Online rheology: faster formulation development

Rapid evaluation of process stabilization – example: polypropylene

Plastic materials are prone to degradation by atmospheric oxygen. These auto-oxidation reactions occur at ambient temperature already but become especially relevant during melt processing.

Adding antioxidants to the polymers results in a pronounced slowing-down of the oxidation processes. Only by this way, the production of the well-known plastic parts by injection moulding, for example, is possible.

Developing new plastic grades is accompanied by lengthy experimental procedures to optimize the antioxidant content. An online rheometer seems to be a promising tool for accelerating such developments.

Antioxidants

Organic matter and therefore also plastics, degrade by auto-oxidation when being in contact with air. This degradation is initiated by elevated temperature or light and propagates as a radical chain reaction which causes cleavage of the polymer chains. The latter are primarily attacked by the OH radical resulting in the formation of hydroperoxide moieties. These trigger follow up reactions leading to regeneration of the OH-radical. For an optimum protection of the polymer, two different types of antioxidants must be added: The primary antioxidant, often containing a phenolic structure, quenches the OH-radical. Secondary antioxidants consist of sterically hindered alkyl-derivatives of functional groups, such as phosphites or thioethers. These react with the hydroperoxide without OH formation. Both types of antioxidant act in a synergistic way, therefore.

A typical commercially available stabilizer package containing both antioxidants in equal amounts was used in the described experiments.

Studying process stabilization

Commercially available virgin plastic grades are typically equipped with appropriate stabilizer packages to be ready for use. For the sake of resource efficiency and economy, the optimum content of process stabilizer must be determined during the development of new plastic grades.

Processing of used plastics to recyclates faces the same problem because the stabilizers have been regularly depleted during the previous life cycle. Compounding the mill charge to recyclates to be used for example in injection molding, requires to add stabilizers adjusted to the respective type of plastics and its stage of aging.

The traditional way to optimize the stabilizer content is based on compounding a series containing varying amounts of antioxidants. The compounds are then characterized offline by means of different tests, such as the melt volume rate (MVR, DIN 1133-1) or the oxidative induction time (OIT, ASTM D3895-19). First reliable results are obtained only after the compounding step.

Research at LBF aims at obtaining an indication regarding the efficacy of the actual stabilizer content already during the compounding step. Towards this goal the viscosity of the melt is used as a response recorded while varying the recipe. This is realized by incorporating an online rheometer behind the screw tips of a twin screw extruder (Fig. 1) to measure the flow curves of the shear as well as the elongational viscosity.

Fig. 1a: Scheme of the experimental setup with twin screw extruder and online rheometer

Characterize the melt online

In the following, experiments on a minimally stabilized virgin polypropylene (PP) grade are described: Different stages of aging were mimicked via extruding at various screw speeds. Increasing the screw speed applies increasing amounts of shear energy which in turn results in the formation of hot spots. These trigger process-induced degradation, which corresponds to an aging of the polymer. The latter becomes manifest as chain cleavage and decreasing melt viscosity. In Fig. 2 it can be seen accordingly that the flow curves of the online measured shear viscosity monotonously trend to lower values with increasing screw speed. This means, the higher the screw speed the higher the degradation.

Fig. 2: Shear viscosity flow curves at various screw speeds.

Fig. 3a: Flow curves of shear viscosity for various amounts of antioxidant (“Stab”). The screw speed was 100 rpm

Fig. 3b: Flow curves of shear viscosity for various amounts of antioxidant (“Stab”). The screw speed was 300 rpm and the flow curve for 100 rpm and 0.2% Stab (dark green curve) is shown too.

Tests with little stabilized virgin material

The flow curves with varying amounts of a typical commercial process stabilizer (“Stab”) are shown for screw speeds of a) 100 rpm and b) 300 rpm in Fig. 3.

Without additives, the curves are essentially the same as those in Fig. 2 for the corresponding screw speeds.

In Fig. 3a), at 100 rpm, a significant shift of the flow curve towards higher viscosity values can be seen when feeding 0.1% of the stabilizer. Doubling this amount results in a further slight increase of the viscosity values, while the curve for 0.3% is virtually the same as for 0.2%.

When applying 300 rpm (Fig. 3b) the flow curves also shift towards higher viscosities with increasing additive content. However, the viscosity values while feeding 0.3 % stabilizer are below those which are recorded at 100 rpm and only 0.2% of the additive.

This allows us to conclude: At processing with 100 rpm screw speed, the process-induced degradation can be virtually suppressed by 0.2 % additive and raising the amount (here: 0.3%) does not have any effect.

At 300 rpm on the other hand, degradation becomes more prominent and cannot be completely suppressed with even 0.3% of stabilizer.

Potential for batch-adapted post-stabilization in real time

This example demonstrates that online rheology provides valuable information regarding the efficacy of a processing stabilizer already during compounding. It can be expected that this approach will help to accelerate the development of new formulations, in particular in the context of the circular plastics economy.

Furthermore, the flow curves of the different polymers are not identical. The information content of a flow curve is therefore much higher than that of a single numerical value from an MVR measurement. In addition, the flow curves of the elongational viscosity can be included in the evaluation. Supported by an appropriate AI based system, online rheology appears to be a very promising tool to implement stabilizing during the production of recyclates with the ability of real-time adjustment to the aging stage of the mill charges.