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Structural evolution of fluorinated aramid fibers with fluorination degree and dominant factor for its adhesion property
Journal of Fluorine Chemistry 188 (2016) 139–146
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u o r
Structural evolution of fluorinated aramid fibers with fluorination degree and dominant factor for its adhesion property Zheng Cheng, Peng Wu, Jie Gao, Xu Wang* , Mengmeng Ren, Baoyin Li, Longbo Luo, Xiangyang Liu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Material and Engineering, Sichuan University, Chengdu 610065, People’s Republic of China
A R T I C L E I N F O
Article history: Received 15 April 2016 Received in revised form 23 June 2016 Accepted 27 June 2016 Available online 27 June 2016 Keyword: Direct fluorination Aramid fibers Surface structures Adhesion property
A B S T R A C T
Fluorination of aramid fibers using F2 is a useful method to improve the adhesion property which is of great important for the mechanical performance of fibers-reinforced composites. In this paper, eight kinds of aramid fibers with different fluorination degree were prepared. With the increasing of fluorination degree, surface morphology and chemical structure of fibers both varied dramatically, such F bond and COOH in the surface and the increased content, among which the as appearance of C C F bond and COOH groups all contributed to the variation of surface roughness and formation of enhancement of pull-out strength of fluorinated aramid fibers from 0.43 to about 0.60 N/tex mm. Based on the contrast analysis, the appearance of chemical group COOH and increasing of its content are the dominant factor for the gradually improving the adhesion properties. Meanwhile, it was found that water soaking treatment after direct fluorination was a feasible method to further increase the COOH content and thus improve the pull-out strength to a higher level of 0.62 N/tex mm. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Aramid fibers such as Kevlar, Twaron and Armos are the high performance organic fibrous materials, which are widely used in commercial and industrial applications [1,2]. However, they offer a relatively inert surface, which limits potential chemical and mechanical interactions with polymeric resin systems and leads to the poor adhesion strength between fibers and epoxy resin [3–5].The various conventional surface modification treatments are difficult to achieve high-effective activation and high-density functional group aggregation without damaging the surface structure [6–10]. Direct fluorination has been currently recognized as an effective method to modify and control physicochemical property of chemically inert polymers due to high reactive capability of fluorine gas [11–17]. The surface nature of activated layer can be controlled by direct fluorination introducing polar groups such as C F bonds and some oxygen-containing groups regardless of whether polymers are hydrophobic or hydrophilic.
* Corresponding author. E-mail addresses: wx2010_fi[email protected] (X. Wang), [email protected] (X. Liu) . http://dx.doi.org/10.1016/j.jfluchem.2016.06.018 0022-1139/ã 2016 Elsevier B.V. All rights reserved.
Peng et al. has reported that the direct fluorination of aramid fibers is a useful surface modification to improve the interlaminer shear strength (ILSS) of fibers/epoxy composites [13]. Further findings from Gao et al.’s study show that direct fluorination leads to the increasing of the surface polarity and roughness, and these chemical and physical changes play an important role in the improvement of interfacial adhesion property [18]. However, the above researches have not given a deep insight into the effect degree caused by each change of surface physical and chemical structure on the adhesion property of fibers. For example, direct fluorination would introduce COOH, C F groups, and induce a change of surface morphology, but each type may have a different contribution to the adhesion property. In order to optimize the adhesion property, it is necessary to identify the dominant factor to guide targeted treatment. In the present work, we obtained the different chemical and physical structures of surface of aramid fibers (chemical structure in Fig. 1) by controlling the fluorination degree. The changing trends of surface structures of fluorinated aramid fibers are investigated. XPS and FTIR showed the transformation of categories and quantities of chemical groups in the process of direct fluorination. SEM and AFM indicated the varying of surface morphology and roughness with fluorination degree. It was found
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Fig. 1. the chemical structure of aramid fiber.
that COOH content of surface was the dominant factor to improve the pull-out strength of fluorinated aramid in comparison with surface roughness and fluorine content in the surface. Therefore, introducing more COOH groups into surface could obtain a better adhesion property. 2. Experimental 2.1. Materials The aramid fibers with a linear density of 100 tex were obtained from BLUESTAR Co., Ltd. The F2/N2 (10 vol% for F2) mixed gas was supplied by Chengdu Kemeite Fluorine Industry Plastic Co., Ltd. Epoxy resin E-51 was obtained from BLUESTAR Co., Ltd. (Chengdu, China). 2.2. Surface modification Direct fluorination of aramid fibers was carried out in sealed stainless steel chamber equipped with a vacuum line, and each sample has been treated together with aramid film. The air in the closed chamber was removed and replaced by nitrogen gas for three cycles to remove residual oxygen and moisture in the chamber. Different partial pressure of F2/N2 (vol%)mixture gas (0.2 kPa, 0.4 kPa, 0.8 kPa, 1.6 kPa, 6.4 kPa, 12.8 kPa, 25.6 kPa and 30 kPa) was introduced into chamber. After the completion of reaction, the gas in the chamber was pumped out, then N2 gas (99.99% purity) was carefully introduced into the chamber until atmospheric pressure was reached, at which point the sample could be extracted. These fluorinated samples were orderly labeled as F-1, F-2, F-3, F-4, F-5, F-6, F-7, and F-8 in a sequence from low to high partial pressure of F2/N2 mixture gas for use. A part of F-4 sample was soaked in water immediately after direct fluorination for 24 h, then we got the fluorinated-water-soaking sample dried and labeled it as WF.
2.3. Pull-out test preparation and characterization The adhesion property of aramid fibers reinforced epoxy composites was characterized by pull-out test of specimen that was prepared by a bundle of fibers axially infiltrating in cylindershaped resin with thickness between 2 to 4 mm. The epoxy resin curing system was made of epoxy resin E-51 (BLUESTAR Co., Ltd), diamine (polyether amine with molecular weight of 230) as curing agent, which was mixed at the ratio of 3:1 by weight, respectively. The curing process was operated at ambient temperature for 48 h. The pull-out strength was tested on a universal testing machine (Instron 4505) using a tensile test method. The specimens were tested at the speed of 25 mm min1. The pull-out strength for the fibers-matrix was calculated according to the following equation (1):
s = F/pd
(1)
where s is the pull-out strength in N/tex mm, F is the debonding force for a given specimen in N, p is the linear density of a bundle fibers in tex, d is the embedded length mm. Every pull-out strength value was the average of more than ten successful measurements. 2.4. Characterization The XPS data were collected on a Kratos ASAM 800 spectrometer (Kratos Analytical Ltd., U.K.) using a non-monochromatic Al Ka (1486.6 eV) X-ray source (a voltage of 15 kV, a wattage of 250 W) radiation. Nicolet 560 FTIR spectrometer (Thermo Electron) using Attenuated Total Reflection (ATR) mode was used to measure the changes in various functional groups on fiber surface. The spectra were measured in a wavenumber range of 4000–600 cm1. SEM (FEI Inspect F, FEI Company, Europe) was carried out on a FEI Inspect F (FEI Company, EU/USA) at 20 kV and the magnification was set at 20,000. AFM (Picoplus, MI Ltd., U.S.A) images of observation were captured by tapping mode and operated under normal atmospheric pressure and temperature. The spring constant is 3 N/m, and the resonance frequency is 75 KHz. For the surface roughness values, each sample was tested for 5 times and the values were averaged 3. Results and discussion 3.1. Adhesion property of aramid fibers to epoxy
Fig. 2. The pull-out strength of fibers/epoxy composites with different fluorination degree.
Adhesion property was tested by pull-out strength measurement. Fig. 2 shows the effect of direct fluorination on the pull-out strength of the fibers/epoxy composites. The pull-out strength was improved significantly after fluorination in comparison with that of the untreated fibers and increased gradually from 0.43 to about 0.59 N/tex mm with fluorination degree. For samples from F-4 to F-8, the pull-out strength fluctuates between 0.57 and 0.60 N/ tex mm, while the adhesion property was improved about 39% improvement by compared to the untreated one. The enhancement of pull-out strength indicates the improvement of adhesion property may be contributed to the change of surface physical and chemical structures due to thedirect
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Fig. 3. The SEM images of untreated Aramid fibers and fluorinated Aramid fibers: (a) Untreated; (b) F-1; (c) F-2; (d) F-3; (e) F-4; (f) F-5; (g) F-6; (h) F-7; (i) F-8.
fluorination. As following, the influence of physical and chemical structures is discussed respectively. 3.2. Surface morphology The surface morphology of untreated and fluorinated Aramid fibers was observed by SEM. As shown in Fig. 3, surface morphology varied significantly with increasing of F2 partial pressure in the process of direct fluorination. Firstly, some lengthwise furrows and grooves appeared in the surface of F-2 and F-3 in comparison with the smooth surface of F-1, and then the number of defect continue to grow by companied with the formation of “ringworm” like flake in the surface of F-4 and F-5. This is attributed to the fact that amide bond was easily decomposed due the high reactivity of element fluorine and the much heat liberated from direct fluorination. Whereas, with further increasing of the F2 pressure, these appeared “defects” seem to be etched by the drastic fluorination, which leads to the decreasing of roughness and the re-appearing of smooth surface for sample F-5, F-6, F-7 and F-8. In order to quantify the surface roughness, aramid films were used as substitution, and the surface roughness information was obtained by AFM. Fig. 4 shows the AFM height images of untreated Aramid film and fluorinated Aramid film. The untreated Aramid film has a smooth surface with a roughness of 3.2 nm rms. After direct fluorination, the roughness increases firstly in case of F-1 to F-4, among which F-4 has the highest roughness of 8.4 nm rms, and then the roughness does not increase any more, but decreases with
the increasing of fluorination degree. The results are consistent with the change observed by SEM images (see Fig. 3). The relationship between pull-out strength and surface roughness are shown in Fig. 5. There is a fine linearity from untreated fibers to F-4, and the pull-out strength is increasing with the surface roughness. However, there are four deviation points which belongs to F-5, F-6, F-7 and F-8. These samples have similar roughness but relatively higher fluorination degree in comparison with untreated fibers. It is worth to note that fibers from F-5 to F-8 have a good or better adhesion property (see Fig. 2) even if their roughness deceases. This means the decline of surface roughness did not lead to the weakening of the adhesion property in this case. That is, surface roughness may influence the adhesion property of fluorinated aramid fibers but is not the dominant factor. Further, it is necessary to analysis the effect of surface chemical structure on adhesion property in this fibers reinforced epoxy composites. 3.3. Surface chemical analysis of fluorinated aramid fibers XPS was applied to investigate surface chemical composition of Aramid fibers. As shown in Table 1, the fluorine content increases from 0% to 22.96%, and F/C atomic ratio increases from 0 to 0.40 after direct fluorination. Meanwhile, the value of O/C atomic ratio increases from 0.18 to 0.25 with increasing of F2 partial pressure. This confirms that fluorine atoms were successfully introduced into the surface of Aramid fibers, companied with the increase of oxygen content, and the fluorination degree was controlled by adjusting the F2 partial pressure.
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Fig. 4. AFM height images of fluorinated Aramid films: (a) Untreated; (b) F-1; (c) F-2; (d) F-3; (e) F-4; (f) F-5; (g) F-6; (h) F-7; (i) F-8.
The thickness of fluorinated layer could be several nanometers [19] while the testing depth of ATR-FTIR is in a range of hundreds of nanometers. However, the ATR-FTIR spectra in Fig. 6 still reveal some obvious variation of chemical structure in the surface of
aramid fibers after direct fluorination. With increasing of fluorination degree, a new band appears at 1720 cm1, which is assigned to the stretching vibration of C¼O in carboxyl group (COOH), and the intensity increases gradually. This indicates the appearance of carboxyl group due to the fluorination. T. Solomun reported that acyl fluoride COF would form derived from the breakage of amide bonds CONH during fluorination, and then COF could hydrolyze to carboxyl group COOH [13]. This ATRFTIR result is consistent with the XPS data which shows the
Table 1 Surface elemental analysis of aramid fibers with different fluorinationdegree. Type
Untreated F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8 Fig. 5. Dependence of the pull-out strength on surface roughness.
Atomic percent (%)
Atomic ratio
C
O
F
F/C
O/C
80.59 78.21 76.79 74.77 70.69 61.01 59.64 58.24 57.44
14.71 14.93 15.33 15.43 15.96 14.67 14.83 14.72 14.62
0 1.52 3.03 4.24 8.72 18.97 19.97 21.22 22.96
0 0.02 0.04 0.06 0.12 0.31 0.33 0.36 0.40
0.18 0.19 0.20 0.21 0.23 0.24 0.25 0.25 0.25
Z. Cheng et al. / Journal of Fluorine Chemistry 188 (2016) 139–146
Fig. 6. ATR-FTIR curves of untreated and eight fluorinated Aramid fibers: 1000– 1900 cm1.
increase of oxygen content in the fluorinated surface. According to previous reports, the stretching vibration of C F covalent bond corresponds to the absorption peak at around 1200 cm1. In the spectra, a broad band at 1200 cm1 can be found and becomes stronger with the increasing of fluorination degree, especially in sample F-8. The whole schematic illustration for the reaction is illustrated in Fig. 7. 3.4. The effect of chemical groups on the pull-out strength By combining of the XPS and ATR-FTIR results, it is confirmed that direct fluorination introduces C F bonds and COOH groups into the surface of Aramid fibers. Their polar groups can form various kinds of physical and chemical interaction with polymeric resin, such as hydrogen bonding, and thus improve the surface activity and adhesion property of fluorinated fibers. However, it is not clear which factor play a dominant role in determining the final adhesion properties of the fluorinated fibers now. This does not facilitate the optimization of chemical structure for the adhesion property to meet the targeted requirement. Here, the relationship between pull-out strength and atomic ratio F/C, O/ C is analyzed. Fig. 8 shows the dependence of pull-out strength on
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atomic ratio F/C and O/C of fibers surface. Since the relative high roughness of F-2, F-3 and F-4 (see Fig. 4) would influence the pullout strength at some extent, these three points (red points in Fig. 8) are ignored for better linear fitting. For other points, both F/C and O/C seem to be linear with the pull-out strength. However, the F/C of F-7 and F-8, respectively increasing 9.1% and 21.2% in comparison with that of F-6, did not lead to any improvement of pull-out strength. In contrast, the variation trends of pull-out strength and O/C ratio are highly consistent. Based on the above ATR-FTIR results and reported literature, the increasing of O/C ratio is mainly attributed to the formation of COOH group in the surface due to the cleavage of CO NH-group. Therefore, the C1s XPS spectra are curve-fitted to analyze the variation trends of COOH/ CONH and CF/ C C ratio in order to compare the influences of COOH group and C F bond on the adhesion property. As shown in Fig. 9, the C1s XPS spectra of Aramid fibers sample are curve-fitted using a peak synthesis procedure which combines Gaussian and Lorentzian functions. The intensity contribution of each functional component peak was estimated by a computer simulation. The positions of carbon peaks are in a range of 280– 295 eV. Binding energies about 284.6 eV, 285.5 eV, 287.5 eV, 288.3 eV, 289.2 eV and 290.4 eV are attributed to the carbon atom of CC , C N , CONH , COOH, C F and the multifluorine-substituted carbon CFn, respectively. The new peaks in the C1s XPS spectra corresponding to C F and COOH confirm that direct fluorination does introduce polar groups COOH and C F into the surface. Table 2 shows the percentage value of different chemical groups obtained from the XPS spectra, and the ratio values of COOH/ CONH and CF/ C C are shown in Table 2 and Fig. 10. As shows in Fig. 10, the ratio of CF/ C C gradually increases from 0 to 1.05. The rate of increasing is slow at low fluorination degree and then speeds up. Whereas ratio of COOH/ CONH increases with a high rate at low fluorination degree and then slows down until reaches to 1.40. It is obvious that the trend of COOH/ CONH is relatively consistent with that of pull-out strength in comparison with C F/ C C ratio. Therefore, COOH group plays a more important role in improving the adhesion property of aramid fibers. In conclusion, direct fluorination leaded to a series of structural change, among which the variation of surface roughness and formation of CF bond and COOH groups all contributed to the enhancement of pull-out strength of fluorinated aramid fibers. Based on the contrasting analysis, it is found that the appearance of
Fig. 7. The schematic illustration for the direct fluorination of aramid fiber.
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Fig. 8. Dependence of the pull-out strength on atomic ratio: (a) O/C and (b) F/C.
Fig. 9. C1s XPS spectra of aramid fibers: (a) Untreated; (b) F-2; (c) F-3; (d) F-4; (e) F-8; (f) All fluorinated samples.
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Table 2 The detail of each C1s peak. Type
Percent of each group (%, 0.02) C C
CN
CONH
COOH
CF1,n
COOH/CONH
CF1,n/CC
Untreated F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8
60.73 54.48 53.8 56.64 52.92 47.87 38.23 36.98 31.76
33.74 35.16 34.83 27.95 23.14 19.02 23.44 22.35 21.56
5.53 6.6 5.47 7.5 8.86 7.99 7.81 7.52 5.54
0 3.76 4.95 7.07 8.79 11.15 10.94 10.53 7.76
0 0 0.95 0.84 6.28 13.97 19.54 22.62 33.38
0 0.57 0.90 0.94 0.99 1.39 1.40 1.40 1.40
0 0 0.02 0.02 0.12 0.29 0.51 0.61 1.05
Group ratio
Fig. 10. The ratio of COOH/CONH, CF1,n/CC and pull-out strength depends on F/C.
chemical group COOH and increasing of its content are the dominant factor for the improving the adhesion properties. 3.5. Water-soaking fluorinated aramid fibers It is well known that the free radicals would survive for a certain time after fluorination [20,21]. Therefore, a series of chemical reactions would occur when H2O introduces in the end of aramid fibers fluorination, it may further increase the COOH content of surface.
As shown in Fig. 11, C1s XPS spectrum of WF has the same characteristic peaks (284.6 eV, 285.5 eV, 287.5 eV, 288.3 eV, 289.2 eV and 290.4 eV are contributed to CC , C N , CONH, COOH, C F and CFn.). Surface elemental changes during water-soaking are demonstrated in Table 3. Statistical analysis reveals that the water-soaking treatment does introduce more COOH groups. Compared with F-4, the O/C increases to 0.28 and COOH/ CONH is 2.51, which are much more than the only-fluorination samples. The pull-out strength of WF-fibers/epoxy composites also increases from 0.58 to 0.62 N/ texmm, which is more excellent than any other fluorinated samples due to the highest ratio value of O/C. The results further confirm the dominant role of COOH group in determining the final adhesion property of fluorinated fibers, and provide a feasible method to dramatically improve the pull-out strength for preparation of advanced fibers reinforced composites. 4. Conclusion
Fig. 11. XPS spectra of water-soaking fluorinated Aramid fibers (WF).
Both physical and chemical structure of aramid fibers surface were changed by direct fluorination, such as the variation of surface roughness and increases of F and O content. The increased oxygen content on surface after direct fluorination was mainly contributed by the formation of COOH group. Based on the contrasting analysis, it is found that the appearance of chemical group COOH and increasing of its content are the dominant factor for the improving the adhesion properties. More COOH groups were introduced on fiber surface by water-soaking treatment after fluorination, and leaded to the further improvement of adhesion property.
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Table 3 Surface elemental analysis of water-soaking fluorinated Aramid fibers. Type
F-4 WF
Atomic percent (%)
Atomic ratio
Group ratio
C
O
F
F/C
O/C
COOH/CONH
70.69 64.68
15.96 18.28
8.72 10.11
0.12 0.16
0.23 0.28
0.99 2.51
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51573105) and the Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) References [1] N.N. Machalaba, K.E. Perepelkin, Heterocyclic aramide fibers–production principles, properties and application, J. Ind. Text. 31 (2002) 189–204. [2] G. Srinivasan, D.H. Reneker, Structure and morphology of small diameter electrospun aramid fibers, Polym. Int. 36 (1995) 195–201. [3] J. Kalantar, L. Drzal, The bonding mechanism of aramid fibres to epoxy matrices, J. Mater. Sci. 25 (1990) 4186–4193. [4] A. Mittelman, H. Harel, I. Roman, G. Marom, The morphology of shear fracture surfaces of Kevlar fibre-reinforced epoxy composites, J. Mater. Sci. Lett. 4 (1985) 1361–1363. [5] A.A. Leal, J.M. Deitzel, S.H. McKnight, J.W. Gillespie, Interfacial behavior of high performance organic fibers, Polymer 50 (2009) 1228–1235. [6] C. Wang, M. Du, J. Lv, Q. Zhou, Y. Ren, G. Liu, D. Gao, L. Jin, Surface modification of aramid fiber by plasma induced vapor phase graft polymerization of acrylic acid. I. Influence of plasma conditions, Appl. Surf. Sci. 349 (2015) 333–342. [7] J. Brown, Z. Mathys, Plasma surface modification of advanced organic fibres: part V Effects on the mechanical properties of aramid/phenolic composites, J. Mater. Sci. 32 (1997) 2599–2604. [8] Y. Zhang, Y. Huang, L. Liu, L. Wu, Surface modification of aramid fibers with g-ray radiation for improving interfacial bonding strength with epoxy resin, J. Appl. Polym. Sci. 106 (2007) 2251–2262. [9] T. Lin, S. Wu, J. Lai, S. Shyu, The effect of chemical treatment on reinforcement/ matrix interaction in Kevlar-fiber/bismaleimide composites, Compos. Sci. Technol. 60 (2000) 1873–1878. [10] L. Liu, Y. Huang, Z. Zhang, Z. Jiang, L. Wu, Ultrasonic treatment of aramid fiber surface and its effect on the interface of aramid/epoxy composites, Appl. Surf. Sci. 254 (2008) 2594–2599.
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[11] A.P. Kharitonov, Direct fluorination of polymers—from fundamental research to industrial applications, Prog. Org. Coat. 61 (2008) 192–204. [12] J. Gao, X. Xu, C. Fan, X. Wang, Y. Dai, X. Liu, Surface modification of fluoroelastomer by direct fluorination with fluorine gas, Mater. Lett. 121 (2014) 219–222. [13] P. Tao, C. Renqin, C. Chaofeng, W. Fengde, L. Xiangyang, W. Bin, X. Jianjun, Surface modification of para-Aramid fiber by direct fluorination and its effect on the interface of Aramid/Epoxy composites, J. Macromol. Sci. B–Phys. 51 (2012) 538–550. [14] T. Solomun, A. Schimanski, H. Sturm, E. Illenberger, Efficient formation of difluoramino functionalities by direct fluorination of polyamides, Macromolecules 38 (2005) 4231–4236. [15] B. Li, M. Li, C. Fan, M. Ren, P. Wu, L. Luo, X. Wang, X. Liu, The wear-resistance of composite depending on the interfacial interaction between thermoplastic polyurethane and fluorinated UHMWPE particles with or without oxygen, Compos. Sci. Technol. 106 (2015) 68–75. [16] E. Jeong, B.H. Lee, S.J. Doh, I.J. Park, Y.S. Lee, Multifunctional surface modification of an aramid fabric via direct fluorination, J. Fluor. Chem. 141 (2012) 69–75. [17] J. Maity, C. Jacob, C. Das, S. Alam, R. Singh, Direct fluorination of Twaron fiber and the mechanical, thermal and crystallization behaviour of short Twaron fiber reinforced polypropylene composites, Compos. Part A: Appl. Sci. Manuf. 39 (2008) 825–833. [18] G. Jie, D. Yunyang, W. Xu, H. Jieyang, Y. Jin, Y. Jin, L. Xiangyang, Effects of different fluorination routes on aramid fiber surface structures and interlaminar shear strength of its composites, Appl. Surf. Sci. 270 (2013) 627– 633. [19] A.P. Kharitonov, R. Taege, G. Ferrier, V. Teplyakov, D.A. Syrtsova, G.H. Koops, Direct fluorination—useful tool to enhance commercial properties of polymer articles, J. Fluor. Chem. 126 (2005) 251–263. [20] R. Florin, L. Wall, Radicals detected by electron spin resonance during fluorination of polymers, J. Chem. Phys. 57 (1972) 1791–1792. [21] W.T. Miller Jr., S.D. Koch Jr., F.W. McLafferty, The mechanism of fluorination. II. Free radical initiation reactions. Fluorine-sensitized chlorination and oxidation, J. Am. Chem. Soc. 78 (1956) 4992–4995.
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / fl u o r
Structural evolution of fluorinated aramid fibers with fluorination degree and dominant factor for its adhesion property Zheng Cheng, Peng Wu, Jie Gao, Xu Wang* , Mengmeng Ren, Baoyin Li, Longbo Luo, Xiangyang Liu* College of Polymer Science and Engineering, State Key Laboratory of Polymer Material and Engineering, Sichuan University, Chengdu 610065, People’s Republic of China
A R T I C L E I N F O
Article history: Received 15 April 2016 Received in revised form 23 June 2016 Accepted 27 June 2016 Available online 27 June 2016 Keyword: Direct fluorination Aramid fibers Surface structures Adhesion property
A B S T R A C T
Fluorination of aramid fibers using F2 is a useful method to improve the adhesion property which is of great important for the mechanical performance of fibers-reinforced composites. In this paper, eight kinds of aramid fibers with different fluorination degree were prepared. With the increasing of fluorination degree, surface morphology and chemical structure of fibers both varied dramatically, such F bond and COOH in the surface and the increased content, among which the as appearance of C C F bond and COOH groups all contributed to the variation of surface roughness and formation of enhancement of pull-out strength of fluorinated aramid fibers from 0.43 to about 0.60 N/tex mm. Based on the contrast analysis, the appearance of chemical group COOH and increasing of its content are the dominant factor for the gradually improving the adhesion properties. Meanwhile, it was found that water soaking treatment after direct fluorination was a feasible method to further increase the COOH content and thus improve the pull-out strength to a higher level of 0.62 N/tex mm. ã 2016 Elsevier B.V. All rights reserved.
1. Introduction Aramid fibers such as Kevlar, Twaron and Armos are the high performance organic fibrous materials, which are widely used in commercial and industrial applications [1,2]. However, they offer a relatively inert surface, which limits potential chemical and mechanical interactions with polymeric resin systems and leads to the poor adhesion strength between fibers and epoxy resin [3–5].The various conventional surface modification treatments are difficult to achieve high-effective activation and high-density functional group aggregation without damaging the surface structure [6–10]. Direct fluorination has been currently recognized as an effective method to modify and control physicochemical property of chemically inert polymers due to high reactive capability of fluorine gas [11–17]. The surface nature of activated layer can be controlled by direct fluorination introducing polar groups such as C F bonds and some oxygen-containing groups regardless of whether polymers are hydrophobic or hydrophilic.
* Corresponding author. E-mail addresses: wx2010_fi[email protected] (X. Wang), [email protected] (X. Liu) . http://dx.doi.org/10.1016/j.jfluchem.2016.06.018 0022-1139/ã 2016 Elsevier B.V. All rights reserved.
Peng et al. has reported that the direct fluorination of aramid fibers is a useful surface modification to improve the interlaminer shear strength (ILSS) of fibers/epoxy composites [13]. Further findings from Gao et al.’s study show that direct fluorination leads to the increasing of the surface polarity and roughness, and these chemical and physical changes play an important role in the improvement of interfacial adhesion property [18]. However, the above researches have not given a deep insight into the effect degree caused by each change of surface physical and chemical structure on the adhesion property of fibers. For example, direct fluorination would introduce COOH, C F groups, and induce a change of surface morphology, but each type may have a different contribution to the adhesion property. In order to optimize the adhesion property, it is necessary to identify the dominant factor to guide targeted treatment. In the present work, we obtained the different chemical and physical structures of surface of aramid fibers (chemical structure in Fig. 1) by controlling the fluorination degree. The changing trends of surface structures of fluorinated aramid fibers are investigated. XPS and FTIR showed the transformation of categories and quantities of chemical groups in the process of direct fluorination. SEM and AFM indicated the varying of surface morphology and roughness with fluorination degree. It was found
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Fig. 1. the chemical structure of aramid fiber.
that COOH content of surface was the dominant factor to improve the pull-out strength of fluorinated aramid in comparison with surface roughness and fluorine content in the surface. Therefore, introducing more COOH groups into surface could obtain a better adhesion property. 2. Experimental 2.1. Materials The aramid fibers with a linear density of 100 tex were obtained from BLUESTAR Co., Ltd. The F2/N2 (10 vol% for F2) mixed gas was supplied by Chengdu Kemeite Fluorine Industry Plastic Co., Ltd. Epoxy resin E-51 was obtained from BLUESTAR Co., Ltd. (Chengdu, China). 2.2. Surface modification Direct fluorination of aramid fibers was carried out in sealed stainless steel chamber equipped with a vacuum line, and each sample has been treated together with aramid film. The air in the closed chamber was removed and replaced by nitrogen gas for three cycles to remove residual oxygen and moisture in the chamber. Different partial pressure of F2/N2 (vol%)mixture gas (0.2 kPa, 0.4 kPa, 0.8 kPa, 1.6 kPa, 6.4 kPa, 12.8 kPa, 25.6 kPa and 30 kPa) was introduced into chamber. After the completion of reaction, the gas in the chamber was pumped out, then N2 gas (99.99% purity) was carefully introduced into the chamber until atmospheric pressure was reached, at which point the sample could be extracted. These fluorinated samples were orderly labeled as F-1, F-2, F-3, F-4, F-5, F-6, F-7, and F-8 in a sequence from low to high partial pressure of F2/N2 mixture gas for use. A part of F-4 sample was soaked in water immediately after direct fluorination for 24 h, then we got the fluorinated-water-soaking sample dried and labeled it as WF.
2.3. Pull-out test preparation and characterization The adhesion property of aramid fibers reinforced epoxy composites was characterized by pull-out test of specimen that was prepared by a bundle of fibers axially infiltrating in cylindershaped resin with thickness between 2 to 4 mm. The epoxy resin curing system was made of epoxy resin E-51 (BLUESTAR Co., Ltd), diamine (polyether amine with molecular weight of 230) as curing agent, which was mixed at the ratio of 3:1 by weight, respectively. The curing process was operated at ambient temperature for 48 h. The pull-out strength was tested on a universal testing machine (Instron 4505) using a tensile test method. The specimens were tested at the speed of 25 mm min1. The pull-out strength for the fibers-matrix was calculated according to the following equation (1):
s = F/pd
(1)
where s is the pull-out strength in N/tex mm, F is the debonding force for a given specimen in N, p is the linear density of a bundle fibers in tex, d is the embedded length mm. Every pull-out strength value was the average of more than ten successful measurements. 2.4. Characterization The XPS data were collected on a Kratos ASAM 800 spectrometer (Kratos Analytical Ltd., U.K.) using a non-monochromatic Al Ka (1486.6 eV) X-ray source (a voltage of 15 kV, a wattage of 250 W) radiation. Nicolet 560 FTIR spectrometer (Thermo Electron) using Attenuated Total Reflection (ATR) mode was used to measure the changes in various functional groups on fiber surface. The spectra were measured in a wavenumber range of 4000–600 cm1. SEM (FEI Inspect F, FEI Company, Europe) was carried out on a FEI Inspect F (FEI Company, EU/USA) at 20 kV and the magnification was set at 20,000. AFM (Picoplus, MI Ltd., U.S.A) images of observation were captured by tapping mode and operated under normal atmospheric pressure and temperature. The spring constant is 3 N/m, and the resonance frequency is 75 KHz. For the surface roughness values, each sample was tested for 5 times and the values were averaged 3. Results and discussion 3.1. Adhesion property of aramid fibers to epoxy
Fig. 2. The pull-out strength of fibers/epoxy composites with different fluorination degree.
Adhesion property was tested by pull-out strength measurement. Fig. 2 shows the effect of direct fluorination on the pull-out strength of the fibers/epoxy composites. The pull-out strength was improved significantly after fluorination in comparison with that of the untreated fibers and increased gradually from 0.43 to about 0.59 N/tex mm with fluorination degree. For samples from F-4 to F-8, the pull-out strength fluctuates between 0.57 and 0.60 N/ tex mm, while the adhesion property was improved about 39% improvement by compared to the untreated one. The enhancement of pull-out strength indicates the improvement of adhesion property may be contributed to the change of surface physical and chemical structures due to thedirect
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Fig. 3. The SEM images of untreated Aramid fibers and fluorinated Aramid fibers: (a) Untreated; (b) F-1; (c) F-2; (d) F-3; (e) F-4; (f) F-5; (g) F-6; (h) F-7; (i) F-8.
fluorination. As following, the influence of physical and chemical structures is discussed respectively. 3.2. Surface morphology The surface morphology of untreated and fluorinated Aramid fibers was observed by SEM. As shown in Fig. 3, surface morphology varied significantly with increasing of F2 partial pressure in the process of direct fluorination. Firstly, some lengthwise furrows and grooves appeared in the surface of F-2 and F-3 in comparison with the smooth surface of F-1, and then the number of defect continue to grow by companied with the formation of “ringworm” like flake in the surface of F-4 and F-5. This is attributed to the fact that amide bond was easily decomposed due the high reactivity of element fluorine and the much heat liberated from direct fluorination. Whereas, with further increasing of the F2 pressure, these appeared “defects” seem to be etched by the drastic fluorination, which leads to the decreasing of roughness and the re-appearing of smooth surface for sample F-5, F-6, F-7 and F-8. In order to quantify the surface roughness, aramid films were used as substitution, and the surface roughness information was obtained by AFM. Fig. 4 shows the AFM height images of untreated Aramid film and fluorinated Aramid film. The untreated Aramid film has a smooth surface with a roughness of 3.2 nm rms. After direct fluorination, the roughness increases firstly in case of F-1 to F-4, among which F-4 has the highest roughness of 8.4 nm rms, and then the roughness does not increase any more, but decreases with
the increasing of fluorination degree. The results are consistent with the change observed by SEM images (see Fig. 3). The relationship between pull-out strength and surface roughness are shown in Fig. 5. There is a fine linearity from untreated fibers to F-4, and the pull-out strength is increasing with the surface roughness. However, there are four deviation points which belongs to F-5, F-6, F-7 and F-8. These samples have similar roughness but relatively higher fluorination degree in comparison with untreated fibers. It is worth to note that fibers from F-5 to F-8 have a good or better adhesion property (see Fig. 2) even if their roughness deceases. This means the decline of surface roughness did not lead to the weakening of the adhesion property in this case. That is, surface roughness may influence the adhesion property of fluorinated aramid fibers but is not the dominant factor. Further, it is necessary to analysis the effect of surface chemical structure on adhesion property in this fibers reinforced epoxy composites. 3.3. Surface chemical analysis of fluorinated aramid fibers XPS was applied to investigate surface chemical composition of Aramid fibers. As shown in Table 1, the fluorine content increases from 0% to 22.96%, and F/C atomic ratio increases from 0 to 0.40 after direct fluorination. Meanwhile, the value of O/C atomic ratio increases from 0.18 to 0.25 with increasing of F2 partial pressure. This confirms that fluorine atoms were successfully introduced into the surface of Aramid fibers, companied with the increase of oxygen content, and the fluorination degree was controlled by adjusting the F2 partial pressure.
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Fig. 4. AFM height images of fluorinated Aramid films: (a) Untreated; (b) F-1; (c) F-2; (d) F-3; (e) F-4; (f) F-5; (g) F-6; (h) F-7; (i) F-8.
The thickness of fluorinated layer could be several nanometers [19] while the testing depth of ATR-FTIR is in a range of hundreds of nanometers. However, the ATR-FTIR spectra in Fig. 6 still reveal some obvious variation of chemical structure in the surface of
aramid fibers after direct fluorination. With increasing of fluorination degree, a new band appears at 1720 cm1, which is assigned to the stretching vibration of C¼O in carboxyl group (COOH), and the intensity increases gradually. This indicates the appearance of carboxyl group due to the fluorination. T. Solomun reported that acyl fluoride COF would form derived from the breakage of amide bonds CONH during fluorination, and then COF could hydrolyze to carboxyl group COOH [13]. This ATRFTIR result is consistent with the XPS data which shows the
Table 1 Surface elemental analysis of aramid fibers with different fluorinationdegree. Type
Untreated F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8 Fig. 5. Dependence of the pull-out strength on surface roughness.
Atomic percent (%)
Atomic ratio
C
O
F
F/C
O/C
80.59 78.21 76.79 74.77 70.69 61.01 59.64 58.24 57.44
14.71 14.93 15.33 15.43 15.96 14.67 14.83 14.72 14.62
0 1.52 3.03 4.24 8.72 18.97 19.97 21.22 22.96
0 0.02 0.04 0.06 0.12 0.31 0.33 0.36 0.40
0.18 0.19 0.20 0.21 0.23 0.24 0.25 0.25 0.25
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Fig. 6. ATR-FTIR curves of untreated and eight fluorinated Aramid fibers: 1000– 1900 cm1.
increase of oxygen content in the fluorinated surface. According to previous reports, the stretching vibration of C F covalent bond corresponds to the absorption peak at around 1200 cm1. In the spectra, a broad band at 1200 cm1 can be found and becomes stronger with the increasing of fluorination degree, especially in sample F-8. The whole schematic illustration for the reaction is illustrated in Fig. 7. 3.4. The effect of chemical groups on the pull-out strength By combining of the XPS and ATR-FTIR results, it is confirmed that direct fluorination introduces C F bonds and COOH groups into the surface of Aramid fibers. Their polar groups can form various kinds of physical and chemical interaction with polymeric resin, such as hydrogen bonding, and thus improve the surface activity and adhesion property of fluorinated fibers. However, it is not clear which factor play a dominant role in determining the final adhesion properties of the fluorinated fibers now. This does not facilitate the optimization of chemical structure for the adhesion property to meet the targeted requirement. Here, the relationship between pull-out strength and atomic ratio F/C, O/ C is analyzed. Fig. 8 shows the dependence of pull-out strength on
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atomic ratio F/C and O/C of fibers surface. Since the relative high roughness of F-2, F-3 and F-4 (see Fig. 4) would influence the pullout strength at some extent, these three points (red points in Fig. 8) are ignored for better linear fitting. For other points, both F/C and O/C seem to be linear with the pull-out strength. However, the F/C of F-7 and F-8, respectively increasing 9.1% and 21.2% in comparison with that of F-6, did not lead to any improvement of pull-out strength. In contrast, the variation trends of pull-out strength and O/C ratio are highly consistent. Based on the above ATR-FTIR results and reported literature, the increasing of O/C ratio is mainly attributed to the formation of COOH group in the surface due to the cleavage of CO NH-group. Therefore, the C1s XPS spectra are curve-fitted to analyze the variation trends of COOH/ CONH and CF/ C C ratio in order to compare the influences of COOH group and C F bond on the adhesion property. As shown in Fig. 9, the C1s XPS spectra of Aramid fibers sample are curve-fitted using a peak synthesis procedure which combines Gaussian and Lorentzian functions. The intensity contribution of each functional component peak was estimated by a computer simulation. The positions of carbon peaks are in a range of 280– 295 eV. Binding energies about 284.6 eV, 285.5 eV, 287.5 eV, 288.3 eV, 289.2 eV and 290.4 eV are attributed to the carbon atom of CC , C N , CONH , COOH, C F and the multifluorine-substituted carbon CFn, respectively. The new peaks in the C1s XPS spectra corresponding to C F and COOH confirm that direct fluorination does introduce polar groups COOH and C F into the surface. Table 2 shows the percentage value of different chemical groups obtained from the XPS spectra, and the ratio values of COOH/ CONH and CF/ C C are shown in Table 2 and Fig. 10. As shows in Fig. 10, the ratio of CF/ C C gradually increases from 0 to 1.05. The rate of increasing is slow at low fluorination degree and then speeds up. Whereas ratio of COOH/ CONH increases with a high rate at low fluorination degree and then slows down until reaches to 1.40. It is obvious that the trend of COOH/ CONH is relatively consistent with that of pull-out strength in comparison with C F/ C C ratio. Therefore, COOH group plays a more important role in improving the adhesion property of aramid fibers. In conclusion, direct fluorination leaded to a series of structural change, among which the variation of surface roughness and formation of CF bond and COOH groups all contributed to the enhancement of pull-out strength of fluorinated aramid fibers. Based on the contrasting analysis, it is found that the appearance of
Fig. 7. The schematic illustration for the direct fluorination of aramid fiber.
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Fig. 8. Dependence of the pull-out strength on atomic ratio: (a) O/C and (b) F/C.
Fig. 9. C1s XPS spectra of aramid fibers: (a) Untreated; (b) F-2; (c) F-3; (d) F-4; (e) F-8; (f) All fluorinated samples.
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Table 2 The detail of each C1s peak. Type
Percent of each group (%, 0.02) C C
CN
CONH
COOH
CF1,n
COOH/CONH
CF1,n/CC
Untreated F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8
60.73 54.48 53.8 56.64 52.92 47.87 38.23 36.98 31.76
33.74 35.16 34.83 27.95 23.14 19.02 23.44 22.35 21.56
5.53 6.6 5.47 7.5 8.86 7.99 7.81 7.52 5.54
0 3.76 4.95 7.07 8.79 11.15 10.94 10.53 7.76
0 0 0.95 0.84 6.28 13.97 19.54 22.62 33.38
0 0.57 0.90 0.94 0.99 1.39 1.40 1.40 1.40
0 0 0.02 0.02 0.12 0.29 0.51 0.61 1.05
Group ratio
Fig. 10. The ratio of COOH/CONH, CF1,n/CC and pull-out strength depends on F/C.
chemical group COOH and increasing of its content are the dominant factor for the improving the adhesion properties. 3.5. Water-soaking fluorinated aramid fibers It is well known that the free radicals would survive for a certain time after fluorination [20,21]. Therefore, a series of chemical reactions would occur when H2O introduces in the end of aramid fibers fluorination, it may further increase the COOH content of surface.
As shown in Fig. 11, C1s XPS spectrum of WF has the same characteristic peaks (284.6 eV, 285.5 eV, 287.5 eV, 288.3 eV, 289.2 eV and 290.4 eV are contributed to CC , C N , CONH, COOH, C F and CFn.). Surface elemental changes during water-soaking are demonstrated in Table 3. Statistical analysis reveals that the water-soaking treatment does introduce more COOH groups. Compared with F-4, the O/C increases to 0.28 and COOH/ CONH is 2.51, which are much more than the only-fluorination samples. The pull-out strength of WF-fibers/epoxy composites also increases from 0.58 to 0.62 N/ texmm, which is more excellent than any other fluorinated samples due to the highest ratio value of O/C. The results further confirm the dominant role of COOH group in determining the final adhesion property of fluorinated fibers, and provide a feasible method to dramatically improve the pull-out strength for preparation of advanced fibers reinforced composites. 4. Conclusion
Fig. 11. XPS spectra of water-soaking fluorinated Aramid fibers (WF).
Both physical and chemical structure of aramid fibers surface were changed by direct fluorination, such as the variation of surface roughness and increases of F and O content. The increased oxygen content on surface after direct fluorination was mainly contributed by the formation of COOH group. Based on the contrasting analysis, it is found that the appearance of chemical group COOH and increasing of its content are the dominant factor for the improving the adhesion properties. More COOH groups were introduced on fiber surface by water-soaking treatment after fluorination, and leaded to the further improvement of adhesion property.
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Table 3 Surface elemental analysis of water-soaking fluorinated Aramid fibers. Type
F-4 WF
Atomic percent (%)
Atomic ratio
Group ratio
C
O
F
F/C
O/C
COOH/CONH
70.69 64.68
15.96 18.28
8.72 10.11
0.12 0.16
0.23 0.28
0.99 2.51
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