CC BY
16
УДК 669.017
M. A. Mikitaev, G. V. Kozlov, G. E. Zaikov, A. K. Mikitaev
THE DEPENDENCE OF PERMEABILITY TO GAS COEFFICIENT OF BLENDS POLY(ETHYLENE TEREPHTHALATE)/POLY(BUTYLENE TEREPHTHALATE) ON GLASS TRANSITION TEMPERATURE
Key words: polymer blend, permeability to gas, glass transition temperature, miscibility, crystalline phase.
The permeability to two gases of blends poly(ethylene terephthalate)/poly(butylene terephthalate) has been studied in the whole compositions range. The dependence ofpermeability to gas coefficient on glass transition temperature of the studied blends has been shown. Gas transport processes are controlled also by both miscibility in amorphous phase and structure of crystalline phase.
Ключевые слова: полимерная смесь, газопроницаемость, температура стеклования, совместимость, кристаллическая фаза.
Исследована газопроницаемость по двум газам для смеси полиэтилентерефталат/полибутилентерефталат во всем интервале составов. Показана зависимость коэффициента газопроницаемости от температуры стеклования исследованных смесей. Процессы газопереноса контролируются также совместимостью в аморфной фазе и структурой кристаллической фазы.
Introduction
At present the most perspective trend in packaging field is the packing, prepared on the basis of poly(ethylene terephthalate) (PET) [1]. Plastic bottles, as well as other varieties of packing from PET, have a wide application in food industry, cosmetics, medicine and cosmic branch. This circumstance is due to obvious advantages of plastic packing. Possesing main properties of glass packing (transparence, stability to atmospheric force and so on), packings on PET basis are much lighter and stable to mechanical forces. Besides, they can be processed for repeated usage.
The permeability to gas is no less important characteristic for packing materials. The authors [2] found the dependence of diffusivity D on glass transition temperature Tg for a number of materials, which in case of oxygen diffusion is described by the following empirical equation:
lg D = -(9.4 - 0.0058Tg) (1)
for testing temperatures 7>298 K. It is easy to see, that the equation (1) predicts D growth at Tg enhancement. It is obvious, that something the like has to be expected for permeability to gas coefficient P as well. As it is known [3], blends poly(ethylene terephthalate)/poly(butylene terephthalate) are miscible in amorphous phase, that defines the only glass transition temperature for them. Therefore the present paper purpose is the study of permeability to gas of blends poly(ethylene terephthalate)/poly(butylene terephthalate) as a function of their composition and, hence, their glass transition temperature.
Experimental
The polymer blends, components of which are copolymer of poly(ethylene terephthalate) and isophthalic acid (PET) of mark SPET 8200A were obtained from OAO "Mogilevkhimvolokno" and poly(butylene terephthalate) (PBT) of mark D201 was obtained from firm Shinity (China). The blends were
prepared by melt mixing on twin-screw extruder of mark Jiangsu Xinda of firm Science and Technology Co., Ltd. (China), having six zones of heating (the ratio of screw length to diameter is equal to 36). The processing temperature by zones made up 503-538 K, rotation rate of screws made up 190-210 rpm. The prepared blends are granulated and extrudated again on a single-screw extruder of model Paladini Roberto (Italy) at the greatest temperature 538 K with flat formative die, allowing to obtain films, having thickness of 0.15-0.20 mm, which are cooled immediately in the water. The blends PET/PBT with weight ratio of components were used: 95/5, 90/10, 80/20, 70/30, 50/50, 40/60 and 20/80.
Before studing thermal physical properties of blends their film samples were maintained at temperature 363 K during 15 min for inner stresses relieving and then dried at 333 K during one hour under vacuum. Thermal physical parameters of the blends PET/PBT, namely, their glass transition temperature Tg, were determined by differential scanning calorimetry (DSC) method using apparatus DSC 4000 of the firm Perkin Elmer (USA). Heating rate of samples in the air medium made up 10 K/min. The conditions of the study by DSC method were the following ones: maintenance at 303 K during 1 min, heating within the range of temperatures with the indicated above heating rate and maintenance at 553 K during 1 min. The values of glass transition temperature of blends, obtained by DSC method, have been shown the extreme dependence on composition.
The special diffusive cell (Fig. 1) was designed for the study of barrier properties of the blends PET/PBT. Before polymer material 3 testing gas from gascontainer 1 comes into the upper chamber of cell 2, cleaning it during 10 min at the open tap 5. At the same time lower chamber 6 is cleaned by inert gas for air removal (position II of valve of tap-dosator). After cleaning the upper chamber is closed hermetically by gas top 5, in which surplus pressure of the tested gas, equal to 1 atm (reading of manometer 4) is created. After this the lower chamber of cell is disconnected
from the flow of gas-carrier (the position I of tap-dosator). In such position gas-carrier through valve A-D comes immediately to chromatograph pump. In the appointed interval of time the gas, passed through the film to a lower chamber, is directed to analysis (position II of tap-dosator). The flow of gas-carriers passes in the direction A-B-lower chamber-C-D and forces out the studied gas into chromatograph of mark Tswet-800, production of Russian Federation.
З^я He
P, cm3/m2-day-atm
2.50-
1.25
50
100
Срвт, wt. %
Fig. 2 -The dependences of permeability to gas coefficients to carbon dioxide (CO2) PCO (1) and
oxygen (O2) PO2 (2) on PBT content CPBT in blends
PET/PBT
Fig. 1 - The scheme of diffusive cell: 1 - gascontainer with tested gas; 2 - upper chamber of cell; 3 - tested polymer film; 4 - controlling manometer; 5 - gas tap; 6 - lower chamber of cell; 7 - tap-dosator; 8 - chromatograph
Results and Discussion
In Fig. 2 the dependences of permeability to gas coefficient by carbon dioxide (CO2) PCO2 and oxygen
(O2) PO2 on composition of the blends PET/PBT,
which is expressed by PBT content CPBT, are adduced. These dependences have two features: first, weak enough minimum of permeability to gas coefficient at CPBT«5 wt. % and, secondly, their maximum at CPBT«50 wt. %. These features are connected to the corresponding structural features, which will be considered in detail lower. The lower absolute values of PCO2 in comparison with PO2 are due to a larger size of
gas-penetrant molecules, which is equal to 0.323 nm for CO2 and 0.30 nm - for O2 [3].
In Fig. 3 the dependence of glass transition temperature Tg on composition of the blends PET/PBT is adduced, which has typical for the indicated blends shape [4]. As it follows from the plot of Fig. 3, the dependence Tg(CPBT) has pronounced minimum at CPBT«50 wt. %, i.e. the dependences P(CPBT) and Tg(CPBT) are antibate, in contradistinction to the dependences D(Tg) [2]. This gives the possibility to describe the correlations P(Tg) by the empirical equations, by its form similar to the equation (1):
(2)
(3)
where PO2 and PCO2 are given in cm3/m2-day-atm and Tg in K.
Tg, K
and
PO2 = 18.1 - 0.0475Tg
PCO2 = 18.1 - 0.0500Tg,
360
35
340
330
0
50
100
CPBT, wt. %
Fig. 3 - The
T
g
dependences of on PBT content
glass transition temperature Tg on PBT content CPBT for blends PET/PBT: 1 - experimental data; 2 - calculation according to the equation (4) (Fox equation)
The comparison of calculated theoretically
according to the equations (2) and (3) values PO2
and
PCO2, correspondingly, and determined experimentally
magnitudes of permeability to gas coefficients by oxygen Pc2 and carbon dioxide PCc2 for blends
PET/PBT is adduced in Fig. 4, which shows a good correspondence of theory and experiment. The theoretical model of the dependences D(Tg) or P(Tg) within the frameworks of fractal analysis was proposed in monograph [5].
Let us consider structural model of permeability to gas in reference to the studied blends PET/PBT. As it is known [3], the glass transition temperature of miscible blends can be described by a well-known Fox equation:
1 Q
PET ^PET
+-
Q
PBT
T TPET rp
g Tg Tg
PBT
(4)
where CPET and CPBT are weight contents of PET and
PBT 'rPET
of PET and PBT, accordingly.
Tg and TgPBT are glass transition temperatures
0
PT, cm3/m2-day-atm
3 -
1 -
0
1
2
3
P, cm3/m2-day-atm
Fig. 4 - The comparison of calculated according to the equations (2) and (3) PT and obtained experimentally P values of permeability to gas coefficient to carbon dioxide (1) and oxygen (2) for blends PET/PBT
The calculated according to the equation (4) dependence Tg(CPBT) is also adduced in Fig. 3 (the stroked line). As one can see, although qualitatively theoretical and experimental dependences of Tg on composition are similar ones, but quantitatively they are differed strongly enough, namely, experimental Tg values are essentially lower than the calculated ones according to the Fox equation. This discrepancy can be explained with the aid of the model [6], in which the
dependence of equation:
Tg on composition is given by the
CPET ln
Tg
T
v g
PET
+ kCPBT ln
Tg vg
PBT
= 0,
(5)
where the coefficient k presents ratio of heat capacity jumps at constant pressure at glass transition
temperatures of components ACp: k= A^^1 / ACpET.
Coefficient k according to the model [6] allows to estimate miscibility level of blend components within the frameworks of the following gradation: for fully miscible polymers k«1.0, for nonmiscible ones k<0.01 and for systems near a phase separation k>0.3. Estimations according to the equation (5) have shown a wide variation of parameter k for the considered blends within the range of 0.31-2.45, i.e. from the systems near phase separation (small CPBT) up to full miscibility (at CPBT>20 wt. %). Let us note, that the value k in the equation (5) is determined by values of glass transition
temperatures Tg, TjET and T^ai , i.e. by characteristics
of amorphous phase. This gives grounds to propose, that the indicated parameter characterizes miscibility of blend components precisely in amorphous phase. Hence, in spite of the blends PET/PBT miscibility in amorphous phase in the whole compositions interval, that indicates unequivocally the only glass transition temperature, but miscibility level can be differed essentially [7]. Let us note, that the greatest values PO
and PCO2 correspond to the greatest values £=1.90-2.45
at CPBT«50-60 wt. % (Fig. 2), i.e. the blends with the greatest level of miscibility in amorphous phase.
PBT
In Fig. 5 DSC thermograms of blends PET/PBT within the range of temperatures T=433-543 K are adduced. The fact that attracts attention, is clearly expressed division of peaks of crystallization, (two different melting temperatures Tm, which are equal to Tm of PET and PBT), begins for blends with PBT content CPBT>30 wt. %, that is typical effect for blends PET/PBT [8]. Let us note, that the smallest values PO2 and P
J2 ----- JCO2
correspond to the indicated CPBT values. Thus, the smallest values PO2 and PCO2 (lower than additive
ones, Fig. 2) correspond to one crystallization peak of blends PET/PBT, that means either combined crystallization of PET and PBT or crystallization of PET only and separate crystallization of PET and PBT (two melting temperatures Tm) - the greatest values (significantly higher than additive ones) of permeability to gas coefficient.
Fig. 5 - DSC thermograms of blends PET/PBT with PBT content: 0 (1), 5 (2), 10 (3), 20 (4), 30 (5), 50 (6), 60 (7), 80 (8) and 100 (9) wt. %
Let us consider possible variants of reduction of permeability to gas coefficient of the studied blends within the frameworks of the proposed model on the example of the materials with equal contents of PET and PBT (Cpet=Cpbt=50 wt. %). If the dependence Tg(CPBT) submitted to the Fox equation, than for the indicated blend PET/PBT the value Tg«348 K and then PO2 «1.55 cm3/m2-day-atm instead of experimental value PO2 =2.05 cm3/m2-day-atm, i.e. lower on 25 %, and P(CO2 =0.68 cm3/m2-day-atm instead of experimental magnitude PCO2 =1.20 cm3/m2-day-atm, i.e. lower on 44 %. The condition PCO2 =0 is realized according to
the equation (3) at Tg=362 K, i.e. according to the equation (5) at k«-1 for blend PET/PBT, contained 50 wt. % PBT. According to the cited above gradation [6], such value k<0.01 means fully nonmiscible blend. Hence, not only the fact of presence or absence of miscibility of blends components can influence on their permeability to gas, but this miscibility level as well.
2
g
Conclusions
Thus, the present paper results have demonstrated, that polymer blends follow the general tendency for polymers as regards their properties at gas transport are defined by glass transition temperature. The empirical equations have been proposed for describing the dependence of permeability to gas coefficient by carbon dioxide and oxygen on glass transition temperature for blends PET/PBT, which have shown the indicated coefficient growth at glass transition temperature reduction. On the process of gas transport influences not only the fact of miscibility of blend components into amorphous phase, through which the indicated processes are realized, but also miscibility level. Weaking of gas transport processes at miscibility level of blend components reduction. Is the common tendency gas permeability coefficient of the considered blends depends also on their crystalline structure.
Work is performed within the complex project on creation of hi-tech production with the participation of the Russian higher educational institution, the Contract of JSC "Tanneta" with the Ministry of Education and Science of the Russian Federation of February 12, 2013 No. 02.G25.31.0008 (Resolution of the Government of the Russian Federation No. 218).
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© M. A. Mikitaev - Ph.D., Senior Researcher, Kh.M. Berbekov Kabardino-Balkarian State University, mikitaev@mail.ru, G. V. Kozlov - Senior Researcher, Kh. M. Berbekov Kabardino-Balkarian State University, i_dolbin@mail.ru, G. E. Zaikov - Doctor of Chemistry, Full Professor, Plastics Technology Department, Kazan National Research Technological University, ov_stoyanov@mail.ru, A. K. Mikitaev - Doctor of Chemistry, Full Professor of Organic Chemistry and Macromolecular Compounds Department, Kh.M. Berbekov Kabardino-Balkarian State University, Nal'chik, Russia, mikitaev@mail.ru.
© М. А. Микитаев - канд. хим. наук, ст. науч. сотр. УНИИД ФГБОУ ВПО «Кабардино-Балкарский государственный университет им. Х.М. Бербекова» (КБГУ), mikitaev@mail.ru; Г. В. Козлов - ст. науч. сотр. УНИИД КБГУ, i_dolbin@mail.ru; Г. Е. Заиков - д-р хим. наук, проф. каф. ТПМ КНИТУ, ov_stoyanov@mail.ru; А. К. Микитаев - д-р хим. наук, проф. каф. органической химии и высокомолекулярных соединений КБГУ, mikitaev@mail.ru.
