PROCESS FOR COAGULATION FINISHING OF POLYMERS

Abstract: A process for coagulation finishing (co)polymer by using polyvinyl alcohol as dispersant, during said coagulation. In particular low viscosity block copolymers, comprising at least one block of predominantly conjugated diene and at least one block of predominantly monovinyl aromatic monomer can be successfully finished. The polyvinylalcohol is characterized by a weight average molecular weight of from 200 to 200,000 and by an average degree of hydrolysis of from 25 to 98 %.

Description: Process for Coagulation Finishing of Polymers The invention relates to a process for coagulation finishing of polymers. More in particular the invention relates to an improvement of the steam coagulation process, finishing block and random copolymers derived from predominantly conjugated diene and\or predominantly monovinyl aromatic. The use of steam coagulation processes for finishing random and block copolymers, from a solution of said copolymers in an organic solvent, after termination of the organo alkali metal initiated anionic polymerisation reaction of predominantly monovinyl aromatic and predominantly conjugated diene, by means of either a proton donating agent in the case of a fully sequentially prepared block copolymer or a coupling agent in the case of an initially prepared intermediate diblock copolymer, was known in principle from e.g. European patent application No. 0413403. From said European patent application and in particular page 6 lines 7,8 and lines 54 - 56, it was known to treat a cement of random copolymers or block copolymers from monovinyl aromatic and conjugated diene and a solvent, comprising for a significant part cyclopentane, by means of steam coagulation. However, a generally appreciated shortcoming of such a process, in particular when applied on block copolymers, derived from monovinyl aromatic and conjugated diene and having a relative low viscosity, was the formation of large agglomerates and severe reactor fouling and/or the finishing of said block copolymers proceeding in very low throughputs, and therefore additional recovery costs. On the other hand, it was generally known to the average person skilled in the art, that alternative finishing methods of block copolymers having a relatively low viscosity and/or a relatively high poly (monovinyl aromatic) content, were involved with much higher capital investments due to additional expensive equipment. It will be appreciated by a person skilled in the art, that just the need for block copolymers derived from predominantly monovinyl aromatic and predominantly conjugated diene having a relatively low viscosity, for several modern block copolymer end use applications, e.g. pressure sensitive adhesives, was still growing, whereas on the other hand the economy of block copolymer finishing was demanding for lower costs, i.e. for higher throughputs, and reduced clean out time due to avoid or diminish fouling, and lower capex. An object of the present invention was therefore to provide an improved process for the coagulation finishing of (co)polymers, and in particular those derived from predominantly conjugated diene and/or predominantly monovinyl aromatic. It will be appreciated that with the term "process for coagulation finishing of (co)polymers", as used throughout the present specification, are meant to be covered in principal all finishing processes wherein a (co)polymer containing organic solvent solution is mixed with an immiscible liquid, wherein the polymer is substantially not soluble, under simultaneous supply of heat to devolatilize a substantial part of the organic sovent by means of a hot fluid, such as steam or a hot inert gas e.g. nitrogen. Of these embodiments the steam coagulation finishing process is the most preferred. As a result of extensive research and experimentation such an improved process aimed at has surprisingly been found. Accordingly the present invention relates to a process for coagulation finishing of (co)polymers, and in particular those derived from predominantly conjugated diene and/or predominantly monovinyl aromatic, by coagulation of said (co)polymer containing cement in the presence of polyvinyl alcohol as dispersant. Preferably the present invention is relating to a process for steam coagulation finishing of (co)polymers and more preferably to a process for steam coagulation finishing of block copolymers comprising at least one block derived from a predominantly conjugated diene and at least one block derived from a predominantly monovinyl aromatic. The polyvinyl alcohol to be used for the process of the present invention has a weight average molecular weight in the range of from 200 to 200,000 and preferably from 3000 to 30,000 and may have an average degree of hydrolysis in the range of from 25 to 98% and preferably from 60 to 90%. The polyvinyl alcohol is added in amounts of from 10 to 10,000 ppm relative to the weight of the coagulation water, and preferably from 25 to 1000 ppm and most preferably from 40 to 100 ppm. According to a preferred embodiment of the present process the polyvinylalcohol is introduced into the contactor by means of a small water stream, but it will be appreciated that according to other embodiments the PVA can be introduced into the contactor via the premixed cement. It has surprisingly been found that the use of polyvinylalcohol as dispersant during a coagulation finishing process and in particular a steam coagulation finishing process for block copolymers, containing at least one block of predominantly conjugated diene and at least one block of predominantly monovinyl aromatic, could prevent the formation of large agglomerates and severe reactor fouling for low viscosity block copolymer grades, whereas the block copolymer concentration in the slurry could be increased significantly e.g. from 1 wt% to 4.5 wt% when applying 50 ppm of polyvinyl alcohol in the coagulation water. The use of polyvinyl alcohol dispersant will allow higher throughputs due to increased block copolymer concentrations in the aguenous slurry and reduced clean out time due to reactor fouling. With the term "predominantly" as used throughout the specification, is meant that the specified monomer can be used in substantially pure form or can be intentionally mixed with certain specific minor amounts of comonomer (< 20wt% ) which may be structurally similar to or structurally different from the major monomer constituent of a block segment. E.g. the main constituent 1, 3-butadiene may be mixed with minor amounts of other conjugated dienes, such as isoprene, 1, 3-pentadiene, 2, 3-dimethyl-l, 3-butadiene, 2-methyl- 1, 3-pentadiene or 2, 3-dimethyl-l, 3-pentadiene or mixtures thereof, or with structurally different comonomer, such as monovinyl aromatic. The monovinyl aromatic monomer main component usually will be selected from styrene, optionally mixed with minor amounts of α-methylstyrene; 3,5 diethylstyrene, 4-n- propylstyrene, 2,4, 6-trimethylstyrene; 4-phenylstyrene, 4-methyl styrene, 3, 5-diphenylstyrene, 1- vinylnaphtalene; 3-ethyl-l-vinylnaphtalene; phenyl-1- vinylnaphtalene or mixtures thereof, or mixed with isoprene, butadiene, 1, 3-pentadiene, 2, 3-dimethyl-l, 3- butadiene, 2-methyl-l, 3-pentadiene or 2, 3-dimethyl-l, 3- pentadiene or mixtures thereof. The copolymer block segment derived from such monomer mixtures may have a substantially random or tapered structure. If any, preferably the major monomer constituent of one specific block segment, is mixed with the main monomer component of the other block segment e.g. butadiene mixed with styrene. It will be appreciated that the process of the present invention is applied on (co)polymer cements, the polymer in which may preferably consist of a predominantly poly(conjugated diene) or of a copolymer of conjugated diene and monovinyl aromatic. More preferably the process is used for finishing of block co-polymers which have a bound poly (monovinylaromatic) content in the range from 15 to 90 wt% and more in particular from 25 to 85 wt%. Another particular category of block copolymer grades to be preferentially finished by the process of the present invention is formed by low viscosity block copolymer grades to be used in adhesive compositions. Said block copolymers category comprises in particular block copoloymers of the type ABA or (ABijn X, wherein the component A represents a predominantly polystyrene block, having an average molecular weight ranging from 5000 to 30,000 wherein B represents a predominantly poly (isoprene) block and/or poly(butadiene) block, which optionally may have been selectively hydrogenated, and having an average molecular weight ranging from 30,000 to 70,000 wherein B^ represents a predominantly poly (isoprene) and/or poly(butadiene) block segment having an average molecular weight ranging from 15,000 to 35,000,and which optionally may have been selectively hydrogenated wherein X is the residue of a non halogen containing coupling agent and n>3, the overall molecular weight whereof ranges from 50,000 to 300,000 and wherein the contents of bound styrene in the block copolymer is in the range of from 10 to 50 wt% relative to the weight of the complete block copolymer. More in particular, the average molecular weight of the B block of said specific ABA triblock copolymers, will range from 30,000 to 50,000, while the A block's average molecular weight will range from 8,000 to 25,000, the overall molecular weight of the copolymer ranges from 50,000 to 200,000 and the A component is present in an amount ranging from 10 to 30 wt%. It will be appreciated that a triblock copolymer ABA or a radial block copolymer (AB^JnX main product may be accompanied by a varying amount of diblock copolymer AB^, which is the inevitable result of coupling of precedingly prepared living block copolymers AB 1 with a coupling agent into (ABl)nX or ABA, due to a coupling efficiency of <95%, or which has intentionally been coproduced with or added to a fully sequential polymerised triblock copolymer in order to have adapted the processability on behalf of the manufacture of hot melt adhesive compositions. Said present diblock copolymers may have an average molecular weight from 40,000 to 80,000. The process of the present invention has been found to be successfully applied on (co)polymer cements comprising a large variety of inert organic solvents, in which the preceding polymerisation took place. More particularly the process has been found to be successfully applied on cements comprising block copolymers containing pure blocks of styrene and blocks of butadiene and/or isoprene. The solvents may usually have been selected from cyclohexane or cyclopentane, optionally mixed with minor amounts (<50 vol%) of solvents selected from linear or branched alkanes,having from 5 to 7 carbon atoms, such as n-hexane, isopentane or n-pentane. The process of the present invention is normally applied by mixing the cement with steam, of a pressure in the range of from 3 to 12 bara and a temperature in the range of from 120 to 200°C in a contactor and the mixture is led to a coagulator, which is usually operated at a temperature in the range from 90 to 120°C and at a pressure from 1.0 to 2.5 bara, and wherein most of the solvent is flashed off and condensed. The resulting rubber slurry contains normally from 3 to 5wt% of polymer and in particular elastomer and approximately 5wt% of residual solvent on polymer which is further removed in one or more further steam flashing steps, operating at a temperature from 95 to 105°C and from 1 to 1.5 bara, until a solvent content of 0.2 wt% or lower. Normally a residence time in each steam flashing vessel is in the range of from 20 to 40 minutes. It is true that a process of dewatering an aqueous slurry of polymer beads, and in particular polyester beads, which slurry contained poly vinyl alcohol) dispersion stabiliser, was known from e.g. US patent 4,154,923. Said process was comprising the steps: (a) precipitation in the slurry of an insoluble organic destabilizer, having a molecular weight of at least 200, from a soluble parent compound containing solubilising ionizable groups, in a sufficient quantity to flocculate and promote dewatering of the slurry and (b) dewatering the slurry so treated by physically separating therefrom water, at a temperature of at least 50°C, to form a soft cake of polymer beads, essentially free of poly (vinyl alcohol) dispersion stabiliser. However the use of poly (vinyl alcohol) was only taught to a person skilled in the art to be disadvantageous, as poly (vinyl alcohol) containing polymer slurries to be recovered needed an additional ' precipitation step with destabilizer to prepare the polymer free of poly (vinyl alcohol) . Therefore a person skilled in the art could only lead away from its application for coagulation finishing. The invention can be further illustrated by the following examples, however without restricting its scope to these embodiments. Example 1 1500 gram of Cariflex TR-1112 was charged to a 25 litre stainless steel reactor equipped with a stirrer and 14.1 litre (11.0 kg) cyclohexane and 9 litre (5.7 kg) iso-pentane was added. The reactor was pressurised with 1 barg nitrogen and stirred during 12 hours. The resulting solution was transported to a vessel and stored whilst stirring under 4 barg nitrogen pressure. A 100 litre coagulation vessel equipped with a stirrer was charged with 30 litre demineralised water. To this reaction was then added 1.616 gram of polyvinylalcohol having an average molecular weight (Mw) in the range of from 9000 g/mole to 10000 g/mole and an average degree of hydrolysis of 80% as a solid. The aqueous solution was stirred whilst heated by high pressure steam and a mixture of high pressure steam and the hydrocarbon thermoplastic elastomer solution were injected into the coagulation vessel at such a rate that a steady stream of distillate, consisting of water and hydrocarbon solvent, was obtained. The content of the coagulation vessel was discharged and collected. The wet polymer was dried using hot air of approximately 45°C during 8 hour. 1480 gram of fine white crumbs were obtained. The coagulation of Cariflex TR-1112 without the added polyvinylalcohol, carried out under similar conditions as described above, resulted in large agglomerates and severe fouling of the coagulation vessel occurred. Example 2 9 litre (7.0 kg) of an experimental thermoplastic elastomer (10 %wt in cyclohexane, branched SBS, Mw step-I.35000 g/mole, 70 %wt polystyrene content) was charged to a vessel and 6 litre (3.7 kg) of iso-pentane was added. The resulting solution was stored whilst stirring under 4 barg nitrogen pressure. A 100 litre coagulation vessel was charged with 30 litre demineralised water. To this reactor was then added 1.50 gram of polyvinylalcohol having an average molecular weight (M) in the range of 9000 g/mole to 10000 g/mole and an average degree of hydrolysis of 80% as a solid. The agueous solution was stirred whilst heated by high pressure steam injection to a temperature in the range of 95°C to 100°C in approximately 30 minutes. A mixture of high pressure steam and the hydrocarbon thermoplastic elastomer solution were injected into the coagulation vessel at such a rate that a steady stream of distillate, consisting of water and hydrocarbon solvent, was obtained. Simultaneously an aqeuous solution containing 0.7 %wt of the above described polyvinylalcohol was added to the high pressure steam/polymer solution mixture with an average rate of 10 ml/min. The content of the coagulation vessel was discharged and collected. The wet polymer was dried using hot air of approximately 45°C during 8 hours. Approximately 700 gram of white crumbs were obtained. The coagulation of the same experimental thermoplastic elastomer (10 %wt in cyclohexane, branched SBS, Mw step-I 35000 g/mole, 70 %wt poltstyrene content) without the added polyvinylalcohol, carried out under similar conditions as described above, resulted in the formation of very large agglomerates leading to problems in discharging the coagulator and in the drying step.

Claims: C L A I M S 1. A process for coagulation finishing (co)polymers, characterized in that the coagulation of said (co)polymer is carried out in the presence of polyvinyl alcohol as dispersant. 2. A process according to claim 1, characterized in that the (co)polymer has been derived from predominantly conjugated diene and/or predominantly monovinylaromatic. 3. A process according to claims 1 or 2, characterized in that a steam coagulation finishing is carried out in the presence of polyvinyl alcohol as dispersant. 4. A process according to claims 1-3, characterized in that block copolymers, containing at least one block of predominantly conjugated diene and at least one block of predominantly monovinyl aromatic and showing a low viscosity, are steam coagulation finished. 5. A process according to claims 1-4, characterized in that the cement is mixed with steam of a pressure in the range of from 3 to 12 bara and a temperature in the range of from 120 to 200°C in contactor and that the mixture is led to a coagulator, operated at a temperature in the range of from 90 to 120°C and at a pressure of from 1.0 to 2.5 bara. 6. A process according to claims 1-5, characterized in that the polyvinyl alcohol has a weight average molecular weight in the range of from 3000 to 30,000. 7. A process according to claims 1-6, characterized in that the polyvinyl alcohol has an average degree of hydrolysis in the range of from 60 to 90%. 8. A process according to claims 1-7, characterized in that the polyvinyl alcohol is added in amounts of 40 to 100 ppm. 9. A process according to claims 1-8, characterized in that the polyvinyl alcohol is introduced into the contactor by means of a small water stream.

Inventors: GROTENHUIS PAULUS ALEXANDER MA, VAN DIJK CORNELIS MARTINUS, GROTENHUIS, PAULUS, ALEXANDER, MARIA, VAN DIJK, CORNELIS, MARTINUS
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1995
Main classification: C08F6
IPC codes: C08C2, C08F6, C08F236, C08F297, C08J3