Author’s Accepted Manuscript A Study on the Tensile Properties of Silicone Rubber/Polypropylene Fibers/Silica Hybrid Nanocomposites Sahar Ziraki, Seyed Mojtaba Zebarjad, Mohammad Jafar Hadianfard www.elsevier.com/locate/jmbbm
PII: DOI: Reference:
S1751-6161(16)00022-9 http://dx.doi.org/10.1016/j.jmbbm.2016.01.019 JMBBM1780
To appear in: Journal of the Mechanical Behavior of Biomedical Materials Received date: 14 November 2015 Revised date: 15 January 2016 Accepted date: 20 January 2016 Cite this article as: Sahar Ziraki, Seyed Mojtaba Zebarjad and Mohammad Jafar Hadianfard, A Study on the Tensile Properties of Silicone Rubber/Polypropylene Fibers/Silica Hybrid Nanocomposites, Journal of the Mechanical Behavior of Biomedical Materials, http://dx.doi.org/10.1016/j.jmbbm.2016.01.019 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 galley proof before it is published in its final citable 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.
A Study on the Tensile Properties of Silicone Rubber/Polypropylene Fibers/Silica Hybrid Nanocomposites Sahar Zirakia, Seyed Mojtaba Zebarjadb1 and Mohammad Jafar Hadianfardc Department of Materials Science and Engineering, School of Engineering, Shiraz University, Zand Avenue, Shiraz, Iran a
[email protected], [email protected], [email protected]
Abstract. Metacarpophalangeal joint implants have been usually made of silicone rubber. In the current study, silica nano particles and polypropylene fibers were added to silicone rubber to improve silicone properties. The effect of the addition of silica nano particles and polypropylene fibers on the tensile behavior of the resultant composites were investigated. Composite samples with different content of PP fibers and Silica nano particles (i. e. 0, 1 and 2 wt%) as well as the hybrid composite of silicone rubber with 1 wt% SiO2 and 1 wt% PP fiber were prepared. Tensile tests were done at constant cross head speed. To study the body fluid effect on the mechanical properties of silicone rubber composites, samples soaked in simulated body fluid (SBF) at 37 ◦C were also tested. The morphology of the samples were studied by scanning electron microscope. Results of analysis revealed that an increase in PP fibers and silica nano particles content to 2 wt%, increases the tensile strength of silicone rubber of about 75% and 42% respectively. It was found out that the strength of the samples decreases after being soaked in simulated body fluid, though composites with PP fibers as the reinforcement showed less property degradation. Keywords: Silicone Rubber (SR), Silica nanoparticles (SiO2), Polypropylene (PP) fiber, Tensile properties, Finger joint implant 1
Corresponding Author (Email: [email protected], Tel/Fax:+987112307293, Postal address: 7134851154) 1
1. Introduction Silicone rubber is a polymer with a backbone of Si-O and this inorganic backbone attaches to the organic groups (frequently methyl) (Mahomed, 2008). Silicone rubber has got many applications owing to its excellent properties, i.e. its thermal and chemical resistivity has led to its usage as an insulator in electrical field, its hydrophobicity has led to its use in aerospace and its inertness together with its biocompatibility has led to its usage as a biomaterial in medical applications and healthcare (Roger De Jaeger, 2009),(Rezaei, M., Ahmadi-Joneidi, I., Parhizgar, A., Kahuri, H., & Sayani, 2013),(Germano et al., 2013),(Thomas, 2007),(Joan, 2007). For example metacarpophalangeal joint implant has been made of silicone rubber for many years. Metacarpophalangeal joint is an important joint in hand which may be hurt by arthritis disease. In such cases, finger joint is replaced by implant. Swanson, Niebauer and sutter are three implants which have been made of silicone and among these implants, Swanson is the most common. In spite of advantages i.e. pain reduction, patient satisfaction, ease of implantation and suitable price, reconstructive surgery is sometimes required due to its property drop (Lester, 2009),(Parkkila, 2007),(Niebauer et al., 1969). So, addition of fillers to silicone rubber with the aim to improve its performance in various applications has been studied. For example, common fillers like SiO2, TiO2 and carbon black and fibers like polyester and cellulose have been added to silicone rubber to improve its properties (Messori, 2011),(El-Hag et al., 2006),(Rajeev, 2008). Nickel particles were added to silicone rubber in aligned direction by Song et al. and based on their results, mechanical and electrical properties and shear storage modulus improved (Song et al., 2013). In Momen et al. review ATH, ZnO, TiO2, CaCO3 and BaTiO3 have been used in silicone rubber matrix to improve electrical and thermal properties (Momen and Farzaneh, 2011). Horn et al. studies showed that polyacrylamide hydrogel/silicone rubber composite is suitable for
2
biomaterial (Hron, P., Šlechtová, J., Smetana, K., Dvořánková, B., & Lopour, 1997). Hosseini et al. studied the mechanical properties and cell behavior of organoclay silicone rubber composites and their results showed that this composite is biocompatible and thus could be used in tissue engineering (Hosseini, M. S., Tazzoli-Shadpour, M., Amjadi, I., Haghighipour, N., Shokrgozar, M. A., & Boroujerdnia, 2012). Recently, Rey et al. added NiTi wire to silicone rubber for use as biomaterial in stents and artificial vessels. In this study plasma surface treating was used for better attachment of NiTi and silicone rubber (Rey et al., 2014). Dacron has also been used in Niebauer implants for better fixation but studies on these implants showed their ulnar deviation and failure (Parkkila, 2007),(Niebauer et al., 1969). The use of PP fibers as reinforcement for silicone rubber has not yet been under attention and the aim of the current study is to add PP fibers and silica nano particles to silicone rubber to investigate their effect on the mechanical behavior of silicone rubber for its use as biomaterial in finger joint implants. Polypropylene is a synthetic hydrocarbon polymer which its fibers are produced by extrusion and hot drawing (Ahmed et al., 2006). PP is a hydrocarbon polymer with many advantages including inexpensiveness, lightness, inertness, chemical resistivity with low modulus of elasticity and high strength (Umemori et al., 2012),(Bei-xing, L., Ming-xiang, C., Fang, C., & Lu-ping, 2004).
2. Material and methods In this study, two parts silicone rubber with 5 mole % vinyl group manufactured by Midgold (China) was used. HTV (High Temperature Vulcanization) silicone rubber was cured at the temperature of about 120◦C for 10 min and curing speed was 1
600
s 1 at constant concentration
of silicone rubber. Precipitated nano particles of silica with a silane coating and a size of about 25 nm and PP fibers with the average diameter of 30 µm were used as the reinforcements of
3
silicone rubber. PP fibers were cut to the length of 10-15 mm by scissor. Fig. 1 and Fig. 2 show microscopic images of utilized silica nanoparticles and PP fibers. Saline solution provided by Daroopakhsh (Iran) was used as the simulated body fluid (SBF). 2.1. Sample preparation and evaluation The procedure used for the preparation of the composite samples is depicted in Fig. 3. Silica/silicone nanocomposites samples were prepared by mixing 1 and 2 wt% of Silica with silicone rubber. A similar procedure was used for the production of PP/silicone composites. For the preparation of hybrid composite, 1 wt% of silica and 1 wt% of PP fibers were simultaneously mixed with silicone rubber. A rectangular die was used to fabricate silicone sheets. The silicone rubber mixture was cured at the temperature of 120 ◦C and the pressure of 2000 kPa for10 min and was then post cured at the temperature of 120 ◦C for 2 h. The sheets were cut to standard dumbbell shape of 33*6*3 mm according to ASTM-D412. Tensile tests were done by Santam universal testing machine under a constant cross head speed of 500 mm/min. To study the effect of the body fluid on implant’s materials, composite samples soaked in saline solution at a temperature of 37 ◦C for 2 days were tested. Three samples of each composite were tested. The deformation mechanisms in tension samples were investigated by Scanning Electron Microscope with accelerating voltage of 15 and 20 kV (Cambridge Stereo Scan).
3. Results and discussions Fig. 4a and b show tensile engineering stress-strain curve of silica and PP/ silicone rubber composites. Silicone rubber has in linear behavior. Stretching of silicone leads to open kinks and
4
large strain is achieved by little stress. Continuous stretching leads to the alignment of chains that causes to increase crystallinity and stress. Presence of reinforcements improves tensile strength of silicone rubber as a result of the interaction of silicone chains with nano particles and fibers. Table 1 shows the details of tensile tests. Silica nano particles have large specific surface area, thus transferring load from matrix to particles occurs over a wide surface and strength increase. A model developed by Pukanszky et al. relates the debonding stress and particle size as follows:
D
C W C1 2 mf R
0.5
T
Eq. 1
Where σD and σT are the debonding and thermal stress, C1 and C2 are constants, R is the radius of particles and Wmf is the work of adhesion. R is in inverse relation with stress of debonding (Pukanszky, B. É. L. A., & VÖRÖS, 1993). Increasing the SiO2 content to 2 wt%, causes to increase tensile strength from 3.94 to 5.6 MPa as can be seen in Table 1. This result is similar to what Kim et al. proposed that by adding 1 wt% of CNT in silicone rubber matrix, tensile strength increased from 5.6 to 8.4 MPa (Kim, H. S., Kwon, S. M., Lee, K. H., Yoon, J. S., & Jin, 2008). Jia et al. also added modified halloysite nanotube (m-HNT) to styrene butadiene rubber (SBR) and observed that tensile strength increased from 2.4 to 3.4 MPa for 10 phr (part per hundred rubber) reinforcement and elongation at break decreased from 610 to 580% (Jia, Z. X., Luo, Y. F., Yang, S. Y., Guo, B. C., Du, M. L., & Jia, 2009). The results of PP composites tensile tests show that an increase in fiber content causes an increase in the tensile strength. The same trend is observed for elongation and toughness. PP fibers with high aspect ratio can transfer load from matrix over a large surface. Although the
5
strength of PP fiber is about 400 MPa, the strength of composite does not increase significantly and the theoretical strength measurements, obtained by rule of mixtures, do not match the experimental results. This could be explained by the fact that the strength of composites strongly depends on the strength of the interface. Since the matrix must transfer the load to the fiber through the interface, an interface which is not sufficiently strong causes the load to be released instead of being transferred to the fiber. The role of the interface could be clarified by microscopic evaluation. Fig. 5 shows SEM micrographs of composite samples with 1 and 2 wt% PP fibers. As seen in the micrographs, the surface of fiber is almost smooth and implies lack of compatibility. Hence, there is a weak bonding between the matrix and the fiber and PP fibers cannot completely play their role in increasing the strength. In this condition, fibers tend to pull out and some voids will be made in the matrix which can produce cracks and hence samples break rapidly and strength decreases. This is why PP fibers cannot improve the strength of silicone drastically. However the fibers pulling out consumes part of the applied load and energy which leads to an increase in the strength and fracture energy (toughness). Saliba et al. reported a similar microscopic image. According to their research short inorganic silexil fibers were distributed in polyurethane and pulling out occurred due to nonsurface treated fibers (Saliba, C. C., Oréfice, R. L., Carneiro, J. R. G., Duarte, A. K., Schneider, W. T., & Fernandes, 2005). The energy required for the pulling out is measured by Eq. 2 as follows:
W 1 d 2 lc f 4
Eq. 2
Where d, lc, and σf are diameter, critical length and strength of fibers, respectively. By increasing these parameters, the energy required for pulling out the fibers increases (N. G. McCrum, C. P. Buckley, 1997). Assuming τ0 = σy/2, the shear yield stress of silicone rubber is 1.97 MPa.
6
Critical length defined as the minimum length required to increase composite strength is measured as follows: d L 1 f 2 0
Eq. 3
The critical length of fiber is achieved 3.05 mm and as the length of fibers used in this study is higher than Lc, fibers are effective in strengthening. By using Eq. 3, fiber pull out energy is to be 0.86 mJ. By measuring the number of fibers (by volume), pull out energy of all fibers in 1 and 2 wt% PP composite are achieved 561123 mJ and 1125565 mJ. The area under stress-strain curve which is equivalent to the absorbed energy are 2505.97 and 2660.89 mJ, but part of this energy relates to matrix material and by subtracting the matrix energy (measured by the area under the curve of pure silicone rubber) from this, absorbed energy of 1 and 2 wt% PP composites are achieved 741.94 and 896.89 mJ, respectively. By comparing these two energy (the energy measured by the fiber pull out and the energy measured by the area under curve), it is observed that these are not the same. These differences can be attributed to the random distribution of fibers in matrix which are partly positioned in the load direction and may not experience pull out. Besides, the assumed value of τ0 is not very accurate. Tensile engineering stress-strain curves of silica and PP/ silicone rubber composites after being soaked in saline solution as a simulated body fluid are illustrated in Fig. 6. Tensile behavior of samples after being soaked in SBF show the same trend as before, but strength decrease slightly and elongation increase in SBF. Table 2 shows the details of tests in SBF. Increasing of elongation may be due to role of solution as a plasticizer between polymer chains which can soften composites (Dhakal et al., 2007). The strengths of the samples before and after being soaked in SBF are depicted in Fig.7. Interaction of reinforcements and solution can be a reason of property drop which leads to the 7
weakening of the reinforcement’s interaction with silicone rubber. This result is in accordance with other studies. Dai et al. observed some voids on the surface of silicone rubber in SEM micrographs after being soaked in saline serum which were because of the detachment of silica from silicone after the interaction by solution (Dai et al., 2006). SEM images of composites in this study after being soaked in SBF for about 3 months are observed in Fig. 8. The figures indicate the debonding and the formation of the repeated voids. These voids attach together and void coalescence occur in the composite. The effect of solution in PP fibers can also be seen in Fig. 8. As the strength drop is one of the disadvantages of biomaterials leading to earlier failure of implant in body, materials selection is an important factor. Results show that the presence of PP fibers leads to less strength drop. In order to compare the tensile strength of the composite and hybrid samples with constant wt% of 2 of reinforcements, Fig. 9 is shown. As can be seen, tensile strength in hybrid composite is more than SR/SiO2 composite, i.e. presence of both fibers and nano particles show better effect on tensile strength than presence of just silica nano particles. It can be explained by the strengthening mechanism of both fibers and nano particles. Effect of aspect ratio in fibers is more than role of nano particles in strength and so hybrid composite can not reach to SR/PP composite (Campbell, 2010). Besides, fibers have lower density than nano particles and in a constant content, volume fraction of PP fibers are more than nano particles. Stress-strain curve of these samples in SBF show same trend as before, but with lower strength and higher elongation. 4. Conclusions Silicone rubber has been used in finger joint replacement. Despite unique properties, silicone rubber has poor mechanical properties which leads to its failure. However the mechanical
8
properties could be improved by the use of fillers. In this study silica nano particles and PP fibers were added to silicone rubber and the following results were found: 1) Tensile behavior of silicone rubber in hybrid composites improved from 5.6 MPa to 6.21 MPa in comparison with 2 wt% silica/ silicone composite. 2) Strength drop of composites in SBF was about 6% and composites with PP fibers showed less strength drop. 3) SEM images showed voids and fiber degradation after the composites were soaked in SBF. References Ahmed, S., Taxila, T., Bukhari, I.A., Qureshi, S.A., 2006. A Study on Properties of Polypropylene Fiber Reinforced Concrete, in: 31st Conference on Our World in Concrete & Structures. Singapore, pp. 1–10. Bei-xing, L., Ming-xiang, C., Fang, C., & Lu-ping, L., 2004. The Mechanical Properties of Polypropylene Fiber Reinforced Concrete. J. Wuhan Univ. Technol. Sci. Ed. 19, 68–71. Campbell, F.C., 2010. Introduction to Composite Materials, Structural Composite Materials. ASM International. Dai, J., Yao, X., Yeh, H.Y., Liang, X., 2006. Moisture Absorption of Filled Silicone Rubber Under Electrolyte. J. Appl. Polym. Sci. 99, 2253–2257. doi:10.1002/app.22403 Dhakal, H., Zhang, Z., Richardson, M., 2007. Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites. Compos. Sci. Technol. 67, 1674–1683. doi:10.1016/j.compscitech.2006.06.019 El-Hag, A.H., Simon, L.C., Jayaram, S.H., Cherney, E. a., 2006. Erosion Resistance of Nano-
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Filled Silicone Rubber. IEEE Trans. Dielectr. Electr. Insul. 13, 122–128. doi:10.1109/TDEI.2006.1593410 Germano, A.D., Seifert, J.M., Kindersberger, J., Muenchen, 2013. Influence of Nanosilica on the Performance of HTV Silcone Rubber for Outdoor Insulation, in: 18th International Symposium on High Voltage Engineering. pp. 3–7. Hosseini, M. S., Tazzoli-Shadpour, M., Amjadi, I., Haghighipour, N., Shokrgozar, M. A., & Boroujerdnia, M.G., 2012. Relationship Between Cell Compatibility and Elastic Modulus of Silicone Rubber/Organoclay Nanobiocomposites. Jundishapur J. Nat. Pharm. Prod. 7, 65– 70. Hron, P., Šlechtová, J., Smetana, K., Dvořánková, B., & Lopour, P., 1997. Silicone RubberHydrogel Composites as Polymeric Biomaterials: IX. Composites Containing Powdery Polyacrylamide Hydrogel. Biomaterials 18, 1069–1073. Jia, Z. X., Luo, Y. F., Yang, S. Y., Guo, B. C., Du, M. L., & Jia, D.M., 2009. Morphology, Interfacial Interaction and Properties of Styrene-Butadiene Rubber/Modified Halloysite Nanotube Nanocomposites. Chinese J. Polym. Sci. 27, 857–864. Joan, H.P.Y., 2007. Plasma Surface Modification of Biomedical Polymers and Metals. University of Sydney. Kim, H. S., Kwon, S. M., Lee, K. H., Yoon, J. S., & Jin, H.J., 2008. Preparation and Characterization of Silicone Rubber/Functionalized Carbon Nanotubes Composites via In Situ Polymerization. J. Nanosci. Nanotechnol. 8, 5551–5554. doi:10.1166/jnn.2008.1312 Lester, L., 2009. The Biomechanical Analysis of the Hand in Rheumatoid Arthritis Patients With Mcp Arthroplasty. University of Birmingham. 10
Mahomed, A., 2008. Properties of elastomers for small-joint replacements. University of Birmingham. Messori, M., 2011. Recent Advances in Elastomeric Nanocomposites, in: V. Mittal, J.K.K. and K.P. (Ed.), Advanced Structured Materials. Springer-Verlag Berlin Heidelberg publishers, Berlin, pp. 307–342. doi:10.1007/978-3-642-15787-5 Momen, G., Farzaneh, M., 2011. Survey of Micro/Nano Filler Use to Improve Silicone Rubber for Outdoor Insulators. Rev. Adv. Mater. Sci. 27, 1–13. N. G. McCrum, C. P. Buckley, C.B.B., 1997. Principles of Polymer Engineering. Oxford Science Publication, New York Tokyo. Niebauer, J.J., Shaw, J.L., Doren, A.W., 1969. Silicone-Dacron Hinge Prosthesis: Design, Evaluation, and Application. Ann. Rheum. Dis. 28, 56–58. Parkkila, T., 2007. Sutter metacarpo-phalangeal arthroplasty in rheumatoid patients. University of Tampere. Pukanszky, B. É. L. A., & VÖRÖS, G., 1993. Mechanism of Interfacial Interactions in Particulate Filled Composites. Compos. Interfaces 1, 411–427. Rajeev, R.S., 2008. Fiber Reinforced Elastomers, in: Bhowmick, A.K. (Ed.), Current Topics in Elastomers Research. CRC Pres, pp. 351–394. Rey, T., Le Cam, J.-B., Chagnon, G., Favier, D., Rebouah, M., Razan, F., Robin, E., Didier, P., Heller, L., Faure, S., Janouchova, K., 2014. An Original Architectured NiTi Silicone Rubber Structure for Biomedical Applications. Mater. Sci. Eng. C. Mater. Biol. Appl. 45, 184–90. doi:10.1016/j.msec.2014.08.062
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Rezaei, M., Ahmadi-Joneidi, I., Parhizgar, A., Kahuri, H., & Sayani, A., 2013. Evaluation of Actual Field Ageing on silicone Rubber Insulator Under Coastal Environment. Life Sci. J. 10, 199–205. doi:10.1017/CBO9781107415324.004 Roger De Jaeger, M.G., 2009. Silicones in Industrial Applications. Nova Science Publishers. Saliba, C. C., Oréfice, R. L., Carneiro, J. R. G., Duarte, A. K., Schneider, W. T., & Fernandes, M.R.F., 2005. Effect of the Incorporation of a Novel Natural Inorganic Short Fiber on the Properties of Polyurethane Composites. Polym. Test. 24, 819–824. doi:10.1016/j.polymertesting.2005.07.008 Song, P., Peng, Z.J., Yue, Y.L., Zhang, H., Zhang, Z., Fan, Y.C., 2013. Mechanical Properties of Silicone Composites Reinforced with Micron- and Nano-Sized Magnetic Particles. Express Polym. Lett. 7, 546–553. doi:10.3144/expresspolymlett.2013.51 Thomas, X., 2007. 17. Silicones in Medical Applications, in: Inorganic Polymers. Nova Science Pub Inc., pp. 61–161. doi:10.1002/jbm.1043 Umemori, M., Taniike, T., Terano, M., 2012. Influences of Polypropylene Grafted to SiO2 Nanoparticles on the Crystallization Behavior and Mechanical Properties of Polypropylene/SiO2 Nanocomposites. Polym. Bull. 68, 1093–1108. doi:10.1007/s00289011-0612-y
Table 1 Tensile properties of silicone composites
Tensile strength (MPa) Elongation (%)
0
1%SiO2
2%SiO2
1%PP
2%PP
3.94 1114.14
4.66 1047.2
5.6 955.1
5.5 1044.62
6.91 910.98
12
Toughness (MJ/m3)
19.1
20.57
21.91
27.52
29.17
Table 2 Tensile properties of silicone composites soaked in SBF
Tensile strength (MPa) Elongation (%) Toughness (MJ/m3)
0
1%SiO2
2%SiO2
1%PP
2%PP
3.85 1131.75 19.6
4.41 1123.96 22.07
5.26 1119.7 25.83
5.31 1106.19 27.65
6.74 954.15 29.3
Fig. 1. TEM micrographs of nano SiO2 powder before sonication. 13
Fig. 2. Short PP fibers.
14
Fig. 3. Flowchart of composites preparation steps.
6
8
Stress (MPa)
5 4
Stress (MPa)
SR S1 S2
3 2
(a)
1 0
SR PP1 PP2
6 4 2
(b)
0 0
2
4
6
8
Strain (mm/mm)
10
12
0
2
4
6
8
Strain (mm/mm)
10
12
Fig. 4. The stress-strain curve of (a) silica/ silicone rubber and (b) PP fibers/ silicone rubber composites.
15
16
a
b
Fig. 5. Scanning electron micrographs of PP/ silicone rubber composites; (a) 1% PP and (b) 2% PP.
6
8
Stress (MPa)
5 4
Stress (MPa)
SR S1 S2
3 2
(a)
1 0
SR PP1 PP2
6 4 2
(b)
0 0
2
4
6
8
Strain (mm/mm)
10
12
0
2
4
6
8
Strain (mm/mm)
10
12
Fig. 6. The stress-strain curve of (a) silica/ silicone rubber and (b) PP fibers/ silicone rubber composites after being soaked in SBF.
17
Tensile Strength (MPa)
8 7
air
saline
6 5 4 3 2 1 0
SR
S1
S2
PP1
PP2
Fig. 7. Comparison of tensile strength of samples before and after being soaked in SBF.
18
a
b
Fig. 8. SEM micrographs of silicone rubber composites; (a) 2 wt% silica and (b) 2 wt% PP after being soaked in SBF.
8 PP2 SPP11 S2
6
PP2
Stress (MPa)
Stress (MPa)
8
4
(a)
2
SPP11
6
S2
4
(b)
2 0
0 0
3
6
Strain (mm/mm)
9
0
12
3
6
Strain (mm/mm)
9
12
Fig. 9. Tensile strength of silicone rubber composites and hybrid composites (a) before and (b) after being soaked in SBF.
19
20
PII: DOI: Reference:
S1751-6161(16)00022-9 http://dx.doi.org/10.1016/j.jmbbm.2016.01.019 JMBBM1780
To appear in: Journal of the Mechanical Behavior of Biomedical Materials Received date: 14 November 2015 Revised date: 15 January 2016 Accepted date: 20 January 2016 Cite this article as: Sahar Ziraki, Seyed Mojtaba Zebarjad and Mohammad Jafar Hadianfard, A Study on the Tensile Properties of Silicone Rubber/Polypropylene Fibers/Silica Hybrid Nanocomposites, Journal of the Mechanical Behavior of Biomedical Materials, http://dx.doi.org/10.1016/j.jmbbm.2016.01.019 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 galley proof before it is published in its final citable 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.
A Study on the Tensile Properties of Silicone Rubber/Polypropylene Fibers/Silica Hybrid Nanocomposites Sahar Zirakia, Seyed Mojtaba Zebarjadb1 and Mohammad Jafar Hadianfardc Department of Materials Science and Engineering, School of Engineering, Shiraz University, Zand Avenue, Shiraz, Iran a
[email protected], [email protected], [email protected]
Abstract. Metacarpophalangeal joint implants have been usually made of silicone rubber. In the current study, silica nano particles and polypropylene fibers were added to silicone rubber to improve silicone properties. The effect of the addition of silica nano particles and polypropylene fibers on the tensile behavior of the resultant composites were investigated. Composite samples with different content of PP fibers and Silica nano particles (i. e. 0, 1 and 2 wt%) as well as the hybrid composite of silicone rubber with 1 wt% SiO2 and 1 wt% PP fiber were prepared. Tensile tests were done at constant cross head speed. To study the body fluid effect on the mechanical properties of silicone rubber composites, samples soaked in simulated body fluid (SBF) at 37 ◦C were also tested. The morphology of the samples were studied by scanning electron microscope. Results of analysis revealed that an increase in PP fibers and silica nano particles content to 2 wt%, increases the tensile strength of silicone rubber of about 75% and 42% respectively. It was found out that the strength of the samples decreases after being soaked in simulated body fluid, though composites with PP fibers as the reinforcement showed less property degradation. Keywords: Silicone Rubber (SR), Silica nanoparticles (SiO2), Polypropylene (PP) fiber, Tensile properties, Finger joint implant 1
Corresponding Author (Email: [email protected], Tel/Fax:+987112307293, Postal address: 7134851154) 1
1. Introduction Silicone rubber is a polymer with a backbone of Si-O and this inorganic backbone attaches to the organic groups (frequently methyl) (Mahomed, 2008). Silicone rubber has got many applications owing to its excellent properties, i.e. its thermal and chemical resistivity has led to its usage as an insulator in electrical field, its hydrophobicity has led to its use in aerospace and its inertness together with its biocompatibility has led to its usage as a biomaterial in medical applications and healthcare (Roger De Jaeger, 2009),(Rezaei, M., Ahmadi-Joneidi, I., Parhizgar, A., Kahuri, H., & Sayani, 2013),(Germano et al., 2013),(Thomas, 2007),(Joan, 2007). For example metacarpophalangeal joint implant has been made of silicone rubber for many years. Metacarpophalangeal joint is an important joint in hand which may be hurt by arthritis disease. In such cases, finger joint is replaced by implant. Swanson, Niebauer and sutter are three implants which have been made of silicone and among these implants, Swanson is the most common. In spite of advantages i.e. pain reduction, patient satisfaction, ease of implantation and suitable price, reconstructive surgery is sometimes required due to its property drop (Lester, 2009),(Parkkila, 2007),(Niebauer et al., 1969). So, addition of fillers to silicone rubber with the aim to improve its performance in various applications has been studied. For example, common fillers like SiO2, TiO2 and carbon black and fibers like polyester and cellulose have been added to silicone rubber to improve its properties (Messori, 2011),(El-Hag et al., 2006),(Rajeev, 2008). Nickel particles were added to silicone rubber in aligned direction by Song et al. and based on their results, mechanical and electrical properties and shear storage modulus improved (Song et al., 2013). In Momen et al. review ATH, ZnO, TiO2, CaCO3 and BaTiO3 have been used in silicone rubber matrix to improve electrical and thermal properties (Momen and Farzaneh, 2011). Horn et al. studies showed that polyacrylamide hydrogel/silicone rubber composite is suitable for
2
biomaterial (Hron, P., Šlechtová, J., Smetana, K., Dvořánková, B., & Lopour, 1997). Hosseini et al. studied the mechanical properties and cell behavior of organoclay silicone rubber composites and their results showed that this composite is biocompatible and thus could be used in tissue engineering (Hosseini, M. S., Tazzoli-Shadpour, M., Amjadi, I., Haghighipour, N., Shokrgozar, M. A., & Boroujerdnia, 2012). Recently, Rey et al. added NiTi wire to silicone rubber for use as biomaterial in stents and artificial vessels. In this study plasma surface treating was used for better attachment of NiTi and silicone rubber (Rey et al., 2014). Dacron has also been used in Niebauer implants for better fixation but studies on these implants showed their ulnar deviation and failure (Parkkila, 2007),(Niebauer et al., 1969). The use of PP fibers as reinforcement for silicone rubber has not yet been under attention and the aim of the current study is to add PP fibers and silica nano particles to silicone rubber to investigate their effect on the mechanical behavior of silicone rubber for its use as biomaterial in finger joint implants. Polypropylene is a synthetic hydrocarbon polymer which its fibers are produced by extrusion and hot drawing (Ahmed et al., 2006). PP is a hydrocarbon polymer with many advantages including inexpensiveness, lightness, inertness, chemical resistivity with low modulus of elasticity and high strength (Umemori et al., 2012),(Bei-xing, L., Ming-xiang, C., Fang, C., & Lu-ping, 2004).
2. Material and methods In this study, two parts silicone rubber with 5 mole % vinyl group manufactured by Midgold (China) was used. HTV (High Temperature Vulcanization) silicone rubber was cured at the temperature of about 120◦C for 10 min and curing speed was 1
600
s 1 at constant concentration
of silicone rubber. Precipitated nano particles of silica with a silane coating and a size of about 25 nm and PP fibers with the average diameter of 30 µm were used as the reinforcements of
3
silicone rubber. PP fibers were cut to the length of 10-15 mm by scissor. Fig. 1 and Fig. 2 show microscopic images of utilized silica nanoparticles and PP fibers. Saline solution provided by Daroopakhsh (Iran) was used as the simulated body fluid (SBF). 2.1. Sample preparation and evaluation The procedure used for the preparation of the composite samples is depicted in Fig. 3. Silica/silicone nanocomposites samples were prepared by mixing 1 and 2 wt% of Silica with silicone rubber. A similar procedure was used for the production of PP/silicone composites. For the preparation of hybrid composite, 1 wt% of silica and 1 wt% of PP fibers were simultaneously mixed with silicone rubber. A rectangular die was used to fabricate silicone sheets. The silicone rubber mixture was cured at the temperature of 120 ◦C and the pressure of 2000 kPa for10 min and was then post cured at the temperature of 120 ◦C for 2 h. The sheets were cut to standard dumbbell shape of 33*6*3 mm according to ASTM-D412. Tensile tests were done by Santam universal testing machine under a constant cross head speed of 500 mm/min. To study the effect of the body fluid on implant’s materials, composite samples soaked in saline solution at a temperature of 37 ◦C for 2 days were tested. Three samples of each composite were tested. The deformation mechanisms in tension samples were investigated by Scanning Electron Microscope with accelerating voltage of 15 and 20 kV (Cambridge Stereo Scan).
3. Results and discussions Fig. 4a and b show tensile engineering stress-strain curve of silica and PP/ silicone rubber composites. Silicone rubber has in linear behavior. Stretching of silicone leads to open kinks and
4
large strain is achieved by little stress. Continuous stretching leads to the alignment of chains that causes to increase crystallinity and stress. Presence of reinforcements improves tensile strength of silicone rubber as a result of the interaction of silicone chains with nano particles and fibers. Table 1 shows the details of tensile tests. Silica nano particles have large specific surface area, thus transferring load from matrix to particles occurs over a wide surface and strength increase. A model developed by Pukanszky et al. relates the debonding stress and particle size as follows:
D
C W C1 2 mf R
0.5
T
Eq. 1
Where σD and σT are the debonding and thermal stress, C1 and C2 are constants, R is the radius of particles and Wmf is the work of adhesion. R is in inverse relation with stress of debonding (Pukanszky, B. É. L. A., & VÖRÖS, 1993). Increasing the SiO2 content to 2 wt%, causes to increase tensile strength from 3.94 to 5.6 MPa as can be seen in Table 1. This result is similar to what Kim et al. proposed that by adding 1 wt% of CNT in silicone rubber matrix, tensile strength increased from 5.6 to 8.4 MPa (Kim, H. S., Kwon, S. M., Lee, K. H., Yoon, J. S., & Jin, 2008). Jia et al. also added modified halloysite nanotube (m-HNT) to styrene butadiene rubber (SBR) and observed that tensile strength increased from 2.4 to 3.4 MPa for 10 phr (part per hundred rubber) reinforcement and elongation at break decreased from 610 to 580% (Jia, Z. X., Luo, Y. F., Yang, S. Y., Guo, B. C., Du, M. L., & Jia, 2009). The results of PP composites tensile tests show that an increase in fiber content causes an increase in the tensile strength. The same trend is observed for elongation and toughness. PP fibers with high aspect ratio can transfer load from matrix over a large surface. Although the
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strength of PP fiber is about 400 MPa, the strength of composite does not increase significantly and the theoretical strength measurements, obtained by rule of mixtures, do not match the experimental results. This could be explained by the fact that the strength of composites strongly depends on the strength of the interface. Since the matrix must transfer the load to the fiber through the interface, an interface which is not sufficiently strong causes the load to be released instead of being transferred to the fiber. The role of the interface could be clarified by microscopic evaluation. Fig. 5 shows SEM micrographs of composite samples with 1 and 2 wt% PP fibers. As seen in the micrographs, the surface of fiber is almost smooth and implies lack of compatibility. Hence, there is a weak bonding between the matrix and the fiber and PP fibers cannot completely play their role in increasing the strength. In this condition, fibers tend to pull out and some voids will be made in the matrix which can produce cracks and hence samples break rapidly and strength decreases. This is why PP fibers cannot improve the strength of silicone drastically. However the fibers pulling out consumes part of the applied load and energy which leads to an increase in the strength and fracture energy (toughness). Saliba et al. reported a similar microscopic image. According to their research short inorganic silexil fibers were distributed in polyurethane and pulling out occurred due to nonsurface treated fibers (Saliba, C. C., Oréfice, R. L., Carneiro, J. R. G., Duarte, A. K., Schneider, W. T., & Fernandes, 2005). The energy required for the pulling out is measured by Eq. 2 as follows:
W 1 d 2 lc f 4
Eq. 2
Where d, lc, and σf are diameter, critical length and strength of fibers, respectively. By increasing these parameters, the energy required for pulling out the fibers increases (N. G. McCrum, C. P. Buckley, 1997). Assuming τ0 = σy/2, the shear yield stress of silicone rubber is 1.97 MPa.
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Critical length defined as the minimum length required to increase composite strength is measured as follows: d L 1 f 2 0
Eq. 3
The critical length of fiber is achieved 3.05 mm and as the length of fibers used in this study is higher than Lc, fibers are effective in strengthening. By using Eq. 3, fiber pull out energy is to be 0.86 mJ. By measuring the number of fibers (by volume), pull out energy of all fibers in 1 and 2 wt% PP composite are achieved 561123 mJ and 1125565 mJ. The area under stress-strain curve which is equivalent to the absorbed energy are 2505.97 and 2660.89 mJ, but part of this energy relates to matrix material and by subtracting the matrix energy (measured by the area under the curve of pure silicone rubber) from this, absorbed energy of 1 and 2 wt% PP composites are achieved 741.94 and 896.89 mJ, respectively. By comparing these two energy (the energy measured by the fiber pull out and the energy measured by the area under curve), it is observed that these are not the same. These differences can be attributed to the random distribution of fibers in matrix which are partly positioned in the load direction and may not experience pull out. Besides, the assumed value of τ0 is not very accurate. Tensile engineering stress-strain curves of silica and PP/ silicone rubber composites after being soaked in saline solution as a simulated body fluid are illustrated in Fig. 6. Tensile behavior of samples after being soaked in SBF show the same trend as before, but strength decrease slightly and elongation increase in SBF. Table 2 shows the details of tests in SBF. Increasing of elongation may be due to role of solution as a plasticizer between polymer chains which can soften composites (Dhakal et al., 2007). The strengths of the samples before and after being soaked in SBF are depicted in Fig.7. Interaction of reinforcements and solution can be a reason of property drop which leads to the 7
weakening of the reinforcement’s interaction with silicone rubber. This result is in accordance with other studies. Dai et al. observed some voids on the surface of silicone rubber in SEM micrographs after being soaked in saline serum which were because of the detachment of silica from silicone after the interaction by solution (Dai et al., 2006). SEM images of composites in this study after being soaked in SBF for about 3 months are observed in Fig. 8. The figures indicate the debonding and the formation of the repeated voids. These voids attach together and void coalescence occur in the composite. The effect of solution in PP fibers can also be seen in Fig. 8. As the strength drop is one of the disadvantages of biomaterials leading to earlier failure of implant in body, materials selection is an important factor. Results show that the presence of PP fibers leads to less strength drop. In order to compare the tensile strength of the composite and hybrid samples with constant wt% of 2 of reinforcements, Fig. 9 is shown. As can be seen, tensile strength in hybrid composite is more than SR/SiO2 composite, i.e. presence of both fibers and nano particles show better effect on tensile strength than presence of just silica nano particles. It can be explained by the strengthening mechanism of both fibers and nano particles. Effect of aspect ratio in fibers is more than role of nano particles in strength and so hybrid composite can not reach to SR/PP composite (Campbell, 2010). Besides, fibers have lower density than nano particles and in a constant content, volume fraction of PP fibers are more than nano particles. Stress-strain curve of these samples in SBF show same trend as before, but with lower strength and higher elongation. 4. Conclusions Silicone rubber has been used in finger joint replacement. Despite unique properties, silicone rubber has poor mechanical properties which leads to its failure. However the mechanical
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properties could be improved by the use of fillers. In this study silica nano particles and PP fibers were added to silicone rubber and the following results were found: 1) Tensile behavior of silicone rubber in hybrid composites improved from 5.6 MPa to 6.21 MPa in comparison with 2 wt% silica/ silicone composite. 2) Strength drop of composites in SBF was about 6% and composites with PP fibers showed less strength drop. 3) SEM images showed voids and fiber degradation after the composites were soaked in SBF. References Ahmed, S., Taxila, T., Bukhari, I.A., Qureshi, S.A., 2006. A Study on Properties of Polypropylene Fiber Reinforced Concrete, in: 31st Conference on Our World in Concrete & Structures. Singapore, pp. 1–10. Bei-xing, L., Ming-xiang, C., Fang, C., & Lu-ping, L., 2004. The Mechanical Properties of Polypropylene Fiber Reinforced Concrete. J. Wuhan Univ. Technol. Sci. Ed. 19, 68–71. Campbell, F.C., 2010. Introduction to Composite Materials, Structural Composite Materials. ASM International. Dai, J., Yao, X., Yeh, H.Y., Liang, X., 2006. Moisture Absorption of Filled Silicone Rubber Under Electrolyte. J. Appl. Polym. Sci. 99, 2253–2257. doi:10.1002/app.22403 Dhakal, H., Zhang, Z., Richardson, M., 2007. Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites. Compos. Sci. Technol. 67, 1674–1683. doi:10.1016/j.compscitech.2006.06.019 El-Hag, A.H., Simon, L.C., Jayaram, S.H., Cherney, E. a., 2006. Erosion Resistance of Nano-
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Filled Silicone Rubber. IEEE Trans. Dielectr. Electr. Insul. 13, 122–128. doi:10.1109/TDEI.2006.1593410 Germano, A.D., Seifert, J.M., Kindersberger, J., Muenchen, 2013. Influence of Nanosilica on the Performance of HTV Silcone Rubber for Outdoor Insulation, in: 18th International Symposium on High Voltage Engineering. pp. 3–7. Hosseini, M. S., Tazzoli-Shadpour, M., Amjadi, I., Haghighipour, N., Shokrgozar, M. A., & Boroujerdnia, M.G., 2012. Relationship Between Cell Compatibility and Elastic Modulus of Silicone Rubber/Organoclay Nanobiocomposites. Jundishapur J. Nat. Pharm. Prod. 7, 65– 70. Hron, P., Šlechtová, J., Smetana, K., Dvořánková, B., & Lopour, P., 1997. Silicone RubberHydrogel Composites as Polymeric Biomaterials: IX. Composites Containing Powdery Polyacrylamide Hydrogel. Biomaterials 18, 1069–1073. Jia, Z. X., Luo, Y. F., Yang, S. Y., Guo, B. C., Du, M. L., & Jia, D.M., 2009. Morphology, Interfacial Interaction and Properties of Styrene-Butadiene Rubber/Modified Halloysite Nanotube Nanocomposites. Chinese J. Polym. Sci. 27, 857–864. Joan, H.P.Y., 2007. Plasma Surface Modification of Biomedical Polymers and Metals. University of Sydney. Kim, H. S., Kwon, S. M., Lee, K. H., Yoon, J. S., & Jin, H.J., 2008. Preparation and Characterization of Silicone Rubber/Functionalized Carbon Nanotubes Composites via In Situ Polymerization. J. Nanosci. Nanotechnol. 8, 5551–5554. doi:10.1166/jnn.2008.1312 Lester, L., 2009. The Biomechanical Analysis of the Hand in Rheumatoid Arthritis Patients With Mcp Arthroplasty. University of Birmingham. 10
Mahomed, A., 2008. Properties of elastomers for small-joint replacements. University of Birmingham. Messori, M., 2011. Recent Advances in Elastomeric Nanocomposites, in: V. Mittal, J.K.K. and K.P. (Ed.), Advanced Structured Materials. Springer-Verlag Berlin Heidelberg publishers, Berlin, pp. 307–342. doi:10.1007/978-3-642-15787-5 Momen, G., Farzaneh, M., 2011. Survey of Micro/Nano Filler Use to Improve Silicone Rubber for Outdoor Insulators. Rev. Adv. Mater. Sci. 27, 1–13. N. G. McCrum, C. P. Buckley, C.B.B., 1997. Principles of Polymer Engineering. Oxford Science Publication, New York Tokyo. Niebauer, J.J., Shaw, J.L., Doren, A.W., 1969. Silicone-Dacron Hinge Prosthesis: Design, Evaluation, and Application. Ann. Rheum. Dis. 28, 56–58. Parkkila, T., 2007. Sutter metacarpo-phalangeal arthroplasty in rheumatoid patients. University of Tampere. Pukanszky, B. É. L. A., & VÖRÖS, G., 1993. Mechanism of Interfacial Interactions in Particulate Filled Composites. Compos. Interfaces 1, 411–427. Rajeev, R.S., 2008. Fiber Reinforced Elastomers, in: Bhowmick, A.K. (Ed.), Current Topics in Elastomers Research. CRC Pres, pp. 351–394. Rey, T., Le Cam, J.-B., Chagnon, G., Favier, D., Rebouah, M., Razan, F., Robin, E., Didier, P., Heller, L., Faure, S., Janouchova, K., 2014. An Original Architectured NiTi Silicone Rubber Structure for Biomedical Applications. Mater. Sci. Eng. C. Mater. Biol. Appl. 45, 184–90. doi:10.1016/j.msec.2014.08.062
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Rezaei, M., Ahmadi-Joneidi, I., Parhizgar, A., Kahuri, H., & Sayani, A., 2013. Evaluation of Actual Field Ageing on silicone Rubber Insulator Under Coastal Environment. Life Sci. J. 10, 199–205. doi:10.1017/CBO9781107415324.004 Roger De Jaeger, M.G., 2009. Silicones in Industrial Applications. Nova Science Publishers. Saliba, C. C., Oréfice, R. L., Carneiro, J. R. G., Duarte, A. K., Schneider, W. T., & Fernandes, M.R.F., 2005. Effect of the Incorporation of a Novel Natural Inorganic Short Fiber on the Properties of Polyurethane Composites. Polym. Test. 24, 819–824. doi:10.1016/j.polymertesting.2005.07.008 Song, P., Peng, Z.J., Yue, Y.L., Zhang, H., Zhang, Z., Fan, Y.C., 2013. Mechanical Properties of Silicone Composites Reinforced with Micron- and Nano-Sized Magnetic Particles. Express Polym. Lett. 7, 546–553. doi:10.3144/expresspolymlett.2013.51 Thomas, X., 2007. 17. Silicones in Medical Applications, in: Inorganic Polymers. Nova Science Pub Inc., pp. 61–161. doi:10.1002/jbm.1043 Umemori, M., Taniike, T., Terano, M., 2012. Influences of Polypropylene Grafted to SiO2 Nanoparticles on the Crystallization Behavior and Mechanical Properties of Polypropylene/SiO2 Nanocomposites. Polym. Bull. 68, 1093–1108. doi:10.1007/s00289011-0612-y
Table 1 Tensile properties of silicone composites
Tensile strength (MPa) Elongation (%)
0
1%SiO2
2%SiO2
1%PP
2%PP
3.94 1114.14
4.66 1047.2
5.6 955.1
5.5 1044.62
6.91 910.98
12
Toughness (MJ/m3)
19.1
20.57
21.91
27.52
29.17
Table 2 Tensile properties of silicone composites soaked in SBF
Tensile strength (MPa) Elongation (%) Toughness (MJ/m3)
0
1%SiO2
2%SiO2
1%PP
2%PP
3.85 1131.75 19.6
4.41 1123.96 22.07
5.26 1119.7 25.83
5.31 1106.19 27.65
6.74 954.15 29.3
Fig. 1. TEM micrographs of nano SiO2 powder before sonication. 13
Fig. 2. Short PP fibers.
14
Fig. 3. Flowchart of composites preparation steps.
6
8
Stress (MPa)
5 4
Stress (MPa)
SR S1 S2
3 2
(a)
1 0
SR PP1 PP2
6 4 2
(b)
0 0
2
4
6
8
Strain (mm/mm)
10
12
0
2
4
6
8
Strain (mm/mm)
10
12
Fig. 4. The stress-strain curve of (a) silica/ silicone rubber and (b) PP fibers/ silicone rubber composites.
15
16
a
b
Fig. 5. Scanning electron micrographs of PP/ silicone rubber composites; (a) 1% PP and (b) 2% PP.
6
8
Stress (MPa)
5 4
Stress (MPa)
SR S1 S2
3 2
(a)
1 0
SR PP1 PP2
6 4 2
(b)
0 0
2
4
6
8
Strain (mm/mm)
10
12
0
2
4
6
8
Strain (mm/mm)
10
12
Fig. 6. The stress-strain curve of (a) silica/ silicone rubber and (b) PP fibers/ silicone rubber composites after being soaked in SBF.
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Tensile Strength (MPa)
8 7
air
saline
6 5 4 3 2 1 0
SR
S1
S2
PP1
PP2
Fig. 7. Comparison of tensile strength of samples before and after being soaked in SBF.
18
a
b
Fig. 8. SEM micrographs of silicone rubber composites; (a) 2 wt% silica and (b) 2 wt% PP after being soaked in SBF.
8 PP2 SPP11 S2
6
PP2
Stress (MPa)
Stress (MPa)
8
4
(a)
2
SPP11
6
S2
4
(b)
2 0
0 0
3
6
Strain (mm/mm)
9
0
12
3
6
Strain (mm/mm)
9
12
Fig. 9. Tensile strength of silicone rubber composites and hybrid composites (a) before and (b) after being soaked in SBF.
19
20
