Xplore’s MC 40 micro-compounder in continuous compounding mode

Part-1: Performance of the new screw design in blending immiscible polymers

‍

Introduction

The MC 15 HT, Xplore’s flagship micro-compounder with 15 ml capacity, offers many advantages, such as high shear, good dispersion, and operational stability. Over time, Xplore was inspired by customers' needs to process larger amounts of material; as a result, our engineers developed an MC 40, which can process a higher compound volume of 40 mL. This sample amount can then enable the usage in subsequent high-volume applications, experimental or analysis steps.    

‍

An MC 40 has a longer overall screw length; therefore, as well as batch processing, it can also potentially enable continuous processing. Proper mixing along the screw's longitudinal axis must be ensured to unlock this potential. In this first demonstration, the effect of the screw geometry on the MC 40 continuous mixing performance has been studied. For this purpose, a new screw geometry was designed. The primary revision incorporates mixing elements to promote dispersion further (Figure 1a).

‍

Dispersive mixing involves particle size reduction of cohesive components, which requires considerably high flow stresses during compounding under both shear and elongational deformation conditions. This way, solid fillers or viscous ingredients can be de-agglomerated, or even immiscible viscous liquid droplets can be broken up.  

In the existing screw design, shear deformation is dominant, and maximum shear conditions occur over the screw flank due to the small gap. Besides, some portion of the melt can also flow backwards over the screw flights, resulting in elongational mixing.

However, this only applies to a minor part of the melt (Figure 1b). In the new screw geometry, to enhance the dispersive mixing further, new mixing elements were incorporated into both mixing screws to promote elongational deformation, which leads to improved dispersive mixing of the melt.

‍

‍

In this work, we compared the mixing efficiency of the conventional (regular) screw of the MC40 with the new design (dispersive) under continuous compounding conditions (Fig. 2). We mainly focused on two critical indicators of the mixing, residence time and mixing quality (particle size reduction and homogeneity). One notes that higher mixing quality means that the particles can be dispersed effectively with the correct shear level, which leads to a higher surface area at the interface.  

Sufficient residence time is crucial in the compounding process to achieve a high mixing quality (also without unmelts),i.e., good dispersion and distribution. The second primary revision made in the new screw design is the shortening of the pitch of the screw. In this way, a longer residence time can be achieved compared to the regular screw design of the MC40 compounder.

‍

‍

In the first part of this study, the residence time of 2 screw designs was compared by processing a colored LDPE incontinuous mode (details are given in the Materials and Testing Section). In the second part, the mixing quality of the two screws was compared. A compatibilized PP/PA6 blend system was chosen as a model system. Due to the difference in the polarities of these two semi-crystalline polymers, they are highly thermodynamically immiscible. Therefore, the mixing quality depends on not only the shear conditions but also the extent of compatibilization reaction where residencetime plays an important role.

‍

To achieve PA6/PP compatibility, maleicanhydride grafted PP (PP-g-MA) was used as a compatibilizer (emulsifier). PP-g-MA preferentially resides at the interface of both polymers and improves interfacial adhesion through the chemical linkage of the anhydride groups withthe polyamide end groups (Figure 3) and, at the same time, through sustaining the thermodynamical compatibility with the PP phase. This reduces the interfacial surface tension between two blend components and leads to a better and more stable blend morphology.

‍

‍

‍

Materials and Methods

Some important experimental parameters are summarized in Table 1. In the first part, LDPE/LDPE-masterbatch system was used to compare the residence time of regular and dispersive screws in continuous mode. LDPE (MFI 10 g/10min) was fed to the MC 40 compounder with an Xplore volumetric continuous feeder at a 10 g/min feed rate.The screw speed was set as either 50 or 100 rpm. The barrel temperature was 200^oC.The recirculation valve was kept open to allow continuous compounding, which means the polymer was allowed to pass one time over both screws, and the extrudate was collected. Once the steady state was obtained, 0.07 gram of colored masterbatch was fed to the micro-compounder from the top at once, and t_iand t_f values were recorded (Figure 4):  -      t_i: time at which the colorappears

-      t_f: time at which the colordisappears

In the second part, PA6/PP/PP-g-MA blends were prepared under continuous compounding conditions to see the effect ofscrew geometry on the blend morphology.

Before processing, PA6 was dried at 80^oC for 16 hours in a vacuum oven to remove the moisture.

PA6/PP/PP-g-MA blends were melt compounded in an Xplore MC40 micro-compounder in continuous mode at 100 rpm screw speed and 230^oC barrel temperature having 30%/70%/2% weight ratio, respectively, by using an Xplore continuous double feeder.

The continuous flow was interrupted with an IM12 Xplore Injection molding machine to obtain tensile test specimens. The injection and holding pressure was kept at 8 bar, and the cycle time was 15 sec.

To compare the mixing quality of the continuous new screw design with batch processing, samples were also prepared with regular screws under batch processc onditions.

In this route, the valve was kept closed, and the blend was sheared for 5 minutes and then transferred to the IM12. The mechanical performance of the blends was investigated by tensile testing.

Additionally, a scanning electron microscope (SEM) accomplished the examination of the blends'morphology.

‍

‍

Results and Discussion

The results of the residence time study are summarized in Table 2.  Both t_i and t_fvalues are higher for dispersive screws than regular screws, indicating longer residence time for a given screw speed.

In the second part, we compared the mixing quality of regular and dispersive screws. Figure 5 shows the SEM images of the PA6/PP/PP-g-MA blends. PP is the matrix, and PA6 is the dispersed phase, which is obviously seen as a particle.

The average dispersed particle size of PA6 was decreased from 2.05 microns to 0.45 microns when the dispersive screws were used instead of regular screws. The dispersive screws provided better uniformity and homogeneity of PA phase dispersion in the PP matrix with a reduction in the size of the domains thanks to better mixing ability and longer residence time.

‍

‍

Since the blending of the PP/PA6 system in the presence of PP-g-MA is are active process, the longer residence time of the new screws compared to the regular ones could promote the grafting reaction of maleic anhydride to the chain ends of PA6 by allowing the polymer chains to find time for the reaction.  

Using the regular mixing screws under batch process conditions is usually expected to promote the dispersion of the blend components in the most effective way. However, the particle size was observed to be 1.12 microns, almost more than double the value (0.45 microns) achieved with dispersive screws. However, it must be remembered that the compounding conditions, either in continuous or batch mode, are not optimized for PA6/PP blends.

One can expect that the longer residence time would lead to better dispersion.  For both screws, increasing the screw speed (and eventually increasing the shear rate) decreased both t_i and t_f values, meaning that a shorter residence time is achieved with a higher screw speed.

Tensile test results given in Figure 6. show that the elongation of the blends is in the order: Dispersive continuous mode> Regular batch mode > Regular continuous mode

This result correlates well with the average particle size measurements in SEM images. The dispersive screw in continuous mode yields the smallest particle size and the highest elongation. In other words, smaller particle size prevents premature failure. Basically, as aligned with the interface phenomena, it can be concluded that the smaller the particle size, the higher the elongation due to the larger interfacial surface area.

‍

‍

‍

Conclusion

There are two critical parameters in the continuous compounding of polymers to achieve good dispersion and distribution. One is having a proper shear and elongational deformation during the compounding.

The other one is the enough residence time under those deformations. A good screw design for a given compounding system can sustain these two essential factors. In this work, we demonstrated that besides well-established batch compounding in the Xplore MC 40, with the new screw design the MC 40 is capable of being operated in continuous mode as well for preparing small batches (app. 250 g – 3,0 kg per hour, depending on the bulk density of the polymer granules, flakes and powder) of polymer compounds.

In addition, in combination with the Xplore water bath and Pro-Pelletizer, it is now possible to make a continuous pelletizing process using an MC 40, as shown in Figure 7.

‍

For further information, contact our experts at Xplore Instruments BV.

‍