What is the Difference Between PET and PETE

We aim, with this blog, to give you a detailed primer about the main factor that differentiates the various grades of PET and what the main variable between them changes in the material properties that defines the right application for each class of the polymer.

PET is one of the more important materials in our service repertoire, and it’s important that our clients understand some of the subtleties of the material so we at Srlon can quickly hone-in on the way to meet your packaging needs.

PET is among the highest volume production of polymers. It is the most commonly used material for beverage packaging, with bottled water accounting for 35% of all PET production 

Introducing Polyethylene Terephthalate (PET)

First let’s dive into what exactly PET is. Polyethylene terephthalate is a type of polymer belonging to the polyester group. It offers excellent physical and chemical properties that make it a go-to material for packaging of a wide array of products.

PET is formed by the polymerization of ethylene and terephthalic acid, delivering excellent clarity, high tensile strength and resistance to impact, resistance to acids, bases, and solvents, and it is easily reprocessed into high grade second-life raw material.

It’s widely used for drink and consumer product bottles, food containers, and it can be uniaxially and biaxially oriented (stretched) for packaging films (BOPET). It also makes great fibers for clothing, upholstery, and industrial applications – fleece and woven fabrics of many weights. It is increasingly used in manufacturing automotive parts, electronics, and appliances by injection molding, though this can be challenging because of its sensitivity to processing conditions.

PET offers some big advantages, particularly in single use applications. It is stiff but light, so it reduces transportation costs and energy consumption compared with alternatives like polystyrene and glass, and is as clear as glass. It’s really versatile and easily processed in sheet and extruded forms, well accepted as safe for food contact and easily recycled into good quality material, referred to as rPET (recycled PET).

PET Grades

Polyethylene terephthalate comes in various grades, each tailored for specific applications based on its properties and performance requirements. There are four broad and not tightly defined classes;

• Bottle grade PET, used for manufacturing bottles and containers. It has high clarity, excellent strength, and good barrier properties and is used for beverage bottles (water, soft drinks, juices), food containers, and other packaging needs. It is typically injection blow molded (IBM)、stretch blow molded（SBM） or extrusion blow molded (EBM) to form liquid containers with narrow, often screw necks.  

• Film grade PET, for films that are thin sheets of PET material that can be as-extruded or stretched (oriented) on one or two axes. This material typically possesses good tensile strength, good thermal stability, and excellent barrier properties and it is used for packaging films, magnetic tapes, photographic media, and electrical insulation.

• Fiber grade PET, used in the textile industry. It is characterized by high tensile strength, durability, and resistance to abrasion and used in clothing, upholstery, carpets, industrial fabrics, and non-woven fabrics.

• Engineering grade PET, used as an engineered component material. It has high mechanical strength, thermal stability, and chemical resistance and is used in automotive parts, electrical components, mechanical parts, and consumer goods.

Various grade definitions are used, and these can fit into more than one of the above classes;

• Recycled PET (rPET) has been recycled and reprocessed from sorted and cleaned waste. It has quite varied properties that depend on the source material, reprocessing methods used and additives that it receives. It typically retains the basic properties of generic PET, but rarely performs well in the more taxing applications.

Its uses are the same as for the typical material when new, but it is often used to emphasize sustainability, rather than for specific property benefits. The most extensive use for rPET is in fiber production.

• PET glycol-modified (PETG) has the glycol added as a copolymer, to improve specific properties – clarity, impact resistance, and ease of processing. In particular, it has a lower melting point than standard PET, making it easier to process as thermoformed products, medical disposable devices, packaging, and signage.

• Amorphous PET (aPET) has an essentially non-crystalline structure. This delivers higher transparency and gloss, better impact resistance, and easy thermoformability due to the lower softening temperature. It is used in general packaging, especially clamshells, blisters, and trays.

• Crystalline PET (cPET) has increased crystallinity, for improved thermal resistance. It is opaque, has higher thermal stability and is used for oven-ready food trays, microwaveable containers, and other moderately heat-resistant packaging.

• Extrusion PET (PETe or PETE) is a group of grades that have been modified to improve their intrinsic viscosity (IV) which renders the material better suited to extrusion processes, as the IV defines the stiffness of the formable, softened material and its lower risk of sag/tearing during extrusion.

What is the Difference Between PET and PETE Grades?

PET is a long chain polymer, and grades consist of linear repeating groups of ethylene terephthalate, of varied molecular weights.

This shows the monomer that repeats to make the PET chain.

The biggest differentiator of properties in PET/PETE is the total length of the repeating chains that together make up the polymer – which defines the molecular weight. The longer the chains, generally the better the quality of the polymer. That doesn’t mean that short chain materials are not useful – for many applications, short chains are quite acceptable.

High intrinsic viscosity (IV) is a crucial property of PETE that quantifies the polymer grade’s high molecular weight and informs us about the mechanical and physical properties of the polymer, when processed. 

The intrinsic viscosity is derived by measuring the polymer’s viscosity in a standardized test in which the polymer is dissolved in a specific solvent and then tested with a viscometer.

It is an indicator of the polymer’s average molecular weight, which equates to the polymer chain length.

Intrinsic Viscosity (IV) is a shear rate in deciliters per gram (dL/g) and this equates to a number-average molecular weight (Mn) that allows comparison of grades and informs the manufacturer about the application suitability of a material sample.

How Does this Inform Manufacturers?

A high IV value indicates a high molecular weight, which correlates with stronger mechanical properties, such as tensile strength, and creep and impact resistance. This variation presents as a higher melt viscosity in processing – and a stronger softened state in extrusion. Higher IV values indicate a tougher material that is suited to higher stress applications.

This is typified by the preference for higher IV value grades for applications like soda bottles, as they must withstand larger forces long term, due to the dissolved Carbon dioxide pressure of the bottle contents. Lower IV value materials are a little easier to blow mold, but will be susceptible to more stretch in longer term storage of pressurized beverages.

Typically, higher IV values result in improved barrier properties as the longer and more tangled chains present a more resilient leakage path for various molecules.

Intrinsic viscosity is typically measured using an Ubbelohde viscometer. The polymer is dissolved in a mixture of phenol and 1,1,2,2-tetrachloroethane, and the flow time of the solution through the viscometer is compared to that of the pure solvent, from which an IV measure is derived to allow comparison with other grades.

Application Relevance

Packaging:

• Bottles – PET with slightly higher IV (PETE) is used for carbonated drink bottles, benefitting from its strength and resistance to gas permeability. Higher IV values suit EBM bottle manufacture. The greater resilience in the softened state makes a better quality parison (extrudedinjection preform) and typically requires greater blow molding pressure.
• Films – for packaging films, a balance of IV is sought to ensure flexibility and adequate barrier properties. Material for thermoforming and biaxial orientation demands higher IV values to give better toughness, transparency and formability.

Textiles:

• Fibers – PETE with higher IV values is used to produce strong and durable textile fibers for clothing, upholstery, and industrial fabrics. Lower grade stock is used for less durable fibers and non woven fabrics such as fleece.

Engineering plastics:

• Mechanical Parts – high IV PETE is used in the manufacture of engineering plastics that require high mechanical strength and stability.
• IBM bottle preforms – these sit between engineering and packaging, requiring the precision of injection molding but the ease of blow molding. Grades/IV levels are selected by application, but higher IV materials pose additional difficulties in injection molding.

Recycling:

• rPET – the IV of recycled PET (rPET) is a critical factor determining its suitability for various applications. Various processes and additives can be used to modify the IV or rPET to make it suitable for higher grade applications.

Overall, the original polymerization process is responsible for the typical chain length that is produced, and manufacturers can control this to a high degree. All processing can degrade the polymer by fracturing chains and altering the molecular weight – and recycled PET (rPET) is quite variable in this, having been through potentially more than one recycling process.

Additives can be introduced that join shorter chains into longer lengths, often in a heat activated and re-extrusion process. This can counter some of the IV/molecular weight loss that the polymer suffers. This can revive the properties of recycled and lower grade materials to deliver higher quality outcomes.

Summary

Extrusion of PET is easier with higher molecular weight (or higher intrinsic viscosity, IV), and this is the primary difference between basic PET and extrusion adapted material or PETE

PETE offers improved melt strength, which is the ability of the molten polymer to resist stretching and deformation under its own weight. This property is crucial during the extrusion process, where the material needs to maintain its shape and uniformity as it is pushed through the die. Lower molecular weight PET has lower melt strength, making it more prone to sagging, tearing, or locally deforming during extrusion, resulting in defects in the final product.

PETE extrusion results in enhanced process stability, maintaining a more consistent flow rate during extrusion for more uniform thickness outcomes. Low molecular weight PET is liable to present fluctuations in flow rate, leading to inconsistencies and variations in the thickness and quality of the extruded material.

Improved thermal stability results from higher molecular weight, meaning the material can withstand higher processing temperatures without degrading. This is important in extrusion, which is typically performed at relatively high temperatures. Low molecular weight PET is more prone to degrade in extrusion, leading to discoloration, poor mechanical properties, and quality issues.

Higher output rates are feasible, due to PETEs improved process stability and melt strength, making the extrusion process more time-efficient and cost-effective.

Conclusion

It’s important to understand the application specifics of the grades of PET available and the production/performance differentials that they offer. We offer our summary to help you understand the range of options and their applications.

At Srlon, we have huge experience in selection and processing of PET grades and we can help you to navigate the difficulties that might otherwise disrupt your progress in specification and production of the packaging you need for your products.