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41 Kunststoffe international 8/2019 www.kunststoffe-international.com Nanoparticles COMPOUNDING Nanoparticle powder: the fineness of the particles can lead to fine dust pollution and agglomer- ation, which makes liquid-compounding with suspensions attractive (© TUM/L. Osthues) S tate-of-the-art of modifying thermo- plastics is using melt compounding on screw machines. After synthesis of the base polymer, compounding represents the first refining step to adjust properties such as color, viscosity, chemical resis- tance and flow behavior [1, 2] as well as stiffness and strength [3]. The tribological performance of plastic composites, e. g. abrasion resistance, can also be increased by fillers [4, 5]. With the aid of suitable additives it is even poss- ible to improve the biodegradability of plastics or to achieve an antimicrobial ef- fect [5, 6]. Customized Properties due to Nanoparticles All these possibilities can be further in- creased through nanoparticles or nano- composite materials. Nanocomposite materials are called plastic matrices filled with nanoscale particles or fibers. Positive effects usually result from particles in the order of 10 to 100 nm [7]. Up to now, conventional melt com- pounding has reached its technical limits in the incorporation of nanoparticles as powders as undesired effects occur dur- ing their use. The small particle size can lead to fine dust pollution and agglomer- ation of the particles in the plastic ma- terial [7]. Such agglomerates reduce the specific surface area of the nanoparticles in the material and can even lead to a de- crease of the material strength [2]. In order to avoid this effect, a homo- genous distribution of the nanoparticles is necessary [2]. A liquid-compounding method by means of suspension devel- oped by Wilhelm et al. represents an ap- proach to solve this problem [8, 9]. The aim of the investigations of the Institute of Medical and Plastics Engineering and SKZ, an institute of the Zuse-Community, was to show the advantages of the liquid- compounding method by analyzing the quality of the particle distribution in plastics produced by different com- pounding techniques. Production of Nanocomposites State-of-the-art for bringing nanopar- ticles into a plastic matrix is the classic powdered incorporation of the additive into the plastic melt. By dispersing nano- particles directly into the matrix, primary particles are incorporated, but also na- noscaled agglomerates are formed. Dur- ing the compounding process, disper- sion takes place in two steps. First, the ag- glomerates are incorporated in the plastic matrix and the enclosed air be- tween the particles is replaced by the plastic material. In the next step, the agglomerates can be disaggregated and distributed in the matrix [5, 6]. An important standard is the master- batch-process. First, a high concentration of the additive powder is processed Agglomerate-Free Compounding Comparison of Two Techniques to Produce Thermoplastic-Nanoparticle-Composites Often pure plastic materials are not able to fulfill the increasing demands of plastic component. It is therefore necessary to adapt the properties of the plastics by adding other polymers or additives. [VEHICLE ENGINEERING] [MEDICAL TECHNOLOGY] [PACKAGING] [ELECTRICAL & ELECTRONICS] [CONSTRUCTION] [CONSUMER GOODS] [LEISURE & SPORTS] [OPTIC] »
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Page 1: Agglomerate-Free Compounding

41

Kunststoffe international 8/2019 www.kunststoffe-international.com

Nanoparticles COMPOUNDING

Nanoparticle powder: the fineness of the particles can lead to fine dust pollution and agglomer-

ation, which makes liquid-compounding with suspensions attractive (© TUM/L. Osthues)

State-of-the-art of modifying thermo-plastics is using melt compounding

on screw machines. After synthesis of the base polymer, compounding represents the first refining step to adjust properties such as color, viscosity, chemical resis-tance and flow behavior [1, 2] as well as stiffness and strength [3].

The tribological performance of plastic composites, e. g. abrasion resistance, can also be increased by fillers [4, 5]. With the aid of suitable additives it is even poss-ible to improve the biodegradability of plastics or to achieve an antimicrobial ef-fect [5, 6].

Customized Properties due to Nanoparticles

All these possibilities can be further in-creased through nanoparticles or nano-composite materials. Nanocomposite materials are called plastic matrices filled with nanoscale particles or fibers. Positive effects usually result from particles in the order of 10 to 100 nm [7].

Up to now, conventional melt com-pounding has reached its technical limits in the incorporation of nanoparticles as powders as undesired effects occur dur-ing their use. The small particle size can lead to fine dust pollution and agglomer-ation of the particles in the plastic ma-terial [7]. Such agglomerates reduce the specific surface area of the nanoparticles in the material and can even lead to a de-crease of the material strength [2].

In order to avoid this effect, a homo-genous distribution of the nanoparticles is necessary [2]. A liquid-compounding method by means of suspension devel-oped by Wilhelm et al. represents an ap-proach to solve this problem [8, 9]. The aim of the investigations of the Institute

of Medical and Plastics Engineering and SKZ, an institute of the Zuse-Community, was to show the advantages of the liquid-compounding method by analyzing the quality of the particle distribution in plastics produced by different com-pounding techniques.

Production of Nanocomposites

State-of-the-art for bringing nanopar-ticles into a plastic matrix is the classic powdered incorporation of the additive into the plastic melt. By dispersing nano-

particles directly into the matrix, primary particles are incorporated, but also na-noscaled agglomerates are formed. Dur-ing the compounding process, disper-sion takes place in two steps. First, the ag-glomerates are incorporated in the plastic matrix and the enclosed air be-tween the particles is replaced by the plastic material. In the next step, the agglomerates can be disaggregated and distributed in the matrix [5, 6].

An important standard is the master-batch-process. First, a high concentration of the additive powder is processed

Agglomerate-Free Compounding

Comparison of Two Techniques to Produce Thermoplastic- Nanoparticle-Composites

Often pure plastic materials are not able to fulfill the increasing demands of plastic component. It is therefore

necessary to adapt the properties of the plastics by adding other polymers or additives.

[VEHICLE ENGINEERING] [MEDICAL TECHNOLOGY] [PACKAGING] [ELECTRICAL & ELECTRONICS] [CONSTRUCTION] [CONSUMER GOODS] [LEISURE & SPORTS] [OPTIC]

»

Page 2: Agglomerate-Free Compounding

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© Carl Hanser Verlag, Munich Kunststoffe international 8/2019

COMPOUNDING Nanoparticles

As reference, a thermoplastic polyure-thane (TPU) with 2 wt. % TiO2 was pro-duced using the masterbatch.

New potential in the production of na-nocomposites on an industrial scale is of-fered by a newly developed processing unit in which the nanoparticles are incor-porated into the plastic by a suspension [8, 9]. Using a suspension avoids agglomerate formation and reduces safety risks while handling nanoparticles. In this process, na-noparticles are not added free-flowing to the plastic material, but in a carrier medi-um such as water, oil, etc. The additive is thus encapsulated in a dispersing medium and separated into nanoscale primary par-ticles. The medium avoids the re-agglom-eration. In a first step, this suspension is in-jected into the plastic melt under high pressure. The carrier medium is then re-moved by a highly effective venting sys-tem. This makes it possible to achieve high dispersion qualities [1, 8, 9].

The incorporation of nanoscale addi-tives in form of a disagglomerated suspen-sion has further advantages in addition to better distribution. For health reasons, the exposure of nanodust should be kept as low as possible. The binding of the nano-particles in a suspension reduces this health risk. In addition, the so-called “blow-up”-effect provides an additional disper-sion when water is used as the dispersing medium and evaporates from the matrix [1]. Studies have also shown that the usage of well dispersed primary particles signifi-cantly reduces the amount of active nano-particles required to achieve the desired property benefits [10–12].

The newly developed system con-figuration used for the first process step consists of a co-rotating twin screw ex-truder (TSE) type ZSK 26 MCC (manufac-turer: Coperion GmbH, Stuttgart, Ger-many). By means of TSE and the devel-

oped injection technology, the suspen-sion is injected under pressure into the hot polymer melt and incorporated into a dispersing zone. Subsequently, in a sec-ond process step, the loaded melt is transferred to a multi-rotation system (MRS) 35 of the manufacturer Gneuss GmbH (Bad Oeynhausen, Germany). The interaction of MRS and the installed vac-uum system effectively and completely remove the carrier medium of the sus-pension. The melt is then formed into a strand, which is cooled by use of a water bath and then granulated. In the scope of these investigations 2 wt. % TiO2 was in-corporated into TPU by suspension.

Analysis of the Extrudated Compounds

The aim of the investigations was to compare both compounding tech-niques (powder and suspension) regard-ing the dispersion quality of nanopar-ticles. Figure 1 shows the used compound-ing systems. TPU Estane 54610 (manufac-turer: Lubrizol Advanced Materials Inc., Cleveland, OH/USA) was used as matrix material and TiO

2 Aeroxide P25 (manu-

facturer: Evonik Industries AG, Essen, Ger-many) as white pigment. The com-pounded granulates were then pro-cessed into test specimens in the extru-sion process. An ashing of ten specimens should provide information on the actual value of the additive content in order to correlate it with the subsequent color and energy dispersive X-ray spectro-scopy (EDX) of the specimens. The Table 1 shows the result of the ashing. Under consideration of the deviation both com-pounding techniques showed the same additive content.

In order to determine the brightness of the various compounds, a color mea -

Fig. 1. Comparison of compounding techniques: feed units and its key data (source:: SKZ, Coperion, Gneuss)

Co-rotating twin screw extruder Mulit-rotation system

Type:ZSK 26 MCC CoperionDo/Di = 1.55

Function: Plastifying Incorporation of suspension Build-up of counterpresser Dispersion

Type:MRS 35 GneussD = 35 mm

Function: Degassing of carrier medium Polycondensation intervention IV/Mw-preservation/structure

The AuthorsTheresa Fischer, M.Sc, is research assist-

ant at the Institute of Medical and Plastic

Engineering, Technical University of

Munich, Germany; [email protected]

Dr.-Ing. Matthias Wilhelm is research

assistant at SKZ, Würzburg, Germany, in

the field of materials, compounding and

extrusion.

Florian Fechter, B.Sc., is student at the

Institute of Medical and Plastic Engineer-

ing at TUM.

Dr. med. Markus Eblenkamp is Deputy

Head of the Institute of Medical and

Plastic Engineering, TUM.

Dipl.-Ing. (FH) Johannes Rudloff is group

leader in the field of compounding and

extrusion at SKZ.

Dr.-Ing. Marieluise Lang is Division

Manager Materials in the field of com-

pounding and extrusion at SKZ.

Dr. rer. nat. Thomas Hochrein is Manag-

ing Director of Education and Research at

SKZ

Prof. Dr.-Ing. Martin Bastian is Institute

Director and Professor of the specialist

field technology of polymer materials at

the University of Würzburg.

ServiceReferences & Digital VersionB You can find the list of references

and a PDF file of the article at www.kunststoffe-international.com//2019–08

German VersionB Read the German version of the

article in our magazine Kunststoffe or at www.kunststoffe.de

© Kunststoffe

into a masterbatch by incorporating it in a polymer matrix. In the next step, the ag-glomerates are separated by a highly dis-persing screw combination. This master-batch can now be processed into gran-ules and then diluted to a required con-centration. As standard, co-rotating twin-screw extruders are used for this purpose. A disadvantage of the compounding is that the dispersion is getting more diffi-cult the smaller the particles are [6].

For the investigations described here, a masterbatch with a nanoparticle content of 10 wt. % titanium dioxide (TiO2) was pro-duced using a co-rotating twin extruder.

Page 3: Agglomerate-Free Compounding

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Kunststoffe international 8/2019 www.kunststoffe-international.com

Nanoparticles COMPOUNDING

Table 1. Additive content and L*-value of the

specimens examined (source: TUM)

Additive content in wt %

L* value

Powder

2.03 ± 0.03

83.79 ± 0.15

Suspension

2.13 ± 0.08

81.08 ± 0.23

surement according to DIN 5033–7 was carried out. As the color and brightness perception depends strongly on subjec-tive influences, the samples were mea -sured with a spectrophotometer in order to obtain quantitative results. It is deter-mined which part of the light falling on the sample is reflected at each wave-length. So color and intensity of flat test samples can be measured exactly. To describe the measured color quanti-tatively, there are various systems to be used. The CIE L*,a*,b*-system is very wide established. In this system, every color can be assigned to a specific color posi-tion in a three-dimensional color spec-trum L*, a*, b*. Thereby L* = 0–100 de-scribes the brightness, a* the colors green to red and b* the colors blue to yellow. In this case, the L*axis perpendicular to the a*-b*-plane is significant [13].

The measurements of the samples were performed with a spectrophoto-meter CM-700d (Konica Minolta, Chiyoda, Japan). During the analysis attention was paid to ensure that the sample was illumi-nated by the ambient light in a uniform way. The test specimens were placed on a black background to prevent light reflec-tion from the substrate and thus falsifica-tion of the measurement. For each com-pound, ten samples were measured and the arithmetic average calculated.

Titanium dioxide nanoparticles ap-pear transparent in visible light [14]. As pure TPU is transparent, a lower L* value can be expected with a nanoscale dis-tribution of the additive using a black measuring background. Samples pro-duced by suspension technique have a lower L* value than those produced from powder (Tab. 1), which indicates a higher transparency of the material and thus a better distribution of the additive in the polymer matrix.

EDX-analyses, the detection and evalu-ation of X-rays generated with a scanning electron microscope (REM), should sup-port these results. Since X-rays are charac-teristic for each element, a mapping can be used to deduce the distribution of tita-nium dioxide within the sample under in-vestigation [15]. First, SEM images were taken at 2500x magnification. Figure 2 shows the SEM images on the left side. Subsequently, an EDX measurement was used to map the element titanium (Ti) in order to track the distribution of titanium dioxide in the materials. The two images on the right in Figure 2 show the distribu-tion of the nanoparticles in yellow.

As can be seen in Figure 2, both com-

Fig. 2. REM-EDX-

figures of the speci-

mens examined

(© TUM)

pounding techniques succeed in distribu-ting nanoparticles throughout the entire material. However, the comparison shows an increased occurrence of agglomerates in powder compounding technology com-pared to suspension technology. These are in the order of a micrometer range and smaller. In the lower image sections it can be seen that a significantly more homogen-ous distribution can be achieved by using the suspension technique.

Conclusion

The analyses of both compounding tech-niques show that the suspension tech-nique is able to distribute nanoparticles more homogenously than this is the case in the masterbatch process. As agglomer-ates lead to a lower specific surface area of the nanoparticles in the plastic, it is theor-etically possible with suspension technol-ogy to achieve the same specific surface area of nanoparticles and thus the same properties in the plastic with a lower addi-tive content. W

The Colorful World of Plastics.www.kunststoffe-international.com


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