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Abstract. This study extends a work started on the new bio-composite materials; polymer/eggshell composites. Polypropylene/ chicken eggshell composites ...
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Thermal and mechanical characterization of polypropylene/eggshell biocomposites

Journal of Reinforced Plastics and Composites 32(6) 402–409 ! The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0731684412470015 jrp.sagepub.com

Tojan Ghabeer, Radwan Dweiri and Shadi Al-Khateeb

Abstract This study extends a work started on the new bio-composite materials; polymer/eggshell composites. Polypropylene/ chicken eggshell composites were chosen and prepared via melt extrusion. The mechanical and thermal behavior of composites containing 10–40 wt% of untreated and strearic acid-treated eggshell were studied. The results show improvements in tensile modulus of composites while the values of tensile strength, strain at break, and impact strength decrease compared to that of neat polypropylene. The crystallization temperature, Tc, is clearly increased by incorporating 10 wt.% eggshell into polypropylene while a margin effect on the degree of crystallinity is seen. It can be concluded that composites need to be further studied and modified for possible use of eggshell as filler without scarifying the mechanical properties of the neat polymer matrix.

Keywords Polypropylene, eggshell, mechanical properties, differential scanning calorimeter properties, microstructure

Introduction Over the past two decades, organic fillers have become a strong competitor to inorganic fillers in polymer composites due to their low densities, very low cost, nonabrasiveness, high filling levels, recyclability, biodegradability, and renewable nature. Eggshell (eggshell) is a bio-mineral filler which is annually generated in thousands or millions of tones in some countries such as USA and China.1,2 A concern in polymer/eggshell composites have been started since the last few years as a new bio-composite material.2–11 A dry ES has been reported to contain approximately 95% (by weight) with a typical mass of 5.5 g in the form of calcite.12 Polypropylene (PP) composites normally use inorganic fillers such as mineral calcium carbonate, silica, and talc13–22 and the mineral calcium carbonate (CaCO3) is considered as one of the most commonly used inorganic filler in PP. The bio-mineral calcium carbonate from ES has been introduced to fill PP in this study. Researchers have investigated the mechanical and morphological properties of PP/ES composites and compared them to that of PP/talc and PP/CaCO3 composites.7–9 In this study, the thermal properties of PP/ ES composites were also investigated besides the mechanical properties. ES were also surface-treated with stearic acid based on the fact that calcium carbonate is

usually coated with stearic acid to improve its dispersion in the polymer matrix. In the literature, Hussein et al.4 treated ES powders with a solution of 5% NaOH based on a patent on the process and use of ES as a biofiller for different PP, which was registered by the Universidad de Chile in 2006.23 Lin et al.9 prepared ES/b-PP composites by using ES modified with pimelic acid (PA) and the results showed that ES modified by PA was a b-nucleating agent and that PA promoted the dispersion and interfacial bonding of ES in PP. Supri et al.10 used polyethylene-grafted maleic anhydride (PE-g-MAH) as a compatibilizer into low-density polyethylene (LDPE)/ ES composites to enhance interfacial adhesion between LDPE and ES. Indeed, there are abundant of polar groups-proteins on the surface of ES particles, which is incompatible with non-polar PP. The amine, carbonyl groups of proteins on the surface of ES particles are potential source of hardening agent.6 Therefore, it is

Department of Materials Engineering, Faculty of Engineering, Al-Balqa’ Applied University-BAU, Al-Salt, Jordan Corresponding author: Radwan Dweiri, Department of Materials Engineering, Faculty of Engineering, Al-Balqa’ Applied University-BAU, 19117 Al-Salt, Jordan. Email: [email protected], [email protected]

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worthwhile to explore the effect of surface treatment of ES with stearic acid on the mechanical, thermal, and morphological properties of PP/ES composites.

Materials and experiments Materials PP which is used in this study was purchased from Sabic company, KSA, with a density of 0.905 gcm-3 and melt flow index (MFI) of 10.5 g/10 min at 230 C. Chicken ESs were collected at home which were then washed in a bath of hot water at a temperature of 80 C. Egghells were dried in an oven at 110 C for 30 min and then ground into a powder using ball mill machine. The ES powder was sieved using a 125-mm sieve size and it was further characterized using a Master Sizer measurement. The ES particle size distribution is found to range between 14 and 157 mm and the mean average particle size which was obtained at the 50% particle cumulative distribution point is 90 mm. Stearic acid 50%, commercial grade, was supplied from Global Bacic Chemicals Sdn Bhd, Malaysia.

Surface treatment of ES particles The chemical treatment method of ES was carried according to Lin et al.14 as follows. First, 10 g of ES was mixed with a 40-mL solution mixture of water and ethanol in a volume ratio of 3:1. The suspension was stirred by a magnetic stirrer for 2 h to completely wet the particles. Then, the suspension was heated to and maintained at 80 C. Stearic acid (6% by weight of the ES amount) dissolved in the ethanol solvent was gradually added to the suspension drop by drop. After 2 h of reaction time, surface-treated ES particles, which settled at the bottom of the beaker, were dried in an oven at 105 C overnight.

Preparation of PP/ES composites Formulations of PP/ES composites with 10, 20, 30, and 40 wt.% ES (shown in Table 1) were prepared using a

Brabender co-rotational twin screw extruder which has a screw diameter of 30 mm, L/D ¼ 35. The temperature profile was set to 170/180/190/195 C. The mixing process were done in two steps: the first was mixing PP and ES at a rotation speed of 40 r/min and the second was by re-mixing at a speed of 100 r/min in an attempt to homogenize the composites. All components were dried at 110 C prior mixing. The resulting compounds of PP/ES composites were preheated at 200 C for 5 min then hot pressed in steps into (100  100  2) mm3 plates for 2–3 min. The plates were air cooled. The compositions of the composites and the samples notations are shown in Table 2.

Characterization of composites Mechanical tests Tensile dumbbell-shaped specimens (2 mm thick and 70 mm long) of composites were prepared according to ASTM D638 standard and the specimens were then tested by using a universal testing machine, EKTRON TS-2000. The width at the middle of the specimens is 6 mm and the gauge length is 25 mm. The cross-head speed was set to 5 mm/min at room

Table 1. Compositions in wt.% of PP/ES composites Sample notation

PP

ES (untreated)

ES (treated)

PP 10ES 20ES 30ES 40ES 10ESt 20ESt 30ESt 40ESt

100 90 80 70 60 90 80 70 60

0 10 20 30 40 0 0 0 0

0 0 0 0 0 10 20 30 40

PP: polypropylene; ES: chicken eggshell.

Table 2. Thermal properties of PP and ES-filled composites Property/sample 

Tm ( C) Hm (J/g) Tc ( C) Hc (J/g) Xc (%)

PP

10ES

20ES

30ES

40ES

10ESt

20ESt

30ESt

40ESt

172 63.5 108 95.4 45.6

167 64.8 118 89.9 47.8

168 54.4 119 74.8 44.7

165 49.2 118 68.6 46.9

169 42.8 119 53.6 42.7

170 57.0 121 87.8 46.7

169 42.2 119 79.5 47.5

166 40.7 119 76.7 52.4

166 36.7 119 62.5 49.8

PP: polypropylene; ES: chicken eggshell.

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temperature and a load of 1000 lbf. The Izod notched impact test was carried out using a Ceast pendulum impact tester at room temperature. A 2 Joule hammer was used for the impact test. The specimens have dimensions of (64  13  2) mm3. The tensile and impact tests were conducted according to ASTM D638 and ASTM D265-88, respectively.

DSC measurements A differential scanning calorimeter (DSC, Perkin Elmer 7) was used to examine the thermal properties of the PP/ES composites. The specimens for DSC analysis were the same as PP/ES composites prior to compression molding. The specimens were placed in sealed aluminum cups with weight varying from 7 to 10 mg for each sample. Samples were first heated from 25 C to 200 C, at a heating rate of 10 C/min and annealed for 5 min before it was cooled to room temperature at a cooling rate of 10 C/min.

Figure 1. Young’s modulus of neat polypropylene (PP) and PP/chicken eggshell (ES) composites with different untreated and treated ES loadings.

Fractography The fractured surface of the samples on PP/ES composites from tensile tests were examined using a scanning electron microscope (SEM) (Philips XL-30 W/TMP) with an operating voltage of 10 KV. Samples were made to be conductive by using a sputter coater to deposit of gold before SEM examination.

Results and discussion Mechanical properties of composites Figures 1–4 represent the results of tensile and impact tests for neat PP and composite systems. Figure 1 shows the values of tensile modulus as a function of ES content. The tensile modulus increases gradually from 504 for neat PP to 736 for 40ES. The inclusion of a rigid phase, such as ES, is able to increase the polymer stiffness. Sivarao and Vijayaram24 recorded an increase in the tensile modulus from 526 MPa for neat PP to 820 MPa for PP with 40 wt.% ES composites. They used ES particles with a size below 160 mm and they added silane as a coupling agent during mixing. Toro et al.7 achieved modulus of 2800 MPa for the same composition of ES compared to 1700 MPa for neat PP, but with ES particle size of 8.4 mm. Young’s modulus for composites containing treated ES are also shown in Figure 1 and the results show higher modulus values than that with untreated ES. The increase of tensile modulus of composites is more pronounced at higher filler loading (i.e. 30ESt and 40ESt) and this may due to the apparent raise in the crystallinity percentage of these composites as will

Figure 2. Tensile strength of neat polypropylene (PP) and PP/ chicken eggshell (ES) composites with different untreated and treated ES loadings.

be shown later. Lin et al.20 stated that the maximum tensile modulus of PP-CaCO3 nanoparticles coated with stearic acid results from the high crystallinity as well as the large interfacial area between the particles and the matrix due to the good dispersion. Supri et al.10 reported higher values of tensile modulus for LDPE/ES composites in the presence of PE-g-MAH which was used as a compatiblizer and they attributed that to better interfacial adhesion. It is observed from Figure 2 that the tensile strength for PP composites decreases with increasing filler content to reach a value of 17 MPa at 40 wt.% ES compared to that of neat PP which is 27 MPa. This result is similar to that reported by Kamarudin et al.2 and Hussein et al.4 They reported that the decrease in tensile strength is due to the poor adhesion of the filler-matrix and the agglomeration of ES particles. Conversely to the modulus, tensile strength is very sensitive to the

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Figure 3. Strain at break of neat polypropylene (PP) and PP/chicken eggshell (ES) composites with different untreated and treated ES loadings.

Figure 4. Izod impact strength of neat polypropylene (PP) and PP/chicken eggshell (ES) composites with different untreated and treated ES loadings.

filler/matrix interfacial adhesion, the interface of which plays a crucial role in transferring the stress from the matrix to the filler. The tensile strength increases slightly for the composite containing 10 wt.% treated ES (27 MPa) compared to that of 10 wt.% untreated ES (25 MPa) and this may be within the measurements error. The incorporation of 20–40 wt.% treated ES into PP lowers the tensile strength of the composites compared to their counterparts which contain untreated ES. Shuhadah and Surpi3 recorded a slight improvement of the tensile strength by involving NaOH-isophthalic acid modified ES to LDPE. Sivarao and Vijayaram24 cited no improvement in tensile strength of PP/ES composites using silane as a coupling agent. The addition of pimelic-modified ES (100 mesh size) slightly decreased tensile strength of b-PP/ES composites compared to unmodified ones and that was mainly attributed to the effect of b-PP with low tensile strength.9 Figure 3 shows the effect of ES filler on the strain at the break for PP/ES composites. The incorporation of

405 the filler results in an abrupt drop in strain at break compared to the strain at break of PP. Increasing ES loading in the PP matrix results in the stiffening and hardening of PP composite. The reduction of the strain at break with the increasing of ES loading indicates the incapability of the ES to support the stress transfer from polymer matrix to filler. The incorporating of treated ES to PP results in more brittle composites than that with untreated ES. Finally, the results of impact strength of PP-filled composites are shown in Figure 4. In general, a worsening of the impact strength is observed for all filler systems compared with the neat polymer matrix (58 J/m). There is a gradual increase in impact strength from 25 J/m for 10ES to 40 J/m for 30ES, and then it decreases to 28 J/m for 40ES. For composites containing treated ES, the impact strength increases from 25 for 10ES to 33 J/m for 10ESt and thereafter, it levels off at a value of 32 J/m. The results obtained in this study are in line with literature. Ghani et al.5 showed that the impact strength increased with the increasing of ES loading for ES-filled epoxy composites. They stated that ES impart the rigidity into the samples and cause them to become more rigid. Toro et al.8 explained the decrease of impact strength at higher filler loading as follows: the neat PP possesses high impact strength and therefore, at higher filler loading, the amount of PP available becomes scarce and this leads to a decrease in toughness of the composites. It is also known that the stiffest composites exhibit the lowest impact properties as the high stress is transferred from the polymer matrix to the filler particles. For composites containing treated ES, the impact strength does insignificant change by increasing ESt content. Deshmukh et al.21 reported no increment in the notched Izod impact strength by incorporating 5 to 30 wt.% of uncoated CaCO3 into poly(butylenes terephthlate), PBT, and its values were constant at 32 J/m. They also reported a value of 32 J/m for impact strength of PBT filled with 5–10 wt.% of CaCO3 coated with stearic acid and its value jumped to 39 J/m at 20 wt.% coated CaCO3. They attributed the increase of impact strength at 20 wt % to the fact that the stearic acid coating imparts hydrophobic characteristics on CaCO3 surface, thereby promoting adhesion between the particles and PBT matrix and thus improving the dispersion and compatibility between phases. Wai25 studied the PP/CaCO3 composites using different coupling agents (stearic acid and titanate) and they reported that a good interfacial adhesion between the matrix and filler will help to improve the toughening of composite. Toro et al.8 reported a gradual decrease in impact strength with increasing in filler content and they attributed that to poor interfacial region between the matrix and the filler. Based on the above discussion, the effect of stearic acid, in this study,

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Figure 5. Differential scanning calorimeter (DSC) melting curves for neat polypropylene (PP) and PP/chicken eggshell (ES) composites with different untreated and treated ES loadings.

for improving the coupling effect between matrix and filler may not be significant and needs further study in the future.

Figure 6. Differential scanning calorimeter (DSC) crystallization curves for neat polypropylene (PP) and PP/chicken eggshell (ES) composites with different untreated and treated ES loadings.

DSC analysis The distinct effect of particulate fillers on the crystalline structure of semi-crystalline thermoplastics is their ability to work as a nucleation agent. Literature studied the effects of crystalline changes on mechanical properties and cited clearly that by increasing crystallinity, the modulus of PP increases and the strength and deformability decrease.18,19,26 However, the nucleation effect differs strongly by filler type, particle size (i.e. surface area), and filler surface treatment. The DSC heating and cooling thermograms are shown in Figures 5 and 6, respectively. The thermal properties such as crystallization temperature (Tc), melting temperature (Tm), enthalpy of melting (Hm), and enthalpy of crystallization (Hc) are listed in Table 2. Since the specimens for DSC analysis were the same as PP and PP/ES composites prior to compression molding, we tried to simulate what was going on during the compression molding of PP and PP composites. Therefore, degree of crystallinity was obtained from the cooling curves and the % of crystallinity (Xc) was estimated using the following equation: Xc ¼

Hc  100 ð1  p ÞH0

where Hc is the heat of crystallization of the sample analysed (J/g) and H0 is a reference value that represent the enthalpy value for a theoretically 100% crystalline PP, taken as 209 J/g.27 With the addition of untreated ES to PP, the percentage of crystallinity of PP varied only slightly and it might be within the experimental error. For composites with treated ES, an increase in the degree of crystallinity is seen at higher filler loadings (i.e. 30 and 40 wt.%). Researchers reported that by modifying the surface chemistry of the filler, nucleation effect may be affected, increased or decreased.19 There is a noticeable increase in Tc from 108 C to 118 C by incorporating 10 wt.% of untreated ES to neat PP and it increases to 121 C for 10 wt.% of treated ES. The Tc then levels off at a value of about 119 C for the rest of untreated and treated composites. However, ES do accelerate crystallization during cooling. Tm of all composites decreases slightly as compared to neat PP. The Tm of the neat PP is found to be 172 C, while Tm of the composites ranges from 165 to 169 C. The small peaks at about 150 C in the melting curves are the endothermic peaks of the b phase of PP. Indeed, the beta crystal form is not clearly seen for all PP/ES composites as observed from DSC

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(a)

(b)

(c)

1

2

1

2

Figure 7. Scanning electron microscope (SEM) tensile fracture images of (a) neat polypropylene (PP) sample, (b) and (c) magnification at areas 1 and 2, respectively.

(b) 1

(a)

(c) 2

2

3 1

(d)

3

(e)

Figure 8. Scanning electron microscope (SEM) tensile fracture images of (a)10ES, (b), (c), and (d) magnification at areas 1, 2, and 3, respectively, (e) 10ESt.

melting curves but the formation of b phase in the presence of ES filler is a result of the nucleation effect of the filler in the matrix crystallization process. This agrees with the literature.15,18,19 Lin et al.9 also reported that the decrease in tensile strength and the increase of impact strength of PP/ES composites are mainly attributed to the effect of b-PP which has low tensile strength and high impact strength.

Morphology SEM micrographs of the tensile fracture surface of neat PP and the composites are shown in Figures 7–9. Figure 7 shows the SEM images of tensile fracture sample of neat PP. The fibrillated ligaments of PP in Figure 7(b) and the smooth plane in Figure 7(c) indicate a cup-cone (ductile) fracture mode of PP sample. The fracture surface of composites with 10 wt.% ES (Figure 8) identify two areas: the plastic deformation at the crack-initiation stage (Figure 8(d)) and the

propagation of the crack stage (Figure 8(c)), which indicate a semi-brittle fracture mode. The dispersion of ES particles in the PP matrix is visible in Figure 8(d). ES particles are poorly welded to PP matrix and fracture lines and air voids are observed at the boarderline of filler, which is formed as a result of detachment of the unembedded particles during fracture, indicating that there is a poor adhesion between the filler and the matrix. Similar observations are reported for composites at higher filler loadings (Figure 9(a) and (b)). No evidence on the particles agglomeration can be seen in the SEM images. For composites with 10 wt.% of treated ES (10ESt), it can be seen that the ES particles are pulled out in the same manner as untreated case during the fracture of the composites (Figure 8(e)), but it seems that the filler is better embedded in the matrix with a finer structure. The compatibility is improved between the two phases as can be seen in Figure 9(d) but still air voids are detected in Figure 9(c) and (d). No sign for plastic

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Journal of Reinforced Plastics and Composites 32(6)

(a)

(c)

(b)

(d)

Figure 9. Scanning electron microscope (SEM) tensile fracture images of (a) and (b) 30ES, (c) and (d) 30ESt.

deformation is detected at higher filler loading of the treated composites.

providing facilities to this work such as materials, experiments, and tests.

Conclusions

Funding

Based on the results that are obtained from the mechanical tests of PP and PP/ES composites, it can be concluded that ES does not provide reinforcement to enhance the strength of the composites. In this study, large particle size of ES was used and there is also a critical amount of stearic acid coating to achieve complete monolayer coverage on the ES particle surface as cited in the literature. The attention must be given to these points in the future, i.e. the use of lower particle size (less than 10 microns are recommended to be used in the future) and determination of stearic acid coverage on ES. The DSC results showed an apparent increase in the crystallization temperature by incorporating ES into PP and the formation of b-PP, which are evident on the effect of ES as a nucleating agent. Finally, it is also recommended to do experiments in the future using ES as filler in a thermoplastic elastomer and to improve the properties of the composites by using new coupling agents and carry out XRD and DMTA for analysis of distribution of the fillers. Acknowledgment The authors thank Al Balqa’ Applied University (BAU) and Royal Scientific Association (RSA) for their support by

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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