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Chapter 6

Melt Stability of Starch-Filled LDPE during Multi-Pass Extrusion Determined by Melt-Flow and Non-Isothermal Thermogravimetric Investigations Marlene J. Cran,1 Stephen W. Bigger,*,1 Richard A. Venditti,2 Joel J. Pawlak,2 Sebastien Livi,3 and Ali Ayoub2 1Institute

for Sustainable Industries and Liveable Cities Victoria University, P.O. Box 14428, Melbourne, 8001, Australia 2College of Natural Resources, Department of Forest Biomaterials, North Carolina State University, Raleigh, North Carolina 27695, United States 3University of Lyon, INSA de Lyon, CNRS, IMP UMR 5223, Villeurbanne 69621, France *E-mail: [email protected].

The melt stability of LDPE containing 50% w/w starch/glycerol filler during multi-pass extrusion was determined by measurements of the melt-flow rate (MFR). The robustness of a polymer processing degradation index (PDI) calculation previously proposed by others in the literature is questioned and corrected before applying it to the polymer formulation under consideration. The decomposition kinetics of the LDPE component in the formulation were determined from non-isothermal thermo-gravimetric analysis of samples taken at different stages during multi-pass extrusion and the results compared with the MFR data. It was found that the PDI is a useful index to characterize melt stability and that the inclusion of the starch/glycerol filler in the LDPE imparts some melt stability to the LDPE component during multi-pass extrusion. Possible reasons for the observed stabilizing effect of the filler are explored and it is proposed that a PDI based on non-isothermal thermogravimetric measurements may be

© 2018 American Chemical Society Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

a more sensitive indicator of melt stability than a PDI that is based on MFR data.

Introduction The realization that the world’s supply of petroleum products is limited, along with increased concerns about the long-term effects of synthetic polymeric materials in the environment, much of which is waste (1, 2), has led to the search for new materials. This is particularly evident in the search for new materials that can be used for food as well as other packaging applications (3–9) and a number of different approaches have been used in this search. For example, bio-based materials that are either: (i) naturally occurring or (ii) chemically modified or further synthesized have been extensively explored in the past (10). Moreover, monomers such as lactic acid that are derived from renewable resources such as starch have been used to produce materials that have comparable physicomechanical properties to the traditional commodity polymers (11–13). Being derived from a natural source, such materials are considered to be readily degradable in the environment by microorganisms and, as such, do not inflict long-term environmental impacts (3, 14). Another approach explored in the search for new materials had originally involved the use of inert fillers in conjunction with synthetic commodity polymers to produce biodegradable composites (15). The incorporation of a filler within a polymer matrix can reduce the amount of polymer in the formulation and can deliver a material that has comparable, or in some cases superior, mechanical strength to the un-filled material at quite often a lower cost (16, 17). The various types of filler have included inorganic species (18) such as glass beads (e.g. glass-filled polypropylene (19), as well as natural materials including cereal grains (20), rice bran (21), wood flour (22), cellulose (23, 24) and starches (25–27). In more recent times, the use of naturally-derived fillers such as kenaf fibre in polymers derived from renewable resources such as poly(lactic acid) (PLA) has been explored (28–30). Amongst the first so-called “biodegradable” plastics that were developed were those comprised of commodity polyolefins that were starch-filled (31–35). These materials typically contained between 6 to 15% w/w of starch and were found to be only bio-disintegrable at very best. It was envisaged that upon starch degradation, the residual plastic material would lose structural integrity and thereby degrade more readily in the environment (36, 37). However, it was found that only the surface starch was biodegraded leaving a recalcitrant polymer matrix that remained resistant to bio degradation (31, 33). However, the incorporation of higher levels of starch into the formulation by the use of ethylene acrylic acid copolymer in place of low-density polyethylene has been found to promote significant biodegradation (38). Coupled with the development of renewable material composites containing naturally-derived fillers is the need to understand their melt processing stability, particularly with regard to the possibility of recycling those materials that are not destined for microbiological degradation in landfill in the first instance. The 116 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

measurement of melt flow rate (MFR) (39) has been widely used, often as a quality control indicator of the polymer molecular weight and its flow characteristics under extrusion conditions (40–42). The MFR (also referred to as the “melt flow index”, MFI) is the mass of polymer flowing in 10 min through a die of specific diameter and length by a pressure applied by a given weight at a given temperature (39). Tochácek and Jancár (43) have pointed out that the conditions of elevated temperatures, limited oxygen and mechanical shear stress experienced by polymer melts during extrusion, even under short residence times, can induce chemical changes in the polymers. These changes, in turn, can significantly affect the properties of the extruded product (44–47). It has been further suggested (43) that laboratory-based, multiple extrusion experiments serve as a useful tool in simulating commercial production conditions and thereby enable polymer processing information to be conveniently acquired using relatively low quantities of material (44, 45, 48–50). In such experiments, it is usual to plot the MFR as a function of the number of extrusions and obtain a “processing curve” from which information on the extent of polymer degradation upon extrusion can be determined. To facilitate a parameter derived from such data that serves as a single indicator of processing stability a so-called “processing degradation index” (PDI) has been proposed (41). Among the many other techniques used to determine polymer stability is that of thermogravimetric (TG) analysis typically performed under non-isothermal conditions (51–56). A recent contribution to this field has been the development of an iterative computer-based analysis technique that enables the kinetic analysis of non-isothermal TG data without the invocation of approximations traditionally used and necessary to solve the associated kinetics equations (57). In particular, the technique was used to investigate a PLA-kenaf composite where it was found the addition of the natural kenaf fibre to the polymer decreased the apparent activation energy for decomposition of the composite when the material was degraded in a nitrogen atmosphere (57). In view of the need and emergence of novel polymer composites that are based on renewable and/or natural resources, as well as the importance of developing new test methods that lead to simplified material characterization parameters that apply to such materials the following work was conducted. It involved the close examination of the proposed PDI (43) and its applicability to a multi-pass extruded, starch-filled, low-density polyethylene (LDPE) formulation as well as the investigation of the material using the iterative numerical TG analysis technique (57). The possible correlation between the PDI and apparent activation energy for decomposition under nitrogen, derived from TG analysis, was also investigated.

Experimental Section Materials Low-density polyethylene (LDPE) was purchased from Sigma Aldrich (CAS No. 9002-88-4) with the following properties: melt index 25 g/10 min (190°C, 2.16 kg), impact strength 45.4 J m-1 (Izod, ASTM D 256, -50°C), melting point 117 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

116°C, Vicat softening temperature 93°C (ASTM D 1525), density 0.925 g mL-1 at 25°C. A native corn starch was purchased from Cargill Incorporated and was comprised of 73% amylopectin and 27% amylose. Multi-Pass Extrusion Samples of neat LDPE and a 50% w/w composite of LDPE with starch that was pre-plasticized with 30% w/w glycerol (LDPE-starch/glycerol composite) were subjected to multi-pass extrusions using a 15 g capacity DSM micro-extruder (Midi 2000 Heerlen, The Netherlands) with two co-rotating conical screws. The formulations were made by extruding LDPE pellets at 120°C and 250 rpm. Five grams of LDPE was injected into the extruder each time and was circulated for 5 min before extruding. The extrudate was then cut into small pellets and, in the cases of multi-pass extrusion, the pellets were injected and extruded again under the same conditions. For samples containing starch plasticized with glycerol, the starch was first premixed with 30% w/w of glycerol at room temperature and conditioned for 24 h to achieve complete mixing. Five grams of LDPE was then injected into the extruder with the extruder in circulating mode, and then 5 g of the premixed starch/ glycerol was immediately added into the extruder. The mixture was circulated for 5 min before extruding. The samples were then cut into pellets for re-extruding. The “pass 0” designates the processing step where the material was transformed from a blend into a molten homogeneous melt in the extruder (residence time = 5 min). The “pass 1” designates the re-extrusion of “pass 0” material; “pass 2” designates the re-extrusion of “pass 1” material, and so on, up to a maximum of 4 re-extrusion passes through the extruder (i.e. “pass 4” material). MFR Experiments The melt flow rate (MFR) measurements were performed in accordance with ASTM method D-1238-04 (190°C, 2.16 kg). Thermal Decomposition Kinetics The thermal decomposition kinetics of the materials was determined using thermogravimetric analysis (TGA) where the samples were decomposed under a nitrogen atmosphere in a Mettler Toledo TGA instrument (TGA/DSC 1 Star System). In these analyses, the samples were heated from 50 to 600°C at a heating rate of 5°C min-1 and under a nitrogen flow rate of 0.2 L min-1. Such non-isothermal techniques for determining kinetic parameters associated with the thermal decomposition of materials has been long established (52). The choice of performing the experiments under nitrogen was made to simulate those conditions encountered during extrusion and the MFR experiments where exposure of the melt to oxygen is restricted. As only the trends in the kinetic parameters were to be investigated, measurements were made at a single heating rate. 118 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

In the current study, the data obtained from the TGA instrument were kinetically analyzed using two computer-based algorithms described elsewhere (57) for the processing of thermogravimetric (TG) data obtained during the degradation of polymeric samples under non-isothermal conditions. The first algorithm identifies the most likely kinetic models to which the experimental TG data can be fitted and where 15 common kinetic models are considered. The second algorithm utilizes an iterative arithmetic method to extract the apparent activation energy from the TG data. In relation to the latter, the software firstly determines the solution of the following equation pertaining to TG analysis:

where α is the degree of conversion at time t in the process, g(α) is a function of α as defined by the kinetic model, A is the Arrhenius A-factor, Ea is the apparent activation energy for the process, R is the ideal gas constant and β is the heating rate. The function p(x) represents the integral:

where x = Ea/RT and T is the absolute temperature. From the solution to Equation (1) a value of the apparent activation energy can be obtained.

Color Measurement Photographs of samples of the extruded LDPE and LDPE-starch blends were taken under identical conditions. Sections of the photographs were extracted and converted to 24-bit bitmap images that measured approximately 20 × 40 pixels. To determine differences in color between the samples, the red (R), green (G) and blue (B) color components of each pixel were determined (58) using an iterative macro programmed in Microsoft Excel 2016. To obtain the RGB value, the values of R, G×256, and B×256×256 were calculated and summed (59).

Scanning Electron Micrography Samples of the LDPE and LDPE-starch blend extrudate were prepared for imaging by cutting thin slices with carbon steel injector blades. Some samples were then placed in a furnace at 250°C and others were heated to 250°C under nitrogen in the Mettler TGA instrument. Samples were mounted on aluminium stubs with double-sided conductive carbon tape and were then coated with iridium using a Cressington 208HR sputter coater. Once coated, the samples were imaged using a Hitachi TM3030 Plus Tabletop SEM using an accelerating voltage of 15 kV.

119 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Results and Discussion Processing Degradation Index The following equation has been derived and proposed as a quantitative measure of the processing stability of polypropylene; it has been termed a “processing degradation index” or “PDI” (43):

where MFR0 is the melt flow rate (MFR) after the compounding (zero) extrusion; MFR1 to MFRn are the melt flow rates after the “degradation” extrusions and n is the number of “degradation” extrusions (i.e. passes through the extruder after the initial processing pass). Careful inspection of the derivation of Equation (3) as published in the original paper reveals it to be flawed. The diagram presented in Figure 1 can be used to derive the correct equation. The proposed PDI is the ratio of areas A and B:

where area A is the total area above the total “baseline area”, (i.e. area B). It can be readily seen from Figure 1 that, if the increment in the number of extruder passes between the successive MFR observations is +1, the area, An, above the corresponding “baseline area” bounded by the (n – 1)th and nth extruder passes is given by:

Figure 1. Schematic representation of the melt flow rate (MFR) as a function of the number of extruder passes. 120 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Hence, the correct equation for the PDI index is given by:

The question remains as to how serious an error is invoked if Equation (3) is used to calculate the PDI instead of the correct Equation (6). It is found that the severity of the error depends on the nature of the variation in the MFR with the number of extruder passes. In particular, some simple modelling of this has shown that: (i) for one extruder pass or (ii) for a system where the MFR is unity and increases linearly with the number of extruder passes, then Equations (3) and (6) produce identical results. However, such a case as the latter is highly idealized and is unlikely to be encountered in practice. Instead, the variation in MFR with the number of extruder passes is usually non-linear with an increment that can, in some cases, increase non-linearly between passes as represented in Figure 1. Such behaviour is also seen in some of the experimental results presented by the proponents of the PDI (43). In cases where the MFR does not increase linearly with the number of extruder passes the estimated variation between PDI values calculated by Equations (3) and (6) can be significant. Figure 2 shows the percentage error that is invoked by the application of Equation (3) as a function of the number of extruder passes for a system whose MFR0 is unity and increases non-linearly with the number of extruder passes n in accordance with the simple equation:

The error is clearly not consistent and is near its maximum value at n = 5 which is the number of extrusion passes utilized by the proponents of the index in their experimental work leading to its proposal. The maximum error of ca. 7% is unacceptably high and may even be higher in other systems where the variation in MFR with the number of extruder passes is more pronounced than that described by Equation (7). Furthermore, the non-linear variation of the deviation between the PDI calculated by Equation (3) and the correct value (calculated using Equation (6)) may prevent a meaningful comparison of PDI values obtained at different extruder passes within the same sample set. Hence, for these reasons it is recommended that Equation (3) not be used to calculate PDI values and that Equation (6), which provides the correct route to this index, be used instead. Figure 3 shows plots of the MFR as a function of the number of extruder passes for the LDPE base material and the 50% w/w LDPE-starch/glycerol composite studied in the current work. The data reveal, at first inspection, a very large overall difference in the melt viscosities of the two formulations with the starch-filled material having a much lower MFR (i.e. higher melt viscosity) than the LDPE. A reduction in the MFR upon the addition of starch plasticized with glycerol (60) to polyethylene formulations has been observed elsewhere (61, 62). In the case of the neat LDPE sample (Figure 3) the MFR appears to reach an upper limit after about two extruder passes. This may be due to crosslinking that causes a persistence in the molecular weight of the polymer upon its continued thermomechanical processing (63) when the amount of 121 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

low-molecular weight material that acts as a plasticizer has been reduced due to volatilization, degradation, etc. (64). There is also some evidence of the MFR reaching an upper limit in the case of the starch-filled material after three extruder passes, again presumably due to cross-linking effects. During melt processing the predominance of such crosslinking over chain scission must take into account the vinyl, vinylidene and possibly t-vinylene concentrations in the polymer as well as the temperature and relative oxidation rate (65) and so it is difficult to state a precise reason for the observed persistence in the molecular weight.

Figure 2. Percentage error invoked in the use of Equation (1) to calculate the processing degradation index, PDI. The PDI of the LDPE calculated according to Equation (6) is 18.2 whereas when calculated using Equation (3) the index is 16.9; the latter equation rendering an unacceptably high difference of 7.1%. The PDI’s of the starch-filled LDPE calculated using Equations (6) and (3) are comparable at 17.9 and 17.6 respectively (i.e. 1.7% difference). The latter result exemplifies the case mentioned above where the two equations produce similar results for systems where the MFR increases almost linearly with the number of extrusion passes. Moreover, the PDI’s of the LDPE and starch-filled LDPE are similar when calculated using Equation (6) with a difference of only ca. 2%. This suggests that these two formulations have similar melt stabilities during processing with the stability of the starch-filled material being slightly higher than that of the LDPE. Whence, the addition of the plasticized starch to the LDPE significantly affects its melt viscosity but does not affect its PDI to any significant extent. It is interesting to note that the use of incorrect Equation (3) to calculate the two PDI’s leads to the incorrect conclusion that the melt stability of the LDPE is slightly higher than that of the starch-filled material whereas the melt stabilities calculated using the correct Equation (6) suggest that the relative melt stabilities are in the reverse order. This, again, highlights the danger of using the incorrect equation to calculate the PDI and subsequently infer the melt behaviour on the 122 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

basis of the result. The slightly higher melt stability of the starch-filled material as revealed by the correct PDI value (Equation (6)) may be due to the presence of the glycerol plasticizing agent that reduces to some extent the thermo-mechanically induced chain scission reactions (44) that would normally occur in the LDPE and starch components of the composite. This, in turn, may be responsible for the MFR of the starch-filled material rising much less abruptly at low extruder passes compared with the LDPE material. Nonetheless, the precise reason for the observed order of the overall melt stabilities, even if these are deemed to be significantly different, cannot be conclusively deduced from the results of the present study.

Figure 3. Plots of MFR for: (a) LDPE (filled circles) and (b) 50% w/w LDPE-starch/glycerol composite (open circles) as a function of the number of extruder passes. In addition to possible crosslinking processes, the persistence in the molecular weight of the LDPE upon thermomechanical processing (see Figure 3) may be due to the presence of a stabilizing package in the base polymer. Evidence for this is that the observed magnitude of the PDI of this material is comparable to that observed for phosphite-stabilized polypropylene (43). Nonetheless, it has been pointed out that the PDI is a relative parameter that is not simply transferable and it depends on the type of processing equipment, screw configuration, screw rotation speed, temperature and the number of extrusions (43). Thermogravimetric Analysis Figure 4 shows plots of the degree of conversion, α, versus temperature for the LDPE and starch-filled LDPE materials for both the “pass 0” and “pass 4” runs. The initial steps between ca. 100°C and 350°C for the starch-filled material corresponds to the degradation of the starch/glycerol component of the composite and corresponds to about 50% of the total mass of the samples, as expected. The 123 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

mass loss at temperatures greater than 350°C in all systems is attributed to the decomposition of the LDPE base polymer. In the case of the neat LDPE, the TGA results indicate that the thermomechanical treatment during multi-pass extrusion has resulted in a slight shift in the TG profile towards higher temperatures. This is consistent with the notion that crosslinking has occurred to some extent during processing which renders the material in a form that does not decompose as readily and volatilize during the TGA experiment as does the less processed material. In the case of the starch-filled LDPE, the thermomechanical treatment (see “pass 4” profile) has clearly decreased the starch/glycerol content by ca. 8% compared with the “pass 0” material as indicated by the lowering of the plateau at ca. 300°C. Due to the two different plateau positions of the starch-filled LDPE materials (see Figure 4) it is difficult to determine from the figure if the thermomechanical treatment has produced a shift in the TG profile of the LDPE component towards higher temperatures, as is more clearly the case for the neat LDPE material. Consequently, a direct comparison of the relative stabilities of the LDPE in the neat LDPE and starch-filled LDPE samples upon thermomechanical treatment cannot be readily made from the data presented in Figure 4. A further analysis must therefore be considered if these are to be compared. Such an analysis is shown in Figure 5 where the normalized TG profiles of the neat LDPE (control) and the LDPE component of starch-filled LDPE between 350 and 550°C are shown for comparison.

Figure 4. Plots of the degree of conversion, α, versus temperature for multi-pass extruded pass 0 (control) and pass 4 samples of LDPE and 50% w/w LDPE-starch/glycerol composite.


It is clear from the data in Figure 5 that the neat LDPE material undergoes a greater shift in its TG profile upon thermomechanical treatment than does the LDPE component of the starch-filled LDPE. This suggests the stability of LDPE in the starch-filled material has been enhanced in some way by its incorporation with the starch/glycerol filler. The observed enhancement of the LDPE stability may be due to the presence of fine particles of carbon black that are produced from the pyrolysis of the starch as the thermal stabilizing properties of carbon black in polyolefins such as polyethylene, under certain conditions, have long been established in the literature (66–71). Although the TG profiles of the starch-filled material (see Figure 4) show that the starch/glycerol component is almost completely degraded and lost from the composite only at temperatures above ca. 350°C that far exceed those encountered under extrusion conditions, there is still appreciable loss of the starch/glycerol component through decomposition well below this temperature. It is conceivable that the additional application of mechanical shear forces encountered in the extruder will also contribute to the degradation of the starch/glycerol phase at temperatures lower than 350°C (72). The notion that carbon black may be produced as one of the degradation products of the starch/glycerol component is also consistent with the decrease in the level of this phase upon multi-pass extrusion (see Figure 4). The data collected during the non-isothermal TGA experiments were subjected to kinetic analyses using the two computer-based algorithms discussed above (57) and a summary of the results is presented in Table 1. Of the 15 common models that are widely used to analyze TG kinetic data the results presented in Table 1 suggest that the first-order (F1) and contracting volume (R3) models consistently fit both the LDPE base material (control) and LDPE component of the starch-filled material across the range of extruder passes that were performed. Such an observation can be made by inspection of the values of the kinetic fit parameter, ρ, listed in the main body of the table. The nature of this parameter is such that it ranges between zero and unity where a value of unity implies a perfect fit to the given kinetic model (57). No significant difference was observed between the overall fits (i.e. fits to both the LDPE and starch-filled materials) to the first-order model (F1, average fitting factor, ρave = 0.971 ± 0.014) and the contracting volume model (R3, ρave = 0.953 ± 0.013). The three-dimensional diffusion model (D3, ρave = 0.986 ± 0.004) produced the best fit of the data but this model did not produce a convergence to an apparent activation energy that was less than 400 kJ mol-1; the reason for this is unclear at present and requires further investigation. The goodness of fit of the data to the F1 model, across the range of samples tested, is demonstrated in Figure 6 that shows selected plots of the function g(α) versus the function p(x) in accordance with Equation (1). When plotted against each other these two highly non-linear functions should produce a straight line that passes through the origin if an appropriate kinetic fit to a given model has been achieved. Clearly, the appropriateness of the kinetic model in fitting the data and the quality of the fit will be reflected in the linearity of these plots and the closeness of the intersection of the straight line to the origin. The linear regression coefficients, R2, for the various plots of g(α) versus p(x) are also given in Table 1. 125 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

The data in Table 1 indicate that the apparent activation energy, Ea, determined under kinetic model F1, for the thermal decomposition of the LDPE in the starchfilled pass 0 (control) material is greater than that of the LDPE pass 0 (control) material. These data suggest that the LDPE (control) in the starch-filled material is thermally more stable than that in the LDPE (control) and is consistent with the inferences made through the PDI values calculated in accordance with Equation (6) above as well as with the TG data discussed above in relation to Figure 5. The Ea values for these samples calculated under the R3 model (see Table 1) also lead to a similar conclusion.

Figure 5. Thermogravimetric profiles for (a) the multi-pass extruded LDPE pass 0 (control) and pass 4 samples and (b) the pass 0 (control) and pass 4 LDPE component of the 50% w/w LDPE-starch/glycerol composite.


Figure 7 shows plots of the apparent activation energy for the thermal decomposition of the LDPE and the LDPE component of the 50% w/w LDPE-starch/glycerol composite as a function of the number of extruder passes. When compared with the MFR plots in Figure 3 it is clear that the apparent activation energy data adhere to similar trends to those observed for the MFR data. In the case of Figure 7 the trends may, once again, be attributed to the effects of thermomechanical processing on the materials, namely those effects resulting in the crosslinking of the LDPE base polymer and the possible production of thermal melt stabilizing by-products of the decomposition of the starch/glycerol component such as carbon black. The TGA experiments in the current study determine the kinetics of the non-isothermal decomposition of the LDPE component in the samples. The rate of decomposition and subsequent volatilization of the polymer under the conditions of the test will therefore reflect to a significant extent: (i) the degree of crosslinking in the polymer caused by its previous thermo-mechanical history and (ii) the overall stability of the melt, in much the same way, perhaps, as do the MFR measurements.

Figure 6. Plots of the functions g(α) versus p(x) for thermal degradation of: (a) multi-pass extruded LDPE pass 1 (control) and pass 4 samples and (b) the pass 0 (control) and pass 2 LDPE component of the 50% w/w LDPE-starch/glycerol composite. 127 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Table 1. Kinetic model fits and associated parameters for the TG analysis of LDPE and a 50% w/w LDPE-starch/glycerol composite Kinetic Fit Parameter, ρ No. extruder passes, LDPE 0

1

2

4

0

1

2

4

Pn Power law

0.151

0.128

0.209

0.189

0.101

0.083

0.101

0.139

E1 Exponential law

0.521

0.522

0.522

0.523

0.521

0.521

0.521

0.522

A2 Avrami-Erofeev

0.844

0.860

0.857

0.867

0.873

0.878

0.875

0.886

A3 Avrami-Erofeev

0.835

0.823

0.860

0.855

0.841

0.840

0.841

0.868

A4 Avrami-Erofeev

0.068

0.044

0.133

0.117

0.051

0.036

0.052

0.103

B1 Prout-Tompkins

0.142

0.122

0.195

0.183

0.137

0.131

0.138

0.185

0.940

0.955

0.962

0.964

0.922

0.918

0.924

0.928

0.947

0.968

0.966

0.977

0.940

0.935

0.942

0.945

D2 Two dimensional

0.765

0.767

0.811

0.804

0.735

0.725

0.737

0.761

D3 Three dimensional

0.955

0.953

0.957

0.953

0.935

0.930

0.935

0.931

D4 Ginstling-Brounshtein

0.989

0.992

0.972

0.985

0.991

0.985

0.991

0.986

Kinetic Model

Acceleratory

Sigmoidal

No. extruder passes, LDPE-Starch Blend

128 Deceleratory Geometrical

R2 Contracting area R3 Contracting volume

Diffusion

D1 One dimensional

Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

Kinetic Fit Parameter, ρ No. extruder passes, LDPE 0

1

2

4

0

1

2

4

0.912

0.918

0.897

0.899

0.901

0.900

0.901

0.885

F2 Second order

0.956

0.959

0.954

0.959

0.983

0.989

0.983

0.988

F3 Third order

0.846

0.850

0.844

0.850

0.880

0.889

0.880

0.887

0.569

0.578

0.557

0.568

0.617

0.629

0.617

0.618

Apparent Ea/kJ mol–1

261

304

299

298

313

317

320

326

R2 for g(α) vs p(x) plot

0.9912

0.9921

0.9944

0.9938

0.9993

0.9997

0.9989

0.9998

Apparent Ea/kJ mol–1

240

263

274

301

261

260

257

276

R2 for g(α) vs p(x) plot

0.9991

0.9974

0.9999

0.9997

0.9974

0.9894

0.9818

0.9949

Kinetic Model

Reaction Order

F1 Model Fit

129 R3 Model Fit

No. extruder passes, LDPE-Starch Blend

F1 First order


If it is assumed that the apparent activation energies for the decomposition of the LDPE as determined in the TGA experiments reflect the melt stability in a way similar to the MFR measurements then it may be proposed that a form of “processing degradation index” can be calculated using the Ea values as well (i.e. PDI′). When applied to the data plotted in Figure 7 (see also Table 1). The melt stabilities as revealed by the calculated PDI′ values are such that the stability of the starch-filled material (PDI′ = 2.2) is significantly greater than the base LDPE (PDI′ = 13.2); the relative order of stabilities being consistent with that deter-mined from the MFR data processed in accordance with Equation (6). It is interesting to note, however, that the TG kinetic analysis appears to be much more sensitive a technique in distinguishing the differences between the two melt stabilities compared with the MFR data analysis. Nonetheless, this requires further investigation in the future.

Figure 7. Plots of the apparent activation energy, Ea, obtained from a kinetic analysis under the first-order (F1) model for the non-isothermal decomposition of: (a) LDPE (filled circles) and (b) the LDPE component of 50% w/w LDPE-starch/glycerol composite (open circles) as a function of the number of extruder passes.


Color and Imaging Color changes during the multi-pass extrusion of polymers is a common phenomena that is attributed to the formation of chromophores (73). This is reflected in Figure 8 which shows the RGB value as a function of the number of extruder passes for LDPE and the LDPE-starch blends. In the case of LDPE, the extrudate samples were opaque white with negligible color changes between processing steps. In the case of the LDPE-starch blends, the appearance was pale yellow and there was some evidence of color change attributed to darkening of the starch-filled material upon continued processing (see Figure 8). The thermal decomposition of starch can result in the formation of a range of carbon forms depending on the temperature (74). This was evident by the significantly greater amount of carbon residue left behind in the sample pan upon the completion of the TGA experiments for the starch-filled material is compared with no residue for the neat LDPE material. Imaging of the samples by SEM revealed the presence of starch particles in the LDPE-starch blends as shown by the arrows in Figure 9. The multi-pass extrusion of the blends improved the dispersion of the starch throughout the LDPE matrix with a concurrent reduction of size of the starch particles (72, 75). The presence of smaller starch particles during the TGA experiment may contribute to the formation of a stable form of carbon analogous to carbon black which is a known stabilizer (66, 68, 69). This may have contributed to the improved thermal stability as evidenced by the higher apparent activation energies obtained from the TGA data.

Figure 8. RGB color values of the multi-pass extruded samples of LDPE (white bars) and LDPE-starch blends (gray bars).


Figure 9. SEM images showing the distribution of starch particles in the LDPE-starch blends. Scale bars are 100 μm.

Conclusions The PDI as originally proposed by others in the literature has been corrected and successfully applied to determine the melt stability of starch-filled LDPE relative to the base LDPE polymer. The PDI data suggest that the relative melt stabilities of the two materials are similar with the starch-filled material being slightly more stable than the base polymer. The PDI results were confirmed by the kinetic analysis of non-isothermal thermogravimetric data obtained for the materials at different stages during multi-pass extrusion. The kinetic data conform to first-order and the contracting volume models, the former model found to give a slightly better fit to the data. The resulting apparent activation energies for the decomposition of the LDPE component of the materials reflect the MFR data used to determine the PDI. As such it can be proposed that a PDI based on the apparent activation energy values can be calculated and it appears that the index calculated in this way is a more sensitive measure of melt stability than that which relies on MFR data. The study has also indicated that the incorporation of the starch/glycerol filler in the LDPE has an apparent stabilizing effect on the LDPE component in the melt during multi-pass extrusion. Although the reasons for this are still unclear at present it is proposed that the decomposition of the starch/glycerol component may produce by-products, amongst which is carbon black that may contribute to the observed stabilizing effect. The study has therefore highlighted the need to explore the PDI and its measurements by more sensitive means than MFR determinations 132 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.

in the future study of the melt stabilities of polymer blends with natural fillers during multi-pass extrusion.

Acknowledgments The authors would like to thank Mr Yuhan Wang from North Carolina State University, Department of Forest Biomaterials for his assistance in the production of the extrudate samples.

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