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epoxy matrix adhesion
Colloids and Surfaces A: Physicochemical and Engineering Aspects 164 (2000) 325 – 336 www.elsevier.nl/locate/colsurfa
Effect of acid–base properties of unsized and sized carbon fibers on fiber/epoxy matrix adhesion Nursel Dilsiz 1, J.P. Wightman * Department of Chemistry, NSF Science and Technology Center for High Performance Polymeric Adhesi6es and Composites, Virginia Polytechnic Institute and State Uni6ersity, Blacksburg, VA 24061, USA Received 25 June 1999; accepted 4 August 1999
Abstract The surface energies and acid–base character of unsized and sized Zoltek fiber were investigated using dynamic contact angle analysis. The surface’ acid–base property were characterized by calculating the work of acid –base interaction according to Fowkes’ and Good’s theory. To evaluate the effect of sizing on carbon fiber surface properties, Ultem® polyimide and polyurethane (PU) sized fibers were studied in comparison with unsized fibers. It was found that the sizing on the carbon fiber tended to reduce the surface energy and cover acid – base sites. The surface of unsized and sized carbon fibers were analyzed by XPS to investigate changes in surface chemistry. XPS analysis indicated that hydroxyl groups on the fiber surface decreased with sizing (Ultem® and polyurethane). These changes in chemistry are reflected in a decrease in the polar g ps and basic g− s contribution of the surface energy of the fibers. Single fiber fragmentation testing of unsized and Ultem® and polyurethane sized fibers in epoxy matrix showed that there is a direct effect of the surface chemical changes on the fiber/matrix adhesion. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Acid–base; Contact angles; X-ray photoelectron spectroscopy; Single fiber fragmentation test; Interfacial shear strength
1. Introduction The interface between reinforcing fibers and polymeric matrices in composite materials is the controlling factor in obtaining optimum mechanical properties from composites [1 – 3]. It has been Paper honoring Professor A.W. Neumann on the occasion of his 65th birthday. * Corresponding author. Tel.: + 1-540-231-5854; fax: + 1540-231-3971. E-mail address: [email protected] (J.P. Wightman) 1 Roketsan, 06780 Elmadag/Ankara, Turkey.
recognized that the function of the interface is to transmit stress from the matrix to the reinforcing fiber. For good bonding and stress transfer, fibers are generally sized or coated with thin polymer film after surface treatment that removes weak boundary layers [4–6]. Sizing has an adhesion and wetting promotion function but is particularly important for facilitating fiber handling during composite manufacture acting as a lubricant to prevent fiber damage. The effectiveness is confirmed by the off-axis strength which is usually
0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 4 0 0 - 8
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close to the strength of the polymer [7,8]. 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 [9,10]. Wetting is a prerequisite to good adhesion [11]. However, good adhesion also requires functional groups, vis-a-vis Lewis acidic and basic sites in the interfacial region between the fiber and the matrix resin. The concepts proposed by Fowkes [12,13], regarding the short-range hydrogen bonding interactions is important in adhesion, make it possible to assess the acid – base character of the surfaces using contact angle measurements. The acid–base nature of the fiber surface is a significant factor in determining the degree of adhesion of these fibers in a given resin matrix. If the acid–base properties of the matrix resin is also determined, it should be possible to choose a fiber/matrix pair to maximize adhesion. The adhesion strength of the interface depends on the thermodynamic work of adhesion that is closely related to the surface energy of the fiber and matrix [14]. Surface energy of fibers have been determined quantitatively by measurement of the contact angles by using Wilhelmy plate technique [15–19]. An excellent comprehensive summary of both the theoretical and experimental aspects of contact angles has been published recently [20]. The objective of the present study was done to evaluate the influences of polyurethane and polyetherimide sizing on both acid – base components and polar dispersive components of the surface energy of Zoltek® carbon fibers. The surfaces of unsized and sized fibers were also analyzed by XPS to determine the elemental compositional changes of the fiber surface. The work of adhesion at the interface between the fiber and epoxy resin matrix was calculated and compared with the interfacial shear strength measured by the single fiber fragmentation test. 2. Theory — surface energy measurements
2.1. Three liquid method: acid – base approach The wetting of a solid surface by a liquid and the concept of contact angle (u) was first formalized by Young [21].
gS − gSL = gL Cos u
(1)
where gL is the surface energy of the liquid, gSL is the interfacial energy of solid/liquid interface, and gS is the surface energy of solid. Fowkes [22,23] proposed that the surface energy of a liquid or a solid has two parts, namely, the dispersion and polar components. gS = g dS + g pS
(2)
where the superscript d refers to the contribution due to London dispersion forces which are common to all materials, and superscript p relates to the Keesom polar contribution, largely made up of hydrogen bonding and dipole–dipole interactions. Owens and Wendt [24] derived the following equation for the interfacial energy between liquid and solid assuming a geometric mean combination of the dispersion and polar components: gSL = gS + gL − 2 (gSdgLd)
1/2
− 2(gSpgLp)
1/2
(3)
Hydrogen bonding has been suggested as the main attraction force at the interfaces according to recent theoretical developments of the surface energy of solids. Good et al. [25–30] wrote the total surface energy as the sum of the Lifshitz– van der Waals gLLW and acid–base gLAB components. For the solid, A gS = g LW S +gS
(4)
and for the liquid AB gL = g LW L +gL
(5)
The Lifshitz–van der Waals component includes London dispersion forces gd, Debye induction gi, and Keesom dipole–dipole forces gm. g LW = gd + gi + gm
(6)
The acid–base component (or hydrogen bonding) includes electron acceptor g + and electron donor g − components, which are not additive and are expressed as + − 1/2 g AB S = 2(g S g S )
(7)
for the solid and as − 1/2 g AB L = 2(gL + g L )
(8)
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for the liquid. Assuming a geometric mean for the interaction, the total interfacial energy between solid and liquid is g12 =g1 +g2 −2(g
g
LW LW 1/2 1 2
− 2[(g1 + g2 −)
1/2
)
+(g2 + g1 −)
1/2
(9)
The Young – Dupre equation [31] of the work of adhesion (Wa) is given as Wa= gS − gSL +gL =gL (1 + Cos u)
(10)
and substituting for (gS −gSL) in Eq. (9), the result is
(11)
of the solid is determined from The value of g LW S the contact angle of an apolar liquid such as diiodomethane on the solid in which case equation [11] reduces to LW 1/2 gL(1+ Cos u) =2(g LW L gS )
(12)
assuming the liquid has negligible acid – base interactions with the solid. With gSLW known and the contact angles obtained using different liquids on the solid, one can get two equations similar to equation [11], which can be solved simultaneously for gS + and gS −.
4.1. Materials Zoltek® carbon fibers were used without and with polyetherimide (Ultem®) and polyurethane (PU) sizings. An amine cured epoxy systems were selected as matrix resin. A difunctional epoxy, diglycidyl ether of bisphenol-A (DGEBA), Epon 828 was cured with a stoichiometric amount of an aromatic amine, 1,3 phenylene diamine (mPDA).
The surfaces of Zoltek® carbon fibers and the neat polymers were analyzed using a Perkin Elmer PHI 5400 X-ray photoelectron spectrometer (XPS). The spectra were collected using a Mg Ka X-ray source (1253.6 eV) and operated at 14 kV and 300 W with an emission current of 25 mA. The pressure inside the chamber was held below 5× 10 − 7 Torr during analysis. Both survey and high resolution XPS spectra were recorded at a 45° take-off angle. A Gaussian–Lorentzian function was used for curve fitting C 1s photopeaks. The C 1s electron binding energy was referenced at 285.0 eV.
5. Contact angle measurements
3. Polar/dispersive surface energy analysis According to Owens and Wendt [24], combining Eqs. (1) and (3) yields a linear equation gL(1+ Cos f) = 2(g dSg dL)1/2) + 2(g pSg pL)1/2
4. Experimental
4.2. Fiber surface analysis
gL(1+ Cos u) LW 1/2 − 1/2 − 1/2 =2(g LW +2[(g+ +(g+ ] L gS ) L gS ) S gL )
327
(13)
where gL, g dL and g pL are known for the test liquids and g dL and g pS can be calculated from measured contact angles. Finally, knowing the dispersive and acid and base components of the fiber and matrix, it is possible to estimate the work of adhesion between epoxy matrix (M) and the fiber (F) at the interface using Eqs. (10) and (12). 1/2 LW 1/2 − 1/2 Wad=2[(g LW +[(g+ +(gM+g − ] F gM ) F g M) F ) (14)
5.1. Wilhelmy plate technique Contact angles were measured using Wilhelmy plate technique with a Cahn dynamic contact angle analyzer. A single carbon fiber was fixed to nichrome wire with a fast curing cyanoacrylate adhesive. Then, the fiber was suspended from microbalance. Wetting force data were recorded at 1 s intervals at a speed of 20 mm min − 1. The advancing wetting forces were used to calculate carbon fiber surface energies. The perimeter of the fiber was determined using the wetting force measured for silicon oil which was assumed to give a zero contact angle with the fiber. Twenty separate carbon fibers were measured to get an average value.
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Methylene iodide (DIM) was used as the probe liquid for Lifshitz – van der Waals interactions; ethylene glycol (EG), formamide (F) and glycerol (G) were used to probe for acid – base interactions. The surface energy components of these probe liquids to be used in the fiber wetting studies are given in Table 1.
5.2. Contact angle goniometer The contact angles of liquids against the cured epoxy resin were measured by the sessile drop method with a Rame-Hart 100-00115 NRL contact angle goniometer. Before surface energy measurement the epoxy resin was cured on the glass slide and it was stored in desicator to eliminate contamination. The epoxy resin was cured at 75°C for 2 h and postcured at 125°C for 2 h. A total of 5 ml drops of liquid were carefully placed on the substrate using a microliter syringe. The contact angles of both sides of each drop were measured. Six separate measurements were done to obtain an average value for the contact angle.
6. Single fragmentation test The capacity of the fiber/matrix interface to transfer stress from matrix to the fiber through the interface can be reflected by the critical length in terms of interfacial shear strength. The strength of the interface and the fracture strength of the embedded fiber determine the length of the fiber fragments obtained. The shear-lag model developed by Kelly – Tyson [32] gives the interfacial shear strength (t or IFSS) according to the following equation:
t= sfd/2Lc
(15)
where d is the fiber diameter, and sf is the fiber tensile strength at Lc, the critical length of fragments. The fiber/matrix interfacial shear strength (IFSS) for the unsized and sized carbon fibers were determined by the single fragmentation test. A single fiber was embedded along the center line of a silicon rubber mold with dog-bone shaped cavity and strained uniaxially along the fiber axis. Epoxy resin was degassed under vacuum and carefully poured into the cavity and filled to the level of mold. The resin was cured at 75°C for 2 h and postcured at 125°C for 2 h. The specimens were then removed from mold and stored in desicator until testing. The specimen was pulled in tension with 5% specimen strain at a speed of 1 mm min − 1 and breaking process was observed under cross polarizing light. The pulling was stopped when no further breaks were observed. The elongation was 5% for carbon fiber/epoxy system (i.e. after 5% strain the fiber fragmentation reached saturation). For single fiber composite samples, the fragments length were measured fragment lengths measured using transmission optical microscope.
7. Tensile strength measurements The tensile strengths of single carbon fibers were measured at a gauge length of 20 mm at cross-head speed of 3 mm s − 1. The fibers were glued to paper tabs at both ends using cyanoacrylate adhesive. After the adhesive cured, one of the tabs was hung to a Mettler PM 300 balance and the other tab was griped by a microvise which
Table 1 Surface energy components (mJ m−2) of probe liquids
Glycerol (G) Ethylene glycol (E) Formamide (F) Diiodomethane (DIM) Silicone oil
gT L
gW L
g AB
g+
g−
gd
gp
Ref.
64 48 58 50.8 18.8
34 29 39 50.8 18.8
30 19 19
3.92 1.92 2.28
57.4 57.4 39.6
34 29 32 48.5
30 19 26 2.3
[25] [16] [38] [25] [16]
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Table 2 XPS surface composition of unsized and sized Zoltek® carbon fibers Carbon fiber
Photopeaks C1s
O1s
N1s
Na1s 1073.7 1.35
Unsized Zoltek®
B.E. (eV) A.C. (%)
285.0 91.2
533.5 6.19
401.4 1.24
Ultem® polymer
B.E. (eV) A.C. (%)
285.0 82.0
532.6 13.3
400.4 4.0
Ultem® sized
B.E. (eV) A.C. (%)
285.0 83.6
532.5 12.6
400.8 2.6
Polyurethane (PU) polymer
B.E. (eV) A.C. (%)
285.0 75.7
532.7 17.1
399.7 5.5
PU sized
B.E. (eV) A.C. (%)
285.0 76.1
532.8 20.6
399.3 2.4
could be moved vertically by an adjustable motor controlled elevator. The paper strips were then cut and the fiber was pulled in tension. The force was measured by the balance. A total of 20 data points were collected for each fiber type.
8. Result and discussion
8.1. Surface composition The surface compositions of unsized, Ultem® and polyurethane sized carbon fibers and Ultem® and polyurethane neat polymers are presented in Table 2. Values of the binding energy (B.E.) and the atomic concentration (A.C.) are listed for each photopeak. Unsized, Ultem® and polyurethane sized carbon fibers all contain varying concentrations of carbon, oxygen, hydrogen and sodium. The Ultem® polymer contains carbon, oxygen, nitrogen and trace amounts of silicon and polyurethane polymers also contains carbon, oxygen, nitrogen and trace amounts of fluorine. The identification of functional groups (COH, CO and – COOH) on the unsized and sized carbon fibers was made by curve fitting the C1s photopeaks and results are shown in Table 3. The
F1s
Si2p
101.9 0.7 1072.2 1.2 688.9 1.7 1071.4 0.9
theoretical composition of polyurethane and Ultem® polymers are also given in Table 3. The abundance of functional groups is given in terms of the percent contribution (P.C.) of each curve fit photopeak to the total C 1s photopeak. The percentage of carboxyl and carbonyl functional groups on the surface of the fiber does not change with sizing, However, the hydroxyl group concentration decreased by nearly a factor of two for polyurethane sized fiber compared to the unsized fiber. For Ultem® sized fiber, hydroxyl group concentration decreased from 20 to 16%. Fig. 1(a–c) shows typical XPS C1s curve fit spectra for unsized and sized carbon fibers. The C1s photopeak for polyurethane sizing fiber shifted to lower bonding energy (283.2 eV) due to non-conductive polyurethane sizing. The shift of C 1s photopeak may indicate that the most of the carbon fiber surface was covered by polyurethane polymer. The diameter of polyurethane sized fiber determined by wetting force is slightly higher than that of Ultem® sized fiber as given in Table 4. This result suggests that the Ultem® sizing is relatively thin. This suggestion is consistent with the thickness measurement reported by Iroh et al. [33].
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8.2. Surface energy
and polyurethane compared to the unsized fibers. This result suggests that high energy sites on the fiber surface are being blocked by the sizing. The similar result was reported for epoxy sizing by Hoecker and Karger-Kocsis [18] and Fitzer and Weiss [34]. The results of surface energy measurements are summarized in Table 6. The g LW component of S fiber surface energy were calculated by using methylene iodide using a rearranged equation [12] for methylene iodide.
8.2.1. Analysis of acid– base components of surface energy The wetting forces of unsized and sized single fibers were determined by the dynamic contact angle analyzer (DCA) that are summarized in Table 4. The error bars associated with determining the wetting force are also given in Table 4. The calculated perimeters and diameters of unsized and sized single fibers from wetting force measurements are presented in Table 5. Diameters of the carbon fibers were also estimated from SEM photomicrographs and the values listed in Table 5. The diameter of polyurethane sized fibers is the largest as determined from either wetting force or SEM. The contact angles (u) of the fibers were calculated from wetting forces (F) of each liquid according to the following equation: F =PgL cos u
2/4 g LW S = gL(1+Cos u)
(17)
The acid–base surface interaction term was determined using polar liquids and Eq. (11). The basic component (g−) of the surface energy decreased from 32 mJ m − 2 for the unsized fiber to 20 and 15 mJ m − 2 for the Ultem® and polyurethane sizing. However, it is not possible to assess the acidity or basicity of these fibers from XPS results since the nature of COH and CO groups are not known. It is important to note that the COOH functional group is clearly acidic. However, this may not be true for hydroxyl group ROH. The OH group can be acidic if the R is a phenyl group or basic if R is an aliphatic group. Similarly, the CO is generally slightly basic, but
(16)
where P is the perimeter of the fiber measured from the wetting force in silicon oil, gL is the surface energy of liquid used. The values of the advancing contact angle of unsized and sized carbon fiber are also given in Table 5. The contact angels were higher after sizing with both Ultem®
Table 3 XPS carbon 1s curve fit results of unsized and sized Zoltek® fibers Carbon fiber
C1s photopeak 1
2
3
4
5
6
287.6 2.3
288.6 7.5
290.3 4.7
292.1 4.2
Unsized
B.E. (eV) P.C. (%)
285.0 61.2
286.5 20.1
Ultem® Polymer
B.E. (eV) P.C. (%)
285.0 71.1
286.5 17.6
Ultem® sized
B.E. (eV) P.C. (%)
285.0 69.0
286.3 16.4
Polyurethane (PU) Polymer
B.E. (eV) P.C (%)
285.0 70.9
286.5 8.3
PU sized
B.E. (eV) P.C. (%)
285.0 65.7
286.5 10.4
–C–C –C–H
–C–OH –C–OR
Peak assignment
287.3 2.64
288.6 8.4
291.4 2.9
288.7 7.3
291.3 4.6
7
288.9 8.6
285.8 12.2
287.4 3.0
288.8 7.5
283.2 13.3
–C= O
–COOH –COOR
O =C = O
Plasmon
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331
Fig. 1. Curve fit of carbon 1s photoelectron peaks of: (a) unsized; (b) Ultem® sized; and (c) polyurethane sized fibers.
if there is an alpha hydrogen, such as CHCO, this hydrogen atom is acidic. The strongly acidic carboxylic groups exhibit a stronger interaction with a basic resin than other groups present on the fiber
surface and thus are the ones principally responsible for adhesion to basic polymers [16]. Total surface energy of the carbon fibers decreased in the order of unsized\Ultem® \ polyurethane.
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Fig. 1. (Continued) Table 4 Wetting forces of Zoltek® carbon fibers measured by dynamic contact angle analyzer Carbon fiber
Wetting force
Unsized standard deviation Ultem® sized standard deviation Polyurethane sized standard deviation
Formamide (F)
Ethylene glycol (E)
Glycerol (G)
Diiodomethane (DIM)
Silicon oil
0.0950 90.008 0.100890.003 0.09289 0.004
0.0880 90.003 0.0978 90.009 0.0926 9 0.005
0.0906 90.001 0.0829 90.005 0.0735 9 0.006
0.1009 9 0.005 0.1100 9 0.005 0.0965 9 0.003
0.0464 9 0.001 0.055 9 0.002 0.057 9 0.002
Table 5 Diameters and contact angles for unsized and sized Zoltek® carbon fibers and epoxy matrix as determined by dynamic contact angle analysis (DCAA) Carbon fiber
Unsized Ultem® sized Polyurethane sized Epoxy matrix
Perimeter (mm)
24.29 1.1 28.691.4 30.191.7
Diameter (mm)
Contact angle (u)
DCAA
SEM
Glycerol (G)
Formamide (F)
Ethylene glycol (EG)
Diiodomethane (DIM)
7.690.3 9.19 0.4 9.6 90.5
7.49 1.1 8.390.1 9.09 1.2
55.1 63.6 68.1
48.4 53.2 58.6
42.0 45.6 51.1
36.4 42.0 50.9
31.6
16.7
30.6
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Table 6 Acid–base surface energy components (mJ m−2) of unsized and sized Zoltek® carbon fibers as calculated by the three liquid approach Ultem® sized
Unsized
Ethylene glycol/formamide Etylene glycol/glycerol Formamide/glycerol Average
Polyurethane sized
g LW S
g− S
g+ S
gT S
g LW S
g− S
g+ S
gT S
g LW S
g− S
g+ S
gT S
41.3 41.3 41.3 41.3
27.6 34.3 35.4 32.4
0.00 0.00 0.00 0.00
41.3 41.3 41.3 41.3
38.6 38.6 38.6 38.6
16.5 20.2 25.0 20.5
0.05 0.02 0.00 0.03
40.4 39.8 38.6 39.6
33.2 33.2 33.2 33.2
15.8 15.4 14.6 15.3
0.10 0.11 0.13 0.11
35.7 35.8 35.9 35.8
8.3. Polar/dispersi6e surface energy analysis The polar and dispersive components of the surface energy of the fibers and the epoxy resin were determined by using equation [13]. The results are given in Fig. 2. Sizing the fibers resulted primarily in a decrease of the polar component of surface energy g p from 9 mJ m − 2 for unsized fiber to 6 mJ m − 2 for the Ultem® and polyurethane sized fibers whereas the dispersive part (g d) did not vary with sizing. This decrease is assumed to be partially due to a decrease in the percentage of –OH functional groups but may also be dependent on the nature of the polymer. The reported [18] value of the surface energy of epoxy resin in the solid state 47 mJ m − 2; the experimentally measured value in the present work is 51.5 mJ m − 2. The surface energy of the epoxy resin was higher than that of the fibers ( 40 mJ m − 2) and also the surface of the epoxy resin showed a basic character. The acidic, basic and Lifshitz –van der Waals components of surface energy for the − LW epoxy resin were g+ S =0, g S =66.8, g S = 46.7 −2 tot and g S =46.7 mJ m . The changes in surface energy closely related to the concentration of surface hydroxyl functional group determined by XPS analysis of the fiber surfaces as shown in Fig. 3. Both the basic components and the total surface energy increased with increasing hydroxyl functional group concentration. Hydroxyl functional groups may increases surface polarity and acid – base reactive sites on the fiber surface. From surface energy data it is concluded that C – OH and CO functional groups contribute to the basic nature of the fiber surface. The unsized fiber showed the highest
overall functionality and basic surface energy component in comparison to the sized fiber.
8.4. Interfacial shear strength The results of the single fragmentation test and tensile strengths of the unsized and sized fibers are summarized in Table 7. The tensile strength of the fibers gradually decreased from 3.2 GPa for the unsized fibers to 2.5 Gpa for the sized fibers. This decrease may be due to fiber damage during sizing process. The critical length of fragments (Lc) increased with sizing from 0.44 to 0.85 mm as noted in Table 7. Small critical lengths are associated with strong adhesion between the fibers and matrix [35]. The decrease in these three factors: (i) basic
Fig. 2. Polar/dispersive components of surface energy for unsized, Ultem® sized and polyurethane sized fibers and epoxy resin.
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Fig. 3. Basic components of surface energy and total surface of unsized, Ultem® sized and polyurethane sized fibers as a function of hydroxyl groups on the. surface of fibers. Table 7 Results of IFSS and tensile strength measurements on unsized and sized Zoltek® carbon fibers Carbon fiber
gST (mJ m−2)
sf Tensile strength (GPa)
i IFSS (MPa)
Lc (mm)
Wad (mJ m−2)
Unsized Ultem® sized Polyurethane sized
41.3 39.6 35.8
3.2 90.5 2.69 0.3 2.59 0.7
28.0 9 6 19.4 93 13.5 9 2
0.44 90.1 0.58 9 0.1 0.85 9 0.1
100.3 99.5 94.2
component; (2) polar component; and (3) hydroxyl functional groups may contribute to the decrease in the work of adhesion. Fig. 4 shows the relationships between interfacial shear strength (IFSS) and the work of adhesion as a function of the total surface energy of fibers. The work of adhesion decreased with a decrease in the total surface energy of fibers. The decrease in the work of adhesion may result from a decrease in the basic character of the fibers in the order unsized\ Ultem® sized \polyurethane sized. The magnitude of work of adhesion changes related to the increases in the fiber surface free energies. The polyurethane sized fiber showed a considerably lower IFSS (13.5 MPa) compared to the unsized fiber (28 MPa). Sizing may create a
weaker interface between sizing and epoxy matrix. This probably arises because the polyurethane and Ultem® sizing is adsorbed onto the fiber surface and therefore cannot readily dissolve in the matrix during fabrication of the composite, which is requirement for the formation of a strong interface by chemical reaction between fibre and matrix functionalities [36]. The adsorption of the sizing may reduce the reactivity of the fibre to the matrix since the reactive functional groups on the fiber surface and within the sizing are involved in the adsorption process and therefore not available for reaction with the matrix. The result is a weak interface between the adsorbed sizing and matrix. These arguments are supported by Yumitori et al. [37] that if the sizing layer existed as a distinct layer, then a low interfacial shear strength may
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335
Fig. 4. The relationships between interfacial shear strength (IFSS) and work of adhesion as a function of the total surface energy of unsized and sized Zoltek® carbon fibers.
have been anticipated. However, penetration of the sizing into matrix could explain the enhancement in interfacial shear strength. The wettability should be enhanced as well as shear strength between sized fiber and matrix if the matrix resin has the same surface energy as the fibers. The results of the present work findings indicate that a good match should be obtained by using an acid resin for the unsized and Ultem® and polyurethane sized. The similar result was reported for IM7 sized graphite fiber by Tsutsumi et al. [14]. The acid–base interactions between the basic sites of fibers and the acidic sites of the resin are expected for nondispersive interaction. Recently, acid–base interactions have been shown to play an important role in adhesion system [38].
9. Summary The surface energy of the epoxy resin (51 mJ m − 2) was higher than that of the fibers ( 40 mJ
m − 2) and in addition, the surface of the epoxy resin was basic. The surface energy components of the fibers slightly decreased with sizing. Unsized fiber showed a more basic character than sized fibers. The atomic percentage of hydroxyl groups on the unsized fiber was higher than on the Ultem® and polyurethane sized fiber. The atomic percentage of other functional groups did not change with sizing. The magnitude of work of adhesion changes related to the increases in the fiber surface free energies. The work of adhesion between carbon fibers and epoxy resin correlated well with the measured interfacial shear strength. The IFSS and tensile strength of single fiber both decreased gradually with sizing (Ultem® and polyurethane) compared to the unsized fiber. The presence of sizing reduced the interfacial shear strength of the composite. It is suggested that compatibility of the deposited sizing on the carbon fiber with the matrix determines the adhesive bond between fiber and matrix and the formation of an interphase region.
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Acknowledgements We acknowledge the support of NSF and Technology Center for High Performance Polymeric Adhesives and Composites under grant DMR9120004 at Virginia Tech. We thank Frank Cromer for his help with XPS measurements.
References [1] P. Bourgeois, T. Davidson, J. Adhes. 45 (1994) 73. [2] Y.L. Zou, A.N. Netravali, J. Adhes. Sci. Technol. 9 (1995) 1505. [3] S. Yumitrori, D. Wang, F.R. Jones, Composites 25 (1994) 698. [4] L.T. Drzal, R.J. Micheal, M.F. Koenig, P.F. Lloyd, J. Adhes. 16 (1983) 133. [5] J.O. Iroh, W. Yuan, Polymer 37 (1996) 4197. [6] L.T. Drzal, SAMPE J. 19 (1983) 7. [7] M.R. Piggott, Carbon 27 (1989) 657. [8] D.M. Blackketter, D. Vpadhyaya, T.R. King, J.A. King, Polym. Compos. 14 (1993) 430. [9] K.J. Huttinger, G. Krekel, Carbon 29 (1991) 1065. [10] T.H. Chang, J. Zhang, S. Yumitori, F.R. Jones, C.W. Anderson, Composites 25 (1994) 661. [11] R.H. Bradley, X. Ling, I. Shutherland, Carbon 31 (1993) 1115. [12] FM. Fowkes, J. Adhes. Sci. Tech. 1 (1987) 7. [13] F.M. Fowkes, D.O. Tischler, J.A. Wolfe, L.A. Lannigan, C.M. Ademu-John, M.J. Halliwell, J. Polym. Sci. Chem. Ed. 22 (1984) 547. [14] K. Tsutsumi, K. Ban, K. Shibata, S. Okazaki, Kogoma, J. Adhes. 57 (1996) 45. [15] L.T. Drzal, M. Madhukar, M.C. Waterbury, Compos. Struct. 27 (1994) 65. [16] D. Chan, M.A. Hozbor, E. Bayramli, R.L. Powell, Carbon 29 (1991) 1091.
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[17] C.V. Le, N.G. Ly, M.G. Stevens, Text. Res. J. 66 (1996) 389. [18] F. Hoecker, J. Karger-Kocsis, J. Appl. Polym. Sci. 59 (1996) 139. [19] Y. Huang, D.J. Gardner, M. Chen, C.J. Biermann, J. Adhes. Sci. Technol. 9 (1995) 1403. [20] A.W. Neumann, J.K. Spelt (Eds.), Applied Surface Thermodynamics, Marcel Dekker, New York, 1996. [21] A.J. Kinloch, Adhesion and Adhesives, Chapman and Hall, London 1987. [22] F.M. Fowkes, J. Phys. Chem. 67 (1963) 2538. [23] F.M. Fowkes, in: R.L. Patric (Ed.), Treatise on Adhesion and Adhesive, vol. 1., Marcel Dekker, New York, 1967. [24] D.K. Owens, R.C. Wendt, J. Appl. Polym. Sci. 13 (1696) 1741. [25] R.J. Good, J. Adhes. Sci. Technol. 6 (1992) 1269. [26] R.J. Good, N.R. Srivatsa, M. Islam, H.T.L. Huang, C.J. Van Oss, J. Adhes. Sci. Technol. 4 (1990) 607. [27] C.J. Van Oss, R.J. Good, M.K. Chaudhury, Lanqmuir 4 (1986) 884. [28] C.J. Van Oss, R.J. Good, M.K. Chaudhury, J. Colloid Interface Sci. 111 (1986) 387. [29] C.J. Van Oss, M.K. Chaudhury, R.J. Good, Adv. Colloid Interface Sci. 28 (1987) 35. [30] R.J. Good, L.K. Shu, H.C. Chiu, C.K. Yeung, J. Adhes. 39 (1996) 25. [31] W. Su, Polymer Interface and Adhesion, Marcel Dekker, New York, 1982. [32] A. Kelley, W.R.V. Tyson, J. Mech. Phys. Solids 13 (1965) 329. [33] J.O. Iroh, W. Yuan, Polymer 37 (1996) 4197. [34] E. Fitzer, R. Weiss, Carbon 25 (1987) 455. [35] L.T. Drzal, M.J.M. Rich, P.F. Lloyd, J. Adhes. 16 (1983) 1. [36] T.H. Cheng, J. Zhang, S. Yumitori, F.R. Jones, C.W. Anderson, Composites 25 (1994) 661. [37] S. Yumitori, D. Wang, F.R. Jones, Composites 25 (1994) 698. [38] F.M. Fowkes, in: K.L. Mittal, E.R. Anderson Jr. (Eds.), Acid – Base Interactions: Relevance to Adhesion Sciences and Technology, Zeist, Utrecht, 1991.
Effect of acid–base properties of unsized and sized carbon fibers on fiber/epoxy matrix adhesion Nursel Dilsiz 1, J.P. Wightman * Department of Chemistry, NSF Science and Technology Center for High Performance Polymeric Adhesi6es and Composites, Virginia Polytechnic Institute and State Uni6ersity, Blacksburg, VA 24061, USA Received 25 June 1999; accepted 4 August 1999
Abstract The surface energies and acid–base character of unsized and sized Zoltek fiber were investigated using dynamic contact angle analysis. The surface’ acid–base property were characterized by calculating the work of acid –base interaction according to Fowkes’ and Good’s theory. To evaluate the effect of sizing on carbon fiber surface properties, Ultem® polyimide and polyurethane (PU) sized fibers were studied in comparison with unsized fibers. It was found that the sizing on the carbon fiber tended to reduce the surface energy and cover acid – base sites. The surface of unsized and sized carbon fibers were analyzed by XPS to investigate changes in surface chemistry. XPS analysis indicated that hydroxyl groups on the fiber surface decreased with sizing (Ultem® and polyurethane). These changes in chemistry are reflected in a decrease in the polar g ps and basic g− s contribution of the surface energy of the fibers. Single fiber fragmentation testing of unsized and Ultem® and polyurethane sized fibers in epoxy matrix showed that there is a direct effect of the surface chemical changes on the fiber/matrix adhesion. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Acid–base; Contact angles; X-ray photoelectron spectroscopy; Single fiber fragmentation test; Interfacial shear strength
1. Introduction The interface between reinforcing fibers and polymeric matrices in composite materials is the controlling factor in obtaining optimum mechanical properties from composites [1 – 3]. It has been Paper honoring Professor A.W. Neumann on the occasion of his 65th birthday. * Corresponding author. Tel.: + 1-540-231-5854; fax: + 1540-231-3971. E-mail address: [email protected] (J.P. Wightman) 1 Roketsan, 06780 Elmadag/Ankara, Turkey.
recognized that the function of the interface is to transmit stress from the matrix to the reinforcing fiber. For good bonding and stress transfer, fibers are generally sized or coated with thin polymer film after surface treatment that removes weak boundary layers [4–6]. Sizing has an adhesion and wetting promotion function but is particularly important for facilitating fiber handling during composite manufacture acting as a lubricant to prevent fiber damage. The effectiveness is confirmed by the off-axis strength which is usually
0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 9 9 ) 0 0 4 0 0 - 8
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close to the strength of the polymer [7,8]. 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 [9,10]. Wetting is a prerequisite to good adhesion [11]. However, good adhesion also requires functional groups, vis-a-vis Lewis acidic and basic sites in the interfacial region between the fiber and the matrix resin. The concepts proposed by Fowkes [12,13], regarding the short-range hydrogen bonding interactions is important in adhesion, make it possible to assess the acid – base character of the surfaces using contact angle measurements. The acid–base nature of the fiber surface is a significant factor in determining the degree of adhesion of these fibers in a given resin matrix. If the acid–base properties of the matrix resin is also determined, it should be possible to choose a fiber/matrix pair to maximize adhesion. The adhesion strength of the interface depends on the thermodynamic work of adhesion that is closely related to the surface energy of the fiber and matrix [14]. Surface energy of fibers have been determined quantitatively by measurement of the contact angles by using Wilhelmy plate technique [15–19]. An excellent comprehensive summary of both the theoretical and experimental aspects of contact angles has been published recently [20]. The objective of the present study was done to evaluate the influences of polyurethane and polyetherimide sizing on both acid – base components and polar dispersive components of the surface energy of Zoltek® carbon fibers. The surfaces of unsized and sized fibers were also analyzed by XPS to determine the elemental compositional changes of the fiber surface. The work of adhesion at the interface between the fiber and epoxy resin matrix was calculated and compared with the interfacial shear strength measured by the single fiber fragmentation test. 2. Theory — surface energy measurements
2.1. Three liquid method: acid – base approach The wetting of a solid surface by a liquid and the concept of contact angle (u) was first formalized by Young [21].
gS − gSL = gL Cos u
(1)
where gL is the surface energy of the liquid, gSL is the interfacial energy of solid/liquid interface, and gS is the surface energy of solid. Fowkes [22,23] proposed that the surface energy of a liquid or a solid has two parts, namely, the dispersion and polar components. gS = g dS + g pS
(2)
where the superscript d refers to the contribution due to London dispersion forces which are common to all materials, and superscript p relates to the Keesom polar contribution, largely made up of hydrogen bonding and dipole–dipole interactions. Owens and Wendt [24] derived the following equation for the interfacial energy between liquid and solid assuming a geometric mean combination of the dispersion and polar components: gSL = gS + gL − 2 (gSdgLd)
1/2
− 2(gSpgLp)
1/2
(3)
Hydrogen bonding has been suggested as the main attraction force at the interfaces according to recent theoretical developments of the surface energy of solids. Good et al. [25–30] wrote the total surface energy as the sum of the Lifshitz– van der Waals gLLW and acid–base gLAB components. For the solid, A gS = g LW S +gS
(4)
and for the liquid AB gL = g LW L +gL
(5)
The Lifshitz–van der Waals component includes London dispersion forces gd, Debye induction gi, and Keesom dipole–dipole forces gm. g LW = gd + gi + gm
(6)
The acid–base component (or hydrogen bonding) includes electron acceptor g + and electron donor g − components, which are not additive and are expressed as + − 1/2 g AB S = 2(g S g S )
(7)
for the solid and as − 1/2 g AB L = 2(gL + g L )
(8)
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for the liquid. Assuming a geometric mean for the interaction, the total interfacial energy between solid and liquid is g12 =g1 +g2 −2(g
g
LW LW 1/2 1 2
− 2[(g1 + g2 −)
1/2
)
+(g2 + g1 −)
1/2
(9)
The Young – Dupre equation [31] of the work of adhesion (Wa) is given as Wa= gS − gSL +gL =gL (1 + Cos u)
(10)
and substituting for (gS −gSL) in Eq. (9), the result is
(11)
of the solid is determined from The value of g LW S the contact angle of an apolar liquid such as diiodomethane on the solid in which case equation [11] reduces to LW 1/2 gL(1+ Cos u) =2(g LW L gS )
(12)
assuming the liquid has negligible acid – base interactions with the solid. With gSLW known and the contact angles obtained using different liquids on the solid, one can get two equations similar to equation [11], which can be solved simultaneously for gS + and gS −.
4.1. Materials Zoltek® carbon fibers were used without and with polyetherimide (Ultem®) and polyurethane (PU) sizings. An amine cured epoxy systems were selected as matrix resin. A difunctional epoxy, diglycidyl ether of bisphenol-A (DGEBA), Epon 828 was cured with a stoichiometric amount of an aromatic amine, 1,3 phenylene diamine (mPDA).
The surfaces of Zoltek® carbon fibers and the neat polymers were analyzed using a Perkin Elmer PHI 5400 X-ray photoelectron spectrometer (XPS). The spectra were collected using a Mg Ka X-ray source (1253.6 eV) and operated at 14 kV and 300 W with an emission current of 25 mA. The pressure inside the chamber was held below 5× 10 − 7 Torr during analysis. Both survey and high resolution XPS spectra were recorded at a 45° take-off angle. A Gaussian–Lorentzian function was used for curve fitting C 1s photopeaks. The C 1s electron binding energy was referenced at 285.0 eV.
5. Contact angle measurements
3. Polar/dispersive surface energy analysis According to Owens and Wendt [24], combining Eqs. (1) and (3) yields a linear equation gL(1+ Cos f) = 2(g dSg dL)1/2) + 2(g pSg pL)1/2
4. Experimental
4.2. Fiber surface analysis
gL(1+ Cos u) LW 1/2 − 1/2 − 1/2 =2(g LW +2[(g+ +(g+ ] L gS ) L gS ) S gL )
327
(13)
where gL, g dL and g pL are known for the test liquids and g dL and g pS can be calculated from measured contact angles. Finally, knowing the dispersive and acid and base components of the fiber and matrix, it is possible to estimate the work of adhesion between epoxy matrix (M) and the fiber (F) at the interface using Eqs. (10) and (12). 1/2 LW 1/2 − 1/2 Wad=2[(g LW +[(g+ +(gM+g − ] F gM ) F g M) F ) (14)
5.1. Wilhelmy plate technique Contact angles were measured using Wilhelmy plate technique with a Cahn dynamic contact angle analyzer. A single carbon fiber was fixed to nichrome wire with a fast curing cyanoacrylate adhesive. Then, the fiber was suspended from microbalance. Wetting force data were recorded at 1 s intervals at a speed of 20 mm min − 1. The advancing wetting forces were used to calculate carbon fiber surface energies. The perimeter of the fiber was determined using the wetting force measured for silicon oil which was assumed to give a zero contact angle with the fiber. Twenty separate carbon fibers were measured to get an average value.
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Methylene iodide (DIM) was used as the probe liquid for Lifshitz – van der Waals interactions; ethylene glycol (EG), formamide (F) and glycerol (G) were used to probe for acid – base interactions. The surface energy components of these probe liquids to be used in the fiber wetting studies are given in Table 1.
5.2. Contact angle goniometer The contact angles of liquids against the cured epoxy resin were measured by the sessile drop method with a Rame-Hart 100-00115 NRL contact angle goniometer. Before surface energy measurement the epoxy resin was cured on the glass slide and it was stored in desicator to eliminate contamination. The epoxy resin was cured at 75°C for 2 h and postcured at 125°C for 2 h. A total of 5 ml drops of liquid were carefully placed on the substrate using a microliter syringe. The contact angles of both sides of each drop were measured. Six separate measurements were done to obtain an average value for the contact angle.
6. Single fragmentation test The capacity of the fiber/matrix interface to transfer stress from matrix to the fiber through the interface can be reflected by the critical length in terms of interfacial shear strength. The strength of the interface and the fracture strength of the embedded fiber determine the length of the fiber fragments obtained. The shear-lag model developed by Kelly – Tyson [32] gives the interfacial shear strength (t or IFSS) according to the following equation:
t= sfd/2Lc
(15)
where d is the fiber diameter, and sf is the fiber tensile strength at Lc, the critical length of fragments. The fiber/matrix interfacial shear strength (IFSS) for the unsized and sized carbon fibers were determined by the single fragmentation test. A single fiber was embedded along the center line of a silicon rubber mold with dog-bone shaped cavity and strained uniaxially along the fiber axis. Epoxy resin was degassed under vacuum and carefully poured into the cavity and filled to the level of mold. The resin was cured at 75°C for 2 h and postcured at 125°C for 2 h. The specimens were then removed from mold and stored in desicator until testing. The specimen was pulled in tension with 5% specimen strain at a speed of 1 mm min − 1 and breaking process was observed under cross polarizing light. The pulling was stopped when no further breaks were observed. The elongation was 5% for carbon fiber/epoxy system (i.e. after 5% strain the fiber fragmentation reached saturation). For single fiber composite samples, the fragments length were measured fragment lengths measured using transmission optical microscope.
7. Tensile strength measurements The tensile strengths of single carbon fibers were measured at a gauge length of 20 mm at cross-head speed of 3 mm s − 1. The fibers were glued to paper tabs at both ends using cyanoacrylate adhesive. After the adhesive cured, one of the tabs was hung to a Mettler PM 300 balance and the other tab was griped by a microvise which
Table 1 Surface energy components (mJ m−2) of probe liquids
Glycerol (G) Ethylene glycol (E) Formamide (F) Diiodomethane (DIM) Silicone oil
gT L
gW L
g AB
g+
g−
gd
gp
Ref.
64 48 58 50.8 18.8
34 29 39 50.8 18.8
30 19 19
3.92 1.92 2.28
57.4 57.4 39.6
34 29 32 48.5
30 19 26 2.3
[25] [16] [38] [25] [16]
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329
Table 2 XPS surface composition of unsized and sized Zoltek® carbon fibers Carbon fiber
Photopeaks C1s
O1s
N1s
Na1s 1073.7 1.35
Unsized Zoltek®
B.E. (eV) A.C. (%)
285.0 91.2
533.5 6.19
401.4 1.24
Ultem® polymer
B.E. (eV) A.C. (%)
285.0 82.0
532.6 13.3
400.4 4.0
Ultem® sized
B.E. (eV) A.C. (%)
285.0 83.6
532.5 12.6
400.8 2.6
Polyurethane (PU) polymer
B.E. (eV) A.C. (%)
285.0 75.7
532.7 17.1
399.7 5.5
PU sized
B.E. (eV) A.C. (%)
285.0 76.1
532.8 20.6
399.3 2.4
could be moved vertically by an adjustable motor controlled elevator. The paper strips were then cut and the fiber was pulled in tension. The force was measured by the balance. A total of 20 data points were collected for each fiber type.
8. Result and discussion
8.1. Surface composition The surface compositions of unsized, Ultem® and polyurethane sized carbon fibers and Ultem® and polyurethane neat polymers are presented in Table 2. Values of the binding energy (B.E.) and the atomic concentration (A.C.) are listed for each photopeak. Unsized, Ultem® and polyurethane sized carbon fibers all contain varying concentrations of carbon, oxygen, hydrogen and sodium. The Ultem® polymer contains carbon, oxygen, nitrogen and trace amounts of silicon and polyurethane polymers also contains carbon, oxygen, nitrogen and trace amounts of fluorine. The identification of functional groups (COH, CO and – COOH) on the unsized and sized carbon fibers was made by curve fitting the C1s photopeaks and results are shown in Table 3. The
F1s
Si2p
101.9 0.7 1072.2 1.2 688.9 1.7 1071.4 0.9
theoretical composition of polyurethane and Ultem® polymers are also given in Table 3. The abundance of functional groups is given in terms of the percent contribution (P.C.) of each curve fit photopeak to the total C 1s photopeak. The percentage of carboxyl and carbonyl functional groups on the surface of the fiber does not change with sizing, However, the hydroxyl group concentration decreased by nearly a factor of two for polyurethane sized fiber compared to the unsized fiber. For Ultem® sized fiber, hydroxyl group concentration decreased from 20 to 16%. Fig. 1(a–c) shows typical XPS C1s curve fit spectra for unsized and sized carbon fibers. The C1s photopeak for polyurethane sizing fiber shifted to lower bonding energy (283.2 eV) due to non-conductive polyurethane sizing. The shift of C 1s photopeak may indicate that the most of the carbon fiber surface was covered by polyurethane polymer. The diameter of polyurethane sized fiber determined by wetting force is slightly higher than that of Ultem® sized fiber as given in Table 4. This result suggests that the Ultem® sizing is relatively thin. This suggestion is consistent with the thickness measurement reported by Iroh et al. [33].
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8.2. Surface energy
and polyurethane compared to the unsized fibers. This result suggests that high energy sites on the fiber surface are being blocked by the sizing. The similar result was reported for epoxy sizing by Hoecker and Karger-Kocsis [18] and Fitzer and Weiss [34]. The results of surface energy measurements are summarized in Table 6. The g LW component of S fiber surface energy were calculated by using methylene iodide using a rearranged equation [12] for methylene iodide.
8.2.1. Analysis of acid– base components of surface energy The wetting forces of unsized and sized single fibers were determined by the dynamic contact angle analyzer (DCA) that are summarized in Table 4. The error bars associated with determining the wetting force are also given in Table 4. The calculated perimeters and diameters of unsized and sized single fibers from wetting force measurements are presented in Table 5. Diameters of the carbon fibers were also estimated from SEM photomicrographs and the values listed in Table 5. The diameter of polyurethane sized fibers is the largest as determined from either wetting force or SEM. The contact angles (u) of the fibers were calculated from wetting forces (F) of each liquid according to the following equation: F =PgL cos u
2/4 g LW S = gL(1+Cos u)
(17)
The acid–base surface interaction term was determined using polar liquids and Eq. (11). The basic component (g−) of the surface energy decreased from 32 mJ m − 2 for the unsized fiber to 20 and 15 mJ m − 2 for the Ultem® and polyurethane sizing. However, it is not possible to assess the acidity or basicity of these fibers from XPS results since the nature of COH and CO groups are not known. It is important to note that the COOH functional group is clearly acidic. However, this may not be true for hydroxyl group ROH. The OH group can be acidic if the R is a phenyl group or basic if R is an aliphatic group. Similarly, the CO is generally slightly basic, but
(16)
where P is the perimeter of the fiber measured from the wetting force in silicon oil, gL is the surface energy of liquid used. The values of the advancing contact angle of unsized and sized carbon fiber are also given in Table 5. The contact angels were higher after sizing with both Ultem®
Table 3 XPS carbon 1s curve fit results of unsized and sized Zoltek® fibers Carbon fiber
C1s photopeak 1
2
3
4
5
6
287.6 2.3
288.6 7.5
290.3 4.7
292.1 4.2
Unsized
B.E. (eV) P.C. (%)
285.0 61.2
286.5 20.1
Ultem® Polymer
B.E. (eV) P.C. (%)
285.0 71.1
286.5 17.6
Ultem® sized
B.E. (eV) P.C. (%)
285.0 69.0
286.3 16.4
Polyurethane (PU) Polymer
B.E. (eV) P.C (%)
285.0 70.9
286.5 8.3
PU sized
B.E. (eV) P.C. (%)
285.0 65.7
286.5 10.4
–C–C –C–H
–C–OH –C–OR
Peak assignment
287.3 2.64
288.6 8.4
291.4 2.9
288.7 7.3
291.3 4.6
7
288.9 8.6
285.8 12.2
287.4 3.0
288.8 7.5
283.2 13.3
–C= O
–COOH –COOR
O =C = O
Plasmon
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331
Fig. 1. Curve fit of carbon 1s photoelectron peaks of: (a) unsized; (b) Ultem® sized; and (c) polyurethane sized fibers.
if there is an alpha hydrogen, such as CHCO, this hydrogen atom is acidic. The strongly acidic carboxylic groups exhibit a stronger interaction with a basic resin than other groups present on the fiber
surface and thus are the ones principally responsible for adhesion to basic polymers [16]. Total surface energy of the carbon fibers decreased in the order of unsized\Ultem® \ polyurethane.
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Fig. 1. (Continued) Table 4 Wetting forces of Zoltek® carbon fibers measured by dynamic contact angle analyzer Carbon fiber
Wetting force
Unsized standard deviation Ultem® sized standard deviation Polyurethane sized standard deviation
Formamide (F)
Ethylene glycol (E)
Glycerol (G)
Diiodomethane (DIM)
Silicon oil
0.0950 90.008 0.100890.003 0.09289 0.004
0.0880 90.003 0.0978 90.009 0.0926 9 0.005
0.0906 90.001 0.0829 90.005 0.0735 9 0.006
0.1009 9 0.005 0.1100 9 0.005 0.0965 9 0.003
0.0464 9 0.001 0.055 9 0.002 0.057 9 0.002
Table 5 Diameters and contact angles for unsized and sized Zoltek® carbon fibers and epoxy matrix as determined by dynamic contact angle analysis (DCAA) Carbon fiber
Unsized Ultem® sized Polyurethane sized Epoxy matrix
Perimeter (mm)
24.29 1.1 28.691.4 30.191.7
Diameter (mm)
Contact angle (u)
DCAA
SEM
Glycerol (G)
Formamide (F)
Ethylene glycol (EG)
Diiodomethane (DIM)
7.690.3 9.19 0.4 9.6 90.5
7.49 1.1 8.390.1 9.09 1.2
55.1 63.6 68.1
48.4 53.2 58.6
42.0 45.6 51.1
36.4 42.0 50.9
31.6
16.7
30.6
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333
Table 6 Acid–base surface energy components (mJ m−2) of unsized and sized Zoltek® carbon fibers as calculated by the three liquid approach Ultem® sized
Unsized
Ethylene glycol/formamide Etylene glycol/glycerol Formamide/glycerol Average
Polyurethane sized
g LW S
g− S
g+ S
gT S
g LW S
g− S
g+ S
gT S
g LW S
g− S
g+ S
gT S
41.3 41.3 41.3 41.3
27.6 34.3 35.4 32.4
0.00 0.00 0.00 0.00
41.3 41.3 41.3 41.3
38.6 38.6 38.6 38.6
16.5 20.2 25.0 20.5
0.05 0.02 0.00 0.03
40.4 39.8 38.6 39.6
33.2 33.2 33.2 33.2
15.8 15.4 14.6 15.3
0.10 0.11 0.13 0.11
35.7 35.8 35.9 35.8
8.3. Polar/dispersi6e surface energy analysis The polar and dispersive components of the surface energy of the fibers and the epoxy resin were determined by using equation [13]. The results are given in Fig. 2. Sizing the fibers resulted primarily in a decrease of the polar component of surface energy g p from 9 mJ m − 2 for unsized fiber to 6 mJ m − 2 for the Ultem® and polyurethane sized fibers whereas the dispersive part (g d) did not vary with sizing. This decrease is assumed to be partially due to a decrease in the percentage of –OH functional groups but may also be dependent on the nature of the polymer. The reported [18] value of the surface energy of epoxy resin in the solid state 47 mJ m − 2; the experimentally measured value in the present work is 51.5 mJ m − 2. The surface energy of the epoxy resin was higher than that of the fibers ( 40 mJ m − 2) and also the surface of the epoxy resin showed a basic character. The acidic, basic and Lifshitz –van der Waals components of surface energy for the − LW epoxy resin were g+ S =0, g S =66.8, g S = 46.7 −2 tot and g S =46.7 mJ m . The changes in surface energy closely related to the concentration of surface hydroxyl functional group determined by XPS analysis of the fiber surfaces as shown in Fig. 3. Both the basic components and the total surface energy increased with increasing hydroxyl functional group concentration. Hydroxyl functional groups may increases surface polarity and acid – base reactive sites on the fiber surface. From surface energy data it is concluded that C – OH and CO functional groups contribute to the basic nature of the fiber surface. The unsized fiber showed the highest
overall functionality and basic surface energy component in comparison to the sized fiber.
8.4. Interfacial shear strength The results of the single fragmentation test and tensile strengths of the unsized and sized fibers are summarized in Table 7. The tensile strength of the fibers gradually decreased from 3.2 GPa for the unsized fibers to 2.5 Gpa for the sized fibers. This decrease may be due to fiber damage during sizing process. The critical length of fragments (Lc) increased with sizing from 0.44 to 0.85 mm as noted in Table 7. Small critical lengths are associated with strong adhesion between the fibers and matrix [35]. The decrease in these three factors: (i) basic
Fig. 2. Polar/dispersive components of surface energy for unsized, Ultem® sized and polyurethane sized fibers and epoxy resin.
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Fig. 3. Basic components of surface energy and total surface of unsized, Ultem® sized and polyurethane sized fibers as a function of hydroxyl groups on the. surface of fibers. Table 7 Results of IFSS and tensile strength measurements on unsized and sized Zoltek® carbon fibers Carbon fiber
gST (mJ m−2)
sf Tensile strength (GPa)
i IFSS (MPa)
Lc (mm)
Wad (mJ m−2)
Unsized Ultem® sized Polyurethane sized
41.3 39.6 35.8
3.2 90.5 2.69 0.3 2.59 0.7
28.0 9 6 19.4 93 13.5 9 2
0.44 90.1 0.58 9 0.1 0.85 9 0.1
100.3 99.5 94.2
component; (2) polar component; and (3) hydroxyl functional groups may contribute to the decrease in the work of adhesion. Fig. 4 shows the relationships between interfacial shear strength (IFSS) and the work of adhesion as a function of the total surface energy of fibers. The work of adhesion decreased with a decrease in the total surface energy of fibers. The decrease in the work of adhesion may result from a decrease in the basic character of the fibers in the order unsized\ Ultem® sized \polyurethane sized. The magnitude of work of adhesion changes related to the increases in the fiber surface free energies. The polyurethane sized fiber showed a considerably lower IFSS (13.5 MPa) compared to the unsized fiber (28 MPa). Sizing may create a
weaker interface between sizing and epoxy matrix. This probably arises because the polyurethane and Ultem® sizing is adsorbed onto the fiber surface and therefore cannot readily dissolve in the matrix during fabrication of the composite, which is requirement for the formation of a strong interface by chemical reaction between fibre and matrix functionalities [36]. The adsorption of the sizing may reduce the reactivity of the fibre to the matrix since the reactive functional groups on the fiber surface and within the sizing are involved in the adsorption process and therefore not available for reaction with the matrix. The result is a weak interface between the adsorbed sizing and matrix. These arguments are supported by Yumitori et al. [37] that if the sizing layer existed as a distinct layer, then a low interfacial shear strength may
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Fig. 4. The relationships between interfacial shear strength (IFSS) and work of adhesion as a function of the total surface energy of unsized and sized Zoltek® carbon fibers.
have been anticipated. However, penetration of the sizing into matrix could explain the enhancement in interfacial shear strength. The wettability should be enhanced as well as shear strength between sized fiber and matrix if the matrix resin has the same surface energy as the fibers. The results of the present work findings indicate that a good match should be obtained by using an acid resin for the unsized and Ultem® and polyurethane sized. The similar result was reported for IM7 sized graphite fiber by Tsutsumi et al. [14]. The acid–base interactions between the basic sites of fibers and the acidic sites of the resin are expected for nondispersive interaction. Recently, acid–base interactions have been shown to play an important role in adhesion system [38].
9. Summary The surface energy of the epoxy resin (51 mJ m − 2) was higher than that of the fibers ( 40 mJ
m − 2) and in addition, the surface of the epoxy resin was basic. The surface energy components of the fibers slightly decreased with sizing. Unsized fiber showed a more basic character than sized fibers. The atomic percentage of hydroxyl groups on the unsized fiber was higher than on the Ultem® and polyurethane sized fiber. The atomic percentage of other functional groups did not change with sizing. The magnitude of work of adhesion changes related to the increases in the fiber surface free energies. The work of adhesion between carbon fibers and epoxy resin correlated well with the measured interfacial shear strength. The IFSS and tensile strength of single fiber both decreased gradually with sizing (Ultem® and polyurethane) compared to the unsized fiber. The presence of sizing reduced the interfacial shear strength of the composite. It is suggested that compatibility of the deposited sizing on the carbon fiber with the matrix determines the adhesive bond between fiber and matrix and the formation of an interphase region.
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Acknowledgements We acknowledge the support of NSF and Technology Center for High Performance Polymeric Adhesives and Composites under grant DMR9120004 at Virginia Tech. We thank Frank Cromer for his help with XPS measurements.
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