Recycling of Chrome Tanned Leather Dust in Acrylonitrile Butadiene Rubber

429KGK · September 2008

Natural rubber (NR) holds a unique place in rubber technology due to its (i) outstanding tack, (ii) strength in the unvulcanized state, (iii) high tensile strength and (iv) crack growth resistance exhibited by vulcanizates. These features are attributed to its ability to crystallize rapidly on straining [1]. NR is pri-marily composed of cis-1,4-polyisoprene and a small amount of non-rubber compo-nents and groups linked to the polymer chain several characteristic properties. The physical and mechanical properties of natu-ral rubber have been found to be enhanced by the presence of long- chain fatty acids and their esters which are constituents of the non-rubber components in the polymer. These compounds are also widely known to have significant influence on the crystalliza-tion behavior of natural rubber [2].To improve the properties of rubbers some ingredients such as accelerators, activators, antioxidants and softeners should be add-ed. The addition of these materials in small quantities with respect to raw rubber could affect the electrical, mechanical and ultra-sonic properties of the mixes. On the other hand, accelerator, activators are used to in-crease the vulcanization rate by activating the accelerator so that, it performs more ef-fectively. It is believed that they react in some manner to form intermediate com-plexes with the accelerator. The complex thus formed is more effective in activating the sulphur present in the mixture, thus in-creasing the cure rate, obtaining the maxi-mum benefits from an acceleration system and improving the final products. Polymers can be modified by introducing of ionic groups. The ionic polymers, also called ionomers, offer great potential in a variety of applications. Ionic rubbers under ambient con-ditions show moderate to high tensile, tear strength and high elongation. The ionic crosslinks are thermolabile and thus the mate-rials can be processed like thermoplastics [3].

The aim of the present work is to study sys-tematically the dielectric, ultrasonic, physi-co-mechanical properties and rheological behavior of NR vulcanized in the presence of ZnO/fatty acid (stearic acid) or in the presence of different concentrations of zinc stearate (salt of fatty acid).

Experimental

MaterialsNatural rubber (NR) : Ribbed smoked sheets (RSS-1) with specific gravity 0.913, Mooney viscosity ML (1+ 4), 60-90 at 100 oC& Tg -75oC ), were supplied by Transport and Engineering Company (Alexandria, Egypt).Zinc stearate, obtained locally, rubber grade, m.p. 128 oC

The other rubber ingredients listed in Ta-ble 1, were the customarily used in rubber industries; the solvents and chemicals were of pure grade.

Techniques for preparation and characterization All rubber mixes were prepared on a labora-tory two-roll mill of 470 mm diameter and 300 mm working distance. The speed of the slow roller was 24 r.p.m., with a 1:1.4 gear

Natural rubber (NR) · Zinc oxide · stearic acid · Zinc streatate · Stress-strain · Swelling · SEM · Ultrasonic · Dielectric properties

The effects of zinc stearate (ZnSt) on the properties of natural rubber NR were studied by means of ultrasonic and dielectric spectroscopy. Incorpora-tion of zinc stearate increases margin-ally stress at break, modulus and enhances the physico-mechanical properties. It also affects the longitudi-nal & transverse ultrasound velocities, elastic modului, hardens and other measured properties according to ultrasonic measurements performed at 2 MHz. The broad-band dielectric relaxation (0.01 Hz-10 MHz) reveal that,

increases in the presence of zinc stearate, comparable to white fillers and zinc stearate acts as a plasticizer for the ionic domains at higher temperatures.

Mechanische, Dielektrische und Ultraschall-Eigenschaften von Naturkautschuk mit unter-schiedlichen Anteilen von Zinkstearat

Naturkautschuk (NR) · Zinkoxid · Stea-rinsaeure. Zinkstearat · Zug-Dehnungs-verhalten · Quellung · SEM · Ultraschall · dielektrische Eigenschaften

Der Einfluss von Zinkstearat (ZnSt) auf die Eigenschaften von Naturkautschuk wurde mit Hilfe von Ultraschall- und dielektrischer Spektroskopie unter-sucht. Das Einbringen von Zinkstearat in Naturkautschuk fuehrt zu einer leichten Erhoehung der Zugfestigkeit sowie des Schubmoduls, und verbessert die physikalischen Eigenschaften. Ultraschallmessungen bei 2 MHz zeigen, dass auch die longitudinale und trans-versale Schallgeschwindigkeit und die elastischen Moduln beeinflusst werden. Die dielektrische Relaxation im Fre-quenzbereich von 0.01 Hz bis10 MHz bei verschiedenen Temperaturen zeigen, mit dem Zinkstearat-Anteil, ähnlich wie bei hellen Fuellstoffen ansteigt und leitfaehige Domaenen bei hohen Temperaturen bildet.

AuthorsA. A. Ward, S. El-Sabbagh , Dokki Cairo (Egypt), N. S. Abd El-Aal, Giza (Egypt)

Corresponding author:Dr. Azza. A. WardNational Research CentreMicrowave Physics and Dielectrics Dept.Physics Section12311 El Behooth St. Dokki, Cairo-EgyptTel. +20/225/79890E-mail: [email protected]

Mechanical, Ultrasonic,Dielectric and Physical Properties of Natural Rubber Different Concentrations of Zinc Stearate

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430 KGK · September 2008

PRÜFEN UND MESSENTESTING AND MEASURING

ratio. The rubber was mixed with ingredi-ents according to ASTM (D15-72) and care-ful control of temperature, nip gap and se-quenced addition of ingredients.Vulcanization was carried out in a single-daylight electrically heated auto controlled hydraulic press at (142 1 oC) and pressure 4 MPa. The compounded rubber and vul-canizates were tested according to stand-ard methods, namely:

Cure characteristics [ASTM D2084-95 (1994)]: for determination of rheometric characteristic using a Monsanto Rheom-eter model 100.Physico-mechanical properties deter-mined using Zwick tensile testing machine (model-1425), [ASTM D412-8a (1998)] Hardness was determined according to [ASTM D 2240-97 (1997)].Fatigue properties were determined us-ing a Monsanto Fatigue Failure Testing Machine, according to ASTM D 3629 (1998).Swelling was determined according to ASTM D 471-97(1998). Swelling experi-ments were carried out with the moulded samples by putting the samples in tolu-ene at room temperature, 25 0C, for 24 hours.Thermal oxidative aging was carried outaccording to ASTM D 573-88 (1994).

Strain energy determinationStrain energy values are obtained by plot-ting stress-strain curves for vulcanized rub-ber samples and integrating the area under the curve up to the particular extension used. To calculate the strain energy, Simp-son’s rule is applied [4].

Scanning electron microscopy The edges morphology of the samples un-der investigation was accomplished using a Joel JSM-T20 scanning electron microscope (SEM). The samples were coated with a very thin layer of gold to avoid electrostatic charging during examination.

Density Measurement The density ( ) of the rubber specimens was calculated by employing the Archimedes principle using oil as buoyant and applying the relation:

P ww w

a

a b

b

Where b is the density of the buoyant, and wa and wb are the sample weights in air and the buoyant respectively. The experiment was repeated three times, and the estimat-ed error in density measurement for all samples was found to be 1 kg/m3

Ultrasonic measurementsThe longitudinal and shear ultrasonic ve-locities were measured for the samples (discs of 1.5 cm diameter and 0.95 cm thick-ness) using the pulse-echo technique (USIP20, Krautkramer, Germany) and stand-ard electronic circuit, oscilloscope ( 54615 B, Hewlett Packard) [5]. The longitudinal and shear ultrasonic velocities were therefore obtained by dividing the round trip distance by the elapsed time according to the rela-tion V 2X t. Where X is the sample thick-

ness and t is the time interval. Measure-ments were performed at 2 MHz and at room temperature. The accuracy of velocity measurements was estimated to be ±16 m/s for longitudinal wave velocity (VL) and ± 15 m/s for shear wave velocity (Vs).Then, the elastic moduli longitudinal (L), shear (G), Bulk (K), young’s (E) and Poisson’sratio ( ) of the investigated samples have been determined from the measured ultra-sonic wave velocities and density measure-ments.

Relative minimum torque as a function of zinc stearate content.

111

Samples Formulations

Ingredients in phr A B C D E F G

NR

ZnO

Stearic acid

CBS

PBN

S

100

5

2

0.6

1

2.5

100

2.5

1

0.6

1

2.5

100

1

0.5

0.6

1

2.5

100

----

----

0.6

1

2.5

100

----

----

0.6

1

2.5

100

----

----

0.6

1

2.5

100

----

----

0.6

1

2.5

Zinc stearate --- 2 3.5 5 7 9 11

1

Rheometric Characteristics at 140 0C

Property A B C D E F G

M (dN.m)

Tc90 (min)

Ts2 (min)

CRI (min-1)

53.5

16.25

5

8.88

51

20.25

8

8.16

47

19.25

7.5

8.51

31.25

14.25

7

13.79

33

14.75

7.25

13.33

34.775

17.75

9.75

12.75

32.25

21.5

10.5

9.09

2

Physico-mechanical properties of the samples at the optimum cure time

Property A B C D E F G

Modulus at 100 %, MPa

Modulus at 200 %, MPa

Tensile strength, MPa

Rupture strength, MPa

Yield strain %

Rupture strain %

Young’s mod. N/ mm2

No. of cycles x 100 until fracture

Hardness, Shore A

Strain energy MJ/m3

1.12

1.76

12.84

12.65

684

684

1.02

635

41

4.21

1.19

2.1

23.5

23.41

863

863

1.07

1233

47

5.45

1.38

2.31

25.47

25.44

954

954

1.09

1236

45

6.4

1.17

1.51

18.1

18

914

915

1.12

1485

45

3.23

1.4

1.81

22.17

21.27

1169

1169

1.37

1493

46

3.29

1.615

2.1

27.96

27.5

1155

1155

1.59

1590

41

3.91

0.886

1.31

20.58

20.58

1139

1139

0.86

1498

43

3.4

3

431KGK · September 2008

Broad- band dielectric relaxations measurementsDielectric permittivity and loss were measured with an impedance analyzer (Sch-lumberger Solartron 1260), an electrometer amplifier and measuring cell, which is de-scribed before [6]. The accuracy in measure-ment of tan is 10-3. The reproducibility of the measurement was tested by re-measur-ing and after performing the experi-ment once again.

Results and discussion

Rheometric characteristics and physico-mechanical propertiesThe rheometric characteristics and physico-mechanical properties of natural rubber compounds loaded with ZnO / stearic acid or with different concentration of zinc stea-rate are given in Tables 2, 3. It is well known that, the difference between maximum and minimum torque is a rough measure of the crosslink density of the samples and usually known as M. From these tables it is noticed that M decreased when ZnO / stearic acid is replaced by zinc stearate. On the other hand, M increased by increasing the ratio of zinc stearate in rubber vulcani-

zates up to 9 phr and then decreased. More-over one can clearly seen that, the scorch time ts2 and optimum cure time tc90 in-creased while cure index CRI decreased with increasing zinc stearate content in the in-vestigated samples.

Concerning the rheometric characteristics results in Table 2, it is revealed that, the rate of vulcanization change is dependent on the ratio of zinc stearate used. In order to confirm these findings, the relative mini-mum torque, DR

min is calculated as a func-tion of zinc stearate loading according to the following equation:

D D

D -1min

R minZ

min0 (1)

where DZmin

is the minimum torque of the rubber compounds loaded with zinc stea-rate and Dmin

0 is the minimum torque of the gum (sample that contains only ZnO / stearic acid ). The results of this calculati-on are plotted graphically in Figure 1.From this figure it is obvious that, the re-lative torque decreased with increasing zinc stearate concentration, up to 9 phr, above this concentration (at 11 phr) unex-pected decreased is obtained. From these results one can directly conclude that, the sample loaded with 9 phr zinc stearate has a strong fatty acids-polymer interac-tion. Moreover the maximum change in curemeter torque during vulcanization in-creases when zinc stearate incorporated into natural rubber compound. The ratio between the torque increase of the loa-ded compound and that of the gum was found to be directly proportional to zinc stearate loading. The slope of the linear plot showing the relative torque increase to be a function of zinc stearate loading and was defined by Wolff [7] as f. These different terms were expressed in equati-on (2) as follows:

Maximum change in curem-eter torque during vulcaniza-tion as a function of mz/mp for zinc stearate.

222

Stress-Strain curve of NR vulcanizates loaded with different concentration of zinc stearate.

333

Swelling characteristics of NR vulcanizates loaded without/with ZnSt

Property A B C D E F G

Eq. Swelling Q%

Mol. Weight g/mole

between two crosslinks

Crosslink density x105

mole/cc

Sol. Fraction%

356

5112

9.78

0.23

353

5035

9.93

0.0664

364

5330

9.38

0.0518

387

5966

8.38

0.02333

422

7002

7.14

0.0471

358

5170

9.67

0.0307

424

7062

7.08

0.0429

4

Variation of: Density ( ), Longitudinal Ultrasonic Velocity(V1) , and shear ultrasonic Velocity(Vs)Longitudinal modulus(L) , shear modulus(S) , Bulk modulus(K), Young’s modulus(E), Poisson’s ratio ( ) , Crosslink density (Nc), with zinc stearate content.

Sample ± 1Kg/m3

VL ± 16m/s

VS ± 15m/s

L ± 0.03(GPa)

S ± 0.007(GPa)

E ± 0.01(GPa)

K ± 0.03(GPa) ± 0.02 NC

A

B

C

D

E

F

G

939.5

941.1

924.7

922.9

924.3

929.6

931.9

1472

1469

1417

1446

1469

1479

1423

701

700

644

673

700

721

678

2.04

2.03

1.86

1.93

1.99

2.03

1.89

0.462

0.461

0.384

0.417

0.452

0.484

0.428

1.25

1.25

1.05

1.14

1.22

1.30

1.16

1.42

1.42

1.35

1.37

1.39

1.39

1.32

0.35

0.35

0.37

0.36

0.35

0.34

0.35

0.4096

0.4096

0.3280

0.3660

0.4096

0.4599

0.4096

5

432 KGK · September 2008

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D D

D D

m

mfz

p

max min

max min0 0

1 (2)

where (Dmax Dmin ) is the maximum change in torque during vulcanization for rubber with zinc stearate, D Dmax min

0 0 is the max-imum change in torque during vulcaniza-tion for gum , mp and mz are the mass of polymer and zinc stearate in the compound respectively. The parameter (mz ) used to characterize the optimum concentration of zinc stearate, (Fig. 1, 2).Moreover, Table 2 shows that, the cure time increased when a small portion of ZnO / stearic acid is replaced by zinc stearate. This increase may be due to the aggregation of the particles of ZnO / stearic acid and zinc stearate forming a coherent gel in the rub-ber matrix [7]. This aggregates prevented physically the rubber from being readily vul-canized and consequently the cure time in-creased. Considering zinc stearate loading, it is noticed that the cure time decreased with the initial loading of zinc stearate and then increased by increasing zinc stearate content because zinc stearate acts as lubri-cant agent. On the other hand, the mechan-ical properties and fatigue life increased when ZnO / stearic acid is replaced by zinc stearate. This finding can be explained on the basis of considering zinc stearate as re-inforcing filler to NR. This in turn may ex-plain the improvement in the mechanical properties with increasing the zinc stearate concentration in the rubber. Also from Table 4, it is observed that as the ratio of zinc stearate increased the equilib-rium swelling increased up to 9 phr. The swelling (Q) decreased and then increased. From the pervious results of the physical properties one found that these results en-hanced in the presence of zinc stearate. To confirm these results, the formation of such crosslinks was determined from the equilib-rium swelling measurements through the molar mass between crosslinks. Mc accord-ing to Flory-Rehner relation [8]:

1

2

1

2

112

0

2

1 3M V

V V V

V Vc

r r r

r r

ln(3)

where is the density of NR g/cm3 , V0 is the molar volume of solvent (toluene) 106.3 cm3/mol, Vr is the volume fraction of the swollen rubber that can be obtained from the mass and density of rubber sam-ples and solvent, is the interaction param-eter between the rubber and toluene which is about 0.393 for NR. The degree of cross-linking is given by [1]:

1 2Mc

The obtained are listed in Table 3, from this table one can notice that increased by replacing some amount of ZnO / fatty acid by 2 phr zinc stearate {sample B}, and de-creased by increasing the zinc stearate up to

9 phr of zinc stearate {sample F}. The in-crease of in the presence of zinc stearate may be due to additional physical and chemical crosslinks. Figure 3 shows stress-strain curves of the NR vulcanizates with and without zinc stea-rate. From this figure it is clear that, the

(a) Retained Values of stress at 100 % strain(b) Retained Values of tensile Strength(c) Retained Values of strain at break

444

Relaxation parameters of NR vulcanizates before and after thermal aging.

Before aging After aging

Sample 1 2 x 1043 x 108

1 2 x 1043 x 108

A

B

C

D

E

F

G

6.477

6.466

6.339

6.339

6.339

5.649

5.638

----

44.28

46.86

50.11

51.51

55.80

58.46

7.55

7.55

7.57

7.585

7.62

7.76

7.73

5.39

5.39

5.40

5.47

5.52

5.52

5.51

-----

42.12

43.52

53.95

55.20

59.11

54.56

7.46

7.43

7.51

7.62

7.89

7.99

7.75

6

433KGK · September 2008

stress increased significantly above 800 %elongation. This increase in stress is attrib-uted to the crystallization of NR rubber on straining [1, 2]. Moreover, the stress at break or green strength for {sample A} which con-tains of ZnO / fatty acid this make the rate of crystallization on stretching slow and therefore low green strength is observed.

While in sample B and C, after replacing ZnO / fatty acid by 2& 3 phr zinc stearate respectively, show an increase in stress as strain increased up to 800 % elongation. This increase in stress continued with in-creasing the concentration of zinc stearate in NR up to 9 phr then followed by a de-crease. From these results one can conclude

that, sample F which contains 9 phr of zinc stearate has the largest value among all measured samples. Moreover these results are in good agreement with that shown in Table 3. Also it is obviously seen that strain energy decreased in the presence of zinc stearate while fatigue life increased. This means that the compounds contain salt of fatty acid are more stable.

Effect of thermal oxidative aging on the mechanical properties The vulcanized samples were heated in an air oven at 90 °C up to 7 days. Then the me-chanical properties were re- measured and plotted graphically in Figure 4a-c. From these figures it is noticed that, the retained values of the measured properties first in-creased, this may be due to further corsslinking formed during aging that counteracted the degradation process in the initial time. On the other hand these values decreased with increasing aging time which may be due to some sort of degradation. The samples containing zinc stearate namely {D, E, F G} exhibited the highest efficiency with aging. The relation-ship between these plotted parameters in figure 4a-c could be expressed by the fol-lowing equation 4:

y AeBx (4)

where y represents either the tensile strength or the strain at break, x is the aging time and B is the slope of the line. The first derivative of this equation dY / dX is the gradient of the curve at any point ; giving the rate of change of the property, i.e equation 5:

dY

dX ABeBx (5)

Consequently, the minimum slope means the maximum rate of change of the prop-erty, which illustrates the efficiency of the zinc stearate or ZnO /stearic acid. Stress at 100 % strain was found to be well correlated with the aging time of the composites with/without zinc stearate in accordance with the power equation 6:

y AxB (6)

where y represents the stress at 100 %strain and X is the aging time of the NR com-pounds, once again dY / dX gave the change of the property, equation 7:

dY

dX ABx B-1 (7)

which illustrated the efficiency of the ZnO / fatty acid or that of salt of the fatty acid used.

SEM photograph of NR loaded with different concentrations of Zinc oxide (ZnO) Stearic acid (St. acid) and Zinc stearate (Zn-St), [Samples A, B, D, F and G].

55

5

Variation of longitudi-nal / shear wave velocity with zinc stearate content.

666

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Scanning electron microscopyFigure 5a, shows the micrographs of NR loaded with ZnO / fatty acid. From this mi-crographs, it is seen that the dispersion is poor due to large agglomerates arising from ZnO / fatty acid networking. By introducing zinc stearate together with ZnO / fatty acid Figure 5b, one can see that the dispersion becomes better. Replacing ZnO / fatty acid by 5phr zinc stearate a remarkable reduc-tion in domain size as can be seen in Fig-ure 5d. Further increase in zinc stearate con-tent resulted in uniform domain size as shown in Figure 5e,f. From these figures clear distribution observed between the matrix and dispersed phase. That is the in-corporation of zinc stearate in NR vulcani-zates improved the interfacial adhesion and reduced the size of dispersed domain. Moreover from these serial micrographs of NR loaded with zinc stearate shown in fig-ures 5 b,c,d,e it is noted that, zinc stearate is an effective accelerator activator, softener especially at concentration of 9 phr , also soluble in rubber and dispersed as reinforc-ing filler. Further addition of zinc stearate (11 phr) increases the domain size and once more bad dispersion (Fig. 5g).

Ultrasonic measurementsExperimental values of density, ultrasonic velocities ( longitudinal& shear) elastic moduli, Poisson’s ratio and crosslink density of the investigated samples of natural rub-ber compounds with ZnO / stearic acid or with different concentration of zinc – stear-ate are listed in Table 5 . Both longitudinal VL

and shear VS wave velocities decrease when replacing ZnO/ stearic acid with zinc stear-ate samples (A-C) while by increasing the ratio of zinc stearate both the velocities in-crease (samples D-F) and then decreased at concentration 11phr zinc strearate. Figure 6 represents the relation between both ultrasonic velocities and of zinc stear-ate content. The increase of ultrasonic wave velocity long & shear is due to the decrease in spaces between chains inside the struc-ture of material .This decrease in spacing is may be attributed to the increase in cross linking between chains which consequently cause the increase in the rigidity of the ma-terial. Table 5 gives the experimentally esti-mated values of the elastic moduli, longitu-dinal modulus (L), shear modulus (S), Young’s modulus (E) and bulk modulus (K). As can be seen from Figure 7 the same trend as in the elastic Young’s modulus (E) and bulk modu-lus (K) decrease with decrease ZnO from 5 to 1 and stearic acid from 2-0.5 ( Sample A,B,C) while by adding Zinc stearate of con-

centration 2, 3.5, 5, 7 and 9phr all the elastic moduli increased Table 5, and then decrease at concentration 11phr ,this indicated that at 9phr of Zinc stearate,strong fatty acids – polymer interaction is expected. In other words in the rubbery state, when the ki-netic elements of adjacent chains have a high mobility, the cross-linkages prevent the moving apart of adjacent chains, increasing the effectiveness of intermolecular interac-tion .It is this factor that results in a growth in the modulus of elasticity and the velocity of sound with an increase in the cross- link-age factor [9]. Variations in Poisson’s ratio for the unfilled and filled samples with zinc stearate are tabulated in table [5] reflects an opposite to the other mechanical properties, it is known that Poisson’s ratio is formally defined for any structure as the ratio of lat-eral to longitudinal strain produced when tensile force are applied . For the tensile stresses applied parallel to the chains, the longitudinal strain produced will be the same irrespective of the degree of direc-tional bonds of the chain. On the other hand, lateral strain decreased with increasing crosslink density. The behavior of Poisson’s ratio with composition ought to be nearly the reverse of the cross- link (Nc) of the net-

work [ 0.28(Nc)-0.25 ] [10], consequently,

the decrease in Poisson’s ratio (samples C-F) is attributed to the increase in the crosslink density as shown in table [5]. Moreover These results are in good agreement with that taken by rheometric characteristic and physico- mechanical properties.

Dielectric measurementsThe permittivity and dielectric loss for the investigated samples were measured in a frequency range of 0.01 Hz up to 10 MHz at temperature ranging from 20 t0 1800C.Example of the obtained data at 20 0C is il-lustrated graphically in Figures 8 a,b . It is noticed that the values of increased in the presence of zinc stearate (Zn-St) together with ZnO and Stearic acid {samples b & c}. Moreover, the values of still increased even in the absence of both ZnO and Stear-ic acid {samples d,e,f and g }. In general, re-inforcing fillers are known to cause an in-crease in of rubbers [11-14]. Accordingly, it is believed that zinc stearate acts not only as crosslink sites, but also as filler providing reinforcement to the matrix [15 -17]. As zinc stearate acts as a reinforcing filler under ambient conditions, so it is suggested that some interaction occurs with rubber mole-

Variation of Young’s modulus and bulk modulus with zinc stearate content.

777

(a) Permittivity versus the frequency for NR loaded with different concentrations of zinc stearate at 20 0C.

(b) Dielectric loss versus the frequency for NR loaded with different concentrations of zinc stearate at 20 0C.

88

8

435KGK · September 2008

cules is responsible for the increase in and with the increase in Zn-St content.

From the same figure, it is evident that the curves relating log and log f are broader than Debye curves [18], indicating that more than one relaxation process is present. Fit-ting the data was preformed by a computer program based on Cole –Cole [19] in addi-

tion to conductivity contribution. Relaxation parameters obtained according to this fit-ting are given in Table6. Examples of the di-electric spectrum and the fitting of the data for sample A, F and G are illustrated graphi-cally in Figure9 The first relaxation process at about 0.0195Hz is found to be present for all the investigated samples. Those low fre-

quency losses may be attributed to Maxwell-Wagner losses arising from interfacial po-larization caused by the multi constituents of the investigated systems. The difference in permittivities and conductivities in the constituents of the investigated materials is considered to be the reason for the presence of such effect. At relatively higher frequen-cies a second process appeared after the ad-dition of zinc stearate and shifted somewhat to lower frequency with increasing zinc stea-rate content which may be attributed to NR chains of lower mobility which are tightly bounded to zinc stearate surface. In other words, this means that, there is some restric-tions of the molecular dynamics of this proc-ess restricting the motion of NR chains from there surrounding environment. However at this point, it is worth to mention the studies given by Datta et al [3]and Hird et al [15] about zinc stearate filled rubber using DMTA technique. In this study they found [from the plot of tan against temperature] that there are two relaxation processes, the first one called the backbone relaxation related to Tg which occur at relatively lower tem-peratures. The second one called high tem-perature relaxation, which became promi-nent in the presence of zinc stearate. This high temperature relaxation peak is believed to be due to onset of restricted motion of chains firmly held by the ionic aggregates called clusters [15]. However on the basis of these studies one can attribute the second process in kHz to the chains of restricted mobility. Moreover these findings are also discussed by other earlier studies of silica filled rubbers [ 14,20]. Additionally the process at about 1.75 MHz is not largely affected by the addition of zinc stearate. This means that a certain por-tion of NR chains in the vulcanizates will remain largely surrounded by NR chains and the motion of these chains will be unaf-fected. These results are and in fair agree-ment with the results obtained before [20].

Effect of temperature on the dielectric properties The variation of the permittivity with tem-perature for samples a, f and g is shown in Figure 10. It is evident that , decreases with increase in temperature to about 100 0C. Also as the temperature increases, there is a decrease in the number of dipoles per unit volume and so decreases with increase in temperature which is a common phenomenon for non-polar and slightly po-lar polymers [21]. A further increase in tem-perature causes a decrease in for sample A. This is because the ionic groups present

Absorption cures of NR load-ed with:(a) NR + 5 phr zinc + 2 phr st. acid(b) NR + 9 phr ZnSt(c) NR + 11 phr ZnSt

999

Permittivity versus tem-perature at 100 Hz for sam-ples: A, F and G.

101010

436 KGK · September 2008

PRÜFEN UND MESSENTESTING AND MEASURING

act as physical crosslinks and restrict the chain mobility which hinders the dipole ori-entation under electric field.On the other hand, concerning sample F &G,

is slightly affected with the increase in temperature to about 100 0C showing good thermal stability in this range. Where as the temperature increases zinc stearate melts and plasticizers the ionic aggregates, there-by breaking down the physical crosslinks. Moreover as shown in the same figure, the zinc stearate-filled system shows an appre-ciable increase in with temperature in the high temperature region.

Effect of thermal aging on the dielectric properties The investigated samples were subjected to thermal oxidative aging at 90 0C for two days in thermal oxidative oven. The permit-tivity and dielectric loss for these sam-ples were re-measured and the obtained data are illustrated graphically in Figure 11.Comparing these results with those for the same unaged samples, it was found that and have affected by aging for some samples, while some have good thermal stability especially the sample containing 9 phr zinc stearate. This result is supported by the calculation of the retained values. Moreover the re-measured data were ana-lyzed by the same way as mentioned before, and the results are given in Table 6 and il-lustrated graphically Figure 12. Although, there were a notable increase in the dielec-tric strength for all relaxation mecha-nisms, no greet change in the values of the relaxation time of the first process is no-ticed, while the relaxation times , have slightly increased for high content of zinc stearate samples. This may be due to due to either the formation of large aggregates or more likely, due to the formation of cross linked molecules during thermal ageing.

ConclusionThe effect of adding different concentra-tions of zinc stearate on the physico-me-chanical, ultrasonic, SEM and dielectric properties has been investigated.The dielectric data on the frequency do-main (0.01 Hz-10 MHz) reveals three re-laxation mechanisms ascribing Maxwell-Wagner polarization, rubber chain of lower mobility which are tightly bound to zinc stearate surface and that for the main chain.Zinc stearate was found to play dual role, firstly, it reinforces the matrix blow its melting point and secondly higher temperature it plasticizes the system.

The samples containing 9 phr was found to be the most promising concentration showing good dispersion and thermal sta-bility on thermal aging.

The authorsDr. Azza. A. Ward and Dr. Salwa EL-Sabbagh are members in National Research Centre, Dokki, Cairo, Egypt. Dr. Nadia S. Abd El-Aal

(i) Permittivity versus the frequency before & after 2-days of thermal aging for NR loaded with different concentrations of ZnO, St. acid and zinc stearate. (ii) Dielectric loss versus the frequency before & after 2-days of thermal aging at 90 0C for NR loaded with different concentrations of ZnO, St. acid and zinc stearate.

1111

11

Absorption cures of NR load-ed with:(d) NR + 5 phr zinc + 2 phr st. acid(e) NR + 9 phr ZnSt(f) NR + 11 phr ZnSt After2-days of thermal aging at 900C

121212

437KGK · September 2008

is a member in National Institute for Stan-dards, Giza, Egypt.

References

[1] A. N. Gent, S. Kawahara and J. Zhao, Rubber chem.Technol. 71 (1998) 668.

[2] S. Kawahara, N. Nishiyama, T. Kakubo and Y. Tan-aka , Rubber chem. Technol. 69 (1996) 600.

[3] S. Datta, S. K. De, E. G. Kontos and J. M. Weffer, J. Appl. Polym. Sci. 61 (1996) 177.

[4] R. s. Rivlin and A,G. Thomas, J. Polym. Sci. 10(1953) 291.

[5] R. Truell, C. Elbaum, B. Chick „Ultrasonic method in solid state physics“ Acdemic Press, NY and London (1969).

[6] A. A. Ward, PhD. Thesis , Uni. Cairo (2003).

[7] S. Wolff, M. J. Wang Rubber Chem. Technol. 65(1992) 329.

[8] G. A. Zhag, M. H. Yhou, J. H. Ma , B. R. Liang J. Appl. Polym. Sci. 90 (2003) 2241.

[9] S. S. Perepechko, L. A. Kracheeva, L. A. Vshakov, A.Ya.Selov, V.A.Grechishkin, Plast massy 8 (1973).

[10] A. A. Hhigazy, B. Bridge, J. Non, Crystalline solids,72 (1985) 81.

[11] I. K. Hakim, A. M. Bishai, F. F. Hanna, J. Appl. Pol. Sci. 21 (1977) 1155.

[12] A. M. Bishai, A. M. Ghoneim and A. A. Ward, Int. J. Pol. Mat. 51(2002) 793.

[13] A. M. Bishai, A. A. M. Ward, A. M. Ghoneim and A. F. Younan, Int. J. Pol. Mat. 52 (2003) 31.

[14] A. A. Ward, B. Stoll, W. von Soden, S. Herming-haus, Augenie M. Bishai and Faika F. Hanna., J. Macromol. Sci., part B – Phs. 42(2003)1265.

[15] B. Hird and A. Eisenberg, Macromolecules 25(1992) 6466.

[16] L. Ibarra, J. Appl. Pol. Sci. 73 (1999) 927.[17] P. Antonz, S. Bandyopadhyay, S. K. De, Pol.

Eng. & Sci. 39 (2004) 963.[18] N. E. Hill; W. E. Vaughan; A. H. Price M. Davies,

“Dielectric Properties and Molecular Behaviour”Van Nostrand, London (1969).

[19] C. P. Smyth, “Dielectric Behavior and Structure”, McGraw-Hill, New York, NY (1955).

[20] A. A. Ward ,A. M. Bishai, F. F. Hanna, A. A. Yehia, B. Stoll, W. Von Soden, S. Herminghaus, A. A. Mansour, KGK, 12 (2006), 654.

[21] N. G. McCrum, B. E. Reed, G. Willimas, An elastic and Dielectric Effects in Polymeric Solids, Wiley, New York, (1967).