POLYMERISATION PROCESS FOR POLYESTER

Abstract: A process for producing a polyester in a stable emulsion state, which comprises thermal polymerisation of a bis(ω-hy­ droxyalkyl)terephthalate or an oligomer thereof in the form of finely divided particles dispersed in a silicone oil by using a graft polymer, as a dispersion stabiliser, constructed of both a poly­ siloxane and an acrylic polymer.

Description: The present invention relates to a polymerization process for polyesters and more particularly to a process for producing a polyester, which comprises thermal polymerization of a bis(ω-hydroxyalkyl)terephthalate or an oligomer thereof in the form of fine particles emulsified in a silicone oil. The bulk melt polymerization process has been most commonly applied to produce polyesters which are typified by polyethylene terephthalate. However, this process has several drawbacks such that as the molecular weight of polyester increases under polymerization, the melt viscosity thereof increases and more agitation power becomes necessary, that the increased melt viscosity makes worse the efficiency of heat transfer through walls of the polymerization vessel and hence the polymer is liable to undergo thermal degradation locally, and that a long time is required for taking out the polymer and this brings about difference in the degree of polymerization between the initially discharge polymer and the finally discharged polymer. To overcome these drawbacks, the present inventors studied polymerizations of bis(ω-hydroxyalkyl) terephthalates (hereinafter referred to as BHT) or oligomer thereof. AS a result, a very effective polymerization process for the polyester has been found out, which comprises dispersing BHT or an oligomer thereof with a dispersion stabilizer to a finely emulsified state in a silicone oil which is a medium inert to BHT or to the oligomer, and reacting the BHT or the oligomer to remove the alkyleneglycol therefrom. This type of polymerisation is known for the preparation of graft copolymers having a polysiloxane backbone. For example, JP 60 141715 and JP 60 141714 disclose preparation of a polyester by polymerisation of a bis (w-hydroxy-alkyl) terephthalate in a particulate form in silicone oil or aromatic hydrocarbon, using as dispersion stabiliser, a graft copolymer having a polysiloxane backbone and polyacrylate branching groups. In addition, EP-4947 discloses graft copolymers of acrylic monomers and vinyl end-blocked polysiloxanes. It is an object of the invention to provide a process for polymerising BHT or an oligomer thereof in emulsified form. It is another object of the invention to provide a process for polymerising BHT or an oligomer thereof in a stabilized emulsion state. The substance of the invention is a process for producing a polyester, which comprises thermal polymerisation of a bis (w-hydroxyalkyl) terephthalate or an oligomer thereof in the form of finely divided particles dispersed in a silicone oil by using a graft polymer, as a dispersion stabilizer, constructed of both a polysiloxane and an acrylic polymer. BHT or its oligomer used in the invention is desired to have 2 to 6 carbon atoms in the alkyl group, and is obtained by reacting terephthalic acid or its ester-formable derivative with alkylene glycol or its ester-formable derivative. The silicone oil used in the invention may by any of those stable physically and chemically at temperatures of at least 150°C and inert to BHT or its oligomer. Such silicone oils include, for example, polydimethylsiloxane, polydiphenylsiloxane, polyphenylmethylsiloxane, and poly (dimethylsiloxane-diphenylsiloxane). The graft polymer used as a dispersion stabilizer in the invention is necessary for dispersing the BHT or its oligomer in the silicone oil to a stable, finely divided particle state and for maintaining this good dispersion state. This graft polymer is constructed of a polysiloxane and an acrylic polymer and comprises acrylic polymer stems and polysiloxane branches. The polysiloxane portion of the graft copolymer preferably consists mainly of at least one member selected from the group consisting of polydimethylsiloxane, polymethylphenylsiloxane and polydiphenylsiloxane. The acrylic polymer portion of the graft copolymer is preferably constituted of an acrylic monomer selected from the group consisting of acrylic esters and methacrylic esters, more preferably selected from ethyl acrylate, methyl methacrylate, benzyl methacrylate and phenyl methacrylate. According to the present invention there is provided a process for producing a polyester which comprises a thermal polymerisation of a bis (w-hydroxy-alkyl) terephtalate or an oligomer thereof in the form of particles of up to 300µm, dispersed in a silicone oil, and using a graft polymer as a dispersion stabilizer constructed of both a polysiloxane and an acrylic polymer and having a stem/branch ratio of 80-20 : 20-80 and wherein the dispersion stabilizer is added in an amount of at least 0.1%, characterised in that the graft polymer used as a dispersion stabilizer is constructed of a stem acrylic polymer and a branch polysiloxane and the molecular weight of the branch polysiloxane is at least 500. One of the preferred types of graft copolymer comprises an acrylic polymer which is an adduct of either a bis (w-hydroxyalkyl) terephthalate or an oligomer thereof to a copolymer of (i) an acrylic monomer selected from the group consisting of alkyl acrylates and alkyl methacrylates and (ii) a functional group-containing acrylic monomer selected from the group consisting of glycidyl acrylate and glycidyl methacrylate. The graft polymer comprising a stem acrylic polymer can be produced by polymerising an acrylic monomer such as acrylic acid, methacrylic acid, acrylic ester, or methacrylic ester in the presence of a radical polymerisation initiator along with a polysiloxane which contains a functional group such as methacryloyloxy, mercapto or vinyl group at one terminal. The polysiloxane containing such a functional group is, for example, a polydimethylsiloxane terminated by a methacryloyloxypropyl, mercaptopropyl or vinyl group. The molecular weight of such a polysiloxane is at least 500, preferably at least 1000. When the molecular weight is lower, the dispersion stability deteriorates. The dispersion stability of BHT or its oligomer, in the present invention, is given by the graft polymer, used as a dispersion stabilizer, which is composed of a polysiloxane having good affinity for the silicone oil and an acrylic polymer having good affinity for BHT or its oligomer. The stem-to-branch weight ratio, i.e. the polysiloxane-to-acrylic polymer weight ratio, in the graft polymer is in the range of 80:20 to 20:80 The ratio out of this range results in deterioration of the dispersion stability. It is also desirable that the molecular weight of the graft polymer be in the range of 1,000 to 100,000. Departing from this range leads to deterioration of the dispersion stability. In the present invention, such a graft polymer constructed of a polysiloxane and an acrylic polymer is used in an amount of at least 0.1% by weight based on BHT or its oligomer. If this amount is less than 0.1% by weight, the dispersion stabilizing effect will be poor and in some cases there is a fear that BHT or its oligomer may form no fine particle or the dispersion in fine particle shapes may be broken and large agglomerates of the polyester may form. In carrying out the present invention, BHT or its oligomer is dispersed in the silicone oil to a finely divided particle state in a suitable way such that the BHT or its oligomer is added to the silicone oil containing the graft polymer and shear is applied to the mixture or the graft polymer is added to the silicone oil containing the BHT or its oligomer in melt form and shear is applied to the mixture. Preferably, BHT or oligomer is added in the form of mixture with a solvent thereof to the silicone oil. The solvent may be any of those which keep the liquid state up to 150°C or higher and are able to dissolve BHT or its oligomer but limited amounts of the silicone oil. Such solvents include; aliphatic hydrocarbons, e.g. nonadecane; phenols, e.g. m-cresol, p-chlorophenol, xylenol, and phenol; and benzylalcohol, nitrobenzene, and tetrachloroethane-phenol mixtures. Such a solvent and either BHT or its oligomer, when mixed and heated to a temperature of at least 150°C, form a nearly uniform solution, whereby the BHT or the oligomer can be trituated more effectively in the silicone oil. This facilitates the diffusion of alkylene glycol to the surface of fine particles of BHT or its oligomer, thus contributing greatly to the efficient polymerization thereof. Particles sizes of BHT or its oligomer dispersed in the silicone oil are maintained up to 300 µm, particulary in the range of 0.05 to 150 µm, in respect to easier elimination of alkylene glycol from the fine particles and in respect to higher dispersion stability of the fine particles in the silicone oil. According to the present invention, acrylic polymer molecules are arranged with high affinity on surfaces of the fine particles of BHT or its oligomer while polysiloxane molecules are arranged on the silicone oil sides of the acrylic polymer layers, thereby forming a very stable nonaqueous emulsion of BHT or its oligomer. This emulsion is heated at a temperature of at least 150°C, preferably from 180 to 300°C, to polymerize the BHT or the oligomer. The polymerization can be accomplished in the presence of no catalyst or a known catalyst such as antimony trioxide. According to the present invention, the fine particles of BHT or its oligomer are dispersed stably and uniformly in the silicone oil, the polymerization of the finely divided particles proceeds effectively, the torque of agitation does not increase with the increasing molecular weight of the resulting polyester, the efficiency of heat transfer from the polymerization vessel is not varied, and the intended polyester of high intrinsic viscosity is yielded in the form of fine particles. The resulting polyester is separated from the medium of the reaction system and thoroughly washed with an organic solvent. The obtained polyester can also be subjected to a molecular weight increasing reaction. The silicone oil separated from the reaction system is filtered, freed from the alkylene glycol by distillation, and recycled for reuse in the reaction system. The process of the present invention can be applied not only to the production of polyethylene terephthalate but also satisfactorily to the production of polybutylene terephthalate or a copolyester comprising an alkylene terephthalate as a main constitutional unit. The following examples illustrate the present invention. In the examples, parts and percentages are all by weight. Reference Example Preparation of dispersion stabilizer (1) Preparation of graft polymer comprising acrylic polymer stems and polysiloxane branches An acrylic monomer and a polysiloxane, as shown in Table 1, and 3 parts of benzoyl peroxide and 500 parts of toluene were charged into a 3-ℓ flask equipped with a stirrer. After replacement of the air in the flask with nitrogen, the monomer was polymerized at 90°C for 8 hours. The resulting polymer-containing solution was poured into a large amount of methanol, and the precipitated polymer was recovered and dried. In this manner, graft polymers No. 1 to No. 17 were prepared. Examples 1 to 21 including Comparative Examples 4 to 6, 18 and Comparative Examples 1 to 2. Polymerization for polyester Each of raw material mixtures shown in Table 3 was charged into a 500-mℓ flask equipped with a stirrer and was heated to 270°C under a nitrogen atmosphere. After bis(ω-hydroxyethyl)terephthalate or its oligomer, i.e. a source of polyester, had melted, the polymerization through the ethylene glycol eliminating reaction was effected in vacuo for 2 hours while stirring. Then the reaction mixture was cooled and the formed precipitate was washed with toluene to recover a polyester resin. The following evaluations were made on each polymerization run. Results of the evaluations are shown in Table 2. (1) Amount of polymer block Amounts of polymer block in the recovered polymers and those clinging to the stirrer were measured. The results were ranked as follows: (2) Shape of polymer particle Shapes of polymer particles sampled during and after each polymerization run were evaluated by microscopic observation. The results were ranked as follows: (3) Size of polymer particle The average particle diameter of the sample polymer was determined by microscopic observation (magnification:100). (4) Intrinsic viscosity The intrinsic viscosity of the sample polymer was determined by measuring viscosities at 25°C of solutions of the polymer in a 50:50 (by weight) phenol/ tetrachloroethane mixture at three different concentrations of 0.5 g/dl, .0.25 g/dl, and 0.125 g/dl. A large amount of polymer block in (1) above and many deformed polymer particles in (2) above indicate low dispersion stability of bis(ω-hydroxyethyl)terephthalate or its olignomer, which results in large agglomerates of polyester.

Claims: A process for producing a polyester which comprises a thermal polymerisation of a bis (w-hydroxy-alkyl) terephthalate or an oligomer thereof in the form of particles of up to 300µm, dispersed in a silicone oil, and using a graft polymer as a dispersion stabilizer constructed of both a polysiloxane and an acrylic polymer and having a stem/branch ratio of 80-20 : 20-80 and wherein the dispersion stabilizer is added in an amount of at least 0.1%, characterised in that the graft polymer used as a dispersion stabilizer is constructed of a stem acrylic polymer and a branch polysiloxane and the molecular weight of the branch polysiloxane is at least 500. The process of claim 1, wherein the polysiloxane consists mainly of at leapt one member selected from the group consisting of polydimethylsiloxane, polymethylphenylsiloxane and polydiphenylsiloxane. The process of any preceding claim wherein the acrylic polymer is constituted of an acrylic monomer selected from the group consisting of acrylic esters and methacrylic esters, preferably selected from ethyl acrylate, methyl methacrylate, benzyl methacrylate and phenyl methacrylate. The process of any preceding claim, wherein the graft polymer is produced by using at least one member, as a graft linking agent, selected from the group consisting of a polysiloxane having one terminal methacryloyloxy group, a polysiloxane having one terminal mercapto group and a polysiloxane having one terminal vinyl group. The process of any preceding claim, wherein the graft polymer used as a dispersion stabilizer has a molecular weight in the range 1000 to 100000. The process of any preceding claim, wherein the bis-(w-hydroxyalkyl) terephthalate or the oligomer thereof is dispersed to particle sizes of 0.05-150µm.

Inventors: MORI, HIROSHI C/O MITSUBISHI RAYON COMP., LTD., SASAKI, ISAO C/O MITSUBISHI RAYON COMP., LTD.
Applicants: MITSUBISHI RAYON CO., LTD.
Application year: 1986
Main classification: C08G63
IPC codes: C08G77, C08F290, C08F283, C08G63, C08L67, C08F299