A process of forming a copolymer

Abstract: A process for forming a copolymer of butadiene and other monomers such as styrene is disclosed wherein the final product is characterized by a low residual butadiene content. The process comprises: (a) charging a reaction vessel with butadiene and styrene; (b) initiating said copolymerization reaction; (c) after the exotherm phase of said reaction, purging said reaction vessel with an inert gas while heating said reaction vessel to a final polymerization temperature.

Description: The present invention relates to a process of forming a copolymer and, more particularly, to a process of forming a copolymer in a reaction vessel from a mixture comprising butadiene and at least one other olefinic co-monomer. In modern electrophotographic equipment, toner particles are automatically recycled many thousands of times over image surfaces while the particles are moving at an extremely high velocity. Further, toner particles, which are deposited in image configurations, must now be fused in extremely short periods of time. Thus, it is necessary for toner materials to possess the proper triboelectric charging properties for electrostatic latent image development and furthermore, they must not agglomerate during storage and transportation. Thus, it is necessary for the toner to endure the harsh environment of high speed electrostatographic copiers and duplicators and be capable of fusion at lower energy levels. Polymers have been developed which exhibit properties meeting the stringent standards of advanced copiers and duplicators. An example of a copolymer used for a toner is a copolymer of styrene butadiene. Such copolymers of styrenebutadiene may be made by various techniques such as solution and emulsion polymerization, however, suspension polymerization has been found to be the most suited for forming copolymers for use in toners. The styrene-butadiene suspension polymerization process as described in U.S. Patent No. 4,558,108 (the '108 patent) uses a series of depressurizations/repressurizations ("venting") of the reactor headspace in order to reduce the residual butadiene in the polymer to levels which are environmentally acceptable. The '108 patent discloses a process for forming in a reaction vessel a copolymer of styrene and butadiene in which there is provided an aqueous suspension phase comprising water, styrene monomer, butadiene monomer, a suspension stabilizing agent, and a chain propagating amount of a free radical polymerization initiator which is insoluble in water, soluble in the styrene monomer, soluble in the butadiene monomer and having a 1 hour half-life between about 50°C and about 130°C, the ratio of the styrene monomer to the butadiene monomer being between about 80:20 and about 95:5 by weight, the weight proportion of water to the combination of the styrene monomer and the butadiene monomer being between about 0.8:1 and about 2:1, and wherein the suspension stabilizing agent consists essentially of a finely-divided, difficultly water-soluble powder; and a vapor phase comprising an inert gas and butadiene monomer. In the process the aqueous phase and the vapor phase are heated to a temperature between about 50°C and about 130°C at a pressure between about 138 kPa (20 psi) and about 965 kPa 140 psi until at least about 90 percent by weight of the styrene monomer and the butadiene monomer are copolymerized to form an aqueous suspension of discrete polymer particles having a Tg value of between about 45°C and about 65°C, a weight average molecular weight of between about 10,000 and about 400,000, a molecular weight distribution of the copolymer between about 2 and about 9 and a butadiene monomer concentration of less than about 10 parts per million by weight. During the process, butadiene monomer is removed from the vapor phase, by venting and repressurization of the reactor vessel, after at least about 75 percent by weight of the butadiene monomer and the styrene monomer in the aqueous phase have been converted to a copolymer and prior to conversion of more than about 98 percent by weight of the butadiene monomer and the styrene monomer to a copolymer in the aqueous phase. Figure 1 represents the standard time/temperature/pressure profile of '108 patent venting process. The steps of the prior process are as follows: First, the reactants are charged in a reactor ("charge phase"; labelled "A" in Figure 1). The reaction is initiated and continues exothermically ("exotherm phase"; labelled "B" in Figure 1) with an initial increase in pressure and temperature, after which the temperature is maintained at a polymerization temperature of 95°C. In the preferred embodiment of the '108 patent, about 165 minutes after the start of the exotherm phase, the reactor is vented ("venting phase"; labelled "C" in Figure 1) with a concomitant decrease in pressure resulting in foam formation in the reactor. The vent is then closed and the pressure is increased until the foam disappears. This depressurization/repressurization venting procedure is repeated several times for approximately 30 minutes. The temperature of the reaction vessel is then raised over a 40 minute period ("heat-up phase"; labelled "D" in Figure 1) to 125°C to complete the reaction process. After maintaining the high polymerization temperature for approximately 75 minutes ("high temperature phase"; labelled "E" in Figure 1), the reactor is then cooled ("cool-down phase"; labelled "F" in Figure 1) for approximately 100 minutes, thus bringing the total reaction time to approximately 400 minutes. Although the method of the '108 patent produces polymer with an acceptable level of residual butadiene, the depressurization/repressurization venting procedure creates some inconvenient processing characteristics, such as: 1) the venting procedure delays the process about 30 minutes before the heat-up phase can begin; 2) foam generation during the depressurization cycles can create disturbances which adversely effect the efficiency of the process; 3) extensive above liquid level fouling of the reactor and reactor appurtenances due to foaming can take place. This is believed to be a major factor in necessitating frequent reactor cleaning. Each of these characteristics of the '108 patent process increases the cost and decreases the efficiency of the polymerization process. Other processes for making these and similar types of polymers exhibit similar inconveniences. It is an object of the present invention to provide a process for polymerization which produces acceptable levels of residual butadiene in the resultant polymer, while overcoming these inconveniences of prior methods. Accordingly, the present invention provides a process of forming a copolymer in a reaction vessel from a mixture comprising butadiene and at least one other olefinic co-monomer, characterised by purging the unreacted butadiene monomer from the vapor space in the reaction vessel by an inert gas stream, following substantial completion of the exotherm phase of the reaction and during at least a portion of the heat-up phase of the reaction. In one embodiment of the invention there is provided.a process for removing residual butadiene from a copolymer composition formed by the reaction of butadiene and at least one second monomer in a reaction vessel, characterised by purging said reaction vessel with an inert gas after the desired degree of polymerization has occurred, and further characterised in that said purge occurs while the reaction vessel is heated to the final polymerization temperature. In a further embodiment of the invention there is provided a process for forming a copolymer of butadiene and styrene, said process including charging a reaction vessel with the ingredients required for a suspension copolymerization procedure including butadiene and styrene, initiating said copolymerization reaction and after the exotherm phase of said reaction, heating said reaction vessel to a final polymerization temperature, characterised by purging said reaction vessel with an inert gas while the reaction vessel is heated to the final polymerization temperature and completing the polymerization at the final reaction temperature. The present invention addresses the above-mentioned problems by avoiding the depressurization/repressurization cycle venting procedure and instead using an inert gas purge which is timed to begin after a substantial selected degree of polymerization has occurred and then is maintained for a period and rate necessary to reduce the butadiene content in the polymer to a very low level. In addition to avoiding the foam formation and the resultant reactor fouling, the purge is applied during the heat up phase, thus eliminating "non-productive" process time associated with prior methods. Further, the purge procedure is not limited to 30 minutes and the purge can be extended to control the residual butadiene levels in the copolymer and thus achieve lower butadiene levels. The present invention stems in part from the observation that when practicing the process of the '108 patent, the polymerization exotherm subsides at approximately 135 minutes after the beginning of the reaction thus indicating a high degree of conversion. Holding the temperature constant for an additional 30 minutes was necessary only to allow completion of the prescribed series of depressurization and repressurization to remove residual butadiene. It was discovered that altering the pressure profile during the standard venting procedure would (1) prevent fouling of the reactor above liquid level by eliminating foaming in the reaction vessel and the resultant transport and deposition of polymer on reactor internals and (2) achieve an acceptable level of residual unreacted butadiene in the polymer product. A process for forming a copolymer of butadiene and styrene is disclosed. The process comprises charging a reaction vessel with butadiene, styrene and possibly other comonomers, together with all other ingredients necessary for the copolymerization reaction; initiating the copolymerization reaction; permitting the reaction to continue for a selected period of time; and thereafter purging the reaction vessel with an inert gas for a limited period of time until the residual unreacted butadiene is substantially removed. The reaction vessel is heated to a final polymerization temperature during the purging step. The purging preferably occurs after the exotherm phase of the reaction has been completed. Most preferably, the reaction vessel is purged after at least 75 percent by weight of the butadiene and the styrene are converted to a copolymer. The purging is therefore used only after a pre-selected conversion. After the reaction vessel is heated to its final polymerization temperature, the reaction vessel is cooled and polymer is collected. In the preferred embodiment of the invention, a continuous inert gas purge allows for a reduction in reactor cycle time by eliminating the depressurization/repressurization procedure of approximately 30 minutes, for example, without introducing a new component in the batch formulation. Foam production is virtually eliminated, thus reducing reactor fouling and increasing the number of batches run between reactor cleanings. This invention further eliminates reaction time as the purge is employed during the heat- up phase of the reaction. Application of the process of the invention to reduce reaction cycle time, control residual butadiene levels and eliminate reactor foaming is not limited to a toner resin production process; it can be used in other styrenebutadiene copolymerization processes, as well as other butadiene copolymerization processes. The invention will be described further, by way of examples, with reference to the accompanying drawings in which:- Figure 1 depicts the time/temperature/pressure profile of the process of the '108 patent employing venting prior to the heat-up phase, Figure 2 depicts the time/temperature/pressure profile of Example 1 wherein a continuous inert gas purge is performed after the exotherm phase of the reaction prior to the heat up phase. Figures 3 and 4 depict the time/temperature/pressure profiles used in Examples 2 and 3, respectively. In all the Figures, (1) time "0" minutes is indicated for each curve by " T o ", and (2) the various phases of the depicted process are labelled as follows: charge phase ("A"), exotherm phase ("B"), venting phase ("C"), heat up phase ("D"), high temperature phase ("E"), cool down phase ("F"), and purge phase ("G"). In a preferred embodiment of the present invention the standard time/temperature/pressure profile (Figure 1) is altered such that the removal of the unreacted volatile monomer is accomplished during, not before, the heat-up phase. Instead of venting the reactor by a series of depressurizations/repressurizations which result in foam formation, the pressure of the reactor is maintained at the pressure observed after the exotherm phase, and an inert gas, for example, nitrogen, purge ("purge phase"; labelled "G" in the Figures) is maintained for a suitable time, preferably 20-30 minutes. In the embodiment not according to the invention shown in Figure 2, the exotherm phase of the reaction is complete once the first initiator is practically consumed (i.e., 135 minutes after the start of the exothermic period). It is important that the removal of the unreacted volatile monomer occurs after the exotherm phase of the reaction at a high conversion of copolymer in order to achieve consistent molecular properties. In the depicted preferred embodiment the purging occurs immediately following the exotherm phase of the reaction and is followed by the heat-up phase. In the preferred embodiment of Figure 3, the reactants are first charged into the reaction vessel during the charge phase. The reaction is initiated and continues exothermically with an initial increase in temperature and pressure during the exotherm phase. The initial polymerization temperature of 95°C is maintained for approximately 130 minutes. The exotherm ends after approximately 135 minutes and the heat-up phase to 125°C begins immediately thereafter (i.e., there is no delay of 30 minutes formerly employed for the venting phase). During the heat-up phase the reactor is purged with an inert gas to remove the unreacted volatile monomer. The reactor temperature is maintained at 125°C (final polymerization temperature) for approximately 60 minutes (high temperature phase) before the cool-down phase. In effect, the time consumed by the venting phase of the standard profile of Figure 1 has been eliminated. Examples 2 and 3 below further illustrate specific features of the invention. Percentages are by weight unless otherwise indicated. In each of the following examples, unless otherwise specified, a 100 gallon reactor was charged with material as follows: The Alkanol (sodium alkylnaphthalenesulfonate, available from E. I. Du Pont de Nemours) and tricalcium phosphate (TCP) were predispersed in 15 L of H₂O for 30 minutes. This dispersion was then added to the remaining water. This final aqueous dispersion was charged into the reactor and then heated to 95°C. The organic materials, i.e., styrene, initiators, benzoyl peroxide (BPO) and oo-t-butyl-o- (2-ethylhexyl)monoperoxycarbonate (TBEC) were then mixed in a separate vessel. The pre-weighed butadiene was then added to the vessel. The organic material was then transferred into the reactor. The polymerization reaction was then allowed to proceed under the specified temperature and pressure profile. Example 1 (not according to the invention) A polymer was prepared in a 100 gallon reactor in accordance with the procedure described in example 15 of the '108 patent. The depressurization/repressurization venting was replaced by an inert gas purge in accordance with the present invention. The following reactants were used in this Example. Styrene 105.3 kg Butadiene 15.4 kg BPO (78%) 4,290.g TBEC 550.1 g H₂0 163.9l TCP 1.927 kg Alkanol 49 g After 137 minutes from the start of the exotherm phase of the reaction, the pressure was noted and the vent controller was set to maintain this pressure. The nitrogen supply valve was opened to admit nitrogen to the reactor by way of the spray ball. The reactor was continuously purged for 30 minutes and then the nitrogen valve was closed as well as the reactor vent valve. The reactor temperature was then raised to about 125°C and the polymerization was completed as described in Example 15 of the '108 patent, by maintaining at 125°C for another hour and then allowing the reactor to cool. Figure 2 represents the time/temperature/pressure profile used in Example 1. Example 2 (according to the invention) A polymer was prepared in a 100 gallon reactor in accordance with the procedure described in example 15 of the '108 patent. The depressurization/repressurization was replaced by an inert gas purge in accordance with the present invention. The purging procedure was performed during the heat-up to 125°C. At 137 minutes from the start of the exotherm phase, the reactor pressure was noted and the pressure controller was set to the reactor pressure and the heating to 125°C was commenced. Nitrogen was admitted through a spray ball at a flow rate of 60 SCFH and after 30 minutes of purging the reactor vent valve was closed and the nitrogen flow was stopped. The polymerization was completed at 125°C as described in example 15 of the '108 patent, by maintaining at 125°C for another hour and then allowing the reactor to cool. Figure 3 represents the time/temperature/pressure profile for Example 2. Example 3 (according to the invention) A polymer was prepared in a 100 gallon reactor in accordance with the procedure described in example 15 of the '108 patent. The depressurization/repressurization was replaced by an inert gas purge in accordance with the present invention. The purging procedure was performed during the heat-up to 125°C. The procedure of Example 2 was repeated except that during the purging, the pressure was increased one psi (1 psi = 6.89 kPa) every 5 minutes. Figure 4 represents the time/temperature/pressure profile for Example 3. The characteristics of polymer materials produced according to the process of the '108 patent and Examples 1 to 3 are summarized in Table 1. As shown in Table I, the methods of the present invention provide polymer product with characteristics commensurate with the process of the '108 patent without the associated process drawbacks.

Claims: A process of forming a copolymer in a reaction vessel from a mixture comprising butadiene and at least one other olefinic co-monomer, characterised by purging the unreacted butadiene monomer from the vapor space in the reaction vessel by an inert gas stream, following substantial completion of the exotherm phase of the reaction and during at least a portion of the heat-up phase of the reaction. A process as claimed in claim 1, characterised in that the purging of said reaction vessel with the inert gas occurs after the desired degree of polymerization has occurred, and further characterised in that said purge occurs while the reaction vessel is heated to the final polymerization temperature. A process as claimed in claim 1 or claim 2, characterised in that the copolymer comprises butadiene and styrene, said process including charging the reaction vessel with the ingredients required for a suspension copolymerization procedure including butadiene and styrene, initiating said copolymerization reaction, and after the exotherm phase of said reaction, heating said reaction vessel to a final polymerization temperature, purging said reaction vessel with the inert gas while the reaction vessel is heated to the final polymerization temperature and completing the polymerization at the final reaction temperature. A process as claimed in any one of claims 1 to 3, characterised in that said reaction vessel is first charged with an aqueous dispersion of sodium alkylnaphthalenesulfonate and tricalcium phosphate, and then said reaction vessel is charged with an organic mixture comprising styrene, benzoyl peroxide, oo-t-butyl-o-(2-ethylhexyl)- monoperoxycarbonate and butadiene. A process as claimed in claim 3, characterised in that said reaction vessel is purged after at least 75 percent by weight of the butadiene and the styrene are converted to a copolymer. A process as claimed in any one of claims 1 to 5, characterised in that the rate and time of said purge is sufficient to reduce the residual butadiene in the recovered polymer to less than 4 parts per million. A process according to claim 1 for forming a copolymer of styrene and butadiene, said method comprising providing an aqueous phase comprising an aqueous mixture comprising water, styrene monomer, butadiene monomer, a suspension stabilizing agent, surfactant, a free radical polymerization initiator insoluble in water, soluble in said styrene monomer, soluble in said butadiene monomer and having a 1 hour half-life between 50°C and 130°C, the ratio of said styrene monomer and said butadiene monomer being between 80:20 and 95:5 by weight, the weight proportion of water to the combination of said styrene monomer and said butadiene monomer is between 0.8:1 and 2:1, said suspension stabilizing agent consisting essentially of a finely-divided, difficultly water-soluble powder and a vapor phase comprising an inert gas and butadiene monomer, heating, concurrent with the removal of the butadiene monomer, said aqueous phase and said vapor phase to a temperature between 50°C and 130°C at a pressure between 138 kPa and 965 kPa (20 psi and 140 psi), characterised by removing butadiene monomer from said vapor phase by purging said reaction vessel with an inert gas after at least 75 percent by weight of said butadiene monomer and said styrene monomer in said aqueous phase are converted to a copolymer, and while purging said reaction vessel heating said aqueous phase at a temperature between 50°C and 130°C at a pressure between 138 kPa and 965 kPa (20 psi and 140 psi) until said styrene monomer and said butadiene monomer are copolymerized to form an aqueous suspension of discrete polymer particles having a Tg value of between 45°C and 65°C, a weight average molecular weight of between 10,000 and 400,000, a molecule weight distribution of said copolymer between 2 and 9 and a butadiene monomer concentration of less than 10 parts per million by weight.

Inventors: DALE WILLIAM J, LIEBERMANN GEORGE, MCQUEEN RICHARD LEE, DALE, WILLIAM J., LIEBERMANN, GEORGE, MCQUEEN, RICHARD LEE
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1991
Main classification: C08F236
IPC codes: C08F2, C08F6, C08F236