WO2016005504A1

WO2016005504A1 - Composite material of uhmwpe and graphene and process for manufacturing thereof

Info

Publication number: WO2016005504A1
Application number: PCT/EP2015/065718
Authority: WO
Inventors: Sanjay Rastogi; Yogesh DESHMUKH; Sarah RONCA; Kangsheng LIU
Original assignee: Maastricht University
Priority date: 2014-07-09
Filing date: 2015-07-09
Publication date: 2016-01-14

Abstract

The present invention pertains to a process for the manufacture of a composite wherein the process comprises the following steps: i) graphene is exfoliated, ii) ultra high molecular weight polyethylene (UHMWPE) is combined with the exfoliated graphene, wherein the UHMWPE has an elastic shear modulus in the plateau region of at most 1.4 MPa and a M_w/M_n ratio of at most 15 and the amount of graphene is 0.5 to 5 wt% based on the combined dry mass of the UHMWPE and the graphene, and the invention also pertains to composites and materials comprising said composites.

Description

Composite material of UHMWPE and graphene and process for manufacturing thereof

The present invention pertains to a process for the manufacture of a composite comprising ultra high molecular weight polyethylene (UHMWPE) and graphene. The invention also pertains to the composite and materials comprising such composite.

For the production of polyolefin materials, addition of UHMWPE is advantageous because it improves the mechanical properties of the polyolefin. Also interesting are polyolefins and in particular UHMWPE composite materials which are modified by the addition of other materials, e.g. reinforcing materials such as graphene.

However, processing of UHMWPE is difficult as the viscosity increases with increasing molecular weight of the polymer. In fact, very high molecular weights of polymers correspond to high melt viscosity, due to the entanglements formed between the chains. For this reason, the most common method that has been used so far for composite preparation is hot pressing of a dried UHMWPE-filler mixture, but strong segregation of filler is usually observed along the grain boundaries of the polymer particles. Increased melt viscosity hinders the processability and necessitates higher energy input e.g. for extrusion mixing of the polymer.

Composites comprising polyethylene and graphene are known.

CN103087386A discloses that dry graphene oxide nanosheets are mixed with deionized water by ultrasonic stirring and UHMWPE and the composite is subsequently hot-pressed under nitrogen protection at a temperature of 250-280°C.

Also Tai et al. (Tribology Letters (2012), 46:55-63) disclose graphene oxide-UHMWPE composites which are produced by a hot-pressing method. In this case, the toluene- assisted mixing of the graphene is optimized. Bhattacharyya et al. (eXPRESS Polymer Letters, Vol. 8, No. 2 (2014), p. 74-84) describes two different processes to prepare nanocomposite films of UHMWPE and graphene. To improve the difficult dispersion of carbon nanotubes (CNT) in UHMWPE, the authors investigate different treatments of the graphene, either the graphene is dispersed in organic solvents and reduced before polymer is added (pre-reduction) or reduction of the graphene is carried out after polymer addition (in situ method).

WO201 1/0821 69 A1 discloses a method of dispersing nanotubes in polypropylene. First, an aqueous solution of CNT is combined with a mixture of any polymeric material in a solvent, this polymer-CNT mixture is precipitated and used as masterbatch which is subsequently mixed with polypropylene. Alternatively, the CNT are dispersed in decaline and slowly added to a hot solution of any polymer. Upon cooling, CNT co- precipitates with the polymer. The precipitate is used as masterbatch for mixing with polypropylene.

EP25971 12 A1 pertains to a process for producing a composite product comprising CNT by combination of solution and melt processing. Exfoliated graphene is dispersed and added to a polymer melt. The current invention provides a solution for improving the processability of UHMWPE and it provides a composite with improved properties.

The invention pertains to a process for the manufacture of a composite wherein the process comprises the following steps:

i) graphene is exfoliated,

ii) ultra high molecular weight polyethylene (UHMWPE) is combined with the exfoliated graphene, wherein the UHMWPE has an elastic shear modulus in the plateau region of at most 1 .4 MPa and a M_w/M_n ratio of at most 15 and the amount of graphene is 0.5 to 5 wt% based on the combined dry mass of the UHMWPE and the graphene. In one embodiment, the amount of graphene is 0.5 to 3 wt% based on the combined dry mass of the UHMWPE and the graphene, or in another embodiment the amount of graphene is 0.5 to 3 wt% based on the dry mass of the composite. The prior art either does not concern UHMWPE for which it is particularly difficult to produce a homogeneous mix with graphene or the prior art describes changes to the process. None of the prior art documents discloses to select a UHMWPE with specific properties to improve the process of dispersing fillers in UHMWPE and to provide improved composites and materials.

Therefore, there is still room for improvement or at least room to provide alternative processes and composites.

In one embodiment, the invention pertains to a process for the manufacture of a composite comprising ultra high molecular weight polyethylene wherein the process is characterized in that graphene is utilized and the process comprises the following steps i) graphene is exfoliated,

ii) ultra high molecular weight polyethylene is combined with the exfoliated graphene to result in a first composite comprising an amount of 0.5 to 3 wt% of graphene based on the dry mass of the composite, wherein the ultra high molecular weight polyethylene has an elastic shear modulus, in the plateau region, of at most 8, which is measured just after the first melting of the as synthesized material.

Optionally and in a further aspect of the invention, the composite is mixed, e.g. in an extruder, with a polyolefin to result in a material comprising an amount of 98 to 2 wt% of the composite and 2 to 98 wt% of a polyolefin, based on the dry mass of the material, preferably the material comprises an amount of 20 to 2 wt% of the composite and 80-98 wt% of a polyolefin.

The ultra high molecular weight polyethylene (referred to as UHMWPE) has an average molecular weight (M_w) of at least 500 000 gram/mol (g/mol), in general between 0.5x10⁶ g/mol and 5x10⁸ g/mol, preferably at least 1 x10⁶ g/mol. The molecular weight distribution and molecular weight averages (M_w, M_n, M_z) of the polymer are determined in accordance with ASTM D 6474-99 at a temperature of 1 60°C using 1 ,2,4- trichlorobenzene (TCB) as solvent. Appropriate chromatographic equipment (PL- GPC220 from Polymer Laboratories) including a high temperature sample preparation device (PL-SP260) may be used. The system is calibrated using sixteen polystyrene standards (M_w/Mn <1 .1 ) in the molecular weight range 5x10³ to 8x10⁶ gram/mol. For molecular weights above 1 x10⁶ g/mol the method described in S Talebi, R Duchateau, S Rastogi, J Kaschta, GWM Peters, PJ Lemstra; Macromolecules 2010; 43 (6); 2780- 2788 may be used.

The UHMWPE used in current invention is a disentangled polyethylene, that means a polyethylene having a low degree of chain entanglements. Specifically selecting disentangled UHMWPE for the process and composite of the invention has unexpected advantages with regard to the processability of the composite. Surprisingly, when such disentangled UHMWPE - instead of regular UHMWPE - is used to manufacture the composite comprising UHMWPE and graphene, the processing is improved and the graphene may be well dispersed. Therefore, the process of this invention is different from the prior art where instead other composite components are adapted or specific process steps are introduced instead of specifically selecting disentangled UHMWPE as a component in the process. Disentangled UHMWPE and processes to manufacture disentangled UHMWPE are known and have been described, e.g. in WO2009/109632, p. 21 -23, in Rastogi et al., Macromolecules (201 1 ), 44, p. 5558-5568 and in Ronca et al. Advances in Polymer Technology (2012), 31 (3), 193-204 which references are expressly incorporated herein. Disentangled UHMWPE usually is synthesised by using a single-site catalytic system.

Disentangled UHMWPE is defined as a polyethylene with the stated average molecular weight, a relatively narrow molecular weight distribution (M_w/M_n ratio) of at most 15 in combination with an elastic shear modulus, in the plateau region, of at most 1 .4 MPa (obtained just after the first melting of the compressed powder), and the fact that the elastic shear modulus of the material increases after first melting. Disentangled UHMWPE has an elastic shear modulus ^N determined directly after melting at 1 60^QC of at most 1 .4 MPa, more in particular at most 1 .0 MPa, still more in particular at most 0.9 MPa, even more in particular at most 0.8 MPa, and even more in particular at most 0.7. The wording "directly after melting" means that the elastic shear modulus is determined as soon as the polymer has melted, in particular within 1 5 minutes after the polymer has melted. For this polymer melt ^N° typically increases from 0.6 to 2.0 MPa in one, two, or more hours, depending on the molar mass. The elastic shear modulus determined directly after melting at 1 60^QC is one of the characterizing features of the disentangled UHMWPE which is used in the present invention. N° is the elastic shear modulus in the rubbery plateau region. It is related to the average molecular weight between entanglements (M_e), which in turn is inversely proportional to the entanglement density. In a thermodynamically stable melt having a homogeneous distribution of entanglements, M_e can be calculated from ^N° via the formula ^{GN ~} S^NP^RT I ^M _{E ; w}here ⁸ⁿ is a numerical factor set at 1 , p is the density in g/cm³, R is the gas constant and T is the absolute temperature in K.

A low elastic shear modulus thus stands for long stretches of polymer between entanglements, and thus for a low degree of entanglement. The adopted method for the investigation on changes in ^N° with the entanglements formation is the same as described in Rastogi, S., Lippits, D., Peters, G., Graf, R., Yefeng, Y. and Spiess, H., "Heterogeneity in Polymer Melts from Melting of Polymer Crystals", Nature Materials, 4(8), August 2005, 635-641 and PhD thesis Lippits, D. R., "Controlling the melting kinetics of polymers; a route to a new melt state", Eindhoven University of Technology, 2007, ISBN 978-90-386-0895-2.

The elastic shear modulus determined directly after melting at 1 60°C is a property of the disentangled UHMWPE which is used as a component or starting material in the process of current invention. A UHMWPE polymer specifically having this property has been selected for this invention and results in composites with surprising advantages compared to composites comprising regular, more entangled UHMWPE and

disentangled UHMWPE without graphene. In particular, the disentangled UHMWPE composite of the invention comprising graphene shows a decrease in storage modulus, which indicates better processability of the composite. Furthermore, the composites of the invention remain (largely) in a disentangled state even over long annealing times at high temperatures, whereas the polymer chains of disentangled UHMWPE without graphene entangle under the same conditions, which is demonstrated by the slower build-up of the storage modulus during annealing in the composites of the invention when compared to disentangled UHMWPE without graphene, and the much slower decline of the second melting peak enthalpy of the composites of the invention during annealing, when compared to disentangled UHMWPE without graphene. Also, the composites of the instant invention have a lower melt viscosity than disentangled UHMWPE without graphene. Therefore, the

composites are easier to process, requiring less energy input during processing, and allowing processing at a lower temperature.

Therefore, the combination of disentangled UHMWPE and graphene results in a composite having surprising advantages over regular, more entangled UHMWPE (with or without graphene) and also over disentangled UHMWPE without graphene.

The molecular weight distribution of the UHMWPE present in the composite according to the invention is relatively narrow. This is expressed by the M_w (weight average molecular weight) over M_n (number average molecular weight) ratio. The disentangled polyethylene used as UHMWPE in the present invention has a M_w/M_n ratio of at most 15, particularly at most 10, in particular at most 8, in particular at most 4, still more in particular at most 3, even more in particular at most 2. The number average molecular weight (M_n) of the UHMWPE may be between 50 000 g/mol and 2 000 000 g/mol.

The disentangled UHMWPE used in the process according to the invention can be a homopolymer of ethylene or a copolymer of ethylene with a co-monomer which is another alpha-olefin or a cyclic olefin both with generally between 3 and 20 carbon atoms. Examples include propene, 1 -butene, 1 -pentene, 1 -hexene, 1 -heptene, 1 -octene, cyclohexene, etc. The use of dienes with up to 20 carbon atoms is also possible, e.g., butadiene or 1 -4 hexadiene. The amount of (non-ethylene) alpha-olefin in the ethylene homopolymer or copolymer used in the process according to the invention preferably is at most 10 mol%, preferably at most 5 mol%, more preferably at most 1 mol%. If a (non- ethylene) alpha-olefin is used, it is generally present in an amount of at least 0.001 mol%, in particular at least 0.01 mol%, still more in particular at least 0.1 mol%.

The disentangled UHMWPE is combined with exfoliated graphene, preferably exfoliated graphene oxide (GO).

During the process to manufacture the composite, the graphene may be reduced by a thermal treatment to result in reduced graphene oxide (rGO).

For exfoliation, for example, graphene, preferably graphene oxide (GO) is agitated in a watery solution, preferably in water.

Graphene is pure carbon in the form of a very thin, nearly transparent sheet. Graphene has excellent mechanical (modulus - 1060 GPa, strength - 20 GPa) properties, compared to other nanoparticles.

Graphene oxide is the preferred nanoparticle used in the present invention. Besides graphene, also other carbon nanomaterials may be used. Carbon nanomaterials include at least one of single-walled nanotubes, double-walled nanotubes, triple-walled nanotubes, multi-walled nanotubes, ultra-short nanotubes, graphene, graphene nanoribbons, graphite, graphite oxide nanoribbons, carbon black, oxidized carbon black, hydrophilic carbon clusters, and combinations thereof.

In one embodiment, graphene oxide (GO) is exfoliated in water by agitation, and subsequently UHMWPE is combined with the water-GO solution, e.g. by adding the polymer as a powder, or a dispersion of the polymer in a solvent or by spraying the GO- water solution on polymer powder.

Subsequently, GO and disentangled UHMWPE are mixed and homogenized (e.g. by sonication), water is removed and the first composite is obtained. The amount of graphene is 0.5 to 5 wt% based on the combined dry mass of the UHMWPE and the graphene. In one embodiment the amount of graphene is 0.5 to 3 wt% of graphene based on the dry mass of the composite, i.e. the dry mixture of UHMWPE and graphene after water and/or solvent have been removed. The amount preferably is between 0.5 and 3 wt% based on the combined dry mass of the UHMWPE and the graphene or alternatively on the dry mass of the composite. The amount preferably is chosen from a concentration of 0.5 to 2 wt%, more preferably from a concentration of 0.8 to 1 .5 wt%. In one embodiment at least one further filler is added in the process according to the invention. This filler is different from the graphene added in the process of the invention. The at least one filler may be selected from inorganic or organic compounds. Fillers in powder, particulate or fibrous form may be used. Examples of inorganic fillers are schungite, kaolin, chalk and talcum. As organic filler, a polyolefin, e.g. high density polyethylene (HDPE) may be used.

HDPE is a polyethylene having a Mw below 500 000 g/mole.

The amount of the at least one further filler may be 0.01 - 40 wt% based on the dry mass of the composite.

The disentangled UHMWPE, the graphene and the at least one further filler may be combined in different ways. In one embodiment, the UHMWPE is combined with the least one filler and the combination is subsequently combined with the exfoliated graphene. Alternatively, the UHMWPE, the exfoliated graphene and the at least one filler are combined in one step, i.e. in step ii) of the process of current invention.

The composite according to the invention comprising UHMWPE and graphene may be thermally treated.

The thermal treatment is carried out at a temperature of 150-250°C, optionally under pressure. For the disentangled UHMWPE of instant invention the temperature preferably is chosen from 1 60-200°C. If pressure is applied, a pressure of at least 1 ton for at least 1 minute is used.

Surprisingly, when disentangled UHMWPE is combined with an amount of graphene or GO in the composite of between 0.5 to 5 wt%, the plateau value of the elastic shear modulus ^N° of the composite at a specific concentration of GO decreases. The decrease in the elastic shear modulus ^N° corresponds to a decrease in the intrinsic viscosity of the first composite. That means that the UHMWPE polymer composite obtained by using disentangled UHMWPE is easier to process.

The decrease in modulus occurs when adding a relatively low amount of graphene, namely a concentration between 0.5 and 5 wt%, in particular 0.5 to 3 wt%.

The decrease in storage modulus of the composite may be up to 50%, corresponding to a decrease in melt viscosity of the polymer composite of 50 % at a fixed frequency (ω), where the melt viscosity is defined as ^Ν° /ω and can be determined as described in William W. Graessley; The Entanglement Concept in Polymer Rheology; Advances in Polymer Science, Volume 1 6, 1974, p. 1 -179 .

The composite according to the invention has improved processability, in particular it shows good miscibility with polyolefins.

The invention also pertains to a composite comprising ultra high molecular weight polyethylene and 0.5 to 5 wt% of graphene based on the combined dry mass of the UHMWPE and the graphene, wherein the UHMWPE has an elastic shear modulus, in the plateau region, of at most 1 .4 MPa and a Mw/Mn ratio of at most 15.

The graphene of the composite preferably is graphene oxide (GO), more preferably reduced graphene oxide (rGO).

In one embodiment the composite according to the invention comprises disentangled UHMWPE, that means ultra high molecular weight polyethylene which has an elastic shear modulus, in the plateau region, of at most 1 .4 MPa and a M_w/M_n ratio of at most 8. In one embodiment, the composite further comprises at least one filler. This filler is different from the UHMWPE and the graphene and may be an inorganic or organic filler. For example, the filler may be selected from talcum and HDPE. In one embodiment the composite of the invention has a storage modulus in rubbery plateau of between 0.5 and 1 .5 MPa.

In another embodiment, the invention pertains to a composite obtainable by one of the processes according to the invention. The composites of the invention are in one aspect characterized by the well-dispersed graphene.

The composite may be used to obtain a material. Therefore, current invention also pertains to a process to obtain a material, where the composite obtained according to the process of current invention is mixed with a polyolefin, e.g. in an extruder.

The polyolefin may be any homopolymer or copolymer produced from olefin monomers

(of the general formula Ο_ηΗ_2η), or blends of such homopolymers and/or copolymers.

Particularly suitable are polyethylene, polypropylene, polymethylpentene or polybutene-

1 . Preferred polyolefins are polyethylene, polypropylene and blends thereof.

To obtain the material, 2-98 wt% of polyolefin and 98-2 wt% of the composite of present invention are mixed, e.g. in an extruder, preferably 80-98 wt% of polyolefin and

20-2 wt% of the composite. The amounts are based on the dry mass of the material.

The mixing of the composite and the polyolefin may be carried out by a melt blending process, The current invention also pertains to a material comprising a blend of the composite of the invention and a polyolefin. In one embodiment the material comprises 2 to 98 wt% of the composite and 98 to 2 wt% of a polyolefin, based on the dry mass of the material. Suitable polyolefins are listed above, preferably the polyolefin is selected from

polyethylene, polypropylene and a mixture thereof. To obtain the material, the composite and the polyolefin are combined, preferably by a melt blending process wherein the combination of composite and polyolefin is heated to above 200°C and mixed in a blender. The composites of the invention have a surprisingly lower melt viscosity and can be easily mixed and processed, e.g. into materials comprising the composite and polyolefins.

Also, the composite and materials made thereof have improved tensile strength and tensile modulus. Furthermore, the abrasion resistance of the composite and material is increased.

The composite allows improved processing of materials comprising the UHMWPE composite and polyolefins. For example, a material comprising the UHMWPE composite of the invention and polypropylene shows enhanced crystallization under flow. For example, the shear or elongational flow causes orientation of long polymer chains, in this case UHMWPE, which act as nucleation sites for the low molar mass component thus enhancing the crystallization rate of the base polymer (the polyolefin combined with the composite).

Furthermore, due to the decreased melt viscosity, the composite itself comprising graphene (preferably GO/rGO) and disentangled UHMWPE and the materials of the invention will show a very homogeneous distribution of the graphene within the

UHMWPE and polyolefin matrix.

The following examples demonstrate the invention further but do not limit its scope. Examples

1 . Materials

Materials for the synthesis of the graphene oxide were purchased from Sigma-Aldrich and used as received. Conventional UHMWPE (indicated as C-PE or c-UHMWPE) powder having molecular characteristics reported in Table 1 was also purchased from Sigma-Aldrich and was used as obtained. Disentangled UHMWPE (indicated as D-PE or d-UHMWPE) with a significantly reduced number of entanglements was synthesised following the method described in Rastogi, S. ; Yao, Y. ; Ronca, S. ; Bos, J. ; van der Eem, J. Macromolecules 2011 , 44 (14), 5558-5568.

Table 1 summarises the molecular and physical characteristics of both polymers. For simplicity, samples are indicated with the notation 'C_PE' or 'C_PE/rGO', where C indicates conventional, i.e. entangled UHMWPE, D indicates disentangled UHMWPE; PE refers to UHMWPE.

Table 1 . Overview of physical properties of the samples of UHMWPE.

2. Determination of molecular weight and molecular weight distribution of the polymers

Weight-average molecular weight M_w and molecular weight distribution (MWD) of both polymers were estimated by rheology using Advanced Rheometrics Expansion System (ARES) as described in Mead, D. J. Rheol. 1994, 38, 1797-1827 and in S Talebi, R Duchateau, S Rastogi, J Kaschta, GWM Peters, PJ Lemstra; Macromolecules 2010; 43 (6); 2780-2788.

3. Preparation of GO and composites The synthesis of GO was done following a modified Hummers Method (Liu, P. ; Gong, K. ; Xiao, P. ; Xiao, M. J. Mater. Chem. (2000), 10, p. 933-935) with additional modifications as follows: after the oxidation reaction, the resultant material was repeatedly vacuum- filtered and washed, 3 times with 5 wt % HCI and a few times with distilled water, until the upper liquid became dark. The suspension became darker with the increasing number of washing steps, suggesting progressive extraction of GO from the bottom layer to the upper suspension. The average number of water washing steps applied was approximately ten, until the pH of the suspension changed from ~ 2 to ~ 7. The dark- liquid portions were combined and dried in a petri dish at 50^QC for 2 days. Films of GO were obtained by peeling them off from the petri dish.

A two-step procedure was carried out to prepare the composites with either

disentangled or conventional UHMWPE: first, the required amount of dried GO was weighed and re-dispersed in 40 ml of water by 15 min ultrasonication, while the required amount of UHMWPE was dispersed in acetone and stirred for 10 min; then, the ultrasonicated homogeneous GO suspension was added to the acetone-suspended UHMWPE under stirring. The mixture was stirred in a fume hood until most of the solvent evaporated and the resulting solid was further dried at 40^QC for 12 hours to remove any residual solvent. The residue, a UHMWPE/GO composite, was obtained in the form of powder. To achieve the reduction of the dispersed GO, the dried composite powders were compressed in a hydraulic press at 230^QC (for C_PE) or 160^QC (for

D_PE), using a combination of pressure and temperature with loads of 1 ton for 1 min, 5 tons for 5 min, 15 tons for 15 min and 20 tons for 5 min at temperatures of 1 60^QC and 230^QC, respectively, with samples having an area of 9.6x 10^"4 m². Following this procedure, composites films of UHMWPE/rGO were prepared. In all samples, 0.7 wt % antioxidant (Irganox 1010, Ciba) was added to avoid oxidation or degradation during the hot press procedure.

For rheological studies ~ 0.7 g of D_PE/GO powder was sintered into discs of 35 mm diameter and 0.6 mm thickness by compression moulding at a constant temperature of 125°C and average force of 20 tons. From the compressed powder films, smaller discs of 12 mm diameter were punched.

4. Rheological characterisation of composites: determination of storage modulus

All rheological measurements were performed in an ARES-LS2 rheometer (TA,

Instruments) using a 12 mm diameter parallel plate geometry. In order to avoid polymer degradation during long time measurements, the samples were kept under nitrogen atmosphere inside a convection oven. The 12 mm polymer disc was placed in the rheometer at 1 10°C and temperature was increased to 1 60°C (approximately 18°C above the equilibrium melting temperature of polyethylene) with a heating rate of 10 ^QC/min. Along this process, an average normal load of 4.0 N was applied to maintain appropriate contact between the sample and plates. Once reached 1 60°C, a small amplitude oscillatory test at a constant frequency of 10 rad/s and strain 0.1 % (within the linear viscoelastic region) was performed to follow the storage modulus (G') build-up. Data acquisition was started 60 seconds after reaching the experimental temperature of 1 60^QC. In the fully equilibrated melt state, frequency sweeps with angular velocity from 0.001 rad/s to 100 rad/s were performed with a constant 0.5% strain in the linear viscoelastic region on the D_PE/rGO samples.

5. Effect of graphene addition to UHMWPE samples The storage modulus build-up curves for D_PE/rGO composites is shown in Fig. 1 . The storage modulus decreases when GO/rGO is added and a composite is prepared. For the used graphene oxide and the specific concentration of GO filler (here: rGO, because the graphene oxide has been reduced), the storage modulus decreases, while it increases again, even above the storage modulus of the polymer-only sample, at higher amounts of filler.

The results show that the UHMWPE composite according to the invention comprising disentangled UHMWPE and GO/rGO filler, shows a clearly decreased storage modulus and thus can be processed easier. The effective concentration of graphene may vary: where less active GO is used, a higher amount might be added (up to 5 wt%) to achieve the decrease in storage modulus.

6. Materials of composites of GO/UHMWPE and polypropylene (PP) or Polyethylene (PE) Using a twin screw mini-compounder (DSM Explore MC-5), having a sample volume capacity of 5 ml, the homogeneous mixing between the composite of GO/d-UHMWPE and the polyolefin PP (or PE) was achieved. For homogeneous mixing in the extruder, PP (or PE) in the powder form was preferred. For the mixing process the extruder was first fed with PP (or PE) powder followed by adding the GO/d-UHMWPE powder composite. The amount of the GO/UHMWPE composite was varied between 2 to 20 wt%, based on the dry mass of the material. All mixing was performed at 230°C. Torque for the mixing was maintained within the limits of the twin screw extruder. After the complete feeding of the extruder, and maintaining residence time for 5 minutes and the rotation speed of 50 rpm (which may be further increased to 100 rpm), the continuous flow of extrudate was obtained. PP (or PE) was acquired from commercial sources. For the examples, polyethylene used is Stamylan HD8621 , DSM and polypropylene used is HD601 CF, Borealis.

7. Thermal analysis of d-PE and c-PE composites The melting and crystallization kinetics of the d-UHMWPE, d-UHMWPE/ 0.8wt% GO composite, c-UHMWPE and c-UHMWPE/ 0.8wt% GO composite was investigated using Differential Scanning Calorimetry (DSC), on a Q-2000 MDSC machine from TA instrument. High precision T-zero pans with lids were used for the experiment. To minimize the thermal lag caused by the samples, the weight was kept within 1 .5± 0.1 mg for each sample. All the measurements have been performed in nitrogen atmosphere. A DSC protocol has been developed to follow melting and crystallization enthalpy and kinetics. The DSC protocol used during these measurements is shown in Figure 2. In the DSC protocol different annealing times are chosen to vary the entanglement density; isothermal crystallization at different temperature, where 128°C differentiates crystallization from entangled and disentangled domains.

The steps of the DSC protocol: (a-b) heating from 50°C to an annealing temperature of 1 60°C which is higher than UHMWPE's equilibrium temperature (141 .5°C) at 10°C/min ; (b-c) annealing for various fixed time (5, 30, 60, 180, 360, 720, 1440 min, respectively); (c-d) cooling to an isothermal crystallization temperature of 128°C at 10°C/min; (d-e) isothermal crystallization temperature for fixed time of 180 min; (e-f) Cooling to 50°C at 10°C/min; (g-h) second heating from 50°C to 1 60°C at 10°C/min.

Fig. 3 shows DSC thermograms for d-UHMWPE (left panel, a) and d-UHMWPE/0.8 wt% GO (right panel, b). The samples including or excluding GO were annealed for different times : 300 seconds, 10800 seconds (3 hours) and 86400 seconds (24 hours). The data show that the enthalpy of the second melting peak (at higher temperature) of the d-UHMWPE/GO composite of the invention remains much higher than the enthalpy of the first melting peak, even at a very long annealing time (24 hours) at 1 60°C. In contrast, at the same annealing time the data of d-UHMWPE without GO show that the enthalpy of the second melting peak decreases, and becomes lower than the enthalpy of the first melting peak, indicating that the polymer has - at least to some extent - lost its disentangled state.

Fig. 4 shows the enthalpy of the second melting peak during the second heating cycle (steps g-h of Fig. 2) of the composite of the invention (d-UHMWPE/0.8 wt% GO) in comparison to a c-UHMWPE/0.8 wt% GO composite and to the c-UHMWPE and d- UHMWPE without GO, obtained from DSC thermograms such as the ones displayed in Figure 3. The melting enthalpy of peak T_h at -141 °C of the -UHMWPE, d-UHMWPE/ 0.8wt% GO, c-UHMWPE and c-UHMWPE/ 0.8wt% GO is presented as a function of the annealing time at 1 60°C.

As can be seen, the enthalpy of the second melting peak of the d-UHMWPE/GO composite of the invention remains high, even after long annealing times indicating that the composite remains (largely) in a disentangled state. In contrast, the enthalpy of the second melting peak of d-UHMWPE without GO decreases as the annealing time increases.

The c-UHMWPE (with and without GO) has a much lower enthalpy of the second melting peak, since it is in the entangles\d state, and the enthalpy of the second melting peak is not noticeably influenced by the addition of GO. 8. Melt viscosity of composite according to invention

Using a twin screw mini-compounder (DSM Explore MC-5), having a sample volume capacity of 5 ml, the processing of d-UHMWPE without and with 0.8 wt% of GO was achieved. All mixing was performed at 230°C. The sample weight for each batch was 2.5 gram. Torque for the mixing was maintained within the limits of the twin screw extruder. After the complete feeding of the extruder, and maintaining residence time for 5 minute and the rotation speed of 50 rpm, the continuous flow of extrudate was obtained. The extruder control system calculates the melt viscosity automatically from the pressure profile inside the extruder.

The values determined are shown in Fig. 5 for the d-UHMWPE/GO composite according to the invention and for d-UHMWPE without GO.

The data show that the composite according to the invention has a lower melt viscosity than d-UHWMPE. Therefore, the composites are easier to process, requiring less energy input during processing, and allowing processing at a lower temperature, which is advantageous in many processes.

Claims

1 . Process for the manufacture of a composite wherein the process comprises the following steps:

i) graphene is exfoliated,

ii) ultra high molecular weight polyethylene (UHMWPE) is combined with the exfoliated graphene, wherein the UHMWPE has an elastic shear modulus in the plateau region of at most 1 .4 MPa and a M_w/M_n ratio of at most 15 and the amount of graphene is 0.5 to 5 wt% based on the combined dry mass of the UHMWPE and the graphene.

2. Process according to claim 1 wherein at least one filler is added.

3. Process according to claim 2 wherein the filler is selected from an inorganic or organic compound.

4. Process according to claims 2 or 3 wherein the UHMWPE is combined with the at least one filler and the combination is subsequently combined with the exfoliated graphene.

5. Process according to claims 2 or 3 wherein the UHMWPE, the exfoliated

graphene and the at least one filler are combined in step ii).

6. The process according to any of the preceding claims wherein the UHMWPE has a molecular weight (M_w) of at least 500 000 g/mol, preferably at least 1 000 000 g/mol.

7. The process according to any one of the preceding claims wherein the graphene is graphene oxide.

8. Composite comprising ultra high molecular weight polyethylene (UHMWPE) and 0.5 to 5 wt% of graphene based on the combined dry mass of the UHMWPE and the graphene, wherein the UHMWPE has an elastic shear modulus, in the plateau region, of at most 1 .4 MPa and a M_w/M_n ratio of at most 15.

9. The composite according to claim 8 wherein the composite further comprises at least one filler.

10. The composite according to claims 8 or 9 wherein the composite has a storage modulus of between 0.5 and 1 .5 MPa.

1 1 . Composite obtainable by a process according to any one of claims 1 to 7.

12. Material comprising a blend of the composite of any of claims 8 to 1 1 and a

polyolefin.

13. Material according to claim 12 comprising 2 to 98 wt% of the composite and 98 to 2 wt% of the polyolefin, based on the dry mass of the material.

14. The material according to claim 12 or 13 wherein the polyolefin is selected from polyethylene, polypropylene and a mixture thereof.