elastomeric composite · electromagne- thermo-oxidative ... · shielding properties will be in...

PRÜFEN UND MESSEN TESTING AND MEASURING

24 KGK · 07-8 2016 www.kgk-rubberpoint.de

Elastomeric composite · electromagne-tic field · thermo-oxidative degradation

Rubber magnetic composites were pre-pared by incorporation of magnetically soft manganese-zinc ferrite H40 and metallic ferrosilicon powder FeSi into rubber matrix based on NBR. The aim was to investigate the influence of fer-rite content on physical-mechanical properties and electromagnetic smog shielding efficiency of prepared rubber composites before and after thermo-oxidative ageing performed in air at-mosphere. at 70oC for 168 hours. Ageing, performed under mentioned conditions, has no significant effect on physical-mechanical properties. All composites containing 200 phr or more of ferrite H40 or of metallic powder FeSi exhibit sufficient absorption shiel-ding of electromagnetic smog. Thermo-oxidation ageing at given parameters has no significant effect on shielding properties of composites.

Thermo-oxidative Alterung magnetischer NBR-Compounds Elastomerkomposit · elektromagneti-sches Feld · thermo-oxidative Alterung

Magnetische Elastomerkomposite sind durch Inkorporieren von magnetischem, weichem Zink-Ferrit H40 und von me-tallischem Eisensilikonpulver FeSi in ei-ne auf NBR basierende Matrix herge-stellt worden. Ziel war es, den Einfluss des Ferrit-Gehalts der Komposite auf die physikalischen-mechanischen Ei-genschaften und auf die Effizienz des Abschirmens von elektromagnetischem Smog, vor und nach thermisch-oxidati-ver Alterung, die bei 70 °C über 168 Std. vorgenommen wurde, zu untersuchen. Die gewählten Alterungsbedingungen haben keinen signifikanten Effekt auf die physikalisch-mechanischen Eigen-schaften. Alle Komposite, die 200 phr oder mehr des Ferrits H40 oder des me-tallischen Pulvers FeSi enthalten, wei-sen eine ausreichende Absorption zur Abschirmung von elektromagnetischem Smog auf. Thermo-oxidative Alterung unter den angegebenen Bedingungen hat keinen signifikanten Effekt auf die Abschirmeigenschaften.

Figures and Tables:By a kind approval of the authors.

1. IntroductionThe vast majority of the now almost 7 billion people population on this planet is born into a world dependent on elec-tricity. Almost none of us can imagine a world without electricity. The contributi-on of electricity to humanity is unquesti-onable, but the flip side is the production of unwanted electromagnetic radiation, also called electromagnetic smog. It is not only the cause of many health prob-lems but this electromagnetic smog also interferes with the operation of other electronic devices, which can lead to their malfunction and failure of their systems, information leakage or even to emergency situations. The World Health Organization states that although expo-sure to electromagnetic fields is not a modern phenomenon, mainly during the 20th century there has been a rapid in-crease in exposure to electromagnetic fields by man. Research shows that long-term exposure to electromagnetic radia-tion has serious effects on human health, especially in the terms of psychological health. There is evidence of neuropsychi-atric disorders, depression, attention de-ficit disorder, hyperactivity of children and suicidal tendencies caused by elec-tromagnetic radiation [1]. Electromagne-tic radiation is very often discussed in terms of male fertility and decrease of sperm production or sperm mobility, par-ticularly in the case of wearing a mobile phone in a pocket near the testicles. The current research shows a clear link bet-ween these two phenomena [2, 3]. Based on these factors, and increasingly con-nection of electromagnetic smog with various health problems, the demand for shielding of electromagnetic radiation is increasing.

There are three main ways how to successfully shield undesirable electro-magnetic radiation – reflection, absorpti-on and multi-reflection. The reflection is nowadays the most used and effective type of shielding mechanism. To achieve good reflection shielding effectiveness high conductivity of material has to be ensured. Therefore as the best reflection based materials are considered those filled with carbon materials and metal particles. From carbon composite materi-

als those based on carbon nanotubes, common or modified carbon fibers [4] or graphene [5]. Similar effects of reflection shielding of incident radiation are achie-ved by using metal particles based on Fe, Al, Mg or their alloy as filler [6] or by using combination of carbon materials and metals and mutual combination of different carbon materials [7]. Since the reflection mechanism only rebound the incident radiation from specific device or person back to environment it does not address the systematic reduction of level of electromagnetic radiation in environ-ment. Another problem, especially with metals, alloys, carbon fibers and graphe-ne is that they are effective mainly in higher frequencies from 8 GHz to 15 GHz of incident electromagnetic radiation, while general electric devices used in households and offices emits electroma-gnetic smog within frequency range 0.8 – 2 GHz. Thus considerable attention is nowadays paid by many research teams, including ours, to the preparation of ma-gnetic composites that are able to shield incident harmful electromagnetic radia-tion mainly by absorption instead of re-flection [8]. The main positive of absorp-tion shielding against reflection is the reduction of overall amount of immedia-tely emitted harmful electromagnetic smog and also electromagnetic radiation

Thermo-oxidative Ageing of ma-gnetic Composites based on NBR

AuthorsRichard Sýkora, Martina Matvejová, Mariana Ušáková, Bratislava, SlovakiaVladimir Babayan, Zlín, Czech Republic Corresponding author:Richard SýkoraDepartment of Plastics and RubberInstitute of Natural and Synthetic PolymersFaculty of Chemical and Food TechnologySlovak University of TechnologyRadlinského 9812 37 Bratislava, Slovakia E-Mail: [email protected]


25KGK · 07-8 2016www.kgk-rubberpoint.de

already accumulated in environment, be-cause during absorption the electromag-netic smog is changed into less harmful forms of energy, such as heat. In the ca-pacity of the fillers in these composites are used soft magnetic materials such as FeSi alloys and ferrites based on nickel, manganese-zinc or lithium [9].

2. TheoreticalFerrites consist of a mixture of iron oxide and oxides of other metals which results in the incorporation of large amounts of oxygen into the material. The high con-centrations of ferrite fillers necessary to obtain sufficient magnetic and shielding properties may affect the long term sta-bility of the rubber materials. Moreover, the iron ions can cause increased oxidati-on and thermo-oxidation of rubber. This is probably due to the catalytic action of iron ions on the decomposition of the hydroperoxides and large amounts of oxygen on the particle surface [10-12]. Another fact is that composites with shielding properties will be in contact with electronic parts, which have higher temperatures, than is the temperature of surrounding environment, due to cons-tant electric current that flow through them. Therefore is very important to know the behavior of composite polymer material containing ferrites or other me-tallic particles in the process of oxidative and thermo-oxidative ageing.

The oxidative degradation of unsatu-rated elastomers, such as acrylonitrile-butadiene rubber, is an autocatalytic, free radical chain reaction. The attack commences on the α-methylenic carbon atom in the chain. A hydrogen atom is extracted and, in the presence of oxygen, an oxidative chain reaction is initiated. The chain reaction then propagates au-to-catalytically [13].

Initiation•• +→ HRRH (1)

Propagation•• →+ ROOOR 2 (2)

•• +→+ RROOHRHROO (3)

If the polymers are constantly exposed to higher temperatures in atmosphere con-taining oxygen they subject to thermo-oxidative degradation. The higher tem-perature acts as a catalyst of oxidative reactions and it significantly increases production of free radicals, which further reacts with oxygen and produce unstab-le peroxides [14].

Initiation•• +→+ HOORORH T

2 (4)222 22 OHRORH T +→+ • (5)

Propagation•• →+ ROOOR 2 (6)

•• +→+ RROOHRHROO (7)

The above mentioned chain reactions terminates, in case of both oxidative and thermo-oxidative aging the same, as fol-lows [15]:

productsradicalNonROO →•2 (8)ROORRROO →+ •• (9)

2121 RRRR →+ •• (10)

The peroxides, which are created during propagation and termination phase of degradation, may decompose or react as follows [15]:

•• +→ OHROROOH (11)OHROOROROOH 22 ++→ •• (12)OHRRORHROOH 2++→+ •• (13)

ROHRRHRO +→+ •• (14)OHRRHOH 2+→+ •• (15)

In addition to these reactions increase of crosslinking density of rubber can be observed as a function of thermo-oxida-tive aging conditions and the mi-crostructure of rubber. This increase leads to increase of toughness of rubber and it´s composites and simultaneously cause decrease of elongation and flexi-bility of these materials [16]. These changes can lead to reduction of mole-cular weight, initiation and propagation of cracks, product spoilage and reduced compliance [17].

3. ExperimentalAs elastomeric matrix, model mixture based on an acrylonitrile-butadiene rub-ber (Zeon Chemicals, USA) was used. As fillers metallic ferrosilicon powder FeSi and manganese-zinc ferrite H40 were used. Mixtures contain only filler and standard semi-EV sulfur curing system consisting of zinc oxide, stearic acid, N-cyclohexyl-2-benzothiazole sulfenamide (CBS) and sulfur. The contents of these raw materials were the same in all mix-tures, only the type and content of mag-netic filler was changed from 0 to 600 phr. The general composition of the mo-del mixtures is listed in Tab. 1. Characte-ristics of both fillers are listed in Tab. 2 and Tab. 3.

The rubber compounds were prepared in the laboratory mixer BRABENDER in two mixing steps. In the first step the rubber and the fillers were compounded (7.5 min, 90°C), in the second step (4 min, 90°C) curing system was added. The pre-pared compounds were cured at 160°C for the optimum cure time tC90 by using the hydraulic press FONTIJNE.

The cross-link density of prepared vul-canizates was determined from equilibri-um swelling in acetone (nch – chemical cross-link density), using the Krause mo-dified Flory-Rehner equation for filled vulcanizates [18]. The resulting values are the average of two measurements.

Physical-mechanical properties of the prepared vulcanizates were measu-red by using Zwick Roell/Z 2.5 appli-ance, in accordance with the valid tech-nical standards, on the double side bla-de specimens (width 6.4 mm, length 800 mm, thickness 2 mm). The resulting values are the average of five measure-ments.

The complex permeability and dielec-tric permittivity spectra of the samples were studied in the frequency range from 1 MHz to 3 GHz, where we emplo-yed the impedance method using Impedance/Material Analyzer (Agilent E4991A). The measurements of complex permeability were performed on toroidal samples with an inner diameter of 3.1 mm and an outer diameter of 8 mm and thickness of 1 mm. The samples we-re cut out of composite plates by a screw press. Calculations of shielding characte-ristics were carried out in the program

1 Composition of the model mixturesComponent NBR Sulfur CBS ZnO Stearic acid Ferrite

Content [phr] 100 1.3 1.5 3 2 0-600

2 Characteristics of manganese-zinc ferrite H40Characteristics Values

Density r [g.cm-3] 5.13Specific surface area [m2. g-1] 1.37

Total porosity [%] 19.23Particle size [μm] 20 - 190

3 Characteristics of metallic powder FeSiCharacteristics Values

Density r [g.cm-3] 6.84Specific surface area [m2. g-1] 0.35

Total porosity [%] 28.53Particle size [μm] 40 - 100



Mathcad, using a programmed algo-rithm, where as input variables served values measured on the device Agilent E4991A.

For thermo-oxidative aging tests the Geer method was used. The measure-ments were performed in air atmosphere at 70°C and exposure time was equal to 168 hours. 4. Results and discussion

4.1 Influence of filler content and ther-mo-oxidative ageing on cross-link den-sity and physical-mechanical properties of prepared vulcanizates From Fig. 1 and Fig. 2 is evident that the presence of both types of fillers in the rubber matrix leads to a decrease of the cross-link density nch of vulcanizates, pro-bably due to the fact that the presented fillers act as a steric barriers to the for-mation of cross-links between rubber macromolecules and this effect is increa-sed with increasing amount of fillers in the vulcanizates. Graph in Fig. 1 compa-

res cross-link density of composites filled with FeSi powder before and after ther-mo-oxidative aging. As can be seen, the exposure of the samples to the thermo-oxidative aging led to slight increase of the cross-ling density of composites compared to the equivalent samples be-fore thermo-oxidation for composites filled in range from 0 to 200 phr. In case of composites filled with 400 and 600 phr of ferrosilicon powder there is no sig-nificant difference in values of cross-link density before and after aging. Fig. 2 displays dependence of the cross-link density from content of ferrite H40 befo-re and after thermo-oxidation. From pic-ture is obvious that the cross-link density of composites filled with manganese-zinc ferrite H40 change after aging ex-actly the same as in the case of composi-tes filled with ferrosilicon powder. The probable cause of increasing of cross-link density nch of vulcanizates after the aging process is the additional crosslinking of rubber compounds caused by residual sulfur and curing additives which have

not reacted in the primary process of vulcanization.

Despite that the values of physical-mechanical properties of vulcanizates were relatively small (the cause are poor physical-mechanical properties of NBR in general), from the experimental data there is obvious, that the presence of both fillers in elastomeric matrix has in-fluence on evaluated characteristics. In Fig. 3 we can see steady decrease of the modulus M300 of composites filled with FeSi powder as a function of ferrite loa-ding. The values of modulus M300 of composites filled with FeSi decrease from 1.6 MPa to approximately 0.9 MPa. It is also visible that aging slightly im-proves modulus M300 and thus stiffness of vulcanizates by about 13%. The reason might be the decrease of flexible sulfur bonds during the process of aging. Simi-lar situation is in case of modulus M300 of materials containing ferrite H40 as filler (Fig. 4). In this case the decrease of stiffness is bigger than in case of com-pounds containing FeSi powder. The de-

Fig. 2: Influence of H40 content and thermo-oxidative aging on chemical cross-link density nch of rubber composites.

2

Fig. 1: Influence of FeSi content and thermo-oxidative aging on chemical cross-link density nch of rubber composites.

1

Fig. 3 Influence of FeSi content and thermo-oxidative aging on modulus M300 of composites.

3

Fig. 4 Influence of H40 content and thermo-oxidative aging on modulus M300 of composites.

4



crease is from 1.6 MPa to about 0.7 MPa in case of original samples and from 1.85 MPa to 0.95 in case of aged samples. Also in here we can observe the increase of modulus of samples after thermos-oxidation.

Tensile strength at break of both FeSi and H40 filled composites (Fig. 5, Fig. 6) shows decrease of its values with incre-asing content of filler. While FeSi com-posites show uniform decrease of their values, basically, H40 composites show abrupt decrease of values at filling le-vels 100 phr and 400 phr. Despite the different type of decrease both types of fillers decrease tensile strength at break from 2.6 MPa for unfilled samples to 1.6 MPa for compounds containing ultima-te amount of filler at level 600 phr. Therefore we can say that only content not type of filler influences tensile strength at break. From both pictures is evident that aging performed under 70°C for 168 hours does not significant-ly affected the values of tensile strength at break since values for original and

aged samples are practically the same in case of all vulcanizates filled with both types of fillers.

The elongation at break of vulcaniza-tes increases with increasing amount of both fillers. The FeSi composites (Fig. 7) also show steady increase of elongation at break, however much smaller compa-red to the composites filled with ferrite H40 - from 470% to around 560%. The elongation at break of H40 composites (Fig. 8) rises from 470% to around 650%. This is evidence that magnetically soft fillers used in this work do not have stif-fening effect on rubber matrix. Elongati-on at break of aged samples shows de-crease in both cases, while decrease of values is more evident in case of compo-sites filled with manganese-zinc ferrite H40. The reason of decrease of samples flexibility is the same as in case of incre-ased stiffness – reduction of flexible po-lysulfidic bonds in the process of aging and their change to shorter less flexible sulfidic bonds (mono-, di- or trisulfidic, etc.).

4.2 Influence of filler content and thermo-oxidative aging on shielding properties and shielding effectivity of vulcanizatesThe most important property that was measured in this work was shielding efficiency of prepared composites. If the shielding efficiency should be considered sufficient, total absorption of interference radiation must be at least 95%. This level of absorption is achieved if the measured return loss is at the level -10 dB or lower. Majority of common electronic devices, such as mobile phones, notebooks, tab-lets, TV etc., emit electromagnetic radiati-on within frequency range 0.8 – 2 GHz. Therefore the prepared rubber composite materials should absorb somewhere within the emitted frequency to ensure effective protection from electromagnetic smog produced by these devices.

Fig. 9 depicts the absorption compo-nent of overall shielding efficiency of prepared acrylonitrile-butadiene rubber composites containing metallic powder FeSi as filler. The results revealed fact

Fig. 6: Influence of H40 content and thermo-oxidative aging on tensile strength at break of composites.

6

Fig. 5: Influence of FeSi content and thermo-oxidative aging on tensile strength at break of composites.

5

Fig. 7: Influence of FeSi content and thermo-oxidative aging on elongation at break of composites.

7

Fig. 8: Influence of H40 content and thermo-oxidative aging on elongation at break of composites.

8



that the shielding effectiveness improves with increasing FeSi powder content up to 400 phr of filler content. However samples with maximum filling level of 600 phr of FeSi powder have deteriorated maximum shielding efficiency and also great shift of effective shielding area to lower frequencies, which are mostly out of desirable levels 0.8 to 2 GHz. Based on the studies carried on, we can conclude that the most promising shielding pro-perties display composites containing 200 and 400 phr of FeSi, since they dis-play reflective loss of -10 dB or more within the frequency range from 0.7 to 1.05 and 1 to 1.15 GHz respectively. The-se frequencies are within the desirable levels mentioned above. The absorption maximum is at the value -21 dB at the frequency 1.07 GHz of the incident radia-tion in case of composite containing 200 phr and -23 dB at frequency 0.8 in case of composite containing 400 phr.

Fig. 9: Influence of FeSi content on shielding properties of com-posites.

9

If we compare the absorption shiel-ding effectiveness of aged FeSi composi-tes (Fig. 10) with original samples (Fig. 9) it is evident that their shielding efficien-cy is exactly the same in terms of effecti-ve shielding frequency range and also absorption maxima levels. Based on this finding we can conclude that aging car-ried under the conditions mentioned in chapter 3 has no effect on absorption shielding of prepared materials filled with ferrosilicon powder.

From the graph displayed in Fig. 11 is clear, that the shielding effectiveness of manganese-zinc filled materials im-proves with increasing ferrite content up to 400 phr similar as in the case of FeSi composites. Same as for FeSi filled materials, sample containing 600 phr of manganese-zinc ferrite, displays a dete-rioration of shielding efficiency. The measurements also show that with the increase of filler content in composites,

the absorption maxima shifts to lower frequencies and this shift is much more significant than in case of FeSi rubber composites. Based on the studies car-ried on, we can say that as the best shielding properties are displayed by shielding material containing 200 phr of filler H40, because this material has a reflective loss of -10 dB or better in the frequency range from 0.6 to 1 GHz. This material has also the best shielding ma-xima at level -36 dB.

Fig. 12 represents absorption shiel-ding of aged composites filled with H40. Here it can be seen that ageing caused moderate shift of effective absorption frequency range by 0.1 GHz to higher frequencies compared to the original samples. However, it can be concluded that aging has no significant effect on overall absorption of incident radiation neither in case of this type of composi-tes.

Fig. 10: Influence thermo-oxidation on shielding properties of composites filled with FeSi.

10

Fig. 11: Influence of H40 content on shielding properties of com-posites

11

Fig. 12: Influence thermo-oxidation on shielding properties of composites filled with H40

12



4. ConclusionIn the present work, rubber magnetic composites were prepared by incorpora-tion of magnetically soft manganese-zinc ferrite H40 and magnetic ferrosili-con powder FeSi into rubber matrix based on in acrylonitrile-butadiene rub-ber (NBR). The content of magnetic filler in the rubber matrices was changed from 0 to 600 phr. The increasing amount of filler content cause further decreasing of rubber modulus M300 and tensile strength at break, however the elongation at break show increase of its values. Therefore we can say that manganese-zinc ferrite H40 and metal-lic powder FeSi16 do not act as reinfor-cing fillers. From thermal oxidation test is obvious that aging has no significant effect on any of measured physical-me-chanical properties. Values of tensile strength at break remain basically the same after the aging of samples. Rubber modulus M300 of aged samples was slightly higher, while elongation at break shows slight decrease of its values compared to the samples before ther-mo-oxidative aging.

All composites containing 200 phr or more of both fillers exhibit sufficient ab-sorption shielding of electromagnetic smog. The results further show that the composites filled with manganese-zinc ferrite H40 are superior to the materials

filled with FeSi, because they absorb radiation wider frequency range from 0.5 GHz to 1.2 and their maximum re-turn loss is better than that of the FeSi composite materials. After thermo-oxi-dative tests we can conclude that aging does not have any significant influence on shielding properties of composites and the levels of return loss as well as effective absorption frequency ranges of prepared composites remain without substantial changes.

Acknowledgement This work was supported by the Slovak Research and Development Agency un-der the contract No. APVV069412.

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DIK The Fall Rubber Colloquium takes place from 22nd to 24th of November 2016 in Hannover (Germany). This will be the twelfth conference on rubber technology in this series. The previous conferences were highly successful and attracted delegates from a variety of disciplines from around the world. Delegates are encouraged to re-gister both from outside and within Germa-ny. The conference highlights the latest and most important scientific concepts and ad-vanced processing techniques in rubber and polymer science and technology. Experts from all over the world will give keynote presentations. Recognized industrial ex-perts will identify future trends in the rub-ber industry. Basic researchers will present studies to understand, and therefore, pre-

dict the properties of final products. The poster session is an additional good oppor-tunity for scientific discussions and know-ledge exchange. Conference topics are: New Polymers, Thermoplastic Elastomers, Rein-forcement, Vulcanization Chemistry, Elasto-mer Physics, Compounding Machinery and Mixing, Extrusion, Calendering, Injection-Moudling, Nanocomposite, Tire Technology, Additives and Modifiers, Frictions and Wear, Aging, Life time prediction, Simulations, Elastomer Analysis, Environment, Green technology, Sustainability. • Organizers: Prof. Dr. Ulrich Giese and Mrs. Trinidad Rodriguez Gallegos, Deut-sches Institut für Kautschuktechnologie, Eupener Str. 33, 30519 Hannover, Phone: +49 511 84201-17, E-Mail: khk@DIKaut-

schuk.de, Internet: www.DIKautschuk.de• Venue: Crowne Plaza Hotel Hannover Schweizerhof, Hinüberstrasse 6, 30175 Hannover (Germany), Phone: +49 511 3495-0, www.schweizerhof-hannover.de, Crowne Plaza Hotel Hannover in Google Maps• Conference Fee: The registration fee, to be paid until November 5, 2016, is as fol-lows: DIK members - 680,- Euro, Nonmem-bers - 850,- Euro, Students - 300,- Euro (*Students have to send a letter verifying their status (fax or postal mail).• Entitlements: The fee covers colloquium attendance and materials, coffee breaks, welcome reception. Additionally, the confe-rence fee includes the Conference Dinner.

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