Carbon fibre in polymer reinforcement

Cm-bon, 1577, Vol. IS. pp. 143-152.

Per@nmo Press.

Printed m Great Britain

CARBON FIBRE IN POLYMER REINFORCEMENT? J. B. DONNET and P. EHRBURGER Centre de Recherches sur la Physico-Chimie des Surfaces Solides, 24, avenue du President Kennedy, 68200 Mulhouse, France (Received 21 Ianuary

1977)

Abstract-The processing and the properties of carbon fibres are reviewed. Next the physical and mechanical properties of carbon and glass fibres are compared. Their use in polymeric composites is discussed.

1.

Wl’RODUCTlON

Within the last decades, the use of composites has made great strides in many fields, and resulted in the de-

velopment of a wide range of composites. Fibres of various kinds, as dissimilar as glass- and boron fibres have been developed and blended into polymer or metal matrixes. A noteworthy example of such development are the carbon fibres and fabric cloth, which constitute outstanding materials for the aeronautic and space industries, owing to their low density and their high theoretical elasticity modulus, which reaches 1200GPa. When the high chemical inertness of carbon is taken into account, it is no wonder that carbon fibres particularly fit these drastic conditions of fatigue and pressure. The high potential performance became the subject of much research work, aiming at economical processes of carbon fibre production. After a- brief review of carbon fibre manufacturing processes, the properties of carbon and glass fibres will be compared. The problems deriving from the incorporation of carbon fibres in polymeric matrixes will be next considered. Finally an attempt will be made at bringing out the current trends in the use of these materials, by starting from the properties of carbon- and glass fibre- reinforced plastics. 2. THISPREPARATIONOFCARBONFIBRJLS Although carbon fibres can be derived from various natural or artificial precursors, rayon has been the main starting material until the last decade. In 1%4 Bacon[ 1,2] prepared high modulus fibres derived from rayon, by stretching carbonized filaments at very high temperatures. The manufacturing of fibres by carbonization of polyacrylonitrile tibres was investigated by Shindo [3] in Japan and Watt [4] in Great Britain at about the same time. More recently, towards the end of the last decade, the need for inexpensive carbon fibres resulted in research, concerned with petroleum pitch as a starting material for high modulus fibres. tInvited paper given at the Carbon 1976Conference in BadenBaden on 30 June 1976.

Nowadays, carbon fibres available in the trade are manufactured from three main precursors: rayon, polyacrylonitrile (PAN) and also to a smaller extent petroleum pitch. A precursor is primarily expected to lead to a sufficient carbon yield. An excessive volatilization or a partial fusion may be avoided by “stabilizing” the precursor through a careful oxidation step. Moreover, the manufacturing of high modulus fibres involves the development and the orientation of the basal planes in the direction of the fibre axis. This orientation may be carried out by a plastic deformation, either of the precursor or of the carbon fibre itself. Rayon fibres have first to be stabilized by carefully pre-oxidizing at a reduced temperature, so as to increase the carbon yield. The fibres are next gradually carbonized at 950°C. The investigation of the carbonization mechanism and of the transient structures showed that the preferential orientation of the cellulosic structure disappears upon heat treatment. It results in an entanglement of the graphitic layers after carbonization. Graphitization under stress provides means for achieving a parallel orientation of the graphitic layers, an indispensable requirement for high modulus fibre. The mechanism of carbon fibre formation from PAN is quite different. Stretching the PAN fibres, the orientation of the polymeric chains is maintained during the first carbonization step by the formation of ladder polymers. This process is carried out under stress, to avoid any polymer relaxation. Extensive research work has been done to specify the structure of ladder polymers, the number of rings formed per chain, the cleavage reactions, the fixation of oxygen and the effect of applied mechanical stress. The investigation of the thermal decomposition products arising from the pyrolysis of PAN fibres demonstrated the presence of several reaction steps. Thus Fitzer [S] showed that hydrocyanic acid release occurs in two main steps: a first one between 200 and 400°C and a second one between 400 and 600°C each one corresponding in fact to different carbonization mechanisms. He also observed that the release of molecular nitrogen takes place only at temperatures above 700°C. Moreover, it has been reported that preoxidizing the 143

J. B. DONNETandP.EHRBURGER

144

polymer affects the pyrolysis mechanisms to a tremendous extent. According to Fitzer[S] hydrocyanic acid release drops greatly between 200 and 400°C as the oxidation time in air increases. This results from a decrease in scission reactions, as ladder polymer formation takes place. Consequently, two main reaction steps are to be distinguished: (if from 200 to 350°C: preoxidation step; (ii) from 300 to 1000°C: the main features of which are the gradual elimination of heteroatoms and the formation of the graphitic layers. The mechanisms of the reactions occurring during the first step (between 200 and 300°C) were extensively studied, both under inert and oxidizing atmospheres 16-121.Huron[l3] recently reported that three main steps are to be distinguished during PAN pyrolysis in this temperature range (Table 1): A thermal step, marked by the polymerization of nitrile groups and the release of liquid or gaseous molecules resulting from scission reactions in uncyclized chains. A stabilizing oxidation process, Ieading to oxygen fixation and to formation of a p~ama~etic, thermostable compound. Ultimately, in the presence of excess oxygen, a reversible oxygen fixation takes place, as well as the disappearance of the paramagnetic character. At the end of this stage , the carbonized polymer may be schematized by the following structure, which has been proposed by Grassie and McGuchanll41: r

I

1) Thermal Step - Desorption of bonded water - Polymerization of the nitrile groups T a 220'C *

[PAN]

IP,I

- Break-down of the uncondensed polymer T >,250°C +

IPI1

[P,l

+vl

fV2

v1 : HCN, nitriles v2 : “heavy” compounds 2) Oxidation step 02

[P,l

+ [P21

-

[P!-jl + n H20

3) Adsorption fin excess of oxygen) [P$

+

x o2 +

[P,l

G

: paramagnetic

p4 : diamagnetic

-j m=O, 1, 2 n=O, 1, 2

HN, m

Table1.Pyrolysis mechanism of PAN between 200and300°C (after Huron[lf])

N; i, CN (CHZ-Y* CN

During the following step (3O~l~‘C), intermolecular condensation reactions between cyclic chains lead to the release of hydrocyanic acid and then of molecular nitrogen. These various steps are schematized in Table 2, according to the mechanisms suggested by Watt et al.[lS]. Concerning the formation of the graphitic structure, X-ray diffraction measurements carried out on the

“pseudo graphitic layers” between 320 and 800°C showed that a slight alignment of the layers in the direction of the fibre axis occurs. However, the coherence length of the organized domains still remains low (La/Q 2.5 nm)[lS]. The mechanisms of pyrolysis and carbonization of pitch are littie understood as yet. Two man~acturi~ processes have been described. The first one is based on the use of isotropic pitches. The pitches are first spun under stress, so as to orientate the aromatic molecules. The thermoplastic fibre is next made infusible by air oxidation, The carbonization and graphitization treatments are carried out under stress, in order to orient further the graphitic layers of the fibre. This process is

Table 2. C~bon~tio~ of PAN fibres (after WattetaL[lS]) Temperature Oxidation at 220 OC

Pyrolysis process Formation of ladder polymer containing OH, CO and COOH groups

220-3OO'C

Break-down of non-ladder polymer

300-4oo"c

Break-down of non-ladder polymer cross-linking starting

400-7oo"c

CH4, S2, NH38 HCN evolution

700-looo"c

HCN and N2 evolution from ladder polymer lengthening and broadening of basal planes

14s

Carbon fibre in polymer reinforcement

not in common use, as the oxidation step is particularly time consuming, together with the fact that the resulting fibres have poor properties. A new pitch spinning process, starting from the mesophase, has been reportedil61. The mesophase can be oriented under the influence of a shear stress occurring during spinning. The preferential orientation is retained later on, since the mesophase-isotropic liquid transition point is higher than the decomposition temperature. The major benefits of this process are the quick stabilization of the fibres and the fact that further stretching under stress at high temperature is not necessary to obtain high modulus fibres. 3.PROPRRTIJB OF CARRONFIRRIB

However different their origin may be, there are two distinctly different types of carbon fibres: Type I fibre, which is important on account of its high modulus when it has been graphitized, and Type II fibre characterized by a lower Young’s elasticity modulus. Properties of the various fibres such as density, tensile strength and Young’s modulus are listed in Table 3. The Young’s modulus of Type I fibre is of the order of magnitude of 400 to NOGPa, which corresponds approximately to half of the theoretical crystal modulus in the basal plane. Values as hi as 880GPa have been achieved with fibres spun by starting from the mesophase[l6]. As a matter of fact values intermediary between Type I and II fibres can be obtained by varying the temperature and stretching conditions. As already mentioned, the high values of the Young’s modulus result from the structural properties of the graphitic layers, and from their preferential orientation in the fibre. A great deal of work has been centered on the elucidation of the microstructure and the texture of the fibres, by use of X-ray diffraction and/or of electron microscopy. The models suggested by Ruland [ 171and by Johnson[l8] seem to be the most relevant ones. Figure 1 depicts Ruland’s model[l7]: the graphitic layers are built up into elongated ribbons forming fibrils, the general direction of which corresponds to the fibre axis. The stacking of these layers is turbostratic. The

Fig. 1.Model of carbon fibre structure according to Ruland[l7].

a high internal porosity. This model however describes neither the joints between the ribbons, nor their arrangement in a section perpendicular to the fibre axis. A different model has been proposed by Johnson and Tyson[ll]. The structure suggested by these authors is schematized on Fig. 2. Crystallites of turbostratic carbon are stacked into columnar arrangements, which may be slightly disconnected from one another, with planes in general oriented in the direction of the fibre axis. The defective stacking of the crystallites, which may be tilted and rotated relative one to another, causes voids to ribbons condition

Table 3. Mechanical properties of carbon fibres

Precursor

T&s

r------I

Tensile Strength GPa

350 - 550

I rayon

/

Young's Modulus GPa

II

30 - 70

I

1.4 - 2.5

345 - 450

II

2.4 - 3.2

200 - 280

I

2.4 - 3.5

200 - 400

II

0.3 - 0.8

25 - 30

PAN

pitch

146

J. B. DONNET and P. EHRWRGER

Fii. 3. Carbon CbreAC, oxidized.

Fig. 2. Model of carbon fibre structureaccordingto Johnsonand

Tyson[l8]. appear, the mean width of which is 10A. A defective and discontinuous stacking thus diiferenciates this structure from the fibrillar model of Ruland, which is generally accepted. Concerning the structure of carbon fibres, evidence has been provided that carbon filaments display a fibrillar structure and that ex-PAN materials contain some very extensive organized areas at 1100°C due to stretching during the preoxidation step. Figure 3 shows a micrograph of an ex-PAN fibre, which has been thinned down by nitric acid etching; extended graphitic areas are apparent on this picture. Similarly, evidence has been given, that after exfoliation of an ex-PAN AG carbon fibre treated at 25WC, that the specific texture of carbon fibres has been retained (Fig. 4)[20]. As to the surface properties, unactivated carbon fibres display, as a rule, low surface areas ranging from 0.1 to 1.5 m*/g. Generally speaking, ex-acryl fibres have a lower surface area than ex-cellulose fibres. This difference arises from the fact that ex-acryl fibres are usually smoother than ex-cellulose fibres. During the graphitization of ex-PAN fibres, the lon-

Fig. 4. Carbon fibre AG, oxidized and de&grated. gitudinal striations on the fibre vanish, leading thus to a smoothening of the surface[21]. 4COMFOSITIONANDSURFACEF'RO~~OFGLASSFIBRES The bulk of the textile glass fibres which can be purchased in the world, is manufactured from E glass, which actually has been designed for electrical applications. The elementary compositions by weight of these glasses are shown in Table 4. All of these fibres belong to the ternary diagram SiOrAIZOrMgO, or to the quaternary diagram SiOrA120j-CaO-MgO. The S and R glass filaments have been particularly

Table 4. Elemental composition of glass fibres E Glass SiO2 %

A Glass

S Glass

R

Glass

54

72

65

60

15

0.8

25

25

CaO %

17

11.5

MgO %

5

3.2

A1203

%

B2°3 % Na20 Other

8

% oxides

21 %

1

12.5

9 10

6

147

Carbon fibre in polymer reinforcement Table 5. Mechanical properties of glass fibres Density g/cm3

Tensile

Strength GPa

Young’s

Modulus GPa

E Glass

2.6

3.5

73

S Glass

2.55

4.5

87

R Glass

2.58

4.4

86

designed to possess better mechanical properties as well as a higher ageing resistance. The mechanical properties of these filaments are shown in Table 5. The tensile strength of glass fibres, and especially of E glass, formed the subject of many investigations[22]. In fact, this property can undergo significant variations as a function of the environment to which the filament is subjected. For example, a virgin filament of E glass, put in a standard atmosphere of 65% relative humidity (RH) at a temperature of 2O”C,loses 21% of its strength within 3 months. Exposed to the same conditions, but submitted in addition to a constant stress amounting to 10% of the ultimate tensile strength, the loss in resistance reaches 36%. Evidence has been obtained that the gradual ad(4 sorption of water vapour on the filament surface accounts for the loss in strength. The presence of water layers bound in a weaker and in a stronger way has been demonstrated[23]. Scanning electron microscopy provides means for following surface changes on glass fibres, as they are tiected by ageing. Evidence has been obtained that virgin glass fibres are perfectly smooth when they emerge from the spinning nozzle [23,24], and that unevennesses occur on fibres which are a few days old, which have been maintained at 20°C and 65% RH. Although the nature of these unevennesses are not welI understood as yet, it would appear that they are corrosion sites resulting from the presence of water vapour. At the time of a subsequent ageing, these sites lead to “surface nuclei”, the size of which may reach 50-1OOnm. This phenomenon is par(b) ticularly obvious on A glass fibres, which are very sensitive to atmospheric corrosion, as a result of their high Fig. 5. (a) Virgin A glass fibre, 6 days after manufacture. sodium oxide content[23] (Figs. Sa and b). (b) Virgin A glass fibre, 3 months after manufacture. Similarly, glass fibres are very temperature-sensitive. By way of example, E glass fibres heated to tem- fibres (Type I) are compared to those of E glass fibre in peratures above 200°C for a few minutes, lose their Table 6. breaking strength, which for instance drops to one half If the original breaking stress of glass fibres is even of the original value at 500°C. higher than in the case of carbon fibres, a tremendous Although S and R glass fibres display higher original difference occurs in favour of carbon fibres with respect properties than E glass fibre, it does not seem possible as to specific moduli. The good thermal conductivity of yet to obtain a Young’s modulus higher than 1OOGPa. carbon fibres, as compared to the case of glass fibres, Thus, there is a fundamental difference with Type I also need be mentioned. carbon fibres which all display a modulus in the vicinity 5. SURFACE TREATMENT OF CARBON FIBRW of 400GPa. Moreover, as already mentioned, ageing brings about a marked decline of glass properties, a It turns out that carbon fibres display outstanding behavioural pattern which does not occur in the case of physical and mechanical properties. However, the percarbon fibres. The main physical properties of carbon formance of a fibre-resin composite is not uniquely

J. B. DONNETand P.

148

EHRBURGER

Table 6. Physical properties of carbon and glass fibres

‘“‘“,.‘:“^I

Property

Density

g/cm3

Tensile

strength

Young’s

Modulus

Specific Elongation Thermal

GPa GPa

Modulus

GPa

to break conductivity

2.6

2

3.5

400

73

200

28 4.8

-lot-1

dependent on the properties of the fibres and of the matrix. In fact, they are widely controlled by the interfacial interactions and the bonding between the components of the composite. In particular, flexural strength interlaminar shear strength and the mode of failure, are directly dependent on the peculiar nature of the interfacial interactions. The first experiments performed with carbon fibres and epoxy resin, showed that the interlaminar shear strength was unsatisfactorily low which implied a faulty fibre-resin bonding. The higher the elasticity modulus of the fibres, the more marked was this effect. Various surface treatments have been devised to overcome this drawback. Chemical vapour deposition (CVD) and surface oxidation in liquid and gas phase are ranking as the most important ones [25,26]. Chemical vapour deposition will be dealt with but only briefly in the scope of this paper. This process is dillicult and costly, but the results are excellent. In fact, using the whisker deposition of silicon carbon or pyrolytic carbon allows to increase the original shear strength by a factor of two or three. This improvement stems largely from the enlargement of the fibre-resin interfacial area. The oxidative treatments in gas phase are primarily carried out with oxygen and air between 400 and 5OO”C, e.g. in the presence of catalysts, such as copper and lead salts. Significant increase in shear strength by a factor in the vicinity of 2-3 can be achieved; however an improvement of the process is difficult, since an unduly high degradation of the fibre might take place. In liquid phase various oxidizing agents such as nitric acid, sodium hypochlorite or potassium permanganate were used. Anodic fibre treatments in various electrolytes have been developed and the processes patented; they appear to be the most commonly used in industry[27-291. Oxidizing agents produce an increase in surface area, as well as in chemical functions. It has generally been observed that oxidative treatments hardly have any effect on the surface area of fibres deriving from PAN[30]. The investigation by scanning electron microscopy of treated fibres showed, in a general way, that oxidative treatments in liquid phase result in surface smoothness[26]. The same behaviour has been observed with anodic treatments, where the smoothness increases as a function of treatment time[30]. Figures 6(a) and 6(b)

Fibres

1.9

1

% cal.s$m

E Glass

9

lb) Fig. 6. (a) Untreated AC fibre. (b) AC fibre after anodic etching. show a view of AC fibres before and after 20 min anodic oxidation, in an HN03 medium. As regards chemical functions, carbon fibre take up oxygenated functions, in particular hydroxylic, carboxylic and carbonylic ones. The nature as well as the amount of fixed functions are dependent on the fibre and on the oxidative treatment. For example, AC ex-PAN fibres (Type II) treated with nitric acid, generally exhibit carbonylic functions, as shown by ESCA spectroscopy (Fig. 7a). On the other hand, high modulus fibres treated with potassium permanganate in sulfuric medium (Hummer’s reagent) exhibit an ESCA spectrum similar to the graphitic oxide one (Fig. 7b).

Carbon libre

149

in polymerreinforcement 2,1 eV _’

F.S.C.A.

G.O.

-.-

------ AC HN$

4

eV

207.7

‘~~............AG Hummers

294

: :

!

* eV

2951 OH

0

0

f 3,5eV

0

@

benzene

cyclohexanehexone

i ;

LAG

initial

294

0

1,9eV

cyclohexanehexol

benzene

(a)

(b)

Fig. 7. (a) ESCA spectrum of oxidized AC fibre. (b) ESCA spectrum of oxidized AG fibre. After fibre oxidation, a tremendous increase in the shear resistance of the composites is obtained (Table 7). The values achieved are of the same order of magnitude as the shear values of glass fibre composites, i.e. from 90 to 100MPa, for equal fibre contents. No simple relationship has been found so far, between the increase in shear strength of composites and the surface properties (chemical functions, surface area). Fibre constitution on one hand and the oxidation mechanism on the other hand, seemingly take a prominent part in the modification of the interface. HerrickBl] Table 7. Interlaminar Carbon

showed thus that the presence of carboxylic functions affects the shear strength of epoxy resins reinforced with excellulose fibres in a most prominent way. However, experiments, carried out again with ex-PAN fibres, showed that the modifications applied to the interfacial structure is only partially caused by the presence of acidic groups[30]. This fact is illustrated by the results shown in Table 8. It concerns untreated fibres and fibres etched in acid and in alkaline medium. In fact, it turns out that the removal of the acidic functions by pyrolysis results in a tremendous decrease of the shear strength of

shear strength of carbon fibre-epoxy

resin composites (uf = 65%) Interlaminate

Fibre

Shear

Strength

MPa AC untreated AC

anodic

(type II)

etching

AG untreated AG

oxidation

AG

anodic

55

NaOH

90 - 95

(type I)

20

with KMn04/H2S04

etching

Table 8.

50 - 60

NaOH

Influence

of

60 - 65

the

acidic

surface

groups

on the

shear

Acidic

Fibre

Surface

grow

AC

untreated

AC anodic

etching

AC anodic

etching

heat treatment

ueq/g

of the composites

Interlaminate

Shear

Strength

MPa

7

55

16

92

0 950 "C

strength

77

J. B.

150

DONNET

and P. EHIWJRGER

the composite. However, the original values for com-

posites reinforced with untreated carbon fibres could not be attained. The surface treatment brings yet another consequence, namely a modification in the mode of failure of the composites. Figures 8(a) and 8(b) show the various aspects of the samples after fracture in a “short beam test” of an epoxy composite reinforced with low modulus AG fibres. It is apparent that after an anodic treatment in nitric acid medium, shear stresses still cause the fibres to slide in the resin. On the other hand, after treatment in alkaline medium. the fibres break at the

Fig. 8. Aspect of the fracture of the composite. (a) Untreated AC Cbre, (b) AC fibre after acidic etching, (c) AC fibre after alkaline etching. interface without sliding out of the matrix. The same processes occur in the case of AG fibres (Type I).

6. PROPER~ AND USESOF CARBON FmRERmNco-

Both carbon and glass fibres are used mostly with the purpose of reinforcing thermosetting and thermoplastic resins. Table 9 collects the properties of various glass fibrereinforced composites. The tabulated values correspond to a 60% fibre volumic content. As expected, the values of the flexural modulus differ widely. Recently Theberge et al.[32] compared the properties of a material reinforced with glass and carbon fibres (fibre content: 40%). Once more the results (Table 10) bring out significant differences for the flexural strength and the flexural modulus. It is noteworthy that carbon fibre-reinforced resins rank also over glass fibre composites as regards fatigue strength. Under similar investigation conditions, when the strength of carbon fibre-reinforced composites decreases by 20%, glass fibre composites lose 50% of their resistance. The use of carbon fibres pertains mostly to areas where mechanical strength and stiffness, as well as a low density are essential, in so far as high cost prices, par-

(‘4

Table 9. Properties of reinforced epoxy resin (uf = 60%)

property

Density Flexural

Strength

Flexural

Modulus

Shear

Strength

Compression

Strength

E Glass

R Glass

Fibre

Fibre

1.6

1.8

1.8

1500

1300

1600

200

44

51

60

60

90

500

700

Carbon

Fibre

800

I

151

Carbon fibre in polymer reinforcement Table 10. Properties of reinforced nylon 6/6 (uf = 40%) Carbon

Property

-3

Density,

g cm

Tensile

Strength,

Flexural

Strength,

Flexural

Modulus,

Shear

Strength,

MPa MPa GPa MPa

Fibre

(after Thebergeet al.[32])

Glass

Fibre

Unreinforced resin

1.34

1.46

1.14

280

220

80

420

300

105

24

11

98

89

with glass fibres, are considered as secondary. The aeronautical and space industries offer them a tremendous field of applications, for local and structural strengthening[33-361. Yet, their utilization and the development of large size structures is kept down by their low impact strength. Outside of the aeronautical industry, their use may be taken up for reinforcement of coachworks, of masts for sailing vessels, of golf clubs, vaulting poles and ski tips. As to the future prospects of glass and carbon fibres, the following trends become apparent: In the case of glass fibres, the investigations are particularly directed towards the improvement of mechanical properties and of the tensile strength. From investigations of the surface compositions of glass fibres, it becomes apparent that their chemical stability is primarily determined by the growth of the surface layers and by the migration of the ions in the silicate lattices [23]. As to the carbon fibres, the endeavours are aiming at obtaining cheaper fibres, particularly starting from pitch, even if ultimately the outstanding mechanical properties are not retained. In this respect pitchspinning by starting from the mesophase is a greatly promising prospect. Provided their price becomes competitive in comparison with glass fibres, the markets for carbon fibres are bound to expand in the near future. It is most likely that carbon fibres will meet with outstanding applications in cases where their uncommon properties can fully be used to advantage. titularly in comparison

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J. B.

DINNET

and P. EHIWJRGER

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