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Influence of thermo-oxidative and ozone ageing on the properties of elastomeric magnetic composites
Polymer Degradation and Stability 97 (2012) 921e928
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Influence of thermo-oxidative and ozone ageing on the properties of elastomeric magnetic composites Ján Kru zelák a, *, Ivan Hudec a, Rastislav Dosoudil b a
Department of Plastics and Rubber, Institute of Polymer Materials, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovakia b Department of Electromagnetic Theory, Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, Ilkovicova 3, 812 19 Bratislava, Slovakia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 February 2012 Received in revised form 12 March 2012 Accepted 16 March 2012 Available online 28 March 2012
Elastomeric magnetic composites are materials with ferromagnetic fillers as one of the constituents and rubber blend as polymer matrix. Ferrites, as commonly used magnetic fillers, might be able to influence the long-term stability of rubber materials, mainly if they are applied in high concentrations. Therefore, the thermo-oxidative and ozone stability of ferrite as well as ferrite and carbon black filled composites based on natural and butadiene rubber were studied by oven ageing and thermogravimetric analysis. The results revealed that ferrite alone also ferrite in combinations with carbon black influences the properties of evaluated materials in various ways. But the thermo-oxidative stability of prepared composites seems not to be influenced by the amount of ferrite even in case of high magnetic filler contents. From thermogravimetric curves it became evident that the thermal degradation of prepared materials also was not influenced by the doping content of ferrite. The ozone ageing tests demonstrated that the influence of ferrite on the ozone stability of natural rubber as well as butadiene rubber based composites was ambiguous. In case of natural rubber based composites filled with combinations of ferrite and carbon black, it can be stated that the more carbon black rubber materials contain, the better is the resistance against ozone degradation. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Elastomeric composite Magnetic filler Thermo-oxidative ageing Thermogravimetric analysis Ozone degradation
1. Introduction In the recent years a rapid interest in smart materials consisting of elastomeric matrix and magnetically polarizable particles has been shown [1e3]. The rheological and mechanical properties of such types of materials can be controlled by an external magnetic field. During the vulcanization, magnetic particles in the rubber matrix are oriented to the direction of applied external magnetic field. The results showed that such structure inside the rubber matrix leads to a strong anisotropic behaviour of prepared composites. These materials have attracted considerable interest for the sake of their applications in many fields of technological and industrial branches [4,5]. Ferrites represent a well established family of magnetic materials. M-types of hexagonal ferrites, which were used in our work, are compounds of iron oxide with the oxides of some other metals of general formula MeO.6Fe2O3 (M is divalent cation such as Sr, Ba, Pb). High values of magneto-crystalline anisotropy and saturation magnetization allow wide application of these materials as permanent magnets. Low price and very good chemical stability * Corresponding author. Tel.: þ421 908478184. E-mail address: [email protected] (J. Kru zelák). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2012.03.025
together with suitable magnetic characteristics include ferrites in the most important magnetic materials which cannot be easily replaced. Ba and Sr ferrites are the most common applied magnetic powdery fillers. It is well known that ageing of rubber materials and vulcanizates is caused by chemical, oxidativeedestructive processes, where oxygen and heat play the crucial influence. Furthermore, iron ions are able to enhance the oxidation of rubber materials, as they initiate the induced dissociation of hydro-peroxides present in rubber materials. Therefore, the aim of this work was to investigate the influence of strontium ferrite on the thermo-oxidative and ozone stability of elastomeric composites based on natural and butadiene rubber by oven ageing and thermogravimetric analysis.
2. Experimental 2.1. Materials The two types of elastomers, natural rubber SMR 20 (Mardec, Malaysia) and cis-1,4-butadiene rubber (Lanxess, Germany) were filled with magnetic particles in order to prepare elastomeric magnetic composites. Anisotropic strontium hexaferrite SrFe12O19,
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922
tlá Hora, type FD 8/24 prepared by wet milling (MAGNETY, Sve Czech Republic) was applied as a magnetic filler. It is a product with additional polyvinyl alcohol, which covers the surface of ferrite particles. Ferrite, which was used in our experiments, was prepared by dissolution of polyvinyl alcohol by extraction in hot water. Ferrite was dosed alone or in combinations with carbon black (N330). In rubber compounds filled only with ferrite, the content of magnetic filler was changed from 0 to 100 phr. In case of rubber compounds filled with combinations of ferrite and carbon black, the content of both fillers was kept constant (85 phr), only the weight fraction of ferrite in combinations of both fillers (wF ¼ mF/ (mF þ mCB)) was changed. For the preparation of rubber compounds filled with fillers combinations, carbon black batch of natural rubber was used. A semi-EV sulphur vulcanization system was introduced in all experiments. The detailed compositions of rubber compounds are shown in Tables 1 and 2.
Table 2 Composition of rubber compounds filled with fillers combinations. Component
NR
ZnO
Stearin
CBS
Sulphur
Ferrite þ carbon black
Content (phr)
100
5
1.5
1.5
1.3
85
s
2 a a2
¼ C1 þ
C2
a
(2)
s e tension (MPa) a e relative extension (%)
C1, C2 e constants
vt ¼ 2C1 =RT
(3)
R ¼ 8.314 J/K.mol T ¼ 293.15 K
2.2. Methods The preparation of elastomeric magnetic composites was carried out in the laboratory mixer BRABENDER in two mixing steps. In the first step the rubber and the filler were compounded (9 min, 90 C), in the second step (4 min, 90 C) the curing system was added. The curing process was performed at 150 C for the optimum cure time under a pressure of approximately 20 MPa by using the hydraulic press FONTUNE. The physicalemechanical properties of prepared composites were measured in accordance with the valid technical standards, on the double side blade specimens (width 6.4 mm, length 10 cm, thickness 2 mm) by using TIRATEST equipment. Magnetic measurements of composites on the magnetometer TVM1 at room temperature and maximum coercivity of 750 kA/m were determined. The basic principle of measurement is induction method of scanning of scattering magnetic flux F induced by magnetic vibrating sample. Magnetic field is generated by means of two cores of Weiss electromagnet at a minimum distance of poles adaptors equals to 7.5 mm. The specimens were of prism shape (8 4 4 mm). The two different methods were used to in order to determine the cross-link density of cured samples. In the first method the chemical cross-link density nch was determined based on equilibrium swelling of samples in xylene. The FloryeRehner equation modified by Krause for filled vulcanizates [6] was used:
Vr0 lnð1 Vr Þ þ Vr þ cVr2 VS V 1=3 V 2=3 0:5Vr
nch ¼
r
(1)
r0
nch e chemical cross-link density (mol/cm3)
Vr0 e volume fraction of rubber in equilibrium swelling sample of vulcanizate in absence of fillers Vr e volume fraction of rubber in equilibrium swelling sample of filled vulcanizate VS e molar volume of solvent (for xylene ¼ 123.45 cm3/mol) c e Huggins interaction parameter (for xylene e natural rubber c ¼ 0.4106, for xylen e e butadiene rubber c ¼ 0.39) In the second method based on deformation measurements, the total cross-link density nt was calculated by means of the MooneyeRivlin equation [7]: Table 1 Composition of rubber compounds filled only with ferrite. Component
NR, BR
ZnO
Stearin
CBS
Sulphur
Ferrite
Content (phr)
100
3
2
1.5
1.3
0e100
The measurements were carried out in the INSPEKT desk 5 kN apparatus (METROTEST), up to 100% deformation, deformation velocity of 10 mm/min. The cross-link structure of vulcanizates was investigated using the thiol-amine method in argon atmosphere as described in [8e10]. There was used propane-2-thiol, which has ability to decompose polysulfidic cross-links and hexane-1-thiol, which is more reactive and effectively decomposes polysulfidic and disulfidic cross-links of vulcanizates. The reactivity and efficiency of thiols is sensitive to different factors, as the structure of thiols (primary, secondary thiols), concentration, reaction time, temperature, size of sample and structure of rubber compounds, too [8]. First, the chemical cross-link density nch of original samples was determined. Then, the samples were treated with solutions of thiols at laboratory temperature in argon atmosphere and the cross-link density was determined again (nPT e the cross-link density of samples after treating them with propane-2-thiol and nHT e the cross-link density of samples after treating them with hexane-1thiol). The content of polysulfidic Sx, disulfidic S2 a monosulfidic S cross-links was then possible to calculate:
Sx ¼ vch vPT ; Sx ¼ ½ðvch vPT Þ,100=vch
(4)
S2 ¼ vPT vHT ; S2 ¼ ½ðvPT vHT Þ,100=vch
(5)
S ¼ vch ðSx þ S2Þ; S ¼ ðvHT =vch Þ,100
(6)
Thermogravimetric analysis was carried out in DERIVATOGRAPH Q-1500D apparatus in temperature range 20e600 C in air atmosphere at a constant heating rate of 5 C/min. For thermo-oxidative ageing tests the Geer method was used. The measurements were performed in air atmosphere at 70 C, exposure time was equal to 72 h. Ozone ageing tests of prepared composites were performed in the ozone chamber at 40 C and ozone concentration of 50 pphm. First, test samples were placed into the special frames (Fig. 1). The upper part of test samples was deformed in 31%, whereas the lower part was without deformation. After 24 h, the frames with test samples were placed into the ozone chamber and in the given time intervals, the formation of ozone ruptures on the surface of the samples was observed. The resistance of composites against ozone was described by means of threshold deformation (TD). It is the biggest value of the static tensile deformation at which the ozone ruptures on the surface of the samples are not spotted yet after the given time. The higher resistance of composites against ozone, the bigger is the value of threshold deformation.
J. Kruzelák et al. / Polymer Degradation and Stability 97 (2012) 921e928
923
TSb of NR-composites (MPa) Fig. 1. Test samples of composites.
2.5
19 2.0
16 13
NR BR 0
20
3. Results and discussion 3.1. Influence of ferrite and thermo-oxidative ageing on physicalmechanical and magnetic properties Elastomeric magnetic composites were prepared based on natural and butadiene rubber without any antioxidants and antiozonants added. Therefore, soft thermal conditions for thermooxidative ageing were chosen, namely 70 C and 72 h. Despite the fact that the values of physicalemechanical properties of butadiene rubber based composites were relatively small, the presence of ferrite in the elastomeric matrix leads to the enhancement of evaluated characteristics. From Fig. 2 it is observable non-linear increase of the tensile strength at break as a function of ferrite content in BR-composites. The increase of the tensile strength at break value of the composite filled with the maximum ferrite content represents more than 75% in comparison with the tensile strength value of the ferrite free sample. The increasing tendency on the magnetic filler content was detected also in case of the elongation at break (Fig. 3). After exposure of test samples to the conditions of thermo-oxidative ageing the values of tensile strength slightly increased, while the specific values of elongation at break decreased. The biggest decline of elongation at break was observed in case of the composite with the maximum ferrite content. On the other hand the elongation at break of composites based on natural rubber showed decreasing tendency with increasing amount of ferrite (Fig. 3). At the maximum ferrite content 17% decrease of the observed property compared to the unfilled sample was spotted. The next decline of elongation at break values was recorded after thermo-oxidative degradation. The decrease of the elongation at break after thermo-oxidative degradation did not exceed 8% of the equivalent elongation at break values of original non-aged composites. In spite of that there is evidently observable the dependence of evaluated characteristics on the magnetic filler content, the tensile strength at break of NR-composites changed only slightly as shown in Fig. 2. One can see that the slight maximum of the tensile strength at break is reached somewhere around medium ferrite contents. When test samples were exposed to the conditions of
60
80
1.5
1.0 100
Ferrite (phr)
(7)
a e the distance between the upper and the lower side of the test sample (cm) b e the distance of the onset of the rupture from the lower side of the test sample (cm) D e deformation (%)
40
NR 72h BR 72h
Fig. 2. Influence of ferrite content and thermo-oxidative ageing on tensile strength at break TSb of composites based on natural and butadiene rubber.
thermo-oxidative ageing the increase of the observed property was detected. The influence of thermo-oxidative ageing was possible to see mainly in case of the reference sample and the sample with the minimum magnetic filler content. With increasing amount of ferrite, the influence of ageing was not so significant. The different character of physicalemechanical properties of both BR- and NR-composites on the ferrite content is attributed to the structure of selected elastomeric matrices. Natural rubber shows high crystallizing capability, while the crystallization of butadiene rubber is very restricted, in spite of the fact that BR implies high content of cis-1,4 structural units. This could be the reason why the values of tensile strength and elongation of natural rubber are much higher in comparison with those of butadiene rubber. From the above mention changes it becomes evident that ferrite does not act as a reinforcing filler in rubber materials. The degree of reinforcement is rather low even at the maximum ferrite content. The reason might be attributed to the small adhesion between ferrite particles and the rubber matrix with many voids existing between the two phases. The inner voids lead to the stress concentration inside the sample, what may result in rupture at low stress. The influence of fillers combinations (ferrite and carbon black) on physicalemechanical properties of natural rubber based composites is illustrated in Fig. 4. From Fig. 4 it becomes apparent
900
160
800
140
700
120
600
100
500
80
400
60
300 NR BR
200 100 0
20
40
60
NR 72h BR 72h 80
40
Eb of BR-composites (%)
b ,D a
3.0
22
10
Eb of NR-composites (%)
TD ¼
25
TSb of BR-composites (MPa)
3.5
28
20 100
Ferrite (phr) Fig. 3. Influence of ferrite content and thermo-oxidative ageing on elongation at break Eb of composites based on natural and butadiene rubber.
J. Kruzelák et al. / Polymer Degradation and Stability 97 (2012) 921e928
30
8 7
5
6 4
5 4
3
3
2
2 1 0 20
40
NR
NR 72h
BR
BR 72h
60
1
0 100
80
Ferrite (phr) Fig. 5. Influence of ferrite content and thermo-oxidative ageing on remanent magnetic induction Br of composites based on natural and butadiene rubber.
filled only with magnetic filler with approximately the same content of ferrite, one can see the similar values of the remanent magnetic induction. Therefore it can be stated that the incorporation of carbon black into the rubber matrix has no substantial influence on magnetic characteristics of elastomeric magnetic composites. Magnetic characteristics are dependent only on the amount of applied ferrite filler regardless of the presence of reinforcing filler. From Figs. 5 and 6 it becomes also clearly evident, that there were almost no changes of the evaluated magnetic characteristic in consequence of thermo-oxidative degradation.
3.2. Influence of ferrite and thermo-oxidative ageing on the crosslink density and cross-link structure The cross-link density and the structure of cross-links in vulcanizates are very important characteristics of all cured rubber systems. Not only the original properties but also the changes of original properties of vulcanizates during their using are dependent on these characteristics. Therefore, the investigation of the magnetic filler influence and thermo-oxidative ageing on the crosslink density of prepared composites was also part of our interest. First, the total cross-link density nt as well as the chemical cross-
6
900
5
25 700
15
500
10 300 5
TSb Eb
0 0
0.2
0.4
0.5
0.6
TSb 72h Eb 72h 0.8
100 1
wF Fig. 4. Influence of ferrite weight fraction and thermo-oxidative ageing on tensile strength at break TSb and elongation at break Eb of composites filled with combinations of ferrite and carbon black.
Br.10 2 (T)
4
20
Eb (%)
TSb (MPa)
6
Br.102 of BR-composites (T)
that the tensile strength at break and the elongation at break of composites were found to increase in the whole examined ferrite concentration range. As a result of thermo-oxidative degradation the values of elongation at break decreased in range from 9 to 13% in comparison with the original values of composites before ageing. The values of tensile strength at break after ageing remained nearly unchanged. The different character of physicalemechanical properties of composites filled with fillers combinations in comparison with the properties of composites filled only with ferrite is attributed mainly to the reducing content of carbon black in combinations with ferrite. Due to very good compatibility and adhesion between carbon black and the rubber matrix, carbon black plays significant role as a reinforcing filler in the rubber materials. Due to very good interactions and binding forces between carbon black and the rubber, the mobility of macromolecular chains is restricted and the total reinforcement of the rubber matrix is then achieved. As a result, the improvement of physicalemechanical properties of composites is observed. Therefore, the increase of the tensile strength at break with decreasing content of carbon black in fillers combinations seems to be very remarkable and surprising. This behaviour is very hard to explain. Based upon the results obtained by the study of influence of thermo-oxidative ageing, it can be concluded that this process affects the evaluated properties in various ways. But almost all evaluated properties of composites after ageing remained the character of their dependences on the content of ferrite, or fillers combinations. The values of physicalemechanical properties of composites after ageing are higher or lower in comparison with corresponding values of composites before thermo-oxidative degradation. Magnetic characteristic of composites were evaluated at a laboratory temperature and maximum coercivity of 750 kA/m. As plotted in Fig. 5 the presence of magnetic filler in the rubber matrix leads to the significant enhancement of the remanent magnetic induction Br. The difference between the values Br of the sample with minimum and maximum ferrite content was approximately 560% in case of NR-composites and more than and 390% in case of BR-composites, respectively. The values of remanent magnetic induction were also found to significantly increase with increasing of the ferrite weight fraction in composites filled with fillers combinations (Fig. 6). When comparison the Br values of composites filled with fillers combinations with the Br values of composites
Br.102 of NR-composites (T)
924
3 2 1
Br
Br 72h
0 0.2
0.4
0.6
0.8
1
wF Fig. 6. Influence of ferrite weight fraction and thermo-oxidative ageing on remanent magnetic induction Br of composites filled with combinations of ferrite and carbon black.
J. Kruzelák et al. / Polymer Degradation and Stability 97 (2012) 921e928
Ferrite (phr)
nt.104 (mol/
nch.104 (mol/ cm3)
nph.104 (mol/ cm3)
0h
72 h
0h
72 h
0h
72 h
1.27 1.37 1.39 1.53 1.74 1.88
1.31 1.44 1.67 1.81 2.05 2.37
1.14 1.14 1.18 1.26 1.29 1.40
1.19 1.25 1.23 1.39 1.37 1.47
0.13 0.23 0.21 0.27 0.45 0.48
0.12 0.19 0.44 0.42 0.68 0.90
cm3)
0 20 40 60 80 100
link density nch were determined. Then, the physical cross-link density nph was possible to calculate. Polymerepolymer physical interactions, polymer-filler physical interactions, also various intramolecular and intermolecular entanglements are involved in physical cross-link density. The results of measurements showed that the total cross-link density nt as well as the chemical cross-link density nch of NRvulcanizates increased with increasing of ferrite content (Table 3). The physical cross-link density nph, which represents the difference between the total and the chemical cross-link density (nph ¼ nt nch), is much lower than nch and was also found to increase with increasing of ferrite content. By contrast, the chemical cross-link density of BR-vulcanizates showed decreasing tendency with doping content of ferrite. The total cross-link density of BR-composites was lower in comparison with the reference sample. However, the content of ferrite has no significant influence on the values of nt (Table 4). The experimentally obtained values of the cross-link density for vulcanizates filled with combinations with ferrite and carbon black are shown in Table 5. One can see that the increase of ferrite weight fraction leads to the consistent linear decrease of the total crosslink density. The difference between the density nt of the sample filled with carbon black only and the sample filled with ferrite only is more than 50%. The same situation was possible to see also in case of the chemical and physical cross-link density, which were also found to gradually decrease in dependence on the ferrite weight fraction increasing. In consequence of mutual interactions between carbon black and the rubber, the physicalechemical adsorption of the rubber to the surface of carbon black particles is ensured. This binding between the rubber and carbon black causes the partial immobilization of the rubber macromolecules and influences the mobility of carbon black particles enwrapped by rubber. The rubber layer closest to the surface of carbon black particles behaves as a polymer in glassy state and is insoluble in solvents suitable for rubber. Polymer mobility increases with distance from the carbon black surface. The overall reduction in polymer mobility associated with
Table 4 Total nt, chemical nch and physical nph cross-link density of vulcanizates based on butadiene rubber before ageing and after ageing. Ferrite (phr)
nt.104 (mol/
nch.104(mol/
nph.104 (mol/ cm3)
0h
72 h
0h
72 h
0h
72 h
1.91 1.82 1.83 1.87 1.83 1.87
2.16 2.05 1.97 2.18 2.15 2.23
1.72 1.66 1.64 1.58 1.57 1.50
1.91 1.81 1.75 1.91 1.85 1.88
0.19 0.16 0.19 0.29 0.26 0.37
0.25 0.24 0.22 0.27 0.30 0.35
cm3)
0 20 40 60 80 100
cm3)
Table 5 Total nt, chemical nch and physical nph cross-link density of vulcanizates filled with combinations of ferrite and carbon before ageing and after ageing. Compound
CB:F CB:F CB:F CB:F CB:F CB:F CB:F
wF
5:0 4:1 3:2 1:1 2:3 1:4 0:5
0 0.2 0.4 0.5 0.6 0.8 1
nt.104 (mol/ cm3)
nch.104 (mol/
nph.104 (mol/
0h
72 h
0h
72 h
0h
72 h
3.59 2.99 2.68 2.79 2.51 2.17 1.89
3.74 3.40 3.39 3.38 3.13 2.67 2.23
2.11 1.81 1.83 1.71 1.68 1.51 1.31
2.53 2.34 2.18 2.08 1.97 1.79 1.46
1.48 1.18 0.85 1.08 0.83 0.66 0.58
1.21 1.06 1.21 1.30 1.16 0.88 0.77
cm3)
cm3)
bound rubber gives a broad relaxation behaviour to the rubber compound. Bound rubber actually consists of physical adsorption of polymer chain as well as chemical adsorption, and it is generally accepted that physical adsorption is of greater importance for rubber reinforcement that chemical adsorption. Only a fraction of the bound rubber actually participates in the chemical adsorption and the majority consists of loops and dangling chains [11e13]. On the other hand, the interactions between the rubber matrix and ferrite are only of weak physical character due to poor bonds and adhesion between the magnetic filler and the rubber matrix. The above mentioned facts are likely responsible for the improvement of the cross-link density of vulcanizates with increasing content of carbon black and decreasing content of ferrite in fillers combinations. As a consequence of thermo-oxidative ageing, the values of cross-link density of both, ferrite as well as ferrite and carbon black filled vulcanizates increased, but the dependences of the cross-link density on the ferrite content, or on the ferrite weight fraction were remained also after ageing (Tables 3e5). The increase of the crosslink density during thermo-oxidative ageing can be attributed to the additional cross-linking of the rubber. This presumption is also supported by the increasing values of the tensile strength at break and decreasing values of the elongation at break of tested composites after thermo-oxidative exposure in comparison with the original values of composites before ageing. Based upon the achieved results, it is possible to claim that the presence of ferrite in rubber materials does not negatively influence the thermooxidative stability of prepared composites. Together, with the study of the cross-link density, the sulphur cross-link structure of vulcanizates was analysed. For this purpose
90
70
Sx (%)
Table 3 Total nt, chemical nch and physical nph cross-link density of vulcanizates based on natural rubber before ageing and after ageing.
925
50
30 0h
72h
10 0
20
40
60
80
100
Ferrite (phr) Fig. 7. Influence of ferrite content and thermo-oxidative ageing on polysulfidic crosslinks Sx of vulcanizates based on natural rubber.
J. Kruzelák et al. / Polymer Degradation and Stability 97 (2012) 921e928
926
Sx
S2
100
S
100
80
80
Sx (%)
Sulfidic cross-links (%)
120
60 40
60 40
20
20
0
0
0h 0
20
40
60
80
100
0
0.2
0.4
Ferrite (phr)
0.8
1
wF
Fig. 8. Influence of ferrite content on sulfidic cross-links of vulcanizates based on butadiene rubber.
the thiol-amine method was used. The measurements were performed in argon atmosphere at a laboratory temperature. The results of measurements showed that by using hexane-1-thiol, which has ability to decompose polysulfidic and disulfidic crosslinks, the analysed vulcanizates based on natural rubber were either completely decomposed or after drying they had powdery form. Simultaneously, small particles of the filler were released from the samples and the weight of samples was reduced. Therefore it might be supposed that NR-vulcanizates practically do not contain, or contain only a very small part of monosulfidic crosslinks S. By using propane-2-thiol, the content of polysulfidic cross-links Sx was quantitatively determined equation (4). With using of equation (5), the ratio of disulfidic cross-links S2 in the network structure of prepared vulcanizates was possible to calculate. In case of NR-vulcanizates filled only with ferrite, the lowest content of polysulfidic cross-links (less than 50% from nch) was spotted in the reference unfilled sample (Fig. 7). The content of Sx non-linearly increased in dependence on the ferrite doping up to 60% approximately. During thermo-oxidative ageing, the content of polysulfidic cross-links increased. The more evident change was shown in the samples with lower ferrite contents. This fact is likely connected with the increase of the cross-link density during thermo-oxidative ageing as a result of additional cross-linking of the rubber matrix. As plotted in Fig. 8 all types of sulfidic cross-links are present in the network structure of BR-vulcanizates with the dominance of
Fig. 10. Influence of ferrite weight fraction and thermo-oxidative ageing on polysulfidic cross-links Sx of vulcanizates filled with combinations of ferrite and carbon black.
polysulfidic cross-links. In consequence of thermo-oxidative ageing the increase of Sx in the network structure of vulcanizates was recorded. The content of disulfidic cross-links decreased, while the content of monosulfidic cross-links increased during ageing period (Fig. 9). The cross-link structure of vulcanizates filled with fillers combinations was, as in case of NR-vulcanizates filled only with ferrite, formed mainly from polysulfidic cross-links (Fig. 10). Neither the change of ferrite weight fraction nor thermo-oxidative ageing affect the network structure of prepared materials. 3.3. Thermogravimetric analysis Figs. 11 and 12 illustrate thermogravimetric curves for ferrite filled NR and BR-composites in air atmosphere at a heating rate of 5 C/min. The results demonstrate that the initial temperature of the thermal decomposition of composites based on natural rubber was 300e315 C. The weight loss 6e8% at the given temperature was detected. The doping content of ferrite particles did not influence the thermal degradation of tested samples. The thermal stability of composites based on butadiene rubber was higher and the initial temperature of the thermal decomposition ranged from 410 to 415 C. The weight loss 10e13% was detected. As mentioned in 120
120
Sx
S2
S
100
100
80
80
Weight (%)
Sulfidic cross-links (%)
0.6
72h
60 40
60 REF
40
20
20
0
0
20 60 100
0
20
40
60
80
100
Ferrite (phr) Fig. 9. Influence of ferrite content on sulfidic cross-links of vulcanizates based on butadiene rubber after thermo-oxidative ageing.
0
150
300
450
600
Temperature (°C) Fig. 11. TG curves of composites based on natural rubber filled with different content of ferrite.
120
120
100
100
80
80
Weight (%)
Weight (%)
J. Kruzelák et al. / Polymer Degradation and Stability 97 (2012) 921e928
60 REF
40
20 60
20
927
CB:F 5:0
60
CB:F 3:2
40
CB:F 1:1 CB:F 2:3
20
CB:F 0:5
100 0
0 0
150
300
450
0
600
150
300
450
600
Temperature (°C)
Temperature (°C) Fig. 12. TG curves of composites based on butadiene rubber filled with different content of ferrite.
previous case, the presence of magnetic filler had almost no influence on the thermal degradation of elastomeric composites. The thermal stability of elastomeric composites filled with combinations of ferrite and carbon black was found to be very similar to that of NR-composites filled only with magnetic filler without the evident influence of both applied fillers on the thermal degradation of evaluated samples (Fig. 13).
3.4. Ozone ageing of composites Rubber materials are predisposed to be exceedingly sensitive to initiatory mechanisms of the thermo-oxidative degradation and to the ozone degradation especially. The values of threshold deformation (TD) for composites based on natural and butadiene rubber filled only with magnetic filler are shown in Table 6. It becomes obvious that the influence of ozone on the degradation of both types of prepared composites is different. In case of composites based on natural rubber, the reference unfilled sample was found to reach the biggest value of threshold deformation in the whole examined temperature range of ozone ageing. With increasing of the magnetic filler content as well as with extension of exposure time, the values of TD were found to decline. The quickest appearance and the quickest broadening of ozone ruptures on the surface of composites were possible to observe in case of the sample with the maximum ferrite content. After 120 min of ozone ageing, the ruptures appeared on the whole surface of the examined sample; TD was equal to zero.
Fig. 13. TG curves of composites filled with combinations of ferrite and carbon black.
On the other hand the presence of magnetic filler in composites based on butadiene rubber seems to positively influence the values of TD. The values of threshold deformation showed increasing tendency with increasing content of ferrite. In contrast to above mentioned composites, the maximum ferrite filled sample of BRcomposites reached the biggest value of TD. It means that this sample showed the best resistance against ozone. In case of both types of elastomeric magnetic composites, the appearance and the broadening of ozone ruptures were more evident at the initial phases of the ozone degradation (up to 2 h). With the extension of exposure time, the appearance of ozone ruptures became less evident. After 4.5 h, the threshold deformation did not change already. The differences in threshold deformations of both types of composites as a consequence of the magnetic filler content are not significant. Therefore it is not possible to clearly quantify the influence of the magnetic filler content on the samples degradation in the given conditions of ozone ageing. After 24 h of ozone ageing, composites based on butadiene rubber were completely decomposed. The surface of composites based on natural rubber was covered with bigger or smaller ruptures, but the total decomposition of examined samples was not observed. Based upon the obtained results it is possible to claim that the type of used butadiene rubber (Buna CB 24) is less stable against the degradation caused by ozone in comparison with natural rubber (SMR 20). The stability of composites filled with combinations of ferrite and carbon black against ozone ageing was clearly different. From Table 7 it becomes evident that the values of threshold deformation tend to significantly decrease with the ferrite weight fraction
Table 6 Threshold deformation of composites based on natural and butadiene rubber. Time (min)
Composites based on NR
Composites based on BR
Ferrite (phr)
Threshold deformation (%)
30 60 90 120 150 210 270 330 1440
Ferrite (phr)
0
20
60
100
0
20
60
100
10.11 7.75 5.56 3.71 3.71 3.03 3.03 3.03 3.03
9.43 5.39 4.21 2.53 2.53 2.53 2.53 2.53 2.53
8.26 5.39 3.71 2.02 2.02 2.02 1.35 1.35 1.35
8.42 4.04 2.02 0.00 0.00 0.00 0.00 0.00 0.00
10.78 7.41 5.22 3.54 3.54 3.20 2.02 2.02 2.02
10.11 7.24 5.05 3.37 3.37 3.37 3.37 3.37 3.37
10.95 9.10 6.74 6.74 6.74 4.21 3.03 3.03 3.03
11.96 9.27 7.41 5.56 5.56 3.71 3.71 3.71 3.71
J. Kruzelák et al. / Polymer Degradation and Stability 97 (2012) 921e928
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Table 7 Threshold deformation of composites filled with combinations of ferrite and carbon black.
Threshold deformation (%)
Time (min)
Carbon black:ferrite 5:0
3:2
1:1
2:3
0:5
30 60 90 120 150 210 270 330 1440
31.00 31.00 29.15 28.30 25.10 24.60 24.60 23.92 18.70
19.54 19.54 17.52 17.52 17.52 17.02 16.51 15.50 14.15
27.29 19.38 17.18 15.16 13.98 13.98 13.98 13.98 12.13
19.04 14.99 12.47 12.47 12.47 11.79 11.79 11.79 9.60
12.97 9.94 8.09 5.73 5.73 4.21 4.21 4.21 4.21
increasing. The sample filled only with carbon black exhibits the biggest value of TD. The appearance of first ruptures on the surface of this sample was recorded after 90 min of ozone ageing. The extension of ozone ageing is connected with the next decrease of TD values of tested samples. The decrease of TD values was more evident at the initial phases. The results revealed that carbon black present in rubber materials plays more significant role when composites are exposed to the conditions of ozone ageing in comparison with ferrite filled rubber composites. The more carbon black rubber composites contain, the better is the resistance against the ozone degradation. This improvement is likely connected with the reinforcing effect of carbon black in rubber materials. 4. Conclusion The aim of the work was the study of influence of ferrite alone, as well as ferrite in combinations with carbon black on the properties of elastomeric composites. The changes in properties in consequence of thermo-oxidative and ozone ageing as well as the thermal stability of prepared materials were considered. The results demonstrate that ferrite incorporated in rubber materials exhibits only low reinforcing effect. Despite of the tensile strength at break of NR and mainly BR-composites increased in the presence of magnetic filler. The increase of the tensile strength at break in dependence on the ferrite weight fraction increasing in composites filled with fillers combinations was also recorded. This seems to be very surprising, as carbon black is proved to be traditional reinforcing filler for rubber materials. The values of physicalemechanical properties of composites after ageing are higher or lower in comparison with corresponding values of composites before thermo-oxidative
degradation. But almost all evaluated properties of composites after ageing remained the character of their dependences on the content of ferrite, or fillers combinations. Thermogravimetric analysis revealed that the initial temperature of the thermal decomposition of evaluated composites was not influenced by the amount of magnetic filler. Based upon results obtained by the study it is possible to claim that ferrite present in rubber matrix does not influence the thermo-oxidative and thermal stability of the prepared composites. The ozone ageing tests showed that the influence of ferrite on the ozone stability of natural rubber as well as butadiene rubber based composites was ambiguous. The change of TD seems not to be considerably dependent on the magnetic filler content. In case of composites filled with combinations of ferrite and carbon black, the resistance against ozone increases with increasing content of carbon black in combinations of both fillers. Acknowledgement This work was supported by grant agency VEGA, project No. 1/ 1163/12 References [1] Zhou GY. Shear properties of a magnetorheological elastomer. Smart Mater Struct 2003;12:139e46. [2] Chen L, Gong XL, Jiang WQ. Investigation on magnetorheological elastomers based on natural rubber. J Mater Sci 2007;42:5483e9. [3] Wang Q, Yang F, Yang Q, Chen J, Guan H. Study on mechanical properties of nano-Fe3O4 reinforced nitrile butadiene rubber. Mater Des 2010;31:1023e8. [4] Chatterjee J, Haik Y, Chen CJ. Polyethylene magnetic nanoparticle: a new magnetic material for biomedical applications. J Magn Magn Mater 2002; 246(3):382. [5] Matsumoto M, Miyata Y. Polymer absorbers containing magnetic particles: effect of polymer permittivity on wave absorption in the quasi-microwave band. J Appl Phys 2002;91(12):9635. [6] Kraus G. Swelling of filler-reinforced vulcanizates. J Appl Polym Sci 1963;7: 861e71. [7] Hamed GR, editor. Materials and compounds: engineering with rubber. New York: Oxford University Press; 1992. [8] Saville B, Watson AA. Structural characterization of sulfur-vulcanized rubber networks. Rubber Chem Tech 1967;40:100e49. [9] Morrison NJ, Porter M. Temperature effects on the stability of intermediates and crosslinks in sulfur vulcanization. Rubber Chem Tech 1984;57:63e86. [10] Warner WC. Methods of devulcanization. Rubber Chem Tech 1994;67: 559e66. [11] White JR, De SK, editors. Rubber technologist’s handbook. Shawbury: Rapra Technology Limited; 2001. [12] Medalia AI. Filler aggregates and their effect on reinforcement. Rubber Chem Tech 1974;47:411e33. [13] Kyselá G, Hudec I, Alexy, editors. Manufacturing and processing of rubber. Bratislava: Slovak University of Technology Press; 2010.
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Influence of thermo-oxidative and ozone ageing on the properties of elastomeric magnetic composites Ján Kru zelák a, *, Ivan Hudec a, Rastislav Dosoudil b a
Department of Plastics and Rubber, Institute of Polymer Materials, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinského 9, 812 37 Bratislava, Slovakia b Department of Electromagnetic Theory, Faculty of Electrical Engineering and Information Technology, Slovak University of Technology, Ilkovicova 3, 812 19 Bratislava, Slovakia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 February 2012 Received in revised form 12 March 2012 Accepted 16 March 2012 Available online 28 March 2012
Elastomeric magnetic composites are materials with ferromagnetic fillers as one of the constituents and rubber blend as polymer matrix. Ferrites, as commonly used magnetic fillers, might be able to influence the long-term stability of rubber materials, mainly if they are applied in high concentrations. Therefore, the thermo-oxidative and ozone stability of ferrite as well as ferrite and carbon black filled composites based on natural and butadiene rubber were studied by oven ageing and thermogravimetric analysis. The results revealed that ferrite alone also ferrite in combinations with carbon black influences the properties of evaluated materials in various ways. But the thermo-oxidative stability of prepared composites seems not to be influenced by the amount of ferrite even in case of high magnetic filler contents. From thermogravimetric curves it became evident that the thermal degradation of prepared materials also was not influenced by the doping content of ferrite. The ozone ageing tests demonstrated that the influence of ferrite on the ozone stability of natural rubber as well as butadiene rubber based composites was ambiguous. In case of natural rubber based composites filled with combinations of ferrite and carbon black, it can be stated that the more carbon black rubber materials contain, the better is the resistance against ozone degradation. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Elastomeric composite Magnetic filler Thermo-oxidative ageing Thermogravimetric analysis Ozone degradation
1. Introduction In the recent years a rapid interest in smart materials consisting of elastomeric matrix and magnetically polarizable particles has been shown [1e3]. The rheological and mechanical properties of such types of materials can be controlled by an external magnetic field. During the vulcanization, magnetic particles in the rubber matrix are oriented to the direction of applied external magnetic field. The results showed that such structure inside the rubber matrix leads to a strong anisotropic behaviour of prepared composites. These materials have attracted considerable interest for the sake of their applications in many fields of technological and industrial branches [4,5]. Ferrites represent a well established family of magnetic materials. M-types of hexagonal ferrites, which were used in our work, are compounds of iron oxide with the oxides of some other metals of general formula MeO.6Fe2O3 (M is divalent cation such as Sr, Ba, Pb). High values of magneto-crystalline anisotropy and saturation magnetization allow wide application of these materials as permanent magnets. Low price and very good chemical stability * Corresponding author. Tel.: þ421 908478184. E-mail address: [email protected] (J. Kru zelák). 0141-3910/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2012.03.025
together with suitable magnetic characteristics include ferrites in the most important magnetic materials which cannot be easily replaced. Ba and Sr ferrites are the most common applied magnetic powdery fillers. It is well known that ageing of rubber materials and vulcanizates is caused by chemical, oxidativeedestructive processes, where oxygen and heat play the crucial influence. Furthermore, iron ions are able to enhance the oxidation of rubber materials, as they initiate the induced dissociation of hydro-peroxides present in rubber materials. Therefore, the aim of this work was to investigate the influence of strontium ferrite on the thermo-oxidative and ozone stability of elastomeric composites based on natural and butadiene rubber by oven ageing and thermogravimetric analysis.
2. Experimental 2.1. Materials The two types of elastomers, natural rubber SMR 20 (Mardec, Malaysia) and cis-1,4-butadiene rubber (Lanxess, Germany) were filled with magnetic particles in order to prepare elastomeric magnetic composites. Anisotropic strontium hexaferrite SrFe12O19,
J. Kruzelák et al. / Polymer Degradation and Stability 97 (2012) 921e928
922
tlá Hora, type FD 8/24 prepared by wet milling (MAGNETY, Sve Czech Republic) was applied as a magnetic filler. It is a product with additional polyvinyl alcohol, which covers the surface of ferrite particles. Ferrite, which was used in our experiments, was prepared by dissolution of polyvinyl alcohol by extraction in hot water. Ferrite was dosed alone or in combinations with carbon black (N330). In rubber compounds filled only with ferrite, the content of magnetic filler was changed from 0 to 100 phr. In case of rubber compounds filled with combinations of ferrite and carbon black, the content of both fillers was kept constant (85 phr), only the weight fraction of ferrite in combinations of both fillers (wF ¼ mF/ (mF þ mCB)) was changed. For the preparation of rubber compounds filled with fillers combinations, carbon black batch of natural rubber was used. A semi-EV sulphur vulcanization system was introduced in all experiments. The detailed compositions of rubber compounds are shown in Tables 1 and 2.
Table 2 Composition of rubber compounds filled with fillers combinations. Component
NR
ZnO
Stearin
CBS
Sulphur
Ferrite þ carbon black
Content (phr)
100
5
1.5
1.5
1.3
85
s
2 a a2
¼ C1 þ
C2
a
(2)
s e tension (MPa) a e relative extension (%)
C1, C2 e constants
vt ¼ 2C1 =RT
(3)
R ¼ 8.314 J/K.mol T ¼ 293.15 K
2.2. Methods The preparation of elastomeric magnetic composites was carried out in the laboratory mixer BRABENDER in two mixing steps. In the first step the rubber and the filler were compounded (9 min, 90 C), in the second step (4 min, 90 C) the curing system was added. The curing process was performed at 150 C for the optimum cure time under a pressure of approximately 20 MPa by using the hydraulic press FONTUNE. The physicalemechanical properties of prepared composites were measured in accordance with the valid technical standards, on the double side blade specimens (width 6.4 mm, length 10 cm, thickness 2 mm) by using TIRATEST equipment. Magnetic measurements of composites on the magnetometer TVM1 at room temperature and maximum coercivity of 750 kA/m were determined. The basic principle of measurement is induction method of scanning of scattering magnetic flux F induced by magnetic vibrating sample. Magnetic field is generated by means of two cores of Weiss electromagnet at a minimum distance of poles adaptors equals to 7.5 mm. The specimens were of prism shape (8 4 4 mm). The two different methods were used to in order to determine the cross-link density of cured samples. In the first method the chemical cross-link density nch was determined based on equilibrium swelling of samples in xylene. The FloryeRehner equation modified by Krause for filled vulcanizates [6] was used:
Vr0 lnð1 Vr Þ þ Vr þ cVr2 VS V 1=3 V 2=3 0:5Vr
nch ¼
r
(1)
r0
nch e chemical cross-link density (mol/cm3)
Vr0 e volume fraction of rubber in equilibrium swelling sample of vulcanizate in absence of fillers Vr e volume fraction of rubber in equilibrium swelling sample of filled vulcanizate VS e molar volume of solvent (for xylene ¼ 123.45 cm3/mol) c e Huggins interaction parameter (for xylene e natural rubber c ¼ 0.4106, for xylen e e butadiene rubber c ¼ 0.39) In the second method based on deformation measurements, the total cross-link density nt was calculated by means of the MooneyeRivlin equation [7]: Table 1 Composition of rubber compounds filled only with ferrite. Component
NR, BR
ZnO
Stearin
CBS
Sulphur
Ferrite
Content (phr)
100
3
2
1.5
1.3
0e100
The measurements were carried out in the INSPEKT desk 5 kN apparatus (METROTEST), up to 100% deformation, deformation velocity of 10 mm/min. The cross-link structure of vulcanizates was investigated using the thiol-amine method in argon atmosphere as described in [8e10]. There was used propane-2-thiol, which has ability to decompose polysulfidic cross-links and hexane-1-thiol, which is more reactive and effectively decomposes polysulfidic and disulfidic cross-links of vulcanizates. The reactivity and efficiency of thiols is sensitive to different factors, as the structure of thiols (primary, secondary thiols), concentration, reaction time, temperature, size of sample and structure of rubber compounds, too [8]. First, the chemical cross-link density nch of original samples was determined. Then, the samples were treated with solutions of thiols at laboratory temperature in argon atmosphere and the cross-link density was determined again (nPT e the cross-link density of samples after treating them with propane-2-thiol and nHT e the cross-link density of samples after treating them with hexane-1thiol). The content of polysulfidic Sx, disulfidic S2 a monosulfidic S cross-links was then possible to calculate:
Sx ¼ vch vPT ; Sx ¼ ½ðvch vPT Þ,100=vch
(4)
S2 ¼ vPT vHT ; S2 ¼ ½ðvPT vHT Þ,100=vch
(5)
S ¼ vch ðSx þ S2Þ; S ¼ ðvHT =vch Þ,100
(6)
Thermogravimetric analysis was carried out in DERIVATOGRAPH Q-1500D apparatus in temperature range 20e600 C in air atmosphere at a constant heating rate of 5 C/min. For thermo-oxidative ageing tests the Geer method was used. The measurements were performed in air atmosphere at 70 C, exposure time was equal to 72 h. Ozone ageing tests of prepared composites were performed in the ozone chamber at 40 C and ozone concentration of 50 pphm. First, test samples were placed into the special frames (Fig. 1). The upper part of test samples was deformed in 31%, whereas the lower part was without deformation. After 24 h, the frames with test samples were placed into the ozone chamber and in the given time intervals, the formation of ozone ruptures on the surface of the samples was observed. The resistance of composites against ozone was described by means of threshold deformation (TD). It is the biggest value of the static tensile deformation at which the ozone ruptures on the surface of the samples are not spotted yet after the given time. The higher resistance of composites against ozone, the bigger is the value of threshold deformation.
J. Kruzelák et al. / Polymer Degradation and Stability 97 (2012) 921e928
923
TSb of NR-composites (MPa) Fig. 1. Test samples of composites.
2.5
19 2.0
16 13
NR BR 0
20
3. Results and discussion 3.1. Influence of ferrite and thermo-oxidative ageing on physicalmechanical and magnetic properties Elastomeric magnetic composites were prepared based on natural and butadiene rubber without any antioxidants and antiozonants added. Therefore, soft thermal conditions for thermooxidative ageing were chosen, namely 70 C and 72 h. Despite the fact that the values of physicalemechanical properties of butadiene rubber based composites were relatively small, the presence of ferrite in the elastomeric matrix leads to the enhancement of evaluated characteristics. From Fig. 2 it is observable non-linear increase of the tensile strength at break as a function of ferrite content in BR-composites. The increase of the tensile strength at break value of the composite filled with the maximum ferrite content represents more than 75% in comparison with the tensile strength value of the ferrite free sample. The increasing tendency on the magnetic filler content was detected also in case of the elongation at break (Fig. 3). After exposure of test samples to the conditions of thermo-oxidative ageing the values of tensile strength slightly increased, while the specific values of elongation at break decreased. The biggest decline of elongation at break was observed in case of the composite with the maximum ferrite content. On the other hand the elongation at break of composites based on natural rubber showed decreasing tendency with increasing amount of ferrite (Fig. 3). At the maximum ferrite content 17% decrease of the observed property compared to the unfilled sample was spotted. The next decline of elongation at break values was recorded after thermo-oxidative degradation. The decrease of the elongation at break after thermo-oxidative degradation did not exceed 8% of the equivalent elongation at break values of original non-aged composites. In spite of that there is evidently observable the dependence of evaluated characteristics on the magnetic filler content, the tensile strength at break of NR-composites changed only slightly as shown in Fig. 2. One can see that the slight maximum of the tensile strength at break is reached somewhere around medium ferrite contents. When test samples were exposed to the conditions of
60
80
1.5
1.0 100
Ferrite (phr)
(7)
a e the distance between the upper and the lower side of the test sample (cm) b e the distance of the onset of the rupture from the lower side of the test sample (cm) D e deformation (%)
40
NR 72h BR 72h
Fig. 2. Influence of ferrite content and thermo-oxidative ageing on tensile strength at break TSb of composites based on natural and butadiene rubber.
thermo-oxidative ageing the increase of the observed property was detected. The influence of thermo-oxidative ageing was possible to see mainly in case of the reference sample and the sample with the minimum magnetic filler content. With increasing amount of ferrite, the influence of ageing was not so significant. The different character of physicalemechanical properties of both BR- and NR-composites on the ferrite content is attributed to the structure of selected elastomeric matrices. Natural rubber shows high crystallizing capability, while the crystallization of butadiene rubber is very restricted, in spite of the fact that BR implies high content of cis-1,4 structural units. This could be the reason why the values of tensile strength and elongation of natural rubber are much higher in comparison with those of butadiene rubber. From the above mention changes it becomes evident that ferrite does not act as a reinforcing filler in rubber materials. The degree of reinforcement is rather low even at the maximum ferrite content. The reason might be attributed to the small adhesion between ferrite particles and the rubber matrix with many voids existing between the two phases. The inner voids lead to the stress concentration inside the sample, what may result in rupture at low stress. The influence of fillers combinations (ferrite and carbon black) on physicalemechanical properties of natural rubber based composites is illustrated in Fig. 4. From Fig. 4 it becomes apparent
900
160
800
140
700
120
600
100
500
80
400
60
300 NR BR
200 100 0
20
40
60
NR 72h BR 72h 80
40
Eb of BR-composites (%)
b ,D a
3.0
22
10
Eb of NR-composites (%)
TD ¼
25
TSb of BR-composites (MPa)
3.5
28
20 100
Ferrite (phr) Fig. 3. Influence of ferrite content and thermo-oxidative ageing on elongation at break Eb of composites based on natural and butadiene rubber.
J. Kruzelák et al. / Polymer Degradation and Stability 97 (2012) 921e928
30
8 7
5
6 4
5 4
3
3
2
2 1 0 20
40
NR
NR 72h
BR
BR 72h
60
1
0 100
80
Ferrite (phr) Fig. 5. Influence of ferrite content and thermo-oxidative ageing on remanent magnetic induction Br of composites based on natural and butadiene rubber.
filled only with magnetic filler with approximately the same content of ferrite, one can see the similar values of the remanent magnetic induction. Therefore it can be stated that the incorporation of carbon black into the rubber matrix has no substantial influence on magnetic characteristics of elastomeric magnetic composites. Magnetic characteristics are dependent only on the amount of applied ferrite filler regardless of the presence of reinforcing filler. From Figs. 5 and 6 it becomes also clearly evident, that there were almost no changes of the evaluated magnetic characteristic in consequence of thermo-oxidative degradation.
3.2. Influence of ferrite and thermo-oxidative ageing on the crosslink density and cross-link structure The cross-link density and the structure of cross-links in vulcanizates are very important characteristics of all cured rubber systems. Not only the original properties but also the changes of original properties of vulcanizates during their using are dependent on these characteristics. Therefore, the investigation of the magnetic filler influence and thermo-oxidative ageing on the crosslink density of prepared composites was also part of our interest. First, the total cross-link density nt as well as the chemical cross-
6
900
5
25 700
15
500
10 300 5
TSb Eb
0 0
0.2
0.4
0.5
0.6
TSb 72h Eb 72h 0.8
100 1
wF Fig. 4. Influence of ferrite weight fraction and thermo-oxidative ageing on tensile strength at break TSb and elongation at break Eb of composites filled with combinations of ferrite and carbon black.
Br.10 2 (T)
4
20
Eb (%)
TSb (MPa)
6
Br.102 of BR-composites (T)
that the tensile strength at break and the elongation at break of composites were found to increase in the whole examined ferrite concentration range. As a result of thermo-oxidative degradation the values of elongation at break decreased in range from 9 to 13% in comparison with the original values of composites before ageing. The values of tensile strength at break after ageing remained nearly unchanged. The different character of physicalemechanical properties of composites filled with fillers combinations in comparison with the properties of composites filled only with ferrite is attributed mainly to the reducing content of carbon black in combinations with ferrite. Due to very good compatibility and adhesion between carbon black and the rubber matrix, carbon black plays significant role as a reinforcing filler in the rubber materials. Due to very good interactions and binding forces between carbon black and the rubber, the mobility of macromolecular chains is restricted and the total reinforcement of the rubber matrix is then achieved. As a result, the improvement of physicalemechanical properties of composites is observed. Therefore, the increase of the tensile strength at break with decreasing content of carbon black in fillers combinations seems to be very remarkable and surprising. This behaviour is very hard to explain. Based upon the results obtained by the study of influence of thermo-oxidative ageing, it can be concluded that this process affects the evaluated properties in various ways. But almost all evaluated properties of composites after ageing remained the character of their dependences on the content of ferrite, or fillers combinations. The values of physicalemechanical properties of composites after ageing are higher or lower in comparison with corresponding values of composites before thermo-oxidative degradation. Magnetic characteristic of composites were evaluated at a laboratory temperature and maximum coercivity of 750 kA/m. As plotted in Fig. 5 the presence of magnetic filler in the rubber matrix leads to the significant enhancement of the remanent magnetic induction Br. The difference between the values Br of the sample with minimum and maximum ferrite content was approximately 560% in case of NR-composites and more than and 390% in case of BR-composites, respectively. The values of remanent magnetic induction were also found to significantly increase with increasing of the ferrite weight fraction in composites filled with fillers combinations (Fig. 6). When comparison the Br values of composites filled with fillers combinations with the Br values of composites
Br.102 of NR-composites (T)
924
3 2 1
Br
Br 72h
0 0.2
0.4
0.6
0.8
1
wF Fig. 6. Influence of ferrite weight fraction and thermo-oxidative ageing on remanent magnetic induction Br of composites filled with combinations of ferrite and carbon black.
J. Kruzelák et al. / Polymer Degradation and Stability 97 (2012) 921e928
Ferrite (phr)
nt.104 (mol/
nch.104 (mol/ cm3)
nph.104 (mol/ cm3)
0h
72 h
0h
72 h
0h
72 h
1.27 1.37 1.39 1.53 1.74 1.88
1.31 1.44 1.67 1.81 2.05 2.37
1.14 1.14 1.18 1.26 1.29 1.40
1.19 1.25 1.23 1.39 1.37 1.47
0.13 0.23 0.21 0.27 0.45 0.48
0.12 0.19 0.44 0.42 0.68 0.90
cm3)
0 20 40 60 80 100
link density nch were determined. Then, the physical cross-link density nph was possible to calculate. Polymerepolymer physical interactions, polymer-filler physical interactions, also various intramolecular and intermolecular entanglements are involved in physical cross-link density. The results of measurements showed that the total cross-link density nt as well as the chemical cross-link density nch of NRvulcanizates increased with increasing of ferrite content (Table 3). The physical cross-link density nph, which represents the difference between the total and the chemical cross-link density (nph ¼ nt nch), is much lower than nch and was also found to increase with increasing of ferrite content. By contrast, the chemical cross-link density of BR-vulcanizates showed decreasing tendency with doping content of ferrite. The total cross-link density of BR-composites was lower in comparison with the reference sample. However, the content of ferrite has no significant influence on the values of nt (Table 4). The experimentally obtained values of the cross-link density for vulcanizates filled with combinations with ferrite and carbon black are shown in Table 5. One can see that the increase of ferrite weight fraction leads to the consistent linear decrease of the total crosslink density. The difference between the density nt of the sample filled with carbon black only and the sample filled with ferrite only is more than 50%. The same situation was possible to see also in case of the chemical and physical cross-link density, which were also found to gradually decrease in dependence on the ferrite weight fraction increasing. In consequence of mutual interactions between carbon black and the rubber, the physicalechemical adsorption of the rubber to the surface of carbon black particles is ensured. This binding between the rubber and carbon black causes the partial immobilization of the rubber macromolecules and influences the mobility of carbon black particles enwrapped by rubber. The rubber layer closest to the surface of carbon black particles behaves as a polymer in glassy state and is insoluble in solvents suitable for rubber. Polymer mobility increases with distance from the carbon black surface. The overall reduction in polymer mobility associated with
Table 4 Total nt, chemical nch and physical nph cross-link density of vulcanizates based on butadiene rubber before ageing and after ageing. Ferrite (phr)
nt.104 (mol/
nch.104(mol/
nph.104 (mol/ cm3)
0h
72 h
0h
72 h
0h
72 h
1.91 1.82 1.83 1.87 1.83 1.87
2.16 2.05 1.97 2.18 2.15 2.23
1.72 1.66 1.64 1.58 1.57 1.50
1.91 1.81 1.75 1.91 1.85 1.88
0.19 0.16 0.19 0.29 0.26 0.37
0.25 0.24 0.22 0.27 0.30 0.35
cm3)
0 20 40 60 80 100
cm3)
Table 5 Total nt, chemical nch and physical nph cross-link density of vulcanizates filled with combinations of ferrite and carbon before ageing and after ageing. Compound
CB:F CB:F CB:F CB:F CB:F CB:F CB:F
wF
5:0 4:1 3:2 1:1 2:3 1:4 0:5
0 0.2 0.4 0.5 0.6 0.8 1
nt.104 (mol/ cm3)
nch.104 (mol/
nph.104 (mol/
0h
72 h
0h
72 h
0h
72 h
3.59 2.99 2.68 2.79 2.51 2.17 1.89
3.74 3.40 3.39 3.38 3.13 2.67 2.23
2.11 1.81 1.83 1.71 1.68 1.51 1.31
2.53 2.34 2.18 2.08 1.97 1.79 1.46
1.48 1.18 0.85 1.08 0.83 0.66 0.58
1.21 1.06 1.21 1.30 1.16 0.88 0.77
cm3)
cm3)
bound rubber gives a broad relaxation behaviour to the rubber compound. Bound rubber actually consists of physical adsorption of polymer chain as well as chemical adsorption, and it is generally accepted that physical adsorption is of greater importance for rubber reinforcement that chemical adsorption. Only a fraction of the bound rubber actually participates in the chemical adsorption and the majority consists of loops and dangling chains [11e13]. On the other hand, the interactions between the rubber matrix and ferrite are only of weak physical character due to poor bonds and adhesion between the magnetic filler and the rubber matrix. The above mentioned facts are likely responsible for the improvement of the cross-link density of vulcanizates with increasing content of carbon black and decreasing content of ferrite in fillers combinations. As a consequence of thermo-oxidative ageing, the values of cross-link density of both, ferrite as well as ferrite and carbon black filled vulcanizates increased, but the dependences of the cross-link density on the ferrite content, or on the ferrite weight fraction were remained also after ageing (Tables 3e5). The increase of the crosslink density during thermo-oxidative ageing can be attributed to the additional cross-linking of the rubber. This presumption is also supported by the increasing values of the tensile strength at break and decreasing values of the elongation at break of tested composites after thermo-oxidative exposure in comparison with the original values of composites before ageing. Based upon the achieved results, it is possible to claim that the presence of ferrite in rubber materials does not negatively influence the thermooxidative stability of prepared composites. Together, with the study of the cross-link density, the sulphur cross-link structure of vulcanizates was analysed. For this purpose
90
70
Sx (%)
Table 3 Total nt, chemical nch and physical nph cross-link density of vulcanizates based on natural rubber before ageing and after ageing.
925
50
30 0h
72h
10 0
20
40
60
80
100
Ferrite (phr) Fig. 7. Influence of ferrite content and thermo-oxidative ageing on polysulfidic crosslinks Sx of vulcanizates based on natural rubber.
J. Kruzelák et al. / Polymer Degradation and Stability 97 (2012) 921e928
926
Sx
S2
100
S
100
80
80
Sx (%)
Sulfidic cross-links (%)
120
60 40
60 40
20
20
0
0
0h 0
20
40
60
80
100
0
0.2
0.4
Ferrite (phr)
0.8
1
wF
Fig. 8. Influence of ferrite content on sulfidic cross-links of vulcanizates based on butadiene rubber.
the thiol-amine method was used. The measurements were performed in argon atmosphere at a laboratory temperature. The results of measurements showed that by using hexane-1-thiol, which has ability to decompose polysulfidic and disulfidic crosslinks, the analysed vulcanizates based on natural rubber were either completely decomposed or after drying they had powdery form. Simultaneously, small particles of the filler were released from the samples and the weight of samples was reduced. Therefore it might be supposed that NR-vulcanizates practically do not contain, or contain only a very small part of monosulfidic crosslinks S. By using propane-2-thiol, the content of polysulfidic cross-links Sx was quantitatively determined equation (4). With using of equation (5), the ratio of disulfidic cross-links S2 in the network structure of prepared vulcanizates was possible to calculate. In case of NR-vulcanizates filled only with ferrite, the lowest content of polysulfidic cross-links (less than 50% from nch) was spotted in the reference unfilled sample (Fig. 7). The content of Sx non-linearly increased in dependence on the ferrite doping up to 60% approximately. During thermo-oxidative ageing, the content of polysulfidic cross-links increased. The more evident change was shown in the samples with lower ferrite contents. This fact is likely connected with the increase of the cross-link density during thermo-oxidative ageing as a result of additional cross-linking of the rubber matrix. As plotted in Fig. 8 all types of sulfidic cross-links are present in the network structure of BR-vulcanizates with the dominance of
Fig. 10. Influence of ferrite weight fraction and thermo-oxidative ageing on polysulfidic cross-links Sx of vulcanizates filled with combinations of ferrite and carbon black.
polysulfidic cross-links. In consequence of thermo-oxidative ageing the increase of Sx in the network structure of vulcanizates was recorded. The content of disulfidic cross-links decreased, while the content of monosulfidic cross-links increased during ageing period (Fig. 9). The cross-link structure of vulcanizates filled with fillers combinations was, as in case of NR-vulcanizates filled only with ferrite, formed mainly from polysulfidic cross-links (Fig. 10). Neither the change of ferrite weight fraction nor thermo-oxidative ageing affect the network structure of prepared materials. 3.3. Thermogravimetric analysis Figs. 11 and 12 illustrate thermogravimetric curves for ferrite filled NR and BR-composites in air atmosphere at a heating rate of 5 C/min. The results demonstrate that the initial temperature of the thermal decomposition of composites based on natural rubber was 300e315 C. The weight loss 6e8% at the given temperature was detected. The doping content of ferrite particles did not influence the thermal degradation of tested samples. The thermal stability of composites based on butadiene rubber was higher and the initial temperature of the thermal decomposition ranged from 410 to 415 C. The weight loss 10e13% was detected. As mentioned in 120
120
Sx
S2
S
100
100
80
80
Weight (%)
Sulfidic cross-links (%)
0.6
72h
60 40
60 REF
40
20
20
0
0
20 60 100
0
20
40
60
80
100
Ferrite (phr) Fig. 9. Influence of ferrite content on sulfidic cross-links of vulcanizates based on butadiene rubber after thermo-oxidative ageing.
0
150
300
450
600
Temperature (°C) Fig. 11. TG curves of composites based on natural rubber filled with different content of ferrite.
120
120
100
100
80
80
Weight (%)
Weight (%)
J. Kruzelák et al. / Polymer Degradation and Stability 97 (2012) 921e928
60 REF
40
20 60
20
927
CB:F 5:0
60
CB:F 3:2
40
CB:F 1:1 CB:F 2:3
20
CB:F 0:5
100 0
0 0
150
300
450
0
600
150
300
450
600
Temperature (°C)
Temperature (°C) Fig. 12. TG curves of composites based on butadiene rubber filled with different content of ferrite.
previous case, the presence of magnetic filler had almost no influence on the thermal degradation of elastomeric composites. The thermal stability of elastomeric composites filled with combinations of ferrite and carbon black was found to be very similar to that of NR-composites filled only with magnetic filler without the evident influence of both applied fillers on the thermal degradation of evaluated samples (Fig. 13).
3.4. Ozone ageing of composites Rubber materials are predisposed to be exceedingly sensitive to initiatory mechanisms of the thermo-oxidative degradation and to the ozone degradation especially. The values of threshold deformation (TD) for composites based on natural and butadiene rubber filled only with magnetic filler are shown in Table 6. It becomes obvious that the influence of ozone on the degradation of both types of prepared composites is different. In case of composites based on natural rubber, the reference unfilled sample was found to reach the biggest value of threshold deformation in the whole examined temperature range of ozone ageing. With increasing of the magnetic filler content as well as with extension of exposure time, the values of TD were found to decline. The quickest appearance and the quickest broadening of ozone ruptures on the surface of composites were possible to observe in case of the sample with the maximum ferrite content. After 120 min of ozone ageing, the ruptures appeared on the whole surface of the examined sample; TD was equal to zero.
Fig. 13. TG curves of composites filled with combinations of ferrite and carbon black.
On the other hand the presence of magnetic filler in composites based on butadiene rubber seems to positively influence the values of TD. The values of threshold deformation showed increasing tendency with increasing content of ferrite. In contrast to above mentioned composites, the maximum ferrite filled sample of BRcomposites reached the biggest value of TD. It means that this sample showed the best resistance against ozone. In case of both types of elastomeric magnetic composites, the appearance and the broadening of ozone ruptures were more evident at the initial phases of the ozone degradation (up to 2 h). With the extension of exposure time, the appearance of ozone ruptures became less evident. After 4.5 h, the threshold deformation did not change already. The differences in threshold deformations of both types of composites as a consequence of the magnetic filler content are not significant. Therefore it is not possible to clearly quantify the influence of the magnetic filler content on the samples degradation in the given conditions of ozone ageing. After 24 h of ozone ageing, composites based on butadiene rubber were completely decomposed. The surface of composites based on natural rubber was covered with bigger or smaller ruptures, but the total decomposition of examined samples was not observed. Based upon the obtained results it is possible to claim that the type of used butadiene rubber (Buna CB 24) is less stable against the degradation caused by ozone in comparison with natural rubber (SMR 20). The stability of composites filled with combinations of ferrite and carbon black against ozone ageing was clearly different. From Table 7 it becomes evident that the values of threshold deformation tend to significantly decrease with the ferrite weight fraction
Table 6 Threshold deformation of composites based on natural and butadiene rubber. Time (min)
Composites based on NR
Composites based on BR
Ferrite (phr)
Threshold deformation (%)
30 60 90 120 150 210 270 330 1440
Ferrite (phr)
0
20
60
100
0
20
60
100
10.11 7.75 5.56 3.71 3.71 3.03 3.03 3.03 3.03
9.43 5.39 4.21 2.53 2.53 2.53 2.53 2.53 2.53
8.26 5.39 3.71 2.02 2.02 2.02 1.35 1.35 1.35
8.42 4.04 2.02 0.00 0.00 0.00 0.00 0.00 0.00
10.78 7.41 5.22 3.54 3.54 3.20 2.02 2.02 2.02
10.11 7.24 5.05 3.37 3.37 3.37 3.37 3.37 3.37
10.95 9.10 6.74 6.74 6.74 4.21 3.03 3.03 3.03
11.96 9.27 7.41 5.56 5.56 3.71 3.71 3.71 3.71
J. Kruzelák et al. / Polymer Degradation and Stability 97 (2012) 921e928
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Table 7 Threshold deformation of composites filled with combinations of ferrite and carbon black.
Threshold deformation (%)
Time (min)
Carbon black:ferrite 5:0
3:2
1:1
2:3
0:5
30 60 90 120 150 210 270 330 1440
31.00 31.00 29.15 28.30 25.10 24.60 24.60 23.92 18.70
19.54 19.54 17.52 17.52 17.52 17.02 16.51 15.50 14.15
27.29 19.38 17.18 15.16 13.98 13.98 13.98 13.98 12.13
19.04 14.99 12.47 12.47 12.47 11.79 11.79 11.79 9.60
12.97 9.94 8.09 5.73 5.73 4.21 4.21 4.21 4.21
increasing. The sample filled only with carbon black exhibits the biggest value of TD. The appearance of first ruptures on the surface of this sample was recorded after 90 min of ozone ageing. The extension of ozone ageing is connected with the next decrease of TD values of tested samples. The decrease of TD values was more evident at the initial phases. The results revealed that carbon black present in rubber materials plays more significant role when composites are exposed to the conditions of ozone ageing in comparison with ferrite filled rubber composites. The more carbon black rubber composites contain, the better is the resistance against the ozone degradation. This improvement is likely connected with the reinforcing effect of carbon black in rubber materials. 4. Conclusion The aim of the work was the study of influence of ferrite alone, as well as ferrite in combinations with carbon black on the properties of elastomeric composites. The changes in properties in consequence of thermo-oxidative and ozone ageing as well as the thermal stability of prepared materials were considered. The results demonstrate that ferrite incorporated in rubber materials exhibits only low reinforcing effect. Despite of the tensile strength at break of NR and mainly BR-composites increased in the presence of magnetic filler. The increase of the tensile strength at break in dependence on the ferrite weight fraction increasing in composites filled with fillers combinations was also recorded. This seems to be very surprising, as carbon black is proved to be traditional reinforcing filler for rubber materials. The values of physicalemechanical properties of composites after ageing are higher or lower in comparison with corresponding values of composites before thermo-oxidative
degradation. But almost all evaluated properties of composites after ageing remained the character of their dependences on the content of ferrite, or fillers combinations. Thermogravimetric analysis revealed that the initial temperature of the thermal decomposition of evaluated composites was not influenced by the amount of magnetic filler. Based upon results obtained by the study it is possible to claim that ferrite present in rubber matrix does not influence the thermo-oxidative and thermal stability of the prepared composites. The ozone ageing tests showed that the influence of ferrite on the ozone stability of natural rubber as well as butadiene rubber based composites was ambiguous. The change of TD seems not to be considerably dependent on the magnetic filler content. In case of composites filled with combinations of ferrite and carbon black, the resistance against ozone increases with increasing content of carbon black in combinations of both fillers. Acknowledgement This work was supported by grant agency VEGA, project No. 1/ 1163/12 References [1] Zhou GY. Shear properties of a magnetorheological elastomer. Smart Mater Struct 2003;12:139e46. [2] Chen L, Gong XL, Jiang WQ. Investigation on magnetorheological elastomers based on natural rubber. J Mater Sci 2007;42:5483e9. [3] Wang Q, Yang F, Yang Q, Chen J, Guan H. Study on mechanical properties of nano-Fe3O4 reinforced nitrile butadiene rubber. Mater Des 2010;31:1023e8. [4] Chatterjee J, Haik Y, Chen CJ. Polyethylene magnetic nanoparticle: a new magnetic material for biomedical applications. J Magn Magn Mater 2002; 246(3):382. [5] Matsumoto M, Miyata Y. Polymer absorbers containing magnetic particles: effect of polymer permittivity on wave absorption in the quasi-microwave band. J Appl Phys 2002;91(12):9635. [6] Kraus G. Swelling of filler-reinforced vulcanizates. J Appl Polym Sci 1963;7: 861e71. [7] Hamed GR, editor. Materials and compounds: engineering with rubber. New York: Oxford University Press; 1992. [8] Saville B, Watson AA. Structural characterization of sulfur-vulcanized rubber networks. Rubber Chem Tech 1967;40:100e49. [9] Morrison NJ, Porter M. Temperature effects on the stability of intermediates and crosslinks in sulfur vulcanization. Rubber Chem Tech 1984;57:63e86. [10] Warner WC. Methods of devulcanization. Rubber Chem Tech 1994;67: 559e66. [11] White JR, De SK, editors. Rubber technologist’s handbook. Shawbury: Rapra Technology Limited; 2001. [12] Medalia AI. Filler aggregates and their effect on reinforcement. Rubber Chem Tech 1974;47:411e33. [13] Kyselá G, Hudec I, Alexy, editors. Manufacturing and processing of rubber. Bratislava: Slovak University of Technology Press; 2010.