Continuous Compounding Mode with the MC 40 Micro-Compounder

Introduction

The MC 15 HT, Nativas Sciences' flagship micro-compounder with a 15 ml capacity, boasts numerous advantages, including high shear, excellent dispersion, and operational stability. Responding to the evolving needs of our customers for processing larger material quantities, our engineers introduced the MC 40, accommodating a higher compound volume of 40 mL. This expanded sample capacity facilitates usage in subsequent high-volume applications, experimental phases, or analytical steps.

The MC 40 features an extended overall screw length, potentially enabling continuous processing in addition to batch processing. To unlock this capability, proper mixing along the screw's longitudinal axis is crucial. In this initial demonstration, we investigated the impact of screw geometry on the MC 40's continuous mixing performance, incorporating a new screw design with mixing elements to enhance dispersion (Figure 1a).

Dispersive mixing involves particle size reduction under high flow stresses during compounding, addressing cohesive components. This process effectively de-agglomerates solid fillers, breaks up immiscible viscous liquid droplets, and ensures good dispersion.

The existing screw design emphasizes shear deformation, with maximum shear conditions along the screw flank. The new screw geometry incorporates additional mixing elements to promote elongational deformation, improving dispersive mixing of the melt (Figure 1b).

This study compares the mixing efficiency of the conventional (regular) screw of the MC 40 with the new design (dispersive) under continuous compounding conditions (Fig. 2). Emphasis is placed on two critical indicators: residence time and mixing quality (particle size reduction and homogeneity). Higher mixing quality indicates effective dispersion, leading to increased surface area at the interface.

Sufficient residence time is vital for achieving high mixing quality without unmelts, ensuring good dispersion and distribution. The new screw design addresses this by shortening the pitch of the screw, prolonging residence time compared to the regular screw design of the MC 40 compounder.

Materials and Methods

Key experimental parameters are summarized in Table 1. In the first part, a colored LDPE was processed in continuous mode to compare the residence time of regular and dispersive screws. The second part involved comparing the mixing quality of regular and dispersive screws using a PA6/PP/PP-g-MA blend system.

Results and Discussion

Residence time study results (Table 2) show that dispersive screws exhibit longer residence times for a given screw speed, offering potential benefits for continuous compounding.

SEM images of the PA6/PP/PP-g-MA blends (Figure 5) reveal a significant reduction in the average dispersed particle size when dispersive screws are used. This results in improved uniformity and homogeneity of the PA phase dispersion in the PP matrix, attributed to better mixing ability and longer residence time.

Tensile test results (Figure 6) indicate that dispersive screws in continuous mode yield the highest elongation, correlating with smaller particle sizes and enhanced interfacial surface area.

Conclusion

In continuous compounding, achieving good dispersion and distribution relies on proper shear and elongational deformation, coupled with sufficient residence time. The demonstrated new screw design for the Nativas Sciences MC 40 allows effective operation in continuous mode, providing versatility for small batches and continuous pelletizing processes. This design enhances dispersive mixing, contributing to improved particle size reduction and overall blending performance.