HA-PP sandwich nano-composites by rolling process

Materials and Design 43 (2012) 549–559

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Fabrication of the new structure high toughness PP/HA-PP sandwich nano-composites by rolling process D. Younesi a, R. Mehravaran b, S. Akbarian c, M. Younesi d,e,⇑ a

Department of Orthopedic Surgery, School of Medicine, Jondishapoor University, Ahvaz, Iran Department of Maxillofacial Surgery, School of Dentistry, Shiraz University of Medicine, Shiraz, Iran c Department of Restorative Dentistry, School of Dentistry, Shiraz University of Medicine, Shiraz, Iran d Department of Materials Science and Engineering, School of Engineering, Shiraz University, Shiraz, Iran e Case Orthopedic Bioengineering Laboratory, Department of Mechanical and Aerospace Engineering, Case Western Reserve University, Cleveland, OH, USA b

a r t i c l e

i n f o

Article history: Received 16 March 2012 Accepted 10 July 2012 Available online 20 July 2012 Keywords: Hydroxyapatite Polypropylene Sandwich composite Rolling process Impact absorbed energy

a b s t r a c t In this study a designed rolling setup was used to fabricate new structure polypropylene/hydroxyapatitepolypropylene (PP/HA-PP) sandwich nano-composites. To check the effect of rolling process and PP layers content on the structure and mechanical properties of these sandwich composites, different mechanical tests and analysis were performed on these composites. Results of tensile, bending and buckling tests show the rolling process improves the strength, modulus and flexural rigidity of composites significantly while with increasing the PP layers content from 10 vol.% to 20 vol.% decreases the stiffness, flexural rigidity and modulus of composites slightly. Results of impact test demonstrate the rolling process and increasing the volume percentage of the PP layers in sandwich composites cause a dramatic improve in impact absorbed energy of the PP/HA-PP sandwich composites. The results of Differential Scanning Calorimetry (DSC) analysis confirm the rolling process increases the crystallinity and molecular alignment of polypropylene in composites. The results of mechanical tests and DSC analysis show the increasing of polypropylene molecular alignment by rolling process is the most dominant reason of improvement the mechanical properties of sandwich composites. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The fracture of bones resulting from trauma or natural aging is a typical type of tissue failure. An operative treatment frequently requires implantation of a temporary or permanent prosthesis, which remains a challenge for orthopedic surgeons, especially in the cases of large bone defects. A quickly aging population and serious drawbacks of natural bone grafts make the situation even worse; therefore, there is a high clinical demand for bone substitutes and investigations of artificial materials for bone tissue repair. This subject area is of particular interest in the field of biomaterials research for clinical applications. Regardless of their composition or application, materials used for body repair must meet both biofunctionality and biocompatibility. Biofunctionality concerns the ability of the implant to perform the purpose for which it was designated. These requirements are: (i) mechanical properties such as tensile strength, fracture toughness, elongation at fracture, Young’s modulus, and fatigue strength; ⇑ Corresponding author at: Case Orthopedic Laboratory, Department of Mechanical and Aerospace Engineering, Case Western Reserve University, Cleveland, OH, USA. Tel.: +1 2163348020. E-mail address: [email protected] (M. Younesi). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.07.022

(ii) physical properties such as density in the case of orthopedic implants, or thermal expansion in the case of bone cement; and (iii) surface chemistry such as degradation resistance, oxidation, corrosion, or bone bonding ability [1]. Polymer based composites provide alternative choice to overcome many short comings of homogenous materials mentioned above. The specific advantages of polymer composites for orthopaedic applications are highlighted in the following passage. One of the major problems in orthopaedic surgery is the mismatch of stiffness between the bone and metallic or ceramic implant. In the load sharing between the bone and implant, the amount of stress carried by each of them is directly related to their stiffness. Therefore, bone is insufficiently loaded compared to the implant. This phenomenon is called ‘‘stress shielding’’ or stress protection. The stress-shielding affects the bone remodeling and healing process leading to increased bone porosity [2–5]. It has been recognized that by matching the stiffness of implant with that of the host tissues reduces the stress-shielding effect and produces desired tissue remodeling. Since the polymer based composites exhibit simultaneously low elastic modulus and high strength, they are proposed for several orthopaedic applications. Another merit of composites materials is that by controlling the volume fractions and local and global arrangement of the reinforcement phase, the

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properties and design of an implant can be varied and tailored to suit the mechanical and physiological conditions of the host tissues. It is, therefore, suggested that composite materials offer a greater potential of structural biocompatibility than the homogenous monolithic materials [6]. Furthermore, the human tissues are essentially composite materials with anisotropic properties, which depend on their roles and structural arrangements. For example, the longitudinal mechanical properties of cortical bone are higher than the transverse direction properties. These similarities have led to the development of polymer composite biomaterials. Nowadays, use of the polymer based composites in biomaterials, especially in the orthopaedic field, has been increasing dramatically. The original concept of a ceramic reinforced polymer composite to be used as a bone substitute was introduced in 1981 by Bonfield et al. [7]. Several approaches [8–15] have been used in order to improve the mechanical properties of PE/HA composites. Special attention has been given to the reinforcement selection and characteristics [8,10–12] (composition, shape, granulometric distribution), and to the optimization of the processing techniques [8,13–15] and respective processing parameters, including compounding, compression and injection molding and extrusion. The molecular orientation of a semi-crystalline polymer can lead to a significant enhancement of the strength and stiffness in specific directions, providing a route for tailoring the mechanical anisotropy of the part [15–23]. Some examples are the approaches of the research groups of Ward [15–18] and Bevis [19–23] that have been successfully applied to several materials including polyethylene based composites [15,16,18,20]. Hydrostatic extrusion [15,16] has been recently used to produce load-bearing PE/HA composites with strength and stiffness in the desired range of human cortical bone and with the added benefit of superior ductility [15]. The use of a polyethylene matrix does not preclude the use of other polymers as matrix materials; indeed, the use of other materials may enhance the properties of the final composite. For this reason, polypropylene has been investigated as a matrix material. Like polyethylene, polypropylene is biologically inert, but unlike polyethylene, polypropylene generally exhibits better performance in fatigue and suffers less reduction in mechanical properties at elevated temperatures. Toughness is a key parameter which influences the performance of polymers in various engineering applications. Polypropylene has a remarkable combination of physical and mechanical properties, but it has poor impact strength, especially at low temperature due to the inherently high glass transition temperature (Tg) and high crystallinity. This problem becomes more fatal when a high percentage of (40 vol.%) reinforcement is added to it. In the last decade, different research groups try to solve this problem by different methods. Blending polypropylene with a high toughness polymer is one of the approaches which are for answering the brittleness problem of the polypropylene [24–26]. In previous works by authors the effect of production processing parameters (pressure and temperature of hot pressing process) and also adding of LDPE particles to PP/HA biocomposites on their mechanical properties (specially impact absorbed energy) were investigated [27,28]. Moreover, the toughness of particulate–polymer composites depends strongly on the particle size and particle– matrix interface adhesion. Many researchers investigate the effect of particle size and coupling agent on mechanical properties of particulate composites [29–37]. They showed reducing the reinforcing particles size and enhancing the strength of particle–matrix interface increase the mechanical properties of particulate composites (especially their toughness). The specific objectives of this experimental study were to prepare new structure PP/HA-PP composites, test their mechanical properties and use different parameter to improve the mechanical properties specially toughness of these composites.

2. Materials and methods 2.1. Materials Pure polypropylene (Grade: Moplen Q30G) and Polypropylene copolymer (Moplen EPD60R) was prepared from Arak Petroleum Co., Iran. Silane coupling agent (3-aminopropyltriethoxy silane) (cas No. 919-30-2) was purchased from Nanjing Capatue Chemical Co., Ltd., China. 2.2. Preparation and surface treatment HA nano-powder Hydroxyapatite was produced from cortical bovine bone according to the method mentioned in previous works [26]. The particles size of hydroxyapatite powder was reduced to nanometer scale by wet milling of hydroxyapatite powder with a high energy attrition mill. The average particle size of hydroxyapatite powder after milling was 150 nm, approximately. Hydroxyapatite powder was surface treated with silane coupling agent (3-aminopropyltriethoxy silane) according to the method in the previous author’s work [27]. Modification of HA surface was carried out using silane coupling agent. Silane was hydrolyzed in mixture of water and ethanol alcohol. The amount of silane used was 2.0% based on weights of HA powders. HA powders were immersed in the silane solutions and left under agitation at room temperature for 3 h. After that the powders were washed and dried at 80 °C for 12 h in an oven. 2.3. Preparation of PP/HA and PP/HA-PP sandwich composites PP/HA composite sheets with dimensions of 300 mm  150 mm  3 mm was produced by mixing, extruding and hot pressing of PP and HA powders according to previous work by Younesi and Bahrololoom [27]. Polypropylene and hydroxyapatite powders were mixed by volume ratio of 60:40 and extrude by an extruder and pelletized by a pelletizer. The pellets were ground by cry-milling and then resulted powder was hot pressed to the sheets under the pressure of 50 MPa and the temperature of 210 °C. The sheets of polypropylene with dimensions of 300 mm  150 mm  1 mm were made by hot pressing of PP pellets. To prepare the final sandwich composites, seven sheets of the PP/HA composites and six pure PP sheets were fixed on each other alternately (Fig. 1a). To have different percentages of pure polypropylene layers in sandwich composites (10 vol.%, 15 vol.%, and 20 vol.%) as toughening phase, the thickness ratio of two types of layers (PP/HA layers and pure PP layers) in composites was changed. In the next step, raw composites were hot pressed at the temperature of 210 °C and the pressure of 50 MPa to make the primary sandwich composites for rolling step. To have maximum biocompatibility of sandwich composites, two outer layers in all of sandwich composites were always PP/HA Composite layers. Afterward, the hot pressed sandwich composites were hot rolled by a designed rolling setup that is schematically shown in Fig. 1b. The thickness of all sandwich composites sheets before hot rolling is near 20 mm. For all the sandwich composites final thickness of composites sheets exactly was 4 mm after four rolling passes. The rolling temperature was about 125 °C. The linear rotation speed of rollers in the rolling process was 3 mm/s. 2.4. Characterization – XRD, SEM, particle size analysis A Bruker, D8 advance X-ray diffractometer with Cu Ka radiation was used to check the phase compositions and purity of the produced hydroxyapatite powder. X-ray diffraction spectra were taken at 40 kV and 40 mA and recorded from 2° to 60° for 2h at a scanning speed of 2.5 deg/min and a step size of 0.02°. A secondary electron microscope, Cambridge S-360 scanning electron microscope (SEM),

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Fig. 1a. Schematic presentation of the arrangement of the different layers in the PP/HA-PP sandwich composites.

performed by a Zwick-5102 impact testing machine. To measure the flexural rigidity of different composites uniaxial compression loading was performed on composite samples. The buckling tests specimens with dimensions of 100 mm  12 mm  4 mm were cut from the sandwich composites sheets. The gauge length of the buckling specimens was 70 mm. For all the mechanical tests, five samples were tested and the average amount of each set was reported. 2.6. DSC analysis To evaluate the effect of rolling process on structure of sandwich composites, Differential Scanning Calorimetry analysis (DSC) was performed on rolled composites using a Mettler Toledo 851E thermal analyzer by changing the temperature from 100 °C to 210 °C at a rate of 3 °C/min in air atmosphere. The weight of the samples for thermal analysis was 30 mg and a-Al2O3 was used as a crucible. 3. Results and discussion

Fig. 1b. Schematic presentation of the rolling setup used for fabrication of the sandwich composites.

by using 15 kV accelerating voltage was used to investigate the fracture mechanism and the morphology of fracture surfaces of mechanical tests specimens. A laser particle size analyzer (Mastersizer 3000, Malvern Instruments) was used to measure the particle size distribution of produced hydroxyapatite powder.

2.5. Mechanical tests For investigating the effect of the hot rolling process on the mechanical properties of sandwich composites, two samples groups for mechanical tests were prepared. One group of specimens was cut parallel to rolling direction from composite sheets and the other one was cut perpendicular to rolling direction. The samples for tensile and impact tests were prepared according to ASTM D-638 [38] and ASTM D-256 [39]. Tensile tests were performed using an Instron TT-CM-L testing machine with a strain rate of 2 mm/min and the stress–strain curves obtained in this way were used to determine the ultimate tensile strength (UTS) and elastic modulus of composites. Same instrument was used for flexural tests. The flexural tests samples were prepared in rectangular shape having dimensions of 50 mm  10 mm  4 mm. Impact tests were

Since the names of all the materials and composites that were used in this study are long, the abbreviations names for all them are mentioned in Table 1. Fig. 2a shows particle size distribution of the HA powder which prepared by hear treatment and milling process. According to this figure, the average particle size of the powders is about 150 nm. Fig. 2b presents the HA particle morphology. This image shows that the HA particles have a semi-round shape. In many studies, researchers claimed that using nano-size reinforcing particles in polymer matrix composites increases the mechanical properties of polymer composites more effectively than micro size particles [32–37]. Nano-particles by increasing the effective surface area in composites and also increasing the crack growth path in composites improve the mechanical properties of composites. In previous work, authors discussed the effect of reinforcing particle size on the mechanical properties of PP/HA composites comprehensively [29]. To confirm the purity and composition of the produced HA powder by the heat treatment and milling process, the XRD analysis was done on the HA powder (Fig. 2c). By comparing the result of the XRD analysis of the produced HA powder in this research with the X-ray diffraction (XRD) pattern of calcium HA SRM 2910 [29], it is seen both Table 1 Abbreviations that are used in this paper. Abbreviated name

Complete name

PP PP-R CPP HA PP/HA PP/HA-R PP/HA-10-R PP/HA-15-R PP/HA-20-R

Polypropylene Rolled polypropylene copolymer Copolymer polypropylene Hydroxyapatite Polypropylene/hydroxyapatite composite Rolled polypropylene/hydroxyapatite Rolled polypropylene/hydroxyapatite with 10 vol.% CPP Rolled polypropylene/hydroxyapatite with 15 vol.% CPP Rolled polypropylene/hydroxyapatite with 20 vol.% CPP

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Fig. 2a. Distribution of the HA particles size which is measured by the Laser Particle Size Analyzer.

Fig. 2b. SEM images of the HA nano-particles that were milled in the high energy wet attrition mill.

Fig. 2c. XRD analysis of the HA nano-powder produced by the high energy attrition milling.

patterns are completely similar in the number and the place of the peaks. It shows the process of preparing the HA powder introduces no impurities in the HA powder. Figs. 3a and 3b present the variations in ultimate tensile strength (UTS) and Young’s modulus of the PP/HA-PP sandwich composites with changing the volume percentage of the PP layers. According to this figure, the increase in the volume percentage of the PP layers percentage improves the tensile strength of the sandwich composites slightly. The results of tensile tests which are presented in this graph show the UTS of the rolled PP/HA-PP with 20 vol.% pure polypropylene layers is 40% more than that of the unrolled PP/HA composites. Also, the rolling process increase the UTS of PP/HA composites and PP about 23% and 86%, respectively. The

results, presented in Fig. 3a, show the strength of the unrolled pure polypropylene is much lower than the strength of the rolled PP and PP/HA composites. Moreover, the rolling process improves the strength of the pure PP more than the PP/HA composites. The rolling process makes the PP molecules oriented in the rolling direction. The orientating of molecules by rolling process is more efficient and easier for pure PP than the PP/HA composites because of its lower viscosity and easier molecule mobility. In the PP/HA composites, the HA particles act like pins avoid easy movement of PP molecules. Therefore, PP molecules cannot move easily and become aligned in the rolling direction. A high HA volume fraction in the PP/HA composites (40 vol.%) makes the aligning of polymer molecules more difficult by the rolling process. Therefore, the orientation of the PP molecules in the PP/HA composite is weaker comparably. Since the strength of the rolled PP sheets is more than the rolled PP/HA composite sheets, by increasing the volume percentage of the PP layers in the sandwich composites, the strength of the PP/HA-PP sandwich composites increases slightly. The Young’s modulus of the sandwich composites followed the same trends as UTS, namely the modulus varied linearly with increasing the volume percent of the pure PP layers, suggesting an agreement with the rule of mixtures. Moreover, the Young’s modulus of the PP/HA composites shows a slightly increase by the rolling process while the Young’s modulus of the PP increases about 190% by the rolling process. As described for UTS of the pure PP and Sandwich composites, the differences in the Young’s modulus of the PP/HA and PP by the rolling process can be attributed to the difference in capability of PP molecules in the PP/HA composite and PP to move and to be aligned in the rolling direction. Phua et al. [40] showed the rolling process is an effective method for improvement the mechanical properties of the poly (butylene succinate) (PBS). They show the rolling process increase the strength and Young’s modulus of the PBS by alignment of the PBS molecules in rolling direction. Our results are in agreement with their results. Fig. 4 shows the effect of the rolling process on the microstructure of PP schematically. According to this figure, the orientation of PP molecules before the rolling process is completely randomly while after the rolling process the most of them are aligned in the rolling direction, which makes big differences in the mechanical properties of the designed sandwich composites. The relation between the molecules orientation in thermoplastic polymers, which is influenced by different manufacturing methods like drawing and extrusion, and their mechanical properties are discussed in different study and their results are in consistent with our results [15,41,42]. The results of the impact tests of the longitudinal and transverse dissected samples from the rolled PP/HA-PP composites sheets are shown in Figs. 5a and 5b. The results of the impact tests illustrate by increasing the volume percentage of the PP layers, the impact resistance of the composites increases dramatically. The rolling process improves the impact absorbed energy of PP/HA composites and pure PP by 21% and 25%, respectively. As it was described in previous sections, the rolling process changes the orientation of the PP molecules which it directly influences the mechanical properties of PP and PP/HA composites. Furthermore, in PP/HA-PP sandwich composites with 20 vol.% PP layers, rolling process increases the impact absorbed energy of the sandwich composites about 120%. According to results of the previous work by Younesi and Bahrololoom [28], adding a high toughness phase (a high toughness polymer) to the PP/HA composites increases the impact resistance of the PP/HA composites significantly. Analyses showed the main toughness mechanisms in the PP/HA composites containing LLDPE particles are predominantly the crack bridging mechanism and lengthening the crack growth path [28]. Since in the previous work distribution of the LLDPE particles in PP/HA composites were completely uniform, the increase

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Fig. 3a. Effect of the rolling process and pure PP content on the ultimate tensile strength of the sandwich composites.

Fig. 3b. Effect of the rolling process and pure PP content on the Young’s modulus of the sandwich composites.

Fig. 4. Schematic presentation of the PP molecular arrangement before (a) and after (b) the rolling process.

in the impact absorbed energy of the composites was related to the probability of passing the cracks through the LLDPE particles. Since in sandwich composites the structure is similar to laminar

composites and the cracks have to pass through the all PP layers, the impact absorbed energy of the composites like the other mechanical properties of the sandwich composites increases with

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Fig. 5a. Effect of the rolling process and pure PP content on the impact absorbed energy of the sandwich composites that were cut in parallel to the rolling direction.

Fig. 5b. Effect of the rolling process and pure PP content on the impact absorbed energy of the sandwich composite samples that were cut in perpendicular to the rolling direction.

increasing the volume percent of PP layers. In PP/HA composites toughened with the LLDPE particles, the strength of LLDPE is much lower than PP/HA composites and increasing the volume percent of the LLDPE phase reduces the other mechanical properties of the composites significantly. However, in sandwich composites, the strength of rolled PP layers is higher than the rolled PP/HA composites. Increasing the volume percent of PP layers increases the impact absorbed energy of the composites without decreasing the other mechanical properties of the sandwich composites. Other researchers have found the adding high toughness particles to rigid polymers increases the toughness of the rigid polymers drastically if the distance between the toughening phase particles in the matrix be lower than a specific amount [43,44]. They considered the stress shielding of the particles as the main toughening mechanism. The superior toughness of the sandwich composites is in agreement with their results, because in the sandwich composites, the toughening phase is the continuous PP layers. However, the main toughening mechanism in the sandwich composites is considered as crack bridging of the PP layers because cracks have to pass through the PP toughening layers. Crack bridging toughening mechanism was described comprehensively in the authors’ previous work [28].

Fig. 6a. SEM image of the fracture surface of the impact test sample. This image shows the HA particles which are glued together by PP matrix.

Fig. 6a shows the fracture surface of tensile test sample of the PP/HA composites. Since these composites have a high volume

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Fig. 6b. SEM image from the cross-section of the sandwich composite sample in a low magnification. The PP/HA and PP layers are distinctive in this image.

Fig. 6c. SEM image of the fracture surface of the impact test sample which presents differences between the fracture morphology of the PP/HA layer and pure PP layer with a high amount of plastic deformation.

percentage of the HA, the PP matrix acts like an adhesive and glue the HA particles together. As it can be seen, HA particles are distributed in PP matrix completely uniform. Fig. 6b illustrates the SEM image of the sandwich composites. Considering the thickness of the layers in this image, it show the darker layer represents the PP layer while the layer with the brighter color is the PP/HA composite layers. The image in Fig. 6c represents the fracture surface of the impact test sample of the PP/HA-PP sandwich composites which was cut in the longitudinal to the rolling direction. This image shows a deformed matrix with regions of brittle failure for the PP/HA composite layers in the sandwich composites in comparison to the completely ductile and deformed fracture surface of the PP layers. Propagation of cracks in PP layers with these high amounts of the deformation requires a high amount of energy which is incorporated to the impact absorbed energy of the PP/HAPP sandwich composites. Therefore, adding the PP layers to the PP/ HA composites improve their impact absorbed energy by increasing the amount of required energy for crack propagating in the sandwich composites. Moreover, in this image, it can be seen that the primary hot pressing and in continue the rolling process make a good adhesion at interface of the PP layers and PP/HA composites layers in sandwich composites. Furthermore, the results of impact tests of the sandwich composites show the impact absorbed energy of the PP and also PP/HA composites shows an increase in the longitudinal direction and a slight decrease in the transverse direction. These changes in the impact absorbed energy of rolled PP and the PP/HA composites are due to changes in the orientation of polypropylene molecules. The rolling process distributes PP molecules ends more uniformly and arranges polymer molecules unidirectionally in the longitudinal direction. As result of these phenomena, cracks in the polymers must pass a longer path due to deviations from the direct growth path and requires more energy because of passing through more polypropylene molecules. Consequently, these changes in the microstructure of polypropylene increase the impact absorbed energy of the sandwich composites in the longitudinal direction. Moreover, as a result of this unidirectional orientation of polypropylene molecules, the impact absorbed energy of the pure PP and PP/HA sandwich composites show a reduction in the transverse direction of rolling process. This reduction can be attributed to easier growing of the cracks

Table 2 Results of flexural buckling test for longitudinal dissected samples from sandwich composites sheets. Type of material

PP/HA PP/HA-R PP PP-R PP/HA-10-R PP/HA-15-R PP/HA-20-R

Average flexural rigidity (N m2)  103

Standard deviation (N m2)  10–3

Average critical load (Pcr) (N)

Standard deviation (N)

23 °C

37 °C

23 °C

37 °C

23 °C

37 °C

23 °C

37 °C

488.32 589.44 88.32 247.67 511.61 507.52 461.44

431.36 516.48 67.20 189.44 439.68 431.36 387.19

15.62 16.50 3.76 7.93 15.13 15.74 11.98

14.67 10.85 2.43 3.03 18.91 14.23 12.00

7703.25 9298.42 1393.25 3907.15 8228.24 8006.13 7279.22

6804.70 8147.47 1060.08 2988.42 6835.95 6804.70 6108.08

390.81 260.34 81.67 175.81 312.66 565.43 422.32

441.17 316.71 105.38 77.68 513.18 258.49 387.84

Table 3 Results of flexural buckling test for transverse dissected specimens from sandwich composites sheets. Type of material

PP/HA PP/HA-R PP PP-R PP/HA-10-R PP/HA-15-R PP/HA-20-R

Average flexural rigidity (N m2)  103

Standard deviation (N m2)  10–3

Average critical load (Pcr) (N)

Standard deviation (N)

23 °C

37 °C

23 °C

37 °C

23 °C

37 °C

23 °C

37 °C

488.32 523.52 88.32 205.44 487.68 412.80 385.28

431.36 516.48 67.20 155.52 458.24 391.04 364.16

15.62 19.63 3.76 9.47 14.81 21.56 26.38

14.68 12.59 2.43 6.21 17.33 17.42 25.79

7703.25 8258.51 1393.25 3240.86 7693.15 6511.49 6077.27

6804.70 7743.63 1060.08 2453.31 7228.74 6168.94 5744.72

390.81 345.71 81.67 249.17 268.87 616.45 574.26

441.17 390.46 105.38 180.32 605.65 474.23 536.49

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Fig. 7a. Effect of the rolling process and pure PP content on the bending strength of the sandwich composites.

Fig. 7b. Effect of the rolling process and pure PP content on the bending modulus of the sandwich composites.

between polymer molecules in transverse to the rolling direction. In a study, Bartczak et al. [45] investigate the effect of the rolling process on molecular orientation of the isotactic polypropylene molecules. They found the rolling process by side constraint increases the molecular alignment of isotactic polypropylene molecules in rolling direction and consequently improve the strength and impact absorbed energy of the polypropylene. Since adding PP layers to PP/HA composites decreases the Young’s modulus and the stiffness of the PP/HA-PP sandwich composites and these decreases directly influence the rigidity and the buckling strength of the PP/HA composites knowing the effect of the laminar structure and also the rolling process on the rigidity and the buckling performance of the PP/HA-PP sandwich composites has a great importance. Buckling is a tendency of slender compression members to bow out, which causes bending. When the combined bending and compressive stress exceed the buckling limit or buckling capacity of the samples, failure occurs. For calculating the theoretical critical buckling load, the Euler formula can be used [46];

Pcr ¼ ðp2 EIÞ=L2

ð1Þ

where Pcr is critical buckling load, E is elastic modulus, I is moment of inertia and L is the length of sample. Since r = P/A, the critical bucking stress is:

rcr ¼ ðp2 EIÞ=AL2

ð2Þ

Eq. (2) can be presented also in the below form;

rcr ¼ ðp2 EÞ=ðKL=rÞ2

ð3Þ

where A is the cross-section of the buckling sample, KL/r is Slenderness ration, r is radius of gyration (r2 = (I/A)) and K is the support factor that its amount depends the end fixation of the buckling sample, as follow; For both ends pinned (hinged, free to rotate), K = 1.0. For both ends fixed, K = 0.50. For one end fixed and the other end pinned, K = 0.699. . . For one end fixed and the other end free to move laterally, K = 2.0.

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Fig. 8. Results of the DSC analysis for: (a) PP copolymer, (b) PP as a matrix in the PP/HA composites, (c) PP/HA composite, (d) PP/HA-10-R, (e) PP/HA-15-R, and (f) PP/HA-20-R.

Since in this experiment both ends of the buckling specimens were fixed, the K value in this equation is 0.5. Tables 2 and 3 demonstrate the results of the buckling tests on the longitudinal and transverse dissected samples from the PP, PP/ HA composites, and PP/HA-PP sandwich composites sheets respectively. The results in Table 2 show the rolling process increase the flexural rigidity of the PP/HA composites and PP about 21% and 180%, respectively. Furthermore, the flexural rigidity of the sandwich composites shows a slight decrease of 9% with increasing the volume percentage of the PP layers from 10 vol.% to 20 vol.%. This reduction can be resulted from replacing the PP/HA composites with the PP layers which have lower stiffness and rigidity comparing to PP/HA composites. In the same trend, the critical buckling loads for the sandwich composites show 10% reduction by increasing the percentage of the PP layers from 10 vol.% to 20 vol.%. By comparing the results in Tables 2 and 3, it can be understood that the flexural rigidity and critical buckling loads of the sandwich composites in the transverse direction of the rolling process are comparably lower than the longitudinal direction. These differences in the critical buckling loads, flexural rigidity, and the other mechanical properties of the sandwich composites in the longitudinal and transverse directions of the rolling process confirm there is a change in the PP structure as matrix of the sandwich composites. The reason for this phenomenon can be explained by the change in the PP molecular orientation. An increase in the PP molecules alignment and its crystallinity would increase both the Young’s modules and strength of the PP. In the unrolled samples, the PP molecules are oriented randomly and uniaxial alignment of PP molecules can be considered close to the zero, so the mechanical properties show an isotropic behavior. The results of the bending test on the sandwich composites show the same trend as the tensile test. Figs. 7a and 7b show the bending strength and bending modulus of the sandwich composites which were dissected in the longitudinal and transverse

directions of the rolling direction. By comparing the results in Fig. 7a and 7b it can be seen that the bending strength and bending modulus of the composites which are dissected in the transverse direction are comparably lower than the longitudinal direction. According to the previous passage, because of the orientation of polymer molecules in the rolling direction, the strength of the composites in the transverse direction of the rolling process is lower than the longitudinal direction. It can be concluded that propagation of cracks along the PP molecules orientation is easier than in the perpendicular direction of polymer molecules. As a result, the growth of cracks in the transverse direction of the rolling process needs lower load and energy, which causes the lower strength for the composites in this direction. All the mechanical tests were performed at room temperature (23 °C) and 37 °C (human body temperature) as the real working temperature of these composites in body. It is completely clear that for thermoplastic composites, increasing the temperature makes easier the moving and slipping of polymer molecules on each other under a specific applied load. As a result, rising the temperature increases the ductility of thermoplastic polymers and their composite and makes a reduction in their strength and modulus. The DSC analysis is a powerful method for investigating the effect of different processing on the structure and molecular orientation of polymers and polymer matrix composites and helps to find a correlation between their microstructure and mechanical properties. The effect of the rolling process on the microstructure of the PP and PP/HA sandwich composites is checked by DSC (Differential Scanning Calorimetry) (Fig. 8). Fig. 8a–c shows the DSC curves of polypropylene copolymer that is used as a toughening phase, polypropylene as a matrix in the PP/HA composites and PP/HA composites before the rolling process, respectively. These curves show an endothermic peak at 145 °C, 158 °C and 156 °C. The lower melting range in Fig. 8a is related to the chemical composition of this polymer as a copolymer in comparison to Fig. 8b for a homopolymer.

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Furthermore, as it can be seen in these curves adding hydroxyapatite to polypropylene decreases its melting point. This small decrease can be resulted from the increasing the surface energy of polypropylene due to the PP/Ha particle interface. Fig. 8c–f shows the results of the DSC analysis for the rolled PP/HA-PP sandwich composites with the different PP contents (10%, 15%, and 20% pure polypropylene). These DSC curves show that the rolling process increases the melting point ranges of the PP and PP/HA-PP sandwich composites. These rises in the melting range of the sandwich composites confirm the increasing in the molecular orientation of PP in the sandwich composites by rolling process. Hugo et al. [15] used this method to check the effect of isostatic extrusion on the orientation of polyethylene molecules in polyethylene/hydroxyapatite composites. They found the isostatic extrusion process increases polyethylene molecules orientation and consequently increases the melting region of polyethylene. Our results are in agreement with their achievements and confirm the rolling process can be used as a good method for improving the mechanical properties of the PP/HA composites and specially PP/HA-PP sandwich composites.

4. Conclusion In this study, the PP/HA-PP sandwich composites were fabricated. Different mechanical tests were done on PP/HA-PP composites to understand the effect of the manufacturing process on mechanical properties of the sandwich composites. The results of the tensile and bending tests showed the rolling process increases the strength of the PP/HA-PP sandwich composites. Adding the PP layers to the PP/HA decreases the bending modulus and Young’s modulus of the composites and consequently makes a slight reduction in the flexural rigidity of the sandwich composites. The results of the impact test show adding the PP layers to PP/HA composite increase their impact absorbed energies dramatically. Comparing the results of the mechanical tests of the sandwich composites show mechanical properties of composites have big differences in the longitudinal and transverse directions of the rolling process. The results of the DSC analysis show the rolling process changes the orientation and arrangement of polypropylene molecules and aligned them in the rolling direction. This induced alignment improves the mechanical properties of the composites in the rolling direction. This good combination of the mechanical properties of sandwich composites presents them as good candidate material for use in orthopedic applications, while the diversity of different parameters which influence the mechanical properties of the sandwich composites demands more study in future.

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