The effect of processing methods on some properties of rice husk-polypropylene composite A preliminary report
B.I.Ugheoke Department of Mechanical Engineering, University Teknologi PETRONAS, Bandar Seri Iskandar, 31750, Tronoh, Perak, Malaysialine
[email protected] R.M Joshua Department of Mechanical Engineering, Federal University of Technology, Yola Nigeria
[email protected] Abstract— Natural fibers fulfill most requirements needed to replace synthetic fibers in thermoplastic composites. However, some disadvantages appear when natural fibers are used for composites. The poor compatibility between the hydrophilic fibers with the hydrophobic thermoplastic matrix leads to a weak interface and hence, poor mechanical properties. In this work, the effects of processing techniques (fiber surface modification and varying compounding pressure) on the mechanical properties of rice huskpolypropylene composite were investigated. The results showed improvement in mechanical properties for the treated composites (tensile strength, impact energy and hardness), with the mercerized sample having the highest improvement. The trend of results suggest that there is a need for further work on the optimization of the fibermatrix interface for improved mechanical properties to be achieved. Keywords- Processing techniques effects, mechanical properties, rice husk polypropylene composites
I.
INTRODUCTION
The use of lignocellulosic materials as reinforcement in composite materials is not new. Man has used this idea for a long time, since the beginning of civilization when grass and straw were used to reinforce mud bricks [1]. However, in the past three decades organic fillers have become a good competitor for inorganic fillers. This trend has been attributed to their low densities, very low cost, high filling levels, non-abrasive nature, high specific mechanical properties, recyclable, biodegradable and renewable nature [2]. Their bio-degradability phenomenon is perceived to have the most appealing effect and attraction, since the phenomenon provides positive environmental benefits with respect to ultimate disposability and raw materials utilization.
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N.O. Namesan Department of Agronomy, Taraba State University, Jalingo, Nigeria
[email protected] R.N. Ufodi Department of Mechanical Engineering, Federal University of Technology, Yola Nigeria
Biodegradable composites (also known as biocomposites) are made from a combination of a polymeric matrix and cellulosic filler of fibrous nature. Thus it is rife to find the use of lignocellulosic fibers like Jute, Sisal, Coir, Pineapple and Banana leaves in bio-compositing [3]. Rice husk, one of several lignocellulosic materials, is an agricultural residue (other examples include bagasse, empty palm oil fruit bunches and wood chips) produced as by-product during rice milling process. It has been used [4,5,6] in polymeric composite manufacturing, showing good properties. As stated in [7], Nigeria’s rice production stood at 3.745 million tons of rice paddies in 2005 and 20 wt. % of this is the husk [8]. The commonest uses of rice husk in Nigeria include bedding material for animals, land filling, but more oftener burned. This may not be unconnected with the fact that of all cereal by-products, rice husk has the lowest percentage of total digestible nutrients, being less than 10% [9]. It is therefore conceivable that the use of rice husk in polymeric composite making would boost the socio-economic status of rice farmers in Nigeria and impliedly, the country in general. However, poor interfacial properties in the form of poor adhesion between the hydrophilic lignocellulosic fibers and hydrophobic polymer matrix often reduce the potential of natural fibers as reinforcing agents [10], since it has been shown [3,11,12] that interface state is an important determinant of the physical and mechanical properties of composites. To promote and improve adhesion between these chemically diverse constituents of polymeric composites, interface properties optimization is needed. This is often achieved either through the use of compatibilizing agents [13],
or through the treatment or modification of fiber surface using appropriate chemicals. Compatibilizers have the ability to react in some manner both with the organic fillers and the polymers, building bridges across the interface. Examples of compatibilizers include silane, peroxide, isocyanate, maleated polypropylene (MAPP), acrylated and latex coating treatments. On the other hand, mercerization and acetylation (grouped as alkali treatment) fibrillate the lignocellulosic fibers, reducing their aspect ratio, increase surface roughness and opening them up for a better matrix penetration resulting in enhanced adhesion [3]. Mercerization on flax fibers has been used to study these effects [14]. In addition to the interface properties, processing techniques such as extrusion and injection molding have impact on the mechanical properties of biocomposites [15]. This may not be unconnected with the difference in working pressure of the processes employed for the composites formation. Attention has been focused on the use of compatibilizers rather than alkali treatment on fiber surface modification to improve mechanical properties of composites. In a recent work [6] compatibilizing agents including MAPP were used to improve interface quality. In this work however, alkali treatment (mercerization and acetylation) together with varying pressure were employed and the results are reported.. II.
MATERIALS AND METHODS
A. Materials Rice husk. Freshly milled husk was collected freely from rice mills in Jimeta metropolis, Yola North LGA of Adamawa State. The collected rice husk was conditioned in an oven for 24 hours at 1050C to expel any associated moisture. The conditioned husk was ground into powder and graded. The grinding was accomplished through the use of a manually operated domestic grinder. The grading exercise was carried out in a universal Shaker (Zwick, Germany) using a designated 40-mesh sieve. The appropriateness of the sieve openings had already been established in a similar work [5]. Rice husk particles that passed through the sieve aperture (termed Rice husk flour, RHF) were used in all the procedures of the work.
combing was used to facilitate the essence of scouring, which is to loosen lignin and other mucilaginous materials from the fiber. Thorough washing under running water followed this and the retted RHF was passed on to the scouring procedure. Scouring followed the retting procedure. In scouring, 10g of retted rice husk powder was weighed into 250ml beaker and 150ml of 2% NaOH was added. Other processes that followed are similar to those of the retting procedure. After scouring, mercerization of the RHF was conducted. This involved measurement of 40ml of 20% NaOH into a 150ml beaker conditioned in an ice bath at 20C. Into this conditioned beaker content were measured 10g of the scoured RHF. The mixture was allowed to stay for 45 minutes. The RHF was thereafter removed and washed several times in tap water. It was thereafter oven dried at 550C for 24 hours. Later the conditioned sample was removed from the oven and stored in polythene bags enclosed with desiccant sachets. Acetylation. 10g of mercerized rice husk was measured into a 400 ml beaker. 15ml acetic acid, 10ml acetic anhydride and 65ml conc. H2SO4 were measured and added to the 400ml beaker containing the 10g of mercerized rice husk. The beaker with its content was warmed gently to 450C on a heating plate for 1 hour. Thereafter 65ml of conc. H2SO4 was again added and the content of beaker was stirred for another 1 hour. The dark-coloured solution was decanted into 50ml water and allowed to stand for 48 hours. The precipitate was dried, the yield collected and oven dried for 24 hours at 550C and then packaged in polythene bags enclosed with desiccant sachets.
Polypropylene (PP). This was used as purchased homopolymer pellets supplied by HANWHA L & C Corp., South Korea and the manufacturer’s indicated melt flow index (MFI) as 12g/10 min (2300C/2160g) with a density of 0.91g/cm3. These were not reconfirmed in the present work, since they were not the primary objectives of the work
Compounding. Each composite comprised 80g of polypropylene and 20g of rice husk were weighed and kept. The weighed quantity of polypropylene was poured into a mould cavity of a press designed for squeeze casting process preheated to a temperature of 2300C. This was allowed to stand for 5 minutes to allow the PP to melt completely. The measured quantity of rice husk was carefully poured into the mould, mixed manually with the molten polypropylene, and stirred with a glass stirrer until the turning effect became steady and physical observation showed a homogenous mix. At this point, pressure was applied to the molten composite in the mould while the heat supply was cut off. The composites were pressure processed at 5, 10 and 15 MPa respectively. Composite samples were prepared under these pressure levels using crude (untreated), mercerized and acetylated rice husk. Each composition had five samples for each of the mechanical properties investigated at each pressure level.
B. Methods Mercerization. Prior to this process, two other procedures: retting and scouring took place. In the retting exercise, 250ml of ammonium oxalate solution was measured into a 400ml beaker. 10g of RHF was gently placed in the solution. The beaker and its content was thereafter placed on a heating mantle and heated to boil. After about 45minutes, manual
Mechanical Tests. These include tensile, impact and hardness tests. The tensile tests were carried out in conformity with the ASTM D638-99 procedure [16], using 100 KN Universal Testing Machine UTM (Instron) with a cross head speed of 10 mm/min. Noted values were ultimate tensile strength (UTS), percentage elongation and percentage reduction in cross sectional area. Izod impact energy test was conducted on notched samples according to ASTM D256-
97[17]. A hardness tester was used to perform Rockwell hardness tests on all samples. Each surface was prepared by grinding using smooth emery cloth and indentation was made and observed gauge readings recorded on the ‘B’ scale of the tester. III.
RESULTS AND DISCUSSIONS
Table 1 gives the results for the ultimate tensile strength (UTS) of the samples. As could be seen from the table, there is an improved variation of UTS with rising pressure, with highest UTS values exhibited by mercerized RHF composite samples. This trend of increasing UTS with increase in pressure could be explained by better compaction amongst the constituents of the composite. This gave rise to better wettability of RHF by the PP matrix, resulting in effective load transfer through the composite system. The mercerized samples did not show much variation in UTS with increased pressure. One reason for this is because the fibers seem to undergo some toughening in the mercerization process as revealed by their post-treatment texture. Greater UTS values shown by mercerized over the acetylated RHF composite samples are due to excessive fibrillation which was caused by the latter process. This led to the decreased mechanical properties of the acetylated RHF compared to the mercerized RHF. It is presumable that a linear relationship between UTS and pressure variation with a positive gradient would be observed for each sample. Non-conformity in this work to this presumed linearity may be due to the manual mixing method used. This may have resulted in pockets of agglomerated RHF within the composite body. The consequences of such agglomerated sites of RHF are poor load transfer and high stress concentration, which early component failure could be predicated upon. IV.
Elongation percent
11.00
2.0
Percentage Reduction in CSA 0.63
ARH (10) ARH (15) AVERAGE
11.85 12.98 11.94
1.8 1.6 1.8
0.63 0.63 0.63
MRH (5) MRH (10) MRH (15) AVERAGE
14.18 14.31 14.52 14.34
3.0 2.2 1.8 2.3
0.67 0.62 0.67 0.65
CRH (5) CRH (10) CRH (15) AVERAGE
11.23 11.44 12.94 11.87
2.0 1.8 1.6 1.8
0.67 0.67 0.67 0.67
A. ARH (5)
ARH (10) ARH (15)
96.0 98.0
MRH (5) MRH (10) MRH (15)
89.5 92.5 103.5
CRH (5) CRH (10) CRH (15)
86.0 92.0 96.5
Table 3: Impact energy of various samples Sample Impact Code Energy (Pressure, (J/mm2) MPa) 5.0 C. ARH
(5) ARH (10) ARH (15) AVERAGE
4.5 4.0 4.5
MRH (5) MRH (10) MRH (15) AVERAGE
5.5 5.0 5.5 5.0
CRH (5) CRH (10) CRH (15) AVERAGE
4.5 4.0 3.5 4.0
TABLE 1: TENSILE TEST RESULTS UTS (N/mm2)
Sample Code (Pressure, MPa)
Table 2: Results of Hardness test Sample Code Rockwell (Pressure, Hardness (HR) MPa) ‘B’ Scale 89.3 B. ARH (5)
Note: ARH= Acetylated Rice Husk, MRH= Mercerized Rice Husk and CRH= Crude Rice Husk. Processing pressure (MPa) is enclosed in parenthesis. IV CONCLUSION The results of this work indicate that it is possible to improve the mechanical properties of biocomposites by varying processing techniques. Ultimate tensile strength increased with processing pressure, which was also the same trend observed for hardness of the composite. Composites formulated with the treated rice husk exhibited higher ultimate tensile strength than the untreated ones. The higher the processing pressure, the more brittle the composites became as indicated by the results of the impact energy tests. Further work is necessary to obtain optimal pressure that would yield improved mechanical properties of the composites.
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