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thermoplastic starch blends
Polymer Testing 58 (2017) 166e172
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Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Degradation and recovery in poly(butylene adipate-co-terephthalate)/ thermoplastic starch blends ria A.D. Marinho, Camila A.B. Pereira, Maria B.C. Vitorino, Aline S. Silva, Vitho Laura H. Carvalho, Eduardo L. Canedo* Department of Materials Engineering, Federal University of Campina Grande, Campina Grande, PB 58429-140, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 November 2016 Accepted 22 December 2016 Available online 26 December 2016
The economic and social impact of the increasing waste disposal problems of conventional plastic materials are well known and promoted the search for better recyclable and biodegradable polymers, blends and compounds. Fully biodegradable blends of poly(butylene adipate-co-terephthalate) (PBAT), a synthetic copolyester, and thermoplastic starch (TPS), a natural polysaccharide, are of technical and economic interest in the quest for eco-friendly polymeric materials to substitute conventional alternatives. One of less desirable characteristics of many new biodegradable materials is their relative thermal instability (degradation) under processing conditions. In the present work, PBAT/TPS blends with up to 30% TPS were processed at different temperatures in a laboratory internal mixer, with and without the incorporation of a chain extender additive (Joncryl). The rate of change of torque during the melt processing stage, adjusted to eliminate minor temperature variations, is a very sensitive indicator of variation of molar mass due to degradation and recovery. It was found that TPS content promotes thermal degradation in the PBAT/TPS blends at levels above those observed in neat components, in a strongly composition and temperature-dependent process. The addition of 1% of the chain extender additive partially reverts the process, especially during processing at high temperature. © 2016 Published by Elsevier Ltd.
Keywords: PBAT TPS Joncryl Degradation during processing
1. Introduction The economic and social importance of biodegradable and ecofriendly materials has grown tremendously over the last decade as increasing public awareness and environmental concerns over the disposal of solid wastes prompted the creation of more stringent laws on the disposal of plastic products, the concept of sustainability, the growth of the recycling industry and the use of biodegradable and/or biopolymers as substitutes of petroleum based plastics, particularly in the packaging industry [1]. There is a strong need to develop biodegradable products for packaging and mulching films as these are high volume plastic applications which are rapidly disposed of while still retaining satisfactory performance [2]. Plastics used for these applications, if synthetic in origin and non-biodegradable, can provoke considerable environmental impact as they cause severe visual pollution once discarded, take a
* Corresponding author. E-mail address: [email protected] (E.L. Canedo). http://dx.doi.org/10.1016/j.polymertesting.2016.12.028 0142-9418/© 2016 Published by Elsevier Ltd.
long time to degrade and also tend to reduce permeability of the soil if used as mulching films. Biodegradable plastic materials (BPMs) would be an ideal choice for these applications as they could be safely and effectively disposed in soil or compost, as their degradation products would not harm the soil, flora or fauna [1,2]. BPMs can be of natural or petrochemical origin. Poly(butylene adipate-co-terephthalate) (PBAT) is an aromatic-aliphatic copolyester and an example of a BPM of petrochemical origin. It is a synthetic semicrystalline thermoplastic copolyester, with mechanical and thermal properties similar to those of some polyethylenes [3]. It is fully biodegradable in municipal dumps (it is “compostable”), may be processed in conventional polymer processing equipment (mixers, extruders, injection molding machines, etc.) and is fairly stable under processing conditions. Moreover, it has some interesting barrier properties for a material that may be formed into films for the food packaging industry. Compared with LDPE, neat PBAT has a lower permeability to oxygen (50%) and a much higher permeability to water vapor (80 times) [4e10]. Although PBAT is fairly stable, it degrades at processing temperatures (140e230 C), as most polyesters do, by hydrolysis of the
V.A.D. Marinho et al. / Polymer Testing 58 (2017) 166e172
ester bond in the presence of humidity and by thermally-induced chain scission, even in the absence of water. As a result, a decrease in average molar mass is observed after processing [11e13]. PBAT applications, in some fields, is limited due to cost. Starch is an example of BPM from natural sources. Thermoplastic starch (TPS) is obtained by plasticization of starch granules with water or glycerol under heating. TPS applications are limited due to its mechanical properties and moisture sensitivity, so it is often used in blends with other polymers to reduce cost [14e16]. The association of TPS with another biodegradable polymer is a means to obtain low cost compostable product, thus, blends of TPS with PBAT, PLA, PCL and other polyesters and poly(hydroxyl alcanoates) have been reported in the literature [17e19]. The main problems associated with these blends are their cost, compatibility and water sensitivity with increasing TPS content. The latter property limits TPS materials in applications where water contact can occur, such as packaging. Studies performed on PBAT/TPS blends indicate that the use of compatibilizers, particularly at high TPS contents is desirable if a good set of properties are to be achieved. Several compounds such as soybean oil, citric, and tartaric acids, maleic anhydride, glycidyl methacrylate, as well as maleated TPS, maleated PBAT, and starch nanoparticles have been used as compatibilizers for this system with different efficiencies [20e24]. In general, thermal and mechanical properties, and biodegradability, were improved [25e27]. Studies have also shown that transesterification reactions do take place during melt processing of PBAT/TPS blends and that, although these reactions are more intense in the presence of maleated TPS, they do take place with TPS which was not derivatized as the free glycerol present in TPS is sufficient to catalyze the reaction [28e30]. No studies dealing specifically with degradation under processing were found in the literature. In the present work, PBAT/TPS blends were processed at different temperatures in a laboratory internal mixer for 15 min. After 10 min of processing, a chain extender, which may also act as compatibilizer, Joncryl PR010, an oligomer with epoxy and methacrylate residues, which is capable to chain extend and compatibilize the PBAT/TPS system, was added to the mixture which was then processed for another 5 min. The torque and temperature were monitored and the data discussed with respect to torque recovery and polymer degradation. 2. Experimental 2.1. Materials Poly(butylene adipate-co-terephthalate) [PBAT], commercialized by BASF (Germany) under the name Ecoflex® F Blend C1200, and thermoplastic starch [TPS], produced by Ingredion (Brazil) as Beneform 4180, were used in the present work. The chain extension additive Polyad PR10 [Joncryl] e a epoxidic oligomer recommended compensate degradation during processing in polyesters and polyamides e was supplied by Polyad Services (Brazil). Thermoplastic starch is a complex material, composed of at least two different polysaccharides e linear amylose and branched amylopectin e and low molecular weight plasticizers. The exact composition of the commercial TPS used in this work is unknown. PBAT/TPS blends are actually ternary blends, even if they are nominally designated as binary.
167
operating at a nominal speed of 60 rpm with chamber wall temperature kept at a constant temperature of a 140, 170 and 200 C. The fill factor of the mixing chamber was 67 ± 1% at ambient temperature, with a total processing time of 15 min. PBAT and TPS were also subjected to the same treatment to obtain a baseline for comparison. Tests were also conducted with the blends and neat components in which 1% Joncryl additive was added after 10 min without interrupting the process.
2.3. Torque rheometry and degradation In tests conducted in an internal mixer at constant rotor speed, during the last processing stage (melt processing), torque Z is directly proportional to the viscosity h of the molten material [31]:
Zfh
(1)
Melt viscosity is very sensitive to changes in molar mass; for a polymer melt of power law index n processed at constant temperature [32]: 2;5þn hfMw
(2)
where Mw is weight average molar mass. For PBAT, we may assume n z 0.8 for processing in an internal mixer at 60 rpm [10]. However, viscosity (hence, torque) is strongly dependent on temperature. Thus, torque variations during terminal processing may be attributed to the combined effect of molar mass and temperature. For minor changes in temperature its dependence may be eliminated using a temperature-corrected value:
Z* ¼ Z expfbðT T*Þg
(3)
where Z* is the adjusted value at the reference temperature T* (a function of molar mass only) and b is the exponential temperature coefficient of the viscosity; b z 0,025 C1 has been measured for PBAT [10] and will be used for PBAT/TPS blends as a first approximation. It should be pointed out that Eq. (3) deals with temperature dependence of molecular mobility and free volume effects on the viscosity. Changes in molar mass are due to chemical reactions, whose rate depends strongly on temperature. Thus, the rate of change of molar mass e and its effect on viscosity and torque e are affected by temperature in ways not covered by Eq. (3). The relative rate of change of the adjusted torque can be taken as a measure of the rate of degradation (if torque decreases with time) or chain extension (if torque increases with time); it is, in fact, the kinetic constant of a first-order rate process associated with the variation of molar mass. In practical terms, choosing a terminal time interval Dt (in the present case, the interval between 12 and 15 min processing time) gives:
RZ ¼
1 DZ * Z * Dt
(4)
where Z * is the mean adjusted torque in the interval Dt. The rate of change of molar mass may be estimated considering the dependence of the melt viscosity with the weight average molar mass, Eq. (2)
1 DMw 1 R z 2; 5 þ n Z M w Dt
2.2. Processing
RM ¼
Blends of PBAT and TPS with 10%, 20%, and 30% starch (by weight) were compounded in a Haake Rheomix 3000 laboratory internal mixer fitted with high intensity (roller type) rotors,
If Dt is expressed in minutes, 100RZ is the percent variation of corrected torque, and 100RM is the percent variation of molar mass (weight average) per minute of processing.
(5)
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V.A.D. Marinho et al. / Polymer Testing 58 (2017) 166e172
Laboratory internal mixers are not high precision instruments; viscosity estimates obtained in this way are only approximate, and cannot substitute intrinsic viscosity measurements or direct determinations of molar mass in the study of polymer degradation and recovery [33,34]. However, the simple method presented in this contribution has definite advantages: (a) melt viscosity is much more sensitive to changes in molar mass than intrinsic viscosity, and (b) estimates are obtained in real time, as polymer degrades under processing. Moreover, laboratory internal mixers are unsophisticated pieces of equipment, readily available in small academic and industrial settings alike. While torque rheometry methods have been explored for these applications in the past [35], their systematic use is fairly recent. Variants of this approach has been reported in the literature, to study chain extension in virgin and recycled PET [36,37] and PBAT [10], and degradation during processing in PBAT/vegetable fiber compounds [38].
3. Results and discussion Fig. 1 shows temperature and torque versus time for the processing of PBAT/TPS blends with 20% TPS content at three different chamber wall temperatures, and Fig. 2 the same results when 1% chain extension additive is added after 10 min processing time. Neat PBAT, TPS, and PBAT/TPS blends with 10% and 30% TPS present similar results. Torque was adjusted according to Eq. (3) using two different choices of reference temperature: (a) the mixing chamber wall temperature of the test, T* ¼ T0, and a fixed temperature, the same for all tests, T* ¼ 170 C, as shown in the example of Fig. 3. Visually, the first choice (Fig. 3a) shows the temperature dependence of the viscosity (higher torque at lower temperature), the second (Fig. 3b) shows more clearly the effect of temperature on the rate of degradation (higher slope at higher temperature); but from an analytical point of view both choices are equivalent. Average melt temperature (T) and adjusted torque (Z *) computed using the chamber wall temperature T0 as reference were estimated for the terminal stage of processing (12e15 min) in the internal mixer. The rates of change (positive: decrease, negative: increase) of the adjusted torque (RZ) and the weight-average
molar mass (ZM) were estimated according to the procedure described in the previous section. These results are shown as bar plots in Figs. 4e6. Numerical values are included as Supplementary Information. Fig. 4 shows graphically the adjusted torque of PBAT and the blends in terms of processing temperature and composition. Since terminal adjusted torque reflects the viscosity of the melts, results will discussed in terms of viscosity. PBAT is more viscous than TPS. The viscosity of the blends was found to be higher than the viscosity of the components at low processing temperatures, and intermediate between them at the higher temperatures. This seemingly puzzling effect may be due to the complex dependence of the ternary blend viscosity on the individual components viscosities, coupled with significant differences in their temperature dependence. Note that terminal melt temperature is 22 C degrees higher than the wall temperature at low temperature (T0 ¼ 140 C), but only 12 C and 6 C at higher temperatures (T0 ¼ 170 C and T0 ¼ 200 C, respectively). However, the viscosity of the blends is virtually independent of composition and the presence or absence of additive. Terminal adjusted torque for the samples processed at the lower temperature (140 C) are around ±5% of the average value 61.0 Nm; at the higher processing temperature (200 C) torque is around ±8% of the average value 12.8 Nm. These differences did not show a significant amount of degradation or recovery. Viscosity comparisons (at least within the precision that may be achieved in a torque rheometer) are not appropriate to study the minor degradation/recovery processes taking place in the PBAT/TPS system. In order to investigate degradation and recovery we need to consider the rate of change of terminal torque. Fig. 6 shows graphically the relative rate of change of adjusted torque in terms of processing temperature and composition. Fig. 5 shows the expected higher rate of degradation of TPS compared with PBAT, and reveals that degradation is independent of temperature at low processing temperature (T0 ¼ 140 C and T0 ¼ 170 C). The effect of the chain extender additive on neat PBAT is striking: torque (hence, viscosity and molar mass) increases sharply and the increase is highly temperature dependent. The additive not only recovered the mild losses due to degradation, but actually extended PBAT chains. The rate of degradation of the
Fig. 1. Temperature (a) and torque (b) versus time during processing of a PBAT/20%TPS blend in the laboratory internal mixer at 60 rpm and three different chamber wall temperatures (indicated).
V.A.D. Marinho et al. / Polymer Testing 58 (2017) 166e172
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Fig. 2. Temperature (a) and torque (b) versus time during processing of a PBAT/20%TPS blend in the laboratory internal mixer at 60 rpm and three different chamber wall temperatures (indicated).
Fig. 3. Adjusted torque versus time during the last stage of processing of the PBAT/30%TPS blend. Torque was adjusted to the chamber wall temperature T* ¼ T0 (a) and to a fixed temperature, T* ¼ 170 C (b).
blends (without additivation) is higher that the rate of degradation of any of the components, and increases significantly with TPS content, especially at the higher processing temperature. Incorporation of the chain extender resulted in a partial recovery, and is highly temperature-dependent, consistent with results previously reported for neat PBAT [10]. The effect of the additive is virtually null at low processing temperature (T0 ¼ 140 C) but is very significant at high temperature (T0 ¼ 200 C). At this temperature, the recovery significantly depends on TPS content: RZ is 47% in PBAT/ 10%TPS blends with additive compared to ones without additive (approximate 2:1 ratio), 36% in PBAT/20%TPS blends (approximate 3:1 ratio) and 16% (approximate 6:1 ratio). Joncryl is highly effective increasing the molar mass of polyesters, such as PBAT, not for polysaccharides, such as TPS. On the other hand, TPS increases the degradation of PBAT. But the more TPS in the blend, the higher the proportion of chain extension
additive relative to PBAT (for a fixed percentage of Joncryl). The combined effect of these effects may be responsible for peculiar pattern of recovery observed as a function of composition. Fig. 6 shows results of the relative molar mass decrease, Eq. (5), for the blends in terms of processing temperature and composition. As expected, RM trends follow exactly RZ trends. However, differences between different compositions and temperatures are considerably flattened, due to the high power of the viscosity dependence on molar mass. Thus, the relative rate of torque variation with time, albeit an indirect measure of molar mass change, is a better tool to study it.
4. Final considerations In this contribution, a powerful method to study polymer degradation and recovery under processing was introduced and
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Fig. 4. Mean corrected terminal torque for PBAT and PBAT/TPS blends with and without additive at three temperatures.
Fig. 5. Rate of change of terminal torque for PBAT and PBAT/TPS blends with and without additive at three temperatures. Negative values of eRZ correspond to torque (molar mass) increase due to chain extension.
V.A.D. Marinho et al. / Polymer Testing 58 (2017) 166e172
171
Fig. 6. Estimated rate of change of molar mass during terminal processing for PBAT and PBAT/TPS blends with and without additive at three temperatures.
applied to the technically and economically significant system of fully biodegradable blends as an example. Some interesting conclusions may be drawn for this exercise: The inability of torque (viscosity) comparisons to reveal the temperature and composition dependence of mild polymer degradation under processing conditions, and the superior performance of relative rate of torque (viscosity) changes in the terminal stage of processing for this purpose. The deleterious effect of TPS on the degradation of PBAT/TPS blends and its significant temperature dependence (and composition dependence at high temperatures). Not only TPS degrades more than PBAT during processing, but it also increases the degradation of PBAT. The effectiveness of the chain extender additive used (Joncryl) at 1% concentrations on the partial recovery of degradation on PBAT/TPS blends, particularly at high processing temperatures. Joncryl is highly effective in recovering e and actually increasing e molar mass of PBAT (a polyester), but not so for TPS (a mixture of polysaccharides). We have purposely refrained from speculating about the structural and morphological significance of the results obtained, showing instead the power of torque rheometry to reveal some quantitative facts on a macroscopic level, upon which such microscopic considerations could be built, when complemented by other observations and measurements.
Acknowledgements The authors wish to thank the Conselho Nacional de Pesquisa ~o de Aperfeiçoamento de Pessoal Superior (CNPq) e Coordenaça (CAPES), Grant # 473622/2013-0, for financial support.
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Contents lists available at ScienceDirect
Polymer Testing journal homepage: www.elsevier.com/locate/polytest
Material Properties
Degradation and recovery in poly(butylene adipate-co-terephthalate)/ thermoplastic starch blends ria A.D. Marinho, Camila A.B. Pereira, Maria B.C. Vitorino, Aline S. Silva, Vitho Laura H. Carvalho, Eduardo L. Canedo* Department of Materials Engineering, Federal University of Campina Grande, Campina Grande, PB 58429-140, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 November 2016 Accepted 22 December 2016 Available online 26 December 2016
The economic and social impact of the increasing waste disposal problems of conventional plastic materials are well known and promoted the search for better recyclable and biodegradable polymers, blends and compounds. Fully biodegradable blends of poly(butylene adipate-co-terephthalate) (PBAT), a synthetic copolyester, and thermoplastic starch (TPS), a natural polysaccharide, are of technical and economic interest in the quest for eco-friendly polymeric materials to substitute conventional alternatives. One of less desirable characteristics of many new biodegradable materials is their relative thermal instability (degradation) under processing conditions. In the present work, PBAT/TPS blends with up to 30% TPS were processed at different temperatures in a laboratory internal mixer, with and without the incorporation of a chain extender additive (Joncryl). The rate of change of torque during the melt processing stage, adjusted to eliminate minor temperature variations, is a very sensitive indicator of variation of molar mass due to degradation and recovery. It was found that TPS content promotes thermal degradation in the PBAT/TPS blends at levels above those observed in neat components, in a strongly composition and temperature-dependent process. The addition of 1% of the chain extender additive partially reverts the process, especially during processing at high temperature. © 2016 Published by Elsevier Ltd.
Keywords: PBAT TPS Joncryl Degradation during processing
1. Introduction The economic and social importance of biodegradable and ecofriendly materials has grown tremendously over the last decade as increasing public awareness and environmental concerns over the disposal of solid wastes prompted the creation of more stringent laws on the disposal of plastic products, the concept of sustainability, the growth of the recycling industry and the use of biodegradable and/or biopolymers as substitutes of petroleum based plastics, particularly in the packaging industry [1]. There is a strong need to develop biodegradable products for packaging and mulching films as these are high volume plastic applications which are rapidly disposed of while still retaining satisfactory performance [2]. Plastics used for these applications, if synthetic in origin and non-biodegradable, can provoke considerable environmental impact as they cause severe visual pollution once discarded, take a
* Corresponding author. E-mail address: [email protected] (E.L. Canedo). http://dx.doi.org/10.1016/j.polymertesting.2016.12.028 0142-9418/© 2016 Published by Elsevier Ltd.
long time to degrade and also tend to reduce permeability of the soil if used as mulching films. Biodegradable plastic materials (BPMs) would be an ideal choice for these applications as they could be safely and effectively disposed in soil or compost, as their degradation products would not harm the soil, flora or fauna [1,2]. BPMs can be of natural or petrochemical origin. Poly(butylene adipate-co-terephthalate) (PBAT) is an aromatic-aliphatic copolyester and an example of a BPM of petrochemical origin. It is a synthetic semicrystalline thermoplastic copolyester, with mechanical and thermal properties similar to those of some polyethylenes [3]. It is fully biodegradable in municipal dumps (it is “compostable”), may be processed in conventional polymer processing equipment (mixers, extruders, injection molding machines, etc.) and is fairly stable under processing conditions. Moreover, it has some interesting barrier properties for a material that may be formed into films for the food packaging industry. Compared with LDPE, neat PBAT has a lower permeability to oxygen (50%) and a much higher permeability to water vapor (80 times) [4e10]. Although PBAT is fairly stable, it degrades at processing temperatures (140e230 C), as most polyesters do, by hydrolysis of the
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ester bond in the presence of humidity and by thermally-induced chain scission, even in the absence of water. As a result, a decrease in average molar mass is observed after processing [11e13]. PBAT applications, in some fields, is limited due to cost. Starch is an example of BPM from natural sources. Thermoplastic starch (TPS) is obtained by plasticization of starch granules with water or glycerol under heating. TPS applications are limited due to its mechanical properties and moisture sensitivity, so it is often used in blends with other polymers to reduce cost [14e16]. The association of TPS with another biodegradable polymer is a means to obtain low cost compostable product, thus, blends of TPS with PBAT, PLA, PCL and other polyesters and poly(hydroxyl alcanoates) have been reported in the literature [17e19]. The main problems associated with these blends are their cost, compatibility and water sensitivity with increasing TPS content. The latter property limits TPS materials in applications where water contact can occur, such as packaging. Studies performed on PBAT/TPS blends indicate that the use of compatibilizers, particularly at high TPS contents is desirable if a good set of properties are to be achieved. Several compounds such as soybean oil, citric, and tartaric acids, maleic anhydride, glycidyl methacrylate, as well as maleated TPS, maleated PBAT, and starch nanoparticles have been used as compatibilizers for this system with different efficiencies [20e24]. In general, thermal and mechanical properties, and biodegradability, were improved [25e27]. Studies have also shown that transesterification reactions do take place during melt processing of PBAT/TPS blends and that, although these reactions are more intense in the presence of maleated TPS, they do take place with TPS which was not derivatized as the free glycerol present in TPS is sufficient to catalyze the reaction [28e30]. No studies dealing specifically with degradation under processing were found in the literature. In the present work, PBAT/TPS blends were processed at different temperatures in a laboratory internal mixer for 15 min. After 10 min of processing, a chain extender, which may also act as compatibilizer, Joncryl PR010, an oligomer with epoxy and methacrylate residues, which is capable to chain extend and compatibilize the PBAT/TPS system, was added to the mixture which was then processed for another 5 min. The torque and temperature were monitored and the data discussed with respect to torque recovery and polymer degradation. 2. Experimental 2.1. Materials Poly(butylene adipate-co-terephthalate) [PBAT], commercialized by BASF (Germany) under the name Ecoflex® F Blend C1200, and thermoplastic starch [TPS], produced by Ingredion (Brazil) as Beneform 4180, were used in the present work. The chain extension additive Polyad PR10 [Joncryl] e a epoxidic oligomer recommended compensate degradation during processing in polyesters and polyamides e was supplied by Polyad Services (Brazil). Thermoplastic starch is a complex material, composed of at least two different polysaccharides e linear amylose and branched amylopectin e and low molecular weight plasticizers. The exact composition of the commercial TPS used in this work is unknown. PBAT/TPS blends are actually ternary blends, even if they are nominally designated as binary.
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operating at a nominal speed of 60 rpm with chamber wall temperature kept at a constant temperature of a 140, 170 and 200 C. The fill factor of the mixing chamber was 67 ± 1% at ambient temperature, with a total processing time of 15 min. PBAT and TPS were also subjected to the same treatment to obtain a baseline for comparison. Tests were also conducted with the blends and neat components in which 1% Joncryl additive was added after 10 min without interrupting the process.
2.3. Torque rheometry and degradation In tests conducted in an internal mixer at constant rotor speed, during the last processing stage (melt processing), torque Z is directly proportional to the viscosity h of the molten material [31]:
Zfh
(1)
Melt viscosity is very sensitive to changes in molar mass; for a polymer melt of power law index n processed at constant temperature [32]: 2;5þn hfMw
(2)
where Mw is weight average molar mass. For PBAT, we may assume n z 0.8 for processing in an internal mixer at 60 rpm [10]. However, viscosity (hence, torque) is strongly dependent on temperature. Thus, torque variations during terminal processing may be attributed to the combined effect of molar mass and temperature. For minor changes in temperature its dependence may be eliminated using a temperature-corrected value:
Z* ¼ Z expfbðT T*Þg
(3)
where Z* is the adjusted value at the reference temperature T* (a function of molar mass only) and b is the exponential temperature coefficient of the viscosity; b z 0,025 C1 has been measured for PBAT [10] and will be used for PBAT/TPS blends as a first approximation. It should be pointed out that Eq. (3) deals with temperature dependence of molecular mobility and free volume effects on the viscosity. Changes in molar mass are due to chemical reactions, whose rate depends strongly on temperature. Thus, the rate of change of molar mass e and its effect on viscosity and torque e are affected by temperature in ways not covered by Eq. (3). The relative rate of change of the adjusted torque can be taken as a measure of the rate of degradation (if torque decreases with time) or chain extension (if torque increases with time); it is, in fact, the kinetic constant of a first-order rate process associated with the variation of molar mass. In practical terms, choosing a terminal time interval Dt (in the present case, the interval between 12 and 15 min processing time) gives:
RZ ¼
1 DZ * Z * Dt
(4)
where Z * is the mean adjusted torque in the interval Dt. The rate of change of molar mass may be estimated considering the dependence of the melt viscosity with the weight average molar mass, Eq. (2)
1 DMw 1 R z 2; 5 þ n Z M w Dt
2.2. Processing
RM ¼
Blends of PBAT and TPS with 10%, 20%, and 30% starch (by weight) were compounded in a Haake Rheomix 3000 laboratory internal mixer fitted with high intensity (roller type) rotors,
If Dt is expressed in minutes, 100RZ is the percent variation of corrected torque, and 100RM is the percent variation of molar mass (weight average) per minute of processing.
(5)
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Laboratory internal mixers are not high precision instruments; viscosity estimates obtained in this way are only approximate, and cannot substitute intrinsic viscosity measurements or direct determinations of molar mass in the study of polymer degradation and recovery [33,34]. However, the simple method presented in this contribution has definite advantages: (a) melt viscosity is much more sensitive to changes in molar mass than intrinsic viscosity, and (b) estimates are obtained in real time, as polymer degrades under processing. Moreover, laboratory internal mixers are unsophisticated pieces of equipment, readily available in small academic and industrial settings alike. While torque rheometry methods have been explored for these applications in the past [35], their systematic use is fairly recent. Variants of this approach has been reported in the literature, to study chain extension in virgin and recycled PET [36,37] and PBAT [10], and degradation during processing in PBAT/vegetable fiber compounds [38].
3. Results and discussion Fig. 1 shows temperature and torque versus time for the processing of PBAT/TPS blends with 20% TPS content at three different chamber wall temperatures, and Fig. 2 the same results when 1% chain extension additive is added after 10 min processing time. Neat PBAT, TPS, and PBAT/TPS blends with 10% and 30% TPS present similar results. Torque was adjusted according to Eq. (3) using two different choices of reference temperature: (a) the mixing chamber wall temperature of the test, T* ¼ T0, and a fixed temperature, the same for all tests, T* ¼ 170 C, as shown in the example of Fig. 3. Visually, the first choice (Fig. 3a) shows the temperature dependence of the viscosity (higher torque at lower temperature), the second (Fig. 3b) shows more clearly the effect of temperature on the rate of degradation (higher slope at higher temperature); but from an analytical point of view both choices are equivalent. Average melt temperature (T) and adjusted torque (Z *) computed using the chamber wall temperature T0 as reference were estimated for the terminal stage of processing (12e15 min) in the internal mixer. The rates of change (positive: decrease, negative: increase) of the adjusted torque (RZ) and the weight-average
molar mass (ZM) were estimated according to the procedure described in the previous section. These results are shown as bar plots in Figs. 4e6. Numerical values are included as Supplementary Information. Fig. 4 shows graphically the adjusted torque of PBAT and the blends in terms of processing temperature and composition. Since terminal adjusted torque reflects the viscosity of the melts, results will discussed in terms of viscosity. PBAT is more viscous than TPS. The viscosity of the blends was found to be higher than the viscosity of the components at low processing temperatures, and intermediate between them at the higher temperatures. This seemingly puzzling effect may be due to the complex dependence of the ternary blend viscosity on the individual components viscosities, coupled with significant differences in their temperature dependence. Note that terminal melt temperature is 22 C degrees higher than the wall temperature at low temperature (T0 ¼ 140 C), but only 12 C and 6 C at higher temperatures (T0 ¼ 170 C and T0 ¼ 200 C, respectively). However, the viscosity of the blends is virtually independent of composition and the presence or absence of additive. Terminal adjusted torque for the samples processed at the lower temperature (140 C) are around ±5% of the average value 61.0 Nm; at the higher processing temperature (200 C) torque is around ±8% of the average value 12.8 Nm. These differences did not show a significant amount of degradation or recovery. Viscosity comparisons (at least within the precision that may be achieved in a torque rheometer) are not appropriate to study the minor degradation/recovery processes taking place in the PBAT/TPS system. In order to investigate degradation and recovery we need to consider the rate of change of terminal torque. Fig. 6 shows graphically the relative rate of change of adjusted torque in terms of processing temperature and composition. Fig. 5 shows the expected higher rate of degradation of TPS compared with PBAT, and reveals that degradation is independent of temperature at low processing temperature (T0 ¼ 140 C and T0 ¼ 170 C). The effect of the chain extender additive on neat PBAT is striking: torque (hence, viscosity and molar mass) increases sharply and the increase is highly temperature dependent. The additive not only recovered the mild losses due to degradation, but actually extended PBAT chains. The rate of degradation of the
Fig. 1. Temperature (a) and torque (b) versus time during processing of a PBAT/20%TPS blend in the laboratory internal mixer at 60 rpm and three different chamber wall temperatures (indicated).
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Fig. 2. Temperature (a) and torque (b) versus time during processing of a PBAT/20%TPS blend in the laboratory internal mixer at 60 rpm and three different chamber wall temperatures (indicated).
Fig. 3. Adjusted torque versus time during the last stage of processing of the PBAT/30%TPS blend. Torque was adjusted to the chamber wall temperature T* ¼ T0 (a) and to a fixed temperature, T* ¼ 170 C (b).
blends (without additivation) is higher that the rate of degradation of any of the components, and increases significantly with TPS content, especially at the higher processing temperature. Incorporation of the chain extender resulted in a partial recovery, and is highly temperature-dependent, consistent with results previously reported for neat PBAT [10]. The effect of the additive is virtually null at low processing temperature (T0 ¼ 140 C) but is very significant at high temperature (T0 ¼ 200 C). At this temperature, the recovery significantly depends on TPS content: RZ is 47% in PBAT/ 10%TPS blends with additive compared to ones without additive (approximate 2:1 ratio), 36% in PBAT/20%TPS blends (approximate 3:1 ratio) and 16% (approximate 6:1 ratio). Joncryl is highly effective increasing the molar mass of polyesters, such as PBAT, not for polysaccharides, such as TPS. On the other hand, TPS increases the degradation of PBAT. But the more TPS in the blend, the higher the proportion of chain extension
additive relative to PBAT (for a fixed percentage of Joncryl). The combined effect of these effects may be responsible for peculiar pattern of recovery observed as a function of composition. Fig. 6 shows results of the relative molar mass decrease, Eq. (5), for the blends in terms of processing temperature and composition. As expected, RM trends follow exactly RZ trends. However, differences between different compositions and temperatures are considerably flattened, due to the high power of the viscosity dependence on molar mass. Thus, the relative rate of torque variation with time, albeit an indirect measure of molar mass change, is a better tool to study it.
4. Final considerations In this contribution, a powerful method to study polymer degradation and recovery under processing was introduced and
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Fig. 4. Mean corrected terminal torque for PBAT and PBAT/TPS blends with and without additive at three temperatures.
Fig. 5. Rate of change of terminal torque for PBAT and PBAT/TPS blends with and without additive at three temperatures. Negative values of eRZ correspond to torque (molar mass) increase due to chain extension.
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Fig. 6. Estimated rate of change of molar mass during terminal processing for PBAT and PBAT/TPS blends with and without additive at three temperatures.
applied to the technically and economically significant system of fully biodegradable blends as an example. Some interesting conclusions may be drawn for this exercise: The inability of torque (viscosity) comparisons to reveal the temperature and composition dependence of mild polymer degradation under processing conditions, and the superior performance of relative rate of torque (viscosity) changes in the terminal stage of processing for this purpose. The deleterious effect of TPS on the degradation of PBAT/TPS blends and its significant temperature dependence (and composition dependence at high temperatures). Not only TPS degrades more than PBAT during processing, but it also increases the degradation of PBAT. The effectiveness of the chain extender additive used (Joncryl) at 1% concentrations on the partial recovery of degradation on PBAT/TPS blends, particularly at high processing temperatures. Joncryl is highly effective in recovering e and actually increasing e molar mass of PBAT (a polyester), but not so for TPS (a mixture of polysaccharides). We have purposely refrained from speculating about the structural and morphological significance of the results obtained, showing instead the power of torque rheometry to reveal some quantitative facts on a macroscopic level, upon which such microscopic considerations could be built, when complemented by other observations and measurements.
Acknowledgements The authors wish to thank the Conselho Nacional de Pesquisa ~o de Aperfeiçoamento de Pessoal Superior (CNPq) e Coordenaça (CAPES), Grant # 473622/2013-0, for financial support.
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