Aug. 29, 1961 N. FLETCHER 2,998,412 PROCESS FOR MANUFACTURE OF POLYETHYLENE TEREPHTHALATE WITH ZINC ACETATE-ANTIMONY TRIFLUORIDE CATALYST SYSTEM Filed May 12, 1958

ReAction times FOR ESTER-FINTERCHANGE AND PolycoNDENsATION PRODUCED BY DFRNt CONCENTRATIONS OF ZINC ACATE AND E. FLUORDE 4. UNDER stANDARD CONDITIONS.

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M1/p/a/v72 a MVaa/7/M / a 7-caya a r (44. Aeter: 6-4- a 77 oat//a/s 2,998,412 United States Patent Office Patented Aug. 29, 1961 1. 2 2,998,412 Apart from their influence on rate of ester-interchange PROCESS FOR MANUFACTURE OF POLYETHYL and polycondensation it is particularly important to con ENE TEREPHTHALATE WITH ZNC ACETATE sider the effect of metal compounds on thermal stability ANTMONY TRFLUORIDE CATALYST SYSTEM since these compounds remain in the polymer. Subse Norman Fletcher, Harrogate, England, assignor to Im quent processing operations such as the spinning of fibres perial Chemical Industries Limited, London, England, a corporation of Great Britain and casting of films are carried out from the polymer Filed May 12, 1958, Ser. No. 734,489 melt at elevated temperatures and high thermal sta Claims priority, application Great Britain Feb. 27, 1953 bility facilitates process control and minimises colour 3 Claims. (C. 260-75) deterioration during such operations. We have found 0. that the thermal stability of polyethylene terephthalate This invention relates to an improved process for the may be conveniently measured by rate of fall of intrinsic manufacture of polyethylene terephthalate, a synthetic viscosity at a standard temperature preferably 282 C. linear polyester having useful fibre and film-forming and may be expressed as a degradation rate constant. properties. In particular, this invention is directed to an A useful catalyst is one which satisfies all of four of improved catalyst system for use in the ester-interchange 5 the criteria set out below. reaction between a dialkyl terephthalate and ethylene (1) Rates of reaction. These must allow a high rate glycol and in the polycondensation of the resultant glycol of productivity and the rates of ester-interchange and ester of terephthalic acid. This application is a continua polycondensation should be such as to keep the two steps tion-in-part of my application, Serial No. 410,457, filed in phase under conditions of routine manufacture. on February 15, 1954, now abandoned. 20 (2) Polymer colour. Not only must the polymer be The preparation of polyesters based on terephthalic acid as clear a colour as possible but if it is to be spun into and a glycol of the series HO(CH)OH where n=2 to textile yarns, these must be of a colour satisfactory to 10 inclusive is described in U.S.P. 2,465,319. A pre the textile trade. ferred method for the preparation of polyethylene tereph (3) Polymer stability. A low degradation rate should thalate takes place in two stages, ester-interchange be 25 be obtained when the polymer is heated in the molten tween ethylene glycol and a dialkyl terephthalate being State. followed by polycondensation of the resultant glycol ter (4) Solubility characteristics. The catalyst must be ephthalate. soluble in the reaction mixture and in the polymer. The catalysis of these processes may be brought about Ready solubility in ethylene glycol in a minimum re by a wide range of metals and metal compounds. U.S.P. 30 quirement. 2,465,319 discloses the following: lithium, sodium, potas We have found that antimony trifluoride is of out sium, calcium, beryllium, magnesium, zinc cadmium, alu standing value in respect of polymer colour, stability and minium, chromium, molybdenum, manganese, iron, solubility characteristics. Unfortunately, however, under nickel, copper, silver, mercury, tin, lead, bismuth, anti atmospheric pressure conditions it catalyses only the poly mony, platinum, palladium, alkali metal alcoholates, al 35 condensation step in which it is capable of producing kaline earth metal alcoholates, magnesium alcoholates, useful rates of reaction when present in concentrations alkali metal carbonates, alkali metal borates, magnesium between 0.005% and 0.04% by weight of ester. Above oxide. this level of concentration little further advantage is A large number of U.S. patents relating to specific gained when the reaction is carried out in a conventional catalysts or catalyst combinations have since been filed. 40 autoclave and the presence of large amounts of anti Their claims may be summarized as follows: mony can have a deleterious effect on polymer colour. In view of the limitation of the effect of antimony fluoride U.S.P 2,518,283-Zinc borate for both stages. to polycondensation it is necessary to combine the fluo U.S.P. 2,534,028-Litharge for both stages. ride with another metal compound which will catalyse U.S.P. 2,578,660-Germanium and germanium oxide for ester-interchange. For this purpose we have found zinc polycondensation. 45 acetate most suitable since it is readily soluble in cold U.S.P. 2,641,592-Cobaltous acetate for both stages. ethylene glycol and, in small quantities, has little adverse U.S.P. 2,643,989-Cerium and cerium oxides for both effect on polymer stability. In order to give a rate of stages. ester-interchange equivalent to that of polycondensation U.S.P. 2,647,885-Antimony trioxide for polycondensa using antimony trifluoride it is desirable to use Zinc tion. 50 acetate in concentration 0.01-0.03% by weight of ester. U.S.P. 2,650,213-Litharge/antimony trioxide/triphenyl In defining this concentration the fact that such concen phosphite for both stages. trations of zinc acetate have also a small positive cata U.S.P. 2,681,360-Mixed lithium hydride-metal organic lytic effect on polycondensation has been taken into carboxylate for ester-interchange. account. U.S.P. 2,711,402-Aluminates for polycondensation. 55 The combined catalyst compounds may be added in U.S.P. 2,729,620-Neodymium chloride for both stages. solution in the cold ethylene glycol charge to the ester U.S.P. 2,739,957-Calcium compound for ester-inter interchange reaction antimony trifluoride having no ad change. Calcium compound/antimony compound for verse effect on the rate of this reaction. In this respect polycondensation. our system shows an advantage over that if Vodonik U.S.P. 2,740,768-Basic ferric acetate for both stages. 60 (U.S.P. 2,681,360) in which the presence of antimony U.S.P. 2,820,023-Lanthanum compounds for both trioxide is found to retard the ester-interchange step. Stages. Our experiments show that such a zinc acetate-anti mony trifluoride catalyst yields polymer markedly su The foregong list shows the wide scope of the Work perior in respect of colour and stability to that containing carried out in this field in which the main aims have 65 compounds of lead, iron, cobalt, rare earth metals and been to achieve rapid rates of reaction and improved alkali metals. The introduction of lithium compounds polymer quality, notably colour. These aims have often facilitates ester-interchange but is deleterious to polymer been conflicting, a fundamental difficulty being that rapid stability. While zinc compounds alone may be used to reaction has tended to give discoloured polymers. It is catalyse both stages a relatively high concentration therefore apparent that the choice of a catalyst System 70 (>0.05%) is required to give a reasonable rate of poly must involve some measure of compromise. condensation and at these levels stability and colour both 2,998,412 3 4. suffer considerable deterioration. Moreover, ester-inter slight amber tint, 2=very pale amber, 3=pale amber, change and polycondensation rates are badly out of phase. 4=amber. Among the compounds disclosed as catalyst in the prior Prior to determination of the thermal stability the art only certain calcium-antimony combinations (U.S.P. polymer ribbon is broken up and dried at 160° C. and 2,739,957) give results at all comparable to those ob 0.2% relative humidity to give a small constant residual tained with zinc acetate-antimony fluoride. The presence moisture content (e.g. y% by weight). The effect of this of calcium compounds, however, results in the precipita moisture which cannot be completely removed in molten tion of insoluble calcium salts during polycondensation polymer is given by the equation: and this produces an undesirable cloudiness in the poly y e. 0. C -- In the examples which follow the basic procedure for the testing of catalysts was as follows (all parts are by 1800k 7xi 70 weight): 100 parts dimethyl terephthalate and 70 parts ethylene where k and c are the constants in the relationship be glycol were melted together in an ester-interchange ap 5 tween intrinsic viscosity and number average molecular paratus consisting of a glass flask fitted with a short frac weight (m=kMn"). These constants may be determined tionating column leading via a distillation head to a con experimentally in a number of ways such as the end denser and graduated receiver. Thermometers were pro group method of Griehl and Neue, Faserforsch, und Text, vided to measure temperature in the reaction flask and 1954, 5,423. in the stillhead. The catalyst under examination was 20 mo=initial intrinsic viscosity. stirred with 1 part ethylene glycol at 140-150° C. and its nx=intrinsic viscosity after hydrolytic degradation (on state of dissolution or otherwise noted before addition to remelting). the reactants. Ester-interchange was then carried out over a temperature gradient rising to 215 C.; the point From this equation we can calculate what the intrinsic at which distillation of methanol commenced being ob 25 viscosity of our dried polymer will be immediately it is served. Progress of the reaction was followed by means remelted. Thermal stability is then measured by holding of the amount of methanol collected. molten polymer at 282° C. for varying lengths of time The ester-interchange products were next charged to a in an inert atmosphere under carefully controlled condi stainless steel autoclave fitted with a stirrer and con tions and recording fall in intrinsic viscosity. A mean denser-receiver arrangement attached to a vacuum pump. 30 value for Ks, the degradation rate constant, is then ob Excess glycol was allowed to distill out under atmospheric tained using the formula: pressure while the temperature rose to 245 C. and under 1. a pressure gradually reduced to 1 mm. Hg while the tem perature rose from 245 to 275 C. Polycondensation K.t= Mk: ---, was then carried out at 275 to 278 C. and pressure 0.2 35 n m." to 0.5 mm. Hg to yield polyethylene terephthalate hav ing an intrinsic viscosity measured in 1% solution in where M=the polymer repeat unit (192) ortho-chlorophenol at 25° C. of about 0.70. The extent m=intrinsic viscosity at zero time. of polycondensation was determined by increase in power m=intrinsic viscosity after time t. required to turn the stirrer (AP), time of polycondensa 40 t-time (hours) tion being that from 275 C./1 mm. to completion. The The table of examples which follows shows the results polyethylene terephthalate was extruded from the auto obtained with a considerable number of catalyst systems, clave on to water cooled nip rollers and so obtained as the attached FIG. I being taken from Examples 1 to 7 in a quenched (amorphous) ribbon. order to further illustrate the effects. On this graph the The polymer was assessed for clarity and for colour 45 vertical lines indicate the limits of concentration of cata on an arbitrary scale 0-4 where 0=water white, 1=very lyst within which it is reasonable to operate.

Initial Solubility Ester- Ester-inter-Polyconden- PolyInter Catalyst System (percent wt. of in inter- change sation Polymer EEE dimethylterephthalate) Ethylene change Time Time Colour KX1031 Glycol Temp. (hrs.) (hrs.) hr.-1 (o C.)

Zi tate Perg, Elysiuoride:nC acetate------... 8:8; } ++ 160 1.0 1.5------1.25------1.8 Eigstifluoride-aic acetate.------0.04.03 --- 60 1.4 1.5------0.75. ------1.75 attituorideinc acetate------no 0.0250.02 -H-I- 160 2.2 18------0.5------1.6 itsinc acetate------rifluori 0.050.025 } ---- 160 3.25 18------0.5------... 6 inc acetate------0.01 Aloysilluoride - - 0.01 } --- 160 3.8 2.6------0.5------... 4 Airesiduoride:inc acetate------0.01g } --- 160 3.8 3.3------1-0------... 4 Ainc acetate.------grifluoride 89:... O } ++ 60 4.0 Ingolipse 1.75------1.7 A.Eagle-1IC a Cetate------88,015 ++ 160 3.25 2.0------0.75------1.6 AREloride:alcium acetate. 0.0888) ++ 160 3.75 2.3------2, cloudy.---- 1.9 Antimonya Cillil acetate.------. trifluoride--- -... - - - 0.088:8; ++ 160 3.9 2.35------<1, cloudy- 1.1

Lithium hydride-- 0.01 Zinc acetate.---- 0.04 ---- 40 0, 66 1.4------2.5 figyllage------trifluo 8..02 8: -- - 1. 55 2.53.2------3.9 Basic ferric acetate----- 0.05 -- 165 4.0 Incomplete 3.5 reaction. gases acetate------ocol -- 140 3.0 1.8------n 2.4 AntimonyelC OX100---...------trifluoride------88) +- 85 4.2.2.0------1.8 See footnotes at end of table.

Initial Solubility Ester- Ester-inter-Polyconden- Polymer Oatalyst System (percent wit, of in inter- change sation Polymer stabilit dimethylterephthalate) Ethylene change Time Time Colour KX10 Glycol Temp. (hrs.) (hrs.) ar. (° C) Percent Lanthanum oxide (LaO).------0.03 180 Incomplete 4------2.45 Lanthanum oxide------0.01 185 reaction. 2.0 OaSSillaE", OOrate. Side ------a 8. 8. Antimony trifluoride 0.02 140 20------26------3.1 Lithium hydride- 0.01 AntimonyZine acetate.------trioxide 0.030.02 1.95------125G------2,75 Zinc acetate------0,02 21------0.75G------1.7 &ESR;EglidealC1, OX1Cle. : Antinaony trioxi 0.03 2.45------1.0, cloudy- 1.2 Zinc acetate------0.04 Incomplete 3.0------2.0 Zinc acetate------0,05 reaction. 2.0 Zinc acetate. 0.08 2.6 Zinc borate------0.05 1.9 NOTES: or(1) insolubility. In column 2 the symbols --- denote ready solubility in cold glycol; --- soluble only on heating;-indicates limited solubility (2) Where incomplete polycondensation reaction is noted this means that the power input to the agitator levelled out before intrinsic viscosity 0.7 was reached and remained steady for at least one hour. (3) In Examples 19 and 20 the suffix G in polymer colour denotes a grey shade. Consideration of these data will show that best results from 150 to 225 C. in the presence of zinc acetate in are obtained using zinc acetate and antimony trifluoride amounts between 0.01% and 0.03% by weight of di in concentrations between 0.02% and 0.025%. methyl terephthalate and thereafter polycondensing the It will, therefore, be seen from the above discussion and 30 ethylene glycol terephthalate at a temperature of from the graph that this invention contemplates a process for 225 to 285 C. and pressure from 0.02 to 2 millimeters making highly polymeric polyethylene terephthalate of mercury in the presence of antimony trifluoride in which comprises reacting ethylene glycol and dimethyl amounts between 0.005% and 0.04% by weight of di terephthalate at a temperature of from 150 to 225 C. methyl terephthalate until a polyester having an intrinsic in the presence of zinc acetate in amounts between about 35 viscosity of at least 0.55 is obtained. 0.01% and 0.03% by weight of dimethyl terephthalate 2. The process of claim 1, wherein the concentration and thereafter polycondensing the ethylene glycol tereph of zinc acetate and antimony trifluoride are each about thalate at a temperature of from 225 to 285 C. and pres 0.025% by weight of dimethyl terephthalate. sure from 0.02 to 2 millimeters of mercury in the pres 3. The process of claim 1, wherein both zinc acetate ence of antimony trifluoride in amounts between about 40 and antimony trifluoride are present throughout both 0.005% and 0.04% by weight of dimethyl terephthalate ester-interchange and polycondensation. till a polyester having an intrinsic viscosity of at least 0.55 is obtained. I claim: References Cited in the file of this patent 1. The process for making highly polymeric poly- 45 UNITED STATES PATENTS ethylene terephthalate which comprises reacting ethylene 2,681350 Yodonik ------June 15, 1954 glycol and dimethyl terephthalate in a temperature of 2,739,957 Billica et al. ------Mar. 27, 1956