Study of fracture toughness and mechanical properties of PP/EPDM/clay nanocomposites

Study of fracture toughness and mechanical properties of PP/EPDM/clay nanocomposites prepared using twin-screw extruder and Friction stir process
 
Abstract:

Polymer nanocomposites have a wide application in automotive and airplane industries, shipbuilding, household appliances and medical apparatus due to their excellent mechanical, thermal and chemical properties. In this research, Polypropylene (PP) / Ethylene-Propylene Diene Monomer (EPDM) nanocompositeswith 5%wtnanoclayare fabricatedby Friction Stir Processing (FSP) and compared to those obtained via a conventional Twin Screw Extruder (TSE) and) in order to determine the effects of process parameters on morphology and mechanical properties.The mixtures and dispersion of nanoclay in PP/EPDM have been characterized by X-ray diffraction, transmission electron microscopy and mechanical property tests.Comparison of the results of these methods indicated that PP/EPDM/clay nanocomposite fabricated by FSP had a better dispersion of nanoclay and mechanical properties. The results show that by addition of 5% wt nanoclay to base material, the tensile strength and tensile modulus increased and elongation at break and impact strength decreased.Under optimal conditions of rotational speed of 1200 rpm, traverse speed of 50 mm/min, shoulder temperature of 100 °C and number passes of 3, simultaneous maximization of tensile strength (19.35 MPa), tensile modulus (643 MPa), impact strength (63 J/m) and elongation at break (101 %) can be obtained.

Keywords: Nanocomposite, Clay, Friction Stir Process, Twin Screw Extruder, Mechanical Properties.

INTRODUCTION

Thermoplastic elastomeric materials (TPEs) aredefined as a group of polymer blends which consist of two-phase materials exhibiting both thermoplastic and elastomeric properties[1, 2]. TPE blends can be divided into dynamically vulcanized blends (TPVs) and polyolefin blends (TPOs). TPOs blendare widely used in airplane, shipbuilding and automotive industries and medical apparatusdue to their excellent mechanical, thermal and chemical properties[3, 4]. Polypropylene (PP) as one of the most important commodity thermoplastic polymer have useful properties such as high thermal stability, low density, chemical resistance, good processability and high strength[5]. The industrial use of this polymer is still limited because of its low tensile modulus and poor impact resistance, especially at high strain rate and low temperatures[6].Blending PP with rubbers such as ethylene-propylene diene monomer (EPDM) give thougher materials suitable for automotive, aerospace and medical applications where excellent mechanical, thermal and chemical properties are needed [7, 8]. The addition of a small amount of clay (less than 10% w/w) further enhances mechanical, thermal and barrier properties [9, 10]. The development of polymersnanocomposite and their manufacturing technologies is one of the important advances in the recent history of plastics. Lately, various process have been developed for fabricating polymer nanocomposites which includes solvent blending, in-situ polymerization and melt compounding[8, 11, 12]. Studies on the preparation and properties of thermoplastic Elastomer (TPEs) nanocomposites based on PP, EPDM and nanoclay are readily available [2, 7].

Friction stir process (FSP) is a novel process developed based on the principle of friction stir welding (FSW) for the fabrication of composites, nanocomposites and microstructural modifications[13, 14].According to Fig. 1(a), in this process, a groove with defined dimensions is machined in the base material sheet and nano particles are placed into this groove. Subsequently a rotating pin first plunged into the base material and then move along the groove (Fig.1 (b) and (c)).The pin produces frictional and plastic deformation heating within the processing zone while also dispersing particles in the base material[15, 16]. FSP are used on a wide variety of materials, including aluminum, magnesium alloys, copper, steel and titanium alloys. Eskandari et al have investigated the effect of nanoparticle on microstructure and mechanical properties of Al/TiB2/Al2O3 nanocomposite fabricated by friction stir process.They concluded that the adition of TiB2/Al2O3 particles increased the microhardness.Also the maxzimum Young’s modulus (E), ultimate yield strength (UYS) and ultimate tensile strength (UTS) were obtained at a rotational speed of 800 rpm. Srinivasan et al studied the effect of FSP processing parameters on mechanical properties of copper nanocomposites reinforced with silicon carbide nanoparticles. They observed that with decreasing rotational speed and increasing processing speed the hardness in nanocomposite increased. To maximize the hardness, rotational speed, processing speed and tilt angle were set at 500 rpm, 50 mm/min and 1°, respectively.Very few reports on fabricating polymeric composite and nanocomposites by FSP are found in the literature. For instance, enhancement of morphology and mechanical properties of polyethylene (PE)/copper composites and high density polyethylene (HDPE)/clay nanocomposites fabricated using FSP have been reported by Azarsa et al and Barmous et al, respectively. They concluded that the FSP is a suitable methodfor fabricating thermoplastic composite and nanocomposites. Their results showed that thesurface appearance and mechanical properties of composite and nanocomposites are strongly affected byFSP parameters such as rotational speed, traverse speed and shoulder temperature[17, 18].In thisstudy, friction stir process and twin screw extruder were employed to fabricated PP/EPDM/Clay nanocomposites. The aim was to compare the morphology, rheological, mechanical properties and fracture thoughness of these nanocomposites. For this purpose, the Taguchi method was applied to planexperiments and optimize processing conditions in FSP and TSE in order tosimultaneously maximize the tensile strength (St), tensile modulus (Mt), elongation at break (Eb) and impact strength (Si). Also the influence of process parameters of FSP(rotational speed, traverse speed, shoulder temperature and number passes) and TSE (screw speed, feed rate and barrel temperature) on mechanical properties of these nanocomposites were investigated. The effect of manufacturing processon fracture toughness behavior of PP/EPDM/Clay nanocomposite were investigated using the essential work of fracture (EWF) method.

EXPERIMENTAL PROCEDURE

Materials

In this work,polypropylene (PP), ethylene-propylene diene monomer (EPDM) and organoclay were usedto prepareTPO nanocomposites with 5% w/w nanoclay ,the grade name, company and characterization of these materials are shown in Table 1.

Sample preparation

Master batch Preparation

TPO composite with 80% PP and 20% EPDM were formed by melt mixing in a Brabender co-rotating twin screw extruder (L/D = 40 and D = 25 mm) whit screw speeds of 100 rpm, feeding rates of 0.5 kg/hand a temperature profile of 185-200 °C and 190 °C from the feeding zone to the die.Physical and mechanical properties of the TPO composite show in Table 2.

Nanocomposite Preparation

TPO nanocomposites samples with 5% wt nanoclay were prepared by TSEand FSP.

2.2.2.1 Twin Screw Extruder (TSE)

A co-rotating twinscrew extruder(L/D = 40 and D = 25 mm) type Brabender was used for mixing of the TPO blend with the nanoclay. In order to investigate the effect of process parameters such as feeding rate, screw speed and barrel temperature, the Taguchi method was employed to design experiments for three factorsat three levels, as shown in Table 3.

2.2.2.2 Friction Stir process (FSP)

The TPO 200×60×10 mm sheets were prepared using a Collin P 200 E-type heat press for FSP experiment.As illustrated onFig. 2, the tool used in this study consisted of a Teflon (PTFE) coated shoulder, a heating system, ball bearing and a cylindrical threaded pin.A groove of 2.25 mm depth and 2mm width(TS), to put the nanoclay samples, was machined in the base sheet as shown in Fig.2. The cross-sectional area of the groove (Ag) is therefore 2.7 mm2. Based on Eq. (1), height of the nanoclay particle (hn)bed is calculated to be 1.2 mm.

h_n= A_g/(T_S×D_r ) (1)

As = Cross-sectional area of the groove

Dr= Density ratio is determined by dividingnanoparticles density by TPO composite density.

The TPO plate was fixed on a Deckel milling machine (M.S.T Co, Iran) and the rotating pin was plunged into the groove till the underside of the shoulder touched the plate surface and the tool were moved along the groove with a constant speed for fabricating TPO nanocomposite (Fig. 3).The FSP parameters levels and their set values in the experimental design are reported inTable 4.

Nanocomposite characterization

2.3.1 Structure and morphological characterization

XRD experiments were carried out with a Philips -X’Pert diffractometer at room temperature in the low angle range of 2θ. The X-ray beam was a CuKα radiation (λ = 1.540598 A) operated at 50 kV voltage and 40 mA current. The scanning rate was 0.5 °/min and the experiments were performed in the angle range of 0 – 10 °.The transmission electron microscopy (TEM) observation was performed on ultrathin sections of cryo-microtemed thin nanocomposite films by a Jeol transmission electron microscopy (JEM-2100F) with an acceleration voltage of 100 kv.

2.3.2 Rheological measurements

Rheological characteristics within the linear viscoelastic region were measured with a stress-controlled rheometer (MCR 3000) using 25 mm diameter parallel plate geometry and gape size of 1mm.Measurements were conducted at a temperature of 180°C and in the frequency rangeof 0.01-80 Hz.

2.3.3 Dynamic Mechanical Analysis

Dynamic mechanical properties of the PP/EPDM and TPO nanocomposites were performed by using a DMA-Triton-Tritec 2000 machine and according to ASTM D4065 in the double cantiliver bending mode. Temperature scanning from -100°C to 60°C wasperformed with a heating rate of 3 °C/min at anoscillation frequency of 1 Hz. The storage modulus (E/) and tan  as a function of temperature were measured.

2.3.4 Differential scanning calorimetry (DSC) and thermogravimetry (TGA)

The melting and crystallization behavior characterization was carried out using a Maia-200F3 (Netzsch Co, USA) differential scanning calorimeter under N2 atmosphere. Each sample (about 10 mg) was first heatedto 200°Cat the heating rate of 10°C/minand held at 200°C for 5 min, then was cooled from this temperature at the same rate. The change in weight of samples as it is heated was investigated using a TA instrument 2960 (New Castle, DE, USA) at a heating rate of 10 °C /min from 25to 600 °C undera N2 atmosphere.

2.3.5 Mechanical properties

For tensile and impact test, specimenswere prepared from the fabricated TPO/clay nanocomposite sheets according to ASTM D638 and D256 standards respectively. The tensile test samples were tested in a Zuker tensile test machine (Zwick Co, Germany) with cross-head speed of 1 mm/min and at room temperature.Impact test was measured with notched specimens; using a Zwick with energy of 1J. The mechanical propertiesof nanocomposite fabricated by extruder and FSP are reported in Tables5 and 6, respectively.

2.3.6Essential work of fracture method

The essentialwork of fracture method (EWF) has been used for the examination of fracture characterizationof ductile polymers, related blends and composites.Thedouble edge notched tension (DDENT)samples to evaluate the essential workof fractureaccording to the ESIS-TC4 protocol , as illustrated in as shown in Fig. 4, were prepared to the dimensions reported in Table 7[19, 20].For each ligament length (l), at least fourDENT specimens were preparedfrom the PP/EPDM, FSPand TSE sheets by compressionmolding according to standard ISO 293.TheEWF tests were carried out atroom temperature with a cross-head speed of 5 mm/minusing the Zuker tensile test machine.The EWF is the total work of fracture (W_f) whichis calculated from the areaunderthe load –displacement curves.According to the Eq.2, W_f consists of two components of W_e and W_p that the essential work of fracture (W_e) related with the inner fractureprocess zone (FPZ), and the non-essential work of fracture (W_p) that corresponds with the plastic work done in the outer process zone (OPZ)[21, 22].

W_f=W_(e )+ W_p= w_e×lt+ w_p× βtl^2 〖 which W_f/lt= w〗_f=w_(e )+ βw_p×l (2)

Wherew_e,w_e and w_p are total specific work of fracture,specific fracture work and specific plastic work, respectively, β is the shape factor related to the form of the outer plastic zone and t is the specimen thickness. According to the energy-partitioned method recommended by Karger-Kocsis, the total work of fracture separates into two components:the work for yielding the ligament region (w_y), and the energy required for necking and tearing ligament area (w_n).

W_f=W_(y )+ W_n (3)

According to Eq. (2) and (3), the EWF formulationcan be rewritten as follows:

w_f=(w_(e,y)+〖β_y w〗_(p,y) l)+(w_(e,n)+〖β_n w〗_(p,n) l) (4)

where w_(e,y) is the specific essential yielding-related work of fracture,w_(e,n)is the specific essential necking and tearing work, w_(p,y)is the volumetric energy dissipated during yielding and w_(p,n) isthe dissipated plastic work during necking and tearing[23-25].

The fracture surface and fracture behavior of PP/EPDM blends wereexamined using scanning electrion microscopy (SEM). As shown inFig 4 (b) and (c) SEM fractographywas applied to the surface and sub-surface of the EWFsamples.The evolution of plastically deformed zones and micro-mechanicaldeformation mechanismsclose to crack – tip were investigated by transmision optical microscopy (TOM). For each blend, the single edge notched (SEN) thin films with dimensions of 80×25 mm and 40 µm thicknesswere prepared by compressionmolding according to standard ISO 293. Samples were notchedusing a sharp razor blade.Thetensile testsof SEN films were carried out at23°Cwith a cross-head speed of 5 mm/minand testing was terminated when force reached to 120 N (Fig. 4 (d)).

RESULTS AND DISCUSSION

3.1 XRD analysis

The results of X-ray diffraction, TPO nanocomposites fabricated by TSE and FSP are shown in Fig.5.The d-spacing of clay and nanocomposites was obtained from Bragg’s law, d= λ/ (2sinϴ). The diffraction peaks of TPO nanocomposites fabricated by TSE and FSP are shifted to lower angles 2.82° (31.3 A) and 2.58° (34.49 A), respectively.The reason for lower diffraction peak of TPO nanocomposite fabricated by FSP compared to nanocomposite made by TSEis attributed to the better dispersion of clay in the matrix and intercalation of polymer chains inside the silicate layers, as previously observedby other researchers[7, 26].

The SEM images of the cryofractured surfaces of three samples are shown in Fig. 6, where thedark holes correspond to etched EPDM particles. Addition of nanoclay to matrix resulted in a smaller size of dispersed EPDM particles because the nanoclay act as hindering for coalescence of elastomer particles[2, 4].A comparison of Fig. 6 (b) and (c) shows that very small EPDM particles are obtained when using FSP to produce nanocomposite. This is attributed tothe higher rotating speed found in FSP process, leading to higher shear stress facilitating the breakage of large elastomer particles.

Figures 7 shows the size distribution (PSD) of EPDM particlesas measured by image analysis of three samples totalizing more than 150 holes. The surface average size of EPDM particle (Dave) and average interparticle distance (IDave)were calculated using Eqs (5) and (6).

D_ave=(∑▒〖n_i 〖d_i〗^2 〗)/(∑▒〖n_i d_i 〗) (5)

〖ID〗_ave=[∛(π/(6ϕ_r ))-1]× D_ave which for 20% wt EPDM ID≈0.4×D_ave (6)

As clearly shown, the average diameter of rubber in the TFE sample decreases from 3.2 to 2.1 mm with the addition of 5 % w/w clay to PP/EPDM blend. While in FSE sample, the average diameter of EPDM particle is 0.68 mm.

The interparticle distance for PP/EPDM blend and PP/EPDM nanocomposite fabricated by TSE and FSP approximately is 1.25, 0.85 and 0.3µm, respectively.

3.2. TEM analysis

The TEM micrograph of TPO nanocomposite with 5% nanoclay which fabricated by FSP andTSE are shown in Fig.8 (a) and (b), respectively.The PP (denser phase) in the micrographs are indicated by gray areas and the light region represents the EPDM phase (lower density), the cross section of clay platelets are seen as black straigth line. The TEM micrograph of the TSE sample (Fig. 8 (b)) showedagglomeration or tactoids of clay in PP/EPDM matrix whereas we can see an overall better dispersion and intercalated and exfoliated nanostructures in the FSPmicrograph(Fig. 8 (a)).The better dispersion comes as an additionnal confirmation of the higher shear rate applied bythe FSP process, hence a more intense breaking action of the clay agglomerates[7, 27].

3.3Effect of process paramrameters on mechanical properties

Figure 9 shows the effect of extruder parameters such as screw speed, feeding rate and barrel temperature on mechanical properties of TPO nanocomposite.

It is clear from this figure that all the three factors have a significant effect on the response. Increasing the barrel temperature or the screw speed produces stiffer materials. Lower viscosity and more intense shear rates favor the dispersion of clay particles, and it is known that the major factor controlling mechanical properties in TPO nanocomposites is the aspect ratio of dispersed organoclay in the matrix[28].The higher level oforganoclay dispersion state leadsto a increase interfacial area between the organo clay and the polymer phase, which in turn favors a load transfer bewtween the matrix and its reinforcing fraction [29, 30]. Increase in feed rate increases the impact strength and elongation but decreases the tensile strength and tensile modulus.The residence time in the extruder decreases with increasing the feed rate which translate by a lesser dispersion of nanoclay inthe TPO matrix[7, 26, 28].

Fig. 10 is a perturbation plot which illustrates the effect of FSP parameters on the mechanical properties in the design space.With increasing rotational speed, shoulder temperatureand number of passes the tensile strength and tensile modulus increased while impact strength and elongation at break decreased.By increasing the number of passes the amount of shear forces and stress in the processed zone are increased and clay nanoparticles dispersed better in the matrix. In a study on Al3Ti/A356 composites, Yang et al. [31]observed a decreasein theaverage size and aspect ratioof Si particleswith increasing number of FSP passes.SEM and TEM micrographs show thatincreasing the number of FSP passes from 1 to 4 led to more than 50% reduction in the size of Si particle.Liu et al [32]reported thatfor CNT/ 2009 Al, dispersion of individualCNTswas achieved in multiple passes FSP. Increasing rotational speed increases shear stress and consequently dispersion of clay nanoparticles in base material can be obtained[17, 18, 32]. The mixing time in the processed zone decreases with increasing the traverse speed which causes lower dispersion of nanoclay inthe TPO matrix and it also decreases the tensile strength and tensile modulus whileenhancing impact strength and elongation at break [8, 14].X-ray diffraction was used to evaluate the extent of clay dispersion in the polymer matrix. Figure11shows the effect of different rotational and traverse speed on the basal spacing of sillicates in the TPOnanocomposites prepared by FSP. It was observed that higherrotational speed/traverse speed (ω/V)ratios lead to alargerbasal spacing of silicates. Thisconfirms a previous investigation by Salehi et al [33, 34] who also concluded that higherω/V ratiogive betterparticlesdispersion and smaller particle size.

3.4 Optimization of mechanical properties

According to Figs. 9 and 10, it can be observed that input parameters for extruder and FSP have opposite effect on mechanical properties.For example, with increasing rotational speedand screw speed the tensile strength and tensile modulus increased while impact strength and elongation decreased.While the criterion for fabrication of TPO nanocomposites is to simultaneously maximize themechanical properties.Table 8 shows the optimal conditions of the process parameters (FSP and extruder) for maximizing the mechanical properties. The mechanical properties shows that the FSP is better than extruder for fabrication of TPO nanocomposites. This attributed to the size reduction of the nanoclay particle due to the higher rotational speed of FSP compare to TSE which lead to higher shear stress on the melt. The higher shear stress leads to higher diffusion of polymer into the clay galleries and better dispersion of nanoclay in the polymer matrix and consequently an improvement in the mechanical properties.

3.5 Crystallization and melting behaviors

The results in section 3.1 to 3.4 shows that theFSP is better method for fabrication of TPO nanocomposites. The TPO nanocomposite with 5 % wt nanoclay fabricated by FSP have better morphology, rheology and thermal propertiescompare to the TPO nanocomposite fabricated by TSE and PP/EPDM blend.The DSC curves of three samplesillustrate in Table 9. It is observed that the meltingpeak temperatures of PP/EPDM and TPO nanocomposite fabricated by TSE and FSP are 165.9, 167.2 and 168.79 °C, respectively.Delta H (ΔH) was defined by dividing the area under peak by the weight of the sample prepared for DSC test.The percent of crystallinity (χ) was calculated as follows (Eq.7):

χ= ∆H/〖∆H〗_m ×100 (7)

In which, ΔH (J/g) is heat of fusion of TPO nanocomposite and PP/EPDM blend, ΔHm is heat of fusion of a pure crystalline PP. Accoding to Table 9, withaddition 5%wt nanoclay to PP/EPDM blend, the crystallization temperature and melting temperature increases whereas ΔH and Crystallinity decreases.The reason for higher crystallization temperature and melting temperature when adding nanoclay to PP/EPDMblend related to the fact that the nanoclay may function like weak nucleating agents being silicate in nature[35, 36].In the FSP sample,the better dispersion of nanoclay resulted inmore interaction between matrix and nanoclay which this reduces the mobility crystallizable chain segmentsand alsodecreased the degree of crystallinity because the silicates acted as nucleating agents.

3.6 Thermogravimetric Analysis

Figure 12 illustratethe TGAscanned results of PP/EPDM blend and TPO nanocomposites fabricated by TSE and FSPin temperature range of 25 to 600 °C. The degradation of PP/EPDM blend started at 240 °C,while in the case of PP/EPDM/clay nanocomposite, the degradation of nanocompsites started at about 265 °C. It is found that the incorporation of nanoclay has improved the thermal stability of the nanocomposites.According to table 10, the temperatures at 10%, 50% and 90 % weight loss increases by almost 30 °C with addition 5%wt nanoclay.The temperature at 10% weight loss for PP/EPDM blend is 290 ℃. However, the addition of 5 wt% nanoclay by TSE has increased this temperature to 308℃ which is 18℃ higher than the base material. This temperature is increased to 320℃for FSP sample which is 30℃ higher than the base material. The reason for higher thermal stability of TPO nanocomposite attributed to the incorporation and dispersion of clay in the matrix which the clay layers act as superior insulation and mass transport barrier to the volatile products generated during decomposition[35, 37].Similar result was also observed by Chiu et al [38].

3.7 Rheology

Figures 13 and 14 shows the complex viscosity and Storage modulusas function of frequencyfor PP, PP/EPDM blend and TPO nanocomposites with 5%wt nanoclay, respectively. It can be clearly seen that the viscosities of PP increase with adding 20%wt of EPDMto PP and also the viscosities of PP/EPDM blend increase with adding 5 %wt of nanoclayin the low frequency region. The reason for higher melt viscosity ofthe TPO nanocomposite fabricated by FSP compared to other samples is attributed to the strong interaction oforganoclay and PP/EPDM blend and also good dispersion of nanoclay in polymer matrix. Increasing the frequency decreased the complex viscosity. This is due to the strong shear thinning behaviour of the TPO nanocomposite and PP/EPDM blend at the melted state. Figure 14 shown a dramatic increase in the storage modulus of TPO nanocompositecompared to PP/EPDM blend at low frequency. The higher storage modulusand smaller slope of the TPO nanocomposite over PP/EPDM blend indicate the formation of three dimensional filler network structure.The storage modulus increase with the increased frequency. This is due to that at low frequency, time islarge enough to unraveling of the entanglements so a largeamount of relaxation occur results in a low value of storagemodulus. However, when a polymer sample isdeformed at large frequency the entanglement chains do nothave time to relax, so modulus goes up[2, 7, 37].

3.8 Dynamic mechanical analysis

Dynamic mechanical analysis was performed to definethe strong modulus (E) andloss factor (tan ) as a function oftemperature.Figure 15 and 16illustrate the storagemodulus and loss factor (tanδ) as a function of temperature of PP/EPDM blend and TPO nanocomposite, in the temperature range -100to 60ºC.It is clear from Fig.15, that the storage modulus of TPO nanocomposite increase with adding 5 %wt of nanoclay to PP/EPDM blend. This is due to the dispersion of clay nanoparticles in polymer matrixproduceda reinforcing effect.From Figure 16, it is observed that two transitions occur in the damping curve, first one occurs at lower temperature of –45ºC considered to be the glass transition of EPDM, another transition occurs at around 7ºC may be taken as the glass transition temperature of polypropylene.Theglass transition temperatureslightly increased in TPO nanocomposites compared to PP/EPDM blend, reflectingthe restriction of the motion of polymer chainsinduced by the mixing of nanoclay. It is clear that the glass transition temperature of a polymer in a polymer nanocompositedepends on the mobility of the chain segment of themacromolecules in the polymer matrix. If the molecularchain is restricted, the motion or relaxation of the chainsegment becomes difficult at the original glass transitiontemperature and a higher temperature is required[39, 40].

3.9 Essential work of fracture

Figure 17illustrates the load–displacement curves of EWF testspecimens cut from PP/EPDM composite and PP/EPDM/Clay nancomposite sheets for different ligament lengths. It is obvious that the same behaviour with similar shapes of load-displacement curves for different ligament lengths is obtained for each sample.From Figure 17 the area under theload-displacement curves increases as the ligament length increases.The self similarity of load-displacement curves is an important prerequisite of the EWF test which shows the fracture mode of the test sample being unchanged with the size of ligament. when comparing different samples, there are quantitative differences during the necking and tearing stages.It isobserved that the maximum forceare increased with addition 5% wt of nanoclay to the PP/EPDM blend by TSE and FSP.According to the Fig. 17, comparing curve of EWF test samples fabricated by TSE and FSP shows that the specimens processed with FSP exhibited superior maximum force and extensioncompared to the TSE.This may be attributed to the size reduction ofthe nanoclayparticle due to the higher shear stress transfer within the bulk and breaking the clay agglomerates.Increased dispersion of nanoclay in matrix of PP/EPDM help achieve good load transfer to the nanofiller network, resulting in higher maximum force and more uniform stress distribution.When adding nanoclay to the PP/EPDM blend, the crack growth rate in the base material decreases due to nanoclay particles could act as crack deviations and therefore could change the direction of crack growth.According to Hill’s plasticity theory, another important validity criteria of the EWF test is the data from DENT samples were obtained under plane stress conditions that for this purpose the σ_max values should be in the range 0.9 – 1.1 σ_m.The maximum of stress (σ_max)that a ligament can endure was calculated by dividing maximum load, as measured in the force – displacement curve,by the initial cross section of the ligament[20, 41]. When the maximum stress (σ_max)was measured then the average of those values (σ_m) for each sample is obtained.Fig. 18illustrates themaximum stress valuesof different ligament lengthsfor three sample,which these values within the stress range of0.9 σ_m<σ_max<1.1 σ_m, indicating the validity of data obtained fromEWF test.

3.9.1 Terms of the essential and non-essential work of fracture

The specific total work of fracture (w_f) defined as the ratio of the area of the load – displacement curves in figure 17to the ligament cross sectional area (lt) for each sample. Then linear regression between the specific total work of fracture and ligament lengths(w_f=w_e+ βw_p×l) is obtained for each samples. The results show that very good linear regression with correlation coefficient (R2) higher than 0.96 between thew_f and l is obtained for these samples. The results of EWF test such as essential (w_e) and non-essential (〖βw〗_p) work parameters for three samples are listed in Table 11. At it is observed from this table, the addition of nanoclay into the PP/EPDM blend results in decreased the w_e whereas increases the 〖βw〗_p.As mentioned in the previous section, the size of EPDM particle decreases with addition nanocaly to PP/EPDM blend by TSE and FSPthat this improve the fracture toughness. Also In the TFE sample, the clay tactoids act as stress concentrators leading to debonding of clay – matrix and/ or breaking the clay tactoids which producing micro and nano cavitation.Similar mechanisms in the fracture toughness of epoxy/ clay nanocompositeshas been reported by Lim et al[42].As a result the combination of these two mechanisms leads to the fracture toughness (w_e) of TSE sample was decreased compared to PP/EPDM blend. This related to the fact that the around nanoclay particles stress concentration point exist so these particles introduce crack initiation point in matrix when the specimen is under external force[43].Panda et al [44]reported that with addition of nanoclay from 0 to 3% wt, fracture toughness increased, but at higher values of nanoclay content, the fracture toughness decreased. They concluded thatdecreases fracture toughness in nanocomposite with 5 % wt nanoclay it due to the nanoclay aggregations.As well as, the nanoclay particles and nanoclay aggregates could inhibit the crack propagation and decreases crack growth rate in the base material because nanoparticles change the direction of crack growth and act as crack deviations.In PP/EPDM blend the large particle size of EPDM are creating large cavitation that these cavities are unstable and increases crack propagation in the surrounding matrix which this decreases 〖βw〗_pparameter.However, similar to results of this research, van der Wal [45] reported that large cavitation have significant effect on fracture of PP/EPR blends so that might change the fracture behaviour these materials. The increase of resistance to crack propagation parameter (〖βw〗_p) of FSP sample compared to THE sample may be due to the more dispersion of nanoparticles in the matrix.Adding of nanoclay to PP/EPDM blend leads to a reduction in the size of spherulites of PP because the nanocaly acting as a nucleating agent. By decreasing the size of spherulites in polymer nanocomposite the ductility of matrix are increased. Therefore, increased ductility leads to higher total fracture energy (w_f).These observations will be further supported by SEM fractography analysis presented in the next sections.

3.9.2 Fracture behaviour and toughening mechanisms

The deformation zones of the crack tip of SENT was studied using a transmission optical microscopy.Fig.19 shows the bright field images of the three samples, wherethe cavitates appear dark in the micrographs. The dark area on the TOM image is due to the stress whiteningphenomenonin the crack tip of SENT.The observed stress whitening zone in the samples is either due to debonding of the rubber particles from the surrounding matrix or to internal rubber cavitation which led to dilatation shear bands.The EPDM particles in the PP matrix act as stress concentrationpoints, resulting in a distribution of strain constraints and improve the fracture toughness[46, 47].According to the Fig. 19, it was observed that the stresswhitening zone of the PP/EPDM nanocomposite fabricated by FSP were significantly larger than the other two samples.It is clear in Fig. 19 (c) that a large number of dilatation shear bands in crack tip of FSP sample have been developed whereas thethickness of these bands isvery smaller than those observed in PP/EPDM blends.The reason for the FSP sample larger stress whitening is attributed to the smaller size of EPDM particles and their uniform distribution in the PP matrix. A better dispersionof EPDM and nanoclay particles brings a moreuniform stressdistributionin the PP matrixwhich in turn causes a larger volume of the material to participate in energy dissipation, shock absorption processand improve facture toughness.A larger deformation zone with smallersize of EPDM particles in the PP matrix has also been reported by Khodabandelou et al [47].

The Fig. 20 shows the SEM micrographs of the EWF test fractured surfaces for PP/EPDM blend and PP/EPDM nanocomposites fabricated by TSE and FSP. From Fig. 20 it is clear that fracture surface is comprised of micro and nano voids and stretched fibrils which reflect a ductile crack growth mechanism. According to Fig. 20 (a) it was observed that in the PP/EPDM sample large voids were formed because of the debonding of EPDM particles from the matrix and also by internal cavitation and tearing of rubber. In this sample the fibrils with larger thickness resulted from thelarger distance betweenrubbers particles. Furthermore, it can be seen from Fig. 20 (b) that the stretched fibrils thickness and size of voids decreases with the addition of nanoclay to PP/EPDM blend.One may hypothesis that the nanoclay acts like a weak nucleating agent owing to itssilicate nature and hence it decreases the size of PP spherulites of PP andthe size of EPDM particles. Similar results were observed by Khosrokhavar et al and Pahlavanpour et al [7, 48].Some voids in fracture surface of TSE sample may be related to debonding of nanoclay aggregations from surrounding matrix because nanoclay particles causes stress concentration in matrix and voids have been formed around of these particle.In this sample it can be seen some stretched fibrils with smaller length that it related to the presence of clay aggregations and larger voids around it in fibrils which leads to earlier tearing this fibrils. Fig. 20 (c) clearly shows that the size of voids and fibrils thickness is much smaller compared to other sample. In this sample length of stretched fibils is higher so that after the fibrils tearing its covers the voids.This is in accordance with the results of EPDM particle size and interparticle distance obtained for three samples.

Fig. 21 illustrates the subsurface SEM micrographs obtained from the coreof ligaments for three samples at two distances from crack plane (marked as FPZ and OPZ) , according to Fig 4 (c).Figs21 (a) and 22(a) illustrate the differences between the FPZ and OPZ for PP/EPDM composite sample. In the EWF test, initial stage is plastic deformation at tip of crack of DENT specimen as demonstrated by stress whitening evidences on the sample. The second step is a full ligament yielding initiated by stress whitening that grows in the plane perpendicular to the loading direction from tip rof crack to middle of ligament, which area formed the OPZ.Also in this step, voids growth in the direction of loading took place and it led to the formation of fibrils.After full ligament yielding, onset of crack propagation is observed, which is the stage where the crack will begin to propagate and continuously grow along the ligament with tearing of fibrils. Finally fracture occures in FPZ with unstable crack propagation in the matrix. Fig 21 (a) – (c) shows deforming mechanism in FPZ which voids growth and fibrils formed. It is clear that with addition of clay to PP/EPDM blend the cavitation, fibrils thickness and voids in the FPZ of the TSE sample decreases and stretched fibrils with longer length can be observed. In PP/EPDM nanocomposite fabricated by FSP the fibrils length is very longer and this thickness is very smaller which this results conform with the larger stretched fibrils in SEM micrographs of fractured surfaces for this sample compared to other samples.Fig 22 (a) – (c) shows the subsurface SEM micrographs from three samples for OPZ which it is clear from the relative comparison of the subsurface zone in PP/EPDM and FSP sample that the small size of rubber particle leads to smaller initial voids in the FSP samples.In PP/EPDM sample it can be observed that the size of voids is larger and number of this voids is low which led to smaller stress whitening and fast crack growth in this sample compared to FSP sample. Similar results werereported by other researchers such as Zebarjad and Khodabandelou et al [47, 49].

Therefore, the failure mechanisms in PP/EPDM blend and PP/EPDM nanocomposite that occurred during EWF test in the followed thesesteps: (1) micro voids formation around of EPDM becauseof the lack of affinity between PP and EPDM droplatesand/or internal void formed in the rubber particle or in the nanocompsoites, as well as micro and nano voids because of debonding of clay particle from surrounding matrix, (2) void growth and formation of fibrils, (3) crazing formation (or craze-like), (4) diffused crazing and crack initiation by tearing of fibrils and (5) unstable crack propagation in the surrounding matrix andfracture.

CONCLUSIONS

In this study, polypropylene (PP) / ethylene-propylene diene monomer (EPDM) nanocomposites with 5%wt nanoclay are fabricated byfriction stir processing (FSP) and compared to those obtained by melt mixing method (extruder).The effect of process parameters on morphology and mechanical properties of TPO nanocomposite is investigated. Based on this investigation, the following conclusions can be derived:

All the process parameters of extruder and FSP are significant factors affecting the mechanical properties.

The mechanical properties and XRD diffraction showed that the FSP is better than extruder for fabrication of polymer nanocomposites.

Under optimal conditions of rotational speed of 800 rpm, traverse speed of 30 mm/min, shoulder temperature of 107 °C and number passes of 1, simultaneous improvement of tensile strength (18.1 %), tensile modulus (13.2 %)are observed.

With addition 5%wt nanoclay to PP/EPDM blend, the crystallization temperature and melting temperature increases whereas crystallinitydecreases.

The temperatures at 10%, 50% and 90 % weight loss increases almost by 40°C with addition nanoclay to PP/EPDM blend by FSP more so than with nanocomposites obtained by TSE.

The TPO nanocomposite with 5%wt nanoclay fabricated by FSP has higher strong modulus, complex viscosity and glass transition temperature compared to PP/EPDM blend.

Source: Essay UK - https://www.essay.uk.com/essays/science/study-of-fracture-toughness-and-mechanical-properties-of-pp-epdm-clay-nanocomposites/


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