Ind. Eng. Chem. Prd.Res. Dev. 1982, 21, 126-130
126
Ester Interchange in Poly(ethy1ene terephthalate) and Related Model Copolyesters Mlchael Droscher Instltut fik h&kromMu&re Chwnk der Univmlt& Frelbug, Hennann-Stadlnger-Mws, Stefan-M&-Strasse 3 1, 7800 Freiburg 1. Br., Federel Repubiic of Germany
The ester-interchange reaction occurrlng In the solid state and in the meit of poly(ethyiene terephthalate) (PET) was investigated. For this purpose, pure, catalyst-free PET and a monodisperse oilgoester which contained di(oxyethy1ene)oxy (DE) units were synthesized. The activation energy of the observed DE unit transfer reaction was 85 W/mol. Regular sequence copolyesters of the type P(Eteo-DET) were synthesked and characterized by 250 MHz 'H NMR spectroscopy. No crystailization-induced ester-interchange reactions were observed for the sequence copolyesters.
Introduction Commercial poly(ethy1eneterephthalate) (PET) is either produced by transesterifkation of dimethyl terephthalate with ethylene glycol or by direct esterification of the terephthalic acid with ethylene glycol. In both processes ester-interchange reactions are of great importance. Recently, Reimschuessel(1980)reported an extended study on the mechanism and kinetics of the direct esterification process, employing a low molecular weight model system (Reimschuesseland Debona, 1979). For the condensation reaction in the polyester melt, it is well known that side reactions occur to a large extent (Buxbaum, 1968). The most important side reaction observed is the formation of diethylene glycol, which is incorporated in the polyester chain as a di(oxyethy1ene)oxy (DE) unit. To study the kinetics of the ester-interchange reaction in a polymer system, these side reactions have to be avoided. Therefore, we have synthesized pure PET in a catalyst-free solid state condensation process which does not contain any DE units (Driischer and Wegner, 1978). The same process can be employed for the synthesis of catalyst-free copolyesters with various DE concentrations (Drkher, 1980). Upon annealing of a mixture of the pure PET with a monodisperse oligoester containing DE units, the rate of ester-interchange reactions involving the polyester and the oligoester can be easily investigated (Droscher and Schmidt, 1981). As shown by Lenz and Go (1974) in their investigation on the ester-interchange behavior of the poly(ethy1ene terephthalate-co-Zmethyl succinate) system, ester-interchange reactions occur as well in the solid state. Here, the driving force for the reaction was the free energy of crystallization. The original random copolyester was transferred into a block structure where longer PET sequences gave rise to a higher degree of crystallinity. Whether such ester-interchange reactions in the solid state occur as well in regular sequence copolyesters has never been investigated. Therefore, we have synthesized and characterized regular copolyesters of the structure P(ET-ET-DET) and P(ET-ET-DET-DET) and utilized these to study the ester-interchange behavior. Nomenclature The following abbreviations are used in this report: T denotes the terephthaloyl group, E the oxyethyleneoxy group, DE the di(oxyethy1ene)oxy group, and Bz the benzyloxy group, respectively.
Synthesis of Pure PET Pure PET, free of catalyst and DE units, was obtained in a two-step procedure (Driischer and Wegner, 1978). In the first step, terephthaloyl dichloride was reacted with an equimolar amount of the bis(2-hydroxyethyl) terephthalate in 1,1,2,2-tetrachloroethane(TCE) in the presence of pyridine (PY). The oligoesters formed contained hydroxyl and acid chloride end groups and precipitated from the reaction mixture after reaching a degree of polymerization of only 2 to 4. This so-called precondensate was purified, dried, and subsequently heated under reduced pressure to the appropriate annealing temperature (245 to 255 "C). To avoid melting of the sample, the heating rate had to be controlled so that, due to end group reactions, the sample temperature rose slower than its melting temperature. Finally, the sample was isothermally annealed for several hours under reduced pressure. The molecular weight of the sample increased linearly with the square root of the annealing time because of end group reactions and ester-interchange reactions. Molecular weights higher than 40 kg/mol could easily be achieved. Due to the route of synthesis, the polyesters contained about 50% carboxylic end groups and 50% hydroxylic end groups. Unlike commercial PET, the samples prepared by our solid-state condensation method can be stored under vacuum at 300 "C for 24 h without any formation of DE units being observed. Therefore, these samples are excellent model compounds for the ester-interchangereaction as described below. Synthesis of the Oligoester BZ-T-E-T-DE-T-E-T-BZ (1) The route of synthesis employed for the oligoester 1 is schematically described by eq 1-4. Here, HAC,TFA, and 2Bz-T-C1+ H-DE-H
PYfTCE
2
3
HBr/HAc/TFA
HO-T-DE-T-OH
Bz-T-DE-T-BZ 3 SOC12/DMF
-2Bz-H
C1-T-DE-T-C1 4
(1)
+ 1OH-E-H
4
(2)
PY f TCE
H-E-T-DE-T-E-H
0196-432118211221-0126$01.25/0 0 1982 American Chemical Society
5
+ 8H-E-H
(3) (4)
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982 127
d
e
a
X
b
Y
volume) is shown in Figure 2. The assignment of the protons x and y is based on the concentration of the ET-DE = DE-T-E and E-T-E triads. A detailed discussion of the spectrum has already been published (Kricheldorf et al., 1981). Equations 6 through 8 describe the synthesis of the
c
I TMS
H-E-T-E-H
PY/TCE
+2Bz-T-C1 2
1. HBr/HAc/TFA-2Bz-H
Bz-T-E-T-E-T-BZ
C1-T-E-T-E-T-C1
2. SOCl,/DMF 9
8
7
6
5
L
3
2
~ . .
(ET-ET-DET-DET)
Figure 1. The 60-MHz 'H NMR spectrum of the olioester Bz-TE-T-DET-E-T-Bz
measured in CDClp
Synthesis of the Sequence Copolyesters The copolyester P(ET-ET-DET) was synthesized according to the scheme in eq 5. The intrinsic viscosity of
+ nC1-T-DE-T-C1 4
PY/TCE
(ET-ET-DET),
(5)
the sample was 0.52 dL/g. Using the Mark-Houwink relation for PET as a fmt-order approximation, this value corresponds to a molecular weight of about 13 kg/mol. The 250-MHz 'H NMR spectrum in CDCl,/TFA (2:l by 10-C H 2 - C H 2
- 0-C @CII
L -
4
I1
0-0
-
-0
- C e C I1
I1
Y
1
- 0- CH2 - CH2 - 0 -CH2 - CH2 - 0 - C a
C-
0-0
0-0
a
X
I
X
-
0 CH2- CH2
,(8)
copolyester P(ET-ET-DET-DET), containing all four possible triads of P(ET-co-DET): E-T-E, DE-T-E = E-T-DE, and DE-T-DE. Consequently, as shown in Figure 3, three aromatic signals were observed in the 250-MHz 'H NMR spectrum. The intrinsic viscosity for this sample was 0.27 dL/g, corresponding to a molecular weight of about 4 kg/mol, again utilizing the Mark-Houwink relation for PET. Annealing of the Sequence Copolyester SemicrystaJline samples of the two sequence copolyeaters were annealed at temperatures below their melting temperatures, 202 and 137 "C,for P(ET-ET-DET) and P(ET-ET-DET-DET), respectively. No crystallizationinduced ester-interchange reactions yielding long ET sequences were observed. This is demonstrated in Figure 4 for the copolyester P(ET-ET-DET). Here, DSC traces are shown for three samples, annealed for 125 min at 170, 180,and 190 OC. The only effect noted is a partial melting of the sample annealed at the highest temperature. This resulted in a low-temperaturemelting peak from the material which had recrystallized upon quenching. We did not observe an increase of the melting temperature which had been reported for the poly(ethylene terephthalateco-2-methyl succinate) by Lenz and Go (1974). As determined from wide-angle X-ray investigations, the copolyesters crystallize in their own structures, which are
DMF denote acetic acid, trifluoroacetic acid, and dimethylformamide, respectively. The oligoester 1 (molecular weight 967.0 g/mol) was monodisperse as confirmed by high-pressure liquid chromatography (HPLC). From differential scanning calorimetry (DSC) a melting temperature of 142 "C was determined. Its 60-MHz proton nuclear magnetic resonance ('H NMR) spectrum in CDC1, is presented in Figure 1 together with the assignment of the peaks. In the case of the aromatic protons of the terephthaloyl group the assignment is only tentative. The pure oligoester did not undergo ester-interchange reactions when annealed at 275 OC for several hours.
nH-E-T-E-H
(7)
7
PY/TCE
n7 + nH-DE-T-DE-H
1 0 tilppm)
b
c
c
b
Y
Y
b
CHCl3
-8
I
7
6 (6)
6
5
C
4
6 (ppm) Figure 2. The 250-MHz 'H NMR spectrum of the sequence copolyester (ET-ET-DET), measured in TFA/CDC13 (1:2) by volume).
128
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982
E-
0 -CH2-CH2 - 0 - C e C I1
0-0
[;-;
I1
II
0-0 b
Y
a
X
0
C-@C-0-
-
- O - C H Z - C H Z - 0 - C -@-C - 0 - C H 2 - CH 2
II
c
o
C H 2 - C H ~ - O - C H ~ - C H Z 0- - C G C - 0 - C H 2 - C H 2 II
II
1 J
0-0
Y
b
8
c
c
b
2
6
7
b
5
c
4
ij(ppm) Figure 3. The 250-MHz 'H NMR spectrum of the sequence copolyester (ET-ET-DET-DET),
E l
310
fl
n
330
350
370
390
(10
430
150
limperalure
L?O tn
490
463 K. Sample weight 2 mg, heating rate 20 K/min.
different from the structure of PET. Ester-Interchange Reaction As already pointed out in the Introduction, the DE transfer from the oligoester 1 to PET is a good model reaction for the ester interchange. Equation 9 is a sche...T-E-T
il
E-T-E-T-E-T-..
1
-
Bz-T-f:
+ T-DE-T-E-T-Bz
BT-E-T
E-T-E-T-E-T-,.
(9)
matical representation for this transesterification process (Driischer and Schmidt 1981). Here, each reaction between an oligoester molecule and the polymer chain leads to an increase of the DE concentration in the part of the sample which is insoluble in chloroform. As mentioned above, this reaction can only be carried out with pure catalyst-free PET, where no new DE moieties are formed during the annealing treatment. The two components were mixed in TFA solution. After the film had been cast on a glass surface, the solvent was evaporated though traces of TFA probably remained in the film. As shown by the DSC trace of a sample containing 30 w t % of 1 in Figure 5, semicrystallinefilms were obtained. The observed melting temperature was in this case 520 K, 10 K lower than for melt-crystallized, pure
n
. 4 0 m
K
Figure 4. DSC traces for samples of the sequence copolyester @"-ET-DET), annealed for 125 min at: (A) 443 K (B)453 K (C)
Bz-T-E-T-DE-T-E-T-Bz
measured in TFA/CDCl, (1:2 by volume).
350
LOO
L50 temperature
5 00
5 50
in K
Figure 6. DSC trace for a PET f i i containing 30 w t % of the oligoester Bz-T-E-T-DET-E-T-Bz as cast from TFA solution. Sample weight 4 mg, heating rate 40 K/min.
PET. The difference was due to the mixing effects. It should be noted that no melting peak of the oligoester 1 was detected, thus implying that the oligoester was fully dissolved in the PET matrix. After annealing of the film, the unreacted oligoester could be extracted with chloroform. Subsequently, the amount of transferred DE units was determined by 'H NMR from the ratio of the peak intensities of the signals c and (a + b) (see Figures 2 and 3) (Frank and Zachmann, 1977; Drbscher, 1980; Kricheldorf et al., 1981). It is well known that the amount of DE units in PET increases during annealing processes near the melting temperature. However, this is due to the remaining ester-interchange catalyst from the preparation procedure. We found that the total amount of DE units in the mixed film remained constant when we used our pure PET, but the DE concentration increased when commercial material was employed. Therefore, our investigations were carried out with pure PET, only. The Kinetics of the Ester-InterchangeReaction At temperatures below the melting point of the mixed film,large amounta of DE units in the polyester were found only after considerably long annealing times: e.g., 1 2 mol
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982 129
r=l W
a X
I
-0
W
0
X
-8
W 0 X
U \
W
a X
t i m e o f a n n e a l i n g in m i n
m 6. Kinetics of the ester-interchangereaction in the melt for a PET f h containihg 30 w t % of the oligoeater Bz-T-ET-DET-ET-Bz. Second-order d o t with resDect to the concentration of the DE units of the unreacted oligoester. Parameter temperature: (A)250 O C ; ( 0 ) 275 "C; ( 0 )3bO "C. F
% after 3780 min at 200 "C and 25 mol % after 850 min at 225 "C. In the melt the ester-interchange reaction proceeded much faster than in the solid state. At 250 "C, which is above the melting temperature of the mixed film, it took only 19 min to transfer 50% of all DE units from the oligoester to the polyester chain. At 275 OC the same conversion was reached after 13 min, and at 300 OC after only 5 min. Thus,mixtures of different PET samples, e.g., perdeuterated PET in normal PET used for neutron scattering experiments, should only be annealed for short times, even at temperatures below the melting point of the mixture. Mixing in the melt is impossible without the formation of block copolyesters. To describe the kinetics we assumed that the amount of insoluble material, i.e., the amount of the polymeric material, remained approximately constant (Droscher and Schmidt, 1981). We also neglected the back reaction, because only those back reactions which occurred near the chain end yielded material soluble in chloroform, and the probability for reactions near the polyester chain end was low for a starting degree of polymerization of 115. Hence, the rate of DE transfer from the oligoester to the insoluble part of the film is given by eq 10.
Here, zDE is the mole fraction of the DE units contained in the insoluble part of the sample, and (%DE).. denotes the maximum conversion. The experimental data was found to fit a second order plot (n = 2) as shown in Figure 6. The reaction mechanisms are not yet known. However, the temperature dependence of the rate constant thus obtained gave an Arrhenius type plot as presented in Figure 7. The corresponding activation energy was 85 kJ/mol. Reimschuessel(1980) found, for his ester-interchange model reaction, an activation energy of 205 kJ/mol for the uncatalyzed case. For the corresponding carbocylic acid catalyzed reaction he observed an activation energy of 91 kJ/mol. Comparing our result with Reimschuessel's data we have to conclude that the reaction reported here was catalyzed by the carboxylic end groups of the polyester and most probably by the residual amount of TFA which could not be removed from the film after the casting process.
-2
x
-
0
-3
-L 1.8
1.9
2.0 1 0 3 1 1 in K - 1
Figure 7. Arrhenius plot for the rate constants from Figure 6.
Experimental Section The synthesis and characterization of pure PET was already described elsewhere (Droscher and Wegner, 1978)) as was the synthesis of the oligoester 1 (Droscher and Schmidt, 1981). Synthesis of the Copolyester P(ET-ET-DET). A total of 4.30 g (0.01 mol) of 4 (Drkher and Schmidt, 1981) was dissolved in 100 mL of TCE and 6 g (0.05 mol) of 4-dimethylaminopyridine;2.35 g (0.0095 mol) of H-E-TE-H in 20 mL of TCE was added dropwise under cooling. The reaction mixture was stirred for 3 days at room temperature, precipitated in acetone, and dried. To extract the 4-dimethylaminopyridine hydrochloride, the product was stirred overnight in water, filtered off, and dried; 4.8 g (81%)polyester was obtained with an intrinsic viscosity of 0.42 dL/g. The product was dissolved in 50 mL of thionyl chloride which contained three drops of DMF and was then refluxed for 1h. The thionyl chloride was distilled 0% 50 mL of toluene was added and again distilled off. The chlorinated product was dissolved in 50 mL of TCE and 200 mg (0.0008mol) of H-E-T-E-H was added. The mixture was again stirred for 1day and the product was precipitated from acetone. After reprecipitating from TCE into water-acetone (50 ~ 0 1 %4.1 ) g of the copolyester was obtained with an intrinsic viscosity of 0.52 dL/g. Synthesis of the Copolyester P(ET-ET-DETDET). A total of 9.7 g (0.038 mol) of H-E-T-E-H was dissolved in 150 mL of TCE and 15 mL of pyridine was added. Subsequently, 23 g (0.084 mol) of Bz-T-Cl was added dropwise under cooling. After stirring for 6 h the reaction mixture was precipitated in acetone and stirred
Ind. Eng. Chem. Prcd. Res. Dev. 1982, 21, 130-134
130
overnight. After recrystallization from 800 mL of toluene, (6) (mp 173 OC) was 20 g (72%) of Bz-T-E-T-E-T-Bz obtained; 20 g (0.027 mol) of 6 was dissolved in 130 mL of TFA and stirred for 4 h after 20 mL of HBr/HAc (40 w t % ) had been added. After precipitating into water, redissolving in dioxane, and again precipitating into water, 13 g (86%) of HO-T-E-T-E-T-OH was isolated. This was transferred into the dichloride 7 with 130 mL of thionyl chloride as described above to yield 13.8 g (99%, 0.023 mol) of 7. The product was dissolved in 100 mL of TCE and 3 mL of pyridine. To this 8.04 g (0.023 mol) of H-DE-T-DE-H dissolved in 60 mL of TCE was added dropwise under stirring. The mixture was stirred for 3 days, precipitated in acetone, redissolved in TFA, and again precipitated into water/acetone to yield 11.4 g of the copolyester. The intrinsic viscosity was determined to 0.23 dug. Model Reaction. Films of pure PET (M, N 22 kg/mol) and 1 were cast from TFA solution (about 5 wt %) by evaporating the solvent. The films were stored in vacuo for at least 3 days. About 200 mg of the film in a porcelain boat was placed into a glass tube which was evacuated and refilled with dry nitrogen three times. Then the evacuated tube was quickly heated to the reaction temperature which was maintained with an accuracy of k0.5 K. Finally, the sample was quenched in air and stirred in CHC13for 1h to dissolve the low molecular weight material. The polymer was filtered off and dried carefully. It was then dis-
solved in TFA to determine the concentration of the DE units by ‘HNMR. ‘HNMR Measurements. The 250-MHz spectra were measured on a Bruker WH-250 spectrometer by W. E. Hull. For details refer to Kricheldorf et al. (1981). For the 60-MHz spectra a JEOL JNM-PM X 60 was used. D S C Measurements. All DSC measurements were carried out with a DSCS (Perkin-Elmer Corp.) in unsealed aluminum pans under a nitrogen atmosphere. For standards, In and Sn were employed. Viscosity Measurements. The intrinsic viscosities were determined in a phenol/TCE mixture (60/40 by wt) at 30 “C in an Ostwald viscosimeter. The molecular weights were calculated according to the relationship given dL/g; a = 0.58. by Cha (1964): k = 2.1 X Literature Cited Buxbaum, L. H. Angew. Chem. 1988, 80, 215. Cha, C. Y., J . polym. Sci. Po@?. Lett. Ed. 1984. 2, 1069. Drhcher, M. Makrorol. (2”.1980, 181, 789. Drbcher, M.; Schmldt, F. G. Poiym. Bull. 1981, 4 , 261. Dr6scI-w. M.; Wegner, G. Po!vmef 1978, 19, 43. Frank, W. P.; Zachmann, H. G. Prog. colyold f ” . Sci. 1977, 62, 88. Krlcheklorf. H. R.; D r h h e r , M.; Hull, W. E. Po@?. & 1981, I .4 , 547. Lenz, R. W.; (k, S. J. f ” . Sci. polym. Chem. Ed. 1974, 12, 1. Relmschuessel, H. K. Ind. Eng. Chem. Rod. Res. D e v . 1980, 19, 117. Relmschuessel, H. K.; Debona. B. T. J . Po(” Sci. Polym. Chem. Ed. 1979, 17, 3241.
Received for review July 30, 1981 Accepted December 2, 1981
Rheological Properties of Coal-Tar Pitches Hlroshl Okarakl Research Department, N@wn Steel chemlcei CO.,LM.,46-51, Sakinohama, Nakabaru, Tobataku, Kltakyushhshi, Fukuoka Prefecture, 804 Japan
The viscosities of 21 coal-tar pitches with softening points in the range of 21.8 to 113.5 O C were determined in the temperature range from 60 to 220 O C . The appliceblltty of two equation forms to these measured values was assessed. An equation relating the simple logarithmic viscosity to the second power of the difference between reciprocal absdute temperature and reciprOCa1 sottenlng point (absolute temperature unit) fits the results well. I t is further shown that the reciprocal Softening point is additive in binary pitch mixture.
Introduction
tried. The resulting relationships were satisfactory.
Coal-tar pitch is currently used for electrode binders, briquette binders, pitch coke, tar-epoxy resins, etc. In the preparation and use of pitches, their levels of softening point and dependence of viscosity on temperature are of the greatest importance. It is therefore desirable to relate viscosity with temperature and softening point by a single equation applicable to the wide range of pitch products. For these purposes Frank and Wegener (1958) and Wallouch et al. (1977) gave an equation relating the double logarithmic viscosity to the logarithm of the temperature/softening point ratio. The latter workers also mentioned that the softening point is additive in binary pitch mixtures. The author examined 21 materials ranging from a briquette binder of 21.8 OC softening point to an electrode binder of 113.5 O C softening point and some mixtures of pitches. As the present investigation proceeded, neither of the above relationships was found to be suitable for the whole range of materials, so other equation forms were
Experimental Section
0196-4321/82/122 1-0130$01.25/0
The softening points of pitches were determined by the ring and ball method (JIS K 2471) using an automatic apparatus (Model NA-285PD of Nippon Oil Shiken Kogyo KK) within an accuracy of k0.5 OC. The viscosities were measured in the temperature range from about 20 O C above the softening point to about 120 “C above the softening point using a Brookfield viscometer (Model B8H of Tokyo Keiki Seisakusho) within an accuracy of about 5%. The measured values of softening points and other properties of pitch samples are given in Table I, and the viscosities are listed in Table 11. The m i x t m of different kinds of pitches were obtained as follows. About 100 g of pitch A and B were weighed into a beaker. While heated in an oil bath of 130 O C , the mixture was stirred with a glass rod for 30 min. The resulting liquid mixture was allowed to flow into a ring for the softening point measurement. 0 1982 American Chemical Society
