MECHANICAL PROPERTIES OF HYBRID COMPOSITES OF PET/POLY(ETHYLENE-CO-METHYL ACRYLATE-CO- GLYCIDYL METHACRYLATE) ELASTOMER / GLASS FIBER

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1 MECHANICAL PROPERTIES OF HYBRID COMPOSITES OF PET/POLY(ETHYLENE-CO-METHYL ACRYLATE-CO- GLYCIDYL METHACRYLATE) ELASTOMER / GLASS FIBER A.L.F. de M. Giraldi 1, L.H. I. Mei 1*, J. A. Sousa 2* 1 Polymer Technology Department, College of Chemical Engineering, Universidade Estadual de Campinas P.O. Box 6066, Campinas SP Brazil- lumei@feq.unicamp.br 2* Materials Engineering Department, Universidade Federal de São Carlos, P.O. BOX 676, São Carlos SP Brazil- jasousa@power.ufscar.br; Abstract - A series of glass fiber-reinforced and rubber-toughened poly(ethylene terephthalate) hybrid composites were twin-screw extrusion compounded with varying total and relative concentrations of a reactive terpolymer of poly(ethylene-co-methyl acrylate-co-glycidyl methacrylate) (E-MA-GMA) as the elastomeric impact modifier ( wt.%) and short glass fiber SGF ( wt.%). Haake torque rheometry mixing data of these composites indicate preferential interactions between the terminal carboxyl groups of the matrix PET with epoxy functionalities of E-MA-GMA. Tensile tests on these hybrid PET composites, at a given SGF content, indicated that the elastic modulus and tensile strength reduce monotonically and Izod impact strength increases substantially, as higher volume fractions of E-MA-GMA substitute PET matrix in the composite. Using the mechanical tests data (tensile and Izod impact) and microscopy analysis (MEV), a discussion is presented on the contributions of the fiber-reinforcement and rubbertoughening mechanisms to the balance in modulus/toughness behaviour of this model hybrid ternary composite of PET, as the total volume concentration of the hybrid reinforcement of SGF/E-MA-GMA increases in the polymer matrix. Introduction Waste bottle-grade PET as a hybrid thermoplastic composite system for up-graded applications such as engineering grade PET can be an attractive industrial proposition, if sufficient elastomeric impact modifier and short glass fiber (SGF) can be incorporated as distinct dispersed phases into the matrix thermoplastic polyester during its recycling process. In order to assure a good balance between engineering properties such as high impact resistance and toughness, high strength and stiffness in this hybrid PET composite, it is important to avoid SGF encapsulation by the elastomer, so that both efficient fiber-reinforcement and rubber-toughening mechanisms can simultaneously prevail in the matrix polymer and, thereby, counter-balancing the drastic deterioration of PET properties, leading to problems such as low melt viscosity, reduced impact strength and tensile ductility [1, 2]. Much of the recent growth in glass fiber reinforced PET (thermoplastic polyester for injection molding) has been found in various industrial applications such as automotive, appliances, furniture, and so on, where PET made parts and structures are gradually replacing steels, light alloys, and in some cases expensive, thermoplastics and thermosets [3]. Many studies dealing with short-fiber reinforced engineering plastics, such as poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) have been reported [4-9], where different aspects related to the correct choice of the silane coupling agent of glass fibers, use of nucleating agents and optimized processing conditions contribute towards improved mechanical performance of these glass-reinforced thermoplastic composites. Recently, there has been active research interest in the study of rubber toughened polymer blends based on poly(ethylene terephthalate) (PET) as the matrix material [3,10-11,12-15]. By blending of PET with rubber in a suitable manner, materials with high toughness can be produced. The effects of compatibility between rubber and PET matrix have been observed to play an important role in the resulting blends behaviour [11, 14-15]. With different rubber types, it has been observed that maleic anhydride grafted styrene-ethylene-butadiene-styrene (SEBS-g-MA) rubber [3], ethylene-propylene-diene elastomer (EPDM) [16] and random terpolymer of ethylene-methyl acrylate glycidyl methacrylate (E-MA-GMA) [17-18] are effective in the toughening of PET, where higher quantities of impact modifier (20%) and smaller particle size of less than 200 nanometers (at an interparticle distance of 50 nanometers) are required to induce supertoughness in polymers such PET [17]. However, the addition of rubber particles decreases the strength and stiffness of PET. For this purpose, recycled PET was extrusion compounded with varying concentrations of SGF pre-treated with epoxisilane supposedly and also with a reactive terpolymer of poly(ethylene-co-methyl acrylate-co-glycidyl methacrylate) (E-MA-GMA), used as the elastomeric impact modifier. During the melt compounding process of PET composite, it is expected that the epoxi functional groups of E-MA-GMA should preferentially react with the terminal carboxyl and hydroxyl groups present in the matrix PET, rather than with the epoxide organic functionalities of the siloxane deposited on SGF surface. Under such conditions, it is expected that no SGF encapsulation by the elastomer should occur and, thereby, leading to a mechanically efficient hybrid PET composite system. The main goal of this investigation was to promote a better understanding on the structure-mechanical properties relationships existing in a model hybrid ternary composite of PET, where the contributions of the fiber-reinforcement and rubber-toughening

2 mechanisms to the balance in modulus and toughness behaviour of this composite shall be analysed, as the total volume concentration of the SGF and E-MA-GMA increases in the polymer matrix. Experimental The poly(ethylene terephthalate) (PET) used in this study was a bottle-grade from A80W resin supplied by Grupo Mossi & Ghisolfi - M&G ( Poços de Caldas Brazil) with density of 1.39 g/cc and intrinsic viscosity ASTM D 4603 of 0.80 dl/g. The toughening rubber (E-MA-GMA) employed is a random terpolymer of ethylene-methyl acrylate glycidyl methacrylate (Lotader AX8900, Arquema Group) with 68 % (wt.) of elastomeric polyethylene (E), 25 % (wt.) of methyl acrylate (MA) and 8 % (wt.) of GMA, MFI (190 C/2,16kg) of 6 g/10min, density of 0.94 g/cc and Melting Point of 60 C. The short glass fiber was supplied by Saint-Gobain Vetrotex Vidros S.A. as a commercial grade EC 952 of 4,5 mm chopped filaments, 10 µm nominal fiber diameter, density of 2.55 g/cc and with an alleged sizing containing an epoxisilane coupling agent. An antioxidant (Irganox B561) produced by Ciba was used in all composites in order to avoid as much as possible the hydrolytic degradation effects caused by moisture during PET processing. Preliminary torque rheometry investigation on the potential reactivity between PET end-funtional groups with the reactive elastomer (E-MA-GMA) was carried out in a Haake torque rheometer (Rheocord 9000 batch mixer), using a mixing chamber of 69 cm 3 at 250 C and rotors speed of 50 rpm for a total mixing time of 10 minutes. The compositions of the PET/ E-MA-GMA elastomer blends were based on several polymer/elastomer ratios. Two types of PET formulations were prepared in this investigation: (1) binary blends of PET and the basic elastomer, and (2) hybrid ternary composites of PET/SGF /E-MA-GMA. A summary of the blends and composites prepared is given in Table 1. Both the binary PET/ E-MA-GMA blends and the ternary PET/SGF /E-MA-GMA hybrid composites were compounded in a twin screw extruder (ZSK-30 - Werner & Pfleiderer, working at 200 rpm and at temperatures in the range o C. Before melt processing all components were carefully dried at temperatures in the range o C for at least 6 h in order to limit the processing-induced hydrolysis. These extruded PET compounds pellets were thoroughly vacuum-oven dried at 120 o C for 4 h. and used for injection molding of mechanical test specimens in an Arburg Allrounder 270V/ machine. The temperature profile in the injection cylinder was o C, and the mold temperature was fixed at 55 o C. The hydraulic injection pressure was 650 bar, whilst the holding pressure was maintained at 350 bar during 12 s. Table 1 - Volume concentrations of the components of PET/SGF/E-MA-GMA ternary hybrid composites. Composition SGF E-MA-GMA PET Testing and characterization PET SGF SGF SGF R R R R10SGF R10SGF R10SGF R15SGF R15SGF R15SGF R20SGF R20SGF R20SGF Mechanical properties characterization of all PET blends and composites was determined by tensile tests, performed on the injection-molded (ASTM D-638) dumbbell test specimens, using a universal testing Instron machine, model 5569 provided with crosshead speed of 10 mm/min. Izod impact tests were carried out according to ASTM D specifications using a Ceast 25 pendulum impact testing machine.

3 Data on the average fiber length and overall fiber length distribution in the composites was determined from ovenincineration residues taken from tensile specimen, using a Leica transmission optical microscope adapted with an image analysis software program Image Pro-Plus 3.0. To investigate the morphological aspects associated with the fracture process of the PET composites, nitrogen freeze-fracture surfaces of tensile specimen were analyzed in a LEO 1530 scanning electron microscope (SEM): Results and Discussion The functional GMA groups of E-MA-GMA elastomer will react with the PET functional end groups, resulting in the formation of graft copolymers across the interface of PET/ E-MA-GMA blends. The overall reaction schemes generally expected are outlined in Figure 1. Figure 1 Chemical reaction schemes between epoxide and PET functional end groups The main reaction taking place between the PET carboxyl end groups and the epoxide functionalities (Figure 1) is believed to be a ring opening reaction according to a nucleophilic substitution mechanism. Due to the higher degree of acidity of the carboxyl group and based on experimental literature data [19,20], it is believed that the preferred compatibilization reaction is the one between the epoxide and the carboxyl functionality of PET end groups. The morphology development during melt-mixing of rubber-toughened polymer blends is highly controlled by the rheological characteristics of the constituting components. Rheological measurements are often used to demonstrate the occurrence of compatibilization reactions, where the mixing device s torque values are known to be related to the viscosity of the blend system within the mixing chamber [21]. Accordingly, simultaneous Haake torque measurements during melt mixing were performed in order to obtain an idea about the occurring reactions and the viscosity changes of PET/E-MA-GMA. A chemical reaction taking place between the reactive blend components will lead to an increase of the blend viscosity (torque) compared to a mixture without any reaction. As shown in Figure 2, PET displays a much lower torque value than de E-MA-GMA rubber, as expected from its thermoplastic nature and consequent lower viscosity as compared to that of the elastomer. A slight but continuous decrease in the torque response of PET during the observed mixing time can be attributed to some thermal and/or oxidative chain degradation of PET at the applied mixing temperature of 280 C [19,20] For the PET blends with increasing E MA GMA content (2,5 to 20 wt. %), all torque curves are positioned outside the torque range between the blend components and above the torque curve of E-MA-GMA elastomer. The observed equilibrium torque of the blends increases with increasing elastomer content. These observations clearly indicate that the previously mentioned compatibilization reactions between the epoxide functionalities of E-MA-GMA and the carboxylic end-groups are effectively taking place. Also a constant torque value at long mixing times in all blends upto 20 wt. % of rubber is a clear indication that no competitive cross-linking reactions are actually taking place, as reported by. P. Martin et al during compatibilization of blends of poly(butylene terephthalate) with epoxide-containing rubber [21]. The lengths of over 500 fibers obtained from the ash contents of all PET composites were counted and the average fiber length was calculated for each composition. It was found that the average fiber length of all composites were in the range of 420 to 600 µm, with the average fiber length decreasing with SGF content, as expected due to viscosity increases associated to fiber content in the composite during the compounding and posterior injection molding operations. However, at a given constant SGF content, the average fiber length increased with the E-MA-GMA rubber content, probably due to reduced fiber attrition during the composite compounding operation, derived from increased melt elasticity of the PET matrix with increasing rubber content. Similar observations were also reported in literature for other hybrid reinforced thermoplastic composites [25].

4 Figure 2. Torque response as a function of the mixing time for the pure blend components and PET/rubber blends. In Figure 3.a are shown the tensile elastic modulus data of PET. /SGF/E-MA-GMA hybrid composites, as a function of SGF and E-MA-GMA elastomer volume concentrations. These results indicate that the elastic modulus of the composites reduces monotonically as higher volume fractions of E-MA-GMA rubber substitute PET matrix in the composite. Similar results were also obtained on the tensile strength (TS) data of these hybrids PET composites (Figure 3.b).In the case of the binary PET/E-MA-GMA blends, the data presented in Figure 3 indicates that both the tensile modulus and strength values decrease with rubber volume content, as would be expected for rubber-toughened thermoplastics [14-18]. Elastic Tensile Modulus (GPa) SGF = 0 wt% SGF = 20 wt% SGF = 25 wt% SGF = 30 wt% E-MA-GMA Volume Fraction in Polymer Matrix ( a )

5 SGF= 0 wt% SGF= 20 wt% SGF= 25 wt% SGF= 30 wt% Tensile Strength (MPa) ,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 E-MA-GMA Volume Fraction in Polymer Matrix ( b ) Figure 3. Tensile modulus (a) and tensile strength (b) data of PET/SGF/E-MA-GMA hybrid composites, as a function of SGF and elastomer concentrations. Now, in order to verify if the observed decrease in tensile modulus and strength values of the ternary hybrid composites of PET. /SGF/E-MA-GMA can be attributed solely to the observed decrease in both the tensile modulus and strength values of the matrix polymer consisting of PET with increasing volume fraction of E-MA-GMA rubber particles, the relative modulus and tensile strength data of all the ternary composites were plotted with respect to the same property values of their corresponding PET + E-MA-GMA matrixes. For this purpose, as the real volume fraction of E-MA- GMA elastomer, at a given constant weight content of elastomer in the ternary composite, actually increases with glass fiber content (vide data in Table 1), it was necessary to estimate the real modulus and strength values of the matrix polymer (PET+E-MA-GMA) for each composition of PET ternary composites, using the estimated properties values obtained from the extrapolated linear fit curve of tensile modulus and strength of the binary PET/E-MA-GMA blends, as shown in Figure 3.a. Thus the relative tensile modulus and strength curves of PET ternary composites, with respect to their corresponding matrix (PET with E-MA-GMA elastomer) modulus and strength values respectively, were plotted as a function of the elastomer content, as shown in the following Figures 4 and 5. Now, analysing the relative modulus data for both binary and ternary composites, with respect to the real matrix (PET with E-MA-GMA elastomer) modulus, as a function of the elastomer content, as shown in Figure 4, it can be seen that these relative modulus values, at a given constant glass fiber content, actually increase with increasing E-MA-GMA volume concentration. Similar results were also obtained for the relative tensile strength data of the PET ternary hybrid composites, as a function of the elastomer content (Figure 5). Both these observations clearly indicate that no GFencapsulation by the elastomer can actually occur, even at the highest level of E-MA-GMA elastomer concentration (20%) used in these hybrid ternary PET composites. Should glass fiber encapsulation by the elastomer actually occur, then it is to be expected that the relative modulus and strength values of these ternary PET composites, at constant glass fiber content, should actually decrease with increasing elastomer content. The fact that these relative tensile modulus actually do increase with increasing elastomer content can be attributed to the expected increase in fibre orientation with the observed substancial increase in melt viscosity of the matrix polymer (PET with E-MA-GMA) in the ternary composite, as verified from the rheometry torque data presented earlier in Figure 2. Also, the observed increase in the relative tensile strength values of the analysed ternary PET composites, at a given constant SGF content, can be again attributed to the above mentioned increase in fiber orientation effect and also to the verified increase in average fiber length with increasing elastomer content, as reported earlier in this paper. Barboza and Sousa observed the same behaviour in relative tensile modulus and strength data of recycled PET/SGF/ E-MA-GMA hybrid composites, analysed at lower elastomer concentrations (up to 10 wt.%) [21]. High-magnication SEM micrographs showing the pulled-out fibers for some of the binary and ternary composite are given in Figure 6. In Figure 6.a, it shows the pulled-out fiber for SGF20. The SGF used in this research (952) was surface pretreated in the supplied form for PET reinforcement. Therefore, it can be seen that a good level of interfacial bonding exists between SGF and PET

6 SGF = 20 wt % SGF = 25 wt % SGF = 30 wt % 6.0 Relative Tensile Modulus E-MA-GMA Vol. Fraction in PET Polymer Figure 4 Relative tensile modulus of PET. /SGF/E-MA-GMA hybrid composites, as a function of SGF and elastomer concentrations. Relative Tensile Strength SGF = 20 wt % SGF = 25 wt % SGF = 30 wt % E-MA-GMA Vol. Fraction in PET Polymer Figure 5 Relative tensile strength of PET/SGF/E-MA-GMA hybrid composites, as a function of SGF and elastomer concentrations. Results from the Izod impact tests for the PET/E-MA-GMA elastomer blends and PET/SGF/E-MA-GMA hybrid composite are summarized in Figure 7. The Izod impact strength for the pristine PET sample is also shown for comparison purposes. Analyzing firstly the Izod impact strength (IIS) data of the PET/E-MA-GMA blends, it can be seen that effective rubber toughening of the analyzed PET polymer occurs only at rubber content greater than 10 wt%, when optimum rubber particles size and critical rubber interparticle distance is achieved, corroborating previous published literature on rubber toughening of PET polymer [16-18]. At 15 and 20 wt% of E-MA-GMA content, the observed IIS values in the range of J/m indicate that a super-tough PET material is obtained. However, the IIS data for the hybrid ternary PET composites fall in the lower range values between J/m. For a given constant SGF content, it was observed that increasing E-MA-GMA rubber content has a small positive influence on the Izod impact strength. However, the general observation is that the impact strength is mainly controlled by the SGF content. Similar minor improvement in impact toughness was also reported for other hybrid thermoplastic composites by Fung et al for PET/SGF/SEBS-g-MA hybrid composites [26], by Tam et al for PP/SGF/rubber blends [15] and by Laura et al. for PA 6/SGF/maleated-EPR [24-25].

7 ( b ) ( c ) Figure 6. SEM micrographs of freeze-fractured surfaces: (a) PET/SGF(20%) composite, (b) PET/SGF(20%)/E-MA- GMA (10%) hybrid composite and (c) PET/SGF(20%)/E-MA-GMA(20%) hybrid composite.

8 1000 SGF = 0 wt% SGF = 20 wt% SGF = 25 wt% SGF = 30 wt% 800 Izod impact strength ( J/m ) Rubber Content ( wt% ) Figure 7. Effect of E-MA-GMA rubber content on the Izod impact strength for binary PET/E-MA-GMA blends and hybrid composites of PET/SGF/E-MA-GMA. Conclusion The incorporation of SGF into PET matrix can give rise to significant improvements in the tensile modulus, tensile strength and Izod impact strength. The improvements appear to increase with increasing SGF content up to the maximum SGF content of 30 wt% used in this study. When E-MA-GMA rubber was also incorporated to form the hybrid SGF/rubber/PET composite, it was observed that both the tensile modulus and tensile strength were reduced. The incorporation of E-MA-GMA rubber and SGF into the PET/SGF/E-MA-GMA hybrid composites contributed towards an excellent combination of tensile strength/rigidity and impact resistance properties. Acknowledgements This work was carried out with the financial support from research funding institutions from Ministerio de Ciencia e Tecnologia, CNPq and PRONEX. The authors sincerely wish to thank Grupo Mossi & Ghisolfi - M&G, Saint Gobain (Vetrotex Brazil), Arquema Group and Ciba for samples for research. References [1] G. P. Karayannidis & E. A. Psalida, J. App. Polym. Sci. 2000, 77, [2] N. B. Dimitris & G. P. Karayannidis, Polym. Degrad. & Stabiliz. 1999, 63, 213. [3] Z.Z. Yu, M.S. Yang, S.C. Dai, Y.-W. Mai, Journal of Applied Polymer Science 2004, 93, [4] VAL A. Kagan, I. Palley, N. Jia Journal of Reinforced Plastics And Composites 23, 15, [5] K. Tóth; T. Czvikovszky; M. Abd-Elhamid Radiation Physics and Chemistry 2004, 69, 143. [6] J. I. Velasco; D. Arencón Journal of Thermoplastic Composite Materials 2002, 15, 317. [7] Z. A. Mohd Ishak,, A. Ariffin and R. Senawi European Polymer Journal 2001,37, 8, [8] A. L. F. de M. Giraldi; R. C. de Jesus; L. H. I. Mei Journal of Materials Processing Technology 2005, 162, 163, 90. [9] A. L. F. de M. Giraldi; J. R. Bartoli; J. I. Velasco ; L. H. I. Mei Polymer Testing 2005, 24, 507. [10] D. E. Mouzakis, N. Papke, J. S. Wu, J. Karger-Kocsis Journal of Applied Polymer Science 2001, 79,842. [11] W. Loyens, G. Groeninckx Polymer 2003, 44, [12] V. Tanrattanakul, A. Hiltner, E. Baer, W.G. Perkins, F.L. Massey, A. Moet Polymer, 1997, 38, [13] N. Papke, J. Karger-Kocsis Polymer 2001, 42, [14] W. Loyens, G. Groeninckx Polymer 2002, 43, [15] W. Loyens, G. Groeninckx Macromol. Chem. Phys. 2002, 203, [16] J. -G. Park, D. -H. Kim, K. -D. Suh Journal of Applied Polymer Science 2000, 78, [17] T. J. Pecorini, D. Calvert, Chaper 9, Toughening of Plastics: testing and advances in modeling and experiments, 2000.

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