Process for the preparation of polyethylene having a broad molecular weight distribution

The present invention relates to a process for preparing polyethylene having a broad molecular weight distribution. More particularly, the present invention relates to the preparation of high density polyethylene, medium and low density having a broad or bimodal molecular weight distribution.

Particularly for high density polyethylene, the distribution of molecular weight distribution (MWD) is a fundamental properties determines the properties of the polymer and thus its applications.

Although it may be difficult to assess the influence of each of property taken independently, it is generally recognized that the molecular weight mostly determines the properties mechanical while the molecular weight dispersion determines mainly the rheological properties.

As an increase in molecular weight normally improves the physical properties of the resins, there is a strong demand for the high molecular weight polyethylene. However it is the highest molecular weight which render the polymers more difficult to transform. On the other hand, an increase in the MWD tends to improve flow high shear rate during the transformation. Thus, in applications requiring a rapid transformation in fairly high die swell as in blow molding techniques and extrusion, broadening the MWD improves the high weight polyethylene processing molecular (= low flow index to melt or low MID₂).

It is believed that when the polyethylene has both a high weight molecular and a wide distribution of molecular weight, transformation is made easier by the low weight section molecular while the high molecular weight portion contributes to good impact resistance of the film. A polyethylene of this type can be transformed into less energy with yields higher.

The MWD may be described completely by the curve obtained by perméacion gel chromatography. However, the MWD is generally described by a value that is a fair assessment, also called the polydispersity index, and which has the ratio between the average molecular weight and weight number average molecular. Depending on the application, the MWD required will vary from 10 to 30.

In order to obtain the advantages of a broad distribution was first proposed to prepare polyethylene having a broad MWD by mixing polyethylenes having different molecular weights. However, results are not proven satisfactory because a simple mixture not behave like an intimate blend of polyethylene prepared in if you. reactions have also been proposed to use two-step in a reactor. Examples of such methods are described in the patent publications GB-A-1174542, GB-A-2020672 and BE-A-883 687.

It was also proposed to use several reactors connected in series. In this respect, there is known a method for preparing a polymodal ethylene polymer in which ethylene is polymerized in the two stages in the presence of an organoaluminum compound containing a halogen, a transition metal compound and different amounts hydrogen in each stage (British Patent No. 1,233,599).

Also known is a process for the preparation of polymers olefin by a two stage polymerization, in which process the high molecular weight polymer is prepared in the first step H with a report₂/ C₂H₄ low and the low molecular weight polymer is prepared in the second step with a ratio H₂/ C₂H₄ high (demand EP-A-57 352). The catalyst used is, inter alia, a organoaluminum compound containing a halogen and the product of reacting an organomagnesium compound containing oxygen, a organotitanium compound containing oxygen (both compounds being in solution) and a halide of aluminum. A similar process is described in patent application EP-A-57 420. It was also suggested a process for polymerizing ethylene in two steps wherein pressure in the second reactor is maintained smaller than the first reactor; the polymerization is carried out in the presence of a Ziegler-Natta catalyst usual such as a metal catalyst transition supported on a solid carrier and a compound organoaluminum. Examples of such methods are described in the US-A-4414369 and US Patent 4,338,424. However, the polymers ethylene obtained by these methods do not bear long side chains, which is unfavorable for their ability to be processed. This is why the Applicant has developed polyethylene production processes by copolymerizing ethylene and one or more alpha-olefin comonomers having 3 to 10 carbon atoms, in two reactors loop polymerization liquid phase connected in series, presence of a catalytic system consisting of the reaction product an organomagnesium compound with a titanium compound, a compound organoaluminum and optionally one or more donors of electrons, a metallocene catalyst with an aluminoxane cocatalyst or a chromium catalyst, which is the subject of Patent Applications EP-A-580 930 and 649 860.

However, these two processes have the major drawback of being based on the production of very high molecular masses copolymerized in the first reactor and of low mass molecular in the second reactor. This configuration is necessary by the fact that the molecular weight is regulated by Hydrogen additions and difficult in two reactors loop polymerization liquid phase connected in series to produce low molecular weight initially and generate hydrogen surplus between the two reactors. In addition, the comonomer not incorporated into the polymer chains produced in the first reactor is driven in the second where it can react and incorporate in low molecular weights.

The object of the present invention is therefore to remedy the aforesaid drawbacks and to provide ethylene polymers high, medium and low density with good aptitude for processing, good physical properties including mechanical and in which there is no incorporation of the comonomer in the low molecular weights.

To this end, there is provided, according to the present invention, a method of polyethylene prepared by the homopolymerization of ethylene or by copolymerizing ethylene and at least one alpha-olefin comonomer having 3 to 10 carbon atoms, in two reactors polymerization in liquid phase loop connected in series presence of a chromium catalyst. The ethylene and the chromium catalyst are first introduced into the first reactor at a temperature of 95 to 110 ° C to produce a ethylene homopolymer of low molecular weight of an index MI fusion₂ at least 1 g / 10 minutes. The homopolymer of ethylene thus obtained is transferred into the second reactor in which is added ethylene and optionally an alpha-olefinic comonomer at a temperature of 80 to 90 ° C for obtain a homopolymer or a high weight ethylene copolymer molecular.

Advantageously, the chromium catalyst is a chromium catalyst on a support, activated, said catalyst preferably containing 1% chromium and its support being preferably based on silica and oxide titanium.

According to an advantageous embodiment of the invention, the catalyst has a surface area of ​​300 to 750 m²/ G, preferably from 400 to 600 m²/ G and a pore volume of 1 to 3cc / g, preferably of 1.3 to 2.7 cc / g.

According to another advantageous embodiment of the invention, the amount of polymer formed in the first reactor is from 45 to 55% by weight of the total polymer and when alpha-olefinic comonomer is injected, it is selected from the group consisting of 1-butene, 1-pentene, 1-hexene, 4-methyl 1-pentene, 1-heptene and 1-octene.

The invention also relates to the broad distribution of polyethylene molecular weight and very good homogeneity obtained.

As was already mentioned, the ethylene supply, chromium catalyst and optionally hydrogen are injected in the first loop reactor in the liquid phase at a temperature of polymerization of from 95 to 110 ° C, preferably from 104 to 109 ° C for produce an ethylene homopolymer. These loop reactors are known in the art and are described, for example, in the patents US-A-3,152,872, 3,242,150 and 4,613,484.

We will use as a catalyst any type of catalyst but the chrome chromium catalysts activated carrier suitable particularly well for this purpose. Thus was obtained the Particularly interesting polyethylene yields with Catalysts containing 1% chromium on silica support and titanium oxide, previously activated before being introduced into the reactor in air at a temperature of 500 to 900 ° C, preferably about 650 ° C and optionally reduced Monoxide carbon at a temperature of 300 to 500 ° C. Be used as catalysts supported catalysts with a surface area of ​​300 750 m²/ G and preferably from 400 to 600 m²/ G and a pore volume of 1-3 cc / g and preferably from 1.3 to 2.7 cc / g. The residence time of ethylene and the chromium catalyst in the first reactor is preferably adjusted to produce 45 to 55% by weight of total polymer.

As was already stated, the first reactor is heated to a temperature of 95 to 110 ° C and preferably at a temperature of 104 to 109 ° C to give a product consisting homopolymer essentially of low molecular weight, high density, and MI melt index₂ at least 1 g / 10 minutes and preferably greater than 2 g / 10 minutes.

According to the invention, the ethylene homopolymer stream obtained in the first polymerisation reactor is then transferred to the second loop reactor, eg via one or more settling legs of the first reactor, as described in Patent Application EP-A-649 860. At this time, the second reactor is also fed with ethylene and optionally with comonomer alpha-olefin such as, for example, 1-butene, 1-pentene, 1-hexene, 4-methyl 1-pentene, 1-heptene, 1-octene or a mixture thereof. The reaction in this second reactor is carried out at a temperature 80 to 90 ° C, preferably 84 to 86 ° C, preferably in the presence of an inert diluent such as, for example, a aliphatic or cycloaliphatic hydrocarbon such as butane, pentane, hexane, heptane, cyclohexane, methylcyclohexane or an aromatic hydrocarbon such as benzene or toluene. A metal alkyl co-catalyst type, preferably aluminum alkyl, or alumoxane or boroxane or alkylboron is preferably added to reaction product of the second polymerization reactor to lower the melt index of the copolymer product obtained.

The operating conditions in the second reactor are adapted to the homopolymer or the copolymer of ethylene has a high weight molecular and melt index and the desired density. It is and the final polymer will preferably have a melt index MID₂ less than 0.1 g / 10 minutes.

Although the polymerization pressure in the first reactor is slightly higher than that of the second reactor, to be used a Generally a pressure of from 30 to 50 bar and a time of stay of the reactants in each reactor between 25 and 90 minutes. Thus it advantageous to use a pressure about 40 bar in the second reactor and slightly higher, such as, for example, 42 bar in the first reactor. In fact, it At a minimum the inert diluent containing dissolved ethylene remains liquid and pressure we could increase if the cost of reactor materials did not become prohibitive.

The process is generally carried out continuously, that is to say ethylene homopolymer composed of low molecular weights and containing the active catalyst is transferred in a continuous manner in the second reactor through a transfer line such as, for example, a settling leg of the first reactor, connecting two reactors.

The major advantage of the method of the invention is that by adapting The operating conditions of polymerization in the first and second reactor, such as in particular the temperature and the pressure of polymerization, the choice of chromium catalyst, the amount of ethylene injected, the addition of the cocatalyst, a polymer was obtained final ethylene of melt index, density, and distribution of desired molecular weight.

The production of low molecular weight ethylene homopolymer in the first reactor solves the problem encountered in patent applications EP-A-580 930 and EP-A-649860 where the production of high molecular weight polyethylene is in the first reactor; the latter arrangement inevitably causes The drive into the second reactor, the unincorporated comonomer in the polymer chains formed in the first reactor.

The following provides two non-limiting examples describing the preparation polyethylene according to the process of the present invention and two comparative examples.

<b>Examples 1 and 2</b>

The copolymerization of ethylene was carried out with 1-hexene in mouth two reactors in liquid phase connected in series, following the polymerization process described above, under the conditions specified in Table 1 below. Isobutane was used as diluent. The concentration of triethylaluminum (Teai) is given by based on the amount of isobutane. The catalyst is a catalyst containing 1% chromium on silica carrier and titanium oxide (4% titanium), activated at 650 ° C. It has the following features:<dl tsize="25" compact="compact"><dt>specific surface</dt><dd>: 443 m²/ G;</dd><dt>pore volume</dt><dd>1.4 cc / g;</dd><dt>average pore diameter</dt><dd>: 125 Å.</dd></dl>

The physical properties such as melt index (MI₂), The index under high load melt (HLMI), the ratio of these indexes HLMI / MI₂(SR) and the density and the mechanical properties (resistance to stress cracking) of the final product are also given in Table 1.

<b>Comparative Examples 3 and 4</b>

Copolymerization of ethylene with 1-hexene was carried out in a single reactor under reaction conditions indicated in the Table 1. monomodal resin obtained properties are Also described in this table.

It is found that the polyethylene resins produced by the process of the invention have a given density, a resistance to the stress cracking (ESCR bell) and a much better value SR much higher. <tables><table><tgroup cols="5"><tbody><row><entry align="left">example</entry><entry align="center">(1)</entry><entry align="center">(2)</entry><entry align="center">(3)</entry><entry align="center">(4)</entry></row><row><entry namest="1" align="left"><b>First Reactor</b></entry><entry namest="2" nameend="3" align="center">Two reactors in series</entry><entry namest="4" nameend="5" align="center">A single reactor</entry></row><row><entry align="left">Volume (L)</entry><entry align="right">70</entry><entry align="right">70</entry><entry align="right">70</entry><entry align="right">70</entry></row><row><entry namest="1" align="left">Catalyst</entry><entry namest="2" nameend="5" align="center">Cr / Support activat ed at 650 °</entry></row><row><entry align="left">T (° C)</entry><entry align="right">106</entry><entry align="right">106</entry><entry align="right">105</entry><entry align="right">103</entry></row><row><entry align="left">TEAL (ppm)</entry><entry align="right">0</entry><entry align="right">0</entry><entry align="right">0</entry><entry align="right">0</entry></row><row><entry align="left">Ethylene (kg / hr)</entry><entry align="right">3.1</entry><entry align="right">3.1</entry><entry align="right">9.3</entry><entry align="right">10</entry></row><row><entry align="left">1-Hexene (cc / h)</entry><entry align="right">0</entry><entry align="right">0</entry><entry align="right">75</entry><entry align="right">87</entry></row><row><entry align="left">H₂ (Nl / h)</entry><entry align="right">0</entry><entry align="right">0</entry><entry align="right">10</entry><entry align="right">10</entry></row><row><entry align="left">Isobutane (kg / h)</entry><entry align="right">thirty</entry><entry align="right">thirty</entry><entry align="right">26</entry><entry align="right">26</entry></row><row><entry align="left">Residence Time (min)</entry><entry align="right">71</entry><entry align="right">71</entry><entry align="right">68</entry><entry align="right">66</entry></row><row><entry align="left">Gas Release</entry></row><row><entry align="left">Ethylene (% by weight)</entry><entry align="right">1.9</entry><entry align="right">1.8</entry><entry align="right">4.3</entry><entry align="right">4</entry></row><row><entry align="left">1-Hexene (% by weight)</entry><entry align="right">0</entry><entry align="right">0</entry></row><row><entry align="left">H₂ (% By volume)</entry><entry align="right">0</entry><entry align="right">0</entry><entry align="right">0.08</entry><entry align="right">0.12</entry></row><row><entry align="left">MID₂ (G / 10 min.)</entry><entry align="right">2.1</entry><entry align="right">2.4</entry><entry align="right">0.09</entry><entry align="right">0.09</entry></row><row><entry align="left">Density (g / cc)</entry><entry align="right">0.964</entry><entry align="right">0.964</entry></row><row><entry align="left">% Polymer Produced in 1st reactor</entry><entry align="right">51</entry><entry align="right">52</entry></row><row><entry align="left"><b>second Reactor</b></entry></row><row><entry align="left">Volume (L)</entry><entry align="right">35</entry><entry align="right">35</entry></row><row><entry align="left">T (° C)</entry><entry align="right">85</entry><entry align="right">84</entry></row><row><entry align="left">TEAL (ppm)</entry><entry align="right">0.5</entry><entry align="right">1.0</entry></row><row><entry align="left">Ethylene (kg / hr)</entry><entry align="right">4.0</entry><entry align="right">4.1</entry></row><row><entry align="left">1-Hexene (cc / h)</entry><entry align="right">393</entry><entry align="right">1073</entry></row><row><entry align="left">H₂ (Nl / h)</entry><entry align="right">0</entry><entry align="right">0</entry></row><row><entry align="left">Isobutane (kg / h)</entry><entry align="right">7</entry><entry align="right">7</entry></row><row><entry align="left">Residence Time (min)</entry><entry align="right">28</entry><entry align="right">28</entry></row><row><entry align="left">Gas Release</entry></row><row><entry align="left">Ethylene (% by weight)</entry><entry align="right">5.8</entry><entry align="right">6.1</entry></row><row><entry align="left">1-Hexene (% by weight)</entry><entry align="right">0.46</entry><entry align="right">1.0</entry></row><row><entry align="left">MID₂ (G / 10min)</entry><entry align="right">0.07</entry><entry align="right">0.09</entry><entry align="right">0.29</entry><entry align="right">0,295</entry></row><row><entry align="left">HLMI</entry><entry align="right">13.2</entry><entry align="right">18.6</entry><entry align="right">25.3</entry><entry align="right">26.9</entry></row><row><entry align="left">SR</entry><entry align="right">189</entry><entry align="right">207</entry><entry align="right">86</entry><entry align="right">91</entry></row><row><entry align="left">Density (g / cc)</entry><entry align="right">.9584</entry><entry align="right">.9567</entry><entry align="right">.9581</entry><entry align="right">0,958</entry></row><row><entry align="left">Bell ESCR F₅₀ (Hrs)</entry><entry align="right">175</entry><entry align="right">F₀&gt; 350</entry><entry align="right">57</entry><entry align="right">54</entry></row><row><entry align="left">50 ° C - 100% Antarox</entry></row></tbody></tgroup></table></tables>