High-energy mechanical milling of ultra-high molecular weight ...

Seventh International Latin American Conference on Powder Technology, November 08-10, Atibaia, SP, Brazil

High-energy mechanical milling of ultra-high molecular weight polyethylene (UHMWPE) Melina C. Gabriel1,a, Luciana B. Mendes1,b , Benjamim de M. Carvalho1,c, Luís A. Pinheiro1,d, José D. T. Capocchi2,e, Evaldo T. Kubaski2,f and Osvaldo M. Cintho1,g 1

2

Universidade Estadual de Ponta Grossa – UEPG – Av. Carlos Cavalcanti, 4748 – Uvaranas, CEP: 84030-900, Ponta Grossa-PR, Brazil

Universidade de São Paulo – USP – Av. Prof. Luciano Gualberto, 380, travessa 3 – CEP: 05508-970 - São Paulo – SP, Brazil a

[email protected], [email protected], [email protected], d [email protected], [email protected], [email protected], [email protected]

Keywords: Ultra-high molecular weight polyethylene, high-energy mechanical milling, xray diffraction. Abstract. Ultra-high molecular weight polyethylene (UHMWPE) is a polyethylene with a very long chain, which provides excellent features, however it makes the processing difficult due to high melt viscosity. Many studies intend to found out means to make its processing easier. Recently, the high-energy mechanical milling has been used for polymeric materials and it was detected that physical and chemical changes occur during milling. In such case, powder of UHMWPE was milled in three types of mills: SPEX, attritor e planetary, in different times of milling. The polymer was characterized by SEM and XRD. Thus, it was observed that the material processed in attritor mill showed larger phase transformation from orthorhombic to monoclinic. This is most likely due to the smaller milling temperature of attritor mill when compared with the other two mills and the high shear force generated during milling.

Introduction Ultra-high molecular weight polyethylene (UHMWPE) is a polyethylene resin with a high polymerization degree. Its high molecular weight (from 3 to 8x106 g/mol) is approximately ten times that of high molecular weight high-density polyethylene (HDPE) resins [1]. UHMWPE has both the highest abrasion resistance and highest impact strength of any polymeric material. Combined with these features, the low coefficient of friction of UHMWPE yields a self-lubricating surface. Moreover, UHMWPE has a negligible water absorption, good chemical and corrosion resistance, biocompatibility and stability in the body. Its average crystallinity is 45%, approximately [1,2]. Despite of so many excellent features, UHMWPE cannot be processed by conventional techniques (as extrusion and injection molding) due to its high molecular weight and, consequently, high viscosity of flow. The most commom methods for fabrication of UHMWPE are ram extrusion and compression molding. In order to study another processing means for UHMWPE, this work proposes the high-energy mechanical milling as an intermediate process and the estimation of structural modifications caused in the material by this method. Mechanical milling (MM) is a powder processing technique that allows production of homogeneous materials starting from elemental powder mixtures and involves repeated welding, fracturing, and rewelding of powder particles in a high-energy ball mill [3]. This technique is

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widely applied to metallic systems, but recently has been studied in blending polymers, as showed Smith et al. [4,5] since the combination of shearing, extension, fracturing and cold-welding of polymeric powder may induce chain scission or hydrogen abstraction and, consequently, free radical formation. This allows the miscibility of immiscible polymers. Castricum et al. [6] presented in their work possible physical and chemical changes in polyethyleneglycol after mechanical milling and observed that, as the milling time increased, the proportion of monoclinic crystalline phase also increased. In general, polyethylene exhibits two main types of unit cells: orthorhombic and monoclinic. The orthorhombic cell is most commom. It is a cuboid, each of its axes having a different length while the angles made by adjoining faces are all 90° [7]. Monoclinic is a metastable phase formed from orthorhombic cells under elongation and deformation conditions [8-12]. It may be present to a small extent in commercial samples, from 5 to 10% [10-12]. Temperatures in excess of 60–70°C cause it to revert to the orthorhombic form [7]. Kiho et al. [9], and later Seto et al. [11], investigated the phase transformation of polyethylene. They suggested that the phase transformation is one of the most important modes of deformation and that, under certain conditions, it takes place with priority to other processes of plastic deformation. The authors also suggested that the monoclinic phase would be deriving from the orthorhombic phase involving a reversible process without diffusion in a way similar to usual twinning in metals. There are three planes on which molecules are densely packed. The most closely packed one is (001). The (001) diffraction of monoclinic phase is the most evident in x-ray diffratograms and corresponds to a d spacing of approximately 4,55 Å or a CuKα diffraction 2θ of approximately 19,5° [8,11,12]. The most evident diffractions of orthorhombic phase are at approximately 21,5°, corresponding to (110) plane and d spacing of 4,13 Å, and at 23,9°, corresponding to (200) plane and d spacing of 3,72 Å [12]. Experimental Mechanical Milling. Two types of UHMWPE reactor powder supplied by Braskem were used: UTEC® 3040 and UTEC® 4040, named P3 and P4 respectively. P3 had a molecular weight of 3 · 106 g/mol and P4 of 4,5 · 106 g/mol. Firstly, mechanical milling was made, following the steps: i) P4 were processed in SPEX mill (8000 mixer/mill, CertiPrep) with a ball-to-powder weight ratio of 7:1 (powder weight of 2,98 g) for 1, 4, 8 and 16 hours; ii) P3 were processed in planetary mill (Pulverisette 6, Fritsch) with rotation speed of 400 rpm and ball-to-powder weight ratio of 20:1 (powder weight of 20 g) for 1, 2, 4, 8, 16 and 24 hours; iii) P4 were processed in attritor mill (01HD, Union Process) with water refrigeration system, rotation speed of 500 rpm and ball-topowder weight ratio of 40:1 (powder weight of 50 g) for 5, 8, 10 and 15 hours. Characterization. Following characterization were made: 1) SEM, scanning electron microscopy – A small sample of UHMWPE was mounted onto a SEM stub and then coated with gold-palladium alloy. UHMWPE powder before and after milling were observed in a Shimadzu SSX-500 microscopy, operated at less than 5,0 kV for secondary electrons; 2) XRD, powder x-ray diffraction was done on a Shimadzu XRD-6000 diffractometer, using nickel-filtered CuKα radiation (λ = 1,5406 Å). The data were collected in the 2θ range of 15-30°, in continual scan mode with scan speed of 1°/min.

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Results and Discussion

Fig. 1 – SEM micrograph of P3 reactor powder. Fig. 2 – SEM micrograph of P3 milled in planetary mill for 8 hours.

Fig. 3 – SEM micrograph of P4 milled in SPEX Fig. 4 – SEM micrograph of P4 milled in attritor mill for 8 hours. mill for 8 hours. Fig. 1 shows the SEM micrograph for non-milled P3. It consists of almost rounded particles. It seems that each particle is itself composed of an agglomeration of smaller particles that are joined by fibrils. Fig. 2, 3 and 4 show SEM micrographs of UHMWPE processed in planetary, SPEX and attritor mills, respectively, for 8 hours. The UHMWPE particles after milling became flat, similar to flake-like shape. This is due to successive collisions between the material and the milling balls. It is observed that the polymer milled in SPEX mill (Fig. 3) shows “broken” edges and the polymer milled in planetary mill (Fig. 2) seems to be the material that bears less morphologic modifications. This is in accordance with Suryanarayana [3] that shows that the SPEX is higher energy mill, while planetary is the lower one. Fig. 5 shows x-ray diffratograms of non-milled P4 and of UHMWPE milled in planetary, SPEX and attritor mills for 8 hours. It can be noted that the diffratogram for non-milled polymer shows two most evident peaks (at approximately 21,5 and 24°) related to the orthorhombic phase. However, the milling in attritor mill generated another very evident peak, left to the main peak, approximately at 19,5°. This peak is characteristic of monoclinic structure. The milling in SPEX mill have not provided a clear appereance of monoclinic peak, while the milling in planetary mill presented a little monoclinic peak. This shows that milling in attritor mill provided a larger phase

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Intensity

transformation, while milling in SPEX mill provided smaller one, maybe due to the smaller temperature achieved into attritor drum and/or high shear forces during attrition milling.

spex 8h non-milled P4 attritor 8h planetary 8h 15

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2θ Fig. 5 – X-ray diffratogram for UHMWPE: non-milled P4 and polymer milled in planetary, SPEX and attritor mills for 8 hours.

Conclusions High-energy mechanical milling provided morphological changes in UHMWPE that varied according to the use of different mills. Powder particles became flake-like shaped. Moreover, mechanical milling caused phase transformation, as showed the x-ray diffratograms. A monoclinic peak approximately at 19,5° appeared more or less intense depending on certain conditions. As the phase transformation is favoured, (001) monoclinic diffraction intensifies while (110) and (200) orthorhombic diffractions loose intensity. The larger phase transformation occurred for the polymer milled in attritor mill, probably due to more intense shear forces between the polymer and the milling balls and/or to the smaller temperature achieved into attritor drum, since it was used a water refrigeration system during milling.

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[5] A. P. Smith; J. S. Shay; R. J. Spontak; C. M. Balik; H. Ade; S. D. Smith and C. C. Koch: Polymer Vol. 41 (2000), p. 6271. [6] H. L. Castricum; H. Yang; H. Bakker and J. H. Van Deursen: Mater. Sci. Forum Vol. 235-238 (1997), p. 211. [7] A. J. Peacock: Handbook of polyethylene: structure, properties and applications (Marcel Dekker, New York 2000). [8] P. J. Hendra; M. A. Taylor and H. A. Willis: Polymer Vol. 26 (1985), p. 1501. [9] H. Kiho; A. Peterlin and P. H. Geil: J. Appl. Phys. Vol 35 (1964), p. 1599. [10] C. Gieniewski and R. S. Moore: Macromolecules Vol. 2 (1969), p. 385. [11] T. Seto; T. Hara and K. Tanaka: J. J. Appl. Phys. Vol 7 (1968), p. 31. [12] K. E. Russell; B. K. Hunter and R. D. Heyding: Polymer Vol. 38 (1997), p. 1409.

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