A TWO-STEP BUTADIENE POLYMERIZATION PROCESS FOR MANUFACTURING
HIGH-CIS 1,4-POLYBUTADIENE
CROSS REFERENCE STATEMENT
This application claims the benefit of U.S. Provisional Application No. 60/385,226, filed May 31, 2002.
FIELD OF THE INVENTION
The invention relates to a two-step method for manufacturing bimodal high-cis polybutadiene rubber compositions which are not a blend of separately polymerized polybutadienes. The invention relates to a process improvement allowing the production of high cis-l,4-polybutadiene which includes both very high and very low solution viscosity grades. Such materials offer greater processability, especially with regards to finishing constraints.
BACKGROUND AND SUMMARY OF THE INVENTION
High-Cis polybutadiene is known for its use in many applications, including high impact polystyrene. It has been discovered that the properties of such materials are affected by the molecular weight of the polybutadiene polymers. Unfortunately some of the desired properties are favored by having smaller molecules and some of the desired properties are favored by having longer molecules. Further it has been shown that polybutadiene material having molecular lengths spanning the desired molecular weight range in a uniform bell curve manner do not offer the same properties as polybutadiene material which has a bimodal distribution, that is where the polybutadiene material comprises one portion of material having molecular weights clustered around a lower value, and one portion of material having molecular weights clustered around a higher value.
Normally a bimodal molecular weight distribution exhibiting high-cis-polybutadiene as a synthetic rubber product is the result of a blend of two separately produced high-cis- polymers each exhibiting different molecular weight characteristics, preferably one having a relatively average molecular weight high and a second having relatively low average molecular weight. Such a blending of two high cis polybutadiene rubbers providing a bimodal MWD has been described in U.S. Patents 3,278,644 and 6,191,226 and in WO 00/69928.
Recently, a series of applications were published in Japan relating to polybutadiene having a high nolecular weight component and a low molecular weight component. These references (JP 99193308, JP99181202, IP 99181026, and JP 00072823 JP 00086852), suggest that the desired polybutadiene with a bimodal molecular weight distribution can, in addition to the blending methods described above, be made in a two-stage linear arrangement where one of the components is made in a first reaction vessel, and a second is made in a different reaction vessel.
DD 251149, teaches a two step polymerization process for providing a mix of high and low Mw high-cis-polybutadiene. This process did not require the separate polymerization and resultant blending of the references mentioned above. However, this proces uses a Nickel based Ziegler-Natta system and achieves its bi modality in terms of weight distribution through the use of additional co-catalyst (a boron trifluoride complex of esters). This system can be improved in terms of controlling gel content, ability to control viscosity ranges, and ability to adjust the linearity/viscosity balance.
Accordingly, the present invention offers a process to create high-cis bi-modal polybutadiene compositions without requiring the blending of separately polymerized components, and without having two use two or more rection vessels. It would also be advantageous to have a process which promotes branching in the polybutadiene so that the rubber properties can be improved. The invention in particular describes the manufacturing of bimodal high-cis- 1 ,4-polybutadiene which comprises a two step butadiene polymerization process generating controlled repartition between a high molecular weight part (kinematic Solution Viscosity of 5% rubber in styrene 200 — 500 mmVs) and a low molecular weight part (kinematic Solution Viscosity of 5% rubber in styrene 10— 150 mm2/s). The process can be tailored to provide differing degrees of branching and intermolecular branching distribution in the product as measured with GPC and viscosity detection. Products produced by the process will have a characteristic bimodal molecular weight distribution (MWD) exhibiting two peak maxima or at least a significant broadening of the MWD (that is, the Mw/Mn ratio is larger than at least 3.5, more preferrably 3.8 and most preferably at least 4) compared with one step process carried out under the same conditions but using the same total amount of cobalt right from the start. The process typically results in the amount of 1,2-vinyl-units in the low molecular weight rubber to be
increased significantly (more than 50 %) as compared to the amount of 1,2 -vinyl units in the high molecular weight rubber.
The process of the present invention relates to the selection of particular Cobalt salts together with particular alkylaluminum chloride compounds together with water for use as a catalyst, particularly suited for the production of high-cis poly(butadiene). A portion of the total amount of catalyst is applied at a second stage of the reaction preferably with the addition of particular chain transfer agents and/or molecular weight modifier like COD or TETAM to provide the low molecular weight part of the product. The Ziegler-Natta- polymerization process of the present invention applies Co-carboxylate/dialkylaluminium- chloride/water catalyst systems and differing amounts of 1,5-cyclooctadiene (1 —20 000 ppm per total feed) and triethanolamine (0.1 to 20 weight % per weight Cobalt) in differing ratios. By adjusting the amounts of the two components in different stages of the reaction, the manufactured rubber material will have the characteristic bimodal MWD and microstructure.
Another aspect of the invention relates to the introduction of a chain transfer agent (CTA) and/or molecular weight modifier together with a second amount of catalyst after a first part of the monomer has been converted regarding the required repartition of high to low molecular weight polybutadiene amount in the product. This procedure is especially effective for the preparation of at least bimodal rubber containing increasing amounts of vinyl units in the low molecular weight part.
The method of the present invention also allows an alternative way to adjust the viscosity of a rubber material. Traditionally, to adjust the viscosity, it was necessary to adjust the molecular weight of the material. With the present method, however, the intermolecular long chain branching can be affected so that different viscosities can be produced for products having the same molecular weight, Alternatively, viscosity can be held constant while the molecular weight can be altered. Thus the present method offers much more flexibility to custom design a bimodal rubber material for particular applications.
In contrast to earlier bimodal products which exhibit a particular solution viscosity for each component the two step process of the present invention gives much more flexibility as each component can be adjusted with respect to molecular weight and the
degree of long chain branching. The process therefore facilitates a cost effective method of producing a more linear high molecular weight component. Similarly, the low molecular weight component can be produced with higher branching levels but without changing the solution viscosity (implying larger molecules). This provides better grafting capability as compared to linear (smaller) molecules having the same solution viscosity. The ability to tailor the branching levels of each component can be used to make materials which were previously not available.
Accordingly, another aspect of the invention is a new polybutadiene material, having bimodal molecular weight distribution characterized by the level of branching. The degree of branching can be characterized by TriSEC (Size exclusion chromatography based on tri-dectors which are sensitive to absolut molecularweight, concentration and viscosity) or rehological testing. The TriSEC method gives a branching number which is related to certain number of carbon atoms or per molecule. The bimodal materials can be analyzed according to the high molecular weight component and the low molecular weight component. In the materials of the present invention, the TriSEC number attributed to the high moleculkar weight component is less than 1.5, more preferably less thanl, and most preferably less than 0.5. For the low molecular weight component, the TriSEC number should be larger than 1 more preferably larger than 2 and most preferably larger than 3. Larger TriSEC values such as 5 or greater are possible and can be advanageously used, however care should be taken to avoid gel formation. For the entire bimodal product, an average TriSECvalue between 1 and 2 will be prefered.
Another emperical test which provides and indication of the level of branching is the ratio of solution viscosity to Mooney viscosity (VS/NM). For the high molecular weight component, the NS/NM ratio should be 5 or greater, more preferably 6 or more and most preferably 7 or more. For the low molecular weight component, the NS/NM ratio should be 2 or less, mor preferably 1.5 or less, most preferably 1.2 or less. The bimodal material as a whole should have a value between 2.4 and 5, more preferably between 2.8 and 4.5, and most preferably between 3.3 and 4.
DETAILED DESCRIPTION OF THE INVENTION
The polymerization process of this invention comprises two step molecular weight controlled in-situ polymerizing of 1 ,3-butadiene in the course of one process using differing
levels of 1- and 2-olefins to control molecular weight in the presence of a catalyst system. The catalyst system can be those typically used in the art, which generally include a transistion metal compound, an organoaluminum compound and a polar compound.
According to the present invention the transition from the first to the second polymerization step exhibits fast and significant changes of reaction conditions due to a second introduction of additional catalyst (from 0.5 until 10 times the amount of the transition metal compound used from the start of the polymerization), optionally together with additional co-catalyst and molecular weight modifiers after a controlled period of butadiene conversion. Accordingly the first polymerization step is followed by a controlled second period of butadiene polymerization under the adjusted conditions. Mainly the monomer/catalyst molar ratio of the first phase (generally in the range of 5*105 - 1*105) decreases, preferably between 2 and 10 times, when compared with the monomer/catalyst molar ratio of the second phase.
The polymerization process due to this invention allows the production of rubber grades ranging from very high Mooney- or solution viscosity to very low Mooney- or solution viscosity. The latter may be particularly desired if they exhibit high levels of cold flow by providing the in-situ mix of the same products in different Mooney/solution viscosity ratios throughout one course of reaction. The resulting product can be produced in a way that it has a medium Mooney- (35 - 60) or solution viscosity (80 - 200 cPoise) exhibiting a multimodal- bimodal or at least broad molecular weight distribution. In addition the same procedure could be used for the preparation of at least bimodal rubber containing increasing amounts of vinyl units in the low molecular weight part. The main application relates to tire and impact-resistant aromatic vinyl resin compositions.
While the present invention may be suitable for any catalyst system known in the art for the polymerization of butadiene, the preferred catalysts system includes a cobalt salt of the formula CoAx, where A is a monovalent or divalent anion and x is 1 or 2; an alkyl aluminum chloride compound of the structure R A1C1, where R is an alkyl group containing 1-8 carbon atoms; a trialkyl aluminum compound of the formula R3A1, where R is an alkyl group containing 1-8 carbon atoms; and a catalytic amount of water. The catalyst system may also preferrably further comprise a ternary alkyl amine or a ternary aryl amine and/or a trialkyl aluminum compound of the formula R3A1 where R is as defined above.
The cobalt salt of the prefered catalyst can be any of those generally known in the art. Examples include cobalt (ii) acetylacetonate, cobalt (II) octoate, cobalt (II) isooctoate, cobalt (II) naphthanate, Cobalt (II) neodecanoate and their cobalt (III) congeners. In general it is preferred that the cobalt salt be anhydrous. Of these, cobalt (II) neodecanoate was observed to give the most activity when using the preferred ethylaluminum sesquichloride/trioctyl aluminum co-catalyst.
The preferred catalyst system of the present invention also includes one or more alkyl aluminum chloride compound of structure R2A1C1 where R is independently an alkyl group containing 1-8 carbon atoms. The R group may be straight or branched. Suitable compounds include diethylaluminum chloride, di-n-butylaluminimum chloride, di-n- octylaluminum chloride, ethyl-n-octylaluminum chloride, ethyl aluminum dichloride, and ethylaluminum sesquichloride. It is preferred that one or more trialkyl aluminum compounds of formula R3A1, where R is as defined above, also be part of the catalyst system. Suitable trialkyl aluminum compounds include triethylaluminum, and trioctyl aluminum. It should be understood that the trialkylaluminum can first be reacted with the alkylaluminum chloride compound to form an intermediate species before combining with the catalyst system. For example, an equimolar mixture of ethyl aluminum sesquichloride together with trioctyl aluminum (which mixture may hereafter be referred to as "EOAC") was shown to give particularly good results in terms of maintaining activity while reducing fouling and branching, when added to the catalyst system, as well as reducing the level of gel formation.
The preferred catalyst system also contains a catalytic amount of water. The amount of water should typically be in the range of 0.1 to 0.8 moles per mole of the alkyl aluminum chloride compound used, with about 0.5 being most preferred. The exclusion of additional moisture can be achieved my maintaining a nitrogen or other inert atmosphere over the liquid when preparing the reaction mixture and carrying out the polymerization.
It has been discovered that the presence of a ternary amine can also boost the performance of the catalyst system. The catalyst system can therefore optionally contain a ternary alkyl/aryl amine. The alkyl groups which may be used in this aspect of the invention may be linear or have branching. Aryl groups can similarly be chosen from all existing materials. It is generally preferred, that the amine be somewhat water soluble, however, as this allows it to me more easily removed in water washes. Thus shorter chain
lengths, such as C6 or less, are generally preferred. It should be understood that the same amine may have alkyl and aryl characteristics. Suitable examples include triethylamine, tributylamine, triphenylamine, dimethylphenylamine, and triethanolamine, with triethylamine and triethanolamine being generally more preferred. The amine should be added in an amount such that the molar ratio of cobalt to nitrogen is in the range of 1:0.1 to 1:10, more preferably in the range of 1 : 1 to 1 : 3. It is especially preferred if these materials are added in the second phase of the process of the present invention.
As is generally known in the art, the preferred catalyst system of the present invention will be added to a mixture comprising 1,3 -butadiene in one or more hydrocarbon materials which act as a solvent at least for the monomer. The solvent can also be useful to control the polymerization temperature by refluxing. In this regard, it should be appreciated that by mixing two or more solvents the desired polymerization temperature can be more precisely achieved. Preferred solvents include aliphatic, cycloaliphatic, aromatic, and monoolefinic hydrocarbons and mixtures thereof. Particularly well suited solvents for use with the catalyst system of the present invention include C4-C8 aliphatic hydrocarbons, C5 to CIO cyclic aliphatic hydrocarbons, C6 to C9 aromatic hydrocarbons, and C4 to C6 monoolefinic hydrocarbons or mixtures thereof. 2-butene, 1 -butene, cyclohexane, benzene, pentane, hexane, heptane, toluene, and xylene are specific examples of such suitable solvents.
The preferred catalyst should be made up of the various components in ratios such that when added to the solvent and monomer at the beginning of the reaction, the cobalt is present in the reaction medium in a ratio of cobalt to Al from approximately 1 :75 to 1:150, with a range of 1 : 90 to 1 : 125 being more preferred. Typical cobalt concentrations in the reaction medium are about 2 ppm, although they can range from 0.2 to 10 ppm. The alkyl aluminum chloride/trialkyl aluminum compounds are added such that the total amount of Al in the reaction system is in the range of 0.002-0.004 molar. It is preferred that the concentration of Al in the final reaction mixture be approximately 0.003 molar. It is preferred that from 10 to 90, more preferably 50 to 75 percent of the total Al come from the alkyl aluminum chloride species.
In conducting the polymerization using this catalyst, many procedures may be filed. This catalyst system appears to be particularly effective in polymerizing 1,3 -butadiene in a feed comprising about 5 to 30 percent, more preferably 15 to 25, most preferably about 20
percent by weight 1,3 -butadiene, 30 to 70, more preferably 45 to 65 and most preferably about 55 percent butenes (1 butene and/or 2-butene), and 20 to 40, more preferably 25 to 35 and most preferably about 30 percent by weight cyclohexane, optionally with benzene. A preferred cyclohexane/benzene mix was such that the ratio of cyclohexane to benzene was about 0.65.
Normally the polymerization is conducted at a temperature in the range of -35° to 100°C, more preferably from -10°C to 50°C, most preferably 0°C to 40°C. The polymerization can be conducted in a pressure autoclave if desired.
The initial polymerization reaction is allowed to proceed for a suitable length of time depending on the partition or high molecular weight component to low molecular weight component desired in the end product. The progress of the reaction can be followed by any suitable method known in the art such as heat evolution or sample analysis. When it has been determined that the reaction has proceeded a suitable amount, a second amount of catalyst is added, preferably with an amount of chain transfer agent (CTA) like 1,5- Cyclooctadiene (COD), 1,2-Butadiene, norbornadiene, or 4-vinylcyclohexene and/or accelerating molecular weight modifiers such as alcoholamines having the general formula (OR)3-XNHX (where R is independently alkyl or aryl with Cl — CIO ) such as Tetraethanolamine (TETAM), or other amines having the formula R3-XNHX (where R is independently alkyl or aryl with Cl — Cl 0). If used, the CTA is desirably added in a molar ratio butadiene/CTA from 2000 to 50 preferably 1500 to 100 most preferably from 1000 to 200.
The polymerization can be advantageously carried out in the following manner: The butadiene feed, water and a mixture of the alkyl aluminum chloride compound with the trialkyl aluminum compound, can be added to the reaction vessel in any order, and mixed together in and before metering into the reactor. The cobalt catalyst can then be added, optionally pre-dissolved in a suitable solvent or solvent mixture. The polymerization is started by adding the first amount of cobalt catalyst followed by a second addition of catalyst together with CTA, if any, plus additional co-catalyst if desired. This additional catalyst, co-catalyst and CTA will provide the changed conditions to create the second step of polymerization. The amount of catalyst added in this second stage is such that the ratio of monomer/catalyst molar ratio of the first phase (generally in the range of 5 * 105 - 1 * 105) decreases, preferably between 2 and 10 times, when compared with the monomer/catalyst
molar ratio of the second phase. If a portion of the co-catalyst is added at this stage it is preferably added such that the total amount of co-catalysts are within the guidelines set out above.
The following examples are presented to further illustrate the invention, however they are not intended to limit the scope of the invention to these particular embodiments.
EXAMPLES 1-43
The following polymerization reactions were carried out in a 5 liter stainless steel stirred reactor equipped with the necessary auxiliaries, like inlets and outlets for nitrogen, solvents, and catalysts, a cooling circuit and a premixing vessel at 25°C. The reactor was charged with 3 liters of a dry feed consisting of 1,3 -butadiene in a range between 20.5 — 26 percent by weight as indicated in Table I, 40 - 60 percent by weight of butenes (only butene-2 or a mix of butene-2 /butene- 1 with ratio which was about 0.3 unless stated otherwise in the Table), and the balance being cyclohexane/benzene (25 percent by weight unless stated otherwise) (ratio of cyclohexane to benzene was about 0.65). At 25°C, a previously prepared mixture of aluminumalkyl (15 percent by weigh aluminum alkyl in cyclohexane) with water was added to the feed so as to obtain a 0.00332 molar solution of Al in the reactor. The water to aluminum ratio has been kept at about 0.4 unless stated otherwise. The polymerization was then initiated by injection of a cobalt solution (10 - 12% by weight cobalt (II) salt neodecanoate in mineral oil) into the reactor so as to obtain a cobalt concentration of 0.2 — 1.5 ppm as reported in the Tables. The progress of the reaction was monitored by GC analysis of the samples taken from the gas phase of the reaction vessel. A second amount of catalyst in the range of 1 to 5 ppm as indicated in Table I was added to the reactor after the reaction was 20 — 50 percent complete (as can be calculated from the conversion and repartition values in the Table). When a molecular modifier like an amine, alcohol amine (triethanolamine (TEA)) or diene (1,5- Cyclooctadiene/COD) as chain-transfer-agent was present it was added together with the cobalt solution. TEA, when added, comprised 4 percent by weight of the cobalt solution and was added in the second phase unless otherwise indicated. The COD was only added during the second phase and in the amounts indicated. When TEA was added it was added as part of the Co solution and was added in an amount equal to approximately 4 percent by weight. All materials were handled in a dry nitrogen atmosphere. The solvents and 1,3- butadiene were dried over alumina columns prior to use.
The conversion of the 1,3 -butadiene to polybutadiene was monitored by GC analysis. At approximately 75 percent conversion, the polymerization was terminated by the addition of 2 ml ethanol to the reactor. The polymer solution was then washed with water and coagulated after addition of a standard hindered phenol antioxidant polymer stabilizer.
The recovered product was then subjected to the following analytical tests. Molecular weight determinations (both Mw and Mn) were carried out with Gel Permeation Chromatography using a Waters GPC system being maintained at an internal temperature of about 30°C, and employing 5 "mixed bed" Styragel™ columns (HT6, HT5, HT4, HR3, HRl) in a series, a differential refractive index (DRI) detector and tetrahydrofuran (THF) as the eluent at a flow rate of 0.8 ml/min. The Mw/Mn value is reported as an indication of polydispersity.
The TriSEC analysis for determining branching was conducted as follows :
The rubber sample was dissolved in THF for 30/45 min at a concentration of 1.5 mg/ml.
The solution was then filtered with "Syringe Driven Filter Units" (Millex-FH) 0.45 μm and analyzed using the following equipment and conditions:
Pump 600 E (Waters)
Flow rate : 0.8 ml/min
Injector WISP 717+ (Waters)
Oven CHR (Waters) Internal Temp. : 30°C
Columns PL Gel (Polymer Laboratories)
10 μm Mixed-B LS (300 x 7.5 mm) number : 3 columns Detectors
• Concentration
RI 2410 (Waters) Internal Temp. : 30°C
• Molecular weight & Molecular density (Light scattering & viscometer)
T60A (Viscotek)
Internal Temp. : Ambiant
The folowing software was used to produce the numbers reported in the tables :
Millenium32 (Waters)
(injection)
TriSEC 3.0 GPC software (Viscotek)
(Acquisition, calculation & reports)
Viscosities of products at five percent by weight in styrene solvent (VS) were determined by conventional viscometric techniques according to ASTM is D0446. Mooney viscosities (VM) were determined according to ASTM 1646, ML 1+4 at 100°C. The Linearity of the BR is a relative estimate of the VS/NM ratio. High values (for example 5) of this ratio for materials having the same Mooney value indicate a less branched product and low values for example 1 represent a high degree of long chain branches in the product. The results are shown in the following two Tables.
A comparative example was done as in Example 1 except that no modifier and no second cobalt addition was performed. The conversion had been kept below 50 percent and the NS(520)/NM(80)-ratio has been measured to be 6.5 what refers to a very linear product with a branching number lower than 0.6. This is an indication that the high molecular weight component of the material produced in the process of the present invention exhibits a drastically different level of branching than the low molecular weight component.
In Example 18 the high and low MW part have been investigated more thoroughly by the TriSEC method to compare branching in both parts. It has been found that the high MW part exhibited a branching number below 1 (0.8) and the low MW part a branching resulted in a TriSEC value of 4. The average branching number was 3. Compared to a commercially available rubber this has been found to be almost twice as high for the high MW part and approximately 1.4 times as high for the low MW part.
Table I
Table I
a) Phase of reaction (1 = time from start of reaction till second addition of catalyst components or other , 2 = from the time when the recipe has been changed by second step addition of catalyst or another component)
Table II
It should be realized by those skilled in the art that the invention is not limited to the exact configuration or methods illustrated above, but that various changes and modifications may be made without departing from the spiπt and scope of the invention as described within the following claims: