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The surface composition and energetics of Type HM graphite fibers
Carbon Vol. 17. pp. 375-382 Pergamon Press Ltd.. 1979. Pnnted in Great Britam
THE SURFACE COMPOSITION AND ENERGETICS OF TYPE HM GRAPHITE FIBERS L. T. DRZAL Surface Interactions and Mechanics Branch, Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH 45433,U.S.A.
J. A. MESCHERand D. L. HALL University of Dayton Research Institute, Dayton, OH 45469,U.S.A. (Received 21 December 1978)
Abstract-The surfaces of Type HM graphite fibers, both treated (HMS) and untreated (HMU), were investigated using low temperature thermal desorption, krypton thermodynamic analysis, Ion Scattering with Positive Secondary Ion Mass Spectroscopy and Contact Angle Polar/Dispersion Analysis techniques. The surface energetics of HM fibers increased with surface treatment. However, vacuum treatment to temperatures of 300°C reduces a significant percentage of the highly energetic portion of the surface. Contact angle measurements on these same surfaces also indicate that the polarity of the HM fiber surface is increased with surface treatment.
1.WR~DUCTION
The molecular interactions between polymer matrix and reinforcing fiber are of paramount importance in under-
standing the role of the fiber-matrix interface in composite mechanical performance and durability. The nature of the reinforcing fiber surface is an important and necessary component of the interface. The purpose of this work is to elucidate the energetic and molecular nature of the graphite fiber surface so that phenomena occurring at the reinforcing fiber-matrix interface may be more fully understood. Graphite reinforcing fibers vary in their properties with processing conditions and precursor material. Commercial fibers can possess a spectrum of properties. A previous study[1] has determined the effect of surface treatment on the surface energetics and surface composition of a low modulus (Type A) polyacrylonitrile (PAN) based fiber. This study is a continuation of that work and is directed at delineating the changes in surface energetics and composition with surface treatment of a high modulus (Type HM) PAN based fiber. A structural model of the Type HM fiber used in this study is shown in Fig. 1[2]. The main elements of the fiber structure are the graphitic lamellar ribbons. The variation in these structural elements depends on the graphitization temperature and processing of the particular fiber. The type HM fiber studied here is processed at high temperatures (2600°C) causing a high degree of axial alignment. The graphitic lamellar ribbons are about 20 basal layers thick and 7nm wide with over 75% of the graphitic basal planes oriented within 12” of the fiber axis. There is a small amount of ribbon intertangling and the graphitic basal planes are oriented radially parallel to the fiber surface. Recent work by Bennett and Johnson[3] presents additional evidence for a thin surface layer (150-250 nm) with larger crystallites and a higher degree of orientation than the interior of the fiber. CARBON
Vol. 17. No. 5/A
Because of the graphitic basal plane orientation, the fiber surface in the untreated case would be expected to be similar energetically to high temperature processed graphite. A more detailed discussion of graphite fiber morphology is available in Ref. [2]. 2.EXPERIMENTAL. 2.1 Experimental techniques The key to understanding the fiber-matrix interactions occurring in composite materials depends on the ability to relate the “clean” graphite fiber surface analyzed in spectroscopic systems to the “real” graphite fiber surface initially in equilibrium with the ambient environment and then subject to increases in temperature (up to 300°C) in contact with the polymeric matrix. In order to accomplish this, a variety of experimental techniques have been employed both of fibers in the “as-received” state and after a 300°C vacuum heat treatment. Thermal desorption in 50°C increments was used to quantify the amount of material present on the fiber surface and mass spectroscopy was used to analyze the desorbed material. Ion scattering spectroscopy (ES) coupled with positive secondary ion mass spectroscopy (+ SIMS) was applied to determine if ionizable surface species, especially alkali metals, were present. Krypton adsorption isotherms at high temperatures were determined in order to quantify surface area changes and enthalpies and entropies of adsorption. Finally, contact angles were measured with the micro-Wilhelmy technique for a variety of liquids, so that the polar and dispersive components of the graphite fiber surface free energy could be calculated. 2.2 Fibers Polyacrylonitrile (PAN) based fibers were obtained from a single batch of material provided by Hercules 375
376
L T. &CAL
et al,
2.3.3 ion scattering spect~scopy (ISS) and positive secondary ion mass spectroscopy (+ SIMS). 20mm samples of graphite fibers were aligned in a stainless steel holder for analysis in a 3 M model 520 ISS unit equipped with a UTI-1OOC quadrupole mass spectrometer biased for f SIMS. 2.3.4 Polar-dispersioncomponentsof fiber surface free energy. Ultimately, the surface free energy of the graphite fiber determines the wetting behavior of the matrix and the thermodynamic state of the resulting fiber-matrix interface. Because significant amounts of material are desorbed from these fiber surfaces at temperatures representing conditions that current and future ,soo& matrices would experience during fabrication, a determination of the fiber surface free energy is desirable under the “as-received” and 300°C vacuum treated conditions. A measure of the fiber wettability is available through evaluation of a critical surface free energy as proposed by Zisman[5], where the contact angle of a series of liquids on the surface of interest is measured and the value of the surface tension of a liquid which has a zero contact angle (spreading) is chosen as the critical surface free energy for wetting. Ail liquids with a value Schematic 3-D Structural Model of Poly~rylonitrile Eased of surface tension less than this value woufd spread on Fiber with Properties Similar to Type HMU (after Diefe~orf and Tokarskyl the substrate while those with a larger value would have a finite contact angle. Fig. 1. The structureof a Type HM fiber (after Ref.121). Kaelbe[6] has shown that for low energy surfaces the surface free energy can be considered to be composed of two parts-a dispersive component (rd) which is a Inc. [4]. One portion of this batch was given a proprietary result of the non-specific interactions between materials surface treatment and was designated HMS (Lot No. and a polar component (yp) which is the result of XA-289-74-2) and the other portion designated HMU specific polar interactions due to polar surface func(Lot No. XA-289-74-4) was left untreated. No additional tionalities. These quantities can be measured for a solid surface treatment sizing was used. The modulus of these surface by measuring the contact angle 8 for a series of fibers was 51.5 x lo&psi. The fibers were handled as little liquids of known polar and dispersive components. The as possibIe to prevent surface con~mination or damage. polar and dispersive components of the sotid can then be evaluated by use of the relations~p 2.3 Expe~men~ul procedure 2.3.1 Thermal desorptionand compositionalanalysis. The fiber sample was heated in - 50°C increments in a calibrated volume system kept at elevated temperature. The amount of material desorbed was calculated from pressure, volume and temperature measurements using the ideal gas law. Molecular analysis of the desorbate was accomplished using standard cracking patterns. The exact details of this technique as well as the Krypton adsorption and ISS/SIMS techniques are given eisewherelll. 2.3.2 tiptoe adso~tio~ studies. The adsorption of krypton, an inert gas, was used to investigate the changes in surface energetics of graphite fibers. Adsorption isotherms were determined volumetrically at two temperatures and the surface area and adsorption thermodynamics were computed. Both fiber surface treatment and vacuum heat treatments effects on the fiber surfaces, were examined in this way. Procedures for preparing the fiber surfaces, determination of the isotherms and calculation of the adsorption thermodynamics are the same as used in the previously reported work [ll.
YL”(l fcos x9)=
ysdfj2+
2Yz.d1’2
5 f
Ii2 ,,&/2 >
where subscript L refers to the contacting liquid, S refers to the solid surface under investigation, yLv is the surface free energy of the contacting liquid and 0 is the contact angle formed between the liquid and the solid surface. (A detailed derivation is available in Ref[4]) #* and ysd” can be measured by plotting yLv(l + cos 8)/2 y~~“~vs yLp”lyLd”* and determining the slope and intercept of the best straight lime through these points. Ex~~men~ly, the contact angle of a drop deposited on a fiber 8m in diameter is very difficult to measure directly. An easier method is the micro-Wilhelmy technique[7,8] were a single fiber is partially immersed in a reservoir of the liquid of interest and the force exerted on the fiber due to the surface tension of the liquid is measured. The following relationship then holds F = Cy,, cos B where F is the force measured corrected for bouyancy, C is the circumference of the wetted fiber, yLv is the
The surfacecompositionand energeticsof type HMgraphitefibers liquid surface tension and 8 is the contact angle. F and yLv are measured. C is the circumference of the fiber calculated from an optical measurement of the fiber diameter for the circular cross section fibers used in this study or measured directly by measuring the force F when the fiber is dipped into a wetting liquid (cos 0 = cos 0” = 1). The contact angle can then be evaluated. The measurements conducted here involved attaching a single fiber to a pure nickel hook with a small amount of cyanoacrylate adhesive. Surface mi~ation of the adhesive along the fiber length below the attachment point ( - 1cm) was checked using a scanning Auger microprobe with a beam size of 2 pm. No evidence of migration to the fiber portion in contact with the liquid was detected. The adhesive was placed on the hook first, and the fiber was brought in contact with it. The portion of the fiber handled was removed prior to measurement. The fiber mounted on a hook was suspended from the arm of a Cahn RH microbalance kept in a constant temperature (72°F 2 IoF), constant humidity (40% t 2%) room. The fiber tip was immersed in the liquid of interest and the equ~ib~um force was recorded to 22~8. The sample was removed and its diameter was measured optically with a calibrated Vickers AEI image-splitting eyepiece. Between five and ten fibers were used for each liquid and were immersed only once. The fiber circumference as determined optically was checked against a determination with n-octane and the agreement was within the experimental error. The reagent grade liquids used in this study are listed in Table 1 along with their respective surface free energies and the polar and dispersive components of their surface free energies as determined from the literature [6,9]. Liquid surface tensions were periodically checked with a ~Nouy tensiometer for a~eement with the literature values. Purified liquids were stored in enclosed pipette equipped bottles to preclude contamination. An acid cleaned PTFE 10ml beaker was used for storage of the liquids during wetting studies and the liquid surface was swept clean with additional liquid by ovefiowing prior to determination for each sample.
377
raised an additional 0.5 mm to eliminate spurious fiber and effects. This force measurement represented the value associated with an equilibrium advancing contact angle. Further immersion of the fiber showed some variation in contact angle along the fiber length associated with circumference changes. Repeat immersions of the same fiber altered the contact angle, therefore each fiber was immersed only once and the average for five to ten fibers was used in the contact angle and surface enrgetic determinations. The cummulative effect of the variations in fiber diameter, immersion force and linear regression analysis of the data is shown in the error bounds on the values for the polar and dispersion components of these fibers listed in Table 4.
3.1 Thermal desorption and compositional analysis The inremental and total amounts of material desorbed from the surfaces of HMU and HMS fibers are shown in Fig. 2 as a function of vacuum heat treatment temperature. The total amounts desorbed as well as the variation in incremental amount desorbed with temperature are approximately the same for both fibers. An equivalent of 0.5 monolayers is removed from both the untreated and surface treated fibers. Significant differences appear, however, in the com-
M-is Faber
:.50 K _m : I .25
The liquid surface was brought into contact with each
fiber by slowly raising its platform vertically until contact with the fiber was made as indicated by a force response displayed on the balance recorder. The liquid was then
Fig. 2. Incremental and total amount of material desorbed from HMU and HMS graphite fiber surfaces duringthermaltreatment.
Table I. Surface free energy components of reference liauids Liquid Water
72.8
21.8
51.0
glycerol
64.0
34.0
30.0
ethylene glycol
48.3
29.3
19.0
polypropylene glycol PG-1200
31.3
24.5
6.8
formamide
58.3
32.3
26.0
hexadecane
27.6
27.6
0
methylene iodide
50.8
48.4
2.4
br~onaphthalene
44.6
44.6
0
378
L. T.
hZAL
position of the volatiles that are thermally desorbed (Fig. 3). The untreated high modulus fiber (HMU) desorbs primarily water (40%) with some additional carbon monoxide (35%). Together they constitute over 70% of the desorbate. After surface treatment, the composition of these desorbed materials changes, however. The HMS fiber thermally desorbs primarily carbon dioxide and carbon monoxide which account for - 70% of its volatiles. Water comprises only an additional 20% of the desorbed gas. This difference in composition must be due to changes brought about by the surface treatment given HMS. The large amount of carbon dioxide may be indicative of the deso~tion of some surface functional groups originally added to the surface. Usually species of this type desorb at higher temperatures[lO]. However, the low temperature at which the CO and CO2 and desorbed may be indicative of a weak adsorption strength for these surface species. 3.2 Surface area analysis Fiber surface area can be an alterable parameter which might have a beneficial effect on graphite-fiber-matrix adhesion. Evaluation of fiber surface area is possible through use of the krypton adsorption isothe~s. Typical adso~tion isotherms for I-TMfibers are shown in Fig. 4. The 20°C VT fibers possess a weak Type II isotherm according to the Braunauer method of classification[ll]. However, the 300°C VT fibers both surface treated and untreated have changed their adsorptive character markedly resulting in the appearance of a pronounced “knee” at low pressures followed by an almost horizontal portion. This type of behavior is indicative of strong monolayer formation and is observed HUU
Fiber
rerrperature,‘C Fig. 3. Mass spectral analysis (mole%) of material desorbed from HMU and MHS fibers during thermal treatment.
et
al.
0 t 0
I
,
I
,
I
20
40
60
60
100
PressureCtori-1 I
1
2
4
6 8 PressurefPaxtd)
IO
12
Fig. 4. Typical krypton adsorption isotherms on HMU and HMS fibers.
on highly graphitic carbons[12]. Ah of these isotherms were evaluated for the monolayer point using the “B” method according to the method proposed by HalseyIl31. Although there may be some question regarding the absolute accuracy of the surface area calculated from this method, relative differences in surface areas for the same sample are reliable. Conventional BET calculations for the 20°C VT isotherms were not applicable because of the “weak knee” at the monolayer point and the resultant low value for the BET “C” constant. BET determination for the 300°C VT isotherms gave values of surface area consistent with the “B” method but slightly higher. The results of the surface area analysis are tabulated in Table 2. The fiber treatment conditions were: as received (20°CVT)-the fiber was evacuated I5 hr at 293°K prior to the adsorption studies and (3OOYYTj-the fiber was evacuated at 573°K for 16 hr prior to the adsorption studies. The surface areas were calculated using an average 2.17 nm* as the molecular area of krypton at the adsorption temperatures as discussed by Drzal and Fort[ltlJ. Compa~ng the treated fiber (HMS) with the untreated fiber (HMU), shows a very slight increase in surface area of 7%. Comparing the HMS fiber with the HMU fiber after the 300°C VHT also shows an increase in surface area of about the same order of magnitude. Even though the absolute area has increased with vacuum heat treatment, the percentage increase in surface area remains the same. Since composite processing generally involves temperatures up to 3OO”C,the true area of interfacial contact between resin and fiber would be between the “as received” and “VHT” conditions. In either case the increase in surface area alone cannot be responsible for the improved strength characteristics of the surface treated over the untreated fiber. These results are similar to those reported in an earlier work on Type A fibers[l]. The manufacturer’s surface treatment did not affect the surface area significantly.
The surface composition and energetics of type HM graphite fibers
379
Table 2. Point “B” surface area analysis of Type HM graphite fibers Fiber
E*(m2/gm)
nm (moles/gm)
HMS
3.2X1O-6
.418
300°C V.T.
3.6x1o-6
.470
300°C V.T., AIR XPO
3.5x10-s
.457
ZO*C V.T.
3.1x10-6
.405
300°C Y.T.
3.4x10-6
.444
2O’C
V.T.
HMU
*2.17 t-d/Krypton molecule
Recently, Rand and Robinson[lS] also reported on extensive investigation into surface area change on graphite fibers with oxidation treatment using nitrogen adsoprtion techniques. Agreement on absolute values for the surface area between those results and those on Rand and Robinson is within 20% and probably reelects the assumptions in molecular area, differences in experimental technique and in sample prep~ation. These samples were continuous filaments would on a spool whereas Rand and Robinson chopped their fiber samples into 10mm lengths. Nevertheless, the slight increase in surface area of 540% with treatment is identical in both cases. The increase in surface area with vacuum heat treatment is difficult to explain. Rand and Robinson noted the same trend and ascribed the increase to opening of internal voids by elimination of carbonization products trapped in the fiber. However, in this case the fibers were not chopped and the surface area increase after 300°C VHT must be due to increased accessibility of the adsorbate to micro cracks and fissures present on the fiber surface. 3.3 Isoteric heat and differential entropies of adsorption 3.3.1 Type HMU fiber. The isosteric heats and differential entropies of adsorption were calculated from the adsorption isotherms by applying the integrated form of the Claussius-Clapyron equation. In all cases replicate adsorption isotherms were determined to check the reproduceability of the data. The monolayer points are indicated by the short vertical lines. The isosteric heat and differential entropies of adsorption are shown in Fig. 5. Curve 20°C VT represents the data for a sample given only a room temperature evacuation at 10d6 Ton for 16 hr and the curve labled 300°CVT represents the fiber evacuated at 300°C for I6 hr. The isosteric heats for the 2O’C VT fiber display a low interaction with krypton. At small coverage values (l&20% of the surface) the heats of adsorption begin at 12 KJ/mole and decrease to IOKJlmole. The isosteric heat remains constant at this value until the monolayer is complete. Thereafter the isosteric heat approaches the heat of liquifaction of krypton. The differential entropy of krypton on this surface is quite high indicating a high degree of adsorbate mobility. The very slight increase in
KR/HMU
r"n
, w $ L F w t cs
20-
0 *
. ..I..........................
,_ 3r
‘L; 300°C
-20
f
..,..,,,.
VT
1 0
1
Amount
2 3 4 Adswbed (moiesigm)x
5 10-O
6
Fig. 5. Isosteric beat and differential entropy of adsorption for Type HMU fibers treated at 20°C and 3WC. the heat of adsorption of krypton in the first monolayer as compared to the krypton heat of liquifaction coupled with the high entropic values indicate that the HMU fiber surface in the “as-received” case is covered with adsorbed material which effectively reduces the strength of the surface electric field. Only the initial small portion appears energetically to be superior to the rest of the surface. Indeed, the amount desorbed in the thermal deso~tion studies was nearly su~cient to cover one-half of the fiber surface. After the HMU fiber has been evacuated at 300°C for 16 hr, its surface has changed in adsorption character. The adsorption isotherms, as discussed earlier, have a pronounced knee. The isosteric heats display a magnitude and variation with coverage which is identical to that shown by highly graphitized carbon such as Graphon on P-33[12]. The isosteric heat gives an indication of some (< 1%) heterogeneity initially but then it increases with increasing coverage until the monolayer point is reached. Thereafter the heats of adsorption
decrease to the heat of liquifaction of krypton as the second monolayer is fihed. The differential entropies likewise are large and positive indicating a mobile adsorbate and one behaving very similar to the 20”CVTHMU surface. The entropy does however display the pronounced minimum at the monolayer which is characteristic of completion of the first adsorbate layer. The removal of the material present on the fiber surface increases the Krypton-HMU interaction from 10 to 14 KJ/mole, In addition, the heterogeneities, observable at low coverages, on the 20°C VT surface are reduced to a very low level after the 300°C VT. This reduction in heterogeneities appears to be irreversible since a subsequent exposure to laboratory air and evacuation at 20°C did not restore the surface to its “as received” condition but allowed it to remain the same as the 300°C VT surface.
3WCVT curve is identical to that obtained on the 300°CVT HMU fiber and is characteristic of adsorption on a homogeneous surface. The nature of the high energy sites on the HMS fiber is indicated by the desorption products previously discussed in Fig. 3. The material thermaily desorbed from the HMS fiber had a much higher content of COZ and ratio of CO&O than the HMU fiber. For the HMS fiber the CO* content was as high as 35% and the C02/C0 ratio was 1.4 while for the HMU fiber the CO, content of the desorbed material was only 15% and the COdCO ratio was 0.25. Apparently the species added to the HM fiber with surface ~eatment are oxygenated species that desorb at low temperature ( < 300°C) and decompose to give primarily CO2 and CO. After 300°CVT only a very small percentage of surface sites remain which are more energetic than the remainder of the fiber surface.
3.3.2 Type HA&i' fiber. The isosteric heat and differential entropy of krypton adsorption on the 20°C VT are shown in Fig. 6. The monolayer points determined by the surface area analysis are indicated by short vertical lines. A large amount of similarity exists between the FINS and HMU data. Except for the data at small fractions of a monolayer, these the~odynamic curves are superimposeable with the untreated fiber data of Fig. 5. The isosteric heat of adsorption is 15 KJ/mole at the first amount adsorbed and decreases to a value slightly greater than lOKJ/mole until the monolayer point is reached whereupon the isosteric heat approaches the heat of liqu~a~tion of krypton. The 300°CVT of the HMS fiber apparently removes the surface groups responsible for the initial high values for the heat of adsorption which were present on the 20°C VT surface. That initial region which was - 0.3 monolayer on the 20°C VT HMS fiber has decreased to less than S% of a monolayer with 3OOYVT. The remainder of the
3.4 ~st~~~teof active swface sites Although the entire fiber surface comes into contact with the matrix in a composite, some portions of the fiber surface are more active in their interaction with the matrix. An estimation of the number of active sites is desirable in order to relate surface properties to adhesion in composites. The adsorption isotherms on the HMU and HMS fibers after 300°C VT exhibit homogeneous adsorbent behavior except for a small amount of heterogeneity. The heterogeneity of highly graphitized carbons has been determined by extrapolation of the low pressure linear portion of the isotherm to zero pressureil61. Figure 7 shows the low pressure region for an HMU and HMS fiber and the extrapolation to zero pressure. Table 3 lists the determination of heterogeneity for the HMU and HMS fibers using this method. All of the extrapolated values agree quite well and give a value of 0.8% for the HMU fiber and 3.8% for the HMS fiber. These figures agree quite well with the estimates of O&0.7% for HMU and i-3% for HMS as determined by oxygen chemisorption at 34O”C[lS].The isosteric heat curves of Figs. 5 and 6 also agree with this estimate of the heterogeneity. Initial high values of Qsr are ascribed to these sites for the 300°C VT surfaces.
Kr/HMS
I
I
I
t
Pressure
I
0
1 Amount
I 2
3 Adsorbt?d
4 hdes/gm)
5
10
6
x 10e6
Fig. 6. Isosteric heat and diierential entropy of adsorption for Type HMS fibers treated at 20°Cand 3OtW.
Fig.
t
0.2
0.1
20 Presswe
0.3
(tori-) 30
, 40
7. Low pressure region of krypton adsorption isotherms on HMU and HMS fibers.
381
The surface composition and energetics of type HM graphite fibers Table 3. Surface heterogeneity of HM fibers from krypton adsorption isotherms HMU
0.8%
HMS
3.8%
Type
HMU
IS5
The 20°C VT surfaces of HMU and HMS are more di&ult to interpret. The isotherms exhibit variable curvature and are not as readiiv analyzed as the 300°CVT surface. If the high initial values of the isosteric heat in Figs. 5 and 6 are due to active surface sites, then a much larger portion of the surface is initially active. Although this type of isotherm does not lend itself to as rigorous an analysis as the isotherm for a homogeneous surface (HMU, 300°CVT) as estimate of N-15% active sites for the HMU surface and 20-30% for the HMS fiber can be obtained from these curves. However, as the previous thermal desorption studies have shown, these surface species are desorbed as CO and CO, during thermal treatment to 3OO’C.Whether or not these sites which are active though ~errn~Iy unstable piay an impost role in fiber-matrix adhesion is an area for further invest~ation. 3.5 Ion scattering spectroscopy (ZSS) and positive secondary ion mass spectroscopy ( + SO4S) Figures 8 and 9 display the ISS and t SIMS spectra for the HMU and HMS fibers. The ISS spectra for both fibers is featureless except for the carbon peaks. The t SIMS spectra both indicate the presence of sodium with higher concentrations on the HMS fiber. However, the amount of sodium present is quite small. The signal shown here is two orders of magnitude lower than on the Type A fibers and is less than 1% of the surface. 3.6 ~ofar-dispersion components of graphite fiber surface free energy The results of the application of Kaelble’s analysis of the contact angle data on the HM fibers is shown in Table 4. Both the HMU and the HMS fibers were analyzed. Each fiber was analyzed at 72°F and 40% relative humidity in the “as-received” state as well as after the 300°C VT. Column 3 list the totai surface free energy for each fiber surface. The HMU fiber has a value of around
6
7
6 EIE,-
9
10
Fig. 8. Ion scattering (ES) and positive secondary ion mass spectroscopy ( + SIMS)of the HMU fiber surface.
Type HMS + SIMS
1ss
6
i 8 E/E,-
9
io
Fig. 9. Ion scattering spectroscopy (ISS) and positive secondary ion mass spectroscopy ( f SIMS)of the HMSfibersurface.
41 m.Jim’. After 300°CVT there is no appreciable change in the surface free energy within experimental error. Likewise the polar (y”) and dispersive (y’) components are not altered by 300°CVT. The magnitude of yP is quite small indicating that the HMU surface has few polar species even in air. This agrees with the surface interpretation determined by krypton isosteric heat variations. The HMS fiber, on the other hand, has a much larger total surface free energy in the “as-received” state. However, 300°CVT reduces this value appreciably. Al-
Table 4. Surface energetics of HM graphite fibers YP(mJ/m2)
YdWm2)
YT(d/mZ)
20°C V.T.
8.15.3.0
33.el.2
41.153.0
300°C V.T.
7.4+0.9
32.0+0.9
39.4+a.9
20°C V.T.
20.7~4.0
28.2tO.3
48.9+4.0
300°C V.T.
12.83.7
30.2+0.4
43.4+1.7
HMU
l%4S
L. T. DRZAL
382
though the dispersive component remains about the same after 300°CVT the polar component shows a significant decrease. The thermal desorption results showed that this fiber loses primarily CO and CO2 which must be associated with the increase in surface polarity brought about by surface treatment. Likewise, the fact that the 300°CVT removes these polar surface species is reflected in the comparison of the HMU and HMS fibers where their isosteric heats are identical except for the small heterogeneity (0.8 vs 3.8%). Most epoxies have a total surface free energy of 40 mJ/m’ or less. Therefore, the thermodynamic criteria for spreading is met, namely that the spreading phase have a lower surface free energy lower than the substrate. Wetting of graphic fibers by epoxies has been reported[17]. Although surface polar sites has been showed to be primarily carboxylic acid groups for highly graphitized fibers similar to the HM fibers studied here[18], the exact nature of these sites is at present unknown. A CONCLUSIONS Fist, the HM type fibers, which undergo a high temperature graphitization, possess an adsorptive surface very similar in nature to the high basal plane content surfaces of highly graphitized carbon blacks, i.e. they have little heterogeneity and are uniform in adsorptive energy. Second, surface treatment improves the number of heterogeneous sites significantly (O&3.8%). Third, the surface sites added with surface treatment are highly energetic and remain energetic even when exposed to air. However, 3OO”CVT removes a great majority of these species which desorb as CO and CO,.
et al.
Fourth, the fiber surface area is not significantly affected by surface treatment. Fifth, surface treatment increases the total surface free energy of the treated fiber, primarily through an increase in the bolar component of the surface free energy.
REFERENCES
L. T. Drzal, Carbon 15, 129(1977). 2. R. J. Diefendorf and W. W. Tokarsky, AFML-TR-72-133 Part I (October 1971). 3. S. C. Bennett and D. J. Johnson, 5rh Znt. Conf. Carbon Graphite, London (1978). 4. Hercules, Inc., Wilmington, Delaware 19899. 5. W. A. Zisman, Adhesion and Cohesion, (Editedby P. Weiss), Elsevier, Amsterdam (1%2). 6. D. H. Kaelble, Physical Chemistry of Adhesion, p. 153. Wiley-Intersciences, New York (1971). 7. G. Mozzo and R. Chaburd, Society of the Plastic Industry, Inc., 23rd Annual Technical Conf. Section 9-C, l-8 (1968). 8. R. L. Bendure, J. Coil. Znt. Sci. 42, 137 (1973). 9. D. H. Kaelble, Proc. XXZZrd Znt. Congress of Pure and Applied Chemistry, Vol. 8, p. 265 (1971). 10. B. R. Puri, S. Singh and 0. P. Mahajan, J. Indian Chem Sot. 42,427 (l%S). 11. S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. 60, 309 (1938). 12. F. A. Putnam and T. Fort Jr., J. Phys. Chem. 79,459 (1975). 13. G. D. Halsey Jr., Dfscussion Faraday Sot. 8.54 (1950). 14. L. T. Drzal and T. Fort Jr., .I. CON.Poly. Sci. 254,795 (1976). 15. B. Rand and Robinson, Carbon 15,257 (1977). 16. D. Graham, J. Phys. Chem. 61, 1310(1957). 17. S. Chwastiak, J. Coil. Znt. Sci. 42, 298 (1973). 18. D. Stuetz, R. E. Sullivan, A. DiEdwardo, G. Hardy, 0. D. Recta, P. McMahon and J. Leal, Eflects of Moisture on Chemical Interactions at a Polymer-Fiber m&face, AFMLTR-77-214 (1977). 1.
THE SURFACE COMPOSITION AND ENERGETICS OF TYPE HM GRAPHITE FIBERS L. T. DRZAL Surface Interactions and Mechanics Branch, Air Force Materials Laboratory, Wright-Patterson Air Force Base, OH 45433,U.S.A.
J. A. MESCHERand D. L. HALL University of Dayton Research Institute, Dayton, OH 45469,U.S.A. (Received 21 December 1978)
Abstract-The surfaces of Type HM graphite fibers, both treated (HMS) and untreated (HMU), were investigated using low temperature thermal desorption, krypton thermodynamic analysis, Ion Scattering with Positive Secondary Ion Mass Spectroscopy and Contact Angle Polar/Dispersion Analysis techniques. The surface energetics of HM fibers increased with surface treatment. However, vacuum treatment to temperatures of 300°C reduces a significant percentage of the highly energetic portion of the surface. Contact angle measurements on these same surfaces also indicate that the polarity of the HM fiber surface is increased with surface treatment.
1.WR~DUCTION
The molecular interactions between polymer matrix and reinforcing fiber are of paramount importance in under-
standing the role of the fiber-matrix interface in composite mechanical performance and durability. The nature of the reinforcing fiber surface is an important and necessary component of the interface. The purpose of this work is to elucidate the energetic and molecular nature of the graphite fiber surface so that phenomena occurring at the reinforcing fiber-matrix interface may be more fully understood. Graphite reinforcing fibers vary in their properties with processing conditions and precursor material. Commercial fibers can possess a spectrum of properties. A previous study[1] has determined the effect of surface treatment on the surface energetics and surface composition of a low modulus (Type A) polyacrylonitrile (PAN) based fiber. This study is a continuation of that work and is directed at delineating the changes in surface energetics and composition with surface treatment of a high modulus (Type HM) PAN based fiber. A structural model of the Type HM fiber used in this study is shown in Fig. 1[2]. The main elements of the fiber structure are the graphitic lamellar ribbons. The variation in these structural elements depends on the graphitization temperature and processing of the particular fiber. The type HM fiber studied here is processed at high temperatures (2600°C) causing a high degree of axial alignment. The graphitic lamellar ribbons are about 20 basal layers thick and 7nm wide with over 75% of the graphitic basal planes oriented within 12” of the fiber axis. There is a small amount of ribbon intertangling and the graphitic basal planes are oriented radially parallel to the fiber surface. Recent work by Bennett and Johnson[3] presents additional evidence for a thin surface layer (150-250 nm) with larger crystallites and a higher degree of orientation than the interior of the fiber. CARBON
Vol. 17. No. 5/A
Because of the graphitic basal plane orientation, the fiber surface in the untreated case would be expected to be similar energetically to high temperature processed graphite. A more detailed discussion of graphite fiber morphology is available in Ref. [2]. 2.EXPERIMENTAL. 2.1 Experimental techniques The key to understanding the fiber-matrix interactions occurring in composite materials depends on the ability to relate the “clean” graphite fiber surface analyzed in spectroscopic systems to the “real” graphite fiber surface initially in equilibrium with the ambient environment and then subject to increases in temperature (up to 300°C) in contact with the polymeric matrix. In order to accomplish this, a variety of experimental techniques have been employed both of fibers in the “as-received” state and after a 300°C vacuum heat treatment. Thermal desorption in 50°C increments was used to quantify the amount of material present on the fiber surface and mass spectroscopy was used to analyze the desorbed material. Ion scattering spectroscopy (ES) coupled with positive secondary ion mass spectroscopy (+ SIMS) was applied to determine if ionizable surface species, especially alkali metals, were present. Krypton adsorption isotherms at high temperatures were determined in order to quantify surface area changes and enthalpies and entropies of adsorption. Finally, contact angles were measured with the micro-Wilhelmy technique for a variety of liquids, so that the polar and dispersive components of the graphite fiber surface free energy could be calculated. 2.2 Fibers Polyacrylonitrile (PAN) based fibers were obtained from a single batch of material provided by Hercules 375
376
L T. &CAL
et al,
2.3.3 ion scattering spect~scopy (ISS) and positive secondary ion mass spectroscopy (+ SIMS). 20mm samples of graphite fibers were aligned in a stainless steel holder for analysis in a 3 M model 520 ISS unit equipped with a UTI-1OOC quadrupole mass spectrometer biased for f SIMS. 2.3.4 Polar-dispersioncomponentsof fiber surface free energy. Ultimately, the surface free energy of the graphite fiber determines the wetting behavior of the matrix and the thermodynamic state of the resulting fiber-matrix interface. Because significant amounts of material are desorbed from these fiber surfaces at temperatures representing conditions that current and future ,soo& matrices would experience during fabrication, a determination of the fiber surface free energy is desirable under the “as-received” and 300°C vacuum treated conditions. A measure of the fiber wettability is available through evaluation of a critical surface free energy as proposed by Zisman[5], where the contact angle of a series of liquids on the surface of interest is measured and the value of the surface tension of a liquid which has a zero contact angle (spreading) is chosen as the critical surface free energy for wetting. Ail liquids with a value Schematic 3-D Structural Model of Poly~rylonitrile Eased of surface tension less than this value woufd spread on Fiber with Properties Similar to Type HMU (after Diefe~orf and Tokarskyl the substrate while those with a larger value would have a finite contact angle. Fig. 1. The structureof a Type HM fiber (after Ref.121). Kaelbe[6] has shown that for low energy surfaces the surface free energy can be considered to be composed of two parts-a dispersive component (rd) which is a Inc. [4]. One portion of this batch was given a proprietary result of the non-specific interactions between materials surface treatment and was designated HMS (Lot No. and a polar component (yp) which is the result of XA-289-74-2) and the other portion designated HMU specific polar interactions due to polar surface func(Lot No. XA-289-74-4) was left untreated. No additional tionalities. These quantities can be measured for a solid surface treatment sizing was used. The modulus of these surface by measuring the contact angle 8 for a series of fibers was 51.5 x lo&psi. The fibers were handled as little liquids of known polar and dispersive components. The as possibIe to prevent surface con~mination or damage. polar and dispersive components of the sotid can then be evaluated by use of the relations~p 2.3 Expe~men~ul procedure 2.3.1 Thermal desorptionand compositionalanalysis. The fiber sample was heated in - 50°C increments in a calibrated volume system kept at elevated temperature. The amount of material desorbed was calculated from pressure, volume and temperature measurements using the ideal gas law. Molecular analysis of the desorbate was accomplished using standard cracking patterns. The exact details of this technique as well as the Krypton adsorption and ISS/SIMS techniques are given eisewherelll. 2.3.2 tiptoe adso~tio~ studies. The adsorption of krypton, an inert gas, was used to investigate the changes in surface energetics of graphite fibers. Adsorption isotherms were determined volumetrically at two temperatures and the surface area and adsorption thermodynamics were computed. Both fiber surface treatment and vacuum heat treatments effects on the fiber surfaces, were examined in this way. Procedures for preparing the fiber surfaces, determination of the isotherms and calculation of the adsorption thermodynamics are the same as used in the previously reported work [ll.
YL”(l fcos x9)=
ysdfj2+
2Yz.d1’2
5 f
Ii2 ,,&/2 >
where subscript L refers to the contacting liquid, S refers to the solid surface under investigation, yLv is the surface free energy of the contacting liquid and 0 is the contact angle formed between the liquid and the solid surface. (A detailed derivation is available in Ref[4]) #* and ysd” can be measured by plotting yLv(l + cos 8)/2 y~~“~vs yLp”lyLd”* and determining the slope and intercept of the best straight lime through these points. Ex~~men~ly, the contact angle of a drop deposited on a fiber 8m in diameter is very difficult to measure directly. An easier method is the micro-Wilhelmy technique[7,8] were a single fiber is partially immersed in a reservoir of the liquid of interest and the force exerted on the fiber due to the surface tension of the liquid is measured. The following relationship then holds F = Cy,, cos B where F is the force measured corrected for bouyancy, C is the circumference of the wetted fiber, yLv is the
The surfacecompositionand energeticsof type HMgraphitefibers liquid surface tension and 8 is the contact angle. F and yLv are measured. C is the circumference of the fiber calculated from an optical measurement of the fiber diameter for the circular cross section fibers used in this study or measured directly by measuring the force F when the fiber is dipped into a wetting liquid (cos 0 = cos 0” = 1). The contact angle can then be evaluated. The measurements conducted here involved attaching a single fiber to a pure nickel hook with a small amount of cyanoacrylate adhesive. Surface mi~ation of the adhesive along the fiber length below the attachment point ( - 1cm) was checked using a scanning Auger microprobe with a beam size of 2 pm. No evidence of migration to the fiber portion in contact with the liquid was detected. The adhesive was placed on the hook first, and the fiber was brought in contact with it. The portion of the fiber handled was removed prior to measurement. The fiber mounted on a hook was suspended from the arm of a Cahn RH microbalance kept in a constant temperature (72°F 2 IoF), constant humidity (40% t 2%) room. The fiber tip was immersed in the liquid of interest and the equ~ib~um force was recorded to 22~8. The sample was removed and its diameter was measured optically with a calibrated Vickers AEI image-splitting eyepiece. Between five and ten fibers were used for each liquid and were immersed only once. The fiber circumference as determined optically was checked against a determination with n-octane and the agreement was within the experimental error. The reagent grade liquids used in this study are listed in Table 1 along with their respective surface free energies and the polar and dispersive components of their surface free energies as determined from the literature [6,9]. Liquid surface tensions were periodically checked with a ~Nouy tensiometer for a~eement with the literature values. Purified liquids were stored in enclosed pipette equipped bottles to preclude contamination. An acid cleaned PTFE 10ml beaker was used for storage of the liquids during wetting studies and the liquid surface was swept clean with additional liquid by ovefiowing prior to determination for each sample.
377
raised an additional 0.5 mm to eliminate spurious fiber and effects. This force measurement represented the value associated with an equilibrium advancing contact angle. Further immersion of the fiber showed some variation in contact angle along the fiber length associated with circumference changes. Repeat immersions of the same fiber altered the contact angle, therefore each fiber was immersed only once and the average for five to ten fibers was used in the contact angle and surface enrgetic determinations. The cummulative effect of the variations in fiber diameter, immersion force and linear regression analysis of the data is shown in the error bounds on the values for the polar and dispersion components of these fibers listed in Table 4.
3.1 Thermal desorption and compositional analysis The inremental and total amounts of material desorbed from the surfaces of HMU and HMS fibers are shown in Fig. 2 as a function of vacuum heat treatment temperature. The total amounts desorbed as well as the variation in incremental amount desorbed with temperature are approximately the same for both fibers. An equivalent of 0.5 monolayers is removed from both the untreated and surface treated fibers. Significant differences appear, however, in the com-
M-is Faber
:.50 K _m : I .25
The liquid surface was brought into contact with each
fiber by slowly raising its platform vertically until contact with the fiber was made as indicated by a force response displayed on the balance recorder. The liquid was then
Fig. 2. Incremental and total amount of material desorbed from HMU and HMS graphite fiber surfaces duringthermaltreatment.
Table I. Surface free energy components of reference liauids Liquid Water
72.8
21.8
51.0
glycerol
64.0
34.0
30.0
ethylene glycol
48.3
29.3
19.0
polypropylene glycol PG-1200
31.3
24.5
6.8
formamide
58.3
32.3
26.0
hexadecane
27.6
27.6
0
methylene iodide
50.8
48.4
2.4
br~onaphthalene
44.6
44.6
0
378
L. T.
hZAL
position of the volatiles that are thermally desorbed (Fig. 3). The untreated high modulus fiber (HMU) desorbs primarily water (40%) with some additional carbon monoxide (35%). Together they constitute over 70% of the desorbate. After surface treatment, the composition of these desorbed materials changes, however. The HMS fiber thermally desorbs primarily carbon dioxide and carbon monoxide which account for - 70% of its volatiles. Water comprises only an additional 20% of the desorbed gas. This difference in composition must be due to changes brought about by the surface treatment given HMS. The large amount of carbon dioxide may be indicative of the deso~tion of some surface functional groups originally added to the surface. Usually species of this type desorb at higher temperatures[lO]. However, the low temperature at which the CO and CO2 and desorbed may be indicative of a weak adsorption strength for these surface species. 3.2 Surface area analysis Fiber surface area can be an alterable parameter which might have a beneficial effect on graphite-fiber-matrix adhesion. Evaluation of fiber surface area is possible through use of the krypton adsorption isothe~s. Typical adso~tion isotherms for I-TMfibers are shown in Fig. 4. The 20°C VT fibers possess a weak Type II isotherm according to the Braunauer method of classification[ll]. However, the 300°C VT fibers both surface treated and untreated have changed their adsorptive character markedly resulting in the appearance of a pronounced “knee” at low pressures followed by an almost horizontal portion. This type of behavior is indicative of strong monolayer formation and is observed HUU
Fiber
rerrperature,‘C Fig. 3. Mass spectral analysis (mole%) of material desorbed from HMU and MHS fibers during thermal treatment.
et
al.
0 t 0
I
,
I
,
I
20
40
60
60
100
PressureCtori-1 I
1
2
4
6 8 PressurefPaxtd)
IO
12
Fig. 4. Typical krypton adsorption isotherms on HMU and HMS fibers.
on highly graphitic carbons[12]. Ah of these isotherms were evaluated for the monolayer point using the “B” method according to the method proposed by HalseyIl31. Although there may be some question regarding the absolute accuracy of the surface area calculated from this method, relative differences in surface areas for the same sample are reliable. Conventional BET calculations for the 20°C VT isotherms were not applicable because of the “weak knee” at the monolayer point and the resultant low value for the BET “C” constant. BET determination for the 300°C VT isotherms gave values of surface area consistent with the “B” method but slightly higher. The results of the surface area analysis are tabulated in Table 2. The fiber treatment conditions were: as received (20°CVT)-the fiber was evacuated I5 hr at 293°K prior to the adsorption studies and (3OOYYTj-the fiber was evacuated at 573°K for 16 hr prior to the adsorption studies. The surface areas were calculated using an average 2.17 nm* as the molecular area of krypton at the adsorption temperatures as discussed by Drzal and Fort[ltlJ. Compa~ng the treated fiber (HMS) with the untreated fiber (HMU), shows a very slight increase in surface area of 7%. Comparing the HMS fiber with the HMU fiber after the 300°C VHT also shows an increase in surface area of about the same order of magnitude. Even though the absolute area has increased with vacuum heat treatment, the percentage increase in surface area remains the same. Since composite processing generally involves temperatures up to 3OO”C,the true area of interfacial contact between resin and fiber would be between the “as received” and “VHT” conditions. In either case the increase in surface area alone cannot be responsible for the improved strength characteristics of the surface treated over the untreated fiber. These results are similar to those reported in an earlier work on Type A fibers[l]. The manufacturer’s surface treatment did not affect the surface area significantly.
The surface composition and energetics of type HM graphite fibers
379
Table 2. Point “B” surface area analysis of Type HM graphite fibers Fiber
E*(m2/gm)
nm (moles/gm)
HMS
3.2X1O-6
.418
300°C V.T.
3.6x1o-6
.470
300°C V.T., AIR XPO
3.5x10-s
.457
ZO*C V.T.
3.1x10-6
.405
300°C Y.T.
3.4x10-6
.444
2O’C
V.T.
HMU
*2.17 t-d/Krypton molecule
Recently, Rand and Robinson[lS] also reported on extensive investigation into surface area change on graphite fibers with oxidation treatment using nitrogen adsoprtion techniques. Agreement on absolute values for the surface area between those results and those on Rand and Robinson is within 20% and probably reelects the assumptions in molecular area, differences in experimental technique and in sample prep~ation. These samples were continuous filaments would on a spool whereas Rand and Robinson chopped their fiber samples into 10mm lengths. Nevertheless, the slight increase in surface area of 540% with treatment is identical in both cases. The increase in surface area with vacuum heat treatment is difficult to explain. Rand and Robinson noted the same trend and ascribed the increase to opening of internal voids by elimination of carbonization products trapped in the fiber. However, in this case the fibers were not chopped and the surface area increase after 300°C VHT must be due to increased accessibility of the adsorbate to micro cracks and fissures present on the fiber surface. 3.3 Isoteric heat and differential entropies of adsorption 3.3.1 Type HMU fiber. The isosteric heats and differential entropies of adsorption were calculated from the adsorption isotherms by applying the integrated form of the Claussius-Clapyron equation. In all cases replicate adsorption isotherms were determined to check the reproduceability of the data. The monolayer points are indicated by the short vertical lines. The isosteric heat and differential entropies of adsorption are shown in Fig. 5. Curve 20°C VT represents the data for a sample given only a room temperature evacuation at 10d6 Ton for 16 hr and the curve labled 300°CVT represents the fiber evacuated at 300°C for I6 hr. The isosteric heats for the 2O’C VT fiber display a low interaction with krypton. At small coverage values (l&20% of the surface) the heats of adsorption begin at 12 KJ/mole and decrease to IOKJlmole. The isosteric heat remains constant at this value until the monolayer is complete. Thereafter the isosteric heat approaches the heat of liquifaction of krypton. The differential entropy of krypton on this surface is quite high indicating a high degree of adsorbate mobility. The very slight increase in
KR/HMU
r"n
, w $ L F w t cs
20-
0 *
. ..I..........................
,_ 3r
‘L; 300°C
-20
f
..,..,,,.
VT
1 0
1
Amount
2 3 4 Adswbed (moiesigm)x
5 10-O
6
Fig. 5. Isosteric beat and differential entropy of adsorption for Type HMU fibers treated at 20°C and 3WC. the heat of adsorption of krypton in the first monolayer as compared to the krypton heat of liquifaction coupled with the high entropic values indicate that the HMU fiber surface in the “as-received” case is covered with adsorbed material which effectively reduces the strength of the surface electric field. Only the initial small portion appears energetically to be superior to the rest of the surface. Indeed, the amount desorbed in the thermal deso~tion studies was nearly su~cient to cover one-half of the fiber surface. After the HMU fiber has been evacuated at 300°C for 16 hr, its surface has changed in adsorption character. The adsorption isotherms, as discussed earlier, have a pronounced knee. The isosteric heats display a magnitude and variation with coverage which is identical to that shown by highly graphitized carbon such as Graphon on P-33[12]. The isosteric heat gives an indication of some (< 1%) heterogeneity initially but then it increases with increasing coverage until the monolayer point is reached. Thereafter the heats of adsorption
decrease to the heat of liquifaction of krypton as the second monolayer is fihed. The differential entropies likewise are large and positive indicating a mobile adsorbate and one behaving very similar to the 20”CVTHMU surface. The entropy does however display the pronounced minimum at the monolayer which is characteristic of completion of the first adsorbate layer. The removal of the material present on the fiber surface increases the Krypton-HMU interaction from 10 to 14 KJ/mole, In addition, the heterogeneities, observable at low coverages, on the 20°C VT surface are reduced to a very low level after the 300°C VT. This reduction in heterogeneities appears to be irreversible since a subsequent exposure to laboratory air and evacuation at 20°C did not restore the surface to its “as received” condition but allowed it to remain the same as the 300°C VT surface.
3WCVT curve is identical to that obtained on the 300°CVT HMU fiber and is characteristic of adsorption on a homogeneous surface. The nature of the high energy sites on the HMS fiber is indicated by the desorption products previously discussed in Fig. 3. The material thermaily desorbed from the HMS fiber had a much higher content of COZ and ratio of CO&O than the HMU fiber. For the HMS fiber the CO* content was as high as 35% and the C02/C0 ratio was 1.4 while for the HMU fiber the CO, content of the desorbed material was only 15% and the COdCO ratio was 0.25. Apparently the species added to the HM fiber with surface ~eatment are oxygenated species that desorb at low temperature ( < 300°C) and decompose to give primarily CO2 and CO. After 300°CVT only a very small percentage of surface sites remain which are more energetic than the remainder of the fiber surface.
3.3.2 Type HA&i' fiber. The isosteric heat and differential entropy of krypton adsorption on the 20°C VT are shown in Fig. 6. The monolayer points determined by the surface area analysis are indicated by short vertical lines. A large amount of similarity exists between the FINS and HMU data. Except for the data at small fractions of a monolayer, these the~odynamic curves are superimposeable with the untreated fiber data of Fig. 5. The isosteric heat of adsorption is 15 KJ/mole at the first amount adsorbed and decreases to a value slightly greater than lOKJ/mole until the monolayer point is reached whereupon the isosteric heat approaches the heat of liqu~a~tion of krypton. The 300°CVT of the HMS fiber apparently removes the surface groups responsible for the initial high values for the heat of adsorption which were present on the 20°C VT surface. That initial region which was - 0.3 monolayer on the 20°C VT HMS fiber has decreased to less than S% of a monolayer with 3OOYVT. The remainder of the
3.4 ~st~~~teof active swface sites Although the entire fiber surface comes into contact with the matrix in a composite, some portions of the fiber surface are more active in their interaction with the matrix. An estimation of the number of active sites is desirable in order to relate surface properties to adhesion in composites. The adsorption isotherms on the HMU and HMS fibers after 300°C VT exhibit homogeneous adsorbent behavior except for a small amount of heterogeneity. The heterogeneity of highly graphitized carbons has been determined by extrapolation of the low pressure linear portion of the isotherm to zero pressureil61. Figure 7 shows the low pressure region for an HMU and HMS fiber and the extrapolation to zero pressure. Table 3 lists the determination of heterogeneity for the HMU and HMS fibers using this method. All of the extrapolated values agree quite well and give a value of 0.8% for the HMU fiber and 3.8% for the HMS fiber. These figures agree quite well with the estimates of O&0.7% for HMU and i-3% for HMS as determined by oxygen chemisorption at 34O”C[lS].The isosteric heat curves of Figs. 5 and 6 also agree with this estimate of the heterogeneity. Initial high values of Qsr are ascribed to these sites for the 300°C VT surfaces.
Kr/HMS
I
I
I
t
Pressure
I
0
1 Amount
I 2
3 Adsorbt?d
4 hdes/gm)
5
10
6
x 10e6
Fig. 6. Isosteric heat and diierential entropy of adsorption for Type HMS fibers treated at 20°Cand 3OtW.
Fig.
t
0.2
0.1
20 Presswe
0.3
(tori-) 30
, 40
7. Low pressure region of krypton adsorption isotherms on HMU and HMS fibers.
381
The surface composition and energetics of type HM graphite fibers Table 3. Surface heterogeneity of HM fibers from krypton adsorption isotherms HMU
0.8%
HMS
3.8%
Type
HMU
IS5
The 20°C VT surfaces of HMU and HMS are more di&ult to interpret. The isotherms exhibit variable curvature and are not as readiiv analyzed as the 300°CVT surface. If the high initial values of the isosteric heat in Figs. 5 and 6 are due to active surface sites, then a much larger portion of the surface is initially active. Although this type of isotherm does not lend itself to as rigorous an analysis as the isotherm for a homogeneous surface (HMU, 300°CVT) as estimate of N-15% active sites for the HMU surface and 20-30% for the HMS fiber can be obtained from these curves. However, as the previous thermal desorption studies have shown, these surface species are desorbed as CO and CO, during thermal treatment to 3OO’C.Whether or not these sites which are active though ~errn~Iy unstable piay an impost role in fiber-matrix adhesion is an area for further invest~ation. 3.5 Ion scattering spectroscopy (ZSS) and positive secondary ion mass spectroscopy ( + SO4S) Figures 8 and 9 display the ISS and t SIMS spectra for the HMU and HMS fibers. The ISS spectra for both fibers is featureless except for the carbon peaks. The t SIMS spectra both indicate the presence of sodium with higher concentrations on the HMS fiber. However, the amount of sodium present is quite small. The signal shown here is two orders of magnitude lower than on the Type A fibers and is less than 1% of the surface. 3.6 ~ofar-dispersion components of graphite fiber surface free energy The results of the application of Kaelble’s analysis of the contact angle data on the HM fibers is shown in Table 4. Both the HMU and the HMS fibers were analyzed. Each fiber was analyzed at 72°F and 40% relative humidity in the “as-received” state as well as after the 300°C VT. Column 3 list the totai surface free energy for each fiber surface. The HMU fiber has a value of around
6
7
6 EIE,-
9
10
Fig. 8. Ion scattering (ES) and positive secondary ion mass spectroscopy ( + SIMS)of the HMU fiber surface.
Type HMS + SIMS
1ss
6
i 8 E/E,-
9
io
Fig. 9. Ion scattering spectroscopy (ISS) and positive secondary ion mass spectroscopy ( f SIMS)of the HMSfibersurface.
41 m.Jim’. After 300°CVT there is no appreciable change in the surface free energy within experimental error. Likewise the polar (y”) and dispersive (y’) components are not altered by 300°CVT. The magnitude of yP is quite small indicating that the HMU surface has few polar species even in air. This agrees with the surface interpretation determined by krypton isosteric heat variations. The HMS fiber, on the other hand, has a much larger total surface free energy in the “as-received” state. However, 300°CVT reduces this value appreciably. Al-
Table 4. Surface energetics of HM graphite fibers YP(mJ/m2)
YdWm2)
YT(d/mZ)
20°C V.T.
8.15.3.0
33.el.2
41.153.0
300°C V.T.
7.4+0.9
32.0+0.9
39.4+a.9
20°C V.T.
20.7~4.0
28.2tO.3
48.9+4.0
300°C V.T.
12.83.7
30.2+0.4
43.4+1.7
HMU
l%4S
L. T. DRZAL
382
though the dispersive component remains about the same after 300°CVT the polar component shows a significant decrease. The thermal desorption results showed that this fiber loses primarily CO and CO2 which must be associated with the increase in surface polarity brought about by surface treatment. Likewise, the fact that the 300°CVT removes these polar surface species is reflected in the comparison of the HMU and HMS fibers where their isosteric heats are identical except for the small heterogeneity (0.8 vs 3.8%). Most epoxies have a total surface free energy of 40 mJ/m’ or less. Therefore, the thermodynamic criteria for spreading is met, namely that the spreading phase have a lower surface free energy lower than the substrate. Wetting of graphic fibers by epoxies has been reported[17]. Although surface polar sites has been showed to be primarily carboxylic acid groups for highly graphitized fibers similar to the HM fibers studied here[18], the exact nature of these sites is at present unknown. A CONCLUSIONS Fist, the HM type fibers, which undergo a high temperature graphitization, possess an adsorptive surface very similar in nature to the high basal plane content surfaces of highly graphitized carbon blacks, i.e. they have little heterogeneity and are uniform in adsorptive energy. Second, surface treatment improves the number of heterogeneous sites significantly (O&3.8%). Third, the surface sites added with surface treatment are highly energetic and remain energetic even when exposed to air. However, 3OO”CVT removes a great majority of these species which desorb as CO and CO,.
et al.
Fourth, the fiber surface area is not significantly affected by surface treatment. Fifth, surface treatment increases the total surface free energy of the treated fiber, primarily through an increase in the bolar component of the surface free energy.
REFERENCES
L. T. Drzal, Carbon 15, 129(1977). 2. R. J. Diefendorf and W. W. Tokarsky, AFML-TR-72-133 Part I (October 1971). 3. S. C. Bennett and D. J. Johnson, 5rh Znt. Conf. Carbon Graphite, London (1978). 4. Hercules, Inc., Wilmington, Delaware 19899. 5. W. A. Zisman, Adhesion and Cohesion, (Editedby P. Weiss), Elsevier, Amsterdam (1%2). 6. D. H. Kaelble, Physical Chemistry of Adhesion, p. 153. Wiley-Intersciences, New York (1971). 7. G. Mozzo and R. Chaburd, Society of the Plastic Industry, Inc., 23rd Annual Technical Conf. Section 9-C, l-8 (1968). 8. R. L. Bendure, J. Coil. Znt. Sci. 42, 137 (1973). 9. D. H. Kaelble, Proc. XXZZrd Znt. Congress of Pure and Applied Chemistry, Vol. 8, p. 265 (1971). 10. B. R. Puri, S. Singh and 0. P. Mahajan, J. Indian Chem Sot. 42,427 (l%S). 11. S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. 60, 309 (1938). 12. F. A. Putnam and T. Fort Jr., J. Phys. Chem. 79,459 (1975). 13. G. D. Halsey Jr., Dfscussion Faraday Sot. 8.54 (1950). 14. L. T. Drzal and T. Fort Jr., .I. CON.Poly. Sci. 254,795 (1976). 15. B. Rand and Robinson, Carbon 15,257 (1977). 16. D. Graham, J. Phys. Chem. 61, 1310(1957). 17. S. Chwastiak, J. Coil. Znt. Sci. 42, 298 (1973). 18. D. Stuetz, R. E. Sullivan, A. DiEdwardo, G. Hardy, 0. D. Recta, P. McMahon and J. Leal, Eflects of Moisture on Chemical Interactions at a Polymer-Fiber m&face, AFMLTR-77-214 (1977). 1.