ENHANCED DISPERSION OF PARTICLE ADDITIVE INTO POLYMERS USING TWIN SCREW EXTRUSION WITH ULTRASOUND ASSISTANCE

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1 ENHANCED DISPERSION OF PARTICLE ADDITIVE INTO POLYMERS USING TWIN SCREW EXTRUSION WITH ULTRASOUND ASSISTANCE Karnik Tarverdi, Kun Qi & Jack Silver Wolfson Centre for Materials Processing Institute of Materials and Manufacturing Brunel University London, United Kingdom Abstract Most plastics products are made from a base polymer mixed with complex blends of materials known collectively as additives, to ensure that the physical, mechanical and surface properties of the final product is optimised in all aspects. This will include safer, cleaner product possessing optimal colour and properties. Fine additives such as fillers or coloured pigments are most widely used and the improved technique for dispersing particles into polymer is highly demanding in industrial practice. A USV (ultra-sonication and vibration) assisted process during twin screw extrusion system was implemented and the dispersion results tested in our labs and the technology transferred to our industrial partner s manufacturing facility. Particle additives such as clay, organic and inorganic pigments were compounded and tested using USV assisted twin screw extrusion. Introduction Ultrasonication has long been used as a pre-treatment method for fine particle size distribution due to the important delamination effect which can improve the accuracy of the particle size or morphology measurement (1), In recent years ultrasonication was used to help enhance and extend polymer extrusion properties, via ultrasonic activation effect, to modify the properties at the interface between the molten polymer material and the metallic screw and barrel, allowing thus high speed processing, and at the same time increasing productivity and product quality (2). For the past decade, development by researches of applications using ultrasonications as high frequency sound waves to enhance injection moulding and extrusion has been carried out. There are many publications in form of scientific papers, (3) (4) (5); and patents (CA A1, Avraam Isayev, WO A1, Paul Beaney, WO A1, and WO A1, Lanty Patrick De) and there is particular interest to try and disperse fine particulates in polymers at high concentrations. Experimental procedure With the help of project partner Sonic Systems, ultrasonication equipment manufacturers USV assisted twin screw extrusion system was implemented and tested in our lab and the results are presented in this work. Particle additives such as clay, organic and inorganic pigments were compounded using a co rotating intermeshing twin screw extruder with specially designed USV system that could be attached to an open port of the extruder barrel where the polymer melt could come in full contact with the ultrasonic horn. Materials Green MB mix was supplied by Colloid lid. Which contained 15% Phthalocyanine Green as pigment in LLDPE (MFR 20 g/10min) as matrix. The LDPE used for film blowing was Dow LDPE 410E, MFI 2.0 Microgranuled clay for non-polar polymers was from BYK-Chemie GmbH, Germany. The extrusion compounding work was carried out on a Thermo-fisher twin-screw extruder (Fig 1). The polymer granules was fed through a gravimetric twin screw feeder (K-tron Deutschland GmbH). Along the extruder barrel three access ports could be used to mount the USV horn. With the control system of the lab scale extruder it was possible to measure and record the screw speed ( rpm), torque (Nm), and melt pressure at the die (bar) during extrusion. The mass input/output of extrusion was controlled by changing the rotation speed (rpm) of the feeder. USV modification was carried out by using a L500 Ultrasonic Process System (bespoke made by Sonic Systems) operating a frequency ranges 19 to 20 khz. SPE ANTEC Anaheim 2017 / 1058

2 With amplitude variable range of 1~ 12 micrometre and the USV energy input power (watt) displayed. Optical microscopy and image analysis were used for the evaluation of the degree of particle dispersion and the size of agglomerates of particles in the polymer compounds. Mechanical and thermal analysis was used to determine the benefit of well dispersed particles in compound. Mastebatch preparation and evaluation Masterbatches of green pigment (15% Phthalocyanine Green G pigment in polyethylene) were produced using seven different USV power levels. LDPE film blowing exercises were carried out using in house Thermo-fisher twin-screw extruder with film blowing line attachment. The base polymer used for film blowing was Dow LDPE 410E, MFI 2.0 with the masterbatch let-down ratio of 1% MB. Pigment particle dispersion analysis The films with pigment were subjected to microscopic observation using a Zeiss Axioskop 2 MAT optical microscope. Film samples cut from extrusion blown film was placed in between microscope glass slides to keep the film. The low-power objective (magnifies X10 and X20 ) was used with transmitted light to capture images. For each film sample more than 20 images were taken. Measurement of mechanical properties Thermogravimetric analysis (TGA) equipment TA was employed to examine the thermal degradation of the samples. Experiments were carried out from 50 to 600 C with a heating rate of 10 C/m. Tensile test bars were injection moulded using DEMAG D 150 NCII injection moulding machine. Tensile test measurement were carried out using a Universal Instron Test rig with Zwick measurement control and testxpert II testing software, followed ISO 527 test methods. Figure 1 USV ultrasonic treatment device setup with twin screw extruder. The USV horn can be attached at different locations alongside the barrel of the extruder. Results and Discussions USV energy transfer during extrusion The ultrasound can be used for mixing through a phenomenon called ultrasonic cavitation. The sound waves that propagate into the liquid media result in alternating high pressure (compression) and low-pressure (rarefaction) cycles. During the low-pressure cycle, highintensity ultrasonic waves create small vacuum bubbles or voids in the liquid. When the bubbles attain a volume at which they can no longer absorb energy, they collapse violently during a high pressure cycle. Polymer material is exposed to ultrasonic cavitation at a controlled intensity, by variate the exposure time through the material feed-though rate. In order to let all material are exposed to identical sonication, the tip of the USV horn is shaped to close fit the contour of the twin screws of the extruder to keep a full contact with the plastic melt, at the same time without any direct contact between them to cause wear or damage to the tooling face. One or more USV probes can be mounted on the three ports on the barrier of the twin screw extruder (Figure 1) which gives us opportunity to explore the efficient of UVS at different stage of the polymer, ie, solid polymer, solid/melt transaction, and polymer melt. The experiments proved there s hardly any USV power been transferred by mount horn at the end next to the feeder of raw material or at the middle point of the barrier, where polymer is either in its solid state or solidliquid transaction (Figure 2). Only place the USV horn at the exit end where polymer melt can see a higher USV power been delivered. When applied to dispersions, higher amplitudes show a higher destructiveness to solid particles. The individual particles tend to agglomerate and are held together by attraction forces of various physical and chemical natures, including van der Waals forces and liquid surface tension. This effect is stronger for higher viscosity polymers melt. The attraction forces must be overcome on order to deagglomerate and disperse the particles polymer melt. The input energy is transformed into melt depends on several important parameters, amplitude, pressure, temperature, viscosity. In the case of extrusion, melt pressure is a very decisive fact for obtain UVS to an active level. Table 1shows the ultrasonic power delivered into the plastic at amplitude changes from 2 to 12 micron, during extrusion of LLDPE. Higher amplitude means a SPE ANTEC Anaheim 2017 / 1059

3 higher power. By changing parameters like the feeding mass of the raw material of during the extrusion, the pressure near USV horn changed, and the USV energy input to extrude be varied from very low inactive to a higher active level. Figure 2 shows the test result, the active SUV only appears at a pressure higher than of 15.3 bar, and not at lower pressure 12.9 bar (Figure 3). The threshold pressure kept same without respects of the amplitude of USV applied. (CLOISITE-20 Technical Data Sheet) In the case of polymer extrusion, temperature at the point where USV horn to be fixed could be well above the 200 C. prolong running of the system could see the problem of drop with power delivery. The water or air cooling system we ve tried will normally falls to overcome the overheat issue. The alternative effective way we adapt is to reconfigure the driver of the USV system for it to be able to track down the wider operating frequency variation from 19.0 khz to 19.6 khz. Figure 2 The USV attachment on the twin-screw extruder with a 4mm rod die Material: Dow LDPE 410E, MFI 2.0 Temperature: 190 C, Screw speed: 50rpm, Torque: 40 Nm, Feeder: 300rpm Table 1 Ultrasonic power into the plastic tested at amplitude from 2 to 12 micron, during extrusion of LLDPE Material: ExxonMobil LLDPE LL6101 Temperature: 190 C, Screw speed: 50rpm Figure 3 dependence of USV energy power with pressure at the exit of die for amplitude of 2~12micron metre Material: ExxonMobil LLDPE LL 6101, Temperature: 150 C, Screw speed: 50rpm Effect of USV on polypropylene/clay composites CLOISITE 20 is one of the additive for plastics and rubber to improve various physical properties, such as reinforcement, CLTE, synergistic flame retardant and barrier. Microgranuled Nanoclay based on a natural mineral for nonpolar polymers having optimized polarity to enable intercalation and possibly exfoliation. The CLOISITE 20 is microscaled in its delivery form and exfoliates to nano scale in the polymer matrix only. Fully use of CLOISITE 20 in the polymers can only be exploited in highly even dispersed state. An even dispersion and de-agglomeration is important to use the full potential of the particles. Compounding clay with PP carried out by extrusion with and without USV. The ultrasonic assisted extrusion SPE ANTEC Anaheim 2017 / 1060

4 technology notably improved the dispersion and exfoliation of nanoclay in polymers during melt mixing. The breakup of the agglomerate structures in polymer melt allows utilizing the full potential of nano size materials. The polypropylene/clay composites were prepared using the ultrasonic twin-screw extruder with 1% and 5% of clay. For extrusion 1% clay /PP, an USV power can reach as higher as 170 Watts at amplitude of 12 micron. Based on pressure transducer readings (Figure 1), we find that the pressure to active the ultrasonic can only be achieved at relative higher feeder speed compare to pure PP. Figure 5 gives the TGA test result of PP and PP/clay with and without USV. It is clear that PP degrades in a single step. By adding clay into PP delayed the polymer degradation. The improvement in thermal stability can be attributed to good matrix clay interaction by USV treatment. As the clay is good thermal conductor easily take up the heat that is applied to the PP/clay composite materials. The dispersion and exfoliation of clay can also directly affect mechanical properties of PP/clay composites. Figures 5 and 6 demonstrate the stress-stain corves and tensile test results of injection moulded test samples of PP and PP/clay composites, respectively. Generally, the clay should be well dispersed at the level of USV power according to our previous experiment. However, with 5% clay the pressure is unable to reach the level which cans trigger the activation of the ultrasonic. Power of USV keeps on a very low level even with a very high feeding speed used. In this case, the clay Cloisite acts more likely a flow promotor which actually reduced resistant. The improvement of dispersion and exfoliation of clay can hardly achieve. Figure 4 TGA tests of pure PP and PP/clay with and without USV. PP/1% clay and PP/5% clay (lower). SPE ANTEC Anaheim 2017 / 1061

5 To prove the concept, blown films using the USV treated masterbatches were subjected to image analyse, where the results demonstrate that there is good indication that when the ultrasonication horn is switched on much finer particle size distribution is achieved by breaking down the larger agglomerated particles. USV activation also improves the mechanical properties of the polymer with the particulate fillers which was demonstrated in this project following injection moulding and mechanical testing of the USV activated compounds. Figure 5. Stress-strain curves of PP and PP/clay composites prepared with and without USV. Figure 6. Tensile strength (upper) and modulus (lower) of PP and PP/clay composites prepared with and without USV. Conclusions Typical tensile stress strain curves of all PP and PP/clay samples exhibit deformation behaviour unrespect to their clay content and USV power used. High intensity ultrasonication is an important alternative technology for fine particle treatment and dispersion when polymer processing. The dispersing and deagglomeration of particle with base polymer, to achieve the full potential of specific particles by dispersion and distribution is an important development in delivering better products with minimum of energy and materials input. USV assisted twin-screw extrusion mixing systems were set up and tested with various polymer compounds. This inline-sonication technology provides step change improvement of the distribution of particle additive in plastics and demonstrates great potential for industrial application where micro particles and fibre distribution is essential. References [1] F. Franco, et al., The effect of ultrasound on the particle size and structural disorder of a well-ordered kaolinite, Journal of Colloid and Interface Science 274, (2004) [2] Sergey Lapshin, A. I. Isayev, Ultrasound-aided extrusion process for preparation of polypropylene clay nanocomposites, 13 (1), (2007) [3] T. Saito, M. Okamoto, Polypropylene-based nanocomposite formation: Delamination of organically modified layered filler via solid-state processing, Polymer 51, 4238(2010) [4] Birgit Bittmann, Preparation of TiO2/epoxy nanocomposites by ultrasonic dispersion and their structure property relationship, Ultrasonics Sonochemistry 18, (2011) [5] Yingzi Chen, et al., Effect of ultrasound on the morphology and properties of polypropylene/inorganic filler composites, Journal of Applied Polymer Science, Volume 97, Issue 4, (2005) [6] MK Ultrasonik, Properties Enhancement of TPNR- MWNTs-OMMT Hybrid Nanocomposites by Using Ultrasonic Treatment, Sains Malaysiana 42(4), 503(2013) [7] Juan G. Mart ınez-colunga, et al., Simultaneous Polypropylene Functionalization and Nanoclay Dispersion in PP/Clay Nanocomposites using Ultrasound, Journal of Applied Polymer Science, 131, (2014) Acknowledgements The research work reported in this paper was funded by Innovate UK Programme (Project ). The authors would also like to thank the companies involved, Nextek, Colloids Ltd. Innovia and Sonic Systems for their involvement and valued input in the project. SPE ANTEC Anaheim 2017 / 1062