WEIGHT POLYETHYLENE FIELD OF THE INVENTION

The invention relates to the production of highly crystalline ultra-high molecular weight polyethylene (UHMWPE). According to the invention, the high crystallinity of polyethylene results from specific polymerisation conditions and not from any post- polymerisation treatment. Additionally, the method according to the invention improves the molecular weight of polyethylene as well.

Typically, high molecular weight polyolefins such as UHMWPE may be used for medical prosthetics, bulletproof vests, or machined parts for instance. BACKGROUND OF THE INVENTION

Typically, thermoplastic polyolefins such as polyethylene or polypropylene can be classified according to their molecular weight and/or density and/or crystallinity since these features may have a direct impact on their properties.

For instance, highly crystalline polyethylene is generally harder and more thermally stable than poorly crystalline polyethylene.

Among thermoplastic polymers, ultra-high molecular weight polyethylene (UHMWPE) is of great interest in view of its unique mechanical properties. Indeed, due to its high impact resistance and its toughness, UHMWPE can substitute more expensive materials such as Kevlar ®.

As a result, the improvement of known methods for increasing the crystallinity of UHMWPE after the reaction step has been the focus of many research and development programs.

For instance, Czaja et al. have reported an electron beam irradiation process that improves the crystallinity of UHMWPE from 46 to 57% (Czaja et al., Radiation Physics and Chemistry, 2011, 80, pages 514-521). The effect of adding zeolite to UHMWPE has also been reported. The crystallinity of UHMWPE can reach a maximum of 47.2% in the presence of 15wt% zeolite (Boon Peng Chang et al, Advanced Materials Research, 2013, Vol. 812, pages 100-106). Typically, in these processes, ethylene is first polymerised to afford UHMWPE. Then, UHMWPE is post treated in order to increase its crystallinity thanks to a high pressure induced orthorhombic to hexagonal phase transition.

Although this kind of treatment affords highly crystalline UHMWPE, it is not satisfactory since it requires a large amount of energy.

The present invention relates to a method for the production of highly crystalline UHMWPE, which, contrary to prior art methods, does not require any costly post treatment.

As opposed to prior art synthesis of UHMWPE, the invention is carried out in the gas phase.

The invention relates to a process that leads to a simultaneous (1) increase in the molecular weight of said polymer to obtain polyethylene of ultra-high molecular weight, and (2) increase in the crystallinity of the resulting polymer.

SUMMARY OF THE INVENTION The invention relates to an efficient and cost effective method for the preparation of semi-crystalline ultra-high molecular weight polyethylene (UHMWPE).

UHMWPE is an ethylene polymer having a molecular weight of at least 1,000,000 g/mol, preferably at least 2,000,000 g/mol.

As opposed to the state of the art, the method according to the invention can be carried out in the reactor wherein ethylene is polymerised to afford UHMWPE having high crystallinity. It requires the presence of a short chain molecule, and preferably the vaporisation of this short chain molecule. More specifically, the invention relates to a method for the preparation of polyethylene having a molecular weight of at least 1 million g/mol, comprising the following steps: introducing the following compounds into a gas phase reactor:

• optionally an inert gas such as nitrogen;

• optionally hydrogen and/or an alpha-olefin comonomer;

• an olefin polymerisation catalyst;

• at least one unreactive compound having a molecular weight of less than 150 g/mol; said unreactive compound being unreactive vis-a-vis ethylene, hydrogen, alpha-olefins and the olefin polymerisation catalyst; introducing ethylene in the reactor;

polymerising ethylene, and optionally hydrogen and/or an alpha-olefin comonomer, in the presence of said unreactive compound;

obtaining polyethylene having a molecular weight of at least 1 million g/mol. The resulting polymer is a ultra-high molecular weight polyethylene (UHMWPE).

The prior art UHMWPE synthesis is generally carried out in a slurry. This is contrary to the invention which involves a gas phase polymerisation. The invention provides a polyethylene having a molecular weight and a crystallinity higher than are obtained in a conventional process using the same catalyst formulation.

Indeed, in the absence of unreactive compound, polyethylene having a molecular weight of 200-300,000 g/mol and a crystallinity of 50% is obtained, while in the same experimental conditions (but in the presence of the unreactive compound), the invention affords polyethylene having a molecular weight of at least 1 million and a crystallinity of over 60%.

As a result, the polyethylene obtained from the method according to the invention can eventually be used "as-is".

If there is any need to increase the crystallinity of the polyethylene even further, then the post reaction high pressure crystallization step is easier and less costly in terms of energy input as compared to prior art polyethylene. The crystallinity improvement is due to the presence of the above mentioned unreactive compound during the polymerisation of ethylene. This technical effect is definitely due to the polymerisation conditions since such crystallinity is not obtained in the absence of said unreactive compound. Without wanting to be bound or limited by one particular explanation, it appears that the crystallinity improvement is due to the plasticisation of the nascent polymer chains, which enhances the degree of disentanglement of the high molecular weight chains, allowing them to form a more ordered structure. It is clear that without the inert compound (i.e. the unreactive compound), the formation of crystals is less.

Although it is known that alkanes may be used in order to cool down the reaction medium in the gas phase preparation of regular polyethylene (i.e. polyethylene having a molecular weight of less than 1000000 g/mol), their use has not been reported in connection with UHMWPE, nor has their use been reported specifically to enhance the properties of the polymer. Indeed, as already pointed out, the industrial prior art synthesis of UHMWPE is carried out in a slurry reaction medium, not in the gas phase. Furthermore, alkanes have not been reported as crystalline enhancer of regular polyethylene. The crystallinity improvement of UHMWPE due to the presence of an unreactive compound such as an alkane is therefore quite surprising and unexpected.

The crystallinity of the UHMWPE resulting from the invention is usually at least 35%. For instance, it may range from 40 to 70%. The invention allows a crystallinity increase of 5 to 10% as compared to prior art UHMWPE synthesis carried out in the absence of the above unreactive compound.

As already mentioned, the unreactive compound has a molecular weight of less than 150 g/mol, preferably between 44 and 114 g/mol, and most preferably between 58 and 86 g/mol. It is preferably a hydrocarbon, and more preferably a C3 to C8 alkane, and even more preferably a C4 to C6 alkane.

Such alkane may be linear, branched or cyclic. For instance, it may be any of the following n-butane, n-pentane, n-hexane, cyclohexane and mixture thereof. The unreactive compound may also be any isomer of these alkanes, for instance isopentane. The unreactive compound may be injected in the reactor in either liquid or vapour form. It may be injected in the reactor either before, during or after the beginning of the polymerisation. According to a particular embodiment, the unreactive compound is a liquid that may be injected, then vaporised. Thereafter, ethylene is injected and polymerised.

According to another particular embodiment, the unreactive compound may be continuously fed to the reactor.

Within the reactor, the molar ratio unreactive compound / ethylene can range from 0.001 to 30, preferably from 0.01 to 20, even more preferably from 0.01 to 10. For instance, it may be 0.12 or less when the unreactive compound is a C6 alkane, or 0.36 or less when the unreactive compound is a C5 alkane. On the other hand, it may be 1.4 or less when the unreactive compound is a C4 alkane, or 5 or less when the unreactive compound is a C3 alkane. These ratios are particularly advantageous when the polymerisation temperature is about 80°C.

In the reactor, and under ethylene polymerisation conditions, the unreactive compound, ethylene and the optional olefin comonomer(s) are preferably all in the vapour phase.

The ethylene polymerisation is preferably carried out in the presence of 1 to 80 bars of ethylene, more preferably between 5 and 35 bars.

The ethylene polymerisation is preferably carried out at a temperature ranging from 10 to 150 °C, more preferably between 40 and 120°C. Since the polymerisation is carried out in a gas phase reactor, the olefin polymerisation catalyst is supported. It can be any well-known catalyst. For instance, it may be selected from the group consisting of conventional Ziegler-Natta catalysts, such as titanium/magnesium Ziegler-Natta catalysts; Phillips-type catalysts such as chromium oxide supported on silica; and supported metallocene-type catalysts supported.

The catalyst support is generally a material such as a solid having a high surface area. It may typically be made of any of carbon, alumina, and silica for instance.

Prior to carrying out the invention, the reactor may be purged with an inert gas (argon or nitrogen for instance) so as to remove oxygen. It can consist of a series of cycles of vacuum and subsequent introduction of inert gas to the reactor. A scavenger may also be introduced in the reactor, for instance an oxygen scavenger.

Ethylene may be added at room temperature. It may also be added at less than 20°C. According to a particular embodiment of the invention, the reactor is continuously fed with ethylene during the ethylene polymerisation.

According to another particular embodiment of the invention, the reactor is continuously fed with ethylene and/or the unreactive compound during the ethylene polymerisation.

According to another particular embodiment of the invention, the reactor can be discontinuously fed with ethylene and/or the unreactive compound during the ethylene polymerisation

The unreactive compound and ethylene are preferably added in the reactor that already contains the catalyst.

As already mentioned, an alpha-olefin comonomer may also be introduced in the reactor. According to this embodiment, ethylene is polymerised in the presence of both said alpha-olefin comonomer and unreactive compound. The alpha-olefin comonomer may be an olefin of formula CH ₂ =CR ¹ R ² , wherein R ¹ is H or CH ₃ , and R ² is a Ci-Cio alkyl, preferably a C ₁ -C4 alkyl. The alpha-olefin comonomer is preferably selected from the group comprising propylene, butene, pentene isomers, and hexene. It may also be octene.

According to another embodiment, the R ² group may comprise a carbon carbon double bond. The comonomer may therefore be a diene such as butadiene or isoprene for instance.

Since all the steps of the method can be carried out directly in the reactor, the invention does not require any additional equipment. The invention affords a low-cost method for increasing the crystallinity and/or the molecular weight of polyethylene, and in particular the crystallinity of gas phase UHMWPE directly in the reactor in which it has been prepared. The invention also relates to the ultra-high molecular weight polyethylene that is directly obtained from this method.

Since UHMWPE is a thermoplastic material combining light weight, a high resistance to wear, low adhesion, and extreme biological inertness, it may be used for medical prosthetics, bullet proof vests, or machined parts with extended wear resistance for instance. Its strength is related to its crystallinity, so directly increasing the level of crystallinity in the reactor is an advantage of the current invention. The invention and its advantages will become more apparent to one skilled in the art from the following figures and examples.

FIGURES Figure 1 relates to the effect of the presence of n-pentane on the crystallinity of UHMWPE.

Figure 2 relates to the effect of presence of n-hexane on the crystallinity of UHMWPE. EXAMPLES

1/ Synthesis of UHMWPE

Four UHMWPE have been prepared, two according to the invention (INV-1 and INV- 2) and two counter examples (CE-1 and CE-2). The same experimental setup has been used for these four syntheses.

The experimental set-up consists of a 2.5 litre spherical stirred-bed gas phase reactor heated by circulating water in a jacket covering the external surface of the reactor. A pressure regulator controls the pressure of ethylene in the reactor.

This gas phase reactor is conditioned at 80°C for at least one hour by five cycles of vacuum and consequent introduction of argon to the reactor.

This is followed by introduction of 1 cm ³ of a 1M solution of TEA (TEA = triethyl aluminum) in heptane into the reactor for scavenging all the remaining traces of water while also acting as the co-catalyst. The ethylene polymerisation catalyst is then introduced into the reactor with catalyst injection cartridge having an inner volume of 100 cm ³ .

The cartridge is filled with the catalyst diluted in dried NaCl, and is pressurised to 10 bars with a small amount of gas to push all the catalyst/salt mixture into the reaction environment.

Finally, the reaction is started by feeding the ethylene gas to the reactor while maintaining its pressure at 7 bars during the polymerisation reaction for two hours at a reaction temperature of 80°C .

In order to stop the reaction, the reactor is degassed from ethylene while being cooled down by circulation of cold water in the external jacket. In order to study the effect of n-hexane as crystallinity enhancer, after conditioning the reactor, first, at room temperature of 25°C, specific amount of liquid n-hexane is injected to bed.

Then the reactor temperature is raised to 80°C assuring all n-hexane inside the reactor is vaporised.

This is followed by catalyst injection and ethylene introduction, respectively.

The reactor powders have been analysed using differential scanning calorimetry (DSC). In the current study, during the first heating cycle, the PE sample was heated at the constant rate of 5 °C min ^"1 from 30 °C to 170 °C. The temperature of the sample was kept at 170 °C for 2 minutes and then cooled down at the constant rate of -20 °C min ^"1 to 30 °C. After keeping the temperature of sample at 30 °C for 2 minutes, the second heating cycle was started by heating the sample at the constant rate of 5 °C min ^"1 from 30 °C to 170 °C. This same procedure was repeated for the third and fourth heating cycles.

The results are shown in Table 1, where an energy balance on the sample is used to calculate the fraction of crystalline material in the overall product. Table 1 : Crystalline fractions of polyethylene

: weight average molecular weight (10 ⁶ g.mol ^"1 )

CE: counter-example

INV: example according to the invention

It can be seen from this analysis that adding 0.3 or 0.6 bars of hexane to the reactor, allows to increase the crystallinity from near 54% (CE-1) to 59% (INV-1) and 62 % (INV-2) (second heat).

It is also interesting to note that while the crystallinity of the samples changes as function of the amount of hexane in the reactor, the melting and crystallisation temperatures do not. The CE-2 polymer has been made in the reactor in the absence of hexane. It has then been put back in the reactor, and 0.6 bars of hexane have been injected.

The powder has been stirred at 80°C for one hour. In this case, the crystallinity actually decreases significantly.

21 The effect of the presence of the unreactive compound on polymer crystallinity

The effect of presence of different unreactive compounds on the degree of crystallinity of polymer particles was investigated by differential scanning calorimetry (DSC) technique. The DSC analysis was performed on at least 8 UHMWPE samples obtained from the polymerization reaction at each specific condition (figures 1 and 2).

The crystallinity of the powders measured during both of the first and second heating cycles were recorded. The first heat determines the crystallinity of the nascent UHMWPE powder in the reactor condition. This is while by heating the sample above its melting temperature and cooling at a controlled rate, the thermal history of the sample inside the reactor will be erased. Therefore, the second heat evaluates the inherit properties of the UHMWPE powder obtained at each specific polymerization condition.

During the first heating cycle, the PE sample was heated at the constant rate of 5 °C min ^"1 from 30 °C to 170 °C. The temperature of the sample was kept at 170 °C for 2 minutes and then cooled down at the constant rate of -20 °C min ^"1 to 30 °C. After keeping the temperature of sample at 30 °C for 2 minutes, the second heating cycle was started by heating the sample at the constant rate of 5 °C min ^"1 from 30 °C to 170 °C.

It must be noted here that it has been verified that the measured crystallinity of polymer samples during the subsequent heating cycles (i.e., third and fourth cycles) did not change in comparison with their crystallinity recorded during the second heating cycle. This indicates and assures that the thermal history of the polymer samples inside the reactor has been completely erased during the first heating cycle.

Figure 1 relates to the effect of the presence of n-pentane on the crystallinity of UHMWPE produced during the gas phase ethylene polymerization on the supported catalyst with 7 bars of ethylene at 80 °C.

Figure 2 relates to the effect of presence of n-hexane on the crystallinity of UHMWPE produced during the gas phase ethylene polymerization on the supported catalyst with 7 bars of ethylene at 80 °C. Figure 1 and figure 2, respectively, demonstrate the effect of presence of «-pentane and «-hexane as the unreactive compound in the gas phase composition on the crystallinity of UHMWPE produced during the polymerization on the supported catalyst with 7 bars of ethylene and at 80 °C. As the general pattern, the crystallinity of produced UHMWPE recorded during both of the first and second heating cycles increases by increasing the partial pressure of the studied unreactive compounds «-pentane and n- hexane in the reaction environment, presented in figure 1 and figure 2, respectively. Nevertheless, it appears that the crystallinity of UHMWPE remains more or less constant while increasing the partial pressure of «-hexane from 0.6 to 0.8 bar, as given in figure 2. The increase in recorded crystallinity of UHMWPE during the second heating cycle is a substantial indicator that the presence of studied unreactive compounds (i.e., «-pentane and «-hexane) in the reactor, not only affects the quality of growth of polymer chains from the active sites and their subsequent crystallization behavior in the nascent polymer particles but also it permanently modifies the quality of organization of polymer chains in the molten state (i.e., the density and distribution of chain entanglements) and consequently the crystallization capability of the produced polymer chains.

The crystallinity of the nascent UHMWPE powder measured during the first heating cycle is always higher than the one recorded for the same sample during the second heating cycle. In the growing polymer particles, the polymer chains initiated from the active sites can align with the other chains in order to form the crystalline phase. Therefore, regarding the gradual nature of the growth and crystallization of the polymer chains, it can be speculated that the polymer chains would have more time to align appropriately with each other in order to form the crystalline domains in the nascent polymer particle leading to its higher recorded crystallinity during the first heating cycle.

However, after being completely melted by the end of the first heating cycle, the molten polymer is cooled down at a fast rate, relatively speaking (see the experimental procedure for the DSC analysis at the beginning of this section). Therefore, while a portion of the molten polymer chains will have enough time to align appropriately with each other to form the crystalline phase, some other portion will be "frozen" in the amorphous state during the cooling process before being able to align and contribute to the crystalline phase. This would, in turn, result in the lower degree of crystallinity recorded during the second heating cycle.

Nevertheless, the crystallization capability of the polymer chains after complete melting of the nascent UHMWPE samples during the first heating cycle still depends on the process condition (i.e., the partial pressure of the unreactive compounds in the reaction environment) at which they are produced. Regarding that all the polymer samples are UHMWPE with a linear microstructure for all the individual polymer chains in the molten state, the reason for the difference in the crystallization capability of the samples from the molten state and consequently the measured crystallinity in the second heating cycle must be sought in their production step and the influence of the presence of the unreactive compound on the quality of growth of polymer chains and their subsequent crystallization in the active polymer particles inside the reactor.