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Manipulation of mechanical properties of short pineapple leaf fiber reinforced natural rubber composites through variations in cross-link density and carbon black loading
Accepted Manuscript Manipulation of mechanical properties of short pineapple leaf fiber reinforced natural rubber composites through variations in cross-link density and carbon black loading Pitchapa Pittayavinai, Sombat Thanawan, Taweechai Amornsakchai PII:
S0142-9418(16)30500-1
DOI:
10.1016/j.polymertesting.2016.07.002
Reference:
POTE 4702
To appear in:
Polymer Testing
Received Date: 26 May 2016 Accepted Date: 2 July 2016
Please cite this article as: P. Pittayavinai, S. Thanawan, T. Amornsakchai, Manipulation of mechanical properties of short pineapple leaf fiber reinforced natural rubber composites through variations in cross-link density and carbon black loading, Polymer Testing (2016), doi: 10.1016/ j.polymertesting.2016.07.002. 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 Material Properties Manipulation of mechanical properties of short pineapple leaf fiber reinforced natural rubber composites through variations in cross-link density and carbon black loading
Polymer Science and Technology Program, Department of Chemistry and Center of
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1
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Pitchapa Pittayavinai1, Sombat Thanawan2, Taweechai Amornsakchai*1, 3, 4
Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University,
2
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Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand Rubber Technology Research Center, Faculty of Science, Mahidol University,
Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand 3
Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University,
4
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Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand Center of Sustainable Energy and Green Materials, Faculty of Science, Mahidol University,
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Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand
* Corresponding author
Tel: (662) 441-9816 ext. 1161 Fax: (662) 441-9322 Email: [email protected]
ACCEPTED MANUSCRIPT Abstract The aim of this paper is to demonstrate that the stress-strain behavior of natural rubber reinforced with short pineapple leaf fiber (PALF) can easily be manipulated by changing the cross-link density and the amount of carbon black (CB) primary filler. This gives more
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manageable control of mechanical properties than is possible with conventional particulate fillers alone. This type of hybrid rubber composite displays a very sharp rise in stress at very low strains, and then the stress levels off at medium strains before turning up again at the
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highest strains. The composites studied here contain a fixed amount of PALF at 10 part (by weight) per hundred rubber (phr) and varying carbon black contents from 0 to 30 phr. To
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change the cross-link density, the amount of sulfur was varied from 2 to 4 phr. Swelling ratio results indicate that composites prepared with greater amounts of sulfur and carbon black have greater cross-link densities. Consequently, this affects the stress-strain behavior of the composites. The greater the cross-link density, the less is the strain at which the stress upturn
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occurs. Variations in the rate of stress increase (although not the stress itself) in the very low strain region, while dependent on fillers, are not dependent on the crosslink density. The effect of changes in crosslinking is most obvious in the high strain region. Here, the rate of
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stress increase becomes larger with increasing cross-link density. Hence, we demonstrate
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that the use of PALF filler, along with the usual carbon primary filler, provides a convenient method for the manipulation of the stress-strain relationships of the reinforced rubber. Such composites can be prepared with a controllable, wide range of mechanical behavior for specific high performance engineering applications.
Keywords: natural rubber; pineapple leaf fiber; natural fiber, hybrid composite; cross-link density
ACCEPTED MANUSCRIPT 1. Introduction Rubber is one of the most versatile materials. With additives, it can be formulated so that its properties span a very wide range from very soft to very hard. This can be done either by changing the vulcanization system [1, 2] or by the addition of various types of filler [3].
carbon black, silica and calcium carbonate [4, 5].
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The most common types of filler used in the rubber industry are particulate materials such as For applications where very large
deformations are undesirable, the rubber may be reinforced with textile fabrics. The use of
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short fiber fillers allows much greater flexibility than when textile fabrics are used, although this is at the expense of strength. Recently, our group has shown that short pineapple leaf
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fiber (PALF) can be used to reinforce nitrile rubber (NBR) very effectively [6-8]. The reinforced rubber shows a very distinct stress-strain curve in which the stress rises sharply on stretching. It has also been shown that properties of this reinforced rubber can be changed by using particulate filler along with the PALF. In order to widen the applications of this type of
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rubber composite, it is important that the composite properties can be adjusted accordingly. Generally, theories for the properties of particulate filled rubber will be different from those for short fiber filled rubber. Properties of the former can be predicted with Cox’s shear
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lag theory [9], its modified theory [10-13] and the Halpin-Tsai equation [14]. However, there
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is no generally accepted theory, covering the whole strain range, for the latter. Short fiber reinforced composite theories [15-18], which are more suitable for small strain deformation, may be used as a guide. For a selected fiber system (type and amount), properties of the composite depend on the shear modulus of the continuous phase. For an isotropic matrix, this is determined by its Young’s modulus. Hence, the easy manipulation of the properties of a matrix rubber, as described above, can yield fiber and particulate filler composites with different properties.
ACCEPTED MANUSCRIPT In this work, the effect of cross-link density and carbon black content on the properties of PALF reinforced natural rubber (NR) composites was studied. Different crosslink densities were obtained by changing the sulfur content. PALF was fixed at 10 phr and
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carbon black content varied from 0 to 30 phr.
2. Experimental 2.1 Materials
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2.1.1 PALF
Pineapple leaf fiber was prepared by the milling technique developed in our
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laboratory [17]. The fresh pineapple leaves were collected from Ban Yang District, Amphoe Nakhon Thai, Phitsanulok Province, Thailand. The fresh leaves were cut across the long axis into 6 mm long pieces. After that, the leaves were fed into a disc milling machine. Then, the fiber bundles and soft tissues were mashed into paste. The paste was dried and sieved to
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separate the PALF from non-fibrous materials. The shape and size distribution this PALF have been reported elsewhere [6, 17-18]. 2.1.2 Rubbers and rubber chemicals
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Natural rubber (NR) was STR5L grade supplied by MBJ Enterprise Co. Ltd.
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Thailand. All rubber chemicals including zinc oxide (ZnO), stearic acid, N-Cyclohexyl-2benzothaiazolesulfenamide (CBS) co-agent and sulfur were commercial grade.
2.2 Composite preparation
NR, PALF and all rubber ingredients were mixed on a two-roll mill. The nip gap, speed and mixing time were kept the same for all formulations. The total mixing time was 17 mins. The formulations of all the compounds are shown in Table 1. The amount of PALF was fixed at 10 phr and carbon black (CB) was varied from 0 to 30 phr. PALF and carbon
ACCEPTED MANUSCRIPT black were denoted as F and CB with the amounts of added fillers in phr. The rubber compounds were vulcanized by compression molding in a hydraulic press with a pressure of
2.3 Characterization
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1500 psi and temperature of 160οC so as to yield a 1 mm thick sheet.
Cure behavior was studied at 160οC with a moving die rheometer (MDR) (Rheo TECH MD+, Alpha Technologies, Akron, USA).
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Tensile properties and tear strength of rubber composites were measured according to ISO 37 and ISO 34 but with thinner test pieces, respectively. Test specimens were cut in
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both longitudinal and transverse fiber orientations. The measurements were carried out on a universal testing machine (Instron 5566, High Wycombe, UK) fitted with a long travel contact-style extensometer. A crosshead speed of 500 mm/min and 1 kN load cell were used. Average values were determined from five samples. Hardness, Shore A, was determined
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according to ISO 7619 with a durometer (Wallace, H17A) using six positions of a specimen in order to determine an average value.
Swelling behavior was determined on a rectangular specimen cut from a vulcanized
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sheet and the value was expressed as percentage weight gain. The specimens were soaked in
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toluene at room temperature for 72 h and weight gain determined. The swelling was not used to quantify the crosslink density, but increases in the crosslinking were assumed to be reflected in decreases in the swelling coefficient. Tensile fracture surfaces of the rubber composites were observed with a scanning
electron microscope (SEM) (Hitachi Tabletop Microscope; model TM 1000, Japan).
3. Results 3.1 Cure characteristics
ACCEPTED MANUSCRIPT The cure characteristics of the various NR compounds containing different amounts of sulfur, co-agent and carbon black are displayed as torque-time curves in Fig 1. Fig. 1(a) serves as a reference comparator with various amounts of sulfur and co-agent CBS, but with no carbon and no fiber. The scorch times become shorter, while the maximum torques
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become higher, with increasing amount of sulfur (and so of crosslinking). The effect of adding various amounts of carbon black, but holding the sulfur and co-agent constant, again with no fiber, is shown in Fig 1(b). In the same way, scorch times become shorter while the
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maximum torque becomes greater with increasing amounts of carbon black. Finally, the cure characteristics of composites with different amounts carbon black, fixed amounts of sulfur
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and coagent, and with 10 phr of fiber are shown in Fig 1(c). The only difference between data in figures 1(b) and 1(c) is the presence of the fiber. The effects of filler, both PALF and carbon black, on the cure characteristics are similar. Addition of 10 phr PALF causes the torque to increase more than that caused by the same amount of carbon black. Addition of
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more carbon black causes a further increase in torque. Nevertheless, for the same total filler content, the composite with PALF displays the higher torque. Also, the torque increase per the amount of carbon black added reduces at the higher levels of carbon black. Numerical
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values of these cure characteristics are listed in Table 2.
3.2 Swelling behavior
The swelling behavior, quantified in terms of the swelling ratio, of cured composites
with and without fiber are shown in Fig 2. The swelling ratios of the composites decrease with increasing amounts of sulfur and carbon black. The lower swelling ratio indicates a higher cross-link density and/or a higher proportion of bound rubber [21, 22] in the rubber matrix. These results agree very well with the observations of maximum torque presented above.
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3.3 Tensile properties Fig. 3 displays the stress-strain curves in the longitudinal direction of PALF/CB-NR composites and of CB-NR composites with different amounts of carbon black.
NR
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composites with only PALF exhibit a sharp rise in stress at low strain region but fail at relatively low tensile strengths. For composites with PALF/CB hybrid filler, the extensibility decreases with increasing carbon black content. The stress displays an upturn at lower strains
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and the tensile stress at break increases.
The effect on the stress-strain curves of the curing systems (CV1, CV2 and CV3) with
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different carbon black loadings is illustrated in Fig. 4. A similar pattern of behavior is seen. With increasing sulfur content, i.e. from CV1 to CV3, the strain at which the stress upturn occurs and strain at break are both reduced. The effect of carbon black loading can be seen by comparing Fig. 4 (a), (b) and (c). Increasing the carbon black content has a similar effect
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to increasing the sulfur content. The stress at break increases at low carbon black content but remains relatively unchanged at higher contents. This exemplifies the effect of the carbon particles in holding bound, and partially bound, rubber molecules [21, 22].
Further
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mechanical properties can be extracted from these stress-strain curves and their average
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values are shown against total filler content in Figs. 5 and 6. Again, the effects achieved by varying the amounts of carbon black and sulfur of PALF/CB-NR composites are superior to those seen in the absence of PALF. This demonstrates that the stress-strain behavior of the fiber-filled composites can more easily be manipulated to suit the intended applications.
3.4 Tear strength Tear strengths of PALF/CB-NR composites in both longitudinal and transverse directions are shown in Fig. 7. Tear strength in the longitudinal direction increases slightly
ACCEPTED MANUSCRIPT with increasing either carbon black or sulfur content, while those in the transverse direction show a more complex response. At low and high sulfur content (CV1 and CV3), tear strength increases with increasing carbon black content and then decreases. Tear strength for medium sulfur content (CV2) does not change much with carbon black content.
Tear
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strength for all sulfur contents seems to reach similar values at the highest carbon black contents. Presumably, this is related to the increase in tensile strength of the matrix rubber
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after the addition of the appropriate particulate filler [7-8, 19].
3.5 Hardness
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Fig. 8 displays the hardness of PALF/CB-NR composites with different amounts of sulfur and carbon black. As expected, the hardness increases with increasing amounts of carbon black and sulfur. This behavior is in accordance with observations of PALF with
3.6 Fracture surface
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silica or carbon black in nitrile butadiene rubber (NBR) composites [7, 8].
Tensile fractured surfaces (in the longitudinal direction) of PALF/CB-NR composites
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(F10CB30CV3) is displayed in Fig 9. Composites with PALF (F10CV3 and F10CB30CV3)
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display similar fracture surfaces with a large number of long protruding fibers. Hence, it can be said that failure occurs predominantly by fiber de-bonding and pullout. Note that the fracture plane is perpendicular to the fiber alignment direction but the pulled-out fibers seem to align themselves to the plane. This should not be taken as the original fiber direction in the composite but the result of sudden retraction after the matrix failure.
4. Discussion
ACCEPTED MANUSCRIPT We now consider the above results in a general interpretation of the effect of crosslink density and carbon black content on stress-strain behavior of PALF/CB-NR composites. Both sulfur and carbon black result in higher effective cross-link densities but via different mechanisms. The former form chemical bonds directly on the rubber network while
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the latter hold and bound rubber molecules onto its surface. These effects impart greater stiffness or hardness to the matrix rubber [1, 3, 5]. According to composite theories [9-14], the increase in stiffness would then, in turn, affect the stress transfer pattern along the
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reinforcing fiber and so the stress-strain behavior, especially in the low strain regions. However, there appear to be counteracting effects of carbon black and sulfur (cf. Fig. 5 (a)).
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At low and high carbon black contents (0 and 30), the modulus at 10% strain increases with increasing sulfur content. This is less apparent for medium carbon black contents. At high strains, where the matrix-fiber interface has been broken, the composite behavior can be explained by consideration of the rubber matrix. By increasing sulfur and carbon black
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contents, the effective cross-link density increases and the chain length between cross-link points decreases. As a result, the strain at which this chain length becomes taut decreases and the stress upturn takes place. The presence of carbon black allows the matrix to extend
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without breaking by blunting the sharp cracks that start to form at de-bonded fiber ends [8,
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20].
5. Conclusions
Mechanical properties, or specifically stress-strain behavior, of PALF/CB-NR
composites can be manipulated by changing the cross-link density. This, in turn, can be achieved by changing either the sulfur (chemical crosslinks) or the carbon black (adsorbed bound rubber) content. While PALF changes the stress-strain curve in the very low strain region, sulfur and carbon black affect the curve in the moderate and high strain regions. An
ACCEPTED MANUSCRIPT increase in either sulfur or carbon black content (or both) shifts the point where the stress upturn occurs to lower strains, thus shortening the mid-strain plateau region of the stressstrain curve. Although the mid-strain effects of sulfur and carbon black are similar, carbon black offers greater advantage in providing greater elongation at break.
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The ability to manipulate the stress-strain curve of the PALF composites offers a great opportunity for designing materials to suit different requirements of engineering applications.
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Acknowledgements
Partial financial support from the Center of Excellence for Innovation in Chemistry
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(PERCH-CIC), and the Office of the Higher Education Commission, Ministry of Education is gratefully acknowledged.
References
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[1] A.Y. Coran, Vulcanization, in: J.E. Mark, B. Erman, F.R. Eirich (Ed.), Science and Technology of Rubber, third ed., Elsevier Academic Press, San Diego, 2005, pp. 321366.
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Conventional and efficient of crosslinking of natural rubber, KGK Kautschuk Gummi Kunststoffe 58 (2005) 638-643. [3] J.B. Donnet, E. Custodero, Reinforcement of Elastomers by Particulate Fillers, in: J.E.Mark, B. Erman, F.R. Eirich (Ed.), Science and Technology of Rubber, third ed., Elsevier Academic Press, San Diego, 2005, pp. 367-400. [4] S. Wolff, Chemical Aspects of Rubber Reinforcement by Fillers, Rubber Chem. Technol. 69 (1996) 325-346.
ACCEPTED MANUSCRIPT [5] M. Plavsic, Interactions of Nanostructured Fillers with Polymer Networks-Transition from Nano-to Macro scale, in: A.M. Spasic, J.P. Hsu (Ed.), Finely dispersed particles Micro-, Nano-, and Atto-Engineering, Taylor and Francis CRC press, United States of America, 2006, pp. 131-155.
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[7] U. Wisittanawat, S. Thanawan, T. Amornsakchai, Remarkable improvement of failure strain of preferentially aligned short pineapple leaf fiber reinforced nitrile rubber
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composites with silica hybridization, Polym. Test. 38 (2014) 91-99.
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[9] H.L. Cox, The elasticity and strength of paper and other fibrous materials, J. Appl. Phys.
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modulus of short-fiber (or Whisker) reinforced metal matrix composites, J. Appl. Mech.
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59 (1992) 176–182.
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ACCEPTED MANUSCRIPT [14] J.C. Halpin, Stiffness and expansion estimates for oriented short fiber composites, J. Compos. Mater. 3 (1969) 732–734. [15] A.P. Foldi, Short-fibre-reinforced rubber: a new kind of composite, in:T.L. Vigo, B.J. Kinzig (Ed.), Composite Applications: the Role of Matrix, Fibre and Interface, VCH,
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New York, 1992, pp. 133-177.
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reinforcement: 1. A novel extraction method for short pineapple leaf fiber, Ind. Crops
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and Technology of Rubber, third ed., Elsevier Academic Press, San Diego, 2005,
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[20] K. Prukkaewkanjana, T. Amornsakchai, An anomalous reinforcement of ordinarily weak synthetic rubber, J. Polym. Res. 22 (2015) 1-6. [21] B. Meissner, Theory of bound rubber, J. Appl. Polym. Sci. 18 (1974) 2483–2491. [22] G. Heinrich, M. Klüppel, T.A. Vilgis, Reinforcement of elastomers, Curr. Opin. Solid State Mater. Sci. 6 (2002) 195–203.
ACCEPTED MANUSCRIPT Table Captions Table 1. Formulations of rubber composites. F and CB represent the amount in phr of PALF and carbon black in the composites.
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Table 2. Cure characteristics of different NR composites.
ACCEPTED MANUSCRIPT Table 1 Formulations of rubber composites. F and CB represent the amount in phr of PALF and carbon black in the composites. Amount (phr) Compound/Ingredient CV1
100
-
CB10-40CV1
PALF ZnO Stearic acid CBS* Sulfur -
5
2
1
2
100 10-40
-
5
2
1
2
F10-30CBCV1
100 10-30
10
5
2
1
2
CV2
100
-
5
2
1.5
3
CBCV2
100 10-40
-
5
2
1.5
3
F10-30CBCV2
100 10-30
10
5
2
1.5
3
CV3
100
-
5
2
2
4
CB10-40CV3
100 10-30
-
5
2
2
4
F10-30CBCV3
100 10-30
10
5
2
2
4
-
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CB
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NR
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* CBS = N-Cyclohexyl-2-benzothaiazolesulfenamide
ACCEPTED MANUSCRIPT Table 2 Cure characteristics of different NR composites MH (dNm)
ts2 (min)
tc90 (min)
CV1
0.05
5.06
2.5
5.5
CV2
0.07
6.91
2.3
4.8
CV3
0.07
9.09
1.9
3.8
F10CV1
0.09
6.84
F10CV2
0.04
10.07
F10CV3
0.09
12.45
F10CB10CV1
0.15
9.82
F10CB10CV2
0.16
12.40
F10CB10CV3
0.16
F10CB20CV1
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ML (dNm)
1.5
2.8
1.6
2.7
1.3
2.2
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Compound
3.2
1.4
2.6
14.71
1.1
2.2
0.17
10.12
1.5
3.1
F10CB20CV2
0.04
12.83
1.2
2.4
F10CB20CV3
0.19
17.08
1.1
2.2
0.13
11.09
1.3
2.9
0.21
16.53
1.1
2.7
0.14
19.61
0.9
2.0
F10CB30CV2
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F10CB30CV3
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F10CB30CV1
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ACCEPTED MANUSCRIPT 1
Figure Captions
2
Fig. 1 Cure curves of various NR compounds. (a) Varying amounts of sulfur and co-agent but with no carbon and no fiber. (b) Varying amounts of carbon black with fixed
4
sulfur and coagent but with no fiber. (c) The same variation in carbon black content
5
as in Fig. 1(b) but with 10 phr of fiber. The composition codes are listed in Table 1.
6 7
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3
Fig. 2 Swelling ratio of PALF/CB-NR composites. The composition codes are listed in Table 1.
Fig. 3 Stress-strain curves of PALF/CB-NR composites (open symbols) and CB-NR
9
composites (closed symbols) in the basic crosslinking formulation (CV1). composition codes are listed in Table 1.
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10
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8
The
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Fig. 4 Stress-strain curves of PALF/CB-NR composites with varying amounts of sulfur. (a)
12
with the carbon content constant at CB10 , with varying sulfur and coagent quantities;
13
open symbols with fiber at 10 phr, closed symbols without fiber (b) the same but with
14
carbon content CB20 and (c) the same but with carbon content CB30.
15
composition codes are listed in Table 1.
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The
Fig. 5 (a) Modulus at 10% strain and (b) tensile strength of PALF/CB-NR and control (no
17
filler) NR composites in the longitudinal direction. In each case with filler the PALF
18
was 10 phr. The three colors represent the three sulfur and coagent formulations.
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19
Fig. 6 (a) Strain at the upturn and (b) strain at break in the longitudinal direction of
20
PALF/CB-NR composites as a function of the total filler content.. In each case with
21 22
filler the PALF was 10 phr. The three colors represent the three sulfur and coagent
formulations. The composition codes are listed in Table 1.
23
Fig. 7 Tear strength of PALF/CB-NR composites as a function of the total filler content (a)
24
in the longitudinal and (b) in the transverse directions. In each case with filler the
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PALF was 10 phr.
The three colors represent the three sulfur and coagent
26
formulations. The composition codes are listed in Table 1. Fig. 8 Hardness of PALF/CB-NR composites as a function of the total filler content. In each
28
case with filler the PALF was 10 phr. The three colors represent the three sulfur and
29
coagent formulations. The composition codes are listed in Table 1.
30 31
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Fig. 9 SEM micrograph of tensile fractured specimen of a representative PALF/CB-NR composite (F10CB30CV3).
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12
12
(a)
(b)
(c)
10
10
8
8
8
6
6
6
4
4 CV1 CV2 CV3
2 0
4 CV1 CB10CV1 CB20CV1 CB30CV1 CB40CV1
2 0
0
2
4
6
8 10
0
2
4
6
8
Time (min)
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0 0
2
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10
CV1 F10CV1 F10CB10CV1 F10CB20CV1 F10CB30CV1
2
4
6
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Torque (dNm)
10
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12
8
10
Fig. 1 Cure curves of various NR compounds. (a) Varying amounts of sulfur and co-agent
36
but with no carbon and no fiber. (b) Varying amounts of carbon black with fixed
37
sulfur and coagent but with no fiber. (c) The same variation in carbon black content
38
as in Fig. 1(b) but with 10 phr of fiber. The composition codes are listed in Table 1.
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ACCEPTED MANUSCRIPT 5
2
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3
CBCV1 FCBCV1 CBCV2 FCBCV2 CBCV3 FCBCV3
1
0 10
20
30
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Swelling ratio
4
40
50
Total filler content (phr) 40 41
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Table 1.
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Fig. 2 Swelling ratio of PALF/CB-NR composites. The composition codes are listed in
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CV1 CB10CV1 CB20CV1 CB30CV1 CB40CV1
35
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25 20 15 10
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Stress (MPa)
30
F10CV1 F10CB10CV1 F10CB20CV1 F10CB30CV1
0 0
100
200
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300
400
500
600
700
800
Strain (%)
45
Fig. 3 Stress-strain curves of PALF/CB-NR composites (open symbols) and CB-NR
47
composites (closed symbols) in the basic crosslinking formulation (CV1).
48
composition codes are listed in Table 1.
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The
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30 25 20 15
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Stress (MPa)
(a)
CB10CV1 CB10CV2 CB10CV3 F10CB10CV1 F10CB10CV2 F10CB10CV3
35
10 5 0
51
40
20 15 10 5
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0 40
(c)
CB30CV1 CB30CV2 CB30CV3 F10CB30CV1 F10CB30CV2 F10CB30CV3
35 30 25
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Stress (MPa)
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Stress (MPa)
30
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(b)
CB20CV1 CB20CV2 CB20CV3 F10CB20CV1 F10CB20CV2 F10CB20CV3
35
20
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15 10 5 0
0
53
100
200
300
400
500
600
Strain (%)
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Fig. 4 Stress-strain curves of PALF/CB-NR composites with varying amounts of sulfur. (a)
55
with the carbon content constant at CB10, with varying sulfur and coagent quantities;
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open symbols with fiber at 10 phr, closed symbols without fiber (b) the same but with
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carbon content CB20 and (c) the same but with carbon content CB30. The
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composition codes are listed in Table 1.
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5
(a)
CV1 CV2 CV3
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4 3 2 1
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Modulus 10% strain (MPa)
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0
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40 CV1 CV2 CV3
30
20 15 10 5
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25
Control
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(b)
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Tensile strength (MPa)
35
Filler 10
Filler 20
Filler 30
Filler 40
Total filler content (phr)
62
Fig. 5 (a) Modulus at 10% strain and (b) tensile strength of PALF/CB-NR and control (no
63
filler) NR composites in the longitudinal direction. In each case with filler the PALF
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was 10 phr. The three colors represent the three sulfur and coagent formulations.
65
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(a)
FCBCV1 FCBCV2 FCBCV3
700 600
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500 400 300 200
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Strain at the upturn (%)
800
100
FCBCV1 FCBCV2 FCBCV3
700 600
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500 400 300 200
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100
(b)
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Strain at break (%)
800
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0 66
0
0
67
10
20
30
40
50
Total filler content (phr)
68
Fig. 6 (a) Strain at the upturn and (b) strain at break in the longitudinal direction of
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PALF/CB-NR composites as a function of the total filler content.. In each case with
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filler the PALF was 10 phr. The three colors represent the three sulfur and coagent
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formulations. The composition codes are listed in Table 1.
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100
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80 60 40
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Tear strength (N/mm)
120
(a)
CV1 CV2 CV3
20 0
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120
CV1 CV2 CV3
60 40 20
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80
(b)
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100
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Tear strength (N/mm)
140
0
Control
73
Filler 10
Filler 20
Filler 30
Filler 40
Total filler content (phr)
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Fig. 7 Tear strength of PALF/CB-NR composites as a function of the total filler content (a)
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in the longitudinal and (b) in the transverse directions. In each case with filler the
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PALF was 10 phr.
77
formulations. The composition codes are listed in Table 1.
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The three colors represent the three sulfur and coagent
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CV1 CV2 CV3
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60 50 40 30 20 10 0 Filler 10
Filler 20
Filler 30
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Control
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Hardness
70
Filler 40
Total filler content (phr) 79
Fig. 8 Hardness of PALF/CB-NR composites as a function of the total filler content. In each
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case with filler the PALF was 10 phr. The three colors represent the three sulfur and
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coagent formulations. The composition codes are listed in Table 1.
85
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84
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83
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80
86 87
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90
composite (F10CB30CV3).
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Fig. 9 SEM micrograph of tensile fractured specimen of a representative PALF/CB-NR
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S0142-9418(16)30500-1
DOI:
10.1016/j.polymertesting.2016.07.002
Reference:
POTE 4702
To appear in:
Polymer Testing
Received Date: 26 May 2016 Accepted Date: 2 July 2016
Please cite this article as: P. Pittayavinai, S. Thanawan, T. Amornsakchai, Manipulation of mechanical properties of short pineapple leaf fiber reinforced natural rubber composites through variations in cross-link density and carbon black loading, Polymer Testing (2016), doi: 10.1016/ j.polymertesting.2016.07.002. 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 Material Properties Manipulation of mechanical properties of short pineapple leaf fiber reinforced natural rubber composites through variations in cross-link density and carbon black loading
Polymer Science and Technology Program, Department of Chemistry and Center of
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1
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Pitchapa Pittayavinai1, Sombat Thanawan2, Taweechai Amornsakchai*1, 3, 4
Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University,
2
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Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand Rubber Technology Research Center, Faculty of Science, Mahidol University,
Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand 3
Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University,
4
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Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand Center of Sustainable Energy and Green Materials, Faculty of Science, Mahidol University,
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Phuttamonthon 4 Road, Salaya, Phuttamonthon District, Nakhon Pathom 73170, Thailand
* Corresponding author
Tel: (662) 441-9816 ext. 1161 Fax: (662) 441-9322 Email: [email protected]
ACCEPTED MANUSCRIPT Abstract The aim of this paper is to demonstrate that the stress-strain behavior of natural rubber reinforced with short pineapple leaf fiber (PALF) can easily be manipulated by changing the cross-link density and the amount of carbon black (CB) primary filler. This gives more
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manageable control of mechanical properties than is possible with conventional particulate fillers alone. This type of hybrid rubber composite displays a very sharp rise in stress at very low strains, and then the stress levels off at medium strains before turning up again at the
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highest strains. The composites studied here contain a fixed amount of PALF at 10 part (by weight) per hundred rubber (phr) and varying carbon black contents from 0 to 30 phr. To
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change the cross-link density, the amount of sulfur was varied from 2 to 4 phr. Swelling ratio results indicate that composites prepared with greater amounts of sulfur and carbon black have greater cross-link densities. Consequently, this affects the stress-strain behavior of the composites. The greater the cross-link density, the less is the strain at which the stress upturn
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occurs. Variations in the rate of stress increase (although not the stress itself) in the very low strain region, while dependent on fillers, are not dependent on the crosslink density. The effect of changes in crosslinking is most obvious in the high strain region. Here, the rate of
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stress increase becomes larger with increasing cross-link density. Hence, we demonstrate
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that the use of PALF filler, along with the usual carbon primary filler, provides a convenient method for the manipulation of the stress-strain relationships of the reinforced rubber. Such composites can be prepared with a controllable, wide range of mechanical behavior for specific high performance engineering applications.
Keywords: natural rubber; pineapple leaf fiber; natural fiber, hybrid composite; cross-link density
ACCEPTED MANUSCRIPT 1. Introduction Rubber is one of the most versatile materials. With additives, it can be formulated so that its properties span a very wide range from very soft to very hard. This can be done either by changing the vulcanization system [1, 2] or by the addition of various types of filler [3].
carbon black, silica and calcium carbonate [4, 5].
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The most common types of filler used in the rubber industry are particulate materials such as For applications where very large
deformations are undesirable, the rubber may be reinforced with textile fabrics. The use of
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short fiber fillers allows much greater flexibility than when textile fabrics are used, although this is at the expense of strength. Recently, our group has shown that short pineapple leaf
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fiber (PALF) can be used to reinforce nitrile rubber (NBR) very effectively [6-8]. The reinforced rubber shows a very distinct stress-strain curve in which the stress rises sharply on stretching. It has also been shown that properties of this reinforced rubber can be changed by using particulate filler along with the PALF. In order to widen the applications of this type of
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rubber composite, it is important that the composite properties can be adjusted accordingly. Generally, theories for the properties of particulate filled rubber will be different from those for short fiber filled rubber. Properties of the former can be predicted with Cox’s shear
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lag theory [9], its modified theory [10-13] and the Halpin-Tsai equation [14]. However, there
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is no generally accepted theory, covering the whole strain range, for the latter. Short fiber reinforced composite theories [15-18], which are more suitable for small strain deformation, may be used as a guide. For a selected fiber system (type and amount), properties of the composite depend on the shear modulus of the continuous phase. For an isotropic matrix, this is determined by its Young’s modulus. Hence, the easy manipulation of the properties of a matrix rubber, as described above, can yield fiber and particulate filler composites with different properties.
ACCEPTED MANUSCRIPT In this work, the effect of cross-link density and carbon black content on the properties of PALF reinforced natural rubber (NR) composites was studied. Different crosslink densities were obtained by changing the sulfur content. PALF was fixed at 10 phr and
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carbon black content varied from 0 to 30 phr.
2. Experimental 2.1 Materials
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2.1.1 PALF
Pineapple leaf fiber was prepared by the milling technique developed in our
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laboratory [17]. The fresh pineapple leaves were collected from Ban Yang District, Amphoe Nakhon Thai, Phitsanulok Province, Thailand. The fresh leaves were cut across the long axis into 6 mm long pieces. After that, the leaves were fed into a disc milling machine. Then, the fiber bundles and soft tissues were mashed into paste. The paste was dried and sieved to
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separate the PALF from non-fibrous materials. The shape and size distribution this PALF have been reported elsewhere [6, 17-18]. 2.1.2 Rubbers and rubber chemicals
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Natural rubber (NR) was STR5L grade supplied by MBJ Enterprise Co. Ltd.
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Thailand. All rubber chemicals including zinc oxide (ZnO), stearic acid, N-Cyclohexyl-2benzothaiazolesulfenamide (CBS) co-agent and sulfur were commercial grade.
2.2 Composite preparation
NR, PALF and all rubber ingredients were mixed on a two-roll mill. The nip gap, speed and mixing time were kept the same for all formulations. The total mixing time was 17 mins. The formulations of all the compounds are shown in Table 1. The amount of PALF was fixed at 10 phr and carbon black (CB) was varied from 0 to 30 phr. PALF and carbon
ACCEPTED MANUSCRIPT black were denoted as F and CB with the amounts of added fillers in phr. The rubber compounds were vulcanized by compression molding in a hydraulic press with a pressure of
2.3 Characterization
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1500 psi and temperature of 160οC so as to yield a 1 mm thick sheet.
Cure behavior was studied at 160οC with a moving die rheometer (MDR) (Rheo TECH MD+, Alpha Technologies, Akron, USA).
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Tensile properties and tear strength of rubber composites were measured according to ISO 37 and ISO 34 but with thinner test pieces, respectively. Test specimens were cut in
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both longitudinal and transverse fiber orientations. The measurements were carried out on a universal testing machine (Instron 5566, High Wycombe, UK) fitted with a long travel contact-style extensometer. A crosshead speed of 500 mm/min and 1 kN load cell were used. Average values were determined from five samples. Hardness, Shore A, was determined
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according to ISO 7619 with a durometer (Wallace, H17A) using six positions of a specimen in order to determine an average value.
Swelling behavior was determined on a rectangular specimen cut from a vulcanized
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sheet and the value was expressed as percentage weight gain. The specimens were soaked in
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toluene at room temperature for 72 h and weight gain determined. The swelling was not used to quantify the crosslink density, but increases in the crosslinking were assumed to be reflected in decreases in the swelling coefficient. Tensile fracture surfaces of the rubber composites were observed with a scanning
electron microscope (SEM) (Hitachi Tabletop Microscope; model TM 1000, Japan).
3. Results 3.1 Cure characteristics
ACCEPTED MANUSCRIPT The cure characteristics of the various NR compounds containing different amounts of sulfur, co-agent and carbon black are displayed as torque-time curves in Fig 1. Fig. 1(a) serves as a reference comparator with various amounts of sulfur and co-agent CBS, but with no carbon and no fiber. The scorch times become shorter, while the maximum torques
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become higher, with increasing amount of sulfur (and so of crosslinking). The effect of adding various amounts of carbon black, but holding the sulfur and co-agent constant, again with no fiber, is shown in Fig 1(b). In the same way, scorch times become shorter while the
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maximum torque becomes greater with increasing amounts of carbon black. Finally, the cure characteristics of composites with different amounts carbon black, fixed amounts of sulfur
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and coagent, and with 10 phr of fiber are shown in Fig 1(c). The only difference between data in figures 1(b) and 1(c) is the presence of the fiber. The effects of filler, both PALF and carbon black, on the cure characteristics are similar. Addition of 10 phr PALF causes the torque to increase more than that caused by the same amount of carbon black. Addition of
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more carbon black causes a further increase in torque. Nevertheless, for the same total filler content, the composite with PALF displays the higher torque. Also, the torque increase per the amount of carbon black added reduces at the higher levels of carbon black. Numerical
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values of these cure characteristics are listed in Table 2.
3.2 Swelling behavior
The swelling behavior, quantified in terms of the swelling ratio, of cured composites
with and without fiber are shown in Fig 2. The swelling ratios of the composites decrease with increasing amounts of sulfur and carbon black. The lower swelling ratio indicates a higher cross-link density and/or a higher proportion of bound rubber [21, 22] in the rubber matrix. These results agree very well with the observations of maximum torque presented above.
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3.3 Tensile properties Fig. 3 displays the stress-strain curves in the longitudinal direction of PALF/CB-NR composites and of CB-NR composites with different amounts of carbon black.
NR
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composites with only PALF exhibit a sharp rise in stress at low strain region but fail at relatively low tensile strengths. For composites with PALF/CB hybrid filler, the extensibility decreases with increasing carbon black content. The stress displays an upturn at lower strains
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and the tensile stress at break increases.
The effect on the stress-strain curves of the curing systems (CV1, CV2 and CV3) with
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different carbon black loadings is illustrated in Fig. 4. A similar pattern of behavior is seen. With increasing sulfur content, i.e. from CV1 to CV3, the strain at which the stress upturn occurs and strain at break are both reduced. The effect of carbon black loading can be seen by comparing Fig. 4 (a), (b) and (c). Increasing the carbon black content has a similar effect
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to increasing the sulfur content. The stress at break increases at low carbon black content but remains relatively unchanged at higher contents. This exemplifies the effect of the carbon particles in holding bound, and partially bound, rubber molecules [21, 22].
Further
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mechanical properties can be extracted from these stress-strain curves and their average
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values are shown against total filler content in Figs. 5 and 6. Again, the effects achieved by varying the amounts of carbon black and sulfur of PALF/CB-NR composites are superior to those seen in the absence of PALF. This demonstrates that the stress-strain behavior of the fiber-filled composites can more easily be manipulated to suit the intended applications.
3.4 Tear strength Tear strengths of PALF/CB-NR composites in both longitudinal and transverse directions are shown in Fig. 7. Tear strength in the longitudinal direction increases slightly
ACCEPTED MANUSCRIPT with increasing either carbon black or sulfur content, while those in the transverse direction show a more complex response. At low and high sulfur content (CV1 and CV3), tear strength increases with increasing carbon black content and then decreases. Tear strength for medium sulfur content (CV2) does not change much with carbon black content.
Tear
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strength for all sulfur contents seems to reach similar values at the highest carbon black contents. Presumably, this is related to the increase in tensile strength of the matrix rubber
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after the addition of the appropriate particulate filler [7-8, 19].
3.5 Hardness
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Fig. 8 displays the hardness of PALF/CB-NR composites with different amounts of sulfur and carbon black. As expected, the hardness increases with increasing amounts of carbon black and sulfur. This behavior is in accordance with observations of PALF with
3.6 Fracture surface
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silica or carbon black in nitrile butadiene rubber (NBR) composites [7, 8].
Tensile fractured surfaces (in the longitudinal direction) of PALF/CB-NR composites
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(F10CB30CV3) is displayed in Fig 9. Composites with PALF (F10CV3 and F10CB30CV3)
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display similar fracture surfaces with a large number of long protruding fibers. Hence, it can be said that failure occurs predominantly by fiber de-bonding and pullout. Note that the fracture plane is perpendicular to the fiber alignment direction but the pulled-out fibers seem to align themselves to the plane. This should not be taken as the original fiber direction in the composite but the result of sudden retraction after the matrix failure.
4. Discussion
ACCEPTED MANUSCRIPT We now consider the above results in a general interpretation of the effect of crosslink density and carbon black content on stress-strain behavior of PALF/CB-NR composites. Both sulfur and carbon black result in higher effective cross-link densities but via different mechanisms. The former form chemical bonds directly on the rubber network while
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the latter hold and bound rubber molecules onto its surface. These effects impart greater stiffness or hardness to the matrix rubber [1, 3, 5]. According to composite theories [9-14], the increase in stiffness would then, in turn, affect the stress transfer pattern along the
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reinforcing fiber and so the stress-strain behavior, especially in the low strain regions. However, there appear to be counteracting effects of carbon black and sulfur (cf. Fig. 5 (a)).
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At low and high carbon black contents (0 and 30), the modulus at 10% strain increases with increasing sulfur content. This is less apparent for medium carbon black contents. At high strains, where the matrix-fiber interface has been broken, the composite behavior can be explained by consideration of the rubber matrix. By increasing sulfur and carbon black
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contents, the effective cross-link density increases and the chain length between cross-link points decreases. As a result, the strain at which this chain length becomes taut decreases and the stress upturn takes place. The presence of carbon black allows the matrix to extend
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without breaking by blunting the sharp cracks that start to form at de-bonded fiber ends [8,
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20].
5. Conclusions
Mechanical properties, or specifically stress-strain behavior, of PALF/CB-NR
composites can be manipulated by changing the cross-link density. This, in turn, can be achieved by changing either the sulfur (chemical crosslinks) or the carbon black (adsorbed bound rubber) content. While PALF changes the stress-strain curve in the very low strain region, sulfur and carbon black affect the curve in the moderate and high strain regions. An
ACCEPTED MANUSCRIPT increase in either sulfur or carbon black content (or both) shifts the point where the stress upturn occurs to lower strains, thus shortening the mid-strain plateau region of the stressstrain curve. Although the mid-strain effects of sulfur and carbon black are similar, carbon black offers greater advantage in providing greater elongation at break.
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The ability to manipulate the stress-strain curve of the PALF composites offers a great opportunity for designing materials to suit different requirements of engineering applications.
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Acknowledgements
Partial financial support from the Center of Excellence for Innovation in Chemistry
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(PERCH-CIC), and the Office of the Higher Education Commission, Ministry of Education is gratefully acknowledged.
References
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[1] A.Y. Coran, Vulcanization, in: J.E. Mark, B. Erman, F.R. Eirich (Ed.), Science and Technology of Rubber, third ed., Elsevier Academic Press, San Diego, 2005, pp. 321366.
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[2] L. GonzaÂlez, A. RodrõÂguez, J. L. ValentõÂn, A. Marcos-FernaÂndez and P.Posadas,
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Conventional and efficient of crosslinking of natural rubber, KGK Kautschuk Gummi Kunststoffe 58 (2005) 638-643. [3] J.B. Donnet, E. Custodero, Reinforcement of Elastomers by Particulate Fillers, in: J.E.Mark, B. Erman, F.R. Eirich (Ed.), Science and Technology of Rubber, third ed., Elsevier Academic Press, San Diego, 2005, pp. 367-400. [4] S. Wolff, Chemical Aspects of Rubber Reinforcement by Fillers, Rubber Chem. Technol. 69 (1996) 325-346.
ACCEPTED MANUSCRIPT [5] M. Plavsic, Interactions of Nanostructured Fillers with Polymer Networks-Transition from Nano-to Macro scale, in: A.M. Spasic, J.P. Hsu (Ed.), Finely dispersed particles Micro-, Nano-, and Atto-Engineering, Taylor and Francis CRC press, United States of America, 2006, pp. 131-155.
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[6] 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) 20-27.
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[7] U. Wisittanawat, S. Thanawan, T. Amornsakchai, Remarkable improvement of failure strain of preferentially aligned short pineapple leaf fiber reinforced nitrile rubber
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composites with silica hybridization, Polym. Test. 38 (2014) 91-99.
[8] K. Prukkaewkanjana, S. Thanawan, T. Amornsakchai, High performance hybrid reinforcement of nitrile rubber using short pineapple leaf fiber and carbon black, Polym. Test. 45 (2015) 76-82.
3 (1952) 72–79.
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[9] H.L. Cox, The elasticity and strength of paper and other fibrous materials, J. Appl. Phys.
[10] S.V. Nair, H.G. Kim, Modification of the shear lag analysis for determination of elastic
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modulus of short-fiber (or Whisker) reinforced metal matrix composites, J. Appl. Mech.
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59 (1992) 176–182.
[11] C.H. Hsueh, A modified analysis for stress transfer in fibre-reinforced composites with bonded fibre ends, J. Mater. Sci. 30 (1995) 219–224. [12] I.J. Beyerlein, C.M. Landis, Shear-lag model for failure simulations of unidirectional fiber composites including matrix stiffness, Mech. Mater. 31 (1999) 331–350. [13] M.J. Starink, S. Syngellakis, Shear lag models for discontinuous composites: fibre end stresses and weak interface layers, Mater. Sci. Eng. 270 (2) (1999) 270–277.
ACCEPTED MANUSCRIPT [14] J.C. Halpin, Stiffness and expansion estimates for oriented short fiber composites, J. Compos. Mater. 3 (1969) 732–734. [15] A.P. Foldi, Short-fibre-reinforced rubber: a new kind of composite, in:T.L. Vigo, B.J. Kinzig (Ed.), Composite Applications: the Role of Matrix, Fibre and Interface, VCH,
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New York, 1992, pp. 133-177.
[16] K. Susheel, B.S. Kaith,, K.Inderjeet, Pretreatments of natural fibers and their
application as reinforcing material in polymer composites—a review, Polym. Eng. Sci.
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49 (2009) 1253–1272.
[17] N. Kengkhetkit, T. Amornsakchai, Utilisation of pineapple leaf waste for plastic
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reinforcement: 1. A novel extraction method for short pineapple leaf fiber, Ind. Crops
[18] N. Kengkhetkit, T. Amornsakchai, A new approach to ‘‘Greening’’ plastic composites using pineapple leaf waste for performance and cost effectiveness, Mater. Des. 55
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(2014) 292–299.
[19] A.N. Gent, Strength of elastomers, in: J.E. Mark, B. Erman, F.R. Eirich (Ed.), Science
pp. 469-475.
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and Technology of Rubber, third ed., Elsevier Academic Press, San Diego, 2005,
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[20] K. Prukkaewkanjana, T. Amornsakchai, An anomalous reinforcement of ordinarily weak synthetic rubber, J. Polym. Res. 22 (2015) 1-6. [21] B. Meissner, Theory of bound rubber, J. Appl. Polym. Sci. 18 (1974) 2483–2491. [22] G. Heinrich, M. Klüppel, T.A. Vilgis, Reinforcement of elastomers, Curr. Opin. Solid State Mater. Sci. 6 (2002) 195–203.
ACCEPTED MANUSCRIPT Table Captions Table 1. Formulations of rubber composites. F and CB represent the amount in phr of PALF and carbon black in the composites.
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Table 2. Cure characteristics of different NR composites.
ACCEPTED MANUSCRIPT Table 1 Formulations of rubber composites. F and CB represent the amount in phr of PALF and carbon black in the composites. Amount (phr) Compound/Ingredient CV1
100
-
CB10-40CV1
PALF ZnO Stearic acid CBS* Sulfur -
5
2
1
2
100 10-40
-
5
2
1
2
F10-30CBCV1
100 10-30
10
5
2
1
2
CV2
100
-
5
2
1.5
3
CBCV2
100 10-40
-
5
2
1.5
3
F10-30CBCV2
100 10-30
10
5
2
1.5
3
CV3
100
-
5
2
2
4
CB10-40CV3
100 10-30
-
5
2
2
4
F10-30CBCV3
100 10-30
10
5
2
2
4
-
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CB
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NR
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* CBS = N-Cyclohexyl-2-benzothaiazolesulfenamide
ACCEPTED MANUSCRIPT Table 2 Cure characteristics of different NR composites MH (dNm)
ts2 (min)
tc90 (min)
CV1
0.05
5.06
2.5
5.5
CV2
0.07
6.91
2.3
4.8
CV3
0.07
9.09
1.9
3.8
F10CV1
0.09
6.84
F10CV2
0.04
10.07
F10CV3
0.09
12.45
F10CB10CV1
0.15
9.82
F10CB10CV2
0.16
12.40
F10CB10CV3
0.16
F10CB20CV1
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ML (dNm)
1.5
2.8
1.6
2.7
1.3
2.2
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Compound
3.2
1.4
2.6
14.71
1.1
2.2
0.17
10.12
1.5
3.1
F10CB20CV2
0.04
12.83
1.2
2.4
F10CB20CV3
0.19
17.08
1.1
2.2
0.13
11.09
1.3
2.9
0.21
16.53
1.1
2.7
0.14
19.61
0.9
2.0
F10CB30CV2
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F10CB30CV3
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F10CB30CV1
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ACCEPTED MANUSCRIPT 1
Figure Captions
2
Fig. 1 Cure curves of various NR compounds. (a) Varying amounts of sulfur and co-agent but with no carbon and no fiber. (b) Varying amounts of carbon black with fixed
4
sulfur and coagent but with no fiber. (c) The same variation in carbon black content
5
as in Fig. 1(b) but with 10 phr of fiber. The composition codes are listed in Table 1.
6 7
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3
Fig. 2 Swelling ratio of PALF/CB-NR composites. The composition codes are listed in Table 1.
Fig. 3 Stress-strain curves of PALF/CB-NR composites (open symbols) and CB-NR
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composites (closed symbols) in the basic crosslinking formulation (CV1). composition codes are listed in Table 1.
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10
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8
The
11
Fig. 4 Stress-strain curves of PALF/CB-NR composites with varying amounts of sulfur. (a)
12
with the carbon content constant at CB10 , with varying sulfur and coagent quantities;
13
open symbols with fiber at 10 phr, closed symbols without fiber (b) the same but with
14
carbon content CB20 and (c) the same but with carbon content CB30.
15
composition codes are listed in Table 1.
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The
Fig. 5 (a) Modulus at 10% strain and (b) tensile strength of PALF/CB-NR and control (no
17
filler) NR composites in the longitudinal direction. In each case with filler the PALF
18
was 10 phr. The three colors represent the three sulfur and coagent formulations.
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16
19
Fig. 6 (a) Strain at the upturn and (b) strain at break in the longitudinal direction of
20
PALF/CB-NR composites as a function of the total filler content.. In each case with
21 22
filler the PALF was 10 phr. The three colors represent the three sulfur and coagent
formulations. The composition codes are listed in Table 1.
23
Fig. 7 Tear strength of PALF/CB-NR composites as a function of the total filler content (a)
24
in the longitudinal and (b) in the transverse directions. In each case with filler the
ACCEPTED MANUSCRIPT 25
PALF was 10 phr.
The three colors represent the three sulfur and coagent
26
formulations. The composition codes are listed in Table 1. Fig. 8 Hardness of PALF/CB-NR composites as a function of the total filler content. In each
28
case with filler the PALF was 10 phr. The three colors represent the three sulfur and
29
coagent formulations. The composition codes are listed in Table 1.
30 31
RI PT
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Fig. 9 SEM micrograph of tensile fractured specimen of a representative PALF/CB-NR composite (F10CB30CV3).
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12
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(a)
(b)
(c)
10
10
8
8
8
6
6
6
4
4 CV1 CV2 CV3
2 0
4 CV1 CB10CV1 CB20CV1 CB30CV1 CB40CV1
2 0
0
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4
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8 10
0
2
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Time (min)
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0 0
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10
CV1 F10CV1 F10CB10CV1 F10CB20CV1 F10CB30CV1
2
4
6
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Torque (dNm)
10
RI PT
12
8
10
Fig. 1 Cure curves of various NR compounds. (a) Varying amounts of sulfur and co-agent
36
but with no carbon and no fiber. (b) Varying amounts of carbon black with fixed
37
sulfur and coagent but with no fiber. (c) The same variation in carbon black content
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as in Fig. 1(b) but with 10 phr of fiber. The composition codes are listed in Table 1.
EP AC C
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TE D
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CBCV1 FCBCV1 CBCV2 FCBCV2 CBCV3 FCBCV3
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0 10
20
30
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Swelling ratio
4
40
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Total filler content (phr) 40 41
TE D
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Table 1.
EP
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Fig. 2 Swelling ratio of PALF/CB-NR composites. The composition codes are listed in
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CV1 CB10CV1 CB20CV1 CB30CV1 CB40CV1
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RI PT
25 20 15 10
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Stress (MPa)
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F10CV1 F10CB10CV1 F10CB20CV1 F10CB30CV1
0 0
100
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M AN U
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300
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Strain (%)
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Fig. 3 Stress-strain curves of PALF/CB-NR composites (open symbols) and CB-NR
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composites (closed symbols) in the basic crosslinking formulation (CV1).
48
composition codes are listed in Table 1.
EP
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AC C
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TE D
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The
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30 25 20 15
RI PT
Stress (MPa)
(a)
CB10CV1 CB10CV2 CB10CV3 F10CB10CV1 F10CB10CV2 F10CB10CV3
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10 5 0
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40
20 15 10 5
TE D
0 40
(c)
CB30CV1 CB30CV2 CB30CV3 F10CB30CV1 F10CB30CV2 F10CB30CV3
35 30 25
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Stress (MPa)
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(b)
CB20CV1 CB20CV2 CB20CV3 F10CB20CV1 F10CB20CV2 F10CB20CV3
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20
AC C
15 10 5 0
0
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100
200
300
400
500
600
Strain (%)
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Fig. 4 Stress-strain curves of PALF/CB-NR composites with varying amounts of sulfur. (a)
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with the carbon content constant at CB10, with varying sulfur and coagent quantities;
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open symbols with fiber at 10 phr, closed symbols without fiber (b) the same but with
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carbon content CB20 and (c) the same but with carbon content CB30. The
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composition codes are listed in Table 1.
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(a)
CV1 CV2 CV3
RI PT
4 3 2 1
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Modulus 10% strain (MPa)
6
0
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40 CV1 CV2 CV3
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20 15 10 5
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Control
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(b)
EP
Tensile strength (MPa)
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Filler 10
Filler 20
Filler 30
Filler 40
Total filler content (phr)
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Fig. 5 (a) Modulus at 10% strain and (b) tensile strength of PALF/CB-NR and control (no
63
filler) NR composites in the longitudinal direction. In each case with filler the PALF
64
was 10 phr. The three colors represent the three sulfur and coagent formulations.
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(a)
FCBCV1 FCBCV2 FCBCV3
700 600
RI PT
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SC
Strain at the upturn (%)
800
100
FCBCV1 FCBCV2 FCBCV3
700 600
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(b)
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10
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Total filler content (phr)
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Fig. 6 (a) Strain at the upturn and (b) strain at break in the longitudinal direction of
69
PALF/CB-NR composites as a function of the total filler content.. In each case with
70
filler the PALF was 10 phr. The three colors represent the three sulfur and coagent
71
formulations. The composition codes are listed in Table 1.
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100
RI PT
80 60 40
SC
Tear strength (N/mm)
120
(a)
CV1 CV2 CV3
20 0
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120
CV1 CV2 CV3
60 40 20
EP
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(b)
TE D
100
AC C
Tear strength (N/mm)
140
0
Control
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Filler 10
Filler 20
Filler 30
Filler 40
Total filler content (phr)
74
Fig. 7 Tear strength of PALF/CB-NR composites as a function of the total filler content (a)
75
in the longitudinal and (b) in the transverse directions. In each case with filler the
76
PALF was 10 phr.
77
formulations. The composition codes are listed in Table 1.
78
The three colors represent the three sulfur and coagent
ACCEPTED MANUSCRIPT 100 90 80
CV1 CV2 CV3
RI PT
60 50 40 30 20 10 0 Filler 10
Filler 20
Filler 30
M AN U
Control
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Hardness
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Filler 40
Total filler content (phr) 79
Fig. 8 Hardness of PALF/CB-NR composites as a function of the total filler content. In each
81
case with filler the PALF was 10 phr. The three colors represent the three sulfur and
82
coagent formulations. The composition codes are listed in Table 1.
85
EP
84
AC C
83
TE D
80
86 87
TE D
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composite (F10CB30CV3).
EP
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Fig. 9 SEM micrograph of tensile fractured specimen of a representative PALF/CB-NR
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M AN U
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