- Email: [email protected]
scrap rubber tire (SRT) mixtures using RSM methodology
Polymer Testing 29 (2010) 572–578
Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Analysis and optimization of polypropylene (PP)/ethylene–propylene– diene monomer (EPDM)/scrap rubber tire (SRT) mixtures using RSM methodology H.M. da Costa a, *, V.D. Ramos a, W.S. da Silva a, A.S. Sirqueira b a
Departamento de Materiais (DEMAT), Instituto Politécnico (IPRJ), Universidade do Estado do Rio de Janeiro (UERJ), Rua Alberto Rangel, s/n., Vila Nova, CEP: 28630-050, Nova Friburgo, RJ, Brazil b Centro Universitário Estadual da Zona Oeste (UEZO), Colegiado de Produção Industrial, Avenida Manuel Caldeira de Alvarenga, 1203, Campo Grande, CEP: 23070-200, Rio de Janeiro, RJ, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 February 2010 Accepted 9 April 2010
Response surface methodology (RSM) is a collection of statistical and mathematical techniques useful for developing, improving and optimizing processes. In this work, RSM was applied to the investigation of ternary mixtures of polypropylene (PP), ethylene– propylene–diene monomer (EPDM) and scrap rubber tires (SRT). After appropriate processing in a co-rotating twin extruder and injection moulding, mechanical properties, such as tensile strength and impact strength, were determined and used as response variables. Scanning electron microscopy (SEM) was also used for investigation of the morphology of the different blends and interpretation of results. With specific statistical tools, a minimum number of experiments allowed development of the response surface model and optimization of the concentrations of the components according to mechanical performance. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
Keywords: Ternary mixtures Response surface methodology Mechanical properties Scrap rubber tire
1. Introduction Because two or more polymer properties can be combined, polymer blends and alloys have been studied widely with the aim of improving physical properties. Polymer alloying and blending is an attractive technique for producing new polymeric materials with desirable properties without synthesizing new material and additional polymerization equipment. The purpose of polymer blending is to obtain materials with additional properties together with minimum loss of their original properties [1,2]. Other advantages are versatility, simplicity and relatively low cost. Among the physical properties of these materials, particular importance is attached to mechanical properties. Polypropylene (PP), due to its intrinsic properties such as high melting temperature, low density and high chemical resistance, finds use in a wide range of applications [3]. * Corresponding author. E-mail address: [email protected] (H.M. da Costa).
Easy incorporation of high loadings of fillers and reinforcing agents, and ability to produce blends with other polymers, including rubbers, makes polypropylene (PP) versatile [4,5]. The poor impact strength of PP, however, poses limitations to its use in some applications. Blending PP with an elastomer (EPDM, BR or SBR, for example), that has a much lower Tg, is extensively reported in the literature to improve low temperature and impact properties [6–9]. At the same time, waste rubber is becoming a worldwide waste disposal problem. Addition of scrap rubber in the form of either ground vulcanized waste or reclaim in polymer compounds gives economic as well as processing advantages. Hence, toughening of various plastics by means of the addition of particles of rubber vulcanizates (scrap tires) is also found in the literature [10–12]. However, in the literature, the study of the different blends of PP and rubbers resembles “scattergun” procedures, where a large number of combinations of the components of the mixture are tried. Such scattergun procedures can require large expenditure in terms of time
0142-9418/$ – see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2010.04.003
H.M. da Costa et al. / Polymer Testing 29 (2010) 572–578
and cost of experimentation and, in most cases, better methods can be employed. Hence, in this work, the general mixture problem is again investigated using a ternary mixture of PP/EPDM/SRT but with the implementation and combination of analysis of variance, response surface methodology (RSM) and other statistical techniques. Using the mechanical properties (tensile strength and impact strength) as response variables and scanning electron microscopy (SEM) for study of the morphology, the proportions of components (PP, EPDM and SRT) were optimized with a minimum number of experiments. In addition, mapping of the response surface over a particular region of interest was done and this allowed prediction of changes in the properties for different compositions of the mixture.
PP 1,0
The polymer used in this investigation was a commercial homopolymer polypropylene (TS-6100), generously supplied by Quattor Petroquímica S.A., with melt flow index of 16.0 g/10 min (ASTM D1238, 230 C, 2.16 kg) and specific gravity of 905 kg/m3. Ethylene–propylene–diene monomer (EPDM) was provided by Branco Indústria e Comércio Ltda. Scrap rubber tires (SRT) ground to 40 mesh (y300 mm), designated as AG-40, was supplied by ArtGoma do Brasil LTDA. The SRT was obtained from whole tires, with metal and polyester cord separated. The approximate composition of the SRT was as follows: 40–55% rubber hydrocarbon, 10– 15% acetone extractables, 25–40% carbon black and 3–6% ash. The exact composition depends on the specific type of tire and from where in the tire the particles originated. Antioxidant Irganox B215 was kindly provided by BASF S.A. and was used to avoid degradation during processing.
PP: 1 SRT: 0 EPDM: 0
PP: 0.75 SRT: 0.25 EPDM: 0.00
PP: 0.75 SRT: 0.00 EPDM: 0.25 0,0
0,0
0,5 SRT
2. Experimental 2.1. Materials
573
PP: 0.50 SRT: 0.25 EPDM: 0.25
0,5
0,5 EPDM
Fig. 1. Extreme vertices for three-component design with both upper and lower constraints for PP, EPDM and SRT.
respectively. In addition, the mixtures design was done covering only a subportion or smaller space within that possible. With this aim, proportions of x1, x2, and x3 were 0.0 x2 0.25; and, restricted to: 0.50 x1 1.0; 0.0 x3 0.25. Fig. 1 shows the extreme vertices for three-component design with both upper and lower constraints – the bold dark lines represent the design space for study. The thirteen black circles represent PP/EPDM/SRT mixtures that must be done to obtain an appropriate response surface by means of a mth-degree polynomial equation. The PP/EPDM/SRT mixtures defined by MINITAB 15.0ÔÒ for our investigation are given in Table 1. 2.3. Mixtures preparation and mechanical tests
2.2. Mixtures design The distinguishing feature of the mixture problem is that the independent or controllable factors (PP, EPDM and SRT) represent proportionate amounts of the mixture rather than unrestrained amounts. The proportions are not negative, and, if expressed as fractions of the mixture, they must sum to unity, especially if they are the only ingredients to be studied comprising the mixtures. Clearly, if we let q represent the number of ingredients (or constituents) in the system under study and if we represent the proportion of the ith constituent in the mixture by xi, then
xi 0;
i ¼ 1; 2; .; q
and q X
x i ¼ x1 þ x 2 þ . þ x q ¼ 1
i¼1
With three components (q ¼ 3), the simplex factor space is an equilateral triangle. The MINITAB 15.0ÔÒ package software was used for mixture design, analysis of mechanical properties data and optimization of components. In this work, PP, EPDM and SRT will be denoted by x1, x2, and x3,
For extrusion of the PP/EPDM/SRT mixtures, a co-rotating twin extruder (Extrusão Brasil) was used. This extruder has five temperature control zones and one melt pressure measuring device. Unblended PP and other blends were prepared with a temperature profile of 180/200/210/220/ 230 C. The screw speed was fixed at 100 rpm. The extrudate Table 1 The thirteen PP/EPDM/SRT mixtures designed. Mixture
Components’ proportions PP (x1)
SRT (x2)
EPDM (x3)
1 2 3 4 5 6 7 8 9 10 11 12 13
1.000 0.875 0.875 0.875 0.750 0.750 0.750 0.750 0.750 0.625 0.625 0.625 0.500
0.000 0.125 0.000 0.0625 0.250 0.000 0.1875 0.0625 0.125 0.250 0.125 0.1875 0.250
0.000 0.000 0.125 0.0625 0.000 0.250 0.0625 0.1875 0.125 0.125 0.250 0.1875 0.250
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H.M. da Costa et al. / Polymer Testing 29 (2010) 572–578
was cooled in a water bath, and later granulated into a form ready for injection molding into impact and tensile test specimens. Injection moulding was carried out with a Battenfeld Plus 35/75 machine at 200 C. After conditioning for 24 h, the mechanical properties were evaluated. Stress–strain data were determined on a Universal Testing Machine, Shimadzu AG-I model, on V-type specimens, according to ASTM D-638. Izod impact strength testing of the notched specimens according to ASTM D-256 was conducted using an impact tester with a 2.7 J pendulum. The average value and standard deviation of the impact and tensile properties were calculated using at least ten samples. 2.4. Analysis of the fracture surface Examination of the fracture surface of some mixtures was carried out on a scanning electron microscope (SEM) model JEOL (JSM-6610LV). The objective was to get an insight into the fracture mode in an attempt to characterise the PP matrix blended with EPDM and/or SRT. The fractured ends of the tensile specimens were mounted on aluminum slabs and sputter coated with a thin layer of gold to avoid electrical charging during examination. 3. Results and discussion Mechanical properties of tensile strength and impact strength were evaluated for different ternary mixtures specified in Table 1. Using analysis of variance provided by the MINITAB 15.0ÔÒ package software, some polynomial equations were tested for construction of response surfaces. The special quartic model for three components (Eq. (1)) was chosen because it provided the best regression results for the experimental data.
J ¼ a1 x1 þ a2 x2 þ a3 x3 þ a12 x1 x2 þ a13 x1 x3 þ a23 x2 x3 þ a1123 x21 x2 x3 þ a1223 x1 x22 x3 þ a1233 x1 x2 x23 ð1Þ
Fig. 2. Maximum strength for ternary mixtures of PP/EPDM/SRT.
transfer of stress from PP matrix to the filler, hence ternary blends rich in SRT exhibit a sharp reduction in this property. Even if the addition of EPDM to the PP/EPDM/SRT blend can increase the adhesion between PP matrix and SRT domains by modifying the interface, as suggested in similar investigations of Ishak and Bakar [13] and Plawky et al. [14], this is not enough to maintain high values of strength. At first, critical values, in keeping with the mathematical model proposed, are reached in compositions close to lowest extreme vertex of Fig. 1 – proportions of PP/EPDM/SRT of 50%/25%/25%, respectively. In Fig. 3, elongation at break for ternary mixtures is illustrated. The behavior observed for mixtures is similar to that for maximum strength, in that addition of scrap rubber tire to the polypropylene (PP) matrix did not result in improvement. Low values for elongation at break (<75%) are found in ternary compositions rich in SRT content, which can be seen in the large black region that appears in the triangle of Fig. 3. SRT particles cannot play a role of
where J is the response variable, a1, a2 etc. are polynomial coefficients, and x1, x2 and x3 are PP, EPDM and SRT proportions in the ternary blend, respectively. Eq. (1) is especially useful for detecting curvature of the surface in the interior of the triangle. Response surfaces were generated from Eq. (1) and they are represented in Figs. 2–6. 3.1. Tensile properties In Fig. 2, maximum strength of PP/EPDM/SRT is shown. It can be observed that maximum strength of the ternary mixtures decreases with increasing EPDM or SRT content. Only when PP content is greater than 85% in the mixture does maximum strength approach values greater than 30 MPa – the small black area represented in the triangle of Fig. 2. It has been reported [10–12] that addition of SRT in powder (filler) form reduces the tensile strength and elongation at break as a consequence of the poor adhesion between phases and stress concentration around the SRT particles. SRT presents poor capability to support the
Fig. 3. Elongation at break for ternary mixtures of PP/EPDM/SRT.
H.M. da Costa et al. / Polymer Testing 29 (2010) 572–578
Fig. 4. Toughness from stress–strain curves for ternary mixtures of PP/ EPDM/SRT.
energy transfer in the composite. If no adhesion exists, voiding will occur at the interface with major crack propagation in tensile testing [15–18]. Compositions where EPDM content is similar to or greater than SRT loading show elongation at break within the 75% up to 150% range – narrow white region in the triangle (Fig. 3). Seemingly, EPDM develops some change in the adhesion between the two dissimilar phases (SRT and PP) so that EPDM should stabilize the distribution of SRT powder in the PP matrix, as suggested by Phadke and De [10]. Toughness is defined as the ability of a material to absorb applied energy without failure. In this work, toughness was estimated by integration of the area below of stress–strain curves. Fig. 4 presents the toughness results for different PP/EPDM/SRT mixtures.
575
Fig. 6. Impact strength for ternary mixtures of PP/EPDM/SRT.
Direct blending of plastic with rubber has been widely used to improve toughness, albeit with reduced stiffness [15–20]. At same time, it has been found in most of the works that the important factors in rubber toughening of PP include (i) rubber content, (ii) rubber particle size and particle size distribution, (iii) degree of crosslinking, (iv) degree of interfacial adhesion, and (v) spherulite size and spherulite boundary of PP [19–22]. Toughness of PP/EPDM/ SRT mixtures decreases progressively when SRT loading increases (black area in Fig. 4, where toughness values are below of 1 J). Undoubtedly, SRT particles show poor adhesion in the PP matrix and microvoids at the interface develop, resulting in deterioration of mechanical properties. Generally, addition of a third component or a compatibilizer into a binary blend should improve compatibility such that the resultant blend displays homogeneous and fine morphology of the minor phase in the matrix polymer [18–22]. This observation is true when EPDM is used – white region in the L inverse form inside of the triangle – where toughness values are from 1 J up to 5 J. Elastic modulus (E) or stiffness of ternary mixtures is shown in the Fig. 5. SRT and EPDM are softer materials than PP and, therefore, as expected, their addition results in decrease in elastic modulus values. This can be observed in the dotted region (E values between 450 and 550 MPa) and in the squared region (E values between 350 and 450 MPa) of the triangle. 3.2. Impact strength
Fig. 5. Elastic modulus (E) for ternary mixtures of PP/EPDM/SRT.
The impact strength of thermoplastic is often increased by the addition of a rubber phase, i.e., the rubber phase helps in toughening the matrix polymer. For incorporation of particulate scrap rubber into a polymer matrix, the particle size of the rubber and the adhesion between the polymer matrix and ground rubber are believed to be major factors controlling the mechanical properties of the composites [23–26]. Fig. 6 shows that addition of scrap
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rubber tire to polypropylene (PP) matrix did not result in improving but rather maintained the impact properties of PP (black region in the triangle). This may be due to insufficient adhesion between the two phases as discussed previously. Surprisingly, when EPDM and SRT contents are fixed near to 25%, higher impact strength values are found (dotted and squared regions – 60 J up to 100 J). Stehling et al. [4] studied the state of dispersion of poly(ethylene-copropylene) (PEP) rubber and high-density polyethylene (HDPE) in polypropylene (PP) blends. They observed that when small quantities of PEP and HDPE are mixed with PP, they combine to yield composite particles with rubber tending to form a shell around a HDPE inclusion. Thus, at modest HDPE loadings, the composite rubber-PP particle should have the craze initiation characteristics of a rubber particle with attendant high impact strength. In another investigation, it had been shown by Setz et al. [26] that PP has good compatibility with the ethylene-butylene (EB) block copolymers because of the repulsion effect of ethane
and but-1-ene segments that might contribute to the improvement of the miscibility of PP with scrap rubber tire. Perhaps EPDM shows the same behavior in the PP/EPDM/ SRT mixtures investigated. If EPDM tends to coat the SRT filler surface, then this coating provides a soft interface, which is responsible for the improvement in the impact strength as observed for PP/EPDM/SRT 50%/25%/25% blend. 3.3. Microscopy Fig. 7 shows SEM micrographs of the fracture surface from tensile testing of the compounds. SEM micrographs of the surfaces of PP/EPDM/SRT blends show the typical morphology of an immiscible compound. However, addition of EPDM generates different features, smooth and polished surfaces, and more homogeneous in appearance than the blends riches in SRT content. In Fig. 7(a), it can be observed that, for the PP/EPDM/SRT 75%/0%/25% mixture without EPDM, there are large domains and larger protrusions of the dispersed phase present. In addition, the
Fig. 7. SEM micrographs of the fracture ends of the tensile specimens of PP/EPDM/SRT mixtures. (a) PP/EPDM/SRT 75%/0%/25%. (b) PP/EPDM/SRT 75.00%/6.25%/ 18.75%. (c) PP/EPDM/SRT 62.50%/18.75%/18.75%. (d1) PP/EPDM/SRT 50%/25%/25%. (d2) PP/EPDM/SRT 50%/25%/25%.
H.M. da Costa et al. / Polymer Testing 29 (2010) 572–578
micrograph reveals voids resulting from pulling out of SRT particles from the PP matrix. This morphology, with poor adhesion between phases, explains the pronounced deterioration of mechanical properties of the compounds rich in SRT. Fig. 7(b) and (c) shows the PP matrix of the compounds containing 6.25% and 18.75% of EPDM, respectively. EPDM seems to develop a compatibilizer role between SRT and the PP matrix. The morphology exhibits a reduction of the SRT particle domains, i.e., narrow distribution of the particle size. This fact represents some increment in the mechanical properties, especially for impact strength. Fig. 7(d.1) and (d.2) represents the PP/EPDM/SRT 50%/ 25%/25% mixture. In this ternary blend, there is evidence of elastic deformation occurring at the interface between PP matrix and scrap rubber particles to form fibrils – see arrow in Fig. 7(d.1). The observation of fibril structure can be used to explain the improvement in impact strength of the compound as reported previously. In addition, the occurrence of voiding and filaments at the interface between the PP and dispersed phases could show that void formation may be due to cavitation localized at the rubber particle/ matrix interfaces. Therefore, it can be concluded that increasing impact strength of compound “compatibilized” with EPDM is due to formation of fibril texture between PP and SRT particles. It can be expected that the improved impact values were achieved because of the good compatibility of components at the interface of polypropylene/scrap rubber tire compound. At same time, the average dimensions of the dispersed phase decreases, and interfacial adhesion between the polypropylene and SRT is also improved. The fracture surface of the PP/SRT “compatibilized” by EPDM etched in 1 M hydrochloric acid for 24 h is shown in Fig. 7(d.2). The morphology shows small holes and better dispersion than in other compounds. This explains the existence of compounds with highest impact values (see Fig. 6), above all, when addition of EPDM is proportional to SRT. The morphology suggests that the EPDM spans the interfaces between regions of SRT and PP, thus enhancing adhesion and compatibilization of the compound as proposed by Horák et al. [16].
4. Conclusions Study of ternary blends, particularly PP/EPDM/SRT mixtures, can be developed using response surface methodology (RSM). This technique is useful to optimize components in the mixture and to obtain equations which are employed for mapping the response surface over a particular region of interest. With the experimental conditions and statistical tools used in this investigation, it can be concluded that: (i) A minimum number of experiments allow prediction of the optimal composition of PP, EPDM and SRT present in ternary compounding. (ii) Mechanical properties show sharp deterioration when SRT particles content increases in the ternary mixtures because of the poor adhesion between SRT and PP matrix.
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(iii) EPDM appears to span the interfaces between regions of SRT and PP, thus enhancing adhesion and compatibilization of the compound. Some tensile properties and, particularly, impact strength together with SEM micrographs proved this observation. (iv) Highest impact strength can be reached, for the experimental conditions used, only with a physical mixture of PP/EPDM/SRT when EPDM and SRT contents are maintained around 25%.
Acknowledgements Authors thank CNPq, CAPES and FAPERJ for financial support. References [1] M. Xanthos, Interfacial agents for multiphase polymer systems: recent advances. Polym. Eng. Sci. 28 (21) (1988) 1392–1400. [2] L.A. Utracki, Commercial Polymer Blends. Chapman and Hall, London, 1998. [3] M. Jaziri, N. Mnif, V.M. Nageotte, H.P. Camby, Rheological, thermal, and morphological properties of blends based on poly(propylene), ethylene propylene rubber, and ethylene-1-octene copolymer that could result from end life vehicles: effect of maleic anhydride grafted poly(propylene). Polym. Eng. Sci. 47 (7) (2007) 1009–1015. [4] F.C. Stehling, T. Huff, C.S. Speed, G. Wissler, Structure and properties of rubber-modified polypropylene impact blends. J. Appl. Polym. Sci. 26 (8) (1981) 2693–2711. [5] P. Galli, S. Danesi, T. Simonazzi, Polypropylene based polymer blends: fields of application and new trends. Polym. Eng. Sci. 24 (8) (1984) 544–554. [6] S.C. Tjong, W.D. Li, R.K.Y. Li, Impact toughening behavior of quaternary PP/HDPE/EPDM/EP blends. Eur. Polym. J. 34 (5–6) (1998) 755–760. [7] H. Yang, B. Li, K. Wang, T. en Sun, X. Wang, Q. Zhang, Q. Fu, X. Dong, C.C. Han, Rheology and phase structure of PP/EPDM/SiO2 ternary composites. Eur. Polym. J. 44 (1) (2008) 113–123. [8] X.G. Tang, R.Y. Bao, W. Yang, B. Xie, M. Yiang, M. Hou, Effect of bphase on the fracture behavior of dynamically vulcanized PP/EPDM blends studied by essential work of fracture approach. Eur. Polym. J. 45 (5) (2009) 1448–1453. [9] J.T. Yeh, S.C. Lin, Optimized processing conditions for the preparation of dynamically vulcanized EPDM/PP thermoplastic elastomers containing PP resins of various melts indexes. J. Appl. Polym. Sci. 114 (5) (2009) 2806–2815. [10] A. Phadke, S.K. De, Effect of cryo-ground on melt flow and mechanical properties of polypropylene. Polym. Eng. Sci. 26 (15) (1986) 1079–1087. [11] S. Tantayanon, S. Juikham, Enhanced toughening of poly (propylene) with reclaimed-tire rubber. J. Appl. Polym. Sci. 91 (1) (2004) 510–515. [12] S.H. Lee, M. Belasubramanian, J.K. Kim, Dynamic reaction inside corotating twin extruder. I. Truck tire model material/polypropylene blends. J. Appl. Polym. Sci. 106 (5) (2007) 3193–3208. [13] Z.A.M. Ishak, A.A. Bakar, An investigation on the potential of rice husk ash as fillers for epoxidized natural rubber (ENR). Eur. Polym. J. 31 (3) (1995) 259–269. [14] U. Plawky, M. Schlabs, W. Wening, The role of styrene–ethylene/ butylene–styrene triblock copolymer as impact modifier in polypropylene–polyethylene blends. J. Appl. Polym. Sci. 59 (12) (1996) 1891–1896. [15] R. Fayte, R. Jerome, P.H. Teyssié, Characterization and control of interfaces in emulsified incompatible polymer blends. Polym. Eng. Sci. 27 (5) (1987) 328–334. [16] Z. Horák, F. Fort, D. Hlavatá, F. Lednický, V. Vecerka, Compatibilization of high-impact polystyrene/polypropylene blends. Polymer 37 (1) (1996) 65–73. [17] H. Ismail, M.N. Nasaruddin, H.D. Rozman, The effect of multifunctional additive in white rice husk ash filled natural rubber compounds. Eur. Polym. J. 35 (8) (1999) 1429–1437. [18] S. Siriwardena, H. Ismail, U.S. Ishiaku, M.C.S. Perera, Mechanical and morphological properties of white rice husk ash filled
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Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Analysis and optimization of polypropylene (PP)/ethylene–propylene– diene monomer (EPDM)/scrap rubber tire (SRT) mixtures using RSM methodology H.M. da Costa a, *, V.D. Ramos a, W.S. da Silva a, A.S. Sirqueira b a
Departamento de Materiais (DEMAT), Instituto Politécnico (IPRJ), Universidade do Estado do Rio de Janeiro (UERJ), Rua Alberto Rangel, s/n., Vila Nova, CEP: 28630-050, Nova Friburgo, RJ, Brazil b Centro Universitário Estadual da Zona Oeste (UEZO), Colegiado de Produção Industrial, Avenida Manuel Caldeira de Alvarenga, 1203, Campo Grande, CEP: 23070-200, Rio de Janeiro, RJ, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 February 2010 Accepted 9 April 2010
Response surface methodology (RSM) is a collection of statistical and mathematical techniques useful for developing, improving and optimizing processes. In this work, RSM was applied to the investigation of ternary mixtures of polypropylene (PP), ethylene– propylene–diene monomer (EPDM) and scrap rubber tires (SRT). After appropriate processing in a co-rotating twin extruder and injection moulding, mechanical properties, such as tensile strength and impact strength, were determined and used as response variables. Scanning electron microscopy (SEM) was also used for investigation of the morphology of the different blends and interpretation of results. With specific statistical tools, a minimum number of experiments allowed development of the response surface model and optimization of the concentrations of the components according to mechanical performance. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
Keywords: Ternary mixtures Response surface methodology Mechanical properties Scrap rubber tire
1. Introduction Because two or more polymer properties can be combined, polymer blends and alloys have been studied widely with the aim of improving physical properties. Polymer alloying and blending is an attractive technique for producing new polymeric materials with desirable properties without synthesizing new material and additional polymerization equipment. The purpose of polymer blending is to obtain materials with additional properties together with minimum loss of their original properties [1,2]. Other advantages are versatility, simplicity and relatively low cost. Among the physical properties of these materials, particular importance is attached to mechanical properties. Polypropylene (PP), due to its intrinsic properties such as high melting temperature, low density and high chemical resistance, finds use in a wide range of applications [3]. * Corresponding author. E-mail address: [email protected] (H.M. da Costa).
Easy incorporation of high loadings of fillers and reinforcing agents, and ability to produce blends with other polymers, including rubbers, makes polypropylene (PP) versatile [4,5]. The poor impact strength of PP, however, poses limitations to its use in some applications. Blending PP with an elastomer (EPDM, BR or SBR, for example), that has a much lower Tg, is extensively reported in the literature to improve low temperature and impact properties [6–9]. At the same time, waste rubber is becoming a worldwide waste disposal problem. Addition of scrap rubber in the form of either ground vulcanized waste or reclaim in polymer compounds gives economic as well as processing advantages. Hence, toughening of various plastics by means of the addition of particles of rubber vulcanizates (scrap tires) is also found in the literature [10–12]. However, in the literature, the study of the different blends of PP and rubbers resembles “scattergun” procedures, where a large number of combinations of the components of the mixture are tried. Such scattergun procedures can require large expenditure in terms of time
0142-9418/$ – see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.polymertesting.2010.04.003
H.M. da Costa et al. / Polymer Testing 29 (2010) 572–578
and cost of experimentation and, in most cases, better methods can be employed. Hence, in this work, the general mixture problem is again investigated using a ternary mixture of PP/EPDM/SRT but with the implementation and combination of analysis of variance, response surface methodology (RSM) and other statistical techniques. Using the mechanical properties (tensile strength and impact strength) as response variables and scanning electron microscopy (SEM) for study of the morphology, the proportions of components (PP, EPDM and SRT) were optimized with a minimum number of experiments. In addition, mapping of the response surface over a particular region of interest was done and this allowed prediction of changes in the properties for different compositions of the mixture.
PP 1,0
The polymer used in this investigation was a commercial homopolymer polypropylene (TS-6100), generously supplied by Quattor Petroquímica S.A., with melt flow index of 16.0 g/10 min (ASTM D1238, 230 C, 2.16 kg) and specific gravity of 905 kg/m3. Ethylene–propylene–diene monomer (EPDM) was provided by Branco Indústria e Comércio Ltda. Scrap rubber tires (SRT) ground to 40 mesh (y300 mm), designated as AG-40, was supplied by ArtGoma do Brasil LTDA. The SRT was obtained from whole tires, with metal and polyester cord separated. The approximate composition of the SRT was as follows: 40–55% rubber hydrocarbon, 10– 15% acetone extractables, 25–40% carbon black and 3–6% ash. The exact composition depends on the specific type of tire and from where in the tire the particles originated. Antioxidant Irganox B215 was kindly provided by BASF S.A. and was used to avoid degradation during processing.
PP: 1 SRT: 0 EPDM: 0
PP: 0.75 SRT: 0.25 EPDM: 0.00
PP: 0.75 SRT: 0.00 EPDM: 0.25 0,0
0,0
0,5 SRT
2. Experimental 2.1. Materials
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PP: 0.50 SRT: 0.25 EPDM: 0.25
0,5
0,5 EPDM
Fig. 1. Extreme vertices for three-component design with both upper and lower constraints for PP, EPDM and SRT.
respectively. In addition, the mixtures design was done covering only a subportion or smaller space within that possible. With this aim, proportions of x1, x2, and x3 were 0.0 x2 0.25; and, restricted to: 0.50 x1 1.0; 0.0 x3 0.25. Fig. 1 shows the extreme vertices for three-component design with both upper and lower constraints – the bold dark lines represent the design space for study. The thirteen black circles represent PP/EPDM/SRT mixtures that must be done to obtain an appropriate response surface by means of a mth-degree polynomial equation. The PP/EPDM/SRT mixtures defined by MINITAB 15.0ÔÒ for our investigation are given in Table 1. 2.3. Mixtures preparation and mechanical tests
2.2. Mixtures design The distinguishing feature of the mixture problem is that the independent or controllable factors (PP, EPDM and SRT) represent proportionate amounts of the mixture rather than unrestrained amounts. The proportions are not negative, and, if expressed as fractions of the mixture, they must sum to unity, especially if they are the only ingredients to be studied comprising the mixtures. Clearly, if we let q represent the number of ingredients (or constituents) in the system under study and if we represent the proportion of the ith constituent in the mixture by xi, then
xi 0;
i ¼ 1; 2; .; q
and q X
x i ¼ x1 þ x 2 þ . þ x q ¼ 1
i¼1
With three components (q ¼ 3), the simplex factor space is an equilateral triangle. The MINITAB 15.0ÔÒ package software was used for mixture design, analysis of mechanical properties data and optimization of components. In this work, PP, EPDM and SRT will be denoted by x1, x2, and x3,
For extrusion of the PP/EPDM/SRT mixtures, a co-rotating twin extruder (Extrusão Brasil) was used. This extruder has five temperature control zones and one melt pressure measuring device. Unblended PP and other blends were prepared with a temperature profile of 180/200/210/220/ 230 C. The screw speed was fixed at 100 rpm. The extrudate Table 1 The thirteen PP/EPDM/SRT mixtures designed. Mixture
Components’ proportions PP (x1)
SRT (x2)
EPDM (x3)
1 2 3 4 5 6 7 8 9 10 11 12 13
1.000 0.875 0.875 0.875 0.750 0.750 0.750 0.750 0.750 0.625 0.625 0.625 0.500
0.000 0.125 0.000 0.0625 0.250 0.000 0.1875 0.0625 0.125 0.250 0.125 0.1875 0.250
0.000 0.000 0.125 0.0625 0.000 0.250 0.0625 0.1875 0.125 0.125 0.250 0.1875 0.250
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was cooled in a water bath, and later granulated into a form ready for injection molding into impact and tensile test specimens. Injection moulding was carried out with a Battenfeld Plus 35/75 machine at 200 C. After conditioning for 24 h, the mechanical properties were evaluated. Stress–strain data were determined on a Universal Testing Machine, Shimadzu AG-I model, on V-type specimens, according to ASTM D-638. Izod impact strength testing of the notched specimens according to ASTM D-256 was conducted using an impact tester with a 2.7 J pendulum. The average value and standard deviation of the impact and tensile properties were calculated using at least ten samples. 2.4. Analysis of the fracture surface Examination of the fracture surface of some mixtures was carried out on a scanning electron microscope (SEM) model JEOL (JSM-6610LV). The objective was to get an insight into the fracture mode in an attempt to characterise the PP matrix blended with EPDM and/or SRT. The fractured ends of the tensile specimens were mounted on aluminum slabs and sputter coated with a thin layer of gold to avoid electrical charging during examination. 3. Results and discussion Mechanical properties of tensile strength and impact strength were evaluated for different ternary mixtures specified in Table 1. Using analysis of variance provided by the MINITAB 15.0ÔÒ package software, some polynomial equations were tested for construction of response surfaces. The special quartic model for three components (Eq. (1)) was chosen because it provided the best regression results for the experimental data.
J ¼ a1 x1 þ a2 x2 þ a3 x3 þ a12 x1 x2 þ a13 x1 x3 þ a23 x2 x3 þ a1123 x21 x2 x3 þ a1223 x1 x22 x3 þ a1233 x1 x2 x23 ð1Þ
Fig. 2. Maximum strength for ternary mixtures of PP/EPDM/SRT.
transfer of stress from PP matrix to the filler, hence ternary blends rich in SRT exhibit a sharp reduction in this property. Even if the addition of EPDM to the PP/EPDM/SRT blend can increase the adhesion between PP matrix and SRT domains by modifying the interface, as suggested in similar investigations of Ishak and Bakar [13] and Plawky et al. [14], this is not enough to maintain high values of strength. At first, critical values, in keeping with the mathematical model proposed, are reached in compositions close to lowest extreme vertex of Fig. 1 – proportions of PP/EPDM/SRT of 50%/25%/25%, respectively. In Fig. 3, elongation at break for ternary mixtures is illustrated. The behavior observed for mixtures is similar to that for maximum strength, in that addition of scrap rubber tire to the polypropylene (PP) matrix did not result in improvement. Low values for elongation at break (<75%) are found in ternary compositions rich in SRT content, which can be seen in the large black region that appears in the triangle of Fig. 3. SRT particles cannot play a role of
where J is the response variable, a1, a2 etc. are polynomial coefficients, and x1, x2 and x3 are PP, EPDM and SRT proportions in the ternary blend, respectively. Eq. (1) is especially useful for detecting curvature of the surface in the interior of the triangle. Response surfaces were generated from Eq. (1) and they are represented in Figs. 2–6. 3.1. Tensile properties In Fig. 2, maximum strength of PP/EPDM/SRT is shown. It can be observed that maximum strength of the ternary mixtures decreases with increasing EPDM or SRT content. Only when PP content is greater than 85% in the mixture does maximum strength approach values greater than 30 MPa – the small black area represented in the triangle of Fig. 2. It has been reported [10–12] that addition of SRT in powder (filler) form reduces the tensile strength and elongation at break as a consequence of the poor adhesion between phases and stress concentration around the SRT particles. SRT presents poor capability to support the
Fig. 3. Elongation at break for ternary mixtures of PP/EPDM/SRT.
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Fig. 4. Toughness from stress–strain curves for ternary mixtures of PP/ EPDM/SRT.
energy transfer in the composite. If no adhesion exists, voiding will occur at the interface with major crack propagation in tensile testing [15–18]. Compositions where EPDM content is similar to or greater than SRT loading show elongation at break within the 75% up to 150% range – narrow white region in the triangle (Fig. 3). Seemingly, EPDM develops some change in the adhesion between the two dissimilar phases (SRT and PP) so that EPDM should stabilize the distribution of SRT powder in the PP matrix, as suggested by Phadke and De [10]. Toughness is defined as the ability of a material to absorb applied energy without failure. In this work, toughness was estimated by integration of the area below of stress–strain curves. Fig. 4 presents the toughness results for different PP/EPDM/SRT mixtures.
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Fig. 6. Impact strength for ternary mixtures of PP/EPDM/SRT.
Direct blending of plastic with rubber has been widely used to improve toughness, albeit with reduced stiffness [15–20]. At same time, it has been found in most of the works that the important factors in rubber toughening of PP include (i) rubber content, (ii) rubber particle size and particle size distribution, (iii) degree of crosslinking, (iv) degree of interfacial adhesion, and (v) spherulite size and spherulite boundary of PP [19–22]. Toughness of PP/EPDM/ SRT mixtures decreases progressively when SRT loading increases (black area in Fig. 4, where toughness values are below of 1 J). Undoubtedly, SRT particles show poor adhesion in the PP matrix and microvoids at the interface develop, resulting in deterioration of mechanical properties. Generally, addition of a third component or a compatibilizer into a binary blend should improve compatibility such that the resultant blend displays homogeneous and fine morphology of the minor phase in the matrix polymer [18–22]. This observation is true when EPDM is used – white region in the L inverse form inside of the triangle – where toughness values are from 1 J up to 5 J. Elastic modulus (E) or stiffness of ternary mixtures is shown in the Fig. 5. SRT and EPDM are softer materials than PP and, therefore, as expected, their addition results in decrease in elastic modulus values. This can be observed in the dotted region (E values between 450 and 550 MPa) and in the squared region (E values between 350 and 450 MPa) of the triangle. 3.2. Impact strength
Fig. 5. Elastic modulus (E) for ternary mixtures of PP/EPDM/SRT.
The impact strength of thermoplastic is often increased by the addition of a rubber phase, i.e., the rubber phase helps in toughening the matrix polymer. For incorporation of particulate scrap rubber into a polymer matrix, the particle size of the rubber and the adhesion between the polymer matrix and ground rubber are believed to be major factors controlling the mechanical properties of the composites [23–26]. Fig. 6 shows that addition of scrap
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rubber tire to polypropylene (PP) matrix did not result in improving but rather maintained the impact properties of PP (black region in the triangle). This may be due to insufficient adhesion between the two phases as discussed previously. Surprisingly, when EPDM and SRT contents are fixed near to 25%, higher impact strength values are found (dotted and squared regions – 60 J up to 100 J). Stehling et al. [4] studied the state of dispersion of poly(ethylene-copropylene) (PEP) rubber and high-density polyethylene (HDPE) in polypropylene (PP) blends. They observed that when small quantities of PEP and HDPE are mixed with PP, they combine to yield composite particles with rubber tending to form a shell around a HDPE inclusion. Thus, at modest HDPE loadings, the composite rubber-PP particle should have the craze initiation characteristics of a rubber particle with attendant high impact strength. In another investigation, it had been shown by Setz et al. [26] that PP has good compatibility with the ethylene-butylene (EB) block copolymers because of the repulsion effect of ethane
and but-1-ene segments that might contribute to the improvement of the miscibility of PP with scrap rubber tire. Perhaps EPDM shows the same behavior in the PP/EPDM/ SRT mixtures investigated. If EPDM tends to coat the SRT filler surface, then this coating provides a soft interface, which is responsible for the improvement in the impact strength as observed for PP/EPDM/SRT 50%/25%/25% blend. 3.3. Microscopy Fig. 7 shows SEM micrographs of the fracture surface from tensile testing of the compounds. SEM micrographs of the surfaces of PP/EPDM/SRT blends show the typical morphology of an immiscible compound. However, addition of EPDM generates different features, smooth and polished surfaces, and more homogeneous in appearance than the blends riches in SRT content. In Fig. 7(a), it can be observed that, for the PP/EPDM/SRT 75%/0%/25% mixture without EPDM, there are large domains and larger protrusions of the dispersed phase present. In addition, the
Fig. 7. SEM micrographs of the fracture ends of the tensile specimens of PP/EPDM/SRT mixtures. (a) PP/EPDM/SRT 75%/0%/25%. (b) PP/EPDM/SRT 75.00%/6.25%/ 18.75%. (c) PP/EPDM/SRT 62.50%/18.75%/18.75%. (d1) PP/EPDM/SRT 50%/25%/25%. (d2) PP/EPDM/SRT 50%/25%/25%.
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micrograph reveals voids resulting from pulling out of SRT particles from the PP matrix. This morphology, with poor adhesion between phases, explains the pronounced deterioration of mechanical properties of the compounds rich in SRT. Fig. 7(b) and (c) shows the PP matrix of the compounds containing 6.25% and 18.75% of EPDM, respectively. EPDM seems to develop a compatibilizer role between SRT and the PP matrix. The morphology exhibits a reduction of the SRT particle domains, i.e., narrow distribution of the particle size. This fact represents some increment in the mechanical properties, especially for impact strength. Fig. 7(d.1) and (d.2) represents the PP/EPDM/SRT 50%/ 25%/25% mixture. In this ternary blend, there is evidence of elastic deformation occurring at the interface between PP matrix and scrap rubber particles to form fibrils – see arrow in Fig. 7(d.1). The observation of fibril structure can be used to explain the improvement in impact strength of the compound as reported previously. In addition, the occurrence of voiding and filaments at the interface between the PP and dispersed phases could show that void formation may be due to cavitation localized at the rubber particle/ matrix interfaces. Therefore, it can be concluded that increasing impact strength of compound “compatibilized” with EPDM is due to formation of fibril texture between PP and SRT particles. It can be expected that the improved impact values were achieved because of the good compatibility of components at the interface of polypropylene/scrap rubber tire compound. At same time, the average dimensions of the dispersed phase decreases, and interfacial adhesion between the polypropylene and SRT is also improved. The fracture surface of the PP/SRT “compatibilized” by EPDM etched in 1 M hydrochloric acid for 24 h is shown in Fig. 7(d.2). The morphology shows small holes and better dispersion than in other compounds. This explains the existence of compounds with highest impact values (see Fig. 6), above all, when addition of EPDM is proportional to SRT. The morphology suggests that the EPDM spans the interfaces between regions of SRT and PP, thus enhancing adhesion and compatibilization of the compound as proposed by Horák et al. [16].
4. Conclusions Study of ternary blends, particularly PP/EPDM/SRT mixtures, can be developed using response surface methodology (RSM). This technique is useful to optimize components in the mixture and to obtain equations which are employed for mapping the response surface over a particular region of interest. With the experimental conditions and statistical tools used in this investigation, it can be concluded that: (i) A minimum number of experiments allow prediction of the optimal composition of PP, EPDM and SRT present in ternary compounding. (ii) Mechanical properties show sharp deterioration when SRT particles content increases in the ternary mixtures because of the poor adhesion between SRT and PP matrix.
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(iii) EPDM appears to span the interfaces between regions of SRT and PP, thus enhancing adhesion and compatibilization of the compound. Some tensile properties and, particularly, impact strength together with SEM micrographs proved this observation. (iv) Highest impact strength can be reached, for the experimental conditions used, only with a physical mixture of PP/EPDM/SRT when EPDM and SRT contents are maintained around 25%.
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