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High performance hybrid reinforcement of nitrile rubber using short pineapple leaf fiber and carbon black
Polymer Testing 45 (2015) 76e82
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Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material properties
High performance hybrid reinforcement of nitrile rubber using short pineapple leaf fiber and carbon black* Kontapond Prukkaewkanjana a, Sombat Thanawan b, Taweechai Amornsakchai a, c, d, * a
Polymer Science and Technology Program, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand b Rubber Technology Research Center, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand c Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand d Center of Sustainable Energy and Green Materials, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 April 2015 Accepted 12 May 2015 Available online 29 May 2015
Rubber composites with very high moduli at low elongation, high elongation at break and high ultimate breaking strength have been developed. The matrix was acrylonitrile butadiene rubber (NBR) and the hybrid (fibrous and particulate) reinforcements were short, fine pineapple leaf fiber (PALF) and carbon black. The amount of PALF was fixed at 10 parts (by weight) per hundred of rubber (phr) while that of carbon black was varied from 0 to 30 phr. Uniaxial NBR composites were prepared. Tensile strength, elongation at break, modulus and tear strength of the hybrid composites were characterized in both longitudinal (parallel to the fiber axis) and transverse (perpendicular to the fiber axis) directions. The addition of carbon black causes the slope of the early part of the stressestrain curve to increase and also extends breaking to greater strains. At carbon black contents of 20 phr and above, the stressestrain relation displays an upturn at high elongations, providing greater ultimate strength. Comparison with the usual carbon black filled rubber shows that the composite behavior at low strains is determined by the PALF, and at high strains by the carbon black. This high performance PALF-carbon black reinforced NBR shows great promise for engineering applications. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Pineapple leaf fiber Carbon black Nitrile rubber Rubber composite Hybrid composite
1. Introduction Short fiber reinforced rubber composites have unique mechanical properties which combine fiber rigidity and rubber elasticity [1]. According to models of short fiber reinforced composites, the mechanical properties of uniaxially aligned composite are determined mainly by the fiber modulus, the fiber volume fraction and the fiber aspect ratio. Usually, these parameters are varied in such a way as to obtain composites with the required properties. To a much lesser degree, the composite properties
*
Part of this work has been presented at ICNF-2015. * Corresponding author. Polymer Science and Technology Program, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand. E-mail address: [email protected] (T. Amornsakchai). http://dx.doi.org/10.1016/j.polymertesting.2015.05.004 0142-9418/© 2015 Elsevier Ltd. All rights reserved.
depend also on the matrix modulus. For rubber matrix composites, different types of particulate filler are commonly used to adjust the composite properties [2]. Certain fillers increase, not only the modulus, but also the strength, of the rubber composites. These are known as reinforcing fillers and a good example of this is carbon black. Previous research has shown that short, fine pineapple leaf fiber can be used successfully to reinforce nitrile rubber. However, the composites fail at relatively low strain [3]. Such a problem is common for rubbers filled with short fibers [4e7]. We have recently shown that, by adding silica to form a hybrid filler in short PALF reinforced nitrile rubber, the elongation at break is increased remarkably [8]. This research is aimed at examining this concept again by using carbon black as the hybrid filler. Carbon black, of course, is the best known filler for rubber reinforcement. Unlike silica, it can easily be used without the need of a coupling agent. The PALF content will be fixed at 10 phr and the content of carbon
K. Prukkaewkanjana et al. / Polymer Testing 45 (2015) 76e82
black varied at 10, 20 and 30 phr. Comparison will be made with the control system containing only carbon black at 10, 20, 30 and 40 phr.
Table 2 Formulations of rubber compounds. CB and F denote carbon black and PALF, respectively. The number following letter CB and F in a compound name indicates the amount of the respective materials in phr. Compound/Ingredient
2. Experimental
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Amount (phr) NBR
CB
PALF
ZnO
Stearic acid
CBSa
Sulfur
100 100 100 100 100 100 100 100 100
e 10 20 30 40 e 10 20 30
e e e e e 10 10 10 10
5 5 5 5 5 5 5 5 5
2 2 2 2 2 2 2 2 2
1 1 1 1 1 1 1 1 1
2 2 2 2 2 2 2 2 2
2.1. Materials 2.1.1. PALF The milling technique developed in our laboratory [9] was used for separating PALF from fresh pineapple leaves. These were collected from the Kok Kwai District, Amphor Ban Rai, Uthai Thani Province, Thailand. The leaves were cut into 6 mm lengths before being fed into a disc milling machine. The cut leaves were crushed into paste. This paste was then dried, ground, and sieved in order to separate PALF from non-fibrous materials. The general characteristics of PALF have been reported elsewhere [9]. 2.1.2. Rubbers NBR was JSR N230SL grade with 35% acrylonitrile content produced by JSR Corporation, Japan. It has a Mooney Viscosity (ML1þ4(100 C)) of 42. 2.1.3. Rubber chemicals All rubber ingredients including ZnO, stearic acid, sulfur, and CBS were commercial grade. 2.2. Composite preparation Compounds were mixed on a laboratory two-roll mill to produce mixtures with fibers aligned in the machine direction, as described previously [3,8]. The sequence of ingredient addition is shown in Table 1. The formulations of the rubber compound are shown in Table 2. The amount of PALF was fixed at 10 phr and that of carbon black (CB) varied from 0 to 30 phr. The abbreviations CB and F denote carbon black and PALF respectively. The numbers following letters CB and F in a compound name indicate the amount of the respective material in phr. The rubber composites were prepared by vulcanizing the rubber compound while compression molding in a hydraulic press. This was done under a pressure of 1500 psi and temperature of 150 C with a 1 mm thick mold.
Unfilled CB10 CB20 CB30 CB40 F10 CB10F10 CB20F10 CB30F10 a
CBS ¼ N-Cyclohexyl-2-benzothiazolesulfenamide.
tear tests, respectively. Five specimens were measured for each formulation and average values determined for the different properties. Hardness measurements were made according to ISO 7619 using a Shore A durometer (Wallace, H17A). An average value from six positions was recorded for each specimen. Swelling percentage weight gains were studied using rectangular specimens cut from vulcanized sheet. The rectangular specimens were soaked in toluene at room temperature for 72 h and the weight gain determined. The shape and size of PALF and fractured surfaces of the composites were observed with a scanning electron microscope (SEM) (Hitachi Tabletop Microscope; model TM 1000, Japan). 3. Results 3.1. Cure characteristics Fig. 1 displays curing characteristics of the NBR compounds filled with PALF/carbon black hybrid and carbon black only. Numerical values for different cure characteristics of the compounds are tabulated in Table 3. Considering the curing behavior of compounds with hybrid fillers (Fig. 1, open symbols), the compounds containing greater amounts of carbon black have shorter scorch times. When these are compared with those of compounds containing carbon black only
2.3. Characterization
Table 1 Sequence of ingredient addition in rubber compound preparation. Order
Ingredient
Mixing time (min)
1 2 3 4 5
NBR Sulfur PALF and/or CB ZnO with stearic acid CBS Total time
0e1 1e2 2e11 11e17 17e20 20
25
unfilled CB10 CB20 CB30 CB30
20
Torque (dNm)
Minimum torque (ML), maximum torque (MH), scorch time (ts2) curing time (tc90) of non-vulcanized rubbers were characterized using a moving die rheometer (MDR), (Rheo TECH MDþ, Alpha Technologies, Akron, USA) at 150 C (ISO 6502). Tensile properties and tear strengths of rubber composites were measured on a universal testing machine (Instron 5566, High Wycombe, UK) with a long travel contact-style extensometer at room temperature, according to ISO 37 and ISO 34, respectively. Type C and type B dies were used to cut the sample for tensile and
F10 CB10F10 CB20F10 CB30F10
15 10 5 0
0
5
10
15
Time (minute) Fig. 1. Cure curves of PALF/CB-NBR (open symbols) and controlled CB-NBR (closed symbols) composites. The number in a legend indicates the amount in phr of the filler and letter F means PALF which is fixed at 10 phr.
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Table 3 Cure characteristics of different compounds. CB and F denote carbon black and PALF, respectively. The number following letter CB and F in a compound name indicates the amount of the respective materials in phr. Compound
ML (dN.m)
MH (dN.m)
ts2 (min)
tc90 (min)
Unfilled CB10 CB20 CB30 CB40 F10 CB10F10 CB20F10 CB30F10
0.27 0.37 0.47 0.64 0.87 0.47 0.55 0.70 0.84
6.94 9.10 11.07 13.75 15.90 9.91 12.18 14.67 16.69
4.51 4.17 3.30 3.28 2.32 3.27 3.19 2.48 2.10
9.59 10.09 10.41 9.37 8.07 7.06 6.04 6.19 5.41
(Fig. 1, closed symbols), it appears that the scorch time depends on the amount of carbon black but not on the presence of PALF. Addition of (10 phr) PALF to NBR (F10) increases the maximum torque. When 10 phr of carbon black is further added to the system (CB10F10), the maximum torque increases again. With 20 and 30 phr carbon black (CB20F10 and CB30F10), the maximum torque increases steadily. For systems containing only carbon black, addition of 10, 20, 30 and 40 phr of carbon black causes the maximum torque to increase in a similar manner. For compounds containing the same amount of total filler, those with hybrid filler have slightly greater maximum torque. 3.2. Tensile properties Fig. 2 displays longitudinal stressestrain curves of PALF/carbon black-NBR and carbon black-NBR composite vulcanizates containing different amounts of carbon black. The PALF-NBR composite with 10 phr PALF (F10) exhibits a sharp rise in stress, reaches a peak value at about 30% strain and then decreases and eventually fails at about 50% strain. With the addition of 10 phr of carbon black (CB10F10), the stress values increase significantly, reaching a peak value before dropping slightly and extending to a strain greater than 100%. When the carbon black content is increased to 20 phr (CB20F10), the stressestrain curve
rises with a similar slope to that of 10 phr carbon black. However, when the stress reaches a peak value it does not drop as much and extends with relatively constant stress to about 300% strain. At this point it displays an upturn in stress. With further increase in carbon black content (CB30F10), the stressestrain curve rises with even greater slope and reaches a higher peak stress. Again, after the peak the stress gradually decreases with increasing strain and then levels off. This level stress is greater than that with 20 phr carbon black. With further extension, an upturn in stress occurs at approximately 250% strain. The failure stress for this CB30F10 hybrid composite is greater (although at lower strain) than that of CB20F10. The increases in the initial slope of the stressestrain curve and in the elongation at break with increasing carbon black are similar to those observed in PALF/silica-NBR hybrid composites [8]. There are, however, two significant points of difference. The first is that the levelling stress in this PALF/carbon black-NBR hybrid system is lower than that in PALF/silica-NBR hybrid system. The second point is that PALF/carbon black-NBR hybrid systems display a clear upturn in stress at high carbon black content. Conversely, at high silica contents, PALF/silica-NBR hybrid systems do not. In order to understand the behavior of the hybrid composites, comparison can be made with the stressestrain curves of the control carbon black-NBR composites. Since carbon black is a particulate filler, no directional property is observed. NBR gum displays the lowest stressestrain curve with an elongation at break of almost 500%. When 10 phr carbon black is added, the stress increases over the whole range and display an upturn at high strain. There is no maximum peak and the failure stress increases significantly. The failure strain remains roughly the same. With further addition of carbon black to 20 phr, the stress again increases over the whole range. The upturn seems to occur at lower strain and the failure strain (elongation at break) deceases. An important point to note is that this high strain part of the stressestrain curve is virtually superimposable on that of the CB20F10 hybrid composite. With an increase in carbon black content to 30 phr, a similar situation is seen, i.e. the high strain part of the stressestrain curve is almost superimposable on that of the CB30F10 hybrid composite.
Fig. 2. Stressestrain curves of PALF/CB-NBR and controlled CB-NBR composites, in the longitudinal direction (a) and the expanded low strain region (b). The number in a legend indicates the amount in phr of the filler and letter F means PALF which is fixed at 10 phr.
K. Prukkaewkanjana et al. / Polymer Testing 45 (2015) 76e82
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However, in this case, the failure stress of the CB30F10 hybrid composite is greater than that of composite containing carbon black only. The stressestrain behavior of PALF/carbon black-NBR composites in the transverse direction (Fig. 3) also displays a dependency on the amount of carbon black. Stress in the low strain region appears to increase with increasing carbon black content. In the high strain region, however, the effect of carbon black is much greater. The stress does display an upturn at high strains, but to a lesser degree than seen in the longitudinal direction. The elongation at break decreases slightly with increasing carbon black content. From the stressestrain curves shown in Figs. 2 and 3, moduli at 10% and 50% strain, tensile strength and elongation at break can be extracted and these are shown in Figs. 4e6, respectively. The composites clearly display mechanical anisotropy, as seen from the differences in modulus at 10% and 50% strain in the two directions (Fig. 4). It should be emphasized again that this property can be attained only in fiber reinforced, and not in particulate filled rubbers. The most striking changes with the addition of carbon black are the increases in the longitudinal direction of the elongation at break (Fig. 5) and the tensile strength (Fig. 6). Fig. 7 displays the stress ratio anisotropy for PALF/CB-NBR composites as a function of strain. The composites display considerable mechanical anisotropy over a certain range of strain. The peak in stress ratio anisotropy
Fig. 4. 10 and 50% moduli (or stress at 10 and 50% strain) of PALF/CB-NBR and controlled CB-NBR composites, in the longitudinal (LD) and transverse direction (TD).
Fig. 5. Tensile strengths of PALF/CB-NBR and controlled CB-NBR composites in the longitudinal (LD) and transverse direction (TD).
Fig. 3. Stressestrain curves of PALF/CB-NBR and controlled CB-NBR composites, in the transverse direction (a) and the expanded low strain region (b). The number in a legend indicates the amount in phr of the filler and letter F means PALF which is fixed at 10 phr.
appears to increase and shift toward lower strain when the amount of carbon black is increased. At high strain, the stress ratio anisotropy becomes smaller and close to unity (no anisotropy). The reinforcing efficiency of PALF in the hybrid composite may be seen in the stress ratios of PALF/CB-NBR in the longitudinal direction and CB-NBR shown in Fig. 8. With increasing carbon black content, the
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K. Prukkaewkanjana et al. / Polymer Testing 45 (2015) 76e82
Fig. 6. Elongations at break of PALF/CB-NBR and controlled CB-NBR composites, in the longitudinal (LD) and transverse direction (TD).
Fig. 7. Stress ratio anisotropies (LD/TD) of PALF/CB-NBR composites. The number in a legend indicates the amount in phr of the filler and letter F means PALF which is fixed at 10 phr.
Fig. 9. Tear strengths of PALF/CB-NBR composites in the longitudinal (LD) and transverse direction (TD).
types of composite have greater tear strength than NBR gum, and the tear strength increases with increasing filler content. At the filler content of 10 phr, PALF-NBR in both directions and carbon black-NBR display very similar tear strength. Carbon black-NBR composites, however, display greater increases in tear strength than do those of PALF/carbon black-NBR hybrid composites with the same total filler content. One important point is that the tear strength of PALF/carbon black-NBR hybrid composites is only slightly greater in the longitudinal than in the transverse direction. This small difference diminishes as the carbon black content is increased. This suggests that the tear strengths at the higher levels of carbon black are determined by the carbon black, not the PALF. The improvement in tear strength of a rubber can be attributed to two main causes [10]. These are the increase in tensile strength and the change in the character of the tear process. For this study, it is proposed that the observed increase in tear strength must be due primarily to the increase in tensile strength. This was shown to be the case in PALF/silica-reinforced NBR [8].
3.4. Hardness Shore A hardness of all PALF/carbon black-NBR and carbon black-NBR composites is displayed in Fig. 10. The carbon black-NBR
Fig. 8. Stress ratios (LD/CB filled) of PALF/CB-NBR composites. The number in a legend indicates the amount in phr of the filler and letter F means PALF which is fixed at 10 phr.
reinforcing efficiency increases with maximum value above 50 times that of the matrix (CB-NBR). Again, the reinforcing efficiency diminishes at very high strains when it approaches unity. 3.3. Tear strength Fig. 9 displays the tear strengths of different composites. All
Fig. 10. Hardness of PALF/CB-NBR composites.
K. Prukkaewkanjana et al. / Polymer Testing 45 (2015) 76e82
composites display a gradual increase in hardness with increasing filler content. PALF/carbon black-NBR hybrid composites display greater hardness than their carbon black-NBR counterparts. The difference between the two series is approximately 12 Shore A units over the whole range. This is similar to the behavior seen in the PALF/silica-NBR system [8].
81
3.5. Fracture surface Fig. 11 displays the tensile (in the longitudinal direction) fracture surfaces of the control 30 phr carbon black filled rubber (CB30) and of the hybrid composites with 10 phr PALF/30 phr carbon black (CB30F10). The plane of fracture is perpendicular to the direction of fiber alignment. The fracture surface of the hybrid composite contains a large number of long protruding fibers. This indicates that failure occurs predominantly by fiber de-bonding and pullout. It is important to note that the directions of long fibers seen in the micrographs do not reflect their directions inside the composites. The seemingly random orientation results when the sudden fiber retraction after matrix failure forces them to lie on the fracture surface. 3.6. Swelling behavior Fig. 12 displays the swelling behavior of PALF-carbon black hybrid and carbon black composites. The swelling ratios of carbon black composites decrease with increasing carbon black content. For PALF-carbon black hybrid composites, similar swelling ratios are seen. 4. Discussion We have drawn similar results into a general interpretation of the effect of PALF on the properties of PALF-NBR composites in previous publications [3,8]. Hence, here we shall consider only the effect of carbon black in the hybrid system. This will be compared with that of silica. We start with the control system of carbon black only. The effect on the rubber (NBR) is very similar to that of silica, but very different from that of PALF. Carbon black affects the stress at low strain only slightly (see Fig. 2(b) and Fig. 4(a)). The effect becomes more and more apparent at increasing strains. Consequently, the greatest effect on stress is seen at the failure point. The tensile strength of carbon black filled NBR seems to approach a limit after addition of 20 phr (Fig. 5) while tear strengths increases with increasing carbon black content (Fig. 9). Elongation at break is not very much altered (Fig. 6). The hybrid systems are significantly different. Addition of carbon black to 10 phr PALF-NBR causes considerable changes in the stressestrain behavior, especially in the very low strain and high
Fig. 11. SEM micrographs of tensile fractured specimens of 30 phr CB-NBR (a), 10 phr PALF-NBR (b) and 10 phr PALF/30 phr CB-NBR (c).
Fig. 12. Swelling ratios of PALF/CB-NBR composites in the longitudinal (LD) and transverse direction (TD).
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strain regions i.e. 10% modulus and elongation at break (Fig. 2). These remarkable changes have not been observed in other hybrid systems [5,11e14]. The effect of carbon black on elongation at break observed here is much greater than that of silica recently reported [8] in a comparable system. Nevertheless, similar to what was reported for silica, the main effect of carbon black on the overall mechanical behavior of the hybrid composites is an improvement in the tear strength. Upon stretching, de-bonding will occur at the fiber ends creating numerous voids. These will act as initiation sites for failure in much the same way as does a nick in a tear-test piece. From Fig. 6, carbon black improves the tear strength of NBR significantly, therefore the matrix of the hybrid composite should have similar resistance to tear. In other words, carbon black blunts the sharp cracks at de-bonded fiber ends allowing the composites to extend without premature breaking. The matrix not only extends, but the accommodated stress also increases significantly on further extension. This effect is not seen with silica. The control silica-only filled NBR displays behavior similar to the carbon-only control, but with much lower tear strength. This could be related to the need, not yet optimized, of a silane coupling agent for silica [15]. The increase in the slope of the stressestrain curve and in the 10% modulus with the addition of carbon black is due to better stress transfer to PALF. The mechanism for this could be due to the increase in the shear modulus of the matrix rubber, not to better adhesion as in the use of a dry bonding agent [3,16e19]. Additionally, the improvement of stress transfer can be caused by contraction of the matrix perpendicular to the fiber axis, which presses the surfaces together upon extension. This is frictional stress transfer and is known as the ‘Chinese finger’ effect [1]. With increasing carbon black content, the matrix becomes stiffer and this frictional stress transfer becomes more effective. This is seen even at lower strains. This improved stress transfer allows increases in the stress up to the medium strain range. However, at very high strains, it appears that the stressestrain behavior of the composites is dominated by carbon black. This is confirmed by the fact that both the stress ratio anisotropy (Fig. 7) and the stress ratio (Fig. 8) become smaller and close to unity at very high strains. 5. Conclusions This work demonstrates the use of particulate carbon black with oriented PALF as hybrid filler to modify the tensile properties of reinforced NBR. The main advantage of adding carbon black to the PALF is to extend the elongation at break and increase the ultimate tensile strength of the composites. So, importantly, these hybrid composites of normally weak synthetic rubbers display the distinct characteristic stressestrain curves of fiber reinforced rubbers but, like natural rubber, with higher elongation at break. Each filler affects the tensile behavior of the composite separately, i.e. the effect of PALF dominates in low strain regions while that of carbon black is important in high strain regions. By comparison with silica fillers, the carbon black filled NBR matrix is less affected by defects and so
can be extended to very high elongations. Hybrid composites also allow further control of the modulus at low strains. Such improved performance will certainly be beneficial in product design and use of synthetic rubbers.
Acknowledgments Partial financial supports from Center of Excellence for Innovation in Chemistry (PERCH-CIC), Office of the Higher Education Commission, Ministry of Education is gratefully acknowledged.
References [1] A.P. Foldi, Short-fibre-reinforced rubber: a new kind of composite, in: T.L. Vigo, B.J. Kinzig (Eds.), Composite Applications: the Role of Matrix, Fibre, and Interface, VCH, New York, 1992, p. 139. [2] J.B. Donnet, E. Custodero, Reinforcement of elastomers by particulate fillers, in: J.E. Mark, B. Erman, F.R. Eirich (Eds.), Science and Technology of Rubber, third ed., Elsevier Academic Press, San Diego, 2005, pp. 367e400. [3] U. Wisittanawat, S. Thanawan, T. Amornsakchai, Mechanical properties of highly aligned short pineapple leaf fiber reinforced e nitrile rubber composite: effect of fiber content and bonding agent, Polym. Test. 35 (2014) 20e27. [4] S.K. Chakraborty, D.K. Setua, S.K. De, Short jute fiber reinforced carboxylated nitrile rubber, Rubber Chem. Technol. 55 (1982) 1286e1307. [5] D.K. Setua, S.K. De, Short silk fibre reinforced nitrile rubber composites, J. Mater. Sci. 19 (1984) 983e999. [6] K. Agarwal, D.K. Satua, G.N. Mathur, Short fibre and particulate-reinforced rubber composites, Def. Sci. J. 52 (2002) 337e346. [7] C. Rajesh, G. Unnikrishanan, E. Purushothaman, S. Thomas, Cure characteristics and mechanical properties of short nylon fiber-reinforced nitrile rubber composites, J. Appl. Polym. Sci. 92 (2004) 1023e1030. [8] U. Wisittanawat, S. Thanawan, T. Amornsakchai, Remarkable improvement of failure strain of preferentially aligned short pineapple leaf fiber reinforced nitrile rubber composites with silica hybridization, Polym. Test. 38 (2014) 91e99. [9] N. Kengkhetkit, T. Amornsakchai, Utilisation of pineapple leaf waste for plastic reinforcement: 1. A novel extraction method for short pineapple leaf fiber, Ind. Crops Prod. 40 (2012) 55e61. [10] A.N. Gent, Strength of elastomers, in: J.E. Mark, B. Erman, F.R. Eirich (Eds.), Science and Technology of Rubber, third ed., Elsevier Academic Press, San Diego, 2005, pp. 469e475. [11] N. Lopattananon, D. Jitkalong, M. Seadan, Hybridized reinforcement of natural rubber with silane-modified short cellulose fibers and silica, J. Appl. Polym. Sci. 120 (2011) 3242e3254. [12] L.I. Rueda, C.C. Anton, M.C.T. Rodriguez, Mechanics of short fibers in filled styreneebutadiene rubber (SBR) composites, Polym. Compos. 9 (1988) 198e203. [13] L. Mathew, S.K. Narayanankutty, Cure characteristics and mechanical properties of HRH bonded nylon-6 short fiber-nanosilica-acrylonitrile butadiene rubber hybrid composite, Polym. Plast. Technol. Eng. 48 (2008) 75e81. [14] W. Bai, K. Li, Partial replacement of silica with microcrystalline cellulose in rubber composites, Compos. Part A-Appl. Sci. Manuf. 40 (2009) 1597e1605. [15] J.W. Ten Brinke, S.C. Debnath, L.A.E.M. Reuvekamp, J.W.M. Noordermeer, Mechanistic aspects of the role of coupling agents in silicaerubber composites, Compos. Sci. Technol. 63 (2003) 1165e1174. [16] H. Ismail, N. Rosnah, H.D. Rozman, Effects of various bonding systems on mechanical properties of oil palm fiber reinforced rubber composites, Eur. Polym. J. 33 (1997) 1231e1238. [17] V.G. Geethamma, K.T. Mathew, R. Lakshminarayanan, S. Thomas, Composite of short coir fibers and natural rubber: effect of chemical modification, loading and orientation of fiber, Polymer 39 (1998) 1483e1491. [18] R.S. Rajeev, A.K. Bhowmick, S.K. De, S. Bandyopadhyay, Short melamine fiber filled nitrile rubber composites, J. Appl. Polym. Sci. 90 (2003) 544e558. [19] L. Mathew, S.K. Narayanankutty, Nanosilica as dry bonding system component and as reinforcement in short nylon-6 fiber/natural rubber composite, J. Appl. Polym. Sci. 112 (2009) 2203e2212.
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material properties
High performance hybrid reinforcement of nitrile rubber using short pineapple leaf fiber and carbon black* Kontapond Prukkaewkanjana a, Sombat Thanawan b, Taweechai Amornsakchai a, c, d, * a
Polymer Science and Technology Program, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand b Rubber Technology Research Center, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand c Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand d Center of Sustainable Energy and Green Materials, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 April 2015 Accepted 12 May 2015 Available online 29 May 2015
Rubber composites with very high moduli at low elongation, high elongation at break and high ultimate breaking strength have been developed. The matrix was acrylonitrile butadiene rubber (NBR) and the hybrid (fibrous and particulate) reinforcements were short, fine pineapple leaf fiber (PALF) and carbon black. The amount of PALF was fixed at 10 parts (by weight) per hundred of rubber (phr) while that of carbon black was varied from 0 to 30 phr. Uniaxial NBR composites were prepared. Tensile strength, elongation at break, modulus and tear strength of the hybrid composites were characterized in both longitudinal (parallel to the fiber axis) and transverse (perpendicular to the fiber axis) directions. The addition of carbon black causes the slope of the early part of the stressestrain curve to increase and also extends breaking to greater strains. At carbon black contents of 20 phr and above, the stressestrain relation displays an upturn at high elongations, providing greater ultimate strength. Comparison with the usual carbon black filled rubber shows that the composite behavior at low strains is determined by the PALF, and at high strains by the carbon black. This high performance PALF-carbon black reinforced NBR shows great promise for engineering applications. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Pineapple leaf fiber Carbon black Nitrile rubber Rubber composite Hybrid composite
1. Introduction Short fiber reinforced rubber composites have unique mechanical properties which combine fiber rigidity and rubber elasticity [1]. According to models of short fiber reinforced composites, the mechanical properties of uniaxially aligned composite are determined mainly by the fiber modulus, the fiber volume fraction and the fiber aspect ratio. Usually, these parameters are varied in such a way as to obtain composites with the required properties. To a much lesser degree, the composite properties
*
Part of this work has been presented at ICNF-2015. * Corresponding author. Polymer Science and Technology Program, Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand. E-mail address: [email protected] (T. Amornsakchai). http://dx.doi.org/10.1016/j.polymertesting.2015.05.004 0142-9418/© 2015 Elsevier Ltd. All rights reserved.
depend also on the matrix modulus. For rubber matrix composites, different types of particulate filler are commonly used to adjust the composite properties [2]. Certain fillers increase, not only the modulus, but also the strength, of the rubber composites. These are known as reinforcing fillers and a good example of this is carbon black. Previous research has shown that short, fine pineapple leaf fiber can be used successfully to reinforce nitrile rubber. However, the composites fail at relatively low strain [3]. Such a problem is common for rubbers filled with short fibers [4e7]. We have recently shown that, by adding silica to form a hybrid filler in short PALF reinforced nitrile rubber, the elongation at break is increased remarkably [8]. This research is aimed at examining this concept again by using carbon black as the hybrid filler. Carbon black, of course, is the best known filler for rubber reinforcement. Unlike silica, it can easily be used without the need of a coupling agent. The PALF content will be fixed at 10 phr and the content of carbon
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black varied at 10, 20 and 30 phr. Comparison will be made with the control system containing only carbon black at 10, 20, 30 and 40 phr.
Table 2 Formulations of rubber compounds. CB and F denote carbon black and PALF, respectively. The number following letter CB and F in a compound name indicates the amount of the respective materials in phr. Compound/Ingredient
2. Experimental
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Amount (phr) NBR
CB
PALF
ZnO
Stearic acid
CBSa
Sulfur
100 100 100 100 100 100 100 100 100
e 10 20 30 40 e 10 20 30
e e e e e 10 10 10 10
5 5 5 5 5 5 5 5 5
2 2 2 2 2 2 2 2 2
1 1 1 1 1 1 1 1 1
2 2 2 2 2 2 2 2 2
2.1. Materials 2.1.1. PALF The milling technique developed in our laboratory [9] was used for separating PALF from fresh pineapple leaves. These were collected from the Kok Kwai District, Amphor Ban Rai, Uthai Thani Province, Thailand. The leaves were cut into 6 mm lengths before being fed into a disc milling machine. The cut leaves were crushed into paste. This paste was then dried, ground, and sieved in order to separate PALF from non-fibrous materials. The general characteristics of PALF have been reported elsewhere [9]. 2.1.2. Rubbers NBR was JSR N230SL grade with 35% acrylonitrile content produced by JSR Corporation, Japan. It has a Mooney Viscosity (ML1þ4(100 C)) of 42. 2.1.3. Rubber chemicals All rubber ingredients including ZnO, stearic acid, sulfur, and CBS were commercial grade. 2.2. Composite preparation Compounds were mixed on a laboratory two-roll mill to produce mixtures with fibers aligned in the machine direction, as described previously [3,8]. The sequence of ingredient addition is shown in Table 1. The formulations of the rubber compound are shown in Table 2. The amount of PALF was fixed at 10 phr and that of carbon black (CB) varied from 0 to 30 phr. The abbreviations CB and F denote carbon black and PALF respectively. The numbers following letters CB and F in a compound name indicate the amount of the respective material in phr. The rubber composites were prepared by vulcanizing the rubber compound while compression molding in a hydraulic press. This was done under a pressure of 1500 psi and temperature of 150 C with a 1 mm thick mold.
Unfilled CB10 CB20 CB30 CB40 F10 CB10F10 CB20F10 CB30F10 a
CBS ¼ N-Cyclohexyl-2-benzothiazolesulfenamide.
tear tests, respectively. Five specimens were measured for each formulation and average values determined for the different properties. Hardness measurements were made according to ISO 7619 using a Shore A durometer (Wallace, H17A). An average value from six positions was recorded for each specimen. Swelling percentage weight gains were studied using rectangular specimens cut from vulcanized sheet. The rectangular specimens were soaked in toluene at room temperature for 72 h and the weight gain determined. The shape and size of PALF and fractured surfaces of the composites were observed with a scanning electron microscope (SEM) (Hitachi Tabletop Microscope; model TM 1000, Japan). 3. Results 3.1. Cure characteristics Fig. 1 displays curing characteristics of the NBR compounds filled with PALF/carbon black hybrid and carbon black only. Numerical values for different cure characteristics of the compounds are tabulated in Table 3. Considering the curing behavior of compounds with hybrid fillers (Fig. 1, open symbols), the compounds containing greater amounts of carbon black have shorter scorch times. When these are compared with those of compounds containing carbon black only
2.3. Characterization
Table 1 Sequence of ingredient addition in rubber compound preparation. Order
Ingredient
Mixing time (min)
1 2 3 4 5
NBR Sulfur PALF and/or CB ZnO with stearic acid CBS Total time
0e1 1e2 2e11 11e17 17e20 20
25
unfilled CB10 CB20 CB30 CB30
20
Torque (dNm)
Minimum torque (ML), maximum torque (MH), scorch time (ts2) curing time (tc90) of non-vulcanized rubbers were characterized using a moving die rheometer (MDR), (Rheo TECH MDþ, Alpha Technologies, Akron, USA) at 150 C (ISO 6502). Tensile properties and tear strengths of rubber composites were measured on a universal testing machine (Instron 5566, High Wycombe, UK) with a long travel contact-style extensometer at room temperature, according to ISO 37 and ISO 34, respectively. Type C and type B dies were used to cut the sample for tensile and
F10 CB10F10 CB20F10 CB30F10
15 10 5 0
0
5
10
15
Time (minute) Fig. 1. Cure curves of PALF/CB-NBR (open symbols) and controlled CB-NBR (closed symbols) composites. The number in a legend indicates the amount in phr of the filler and letter F means PALF which is fixed at 10 phr.
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Table 3 Cure characteristics of different compounds. CB and F denote carbon black and PALF, respectively. The number following letter CB and F in a compound name indicates the amount of the respective materials in phr. Compound
ML (dN.m)
MH (dN.m)
ts2 (min)
tc90 (min)
Unfilled CB10 CB20 CB30 CB40 F10 CB10F10 CB20F10 CB30F10
0.27 0.37 0.47 0.64 0.87 0.47 0.55 0.70 0.84
6.94 9.10 11.07 13.75 15.90 9.91 12.18 14.67 16.69
4.51 4.17 3.30 3.28 2.32 3.27 3.19 2.48 2.10
9.59 10.09 10.41 9.37 8.07 7.06 6.04 6.19 5.41
(Fig. 1, closed symbols), it appears that the scorch time depends on the amount of carbon black but not on the presence of PALF. Addition of (10 phr) PALF to NBR (F10) increases the maximum torque. When 10 phr of carbon black is further added to the system (CB10F10), the maximum torque increases again. With 20 and 30 phr carbon black (CB20F10 and CB30F10), the maximum torque increases steadily. For systems containing only carbon black, addition of 10, 20, 30 and 40 phr of carbon black causes the maximum torque to increase in a similar manner. For compounds containing the same amount of total filler, those with hybrid filler have slightly greater maximum torque. 3.2. Tensile properties Fig. 2 displays longitudinal stressestrain curves of PALF/carbon black-NBR and carbon black-NBR composite vulcanizates containing different amounts of carbon black. The PALF-NBR composite with 10 phr PALF (F10) exhibits a sharp rise in stress, reaches a peak value at about 30% strain and then decreases and eventually fails at about 50% strain. With the addition of 10 phr of carbon black (CB10F10), the stress values increase significantly, reaching a peak value before dropping slightly and extending to a strain greater than 100%. When the carbon black content is increased to 20 phr (CB20F10), the stressestrain curve
rises with a similar slope to that of 10 phr carbon black. However, when the stress reaches a peak value it does not drop as much and extends with relatively constant stress to about 300% strain. At this point it displays an upturn in stress. With further increase in carbon black content (CB30F10), the stressestrain curve rises with even greater slope and reaches a higher peak stress. Again, after the peak the stress gradually decreases with increasing strain and then levels off. This level stress is greater than that with 20 phr carbon black. With further extension, an upturn in stress occurs at approximately 250% strain. The failure stress for this CB30F10 hybrid composite is greater (although at lower strain) than that of CB20F10. The increases in the initial slope of the stressestrain curve and in the elongation at break with increasing carbon black are similar to those observed in PALF/silica-NBR hybrid composites [8]. There are, however, two significant points of difference. The first is that the levelling stress in this PALF/carbon black-NBR hybrid system is lower than that in PALF/silica-NBR hybrid system. The second point is that PALF/carbon black-NBR hybrid systems display a clear upturn in stress at high carbon black content. Conversely, at high silica contents, PALF/silica-NBR hybrid systems do not. In order to understand the behavior of the hybrid composites, comparison can be made with the stressestrain curves of the control carbon black-NBR composites. Since carbon black is a particulate filler, no directional property is observed. NBR gum displays the lowest stressestrain curve with an elongation at break of almost 500%. When 10 phr carbon black is added, the stress increases over the whole range and display an upturn at high strain. There is no maximum peak and the failure stress increases significantly. The failure strain remains roughly the same. With further addition of carbon black to 20 phr, the stress again increases over the whole range. The upturn seems to occur at lower strain and the failure strain (elongation at break) deceases. An important point to note is that this high strain part of the stressestrain curve is virtually superimposable on that of the CB20F10 hybrid composite. With an increase in carbon black content to 30 phr, a similar situation is seen, i.e. the high strain part of the stressestrain curve is almost superimposable on that of the CB30F10 hybrid composite.
Fig. 2. Stressestrain curves of PALF/CB-NBR and controlled CB-NBR composites, in the longitudinal direction (a) and the expanded low strain region (b). The number in a legend indicates the amount in phr of the filler and letter F means PALF which is fixed at 10 phr.
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However, in this case, the failure stress of the CB30F10 hybrid composite is greater than that of composite containing carbon black only. The stressestrain behavior of PALF/carbon black-NBR composites in the transverse direction (Fig. 3) also displays a dependency on the amount of carbon black. Stress in the low strain region appears to increase with increasing carbon black content. In the high strain region, however, the effect of carbon black is much greater. The stress does display an upturn at high strains, but to a lesser degree than seen in the longitudinal direction. The elongation at break decreases slightly with increasing carbon black content. From the stressestrain curves shown in Figs. 2 and 3, moduli at 10% and 50% strain, tensile strength and elongation at break can be extracted and these are shown in Figs. 4e6, respectively. The composites clearly display mechanical anisotropy, as seen from the differences in modulus at 10% and 50% strain in the two directions (Fig. 4). It should be emphasized again that this property can be attained only in fiber reinforced, and not in particulate filled rubbers. The most striking changes with the addition of carbon black are the increases in the longitudinal direction of the elongation at break (Fig. 5) and the tensile strength (Fig. 6). Fig. 7 displays the stress ratio anisotropy for PALF/CB-NBR composites as a function of strain. The composites display considerable mechanical anisotropy over a certain range of strain. The peak in stress ratio anisotropy
Fig. 4. 10 and 50% moduli (or stress at 10 and 50% strain) of PALF/CB-NBR and controlled CB-NBR composites, in the longitudinal (LD) and transverse direction (TD).
Fig. 5. Tensile strengths of PALF/CB-NBR and controlled CB-NBR composites in the longitudinal (LD) and transverse direction (TD).
Fig. 3. Stressestrain curves of PALF/CB-NBR and controlled CB-NBR composites, in the transverse direction (a) and the expanded low strain region (b). The number in a legend indicates the amount in phr of the filler and letter F means PALF which is fixed at 10 phr.
appears to increase and shift toward lower strain when the amount of carbon black is increased. At high strain, the stress ratio anisotropy becomes smaller and close to unity (no anisotropy). The reinforcing efficiency of PALF in the hybrid composite may be seen in the stress ratios of PALF/CB-NBR in the longitudinal direction and CB-NBR shown in Fig. 8. With increasing carbon black content, the
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Fig. 6. Elongations at break of PALF/CB-NBR and controlled CB-NBR composites, in the longitudinal (LD) and transverse direction (TD).
Fig. 7. Stress ratio anisotropies (LD/TD) of PALF/CB-NBR composites. The number in a legend indicates the amount in phr of the filler and letter F means PALF which is fixed at 10 phr.
Fig. 9. Tear strengths of PALF/CB-NBR composites in the longitudinal (LD) and transverse direction (TD).
types of composite have greater tear strength than NBR gum, and the tear strength increases with increasing filler content. At the filler content of 10 phr, PALF-NBR in both directions and carbon black-NBR display very similar tear strength. Carbon black-NBR composites, however, display greater increases in tear strength than do those of PALF/carbon black-NBR hybrid composites with the same total filler content. One important point is that the tear strength of PALF/carbon black-NBR hybrid composites is only slightly greater in the longitudinal than in the transverse direction. This small difference diminishes as the carbon black content is increased. This suggests that the tear strengths at the higher levels of carbon black are determined by the carbon black, not the PALF. The improvement in tear strength of a rubber can be attributed to two main causes [10]. These are the increase in tensile strength and the change in the character of the tear process. For this study, it is proposed that the observed increase in tear strength must be due primarily to the increase in tensile strength. This was shown to be the case in PALF/silica-reinforced NBR [8].
3.4. Hardness Shore A hardness of all PALF/carbon black-NBR and carbon black-NBR composites is displayed in Fig. 10. The carbon black-NBR
Fig. 8. Stress ratios (LD/CB filled) of PALF/CB-NBR composites. The number in a legend indicates the amount in phr of the filler and letter F means PALF which is fixed at 10 phr.
reinforcing efficiency increases with maximum value above 50 times that of the matrix (CB-NBR). Again, the reinforcing efficiency diminishes at very high strains when it approaches unity. 3.3. Tear strength Fig. 9 displays the tear strengths of different composites. All
Fig. 10. Hardness of PALF/CB-NBR composites.
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composites display a gradual increase in hardness with increasing filler content. PALF/carbon black-NBR hybrid composites display greater hardness than their carbon black-NBR counterparts. The difference between the two series is approximately 12 Shore A units over the whole range. This is similar to the behavior seen in the PALF/silica-NBR system [8].
81
3.5. Fracture surface Fig. 11 displays the tensile (in the longitudinal direction) fracture surfaces of the control 30 phr carbon black filled rubber (CB30) and of the hybrid composites with 10 phr PALF/30 phr carbon black (CB30F10). The plane of fracture is perpendicular to the direction of fiber alignment. The fracture surface of the hybrid composite contains a large number of long protruding fibers. This indicates that failure occurs predominantly by fiber de-bonding and pullout. It is important to note that the directions of long fibers seen in the micrographs do not reflect their directions inside the composites. The seemingly random orientation results when the sudden fiber retraction after matrix failure forces them to lie on the fracture surface. 3.6. Swelling behavior Fig. 12 displays the swelling behavior of PALF-carbon black hybrid and carbon black composites. The swelling ratios of carbon black composites decrease with increasing carbon black content. For PALF-carbon black hybrid composites, similar swelling ratios are seen. 4. Discussion We have drawn similar results into a general interpretation of the effect of PALF on the properties of PALF-NBR composites in previous publications [3,8]. Hence, here we shall consider only the effect of carbon black in the hybrid system. This will be compared with that of silica. We start with the control system of carbon black only. The effect on the rubber (NBR) is very similar to that of silica, but very different from that of PALF. Carbon black affects the stress at low strain only slightly (see Fig. 2(b) and Fig. 4(a)). The effect becomes more and more apparent at increasing strains. Consequently, the greatest effect on stress is seen at the failure point. The tensile strength of carbon black filled NBR seems to approach a limit after addition of 20 phr (Fig. 5) while tear strengths increases with increasing carbon black content (Fig. 9). Elongation at break is not very much altered (Fig. 6). The hybrid systems are significantly different. Addition of carbon black to 10 phr PALF-NBR causes considerable changes in the stressestrain behavior, especially in the very low strain and high
Fig. 11. SEM micrographs of tensile fractured specimens of 30 phr CB-NBR (a), 10 phr PALF-NBR (b) and 10 phr PALF/30 phr CB-NBR (c).
Fig. 12. Swelling ratios of PALF/CB-NBR composites in the longitudinal (LD) and transverse direction (TD).
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strain regions i.e. 10% modulus and elongation at break (Fig. 2). These remarkable changes have not been observed in other hybrid systems [5,11e14]. The effect of carbon black on elongation at break observed here is much greater than that of silica recently reported [8] in a comparable system. Nevertheless, similar to what was reported for silica, the main effect of carbon black on the overall mechanical behavior of the hybrid composites is an improvement in the tear strength. Upon stretching, de-bonding will occur at the fiber ends creating numerous voids. These will act as initiation sites for failure in much the same way as does a nick in a tear-test piece. From Fig. 6, carbon black improves the tear strength of NBR significantly, therefore the matrix of the hybrid composite should have similar resistance to tear. In other words, carbon black blunts the sharp cracks at de-bonded fiber ends allowing the composites to extend without premature breaking. The matrix not only extends, but the accommodated stress also increases significantly on further extension. This effect is not seen with silica. The control silica-only filled NBR displays behavior similar to the carbon-only control, but with much lower tear strength. This could be related to the need, not yet optimized, of a silane coupling agent for silica [15]. The increase in the slope of the stressestrain curve and in the 10% modulus with the addition of carbon black is due to better stress transfer to PALF. The mechanism for this could be due to the increase in the shear modulus of the matrix rubber, not to better adhesion as in the use of a dry bonding agent [3,16e19]. Additionally, the improvement of stress transfer can be caused by contraction of the matrix perpendicular to the fiber axis, which presses the surfaces together upon extension. This is frictional stress transfer and is known as the ‘Chinese finger’ effect [1]. With increasing carbon black content, the matrix becomes stiffer and this frictional stress transfer becomes more effective. This is seen even at lower strains. This improved stress transfer allows increases in the stress up to the medium strain range. However, at very high strains, it appears that the stressestrain behavior of the composites is dominated by carbon black. This is confirmed by the fact that both the stress ratio anisotropy (Fig. 7) and the stress ratio (Fig. 8) become smaller and close to unity at very high strains. 5. Conclusions This work demonstrates the use of particulate carbon black with oriented PALF as hybrid filler to modify the tensile properties of reinforced NBR. The main advantage of adding carbon black to the PALF is to extend the elongation at break and increase the ultimate tensile strength of the composites. So, importantly, these hybrid composites of normally weak synthetic rubbers display the distinct characteristic stressestrain curves of fiber reinforced rubbers but, like natural rubber, with higher elongation at break. Each filler affects the tensile behavior of the composite separately, i.e. the effect of PALF dominates in low strain regions while that of carbon black is important in high strain regions. By comparison with silica fillers, the carbon black filled NBR matrix is less affected by defects and so
can be extended to very high elongations. Hybrid composites also allow further control of the modulus at low strains. Such improved performance will certainly be beneficial in product design and use of synthetic rubbers.
Acknowledgments Partial financial supports from Center of Excellence for Innovation in Chemistry (PERCH-CIC), Office of the Higher Education Commission, Ministry of Education is gratefully acknowledged.
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