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PLLA composites after immersion in simulated body environments
Composites Science and Technology 70 (2010) 1820–1825
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Strain rate dependency of mechanical properties of TCP/PLLA composites after immersion in simulated body environments Satoshi Kobayashi *, Shusaku Yamadi Department of Mechanical Engineering, Graduate School of Science and Engineering, Tokyo Metropolitan University, Japan
a r t i c l e
i n f o
Article history: Available online 1 July 2010 Keywords: A. Particle-reinforced composites B. Environmental degradation B. Debonding D. Fractography
a b s t r a c t In order to clarify strain rate dependency of mechanical properties of b-tricalcium phosphate (TCP)/ poly(L-lactic acid) (PLLA) composites after immersion in simulated body environment, tensile tests at various loading rate were conducted on the TCP/PLLA specimens with and without immersion. TCP contents in the composite were 5, 10 and 15 wt%. Phosphate buffered solution was selected as simulated body environment and immersion periods were 8, 16 and 24 weeks. Young’s modulus and tensile strength increased with increasing strain rates. However, the strain rate dependencies decreased with immersion. Swelling and cracks around TCP agglomerations were observed in the cross-section of 15 wt% specimen after 24 weeks immersion. From the fracture surface observation, voids existed only in the ductile fracture surface of the specimen without immersion, whereas they existed in both ductile and brittle surface of the specimen with immersion. These results indicated that diffused water through the interfaces between TCP and PLLA hydrolyzed and weakened the interfaces and/or matrix near the interfaces. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Metallic bone fixation devices require re-operation to remove from body because of bone absorption caused by stress shielding and inflammatory. In order to improve the quality of life of patients, bioresorbable bone fixation devices with no requirement of re-operation have been developed. Poly(L-lactic acid) (PLLA) has been used for this types of fixations. PLLA is a bioresorbable plastics and is a polymer of L-lactic acid within living body so that PLLA has high biocompatibility. However, the stiffness of PLLA is lower. In order to overcome this weakness, bioactive ceramics, such as hydroxyapatite (HA) and b-tricalcium phosphate (TCP) have been used for the reinforcements. In the previous studies, the experimental characterizations of in vitro and in vivo bioresorbability and associated variations in mechanical properties have been conducted on bioactive ceramics/PLLA composites. Verheyen et al. reported that HA/PLLA composites lost their 50% of flexural strength with 3 weeks in vitro and in vivo [1]. Zang and Ma conducted compressive tests on the porous HA/PLLA composites created by biomimetic process and concluded that the compressive modulus are almost constant after 60 days immersion in a Tris buffer [2]. Ignatius et al. fabricated TCP/poly(L,DL-lactide)
* Corresponding author. Address: Department of Mechanical Engineering, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan. Tel.: +81 42 677 2704; fax: +81 42 677 2701. E-mail address: [email protected] (S. Kobayashi). 0266-3538/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2010.06.008
composite pins and conducted in vitro degradation tests up to 78 weeks [3]. They reported that the blending 10% or 30% TCP decreased the initial mechanical properties and led to an accelerated degradation rate, i.e. the pins with 30 wt% TCP lost half of their strength after 16 weeks faster than the unmodified pins (40 weeks). In contrast, Bleach et al. reported that unfilled samples absorbed more water and show greater mass loss than the samples containing TCP fillers [4]. Kikuchi et al. conducted three-point bending tests on copolymerized PLLA (CPLA) and TCP/CPLA composites [5]. They found that the bending strength of pure CPLA was constant for 4 weeks but subsequently decreased rapidly, whereas that of composite declined immediately. Niemelä conducted shear testing on self-reinforced PLLA (SRPLA) and TCP/ SRPLA composites after in vitro hydrolysis tests [6]. All SRPLA rods retained their initial shear strength virtually unchanged up to 30– 36 weeks. After that the shear strength of SRPLA decreased rapidly, being only about 15% of initial value after 52 weeks in vitro, whereas the TCP/SRPLA composites still have about 60–70% left. Ehrenfried et al. reported that a 40 wt% TCP addition retard the degradation of TCP/polylactide-co-glycolide composites [7]. As shown above, many contradictable results exist, because bioresorption of calcium phosphate/PLLA composites depends on both chemical structure (molecular weight, crystallization, orientation, etc.) and sample geometries, which are associated with water diffusion and release of hydrolyzed products. Although the components of the composite are not only matrix but also filler and interface between filler and matrix, most of those discuss the variation in the property from the point of degradation of
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matrix PLLA. In addition, these studies were conducted at strain rate of less than 10 4 s 1. Considering the strain rate dependency of polymer materials and practical usage up to 100 s 1, the characterizations at higher strain rate are necessary. Dynamic fracture toughness of poly(lactic acid) [8] and poly(lactic acid)/poly(caprolacton) polymer blends [9,10] and strain rate dependency of mechanical properties of PLLA [11] were reported, however, the investigation on calcium phosphate/PLLA composites are limited [12]. The clarification of effect of hydrolysis on the strain rate dependency of basic mechanical properties, such as strength and modulus, is also important. The objective of the present study is to clarify the effect of strain rate on the mechanical behavior of hydrolyzed TCP/PLLA composites toward materials design for bioresorbable therapeutic materials considering strain rate effect. The biodegradation mechanism is also discussed. 2. Experiments 2.1. Materials Poly-L-lactide, Lacty#5000 (Shimadzu Co. Ltd., Japan) and b-tricalcium phosphate powder, b-TCP (Taihei Chemical Industrial Co. Ltd., Japan) were used as the matrix and the filler materials, respectively. The diameter of TCP particle is less than 2.0 lm with spherical shape. Specific surface area is 50–60 m2/g. The PLLA pellets and TCP powder were mixed in a polyethylene bottle in dry condition. TCP/PLLA mix proportions were 5/95, 10/90, and 15/85 in weight. Rectangular specimens (100 mm 10 mm 4 mm) of the TCP/ PLLA composites were then injection molded from the mixtures under the conditions listed in Table 1. Since low contents composites were dealt in the present study, kneading of TCP and PLLA was processed only in the cylinder of the injection molding machine. Pure PLLA specimens were also molded for comparison. The number of specimens is at least six for each condition. 2.2. Immersion tests In the present study, 1/15 mol/l phosphate buffered solution (PBS) was selected as simulated body environment. The pHs of PBS was 7.4. 500 ml PBS were used for 15 samples, which results in 6.6 ml/g samples. PBS was poured into a tray and the both ends of the specimens were placed on aluminum plates to immerse the whole of the specimen. The trays were located in an incubator. Temperature in the incubator was kept at 37 °C. The immersion periods were 8, 16, and 24 weeks. After immersion tests, PBS solution at the surface of the specimen was wiped up and dried in the vacuum desiccator for 48 h.
10, 100 and 1000 mm/min, which result in the strain rate of 10 3, 10 2 and 10 1 s 1, respectively. Six specimens per TCP fraction and strain rate were tested. After tensile tests, fracture surfaces were observed with a scanning electron microscope (SEM) (HITACHI, Ltd., Tokyo, Japan) to clarify the degradation and fracture process. In the rest of the paper, we discuss tendency of tensile properties quantitatively based on t-test with significance level of 5%. 3. Results and discussion Fig. 1 shows the surface view of 15 wt% specimen with and without immersion in the simulated body environment. After 24 weeks immersion, swelling of TCP agglomerations around the specimen surface was observed. Fig. 2 shows the cross-section of the specimen around an agglomeration observed by scanning electron microscopy. Cracks were observed around the agglomeration, which is also considered to weaken the strength of the composites. Fig. 3 shows the relation between the weight variation ratio and immersion days. Water uptake increased with TCP contents. This result indicates the water diffused into TCP and/or TCP/PLLA interface. Fig. 4 shows cryogenic fracture surface of the 15 wt% composites with and without immersion. Adhesion between TCP and PLLA was good without immersion, whereas gaps between TCP and PLLA exist with 24 weeks immersion. This means no interfaces between TCP and PLLA or very low interfacial strength due to hydrolysis. This result also indicated the water diffused via TCP/PLLA interface. Fig. 5 shows stress–strain curves of TCP/PLLA composites with and without immersion in the simulated body environment. In all specimens, the strain at the nonlinear behavior initiation increased with increasing strain rate in each immersion period. Fracture strain decreased with increasing TCP contents. The fracture strain decreased with increasing strain rates, which is mainly attributed the strain rate dependency of the matrix [11]. The fracture strain increased with increasing immersion period in 5 wt% specimen. In 10 and 15 wt% specimens, the increases in fracture strain were observed at 8 weeks. At the initial stage of immersion, water absorption toughened PLLA, which result in the improvement in the elongation followed by degradation of PLLA by hydrolysis [13]. From Fig. 4, decreasing rate with immersion was larger in 15 wt% specimen. This result means the
2.3. Tensile tests The specimens were polished with 180 and 800 grits abrasive papers in order to obliterate imprint of an extruded rod introduced during the injection molding process. Then aluminum tabs and strain gauges were bonded on the ends and middle of specimens. These specimens were tensile tested at room temperature using Autograph AG-IS 50kNE (SHIMAZU CO., Ltd., Japan) at testing rate
(a) 0weeks
Table 1 Molding conditions. Mold clamping force (kN) Injection pressure (MPa) Fusion temperature (°C) Mold temperature (°C) Injection time (s) Cooling time(s)
69 114 200 50 40 30
(b) 24weeks 10mm Fig. 1. Surface view of 15 wt% b-TCP/PLLA composites.
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A
20 µm
3 mm (a) Cross Section
(b) Enlargement of A
Fig. 2. Cross section of 15 wt% b-TCP/PLLA composites after 24 weeks immersion in simulated body environment.
Weight Variation Ratio (%)
2.0 5wt%
10wt%
15wt%
1.5
1.0
0.5
0.0
0
50
100
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200
Immersion Period (Days) Fig. 3. Relationship between weight variation ratio and immersion period.
hydrolysis was faster in specimen with larger TCP contents. The larger area of interface between TCP and PLLA in the specimens
(a) 0 Week
with larger TCP contents is considered to act as the path of water diffusion. Fig. 6 shows Young’s modulus as functions of the immersion period and strain rate. Young’s modulus increased with increasing strain rate. Little effect of immersion on modulus was observed in 5 wt% specimens at each strain rate, whereas the decreases with immersion were observed in the 10 and 15 wt% specimens. Especially, Young’s modulus of 15 wt% specimen decreased significantly. Fig. 7 shows result of Young’s modulus of pure PLLA under strain rate of 10 3 s 1. There are not many differences with immersion. This result suggests that the degradation is mainly due to TCP and/or TCP/PLLA interface. That is to say, the damages such as the interface debonding between TCP and PLLA and/or hydrolysis of PLLA near the TCP/PLLA interface, as shown in Figs. 2 and 4 were considered to prevent the stress transfer to TCP particles with higher modulus so that the TCP particles lost the load carrying capacity and net TCP contents in the composites decreased. For this reason, Young’s modulus decreased significantly in larger TCP contents.
(b) 24 Week
Fig. 4. Cryogenic fracture surfaces of 15 wt% b-TCP/PLLA composites before and after immersion.
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S. Kobayashi, S. Yamadi / Composites Science and Technology 70 (2010) 1820–1825
5
0 0
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Strain [%]
2
Strain [%]
(c) 15wt%, 10-3 /s
(f) 15wt%, 10-1 /s
Fig. 5. Stress–strain curves of b-TCP/PLLA composites.
Fig. 8 shows tensile strength as functions of the immersion period and strain rate. Tensile strength increased with increasing strain rate. A clear decrease in strength was observed only in the 15 wt% specimen under strain rate of 10 1. This is considered as the effect of the embrittlement of matrix due to hydrolysis [13] especially at higher strain rate. For the other case, effect of immersion was not observed clearly. This tendency was observed in the result of pure PLLA as shown in Fig. 9. These indicate that the little hydrolysis occurred in the specimen with lower TCP contents. Considering decreasing strength with TCP contents, the TCP particles debonded at the initial stage of loading and acted as defects, rather than reinforcements. In order to improve the strength, some Interfacial treatments are necessary [14]. Fig. 10 shows fracture surfaces of 15 wt% specimens observed by scanning electron microscopy. In our previous study, it is clarified that the fracture surfaces were classified into two different
regions, such as the region where debondings between TCP and PLLA have grown into the void shapes (ductile surfaces) and the region without debonding around particles (brittle surfaces) [15]. Similar surfaces were observed in the specimen after immersion. The number of voids increased with lower strain rates (Fig. 10a and d). Comparing to specimen without immersion, more voids were observed in the ductile fracture surfaces of the specimen after immersion (Fig. 10b and e). Voids also existed in the brittle fracture surface with immersion (Fig. 10c and f), whereas voids did not exist in the brittle fracture surfaces of the specimens without immersion. The existence of two types of fracture aspects in the fracture surface, with and without voids, is attributed to the crack growth rate. Debondings formed at the TCP/PLLA interface where the energy release rate reaches the critical values. When the energy release rate at the matrix in front of the process zone reaches the fracture toughness of PLLA with successive loading, the specimen
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80
4.25 10 -3 /s 4.00
10 -2 /s 10 -1 /s
3.75
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(c) 15wt% Fig. 6. Young’s modulus of b-TCP/PLLA composites as functions of the immersion period and strain rate.
Fig. 8. Tensile strength of b-TCP/PLLA as functions of the immersion period and strain rate.
100
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Young's Modulus (GPa)
4.0
3.5
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3
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as a function of
does not carry more load and matrix catastrophic failure occurs without TCP/PLLA debondings [15]. In order to form the voids in the brittle fracture surface, lower critical energy release rate associated with TCP/PLLA debonding must be necessary. As mentioned above, the interfaces between TCP and PLLA acted as the water diffusion paths and the diffused water hydrolyzed and weakened the interfaces and/or matrix near the interfaces. As a result, voids formed in the brittle fracture surface.
8
16
24
Immersion Period (weeks) Fig. 9. Tensile strength of pure PLLA under strain rate of 10 the immersion period.
3
s
1
as a function of
4. Conclusion The immersion tests in simulated body environment and tensile tests at various loading rates were conducted on TCP/PLLA composites with TCP contents of 5, 10 and 15 wt%. Young’s modulus and tensile strength increased with increasing strain rate at each immersion period. The strain rate dependencies, however, de-
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1825
Fig. 10. Fracture surfaces of 15 wt% b-TCP/PLLA composites with and without immersion in simulated body environment.
creased with immersion period in the specimen with larger TCP contents. From the cross-sectional observation after immersion, cracks around TCP agglomerations were observed. Decreasing rate of fracture strain with immersion was larger in 15 wt% specimen. Young’s modulus and tensile strength in the specimen with larger TCP contents also decreased with immersion. From the fracture surface observation, two types of region, which consisted with ductile and brittle surfaces were observed. Voids existed in the ductile surface in the specimens without immersion, whereas they existed in both ductile and brittle surface in the specimens with immersion. These results indicate that the interfaces between TCP and PLLA acted as the water diffusion paths and the diffused water hydrolyzed and weakened the interfaces and/or matrix near the interfaces.
References [1] Verheyen CCCPM, de Wijin JR, van Blitterswijk CA, de Groot K. Evaluation of hydroxyapatite/poly(L-lactide) composites: mechanical behavior. J Biomed Mater Res 1992;26:1277–96. [2] Zhang R, Ma PX. Porous poly(L-lactic acid)/apatite composites created by biomimetic process. J Biomed Mater Res 1999;45:285–93. [3] Ignatius AA, Augat P, Claes L. Degradation behavior of composite pins made of tricalcium phosphate and poly(L,DL-lactide). J Biomater Sci Polym Ed 2001;12:185–94.
[4] Bleach NC, Tanner KE, Kellomäki M, Törmälä P. Effect of filler type on the mechanical properties of self-reinforced polylactide–calcium phosphate composites. J Mater Sci: Mater Med 2001;12:911–5. [5] Kikuchi M, Koyama Y, Takakuda K, Miyairi H, Shirahama N, Tanaka J. In vitro change in mechanical strength of b-tricalcium phosphate/copolymerized polyL-lactide composites and their application for guided bone regeneration. J Biomed Mater Res 2002;62:265–72. [6] Niemelä T. Effect of b-tricalcium phosphate addition on the in vitro degradation of self-reinforced poly-L,D-lactide. Polym Degrad Stab 2005;89:492–500. [7] Ehrenfried LM, Patel MH, Cameron RE. The effect of tri-calcium phosphate (TCP) addition on the degradation of polylactide-co-glycolide (PLGA). J Mater Sci: Mater Med 2008;19:459–66. [8] Park SD, Todo M, Arakawa K. Effect of annealing on the fracture toughness of poly(lactic acid). J Mater Sci 2004;39:1113–6. [9] Takayama T, Todo M. Improvement of impact fracture properties of PLA/PCL polymer blend due to LTI addition. J Mater Sci 2006;41:4989–92. [10] Takayama T, Todo M, Tsuji H, Arakawa K. Effect of LTI content on impact fracture property of PLA/PCL/LTI polymer blends. J Mater Sci 2006;41:6501–4. [11] Yamadi S, Kobayashi S. Effect of strain rate on the mechanical properties of crystallized poly(L-lactide). J Biomech Sci Eng 2008;3:453–60. [12] Yamadi S, Kobayashi S. Effects of strain rate on the mechanical properties of tricalcium phosphate/poly(L-lactide) composites. J Mater Sci: Mater Med 2009;20:67–74. [13] Kobayashi S, Todo M. The effect of hydrolysis on the mechanical properties of injection-molded poly(L-lactic acid). J Solid Mech Mater Eng 2008;2:8–14. [14] Qiu X, Hong Z, Hu J, Chen L, Chen X, Jing X. Hydroxyapatite surface modified by L-lactic acid and its subsequent grafting polymerization of L-lactide. Biomacromolecules 2005;6:1193–9. [15] Kobayashi S, Sakamoto K. Experimental and analytical characterization of btricalcium phosphate particle reinforced poly-L-lactide composites. JSME Int J A 2006;49:314–20.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Strain rate dependency of mechanical properties of TCP/PLLA composites after immersion in simulated body environments Satoshi Kobayashi *, Shusaku Yamadi Department of Mechanical Engineering, Graduate School of Science and Engineering, Tokyo Metropolitan University, Japan
a r t i c l e
i n f o
Article history: Available online 1 July 2010 Keywords: A. Particle-reinforced composites B. Environmental degradation B. Debonding D. Fractography
a b s t r a c t In order to clarify strain rate dependency of mechanical properties of b-tricalcium phosphate (TCP)/ poly(L-lactic acid) (PLLA) composites after immersion in simulated body environment, tensile tests at various loading rate were conducted on the TCP/PLLA specimens with and without immersion. TCP contents in the composite were 5, 10 and 15 wt%. Phosphate buffered solution was selected as simulated body environment and immersion periods were 8, 16 and 24 weeks. Young’s modulus and tensile strength increased with increasing strain rates. However, the strain rate dependencies decreased with immersion. Swelling and cracks around TCP agglomerations were observed in the cross-section of 15 wt% specimen after 24 weeks immersion. From the fracture surface observation, voids existed only in the ductile fracture surface of the specimen without immersion, whereas they existed in both ductile and brittle surface of the specimen with immersion. These results indicated that diffused water through the interfaces between TCP and PLLA hydrolyzed and weakened the interfaces and/or matrix near the interfaces. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Metallic bone fixation devices require re-operation to remove from body because of bone absorption caused by stress shielding and inflammatory. In order to improve the quality of life of patients, bioresorbable bone fixation devices with no requirement of re-operation have been developed. Poly(L-lactic acid) (PLLA) has been used for this types of fixations. PLLA is a bioresorbable plastics and is a polymer of L-lactic acid within living body so that PLLA has high biocompatibility. However, the stiffness of PLLA is lower. In order to overcome this weakness, bioactive ceramics, such as hydroxyapatite (HA) and b-tricalcium phosphate (TCP) have been used for the reinforcements. In the previous studies, the experimental characterizations of in vitro and in vivo bioresorbability and associated variations in mechanical properties have been conducted on bioactive ceramics/PLLA composites. Verheyen et al. reported that HA/PLLA composites lost their 50% of flexural strength with 3 weeks in vitro and in vivo [1]. Zang and Ma conducted compressive tests on the porous HA/PLLA composites created by biomimetic process and concluded that the compressive modulus are almost constant after 60 days immersion in a Tris buffer [2]. Ignatius et al. fabricated TCP/poly(L,DL-lactide)
* Corresponding author. Address: Department of Mechanical Engineering, Graduate School of Science and Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan. Tel.: +81 42 677 2704; fax: +81 42 677 2701. E-mail address: [email protected] (S. Kobayashi). 0266-3538/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2010.06.008
composite pins and conducted in vitro degradation tests up to 78 weeks [3]. They reported that the blending 10% or 30% TCP decreased the initial mechanical properties and led to an accelerated degradation rate, i.e. the pins with 30 wt% TCP lost half of their strength after 16 weeks faster than the unmodified pins (40 weeks). In contrast, Bleach et al. reported that unfilled samples absorbed more water and show greater mass loss than the samples containing TCP fillers [4]. Kikuchi et al. conducted three-point bending tests on copolymerized PLLA (CPLA) and TCP/CPLA composites [5]. They found that the bending strength of pure CPLA was constant for 4 weeks but subsequently decreased rapidly, whereas that of composite declined immediately. Niemelä conducted shear testing on self-reinforced PLLA (SRPLA) and TCP/ SRPLA composites after in vitro hydrolysis tests [6]. All SRPLA rods retained their initial shear strength virtually unchanged up to 30– 36 weeks. After that the shear strength of SRPLA decreased rapidly, being only about 15% of initial value after 52 weeks in vitro, whereas the TCP/SRPLA composites still have about 60–70% left. Ehrenfried et al. reported that a 40 wt% TCP addition retard the degradation of TCP/polylactide-co-glycolide composites [7]. As shown above, many contradictable results exist, because bioresorption of calcium phosphate/PLLA composites depends on both chemical structure (molecular weight, crystallization, orientation, etc.) and sample geometries, which are associated with water diffusion and release of hydrolyzed products. Although the components of the composite are not only matrix but also filler and interface between filler and matrix, most of those discuss the variation in the property from the point of degradation of
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matrix PLLA. In addition, these studies were conducted at strain rate of less than 10 4 s 1. Considering the strain rate dependency of polymer materials and practical usage up to 100 s 1, the characterizations at higher strain rate are necessary. Dynamic fracture toughness of poly(lactic acid) [8] and poly(lactic acid)/poly(caprolacton) polymer blends [9,10] and strain rate dependency of mechanical properties of PLLA [11] were reported, however, the investigation on calcium phosphate/PLLA composites are limited [12]. The clarification of effect of hydrolysis on the strain rate dependency of basic mechanical properties, such as strength and modulus, is also important. The objective of the present study is to clarify the effect of strain rate on the mechanical behavior of hydrolyzed TCP/PLLA composites toward materials design for bioresorbable therapeutic materials considering strain rate effect. The biodegradation mechanism is also discussed. 2. Experiments 2.1. Materials Poly-L-lactide, Lacty#5000 (Shimadzu Co. Ltd., Japan) and b-tricalcium phosphate powder, b-TCP (Taihei Chemical Industrial Co. Ltd., Japan) were used as the matrix and the filler materials, respectively. The diameter of TCP particle is less than 2.0 lm with spherical shape. Specific surface area is 50–60 m2/g. The PLLA pellets and TCP powder were mixed in a polyethylene bottle in dry condition. TCP/PLLA mix proportions were 5/95, 10/90, and 15/85 in weight. Rectangular specimens (100 mm 10 mm 4 mm) of the TCP/ PLLA composites were then injection molded from the mixtures under the conditions listed in Table 1. Since low contents composites were dealt in the present study, kneading of TCP and PLLA was processed only in the cylinder of the injection molding machine. Pure PLLA specimens were also molded for comparison. The number of specimens is at least six for each condition. 2.2. Immersion tests In the present study, 1/15 mol/l phosphate buffered solution (PBS) was selected as simulated body environment. The pHs of PBS was 7.4. 500 ml PBS were used for 15 samples, which results in 6.6 ml/g samples. PBS was poured into a tray and the both ends of the specimens were placed on aluminum plates to immerse the whole of the specimen. The trays were located in an incubator. Temperature in the incubator was kept at 37 °C. The immersion periods were 8, 16, and 24 weeks. After immersion tests, PBS solution at the surface of the specimen was wiped up and dried in the vacuum desiccator for 48 h.
10, 100 and 1000 mm/min, which result in the strain rate of 10 3, 10 2 and 10 1 s 1, respectively. Six specimens per TCP fraction and strain rate were tested. After tensile tests, fracture surfaces were observed with a scanning electron microscope (SEM) (HITACHI, Ltd., Tokyo, Japan) to clarify the degradation and fracture process. In the rest of the paper, we discuss tendency of tensile properties quantitatively based on t-test with significance level of 5%. 3. Results and discussion Fig. 1 shows the surface view of 15 wt% specimen with and without immersion in the simulated body environment. After 24 weeks immersion, swelling of TCP agglomerations around the specimen surface was observed. Fig. 2 shows the cross-section of the specimen around an agglomeration observed by scanning electron microscopy. Cracks were observed around the agglomeration, which is also considered to weaken the strength of the composites. Fig. 3 shows the relation between the weight variation ratio and immersion days. Water uptake increased with TCP contents. This result indicates the water diffused into TCP and/or TCP/PLLA interface. Fig. 4 shows cryogenic fracture surface of the 15 wt% composites with and without immersion. Adhesion between TCP and PLLA was good without immersion, whereas gaps between TCP and PLLA exist with 24 weeks immersion. This means no interfaces between TCP and PLLA or very low interfacial strength due to hydrolysis. This result also indicated the water diffused via TCP/PLLA interface. Fig. 5 shows stress–strain curves of TCP/PLLA composites with and without immersion in the simulated body environment. In all specimens, the strain at the nonlinear behavior initiation increased with increasing strain rate in each immersion period. Fracture strain decreased with increasing TCP contents. The fracture strain decreased with increasing strain rates, which is mainly attributed the strain rate dependency of the matrix [11]. The fracture strain increased with increasing immersion period in 5 wt% specimen. In 10 and 15 wt% specimens, the increases in fracture strain were observed at 8 weeks. At the initial stage of immersion, water absorption toughened PLLA, which result in the improvement in the elongation followed by degradation of PLLA by hydrolysis [13]. From Fig. 4, decreasing rate with immersion was larger in 15 wt% specimen. This result means the
2.3. Tensile tests The specimens were polished with 180 and 800 grits abrasive papers in order to obliterate imprint of an extruded rod introduced during the injection molding process. Then aluminum tabs and strain gauges were bonded on the ends and middle of specimens. These specimens were tensile tested at room temperature using Autograph AG-IS 50kNE (SHIMAZU CO., Ltd., Japan) at testing rate
(a) 0weeks
Table 1 Molding conditions. Mold clamping force (kN) Injection pressure (MPa) Fusion temperature (°C) Mold temperature (°C) Injection time (s) Cooling time(s)
69 114 200 50 40 30
(b) 24weeks 10mm Fig. 1. Surface view of 15 wt% b-TCP/PLLA composites.
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A
20 µm
3 mm (a) Cross Section
(b) Enlargement of A
Fig. 2. Cross section of 15 wt% b-TCP/PLLA composites after 24 weeks immersion in simulated body environment.
Weight Variation Ratio (%)
2.0 5wt%
10wt%
15wt%
1.5
1.0
0.5
0.0
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150
200
Immersion Period (Days) Fig. 3. Relationship between weight variation ratio and immersion period.
hydrolysis was faster in specimen with larger TCP contents. The larger area of interface between TCP and PLLA in the specimens
(a) 0 Week
with larger TCP contents is considered to act as the path of water diffusion. Fig. 6 shows Young’s modulus as functions of the immersion period and strain rate. Young’s modulus increased with increasing strain rate. Little effect of immersion on modulus was observed in 5 wt% specimens at each strain rate, whereas the decreases with immersion were observed in the 10 and 15 wt% specimens. Especially, Young’s modulus of 15 wt% specimen decreased significantly. Fig. 7 shows result of Young’s modulus of pure PLLA under strain rate of 10 3 s 1. There are not many differences with immersion. This result suggests that the degradation is mainly due to TCP and/or TCP/PLLA interface. That is to say, the damages such as the interface debonding between TCP and PLLA and/or hydrolysis of PLLA near the TCP/PLLA interface, as shown in Figs. 2 and 4 were considered to prevent the stress transfer to TCP particles with higher modulus so that the TCP particles lost the load carrying capacity and net TCP contents in the composites decreased. For this reason, Young’s modulus decreased significantly in larger TCP contents.
(b) 24 Week
Fig. 4. Cryogenic fracture surfaces of 15 wt% b-TCP/PLLA composites before and after immersion.
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S. Kobayashi, S. Yamadi / Composites Science and Technology 70 (2010) 1820–1825
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0 0
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Strain [%]
2
Strain [%]
(c) 15wt%, 10-3 /s
(f) 15wt%, 10-1 /s
Fig. 5. Stress–strain curves of b-TCP/PLLA composites.
Fig. 8 shows tensile strength as functions of the immersion period and strain rate. Tensile strength increased with increasing strain rate. A clear decrease in strength was observed only in the 15 wt% specimen under strain rate of 10 1. This is considered as the effect of the embrittlement of matrix due to hydrolysis [13] especially at higher strain rate. For the other case, effect of immersion was not observed clearly. This tendency was observed in the result of pure PLLA as shown in Fig. 9. These indicate that the little hydrolysis occurred in the specimen with lower TCP contents. Considering decreasing strength with TCP contents, the TCP particles debonded at the initial stage of loading and acted as defects, rather than reinforcements. In order to improve the strength, some Interfacial treatments are necessary [14]. Fig. 10 shows fracture surfaces of 15 wt% specimens observed by scanning electron microscopy. In our previous study, it is clarified that the fracture surfaces were classified into two different
regions, such as the region where debondings between TCP and PLLA have grown into the void shapes (ductile surfaces) and the region without debonding around particles (brittle surfaces) [15]. Similar surfaces were observed in the specimen after immersion. The number of voids increased with lower strain rates (Fig. 10a and d). Comparing to specimen without immersion, more voids were observed in the ductile fracture surfaces of the specimen after immersion (Fig. 10b and e). Voids also existed in the brittle fracture surface with immersion (Fig. 10c and f), whereas voids did not exist in the brittle fracture surfaces of the specimens without immersion. The existence of two types of fracture aspects in the fracture surface, with and without voids, is attributed to the crack growth rate. Debondings formed at the TCP/PLLA interface where the energy release rate reaches the critical values. When the energy release rate at the matrix in front of the process zone reaches the fracture toughness of PLLA with successive loading, the specimen
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(c) 15wt% Fig. 6. Young’s modulus of b-TCP/PLLA composites as functions of the immersion period and strain rate.
Fig. 8. Tensile strength of b-TCP/PLLA as functions of the immersion period and strain rate.
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s
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does not carry more load and matrix catastrophic failure occurs without TCP/PLLA debondings [15]. In order to form the voids in the brittle fracture surface, lower critical energy release rate associated with TCP/PLLA debonding must be necessary. As mentioned above, the interfaces between TCP and PLLA acted as the water diffusion paths and the diffused water hydrolyzed and weakened the interfaces and/or matrix near the interfaces. As a result, voids formed in the brittle fracture surface.
8
16
24
Immersion Period (weeks) Fig. 9. Tensile strength of pure PLLA under strain rate of 10 the immersion period.
3
s
1
as a function of
4. Conclusion The immersion tests in simulated body environment and tensile tests at various loading rates were conducted on TCP/PLLA composites with TCP contents of 5, 10 and 15 wt%. Young’s modulus and tensile strength increased with increasing strain rate at each immersion period. The strain rate dependencies, however, de-
S. Kobayashi, S. Yamadi / Composites Science and Technology 70 (2010) 1820–1825
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Fig. 10. Fracture surfaces of 15 wt% b-TCP/PLLA composites with and without immersion in simulated body environment.
creased with immersion period in the specimen with larger TCP contents. From the cross-sectional observation after immersion, cracks around TCP agglomerations were observed. Decreasing rate of fracture strain with immersion was larger in 15 wt% specimen. Young’s modulus and tensile strength in the specimen with larger TCP contents also decreased with immersion. From the fracture surface observation, two types of region, which consisted with ductile and brittle surfaces were observed. Voids existed in the ductile surface in the specimens without immersion, whereas they existed in both ductile and brittle surface in the specimens with immersion. These results indicate that the interfaces between TCP and PLLA acted as the water diffusion paths and the diffused water hydrolyzed and weakened the interfaces and/or matrix near the interfaces.
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