Effect of Talc on Crystallization and Properties of Polypropylene

Transcription

1 American Journal of Materials Research 2015; 2(4): Published online July 10, 2015 ( ISSN: Effect of Talc on Crystallization and Properties of Polypropylene Wei Pan 1, Xuejia Ding 1, *, Hanqing Gu 2, Guoying Hu 2 1 Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing, China 2 Tianjin Institute of Urology, Tianjin Medical University, Tianjin, China address dingxj2011@126.com (Xuejia Ding) Keywords Polypropylene, Talc, Crystallization, Mechanical Properties Received: June 26, 2015 Revised: June 27, 2015 Accepted: June 28, 2015 Citation Wei Pan, Xuejia Ding, Hanqing Gu, Guoying Hu. Effect of Talc on Crystallization and Properties of Polypropylene. American Journal of Materials Research. Vol. 2, No. 4, 2015, pp Abstract Composites of PP and talc were prepared by melt-blending and effects of talc on crystallization and mechanical properties of PP were systematically studied in this paper. When talc was added with a small amount, the crystallization properties and structure were studied by DSC and polarizing microscope. Study found that talc can make good function of nucleating agent, and the crystallization onset temperature and the crystallization peak temperature were both improved, and the degree of crystallinity reached up to the maximum 53.0%. When talc was added with a large amount, mechanical properties such as notched charpy impact strength and tensile strength were improved. Talc can also improve thermal stability of the PP/talc composites which was performed by TGA, and the onset temperature of degradation was improved by 18.2 C when 25phr talc was added. The study found that talc could make good performance both as nucleating agent and inorganic fillers. 1. Introduction Polypropylene (PP) is prepared by propylene polymerization, which is one of the preferred commodity polymers for a wide range of modern technical applications. According to the position of methyl, PP can be divided into isotactic polypropylene, atactic polypropylene and syndiotactic polypropylene. PP is widely used in a number of industries and fields, including construction, upholstery, automobile and medical applications [1~2]. Although polypropylene has low density, good electrical insulation, chemical stability, excellent mechanical properties and good transparency and many other advantages, but the defects of low strength, poor heat resistance and creep resistance also greatly limit the application as high performance engineering plastic [3~4]. Talc is a three-layered magnesium sheet with lubricant properties; it is rarely found as a pure entity in nature [5]. The chemical formula of talc is 3MgO 4SiO 2 H 2 O, and the crystal structure was single crystal system with layered structure [6] ; the basic structure cell of talc is shown in Fig.1 and Fig.2. Among all inorganic powder, talc has the minimum hardness with Mohs hardness of 1.and stable chemical property; it can be heated to more than 800 C when starts to lose crystallization water. Talc has been one of the most commonly used fillers in thermoplastic. Talc has numerous applications because of its chemical and thermal stabilities and lamellar morphology. In particular, talc has become one of the most important fillers used in PP, and it can improve mechanical properties through imparting stiffness [7]. The more important is that talc can be used as a nucleating agent for PP to improve the degree of

2 36 Wei Pan et al.: Effect of Talc on Crystallization and Properties of Polypropylene crystallinity, crystallization temperature, make grain refined and finally reduce cycle time in moulding applications. The filled polymer properties are related to the filler content, its specific surface area and finally to the surface characteristics of the filler which account for the physical and chemical interaction possibilities within the polymer matrix [8]. In this study, we investigated the effect of using talc for PP as nucleating agent and fillers respectively, and the PP/talc composites were prepared by melt-blending. In the talc using as nucleating agent system, talc was added a few thousandths; while in the talc using as inorganic fillers system, talc was added from 5phr to 25phr. composite. Polypropylene (PPH-T30S) was obtained from Sinopec Tianjin Company, Tianjin, and China, with melt flow index (MFI) of 4.8 g (10min)-1 at 230 C and density 0.9 g cm-3. Talc (Sl-3005), which average particle size d50 was 3.86um (3000 mesh), was supplied by Star Better chemical materials Co., Ltd, Beijing, China. Polypropylene was delivered in the form of pellets while talc was in the form of powder Preparation of Talc Filled PP Composites PP and talc were dried for 4h respectively at 80 C and 140 C. Then, the mixes were melt-blended according to the formulations given in Table1 using a co-rotating twin-screw extruder (ZSK-25WLE) with L/D ratio of 40. The screw speed was 150rpm and the temperature of the extruder zones was 190 C-200 C -200 C-190 C. After extruding, the resulting pellets were dried at 80 C for 4h and then moulded using injection molding machine (JPH-10) into various shapes corresponded to the characterizations discussed below. The injection temperature profile was 185 C-190 C-195 C-200 C and holding pressure time was 30s Characterization Mechanical Testing Tensile strength and percentage tensile elongation at break (ultimate elongation) were determined at 25 C according to Chinese standard GB/T using a universal testing machine (LR30K, Ametek Co., England). And the tensile speed was 50 mm min-1. Bending strength was measured at 25 C according to Chinese standard GB/T using the universal testing machine as above. Notched Charpy impact strength was carried out at 25 C using a resil impactor (Ceast Co, Italy) according to Chinese standard GB/T Ten specimens of each composition were tested and the average values were recorded. Fig. 1. Basic structure cell of talc Differential Scanning Calorimeter (DSC) The crystallization behavior of PP/Talc composites was determined using the differential scanning calorimeter (DSC) performed on a DSC Q20 (TA) apparatus. DSC measurements were carried out in the range of 40 C to 200 C at a rate of 50 C min-1 under nitrogen flow of 50 ml min-1. The samples were kept for 5 minutes at 200 C to erase the thermal history before being heated, and then cooled at a rate of 10 C min-1. The sample mass was 5 mg. For calculations of the crystalline content of the matrix, the enthalpy of corresponding to % crystalline PP is 209 J/g [9]. Fig. 2. SEM photomicrographs ( 00) of talc. 2. Experimental Section 2.1. Raw Materials Commercial polypropylene was used as the matrix of the Table 1. Compositions of the Formulations. Components(phr) Material A0 PP Talc 0 Mineral oil 0 A1 A2 0.2 A3 0.3 A4 0.4 A5 B1 5 B2 10 B3 15 B4 20 B5 25

3 American Journal of Materials Research 2015; 2(4): Polarizing Microscope (PLM) The amount of 0.05g of each PP/talc composites sample was placed on a hot stage and melted at 230 C. Each PP sample was pressed between glass slides and coverslips, and kept there for 10 min to remove all the memory of previous thermal and mechanical history, then quickly transferred to another hot stage and kept at a constant temperature of 130 C for 30min. After that, they were cooled to room temperature and the morphology of sperulites was observed by a polarization microscope, MOTIC B1SERIES Thermo Gravimetric Analyzer (TGA) Thermo-gravimetric analysis (TGA) was performed on TGA Q500 (TA) apparatus to investigate the thermal decomposition of the PP/talc composites under N 2 atmosphere. Samples of 5 mg were heated from 40 C up to 550 C with heating rate of 10 C min Scanning Electron Microscopy (SEM) The morphology of fracture surface of impacted pieces was observed using a JSM-6701F scanning electron microscope (SEM), at an accelerate voltage of 5.0KV. The morphology of talcum powder was observed using a HITACHI US-1510 scanning electron microscope, at an accelerate voltage of 10.0KV. The surfaces of the samples used for SEM all were gold-sputtered with a conductive layer before observation. 3. Results and Discussion 3.1. PP/Talc Composites with a Small Amount of Talc Mechanical Properties Fig. 3. Mechanical properties of PP with different talc dosage (a) tensile strength and elongation at break (b) notched charpy impact strength and bending strength. PP with various concentration of talc from to phr (Table 1) were prepared to investigate talc s effects as nucleating agent on mechanical properties such as tensile, impact and bending strength. The results and comparison diagrams were showed in Fig. 3. It was shown that all the mechanical properties investigated such as tensile strength, notched charpy impact strength, etc. increased obviously comparing to those of pure PP. PP/talc composites achieved the best comprehensive properties in presence of 0.3phr talc, at which point the notched charpy impact strength increased by 40.7% in the range from 2.1MPa to2.9mpa, bending strength increased from 39.5MPa to 43.89MPa, tensile strength increased from 30.8MPa to 32.9MPa, and the elongation at break increased form 319.5% to 337.5%. This result may due to the function of talc as nucleating agent, which can uniformly disperse into the resin matrix, play the role of heterogeneous nucleation in the crystallization process, make grain refined and the crystallinity increased. Because all was mentioned above, the performance of mechanical properties were improved at the grandest scale [10].But when we continued to increase the amount of talc by 0.4phr and phr, the mechanical properties began to decrease due to the talc aggregation and the number of larger talc particle increasing, which may cause the formation of crystal defects and hinder the crystal growth DSC Measurement Table 2. DSC parameters of PP and PP/talc composites. Sample T on c / C T p c / C T c / C H /J g -1 X c /% A A A A A A To assess the incidence of talc on PP crystallization properties, a systematic DSC study was carried out. Thermograms were recorded with the same thermal history and cooling/heating rate for PP and the composites. Table 2 and Fig.4 showed the temperatures, enthalpies of crystallization and the degree of crystallinity of PP/talc composites. It s clear that the crystallization onset temperature (T c on ) and crystallization peak temperature (T c p ) shifted to higher temperatures after the addition of talc. With

4 38 Wei Pan et al.: Effect of Talc on Crystallization and Properties of Polypropylene the increase of talc content, the peak area became larger, and the peak became narrower, which can reflect the increase of crystallinity (Xc= H/ H0, H0=209 J g-1)[9] and reaction rate of crystallization. Table 2 showed that the Tcon and Tcp increased with the increase of talc content, which improved from C and C to C and C respectively. Tc ( Tc =Tcon-Tcp) which can reflect the crystallization rate of PP/talc composites to some degree was significantly decreased. So the results could tell us that the addition of talc can improve the reaction rate of crystallization. The degree of crystallinity was reached up to 53.0% from 46.2% in presence of 0.3phr talc; and when we continued to increase the content of talc after 0.3phr, the degree of crystallinity began to decrease. It s because that talc was inorganic and was infusible matter in PP melt, and when PP was cooled from the melt, talc acted as nucleating center to improve the nucleation rate. The rate was controlled by the nucleation in the heterogeneous nucleation process, and then accelerated the crystallization rate and degree. When a small amount of talc was added, which can be uniformly dispersed in the melt, and the formation of more effective nucleation centers promoted PP to form more uniform size spherulites. With the increasing content of talc, its dispersion decreased and formed aggregation, and the content of talc that can act as effective nucleating centers decreased relatively. The DSC result is consistent with the test results of mechanical properties. DSC measurement confirms that talc can act as nucleating agent, as previously mentioned in the literatures [11~12]. Fig. 4. DSC cooling curves of non-isothermal crystallization samples PLM Observations

5 American Journal of Materials Research 2015; 2(4): Fig. 5. PLM micrographs of PP/talc composites. (a) pure PP (b) with phr talc (c) with 0.2phr talc (d) with 0.3phr talc (e) with 0.4phr talc (f) with phr talc. Fig. 6. Mechanical properties of PP with different talc dosage (a) tensile strength and elongation at break (b) notched charpy impact strength and bending strength. Fig.5 showed the PLM micrograghs of isothermal crystallization samples. In Fig.5 (a), the spherulitic morphology of the pure PP sample predominantly consisted of α-spherulites, and the spherulites size was about um. In comparison to the pure PP sample, the spherulites size of PP/talc composites decreased obviously with the presence of talc, which was in agreement with the results of DSC and mechanical testing. When phr talc was introduced into PP [Fig.5 (b)], the spherulites size of PP/talc composites decreased to about 20um, and with the increasing content of talc, the spherulites size of PP/talc composites decreased so greatly that it was hard to be figured out. The increasing talc content had not only obviously remarkable effect on spherulites size but also refined spherulites and improved spherulites content in PP/talc composites. We can see the PP/talc composites with o.3phr talc had the best crystal morphology and the least crystal defects among all above, that may be why PP/talc composites with o.3phr talc had the best mechanical properties. So the perfection of PP crystal, as well as its degree of crystallinity is highly sensitive to the presence of talc particles which can perform well as nucleating agent PP/Talc Composites with a Large Amount of Talc Mechanical Properties PP with various concentrations of talc from 5phr to 25phr (Table 1) were prepared to investigate the effect of using talc as fillers on mechanical properties. The results and comparison diagram were showed in Fig.6. We can see that the mechanical properties were improved with the filling of talc. The notched charpy impact strength increased greatly up to 4.0MPa at the presence of 10phr talc comparing to 2.1MPa of pure PP, and then it performed downward trend with the dosage of talc. This would be because talcum powder particles in the case of appropriate amount can be evenly dispersed, and when the material was subjected to external force, the particles could become the stress concentration with producing a large amount of crazing and shear zone to absorb a large number of impact energy; but when the amount of talc exceeded the saturation value, the impact strength began to decrease because of the agglomeration of a large amount of inorganic superfine rigid particles [13]. The tensile strength achieved the maximum value in presence of 10phr talc and increased by 9.9% from 30.8MPa to 33.9MPa,

6 40 Wei Pan et al.: Effect of Talc on Crystallization and Properties of Polypropylene and bending strength increased gradually with the increase dosage of talc, so talc can make both strengthening and toughening effects on PP/talc composites as the rigid inorganic fillers. But because of the poor compatibility between talc and the organic resin, the elongation at break decreased dramatically with the increasing content of fillers. agglomeration, and the continuity of PP matrix became worse, so that thermal stability of PP/talc composites began to decline. In a word, the thermal stability was improved by appropriate addition of talc. Table 3. TG parameters of PP and PP/talc composites. Sample A0 B1 B2 B3 B4 B5 Tonset ( C) T10wt%( C) T50wt% ( C) Loss residual(%) TGA Measurement The thermal stability of the PP/talc composites was estimated by TGA through measuring the onset temperature of degradation Tonset determined at 1% of degradation, and the temperature at 10% of degradation and 50% of degradation respectively, noted T10wt% and T50wt%. The data for all of composites were reported in Table 3 and Fig 5. All the measurements were in a heating rates of 10 C min-1, with a controlled dry nitrogen flow of 40 cm3 min-1. PP decomposes at a lower temperature than those PP/talc composites, with the Tonset C below that of composites (380.4 C, C, C, C, C), Tonset was improved by 18.2 C when 25phr talc was added. T10wt% and T50wt% were also improved from C and C to C and C when 15phr talc was added, which reached the maximum. It s because that talc was inorganic mineral, and it s more stable than organisms like PP, meanwhile, talc can improve crystallinity of PP to form better heat-resistant crystal to a certain extent and so it could improve thermal stability [14]. When the dosage of talc was more than 15phr, T10wt% and T50wt% began to decrease, but still higher than those of pure PP. It s because that when the high content of talc was added, the distribution of talc powder became uneven, formed more and more Fig. 7. TGA curves of pure PP and PP/talc composites in N2 atmosphere SEM Observations In order to observe the dispersion and compatibility between talc and PP matrix, SEM micrographs of the fracture surface were carried out and showed in Fig.8. When the content of talc was high, the inorganic filler effect of talc would become the dominant factor. In Fig.8, talcum powder can be uniformly dispersed in PP matrix, with a certain direction, and showed some better compatibility between talcum powder and PP matrix. As the fillers loading increased such as the dosage was 25phr, significant filler agglomeration was also indicated. All these factors contributed to the poor stress transfer from matrix to fillers resulting in the decreasing of properties, which were still better than those of pure PP before saturation. It was clear that the fracture surface of PP matrix was clear and the number of craze was limited, so the fracture of PP matrix was brittle fracture. But the fracture surface of PP/talc composites was rough, and the number of craze increased with the increasing dosage of talc.

7 American Journal of Materials Research 2015; 2(4): Fig. 8. SEM micrographs of the fracture surface (a) pure PP (b) with 5phr talc (c) with 15phr talc (d) with 25phr talc. 4. Comparison of PP/Talc Composites with Higher and Lower Amount of Talc On the basis of the experimental works above it was shown that lower amount of talc addition improved crystallization whereas higher amount improved mechanical strength. Comparison of PP/talc composites with higher and lower amount of talc was studied through DSC measurement, so the main effect of higher and lower amount of talc on PP can be analyzed respectively. As is shown in Fig.9, it s clear that crystallization peak shifted to higher temperature after the addition of talc. The Tcon of talc/pp composites increased from C to C and C when o.5phr and 25phr talc was added, and the Tcp increased to C and

8 42 Wei Pan et al.: Effect of Talc on Crystallization and Properties of Polypropylene from C in the same case. The degree of crystallinity was reached up to 49.8% from 46.2% in presence of phr talc, but it decreased to 36.53% when 25phr talc was added. This result showed that lower amount of talc such as a few thousandths can make good function of nucleating agent of PP, and it can increase the degree of crystallinity to improve properties. But the improvement of macro property is slightly because of limited increase of the degree of crystallinity, for an example, the impact strength was only increased by Mpa when phr talc was added as mentioned in front. When the higher amount of talc was added into PP, the content of talc that can act as effective nucleating centers decreased because of aggregation, and too much out-phase matter can cause more defects, both of which would hinder the spherocrystal formation, but in this case, talc as rigid inorganic fillers that was uniformly dispersed can make the main function of strengthening and toughening factors on PP, and it can improve the performance of composies more obviously, such as the impact strength increased by 1.4Mpa about at the presence of 25phr talc. So the higher addition amount of talc was the more effective way to improve the properties of PP. Table 4. DSC parameters of PP and PP/talc composites. Sample T c on / C T c p / C T c / C H /J g -1 X c /% A A B degree of crystallinity was reached up to 53.01% at presence of 0.3phr talc. The improvement of crystallinity improved mechanical properties, such as the notched charpy impact strength, tensile strength and so on, which were corresponding to crystallization analysis. Talc can make good performance as nucleating agent of PP. When talc was added with a large amount, talc as rigid inorganic fillers can make the strengthening and toughening effects on PP/talc composites. The appropriate dosage of talc was 10phr, at which point the notched charpy impact strength and tensile strength achieved the maximum value. The bending strength increased gradually with the increasing dosage of talc, but the elongation at break decreased dramatically because of the poor compatibility between talc and the organic resin. TGA results revealed that talc can improve thermal stability of the PP/talc composites, and the onset temperature of degradation was improved when talc was added. SEM micrographs revealed talc can be uniformly dispersed in PP matrix and the fracture mechanism changed from brittle fracture to ductile fracture. Talc can make good performance as inorganic fillers of PP. So talc can play a role of heterogeneous nucleation agent on polypropylene when added a small amount, and can play a role of inorganic fillers to modify the resin when added a large amount. Through both of the two approaches talc can play the role well in improving the performance of polypropylene, but the role of inorganic fillers when talc was added a large amount was the more effective one. Acknowledgments The authors thank the National Science and Technology Infrastructere Program (2014BAI1B12) for financial support. References [1] Li M, Li G, Jiang J, et al. Ultraviolet Resistance and Antimicrobial Properties of ZnO in the Polypropylene Materials: A Review [J]. Journal of Materials Science & Technology, 2015, 31(4): Fig. 9. DSC cooling curves of non-isothermal crystallization samples. 5. Conclusion The effects of talc on crystallization and properties of PP as nucleating agent or inorganic fillers were investigated. On the basis of the experimental works and test results, the following conclusions were drawn: When talc was added with a small amount, the addition of talc obviously improved crystallinity, crystallization temperature, and PLM micrographs revealed that talc made grains refined and crystal regularity. DSC results revealed the crystallization onset temperature increased by 7.83 C and the crystallization peak temperature increased by 8.05 C, and the [2] Jahani Y. Comparison of the effect of mica and talc and chemical coupling on the rheology, morphology, and mechanical properties of polypropylene composites [J]. Polymers for Advanced Technologies, 2011, 22(6): [3] Pandey J K, Reddy K R, Kumar A P, et al. An overview on the degradability of polymer nanocomposites [J]. Polymer degradation and stability, 2005, 88(2): [4] Gafur M A, Nasrin R, Mina M F, et al. Structures and properties of the compression-molded istactic-polypropylene/talc composites: Effect of cooling and rolling [J]. Polymer Degradation and Stability, 2010, 95(9): [5] Aronson J K. Meyler's side effects of drugs: The international encyclopedia of adverse drug reactions and Interactions [M]. 15th ed. Elsevier, 2006: [6] Tian C X. Study on surface modification of talc etc and PP filled with them [D]. Da Lian University of Technology, Da Lian, Liao Ning, China, 2006.

9 American Journal of Materials Research 2015; 2(4): [7] Castillo L A, Barbosa S E, Capiati N J. Surface modified talc particles by acetoxy groups grafting: Effects on mechanical properties of polypropylene/talc composites [J]. Polymer Engineering & Science, 2013, 53(1): [8] Premalal H G B, Ismail H, Baharin A. Comparison of the mechanical properties of rice husk powder filled polypropylene composites with talc filled polypropylene composites [J]. Polymer Testing, 2002, 21(7): [9] Marinelli A L, Bretas R E S. Blends of polypropylene resins with a liquid crystalline polymer. I. Isothermal crystallization[j]. Journal of Applied Polymer Science, 2003, 87(6): [10] Cui Z, Zhang Y P, Zhang S, et al. Study on crystallization behavior, thermal and mechanical properties of PET/ talc composites [J].China Plastics, 2009 (11): [11] Frihi D, Masenelli Varlot K, Vigier G, et al. Mixed percolating network and mechanical properties of polypropylene/talc composites [J]. Journal of applied polymer science, 2009, 114(5): [12] Duan J, Dou Q. Investigation on β polypropylene/pp g MAH/surface treated talc composites [J]. Journal of Applied Polymer Science, 2013, 130(1): [13] Eiras D, Pessan L A. Mechanical properties of polypropylene/calcium carbonate nanocomposites [J]. Materials Research, 2009, 12(4): [14] Yousfi M, Livi S, Dumas A, et al. Use of new synthetic talc as reinforcing nanofillers for polypropylene and polyamide 6 systems: thermal and mechanical properties [J]. Journal of colloid and interface science, 2013, 403: