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Design and manufacturing of new elastomeric composites: Mechanical properties, chemical and physical analysis
Accepted Manuscript Design and manufacturing of new elastomeric composites: Mechanical properties, chemical and physical analysis D. Zaimova, E. Bayraktar, I. Miskioglu PII:
S1359-8368(16)30854-X
DOI:
10.1016/j.compositesb.2016.05.061
Reference:
JCOMB 4344
To appear in:
Composites Part B
Received Date: 29 July 2015 Accepted Date: 30 May 2016
Please cite this article as: Zaimova D, Bayraktar E, Miskioglu I, Design and manufacturing of new elastomeric composites: Mechanical properties, chemical and physical analysis, Composites Part B (2016), doi: 10.1016/j.compositesb.2016.05.061. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Design and manufacturing of new elastomeric composites: mechanical properties, chemical and physical analysis
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D. Zaimova1, 2, E. Bayraktar2*, I. Miskioglu3
UCTM, University of Chemical Technology and Metallurgy, Sofia, Bulgaria
Supmeca-Paris, School of Mechanical and Manufacturing Engineering, France Michigan Technological University ME-EM Department, Houghton, MI-USA
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*Corresponding author: [email protected];
Abstract
Filler-reinforced vulcanized rubber and its blends have been used for engineering applications for over a century. Traditional applications include tires, seals, bushings, and engine mounts. The rubbers for tire manufacturing
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must have high elasticity, good frictional properties and high load bearing capacity. Conforming to these needs, rubbers are vulcanized by various materials such as sulphur, carbon black, accelerators, and retardants under different conditions. The reactivity of sulphur vulcanization and physical properties of the final product are
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affected by the chemical structure, molecular weight, and conformation of the base elastomers. The aim of this study is to investigate the influence of accelerator-vulcanizing agent system and the
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vulcanization temperature on the mechanical and aging properties of vulcanizates based on natural rubber/Polybutadiene rubber (NR/BR) composites. Two ratios of accelerator to sulphur were used for the preparation of the composites also known as conventional vulcanization system (CV), where the quantity of sulphur is more and efficient vulcanization system (EV), where the quantity of accelerator is more.This preliminary study will allow optimization of the composition/processing conditions to improve the mechanical properties and resistance to damage. NR/BR based composites at different vulcanization temperatures and using different curing systems were manufactured. Tensile strength, elongation at break, tensile modulus at 100% (M100) and at 300% (M300)
ACCEPTED MANUSCRIPT deformation hardness (Shore A) of the composites were determined. Scanning electron microscopy was used to study the microstructure of the fracture surfaces. The main findings are that CV gives more reliable results for multi-purpose applications. Effective vulcanization system is suitable for application in areas where good aging resistance is needed and also in applications where low deformations are expected. Vulcanization temperature
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does not play significant role although most of the results are in favour of the lower temperature (140oC).
INTRODUCTION
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Keywords: Elastomer composites; mechanical properties; chemical and physical analysis; SEM study
Vulcanized rubber and its blends reinforced with fillers have been traditionally used in tires, seals, bushings, and engine mounts (Morawetz, 2000). Depending on the application, the rubbers must have high elasticity, good frictional properties, good wear characteristics as well as high load bearing capacity (Smith, 1961). The sulphur vulcanization with unsaturated rubbers occurs through complicated radical substitution in the form of mono-, di-, or polysulphide bridges and sulphur
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containing intracyclization of the polymer molecules. The cross-link density and distribution affect the physical properties and stability of rubber on aging. They depend not only on type and ratio of accelerator to sulphur, but also on the reaction temperature and duration (Nasir and Teh, 1988).
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The rate of vulcanization of rubber is closely related to the efficient manufacturing of elastomer parts. Hence, it is desirable to increase the vulcanization rate by raising temperature. However, at higher temperatures the sulphur cross-links
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are less effective and the physical properties are sacrificed due to dissociation of sulphur bonds and rubber chains (Bhowmick et al., 1979). Instead of increasing temperature, the vulcanization reactivity may be controlled by additional amounts of sulphur and accelerator. Although the addition of sulphur increases cross-link density and the amount of polysulphide linkage, it may decrease the stability of the cross-links, especially upon aging (Studebaker and Beatty, 1972). By increasing the amount of accelerators, the effectiveness of sulphur vulcanization with mono- and disulphide linkages is improved which in turn results in more stable characteristics upon aging at the cost of degrading the dynamic properties of the product (Bayraktar et al., 2008a). The optimum conditions must be chosen to obtain the best properties for products and productivity. Hence the goal of this study is to investigate the influence of accelerator-vulcanizing agent system and the
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ACCEPTED MANUSCRIPT vulcanization temperature on the mechanical and aging properties of vulcanizates based on Natural rubber/Polybutadiene rubber (NR/BR) composites. For this study two NR/BR based composites processed with two different curing systems (conventional and efficient) and vulcanized at two different temperatures (140ºC and 160ºC) were manufactured. The curing characteristics and the
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mechanical properties of the manufactured composites were investigated. The curing characteristics of the rubber composites were studied by using the Monsanto MDR 2000 rheometer. The investigated mechanical properties were tensile strength, elongation at break, tensile modulus at 100% (M100) and at 300% (M300) deformation. Shore A hardness and molecular mass of the samples were also determined. Scanning electron microscopy was used to study the microstructure of
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the fracture surfaces of the tensile samples.
In this paper partial results of the applied project supported by French-Bulgarian research cooperation are presented.
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The exact composition of the blends used in this study is proprietary, hence will not be discussed in this paper.
EXPERIMENTAL PROCEDURE
The two composites were mixed in a laboratory scale two roll mixer. The moulding conditions were determined from the
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torque data from a moving die rheometer MDR 2000 (Alpha Technologies) for temperatures 140°C and 160°C. At the end, four composites (A, B, C, D) based on sulphur vulcanized Natural rubber/Polybutadiene rubber blends were prepared containing certain fillers and/or reinforcements in order to investigate their deformation behaviour. Two ratios of accelerator
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to sulphur were used for the preparation of the composites also known as conventional vulcanization system (CV), where the quantity of sulphur is more and efficient vulcanization system (EV), where the quantity of accelerator is more. Composites A and B were vulcanized using conventional vulcanization system (CV) and at vulcanization temperatures
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140ºC and 160ºC respectively. Composites C and D were vulcanized using the efficient vulcanization system (EV) respectively at vulcanization temperatures 140ºC and 160ºC. Cured sheets were prepared by compression moulding at 140°C and 160°C and at a pressure of 100 kg/cm2. The behavior of these composites in harsh environmental conditions was tested by means of swelling experiments. The swelling tests were carried out by soaking the samples in toluene at room temperature. At the conclusion of these tests, the crosslink densities for each composite were calculated according to the molecular mass between two crosslinks. All of the crosslink densities were calculated according to the Flory-Rehner relation (Luong et al., 2007):
ACCEPTED MANUSCRIPT ଵ ሺଵିೝ ሻାೝ ାఓೝమ ቈ భ/య భ ଶఘబ ೝ ି ೝ
=
ߥ=
ଶெ
మ
ଵ
(2)
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where; Mc- molecular mass between crosslinks; ρ- Density of the rubber; V0-molar volume of the solvent; Vr - volume fraction of the swollen rubber; µ- interaction parameter between the rubber sample and the solvent;
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ν- Crosslink density.
(1)
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ଵ ଶெ
The nominal thickness of the sheets was set at 2 mm. The mechanical properties were determined before and after aging in accordance with ASTM D 412aε2 (2010) (ASTM: and 412aε2, 2010). The aging test was performed in a batch oven with forced convection at 70ºC for 21 days. Second type of aging was performed in an ultraviolet oven for 21 days. Dumbbell
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specimens for tensile testing were cut using standard die C from the moulded sheets. The sample length and thickness were measured. Tensile tests were performed using an Instron (model 4507). A minimum of 3 specimens were tested for each composite. Testing was done at room temperature with a crosshead speed of 500 mm/min. Shore A hardness of the
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specimens was measured according to ASTM D 2240-05 (2010) (ASTM: and 2240-05, 2010). Dynamic Mechanical Thermal Analysis (DMTA) was performed with the Dynamic Mechanical Analyzer MK III system (Rheometric Scientific). As a mode of deformation, single cantilever beam in bending was used at a frequency of 5 Hz, with a deformation of 64 µm.
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The tests were conducted in the temperature range from -80 to 80°C at a heating rate of 2°C/min. CSM Micro Indentation Tester with a Vickers diamond indenter was used to measure indentation hardness. Nine tests were performed for each composite and then averaged. The samples’ thickness were approximately 2mm. The maximum indentation load (Fmax) was 250 mN, the rate of loading - unloading was 500 mN/min. The load was held at Fmax for 50s. The indentation hardness (HIT) was determined by Oliver and Pharr method (Oliver and Pharr, 1992).
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RESULTS AND DISCUSSION
3.1
Vulcanization characteristics
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The rheometer, a convenient instrument to evaluate the effects of carbon black-rubber interactions on rate of cure and crosslinking, was employed for the purpose of characterizing critical parameters related to the vulcanization process: minimum torque (ML), maximum torque (MH), the difference ∆M between maximum (MH) and minimum (ML) torque, optimal
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vulcanization time (T90), the scorch time (Ts2) and rate of vulcanization (V).
Table I. Vulcanization characteristics of four composites studied in this research project Blend №
ML, dNm MH, dNm ∆M= ( MH – ML )
C
D
2.9
2.7
2.7
2.5
36.5
33.9
32.3
31.7
33.5
31.3
29.5
29.1
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T90, min:s
B
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Characteristic
A
6:18
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9:34
2:41
V, %min
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34
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Ts2, min:s
Fig. 1 Crosslink density determined by swelling
ACCEPTED MANUSCRIPT The minimum torque (ML) in a rheograph is related to the viscosity of the vulcanizates, and the maximum torque (MH) is generally correlated to the stiffness and crosslink density. It is well known that the difference ∆M between maximum (MH) and minimum (ML) torque is a rough measure of the crosslink density of the samples. Table I shows The vulcanization characteristics of the four composites used in this study.
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As can be seen in Table I the highest value of ∆M is for composite A. Hence the crosslink density of composite A is larger than the other three composites. This is also confirmed by the results for the crosslink density obtained by the swelling tests (Figure 1).The carbon black particles whose surfaces are covered by entangled rubber chains can be considered as physical crosslinks. The physical crosslinking hinders the mobility of rubber chains and restrains the deformation of rubber therefore
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leading to higher torque measurements. In a swelling test this type of crosslinks cannot be detected, but the high maximum torque for composite A can be considered as an indication of high number of physical crosslinks for this composite.
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The scorch time (Ts2) is the time required for the minimum torque value to increase by two units and is a measure of premature vulcanization of the material. It should be noted that that the composites with the CV system exhibited shorter Ts2 values than the ones with the EV system, therefore more crosslinks are formed in the CV systems in shorter time. These results are expected because the greater quantity of the accelerator (in composites C and D) should provide higher values for Ts2. It is possible this result to be due to the nature of sulphenamide class accelerators (delayed action in the
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beginning of vulcanization) and on the other hand the higher content of sulfur in composites “A” and “B”. The overall rate (V) of vulcanization is slightly in favor of the composite cured with EV.
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Mechanical properties before aging
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Tensile properties are commonly used to measure the degradation behavior of elastomers. Figure 2 shows the typical stressstrain curves and Figure 3 shows the comparison of modulus at 100% and 300% deformation, tensile strength and
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elongation at break for the composites A, B, C and D. Results for modulus at 300% deformation are not available for composites A, B and C. These specimens failed before reaching 300% deformation. As depicted in Figure 3 (a and b) all composites show similar values of modulus at 100% (M100) and 300% (M300) deformation. Obviously the temperature of vulcanization does not have a significant effect on M100 and M300.
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Fig. 2 Stress-strain curve for a) Composite A, b)Composite B, c)Composite C, d)Composite D
On the other hand the tendency is not the same for tensile strength (Figure 3c) and the elongation at break (Figure 3d). The composites vulcanized by CV yielded higher tensile strength and higher elongation at break under non-aged and aged conditions compared with the composites vulcanized by EV. All composites have the ability to exhibit crystallization under
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stress thanks to NR but according to literature the superior number of di- and poly- sulphidic linkages (result from CV) improves the elasticity of the composite which explains the higher values of elongation at break. In addition CV gives more homogeneous structure which plays positive role for the strength of vulcanizates. Once more the temperature of
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vulcanization does not play significant role for the tensile strength and elongation at break.
Mechanical properties after aging
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The mechanical properties of the composites after aging are also investigated. On Figures 2 the stress-strain curves before and after 21 days of aging (in air oven and under UV light) are compared. It can be seen that UV light does not affect significantly the mechanical properties of the studied composites. The same relation is observed for M100, M300, tensile strength and elongation at break.
These results can be explained with the ability of rubber composite to form additional crosslinks under the influence of temperature and UV light. Although the overcrosslinking makes the vulcanizate brittle and reduces its elasticity apparently the period of exposure was not sufficient to create overcrosslinked vulcanizates. Probably the number of crosslinks created during UV aging compensates the number of the chains destroyed by aging. The thermal aging however (21days in air oven
ACCEPTED MANUSCRIPT at 70ºC) causes more severe degradation of the strength and elongation at break values of the studied composites. Under low level of deformation (100%) thermal aging has positive effect for the same reason as explained before-under the influence of the temperature additional crosslinks are formed which increases the modulus at this level of deformation . At this stage, crosslink scissions and the formation of new crosslinks into the networks affected these properties. But under
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high stress and deformation the destruction of the crosslinks is much more significant and leads to failure of the specimen. The results shown in Figure 3 also reveal that the CV and EV vulcanization systems used in the processing of the four composites did not affect the mechanical properties consaidered. The CV system vulcanizates (A and B) has a high content of polysulphide linkages with low bond strength. By contrast, monosulphide linkages have relatively strong bond strength
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and are dominant in the EV system vulcanizates (Morrison, 1984). As a result, the CV system exhibits a weaker thermal aging resistance than polysulphide crosslinks. For all composites the extended period of heating, the excessive main chain
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scission and/or modification resulted in a reduced tendency to crystallize at high elongation. Again, it is observed that the
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effect of the temperature of vulcanization is not significant on the mechanical response of the four compunds after aging.
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Fig. 3 Comparison of a) M100, b) M300, c) Tensile strength, d) Elongation at break for Composites A, B, C, D in normal and aging conditions
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3.4 Hardness-Shore ”A” test evaluation The hardness (Shore A) results are presented in Figure 4. The hardness is relatively high for NR composite but this is probably due to the presence of BR in the mixture. The results do not show any dependence on the vulcanization system or
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vulcanization temperature. For most of the composites UV and thermal aging leads to some reduction in the hardness values. However, there is almost no difference between the effect of UV aging and thermal aging similar to what was
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observed for the mechanical properties.
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Fig. 4 Comparison Hardness (Shore A) for Composites A, B, C, D in normal and aging conditions
3.5 Dynamic Mechanical Thermal Analysis (DMTA)
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The variation of storage modulus (E´) and loss factor (Tan Delta) with the temperature of vulcanization and vulcanization system were studied in the temperature interval from −80ºC to +80ºC. The loss factor Tan Delta is defined
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as Tan Delta = E"/ E´ where E" is the loss modulus. As shown in Figure 5, in the range −80°C to −40°C there are no considerable changes that occurred in the storage modulus (E´). There is a slight difference of the maximum values among composites B, C and D. What makes an impression is that the maximum of storage modulus for composite A is considerably lower compared to the other three composites. The decrease of storage modulus (E´) with the increasing temperature, in other words, the transition from the glassy to the high elastic state occurs at about −49°C. In the interval from -40 to 80°C, when the vulcanizates are in high elastic state, the increase in storage modulus (E´) can be related to the limited mobility of rubber molecules, being immobilized by the carbon black surface. According to (Zaper and Koenig, 1987), this effect may be used as a measure of the filler reinforcing activity, because the greater reinforcing activity of the filler, the lesser the mobility and the higher
ACCEPTED MANUSCRIPT storage modulus (E´) values are. All the curves presented for (E´) on Figure 5 are similar leading to the conclusion that the reinforcing effect of the filler is the same for all four composites. The loss factor of rubber materials is related to the energy lost due to energy dissipation as heat under an oscillating force. The mechanical loss angle tangent is the ratio between the dynamic loss modulus (E") and the dynamic storage modulus
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(E´) (Tan Delta = E"/ E´). It represents naturally the macromolecules mobility of the chains and polymers phase transitions (Zulkifli et al., 2002; Ramier et al., 2007; J.A., 1988; Gonzalez et al., 2005; Bessri et al., 2010; Bayraktar et al., 2008b) It is accepted that the higher the Tan Delta is the greater the mechanical losses; these losses are related to high energy input required for the motion of the molecular chains of the polymer as the transition is being approached (Zaimova et al., 2011;
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Varghese et al., 2003; Sombatsompop, 1999; Botelho et al., 2007; Botelho and Bayraktar, 2009; Bayraktar et al., 2006; AlHartomy et al., 2011). The position of the Tan Delta peak in the loss factor versus temperature curve can be used to identify
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the Tg of the rubber materials.
It is observed that Tg of the studied composites does not change considerably (within 2-3ºC). The intensity of the peak is also similar. Obviously the vulcanization system and vulcanization temperature does not play role in this particular case. This is not in accordance with our previous result (Zaimova et al., 2012), where only the temperature does not play a role for tan delta and storage modulus. The results for tan delta suggest that despite the difference in crosslink density and the
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different vulcanization mechanism the interactions rubber-rubber and rubber-filler are similar for all the composites.
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Fig. 5 Storage modulus and Tan Delta as a function of the temperature a)Composite A, b)Composite B, c)Composite C, d)Composite D.
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3.6 Microindentation analysis
The microindentation test has become a popular technique due to its simplicity and to the fact that it provides valuable
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information about the morphology and mechanical properties of polymeric materials. Microindentation appears to be suitable tool for micromechanical investigation of polymer blends (Kohl and Singer, 1999). First of all, microindentation differs from classical measurement of hardness, where the impressions are generated and then imaged with a microscope. In a microindentation test load and associated penetration depths are recorded simultaneously during both loading and
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unloading, producing a force-displacement diagram as shown schematically in Figure 6a.
The viscoelastic properties of polymers are markedly dependent on the type of crosslinks and the degree of crosslinking.
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Crosslinking can raise the glass-transition temperature (Tg) of a polymer by introducing constraints on the molecular
b)
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motions of the chain.
Fig. 6 a) Schematic representation of the force-depth curve for microindentation procedure b) Comparison of Indentation
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hardness for Composites A, B, C, D in normal and aging conditions
Low degrees of crosslinking in normal vulcanized rubbers act in a similar way to entanglements and raise Tg only slightly above that of the crosslinked polymer. The effect of crosslinking is the most important and best understood in elastomers. Sulphur crosslinking of NR produces a variety of crosslinking types and crosslink lengths (Singer et al., 2000; Nielsen and Landel, 1993; Kohl et al., 2006; Kohl et al., 2000; Hofmann, 1989; Chapman and Porter, 1988; Brady and Singer, 2000).
ACCEPTED MANUSCRIPT It is well known that polysulfide linkages predominate with conventional sulphur vulcanization system whereas monosulfide and disulfide crosslinks are formed with an efficient vulcanization system, which has a higher accelerator/sulphur ratio. Sulphur also introduces main chain modifications either in the form of pendant groups or as cyclic sulfide linkages that have a large influence on the viscoelastic properties (Singer et al., 2000; Nielsen and Landel, 1993; Ngan et al., 2005;
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Kohl et al., 2006; Kohl et al., 2000; Hofmann, 1989; Fischer-Cripps, 2004; Chapman and Porter, 1988). Networks containing high proportions of polysulfide crosslinks display different mechanical properties from those containing mono-sulfides crosslinks. It is shown in the literature that the increase in the accelerator level only resulted in small changes of the Tg and that the rubbery tensile modulus was dependent on the crosslink density but almost independent
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of the crosslink type (Singer et al., 2000; Nielsen and Landel, 1993; Kohl et al., 2000; Hofmann, 1989; Chapman and Porter, 1988; Brady and Singer, 2000).
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Normal and aged (air and UV) specimens of the elastomer based composites manufactured were tested by micro indentation and their hardness and viscoelastic responses were compared. Figure 6b shows the comparison of indentation hardness for composites A, B, C, D for normal and aged specimens. For composites A and B processed using the CV system, the effect of aging on hardness is apparent. On the other hand for composites C and D processed using the EV system there is no effect of aging on hardness.
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Figure 7 shows microindentation depth at maximum load as a function of time for the composites A, B, C and D respectively under normal and aging conditions. It is revealed that samples with composite C have a higher viscoelastic deformation in normal conditions than the corresponding samples with composites A, B and D due to improved chain mobility (moderately due to subdivision and allow more relaxation after loading.
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In the same way, thermal aging (air) influences very strongly the structure of the composites A and B and interesting results were observed in case of aging effect by UV on the composites of C and D as indicated in the Figure 7 c and d. All of these
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results will be used for modelling the viscoelastic behaviour of the elastomeric composites that will be presented in an upcoming paper.
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Fig. 7 Comparison of Microindentation depth at maximum load as a function of time a)Composite A, b)Composite B,
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c)Composite C, d)Composite D.
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3.7 Damage and fracture surface analysis by means of Scanning Electron Microscopy
Fracture surfaces analysis of tensile specimens observed by means of the Scanning Electron Microscopy (SEM) should give
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more realistic information to understand the damage behavior of these four composites. Because these observations show that the fracture energy and stability are exceedingly reliant on the composition and morphology of these new elastomeric composites.
Fracture surfaces are shown in Figure 8a-c as low and high magnification. In reality, elastomeric composites do not exhibit the same fracture type as metallic materials. At high magnifications, it is always possible to see tearing deformation lines in films and deformed fibrils, occasionally one may see on each specimen. Elliptical cap, steps and striped patterns can also be observed. These microstructures that are located in narrow stressed layers of the fracture surface give obvious information about the initiation peak and propagation and fracture directions. Typically, these entire composites show similar crack initiation zone called “cavitation” very similar to the “fish eye formation”. It means that the failure initiates from a particle
ACCEPTED MANUSCRIPT (Energy-dispersive X-ray spectroscopy-“EDS” analysis gives basically a small metallic and/or metallic oxide such as Zn) and propagates around these particles by forming a circle shape after that, at the final stage, a catastrophic failure occurs very rapidly by showing regular tearing lines at the fracture surface (very similar to the failure bands due to the tearing crossing over heterogeneities).
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Typical tongue formation at the fracture surfaces were observed in entire composites (an example is given here Figure 8b with cartography of the composite C. This phenomenon is due to the behavior of the natural rubber during the deformation. This behavior can be observed very often in entire composites from elastomeric composites. Similar results carried out on the four composites designed here can be found in the literature (Luong et al., 2007; Bayraktar et al., 2008b; Bayraktar et
a) Composite “A”
Composite “B
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al., 2008a).
Composite “C”
Composite “D”
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b) Composite “C”, cartography (left column) and tongue formation at the fracture surface, (right column)
c) Composite “B” after thermal-airflow aging (left col.) and Composite “D” after thermal-airflow aging (right column) Fig. 8 Typical Scanning Electron Microscopy (SEM) fracture surface images, at low and high magnifications (a, b, c)
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4.
CONCLUSION
In the frame of the common research project going on, the processing, mechanical and viscoelastic parameters of new
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elastomeric composites containing mixture of NR-BR vulcanizates with different additives/reinforcements have been studied. The effect of the processing and aging parameters on the mechanical and viscoelastic properties were discussed and results obtained for different composites were compared.
The findings are that CV gives more reliable results for multi-purpose application thanks to its ability to produce more
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homogeneous network and good elasticity (because of the polysulphidic linkages). Effective vulcanization system is suitable for application in areas where good aging resistance is needed and also in applications where low deformations are
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applied. Vulcanization temperature does not play significant role on the material properties. This means that the addition of butadiene rubber in the mixture of natural rubber gives the possibility to increase the vulcanization temperature up to 160⁰C (normally 140-150⁰C for natural rubber) hence to decrease the production time without lowering the quality of the product.
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Hofmann W. (1989) Rubber Technology Handbook, New York: Hanser Publishers. J.A. B. (1988) Natural Rubber Rubber Materials and Their Composites. New York: Elsevier Science Publishers LTD, 69. Kohl JG and Singer IL. (1999) Pull-off behavior of epoxy bonded to silicone duplex coatings. Progress in Organic Coatings 36(1-2): 15-20.
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ACCEPTED MANUSCRIPT Luong R, Isac N and Bayraktar E. (2007) Damage initiation mechanism in rubber sheet composites during the static loading. Archives of Materials Science and Engineering. Morawetz H. (2000) History of rubber research. Rubber Chemistry and Technology 73(3): 405-426. Morrison NJ. (1984) The Reactions of Crosslink Precursors in Natural Rubber. Rubber Chemistry and Technology 57(1):
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86-96. Nasir M and Teh GK. (1988) The effects of various types of crosslinks on the physical properties of natural rubber. European Polymer Journal 24(8): 733-736.
Ngan AHW, Wang HT, Tang B, et al. (2005) Correcting power-law viscoelastic effects in elastic modulus measurement
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using depth-sensing indentation. International Journal of Solids and Structures 42(5-6): 1831-1846.
Nielsen LE and Landel LE. (1993) Mechanical Properties of Polymers and Composites, New York: Marcel Dekker.
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Oliver WC and Pharr GM. (1992) AN IMPROVED TECHNIQUE FOR DETERMINING HARDNESS AND ELASTICMODULUS USING LOAD AND DISPLACEMENT SENSING INDENTATION EXPERIMENTS. Journal of Materials Research 7(6): 1564-1583.
Ramier J, Gauthier C, Chazeau L, et al. (2007) Payne effect in silica-filled styrene-butadiene rubber: Influence of surface treatment. Journal of Polymer Science Part B-Polymer Physics 45(3): 286-298.
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Singer IL, Kohl JG and Patterson M. (2000) Mechanical aspects of silicone coatings for hard foulant control. Biofouling 16(2-4): 301-309.
Smith FB. (1961) Response of Elastomers to High Temperature Cure. Rubber Chemistry and Technology 34(2): 571-587. Sombatsompop N. (1999) Dynamic mechanical properties of SBR and EPDM vulcanisates filled with cryogenically
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pulverized flexible polyurethane foam particles. Journal of Applied Polymer Science 74(5): 1129-1139. Studebaker ML and Beatty JR. (1972) The Oxidative Hardening of SBR. Rubber Chemistry and Technology 45(2): 450-
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466.
Varghese S, Karger-Kocsis J and Gatos KG. (2003) Melt compounded epoxidized natural rubber/layered silicate nanocomposites: structure-properties relationships. Polymer 44(14): 3977-3983. Zaimova D, Bayraktar E and Dishovsky N. (2011) State of cure evaluation by different experimental methods in thick rubber parts. Journal of Achievements in Materials and Manufacturing Engineering 44(2): 161-167. Zaimova D, Katundi D, Bayraktar E, et al. (2012) Elastomeric matrix composites: effect of processing conditions on the physical, mechanical and viscoelastic properties. Journal of Achievements in Materials and Manufacturing Engineering 50 (2): 81-91.
ACCEPTED MANUSCRIPT zaper am and koenig jl. (1987) solid-state c-13 nmr-studies of vulcanized elastomers .3. accelerated sulfur vulcanization of natural-rubber. rubber chemistry and technology 60(2): 278-297. Zulkifli R, Fatt LK, Azhari CH, et al. (2002) Interlaminar fracture properties of fibre reinforced natural
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rubber/polypropylene composites. Journal of Materials Processing Technology 128(1-3): 33-37.
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S1359-8368(16)30854-X
DOI:
10.1016/j.compositesb.2016.05.061
Reference:
JCOMB 4344
To appear in:
Composites Part B
Received Date: 29 July 2015 Accepted Date: 30 May 2016
Please cite this article as: Zaimova D, Bayraktar E, Miskioglu I, Design and manufacturing of new elastomeric composites: Mechanical properties, chemical and physical analysis, Composites Part B (2016), doi: 10.1016/j.compositesb.2016.05.061. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Design and manufacturing of new elastomeric composites: mechanical properties, chemical and physical analysis
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D. Zaimova1, 2, E. Bayraktar2*, I. Miskioglu3
UCTM, University of Chemical Technology and Metallurgy, Sofia, Bulgaria
Supmeca-Paris, School of Mechanical and Manufacturing Engineering, France Michigan Technological University ME-EM Department, Houghton, MI-USA
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*Corresponding author: [email protected];
Abstract
Filler-reinforced vulcanized rubber and its blends have been used for engineering applications for over a century. Traditional applications include tires, seals, bushings, and engine mounts. The rubbers for tire manufacturing
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must have high elasticity, good frictional properties and high load bearing capacity. Conforming to these needs, rubbers are vulcanized by various materials such as sulphur, carbon black, accelerators, and retardants under different conditions. The reactivity of sulphur vulcanization and physical properties of the final product are
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affected by the chemical structure, molecular weight, and conformation of the base elastomers. The aim of this study is to investigate the influence of accelerator-vulcanizing agent system and the
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vulcanization temperature on the mechanical and aging properties of vulcanizates based on natural rubber/Polybutadiene rubber (NR/BR) composites. Two ratios of accelerator to sulphur were used for the preparation of the composites also known as conventional vulcanization system (CV), where the quantity of sulphur is more and efficient vulcanization system (EV), where the quantity of accelerator is more.This preliminary study will allow optimization of the composition/processing conditions to improve the mechanical properties and resistance to damage. NR/BR based composites at different vulcanization temperatures and using different curing systems were manufactured. Tensile strength, elongation at break, tensile modulus at 100% (M100) and at 300% (M300)
ACCEPTED MANUSCRIPT deformation hardness (Shore A) of the composites were determined. Scanning electron microscopy was used to study the microstructure of the fracture surfaces. The main findings are that CV gives more reliable results for multi-purpose applications. Effective vulcanization system is suitable for application in areas where good aging resistance is needed and also in applications where low deformations are expected. Vulcanization temperature
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does not play significant role although most of the results are in favour of the lower temperature (140oC).
INTRODUCTION
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Keywords: Elastomer composites; mechanical properties; chemical and physical analysis; SEM study
Vulcanized rubber and its blends reinforced with fillers have been traditionally used in tires, seals, bushings, and engine mounts (Morawetz, 2000). Depending on the application, the rubbers must have high elasticity, good frictional properties, good wear characteristics as well as high load bearing capacity (Smith, 1961). The sulphur vulcanization with unsaturated rubbers occurs through complicated radical substitution in the form of mono-, di-, or polysulphide bridges and sulphur
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containing intracyclization of the polymer molecules. The cross-link density and distribution affect the physical properties and stability of rubber on aging. They depend not only on type and ratio of accelerator to sulphur, but also on the reaction temperature and duration (Nasir and Teh, 1988).
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The rate of vulcanization of rubber is closely related to the efficient manufacturing of elastomer parts. Hence, it is desirable to increase the vulcanization rate by raising temperature. However, at higher temperatures the sulphur cross-links
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are less effective and the physical properties are sacrificed due to dissociation of sulphur bonds and rubber chains (Bhowmick et al., 1979). Instead of increasing temperature, the vulcanization reactivity may be controlled by additional amounts of sulphur and accelerator. Although the addition of sulphur increases cross-link density and the amount of polysulphide linkage, it may decrease the stability of the cross-links, especially upon aging (Studebaker and Beatty, 1972). By increasing the amount of accelerators, the effectiveness of sulphur vulcanization with mono- and disulphide linkages is improved which in turn results in more stable characteristics upon aging at the cost of degrading the dynamic properties of the product (Bayraktar et al., 2008a). The optimum conditions must be chosen to obtain the best properties for products and productivity. Hence the goal of this study is to investigate the influence of accelerator-vulcanizing agent system and the
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ACCEPTED MANUSCRIPT vulcanization temperature on the mechanical and aging properties of vulcanizates based on Natural rubber/Polybutadiene rubber (NR/BR) composites. For this study two NR/BR based composites processed with two different curing systems (conventional and efficient) and vulcanized at two different temperatures (140ºC and 160ºC) were manufactured. The curing characteristics and the
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mechanical properties of the manufactured composites were investigated. The curing characteristics of the rubber composites were studied by using the Monsanto MDR 2000 rheometer. The investigated mechanical properties were tensile strength, elongation at break, tensile modulus at 100% (M100) and at 300% (M300) deformation. Shore A hardness and molecular mass of the samples were also determined. Scanning electron microscopy was used to study the microstructure of
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the fracture surfaces of the tensile samples.
In this paper partial results of the applied project supported by French-Bulgarian research cooperation are presented.
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The exact composition of the blends used in this study is proprietary, hence will not be discussed in this paper.
EXPERIMENTAL PROCEDURE
The two composites were mixed in a laboratory scale two roll mixer. The moulding conditions were determined from the
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torque data from a moving die rheometer MDR 2000 (Alpha Technologies) for temperatures 140°C and 160°C. At the end, four composites (A, B, C, D) based on sulphur vulcanized Natural rubber/Polybutadiene rubber blends were prepared containing certain fillers and/or reinforcements in order to investigate their deformation behaviour. Two ratios of accelerator
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to sulphur were used for the preparation of the composites also known as conventional vulcanization system (CV), where the quantity of sulphur is more and efficient vulcanization system (EV), where the quantity of accelerator is more. Composites A and B were vulcanized using conventional vulcanization system (CV) and at vulcanization temperatures
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140ºC and 160ºC respectively. Composites C and D were vulcanized using the efficient vulcanization system (EV) respectively at vulcanization temperatures 140ºC and 160ºC. Cured sheets were prepared by compression moulding at 140°C and 160°C and at a pressure of 100 kg/cm2. The behavior of these composites in harsh environmental conditions was tested by means of swelling experiments. The swelling tests were carried out by soaking the samples in toluene at room temperature. At the conclusion of these tests, the crosslink densities for each composite were calculated according to the molecular mass between two crosslinks. All of the crosslink densities were calculated according to the Flory-Rehner relation (Luong et al., 2007):
ACCEPTED MANUSCRIPT ଵ ሺଵିೝ ሻାೝ ାఓೝమ ቈ భ/య భ ଶఘబ ೝ ି ೝ
=
ߥ=
ଶெ
మ
ଵ
(2)
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where; Mc- molecular mass between crosslinks; ρ- Density of the rubber; V0-molar volume of the solvent; Vr - volume fraction of the swollen rubber; µ- interaction parameter between the rubber sample and the solvent;
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ν- Crosslink density.
(1)
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ଵ ଶெ
The nominal thickness of the sheets was set at 2 mm. The mechanical properties were determined before and after aging in accordance with ASTM D 412aε2 (2010) (ASTM: and 412aε2, 2010). The aging test was performed in a batch oven with forced convection at 70ºC for 21 days. Second type of aging was performed in an ultraviolet oven for 21 days. Dumbbell
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specimens for tensile testing were cut using standard die C from the moulded sheets. The sample length and thickness were measured. Tensile tests were performed using an Instron (model 4507). A minimum of 3 specimens were tested for each composite. Testing was done at room temperature with a crosshead speed of 500 mm/min. Shore A hardness of the
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specimens was measured according to ASTM D 2240-05 (2010) (ASTM: and 2240-05, 2010). Dynamic Mechanical Thermal Analysis (DMTA) was performed with the Dynamic Mechanical Analyzer MK III system (Rheometric Scientific). As a mode of deformation, single cantilever beam in bending was used at a frequency of 5 Hz, with a deformation of 64 µm.
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The tests were conducted in the temperature range from -80 to 80°C at a heating rate of 2°C/min. CSM Micro Indentation Tester with a Vickers diamond indenter was used to measure indentation hardness. Nine tests were performed for each composite and then averaged. The samples’ thickness were approximately 2mm. The maximum indentation load (Fmax) was 250 mN, the rate of loading - unloading was 500 mN/min. The load was held at Fmax for 50s. The indentation hardness (HIT) was determined by Oliver and Pharr method (Oliver and Pharr, 1992).
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RESULTS AND DISCUSSION
3.1
Vulcanization characteristics
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The rheometer, a convenient instrument to evaluate the effects of carbon black-rubber interactions on rate of cure and crosslinking, was employed for the purpose of characterizing critical parameters related to the vulcanization process: minimum torque (ML), maximum torque (MH), the difference ∆M between maximum (MH) and minimum (ML) torque, optimal
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vulcanization time (T90), the scorch time (Ts2) and rate of vulcanization (V).
Table I. Vulcanization characteristics of four composites studied in this research project Blend №
ML, dNm MH, dNm ∆M= ( MH – ML )
C
D
2.9
2.7
2.7
2.5
36.5
33.9
32.3
31.7
33.5
31.3
29.5
29.1
17:22
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17:24
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T90, min:s
B
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Characteristic
A
6:18
1:23
9:34
2:41
V, %min
9
34
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Ts2, min:s
Fig. 1 Crosslink density determined by swelling
ACCEPTED MANUSCRIPT The minimum torque (ML) in a rheograph is related to the viscosity of the vulcanizates, and the maximum torque (MH) is generally correlated to the stiffness and crosslink density. It is well known that the difference ∆M between maximum (MH) and minimum (ML) torque is a rough measure of the crosslink density of the samples. Table I shows The vulcanization characteristics of the four composites used in this study.
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As can be seen in Table I the highest value of ∆M is for composite A. Hence the crosslink density of composite A is larger than the other three composites. This is also confirmed by the results for the crosslink density obtained by the swelling tests (Figure 1).The carbon black particles whose surfaces are covered by entangled rubber chains can be considered as physical crosslinks. The physical crosslinking hinders the mobility of rubber chains and restrains the deformation of rubber therefore
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leading to higher torque measurements. In a swelling test this type of crosslinks cannot be detected, but the high maximum torque for composite A can be considered as an indication of high number of physical crosslinks for this composite.
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The scorch time (Ts2) is the time required for the minimum torque value to increase by two units and is a measure of premature vulcanization of the material. It should be noted that that the composites with the CV system exhibited shorter Ts2 values than the ones with the EV system, therefore more crosslinks are formed in the CV systems in shorter time. These results are expected because the greater quantity of the accelerator (in composites C and D) should provide higher values for Ts2. It is possible this result to be due to the nature of sulphenamide class accelerators (delayed action in the
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beginning of vulcanization) and on the other hand the higher content of sulfur in composites “A” and “B”. The overall rate (V) of vulcanization is slightly in favor of the composite cured with EV.
3.2
Mechanical properties before aging
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Tensile properties are commonly used to measure the degradation behavior of elastomers. Figure 2 shows the typical stressstrain curves and Figure 3 shows the comparison of modulus at 100% and 300% deformation, tensile strength and
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elongation at break for the composites A, B, C and D. Results for modulus at 300% deformation are not available for composites A, B and C. These specimens failed before reaching 300% deformation. As depicted in Figure 3 (a and b) all composites show similar values of modulus at 100% (M100) and 300% (M300) deformation. Obviously the temperature of vulcanization does not have a significant effect on M100 and M300.
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Fig. 2 Stress-strain curve for a) Composite A, b)Composite B, c)Composite C, d)Composite D
On the other hand the tendency is not the same for tensile strength (Figure 3c) and the elongation at break (Figure 3d). The composites vulcanized by CV yielded higher tensile strength and higher elongation at break under non-aged and aged conditions compared with the composites vulcanized by EV. All composites have the ability to exhibit crystallization under
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stress thanks to NR but according to literature the superior number of di- and poly- sulphidic linkages (result from CV) improves the elasticity of the composite which explains the higher values of elongation at break. In addition CV gives more homogeneous structure which plays positive role for the strength of vulcanizates. Once more the temperature of
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vulcanization does not play significant role for the tensile strength and elongation at break.
Mechanical properties after aging
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The mechanical properties of the composites after aging are also investigated. On Figures 2 the stress-strain curves before and after 21 days of aging (in air oven and under UV light) are compared. It can be seen that UV light does not affect significantly the mechanical properties of the studied composites. The same relation is observed for M100, M300, tensile strength and elongation at break.
These results can be explained with the ability of rubber composite to form additional crosslinks under the influence of temperature and UV light. Although the overcrosslinking makes the vulcanizate brittle and reduces its elasticity apparently the period of exposure was not sufficient to create overcrosslinked vulcanizates. Probably the number of crosslinks created during UV aging compensates the number of the chains destroyed by aging. The thermal aging however (21days in air oven
ACCEPTED MANUSCRIPT at 70ºC) causes more severe degradation of the strength and elongation at break values of the studied composites. Under low level of deformation (100%) thermal aging has positive effect for the same reason as explained before-under the influence of the temperature additional crosslinks are formed which increases the modulus at this level of deformation . At this stage, crosslink scissions and the formation of new crosslinks into the networks affected these properties. But under
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high stress and deformation the destruction of the crosslinks is much more significant and leads to failure of the specimen. The results shown in Figure 3 also reveal that the CV and EV vulcanization systems used in the processing of the four composites did not affect the mechanical properties consaidered. The CV system vulcanizates (A and B) has a high content of polysulphide linkages with low bond strength. By contrast, monosulphide linkages have relatively strong bond strength
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and are dominant in the EV system vulcanizates (Morrison, 1984). As a result, the CV system exhibits a weaker thermal aging resistance than polysulphide crosslinks. For all composites the extended period of heating, the excessive main chain
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scission and/or modification resulted in a reduced tendency to crystallize at high elongation. Again, it is observed that the
b)
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effect of the temperature of vulcanization is not significant on the mechanical response of the four compunds after aging.
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Fig. 3 Comparison of a) M100, b) M300, c) Tensile strength, d) Elongation at break for Composites A, B, C, D in normal and aging conditions
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3.4 Hardness-Shore ”A” test evaluation The hardness (Shore A) results are presented in Figure 4. The hardness is relatively high for NR composite but this is probably due to the presence of BR in the mixture. The results do not show any dependence on the vulcanization system or
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vulcanization temperature. For most of the composites UV and thermal aging leads to some reduction in the hardness values. However, there is almost no difference between the effect of UV aging and thermal aging similar to what was
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observed for the mechanical properties.
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Fig. 4 Comparison Hardness (Shore A) for Composites A, B, C, D in normal and aging conditions
3.5 Dynamic Mechanical Thermal Analysis (DMTA)
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The variation of storage modulus (E´) and loss factor (Tan Delta) with the temperature of vulcanization and vulcanization system were studied in the temperature interval from −80ºC to +80ºC. The loss factor Tan Delta is defined
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as Tan Delta = E"/ E´ where E" is the loss modulus. As shown in Figure 5, in the range −80°C to −40°C there are no considerable changes that occurred in the storage modulus (E´). There is a slight difference of the maximum values among composites B, C and D. What makes an impression is that the maximum of storage modulus for composite A is considerably lower compared to the other three composites. The decrease of storage modulus (E´) with the increasing temperature, in other words, the transition from the glassy to the high elastic state occurs at about −49°C. In the interval from -40 to 80°C, when the vulcanizates are in high elastic state, the increase in storage modulus (E´) can be related to the limited mobility of rubber molecules, being immobilized by the carbon black surface. According to (Zaper and Koenig, 1987), this effect may be used as a measure of the filler reinforcing activity, because the greater reinforcing activity of the filler, the lesser the mobility and the higher
ACCEPTED MANUSCRIPT storage modulus (E´) values are. All the curves presented for (E´) on Figure 5 are similar leading to the conclusion that the reinforcing effect of the filler is the same for all four composites. The loss factor of rubber materials is related to the energy lost due to energy dissipation as heat under an oscillating force. The mechanical loss angle tangent is the ratio between the dynamic loss modulus (E") and the dynamic storage modulus
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(E´) (Tan Delta = E"/ E´). It represents naturally the macromolecules mobility of the chains and polymers phase transitions (Zulkifli et al., 2002; Ramier et al., 2007; J.A., 1988; Gonzalez et al., 2005; Bessri et al., 2010; Bayraktar et al., 2008b) It is accepted that the higher the Tan Delta is the greater the mechanical losses; these losses are related to high energy input required for the motion of the molecular chains of the polymer as the transition is being approached (Zaimova et al., 2011;
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Varghese et al., 2003; Sombatsompop, 1999; Botelho et al., 2007; Botelho and Bayraktar, 2009; Bayraktar et al., 2006; AlHartomy et al., 2011). The position of the Tan Delta peak in the loss factor versus temperature curve can be used to identify
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the Tg of the rubber materials.
It is observed that Tg of the studied composites does not change considerably (within 2-3ºC). The intensity of the peak is also similar. Obviously the vulcanization system and vulcanization temperature does not play role in this particular case. This is not in accordance with our previous result (Zaimova et al., 2012), where only the temperature does not play a role for tan delta and storage modulus. The results for tan delta suggest that despite the difference in crosslink density and the
a)
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different vulcanization mechanism the interactions rubber-rubber and rubber-filler are similar for all the composites.
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Fig. 5 Storage modulus and Tan Delta as a function of the temperature a)Composite A, b)Composite B, c)Composite C, d)Composite D.
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3.6 Microindentation analysis
The microindentation test has become a popular technique due to its simplicity and to the fact that it provides valuable
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information about the morphology and mechanical properties of polymeric materials. Microindentation appears to be suitable tool for micromechanical investigation of polymer blends (Kohl and Singer, 1999). First of all, microindentation differs from classical measurement of hardness, where the impressions are generated and then imaged with a microscope. In a microindentation test load and associated penetration depths are recorded simultaneously during both loading and
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unloading, producing a force-displacement diagram as shown schematically in Figure 6a.
The viscoelastic properties of polymers are markedly dependent on the type of crosslinks and the degree of crosslinking.
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Crosslinking can raise the glass-transition temperature (Tg) of a polymer by introducing constraints on the molecular
b)
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motions of the chain.
Fig. 6 a) Schematic representation of the force-depth curve for microindentation procedure b) Comparison of Indentation
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hardness for Composites A, B, C, D in normal and aging conditions
Low degrees of crosslinking in normal vulcanized rubbers act in a similar way to entanglements and raise Tg only slightly above that of the crosslinked polymer. The effect of crosslinking is the most important and best understood in elastomers. Sulphur crosslinking of NR produces a variety of crosslinking types and crosslink lengths (Singer et al., 2000; Nielsen and Landel, 1993; Kohl et al., 2006; Kohl et al., 2000; Hofmann, 1989; Chapman and Porter, 1988; Brady and Singer, 2000).
ACCEPTED MANUSCRIPT It is well known that polysulfide linkages predominate with conventional sulphur vulcanization system whereas monosulfide and disulfide crosslinks are formed with an efficient vulcanization system, which has a higher accelerator/sulphur ratio. Sulphur also introduces main chain modifications either in the form of pendant groups or as cyclic sulfide linkages that have a large influence on the viscoelastic properties (Singer et al., 2000; Nielsen and Landel, 1993; Ngan et al., 2005;
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Kohl et al., 2006; Kohl et al., 2000; Hofmann, 1989; Fischer-Cripps, 2004; Chapman and Porter, 1988). Networks containing high proportions of polysulfide crosslinks display different mechanical properties from those containing mono-sulfides crosslinks. It is shown in the literature that the increase in the accelerator level only resulted in small changes of the Tg and that the rubbery tensile modulus was dependent on the crosslink density but almost independent
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of the crosslink type (Singer et al., 2000; Nielsen and Landel, 1993; Kohl et al., 2000; Hofmann, 1989; Chapman and Porter, 1988; Brady and Singer, 2000).
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Normal and aged (air and UV) specimens of the elastomer based composites manufactured were tested by micro indentation and their hardness and viscoelastic responses were compared. Figure 6b shows the comparison of indentation hardness for composites A, B, C, D for normal and aged specimens. For composites A and B processed using the CV system, the effect of aging on hardness is apparent. On the other hand for composites C and D processed using the EV system there is no effect of aging on hardness.
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Figure 7 shows microindentation depth at maximum load as a function of time for the composites A, B, C and D respectively under normal and aging conditions. It is revealed that samples with composite C have a higher viscoelastic deformation in normal conditions than the corresponding samples with composites A, B and D due to improved chain mobility (moderately due to subdivision and allow more relaxation after loading.
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In the same way, thermal aging (air) influences very strongly the structure of the composites A and B and interesting results were observed in case of aging effect by UV on the composites of C and D as indicated in the Figure 7 c and d. All of these
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results will be used for modelling the viscoelastic behaviour of the elastomeric composites that will be presented in an upcoming paper.
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Fig. 7 Comparison of Microindentation depth at maximum load as a function of time a)Composite A, b)Composite B,
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c)Composite C, d)Composite D.
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3.7 Damage and fracture surface analysis by means of Scanning Electron Microscopy
Fracture surfaces analysis of tensile specimens observed by means of the Scanning Electron Microscopy (SEM) should give
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more realistic information to understand the damage behavior of these four composites. Because these observations show that the fracture energy and stability are exceedingly reliant on the composition and morphology of these new elastomeric composites.
Fracture surfaces are shown in Figure 8a-c as low and high magnification. In reality, elastomeric composites do not exhibit the same fracture type as metallic materials. At high magnifications, it is always possible to see tearing deformation lines in films and deformed fibrils, occasionally one may see on each specimen. Elliptical cap, steps and striped patterns can also be observed. These microstructures that are located in narrow stressed layers of the fracture surface give obvious information about the initiation peak and propagation and fracture directions. Typically, these entire composites show similar crack initiation zone called “cavitation” very similar to the “fish eye formation”. It means that the failure initiates from a particle
ACCEPTED MANUSCRIPT (Energy-dispersive X-ray spectroscopy-“EDS” analysis gives basically a small metallic and/or metallic oxide such as Zn) and propagates around these particles by forming a circle shape after that, at the final stage, a catastrophic failure occurs very rapidly by showing regular tearing lines at the fracture surface (very similar to the failure bands due to the tearing crossing over heterogeneities).
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Typical tongue formation at the fracture surfaces were observed in entire composites (an example is given here Figure 8b with cartography of the composite C. This phenomenon is due to the behavior of the natural rubber during the deformation. This behavior can be observed very often in entire composites from elastomeric composites. Similar results carried out on the four composites designed here can be found in the literature (Luong et al., 2007; Bayraktar et al., 2008b; Bayraktar et
a) Composite “A”
Composite “B
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al., 2008a).
Composite “C”
Composite “D”
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b) Composite “C”, cartography (left column) and tongue formation at the fracture surface, (right column)
c) Composite “B” after thermal-airflow aging (left col.) and Composite “D” after thermal-airflow aging (right column) Fig. 8 Typical Scanning Electron Microscopy (SEM) fracture surface images, at low and high magnifications (a, b, c)
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4.
CONCLUSION
In the frame of the common research project going on, the processing, mechanical and viscoelastic parameters of new
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elastomeric composites containing mixture of NR-BR vulcanizates with different additives/reinforcements have been studied. The effect of the processing and aging parameters on the mechanical and viscoelastic properties were discussed and results obtained for different composites were compared.
The findings are that CV gives more reliable results for multi-purpose application thanks to its ability to produce more
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homogeneous network and good elasticity (because of the polysulphidic linkages). Effective vulcanization system is suitable for application in areas where good aging resistance is needed and also in applications where low deformations are
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applied. Vulcanization temperature does not play significant role on the material properties. This means that the addition of butadiene rubber in the mixture of natural rubber gives the possibility to increase the vulcanization temperature up to 160⁰C (normally 140-150⁰C for natural rubber) hence to decrease the production time without lowering the quality of the product.
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