Mixing in a single screw extruder with an EFM^TM equivalent to, even outperforming that in a twin screw extruder for certain applications

 

Eliminating gel, pit, or unmelt particles in films, profiles, pipes and bottles.

 

Incorporating elastomers, lubricants and other additives in plastics with superior properties.

 

Good dispersive and distributive mixing especially valuable for multiphase plastics systems having high viscosity ratio, such as plastics alloys and blends, recycled plastics, master batches, etc.

 

Less energy per unit volume consumed in an EFM^TM so that low temperature mixing can be maintained, as well as low costs.

The industries the EFM^TM serves are: packaging, automobile, electronics, medical, construction, chemical and every industry involving plastics, where quality, high performance and low costs are concerned. The EFM^TM can also be used in different industries to disperse and homogenize liquids, solutions, and emulsions, in chemical, food, adhesives, and other industries.

Application Case #2: Making Mixing in Single Screw Extruder to Match That in Twin Screw

The above case has shown that a single screw extruder with EFM can effectively reduce gels comparable to, or even better than a twin screw.

Better blending different polymers, such as LDPE w/LLDPE, PS/Polyolefin, PP/PE, HDPE/EVOH, and UHMWPE/HDPE, and incorporating elastomers (PP/Ethylene-propylene elastomer), etc. EFM is especially effective to blend polymeric systems with very high viscosity ratio of components.

The main reason for using blends is to enhance product performance by utilizing different plastics’ properties, to reduce processing and product costs and to increase productivity. For example, using 15 wt% of LLDPE in a LDPE blend can improve LDPE film’s tensile and impact strength and increase production speed. There is a growing trend to use more blends in the plastics industries. The EFM is especially suitable for the preparation of blends in which the viscosity of the dispersed phase (at constant shear stress) is at least four-times higher than that of the matrices. Note that these systems cannot be mixed in the shear mixing devices.

Experiments shows that the EFM added to a single-screw extruder can provide an inexpensive alternative to a twin-screw extruder and outperform twin-screw extruder in generating finer dispersion and improving the blends’ toughness.

For some blends, such as UHMWPE and HDPE, even twin screw extruder may not produce a good dispersion. For the UHMWPE/HDPE blend shown in the application case #3, it is shown clearly that an excellent mixing has been achieved.

On the other hand, we have to caution that the advantages of EFM with a single-screw extruder over a twin-screw is only limited to mixing! Twin-screw extruder has several functions that single-screw with EFM will never duplicate! For example, TSE is an excellent reactor that can be well compartmentalized into sections that perform specific functions: melting, drying, chemical reaction, devolatilization, mixing, pumping, etc. Since often all these functions require longer barrel length than physically possible, Twin-screw extruder with EFM can help twin-screw to perform the required tasks.

In the Application#2 and many other blend systems we have worked, the viscosity ratio of the two polymers are high. In the case of UHMWPE/HDPE, the viscosity ratio l > 100. When the viscosity ratios exceeded 4, i.e., more viscous drops in a shear flow field could be deformed but not broken ["Polymer Blends Handbook", L. A. Utracki,  Kluwer Academic Publishers, Dordrecht (1999)]. While in the elongation field, drops can be broken without the viscosity ratio limit. We has successfully tested EFM with a blend of a viscosity ratio > 80000.

LLDPE is currently taking a back seat to metallocene LLDPE. Our industrial experiments indicate, however, that LLDPE film extruded by a single screw extruder with an EFM has superior properties than the same film without EFM. The properties even match those of metallocene LLDPE! This observation results from the fact that EFM can make a much better dispersion of the high molecular weight tail of the broader molecular weight distribution (MWD) of LLDPE as if the LLDPE were mLLDPE with the tail functioned as reinforcement to the mLLDPE.

Results of DOWLEX 2070D film produced by a 2.5" extruder with an EFM shows significantly increases in the needed properties as listed below over the original properties given by Dow Chemical (Form no. 305-02691-899X). The film becomes much tougher with slightly lower stiffness. Please check the data sheet for the detailed test results.

Improper mixing, or melting has to force people to raise processing temperature or slow down the extruder. Raising temperature may change the absolute values of the viscosities of different polymers in a blend, Unfortunately, it can not change the viscosity ratio at all. Therefore, the mixing can not be improved by increasing temperature. The increased temperature could result in degradation, reduced productivity due to extra cooling is needed and inceasing energy costs. Sometimes, it is impossible to raise temperature, such as in the cases of many thermal sensitive resins.

EFM mixers can improve mixing at relatively low temperature without extra shearing or heating. The benefits are colder and faster processing, lower energy costs, and versatility of adding thermal sensitive additives to achieve additional properties for the products. The last benefit above may not be possible at a higher temperature.

Sometimes, for specific applications, the sizes of particles of the dispersed phase needs to be controlled within certain range. Our EFM mixers have successfully tailored the particle size to different applications. 

The following morphology images show different sizes of PP/mPE have been achieved, which all have the same blend of PP/mPE.