A cleaner production of rice husk-blended polypropylene eco-composite by gas-assisted injection moulding

A cleaner production of rice husk-blended polypropylene eco-composite by gas-assisted injection moulding

Journal of Cleaner Production 67 (2014) 277e284 Contents lists available at ScienceDirect Journal of Cleaner Production journal homepage: www.elsevi...

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Journal of Cleaner Production 67 (2014) 277e284

Contents lists available at ScienceDirect

Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

A cleaner production of rice husk-blended polypropylene ecocomposite by gas-assisted injection moulding R.C.M. Yam*, D.M.T. Mak Department of System Engineering and Engineering Management, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong

a r t i c l e i n f o

a b s t r a c t

Article history: Received 29 May 2013 Received in revised form 13 December 2013 Accepted 15 December 2013 Available online 22 December 2013

Gas-assisted injection moulding processes are widely applied in the plastics industry to improve moulding quality by using fewer polymers. This process is achieved by hollowing out the internal section of the mould to reduce the usage for conventional polymers and is particularly good for thick, moulded products. The eco-composites polymers, such as rice husk-filled polymers are environmental friendly materials to replace polymers. A cleaner production can be achieved by using rice husk-blended polypropylene RH/PP eco-composite in the gas-assisted injection moulding process. No successful case is reported in literature, as the increased shear viscosity of the non-petrochemical and natural-based polymers make it difficult for the eco-composite to flow inside the moulds. In this study, gas-assisted injection moulding technology was successfully applied to the rice husk-filled polypropylene ecocomposites polymers in different compositions. Different stages of injection pressure and delays of gas pressure were applied in order to improve the flow characteristics. In addition, the internal wall surface of the mould must be polished to ‘Mold-Tech’ SPI A2, an industry standard textured finishes. The new approach uses fewer petrochemical polymers with improved moulding quality, especially for thick, moulded parts. The new method is also an environmentally-friendly approach as it uses less injection pressure and clamping force. This has created a good foundation for further research in cleaner production of different kinds of eco-composites material by gas-assisted injection moulding. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Eco-composite Rice husk Gas-assisted injection moulding Polypropylene

1. Introduction The continued consumption of petrochemical-based polymers together with the unappropriated disposal of the huge amount of rice husk, mostly by burning, have caused significant concerns to the environment (Bevilaqua et al., 2013). Thus, there is an immediate need to develop environmentally-friendly eco-composite materials through innovative moulding processes to reduce the demand on conventional, petrochemical-based polymers. However, rice husk-blended polypropylene eco-composite, has several weaknesses including: low melt-flow indexes, which do not flow effectively inside the moulds in conventional injection moulding, weak interfacing bonding between the hydrophilic natural filler and hydrophobic polymer (Fung et al., 2002) and increased shear viscosity with an increase in natural and agricultural filler content (Fung et al., 2003), such as rice husks, and the relatively lower thermal degradation temperature of natural fillers compared with

* Corresponding author. E-mail address: [email protected] (R.C.M. Yam). 0959-6526/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jclepro.2013.12.038

polymer resins (Albano et al., 1999). The worldwide annual output of rice husk is 800,000 million tons in which 50% are coming from China (Liu et al., 2012). Rice husks are inexpensive, biodegradable, environmentally friendly. The main component of the rice husk is silica (52%) (Liu et al., 2012) which has made rice husk possess good mechanical and fire retardancy properties (Zhao et al., 2009). Furthermore, rice husks are natural waste materials that need to be disposed of without causing damage to the environment. Hence this natural filler has attracted significant research interest for ecocomposite developments (Zhao et al., 2008). While there are a number of findings and research reports on the injection moulding process, studies on gas-assisted injection moulding of ecocomposite are rarely reported. Serrano (Serrano et al., 2013) reports the cleaner use of the old recycled newspaper to replace glass fibres by conventional injection moulding process. The major focus of Serrano’s work is on old newspaper through the convention method which is different from the rice husk and the gas-assisted process used in the current study. Gas-assisted injection moulding process is a cleaner production moulding technology. The principle of gas-assisted injection moulding is to hollow out the internal, thick sectional area by

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injecting gas at high pressure during injection moulding to further reduce the use of polymers than the conventional method. The gas forces the molten resin forward and then holds it at high pressure during cooling. The resin begins to shrink, thus leaving a hollow section inside the moulded parts. The process is capable of introducing empty space. This is basically an injection moulding process in which the moulded part is first partly filled with molten polymer and then gas is injected into the cavities by filling up 75%e98% of the space through gas pins or nozzles designed and positioned separately from the injection gating of the mould. After a short time delay, compressed nitrogen hollows the internal section of the molten polymeric material, thus pushing the material towards the inner wall surface of the cavity of the mould. Based on the heat exchange through the mould and circulating water flowing through the cooling channels inside the moulds, heat is transferred from the molten plastic to the mould, the resin nearest to the mould surface is actually cooler than the inner section, and the injected gas stays in the middle section of the plastic forming cavities. The conventional injection moulding technology has difficulties to mould thick products causing defects such as warping and sink marks, which are basically the result of shrinkage. Over the years, gas-assisted injection moulding research focused on filling simulations (Chen et al., 2008), gas channel design (Marcilla et al., 2006), as well as faster cooling (Gao et al., 1997). Research also compared the advantages of gas-assisted injection moulding over the conventional moulding processes in terms of material usage (Parvez et al., 2002) and moulding quality (Castany et al., 2003), e.g. improving warping and sink marks. However, all these previous studies on gas-assisted injection moulding were based on conventional plastic resins (Chen et al., 1996). This study is the first attempt to extend the gas-assisted injection moulding technology to the eco-composite material. Farmers commonly burn agricultural waste, such as rice husks, and this has adversely polluted the environment. Collecting agriculture wastes and blending them with petrochemical polymers in different compositions to produce moulded parts for various industries, can make the environment cleaner. Several gas-assisted injection moulding methods were involved in this study: SHORT SHOT PROCESS is a standard, internal gas pressure process in which the cavity is pre-filled partially with molten plastic, and then the gas is injected into the cavity of the mould, displacing the melt until the cavity is completely filled. The gas pressure is then maintained as the holding process. FULL SHOT PROCESS, also known as the shrinkage compensation process, is a process in which the cavity of the mould is completely filled with molten plastic. The gas is then injected as the holding process to prevent shrinkage. The gas inside the cavity forms gas channels in the moulded section to provide holding pressure during the cooling cycle, and the plastic begins to solidify. Meanwhile, the gas pressure inhibits sink marks and the gas releases through a valve, thus relieving gas pressure before the mould opens. In the OVER FLOW PROCESS, the cavity is filled completely with molten plastic and then holding pressure is applied to the melt. The gas is then injected into the cavity, displacing the plastic from the cavity into the overflow. The gas pressure is maintained as the holding pressure throughout the entire cooling cycle in order to inhibit shrinkage. The gas pressure is then released before the mould opens. There are many advantages of gas-assisted injection moulding technology over the conventional approach. Some of them are listed as follows (www.ua.es):  For thick products, the gas-assisted approach only requires one moulding process instead of two in the conventional approach. This will reduce energy consumption.

 Due to the packing out of the pressured gas, lower injection pressure is needed;  Less warping and shrinkage leading to less sink marks  Reduced cycle time; and  High strength to weight ratio There are different cleaner production processes for rice husk reported in literature recently. Liu (Liu et al., 2012) reports the simultaneous production of silica and activated carbon from rice husk ash through a simple, environmental-friendly and economical-effective synthetic procedure. This has provided an alternative means to use the rice husk through an enhanced green, less toxic and sustainable chemical process. Liu has found that substantial amount of silica could be extracted from the rice husk ash. This is why the rice husk eco-composite in our study has good mechanical and fire retardancy properties. Bevilaqua (Bevilaqua et al., 2013) suggests another method to use the residual rice husk for producing levulinic acid which is important for the pharmaceutical and food industries. At present, levulinic acid is mostly produced via an environmentally unfriendly petrochemical synthetic route from fossil oil. Replacing it by a renewable biomass, i,e rice husk, through a cleaner production could save the use of the fossil resources. These two studies emphasize the importance of the clean use of the rice husk by different cleaner production methods. The current study supplements their findings by using the gas-assisted injection moulding process. 2. Experimental 2.1. Materials and preparation of composites Rice husk particles (in the form of powder: 80 mesh size) was selected and purchased from Tengzhou City, Shandong Province, People’s Republic of China (PRC), Fig. 1a. Smaller particle sizes of rice husk were selected to minimize air bubble defects in the ecocomposites (Mullin, 1993). Mesh sizes 80 to 100 are the appropriate particle sizes for blending. Before blending with PP, the selected rice husks were filtered through a Mesh 80 Filter to make sure they were close to an 80-mesh size. 2.2. Preparation of eco-composites Polypropylene (PP), commercial product code PP PP332K from Samsung, with a melt flow index of 5g/10 min was purchased. Coupling agents and additives, such as inorganic fillers, maleic anhydride grafted polypropylene, dispersion oil and pigments (provided by Plastique Aveu Maize Limited) were mixed together. The cell wall structure of agricultural wastes, such as rice husks, contains many micropores. These micropores trap moisture that could cause manufacturing defects in the ecocomposites, such as interfacial failure and air pockets (Allan et al., 1991). In order to remove moisture, rice husk particles were pre-dried in a 50 kg (kg) hopper at 110  C for 4 h. Applying the design of experiment approach (Douglas, 2005), the pre-dried husks were mixed with PP in various ratios of 10%, 20% and 30%, i.e., with a 10% increase in rice husk content for each composition. In order to determine the appropriate ratio of coupling agents, different weight percentage of coupling agent and a fixed ratio of various additives (recommended by suppliers) were mixed with the rice husks and PP accordingly, until quality pellets were produced. Tables 1e3 show the designation of coupling agent, additives and polypropylene for different weight percentage of rice husks content.

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2.4. Gas-assisted injection moulding

Fig. 1. a) Rice husk particles (mesh size 80) from Shandong Province, China. b). Pellets of rice husk-blended polypropylene eco-composite.

The eco-composites were blended successfully in various ratios of 10/85, 20/71.9 and 30/58.8, as shown in Table 4. The mixture was fed into the twin-screw extrusion machine Model Number: SHJ-6 (L/D, length to diameter, ratio of 32/52 and screw diameter of 62.40 mm). Based on recommendations from the PP supplier, each temperature control zone was set between 185 and 195  C. The screw speed of the extrusion machine was then set at 500 revolutions per minute (RPM). As suggested by the extrusion machine manufacturer, the same extrusion die, with a hole diameter of 2.00 mm, was used to prepare the eco-composites (rice husk-filled PP). The same rotational speed of the cutting blade (30 RPM) was used. Batches of RH/PP eco-composites were then prepared as shown in Fig. 1b.

2.3. Preparation of tensile bars The greatest weight percentage of rice husk blended ecocomposites (RH/PP-30/58.8) was chosen to prepare the tensile bars as per Fig. 2 for further material properties measurement.

The blended composites were injection moulded in a 100-ton Battenfeld Injection Moulding Machine, Model Number HM1000/ 525S. A gas-assisted modular unit, a Wittman Air-module control system, was connected to a nitrogen gas cylinder and installed on the injection moulding machine. The Air-module control Unit was then connected to the B4 Controller of the moulding machine. The Air-module control system manufactured by Wittman Battenfeld was selected and used with gas-assisted injection moulding for this research. The operating pressure for this system only works in the gas pressure range between 0 and 350 bars. Battenfeld recommends that the initial pressure be set at 0e50 bars for the initial ramp time and held at 50 bars for the entire study. As a result, incomplete filling occurred due to over-packing from the gas valve pressure. The gas pressure was reduced by 5 bars per setting. Quality moulded specimens were obtained. Previous research on gas-assisted injection moulding revealed that there is an inherent relationship between gas process variables and gas channel geometries. There are some useful design guidelines for the gas valve locations and it is recommended to position the gas valve at the gating area (Parvez et al., 2002). It is also recommended that the gate should be located at one end for any circular geometries (Ehritt and Schroder, 1998). Moulded specimens (dimension 25.4 mm in diameter and 150 mm in length) were designed, and a single cavity mould was fabricated. By applying the full-shot process, the barrel temperature was set in the range from 185 to 195  C. The injection pressure was then set on three stages between 1000 and 1200 bars. Mould temperature was maintained at 50  C. Without turning on the air release valve, the air-mould modular unit produced solid, circular, moulded specimens (Fig. 3). Without switching on the gas release valve, solid specimens without any hollow internal sections were successfully obtained. The composites were further injection-moulded with the gasassisted injection moulding valve turned on. The aim of preparing solid specimens was to compare how many hollow sections could be obtained and the amount of material saved between gasassisted, moulded specimens. When applying the short-shot process, the gas valve was turned on, but the RH/PP composites experienced moulding difficulties in gas filling and incompletely filled specimens were obtained (Fig. 4a). This was primarily because the gas pressure did completely fill the cavities. When applying the short-shot process, only 80% of the molten eco-composites filled the cavities. The gas pressure was actually set at 45 bars with a holding pressure time of 10 s. As a result, the pressured gas did not fill up the internal cavities of the specimen properly and the gas pin valve was blocked after four to five shots. In addition, the molten RH/PP was pushed backwards resulting incomplete filling during the first experiment. Consequently, the gas pressure was reduced to 40 bars and the holding time was

Table 1 Designation of coupling agent for 10% rice husks content. Rice husk weight percentage (%)

Inorganic fillers weight percentage (%)

Dispersion oil weight percentage (%)

Additives weight percentage (%)

Coupling agent (maleic anhydride grafted polypropylene weight percentage (%)

PP Code:332K weight percentage (%)

10 10 10 10 10

0 0 0 0 0

0.2 0.2 0.2 0.2 0.2

0.8 0.8 0.8 0.8 0.8

0 1 2 3 4

89 88 87 86 85

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Table 2 Designation of coupling agent for 20% rice husks content. Rice husk weight percentage (%)

Inorganic filler weight percentage (%)

Dispersion oil weight percentage (%)

Additives weight percentage (%)

Coupling agent (maleic anhydride grafted polypropylene weight percentage (%)

PP Code:332K weight percentage (%)

20 20 20 20 20 20 20

1 1 1 1 1 1 1

0.2 0.2 0.2 0.2 0.2 0.2 0.2

0.9 0.9 0.9 0.9 0.9 0.9 0.9

0 1 2 3 4 5 6

77.9 76.9 75.9 74.9 73.9 72.9 71.9

Table 3 Designation of coupling agent for 30% rice husks content. Rice husk weight percentage (%)

Inorganic filler weight percentage (%)

Dispersion oil weight percentage (%)

Additive weight percentage (%)

Coupling agent (maleic anhydride grafted polypropylene weight percentage (%)

PP Code:332K weight percentage (%)

30 30 30 30 30 30 30 30 30

2 2 2 2 2 2 2 2 2

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

1 1 1 1 1 1 1 1 1

0 1 2 3 4 5 6 7 8

66.8 65.8 64.8 63.8 62.8 61.8 60.8 59.8 58.8

Table 4 Composition of rice husk-blended polypropylene. Sample batch

Rice husk (wt.%)

PP (wt.%)

Inorganic fillers (wt.%)

Maleic anhydride grafted polypropylene (wt.%)

Dispersion oil (wt.%)

Additives (wt.%)

RH/PP-10/85 RH/PP-20/71.9 RH/PP-30/58.8

10 20 30

85 71.9 58.8

0 1 2

4 6 8

0.2 0.2 0.2

0.8 0.9 1.0

increased to 15 s. There was no breakage. Again, the cavities did not completely fill (Fig. 4b). The outcome indicated that the injection gas pin needed to be relocated and the specimen geometry needed to be modified in order to improve the filling, and the gas pressure inside the mould cavity needed to be released.

2.5. Modification

Fig. 2. Dimensions of specimens for the mechanical properties test according to ASTM D638 and ASTM D790.

Fig. 3. Specimen of rice husk-blended polypropylene (10% rice husks and 85% PP) without gas-assisted injection moulding valve opened.

After the first round of study, no successful moulded specimens were obtained due to over-packing of gas pressure. The equipment manufacturer (Battenfeld) recommended the addition of over-flow

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Fig. 6. Schematic showing the modified mould cavity design for successful GAIM of the rice husk blended polypropylene eco-composites.

2.6. Moulding and process parameters

Fig. 4. a) Improper filling of gas inside the cavity for specimen of RH/PP-10/85. b) Defects observed from gas-assisted injection moulded specimens.

channels and a relocation of gas valve. The gas valve was then relocated 2.00 mm away from the sprue area. The specimen was modified to have a diameter of 25.50 mm and a length of 75.00 mm. The gas valve was relocated by shifting it 2.00 mm (as indicated in Figs. 5 and 6) to the runner to eliminate over-pressure. Additionally, a cold well was added to the end of the mould cavity to allow for gas pressure and eco-composite overflow. New specimens were prepared.

Fig. 5. Schematic showing the runner, gating and moulding for the GAIM before modification.

After the tooling modification and the length reduction, the airmould module unit was re-connected to both the nitrogen gas cylinder and the test mould. Moulding parameters were set and selected based on recommendations by the machine and material manufacturers. The second round of experiments was first initiated without turning on the air module unit for the rice husk-filled polypropylene (PP). The mould temperature was maintained at 50  C. Solid, moulded specimens without any hollow sections were successfully acquired and they were weighed for further comparison with the gas-assisted, moulded specimens. The gas valve was turned on, with the input gas pressure at 45 bars. Mould temperature was maintained at 50  C. The time delay for the gas-assisted pressure was set to 2.5 s with 10 s holding time using the full-shot process. This resulted in a good, moulded specimen (Fig. 7). 2.7. Weight analysis for moulded specimens Five samples (circular-shaped specimens) of each ecocomposition were selected, and their average net weight (without

Fig. 7. All specimens were filled properly after modifications.

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sprue and runner) was compared with non-gas assisted and gasassisted specimens (Table 5).

Table 6 Properties of rice husk eco-composite, RH/PP-30/58.8 and the commercial polymer, Moplen EP332K (www.ides.com). Properties

Test method

Unit

Flexural modulus Tensile strength Impact resistance Elongation at break Melt mass-flow rate

ASTM ASTM ASTM ASTM ASTM

MPa 1859 MPa 27.4 J/m 26.5 % 7.1 g/10 min. 13.7

3. Results and discussion The aim of this study was to evaluate the feasibility of using gasassisted injection moulding techniques with the rice husk-filled eco-composites. Inspection of all the modified specimens revealed that all surfaces were good and no sink marks occurred on the outer surfaces. Material properties of the rice husk-blended polypropylene RH/ PP-30/58.8, i.e. the highest rice husk content of 30% in our study, were tested. Table 6 shows the comparison of the mechanical and physical properties between RH/PP-30/58.8 and one of the popular commercial-grade polypropylene copolymers, Moplen EP332K. Table 6 shows that the flexural modulus and the tensile strength for RH/PP-30/58.8 are both higher than the Moplen EP332K’s. However, the impact resistance of the eco-composite is lower than the EP332K’s due to the natural features of the rice husk and the ultra-high toughness property of EP332K. The impact resistance for some other general purpose commercial grade polypropylene copolymers, e.g. Generic PP Copolymer, is in the range of 8.0e140 J/ m for notched izod impact resistance (www.ides.com). RH/PP-30/ 58.8 is therefore still suitable to substitute many general purpose commercial grade PP co-polymers. In particular, for environmentally friendly products, e.g. accessories for mobile phone, electronic and computer products etc., the high impact resistance value is not that essential. The elongation at break for the eco-composite is only slightly lower than the EP332K’s. This would not affect much of the industrial and commercial applications. For the melt mass-flow rate, the eco-composite value of 13.7 g/10 min is higher than the commercial-grade copolymer’s. However, after adding appropriate quantity of additives, this will not affect much of the flow ability and manufacturability of the eco-composite. Furthermore, different from other Generic PP Copolymers and Moplen EP332K, the eco-composites material required additional pre-drying for 4-hours at 110  C. Otherwise, the moisture trapped inside the rice husk would cause moulding defects. Most of the commercial-grade polypropylene does not require this pre-drying before moulding. In summary, most of the physical and mechanical properties of the rice husk eco-composite (RH/PP-30/58.8) are comparable with the popular commercial-grade petrochemical-based polymer. The specimens were sectioned further into halves, and both outer and inner diameters were measured and compared. Fig. 8 shows the wall thickness differences among the compositions. For the RH/PP-10/85 sample, the inner diameter was in the regions of 13.90e14.20 mm and the outer diameter was at 25.40 mm (Fig. 9a). Fig. 9b shows that for the RH/PP-20/71.9 sample, the inner diameter was in the range of 13.90e14.50 mm with a consistent outer diameter at 25.40 mm. Fig. 9c indicates that the inner section for RH/PP-30/58.8 (30% rice husks, 58.8% PP and 11.2% additives) was inconsistently filled with eco-composites and this suggests that the increased rice husk

D790 D638 D256 D638 D1238

Eco-composite Commercialgrade EP332K 1130 24.5 150 9.0 5

content affected the airflow into the cavity, thus minimizing the formation of hollow sections inside the moulded specimen. Fig. 10 shows a magnified cross-sectional view for the ecocomposite of RH/PP-10/85 (10% rice husks, 85% PP and 5% additives) at the inner wall section, indicating that air pockets existed mostly in the inner wall. Furthermore, the weights of the non-gas-assisted and gasassisted moulded eco-composite specimens were compared in Table 5. The weight reduction ratio for RH/PP-10/85 was at 27.78%, while RH/PP-20/71.9 was at 28.48% and RH/PP-30/58.8 was at 30.18%. From the weight analysis, there is an average of 28.81% material saving by the gas-assisted injection moulding over the conventional approach. As far as the process, the mould was modified and an over-flow cavity was added. This helped the molten eco-composites from flowing backward and further pushed the eco-composites forward the gas pressure built up inside the cavity. With the application of ‘full-shot’ and ‘over-flow’ processes, molten eco-composites entered the cavity successfully. In addition, with a time delay of 2.5 s and a gas pressure of 10, the entire cycle consistently produced uniform moulded parts, and hollow sections for RH/PP-10/85 and 20/71.9 were acquired.

Table 5 Net weight comparison between non-air assisted and air assisted specimens. Compositions

No gas-assisted (net)

With gas-assisted (net)

Ratio-weight reduction

RH/PP-10/85 RH/PP-20/71.9 RH/PP-30/58.8

32.11 g 34.28 g 34.76 g

23.19 g 24.52 g 24.27 g

27.78% 28.48% 30.18%

Fig. 8. All specimens were sectioned into halves and thicknesses were measured and compared.

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Fig. 10. Magnified cross-sectional view for eco-composite with air pockets contains 10% rice husks, 85% PP and 5% additives.

prevention of sink marks for the moulding of thick parts, the use of fewer plastic resins in the moulding, lower mould clamping force and lower injection pressure are required. In this study, gasassisted technology actually reduced the packing force from the injection process, a 20% savings of energy. The result illustrates that fewer energy and materials are used in the GAIM process. Due to the particle-like morphology of rice husks, the resulting GAIM product will be more isotropic. Furthermore, rice husks are a natural waste material that needs to be disposed of without causing damage to the environment. Therefore, it is a natural filler which has attracted significant research interest for eco-composite developments. Results from this study can provide some guidelines for the future research and commercial applications of natural fibre-filled eco-composites. 4. Conclusions

Fig. 9. a) Cross-sectional view for eco-composite containing 10% rice husks, 85% PP and 5% additives. b) Cross-sectional view for eco-composite containing 20% rice husks, 71.9% PP and 8.1% additives. c) Cross-sectional view for eco-composite containing 30% rice husks, 58.8% PP, and 11.2% additives.

For the experiment, the third stage of pressure was only set to 1200 bars (instead of 1500 bars). Therefore, less injection pressure was used and less clamping force was required. Compared to the conventional injection process, 300 bars of pressure were reduced, thus 20% of the clamping force was saved during the entire process. This implied an energy savings of 20%. In this study, it is observed that both the 10/85 and 20/71.9 specimens had consistent, internal, hollow sections. The moulded parts from gas-assisted injection moulding technology proved to be more isotropic. The net weight analysis indicated that rice huskblended PP has an average of 70.85% aspect ratio between nongas-assisted and gas-assisted specimens. The results prove that it is possible to use fewer materials, resulting in a reduced cost. Gas-assisted injection moulding (GAIM) is a cleaner production technology that possesses the following unique features:

This study focused on the blending of agricultural waste (rice husks) with PP. Weight percentages up to 30% rice husks and 58.8% PP, was blended successfully and processed through gas-assisted injection moulding. To echo with the concern of the decrease of availability of fossil sources reported by Serrano (Serrano et al., 2013), this research has proposed means to reduce the reliance on petrochemical-based polymers by 30% through gas-assisted injection moulding. The use of 30% rice husk could also reduce the burning of the agricultural waste. This has enlightened the growing environmental concern of the huge quantities of rice husks being unsuitably disposed (Bevilaqua et al., 2013). The aggregate result of using less polymer and more rice husk will reduce the adverse impact to the environment. Table 5 shows that, because of the hollow section inside the moulded part, the weight ratio of the gas-assisted product is about two third (weight reduced by 27.78%e 30.18%) of the conventional injection moulded ones. This would further reduce the use of petro-chemical-based polymer. As discussed in section 3, with the lower mould clamping force and lower injection pressure used in the gas-assisted process, there is also a 20% saving in energy cost over the conventional process. The use of fewer petro-chemical polymer, more rice husk and the saving in energy and cost have both environmental and economic benefits to the manufacturers and the community. This study has shown that the gas-assisted injection moulding process for eco-composite is a much cleaner production technology with a significant environmental and economic achievement which has not been previously reported in literature.

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As stated in the introduction section, there are many major concerns of using natural fibre-filled eco-composites in conventional injection moulding process. All these problems were mostly resolved in this study through the gas-assisted injection moulding process. For instance, the weak interfacing bonding of the ecocomposite was overcome by adding appropriate additives to improve the shear viscosity and consistency during gas injection moulding. This has removed the major considerations preventing the use of natural materials, i.e. rice husk, in the gas-assisted injection moulding process. The success of the study would therefore attract additional research interest to further develop ecocomposite. This study has also confirmed that gas-assisted injection moulding is suitable to mould eco-composite products even with thick-wall sections. The successful application of eco-composites in gas-assisted injection moulding technology can reduce the adverse environmental impacts for the plastics industry. This cleaner production process is likely to become one of the greatest strategic choices for moulding companies to enhance their competitiveness. Introducing the eco-composite material to reduce the use of petrochemical polymers through the cleaner gas-assisted injection moulding production can offer an environmental friendly niche for the plastics industry. References Albano, C., Gonzalez, J., Ichazo, M., Kaiser, D., 1999. Thermal stability of blends of polyolefins and sisal fiber. Polym. Degrad. Stab. 66, 179e190. Allan, G.G., Ko, Y.C., Ritzenthaler, P., 1991. The microporosity of pulp. Tappi J. 74, 205. Bevilaqua, D.B., Rambo, M.K.D., Rizzetti, T.M., Cardoso, A.L., Martins, A.F., May 2013. Cleaner production: levulinic acid from rice husks. J. Clean. Prod. 47, 96e101. Castany, F.J., Serraller, F., Claveria, I., Javierre, C., 2003. Methodology in gas assisted moulding of plastics. J. Mater. Process. Technol. 143e144, 214e218.

Chen, S.C., Hsu, K.F., Hsu, K.S., 1996. Polymer melt flow and gas penetration in gasassisted injection moulding of a thin part with gas channel design. Int. J. Heat Mass Transfer 39, 2957e2968. Chen, L., Li, J., Zhou, H., Li, D., He, Z., Tang, Q., 2008. A study on gas-assisted injection moulding filling simulation based on surface model of a contained circle channel part. J. Mater. Process. Technol. 208, 90e98. Douglas, C.M., 2005. Design and Analysis of Experiments (Chapter 3), sixth ed. John Wiley & Sons, pp. 60e112 (Chapter 6), pp. 203e254. Ehritt, J., Schroder, K., 1998. Gas Injection and Two-component Injection Moulding. Praxis Huthig Battenfeld, pp. 8e47. Fung, K.L., Li, R.K.Y., Tjong, S.C., 2002. Interface modification on the properties of sisal fiber reinforced polypropylene composites. J. Appl. Polym. Sci. 85, 169e176. Fung, K.L., Xing, X.S., Li, R.K.Y., Tjong, S.C., Mai, Y.W., 2003. An investigation on the processing of sisal fiber reinforced polypropylene composites. J. Comp. Sci. Technol. 63, 1255e1258. Gao, D.M., Nguyen, K.T., Garcia-Region, A., Salloum, G., 1997. Optimization of the gas-assisted injection moulding process using multiple gas-injection systems. J. Mater. Process. Technol. 69, 282e288. http://www.ides.com/prospector. Liu, Y., Guo, Y., Wang, W.G.Z., Ma, Y., Wang, Z., September 2012. Simultaneous preparation of silica and activated carbon from rice husk ash. J. Clean. Prod. 32, 204e209. Marcilla, A., Odjo-Omoniyi, A., Ruiz-Femenia, R., Garcia-Quesada, J.C., 2006. Simulation of the gas-assisted injection moulding process using a mid-plane model of a contained-channel part. J. Mater. Process. Technol. 178, 350e357. Mullin, J.W., 1993. Crystallization, third ed. Butterworth-heinemann Limited, Oxford, England, p. 527. Parvez, M.A., Ong, N.S., Lam, Y.C., Tor, S.B., 2002. Gas-assisted injection moulding: the effects of process variables and gas channel geometry. J. Mater. Process. Technol. 121, 27e35. Serrano, A., Espinach, F.X., Tresserras, J., Pellicer, N., Alcala, M., Mutje, P., 2014. Study on the technical feasibility of replacing glass fibers by old newspaper recycled fibers as polypropylene reinforcement. J. Clean. Prod. 65, 489e496. http://www.ua.es/otri/es/areas/ttot/docs/moldeo-inyeccion_gas_ENGL.pdf. Zhao, Q., Tao, J., Yam, R.C.M., Mok, A.C.K., Li, R.K.Y., Song, C.J., 2008. Biodegradation behaviour of polycaprolactone/rice husk ecocomposites in simulated soil medium. Polym. Degrad. Stab. 93, 1571e1576. Zhao, Q., Zhang, B.Q., Quan, H., Yam, R.C.M., Yuen, R.K.K., Li, R.K.Y., 2009. Flame retardancy of rice husk-filled high-density polyethylene ecocomposites. J. Comp. Sci. Technol. 69, 2675e2681.