Recycling of Carbon Fibres From Epoxy Composites · Recycling of Carbon Fibres From Epoxy Composites João Pedro dos Santos Carvalho Thesis to obtain the Master of Science Degree

Recycling of Carbon Fibres From Epoxy Composites

João Pedro dos Santos Carvalho

Thesis to obtain the Master of Science Degree in

Chemical Engineering

Supervisors

Supervisor: Professora Doutora Ana Paula Vieira Soares Pereira Dias

External Co-Advisor: Doutor Ricardo José Nunes dos Reis

Examination Committee

Chairperson: Professor Doutor Sebastião Manuel Tavares da Silva Alves

Supervisor: Professora Doutora Ana Paula Vieira Soares Pereira Dias

Members of the Committee: Professor Doutor Luís Filipe da Silva dos Santos

May 2016

ii

Acknowledgements

Firstly I would like to thank my supervisor, Prof. Ana Paula Dias, for the opportunity to work in this

project. Her advice, help, trust and support were essential to the conclusion of this thesis.

To Eng. Ricardo Reis I would like to thank for all the help he gave and for the availability to give me

the tour of the factory.

A special thanks to Cristina Marques for all the support, help, interest and patience throughout all the

development of this thesis.

I would also like to thank Tiago Ribeiro for all the help he gave me in the DRX and Raman

spectroscopy.

I would also like to thank all my friends and family for all the help, support and friendship they gave me

throughout the work.

And a special thanks for my parents for never stopping believing in me and giving all the support they

could throughout this project.

iii

Resumo

Este trabalho teve como objectivo a recuperação de fibras de carbono a partir de um compósito

proveniente de peças de aviação. Foi utilizado o método de solvólise ácida com ácido nítrico. Foram

testadas várias concentrações de ácido nítrico com várias razões de massa de compósito/volume de

solução. As amostras foram também expostas a tratamentos de ultra-violeta de modo a simular as

condições atmosféricas.

Após a recuperação das fibras, estas foram submetidas a vários testes de caracterização de modo a

perceber qual a melhor combinação de tratamentos a utilizar de modo a obter fibras de carbono

comparáveis com fibras de carbono virgens. As fibras foram submetidas a espectroscopia de Raman,

termogravimetria, difracção de raios-X, espectroscopia de infra-vermelho, testes mecânicos e

microscopia electrónica de varrimento.

Submeteu-se a alguns destes testes uma amostra de fibras virgens e os resultados obtidos foram

comparados com os resultados das fibras recuperadas.

Observou-se que o ultra-violeta degrada a resina do compósito e danifica ligeiramente a fibra.

Observou-se também que as fibras com melhores resultados foram as que foram submetidas a uma

solução de ácido nítrico de concentração 4M e com uma razão de massa de compósito/volume de

solução igual a 4.

Palavras-chave: Fibras de carbono, solvólise ácida, ácido nítrico, compósito, UVB

iv

Abstract

The aim of this work was the recuperation of carbon fibres from an aviation composite. The solvolysis

with nitric acid was the method chosen. Various concentrations of nitric acid were utilised to various

composite weight/volume of solution ratio of composite. The samples were also exposed to ultraviolet

treatments to simulate atmospheric conditions.

After recuperating the carbon fibres, these were submitted to various characterization tests in order to

conclude which method is better to obtain carbon fibres similar to virgin carbon fibres. The carbon

fibres were submitted to Raman spectroscopy, thermogravimetry, X-ray diffraction, infrared

spectroscopy, mechanical tests and scanning electronic microscopy.

Virgin fibre samples were submitted to some of the characterization tests and the results were

compared to the recuperated carbon fibres.

It was observed that ultraviolet rays degrade the resin in the composite and slightly damage the

carbon fibre. It was also observed that the fibres submitted to the combination nitric acid at 4M and a

ratio of composite weight/volume of solution equal to 4 obtained better results in the characterization

tests.

Keywords: Carbon fibres, acid solvolysis, nitric acid, composite, UVB

v

Index

Acknowledgements .................................................................................................................................. ii

Resumo ................................................................................................................................................... iii

Abstract.................................................................................................................................................... iv

Table Index .............................................................................................................................................. vi

Figure Index ............................................................................................................................................ vii

Abbreviations ........................................................................................................................................... ix

1. Introduction .......................................................................................................................................... 1

1.1. Introduction to Embraer ................................................................................................................ 1

1.2. Carbon Fibre ................................................................................................................................. 2

1.2.1. Synthesis ............................................................................................................................... 2

1.2.3. Properties .............................................................................................................................. 3

1.3. Carbon Fibre composites ............................................................................................................. 4

1.3.1. Properties ............................................................................................................................. 4

1.3.2. Problematic ........................................................................................................................... 5

2. Bibliographic research ......................................................................................................................... 6

3. Experimental method ......................................................................................................................... 11

4. Results and Discussion ..................................................................................................................... 13

4.1. Results ........................................................................................................................................ 13

4.1.1. X-Ray Diffraction ................................................................................................................. 13

4.1.2. Scanning Electronic Microscopy ......................................................................................... 16

4.1.3. Mechanical Tests ................................................................................................................. 19

4.1.4. Raman Spectroscopy .......................................................................................................... 22

4.1.5. Thermogravimetry ............................................................................................................... 26

4.1.6. Infrared Spectroscopy ......................................................................................................... 30

4.2. Discussion of the results ............................................................................................................ 34

5. Conclusions and Future Work ........................................................................................................... 36

5.1. Conclusions ................................................................................................................................ 36

5.2. Future Work ................................................................................................................................ 37

Bibliography ........................................................................................................................................... 38

Appendix A – SEM Images.................................................................................................................... 40

vi

Table Index

Table 1 – Mechanical properties of commercial available carbon fibres ................................................ 3

Table 2 – Conditions of each experiment .............................................................................................. 12

Table 3 – XRD results of CFs treated under different reaction conditions ............................................ 15

Table 4 – Mechanical properties of the 4M 2α sample ......................................................................... 20



Table 7 – Raman results of CFs treated under different reaction conditions ........................................ 22

Table 8 – Degradation temperatures of the carbon fibres ..................................................................... 26

Table 9 – Residual mass after TGA analysis ........................................................................................ 26

vii

Figure Index

Fig. 1 – Plausible reaction mechanism for solubilisation of epoxy resin using PEG-200/NaOH

proposed by Yang et al (2012) ................................................................................................................ 6

Fig. 2 – Solubilisation product obtained by PEG-200/NaOH reaction proposed by Yang et al (2012) ... 7

Fig. 3 – Glass vessel with a solution of nitric acid and samples of CFRP ............................................ 11

Fig. 4 – XRD of carbon fibres with different treatments ........................................................................ 14

Fig.5 – SEM image of Carbon fibre composite (x1500) ........................................................................ 16

Fig.7 - SEM image of Carbon fibre treated with a 4M solution of HNO3 and 2α (x4000) ...................... 17

Fig.8- SEM image of Carbon fibre treated with 120 cycles of UV, a 4M solution of HNO3 and 2α (x3000)

............................................................................................................................................................... 18

Fig.9- SEM image of Carbon fibre with the diameter measured ........................................................... 18

Fig.10- Card specimen holder with a sample of recovered carbon fibre ............................................... 19

Fig.11- Mechanical test result of a specimen of carbon fibre ................................................................ 19

Fig.12 – Raman spectra of the carbon fibres ........................................................................................ 23

Fig.13- Kubelka-Munk Raman Spectra of the carbon fibres ................................................................. 24

Fig.14- Raman Spectra of the composite and of a polymer residue ..................................................... 25

Fig.15- Thermogravimetric analysis spectra of the recuperated carbon fibres ..................................... 27

Fig.16- DTG spectra of the recuperated carbon fibres .......................................................................... 28

Fig.17- Comparison of the degradation temperature of a sample of recuperated carbon fibre with the

virgin carbon fibre .................................................................................................................................. 29

Fig.18- Infrared adsorption spectra of the carbon fibres (virgin and recuperated) ................................ 31

Fig.19- Infrared adsorption spectra of the carbon fibres (virgin and recuperated) between 2250 cm-1

and 2500 cm-1

........................................................................................................................................ 32


and

1800 cm-1

............................................................................................................................................... 33

Fig.1A - SEM image of Carbon fibre composite (x4000) ....................................................................... 40

Fig.2A - SEM image of Carbon fibre treated with a 6M solution of HNO3 and 2α (x1500) .................... 40


Fig.4A - SEM image of Carbon fibre treated with 309 cycles of UV, a 2M solution of HNO3 and 2α

(x1500)................................................................................................................................................... 41


(x4000)................................................................................................................................................... 42

viii


(x1500)................................................................................................................................................... 42


Fig.8A - SEM image of Carbon fibre treated a 4M solution of HNO3 and 2α (x2000) ........................... 43

Fig.9A - SEM image of Carbon fibre treated with 120 cycles of UV and a 4M solution of HNO3 (x2000)

............................................................................................................................................................... 44

ix

Abbreviations

CFRP – Carbon fibre reinforced plastic

XRD – X-ray diffraction

SEM – Scanning electronic microscopy

UV – Ultraviolet radiation

GC-MS – Gas chromatography mass spectrometry

PAN - Polyacrylonitrile

FTIR – Fourier transform infrared spectroscopy

HATR – Horizontal attenuated total reflectance

1

1. Introduction

1.1. Introduction to Embraer

Created on August 19, 1969, Embraer - Empresa Brasileira de Aeronáutica, was a mixed capital

company under government control. With the support of the Brazilian Government, the Company

would transform science and technology into engineering and industrial capability. In addition to

starting the production of the Bandeirante, Embraer was commissioned by the Brazilian Government

to manufacture the EMB 326 Xavante, an advanced trainer and ground attack jet, under license of

Italian company Aermacchi.

By the end of the 70’s, the development of new products, such as the EMB 312 Tucano and the EMB

120 Brasilia, followed by the AMX program in cooperation with the companies Aeritalia (now Alenia)

and Aermacchi, allowed Embraer to reach a new technological and industrial level.

The entry into service of the new EMBRAER 170/190 family of commercial aviation in 2004, the

confirmation of Embraer’s definitive presence in the executive aviation market with the launch of new

products. Embraer is one of Brazil's largest exporters, working through three business units:

Commercial Aviation, Executive Aviation, and Defense & Security.

In 2008, Embraer announced the deployment of two new plants in Portugal, both based in the city of

Évora, east of Lisbon. The construction of the plants started in November 2010 and they were

inaugurated in September 2012. The units occupy 30 thousand cubic meters and are dedicated to

manufacture machined metal structures and composite material assemblies.

In November of 2012 Embraer’s composites factory in Évora completed its first shipset – a mostly

composite empennage for the Legacy 500 midsize twinjet. The Legacy 500 was the first Embraer

aircraft to feature a composite fuselage section.

The carbon fibre empennage was assembled in a semi-automatic line in one of two Embraer plants in

Évora, allowing for higher efficiency in operations such as drilling and joining primary

composite structures.[1]

2

1.2. Carbon Fibre

1.2.1. Synthesis

The synthesis process begins with a precursor. About 90% of the carbon fibres produced are

manufactured from polyacrylonitrile (PAN), the remaining 10% are manufactured from rayon or a

mesophase pitch.

A acrylonitrile plastic powder is mixed with another plastic, like methyl acrylate or methyl methacrylate,

and is reacted with a catalyst in a conventional suspension or solution polymerization process to form

a polyacrylonitrile plastic. The polymer is then spun into fibres. The spinning step is vital because the

internal atomic structure of the fibre is shaped during this process. The fibres are then washed and

stretched to the desired fibre diameter. The stretching helps align the molecules within the fibre and

provide the basis for the formation of the tightly bonded carbon crystals after carbonization.

The fibres are then heated in air to 200o-300

oC for 30-120 minutes. This allows the fibres to pick up

oxygen molecules from the air and rearrange their atomic bonding pattern, and thus becoming more

thermally stable.

Once the fibres are stabilized they need to be carbonized. This is accomplished by heating the fibres

to a temperature of about 1000o-3000

oC for several minutes in a furnace with an inert atmosphere. As

the fibres are heated, they begin to lose their non-carbon atoms, plus a few carbon atoms, in the form

of various gases including water vapour, ammonia, carbon monoxide, carbon dioxide, hydrogen,

nitrogen, and others. As the non-carbon atoms are expelled, the remaining carbon atoms form tightly

bonded carbon crystals that are aligned more or less parallel to the long axis of the fibre.

After carbonizing, the fibres’ surface does not bond well with epoxies and other materials used in

composite materials. To give the fibres better bonding properties, the surface is slightly oxidized. The

surface treatment process must be carefully controlled to avoid forming tiny surface defects, such as

pits, which could cause fibre failure.

After the oxidation of the fibre surface, the fibres are coated to protect them from damage during

winding or weaving. This process is called sizing. Coating materials are chosen to be compatible with

the adhesive used to form composite materials. Typical coating materials include epoxy, polyester,

nylon, urethane, and others.[2, 3]

3

1.2.3. Properties

Depending upon the precursor to make the fibre, carbon fibre may be turbostratic or graphitic, or have

a hybrid structure with both graphitic and turbostratic parts present. In turbostratic carbon fibre the

sheets of carbon atoms are crumpled together. Carbon fibres derived from PAN are turbostratic,

whereas carbon fibres derived from mesophase pitch are graphitic after heat treatment at

temperatures exceeding 2200oC. Turbostratic carbon fibres tend to have high tensile strength,

whereas heat-treated mesophase-pitch-derived carbon fibres have high Young's modulus and

high thermal conductivity. In the table below it is shown some mechanical properties of some

commercial available fibre.[2]

Table 1 – Mechanical properties of commercial available carbon fibres

Type Manufacturer Model Tensile

Strength (MPa) Young's

Modulus (GPa)

PAN Hercules Inc.

(U.S.A.) AS-4 4000 235

PAN Torey Indust.

(Japan) T300 3530 230

PAN Amoco Corp.

(U.S.A.) Thornel T600 4160 241

PAN Torey Indust.

(Japan) T1000 7060 294

Rayon Amoco Corp.

(U.S.A.) Thornol 75 2520 517

Rayon Hitco

(U.S.A.) HMG 50 0.7 345

Pitch Osaka Gas

(Japan) Danacarb F140 1800 140

Pitch Kureha (Japan)

Kureca KCF100 900 38

http://en.wikipedia.org/wiki/Polyacrylonitrile

http://en.wikipedia.org/wiki/Mesophase

http://en.wikipedia.org/wiki/Pitch_(resin)

http://en.wikipedia.org/wiki/Tensile_strength

http://en.wikipedia.org/wiki/Young%27s_modulus

http://en.wikipedia.org/wiki/Thermal_conductivity

4

1.3. Carbon Fibre composites

1.3.1. Properties

Carbon fibre reinforced plastic (CFRP) is an extremely strong and light fibre-reinforced polymer which

contains carbon fibres.

This composite can be very expensive to manufacture but it is very commonly used whenever a high

strength-to-weight ratio and rigidity is needed, for example, in aerospace, in the automotive industry, in

sports goods and in civil engineering.

Carbon fibre composites are composed by two parts: a matrix and a reinforcement. The matrix is,

usually, a resin (e.g., epoxy) to bind the reinforcements together. Carbon fibres are usually the

reinforcement, which gives the composite its strength and rigidity.

Despite its high strength-to-weight ratio, CFRP lacks a definable fatigue endurance limit, so it is

needed to be designed in considerable strength safety margins to provide suitable component

reliability over its service life.

The properties of a carbon fibre part are dependent on a number of factors. One important

consideration is knowing the direction of the property of interest. Unlike metals, carbon fibre, and

composites in general, are called anisotropic materials. That means the properties of the material

are directionally dependent. The strength of the carbon fibre is dependent on the orientation of the

fibre. On the other hand, metals, plastics, and most common materials have the same properties in

every direction. They are called isotropic materials.

In addition to being directionally dependent (anisotropic), there are many other factors in determining

the properties of carbon fibres composites. Some of the key factors are:

Type of carbon fibre and resin;

Fibre to resin ratio (fibre amount, fibre volume);

Fibre form - unidirectional, fabric, braid, chopped;

Fibre orientation - fibre layup design;

Quality - Uniformity of fibre distribution, voids, etc.

All of the factors above, or design options, are a unique advantage of carbon fibre and advanced

composite materials. A part using carbon fibre can be tailored and designed for a specific

application. The fibre type, fibre amount, fibre orientation, etc. can all be changed to achieve certain

properties, whether for mechanical reasons (strength, stiffness) or for other reasons, such as low CTE

(coefficient of thermal expansion). Metals and other materials do not have this tailor ability. For

example, on a metal part, the only thing that can be changed is the grade or alloy of metal and its

thickness or shape.

Because there are literally thousands of options when it comes to CFRPs, it is impossible to list and

explain all of the potential properties of CFRPs.[4]

5

1.3.2. Problematic

CFRP is lightweight and easier to manipulate into shapes than other materials, including various

metals, and it lasts longer than any other material. That is actually a great problem. Carbon fibre,

which at its most basic form is carbon graphite, will last virtually forever. The material is typically not

biodegradable or photo-degradable. The problem is made worse by the fact that unlike plastics,

aluminium and many other materials, CFRPs will not degrade, and it is extremely difficult to recycle.

There are, however, industries that have methods of recycling CFRPs, but the results are still

unsatisfactory.

When free of vinyl and other halogenated polymers, a pyrolysis can be used to recover the fibres.

Pyrolysis, the thermal decomposition of organic molecules in an inert atmosphere (e.g. N2), is one of

the most widespread recycling processes for CFRP. During this process, the CFRP is heated up

between 450oC to 700

oC in the (nearly) absence of oxygen; the polymeric matrix is volatilized into

lower weight molecules, while the CFs remain inert and are eventually recovered. The main

drawbacks of this method is that there is a possible deposition of char on the fibre surface and the

release of hazardous gases.[5, 6]

Solvolysis is another method for recovering carbon fibres. Using reactive solvents to break down

chemical bonds in thermoset resins, has been regarded as one of the most promising recycling

methods. Nitric acid is capable to decompose epoxy resin; however, quite long reaction time (100-400

h) and high concentration nitric acid (4-8 mol L-1

) makes this process difficult to handle. On the other

hand, monoethanolamine (MEA) titanium(IV) n-butoxide/diethyleneglycol (TBT/DEG) could also

enhance the solvolysis of epoxy resin, but the solubilisation efficiency is still unsatisfactory. Recently, it

is found that the solvolysis of thermosets using subcritical/supercritical fluids such as water, ethanol,

or propanol possesses high decomposition efficiency, but they must be carried out in high temperature

and pressures that require the use of expensive facilities.[6, 7, 8, 9, 10, 11]

Therefore, developing a new process with high efficiency under mild reaction conditions is of

considerable importance.

6

2. Bibliographic research

This thesis is based on the paper by Liu et al. (2004). Liu et al., through orthogonal experiments,

examined the recycling conditions for carbon fibres while varying the concentration of nitric acid (4M,

6M and 8M), the reaction temperature (70oC, 80

oC and 90

oC) and the weigh to volume of solution ratio

(2g:100mL, 4g:100mL and 6g:100mL). They concluded that the best combination was a

decomposition temperature at 90oC, a nitric acid solution concentration of 8M and a ratio of the

sample weight to the nitric acid solution volume of 4g:100mL. Gas chromatography-mass

spectrometry (GC-MS) and gel permeation chromatography (GPC) showed that the epoxy resin

decomposed to low molecular weight compounds, such as 2,4-dinitrophenol and 2-nitro-4-

carboxylphenol, and mechanical tests showed that the recovered fibres lost about 1.1% of single fibre

tension strength.[8]

Yang et al. (2012) verified that using poly(ethylene glycol) (PEG)/NaOH is highly efficient at recycling

epoxy resins. They state that with 40mL of PEG-200, 0.8g of NaOH at 180oC for 50 minutes the epoxy

resin is completely dissolved. They conclude that the main reaction mechanism is a ester hydrolysis

accompanied by a transetherification process and the most plausible mechanism is as follows:

Fig. 1 – Plausible reaction mechanism for solubilisation of epoxy resin using PEG-200/NaOH

proposed by Yang et al (2012)

7

The exact solubilisation products were investigated using ESI-MS (electrospray ionization-mass

spectrometry) that mainly include:

Fig. 2 – Solubilisation product obtained by PEG-200/NaOH reaction proposed by Yang et al (2012)

Yang et al. also stated that the solubilisation rate in this process is better than the solubilisation rate

obtained by Gersifi et al. (2003).[11]

Gersifi et al. (2003) stated that the association of titanium (IV) n-butoxide with diethyleneglycol proved

effective in solubilising epoxy resins. The depolymerisation of the model matrix and the composite

waste are pushed to the monomer stage and are mainly constituted by esterdiols, tetraalcohols and

excess of glycol. It was also studied the solvolysis with monoethanolamine and it was observed that

the reaction is more effective but its products are solid. Characterization of the depolymerisation

products by NMR (nuclear magnetic resonance) and MALDI-TOF (matrix-assisted laser

desorption/ionization - time of flight mass spectrometer) has confirmed the reaction mechanism of

transesterification, similar to those observed by Yang et al. (2012), and the presence of other

alcoholysis side reactions.[7]

Palmer et al. (2009) and Pickering et al. (2006) used the mechanical recycling approach, which

involves breaking down the composite by shredding, crushing, milling or other similar mechanical

processes and segregating the resulting scrap pieces, by sieving, into powdered products (rich in resin)

and fibrous products (rich in fibres). These recycled composites are usually re-incorporated in new

composites, as filler or reinforcement, and used in construction industry, as fillers for asphalt or as

mineral sources for cement. However, this method reduces significantly the mechanical properties of

the fibres and limits the possibilities for re-manufacturing.[12, 13]

Meyer et al. (2009) used pyrolysis as a recycling method. During pyrolysis, the composite is heated up

to 450oC to 700

oC in an inert atmosphere. While the fibres are inert and are eventually recovered, the

resin is volatilized into lower weight molecules. These molecules, however, are environmentally

hazardous and there is a possible deposition of char on the fibre surface that can alter the properties

of the fibres.[5, 14]

Pinero-Hernanz et al. (2008) tried various supercritical fluids to dissolve the resin and obtained

interesting results. They recovered carbon fibres with virtually no mechanical degradation, especially

8

when using propanol, and recovered useful chemicals from the matrix. However this method is very

expensive to use in industry, because of the high temperature and the high pressure that is needed to

achieve the supercritical stage of a fluid.[9, 10]

Kumar et al. (2002) observed extensive erosion of the epoxy resin when CFRPs are exposed to

ultraviolet radiation and/or condensation. They concluded that matrix dominated properties are

affected the most, with the transverse tensile strength decreasing by 29% after only 1000 h of cyclic

exposure to UV radiation and condensation. While the longitudinal fibre-dominated properties are not

affected for the exposure durations investigated, it was noted that extensive matrix erosion would

ultimately limit effective load transfer to the reinforcing fibres and lead to the deterioration of

mechanical properties even along the fibre dominated material direction.[15]

Lee et al. (2011) proposed and adapted a circulating flow reactor to a recycling process for carbon

fibre from carbon fibre reinforced epoxy composite. Lee at al. proposed a recycling system that was

composed of hexahedral circulating flow reactor made of quartz, teflon supporter, acid resistance

pump and auxiliaries. They concluded that the circulating flow reactor most effectively recycled carbon

fibre at 12M of concentration of nitric acid aqueous solution, 90ºC of decomposition temperature, 1.8L

of nitric acid aqueous solution per 100g composite and 1.0cm/sec of linear flow rate of circulating

solution. They observed that the liquid phased decomposition product was a mixture of nitrated

compounds and the tensile strength loss of the recycled single carbon fibre was 2.91%.[16]

Feraboli et al. (2011) bathed the composites in boiling sulphuric acid to separate the fibres from the

resin. They observed that the recycled reinforcement consisted of long fibres arranged in a random,

entangled mat. Then, using the same epoxy matrix and infusion materials and process, they reutilized

the recycled fibres to manufacture solid laminates. They characterized the laminates, by means of

pulse-echo ultrasound, visual microscopy, fibre volume content and tested the mechanical properties

of the laminates, and concluded that the recycled panels show trends and traits similar to those of

advanced carbon fibre and that with further development efforts, it is possible to realign the recycled

long fibres and re-establish their original performance.[17]

Shi et al. (2012) recycled glass and carbon fibres using superheated steam. These fibres were then

surface treated for reuse as fibre-reinforced polymer composite. The treated fibres were characterized

by scanning electron microscopy and remanufactured by vacuum-assisted resin transfer moulding.

They observed that most residual resin impurities were removed by the surface treatment and that the

treatment did no adverse effect on bending strength. The mechanical properties of the treated

composites were determined and compared with those of recycled reinforced polymers. The bending

strengths of recycled reinforced polymers were very low, compared to that of virgin glass fibre-

reinforced polymer and that of virgin carbon fibre-reinforced polymer. The bending strength of treated

glass fibre polymer composites was improved to about 90% of that of the virgin glass fibre one, and

the bending strength of the treated carbon fibre polymer composites was improved to about 80% of

that of the virgin carbon fibre one.[18]

9

Akonda et al. (2011) used chopped recycled carbon fibres from Recycled Carbon Fibre Ltd., UK, to

produce yarns of comingled carbon fibre and polypropylene blended with matrix polypropylene staple

fibres using a modified carding and wrap spinning process. They observed with microscopic analysis

that more than 90% of the recycled carbon fibres were aligned along the yarn axis and thermoplastic

composite test specimens fabricated from the wrap-spun yarns had 15–27.7% recycled carbon fibre

volume content. They also observed that similar to the yarn, greater than 90% of the recycled carbon

fibres comprising each composite sample made, showed a parallel alignment with the axis of the test

specimens. The mechanical tests showed that the average values obtained for tensile, and flexural

strengths were 160 MPa and 154 MPa, respectively for composite specimens containing 27.7%

recycled carbon fibres by volume. Akonda et al. concluded that with such mechanical properties,

thermoplastic composites made from recycled carbon fibres could be used as low cost materials for

many non-structural applications.[19]

McNally et al. (2007) used reclaimed carbon fibres obtained from the CARBON-CLEAN® technology,

a thermochemical process that recovers 95% of the CF, to manufacture a composite with polyethylene.

The morphology of the composites and the degree of dispersion of the carbon fibres in the matrix was

examined using scanning electron microscopy, and revealed the carbon fibres to be highly dispersed

at all loadings and strong interfacial adhesion to exist between the carbon fibres and the matrix.

Raman and FTIR spectroscopy were used to characterize the surface chemistry and potential bonding

sites of recycled carbon fibres. They observed that both the Young’s modulus and ultimate tensile

stress increased with increasing carbon fibre loading, but the percentage stress at break was

unchanged up to 5 wt % loading, then decreased with further successive addition of carbon fibres.[20]

McNally et al. (2007) concluded that carbon fibres reclaimed from carbon fibre filled epoxy prepregs

can be readily melt blended with polyethylene, the carbon fibre was highly dispersed in the matrix at

all loadings and that the Young’s modulus and tensile strength of the composites increased by 180

and 27.5%, respectively, with increasing carbon fibre loading, suggesting that some degree of

interfacial adhesion is present between the carbon fibre and polyethylene. They also concluded that

good interfacial adhesion and wetting between polyethylene and the recycled carbon fibres is possible,

in part, due to the presence of polar functional groups, observed in the FTIR spectrum of this carbon

fibre, along the length of the carbon fibre.[20]

Dannenhauer et al. (2003) proposed a process for recycling composite materials that comprise fibres

and a matrix by exposing the composite material to electromagnetic waves in the form of microwaves.

The electromagnetic waves induce an introduction of energy into the composite material, leading to a

separation of the fibres and the matrix. This method is assisted by suitable solvent, and this solvent

has to be polar. The fibres and the solvent are put in a TEFLON® container and put inside a

microwave oven with a frequency used between 300 MHz and 300 GHz. They observed that the resin

was completely removed from the fibres.[21]

Adam et al. (2014) proposed a method of extracting and recycling carbon fibres with furan-2-

carbaldehyde. This method consists in bathing the composite in the solvent and a swelling agent at a

10

temperature between 50oC and 90

oC during about 1 hour to 24 hours. It was observed that the matrix

was removed and that can be reutilized, and that the fibres were reusable as well.[22]

11

3. Experimental method

The method chosen in this work was the solvolysis approach[7]

. In this case, different concentrations of

nitric acid were used in different weight to volume ratios and with different UV light treatments.[15]

The UV light treatment simulates the environmental conditions so that we can study the effect of the

natural degradation that the carbon fibres suffer over time and the effect it has on the nitric acid

treatment. This treatment was performed in a POL-EKO Apartura UVB oven. Each cycle is composed

by three steps:

Heating to 30oC and stabilization at that temperature for 30 minutes;

Heating to 60oC and stabilization at that temperature and at 60% humidity for 2 hours and 30

minutes;

Cooling to 25oC and returning to 0% humidity and stabilizing at that temperature for 30

minutes.

The method is carried out in a glass vessel with a magnetic agitator, at 95oC for 12 hours with a 250ml

of a nitric acid solution. The nitric acid solutions were made with a Panreac nitric acid at 65% (w/w).

Fig. 3 – Glass vessel with a solution of nitric acid and samples of CFRP

Various experiments were carried out to optimize the method. The variables that were altered during

the optimization process are:

The concentration of nitric acid solution;

The mass of composite to volume of acid ratio;

12

The number of cycles of the UV treatment.

The following table describes the specific conditions of each experiment.

Table 2 – Conditions of each experiment

[HNO3] (M) α (g of composite/100mL of solution) UVB cycles

Experiment 1 4 4 0

Experiment 2 4 2 0

Experiment 3 6 2 0

Experiment 4 4 2 120



Experiment 7 8 6 0

During the experiments, some changes were observed. At 10 minutes into the reaction, the liquid

solution started showing a light yellow colour. At 30 minutes into to the reaction, the solution is bright

yellow shows signs that the viscosity rises. At approximately 1h30 min. the first loose fibres are

starting to show in the liquid, and at approximately 4 hours most of the fibres appear loose from the

matrix.

After the 12 hour reaction is over, the mixture is cooled to room temperature and then is filtered. The

fibres are then washed with running water. When the water washing the fibres is no longer coming out

yellow, the fibres are considered clean and then they dried in a stove at 120oC for two days. After this

the fibres were characterized.

13

4. Results and Discussion

4.1. Results

After the experiments, the recovered carbon fibres were submitted to various tests so that it was

possible to characterize this fibres and the polymer that was in the liquid residue.

The fibres were submitted to x-ray diffraction (XRD) (Fig. 4), scanning electronic microscopy (SEM)

(Fig. 5-9), mechanical tests (Table 4-6), Raman spectroscopy (Fig. 12-14; Table 7), thermogravimetry

analysis (Fig. 15-17; Table 8 and 9) and infrared spectroscopy (Fig. 18-20).

4.1.1. X-Ray Diffraction

The fibres were analysed by X-ray diffraction (XRD) with a Philips PW3020, at room temperature,

using Cu Kα radiation (λ= 1.541874 Å) generated at 40 kV and 30 mA, in the range (2θ) 10 to 35o, with

a step of 3 o/min. with an aluminium support. The results are shown bellow in Fig. 4.

The X-ray diffractogram is used to test the structure of CFs as shown in Fig. 4. The detailed data are

listed in Table 3.

It is seen that a strong peak ((002) plane) exists at approximately 2θ = 25.5o. The interlayer spacing

(d002) and apparent crystallite thickness (Lc) derived from (002) reflection are also used as an

experimental parameter to assess the structural properties of the CF’s Layers of the stacked crystallite

are used to measure the degree of graphitization of CF determined by Lc/d002. [23, 24]

The value of d002 was calculated using Bragg’s law (1), and the crystallite size Lc was calculated using

Scherrer’s formula (2):

λ

θ

λ

θ

Where θ is the scattering angle, λ is the wavelength of the X-rays used (1.541874 Å), and is the full

width at half maximum intensity (FWHM). The form factor K is 0.9.[23, 24]

(1)

(2)

14

Fig. 4 – XRD of carbon fibres with different treatments

0

500

1000

1500

2000

2500

3000

3500

10 15 20 25 30 35

Inte

nsi

ty

2θ(o)

C epoxi

C.F. 4M 2α

C.F. 4M 2α 249UV

C.F. 6M 2α

C.F. 4M 4α

Virgin Fiber

V. F. Pretreatment

15

Table 3 – XRD results of CFs treated under different reaction conditions

Sample β (o) 2θ(o) Lc (nm) d002 (nm) Lc/d002

Virgin fibres 4.83 26.5 1.7 0.34 5.00

V.F. Pre-treatment 5.41 25.6 1.5 0.35 4.28

C. epoxy 4.13 25.4 2.0 0.35 5.71

C.F. 4M 2α 5.92 25.9 1.5 0.34 4.41

C.F. 4M UV249 2α 5.39 25.7 1.5 0.35 4.28

C.F. 4M 4α 5.04 26.2 1.6 0.34 4.70

C.F. 6M 2α 5.56 25.7 1.5 0.35 4.28

16

4.1.2. Scanning Electronic Microscopy

The surface images of the carbon fibres were obtained with a JEOL 7001F Field Emission Scanning

Electron Microscope (FEG-SEM) in secondary and back-scattered electron modes. A thin layer of

chromium (~ 15 nm) was deposited on the surface of the films in order to minimize charging effects.

Firstly the composite was analyzed and was observed some alignment of the fibres and the matrix is

clearly shown in Fig. 5.

Fig.5 – SEM image of Carbon fibre composite (x1500)

Afterwards the fibres that were recovered from the various experiments were also analyzed.

There is signs of polymer left (white spots in the fibres) and the scales that appear in the fibres show

that the fibres were damaged during the polymeric matrix removal processes.

17

Fig.6 - SEM image of Carbon fibre treated with a 4M solution of HNO3 and 2α (x 1500)

Fig.7 - SEM image of Carbon fibre treated with a 4M solution of HNO3 and 2α (x4000)

18

Fig.8- SEM image of Carbon fibre treated with 120 cycles of UV, a 4M solution of HNO3 and 2α (x3000)

After all the fibres were analysed, the diameter of the fibres was measured (fig. 9)

Fig.9- SEM image of Carbon fibre with the diameter measured

As shown in the image above, the diameter measured is approximately 5.5 µm and is assumed that

this diameter is the same among all of the recovered fibres.

19

4.1.3. Mechanical Tests

A Instron 5566 with 10N claws was used to do the mechanical tests on the fibres.

Due to limitations of the machine, it was only possible to test fibres which were 5 centimetres long or

more.

Firstly the carbon fibres were glued to a card specimen holder (Fig. 10). Then these specimens were

placed in the equipment and underwent a strain test. The samples were subjected with a 0.08 mm/s

extension, with a 0.1s step, until the sample was broken, at the same time the tension exerted on the

sample was recorded. The results are exemplified in the image below (Fig. 11).

Fig.10- Card specimen holder with a sample of recovered carbon fibre

Fig.11- Mechanical test result of a specimen of carbon fibre

0

0,5

1

1,5

2

2,5

3

3,5

0 0,5 1 1,5

Load

(N

)

Extension (mm)

CF 4M 2α test 4

20

After all the specimens were tested, it estimated the number of fibres in all the specimen holders so

that it was possible to calculate the Young’s modulus and the ultimate tensile strength of a single

carbon fibre.

The ultimate tensile strength corresponded with the maximum load value and the Young’s modulus

corresponded with the slope in the Tensile vs Axial Extension chart.

The values of the ultimate tensile strength and the Young’s modulus are displayed below in the tables

4 through 6 . The values that are strickenthrough were not considered for the calculation of the

average.

Table 4 – Mechanical properties of the 4M 2α sample

Sample Tensile strength

(Mpa) Young's Modulus

(Gpa)

1 3297 76

2 3173 156

3 3213 138

4 3190 154

5 3166 151

6 3138 149

7 3157 146

8 3180 135

9 3167 168

10 3200 151

Average 3176 150




(Gpa)

1 4188 189

2 4293 129

3 5051 111

4 5472 70

5 4642 142

6 4183 190

7 4546 235

8 4420 195

9 4798 135

10 4098 212

Average 4569 161

21




(Gpa)

1 4510 196

2 4525 313

3 4299 249

4 4198 219

5 4156 170

6 4191 200

7 4162 187

8 4339 212

9 4359 233

10 4341 226

Average 4308 221

22

4.1.4. Raman Spectroscopy

The Raman spectra were recorded with a Labram HR 800 Evolution micro-Raman spectrometer

(Horiba Jobin– Yvon), using a 532 nm diode laser for excitation and a 600 grove/mm grating. The

laser power on the samples was 10 mW and the data were collected in the range of 100 - 3500 cm-1

at

a resolution of 4 cm-1

.

Raman spectrogram combines a prominent surface selectivity with an exceptional sensitivity to the

degree of structural disorder. Carbon mainly shows two peaks in the first order (1000–2000cm-1

). One

near 1580cm-1

is corresponding to an ideal graphitic lattice called G band while the other near

1360cm-1

is the characteristic peak of sp3 state of C called D band which is due to the existence of

structural disorder.[23, 24, 25, 26]

Table 7 – Raman results of CFs treated under different reaction conditions

Sample Raman

Shift (cm-1)

Area (x10-5) ID/IG

CF 2M 2α 1347 6.8

2.08 1582 3.3

CF 4M 2α 1350 5.1

1.33 1580 3.8

CF 4M 120UV 2α 1348 6.4

1.30 1576 4.9

CF 4M 4α 1359 5.8

1.18 1583 4.9

CF 6M 2α 1347 9.0

1.35 1571 6.6

CF 8M 6α 1358 7.3

1.32 1576 5.5

Virgin Fibre 1345 5.5

1.32 1569 4.1

V. F. Pre-Treatment

1342 7.1 1.20

1574 5.9

After the Raman spectra were obtained, it was used the Kubelka-Munk function to aid in the analysis

of the spectra, and the results are shown in Fig. 13 and Fig. 14.

The D to G band integrated intensity ratio (ID/IG) is a parameter to quantify the degree of disorder.

Usually, the smaller value of ID/IG indicates the higher degree of graphitization of the CF’s.[23, 24, 25, 26]

23

Fig.12 – Raman spectra of the carbon fibres

0 500 1000 1500 2000 2500 3000 3500

Inte

nsi

ty [

a.u

.]

Wavenumbers [1/cm]

CF 2M

CF 4M UV 120

CF 4M

CF 4M 4alfa

CF 6M

CF 8M

Virgin CF

V. F. Pre-treatment

2α

2α

2α

4α

2α

6α

24

Fig.13- Kubelka-Munk Raman Spectra of the carbon fibres

0 500 1000 1500 2000 2500 3000 3500

Inte

nsi

ty (

a.u

.)

Wavenumbers [1/cm]

CF 2M 2α 309UV

CF 4M 2α 120UV

CF 4M 2α

CF 4M 4α

CF 6M 2α

CF 8M 6α

25

Fig.14- Raman Spectra of the composite and of a polymer residue

0 500 1000 1500 2000 2500 3000 3500 4000

Inte

nsi

ty (

a.u

.)

Wavenumbers [1/cm]

Composite

CF 8M poly

26

4.1.5. Thermogravimetry

The carbon fiber samples were characterized by thermogravimetry (Netzsch STA 409 PC equipment),

under air, to evaluate the thermal stability after the recuperation process .The fibres were heated from

30oC to 1100

oC at 30

oC/min using alumina crucibles. The DTG profiles, the weight losses and the

degradation temperatures of the carbon fibres were computed using the equipment software (Table 8

and 9; Fig. 15, 16 and 17).

Some samples were analyzed twice, one test with the fibres showing less polymer (A) and one test in

which the polymer was not totally removed from the fibre (B).

Table 8 – Degradation temperatures of the carbon fibres

Sample Degradation Temperature (oC)

Virgin CF 767

CF 4M 4α 650

CF 4M 120UV 2α A 704

CF 4M 120UV 2α B 754

CF 8M 6α A 638

CF 8M 6α B 654

The degradation temperature of the composite was also calculated. The composite starts

decomposing at 399,7oC.

Table 9 – Residual mass after TGA analysis

Sample Residual mass (%)

Virgin CF 86.5

CF 4M 4α 74.3

CF 4M 120UV 2α A 68.0

CF 4M 120UV 2α B 68.0

CF 8M 6α A 78.1

CF 8M 6α B 79.7

Composite 55.9

27

Fig.15- Thermogravimetric analysis spectra of the recuperated carbon fibres

50

100

0 100 200 300 400 500 600 700 800 900 1000 1100

Weig

ht (%

)

Temperature (oC)

CF 4M 4α

Composite

CF 4M 2α 120UV A

CF 4M 2α 120UV B

CF 8M 6α A

CF 8M 6α B

Virgin CF

28

Fig.16- DTG spectra of the recuperated carbon fibres

-6,00E+00

-5,00E+00

-4,00E+00

-3,00E+00

-2,00E+00

-1,00E+00

0,00E+00

1,00E+00

-2,00E+00

-1,50E+00

-1,00E+00

-5,00E-01

0,00E+00

5,00E-01

1,00E+00

0 200 400 600 800 1000 1200

CF 4M 4α

CF 4M 2α 120UV A

CF 8M 6α A

CF 8M 6α B

Virgin CF

Composite

29

Fig.17- Comparison of the degradation temperature of a sample of recuperated carbon fibre with the virgin carbon fibre

Virgin CF

CF 4M 4α

30

4.1.6. Infrared Spectroscopy

The recycled carbon fibers were characterized by HATR-FTIR spectroscopy. The spectra in mid

infrared wave numbers range (4000-600 cm-1

) were collected with a resolution of 16 cm− 1,

using a FT-

MIR equipment from BOMEN (FTLA2000-100, ABB) with a DTGS detector. A horizontal total

attenuated reflection accessory (HATR), from PIKE Technologies, with a ZnSe crystal was used.

Sixty-four scans were accumulated for each spectrum to obtain an acceptable signal-to-noise ratio.

Furthermore, the spectra were recorded in duplicate for each sample and the average of the two

measurements was used.

The FTIR spectre of a virgin carbon fibre doesn’t show any bands, so any bands showing in a

recovered carbon fibre FTIR spectre is proof of organic residue of the epoxy resin left over from the

recovery process. The FTIR spectra are shown in the Fig. 18-20.

31

Fig.18- Infrared adsorption spectra of the carbon fibres (virgin and recuperated)

0

0,5

1

1,5

2

800 1300 1800 2300 2800

Ab

sorb

ance

Wavenumbers [1/cm]

CF 4M 4α

CF 6M 2α

CF 8M 6α

CF 8M 6α Poly

CF 2M 2α 309UV

CF 4M 2α

CF 4M 2α 120UV

Virgin CF

32


and 2500 cm-1

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

2250 2300 2350 2400 2450 2500

Ab

sorb

ance

Wavenumbers [1/cm]

CF 4M 4α

CF 6M 2α

CF 8M 6α

CF 8M 6α Poly

CF 2M 2α 309UV

CF 4M 2α

CF 4M 2α 120UV

Virgin CF

33


and 1800 cm-1

0

0,5

1

1,5

2

800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800

Ab

sorb

ance

Wavenumbers [1/cm]

CF 4M 4α

CF 6M 2α

CF 8M 6α

CF 8M 6α Poly

CF 2M 2α 309UV

CF 4M 2α

CF 4M 2α 120UV

Virgin CF

34

4.2. Discussion of the results

The FTIR spectra shows the organic residue in the fibres, as well as evidence of CO2 adsorbed in the

fibres. Generally, the recovered carbon fibres show adsorption bands around 2900 cm-1

and 1700 cm-1

which are derived from carboxylic acid and carbonyl functionality respectively. These groups are

formed due to the degradation of the epoxy resin during the recovery process[20]

.

The adsorption band present in all the recuperated fibres at around 2350 cm-1

corresponds to the CO2

adsorbed. Generally, the longer the UV treatment, the value of the CO2 adsorbed in the fibres is higher.

This adsorption band can also correspond to the C-N triple bond resulting from the nitric acid attack on

the epoxy resin[9]

.

Some fibres show an adsorption band at around 850 cm-1

, which represents evidence of epoxy

groups[9]

.

The TGA analysis shows that the fibres treated with UV lose more mass than the fibres which are not

treated with UV, however, they show more thermal stability than the other recuperated fibres. This

may be because, during the UV treatment, the graphite crystallises thus increasing the resistance to

the oxidation process.

Comparing the weight loss of the recovered fibres and the virgin fibres, it is shown that all the

recovered fibres have a higher weight loss, this is because all of the recovered fibres have a lower

thermal stability in comparison with the virgin fibres.

All the samples analyzed in the Raman show a D to G band integrated intensity ratio similar to the

virgin carbon fibres, except the samples recovered with the two treatments of 2M concentration of

nitric acid and with 309 cycles of UV treatment, which shows a higher ratio, indicating a lower degree

of graphitization. The sample recovered with the treatment of 4M concentration of nitric acid and an

alpha of 4 shows the lowest integrated ratio, which indicates a higher degree of graphitization. The

Raman results of this fibre show that it has less damage and less residue to the surface[27]

.

Comparing the Raman of the virgin fibres and the virgin fibres with a thermal pre-treatment, it is shown

that the fibres with the treatment show a higher degree of graphitization, thus the treatment is shown

to increase the strength of the carbon fibre. The recovered fibre which was treated with a 4M solution

of nitric acid and an alpha of 4 has a similar integrated intensity ratio in comparison to the virgin fibre

with the thermal pre-treatment, this tells us that the degree of graphitization is similar to the virgin fibre

with the thermal pre-treatment.

XRD spectra were also used to test the structure of CFs. The detailed data are listed in Table 3. It is

seen that a strong peak ((002) plane) exists at a 2θ = 25.5o. It is found that the layers of the stacked

crystallite of the recovered CFs are comparable with the virgin ones. It reveals that the recovered CFs

retains high degree of graphitization of the virgin CFs. These results indicate that the structure of CFs

is damaged slightly during the recycling process. Combining the SEM, Raman and XRD results, it is

35

believed that the structure of CFs is not damaged dramatically, and this can be testified by the tensile

strength[27]

.

The mechanical tests show that all the samples are in the gamma of the tabulated values for virgin

fibres and the fibres treated with 4M of nitric acid and an alpha of 4 have a higher ultimate tensile

strength. This result backs up the conclusions of the XRD and the Raman, however the sample

treated with the 6M of nitric acid shows a higher Young’s modulus.

In the SEM images the results back up the previous conclusions; the more UV cycles used, the more

resin is removed. But the SEM images also show that the more cycles used, the more damage the

fibres suffer. It also shows that, the more concentrated the solution, more resin is removed and more

damage appears on the fibres.

Without the UV treatment, the fibres treated with the 4M solution show less damage on the surface

than those treated with the 6M and 8M solutions, and the loss of resin is almost the same.

With the UV treatment, the fibres treated with 309 cycles of UV light, even though they were treated

with a 2M solution, show a lot more damage than those with less cycles and a 4M solution.

36

5. Conclusions and Future Work

5.1. Conclusions

An experimental investigation was conducted to recover carbon fibres from a epoxy resin. Specimens

of carbon fibre reinforced plastic were subjected to four different concentrations of nitric acid, three

different treatments of UV cycles and three different initial weights of composite. The recovered fibres

were then physically, chemically and mechanically characterized.

From the SEM images, it was concluded that the UV treatments help in the removal of the epoxy resin,

but it also damages the carbon fibres. Also, it was shown that higher concentrations of nitric acid

damages the carbon fibres and the effectiveness of the removal of the epoxy resin does not justify the

usage of such high concentrations.

From the FTIR results, it was observed that all the recovered carbon fibres contain residues of the

epoxy resin, but the treatment with a concentration of 4M of nitric acid and 4α and the treatment with a

concentration of 6M of nitric acid and 2α were the more effective treatments in the removal of the

epoxy resin.

From the Raman and the XRD results it was concluded that the recuperated carbon fibres do not lose

much of their graphitization degree, which means that there was practically no significant damage to

the surface of the carbon fibres. The treatments with the best results were the fibres treated with a

concentration of 4M of nitric acid and 4α.

From the mechanical tests it was concluded that the recuperated carbon fibres maintain their tensile

strength, comparing it with tabulated values. The recuperated carbon fibre with the highest tensile

strength was treated with a concentration of 4M of nitric acid and 4α.

From the TGA analysis it was shown that the recuperated carbon fibres lose some of their thermal

stability and that the carbon fibres treated with the UV lights have a higher thermal stability due to the

crystallization of the graphite.

In conclusion, it is possible to recuperate carbon fibres from an epoxy resin with nitric acid and that the

best concentrations of nitric acid are between 4M and 6M, the best α is between 2 and 4 and that the

UVB light treatment is helpful in removing the epoxy resin, but too much exposure can damage the

carbon fibres.

37

5.2. Future Work

Despite the advances made in this thesis, the continuation of this project is necessary. There are still

many ways to develop and improve, such as those shown hereinafter.

Obtain the tensile strength of all recuperated carbon fibres, so that it is possible to ascertain which is

the better treatment option.

The comparison of the mechanical tests with virgin fibres is invaluable, so that we can see if there is a

significantly loss of strength in the recovery methods.

Analyzing the liquid residue is important, so that we can see if it is possible to reuse the polymer.

Optimize the number of UV cycles. It was shown that the UV treatment helps in the removal of the

resin, but it also damages the fibres. With the optimization it will be possible to minimize the damage

of the fibres and obtain good results in the removal of the resin.

A alternative to nitric acid was encountered during this thesis. The use of polyethylene glycol with

sodium hydroxide shows some promise and a comparison of methods could prove interesting if this

method is capable of recovering the carbon fibres.

The energy costs are important to the industry, so it is needed to see if these methods are too energy

consuming and it is needed to choose the best method with the lowest energy cost.

The scale up of the best method to a pilot scale is the most important improvement that is needed to

approach, since is the last step before introducing it to the industry and there is still variables that were

not approached in this thesis, such as pumps, heat exchangers and the materials to use in all the

pipes and reactor at full scale.

38

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40

Appendix A – SEM Images

Fig.1A - SEM image of Carbon fibre composite (x4000)

Fig.2A - SEM image of Carbon fibre treated with a 6M solution of HNO3 and 2α (x1500)

41



(x1500)

42


(x4000)


(x1500)

43


Fig.8A - SEM image of Carbon fibre treated a 4M solution of HNO3 and 2α (x2000)

44

Fig.9A - SEM image of Carbon fibre treated with 120 cycles of UV and a 4M solution of HNO3 (x2000)

Documents

Recycling of Carbon Fibres From Epoxy Composites · Recycling of Carbon Fibres From Epoxy Composites João Pedro dos Santos Carvalho Thesis to obtain the Master of Science Degree