Polyolefin Composites Filled with Magnesium Hydroxide

Transcription

1 Polyolefin Composites Filled with Magnesium Hydroxide Polyolefin Composites Filled with Magnesium Hydroxide Song Zhu, Yong Zhang*, Yinxi Zhang Research Institute of Polymer Materials, Shanghai Jiao Tong University, Shanghai , People s Republic of China Received: 3 November 2001 Accepted: 21 March 2002 SUMMARY In this study, modified and non-modified composites of polypropylene (PP) and linear low-density polyethylene (LLDPE) filled with magnesium hydroxide (Mg(OH) 2 ) were investigated, and maleic acid anhydride-grafted PP or LLDPE (MAH-g-PP, or MAH-g- LLDPE) were used as polymer modifiers. In the composites, when the LLDPE was partially replaced by MAH-g-LLDPE, the notched Izod impact strength, tensile strength, and flexural strength of the composites increased, while the modulus decreased. When the PP was partially replaced by MAH-g-PP, the tensile strength and flexural strength of the composites increased, and the impact strength and modulus changed slightly. The phase structure of the composites was characterized using scanning electron microscopy (SEM), dynamic mechanical thermal analysis (DMTA), and differential scanning calorimeter (DSC). INTRODUCTION The poor impact strength of isotactic polypropylene (PP) is well known. Much attention has been paid to ternary PP composites containing an olefinic elastomer and a filler to increase the impact strength of the composites. The elastomer typically includes ethylene propylene random block copolymer (EPR), ethylenevinyl acetate (EVA), and ethylene-octene copolymer (POE), etc. The relationships between the morphology and the mechanical properties of the ternary composites have been studied extensively in recent years 1-6. In those studies, two kinds of structure were designed depending on the polarity of the PP and elastomeric phase, e.g. separation of elastomer and filler particles, or encapsulation of filler by elastomer 5. Fire safety concerns bring about an additional requirement for reduced flammability, especially in the mass transport and construction industries. Magnesium hydroxide (Mg(OH) 2 ) has been shown to be an effective halogen-free flame-retardant and a smoke suppressing additive for polymer materials at sufficiently high levels of addition 7. Compared with aluminium hydroxide with its relatively low decomposition temperature, Mg(OH) 2 is stable to higher temperatures. This makes possible its incorporation in a wider range of polymers 8. * Corresponding author Both the stiffness and the toughness of PP should be considered in engineering applications. Because the introduction of the elastomers results in a lower rigidity in PP composites, the amount of elastomer should be kept low. This ensures a sufficiently high modulus of the composites. Compared with the elastomers mentioned above, linear low-density polyethylene (LLDPE) has a relatively high modulus. In our previous work it has been shown that the impact strength of highly filled LLDPE can be improved remarkably by using a small amount of a grafted copolymer containing polar groups 9. In this work, LLDPE was used as the third component of the ternary composites of PP. Maleic anhydride (MAH)- grafted PP and MAH-grafted LLDPE were added as polymeric modifiers to enhance the interaction between the filler and the polymer. EXPERIMENTAL Materials Commercial PP Y1600 was produced by the Shanghai Jinshan Petroleum Chemical Co., Ltd. Shanghai, China, with melt flow index (MFI) of 16.0 g/10 min (190 o C, 2.16 kg) and density of 0.905g/cm 3. Commercial LLDPE 218W was produced by Saudi Basic Industrial Company (SABIC), Saudi Arabia, with melt flow index (MFI) of 1.2 g/10 min (190 o C, Polymers & Polymer Composites, Vol. 10, No. 6,

2 Song Zhu, Yong Zhang, Yinxi Zhang 2.16 kg) and density of 0.915g/cm 3. PP and LLDPE were used as the matrix polymers. Mg(OH) 2, made in Haihua Co., Ltd., Shandong, China, with density of 2.39 g/cm 3 and mean particle size of 3µm, was used as the filler. MAH-g-PP and MAH-g-LLDPE were prepared by melt grafting in a HAAKE rheometer RC90. Powder isotactic PP, supplied by Shanghai Petrochemical Complex, China, with melt flow index (MFI) of 6.7 g/ 10min (230 o C, 2.16 kg) and LLDPE 218W were used as the start material. Dicumyl peroxide (DCP), MAH, Acrylamide, and Dimethyl Formamide were all reagent grades and used without any further purification. PP was dry blended with desired amounts of other additives, while LLDPE was blend with the solution of acetone. The mixer was fed into the mixer. The reaction time and mixing speed were maintained at 6 min and 100 rpm, respectively. The reaction temperature was 180 o C. The reaction products were pulverized and dried under vacuum at 85 o C for 10 hr. The grafting degree is determined by titration according to the literature 10. The details of the graft polymers were shown in Table 1. The third additive in the formulations is to suppress the degradation of PP 11 and crosslink of LLDPE 10. Compounding Mg(OH) 2 filler was used in untreated form. The components were mixed in the chamber of a HAAKE rheometer RC90 at 185 o C and a rotor speed of 64 rpm for 12 min. Specimen preparation Test specimens used for morphology observation and mechanical property testing were prepared by compression moulding sheets of 1.0 mm or 3.0 mm thickness at 185 o C and a pressure of 15 MPa for 7 min and then at room temperature under the same pressure of 15 MPa. Mechanical properties Tensile dumbbell specimens were cut from the 1 mmthick sheets and were tested using an Instron 4465 tensile tester according to ASTM D at a crosshead speed of 50 mm/min. Three point bending flexural tests were performed according to ASTM D790M using the same tester at a crosshead speed of 14 mm/ min and the specimen size was 62.5x10x3mm. Notched specimens 3 mm thick were tested in a Ray-Ran impact tester according to ASTM D at a hammer speed of 3.5 m/s and a pendulum weight of Kg. Scanning Electron Microscope (SEM) observation A Hitachi S-2150 scanning electron microscope was used for the morphology observation. Impact specimens were immersed in liquid nitrogen and then fractured by the Ray-Ran impact tester. The fractured surfaces were etched in xylene at 95~100 o C, with slight stirring, prior to the SEM observations. All of the specimens were coated with a thin layer of gold. Differential Scanning Calorimeter (DSC) analysis The melting and crystallization behavior of the composites were studied using a Perkin-Elmer DSC Pyris 1. Samples were first heated to 200 o C, and maintained at 200 o C for 5 min before cooling to 50 o C at 5 o C /min. Dynamic Mechanical Thermal Analysis (DMTA) The dynamic mechanical properties of the composites of were determined using a Rheometric Scientific TM DMTA TM. The dimensions of the specimens were about 20mm in length, 4mm in width and 0.9 mm in thickness. Testing was carried out in the dual cantilever mode over a temperature range from 30 to 100 o C at a frequency of 1Hz. Table 1 The details of the grafted PP and LLDPE Polyolefin Dicumyl Peroxide MAH 3 rd additive Grafting degree (%) PP Acrylamide/ LLDPE Dimethyl Formamide/ PP or LLDPE is 100 by weight 448 Polymers & Polymer Composites, Vol. 10, No. 6, 2002

3 Polyolefin Composites Filled with Magnesium Hydroxide RESULTS AND DISSCUSSION Morphology observation PP, MAH-g-PP, LLDPE, and MAH-g-LLDPE plaques having weights of about 40 mg were immersed in xylene at 97 ± 2 o C. In 20 min time range, LLDPE and MAH-g-LLDPE plaques showed swelling and about 20% weight loss. LLDPE and MAH-g-LLDPE were dissolved completely after immersed in xylene in one hour. After the PP and MAH-g-PP plaques were immersed in xylene for more than 24 hours, they were still rigid and had little change in weight. So it is feasible to etch the LLDPE and to keep the PP unetched using xylene, under the controlled condition. Figure 1 Cryogenic fractured and etched surfaces of the PP/ LLDPE/Mg(OH) 2 composites. Compositions: polyolefin filled with 36 vol % of Mg(OH) 2 (a) PP/LLDPE 70/30 (xylene etched for 60 min) (b) PP/LLDPE/MAH-g-PP 60/30/10 (xylene etched for 60 min) (c) PP/LLDPE/MAH-g-LLDPE 70/20/10 (xylene etched for 20 min) (d) PP/LLDPE/MAH-g-LLDPE 70/20/10 etched for 60 min (xylene etched for 60 min) (a) Figure 1 shows an SEM micrograph of the fractured surface of the PP composites. In Figure 1(a), the etch time was 60 min and the filler particles can be found easily in the composite without grafted copolymer, which means that there is no strong adhesion between polymer matrix and filler particles. For the composite containing MAH-g-PP, the adhesion between PP matrix and Mg(OH) 2 filler particles should have been improved, so fewer filler particles could be found in Figure 1(b) when the etch time was 60 min. MAH-g- LLDPE can also encapsulate the filler particles. No filler particle could be observed except for some protuberances and holes in Figure 1(c) after the composite containing MAH-g-LLDPE was etched for 20 min, which is explained by the adhesion of MAHg-LLDPE to the filler particles. As shown in Figure 1(d), when the etched time was over 60 min, the LLDPE should have dissolved completely, so only distinct holes in the smooth surface can be observed because the filler particles leave as the LLDPE and MAH-g-LLEPE are lost. From the observations above it can be concluded that the encapsulation of filler particles by PP or LLDPE can be achieved by the introduction of MAH-g-PP or MAH-g-LLDPE. (b) (c) Mechanical properties The impact, flexural, and tensile properties of the composites containing 36% volume fraction of Mg(OH) 2 are shown in Figure 2. In the composites, the amount of LLDPE in the matrices varies from 0 to 100 vol % when there is no grafted copolymer (MAHg-LLDPE or MAH-g-PP) or 0 to 90% when PP is partially replaced by MAH-g-PP, or 10 to 100% when LLDPE is partially replaced by MAH-g-LLDPE. The notched Izod impact strength of PP/LLDPE blends containing no filler is also shown in Figure 2 (a). (d) Polymers & Polymer Composites, Vol. 10, No. 6,

4 Song Zhu, Yong Zhang, Yinxi Zhang Figure 2 Effect of LLDPE content on the mechanical properties of the composites. Composition: polyolefin filled with 36 vol. % of Mg(OH) 2, The LLDPE content is the vol. % of LLDPE and MAH-g-LLDPE in the polyolefin. (a) Notched Izod impact strength (b) Tensile strength 450 Polymers & Polymer Composites, Vol. 10, No. 6, 2002

5 Polyolefin Composites Filled with Magnesium Hydroxide Figure 2 Continued (c) Flexural strength (d) Flexural modulus In EPR toughed PP containing inorganic fillers, the impact strength of the filled PP/ EPR composites with either of the limiting morphologies, e.g. the complete dispersion, or complete encapsulation of rubber conclusions is increased 3,5. In our study, unlike the filled PP/EPR composites, the notched impact strength of PP/LLDPE/Mg(OH) 2 can reach a relatively high value composites only with the latter morphology. As shown in Figure 2(a), the notched Izod impact strength of the PP/LLDPE composite changes a little when the LLDPE content increased from 0 to 100%. When LLDPE is partially replaced by MAH-g-LLDPE, Polymers & Polymer Composites, Vol. 10, No. 6,

6 Song Zhu, Yong Zhang, Yinxi Zhang the impact strength changes dramatically. For the composite without PP, the impact strength is more than 400 J/m. However, the relationship between the impact strength and LLDPE content is non-linear. When the LLDPE content is more than 50%, the notched Izod impact strength is lower than 100 J/m. On the other hand, the substitution of PP by MAH-g- PP does not have much influence on the impact strength. The most interesting thing is that the notched impact strength of filled PP/LLDPE/MAH-g-LLDPE is higher than that of non-filled blends. The effect of LLDPE content on the tensile strength is shown in Figure 2(b). The incorporation of LLDPE into the PP composites causes a reduction in the tensile strength of the composites, as showed in the Figure, because the strength of LLDPE is lower than that of PP. Because MAH-g-PP and MAH-g-LLDPE can enhance the adhesion between the polymer matrix and the filler particles, the tensile strength increases as the two graft copolymers are introduced. It is shown in Figure 2(b) that the tensile strength of the composites containing MAH-g-PP is a little higher than that those containing MAH-g-LLDPE. MAH-g- LLDPE and MAH-g-PP also increase the flexural strength of the composites through better adhesion between filler and polymer matrix, as shown in Figure 2(c). Mg(OH) 2 can increase the stiffness of the composites. On the other hand, because the modulus of LLDPE is lower than that of PP, LLDPE inclusion decreases the flexural modulus. The encapsulation of MAH-g-LLDPE around filler particles changes the characteristics of the inclusions from rigid to soft, and the reinforcing effect of Mg(OH) 2 will be suppressed by the encapsulation. The flexural modulus of the composite containing MAH-g-LLDPE, as shown in Figure 2(d), is lower than that of the composites without MAH-g-LLDPE. When PP is partially replaced by the MAH-g-PP, the modulus of the composites increases because the better reinforcing effect of Mg(OH) 2 can be fulfilled by the encapsulation of MAH-g-PP. The effects of Mg(OH) 2 on the mechanical properties are shown in Figures 3 (a) and (b). In Figure 3 (a), the notched Izod impact strength of the PP/LLDPE/MAHg-LLDPE composites keeps rising as the amount of Mg(OH) 2 increases. It is very important because a high loading of Mg(OH) 2 is necessary for the optimum flame-retardant performance. The modulus of the particalate-filled composites can be described by Nielsen s expression for polymers filled with rigid spheres 12 : where E c 1 + ABφ f = E m 1 B ψφ f f f f (1) E / E 1 f m B = E E + A and A = K 1 φ max 1, ψ = 1 + ( ) φ / φ f m E f f In the above expressions, the subscripts f, c, and m refer to the filler, composites, and matrix (e.g. the unfilled polyolefinic blend), respectively; φ is the volume fraction, φ max is the maximum volume fraction of filler, and K E is the Einstein coefficient, which is equal to 2.5 for spherical inclusions. A deviation from theoretical calculation on the flexural modulus is also shown in Figure 3 (b). The measured modulus of the PP/LLDPE/MAH-g-LLDPE/Mg(OH) 2 composites is lower than the predicted value using Nielsen s equation, while the modulus of PP/LLDPE composites follows the equation very well. It indicates that the reinforcing efficiency of Mg(OH) 2 was suppressed by the surrounding LLDPE layer because of the core-shell structure. DSC and DMTA analyses The effects of the Mg(OH) 2 and the grafted copolymers on the crystallization behaviors of the PP/LLDPE blends and PP/LLDPE/Mg(OH) 2 composites are illustrated in Table 2, where T m is the melt temperature, T c is the peak temperature of crystallization, and the T c,onset is the onset temperature of crystallization. Mg(OH) 2 acted as a nucleating agent promoting crystallization of PP at the particles surface and hence led to a composite with higher T c and T c,onset, while the T c and T c,onset of several blends containing no filler change a little. These indicate that the incorporated filler had more influence on the crystallization of PP than LLDPE or the grafted copolymers. In the filled composites, T c and T c,onset of PP in filled PP/LLDPE slightly decreased compared with filled PP, which means that the nucleating effect of Mg(OH) 2 on the nucleation is suppressed a little by the introduction of LLDPE. When the LLDPE is partially replaced by MAH-g-LLDPE, the suppression is enlarged and the T c and T c,onset are lower that that of filled PP about 7 o C, which indicated encapsulation of filler particles by LLDPE and MAH-g-LLDPE. The max 452 Polymers & Polymer Composites, Vol. 10, No. 6, 2002

7 Polyolefin Composites Filled with Magnesium Hydroxide Figure 3 Effect of volume fraction of Mg(OH) 2 on mechanical properties. Composition: LLDPE and MAH-g-LLDPE content in the matrices is 30% (a) Notched Izod impact strength Flexural modulus Polymers & Polymer Composites, Vol. 10, No. 6,

8 Song Zhu, Yong Zhang, Yinxi Zhang Table 2 Effect of grafted copolymers and Mg(OH) filler on the crystallization behaviour of PP 2 PP/LLDPE/grafted copolymer N on-filled illed with 36 Vol.% Mg(OH) F 2 T m, C T, C T c,onset c, C T m, C T, C T c,onset c, C 100/0/ /30/ /20/10 (MAH-g-LLDPE) /30/10 (MAH-g-PP) effect of MAH-g-PP on the crystallization behavior is the most remarkable since the T c and T c,onset of PP rise more than 10 o C. It strongly indicates the encapsulation of the PP on the surface of filler particles. Figure 4 shows the temperature dependencies of tan δ for various composites. A peak of tan d can be seen at about 10 o C corresponding to the glass transition temperature (T g ) of PP 13. The peak of tan δ for the composite containing MAH-g-LLDPE is broader than that of the composite containing MAH-g-PP or containing no grafted copolymer, which means that the compatibility between PP and LLDPE is improved. Perhaps that is the other reason for the increased impact strength of the composites containing MAH-g-LLDPE. CONCLUSION The morphology and mechanical properties of the Mg(OH) 2 /PP/LLDPE composites are affected by the addition of MAH-g-PP or MAH-g-LLDPE. An encapsulation of MAH-g-LLDPE around the Mg(OH) 2 filler particles improves the notched Izod impact strength, tensile strength, and flexural strength of the composites when compared with the composites without any graft polymer. When the PP is partially replaced by MAH-g-PP in the composites, the tensile strength and flexural strength and modulus increase, although the impact strength changes little. Even at very high loading, Mg(OH) 2 can increase the notched Izod impact strength of the PP/LLDPE/MAH-g-LLDPE composites. SEM and DSC evidence suggest that an Figure 4 Temperature dependency of tand for PP/LLDPE/Mg(OH) 2 composites. Composition: polyolefin filled with 36 vol.% of Mg(OH) Polymers & Polymer Composites, Vol. 10, No. 6, 2002

9 Polyolefin Composites Filled with Magnesium Hydroxide encapsulation of LLDPE or PP around Mg(OH) 2 filler particles in the PP/LLDPE composites containing MAH-g-LLDPE or MAH-g-PP. REFERENCE 1. Comitov P. G., Nicolova Z. G., Simeonov I. S. Eur. Polym. J., 20 (1984) Hornsby P. R., Premphet K. J. Appl. Polym. Sci., 70 (1998) Jancar J., Dibenedetto A. T. Polym. Commun., 31 (1990) Wang J., Tung J. F., Ahmad M. Y. J. Appl. Polym. Sci., 60 (1996) Jancar J., Dibenedetto A. T. J. Mater. Sci., 30 (1995) Premphet K., Horanont, P. Polym. 41, (2000) Plastics, Additives and Compounding 2, 5, (2000) Hornsby P. R. and Watson C. L. Plast. Rubb. Proc. Appl., 6 (1986) Zhu S., Zhang Y., Zhang Y. X. J. Appl. Polym. Sci., 83 (2002) Gaylord N. G., Mehta R., Mohan D. R. J. Appl. Polym. Sci., 44 (1992) Huang, H., Liu, N.C. J. Appl. Polym. Sci. 67 (1998) Nielsen L. E., J. Appl. Phys., 41 (1970) McGrum N. G, Read B. E, Williams G. Anelastic and Dielectric Effects in Polymeric Solids. London: Wiley, Polymers & Polymer Composites, Vol. 10, No. 6,

10 Song Zhu, Yong Zhang, Yinxi Zhang 456 Polymers & Polymer Composites, Vol. 10, No. 6, 2002