Improving the Flame Retardancy of Polypropylene /Rice Husk Composites using Graphene Nanoplatelets and Metal Hydroxide Flame Retardants

Transcription

1 Improving the Flame Retardancy of Polypropylene /Rice Husk Composites using Graphene Nanoplatelets and Metal Hydroxide Flame Retardants Hsiao-Ching Chang 1, SSu-Hsuan Yang 2, Yi-Syun Liao 2, Chris C. C. Yen 3, and Shu-Kai Yeh 2* 1. Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan 2. Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan 3. Miniwiz Co. Ltd. Taipei, Taiwan Corresponding author: Dr. Shu-Kai Yeh, Abstract In this study, rice husk/polypropylene composites filled with graphene nanoplatelets and two kinds of metal hydroxide flame retardants, aluminum hydroxide (ATH) and magnesium hydroxide (MH), were compounded using a Brabender Plasticorder. The flammability and mechanical properties of natural fiber composites of different formulations were evaluated. The horizontal burning test results showed that plain 50 wt% PP/rice husk composites demonstrated a horizontal burning rate of mm/min. When flame retardant or nanographite was added to the composite, the burning rate reduced to mm/min. On the other hand, a synergetic effect was observed when graphene nanoplatelets were used in conjunction with aluminum hydroxide (ATH) or magnesium hydroxide (MH). Horizontal burning rates were significantly reduced. Additionally, materials self-extinguished during the testing period under some circumstances. The horizontal burning rate of these samples was as low as 5.66 mm/min. The results of mechanical testing showed that adding graphene nanoplatelets not only improves the flame retardancy, the stiffness of the composites increases as well. Introduction Natural fiber composites (NFCs) possess excellent properties such as low density, low cost, and they are biodegradable. With the increasing awareness of environmental protection, the applications of NFCs have attracted more attention. NFCs have been, and are, used in many applications including construction, automotive, and packaging. At the moment, natural fibers such as sisal, jute, flex, hemp, cotton, coir, bagasse and bamboo fibers are used in NFCs [1-3]. Due to the high energy and transportation costs the use of local natural fiber resources to produce NFCs is encouraged [2]. A survey conducted by the Food and Agriculture Organization of the United Nations in 15 indicated that global rice paddy production was million tons over an area of million hectares, of which million tons was produced in Asia [4]. This means there is an abundance of agricultural waste, such as rice husk, leaf and straw, available. Most of the agricultural waste in mainland China is incinerated and causes air pollution. The use of rice husk as a reinforcement for composite material has become the preferred method to solve this problem. It enhances the biodegradability and reduces the price of the composites. Asia is the world s primary rice growing region and the application of rice husks as the fibers of NFCs has become one of the most important research topics. However, NFCs have some drawbacks. The main issues include inconsistent fiber quality, poor mechanical properties, poor weather durability and flammability. The application of NFCs is therefore currently limited. Previous research has addressed the issue of improving the mechanical properties and durability of NFCs using nanotechnology [5-9]. Since NFCs are widely applied in transportation and construction, flame retardancy is a critical requirement. In this study, the flame retardancy of NFCs is investigated. Based on the retarding mechanisms, flames may be blocked physically or chemically. The physical actions include cooling, forming a protective layer, or by gas dilution. The chemical actions involve reactions between the gas and solid phases. Gas phase reactions usually associate with radical quenching. Solid phase reactions involve forming a protective layer []. The flame retardancy of polymer composites may be improved by adding additives. The flame retardants can be categorized as additive or reactive retardants [11]. Additive flame retardants do not react with polymers. They hydrolyze upon heating and the reactions are endothermic. During the thermal decomposition, heat is absorbed and moisture is released. Additive flame retardants are usually applied in thermoplastic polymers. Most commercial flame retardants, such as aluminum hydroxide (ATH), magnesium hydroxide (MH) and chlorinated paraffin, are additive retardants. The major challenges of using additive flame retardants are dispersion and their compatibility with the polymers. On the other hand, reactive retardants react and bond with polymers. The high price and limited choices are the shortcomings of reactive flame retardants. In the past, the most common flame retardants were halogen-based which effectively enhance flame retardancy by radical quenching. However, halogen flame retardants produce hydrogen halides and heavy fumes during thermal decomposition. These gases are harmful to human health and halogen ions may leach out and cause environmental problems. Moreover, they cannot be SPE ANTEC Anaheim 17 / 636

2 recycled. Thus, halogen-free flame retardant has become an important research topic. In recent years, aluminum, boron, sulfur, nitrogen and phosphorus have been used to replace halogen elements in order to develop low toxicity flame retardants. With the development of nanotechnology, nanomaterials such as expanded graphite, carbon nanotubes and nanoclay are applied as flame retardants [12, 13]. It is known that nanomaterials alone may not improve flame retardancy significantly. However, when they are applied in conjunction with conventional flame retardants, synergistic effects can be observed and the loading level of flame retardants may be reduced. Synergistic effect means that adding two or more additives produces a greater effect than the sum of their individual outcomes. For example, Bourbigot et al. observed that adding zinc borate, ATH and MH could have a synergistic effect on the flame retardancy of ethylene-vinyl acetate [14]. Similar effects were also observed in nanomaterials. Yen et al. found a synergistic effect between nanoclay and ATH / MH. The flame retardancy of ethylene-vinyl acetate copolymer composites was improved [15]. These observations are also true for carbon nanomaterials. Wang et al. added graphene together with an intumescent flame retardant (a mixture of ammonium polyphosphate and melamine) in poly(butylene succinate). A synergistic effect on both flame retardancy and an anti-dripping effect were recognized [16]. Studies have also shown that cellulose materials could enhance not only the stiffness but also the flame retardancy of composites [17]. Zhao et al. showed that adding rice husks into high density polyethylene enhances the flame retardancy [18]. During the combustion of the composites, silicon dioxide in rice husks is exposed to form a heat shield and cellulose forms a char layer. In this study, rice husks were added as a filler to form PP/rice husk composites. ATH, MH and graphene nanoplatelets were added as the flame retardants. The mechanical properties and flame retardancy of the composites were investigated. The flame retardancy of the composites was characterized by the horizontal burning tests (ASTM D635). Experiment Materials Polypropylene, PP K23, was purchased from Formosa Chemicals & Fiber, Taiwan. The melt flow index of PP K23 is 25 g / min at 230 C and 2.16 kg. Forty mesh rice husks were provided by Miniwiz Co., Ltd. Exfoliated graphene nanoplatelets, x-gnp H25 (H25), were purchased from XG Sciences, Inc., USA. The average diameter of H25 is 25 μm. Aluminum hydroxide (ATH) with greater than 98% purity was purchased from J. T. Baker. The average particle size of ATH is 50 μm. Magnesium hydroxide (MH) with 95-0% purity was purchased from Alfa Aesar. ATH and MH were used as flame retardants. The rice husk content in all experiments was maintained at 50 wt%. Preparation of PP / RH composites In this study, the PP / RH composites were compounded according to the matrix listed in Table 1. To make the table more readable, A and M are used in place of ATH and MH, while G is used to denote H25. The base formulation was PP and 50wt% RH. The flame retardant, ATH or MH, was added based on the total weight of the composites (phr). That is, we assume the weight of PP/RH composites as 0%. The loading level of ATH and MH was controlled at, or 30 phr. The phr loading level means 0 parts of PP/RH composites plus parts of the additive. On the other hand, the loading level of H25 was fixed at 1, 3 or 5 phr. Samples containing both metal hydroxide flame retardants and graphene nanoplatelets are listed in Table 2. The maximum metal hydroxide loading level in these experiments was phr. It is interesting to see if the addition of graphene nanoplatelets will replace a certain amount of metal hydroxide flame retardants. All materials were dried at 80 C for 12 hours to remove the moisture. The dried rice husks were compounded with polypropylene and flame retardants by using a plasticoder (Brabender PLE-331). The temperature and rotator speed were maintained at 180 C and 50 rpm, respectively. The compounding time was 5 minutes. The compounded NFCs were immediately quenched in water to prevent degradation. The composites were then ground using a pulverizing machine (Rong Tsong model RT-02A). The pulverized composites were dried at 80 C for 12 hours before any post processing was carried out. Composites were injection molded into ISO Type 5A dumbbell-shaped specimens and mm bars for tensile and impact testing. The cylinder and mold temperature were set at 190 C and 0 C, respectively. The injection pressure was controlled at 750 bar and the holding pressure was maintained at 300 bar for 15 seconds. Additionally, composites were compression molded into mm plates for horizontal burning tests. The molding temperature was set at 170 C. The specimens were prepared according to ASTM D635. Horizontal Burning Tests The flame retardancy of the composites was characterized by the rate of burning in a horizontal position (ASTM D635). Two reference marks were drawn on the specimen. The first and second reference marks were 25 mm and 0 mm from the free end of the specimen, respectively. The specimen was held by a test fixture horizontally. A -mesh wire gauze was put under the specimen to support burning particles dripping from the specimens. The flame was supplied by a laboratory burner at the other end of the specimen. The flame was SPE ANTEC Anaheim 17 / 637

3 applied for 30 seconds and then removed. The burning time was measured after the flame front had traveled from the first reference mark to the second reference mark. The burning rates were calculated by equation (1), where V is the linear burning rate in millimeters per minute, L is the burned length in millimeters, and t is the time in seconds for the flame to pass the distance L. The average burning rate was determined when (1) three specimens had burned beyond the second reference mark, or (2) until ten specimens had been tested. In our case, sometimes the flame was self-extinguished before reaching the first reference mark. In these cases the linear burning rate was recorded as zero. V=60L/t (1) Mechanical Properties Tests The tensile properties of composites were examined using a Tinius Olsen H5K5 universal tensile testing machine. The injection molded ISO Type 5A dumbbellshaped samples were used for testing. The tests were carried out at a crosshead speed of 1 mm/min. For each formulation, samples were tested. The results are based on the average and standard deviation of the samples. Results and Discussion Horizontal Burning Tests The horizontal burning test results of PP/RH composites filled with 1, 3 or 5 phr H25 are shown in Table 3, in which sample neat represents PP/RH composites. It seems like the horizontal burning rate of polypropylene was the lowest at mm/min. However, the burning polymer kept dripping from the samples, which is highly undesirable. The average burning rate of PP/RH composite was mm/min. The burning rate was significantly higher than neat PP. However, no polymer dripping was observed. It confirmed that cellulose is a char forming material and may help improve the flame retardancy of plastics [19]. When H25 was added to the neat composite, horizontal burning rates were reduced with increasing H25 content. With the addition of 5 wt% H25, the average burning rate reduced to mm/min. Compared to PP / RH composite, the value was decreased by 38.5%. Adding H25 significantly reduced the burning rate. The horizontal burning rates were reduced with increasing ATH and MH content. Among composites with ATH, the horizontal burning rates of composites with 30 phr ATH and 30 phr MH were mm/min and.58 mm/min, respectively. Compared with PP/RH composite, the burning rate decreased by 36.7% and 42.9%. It seems like both ATH and MH are effective flame retardants and the effect of flame retardancy with MH is more pronounced. However, the high loading level at 30 phr would make the composite viscous and difficult to process. As previously mentioned, adding less than 5 phr H25 would improve the flame retardancy of NFCs. It is of interest to see whether adding a small amount of H25 into a PP/RH composite may reduce the loading level of ATH or MH while obtaining similar burning rates. The burning test results of PP/RH composites filled with 1, 3 or 5 phr H25 and, phr ATH or MH are shown in Table 4. Here, the sample code G1A represents PP/RH composites containing 1 phr H25 and phr ATH and so on. As can be seen in Table 4, a synergistic effect was observed between ATH and H25. The burning rates of samples G1A and G3A were about mm/min. This is lower than the samples that contained 30 phr ATH and even lower than pure PP. In some ways, adding only 1 phr H25 may replace phr ATH. As the amount of H25 increased to 5 phr, some of the samples were selfextinguished during the test or before the flame reached the first reference mark. In order to quantify the experimental results, we set the burning rate of the sample self-extinguished before the first reference mark as zero. If the flame passed through the first reference mark and self-extinguished during the test, the burning rate was determined using equation (1). Thus, the burning rate was further reduced to mm/min. When the loading level of ATH increased to phr, the synergistic effect was even more apparent. Though the burning rates of samples G1A and G1A are not so different, the burning rate of sample G3A dropped to mm/min. In the case of G5A sample, all samples self-extinguished. Four of the five samples were extinguished before the flame reached the first reference mark and one sample was extinguished between the first and second reference marks. The burning rate of G5A dropped to 5.66 mm/min. Similar synergistic flame retardant effects were observed in H25 / MH samples. For example, the burning rate of G5M was recorded as mm/min. In the case of G3M and G5M, all of the samples extinguished during the test. The burning rate dropped to 5.98 mm/min and 8.95 mm/min, respectively, which can be translated into an 83.4% and 75.2% decrease in burning rate. These results also indicate that too much H25 may not be helpful. A loading level of 3 phr H25 seems to be the optimized content. It is worth noting that at the same H25 loading level, materials dripped from composites containing phr metal hydroxide selfextinguished right away. On the other hand, materials dripped from composites containing phr metal hydroxide sustained burning for several seconds. Such a difference could be fatal in a fire accident. In summary, our results demonstrated an apparent synergistic effect between graphene nanoplatelets and metal hydroxides. The synergistic effects are more significant at high graphene nanoplatelets loading level and adding graphene nanoplatelets may replace a meaningful amount of metal hydroxide flame retardants. Adding nanomaterials such as graphene SPE ANTEC Anaheim 17 / 638

4 nanoplatelets, nanoclay, or graphene is known to improve the modulus of the composites [6]. In our case, H25 may have increased the modulus of composites as well. To confirm the benefits of adding graphene nanoplatelets, tensile tests were conducted. Since the minimum content of ATH and MH is more than phr and the composites already contain 50 wt% of RH, it may not make sense to compare the mechanical properties for samples such as G1A with G1. The filler content of G1A is way above 50 wt%. Therefore, in this section, we only compare the mechanical properties of PP/RH samples with either metal hydroxide flame retardants or graphene nanoplatelets. The modulus, strength and elongation at break of samples containing 1, 3 and 5 phr of H25 are listed in Figures 1 and 2. Although it is suggested that the more dispersion steps are needed to disperse H25 in PP [], as can be seen in Figure 1, the modulus of composite increased from 2653 MPa to 3614 MPa when the loading level of H25 increased to 5 phr, which can be translated into an increase of 21.2%. On the other hand, a high modulus material such as graphene nanoplatelets into polymer may be pulled out from the polymer matrix during the tension tests and thus decrease the elongation at break of the composite. In our case, the elongation at break decreased from 1.56% to 1.06%. The tensile properties of RH samples containing, and 30 phr ATH (A, A, and A30 sample) are shown in Figures 3 and 4. The Young's modulus slightly increases with increasing ATH content, but the tensile strength in general remained unchanged. The elongation at break also remained unchanged as well only slightly decreasing from 1.68% to 1.34%. Similar to adding ATH, adding MH showed a similar trend. The mechanical properties of M, M, and M30 are shown in Figures 5 and 6. The higher the MH content, the lower the tensile strength and elongation at break are, but the modulus increases with increasing MH content. In general, unlike H25, both flame retardants showed little impact on the mechanical properties of the composites. In contrast, adding only 5 phr H25 increased the modulus of the composite by more than %. Conclusion In summary, the results of these experiments showed that adding graphene nanoplatelets together with metal hydroxide flame retardants, such as ATH and MH, to PP/RH composites demonstrated a very significant synergistic effect. Adding metal hydroxide or nanograhite alone to the composite only decreased the horizontal burning rate from 36 mm / min to mm / min. However, when both flame retardants were added, the horizontal burning rate of the composites could be further decreased to 5.66 mm / min. The best results happened in the G5A sample, in which 4 out of 5 samples selfextinguished before the test started. Additionally, adding graphene nanoplatelets also improves the mechanical properties of the composites. Tension test results showed that the Young's modulus increases by more than % with the addition of 5 wt% H25, indicating that nanoparticles not only improve the flame retardancy but further enhance the stiffness of the composites even without additional dispersion steps. In contrast, the modulus and strength of the composites remained unchanged in the presence of either ATH or MH. Therefore, adding graphene nanoplatelets together with the metal hydroxide not only can improve the flame retardancy of the composite, the mechanical properties also increased significantly in the presence of graphene nanoplatelets. Acknowledgements This work was funded by contract number NSC E MY3 from the National Science Council, Taiwan. References 1. K. L. Pickering, M. G. A. Efendy, and T. M. Le, Composites Part A: Applied Science and Manufacturing, 83, 98 (16). 2. D. B. Dittenber and H. V. S. GangaRao, Composites Part A: Applied Science and Manufacturing, 43, 1419 (12). 3. T. Gurunathan, S. Mohanty, and S. K. Nayak, Composites Part A: Applied Science and Manufacturing, 77, 1 (15). 4. "Trade and Markets Division Food and Agriculture Organization of the United Nations. FAO rice market monitor ", December S. K. Yeh, K. J. Kim, and R. K. Gupta, Journal of Applied Polymer Science, 127, 47 (13). 6. S. K. Yeh and R. K. Gupta, Polymer Engineering and Science, 50, 13 (). 7. S. K. Yeh, S. Agarwal, and R. K. Gupta, Composites Science and Technology, 69, 2225 (09). 8. S. K. Yeh and R. K. Gupta, Composites Part a-applied Science and Manufacturing, 39, 1694 (08). 9. S.-K. Yeh, A. Al-Mulla, and R. K. Gupta, Journal of Polymer Engineering, 26, 783 (06).. T. R. Hull and B. K. Kandola, "Fire retardancy of polymers : new strategies and mechanisms", Royal Society of Chemistry, Cambridge, USEPA, "Furniture Flame Retardancy Partnership: Environmental Profiles of Chemical Flame-Retardant Alternatives for Low-Density Polyurethane Foam (Volume 1)" 12. A. B. Morgan and C. A. Wilkie, "Flame retardant polymer nanocomposites", Wiley-Interscience, Hoboken, N.J., Y. A. P. M. Visakh, "Flame Retardants Polymer Blends, Composites and Nanocomposites", Springer International Publishing S. Bourbigot, M. L. Bras, R. Leeuwendal, K. K. Shen, and D. Schubert, Polymer Degradation and Stability, SPE ANTEC Anaheim 17 / 639

5 64, 419 (1999). 15. Y.-Y. Yen, H.-T. Wang, and W.-J. Guo, Polymer Degradation and Stability, 97, 863 (12). 16. X. Wang, L. Song, H. Yang, H. Lu, and Y. Hu, Industrial & Engineering Chemistry Research, 50, 5376 (11). 17. B. K. Kandola, A. R. Horrocks, D. Price, and G. V. Coleman, Journal of Macromolecular Science - Reviews in Macromolecular Chemistry and Physics, 36, 721 (1996). 18. Q. Zhao, B. Zhang, H. Quan, R. C. M. Yam, R. K. K. Yuen, and R. K. Y. Li, Composites Science and Technology, 69, 2675 (09). 19. I. Milosavljevic, V. Oja, and E. M. Suuberg, Industrial & Engineering Chemistry Research, 35, 653 (1996).. K. Kalaitzidou, H. Fukushima, and L. T. Drzal, Composites Science and Technology, 67, 45 (07). Table 1. Formulation and code of samples without graphene nanoplatelets Sample Code* Filler (phr) Flame Retardant (phr) xgnp-h25 ATH MH PP Neat G G G A - - A - - A M - - M - - M *Base material is 50 wt% PP and 50 wt% RH Table 2 Formulation and code of samples with graphene nanoplatelets Sample Code* Flame Filler (phr) Retardant (phr) H25 ATH MH G1A 1 - G3A 3 - G5A 5 - G1A 1 - G3A 3 - G5A 5 - G1M 1 G3M 3 G5M 5 G1M 1 G3M 3 G5M 5 Table 3. Horizontal burning rates and standard deviation of PP and PP/RH composites containing graphene nanoplatelets Sample Code Burning rate(mm/min) Standard Deviation PP Neat G G G A A A M M M Table 4 Horizontal burning rates of samples containing both graphene nanoplatelets and metal hydroxides Sample Code Tensile Strength(MPa) Buring Rate (mm/min) STDEV Tensile Strength Young's modulus Number of selfextinguished sample Before mark After mark G1A G3A G5A G1M G3M G5M G1A G3A G5A G1M G3M G5M H25(wt%) Figure 1 Modulus and strength of PP/RH samples containing different amounts of graphene nanoplatelets Young s modulus(mpa) SPE ANTEC Anaheim 17 / 640

6 Elongation at break (%) H25(wt%) Figure 2 Elongation at break of PP/RH samples containing different amounts of graphene nanoplatelets Tensile Strength(MPa) Tensile Strength Young's modulus ATH(wt%) Figure 3 Modulus and strength of PP/RH samples containing different amounts of ATH Elongation at break(%) ATH(wt%) Figure 4 Elongation at break of PP/RH samples containing different amounts of ATH Young s modulus(mpa) Tensile Strength(MPa) Elongation(%) Tensile Strength Young's modulus MH(wt%) Figure 5 Modulus and strength of PP/RH samples containing different amounts of MH MH(wt%) Figure 6 Elongation at break of PP/RH samples containing different amounts of MH Young s modulus(mpa) SPE ANTEC Anaheim 17 / 641