- Email: [email protected]
SiO2-nanoparticles in NBR
Wear 271 (2011) 1066–1071
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Reducing friction with Al2 O3 /SiO2 -nanoparticles in NBR L. Busse a , K. Peter b , C.W. Karl a , H. Geisler a , M. Klüppel a,∗ a b
Deutsches Institut für Kautschuktechnologie e.V., Eupener Straße 33, 30519 Hannover, Germany DWI an der RWTH Aachen e.V., Pauwelsstr. 8, 52074 Aachen, Germany
a r t i c l e
i n f o
Article history: Received 6 July 2010 Received in revised form 21 April 2011 Accepted 10 May 2011 Available online 17 May 2011 Keywords: Sliding friction PAOS Nanoparticles Tribology NBR elastomer Rubber
a b s t r a c t Friction is a phenomenon that is sometimes desired and sometimes to be avoided. In the past, material development grants have not focused on this task. In this article, we present a new and alternative method of reducing elastomer friction when sliding on different kinds of substrates by inducing hard alumina/silica nanostructures on the elastomer surface. The formation of Al2 O3 /SiO2 nanostructures is based on an in situ sol–gel process of a dispersion of spherical aluminum oxide nanoparticles in a silica liquid precursor polymer – hyperbranched polyalkoxysiloxane (PAOS) – in an elastomer mixture. The influence of variable amounts of Al2 O3 /PAOS dispersion, variable amounts of carbon black fillers and annealing time on the friction coefficient is studied for a large velocity range. Experiments were conducted with and without lubricant and discussed with regard to material and surface properties of the samples. It is found that sufficient PAOS concentrations significantly decrease friction for all systems and prevent stick-slip phenomena. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Our goal is to reduce friction for elastomers sliding over solid surfaces such as granite, steel and glass, which can be either dry or wet. Fillers such as PU precursors, organomodified nanoclays and carbon nanofibers have been used by previous authors to produce elastomer nanocomposites containing HNBR or NR [1,2]. Latest investigations have shown that inorganic fullerene like materials (IFLM) can be used as nanomaterials for reduction of friction [3]. In this article, we focus on NBR samples (acrylnitril butadiene copolymers) because of their stability against oil, heat and abrasion, which offers broad usage as gaskets, hoses and rubberized fabric with small friction coefficients. A model developed at the DIK [4–10] has shown that two effects contribute to friction: The first is a hysteresis part that is due to the energy loss when the rubber is squeezed into the substrate surface and deformed by the lateral movement according to its viscoelastic properties. Consequently, temperature is important for friction behavior [11]. Neglecting internal heating effects, hysteresis friction increases with sliding velocity. The other effect is caused by molecular forces of the direct contact of elastomer and substrate in the absence of lubricant (dry friction) [12]. The adhesion occurring in this case is directly related also to the real area of contact, which cannot be established accurately when sliding fast, and which depends on the surface tensions
∗ Corresponding author. Tel.: +49 511 8420127; fax: +49 511 8386826. E-mail address: [email protected] (M. Klüppel). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.05.017
of rubber and substrate [13,14]. To achieve minimal friction, it is useful to minimize the real area of contact [15–18], which plays a major role, especially for dry friction, thus reducing abrasion effects [19]. In every system, friction is increased when the elastomer is forced into substrate cavities. Hard structures with small cavities will hinder this process, and reduce the total friction coefficient. In nature, this is shown by surface nano structures of the Sandfish (scincus scincus) [20]. To produce such a sample, hard spherical nanoparticles were formed at the surface of the rubber matrix. These particles consist of silica that arises from the silica liquid precursor hyperbranched polyalkoxysiloxane in an in situ sol–gel process and aluminum oxide. Compared to the monomeric sol–gel precursor tetraethoxysilane (TEOS) PAOS is a well defined precursor polymer (Fig. 1) with preferable properties [21]: A high resistance to hydrolysis due to the absence of OH groups. Completely covered by organic end groups, PAOS shows hydrophobic properties combined with solubility in most organic solvents. It can easily be modified for a better compatibility to different polymers. For example, incorporation of long alkyl chains, i.e. hexadecyl alkyl chains, increases the hydrophobicity of such modified PAOS. Recently, PAOS was employed in thermoplastic processing aimed at the development of a solvent-free sol–gel technology for the in situ formation of nanoscale filler particles in polypropylene [22,23]. Applied as an additive without addition of a catalyst for conversion to silica, PAOS being throughout the sample is surface active and enriches at polymer surfaces after processing. There it is converted to silica by atmospheric moisture. PAOS is also a suitable dispersant for silica or alumina nanoparticles.
L. Busse et al. / Wear 271 (2011) 1066–1071
OEt
EtO
O
O
O
O
Si
O
O
O
EtO
OEt
Si
OEt
O Si
Si
1. mixing of an NBR batch, including various parts of PAOS and carbon black; 2. vulcanization until t90 time; 3. if annealing is indicated, the sample was exposed to a 90% humid atmosphere at 60 ◦ C for 24 h to ensure a complete process of silica building on the surface.
OEt O
EtO
Si
R
OEt
Si
Si
O
Si
R
OEt
R
OEt
When not in use, samples were kept in a desiccator to prevent further silica formation. To protect the surface, clean conditions were applied to vulcanization and further handling.
OEt Fig. 1. Structure of hyper branched polyalkoxysiloxane (PAOS), with R: –C16 H33 .
2.2. Material properties First, the influence of PAOS and of annealing on the material properties was investigated. Increasing amounts of PAOS prolonged the vulcanization time. They also increased the shear moduli G and G in the high temperature range, as shown by DMA measurements (dynamic mechanical analysis). Consequently, the maximum of the loss angle tan ı = G /G is diminished without changing the associated temperature of the maximum (see Fig. 2)—PAOS behaves like a non-active filler without coupling to the polymer. As expected, carbon black filled samples show higher shear moduli with a decreased maximum of tan ı. Annealing had only minor effects on the viscoelasticity. Mechanical properties were determined by PAOS amount, carbon black amount and annealing (Table 1). The combination of
2. Preparation, material and surface properties 2.1. Preparation To form alumina/silica nanostructures on an elastomer surface, hydrophobically modified Al2 O3 particles (diameter: 100–120 nm, 10 wt. % in relation to PAOS) dispersed in a hexadecyl-modified PAOS (Fig. 1) were added to the mixing process. We will abbreviate this dispersion here simply as “PAOS”. The nanostructures were formed when the sample surface was treated with a heated humid atmosphere, which will be called annealing in this article. The NBR sample composition is made of 100 phr (“per hundred rubber”) Perbunan NT 2870F, 5 phr 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0
G''/G'
a
no cb, 0PAOS no cb, 10 PAOS no cb, 20 PAOS 50 cb, 0 PAOS 50 cb, 10 PAOS 50 cb, 20 PAOS
tan δ (not annealed) T [°C] -80 -60 -40 -20
0
20
40
60
80
100
b
1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0
G''/G'
R
ZnO, 1 phr stearic acid, 1.5 phr sulphur, 2 phr N-cyclohexyl2-benzthiazylsulfenamide (CBS) and 0.5 phr diphenylguanidine (DPG) for unfilled samples, or additionally 50 phr carbon black N 330 for filled samples. Both filled and unfilled samples were prepared at 50 ◦ C in an industrial type mixer either without PAOS, with 10 phr or with 20 phr PAOS, which had been produced at the DWI Aachen. The following steps were carried out to prepare the samples:
R
Si
1067
no cb, 0 PAOS no cb, 10 PAOS no cb, 20 PAOS 50 cb, 0 PAOS 50 cb, 10 PAOS 50 cb, 20 PAOS
tan δ (annealed) T [°C] -80 -60 -40 -20
0
20
40
60
80 100
Fig. 2. Viscoelastic properties (shown as tan ı) of unfilled and filled samples with different amounts of PAOS before (left) and after annealing (right).
Table 1 Mechanical properties. Carbon black [phr]
PAOS [phr]
Anneal time [h]
IRHD hardness
Rebound [%]
Tensile strength
Elongation at break [%]
Stress 200% [MPa]
DIN abrasion [mm3 ]
Density [g/cm3 ]
0 0 0 0 0 0 50 50 50 50 50 50
0 10 20 0 10 20 0 10 20 0 10 20
0 0 0 24 24 24 0 0 0 24 24 24
54 55 56 54 58 64 75 76 75 74 78 80
53 53 54 55 56 56 33 36 39 33 36 38
5.0 3.8 2.2 4.3 5.7 2.7 27.9 23.3 18.6 24.7 24.7 17.8
344 347 336 357 381 385 294 272 248 262 268 231
2.3 2.2 1.4 2.2 2.5 1.7 16.8 14.9 13.4 16.7 16.1 14.4
67 100 143 65 151 263 71 87 141 77 105 174
1.01 1.02 1.03 1.01 1.02 1.03 1.17 1.17 1.16 1.17 1.17 1.17
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L. Busse et al. / Wear 271 (2011) 1066–1071
Fig. 3. Light microscopical images (DIAS) for 0 (left), 10 (middle), 20 (right) phr annealed PAOS samples, with (above) and without (below) carbon black, image size 640 m × 460 m.
annealing and addition of Al2 O3 /PAOS increased hardness (IRHD) and rebound, whereas tensile strength, elongation at break and the stress at 200% strain dropped considerably for high PAOS contents. Some of these property changes (hardness, elongation) are typical for fillers, comparable to the effect of the carbon black, which has been well studied [16]. Others are rather connected to softeners, distinguishing PAOS from carbon black. Density increased with filler content, but when the total percentage of carbon black was replaced by PAOS, density remained constant. Abrasion is considerably increased, especially for annealed samples without carbon black. This is due to a lack of cross-linking. PAOS can be detected even on abraded surfaces.
2.3. Surface properties The surface morphology was characterized with several methods. The Dispersion Index Analysis System (DIAS) shows how microscopic structures of PAOS filled samples arise. The PAOS is added to the samples prior to annealing. For samples containing more than 10 phr PAOS without carbon black and more than 20 phr PAOS with carbon black clustered patterns appear (Fig. 3). The clustered patterns expected to minimize contact, thus minimizing friction. Structures also appear in AFM (atomic force microscopy) images of differently treated samples in Fig. 4. Starting with 20 phr PAOS
Fig. 4. AFM images of differently treated samples, each differing with respect to central picture in the single way indicated.
L. Busse et al. / Wear 271 (2011) 1066–1071
Friction Force [N]
a
1069
b
70 60
FN
10mm/s 1mm/s
50
elastomer
0,1mm/s
40 0,01mm/s
30 20
substrate
10
Ffr
lubricant
0 0
10
20
30
40
50
60
Distance [mm] Fig. 5. Typical friction forces in relation to sliding distance for selected velocities of a carbon black filled sample with 20 phr PAOS on granite (left); experimental set-up (right).
a
1,6
b
1,2
0,8
μ
μ
1,2
dry granite no carbon black not annealed 00phr PAOS 10phr PAOS 20phr PAOS
0,4
0,0
1,6
1E-5
1E-4
1E-3
0,01
0,1
0,8 dry granite no carbon black 24 h annealed 00phr PAOS 10phr PAOS 20phr PAOS
0,4
0,0
1E-5
1E-4
v [m/s]
1E-3
0,01
0,1
v [m/s]
Fig. 6. Friction of samples without carbon black before (left) and after annealing (right) on dry granite.
and no carbon black in a new sample (middle of figure), the effects of changing single parameters can be seen separately on the surrounding images. The addition of carbon black sharpened the structures; the absence of PAOS prevented the formation of large structures. Annealing caused the structures to agglomerate, and abrasion flattened the structures. Roughness analysis shows that fillers are well diffused in PAOS and carbon black filled samples, as indicated by a smooth surface. Annealing makes the surface rougher, as more alumina/silica particles are clustered on the surface. Surface tension was measured using the (static) sessile drop method with water, diiodmethane and ethylene glycol on samples cleaned with isopropanol, which was found to have no effect on surface roughness. The disperse and polar components of the surface tension have been evaluated according to Owens et al. [24] in Table 2. As expected, annealing had little effect on samples without PAOS. The addition of PAOS significantly decreased the polar part and improved dispersion in carbon black-filled samples. The total
Table 2 Surface tensions of cleaned samples. Carbon black [phr]
PAOS [phr]
Annealing [24 h]
0 0 0 0 50 50 50 50
0 0 20 20 0 0 20 20
Before After Before After Before After Before After
Surface tension [mN/m] Total
Polar
Disperse
16.62 16.41 11.33 15.05 17.32 17.09 18.84 19.94
2.04 1.92 0.29 0.17 1.7 1.93 0.59 0.99
14.59 14.49 11.03 14.89 15.61 15.16 18.26 18.95
tension is increased by carbon black. Annealing increases the total tension for PAOS filled samples. 3. Friction properties 3.1. Experimental set-up Friction has been tested for all sample types on different dry and wet substrates. Rectangular samples of 5 mm × 5 mm with 2 mm thickness were put under a normal constant force FN , resulting in a pressure of 12.3 kPa and drawn at various stationary velocities horizontally over the substrate, as seen in Fig. 5. The friction force Ffr was measured and is displayed in the left part of the figure as curves depending on sliding distance. In order to put all surfaces into a comparable state, samples were moved over the substrates for a limited but defined time until further friction did not change the surface behavior significantly. Friction measurements were conducted on dry granite, wet granite, on steel and on glass. Samples were cleaned from isopropanol. Abrasion debris was removed with dry dust free tissues. 3.2. Results and discussion The friction coefficient , defined as the ratio of friction force to normal force = Ffr /FN , generally increases with velocity for all friction systems investigated here. At high velocities the dry friction in some cases becomes almost constant. This plateau behavior has already been observed for other systems [25–27] and is explained as a superposition of an increasing hysteresis friction and a decreasing adhesion friction, as defined in the introduction of the paper. Consequently, elastic samples without carbon black display higher
1070
L. Busse et al. / Wear 271 (2011) 1066–1071
a
b
1,6
1,2
μ
1,2
μ
1,6
0,8
dry granite 50 phr carbon black not annealed 00phr PAOS 10phr PAOS 20phr PAOS
0,4
0,0 1E-5
1E-4
1E-3
0,01
0,8 dry granite 50 phr carbon black 24h annealed 10phr PAOS 20phr PAOS
0,4
0,0 1E-5
0,1
1E-4
1E-3
0,01
0,1
v [m/s]
v [m/s]
Fig. 7. Friction of samples with carbon black before (left) and after annealing (right) on dry granite.
0,8
b
wet granite 50 phr carbon black 00phr PAOS, 00h 00phr PAOS, 24h 20phr PAOS, 00h 20phr PAOS, 24h
1,6
1,2
μ
μ
a
0,4
0,8
wet granite no carbon black not annealed 00phr PAOS 20phr PAOS
0,4
0,0
0,0 1E-5
1E-4
1E-3
0,01
1E-5
0,1
1E-4
1E-3
0,01
0,1
v [m/s]
v [m/s]
Fig. 8. Friction on samples with (left) and without carbon black (right) on wet granite.
friction values, especially on dry substrates, because they can penetrate the cavities more deeply and cover a higher area of real contact. As expected, the presence of PAOS lowers the friction on dry granite. This is true either with (Fig. 7) and also without (Fig. 6) carbon black fillers and fits well to the observed reduction of surface tension. The effect was generally amplified by annealing, which not only reduces friction, but also made it less dependent on velocity, even without the addition of PAOS. Annealed samples containing 20 phr PAOS showed more than 20% less friction at high velocities. This reduction in friction was not apparent at low velocities.
a
The increase of high velocity friction is caused by hysteresis effects (present both for dry and wet surfaces), so this behavior should apply even more with lubricated sliding and also for lower velocities, which is confirmed in Fig. 8. Apart from PAOS influence, friction is considerably lower on lubricated substrates; especially for carbon black filled samples (again up to 20% for 20 phr PAOS). Annealing enhances the effect of PAOS. Friction is reduced not only on granite but also on polished steel, as depicted in Fig. 9. The very high initial friction coefficient can be significantly reduced by PAOS. In contrast to granite, the reduction factor is non-linearly dependent on the velocity. Another differ-
b
2,4
2,8
2,0
2,4
1,6
2,0
μ
μ
3,2
1,2 dry steel no carbon black 00 phr PAOS 00 phr PAOS 20 phr PAOS 20 phr PAOS
0,8 0,4 0,0 1E-5
1E-4
1E-3
0,01
1,6 1,2
00 h 24 h 00 h 24 h 0,1
dry glass no carbon black 24 h annealed 00 phr PAOS 20 phr PAOS
0,8 0,4 0,0 1E-5
1E-4
v [m/s] Fig. 9. Friction on dry steel (left) and dry glass (right).
1E-3
v [m/s]
0,01
0,1
L. Busse et al. / Wear 271 (2011) 1066–1071
1071
ence, a slight increase of friction after annealing, is also found in the data for surface tension. For highest velocities, stick-slip behavior is observed in absence of PAOS, so the periodical sliding and resting [28] of the sample cannot be attributed to friction coefficients in this case. Friction on glass is even higher than on steel since the very flat surface allows a large area of real contact between the friction partners. On this substrate, stick-slip is extremely pronounced for systems without PAOS above a critical velocity (0.1 mm/s), though not for the whole friction curve, i.e. resonance must be established over the sliding distance. The drop in friction above that velocity is a characteristic effect for stick-slip systems, as contact is temporarily lost. Usage of 20 phr PAOS reduces friction to approximately 50% in the low velocity range and completely removes stick-slip in the high velocity range. This means a significant advantage as stick-slip is usually regarded as undesirable in applications.
with financial support from “Bundesministerium für Wirtschaft und Technologie” (BMWi) and “Arbeitsgemeinschaft Industrieller Forschungsvereinigungen” (AIF), in cooperation with “Vereinigung zur Förderung des Instituts für Kunststoffverarbeitung in Industrie und Handwerk an der RWTH Aachen” (IKV) and “Deutsche Kautschukgesellschaft” (DKG).
4. Summary
[10] [11] [12] [13] [14] [15] [16] [17]
NBR samples with different degrees of PAOS with Al2 O3 particles have been prepared and analyzed regarding their mechanical, surface morphological and friction behavior. PAOS concentrations of 20 phr cause a decrease of friction by up to 20%, depending on velocity, for all samples (with and without carbon black) on dry and wet granite, steel and glass. Additionally, PAOS can prevent stick-slip phenomena that would otherwise appear on very flat substrates. Though the increased DIN abrasion indicates a low resistance against catastrophic crack propagation, the advantages make PAOS induced surface structures an ideal treatment for NBR parts requiring low friction, such as gaskets, hoses and rubber covered surfaces. Acknowledgements This work was supported by the DECHEMA “Gesellschaft für Chemische Technik und Biotechnologie” as AIF-Nr. 196 ZN in the program “Industrielle Gemeinschaftsforschung” (IGF),
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
M. Bhattacharya, A.K. Bhowmick, Wear 269 (2010) 152. J. Karger-Kocsis, D. Felhös, D. Xu, Wear 268 (2010) 464. S. Jacobson, S. Hogmark, Wear 266 (2009) 370. M. Klüppel, G. Heinrich, Rubber Chem. Technol. 73 (2000) 578. J. Greenwood, J. Williamson, Proc. R. Soc. London A295 (1966) 300. A. Müller, J. Schramm, M. Klüppel, Kautschuk Gummi Kunstst 55 (2002) 432. A. Le Gal, X. Yang, M. Klüppel, Lecture Notes in Applied and Computational Mechanics, vol. 27, Springer, 2006, pp. 253–260. A. Le Gal, M. Klüppel, J. Phys. Condens. Matter 20 (2008) 015007. T. Meyer, L. Busse, A. Le Gal, M. Klüppel, Constitutive Models for Rubber V, Taylor & Francis Group, London, 2008, p. 351. M.J. Klüppel, Phys. Condens. Matter 21 (2009) 035104. B.J. Persson, Phys. Condens. Matter 18 (2006) 7789. H. Kummer, Rubber, Chem. Technol. 41 (1968) 895. P. De Gennes, Langmuir 12 (1996) 4497. A. Le Gal, M. Klüppel, Kautschuk Gummi Kunstst 59 (2006) 308. A. Le Gal, X. Yang, M. Klüppel, J. Chem. Phys. 123 (2005) 014704. A. Le Gal, L. Guy, G. Orange, Y. Bomal, M. Klüppel, Wear 264 (2007) 606. A. Le Gal, M. Klüppel, Constitutive Models for Rubber IV, Taylor & Francis Group, London, 2005, p. 509. A. Le Gal, Ph.D. Thesis, University of Hannover, 2007. B. Persson, E. Brener, Phys. Rev. E 71 (2005) 036123. W. Baumgartner, F. Saxe, A. Weth, D. Hajas, D. Sigumonrong, J. Emmerlich, M. Singheiser, W. Bohme, J.M. Schneider, J. Bionic Eng. 4 (2007) 1. X. Zhu, M. Jaumann, K. Peter, M. Möller, C. Melian, A. Adams-Buda, D.E. Demco, B. Blümich, Macromolecules 39/5 (2006) 1701. Q. Dou, X. Zhu, K. Peter, D.E. Demco, Möller M, J. Sol–Gel Sci. Technol. 48 (2008) 51. X. Zhu, K. Peter, M. Moeller, C. Melian, A. Adams-Buda, D.E. Demco, Polym. Preprints 47 (2006) 1133. D. Owens, R. Wendt, J. Appl. Polym. Sci. 13 (1969) 1741. K. Grosch, Proc. R. Soc. A 274 (1963) 2. L. Busse, M. Klüppel, Constitutive Models for Rubber VI, Taylor & Francis Group, London, 2010, p. 295. L. Busse, A. Le Gal, M. Klüppel, Lecture Notes in Applied and Computational Mechanics, Springer, 2010. T. Papatheodorou, Kautschuk Gummi Kunstst 60 (2007) 586.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Reducing friction with Al2 O3 /SiO2 -nanoparticles in NBR L. Busse a , K. Peter b , C.W. Karl a , H. Geisler a , M. Klüppel a,∗ a b
Deutsches Institut für Kautschuktechnologie e.V., Eupener Straße 33, 30519 Hannover, Germany DWI an der RWTH Aachen e.V., Pauwelsstr. 8, 52074 Aachen, Germany
a r t i c l e
i n f o
Article history: Received 6 July 2010 Received in revised form 21 April 2011 Accepted 10 May 2011 Available online 17 May 2011 Keywords: Sliding friction PAOS Nanoparticles Tribology NBR elastomer Rubber
a b s t r a c t Friction is a phenomenon that is sometimes desired and sometimes to be avoided. In the past, material development grants have not focused on this task. In this article, we present a new and alternative method of reducing elastomer friction when sliding on different kinds of substrates by inducing hard alumina/silica nanostructures on the elastomer surface. The formation of Al2 O3 /SiO2 nanostructures is based on an in situ sol–gel process of a dispersion of spherical aluminum oxide nanoparticles in a silica liquid precursor polymer – hyperbranched polyalkoxysiloxane (PAOS) – in an elastomer mixture. The influence of variable amounts of Al2 O3 /PAOS dispersion, variable amounts of carbon black fillers and annealing time on the friction coefficient is studied for a large velocity range. Experiments were conducted with and without lubricant and discussed with regard to material and surface properties of the samples. It is found that sufficient PAOS concentrations significantly decrease friction for all systems and prevent stick-slip phenomena. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Our goal is to reduce friction for elastomers sliding over solid surfaces such as granite, steel and glass, which can be either dry or wet. Fillers such as PU precursors, organomodified nanoclays and carbon nanofibers have been used by previous authors to produce elastomer nanocomposites containing HNBR or NR [1,2]. Latest investigations have shown that inorganic fullerene like materials (IFLM) can be used as nanomaterials for reduction of friction [3]. In this article, we focus on NBR samples (acrylnitril butadiene copolymers) because of their stability against oil, heat and abrasion, which offers broad usage as gaskets, hoses and rubberized fabric with small friction coefficients. A model developed at the DIK [4–10] has shown that two effects contribute to friction: The first is a hysteresis part that is due to the energy loss when the rubber is squeezed into the substrate surface and deformed by the lateral movement according to its viscoelastic properties. Consequently, temperature is important for friction behavior [11]. Neglecting internal heating effects, hysteresis friction increases with sliding velocity. The other effect is caused by molecular forces of the direct contact of elastomer and substrate in the absence of lubricant (dry friction) [12]. The adhesion occurring in this case is directly related also to the real area of contact, which cannot be established accurately when sliding fast, and which depends on the surface tensions
∗ Corresponding author. Tel.: +49 511 8420127; fax: +49 511 8386826. E-mail address: [email protected] (M. Klüppel). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.05.017
of rubber and substrate [13,14]. To achieve minimal friction, it is useful to minimize the real area of contact [15–18], which plays a major role, especially for dry friction, thus reducing abrasion effects [19]. In every system, friction is increased when the elastomer is forced into substrate cavities. Hard structures with small cavities will hinder this process, and reduce the total friction coefficient. In nature, this is shown by surface nano structures of the Sandfish (scincus scincus) [20]. To produce such a sample, hard spherical nanoparticles were formed at the surface of the rubber matrix. These particles consist of silica that arises from the silica liquid precursor hyperbranched polyalkoxysiloxane in an in situ sol–gel process and aluminum oxide. Compared to the monomeric sol–gel precursor tetraethoxysilane (TEOS) PAOS is a well defined precursor polymer (Fig. 1) with preferable properties [21]: A high resistance to hydrolysis due to the absence of OH groups. Completely covered by organic end groups, PAOS shows hydrophobic properties combined with solubility in most organic solvents. It can easily be modified for a better compatibility to different polymers. For example, incorporation of long alkyl chains, i.e. hexadecyl alkyl chains, increases the hydrophobicity of such modified PAOS. Recently, PAOS was employed in thermoplastic processing aimed at the development of a solvent-free sol–gel technology for the in situ formation of nanoscale filler particles in polypropylene [22,23]. Applied as an additive without addition of a catalyst for conversion to silica, PAOS being throughout the sample is surface active and enriches at polymer surfaces after processing. There it is converted to silica by atmospheric moisture. PAOS is also a suitable dispersant for silica or alumina nanoparticles.
L. Busse et al. / Wear 271 (2011) 1066–1071
OEt
EtO
O
O
O
O
Si
O
O
O
EtO
OEt
Si
OEt
O Si
Si
1. mixing of an NBR batch, including various parts of PAOS and carbon black; 2. vulcanization until t90 time; 3. if annealing is indicated, the sample was exposed to a 90% humid atmosphere at 60 ◦ C for 24 h to ensure a complete process of silica building on the surface.
OEt O
EtO
Si
R
OEt
Si
Si
O
Si
R
OEt
R
OEt
When not in use, samples were kept in a desiccator to prevent further silica formation. To protect the surface, clean conditions were applied to vulcanization and further handling.
OEt Fig. 1. Structure of hyper branched polyalkoxysiloxane (PAOS), with R: –C16 H33 .
2.2. Material properties First, the influence of PAOS and of annealing on the material properties was investigated. Increasing amounts of PAOS prolonged the vulcanization time. They also increased the shear moduli G and G in the high temperature range, as shown by DMA measurements (dynamic mechanical analysis). Consequently, the maximum of the loss angle tan ı = G /G is diminished without changing the associated temperature of the maximum (see Fig. 2)—PAOS behaves like a non-active filler without coupling to the polymer. As expected, carbon black filled samples show higher shear moduli with a decreased maximum of tan ı. Annealing had only minor effects on the viscoelasticity. Mechanical properties were determined by PAOS amount, carbon black amount and annealing (Table 1). The combination of
2. Preparation, material and surface properties 2.1. Preparation To form alumina/silica nanostructures on an elastomer surface, hydrophobically modified Al2 O3 particles (diameter: 100–120 nm, 10 wt. % in relation to PAOS) dispersed in a hexadecyl-modified PAOS (Fig. 1) were added to the mixing process. We will abbreviate this dispersion here simply as “PAOS”. The nanostructures were formed when the sample surface was treated with a heated humid atmosphere, which will be called annealing in this article. The NBR sample composition is made of 100 phr (“per hundred rubber”) Perbunan NT 2870F, 5 phr 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0
G''/G'
a
no cb, 0PAOS no cb, 10 PAOS no cb, 20 PAOS 50 cb, 0 PAOS 50 cb, 10 PAOS 50 cb, 20 PAOS
tan δ (not annealed) T [°C] -80 -60 -40 -20
0
20
40
60
80
100
b
1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0
G''/G'
R
ZnO, 1 phr stearic acid, 1.5 phr sulphur, 2 phr N-cyclohexyl2-benzthiazylsulfenamide (CBS) and 0.5 phr diphenylguanidine (DPG) for unfilled samples, or additionally 50 phr carbon black N 330 for filled samples. Both filled and unfilled samples were prepared at 50 ◦ C in an industrial type mixer either without PAOS, with 10 phr or with 20 phr PAOS, which had been produced at the DWI Aachen. The following steps were carried out to prepare the samples:
R
Si
1067
no cb, 0 PAOS no cb, 10 PAOS no cb, 20 PAOS 50 cb, 0 PAOS 50 cb, 10 PAOS 50 cb, 20 PAOS
tan δ (annealed) T [°C] -80 -60 -40 -20
0
20
40
60
80 100
Fig. 2. Viscoelastic properties (shown as tan ı) of unfilled and filled samples with different amounts of PAOS before (left) and after annealing (right).
Table 1 Mechanical properties. Carbon black [phr]
PAOS [phr]
Anneal time [h]
IRHD hardness
Rebound [%]
Tensile strength
Elongation at break [%]
Stress 200% [MPa]
DIN abrasion [mm3 ]
Density [g/cm3 ]
0 0 0 0 0 0 50 50 50 50 50 50
0 10 20 0 10 20 0 10 20 0 10 20
0 0 0 24 24 24 0 0 0 24 24 24
54 55 56 54 58 64 75 76 75 74 78 80
53 53 54 55 56 56 33 36 39 33 36 38
5.0 3.8 2.2 4.3 5.7 2.7 27.9 23.3 18.6 24.7 24.7 17.8
344 347 336 357 381 385 294 272 248 262 268 231
2.3 2.2 1.4 2.2 2.5 1.7 16.8 14.9 13.4 16.7 16.1 14.4
67 100 143 65 151 263 71 87 141 77 105 174
1.01 1.02 1.03 1.01 1.02 1.03 1.17 1.17 1.16 1.17 1.17 1.17
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Fig. 3. Light microscopical images (DIAS) for 0 (left), 10 (middle), 20 (right) phr annealed PAOS samples, with (above) and without (below) carbon black, image size 640 m × 460 m.
annealing and addition of Al2 O3 /PAOS increased hardness (IRHD) and rebound, whereas tensile strength, elongation at break and the stress at 200% strain dropped considerably for high PAOS contents. Some of these property changes (hardness, elongation) are typical for fillers, comparable to the effect of the carbon black, which has been well studied [16]. Others are rather connected to softeners, distinguishing PAOS from carbon black. Density increased with filler content, but when the total percentage of carbon black was replaced by PAOS, density remained constant. Abrasion is considerably increased, especially for annealed samples without carbon black. This is due to a lack of cross-linking. PAOS can be detected even on abraded surfaces.
2.3. Surface properties The surface morphology was characterized with several methods. The Dispersion Index Analysis System (DIAS) shows how microscopic structures of PAOS filled samples arise. The PAOS is added to the samples prior to annealing. For samples containing more than 10 phr PAOS without carbon black and more than 20 phr PAOS with carbon black clustered patterns appear (Fig. 3). The clustered patterns expected to minimize contact, thus minimizing friction. Structures also appear in AFM (atomic force microscopy) images of differently treated samples in Fig. 4. Starting with 20 phr PAOS
Fig. 4. AFM images of differently treated samples, each differing with respect to central picture in the single way indicated.
L. Busse et al. / Wear 271 (2011) 1066–1071
Friction Force [N]
a
1069
b
70 60
FN
10mm/s 1mm/s
50
elastomer
0,1mm/s
40 0,01mm/s
30 20
substrate
10
Ffr
lubricant
0 0
10
20
30
40
50
60
Distance [mm] Fig. 5. Typical friction forces in relation to sliding distance for selected velocities of a carbon black filled sample with 20 phr PAOS on granite (left); experimental set-up (right).
a
1,6
b
1,2
0,8
μ
μ
1,2
dry granite no carbon black not annealed 00phr PAOS 10phr PAOS 20phr PAOS
0,4
0,0
1,6
1E-5
1E-4
1E-3
0,01
0,1
0,8 dry granite no carbon black 24 h annealed 00phr PAOS 10phr PAOS 20phr PAOS
0,4
0,0
1E-5
1E-4
v [m/s]
1E-3
0,01
0,1
v [m/s]
Fig. 6. Friction of samples without carbon black before (left) and after annealing (right) on dry granite.
and no carbon black in a new sample (middle of figure), the effects of changing single parameters can be seen separately on the surrounding images. The addition of carbon black sharpened the structures; the absence of PAOS prevented the formation of large structures. Annealing caused the structures to agglomerate, and abrasion flattened the structures. Roughness analysis shows that fillers are well diffused in PAOS and carbon black filled samples, as indicated by a smooth surface. Annealing makes the surface rougher, as more alumina/silica particles are clustered on the surface. Surface tension was measured using the (static) sessile drop method with water, diiodmethane and ethylene glycol on samples cleaned with isopropanol, which was found to have no effect on surface roughness. The disperse and polar components of the surface tension have been evaluated according to Owens et al. [24] in Table 2. As expected, annealing had little effect on samples without PAOS. The addition of PAOS significantly decreased the polar part and improved dispersion in carbon black-filled samples. The total
Table 2 Surface tensions of cleaned samples. Carbon black [phr]
PAOS [phr]
Annealing [24 h]
0 0 0 0 50 50 50 50
0 0 20 20 0 0 20 20
Before After Before After Before After Before After
Surface tension [mN/m] Total
Polar
Disperse
16.62 16.41 11.33 15.05 17.32 17.09 18.84 19.94
2.04 1.92 0.29 0.17 1.7 1.93 0.59 0.99
14.59 14.49 11.03 14.89 15.61 15.16 18.26 18.95
tension is increased by carbon black. Annealing increases the total tension for PAOS filled samples. 3. Friction properties 3.1. Experimental set-up Friction has been tested for all sample types on different dry and wet substrates. Rectangular samples of 5 mm × 5 mm with 2 mm thickness were put under a normal constant force FN , resulting in a pressure of 12.3 kPa and drawn at various stationary velocities horizontally over the substrate, as seen in Fig. 5. The friction force Ffr was measured and is displayed in the left part of the figure as curves depending on sliding distance. In order to put all surfaces into a comparable state, samples were moved over the substrates for a limited but defined time until further friction did not change the surface behavior significantly. Friction measurements were conducted on dry granite, wet granite, on steel and on glass. Samples were cleaned from isopropanol. Abrasion debris was removed with dry dust free tissues. 3.2. Results and discussion The friction coefficient , defined as the ratio of friction force to normal force = Ffr /FN , generally increases with velocity for all friction systems investigated here. At high velocities the dry friction in some cases becomes almost constant. This plateau behavior has already been observed for other systems [25–27] and is explained as a superposition of an increasing hysteresis friction and a decreasing adhesion friction, as defined in the introduction of the paper. Consequently, elastic samples without carbon black display higher
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a
b
1,6
1,2
μ
1,2
μ
1,6
0,8
dry granite 50 phr carbon black not annealed 00phr PAOS 10phr PAOS 20phr PAOS
0,4
0,0 1E-5
1E-4
1E-3
0,01
0,8 dry granite 50 phr carbon black 24h annealed 10phr PAOS 20phr PAOS
0,4
0,0 1E-5
0,1
1E-4
1E-3
0,01
0,1
v [m/s]
v [m/s]
Fig. 7. Friction of samples with carbon black before (left) and after annealing (right) on dry granite.
0,8
b
wet granite 50 phr carbon black 00phr PAOS, 00h 00phr PAOS, 24h 20phr PAOS, 00h 20phr PAOS, 24h
1,6
1,2
μ
μ
a
0,4
0,8
wet granite no carbon black not annealed 00phr PAOS 20phr PAOS
0,4
0,0
0,0 1E-5
1E-4
1E-3
0,01
1E-5
0,1
1E-4
1E-3
0,01
0,1
v [m/s]
v [m/s]
Fig. 8. Friction on samples with (left) and without carbon black (right) on wet granite.
friction values, especially on dry substrates, because they can penetrate the cavities more deeply and cover a higher area of real contact. As expected, the presence of PAOS lowers the friction on dry granite. This is true either with (Fig. 7) and also without (Fig. 6) carbon black fillers and fits well to the observed reduction of surface tension. The effect was generally amplified by annealing, which not only reduces friction, but also made it less dependent on velocity, even without the addition of PAOS. Annealed samples containing 20 phr PAOS showed more than 20% less friction at high velocities. This reduction in friction was not apparent at low velocities.
a
The increase of high velocity friction is caused by hysteresis effects (present both for dry and wet surfaces), so this behavior should apply even more with lubricated sliding and also for lower velocities, which is confirmed in Fig. 8. Apart from PAOS influence, friction is considerably lower on lubricated substrates; especially for carbon black filled samples (again up to 20% for 20 phr PAOS). Annealing enhances the effect of PAOS. Friction is reduced not only on granite but also on polished steel, as depicted in Fig. 9. The very high initial friction coefficient can be significantly reduced by PAOS. In contrast to granite, the reduction factor is non-linearly dependent on the velocity. Another differ-
b
2,4
2,8
2,0
2,4
1,6
2,0
μ
μ
3,2
1,2 dry steel no carbon black 00 phr PAOS 00 phr PAOS 20 phr PAOS 20 phr PAOS
0,8 0,4 0,0 1E-5
1E-4
1E-3
0,01
1,6 1,2
00 h 24 h 00 h 24 h 0,1
dry glass no carbon black 24 h annealed 00 phr PAOS 20 phr PAOS
0,8 0,4 0,0 1E-5
1E-4
v [m/s] Fig. 9. Friction on dry steel (left) and dry glass (right).
1E-3
v [m/s]
0,01
0,1
L. Busse et al. / Wear 271 (2011) 1066–1071
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ence, a slight increase of friction after annealing, is also found in the data for surface tension. For highest velocities, stick-slip behavior is observed in absence of PAOS, so the periodical sliding and resting [28] of the sample cannot be attributed to friction coefficients in this case. Friction on glass is even higher than on steel since the very flat surface allows a large area of real contact between the friction partners. On this substrate, stick-slip is extremely pronounced for systems without PAOS above a critical velocity (0.1 mm/s), though not for the whole friction curve, i.e. resonance must be established over the sliding distance. The drop in friction above that velocity is a characteristic effect for stick-slip systems, as contact is temporarily lost. Usage of 20 phr PAOS reduces friction to approximately 50% in the low velocity range and completely removes stick-slip in the high velocity range. This means a significant advantage as stick-slip is usually regarded as undesirable in applications.
with financial support from “Bundesministerium für Wirtschaft und Technologie” (BMWi) and “Arbeitsgemeinschaft Industrieller Forschungsvereinigungen” (AIF), in cooperation with “Vereinigung zur Förderung des Instituts für Kunststoffverarbeitung in Industrie und Handwerk an der RWTH Aachen” (IKV) and “Deutsche Kautschukgesellschaft” (DKG).
4. Summary
[10] [11] [12] [13] [14] [15] [16] [17]
NBR samples with different degrees of PAOS with Al2 O3 particles have been prepared and analyzed regarding their mechanical, surface morphological and friction behavior. PAOS concentrations of 20 phr cause a decrease of friction by up to 20%, depending on velocity, for all samples (with and without carbon black) on dry and wet granite, steel and glass. Additionally, PAOS can prevent stick-slip phenomena that would otherwise appear on very flat substrates. Though the increased DIN abrasion indicates a low resistance against catastrophic crack propagation, the advantages make PAOS induced surface structures an ideal treatment for NBR parts requiring low friction, such as gaskets, hoses and rubber covered surfaces. Acknowledgements This work was supported by the DECHEMA “Gesellschaft für Chemische Technik und Biotechnologie” as AIF-Nr. 196 ZN in the program “Industrielle Gemeinschaftsforschung” (IGF),
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