process chain • liquid rubber • planetary mixer • viscotec

MASCHINEN UND ANLAGEN MACHINERY AND EQUIPMENTS

27KGK · 4 2021www.kgk-rubberpoint.de

3D-Printing • Additive Manufacturing • Rapid Prototyping • process chain • liquid rubber • planetary mixer • ViscoTec

This publication shows the investigati-on of formulations and the production of sulfur-crosslinked compounds based on liquid rubbers for processing by me-ans of additive manufacturing in the „Direct-Ink-Writing” process. This work follows on from the system engineering setup explained in Part 1 [1]. Compared with the use of high-viscosity conventi-onal bale rubbers, the primary use of li-quid rubbers places extended require-ments on the design of formulation and mixing procedures. Formulations with different crosslinking kinetics, pro-cessing-relevant rheological and me-chanical properties are presented, and the mixing process developed for the dispersion of fillers in low-viscosity pas-tes is explained.

Additive Fertigung von Form-teilen auf Basis von Flüssig-kautschuken -

Teil 2: Entwicklung niedervis-koser Mischungsrezepturen zum Einsatz in der additiven Fertigung 3D-Druck • Additive Fertigung • Rapid Prototyping • Prozesskette • Flüssigkau-tschuk • Planetenmischer • ViscoTec Die vorliegende Arbeit beschreibt die Entwicklung von Rezepturen und die Herstellung von schwefelvernetzten Gummimischungen auf Basis von Flüs-sigkautschuken für die Verarbeitung mittels additiver Fertigung im Direct-Ink-Writing-Verfahren“. Diese Arbeit folgt auf den in Teil 1 [1] erläuterten systemtechnischen Aufbau. Gegenüber der Verwendung von hochviskosen konventionellen Ballenkautschuken stellt der primäre Einsatz von Flüssig-kautschuken erweiterte Anforderungen an das Design von Rezeptur und Misch-prozedere. Rezepturen mit unterschied-lichen vernetzungskinetischen, verar-beitungsrelevanten rheologischen und mechanischen Eigenschaften werden vorgestellt und der entwickelte Misch-prozess zur Dispersion von Füllstoffen in niedrigviskosen Pasten erläutert. Figures and Tables: By a kind approval of the authors.

IntroductionAdditive manufacturing of rubber-based parts is a steadily growing branch of re-search. Depending on the dosage form of the rubber compound (e.g. as granu-les, as filaments or, as in this case, as a low-viscosity paste), there are different options for designing the processing-re-levant hardware and developing rubber-based compound formulations that can be processed with the respective machi-ne setup.

A first approach to transfer the additi-ve manufacturing process of a typical FFF-process (Fused Filament Fabrication) to high-viscous carbon black filler rubber compounds was done by Wittek et al. [2] according to the so called AME-process (Additive Manufacturing of Elastomers). In this case a 9 mm twin screw extruder (L/D = 20) was used for additive manu-facturing of the rubber part from rubber filaments as raw material. This setup has been installed and already been proven by Sundermann et al. [3] for typical NR-, NBR-, HNBR- and EPDM-based recipes.

The system design presented by Thiel et al. [1] in Part 1 of this publication, on the other hand, follows a completely dif-ferent concept. The use of low-viscosity formulations based on liquid rubber re-quires a different dosing unit to realize an additive manufacturing process for such types of rubber compounds. The extrusion of low-viscosity media can be realized using syringe pumps, piston pumps, peristaltic pumps or other me-chanically pressurized or pneumatic sys-tems [4]. Most solutions, however, are mainly designed for the dosing of extre-mely low-viscosity liquids. These media can be described as inks rather than pastes and have viscosities of less than 1 Pa*s. However, liquid rubber polymers, in this viscosity range are not available and are more in the range from 100 to 10.000

Pa*s and even higher viscosities will be reached when a certain concentration of filler will be added to the polymer. In this case the “vipro-HEAD 5” by ViscoTec sys-tem is installed, offering a dosing volume flow up to 6 ml/min and dosing accuracy of ± 1 %. To print relatively thin rubber layers to increase the resolution and the bonding between the printed layers, the dosing unit is equipped with a precision needle die of 0.4 mm in diameter. Other precision needle dies are available in dia-meters between 0.15 to 1.2 mm from ViscoTec.

The second part of the publication describes the challenges in the develop-ment and composition of compound for-mulations based on liquid rubber. Here, the final compound has to be, if possible at room temperature, in a viscosity range that allows dosing with the “vipro-HEAD 5” unit. At the same time, it is important to ensure that the developed compound formulations respectively the produced compounds results in mechanical pro-perties of the vulcanized parts that at least approximate the industrial stan-dards of conventionally manufactured molded parts. Special attention is also paid to the mixing process and the use of conventional mixing units, since the low shear-induced energy input due to visco-sity impairs the dispersion of the fillers.

Additive Manufacturing of Rubber Parts based on liquid Rubber Polymers - Part 2: Development of low- viscosity Compound Formulations for use in additive Manufacturing

AuthorsR. Thiel, B. Klie, U. Giese, Hannover, Germany

Corresponding Author:Dr.-Ing. Benjamin KlieDeutsches Institut für Kautschuktechnologie e. V.Eupener Str. 3330519 HannoverE-Mail: [email protected]

KGK_2021_4__27_WI_M_B. Klie U. Giese_Teil 2_AS_MUSS.indd 27 05.08.2021 08:53:22


28 KGK · 4 2021 www.kgk-rubberpoint.de

Theoretical contextIn a patent filed in 1978 from Masuko et al. [5] an approach for mixing and disper-sing different, unspecified, carbon black types in liquid butadiene rubber R-45 HT (mol. wt. = 2750; η = 50 Pa*s) in a modi-fied multi-purpose mixer is explained. In the illustrated method, the carbon black is dispersed in a part of the liquid rubber in a first mixing step, with concentra-tions between 60-120 phr carbon black (CB) being used. The first stage of the mixing process here lasts from 30 minu-tes to 4 hours, the time required to achie-ve a maximum of dispersion decreases as the filler concentration increases. Af-ter the dispersion of CB the mixture is further diluted by liquid rubber and the other components of the mixture are in-corporated.

The apparatus for a continuous mi-xing process of carbon black in liquid rubber was explained in detail by Hum-pidge et al. [6]. During the process car-bon black is predispersed in an an-techamber by a rotary mixing device. Af-ter sufficient dispersion, the masterbatch is fed hydraulically or pneumatically to the screw chamber. Other components are then added and completely dispersed and distributed by the shear induced energy input of the screw.

In a contribution with application ex-amples from Kuraray, Kilian et al. [7] de-scribe the use of Kuraray Liquid Rubber (KLR) L-IR-390 with a concentration of 100 phr or a blend of 60 phr L-IR-50 with 40 phr BR as corrosion protection coa-tings. Particularly interesting is the possi-bility of sulfur crosslinking with the pre-sented recipe with 4 phr zinc oxide (ZnO),0.5 phr stearic acid (SA), 3 phr di-benzothiazyl disulfide (MBTS), 2 phr o-tolyl biguanide (OTDG), 5 phr sulphur (S) and further antioxidants, process oils (TDAE) and up to 200 phr of calcium car-bonate (CaCO3). For the application in additive manufacturing using the “vipro-HEAD 5” unit the high CaCO3 content is not suitable due to increased compound viscosity, but the crosslinking system de-monstrated is of interest.

In a patent from Hochgesang [8] from 2004, various formulations for mixtures based on liquid NBR (Nipol 1312 / 1312LV) are presented. The mixtures are mainly crosslinked based on peroxides, but a recipe with 3.0 phr dibenzothiazyl disulfide (MBTS), 1.6 phr tetramethylthi-uram disulfide (TMTD), 1.6 phr dipenta-methylene thiuram hexasulfide (DPTH) and 3.0 phr sulfur (S) is also suggested.

Sufficient crosslinking cannot be repro-duced with Nipol 1312LV with this reci-pe. The need for higher sulfur concentra-tions, even in combination with sulfur donors such as dipentamethylene thi-uram tetrasulfide (DPTT) and dipenta-methylene thiuram hexasulfide (DPTH), is determined in preliminary tests. All industry-standard mixing devices are na-med for the mixing process. The sole production using the internal mixer and the double roll mill is not reproducible and does not provide any controllable results.

Experimental

Raw materials and compoundingMixtures based on three different liquid rubber polymers from different manu-facturers are used as the base material for the experiments (Table 1). Each com-pound contains 80 phr of the liquid rub-ber “Nipol 1312LV” (Zeon Chemical L.P.), “L-IR-390” (Kuraray Europe GmbH.) and “Polyvest 130” (Evonik Industries AG) re-spectively. The liquid rubber is combined with 20 phr of chemically comparable bulk rubber of the same polymer class. Mixtures with 100 phr of bulk rubber serve as reference mixtures. The bulk rubber used are NBR “Perbunan 2845 F” (Arlanxeo B.V.), IR “Natsyn 2200” (The Goodyear Tire and Rubber Company) and BR “BUNA CB 24” (Arlanxeo B.V.). 30 phr “Corax N550” (Orion Engineered Carbon GmbH) with an STSA of 39 g/m² and OAN

of 121 ml/100 g is used as filler. “Irganox 1010” (BASF) is added as an antioxidant in a proportion of 2.0 phr. “Struktol WB42” (Schill + Seilacher „Struktol“ GmbH), a mixture of fatty acid derivati-ves which is specifically suitable to opti-mize rubber for processing in extrusion [9], is added as a processing aid with 3.0 phr. Compound additives for activation of the crosslinking reaction are stearic acid “Edenor ST 4 A“ (Emery Oleochemi-cals GmbH) and zinc oxide of the Rotsie-gel type (Grillo Werke AG).

In conclusion, when it comes to for-mulation development, liquid rubber based formulations require significantly higher sulfur concentrations to form a resilient network to the polymer for ef-fective vulcanization. For the reference compound based on conventional bulk rubber, this generally means significant overconcentrations. The design of the crosslink system based on polymer-spe-cific sulfur-accelerator-systems of diffe-rent material composition and concent-ration is based on a literature study and preliminary tests and is specifically desi-gned for effective vulcanization of the li-quid rubber based formulations.

Mixing ProcessAccording to the results of the prelimi-nary tests, the compounds are made in a multi-stage process. Exclusive mixing in the internal mixer or on a double roll mill is, due to the low viscosity of the compound, not possible in a reproducib-

1 Table 1: Designed compound recipes based on liquid rubberIngredients 1.001 1.002 1.003 1.004 1.005 1.006

Content [phr]L-NBR (Nipol 1312LV 26,5% ACN) 80 - - - - -NBR (Perbunan 2845F 28% ACN) 20 100 - - - -L-IR (L-IR-390) - - 80 - - -IR (Natsyn 2200) - - 20 100 - -L-BR (Polyvest 130) - - - - 80 -BR (BUNA CB 24) - - - - 20 100Carbon black N 550 30 30 30 30 30 30Stearic acid (SA) 2 2 2 2 2 2Zinc oxide (ZnO) 3 3 3 3 3 3Phenolic antioxidant (Irganox 1010) 2 2 2 2 2 2Fatty acids (Struktol WB42) 3 3 3 3 3 3CBS (Rhenogran CBS80) 2.5 2.5 2.5 2.5 1.25 1.25TBzTD (Rhenogran TBzTD70) 2.86 2.86 - - 2.14 2.14CLD (Vulcofac CLD MB80) 1.88 1.88 - - - -MBTS (Rhenogran MBTS80) - - 0.63 0.63 - -ZBEC (Rhenogran ZBEC70) - - - - 2.14 2.14Sulphur (Rhenogran S80) 15 15 6.25 6.25 6.25 6.25

CBS N-Cyclohexyl-2-benzothiazole sulfenamide; TBzTD Tetrabenzylthiuram disulfide; CLD Caprolactam disulfide; MBTS Diben-zothiazyl disulfide; ZBEC Zinc dibenzyl dithiocarbamate




le procedure so an additional mixing step using a planetary mixer was added (Table 2).

The first mixing step is figured out in a tangential internal mixer “HAAKE Rheomix 3000” (Thermo Fisher Scienti-fic) equipped with Banbury rotors. To re-alize effective dispersive mixing only 20 of 80 phr of liquid rubber is combined with 20 phr of bulk rubber and further masterbatch ingredients in mixing step 1. The rotational speed is set to 80 rpm and chamber/rotors are heated up to 60 °C. Sulfur (Rhenogran S80), sulfur do-nor caprolactam disulfide (Vulcofac CLD MB80) and accelerators are used in pre-dispersed granulated form, incorporated on the double roll mill for 5 minutes. For the final mixing stage the granulated masterbatch is added into a planetary mixer (LPV 1A40 type 420-16) together with the residual amount of 60 phr of li-quid rubber and mixed at moderate rotor speeds to slowly incorporate and dissol-ve the masterbatch in the liquid rubber phase. After the viscosity and the com-pound temperature have increased due to shear induced dissipative heating, the closed chamber is set under vacuum (<10 mbar) and the compound is homoge-nized at higher rotor speed.

The exact same planetary mixer was already used by Omelan et al. [10] to dis-perse carbon nanotubes (CNTs) in polydi-methylsiloxane (PDMS), where agglome-

rates of CNTs were optically measured smaller than 180 nm after the mixing process (10 min at 300 rpm). Therefor this mixer should have adequate shear force for the application with liquid rub-ber compounds. The compound tempe-rature at the end of the mixing process is 70-80 °C, depending on the components of the mixture. The final batch is imme-diately removed from the planetary mi-xer while being still warm and is filled in this “liquefied state” into in a syringe barrel system by “Vieweg GmbH” (barrel series 8000) as material reservoir for ad-ditive manufacturing. These syringe bar-rels can hold 55 ml of material and fit into the material heating unit of the “vi-pro-HEAD 5” dosing unit.

Characterization

Rheological analysis of the non-vulcanized materialA TA Instruments rubber process analy-zer of type „RPA elite“ is employed for the rheological analysis to determine the dynamic viscosity of the rubber com-pound. For this purpose, the compounds are measured at isothermal temperatu-res of 30 °C, 50 °C and 70 °C (“vipro-HEAD 5” unit can be heated up to max. 70 °C) with an angular frequency in the range from 0.314 s-1 to 314.159 s-1 (corresponds to 0.05 and 50 Hz) and an amplitude de-flection of 6.975 %.

Vulcanization behavior and sample preparationThe crosslinking properties of the com-pounds are characterized at different isothermal temperatures for at least 30 minutes on a moving die rheometer “MDR 3000 Basic“ from MonTech Materi-alprüfmaschinen GmbH. The temperatu-res vary between 150 and 180 °C, the measurement time is only extended if the sample still shows a marching modu-lus after 30 minutes. In addition, the t90 times for the production of vulcanized test plates are evaluated from this inves-tigation. The compounds are then vulca-nized in a laboratory press at 280 bar and 150 °C, corresponding to the respective t90 time adding 60 s for each mm of slab thickness. The vulcanized slabs measure 180 x 80 x 2 mm (length x width x thick-ness). These slabs are prepared for the physical-mechanical analysis.

Physical parametersThe tensile test on the vulcanizates is carried out according to DIN 53504 on a Zwick Z010 with type 2A test specimens. The initial load was set to 1 N (for the L-BR based recipe 1.005 the initial load had to be reduced to 0.3 N) and the testing speed was 200 mm/min. The Shore A hardness measurement is carried out in accordance with DIN 53505 on a Frank-Bareiss HP-04 hardness testing device.

Results

Rheological analysisThe rheological investigations are inten-ded to characterize the dependence of the dynamic viscosity respectively the rheological behavior of the material on shear rate and temperature and therefo-re the possible use of these compounds in additive manufacturing using the mentioned 3D-printer setup [1].

The results of the measurements for the dynamic viscosity η* are evaluated for 30 °C, 50 °C and 70 °C above the an-gular frequency ω for each compound (Figure 1). A clearly pronounced shear thinning effect (typical non-Newtonian behavior of decreased viscosity with in-creased shear rate) can be demonstrated for all materials combined with a tempe-rature dependency (the higher the tem-perature the lower the viscosity at cons-tant shear rate). The NBR based recipes show a pronounced effect in both cases. For additive manufacturing of these compounds using the “vipro-HEAD 5” unit the viscosity of the material to be

2 Table 2: Combined mixing processMixing time (cum.) ActionStage 1 (Internal mixer)

0”00’ 1/1 bulk rubber, 2/3 liquid rubber0”15’ ram lowered

1”00’ ram up; 2/3 carbon black1”15’ ram lowered2”00’ ram up; 1/3 liquid rubber, 1/3 carbon black

2”15’ ram lowered4”00’ ram up; admixture of stearic acid, ZnO, antioxidant, WB424”15’ ram lowered7”00’ ejection of masterbatch (MB)Stage 2 (Double roll mill)

0”00’ admixture of MB1”00’ admixture of S and accelerants3”30’ upturning (6x)5”00’ drop off sheetStage 3 (Planetary mixer; only liquid rubber compounds)

00”00’ adding MB and 60 phr liquid rubber01”00’ dissolving of MB in liquid rubber (200-250 rpm)

15”00’ homogenizing (400-500 rpm), vacuum45”00’ ejection and filling in cartridges




dosed is limited to 105 Pa*s. Moreover the unit has a heated screw-zone and material supply which is limited to 70 °C. That means for the bulk rubber based reference compounds that the viscosity of the material provided in the material reservoir without shear stress being pre-sent is too high (up to 106 Pa*s) for all relevant temperatures. On the other hand, all compounds that are based on high proportions of liquid rubber (1.001, 1.003 and 1.005) are well within the pro-cessing/viscosity range for the selected formulations independent on shear rate, even at room temperature. The results show, that there is enormous potential in further optimization of the recipes for specific applications because higher amounts of conventional bulk rubber or

filler concentrations seem to be possible within the compound without exceeding the system based limitations.

Overall the additive processing in the extrusion unit of the 3D printer should take place at higher temperatures accor-ding to the results. Compared to proces-sing at room temperature, the viscosity and the required energy can be signifi-cantly reduced, which will also help to avoid scorch effects during processing especially when thin nozzle diameters are used to print the layers with high re-solution. During additive manufacturing the conveying capacity and thus the discharged volume flow and initiated shear rate are limited by the geometry of the screw, technical implementation of the metering through eccentric screw

movement and the layer thickness in the printing process. The shear thinning ef-fect of the compounds can obviously be an advantage, but the maximum shear rate occurred while printing depends on the technical conditions of the dosing unit and cannot be further increased. On request the manufacturer ViscoTec con-firms the generally low shear rate within the pump mechanism. The possibly high-est shear rate occurs here in the nozzle area and is therefore dependent on the nozzle diameter selected. As a rough guide value, shear rates between 1 and 3 s-1 are mentioned [11]. Whether this value is realistic for the processing of li-quid rubber-based compounds remains to be answered and must be investigated in further studies.

Figure 1: Results of the rheological analysis by means of the RPA of the dynamic viscosity over ang. frequency ω for liquid rubber and refe-rence bulk rubber samples

1

Figure 2: Isothermal rheometer curves for different temperatures for liquid rubber and reference bulk rubber samples (left: NBR based compounds, center: IR based compounds, right: BR based compounds)

2




Vulcanization kineticsThe crosslinking kinetics are determined over a period of 30 minutes at isother-mal temperatures between 150 °C and 180 °C. The results are shown as torque over time in Figure 2. Each diagram com-pares the polymer specific reference compound without liquid rubber with the results of the liquid rubber based samples.

The results for the NBR based com-pounds 1.001 (L-NBR Nipol 1312LV) and 1.002 (Perbunan 2845 F) are shown in Figure 2, left. It can be seen that an incre-ased curing temperature leads to a shor-ten incubation time, a faster cure behavi-or reaching a plateau in the crosslink density for the reference bulk rubber based compound. For the liquid rubber based compound the cure kinetics is sig-nificantly slower showing a strong mar-ching modulus after 30 min of curing. This will lead to uneconomically long cycle times in the vulcanization of rubber based parts. Furthermore, increasing the vulcanization temperature leads to a well increased crosslink density where the possible maximum seems to be well

below the bulk rubber based compound despite the comparable ACN-content as reactive groups for the sulphur based vulcanization.

The results for the IR based com-pounds 1.003 (L-IR-390) and 1.004 (Nat-syn 2200) are shown in Figure 2, center. It can be seen that the material behavior of IR rubber differ significantly from the re-sults for NBR rubber. An increased curing temperature leads again to a shorten in-cubation time and a faster cure behavior. However, the achieved crosslink density is higher the lower the vulcanization temperature. The differences in the achieved crosslink density between bulk rubber and liquid rubber based com-pound are less significant than for NBR. It is noticeable that the liquid rubber based compound even achieves higher crosslink densities than the bulk rubber based compound. This contradicts the physical parameters determined. One re-ason for this can be a phase separation after the mixing process during storage. L-IR-390 is a copolymer of isoprene and butadiene rubber, which only partially dissolves in isoprene bulk rubber. In an

investigation on the DSC, this phase se-paration can be demonstrated by the formation of two glass transition tempe-ratures in the areas of the rubbers of the mixture. Mixtures of L-IR-390 without other bulk rubber types show a high gra-de of vulcanization on MDR tests.

After reaching a plateau a pronounced reversion respectively degradation of the sulphur network takes place with increa-sing cure temperature which is pro-nounced for the bulk rubber based com-pounds and which makes effective cu-ring processes more ambitious. Thermal aging of synthetically produced polyisop-rene units and desulfurization of the network increase with vulcanization time and temperature.

The results for the BR based com-pounds 1.003 (L-BR Polyvest 130) and 1.004 (Buna CB 24) are shown in Figure 2, right. It can be seen that an increased cu-ring temperature leads to a shorten incu-bation time, a faster cure behavior re-aching a plateau in the crosslink density for both the reference bulk rubber and the liquid rubber based compound. The achie-ved crosslink density is higher the lower the vulcanization temperature. No rever-sion or degradation processes can be se-en. For the liquid rubber based compound, it should be noted that the maximum crosslink density achieved is very low.Regarding additive manufacturing of the liquid rubber based compounds with the “vipro-HEAD 5” unit and the recipe/poly-mer dependent cure kinetic behavior, no processing-restricting effects are expec-ted. Due to the limited temperature set-tings (70 °C), the probably low shear ra-tes occurred in dependence of the nozzle diameter and the material dependent shear induced heating and the short resi-dence time of the material in the extrusi-on unit, the mass temperatures will not exceed 100-120 °C. It can therefore be assumed that crosslinking of the com-pounds during processing can be ruled out.

Mechanical propertiesThe results clearly show that the solid rubber-based vulcanizates achieve signi-ficantly higher tensile strength than their liquid rubber-based equivalents be-cause of their higher molecular weights. The formulations based on NBR (1.002) and IR (1.004) show significantly higher tensile strengths than the BR-based for-mulation (1.006). The elongation behavi-or is most pronounced for L-BR based compound (1.005).

Figure 4: Shore A hardness depen-ding on sample no.

4

75,9

51,2

68,5 64,7

17,4

71,1

Sample

1.001 1.0061.0051.0041.0031.002

Har

dnes

s [Sh

ore

A]

90

0

10

20

30

40

506070

80

Figure 3: Tensile strength and elongation at break depending on sample no.

3

Sample no. Sample no.

7,7

4,22,6 3,0

15,1 14,1

1.001 1.002 1.0061.0051.0041.003 1.001 1.002 1.0061.0051.0041.003

67,5

172,3

349,4

274,0

105,1141,9

20

10

0

5

15

Tens

ile st

reng

th [M

Pa]

Elon

gatio

n at

bre

ak [

%]

400

050

100

150200

300350

250




HardnessThe results show a significantly increa-sed Shore A hardness for the bulk rubber based NBR and BR compounds (1.002 and 1.006) compared to their liquid rub-ber based equivalents (1.002 and 1.005). The very low hardness of the L-BR based formulation is noticeable (1.005). For the IR rubber based samples there is no pro-nounced difference between the bulk rubber and the liquid rubber based sam-ple (1.003 and 1004).

With regard to the application of li-quid rubber-based compounds for additi-ve processing, it can generally be stated that all compounds could be cured and result in crosslinked parts with specific mechanical properties. The results of the rheological characterization have shown that the limiting factor on the processing side, the compound viscosity (the com-pound must be processible with the dis-penser unit), still offers potentials to im-prove the performance of the formulati-ons (e.g. through higher filler or bulk rubber concentrations).

Handling and storage of the liquid rub-ber based compoundsAfter production of the liquid rubber based compounds using the planetary mixer for the last mixing step, the com-pounds are filled while still warm and with a reduced viscosity in cartridge sys-tems from Vieweg (Basic 8000 Series) of-fering a max. fill volume of 55 ml. The Lu-er-Lock connection can be closed airtight for storage and mounted directly on the “vipro-HEAD 5” extrusion unit for additive processing of the material. The back of the cartridges is provided with a stopper, can be labeled and stored in the freezer for later use. Scorch effects, migration and blooming of components as well as oxida-tion can thus be avoided. The cartridges are made of polyethylene and are desig-ned for single use. But due to their stable geometry and resilient material, they can be easily cleaned and reused.

ConclusionThe results show the potential use of li-quid rubber based carbon black filled compounds for the additive manufactu-ring process based on the machinery se-tup which was published in part 1 of this publication [1]. The compound formula-tions are based on liquid NBR, IR or BR and were compared to bulk rubber based reference compounds. The mixing pro-cess presented in this thesis combines the mixing step with high shear and dis-sipative energy input in the internal mi-xer for an effective dispersion of the filler with an additional dilution step to incor-porate and homogenize the master batch in liquid rubber using the planeta-ry mixer. Although it is a discontinuous process, up to 350 g of material can be produced per cycle in the planetary mi-xer. The “vipro HEAD 5” extrusion unit in combination with the cartridge system as material reservoir which provides the liquid rubber based materials for additi-ve manufacturing can be heated up to 70°C. The practical limitations of the do-sing unit are given by the viscosity of the liquid media to be processed which is li-mited to values of 100 kPa*s. The viscosi-ties determined with the RPA are tempe-rature and shear dependent and show, especially at elevated temperature of 70 °C, viscosities less than 10 kPa*s. The cure kinetics are comparable to bulk rubber based compounds offering for liquid NBR and BR based compounds a significantly lower crosslink density. Due to the shor-ter chain length and chemical structure, the mechanical properties of compounds based on liquid rubber all show (signifi-cantly) lower results for tensile strength than the corresponding reference bulk rubber based compounds. Improved reci-pe design (higher filler and/or bulk rub-ber concentrations) to increase the mate-rial properties as well as experimental investigations regarding the additive manufacturing process will be done and published in further studies.

AcknowledgementThis publication evolved as part of a “ZIM – Zentrales Innovationsprogramm Mittelstand”-research project in coope-ration with Conspir3D GmbH (support-code: ZF 4369807BA9) and is sponsored by the Federal Ministry for Economic Af-fairs and Energy (BMWi). The material samples used were kindly provided by Evonik Industries AG (Polyvest), Zeon Chemical L.P. (NIPOL) and Kuraray Europe GmbH (Kuraray Liquid Rubber).

Figure 5: Cart-ridge system to provide liquid rubber based samples for ad-ditive manufac-turing.

5

REFERENCES[1] R. Thiel, B. Klie, U. Giese, “Part 1: Design and construction of an additive manufacturing unit for 3D-printing,” KGK Kautschuk Gummi Kunst-stoffe, 3-2021, 26.[2] H. Wittek, B. Klie, U. Giese, S. Kleinert, L. Bindszus, L. Overmeyer, “Approach for additive Manufacturing of high-vioscosity, curable Rub-bers by AME Processing (Additive Manufactu-ring of Elastomers) - Rubber 3D,“ KGK Kautschuk Gummi Kunststoffe, 06-2019, 53.[3] L. Sundermann, B. Klie, U. Giese, S. Leinewe-ber, L. Overmeyer, “Development, Construction and Testing of a 3D-Printing-System for Additive Manufacturing of Carbon Black filled Rubber Compounds,“ KGK Kautschuk Gummi Kunst-stoffe, 10-2020, 30.[4] S. Walker, O.D. Yirmibesoglu, U. Daalkhaijav, Y. Mengüc, “Additive manufacturing of soft ro-bots,” 2019, doi: 10.1016/B978-0-08-102260-3.00014-7.[5] T. Masuko, O. Yanagida, S. Yamamoto, “Pro-cessing for blending liquid rubber and carbon black,“ Japan Patent US 4,098,715, 4 Jul 1978.[6] R. T. Humpidge, D. Matthews, S. H. Morrell, J. R. Pyne, “Processing and Properties of Liquid Rubbers,” Rubber Chemistry and Technology 46. (1), 148 01 Mar 1973; doi: 10.5254/1.3545006[7] D. Kilian, R. Boehm und M. Maeda, “Funktio-nalisierte flüssige Kautschuke,“ Gummi Fasern Kunststoffe, 09-2010, 540.[8] P. J. Hochgesang, “Solventless liquid nitrile compounds.“ U.S. Patent 6,812,294 B2, issued Nov 2, 2004.[9] Schill + Seilacher „Struktol“ GmbH, “Struktol Kautschuk Additive“, 2015.[10] M. C. V. Omelan, A. Diekmann, U. Giese, “Development of soft electrical conductive PDMS/CNT-Composites with extremely low CNT Content,“ KGK Kautschuk Gummi Kunststoffe, 08-2020, 22.[11] ViscoTec Pumpen- u. Dosiertechnik GmbH, eMail correspendence 02 Mar 2021.


process chain • liquid rubber • planetary mixer • viscotec

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