epoxy filament wound composites

COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 2261–2270 www.elsevier.com/locate/compscitech

Effect of new epoxy matrix for T800 carbon fiber/epoxy filament wound composites Weiming Chen 1, Yunhua Yu 1, Peng Li, Chengzhong Wang, Tongyue Zhou, Xiaoping Yang * The Key Laboratory of Beijing City on the Preparation and Processing of Novel Polymers, Beijing University of Chemical Technology, P.O. Box 34, Beijing 100029, China Received 29 April 2006; received in revised form 30 September 2006; accepted 23 January 2007 Available online 11 February 2007

Abstract A new epoxy resin matrix with good adherence to T800 carbon fibers (T800 CFs) in filament winding was developed by addition of hardener and resin diluter. Interfacial behavior of the T800 CF/epoxy composites was analyzed according to the Naval Ordnance laboratory (NOL) ring test, short-beam-shear test and fracture surface observation. Meanwhile, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) were used in analysis of the interfacial behavior. The interfacial properties of the T800 CF/epoxy filament wound composites were improved by optimizing the matrices through increasing the toughness and reducing the viscosity, which is an important factor in influencing the wettability of T800 CFs. The Interlaminar shear strength (ILSS) of the unidirectional T800 CF/epoxy composites and the tensile strength of NOL-ring in this work reached to 123 and 2570 MPa, respectively. Also, the interfacial adhesion was much improved by the chemical reactions between the new matrix and the sizing on the T800 CFs.  2007 Elsevier Ltd. All rights reserved. Keywords: A. T800 carbon fiber; E. Filament winding; A. Composites; B. Interfacial properties; A. Resin matrix

1. Introduction Mechanical performance of carbon fiber reinforced polymer (CFRP) composites depends not only on the properties of reinforcing fiber and matrix, but also on the fiber/ sizing and sizing/matrix interfacial properties [1]. Therefore, many scientific efforts have been devoted to modify carbon fibers by a variety of methods such as gas-phase, liquid-phase and continuous anodic oxidation [2–6], and then to apply a very thin (usually on nanometer scale) coating of a prepolymer or resin to the modified carbon fiber surface [7–10] for the purpose of improving the interfacial properties between the carbon fibers and matrix [11,12], *

Corresponding author. Tel.: +86 10 6441 2084. E-mail addresses: [email protected] (W. Chen), [email protected]. edu.cn (Y. Yu), [email protected] (X. Yang). 1 Tel.: +86 10 6442 7698. 0266-3538/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.01.026

and meanwhile to prevent the fibers from damage through the process of manufacture. Many investigations on the nature of the fiber/sizing and sizing/matrix interphases have proved that the nanometer scale sizing plays a dominant role in the interfacial adhesion of the carbon fiber to resin matrix [13,14]. The functional groups of the sizing can react and/or interact with the matrix, which give rise to strong adhesion between the fiber and the matrix. The filament winding process is an efficient and viable technique, and commonly using in the mass production of advanced fiber reinforced composites such as pressure vessels, pipes and shafts. Many of these kinds of filament wound composites are also using in such applications as aviation and spaceflight for their high strength, light weight and high stiffness. However, owing to the extreme inertness of surface caused by the alignment of graphitic crystallites [15,16], high performance carbon fibers such as T800/ T1000 or M50J/M60J always failed to reach the expected

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epoxy resin was provided by Tianjin Jindong chemical factory (epoxy value, 0.85), and the tetraglycidyl diaminodiphenyl methane (TGDDM) type epoxy resin was supplied by Shanghai Institute of Synthetic Resins (epoxy value, 0.80). MeTHPA, DDM and 2,4-EMI, bought from Tianjin Synthetic Material Research Institute, China, were used as hardeners, 1 phr of 2,4-EMI was added into matrix when the hardener was MeTHPA. The unit phr is an abbreviation of part per one hundred base resins. A mixture hardener of two diethyltoluene diamine (DETDA) isomers (74–80% 2,4-isomer and 18–24% 2,6-isomer) was purchased from Lonza, Switzerland. The chemical structures of the resins and hardeners are shown in Table 1. The liquid aromatic diamine (DETDA) was selected to be a less reactive hardener with an epoxy resin [17]. So in this work, it was mixed with the hardener DDM in a specific ratio to prolong the shelf life of the matrix. A diglycidyl ether type diluter with a low molecular weight was synthesized by authors to reduce the viscosity of resins. Both T300 and T800 CFs were obtained from Toray Co., Japan. T800 CFs possess a diameter of 5 lm, a tensile strength of 5.5 GPa, a modulus of 294 GPa and an elongation of 1.9%.

properties in their reinforced composites. Therefore, modification of the interfacial behaviors between carbon fiber and matrix, and between matrix and sizing is a feasible way to improve the mechanical properties of the high performance CFRP. In this research, development of new epoxy resins is tried to obtain high performance T800 CF/epoxy composites. For this purpose, a combination of hardeners was carried out and a diluter was added to the combined matrix, and then the effect of the new epoxy matrix was investigated according to the NOL-ring test and other mechanical property test methods. SEM, FT-IR, XPS and AFM were used to analyze the interfacial behaviors. Also, sizing effect was studied in consideration of chemical reactions between sizing and matrix, which could improve the performance of composites. 2. Experimental 2.1. Materials Three types of epoxy resins were used in this work. The diglycidyl ether of bisphenol A (DGEBA) type epoxy resin was supplied by Yueyang resin factory China (epoxy value, 0.51), the diglycidyl ester of aliphatic cyclo (DGEAC) type Table 1 Chemical structures of the materials Materials

Chemical structures CH3

DGEBA CH2 CH CH2 O O

DGEAC

CH 3

C

O

CH2 CH

CH3

O

CH 2

O

OH

CH3

O

C O CH2 CH CH2

O

C O CH2 CH CH2 O

O

TGDDM

O

O CH2 CH CH2

CH2 CH CH2 N

CH2

N CH2 CH CH2 O

CH2 CH CH2 O

MeTHPA

O

H3C

O O

DDM H2 N

DETDA

CH 2

H2N H3C

(2,6-isomer)

H2N

2-methyl-4-eshyl-EMI

NH 2

C2 H5

C2H5

+

H3C

C2H5

H N C2H5 H 3C

Diluter

N

CH2-CH-CH2O-(CH2) 4-O-CH2-CH-CH2 O O

H 2N

C

n

NH2 (2,4-isomer) C2H5

O CH2

CH CH2 O

W. Chen et al. / Composites Science and Technology 67 (2007) 2261–2270

A

150 mm

A

Section A Fig. 1. Schematic of NOL-ring specimens.

2.2. Specimen preparation and testing The resin casts and unidirectional composite laminates with 60% volume content of carbon fibers were cured at 80 C/2 h + 120 C/2 h + 150 C/4 h when the hardener was MeTHPA and at 80 C/1 h + 120 C/2 h + 150 C/ 3 h + 180 C/1 h when the hardener was amine. A NOL-ring, as shown in Fig. 1, is one kind of filament wound composite sample. Not only can it reflect the capability of the composite to transfer load, but also assess the interfacial adhesion of the composite [18,19]. In this work, NOL-rings were produced on a filament winding machine (MAW20-LS1-6, Mikrosam Co., Macedonia) with a winding tension of 25 N. The cure cycles were the same as that of the unidirectional composite laminates. The tensile strength of the NOL-ring was tested on an Instron-1196 universal testing machine at a rate of 5 mm/min [20]. It should be noted that the tensile strength (r) can be determined as r ¼ p=2tw; where p is the ultimate burst force recorded in Newton (N), t and w are the thickness and width of the NOL-ring in millimeter (mm), respectively. Six specimens were tested for each experiment to get an average value.

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According to the ASTM D2344, short-beam-shear test of the unidirectional composites was carried out on an Instron-1196 universal testing machine at a span-to-depth ratio of 6/1. More than six composite specimens with dimensions of 20 mm · 6 mm · 2 mm were selected for each ILSS test. The commercially sized fibers were desized according to the method as reported [21,22]. Firstly, the carbon fibers were immersed in dichloromethane (CH2Cl2) for 1 week, then washed with distilled water for 2 days, and then dried under vacuum at 110 C for 24 h. Finally, the CH2Cl2 was evaporated for residue analysis. 2.3. Analysis and characterization FT-IR spectra were taken with a Magna IR TM Spectrometer 550 (Nicolet) in the reflectance mode. Data acquisition was performed automatically using an inter-faced computer and a standard software package. The samples were dried under vacuum at 110 C. The sample chamber was kept purged with nitrogen during the whole experiment. The surface of carbon fibers was analyzed using a VG Scientific LAB MK-II X-ray photoelectron spectrometer (XPS). The spectra were collected using a Mg Ka X-ray source (1253.6 eV). The pressure in the chamber was held below 5 · 108 Torr during analysis. All XPS spectra were recorded at a 45 take-off angle. The samples were observed with Cambridge Stereoscan 250MK3 scanning electron microscope (SEM). All samples were coated with gold to avoid the electric charge. AFM measurement was performed on a Nanoscope III instrument by fastening a carbon fiber filament to a steel sample mount using double sided tape to observe the surficial morphology of the carbon fiber. All images were collected in air using the tapping mode with a silicon nitride probe. 3. Results and discussion 3.1. Mechanical properties of resin casts The mechanical properties of the three kinds of resin casts (DGEBA, DGEAC and TGDDM) are shown in Table 2. It was seen that the three kinds of resins with

Table 2 Tensile properties of resin casts Resin/Hardener

Tensile strength (MPa)

Elongation (%)

Tensile modulus (GPa)

DGEBA

MeTHPA DDM DDM/DETDA

60 ± 5 70 ± 5 72 ± 6

2.1 ± 0.2 2.3 ± 0.1 2.5 ± 0.2

2.2 ± 0.2 2.2 ± 0.1 2.0 ± 0.2

DGEAC

MeTHPA DDM DDM/DETDA

62 ± 5 88 ± 4 98 ± 5

2.5 ± 0.1 3.1 ± 0.2 3.7 ± 0.2

3.2 ± 0.2 3.6 ± 0.3 3.2 ± 0.2

TGDDM

MeTHPA DDM DDM/DETDA

65 ± 6 71 ± 4 75 ± 6

3.0 ± 0.3 2.9 ± 0.3 3.1 ± 0.4

2.4 ± 0.2 2.7 ± 0.5 2.8 ± 0.4

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109

ILSS (MPa)

102

101

104

91 80 67

69 72

77

40

0 DGEBA

DGEAC

TGDDM

Fig. 2. ILSS of T800 CF composites with different matrices.

the hardener of DDM or DDM/DETDA showed higher tensile properties than the resins with the hardener of MeTHPA. For the DGEBA resin casts, no matter what hardener was, all of their tensile properties were lower than those of the other two resin casts. With regard to the TGDDM resin casts, they showed tensile strengths close to those of the DGEBA resin casts but higher elongations. While the DGEAC resin casts with the mixture hardeners of DDM and DETDA showed the highest tensile strength and modulus. Therefore, DGEAC/DDM/DETDA system is recommended to a promising matrix in this work.

3.2. The ILSS and NOL-ring tensile strength of T800 CF/epoxy composites Fig. 2 shows the ILSS of the unidirectional T800 CF composites with the three kinds of resin matrices. For the DGEBA resin, no matter what kind of hardener was used, the T800 CF composites showed the lowest ILSS. The main reason is that T800 CF, like other high toughness carbon fibers [23], has an elongation of 1.9%, which needs higher toughness of matrix. However, as shown in Table 2, the DGEBA resin casts only had an elongation of the order of 2.1–2.5%, which will result in premature failure in the interphase of the T800 CF composites. In contrast, all of the T800 CF reinforced TGDDM composites have a ILSS over 100 MPa. However the T800 CF reinforced DGEAC/ DDM/DETDA composites showed the highest ILSS. Combining the data of Table 2 and Fig. 2, a good relationship between the toughness of resin casts and the ILSS of T800 CF reinforced composites is found, that is, the higher toughness of matrix is, the higher ILSS of composites will be. SEM observations of the interphase regions of the T800 CF composites after ILSS test are shown in Fig. 3(a)–(c). Compared with the T800 CF/DGEBA/MeTHPA composites [Fig. 3(a)], both T800 CF/DGEAC/DDM/DETDA and T800 CF/TGDDM/MeTHPA composites had stronger interfacial adhesion, with carbon fibers well covered by matrices. However, it is interesting to see that the changes in the tensile strength of T800 CF NOL-ring composites shown in Fig. 4 are different from the changes in the ILSS of

Fig. 3. SEM photographs of (a) T800 CF/DGEBA/MeTHPA, (b) T800 CF/DGEAC/DDM/DETDA and (c) T800 CF/TGDDM/MeTHPA.

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2265

4000

MeTHPA DDM/DETDA

1

3000

2400

2220 1889

1994

2030

2119

2021

Strength (MPa)

Tensile strength (MPa)

3200 2500

2570 2430

2510

0

2355 2220

2230

2245

-1

2000 125

1600

-2

122

123 118

116

117

110

109

-3

Viscosity (Pa⋅s/25ºC)

Viscosity Tensile strength of NOL rings ILSS

100 -4

800 0

5

10

15

20

25

30

Diluter (phr) 0 DGEBA

DGEAC

TGDDM

Fig. 4. Tensile strength of T800 CF NOL-ring.

T800 CF composites shown in Fig. 2. Because of high viscosity and short shelf life, the epoxy/DDM system was not used anymore as the matrix to make NOL-rings in filament winding. As shown in Fig. 4, the T800 CF NOL-ring composite with the DGEAC/DDM/DETDA system as matrix showed the highest tensile strength. On the other hand, the T800 CF/TGDDM NOL-ring composites did not show high enough tensile strength in spite of high ILSSs due to the high viscosity of resin, which has adversely influenced the wettability to carbon fibers in the filament winding [24]. Therefore, unlike the static process of unidirectional composites, a matrix with low viscosity and good wettability to carbon fibers is more necessarily recommended for the dynamic filament winding process. 3.3. The ILSS and NOL-ring tensile strength of T800 CF/toughened-epoxy composites In order to satisfy the requirements of the viscosity of the resin matrix and the wettability to carbon fibers in filament winding process, a viscosity diluter, which was synthesized by authors, was added to the DGEAC/DDM/ DETDA system. Table 3 shows the mechanical properties of the DGEAC/DDM/DETDA system with different viscosity. The addition of the diluter to the DGEAC/DDM/ DETDA system has reduced the viscosity of the matrix obviously. In addition, both tensile strength and elongation

Fig. 5. Change in mechanical properties of T800 CF/DGEAC/DDM/ DETDA composites with matrix viscosity.

were improved while the modulus was reduced with addition of the diluter. Fig. 5 shows the effect of matrix viscosity on the mechanical properties of T800 CF reinforced DGEAC/DDM/ DETDA composite. Both the ILSS and tensile strengths of NOL-rings were improved when the diluter was added. With 25 phr diluter, the ILSS of the T800 CF composite has reached to 122 MPa and the tensile strength of NOLring increased to 2570 MPa. In consideration of the commercial T300 or T700 CF composites with only about 90 MPa of ILSS [19,21–23], T800 CF composites show about 36% higher in ILSS value with small amount of diluter addition. These results suggested that the optimum viscosity of matrix is necessary to increase the interfacial properties of T800 CF/epoxy filament wound composites. Fig. 6 is the fracture pictures of T800 CF NOL-rings for DGEBA/MeTHPA, DGEAC/DDM/DETDA/diluter and TGDDM/MeTHPA after burst tests. Each picture on the right is the amplified section of the NOL-ring shown in the left one. As shown in Fig. 6(a), the T800 CF/ DGEBA/MeTHPA NOL-ring composite showed a fracture of delamination, while fractures of the other two NOL-rings showed that all the T800 CFs carried the load synchronously and ruptured together. These kinds of broken modes have been reported to identify whether the interfacial adhesion of NOL-ring is good or not [20]. The results confirm that DGEAC/DDM/DETDA/diluter or TGDDM/MeTHPA matrix has a better interfacial adhesion to T800 CFs. The results can also be convinced by

Table 3 Mechanical properties of the DGEAC/DDM/DETDA system with different viscosity Diluter (Phr)

Viscosity (Pa s/25 C)

Tensile strength (MPa)

Elongation (%)

Tensile modulus (GPa)

0 5 10 15 20 25 30

1.40 1.11 0.68 0.61 0.57 0.51 0.44

98 ± 5 100 ± 3 102 ± 4 101 ± 3 103 ± 5 102 ± 4 101 ± 5

3.7 ± 0.2 3.8 ± 0.2 3.8 ± 0.3 3.9 ± 0.2 4.1 ± 0.1 4.4 ± 0.1 5.0 ± 0.2

3.2 ± 0.2 2.9 ± 0.3 2.6 ± 0.2 2.5 ± 0.2 2.3 ± 0.4 2.2 ± 0.2 1.9 ± 0.4

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Fig. 6. Failure sections of NOL specimens of (a) T800 CF/DGEBA/MeTHPA, (b) T800 CF/DGEAC/DDM/DETDA/Diluter and (c) T800 CF/TGDDM/MeTHPA.

Fig. 7. SEM photographs of failure section for NOL-ring of (a) T800 CF/DGEBA/MeTHPA, (b) T800 CF/DGEAC/DDM/DETDA/Diluter and (c) T800 CF/TGDDM/MeTHPA.

W. Chen et al. / Composites Science and Technology 67 (2007) 2261–2270

the SEM photographs of the failure sections of NOL-ring composites after burst test as shown in Fig. 7. It can be seen from Fig. 7(b) and (c) that carbon fibers are well adhered by the matrix, while in Fig. 7(a) some carbon fibers are separated from matrix and pulled out with an indicator of poor adhesion.

ferent from the bands of DGEBA. The downward convex bands in 3700–3100 and 1740 cm1 are due to the vibration of O–H and C@O which may come from the curing reaction of epoxy resin and carboxylic acid anhydride [33,34]. Therefore, the sizing on T800 CFs may be the matter of DGEBA with anhydride added to form certain cross-linking. That is to say, the sizing may contain high molecular weight of DGEBA. Since all the DGEAC, the diluter and the sizing contain epoxy groups, a three-dimensional network can be formed between the sizing and the DGEAC/DDM/DETDA/ diluter matrix system by taking epoxy–amine curing reaction. The mechanism of epoxy–amine curing reactions has been widely analyzed [35–37]. The complex epoxy–amine curing reactions and the typical autocatalytic behavior make it very difficult to quantitatively characterize the structures of the interphase and the bulk resin in the T800 CF/DGEAC/DDM/DETDA/ diluter system. However, some qualitative analysis can be used to investigate the compatibility of the T800 CF with the DGEAC/DDM/DETDA/diluter matrix. The elemental compositions for three kinds of T800 CFs, which can be obtained from XPS spectrum are shown in Fig. 9. It is generally accepted that the active functional groups (C–OH; C@O; COOH) on the carbon fiber surface contribute an important role in fiber/matrix adhesion because of the possibility of forming a chemical bond between fiber and matrix. Therefore, O1s/C1s composition ratio was used to predict the adhesion between fiber and matrix [8,38]. It was seen in Fig. 9 that the unsized T800 CF showed the lowest O1s/C1s ratio, while the commercially sized T800 CF showed a higher O1s/C1s composition ratio than the unsized T800 CF, indicating that sizing is helpful to improve the interfacial adhesion between T800 CFs and matrix.

3.4. The interfacial behavior analysis of T800 CF/epoxy composites As well-known, the interface/interphase plays a vital role in stress transfer from one fiber to another in the matrix of a composite. An optimal interfacial adhesion is required for taking advantage of the excellent mechanical properties of T800 CFs in composites. For good bonding with matrix and facilitating handling during composite manufacture, carbon fibers are generally sized with different polymers after surface-treatment [25–28]. Among the different commercial sizings of CFs, DGEBA is the most common [21,22,29,30]. It has been also reported that the presence of sizing may improve the wetting of the fiber by the matrix resin and protect its reactivity [31,32]. Therefore, to analyze the interfacial behavior of the T800/ DGEAC/DDM/DETDA/diluter composite with good interfacial adhesion, sizing was analyzed first. Fig. 8 shows the FT-IR results of DGEBA and the sizings on T800 CF and T300 CF, respectively. The spectra of the sizings on T800 CF and T300 CF are almost same and also similar with that of DGEBA. As reported [21,22], the sizing obtained from T300 CF was DGEBA, with an aliphatic ether added. Therefore, it can be deduced that the sizing on the T800 CFs is also the matter of DGEBA, with an aliphatic ether added. However, it should be noted that the bands of T300 and T800 CFs in the 3700–3100 and 1740 cm1 are obvious dif-

DGEBA

2267

sizing of T800

sizing of T300

3500

3000

2500

2000

1500

1000

Wavenumber (cm-1) Fig. 8. FT-IR spectra of DGEBA resin and sizings on carbon fibers.

500

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W. Chen et al. / Composites Science and Technology 67 (2007) 2261–2270 0.4

C1S O 1S O 1S /C 1S

0.3 90

74.40

71.02

69.67

0.2

60

30

0

20.01

unsized T800

21.03

sized T800

22.11

T800 in composites

O1S /C1S

Elemental composition (%)

120

0.1

0.0

Fig. 9. Elemental composition and O1s/C1s ratio of carbon fibers.

In addition, compared with the commercially sized T800 CF, the T800 CF in the T800 CF/DGEAC/DDM/ DETDA/diluter composites showed higher content of O1s and higher ratio of O1s/C1s which possibly arose from the carbon fiber surface and the epoxy rings of matrix. This is due to the strong chemical reaction between matrix and sizing which resulted in good interfacial adhesion of T800 CF/DGEAC/DDM/DETDA/diluter composite. Therefore, we suggest that the interface/interphase adhesion of T800 CF composites is one model of ‘‘double interface/interphase’’. One is the adhesion between carbon fiber and sizing, another is that between the sizing and matrix. Because T800 carbon fibers like T300 or T700 CFs is one of commercial carbon fibers, they have been already surface treated. These treatments increase the sur-

face-active sites and improve bonding between the fibers and the resin matrix [11–14]. However, strong treatment may damage the carbon fiber surface, and decrease its properties. Some researchers reported that the treatment on the commercial carbon fiber surface did not improve the fiber/matrix adhesion well [21,22]. So, it indicates that the interface/interphase adhesion formed through the chemical reactions between the sizing and the matrix is more important in the interfacial behaviors of T800 CF composites than the adhesion formed by the T800 CFs and the sizing. In order to improve the interfacial adhesion of T800 CF composites, emphasis should be placed on the chemical interface/interphase between the sizing and the matrix rather between the T800 CFs and the sizing by developing functional groups. AFM was used to characterize the roughness of the three T800 CF surfaces. Resultant AFM images of 2 lm · 2 lm are shown in Fig. 10. Like other PAN-based CFs [38–40], all the three T800 CF surfaces have clear ridges and striations running along the axises of the fibers. As seen in Fig. 10a and b, the longitudinal ridges and the striations of unsized T800 CF are less well defined than commercial sized T800 CF. This difference presented that the original features of the surface topography were changed by the sizing [8,10]. In addition, Fig. 10c reveals that the longitudinal ridges and striations of T800 CF are covered by matrix on a nanometer scale. This is presumably due to the chemical reactions between the sizing and the matrix to form a coating along the carbon fiber [41]. These assertions obtained from Fig. 10 were corroborated with the roughness parameters in Table 4.

Fig. 10. Three-dimensional AFM topographical images of (a) unsized T800 CF, (b) commercial sized T800 CF, (c) T800 CF in DGEAC/DDM/DETDA/ diluter matrix composite.

W. Chen et al. / Composites Science and Technology 67 (2007) 2261–2270 Table 4 Main roughness parameters of the T800 CFs measured by AFM on images of 2 lm · 2 lm Carbon fibers

Ra (nm)

Rmax (nm)

Unsized T800 CF Commercial sized T800 CF T800 CF in composites

12.66 25.84 42.49

69.83 115.40 188.30

Table 4 summarizes the results of the roughness analysis of T800 CF as obtained from AFM, where Ra is the mean surface roughness value and Rmax is the maximum height roughness value. Like some other carbon fibers [8,10], the result in this work revealed that the roughness of the commercial sized T800 CF was twice higher than that of the unsized T800 CF. Comparison the roughness values of the commercial sized T800 CF with T800 CF in DGEAC/DDM/DETDA/diluter/T800 composites, the interfacial reaction between the sizing and the matrix changes the surface topography very much. The Ra and the Rmax values increased from 25.84 to 42.49 nm and from 115.40 to 188.30 nm, respectively. It was reported that the improved interfacial adhesion was strongly related to the surface structure, while the chemical component of the surface had little influences on the interfacial adhesion [42,43]. In contrast, many investigators have recognized that the surface structure does not play significant role in the adhesion and the chemical component is underlined [44,45]. Therefore, the results obtained from AFM analysis in this study suggest that the chemical interface/interphase between the sizing and the matrix plays an important role in the interfacial adhesion and composite mechanical properties. This result is in good agreement with the previous XPS and FT-IR analysis. 4. Conclusion The new epoxy matrix, DGEAC/DDM/DETDA/ diluter, has a good interfacial adhesion to T800 carbon fibers. The ILSS and tensile strength of NOL-ring of this T800 CFs reinforced composites could reach up to 123 MPa which is about 36% higher than commercial T300 or T700 composites and 2570 MPa, respectively. The chemical reaction of T800 CF/DGEAC/DDM/DETEDA/diluter was proved as an epoxy–amine reaction. Also, sizing the carbon fiber is very important to increase the performance of composite because the sizing and the matrix makes another chemical reaction and resulting in the ‘‘double interface/interphase’’ adhesion in the composites. References [1] Donnet JB, Bansal RC. Carbon fiber. 3rd ed. New York: Marcel Dekker; 1998. [2] Fitzer E, Weiss R. Effect of surface treatment and sizing of carbon fibers on the mechanical properties of CFR thermosetting and thermoplastic polymers. Carbon 1987;25(4):455–67.

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