Enhancing the properties of low-density polyethylene composites
10.2417/spepro.006114
Enhancing the properties of low-density polyethylene composites Tshwafo Motaung, Thabang Mokhothu, Teboho Mokhena, Mokgaotsa Mochane, Thollwana Makhetha, Rantooa Moji, and Setumo Motloung
Mechanical treatment, with the use of a Supermasscolloider, of sugar cane bagasse significantly improves the mechanical and thermal characteristics of composites made with this material. Mechanical defibrillation of natural fibers is a method that can be used to advance the morphology and application of the fibers. In recent years the so-called Supermasscolloider (SMC), for ultra-fine grinding, has received much attention because of its good accessibility, ease of use, and affordability. Indeed, its potential for the generation of nanofibrils from pulp has been demonstrated.1, 2 Cellulose nanofibrils were successfully produced by fibrillation of softwood pulp using a stone grinding method with the SMC.1 A similar approach has also been adopted for the fibrillation of eucalyptus pulp.3 It was determined that both lignin content and mechanical treatment significantly affected defibrillation of these materials. Furthermore, two major structures were identified from scanning electron microscope (SEM) images of the substances. Highly kinked, naturally helical, and untwisted fibrils were found to serve as backbones of carbon nanofiber (CNF) networks. In addition, entangled, less-distinctively kinked (or curled), and twisted nanofibrils were observed. These two major structures appeared in different features of the CNF networks (e.g., trees, nets, flowers, and single fibrils). Prolonged fibrillation, however, can break the nanofibrils from untwisted fibrils, with high crystallinity, into nanowhiskers. Researchers have thus been investigating other ways to reinforce polymers. Several teams have been working to develop using sugercane bagasse (SB)—the fibrous material that remains after sugercane is crushed—as an essential reinforcement for several polymeric materials.4–9 For instance, composites that are based on recycled highdensity polyethylene and on SB, which has been modified with maleic anhydride (through melt blending and compression molding), have been studied.4 Other researchers have focused on extracting cellulose from the bagasse before the SB is incorporated—by melt mixing—into
Figure 1. Scanning electron microscope (SEM) images of (A) untreated sugercane bagasse (SB) and (B) mechanically treated SB. The mechanically treated SB (SB500) has been passed through a Supermasscolloider (SMC) 500 times.
high-density polyethylene.6, 9 The correlation between the the tensile strength of SB and low-density polyethylene (LDPE) composites and their compression molding temperatures has also been investigated.8 The results of these studies indicate that mechanical properties generally improve in the presence of SB, which is attributed to a reinforcement effect. A significant influence on the mechanical properties of the composites was also observed after surface modification of the SB fibers, which is thought to be caused by improved interfacial interactions. In our work we have investigated how mechanical treatments can affect the morphology as well as the thermal and mechanical properties of LDPE-SB composites.10 We thus mechanically treated agro-based SB by passing it (100–500 times) through an SMC. We then used this mechanically treated SB to reinforce our LDPE composites. We passed the sugarcane through the SMC that consisted of two rotating grinding stones and then prepared the LDPE-SB composites—with 95/5w/w (i.e., mass fraction)—by melt compounding.
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We obtained SEM images of untreated SB, as well as our mechanically treated SB500 (SB that had been passed through the SMC 500 times), as shown in Figure 1. Microspores that are arranged orderly and in light, soft clumps can be observed in the images of the untreated SB. These microspores occupy parts of the fiber that have a diameter of 100m. We attribute these lumps to the dispersal of pectin, wax, lignin, and hemicellulose, which are normally trapped within the cell walls of plants. After our mechanical treatment of the SB, more lumps are observed in association with non-lignocellulose materials. These lumps occur in random orientations on the surface of the fibers and between fibers that have average diameters of 20m. These observations are a clear indication that our mechanical treatment approach is sufficient for breaking the plant cell wall. We have also acquired SEM images of our LDPE-SB composites (see Figure 2). These micrographs show that the fiber diameter decreases with the increasing number of times the SB was passed through the SMC. In addition, fiber pullout and fracture are evident in the images of the composites, and these seem to decrease in magnitude and number with increasing numbers of passes. Although in our LDPESB500 composite most fibers appear to be strongly attached to the outside of the surface, in the LDPE-SB200 samples the fibers appear to be loosely attached. We think a possible reason for this is the increased number of passes through the SMC, which increases the surface area of the fibers and promotes better interfacial interaction.
Figure 3. (A) Thermogravimetric analysis and (B) differential thermogravimetric (DTG) curves of LDPE, LDPE-SB, LDPE-SB100 (SB has been passed through the SMC 100 times), LDPE-SB200, and LDPESB500 composites.
Our experimental results also indicate that the presence of SB in LDPE composites causes a reduction in crystallinity index (CI). With several treatments, however, there is a resultant increase in CI that almost matches that of the LDPE. Furthermore, the increased surface area of the fibers gives rise to trans-crystallization and our results are in line with differential scanning calorimetry, within experimental uncertainty. We measured the Young’s modulus of the LDPE-SB500 composite, which confirmed our SEM and x-ray diffraction observations, i.e., that SB500 has the highest surface area—with the minimum number of defects—and better interfacial interaction. These characteristics promote nucleation of LDPE and significantly improve the tensile modulus of the composite. In addition, we observed increased thermal stability of the composites with increasing numbers of mechanical passes for the SB (see Figure 3). Our thermal stability results suggest that the SMC was able to break the cell wall of the SB and cause partial removal of lignin and hemicellulose. A better interfacial interaction was thus created and caused the enhanced thermal stability. We have conducted a series of tests to investigate how mechanical treatment (i.e., grinding) of SB can affect the thermal and mechanical properties of LDPE-SB composites. From our results, we find that significant improvements to the thermal stability and mechanical properties of the composites can be achieved with increasing levels of mechanical treatment. We also see a similar trend for CI values, i.e., composites made with more treated SB have the highest crystallinities. In our future work, we will continue to explore the mechanical treatment of different natural biomass fibers so that we can optimize the properties of polymer composites.
Figure 2. SEM images of various composites made of low-density polyethylene (LDPE) and SB. (A) LDPE-SB (untreated SB). (B) LDPESB200 (SB has been passed through an SMC 200 times). (C) LDPESB500. Continued on next page
10.2417/spepro.006114 Page 3/3
Author Information Tshwafo Motaung Department of Chemistry University of Zululand KwaDlangezwa, South Africa Tshwafo Motaung is an associate professor and is involved with various projects in the fields of chemistry, physics, polymer chemistry, and composites. Thabang Mokhothu and Teboho Mokhena Materials Science and Manufacturing Council for Scientific and Industrial Research Port Elizabeth, South Africa Mokgaotsa Mochane, Thollwana Makhetha, Rantooa Moji, and Setumo Motloung Department of Chemistry University of the Free State Bloemfontein, South Africa
References 1. R. G. Raj and B. V. Kokta, The effect of processing conditions and binding material on the mechanical properties of bagasse fibre composites, Euro. Polym. J. 27, pp. 1121– 1123, 1991. 2. I. C. Hoeger, S. S. Nair, A. J. Ragauskas, Y. Deng, O. J. Rojas, and J. Y. Zhu, Mechanical deconstruction of lignocellulose cell walls and their enzymatic saccharification, Cellulose 20, pp. 807–818, 2013. 3. Q. Q. Wang, J. Y. Zhu, R. Gleisner, T. A. Kuster, U. Baxa, and S. E. McNeil, Morphological development of cellulose fibrils of a bleached eucalyptus pulp by mechanical fibrillation, Cellulose 19, pp. 1631–1643, 2012. 4. A. Moubarik, N. Grimi, and N. Boussetta, Structural and thermal characterization of Moroccan sugar cane bagasse cellulose fibers and their applications as a reinforcing agent in low density polyethylene, Compos. Part B: Eng. 52, pp. 233–238, 2013. 5. Y. Lei, Q. Wu, F. Yao, and Y. Xu, Preparation and properties of recycled HDPE/natural fiber composites, Compos. Part A: Appl. Sci. Manufact. 38, pp. 1664–1674, 2007. 6. E. F. Cerqueira, C. A. R. P Baptista, and D. R. Mulinaria, Mechanical behaviour of polypropylene reinforced sugarcane bagasse fibers composites, Procedia Eng. 10, pp. 2046–2051, 2011. 7. M. P¨oll¨anen, M. Suvanto, and T. T. Pakkanen, Cellulose reinforced high density polyethylene composites—morphology, mechanical, and thermal expansion properties, Compos. Sci. Technol. 76, pp. 21–28, 2013. 8. S. M. Luz, A. Caldeira-Pires, and P. M. C. Ferr˜ao, Environmental benefits of substituting talc by sugarcane bagasse fibers as reinforcement in polypropylene composites: ecodesign and LCA as strategy for automative components, Resources Conservation Recycling 54, pp. 1135–1144, 2010. 9. T. E. Motaung, Z. Gqokoma, L. Z. Linganiso, and M. J. Hato, The effect of acid content on the poly(furfuryl) alcohol/cellulose composites, Polym. Compos., 2015. First published online: 11 April 2015. doi:10.1002/pc.23428 10. T. Motaung, M. Mochane, T. Makhetha, S. Motloung, T. Mokhothu, T. Mokhena, and R. Moji, Effect of mechanical treatment on morphology and thermal and mechancial properties of sugar cane bagasse–low density polyethylene composites, Polym. Compos., 2015. First published online: 25 July 2015. doi:10.1002/pc.23717
c 2015 Society of Plastics Engineers (SPE)
Enhancing the properties of low-density polyethylene composites Tshwafo Motaung, Thabang Mokhothu, Teboho Mokhena, Mokgaotsa Mochane, Thollwana Makhetha, Rantooa Moji, and Setumo Motloung
Mechanical treatment, with the use of a Supermasscolloider, of sugar cane bagasse significantly improves the mechanical and thermal characteristics of composites made with this material. Mechanical defibrillation of natural fibers is a method that can be used to advance the morphology and application of the fibers. In recent years the so-called Supermasscolloider (SMC), for ultra-fine grinding, has received much attention because of its good accessibility, ease of use, and affordability. Indeed, its potential for the generation of nanofibrils from pulp has been demonstrated.1, 2 Cellulose nanofibrils were successfully produced by fibrillation of softwood pulp using a stone grinding method with the SMC.1 A similar approach has also been adopted for the fibrillation of eucalyptus pulp.3 It was determined that both lignin content and mechanical treatment significantly affected defibrillation of these materials. Furthermore, two major structures were identified from scanning electron microscope (SEM) images of the substances. Highly kinked, naturally helical, and untwisted fibrils were found to serve as backbones of carbon nanofiber (CNF) networks. In addition, entangled, less-distinctively kinked (or curled), and twisted nanofibrils were observed. These two major structures appeared in different features of the CNF networks (e.g., trees, nets, flowers, and single fibrils). Prolonged fibrillation, however, can break the nanofibrils from untwisted fibrils, with high crystallinity, into nanowhiskers. Researchers have thus been investigating other ways to reinforce polymers. Several teams have been working to develop using sugercane bagasse (SB)—the fibrous material that remains after sugercane is crushed—as an essential reinforcement for several polymeric materials.4–9 For instance, composites that are based on recycled highdensity polyethylene and on SB, which has been modified with maleic anhydride (through melt blending and compression molding), have been studied.4 Other researchers have focused on extracting cellulose from the bagasse before the SB is incorporated—by melt mixing—into
Figure 1. Scanning electron microscope (SEM) images of (A) untreated sugercane bagasse (SB) and (B) mechanically treated SB. The mechanically treated SB (SB500) has been passed through a Supermasscolloider (SMC) 500 times.
high-density polyethylene.6, 9 The correlation between the the tensile strength of SB and low-density polyethylene (LDPE) composites and their compression molding temperatures has also been investigated.8 The results of these studies indicate that mechanical properties generally improve in the presence of SB, which is attributed to a reinforcement effect. A significant influence on the mechanical properties of the composites was also observed after surface modification of the SB fibers, which is thought to be caused by improved interfacial interactions. In our work we have investigated how mechanical treatments can affect the morphology as well as the thermal and mechanical properties of LDPE-SB composites.10 We thus mechanically treated agro-based SB by passing it (100–500 times) through an SMC. We then used this mechanically treated SB to reinforce our LDPE composites. We passed the sugarcane through the SMC that consisted of two rotating grinding stones and then prepared the LDPE-SB composites—with 95/5w/w (i.e., mass fraction)—by melt compounding.
Continued on next page
10.2417/spepro.006114 Page 2/3
We obtained SEM images of untreated SB, as well as our mechanically treated SB500 (SB that had been passed through the SMC 500 times), as shown in Figure 1. Microspores that are arranged orderly and in light, soft clumps can be observed in the images of the untreated SB. These microspores occupy parts of the fiber that have a diameter of 100m. We attribute these lumps to the dispersal of pectin, wax, lignin, and hemicellulose, which are normally trapped within the cell walls of plants. After our mechanical treatment of the SB, more lumps are observed in association with non-lignocellulose materials. These lumps occur in random orientations on the surface of the fibers and between fibers that have average diameters of 20m. These observations are a clear indication that our mechanical treatment approach is sufficient for breaking the plant cell wall. We have also acquired SEM images of our LDPE-SB composites (see Figure 2). These micrographs show that the fiber diameter decreases with the increasing number of times the SB was passed through the SMC. In addition, fiber pullout and fracture are evident in the images of the composites, and these seem to decrease in magnitude and number with increasing numbers of passes. Although in our LDPESB500 composite most fibers appear to be strongly attached to the outside of the surface, in the LDPE-SB200 samples the fibers appear to be loosely attached. We think a possible reason for this is the increased number of passes through the SMC, which increases the surface area of the fibers and promotes better interfacial interaction.
Figure 3. (A) Thermogravimetric analysis and (B) differential thermogravimetric (DTG) curves of LDPE, LDPE-SB, LDPE-SB100 (SB has been passed through the SMC 100 times), LDPE-SB200, and LDPESB500 composites.
Our experimental results also indicate that the presence of SB in LDPE composites causes a reduction in crystallinity index (CI). With several treatments, however, there is a resultant increase in CI that almost matches that of the LDPE. Furthermore, the increased surface area of the fibers gives rise to trans-crystallization and our results are in line with differential scanning calorimetry, within experimental uncertainty. We measured the Young’s modulus of the LDPE-SB500 composite, which confirmed our SEM and x-ray diffraction observations, i.e., that SB500 has the highest surface area—with the minimum number of defects—and better interfacial interaction. These characteristics promote nucleation of LDPE and significantly improve the tensile modulus of the composite. In addition, we observed increased thermal stability of the composites with increasing numbers of mechanical passes for the SB (see Figure 3). Our thermal stability results suggest that the SMC was able to break the cell wall of the SB and cause partial removal of lignin and hemicellulose. A better interfacial interaction was thus created and caused the enhanced thermal stability. We have conducted a series of tests to investigate how mechanical treatment (i.e., grinding) of SB can affect the thermal and mechanical properties of LDPE-SB composites. From our results, we find that significant improvements to the thermal stability and mechanical properties of the composites can be achieved with increasing levels of mechanical treatment. We also see a similar trend for CI values, i.e., composites made with more treated SB have the highest crystallinities. In our future work, we will continue to explore the mechanical treatment of different natural biomass fibers so that we can optimize the properties of polymer composites.
Figure 2. SEM images of various composites made of low-density polyethylene (LDPE) and SB. (A) LDPE-SB (untreated SB). (B) LDPESB200 (SB has been passed through an SMC 200 times). (C) LDPESB500. Continued on next page
10.2417/spepro.006114 Page 3/3
Author Information Tshwafo Motaung Department of Chemistry University of Zululand KwaDlangezwa, South Africa Tshwafo Motaung is an associate professor and is involved with various projects in the fields of chemistry, physics, polymer chemistry, and composites. Thabang Mokhothu and Teboho Mokhena Materials Science and Manufacturing Council for Scientific and Industrial Research Port Elizabeth, South Africa Mokgaotsa Mochane, Thollwana Makhetha, Rantooa Moji, and Setumo Motloung Department of Chemistry University of the Free State Bloemfontein, South Africa
References 1. R. G. Raj and B. V. Kokta, The effect of processing conditions and binding material on the mechanical properties of bagasse fibre composites, Euro. Polym. J. 27, pp. 1121– 1123, 1991. 2. I. C. Hoeger, S. S. Nair, A. J. Ragauskas, Y. Deng, O. J. Rojas, and J. Y. Zhu, Mechanical deconstruction of lignocellulose cell walls and their enzymatic saccharification, Cellulose 20, pp. 807–818, 2013. 3. Q. Q. Wang, J. Y. Zhu, R. Gleisner, T. A. Kuster, U. Baxa, and S. E. McNeil, Morphological development of cellulose fibrils of a bleached eucalyptus pulp by mechanical fibrillation, Cellulose 19, pp. 1631–1643, 2012. 4. A. Moubarik, N. Grimi, and N. Boussetta, Structural and thermal characterization of Moroccan sugar cane bagasse cellulose fibers and their applications as a reinforcing agent in low density polyethylene, Compos. Part B: Eng. 52, pp. 233–238, 2013. 5. Y. Lei, Q. Wu, F. Yao, and Y. Xu, Preparation and properties of recycled HDPE/natural fiber composites, Compos. Part A: Appl. Sci. Manufact. 38, pp. 1664–1674, 2007. 6. E. F. Cerqueira, C. A. R. P Baptista, and D. R. Mulinaria, Mechanical behaviour of polypropylene reinforced sugarcane bagasse fibers composites, Procedia Eng. 10, pp. 2046–2051, 2011. 7. M. P¨oll¨anen, M. Suvanto, and T. T. Pakkanen, Cellulose reinforced high density polyethylene composites—morphology, mechanical, and thermal expansion properties, Compos. Sci. Technol. 76, pp. 21–28, 2013. 8. S. M. Luz, A. Caldeira-Pires, and P. M. C. Ferr˜ao, Environmental benefits of substituting talc by sugarcane bagasse fibers as reinforcement in polypropylene composites: ecodesign and LCA as strategy for automative components, Resources Conservation Recycling 54, pp. 1135–1144, 2010. 9. T. E. Motaung, Z. Gqokoma, L. Z. Linganiso, and M. J. Hato, The effect of acid content on the poly(furfuryl) alcohol/cellulose composites, Polym. Compos., 2015. First published online: 11 April 2015. doi:10.1002/pc.23428 10. T. Motaung, M. Mochane, T. Makhetha, S. Motloung, T. Mokhothu, T. Mokhena, and R. Moji, Effect of mechanical treatment on morphology and thermal and mechancial properties of sugar cane bagasse–low density polyethylene composites, Polym. Compos., 2015. First published online: 25 July 2015. doi:10.1002/pc.23717
c 2015 Society of Plastics Engineers (SPE)
