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Effect of direct fluorination on the mechanical and scratch performance of nitrile butadiene rubber
Wear 376-377 (2017) 1314–1320
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Effect of direct fluorination on the mechanical and scratch performance of nitrile butadiene rubber Zhongmeng Zhu, Yan Xia, Guangguang Niu, Jie Liu, Chaoming Wang, Han Jiang n Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu 610031, China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 September 2016 Received in revised form 19 November 2016 Accepted 8 December 2016
To explore the possibility of direct fluorination to enhance the scratch performance of rubber materials, the effect of direct fluorination on the mechanical and surface properties and scratch performance of nitrile butadiene rubber (NBR) was studied. The bulk mechanical performance of NBR showed no apparent change after fluorination except the fair alteration of the type B tear strength, while the surface properties experienced substantial variations. The surface of the fluorinated samples appears to be cleaner, accompanied with the decreased surface roughness. The friction coefficient decreased substantially with fluorination treatment for only 30 minutes. Obvious changes were observed for the surface modulus and surface energy of the fluorinated NBR. The onset of the scratch groove was significantly delayed under certain fluorination conditions. However, the surface whitening did not show significant improvement. The correlations between the scratch resistance and the bulk mechanical and surface properties were analyzed systematically. The findings demonstrated the surface fluorination can be an efficient method to enhance the rubber's resistance to scratch groove. & 2016 Elsevier B.V. All rights reserved.
Keywords: Direct fluorination Scratch Rubber Mechanical and surface properties
1. Introduction For rubber products such as tires or sealing rings, the scratch on their surface is unpleasant because it not only harms the aesthetics, but also severely damages the functionalities. The stress concentration at the region of scratches may even cause premature failure of the rubber components. Therefore, excellent scratch resistance is critical for the normal use and properties sustaining of rubber products. The surface scratch behaviors of polymeric materials have been studied by various researchers [1–3]. For rubber materials, scratch tests have been performed on natural rubber (NR) [4–6], styrene butadiene rubber (SBR) [7] and hydrogenated nitrile butadiene rubber (HNBR) etc [8]. under different loading modes. The pioneering work by Schallamach studied the needle friction on NR in a single pass and demonstrated that the lateral tears on the sample were produced by the tensile stress in the sample behind the needle [4–6]. Briscoe et al. displayed severe scratch damages of the NR, SBR and butadiene rubber (BR) resulted from the decreasing of the cone angle of the scratch tip [7,9]. Low et al. found out that the scratch damage of ethylene propylene diene monomer rubber (EPDM), neoprene and SBR was aggravated with the increasing n
Corresponding author. E-mail address: [email protected] (H. Jiang).
http://dx.doi.org/10.1016/j.wear.2016.12.013 0043-1648/& 2016 Elsevier B.V. All rights reserved.
scratch velocity while reinforcing the rubber materials with carbon black could enhance their scratch performance [10]. Zhu et al. observed two distinct scratch damage modes, namely flake peeling and the helical-form damage, for HNBR, under a linearly increasing scratch depth. They found a strong correlation exists between the bulk tear strengths and the scratch resistance [8]. Recently, the surface fluorination was used to modify the surface properties of rubbers due to its high efficiency, simple treatment process and low costs. For example, the surface modulus of wiper blade can be increased by fluorination to obstacle the possibility of forming high friction on wet surfaces [11]. By treating the rubbers either with F2, namely direct fluorination, or fluoride, the fluorine element can be attached onto the rubber surface with the formation of -CF, -CF2 or -CF3 groups by addition reactions occurring at the C ¼ C bounds or substitutions of hydrogen atoms [12–14]. The work by Fan demonstrated that the direct fluorination can be used as an effective way to substantially increase the crosslinking density of the fluoroelastomer film by extending fluorination time at 100 °C atmosphere [12]. Systematical works by Kharitonov et al. demonstrated that direct fluorination with F2 remarkably improved the chemical resistance and adhesive property of polymers meanwhile reduced their electrostatic charging and the friction coefficient [13,16,17]. Nazarov et al. experimentally found the wear properties during a repeated abrasion of rubbers were enhanced by direct fluorination [18]. Schlögl et al. observed that direct fluorination decreased the wettability
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and the surface tension of NR films, however the oxy-fluorination led to an increase of hydrophilicity and increased the surface tension [14]. Gao et al. detected a great increase of the contact angle and a remarkable reduction of surface roughness and friction coefficient of fluoroelastomer after fluorination at 50 °C for 2 hours [22]. With the alteration of the surface properties, the scratch performance of rubber materials was expected to be amended. Therefore, there's a considerable probability to use direct fluorination as a surface modification method to enhance the scratch resistance of rubber materials. However, few studies have addressed this issue. In the present paper, the influence of direct fluorination on the bulk and surface properties of the nitrile butadiene rubber (NBR) was studied experimentally. Then how the direct fluorination influencing the scratch behavior of NBR was discussed. The results demonstrated that the direct fluorination can be an efficient method to enhance the scratch performance of the rubbery materials.
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measured using an atomic force microscope (MultiMode-8 h, Bruker) with Peak Force Quantitative Nanomechanical Mapping (PFQNM) mode. A spring constant of 6N/m and a resonance frequency of 105 kHz were used for measuring in air. The calibration was conducted on the PDMS-SOFT-2-12M standard surface (3.5 MPa) before the surface modulus measurements. The modulus was calculated from the force-separation curve using the DMT model. 2.3. Bulk mechanical properties measurements
2. Experiment
Uniaxial tensile tests were conducted using an electronic universal testing machine (AGS-J, SHIMADZU) under the ambient temperature (about 25 °C). A 500mm/min loading rate was used following the requirement of ASTM D412-06a (2013) [23] to measure the tensile strength and the fracture strain (also known as elongation-at-break) of the samples before and after fluorination. A universal testing machine (AGS-J, SHIMADZU) was utilized to conduct the tearing tests at about 25 °C. The tear strengths of the type T and type B specimens were measured with a crosshead speed of 50 mm/min and 500 mm/min respectively according to ASTM D624-00 (2012) [24].
2.1. Material and fluorination experiments
2.4. Scratch tests
The nitrile butadiene rubber (NBR) plates in thickness of 2 and 6mm were provided by Kaidi Northwest Rubber Co., Ltd. Rubber plates in 2 mm thickness were adopted for the tensile and tear tests following ASTM D412-06A (2013) [23] and ASTM D624-00 (2012) [24]. To prevent accident penetration, plates in 6 mm thickness were used for scratch tests. The samples containing 29% acrylonitrile were reinforced with 60 fast extruding furnace black (FEF). The mixed treatment gas containing 10 vol% fluorine (99.99% in purity) diluted with 90 vol% nitrogen was provided by Sichuan Juhe Technology Co., Ltd. The direct fluorination was performed in a closed reaction chamber, which was evacuated at first and then filled with the F2/N2 mixture gas until the pressure in the vessel reached 30kPa. The rubber samples were hung in the reaction chamber to allow the air well circulated. The fluorination treatment was conducted at room temperature (25 °C7 2 °C) and 45 °C respectively and the reaction duration was set at either 0.5 or 1.0 hour. 2.2. Surface properties measurements The surface morphology of the virgin and fluorinated samples was observed by a super depth digital microscope (VHX-1000, Keyence). The surface roughness Ra of the samples was calculated from the 3D height profile obtained with a confocal laser scanning microscopy (VK-9700, Keyence) according to ISO 25178-2 (2012) [25] within an analyzed area of 2 mm 2 mm. The friction coefficients of the virgin and fluorinated samples were measured using a home-made scratch equipment (detail information about the scratcher can be found in [9]) under the ambient temperature. A squared flat stainless steel smooth counterpart with dimensions of 10 mm 10 mm was used. A constant normal load of 10N was applied over a distance of 100 mm at a speed of 25 mm/s. The tangential friction force was recorded simultaneously. The contact angle measurements were carried out using a drop shape analysis system (DSA 100, Krüss). Deionized-water and diiodomethane were used as the test liquids. The contact angles were determined at 10 locations on the same specimen surface then the average was recorded. The surface energy was then estimated based on the data of the contact angle according to the conventional Owens method [26]. The Young's modulus of the local surface layer of the samples was
The scratch tests were carried out using a home-made scratch equipment same as mentioned in Section 2.2 under the ambient temperature. The stainless steel indenter used in this work was conical (half cone angle: 22.5°) with a spherical indenter tip (diameter: E0.3 mm). The scratch depth (D) was increased linearly from 0 to 3 mm on a 100 mm scratch length with a constant scratch speed of 25 mm/s. At least three duplicated scratch tests were performed on the samples with same fluorination degree. The overall surface morphology of the samples was scanned using a high resolution (2400 2400 dpi) optical scanner (LiDE 210, Canon). The detailed topology of the scratches was observed using a super depth digital microscope (VHX-1000, Keyence). The in-situ depth d at a given scratch distance x can be calculated using the following equation:
d = Dx/L
(1)
where D (D¼ 3 mm) is the final depth of the scratch process and L (L ¼100 mm) is the scratch length. The residual depth can be obtained from the 3D height profile obtained with a super depth digital microscope. In the present paper, the depth of the scratch damage was the in-situ depth.
3. Results and discussion 3.1. Effect of direct fluorination on surface properties The surface morphology of the NBR samples fluorinated under various conditions is shown in Fig. 1. Since the impurities on the virgin sample surface were etched by the fluorine, cleaner sample surfaces were observed after fluorination. The density of the surface layer is substantially increased due to the attachment of fluorine groups. The resulted difference between the densities of fluorinated surface layer and that of untouched substrate layer would lead to an uneven shrinkage of the rubber material which causes larger but shallower pits and furrows [14,15]. A more homogeneous sample surface was gradually formed with the shoaling of the pits and furrows. The evolution of the surface morphology was accompanied with the change of surface roughness. A decline of the surface roughness from 6.47 μm to 5.61 μm and 5.86 μm was observed for the sample fluorinated under room
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Fig. 1. Surface morphology and roughness of the NBR samples fluorinated under various conditions (the temperature and duration of fluorination is indicated at the top left corner of each image).
temperature and 45 °C respectively for 1 hour. This was consistent with more regular surface morphology. The friction coefficients of the samples under various fluorinated conditions are provided in Fig. 2. A substantial decrease can be found after the fluorination treatment for only 30 minutes under room temperature, which is in agreement with the previous results [13,16,17]. Although a saturated influence on the friction coefficient was observed for the further fluorination process, a 50% drop of friction coefficient is expected to sufficiently improve the wear property of rubbers [18–21]. The contact angles of the rubber samples tested with
Fig. 3. Variation of the contact angles of the samples with fluorination under RT and 45 °C.
Fig. 2. Variation of the friction coefficient of the samples with fluorination under RT and 45 °C.
deionized-water and diiodomethane are shown in Fig. 3. The deionized-water contact angle showed little variation after fluorination under room temperature but slightly decreased with fluorination at 45 °C atmosphere, showing an improvement of the hydrophilicity of the surface layer at a relative high temperature. Same tendency was found for diiodomethane as well, which indicated that high temperature can boost the improvement of the absorptivity of the tested liquids. According to the Wenzel model, the decline of the surface roughness also led to a decrease of the contact angle for the hydrophobic materials [27]. Another possible cause of the declining of the contact angle
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Fig. 4. Variation of the surface energy of the samples with fluorination under RT and 45 °C.
Fig. 6. Variation of the tensile properties of NBR samples with fluorination under RT and 45 °C.
might be the existence of the residual oxygen in the reaction chamber during fluorination treatment, as Schlögl found that oxy-fluorination could considerably decrease the water contact angle of the NR film [14]. The surface energy of the samples, calculated based on the contact angle data, is shown in Fig. 4. Due to the attaching of fluorine groups, the surface polarity of NBR was improved after fluorination treatment under both temperatures. Meanwhile, a significant drop of the dispersive fraction of the surface energy was obtained for the samples fluorinated at a moderate condition. For NBR fluorinated at 45 °C for 1 hour, the dispersive fraction of its surface energy increased dramatically with the extending fluorination time because of the formation of a great deal of -CF2 under such a rigorous reaction condition. The surface modulus of the samples fluorinated under various conditions obtained from AFM measurements are provided in Fig. 5. Because of the increased crosslinking density of the surface layer resulted from the introduction of fluorine element [12], the modulus of the surface layer increased dramatically after fluorination. The results showed that an exposure of 30 min under room temperature was sufficient to provide a substantial enhancement (430%) of the surface modulus. The further increase of the fluorination duration from 30 to 60 minutes showed a saturated influence on the surface modulus. The elevated temperature speeded up the improvement of the surface modulus. After fluorinated for 1 hour, 450% and 540%
increase of the surface modulus of the samples were obtained under room temperature and 45 °C respectively.
While the surface properties of NBR samples changed substantially, tiny alterations of their bulk tensile properties were observed as shown in Fig. 6. The tensile strength and the fracture strain of the samples were well sustained even under a harsh fluorination condition of 45 °C-1 h. It can be seen that although the fluorinated surface layer owned substantially enhanced modulus, it had little effect on the bulk properties of the samples due to its small thickness, usually in tens of micrometers. The type T and type B tear strengths of the NBR samples are shown in Fig. 7. The type T tear strength of NBR maintained well during fluorination under both temperatures. As for the type B tear strength, an enhancement was observed after the room temperature fluorination treatment for both 0.5 and 1 hour due to the diminishing of the surface flaw such as voids and cracks, which could cause a complete failure [28,29]. However, a slight decline occurred when the sample was over-fluorinated by exposing in the F2 of 1 hour at 45 °C. These indicated that the type B tear strength corresponding to the mode I fracture might be easier to be affected by the properties variation of the surface layer. Since only the top layer material has been effectively fluorinated, the actual improvement
Fig. 5. Variation of the surface modulus of the samples with fluorination under RT and 45 ⁰C.
Fig. 7. Variation of the tear strengths of NBR samples with fluorination under RT and 45 °C.
3.2. Effect of direct fluorination on bulk mechanical properties
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Fig. 8. Scratch morphology of NBR samples (a) optical scan image of scratched specimen fluorinated under various conditions (b) micro image of the surface whitening of the specimen fluorinated under 45 °C for 1 hour (surface cracks in the red circle) (c) 3D profile of the scratch groove of the specimen fluorinated under 45 °C for 1 hour.
on type B tear strength of the fluorinated surface may be more significant than the bulk parameter appeared in Fig. 7. 3.3. Effect of direct fluorination on scratch performance The scanned images of the scratched NBR samples under various fluorination conditions are shown in Fig. 8(a). Two distinct scratch damage modes, i.e., the whitening phenomenon (Fig. 8(b)) and scratch groove (Fig. 8(c)), can be observed sequentially on the sample with the increase of the scratch depth. For both fluorination temperatures, it was observed that the whitening phenomenon appeared early after the fluorination treatment, while the onset of the scratch groove delayed with the extending fluorination time under room temperature. However, under 45 °C, the onset of the scratch groove postponed only for the half-hour fluorination treatment but dropped substantially with the longer treatment duration. During the scratching process, the normal and tangential scratch loads were real-time recorded. The scratch coefficient of friction (SCOF) was calculated based on these forces data. The plot of SCOF against the scratch distance is shown in Fig. 9. It can be found that the SCOF generally increased with the increase of the scratch depth. Under a lower scratch depth, the material deformation induced by scratch was subtle and the tangential force was mainly induced from the surface friction. With the increasing scratch depth, the scratch tip induced severer deformation thus the inherent material resistance force contributed to a larger SCOF. When the scratch depth reaches a critical value, the resultant force was sufficient to allow the scratch tip to penetrate the sample surface and the onset of residual scratch groove will occur. The whitening or mar damage occurring at the initial stage of scratch were generally recognizable due to the alteration of the light refraction or haze of the sample surface. Since little surface cracks or voids were observed at the whitening zone as shown in Fig. 8(b), the macro-level defect was not the fundamental reason for the observable whitening phenomenon. Here, the whitening was believed to be related to the surface morphology evolution before and after the fluorination treatment. The correlations between the onset of whitening and the surface properties such as
Fig. 9. Scratch coefficient of friction (SCOF) vs. scratch distance for NBR samples fluorinated under various conditions.
the surface modulus and surface roughness were displayed in Fig. 10. Although the surface modulus between 20 and 40 N/mm2 was difficult to obtain since the direct fluorination procedure is very active and violent, a high negative correlation can still be found between the onset of whitening and the surface modulus as shown in Fig. 10(a), which is in good agreement with the reference [30,31]. As shown in Fig. 10(b), the surface roughness was also highly correlated with the onset of whitening. The increased real contact area between the scratch tip and the sample caused the increasing of friction coefficient thus contributed to the early happening of whitening [32]. As the scratch depth increased, the scratch tip eventually pierced into the substrate. Then a scratch groove appeared as shown in Fig. 8(c). The scratch groove was straight and narrow, which was similar to a cutting process. The depth of the scratch groove was about 50 μm, much shallower than the recorded insitu scratch depth (2.7 mm). This phenomenon was due to the rubber's large hyperelastic recovery.
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Fig. 10. Correlation between the onset of the whitening and the surface properties a) onset depth of whitening vs. surface modulus b) onset depth of whitening vs. surface roughness.
analysis with a viscoelastic constitutive model [35,36] can be combined in this case to quantitatively investigate the viscoelasticity effect. Moreover, other roughness parameters, such as the roughness power spectrum [37,38], might be a good alternative index for comprehensively understanding the effect of fluorination on the surface morphology, which also is the subject of the further research.
4. Summary To explore the possibility of direct fluorination to enhance the scratch performance of rubber materials, this work experimentally studied the effect of surface fluorination on the mechanical and surface properties and scratch performance of NBR. The following conclusions were found:
Fig. 11. In-situ onset depth of scratch groove vs. type B tear strength under both temperatures.
Fig. 11 shows that both the onset depth of the scratch groove and the type B tear strength altered with the increase of the fluorination time under both temperatures. The type B tear strength increased after fluorination treatment under moderate conditions; however, an over-fluorination resulted in a substantial decrease due to the embrittlement and degradation of the surface layer. Similar tendency was shown for the onset depth of the scratch groove under both temperatures except a more significant declining trend under a longer treatment duration. The more significant declining may be attributed to the great influence from the higher surface modulus, which gave rise to a more severe stress state [30,31]. It is clear that the type B tear strength can be correlated with the onset of the scratch groove of NBR. Similar correlation was also found by Gent who validated that the cutting energy of polyethylene was linearly correlated with its tearing energy [33]. Now it's clear that fluorination treatment showed great effect on the scratch behavior of NBR. The detailed tribomechanisms of the surface behavior and their relation to the effects of fluorination can be studied in the future. Considering the remarkable viscosity of rubber materials, the effect of their viscoelastic behavior on the scratch performance is crucial and needed to be revealed. Experimental study [34] and finite element
(1) The surface direct fluorination modified the surface properties of NBR including the smoother surface morphology, decreased surface roughness and friction coefficient and increased surface modulus. The type B tear strength of bulk sample was also increased by this surface treatment. (2) The scratch resistance corresponding to the scratch groove is found to be correlated with the type B tear strength. The resistance can be dramatically strengthened by the surface fluorination treatment under certain conditions such as being fluorinated under 45 °C for 0.5 hour. (3) The surface modulus and roughness are the dominant influencing factors for the onset of surface whitening during scratch. Direct fluorination may accelerate the occurrence of the surface whitening, which would degenerate the aesthetics of NBR samples. Thus the fluorination is not always suitable, at least under the studied range, to enhance the scratch resistance to surface whitening of NBR. It is believed that the findings in this paper would provide good guidance to enhance the rubber materials’ resistance to the scratch groove by surface direct fluorination.
Acknowledgments The authors thank the financial support from the National Natural Science Foundation of China (11472231) and State Key Laboratory for Strength and Vibration of Mechanical Structures
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(SV2016-KF-02). The kind support from Kaidi Northwest Rubber Co.; Ltd. is greatly appreciated. Special thanks are also given to the Center for high-performance materials and devices (CMD) at Southwest Jiaotong University for their generous supply of equipment for this research.
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Effect of direct fluorination on the mechanical and scratch performance of nitrile butadiene rubber Zhongmeng Zhu, Yan Xia, Guangguang Niu, Jie Liu, Chaoming Wang, Han Jiang n Applied Mechanics and Structure Safety Key Laboratory of Sichuan Province, School of Mechanics and Engineering, Southwest Jiaotong University, Chengdu 610031, China
art ic l e i nf o
a b s t r a c t
Article history: Received 2 September 2016 Received in revised form 19 November 2016 Accepted 8 December 2016
To explore the possibility of direct fluorination to enhance the scratch performance of rubber materials, the effect of direct fluorination on the mechanical and surface properties and scratch performance of nitrile butadiene rubber (NBR) was studied. The bulk mechanical performance of NBR showed no apparent change after fluorination except the fair alteration of the type B tear strength, while the surface properties experienced substantial variations. The surface of the fluorinated samples appears to be cleaner, accompanied with the decreased surface roughness. The friction coefficient decreased substantially with fluorination treatment for only 30 minutes. Obvious changes were observed for the surface modulus and surface energy of the fluorinated NBR. The onset of the scratch groove was significantly delayed under certain fluorination conditions. However, the surface whitening did not show significant improvement. The correlations between the scratch resistance and the bulk mechanical and surface properties were analyzed systematically. The findings demonstrated the surface fluorination can be an efficient method to enhance the rubber's resistance to scratch groove. & 2016 Elsevier B.V. All rights reserved.
Keywords: Direct fluorination Scratch Rubber Mechanical and surface properties
1. Introduction For rubber products such as tires or sealing rings, the scratch on their surface is unpleasant because it not only harms the aesthetics, but also severely damages the functionalities. The stress concentration at the region of scratches may even cause premature failure of the rubber components. Therefore, excellent scratch resistance is critical for the normal use and properties sustaining of rubber products. The surface scratch behaviors of polymeric materials have been studied by various researchers [1–3]. For rubber materials, scratch tests have been performed on natural rubber (NR) [4–6], styrene butadiene rubber (SBR) [7] and hydrogenated nitrile butadiene rubber (HNBR) etc [8]. under different loading modes. The pioneering work by Schallamach studied the needle friction on NR in a single pass and demonstrated that the lateral tears on the sample were produced by the tensile stress in the sample behind the needle [4–6]. Briscoe et al. displayed severe scratch damages of the NR, SBR and butadiene rubber (BR) resulted from the decreasing of the cone angle of the scratch tip [7,9]. Low et al. found out that the scratch damage of ethylene propylene diene monomer rubber (EPDM), neoprene and SBR was aggravated with the increasing n
Corresponding author. E-mail address: [email protected] (H. Jiang).
http://dx.doi.org/10.1016/j.wear.2016.12.013 0043-1648/& 2016 Elsevier B.V. All rights reserved.
scratch velocity while reinforcing the rubber materials with carbon black could enhance their scratch performance [10]. Zhu et al. observed two distinct scratch damage modes, namely flake peeling and the helical-form damage, for HNBR, under a linearly increasing scratch depth. They found a strong correlation exists between the bulk tear strengths and the scratch resistance [8]. Recently, the surface fluorination was used to modify the surface properties of rubbers due to its high efficiency, simple treatment process and low costs. For example, the surface modulus of wiper blade can be increased by fluorination to obstacle the possibility of forming high friction on wet surfaces [11]. By treating the rubbers either with F2, namely direct fluorination, or fluoride, the fluorine element can be attached onto the rubber surface with the formation of -CF, -CF2 or -CF3 groups by addition reactions occurring at the C ¼ C bounds or substitutions of hydrogen atoms [12–14]. The work by Fan demonstrated that the direct fluorination can be used as an effective way to substantially increase the crosslinking density of the fluoroelastomer film by extending fluorination time at 100 °C atmosphere [12]. Systematical works by Kharitonov et al. demonstrated that direct fluorination with F2 remarkably improved the chemical resistance and adhesive property of polymers meanwhile reduced their electrostatic charging and the friction coefficient [13,16,17]. Nazarov et al. experimentally found the wear properties during a repeated abrasion of rubbers were enhanced by direct fluorination [18]. Schlögl et al. observed that direct fluorination decreased the wettability
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and the surface tension of NR films, however the oxy-fluorination led to an increase of hydrophilicity and increased the surface tension [14]. Gao et al. detected a great increase of the contact angle and a remarkable reduction of surface roughness and friction coefficient of fluoroelastomer after fluorination at 50 °C for 2 hours [22]. With the alteration of the surface properties, the scratch performance of rubber materials was expected to be amended. Therefore, there's a considerable probability to use direct fluorination as a surface modification method to enhance the scratch resistance of rubber materials. However, few studies have addressed this issue. In the present paper, the influence of direct fluorination on the bulk and surface properties of the nitrile butadiene rubber (NBR) was studied experimentally. Then how the direct fluorination influencing the scratch behavior of NBR was discussed. The results demonstrated that the direct fluorination can be an efficient method to enhance the scratch performance of the rubbery materials.
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measured using an atomic force microscope (MultiMode-8 h, Bruker) with Peak Force Quantitative Nanomechanical Mapping (PFQNM) mode. A spring constant of 6N/m and a resonance frequency of 105 kHz were used for measuring in air. The calibration was conducted on the PDMS-SOFT-2-12M standard surface (3.5 MPa) before the surface modulus measurements. The modulus was calculated from the force-separation curve using the DMT model. 2.3. Bulk mechanical properties measurements
2. Experiment
Uniaxial tensile tests were conducted using an electronic universal testing machine (AGS-J, SHIMADZU) under the ambient temperature (about 25 °C). A 500mm/min loading rate was used following the requirement of ASTM D412-06a (2013) [23] to measure the tensile strength and the fracture strain (also known as elongation-at-break) of the samples before and after fluorination. A universal testing machine (AGS-J, SHIMADZU) was utilized to conduct the tearing tests at about 25 °C. The tear strengths of the type T and type B specimens were measured with a crosshead speed of 50 mm/min and 500 mm/min respectively according to ASTM D624-00 (2012) [24].
2.1. Material and fluorination experiments
2.4. Scratch tests
The nitrile butadiene rubber (NBR) plates in thickness of 2 and 6mm were provided by Kaidi Northwest Rubber Co., Ltd. Rubber plates in 2 mm thickness were adopted for the tensile and tear tests following ASTM D412-06A (2013) [23] and ASTM D624-00 (2012) [24]. To prevent accident penetration, plates in 6 mm thickness were used for scratch tests. The samples containing 29% acrylonitrile were reinforced with 60 fast extruding furnace black (FEF). The mixed treatment gas containing 10 vol% fluorine (99.99% in purity) diluted with 90 vol% nitrogen was provided by Sichuan Juhe Technology Co., Ltd. The direct fluorination was performed in a closed reaction chamber, which was evacuated at first and then filled with the F2/N2 mixture gas until the pressure in the vessel reached 30kPa. The rubber samples were hung in the reaction chamber to allow the air well circulated. The fluorination treatment was conducted at room temperature (25 °C7 2 °C) and 45 °C respectively and the reaction duration was set at either 0.5 or 1.0 hour. 2.2. Surface properties measurements The surface morphology of the virgin and fluorinated samples was observed by a super depth digital microscope (VHX-1000, Keyence). The surface roughness Ra of the samples was calculated from the 3D height profile obtained with a confocal laser scanning microscopy (VK-9700, Keyence) according to ISO 25178-2 (2012) [25] within an analyzed area of 2 mm 2 mm. The friction coefficients of the virgin and fluorinated samples were measured using a home-made scratch equipment (detail information about the scratcher can be found in [9]) under the ambient temperature. A squared flat stainless steel smooth counterpart with dimensions of 10 mm 10 mm was used. A constant normal load of 10N was applied over a distance of 100 mm at a speed of 25 mm/s. The tangential friction force was recorded simultaneously. The contact angle measurements were carried out using a drop shape analysis system (DSA 100, Krüss). Deionized-water and diiodomethane were used as the test liquids. The contact angles were determined at 10 locations on the same specimen surface then the average was recorded. The surface energy was then estimated based on the data of the contact angle according to the conventional Owens method [26]. The Young's modulus of the local surface layer of the samples was
The scratch tests were carried out using a home-made scratch equipment same as mentioned in Section 2.2 under the ambient temperature. The stainless steel indenter used in this work was conical (half cone angle: 22.5°) with a spherical indenter tip (diameter: E0.3 mm). The scratch depth (D) was increased linearly from 0 to 3 mm on a 100 mm scratch length with a constant scratch speed of 25 mm/s. At least three duplicated scratch tests were performed on the samples with same fluorination degree. The overall surface morphology of the samples was scanned using a high resolution (2400 2400 dpi) optical scanner (LiDE 210, Canon). The detailed topology of the scratches was observed using a super depth digital microscope (VHX-1000, Keyence). The in-situ depth d at a given scratch distance x can be calculated using the following equation:
d = Dx/L
(1)
where D (D¼ 3 mm) is the final depth of the scratch process and L (L ¼100 mm) is the scratch length. The residual depth can be obtained from the 3D height profile obtained with a super depth digital microscope. In the present paper, the depth of the scratch damage was the in-situ depth.
3. Results and discussion 3.1. Effect of direct fluorination on surface properties The surface morphology of the NBR samples fluorinated under various conditions is shown in Fig. 1. Since the impurities on the virgin sample surface were etched by the fluorine, cleaner sample surfaces were observed after fluorination. The density of the surface layer is substantially increased due to the attachment of fluorine groups. The resulted difference between the densities of fluorinated surface layer and that of untouched substrate layer would lead to an uneven shrinkage of the rubber material which causes larger but shallower pits and furrows [14,15]. A more homogeneous sample surface was gradually formed with the shoaling of the pits and furrows. The evolution of the surface morphology was accompanied with the change of surface roughness. A decline of the surface roughness from 6.47 μm to 5.61 μm and 5.86 μm was observed for the sample fluorinated under room
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Fig. 1. Surface morphology and roughness of the NBR samples fluorinated under various conditions (the temperature and duration of fluorination is indicated at the top left corner of each image).
temperature and 45 °C respectively for 1 hour. This was consistent with more regular surface morphology. The friction coefficients of the samples under various fluorinated conditions are provided in Fig. 2. A substantial decrease can be found after the fluorination treatment for only 30 minutes under room temperature, which is in agreement with the previous results [13,16,17]. Although a saturated influence on the friction coefficient was observed for the further fluorination process, a 50% drop of friction coefficient is expected to sufficiently improve the wear property of rubbers [18–21]. The contact angles of the rubber samples tested with
Fig. 3. Variation of the contact angles of the samples with fluorination under RT and 45 °C.
Fig. 2. Variation of the friction coefficient of the samples with fluorination under RT and 45 °C.
deionized-water and diiodomethane are shown in Fig. 3. The deionized-water contact angle showed little variation after fluorination under room temperature but slightly decreased with fluorination at 45 °C atmosphere, showing an improvement of the hydrophilicity of the surface layer at a relative high temperature. Same tendency was found for diiodomethane as well, which indicated that high temperature can boost the improvement of the absorptivity of the tested liquids. According to the Wenzel model, the decline of the surface roughness also led to a decrease of the contact angle for the hydrophobic materials [27]. Another possible cause of the declining of the contact angle
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Fig. 4. Variation of the surface energy of the samples with fluorination under RT and 45 °C.
Fig. 6. Variation of the tensile properties of NBR samples with fluorination under RT and 45 °C.
might be the existence of the residual oxygen in the reaction chamber during fluorination treatment, as Schlögl found that oxy-fluorination could considerably decrease the water contact angle of the NR film [14]. The surface energy of the samples, calculated based on the contact angle data, is shown in Fig. 4. Due to the attaching of fluorine groups, the surface polarity of NBR was improved after fluorination treatment under both temperatures. Meanwhile, a significant drop of the dispersive fraction of the surface energy was obtained for the samples fluorinated at a moderate condition. For NBR fluorinated at 45 °C for 1 hour, the dispersive fraction of its surface energy increased dramatically with the extending fluorination time because of the formation of a great deal of -CF2 under such a rigorous reaction condition. The surface modulus of the samples fluorinated under various conditions obtained from AFM measurements are provided in Fig. 5. Because of the increased crosslinking density of the surface layer resulted from the introduction of fluorine element [12], the modulus of the surface layer increased dramatically after fluorination. The results showed that an exposure of 30 min under room temperature was sufficient to provide a substantial enhancement (430%) of the surface modulus. The further increase of the fluorination duration from 30 to 60 minutes showed a saturated influence on the surface modulus. The elevated temperature speeded up the improvement of the surface modulus. After fluorinated for 1 hour, 450% and 540%
increase of the surface modulus of the samples were obtained under room temperature and 45 °C respectively.
While the surface properties of NBR samples changed substantially, tiny alterations of their bulk tensile properties were observed as shown in Fig. 6. The tensile strength and the fracture strain of the samples were well sustained even under a harsh fluorination condition of 45 °C-1 h. It can be seen that although the fluorinated surface layer owned substantially enhanced modulus, it had little effect on the bulk properties of the samples due to its small thickness, usually in tens of micrometers. The type T and type B tear strengths of the NBR samples are shown in Fig. 7. The type T tear strength of NBR maintained well during fluorination under both temperatures. As for the type B tear strength, an enhancement was observed after the room temperature fluorination treatment for both 0.5 and 1 hour due to the diminishing of the surface flaw such as voids and cracks, which could cause a complete failure [28,29]. However, a slight decline occurred when the sample was over-fluorinated by exposing in the F2 of 1 hour at 45 °C. These indicated that the type B tear strength corresponding to the mode I fracture might be easier to be affected by the properties variation of the surface layer. Since only the top layer material has been effectively fluorinated, the actual improvement
Fig. 5. Variation of the surface modulus of the samples with fluorination under RT and 45 ⁰C.
Fig. 7. Variation of the tear strengths of NBR samples with fluorination under RT and 45 °C.
3.2. Effect of direct fluorination on bulk mechanical properties
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Fig. 8. Scratch morphology of NBR samples (a) optical scan image of scratched specimen fluorinated under various conditions (b) micro image of the surface whitening of the specimen fluorinated under 45 °C for 1 hour (surface cracks in the red circle) (c) 3D profile of the scratch groove of the specimen fluorinated under 45 °C for 1 hour.
on type B tear strength of the fluorinated surface may be more significant than the bulk parameter appeared in Fig. 7. 3.3. Effect of direct fluorination on scratch performance The scanned images of the scratched NBR samples under various fluorination conditions are shown in Fig. 8(a). Two distinct scratch damage modes, i.e., the whitening phenomenon (Fig. 8(b)) and scratch groove (Fig. 8(c)), can be observed sequentially on the sample with the increase of the scratch depth. For both fluorination temperatures, it was observed that the whitening phenomenon appeared early after the fluorination treatment, while the onset of the scratch groove delayed with the extending fluorination time under room temperature. However, under 45 °C, the onset of the scratch groove postponed only for the half-hour fluorination treatment but dropped substantially with the longer treatment duration. During the scratching process, the normal and tangential scratch loads were real-time recorded. The scratch coefficient of friction (SCOF) was calculated based on these forces data. The plot of SCOF against the scratch distance is shown in Fig. 9. It can be found that the SCOF generally increased with the increase of the scratch depth. Under a lower scratch depth, the material deformation induced by scratch was subtle and the tangential force was mainly induced from the surface friction. With the increasing scratch depth, the scratch tip induced severer deformation thus the inherent material resistance force contributed to a larger SCOF. When the scratch depth reaches a critical value, the resultant force was sufficient to allow the scratch tip to penetrate the sample surface and the onset of residual scratch groove will occur. The whitening or mar damage occurring at the initial stage of scratch were generally recognizable due to the alteration of the light refraction or haze of the sample surface. Since little surface cracks or voids were observed at the whitening zone as shown in Fig. 8(b), the macro-level defect was not the fundamental reason for the observable whitening phenomenon. Here, the whitening was believed to be related to the surface morphology evolution before and after the fluorination treatment. The correlations between the onset of whitening and the surface properties such as
Fig. 9. Scratch coefficient of friction (SCOF) vs. scratch distance for NBR samples fluorinated under various conditions.
the surface modulus and surface roughness were displayed in Fig. 10. Although the surface modulus between 20 and 40 N/mm2 was difficult to obtain since the direct fluorination procedure is very active and violent, a high negative correlation can still be found between the onset of whitening and the surface modulus as shown in Fig. 10(a), which is in good agreement with the reference [30,31]. As shown in Fig. 10(b), the surface roughness was also highly correlated with the onset of whitening. The increased real contact area between the scratch tip and the sample caused the increasing of friction coefficient thus contributed to the early happening of whitening [32]. As the scratch depth increased, the scratch tip eventually pierced into the substrate. Then a scratch groove appeared as shown in Fig. 8(c). The scratch groove was straight and narrow, which was similar to a cutting process. The depth of the scratch groove was about 50 μm, much shallower than the recorded insitu scratch depth (2.7 mm). This phenomenon was due to the rubber's large hyperelastic recovery.
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Fig. 10. Correlation between the onset of the whitening and the surface properties a) onset depth of whitening vs. surface modulus b) onset depth of whitening vs. surface roughness.
analysis with a viscoelastic constitutive model [35,36] can be combined in this case to quantitatively investigate the viscoelasticity effect. Moreover, other roughness parameters, such as the roughness power spectrum [37,38], might be a good alternative index for comprehensively understanding the effect of fluorination on the surface morphology, which also is the subject of the further research.
4. Summary To explore the possibility of direct fluorination to enhance the scratch performance of rubber materials, this work experimentally studied the effect of surface fluorination on the mechanical and surface properties and scratch performance of NBR. The following conclusions were found:
Fig. 11. In-situ onset depth of scratch groove vs. type B tear strength under both temperatures.
Fig. 11 shows that both the onset depth of the scratch groove and the type B tear strength altered with the increase of the fluorination time under both temperatures. The type B tear strength increased after fluorination treatment under moderate conditions; however, an over-fluorination resulted in a substantial decrease due to the embrittlement and degradation of the surface layer. Similar tendency was shown for the onset depth of the scratch groove under both temperatures except a more significant declining trend under a longer treatment duration. The more significant declining may be attributed to the great influence from the higher surface modulus, which gave rise to a more severe stress state [30,31]. It is clear that the type B tear strength can be correlated with the onset of the scratch groove of NBR. Similar correlation was also found by Gent who validated that the cutting energy of polyethylene was linearly correlated with its tearing energy [33]. Now it's clear that fluorination treatment showed great effect on the scratch behavior of NBR. The detailed tribomechanisms of the surface behavior and their relation to the effects of fluorination can be studied in the future. Considering the remarkable viscosity of rubber materials, the effect of their viscoelastic behavior on the scratch performance is crucial and needed to be revealed. Experimental study [34] and finite element
(1) The surface direct fluorination modified the surface properties of NBR including the smoother surface morphology, decreased surface roughness and friction coefficient and increased surface modulus. The type B tear strength of bulk sample was also increased by this surface treatment. (2) The scratch resistance corresponding to the scratch groove is found to be correlated with the type B tear strength. The resistance can be dramatically strengthened by the surface fluorination treatment under certain conditions such as being fluorinated under 45 °C for 0.5 hour. (3) The surface modulus and roughness are the dominant influencing factors for the onset of surface whitening during scratch. Direct fluorination may accelerate the occurrence of the surface whitening, which would degenerate the aesthetics of NBR samples. Thus the fluorination is not always suitable, at least under the studied range, to enhance the scratch resistance to surface whitening of NBR. It is believed that the findings in this paper would provide good guidance to enhance the rubber materials’ resistance to the scratch groove by surface direct fluorination.
Acknowledgments The authors thank the financial support from the National Natural Science Foundation of China (11472231) and State Key Laboratory for Strength and Vibration of Mechanical Structures
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(SV2016-KF-02). The kind support from Kaidi Northwest Rubber Co.; Ltd. is greatly appreciated. Special thanks are also given to the Center for high-performance materials and devices (CMD) at Southwest Jiaotong University for their generous supply of equipment for this research.
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