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The surface of carbon fibres continuously treated by cold plasma
Composites Science and Technology 34 (1989) 353-364
The Surface of Carbon Fibres Continuously Treated by Cold Plasma
Sun Mujin, Hu Baorong, Wu Yisheng, Tang Ying, Huang Weiqiu & Da Youxian Institute of Chemistry, Academia Sinica, Beijing, People's Republic of China (Received 16 December 1987; accepted 6 August 1988)
A BS TRA C T The surface of carbon .fibres prepared from polyacrylonitrile ( P A N ) precursor have been continuously treated by means of a cold plasma. The interlaminar shear strength of carbon f b r e reinforced epoxy composites containing fibres so treated was increased from about 60 MPa to 100 MPa. There are .four possible mechanisms for this increase: (1)
Higher reactivity between fibre surface and matrix as a result of a n I
increase of
I
COOH, - - C - - O H , and z C z O [
groups on the fibre
J
surface. (2)
The surface constitution was changed by the plasma treatment so as to improve the wetting properties offibre surface. The contact angle 0 between water and the carbon f b r e was decreased from 75° to 61 °. (3) Electron micrographs of the surJace of carbon fibre show that the surface striations and surface roughness were changed increasingly on fibre surfaces after plasma etching. This increases the interracial adhesion and the effect of mechanical interlocking. (4) The strength of the carbon fibre was decreased very little (about 1"6%) by this surface treatment method. 353 Composites Science and Technology 0266-3538/89/$03"50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain
354
Sun Mujin et al. Analysis of fracture morphology by scanning electron microscopy ( S E M ) indicates tha¢ debonding and fibre pull-out between fibre and matrix do not occur in the treated carbon fibre reinforced epoxy composite. These results all show that adhesion between fibre and matrix is very strong. In addition, the plasma treatment technology is very simple and the cost is low. This treatment process produces no environmental pollution and has promising future for engineering applications.
1 INTRODUCTION Carbon fibre reinforced plastic (CFRP) is a composite material possessing excellent properties including high specific strength, specific modulus, corrosion resistance, etc. For this reason, the composite has wide-ranging applications not only in the aerospace industries but also in the atomic energy industry, etc. Most developed industrial countries are therefore actively involved in research and development and investigations of new applications for CFRP. Being superficially inert, there is usually poor adhesion between the carbon fibres and c o m m o n resins, resulting in composites with low interlaminar shear strength, and the application of C F R P is therefore affected. In order to improve the interlaminar shear strength of the composites, universities and industrial organisations at home and abroad have carried out research on the surface treatment of carbon fibres for improvement of interfacial adhesion. Reports of work on carbon fibre surface treatment relate either to oxidation methods, 1-3 including oxidation in aqueous solutions such as concentrated nitric acid, the use of dry gaseous oxidants such as air, 02, 03, etc., and anodic oxidation, or coating methods, 4'5 such as chemical coatings, electro-polymerisation, layer and gaseous phase precipitated carbon coatings, and so forth. Surface grafting and electron beam radiation techniques have also been used, when the surfaces of carbon fibres are treated by these methods, the interlaminar shear strength of C F R P made from the fibres is usually 70 MPa or so. But the technologies for these treatment methods are very complicated. For example, when fibres are oxidised in concentrated nitric acid, the equipment used must have good corrosion resistance and the acid adsorbed onto the fibre surface must be properly removed. This is timeconsuming and troublesome, and, in addition, the carbon fibres lose strength as a result of the treatment. These treatment processes also produce environmental pollution, so that they have little future lbr engineering application. Consequently it is necessary to seek other treatment methods with better prospects for industrial applications.
The surface of carbonfibres continuously treated by cold plasma
355
2 EXPERIMENTAL DETAILS AND RESULTS 2.1 Materials and equipment (1)
(2)
(3)
The carbon fibres used were produced by the Shanghai Carbon Factory. They are a high strength I-type (1000 fibres per bundle) and are unsized. The Sb-1 resin matrix is mainly composed of a polyfunctional epoxy resin of good thermal resistance with 4,4'-diaminodiphenyl sulfone as the curing agent. The glass transition temperature, Tg, of the cured product is 224°C. The plasmatron equipment operates at a frequency of about 20MHz. Power is from 0 to 300W, and degree of vacuum 0"10-001 m m Hg.
2.2 Treatment process The apparatus used for these plasma treatments is illustrated in Fig. 1. Treatment procedure. Five bundles of carbon fibres (1000 fibres per bundle) were treated by plasma 02, being passed at constant velocity into the plasmatron. Composite technology. Fifteen grams of treated carbon fibres were immersed in an acetone solution of SB-1 matrix and converted into pre-preg. This was pressed into a 2 × 200 × 3 0 m m 3 unidirectionally reinforced composite in accordance with laid down moulding technology. Measurement of interlaminar shear strength. Samples were prepared and tested according to the GB 3357-82 standard. There were ten samples in each group. The samples were tested at a cross-head speed of 2 m m / m i n with a loading head radius of 2-0 _+ 0-2 mm. Test results obtained on these materials by several collaborating laboratories are shown in Table 1.
// / B
/3 4
Fig.
!.
Schemeof carbon fibre surface treated with cold plasma. 1, Carbon fibre; 2, raw fibre supply wheel; 3, Tesla coil; 4, vacuum pump; 5, treated fibre receipt wheel.
Mean strength (M Pa) Standard deviation (MPa) Coefficient of variation
\
Prop~ty ~
~lterial
Testing organisation
4"30 4.11
7.45
104-7
5"40
60.4
Composite materials with treated carbon fibres
Institute ~[" AstronauticsTechnology
Composite materials with untreated carbon fibres
TABLE
!
16-0
8.40
54"3
Composite materials with untreated carbon fibres
5-10
4.56
90"2
Composite materials" with treated carbon fibres
Beijing Aeronautics and Astronautics Institute
20"6
9"90
48' 1
Composite materials' with untreated carbon fibres
9.00
8"60
92"0
Composite materials with treated carbon fibres
Qinghua UniversiO'
The Interlaminar Shear Strength of Carbon Fibre Composite Materials
8'90
4-70
52"8
Composite materials with untreated carbon fibres
5'44
5-47
100.5
Composite materials with treated carbon fibres
Institute of Chemisto' Academia Sinica
.=_
~_ ~"
The surface of carbon fibres continuously treated by cold plasma
357
TABLE 2
Wetting Properties of Carbon Fibre Surface
~
roperties Fibre condition
01(degree) 02(degree) 03(degree)
r,(erg/cm 2) rs
Untreated carbon fibres Treated carbon fibres
77
75
52
28-77
8.04
36.81
65
61
47
29"40
15.60
45.00
0t is the contact angle of carbon fibre on water measured by the wicking method. 02 is the contact angle of carbon fibre on water measured by contact angle goniometer. 03 is the contact angle of carbon fibre on CH2I 2 measured by contact angle goniometer.
2.3 Physico-chemical property measurements on fibres and composites
Wetting characteristics of fibre surfaces A microbalance and contact angle goniometer were used to measure changes in the wetting parameter and wetting velocity following fibre surface treatment. Contact angles were measured for water and CH2I 2, and the surface energy o f the carbon fibre was calculated from the geometric mean equation. The results are shown in Fig. 2 and Table 2.
Measurement of fibre strength and diameter The tensile strengths o f 20 fibres were measured to obtain the mean value to an accuracy o f breaking load of ___0.01 g. The fibre diameter was determined 60O
~
400
~ "
400
E
E >
200 .~_
0
5
10 15 20 25 Wetting time (s)
30
Fig. 2. Relationship between wetting parameter, wetting velocity and time. Experimental temperature, 15°C; wetting agent, deionised water. (a) Untreated carbon fibre; (b) treated carbon fibre.
358
Sun Mujin et al.
(al
(b~
Fig. 3.
(a) Photograph of untreated carbon fibre ( x 15000); (b) photograph of treated carbon tibre ( x 15 000).
The surface of carbon fibres continuously treated by cold plasma
359
TABLE 3 Tensile Strength and Diameter of Fibres
ous conditions
Mean strength (GPa) Standard deviation (GPa) Coefficient of variation (%) Diameter (#m)
Untreated .fibre
Treated .fibre
Reduction
3"12 0"465 14"9 6-45
3'07 0"810 26-3 6"30
1"6
(%)
-2"3
with a Zeiss projection instrument of a magnification of × 500 with an accuracy of -I-1%. The results are shown in Table 3.
Electron micrograph analysis of the surface morphology of carbon fibres Untreated and treated carbon fibres were examined by means of a S-530 Type SEM. The results are shown in Fig. 3.
Chemical group analysis of carbon fibre surfaces. Owing to the fact that the concentration of surface groups is of the order of only a few #eq./g, the FT-IR and photoacoustic spectroscopy techniques used for these analytical studies gave no satisfactory results we therefore had no choice but to make measurements by X-ray photoelectron spectroscopy (XPS) which is very sensitive. The measurements were carried out in a Shimadzu ESCA-650B XPS spectrometer with a magnesium anode (EX = 1253-6 eV) in connection with a ESCA-PAC 660 Computer, a curve analyser and an automatically controlled gold-plating apparatus. The instrument had been calibrated so that the meter reading was the binding energy (Eb) of photoelectrons. The vacuum in the instrument was 4 × 10-8 torr, the half-width height of the Au4f½ peak was 1"2 eV. The Cls, Ols and N 1s photoelectron peaks were measured one after another and the C 1s peak was analysed by computer, as shown in Fig. 4. The binding energy is 83-8 eV based on Au4f½ of a gold-plated sample as standard. All the results are listed in Table 4.
Measurement of resin content of the composites The resin contents, measured according to the national standard of the People's Republic of China Test Method of Resin Contents of CFRP, are shown in Table 5.
Analysis of the fracture morphology of composites by SEM Fracture morphologies of sections of composites containing untreated and
Sun Mujin et al.
360
C1s 15
292 288
284 280
293
289
Eb(eV) Fig. 4.
285 281 Eb(eV)
Cls peak fitted by computer. (a) Untreated carbon fibre; (b) treated carbon fibre.
treated carbon fibres were studied in the S-530 SEM. The results are shown in Fig. 5.
Measurement of thermo-mechanical properties Samples of the composites of untreated and treated carbon fibres were cut to dimensions 25 × 2 x 6 m m and mounted in a RJY-80 Type thermomechanical analyser with load of 5 kg/cm 2 and heating rate of 2°C/min to obtain measurements of the deformation versus temperature curve. Untreated samples gave a heat distortion temperature (HDT) of 252°C and the treated samples gave a value of 260°C.
Effects of boiling water on composite strength A group of samples of dimensions 23 x 2 x 23 mm 3 of composites made from untreated and treated carbon fibres was refluxed in distilled water for TABLE 4 XPS Measurements of Carbon Fibre Surface Groups (Cls(eV)) I Fibres
COO
('00tl
V~b (eV) Content
E b (eV)
(% )
Unlrealed carbon (ibre Treated carbon fibre
291"9
1-75
("
('ontent
E b (eV)
1%, )
I 0II
Content
('
E b (eV)
(% )
0
('ontent
('
Eh (e!/)
q% )
tl
Content (% )
290
4'44
285"6
3345
287"1
10-67
284"2
5142
290
680
285"6
36-94
287"1
14-40
284'2
40"08
The surface of carbonfibres continuously treated by coM plasma
361
(a)
(bj Fig. 5. (a) Photograph of composite fracture morphology of untreated carbon fibre surface; (b) photograph of composite fracture morphology of treated carbon fibre surface.
Sun Mujin et al.
362
TABLE 5 Measurement of Fibres and Resin Contents
Composites with treated fibres
Composites with untreated fibres wt% of fibre
wt% of resin
vol% of fibre
vol% of resin
wt% of fibre
wt% of resin
vol% of fibre
vol% of resin
62"57
37'43
55"65
44"35
68'39
31"61
61"89
38"11
6 h in a round-bottom flask equipped with a condenser. The interlaminar shear strength was measured immediately after removal of the samples from the flask. These results are shown in Table 6.
DISCUSSION 1
As seen from the XPS data, the concentration o f - - C - - O H , z C ~ O and - - C O O H groups has increased after the fibre surface is treated by plasma. The surface composition of carbon fibre was changed so as to favour wetting of the surface by polar liquids, and the wettability of treated carbon fibres is increased by a factor of four. The reactivity of the surface towards epoxy resin is also increased, and these two factors, together, have led to the observed increase in the ILSS of the composites. From the SEM images of the carbon fibre surfaces we know that the surface striations and surface roughness were augmented after the fibre
TABLE 6 ILSS OF CFRP after 6 h in Boiling Water
Measured property
Institute of Astronautic Technology
Institute of Chemistr)' Academia Sinica
Composites of treated carbon fibres"
Composites of untreated carbon fibres'
No boiling Mean strength (MPa) Standard deviation (MPa) Coefficient of variation (%) Strength retained (%)
104.7 4-30 4.11
Boiling 90.2 4-56 5' 10
86-15
No boiling 52-8 4-70 8-90
Boiling 44"7 4.00 8'95
84-65
The surface of carbonfibres continuously treated by cold plasma
363
surface was etched and bombarded with plasma. This increases the surface area of the adhesive interface and causes augmented interlocking, thus raising the strength of the interfacial bond in the composites. However, it is clear that this effect may also lead to a reduction in the carbon fibre strength. The extent of any such loss must be kept as small as possible, since otherwise the overall performance of the composite will be impaired rather than improved. From the SEM fracture morphology studies, we know that much fibre/matrix debonding and fibre pull-out occur in the untreated carbon fibre reinforced epoxy composite, but these mechanisms do not appear to occur in tensile fracture of the treated fibre composites. This indicates that the interfacial bond between treated fibres and epoxy matrix is good, since if the interface adhesive strength is high debonding and fibre pull-out do not occur when the composite is broken. From the point of view of current surface analytical technology, the concentration of chemical groups on the fibre surface is of the order of only a few /teq/g and of the available methods only X-ray photoelectron spectroscopy is sufficiently sensitive for the purpose. F T I R and photoacoustic spectroscopy are inadequate. However, only relative changes in concentration can be obtained by XPS, and the problem of absolute quantitative analyses remains to be solved. The curves of deformation versus temperature for composites of untreated and treated fibres were obtained from a thermo-mechanical test. The heat distortion temperature (HDT) of the former was 252°C and that of the latter was 260°C. The strength retention ratio for the untreated fibre composites was 84.65% and for the treated fibre composites 86.15% following the boiling water test. This difference is probably due to the higher level of adhesion, resulting from improved interfacial bonding, in the surface treated fibre composites, which would lead to an improved level of thermal and moisture resistance. The contact angles obtained from the two methods used were similar, and satisfactorily capable of characterising the wetting properties of the carbon fibre surfaces.
CONCLUSIONS 1. The surface of carbon fibre has been continuously treated by cold plasma. The interlaminar shear strength of carbon fibre reinforced epoxy composites is increased from about 60 MPa to 100 MPa by this treatment, which is a very satisfactory result. 2. The plasma treatment technology is very simple and the cost is low.
364
Sun Mujin et al.
Furthermore, the treatment process produces no environmental pollution, unlike some other treatment methods currently in use. This technique therefore has considerable potential for engineering applications. 3. In addition to its use for the treatment o f carbon fibre surfaces, the method is also applicable to other inorganic or organic fibres, wool, etc. It also improves wetting, printing and dyeing of textiles. 4. The improvements brought about by this technique are due to three factors. First, the strength o f the carbon fibre is little affected by the treatment. Second, after plasma etching the fibre surface shows an increased level of surface striation and roughness. This therefore extends the adhesive interface and the effect of mechanical interlocking. Third, the surface of treated carbon fibres show an increase in the concentration of oxygencontaining groups, which improves wettability and reactivity o f the fibre towards epoxy resin, thus raising the interlaminar shear strength of epoxybased composites made from the treated fibres.
REFERENCES 1. Fitzer, E., Geigl, K. H. & Manocha, L. M., Surface chemistry of C-fiber and its influence on mechanical properties of phenic based composites. The 14 Biennial Conf. on Carbon. Extended Abstracts and Program, 1979, p. 220. 2. Zhang Zhiqian, Sun Mujin & Wu Renjie, Study on properties of carbon fibre surface. Chemistry and A dhesion, 2 (1981) 96. 3. Ehrburger, P., Hergue, J. J. & Donnet, J. B., Recent development in carbon fibre treatment. ACS Symposium Series, Number 21, Petroleum derived carbon, 1975, pp. 324-34. 4. Li Bing, Shen Sitang & Zhang Fengfan, The modification of carbon fibre surface by electropolymerization. Proc. 3rd National Composite Conference, Hongzhou, People's Republic of China, Vol. l, 1984, pp. 81-91. 5. Bai Shuzhen & Zhang Fusheng, Electropolymerization on carbon fibre and its effects on composite properties. Proc. 3rd National Composite Conference, Hongzhou, People's Republic of China, Vol. 1, 1984, pp. 92-100. 6. Kaelble, D. H., Physical Chemistry of Adhesion, Wiley-Interscience, New York, 1971, pp. 153 70.
The Surface of Carbon Fibres Continuously Treated by Cold Plasma
Sun Mujin, Hu Baorong, Wu Yisheng, Tang Ying, Huang Weiqiu & Da Youxian Institute of Chemistry, Academia Sinica, Beijing, People's Republic of China (Received 16 December 1987; accepted 6 August 1988)
A BS TRA C T The surface of carbon .fibres prepared from polyacrylonitrile ( P A N ) precursor have been continuously treated by means of a cold plasma. The interlaminar shear strength of carbon f b r e reinforced epoxy composites containing fibres so treated was increased from about 60 MPa to 100 MPa. There are .four possible mechanisms for this increase: (1)
Higher reactivity between fibre surface and matrix as a result of a n I
increase of
I
COOH, - - C - - O H , and z C z O [
groups on the fibre
J
surface. (2)
The surface constitution was changed by the plasma treatment so as to improve the wetting properties offibre surface. The contact angle 0 between water and the carbon f b r e was decreased from 75° to 61 °. (3) Electron micrographs of the surJace of carbon fibre show that the surface striations and surface roughness were changed increasingly on fibre surfaces after plasma etching. This increases the interracial adhesion and the effect of mechanical interlocking. (4) The strength of the carbon fibre was decreased very little (about 1"6%) by this surface treatment method. 353 Composites Science and Technology 0266-3538/89/$03"50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain
354
Sun Mujin et al. Analysis of fracture morphology by scanning electron microscopy ( S E M ) indicates tha¢ debonding and fibre pull-out between fibre and matrix do not occur in the treated carbon fibre reinforced epoxy composite. These results all show that adhesion between fibre and matrix is very strong. In addition, the plasma treatment technology is very simple and the cost is low. This treatment process produces no environmental pollution and has promising future for engineering applications.
1 INTRODUCTION Carbon fibre reinforced plastic (CFRP) is a composite material possessing excellent properties including high specific strength, specific modulus, corrosion resistance, etc. For this reason, the composite has wide-ranging applications not only in the aerospace industries but also in the atomic energy industry, etc. Most developed industrial countries are therefore actively involved in research and development and investigations of new applications for CFRP. Being superficially inert, there is usually poor adhesion between the carbon fibres and c o m m o n resins, resulting in composites with low interlaminar shear strength, and the application of C F R P is therefore affected. In order to improve the interlaminar shear strength of the composites, universities and industrial organisations at home and abroad have carried out research on the surface treatment of carbon fibres for improvement of interfacial adhesion. Reports of work on carbon fibre surface treatment relate either to oxidation methods, 1-3 including oxidation in aqueous solutions such as concentrated nitric acid, the use of dry gaseous oxidants such as air, 02, 03, etc., and anodic oxidation, or coating methods, 4'5 such as chemical coatings, electro-polymerisation, layer and gaseous phase precipitated carbon coatings, and so forth. Surface grafting and electron beam radiation techniques have also been used, when the surfaces of carbon fibres are treated by these methods, the interlaminar shear strength of C F R P made from the fibres is usually 70 MPa or so. But the technologies for these treatment methods are very complicated. For example, when fibres are oxidised in concentrated nitric acid, the equipment used must have good corrosion resistance and the acid adsorbed onto the fibre surface must be properly removed. This is timeconsuming and troublesome, and, in addition, the carbon fibres lose strength as a result of the treatment. These treatment processes also produce environmental pollution, so that they have little future lbr engineering application. Consequently it is necessary to seek other treatment methods with better prospects for industrial applications.
The surface of carbonfibres continuously treated by cold plasma
355
2 EXPERIMENTAL DETAILS AND RESULTS 2.1 Materials and equipment (1)
(2)
(3)
The carbon fibres used were produced by the Shanghai Carbon Factory. They are a high strength I-type (1000 fibres per bundle) and are unsized. The Sb-1 resin matrix is mainly composed of a polyfunctional epoxy resin of good thermal resistance with 4,4'-diaminodiphenyl sulfone as the curing agent. The glass transition temperature, Tg, of the cured product is 224°C. The plasmatron equipment operates at a frequency of about 20MHz. Power is from 0 to 300W, and degree of vacuum 0"10-001 m m Hg.
2.2 Treatment process The apparatus used for these plasma treatments is illustrated in Fig. 1. Treatment procedure. Five bundles of carbon fibres (1000 fibres per bundle) were treated by plasma 02, being passed at constant velocity into the plasmatron. Composite technology. Fifteen grams of treated carbon fibres were immersed in an acetone solution of SB-1 matrix and converted into pre-preg. This was pressed into a 2 × 200 × 3 0 m m 3 unidirectionally reinforced composite in accordance with laid down moulding technology. Measurement of interlaminar shear strength. Samples were prepared and tested according to the GB 3357-82 standard. There were ten samples in each group. The samples were tested at a cross-head speed of 2 m m / m i n with a loading head radius of 2-0 _+ 0-2 mm. Test results obtained on these materials by several collaborating laboratories are shown in Table 1.
// / B
/3 4
Fig.
!.
Schemeof carbon fibre surface treated with cold plasma. 1, Carbon fibre; 2, raw fibre supply wheel; 3, Tesla coil; 4, vacuum pump; 5, treated fibre receipt wheel.
Mean strength (M Pa) Standard deviation (MPa) Coefficient of variation
\
Prop~ty ~
~lterial
Testing organisation
4"30 4.11
7.45
104-7
5"40
60.4
Composite materials with treated carbon fibres
Institute ~[" AstronauticsTechnology
Composite materials with untreated carbon fibres
TABLE
!
16-0
8.40
54"3
Composite materials with untreated carbon fibres
5-10
4.56
90"2
Composite materials" with treated carbon fibres
Beijing Aeronautics and Astronautics Institute
20"6
9"90
48' 1
Composite materials' with untreated carbon fibres
9.00
8"60
92"0
Composite materials with treated carbon fibres
Qinghua UniversiO'
The Interlaminar Shear Strength of Carbon Fibre Composite Materials
8'90
4-70
52"8
Composite materials with untreated carbon fibres
5'44
5-47
100.5
Composite materials with treated carbon fibres
Institute of Chemisto' Academia Sinica
.=_
~_ ~"
The surface of carbon fibres continuously treated by cold plasma
357
TABLE 2
Wetting Properties of Carbon Fibre Surface
~
roperties Fibre condition
01(degree) 02(degree) 03(degree)
r,(erg/cm 2) rs
Untreated carbon fibres Treated carbon fibres
77
75
52
28-77
8.04
36.81
65
61
47
29"40
15.60
45.00
0t is the contact angle of carbon fibre on water measured by the wicking method. 02 is the contact angle of carbon fibre on water measured by contact angle goniometer. 03 is the contact angle of carbon fibre on CH2I 2 measured by contact angle goniometer.
2.3 Physico-chemical property measurements on fibres and composites
Wetting characteristics of fibre surfaces A microbalance and contact angle goniometer were used to measure changes in the wetting parameter and wetting velocity following fibre surface treatment. Contact angles were measured for water and CH2I 2, and the surface energy o f the carbon fibre was calculated from the geometric mean equation. The results are shown in Fig. 2 and Table 2.
Measurement of fibre strength and diameter The tensile strengths o f 20 fibres were measured to obtain the mean value to an accuracy o f breaking load of ___0.01 g. The fibre diameter was determined 60O
~
400
~ "
400
E
E >
200 .~_
0
5
10 15 20 25 Wetting time (s)
30
Fig. 2. Relationship between wetting parameter, wetting velocity and time. Experimental temperature, 15°C; wetting agent, deionised water. (a) Untreated carbon fibre; (b) treated carbon fibre.
358
Sun Mujin et al.
(al
(b~
Fig. 3.
(a) Photograph of untreated carbon fibre ( x 15000); (b) photograph of treated carbon tibre ( x 15 000).
The surface of carbon fibres continuously treated by cold plasma
359
TABLE 3 Tensile Strength and Diameter of Fibres
ous conditions
Mean strength (GPa) Standard deviation (GPa) Coefficient of variation (%) Diameter (#m)
Untreated .fibre
Treated .fibre
Reduction
3"12 0"465 14"9 6-45
3'07 0"810 26-3 6"30
1"6
(%)
-2"3
with a Zeiss projection instrument of a magnification of × 500 with an accuracy of -I-1%. The results are shown in Table 3.
Electron micrograph analysis of the surface morphology of carbon fibres Untreated and treated carbon fibres were examined by means of a S-530 Type SEM. The results are shown in Fig. 3.
Chemical group analysis of carbon fibre surfaces. Owing to the fact that the concentration of surface groups is of the order of only a few #eq./g, the FT-IR and photoacoustic spectroscopy techniques used for these analytical studies gave no satisfactory results we therefore had no choice but to make measurements by X-ray photoelectron spectroscopy (XPS) which is very sensitive. The measurements were carried out in a Shimadzu ESCA-650B XPS spectrometer with a magnesium anode (EX = 1253-6 eV) in connection with a ESCA-PAC 660 Computer, a curve analyser and an automatically controlled gold-plating apparatus. The instrument had been calibrated so that the meter reading was the binding energy (Eb) of photoelectrons. The vacuum in the instrument was 4 × 10-8 torr, the half-width height of the Au4f½ peak was 1"2 eV. The Cls, Ols and N 1s photoelectron peaks were measured one after another and the C 1s peak was analysed by computer, as shown in Fig. 4. The binding energy is 83-8 eV based on Au4f½ of a gold-plated sample as standard. All the results are listed in Table 4.
Measurement of resin content of the composites The resin contents, measured according to the national standard of the People's Republic of China Test Method of Resin Contents of CFRP, are shown in Table 5.
Analysis of the fracture morphology of composites by SEM Fracture morphologies of sections of composites containing untreated and
Sun Mujin et al.
360
C1s 15
292 288
284 280
293
289
Eb(eV) Fig. 4.
285 281 Eb(eV)
Cls peak fitted by computer. (a) Untreated carbon fibre; (b) treated carbon fibre.
treated carbon fibres were studied in the S-530 SEM. The results are shown in Fig. 5.
Measurement of thermo-mechanical properties Samples of the composites of untreated and treated carbon fibres were cut to dimensions 25 × 2 x 6 m m and mounted in a RJY-80 Type thermomechanical analyser with load of 5 kg/cm 2 and heating rate of 2°C/min to obtain measurements of the deformation versus temperature curve. Untreated samples gave a heat distortion temperature (HDT) of 252°C and the treated samples gave a value of 260°C.
Effects of boiling water on composite strength A group of samples of dimensions 23 x 2 x 23 mm 3 of composites made from untreated and treated carbon fibres was refluxed in distilled water for TABLE 4 XPS Measurements of Carbon Fibre Surface Groups (Cls(eV)) I Fibres
COO
('00tl
V~b (eV) Content
E b (eV)
(% )
Unlrealed carbon (ibre Treated carbon fibre
291"9
1-75
("
('ontent
E b (eV)
1%, )
I 0II
Content
('
E b (eV)
(% )
0
('ontent
('
Eh (e!/)
q% )
tl
Content (% )
290
4'44
285"6
3345
287"1
10-67
284"2
5142
290
680
285"6
36-94
287"1
14-40
284'2
40"08
The surface of carbonfibres continuously treated by coM plasma
361
(a)
(bj Fig. 5. (a) Photograph of composite fracture morphology of untreated carbon fibre surface; (b) photograph of composite fracture morphology of treated carbon fibre surface.
Sun Mujin et al.
362
TABLE 5 Measurement of Fibres and Resin Contents
Composites with treated fibres
Composites with untreated fibres wt% of fibre
wt% of resin
vol% of fibre
vol% of resin
wt% of fibre
wt% of resin
vol% of fibre
vol% of resin
62"57
37'43
55"65
44"35
68'39
31"61
61"89
38"11
6 h in a round-bottom flask equipped with a condenser. The interlaminar shear strength was measured immediately after removal of the samples from the flask. These results are shown in Table 6.
DISCUSSION 1
As seen from the XPS data, the concentration o f - - C - - O H , z C ~ O and - - C O O H groups has increased after the fibre surface is treated by plasma. The surface composition of carbon fibre was changed so as to favour wetting of the surface by polar liquids, and the wettability of treated carbon fibres is increased by a factor of four. The reactivity of the surface towards epoxy resin is also increased, and these two factors, together, have led to the observed increase in the ILSS of the composites. From the SEM images of the carbon fibre surfaces we know that the surface striations and surface roughness were augmented after the fibre
TABLE 6 ILSS OF CFRP after 6 h in Boiling Water
Measured property
Institute of Astronautic Technology
Institute of Chemistr)' Academia Sinica
Composites of treated carbon fibres"
Composites of untreated carbon fibres'
No boiling Mean strength (MPa) Standard deviation (MPa) Coefficient of variation (%) Strength retained (%)
104.7 4-30 4.11
Boiling 90.2 4-56 5' 10
86-15
No boiling 52-8 4-70 8-90
Boiling 44"7 4.00 8'95
84-65
The surface of carbonfibres continuously treated by cold plasma
363
surface was etched and bombarded with plasma. This increases the surface area of the adhesive interface and causes augmented interlocking, thus raising the strength of the interfacial bond in the composites. However, it is clear that this effect may also lead to a reduction in the carbon fibre strength. The extent of any such loss must be kept as small as possible, since otherwise the overall performance of the composite will be impaired rather than improved. From the SEM fracture morphology studies, we know that much fibre/matrix debonding and fibre pull-out occur in the untreated carbon fibre reinforced epoxy composite, but these mechanisms do not appear to occur in tensile fracture of the treated fibre composites. This indicates that the interfacial bond between treated fibres and epoxy matrix is good, since if the interface adhesive strength is high debonding and fibre pull-out do not occur when the composite is broken. From the point of view of current surface analytical technology, the concentration of chemical groups on the fibre surface is of the order of only a few /teq/g and of the available methods only X-ray photoelectron spectroscopy is sufficiently sensitive for the purpose. F T I R and photoacoustic spectroscopy are inadequate. However, only relative changes in concentration can be obtained by XPS, and the problem of absolute quantitative analyses remains to be solved. The curves of deformation versus temperature for composites of untreated and treated fibres were obtained from a thermo-mechanical test. The heat distortion temperature (HDT) of the former was 252°C and that of the latter was 260°C. The strength retention ratio for the untreated fibre composites was 84.65% and for the treated fibre composites 86.15% following the boiling water test. This difference is probably due to the higher level of adhesion, resulting from improved interfacial bonding, in the surface treated fibre composites, which would lead to an improved level of thermal and moisture resistance. The contact angles obtained from the two methods used were similar, and satisfactorily capable of characterising the wetting properties of the carbon fibre surfaces.
CONCLUSIONS 1. The surface of carbon fibre has been continuously treated by cold plasma. The interlaminar shear strength of carbon fibre reinforced epoxy composites is increased from about 60 MPa to 100 MPa by this treatment, which is a very satisfactory result. 2. The plasma treatment technology is very simple and the cost is low.
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Furthermore, the treatment process produces no environmental pollution, unlike some other treatment methods currently in use. This technique therefore has considerable potential for engineering applications. 3. In addition to its use for the treatment o f carbon fibre surfaces, the method is also applicable to other inorganic or organic fibres, wool, etc. It also improves wetting, printing and dyeing of textiles. 4. The improvements brought about by this technique are due to three factors. First, the strength o f the carbon fibre is little affected by the treatment. Second, after plasma etching the fibre surface shows an increased level of surface striation and roughness. This therefore extends the adhesive interface and the effect of mechanical interlocking. Third, the surface of treated carbon fibres show an increase in the concentration of oxygencontaining groups, which improves wettability and reactivity o f the fibre towards epoxy resin, thus raising the interlaminar shear strength of epoxybased composites made from the treated fibres.
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