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calcium carbonate nanocomposites
Accepted Manuscript Effect of Thermal Treatment on Impact Resistance and Mechanical Properties of Polypropylene/Calcium Carbonate Nanocomposites Estêvão Mestres do Nascimento, Daniel Eiras, Luiz Antonio Pessan PII:
S1359-8368(16)00049-4
DOI:
10.1016/j.compositesb.2015.12.040
Reference:
JCOMB 3987
To appear in:
Composites Part B
Received Date: 5 May 2015 Revised Date:
2 December 2015
Accepted Date: 26 December 2015
Please cite this article as: do Nascimento EM, Eiras D, Pessan LA, Effect of Thermal Treatment on Impact Resistance and Mechanical Properties of Polypropylene/Calcium Carbonate Nanocomposites, Composites Part B (2016), doi: 10.1016/j.compositesb.2015.12.040. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of Thermal Treatment on Impact Resistance and Mechanical Properties of Polypropylene/Calcium Carbonate Nanocomposites
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Estêvão Mestres do Nascimento1, Daniel Eiras1*# and Luiz Antonio Pessan1 1Department of Materials Engineering, Federal University of São Carlos, 13.565-905 São Carlos, São Paulo, Brazil *[email protected]
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Abstract
Heat treatment was applied in polypropylene/calcium carbonate
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nanocomposite and its effects on the structure and impact resistance of the materials were studied. The nanocomposite was prepared by melt blending in a twin screw extruder and subsequently molded into tensile specimens by injection molding. Polypropylene and the
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nanocomposite were characterized through tensile and impact tests, wide angle x-ray diffraction (WAXD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and
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dynamic mechanical thermal analysis (DMA). The results show an
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increase in impact strength of the nanocomposite after heat treatment which was accompanied by an increase in elastic modulus. DSC and WAXD results show an increase in the crystallinity of the polymer matrix of the nanocomposite but not in the neat polymer. Analysis of the morphology of PP and the nanocomposite PP/CaCO3 revealed that the nanocomposite has a non-spherulitic morphology. Results from DMA indicate that thermal treatment affects the rigidity of the amorphous phase of polypropylene. A mechanism was proposed to
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ACCEPTED MANUSCRIPT explain the influence of thermal treatment in the morphology of PP/CaCO3 nanocomposites and its relation with the impact strength. Keywords: A. Particle-reinforcement; B. Impact behaviour; D. Mechanical
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testing; E. Extrusion
# Rodovia Washington Luiz, km 235 – CEP:13.565-905 São Carlos-
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1. Introduction The application of calcium carbonate nanoparticles to enhance the impact strength of
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polymeric materials has received significant attention recently due to the possibility of increasing the impact strength and the elastic modulus of the polymer simultaneously.[1–4] The toughening mechanism in these systems has been described as the result of debonding of
polymer/particle interface due to stress concentration followed by the occurrence of deformation
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mechanisms such as shear yielding and crazing. A more detailed mechanism suggested by Wu [5,6] involves a critical interparticle distance that is necessary for a polymer composite or blend
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to be tougher than its matrix. If the distance between particles is greater than critical distance no toughening effect is observed but when the distance between the particles is smaller than the critical distance cooperative toughening mechanisms are observed. In an attempt to better explain the mechanism proposed by Wu, Muratoglu et al [7] proposed that the actual influence
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of a second phase in polymeric materials is related to the crystallization of the polymer. According to this concept when the crystallization of the polymer is nucleated from the incoherent surface of a second phase the polymer crystals are formed with a preferential orientation of the lamellae perpendicular to the surface of the disperse particle. The
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crystallographic planes in this lamellae are specific planes with low shear resistance which
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favors the initiation of the deformation mechanisms and the toughening of the polymer. If the interparticle distance is greater than the critical distance the oriented crystals are separated between amorphous material. On the other hand if the interparticle distance is equal or smaller than the critical distance the crystals superpose an toughening is observed. In terms of polypropylene crystallization it has been observed that the crystalline morphology might be different depending on the way the polymer is crystallized.[8] When crystallized from the melt polypropylene presents a spherulitic morphology while the
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ACCEPTED MANUSCRIPT crystallization during quenching creates a non-spherulitic morphology that coincides with the morphology proposed by Muratoglu et al [7] and Bartczak et al.[9,10] Recently the authors have shown that the simple incorporation of calcium carbonate nanoparticles in polypropylene leads to a decrease of brittle-to-ductile transition temperature
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and an increase in the elastic modulus compared to the neat polymer matrix.[11] The
nanocomposite also presented a high impact strength at low temperatures although the impact strength at room temperature was practically the same of the neat polymer. In another work [12]
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polarized optical microscopy have indicated that the incorporation of calcium carbonate nanoparticles strongly reduces the sizes of polypropylene spherulites and increases the
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crystallinity of polypropylene and also affects the crystallization kinetics of the polymer. The most recent contribution to the field has presented high increases in the impact strength of polypropylene/calcium carbonate nanocomposites after thermal treatment.[13,14] The authors suggested that heat treatment leads to recrystallization of the polymer between the particles and enable the superposition of lamellae between the particles. The authors have also
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suggested that the formation of polypropylene´s cross-hatched structure is responsible for increasing the resistance of the fibrils inside crazes and enhance the impact strength of the nanocomposites.
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The effect of thermal treatment in the crystallization of polypropylene has been
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discussed in previous work in the literature.[15–17] If the polymer structure is described by a three phase model composed of an amorphous phase, a rigid amorphous phase and a crystalline phase there are evidences that thermal treatment leads to an increase in crystallinity due to the enhanced mobility of the amorphous phase at high temperatures. As a result of this enhanced mobility the amount of crystalline phase is increased at the expense of the amorphous phase. Moreover, in the case of polypropylene the rigid amorphous phase crystallizes creating the cross-hatched structure of polypropylene.
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ACCEPTED MANUSCRIPT Other strategies like polymer blending, copolymerization and application of nucleating agents have been used to improve mechanical and fracture behavior of polypropylene. Recent work from Wu H. and coworkers [18] used β nucleating agents and thermal treatment to achieve strengthen and toughening of polypropylene/clay nanocomposites. The authors have established
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that the formation of β phase was responsible for the improvement in polypropylene/clay nanocomposites.
Motamedi P. and Bagheri R. [19] prepared polypropylene/polyamide/layered silicate
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ternary nanocomposites and demonstrated the relationship between processing, dispersion of the layered silicates and the mechanical properties of the nanocomposites. In general the
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improvement in dispersion leads to and improvement in strength and modulus but decreases the impact resistance of the material showing that the balance between mechanical resistance and impact resistance is difficult to be achieved.
Yuiling M. and coworkers [20] have developed super tough polypropylene/carbon nanotubes nanocomposites by the incorporation of a third phase composed of polyethylene–
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octene copolymer (POE). Although other materials were incorporated as third phase POE was the only one capable of increasing impact strength and good electrical conductivity. In this context, and considering the importance of polypropylene in various industrial
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sectors and the importance of increasing its impact resistance without losing rigidity the effect
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of thermal treatment in polypropylene/calcium carbonate nanocomposites was studied in terms of the impact and tensile properties and the crystallization of the polymer and of the nanocomposites. The morphology of the nanocomposite before and after thermal treatment was compared to the morphology of the neat polymer and evidences of a non spherulitic morphology in the nanocomposite were obtained. Finally a mechanism to explain the influence of thermal treatment in the crystalline morphology of polypropylene/calcium carbonate nanocomposites was proposed and related to the impact improvements in impact strength.
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ACCEPTED MANUSCRIPT 2. Materials and Methods 2.1 Materials Polypropylene homopolymer H501-HC with melt index of 3,5g/10min (230oC and
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2,16Kg) was supplied by Braskem and used as received. Calcium carbonate nanoparticles YH 303 were supplied by YH-nano.
2.2 Methods
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2.2.1 Preparation of polypropylene/calcium carbonate nanocomposites
Polypropylene was processed and the nanocomposites were prepared in a B&P equipment co-rotational twin screw extruder. Temperature profile was 170/190/190 190/190/195oC and the screw rotation was 160rpm. One composition of polypropylene/calcium carbonate nanocomposite was prepared containing 20wt% of calcium carbonate (PPC-20).
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The materials were molded into tensile specimen in an Arburg Alrounder 270V injection molding machine. The dimensions of the samples followed the dimensions of ASTM D638. The temperature profile was 215/220/235/245/250oC. The samples for impact tests were
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cut from tensile specimens according to the dimensions of ASTM D256.
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2.2.2 Thermal treatment of polypropylene and polypropylene/calcium carbonate nanocomposites
Polypropylene and the nanocomposites test bars were annealed at 150oC for 2 hours in order to verify the effect of annealing on their mechanical properties. Annealing was performed in a digestion block. The system was heated and the temperature stabilized before the introduction of the samples.
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2.2.3 Characterization of the polypropylenene and the nanocomposites
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2.2.3.1 Tensile Tests
Tensile tests were performed at room temperature in an Instron Universal Testing
Machine with a testing speed of 5mm/min. Before testing the samples were conditioned at 23oC
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and 50% maximum humidity according to ASTM D638.
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2.2.3.2 Impact Tests
Impact tests were performed in a CEAST impact testing machine. Before testing the samples were notched according to ASTM D256 and conditioned at 23oC and 50% maximum
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humidity.
2.2.3.3 Diferential Scanning Calorimetry
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The crystallization behavior of polypropylene and the nanocomposites was
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characterized by DSC in a Q2000 TA calorimeter. The tests were performed by heating the samples from 40oC to 190oC, cooling it from 190oC to 40oC and then heating again to 200oC. Both heating and cooling rates were 10oC/min. The degree of crystallization (%Cryst.) was calculated using equation 1. Em que ∆Hf é a entalpia de fusão do polímero, ∆Hf,100% é a entalpia de fusão de um material 100% cristalino e χ é a fração mássica de nanopartículas no nanocompósito.
2.2.3.4 Dynamic Mechanical Analysis
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2.2.3.5 Characterization of the dispersion of calcium carbonate (TEM)
Transmission electron microscopy of the nanocomposites was used to verify the
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dispersion of calcium carbonate nanoparticles in polypropylene. The samples for TEM analyses were prepared by cutting 40nm slices in an ultracryomicrotome Leica. The samples were
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stained with RuO4 for 24hours and analyzed in a Philips CM-120 transmission electron microscope and in a FEI Magellan 400L in STEM mode.
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2.2.3.6 Wide Angle X-Ray diffraction (WAXD)
WAXD was used to characterize the crystalline structure of PP and the nanocomposites before and after annealing in a Siemens D5005 X-Ray Difratometer using Cu Kα radiation, with
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2θ varying from 5 to 90oC.
3. Results and Discussions
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3.1 Characterization of polypropylene and the nanocomposites
Table 1 shows the mechanical properties for the polypropylene and the nanocomposite
before and after thermal treatment. The results of impact tests show that the simple incorporation of calcium carbonate nanoparticles do not affect the impact strength of polypropylene. The nanocomposite shows a slightly higher impact strength but it is close to the standard deviation associated with the test. After thermal treatment the impact resistance of
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ACCEPTED MANUSCRIPT polypropylene has not changed considering the standard deviation of the test, but the impact strength of the nanocomposite has increased approximately 30% which represents an increase of 44% compared to the polypropylene. The elastic modulus of polypropylene was improved after incorporation of calcium
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carbonate nanoparticles. Before thermal treatment the nanocomposite elastic modulus is 50%
greater than the modulus of untreated polypropylene. After thermal treatment this difference has reached 107%.
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The increase in elastic modulus might explain in part the increase in impact strength of the nanocomposite PPC-20. It is well known that higher elastic modulus leads to stronger fibrils
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inside crazes and higher impact resistance due the increase in the amount of energy necessary to break the fibrils that suppress the coalescence of fibrils and the formation of cracks.[21] Besides the increase in the strength of fibrils other effects related to the crystallinity and the crystalline morphology of the polymer can affect the impact strength of the nanocomposites.
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Table 2 shows the values of crystallinity of polypropylene and the nanocomposites obtained with both DSC and WAXD. The results correlates with the observations from mechanical properties that thermal treatment affects only the properties of the nanocomposites.
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The crystallinity of polypropylene was not affected by thermal treatment while the crystallinity of the nanocomposite PPC-20 has increased.
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Figure 1 shows the difratograms of the nanocomposite PPC-20 before and after thermal treatment. It can be observed that after thermal treatment the ratio between the intensities of the peaks change significantly. Before thermal treatment the nanocomposite PPC-20 presents the peaks of the crystallographic planes (110), (040) and (130) with the most intense peak being the (040) plane. After thermal treatment the same three peaks are presented but their intensity is about the same with the peak representing the plane (110) being the most intense. The increase in the intensity of the peak (110) is an evidence of the formation of the cross-hatched structure of PP. It has been reported that the cross-hatched structure may be created after thermal
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ACCEPTED MANUSCRIPT treatment as a consequence of the crystallization of rigid amorphous phase of polypropylene. [15]The formation of the cross-hatched structure has been reported in different works and associated with thermal treatment. Some authors attribute the increase in impact strength of polypropylene/calcium carbonate nanocomposites to the formation of the cross-hatched
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structure.[14]
WAXD and DSC results evidence that the morphological evolution that takes place and influences the mechanical properties of the nanocomposite PPC-20 can be associated with the
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recrystallization of the polymer in the nanocomposites. Moreover, it is possible that this morphological evolution leads to the formation of the cross-hatched morphology of
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polypropylene in the nanocomposite. The high crystallinity explains the increase in elastic modulus which also contributes to the increase in impact resistance.[21] The crystalline morphology can be visualized by microscopic methods. In previous work in our group it was observed that the incorporation of calcium carbonate nanoparticles strongly reduces the size of polypropylene spherulites.[12] In this work the crystalline
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morphology was observed with higher magnification in order to evaluate the effects of calcium carbonate nanoparticles. Figure 2 shows micrographs of polypropylene (Figure 2a) and the nanocomposite before (Figure 2b) and after (Figure 2c) thermal treatment.
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Polypropylene morphology shows clear evidence of the presence of spherulites. This is
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consistent with previous observations and with the morphology of polypropylene crystallized from the melt. [8]The nanocomposite PPC-20 on the other hand does not show any evidence of the presence of spherulites. The absence of spherulites in polypropylene is characteristic of crystallization during quenching. [8] It is also possible to obtain a non-spherulitic morphology of polypropylene in composites and nanocomposites due to the influence of the nanoparticles in the crystallization of the polymer. Instead of spherulites the nanocomposite morphology shows thin lines that are almost invisible before thermal treatment. After thermal treatment this thin lines are more evident and
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ACCEPTED MANUSCRIPT thicker. In both cases the observed morphology appears close to the particles and in the region between the particles. The change in morphology after thermal treatment suggests that during thermal treatment the amount of the crystalline regions in the nanocomposite increases which is in agreement with the results obtained in DSC and WAXD results.
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This morphological evolution could explain the mechanical properties of the
nanocomposite after thermal treatment. The thin lines are evidence of the lamellar structure
which is consistent to the morphology of high impact strength polypropylene/calcium carbonate
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nanocomposites. [7,9,10]The difference between the morphology presented here and the
morphology reported in the literature is the thickness of the lamellae because in Figure 2c the
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lamellae is much thinner than the results presented in the literature.
A further understand of the effects of thermal treatment in polypropylene and the nanocomposites was obtained by DMA. DMA gives information about two different amorphous regions in polypropylene. [8,22,23] The β transition of polypropylene (Tβ) is related to the relaxation of the amorphous phase of polypropylene, representing the glass transition
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temperature of polypropylene and appears at temperatures between 0 and 5oC. The α transition appears at temperatures between 30 and 100oC and is associated to the relaxation of a portion of polypropylene´s amorphous phase that is influenced by crystalline phase and has some degree
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of order. [8] The α transition observation also depends on the crystalline morphology of polypropylene.[8] For a spherulitic morphology thermal treatment improves the perfection of
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spherulites surfaces which leads to high increases in the temperature associated with the α transition. In a non-spherulitic morphology two components of the α transition have been identified: the first between 30 and 80oC represents the stress relaxation of molecular segments which belong to the noncrystalline regions and possess local short-distance ordering. The second component is associated with the orientation relaxation of crystallites to the deformation mechanism and appears above 80oC. [8] Figure 4 shows the loss and storage modulus of polypropylene and the nanocomposite PPC-20 before and after thermal treatment. It is clear from Figure 4 that thermal treatment
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ACCEPTED MANUSCRIPT affects the α transition in the neat polypropylene and in the nanocomposite PPC-20. In both cases the α transition shifts to high temperature and becomes broader. The temperatures corresponding to each transition are displayed in Table 3. The glass transition temperature (Tβ) is not affected by the incorporation of nanoparticles and the α transition is only slightly higher
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for the nanocomposite. The glass transition temperature decreases after thermal treatment which evidences the relaxation of polypropylene´s amorphous phase. On the other hand the α
transition increased after thermal treatment and the temperature of polypropylene´s α transition
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became higher than the temperature of the nanocomposite. For the nanocomposite the fact that the α transition temperature is below 80oC suggests that it represents the low temperature
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component of non spherulitic polypropylene which is associated with amorphous phase that possess some degree of order. The increase this transition temperature could be explained by an increase in the degree of order of this amorphous regions during thermal treatment. For polypropylene the increase in the α transition temperature could be evidence of the improved perfection of the spherulites surface boundaries proposed by Puta and Kryszewski. [8]
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Moreover it is possible to associate the observations of DMA results with the crystallinity calculated from DSC and WAXD results. The fact that the crystallinity of the nanocomposite has increased with thermal treatment could be associated with the increase in molecular
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mobility of the amorphous phase associated with the α transition during thermal treatment, i.e. the mobility of the amorphous phase is increased during thermal treatment which enables the
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increase in the local segment ordering leading to the crystallization of a portion of this amorphous phase. The portion that does not crystallize has greater degree of ordering and is represented by the α transition that appears at higher temperatures after thermal treatment. In the case of polypropylene thermal treatment improves spherulite perfection instead of increasing the polymer crystallinity.
3.2 Mechanism of improvement of impact strength during thermal treatment.
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ACCEPTED MANUSCRIPT The results obtained in this work show strong evidence that the mechanism that allows the improvement of mechanical properties of polypropylene/calcium carbonate nanocomposites by the application of thermal treatment is related to the crystalline morphology of the polymer matrix in the nanocomposite. By considering the toughening mechanisms proposed by Wu [5]
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and Muratoglu [7] and considering the difference in morphology of the nanocomposite and the polymer it seems very important that the nanocomposite have a non-spherulitic morphology in order to toughening to happen. This non-spherulitic morphology is obtained in polypropylene
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by crystallization from the glassy state during quenching [8] and is also obtained by the
presence of incoherent surface of a second phase like calcium carbonate nanoparticles. [7,10]
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Moreover it is necessary that the crystallites that grow from the surface of the particles to have a preferential orientation and to superpose in order to eliminate the amorphous material between them. It is in this superposition mechanism that thermal treatment is important. Because the simple incorporation of calcium carbonate nanoparticles is not enough to create this superposition thermal treatment is necessary to allow the crystallization of the amorphous phase
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that exists between two crystalline phases in the interparticle region. In this process is not clear how injection molding affects the crystallization of the polymer in the nanocomposite but it is speculated that there is a relation between the injection molding process and the development
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of this crystalline morphology. Finally it is possible to obtain further improvement in the impact strength of the nanocomposite if the lamellae become thicker what is possible to obtain with
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thermal treatment at higher temperatures and longer periods of time. Figure 4 shows a schematic representation of the mechanism of recrystallization during annealing and the final morphology that is desired for toughening. The rigid amorphous phase between two crystalline phases is represented in DMA results by the α transition. It is also possible that this phase crystallizes as a cross-hatched structure. The final ideal morphology (Figure 4b) is composed of a series of oriented lamellae between the nanoparticles.
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4. Conclusions Polypropylene/Calcium carbonate nanocomposites with enhanced impact strength and
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elastic modulus were prepared and characterized. The increase in impact strength compared to neat polypropylene was obtained only after thermal treatment at high temperatures. The thermal treatment affected the crystallinity and mechanical properties of the nanocomposite but did not affect the properties or crystallinity of polypropylene matrix. Evidences of morphological
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evolution during annealing associated with the crystalline phase of polypropylene were found in DMA results of neat PP and the nanocomposite PP/CaCO3. A mechanism to explain the
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influence of thermal treatment in the crystallization and morphology of the nanocomposite was proposed and related to the improvements in the impact strength.
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5. Acknowledgments
The authors would like to thank Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for funding. Braskem - Brazil for supplying polypropylene resin and YH-nano
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China for supplying CaCO3 nanoparticles.
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TABLES Table 1: Mechanical properties of polypropylene and polypropylene/calcium carbonate nanocomposite. Impact Strength (J/m)
Young´s Modulus (GPa)
uPP
34 ± 2
1.4
uPPC-20
38 ± 1
aPP
36 ± 1
aPPC-20
49 ± 0
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Sample
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2.1 1.5 2.9
Table 2: Crystallinity of polypropylene and polypropylene/calcium carbonate
uPP
%Cryst. (DSC)
%Cryst. (WAXD)
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Sample
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nanocomposite obtained from DSC and WAXD. %Cryst. α phase (WAXD)
%Cryst. β phase (WAXD)
Amorphous Phase (%)
48
53
41
12
47
51
53
47
6
47
uPPC-20
50
50
44
6
50
tPPC-20
61
73
73
0
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tPP
Table 3: Temperatures of the transitions of polypropylene and the nanocomposite PPC20 obtained from DMA. Sample
Tβ (oC)
Tα (oC)
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69
tPP
1
81
uPPC-20
3
71
tPPC-20
0
76
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uPP
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FIGURES
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Figure 1: Difratogram of nanocomposite PPC-20 before and after thermal treatment.
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Figure 2: Morphology of (a) Polypropylene, (b) nanocomposite PPC-20 before thermal treatment and (c) nanocomposite PPC-20 after thermal treatment.
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(b)
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(a)
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PPC-20. (□) before thermal treatment (●) after thermal treatment
Figure 4: Schematic representation of the mechanism of recrystallization proposed for polypropylene/calcium carbonate nanocomposites during thermal treatment. (a) Morphological
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evolution during annealing and (b) Final morphology desired for toughening.
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S1359-8368(16)00049-4
DOI:
10.1016/j.compositesb.2015.12.040
Reference:
JCOMB 3987
To appear in:
Composites Part B
Received Date: 5 May 2015 Revised Date:
2 December 2015
Accepted Date: 26 December 2015
Please cite this article as: do Nascimento EM, Eiras D, Pessan LA, Effect of Thermal Treatment on Impact Resistance and Mechanical Properties of Polypropylene/Calcium Carbonate Nanocomposites, Composites Part B (2016), doi: 10.1016/j.compositesb.2015.12.040. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of Thermal Treatment on Impact Resistance and Mechanical Properties of Polypropylene/Calcium Carbonate Nanocomposites
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Estêvão Mestres do Nascimento1, Daniel Eiras1*# and Luiz Antonio Pessan1 1Department of Materials Engineering, Federal University of São Carlos, 13.565-905 São Carlos, São Paulo, Brazil *[email protected]
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Abstract
Heat treatment was applied in polypropylene/calcium carbonate
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nanocomposite and its effects on the structure and impact resistance of the materials were studied. The nanocomposite was prepared by melt blending in a twin screw extruder and subsequently molded into tensile specimens by injection molding. Polypropylene and the
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nanocomposite were characterized through tensile and impact tests, wide angle x-ray diffraction (WAXD), transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and
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dynamic mechanical thermal analysis (DMA). The results show an
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increase in impact strength of the nanocomposite after heat treatment which was accompanied by an increase in elastic modulus. DSC and WAXD results show an increase in the crystallinity of the polymer matrix of the nanocomposite but not in the neat polymer. Analysis of the morphology of PP and the nanocomposite PP/CaCO3 revealed that the nanocomposite has a non-spherulitic morphology. Results from DMA indicate that thermal treatment affects the rigidity of the amorphous phase of polypropylene. A mechanism was proposed to
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ACCEPTED MANUSCRIPT explain the influence of thermal treatment in the morphology of PP/CaCO3 nanocomposites and its relation with the impact strength. Keywords: A. Particle-reinforcement; B. Impact behaviour; D. Mechanical
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testing; E. Extrusion
# Rodovia Washington Luiz, km 235 – CEP:13.565-905 São Carlos-
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1. Introduction The application of calcium carbonate nanoparticles to enhance the impact strength of
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polymeric materials has received significant attention recently due to the possibility of increasing the impact strength and the elastic modulus of the polymer simultaneously.[1–4] The toughening mechanism in these systems has been described as the result of debonding of
polymer/particle interface due to stress concentration followed by the occurrence of deformation
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mechanisms such as shear yielding and crazing. A more detailed mechanism suggested by Wu [5,6] involves a critical interparticle distance that is necessary for a polymer composite or blend
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to be tougher than its matrix. If the distance between particles is greater than critical distance no toughening effect is observed but when the distance between the particles is smaller than the critical distance cooperative toughening mechanisms are observed. In an attempt to better explain the mechanism proposed by Wu, Muratoglu et al [7] proposed that the actual influence
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of a second phase in polymeric materials is related to the crystallization of the polymer. According to this concept when the crystallization of the polymer is nucleated from the incoherent surface of a second phase the polymer crystals are formed with a preferential orientation of the lamellae perpendicular to the surface of the disperse particle. The
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crystallographic planes in this lamellae are specific planes with low shear resistance which
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favors the initiation of the deformation mechanisms and the toughening of the polymer. If the interparticle distance is greater than the critical distance the oriented crystals are separated between amorphous material. On the other hand if the interparticle distance is equal or smaller than the critical distance the crystals superpose an toughening is observed. In terms of polypropylene crystallization it has been observed that the crystalline morphology might be different depending on the way the polymer is crystallized.[8] When crystallized from the melt polypropylene presents a spherulitic morphology while the
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ACCEPTED MANUSCRIPT crystallization during quenching creates a non-spherulitic morphology that coincides with the morphology proposed by Muratoglu et al [7] and Bartczak et al.[9,10] Recently the authors have shown that the simple incorporation of calcium carbonate nanoparticles in polypropylene leads to a decrease of brittle-to-ductile transition temperature
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and an increase in the elastic modulus compared to the neat polymer matrix.[11] The
nanocomposite also presented a high impact strength at low temperatures although the impact strength at room temperature was practically the same of the neat polymer. In another work [12]
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polarized optical microscopy have indicated that the incorporation of calcium carbonate nanoparticles strongly reduces the sizes of polypropylene spherulites and increases the
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crystallinity of polypropylene and also affects the crystallization kinetics of the polymer. The most recent contribution to the field has presented high increases in the impact strength of polypropylene/calcium carbonate nanocomposites after thermal treatment.[13,14] The authors suggested that heat treatment leads to recrystallization of the polymer between the particles and enable the superposition of lamellae between the particles. The authors have also
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suggested that the formation of polypropylene´s cross-hatched structure is responsible for increasing the resistance of the fibrils inside crazes and enhance the impact strength of the nanocomposites.
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The effect of thermal treatment in the crystallization of polypropylene has been
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discussed in previous work in the literature.[15–17] If the polymer structure is described by a three phase model composed of an amorphous phase, a rigid amorphous phase and a crystalline phase there are evidences that thermal treatment leads to an increase in crystallinity due to the enhanced mobility of the amorphous phase at high temperatures. As a result of this enhanced mobility the amount of crystalline phase is increased at the expense of the amorphous phase. Moreover, in the case of polypropylene the rigid amorphous phase crystallizes creating the cross-hatched structure of polypropylene.
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ACCEPTED MANUSCRIPT Other strategies like polymer blending, copolymerization and application of nucleating agents have been used to improve mechanical and fracture behavior of polypropylene. Recent work from Wu H. and coworkers [18] used β nucleating agents and thermal treatment to achieve strengthen and toughening of polypropylene/clay nanocomposites. The authors have established
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that the formation of β phase was responsible for the improvement in polypropylene/clay nanocomposites.
Motamedi P. and Bagheri R. [19] prepared polypropylene/polyamide/layered silicate
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ternary nanocomposites and demonstrated the relationship between processing, dispersion of the layered silicates and the mechanical properties of the nanocomposites. In general the
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improvement in dispersion leads to and improvement in strength and modulus but decreases the impact resistance of the material showing that the balance between mechanical resistance and impact resistance is difficult to be achieved.
Yuiling M. and coworkers [20] have developed super tough polypropylene/carbon nanotubes nanocomposites by the incorporation of a third phase composed of polyethylene–
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octene copolymer (POE). Although other materials were incorporated as third phase POE was the only one capable of increasing impact strength and good electrical conductivity. In this context, and considering the importance of polypropylene in various industrial
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sectors and the importance of increasing its impact resistance without losing rigidity the effect
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of thermal treatment in polypropylene/calcium carbonate nanocomposites was studied in terms of the impact and tensile properties and the crystallization of the polymer and of the nanocomposites. The morphology of the nanocomposite before and after thermal treatment was compared to the morphology of the neat polymer and evidences of a non spherulitic morphology in the nanocomposite were obtained. Finally a mechanism to explain the influence of thermal treatment in the crystalline morphology of polypropylene/calcium carbonate nanocomposites was proposed and related to the impact improvements in impact strength.
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ACCEPTED MANUSCRIPT 2. Materials and Methods 2.1 Materials Polypropylene homopolymer H501-HC with melt index of 3,5g/10min (230oC and
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2,16Kg) was supplied by Braskem and used as received. Calcium carbonate nanoparticles YH 303 were supplied by YH-nano.
2.2 Methods
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2.2.1 Preparation of polypropylene/calcium carbonate nanocomposites
Polypropylene was processed and the nanocomposites were prepared in a B&P equipment co-rotational twin screw extruder. Temperature profile was 170/190/190 190/190/195oC and the screw rotation was 160rpm. One composition of polypropylene/calcium carbonate nanocomposite was prepared containing 20wt% of calcium carbonate (PPC-20).
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The materials were molded into tensile specimen in an Arburg Alrounder 270V injection molding machine. The dimensions of the samples followed the dimensions of ASTM D638. The temperature profile was 215/220/235/245/250oC. The samples for impact tests were
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cut from tensile specimens according to the dimensions of ASTM D256.
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2.2.2 Thermal treatment of polypropylene and polypropylene/calcium carbonate nanocomposites
Polypropylene and the nanocomposites test bars were annealed at 150oC for 2 hours in order to verify the effect of annealing on their mechanical properties. Annealing was performed in a digestion block. The system was heated and the temperature stabilized before the introduction of the samples.
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2.2.3 Characterization of the polypropylenene and the nanocomposites
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2.2.3.1 Tensile Tests
Tensile tests were performed at room temperature in an Instron Universal Testing
Machine with a testing speed of 5mm/min. Before testing the samples were conditioned at 23oC
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and 50% maximum humidity according to ASTM D638.
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2.2.3.2 Impact Tests
Impact tests were performed in a CEAST impact testing machine. Before testing the samples were notched according to ASTM D256 and conditioned at 23oC and 50% maximum
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humidity.
2.2.3.3 Diferential Scanning Calorimetry
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The crystallization behavior of polypropylene and the nanocomposites was
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characterized by DSC in a Q2000 TA calorimeter. The tests were performed by heating the samples from 40oC to 190oC, cooling it from 190oC to 40oC and then heating again to 200oC. Both heating and cooling rates were 10oC/min. The degree of crystallization (%Cryst.) was calculated using equation 1. Em que ∆Hf é a entalpia de fusão do polímero, ∆Hf,100% é a entalpia de fusão de um material 100% cristalino e χ é a fração mássica de nanopartículas no nanocompósito.
2.2.3.4 Dynamic Mechanical Analysis
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2.2.3.5 Characterization of the dispersion of calcium carbonate (TEM)
Transmission electron microscopy of the nanocomposites was used to verify the
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dispersion of calcium carbonate nanoparticles in polypropylene. The samples for TEM analyses were prepared by cutting 40nm slices in an ultracryomicrotome Leica. The samples were
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stained with RuO4 for 24hours and analyzed in a Philips CM-120 transmission electron microscope and in a FEI Magellan 400L in STEM mode.
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2.2.3.6 Wide Angle X-Ray diffraction (WAXD)
WAXD was used to characterize the crystalline structure of PP and the nanocomposites before and after annealing in a Siemens D5005 X-Ray Difratometer using Cu Kα radiation, with
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2θ varying from 5 to 90oC.
3. Results and Discussions
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3.1 Characterization of polypropylene and the nanocomposites
Table 1 shows the mechanical properties for the polypropylene and the nanocomposite
before and after thermal treatment. The results of impact tests show that the simple incorporation of calcium carbonate nanoparticles do not affect the impact strength of polypropylene. The nanocomposite shows a slightly higher impact strength but it is close to the standard deviation associated with the test. After thermal treatment the impact resistance of
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ACCEPTED MANUSCRIPT polypropylene has not changed considering the standard deviation of the test, but the impact strength of the nanocomposite has increased approximately 30% which represents an increase of 44% compared to the polypropylene. The elastic modulus of polypropylene was improved after incorporation of calcium
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carbonate nanoparticles. Before thermal treatment the nanocomposite elastic modulus is 50%
greater than the modulus of untreated polypropylene. After thermal treatment this difference has reached 107%.
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The increase in elastic modulus might explain in part the increase in impact strength of the nanocomposite PPC-20. It is well known that higher elastic modulus leads to stronger fibrils
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inside crazes and higher impact resistance due the increase in the amount of energy necessary to break the fibrils that suppress the coalescence of fibrils and the formation of cracks.[21] Besides the increase in the strength of fibrils other effects related to the crystallinity and the crystalline morphology of the polymer can affect the impact strength of the nanocomposites.
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Table 2 shows the values of crystallinity of polypropylene and the nanocomposites obtained with both DSC and WAXD. The results correlates with the observations from mechanical properties that thermal treatment affects only the properties of the nanocomposites.
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The crystallinity of polypropylene was not affected by thermal treatment while the crystallinity of the nanocomposite PPC-20 has increased.
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Figure 1 shows the difratograms of the nanocomposite PPC-20 before and after thermal treatment. It can be observed that after thermal treatment the ratio between the intensities of the peaks change significantly. Before thermal treatment the nanocomposite PPC-20 presents the peaks of the crystallographic planes (110), (040) and (130) with the most intense peak being the (040) plane. After thermal treatment the same three peaks are presented but their intensity is about the same with the peak representing the plane (110) being the most intense. The increase in the intensity of the peak (110) is an evidence of the formation of the cross-hatched structure of PP. It has been reported that the cross-hatched structure may be created after thermal
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ACCEPTED MANUSCRIPT treatment as a consequence of the crystallization of rigid amorphous phase of polypropylene. [15]The formation of the cross-hatched structure has been reported in different works and associated with thermal treatment. Some authors attribute the increase in impact strength of polypropylene/calcium carbonate nanocomposites to the formation of the cross-hatched
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structure.[14]
WAXD and DSC results evidence that the morphological evolution that takes place and influences the mechanical properties of the nanocomposite PPC-20 can be associated with the
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recrystallization of the polymer in the nanocomposites. Moreover, it is possible that this morphological evolution leads to the formation of the cross-hatched morphology of
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polypropylene in the nanocomposite. The high crystallinity explains the increase in elastic modulus which also contributes to the increase in impact resistance.[21] The crystalline morphology can be visualized by microscopic methods. In previous work in our group it was observed that the incorporation of calcium carbonate nanoparticles strongly reduces the size of polypropylene spherulites.[12] In this work the crystalline
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morphology was observed with higher magnification in order to evaluate the effects of calcium carbonate nanoparticles. Figure 2 shows micrographs of polypropylene (Figure 2a) and the nanocomposite before (Figure 2b) and after (Figure 2c) thermal treatment.
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Polypropylene morphology shows clear evidence of the presence of spherulites. This is
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consistent with previous observations and with the morphology of polypropylene crystallized from the melt. [8]The nanocomposite PPC-20 on the other hand does not show any evidence of the presence of spherulites. The absence of spherulites in polypropylene is characteristic of crystallization during quenching. [8] It is also possible to obtain a non-spherulitic morphology of polypropylene in composites and nanocomposites due to the influence of the nanoparticles in the crystallization of the polymer. Instead of spherulites the nanocomposite morphology shows thin lines that are almost invisible before thermal treatment. After thermal treatment this thin lines are more evident and
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This morphological evolution could explain the mechanical properties of the
nanocomposite after thermal treatment. The thin lines are evidence of the lamellar structure
which is consistent to the morphology of high impact strength polypropylene/calcium carbonate
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nanocomposites. [7,9,10]The difference between the morphology presented here and the
morphology reported in the literature is the thickness of the lamellae because in Figure 2c the
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lamellae is much thinner than the results presented in the literature.
A further understand of the effects of thermal treatment in polypropylene and the nanocomposites was obtained by DMA. DMA gives information about two different amorphous regions in polypropylene. [8,22,23] The β transition of polypropylene (Tβ) is related to the relaxation of the amorphous phase of polypropylene, representing the glass transition
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temperature of polypropylene and appears at temperatures between 0 and 5oC. The α transition appears at temperatures between 30 and 100oC and is associated to the relaxation of a portion of polypropylene´s amorphous phase that is influenced by crystalline phase and has some degree
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of order. [8] The α transition observation also depends on the crystalline morphology of polypropylene.[8] For a spherulitic morphology thermal treatment improves the perfection of
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spherulites surfaces which leads to high increases in the temperature associated with the α transition. In a non-spherulitic morphology two components of the α transition have been identified: the first between 30 and 80oC represents the stress relaxation of molecular segments which belong to the noncrystalline regions and possess local short-distance ordering. The second component is associated with the orientation relaxation of crystallites to the deformation mechanism and appears above 80oC. [8] Figure 4 shows the loss and storage modulus of polypropylene and the nanocomposite PPC-20 before and after thermal treatment. It is clear from Figure 4 that thermal treatment
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ACCEPTED MANUSCRIPT affects the α transition in the neat polypropylene and in the nanocomposite PPC-20. In both cases the α transition shifts to high temperature and becomes broader. The temperatures corresponding to each transition are displayed in Table 3. The glass transition temperature (Tβ) is not affected by the incorporation of nanoparticles and the α transition is only slightly higher
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for the nanocomposite. The glass transition temperature decreases after thermal treatment which evidences the relaxation of polypropylene´s amorphous phase. On the other hand the α
transition increased after thermal treatment and the temperature of polypropylene´s α transition
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became higher than the temperature of the nanocomposite. For the nanocomposite the fact that the α transition temperature is below 80oC suggests that it represents the low temperature
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component of non spherulitic polypropylene which is associated with amorphous phase that possess some degree of order. The increase this transition temperature could be explained by an increase in the degree of order of this amorphous regions during thermal treatment. For polypropylene the increase in the α transition temperature could be evidence of the improved perfection of the spherulites surface boundaries proposed by Puta and Kryszewski. [8]
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Moreover it is possible to associate the observations of DMA results with the crystallinity calculated from DSC and WAXD results. The fact that the crystallinity of the nanocomposite has increased with thermal treatment could be associated with the increase in molecular
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mobility of the amorphous phase associated with the α transition during thermal treatment, i.e. the mobility of the amorphous phase is increased during thermal treatment which enables the
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increase in the local segment ordering leading to the crystallization of a portion of this amorphous phase. The portion that does not crystallize has greater degree of ordering and is represented by the α transition that appears at higher temperatures after thermal treatment. In the case of polypropylene thermal treatment improves spherulite perfection instead of increasing the polymer crystallinity.
3.2 Mechanism of improvement of impact strength during thermal treatment.
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ACCEPTED MANUSCRIPT The results obtained in this work show strong evidence that the mechanism that allows the improvement of mechanical properties of polypropylene/calcium carbonate nanocomposites by the application of thermal treatment is related to the crystalline morphology of the polymer matrix in the nanocomposite. By considering the toughening mechanisms proposed by Wu [5]
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and Muratoglu [7] and considering the difference in morphology of the nanocomposite and the polymer it seems very important that the nanocomposite have a non-spherulitic morphology in order to toughening to happen. This non-spherulitic morphology is obtained in polypropylene
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by crystallization from the glassy state during quenching [8] and is also obtained by the
presence of incoherent surface of a second phase like calcium carbonate nanoparticles. [7,10]
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Moreover it is necessary that the crystallites that grow from the surface of the particles to have a preferential orientation and to superpose in order to eliminate the amorphous material between them. It is in this superposition mechanism that thermal treatment is important. Because the simple incorporation of calcium carbonate nanoparticles is not enough to create this superposition thermal treatment is necessary to allow the crystallization of the amorphous phase
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that exists between two crystalline phases in the interparticle region. In this process is not clear how injection molding affects the crystallization of the polymer in the nanocomposite but it is speculated that there is a relation between the injection molding process and the development
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of this crystalline morphology. Finally it is possible to obtain further improvement in the impact strength of the nanocomposite if the lamellae become thicker what is possible to obtain with
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thermal treatment at higher temperatures and longer periods of time. Figure 4 shows a schematic representation of the mechanism of recrystallization during annealing and the final morphology that is desired for toughening. The rigid amorphous phase between two crystalline phases is represented in DMA results by the α transition. It is also possible that this phase crystallizes as a cross-hatched structure. The final ideal morphology (Figure 4b) is composed of a series of oriented lamellae between the nanoparticles.
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4. Conclusions Polypropylene/Calcium carbonate nanocomposites with enhanced impact strength and
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elastic modulus were prepared and characterized. The increase in impact strength compared to neat polypropylene was obtained only after thermal treatment at high temperatures. The thermal treatment affected the crystallinity and mechanical properties of the nanocomposite but did not affect the properties or crystallinity of polypropylene matrix. Evidences of morphological
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evolution during annealing associated with the crystalline phase of polypropylene were found in DMA results of neat PP and the nanocomposite PP/CaCO3. A mechanism to explain the
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influence of thermal treatment in the crystallization and morphology of the nanocomposite was proposed and related to the improvements in the impact strength.
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5. Acknowledgments
The authors would like to thank Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) for funding. Braskem - Brazil for supplying polypropylene resin and YH-nano
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China for supplying CaCO3 nanoparticles.
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TABLES Table 1: Mechanical properties of polypropylene and polypropylene/calcium carbonate nanocomposite. Impact Strength (J/m)
Young´s Modulus (GPa)
uPP
34 ± 2
1.4
uPPC-20
38 ± 1
aPP
36 ± 1
aPPC-20
49 ± 0
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Sample
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2.1 1.5 2.9
Table 2: Crystallinity of polypropylene and polypropylene/calcium carbonate
uPP
%Cryst. (DSC)
%Cryst. (WAXD)
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Sample
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nanocomposite obtained from DSC and WAXD. %Cryst. α phase (WAXD)
%Cryst. β phase (WAXD)
Amorphous Phase (%)
48
53
41
12
47
51
53
47
6
47
uPPC-20
50
50
44
6
50
tPPC-20
61
73
73
0
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tPP
Table 3: Temperatures of the transitions of polypropylene and the nanocomposite PPC20 obtained from DMA. Sample
Tβ (oC)
Tα (oC)
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69
tPP
1
81
uPPC-20
3
71
tPPC-20
0
76
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uPP
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FIGURES
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Figure 1: Difratogram of nanocomposite PPC-20 before and after thermal treatment.
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Figure 2: Morphology of (a) Polypropylene, (b) nanocomposite PPC-20 before thermal treatment and (c) nanocomposite PPC-20 after thermal treatment.
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(b)
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(a)
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SC
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PPC-20. (□) before thermal treatment (●) after thermal treatment
Figure 4: Schematic representation of the mechanism of recrystallization proposed for polypropylene/calcium carbonate nanocomposites during thermal treatment. (a) Morphological
AC C
EP
TE D
evolution during annealing and (b) Final morphology desired for toughening.
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