic stirring bar and roughly 25 mL of DCA were introduced. The flask was then sealed.
The mixture was stirred on a hot plate (set at 100 °C) for 60 minutes. Given some undissolved residues at 60 minutes, the duration extended to 120 minutes.
Once removed from heat, the flask was allowed to cool to room temperature.

Tip: Always ensure that the polymer solution is clear without any residues, as dissolution times may differ based on polymer types and granulate size.

After removing the stirring bar, DCA was added to the flask until a 50 mL volume was achieved, resulting in a concentration of 0.005 g/mL. The flask was then shaken well before analysis.
A solvent blank underwent the same preparation process. Each sample underwent one dissolution.

Tip: Precision during sample preparation is paramount to avoid inaccuracies or repeatability issues.

Measurement Process

Instrumental Configuration The Lovis 2000 M in flow-through mode was utilized for measurements. Here are its key features:

Capillary: 1.8 mm glass
Ball material: Gold-coated steel
O-rings: Kalrez
For increased chemical resistance, a kit with gold-coated balls and Kalrez® O-rings can be obtained.

Method Settings

Measurement mode: Polymer
Temperature: 25 °C
Measurement cycles: 3
Manual angle: 30°
Measuring distance: Long
Variation Coefficient: 0.35 %

The above steps and methods ensure the accurate measurement of PET's intrinsic viscosity for quality bottle production.

Measurement and Analysis

Each polymer solution was tested three times, with thorough cleaning between each test. The parameters of the polymer were derived from the runtimes of the pure solvent and polymer solution. The results for the three PET samples, in terms of relative and intrinsic viscosity, are shown below.

Table 3: Viscosity Analysis of Raw Material and Final Product

Sample	Relative Viscosity [Mean ± 1 σ (RSD %)]	Intrinsic Viscosity [mL/g, Mean ± 1 σ (RSD %)]
PET 1	1.48 ± 0.001 (0.07)	82.1 ± 0.1 (0.12)
PET 2	1.49 ± 0.001 (0.07)	85.5 ± 0.1 (0.12)
PET 3	1.50 ± 0.001 (0.07)	85.0 ± 0.1 (0.12)

For quality assurance, the FW/BW deviation (the difference between forward and backward measurements) and the Variation Coefficient (measuring the consistency between cycles) were evaluated.

Table 4: Lovis Measurement Quality Indicators

Sample	Variation Coefficient [%]	FW/BW Deviation [%]
PET 1	≤ 0.08	≤ 1.37
PET 2	≤ 0.05	≤ 1.37
PET 3	≤ 0.02	≤ 1.37

Note: If FW/BW deviation limits are surpassed, a warning will be displayed. The default limit for the Variation Coefficient is 0.35% for polymer solutions.

Conclusion

The Lovis 2000 M offers an accurate method for grading PET via intrinsic viscosity readings. To enhance the measurement experience, consider adding the Xsample™ 340. This device facilitates automated sample input and cleaning, minimizing user exposure to potential toxins.

Advantages of Monitoring and Regulating IV in PET Processing & Quality

Utilizing a low or unidentified IV grade PET can lead to unfavorable outcomes, such as inconsistent extrusion pressure and melt viscosity. These inconsistencies might manifest as variations in sheet thickness or a brittle end product. Moreover, such discrepancies become more evident during the thermoforming phase, resulting in high variations in the process and increased scrap rates.

PET processing is inherently challenging. Introducing recycled PET (RPET) adds to this complexity. The integration of RPET modifies the IV. Over time, the metallic catalysts from the original polymerization lose their effectiveness, decreasing IV. This is especially pertinent in our sustainability-driven world, where the inclusion of 10% to 100% RPET is becoming standard.

Other factors influencing IV include drying processes and parameters. PET's hygroscopic nature means it easily absorbs moisture and requires thorough drying before extrusion. If inadequately dried PET or RPET is processed, the moisture content can lead to a lower IV product.

In essence, using PET with an inconsistent or unknown IV can be economically disadvantageous. It can negatively impact the bottom line by causing unnecessary waste, additional processing, and increased costs for both the extrusion and thermoforming stages.

Enhancing Quality with Real-time IV Monitoring in Sheet & Rollstock Extrusion Processes

The mechanical properties of PET are directly tied to its molecular weight, known as IV (Intrinsic Viscosity). Higher IV translates to enhanced PET characteristics, while lower IV detracts from its quality. If the IV of extruded PET sheets falls below the desired range, it jeopardizes the end product's structural integrity, causing subsequent issues in thermoforming stages.

Historically, extrusion processors depended on external labs for IV assessments. This often meant waiting several days, if not weeks, for results. Unfortunately, by the time these results were in hand, quality discrepancies were typically already identified, leading to material rejections.

Enter the game-changer: real-time IV monitoring during extrusion. Impact Plastics' advanced PET sheet extrusion system can dynamically track and adjust the IV levels on-the-go. This not only spots processing anomalies swiftly but also initiates immediate corrections, guaranteeing consistent product quality and streamlined production. All results align closely with the ASTM Solution Test Method for IV measurements, boasting an impressive accuracy of +/- 0.02 dl/g.