Oxidized (GO) and expanded (G-Exp) graphite were employed to prepare composites with ultrahigh molecular weight polyethylene (UHMWPE) matrix using masterbatches of polyethylene with different compositions. The materials and a blend of UHMWPE/HDPE were prepared by extrusion and their properties were evaluated. The effect of carbon fillers on the crystalline structure, thermo dynamic-mechanical (DMTA) and thermal properties (melting and crystallization temperatures) of the composites were discussed. The thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) measurements showed that the addition of masterbatch with GO and G-Exp significantly increased the crystallite size of composites, increasing the temperatures of melting, degradation, glass transition and the degree of crystallinity of polyethylene. DMTA analysis indicated the storage and loss moduli of composites in relation to neat UHMWPE, the blend and UHMWPE/composites. SEM micrographs showed a flatter, continuous and uniform surface meaning a compact lamellar structure. The present work resulted in interesting findings on the effects of GO on the crystalline structures, mechanical and thermal properties of UHMWPE, which can lead to generalizations useful for future work.
Ultra high molecular weight polyethylene (UHMWPE) possesses numerous excellent material characteristics, including outstanding chemical, abrasion, wear and impact resistance, and it is biocompatible. It has extremely long chains, with a molar mass usually between 3.5 and 7.5 million g/mol. Its main existing applications are in lightweight high-strength fibers, wire and cables, and in the biomedical area, as implants and joint replacement components. However, it is difficult to process pure UHMWPE by injection molding, blow molding, or conventional screw extrusion [
Polymer composites and nanocomposites with improved mechanical, electrical or thermal properties have been investigated intensively, and they have become one of the important classes of materials [
Focke et al. proposed the use of expanded graphite, which is prepared by partial oxidation of the graphite flakes with simultaneous intercalation (i.e., insertion) of charge- neutralizing guest species (e.g., sulfuric acid anions) in-between the stacked graphene layers. Upon exposure to high temperatures, the intercalated guest molecules decompose into gaseous species that make the graphene sheets to expand rapidly in a worm- like morphology [
According to Lu et al., the exceptional thermomechanical and electrical properties of graphitic materials have led to their incorporation into a variety of polymer matrices to form high performance composites [
In the present work, we prepared masterbatches of HDPE/GO, HDPE-g-MA/GO and HDPE/G-Exp containing 10 wt% of carbon particles and then we produced composites adding theses masterbatches to UHMWPE. The final content of graphitic fillers on matrix (UHMWPE) was fixed at 0.5 wt%. Mixtures were prepared in a twin-screw extruder. The present work resulted in interesting findings on the effects of GO on the crystalline structures, mechanical and thermal properties of UHMWPE, as well, the dispersion and distribution of the fillers which can lead to generalizations useful for future work.
Antioxidant Irganox 1010 was supplied from CIBA-GEIGY, UHMWPE was obtained from Sigma-Aldrich, Mw = 3000.000 - 6000.000, HDPE was provided by Polimeri Europa (Eraclene® MP 90), and the compatibilizer maleated polyethylene (HDPE-g-MA) by DuPont Packaging & Industrial Polymers (Fusabond® E100). Expanded graphite (G-Exp) type 9850300 LTE, was supplied by FAIMA, Italy. Natural graphite flakes were provided by the Nacional de Grafite Ltda, Brazil, with average particle size around 150 mm and carbon contents between 87% and 99%. Concentrated sulfuric acid, concentrated phosphoric acid, hydrogen peroxide 30%, potassium permanganate and ethanol were supplied by Sigma-Aldrich.
Two graphite fillers were employed: oxidized graphite (GO) and expanded GO (G-Exp). GO was obtained similarly to the Hummers’ method [
A 9:1 mixture of concentrated H2SO4/H3PO4 (360:40 mL) was added to a mixture of graphite flakes (3.0 g, 1 wt. equiv.) and KMnO4 (18.0 g, 6 wt. equiv.). The reaction was then heated to 80˚C and stirred for 24 h. The product was cooled to room temperature and drops of 30% H2O2 (3 mL) was poured under ice bath. The mixture was sifted through a metal sieve set (U.S. Standard testing sieve, W.S. Tyler, 200 µm). Then the filtrate was centrifuged (4000 rpm for 4 h), and the supernatant was decanted away. The remaining solid material was then washed in succession with 200 mL of water until pH 7.0. The sample was sonicated for 30 minutes in 200 mL of ethanol. The solid obtained was dried under vacuum overnight at room temperature, obtaining 0.300 g of product.
Expanded GO (G-Exp) was commercially obtained and used without further treatment.
5 g of UHMWPE was mixed with 0.2 wt% stabilizer (Irganox 1010) and charged into a double-screw mini extruder (Haake Minilab) in contra-rotative mode, at 200˚C and 20 rpm. It was not possible to push the entire polymer sample into the bypass channel as, during the charging of the powder, the maximum allowed pressure inside the extruder was reached and the machine stopped. It was not possible to charge more than 2.5 g into the extruder. The permanence time into the extruder was very short. A direct extrusion (“flush” mode) was also tested, however the polymer comes out of the extruder not well mixed (like a sintered powder).
5 g of a blend of UHMWPE with 5 wt% of high density polyethylene (UHMW + 5% HDPE) and antioxidant was prepared at the same conditions (200˚C, 20 rpm) and used as a reference material. It was possible to load up to 3 grams of this blend into the extruder, without reaching the alarm pressure, so that the rotation of the screws was maintained for 10 minutes. Recirculation was still not entirely obtained but the polymer entered the bypass channel and at the end of the screws, it was fairly well molten and homogeneous.
Three masterbatches (BATCH1 to BATCH3) were prepared (
These masterbatches were pelletized and mixed (at a concentration of 5 wt%) with UHMWPE, loading 3 g at 200˚C, 20 rpm, for 10 minutes. Recirculation of these materials in the extruder was still not well achieved. The final materials contained 0.5 wt% of carbon fillers (GO or G-Exp). The materials obtained were characterized by WAXD, DMTA, TGA, DSC and SEM analyses.
The equipment used was a Rigaku model Miniflex, scattering profile at the angles 2θ = 2˚ - 35˚ with a radiation wavelength of CuKα, 0.154 nm. Full width at half maximum (FWHM) was obtained according literature [
To characterize the effect of fillers on crystalline structure of masterbatches, the average crystallite size (L) in the direction perpendicular to the set of lattice planes was calculated by the Scherrer Equation [
where β is full width at half maximum (FWHM) of the related peak, K ~ 0.9 is the constant crystal lattice, λ = 0.154 nm is the X-ray wavelength of CuKα, θ is Bragg angle and L is average crystallite size, a correlation between the peak broadening and the crystallite size can be obtained.
The space between different diffraction planes (d) was obtained by Bragg’s equation [
Code | HDPE wt% | HDPE-g-MA wt% | GO wt% | G-Exp wt% |
---|---|---|---|---|
BATCH1 | 90 | 10 | ||
BATCH2 | 90 | 10 | ||
BATCH3 | 90 | 10 |
where d is interplanar distance between layers, λ is the wavelength of X-ray, θ is Bragg angle.
The thermal stability of the materials was analyzed using TG analysis. For the determination of the initial degradation temperature (Tonset) and that at the maximum degradation rate, Tmax, the samples were analyzed under nitrogen, from room temperature to 700˚C at a heating rate of 10˚C/min and samples weighed to 6 - 7 mg, according to the method of Martinez-Morlanes et al. [
The Instrument was a DMA Q800 V7.5 Build 127. Module DMA multi-frequency- strain, run serial 1398. The analysis conditions were: clamp tension: Film. Geometry rectangular. Sample size (length, width, thickness): 13.26 × 6.29 × 1.51 mm. Frequency torsional load: 1 Hz. Heating rate: 3˚C/min. Range of temperature: from −140˚C to 150˚C.
DSC apparatus was a TA Instruments Q-1000. The experiments were performed under nitrogen atmosphere at a rate of 80 ml/min and samples weighed between 6 and 7 mg. Samples were subjected to a heating-cooling-heating scan to eliminate thermal history between room temperature and 200˚C at 10˚C/min. The crystallinity of the samples was calculated according to Equation:
where ΔH is the total heat energy per mass unit and ΔH100 is the enthalpy of fusion of 100% crystalline polyethylene, 289 J/g [
After extrusion of the masterbatches, the samples were characterized by WAXD (
lated to polyethylene crystals are considerable broader than those in the other masterbatches because the grafting of maleic anhydride decreases the crystallinity of HDPE [
The structural parameters of crystals in GO, G-Exp and the masterbatches were obtained from the WAXD patterns and are listed in
The 2θ values of (110), (200) and (002) crystal planes in
These masterbatches were pelletized and mixed with UHMWPE at 200˚C, 20 rpm for 10 minutes. The final materials contained 0.5 wt% of carbon particles (GO or G-Exp).
The WAXD data were used to evaluate the organization of the UHMWPE chains after extrusion. Each peak was fitted with the best Gaussian distribution and its full-width at half-maximum (FWHM) were calculated.
According to
Sample | 2θ110 (˚) | 2θ200 (˚) | 2θ002 (˚) | d110 (nm) | d200 (nm) | d002 (nm) | L110 (nm) | L200 (nm) | L002 (nm) | β110 (nm) | β200 (nm) | β002 (nm) |
---|---|---|---|---|---|---|---|---|---|---|---|---|
G-Exp | - | - | 26.15 | - | - | 0.34 | - | - | 5.12 | - | - | 1.7720 |
GO | - | - | 25.61 | - | - | 0.35 | - | - | 4.24 | - | - | 2.1973 |
BATCH1 | 21.71 | 24.06 | 26.39 | 0.41 | 0.37 | 0.34 | 22.50 | 18.61 | 5.73 | 0.3997 | 0.4853 | 1.5844 |
BATCH2 | 22.38 | 24.78 | 27.24 | 0.40 | 0.36 | 0.33 | 9.32 | 13.19 | 2.40 | 0.9658 | 0.6858 | 3.7854 |
BATCH3 | 21.63 | 23.98 | 26.61 | 0.41 | 0.42 | 0.34 | 24.44 | 21.46 | 33.81 | 0.3680 | 0.4208 | 0.2685 |
Sample | 2θ110 (˚) | 2θ200 (˚) | dl110 (nm) | d200 (nm) | L110 (nm) | L200 (nm) | β110 (nm) | β200 (nm) |
---|---|---|---|---|---|---|---|---|
NEAT UHMWPE | 21.75 | 23.92 | 0.41 | 0.36 | 6.42 | 8.64 | 1.4006 | 1.0448 |
UHMW + 5% HDPE | 22.23 | 24.58 | 0.39 | 0.36 | 13.20 | 12.53 | 0.6820 | 0.7216 |
UHMWPE/BATCH1 | 22.21 | 24.50 | 0.40 | 0.36 | 11.74 | 12.44 | 0.7670 | 0.7269 |
UHMWPE/BATCH2 | 22.39 | 24.69 | 0.39 | 0.36 | 13.88 | 13.16 | 0.6478 | 0.6864 |
UHMWPE/BATCH3 | 22.08 | 24.64 | 0.40 | 0.36 | 17.03 | 14.82 | 0.5277 | 0.6091 |
Both 2θ values of (110) and (200) crystal planes of polyethylene (
The loss modulus plots for the neat UHMWPE, blend UHMW + 5% HDPE and the composites UHMWPE/carbon particles are presented in
Sample | γ-relaxation (˚C) | β-relaxation (˚C) | α-relaxation (˚C) |
---|---|---|---|
NEAT UHMWPE | −106 | −9 | 62 |
UHMW + 5% HDPE | −106 | −24 | 61 |
UHMWPE/BATCH1 | −107 | −48 | 60 |
UHMWPE/BATCH2 | −97 | −31 | 62 |
UHMWPE/BATCH3 | −98 | −38 | 61 |
may be due to the low adhesion of carbon fillers in the polyethylene matrix. On the other hand, both UHMWPE/BATCH2 and UHMWPE/BATCH3 presented higher Tg. Probably, the nanofiller occupied the free volume in the amorphous region decreasing molecular motion in the amorphous phase.
In literature, the β-relaxation peak normally appears in the range of −30˚C to 10˚C [
Kawai et al. have discussed the β-relaxation as assigned to interlamellar grain boundary phenomena associated with orientational and distortional dispersions of noncrystalline materials between oriented lamellae [
Therefore, it is reasonable to assign β-relaxation around −9˚C (
The α-relaxation of polyethylene is the result of motions or deformations within the interfacial regions (tie molecules, folds, loops, etc.), which are activated because of chain mobility in the crystalline region [
In
The storage modulus of materials was plotted as a function of temperature (
value of storage modulus indicates the material’s ability to store the energy of external forces without permanent strain deformation. Therefore, higher storage modulus is associated with a higher elastic property of material [
The damping factor (Tanδ), being the ratio of the dynamic loss (or viscous) to the storage (or elastic) moduli, provides information on the relative contributions of the viscous and elastic components of a viscoelastic material [
The thermal stability of composites was evaluated by thermogravimetric analysis. The data are presented in
The TG profiles apparently showed similar behavior for the materials with one degradation event. The values of weight loss, Tonset and Tmax are presented in
The initial thermal decomposition temperature of UHMWPE decreased in the blend with HDPE, due to the lower molar mass of HDPE. The prepared composites of UHMWPE have slightly increased Tonset in relation to the polymer blend. The com-
Sample | aStorage Modulus (MPa) | aLoss Modulus (MPa) | aTanδ |
---|---|---|---|
NEAT UHMWPE | 958 46.3 | 0.048 | |
UHMW + 5% HDPE | 1184 113.7 | 0.096 | |
UHMWPE/BATCH1 | 963 78.2 | 0.080 | |
UHMWPE/BATCH2 | 1078 55.5 | 0.051 | |
UHMWPE/BATCH3 | 1052 67.8 | 0.064 |
aRoom temperature.
Sample | Weight loss (%) | Tonset (˚C) | Tmax (˚C) |
---|---|---|---|
NEAT UHMWPE | 100 | 447.3 | 459.5 |
UHMW + 5% HDPE | 100 | 442.3 | 458.9 |
UHMWPE/BATCH1 | 99.9 | 444.2 | 460.2 |
UHMWPE/BATCH2 | 100 | 443.9 | 461.4 |
UHMWPE/BATCH3 | 99.7 | 443.2 | 460.8 |
posite UHMWPE/BATCH2, with HDPE-g-MA, has a relatively higher Tmax, and therefore, higher heat capacity and thermal conductivity than neat UHMWPE.
The Tm peaks of the all composites are slightly higher than for neat UHMWPE. This can be attributed to the recrystallization of imperfect lamellae in the composites to larger crystals as the samples are being cooled. This led to a slight increase in Tm value [
Previous studies have shown that the improvement of UHMWPE properties is influenced by its degree of crystallinity. In the case of polymer composites, it has been hypothesized that the addition of particles in the preparation of UHMWPE composites can positively affect the degree of crystallinity through nucleation and crystal growing processes. However, if the filler content is not optimum, the fillers start to act as obstacles, hindering the mobility of polymer chains in the crystal growth, and leading to a lower degree of crystallinity [
Sample | Tm (˚C) | Tc (˚C) | Xc (%) DSC | Xc (%) XRD |
---|---|---|---|---|
NEAT UHMWPE | 131.6 | 120.3 | 40.0 | 67.5 |
UHMW + 5% HDPE | 133.6 | 118.1 | 56.7 | 67.2 |
UHMWPE/BATCH1 | 133.2 | 118.4 | 55.5 | 66.5 |
UHMWPE/BATCH2 | 132.7 | 118.7 | 40.2 | 67.6 |
UHMWPE/BATCH3 | 132.9 | 118.7 | 51.4 | 71.4 |
This occurs especially in the samples of neat UHMWPE and the composite with HDPE-g-MA. Moreover, the highest Xc observed by WAXD measurements was obtained in sample UHMWPE/BATCH3, containing G-Exp. In this study, the addition of graphitic fillers was fixed at 0.5 wt% to optimize the mixing technique. However, it was not sufficient to improve the mechanical property in comparison with the blend UHMW + 5% HDPE, which showed the highest storage modulus at room temperature.
As can be seen in
The fractured surface significantly changed with the GO addition. The fractured surface changed for UHMWPE/BATCH1 and UHMWPE/BATCH2 to become flatter and there emerged some filaments in a mesh structure, which is due to the interaction between GO and UHMWPE matrix. The same behavior was observed in UHMWPE BATCH3 (with G-exp and HDPE), which became the most continuous, uniform and compact surface.
Three differently masterbatches were prepared by employing oxidized and expanded graphitic materials (GO and G-Exp) as fillers. Through WAXD it was observed that masterbatch with 10 wt% of G-Exp (BATCH3) showed the highest crystallite size for the planes (110), (200) of polyethylene crystals, as well as for the plane (002) of the G-Exp graphitic filler. Apparently, the presence of G-Exp filler significantly augmented the average crystallite size of polyethylene. The glass transition temperature (Tγ) increased significantly for UHMWPE containing HDPE-g-MA/GO (BATCH2) and HDPE/G-Exp (BATCH3) in comparison with neat UHMWPE. However, β-relaxation temperature sharply reduced. It is possible to affirm that the lower crystals size could affect the α-relaxation behavior of the composite UHMWPE/BATCH1 that showed lower crystallite size and therefore, the lowest α-relaxation temperature. The blend UHMW + 5% HDPE presented high value of storage modulus and low Tanδ at room temperature determined by DMTA due to the higher crystallinity of UHMWPE in the presence of HDPE. In particular, the loss modulus of the blend presented the highest value at room temperature, and diminished for UHMWPE/BATCH2 and UHMWPE/ BATCH3 indicating the reinforcement of graphitic fillers in the polyethylene. Moreover, the degradation temperature of the blend decreased, as expected, but this property was recovered for UHMWPE/BATCH2 and UHMWPE/BATCH3. In comparison with the neat UHMWPE, the blend with 5% HDPE has higher Tm while the composite UHMWPE/BATCH3 presented the highest degree of crystallinity. It was observed by SEM micrographs that this last composite presents morphology with continuous, uniform, and compact surface. Therefore, the addition of HDPE and graphitic materials is an efficient way to obtain an overall improvement on properties of UHMWPE.
This work was financially supported by the Brazilian National Council for Scientific and Technological Development (CNPq), FAPERJ (Brazil), and the European project FP7-PEOPLE-IRSES-2011-295262-VAIKUTUS. Special thanks to Dr. Maurizio Avella, Dr. Gennaro Gentile, Dr. Maria Emanuela Errico and Dr. Roberto Avolio, from IPCB-CNR, Italy.
Rocha, L.F.M., Cordeiro, S.B., Ferreira, L.C., Ramos, F.J.H. and Marques, M.F. (2016) Effect of Carbon Fillers in Ultrahigh Molecular Weight Polyethylene Matrix Prepared by Twin-Screw Extrusion. Materials Sciences and Applications, 7, 863-880. http://dx.doi.org/10.4236/msa.2016.712066