INVESTIGATION OF PVC/PLLA BLENDS
Matko ERCEG, Tonka KOVAČIĆ, Ivka KLARIĆ
Department of Organic Chemical Technology, Faculty of Chemical Technology, University of Split, Teslina 10/V, 21 000 Split, Croatia
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ABSTRACT
Poly(vinyl chloride) (PVC) is one of the most important and the most used polymer. Among other things, it is very often used for packaging. It is commonly known that today mostly biologically non-degradable polymers, such as PVC, are used for various packaging. This results in tremendous amount of plastic waste that is considered to be one of the greatest ecological problems of modern society. By blending these commodity polymers with biodegradable ones, usually with polyesters, ecologically and technologically acceptable polymeric materials can be obtained. In this work biodegradable polyester, poly(L-lactide) (PLLA), which can be produced from renewable resources (corn or sugar) and which can successfully substitute some commodity non-degradable polymers, is used. The main drawback of this polymer is its high price compared to commodity polymers. So, blending of PLLA with commodity polymers could be commercially advantageous in order to obtain biodegradable materials with wide variety of properties.
In this investigation PVC/PLLA blends of different mass fraction of polymers were prepared by solution blending method and precipitating in non-solvent. Differential scanning calorimetry was used to study the thermal properties of the blends. The glass transition temperatures, melting temperatures, enthalpies of fusion and enthalpies of crystallisation were determined. DSC curves of the blends show neither a shift of the PVC glass transition temperature nor a shift of PLLA glass transition and melting temperature, which means that these blends are heterogeneous. Therefore, we should find a suitable biodegradable compatibilizer, which would also act as a plasticizer, since both PVC and PLLA are stiff and brittle polymers.
Thermal degradation of the blends was carried out thermogravimetrically in the temperature range 30···750ºC. The nitrogen flow was 30 cm-3 min-1 and the heating rates were 2.5, 5, 10 and 20ºC min-1. It can be noticed that thermal stability of each polymer in the blend is not significantly changed, so there is no interaction of these polymers or their degradable products in the blends.
Key words: PVC/PLLA blends, miscibility, DSC curves, dynamic TG analysis, thermal stability
INTRODUCTION
World wide the problems associated with the production of large amounts of waste are one of the most important challenges to face in future. We have to face the fact that only in Europe 34 million tons of plastic waste has been produced in 1998.1 A significant proportion of this waste are packaging materials that are mostly produced from biologically non-degradable, petrochemically based polymers. Poly(vinyl chloride) (PVC) is one of the most important and the most used commodity polymer, which is very often used for packaging.
All this polymer waste has to be disposed of some day. The preferred solution up to now is recycling, though often conducted as incineration or downcycling. However, introducing biodegradable polymers as packaging materials has made the most important technological improvement. They are produced from renewable resources and can be completely composted into biomass after use, or incinerated without any harm to environment. Poly(L-lactid) (PLLA) is the best example of biodegradable polymers that may substitute commodity polymers in a broad range of applications. PLLA is a linear aliphatic thermoplastic produced from corn or sugar and readily biodegradable.2 Main drawback of this biodegradable polymer is its high price.3 So, blending of biodegradable polymer, PLLA, with commodity polymer, PVC, could be commercially advantageous in order to obtain biodegradable material with wide variety of properties.
EXPERIMENTAL
The materials used in this study were suspension-grade PVC, Ongrovil S-5064, (K value = 63, g mol-1, 86 cm3 g-1 in cyclohexanone) supplied by BorsodChem, (Hungary) and PLLA ( g mol-1, 165 cm3 g-1 in chloroform) supplied by Biomer (Krailling, Germany). The PVC/PLLA blends with different mass fraction of polymers (100/0, 90/10, 80/20, 70/30, 60/40, 50/50, 30/70, and 0/100) were prepared by solution blending method and precipitation in non-solvent. Dichloroethane was used as a solvent and methanol as a non-solvent.
Differential scanning calorimetry (DSC) was used to study thermal properties (the glass transition temperatures, melting temperatures, enthalpies of fusion and enthalpies of crystallisation) of the blends. DSC measurements were performed on a Perkin-Elmer DSC-4 differential scanning calorimeter with Model 3600 Data Station (TADS). The samples of 6-9 mg pressed in aluminium pans were heated at a rate of 10ºC min-1 from 30 to 200ºC (the first heating run) in the nitrogen atmosphere (30 cm-3 min-1) and kept at 200ºC for 10 min. After that the samples were cooled down to 30ºC at 10ºC min-1 and then reheated to 200°C at a rate of 10ºC min-1 (the second heating run). The second heating run was registered. Indium was used as the standard for calibrating the temperature axis and enthalpy output.
The thermal degradation of the blends (mass 2,3 ± 0,2 mg) was carried out thermogravimetrically in the temperature range from 30 to 750ºC using a Perkin-Elmer TGS-2 system with Model 3600 Data Station. The nitrogen flow rate was 30 cm-3 min-1 and the heating rates were 2.5, 5, 10 and 20ºC min-1.
RESULTS AND DISCUSSION
The DSC thermograms of the investigated PVC/PLLA blends are shown in Figure 1 as normalized DSC curves.
Figure 1. Normalized DSC curves for PVC/PLLA blends.
The DSC curves of PVC/PLLA blends show that the glass transition temperatures of PVC (Tg = 83ºC), the glass transition temperatures of PLLA (Tg = 59ºC) and the melting temperatures of PLLA (Tm = 167ºC) in the blends are not significantly changed with the change of the blend composition. That means that for all investigated compositions these PVC/PLLA blends are heterogeneous.
The dynamic thermogravimetric (TG) curves for heating rate 2,5°C min-1 are shown in Figure 2. The TG curves scanned at higher heating rates (5, 10, 20°C min-1) are shifted to higher temperatures. Thermal degradation of PVC occurs through two basic degradation steps: the first one presents the process of dehydrochlorination of PVC, while the second one presents the total degradation of dehydrochlorinated residues.4 Thermal degradation of PLLA is a single stage degradation process.5 Furthermore, PLLA is almost entirely degraded during the first basic degradation step.
PVC/PLLA blends also degrade within two basic degradation steps. This means that process of PVC dehydrochlorination and process of total PLLA degradation in the PVC/PLLA blends take place in the first basic degradation step process. The second basic degradation step of PVC/PLLA blends one presents the total degradation of dehydrochlorinated residues.
Figure 2. Dynamic TG curves for PVC/PLLA blends of different compositions; heating rate of 2,5°C min-1.
The characteristics of TG curves6,7, for the first basic degradation step were obtained by applying the Perkin-Elmer Standard Program and they are: onset temperature, (T10) (the intersections of extrapolated base lines with tangents drawn in the inflection points of the TG curve), the temperatures for a 1% degree of conversion (T1%), the temperature for a 5% degree of conversion (T5%), the temperature (T1m) and the degree of conversion (a1m) at the maximal rate of PVC dehydrochlorination, and the mass loss at the end of the first basic degradation step (Dm1). In the Figure 3 the dependence of these characteristics on the PLLA ratio in the blend is shown for the heating rate 2,5°C min-1. By increasing the PLLA ratio from 0 to 90 mass %, the temperatures T10 are almost unchanged. The temperatures T1m, T1% and T5% are not significantly changed, too. Furthermore, onset temperature (T20) and the temperature of the maximal rate of degradation (T2m) of the second basic degradation step are unchanged with increasing PLLA ratio in the blends. At the same time Dm1 increases and a1m decreases linearly with increasing PLLA ratio in the blends.
Figure 3. Effect of PVC/PLLA blend composition on the first and the second basic degradation step characteristics.
CONCLUSIONS
The PVC/PLLA blends of all the investigated compositions are heterogeneous since their DSC curves show neither shift of PVC glass transition temperature nor a shift of PLLA glass transition and melting temperature. The dynamic thermal degradation of PVC/PLLA blends in the temperature range of 30···750°C occurs through two basic degradation steps where the first basic step consists of two substeps, with dehydrochlorination of PVC and total degradation of PLLA as the main degradation process. By increasing the ratio of PLLA in the blends the maximal rate of PVC dehydrochlorination in blends lowers and the maximal rate of PLLA increases. By increasing the ratio of PLLA in the blends, the temperatures T10, T1m, T1%, T5%, T20 and T2m are not significantly changed. This means that there are no interactions of PVC and PLLA in the blends. Therefore, we should find a suitable biodegradable compatibilizer, which would also act as a plasticizer, since both PVC and PLLA are stiff and brittle polymers.
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