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11
UDC 541.64
DOI: 10.33184/bulletin-bsu-2022.1.5
THE POSSIBILITY OF INVOLVING SECONDARY POLYETHYLENE TEREPHTHALATE IN RECYCLING
© V. V. Chernova1, M. S. Kotyashov2, M. Yu. Lazdina1, E. I. Kulish1*
'Bashkort State University 32 Zaki Validi Street, 450076 Ufa, Republic of Bashkortostan, Russia.
2Ufa State Petroleum Technological University '4 Pervomayskaya Street, 450000 Ufa, Republic of Bashkortostan, Russia.
*Email: [email protected]
The possibility of involving secondary polyethylene terephthalate in reuse was considered. The primary PET produced by SIBUR and secondary PET obtained from used plastic containers were used in the work. Modeling of the process of conversion polymer materials and obtaining composites based on mixtures of primary and secondary PET was carried out in the melt at the laboratory station (plastograph) "PlastographEC" (Brabender, Germany). The deformation and strength properties of the material were determined on pressed samples of material with a thickness of ' mm. Pressing was carried out on an automatic hydraulic press "AutoMH-NE" (Carver, USA). The physical and mechanical properties of polymer composites at rupture were determined according to GOST '' 262 on the ShimadzuAGS-X bursting machine (Shimadzu, Japan). It is shown that the real way of recycling containers made ofpolyethylene terephthalate is its joint processing according to a typical technological scheme with a primary polymer. It is asserted that the addition of a secondary polymer to the primary one generally leads to a decrease in the values of elastic modulus, breaking stress and breaking elongation. However, the ratio of components in the mixture containing no more than 30-40% of the secondary polymer makes it possible to create materials that in their physico-mechanical properties are inferior to materials from the primary polymer by no more than '0%.
Keywords: polyethylene terephthalate, processing, recycling, secondary polymer raw materials.
Introduction
Polyethylene terephthalate (PET) occupies one of leading places in the global production and consumption of structural plastics [1-3]. Only in Russia, at least 2 million tons of used plastic containers have already been accumulated at landfills of solid household waste [4]. Therefore, the relevance of PET containers and packaging recycling issue is beyond doubt [5-8].
Currently, a very common method of processing obsolete PET products is methanolysis to produce dimethyl terephthalate [9-14]. The cleavage products can be used again as raw materials for the PET polycondensation process, however, the impurities present in these products allow the received polymer to be used mainly for the manufacture of fusible and soluble adhesives [15-17]. Therefore, a very promising way of processing PET may be to involve it in recycling, using it as an additive to primary PET. The purpose of this work was to obtain a composition consisting of primary and secondary PET obtained on the basis of used containers, and to create materials based on it with satisfactory performance characteristics.
Experimental
The primary PET produced by SIBUR and secondary PET obtained from used plastic containers were used in the work. Modeling of the process of conversion polymer materials and obtaining composites based on mixtures of primary and secondary PET was carried out in the melt at the laboratory station (plastograph) "PlastographEC" (Brabender, Germany) for 15 minutes
at a load of 200 N. The temperature in the mixing zone varied from 250 to 270 °C. The rotation speed of the rotors ranged from 10 to 50 rpm. The amount of polymer to be loaded (or a mixture of polymers) was 25 g.
The deformation and strength properties of the material were determined on pressed samples of material with a thickness of 1 mm. Pressing was carried out on an automatic hydraulic press "AutoMH-NE" (Carver, USA) at 250 oC and holding at a pressure of 5 000 kg for 3 min. The physical and mechanical properties of polymer composites at rupture were determined according to GOST 11 262 on the ShimadzuAGS-X bursting machine (Shimadzu, Japan) at the temperature of 20 °C and the speed of movement of the movable grip of the bursting machine 1 mm/min. Melt flow index (MFI) was determined at 250 °C and a load weight of 2.16 kg.
Results and discussion
The traditional scheme of obtaining materials from recycled polymer raw materials from used products includes the following stages: pre-sorting and cleaning, grinding, washing, drying, and processing [18-22]. At the same time, it is the stage of processing that is characterized by the most intense mechanical and thermal effects on the material, which can contribute to the processes of thermal oxidative degradation and, as a result, worsen the operational characteristics of materials obtained from a secondary polymer [23-25]. Therefore, the work's initial stage was the assessment of the PET processing conditions im-
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Thus, from the point of view of polymer processing, the most sparing conditions for PET plasticiza-tion are the temperature of 250 °C (PET processing at lower temperatures cannot be carried out, since 250 °C is the melting point of the crystalline phase of PET) and the mixing speed of the components in the plastograph is no more than 20 revolutions per minute. It is under these conditions that the polymer melt is characterized by stable rheological parameters, and therefore, further production of composites based on mixtures of primary and secondary PET was carried out under these conditions.
The representation of the obtained data in the form of a diagram allows us to state the fact that the change in both rheological (MFI) (Figure 1A) and physico-mechanical (the value of the elastic modulus E, breaking stress c, and breaking elongation l) indicators (Figures 1B, 2A, and 2B) of the obtained composites does not occur additively. With a content of 40% secondary PET in the composition, phase reversal takes place.
Table
The values of the melt flow index of secondary and primary PET from the processing conditions
The test sample Rotation spe ;ed of the rotors, min"1 Processing temperature, °C Melt flow index, g/10 min
- - 20.0
10 250 25.1
20 250 25.2
30 250 25.3
40 250 27.7
50 250 29.9
10 260 26.0
Primary PET 20 260 26.1
30 260 26.2
40 260 32.2
50 260 35.5
10 270 30.0
20 270 30.2
30 270 30.4
40 270 37.4
50 270 39.2
- - 24.1
10 250 27.8
20 250 28.0
30 250 28.2
40 250 33.5
50 250 35.6
10 260 31.8
Secondary PET 20 260 32.3
30 260 32.5
40 260 38.2
50 260 41.8
10 270 34.3
20 270 34.8
30 270 36.7
40 270 45.4
50 270 48.6
pact on the properties of formed materials based on the analysis of MFI data.
The following should be noted. Firstly, the passage of the polymer processing stage on the Brabender plastograph is unambiguously accompanied by an increase in the values of both primary and secondary PET (table), which indicates the course of destructive processes. Thus, the MFI values of the initial primary and secondary PET samples that have not passed the plasticization stage are 20.0 and 24.1 g/10 min, respectively. At the same time, the passage of polymer samples through the plasticization stage even under conditions of minimal exposure (temperature in the plastograph chamber - 250 °C, screw rotation speed -10 revolutions per minute), leads to an increase in the values of MFI to 25.1 and 27.8 g/10 min, respectively. Secondly, with an increase in the mixing speed of the components during the processing of PET samples, there is also an increase in the values of MFI, especially when the rotor speed exceeds 40 revolutions per minute. Thirdly, the higher the temperature in the mixing chamber, the more intense the increase in MFI values.
А)
Fig. 1. Dependence of the values of MFI (A) and discontinuous elongation (B) on the composition of the mixture.
A) ............B)
Fig. 2. Dependence of elastic modulus (A) and breaking stress (B) on the ratio of components in the mixture.
The deviation of the values of the rheological and physico-mechanical properties of the composition from the additive values most likely indicates an increased interaction between the components of the mixture, due to their affinity for each other. In turn, the interaction of the components finds its natural reflection on the values of the physico-mechanical parameters of the polymer composition.
As it can be seen from the generalizing dependences on the change in tensile elongation, elastic modulus, and tensile stress shown in Figures 1B, 2A, and 2B, respectively, for a mixed composition, only the addition of more than 40% of secondary PET results in a sharp drop in the elastic modulus and stress values at break. When the portion of secondary PET added to the primary is from 10 to 40%, the main physical and mechanical parameters remain at the level of primary polymer. The values of the elongation at break also decrease significantly only when the secondary PET content is more than 40%.
Thus, the real way to dispose of PET containers, valuable polymer raw materials, is its joint processing according to a typical technological scheme with a primary polymer. On the basis of this material it is possible to create composite materials that, by their qualities, relate to structural plastics of general technical purpose.
Conclusions
1. Optimal conditions for plasticization of both primary and secondary PET have been established: the
temperature is 250 °C; the mixing speed of the components is no more than 20 revolutions per minute.
2. It is shown that the addition of a secondary PET to the primary leads, in general, to a decrease in the values of the elastic modulus, breaking stress, and breaking elongation.
3. The ratio of components with a secondary PET content of no more than 30-40% was found, having basic physical and mechanical parameters that differ from the properties of materials obtained from primary PET by no more than 10%.
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Received 08.02.2022.
