Investigation on Mechanical and Wear Properties of Composites from Recycled Polymer for Gears with Optimized Compression Moulding

Abstract

Cost of a product can be visualized as sum of the cost of materials and process cost. In a competitive world, producing quality products at low cost is need of the day. In reducing the process cost, the use of off line techniques like Design for manufacturing (DFM), design of experiments (DOE), Six Sigma optimization and process modeling could be adopted. Further, judicious development of low cost materials, will help in bringing down the total cost of the product. In view that polymer consumption is growing at a fast pace, reusing post consumer polymers could help reducing the material cost component of the product. Recycling engineering plastic such as post consumed Polyethylene Terephthalate (PET) provides not only a cheap and abundantly available source of material but also expands the sphere of application for recycled PET (r-PET). However, owing to reduction in the properties due to recycling procedures, the plastic need to be developed suitably, to meet the requirements of an application. Reinforcing r-PET with suitable material could address this issue. Fly ash cenospheres are low cost material that could be useful in improving the properties of recycled polymers. The cost of the product developed from low cost recycled materials could be further reduced by developing suitable low cost process. Compression moulding could be a low cost process. The process however needs to be optimized for moulding product with appreciable quality. Thus the present study is focused on developing r-PET based composites with FA cenospheres as reinforcement and compression moulding as a manufacturing process in order to cater the requirements of an industry to produce low cost engineering components such as gears. In the process of optimization, Six Sigma based DMAIC/DMADV methodology along with Taguchi’s method and RSM are utilized where as development of r-PET/FAC composite is carried out using design of experiments (DOE). Entire work is envisaged in five stages. The first stage of the experimentation is carried out with an intention to establish a thermo-mechanical moulding process for r-PET. Six Sigma DMADV methodology is utilized along with Failure Modes and Effects Analysis (FMEA) for successive improvement in the moulding procedure. A reduction in risk priority number (RPN) from 900 to 315 and finally to 8 is achieved on successive improvement in the process using FMEA. At the end of its successful application, a good, repeatable sample quality is achieved. In the second stage, R-PET, reinforced with FAC is studied for a set of compression moulding process variables and material variables using DOE as statistical tool. Five factors, critical to quality (CTQs) viz. moulding pressure (5, 10, 15 MPa); moulding time (5, 10, 15 min.); mould cooling (water, air, water and air); moulding temperature (50, 100, 150 ˚C) and weight fraction of cenospheres (5, 10, 15%), are considered at three levels. The DOE methodology adapted for such investigations showed a down ward trend for FAC content. The cause investigated using fractographic analysis, concludes debonding of FAC from the matrix due to improper interfacial characteristics. Further, the composite underwent brittle fracture making it not much useful for gear applications. The remedy considered for developing r-PET composite that makes it suitable for the gear application is to blend the matrix with an appropriate recycled polymer and to improve interfacial interactions of FAC with matrix by suitably treating FAC with (3-Aminopropyl) trimethoxy silane (3APTMS). The wear property of the composite however, proves promising as FAC reduced Specific wear rate (SWR). The preliminary work in developing the matrix, in the third stage, involves blending rPET with five softer polymers from recycled regime. R-LLDPE, r-LDPE, r-HDPE, r-PP and r-Nylon are these five polymers. Experimentation of r-PET blends suitably selects rLDPE as better suited polymer owing to its flexural and wear properties. . Blending rPET with 10% r-LDPE improves the toughness by 100% and by 112% at 30%. This is followed by r-HDPE that shows 25% and 100% increase at respective composition. Flexural strength and SWR of r-PET/r-LDPE blends affected marginally whencompared to the plastics considered in this study. The next part of third stage involves developing the composite with matrix blended with r-LDPE (30% by wt.), reinforced with FAC (5, 10, 15% by wt). An improvement in the fracture strain, over 87 % is noted at 30% of r-LDPE and 15% FAC. An improvement in toughness by about 66 MPa at 5% FAC and 13 MPa at 30% of FAC is observed. Thus studies on matrix blending conclude that blending r-PET with r-LDPE helps in reducing the brittleness of r-PET. Further, 3APTMS (6, 8, 10% by wt.) is used for treating FAC and to improve the interface. Reinforcing r-PET with 3APTMS (10% by wt.) treated FAC (T-FAC) improved flexural strength of r-PET/T-FAC composite. An increase of 34% strength at 5% T-FAC, 57% increase at 10% and 120% improvement in strength at 15 % of T-FAC is observed owing to surface treatments given to FAC. Such an increase in the properties leads to improvement in the toughness of the composite. Toughness improves by 95% at 5% of T-FAC, 200% at 10% and an increase of 271% in toughness when r-PET is reinforced with 15% T-FAC is observed. Owing to blending and treating of reinforcement, flexural and wears properties improved significantly. Further M-r-PET/T-FAC composite is also tested for their properties. The results of M-r-PET/T-FAC composite conclude favorably for developing low cost material from recycled means. In the next stage, the process and material thus developed are optimised for flexural and wear properties. In the fourth stage the process and material parameters are optimized for improved properties of the composite. Six Sigma DMAIC optimisation tool, Analysis of variance (ANOVA), Response surface methodology (RSM) are used to determine the optimum values. The final optimum parameters for moulding r-PET reinforced with FAC are Moulding pressure – 11.2 MPa, 3APTMS -7.9 % by wt., r-LDPE - 29 % by wt., moulding Temperature - 52.6 ºC and FAC – 12.5%. Confirmation experiments for these optimum values are done to verify the validity of the process adopted. In the fifth and final stage the material developed in the previous stages is moulded into gears with optimized compression moulding and their performance is evaluated on an indigenously designed and fabricated gear test rig. The increase in the gear life by about275% w. r .t the starting composite (r-PET/FAC) seems good for the applications sought for them. The gear can handle a load of about 30.5 N and can take about 50,000 revolutions. With such an improvement shown by the composite material developed in this work, it could be considered as an alternative to the existing gears made from neat polymers for lower loading applications. Thus the objectives set for this research work that to develop a low cost composite material from environment hazardous waste materials with optimized compression moulding for gear applications are met with the systematic application of Six Sigma methodologies as explained in this work.