CARBON PERGAMON Carbon4l(2003)1193-1203 Effect of carbon fiber surface functional groups on the mechanical properties of carbon-carbon composites with HTT SR Dhakate.OP Bahl Carbon Technology Unit, Engineering Materials Division, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, Received 18 March 2001; accepted 25 January 2003 Abstract The present investigation describes the itative measurement of surface functional groups present on commerciall available different pan based carbon fibers, their effect on the development of interface with resol-type phenol stages of heat treatment. An ESCa study of the carbon fibers has revealed that high strength(ST-3 )carbon fibers posse formaldehyde resin matrix and its effect on the physico-mechanical properties of carbon-carbon composites at variou almost 10% reactive functional groups as compared to 5.5 and 4.5% in case of intermediate modulus(IM-500)and high modulus(HM-45)carbon fibers, respectively. As a result, ST-3 carbon fibers are in a position to make strong interactions with phenolic resin matrix and HM-45 carbon fibers make weak interactions, while IM-500 carbon fibers make intermediate interactions. This observation is also confirmed from the pyrolysis data(volume shrinkage)of the composites. Bulk density and kerosene density more or less increase in all the composites with heat treatment up to 2600C. It is further observed that bulk density is minimum and kerosene density is maximum upon heat treatment at 2600C in case of sT-3 based composites compared to HM-45 and IM-500 composites. It has been found for the first time that the defection temperature(temperature at which the properties of the material start to decrease or increase)of fiexural strength as well as interlaminar shear strength is different for the three composites(A, b and C)and is determined by the severity of interactions established at the polymer stage. Above this temperature, flexural strength and interlaminar shear strength increase in all the composites up to 2600C he maximum value of fiexural strength at 2600C is obtained for HM-45 composites and that of Ilss for ST-3 composites o 2003 Published by Elsevier Science Ltd Keywords: A. Carbon fibers; C. Electron spectroscopy; D. Functional groups, Mechanical properties 1. Introduction in carbon-carbon composites between fiber and matrix, within fiber bundles between the different microstructures Carbon-carbon composites are used in a number of which may exist within the matrix. It is well known that demanding applications such as space, defense, turbine the fiber-matrix interaction depends upon the surface blades, etc. [1, 2]. Their performance is known to depen functional groups of the carbon fibers and the matrix on the type of carbon fibers, matrix precursors, nature of precursor 3, 4]. In particular, the mechanical properties of bonding between fiber and matrix(fiber-matrix interface) carbon-carbon composites are very sensitive to the bond- and processing conditions [3, 4]. The nature of the interface ing between fibers and the matrix and its stress transfer and its influence on physico-mechanical properties is capability. In this respect, continuous research and de- extremely complex. Different types of interface may exist velopment work is going on from the last 3-4 decades, and especially regarding the role of interface on controlling the overall performance of carbon-carbon composites [5-10] Corresponding author. Tel:+91-11-574-6290 fax:+91-11- A qualitative correlation between the amount of 572-6952 functional groups, the nature of the interface and E-mail address. dhakate @csnpl ren nic, in(SR. Dhakate) site properties has been reported by Fitzer et al. [7 0008-6223/03/S-see front matter 2003 Published by Elsevier Science Ltd doi:10.1016/S0008-6223(03)00051-4
Carbon 41 (2003) 1193–1203 E ffect of carbon fiber surface functional groups on the mechanical properties of carbon–carbon composites with HTT S.R. Dhakate , O.P. Bahl * Carbon Technology Unit, Engineering Materials Division, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India Received 18 March 2001; accepted 25 January 2003 Abstract The present investigation describes the quantitative measurement of surface functional groups present on commercially available different PAN based carbon fibers, their effect on the development of interface with resol-type phenol formaldehyde resin matrix and its effect on the physico–mechanical properties of carbon–carbon composites at various stages of heat treatment. An ESCA study of the carbon fibers has revealed that high strength (ST-3) carbon fibers possess almost 10% reactive functional groups as compared to 5.5 and 4.5% in case of intermediate modulus (IM-500) and high modulus (HM-45) carbon fibers, respectively. As a result, ST-3 carbon fibers are in a position to make strong interactions with phenolic resin matrix and HM-45 carbon fibers make weak interactions, while IM-500 carbon fibers make intermediate interactions. This observation is also confirmed from the pyrolysis data (volume shrinkage) of the composites. Bulk density and kerosene density more or less increase in all the composites with heat treatment up to 2600 8C. It is further observed that bulk density is minimum and kerosene density is maximum upon heat treatment at 2600 8C in case of ST-3 based composites compared to HM-45 and IM-500 composites. It has been found for the first time that the deflection temperature (temperature at which the properties of the material start to decrease or increase) of flexural strength as well as interlaminar shear strength is different for the three composites (A, B and C) and is determined by the severity of interactions established at the polymer stage. Above this temperature, flexural strength and interlaminar shear strength increase in all the composites up to 2600 8C. The maximum value of flexural strength at 2600 8C is obtained for HM-45 composites and that of ILSS for ST-3 composites. 2003 Published by Elsevier Science Ltd. Keywords: A. Carbon fibers; C. Electron spectroscopy; D. Functional groups, Mechanical properties 1. Introduction in carbon–carbon composites between fiber and matrix, within fiber bundles, between the different microstructures Carbon–carbon composites are used in a number of which may exist within the matrix. It is well known that demanding applications such as space, defense, turbine the fiber–matrix interaction depends upon the surface blades, etc. [1,2]. Their performance is known to depend functional groups of the carbon fibers and the matrix on the type of carbon fibers, matrix precursors, nature of precursor [3,4]. In particular, the mechanical properties of bonding between fiber and matrix (fiber–matrix interface) carbon–carbon composites are very sensitive to the bondand processing conditions [3,4]. The nature of the interface ing between fibers and the matrix and its stress transfer and its influence on physico–mechanical properties is capability. In this respect, continuous research and deextremely complex. Different types of interface may exist velopment work is going on from the last 3–4 decades, and especially regarding the role of interface on controlling the overall performance of carbon–carbon composites [5–10]. *Corresponding author. Tel.: 191-11-574-6290; fax: 191-11- A qualitative correlation between the amount of surface 572-6952. functional groups, the nature of the interface and compoE-mail address: dhakate@csnpl.ren.nic.in (S.R. Dhakate). site properties has been reported by Fitzer et al. [7]. Also, 0008-6223/03/$ – see front matter 2003 Published by Elsevier Science Ltd. doi:10.1016/S0008-6223(03)00051-4
1194 S.R. Dhakate, O.P. Bahl/ Carbon 41(2003)1193-1203 Table I Properties of carbon fibers used (from manufacturer's data sheet) Diameter Density Tensile strength Tensile modulus Strain to failure (GPa) ST-3 6.0 IM-500 5 HM-45 191 2200 0.5 there are plenty of data available on the analysis of surface The composites were coded as follows. functional groups of carbon fibers and their influence on the development of interface and mechanical properties of lymer matrix composites. In addition to this, some data (A)ST-3 carbon fiber composites are available in the literature describing the effect of heat (B)IM-500 carbon fiber composites. treatment temperature(HTT)on the mechanical properties (C)HM-45 carbon fiber composites lyarylacetylene(PAA)and furfuryl alcohol based arbon-carbon composites [11-13]. Some studies are also available on the influence of fiber surface functional The polymer composites were heat-treated to 400- 2600C under an inert atmosphere. The heat-treated groups and various types of surface treated carbon fibers composites were characterized for volume shrinkage on the development of interface with phenolic resin matrix density and mechanical properties during each stage of [7,14,15 heat treatment. Flexural strength was measured by the In the present investigation, a systematic approach was three-point bending technique on a universal Instron adopted to understand the influence of carbon fiber surface testing machine(Model 4411, ASTM standard D-790-80) functional groups on interface development by measuring with a span length to depth ratio of 30: 1. The interlaminar the surface functional groups quantitatively and their shear strength(ILSS)was measured using ASTM standard D-2344-74 with span length to depth ratio of 8: 1. The kerosene density was measured by the kerosene pickup method using the Archimedes principle. The transverse 2. Composite preparation and characterization coefficients of thermal expansion were measured using a thermo-mechanical analyzer(TMA)attached to a Mettler Three types of commercially available PAN-based car hermal system TA-3000, in the range 50-900C under an bon fibers, manufactured by Toho Beslon Inc, and ert atmosphere. The optical micrographs of composites orayca Industries Inc, Japan, were used as reinforcement. heat treated at 2600C were observed using polarized light (a) ST-3(high strength, likely HTT-1200-1500C) (b)IM-500(intermediate modulus, likely HTT <1600- (c)HM45( high modulus, likely HTT>2200°℃C) 3. Results and discussion Since these fibers are heat-treated to different tempera. 3. 1. ESCA studies of carbon fibers tures, they exhibit different physical and mechanical properties (see Table 1)as well as different types of The surface composition of carbon fibers obtained by surface functional groups. The surface functional groups present were measured by ESCA, using an SSI 301 Ko radiation(spot diameter 300 um, 80 W; radiation able energy 14866 ev)under a residual pressure of 5x10-8 Surface composition bon fibers Torr (I Torr=133.322 Pa) Fiber type nidirectional polymer composite samples(150 mmx element N(1s) 4.0 mmX4.5 mm) were prepared using the wet winding and match mold die technique [16] with 45+2% fiber T-3 volume. The resol type phenol formaldchyde resin was HM.-45 9223 used as the matrix precursor for composite preparation. HM-4
1194 S.R. Dhakate, O.P. Bahl / Carbon 41 (2003) 1193–1203 Table 1 Properties of carbon fibers used (from manufacturer’s data sheet) Fiber Diameter Density Tensile strength Tensile modulus Strain to failure 3 type (mm) (g/cm ) (MPa) (GPa) (%) ST-3 6.0 1.75 4200 250 1.68 IM-500 5.5 1.77 4700 300 1.57 HM-45 6.0 1.91 2200 440 0.5 there are plenty of data available on the analysis of surface The composites were coded as follows. functional groups of carbon fibers and their influence on the development of interface and mechanical properties of (A) ST-3 carbon fiber composites. polymer matrix composites. In addition to this, some data (B) IM-500 carbon fiber composites. are available in the literature describing the effect of heat (C) HM-45 carbon fiber composites. treatment temperature (HTT) on the mechanical properties of polyarylacetylene (PAA) and furfuryl alcohol based carbon–carbon composites [11–13]. Some studies are also The polymer composites were heat-treated to 400– available on the influence of fiber surface functional 2600 8C under an inert atmosphere. The heat-treated groups and various types of surface treated carbon fibers composites were characterized for volume shrinkage, on the development of interface with phenolic resin matrix density and mechanical properties during each stage of [7,14,15]. heat treatment. Flexural strength was measured by the In the present investigation, a systematic approach was three-point bending technique on a universal Instron adopted to understand the influence of carbon fiber surface testing machine (Model 4411, ASTM standard D-790-80) functional groups on interface development by measuring with a span length to depth ratio of 30:1. The interlaminar the surface functional groups quantitatively and their shear strength (ILSS) was measured using ASTM standard influence on composite properties as a function of HTT. D-2344-74 with span length to depth ratio of 8:1. The kerosene density was measured by the kerosene pickup method using the Archimedes principle. The transverse 2. Composite preparation and characterization coefficients of thermal expansion were measured using a thermo-mechanical analyzer (TMA) attached to a Mettler Three types of commercially available PAN-based car- thermal system TA-3000, in the range 50–900 8C under an bon fibers, manufactured by Toho Beslon Inc., and inert atmosphere. The optical micrographs of composites Torayca Industries Inc., Japan, were used as reinforcement. heat treated at 2600 8C were observed using polarized light microscopy. (a) ST-3 (high strength, likely HTT |1200–1500 8C). (b) IM-500 (intermediate modulus, likely HTT |1600– 1800 8C). (c) HM-45 (high modulus, likely HTT .2200 8C) 3. Results and discussion Since these fibers are heat-treated to different tempera- 3 .1. ESCA studies of carbon fibers tures, they exhibit different physical and mechanical properties (see Table 1) as well as different types of The surface composition of carbon fibers obtained by surface functional groups. The surface functional groups present were measured by ESCA, using an SSI 301 spectrometer employing monochromatic and focused Al K Table 2 a radiation (spot diameter 300 mm, 80 W; radiation28 Surface composition of carbon fibers energy 1486.6 eV) under a residual pressure of 5310 Torr (1 Torr5133.322 Pa). Fiber type Atomic % Unidirectional polymer composite samples (150 mm3 \element C (1s) O (1s) N (1s) Si (2p) 4.0 mm34.5 mm) were prepared using the wet winding ST-3 86.63 8.19 2.35 1.86 and match mold die technique [16] with 4562% fiber IM-500 92.28 7.23 0.00 0.49 volume. The resol type phenol formaldehyde resin was HM-45 92.21 6.18 0.00 1.60 used as the matrix precursor for composite preparation
S.R. Dhakate, O.P. Bahl/ Carbon 41(2003)1193-1203 220 294 2892286.82844282 2916 2844 B80 660 2868 BINDING ENERGY A Fig 1.(a) ESCA spectra of (a)ST-3, (b) IM-500 and (c) HM-45 carbon fibers (b) Deconvolution spectra of o Is for (a)ST-3, (b)IM-500 and(c)HM-45 carbon fibers. Relative percentage of functional groups on the carbon fibers obtained by ESCA Functional group (28418) (284.05,28483) Phenolic or hydroxyl (B. E, ev) Carboxylic B. E. eV (290.48) (290.92) (B. E, ev) (39802,400.43)
S.R. Dhakate, O.P. Bahl / Carbon 41 (2003) 1193–1203 1195 Fig. 1. (a) ESCA spectra of (a) ST-3, (b) IM-500 and (c) HM-45 carbon fibers. (b) Deconvolution spectra of O 1s for (a) ST-3, (b) IM-500 and (c) HM-45 carbon fibers. Table 3 Relative percentage of functional groups on the carbon fibers obtained by ESCA Functional group Relative atomic percentages of functional groups ST-3 IM-500 HM-45 Graphitic carbon 69.9 61.9 90.88 (B. E., eV) (284.18) (284.06) (284.05, 284.83) Phenolic or hydroxyl 18.76 28.6 – (B. E., eV) (285.42) (285.08) Carbonyl 7.01 5.44 4.3 (B. E., eV) (287.34) (287.90) (288.49) Carboxylic 4.31 4.05 4.8 (B. E., eV) (290.13) (290.48) (290.92) Nitrogen containing 2.35 – – (B. E., eV) (398.02, 400.43)
1196 S.R. Dhakate, O.P. Bahl/ Carbon 41(2003)1193-1203 the ESCa studies is presented in Table 2. As expected 3.2. Interactions of fibers with matrix [17 the oxygen content on the surface decreases from ST-3 to HM-45 and the C/o ratio increases from St-3 The specific interactions are postulated to be Lewis HM-45, confirming the well known fact that the oxygen- acid-base type interactions or electron acceptor-donor ontaining functional groups, and consequently the reac- interactions [30, 31]. The polymer used here is a thermoset- tivity of carbon fiber surface, decrease with increasing ting phenolic resin which has acidic functional groups HTT. The nitrogen-containing functional groups are pres- whereas fibers possess both acidic and basic functional ent only in ST-3 fibers, from the residual nitrogen present groups. It is well known that carboxylic groups are in the precursor fibers responsible for strong interactions with a polymer matrix Fig. la shows the ESCA spectra of the three carbon having basic functional groups [ 18, 32]. Therefore, in the fibers. The three major peaks observed between binding present case these groups are not likely to play an energies 200 to 600 ev of carbon(C Is), oxygen(o Is) important role. It is found that ST-3 fibers possess maxi and nitrogen(N Is)correspond to graphite-like carbon and mum(9.46 relative percentage) functional groups which various functional groups. The peak around 101 ev are basic in character and would make strong interactions corresponds to silicon Si(2p). The C Is peak at 284. 24 ev whereas HM-45 fibers possess minimum reactive func- assigned to the carbon element only and between 284.05 tional groups (4.3 relative percentage) and would thus and 284.83 ev to graphitic carbon, 285.08-285.42 eV to make weak interaction with the phenolic resin matrix On hydroxyl (C-OH), 287.34-288.49 ev to carbonyl the other hand, the IM-500 fibers possess a maximum (C=O)and 290.13-290.92 ev to carboxylic(COoH) amount of total functional groups(38.09 relative per- functional groups of three different carbon fibers is differ- relative percentage (of carbonyl), which can make interac- ent(Fig. 1b, Table 3), which may be due to the structural ions with phenolic resin matrix intermediate between ST-3 difference of the carbon fibers surfaces. The fibers used in and HM-45 carbon fibers [ 33] the investigation are heat-treated at increasing temperature ST-3 to HM-45) as described in Section 2. As a conse- quence, orientation of carbon layers preferentially im- 3.3. Volume shrinkage vs HTT of composites proves parallel to fiber axis leading to different degrees of graphitization and therefore different electrical conduc- During heat treatment, carbon fiber reinforced polymer tivities of the fibers. The ESCA spectrum for the ST-3 opposites show changes in all the three directions of carbon fibers shows a very weak n Is peak as compared to composites, i. e, length, width and thickness, which are due the c Is and o Is peaks, which arises from the residual to the thermal degradation and shrinkage of polymer nitrogen present in the fibers. Nitrogen-containing func matrix [7, 34]. In unidirectional composites, the changes tional groups appear in ST-3 fibers at a binding energy of taking place in the direction parallel to the fiber axis are 398.02 ev, corresponding to the aromatic amines and controlled by the longitudinal thermal expansion of the piperidine structure, at 400. 43 ev to aliphatic amines, carbon fibers themselves. The fiber thermal expansion nitrile and amides [21, 22], while in the cases of IM-500(positive or negative) is very small [35]. As a consequence. and HM-45 carbon fibers these groups are not detected Deconvolution of these peaks gives relative percentages of functional groups present and these are compiled in Table The high strength carbon fibers (ST-3) possess the highest number of surface functional groups and as a result the surface of these fibers may be more disordered possessing comparatively high active surface area. The ST-3 fibers possess carboxylic, phenolic and hydroxyl 25 functional groups which are acidic in nature while car- 20 bonyl and some of nitrogen containing groups are basic in character [23-29]. The high modulus HM-45 carbon fiber, on the other hand, possesses a minimum number of surface functional groups and a better ordered graphite-like carbon fiber surface. Therefore such fibers should have the lowest active surface area. The intermediate modulus(IM-500) carbon fibers possess the greatest number of hydroxyl 04008001200 200024002800 functional groups. The contribution of carboxylic groups is HTT(C) almost the same in all the three fibers while carbony olume shrinkage observed with heat treatment tempera- groups decrease from ST-3 to HM-45 fibers ture of composite
1196 S.R. Dhakate, O.P. Bahl / Carbon 41 (2003) 1193–1203 the ESCA studies is presented in Table 2. As expected 3 .2. Interactions of fibers with matrix [17], the oxygen content on the surface decreases from ST-3 to HM-45 and the C/O ratio increases from ST-3 to The specific interactions are postulated to be Lewis HM-45, confirming the well known fact that the oxygen- acid–base type interactions or electron acceptor–donor containing functional groups, and consequently the reac- interactions [30,31]. The polymer used here is a thermosettivity of carbon fiber surface, decrease with increasing ting phenolic resin which has acidic functional groups HTT. The nitrogen-containing functional groups are pres- whereas fibers possess both acidic and basic functional ent only in ST-3 fibers, from the residual nitrogen present groups. It is well known that carboxylic groups are in the precursor fibers. responsible for strong interactions with a polymer matrix Fig. 1a shows the ESCA spectra of the three carbon having basic functional groups [18,32]. Therefore, in the fibers. The three major peaks observed between binding present case these groups are not likely to play an energies 200 to 600 eV of carbon (C 1s), oxygen (O 1s) important role. It is found that ST-3 fibers possess maxiand nitrogen (N 1s) correspond to graphite-like carbon and mum (9.46 relative percentage) functional groups which various functional groups. The peak around 101 eV are basic in character and would make strong interactions, corresponds to silicon Si (2p). The C 1s peak at 284.24 eV whereas HM-45 fibers possess minimum reactive funcis assigned to the carbon element only and between 284.05 tional groups (4.3 relative percentage) and would thus and 284.83 eV to graphitic carbon, 285.08–285.42 eV to make weak interaction with the phenolic resin matrix. On hydroxyl (.C–OH), 287.34–288.49 eV to carbonyl the other hand, the IM-500 fibers possess a maximum (.C=O) and 290.13–290.92 eV to carboxylic (–COOH) amount of total functional groups (38.09 relative perfunctional groups [18–20]. The chemical shift for all the centage) but the reactive functional groups are only 5.44 functional groups of three different carbon fibers is differ- relative percentage (of carbonyl), which can make interacent (Fig. 1b, Table 3), which may be due to the structural tions with phenolic resin matrix intermediate between ST-3 difference of the carbon fibers surfaces. The fibers used in and HM-45 carbon fibers [33]. the investigation are heat-treated at increasing temperature (ST-3 to HM-45) as described in Section 2. As a consequence, orientation of carbon layers preferentially im- 3 .3. Volume shrinkage vs. HTT of composites proves parallel to fiber axis leading to different degrees of graphitization and therefore different electrical conduc- During heat treatment, carbon fiber reinforced polymer tivities of the fibers. The ESCA spectrum for the ST-3 composites show changes in all the three directions of carbon fibers shows a very weak N 1s peak as compared to composites, i.e., length, width and thickness, which are due the C 1s and O 1s peaks, which arises from the residual to the thermal degradation and shrinkage of polymer nitrogen present in the fibers. Nitrogen-containing func- matrix [7,34]. In unidirectional composites, the changes tional groups appear in ST-3 fibers at a binding energy of taking place in the direction parallel to the fiber axis are 398.02 eV, corresponding to the aromatic amines and controlled by the longitudinal thermal expansion of the piperidine structure, at 400.43 eV to aliphatic amines, carbon fibers themselves. The fiber thermal expansion nitrile and amides [21,22], while in the cases of IM-500 (positive or negative) is very small [35]. As a consequence, and HM-45 carbon fibers these groups are not detected. Deconvolution of these peaks gives relative percentages of functional groups present and these are compiled in Table 3. The high strength carbon fibers (ST-3) possess the highest number of surface functional groups and as a result the surface of these fibers may be more disordered possessing comparatively high active surface area. The ST-3 fibers possess carboxylic, phenolic and hydroxyl functional groups which are acidic in nature while carbonyl and some of nitrogen containing groups are basic in character [23–29]. The high modulus HM-45 carbon fiber, on the other hand, possesses a minimum number of surface functional groups and a better ordered graphite-like carbon fiber surface. Therefore such fibers should have the lowest active surface area. The intermediate modulus (IM-500) carbon fibers possess the greatest number of hydroxyl functional groups. The contribution of carboxylic groups is almost the same in all the three fibers while carbonyl Fig. 2. Volume shrinkage observed with heat treatment temperagroups decrease from ST-3 to HM-45 fibers. ture of composites
S.R. Dhakate, O.P. Bahl/ Carbon 41(2003)1193-1203 these composites exhibit very little change parallel to shrinks onto the fibers when the interaction is maximum carbon fibers. However, appreciable changes in the width (as A). Such composites should exhibit nd thickness direction of shrinkage, as is observed Fig 2 shows the volume shrinkage of polymer compos ix interactions are weak(see Section 3.2) ites with heat treatment temperature. The amount of and as a result there is minimum shrinkage shrinkage depends upon the type of polymer matrix used in the fabrication of composites and fiber-matrix interactions ]. During heat treatment, chemical bonding between the 3.4. Bulk density vs HTT of composites fiber surface and the free functional groups of the resin undergoes rearrangement. Simultaneously, the molecular The bulk or apparent density is the density of compos- chains in the polymer matrix undergo pyrolysis and their ites containing voids and porosity. Table 4 shows the rearrangement results in matrix shrinkage. Up to 400C changes in density observed with HTT of the composites the shrinkage pattern of all the composites does not show These changes in density may be due to two factors much difference, but with increasing HTT shrinkage is maximum during the whole range of temperature and ()Dimensional changes due to shrinkage during (ii) Weight loss due to evolution of volatile product composites B and C it is 11-12%. The higher shrinkage in olysIs the case of composite A is attributed to strong fiber-matrix interactions. With increase in heat treatment temperature shrinkage increases continuously in all the composites and The initial density of the polymer composites depend it is -25% in composite A, 22% in composite B and 19% upon the density of( fibers used. its volume fraction in composite C at 1800C. Above 1800C, the conversion (which is kept the same in all three composites)and of non-graphitic carbon into a graphite-like carbon struc- compactness of the composites. The bulk density of ture(in the matrix), i.e., a reorientation of graphitic planes, lymer composites varied between 1.42 and 1.52 g/cm' starts taking place thus resulting in further composite pon heat treatment to 600C, a gradual density decreas shrinkage [11]. Further, with a continuous increase in heat is observed due to evolution treatment temperature to 2600C, the reorientation of formation of pores which results in volume expansion [36] graphitic planes takes place more extensively and the Above 600C, product evolution decreases to a large extent of shrinkage observed depends on the extent of extent as pyrolysis of the resin matrix is almost complete reorientation. The incremental shrinkage between 2000 and and there is an increase in density up to 1400C which is 2600C is as high as 6-7% in composite A and only 4% due to structural changes. The small density decreas in composite C. It is important to mention here that the between 1400 and 1800"C in composites A and B is due initial fiber volume(45+ 2%)is kept the same in all three to the formation of closed microporosity [37, 38]because cases. This brings out very clearly the effect of reactive of evolution of nitrogen-containing reaction products urface functional groups on the the fiber-matrix interac- tions. Consequently, during heat treatment the matrix m gto gs 4001800220026003000 HTT(C) Fig. 3. Change in kerosene density of composites with stages of
S.R. Dhakate, O.P. Bahl / Carbon 41 (2003) 1193–1203 1197 these composites exhibit very little change parallel to shrinks onto the fibers when the interaction is maximum carbon fibers. However, appreciable changes in the width (as in composite A). Such composites should exhibit a and thickness directions are observed. maximum amount of shrinkage, as is observed. In compoFig. 2 shows the volume shrinkage of polymer compos- site C, fiber–matrix interactions are weak (see Section 3.2) ites with heat treatment temperature. The amount of and as a result there is minimum shrinkage. shrinkage depends upon the type of polymer matrix used in the fabrication of composites and fiber–matrix interactions [9]. During heat treatment, chemical bonding between the 3 .4. Bulk density vs. HTT of composites fiber surface and the free functional groups of the resin undergoes rearrangement. Simultaneously, the molecular The bulk or apparent density is the density of composchains in the polymer matrix undergo pyrolysis and their ites containing voids and porosity. Table 4 shows the rearrangement results in matrix shrinkage. Up to 400 8C changes in density observed with HTT of the composites. the shrinkage pattern of all the composites does not show These changes in density may be due to two factors: much difference, but with increasing HTT shrinkage increases linearly in all the composites. For composite A it is maximum during the whole range of temperature and (i) Dimensional changes due to shrinkage during minimum in case of composite C. Up to 800 8C, 15% pyrolysis. shrinkage is noticed in the case of composite A, whereas in (ii) Weight loss due to evolution of volatile products composites B and C it is 11–12%. The higher shrinkage in during pyrolysis. the case of composite A is attributed to strong fiber–matrix interactions. With increase in heat treatment temperature, shrinkage increases continuously in all the composites and The initial density of the polymer composites depends it is |25% in composite A, 22% in composite B and 19% upon the density of carbon fibers used, its volume fraction in composite C at 1800 8C. Above 1800 8C, the conversion (which is kept the same in all three composites) and of non-graphitic carbon into a graphite-like carbon struc- compactness of the composites. The bulk density of3 ture (in the matrix), i.e., a reorientation of graphitic planes, polymer composites varied between 1.42 and 1.52 g/cm . starts taking place thus resulting in further composite Upon heat treatment to 600 8C, a gradual density decrease shrinkage [11]. Further, with a continuous increase in heat is observed due to evolution of reaction products and treatment temperature to 2600 8C, the reorientation of formation of pores which results in volume expansion [36]. graphitic planes takes place more extensively and the Above 600 8C, product evolution decreases to a large extent of shrinkage observed depends on the extent of extent as pyrolysis of the resin matrix is almost complete reorientation. The incremental shrinkage between 2000 and and there is an increase in density up to 1400 8C which is 2600 8C is as high as 6–7% in composite A and only 4% due to structural changes. The small density decrease in composite C. It is important to mention here that the between 1400 and 1800 8C in composites A and B is due initial fiber volume (456 2%) is kept the same in all three to the formation of closed microporosity [37,38] because cases. This brings out very clearly the effect of reactive of evolution of nitrogen-containing reaction products. surface functional groups on the the fiber–matrix interactions. Consequently, during heat treatment the matrix Table 4 Variation in bulk density of composites 3 HTT Bulk density (g/cm ) (8C) A BC 150 1.42 1.46 1.53 400 1.37 1.42 1.45 600 1.36 1.40 1.44 800 1.41 1.45 1.50 1000 1.45 1.50 1.53 1400 1.51 1.55 1.55 1800 1.50 1.53 1.57 2200 1.53 1.55 1.58 Fig. 3. Change in kerosene density of composites with stages of 2600 1.55 1.57 1.61 heat treatment