Abrasive wear of HDPE/UHMWPE blends

Wear 270 (2011) 576–583

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Abrasive wear of HDPE/UHMWPE blends Alessandra de A. Lucas a,∗ , José D. Ambrósio b , Harumi Otaguro b , Lidiane C. Costa b , José A.M. Agnelli a a b

Department of Materials Engineering – DEMa, Federal University of São Carlos – UFSCar, SP, Brazil Materials Characterization and Development Center – CCDM, DEMa/UFSCar, SP, Brazil

a r t i c l e

i n f o

Article history: Received 6 April 2010 Received in revised form 11 January 2011 Accepted 19 January 2011 Available online 26 January 2011 Keywords: Polymer Abrasion Solid lubricants Wear testing Electron microscopy

a b s t r a c t Blends of high density polyethylene (HDPE) with ultra-high molecular weight polyethylene (UHMWPE) were obtained by mixing in a melted state at concentrations ranging from 10 to 30% by weight in an intermeshing co-rotating twin screw extruder (ICTSE). The abrasive resistance of the blends was evaluated according to the DIN53516 standard, and it was observed that the volumetric loss of the blends decreased with increasing concentration of UHMWPE. The mechanical properties of the samples were analyzed in terms of flexural, tensile, and impact strength; in general, the HDPE/UHMWPE blends had a good set of properties, most of which were better than the properties of pure HDPE. Thermal analysis of samples was made by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), and no significant difference was observed between the blends and pure HDPE. The morphological analysis conducted by scanning electron microscopy (SEM) showed that the UHMWPE is present in the HDPE matrix as a second phase, but there is a good interface between the two. We found an inverse relationship between the volumetric loss in the abrasive wear of the samples and the product between the tensile strength at yielding and strain at yielding, y εy , as proposed by Lancaster and Ratner (Sinha and Briscoe, 2009 [1]). © 2011 Elsevier B.V. All rights reserved.

1. Introduction High density polyethylene (HDPE) is a polymer commonly used in pipes for water supply, sewage, and the three-layer coating of steel pipes, with the primary function of protecting the pipes from damage caused by corrosion, transportation, and pipe installation (e.g., impact, bending, and stress). These buried (in humid or dry environments) or underwater (fresh or salt water) pipelines are used for the transport of gas, oil, minerals, and water, among other uses. The temperature range of use varies between 40 and 80 ◦ C [2]. For this purpose, the HDPE that is used must have a good set of properties, such as resistance to abrasive wear, impact resistance, and tensile and flexural strength. Wear resistance becomes an important factor since the coating is always in contact with sand soil, or with solid particles suspended on water or rubbing against solid particles located at the contact between the surface of the pipe, coated or not, and other surfaces during storage, transportation, installation and usage. HDPE has good flow properties and is widely used in commodity markets, as it can be transformed by traditional processes of extrusion, blow molding, injection molding, and rotational mold-

∗ Corresponding author at: DEMa/UFSCar, Rodovia Washington Luis, km 235, São Carlos, SP CEP13565-905, Brazil. Tel.: 55 16 3351 8532; fax: 55 11 3351 8850. E-mail address: [email protected] (A.A. Lucas). 0043-1648/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2011.01.011

ing. In addition, it is sold at a low price. Ultra-high molecular weight polyethylene (UHMWPE) has the remarkable properties of excellent abrasion resistance, impact resistance, fatigue resistance, and chemical resistance, and it is commonly compared to engineering polymers [3–5]. However, the high molecular weight of UHMWPE (in the range of 3–10 million g/mol) gives it a very high viscosity, which prevents its flow in conventional techniques of polymer processing. This is because in practice this material does not flow, thereby limiting its applications. In this context, the mix between these two polymers is a very interesting alternative since it has the potential to combine the excellent properties of UHMWPE and the good processability of HDPE. The basic idea is to add as much UHMWPE to HDPE as possible, taking the advantage of both: the processability of HDPE under conventional techniques and the outstanding wear and mechanical performance of UHMWPE. For extrusion, for example, the viscosity of the blends will be a limitant factor. These blends have been extensively studied recently [3–14], which adds to the knowledge of various aspects of the behavior of UHMWPE under abrasion and wear in various conditions, especially for applications such as biomaterials in orthopedic implants. Abrasive wear is caused by hard asperities on the counterface and/or hard particles that move over the polymer surface. This mode of wear occurs when the roughness is the determinative parameter in friction [1]. Abrasion results in scratches, gouges, and scoring marks on the worn surface. The phenomenon that controls the resistance of plastics is not yet fully understood. The classic

A.A. Lucas et al. / Wear 270 (2011) 576–583

relationship for the abrasive wear of metals is that wear is inversely proportional to the hardness of the material; the harder the material, the lower the abrasion rate. There is also an empirical approach to the description of abrasive wear, which was proposed by Lancaster and Ratner: experiments have shown that the abrasive wear rate is proportional to 1/ u εu , where u is the ultimate tensile stress and εu is the corresponding strain [15]. Since HDPE sheeting is known to have only about 5–10% of the abrasion resistance of UHMWPE sheeting in numerous abrasion tests [16], it can be expected that the addition of UHMWPE to HDPE will increase its abrasion resistance. According to Bauman [16], HDPE and UHMWPE have good compatibility since they have the same chemical composition. Budinski [17] concluded that UHMWPE is the material that presented the best performance among 17 plastics and 4 rubbers in the dry rubber wheel abrasion test. The explanation for its excellent abrasion resistance appears to be in its ability to absorb energy by deforming itself and its low coefficient of friction when in contact with several other types of materials. The authors suggest that a model for predicting the abrasion resistance should include the fracture tenacity of materials since more fragile materials lose matter by fragile fracture in tests of abrasion and scratch resistance [17]. From a study of rheological and impact properties of HDPE/UHMWPE blends and its morphology, Boscoleto et al. [18] reported that up to about 3% of UHMWPE can be partially dissolved in the HDPE matrix, thus explaining the good interface observed by SEM and the increase in impact strength of samples containing up to 20% of UHMWPE by weight. Although several tribological test results were reported in the literature both for these polymers alone and for HDPE/UHMWPE systems [10,13,14], testing of the abrasion resistance against sandpaper (abrasive paper) according to the DIN53516 standard has not been found so far in the literature. Since it is a very common and easy running industrial test, DIN53516 can be used to analyze the wear resistance of polymers under abrasive conditions. Besides the wear behavior, the mechanical properties of HDPE under tensile, flexural and impact loading must remain balanced. In this context, the purpose of this article was to evaluate the resistance of HDPE blends with UHMWPE against wear using sandpaper and to correlate the results with their mechanical properties and morphology.

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180 ◦ C, as proposed by the manufacturer for this HDPE grade. The material dosage was controlled through the torque registered in the equipment, and the feed rate was of 1000 g h−1 . 2.2. Abrasive wear testing Abrasion, in mm3 , refers to the volume loss under the specified test conditions. Experiments were conducted according to DIN53516. In this test method, small cylindrical specimens (16.0 ± 0.2 mm in diameter and minimum thickness of 6 mm) are abraded in pure sliding motion against a grade 60 corundum abrasive cloth, covering a rotating cylinder (150 ± 0.2 mm and length approximately 500 mm), rotating at (40 ± 1) min−1 . The specimen holder arm moves parallel to the (horizontal) cylinder axis and is loaded by a 1000 g mass, providing a contact normal force of 10 ± 0.2 N. After an abraded distance of 40 m is reached, the specimen is automatically lifted from the equipment and weighted. Three test specimens of each composition were tested. The volumetric loss (mm3 ) = [(m × S0 )/( × S)], where m = mass loss (mg); = density (mg/mm3 ); S0 = value of nominal abrasive power (200 mg); S = average abrasive power (mg). S0 and S are obtained from the calibration of the equipment with a standard rubber. 2.3. DSC and TGA analysis Thermal properties, such as the melting and crystallization temperatures and the degree of crystallinity, were determined using a heat flow differential scanning calorimeter (DSC). Samples of about 10.0 mg were scanned from 25 ◦ C up to 170 ◦ C at a rate of 10 ◦ C min−1 in a nitrogen atmosphere, and they were held for 5 min before cooling them down at 10 ◦ C/min to a temperature of 30 ◦ C. The second scanning session was performed following the same heating rate up to 200 ◦ C. Thermogravimetric analysis (TGA) was performed with a thermogravimetric analyzer, in a platinum pan using a heating rate of 20 ◦ C min−1 in an inert atmosphere (flow rate = 50 mL min−1 ), with temperatures ranging from 25 ◦ C to 550 ◦ C. In the range of 550 ◦ C up to 850 ◦ C, the experiment was performed in an oxidative atmosphere (O2 ) with a flow rate of 50 mL min−1 . 2.4. Mechanical testing

2. Materials and methods 2.1. Materials and sample preparation Pellets of the HDPE bimodal copolymer with a melt index of 0.35 g/10 min (ASTM D1238) and UHMWPE in the powder form were used. UHMWPE resin has a molecular weight of 4 × 106 g/mol and a mean particle size of about 190 ␮m. Both materials were supplied by Braskem Petrochemical, RS, Brazil. HDPE/UHMWPE blends were prepared by direct blending in an intermeshing co-rotating twin screw extruder (L/D = 25, diameter of 19 mm). The contents of UHMWPE blended with HDPE were 0, 10, 20, and 30 wt%. Table 1 shows the labels for the different blend compositions. The processing was carried out at a screw speed of 170 rpm according to the following temperature profile: Zone 1: 170 ◦ C, Zone 2: 170 ◦ C, Zone 3: 170 ◦ C, Zone 4: 170 ◦ C, and Zone 5: Table 1 Composition of the HDPE/UHMWPE blends and material coding. Samples

Coding

HDPE HDPE + 10 wt% UHMWPE HDPE + 20 wt% UHMWPE HDPE + 30 wt% UHMWPE

100/0 90/10 80/20 70/30

Tensile measurements were carried out on injection-molded specimens at room temperature using a 5569 model Instron machine with a crosshead speed of 50 mm min−1 according to ASTM D638, 2003. The flexural tests were performed according to ASTM D790, 2003, in an Universal Testing Machine. The span length was fixed at 50.0 mm. Determination of the Izod impact strength with notched samples was performed at room temperature with a pendulum impact equipment, using a pendulum of 4 J, in accordance with the ASTM D256, 2003 standard. 2.5. Scanning electron microscopy (SEM) An SEM with an accelerating voltage of 20 kV to observe the morphology of the abrasion and the cryogenic fractured surfaces of the HDPE/UHMWPE samples. The samples were sputter-coated with a thin layer of gold prior to SEM examination. 2.6. Hardness The Shore D Hardness of the samples was evaluated according to ASTM D2240-00 with six replicates for each composition.

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Table 2 Effect of UHMWPE content on the onset and peak melting temperatures and crystallinity degree for HDPE/UHMWPE blends. 1st heating

2nd heating

Samples

Tonset ( C)

Tm,peaks ( C)

Hm (J/g)

(%)

Tonset (◦ C)

Tm,peak (◦ C)

Hm (J/g)

a (%)

100/0 90/10 80/20 70/30

120.3 123.9 121.3 119.9

133.9/136.1 134.8/139.8 133.4/139.2 133.5/136.5

188.7 189.9 187.6 186.7

65.7 66.1 65.3 65.0

121.5 122.7 122.0 121.9

132.1 134.8 134.9 138.4

205.7 208.9 198.2 199.7

71.6 72.7 69.0 69.5

a

◦

◦

a

0 Assuming HE,PE = 287.3 J/g [7].

3. Results and discussion 3.1. Thermal analysis From the first and second heating and cooling thermograms of the HDPE/UHMWPE samples, it could be observed that not significant small changes in the onset melting temperatures (Tonset ) and peak melting temperatures (Tm ), as well as the enthalpy involved in the processes of melting and crystallization were observed, as presented in Tables 2 and 3, respectively. From TGA analysis, it was observed that blends showed a lower thermal stability than that of pure HDPE, and onset temperatures of thermal degradation moved to lower temperatures than those of pure HDPE. This decrease in the thermal stability of HDPE may have occurred because of a probable high viscous heating due to the significant increase in viscosity that occurs in the blend due to the presence of UHMWPE [18]. During the mixing in the extruder and injection, this increase in temperature due to viscous heating may have degraded the HDPE, reducing its thermal stability at high temperatures. However, these changes are not significant in relation to the final application of the blends and pipes since the onset temperature of degradation for all samples was above 400 ◦ C.

strength to the greater capacity of UHMWPE to absorb impact energy, which is associated with the good diffuse interface of the two polymers in the blend samples. Lim et al. [4] established that due to its high tenacity, UHMWPE particles can counteract the force applied at the time of the application of impact energy, thereby slowing the spread of cracks. 3.3. Morphological analysis of cryofractured samples The morphologies of the fractured samples under liquid nitrogen obtained by SEM under different magnifications are shown

3.2. Mechanical testing The addition of UHMWPE to the HDPE provided an increase in its tensile strength at break, strength at yielding, and elongation at yielding. A decrease in rupture strain was also observed, as shown in Figs. 1 and 2. The tensile strength increased by about 30% in the blend containing 30% UHMWPE by weight. The increase in the tensile strength at yielding and at deformation at yielding was around 20%. The increase in these properties can be explained by the good compatibility between the two polymers, which can be evidenced by the good interface observed in the micrographs obtained by SEM, as will be discussed below. The Young’s modulus of the blends slightly decreased with the addition of up to 20% of UHMWPE by weight, equaling that of pure HDPE when the UHMWPE content reached 30%, as shown in Fig. 3. The results of the flexural strength tests are shown in Fig. 4 (no significant changes were observed). Fig. 5 presents the results of the impact resistance tests. An increase in impact strength of the blend samples was observed in comparison to the pure HDPE. These results are similar to those found by Boscoleto et al. [18], who attributed this increase in impact

Table 3 Effect of UHMWPE content on the onset and peak crystallization temperatures and crystallinity degree for HDPE/UHMWPE blends. Samples

Tonset (◦ C)

Tc,peak (◦ C)

Hm (J/g)

a (%)

100/0 90/10 80/20 70/30

121.4 120.7 122.4 121.8

119.6 116.0 117.0 113.5

210.6 203.7 196.2 201.4

73.3 70.9 68.3 70.1

a

0 Assuming Hm,PE = 287.3 J/g [7].

Fig. 1. Effects of UHMWPE content on (a) tensile strength and (b) elongation at break.


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Fig. 4. Effects of UHMWPE content on the (a) flexural tensile and (b) flexural modulus of the HDPE/UHMWPE blends. Fig. 2. Effects of UHMWPE content on (a) tensile strength and (b) elongation at yield.

Fig. 3. Effects of UHMWPE content on the Young’s modulus.

Fig. 5. Effects of UHMWPE content on the impact resistance of the HDPE/UHWMPE blends.

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Fig. 6. SEM micrograph images of the fractured surfaces of HDPE and HDPE/UHMWPE blends. (a) HDPE, (b) HDPE/UHMWPE-10 (c) HDPE/UHMWPE-20, and (d) HDPE/UHMWPE-30.

in Fig. 6. There is clearly a presence of two phases, which are the dispersed UHMWPE particles in the HDPE matrix. It was also observed that even at temperatures below room temperature, the sample fractures occurred through the central region of some of the UHMWPE particles, breaking them in half, and not around the interface between HDPE and UHMWPE, indicating a very resistant interface. According to Boscoleto et al. [18], an amount of up to 3% of UHMWPE by weight can be solubilized in the HDPE matrix via mixing in the molten state. His dissolution may be responsible for the good interface that has

been verified and, therefore, the good impact resistance and other mechanical properties of the blend samples. Fig. 7a and b presents in detail the cryogenic fractures of some of the UHMWPE particles. The presence of concentric rings can be observed, as was observed by Boscoleto et al. [18] in samples fractured under impact at room temperature, confirming the model proposed by the authors (presented in Fig. 7b) that the fracture of these particles occurs from the outside, with a significant contribution by UHMWPE in absorbing energy and resisting crack propagation.


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Fig. 7. (a) Details of the fracture of UHMWPE particles, showing circular rings and (b) the mechanism proposed by Boscoleto et al. [18] to explain the existence of such rings, as well as the increased fracture tenacity in the presence of UHMWPE.

3.4. Abrasive wear The abrasion tests (according to the DIN53516 standard) revealed a significant improvement in abrasion resistance of the blends compared to pure HDPE, as shown in Fig. 8. A very good linear trend of the data was obtained (R = −0.99) for concentrations of up to 30% UHMWPE. A synergistic effect was also observed, with the volumetric loss of the obtained blends being experimentally

Fig. 8. Effect of UHMWPE content on the wear properties of the blends.

Fig. 9. Comparison of experimental results obtained for the volumetric losses of the blend samples with those derived from the mixtures law.

less than that due to the simple Mixture Law, considering the reference value for the volumetric loss of pure UHMWPE (about 54 mm3 [16]). This comparison is shown in Fig. 9. This synergy comes from the good interface between the two phases, as observed in previously presented micrographs obtained by SEM, which hinders the pullout of entire UHMWPE particles, allowing this phase to contribute to the excellent resistance of the material to abrasive wear.

Fig. 10. Volumetric loss as a function of the impact resistance of the samples.

Fig. 11. Volumetric loss as a function of the product between the tensile strength at yielding and elongation at yielding, y εy .

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In order to correlate the abrasion resistance of the HDPE/ UHMWPE blends with their mechanical properties, a graph was constructed in Fig. 10 of volume loss as a function of impact resistance of the samples. A quasi-linear negative relationship was observed (coefficient of adjustment 0.91), i.e., the samples with greater abrasion resistance, or lower volume loss, are those that have a greater impact resistance. This result is in accordance with the provisions of Budinski [17], who predicted that abrasion resistance is related to the ability of the sample to deform plastically before being pulled out by the abrasive particle. Fig. 11 shows that the volumetric loss depends on the product between the tensile strength at yielding and the elongation at yielding, as proposed by Lancaster and Tadner [1], since these values represent the material’s resistance to plastic deformation before

they begin to be pulled out of the sample under abrasive wear. As can be seen, there is a reasonable inverse linear relationship, as proposed by the equation; that is, the higher the product value of y εy , the greater the abrasion resistance of the sample and the lower its volumetric loss. This result is related to the tenacity of the material, i.e., its ability to absorb energy before being deformed and pulled out. It is noteworthy that all samples had the same Shore D Hardness of 65 ± 1, revealing that this property cannot be used for a comparison with the abrasive wear behavior of these samples. Fig. 12 shows the micrographs of the samples after abrasive wear testing. It is not possible to distinguish between HDPE and UHMWPE phases. Besides scratches (scratches, gouges and scoring marks), the images of the worn surfaces show the formation of fibrils oriented in the direction of sliding. The number of fibrils in

Fig. 12. SEM images of wear surfaces of HDPE and HDPE/UHMWPE blends: (a) HDPE, (b) HDPE/UHMWPE-20, and (c) HDPE/UHMWPE-30.

Fig. 13. SEM images of wear surfaces of HDPE and HDPE/UHMWPE blends: (a) HDPE, (b) HDPE/UHMWPE-20, and (c) HDPE/UHMWPE-30.


samples of pure HDPE in comparison with that of samples of the HDPE/UHMWPE blends appears to be lower. The high molecular weight of UHMWPE may increase the resistance of these fibrils, thereby increasing the energy required to deform them and take them out of the surface being eroded, thus reducing the volumetric loss of samples, as observed in abrasive wear tests. Fig. 13 shows the circular highlighted regions of the micrographs shown in Fig. 12, at a higher magnification. It can be observed that the fibrils formed in blends are more branched into microfibrils with smaller diameters compared to those of pure HDPE, as indicated by the arrows in Fig. 12b and c. 4. Conclusions From the results presented in this study, the following conclusions can be made: 1. The addition of UHMWPE to HDPE improves its abrasion resistance to abrasive paper (sandpaper) in accordance with the DIN53516 standard. The wear mechanism involves the formation of fibrils, and this process is stronger and contains higher levels of branching in samples containing UHMWPE, as observed by SEM. 2. The mechanical properties were also improved, including the Izod impact strength, tensile strength, and deformation at yielding. 3. An inverse relationship was established between volumetric loss and the product of tensile strength at yielding and deformation at yielding, y εy , as proposed by Ratner and Lancaster [1], i.e., it takes more energy to deform and wear out samples containing UHMWPE. 4. No correlation between the hardness of the samples and other properties was obtained since the hardness remained unchanged. 5. Morphological analysis by SEM of the samples after cryogenic fracture showed separation of the HDPE (matrix) and UHMWPE (dispersed phase) phases and the presence of a good interface. Acknowledgments The authors would like to thank the Brazilian Foundation CNPq for financial support by means of Grant CT-PETRO Nr 50.4994/20043. A.A. Lucas would also like to acknowledge the scholarship registered by grant number 360344/2005-4, from CNPq. Finally, we are grateful to Hausthene Polyurethanes Company, SP, Brazil,

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