Ideal for the injection molding process

How the properties of rPET can be enhanced in a targeted manner

The Kunststoff-Zentrum in Leipzig gGmbH (KUZ) is using BYK’s newly developed additive SCONA TPPET 4214 PA in reactive compounding to enhance the properties of post-consumer rPET. This step ensures that technical lightweight construction components are processed safely.

How is non-recyclable rPET put to use?

Examples of post-consumer waste made from polyethylene terephthalate (PET) include clear packaging films, thermoformed trays, presentation packaging, blister box inserts, and other packaging. These products are collected from consumers using the Dual System recycling scheme and are sent from there to large-scale processing facilities. The material that is recovered from this process is a polyethylene terephthalate recyclate (rPET) and takes the form of a millbase (flakes). These flakes may still contain small quantities of contaminants due having come into contact with foodstuffs and other household waste. When combined with factors such as thermo-mechanical and hydrolytic decomposition reactions, this rPET cannot be reintroduced into bottle production processes. This also applies to remnants of discarded single-use and reusable bottles, which are no longer suitable for closed recycling loops due to their low-quality properties.

In order to use this type of rPET in injection molding processes, especially foam injection molding, it is necessary to harmonize nonuniform rheological properties that result in insufficient melt strength, an inhomogeneous foam structure, and mechanical and visual flaws, by “upcycling” the virgin material. Reactive compounding using SCONA TPPET 4214 PA from BYK can be used to achieve this.

Special features and benefits of SCONA TPPET 4214 PA

BYK is renowned for its broad range of products based on functionalized polymers, which are sold under the brand name SCONA. These are mostly non-polar polymers that are modified with additional functional monomers via chemical grafting. Typically, unsaturated monomers such as maleic anhydride (MSA), 2,3-epoxypropyl methacrylate (GMA), and methacrylic anhydride (AA), which contain additional functional groups, are grafted onto the polymer using a radical addition mechanism. This method creates a wide range of functional polymers with a wide variety of different properties. The length of the polymer and the quantity of grafted monomers can also be controlled during the reaction. The product used here is SCONA TPPET 4214 PA (see Figure 2), a new high-performance polymer modifier based on a polyethylene terephthalate (PET) functionalized with GMA.

Due to its reactive epoxide groups, the additive works as a viscosity enhancer in polycondensation polymers. The higher molecular weight also increases viscosity during incorporation into the base polymer. In this way, excellent processability is achieved during extrusion, thermoforming, and even foam injection molding. Because the additive is PET-based, it is especially suitable for use in PET-based compounds. Compared to conventional chain extenders such as pyromellitic dianhydride (PMDA) (see Figure 3) or tetraglycidyl-diaminodiphenylmethane (TGDDM) (see Figure 4), the product has been developed by BYK to have a significantly higher molecular weight. In addition to the SCONA TPPET 4214 PA used here, BYK also offers products of this type that are grafted with MSA or AA.

Compounding – cost-effective configuration and parameters

Reactive processes such as these are typically used in polymer manufacturing, but can also be used for targeted polymer modification in the fields of compounding and material modification. The incorporation of SCONA TPPET 4214 PA is carried out at KUZ using a (ZE) ZE25Ax47D-UTXi-UG co-rotating twin screw extruder (Æ of screw: 25 mm, L/D: 47, L: 1175 mm) from KraussMaffei Extrusion GmbH.

The PET flakes are dried for 4–6 hours at 120 °C in a dry-air dryer to remove any remaining water. The screw mandrels are only equipped with conveyor elements to ensure that the polymers are subjected to minimal thermo-mechanical stress, preventing damage to the materials during preparation (see Figure 5). In comparison, a preliminary test of the effectiveness of a different screw configuration with conveying kneading blocks and a back-pressure element upstream from the degassing unit demonstrated a negative impact on the quality of the properties.

The dosage of the SCONA TPPET 4214 PA additive is 10 % and it is dosed gravimetrically via the main feed using a FlexWall DDW-M-F40 feeder from Brabender GmbH & Co. KG. The seven heating zones are at a temperature of 260 °C, while the temperature at the feed point is 40 °C. The granulate is manufactured via an EWA10 underwater granulator from Econ GmbH using a single-jet injection nozzle plate. The blade rotation speed of the underwater granulator is 2400 rpm, and the temperature of the water tank is 16 °C.

Reactive compounding keeps properties under control

The addition of SCONA TPPET 4214 PA results in a significant increase in melt viscosity throughout the compounding line. In order to obtain polymer materials for viscosity measurements, the filled screws can be removed from the processing section using an electric motor (the ZE’s UltraGlide function).

Info box

IV value

The polymer chain length and molecular weight of the material is expressed in the PET processing industry as the intrinsic viscosity (IV) in deciliters per gram (dl/g). The IV value is the decisive quality criterion for monitoring incoming goods and production processes. The level and consistency of IV allows quality control to reach important conclusions regarding processability, crystallinity, and mechanical properties. For large-scale bottle production, for example, an IV value of 0.8 dl/g is considered the benchmark for processing.

This four-fold sample collection is obtained by starting at the feed and working backward at a spacing of approx. 9.5D. The IV feed value of the rPET flakes is 0.64 dl/g and increases along the compounding line to 0.79 dl/g (see Figure 6). Figure 7 shows the effect of reactive compounding on the intrinsic viscosity (IV) of both pure flakes and flakes that have been compounded using the additive, in comparison to the virgin material. Both the level and dispersion of the IV value are harmonized in the new product.

The characteristic value for the mechanical properties is the impact strength, which is determined via a Charpy impact test in accordance with DIN EN ISO 179. For this purpose, the multipurpose specimen is injected and tested in accordance with DIN EN ISO 3167, type A. Figure 8 shows that the low level of impact strength in pure flakes can be harmonized to the level of virgin material using reactive compounding.

Improved foam structure in lightweight construction components

Thermo-mechanical and hydrolytic decomposition reactions in rPET flakes not only have a negative impact on mechanical properties. A reduction in the IV value causes nonuniform rheological properties in the melt, which in turn results in insufficient melt strength. In the injection molding process, and especially in foam injection molding, this has an extremely negative effect. For example, insufficient foam formation with large cavities or a reduced number of air bubbles due to bubble coagulation, delamination, an uneven, blistered surface, and yellowing as a result of degradation mechanisms may occur (see Figure 9).

Reactive compounding with SCONA TPPET 4214 PA results in harmonized IV values and improved melt strength, which prevents the cells from collapsing or joining together. The foamed components that result from the prepared compound demonstrate an enhanced foam structure and a smooth surface without yellowing. The appearance is significantly improved. It creates a fine-cell foam, as can be seen in the enlarged section of an REM image (see Figure 9 below). The bubble size is approx. 100 µm.

This means that non-recyclable rPET can be processed into technical lightweight construction components via an injection molding or foam injection molding process.

Free product samples can be ordered here: SCONA TPPET 4214 PA

Get to know more about SCONA

Authors

Annerose Hüttl (Kunststoff-Zentrum in Leipzig gGmbH (KUZ),

Jörg Garlinsky (BYK), Dr. Andre Rapthel (BYK)