Recycling of thermosets and their composites

CHAPTER

Recycling of thermosets and their composites

20 R. Morales Ibarra

Universidad Auto´noma de Nuevo Leo´n, San Nicola´s de los Garza, Mexico

20.1 INTRODUCTION Is recycling a viable and feasible option for thermosets and their composites? Short answer, yes, indeed, it is in fact the only option going forward on thermoset waste management. There is a clear need to develop a more sustainable waste management of thermoset materials. The market of thermoset materials is rapidly increasing, mainly due to the very specific features in design, manufacturing, mechanical properties, and high performance of thermoset composite materials. In that matter, there is a deeply permeated knowledge, even among materials engineering specialists, about the “null” recyclability of thermosets, which might be a result of an oversimplified interpretation of the arguments presented in materials science literature where, at the beginning of the thermoset definition and as a part of the definition itself, it mentioned the “difficulty of reprocessing of thermosets after cross-linking reaction” or about the “irreversibility of the cross-linking chemical reaction” or the worst of all “thermoset polymers are not recyclable”; none of these cases present the relativity of the asseverations. These arguments and their respective erroneous interpretations generate an incomplete knowledge of the subject as it poses little possibilities of technological exploration at engineering for the recycling of these materials. The objective of this analysis it is not to discredit the important input of basic materials science and engineering literature but to question ourselves about the approach that engineering society has toward waste management of thermoset materials and draw a baseline that at the current state of technology, it is not developed enough at industrial level. About thermoset polymers recyclability, it is an incontrovertible fact that there is the technical feasibility demonstrated in the effectiveness of the very diverse technologies revisited in this chapter. Indeed, some of the presented recycling methods have complex parameters and characteristics, but many of those conditions can be easily escalated to pilot plants and/or industrial levels with conditions reachable for the potential industries that in congruence with all of the recyclability factors such as economical offer and demand waste management logistics and legislation Thermosets. https://doi.org/10.1016/B978-0-08-101021-1.00020-4 # 2018 Elsevier Ltd. All rights reserved.

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regulating and incentivizing the social, economical, and political interests that would integrate the recycling of thermosets into a sustainable life cycle of materials. The technologies for recycling thermosets and thermoset composite materials have a level of readiness prepared for escalation that is thoroughly demonstrated in these pages. The European Union is leading in legislation practices, though the end-of-life directive is demanding 95% recyclability of materials in all types of vehicles; research has extensively prepared the best techniques for a sustainable waste management of thermoset materials. It is only a question of how fast the rest of the world adapts its own legislation, how fast do they transfer the technologies and best practices to generate new markets and new products.

20.2 RECYCLABILITY OF THERMOSETS AND THEIR COMPOSITES Recyclability is a concept that takes into account not only the properties of the material but also the whole set of factors that can promote the natural course of action of a potential recycling market. As a definition, it is the capability of a material to be recycled through its life cycle among the economical, technical, legislative, and waste management circumstances that integrate the material to the recycling industry. In Fig. 20.1, the factors affecting the recyclability of thermosets and their composites are shown. The waste management of thermosets must assure the feedstock, logistics, separation of materials, and technological deployments for recycling; from the economical factors, offer either from postconsumer or postindustrial and demand basically from industrial sectors where the use of recycled material can be validated, we can infer that a market must be in place for a realistic approach of an integrated waste management of thermosets in the life cycle of components such as investments are made and decision-making can have an tangible economical value for recycling plants and infrastructure; legislation is a factor that will have a major role in the rapidness of the assimilation of technology by regulation of practices either by subsidies or penalization.

20.2.1 WASTE MANAGEMENT HIERARCHY A model for waste management hierarchy is proposed in Fig. 20.2, integrating both the European and American models, based on the Directive 2008/98/EC on Waste Framework Directive and the American Framework for waste hierarchy. As a point of interest, this model includes the implications and technologies for each level of preference in the hierarchy. Disposal and landfill are the least preferred level of the waste management hierarchy in which there is no recovery of energy or materials; specifically for thermosets synthesized in a cross-linked molecular structure, this means that nonbiodegradable materials are being disposed that have a negative impact on the environment.

20.2 Recyclability of thermosets and their composites

Waste management

Reciclability of thermosets

Legislation

Economical

Technical

FIG. 20.1 Recyclability of thermosets and their composites.

Implications

Most preferred

Cleaner production and efficiency

Reduction

Reuse

Repairing, refurbishing, validation

Technology readiness, new products, validation, recovery of resins, recovery of fibrous fraction of composites

Technologies Lean manufacturing, sustainable thermosets and their composites

Design of components with end-of-life materials

Recycling

Fluidized bed, mechanical pulverization for use as fillers, solvothermal, hydrothermal

Recovery of energy

Recovery

Combustion

No recovery

Disposal

Landfill

Least preferred

FIG. 20.2 Waste management hierarchy model.

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Recovery is the next preferred level in the model; combustion technology is used with and without recovery of energy that in fact raises some environmental issues about emissions of toxic dioxins, CO2, and other greenhouse gases. Recycling of thermosets and their composites comes next in the proposed waste management model, and as it will be demonstrated throughout this chapter, recycling technologies have such an effectiveness translated to a readiness level that has been extensively researched and developed at laboratory scale, preparing the technology to be transferred to pilot plants at industry. At this point, it is possible to recover both the thermoset resin and fibrous fraction from composite materials, while validation of properties of these recycled polymers and reinforcements is the baseline for potential reuse in new components. Some recycling technologies are capable of also recovering energy while recovering reinforcements, that is, fluidized bed; some technologies are being developed with alternate and more efficient sources of power, that is, microwave-assisted pyrolysis; other technologies present very interesting ecofriendly deployments of science and technology for recovering fibers and depolymerized compounds, that is, hydrothermal and solvothermal supercritical fluids; some procedures have very interesting outputs, that is, chemical remotion of matrix from thermoset materials and composites that results in repolymerized epoxy resins with higher mechanical properties than those of the virgin resins; also, very interesting findings have been developed in the reuse of pulverized materials as fillers in new components. With reuse at the next preferred level of waste management, it is difficult to imagine products and components that have not being designed taking into account the possibility of refurbishing, repairing, and revalidating of components, but when it comes down to market and consumption of products, components are designed based on reliability engineering that has the downside of, if not in all cases, producing goods that reach end of life at barely over acceptable levels as perceived per consumers, reaching for high revenue without consideration of the environment. Consumer perception of reliability of products is a subject that raises concerns in several areas, mainly in awareness of anthropogenic impacts; in general, to date, thermoset materials and their composites are an increasing market, and what that means for the most preferred level of waste management, reduction, is that the challenges are also more complex at an increasing rate of consumption. Reduction implies also an efficient and cleaner management of production of thermosets and their composites. On the most positive side of reduction, research is being developed to synthesize more sustainable thermoset materials.

20.2.2 ECONOMICAL DRIVERS: THERMOSETS AND THEIR COMPOSITES MARKETS With the introduction of new high-performance thermosets and thermoset composite materials, for example, carbon fiber epoxy resin composites, the industry is increasing due to the great advantages that these materials have; in 2000, the European consumption of thermoset composites reached 106 t/year; in 2005, plastic production in Japan was over 6.1 million tons and more than 210 million tons worldwide, and in

20.2 Recyclability of thermosets and their composites

2008, the global demand of carbon fibers reached 20,000 t/year [1]. The carbon fiber composite production represents around 2130 t, while the glass fiber composite production represents 144,000 t solely in the United Kingdom, and it was estimated at approximately 1053 million tons in Europe in 2010 [2]. General numbers of increasing markets show that the impact of end of life of thermoset components will be driving both environmental concerns and economical opportunities for the development of potential thermoset and composites recycling industry.

20.2.3 TECHNICAL DRIVERS Most of the technical issues for a recycling industry of thermosets and their composites are not different than general industries; contamination of materials must be controlled in order to consolidate the expected properties of the recycled material; the logistics must be taken into account for a well-distributed supply chain; feedstock of waste material must be assured before the decision-making of infrastructure investment, but the one technical factor that affects the recycling industry in a major manner is separation of materials; nevertheless, present-day waste management practices can be applied for a successful separation of materials.

20.2.4 LEGISLATION DRIVERS It is only fair to say that the European Union is leading in the legislation drivers about environmental issues that can help develop the international legislation for recycling of thermosets and thermoset composite materials. Legislation in the states forming the European Union has been developed by international agreements based on the Environment Action Program, with the latest seventh EAP entered in force in January 2014. Each program is specific to a particular environmental issue. The EAPs are not legally binding agreements that help develop the legislation taken into account a global agenda. Some results of the EAPs are the European Directive 2008/98/EC [3] on Waste Framework Directive and the End-of-Life Vehicles Directive 2000/53/EC [4] that are the baseline for waste management on one of the most important industries worldwide, transportation. The focus of this legislation was at first to reduce waste and consider reuse and recycling and energy recovery; nevertheless, this applies to all components and materials used in vehicles. The original regulations set out targets for reuse and recycling as follows: No later than 1 January 2006, the reuse and recovery rate for all ELVs will be at least 85% (average weight per vehicle and year), and by no later than 1 January 2015, all ELVs will need to have a reuse and recovery rate of no less than 95%. As such, governments are capable of developing the recycling market through legislation. Penalizing can disincentivize irregular and nondesired practices, for example, landfill and combustion, increasing landfill and/or combustion taxes making economically unproductive encouraging recycling. Legislation can also

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incentivize through subsidies to the best recycling practices either supporting investments for recycling plants or taxes subsidies to recycling processes activating the potential recycling industry.

20.3 WASTE MANAGEMENT AND RECYCLING TECHNOLOGIES FOR THERMOSETS AND THEIR COMPOSITES The main objective of the technological development of recycling processes is the sustainable use of materials that addresses at the same time two factors: a reduced environmental impact of thermoset industry and the recovery of economical value from waste and scrap materials. Since the thermoset market is increasing and the recycling technology is being developed, it is more likely that the economical value of the recycled materials could be the main driver of the reduced environmental impact, which would be in fact the natural course of action from the point of view of industry and the market; therefore, thermosets recycling processes and technologies have to be ready to deliver high value recycled materials to be not only cost effective but to be implemented as a logical part of the production chain. Recycling technologies to this day have an approach to recycling thermosets and their composites with energy recovery by thermal methods breaking the chemical bonds while taking advantage of their caloric value; moreover, depolymerization and repolymerization have been developed by chemical methods using decomposition reactions; recovered materials can then be treated to repolymerize in new parts and components; also, mechanical methods are used to reduce the size of scrap and waste parts for a better handling/treatment in further recycling processes; furthermore, pulverized materials can be reused as fillers in new components; such technologies are presented in Fig. 20.3. A sustainable approach to thermoset materials recycling is related to energy and efficiency; the use of renewable sources of energy to power all recycling technologies and machinery is highly recommended since the whole scheme of recycling

Recycling technologies

Mechanical

Pulverization

Thermal

Combustion

Fluidized bed

Chemical

Pyrolisis

Chemical decomposition

FIG. 20.3 Recycling technologies for thermoset materials and their composites.

Supercritical fluids

20.3 Waste management and recycling technologies

is to reduce the environmental impact while taking advantage of the recovered materials. This can be achieved through the efficient use of alternative and renewable means of energy. Important efforts have been made in researching and communicating the whole panorama of thermosets and thermoset composite materials recycling [2,5–10], and there seems to be a consensus about the importance of recycling thermosets and their composites reducing the negative environmental impacts while proposing recycling models driven by legislation and a potential industry of recycled materials; there is also relevant information about the technological readiness and optimization [11] for implementation of these technologies for pilot plants and industrial levels integrated in the life cycle of the composites industry that can be defined in terms of energy [12] of the lifetime of thermosets and their composites. The main difference between sole thermoset recycling technologies and those used to recycle composite materials is the output that in the case of composite materials is necessary in separation of phases and recovery of the solid reinforcement fraction of the composite material, which in some cases, like carbon fiber reinforcement, is the most valuable fraction of the composite. The same technologies can be used then to treat alike thermosets and thermoset composite materials. In this part of the chapter, a discussion and comparison of general recycling methods for thermosets and their composites is presented. Thermal, chemical, and mechanical recycling technologies are the most extensively used to this day for thermosets and their composites; those recycling technologies are also the most promising routes for a sustainable way of using thermoset materials.

20.3.1 MECHANICAL: PULVERIZATION Mechanical technologies can easily handle a wide range of thermosets and thermoset composite materials. Mechanical technologies can be used as pretreatment or preparation of waste materials to be recycled by any other technologies or even for disposal. Furthermore, the output of pulverization of waste materials can be used as fillers or reinforcements for new components. Mechanical recycling processing includes several types of shredders and granulators, each, designed to process different sizes of inputs and reducing the size and changing the morphology of the output. Some first stages of shredding can reduce size of entire components to smaller particles varying up to 50 mm. Some of the most important factors in shredders are blade quantity, blade size, and torque that are also related to energy and efficiency. Very specific endeavors have been developed in research about energy and efficiency for mechanical recycling of thermoset glass fiber-reinforced materials [13] and mechanical recycling of carbon fiber composite materials [14]. In next stages of mechanical recycling, granulators and high-performance blenders can further reduce the input to flakes and powders, depending basically on the design of screen separators and operating speeds. The desired output size is also related to the energy necessary to process the materials that must be observed for economical and ecological objectives.

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20.3 Waste management and recycling technologies

(A)

(B)

FIG. 20.5 Polymeric concrete with addition of pulverized fiberglass-polyester materials recovered from end-of-life components. (A) Polymeric concrete preparation with polyester resin, silica sand, calcium carbonate, and recycled powder obtained from end-of-life pulverized fiberglasspolyester composites. (B) Cured polymeric concrete samples for flexural and compression tests [16].

60 50

Avg 1

Stress(MPa)

Avg 2 40

Avg 3 Avg 4

30

Avg 5 Avg 6

20

Avg 7 Avg 8

10 0 0

0.02

0.04 Strain (mm/mm)

0.06

0.08

FIG. 20.6 Stress/strain behavior of polymeric concrete with the addition of pulverized waste of fiberglass-polyester composite materials as filler [16].

logistics. As mentioned before, mechanical processing delivers the interesting option of developing new products using the powders and recovered fibers into new extruded or molded parts; nevertheless, it must be taken into account that even when this kind of processing is relatively simple, the recovered shredded materials and

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reinforcement fibers are degraded through this treatment, and depending on the market on which the new products are to be offered, verification of properties could be mandatory on new components that can also be an issue if high mechanical performance is desired.

20.3.2 COMBUSTION Combustion is one of the thermal technologies for recycling that has been implemented to industrial level mainly for the simplicity of the technology. From a sustainable point of view, combustion is the least preferred technology that can be used for recycling thermosets and their composites. Incineration does not recover materials for reuse, and of course, emission of CO2 and other greenhouse gases must be addressed as they are the cause of climate change due to global warming; there is also a major environmental concern about emissions when it comes to waste feed of composites that have been treated to be fire retardant, since brominated and other toxic fireretardant chemical compounds have been identified to be produced in thermal decomposition of these thermosets and composites. This is a concern that is also implied in other thermal recycling technologies. Thermoset polymers have a calorific value that can be recovered as a source of energy for industrial processes. Basically, most of thermoset resins have a calorific value of approximately 30,000 kJ/kg [9]. As can be seen in Fig. 20.7, the calorific value of thermoset materials and composites is reduced when there is an addition of fillers and reinforcements and further reduced with the increase of proportion of incombustible materials such as reinforcement fibers and calcium carbonate; both 35000 30000 Calorific value [kJ/kg]

648

25000 20000 15000 10000 5000 0 0

20 40 60 80 Proportion of incombustible materials [%]

FIG. 20.7 Calorific value of thermoset composites [9].

100

20.3 Waste management and recycling technologies

of them are largely used in the thermosets and their composite components; structural composite materials used in automotive, aeronautical, and related industries can reach up to more than 50% in volume fraction; moreover, fire retardants are added in many components that can also absorb a small amount of energy, resulting in a reduced amount of energy that can be effectively recovered with this treatment. Combustion is a technology largely used in developing countries that have a lack of regulation on CO2 and other greenhouse and toxic emissions; international agreements such as the Kyoto Protocol and the Paris Agreement of the COP 21 are rising awareness in such practices since regulation is coming from a bottom-up perspective in which each nation proposes its own collaboration to reach the goals of reduction in emission of the mentioned greenhouse and toxic gases.

20.3.3 FLUIDIZED BED If reconsidered with the addition of sustainable technologies, fluidized bed is an interesting deployment orientated for thermoset composite material recycling that combines the advantages of recovering the calorific value of the resin while recovering the fibrous fraction of the material, that is, carbon fibers recovered from carbon-epoxy systems. Some of the most important advantages of the use of a fluidized-bed recycling process are the recovery of clean fibers, and if this recycling technology is controlled in detail, the degradation of the mechanical properties of the recovered fibers can be reduced optimizing the overall parameters of the process [17]. In Fig. 20.8, the schematics of a fluidized-bed recycling process is shown; the basic process is as follows: In a fluidized bed of silica sand, preheated air is injected through, with temperatures between 450°C and 550°C depending on the type of resin Clean flue gas

Scrap FRP Afterburner

Cyclone (to separate fibres)

Induced draught fan Recovered fibres Air preheating elements Air inlet

FIG. 20.8 Schematics of fluidized-bed recycling process [9].

Fluidised bed Air distributor plate

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that will be treated since polyester resins decompose at much lower temperatures than epoxy resins; then, scrap and/or waste composite materials, which have been in many cases pretreated by mechanical processing for better handling, are distributed to the fluidized bed; once the polymer is oxidized they are carried up by the gas and air stream; next in the process, a cyclone or any other gas-solid separation device is used to recover the fibrous fraction of the composite. Non-fully oxidized gases and polymers can then be burned in a secondary combustion chamber and their calorific value recovered as energy. Further optimization can be applied for separation of fibrous fraction from thermoset composite materials [18]. In a successful application of the fluidized-bed recycling process [19] at a temperature of 450°C and air injected at a velocity of 1.3 m/s, short glass fibers were recovered with an average length of up to 5 mm; tensile strength of the recovered fibers resulted in a 50% reduction of properties; nevertheless, the elastic modulus had little or no change after the recycling process that is an important characteristic of the output of this method since the fibers are reusable immediately after as reinforcement in new composites. In Fig. 20.9, recovered glass fibers from end-of-life composites as a product of fluidized-bed technology are shown. The resulting fiber length is majorly dependent on the scrap/waste composite particulate feed and the degradation that the fiber suffers from the fluidized-bed process, which also affects the mechanical properties of the recovered fibers. Due to the recovering process by cyclone and the fluidized bed itself, the fibers are recovered in bundles that can be then treated by compression to produce new preforms for new composite materials. The gaseous by-products of full oxidation are basically CO2 and water, although, when fire retardants are added to the composites for safety issues and product compliance, the gaseous by-products include acid gases that must be treated and captured

FIG. 20.9 Recovered glass fibers from end-of-life composites with fluidized-bed technology [19].

20.3 Waste management and recycling technologies

before any emissions are made to the atmosphere; in fact, from a sustainable perspective being the CO2 considered as an important greenhouse gas, several technical adaptations can be proposed, for example, to a proper scale of a fluidized-bed recycling process; there can be an interesting amount of energy recovery to power at least partially the system itself; moreover, taking into account that a carbon capture and sequestration deployment might not be feasible in a first approach of a fluidized-bed recycling plant due mainly to costs of its implementation, it is still a technology that could reduce the negative impact of the CO2 emissions of this process. All in all, the fluidized-bed recycling process is a feasible technology, with a high level of readiness at industrial scale for recovering fibers from thermoset composites while recovering energy from the calorific value of burned polymers; through a sustainable waste management of the input of this process and a responsible control over emissions, the fluidized-bed recycling method could be a sustainable and integral technology for recycling thermosets and their composites.

20.3.4 PYROLYSIS As a thermal process for recycling, pyrolysis is a thermal decomposition reaction of the thermoset material in an inert atmosphere by rupture of the chemical bonds in the polymers. The specifics of a pyrolysis reactor must take into account the parameters of the process to be applied and the characteristics of the waste/scrap feed that will be treated and the desired output phase (solid, gas, and liquid). The temperatures are also related to the desired output since high temperatures can result in formation of char and carbonization of the materials. When in a stage of rigorous control applied as a process for thermosets and thermoset composite materials recycling, pyrolysis can have an interesting output in which potentially the solid, gas, and liquid phases are recovered; solid phase in the form of recovered fibers and other reinforcements can be then reused in new components; extensive thermogravimetric analysis has been done to report the composition of the resulting gases from pyrolysis of thermoset polymers [20], while gas and liquid by-products can have enough calorific value for energy recovery used in the form of fuels [21]. The gaseous products of pyrolysis reactions from thermoset materials are dependent on the material that is being decomposed through this method; the chemical compounds observed by gas chromatography-mass spectroscopy are directly related to the building subunits of the polymer, either from the cross-linking curing agent or the monomer present in the resin. Polyester resins subjected to pyrolysis, which is decomposed between 300°C and 370°C, produced chemical compounds from the styrene cross-links and the polyester chains such as diacids and diols, while cinnamate esters confirmed degradation, and phthalate esters suggested more than one degradation mechanism [22]; epoxy resin, which is decomposed between 370°C and 460°C in pyrolysis, released volatile compounds and bisphenol A; phenolic resin, which is decomposed at temperatures between 450°C and 580°C in pyrolysis, resulted in phenol and cresol isomers as the main products of the process [23].

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Toxic emissions of brominated and alike chemical compounds related to fireretardant thermoset materials and their composites must not be forgotten as an environmental concern of the application of this process. In a successful application of a pyrolysis recycling process, clean carbon fibers were recovered from a thermoset epoxy matrix for potential reuse in new components using microwave as a source of power. In Fig. 20.10, the schematics of the microwave-assisted pyrolysis are shown. In this experimental deployment, clean carbon fibers were recovered from a carbonepoxy composite material in a multimode microwave cavity powered for 8 s at 3 kW. A stream of nitrogen gas was used to control an inert atmosphere. The output of the experimental development is shown in Fig. 20.11; the resulting carbon fibers have a relatively clean appearance when compared with virgin fibers; moreover, the mechanical properties of the recovered carbon fibers resulted in

Glass wool packing

N2 outlet

Sand & CF sheets

N2

FIG. 20.10 Schematics of the experimental deployment of microwave-assisted pyrolysis [24].

0.65 kx

20 kv 119

FIG. 20.11 Scanning electron microscopy of recovered carbon fibers [24].

20.3 Waste management and recycling technologies

3.26 GPa in tensile strength and 210 GPa in tensile modulus versus 4.09 and 242 GPa, respectively, from virgin fibers [24]; recovering clean carbon fibers while maintaining the mechanical properties is advantageous for a potential reuse in new components. The use of microwaves as source of power can be observed in latest research [25] for recovering and recycling fibers from thermoset composites. As mentioned before, pyrolysis used as a recycling technology has the advantage of recovering most of the material introduced in the reactor in the forms of solid, gas, and liquids. Clean fibers can be recovered for potential reuse in new composites, while gases and liquids can have a potential reuse as chemical compounds. This technology, particularly applied for recovering carbon fibers from carbon-epoxy composites, is interesting since the carbon fibers themselves are relatively good susceptors of microwave radiation. An interesting deployment and application of this technology would be an adaptation of monomode microwave heating as means for carbon fiber recovery. A logistic approach of waste management and feedstock must also be considered due to the very specific differences of gaseous products resulting from different thermoset resins, which in some cases raise environmental concerns about the emission of bromide and alike toxic chemical compounds.

20.3.5 CHEMICAL DECOMPOSITION OF THERMOSETS Chemical decomposition of thermosets and their composites is an interesting recycling technology that was developed as a by-product of the research for chemical resistance of amine-cured epoxy components and other thermosets to acid solutions with the objective to observe product compliance. The results suggested the technological feasibility for chemical decomposition of thermosets and remotion of matrix from composite materials with an organic decomposed output that, after neutralization, can be repolymerized into new thermosets; recovering fiber reinforcement from thermoset composite materials is also feasible with this technology. In a successful application of this technology, epoxy resin was decomposed in nitric acid solution at 80 °C [26]. In Fig. 20.12, the proposed chemical decomposition reaction is shown, in which basically two major actions are taking place: the +

C-N bond breaking CH2

CH2

H H2 O C C C H2 OH H O C C C H2 H OH 2

CH2 CH3 N

N

C CH3

CH3

O=N=O Attacking benzene ring NO2

OH C H

C O H2

C H2

H O C C CH2 H2 OH

C O H2

C H2

H O C C CH2 H2 OH

OH CH2

FIG. 20 .12 Proposed chemical decomposition reaction [26].

C H

Nitration

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O

O

+

O

+

NR3

C NR3

(1) C O−

O

O

O

O

+

NR3

C

C O−

O

+

H2C

(2)

CH CH2 – O C OCH2 – CH CH2 – O

O O

+

C NR3

O−

O O

O

+

C NR3

+

(3)

O

C OCH2 – CH CH2 – O O−

O

+

C NR3 C OCH2 – CH CH2 – O

O

O

O O C O

O

O

C OH O O

+

(4)

R1OH

C O O−

C OR1 O

FIG. 20.13 Recycled epoxy resin repolymerization reaction [26].

carbon-nitrogen (C-N) covalent bonds are broken, and the benzene ring is nitrated under the attack of nitric acid. After extract neutralization, the resulting chemical structure is then similar to bisphenol F giving place to the potential of recycling the epoxy resin through repolymerization. In Fig. 20.13, the recycled epoxy resin repolymerization reaction is shown, in which a tertiary amine catalyst initiates the reaction opening the cyclic anhydride (1), the previously formed carbonium opens the epoxide ring (2), the regenerated hydroxyl oxygen reacts with anhydride (3), and the resulting extract of decomposition contains hydroxyl groups that react to form polyester chains (4); after these reactions take place, in the presence of tertiary amine compounds, the new groups are cross-linked resulting in a relatively rapid curing reaction and a higher density polymer network than the original epoxy resin under the same reaction conditions that in fact in some experiments also have higher thermal and mechanical properties than the virgin resin. A similar process was then applied to thermoset composite materials with the objective of recycling both the reinforcement and the matrix through chemical decomposition of the resin followed by the recovery of the fibers and further repolymerization of the resin [27]. Glass fiber-reinforced epoxy resin was used for this purpose. After the experimental process mentioned before was effectively applied, clean glass fibers were recovered and the epoxy resin effectively repolymerized. There was a weight loss observed in the recovered glass fiber after the treatment was applied when compared with initial weight of the fibrous fraction of the

20.3 Waste management and recycling technologies

composite material suggesting degradation of the fibers related to dissolution of some of the components of the glass fibers such as aluminum oxide (Al2O3), calcium oxide (CaO), and silicon oxide (SiO2). Similar endeavors have been made in bisphenol A recovery [28]. With a responsible handling of materials that can be considered dangerous or toxic, this chemical approach to recycling thermosets and their composites remains an interesting deployment since it makes possible the repolymerization of residue for new components while recovering the reinforcement of composites. Altogether, chemical decomposition of thermosets and their composites is an effective technology that can recycle both fibers and resins.

20.3.6 SUPERCRITICAL FLUIDS: HYDROTHERMAL AND SOLVOTHERMAL DECOMPOSITION OF THERMOSETS

Pressure

A supercritical fluid is a state of matter reached only when pressure and temperature are above the critical point where distinct phases of liquid and gas do not exist; instead, the supercritical fluids have properties between those of gases and liquids. A supercritical fluid can also be defined as a substance above its critical pressure (PC) and critical temperature (TC), which are related to the substance itself. At the critical point, the phases of liquid and vapor are in equilibrium; the supercritical fluid area is shown in Fig. 20.14, pressure-temperature phase diagram for supercritical fluids over

Solid phase

Compressible liquid

Supercritical fluid

Critical pressure PC

Critical point Liquid phase

Triple point Vapor Critical temperature TC Temperature

FIG. 20.14 Pressure-temperature phase diagram for supercritical fluids.

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the critical point; in this case, the green dotted line is also depicting the behavior of water. A supercritical fluid has gas-like effusion properties with liquid-like density resulting in high dissolving characteristics. Subcritical and supercritical fluids are excellent reaction media for decomposition of thermoset materials and thermoset composite materials. Depolymerization with this technology proceeds rapidly compared with conventional and experimental methods [6]. Hydrothermal decomposition of thermosets as a recycling technology uses water as solvent, what makes the operation of the process very simple in principle. Depolymerization by subcritical and supercritical water recycling technology has the potential to be new approach for recycling thermoset materials. The high pressure and high temperature inside the decomposition reactor lead the supercritical water to decompose the thermoset polymers. Research on supercritical fluids has shown great results at lab scale and should then contribute to the development of this process at industrial scale in a near future [7]. Very clean fibers can be recovered from thermoset composite materials; in batch reactors, it is possible to recover liquid and solid phases from the decomposition [29]; a 79.3% decomposition rate can be achieved using hydrothermal decomposition; the decomposition reaction can be further improved to 95.4% by adding KOH as a catalyst [30,31], while fibers can retain up to 98% of their tensile strength compared with virgin fibers [32]; with the use of proper additives and processes, recovered fibers could also have even better properties after supercritical fluids recycling treatment [33]. The reuse of carbon fibers in several applications with nonwoven short fiber fabric in new composite materials has been reported [34]. Using supercritical water for decomposition in the presence of oxygen results in carbon fibers that can retain mechanical properties as high as the virgin fibers with decomposition rates between 94% and 96% [33]. Subcritical and supercritical alcohols are also excellent reaction media for decomposition of thermoset materials and thermoset composite materials. Decomposition of the thermoset matrix of composites proceeds selectively [6]. Solvothermal decomposition of thermosets as a recycling technology uses alcohols and other solvents as reaction media, the use of benzyl alcohol is of particular interest due to its low critical and vapor pressure. Very clean fibers can also be recovered from thermoset composite materials with this technology reaching a decomposition rate of up to 95% with supercritical alcohols in semicontinuous flow systems [35]. n-Propanol is also being used as a solvent [36]. Some research has found fibers resulting in Young’s modulus of 205 GPa retaining 85% of tensile strength compared with virgin fibers [30,31] and 225 GPa retaining 98% of tensile strength compared with virgin fibers [36]. In a successful application of the supercritical fluids, hydrothermal and solvothermal decomposition reactions were used for recycling of thermosets and their composites with the main objective of the recovery and characterization of carbon fibers from carbon-epoxy composite systems. Water and benzyl alcohol can be considered as eco-friendly solvents, due to low potential toxicity and the ability to dissolve epoxy compounds. The research was conducted to focus in the decomposition rate (DR) of composite materials and the characterization of the resulting fibers by scanning electron microscopy [1]. A schematic diagram of the experimental apparatus (AKICO Co. Japan) is shown in Fig. 20.15.

20.3 Waste management and recycling technologies

(A)

(B)

(C) FIG. 20.15 Schematics of experimental batch reactor. (A) Batch reactor, (B) electric furnace, and (C) cyclic horizontal mechanical movement [1].

In Fig. 20.16, SEM micrographs of recovered carbon fibers with supercritical water at 400°C are shown. Fig. 20.16A shows the recovered sample after 15 min decomposition reaction, which is consistent with a DR of 24%. Cracking of the resin is visible, and solid residue particles are found on the surface of the sample. Based on these results, delamination of the composite samples occurs first, and then, the resin holding the fibers together in a single layer begins to decompose. Fig. 20.16B shows clean carbon fibers after 30 min treatment. Fig. 20.16C shows the recovered carbon fibers after 1 h. In this sample, some epoxy resin residue is visible on the fiber surfaces. Fig. 20.16D shows clean recovered carbon fibers after 2 h. Fig. 20.16E shows clean recovered carbon fibers after 4 h. Fig. 20.16F shows the electron dispersive X-ray spectroscopy (EDS) explaining that in supercritical water, the original surface treatment of the carbon fibers (sizing) with sulfur is retained after treatment. Water under subcritical and supercritical conditions is a good solvent for chemical recycling of thermoset composite materials, resulting in the recovery of clean carbon fibers after treatment, improving the DR results when K3PO4 is added as catalyst. In Fig. 20.17, SEM micrographs of recovered carbon fibers by subcritical benzyl alcohol at 425°C are shown. Fig. 20.17A–E shows clean carbon fibers recovered by this method at 15 min, 30 min, 1 h, 2 h, and 4 h, respectively; loose carbon fibers were found even at very short decomposition reactions, and practically, no solid residue was found on the samples by SEM analysis of these recovered fibers. EDS of recovered carbon fibers after 15 min in decomposition reaction with subcritical benzyl alcohol at 425°C is shown in Fig. 20.17F. No peak for sulfur was detected in EDS; the sizing in carbon fibers seems to be also removed by subcritical benzyl alcohol. Benzyl alcohol, even at subcritical conditions, is an excellent solvent for thermoset materials recycling; improving the DR results when K3PO4 is added as catalyst. Water was found in the liquid residue of the benzyl alcohol experiments at 425°C as a

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FIG. 20.16 SEM micrographs of recovered carbon fibers by supercritical water at 400°C after (A) 15 min, (B) 30 min, (C) 1 h, (D) 2 h, and (E) 4 h; (F) EDS of recovered carbon fibers with water at 400°C after 15 min.

20.3 Waste management and recycling technologies

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FIG. 20.17 SEM of recovered carbon fibers by subcritical benzyl alcohol at 425°C after (A) 15 min, (B) 30 min, (C) 1 h, (D) 2 h, and (E) 4 h; (F) EDS of carbon fibers recovered with benzyl alcohol at 425°C after 15 min.

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Input

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FIG. 20.4 Schematics of mechanical technologies for thermoset materials recycling. (A) General schematic of shredder. (B) General schematic of a granulator.

General schematics of shredders, granulators, and mechanical recycling are shown in Fig. 20.4. A successful application of mechanical treatment of waste materials and reuse as reinforcements in new components is the addition of pulverized waste of composite materials as fillers in concretes [15] and polymeric concrete [16]. Polymeric concrete is a thermoset composite material with low cost of production and relatively good structural properties. A fraction of the materials used in polymeric concrete, such as polyester resin, silica sand and calcium carbonate, were substituted with pulverized fiberglass-polyester materials recovered from end-of-life components as shown in Fig. 20.5A and B. The experimental development of polymeric concrete resulted in increased mechanical properties of the materials, up to 50 MPa (Avg Line 2) in compression versus the 49 MPa (Avg Line 1) in the baseline production formulation (with no recycled material added) as shown in Fig. 20.6. Each line represents the average behavior of an experiment. As can be seen in the figure, the lowest obtained compression strength is 35 MPa, while most of the experiments show relatively good properties between 40 and 45 MPa. The addition of recycled material reduces the fraction of polyester used in the components, reducing thus manufacturing costs while reducing also the environmental impact. The prefabricated polymeric concrete building systems can then be used in the construction or any other industry [16]. There are many details in mechanical treatment of waste materials; as already mentioned, size of input and desired output are some of the most important factors for shredders design and operational costs; basically, the process is the same throughout the range of machines used for mechanical treatment. Waste is shredded and/or granulated into more manageable materials, which is an advantage for transport and

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subproduct of the decomposition reaction; water in those experiments creates a more complex decomposition, as water at that temperature can increase the pressure above of critical pressure of benzyl alcohol. Water and benzyl alcohol under subcritical and supercritical conditions are good solvents for chemical recycling of thermoset composite materials, resulting in very clean carbon fibers after treatment. Benzyl alcohol showed higher DR results, over 90%. Water resulted in a DR of just over 80%. Higher DRs compared with published data could be obtained using this method, for example, 89.1% after 1 h treatment with supercritical water. Moreover, a DR of 93.7% after 1 h of treatment at 400°C with subcritical benzyl alcohol has been achieved, which is better than results published from experiments conducted in semicontinuous flow systems using organic solvents in the presence of catalysts. Recovery of clean carbon fibers by subcritical benzyl alcohol has been corroborated by SEM analysis.

20.4 ADVANCES IN SUSTAINABLE DESIGN OF THERMOSETS AND THEIR COMPOSITES Reiterating over a more sustainable approach of using thermoset and thermoset composite materials, there is more than waste management that can effectively support the recycling processes discussed previously: sustainable design, bio-based biodegradable thermosets, and efficient manufacturing of thermosets. The design of components manufactured with thermoset materials is driven by mechanical and chemical solicitation of application, while composite materials are orientated toward low-density components ensuring high performance throughout the life cycle of the product; many other factors must be taken into account in the design phase, for example, optimization of manufacturing and assembly of components. Recycling must be taken into account for a sustainable design of thermosets and their composites.

20.4.1 ECO-FRIENDLY BIO-BASED DEGRADABLE THERMOSETS AND EFFICIENT DESIGN The design of eco-friendly bio-based degradable thermosets is a relatively new line of research that is drawing a lot of attention since it offers interesting solutions to negative environmental impacts caused by the nonbiodegradability of the currently most used thermosets and their composites. The objective is to develop thermoset resins capable of sustaining the current mechanical and chemical solicitations for use in products that are actually in the market with high contents of bio-based chemical compounds and low contents of toxic raw materials. Monomers like lactic acid, obtained from plants and vegetable oils, sugarcane, and fermentation of corn, potato, and other eco-friendly sources, are of interest for ecological reasons. The use of natural fibers as reinforcements in composites has also been researched resulting in several advantages such as low cost and low ecological impact since many fibers are obtained as by-products of several economical activities; high recyclability is another characteristic of natural fibers and most important biodegradability.

20.4 Advances in sustainable design of thermosets and their composites

The mechanical properties of natural fibers, such as flax, hemp, jute, and sisal, can be considered to compete with glass fibers in composite materials [37]. The design of high-performance biodegradable materials based on natural fiber reinforcements and bio-based resins is one of the most important goals that must be achieved for a sustainable use of green thermosets and their composites. Sustainable design of thermosets and their composites is orientated toward recycling of materials and reuse in new components with a lower level of specifications. Recycling can also be considered in the design of products proposing simple guidelines: dismantling mechanical assembly is preferred over miscellaneous mechanical adhesive unions, reduced use of metallic inserts and core materials, and noncombined use of resins and fibers in composite materials for easy separation and segregation of materials [38]. In Fig. 20.18, a multicircular representation of the thermoset carbon fiber composites life cycle is shown, in which the integration of recycled carbon fiber in nG carbon fiber (new material: n = 1)

z (level of specifications i.e. number of constraints)

nG composite end-of-life Raw material (nG carbon fiber)

Carbon fiber end of life

Aeronautics

Use Maintenance Distribution Transport Design Process Production nG composite life cycle

Fiber recycling (n → n + 1) or material recovery (n → n ) Automotive

(n + 1)G composite life cycle

Fiber recycling (n → n + 1) or material recovery (n → n ) Leisure and sports (n + 2)G composite life cycle

Energetic valorization (incineration) Landfill (banned) q (time [cycle duration])

FIG. 20.18 Multicircular representation of the CF life cycle [38].

r (number of potential uses)

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successive product life cycles is indicated, depending on their level of specifications, for example, mechanical, chemical, and high-performance solicitations according to type of products of that level. The radius r of each life cycle depends on the number of potential uses for the recycled carbon fiber. Efficient manufacturing must be considered not only for cost-related concerns but also for environmental concerns, reiterating over the approach that engineering society has toward waste management of thermoset materials, meaning that low-cost thermoset resins and low-cost fiber reinforcements must not be produced in unsustainable manners just because the industry is capable of handling those minimal production costs. Lean manufacturing and like practices are highly recommended in combination of awareness of environmental impacts for industry development.

20.5 CONCLUSIONS Recyclability is a concept that must be observed and considered as the baseline for design, production, and waste management of thermosets and their composites. Not only waste management of thermosets does depend on technological deployments, which it is in fact one of the factors that is already solved and demonstrated in this chapter, but also recyclability depends on the natural course of action of markets driven by the market (offer-demand) behavior and legislation. In the waste management hierarchy model proposed in this chapter, it is possible to identify the technologies and implications at the different levels of preferred action. While reduction is the most preferred level, the most used level is disposal, with no regulations in developing countries about landfill practices, waste management of thermosets and their composites is behind schedule. Nevertheless, recycling as an objective level presents many economical and ecological advantages. General numbers of thermosets and their composites markets can only be interpreted as an increasing industry that raises the environmental concerns about end of life cycle of the produced components; nevertheless, this increasing production of thermosets and thermoset composites is the material feedback that the recycling industry needs in order to secure an economically viable recycling market. As mentioned in several occasions in this chapter, legislation will have a major role in solving the waste management concerns of thermosets and their composites by incentivizing and penalizing practices and contributing to lay the ground rules and specific circumstances for recycling of thermosets as an economically and ecologically viable solution, thus promoting the rate at which nations will respond to the waste management concerns. Mechanical, thermal, and chemical recycling technologies were revisited in this chapter, showing a whole spectrum of possibilities to address the waste management of thermosets and their composites. Important efforts have been made in researching and communicating the whole panorama of thermosets and thermoset composite materials recycling resulting in the current status of readiness for technological implementation. While mechanical technologies can manage a broad range of

20.5 Conclusions

materials, energy is an important factor of this technology, and its most important output is the better handling of recycled materials and the reuse of powders as fillers in new components. Combustion is one of the thermal technologies that it is being largely used, and in some cases, it is used as an energy recovery technology; still, many concerns are raised about emission of toxic dioxins, CO2, and other greenhouse gases. Fluidized bed remains as an interesting thermal technology with possibilities of recovering energy and clean fibers from composites, while specific equipment can be used for scrubbing and capturing of nondesired toxic and greenhouse gases; fluidized bed, as mentioned before, has the advantage of the potential reuse of the recovered fibers in new components. Pyrolysis used as a recycling technology has the advantage of recovering most of the material introduced in the reactor in the forms of solid, gas, and liquids, while microwave-powered devices remain an interesting alternative source of energy for this technological deployment. Chemical treatment of thermosets and thermoset composites is an interesting form of recycling resulting in some cases in the possibility of repolymerization of decomposed chemical compounds that can be in fact as resistant as the virgin resins. Hydrothermal and solvothermal processes are feasible for industrial scale. Benzyl alcohol decomposition reactions could be safer for possible implementation in industry due to the relatively low critical pressure in the reactor compared with supercritical water. However, these methods have to be adjusted for industrial purposes of carbon fiber recovery, in accordance with other factors such as cost, type of reactors, environmental issues, and properties of recovered fibers. The outlook of these processes seems positive for industrialization from the standpoint of having an increasing lot of waste material confined in disposal with an interesting high intrinsical value and a recycling technology advancing for newer and better results recovering carbon fibers. It is acknowledgeable the importance of the possibility of reusing the recovered carbon fibers and the mechanical properties of the fibers, specifically the tensile strength. All the revisited technologies shown in this chapter have been effectively deployed at laboratory level; some of them are in pilot plant scale or even industrial level. Proliferation of these technologies starts with the right divulgation of their benefits that will play a major role for global implementation of thermoset materials recycling. Economical factors are relevant for recyclability, and even that this analysis does not focus on costs of implementation, we can observe that there is no economical objective that can be pursued without the benefits of the environment sustaining the human civilization activities; furthermore, it must not be forgotten that many global technological deployments start with high economical costs that can be downgraded with the increase of the implementation itself; it is only foreseeable that the recycling technologies will be more feasible and available to industrial levels; moreover, the right implementation of these technologies gives place to new business models, jobs, and development of intermediaries and new products; legislation will take part making the recycling of thermoset and their composites the only sustainable and possible way to continue with the economical productivity of thermoset polymers. The effective and efficient implementation of the technologies, which in

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many cases can be still further improved, will serve as a baseline for a more sustainable waste management of thermoset and thermoset composite materials. Recycling must be taken into account for a sustainable use of thermosets and their composites, while the use of bio-based and eco-friendly materials is an objective of technological development. Recycling of thermosets and their composites is a viable and feasible option and as reiterated throughout this chapter, recycling is in fact the only option going forward on thermosets waste management.

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FURTHER READING [1] United States Environmental Protection Agency, Sustainable materials management: nonhazardous materials and waste management hierarchy. 2016.