PROPER SELECTION OF POLYMERIC FIBERS

REINFORCING EPOXY

Ali A. S.¹ and Ali W. Y. ²

¹Mechanical Engineering Dept., Faculty of Engineering, Suez Canal University, EGYPT.

²Department of Production engineering and Mechanical Design, Faculty of Engineering,

Minia University, El-Minia, EGYPT.

ABSTRACT

The proper selection of polymeric fibers reinforcing epoxy based on the tribological properties isexperimentally investigated in the present work. The effect of reinforcing epoxy matrix by continuous polyamide (PA), Kevlar and polyester (PET) fibers on the friction and wear at dry sliding against steel surface is tested.

It was observed that PA reinforcing epoxy matrix displayed the highest friction coefficient and the lowest wear. The favorite behavior of polyamide can be explained on the basis of the triboelectrification of the tested composites during friction with steel surface. It was found that ESC generated from the friction of PET and steel was higher than that generated from steel and epoxy. The nature of PA fibers influenced the sign of ESC built up on steel counterface, where the resultant ESC showed lower values than that observed for PET fibers.

In the presence of Kevlar as reinforcement, ESC showed higher values than that observed for PET due to the rank of the two sliding materials in the triboelectric series.The attractive force between steel and Kevlar is much higher than that expected for steel and epoxy, where the intensity of ESC controls the strength of the attractive force. Adhesion of Kevlar into steel surface was stronger than the adhesion of epoxy into steel. Consequently, friction force increased with increasing adhesion between the two contact surfaces. As the surface area covered by Kevlar increased, friction increased more than that displayed by the surface area covered by epoxy. Proper selection of fibers in epoxy matrix based on their triboelectrification can control polymer transfer into steel surface and influence both friction and wear.

KEYWORDS

Friction coefficient, wear, epoxy, polyamide, Kevlar, polyester fibers, reinforcement, electrostatic charge.

INTRODUCTION

The increased use of fiber reinforced epoxy composites for the low costs as well as high mechanical and tribological properties accelerates their development. The effect of reinforcing epoxy matrix by polyamide and polyester fibers of different diameters on the friction and wear, at dry sliding against steel surface, was studied, [1, 2], where friction coefficient displayed by the tested composites drastically decreased as the polymeric content increased and significantly increased up to maximum then drastically decreased with increasing polymeric fiber diameter. Wear increased up to maximum then slightly decreased with increasing fiber diameter. At constant content of polymers, fibers of relatively low diameter showed the lowest wear. Wear mechanism of the tested composites is based on the triboelectrification of the sliding surfaces.

Epoxy composites reinforced by fibers are applied in different industrial applications, [3, 4]. Glass fiber reinforced epoxy resin showed wear increase with increasing load and velocity, [5], where fiber orientation affected wear mechanism. The reinforcements are reinforcing epoxy matrix to develop the strength and increase lifetime, [6, 7]. Carbon fibers (CF) reinforced epoxy composites have lightweight, high mechanical strength and chemical resistance, [8 - 15]. Woven fiber reinforced epoxy was filled by Nano-silica particles, [16, 17] to enhance interfacial stress. Fibers of glass, carbon and Kevlar were commonly used to reinforce epoxy composites, [18 - 21], where Kevlar fibers increased the mechanical property.  Besides, multi-walled carbon nanotubes can improve the tensile strength of epoxy, [22 – 24]. The mechanisms of triboelectrification are electron transfer, ion transfer and material transfer, [25 - 27]. For polymers, the electron transfers only happen on their surfaces, [28 – 30]. According to the triboelectric series the polarity of the charge that is transferred from one surface to another can be to predicted, [31]. At relatively higher load, the prevailing mechanism is material transfer, where the sign of ESC charge is frequently changed. Engineering materials including polymers can be arranged in a ‘‘triboelectric series’’ which lists the materials in the order of their relative polarity. In the triboelectric series the higher positioned materials will acquire a positive charge when contacted with a material at a lower position along the series, [32]. The triboelectric series can be used to estimate the relative charge polarity of the materials.

In the present work, effect of reinforcing epoxy by PA, Kevlar and PET fibers on friction coefficient and wear when sliding against steel is investigated.

EXPERIMENTAL

Fig. 1 Arrangement of the test rig.

Experiments were carried out using pin-on-disc wear tester. It consists of a rotary horizontal steel disc driven by variable speed motor. The details of the wear tester are shown in Fig. 1. The pin made of the tested composites is held in the specimen holder that fastened to the loading lever. Friction force can be measured by means of the load cell, fastened to the rotating disc.

Friction tests were carried out under 2.0 m/s sliding velocity. The load values were 8, 10. 12, 14 and 16 N. They lasted for 30 minutes. All measurements were performed at 25 ± 5 ºC and 30 ± 10 % humidity. The test specimen, in the form of a cylinder, is 10 mm diameter and 30 mm height. The diameter is reduced to 6 mm to contact the steel disc. The polyamide, Kevlar and polyester continuous fibers of 0.20 mm diameter and 12.5 vol. % volumetric content were used to reinforce epoxy matrix (KEMAPOXY 150A).

RESULTS AND DICUSSION

The results of friction tests are shown in Figs. 2 and 3. It was observed that a drastic decrease in friction coefficient as the polymeric fiber volumetric content increased, Fig. 2. The highest value of friction coefficient was observed for epoxy reinforced by polyamide fibers. Reinforcing epoxy by Kevlar displayed the lowest friction. Friction coefficient displayed by the tested composites reinforced by the tested polymeric fibers significantly decreased with increasing applied load, Fig. 3. The dependency of friction coefficient on load can be explained considering that as the load increased, the plasticity of the epoxy asperities contacting steel increased, so that the shear strength decreased causing the decrease of friction coefficient. The accumulation of the layers of the transferred epoxy may display the relatively high friction coefficient. It was observed at the beginning of the experiment the tested composites experienced relatively lower values of friction coefficient. As the epoxy transfer film deposited on the steel surface, friction coefficient increased indicating that both epoxy and steel suffered from severe stick-slip.

Fig. 2 Friction coefficient displayed by epoxy reinforced by polymeric fibers

and sliding against steel at 16 N load.

Fig. 3 Friction coefficient displayed by epoxy reinforced by 12.5 vol. % polymeric fibers content and sliding against steel.

Wear of the tested composites reinforced by the tested polymeric fiber decreased with increasing fiber content, Fig. 4. The observations in wear tests confirmed the role of reinforcing fibers that have relatively higher wear resistance than epoxy in decreasing wear. The wear mechanism observed in the present work can be explained based on epoxy transfer onto the steel counterface forming an adherent layer. During friction, the relatively softer epoxy and fibers transferred to the steel counterface. The deposit then back transferred fractionally to the tested composites. An equilibrium state appears to reach as far as the amount of transfer in both directions is concerned. The accumulation of the layers of the transferred material may form the layer that was adhered to the counterface by the action of the contacting asperities then removed from the surface when the shear stress exceeds the adherence between the transferred layer and the steel counterface. Transferred materials are mainly epoxy and polymeric fibres contaminated by tiny steel particles.

The relationship between wear of the tested composites and applied load is shown in Fig. 5. Wear remarkably increased with increasing load. Polyamide showed the lowest wear followed by Kevlar and polyester fibers. During wear process, epoxy worn from the tested composites and adhered to the steel counterface formed thin layer. During sliding, relatively hard steel asperities penetrated the surface of the tested composites, where the stresses at the point of contact were high and caused localized plastic deformation. Then, sliding of the contacting materials was accompanied by repeated extensive deformation of the thin surface layer of epoxy leading to the deformation of the surface layer and wear particles. The polymeric material transfers back to the parent composites. It is expected that the transfer film generated from epoxy is considerably thicker than that generated from fibers. The transfer film of epoxy was accumulated to form thicker film adhered to the steel surface and followed by excessive shear stress that caused considerable plastic flow of the deposited film.

Fig. 4 Wear of epoxy reinforced by polymeric fibers at 16 N load.

Fig. 5 Wear of epoxy composites reinforced by 12.5 vol. % polymeric fibers.

Fig. 6 Distribution of ESC on the epoxy reinforced by PET fibers

 and sliding against steel counterface.

Fig. 7 Distribution of ESC on the epoxy reinforced by PA fibers

 and sliding against steel counterface.

Based on the experimental observation, it was observed that polyamide fibers exhibited the highest friction coefficient and lowest wear. That behavior can be explained on the triboelectrification of the tested composites during friction with steel surface.  Figures 6 - 8 illustrate the distribution of ESC generated on steel counterface and the tested composites surfaces. Figure 6 illustrates the distribution of ESC on the contact area of polyester fibers reinforcing epoxy matrix. It is shown that most of the contact area is charged by double layer of ESC of different charge due to the position of polyester fiber in the triboelectric series, Fig. 9. Consequently, layers of epoxy and polyester can transfer and adhere to the steel counterface, where the contact will be epoxy/epoxy and polyester/polyester rather than epoxy/steel or polyester/steel. That contact condition was responsible for the friction increase.  Polyester is ranked as negative charged material. It is obvious that ESC plays major role in adhesion energy and alters friction by the effect of the trapped charges and, consequently on the presence of surface defects introduced during friction.

Fig. 8 Distribution of ESC on the epoxy reinforced by Kevlar fibers

 and sliding against counterface.

Fig. 9 Illustration of triboelectric series for tested materials.

On the other side, higher fraction of the contact area of steel will be electrified by negative charge. It is expected that ESC generated from the friction of polyester and steel will be higher than that generated from steel and epoxy. This behaviour could be attributed to the fact that epoxy, polyester and steel are different materials and according to the triboelectric series, friction between two surfaces causes the object in the upper position of the series to be charged positively (steel) and that in the lower position to be charged negatively (polyester and epoxy). It is known that different polarity means attraction. Besides, it could be attributed to that, the long distance gives higher chance to exchange more electrons between the two different materials rubbing each other. Based on that, polyester wear particles in form of film will be strongly adhered to the steel surface attracting layers of epoxy of negative charge to be accumulated to form thicker polymeric layer. In that condition due to the transfer of polyester and epoxy into the steel counterface both friction coefficient and wear increased.

The distribution of ESC on the contact area of polyamide fibers reinforcing epoxy matrix is shown in Fig. 7. The nature of polyamide fibers influenced the sign of ESC built up on steel counterface, where the resultant showed lower charge than that observed for PET fibers. After sliding, epoxy and PA transferred into steel surface, where friction coefficient depended on the area covered by epoxy and PA as well as the adhesion between both of epoxy and PA and steel surface. When epoxy matrix was reinforced by PA, epoxy transfer into steel would be easier leading to significant increase in the steel area covered by epoxy. In that condition, the contact would be between PET and epoxy, where the ESC would be lower.

In the presence of Kevlar as reinforcement, ESC showed higher values than that observed for PET reinforcing epoxy due to the rank of the two sliding materials in the triboelectric series.The attractive force between steel and Kevlar is much higher than that expected for steel and epoxy due to the position of those materials in the triboelectric series, where the intensity of ESC controls the strength of the attractive force. Adhesion of Kevlar into steel surface would be stronger than the adhesion of epoxy into steel. It is commonly known that friction force increases with increasing adhesion between the two contact surfaces. As the surface area covered by Kevlar increases, friction increases more than that displayed by the surface area covered by epoxy. Proper selection of fibers in epoxy matrix would control polymer transfer into steel surface.

CONCLUSIONS

1. Friction coefficient displayed by the tested composites drastically decreased with increasing applied load and fiber content.

2. The highest value of friction coefficient was observed for epoxy reinforced by polyamide followed by PET and Kevlar fibers.

3. Wear of the tested composites decreased with increasing fiber content, while remarkably increased with increasing load. Polyamide showed the lowest wear followed by Kevlar and polyester fibers.

4.Proper selection of fibers in epoxy matrix should be based on their triboelectrification.

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