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A Novel Sonochemical Approach for Enhanced Recovery of Carbon Fiber from CFRP Waste using Mild Acid-Peroxide Mixture Mohan Das, and Susy Varughese ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01497 • Publication Date (Web): 19 Feb 2016 Downloaded from http://pubs.acs.org on February 22, 2016

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ACS Sustainable Chemistry & Engineering

A Novel Sonochemical Approach for Enhanced Recovery of Carbon Fiber from CFRP Waste using Mild Acid-Peroxide Mixture Mohan Das and Susy Varughese* Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai, India 600036 KEYWORDS: Sonochemical, Recycling, Composites, CFRP

ABSTRACT: A novel sonochemical method is proposed for the recovery of carbon fiber from Carbon Fiber Reinforced Polymer (CFRP) composites by treatment with a mixture of dilute nitric acid and hydrogen peroxide in the presence of ultrasound. A maximum resin decomposition ratio of 95% could be obtained for the epoxy matrix. SEM and TGA data showed that the recovered fibers had very little or no epoxy resin and their tensile strength was comparable to that of the virgin fiber. ATR-FTIR, MALDI-TOF/MS and 1H-NMR analysis of the recovered solid and liquid byproducts showed the formation of decomposition products of crosslinked epoxy resin through nitration and oxidation reactions. This process opens the possibilities of an environmentally more benign process for CFRP waste decomposition and recovery of carbon fiber in the presence of ultrasound and H2O2 by eliminating the need for strong chemicals and high reaction temperatures and pressures.

ACS Paragon Plus Environment

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Introduction

Carbon Fiber Reinforced Polymers (CFRPs) are used widely in industries like aerospace, automotive, construction, sports, transportation and other consumer goods manufacturing. Aerospace industry is one of the major consumers of carbon fiber based composites. Carbon fiber manufacturing is expected to reach approximately 1 million MT by 2020. This trend inevitably would lead to the problem of more waste generation from CFRP manufacturing processes as well as from end-of-life products in addition to more demand on creating more resources.1-5 Thus recycling of these materials is imperative for controlling CFRP waste generation. This would also bridge the widening gap between the supply and demand for fresh carbon fiber by using recycled fibers.6 Recycling processes for CFRP can be classified broadly into three different categories, viz. mechanical comminution and separation, thermal and chemical processes.7-8 Mechanical processes rely on size reduction of the waste CFRPs and separation of the fractions rich in fibers and pulverized resin with the help of classifiers.9-12 One of the major disadvantages of these processes is their inability to recover individual long fibers free of resin. Hence the crushed composite mass is used as a reinforcing or non–reinforcing filler in other material matrices.13-14 Thermal processes use high temperature (450–550 °C) to decompose the resin part of the crushed CFRP either in the presence or absence of oxygen and eventually separate the resin from the fiber.15 This process results in fibers which are comparatively short and retain only 70-75% of their original strength. Chemical recycling methods using concentrated nitric acid have shown complete resin decomposition and recovery of fibers of higher length and strength retention upto 98%.16-20 This method can lead to the production of NOx during the epoxy resin degradation. However, the concentration, the amount of acid used and the treatment time need to be reduced to reduce the environmental impact of


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this process. Greener approaches using acetone and hydrogen peroxide also have demonstrated promising results,21 but their effectiveness on different epoxy compositions needs to be explored. A more recent technology is the use of solvolysis process which employs solvents like water and common alcohols at their supercritical conditions for decomposing epoxy resin.22-27 This process however requires high temperature (300–400 °C) and pressure (5–15 MPa) for the solvents to attain their supercritical state.

Ultrasound assisted chemical reactions or sonochemical reactions have been used for enhancing the rates of many homogeneous and heterogeneous reactions such as material synthesis (organic, organometallic and inorganic), advanced oxidation process for chemical degradation

of

organic

aqueous

pollutants,

washing

and

cleaning,

extraction,

disinfection/sterilization, and particle aggregation.28-31 Use of ultrasound in aqueous solutions leads to the phenomenon of cavitation.32-33 The implosion of microbubbles created in the liquid as a result of ultrasonic frequency leads to localized release of high energy, temperature (5000 K) and pressure (1000 atm) which in the case of aqueous solutions, can help in the formation of radical species like H*, *OH and HOO*, where *OH is a very strong oxidizer. Hydrogen peroxide is used as a secondary source for hydroxyl radicals in the sonochemical degradation of aqueous pollutants.

The following reactions are reported to occur during the sonolysis of aqueous solutions34,

H O → H ∗ + OH ∗ H O → 2HO∗


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2HO∗ → H O O → 2O∗ O∗ + H O → 2OH ∗ H ∗ + H O → OH ∗ + H H ∗ + O → ∗OOH 2 ∗OOH → H O + O H O + ∗OH → H O + HO∗ HO∗ + H O → H O + ∗OH + O

(1)

From the reactions given in eqn (1) it can be observed that water molecules break down into hydroxyl radicals which react with each other to form H2O2 which further breaks down to form hydroxyl radicals.

Sonochemical reactions may lead to higher reaction rates in the case of solid–liquid reactions, first because of continued solid surface erosion caused by fast moving (400 km/h) liquid microjets and shock waves formed by the imploding bubbles near the solid surface.35 This coupled with rapid fluid motion around the sample continuously provides fresh solid surface for reaction, increasing the mass transfer rate. Another reason for a higher reaction rate is the higher number of reaction sites available. The reactions occur at three sites: (a) inside the cavity, (b) at


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gas – liquid interface and (c) in the bulk liquid. Inside the cavity the substrate can react with the *

OH and *H radicals formed or can get pyrolysed. A similar reaction occurs at the interfacial

region. In the bulk phase the reaction is between the substrate and *OH or H2O2 and other reactants. The C-C, C=C, C-N and C-O bonds can be broken at very high temperatures (~5000 K) caused by the bubble implosion and *OH helps the free radical reaction to occur simultaneously.36

However, it is not yet understood whether the potential of the sonochemical processes could be explored for the separation of fibers from polymer composites. We see a huge potential in the use of this method for developing a sustainable process for recycling CFRP. Hence, the aim of the present work is to study the sonochemical degradation of CFRP waste in an aqueous mixture of dilute nitric acid and hydrogen peroxide and compare it with conventional solid– liquid reaction process. Also, to explore the possibility of the proposed method to minimize the use of environmentally harmful chemicals in thermoset recycling processes.

Materials and methods

Post-fabrication and post-test CFRP waste and virgin carbon fibers in mat form were provided by National Aerospace Laboratories, Bengaluru, India. The composition of the resin, precursor, sizing and the wt% of fiber and resin in the composite were not known. We used these to simulate the recycling of CFRP waste of unknown origin. 70 wt% (or 15.6 M) nitric acid and 30 wt% (or 9M) hydrogen peroxide supplied by Merck Specialities Pvt. Ltd., India were used for the experiments. Nitric acid was diluted with distilled water to 2M solution for the experiments.


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Ultrasonic tanks of frequencies 40 kHz and 470 kHz of capacities 24 L and 22 L respectively (Crest Ultrasonics, USA) were available and compared for the sonochemical function using various parameters such as input power, cavitation intensity and temperature rise.They were filled till 75% of their respective capacities with water at room temperature. The change in cavitation in water was measured at the four corners and the center of the tank at midway from the tank bottom at regular intervals using an Ultrasonic energy meter (PPB Megasonics PB500). Water temperature was measured at the same time using a mercury thermometer. Temperature measurements during these intervals were carried out after the ultrasonic generator was stopped for a brief period. This was done to avoid various instabilities such as, viscous heating, acoustic streaming and non-uniform temperature distribution caused by ultrasound around the thermometer bulb. The thermometer had an accuracy of ±1 °C.

The input power to the liquid in the tank was calculated using calorimetric method31 from which the calorimetric power of the liquid PCal was obtained from,

PCal = dT dt C M P

(2)

where dT/dt is the rate of change of liquid temperature, CP is the specific heat-capacity of the liquid used (4.2 J/gK in this case) and M is the mass of the liquid.

A two-stage process comprising of a pre-treatment stage followed by a sonochemical stage was carried out. Composite samples were cut into pieces of dimension 30 x 25 x 2 mm3using a band saw cutter. In the first stage, these samples were immersed for pre-treatment in an aqueous mixture of 2M nitric acid and 9M hydrogen peroxide of varying compositions (Table 1) in glass bottles with excess volume of fluid. Solid to fluid ratio was maintained at 1:60 for


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optimum cavitation efficiency during sonication. In the second stage, sonication was carried out by placing these bottles in the Ultrasonication tank. Samples were also sonicated in distilled water to compare the effect of cavitation on the composite samples in the absence of a reaction medium. The required pre-treatment time was determined by measuring the mass uptake of the solution by the sample with immersion time. When the composite samples were fully swollen (equilibrium mass uptake) , the glass bottles with the samples and the solution were placed in a 470 kHz Ultrasonication tank maintained at 65 °C using a water bath (Figure S1). 65 °C was maintained to avoid excessive evaporation of water from the tank. Lower tank temperatures lead to slower reaction rates and hence were not considered further. 470 kHz tank was chosen based on its sonochemical performance in comparison to 40 kHz tank. The bottle was removed every hour after the commencement of ultrasonication, cooled to room temperature and then the mixture of decomposed resin and the solution was filtered using a Whatman1 filter paper. The filtrate was ultrasonicated further along with the sample and the process was repeated for 8 h. The resultant carbon fiber mat which is free of epoxy resin was removed from the solution and washed with distilled water until neutral pH. This was followed by washing with acetone and drying at 100 °C in an air oven for 4 h. Individual fibers could be recovered at this stage. Other filtration residues were recovered after drying the filter papers. The remaining solution was neutralized using saturated Na2CO3 solution.16 The degradation reaction process was monitored by measuring the change in the absorbance of the solution with reaction time using a UV/Vis Spectrophotometer (Jasco V630). Gases evolved during the process were collected and analysed using Balzers Thermostar Mass Spectrometer.


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The above mentioned procedure was repeated for waste composite samples without ultrasonication and also in distilled water with ultrasonication for comparison. The results were compared using the resin decomposition ratio, defined by16,

( )

= ( × ) × 100

(3)

Where m1 is the mass of the untreated sample, m2 is the mass of the dried sample remaining after treatment and Wm is the mass fraction of resin in the sample which was determined using ASTM D3171.

The as received carbon fibers (virgin) and recovered fibers were studied using High Resolution Scanning Electron Microscope (Hitachi S–4800). An operating voltage of 1-3 kV was used. Energy Dispersive X–Ray Spectroscopy (EDS) analysis using HR-SEM and XPS analysis using Esca Probe 125 (Omicron Nano Technology) X-Ray Photoelectron Spectroscopy were carried out to understand changes in the surface composition of the fibers. Thermo Gravimetric Analysis (TA SDT Q600) was carried out on both virgin and recovered fibers at a heating rate of 10 °C/min from 25-800 °C to assess the remaining resin content. Tensile strength of single fiber was measured using Universal Testing Machine (Zwick/Roell) with a 1 kN load cell as per the test conditions specified by ASTM C1557–03. At least 20 fiber samples each for virgin as well as recovered fibers were tested. ATR-FTIR analysis (Cary 630 FT–IR Spectrophotometer, Agilent Technologies) was carried out for the neutralized filtrate as well as for the dried solid residue and cured epoxy powder obtained from crushed CFRP samples. Molecular weight of the neutralized filtrate was determined using MALDI-TOF/MS [Voyager– DE PRO, Applied Biosystems, laser: N2 laser (337nm)] with DHB (2, 5–dihydrobenzoic acid)


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matrix and its structure was predicted using Bruker 500 MHz NMR spectrometer with Acetoned6 as the solvent, where all the residues dissolved.

Results

CFRP waste degradation and carbon fiber recovery studies were carried out in dilute nitric acid in the presence of H2O2 and ultrasound. The resultant solid and liquid products as well as the recovered carbon fiber were analysed using various techniques such as ATR-FTIR, UV-Vis, and Mass spectroscopy, SEM and TGA. Prior to the ultrasonication process the samples were immersed for a specific period in the treatment solution to enhance the diffusion of the solvent into the composite.

Small pieces of the CFRP composites were kept immersed in solvents of various compositions (Table 1). Samples attained equilibrium swelling after 24 h immersion in the solutions (Figure S2) which was considered as the optimal time for pre-treatment of the samples before commencing the sonochemical reaction.

Selection of ultrasonication tank

Variation in the cavitation intensity with time for 40 kHz and 470 kHz tanks (Figure 1) shows an initial rise and then a reduction above a temperature of 60 °C (at 20 min). The rise in cavitation corresponds to the degassing process after which the effect of temperature takes over and the cavitation intensity decreases.33 It was observed that for 40 kHz tank the cavitation intensity reduces to zero as the water temperature reaches 60 °C. However, in the case of 470 kHz tank there was considerable amount of cavitation intensity even at 80 °C which shows that a higher


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frequency tank can generate a standing wave of higher frequency and more number of cavitation bubbles at higher temperatures. The energy and temperature released by bubble implosion can help in breaking down the resin matrix surrounding the fiber. The bubble surface is hydrophobic and hence hydrophobic (organic) compounds can be pyrolysed inside the bubble. The rate of temperature rise was higher for 470 kHz tank (0.53 °C/min) compared to 40 kHz tank (0.22 °C/min) and their corresponding calorimetric powers [eqn (2)] were 610 W and 272 W respectively. This implies that 470 kHz tank will have a higher impact on the decomposition of the composite sample. It has also been reported that tanks with frequencies between 200 kHz and 1000 kHz produce substantial amounts of free radicals and the sonochemical activity is optimal around 300-600 kHz range.37 Based on this and the performance of tanks, the 470 kHz tank was selected as the sonochemical reactor for the present work. In the present process, 9M solution of H2O2 which was added to the 2M nitric acid solution at different vol% acted as a secondary source for hydroxyl radicals apart from the water already present in the nitric acid solution. Since the mechanism of decomposition of cured epoxy resin by nitric acid is known,16 it is expected that the combination of dilute nitric acid with a strong oxidizing agent such as H2O2 will have a stronger degrading effect on the resin matrix due to their synergistic reactions.


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Figure 1 Variation in cavitation intensity and temperature with time for 40 kHz and 470 kHz tanks Effect of ultrasound on composite degradation Considerable degradation of the resin matrix was observed on the composite surface with the progression of the sonolysis process (Figure 2). Resin decomposition was pronounced when ultrasound was used in the process. The composite inter-layer separation began after 4 h. After 8 h the clear surface of the carbon fabric was visible in the case of solutions A98H2 and A95H5. The resin decomposition ratio (


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Table ) shows that solution A98H2 was most effective. In the case of ultrasonication in distilled water, no change was observed in the resin decomposition ratio, DR showing ultrasound alone will not help in the resin degradation To distinguish the contribution of the ultrasound in the reaction, experiments were also carried out without ultrasonication. The results given in Table 1 show that the process without ultrasonication gave lower decomposition ratio indicating the role of both ultrasound and H2O2 in intensifying the decomposition process. Pre-treatment stage (soaking) and stage wise filtration process also show a positive effect on the resin decomposition (Figure 3). Pre-treatment stage helps in better diffusion of the solution into the composite. The filtration process helps in the removal of solid residues of the decomposed resin matrix and makes the solution available for the remaining matrix material on the sample. In the case of solution A98H2 which showed 95% resin degradation, 19 wt% of the resin could be recovered in solid form.

Figure 2 Microscopic images of the sample surface (top) and the cross – section (below) showing the effects of ultrasound on sample degradation in solution A98H2


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Table 1 Comparison of resin decomposition ratio for specific processes for different nitric acid – hydrogen peroxide mixtures

Solution

Amount of 2M nitric acid

Amount of 9 M H2O2

(vol%)

(vol%)

Resin decomposition ratio, DR (%) at 65 °C With ultrasound

Without ultrasound

A100

100

0

69

57

A98H2

98

2

95

31

A95H5

95

5

88

29

A92H8

92

8

78

29

A90H10

90

10

66

7

Water

-

-

No change

No change


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Figure 3 Change in resin decomposition ratio, DR under different conditions in A98H2 solution The effect of H2O2 in the reaction mixture on resin degradation is further evident from the UV-Vis absorbance data (Figure 4). The peak absorbance at 350 nm corresponding to aromatic nitro compounds resulting from the degradation of cured epoxy resin by nitric acid increases with reaction time.38 Also, the peak absorbance values were higher for the process with ultrasonication when compared to the process without ultrasound. Considering the linearity of the curves in Figure 4 (a), rate of reaction (dAbs/dt) was calculated (Table 2) which shows a similar trend as in the case of DR (Table 1). Another observation was that the absorbance peak shifts from 350 nm to 290 nm in the case of solutions A92H8 and A90H10 which have higher H2O2 content by volume indicating the formation of oxidation products like phenolics39 which too show increased absorbance with reaction time. The resin decomposition ratio DR was lower for these solutions.

Table 2 Comparison of rate of reaction for different solutions


14

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Solution

Rate of reaction, dAbs/dt (h-1) With ultrasound

Without ultrasound

A100

1.16 x 10-1

2.98 x 10-2

A98H2

1.36 x 10-1

4.52 x 10-2

A95H5

9.35 x 10-2

1.97 x 10-2

(a)

(b) Figure 4 UV/Vis absorbance variation of the solution with ultrasonication time (a) at 350 nm and (b) at 290 nm Properties of the recovered fibers


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The surface of the recovered fibers from A98H2 and A95H5 solutions show very little resin remaining on them (Figure 5). Also, as the H2O2 content increased in the solutions, the brittleness of the fibers increased and the resin remaining on the fiber surface appeared charred. EDS analysis showed the presence of small amounts of oxygen on the surface of the fibers recovered from solution A92H8 (Figure S5). XPS analysis performed on fiber recovered from solution A98H2 shows low oxygen on the surface when compared to the fibers recovered from solution A90H10 (Figure S6). The oxygen found on the fiber surface could be due to the surface oxidation of fiber and residual resin by hydrogen peroxide.40

Figure 5 HR-SEM images showing single carbon fiber (a) virgin fiber (b) recovered from A98H2 (c) recovered from A95H5 and (d) recovered from A92H8 The TGA (Figure S8) showed 7-8%weight loss from the fiber surface in the case of recovered carbon fibers (rCF) in comparison to virgin fibers (vCF) indicating the presence of small amounts of residual resin on the recovered carbon fibers. Tensile strength measurement performed on single fibers (Figure S9) shows good strength retention for fibers recovered from sonochemical reaction. The tensile strength of recovered fibers were comparable to the virgin fibers indicating minimum damage to the fibers during the process.


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Analysis of the decomposition products IR spectra of the dried, neutralized filtrates are identical in their chemical composition (Figure 6). The peak intensity at 1110 cm-1corresponding to C–N bond16increases as the peroxide content increases in the solutions indicating decreasing breakdown of the C-N crosslinks in the cured epoxy resin by nitric acid. Peaks at 1350 cm-1 and 1540 cm-1 show the presence of nitro and aromatic nitro groups16 respectively due to the nitration of the cured epoxy resin by the nitric acid. The intensity of the peaks also increased with increase in the amount of acid used. C-H stretching vibrations corresponding to aliphatic species (2920 cm-1) increases with H2O2 content of the solution. However, the increase in intensity of the peak at 1720 cm-1 shows the presence of carbonyl groups41 which may be the product of hydrolysis of the C–N bonds. Solutions with higher H2O2 content show peaks with higher intensity for carbonyl groups as well as for C-N bond. It can be observed that if the concentration of the hydroxyl radicals in the mixture is higher, then oxidation by hydroxyl radicals becomes predominant and not the C-N bond cleavage. Hence increasing the hydrogen peroxide content in the solutions reduces the epoxy decomposition process. Therefore, it can be concluded that solutions A98H2 and A95H5 which have optimum concentrations of hydroxyl radicals and nitric acid gave higher resin decomposition ratio, DR when compared to solutions A100, A92H8 andA90H10.


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Figure 6 ATR-FTIR Spectra of dried neutralized filtrate from different solutions Molecular weight determination of the neutralized filtrate from A98H2 solution using MALDI/TOF-MS shows the presence of low molecular weight species (

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