The effects of particle size and content on Morphology and Mechanical Properties of Rice Straw and Coal Fly Ash filled-Polypropylene Composites

Polypropylene is a superior type of polymer due to its properties including flexibility, semi-transparent appearance, easily dyed, heat-retaining and hydrophobia. The percentage of recycled polypropylene is minimal which cause environmental pollution. To overcome the environmental problems caused by polypropylene waste, it can be utilized to produce efficient and economically-added product such as thermal insulation composites. The paper is focused on obtaining the technique and operating condition of injection molding to manufacture composite from polypropylene matrix, the influence of composition and particle size of fillers on composite mechanical properties


Introduction
Polypropylene is a superior type of polymer due to its properties including flexibility, semitransparent appearance, easily dyed, heat-retaining and hydrophobia. Therefore, it is widely used for various products. The percentage of recycled polypropylene is minimal which cause environmental pollution.
To overcome the environmental problems caused by polypropylene waste, it can be utilized to produce efficient and economically-added product such as thermal insulation composites. The ideal insulation material has such characteristics as capable to withstand medium heat and rigidity. To obtain the stiffness and thermal insulation properties of polypropylene, it is necessary to add filler material (Grozdanov et al., 2006). In order to obtain composites with superior properties, including strong, lightweight, non-abrasive, able to withstand heat, cheap and environmentally friendly (Garkhail et.al, 2000 andSahu, 2014), Saranya (2014) studied the effect of filler levels (fly ash and fiber) on the insulation properties of polypropylene composites, without explanation of filler's particle size. Zhang (2011) states that smaller particle size of Al2O3 filler increases thermal insulation properties of HDPE (high density polyethylene) composites. Srisawat et al. (2009) uses silica (SiO2) as filler on neat polypropylene, which produces composite with better thermal stability due to strong adhesion between polypropylene and silica. The function of silica as a filler can be replaced by coal fly ash which has high silica content (40% to 60%), relatively low density, spherical particle shape with smooth surface, small particle size, evenly distributed internal stress on the matrix, and its low cost (Sreekanth et al., 2009). The paper is focused on obtaining the technique and operating condition of injection molding to manufacture composite from polypropylene matrix, the influence of composition and particle size of fillers on composite mechanical properties.

Materials
Neat Polypropylen Resin type HI10HO with specific gravity of 0.9 and melt flow rate of 10 g/10 min was obtained from PT Candra Asri Petrochemical, rice straw was obtained from rice field in Cimahi, West Java, coal bottom ash was obtained from textile industry in Cimahi, West Java. Xylene (technical grade) was obtained from a chemical store in Bandung (Bratachem), Indonesia.

Methods
Rice straw from the rice field was soaked in water for two days. Afterwards, it was dried with oven at 105oC and was ground to a certain size. The composites were manufactured with various of filler and matrix composition, i.e. fly ash content (10-20%), fine straw content (10-20%). The rice straw, coal fly ash, polypropylene at a certain proportion and xylen (5 times weight of PP ) were placed into a mixer and heated to the melting temperature of polypropylene (173°C), then the hot agglomerate were transferred to the shallow cast (2 mm deep) where hot agglomerate were cooled down to room temperature in the fume hood, and let the rest of xylene to evaporate.
The dried sheet of agglomerate was cut to 1-2 mm. The agglomerate flake was fed into the feed container of the injection molding apparatus, was heated to 170-190 ° C for 15 minutes, and molded with a pressure of 5 bars to form specimens of a certain shape and dimension according to the testing standard. Cooling of the specimen was done at room temperature, then the specimen was removed from the mold. Density measurement and tensile tests using ASTM E-8 standard were carried out for characterization of composites.

Result and Discussion
The injection molding is a combination of heating and pressing methods. The heating of the feed uses a belt heater where heat is passed through a jacket encasing the feed tube (conductive heat transfered through the tube wall). Compression of feed was carried out using hydraulic cylinder moved by air compressor, with maximum pressure of 6 bars.

Effect of Filler Composition on Tensile Strength of Composite
To study the effect of filler composition on composite tensile strength, the experiment was carried out by varying fillers composition (PP 60-100%, Straw 10% 20%, CFA 10-30%). The result of composite tensile test is presented in Figure 1. 0% on 10% v. filler loading. This phenomenon can be explained because small diameter particles have large surface area. On a large contact surface area the tensile strength is also great because of the efficient power transfer mechanism.
In a study conducted by Nath et al. (2009) on the composite of PP-CFA (without mentioning particle size), it was found that in the CFA fraction of 20%, 45% and 60% there was a continuous decline of tensile strength. Tensile strength tests performed at 50 ° C and 70 ° C showed an increase in composite tensile strength at a 20% of CFA loading. This can be explained because at elevated temperatures the thermoplastic polymer enhanced free volume, which being filled up by the CFA, and causing a better wetting between PP and CFA.
While Zhang et al.(2011) made a composite of high density poly ethylene (HDPE) with filler of Al2O3 with particle size of 0.1 μm to 10 μm, found that the size of the filler particles on the micrometer order gave a less strong bond, and reduction of the tensile strength was proportional to the increasement in particle size at the same volume fraction, i.e. 0.8%, 41.5%, and 53.7% for particles with a diameter of 0.5 μm, 4.7 μm and 10 μm for 30% of Al2O3 volume. The tensile strength of composites increases with the decrease of the alumina particle size. When the Al2O3 of 0,01 μm loading is 30 % vol, the tensile strength enhanced by 12 % higher than the neat HDPE. To study the effect of particle size on composite density, two composites of different CFA particle size was manufactured (Dp 45 μm and 0.75 mm).

Effect of Filler'sParticle Diameter on the Density of Composite
There was a difference in the density of the same CFA loading but different particle size, due to the difference in the density of different particle sizes of CFA (2.03 g/cm3 and 3.36 g/cm3 for Dp 0.75 mm the calculated density.

Figure 2. Effect of Particle Diameter and CFA Loading on Composite Density
This phenomenon indicates that the adhesion between CFA and PP is less strong, resulting in a cavity between the filler and the matrix and enlarge the composite volume.
In the study of Onuegbu et al. (2011) on polypropylene composites with filler of snail shell powder, it was found that the composite density decreases by the increase of the particle size of snail shell powder for a certain filler loading. This is due to the fine particle size of filler distributed more evenly within the matrix.

Effect of Molding Temperature on Composite Density
To study the effect of molding temperature on composite density, the molding temperature was varied for CFA: PP = 30% : 70%, with molding pressure of 5 bar, and the result was presented in Figure  3.  Figure 3 shows that an increase in molding temperature from 175oC to 195oC provided a higher composite density up to 9.07%. This indicates that the higher the heating temperatures enhanced melting process and decreased the viscosity of PP, resulting in a denser composite The resulted composite was a homogeneous product, obtained by polypropylene (PP) grafting method with xylene, optimum injection molding achieved at temperature of 195oC and pressure of 5 bars. Composites with CFA and rice straw fillers with particle size o obtained the best composition at PP: CFA: RS (w) = 6: 1: 3, with the smallest tensile strength drop of 36%, tensile strength of 2.24 kg / mm2 and thermal conductivity (calculation) of 0.1493 W / m.K.