FRICTION BEHAVIOUR OF POLYMETHYL METHACRYLATE REINFORCED BY MULTI-WALLED CARBON NANOTUBES

FRICTION BEHAVIOUR OF POLYMETHYL METHACRYLATE REINFORCED BY MULTI-WALLED CARBON NANOTUBES

Ameer A. K., Mousa M. O. and Ali W. Y.

Department of Production Engineering & Mechanical Design, Faculty of Engineering,

Minia University, El-Minia – 61111, EGYPT.

ABSTRACT

The purpose of the current work is to show the effect of multi-walled carbon nanotubes(MWCNTs) of 0.1, 0.2, 0.3, 0.4 and 0.5 wt. % contents reinforcing polymethyl methacrylate (PMMA) on the frictional behavior of this composite when sliding against emery paper. The hardness of the intended composites is measured by Shore D Durometer on the surface and side of the test specimen. Friction coefficient of MWCNTs/PMMA composites is measured by a reciprocating sliding apparatus at room temperature.

Based on the experiments, results display that the hardness of hot cured tested composites increased gradually by increasing MWCNTs content unlike the cold cured composites. In addition to that, it is shown that friction coefficient of hot and cold cured composites decreased by adding 0.1, 0.2, 0.3, 0.4, and 0.5 wt. % of MWCNTs. Also, friction coefficient of the tested composites was found to be significantly affected by the normal load. Based on these results, it may be stated that hardness of the tested composites is dependent on curing type and MWCNTs content. Besides, friction coefficient of the tested composites is dependent on MWCNTs content and normal load. It can be concluded that hot cured composites are better than cold cured composites to be used in different engineering application.

KEYWORDS

Polymethyl methacrylate (PMMA), Multi-walled carbon nanotubes (MWCNTs), friction coefficient, hardness, normal load, reciprocating sliding.

 INTRODUCTION

Most of polymeric materials exhibit low resistance to scratching and marring, and so are not suited to many industrial applications. Consequently, there is certainly need in bettering properties of surface (i.e. resistance to scratch etc.) and coating is an interesting solution, [1-3]. Certainly there is no polymer in an original form that offers a reasonable low working wear rate with an optimum coefficient of friction. For this reason polymeric coatings that used in tribological applications are improved by appropriate fillers to decrease the friction coefficient (wear rate), to improve their elastic modulus and their strength. These fillers are made to produce an elastic influence into the plastic behavior or to increase the elastic component in an elastic-plastic behavior of the coating, [4]. Poly methyl methacrylate (PMMA) is an extensively used polymer in architecture, automotive or railways glazing, as well as biomedical sector due to its mechanical properties, good optical and biocompatibility properties.

Carbon nanotubes (CNTs) which were discovered by Iijima in 1991, [5, 6] have excellent mechanical properties that could be professionally used as nanofillers. The great aspect ratio of nanofillers causes very large effective surface area and favors their use as reinforced particles in a polymer matrix (nanocomposite). Polymer nanocomposites do not always have mechanical properties improvement in comparison to the neat polymer but a low reinforced CNTs content can be sufficient providing the dispersion of these fillers is homogeneous and interaction between the matrix and the nanoparticules is sufficient, [7, 8]. In such case, the usage of a low percentage of CNTs in the nanocomposite's coating specialized in tribological applications is interesting to keep the transparency at best [3]. The tricky point is therefore to find the best chemical way to modify the CNTs surfaces to avoid both CNTs aggregation and weak bonds with the host matrix. Two traditional approaches are conceivable: covalent or non-covalent surface modifications. Noncovalent bonding can be performed by using surfactants or block copolymers [9, 11], or by solvent free ionic liquid dispersion, [12, 13]. Covalent bonding can be generated on MWCNTs with applied groups like amine, COOH, silane, etc., [14]. Addition of CNTs application in a number of fields including, catalysis and energy storage, composites [15, 19]. Additionally, their compatibility with polymer matrices was significantly improved by the direct introduction of macromolecules onto the surface of CNTs, [20]. For the case of polymethyl methacrylate, a number of ways are reported. A novel in situ atom transfer radical polymerization (ATRP) (“grafting from” approach) to functionalize MWCNTs with PMMA chains was presented, [21]. The PMMA-grafted-MWCNT displayed a relatively good solubility in none or weakly polar solvents such as tetrahydrofuran (THF) and chloroform (CHCl3) and a poor solubility in strong polar solvents such as dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Remarkably, it is stated that the incorporation of PMMA-grafted-MWCNTs to commercial PMMA, realized by dissolution techniques, causes an improvement dispersion of the filler in the matrix and an increase in bulk mechanical properties, in addition to measuring an important rise of the storage modulus (1100%) for PMMA filled with 20 wt. % PMMA-grafted-MWCNT at 20 ºC, [22]. The matrix/nanoparticles interaction was so strong and confirmed a load transfer from the polymer matrix to the MWCNTs. The incorporation of low percentage of PMMA-grafted-MWCNTs [0.5-2 wt. %] to poly (styrene-co-acrylonitrile) was performed and realized by solution casting from THF [23]. For instance, at 40 °C this development was of a factor 2 for the elastic modulus and a factor 6 for the toughness. Their reports also led to the conclusion that best mechanical improvement is reached for an optimal MWCNTs content. For lower PMMA-grafted-MWCNTs amount (i.e. lower than 0.5 wt. %). It was studied that the Young's modulus, toughness and tensile strength were enhanced by 1.9, 4.6, and 13.7 times, respectively, [24]. Actually at low percentage of fillers grafted with macromolecules, it is possible to improve the bulk mechanical properties of a nanocomposite. The question which still waiting for an answer is does such nano-composite still ensures good tribological properties? A huge amount of reports on the tribological behavior of polymeric nanocomposites exist, nevertheless fewer deals with tribological performances of acrylic-based nanocomposites coatings. A significant decrease of the friction coefficient was measured (~30 % for 2.5 weight content with according to the monomer) and the wear rate for 1% of CNTs incorporated in a PMMA matrix by an in situ polymerization process, [25]. Tribological experiments were performed on polyimide-based coatings reinforced with 1, 3 and 5 wt. % PMMA grafted- MWCNT, [26]. These coatings were limited between a steel ball and an aluminum substrate at 0.9 GPa contact pressure and sheared at 0.02 m/s sliding speed. Only the one containing 3wt. % PMMA-grafted-MWCNT provided valuable effects (some delamination happened on the other ones). As the coating thickness is not known, it does not give information on the elastic component brought by the reinforcement fillers.

Several studies have examined the effect of normal load on the coefficient of friction. Some researches revealed that there is a gradual reduction in the coefficient of friction of the polymer based nanocomposites over a wide range of applying loads [27-28]. It was discovered that the friction coefficient depend on the normal load, as normal load increases, the value of friction coefficient increases, [29, 30]. On the other hand, friction coefficient reduces with the increase in normal load for glass fiber, PTFE and nylon, [31]. The influence of titanium dioxide nanoparticles on wear of short fiber reinforced epoxy under various load conditions was investigated, [32]. They noted that the adding of 5 vol. % of TiO₂ nanoparticles can importantly decrease the coefficient of friction of epoxy composites than padding only by traditional fillers.

In the current study, the influence of multi-walled carbon nanotubes (MWCNTs) on the friction behaviour and hardness of polymethyl methacrylate (PMMA) is examined.

EXPERIMENTAL

MATERIALS

The matrix materials used in this study is polymethyl methacrylate (PMMA) while the fiber is carbon nanotubes (MWCNTs). PMMA is used in two types; one as cold cured acrylic reins and the other one as heat cured acrylic reins. Table 1 shows the properties of PMMA as acrylic resins, while table 2 shows the details of MWCNTs.

PREPARATION OF TEST SPECIMEN

Test specimens have been made of PMMA. The MWCNTs were added in different contents of 0.1, 0.2, 0.3, 0.4 and 0.5 wt. %. Three specimens have been fabricated from PMMA (as received)  and the other specimens have been fabricated by adding the MWCNTs contents to the PMMA powder in a glass beaker, then mixed for 20 second and added to the mold of specimens of cylindrical shape as shown in Figs. 1, 2.  The molds were put in water bath at 100 °C for 30 second and then ejected and left for bench cooling.  Figures 3, 4 show samples of PMMA and PMMA/ MWCNTs composites.

Table 1 Typical Properties of Acrylic PMMA

Table 2 Details of MWCNTs.

TEST METHOD

HARDNESS TEST

Shore D Durometer instrument was used. Hardness was measured in three positions. The first one on the surface of the specimens (top and bottom), the second was on the side of the specimens and the third was on the surface in radial distance from center to the edge.

FRICTION TEST

To evaluate the friction coefficient of the specimens, cylindrical specimens (6 mm in diameter and 10 mm in length) of each condition are made in plastic tubes of 6 mm diameter and 10 mm height. Friction force is evaluated through subjecting the specimens to friction test at different normal loads against emery paper (1000 grit size) counterface using reciprocating sliding apparatus as shown in Figure 5. For each normal load, the friction coefficient is determined using the relationship:

                            μ = F / N                        [1]

Where μ is the friction coefficient, F friction force and N normal load. The test conditions are; Velocity = 60 strokes/min., load = 6, 8, 10, 12 and 14 N and time = 1 min

RESULTS AND DISCUSSION

HARDNESS

Figures 6, 7 show theeffect of MWCNTs contents on the hardness of surface and side of MWCNTs/PMMA composites. It can be noticed that the hardness of the hot composites increased by increasing the content of MWCNTs, this improvement in hardness may be due to the high strength and Young’s modulus of the MWCNT reinforcement and the heat treatment that increased hardness values due to an overlap and stacking, which reduced the movement of polymer molecules, and increased the resistance of material to scratch, cut, and become more resistant to plastic deformation.

Figures 8, 9 show the hardness of the cold MWCNTs / PMMA composites in radial distance from the surface of the composites to the edge, while Figures 10, 11 show the hardness of the hot MWCNTs / PMMA composites in radial distance from the surface of the composites to the edge. It can be seen that the hardness increased at the edge in both hot and cold composites but decreased from the edge to the core of composites where the surface is cooled rapidly than the core of composites. For hot composites the hardness is higher than the cold one because of the reasons mentioned before.

FRICTION

EFFECT OF MWCNTs CONTENT ON FRICTION

Figures 12, 13, 14, 15 and 16 show the effect of MWCNTs contents on friction coefficient (µ) of MWCNTs / PMMA composites for both cold and hot composites under loads of 6, 8, 10, 12 14 N.  It is noticed that friction coefficient for both cold and hot composites decreased gradually by increasing MWCNTs contents, where, the highest values of friction coefficient is observed for the unfilled test composites. Specimens filled with MWCNTs showed that µ decreases gradually down to minimum at 0.5 wt. % for both cold and hot composites. This may due to that MWCNTs work as solid lubricant between the sliding surfaces so that the value of friction coefficient is dependent on MWCNTs contents.       

EFFECT OF NORMAL LOAD ON WEAR

Figures 17, 18 show the effect of normal loads on µ of cold composites under different contents of MWCNTs. Figures 19, 20 show the effect of normal load on µ of hot composites. Both cold and hot tested composites showed gradual decrease of µ as the normal load increases at any content of MWCNTs. This can be explained on the basis that with increasing normal load the contact area increases, hence that causes particles produced from the two sliding surfaces of the two bodies, work as a solid lubricant that decreases the friction coefficient.  

CONCLUSIONS

From this study the followings can be concluded:

1. Hardness of hot cured MWCNTs/PMMA composites increases with increasing MWCNTs contents.

2. Hardness of cold cured MWCNTs/PMMA composites decreases with increasing MWCNTs contents.

3. Friction coefficient for both cold and hot composites decreases gradually by increasing MWCNTs contents and normal load.

5. Hot cured MWCNTs/ PMMA composites are recommended for engineering applications.

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