Assessment of Composite Waste Disposal in ... - ScienceDirect

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ScienceDirect Procedia Environmental Sciences 35 (2016) 563 – 570

International Conference on Solid Waste Management, 5IconSWM 2015

Assessment of Composite Waste Disposal in Aerospace Industries N. Vijaya,*, V Rajkumaraa, P Bhattacharjeeb* a b

Scientist D, DRDL.Hyderabad, India Scientist G, DRDL. Hyderabad, India

Abstract Composite materials are playing a vital role in aerospace industries due to its attractive thermal, mechanical and environmental properties. Especially, in aerospace applications where the weight factor is a concern, it offers good strength-toweight ratio over metals, thus making a viable alternative. Apart from this, it gives high fatigue strength, light weight, increased corrosion resistance, improved fire resistance and also provides an ability to manufacture complex shapes. The steady increase in the use of Glass fiber/ carbon fiber composites has brought tremendous changes in aerospace industries. The diversified application of composite materials motivated the scientists to use in different fields where its predominant properties have given value addition to the product. However, it generates waste composite material during manufacturing as well as end of life. The composites waste should be collected, segregated and safely disposed as per the environmental legislation available in this country. Further, the waste generated by aerospace (defence& space) industry is minimum compared to the composite waste generated by the commercial industries. Composite waste disposal is relatively new area in India which is necessary to discuss for protecting the environment. Hence, selection of suitable environmental friendly as well as cost effective composite disposal method is necessary at this stage for aero space industries. In this paper, an attempt has been made to assess the existing disposal methods in the world and suggest suitable disposal method which is applicable for aerospace Industries. © by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2016 2016Published The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility ofthe organizing committee of 5IconSWM 2015. Peer-review under responsibility of the organizing committee of 5IconSWM 2015 Keywords:Glass Fiber, Carbon Fiber, Composite waste, Co - Processing, Cement Kiln;

1.0 Introduction Composite structures have been developed and applied for military applications over 50 years. At present the aerospace industrial applications are rapidly progressing from metallic parts and structures to composite materials

* Corresponding author. E-mail address:[email protected]

1878-0296 © 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 5IconSWM 2015 doi:10.1016/j.proenv.2016.07.041

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N. Vijay et al. / Procedia Environmental Sciences 35 (2016) 563 – 570

due to its high strength to weight ratio. In general, three types of composite materials are developed and widely used in numerous kinds of engineering applications: polymer–matrix composites (PM), metal–matrix composites (MM), and ceramic–matrix composites (CM) (Yanget al.2011). According to the reinforcement types, composite materials can be classified into particulate composites, fiber reinforced composites, and structural composites. Two types of classifications are illustrated in Figure 1. For all types of composite materials, polymer–matrix is dominating the market, among which thermoset composites account for more than two thirds, however the thermoplastics composites are growing more rapidly in recent years. Fiber reinforced plastics (FRP) are lightweight, have high specific mechanical properties, are corrosion resistant, have long life cycles and are easy to manufacture in different shapes. For these attractive properties FRP are increasingly being used in structural components, transportation/automobile industry and sporting goods. Even though, composite materials are having excellent properties, it produces hazardous gases and solid during disposal.

Fig. 1. Types of composite materials

2.0 Hazards of Composite Waste Disposal The composite wastes are mostly disposed by incineration without knowing the hazardous nature of the ingredients of the composite material. The disposal process evolves toxic substances which may affect the living organism as well as the environment. Recent studies have identified a large number of hazardous chemicals that are adsorbed on particulates generated during incineration of material. Although the exact composition of chemical products is specific to the burning material, the spectrum of organic compounds includes nitrogenous aromatics, and phenolics. Several of these chemicals are known mutagens and carcinogens in animals, however, little is known about their toxicity when inhaled with particulate matter The toxic chemicals produced from the combustion of the organic resin from composites may be adsorbed on respirablefibers and enter the respiratory system with acute or chronic effects. Detailed toxicological studies are needed to assess the long-term health effects from exposure to single high dose of fibrous particulates and any synergistic interactions with the organic chemicals (Gupta 2009). Boeing also addressed another issue for recycling and disposal of carbon fiber composites coated with hexavalent chromium primer. These composites are coated with hexavalent chromium and can be classified as hazardous waste and thus may/should not be disposed on land due to possible leaching of the chrome into the ground. This makes the recycling of such coated composite more challenging. It is clear and important that

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recyclability or disposal of the current composite material is a burning issue and driving force for the development of the more recyclable replacement materials. The composite disposal issue imposes a requirement of complete life cycle assessment (LCA) of new technology as well as product which are in advantages in nature with the existing material / technology (George et al.2003). 3.0 Recycling Methodology Recycling of engineering materials will contribute to the sustainability and sustainable development of industrial processes. Nowadays, metals, glass, thermal plastics and many other engineering materials are recycled to a great extent. However, composite materials, as a special category of engineering materials have not yet been properly recycled (both for the matrix and for the reinforcement materials). This is mainly due to their inherent heterogeneous nature of the matrix and the reinforcement, leading to poor materials recyclability, in particular the thermoset based composites. The current and future waste management and environmental legislations require all engineering materials to be properly recovered and recycled, from end-of-life (EOL) products (Yang et al.2011). Recycling will eventually lead to resource and energy savings for production of re-enforcement and matrix materials. At present there are very limited commercial recycling operations for main stream composite materials, due to technological and economic constraints. Basic problem is the difficulty to liberate homogeneous particles from the composite material. Figure 2 shows the recycling methodology adopted in the waste disposal hierarchy. Composite recycling is hindered both by the fiber and other types of reinforcement, and by matrix or binders in particular the thermoset type. Because of these challenges, most of the recycling activities for composite materials are limited to the down recycling such as energy or fuel recovery with little materials recovery such as reinforcement fibers[ (Yang et al.2011). Extensive R&D activities have been conducted, and various technologies, yet to be commercialized, have been developed basically in three categories: mechanical recycling, thermal recycling, and chemical recycling. All three types of recycling methods have been widely investigated for thermoset matrix composites. Some of these recycling methods are used for energy recovery as well as fiber recovery. But, the fibers recovered from recycling process is not having original strength compared with the virgin material.

End of Life Products

Composite Recycling

Manufacturing Scrap

Fig. 2. Recycling Methodology

3.1 Mechanical Recycling: Mechanical recycling process starts with the size reduction of the composite scrap by low speed cutting or crushing (50-100mm). The size is then further reduced down to 10 mm to 50mm through a hammer mill or other high speed millings for fine grinding. Afterwards the fine particles of the waste composites are classified with cyclones and sieves to fiber-rich (coarser) and matrix-rich (finer) fractions. A recent research was published for

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investigation of the potential use of recycled glass fibre composite materials as a replacement for virgin reinforcing materials in new thermoset composites. The properties of these recyclatefibersto be weaker and have a poorer interface with the polyester matrix than the virgin glass fibers. Mechanical recycled fibers can be energy intensive and is only able to produce short milled fibers with poor mechanical properties used as filler reinforcement materials. 3.2 Thermal Recycling Thermal recycling of composites involves the processing at high temperatures. Thermal processing of the composite waste can include 3 types of operations: (a) Incineration or combustion for energy recovery only. (b) Combustion for fiber and filler recycling with energy recovery. (c) Pyrolysis with both fiber and fuel recovery. 3.2.1 Incineration Incineration is another option for composite disposal, but if the composite is carbon fiber . then it cannot be incinerated without taking the proper precautions, due to the potential release of small electrically conductive fibers in to the environment that, if not captured , can cause electrical interference issues. Incineration and combustion for energy recovery do not involve materials recovery, it is not classified as a recycling technology although the inorganic residues after combustion could be potentially used in the cement industry. However, this method is also considered as one of the composite waste disposal method. But, In thermal recycling methods, where the fluidised-bed recycling process has been mostly studied for both combustion and pyrolysis with promising perspectives. 3.2.2 Fluidized - bed combustion recycling process

Fig. 3. Schematic Layout of Fluidized Bed Process

Fluidized-bed recycling process developed at the University of Nottingham is used to combust the resin matrix as energy and to recover the glass or carbon fibers. At the University of Hamburg a fluidized-bed pyrolysis process is used to recover both reinforcement fibres and secondary fuels from the depolymerisation process. Fluidized-bed technology was investigated to recover the glass or carbon fibres, and the organic resins are used as energy source and the combustion heat is recovered through waste-heat recovery system [4]. Figure 4 illustrates the schematic layout of fluidized-bed recycling process. The composite scrap is firstly broken to 25 mm size before feeding into the fluidized-bed reactor operated with a sand-bed and preheated air. The reactor is operated at 450oC for polyester resin composites and up to 550 oC for epoxy resin composites. The recovered fibers are clean and have a mean length of 6–10 mm.


3.2.3 Pyrolysis Recycling Process Pyrolysis is a thermal decomposition of polymers or depolymerisation at high temperatures of 300–800oC in the absence of oxygen, allowing for the recovery of long, high modulus fibers. A higher temperature of 1000oC can be applied but the resulting fibre products will be more seriously degraded. It can be used for the treatment of polymers and polymer matrix composites. Figure 4 shows pyrolysis recycling process. In the case of polymer matrix composites, both the reinforcement fibre and the matrix materials are recovered in the pyrolysis.

Fig. 4.Pyrolyis Recycling Process

Fig. 5. Cross sectional view of Land fill

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3.3 Land Filling of Composite Materials Unlike thermoplastics thermosets cannot be reprocessed due to the formation of permanent cross links during curing; therefore, the waste generated from thermoset FRP ends up in landfills and is dangerous to environmental health. Figure 5 shows the cross sectional view of land fill site. Various measures are being taken worldwide by concerned authorities to limit the environmental impact of FRP waste which in turn escalates the landfill cost which motivates the industries to investigate alternative disposal methods. 3.3.1 Disadvantages of Land fill Landfills are no longer thought to be the best method of waste disposal; they not only use up vast amounts of land but also pose health hazards. Landfill gas (LFG) contains carbon dioxide, methane, volatile organic compounds (VOC’s), hazardous air pollutants (HAP’s) and odorous compounds that can adversely affect public health and the environment. These dangerous gases are vented slowly into the atmosphere and groundwater and are one of the main contributors to the greenhouse effect. Gases that are associated with landfills have been linked to cancer and respiratory diseases. Thermoset polymer resins often contain VOC’s and are non-biodegradable. The increase in production of these materials directly affects the environmental concerns associated with landfills 4.0 Composite Waste Disposal initiatives in Defence R&D Composites are extensively used in defence and space industries due to its inherent properties compared with metal. Composite materials are designed to provide mechanical strength, chemical resistance, and durability. The diversified application of composite materials motivated the scientists for the past 30 years to use in different fields where its predominant properties have given value addition to the product. In Defence R&D industries, composite materials are used in fabrication of rocket motor casings, igniter casings, insulation materials, re-entry components, nozzles, canisters and missile containers. However, it generates waste composite material during manufacturing as well as end of life. Due to the environmental legislations and to protect the earth from hazardous substance an attempt has been made to find out suitable method to dispose the composite material. The waste generated by aerospace (defence & space) industry is minimum compared to the composite waste generated by the commercial industries. Consequently, by definition they cannot be easily re-converted into their original raw materials. Co - Processing of composite waste through cement kiln route is one of the disposal method which can recover the energy and dispose the waste without effecting environment. 4.1 Co - Processing Co-Processing is the use of waste as raw material, as a source of energy, or both to replace natural mineral resources and fossil fuels such as coal, petroleum and gas (energy recovery) in industrial processes, mainly in energy intensive industries (EII) such as cement, lime, steel, glass, and power generation. Waste materials used for Co –processing are referred to as alternative fuels and raw materials (AFR). Co processing is having certain advantages which are mentioned below: x x x x x

Upgrades waste management within the waste hierarchy Reduces global environmental impacts Decreases (largely) the costs of waste management Creates regional job in waste collection, pretreatment, etc. Is a zero-emission technology

Recycling through co-processing in cement kilns has been technically and commercially demonstrated. Hence, recycling through co-processing in cement kilns is increasingly used for managing composite regrind because of its technological potential, environmental benefits and cost effectiveness (CPC 2010).


4.1.1 Incineration Vs Co Processing Incineration is one of the common method which is used to dispose any burning material. But, the incinerated gases should satisfy the environmental legislations. Apart from legal requirement, the cost of providing incinerator would depend on its capacity ranging from Rs 10 crores to 30 crores. Assuming disposal cost of Incinerable hazardous waste is about Rs. 16,000/- per MT, it may roughly be estimated that additionally about Rs. 640 crore / annum would be incurred in incinerating hazardous waste in our country. Besides, incinerator if not operated optimally may contribute to emissions including toxic gases. This coupled with resource conservation and reduced carbon emissions make a strong case for considering co-processing as a sound and better alternative for hazardous wastes disposal in general and Incinerable waste in particular. 4.1.2 Composite Disposal by Co-Processing Co processing is a combination of mechanical and thermal recycling method in which composites are shredded into pieces and fed into the cement kiln where the temperature is around 1200ºC to 1800ºC with residence period of 12-15 seconds. Figure 6 shows the CpProsessing through cement kiln. With this temperature the fibers are converted into ashes and mixes with clinkers. At the same time, the resin is converted into organic matter which supports the burning process (CPC 2010). The co-processing of hazardous substances in cement industry is much beneficial option, whereby composite wastes are not only destroyed at a higher temperature of around 1400ºC and longer residence time but its inorganic content gets fixed with the clinker apart from using the energy content of the wastes. Apart from this, no residues are left, which in case of incineration still requires to be land filled as incinerator ash. Further the acidic gases, if any generated during co-processing gets neutralized, since the raw material is alkaline in nature. Such phenomenon also reduces resource requirement such as coal and lime stone. Thus utilization of composite wastes for co-processing makes viable solution where the quantum of waste is less or high.

5.0 Conclusion Safe disposal of glass/carbon fiber reinforced thermosets has been studied extensively since many years. But finding out the suitable disposal/ recycling method for the waste composite material which is generated during manufacturing as well as end of life products is one of the difficult tasks till today. In this paper, we have discussed mechanical and thermal recycling methods are in detail. But every method is having their own merits and demerits. To set up mechanical recycling plant the amount of waste requirement should be continuous to run the plant as well as to produce products from the recyclates. Hence, mechanical recycling is not a viable solution if the amount of waste generated is minimum. In thermal recycling, incineration of composite is not considered as safe disposal due to hazardous gas evolved during burning of epoxy materials. To setup fluidized bed process, the infrastructure needs to be build and maintained which is not cost effective compared with the waste generated by aerospace R&D industries. Landfills are no longer thought to be the best method of waste disposal due to its environmental issues. Hence, Co processing through cement kiln is considered to be the best suitable method for the waste generated by aerospace industries.

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Fig. 6. Co Processing through Cement Kiln

Acknowledgements The authors would like to thank ACC limited for their initiative on Co-Processing in India to dispose the solid waste through the cement kiln. The authors would also like to acknowledge the effort taken by Central Pollution Control Board, Ministry of Environment & Forests, Govt. of India, PariveshBhawan East Arjun Nagar, Delhi, India for providing the legal guidelines to dispose the hazardous waste through cement kiln. The authors acknowledge to Director, DRDL for his constant encouragement and support to undertake this study. Reference: 1) 2) 3) 4) 5) 6) 7) 8)

Blazsó,M (2010), Pyrolysis for recycling waste composites, in: V. Goodship (Ed.), Management, Recycling and Reuse of Waste Composites, WP and CRC Press, Cambridge, UK, pp. 102–121. George Giulvezan (2003), Composite Recycling and Disposal An Environmental R&D Issue, Retrived from http://www.boeing.com.html. Guidelines of Co –Processing in Cement / Power/Steel Industry (2010)., Central Pollution Control Board, Ministry of Environment & Forests, Govt. of India, PariveshBhawan, East Arjun Nagar, Delhi, India – 110 032 Henshaw, J.M, W. Han, A.D. Owens (1996), An overview of recycling issues for composite materials, Journal of Thermoplastic Composite Materials pp.4-20. Job,s.( 2010), Composite recycling - summary of recent research and development, Materials KTN Report,. Retrived from http:// www.compositesuk.co.uk.html. Meenakshi Gupta (2009),Combustion of Composite Material Assessment &d Quantification of Hazards, DRDO Science Spectrum,DESIDOC, India, 27-30. Pickering,S.J.(2010), Thermal methods for recycling waste composites, in: V. Goodship (Ed.), Management, Recycling and Reuse of Waste Composites, WP and CRC Press, Cambridge, UK,. 65–101. Yongxiang, Y, Rob Boom, Brijan Irion, Derk - Jan Van Heerden, Pieter Kuiper, Hans De Wit ( 2011). Recycling of Composite Materials Chemical Engineering and Processing: Process Intensification Retrived from http:// www.elsevier.com/locate/cep.html