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Comparison of the mechanical properties at similar hardness level of natural rubber filled with various reinforcing-fillers
Polymer Testing 28 (2009) 8–12
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Comparison of the mechanical properties at similar hardness level of natural rubber filled with various reinforcing-fillers N. Rattanasom a, b, *, S. Prasertsri c, T. Ruangritnumchai c a
Institute of Science and Technology for Research and Development, Mahidol University, Salaya, Nakhon Pathom 73170, Thailand Centre for Rubber Research and Technology, Mahidol University, Salaya, Nakhon Pathom 73170, Thailand c Department of Chemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailand b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2008 Accepted 13 August 2008
Commercially, the alteration of a rubber formulation is usually made in such a way as to keep the hardness of the rubber product constant. This is because a specific hardness of the rubber product sets the limit to its practical applications. Therefore, in this paper, natural rubber (NR) vulcanizates containing various fillers were prepared to have the same hardness level, and their mechanical properties were compared and related to the degree of filler dispersion. The results show that higher amounts of carbon black (CB) and silica are needed for CB- and silica-filled natural rubber vulcanizates to achieve the same hardness value as a NR vulcanizate containing 6 phr of montmorillonite clay. At equal loading of fillers, clay-filled vulcanizate exhibits higher modulus, hardness, tensile strength and compression set, but lower heat build-up resistance and crack growth resistance than those of the vulcanizates containing conventional fillers. For the vulcanizate having the same hardness value, CB-filled vulcanizate gives the better overall mechanical properties followed by the clay-filled and silica-filled vulcanizates, respectively. The explanation is given as the better dispersion of carbon black, as can be seen in the SEM micrograph. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Natural rubber Silica Carbon black Clay Mechanical properties Filler dispersion
1. Introduction Natural rubber (NR) exhibits outstanding properties such as green strength and tensile strength because it can crystallize spontaneously when it is strained. However, some properties of natural rubber such as modulus, hardness and abrasion resistance need to be improved for some specific applications. Carbon black and silica are the conventional reinforcing-fillers used to enhance the mechanical properties of various rubbers. In general, a carbon black-reinforced rubber exhibits higher modulus than a silica-reinforced one. However, silica provides a unique combination of tear strength, aging resistance and adhesion properties [1].
In recent years, nanoclays have attracted much attention because of their ability to enhance the mechanical properties of rubber vulcanizates [2–9]. The improvement in properties can be achieved at remarkably low clay content (less than 10 phr) if the clay silicate layers are able to disperse into the polymer matrix at the nanoscale level [7,8]. It has been reported that the incorporation of a small amount of clay gives rise to a more rigid material, which is reflected in a marked increase of hardness and modulus. The nanocomposites of natural rubber have been found to give optimum tensile strength and tear strength when containing 5–8 phr of organomodified montmorillonite [9]. Although, the effect of types of filler on the properties of rubber vulcanizates has been extensively investigated at
* Corresponding author. Institute of Science and Technology for Research and Development, Mahidol University, Salaya, Nakhon Pathom 73170, Thailand. Tel.: þ66 81 4313039; fax: þ66 2 4410511. E-mail address: [email protected] (N. Rattanasom). 0142-9418/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2008.08.004
N. Rattanasom et al. / Polymer Testing 28 (2009) 8–12
equal loading [9–11], it is also interesting to elucidate the effect of types of filler on the mechanical properties of the vulcanizates at the same hardness level. This is because a specific hardness is required for numerous industrial products. In this study, vulcanizates having the same hardness were prepared by adjusting the amount of each filler. Then, the mechanical properties of the vulcanizates were determined. Also, the mechanical properties of natural rubber vulcanizates filled with equal amounts of carbon black, silica and montmorillonite clay (6 phr) were compared. The degree of filler dispersion of all vulcanizates was also examined and related to their properties. 2. Experimental 2.1. Preparation of rubber/clay composite Concentrated natural rubber latex (60% dry rubber content, high ammonia) and the clay (Na-montmorillonite), with a cationic exchange capacity of 119 meq/100 g were used to prepare the rubber/clay composite. Concentrated NR latex was diluted with water to 30% dry rubber content. The amount of clay used was calculated such that the dried natural rubber would contain 6 phr of clay. The clay suspension was prepared before mixing with NR latex. The clay was added into the excess water (2 g per 100 ml of water) and was stirred vigorously for 4 h in order to achieve good dispersion of the silicate layers in the medium. Then, the mixture of clay suspension and NR latex was stirred gently for 30 min. Thereafter, the mixture was cast in a stainless steel tray and left at room temperature for a day. Finally, it was further dried in an oven at 50 C. The rubber/ clay obtained was further mixed with other ingredients to prepare the rubber compound. 2.2. Preparation of NR compounds and NR vulcanizates The compound formulations are given in Table 1. The conventional fillers used in this experiment were carbon black (N330) and precipitated silica (Hisil-233). The samples were designated as gum, C6, CB6, CB14, S6, S35. The letters ‘C’, ‘CB’ and ‘S’ refer to clay, carbon black and silica, respectively. The number followed the letters indicates the amount of filler in phr. The gum sample was prepared for comparing
Table 1 NR compound formulations Ingredients
Gum
C6
CB6
CB14
S6
S35
NR Clay Carbon black (N330) Silica (Hisil-233) Zinc oxide Stearic acid CBSa TMTDb Sulfur PEGc
100 – – – 5 2 1 0.1 2 –
100 6 – – 5 2 1 0.1 2 –
100 – 6 – 5 2 1 0.1 2 –
100 – 14 – 5 2 1 0.1 2 –
100 – – 6 5 2 1 0.1 2 2
100 – – 35 5 2 1 0.1 2 2
a b c
N-Cyclohexyl-2-benzothiazolesulfenamide. Tetramethylthiuram disulfide. Polyethylene glycol.
9
properties with those of the filled samples. All ingredients, except the curatives, were mixed with rubber (or rubber/ clay) in a laboratory-size internal mixer at a set temperature of 50 C with a rotor speed of 50 rpm and a fill factor of 0.7. The total mixing time in the internal mixer was 5.5 min. After discharging, the compound was mixed on a two rollmill for 1 min. Then, the curatives were added and further mixed for 4 min. Finally, 10 end-roll passes were made before sheeting off. The compounds were finally compression molded at 150 C. The cure time used for preparing the NR vulcanizates was the time at which the rheometer torque increased to 90% of the total torque change on the cure curve. The mechanical properties, hardness, tensile strength, tear strength, heat build-up resistance and crack growth resistance, were measured on vulcanizates, a) containing equal amounts of conventional fillers and montmorillonite clay (6 phr), and b) containing different amounts of each filler such as to give the same hardness level. 2.3. Mechanical property measurement The hardness was measured using a Wallace Shore A durometer, according to ISO 7619-1. Compression molded sheets having a thickness of about 1.2 mm were used for tear and tensile testing. Tear and tensile properties of the specimens were measured following ISO 34-1 and ISO 37, respectively. Crescent test pieces were used for determining the tear strength. The measurements were carried out using an Instron Universal Tester (Model 4301) with a crosshead speed of 500 mm/min and initial clamp separation of 65 mm. The values of tear and tensile properties were the average of 4–5 specimens. The heat build-up and crack growth resistance of the vulcanizates were measured, in accordance with ISO 4666 and ISO 132 using Goodrich flexometer and De Mattia type machines, respectively. Dynamic compression set was evaluated using the same specimens as for heat build-up testing. The original height of the specimen was measured prior to heat build-up testing and the specimen was left at 25 C for 1 h after the heat build-up test. Thereafter, its final height was measured and the compression set was calculated using the following equation.
Dynamic compression setð%Þ ¼
Ho Hf
Ho 100
where Ho ¼ original height (mm). Hf ¼ final height (mm). The fracture surfaces of the vulcanizates were examined using a scanning electron microscope (SEM, JEOL JSM6301) in order to view the degree of filler dispersion. The samples were sputtered with gold before examination to prevent charging on the surface. 3. Results and discussion Hardness and 300% modulus of all vulcanizates are illustrated in Figs. 1 and 2, respectively. As expected, the gum gives the lowest hardness and modulus while hardness and modulus increase noticeably when 6 phr of clay is added to the NR. At equal amounts of filler, the clay-filled vulcanizate exhibits the highest stiffness, followed by
10
N. Rattanasom et al. / Polymer Testing 28 (2009) 8–12
35
Tensile strength (MPa)
Hardness (Shore A)
60
55
50
45
40
30 25 20 15 10 5 0
Gum
C6
CB6
CB14
S6
S35
Gum
C6
CB6
CB14
S6
S35
Fig. 1. Hardness of NR vulcanizates filled with various fillers at various contents.
Fig. 3. Tensile strength of NR vulcanizates filled with various fillers at various contents.
CB-filled and silica-filled, respectively. In addition, the results show that 14 phr of CB or 35 phr of silica is required for the vulcanizates to reach the same hardness level as the vulcanizate containing 6 phr of clay. Compared to CB, a higher amount of silica is needed in order to achieve the same hardness level as the clay-filled vulcanizate. This is thought to be due to the decrease in crosslink density when high silica loading is used. In a previous study, crosslink density of NR vulcanizates gradually decreases when silica loading is more than 20 phr [12]. The explanation is given as the adsorption of zinc complex on the silica surface, thus lowering the sulfur vulcanization efficiency. Tensile strength of all vulcanizates is shown in Fig. 3. As can be seen, clay-filled vulcanizate exhibits the highest tensile strength while tensile strength of the other vulcanizates is not much different. Likewise, tensile strength of gum and filled NR vulcanizates containing 50 phr of CB is shown to be similar [13]. The strain induced crystallization is known to be responsible for the high strength of gum NR vulcanizates. Fig. 4 displays the tear strength of various NR vulcanizates. The gum exhibits the lowest tear strength while the vulcanizates having equal amounts of fillers give similar tear strength. Although, it is established that spherical particles can blunt the crack tip more effectively than the plate-shaped filler having high aspect ratio particles [14], such a small amount of CB and silica (6 phr) used in this experiment may not be sufficient to effectively blunt the tear tip. However, the tear strength of CB- and silica-filled NR vulcanizates markedly increases when they were prepared to have similar hardness to that of clay-filled
vulcanizate. At similar hardness level, CB-filled sample exhibits similar tear strength to that of silica-filled vulcanizate, but much higher than that of the clay-filled vulcanizate. The higher amount of fillers in CB- and silica-filled vulcanizates may obstruct the tear path more effectively than that for the clay-filled vulcanizate. It has also been reported that if the clay platelets are aligned in the perpendicular direction to the applied force, the stress may be concentrated at the sharp edges of particles and promote earlier failure compared to spherical fillers [14]. Crack growth resistance, expressed as the length of crack growth (lower length of crack growth indicates higher crack growth resistance), is shown in Fig. 5. In this experiment, the gum exhibits the highest crack growth resistance. The explanation is given as its lower modulus resulting in the lower stress concentration at its crack tip [13,15]. For the vulcanizates having equal amounts of filler, clay-filled vulcanizate shows the lowest crack growth resistance followed by CB- and silica-filled vulcanizates, respectively. It is apparent that these results correspond well with their moduli. The highest modulus of C6 leads to the highest stress concentration at its crack tip compared to CB6 and S6. On the other hand, a decrease in crack growth resistance is observed for S35 although its modulus is similar to that of C6 and CB14. It is thought that the lower crack growth resistance of S35 results from its poor silica dispersion or lower filler-rubber interaction, which overrides the effect of the modulus. The fracture surfaces of NR vulcanizates filled with various fillers are illustrated in Fig. 6. As can be seen, the 180
Tear strength (N/mm)
300% Modulus (MPa)
8
6
4
2
0 Gum
C6
CB6
CB14
S6
S35
Fig. 2. 300% Modulus of NR vulcanizates filled with various fillers at various contents.
150 120 90 60 30 0
Gum
C6
CB6
CB14
S6
S35
Fig. 4. Tear strength of NR vulcanizates filled with various fillers at various contents.
Lenght of crack growth (mm)
N. Rattanasom et al. / Polymer Testing 28 (2009) 8–12
16
12
8
4
0
Gum
C6
CB6
CB14
S6
S35
Fig. 5. Crack growth resistance at 20 kilocycles of NR vulcanizates filled with various fillers at various contents.
11
dispersion of various fillers in NR is not much different when using 6 phr of filler. However, at the same hardness level, S35 has the poorest filler dispersion because the aggregate of silica can be readily observed in Fig. 6(e). It has been also shown in the previous study that filled rubber vulcanizate having poor silica dispersion exhibits lower crack growth resistance despite its lower modulus compared to other vulcanizates [12]. Dynamic compression set results for various NR vulcanizates are shown in Fig. 7. It is evident that compression set is low for the gum. When a low loading of CB or silica (6 phr) is added, the compression set is not significantly changed. Nevertheless, clay-filled vulcanizate gives somewhat higher compression set than that of CB- and silicafilled vulcanizates when equal amounts of fillers are used. At the same hardness level, silica-filled vulcanizate (S35)
Fig. 6. SEM micrographs of NR vulcanizates filled with various fillers at various contents; (a) C6, (b) CB6, (c) S6, (d) CB14 and (e) S35.
N. Rattanasom et al. / Polymer Testing 28 (2009) 8–12
6
12
5
10
Heat buildup (°C)
Compression set (%)
12
4 3 2 1 0 Gum
C6
CB6
CB14
S6
S35
8 6 4 2 0
Gum
C6
CB6
CB14
S6
S35
Fig. 7. Dynamic compression set of NR vulcanizates filled with various fillers at various contents.
Fig. 8. Heat build-up of NR vulcanizates filled with various fillers at various contents.
shows the highest compression set when compared to that of C6 and CB14. This is attributed to the poor filler-rubber interaction or poor filler dispersion when the silica loading is high. Another possible explanation is the decrease in crosslink density when silica loading is high. This result also agrees well with the previous study showing that the dynamic compression set tends to increase when the silica loading is more than 30 phr [12]. It can be seen in Fig. 8 that heat build-up of the vulcanizates containing a small amount of filler is not much different from that of the gum. However, it should be noted that the clay-filled sample shows slightly higher heat buildup than CB-filled and silica-filled vulcanizates when using equal amounts of filler. When higher amounts of CB or silica is added to prepare vulcanizates with similar hardness to that of C6, CB-filled sample (CB14) exhibits the same heat build-up value as that of C6, while heat build-up of silicafilled sample (S35) increases considerably. The possible explanation is also given as the combined effects of the poor silica dispersion and lower crosslink density of silicafilled sample when a high amount of silica is loaded. Heat build-up of a vulcanizate is reported to increase with decreasing crosslink density [13].
At the same hardness value, CB-filled vulcanizate gives the better overall mechanical properties followed by the clayfilled and silica-filled vulcanizates, respectively. The poor silica dispersion and lower crosslink density of silica-filled vulcanizate (S35) are thought to be the causes of its poorer properties compared to the other vulcanizates.
4. Conclusions The results show that modulus, hardness and tear strength of NR filled with conventional fillers and clay are superior to those of the gum. Hardness and modulus increase noticeably when 6 phr of clay is added. At equal loading of fillers, clay-filled NR vulcanizate exhibits higher modulus, hardness, tensile strength compression set but lower heat build-up resistance and crack growth resistance than those of NR vulcanizates containing conventional fillers. However, their tear strength is not much different. Compared to CB, a higher amount of silica is needed to achieve the same hardness as that of the clay-filled sample.
Acknowledgements The authors gratefully acknowledge Mr. Woothichai Thaicharoen for providing the clay/rubber composite used in this experiment. Sincere appreciation is extended to staff of National Metal and Materials Technology Center for carrying out the SEM measurement. References [1] A.S. Hashim, B. Azahari, Y. Ikeda, S. Kohjiya, Rubb. Chem. Technol. 71 (1998) 289–299. [2] L. Zhang, Y. Wang, Y. Wang, Y. Sui, D. Yu, J. Appl. Polym. Sci. 78 (2000) 1873–1878. [3] M. Kato, A. Tsukigase, H. Tanaka, A. Usuki, I. Inai, J. Polym. Sci. Part A: Polym. Chem. 44 (2006) 1182–1188. [4] H. Tan, A.I. Isayev, J. Appl. Polym. Sci. 109 (2008) 767–774. [5] S. Vargese, J. Karger-Kocsis, J. Appl. Polym. Sci. 91 (2004) 813–819. [6] M. Maiti, S. Sadhu, A.K. Bhowmick, J. Appl. Polym. Sci. 96 (2005) 443–451. [7] Y.T. Vu, J.E. Mark, L.H. Pham, M. Engelhardt, J. Appl. Polym. Sci. 82 (2001) 1391–1403. [8] P.L. Teh, Z.A. Mohd Ishak, A.S. Hashim, J. Karger-Kocsis, U.S. Ishiaku, J. Appl. Polym. Sci. 100 (2006) 1083–1092. [9] Y. Wang, H. Zhang, Y. Wu, J. Yang, L. Zhang, J. Appl. Polym. Sci. 96 (2005) 318–323. [10] S. Siriwardena, H. Ismail, U.S. Ishiaku, Polym. Int. 50 (2001) 707–713. [11] H.E. Trexler, M.C.H. Lee, J. Appl. Polym. Sci. 32 (1986) 3899–3912. [12] N. Rattanasom, T. Saowapark, C. Deeprasertkul, Polym. Test. 26 (2007) 369–377. [13] N. Rattanasom, U. Thammasiripong, K. Suchiva, J. Appl. Polym. Sci. 97 (2005) 1139–1144. [14] A.M. Riley, C.D. Paynter, P.M. McGenity, J.H. Adams, Plast. Rubb. Proc. Appl. 14 (1990) 85. [15] N. Rattanasom, O. Chaikumpollert, J. Appl. Polym. Sci. 90 (2003) 1793–1796.
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Comparison of the mechanical properties at similar hardness level of natural rubber filled with various reinforcing-fillers N. Rattanasom a, b, *, S. Prasertsri c, T. Ruangritnumchai c a
Institute of Science and Technology for Research and Development, Mahidol University, Salaya, Nakhon Pathom 73170, Thailand Centre for Rubber Research and Technology, Mahidol University, Salaya, Nakhon Pathom 73170, Thailand c Department of Chemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailand b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 June 2008 Accepted 13 August 2008
Commercially, the alteration of a rubber formulation is usually made in such a way as to keep the hardness of the rubber product constant. This is because a specific hardness of the rubber product sets the limit to its practical applications. Therefore, in this paper, natural rubber (NR) vulcanizates containing various fillers were prepared to have the same hardness level, and their mechanical properties were compared and related to the degree of filler dispersion. The results show that higher amounts of carbon black (CB) and silica are needed for CB- and silica-filled natural rubber vulcanizates to achieve the same hardness value as a NR vulcanizate containing 6 phr of montmorillonite clay. At equal loading of fillers, clay-filled vulcanizate exhibits higher modulus, hardness, tensile strength and compression set, but lower heat build-up resistance and crack growth resistance than those of the vulcanizates containing conventional fillers. For the vulcanizate having the same hardness value, CB-filled vulcanizate gives the better overall mechanical properties followed by the clay-filled and silica-filled vulcanizates, respectively. The explanation is given as the better dispersion of carbon black, as can be seen in the SEM micrograph. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Natural rubber Silica Carbon black Clay Mechanical properties Filler dispersion
1. Introduction Natural rubber (NR) exhibits outstanding properties such as green strength and tensile strength because it can crystallize spontaneously when it is strained. However, some properties of natural rubber such as modulus, hardness and abrasion resistance need to be improved for some specific applications. Carbon black and silica are the conventional reinforcing-fillers used to enhance the mechanical properties of various rubbers. In general, a carbon black-reinforced rubber exhibits higher modulus than a silica-reinforced one. However, silica provides a unique combination of tear strength, aging resistance and adhesion properties [1].
In recent years, nanoclays have attracted much attention because of their ability to enhance the mechanical properties of rubber vulcanizates [2–9]. The improvement in properties can be achieved at remarkably low clay content (less than 10 phr) if the clay silicate layers are able to disperse into the polymer matrix at the nanoscale level [7,8]. It has been reported that the incorporation of a small amount of clay gives rise to a more rigid material, which is reflected in a marked increase of hardness and modulus. The nanocomposites of natural rubber have been found to give optimum tensile strength and tear strength when containing 5–8 phr of organomodified montmorillonite [9]. Although, the effect of types of filler on the properties of rubber vulcanizates has been extensively investigated at
* Corresponding author. Institute of Science and Technology for Research and Development, Mahidol University, Salaya, Nakhon Pathom 73170, Thailand. Tel.: þ66 81 4313039; fax: þ66 2 4410511. E-mail address: [email protected] (N. Rattanasom). 0142-9418/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2008.08.004
N. Rattanasom et al. / Polymer Testing 28 (2009) 8–12
equal loading [9–11], it is also interesting to elucidate the effect of types of filler on the mechanical properties of the vulcanizates at the same hardness level. This is because a specific hardness is required for numerous industrial products. In this study, vulcanizates having the same hardness were prepared by adjusting the amount of each filler. Then, the mechanical properties of the vulcanizates were determined. Also, the mechanical properties of natural rubber vulcanizates filled with equal amounts of carbon black, silica and montmorillonite clay (6 phr) were compared. The degree of filler dispersion of all vulcanizates was also examined and related to their properties. 2. Experimental 2.1. Preparation of rubber/clay composite Concentrated natural rubber latex (60% dry rubber content, high ammonia) and the clay (Na-montmorillonite), with a cationic exchange capacity of 119 meq/100 g were used to prepare the rubber/clay composite. Concentrated NR latex was diluted with water to 30% dry rubber content. The amount of clay used was calculated such that the dried natural rubber would contain 6 phr of clay. The clay suspension was prepared before mixing with NR latex. The clay was added into the excess water (2 g per 100 ml of water) and was stirred vigorously for 4 h in order to achieve good dispersion of the silicate layers in the medium. Then, the mixture of clay suspension and NR latex was stirred gently for 30 min. Thereafter, the mixture was cast in a stainless steel tray and left at room temperature for a day. Finally, it was further dried in an oven at 50 C. The rubber/ clay obtained was further mixed with other ingredients to prepare the rubber compound. 2.2. Preparation of NR compounds and NR vulcanizates The compound formulations are given in Table 1. The conventional fillers used in this experiment were carbon black (N330) and precipitated silica (Hisil-233). The samples were designated as gum, C6, CB6, CB14, S6, S35. The letters ‘C’, ‘CB’ and ‘S’ refer to clay, carbon black and silica, respectively. The number followed the letters indicates the amount of filler in phr. The gum sample was prepared for comparing
Table 1 NR compound formulations Ingredients
Gum
C6
CB6
CB14
S6
S35
NR Clay Carbon black (N330) Silica (Hisil-233) Zinc oxide Stearic acid CBSa TMTDb Sulfur PEGc
100 – – – 5 2 1 0.1 2 –
100 6 – – 5 2 1 0.1 2 –
100 – 6 – 5 2 1 0.1 2 –
100 – 14 – 5 2 1 0.1 2 –
100 – – 6 5 2 1 0.1 2 2
100 – – 35 5 2 1 0.1 2 2
a b c
N-Cyclohexyl-2-benzothiazolesulfenamide. Tetramethylthiuram disulfide. Polyethylene glycol.
9
properties with those of the filled samples. All ingredients, except the curatives, were mixed with rubber (or rubber/ clay) in a laboratory-size internal mixer at a set temperature of 50 C with a rotor speed of 50 rpm and a fill factor of 0.7. The total mixing time in the internal mixer was 5.5 min. After discharging, the compound was mixed on a two rollmill for 1 min. Then, the curatives were added and further mixed for 4 min. Finally, 10 end-roll passes were made before sheeting off. The compounds were finally compression molded at 150 C. The cure time used for preparing the NR vulcanizates was the time at which the rheometer torque increased to 90% of the total torque change on the cure curve. The mechanical properties, hardness, tensile strength, tear strength, heat build-up resistance and crack growth resistance, were measured on vulcanizates, a) containing equal amounts of conventional fillers and montmorillonite clay (6 phr), and b) containing different amounts of each filler such as to give the same hardness level. 2.3. Mechanical property measurement The hardness was measured using a Wallace Shore A durometer, according to ISO 7619-1. Compression molded sheets having a thickness of about 1.2 mm were used for tear and tensile testing. Tear and tensile properties of the specimens were measured following ISO 34-1 and ISO 37, respectively. Crescent test pieces were used for determining the tear strength. The measurements were carried out using an Instron Universal Tester (Model 4301) with a crosshead speed of 500 mm/min and initial clamp separation of 65 mm. The values of tear and tensile properties were the average of 4–5 specimens. The heat build-up and crack growth resistance of the vulcanizates were measured, in accordance with ISO 4666 and ISO 132 using Goodrich flexometer and De Mattia type machines, respectively. Dynamic compression set was evaluated using the same specimens as for heat build-up testing. The original height of the specimen was measured prior to heat build-up testing and the specimen was left at 25 C for 1 h after the heat build-up test. Thereafter, its final height was measured and the compression set was calculated using the following equation.
Dynamic compression setð%Þ ¼
Ho Hf
Ho 100
where Ho ¼ original height (mm). Hf ¼ final height (mm). The fracture surfaces of the vulcanizates were examined using a scanning electron microscope (SEM, JEOL JSM6301) in order to view the degree of filler dispersion. The samples were sputtered with gold before examination to prevent charging on the surface. 3. Results and discussion Hardness and 300% modulus of all vulcanizates are illustrated in Figs. 1 and 2, respectively. As expected, the gum gives the lowest hardness and modulus while hardness and modulus increase noticeably when 6 phr of clay is added to the NR. At equal amounts of filler, the clay-filled vulcanizate exhibits the highest stiffness, followed by
10
N. Rattanasom et al. / Polymer Testing 28 (2009) 8–12
35
Tensile strength (MPa)
Hardness (Shore A)
60
55
50
45
40
30 25 20 15 10 5 0
Gum
C6
CB6
CB14
S6
S35
Gum
C6
CB6
CB14
S6
S35
Fig. 1. Hardness of NR vulcanizates filled with various fillers at various contents.
Fig. 3. Tensile strength of NR vulcanizates filled with various fillers at various contents.
CB-filled and silica-filled, respectively. In addition, the results show that 14 phr of CB or 35 phr of silica is required for the vulcanizates to reach the same hardness level as the vulcanizate containing 6 phr of clay. Compared to CB, a higher amount of silica is needed in order to achieve the same hardness level as the clay-filled vulcanizate. This is thought to be due to the decrease in crosslink density when high silica loading is used. In a previous study, crosslink density of NR vulcanizates gradually decreases when silica loading is more than 20 phr [12]. The explanation is given as the adsorption of zinc complex on the silica surface, thus lowering the sulfur vulcanization efficiency. Tensile strength of all vulcanizates is shown in Fig. 3. As can be seen, clay-filled vulcanizate exhibits the highest tensile strength while tensile strength of the other vulcanizates is not much different. Likewise, tensile strength of gum and filled NR vulcanizates containing 50 phr of CB is shown to be similar [13]. The strain induced crystallization is known to be responsible for the high strength of gum NR vulcanizates. Fig. 4 displays the tear strength of various NR vulcanizates. The gum exhibits the lowest tear strength while the vulcanizates having equal amounts of fillers give similar tear strength. Although, it is established that spherical particles can blunt the crack tip more effectively than the plate-shaped filler having high aspect ratio particles [14], such a small amount of CB and silica (6 phr) used in this experiment may not be sufficient to effectively blunt the tear tip. However, the tear strength of CB- and silica-filled NR vulcanizates markedly increases when they were prepared to have similar hardness to that of clay-filled
vulcanizate. At similar hardness level, CB-filled sample exhibits similar tear strength to that of silica-filled vulcanizate, but much higher than that of the clay-filled vulcanizate. The higher amount of fillers in CB- and silica-filled vulcanizates may obstruct the tear path more effectively than that for the clay-filled vulcanizate. It has also been reported that if the clay platelets are aligned in the perpendicular direction to the applied force, the stress may be concentrated at the sharp edges of particles and promote earlier failure compared to spherical fillers [14]. Crack growth resistance, expressed as the length of crack growth (lower length of crack growth indicates higher crack growth resistance), is shown in Fig. 5. In this experiment, the gum exhibits the highest crack growth resistance. The explanation is given as its lower modulus resulting in the lower stress concentration at its crack tip [13,15]. For the vulcanizates having equal amounts of filler, clay-filled vulcanizate shows the lowest crack growth resistance followed by CB- and silica-filled vulcanizates, respectively. It is apparent that these results correspond well with their moduli. The highest modulus of C6 leads to the highest stress concentration at its crack tip compared to CB6 and S6. On the other hand, a decrease in crack growth resistance is observed for S35 although its modulus is similar to that of C6 and CB14. It is thought that the lower crack growth resistance of S35 results from its poor silica dispersion or lower filler-rubber interaction, which overrides the effect of the modulus. The fracture surfaces of NR vulcanizates filled with various fillers are illustrated in Fig. 6. As can be seen, the 180
Tear strength (N/mm)
300% Modulus (MPa)
8
6
4
2
0 Gum
C6
CB6
CB14
S6
S35
Fig. 2. 300% Modulus of NR vulcanizates filled with various fillers at various contents.
150 120 90 60 30 0
Gum
C6
CB6
CB14
S6
S35
Fig. 4. Tear strength of NR vulcanizates filled with various fillers at various contents.
Lenght of crack growth (mm)
N. Rattanasom et al. / Polymer Testing 28 (2009) 8–12
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12
8
4
0
Gum
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CB6
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Fig. 5. Crack growth resistance at 20 kilocycles of NR vulcanizates filled with various fillers at various contents.
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dispersion of various fillers in NR is not much different when using 6 phr of filler. However, at the same hardness level, S35 has the poorest filler dispersion because the aggregate of silica can be readily observed in Fig. 6(e). It has been also shown in the previous study that filled rubber vulcanizate having poor silica dispersion exhibits lower crack growth resistance despite its lower modulus compared to other vulcanizates [12]. Dynamic compression set results for various NR vulcanizates are shown in Fig. 7. It is evident that compression set is low for the gum. When a low loading of CB or silica (6 phr) is added, the compression set is not significantly changed. Nevertheless, clay-filled vulcanizate gives somewhat higher compression set than that of CB- and silicafilled vulcanizates when equal amounts of fillers are used. At the same hardness level, silica-filled vulcanizate (S35)
Fig. 6. SEM micrographs of NR vulcanizates filled with various fillers at various contents; (a) C6, (b) CB6, (c) S6, (d) CB14 and (e) S35.
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12
5
10
Heat buildup (°C)
Compression set (%)
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4 3 2 1 0 Gum
C6
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8 6 4 2 0
Gum
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Fig. 7. Dynamic compression set of NR vulcanizates filled with various fillers at various contents.
Fig. 8. Heat build-up of NR vulcanizates filled with various fillers at various contents.
shows the highest compression set when compared to that of C6 and CB14. This is attributed to the poor filler-rubber interaction or poor filler dispersion when the silica loading is high. Another possible explanation is the decrease in crosslink density when silica loading is high. This result also agrees well with the previous study showing that the dynamic compression set tends to increase when the silica loading is more than 30 phr [12]. It can be seen in Fig. 8 that heat build-up of the vulcanizates containing a small amount of filler is not much different from that of the gum. However, it should be noted that the clay-filled sample shows slightly higher heat buildup than CB-filled and silica-filled vulcanizates when using equal amounts of filler. When higher amounts of CB or silica is added to prepare vulcanizates with similar hardness to that of C6, CB-filled sample (CB14) exhibits the same heat build-up value as that of C6, while heat build-up of silicafilled sample (S35) increases considerably. The possible explanation is also given as the combined effects of the poor silica dispersion and lower crosslink density of silicafilled sample when a high amount of silica is loaded. Heat build-up of a vulcanizate is reported to increase with decreasing crosslink density [13].
At the same hardness value, CB-filled vulcanizate gives the better overall mechanical properties followed by the clayfilled and silica-filled vulcanizates, respectively. The poor silica dispersion and lower crosslink density of silica-filled vulcanizate (S35) are thought to be the causes of its poorer properties compared to the other vulcanizates.
4. Conclusions The results show that modulus, hardness and tear strength of NR filled with conventional fillers and clay are superior to those of the gum. Hardness and modulus increase noticeably when 6 phr of clay is added. At equal loading of fillers, clay-filled NR vulcanizate exhibits higher modulus, hardness, tensile strength compression set but lower heat build-up resistance and crack growth resistance than those of NR vulcanizates containing conventional fillers. However, their tear strength is not much different. Compared to CB, a higher amount of silica is needed to achieve the same hardness as that of the clay-filled sample.
Acknowledgements The authors gratefully acknowledge Mr. Woothichai Thaicharoen for providing the clay/rubber composite used in this experiment. Sincere appreciation is extended to staff of National Metal and Materials Technology Center for carrying out the SEM measurement. References [1] A.S. Hashim, B. Azahari, Y. Ikeda, S. Kohjiya, Rubb. Chem. Technol. 71 (1998) 289–299. [2] L. Zhang, Y. Wang, Y. Wang, Y. Sui, D. Yu, J. Appl. Polym. Sci. 78 (2000) 1873–1878. [3] M. Kato, A. Tsukigase, H. Tanaka, A. Usuki, I. Inai, J. Polym. Sci. Part A: Polym. Chem. 44 (2006) 1182–1188. [4] H. Tan, A.I. Isayev, J. Appl. Polym. Sci. 109 (2008) 767–774. [5] S. Vargese, J. Karger-Kocsis, J. Appl. Polym. Sci. 91 (2004) 813–819. [6] M. Maiti, S. Sadhu, A.K. Bhowmick, J. Appl. Polym. Sci. 96 (2005) 443–451. [7] Y.T. Vu, J.E. Mark, L.H. Pham, M. Engelhardt, J. Appl. Polym. Sci. 82 (2001) 1391–1403. [8] P.L. Teh, Z.A. Mohd Ishak, A.S. Hashim, J. Karger-Kocsis, U.S. Ishiaku, J. Appl. Polym. Sci. 100 (2006) 1083–1092. [9] Y. Wang, H. Zhang, Y. Wu, J. Yang, L. Zhang, J. Appl. Polym. Sci. 96 (2005) 318–323. [10] S. Siriwardena, H. Ismail, U.S. Ishiaku, Polym. Int. 50 (2001) 707–713. [11] H.E. Trexler, M.C.H. Lee, J. Appl. Polym. Sci. 32 (1986) 3899–3912. [12] N. Rattanasom, T. Saowapark, C. Deeprasertkul, Polym. Test. 26 (2007) 369–377. [13] N. Rattanasom, U. Thammasiripong, K. Suchiva, J. Appl. Polym. Sci. 97 (2005) 1139–1144. [14] A.M. Riley, C.D. Paynter, P.M. McGenity, J.H. Adams, Plast. Rubb. Proc. Appl. 14 (1990) 85. [15] N. Rattanasom, O. Chaikumpollert, J. Appl. Polym. Sci. 90 (2003) 1793–1796.